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Home , Catalytic oxidation, Contact process, Oxidation state, Sulfur trioxide

Today 51 (1999) 301±318

Oxidation of dioxide over supported vanadia catalysts: molecular structure ± reactivity relationships and reaction kinetics

Joseph P. Dunna, Harvey G. Stenger Jr.b, Israel E. Wachs*,b

aBOC Gases Technology, 100 Mountain Avenue, Murray Hill, NJ 07974, USA bZettlemoyer Center for Surface Studies, Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015, USA

Abstract

The oxidation of to over supported catalysts occurs as both a primary and secondary reaction in many industrial processes, e.g., the manufacture of sulfuric , the selective catalytic reduction of NOx with and the regeneration of petroleum re®nery cracking catalysts. This paper discusses the fundamental information currently available concerning the molecular structure and sulfur dioxide oxidation reactivity of surface vanadia species on oxide supports. Comparison of the molecular structure and reactivity information provides new fundamental insights on the following topics related to the catalytic properties of surface vanadia species during the sulfur dioxide oxidation reaction: 1. role of terminal V=O, bridging V±O±V and bridging V±O-support bonds,

2. number of surface vanadia sites required to perform SO2 oxidation, 3. influence of oxide additives, 4. generation and influence of the surface overlayer, 5. effect of surface acidity on the reaction turnover frequency,

6. competitive adsorption between SO2 and SO3 and 7. reaction kinetics.

# 1999 Elsevier Science B.V. All rights reserved.

Keywords: Oxidation; Sulfur dioxide; Vanadia catalyst; Sulfur trioxide

1. Introduction ciated sulfur oxide, SOx, emissions. Approximately two-thirds of the 50 billion pounds of sulfur

Sulfur dioxide, SO2, is formed from both the oxida- released annually in the United States are emitted from tion of sulfur contained in fossil fuels and industrial coal ®red power plants [1]. Industrial fuel processes that treat and produce sulfur-containing and industrial processes (primarily man- compounds. The of sulfur dioxide ufacture, petroleum re®ning and smelting of non- appears in numerous industrial processes and has a ) account for the remainder of the signi®cant environmental impact because of the asso- emissions. Sulfuric acid is the largest volume chemical cur- *Corresponding author. Tel.: +1-610-758-4274; fax: +1-610- rently produced in the world, approximately 95 billion 758-6555; e-mail: [email protected] pounds per year [2], and is manufactured by the

0920-5861/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0920-5861(99)00052-8 302 J.P. Dunn et al. / Catalysis Today 51 (1999) 301±318 , which involves the high temperature 2. savings in operating costs generated by lowering catalytic oxidation of sulfur dioxide to sulfur trioxide SCR temperatures, without the worry of ammo- over a unique supported liquid phase catalyst (silica nium sulfate production and deposition. supported vanadium pyrosulfate with alkali promo- Two new NO /SO removal techniques, SNO (Hal- ters). Under reaction conditions, 450±6108C, the x x x dor Topsoe) and DeSONO (Degussa), combine SCR active vanadia component of the catalyst exists as a x technology with sulfuric acid production [12,13]. Flue molten forming a very thin liquid layer on the gas is heated to 3808C and oxides are surface of the silica support (only 100±1000 AÊ thick). removed via conventional SCR technology. The pro- Sulfur dioxide oxidation to sulfur trioxide proceeds on ducts are further heated to 4208C and the SO is both the active sites located in the interior of the liquid 2 oxidized to SO over a sulfuric acid contact catalyst. ®lm and on the boundary between the ®lm and the 3 The sulfur trioxide is then contacted with water pro- surface of the silica support [3±5]. Thermodynami- ducing concentrated sulfuric acid. Any unreacted cally controlled equilibrium limitations exist at the ammonia from the SCR reactor is oxidized to NO temperatures necessary for the catalyst to activate, x over the second catalyst bed, consequently, avoiding T>4208C, and the conversion of the sulfur dioxide feed the formation of . is incomplete with the very small unreacted portion Sulfur oxide emissions from ¯uid catalytic cracking typically being directly emitted into the environment. (FCC) units account for a sizable fraction of annual In contrast to the sulfuric acid contact process, the SO emissions and are increasingly being targeted by oxidation of SO to SO is undesirable during the x 2 3 the Environmental Protection Agency (EPA). The selective catalytic reduction (SCR) of nitrogen oxides amount of SO emitted from a FCC unit regenerator (NO ) found in the ¯ue gas of power plants. SCR x x is a function of the quantity of sulfur in the feed, coke removes NO in the ¯ue gas by reacting the nitrogen x yield and conversion [14]. Typically, 45±55% of feed oxides with ammonia and to form nitrogen and sulfur is converted to sul®de in the FCC water at approximately 3708C over titania supported reactor, 35±45% remains in the liquid products, and vanadia catalysts (e.g., V O /WO ±MoO /TiO ). 2 5 3 3 2 about 5±10% is deposited on the catalyst in the coke Under typical SCR design and operating conditions, [14,15]. The sulfur in the coke is oxidized to SO NO reduction ef®ciency is directly proportional to the 2 x (90%) and SO (10%) in the FCC regenerator. Tradi- NH :NO ratio up to NO reduction levels of about 3 3 x x tional techniques of SO control such as ¯ue gas 80%. Operating with too high of a NH :NO ratio can x 3 x scrubbing and feedstock hydrodesulfurization are to unreacted ammonia bypassing the reactor, effective, but are labor and cost intensive. The least ammonia slip, where it readily combines with SO 3 costly alternative is the use of a SO -reduction catalyst at temperatures below 2508C to form ammonium x as an additive to the FCC catalyst inventory. The sulfates, which can the catalyst's pores and foul catalyst must be able to: downstream heat exchangers [6]. This problem is so serious that industrial speci®cations for SCR pro- 1. oxidize SO2 to SO3 in the FCC regenerator, cesses include upper limits for the outlet concentration 2. chemisorb the SO3 in the FCC regenerator, and of sulfur trioxide corresponding to approximately 1± then 2% sulfur dioxide conversion. Several studies have 3. release it as hydrogen in the reducing FCC investigated the development of catalysts capable of reactor. simultaneously suppressing the oxidation of sulfur dioxide to sulfur trioxide while ef®ciently promoting Supported vanadia catalysts, such as Amoco's DeSOx the selective catalytic reduction of [7±11]. catalyst (V2O5/CeO2/Mg2Al2O5), have demonstrated The advantages of such catalysts would be: high activity towards these reactions. In addition, petroleum re®ning operations such as 1. the ability to install more intrinsically active SCR FCC and hydrodesulfurization (HDS) yield hydrogen catalysts (e.g., higher vanadia loadings) without sul®de as an undesired product. The hydrogen sul®de

the fear of simultaneously increasing SO3 produc- is typically concentrated and fed to a Claus plant to tion and produce elemental sulfur. However, due to equilibrium J.P. Dunn et al. / Catalysis Today 51 (1999) 301±318 303 limitations only 97% of the sulfur is recovered in the this report due to the limited information available in Claus plant and the tail gas, therefore, needs to be the literature concerning the surface arrangement of treated before being released to the atmosphere. The these catalysts. mobil oil SOx treatment (MOST) process involves combusting the Claus tailgas with air, converting all of the sulfur species to SO2 and SO3.TheSOx is 2. Synthesis of supported vanadia catalysts sorbed onto a V2O5/CeO2/Mg2Al2O5 spinel where it is later regenerated to produce concentrated H2S and Many different synthesis methods have been used in SO2, which is recycled to the Claus plant for further the preparation of supported vanadia catalysts: vapor processing [16]. phase grafting with VOCl3 [19±21], non-aqueous In spite of the industrial importance and environ- impregnation with vanadium alkoxides [22,23] and mental consequences of the above catalytic oxidation vanadium acetate [24], aqueous impregnation of vana- processes involving sulfur dioxide, few fundamental dium oxalate [25], as well as dry impregnation with studies have been performed on the kinetics and crystalline V2O5 [26±29]. However, all the catalysts mechanism of sulfur dioxide oxidation with the excep- were found to contain the same surface vanadia spe- tion of the unique sulfuric acid contact catalyst [3±5]. cies independent of the initial synthesis method after However, the studies on commercial sulfuric acid prolonged calcination [30]. The absence of a ``pre- catalysts are not applicable to the environmental paration memory effect'' is due to the high mobility of oxidation reactions over conventional solid metal V2O5 (Tamman temperature of 3708C) and the strong oxide catalysts since the contact catalyst contains driving force of the mixed oxide system to lower its the active vanadia/alkali/sulfate component as a mol- surface free energy by forming a monolayer of surface ten salt on the silica support. Current research into the vanadia species on the high surface free energy oxide oxidation of SO2 to SO3 over conventional supported support [26±29]. The formation of a monolayer is even vanadia catalysts has focused on establishing the observed during hydrocarbon oxidation reactions over fundamental kinetics and molecular structure-reactiv- physical mixtures of V2O5 and TiO2 [31]. Thus, the ity relationships of the oxidation reaction [17,18]. It is same thermodynamically stable surface metal oxide hoped that the insights generated from these studies species are formed independent of the speci®c synth- will assist in: esis method. In the case of silica supported vanadia catalysts, where different preparation methods can 1. the design of SCR DeNO catalysts with minimal x affect the surface coverage of the surface vanadia SO oxidation activity, 2 species, the same isolated surface vanadia species 2. the development of catalysts for low temperature results from all the different syntheses. Commercial (200±3008C) oxidation of SO to SO during sul- 2 3 preparations usually employ aqueous impregnation furic acid manufacture and, with vanadium oxalate because of its high solubility 3. improvements in additives for the simultaneous in water and the absence of undesirable volatile oxidation/sorption of sulfur oxides in petroleum organic solvents [32±36]. refinery operations. This report will attempt to summarize the current fundamental understanding of the scienti®c concepts 3. Characterization of supported vanadia guiding the oxidation of SO2 to SO3 over supported catalysts vanadia catalysts by correlating pertinent molecular- level structural information with the observed reaction Surface vanadium oxide phases are formed when kinetics. In addition, it is hoped that the information vanadium oxide is deposited on an oxide support (e.g., contained herein will lay the foundation for future SiO2,Al2O3,TiO2, ZrO2,CeO2 and MgO) [37,38]. studies of SO2 oxidation over pollution control cata- The supported vanadium oxide phase, like other sup- lysts. Although widely used in petroleum re®nery ported metal oxide phases, can simultaneously exist in operations, vanadia supported on alumi- several different structural states (bulk crystallites, nate spinels (MgxAl2O3‡x) will not be highlighted in mixed phases with the support, or two-dimensional 304 J.P. Dunn et al. / Catalysis Today 51 (1999) 301±318 overlayers). Recent developments in molecular char- about the theory and application of solid state acterization techniques have successfully demon- 51V NMR to supported vanadium oxide catalysts strated the capability to discriminate between the can be found elsewhere [47]. multiple vanadium oxide structures present in sup- X-ray absorption spectroscopy takes advantage of ported vanadia catalysts and provide information intense synchrotron X-ray sources to determine the required to develop a molecular level understanding and chemical environment around a of these complex catalytic materials. speci®c element in the sample. X-ray absorption Several studies have shown that Raman spectro- techniques (EXAFS/XANES) have been used to deter- scopy can readily discriminate between different mine the molecular structures of the supported vana- vanadium oxide structures with different coordina- dium oxide phase with some success [49±51]. tions and bond lengths [32,39,40]. Raman spectro- Furthermore, X-ray absorption spectroscopy can be scopy is an optical technique and, therefore, can be used under in situ conditions. The disadvantage of the applied to study the structural changes of the surface X-ray absorption techniques is that only an average vanadia species under in situ conditions where the signal of the different vanadium oxide structures is environment around the catalyst is controlled (tem- observed and hence, only the average structure can be perature, pressure and gas composition). The theory obtained. and application of Raman spectroscopy to catalytic X-ray photoelectron spectroscopy (XPS) provides material can be found elsewhere [41,42]. information on the oxidation state and dispersion in Infrared spectroscopy can also discriminate the surface region (2 to 20 atomic layers) of the between various vanadium oxide structures. However, catalyst [47]. However, molecular structural informa- many of the oxide supports obscure the vibrations tion of the surface vanadium oxide phase cannot be arising from the supported vanadium oxide phase obtained using XPS since it cannot discriminate because they absorb the infrared signal (especially between different vanadium oxide structures posses- below 1000 cm1). For some systems it is at least sing the same oxidation state. Furthermore, XPS possible to obtain the highest frequency of the surface requires high vacuum conditions for its operation vanadia species, which usually corresponds to the and cannot be used under in situ conditions, but is vanadium±oxygen terminal bonds [42±44]. Similar especially useful when reduced vanadium oxide states to Raman spectroscopy, infrared spectroscopy can are present. be applied to study the structural changes of the paramagnetic (EPR) is also a surface vanadia species under in situ conditions. bulk characterization technique that is effective in Adsorption of basic molecules (e.g., ammonia and determining the nature of paramagnetic species, such ) provides information about the distribution as vanadium in the 4‡ oxidation state, immersed in of surface Lewis and Bronsted acid sites [45,46]. IR diamagnetic matrices [52]. This is possible since the g also provides direct information about the interaction factor and hyper®ne splitting, arising from an inter- of vanadia species with the surface hydroxyls of oxide action of the unpaired electron with the 51V nucleus, supports. Additional information about the theory and are extremely sensitive to the chemical environment application of infrared spectroscopy to catalytic mate- around the paramagnetic V4‡ . However, EPR rial can be found elsewhere [47]. studies are usually not amenable to in situ studies The 51V nucleus possesses excellent characteristics under reaction environments due to signal broadening for NMR experiments due to its natural abundance at elevated temperatures [53]. An additional limitation (99.78%). These advantageous characteristics have of the EPR technique is that it is ideally applicable resulted in a number of recent studies using solid for isolated V4‡ due to problems associated state 51V nuclear magnetic resonance (NMR) spectro- with signal broadening and spin±spin coupling for scopy for the characterization of supported vanadium paired V4‡ ions. In spite of the above potential oxide catalysts [32,34,35,48]. The limitation of solid complications, EPR is one of the few characterization state 51V NMR is that vanadium in the 4‡ oxidation techniques that can provide molecular structural infor- state broadens the NMR signal, and consequently, the mation about reduced vanadium (IV) oxide species signal due to the V5‡ nucleus can be lost. Information [54±59]. J.P. Dunn et al. / Catalysis Today 51 (1999) 301±318 305

Ultra violet and visible (UV-vis) spectroscopy takes studies have revealed that the surface structures and advantage of the different electronic transitions of oxidation states of the surface vanadia species on metal ions that depend on the symmetry of the envir- various supports (e.g., SiO2,Al2O3,TiO2, ZrO2, onment. The application of UV-vis spectroscopy for CeO2 and MgO) are dynamic and strongly dependent the study of supported vanadium oxide catalysts has on the particular environment (oxidizing and reducing also received some attention [60±62]. However, the atmospheres, moisture and temperature). Unfortu- origin of the speci®c electronic transition is sometimes nately, the dynamic nature of the surface vanadia dif®cult to isolate since the electronic transition species has resulted in some confusion in the pub- depends on various parameters: local symmetry, over- lished literature since many studies have compared all symmetry, condensation degree, polarization effect measurements taken under different experimental and size of the counter cation and dispersion on a conditions. In order to minimize such problems, the support [63]. The broad UV-vis bands can also present paper will only focus on experimental results decrease the sensitivity for certain species. However, obtained under well de®ned conditions with mono- UV-vis spectroscopy can be successfully applied to layer and sub-monolayer loaded vanadia catalysts. supported vanadium oxide catalysts when used in conjunction with other characterization techniques 4.1. Dehydrated conditions (e.g., Raman and NMR). In, summary, the physical characterization techni- Dehydrated conditions are created by heating the ques (e.g., Raman, NMR and IR) are essentially bulk catalyst to elevated temperatures, from 3008Cto techniques that take advantage of the surface nature of 7008C, in a ¯owing oxygen-containing stream for at the supported vanadium oxide phase. Of these tech- least 30 min. Such treatment desorbs adsorbed moist- niques, Raman and solid state 51V NMR spectroscopy ure from the catalyst surface and maintains the surface are best suited for molecular characterization of vana- vanadia species in the fully oxidized V5‡ oxidation dium (V) oxide. X-ray absorption (EXAFS/XANES) state [35]. and IR spectroscopies are good complimentary tech- niques when used in conjunction with Raman and 4.1.1. Acid/base properties of supported vanadia NMR. XPS can be used only to study the oxidation catalysts state and dispersion of the supported vanadium oxide Most support oxides of interest in catalysis (e.g., phase. EPR and X-ray absorption spectroscopy can SiO2,Al2O3,TiO2, ZrO2, CeO2 and MgO) contain a provide structural information about reduced V4‡ variety of surface sites including surface hydroxyl species. UV-vis, however, is not a very informative groups and coordinatively unsaturated Lewis acid/ characterization technique for supported vanadia cat- base sites [64]. The site mixture depends on the nature alysts when applied independently. Characterization of the oxide, but it can also vary according to the techniques most suitable for in situ reaction studies are preparative method used, conditions of activation and Raman, IR, X-ray absorption (EXAFS/XANES), and on the presence of foreign impurities. Upon addition, to a limited extent 51V NMR, EPR and UV-vis. vanadia species titrate the surface hydroxyls and become anchored to the supporting oxide through bridging vanadium±oxygen±support cation bonds. 4. Molecular structures of supported vanadia The surface vanadia species possess both Lewis and catalysts Bronsted acid properties and the ratio of Bronsted acidity to Lewis acidity increases with surface vanadia To fully understand the molecular structure-reac- coverage [65,66]. The basicity of the surface vanadia tivity relationships for the oxidation of sulfur dioxide species is negligible [67±69]. over supported vanadia catalysts, a detailed knowl- edge of the molecular structures of the vanadia surface 4.1.2. V2O5 /SiO2 catalysts species is required. The wide array of spectroscopies There is a unanimous agreement among all mentioned above have been applied to the character- researchers that the dehydrated surface vanadia spe- ization of the supported vanadia catalysts. These cies on silica consist of isolated VO4 units consisting 306 J.P. Dunn et al. / Catalysis Today 51 (1999) 301±318

the two-dimensional surface vanadia overlayer (i.e., the isolated and polymerized surface vanadia species).

4.1.4. V2O5/MgO catalysts Unlike the above supported vanadia catalysts, the magnesia supported vanadia catalyst system cannot form a complete close-packed surface vanadia mono- layer because of the strong acid±base reaction between acidic vanadia and basic magnesia. The interaction between vanadia and magnesia results in the formation of a mixed metal oxide rather than a stable surface vanadia overlayer on the magnesia support [72±74]. The vanadia coordination in bulk V±Mg±O mixed metal oxide catalysts consists of VO ,VO and VO units. Thus, the magnesia sup- Fig. 1. Proposed structure of vanadium oxide species on Al2O3, 4 5 6 TiO2, ZrO2 and CeO2 supports under dehydrated conditions. ported vanadia catalyst system possesses both surface and bulk vanadia species. of one terminal V=O bond and three bridging V±O±Si 4.1.5. Influence of metal oxide additives on bonds (trigonal pyramidal con®guration). The maxi- supported vanadia catalysts mum surface coverage achieved to date on silica Metal oxide additives present in multicomponent without formation of microcrystalline V2O5 particles, vanadia catalysts can be classi®ed into two broad 4 V /nm2, corresponds to that expected for a categories: monolayer of isolated surface VO units [70]. 4 1. non-interacting additives, which preferentially coordinate with the oxide support rather than the 4.1.3. V O /(CeO ,ZrO,TiO and Al O ) catalysts 2 5 2 2 2 2 3 surface vanadia species under dehydrated condi- The dehydrated surface vanadia species on other tions and oxide supports (ceria, zirconia, titania and alumina) 2. interacting additives, which preferentially coordi- possess essentially identical molecular structures nate with the surface vanadia species rather than [37,38,43,44,71±73]. The dehydrated surface vanadia the oxide support under dehydrated conditions. species on these oxide supports are primarily present as both isolated (low coverages) and polymerized Typical non-interacting additives are surface oxides

(high coverages) VO4 units with similar ratios of of Co, Fe, Mo, Nb, Ni, S and W [75±79]. Non- the species at any given vanadium oxide surface interacting additives can only indirectly affect the coverage. As shown in Fig. 1, the molecular structures molecular structure of the vanadia species via lateral of the surface vanadia species are tentatively thought interactions. Such lateral interactions have been found to consist of a terminal V=O and three bridging to increase the ratio of polymerized to isolated surface vanadium±oxygen±support (V±O±M) bonds for the vanadia species in supported metal oxide catalysts: isolated species, and a terminal V=O bond with one or Fe>Ni±Co>S>Mo±W>Nb. two bridging V±O±M and one or two bridging V±O±V Typical interacting additives are oxides of P and Zn bonds for the polymerized species. There may also be and alkali/alkaline earth oxides (Ca, Na, K, etc.) that a trace of surface VO6 units (octahedral coordination) tend to complex with the acidic surface vanadia present at monolayer coverages, but additional studies species [78,79]. The basic alkali/alkaline earth addi- are required to establish this possibility. At coverages tives do not change the trigonal±pyramidal con®gura- exceeding a monolayer (approximately 13 mmol tion of the dehydrated surface vanadia species on most 5‡ 2 2 V /m or 8 V atoms/nm for CeO2, ZrO2,TiO2 supports, but do affect the V±O bond lengths by and Al2O3), microcrystalline V2O5 particles are signi®cantly increasing the V±O and, formed as a separate three-dimensional phase on consequently, decreasing the bridging V±O bond J.P. Dunn et al. / Catalysis Today 51 (1999) 301±318 307 lengths [72,73,78,79]. Thus, the basic surface addi- been extensively investigated in recent years with tives do not form three-dimensional mixed numerous spectroscopic (e.g., IR [83±89], Raman compounds, but just alter the V±O bond lengths of the [90±93] and XPS [92,94±96]) and thermal (e.g., surface vanadia species. However, the interaction TGA [83,86,87,89,91], TPD [83,91,97] and DTA between P and surface vanadia species can result in [67±69]) techniques. MgO, CeO2 and Al2O3 have the formation of crystalline VOPO4 phases, at the received much attention due to their ability to act expense of the surface vanadia phase, after calcination as SOx transfer catalysts [98±101]. Sulfated TiO2 and due to the strong interaction between the two oxides ZrO2 are representatives of a class of metal oxides [78,80,81]. The formation of crystalline VOPO4 known as solid-superacids, which are highly reactive phases can only be avoided if the surface phosphorous for the isomerization of hydrocarbons and dehydration oxide species is ®rst anchored to the oxide support of [102]. In contrast, fewer studies have surface via calcination before the introduction of the focused on adsorption of SO2/SO3 on supported vana- vanadia precursor [80,81]. Thus, interacting additives dia catalysts [67±69,93,103]. Supported vanadia cat- can both modify the local structure of the surface alysts, e.g., V2O5/WO3±MoO3/TiO2, are used vanadia species, affecting the V±O bond lengths, as extensively in the selective catalytic reduction of well as form crystalline mixed metal oxide phases NO with NH3 where they are exposed to moderate because of their chemical af®nity for vanadia. levels of SOx (500 ppm of SOx) [6]. Bronsted acid sites, which promote the adsorption of ammonia dur- 4.2. Water vapor containing environments ing the SCR reaction, are created from the interaction of adsorbed sulfate species and water [104,105]. The in¯uence of water vapor/oxygen environments at elevated temperatures, 120±5008C, upon the dehy- 4.3.1. Unpromoted metal oxide supports drated surface vanadia species has recently been Upon exposure to SO2, in the absence of gas phase investigated due to the high levels of moisture (2± oxygen, either physisorbed sulfur dioxide SO2 † or 10 vol%) present in many air pollution control appli- chemisorbed sul®te SO3 † surface species are formed cations [82]. For the silica supported vanadia catalysts, on metal oxide surfaces, depending on the adsorption moisture has no effect on structure of the surface temperature and the speci®c metal oxide support vanadia species, revealing silica's hydrophobic nature. [83,106]. The mildly acidic sulfur dioxide acts as

For the other supported vanadia catalysts (V2O5 on an electron acceptor and adsorbs on basic surface CeO2, ZrO2,TiO2 and Al2O3), the presence of water sites, but fails to interact with neutral and acidic vapor does have an effect on the surface vanadia surface sites. Since only a fraction of the support species. Above 3008C, the surface vanadia species hydroxyls meet the criteria necessary for SO2 adsorp- retain their structures despite small amounts of tion, maximum surface coverages of SO2 and SO3 adsorbed moisture present on the catalyst surface. species correspond to less than a theoretical mono- Isotopic oxygen-18 exchange experiments reveal that layer (4 S atoms/nm2 for isolated species) [71]. This the reversibly adsorbed moisture is able to rapidly selective titration allows the concentration of chemi- undergo oxygen exchange with the terminal V=O sorbed sul®te species to be used as a measure of the bond of the surface vanadia species at these high relative basicity of the supports: MgO±CeO2 temperatures. At approximately 2008C and below, ZrO2>TiO2±Al2O3SiO2 [70]. monolayer quantities of adsorbed moisture are present Sulfates SO4 † are produced when surface sul®te on the supported vanadia catalysts, extensively solvat- (SO3 ) species titrate adjacent sites and form addi- ing the surface vanadia species (and any other surface tional bonds to the metal oxide support. The transfor- metal oxide species). mation of surface sul®te species into the more thermodynamically stable sulfate species will proceed 4.3. Sulfur oxide containing environments on all supports upon exposure to gas phase oxygen at temperatures above 2008C [83,106]. The creation of a The adsorption and oxidative adsorption of sulfur surface sulfate overlayer may also occur at tempera- dioxide onto the various metal oxide supports have tures above 2008C during the adsorption of sulfur 308 J.P. Dunn et al. / Catalysis Today 51 (1999) 301±318 trioxide, oxidative adsorption of sulfur dioxide and hydrogen sul®de [86,94,95,107].

4.3.1.1. Sulfated SiO2. SiO2 does not contain basic surface hydroxyl functionalities and, therefore, does not form stable surface sulfate overlayers in the presence of sulfur oxides and excess oxygen at elevated temperatures [84]. Fig. 2. Proposed structure of sulfated supported vanadium oxide catalysts under dehydrated conditions (vanadia surface coverage <0.3 monolayers). 4.3.1.2. Sulfated Al2O3,TiO2 and ZrO2. Oxide supports such as Al2O3,TiO2 and ZrO2 contain basic surface hydroxyls and can form stable sulfate overlayers by heating in the presence of either sulfur Two kinds of sulfate species were shown to form on trioxide, sulfur dioxide or and excess MgO depending on the sample surface area and the oxygen. Tridentate SO4 surface species, i.e., (Ti± amount of SO2 introduced. On a low surface area 2 O)3S=O and (Al±O)3S=O, have been observed on sample (10 m /g), bulk-like sulfate was the major sulfated TiO2 and Al2O3 under dehydrated product, whereas, both surface and bulk-like sulfates 2 conditions [85]. In addition, two tridentate SO4 were formed on a high surface area sample (290 m / species and a polymeric S2O7 species were shown g). Oxygen-18 exchange experiments indicated that to exist on sulfated ZrO2 [64]. the surface sulfate has a bidentate con®guration with a Upon exposure to water vapor, at temperatures trigonal O=S=O terminal , i.e., (Mg±O)2(S=O)2 between 2008C and 4008C, the tridentate sulfates [108]. present on alumina, titania and zirconia supports are converted into protonated bidentate surface species 4.3.2. Supported vanadia catalysts (sub-monolayer [85]. This lowers the stability of the surface sulfate vanadia coverages) species and increases the Bronsted acidity of the As shown in Fig. 2, the surface sulfate species support via the production of moderately acidic S± present on sub-monolayer supported vanadia catalysts O±H surface functionalities [104,105]. preferentially coordinate with the oxide supports rather than the surface vanadia species under dehy-

4.3.1.3. Sulfated CeO2 and MgO. On the oxide drated conditions and, consequently, can only indir- supports possessing the most basic surface ectly affect the molecular structures of the supported hydroxyls (CeO2 and MgO), sulfate groups form vanadia species via lateral interactions [93]. In addi- easily upon exposure to sulfur trioxide or either tion, the amount of adsorbed sulfate appears to sulfur dioxide or hydrogen sulfide and excess decrease more rapidly than one may expect from oxygen. Strong interactions between the adsorbed simple geometrical coverage by surface vanadate sulfate groups and the supporting oxide lead to a species [67±69,93]. Similar coverage effects have wide range of surface sulfate and bulk sulfur±oxy± been made for sulfated MoO3/Al2O3 catalysts and support species being formed. Sulfation of the highly can be explained by the observation that while surface basic CeO2 support produces both surface and bulk- sulfate species titrate only the most basic hydroxyls like sulfate species [87,90]. The former is thought to (IR bands at 3785, 3740 and 3705 cm1), surface have a tridentate SO4 structure, i.e., (Ce±O)3S=O, species titrate both basic and neutral hydro- while bulk-like species have been shown to have a xyls (IR bands at 3785, 3740, 3705 and 3590 cm1) more ionic character [87]. In the absence of gas phase forming a complete monolayer [92]. At low MoO3 and oxygen, strong interactions between the CeO2 support V2O5 loadings, the molybdena and vanadia surface and chemisorbed sulfur dioxide promote the oxidation species preferentially titrate the basic hydroxyls, con- of species to both surface and bulk-like sulfates suming the sites capable of sulfur dioxide adsorption. with an accompanying reduction of CeO2 into Ce2O3 The increased Bronsted acidity exhibited by the sul- [87]. fated zirconia, titania and alumina supports in the J.P. Dunn et al. / Catalysis Today 51 (1999) 301±318 309 presence of water vapor is also found for sulfated reduced during the reduction of the surface vanadia supported vanadia catalysts [109±112]. species with the exception of CeO2, which exhibits the presence of minor amounts of reduced Ce2O3 4.3.3. Supported vanadia catalysts (monolayer [119,120]. The average oxidation states of the reduced vanadia coverage) surface vanadia species have been determined by Vanadium oxide±sulfur oxide surface complexes temperature programmed reduction (TPR) and ther- must be formed during the sulfur dioxide oxidation mogravimetric measurements (primarily for alumina reaction. The dif®culties in detecting the sulfation of and silica catalysts) [113,114]. More speci®c informa- surface vanadia species have been demonstrated in tion about the distribution of oxidation states has been several recent studies. Following oxidative adsorption obtained with XPS studies [115±118]. However, there of sulfur dioxide at 1758C on a monolayer V2O5/TiO2 currently appears to be a lack of consensus among (13 mmols V5‡/m2) catalyst the uptake of sulfur these publications as to the distribution of V4‡ and 2 3‡ dioxide was undetectable (<0.05 mmols SO2/m ) V species. In addition, there also does not appear to [103]. Similarly, sulfur dioxide has been employed be any consensus about the structural assignments of 4‡ as a probe for basic sites present on V2O5/Al2O3 the V species in supported vanadia catalysts: dis- catalysts at 808C. As vanadia surface coverage torted VO6, square±pyramidal VO5 or trigonal±pyr- 5‡ 2 4‡ approached a monolayer (13 mmols V /m ), the amidal VO4 V species [54±59]. Thus, dependable volumetric uptake of sulfur dioxide fell below the studies addressing the molecular structures of the 2 limit of detection (<0.09 mmols SO2/m ) of the reduced surface vanadia species are rare at present adsorption chamber used in the study [67±69]. The and a more systematic series of experiments are same study failed to detect the adsorption of sulfur required to determine the in¯uence of various vari- dioxide onto bulk V2O5 and monolayer V2O5/SiO2 ables (speci®c oxide support, speci®c reducing gas, catalysts. steam, temperature, etc.). Unfortunately, no in situ spectroscopic studies attempting to identify vanadium oxide±sulfur oxide reaction intermediates have been performed. How- 5. SO2 oxidation reactivity of surface vanadia ever, it is doubtful that presently available spectro- species scopic techniques could detect the extremely small concentration of surface complexes believed to exist 5.1. Molecular structure±reactivity relationships under reaction conditions, since the quantity of basic sites capable of adsorbing sulfur dioxide on mono- The catalytic properties of several well-de®ned layer supported vanadia catalysts is negligible. In model supported vanadia catalysts have recently been addition, Raman and IR spectroscopy studies of dehy- probed with the oxidation of sulfur dioxide to sulfur drated sulfate promoted monolayer supported vanadia trioxide [17,121]. The reactivity properties of these catalysts fail to detect spectral bands characteristic of catalysts are compared with the corresponding struc- vanadium oxide±sulfur oxide surface complexes [93]. tural information presented above in order to develop The interaction of sulfur trioxide and monolayer molecular structure±reactivity relationships for sup-

V2O5/CeO2 at elevated temperatures has been shown ported vanadia catalysts during the sulfur dioxide to disturb the surface VO4 tetrahedra and results in the oxidation reaction. generation of bulk vanadate and cerium sulfate [93]. 5.1.1. Role of bridging V±O±V bonds Several recent in situ Raman studies have demon- 4.4. Reducing environments strated that the ratio of bridging V±O±V bonds to terminal V=O bonds increases with surface vanadia 5‡ Exposure of the surface V species to reducing H2, coverage on oxide supports, with the exception of H2S and CO environments results in the formation of V2O5/SiO2 which only possesses isolated surface surface V4‡ and V3‡ species [54±59,113±118]. The vanadia species and no bridging V±O±V bonds oxide supports (ZrO2,TiO2,Al2O3 and SiO2) are not [37,38]. Consequently, the role of V±O±V bridging 310 J.P. Dunn et al. / Catalysis Today 51 (1999) 301±318 bonds in oxidation reactions can be chemically probed SO2 oxidation TOFs varied by more than an order of by examining the reaction turnover frequency, TOF ± magnitude: V2O5/CeO2>V2O5/ZrO2,V2O5/TiO2> the number of SO2 molecules oxidized per surface V2O5/Al2O3 [17]. Consequently, the SO2 oxidation vanadia site per second, as a function of surface TOFs do not correlate with the characteristics of the vanadia coverage. The sulfur dioxide oxidation activ- terminal V=O bond and suggests that this bond does ity of V2O5/TiO2 catalysts of variable loading (1±6% not play a critical role in this oxidation reaction. A V2O5, corresponding to 0.16±1.0 theoretical mono- similar lack of correlation between the terminal V=O layers (1.5±8 V atoms/nm2) for a 50 m2/g Degussa bond characteristics and , and metha-

P-25 TiO2 support) was determined between 2008C nol oxidation TOFs was also previously found [122± and 4008C [17]. The magnitude of the turnover fre- 124]. 6 1 quency for 1% V2O5/TiO2 varied from 3Â10 s at 2008Cto1Â104 s1 at 4008C. The results showed 5.1.3. Role of bridging V±O±support bonds that the turnover frequency is approximately constant The sulfur dioxide oxidation reactivity is apparently as the surface vanadia loading is varied up to mono- related to the bridging V±O±M bond since changing layer coverage. Therefore, the reactivity of titania the speci®c oxide support alters the turnover supported vanadia catalysts for the oxidation of sulfur frequency by more than an order of magnitude [17]. dioxide to sulfur trioxide is independent of the surface The only signi®cant differences between the surface of bridging V±O±V bonds, which indicates vanadia species on the various oxide supports are the that these bonds do not play a critical role in the oxide support (e.g., Ce, Zr, Ti, or Al). The overall SO2 oxidation kinetics. In addition, a reaction of the oxide support cation affects requires two or more surface vanadia sites to proceed the electron density on the bridging V±O±M oxygen: a would exhibit higher turnover frequencies at higher lower cation electronegativity will result in a slightly surface coverages due to the increasing number of higher electron density (more basic V±O±M oxygen) polymerized surface vanadia species present at higher and a higher cation electronegativity will result in a surface coverages. Thus, the observation that the SO2 slightly lower electron density (less basic V±O±M oxidation turnover frequency does not increase with oxygen). An inverse correlation between the Sander- increasing surface coverage by surface vanadia spe- son [125] of the oxide support cies indicates that only one surface vanadia site is cations and the sulfur dioxide oxidation turnover necessary for the SO2 oxidation to proceed. In sum- frequencies appears to exist; the lower the oxide mary, the bridging V±O±V bonds do not play a critical support cation electronegativity the higher the sulfur role in SO2 oxidation kinetics and only one surface dioxide oxidation turnover frequency [17]. Therefore, vanadia site appears to be required for this reaction. the more basic the bridging V±O±M bond the higher

Similar trends in TOF versus surface vanadia cov- the activity towards SO2 adsorption and subsequent erages were shown to exist for ceria, zirconia, alumina oxidation of the acidic sulfur dioxide molecule. Con- and silica supported vanadia catalysts. versely, a less basic bridging oxygen depresses the adsorption of sulfur dioxide and its oxidation. Thus, it 5.1.2. Role of the terminal V=O bond appears that the catalysts exhibiting higher turnover The terminal V=O bonds can also be directly frequencies contain a higher percentage of surface monitored with in situ Raman spectroscopy during vanadia sites adsorbing sulfur dioxide and, subse- oxidation reactions. The vibrational frequency of the quently, undergoing cycles under reaction con- terminal V=O is directly related to its bond strength ditions. (stronger or shorter bonds vibrate at higher cm1 and The above analysis may also be applied to other weaker or longer bonds vibrate at lower cm1) catalytic oxidation reactions over supported vanadia [37,38]. Sulfur dioxide oxidation experiments over catalysts. The mechanism and kinetics of several well characterized supported vanadia catalysts oxidation to over vanadia catalysts have

(V2O5/(Al2O3,TiO2, ZrO2 or CeO2)) revealed that been extensively examined in recent years [126]. It has while the catalysts possessed essentially the same been proposed that methanol adsorbs at a bridging terminal V=O bond strength (1025±1031 cm1), the vanadium±oxygen±support (V±O±M) bond via proto- J.P. Dunn et al. / Catalysis Today 51 (1999) 301±318 311 nation of the bridging oxygen (H±O±M) and forma- (surface vanadia, chromia, molybdena, tungsta and tion of a methoxy (V±OCH3) intermediate. The rate niobia). Four-fold coordination is preferred for surface determining step is the subsequent breaking of a oxide, oxide and vanadium oxide methyl C±H bond to form formaldehyde. The trend species, while six-fold coordination is preferred for in turnover frequencies for oxidation of the mildly surface oxide, oxide, oxide acidic methanol molecule (V/Ce>V/Zr, V/Ti>V/ and oxide species. The observation that the

Nb>V/AlV/Si) follows the same pattern as that turnover frequency for SO2 oxidation over all of these observed for sulfur dioxide oxidation and inversely catalysts is approximately the same at both low and correlates with the Sanderson electronegativities of high surface coverages indicates that the mechanism the support cations. of sulfur dioxide oxidation is not sensitive to the coordination of the surface metal oxide species.

5.1.4. Influence of metal oxide additives Furthermore, SO2 oxidation occurs at similar rates The effect of various metal oxide additives on the over both isolated and polymerized surface metal

SO2 oxidation activity of supported vanadia catalysts oxide species, which is expected for a reaction requir- depends on the nature of the additive. The negative ing only one active site. effect of interacting additives, such as GeO2 and ZnO, The redox properties of the titania supported metal on SO2 oxidation TOFs over V2O5/TiO2 catalysts oxide catalysts have also been probed with the partial has been reported in the literature [7] and is most oxidation of methanol to formaldehyde and followed likely due to vanadia±additive compound formation. the trend V2O5±Re2O7>CrO3±MoO3>Nb2O5±WO3 oxide has been shown to substantially [78,128±130]. The yield of selective oxidation pro- retard the SO2 oxidation activity of V2O5/TiO2 cata- ducts (e.g., formaldehyde, methyl formate, dimethoxy lysts through direct interaction of the K2O with the ) for supported niobium oxide and tungsten surface vanadia species and the reduction of its redox oxide catalysts were approximately two orders of potential [127]. magnitude less than for supported vanadium oxide Non-interacting additives provide additional sur- and rhenium oxide. Tungsten oxide was shown to face sites upon which the oxidation of SO2 to SO3 increase the yield of acid products (e.g., dimethyl can proceed and, therefore, increase the catalytic ether). In addition, a very similar trend was observed activity of the promoted catalyst. This effect only for methanol oxidation over niobia supported metal lasts as long as the total surface coverage of (vana- oxide catalysts (V2O5>CrO3>Re2O7>MoO3>WO3) dia‡additive) is less than a monolayer and the redox [131]. These trends indicate that V2O5/TiO2,Re2O7/ activity of the additive is greater than that of the TiO2, CrO3/TiO2, MoO3/TiO2 and to a lesser degree supporting oxide it covers. Nb2O5/TiO2 and WO3/TiO2 possess surface redox Dunn et al. [127] measured the SO2 oxidation sites which can ef®ciently catalyze sulfur dioxide activity of several well characterized binary (e.g., oxidation to sulfur trioxide.

V2O5/TiO2,Fe2O3/TiO2,Re2O7/TiO2, CrO3/TiO2, The reactivity studies of the binary catalysts suggest Nb2O5/TiO2, MoO3/TiO2 and WO3/TiO2) and ternary that all of the surface species present in the ternary (e.g., V2O5/Fe2O3/TiO2,V2O5/Re2O7/TiO2,V2O5/ catalysts (i.e., oxides of V, Fe, Re, Cr, Nb, Mo and W) CrO3/TiO2,V2O5/Nb2O5/TiO2,V2O5/MoO3/TiO2 can undergo redox cycles and oxidize sulfur dioxide to and V2O5/WO3/TiO2) supported metal oxide catalysts sulfur trioxide to some extent. A comparison of the at 4008C. At low surface coverages (<0.2 monolayer) activities of the ternary catalysts with the correspond- the dehydrated surface vanadium, chromium, rhe- ing binary catalysts indicates that the vanadium oxide nium, niobium, molybdenum and tungsten oxide spe- and the additive supported metal oxide surface redox cies supported on titania generally tend to possess sites are acting independently without synergistic four-fold coordination and are predominately isolated. interactions, since the sum of the activities of the Whereas, at high surface coverages (>0.6 monolayer), individual binary catalysts may be added to yield the coordination of the dehydrated surface metal oxide the activity of the corresponding ternary catalyst species depends on the speci®c metal oxide and system. The absence of synergistic interactions is polymerized surface species are also usually present expected for single-site reactions such as sulfur 312 J.P. Dunn et al. / Catalysis Today 51 (1999) 301±318 dioxide oxidation, whereas dual site reactions, e.g., structures of the supported vanadia species via lateral

SCR of NOx, will exhibit an increase in turnover interactions [93]. Surface sulfate species can also frequency as surface coverage increases. undergo redox cycles and promote the oxidation of

The results found by Dunn et al. con®rm the SO2 to SO3 at elevated temperatures. However, for observation of Morikawa et al. [7] that V2O5/TiO2 reaction temperatures below 4008C, the surface sul- catalysts promoted by WO3 or MoO3 exhibit higher fates can be regarded as spectator species since they do rates of sulfur dioxide oxidation than unpromoted not undergo redox cycles at any appreciable rate. In catalysts. However, the TOFs for WO3/TiO2 and addition, surface sulfates may volatilize upon increas- MoO3/TiO2 are signi®cantly lower than the TOF for ing temperature before any tangible enhancement in V2O5/TiO2. oxidation activity is observed [98]. In contrast to the results of Dunn et al., Sazonova et The formation of sulfate overlayers on sub-mono- al. [9] reported that the addition of high loadings of layer supported vanadia catalysts requires that special tungsten oxide to a V2O5/TiO2 catalyst substantially care be taken in measuring sulfur dioxide oxidation suppresses sulfur dioxide oxidation activity. However, kinetics over these catalysts. The measured rate of SO2 Sazonova et al. failed to recognize that for the loadings oxidation over fresh (non-sulfated) catalysts may be of surface vanadia (3 monolayers) and tungsten either over or underestimated, depending on the oxide (6 monolayers) species used in their study, experimental methodology, for the ®rst several hours the surface species are no longer molecularly dis- of reaction time due to the in situ development of the persed and form WO3 and V2O5 crystallites. Since surface sulfate overlayer [121]. no information about the structures or dispersions of the metal oxides was presented, it is not possible to 5.2. Kinetics of SO2 oxidation clearly identify the reason for the decrease in oxida- tion activity, but is most likely due to the presence of 5.2.1. Effect of low SO2 adsorption probability on the metal oxide crystalline phases, which are not very reaction rate active for redox reaction [37,38,128]. The reason for the extremely low sulfur dioxide It has been proposed that electronic interactions oxidation turnover frequencies exhibited by mono- between neighboring surface vanadia and tungsten layer supported vanadia catalysts (3.7Â106 s1 for 4 1 oxide sites on a titania support may lead to an increase V2O5/SiO2±2.2Â10 s for V2O5/CeO2 at 4008C in sulfur dioxide oxidation activities at temperatures [17]) may be either that: below 2308C [131]. However, more recent studies 1. the quantity of vanadium oxide±sulfur oxide probing the redox properties of ternary V O /WO / 2 5 3 surface complexes formed under reaction condi- TiO catalysts at 2008C and 2308C by the single-site 2 tions is very low, however, once formed the sulfur dioxide oxidation and selective oxidation of surface complexes readily decompose to produce methanol to formaldehyde reactions failed to detect sulfur trioxide (low adsorption probability and fast an increase in redox activity for the ternary catalyst rate determining step), with respect to the corresponding binary catalysts 2. large numbers of stable surface complexes are [17]. Therefore, there does not appear to be any formed, which is slowly react to produce sulfur evidence that electronic interactions between surface trioxide (high adsorption probability and slow rate vanadia and surface tungsten oxide species of ternary determining step), or catalysts allow redox reactions to proceed more 3. small numbers of stable surface complexes are ef®ciently. formed (low adsorption probability and slow rate determining step). 5.1.5. Influence of the sulfate overlayer

The surface sulfate species present on sub-mono- As long as the surface coverage of adsorbed SO2 layer supported vanadia catalysts preferentially coor- species is not so high as to be limited by steric factors, dinate with the oxide supports rather than the surface the number of vanadium oxide±sulfur oxide surface vanadia species under dehydrated conditions and, complexes formed at a certain temperature, as deter- consequently, can only indirectly affect the molecular mined by adsorption experiments, can be viewed as J.P. Dunn et al. / Catalysis Today 51 (1999) 301±318 313 the maximum number of surface vanadia sites capable oxygen and the surface vanadia species are of simultaneously undergoing redox cycles at that essentially present in the V5‡ state possessing temperature. The observation that the adsorption dehydrated isolated and polymerized VO4 structures and oxidative adsorption of SO2 onto monolayer (see Section 4.1 above). supported vanadia catalysts at temperatures between 808C and 1508C is negligible rules out option (2) [67± 5.2.2.2. Effect of sulfur oxides. The influence of sulfur 69,103]. Thus, consistent with options (1) and (3) dioxide partial pressure on the rate of oxidation above, it appears that the low SO2 oxidation turnover requires a more complex analysis due to the frequencies of monolayer supported vanadia catalysts presence of the product sulfur trioxide, which may may be due to a lack of surface vanadia sites adsorbing compete with sulfur dioxide for adsorption on the sulfur dioxide and undergoing redox cycles under surface vanadia species. The basicity of the reaction conditions (low adsorption probability). bridging oxygen in the V±O±M bond appears to be Since no data regarding the fundamental rate of responsible for influencing the adsorption of acidic vanadium oxide±sulfur oxide surface complex decom- molecules on the surface vanadia species. The position to products are currently found in the litera- electronic structures of sulfur dioxide and sulfur ture, it is not possible to distinguish between the trioxide molecules in the gas phase indicate the options of fast or slow rate determining step. electron deficiency of the sulfur and, consequently, the acidity of the sulfur in the 5.2.2. Effect of reaction environment resonance hybrid structure for sulfur trioxide (‡2) Qualitative data detailing the effects of complicated is higher than that for sulfur dioxide (‡1). As a result, reaction environments on SO2 oxidation kinetics over sulfur trioxide will experience a greater attraction to industrial-type catalysts can be found in the literature the of the bridging oxygen of the V±O±M [18]. Unfortunately, limited information is presented bond, resulting in a preferential adsorption of sulfur in these studies as to the effects of the various reaction trioxide. This results in a stronger bonding of sulfur gases on the structure of the surface vanadia species. trioxide to the surface vanadia species and, Therefore, in an effort to relate reaction kinetics with consequently, competitive adsorption with sulfur well characterized catalytic systems, the effects of dioxide [17]. only oxygen, water vapor and sulfur oxide partial When product inhibition by sulfur trioxide is not pressures on SO2 oxidation kinetics will be addressed taken into account, the apparent reaction rate depen- in detail since the effects of these gases on the dence on gas phase sulfur dioxide concentration for molecular structures of supported vanadia catalysts vanadia supported catalysts can be crudely ®tted to a are well understood. half-order dependence. However, assuming competi- tive adsorption of sulfur trioxide the rate dependence 5.2.2.1. Effect of oxygen. Several studies using a on sulfur dioxide concentration was shown to be ®rst- reaction gas mixture containing approximately order with a negative ®rst-order dependence on sulfur

1000 ppm of SO2 have confirmed that when oxygen trioxide. Thus, the SO2 oxidation reaction over sup- partial pressures are varied above 1 vol%, the rate of ported vanadia catalysts exhibits a ®rst-order depen-

SO2 oxidation over dehydrated vanadia catalysts is dence on SO2 partial pressures and a negative ®rst- nearly constant (i.e., the reaction rate is independent order dependence on SO3 partial pressures [17]. (zero-order) of the gas phase oxygen partial pressure) [8,17,18]. When the oxygen partial pressure is varied 5.2.2.3. Effect of water vapor. The addition of water between 0.1 and 1 vol% oxygen, the dependence of the to the reaction mixture of SO2,O2 and SO3 results in a rate of oxidation on the gas phase oxygen partial significant decrease in SO2 conversion over supported pressure was seen to be approximately half-order vanadia catalysts [18]. For temperatures above 3008C, [17]. Thus, in the case of industrial conditions the inhibiting effect of water results in an approximate traditionally experienced by supported vanadia 50% decrease in reaction rate. The rate of reaction was catalysts (e.g., 2±6 vol% oxygen), the catalyst shown to be practically independent of the surface is essentially saturated with adsorbed concentration of water vapor in the range of 314 J.P. Dunn et al. / Catalysis Today 51 (1999) 301±318 practical interest in many air pollution control negative first-order in SO3: applications (e.g., 5±15 vol%). If the flow of water k‰SO Š‰O Š0 is stopped the original activity measured in the r ˆ 2 2 (1) SO2 absence of water vapor is restored within ‰SO3Š approximately 1 h and then remains constant. The The apparent activation energy of 21Æ2 Kcal/mol inhibition by water is likely explained in terms of was approximately constant as surface vanadia reversible site blockage of the surface vanadia active coverage on a titania supported catalysts was varied sites for sulfur dioxide adsorption/oxidation. At between 0.16 and 1.0 theoretical monolayers (1.5±8 V reaction temperatures between 2008C and 3008C atoms/nm2) [17]. Essentially the same activation sub-monolayer quantities of adsorbed water form on energy was found for SO2 oxidation over ceria, the catalytic surface and are expected to drastically zirconia, alumina and silica supported vanadia decrease the oxidation rate [82]. The oxidation activity catalysts. will not be restored until the moisture is desorbed at temperatures above 3008C. 5.3. Mechanism of SO2 oxidation

5.2.2.4. Effect of other gases (NOx and NH3). A Numerous mechanisms for the oxidation of SO2 comprehensive study on the effects of gases over supported V2O5/TiO2 catalysts have been pro- typically found in SCR DeNOx applications on the posed [8,17,18]. Forzatti and coworkers [18] proposed SO2 oxidation activity of an industrial V2O5/WO3/ that the active site for the oxidation of sulfur dioxide TiO2 catalyst was conducted by Forzatti and co- over V2O5/TiO2 catalysts involved a dimeric vanadia workers [18]. The addition of low concentrations of sulfate species, which is in disagreement with the ammonia (100 ppm NH3) to a reaction stream ®ndings of Wachs and coworkers [17] that SO2 oxida- containing 1000 ppm of SO2 and 2% O2 was found tion requires only a single vanadia surface site. The to strongly inhibit SO2 oxidation at 3308C and resulted inability of the currently available spectroscopic tech- in a 90% reduction in SO2 oxidation activity. When the niques to elucidate the molecular structure of the flow of ammonia was stopped the SO2 oxidation vanadium oxide±sulfur oxide surface complexes, activity returned to about 90% of its original value which must be formed under reaction conditions, within 3 h. The inhibition by ammonia is likely to prevents the validation of any one of these mechan- explained in terms of reversible site blockage of the isms. Furthermore, the failure of thermal techniques surface vanadia sites responsible for sulfur dioxide (e.g., TGA and TPD) to provide information regarding adsorption/oxidation. Nitric oxide was shown to have the rates of adsorption and desorption of SO2 and SO3 a much smaller effect and neither significantly on the active vanadia sites prevents a clear determina- increased nor decreased the catalyst's SO2 oxidation tion of the rate determining step of the SO2 oxidation activity. This indicates that under the prescribed reaction over these catalysts. It has been proposed that experimental conditions nitric oxide, unlike water in situ spectroscopic studies of the SO2 oxidation and ammonia, does not block the active vanadia reaction over monolayer supported vanadia catalysts sites. The simultaneous addition of NH3 and NO to using isotope-exchanged reactants (e.g., 18 18 18 the reaction stream yielded a net-inhibiting effect for O2; S O2; S O3, etc.) may be able to clarify some NH3:NO ratios greater than 0.4. of the issues that must be addressed before a sound proposal for a mechanism can be made [121]. 5.2.2.5. Effect of reaction temperature. The However, based on the currently available kinetic temperature dependence of the oxidation of SO2 to information it is possible to theorize a general reaction SO3 over a series of supported vanadia catalysts was pathway for SO2 oxidation over supported vanadia investigated using a reactant mixture of 1000 ppm catalysts [17]. Sulfur dioxide may adsorb and coordi-

SO2, 18% O2 and balance He. In view of the results nate onto the vanadium±oxygen±support (V±O±M) previously discussed, the data were analyzed by bond of either isolated or polymerized surface (M± ‡5 ‡5 assuming a differential plug-flow reactor model and O)3V =O sites resulting in either the (V )ÁSO2-ads ‡3 a rate equation first-order in SO2, zero-order in O2 and or the (V )ÁSO3-ads state. This is followed by the J.P. Dunn et al. / Catalysis Today 51 (1999) 301±318 315

‡5 cleavage of the V ±O±SO2 or M±O±SO2 bond and A systematic catalytic investigation of the SO2 formation of SO3(g), which represents the rate deter- oxidation reactivity of several binary (MxOy/TiO2) mining step. The electron de®ciency and, conse- and ternary (V2O5/MxOy/TiO2) supported metal oxide quently, the acidity of the sulfur in the resonance catalysts revealed that the SO2 oxidation turnover hybrid structure for SO3 (i.e., ‡2) is higher than that frequencies of the binary catalysts were all within for SO2 (i.e., ‡1). As a result, sulfur trioxide will an order of magnitude (V2O5/TiO2>Fe2O3/TiO2> experience a greater attraction to the electrons of the Re2O7/TiO2±CrO3/TiO2±Nb2O5/TiO2>MoO3/TiO2± bridging oxygen of the V±O±M bond, resulting in a WO3/TiO2). The observation that the turnover fre- preferential adsorption of SO3. This results in a stron- quency for SO2 oxidation over all of these catalysts ger bonding of SO3 to the surface vanadia species and, is approximately the same at both low and high surface consequently, competitive adsorption on (V‡5) sites. coverages indicates that the mechanism of sulfur The reduced vanadia site is then reoxidized by dis- dioxide oxidation is not sensitive to the coordination sociatively adsorbed oxygen, thereby regenerating the of the surface metal oxide species. Furthermore, SO2 active (V‡5) sites. oxidation occurs at similar rates over both isolated and polymerized surface metal oxide species, which is expected for a reaction requiring only one active 6. Conclusions surface site. A comparison of the activities of the ternary cata- Comparison of the surface vanadia molecular struc- lysts with the corresponding binary catalysts suggests tural information with the corresponding SO2 oxida- that the surface vanadium oxide and the additive tion reactivity data has provided some insights into the surface oxide redox sites act independently without reactivity properties of the surface vanadia species for synergistic interactions, since the sum of the indivi-

SO2 oxidation to SO3. At low vanadia loadings, sur- dual activities of the binary catalysts quantitatively face vanadia preferentially exists on aluminum, tita- correspond to the activity of the corresponding ternary nium, and cerium oxide support surfaces as catalyst. These observations are consistent with struc- ‡5 isolated (M±O)3V =O species. At higher vanadia tural characterization studies, which showed very ‡5 loadings, the isolated (M±O)3V =O species also limited interactions among the surface metal oxide polymerize on the oxide support surface breaking species and the one site requirement of the SO2 V±O±M bonds and forming V±O±V bridging bonds. oxidation reaction. The bridging V±O±V and terminal V=O bonds do not The rate of sulfur dioxide oxidation over supported appear to critically in¯uence the reactivity properties vanadia catalysts is zero-order in oxygen, ®rst-order in of the surface vanadia species during SO2 oxidation. sulfur dioxide and inhibited by sulfur trioxide under The bridging V±O±support bond, however, appears to typical industrial conditions. The apparent activation be the most critical bond since its properties can energy for sulfur dioxide oxidation over these sup- change the SO2 oxidation turnover frequency by more ported vanadia catalysts was approximately 21 Kcal/ than an order of magnitude. The basicity of the brid- mol. The SO2 oxidation reaction is also inhibited by ging V±O±M oxygen appears to be responsible for gaseous molecules that readily adsorb on the surface in¯uencing the adsorption and subsequent oxidation vanadia sites (e.g., H2O, NH3, etc.). of the acidic SO2 molecule. The turnover frequency for SO2 oxidation is very Acknowledgements low, 104±105 s1 at 4008C, due to the inability of the acidic surface vanadia species to ef®ciently adsorb Financial support of National Science Foundation

SO2 under reaction conditions and the inhibition of Grant No. CTS-9626893 is gratefully acknowledged. product SO3. The observation that the SO2 oxidation turnover frequency does not increase with increasing References surface coverage by vanadia species suggests that only one surface vanadia site is necessary for the SO2 [1] C.D. Cooper, F.C. Alley, Air Pollution Control: A Design oxidation to proceed. Approach, Waveland Press, Prospect Heights, IL, 1994. 316 J.P. Dunn et al. / Catalysis Today 51 (1999) 301±318

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