The NOx storage-reduction on PtK/Al2O3 Lean NOx Trap catalyst

Journal of Catalysis 276 (2010) 335–350

Contents lists available at ScienceDirect

Journal of Catalysis

journal homepage: www.elsevier.com/locate/jcat

The NO storage-reduction on PtAK/Al O Lean NO Trap catalyst

x

2

3

x

a

a,

⇑

a

b

L. Castoldi , L. Lietti , P. Forzatti , S. Morandi , G. Ghiotti , F. Vindigni

a

Dipartimento di Energia, Laboratory of Catalysis and Catalytic Processes and NEMAS, Centre of Excellence, Politecnico di Milano, P.zza L. da Vinci 32, Milano, Italy

Dipartimento di Chimica I.F.M. and NIS Centre of Excellence, Università di Torino, via P. Giuria 7, 10125 Torino, Italy

b

a r t i c l e i n f o

a b s t r a c t

Article history:

Received 24 May 2010

Revised 17 September 2010

Accepted 25 September 2010

Available online 4 November 2010

2 3

The nature of stored NO and mechanistic aspects of the reduction of NO stored over a model PtAK/Al O

x

2 3

catalyst sample are investigated in this paper, and a comparison with a model PtABa/Al O catalyst is also

made.

It is found that at 350 °C on both the catalysts the storage proceeds with the initial formation of nitrites,

followed by the oxidation of nitrites to nitrates. A parallel pathway involving the direct formation of

nitrates species is also apparent; at saturation, only nitrates are present on the catalyst surface over both

Keywords:

PtAK/Al

2

O

3

and PtABa/Al

2 3

O . However, whereas bidentate nitrates are present in remarkable amounts on

2 3

PtAK/Al O catalyst

2

3

, along with ionic nitrates, only very small amounts of bidentate nitrates were observed on

NO reduction mechanism

x

PtABa/Al

2 3

O .

NSR catalysts

LNT systems

Under nearly isothermal conditions, the reduction of the stored NO with H occurs via an in series two-

x 2

Temperature Programmed Surface Reaction

FT-IR spectroscopy.

steps Pt-catalysed molecular process involving the formation of ammonia as an intermediate, like for the

PtABa/Al O catalyst sample. However, higher N₂selectivity is observed in the case of the PtAK/Al O₃

2

3

2

catalyst due to the similar reactivity of the H

nia, once formed, readily reacts with surface nitrates to give N

reduction process to N . Notably, a strong inhibition of H on the reactivity of NH

pointed out, due to a competition of H and NH for the activation at the Pt sites. Finally, the effect of

water and CO on the reduction process is also addressed. Water shows a slight promotion effect on

the reduction of the nitrates by H , and no signiﬁcant effect on the reduction by ammonia, whereas

CO has a strong inhibition effect due to poisoning of Pt by CO formed upon CO hydrogenation.

2

+ nitrate and NH

3

+ nitrate reactions. Accordingly ammo-

2

, and this drives the selectivity of the

towards nitrates is also

2

3

2

3

2

1

. Introduction

Toyota in the mid 90s [1], works in a cyclic manner [2]. Long lean

phases, during which NO are stored on the catalyst, are alternated

with short rich periods during which the exhaust is deliberately

made rich to reduce the trapped NO to N , thereby regenerating

the catalyst. For vehicle applications, the trapping or lean phase

is typically 60–90 s long, while the regeneration or the rich phase

is of the order of few seconds. NSR catalysts are composed of a high

x

The transportation sector, and in particular diesel-equipped

vehicles, represents a primary source of NO emissions. For this

reason, regulations to control NO emissions are becoming very se-

vere: in Europe, the next Euro 6 (2014) regulations require a dras-

tic decrease of NO emission for diesel passenger cars, from the

x

2

x

current 0.18 g/km set by Euro 5 down to 0.08 g/km.

surface area support material (often

c-alumina or stabilized alu-

Three Way Catalytic (TWC) converters currently used for stoi-

chiometric gasoline engines are not effective in the reduction of

mina), precious metals (usually a combination of Pt and Rh) and

a basic component, such as Ba or K phases, which acts as NO

age material [5,6].

x

stor-

NO

solutions for the control of NO

nique, which accomplishes the NO

precursor of NH ) in the exhaust gases, and the NO

tion (NSR) or Lean NO Trap (LNT) system [1–3].

x

for Diesel engines, which work under lean conditions. Viable

in this case are the urea-SCR tech-

reduction by injecting urea (a

Storage Reduc-

x

Several studies concentrated on the reactivity and characteris-

tics of Ba-containing catalysts [7–13], but only few reports on

the speciﬁc behaviour of K-based catalysts have been published

in the literature. Toops and co-workers analysed by in situ DRIFT

x

3

x

Although the urea-SCR technology is preferred for heavy-duty

vehicles and minivans, LNTs have been speciﬁcally developed for

small engines [4]. The NSR technology, at ﬁrst introduced by

the NO

x

storage on PtAK/Al

2

O

3

and the effect of H

2 2

O and CO on

this step [14,15]. They concluded that NO

x

is stored primarily in

the form of ionic nitrates on the K component; nitrites are also ob-

served at low temperatures (below 200 °C), for short adsorption

times [16]. The nature of stored species (i.e. nitrites and nitrates)

and the proposed adsorption mechanism parallel those already de-

⇑

Corresponding author. Fax: +39 02 7063/8173.

E-mail address: luca.lietti@polimi.it (L. Lietti).

2 3

scribed in the case of PtABa/Al O catalysts [11,17].

0

doi:10.1016/j.jcat.2010.09.026

3

36

L. Castoldi et al. / Journal of Catalysis 276 (2010) 335–350

Investigations on the reduction of NO

catalysts are also rather scarce. On the other hand, in recent papers

of various groups, mechanistic aspects of the reduction of NO

stored over Ba-based NSR catalytic systems have been reported

18–21]. Some of us previously showed that the reduction of stored

nitrates by hydrogen under nearly isothermal conditions does not

involve the thermal decomposition of the adsorbed NO species as

a preliminary step [22,23], but instead occurs via a low tempera-

ture Pt-catalysed route. In particular, it has been suggested that

the regeneration of Ba-based NSR catalysts proceeds exclusively

according to a two-steps in series molecular pathway in which

the ﬁrst step is the fast reaction of hydrogen with nitrates produc-

x

stored on K-containing

powder has been dried at 80 °C and calcined in air at 500 °C for

5 h after each impregnation step [10]. The impregnation order

(ﬁrst Pt and then K) has been selected in order to ensure a good dis-

persion and stability of the noble metal and of the alkaline compo-

nent on the alumina support, in line with recipes of Toyota patents

[28].

The prepared catalyst sample has been characterized by XRD,

HRTEM, surface area, pore size distribution and Pt dispersion

measurements. XRD diffraction spectra have been recorded with

a Philips PW 1050/70 diffractometer equipped with a vertical goni-

x

[

x

ometer using a Ni-ﬁltered Cu Ka radiation. HRTEM measurements

were performed using a side entry Jeol JEM 3010 (300 kV) micro-

ing ammonia, followed by a slower reaction of NH

species downstream to give N [24–26]. The occurrence of such a

two-steps in series pathway, along with the development of an

front travelling in the reactor [24,27], is able to account for

3

with nitrate

scope. Surface area and pore size distribution have been deter-

mined by N adsorption–desorption at 77 K with the BET method

2

using a Micromeritics TriStar 3000 instrument. The Pt dispersion

has been estimated by hydrogen chemisorption at 0 °C, upon

2

H

2

the temporal sequence of products obtained at the reactor exit

during the regeneration of NSR systems, with nitrogen being ini-

tially produced with very high selectivity, followed by ammonia

evolution.

reduction in H

Instrument.

2

at 300 °C, using a TPD/R/O 1100 Thermo Fischer

2.2. Catalytic tests

The work here presented focuses on the analysis of the reduc-

tion steps when K replaces Ba as the storage component and

hydrogen is used as a reductant. In particular, this study addresses

the speciﬁc role of potassium in the reduction chemistry and spe-

ciﬁcally on the occurrence of (i) the Pt-catalysed route previously

Catalytic tests have been performed in a micro-reactor consist-

ing of a quartz tube (7 mm I.D.), loaded with 60 mg of catalyst

3

powder (70–100

l

m). A total ﬂow of 100 cm /min (at 1 atm and

0 °C) has been used in the experiments. The reactor was directly

suggested for the reduction of stored NO

and (ii) the in series two-steps molecular pathway involved in N

formation [25]. For this purpose, the reduction with H of NO

stored over a model PtAK/Al catalyst has been analysed by

combined microﬂow transient reactivity experiments and FT-IR

spectroscopy. Accordingly, NO have been adsorbed on the catalyst

surface at 350 °C starting from NO/O ; then the reactivity of the

stored NO with H has been investigated by means of Tempera-

x

in Ba-containing samples

connected to a mass spectrometer (Thermostar 200, Pfeiffer), an

UV-NO analyzer (LIMAS 11HW) and a micro-gas chromatograph

x

for analysis of the reaction products [10,11,29].

Prior catalytic activity runs, the catalyst sample has been condi-

tioned by performing few storage/regeneration cycles. For this pur-

2

x

2

O

3

x

pose, NO

step feed of NO (1000 ppm) in ﬂowing He + 3% v/v O

saturation. Then the NO and O concentrations have been stepwise

decreased to zero, followed by a He purge at the same temperature

(350 °C). This leads to the desorption of weakly adsorbed NO spe-

cies. After the He purge, catalyst regeneration (rich phase) has been

carried out with H (2000 ppm in He). Conditioning lasted until a

x

have been adsorbed at 350 °C by imposing a rectangular

2

until catalyst

x

2

ture Programmed Surface Reaction experiments (TPSR) and Iso-

thermal Step Concentration experiments (ISC). The reduction has

been carried out under nearly isothermal conditions with limited

x

concentration of H

with release of NO

2

to prevent thermal decomposition of nitrates

in the gas phase during the lean-rich switch.

2

x

reproducible behaviour was obtained; this typically required 3–4

adsorption/reduction cycles.

x 3

The reduction of the stored NO with NH has also been considered

to clarify the potential role of this species as intermediate in N

formation.

In parallel with reactivity experiments, FT-IR spectroscopy was

employed to obtain information about the nature, reactivity and

evolution of surface species. Experiments have been accomplished

2

After catalyst conditioning at 350 °C, Temperature Programmed

Surface Reaction (TPSR) and Isothermal Step Concentration (ISC)

experiments have been performed.

x

(a) TPSR. In a typical TPSR experiment, after NO adsorption at

in the presence and in the absence of water and of CO

the effect of these species on the reduction mechanism and on the

reactivity of the stored NO . The results obtained over the PtAK/

Al catalyst sample in this study have been compared with those

collected in the case of a correspondent PtABa/Al catalyst [25]

in order to highlight possible similarities and differences.

2

, to address

350 °C followed by a He purge at the same temperature,

the catalyst has been cooled to room temperature (RT) under

x

He ﬂow. Then a rectangular step feed of H

2

(2000 ppm in He)

(1000 ppm in He) has been admitted to the reactor at

- and NH -TPSR, respectively) and the catalyst tem-

2

O

3

or NH

RT (H

3

2

O

3

2

3

perature has been linearly increased to 500 °C (heating rate

0 °C/min), while monitoring the concentration of the prod-

1

ucts exiting the reactor.

2

. Experimental

This procedure leads to a complete removal of the

stored NO

the amounts of NO

x

,

as pointed out by the comparison of

adsorbed during the lean phase

2.1. Catalysts preparation and characterization

x

and the amounts of the N-containing species evolved during

the TPSR run.

(b) ISC. In a typical Isothermal Step Concentration experiment

A homemade PtAK/Al

used in this study. This sample has molar amount of K comparable

to that contained in the PtABa/Al (1/20/100 w/w) model cata-

lyst previously used for similar studies [25] (0.146 mol K or Ba/

00 g of Al ).

The catalytic system has been prepared by incipient wetness

2 3

O (1/5.4/100 w/w) catalyst has been

2

O

3

(ISC), after NO

NO stored species has been carried out at selected temper-

atures, in the range 100–350 °C. Accordingly, after NO stor-

age at 350 °C followed by He purge at the same

temperature, the catalyst temperature has been set at the

desired value and a rectangular step feed of H (2000 ppm

in He) or NH (1000 ppm in He) has been admitted to the

reactor, thus leading to the reduction of the stored NO . At

x

adsorption at 350 °C, the reduction of the

x

1

2

O

3

x

a

impregnation of a commercial alumina sample (Versal 250 from

UOP) with aqueous solutions of dinitro-diammine platinum

2

(

Stream Chemicals, 5% Pt in ammonium hydroxide) and subse-

3

quently with a solution of potassium acetate (Aldrich, 99%). The

x

L. Castoldi et al. / Journal of Catalysis 276 (2010) 335–350

337

the end of the regeneration procedure, the catalyst was

3. Results and discussion

heated up to 350 °C under He ﬂow and eventually hydrogen

was added to the reactor to complete the reduction of unre-

3.1. Catalyst characterization

acted stored NO

ﬁcation of the residual NO

various investigated temperature.

x

, if any. This procedure allowed the quanti-

x

species left after reduction at the

The catalyst characterization has been already reported in a

previous paper [30]. Here, we only recall some useful information.

The speciﬁc surface area of the commercial alumina sample after

2

The role of water and of CO

has also been addressed by performing the experiments in the

presence of water vapour (1% v/v) and of both H O (1% v/v) and

(0.3% v/v). Unless otherwise speciﬁed, water or water and

were present during the regeneration phase only (i.e., not dur-

storage).

During the regeneration with H

served as primary reduction products, along with minor amounts

of NO. No N O has been observed. The reactor outlet products’ con-

2

in the reduction of the stored NO

x

calcination at 700 °C is near 200 m /g, while a slightly lower value

2

has been measured for the PtAK/Al O sample (176 m /g). The sur-

2

3

2

face area contraction was not accompanied by signiﬁcant changes

3

CO

2

in the pore volume (0.96 cm /g for

2 3

c-Al O and 0.90 cm /g for

PtAK/Al O ). The pore radius is in the range 90–100 Å.

2

3

ing the NO

x

The XRD patterns of the pure alumina support (not shown) ex-

hibit the reﬂections typical of microcrystalline -Al O (JCPDS 10-

425). In the XRD patterns of the calcined PtAK/Al O catalyst, the

2

, N

2

and NH

3

have been ob-

c

2

3

2

3

2

2 3

characteristic peaks of c-Al O have been detected; monoclinic

centration changes with time during the regeneration phase, and

hence the selectivity also changes with time. Accordingly, the

K₂CO₃(JCPDS 16-820) and cubic K O (JCPDS 23-493) phases have

been hardly recognized, thus suggesting that the potassium phase

2

time-weighted average N

according to Eq. (a):

2

selectivity (SN2) has been estimated,

is mainly amorphous and well dispersed. Also FT-IR spectra [30]

revealed that carbonate species are present on the fresh PtAK/

2 3

Al O sample; these species are almost completely removed by

outgassing at 500 °C or after a storage-reduction cycle at 350 °C.

Signiﬁcant coverage of the alumina support by the K component

is attained in the prepared sample, as conﬁrmed by FT-IR analysis

in the OH stretching region [30] that showed the disappearance of

2n

N2

N2 þ nNO þ nNH3

S

N2 ¼ ₂_n

ðaÞ

where nN2, nNO and nNH3 are the total molar amounts of N

2

, NO

hydroxyl bands characteristic of the

c

-Al

2

O

3

surface outgassed at

new weak band at

716 cm , reasonably assignable to hydroxyls on the K phase.

The Pt dispersion measured by H chemisorption is near 65%.

and NH

phase.

3

, respectively, evolved during the entire regeneration

5

3

00–550 °C and the appearance of

a

ꢀ1

2

.3. FT-IR analysis

HRTEM images of calcined sample freshly prepared revealed that

the Pt particle size distribution ranges between 0.7 and 2.5 nm

(the mean size being d_P_t= 1.5 nm) and that the K phase is well dis-

persed on the alumina support [30].

Absorption/transmission IR spectra have been obtained on a

Perkin-Elmer FT-IR System 2000 spectrophotometer equipped

with a Hg-Cd-Te cryo-detector, working in the range of wavenum-

ꢀ1

bers 7200–580 cm at a resolution of 1 cm (number of scans

10). For IR analysis, powder catalyst has been compressed in

3.2. ISC experiments at 350 °C and FT-IR analysis

ꢁ

ꢀ2

self-supporting disc (of about 10 mg cm ) and placed in a com-

mercial heatable stainless steel cell (Aabspec) allowing thermal

treatments in situ under vacuum or controlled atmosphere. Pellet

has been activated by heating in vacuum at 500 °C in order to re-

move carbonate surface species formed during the acetate decom-

position. Besides, on catalyst left in the laboratory atmosphere for

few months, the amount and thermal stability of the carbonates

species slightly increase and a storage-reduction cycle, performed

In Fig. 1, the results obtained during a NO

reduction (Section B) cycle carried out at 350 °C over the PtAK/

Al conditioned catalyst are presented. Fig. 1C and D shows,

for comparison purposes, the results obtained under the same

experimental conditions over the PtABa/Al catalyst sample.

x

storage (Section A)-

2 3

O

2 3

O

3.2.1. NO

NO is admitted to the reactor at t = 0 s; after 20 s, the break-

through of NO is observed, while the evolution of NO is seen start-

ing from 100 s. The outlet concentrations of both NO and NO

increase with time and tend to a stationary level, indicating that

saturation of the catalyst surface with NO is almost attained. At

the end of the storage, the NO concentration is far from that corre-

sponding to the equilibrium of the NO oxidation reaction to NO in

= 210 ppm vs. NO2eq = 555 ppm at

x

storage (Fig. 1A)

2 2

at 350 °C by admitting NO and subsequently H , is necessary for

their complete elimination [30]. Afterward, the catalyst has been

oxidized in dry oxygen at 500 °C, and cooling in oxygen down to

the selected temperature.

2

NO

freshly prepared NO/O

50 °C. IR spectra have been recorded at the same temperature at

x

storage experiments have been performed by admitting

x

2

mixture (PNO = 5 mbar; PO2 = 20 mbar) at

2

3

2

increasing exposure times. After the storage phase, the catalyst

2 2

the presence of 3% v/v O (NO

has been evacuated at 350 °C and cooled down to room tempera-

350 °C).

ture (RT). The interaction of H

2

O (from RT up to 350 °C) or CO

(only at RT) with stored NO has

2

Then, at 1200 s, the NO feed is switched off and a tail is observed

in the NO and NO concentration proﬁles, due to the desorption of

weakly adsorbed NO species. An additional small NO desorption

(

from RT up to 350 °C) or NH

3

x

2

been then carried out. Alternatively, after a storage phase, the cat-

x

alyst has been evacuated at 350 °C and cooled down to 100 °C. The

(not shown in the ﬁgure) is detected upon switching off the oxygen

ﬂow, as expected in view of the effect of the oxygen partial pres-

sure on the stability of nitrate species, and in agreement with what

reduction of the stored NO

dry H (5 mbar) or wet H

NH (5 mbar) or wet NH

increasing temperature up to 350 °C. Moreover, the reduction of

stored NO has been carried out by admitting dry H along with

CO (PH2 = 5 mbar; PCO2 = 10 mbar) at increasing temperature up

to 350 °C.

Further details on experimental apparatus and procedures can

be found elsewhere [10,11,25,29].

x

has been then carried out by admitting

(PH2 = 5 mbar; PH2O = 10 mbar), and dry

(PNH3 = 5 mbar; PH2O = 10 mbar) at

2

3

observed in the case of PtABa/Al

The results obtained in the case of the storage carried out on the

PtAK/Al catalyst sample (Fig. 1A) are similar to those collected

on the PtABa/Al catalyst (Fig. 1C). During the storage phase, the

NO concentration trace is similar although the PtBa catalyst shows

a NO breakthrough higher than the PtK sample; besides the NO/

NO ratio is different. This could be ascribed to the different

2 3

O catalyst [20].

x

2

2 3

O

2

2 3

O

x

2

3

38

L. Castoldi et al. / Journal of Catalysis 276 (2010) 335–350

1

200

000

1200

NO_i_n

H₂_i_n

A

B

1000

^N^Ox

H *0.5

2

8

6

4

2

00

0

NO

800

600

400

200

0

N₂

NO₂

NH *2

3

NO*2

0

200

400

600

800

1000 1200 1400 1600

0

200

400

600

800

1000

1

200

1200

1000

800

600

400

200

0

^N^Oin

C

H₂_i_n

D

1000

NO_x

8

6

4

2

00

0

NO

H *0.5

2

N₂

NO₂

^N^H3

NO*5

0

200

400

600

800

1000 1200 1400 1600

0

200

400

600

Time, s

800

1000

Time, s

2 3 2 3 2

Fig. 1. Typical storage (A and C) and reduction (B and D) cycle at 350 °C over PtAK/c-Al O (A and B) and PtABa/c-Al O (C and D) catalysts (storage phase: NO 1000 ppm + O

3

% v/v in He; reduction phase: H 2000 ppm in He).

2

reactivity of Pt in the NO oxidation, being its reactivity modiﬁed by

the presence of storage components (K vs. Ba) having different

electronic properties, in line with the results of FT-IR study show-

ing very different surface properties of the noble metal [30,31].

storage by Toops and co-worker [14] on a similar catalyst. It is

worth of note that all the NO adsorbed species are related to the

K phase, as previously reported [30], since the K phase completely

covers alumina and NO species on the Al support are different

from that formed on the PtAK/Al catalyst.

Fig. 2B shows, for comparison purposes, the results obtained

under the same experimental conditions over the PtABa/Al

catalyst, and already presented in previous papers (see for example

Refs. [11,18]). On both PtAK/Al and PtABa/Al catalysts,

nitrites species are formed at the beginning of the storage, being

oxidized to nitrates in few minutes. Then only nitrates increase

up to the saturation. These results indicate that on both the cata-

lysts the storage proceed following the same pathways demon-

x

2 3

O

The amounts of NO

very similar, being near 5.1 ꢂ 10 and 5.9 ꢂ 10 mol NO

after He purge for the model PtAK/Al and PtABa/Al cata-

x

stored over the two catalytic systems are

2 3

O

ꢀ4

x

/gcat

2

O

3

2

O

3

2 3

O

lysts, respectively. Due to the similar molar loadings of the storage

elements and the different stoichiometry of the K- and Ba-nitrates,

the overall K and Ba utilization (i.e. the fraction of K and Ba sites

2

O

3

2 3

O

involved in the storage of NO

and 24% for K and Ba, respectively.

In situ FT-IR spectra of NO/O

PtAK/Al catalyst at increasing exposure times are reported in

x

) is different, being close to 40%

2

interaction at 350 °C with the

2

O

3

strated for PtABa/Al

2 3

O [11,32]. In particular, a pathway leads to

Fig. 2A: an initial formation of nitrite species is observed that rap-

idly evolve to nitrates. In particular, at low exposure times (10 s,

the formation of nitrites which are then oxidized to nitrates

2

ꢀ

2NO þ 0:5O

2

^þ^Os ^!²^N^O2

ð1Þ

curve a), both nitrites, of linear (

m

NAO mode, broad band at 1100–

ꢀ1

1

000 cm

;

m

N@O mode at 1490 and 1536 cm ) and chelating

ꢀ

ꢀ1

(m

OANAO,asym mode at 1230 cm ) type, and ionic nitrates (

m

NO3,asym

2NO

2

þ O

2

! 2NO

3

ð2Þ

ꢀ1

and mNO3,sym modes at 1370 and 1040 cm , respectively) are pres-

and another one leads to the formation of NO

ing a dismutation process:

2

, then stored follow-

ent at the catalyst surface. At higher exposure times, the bands due

to nitrite species decrease in intensity and eventually disappear

after 2 min of exposure (curve e), simultaneously ionic nitrates in-

NO þ 0:5O

2

! NO

2

ð3Þ

ð4Þ

crease and chelating bidentate nitrates (

N O O N

m @ , m A AO,asym and

ꢀ1

m

O

A

N

AO,sym modes at 1540, 1310 and 1010 cm , respectively) ap-

þ O²_s^ꢀ! 2NO^ꢀ₃þ NO

3NO

2

pear and increase as well. After long exposure times (35 min, line

g), corresponding to the surface saturation, only nitrates, both of

the ionic and bidentate types, are present on the catalyst surface.

Apart from the different ratio between the amounts of ionic and

bidentate nitrates, they are the same species observed during the

Actually, as mentioned above, these conclusions have been

carefully demonstrated by comparison of FT-IR results obtained

during NO/O and NO storage at 350 °C on both PtABa/Al

and Ba/Al samples [11,32]: brieﬂy, during the NO storage,

2

2 3

O

2

O

3

2

L. Castoldi et al. / Journal of Catalysis 276 (2010) 335–350

339

for roughly 300 s and then decreases, in correspondence to H

breakthrough. The reaction is fast and is limited by the concentra-

tion of H ; the overall consumption of 2000 ppm of H and the for-

mation of ca. 400 ppm N well correspond to the stoichiometry of

the reaction between nitrates and H

2

ionic nitrates

A

bidentate nitrates

1

0

.2

.8

.4

2

:

2KNO þ 5H ! N þ 2KOH þ 4H

3

2

O

ð5Þ

d

The reduction of NO also produces water (not shown in the ﬁgure);

x

accordingly, in reaction (5), the formation of KOH has been sug-

gested due to the adsorption of water produced during the reduc-

tion, but the formation of K O cannot be excluded.

2

g

chelating nitrites

In addition to the nitrogen production, at the beginning of the

reduction phase nonnegligible amounts of NO are also detected

a

linear nitrites

at the reactor outlet, possibly due to the desorption of NO

adsorbed species and/or to the unselective reduction of the stored

x

0

.0

.6

1

nitrates. Notably, no appreciable amounts of NH

observed at the reactor outlet.

At the end of the reduction step, the nitrates which have been

stored during the previous adsorption cycle have been fully

removed from the catalytic surface, as conﬁrmed by the N-balance

3 2

and/or N O are

B

1

0

.2

.8

.4

.0

ionic nitrates

x

performed at the end of the cycle (the amounts of stored NO equal

g

f

that of the N-containing species formed during the reduction

phase). This observation is also in line with the FT-IR experiments

showing the complete and fast removal of nitrates species stored

onto the catalytic surface upon admission of hydrogen at 350 °C

(not reported).

e

c

1

210

The reduction of NO

Fig. 1D) shows slightly distinct features and occurs with formation

of N ﬁrst and of an appreciable amount of NH later on. Very small

x

stored over Ba-containing catalyst

b

(

2

3

a

amounts of NO are also detected at the beginning of the reduction

phase.

1

800

1600

1400

1200

1000

-

1

Wavenumbers (cm )

The following overall stoichiometries are hence involved in the

reduction of the stored NO

x

in the case of the PtABa/Al

2

O

3

catalyst:

Fig. 2. Results of NO/O

PtABa/Al catalysts. Spectra recorded after 10 s (curve a), 30 s (curve b), 1 min

curve c), 1 min 30 s (curve d), 2 min (curve e), 5 min (curve f) and 30 min (curve g)

of exposure to NO/O mixture (1:4, pNO = 5 mbar) at 350 °C. Each spectrum is

reported as difference from the spectrum run before NO/O admission.

2 2 3

adsorption FT-IR experiments over (A) PtAK/Al O and (B)

2

O

3

BaðNO

3

Þ₂þ 5H

2

! N

2

þ BaðOHÞ þ 4H

2

O

ð6Þ

2

(

2

BaðNO

^Þ2 ^þ⁸^H

2

! 2NH

3

þ BaðOHÞ þ 4H

2

O

ð7Þ

2

In particular, over the PtABa/Al

stored NO is initially fully selective to N

high selectivity to NH is observed at the end of the reduction pro-

cess (reaction (7)). The time-weighted average N selectivity mea-

sured over the entire rich phase is however high, being near 93%.

The formation of both N and NH upon reduction of the stored

NO over the PtABa/Al model catalyst has been ascribed in pre-

vious studies to the occurrence of a two-steps in series molecular

process [24–26]. Brieﬂy, it has been suggested that H readily re-

2

O

3

sample, the reduction of the

(reaction (6)), whereas

even at low contact times, only nitrate formation and no nitrites

as stable intermediates were observed both in presence or in ab-

x

2

3

sence of Pt. For PtAK/Al

ments performed in our laboratories lead to the same conclusions

33].

It is worth noting the different variety of NO

formed during the NO/O storage on the two catalysts. On PtAK/

Al catalyst both chelating and linear nitrites are observed,

differently from the case of PtABa/Al , where only chelating

2 3 2 3

O and K/Al O catalysts, similar experi-

2

[

2

3

surface species

x

2 3

O

x

2

2 3

O

2

acts with nitrates to give ammonia (ﬁrst step, reaction (7)),

whereas in a second slower step the formed ammonia reacts with

residual stored nitrates to give N :

2

2 3

O

ꢀ

1

nitrites (

more, at the saturation bidentate nitrates are present in remark-

able amounts on PtAK/Al , while mainly nitrates of ionic type

NO3,asym mode split at 1410 and 1320 cm , and mNO3,sym mode

O N

m A AO,asym mode at 1210 cm ) were observed. Further-

2 3

O

3BaðNO

3

^Þ2 ^þ¹⁰^N^H

3

! 8N

2

þ 3BaðOHÞ þ 12H

2

O

ð8Þ

ꢀ1

2

(m

ꢀ1

at 1010 cm ) and minor amounts of bidentate nitrates (

mode at 1550, the only visible one) were observed on PtABa/

Al . In particular, in the following an evaluation of the relative

m

N

@

O

Due to the fast reduction of stored nitrates by hydrogen to give

ammonia and to the integral behaviour of the plug-ﬂow reactor,

an H₂front develops which travels along the reactor [24–26].

Accordingly, NH₃formed at the H₂front according to reaction (7)

reacts with NO stored downstream the front, leading to N forma-

2 3

O

amount of ionic and bidentate nitrates will be done. We will show

that the two types of nitrates are coarsely present in comparable

amounts on the surface over K-based catalyst (Fig. 2A).

x

2

tion (reaction (8)). In line with our previous results [25] and with

other literature reports as well [27,34], the occurrence of this path-

way accounts for the observed temporal evolution of products dur-

ing the reduction, with nearly complete nitrogen selectivity at the

beginning of the rich phase followed by signiﬁcant ammonia forma-

tion near the end of the regeneration stage (see Fig. 1D). This has

also quantitatively accounted for by recent kinetic studies carried

out under nearly isothermal conditions by some of us and by other

groups as well [35,36].

3.2.2. Reduction phase

Fig. 1B shows the results obtained for the PtAK/Al

2

O

3

catalyst

species stored

addition to the reactor (t = 0 s),

are readily reduced to N . H is completely con-

sumed and the N outlet concentration increases immediately to

a level near 400 ppm. The N concentration keeps almost constant

during the reduction carried out at 350 °C of the NO

at the same temperature. Upon H

the stored NO

x

2

x

2

3

40

L. Castoldi et al. / Journal of Catalysis 276 (2010) 335–350

Notably, in the case of the PtAK/Al

small NH formation is observed during the reduction of the stored

NO (see Fig. 1B), as opposite to PtABa/Al . Accordingly, it is of

interest to analyse the potential role of ammonia as intermediate

in N formation in the case of the PtAK/Al catalyst sample as

well. For this purpose, the reduction of the stored NO

and NH

2

O

3

catalyst sample, a very

2.5

2.0

3

A

x

2 3

O

b

2

2 3

O

h

x

with H

2

1

0

.5

.0

.5

3

has been performed by means of Temperature Pro-

grammed Surface Reaction (TPSR) experiments.

Moreover, as we want to study the effect of H

2

O and CO on the

reduction process, the interaction of these molecules with the

stored NO (i.e. with the bidentate and ionic nitrates present at

x

a

the surface) has been preliminary examined by FT-IR spectroscopy,

and results are given in the following paragraph.

h

b

3.3. Preliminary FT-IR study on the interactions of H

2

O, CO

2

and NH

3

b

.0

1

with the stored NO

x

800

1600

1400

1200

1000

-1

Wavenumbers (cm )

Before starting the discussion, it is important to point out that

the No3,asym mode of free nitrate is double degenerate. The

degeneration is maintained for the surface ionic species at high

m

1.0

0.8

B

temperature. However, by decreasing the temperature to RT, the

ꢀ1

b

m

NO3,asym mode splits into two bands at 1392 and 1368 cm

showing a decrease in the symmetry of this adsorbed species

see curves a, both in Figs. 3A, B and 4A, B). This is reasonable, be-

,

(

0

.6

.4

.2

.0

cause a decrease of thermal energy decreases the mobility of the

surface species.

In Fig. 3, the interaction of water (10 mbar) with the stored NO

x

at increasing temperature is shown (nitrate region in Section A,

and hydroxyl region in Section B). Curve a is the spectrum recorded

at RT after the storage phase and evacuation at 350 °C. Notably, the

evacuation causes the formation of small amount of chelating ni-

h

a

ꢀ

1

trites (mode at about 1270 cm ). Immediately after the H

2

O

admission at RT (curve b), the bands of bidentate nitrates (m @

N O

ꢀ1

and

m

O

A

N

A

O,asym modes at 1548 and 1314 cm , respectively) dis-

NO3,asym mode split at 1392 and

368 cm ) suddenly increase in intensity. The absorption related

to the bending mode of the molecular adsorbed water is observed

3600

3300

3000

2700

appear and those of ionic nitrates (

1

m

-1

Wavenumbers (cm )

ꢀ1

Fig. 3. FT-IR results on the changes induced by interaction, at different tempera-

ture, between H O and NO_xpreviously stored over the PtAK/Al O₃catalyst. (A) NO_x

stretching region and (B) OH stretching region. Curves a, spectra of the NO

by NO/O at 350 °C, evacuated at 350 °C and cooled down to RT; curve b, after

0 min of exposure upon admission of H O (10 mbar) at RT; curve c–h, after 10 min

of exposure at increasing temperature by step of 50 °C between 100 °C and 350 °C.

ꢀ1

2

at about 1643 cm , the associated stretching mode being the

ꢀ

1

x

stored

broad band centred at 3300 cm

Fig. 3B). The rise of the temperature at 100 and 150 °C (curves c

and d) causes a consistent desorption of molecular water (erosion

in the OH stretching region

2

(

1

2

ꢀ1

Each spectrum is reported as difference from the spectrum run before NO/O

storage.

2

of the bands at 1643 and 3300 cm , Fig. 3B), but no signiﬁcant

changes in the NO region (Fig. 3A). Further temperature increase

x

provokes the decrease of the ionic nitrate peaks, while those of

bidentate ones starts to reappear (Fig. 3A, curves e–h) simulta-

neously with surface dehydration. The molecular water disappears

at about 200 °C. However, at 350 °C, the surface is still markedly

hydrated, showing an amount of OH bonded group bigger than

in the case of the initial situation (compare curve a and h in

Fig. 3B), and the modes of bidentate nitrates are only poorly

reconstructed.

ꢀ

1

1655 and 1627 cm are assignable to the dasym modes, the band at

ꢀ1

1110 cm to the dsym modes and the envelope between 3400 and

ꢀ

1

3200 cm

spectrum after the NO

adsorbed at RT after the NO

to the stretching modes. Curve (a) in Fig. 4 is the

storage at 350 °C; when NH (10 mbar) is

storage (curve b), in the 1700–

x

3

x

ꢀ1

1000 cm spectral region (Section A) a strong decrease of biden-

tate nitrate bands and a noticeable increase of those of ionic ones

are observed simultaneously with an intensiﬁcation and decrease

The experiments clearly demonstrate that the surface hydration

transforms the bidentate nitrates in the ionic ones and is also con-

ﬁrmed by the isosbestic points visible in Fig. 3A.

in resolution of the dasym(NH

3

) modes (only one broad peak is

ꢀ1

The effect of ammonia is now considered. We have studied the

now present at 1627 cm ), a decrease in intensity of the dsym(NH

3

)

ꢀ

1

adsorption of NH

NO storage, being conﬁdent that no reduction by NH

tive at RT and the results are reported in Fig. 4. When NH

10 mbar) is adsorbed at RT before the NO storage (curve a’), the

following observations can be done: (i) in the 1700–1000 cm

spectral region (Section A), the appearance of three bands at

3

at RT on the catalyst surface before and after the

mode at 1110 cm , and a decrease in the resolution of the

x

3

was effec-

masym(NH ) modes of adsorbed ammonia. Notably, the adsorption

3

of ammonia strongly perturbs the stored nitrates, in a way very

similar to the perturbation produced by water, i.e. transforming

the bidentate nitrates in the ionic ones. It is also clear that in the

presence of the stored NO the modes of adsorbed NH are modi-

x 3

ﬁed, indicating some changes due to its interaction with surface

nitrates.

The effect of water and ammonia allows to evaluate the ratio

between the amounts of bidentate and ionic nitrates at the end

of a storage step. The calculation described in the following was

(

x

ꢀ1

ꢀ

1

2

655 (weak), 1627 (weak), and 1110 cm and (ii) in the 3800–

ꢀ1

700 cm spectral region (Section B), the appearance of an enve-

lope of bands between 3400 and 3200 cm . All these bands are

ꢀ

1

+

assignable to the modes of adsorbed NH

3

polarized on the K ions

or bonded to the surface hydroxyls. In particular, the absorptions at

L. Castoldi et al. / Journal of Catalysis 276 (2010) 335–350

341

2

1

0

.5

.0

.5

.0

.5

1

200

A

900

H *0.5

2

a

6

3

00

0

^N^H3

b

N₂

a

X 2

5

0

100

150

200

250

300

350

400

a'

1

800

500

200

.0

1

B

800

1600

1400

1200

1000

-1

Wavenumbers (cm )

NH₃

0

.50

.25

N₂

9

6

3

00

0

a

B

H₂

5

0

100

150

200

250

300

350

400

b

Temperature, °C

Fig. 5. (A) H

adsorption at 350 °C over the PtAK/Al

2 3

refers to H or NH concentration in the case of PtABa/Al O

2

-TPSR (2000 ppm in He); (B) NH

catalyst. Note: trace a in both the panels

catalyst.

3 2

-TPSR (1000 ppm in He) after NO/O

a'

a

2 3

O

2

3

experiments carried out for the PtABa/Al

displayed for comparison purpose (trace a, in the ﬁgure).

The H proﬁle has a complex shape: its concentration decreases

starting from 110 °C, shows a minimum near 150 °C with full H

consumption and a second consumption peak with a minimum

at 210 °C. H consumption is accompanied by the evolution of N

and NH . N is detected starting from 120 °C, its concentration at

ﬁrst grows slowly and then increases rapidly to the maximum va-

lue of 450 ppm. Then the N concentration decreases and NH pro-

2 3

O catalyst [25] is also

.00

3

800

3600

3400

3200

3000

2800

2

-1

Wavenumbers (cm )

2

Fig. 4. FT-IR spectra obtained by interaction of NH

catalyst: curves a’, NH admission before the NO storage (PNH3 = 10 mbar); curves

b, NH admission after the NO storage (PNH3 = 10 mbar). Curve a is the spectrum of

the NO stored by NO/O at 350 °C, evacuated at 350 °C and cooled down to RT. Each

spectrum is reported as difference from the spectrum run before NO/O storage. (A)

Spectral region of NO stretching and NH bending modes. (B) Spectral region of

NH stretching modes.

3 2 3

at RT with the PtAK/Al O

3

x

2

3

x

3

2

x

2

3

x

3

duction is observed, in correspondence with the second H

2

3

consumption peak centred at 210 °C.

During the reduction of the stored nitrates, water (not shown in

the ﬁgure) is also formed. Neither NO nor N

appreciable amounts during the run.

2

O has been detected in

performed on the spectra shown for NH

the spectra obtained for H O interaction are affected by noise in

the ionic nitrate region, due to the thickness of the pellet. The inte-

3

interaction (Fig. 4A), since

2

The data in Fig. 5A prove that nitrates stored at 350 °C can be

reduced by H already slightly above 100 °C, i.e. at temperatures

2

ꢀ

1

grated intensities of the bands at 1392 and 1368 cm related to

well below those corresponding to their thermal decomposition

(350 °C). In fact, a comparison with the results of TPD experiments

where the stability of the nitrates stored at 350 °C over the PtAK/

Al O sample has been investigated (results not reported for the

2 3

sake of brevity) pointed out that in the absence of H₂nitrates

decompose only starting from the temperature of adsorption.

NH3

ionic nitrates before and after NH

3

interaction (Iion ^aⁿ^d^Iion , respec-

NH3

^tⁱ^v^e^l^y⁾^w^e^r^e^c^o^a^r^s^e^l^y^e^s^tⁱ^m^a^t^e^d^.^T^h^e^r^a^tⁱ^o^Iion =Iion is about 2. Tak-

ing into account that almost all bidentate nitrates are transformed

NH3

.

ⁱⁿ^t^oⁱ^oⁿⁱ^c^oⁿ^e^s^,^t^h^e^v^a^l^u^e^o^f^t^h^e^r^a^tⁱ^o^Iion =Iion shows that the

amounts of bidentate and ionic nitrates originally present on the

surface are comparable.

Moreover, a comparison with the results of H

carried out over a Pt-free sample (i.e. K/Al

the reduction of the stored nitrates is catalysed by Pt, since over

K/Al the reduction is observed starting from the temperature

of the thermal decomposition of the stored NO , i.e. 350 °C. This

clearly indicates that over PtAK/Al a Pt-catalysed route is

responsible for the reduction of the stored nitrates, similarly to

the case of PtABa/Al [24,25].

2

-TPSR experiments

2 x

Finally, the interaction of CO at RT with the stored NO has

2 3

O ) pointed out that

been considered. No effects on the nitrates are observable in this

case (spectra not reported). A minor amount of carbonates is

formed on a small fraction of surface sites free from nitrates.

2 3

O

x

2 3

O

x 2 3

3.4. TPSR of stored NO with H or NH and FT-IR analysis

2 3

O

Fig. 5A shows the results obtained in the case of the H

experiment after NO adsorption at 350 °C. The H , N and NH

concentration proﬁles are displayed as a function of tempera-

ture; the H concentration proﬁle measured in the case of TPSR

2

-TPSR

The overall consumption of hydrogen observed during the TPSR

run and the corresponding production of nitrogen and of ammonia

are in line with the stoichiometries of reactions (5) and (9),

respectively:

x

2

3

2

3

42

L. Castoldi et al. / Journal of Catalysis 276 (2010) 335–350

2

KNO

3

þ 5H

2

! N

2

þ 2KOH þ 4H

2

O

ð5Þ

2.0

A

2

KNO

þ 8H

! 2NH

3

þ 2KOH þ 4H

2

O

ð9Þ

In reactions (5) and (9), the formation of KOH is envisaged, due to

the presence of water, although K O may also be present.

The presence of two H consumption peaks in the TPSR trace

may suggest the involvement of stored NO species having differ-

ent reactivity. As a matter of fact, FT-IR data reported in the follow-

ing show a different reactivity between ionic and bidentate

1.5

2

x

1

0

.0

.5

.0

a

2

nitrates. These H consumption peaks are associated with the for-

mation of different products, i.e. N

2

in the ﬁrst peak and NH

3

in the

shoulder at 210 °C.

Worth to note that calculations showed that the H

tion observed in the low temperature range during the reduction

in H (i.e. below 130–140 °C of Fig. 5A) is slightly higher than that

corresponding to N production based on the stoichiometry of

reaction (5). However, the overall H consumption and the N for-

mation monitored during the TPSR run before NH formation (i.e.

2

consump-

1

567

f

1

324

2

b

c

3

B

below 170 °C) is in agreement with stoichiometry of the same

reaction (5). This suggests that the reduction of the stored nitrates

implies the formation of reduced intermediate adspecies whose

2.0

d

2 2

formation involves the consumption of H but not N evolution.

1

0

.5

.0

.5

e

These partially reduced species might be tentatively associated

with nitrites, formed from the stored nitrates at the beginning of

the reduction process. This point will be discussed later on.

a

The comparison of the H

tained in the case of the PtABa/Al

2

concentration trace with that ob-

catalyst sample (trace a in

2 3

O

Fig. 5A) and discussed elsewhere [25] points out the greater reac-

tivity of the K-containing sample in the reduction of the stored ni-

a

d

f

trates by H

at lower temperature (110 °C vs. 145 °C).

Fig. 5B shows the results of a NH -TPSR experiment performed

with NH (1000 ppm in He), after NO adsorption at 350 °C. A com-

plex shape in the NH consumption is observed with desorption at

low temperature and consumption at higher temperatures; the

onset temperature of ammonia consumption (pointed out by N

formation) is observed near 105 °C and is very close to that

detected for H . Then the NH consumption proceeds quite rapidly,

and the ammonia concentration shows a minimum of 100 ppm

near 170 °C. Above 175 °C, the NH concentration slowly increases

2 2

, since the H consumption peak is observed in this case

b

0.0

1

3

800

1600

1400

1200

1000

3

x

-1

Wavenumbers (cm )

3

Fig. 6. FT-IR spectra recorded during the NO_xreduction over the PtAK/Al O₃

2

catalyst at different temperatures. (A) Reduction in dry H

spectrum of the NO stored by NO/O at 350 °C, evacuated at 350 °C and cooled

down to 100 °C. Curves b–f, upon admission of dry H at: 100 °C for 10 min (curve

b), 120 °C for 10 min (curve c), 150 °C for 10 min (curve d), 200 °C for 10 min (curve

e), 250 °C for 5 min (curve f). (B) Reduction in dry NH (PNH3 = 5 mbar): curve a,

spectrum of the NO stored by NO/O at 350 °C, evacuated at 350 °C and cooled

down to 100 °C. Curves b–f, upon admission of dry NH at: 100 °C for 10 min (curve

b), 150 °C for 10 min (coincident with curve b), 200 °C for 10 min (curve d), 250 °C

for 10 min (curve e), 300 °C for 10 min (curve f). Each spectrum is reported as

2

difference from the spectrum run before NO/O storage.

2

(PH2 = 5 mbar). Curve a,

2

x

2

3

x

2

and eventually decreases again above 300 °C.

Ammonia consumption is accompanied by N formation in the

2

x 3

range 105 °C–230 °C due to the reduction of the stored NO by NH ,

3

reaction (10):

2 3

upon the admission of H or NH at increasing temperatures, by

steps of 20–50 °C, from 100 °C up to 350 °C, the exposure time at

each temperature being 5–10 min.

6

KNO

3

þ 10NH

3

! 8N

2

þ 6KOH þ 12H

2

O

ð10Þ

2

and H evolution is ob-

At higher temperature, above 300 °C, N

served due to the decomposition of ammonia to nitrogen and

hydrogen according to the reaction:

2

In the case of H (Fig. 6A), no sensible changes after exposure of

1

0 min at 100 °C are observed. Conversely, at 120 °C, the intensity

of the bidentate nitrate peaks starts to decrease, the decrease being

more marked after 10 min at 150 °C. However, at 150 °C, the bands

related to ionic nitrates slightly increase in intensity; simulta-

neously, in the 3800–3000 cm spectral region (not reported in

the ﬁgure), a band related to OH stretching modes of surface

2

NH

3

! N

2

þ 3H

2

ð11Þ

During the reduction with ammonia, water (not showed in the ﬁg-

ure) is also formed. Neither NO nor N O has been detected in appre-

ꢀ1

2

ciable amounts during the NH

The comparison of the NH

the PtAK/Al and PtABa/Al

out that the reactivity of NH

on the PtAK/Al catalyst. In fact, over this catalyst, the minimum

in the consumption of NH is seen at much lower temperatures, in

correspondence with the formation of N (not shown in the case of

the PtABa/Al catalyst).

The reduction of the NO

3

-TPSR experiment.

3

consumption curves obtained over

(curve a) samples clearly points

towards adsorbed nitrates is higher

hydroxyls appears. The temperature increase at 200 °C in dry H

2

O

3

2

O

3

provokes the complete reduction of bidentate nitrates and of the

90% of the ionic ones and, after 5 min at 250 °C, the reduction of

nitrates is complete. These features can be easily explicated as

follows: the bidentate nitrates initially present on the catalyst sur-

face in absence of water begin to be reduced already at 120–150 °C,

in accordance with the onset (110 °C) and the maximum (150 °C)

3

2 3

O

3

2

2 3

O

x

stored at 350 °C with H

2

and NH

3

has

2

of H consumption observed in the TPSR experiment. However,

also been investigated by FT-IR spectroscopy (Fig. 6A and B). After

the storage phase, the catalyst has been evacuated at 350 °C and

cooled down to 100 °C (curves a), then FT-IR spectra were recorded

at these temperatures, the water produced by their reduction is

adsorbed in dissociative way at the surface, causing the transfor-

mation of a small fraction of them still present on the surface in

L. Castoldi et al. / Journal of Catalysis 276 (2010) 335–350

343

ionic nitrates, according with experiments illustrated in Fig. 3. Io-

nic nitrates become to be reduced only at 200 °C. These results

put in evidence the different resistance to the reduction of the

two types of surface nitrates, being the bidentate species the most

1

200

A

H *0.5

2

9

00

2

reactive one, according to the two H consumption peaks observed

in the TPSR experiment.

a

FT-IR spectra obtained upon admission of dry NH

3

on stored

600

^N2

NO are now considered (Fig. 6B). Immediately after the admission

x

at 100 °C, the modes of bidentate nitrates markedly decrease while

3

00

NH₃

those of the ionic nitrates markedly increase and a weak band at

ꢀ

1

3

628 cm

appears. Simultaneously, an envelope of bands at

ꢀ1

400–3200 cm appears (not reported). No sensible changes after

0

exposure of 10 min at 100 or 120 °C are observed. At 150 °C and

00 °C, it is possible to appreciate the decrease of the weak band

2

ꢀ1

50

100

150

200

250

300

350

400

at 1628 cm and of those in the 3400–3200 cm region, accom-

panied by an increase of bidentate nitrates and a decrease of the

ionic ones. After 10 min at 250 °C, both bidentate and ionic nitrate

peaks are reduced. A further increase of temperature to 300 °C en-

hances the reduction that is completed at higher temperatures.

1200

B

NH₃

a

The FT-IR results obtained during the reduction of stored NO

with dry NH can be explained as follows: (i) at 100 °C, the effect

x

900

N₂

3

of the ammonia already described for the interaction at RT (see

Fig. 4) is observed; (ii) the increase of the temperature up to

6

00

0

2

00 °C mainly provokes the ammonia desorption; and (iii) the

reduction starts at 200 °C and is appreciable at 250 °C. As previ-

ously discussed, the presence of ammonia strongly perturbs the

stored nitrates and these species modify the modes of adsorbed

^H2

3

NH ; in this condition, it could be difﬁcult to evaluate the exact on-

set temperature of the reduction. Furthermore, differently from the

reduction in hydrogen, in the case of ammonia both bidentate and

ionic nitrates start to be reduced together, showing comparable

reactivity on increasing the temperature. This result is in agree-

ment with the shape of ammonia consumption in the TPSR exper-

5

0

100

150

200

250

300

350

400

Temperature, °C

2 2 3

Fig. 7. (A) H -TPSR (2000 ppm + 1% v/v H O in He); (B) NH -TPSR (1000 ppm + 1%

iment that does not show the shoulder observed for H

2

-TPSR.

v/v H O in He) after NO/O₂adsorption at 350 °C over the PtAK/Al O₃catalyst. Note:

2

trace a in both the panels refers to H

2

or NH

3

concentration in the case of PtABa/

2 3

Al O catalyst.

3.5. TPSR of stored NO

with H or NH in the presence of water and

x 2 3

FT-IR analysis

present in the feed stream. Neither NO nor N

in appreciable amounts.

2

O has been detected

Fig. 7A shows the results obtained in the case of the wet H

TPSR experiment carried out after NO adsorption at 350 °C in

dry conditions; the H concentration proﬁle obtained in the case

of a TPSR experiment carried out over the PtABa/Al catalyst

25] is also displayed for comparison purpose (trace a in the

ﬁgure).

The concentration of H

2

-

The comparison of the H

tained in the case of the PtABa/Al

Fig. 7A) points out the greater reactivity of the Ba-containing sam-

ple in the reduction of the stored nitrates by H , since the threshold

for H consumption is observed in this case at lower temperatures

2

concentration trace with that ob-

x

2 3

O

catalyst sample (trace a in

2

2 3

O

[

2

(below 50 °C vs. 80 °C), as opposite to what observed in the ab-

sence of water.

2

decreases starting from 80 °C, at ﬁrst

slowly and then very rapidly so that the consumption is almost

complete in the temperature range 140–170 °C. Then a shoulder

Comparing Fig. 5A (dry H

appears that in the case of the PtAK/Al

slightly favours the reduction of the stored nitrates by H

in the presence of water the reaction is observed at slightly lower

temperatures (80–100 °C vs. 110 °C). However, the presence of

water does not apparently affect the high-temperature H con-

2

sumption, corresponding to the shoulder near 210 °C. The reason

for which water slightly favours the reduction of the stored ni-

2

-TPSR) with Fig. 7A (wet H

catalyst sample water

, since

2

-TPSR), it

with a minimum near 220 °C is also observed; eventually, the H

concentration increases to the inlet value of 2000 ppm. H con-

sumption is accompanied by the evolution of NH and of N at ﬁrst,

of N only later on and ﬁnally of NH only again at high tempera-

ture. The evolution of these products is observed starting from

10 °C, i.e. at temperatures slightly higher than that of H con-

sumption. The NH concentration proﬁle is rather complex, show-

ing a peak near 140 °C and a second maximum near 210 °C, this

last corresponding to the shoulder observed in the H concentra-

tion proﬁle. On the other hand, N evolution shows an initial slow

increase, near 120 °C, followed by a sharp increase near 140 °C,

when the H consumption is complete. The sharp increase in the

formation corresponds to the sudden decrease in the NH con-

centration: as will be discussed in the following, this suggests that

the two-steps in series process for N formation invoked in the case

of the PtABa/Al catalyst, involving ammonia as intermediate, is

operating in the case of the PtAK/Al sample as well.

2

2 3

O

2

3

2

3

1

2

3

trates by H

where [25], a suggested pathway for nitrate reduction by H

involves the activation of the H molecule over Pt, followed by H

2

has not been clariﬁed so far. As already discussed else-

2

spillover towards nitrates. In this respect, it has been suggested

that water increases the rate of reduction by favouring hydrogen

spillover [25,37]. A signiﬁcant enhancement is expected in this

case in the reduction of nitrates stored far away from Pt sites,

whose reaction implies the mobility of H adspecies on the surface.

Speciﬁc effects of water on the mobility of adsorbed nitrates, a fac-

tor which may play a role in the reaction, can be also suggested,

although this has been apparently ruled out by a recent study of

Epling and co-workers [37] based on the results obtained upon

2

N

2

3

2

2 3

O

2 3

O

Finally, during the reduction of the stored nitrates, water (not

shown in the ﬁgure) is also formed, in addition to that already

3

44

L. Castoldi et al. / Journal of Catalysis 276 (2010) 335–350

reduction of crystalline Ba(NO

3

)

2

doped with Pt. We note however

2.5

2.0

1.5

1.0

that in the case of our catalysts, water provokes signiﬁcant changes

in the nature of the adsorbed nitrates, already at RT, being biden-

tate nitrates transformed into ionic species. These aspects are pres-

ently under investigation in our laboratories.

A

c

b

The results obtained in the reduction with NH

of water of the nitrates stored at 350 °C (wet NH

shown in Fig. 7B. The H , N and NH concentration proﬁles are

displayed as a function of temperature, along with the NH con-

centration proﬁle (trace a) obtained in a similar experiment car-

ried out over the PtABa/Al sample. The onset for ammonia

consumption is observed in this case near 110 °C. Then the con-

sumption of NH proceeds quite rapidly and is almost complete

near 170 °C. Above 190 °C, the NH concentration slowly in-

creases, shows a shoulder and eventually decreases again above

50 °C. Worth to note that, as in the case of the NH -TPSR carried

out in dry conditions, during the wet NH -TPSR only N has been

observed among the reaction products, along with water (not

3

in the presence

3

-TPSR) are

2

3

2 3

O

d

e

a

0.5

3

1648

3

b

c

0

.0

2

3

2

B

showed in the ﬁgure), since neither NO nor N

tected in appreciable amounts.

2

O has been de-

1380

b

d

The NH

nied by N

3

consumption peak observed below 250 °C is accompa-

evolution, in line with the stoichiometry of reaction (10)

1.5

2

e

f

already reported. At higher temperatures, above 300 °C, decompo-

sition of ammonia according to the reaction (11) is observed. The

comparison with the dry NH

out that NH is only slightly inhibited by the presence of water,

as opposite to H . Indeed, under dry conditions, N formation from

NH is observed near 105 °C, whereas in the presence of H O it is

3

-TPSR experiments (Fig. 5B) points

1.0

a

g

3

2

3

2

0

.5

detected starting from 110 °C.

1

624

1

648

a

FT-IR spectra recorded in wet H

2

and in wet NH

3

at increasing

h

x

temperature after NO adsorption at 350 °C are reported in

Fig. 8A and B, respectively. After the storage phase, the catalyst

has been evacuated at 350 °C and cooled down to 100 °C (curves

.0

1

800

1600

1400

1200

1000

a). In the case of the reduction in wet H

after wet H admission (curve b), the effect of the hydration is ob-

served: the modes of bidentate nitrates disappear while those of

the ionic nitrates markedly increase, and weak band at

648 cm appears. No sensible changes after exposure of 10 min

2

(Fig. 8A), immediately

_W_a_v_e_n_u_m_b_e_r_s₍_c_m-1₎

2

Fig. 8. FT-IR spectra recorded during the NO_xreduction over the PtAK/Al O₃

2

a

catalyst at different temperatures. (A) Reduction in wet

P_H₂_O= 10 mbar) and (B) in wet NH₃(P_N_H₃= 5 mbar, P_H₂_O= 10 mbar). Curves a,

spectra of the NO stored by NO/O at 350 °C, evacuated at 350 °C and cooled down

to 100 °C. Curves b–h, spectra recorded upon admission of reducing mixtures at:

00 °C for 10 min (curve b), 150 °C for 10 min (curve c), 200 °C for 10 min (curve d),

250 °C for 10 min (curve e), 300 °C for 5 min (curve f), 300 °C for 10 min (curve g),

00 °C for 30 min (curve h). Each spectrum is reported as difference from the

spectrum run before NO/O storage.

2

H (PH2 = 5 mbar,

ꢀ1

1

x

2

at 100 °C are observed. After 10 min at 150 °C (curve c), the only

change clearly appreciable is the decrease of the weak band at

1

ꢀ1

1

648 cm , while after 10 min at 200 °C (curve d) this band disap-

3

pears and ionic nitrate peaks are markedly reduced. A further in-

crease of temperature to 250 °C (curve e) enhances the reduction

that is only completed at higher temperatures. Due to the effect

of water, it is difﬁcult to evaluate at which temperature the reduc-

tion starts: in fact, between 150 and 200 °C surface dehydration is

observed and bidentate nitrates should be reformed in small

amount (see Fig. 3). However, in the presence of hydrogen, biden-

tate nitrates are immediately reduced and never observed in

2

Upon comparing Figs. 6A and 8A, it appears that in absence of

water, H begins to reduce bidentate nitrates at 150 °C, while both

in wet or dry H the reduction of ionic nitrates (the only present in

2

wet conditions) begins at 200 °C. As mentioned above, due to the

Fig. 8A after H /H

2 2

O admission. Nevertheless, this experiment con-

effect of water on the stored nitrates (see Fig. 3), in the case of

ﬁrms that, also in presence of water, ionic nitrates are markedly re-

2

wet H , it is difﬁcult to evaluate at which temperature the reduc-

duced only at 200 °C.

tion starts. Nevertheless, from the experiments in dry and wet con-

ditions, it is clear that bidentate and ionic nitrates show a different

reactivity, the bidentate being the more reactive ones. Note that in

the absence of water at 250 °C also the residual surface carbonates

When the reduction of the stored NO

x 3

is carried out in wet NH

(Fig. 8B), the admission of the NH /H O mixture immediately pro-

3

2

vokes the disappearance of the bidentate nitrate absorptions and

the increase of those of the ionic ones. No sensible changes are ob-

served after 10 min of exposure at 100 °C or 150 °C. After 10 min of

exposure at 200 °C, the intensity of ionic nitrates is reduced (curve

ꢀ1

are removed (negative bands at 1567 cm

and 1324 cm ,

observed after reduction at 250 °C in Fig. 6A). This feature was

not observed in the reduction in presence of water (see Fig. 8A).

On the other hand, in the reduction with ammonia, it is not well

clear what happens in the presence of water under 300 °C (Fig. 8B),

i.e. if the decrease of the bands of ionic nitrates is due to the dehy-

dration and/or to the ammonia desorption, or to reduction. Indubi-

tably, the reduction is effective at 300 °C but is rather slow.

Conversely, in absence of water (Fig. 6B), the reduction begins at

250 °C, even at very slow rate, and is markedly increased at

300 °C at rate higher than in presence of water. This is in line with

2

d) but, differently than for the wet H , the modes of bidentate

nitrates appear, indicating that the intensity decrease of the ionic

nitrates can be due in part to their conversion into the bidentate

ones; a further increase of temperature at 250 °C leaves the surface

situation almost unaltered (curve e). Finally, at 300 °C, the reduc-

tion of both bidentate and ionic nitrates by NH

curves f-h for increasing time), but it is rather slow and it will

be completed only at 350 °C.

3

is well evident

(

L. Castoldi et al. / Journal of Catalysis 276 (2010) 335–350

345

the effect of water (slight inhibition) pointed out by NH

runs.

3

-TPSR

tion is observed near 160 °C, i.e. at much higher temperatures if

compared to both the H - and NH -TPSR experiments. In particular

during the H /NH -TPSR experiment, the reactivity of ammonia

with nitrates is inhibited by the presence of H in the gas stream,

while the reactivity of H is not or only weakly affected by the

presence of NH . Second, it is well apparent in the H /NH -TPSR

(Fig. 9A) that ammonia reacts with adsorbed NO only in a strict

temperature range (160–180 °C), i.e. when the H concentration

at the reactor outlet is null. This is a much higher temperature

range if compared to the threshold observed for the NH + nitrate

reaction during the NH -TPSR experiments (110 °C, Fig. 7B). Since

it has been previously clariﬁed that the reaction of ammonia with

stored nitrates leading to N is catalysed by Pt [25], it is speculated

that H competes with ammonia for adsorption and activation on

the Pt active sites, thus inhibiting NH reactivity. Third, N forma-

tion is observed during the H /NH -TPSR concomitantly with NH

2

3

2

3

.6. TPSR of stored NO

x

with H

2

/NH

3

mixtures in the presence of water

and NH with

2

In order to further investigate the reactivity of H

stored over the K-containing catalyst, TPSR experiment has

/NH mixtures in the presence

-TPSR), and results are presented in Fig. 9A.

In the co-presence of H and NH , H is consumed above 80 °C,

i.e. at a temperature that is similar to that observed in the case of

the wet H -TPSR previously discussed (Fig. 7A), and is accompa-

nied by NH formation due to the occurrence of reaction (9). The

rate of H consumption (and of the corresponding NH production)

is initially very slow; then, a sharp consumption peak is observed

centred at 170 °C, accompanied by NH consumption as well and

by N formation. A second broad H consumption peak is observed

at higher temperatures (above 200 °C), accompanied by ammonia

production. This broad H consumption peak well corresponds to

the H consumption peak observed near 210 °C in the wet H -TPSR

run previously discussed (Fig. 7A). On the other hand, NH is con-

sumed at temperatures well above those measured during the

NH -TPSR.

The co-presence of NH

of N formation with respect to the experiments in which H

NH were separately fed to the reactor (compare Fig. 9A with

Fig. 7A and B). First of all, during the H

2

3

2

3

NO

x

been carried out by co-feeding H

of 1% v/v H O (H /NH

2

3

2

3

2

3

2

3

2

3

2

3

2

3

2

3

2

3

2

consumption: this clearly indicates that NH

formation.

3

is involved in N

2

In order to further investigate these aspects, and in particular (i)

the presence of adsorbed NO species having different reactivity

(see the H consumption peak near 200 °C) and (ii) the hydrogen

inhibition on ammonia reactivity, additional experiments have

been carried out. In particular, H /NH -TPSR runs have been per-

formed during which the NH + H ﬂow has been stopped near

2

x

3

2

3

2

3

and H

2

signiﬁcantly affects the features

and

3

2

170 °C, after the nitrogen evolution (see the arrow in Fig. 9A). Then

the reactor has been cooled at RT in ﬂowing He in order to preserve

2

3

/NH

3

-TPSR, the N

2

produc-

the NO

quent H

NH , respectively, to analyse the reactivity of the residual NO

sorbed species. The results are shown in Fig. 9B (H

(NH -TPSR).

In the case of the H

shows a minimum near 210 °C, accompanied by NH

Notice that the H consumption is never complete and the mini-

mum corresponds to that observed at the same temperature in

the H concentration proﬁles during both the H - and H /NH -TPSR

experiments previously discussed (Figs. 7A and 9A, respectively).

A different picture is apparent in the case of the NH -TPSR run

Fig. 9C). Indeed, in this case, NH consumption is observed at low

x

species left unreacted onto the catalyst surface. Subse-

2

- and NH

3

-TPSR runs have been carried out with H

2

and

ad-

1

800

500

200

3

x

2

-TPSR) and C

A

NH₃

3

2

-TPSR run (Fig. 9B), the H

2

concentration

evolution.

3

9

6

3

00

0

H *0.5

2

3

N₂

3

(

3

1

200

temperatures, starting from 110 °C; ammonia consumption is

accompanied by nitrogen evolution only.

B

9

6

3

00

0

Hence, it appears that the treatment with H

leaves on the catalyst surface adsorbed NO species characterized

by poor reactivity towards H , and leading to the formation of

NH only, as expected. Indeed, as shown above, N formation

involves NH : however, due the presence of high H concentration,

the reactivity of the formed NH in the production of N is inhib-

ited, and accordingly NH is detected unreacted at the reactor

outlet. At variance, NH alone results less sensitive to the nature

of the adsorbed NO species that are always reduced leading to

the formation of N

On the basis of these results, if we look backward to dry and wet

-TPSR (Figs. 5A and 7A, respectively), the complete NH selectiv-

ity which is observed in correspondence of the high-temperature

consumption peak is related to the H inhibition on the

+ nitrate reaction, being in this temperature range the H con-

centration not null. The important conclusion is that the reduction

of the less reactive NO species towards H (shoulder at 200–220 °C

in the H concentration proﬁle) gives only NH

completely consumed and inhibits ammonia reactivity.

2 3

/NH up to 170 °C

H *0.5

2

x

2

3

2

3

2

NH₃

3

2

^N2

3

x

1

200

C

2

.

9

6

3

00

0

NH₃

H

2

3

H

2

^H2

N₂

NH

3

2

x

2

3

because H

2

is not

100

150

200

250

300

350

400

Temperature, °C

3.7. TPSR of stored NO

x

with H

2

or NH

3

in the presence of CO

2

and

water

Fig. 9. (A) Competitive H

PtAK/Al catalyst after NO/O

v/v H O in He); and (C) NH -TPSR (1000 ppm + 1% v/v H

competitive H + NH -TPSR (2000 ppm + 1% v/v H O in He) stopped near 170 °C.

2

+ NH

adsorption at 350 °C; (B) H

O in He) subsequent to a

3

-TPSR (2000 ppm + 1% v/v H

2

O in He) over the

2

O

3

2

-TPSR (2000 ppm + 1%

In the previous study over PtABa/Al

found that CO inhibits both the initial reduction of nitrates to

2 3

O catalyst [25], it has been

2

3

2

3

2

3

46

L. Castoldi et al. / Journal of Catalysis 276 (2010) 335–350

ammonia by H

with the stored nitrates as well (step 2).

To investigate the effect of the co-presence of carbon dioxide

and water on the reduction of the stored nitrates over PtAK/

2

(step 1) and the subsequent reaction of ammonia

NO

the case of H

atures are shifted towards higher temperature, from 80 °C to

130 °C for H , and from 105 °C to 130 °C for NH

The effect of CO on the reduction of the stored NO

been investigated by FT-IR measurements upon admission of H

(5 mbar) and CO (10 mbar) at increasing temperature on NO

x

with NH

3

is slightly inhibited in the presence of CO

2

, as in

2

. Indeed, the H and NH consumption onset temper-

2

3

2

3

.

Al

formed in the presence of CO

In the case of the H -TPSR experiment (Fig. 10A), hydrogen is

2

O

3

catalyst, H

2

- and NH

3

-TPSR experiments have been per-

2

x

has also

2

(3000 ppm) and H O (1% v/v).

2

x

consumed starting from 130 °C and shows an almost complete

consumption at 200–210 °C. Then its concentration increases,

and a second consumption peak is apparent with minimum of

stored at 350 °C, and results are shown in Fig. 11A. Note that these

experiments have been performed in the absence of water for sake

of clarity, being modest the promoting effect of water with respect

1

300 ppm near 265 °C. The shape of the H

and the temporal evolution of the reduction products are similar

to that described in the case of H -TPSR experiment carried out

in the absence of CO (Fig. 7A), but are shifted towards higher tem-

peratures. This indicates that also over PtAK/Al catalyst, the

presence of CO inhibits the reaction of the stored nitrates with

. This result is conﬁrmed by FT-IR measurements discussed later

on.

2

concentration trace

2

to inhibiting effect of CO .

Upon heating at 100 °C, no sensible changes are observed in the

spectra of the stored nitrates (data not reported). Conversely, at

120 °C, the intensity of the bidentate nitrate peaks

mOANAO,asym and mOANAO,sym modes at 1536, 1315 and 1010 cm

respectively) starts to decrease (curve b), the decrease being more

marked at 150 °C (curve c). Simultaneously, the bands related to

2

(

m

N@O

,

ꢀ

1

2

O

3

2

H

2

ꢀ1

ionic nitrates (

NO3,sym mode at 1045 cm ) slightly increase in intensity, due to

an increase of the surface hydration related to the reduction pro-

cess, as observed in the case of H alone (Fig. 9A). However, differ-

mNO3,asym mode split at 1392 and 1368 cm and

ꢀ

1

The results of the NH

water and CO are shown in Fig. 10B. A NH

seen starting from near 130 °C, as apparent from N

3

-TPSR run carried out in the presence of

consumption peak is

evolution, with

m

2

3

2

a broad minimum centred at 200 °C. A correspondent broad evolu-

tion of nitrogen is observed in the range 130–280 °C, which obeys

the stoichiometry of reaction (10). At high temperatures, above

ently from the case of pure hydrogen, the temperature increase at

3

00–350 °C, the decomposition of NH

observed.

The comparison with the TPSR experiment carried out in the ab-

3

to give H

2

and N

2

is

2.0

A

sence of CO

2

(Fig. 7B) shows that also the reduction of the stored

1.6

a

1

0

.2

.8

1200

d

A

a

d

9

6

3

00

0

c

e

H *0.5

0.4

2

f

g

0.0

1800

1600

1400

1200

1000

^N2

_W_a_v_e_n_u_m_b_e_r_s₍_c_m-1₎

NH₃

0

.08

.06

.04

.02

B

50

100

150

200

250

300

350

400

1

200

B

^N^H3

9

6

3

00

0

g

N₂

a

H₂

.00

2

100

2000

1900

1800

-1

Wavenumbers (cm )

50

100

150

200

250

300

350

400

Fig. 11. FT-IR spectra recorded upon admission of H₂(5 mbar) and CO₂(10 mbar)

mixture at increasing temperature on NO stored at 350 °C. (A) Nitrate region, (B)

x

Temperature, °C

Pt-carbonyl region. Curve a: spectrum of the NO_xstored by NO/O₂at 350 °C,

evacuated at 350 °C and cooled down to 100 °C. Curves b–g, spectra recorded upon

Fig. 10. (A) H

NH /H OACO

adsorption at 350 °C over the PtAK/Al

2

/H

2

OACO

2

-TPSR (2000 ppm + 1% v/v H

O + 0.3% v/v CO

catalyst.

2

O + 0.3% v/v CO

2

in He); (B)

2 2

admission of H /CO mixture at: 120 °C for 10 min (curve b), 150 °C for 10 min

(curve c), 200 °C for 10 min (curve d), 250 °C for 10 min (curve e), 300 °C for 10 min

(curve f), 350 °C for 10 min (curve g).

3

2

-TPSR (1000 ppm + 1% v/v H

2

in He) after NO/O

2

2 3

O

L. Castoldi et al. / Journal of Catalysis 276 (2010) 335–350

347

2

00 °C (curve d) does not cause the complete reduction of biden-

1

00

tate nitrates and ionic ones are almost unaffected. At 250 °C, biden-

tate nitrates are no more observable, but the reduction of ionic

nitrates is not yet complete. Contemporary to the consumption

A

8

6

4

0

PtK

v

C@O

of nitrates, chelating bidentate carbonates ( , vOACAO,asym and

ꢀ1

v

OACAO,sym modes at 1595, 1325 and 1068 cm , respectively) are

formed on the sites left free by the nitrates. It is necessary to raise

the temperature at 300 °C to complete the nitrate reduction, while

with pure H

2

the reduction is complete already at 250 °C.

20

PtBa

These observations are in agreement with the TPSR measure-

ments previously discussed (Fig. 10A) that pointed out an inhibit-

H₂

0

ing effect of CO

case of PtABa/Al

CO on the reduction steps are related to the poisoning of CO on

2

on the reduction step. As already proposed in the

100

B

2 3

O

catalyst [25], the observed inhibiting effects of

2

80

60

PtK

the Pt sites, CO being formed upon reduction of CO

ing to the Reverse WGSR:

2

by H

2

accord-

CO

2

þ H

2

! CO þ H

2

O

ð12Þ

40

As a matter of fact, examining the Pt-carbonyl region (Fig. 11B), two

ꢀ1

20

bands at 2010 and 1910 cm both assignable to CO linear bonded

to Pt [30] increase on increasing temperature starting from 150 °C.

In particular, the observed frequencies correspond to low Pt cover-

age by CO. A similar effect was also pointed out by Choi et al. [38].

H +H O

0

2

100

C

80

60

40

20

0

3.8. Mechanistic aspects of the reduction of the stored nitrates over

PtAK/Al

2 3

O

PtK

PtBa

The results obtained from the combined use of transient reac-

tivity experiments and FT-IR spectroscopy point out that nitrates

are stored over PtAK/Al

in wet or dry conditions near 100 °C, leading initially to the for-

mation of ammonia (Fig. 7A). Then, the formed ammonia produces

by reaction with the residual stored nitrates. Notably, complete

selectivity to nitrogen is observed in the reduction of nitrates with

NH . These data are consistent with the in series 2-steps molecular

pathway already proposed for N formation in the case of PtABa/

Al [24–27,35,36] and involving at ﬁrst the formation of ammo-

nia upon reaction of nitrates with H , followed by the reaction of

the so-formed ammonia with the residual stored nitrates leading

to the selective formation of N . The sum of these reactions leads

to the overall stoichiometry for the reduction of nitrates by H ob-

served during ISC experiments, i.e. reaction (5) in the case of PtAK/

Al catalyst (Fig. 1).

In the case of the PtABa/Al

previous works [24–26], the reduction of the stored nitrates with

hydrogen to give ammonia (reaction (7)) has been found to be very

fast, being observed slightly above the ambient temperature in the

presence of water. On the other hand, the subsequent reaction of

ammonia with the residual stored nitrates to give nitrogen is much

slower (reaction (8)), and hence has been considered the rate

determining in the formation of nitrogen. In line with the sug-

2 3

O at 350 °C and start to be reduced by

H

2

H +H O+CO

2

N

2

1

00

150

200

250

300

350

Temperature, °C

3

2

Fig. 12. N

2

selectivity for PtAK/Al

+ H

temperature in the range 100–350 °C.

2

O

3

and PtABa/Al

O, (B) H and (C) H

2

O

3

catalysts during isothermal

+ H O + CO at different

2 3

O

reduction performed in (A) H

2

As a result, ammonia is by far the major reaction product and the

nitrogen selectivity is very small. Upon increasing the reduction

2

O

3

temperature, the reactivity of NH

comes appreciable and this drives the selectivity to N

the case of the PtABa/Al catalyst sample H is by far more reac-

tive than NH towards surface nitrates, NH reacts only with ni-

trates located downstream the front, i.e. when the

concentration is null. Accordingly, N is observed at ﬁrst at the

3

with nitrates (reaction (8)) be-

2

O

3

catalyst sample considered in

2

. Since in

2

O

3

2

3

H

2

H

2

reactor outlet (Fig. 1D) and the selectivity to NH

3

decreases upon

increasing the temperature (Fig. 12).

In the case of the PtAK/Al

NH -TPSR runs pointed out that the proposed in series two-steps

molecular pathway for N formation is operating as well. Like the

PtABa/Al sample, the occurrence of a fast reduction step of

the adsorbed nitrates by H to give ammonia and the integral

‘‘plug-ﬂow” behaviour of the reactor imply the complete consump-

tion of the reductant H and the formation of an H front travelling

2 3 2

O sample, the results of H - and

gested two-steps mechanism for N

PtABa/Al catalyst sample, a signiﬁcant increase in the N

tivity is observed upon increasing the temperature [24–26]. This is

shown in Fig. 12, which reports the N selectivity values obtained

during a number of ISC experiments carried out with H as reduc-

tant over the PtABa/Al catalyst at various temperatures and un-

der various atmospheres (He, He + H O, He + H O + CO ). The N

selectivity values are compared with those obtained in the case

of the PtAK/Al catalyst used in this study under the same

experimental conditions. From the ﬁgure, it clearly appears that

in the case of the PtABa/Al catalyst very poor N selectivity is

obtained when the regeneration of the trap is carried out at low

temperatures. In fact over PtABa/Al at low temperature, H re-

acts with surface nitrates to give NH (reaction (7)), but ammonia

can hardly react with the stored nitrates to form N (reaction (8)).

2

formation, in the case of the

3

2

O

3

2

selec-

2

2 3

O

2

O

3

2

along the reactor axis [24–26] (Fig. 13). Accordingly, different

zones are present in the trap at a given time of the rich phase,

2

O

3

i.e. (i) a zone upstream the H

been regenerated; (ii) the zone corresponding to the development

of the H front where the formation of ammonia takes place; (iii) a

zone immediately downstream the H front where nitrates react

2

front where the trap has already

2

O

3

2

O

3

2

with ammonia formed upstream leading to nitrogen formation;

and (iv) the last part of the trap with the initial nitrates loading.

However, at variance with that observed in the case of the

3

2

3

48

L. Castoldi et al. / Journal of Catalysis 276 (2010) 335–350

K sites

Pt sites

H2 gas in

H

2

NO ^-

NH

3

axial coordinate (z)

NO ^-

3

N

2

Gas out

H

2

concentration

Fig. 13. The ‘‘H

2

front” model for the reduction of the stored NO .

x

PtABa/Al

leading to ammonia (step 1) is observed at higher temperatures if

compared to PtABa/Al in the presence of water and at lower

temperatures in the absence of water (see Figs. 7A and 5A, respec-

tively), whereas the threshold for the NH + nitrate reaction lead-

ing to N (step 2) is always seen at lower temperatures (Fig. 7B

2

O

3

catalyst sample, the onset for the H

2

+ nitrate reaction

spectroscopy being very reactive in the further reduction to NH

or N . As a matter of fact, nitrite intermediate surface species have

been pointed out upon reduction of the stored nitrates by CO over

the same PtABa/Al catalyst [39]. Being CO less effective than H

2

3

2

2 3

O

2

O

3

in the reduction of the stored nitrates, this allowed the detection of

surface intermediates. Notably, the features observed after interac-

tion of ammonia at RT with the catalyst surface suggest the adsorp-

2

and 5B, respectively). As a result, the temperature thresholds for

steps 1 and 2 are very close for the K-containing catalyst: accord-

+

2ꢀ

3

tion of NH on the K ions (i.e. K O pairs with strongly basic O ).

ingly ammonia, once formed upon reaction of H

readily reacts with residual nitrates leading to the formation of

, and this drives the selectivity to N of the reduction process.

This explain why the N selectivity obtained in the case of the

PtAK/Al catalyst sample is always much higher than that ob-

served over PtABa/Al (Fig. 12).

It is worth to note that H shows a strong inhibiting effect to-

wards the reaction of ammonia with nitrates. Hence the reaction

between NH and the stored nitrates (leading to N formation) is

conﬁned only downstream the H front (see Fig. 13), i.e. when

the H concentration is nil. Since an H front is established during

the ISC runs (Fig. 1), N is readily and extensively produced in these

runs; on the other hand, a higher NH formation is observed during

consumption is observed only

2

with nitrates,

It is suggested that this activated ammonia species reacts with the

suggested nitrite species (or with N-adspecies having nitrogen oxi-

dation state +3, like nitrosonium ions, NO ) giving rise to an ammo-

nium nitrite or nitrosamide intermediate, respectively, that then

decomposes to nitrogen and water, in line with the chemistry pro-

+

N

2

2 3

O

2

O

3

posed in the case of NH

The different reactivity of PtAK/Al

the activation of both hydrogen and ammonia in the reduction of

the stored NO could be related either to a different reactivity of

the adsorbed NO species or to a change in the activity of the Pt ac-

3

-SCR reaction [40,41].

2

O

3

2 3

and PtABa/Al O towards

3

2

x

2

x

2

tive sites in the reduction process due to the presence of the differ-

ent storage component (K vs. Ba), or both. A comparison of the

2

3

features of the NO

those stored on PtAK/Al

tween the NO species stored on the two investigated catalytic sys-

x

2 3

species adsorbed over PtABa/Al O [8,11] with

TPSR experiments since complete H

in a narrow temperature range.

2

2 3

O

points out signiﬁcant differences be-

x

Detailed mechanistic aspects of the reduction of adsorbed ni-

trates with H and with NH are not yet understood. Being the

reaction catalysed by Pt, it is speculated that H is activated over

the Pt sites, and spills-over towards the stored nitrates which are

reduced to ammonia. How nitrates are reduced by H adspecies

and the role of Pt in this reaction are still under discussion, along

with mechanistic details for the Pt-catalysed reaction between

NH and nitrates leading to N . Moreover, the diffusion of nitrates

3 2

towards reduced Pt sites, as discussed above, cannot be excluded.

However the observation that during the initial part of the TPSR

tems. Along similar lines, recent FT-IR characterization studies

[11,42] point out that the Pt active sites are modiﬁed by the pres-

ence of K or Ba due to the strong electronic interaction between the

alkaline/alkaline-earth oxide and the noble metal. Spectroscopic

studies performed in our laboratories by using CO as adsorbate

to probe the state of the Pt sites, pointed out a strong interaction

of Pt with the basic K sites. Indeed, the presence of three bands

markedly red-shifted with respect to carbonyl bands seen on Pt/

2

3

2

ꢀ1

2 3

Al O (from 2060 cm to 2015 cm due to linear carbonyls on

0

ꢀ1

Pt , from 1845 cm to 1745 cm due to bridged CO) has been ob-

served. The inﬂuence of the K phase, that enhances the electron

density on Pt, favours the formation of bridged carbonyls towards

linear ones, as reported in the literature [30,31]. Worth to note that

2 3

experiments H and NH are consumed with no evolution of gas-

eous species suggests the involvement of short-lively reduced

intermediate surface species, e.g. nitrites, undetected by FT-IR

L. Castoldi et al. / Journal of Catalysis 276 (2010) 335–350

349

the interaction between Pt and Ba sites results in a band due to lin-

ear Pt carbonyls that is shifted to lower frequencies with respect to

metal sites. In fact, only when the H

nia reacts with nitrates and N

inhibition on the N formation via step 2 does not imply a low

selectivity during the isothermal reduction of nitrates since

reacts with NO stored downstream the H front, where

the H concentration is null. Notably, the competition of H and

NH on the activation on the Pt sites is not clearly apparent in

the case of the PtABa/Al sample due to the very large differ-

ence in the temperature thresholds for the reaction of H and

NH with the stored nitrates.

Finally, the effects of water and CO

are also addressed. Water showed a slight promoter effect on the

onset temperature of H consumption and no effect on the subse-

quent reaction of ammonia with stored nitrates, whereas CO has

2

concentration is null ammo-

formation is apparent. The H

2

ꢀ1

carbonyl bands seen on Pt/Al

2

O

3

(from 2090 to 2046 cm ) [31],

2

but this red-shift is lower than in the case of K-system. Also in this

case, the appearance of bands assigned to multi-bridged CO species

N

2

NH

3

x

2

ꢀ1

(

band near 1750 cm ) could be a further indication of Pt atoms

2

strongly interacting with the barium phase, probably with partial

negative charge. These FT-IR evidences together with the shifts ob-

served in the band positions seems to indicate that K has a stronger

electronic effect over Pt sites than Ba.

Accordingly, it is likely that the different reactivity between

2 3 2 3 2 3

PtAK/Al O and PtABa/Al O towards H and NH is explained, at

least partially, by the electronic state of Pt. It is indeed well known

that the activation of hydrogen goes through a dissociative adsorp-

tion on Pt sites by weakening the HAH bond and forming a PtAH

3

2 3

O

2

3

2

on the reduction process

2

a strong inhibition effect on both the reduction of nitrates by H

and ammonia, due to poisoning of Pt by CO formed upon CO

hydrogenation.

2

2 3

bond. The competition between H and NH for the activation on

2

Pt sites also suggests that the activation of ammonia goes through

the formation of a PtAH bond by weakening the NAH bond. Hence,

the higher electron density on Pt in the case of K-containing cata-

lyst than on Ba-containing sample may govern the reactivity of the

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Article Doi

The NOx storage-reduction on PtK/Al2O3 Lean NOx Trap catalyst

DOI: 10.1016/j.jcat.2010.09.026

Source and publish data:

Authors:

Article abstract of DOI:10.1016/j.jcat.2010.09.026

Full text of DOI:10.1016/j.jcat.2010.09.026

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