Progress on the mechanistic understanding of SO2 oxidation catalysts

For almost a century vanadium oxide based catalysts have been the dominant materials in industrial processes for sulfuric acid production. A vast body of information leading to fundamental knowledge on the catalytic process was obtained by Academician [G.K. Boreskov, Catalysis in Sulphuric Acid Production, Goskhimizdat (in Russian), Moscow, 1954, p. 348]. In recent years these catalysts have also been used to clean flue gases and other SO⁻₂ containing industrial off-gases. In spite of the importance and long utilization of these industrial processes, the catalytic active species and the reaction mechanism have been virtually unknown until recent years.

It is now recognized that the working catalyst is well described by the molten salt/gas system M₂S₂O₇–MHSO₄–V₂O₅/SO₂–O₂–SO₃–H₂O–CO₂–N₂ (M=Na, K, Cs) at 400–600°C and that vanadium complexes play a key role in the catalytic reaction mechanism.

A multiinstrumental investigation that combine the efforts of four groups from four different countries has been carried out on the model system as well as on working industrial catalysts. Detailed information has been obtained on the complex and on the redox chemistry of vanadium. Based on this, a deeper understanding of the reaction mechanism has been achieved.     

An infrared study of the behavior of SO2 and NOx over carbon and carbon-supported catalysts

The heterogeneous reactivity of NO_x and SO₂ with carbon has been investigated with FT-IR spectroscopy. The interaction between NO and SO₂ on carbon surface have been studied in the presence and in the absence of oxygen. Thermal stabilities of surface structures, formed as a result of NO_x and SO₂ chemisorption have been determined by means of FT-IR spectroscopy. During the reaction of NO/O₂ mixture with carbon the surface species, including C–NO₂, C–ONO, C–NCO and anhydride structures are formed. It has been found that SO₂ retards the oxidation reaction of carbon by oxygen. The oxidation of SO₂ on carbon was found to be greatly enhanced by the presence of NO + O₂ mixture. The adsorption capacity of cellulose based carbon, catalytic NO_x decomposition and TPD was studied using a fixed bed flow reactor.     

EPR spectroscopic characterization of DeNOx and SO2 oxidation catalysts and model systems

The industrial SO₂ oxidation catalyst VK69 deactivates at around 440°C in a 10% SO₂, 11% O₂, 79% N₂ gas mixture. In situ EPR measurements show that the deactivation is caused by the precipitation of V(IV) compounds. DeNO_x catalysts based on V₂O₅/TiO₂, the TiO₂ support, analytical grade anatase and transition metal-exchanged Al-PILCs (pillared clay) have been characterized by EPR spectroscopy and the catalytic activity of the catalysts monitored up to 500°C. Depending on the exchanged metal ion, a relatively large temperature range for the catalytic activity towards the SCR reaction was observed.     

NOx storage and reduction characteristics of Pd/MnOx–CeO2 at low temperature

The reaction between hydrogen and NO was studied over 1 wt.% Pd supported on NO_x-sorbing material, MnO_x–CeO₂, at low temperatures. The result of pulse mode reactions suggest that NO_x adsorbed as nitrate and/or nitrite on MnO_x–CeO₂ was reduced by hydrogen, which was spilt-over from Pd catalyst. The NO_x storage and reduction (NSR) cycles were carried out over Pd/MnO_x–CeO₂ in a conventional flow reactor at 150 °C. In a storage step, NO was removed by the oxidative adsorption from a stream of 0.04–0.08% NO, 5–10% O₂, and He balance. This was followed by a reducing step, where a stream of 1% H₂/He was supplied to ensure the conversion of nitrate/nitrite to N₂ and thus restore the adsorbability. It was revealed that the NSR cycle is much more suitable for the H₂–deNO_x process in excess O₂, compared to a conventional steady state reaction mode.     

NO reduction by CH4 in the presence of excess O2 over Mn/sulfated zirconia catalysts

Selective catalytic reduction (SCR) of NO with methane in the presence of excess oxygen has been investigated over a series of Mn-loaded sulfated zirconia (SZ) catalysts. It was found that the Mn/SZ with a metal loading of 2–3 wt.% exhibited high activity for the NO reduction, and the maximum NO conversion over the Mn/SZ catalyst was higher than that over Mn/HZSM-5. NH₃–TPD results of the catalysts showed that the sulfation process of the supports resulted in the generation of strong acid sites, which is essential for the SCR of NO with methane. On the other hand, the N₂ adsorption and the H₂–TPR of the catalysts demonstrated that the presence of the SO₄²⁻ species promoted the dispersion of the metal species and made the Mn species less reducible. Such an increased dispersion of metal species suppressed the combustion reaction of CH₄ by O₂ and increased the selectivity towards NO. The Mn/SZ catalysts prepared by different methods exhibited similar activities in the SCR of NO with methane, indicating the importance of SO₄²⁻. The most attractive feature of the Mn/SZ catalysts was that they were more tolerant to water and SO₂ poisoning than Mn/HZSM-5 catalysts and exhibited higher reversibility after removal of SO₂.     

Simultaneous SO2/NOx removal by a powder-particle fluidized bed

Simultaneous dry removal of SO₂ and NO_x from flue gas has been investigated using a powder-particle fluidized bed. In a process of flue gas desulfurization by use of solid sorbents such as FeO (dust from a steel plant) and CuO, the smaller the particle size of sorbents, the higher the expected SO₂ conversion. In a powder-particle fluidized bed (PPFB), fine particles less than 40 μm in diameter fed into the bed are fluidized with coarse particles. But only the fine particles are entrained from the bed, and their residence time in the bed is remarkably long.

The reduction of NO_x with NH₃ in the fluidized bed is catalyzed by coarse particles or both coarse and fine particles. In this study, PPFB was applied to simultaneous dry SO₂/NO_x removal process, and several kinds of sorbents or catalysts were evaluated in a PPFB. Using the selected sorbents and catalysts, kinetic measurements were made in the temperature range of 300 to 600°C. SO₂ removal efficiencies were affected by reaction temperature, sorbent/S ratio, and static bed height. NO_x removal efficiencies in excess of 95% were achieved at NH₃/NO_x mole ratio of 1.0. When FeO was used as sorbent, SO₂ conversion increased with increasing temperature and reached 80% at 600°C.     

Sulfuric acid formation over ammonium sulfate loaded V2O5–WO3/TiO2 catalysts by DeNOx reaction with NOx

Operating the SCR DeNO_x reactor at temperatures below 200 °C results in a considerable saving in operating costs. Plant experience shows that on the catalysts in these second generation DeNO_x plants, even for flue gases with SO₂ concentration below 10 mg/m³, over 1–2 years operating time sizeable quantities of ammonium sulfates accumulate. Ammonium sulfates deposited on V₂O₅–WO₃/TiO₂ catalysts react with NO_x to nitrogen and sulfuric acid. Second-order rate constants of this reaction for temperatures of 170 °C have been derived. It could be shown that the sulfuric acid formed on the catalyst is displaced by water vapour and desorbs resulting in gas phase concentrations of up to 6.5 mg acid/m³ flue gas. Plant equipment downstream of the ammonium sulfate containing low temperature DeNO_x catalysts has to be protected against the corrosive action of the sulfuric acid in the flue gases leaving the DeNO_x reactor.     

A study on the characteristics of the SO2 reduction using coal gas over SnO2-ZrO2 catalysts

SO₂, which is an air pollutant causing acid rain and smog, can be converted into elemental sulfur in direct sulfur recovery process (DSRP). SO₂ reduction was performed over catalyst in DSRP. In this study, SnO₂-ZrO₂ catalysts were prepared by a co-precipitation method, and CO and coal gas, which contains H₂, CO, CO₂ and H₂O, were used as reductants. The reactivity profile of the SO₂ reduction over the catalysts was investigated at the various reaction conditions as follows: reaction temperature of 300–550 °C, space velocity of 5000–30,000 cm³/g_-cat. h, [reductant]/[SO₂] molar ratio of 1.0–4.0 and Sn/Zr molar ratio of SnO₂-ZrO₂ catalysts 0/1, 2/8, 3/5, 5/5, 2/1, 3/1, 4/1 and 1/0. SnO₂-ZrO₂ (Sn/Zr = 2/1) catalyst showed the best performance for the SO₂ reduction in DSRP on the basis of our experimental results. The optimized reaction temperature and space velocity were 325 °C and 10,000 cm³/g_-cat. h, respectively. The optimal molar ratio of [reductant]/[SO₂] varied with the reductants, that is, 2.0 for CO and 2.5 for coal gas. SO₂ conversion of 98% and sulfur yield of 78% were achieved with the coal gas.     

Reactivity of NO/NO2–NH3 SCR system for diesel exhaust aftertreatment: Identification of the reaction network as a function of temperature and NO2 feed content

We present a systematic study of the NH₃-SCR reactivity over a commercial V₂O₅–WO₃/TiO₂ catalyst in a wide range of temperatures and NO/NO₂ feed ratios, which cover (and exceed) those of interest for industrial applications to the aftertreatment of exhaust gases from diesel vehicles. The experiments confirm that the best deNO_x efficiency is achieved with a 1/1 NO/NO₂ feed ratio. The main reactions prevailing at the different operating conditions have been identified, and an overall reaction scheme is herein proposed.

Particular attention has been paid to the role of ammonium nitrate, which forms rapidly at low temperatures and with excess NO₂, determining a lower N₂ selectivity of the deNO_x process. Data are presented which show that the chemistry of the NO/NO₂–NH₃ reacting system can be fully interpreted according to a mechanism which involves: (i) dimerization/disproportion of NO₂ and reaction with NH₃ and water to give ammonium nitrite and ammonium nitrate; (ii) reduction of ammonium nitrate by NO to ammonium nitrite; (iii) decomposition of ammonium nitrite to nitrogen. Such a scheme explains the peculiar deNO_x reactivity at low temperature in the presence of NO₂, the optimal stoichiometry (NO/NO₂ = 1/1), and the observed selectivities to all the major N-containing products (N₂, NH₄NO₃, HNO₃, N₂O). It also provides the basis for the development of a mechanistic kinetic model of the NO/NO₂–NH₃ SCR reacting system.     

Activity and stability of Ag–alumina for the selective catalytic reduction of NOx with methane in high-content SO2 gas streams

In this work, we investigated the activity and stability of Ag–alumina catalysts for the SCR of NO with methane in gas streams with a high concentration of SO₂, typical of coal-fired power plant flue gases. Ag–alumina catalysts were prepared by coprecipitation–gelation, and dilute nitric-acid solutions were used to remove weakly bound silver species from the surface of the as prepared catalysts after calcination. SO₂ has a severe inhibitory effect, essentially quenching the CH₄-SCR reaction on this type catalysts at temperatures <600 °C. SO₂ adsorbs strongly on the surface forming aluminum and silver sulfates that are not active for CH₄-SCR of NO_x. Above 600 °C, however, the reaction takes place without catalyst deactivation even in the presence of 1000 ppm SO₂. The reaction light-off coincides with the onset of silver sulfate decomposition, indicating the critical role of silver in the reaction mechanism. SO₂ is reversibly adsorbed on silver above 600 °C. While alumina sites remain sulfated, this does not hinder the reaction. Sulfation of alumina only decreases the extent of adsoption of NO_x, but adsorption of NO_x is not the limiting step. Methane activation is the limiting step, hence the presence of sulfur-free Ag–O–Al species is a requirement for the reaction. Strong adsorption of SO₂ on Ag–alumina decreases the rates of the reaction, and increases the activation energies of both the reduction of NO to N₂ and the oxidation of CH₄, the latter more than the former. Our results indicate partial contribution of gas phase reactions to the formation of N₂ above 600 °C. H₂O does not inhibit the reaction at 625 °C, and the effect of co-addition of H₂O and SO₂ is totally reversible.     

The combustion of carbon particulates using NO/O2 mixtures: The influence of SO4 and NOx trapping materials

The activity of several catalysts are studied in the soot combustion reaction using air and NO/air as oxidising agents. Over Al₂O₃-supported catalysts NO_(g) is a promoter for the combustion reaction with the extent of promotion depending on the Na loading. Over these catalysts SO₄²⁻ poisons this promotion by preventing NO oxidation through a site blocking mechanism. SiO₂ is unable to adsorb NO or catalyse its oxidation and over SiO₂-supported Na catalysts NO_(g) inhibits the combustion reaction. This is ascribed to a competition between NO and O₂. Over Fe-ZSM-5 catalysts the presence of a NO_x trapping component does not increase the combustion of soot in the presence of NO_(g) and it is proposed that this previously reported effect is only seen under continuous NO_x trap operation as NO₂ is periodically released during regeneration and thus available for soot combustion. Experiments during which the [NO]_(g) is varied show that CO, rather than an adsorbed carbonyl-like intermediate, is formed upon reaction between NO₂ (the proposed oxygen carrier) and soot.     

Co3O4 based catalysts for NO oxidation and NOx reduction in fast SCR process

Reaction activities of several developed catalysts for NO oxidation and NO_x (NO + NO₂) reduction have been determined in a fixed bed differential reactor. Among all the catalysts tested, Co₃O₄ based catalysts are the most active ones for both NO oxidation and NO_x reduction reactions even at high space velocity (SV) and low temperature in the fast selective catalytic reduction (SCR) process. Over Co₃O₄ catalyst, the effects of calcination temperatures, SO₂ concentration, optimum SV for 50% conversion of NO to NO₂ were determined. Also, Co₃O₄ based catalysts (Co₃O₄-WO₃) exhibit significantly higher conversion than all the developed DeNO_x catalysts (supported/unsupported) having maximum conversion of NO_x even at lower temperature and higher SV since the mixed oxide Co-W nanocomposite is formed. In case of the fast SCR, N₂O formation over Co₃O₄-WO₃ catalyst is far less than that over the other catalysts but the standard SCR produces high concentration of N₂O over all the catalysts. The effect of SO₂ concentration on NO_x reduction is found to be almost negligible may be due to the presence of WO₃ that resists SO₂ oxidation.     

Improvement of catalytic activity and sulfur-resistance of Ag/TiO2–Al2O3 for NO reduction with propene under lean burn conditions

Ag-based catalysts supported on various metal oxides, Al₂O₃, TiO₂, and TiO₂–Al₂O₃, were prepared by the sol–gel method. The effect of SO₂ on catalytic activity was investigated for NO reduction with propene under lean burn condition. The results showed the catalytic activities were greatly enhanced on Ag/TiO₂–Al₂O₃ in comparison to Ag/Al₂O₃ and Ag/TiO₂, especially in the low temperature region. Application of different characterization techniques revealed that the activity enhancement was correlated with the properties of the support material. Silver was highly dispersed over the amorphous system of TiO₂–Al₂O₃. NO₃⁻ rather than NO₂⁻ or NOx reacted with the carboxylate species to form CN or NCO. NO₂ was the predominant desorption species in the temperature programmed desorption (TPD) of NO on Ag/TiO₂–Al₂O₃. More amount of formate (HCOO⁻) and CN were generated on the Ag/TiO₂–Al₂O₃ catalyst than the Ag/Al₂O₃ catalyst, due to an increased number of Lewis acid sites. Sulfate species, resulted from SO₂ oxidation, played dual roles on catalytic activity. On aged samples, the slow decomposition of accumulated sulfate species on catalyst surface led to poor NO conversion due to the blockage of these species on active sites. On the other hand, catalytic activity was greatly enhanced in the low temperature region because of the enhanced intensity of Lewis acid site caused by the adsorbed sulfate species. The rate of sulfate accumulation on the Ag/TiO₂–Al₂O₃ system was relatively slow. As a consequence, the system showed superior capability for selective adsorption of NO and SO₂ toleration to the Ag/Al₂O₃ catalyst.     

Reduction of SO2 by CO to elemental sulfur over Co3O4–TiO2 catalysts

Mixed oxides of Co₃O₄–TiO₂ have shown the highest catalytic activity for the reduction of SO₂ by CO among catalysts that have been developed so far. Almost zero conversion was observed with cobalt alone, whereas a high conversion was obtained with TiO₂ especially at high temperatures. There existed a strong synergistic promotional effect in the conversion of SO₂ when cobalt was mixed with TiO₂. The synergistic effect observed with mixed oxides is caused by simultaneous contributions from two different reaction routes via COS intermediate mechanism and modified redox mechanism. The synergistic effect that is caused by the COS mechanism has a smaller amount of contribution in the conversion increase and remains almost constant with an increase in the reaction temperature. A larger portion of the synergistic effect is contributed from the modified redox mechanism especially at low temperatures, but the effect disappears at temperatures above 450°C. It is found that the introduction of cobalt into TiO₂ produces COS by the reaction between sulfided CoS₂ and CO even at low temperatures. The COS intermediate can react with SO₂ to produce an additional sulfur via the COS intermediate mechanism, and also behaves as a strong reductant to keep oxygen vacancies on the TiO₂ in a high concentration for the production of sulfur via modified redox mechanism.     

TiO2-SiO2 and V2O5/TiO2-SiO2 catalyst: Physico-chemical characteristics and catalytic behavior in selective catalytic reduction of NO by NH3

TiO₂-SiO₂ with various compositions prepared by the coprecipitation method and vanadia loaded on TiO₂-SiO₂ were investigated with respect to their physico-chemical characteristics and catalytic behavior in SCR of NO by NH₃ and in the undesired oxidation of SO₂ to SO₃, using BET, XRD, XPS, NH₃-TPD, acidity measurement by the titration method and activity test. TiO₂-SiO₂, compared with pure TiO₂, exhibits a remarkably stronger acidity, a higher BET surface area, a lower crystallinity of anatase titania and results in allowing a good thermal stability and a higher vanadia dispersion on the support up to high loadings of 15 wt% V₂O₅. The SCR activity and N₂ selectivity are found to be more excellent over vanadia loaded on TiO₂-SiO₂ with 10–20 mol% of SiO₂ than over that on pure TiO₂, and this is considered to be associated with highly dispersed vanadia on the supports and large amounts of NH₃ adsorbed on the catalysts. With increasing SiO₂ content, the remarkable activity decrease in the oxidation of SO₂ to SO₃, favorable for industrial SCR catalysts, was also observed, strongly depending on the existence of vanadium species of the oxidation state close to V⁴⁺ on TiO₂-SiO₂, while V⁵⁺ exists on TiO₂, according to XPS. It is concluded that vanadia loaded on Ti-rich TiO₂-SiO₂ with low SiO₂ content is suitable as SCR catalysts for sulfur-containing exhaust gases due to showing not only the excellent de-NO_x activity but also the low SO₂ oxidation performance.     

Preparation and characterization of LaFe1−xMgxO3/Al2O3/FeCrAl: Catalytic properties in methane combustion

MnO_x–CeO₂ mixed oxides prepared by sol–gel method, coprecipitation method and modified coprecipitation method were investigated for the complete oxidation of formaldehyde. Structure analysis by H₂-TPR and XPS revealed that there were more Mn⁴⁺ species and richer lattice oxygen on the surface of the catalyst prepared by the modified coprecipitation method than those of the catalysts prepared by sol–gel and coprecipitation methods, resulting in much higher catalytic activity toward complete oxidation of formaldehyde. The effect of calcination temperature on the structural features and catalytic behavior of the MnO_x–CeO₂ mixed oxides prepared by the modified coprecipitation was further examined, and the catalyst calcined at 773 K showed 100% formaldehyde conversion at a temperature as low as 373 K. For the samples calcined below 773 K, no any diffraction peak corresponding to manganese oxides could be detected by XRD measurement due to the formation of MnO_x–CeO₂ solid solution. While the diffraction peaks corresponding to MnO₂ phase in the samples calcined above 773 K were clearly observed, indicating the occurrence of phase segregation between MnO₂ and CeO₂. Accordingly, it was supposed that the strong interaction between MnO_x and CeO₂, which depends on the preparation route and the calcination temperature, played a crucial role in determining the catalytic activity toward the complete oxidation of formaldehyde.     

SO2 promoted oxidation of ethyl acetate, ethanol and propane

The effect of SO₂ addition on the oxidation of ethyl acetate, ethanol, propane and propene, over Pt/γ-Al₂O₃ and Pt/SiO₂ has been investigated. The reactants (300–800 vol. ppm) were mixed with air and led through the catalyst bed. The conversions below and above light-off were recorded both in the absence and in the presence of 1–100 vol. ppm SO₂. For the alumina-supported catalyst, the conversion of ethyl acetate, ethanol and propane was promoted by the addition of SO₂, while the conversion of propene was inhibited. The effect of SO₂ was reversible, i.e. the conversion of the reactants returned towards the initial values when SO₂ was turned off. However, this recovery was quite slow. The oxidation of propane was inhibited by water, both in absence and presence of SO₂. For the silica-supported catalyst no significant effect of SO₂ could be observed on the conversion of ethyl acetate, ethanol or propane, whereas the conversion of propene was inhibited by the presence of SO₂. In situ FTIR measurements revealed the presence of surface sulphates on the Pt/γ-Al₂O₃ catalyst with and after SO₂ addition. It is proposed that these sulphate groups enhance the oxidation of propane, ethyl acetate and ethanol by creating additional reaction pathways to Pt on the surface of the Pt/γ-Al₂O₃ catalyst.     

Characterization and performance of Pt-USY in the SCR of NOx with hydrocarbons under lean-burn conditions

Pt-USY was used for the selective catalytic reduction of NO_x with hydrocarbons in the presence of excess oxygen. The catalyst was prepared by an ion-exchange method and characterized by XRD, TEM, CO chemisorption, and Ar adsorption at 87 K. The platinum particle size distribution was found to be broad (2–20 nm), with no apparent sintering of the active phase during the HC-SCR process after 25 h time-on-stream. Generally, large metal clusters (>15 nm) are situated at the external surface of the zeolite, while the smaller ones are located in the pores of the support. Pt-USY shows an excellent activity in the deNO_x reaction (molar NO_x conversion 90% at 475 K) with propene as the reductant in 5 kPa O₂, as well as stable operation during time-on-stream. Propane only yields a low NO_x conversion compared to propene. The presence of high oxygen contents (5–10 kPa O₂) slightly inhibits the reaction. No significant decrease in deNO_x activity was observed at high space velocities (up to 100,000 h⁻¹). The presence of SO₂ and H₂O in the feed stream did not significantly affect the deNO_x activity. Pt-USY performs better under lean-burn conditions than other Pt-catalysts supported on e.g. ZSM-5, Al₂O₃, or SiO₂. The selectivity to N₂ was similar to the other Pt-based catalysts (30%), the other major product being N₂O.     

SO2 resistant antimony promoted V2O5/TiO2 catalyst for NH3-SCR of NOx at low temperatures

To get the low temperature sulfur resistant V₂O₅/TiO₂ catalysts quantum chemical calculation study was carried out. After selecting suitable promoters (Se, Sb, Cu, S, B, Bi, Pb and P), respective metal promoted V₂O₅/TiO₂ catalysts were prepared by impregnation method and characterized by X-ray diffraction (XRD) and Brunner Emmett Teller surface area (BET-SA). Se, Sb, Cu, S promoted V₂O₅/TiO₂ catalysts showed high catalytic activity for NH₃ selective catalytic reduction (NH₃-SCR) of NO_x carried at temperatures between 150 and 400 °C. The conversion efficiency followed in the order of Se > Sb > S > V₂O₅/TiO₂ > Cu but Se was excluded because of its high vapor pressure. An optimal 2 wt% ‘Sb’ loading was found over V₂O₅/TiO₂ for maximum NO_x conversion, which also showed high resistance to SO₂ in presence of water when compared to other metal promoters. In situ electrical conductivity measurement was carried out for Sb(2%)/V₂O₅/TiO₂ and compared with commercial W(10%)V₂O₅/TiO₂ catalyst. High electrical conductivity difference (ΔG) for Sb(2%)/V₂O₅/TiO₂ catalyst with temperature was observed. SO₂ deactivation experiments were carried out for Sb(2%)/V₂O₅/TiO₂ and W(10%)/V₂O₅/TiO₂ at a temperature of 230 °C for 90 h, resulted Sb(2%)/V₂O₅/TiO₂ was efficient catalyst. BET-SA, X-ray photoelectron spectroscopy (XPS) and carbon, hydrogen, nitrogen and sulfur (CHNS) elemental analysis of spent catalysts well proved the presence of high ammonium sulfate salts over W(10%)/V₂O₅/TiO₂ than Sb(2%)/V₂O₅/TiO₂ catalyst.     

The effects of SO2 on the oxidation of CO and propane on supported Pt and Au catalysts

The catalytic activities of Pt and Au supported on TiO₂ were compared with respect to the oxidation of CO and propane. While the Au catalysts showed higher activities for CO oxidation, the Pt catalysts were more active for propane combustion. A strong de-activation of the CO oxidation activity by SO₂ was observed only over the TiO₂-supported Au catalyst, indicating that SO₂ can block the active sites for CO oxidation over Au catalysts. The results are consistent with a model in which the perimeter sites have a special role in the CO oxidation reaction over Au catalysts.