The Effects of Platinum Dispersion and Pt State on Catalytic Properties of Pt/Al2O3 in NH3 Oxidation

Article abstract of DOI:10.1002/cctc.202001320

Dependence of NH₃ oxidation on the state and dispersion of Pt species in Pt/γ-Al₂O₃ catalysts was investigated. Prereduced Pt/γ-Al₂O₃ catalysts containing Pt⁰ nanoparticles exhibited significantly higher activity than preoxidized ones with the same Pt dispersion. The most significant improvement of the catalytic activity (TOF increased by 30 times) was observed when the size of Pt⁰ particles increased from ～1 to ～8 nm. N₂ selectivity was found to be mainly determined by the reaction temperature, with a minor influence of Pt particle size. Preoxidized catalysts containing ionic Pt were activated by the reaction medium, while partial deactivation was observed for the prereduced ones. The activity improvement was associated with the presence of Pt⁴⁺/Pt²⁺ species on the surface of preoxidized catalysts. The activity decrease of the prereduced catalysts was due to the partial oxidation and subsequent redispersion of Pt particles. Introduction of H₂O and CO₂ to the reaction mixture only moderately influenced NH₃ oxidation activity shifting NH₃ conversion curves by about +15 °C.

Full text of DOI:10.1002/cctc.202001320

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: The effects of platinum dispersion and Pt state on catalytic
properties of Pt/Al2O3 in NH3 oxidation
Authors: Andrei I. Boronin, Elena Slavinskaya, Lidiya Kibis, Olga
Stonkus, Dmitry Svintsitskiy, Andrei Stadnichenko, Elizaveta
Fedorova, Anatolii Romanenko, Vasyl Marchuk, and Dmitry
Doronkin
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ChemCatChem
10.1002/cctc.202001320
FULL PAPER
The effects of platinum dispersion and Pt state on catalytic
properties of Pt/Al
2
O
3
in NH oxidation
3
[
a]
[a]
[a]
[a]
Elena M. Slavinskaya, Lidiya S. Kibis, Olga A. Stonkus, Dmitry A. Svintsitskiy, Andrei I.
[
a]
[a]
[a]
[b]
Stadnichenko, Elizaveta A. Fedorova, Anatolii V. Romanenko, Vasyl Marchuk, Dmitry E.
Doronkin,[b,c] Andrei I. Boronin*[a]
[a]
[b]
[c]
Dr. E.M. Slavinskaya, Dr. L.S. Kibis, Dr. O.A. Stonkus, Dr. D.A. Svintsitskiy, Dr. A.I. Stadnichenko, Dr. A.V. Romanenko, E.A. Fedorova, Prof. Dr. A.I.
Boronin
Boreskov Institute of Catalysis SB RAS
Pr. Lavrentieva 5, 630090 Novosibirsk, Russia
E-mail: boronin@catalysis.ru
V. Marchuk, Dr. D.E. Doronkin,
Institute for Chemical Technology and Polymer Chemistry
Karlsruhe Institute of Technology (KIT),
Engesserstr. 20, 76131 Karlsruhe, Germany
E-Mail: dmitry.doronkin@kit.edu
Dr. D.E. Doronkin
Institute of Catalysis Research and Technology
Karlsruhe Institute of Technology (KIT)
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
Supporting information for this article is given via a link at the end of the document.
Abstract: Dependence of NH
of Pt species in Pt/γ-Al
Pt/γ-Al
3
oxidation on the state and dispersion
The state-of-art ASC is a complex system comprising catalyst
O
2 3
catalysts was investigated. Prereduced
layers for ammonia oxidation (AMOX) and selective NO
x
0
[6]
O
2 3
catalysts containing Pt
nanoparticles exhibited
3 2 3
reduction (NH -SCR). For the oxidation of ammonia, a Pt/Al O
significantly higher activity than preoxidized ones with the same Pt
catalyst is usually applied providing high low-temperature
activity.[6b, 7] However, such a catalyst possesses relatively low
dispersion. The most significant improvement of the catalytic activity
0
(TOF increased by 30 times) was observed when the size of Pt
N
2
selectivity due to the formation of N
250°C and the formation of NO
above 250°C.[7a, 8] To achieve
maximal activity and reduce contribution from the by-products
(N O, NO ), it is necessary to establish factors governing the
operation of Pt/Al during ammonia oxidation. At the same
2
O at temperatures below
particles increased from ~1 to ~8 nm. N
2
selectivity was found to be
x
mainly determined by the reaction temperature, with a minor
influence of Pt particle size. Preoxidized catalysts containing ionic Pt
were activated by the reaction medium, while partial deactivation
was observed for the prereduced ones. The activity improvement
2
x
2
O
3
time it is important to reach the highest degree of Pt usage thus
reducing the cost of aftertreatment systems.[9]
Activity and selectivity of the Pt/Al O catalyst are affected by Pt
2 3
4+
2+
was associated with an increase in the ratio of Pt /Pt species on
the surface of preoxidized catalysts. The activity decrease of the
prereduced catalysts was due to the partial oxidation and
dispersion because of the difference in the physicochemical
properties of small and large Pt particles. For example, Pt
metallic particles smaller than 3 nm exhibit semiconductor and
dielectric properties due to the modification of electronic
structure in comparison with bulk platinum.[10] Furthermore, the
ratio of surface adsorption sites (vertices, edges, terraces)
determining catalytic properties is different for small and large
nanoparticles. In case of small particles, the fraction of surface
atoms increases, while coordination numbers decrease.[10b] Note
that tolerance towards reaction medium exposure (for example,
subsequent redispersion of Pt particles. Introduction of Н
to the reaction mixture only moderately influenced NH
2
О and СО
oxidation
2
3
3
activity shifting NH conversion curves by about +15°C.
Introduction
Suppressing NO
due to a lack of reducing agents in exhaust gas.[1] To solve this
problem, the NH -SCR process (selective catalytic reduction of
NO by ammonia), previously developed to remove NO from
stationary sources, is effectively applied.[2] However, the excess
x
emissions from diesel engines is a challenge
3
2
excess of O ) substantially depends on the initial size of
platinum nanoparticles.
x
x
of ammonia is used under NH
conversion which results in environment pollution by NH
Integrated Heavy Duty Diesel (HDD) aftertreatment systems
include diesel oxidation catalyst, catalyzed soot filter, NH -SCR
3
-SCR conditions to maximize NO
x
.[3]
The strong influence of Pt crystallite size on catalytic properties
of platinum catalysts in ammonia oxidation has been reliably
established in the literature.[8, 11] It is known that small Pt
particles exhibit lower activity compared to large crystallites. At
3
3
component, and an ammonia slip catalyst (ASC), used to reduce
ammonia emissions.[4] ASCs are also used to remove residual
the same time, N
particle size. In the available reports [
temperature range was studied, where NO and NO
formed. Besides, catalysts with different Pt content were
compared. The catalytic data were obtained at high ratios of
2
selectivity increases with decreasing Pt
8,
11a, 11b]
only a narrow
were not
ammonia during the implementation of the DeNO
example, in case of neutralization of NO produced by large
x
process, for
2
x
utility boilers, industrial boilers, and municipal solid waste
incinerators.[5] In this case, the use of the ASC catalyst should
NH :O2 (1:1 or 1:3, not relevant for exhaust aftertreatment)
3
and/or at low temperatures under stationary conditions, which
led to catalyst deactivation.[8b] Catalysts containing small Pt
crystallites are most prone to deactivation due to oxidation of
allow ammonia concentration in the exhaust gases of less than
1
0 ppm.[
5]
1
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ChemCatChem
10.1002/cctc.202001320
FULL PAPER
platinum to PtO
x
, where x depends on the crystallite size.[8b]
of metallic platinum. In this case, using Rietveld method, the
metallic Pt phase was added and its structural parameters were
estimated. The fitting of the experimental pattern with an
additional platinum phase and the calculated pattern for metallic
platinum are shown in Fig. S1c.
Thus, there is a need to investigate in detail catalysts with
different particle sizes of Pt, but with the same platinum content.
Moreover, the catalytic measurements should be performed at
3 2
low NH :O ratios relevant for practical application over a wide
temperature range. Note that under the conditions of diesel
exhaust aftertreatment systems, it is important to study the effect
TEM data
of H
2
O and CO
2
presence in reaction mixture.
According to TEM data, alumina support contains needle- and
The effect of Pt particles size on their catalytic properties in NH
3
sheet-shaped γ-Al O nanoparticles with d = 3-5 nm and l = 10-
2
3
oxidation in the temperature range of 150–550°С was studied
under conditions relevant for diesel exhaust aftertreatment.[12] It
was shown that an increase in the average Pt particle size from
2 3
80 nm. Particles of γ-Al O form porous crystalline aggregates
with sizes of 50-300 nm.
In the N-Ox-400 catalyst, platinum nanoparticles are uniformly
distributed over the support surface (Fig. 1a-d). The average Pt
particle size is 1.1 nm (Fig. 1c). Note that the contribution of
particles with sizes of less than 1 nm is quite large. In HAADF-
STEM images (Fig. 1d) individual Pt atoms and small clusters
with sizes less than 0.3 nm are detected (marked with arrows).
In case of the N-Ox-600 catalyst, larger platinum particles with
an average size of 22.6 nm are observed (Fig. 1e-h). Such
particles are characterized by isometric or irregular shape. The
surface of well-crystallized particles is represented by extended
{111} faces. After calcination at 600°C, highly-dispersed
platinum species with the size less than 0.5 nm can still be
detected in HAADF-STEM images (Fig. 1f, marked with arrows).
Moreover, high-resolution HAADF-STEM imaging allowed us to
detect single Pt atoms on alumina surface (Fig. 1h).
1.3 to 18.7 nm was accompanied by a decrease in the T50 value
from 233 to 215°С. With the increase of particle size to 200 nm,
the T50 value decreased to ~200°C. The size effect on the yield
of the main reaction products was negligible. A tendency to
increase the N
large Pt particles was also shown. The influence of Pt particle
size on the NO /NO ratio was found. The highest value of
NO /NO ~ 0.65 was observed for particles with a size of 2.7 nm.
The author concluded that it is possible to control the NO /NO
ratio (important for the NH -SCR process) by varying the Pt
particle size. For a catalyst exhibiting high N selectivity, the
most favorable ratio of NO /NO = 0.5 was achieved with a Pt
particle size in the range of 2-8 nm. It should be noted that in
work[12] the catalytic data were compared using NH
conversion
2 2
yield and decrease the N O yield in case of
2
x
2
x
2
x
3
2
2
x
[
6b]
3
curves measured during cooling in the reaction mixture.
However, upon the first heating to 550°C, the initial state of the
catalyst (including the particle size of Pt) could change
significantly. A comparison of the curves of the 1st and 2nd
heating/cooling cycles shows a significant decrease in the T50
value for the most dispersed particles during the 2nd cycle. The
authors do not discuss the reasons for this activation.
In the case of Cl-Red catalysts, spherical Pt nanoparticles are
uniformly distributed over the entire support surface (Fig. 2a-c).
Variation of calcination temperature in Ar allowed us to control
the average particle size of Pt in Cl-Red samples. Cl-Red-400
sample contains Pt with the average size of 1.0 nm (Fig. 2g).
Both nanoparticles and atomic Pt species are observed in the
HAADF-STEM image (Fig. 2d). Argon calcination at 550°C leads
to an increase in the average size to 2.6 nm (Cl-Red-550, Fig.
2h), while heating to 600°C - to 8.1 nm (Cl-Red-600, Fig. 2i).
The number of observed highly dispersed Pt species decreases
with the increase in the calcination temperature. Such species
are not reliably detected in case of the Cl-Red-600 sample.
Quantitative XRD and TEM data for all studied samples are
shown in Table 1. Thus, presented XRD and TEM data show
The aim of this work was to systematically study the effect of
particle size and degree of Pt oxidation on the properties of
Pt/Al
model (NH
CO + H
2
O
3
catalysts in the reaction of ammonia oxidation under
+ O /He mixture) and realistic conditions (NH + O
O/N mixture).
3
2
3
2
+
2
2
2
Results and Discussion
2 3
that the used preparation methods allow synthesizing Pt/Al O
catalysts with a variation of the average Pt particle size in a wide
range (from ~1 to ~25 nm). The presence of atomic Pt has also
been revealed.
XRD data
X-ray diffraction patterns for all prepared catalysts and the γ-
Al support are shown in Fig. S1a (see Supporting
information). Samples N-Ox-400, Cl-Red-400, and the support
show reflections of only the γ-Al phase (ICDD PDF-2 #29-
063), while reflections from Pt-containing compounds are not
XPS data
2
O
3
Fig. 3 presents Pt4f spectra curve-fitted into the individual
components (doublets) for all catalysts. The resulting E (Pt4f7/2)
b
values of the different platinum components and their
contributions to the overall Pt4f spectra are given in Table 2.
The Pt4f spectrum of the N-Ox-400 sample can be fitted by two
doublets with the E (Pt4f7/2) values of 72.3 eV and 74.7 eV,
b
which are typical for Pt /Pt and Pt4+ species, respectively.[13]
For the N-Ox-600 catalyst the main doublet is characterized by a
2
O
3
0
observed. This indicates a highly dispersed state of platinum
species. In case of N-Ox-600, Cl-Red-550, Cl-Red-600 samples,
in addition to the γ-Al
observed (ICDD PDF-2 #04-0802). The sizes of Pt crystallites
determined by the Rietveld method) were 17, 2.2, and 6.4 nm
2 3
O phase, reflections from metallic Pt were
δ+
2+
(
for N-Ox-600, Cl-Red-550, Cl-Red-600 catalysts, respectively.
To calculate the size of Pt crystallites in the obtained catalysts,
b
maximum of Pt4f7/2 peak at 71.5 eV. Such E (Pt4f7/2) value is a
bit higher than the one typical for bulk metallic Pt0
first the γ-Al
X-ray diffraction pattern of the γ-Al
obtained values (peak positions and widths) were used to
calculate X-ray diffraction patterns for the Pt/γ-Al catalysts.
2
O
3
phase was described by a set of peaks from the
(
E
b
(Pt4f7/2)~71.1 eV).[13a, 14] The shift to higher E
b
values can be
2
O
3
support. Then the
explained by formation of small metallic Pt particles.[15] The
0
b
second doublet component with E (Pt4f7/2) = 74.3 eV can be
2
O
3
attributed to Pt4+ species. Thus, the Pt4f spectra of the N-Ox
catalysts show the presence of a substantial amount of oxidized
Pt species.
The theoretically calculated X-ray diffraction pattern describes
the experimental X-ray pattern for the Cl-Red-550 sample poorly
(
Fig. S1b). The discrepancy is observed in the region of reflexes
2
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Figure 1. TEM micrographs for (a-d) N-Ox-400 and (e-h) N-Ox-600 catalysts: a, e - BF-TEM images; b, d, f, h - HAADF-STEM images; c, g - the Pt particle size
distributions. Individual Pt atoms and small clusters with sizes less than 0.3 nm are marked with arrows.
Figure 2. TEM micrographs for (a, d, g) Cl-Red-400, (b, e, h) Cl-Red-550 and (c, f, i) Cl-Red-600 catalysts: a-c - BF-TEM images; d-f - HAADF-STEM images; g-i -
the Pt particle size distributions. Individual Pt atoms and small clusters with sizes less than 0.3 nm are marked with arrows.
3
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2 3
Table 1. The XRD and TEM data on Pt particle sizes and Pt dispersion for all synthesized Pt/Al O catalysts
Precursor
Treatment conditions
D(Pt),
nm
XRD
D(Pt),
nm
HRTEM
Dispersion
HRTEM,
%
Catalyst
air/4h/T
400°C
600°C
400°C
400°C
400°C
H
-
2
/2h/T
Ar/6h/T
-
nitrate Pt
nitrate Pt
-
1.1
22.6
1.0
2.6
8.1
87.1
6.4
N-Ox-400
-
-
17 (1)
-
N-Ox-600
H
H
H
2
PtCl
2
PtCl
2
PtCl
6
6
6
350°C
400°C
550°C
600°C
87.1
46.9
17.2
Cl-Red-400
Cl-Red-550
Cl-Red-600
2.2 (2)
6.4 (3)
0
0
Figure 3. Pt4f spectra of (a) the N-Ox catalysts: (1) N-Ox-400 (PtOx, 1.1 nm), (2) N-Ox-600 (Pt , 22.6 nm); (b) the Cl-Red catalysts: (1) Cl-Red-400 (Pt , 1.0 nm),
(
0
0
2) Cl-Red-550 (Pt , 2.6 nm); (3) Cl-Red-600 (Pt , 8.1 nm). (Pt4f spectra of all samples were normalized to the common Al2p intensity).
b
Table 2. XPS data: Ptat/Alat ratio, E (Pt4f7/2) values of the different Pt components and their contribution to the overall Pt4f spectrum.
Sample
Ptat/Alat
%
,
Е
b
(Pt4f7/2
)
I(Pto),
%
I(Pt+/Pt2+),
%
I(Pt4+),
%
Pto
Pt+/Pt2+
Pt4+
74.7
74.3
74.1
73.5
-
N-Ox-400
1.0
0.6
0.5
0.4
0.2
-
72.3
-
50
-
50
30
10
20
-
N-Ox-600
71.5
71.5
71.2
71.4
-
-
-
-
70
90
80
100
Cl-Red-400
Cl-Red-550
Cl-Red-600
-
-
-
In case of the Cl-Red catalysts the main state in the Pt4f spectra
ratios in the N-Ox samples are at least 2 times higher than the
ones in the Cl-Red samples. Probably, the oxidative
has an E
b
(Pt4f7/2) value of 71.4±0.1 eV which can be attributed
0
to small Pt particles. The doublet component with E
b
(Pt4f7/2) ~
pretreatment (in 20%O
dispersed Pt species on the Al
Calcination of the N-Ox sample at 600°С might also induce
sintering of Pt particles due to high volatility of PtO species
2
/He) induces formation of highly
4
+
74.1 eV typical for Pt species is also observed but its intensity
2 3
O
surface in the N-Ox samples.
4
+
is very low. Contribution of the Pt component to the overall Pt4f
spectrum does not exceed 15%. Hence, XPS reveals more
reduced Pt species in the case of the Cl-Red samples than in
the N-Ox catalysts.
2
resulting in Ostwald ripening. As a result, the Ptat/Alat ratio
decreases in comparison with the N-Ox-400 catalyst. The TEM
data for the N-Ox-600 catalyst reveal appearance of Pt
nanoparticles with the average size of 22.6 nm. However, highly
dispersed Pt species can still be observed (Fig. 1 f, h),
Quantitative evaluation of the XPS data were performed to
demonstrate different Ptat/Alat ratios obtained for the N-Ox and
Cl-Red catalysts (see Table 2) with the same Pt loading. Ptat/Alat
4
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explaining quite high Ptat/Alat ratio of 0.6. Based on the
comparison of Ptat/Alat values between Cl-Red and N-Ox
catalysts it can be concluded that calcination in Ar decreases the
number of highly dispersed Pt species. It results in the
decreased intensity of the Pt4f signal. Low Ptat/Alat ratios for Cl-
Red-550 and Cl-Red-600 catalysts in contrast to Cl-Red-400
sample might also be caused by Pt sintering in agreement with
the XRD and HRTEM data. Noteworthy, dispersion calculated
based on the XRD and TEM data does not account for
atomically dispersed Pt species (single sites) as they cannot be
reliably identified and counted. Complementary surface-sensitive
techniques are required for evaluating the total surface area of
Pt including both nanoparticles and single sites. In this respect,
the use of Pt/Al ratio determined from XPS, similarly to CO
chemisorption, allows evaluating the total surface of both single
atoms and nanoparticles. However, since single sites
is negative due to NH
adsorption capacity of Al
our previous work.[16] Temperature of 50% NH
for the N-Ox-400 catalyst is ca. 237°C, while for N-Ox-600 there
is a noticeable increase in activity (T50 ~ 160°C). After the first
heating-cooling cycle in the reaction mixture the N-Ox-400
3
desorption during heating. The high
2
O
3
-based catalysts was discussed in
conversion (T50
3
)
3
sample becomes more active (NH conversion curve shifts by
5°C towards low temperatures, T50 = 232°C) and the N-Ox-600
catalyst slightly deactivates (the conversion curve shifts by 15°C
towards high temperatures, T50 ~ 175°C).
The highest N
the N-Ox-400 and N-Ox-600 catalysts, respectively (Fig. 4b).
O concentration profile over the N-Ox-600 catalyst (Fig. 4d)
shows two maxima at ~175 and ~260°С. For the N-Ox-400
catalyst, only one N O concentration maximum is observed at
about 260°C. Fig. 4c presents N selectivity (SN2) data for N-Ox-
2
concentration is detected at ~270 and ~170°C for
N
2
2
2
demonstrate much lower activity in NH
nanoparticles, the dispersion based on TEM is mainly discussed
in this work.
3
oxidation than
400 and N-Ox-600 catalysts. The maximal SN2 for the N-Ox-400
catalyst is higher (90%) than for the N-Ox-600 (80%). The
difference is caused by the N
N-Ox-600 sample (Fig. S2). N
increasing temperature and becomes similar for both N-Ox-400
and N-Ox-600 catalysts above 315°C. At the same time NO is
detected at temperatures above 225-275°C (Fig. S3) leading to
decreased selectivities to N and N O. Note that the difference in
N2 values measured during the 1st and 2nd heating is
negligible (Fig. 4c).
2
O release below 200°C from the
2
selectivity decreases with
Catalytic properties in the NH
3
oxidation (model mixture)
Fig. 4a presents NH conversion curves for N-Ox catalysts with
3
x
different air-calcination temperatures. The curves were
measured during the first and the second heating in the model
2
2
reaction mixture (1000ppm NH
h ). At low temperatures (<150°C), the apparent NH
3
, 4% O
2
, He, GHSV=120,000
conversion
S
-1
3
Figure 4. a) – NH
3 2 2 2
conversion curves, b) and d) – N and N O concentrations respectively, c) – N selectivity for N-Ox-400 (black lines) and N-Ox-600 catalysts
(red lines). Data collected during the first (solid lines, closed symbol) and the second (dashed lines, open symbol) heating in the reaction mixture.
5
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Fig. S4 presents amount of reaction products per nitrogen atoms
species, which are not visible in HRTEM of the N-Ox-400
catalyst and have low activity.
N and N O concentration curves are similar for the N-Ox-400
2 2
and Cl-Red-400 catalysts above 250°C (Fig. 5b, d), while
significant differences are observed below 250°C. In case of the
depending on temperature. Upon reaching 100% NH
conversion, the amount of N-containing products for the N-Ox-
00 catalyst is 30% higher than the concentration of NH in the
feed. It might be related to the increased amount of NH
3
6
3
3
adsorbed by the catalyst during its pretreatment in the reaction
mixture at room temperature. During the subsequent heating,
Cl-Red-400 sample, N
temperatures than for the N-Ox-400 catalyst. Note that the N
and N O concentration curves for Cl-Red-400 have additional
2 2
and N O are first detected at lower
2
part of the adsorbed NH
3
can be converted to N
2
and N
2
O,
2
providing their overstoichiometric amounts at the outlet of the
reactor. For the N-Ox-400 catalyst, the amount of the released
reaction products is practically equal to the initial concentration
peaks in the temperature range 180-230°C. The appearance of
these narrow peaks (similar to the data from Fig. S2) might be
associated with two reasons. The first reason is related with
desorption of overstoichiometric amounts of reaction products
3
of NH .
Fig. 5 gives a comparison of the catalytic data for catalysts with
similar Pt dispersion but different degrees of Pt oxidation (N-Ox-
formed due to NH
surface. The second one can be related to the reaction of NH
3
adsorption at low temperatures at the Pt
3
4
00 and Cl-Red-400). T50 values measured during the first
heating of N-Ox-400 and Cl-Red-400 catalysts are 237 and
80°C, respectively. During the second heating, shifts of the NH
desorbing from the alumina support (see exemplary NH
profile in Fig. S5).
The calculated SN2 values are shown in Fig. 5c. At NH
3
3
-TPD
1
3
conversion curves by ~5-10°C, towards lower (for N-Ox-400)
and higher (for Cl-Red-400) temperatures, are observed. Taking
into account the XPS data (Fig. 3) and the similar Pt dispersion
for the N-Ox-400 and Cl-Red-400 samples (Table 1), it can be
conversions of less than 100%, the SN2 values reach 90% and
80% for the pre-oxidized N-Ox-400 and the pre-reduced Cl-Red-
400 catalysts, respectively. At 100% NH
270°C), the SN2 values decrease for both catalysts to ~40%. The
N2 curves measured during the 1st and 2nd heating are similar.
NO formation is observed at temperatures above 250°C (Fig.
S6a). The NO /NO ratio at T <375°C is significantly higher for
3
conversion (above
concluded that Pt0 metallic particles on the Al
demonstrate significantly higher activity in the ammonia
oxidation reaction than oxidized PtO species. Note that the XPS
O
3
surface
S
2
x
x
2
x
data show that Pt/Al ratio (table 2) in the N-Ox-400 catalyst is
two times higher than in the Cl-Red-400 catalyst, indicating a
higher number of Pt sites. However, this increase in the number
of Pt sites did not lead to an increase in catalytic activity. This is
due to the large fraction of atomically dispersed oxidized PtOx
the Cl-Red-400 sample than for the N-Ox-400 catalyst (Fig.
S6b). This ratio reaches ~0.7 for Cl-Red-400, indicating the
2
predominant formation of NO on the surface of metallic
platinum.
3 2 2 2
Figure 5. a) NH conversion curves, b) N and d) N O concentration, and c) N selectivity for N-Ox-400 (black lines) and Cl-Red-400 (red lines) catalysts. Data
collected during the first (solid lines, closed symbol) and the second (dashed lines, open symbol) heating in the reaction mixture.
6
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Figure 6. (a, c) NH
Vertical lines in Fig. 6a, c correspond to the T50 values for the first heating curves. (b) N
dashed lines, open symbol) heating of the catalysts in the reaction mixture. (d) A plot of TOF versus temperature during the first heating of Cl-Red-400, Cl-Red-
50, Cl-Red-600, and N-Ox-600 catalysts.
3
conversion curves obtained over Cl-Red catalysts during a) the first (solid lines) and c) second (dashed lines) heating in the reaction mixture.
2
selectivity during the first (solid lines, closed symbol) and the second
(
5
Catalytic properties of the Cl-Red catalysts containing metallic Pt
particles of various sizes are compared in Fig. 6. The T50 values
for Cl-Red-400, Cl-Red-550, and Cl-Red-600 catalysts during
the first heating (Fig. 6a) are ~180, 165, and 155°C,
respectively. During the second heating, the T50 value remains
unchanged for the Cl-Red-550 catalyst, while in case of Cl-Red-
Figure 6d shows TOF values calculated for the first heating
curves taking into account Pt dispersion. These data show that
lowering Pt dispersion leads to a sharp increase in the rate of
ammonia oxidation. For instance, an increase in Pt sizes from
2.6 nm (Cl-Red-550) to 8.1 nm (Cl-Red-600) is accompanied by
an increase in the TOF value by 30 times (at 140°C) at similar
activation energies of 42 and 45 kcal/mol, accordingly (the
corresponding Arrhenius plots are presented in Fig. S10).
4
00 and Cl-Red-600 the NH
towards high temperatures reaching the T50 value of 190 and
65°C, respectively. In a similar way, for the N-Ox-600 catalyst
3
conversion curves are shifted
1
Investigation of the catalysts after reaction by XPS and
TEM
containing large metallic Pt particles (22.6 nm), the Т50 increases
from 160 to 175°С between the first and second heating curves.
The N
shown in Fig. S7. The N
becomes clearer when N
are analyzed (Fig. S8). N
2
and N
2
O concentration curves for Cl-Red catalysts are
O and N curves show two maxima, this
profiles calculated from the FTIR data
selectivity is mainly determined by the
To shed light on the reasons for different activity during the first
and the second heating in the NH + O mixture, the catalysts
after reaction (denoted as AR = after reaction) were analyzed by
HRTEM and XPS (Fig. 7, S11).
2
2
3
2
2
2
reaction temperature, with the minor influence of the size of Pt
particles or the pretreatment conditions (Fig. 6b). At higher
The size of Pt particles for the N-Ox-400 sample changes only
slightly after catalysis. A large number of atomically dispersed Pt
temperature (>250°C), N
2
O concentration decreases leading to
species is observed in the N-Ox-400-AR sample with a
an increase in SN2 to 60-70%. A further increase in temperature
leads to a decrease in SN2 to 30-45% and increase of the
predominance of subnanometer particles. The average particle
size is decreased to 0.8 nm while the overall range of particle
sizes is maintained after catalysis testing (Figs. 1c and 7c).
A bimodal Pt particle size distribution is observed for the N-Ox-
600-AR catalyst (Fig. 7d). Pt particle faceting becomes less
rough due to truncation of sharp angles between the {111}
planes. The sample contains many particles with round and
irregular shapes. In addition to large particles with an average
contribution from NO
lower for Pt particles of about 1 nm (Cl-Red-400) due to the
increased contribution from NO (Fig. S9a). The NO /NOx ratio
x
(Fig. S9a). Above 325°С, the SN2 value is
x
2
is almost independent of the metallic Pt particle size and limited
by thermodynamic equilibrium (Fig. S9b).
7
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size of 17.5 nm, a lot of particles with an average size of 1.4 nm
are observed (inset in Fig. 7d). Note that the initial N-Ox-600
sample contained some subnanometer Pt and atomic Pt species
fixed on the support surface (Fig. 1g). Apparently, during the
catalytic reaction these species grow in size due to the transfer
of platinum from the surface of large crystalline particles. This
leads to a decrease in the average platinum particle size in the
N-Ox-600-AR catalyst compared to the as-synthesized sample.
dispersed Pt species in case of the Cl-Red-400 are preserved
after catalytic tests. However, the particle size distribution
broadens up to 4 nm. Such broadening of the particle size
distribution may occur due to Ostwald ripening. At elevated
temperature and high О
formed.[17] The transfer of such species to the surface of
platinum or Al support leads to growth of existing Pt particles
2 2
concentration volatile PtO species are
2
O
3
or appearance of additional highly dispersed species.
A similar process takes place in other Cl-Red samples.
According to TEM (Fig. 8), a slight increase in the average
particle size (from 2.6 nm to 3.0 nm) is observed in the Cl-Red-
550 catalyst after reaction. On the contrary, in the Cl-Red-600-
AR the calculated average particle size decreases from 8.1 nm
to 7.0 nm. Apparently, the resulting change in the Pt size is
determined by the transfer of PtO to the surface of other
x
platinum particles or to the defects on alumina surface. This, in
turn, directly depends on the initial size of platinum nanoparticles
and the number of highly-dispersed platinum species on the
surface which can act as seeds for growing nanoparticles.
The Pt4f spectra of the catalysts after reaction are presented in
Fig. S11. Note that after the second heating in the reaction
medium, the catalysts were cooled to room temperature in He.
XPS data on the surface Pt content (given as Ptat/Alat ratio) and
the amount of various platinum states for the pristine and used
catalysts are presented in Table 3. In case of the N-Ox-400
δ+
2+
sample, a slight decrease in the amount of Pt /Pt and
corresponding growth of the contribution from the Pt takes
Figure 7. (a-b) HAADF-STEM images and (c-d) the diagrams of Pt particle
size distribution for used (a, c) N-Ox-400-AR, (b, d) N-Ox-600-AR catalysts.
Inset in Fig. 7d shows Pt particle size distribution in the narrow range of 1-4
nm.
4+
place after the catalytic tests. For the N-Ox-600 sample, an
o
increase in the amount of the reduced Pt species is observed
for N-Ox-600-AR. The surface platinum content for N-Ox
catalysts decreases slightly after reaction. In accordance with
TEM data, for N-Ox-600 sample it might be explained by the
decrease of the number of the atomically dispersed Pt species
during the reaction (Figs.1 and 7).
On the contrary, in the Cl-Red-400 catalyst a slight enlargement
of platinum nanoparticles from 1.0 to 1.3 nm occurs under
reaction conditions (see Figs. 2g and 8d). The atomically
Figure 8. (a-c) HAADF-STEM images and (d-f) the diagrams of Pt particle size distribution for used (a, d) Cl-Red-400-AR, (b, e) Cl-Red-550-AR and (c, f) Cl-Red-
00-AR catalysts.
6
8
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Table 3. XPS data for the pristine and used catalysts: Ptat/Alat ratio, the E
b
(Pt4f7/2) values of the different Pt components and their contribution to the overall Pt4f
spectrum.
Sample
Ptat/Alat
%
,
Е
b
(Pt4f7/2
)
I(Pto),
%
I(Pt+/Pt2+),
%
I(Pt4+),
%
Pto
Pt+/Pt2+
Pt4+
74.7
74.6
74.3
74.4
74.1
73.7
74.1
74.4
-
N-Ox-400
1.0
0.9
0.6
0.4
0.5
0.6
0.4
0.5
0.2
0.2
-
72.3
-
50
45
-
50
55
30
25
15
20
15
10
-
N-Ox-400-AR*
N-Ox-600
-
72.5
-
71.5
71.5
71.5
71.6
71.3
71.3
71.4
71.1
-
-
-
70
75
85
80
85
90
100
90
N-Ox-600-AR
Cl-Red-400
-
-
Cl-Red-400-AR
Cl-Red-550
-
-
-
Cl-Red-550-AR
Cl-Red-600
-
-
-
-
Cl-Red-600-AR
74.1
-
10
*
3
AR – after catalytic reaction of NH oxidation (data is presented in Figs.4-6)
Figure 9. NH
3 2
conversion and N selectivity curves for N-Ox-400, Cl-Red-400 and Cl-Red-600 catalysts obtained during three consecutive heating in model (Up-1,
Up-2) and realistic (Up-3) a reaction mixture.
9
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For the Cl-Red-400-AR sample an increase in the contribution of Discussion
the oxidized Pt4+ species from 15 to 20% is observed. TEM data
show an increase in the amount of highly dispersed (<1 nm) Pt
species for this sample (Fig.8). Such species can be easily
oxidized to Pt upon exposure to O . Pt can be additionally
2
A study of Pt/Al
2
O
3
ammonia slip catalysts was carried out using
a combination of structural and spectral techniques. Prepared
catalysts contained metallic or oxidized Pt particles with varied
Pt dispersion. The effects of catalyst pretreatment, Pt particle
size, and the exposure to the reaction mixture on the activity and
selectivity of Pt/Al
discussed below.
4
+
4+
formed as a result of oxidation of the surface of metallic
particles. Also for the Cl-Red-600 catalyst appearance of the
Pt4+ on the surface (~10% contribution, Table 3) is revealed by
XPS. Hence, partial oxidation of Pt to the Pt4+ state during the
reaction can be proposed. For the Cl-Red-550 sample before
and after catalytic measurements, quantitative distributions of
platinum species are similar. The Ptat/Alat ratios for the Cl-Red
samples increase after catalysis which may indicate higher Pt
dispersion.
2 3
O catalysts in ammonia oxidation are
Effect of the catalyst pretreatment
According to the TEM (Figs. 1 and 2) and XPS (Fig. 3) data,
calcination in air at 400°С leads to the formation of oxidized ~1
nm PtO particles. Such PtO particles can be reduced by H in
x
x
2
2
the temperature range of 40-140°C.[ The preoxidized Pt/Al
22]
O
3
Catalytic properties in the NH
mixture)
3
oxidation (realistic
catalyst shows low activity in ammonia oxidation (T50 value
240°C). Calcination of this catalyst in air at 600°C results in
~
N-Ox-400 and Cl-Red-400 catalysts with similar average size of
Pt particles (~1 nm) but different oxidation state of Pt, as well as
the Cl-Red-600 catalyst containing large metallic Pt particles
sintering of Pt with the formation of metallic Pt particles with an
average size of 22.6 nm (Fig.2) in line with the previous
reports.[23] An increase in Pt particle size simultaneously with the
Pt reduction leads to a significant activation of the catalyst (T50
decreases to 175°C, Fig. 4).
(
8.1 nm) were selected for additional tests in the realistic
catalytic mixture containing CO and H O (500ppm NH , 13%
, 5%H O, and 10%CO in N , GHSV=600,000 h ). H O and
CO were added to the reaction mixture at 50°C after two
2
2
3
-1
O
2
2
2
2
2
Also the catalysts reduced in H
2
were prepared and studied after
PtCl as a
2
pretreatment in inert atmosphere. The use of H
2
6
heating – cooling cycles in NH
catalytic run.
3 2
Figure 9a-c shows NH conversion and N selectivity curves
obtained during three consecutive heating-cooling cycles in the
model (1st and 2nd cycles) and realistic (3rd cycle) reaction
mixtures at high GHSV=600,000 h . In the case of the N-Ox-400
catalyst, activation is observed after the 1st cycle (T50 decreases
from 275 to 250°C), while deactivation takes place for Cl-Red-
3
+O
2
atmosphere before the third
precursor allowed preserving the Pt dispersion during the
subsequent reduction treatment,[16] while use of platinum nitrate
as a precursor leads to a bimodal distribution of Pt particles with
sizes of about 1 and 2 nm.[24] XPS data show different platinum
states in the preoxidized (N-Ox-400) and prereduced (Cl-Red-
400) catalysts (Fig. 3) with similar Pt dispersion (Figs. 1 and 2).
In case of the prereduced catalyst, metallic Pt is the main state
-1
4
+
(~87%), while the contribution of oxidized species (Pt ) is close
to 13%. A combination of Pt /Pt and Pt4+ species is found on
δ+
2+
4
1
00 and Cl-Red-600 catalysts (T50 values increase from 230 and
90°C to 235 and 207°C, respectively). The data are completely
the surface of the preoxidized catalyst.
consistent with the results obtained in the model mixture at lower
GHSV. During the third heating in the realistic reaction mixture,
an increase in T50 by 10-15°C is observed for all studied
2 3
The catalytic activity of the prereduced Pt/Al O catalyst is
significantly higher than the activity of the preoxidized one with
the difference in the T50 values ΔT50~55°С (Fig.5). Therefore, it
can be reliably concluded that for samples with similar Pt
catalysts. Thus, introduction of CO
mixture leads to a moderate inhibition of NH
Additional tests with adding only H O or CO to the gas feed (not
2
and H
2
O into the reaction
0
3
oxidation.
dispersion metallic Pt species demonstrate improved activity in
2
2
NH
is in agreement with literature data.[ The increased activity is
associated with the formation of reactive intermediate NH
3 x
oxidation in comparison with PtO species. This conclusion
25]
shown) suggest that the effect can be attributed to water
addition. Figure 9d-f presents the selectivities to different
products versus temperature for three successive heating runs
in the reaction mixture. Selectivity is calculated using the data
presented in Fig. S14-S16. Addition of water only slightly affects
selectivities by increasing SN2O and decreasing SN2 at medium
temperatures. As demonstrated by Shrestha et al., water is not
directly accounted for in the kinetic model of ammonia
oxidation.[ However, water is known to compete for ammonia
x
species as a result of breaking of N-H bonds during the oxidative
ammonia dehydrogenation on metallic surface. This process
proceeds efficiently with the participation of atomic oxygen
formed by the O dissociation. It was established that adsorbed
2
oxygen on the surface of Pt plays the main role in the
mechanism of the oxidative ammonia dehydrogenation. The
18]
activation of NH
3
molecule proceeds much more efficiently on
[
19]
[11b, 25a]
adsorption sites on acidic supports
in this way diminishing the
and slowing down
the surface of metallic platinum than on the oxidized Pt.
We also note that the mechanism of NH
clusters and PtO nanoparticles may differ from that for metallic
[
20]
NH
3
surface coverage (θNH3
)
N
ads
3
oxidation over oxidized
intermediate formation. On the other hand, water does hinder
oxygen activation on the most common Pt surfaces at ASC
operation temperatures [21] leading to lower Nads/Oads ratio and
resulting in higher SN2O in the medium temperature range. At
x
platinum.[26] This assumption is confirmed by a significant
difference (>50°C, Fig. 5) in the onset temperature of ammonia
oxidation reaction on preoxidized and prereduced catalysts.
higher temperatures (T>250°C) water effect on NH
3
adsorption
Different reaction mechanisms can occur in Pt/Al
with metallic or oxidic (ionic) state of platinum. This proposal is
2 3
O catalysts
on the catalysts is low [19a] and no effect on the reaction is
observed (equally high SNOx are observed).
2 3
based on results obtained by Sun et al. who compared Pt/Al O
and Pt/CeZrO
2
catalysts.[ These catalysts displayed different
26]
trends in dissociation of O
facets in Pt/Al provided sites for the formation of active
atomic oxygen species. Oxidized PtO species were established
as active sites in Pt/CeZrO . Formation of different oxygen
2 3
and interaction with NH . Pt(111)
2
O
3
2
1
0
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species resulted in different ammonia oxidation reaction
coordination number, ensuring appearance of terraces on the
surface. A change in the particle size from 2.5 to 4.2 nm is
accompanied by the increase in the amount of terrace-like sites
by 50%.[32] Moreover, the largest changes in the number of
terraces on the surface of Pt nanoparticles take place in the size
range from ~2 to ~3 nm. Further growth of Pt particles leads to
an increase in the degree of crystallinity with the appearance of
isometric crystal faceting due to the development of {111} faces.
mechanisms over Pt/CeZrO
oxidation reaction over Pt/Al
whereas the N mechanism and HNO mechanism coexisted
over Pt/CeZrO . The NH mechanism would easily lead to the
formation of byproduct NO, while N was promoted by N and
2
and Pt/Al
2 3
O . The ammonia
2
3
O followed the NH mechanism,
2
H
4
2
2
2 4
H
HNO mechanism.[26] The stepwise ammonia activation by
adsorbed atomic oxygen is lumped into one stoichiometric
reaction assuming the first step (NH
determining and a first-order reaction with respect to NH
over metallic platinum.[27]
3
+O) to be the rate
Such behavior is observed for the Pt/Al
600°C (Figs.1e-f).
2 3
O catalyst calcined at
3
and O
An increase in catalytic activity along with the growth of Pt0
In our study we observed rather high apparent activation
energies (Fig. S10) with a steep light-off curve, similar to the one
observed in CO oxidation studies,[11c] which may point to surface
poisoning by reactants or intermediates limiting the low
temperature activity. According to Kraehnert and Baerns, the
reaction may indeed be poisoned by adsorbed intermediates,
especially NOads competing with O for adsorption sites.[28]
Further spectroscopic studies would be required to confirm this
theory.
2
crystallites can be related to the effective activation of O on
platinum terraces.[ In case of the highly dispersed platinum,
33]
strong oxygen adsorption on coordinatively unsaturated sites
(edges, kinks, corners etc.) facilitates the formation of PtO
x
species.[34] Niwa et al.[35] and Briot et al. [36] demonstrated that
weakly bound oxygen on the catalyst surface provides higher
activity and the reactivity of chemisorbed oxygen increases with
0
the growth of Pt particles.
A similar relationship between Pt crystal size and the catalytic
0
Drawing an analogy with our catalysts, it can be assumed that
2 3
activity was observed in benzene oxidation over Pt/Al O
NH
3
oxidation over N-Ox-600, Cl-Red-550, and Cl-Red-600
occurring in the same temperature range (25-400°С) [37]. It was
established that the number of Pt-O bonds with lower binding
energy (or higher oxygen reactivity) was greater for large Pt
particles. Additionally, 2–3 nm Pt particles with the largest
contribution of surface atoms (with low coordination numbers)
were found to be the most active in the oxidation of CO under
lean conditions.[11c] Moreover, oxidation of NO occurred more
efficiently on large particles with predominant extended facets.
It is known that TOF increases with the growth of Pt particle size
catalysts with predominantly metallic platinum particles occurs
via the NH mechanism. On the contrary, over the N-Ox-400
catalyst with oxidized platinum species the N
mechanisms are dominating, similar to that on Pt/CeZrO
PtO species.[26] Since О
is not activated on ionic platinum, it can
be assumed that activation requires reduction of PtO to Pt with
abstraction of a hydrogen atom from NH . In this case oxygen
2
H
4
and HNO
2
with
2
0
3
supply and reoxidation of the reduced Pt sites may be the rate
determining step as is the case for the CO oxidation.[29]
due to effective dissociation of O
2
with the formation of reactive
oxygen species on large particles.[ This was confirmed by our
results, according to which an increase in Pt particle size from
2.6 to 8.1 nm leads to an increase in the TOF value by 30 times
(Fig.6d). Note that further particle growth to 22.6 nm does not
change TOF.
8a]
Effect of Pt particle size
Pt/Al
2 3
O catalysts with different Pt dispersion were prepared
(
average particle sizes of ~1, ~2.5, and ~8, Fig. 2). To control
the particle size of Pt, the prereduced catalyst was calcined in
inert atmosphere at different temperatures. According to the
XRD and TEM data, calcination in Ar at 550-600°С leads to the
transition of platinum from the atomically dispersed to the
nanoscale state (Fig. 2 and Table 1), while the charge state of
platinum changes negligibly (XPS data, Fig. 3). Note that in the
case of the preoxidized catalyst, calcination in air at 600°С
results in the growth of Pt particles and reduction of platinum
Effect of the reaction mixture
For each catalyst two (or three) heating-cooling cycles in the
reaction mixture were carried out during catalytic tests. In case
0
of the Pt/Al
2
O
3
catalyst with large Pt particles (~22.6 nm, Fig.
1g), a decrease in activity is observed during the second heating
(the conversion curve shifts towards higher temperatures, Fig.
4). This is accompanied by an increase in Pt dispersion and a
change in faceting of Pt particles, leading to a decrease in the
number of terrace sites and truncation of sharp angles between
{111} faces (TEM data, Fig. 7).
(
Figs. 1 and 3). The influence of both factors ensures evident
activation of the catalyst (a decrease in the T50 value by 77°С,
Fig. 4), while an increase in the size of metallic Pt particles is
accompanied by a shift of the NH
lower temperatures by only ~25°С (Fig. 6).
The most active Pt/Al
catalysts contain crystalline Pt0
3
conversion curve towards
In case of the highly dispersed (~1 nm) PtO
x
particles on the
slight
2
O
3
surface of the preoxidized Pt/Al catalyst,
2
O
3
a
particles with average sizes of ~8 and ~23 nm. Such catalysts
display similar T50 values. This indicates that the most significant
changes in activity take place when the Pt particle size changes
from ~1 to ~8 nm.
improvement in activity takes place after exposure to the
reaction medium (Fig. 4). The reduction of the oxidized Pt
species during the reaction might result in the improvement of
the activity. However, XPS data reveal only Pt4+ and Pt2+ species
on the surface of the sample after reaction without appearance
The low activity of the catalysts containing highly dispersed
0
metallic Pt is associated with the facile formation of PtO
directly under reaction conditions.[30] The appearance of such
species hinders activation of O
, in contrast to crystalline Pt0
nanoparticles. In the latter case, only a thin surface PtO layer is
2
species
of metallic Pt (Fig. S10, Table 3). We can propose that reduced
Pt species formed during the reaction are reoxidized upon
contact of the sample with O
2
. The presence of only ionic Pt4+
2
and Pt2 species confirms preservation of the highly dispersed
platinum particles during the reaction.
+
x
formed. It was found that chemisorbed oxygen on the surface of
metallic Pt shows significantly higher catalytic activity than
oxygen from the bulk platinum oxide.[30-31]
An increase in the size of Pt nanoparticles from ~1 to ~8 nm
leads to an increase in the number of platinum atoms with a high
In contrast to the preoxidized PtO
x
/Al
2
O
3
samples, the
0
prereduced Pt /Al
2
O
3
catalysts with platinum particle sizes of ~1
0
and ~8 nm are deactivated during catalytic tests (Fig. 6). For Pt
particles with a size of ~1 nm, an increase in the number of Pt4+
1
1
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10.1002/cctc.202001320
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sites is observed after the reaction (Table 3). The appearance of
Pt4+ species after catalytic tests is also observed for platinum
particles of ~8.1 nm (Table 3). Thus, the deactivation of
prereduced catalysts correlates with an increase in the
Implications for NH
3
oxidation catalyst development.
The obtained results suggest that in order to obtain active and
stable ammonia slip catalysts metallic Pt nanoparticles larger
than 1 nm (at the same time, not too large, ideally of approx. 3
nm to use Pt efficiently [ ) must be prepared. Sintering and
redispersion of Pt (which would result in oxidation of formed
small Pt species making them inactive) should be prevented.
Ways of achieving this are required to ensure stability of the
catalysts. Also keeping metallic Pt surface free of potentially
poisoning surface species is vital to achieve high activity at low
temperatures. Both conditions can be fulfilled via encapsulating
Pt NPs in inorganic oxide supports. One strategy to produce
such catalysts involves direct NP synthesis and stabilization in
the pores of hierarchical (micro-mesoporous) zeolites as
4
+
contribution of Pt species, which are significantly less active
than metallic Pt. The decrease in the activity of Pt catalysts can
11c]
2
be related to the effective formation of PtO species during the
reaction.[8b] Such species are formed as a result of the local
overheating of the catalyst grain due to the exothermic effect of
the oxidation reaction but easy oxidation of undercoordinated Pt
sites.
It may be concluded that the growth of the Pt4+ surface content
is accompanied by a decrease in the contribution of metallic Pt,
which activates O
oxidation activity. The catalytic activity of the prereduced
Pt/Al
sample with Pt0 particles of ~2.6 nm remains
unchanged due to the preservation of Pt surface state. The
appearance of small PtO species, as well as efficient oxidation
of Pt surface might be considered as the main reason for the
deactivation of Pt supported catalysts in NH oxidation.
2 3 3
and NH . This results in the drop of the NH
demonstrated for CO oxidation Pt [40] and CH
oxidation Pd
2
O
3
4
catalysts.[41] Another viable strategy involves synthesis of a
protective zeolite shell on top of Pt nanoparticles rendering an
active and stable core-shell catalyst.[42] In both cases the zeolite
x
3
3
can be functionalized with transition metals active in NH -SCR
and play a role of the SCR catalyst layer in close proximity to Pt
sites reducing the complexity of the monolith washcoating
procedure. Both strategies are proved to be working, however
the catalyst synthesis procedure becomes more complex.
Whether benefits of more stable catalysts and simpler
washcoating actually outweigh the cost of the more complex
catalyst synthesis depends on the exact application scenario.
Effect of Pt dispersion and Pt state on selectivity in the
NH oxidation
3
For the preoxidized and prereduced Pt/Al
similar Pt dispersion (~1 nm), the N and N
profiles are clearly different (Fig. 5). In case of the prereduced
2
O
3
catalysts with
2
2
O concentration
sample, the N
narrow low-temperature peaks at T<250 C, while for the
preoxidized catalyst, the corresponding curves contain one wide
2
/N
2
O concentration curves show additional
o
peak. Low-temperature peaks are presumably associated with Conclusions
the removal of N, NH , NO intermediate species that may inhibit
adsorption. Indeed, the platinum surface was found to be
A study of Pt/Al O catalysts for NH oxidation was performed
2 3 3
x
O
2
deactivated at low temperatures due to the formation of strongly
using a combination of structural, spectral, and catalytic
, NO and OH species.[
5, 38]
Atomic oxygen reacts
bonded N, NH
with NH
formed after abstraction of all H atoms from the adsorbed NH
x
methods. Catalysts with varied Pt state (metallic or oxidized) and
Pt dispersion (particle size from ~1 to ~23 nm) were prepared.
It has been shown that a prereduced catalyst containing metallic
Pt particles has significantly higher activity in the ammonia
oxidation than the preoxidized one with a similar dispersion of
Pt. The difference in the T50 value for oxidized and metallic
platinum particles was more than 50°C. It was established that
N2 selectivity (SN2) at T>270°C (corresponding to 100% NH3
conversion) weakly depended on the Pt state. Below 270°С, the
preoxidized PtO /γ-Al O catalyst showed higher S value in the
species to form OH groups,[25a, 25b, 26, 39] which are
x
3
molecule.[38] Molecular nitrogen is formed as a result of
recombination of Nad (ad = adsorbed) species, while the
interaction of Nad and Oad leads to the appearance of NOad.[38] An
increase in temperature removes adsorbed oxygen and NH
species with the formation of N and N O products (Fig. 4, 5, S7
x
2
2
and S8). The ratio of Nad and NOad species depends on the
reaction temperature. Nad species prevail on the platinum
x
2
3
N2
0
surface, leading to the predominant formation of N
2
at low
2 3
comparison with the prereduced Pt /γ-Al O . This difference in
temperatures. At high temperatures, the amount of NOad grows
SN2 can be explained by the increased contribution of N O
2
formed during desorption of surface intermediates of oxidative
ammonia dehydrogenation from the metallic surface. Such
[
25c, 38]
increasing the N
reaction scheme described above, it might be concluded that the
low-temperature peaks in the N /N O concentration curves are
2
O yield (Fig. S2). Considering the
2
2
intermediates may inhibit adsorption of O , while their removal
2
associated with the desorption of Nad and NOad intermediates in
the form of products, while the second peak is due to the steady-
state reaction taking place at higher temperature.
provides Pt surface sites available for the efficient NH oxidation
3
with the formation of N and N O at relatively low temperatures.
2
2
It was shown that the most significant improvement in catalytic
activity took place when the size of Pt metallic particles was
increased from ~1 to ~8 nm. In this case, the T50 value
decreased by 28°C, while the TOF value increased by 30 times.
A further increase in the size of metallic Pt particles (up to ~23
nm) did not lead to additional activity improvement. It was found
The effect of Pt crystallite size on N
oxidation reaction is rather weak (Fig. 6). The difference in SN2
values in the range of ~275–400°С is 10% for Pt/Al catalysts
with platinum particles of ~1, ~2.6, and ~8 nm. Larger Pt
particles show higher N selectivity due to a large number of
2 3
selectivity in the NH
2
O
3
2
terraces on the surface.[
<
25c]
Note that higher SN2 was found at T
that the N2 selectivity during NH3 oxidation over Pt/Al O3
2
catalysts was mainly determined by the reaction temperature. At
temperatures above 225°C, large Pt0 particles (8-23 nm)
demonstrated 10% higher N2 selectivity than ~1 nm particles.
Below 225°C, a slight increase in SN2 along with the decreasing
Pt crystallite size was observed.
200°C over smaller Pt crystallites.[8a] It was explained by a
decreased concentration of active oxygen on the surface of
small crystallites, leading to a decreased specific activity and an
increased N
of N selectivity on the particle size Pt revealed the opposite
trend.
2
selectivity. However, at T>200°C, the dependence
2
1
2
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ChemCatChem
10.1002/cctc.202001320
FULL PAPER
The influence of the reaction mixture on the catalytic properties
has been studied through repeated heating/cooling cycles. In the
LVol-IB values (i.e., volume-weighted mean column lengths based on
integral breadth).
2 3
case of the preoxidized Pt/Al O catalyst with highly dispersed Pt
Transmission Electron microscopy (TEM)
species, an increase in activity as a result of the exposure to the
reaction mixture was revealed. It was associated with the
TEM investigation was performed using
a
Cs-corrected JEOL
4
+
2+
presence of Pt /Pt species on the catalyst surface. In this
case, the metallic Pt was not observed, and the average size of
JEM-2200FS electron microscope installed at the “VTAN” resource
center of Novosibirsk State University, Russia; and a Thermo Fisher
Scientific Themis Z double Cs-corrected electron microscope installed at
the Boreskov Institute of Catalysis, Russia. Both microscopes operated
at an accelerating voltage of 200 kV, the spatial lattice resolution was 1.0
Å and 0.7 Å, correspondingly. Bright field images (BF-TEM) were
acquired in traditional transmission mode. Dark field images were
acquired in Scanning mode (STEM) using High-Angle Annular Dark-Field
detectors (HAADF). Particle size distribution diagrams were obtained
using “ImageJ” software.[45] To determine the size of the irregularly
shaped particles, the diameter of a circle having the same projected area
as the particle was determined. The samples for the TEM study were
dispersed in ethanol in an ultrasonic bath and supported on a holey
carbon film mounted on a copper grid.
Pt particles was almost unchanged after tests. In the case of
prereduced catalysts containing Pt0 particles, deactivation was
observed during catalytic measurements due to the increase of
Pt dispersion and partial Pt oxidation. The redispersion of large
metallic particles may occur due to formation of volatile PtO
species. PtO species can result from oxidation of surface of
metallic particles at elevated temperature and under high О
pressure. In this case the volatile PtO species can be trapped
2
2
2
2
by defect alumina surface leading to the appearance of
additional highly dispersed species.
Additional measurements were carried out in a realistic reaction
mixture containing H
was accompanied by a slight shift of the NH
towards higher temperatures (∆T50 <15°C), while the trends of
the activity and selectivity of Pt/Al catalysts on the Pt
2
O and CO
2
. The addition of Н
2 2
О and СО
X-ray photoelectron spectroscopy (XPS)
3
conversion curves
The X-ray photoelectron spectroscopy (XPS) was applied using an ES-
300 (KRATOS Analytical, UK) spectrometer. A non-monochromatic
2
O
3
dispersion and Pt state were completely preserved.
MgKα source (hν=1253.6 eV) was used for electron emission. The core-
level spectra of bulk metallic gold and copper with reference values
E
b
b
(Au4f7/2) = 84.0 eV and E (Cu2p3/2) = 932.7 eV were used for the
spectrometer calibration [ . In accordance with literature data for -
46]
Experimental Section
2 3 b
Al O , the Al2p line with a binding energy E =74.5 eV was used as an
internal standard for calibration of the spectra [46].
Catalyst Preparation
The ratio of aluminum and platinum atomic concentrations on the surface
(
Ptat/Alat) was estimated from the areas of the corresponding core-level
2
wt.% Pt/γ-Al
2
O
3
catalysts were prepared by incipient wetness
[
47]
spectra taking into account the atomic sensitivity factors . Cl2p spectra
were acquired to check for the residual chlorine in the catalysts prepared
using H PtCl . For all samples surface Cl content was negligible. Pt4f
2 6
spectra were used to analyze the charge state of platinum after the
subtraction of the Al2p signal. The detailed description of the extraction
of Pt4f signal from overlapping Pt4f+Al2p spectral lines can be found
elsewhere
program, developed at the Boreskov Institute of Catalysis, and tested
previously on a number of the metal-oxide catalytic systems [16, 48]
impregnation using aqueous solutions of hexachloroplatinic acid or
platinum nitrate. Alumina support was obtained by air-calcination of Pural
SCF-55 aluminum hydroxide (Boehmite Al(OH)O, Sasol) at 750°C for 4
hours.
A solution of platinum nitrate was obtained by dissolving H
water at 50-80°C using nitrogen dioxide.[43] A weak stream of NO
passed through a pale yellow suspension of H [Pt(OH) ] in water at 50-
0°C with constant stirring. The NO flow was not stopped until the
2
[Pt(OH)
6
] in
2
was
[
24]
.
Spectral processing was performed using XPS-Calc
2
6
7
2
.
precipitate was completely dissolved and the color of the solution turned
greenish. After 24 hours, the solution color changed to yellow-orange due
to the oxidation of nitrosyl ligands in the platinum complex. No further
changes to the solution occurred. The impregnation was carried out at
room temperature (RT) with subsequent drying at RT (for 16 h), 60°C (for
Measurement of catalytic activity in a model reaction
mixture, TPD-NH
3
The measurements were carried out in an automated setup (setup #1)
with a plug flow quartz reactor using a temperature-programmed reaction
of NH
desorption of NH
Concentrations of NH
FTIR spectrometer (MIDAC corp., USA). Concentrations of O
were determined using gas chromatography (Crystal-2000M,
CHROMATEC, Russia). Before measurements, the catalysts were
2
pretreated in a mixture of 20%O /He or in pure He at 400°С for 2 h. The
pretreatment reaction medium was 20%O
and helium for Ar-calcined samples.
The catalyst weight was 0.145 g for catalytic activity study. The initial
reaction mixture containing 0.1 vol. % NH
was supplied at a flow rate of 500 cm /min at room temperature. The
space velocity was 120,000 h . The sample was heated twice in the
reaction mixture to 400°C at a rate of 10°C/min with intermediate cooling
to room temperature.
The catalyst weight used for TPD-NH
reaction mixture containing 0.1 vol. % NH
a flow rate of 500 cm /min at room temperature. After NH
reached the initial value, the sample was purged with helium (500
cm /min) to a residual NH
heated in He to 400°C at a rate of 10°C/min.
1 h) and 120°C (for 2 h). Then, the obtained powder was air-calcined at
400°C (for 4 h). Part of the catalyst was additionally calcined in air at
600°C (for 4 h).
3
oxidation (TPR-NH
(TPD-NH ). The catalyst grain size was 0.14-0.25 mm.
, N O, NO, NO were determined using an I1801
and N
3
+O
2
)
and temperature-programmed
3
3
Similarly, catalysts were prepared using H
by calcination in air at 400°C). The air-calcined samples were prereduced
in Н at 350°С (for 2 h) and calcined at 400, 550, or 600°С in Ar (for 6 h).
Then, the catalysts were washed from chlorine ions with aqueous
ammonia and distilled water. The washed catalysts were sequentially
dried in air at RT (for 16 h), 60°C (for 1 h) and 120°C (for 2 h) and, finally,
calcined in argon at 400°C (for 2 h).
The final catalysts are denoted as N/Cl-Ox/Red-T, where N/Cl is Pt(NO
or H PtCl , Ox/Red is the oxidative or reductive pretreatment, and T is
the temperature of the final pretreatment.
2 6
PtCl as a precursor (followed
3
2
2
2
2
2
2
/He for air-calcined samples
3 x
)
2
6
3
and 4 vol. % O
2
(He balance)
3
-
1
X-ray diffraction (XRD)
X-ray diffraction patterns were obtained on a Bruker D8 diffractometer
Germany) using CuK radiation. To record the signal, a one-dimensional
3
study was 0.2 g. The initial
3
(He balance) was supplied at
(
α
LynxEye detector was used. The measurement was carried out in the
range of angles 2θ = 15-90° with a step of 0.05° and acquisition time per
point of 5 s. For phase analysis, the ICDD PDF-2 database was used.
The refinement of the structure and profile analysis were performed in
3
3
concentration
3
3
concentration below 5 ppm. The sample was
the TOPAS software package.[44] The γ-Al
O
2 3
phase was described by a
set of corresponding reflections; the phase of metallic Pt was described
by the Rietveld method. The crystallite size (D) was calculated using
Catalytic measurements in a realistic reaction mixture
1
3
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.202001320
FULL PAPER
Tests were also carried out in a plug flow quartz reactor in setup #2. The
[7]
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Il'chenko, Russ. Chem. Rev. 1976, 45, 1119-1134.
initial reaction mixture contained 500 ppm NH
3 2
and 13%O (balanced by
2
N ). Additionally, tests were carried out in a mixture containing 500 ppm
3
NH , 13%O
2
, 5%H
2
O, and 10%CO
2
2
(balanced by N ). The feed rate of
3
the reaction mixture of 1050 cm /min corresponded to a space velocity of
-
1
6
00,000 h . 25.2 mg of catalyst were diluted with SiC. The samples were
heated twice in the reaction mixture of NH + O from room temperature
to 400°C at a rate of 3°C/min with intermediate cooling to 50°C. In the
third heating-cooling cycle, H O and CO were added to the initial
3
2
[8]
[9]
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1976, 41, 277-292.
2
2
mixture. The analysis of the reaction composition at the outlet of the
reactor was carried out using Multigas 2030 FTIR (MKS Instruments,
M. Machida, Y. Tokudome, A. Maeda, Y. Kuzuhara, T. Hirakawa, T.
Sato, H. Yoshida, J. Ohyama, K. Fujii, N. Ishikawa, ACS Catal. 2020,
10, 4677-4685.
USA). N
balance for all reaction products.
The calculation of NH conversion, product selectivity (for N
and NO ) was carried out according to the following expressions:
2
concentration was calculated taking into account the total N
3
2 2
O, N , NO,
[10] (a) S. Gan, Y. Liang, D. R. Baer, M. R. Sievers, G. S. Herman, C. H. F.
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2
푖푛푙푒푡
ꢂ
표푢푡푙푒푡
ꢂ
퐶ꢀꢁ −퐶ꢀꢁ
푋
푁퐻3
=
,
푖푛푙푒푡
퐶
ꢀ
ꢁꢂ
ꢃ×퐶ꢀ2
2ꢄ+ꢃ×퐶ꢀ2+퐶ꢀꢄ+퐶ꢀꢄ
ꢃ×퐶ꢀ2
2ꢄ+ꢃ×퐶ꢀ2+퐶ꢀꢄ+퐶ꢀꢄ
퐶ꢀꢄ
2ꢄ+ꢃ×퐶ꢀ2+퐶ꢀꢄ+퐶ꢀꢄ
ꢄ
푆푁2푂 = ꢃ×퐶ꢀ
푆푁2 = ꢃ×퐶ꢀ
푆푁푂 = ꢃ×퐶ꢀ
푆푁푂2 = ꢃ×퐶ꢀ
× 100%
,
2
[11] (a) A. C. M. van den Broek, Technische Universiteit Eindhoven
(Eindhoven), 1998; (b) D. P. Sobczyk, E. J. M. Hensen, A. M. de Jong,
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L. J. Lemus-Yegres, J.-D. Grunwaldt, Appl. Catal. B: Environ. 2012,
126, 315-325.
× 100%
,
2
× 100%
,
2
퐶ꢀꢄ2
× 100%,
2ꢄ+ꢃ×퐶ꢀ2+퐶ꢀꢄ+퐶ꢀꢄ2
[
12] T. K. Hansen, Ph.D. Thesis thesis, Technical University of Denmark
(DTU) 2017.
ꢆꢇꢈꢉꢊ
ꢋꢌꢊꢈꢉꢊ
where 푋푁퐻3 – NH
3
conversion
,
ꢅ
– inlet NH
3
concentration, ꢅ푁퐻ꢂ
concentration, ꢅ푁2
concentration.
–
푁퐻ꢂ
outlet NH
3
concentration, ꢅ푁2푂
–
2
N O
-
N
2
[13]
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concentration, ꢅ푁푂 – NO concentration, ꢅ푁푂2 – NO
2
Rate of the catalytic reaction was calculated from the catalytic data
obtained at low NH conversions (from 0 to 20%) using the following
3
expression:
푖푛푙푒푡
푚ꢋꢈ
퐶ꢀꢁ ×ꢍ×푉푅푀
ꢂ
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푊 (
) =
,
푚ꢋꢈ×푠
푃ꢊꢎ푚ꢋꢈꢏ
ꢆꢇꢈꢉꢊ
where ꢅ푁퐻ꢂ is the initial concentration of NH
the NH conversion, VRM is the reaction mixture rate (in mol/s), and
Pt(mol) – Pt content (in moles).
Estimation of catalytic activity taking into an account Pt dispersion
3
(vol.% divided by 100), X is
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(
turnover frequency - TOF) was calculated using the next expression:
ꢓ
푇ꢐ퐹ꢎꢑ−ꢒꢏ =
.
퐷ꢔ퐸푀
TEM-derived dispersion (DTEM) were obtained using the equation:[49]
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17] (a) C. B. Alcock, G. W. Hooper, Proceedings of the Royal Society of
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ꢃ
〈
푑 〉
〈푑〉 0.1ꢜ1
ꢕꢖꢗꢘ = ꢎ1.48ꢙ ×
ꢚ 0.7ꢙꢙ ×
〈푑3〉 〈푑3〉 ꢏ ∗ 100
ꢛ
3
〈
푑 〉
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ChemCatChem
10.1002/cctc.202001320
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The fundamental for heterogeneous catalysis factors such as dispersion and metallic/oxidized states of platinum were investigated in
0
the NH
3
oxidation over Pt/Al
2
O
3
catalysts. For Pt , the catalytic activity can be improved by the increase of average particle size from
~
1 to ~8 nm. PtO
x
species demonstrate significantly lower activity in the comparison with metallic Pt.
1
6
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