In situ FTIR spectroscopy of highly dispersed FeOx catalysts for NO reduction: Role of Na promoter

Article abstract of DOI:10.1016/j.cattod.2015.12.014

The effect of Na on highly dispersed FeOx impregnated onto CeO₂ via the unique precursor Na/Fe-ethylenediaminetetraacetate (NaFeEDTA) was investigated by comparison to a series of well-defined catalysts synthesized by the traditional precursor Fe(NO₃)₃ both with and without Na addition. Catalysts were evaluated for steady-state NO reduction by CO and activities varied based on synthesis method and Fe:Na ratio. Na contributed a promoter effect when added at a stoichiometric ratio (Fe:Na = 1), providing an explanation for the higher activity of NaFeEDTA/CeO₂ for NO reduction by CO. Activity decreased when excess Na was present in Fe(NO₃)₃ catalysts, but the stoichiometric promoter effect persisted up to ～4.0 Fe/nm². In situ Fourier transform infrared (FTIR) spectroscopy during NO adsorption revealed the presence of unique NO adsorption species (1460 cm^-1) on the NaFeEDTA/CeO₂, suggesting enhanced NO adsorption due to Na. At reaction temperature, FTIR bands of bulk nitrates on CeO₂ were quantitatively shown to more rapidly undergo NO reduction catalytic transformations over NaFeEDTA/CeO₂. These results increase understanding of mechanistic effects of Na on NO reduction over FeOx/CeO₂ catalysts and serve to guide future design of oxide-based emission control catalysts that are free of Pt-group metals.

Full text of DOI:10.1016/j.cattod.2015.12.014

Catalysis Today 267 (2016) 56–64
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
In situ FTIR spectroscopy of highly dispersed FeOx catalysts for NO
reduction: Role of Na promoter
a,∗
b
b
a
Charles A. Roberts b , Louisa Savereide , David J. Childers , Torin C. Peck ,
Justin M. Notestein
Toyota Research Institute of North America, 1555 Woodridge Ave., Ann Arbor, MI 48105, United States
a
b
Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Rd., Evanston, IL 60208, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 October 2015
Received in revised form 4 December 2015
Accepted 15 December 2015
Available online 15 January 2016
The effect of Na on highly dispersed FeOx impregnated onto CeO2 via the unique precursor Na/Fe-
ethylenediaminetetraacetate (NaFeEDTA) was investigated by comparison to a series of well-defined
catalysts synthesized by the traditional precursor Fe(NO3)3 both with and without Na addition. Catalysts
were evaluated for steady-state NO reduction by CO and activities varied based on synthesis method and
Fe:Na ratio. Na contributed a promoter effect when added at a stoichiometric ratio (Fe:Na = 1), provid-
ing an explanation for the higher activity of NaFeEDTA/CeO2 for NO reduction by CO. Activity decreased
when excess Na was present in Fe(NO3)3 catalysts, but the stoichiometric promoter effect persisted up
Keywords:
Nitric oxide
Iron oxide
FTIR
In situ
Sodium promoter
Carbon monoxide
2
to ∼4.0 Fe/nm . In situ Fourier transform infrared (FTIR) spectroscopy during NO adsorption revealed
−1
the presence of unique NO adsorption species (1460 cm ) on the NaFeEDTA/CeO2, suggesting enhanced
NO adsorption due to Na. At reaction temperature, FTIR bands of bulk nitrates on CeO2 were quanti-
tatively shown to more rapidly undergo NO reduction catalytic transformations over NaFeEDTA/CeO2.
These results increase understanding of mechanistic effects of Na on NO reduction over FeOx/CeO2 cata-
lysts and serve to guide future design of oxide-based emission control catalysts that are free of Pt-group
metals.
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
ity [8–10]. The traditional synthesis approach of incipient wetness
impregnation (IWI) of metal nitrate precursors, however, is not
conducive to the formation of homogeneous surface oxide struc-
tures [11,12], limiting the ability to design supported metal oxides
with NO reduction catalytic active sites analogous to those in the
zeolites. This issue is not new to supported metal oxide catalyst
synthesis, and significant effort has been made to improve uniform
nanostructuring of surface metal oxide active sites [13,14].
The removal of NOx from automotive exhaust remains an impor-
tant challenge for design of catalytic materials. While the three-way
catalyst is a remarkable example of successful design, it remains
dependent on scarce platinum group metals (PGM) such as Pt, Pd,
and Rh [1]. It is, therefore, desirable to discover new NO removal
catalysts utilizing earth-abundant materials. Over the past two
decades, extensive research on Fe-exchanged zeolites has shown
good activity for selective catalytic reduction (SCR) of NO [1–7],
demonstrating the potential utility of earth-abundant base-metal
oxides in NO reduction reactions. The active sites responsible for the
NO reduction activity are generally accepted to be low-coordinated
FeOx species [3–7].
Recently, Prieto-Centurion et al. have improved on the abil-
ity of IWI to create
surface FeOx species on CeO2 [15,16]. By utilizing a Na/Fe-
ethylenediaminetetraacetate (NaFeEDTA) precursor, higher
a more homogeneous distribution of
a
density of redox cycling FeOx sites could be produced when
compared to catalysts synthesized by the traditional Fe(NO ) pre-
3
3
This deeper understanding of the zeolite system suggests the
possibility of creating similar surface metal oxide active sites on
cursor. The fraction of redox cycling Fe was directly correlated with
activity for NO reduction by both H2 and CO, and correspondingly,
the activity of catalysts prepared via NaFeEDTA was higher than
those prepared via Fe(NO ) [16,17]. It is notable, however, that use
non-zeolitic oxide supports such as CeO , ZrO , and SiO , which
2
2
2
can introduce a variety of support effects such as increased stabil-
3
3
of the NaFeEDTA precursor necessarily deposits Na onto the CeO2
surface at a Fe:Na ratio of 1:1 during IWI, which introduces ambi-
guity into the unique role of the organic ligand to improve catalysts
synthesis and activity. While previous studies sought to account for
∗ Corresponding author. Fax: +1 734 995 2549.
E-mail address: charles.roberts@toyota.com (C.A. Roberts).
http://dx.doi.org/10.1016/j.cattod.2015.12.014
0
920-5861/© 2016 Elsevier B.V. All rights reserved.

C.A. Roberts et al. / Catalysis Today 267 (2016) 56–64
57
Table 1
this difference by synthesizing Na-modified CeO prior to Fe(NO )
3 3
2
Characteristics of catalysts in this study prepared using various precursors.
impregnation, the role of Na in the NO reduction reaction remained
unaddressed as catalysts without Na were not compared.
Fe precursor
Na precursor
Support
Fe loadinga (Fe/nm2)
Fe:Na ratio
The role of Na or other alkali atoms in deNOx catalysis has
been found to vary. Reports of improved NO reduction activity in
the presence of soot due to alkali promotion were attributed to
improved NO adsorption properties [18,19]. This finding is similar
to those made for alkali promotion of Pd supported on yttria-
stabilized-zirconia and Rh on zeolite Y catalysts for NO reduction
by CO, where the alkali introduced enhanced NO surface bonding
or resulted in desirable bridge bonded NO [20,21]. Conversely, for
–
–
–
CeO2
CeO2
CeO2
Na-CeO2
0
0:0
1:0
1:1
1:1
1:2
1:3
1:4
1:1
1:1
2:1
4:1
Fe(NO3)3
NaFeEDTA
Fe(NO3)3
1.11
0.94
0.98
NaHCO3
1
1
1
.02
.02
.02
2.04
4
2
4
.03
.05
.03
NH -SCR, poisoning of Brønsted acid sites in VOx catalysts by alkali
3
ions competes with the adsorption of NH3 [22–24].
a
Loadings confirmed by ICP-OES. Surface density calculated using the BET surface
The current study seeks to determine the role of Na as a poison
or promoter in supported FeOx/CeO2 catalysts for NO reduction
by CO by first replicating previous results utilizing the NaFeEDTA
precursor and comparing activity to reference catalysts without
Na and with various ratios of Fe:Na. Once the role of Na is estab-
lished, deeper insight into its mechanistic effect on NO reduction
is explored by in situ NO adsorption FTIR spectroscopy. This tech-
nique will facilitate understanding of the effect on NO adsorption;
a process that is typically thought to be affected by Na addition.
Finally, the reactivity of NO adsorption species is determined with
time-resolved in situ FTIR spectroscopy under model NO reduction
by H2 reaction conditions. Understanding of the role of Na in NO
reduction over FeOx/CeO2 catalysts is expected to support future
rational design of improved supported base-metal oxide catalysts.
2
area of neat CeO2, 101 m /g.
of the catalyst samples used in this study are summarized in Table 1.
The final catalysts prepared by NaFeEDTA, Fe(NO3)3 and NaHCO3,
and only Fe(NO3)3 are denoted as NaFeEDTA/CeO2, Fe(NO3)3/Na-
CeO2, and Fe(NO3)3/CeO2, respectively.
2.2. Steady-state catalytic activity
Steady-state catalytic activities were measured using a fixed bed
quartz tubular reactor with temperature control and gas flow con-
trol. The reactor was loaded with ∼10–20 mg of catalyst powder
that was mixed with quartz sand as a diluent in order to achieve
−
1
a constant gas hourly space velocity (GHSV) of ∼15,000 h . The
◦
catalyst was first pretreated in 30 sccm of 10% O /He at 500 C for
2
2
. Materials and methods
30 min, and then cooled in flowing UHP He. The reduction of NO
◦
◦
by CO was monitored at 250 C and 450 C for 45 min each and
a total flowrate of 100 sccm with a stoichiometric composition of
4000 ppm NO, 4000 ppm CO, 8000 ppm Ar, and He balance. Mass
spectrometry of the gas phase product stream was obtained contin-
uously using a quadrupole mass spectrometer (MKS Instruments,
Inc., Cirrus-2). The Ar present in the reactant stream acted as a tracer
of constant concentration and the Ar signal at m/z = 40 was used to
normalize each of the mass spectrum traces.
2.1. Catalyst synthesis, nitrogen physisorption, and elemental
analysis
Incipient wetness impregnation (IWI) of non-porous CeO2 (C.I.
2
Kasei Co., Ltd., NanoTek powder, BET SSA = 101 m /g) was per-
formed using various Fe-containing precursors. The precursor
sodium Fe(III) ethylenediaminetetraacetate (NaFeEDTA) was pre-
pared following the method of Prieto-Centurion et al. [15] which
is based on that by Meier and Heinemann [25]. Due to solubility
limits of the NaFeEDTA, multiple cycles of impregnation, drying
overnight, and re-impregnation (1–3 cycles prior to calcination)
were used to achieve the desired total Fe surface density. Cal-
The 100% and 0% conversion of NO was defined as the normal-
ized mass spectrum intensity at m/z = 30 under inert flow only, and
in the presence of the NO + CO reaction mixture bypassing the reac-
tor, respectively. Linear interpolation determined the resulting NO
conversions at steady state temperatures. No significant amount of
NO2 was produced, based on the absence of fragments observed at
◦
cination was performed in air at 550 C for ∼1 h to remove the
◦
precursor ligand and form the highly dispersed Fe species. Using
NaFeEDTA necessarily also impregnates Na onto the CeO2 support
in a 1:1 atomic ratio of Fe:Na. Other catalysts were synthesized
using Fe(NO ) ·9H O as the source of Fe and NaHCO as the source
m/z = 46. Rates at 250 C are at differential NO conversions (<15%).
The steady state NO conversion percentage for each of the catalysts
is reported in Table S1. The reported activity is normalized by total
mass of catalyst.
3
3
2
3
of Na. The CeO support was first impregnated with NaHCO3
2
and dried overnight to yield Na-modified CeO . The modified Na-
2.3. In situ transmission FTIR spectroscopy
2
CeO support was then impregnated with Fe(NO ) ·9H O. Catalysts
2
3
3
2
2
2
were synthesized with up to 4 Fe/nm or 4 Na/nm and varying
Fe:Na atomic ratios (Table 1). Characterization by X-ray diffraction,
Raman spectroscopy, and Mössbauer spectroscopy was reported
previously by several of the authors [17], and confirmed the nature
of the catalysts as highly dispersed, non-crystalline, surface FeOx
species on the CeO2 support, regardless of preparation method.
The Harrick High Temperature Cell with environmental (gas
flow) and temperature control was used for in situ transmission
FTIR spectroscopy. Spectra were recorded using a Thermo Scien-
tific Nicolet 8700 Research FT-IR Spectrometer equipped with a
liquid N2 cooled MCT detector. Spectra were obtained with a reso-
lution of 2 cm and by averaging 16 scans. The optical scan velocity
was set to 3.1647 cm/s, which resulted in a collection time of 4.55 s
per spectrum. Catalyst pellets were prepared by first diluting the
catalyst with spectroscopy grade KBr at a ratio of 1:5 catalyst:KBr.
Then ∼50 mg of catalyst were pressed at ∼1 tonnes for 1 min using
a Specac 13 mm Pellet Die and Manual Hydraulic Press.
−
1
Nitrogen physisorption isotherms were obtained using
a
Micromeritics ASAP 2010 instrument. Samples were degassed for
◦
6
h at <5 mTorr and 120 C to release absorbed water. Specific sur-
face areas were calculated from the N2 physisorption isotherms
using the BET equation. Elemental analysis was performed using
a Varian MPX ICP-OES instrument. Samples were prepared by dis-
solving 50 mg of solids in 1 mL of HF and subsequently diluting to
In situ transmission FTIR spectra were collected during NO
◦
adsorption in the presence of O2 at 50 C. Prior to NO + O adsorp-
2
◦
5
0 mL with H O. Metal contents were quantified by comparison
tion, the sample was first pretreated at 375 C in 20 sccm of 10%
2
against standards of known concentration. The resulting analyses
O /He. The background spectrum (64 scans) was of the catalyst
2

5
8
C.A. Roberts et al. / Catalysis Today 267 (2016) 56–64
◦
Fig. 2. Mass normalized steady-state activity for NO reduction by CO at 250 C over
FeOx/Na-CeO2 catalysts synthesized with Fe(NO3)3 with constant Fe surface density
◦
2
2
Fig. 1. Mass normalized steady-state activity for NO reduction by CO at 250 C over
FeOx/CeO2 catalysts synthesized with various precursors, similar Fe surface den-
sities, and with and without Na. The blank reactor and the bare CeO2 support are
shown for reference.
(1.0 Fe/nm ) and increasing Na surface density (∼1.0–4.0 Na/nm ). The bare CeO2
support is shown for reference.
◦
after cooling to 50 C in 50 sccm UHP He. Adsorption of NO + O was
2
achieved by flowing 50 sccm of 3000 ppm NO and 1% O2 (15 sccm
1
.0% NO/He, 5 sccm 10% O /He, 30 sccm UHP He) over the catalyst
2
pellet for 20 min. Adsorption of NO + O was allowed to proceed for
2
2
0 min while spectra were obtained every minute using a series
collection. To compare peak intensities among different catalyst
samples, the adsorption spectra were normalized to the NO gas
−1
phase peak at ∼1874 cm
.
Next, in situ transmission FTIR spectra were collected during
NO reduction by H2 under lean and stoichiometric cycles. After
the same pretreatment performed above, the sample was cooled
◦
to the reaction temperature of 350 C in 50 sccm of UHP He and
the background spectrum was obtained. Lean NO + O2 adsorption
◦
was carried out at 350 C using the same conditions and spectra
◦
Fig. 3. Mass normalized steady-state activity for NO reduction by CO at 450 C over
collection rate as above. Stoichiometric (Stoichi) NO + H2 reaction
was carried out in the presence of oxygen by flowing 50 sccm of
000 ppm NO, 2.4% H , and 1% O2 (15 sccm 1.0% NO/He, 30 sccm
.0% H /He, 5 sccm 10% O /He). Spectra were then collected every
.55 s during cycles of Stoichi/Lean/Stoichi for 5/10/10 min.
FeOx/Na-CeO2 catalysts synthesized with Fe(NO3)3 with constant Fe surface density
∼1.0 Fe atoms/nm ) and increasing Na surface density. The bare CeO2 support is
2
(
3
4
4
shown for reference.
2
2
2
the two Na-containing synthesis routes are the step-wise vs. simul-
taneous impregnation of Na and Fe for Fe(NO3)3 and NaFeEDTA
precursors, respectively, and the use of the chelating EDTA lig-
and vs. the nitrate salt. Co-depositing Na and Fe via the NaFeEDTA
precursor may create an effect similar to locally high densities of
the two cations, and this hypothesis is tested by increasing the Na
and Fe surface densities in classically-prepared Fe(NO3)3/Na-CeO2
catalysts.
3
. Results and discussion
3
.1. Effect of Na on NO + CO activity—presence or absence of Na
The mass normalized steady-state NO reduction by CO activ-
ity at 250 C for several FeOx/CeO catalysts with similar Fe surface
◦
2
2
densities (Fe/nm ∼1.0) is summarized in Fig. 1. These results repli-
cate a previous demonstration that NaFeEDTA/CeO2 had higher
activity (1.93 ␮mol NO/g/s) than a Na-promoted FeOx/Na-CeO2
catalyst synthesized by the more traditional Fe(NO3)3 precur-
sor (1.43 ␮mol NO/g/s) [17]. Previous H2 temperature controlled
reduction (H -TPR) analysis of the Na-promoted Fe(NO ) /Na-
3.2. Effect of Na on NO + CO activity—Fe:Na ratio
◦
Steady-state NO reduction by CO activity at 250 C for
FeOx/Na/CeO catalysts with constant Fe surface densities (Fe/nm
2
2
2
∼1.0) but increasing Na surface densities (∼1.0–4.0 Na/nm ) is
2
3 3
CeO2 and NaFeEDTA/CeO catalysts quantified the fraction of
summarized in Fig. 2. Adding excess Na relative to Fe does not
significantly improve the overall activity of the system, and at
the highest loadings of Na, activity decreases slightly from 1.5 to
1.2 ␮mol/g/s.
2
3
+
2+
redox cycling Fe /Fe sites [16]. It was found that NaFeEDTA-
2
synthesized catalysts at ∼1.0 Fe/nm contained 22% more of these
sites, and this factor was attributed to the increased activity for NO
reduction by H2 or CO [16,17].
◦
Steady-state NO reduction by CO activity at 450 C for FeOx/Na-
To build on the previous study, a catalyst without Na promotion
was compared. Without Na, the activity of Fe(NO ) /CeO drops
by nearly a factor of 2 to 0.55 ␮mol NO/g/s (Fig. 1). The activity of
CeO2 catalysts with increasing Fe surface densities (Fe/nm2
∼1.0–4.0) and constant or increasing Na surface densities is sum-
3
3
2
◦
marized in Fig. 3. Activity is shown at 450 C in order to improve the
the non-promoted Fe(NO ) /CeO catalyst is above that of the bare
CeO2 support (0.43 ␮mol NO/g/s), but the Na is clearly acting as a
promoter for NO reduction by CO.
clarity of the trends. Even though the steady-state NO conversion
exceeded the typical differential conversions, similar trends were
seen at 250 C but are less pronounced due to the absolute magni-
3
3
2
◦
From this data set alone, it is not possible to determine how Na
promotes the catalytic activity. The primary differences between
tude of rates (see Table S1 and Fig. S1 in Supplementary material).
At the 1:1 stoichiometry, increasing Na and Fe increases activity

C.A. Roberts et al. / Catalysis Today 267 (2016) 56–64
59
up until the highest surface density (Fe/nm = Na/nm2 ∼4.0), at
2
This observation is consistent with the relatively low Fe content
compared to Ce. The presence of Fe in the Fe(NO3)3/CeO2 cata-
lyst is confirmed, however, by the peak at 1792 cm , which also
which point activity falls. In contrast, as long as the Na surface den-
2
−1
sity is maintained at ∼1.0 Na/nm , catalytic activity increases at
2
the highest loading 4 Fe/nm . Up until this highest surface density,
appears to contain a second feature as evidenced by the shoulder at
−
1
however, materials synthesized with the 1:1 stoichiometry out-
perform materials synthesized with excess Fe. Overall, high rates
appear to be associated with stoichiometric Fe:Na, with no benefit
afforded from excess Na and an apparent limit of 4 Na/nm2 before
reactivity falls off. The fact that Na is deposited before Fe during IWI
of Fe(NO ) could account for this behavior in a number of ways. As
some of us have previously proposed, stoichiometric Na already on
the surface likely acts as a structural promoter, helping disperse the
Fe [15,16]. However, at the highest Na loadings, the Na may block
surface sites where Fe could adsorb to the ceria. We have previ-
ously argued for the importance of the Fe–O–Ce interface for NO
reduction by CO, which is based on experimental X-ray absorption
spectroscopy results [17] and is in agreement with DFT calculations
1764 cm . Adsorbed nitrosyl vibrations in this region are generally
2
+
2+
3+
assigned to a mononitrosyl on Fe (Fe -NO). In general Fe -NO
is not expected, but in the case of supported Fe supported systems
3+
such as Fe on ZrO2 or Al2O3, Fe -NO was assigned to this region
[32,33]. Therefore, the presence of Fe2+-NO and Fe -NO likely con-
3+
−
1
tributes to the features at 1764 and 1792 cm . The presence of
3
3
2+
Fe species is in agreement with previous characterization of the
FeOx/CeO2 catalysts by XAS and Mössbauer spectroscopy, which
found the as-prepared catalysts contained a fraction, albeit small,
of Fe2+ [16,17].
When Na promotion (Fe:Na = 1:1) is added to the FeOx/CeO2
catalyst through synthesis by NaFeEDTA, it is again observed that
the NO adsorption FTIR spectrum is largely dominated by the fea-
tures characteristic of adsorbed species found on the CeO2 support
for FeOx/CeO [26]. Likewise, in the presence of excess Na, extended
2
−
1
alkali metal oxides may form, which are less reactive than the cor-
responding electronically-promoted ceria [27]. It is also possible
that during reaction with CO, excess alkali leads to the formation
of unreactive carbon on the surface that blocks access to active sites
during the first few turnovers [28]. Further investigations into the
role of the Na promoter were carried out with FTIR spectroscopy.
(Fig. 4a). The Fe nitrosyl peak around 1792 cm
has decreased
dramatically for this sample compared to the other Fe-containing
catalyst. The NaFeEDTA/CeO2 has nearly the same Fe surface den-
2
sity (∼1.0 Fe/nm ) as the unpromoted Fe(NO3)3/CeO2 catalyst,
however, the significantly lower intensity of the Fe nitrosyl band
implies less overall NO adsorption or NO adsorption of a different
kind. The latter explanation appears most likely because the loss of
the Fe nitrosyl band coincides with the emergence of a new feature
−
1
3.3. In situ NO adsorption FTIR spectroscopy
centered at 1460 cm
.
The assignment of the band at 1460 cm on the 0.94 Fe/nm2
NaFeEDTA/CeO2 catalyst (Fe:Na = 1:1) has proved problematic due
the intense and broad nature of the band itself and of the spectral
−
1
◦
3
.3.1. NO adsorption at 50 C
The role of the Na promoter in FeOx/CeO catalysts in the mech-
2
anism of NO reduction was studied by in situ FTIR spectroscopy
during NO adsorption. The NO molecule is often used as a chemical
probe in FTIR, especially at low temperatures, where in addition
features that arise from NO adsorption on CeO . Many of the corre-
sponding ␯1 or ␯as stretches that typically aid spectral assignment
2
fall in the same region as the CeO surface nitrate band centered at
2
−
−
−1
−1
to expected nitrate (NO3 ) and nitrite (NO2 ) intermediates, sur-
face nitrosyl (M–NO, M = metal) species are present [29]. For the
catalysts in the current study, NO adsorption FTIR spectroscopy
was performed to determine if the presence of the Na promoter
caused significant changes in the NO adsorption spectrum. Such
an observation would indicate that Na changes the interaction of
the reactant molecule with the surface, i.e., causes the formation
of unique intermediates. Differing intermediates would suggest an
alternate pathway to NO reduction and provide and explanation for
1179 cm . The 1460 cm is prominent and the species causing its
presence appears to be preferred over NO adsorption as Fe nitrosyl.
The fact that it appears only when the Na promoter is present sug-
gests that NO adsorption directly on Na cations is responsible for the
band’s appearance. While adsorption of NO on alkali metal contain-
ing materials has been studied, the vibrations do not overlap with
−
1
−1
the one at 1460 cm [34–36] The region between 1516–1435 cm
has previously been attributed to the ␯(N O) stretch of bulk com-
−
pounds containing bridging nitro–nitrito species, where the NO2
the increased activity when FeOx/CeO catalysts are Na-promoted.
anion is bound simultaneously by its N and O atom [37]. It is sug-
gested herein that the presence of Fe and Na on the surface of
CeO2 in the 0.94 Fe/nm2 NaFeEDTA/CeO2 catalyst could create a
surface site amenable to the formation of such a bridging nitro-
nitrito species. Nitro-nitrito species in bulk compounds are also
2
2
Bare CeO , 1.11 Fe(NO ) /CeO , and 0.94 Fe/nm NaFeEDTA/CeO2
2
3
3
2
catalysts were chosen for FTIR analysis as representative samples
for the support, an unpromoted catalyst, and a promoted catalyst
with an Fe:Na ratio of 1:1, respectively.
−
1
Fig. 4a displays the transmission FTIR spectra of several cat-
expected to have a corresponding ␯(N–O) stretch at 1200 cm
,
◦
−1
alysts after 20 min of NO adsorption at 50 C. The spectrum of
but once again, the surface nitrites on CeO2 (1179 cm ) prevent
resolution of such a feature. It is notable, although, that the cen-
ter of the broad band in the NaFeEDTA/CeO2 is significantly blue
the bare CeO2 support contains distinct surface NO adsorption
−
1
features at 1179 and 1607 cm . The location of the relatively
−
1
−1
−1
broad band at 1179 cm is in agreement with an assignment of
shifted to 1206 cm . An increase in intensity at 1200 cm due to
the ␯(N–O) stretch of a bridging nitro–nitrito species would cer-
tainly contribute to the observed shift of the center of the broad
the ␯s(NO ) of a chelating bidentate nitrite on a CeO surface,
2
2
which is also confirmed by the presence of the ␯as(NO ) band at
2
−
1
−1
−1
−1
∼
1280 cm
(not labeled) [29–31]. The feature at 1607 cm
is
feature to 1206 cm . Therefore, the band at 1460 cm is tenta-
tively assigned to the ␯(N O) stretch of a bridging nitro–nitrito
species spanning a Fe and Na surface atom.
assigned to the ␯as(NO ) of a bridging nitrate species. The matching
2
␯
s(NO ) of the bridging nitrate ␯3 split could not be resolved as it
2
−1
is expected between 1225–1170 cm and likely contributes to the
broad nature of the band centered at 1179 cm . Finally, a band
is observed at 1383 cm , which closely matches the ␯3 stretch
of a free NO3 ion. Therefore, this feature has been assigned to
The unique bridging nitro–nitrito ␯(N O) stretch is present only
in the 0.94 Fe/nm2 NaFeEDTA/CeO2 catalyst (Fe:Na = 1:1), which is
the catalyst exhibiting the highest activity for NO reduction by CO
(see Fig. 1). Thus, this surface species could be a surface reaction
intermediate in a more active pathway for NO reduction. The in situ
−
1
−1
−
bulk cerium nitrate species that form due to the relatively long NO
adsorption time.
◦
NO adsorption FTIR spectroscopy at 50 C is beneficial to identify
When FeOx surfaces species are impregnated on the CeO2 sur-
the existence of catalytic surface species. However, because it is
performed far below the reaction temperature, it cannot confirm
face, as in the 1.11 Fe/nm2 Fe(NO ) /CeO , the NO adsorption FTIR
3
3
2
spectrum remains similar to that of the bare CeO support (Fig. 4a).
2

6
0
C.A. Roberts et al. / Catalysis Today 267 (2016) 56–64
Fig. 4. In situ FTIR spectra during lean NO adsorption (NO + O2) over 0.94 Fe/nm NaFeEDTA/CeO2, unpromoted 1.11 Fe/nm2 Fe(NO3)3/CeO2, and bare CeO2 catalysts at (a)
2
◦ ◦
0 C and (b) 350 C. The spectrum of the catalyst sample under He flow just prior to NO adsorption was used as the background.
5
the role of the observed NO adsorption species as reactive interme-
diates.
support, it must be concluded that the only spectroscopically rel-
evant NO adsorption species at 350 C are adsorbed on the CeO2
◦
support. With the disappearance of the other spectral features due
−
◦
to adsorbed NO species, the bulk NO3 band can now be further
3
.3.2. NO adsorption at 350 C
◦
resolved. Shoulders on either side of the main peak at 1311 and
437 cm are observed. These vibrations are also problematic to
assign, but are not likely due to surface nitrates or nitrites because
they do not appear in the expected spectral range and do not exhibit
both the necessary ␯s and ␯as. Therefore, the three main features
at 1311, 1378, and 1437 cm are all tentatively assigned to the ␯3
stretch of bulk NO3 stored in CeO . The differences in wavenum-
ber are attributed to the inhomogeneous nature of the surface of
the CeO2 support, causing a broad feature containing these three
bands.
In situ NO adsorption transmission FTIR spectroscopy at 350 C
was performed to investigate the catalytic relevance of the surface
NO adsorption species observed at 50 C. When the temperature
−
1
1
◦
is increased, two primary outcomes are expected with respect to
a spectroscopically relevant species. First, the species persists and
is a potential candidate as the most abundant reactive interme-
diate and may be involved in the rate-determining-step of the
catalytic mechanism at that temperature. Second, the species is
no longer spectroscopically relevant and is likely not involved
in the rate-determining-step. Fig. 4b reveals the second outcome
to be true of the ␯(N O) stretch of the bridging nitro–nitrito
on the 0.94 Fe/nm NaFeEDTA/CeO2 cata-
lyst (Fe:Na = 1:1). Additionally, at 350 C, the spectral features that
do persist in the NaFeEDTA/CeO2 and Fe(NO ) /CeO catalysts are
exactly the same as those for the bare CeO support. Therefore,
the adsorbed NO species at 350 C are all attributable to the CeO2
−
1
−
2
◦
Thus, at 350 C, there appears to be potential NO storage on the
CeO2. It remains to be determined if any of the three species con-
tributing to the bulk NO3 band are simply spectators, or if there is
an additional mechanism for NO reduction at 350 C that includes
−1
2
species at 1460 cm
−
◦
◦
3
3
2
the participation of NO stored on CeO2.
2
◦
surface and nothing can be said about the role of NO intermedi-
ates adsorbed on FeOx (or Na) in the rate-determining-step for NO
reduction by CO.
3.4. Stoichiometric/lean cycles during FTIR spectroscopy
3.4.1. Behavior of surface NO adsorption species
−
It is still possible that the more active NO reduction mechanism
The mechanistic role of the stored NO3 species can be inves-
2
over the 0.94 Fe/nm NaFeEDTA/CeO2 catalyst proceeds through
tigated by in situ FTIR spectroscopy under NO reduction reaction
conditions. To avoid the significant contribution of carbonate vibra-
tions to the relevant spectral region, the reductant molecule chosen
a bridging nitro-nitrito intermediate, but this intermediate would
not be involved in the rate-determining-step. Indeed, the disap-
−
1
pearance of the 1460 cm
band or the lack of any resolved NO
for the in situ FTIR spectroscopy was H . A previous result has shown
2
3+
2+
adsorption species on Fe is consistent with the previously pro-
posed rate-determining-step. X-ray absorption spectroscopy with
simultaneous gas-phase product monitoring revealed that reduc-
that increased redox cycling between Fe and Fe correlates with
activity and is related to the rate-determining-step of NO reduc-
tion [16,17]. Furthermore, this correlation holds regardless of the
reductant molecule identity, H2 or CO. Therefore, H2 was consid-
ered a suitable reductant molecule to investigate the reduction of
3
+
2+
tion of Fe to Fe at the Fe–O–Ce interfacial bond was necessary
before NO reduction by CO could proceed. If this step is indeed
rate-determining, then the reaction should proceed downhill ener-
getically, and surface NO intermediates should readily undergo
catalytic transformations.
−
the stored NO3 on CeO2 by in situ FTIR spectroscopy.
Cycles of lean adsorption and stoichiometric NO reduction by
H2 in the presence of O2 were studied. It was found that if the lean
NO adsorption gas flow of NO + O2 was stopped and the in situ cell
It is still of interest to examine the catalytic relevance of the NO
◦
−
adsorption species that persisted in the FTIR spectra at 350 C. A low
was flushed with flowing UHP He, the NO3 peaks would slowly
−
1
◦
intensity feature around ∼1600 cm ␯as(NO ) is due to the persis-
decrease and disappear in time at 350 C. This result indicated that
the NO3 species are not strongly stored or stable. For this reason,
2
−
tence of a small amount of the bridging nitrate species on CeO .
2
The main peak in the spectra for all three catalysts is that of the
bulk NO3 on cerium at 1378 cm . Unfortunately, nothing can be
stoichiometric NO + H + O2 conditions were chosen rather than
2
−
−1
switching to a simulated rich reactant mixture containing H2 only
−
said of any adsorbed NO species on FeOx. Because the spectral fea-
to determine if the NO3 species were reactive.
2
2
tures observed for the 1.11 Fe/nm Fe(NO ) /CeO and 0.94 Fe/nm
Fig. 5 displays contour maps of the time resolved in situ FTIR
3
3
2
NaFeEDTA/CeO catalysts are at the same positions as the bare CeO2
spectra of the FeOx/CeO catalysts during cycles of lean NO adsorp-
2
2

C.A. Roberts et al. / Catalysis Today 267 (2016) 56–64
61
−
1
Fig. 5. Top: contour maps of time resolved in situ FTIR spectra from 2000 to 1100 cm during stoichiometric (S) and lean (L) cycles of NO reduction by H2 reaction gas
mixtures (NO + H2 + O2) over CeO2 (left), unpromoted 1.11 Fenm Fe(NO3)3/CeO2 (middle), and 0.94 Fe/nm2 NaFeEDTA/CeO2 (right) catalysts. Highest absorbance is indicated
2
−
1
by dark red and lowest absorbance by black. Bottom: absorbance as a function of time and reaction condition for bulk nitrate FTIR peaks at 1311, 1378, and 1437 cm for
the same catalysts as Top. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
−
1
Fig. 6. Normalized absorbance as a function of time for nitrate FTIR peaks at (a) 1311, (b) 1378, and (c) 1437 cm during the first stoichiometric cycle of NO reduction by H2
2
2
(
NO + H2 + O2) over bare CeO2, unpromoted 1.11 Fe/nm Fe(NO3)3/CeO2, and 0.94 Fe/nm NaFeEDTA/CeO2 catalysts.
−
1
tion and stoichiometric NO reduction by H in the presence of O at
ric cycles between 1311, 1378, and 1437 cm are observed. The
2
2
◦
−
−1
3
50 C. The general behavior of the three NO peaks at 1311, 1378,
1437 cm band did not follow the general behavior as closely as
the other two bands, especially for the NaFeEDTA/CeO2.
The raw absorbance varied between catalyst samples, so the
spectra were normalized to the maximum intensity of each NO3
peak prior to the first stoichiometric NO + O + H cycle. Fig. 6 com-
3
−1
and 1437 cm was the same for the bare CeO2 support and the
Fe(NO ) /CeO and NaFeEDTA/CeO2 catalysts. Following lean NO
3
3
2
−
adsorption, the first exposure to the stoichiometric reaction condi-
tions (NO + H + O ) causes a significant decrease in peak intensity
2
2
2
2
(
0–5 min). The observation is reproducible as demonstrated by the
pares the decay of the normalized absorbance of each peak as a
function of reaction time for the first 5 min of exposure to the sto-
ichiometric reaction gas mixture. There is a time delay of ∼1.7 min
before the reaction mixture reaches the sample in the in situ cell, so
the first data point begins at experiment time t = 1.7 min. It is now
possible to compare the decay rate of the peaks for each catalyst. It
subsequent cycle of lean NO adsorption (5–15 min) and stoichio-
metric NO + H + O (15–25 min). The contour map indicates some
changes also took place in the 2000–1100 cm
however, these changes were difficult to resolve as the intensity of
the features was comparable to the noise level of the spectra. The
three components comprising the peak for the bulk NO3 on CeO2
2
2
−
1
spectral region,
−
−
is qualitatively observed that all three NO3 peaks gradually decay
were examined in more detail in the bottom of Fig. 5. The trends
with stoichiometric/lean/stoichiometric cycles are confirmed, and
qualitative differences in the rate of decay during the stoichiomet-
over the CeO support. However, when FeOx is added to the catalyst
2
in the Fe(NO ) /CeO and NaFeEDTA/CeO catalysts, the decay rate
3
3
2
2
−
1
of the 1311 and 1378 cm peaks increases dramatically (Fig. 6a

6
2
C.A. Roberts et al. / Catalysis Today 267 (2016) 56–64
Table 2
Slope of the decay of the NO3 FTIR peaks in the initial region of linear decay (t = 1.7–2.7 min). Adjusted R2 values were all better than a minimum of 0.95, except for 1437 cm
−1
−
on NaFeEDTA/CeO2, where a linear fit was not obtained.
FTIR peak
Slope of decay
CeO2
Fe(NO3)3/CeO2 1.11 Fe/nm2
NaFeEDTA/CeO20.94 Fe/nm2 (Fe:Na = 1:1)
−
−
−
1
1311 cm
1378 cm
1437 cm
−0.14
−0.09
−0.13
−0.34
−0.14
−0.22
−0.48
−0.25
n/a
1
1
and b), implying that the surface intermediate is more rapidly con-
sumed. This observation is in agreement with the higher activities
for NO reduction by H2 or CO found for the FeOx containing cata-
lysts.
slopes of decay with the highest magnitude at both the 1311
and 1378 cm peaks was the NaFeEDTA/CeO2 catalyst. This find-
−
1
ing quantitatively demonstrates that the NaFeEDTA/CeO , with
2
FeOx and Na promoter cations present, more efficiently reacts
−
−1
−
The NO3 peak at 1437 cm decays with stoichiometric reac-
the adsorbed bulk NO3 intermediates during stoichiometric NO
2
tion time over CeO and more so over the unpromoted 1.11 Fe/nm
reduction reaction conditions. The initial slope of decay of the
2
−
1
Fe(NO ) /CeO catalyst, but the NaFeEDTA/CeO2 catalyst does not
1437 cm peak is not reported for the NaFeEDTA/CeO2 catalyst as
a satisfactory linear fit was not found. The lack of fit is not surprising
since this peak’s behavior did not correlate with the changing reac-
tion conditions (stoichi/lean/stoichi). Once again, it is suggested
3
3
2
2
follow the trend (Fig. 6c). The 0.94 Fe/nm NaFeEDTA/CeO2 cata-
lyst is, once again, the only catalyst that contains the Na promoter
(
Fe:Na = 1:1), suggesting that the difference in the general behavior
−
is related to the promoter causing a more efficient use of the other
that the Na promotion leads to more efficient removal of the NO3
−
−1
adsorbed NO3 species. Indeed, closer examination of Fig. 6a and
species that give rise to the 1311 and 1378 cm features, there-
fore, the species that gives rise to 1437 cm is merely a spectator
when Na is present.
−
1
b reveals that the NaFeEDTA/CeO2 has a higher initial decay rate. If
the region of initial linear decay is defined as the experiment time
from t = 1.7 min to t = 2.7, linear fits can be obtained with a mini-
2
mum adjusted R of 0.95 (see Fig. S2 for plots of all linear fits). The
3
.4.2. Correlation of product formation with intermediate
consumption
In situ FTIR spectroscopy during lean and stoichiometric cycles
resulting slopes of the linear region of initial decay are summarized
in Table 2.
The CeO catalyst yielded the lowest magnitude slopes of decay
2
−1
of NO reduction by H2 was utilized successfully to establish that
for 1311 and 1378 cm
confirming that the bare support alone
−
−
adsorbed bulk NO3 species on CeO2 are more efficiently removed
removes NO3 intermediates at a measurably lower rate, thus
establishing the necessity of FeOx surface species for NO reduc-
tion reactivity in the FeOx/CeO2 catalysts. The catalyst exhibiting
by an FeOx/CeO catalyst with Na promotion. The disappearance of
2
an FTIR band alone, however, is insufficient to prove whether the
surface NO species is simply desorbing or truly undergoing a cat-
−
1
Fig. 7. Top: contour maps of time resolved in situ FTIR spectra from 2300 to 2100 cm during stoichiometric (S) and lean (L) cycles of NO reduction by H2 reaction gas
mixtures (NO + H2 + O2) over CeO2 (left), unpromoted 1.11 Fe/nm Fe(NO3)3/CeO2 (middle), and 0.94 Fe/nm2 NaFeEDTA/CeO2 (right) catalysts. Highest absorbance is indicated
2
−
1
by dark red and lowest absorbance by black. Bottom: absorbance as a function of time and reaction condition for N2O FTIR product peaks at 2202 and 2237 cm for the same
catalysts as Top. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

C.A. Roberts et al. / Catalysis Today 267 (2016) 56–64
63
−
1
Fig. 8. Normalized absorbance of product N2 O FTIR peaks at 2202 (black square) and 2237 cm (red circle) as a function of normalized absorbance of the intermediate
nitrate FTIR peak at 1378 cm from 1.7 to 4.5 min of the first stoichiometric cycle of NO reduction by H2 over (a) bare CeO2, (b) unpromoted 1.11 Fe/nm2 Fe(NO3)3/CeO2,
and (c) 0.94 Fe/nm2 NaFeEDTA/CeO2 catalysts. All R2 values were better than 0.95. (For interpretation of the references to color in this figure legend, the reader is referred to
the web version of this article.)
−1
alytic transformation. While hyphenated spectroscopic techniques
such as FTIR-MS (mass spectrometer) are typically preferred to
establish reactivity, correlations between the time-resolved disap-
pearance of surface intermediate peaks and appearance gas phase
product peaks in the FTIR spectra are also suitable.
observation made during the in situ FTIR spectroscopy experiments
were related to catalytic transformations. The same conclusion is
−
reached when correlating the N O product peaks with the NO
2
3
−
1
peak at 1311 cm (see Fig. S3 of Supplementary material). For the
−
−1
NO3 peak at 1437 cm (see Fig. S4 of Supplementary material),
Fig. 7 (top) displays contour maps of the time resolved in situ
the same holds true with the single exception being the 0.94 Fe/nm2
−1
FTIR spectra between 2300 and 2100 cm of the FeOx/CeO2 cata-
NaFeEDTA/CeO (Fe:Na = 1:1), which once again did not display the
2
lysts during cycles of lean NO adsorption and stoichiometric NO
reduction by H2 in the presence of O2 at 350 C. The preferred
same general behavior (see Fig. S4c of Supplementary material).
Finally, a comparison of the size of the data range used for
the linear fits in Fig. 8 for each of the three materials reveals a
noteworthy trend. The size of the linear regions correspond to dura-
tions of time of 1.6, 2.0 and 2.4 min for CeO , Fe(NO ) /CeO , and
◦
product for NO reduction is N , which is not IR active in the gas
2
phase. However, the ␯(N–N) stretching frequency may become IR
active when adsorbed, which is possible due to the molecule’s
very weak Lewis basicity [30]. Dinitrogen has been detected in
2
3
3
2
NaFeEDTA/CeO , respectively (see text in Fig. 8a–c). The increased
2
the 2299–2290 cm region on Cu+ in zeolites [33,38–41]. Unfor-
−1
duration of the inverse linear correlation for the NaFeEDTA sug-
−
tunately, such a N2 product peak was not observed for the current
gests a more efficient catalytic transformation of the NO
on
3
◦
−1
−1
catalyst systems because 350 C is likely too high a temperature to
CeO2 (1378 cm ) to the product N O (2202 or 2237 cm ). Once
2
observe such a weakly bound species. Therefore, the current study
does not establish the complete catalytic reduction of the stored
NO3 species on CeO2 to N2.
again, this finding is consistently with the higher activity of
NaFeEDTA/CeO2 catalysts for NO reduction by H2 or CO (see Fig. 1)
−
over Fe(NO ) /CeO without Na promotion or the bare CeO2 sup-
3 3 2
The less desirable product of NO reduction is N O, but the detec-
port. Additionally, the participation of CeO2 in this potential NO
reduction pathway supports the finding that excess Na does not
increase activity (see Fig. 2), as the active CeO2 surface might be
covered when the Fe:Na ratio is 1:2, 1:3, or 1:4.. The longer dura-
tion of the linear region for the Fe(NO ) /CeO over the bare CeO2
2
tion of this product can still be used to establish the reactivity
−
of the bulk NO3 species on CeO . The formation of N O is read-
2
2
ily detected by the presence of the ␯(N–N) stretching frequency
−
1
in the 2290–2210 cm region [42,43]. This assignment is consis-
3
3
2
−
1
tent with the location of the two peaks at 2237 and 2202 cm
(compare Fig. 8a and b) confirms the catalytic nature of the FeOx
− ◦
species in conversion of bulk NO3 species on CeO to N O at 350 C,
2 2
observed in the contour maps of the FTIR spectra during stoichio-
metric NO reduction by H2 in the presence of O2 (see Fig. 7, top).
The existence of two peaks is due to simultaneous adsorption of
i.e., the CeO support alone is less active (see Fig. 1). These observa-
2
−
tions suggest a more efficiently shuttling of the bulk NO3 to active
N- and O-bonded molecules, and/or N O molecules adsorbed on
FeOx centers when Na is present.
2
different sites [43]. The bottom of Fig. 7 reveals the expected trend
in N O product formation: peak intensity increased with H pres-
2
2
ence under stoichiometric conditions and decreased when H2 is
removed under lean conditions. The result strongly suggests that
4. Conclusions
catalytic transformations to N O are occurring during the in situ
FTIR spectroscopy.
2
The role of Na as a promoter for NO reduction by CO was
established for highly dispersed FeOx impregnated on CeO2 via
NaFeEDTA and Fe(NO3)3 precursors. At a constant Fe surface den-
To obtain a more quantitative confirmation that the N O forma-
2
−
−
tion is the product of the catalytic transformation of the bulk NO3
species on CeO , the N O formation was correlated with NO
3
2
◦
sity of ∼1.0 Fe/nm , steady-state activity at 250 C was shown
to increase with addition of a stoichiometric amount of Na
(Fe:Na = 1:1). Excess Na, however, did not increase activity for
1.0 Fe/nm2 Fe(NO3)3/Na-CeO2 catalysts. These two observations
provide an explanation for the highest activity of the 0.94 Fe/nm2
NaFeEDTA/CeO2 catalyst, which necessarily impregnates Fe and Na
onto CeO2 at Fe:Na = 1. At the highest Fe surface density studied
2
2
intermediate consumption during the first ∼3 min (experiment
time, t = 1.7–4.5 min) of stoichiometric NO reduction by H . Fig. 8
2
−
1
plots the normalized absorbance of both the 2237 and 2202 cm
peaks as a function of the normalized absorbance of the main NO
−
3
−1
intermediate peak at 1378 cm . For all three catalysts, a region of
strong inverse linear proportionality was identified, confirming the
−
2
relationship between the consumption of NO3 on CeO2 and the
(∼4.0 Fe/nm ), the stoichiometric Na promotion ceases to increase
formation of the NO reduction product N O. Although N O is not
the activity of NO reduction by CO, suggesting the importance of the
simultaneous impregnation of Fe and Na by NaFeEDTA to prevent
unwanted titration of favorable CeO2 surface sites.
2
2
the preferred product, it can be concluded that the experimental

6
4
C.A. Roberts et al. / Catalysis Today 267 (2016) 56–64
In situ NO adsorption FTIR yielded insight into possible mech-
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Acknowledgements
The authors are grateful to Hirohito Hirata, Keiichi Minami,
Shogo Shirakawa, and the catalyst researchers of Toyota Motor
Corporation for their beneficial discussion and support of this col-
laborative research. The ongoing advice of Naoki Takahashi of
Toyota Central Research and Development Laboratories is kindly
acknowledged. Paul T. Fanson and Hongfei Jia of Toyota Motor Engi-
neering and Manufacturing North America are thanked for their
support in executing the experiments and their discussion.
[
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