A Perspective on the Selective Catalytic Reduction (SCR) of NO with

Perspective Cite This: ACS Catal. 2018, 8, 6537−6551

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A Perspective on the Selective Catalytic Reduction (SCR) of NO with NH3 by Supported V2O5−WO3/TiO2 Catalysts Jun-Kun Lai and Israel E. Wachs*

ACS Catal. 2018.8:6537-6551. Downloaded from pubs.acs.org by UNIV DE BARCELONA on 01/16/19. For personal use only.

Operando Molecular Spectroscopy & Catalysis Laboratory, Department of Chemical and Biomolecular Engineering, Lehigh University, Bethlehem, Pennsylvania 18015, United States ABSTRACT: The selective catalytic reduction (SCR) of NOx with NH3 to harmless N2 and H2O plays a crucial role in reducing highly undesirable NOx acid gas emissions from large utility boilers, industrial boilers, municipal waste plants, and incinerators. The supported V2O5−WO3/TiO2 catalysts have become the most widely used industrial catalysts for these SCR applications since introduction of this technology in the early 1970s. This Perspective examines the current fundamental understanding and recent advances of the supported V2O5−WO3/ TiO2 catalyst system: (i) catalyst synthesis, (ii) molecular structures of titaniasupported vanadium and tungsten oxide species, (iii) surface acidity, (iv) catalytic active sites, (v) surface reaction intermediates, (vi) reaction mechanism, (vii) ratedetermining-step, and (viii) reaction kinetics. KEYWORDS: catalyst, V2O5, WO3, TiO2, SCR, NO, NH3

1. INTRODUCTION Most industrial chemical processes are driven by thermal energy and combustion of fossil fuels accounts for main energy source. The high-temperature combustion also forms NOx (NO and NO2) pollution from the reaction between O2 and N2 present in air. The EPA established the emission limit of NOx using recommendations from the Ozone Assessment Transport Group (OTAG)1−3 (NOx emission rate of ∼0.15 lbs/mmBtu (124 ppm) that represents ∼85% reduction from the 1990 emission rate). The current industrial SCR (selective catalytic reduction) of NOx processes can efficiently reduce 80−100% NOx emissions.4Ammonia is the most commonly used reducing agent for the selective catalytic reduction of flue gas NOx to harmless N2 and H2O. The selective catalytic reduction (SCR) of NOx (NO and NO2) with NH3 to harmless N2 and H2O proceeds by the overall reactions 4NO + 4NH3 + O2 → 4N2 + 6H 2O

(1)

2NO2 + 4NH3 + O2 → 3N2 + 6H 2O

(2)

greater thermal stability, and the titiania-based catalysts are used at medium temperatures because of their high selectivity (as much as ∼95−100%) and lower costs. Mn-,Cu-based oxides catalysts have also been reported to efficiently catalyze SCR reactions at low temperatures similarly to V-based oxide catalysts.10−12 This Perspective will focus on titania-supported vanadia-tungsta catalysts, especially supported V2O5/TiO2 and V2O5−WO3/TiO2, since these are the most extensively used catalysts for SCR of NOx with NH3. The supported V2O5− WO3/TiO2 catalysts are widely used in commercial applications because of their excellent thermal stability and lower oxidation activity for the conversion of SO2 to SO3, the latter is strictly regulated because highly reactive SO3 readily converts to H2SO4 in the presence of moisture.13,14 Although the selective catalytic reduction (SCR) of NOx has been successfully applied in industry for almost 50 years, many fundamental details about the nature of the catalytic active sites, surface reaction intermediates, reaction mechanism, ratedetermining-step and kinetic model have long been debated in the scientific literature. This Perspective summarizes the reported literature for SCR of NOx with NH3 by supported V2O5−WO3/TiO2 catalysts and critically examines the issues being debated.13,15 The objective of this article is to provide a modern perspective on this important environmental catalytic reaction and establish a sound fundamental foundation based on supporting data that will be able to guide the rational design of improved SCR catalysts.16

and has primarily been applied to control NOx emissions from large utility boilers, industrial boilers, and municipal solid waste boilers since this technology was initially developed by Hitachi Zosen in Japan at ∼1970.5,6 More recent applications include diesel engines, such as those found on large ships, diesel locomotives, gas turbines, and even automobiles.7−9 The catalysts employed for boilers are supported V2O5/TiO2, V2O5−WO3/TiO2, and V2O5−MoO3/TiO2 and for the more recent applications also consist of zeolite-supported Cu and Fe.4 The zeolite-based catalysts can be used at higher temperatures in the more recent applications because of their © 2018 American Chemical Society

Received: April 6, 2018 Revised: May 30, 2018 Published: June 4, 2018 6537

DOI: 10.1021/acscatal.8b01357 ACS Catal. 2018, 8, 6537−6551

Perspective

ACS Catalysis

2. SUPPORTED V2O5/TIO2, WO3/TIO2, AND V2O5−WO3/TIO2 CATALYSTS 2.1. Synthesis. The catalyst preparation method is a critical factor in determining the interaction between the active components (vanadium and tungsten oxides) and the support (titania). It is, thus, important to carefully compare the characterization findings and SCR performance of catalysts prepared by different synthesis methods. For supported V2O5/ TiO2 catalysts prepared by impregnation of a V-precursor on a crystalline TiO2 support, it has been demonstrated that the coordination of the hydrated surface vanadium oxide species are thermodynamically controlled and cannot be controlled by the specific V-precursor or preparation method.17,18 For a series of supported V2O5/TiO2 catalysts synthesized by a variety of Vprecursors (V-oxalate, ammonium metavanadate, grafting of VOCl3, grafting of V-alkoxide, and even thermal spreading of crystalline V2O5 onto TiO2), all the resulting hydrated surface VOx possess the same surface vanadia species.19−21A similar trend was found for supported WO3/TiO2 catalysts synthesized by impregnated of W-precursors onto crystalline TiO2.22,23 (aqueous solutions of tungstate salts (ammonium metatungstate) or grafting of tungsten alkoxides onto crystalline TiO2(anatase) supports).24−26 Information about coimpregenation of V- and W-precursors onto crystalline TiO2 supports has not received as much attention, but it has been demonstrated that the order of V- and W-precursor impregnation does not affect the hydrated VOx and WOx molecular structures of calcined catalysts.22,27 Although supported V2O5−WO3/TiO2 catalysts have not been extensively examined, some publications claim that the nature of the supported vanadium oxide sites depends on the specific precursors and preparation methods,21,23,28−32 but these claims were not backed with supporting information. Supported V2O5−WO3/TiO2 catalysts prepared by coprecipitation of noncrystalline TiO(OH)2 with ammonium metatungstate and ammonium metavanadate were found to result in new surface VOx and WOx sites on the TiO2 support.17,18,23,28 In addition to the typical surface VOx and WOx sites formed by impregnation of the crystalline TiO2 support, new surface VOx and WOx sites were also present that have been assigned to surface VOx and WOx sites coordinated to defective surface sites on the TiO2 support. Interestingly, both the VOx and WOx components almost exclusively segregated to the surface of the TiO2 support upon calcination that reflects on their mobility in such mixed-oxide systems.23,28 The sol−gel33−35 and flame pyrolysis36 synthesis methods have also been examined, but there is no evidence that the resulting structures of the surface VOx and WOx sites on TiO2 are different than those formed by conventional impregnation. Given the tendencies of both VOx and WOx to surface segregate and the thermal spreading of crystalline V2O5 into surface VOx sites on TiO2 upon calcination, indicating that surface VOx and WOx on TiO2 are the preferred thermodynamic states, it is highly unlikely that any other synthesis methods can result in any new surface WOx and VOx structures on the TiO2 support. The major difference appears to be between impregnation of V/W-precursors onto crystalline TiO2 and coprecipitation of V- and W-precursors with TiO(OH)2 that result in new surface VOx and WOx sites thought to be associated with surface defects of the TiO2 support. It is important to be aware that impregnation procedures involving poorly soluble precursors (e.g., aqueous ammonium

metavanadate or crystalline V2O5-aqueous oxalic acid mixtures) onto crystalline TiO2 can also lead to the formation of crystalline V 2 O 5 and WO 3 nanoparticles much below monolayer coverage in addition to the standard surface VOx and WOx sites.17 Another deviating situation can occur when V-precursors are impregnated onto crystalline TiO2(rutile) that can incorporate some V+4 species into its bulk lattice.37,38 Calcination in an oxygen-containing environment, however, tends to prevent V+4 diffusion into the bulk TiO2(rutile) lattice and maintain surface VOx in the V+5 oxidation state.37−40 The final surface area of the supported V2O5−WO3/TiO2 catalyst is dependent on the specific preparation procedure. Impregnation and calcination with W-precursors before or simultaneously with V-precursors yields the highest surface areas.7,22,41,42 This is related to the thermal stabilizing effect of surface WOx on TiO2 and V2O5/TiO2 catalysts.23,28 Thus, in practice the W-precursor is always introduced before or with the V-precursor to optimize the final surface area of the TiO2 support. In summary, the catalyst synthesis method (titania source, precursor, impregnating solvent, vanadia and tungsta loadings, drying and calcination treatments, etc.) affects the final catalyst structure (titania phase, BET surface area, etc.) and dispersion of the vanadia and tungsta components (surface VOx/WOx species and V2O5/WO3 NPs). Amorphous titania starting materials also result in surface defects that stabilize new surface VOx and WOx sites on the TiO2 support. 2.2. Structures of Supported V2O5/TiO2 Catalysts. Under ambient conditions, the surface vanadia phase is hydrated by a thin aqueous film created by adsorption of moisture from ambient air. Deo et al. demonstrated that under ambient conditions the molecular structures of hydrated surface VO x species are similar to those present in aqueous solutions.18,43−46 The hydrated surface VOx species equilibrate at the pH of the thin aqueous film present on the catalyst, which is determined by the point of zero charge (PZC − net zero charge of the hydrated surface). For TiO2 supports, the PZC is pH ∼ 6−7. For low vanadia loading, the PZC is controlled by the TiO2 support and metavanadate oligomers, (VO3)n, are the dominant hydrated vanadates.18 For high vanadia loading, the PZC of the thin aqueous film is controlled by both the TiO2 support and the vanadia phase (pH ∼ 0.5), which significantly lowers the net PZC, and vanadate clusters are the dominant hydrated vanadates.18 Under SCR reaction conditions, however, the supported VOx phase on TiO2 is present in a relatively dehydrated state because of the rapid desorption of moisture at elevated temperatures.43,47 Under oxidizing conditions, the supported vanadium oxide phase is present as two-dimensional surface V+5Ox species below monolayer coverage (8 V atoms/nm2) as schematically depicted in Figure 1. Extensive characterization studies with IR,13 Raman,18,22,43,48 X-ray absorption spectroscopy (XANES/ EXAFS),49 solid-state51V NMR50 and UV−vis44,51 spectroscopy have established the molecular structures of the surface VOx sites on TiO2. These studies revealed that the surface VOx sites possess VO4 coordination with one terminal VO bond as shown in Figure 1. At low surface coverage (8 V atoms/nm2), crystalline V2O5 nanoparticles are also present and reside on top of the surface vanadia monolayer as indicated in Figure 1c. The nature of the dehydrated supported vanadia phase on the crystalline TiO2 support as a function of surface vanadia coverage is reflected by the in situ Raman spectra of supported V2O5/TiO2 catalysts presented in Figure 2. The Raman band at

Figure 3. Structures of the dehydrated surface tungsta phase on TiO2; (a) isolated mono-oxo WO5, (b) oligomeric mono-oxo WO5/6, and (c) crystalline WO3 nanoparticles on top of the surface tungsta monolayer.

IR,13 Raman,22,47,59 X-ray absorption spectroscopy (XANES/ EXAFS)47,60−62 and UV−vis59,63 spectroscopy have established the molecular structure of the dehydrated surface WOx sites. These studies revealed that the surface WOx sites possess WO5/6 coordination and one terminal WO bond as shown in Figure 3. The lower surface W density at monolayer coverage of the surface WO5/6 sites compared to surface VO4 sites at monolayer coverage is related to the greater number of oxygen atoms surrounding the surface WO5/6 sites than the surface VO4 sites. At low surface coverage (5%).13,139 The inhibiting effect of water has been proposed to arise from (1) competitive adsorption with the reactants (NO or NH3)119,123,139−143 and/or (2) inhibition of reaction between NO and adsorbed ammonia.144−146 Topsøe et al. investigated the effect of H2O (1.7%) during SCR at 390 °C by supported V2O5/TiO2 catalysts with in situ IR spectroscopy and found that water increases the concentration of surface NH4+* species on Brønsted acid sites and only slightly decreases the SCR activity.145 The enhanced concentration of surface NH4+* intermediates may just reflect the conversion of Lewis to Brønsted acid sites in the presence of moisture.39 Importantly, water improves the SCR selectivity by reducing formation of undesirable N2O (from ∼16% to ∼2% at 390 °C and ∼4% to ∼1% at 325 °C) by supported V2O5/TiO2 catalysts.140,145 It was proposed that the suppression of N2O formation occurs because hydroxylated surfaces favor dehydration over dehydrogenation of the surface NHxNO reaction intermediates, but supporting evidence was not given. It has also been claimed that pretreating the catalyst with moisture at 650 °C for 2 h enhances the SCR reaction rate by increasing the number of surface Brønsted acid sites, but additional information is required to determine the origin of this interesting effect.147,148 Thus, H2O has a significant effect on both the supported V2O5/TiO2 catalysts and SCR reaction, and more detailed studies about the role of water are needed. Studies on the influence of moisture on supported WO3/TiO2 and V2O5−WO3/TiO2 SCR catalysts have not been reported. 7.2. Effect of SO2. Flue gases emissions from power plants always contain SO2 (200−1500 ppmw)137,149 that can be further oxidized to SO3 by the supported V2O5−WO3/TiO2 catalysts. The much higher reactivity of SO3 relative to SO2 has led to its emissions being tightly regulated because SO3 readily reacts with moisture to form H2SO4 (acid rain). In addition, SO3 is corrosive to stainless steel SCR reactor components and readily reacts with ammonia to form ammonium sulfate/ bisulfate that deposit on the catalysts causing deactivation and on the walls of heat exchangers to reduce their efficiency. Industrial upper emission limits of sulfur trioxide for SCR processes corresponds to approximately 1−2% sulfur dioxide conversion.14,150,151 SCR catalysts, thus, need to both suppress oxidation of SO2 to SO3 and efficiently promote the SCR reaction to N2. The chemisorption of SO2 and SO3 on TiO2 and supported V2O5/TiO2 has received much attention. In the absence of gas phase molecular O2, SO2 adsorbs on TiO2 as either physisorbed sulfur dioxide (SO2) or chemisorbed sulfite (SO3) surface species, with the ratio depending on the adsorption temper-

oxidation of partially reduced surface vanadia sites during SCR is not a rate-determining-step and is consistent with a Mars− van Krevelen reaction mechanism involving oxygen from the catalyst surface lattice. Below 1% O2, which is below the O2 partial pressure typically found in flue gas emissions, the reaction rate is dependent on O2 partial pressure because the surface vanadia sites are not maintained in their fully oxidized state.67,68,123,124 The SCR reaction kinetics are inhibited at low water partial pressures (1−5% H2O) and is not further affected by water at higher partial pressures.102,103,105,116 In contrast to the reported empirical kinetics, SCR kinetic expressions based on Eley−Rideal68,71,94,109,112 and Langmuir− Hinshelwood68,85,119,130,134 models have also been proposed. Given the strong adsorption of NH3 and weak or absence of NO adsorption on the catalyst, most researchers have invoked the Eley−Rideal reaction model. The Eley−Rideal model implies that NO is not adsorbed on the catalyst surface and directly reacts from the gas phase with the surface NH3*/ NH4+* species. The simple kinetic expression developed by Dumesic et al. is the most widely accepted model because it gives kinetic expressions for both dual-site or single-site mechanisms.13,43,117,133,135 Single-site mechanism: step 1: NH3 + * → NH3 − *

(12)

step 2: NO + NH3 − * → product + *

(13)

rate = k 2 × PNO ×

K NH3 × PNH3 1 + K NH3 × PNH3

(14)

(* is ammonia adsorption site) PNO and PNH3 are the partial pressure of NO and NH3, KNH3 is the equilibrium adsorption constant for NH3 and k2 is the Arrhenius reaction rate constant. Dual-site mechanism: step 1: NH3 + * → NH3 ‐*

(15)

step 2: NH3 ‐* + S → NH3‐S + *

(16)

step 3: NO + NH3‐S → product + S

(17)

(* is ammonia adsorption site and S is reactive site) rate = k 3 × PNO ×

K1 × 1 + K1 ×

PNH3 PNO

+

PNH3 PNO

K2 PNO

+ K NH3 × PNH3 (18)

with K1 = k2 × KNH3/k3, K2 = k−2/k3 and k values representing Arrhenius reaction rate constants. Fitting of the kinetic data with reaction rate expressions, however, is not proof of a reaction mechanism because of the simplified assumptions that are invoked.136 For example, weak adsorption of NO can simplify the Langmuir−Hinshelwood kinetic expression to approach that of the Eley−Rideal kinetic expression. Furthermore, neither the L−H nor the E−R mechanism address the Mars−van Krevelen mechanism found to take place during the SCR reaction. It is only through isotopic labeling and in situ spectroscopy studies during SCR that the details of the reaction pathways have been identified. Furthermore, the SCR reaction rates by supported vanadia catalysts vary by a factor of ∼102 (ZrO2 ∼ TiO2 > Al2O3 > 6544


Perspective

ACS Catalysis ature.14,152 Monolayer surface coverage of SO2 and SO3 species are estimated to correspond to ∼4 S atoms/nm2 for isolated species.153,154 In the presence of gas phase molecular O2 and temperatures above 200 °C, the surface sulfite (SO3) species transform into the more thermodynamically stable surface sulfate (SO4) species.14,152 Surface sulfates (SO4) form when surface sulfite (SO3) species titrate adjacent sites and form additional bonds to the titania support. The surface sulfates on TiO2 have been demonstrated to be present as mono-oxo surface OS(−O−Ti)3 species with vibrational spectroscopy under dehydrated conditions.155 Upon exposure to water vapor, the surface SO4 species convert to protonated bidentate surface species,155 which increase the Brønsted acidity by formation of acidic S−OH groups.81,156 For supported V2O5/TiO2 catalysts, the dehydrated surface SO4 species prefer to coordinate to the TiO2 support instead of the surface vanadia species because of repulsive interactions between the surface vanadia and sulfate species.154,157 At half monolayer coverage of surface vanadia species (∼4 V atoms/nm2), surface sulfate species are no longer able to form on TiO2 because of the repulsion between the surface sulfate and vanadia species. As for supported V2O5/ TiO2 catalysts, the sulfated supported V2O5/TiO2 catalysts with less than a half monolayer of surface vanadia in the presence of moisture were also found to increase the number of surface Brønsted acid sites.147,148,158 It was proposed that surface Brønsted acid sites in the presence of H2O and SO2 at high temperatures (>350 °C) increased the SCR reaction rate, but the relative contributions of Brønsted acid sites from the surface VO4 and SO4 sites were not determined.142,157,159 The oxidation of SO2 to SO3 by supported vanadia catalysts has recently been investigated.160 It was found that the turnover frequency (TOF) for SO2 oxidation is independent of surface vanadia coverage indicating that only one surface vanadia site participates in this oxidation reaction.48,98,160,161 The TOF for SO2 oxidation, however, varies by more than an order of magnitude with the specific oxide support (e.g.,TiO2, ZrO2, Al2O3, etc.) and is mainly related to the bridging V−O− Support bond.160,162 This indicates that the oxygen from the bridging V−O−Support bond controls the specific reactivity (TOF). It has been proposed that the active surface vanadia site for the oxidation of SO2 by supported V2O5/TiO2 catalysts consists of dimeric vanadia-sulfate species,103,144,163 but supporting structural information is still lacking. The kinetics of SO2 oxidation by supported V2O5/TiO2 catalysts have been found to follow first-order dependence on SO2, zero-order dependence on O2 and negative first-order dependence on SO3.160,163,164 Additionally, water has been found to decrease the reaction rate by as much as 50%. Although studies of the SO2 oxidation mechanism by supported V2O5/TiO2 catalysts are rare, Ji et al.165 proposed that adsorbed SO2 is oxidized to SO3 by the terminal VO bond and the reduced vanadia site is subsequently oxidized by gas phase molecular O2. This proposed reaction mechanism, however, is not in agreement with the demonstrated strong dependence of the reaction rate on the oxygen from the bridging Ti−O−V bond and lack of dependence of TOF on the terminal VO bond for the SO2 oxidation TOF.160 Additional studies are needed to fully resolve the SO2 oxidation mechanism by supported V2O5/TiO2 catalysts. At elevated temperatures, SO2 is oxidized by both the surface vanadia and surface tungsta sites with the surface vanadia sites exhibiting an order of magnitude greater specific activity (TOF) than the surface tungsta sites.96,97,103,105,143,166 The overall SO2

oxidation activity is just the simple sum of SO2 oxidation on individual surface vanadia and tungsta sites, which indicates the absence of any synergistic interactions between these two surface metal oxide sites for SO2 oxidation. The participation of only one surface metal oxide site in the SO2 oxidation reaction is consistent with the independent behavior of the surface vanadia and tungsta sites on TiO2. Both surface vanadia and tungsta sites, however, contribute equally to the overall SO2 oxidation reaction because the ratio of surface W/V is typically ∼3−4.14

8. DENSITY FUNCTIONAL THEORY (DFT) Density functional theory (DFT) calculations have recently been applied to gain additional fundamental insights into the SCR reaction. The majority of these DFT studies, however, employed Ti-free models as the catalytic surface: V2O9H cluster,167,168 V6O20H10 cluster,169 V4O16H12 cluster,170 V2O9H8 cluster,171 slab model of V8O20 (001),172 and periodic slab of the (010) plane of bulk V2O5173,174 with finite cluster to an infinite surface computational methods.167,168 The DFT model with unsupported gas phase VOx clusters concluded that surface NH4+* species on Brønsted sites are more energetically favored than surface NH3* species on Lewis sites to reduce NO. In contrast, only a few DFT simulations concluded that surface NH3* species on Lewis acid sites are the reactive surface intermediates in the SCR reaction.172,175,176 These DFT studies, however, did not examine the SCR reaction with realistic surface VO4 active sites on TiO2 because of the more challenging undertaking required to model the structure of the amorphous monolayer of surface VO4 species than well-defined vanadia structures present in clusters and crystalline planes of bulk V2O5. DFT calculations with an isolated surface mono-oxo OVO4H cluster on TiO2(001) have recently been reported for the SCR reaction and suggested involving oxygen from two bridging Ti−O−V oxygen bonds.177 According to this model, one of the bridging oxygens forms a hydroxyl that contributes to the formation of surface NH4+* species on Brønsted acid sites while the second bridging oxygen accepts a proton to stabilize the reduced vanadia active site. The resulting surface NH2* species react with weakly adsorbed NO to form the surface H2NNO* intermediate complex that spontaneously decomposes to N2 and H2O. Additional DFT calculations for more complex titania-supported vanadia sites are still needed to fully understand the fundamentals of the SCR reaction (e.g., influence of oligomeric surface vanadia sites, isolated and oligomeric surface tungsta sites, moisture, sulfur oxides, etc.). 9. PERSPECTIVE OF ISSUES REQUIRING RESOLUTION In spite of the significant recent progress achieved with regards to the fundamental understanding of SCR of NO with NH3 by supported V2O5−WO3/TiO2 catalysts, there are still issues requiring complete resolution (e.g., isolated vs. oligomeric surface vanadia sites, influence of surface tungsta sites on the surface vanadia sites, surface NH4+* vs. NH3* species, nature of surface reaction intermediate complex, reaction pathways for formation of N2O and effect of moisture, SO2, NO2 and N2O). Additional progress will come from studies with catalysts being characterized under reaction conditions given the dynamics of this catalytic system in different environments (much of the confusion in past studies have arisen from the different reaction conditions being employed in many studies), High Field 51V MAS NMR spectroscopy178 to determine the structures of 6545


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ACS Catalysis surface vanadia species will provide new molecular level insights (influence of synthesis, titania support, surface tungsten oxide sites, and reaction environments), fast and modulation spectroscopy that will allow detecting the surface reaction intermediate complex and more advanced DFT calculation studies.

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10. CONCLUSIONS This Perspective highlights the trends that are well-established as well as issues that still need to be resolved for the SCR of NOx with NH3 by supported V2O5−WO3/TiO2 catalysts. The conclusions reached in this Perspective are (i) The catalyst synthesis method (titania source, precursor, impregnating solvent, vanadia and tungsta loadings, drying and calcination treatments, etc.) affects the final catalyst structure (titania phase, BET surface area, pore size, porosity, etc.) and dispersion of the vanadia and tungsta components (surface vanadia species, surface tunsgta species, V2O5 NPs and WO3 NPs). Amorphous titania starting materials also introduce surface defects that stabilize new surface vanadia and tungsta sites. (ii) The surface vanadia and tungsta sites on TiO2 form both isolated and oligomeric surface VO4 and WO5/6 species. The same surface vanadia and tungsta species form on TiO2, anatase as well as rutile, independent of the synthesis method since the synthesis method only determines the metal oxide dispersion and presence of surface titania defects. (iii) The total number of surface acid sites is independent of the surface metal oxide coverage with the concentration of surface Brønsted acid sites increasing linearly with surface vanadia and tungsta coverage. The surface vanadia sites mostly exhibit Brønsted acidity while the surface tungsta sites possess comparable amounts of Brønsted and Lewis acidity. In the presence of moisture at temperatures of 250 °C and higher, the surface Lewis acid sites are converted to surface Brønsted acid sites. (iv) The surface vanadia species, in the V+5 oxidation state, are the catalytic active sites for the SCR reaction; the surface tungsta species are not active for the SCR reaction, but promote the surface vanadia species by their oligomerization and/or crowding, but not by an electronic effect. (v) The most abundant surface intermediate is surface NH4+* species on Brønsted acid sites since surface NH3* species convert to surface NH4+* species in the presence of moisture present under SCR reaction conditions. Surface NHxNO reaction intermediates have not been detected with IR either due to their low concentrations or rapid decomposition to N2 and H2O during SCR reaction conditions. (vi) The SCR of NO with NH3 proceeds via a surface Mars− van Krevelen mechanism involving oxygen from the surface vanadia sites with the surface region of the TiO2 support also supplying oxygen to the surface vanadia sites. The reaction mechanism involves the participation of weakly adsorbed NO with surface NHx species. (vii) The rate-determining-step involves breaking of an N−H bond during the course of formation or decomposition of the surface NO−NHx reaction intermediate. (viii) Although empirically the SCR reaction is first-order in NO and zero-order in both ammonia and molecular O2,

a kinetic expression based on the elementary steps of the reaction mechanism, surface reaction intermediate complex and rate-determining-step is still lacking. Kinetic studies investigating the influence of surface vanadia and tungsta coverage are still absent from the literature.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Israel E. Wachs: 0000-0001-5282-128X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of UNCAGE-ME, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0012577.

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A Perspective on the Selective Catalytic Reduction (SCR) of NO with

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