Total oxidation catalysts based on manganese or copper oxides and platinum or palladium II: Activity, hydrothermal stability and sulphur resistance

Deactivation of catalysts based on either manganese oxides, copper oxides, platinum, palladium or combinations of these metal oxides and noble metals supported on γ-alumina was studied. The activity of the catalysts for the oxidation of carbon monoxide, naphthalene and methane, in a mixture resembling the flue gases from wood combustion, was measured before and after exposure of the catalysts either to a temperature of 900°C in the presence of steam or to sulphur dioxide. Most of the mixed catalysts were more resistant to hydrothermal and sulphur treatments than the catalysts with a single active component. After the hydrothermal treatment the activity of the MnO_x catalyst was enhanced. When Pt is combined with MnO_x or CuO_x, the loss of activity of Pt was decreased during the hydrothermal treatment. Also, the hydrotreated mixed MnO_x–Pd and CuO_x–Pd catalysts were more active than the treated Pd catalyst for the oxidation of methane. After sulphur treatment, the activities of the mixed MnO_x–Pt (Pt: 0.05 mol%), MnO_x–Pd and CuO_x–Pd catalysts were improved for the oxidation of carbon monoxide and naphthalene. Among the catalysts studied, the MnO_x–Pt, CuO_x–Pt and CuO_x–Pd catalysts, with a metal oxide and a noble metal loading of 10 and 0.1 mol%/γ-alumina, respectively, had the best combination of activity, thermal stability and resistance to sulphur treatment.     

Pt/MnOx–CeO2 catalysts for the complete oxidation of formaldehyde at ambient temperature

MnO_x–CeO₂ mixed oxides with a Mn/(Mn + Ce) molar ratios of 0–1 were prepared by a modified coprecipitation method and investigated for the complete oxidation of formaldehyde. The MnO_x–CeO₂ with Mn/(Mn + Ce) molar ratio of 0.5 exhibited the highest catalytic activity among the MnO_x–CeO₂ mixed oxides. Structure analysis by X-ray powder diffraction and temperature-programmed reduction of hydrogen revealed that the formation of MnO_x–CeO₂ solid solution greatly improved the low-temperature reducibility, resulting in a higher catalytic activity for the oxidation of formaldehyde. Promoting effect of Pt on the MnO_x–CeO₂ mixed oxide indicated that both the Pt precursors and the reduction temperature greatly affected the catalytic performance. Pt/MnO_x–CeO₂ catalyst prepared from chlorine-free precursor showed extremely high activity and stability after pretreatment with hydrogen at 473 K. 100% conversion of formaldehyde was achieved at ambient temperature and no deactivation was observed for 120 h time-on-stream. The promoting effect of Pt was ascribed to enhance the effective activation of oxygen molecule on the MnO_x–CeO₂ support.     

Direct decomposition of nitric oxide over Ba catalysts supported on CeO2-based mixed oxides

The direct decomposition of nitric oxide (NO) over barium catalysts supported on various metal oxides was examined in the absence and presence of O₂. Among the Ba catalysts supported on single-component metal oxides, Ba/Co₃O₄ and Ba/CeO₂ showed high NO decomposition activities, while Ba/Al₂O₃, Ba/SiO₂, and Ba/TiO₂ exhibited quite low activities. The effect of an addition of second components to Co and Ce oxides was further examined, and it was found that the activities were significantly enhanced using Ce–Mn mixed oxides as support materials. XRD results indicated the formation of CeO₂–MnO_x solid solutions with the cubic fluorite structure. O₂-TPD of the CeO₂–MnO_x solid solutions showed a large desorption peak in a range of relatively low temperature. The BET surface areas of the CeO₂–MnO_x solid solutions were larger than those of pure CeO₂ and Mn₂O₃. These effects caused by the addition of Mn are responsible for the enhanced activities of the Ba catalysts supported on Ce–Mn mixed oxides.     

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.     

Re-investigating the CO oxidation mechanism over unsupported MnO, Mn2O3 and MnO2 catalysts

Kinetic study of CO oxidation in combination with experiments of temperature-programmed oxidation (TPO) and reduction (TPR) have been performed on various unsupported crystalline manganese oxides (MnO_x); while the reactivity shows an order of MnO ≤ MnO₂ < Mn₂O₃ in a mixture of unit ratio of O₂/CO at/below 523 K. We propose that under the current conditions the interaction of adsorbed CO and O is mainly responsible for CO₂ formation on Mn₂O₃ and MnO₂ catalysts, following either the Langmuir–Hinshelwood mechanism or Eley–Rideal mechanism. Meanwhile, direct evidence from transient CO oxidation suggests that the Mars-van-Krevelen mechanism may occur for all catalysts simultaneously, especially, it is predominant for the MnO catalyst. The evidence of structural modifications during reaction was confirmed by Raman spectra obtained from used MnO.     

Oxides of copper, ceria promoted copper, manganese and copper manganese on Al2O3 for the combustion of CO, ethyl acetate and ethanol

Combustion of CO, ethyl acetate and ethanol was studied over CuO_x/Al₂O₃, CuO_x–CeO₂/Al₂O₃, CuMn₂O₄/Al₂O₃ and Mn₂O₃/Al₂O₃ catalysts. It was found that modification of the alumina with ceria before subsequent copper oxide deposition increases the activity for combustion of CO substantially, but the effect of ceria was small on the combustion of ethyl acetate and ethanol. The activity increases with the CuO_x loading until crystalline CuO particles are formed, which contribute little to the total active surface. The CuO_x–CeO₂/Al₂O₃ catalyst is more active than the CuMn₂O₄/Al₂O₃ catalyst for the oxidation of CO but the CuMn₂O₄/Al₂O₃ catalyst is more active for the combustion of ethyl acetate and ethanol.

Thermal ageing and water vapour in the feed caused a modest decrease in activity and did not affect the CuO_x–CeO₂/Al₂O₃ and CuMn₂O₄/Al₂O₃ catalysts differently. In addition, no difference in intermediates formed over the two catalysts was observed.

Characterisation with XRD, FT-Raman and TPR indicates that the copper oxide is present as a copper aluminate surface phase on alumina at low loading. At high loading, bulk CuO crystallites are present as well. Modification of the alumina with ceria before the copper oxide deposition gives well dispersed copper oxide species and bulk CuO crystallites associated to the ceria, in addition to the two copper oxide species on the bare alumina. The distribution of copper species depends on the ceria and copper oxide loading. The alumina supported copper manganese oxide and manganese oxide catalysts consist mainly of crystalline CuMn₂O₄ and Mn₂O₃, respectively, on Al₂O₃.     

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.     

Catalytic combustion of hydrocarbons with Mn and Cu-doped ceria–zirconia solid solutions

The catalytic activity of a series of CeO₂–ZrO₂ mixed oxides in the total oxidation of methane and light hydrocarbons has been investigated. The influence of dopants like Mn and Cu has also been studied. It is shown that both MnO_x and CuO at low loading dissolve within the ceria–zirconia lattice. This strongly influences the redox behaviour of the catalysts by promoting low-temperature reduction of Ce⁴⁺. In addition, the ternary oxides show better stability to repeated redox cycles, which is attributed to the presence of ZrO₂. The catalytic activity of pure CeO₂ is also enhanced in the presence of ZrO₂, reaching a maximum with Ce_0.92Zr_0.08O₂; a further promotion of activity is observed with the introduction of MnO_x and CuO dissolved into CeO₂–ZrO₂ lattice.     

The selective catalytic reduction of nitrogen oxides by methane on noble metal-loaded sulfated zirconia

The selective catalytic reduction of NO_x by methane on noble metal-loaded sulfated zirconia (SZ) catalysts was studied. Ru, Rh, Pd, Ag, Ir, Pt, and Au-loaded sulfated zirconia catalysts were compared with the intact sulfated zirconia. For the NO–CH₄–O₂ reaction, Ru, Rh, Pd, Ir, and Pt showed promotion effect on NO_x reduction, while for the NO₂–CH₄–O₂ reaction, only Rh and Pd showed promotion effect. Over intact and Rh, Pd, Ag, and Au-loaded sulfated zirconia, NO_x conversion in NO₂–CH₄–O₂ reaction was significantly higher than that in NO–CH₄–O₂ reaction, while clear difference was not observed over Ru, Ir, and Pt-loaded sulfated zirconia. Comparison of [NO₂]/([NO]+[NO₂]) in the effluent gases in NO–O₂ and NO₂–O₂ reactions showed that Ru, Ir, and Pt has high activity for NO oxidation under the reaction conditions. These facts suggest that effects of these metals toward NO_x reduction by methane can be categorized into the following three groups: (i) low activity for NO oxidation to NO₂, and high activity for NO₂ reduction to N₂ (Pd, Rh); (ii) high activity for NO oxidation to NO₂, and low activity for NO₂ reduction to N₂ (Ru, Ir, Pt); (iii) low activity for both reactions (Ag, Au). To confirm these suggestions, combination of these metals were investigated on binary or physically-mixed catalysts. The combination of Pd or Rh with Pt or Ru gave high activity for the selective reduction of NO_x by methane.     

Synthesis of CeO2–MnOx mixed oxides and catalytic performance under oxygen-rich condition

Ce_0.5Zr_0.5O₂, Ce_0.5Zr_0.2Mn_0.3O₂ and Ce_0.5Mn_0.5O₂ were prepared by citric acid sol–gel method. The effect of manganese on the structural and redox properties of ceria-based mixed oxides was investigated by means of powder X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller analyses, temperature-programmed reduction and catalytic activity evaluation in the presence of excess O₂. The results showed that some Mn cations could enter into the ceria lattice to form solid solutions. Mn₃O₄ appeared due to the instability of the mixed oxides with increment of the Mn doping ratio while another oxide Mn₂O₃ is detected in the physical mixture of ceria and manganese oxide. These Mn-doped mixed oxides, especially Ce_0.5Mn_0.5O₂, presented better catalytic activities than Ce_0.5Zr_0.5O₂ and even Pt-loaded catalyst for total oxidation of C₃H₈ and oxidative sorption of NO in the presence of excess oxygen. The oxidation ability of Mn and the strong interaction between Mn and Ce were suggested to promote the oxygen storage/transport capacity of the mixed oxides as well as reactive adsorption of nitric oxide and hydrocarbons.     

Thermal ageing of Pt on low-surface-area CeO2–ZrO2–La2O3 mixed oxides: Effect on the OSC performance

This work aims at exploring the thermal ageing mechanism of Pt on ceria-based mixed oxides and the corresponding effect on the oxygen storage capacity (OSC) performance of the support material. Pt was supported on low-surface-area CeO₂–ZrO₂–La₂O₃ mixed oxides (CK) by impregnation method and subsequently calcined in static air at 500, 700 and 900 °C, respectively. The evolutions of textural, microstructural and redox properties of catalysts after the thermal treatments were identified by means of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and high-resolution transmission electron microscope (HRTEM). The results reveal that, besides the sintering of Pt, encapsulation of metal by the mixed oxides occurs at the calcination temperature of 700 °C and above. The burial of Pt crystallites by support particles is proposed as a potential mechanism for the encapsulation. Further, the HRTEM images show that the distortion of the mixed oxides lattice and other crystal defects are distributed at the metal/oxides interface, probably indicating the interdiffusion/interaction between the metal and mixed oxide. In this way, encapsulation of Pt is capable to promote the formation of Ce³⁺ or oxygen vacancy on the surface and in the bulk of support. The OSC results show that the reducibility and oxygen release behavior of catalysts are related to both the metal dispersion and metal/oxides interface, and the latter seems to be more crucial for those supported on low-surface-area mixed oxides. Judging by the dynamic oxygen storage capacity (DOSC), oxygen storage capacity complete (OSCC) and oxygen releasing rate, the catalyst calcined at 700 °C shows the best OSC performance. This evident promotion of OSC performance is believed to benefit from the partial encapsulation of Pt species, which leads to the increment of Ce³⁺ or oxygen vacancies both on the surface and in the bulk of oxides despite a loss of chemisorption sites on the surface of metal particles.     

Oxidative degradation properties of Co-based catalysts in the presence of ozone

Four series of cobalt-based catalysts, such as bare Co₃O₄ and CoO, CoO_x–CeO₂ mixed oxides, CoO_x supported over alumina and alumina–baria and CoMgAl and CoNiAl hydrotalcites have been synthesized and investigated for the oxidative degradation of phenol in the presence of ozone. Characterizations were obtained by several techniques in order to investigate the nature of cobalt species and their morphological properties, depending on the system. Analyses by XRD, BET, TPR, UV–visible diffuse reflectance spectroscopy and TG/DT were performed.

The CoNiAl hydrotalcite exhibits, after 4 h of reaction, the highest phenol ozonation activity followed by Co(3 wt%)/Al₂O₃–BaO and CoMgAl. The samples Co(1 wt%)/Al₂O₃–BaO and Co(1 and 3 wt%)/Al₂O₃ show a comparable medium activity, while the oxidation properties of bare oxides Co₃O₄, CoO and CoO_x–CeO₂ are really low. Leaching of cobalt ions in the water solution was detected during the reaction, the amount varied depending on the nature of catalysts. A massive release was observed for the CoMgAl and CoNiAl hydrotalcites, while cobalt catalysts over alumina and alumina–baria look much more stable. The recycle of CoO_x/Al₂O₃ and CoO_x/Al₂O₃–BaO was studied by performing three consecutive cycles in the phenol oxidation. Because of the potential interest of the cobalt-supported catalysts in the ozonation process, the oxidative degradation of naphtol blue black was also investigated.

On the basis of TPR and UV–visible results it appears that highly dispersed Co²⁺ ions especially present over Co(3 wt%)/Al₂O₃–BaO are the main active sites for phenol and naphtol blue black oxidative degradation by ozone.     

Isomerization of n-hexane over mono- and bimetallic Pd–Pt catalysts supported on ZrO2–Al2O3–WOx prepared by sol–gel

The catalytic performance of mono- and bimetallic Pd (0.6, 1.0 wt.%)–Pt (0.3 wt.%) catalysts supported on ZrO₂ (70, 85 wt.%)–Al₂O₃ (15, 0 wt.%)–WO_x (15 wt.%) prepared by sol–gel was studied in the hydroisomerization of n-hexane. The catalysts were characterized by N₂ physisorption, XRD, TPR, XPS, Raman, NMR, and FT-IR of adsorbed pyridine. The preparation of ZrW and ZrAlW mixed oxides by sol–gel favored the high dispersion of WO_x and the stabilization of zirconia in the tetragonal phase. The Al incorporation avoided the formation of monoclinic-WO₃ bulk phase. The catalysts increased their S_BET for about 15% promoted by Al₂O₃ addition. Various oxidation states of WO_x species coexist on the surface of the catalysts after calcination. The structure of the highly dispersed surface WO_x species is constituted mainly of isolated monotungstate and two-dimensional mono-oxotungstate species in tetrahedral coordination. The activity of Pd/ZrW catalysts in the hydroisomerization of n-hexane is promoted both with the addition of Al to the ZrW mixed oxide and the addition of Pt to Pd/ZrAlW catalysts. The improvement in the activity of Pd/ZrAlW catalysts is ascribed to a moderated acid strength and acidity, which can be correlated to the coexistence of W⁶⁺ and reduced-state WO_x species (either W⁴⁺ or W⁰). The addition of Pt to the Pd/ZrAlW catalyst does not modify significantly its acidic character. Selectivity results showed that the catalyst produced 2MP, 3MP and the high octane 2,3-dimethylbutane (2,3-DMB) and 2,2-dimethylbutane (2,2-DMB) isomers.     

Catalyst screening for oxidative desulfurization using hydrogen peroxide

Oxidation of a mixture of thiophene, benzothiophene and dibenzothiophene with hydrogen peroxide using supported Pd, Cr₂O₃, unsupported manganese oxides and a commercial Co-Mo/Al₂O₃ as catalysts has been studied in a mixture of hexadecane and acetonitrile. Based solely on the conversion of each organic sulfur compound, the ranking of catalyst efficiency found was: supported Pd > Cr₂O₃ ≈ manganese oxides ≈ Co-Mo/Al₂O₃. The influence of the calcination temperature on synthesized manganese oxides was also investigated. Mn₃O₄, amorphous manganese compounds, Mn₂O₃ and MnO₂ showed a similar catalytic activity independent of the hydrogen peroxide concentration. According to these preliminary results, it seemed that the catalyzed decomposition of the hydrogen peroxide competes with the oxidative desulfurization, however, at short reaction time (10 min) conversions at around 60–70% of thiophene were reached.     

Modification of the oxygen storage capacity of CeO2–ZrO2 mixed oxides after redox cycling aging

High surface area CeO₂–ZrO₂ mixed oxides were treated at 900–950°C either under wet air or under successive reducing and oxidizing atmospheres in order to study the evolution of the oxygen storage capacity (OSC) of these solids after different aging treatments. Several complementary methods were used to characterize the redox behavior: temperature programmed reduction (TPR) by H₂, TPO, magnetic susceptibility measurements to obtain the Ce³⁺ content, FT-IR spectroscopy of adsorbed methanol and a method to compare the oxygen buffering capacity (OBC) of the oxides.

All the results confirm that the mixed oxides exhibit better redox properties than pure ceria, particularly after aging. The enhancement in the OSC at moderate temperature has to be related to a deeper penetration of the reduction process from the surface into the under-layers. Redox cycling aging promotes the reduction at low temperature of all the mixed oxides, the improvement being much more important for low surface area aged samples. The magnitude of this effect does not depend on the BET surface areas which have similar values after cycling. This underlines the critical influence that the preparation and activation procedure have on the final OSC behaviors of the ceria–zirconia mixed oxides.     

Oxidation of methane over Pd/mixed oxides for catalytic combustion

Palladium catalysts supported on mixed oxides (Pd/Al₂O₃–MO_x; M=Co, Cr, Cu, Fe, Mn, and Ni) were investigated for the low-temperature catalytic combustion of methane. Although the surface area decreased with increasing NiO in Pd/mAl₂O₃–nNiO, Pd/Al₂O₃–36NiO demonstrated an excellent activity due to the small particle size of palladium. Also, the catalytic activity strongly depended on the composition of the support. Temperature-programmed desorption of oxygen revealed that the catalytic activity in the low-temperature region depends on the adsorption state of oxygen on palladium. The activity was enhanced when the amount of adsorbed oxygen increased. In-situ XRD analysis indicated that the PdO phase was thermally stabilized on Pd/Al₂O₃–36NiO.     

Relative rates of various steps of NO–CH4–O2 reaction catalyzed by Pd/H-ZSM-5

Reaction mechanism of the reduction of nitrogen monoxide by methane in an oxygen excess atmosphere (NO–CH₄–O₂ reaction) catalyzed by Pd/H-ZSM-5 has been studied at 623–703 K in the absence of water vapor, in comparison with the mechanism for Co-ZSM-5. Kinetic isotope effect for the N₂ formation in NO–CH₄–O₂ vs. NO–CD₄–O₂ reactions was 1.65 at 673 K and decreased with a decrease in the reaction temperature. In addition, H–D isotopic exchange took place significantly in NO–(CH₄+CD₄)–O₂ reaction. These results are in marked contrast with the case of Co-ZSM-5, for which the C–H dissociation of methane is the only rate-determining step, and show that the C–H dissociation is slow but not the only rate-determining step in the case of Pd/H-ZSM-5.

A reaction scheme was proposed, in which the relative rates of the three steps ((i)–(iii) below) vary depending on the reaction conditions.

Further, in contrast to Co-ZSM-5, NO_x–CH₄–O₂ reaction was much slower than CH₄–O₂ reaction for Pd/H-ZSM-5; the presence of NO_x retards the reaction of CH₄ over the latter catalyst, while it accelerates the reaction over the former. It is suggested that CH₄ is activated directly by the Pd atoms in the case of Pd/H-ZSM-5, but by NO₂ strongly adsorbed on Co ion for Co-ZSM-5. The reaction order of the NO–CH₄–O₂ reaction with respect to NO pressure was consistent with this mechanism; 1.05 for Pd/H-ZSM-5 and 0.11 for Co-ZSM-5.     

Oxidation of ethanol over supported manganese catalysts—effect of the carrier

MnO_x catalysts supported on alumina, titania or yttria-stabilised zirconia were studied in ethanol oxidation. Catalysts were characterized by determining their XRD, TPR, TPD-O₂ and TPD-NH₃ properties and light-off behavior. The effect of kind of carrier on activity in the ethanol oxidation and on selectivity to acetaldehyde (ACA) was determined. Relation between the TPR properties of the catalysts and their activity in ethanol conversion is suggested. The maximum of selectivity to ACA appears in the same sequence of temperatures as the first peak of oxygen desorption from supported MnO_x catalysts.     

CO oxidation over promoted Pt catalysts

A study of CO oxidation by O₂ over Pt catalysts, promoted by MnO_x and CoO_x, is described. The activities of Pt/SiO₂, Pt/MnO_x/SiO₂ and Pt/CoO_x/SiO₂ are compared with commercial Pt/Al₂O₃, Pt/Rh/Al₂O₃ and Pt/CeO_x/Al₂O₃ catalysts. Since these catalysts differ in dispersion and weight loading of platinum, the turnover frequencies are also compared. The following order in activity in CO oxidation after a reductive pretreatment is found: Pt/CoO_x/SiO₂ > Pt/MnO_x/SiO₂, Pt/CeO_x/Al₂O₃ > Pt/Al₂O₃, Pt/Rh/Al₂O₃, Pt/SiO₂. Over Pt/CoO_x/SiO₂ CO is already oxidised at room temperature. Possible models to account for the high activity of Pt/CoO_x/SiO₂ in the CO/O₂ reaction are presented and discussed. Partially reduced metal oxides are necessary to increase the activity of the Pt/CoO_x/SiO₂, Pt/MnO_x/SiO₂ or Pt/CeO_x/Al₂O₃ catalysts. It was shown that mild ageing treatments did not affect the activity of the Pt/CoO_x/SiO₂ catalyst in CO oxidation.     

Selective oxidation of hydrogen sulfide containing excess water and ammonia over vanadia–titania aerogel catalysts

A series of V₂O₅–TiO₂ aerogel catalysts were prepared by sol–gel method with subsequent supercritical drying with CO₂. The aerogel catalysts showed much higher surface areas and total pore volumes than V₂O₅–TiO₂ xerogel and impregnated V₂O₅–TiO₂ catalysts. Two species of surface vanadium in the aerogel catalysts were identified by Raman measurements: monomeric vanadyl and polymeric vanadates. The selective oxidation of hydrogen sulfide in the presence of excess water and ammonia was studied over these catalysts. Aerogel catalysts showed very high conversion of H₂S without harmful emission of SO₂. Temperature programmed reduction (TPR), XRD and Raman analyses revealed that the high catalytic performance of the aerogel catalysts originated from their highly dispersed VO_x species and high reducibility.