Selective catalytic reduction of NO with NH3 over supported vanadia catalysts

Article abstract of DOI:10.1006/jcat.1996.0179

The selective catalytic reduction (SCR) of NO with NH₃ was systematically investigated over a series of supported vanadia catalysts to obtain additional insight into this important industrial reaction. The influence of surface vanadia coverage, promoters (surface tungsten oxide, niobium oxide, and sulfate species), and the specific oxide support (TiO₂, Al₂O₃, and SiO₂) was examined. The molecular structures of the surface metal oxide species were determined by in situ Raman spectroscopy, and the corresponding surface acidity properties were monitored with infrared spectroscopy employing pyridine adsorption. The redox properties of the surface metal oxide species were probed with the sensitive methanol oxidation reaction and temperature-programmed reduction. The SCR reactivity of the various catalysts was determined over a wide temperature range. The current findings suggest that a dual-site (a surface vanadia redox site and an adjacent nonreducible metal oxide site) mechanism is required for the efficient selective catalytic reduction of NO with NH₃ over supported vanadia catalysts. The SCR reaction is sensitive to the immediate environment of the surface vanadia species: overall surface coverage of the metal oxide overlayer (factor of 5 in turnover frequency), nature of adjacent surface metal oxide species (factor of 10 in turnover frequency) and oxide support ligands (factor of 3 in turnover frequency). The SCR reaction, however, does not appear to depend on the specific structure of the surface vanadia species. The SCR selectivity toward N₂ formation also varies with the immediate environment of the surface vanadia species. The selectivity depends on the specific oxide support (TiO₂ > Al₂O₃ > SiO₂), temperature (decreases at higher temperature due to oxidation of NH₃ and NO to N₂O), and surface concentration of redox sites (decreases with the concentration of pairs of redox sites). The SCR reaction is not related to the properties of the terminal V=O bond since in situ Raman studies during SCR, employing V=¹⁸O, demonstrate that this bond is relatively stable under reaction conditions (possessing a lifetime that is ～10 times the characteristic reaction time). Thus, the bridging V-O-support bond appears to be involved in the rate-determining step.

Full text of DOI:10.1006/jcat.1996.0179

JOURNAL OF CATALYSIS 161, 211–221 (1996)
ARTICLE NO. 0179
Selective Catalytic Reduction of NO with NH3 over Supported
Vanadia Catalysts
,1
Israel E. Wachs, Goutam Deo, Bert M. Weckhuysen, ,2 Amedeo Andreini,† Michael A. Vuurman,†
ꢀ
ꢀ
ꢀ
Michiel de Boer,† and Michael D. Amiridis‡
ꢀ
Zettlemoyer Center for Surface Studies, Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015; †Department of
Chemical Technology, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV, Amsterdam, The Netherlands; and ‡Department of
Chemical Engineering, Swearingen Engineering Center, University of South Carolina, Columbia, South Carolina 29208
Received July 6, 1995; revised February 6, 1996; accepted February 13, 1996
INTRODUCTION
The selective catalytic reduction (SCR) of NO with NH3 was
systematically investigated over a series of supported vanadia cata-
lysts to obtain additional insight into this important industrial reac-
tion. The influence of surface vanadia coverage, promoters (surface
tungsten oxide, niobium oxide, and sulfate species), and the specific
oxide support (TiO2, Al2O3, and SiO2) was examined. The molecu-
lar structures of the surface metal oxide species were determined by
in situ Raman spectroscopy, and the corresponding surface acidity
properties were monitored with infrared spectroscopy employing
pyridine adsorption. The redox properties of the surface metal oxide
species were probed with the sensitive methanol oxidation reaction
and temperature-programmed reduction. The SCR reactivity of the
various catalysts was determined over a wide temperature range.
The current findings suggest that a dual-site (a surface vanadia
redox site and an adjacent nonreducible metal oxide site) mech-
anism is required for the efficient selective catalytic reduction of
NO with NH3 over supported vanadia catalysts. The SCR reaction
is sensitive to the immediate environment of the surface vanadia
species: overall surface coverage of the metal oxide overlayer (fac-
tor of 5 in turnover frequency), nature of adjacent surface metal
oxide species (factor of 10 in turnover frequency) and oxide sup-
port ligands (factor of 3 in turnover frequency). The SCR reaction,
however, does not appear to depend on the specific structure of the
surface vanadia species. The SCR selectivity toward N2 formation
also varies with the immediate environment of the surface vana-
dia species. The selectivity depends on the specific oxide support
(TiO2 > Al2O3 > SiO2), temperature (decreases at higher tempera-
ture due to oxidation of NH3 and NO to N2O), and surface concen-
tration of redox sites (decreases with the concentration of pairs of
redox sites). The SCR reaction is not related to the properties of
The selective catalytic reduction of NOx with NH3 over
supported V2O5–WO3/TiO2 catalysts is an important com-
mercial technology for the reduction of NOx emissions from
powerplants. Theindustrialsuccessoftheselectivecatalytic
reduction (SCR) reaction has generated many investiga-
tions, but many fundamental questions about the nature of
the catalytic active site and mechanism of this reaction still
remain.
Miyamoto et al. proposed that NH3 is strongly adsorbed
+
==
adjacent to V O sites as NH4 and that the reaction rate
==
is directly proportional to the number of surface V O
bonds which were determined by a rectangular pulse tech-
nique (1). Janssen et al. employed oxygen isotope ex-
periments to probe the surface vanadia species and pro-
== – – ==
posed that a pyrovanadate structure, O V O V O, was
the most likely structure for the active site (2). Went
et al. characterized the surface vanadia species with in situ
Raman spectroscopy as well as temperature-programmed
reduction/temperature-programmed oxidation and con-
cluded that both monomeric vanadyl and polymeric vana-
date surface species were present on the titania support
(3); corresponding SCR studies suggested that the poly-
meric species were about 10 times more active than the
monomeric species, but the polymeric species were less se-
lective toward N2 formation (4). The increase in the SCR
specific activity with surface vanadia coverage on titania has
also been reported by other investigators (5, 6). Lietti et al.
proposed that the redox properties of the catalyst are a ma-
jor factor governing activity and that the reaction involved
primarily a coordinated ammonia species on Lewis acid
sites (6, 7). The potential participation of a protonated am-
monium on a surface Brønsted acid site was also proposed
because of the possible interconversion of these species (6,
7). Ramis et al. also proposed that promoters affect the
==
the terminal V O bond since in situ Raman studies during SCR,
==18
employing V
O, demonstrate that this bond is relatively stable
under reaction conditions (possessing a lifetime that is ꢀ10 times
the characteristic reaction time). Thus, the bridging V–O–support
c
bond appears to be involved in the rate-determining step. ꢁ 1996
Academic Press, Inc.
==
Lewis acidity of V O bonds which affects the adsorption
1
To whom correspondence should be addressed.
Current address: Centrum voor Oppervlaktechemie en Katalyse,
2
of ammonia, the first step in the SCR reaction (8). Based
K. U. Leuven, Kardinaal Mereierlaan 92, B-3001 Heverlee, Belgium.
on in situ FT-IR studies, Topsoe proposed that Brønsted
211
0021-9517/96 $18.00
c
Copyright ꢁ 1996 by Academic Press, Inc.
All rights of reproduction in any form reserved.

212
WACHS ET AL.
acid sites are the main active sites for the SCR reaction with NH3 provided additional fundamental insights into the
over titania-supported vanadia catalysts (9). The role of nature of the active sites present in SCR catalysts.
Brønsted acidity in the SCR reaction over supported vana-
dia catalysts was also suggested by other research groups
(10, 11). Model SCR studies with unsupported V2O5 crys-
tals concluded that the crystallographic planes possessing
EXPERIMENTAL
Preparation of Supported Vanadium Oxide Catalysts
– –
–
==
V O V or V OH bonds rather than V O bonds were
the selective sites for the SCR reaction (12, 13). In a re-
cent series of detailed studies over vanadia–titania catalysts,
Topsoe et al. combined in situ FT-IR and on-line mass spec-
trometry studies, transient as well as steady state, to provide
convincing evidence that both surface Brønsted acid sites
The oxide supports used for the preparation of supported
vanadium oxide catalysts were TiO2 (50 m2/g, Degussa),
Al2O3 (180 m2/g, Harshaw), and SiO2 (300 m2/g, Cabot).
The vanadium oxide precursor used for this study was
vanadium triisopropoxide (Alfa, 95–98% purity), and the
method used for the preparation was incipient wetness
impregnation. The moisture- and air-sensitive nature of
this precursor required that the prepation be performed
in a nitrogen environment and using nonaqueous solvents.
Known amounts of the precursor and methanol (Fisher-
certified ACS, 99.9% pure), corresponding to incipient wet-
ness impregnation volume and final amount of vanadium
oxide required, were prepared in a glove box filled with ni-
trogen, mixed thoroughly with the oxide support, and let to
stand for 16 h. This was followed by various heat treatments
in nitrogen and the final calcination was performed at 723 K
for all the supported vanadium oxide catalysts (19).
==
and surface V O sites are involved in the SCR catalytic
cycle (14, 15).
The mechanism of the selective catalytic reduction of NO
with ammonia over vanadia–titania catalysts has been pro-
posed to occur both by an Eley–Rideal mechanism (1, 2, 6,
7, 12, 15) and by a Langmuir–Hinshelwood mechanism (4,
16, 17). However, significant amounts of adsorbed NO are
not found on the vanadia–titania catalyst surface under re-
action conditions (15). Furthermore, detailed microkinetic
analysis of the SCR reaction by Dumesic et al. suggests
that a simple two-step Eley–Rideal mechanism involving
reaction between adsorbed NH3 and gaseous (or weakly
adsorbed) NO is also not consistent with the data (18). The
SCR kinetic data could quantitatively be described by a ki-
netic model involving a three-step mechanism: equilibrated
ammonia adsorption, activation of adsorbed ammonia, and
reaction between activated ammonia and NO (gaseous or
weakly adsorbed). The surface sites associated with these
three reaction steps have been proposed to involve surface
Preparation of Promoted 1% V2O5/TiO2 Catalysts
The oxides of tungsten, niobium, and sulfur were added
to 1% V2O5/TiO2 to study the effect of the promoters on
supported vanadium oxide catalysts. The precursors and
solvent used for tungsten oxide, niobia, and sulfur ox-
ide were ammonium metatungstate and water, niobium
ethoxide (moisture and air sensitive) and propanol, and am-
monium sulfate and water, respectively. Known amounts of
precursor and solvent were thoroughly mixed with a pre-
viously prepared 1% V2O5/TiO2 in a nitrogen-filled glove
box (niobium ethoxide) or in the ambient (ammonium
metatungstate and ammonium sulfate) and let to sit for
16 h (20, 21). The niobium oxide-promoted sample was then
heated similar to vanadium oxide-supported samples. The
tungsten oxide- and sulfur oxide-promoted samples were
gradually heated in oxygen (or air) up to 723 K.
==
Brønsted acid sites, surface V O sites, and reduced surface
==
V O sites (15).
This investigation was initiated to obtain additional fun-
damental insights into the nature of the active sites of sup-
ported vanadia catalysts employed in the selective catalytic
reduction of NO with NH3. A major objective was to deter-
mine how the structure and reactivity of the surface vana-
dia species are influenced by surface coverage, promoters
(surface tungsten oxide, niobium oxide, silica, and sulfate
species), the specific oxide support (TiO2, Al2O3, and SiO2),
and temperature. The dehydrated molecular structures of
the surface metal oxide species were determined by Raman
spectroscopy. The surface Brønsted and Lewis acidity were
determined by IR experiments employing pyridine adsorp-
tion as a probe molecule. The redox potential of the surface
metal oxide species was probed by the methanol oxidation
reaction and temperature-programmed reduction (TPR).
The DeNOx SCR reactivity of the catalysts was expressed
as turnover frequencies (TOFs) to allow for a fundamental
comparison between the molecular structure and the spe-
cific reactivity of the active sites present in these catalysts.
The systematic variation of the parameters present in cata-
lysts employed for the selective catalytic reduction of NO
Raman Spectrometer
Laser Raman spectra were obtained with an Ar+ laser
(Spectra Physics, Model 165). The incident laser was tuned
to 514.5 nm and delivered 50–100 mW of power measured
at the sample. The scattered radiation from the sample was
collected at right angles to the laser beam and directed into
an OMA III (Princeton Applied Research, Model 1463 op-
tical multichannel analyzer) with a photodiode array detec-
tor thermoelectrically cooled to 238 K. About 100–200 mg
of the pure catalyst was made into a wafer and placed in
the in situ cell. The in situ cell contained a rotating cell,
thermocouple probe, and inlet and outlet gas connections.

SELECTIVE CATALYTIC REDUCTION OF NO WITH NH3
213
The arrangment of the in situ cell has been outlined else- reactors and the seventh stream, consisting of excess mix-
where (22). To obtain the Raman spectra under dehydra- ture, wasventedthroughabackpressurecontroller(Tescom
1
2
tion conditions the in situ cell was heated to 623 K for
h
mod. 26-2321-24) to maintain a constant pressure in the sys-
to desorb the surface moisture and then cooled to room tem. The reactor section is made up of a water-cooled oven
temperature to enhance the signal. The entire procedure controlled by a temperature-programmed controller (West
was performed in a stream of flowing oxygen (Linde Spe- 2050) carrying six glass reactors (Duran 50) which could be
ciality Grade, 99.99% purity). The Raman spectra of the simultaneously tested. Within the oven, the reactor tubes
catalysts were also obtained under ambient conditions and extended through a cylindrical aluminum block designed to
checked for the effect of hydration–dehydration and, con- ensure a uniform temperature profile. The temperature in
sequently, compound formation. No compound formation the oven was measured by means of a chromel–alumel ther-
was observed for any of the samples.
In situ Raman spectroscopy for the selective catalytic that prevailing inside the reactors under the test conditions.
reduction of NO with NH3 was performed at 300ꢂC on a
The analysis of the reactants and products were per-
mocouple. The temperature was not found to differ from
4% V2O5/ZrO2 sample. The fresh catalyst was dehydrated formed with a mass spectrometer interfaced with a IBM-
in situ in a O2 stream (Linde Speciality Grade, 99.99% pu- PC/AT computer. An air-actuated multiposition valve
1
rity) at 500ꢂC for h and then coooled to 450ꢂC. The dehy- (Valco SD type) selects one of the product streams for
2
drated sample was then reduced in a stream of C4H10/He admission to the mass spectrometer. This took place by
gas mixture and reoxidized in a stream of 18O2/He mix- differential pumping through a capillary tube (L = 1.10 m,
ture (JWS Technologies, Inc., Matheson, 18O2/He = 3/97) at i.d. = 0.15 mm) and a small orifice. Calibration gas mixtures
450ꢂC. Repeating the reduction (in C4H10/He) and reoxida- were used to quantify product concentrations. An internal
tion (in 18O2/He) at 450ꢂC maximized the degree of oxygen- standard technique was also applied to improve the accu-
18 isotopic exchange. The selective catalytic reduction ex- racy of the measurements. The TOF for the SCR of NOx
periments were then performed in situ using a reaction with NH3 was defined as the molecules of NO converted
mixture of 500 ppm NO, 500 ppm NH3, 50,000 ppm 16O2, per surface vanadium oxide site per second. Raman spec-
and a balance of He. The entire dehydration–reduction– troscopy demonstrated that vanadium oxide was 100% dis-
reoxidation–reaction procedure was performed in situ with- persed below monolayer coverages (6% V2O5/TiO2). Ad-
out ever exposing the sample to ambient conditions.
ditional details of the DeNOx reaction system can be found
elsewhere (23).
Methanol Oxidation Reaction
Acidity Measurements
The methanol oxidation reaction was performed in a dif-
ferential upflow reactor using a CH3OH/O2/He gas mixture
For the acidity measurements, pyridine was adsorbed at
in the molar ratio of ꢃ6/13/81 and flowing at ꢃ100 sccm. 423 K followed by an hour evacuation of the sample at ap-
The products from the reactor were analyzed by a gas chro- proximately 773 K. A saturated pyridine-in-helium mixture
matograph (HP5840A) using two TCDs and a FID with was created by passing a helium stream through a pyridine
two packed columns (Poropak R and Carbosieve SII) con- container maintained at 273 K. The samples were exposed
nected in parallel. The amount of catalyst was controlled to to this mixture for 2 h, and then evacuated at 453 K to re-
achieve less than 10% methanol conversion. The TOF for move the weakly held pyridine. The diffuse reflectance IR-
methanol oxidation was defined as molecules of methanol FT spectra were collected with a Nicolet 60SX instrument
converted per molecule surface vanadium oxide site per equipped with a standard DRIFTS accessory (ꢄ1 cmꢅ1
second. Raman spectroscopy demonstrated that vanadium resolution). The instrument was not calibrated for the num-
oxide was 100% dispersed below monolayer coverage (6% ber of Lewis and Brønsted acid sites and, consequently, only
V2O5/TiO2). Additional details regarding the methanol ox- relative amounts of Lewis and Brønsted acid sites are re-
idation reaction setup can be found elsewhere (19).
ported. The relative amount of acid sites is defined as the
amount of acid sites in the promoted samples as a fraction
of the amount of acid sites in the unpromoted samples.
DeNOx Reaction
The DeNOx reaction was carried out using a O2 (2%),
NO (500 ppm), NH3 (550 ppm), and balance He (supplied
by UCAR—Union Carbide) gas mixture. The gases were
Temperature-Programmed Reduction
The TPR experiments were performed by initially heat-
fed to a static gas mixing tube (MT) through four mass con- ing the catalysts to 500ꢂC for 1 h in an O2/He stream to
trollers (Hi-Tec MFC 201) to provide a total flow of 50 cm3- desorb adsorbed moisture. The samples were then cooled
/min. The gas mixing section was made up of stainless-steel to room temperature and the gas was switched to a 5%
tubing maintained at 383 K. The reactant gas mixture was H2/Ar stream. Samples were heated at a rate of 5ꢂC/min,
divided into seven streams in a splitter: six streams, ad- and H2 consumption was measured with a thermal conduc-
justed by six mass flow controllers, were admitted to the six tivity detector.

214
WACHS ET AL.
RESULTS
Raman Spectroscopy
The Raman spectra of dehydrated titania-supported
vanadia catalysts are shown in Fig. 1, from 700 to 1200 cmꢅ1
,
asafunctionofvanadialoading. TheTiO2 supportpossesses
a Raman band at 790 cmꢅ1 which diminishes in intensity
with increasing vanadia loading due to the absorption of
the laser light by the yellow-orange-colored vanadia com-
ponent. Addition of vanadia to the titania support results
in the appearance of two new Raman bands at ꢃ1030 and
ꢃ930 cmꢅ1 which have previously been assigned to isolated
and polymerized surface vanadia species, respectively (3,
24). The sharp Raman band of the isolated surface vana-
dia species shifts from 1025 to 1030 cmꢅ1 as a function of
surface vanadia coverage. The ratio of polymerized to iso-
latedsurfacevanadiaspeciesincreaseswithvanadialoading
up to monolayer coverage (approximately 6% V2O5/TiO2,
which corresponds to 13.2 ꢀmol V+5/m2). At higher vana-
dia loadings on titania (8% V2O5/TiO2), microcrystalline
V2O5 particles are formed as separate phases on the two-
dimensional surface vanadia overlayer and give rise to a
strong Raman band at 994 cmꢅ1 (24). Therefore, two sur-
face vanadia species, isolated and polymerized, as well as
a microcrystalline phase, V2O5 particles, are present in the
titania-supported vanadia catalysts and their relative con-
centrations depend on the surface vanadia coverage.
The influence of promoters (tungsten oxide, niobium
oxide, and sulfate) on the Raman spectrum of a 1%
V2O5/TiO2 catalyst under dehydrated conditions is shown
in Fig. 2 in the region 700–1200 cmꢅ1. The unpromoted 1%
V2O5/TiO2 catalyst exhibits a sharp Raman band at 1025
cmꢅ1 which is shifted toward 1030 cmꢅ1 on the addition
of tungsten and niobium oxide promoters. This slight shift
reflects the higher surface coverages brought about by the
addition of tungsten and niobium oxides, which are present
FIG. 2. Raman spectra of promoted 1% V2O5/TiO2 catalysts under
dehydrated conditions. (a) 1% V2O5/TiO2, (b) 1.1% S O 4 /1% V2O5/TiO2,
(c) 6% Nb2O5/1%V2O5/TiO2, (d) 7% WO3/1% V2O5/TiO2.
as surface metal oxide species (see below). The addition of
1.1% S O 4 to the 1% V2O5/TiO2 catalyst results in a down-
ward shift and decrease in relative intensity of the 1025
cmꢅ1 Raman band. The total surface coverages of the mixed
metal oxide overlayers were maintained below monolayer
coverages and are presented in Table 1. Monolayer surface
coverage on the titania support for the promoters corre-
sponds to 7% Nb2O5/TiO2 (10.5 ꢀmol Nb+5/m2) (25) and
8% WO3/TiO2 (6.9 ꢀmol W+6/m2) (24), and only 1.1% S O 4
(2.2 ꢀmol S+6/m2) is achievable for the surface sulfate be-
cause of its volatility.
The tungsten oxide and niobium oxide promoters only
mildly influenced the 925–930 cmꢅ1 Raman bands associ-
ated with the polymerized surface vanadia species. For the
1.1% S O 4 /1% V2O5/TiO2 catalyst the increase in intensity of
the925–930cmꢅ1 Ramanbandismoreobviousbutnotcom-
parable to the increase observed when the vanadium oxide
loading is increased (compare Figs. 1 and 2). The promoters
only formed surface metal oxide phases since the Raman
bands of crystalline WO3 (24) and Nb2O5 (25) phases were
absent. The surface tungsten oxide species gave rise to a
Raman band at 1011 cmꢅ1 (24) (see Fig. 2), and the surface
sulfate species exhibited a Raman band at 1370 cmꢅ1 (not
TABLE 1
Surface Coverages of Mixed Metal Oxide Catalysts
V
Promoter atom
Total
Catalyst
1% V 2 O 5 /TiO2
7% WO3/1% V2O5/TiO2
6% Nb2O5/1% V2O5/TiO2
1.1% S O 4 /1% V2O5/TiO2
(ꢀmol/m2)
(ꢀmol/m2)
(ꢀmol/m2)
2.2
2.2
2.2
2.2
—
6.0
9.0
2.3
2.2
8.2
11.2
4.5
FIG. 1. Raman spectra of V2O5/TiO2 catalysts under dehydrated con-
ditions as a function of vanadia loading.

SELECTIVE CATALYTIC REDUCTION OF NO WITH NH3
215
sites significantly increases as the surface vanadia loading
is increased on the titania support (29).
The effect of promoters on the acidity measured by pyri-
dine adsorption of 1% V2O5/TiO2 depends on the specific
promoter used. The influence of the promoters on the acid-
ity of the 1% V2O5/TiO2 catalyst is shown in Table 2. The
additionofthesurfaceniobiumoxideandsulfatepromoters
does not have an influence on the surface Brønsted acid-
ity characteristics of the V2O5/TiO2 catalyst, but the Lewis
acidity appears to remain unchanged for the surface nio-
bium oxide species and decreases for the surface sulfate
species. However, the addition of the surface tungsten ox-
ide promoter to the vanadia–titania catalyst has a dramatic
influence on the surface Brønsted acidity of the catalyst.
These findings are in agreement with the intrinsic acidities
of these promoters on a titania support since Nb2O5/TiO2
does not possess surface Brønsted acidity (30), SO4/TiO2
possesses surface Brønsted acidity only at high surface cov-
erages (31), and WO3/TiO2 possesses a significant amount
of surface Brønsted acidity (especially for the high surface
coverages employed in the present investigation) (32).
The acidity of the 1% V2O5 supported on different oxides
is determined primarily by the acidic characteristics of the
oxide support because of the very low surface vanadia cov-
erages. Essentially no surface acid sites were present for the
vanadia–silica catalyst, only surface Lewis acid sites were
presentforthevanadia–aluminacatalyst(33), andprimarily
surface Lewis acid sites with a trace of surface Brønsted acid
sites were present for the vanadia–titania catalyst (Table 2).
Therefore, surface Brønsted acidity (measured by pyri-
dine adsorption) is essentially not present at low surface
vanadia coverages on oxide supports, but increases with
surface vanadia coverage and specific promoters such as
surface tungsten oxide species. More recent studies indicate
that NH3 adsorption does give rise to Brønsted acidity for
samples that do not show increased Brønsted acidity with
pyridine as the base, for example, the Nb2O5/V2O5/TiO2
catalysts (34). However, pyridine adsorption detects only
the stronger Brønsted acid sites because it is a weaker base
than NH3, and pyridine adsorption also readily discrimi-
nates between Brønsted and Lewis acid sites than NH3 ad-
sorption.
FIG. 3. Raman spectra of 1% V2O5 supported on (a) alumina, (b) sil-
ica, and (c) titania under dehydrated conditions.
shown in Fig. 2) (21). The surface niobium oxide species
possesses Raman bands at 985 and 930 cmꢅ1, but is over-
shadowed by the much stronger Raman band of the surface
vanadia species (25). Thus, the promoters increase the ap-
parent surface coverage of the surface vanadia species and
have only a mild (tungsten oxide and niobium oxide) to
moderate (sulfate) influence on the ratio of polymerized to
isolated surface vanadia species.
The Raman spectra of 1% V2O5 supported on different
oxides (alumina, titania, and silica) under dehydrated cond-
tions are presented in Fig. 3 in the region 700–1200 cmꢅ1
.
At these low surface coverages, the isolated surface vanadia
species is the predominant vanadia species present on the
different oxide supports and gives rise to a sharp Raman
band at 1016, 1025, and 1039 cmꢅ1 for alumina, titania, and
silica, respectively. For the 1% V2O5/Al2O3 a Raman band
at 880 cmꢅ1 of lesser relative intensity is also observed, but
is a minor component. The 970 and 800 cmꢅ1 bands in spec-
trum b and 790 cmꢅ1 band in spectrum c of Fig. 3 arise from
the SiO2 and TiO2 supports, respectively. The shift in po-
sition of the Raman band of the surface vanadia species is
˚
due to slight changes, approximately 0.02 A, in the length of
==
== – –
the terminal V O bond (26) of the surface O V( O S)3
species (27, 28), where S represents the oxide support cation
(Al, Ti, or Si).
TABLE 2
Surface Acidity Characteristics of Promoted
1% V2O5/TiO2 Catalysts
Surface Acidity
Relative Brønsted
acidity
(arbitrary units)
Relative Lewis
acidity
(arbitrary units)
The distribution of surface acid sites in the titania-
supported vanadia catalysts as a function of vanadia loading
has previously been reported in the literature (29). The ti-
tania support possesses only weak surface Lewis acid sites
and no surface Brønsted acid sites as measured by pyridine
adsorption (30). The number of surface Lewis acid sites
slightly decreases and the number of surface Brønsted acid
Catalysts
1% V 2 O 5 /TiO2
6% WO3/1% V2O5/TiO2
3% Nb2O5/1% V2O5/TiO2
1.1% S O 4 /1% V2O5/TiO2
1
2.9
ꢃ1
ꢃ1
1
0.3
0.9
0.3

216
WACHS ET AL.
Redox Properties
The redox properties of the titania-supported vanadia
catalysts were determined as a function of surface vana-
dia coverage and specific promoters by the selective oxi-
dation of methanol at 503 K. This reaction probe was
selected because methanol oxidation to formaldehyde re-
quires only one surface redox site and yields dimethyl
ether over surface acid sites (19). The selectivity toward
formaldehyde varied from 95 to 99% and the major by-
product was dimethoxymethane, which is also a redox prod-
uctofmethanoloxidation. Onlytracequantitiesofdimethyl
ether were produced, indicating that surface acid sites were
essentially not participating during the methanol oxidation
reaction over the vanadia–titania catalysts. The TOFs for
the titania-supported vanadia catalysts are presented in
Figs. 4 and 5 as a function of vanadia loading and promot-
ers, respectively. Within experimental error, the methanol
oxidation TOF does not appear to depend on the surface
vanadia coverage, over the 1–7% V2O5 loading, or the pres-
ence of the selected promoters (surface tungsten oxide,
surface niobium oxide, and surface sulfate). Independent
methanol oxidation studies revealed that surface tungsten
oxide, surface niobium oxide, and surface sulfate species on
titania did not possess redox characteristics since they did
not yield any formaldehyde at 503 K for the chosen reaction
conditions (20, 21). The 7% V2O5/TiO2 sample contained a
small amount of microcrystalline V2O5 particles; the overall
dispersion or fraction exposed was estimated to be about
90%, which did not significantly affect the apparent TOF.
Higher concentrations of crystalline V2O5 particles present
at a higher vanadia loading do in fact decrease the appar-
ent TOFs of the vanadia–titania catalysts. Thus, the specific
redox properties of the titania-supported vanadia catalysts
are not a function of the surface vanadia coverage or the
presence of the selected promoters (surface tungsten oxide,
niobium oxide, and sulfate).
FIG. 5. Redox reactivity of promoted V2O5/TiO2 catalysts dur-
ing methanol oxidation (503 K). (a) 1% V2O5/TiO2, (b) 1.1% S O 4 /1%
V2O5/TiO2, (c) 6% Nb2O5/1% V2O5/TiO2, (d) 7% WO3/1% V2O5/TiO2.
The influence of the oxide support on the redox prop-
erties of the supported vanadia catalysts was also exam-
ined and the methanol oxidation redox TOFs are shown
in Fig. 6. The selectivity toward formaldehyde was reduced
by the participation of the alumina and silica supports in
the methanol oxidation reaction which resulted in the for-
mation of dimethyl ether and dimethyl ether/carbon ox-
ides, respectively. However, the redox TOF only measures
the reactivity of the surface vanadia sites toward formalde-
hyde production and, consequently, neglects the reactivity
of the support. The specific oxide support has a dramatic
effect on the methanol oxidation redox TOF and varies
by approximately three orders of magnitude (V2O5/TiO2 ꢆ
V2O5/Al2O3 >V2O5/SiO2).
The influence of the specific oxide support on the redox
properties of the surface vanadia species was also probed
with TPR in a hydrogen/helium mixture. The reduction
FIG. 4. Redox reactivity of V2O5/TiO2 catalysts during methanol
FIG. 6. Redox reactivity of 1% V2O5 supported on (a) SiO2, (b) Al2O3,
oxidation as a function of V2O5 loading (503 K).
and (c) TiO2 during methanol oxidation (503 K).

SELECTIVE CATALYTIC REDUCTION OF NO WITH NH3
217
TABLE 3
TPR Peak Maxima for 1% V2O5 Supported on
Different Oxides
Catalyst
Tp (K)
1% V 2 O 5 /TiO2
1% V 2 O 5 /Al2O3
1% V 2 O 5 /SiO2
654
714
721
peak maxima during TPR for the different supported vana-
dia catalysts are presented in Table 3. The differences in
the TPR results are not as dramatic as the methanol ox-
idation TOFs, but exhibit the same trend (V2O5/TiO2 >
V2O5/Al2O3 > V2O5/SiO2). The TPR results confirm that
the specific oxide support does have an effect on the redox
properties of the surface vanadia species.
FIG. 8. SCR reactivity of promoted 1% V2O5/TiO2 catalysts (473 K).
(a) 1% V2O5/TiO2, (b) 1.1% S O 4 /1% V2O5/TiO2, (c) 6% Nb2O5/1%
V2O5/TiO2, (d) 7% WO3/1% V2O5/TiO2.
The influence of the promoters on the SCR TOF is pre-
sented in Fig. 8. The selectivity toward N2 production is
essentially 100% for this series of catalysts at 473 K under
the chosen reaction conditions. The introduction of the sur-
face sulfate species increased the SCR TOF by a factor of 2,
and the introduction of surface tungsten oxide and niobium
oxide species increased the SCR TOF by approximately an
orderofmagnitude. Consequently, thespecificSCRreactiv-
ity of the titania-supported vanadia catalysts is dependent
on the surface vanadia coverage and the presence of the se-
lected promoters (surface tungsten oxide, niobium oxide,
and sulfate species).
The selectivity of SCR DeNOx catalysts at 100% NO
conversion is also an important parameter. All the titania-
supported vanadia SCR catalysts achieved 100% NO con-
version at 623 K. The SCR selectivities toward N2 formation
at 100% NO conversion are listed in Table 4 as a function
of vanadium oxide loading and promoters. Increasing the
vanadium oxide loading beyond 3% results in a drop in N2
selectivity from 100 to 73%. Furthermore, the addition of
tungsten oxide and sulfate decrease the selectivity to N2
formation, 91 and 94%, respectively, whereas the addition
of niobium oxide maintains the selectivity to N2 at 100%.
DeNOx Properties
The TOFs for the selective catalytic reduction of NO with
NH3 over titania-supported vanadia catalysts as a func-
tion of vanadia loading are shown in Fig. 7. The selec-
tivity toward N2 production is essentially 100% for this
series of catalysts at 473 K and chosen experimental con-
ditions. The SCR TOF increases with increasing vanadia
content by a factor of 5 up to monolayer surface coverage,
6% V2O5/TiO2, and decreases above monolayer coverage
where microcrystalline V2O5 particles are present. The 40%
decrease in the SCR TOF above monolayer coverage is
significantly greater than the corresponding decrease in the
dispersion of the vanadia species (from 100% at monolayer
coverage toapproximately80% forthe8%V2O5/TiO2 cata-
lyst). This suggests that the microcrystalline V2O5 particles
are less reactive than the surface vanadia species as well as
interfere with the reactivity of the surface vanadia species
during the SCR reaction.
TABLE 4
SCR Selectivities at 100% NO Conversion (623 K)
Catalyst
1% V 2 O 5 /TiO2
3% V2O5/TiO2
4.5% V2O5/TiO2
N2 selectivity (%)
100
100
85
6% V2O5/TiO2
78
8% V2O5/TiO2
73
6% Nb2O5/1% V2O5/TiO2
7% WO3/1% V2O5/TiO2
1.1% S O 4 /1% V2O5/TiO2
100
91
94
FIG. 7. SCR reactivity of V2O5/TiO2 catalysts as a function of V2O5
loading (473 K).

218
WACHS ET AL.
FIG. 9. SCR reactivity of 1% V2O5 supported vanadia catalysts on
different oxide supports (473 K). Note: same scale as in Fig. 7.
Therefore, at 100% conversion of NO, increasing the sur-
face vanadia coverage lowers the selectivity toward N2 due
to formation of N2O and the specific promoters can further
modify the selectivity.
The TOFs for the SCR of NO with NH3 over 1% V2O5
on different oxides are presented in Fig. 9. The oxide sup-
ports, in the absence of vanadia, exhibited negligible con-
version during the SCR reaction. The TOF for the vanadia–
titania catalyst was approximately three times greater than
those for the vanadia–alumina and vanadia–silica catalysts.
This trend was also observed for the alumina and titania
systems at monolayer coverage of surface vanadia (factor
of 4). Monolayer surface coverage of vanadia on silica is
not achievable (28) and, consequently, could not be mea-
sured. The SCR selectivities of the different 1% V2O5 cata-
lysts are presented in Table 5. The SCR selectivity of 1%
V2O5/TiO2 is found to be better than 1% V2O5/Al2O3 and
1% V 2 O 5 /SiO2 at 473 and 623 K. The selectivity to N2 of 1%
V2O5/Al2O3 and 1% V2O5/SiO2 decreases further at 623 K
but remains 100% for 1% V2O5/TiO2. Thus, the oxide sup-
port affects both the specific reactivity and selectivity of
the selective catalytic reduction of NO with NH3 over sup-
ported vanadia catalysts.
FIG. 10. In situ Raman spectra of 4% V2O5/ZrO2 during SCR reaction
at 300ꢂC. (a) After 25 reduction–reoxidation cycles with butane and 18O2,
respectively and cool down to 300ꢂC. (b) After selective catalytic reduction
of NO with NH3 for 1400 s. (c) After selective catalytic reduction of NO
with NH3 for 1800 s.
==
V O bond during this reaction. The terminal bond was
isotopically labeled with oxygen-18 via a series of succes-
sive butane reduction and 18O oxidation cycles. The heav-
ier mass of oxygen-18 relative to oxygen-16 shifted the
corresponding Raman band from approximately 1030 to
990 cmꢅ1 (see Fig. 10). The surface vanadia monolayer
coverage was deposited on a zirconia support (Degussa,
39 m2/g) because preliminary experiments revealed that
oxygen diffuses less readily through zirconia than titania,
and, consequently, the zirconia support minimized the con-
sumption of the expensive oxygen-18. The time required
==18
==16
O during
to exchange the terminal V
O bond to V
the SCR reaction (containing only 16O2 and NO) was mon-
itored by Raman spectroscopy (see Fig. 10). This exchange
process took approximately 1800 s to complete. Compa-
rison of the exchange time, tex = 1800 s, with characteristic
reaction time, trx = 1/TOF = 182 s, reveals that it took ap-
proximately 10 reaction cycles to completely exchange the
==18
In Situ Raman Spectroscopy during DeNOx
In situ Raman experiments during the SCR reaction were
also undertaken to monitor the stability of the terminal
isotopically labeled V
O terminal bond. This novel ex-
TABLE 5
==
periment suggests that the terminal V O bond is too stable
during the SCR reaction to be directly involved in the rate-
determining step.
SCR Selectivities for 1% V 2 O 5 on Different Oxide Supports
N2 selectivity (%)
Catalyst
473 K
623 K
DISCUSSION
1% V 2 O 5 /TiO2
1% V 2 O 5 /Al2O3
1% V 2 O 5 /SiO2
100
94
91
100
91
85
Increasing the surface vanadia coverage up to mono-
layer coverage increased the density (active sites/m2) of
surface redox sites, the ratio of polymerized to isolated

SELECTIVE CATALYTIC REDUCTION OF NO WITH NH3
219
surface vanadia species, the number of surface Brønsted properties. Only a factor of 2 increase in the SCR TOF was
acid sites, and the SCR DeNOx TOF, but did not affect obtained when the surface sulfate promoter was added. For
the specific redox properties of the surface vanadia species. the sulfated sample additional sulfate could not be added
The constant methanol oxidation TOF, the redox probe re- because it is not possible to achieve a high surface cover-
action, with surface vanadia coverage reflects the one redox age of this surface species due to its volatility (see Table 1).
site requirement of this unimolecular reaction (19, 35) and The addition of surface niobium oxide and tungsten oxide
the constant specific redox potential of the surface vana- species resulted in almost an order of magnitude increase
dia species at all coverages. The increasing SCR TOF with in the SCR TOFs because of the high surface coverages of
surface vanadia coverage reflects that the bimolecular De- these promoters (see Table 1). The relative influence of the
NOx reaction requires another site in addition to the redox promoters, per promoter atom, on the SCR reaction are
ꢃ
=
ꢃ
ꢃ
site to proceed. This requirement of an additional surface
W
1.7, Nb 1.1, and S 0.9. The surface tungsten oxide
=
=
site suggests that the immediate environment of the surface species possessed Brønsted acid sites, and the increase in
redox site is critical. The increase in the SCR TOF with sur- the SCR TOF reveals the promoting effect of such sites.
face vanadia coverage could originate from the need for the Comparison of the SCR TOF enhancements by the sur-
DeNOx reaction to have two adjacent surface redox sites, face niobium oxide and tungsten oxide promoters suggests
a surface redox site with an adjacent surface acid site, or that surface Brønsted acid sites are the most efficient in
the greater specific activity of polymerized surface vana- promoting the SCR reaction since fewer surface tungsten
dia species relative to the isolated surface vanadia species. oxide species resulted in a comparable TOF. Topsoe et al.
However, it is not straightforward to discriminate among recently provided in situ spectroscopic evidence that both
==
these different possibilities by varying the surface vanadia surface Brønsted acid sites, and surface V O sites are in-
coverage on titania since all of the above scenarios increase volved in the SCR catalytic cycle over vanadia–titania cata-
monotonically with surface vanadia coverage.
lysts (14, 15). However, it should be noted that the presence
The introduction of surface promoters to the titania sup- of Brønsted acidity (as measured by pyridine adsorption)
ported vanadia catalyst system introduces an independent is not entirely necessary for the enhancement of the SCR
variable which provides additional fundamental insight into reaction (e.g., niobium oxide).
the SCR DeNOx reaction and allows for better discrimina-
The active surface sites for the SCR DeNOx reaction are
tion among the above possible scenarios. The addition of also influenced by the underlying oxide support, and the
surface tungsten oxide, niobium oxide, and sulfate species TOFs vary by a factor of approximately 3 at 473 K and the
to the 1% V2O5/TiO2 catalyst increased the SCR TOF up to chosen experimental conditions. The redox properties of
an order of magnitude at 473 K, with the maximum TOF ob- the surface vanadia species are influenced by the underly-
servedforthetungstenoxide-andniobiumoxide-promoted ing oxide support as shown by the very sensitive methanol
catalyst. The methanol oxidation probe reaction studies re- oxidationreactionandTPRmeasurements:surfacevanadia
vealed that none of these surface promoter species were reduces more readily on TiO2 ꢆ Al2O3 > SiO2. The acidic
redox sites under these reaction conditions and infrared properties of the surface vanadia species are slightly influ-
studies showed that the tungsten oxide promoter was a sur- enced by the oxide support since 1% V2O5 on titania pos-
face Brønsted acid site. The niobium oxide and sulfate pro- sesses a small amount of surface Brønsted acid sites and
moters did not appear to substantially change the acidity surface Brønsted acid sites are not present on the corre-
of the 1% V2O5/TiO2 catalyst. These observations suggest sponding 1% V2O5 on alumina and silica supports. How-
that the addition of surface non-reducible metal oxide sites ever, comparable numbers of surface redox and Brønsted
to surface redox sites substantially enhances the SCR TOF acid sites (per square meter) are present at monolayer cov-
of titania-supported vanadia catalysts. It also demonstrates erages for alumina- and titania-supported vanadia catalysts
that the DeNOx reaction does not necessarily require two (29, 33) and the difference in TOF is still present. This sug-
adjacent surface redox sites to proceed. Examination of the gests that the specific redox properties of the surface vana-
Raman spectra in Fig. 2 and the SCR data in Fig. 8 shows dia species are primarily responsible for the influence of
that the introduction of the surface promoter species has the oxide support on the SCR TOF. Furthermore, the spe-
only a minor effect on the ratio of polymerized to isolated cific oxide support also has an influence on the selectivity
surface vanadia species but an enhancement of the SCR toward N2 formation (TiO2 > Al2O3 > SiO2) as shown in
TOF is observed. Thus, it appears that the DeNOx reaction Table 5 which may also be related to specific redox prop-
occurs most efficiently over a pair of surface sites containing erties of the surface vanadia species. Similar observations
a surface redox site and an adjacent surface nonreducible about the influence of the oxide support on the SCR reac-
metal oxide site.
The different enhancements of the surface promoters Nickl et al. (36).
on the DeNOx reaction over the 1% V2O5/TiO2 catalyst Comparison of the SCR TOFs with the terminal V O
are due to their different surface concentrations and acidic bond lengths or strengths, as directly measured by Raman
tivity of the surface vanadia species were also reported by
==

220
WACHS ET AL.
spectroscopy (26), also reveals that a correlation between pating as redox sites in the DeNOx reaction at 623 K since
these two parameters does not appear to exist. The most lower selectivities would be expected. At elevated temper-
==
active vanadia–titania catalyst exhibited a V O Raman atures, high concentrations of surface redox sites, or pairs
band 1025 cmꢅ1, while the less active vanadia–alumina and of surface redox sites, appear to result in overoxidation of
==
vanadia–silica catalysts exhibited V O Raman bands at NH3 and NO to N2O and lower the DeNOx selectivity.
1016 and 1037 cmꢅ1, respectively. The terminal V O bond
In summary, the data presented above suggest that se-
==
is clearly perturbed by the reduction as well as adsorption lective catalytic reduction of NOx with NH3 proceeds most
of hydrogen-containing gases such as NH3 (4, 14, 15) and efficiently over a dual-site mechanism involving a surface
H2O (24, 28) due to hydrogen bonding, but these obser- vanadia redox site and an adjacent surface nonreducible
vations do not prove that it is directly involved in the rate- metal oxide site. The SCR reaction appears to be faster
determiningstepduringtheDeNOxreaction. Theinfluence when the surface acid site is a Brønsted acid site; it can also
of the oxide support on the surface vanadia redox proper- proceed with a surface nonreducible metal oxide site, but
ties and the SCR TOF suggests that the bridging V–O– the efficiency of the DeNOx reaction is usually reduced.
support bond may contribute to the rate determining step When the adjacent surface site behaves as a redox site, the
of the DeNOx reaction. This conclusion is consistent with selectivity to N2 may be reduced by ammonia oxidation to
model SCR studies with unsupported V2O5 crystals that re- N2O at elevated temperatures. The above dual-site mech-
– –
ported that the crystallographic planes possessing V O V anism for the SCR reaction has many features in common
–
==
or V OH bonds rather than V O bonds were the selec- with prior mechanisms proposed in the literature. The need
tive sites for the SCR reaction (12, 13). The in situ Raman fortwositesfortheDeNOxreactionhasalsobeenproposed
experiments during the SCR reaction further support the by Miyamoto et al. (1) (ammonia is adsorbed as NH+4 ad-
==18
==
above conclusion since V
stable during the SCR reaction (tex/trx ꢃ 10).
O was formed to be relatively jacent to V O sites) and Janssen et al. (2) (pyrovanadate
== – – ==
structure, O V O V O, is the most likely active site). The
The most efficient catalyst for the DeNOx reaction ap- adsorption of NH3 primarily on surface Lewis acid sites was
pears to be the titania-supported vanadia catalyst since it proposed by Lietti et al. (6, 7) and Went et al. (4). More
yields the highest TOF and selectivity in comparison to the recently, Topsoe et al. provided convincing in situ spectro-
corresponding alumina and silica catalyst systems. The se- scopic evidence that both surface Brønsted acid sites and
lectivity toward N2 is 100% for the vanadia–titania system, surface vanadia sites are involved in the SCR catalytic cycle
and is not affected by the crystalline V2O5 or promoters over titania-supported vanadia catalysts (14, 15). The con-
at 473 K. However, at 623 K the selectivities deviated from clusions reached above may have to be slightly modified
100% N2 production with surface vanadia coverage and the when extrapolating to the more severe industrial DeNOx
introduction of promoters (see Table 4). The selectivity to- conditions where higher temperatures, significant amounts
ward N2 remained 100% up to 0.5 monolayer coverage and of moisture, sulfur oxides, and poisons are encountered.
decreased with increasing surface vanadia coverage up to
monolayer coverage. This suggests that a high surface con-
centration of surface vanadia redox sites, especially pairs
of surface vanadia sites, may lead to overoxidation of NH3
and NO to N2O during the DeNOx reaction at elevated
temperatures. The introduction of surface sulfate to the 1%
V2O5/TiO2 catalyst decreased the N2 selectivity from 100
to 94%. Additional DeNOx studies with sulfated titania, in
the absence of surface vanadia, demonstrated that the sur-
face sulfate species becomes active as a redox site at 623 K,
and results in 50% conversion of NO and a 93% selecti-
vity to N2 for comparable amounts of catalysts. Similarly,
the surface tungsten oxide species on titania also becomes
active as a redox site at elevated temperatures (37). The
surface niobium oxide species on titania is very stable to
reduction by ammonia (38). An 8.8% Nb2O5/TiO2 catalyst
resulted in 42% conversion of NO and an N2 selectivity of
83% at 623 K (39), but this catalyst had more than a mono-
layer of surface niobium oxide species, which corresponds
to 7% Nb2O5/TiO2. The 100% N2 selectivity for the niobium
oxide-promoted 1% V2O5/TiO2 catalyst suggests that the
surface niobium oxide species are not significantly partici-
CONCLUSIONS
The structure and reactivity of supported vanadia cata-
lysts were investigated as a function of surface coverage,
promoters, specific oxide support, and temperature. The
molecular structures of the surface vanadia species were
determined with Raman spectroscopy and the distribution
of surface Brønsted and Lewis acid sites was measured by
infrared spectroscopy employing pyridine adsorption. The
redox potential of the surface vanadia species was moni-
tored by methanol oxidation and TPR. The reactivity of the
various catalysts for the selective catalytic reduction of NOx
with NH3 was measured over a wide temperature range.
Comparison of the structure and reactivity properties of the
supported vanadia catalysts suggests that the reaction pro-
ceeds via a dual-site mechanism, involving a surface vana-
dia redox site and an adjacent surface nonreducible metal
oxide site. Such dual sites are generated with increasing sur-
face coverage of surface vanadia species or the addition of
promoters, such as surface oxides of tungsten, niobium, and
sulfur, and result in a 5- to 10-fold increase in the SCR TOF.

SELECTIVE CATALYTIC REDUCTION OF NO WITH NH3
221
The specific oxide support also influences the redox prop- 13. Ozkan, U. S., Cai, Y., and Kumthekar, M., Appl. Catal. A 96, 365
(1993).
erties of the surface vanadia species, which can affect the
SCR TOF by a factor of approximately 3 as well as the se-
14. Topsoe, N.-Y., Topsoe, H., and Dumesic, J. A., J. Catal. 151, 226 (1995).
15. Topsoe, N.-Y., Dumesic, J. A., and Topsoe, H., J. Catal. 151, 241 (1995).
16. Odriozola, J. A., Heinemann, H., Somorjai, G. A., Garcia de la Banda,
lectivity toward N2 production. This observation along with
==18
the stability of the terminal V
O bonds during the SCR
J. F., and Pereira, P., J. Catal. 119, 71 (1989).
– –
reaction suggests that the bridging V O support bond is 17. Takagi, M., Kawai, T., Soma, M., Onishi, T., and Tamaru, K., J. Catal.
57, 528 (1979).
involved in the rate-determining step. At elevated temper-
atures, the selectivity to N2 may be compromised by ammo-
nia oxidation to N2O when a high concentration of surface
redox sites, especially pairs of surface redox sites, is present.
18. Dumesic, J. A., Topsoe, N.-Y., Slabiak, T., Morsing, P., Clausen, B. S.,
Tornqvist, E., and Toposoe, H., in “Proceedings, 10th International
Congress on Catalysis, Budapest, 1992” (L. Guczi, F. Solymosi, and
P. Tetenyi, Eds.), p. 1325. Akade´miai Kiado´, Budapest, 1993.
The fundamental insights generated from this investigation 19. Deo, G., and Wachs, I. E., J. Catal. 146, 323 (1994).
20. Deo, G., and Wachs, I. E., J. Catal. 146, 335 (1994).
21. Jehng, J. M., and Wachs, I. E., to be published.
22. Jehng, J.-M., Deo, G., and Wachs, I. E., J. Mol. Catal., in press.
23. Andreini, A., de Boer, M., Vuurman, M. A., Deo, G., and Wachs,
have resulted in a better understanding of the nature of the
active sites involved in the selective catalytic reduction of
NOx with NH3.
I. E., J. Chem. Soc., Faraday Trans.
24. Vuurman, M. A., Wachs, I. E., and Hirt, A. M., J. Phys. Chem. 95, 9928
(1991).
ACKNOWLEDGMENT
The financial assistance of DOE (Office of Basic Energy Sciences)
Grant DE-FG02-93ER14350 is gratefully acknowledged by G.D. and
I.E.W.
25. Jehng, J. M., and Wachs, I. E., J. Phys. Chem. 95, 7373 (1991).
26. Hardcastle, F. D., and Wachs, I. E., J. Phys. Chem. 95, 5031 (1991).
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28. Das, N., Eckert, H., Hu, H., Wachs, I. E., Walzer, J. F., and Feher, F. J.,
J. Phys. Chem. 97, 8240 (1993).
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