Au/ZnO and Au/Fe2O3 catalysts for CO oxidation at ambient temperature: comments on the effect of synthesis conditions on the preparation of high activity catalysts prepared by coprecipitation

The preparation of Au/ZnO and Au/Fe₂O₃ catalysts using two coprecipitation methods is investigated to determine the important factors that control the synthesis of high activity catalysts for the oxidation of carbon monoxide at ambient temperature. In particular, the factors involved in the preparation of catalysts that are active without the need for a calcination step are evaluated. The two preparation methods differ in the manner in which the pH is controlled during the precipitation, either constant pH throughout or variable pH in which the pH is raised from an initial low value to a defined end point. Non-calcined Au/ZnO catalysts prepared using both methods are very sensitive to pH and ageing time, and catalysts prepared at a maximum pH = 5 with a short ageing time (ca. 0–3 h) exhibit high activity. Catalysts prepared at higher pH give lower activity. However, all catalysts require a short operation period during which the oxidation activity increases. In contrast, the calcined catalysts are not particularly sensitive to the preparation conditions. Non-calcined Au/Fe₂O₃ catalysts exhibit high activity when prepared at pH ≥ 5. Calcined Au/Fe₂O₃ prepared using the controlled pH method retain high activity, whereas calcined catalysts prepared using the variable pH method are inactive. The study shows the immense sensitivity of the catalyst performance to the preparation methods. It is therefore not surprising that marked differences in the performance of supported Au catalysts for CO oxidation that are apparent in the extensive literature on this subject, particularly the effect of calcination, can be expected if the preparation parameters are not carefully controlled and reported.     

The influence of alkali metals on the activity of supported ruthenium catalysts for the water-gas shift reaction

This paper presents the results of the studies on the influence of alkali metals on the activity of ruthenium catalysts, which were supported on the products of α- and δ-iron oxide-hydroxide calcination. Modification of these supports with alkali metals, in particular potassium, rubidium and cesium, prior to deposition of ruthenium was found to increase the activity of Ru/Fe₂O₃ catalysts in the water-gas shift reaction. It was also established that the activity and stability of catalysts prepared on iron oxide obtained from α-FeOOH increased when the latter was modified with sodium ions. The favourable effect of sodium on the activity of the catalysts was shown to appear at a certain proportion of the base to the active component, i.e. ruthenium. This revised version was published online in July 2006 with corrections to the Cover Date.     

Preservation of the Activity of Supported Gold Catalysts for CO Oxidation

Procedures leading to the preservation of activity of supported gold catalysts for CO oxidation are reviewed. The inclusion of iron as Fe(OH)₃ in preparing catalysts using tin oxide, ceria and zirconia as supports gives better activity and much improved stability with time-on-stream. In the case of Au/Fe-SnO₂ (0.5–0.9% Au), the effect is maximal with ~4% Fe. The stability of catalysts based on ceria as support is also much better when small amounts of either iron or lanthanum during preparation of the support by thermal decomposition of nitrates. Au/SnO₂ catalysts often suffer initial deactivation followed by an increase in activity with time-on-stream; a period of refrigeration (7d) induces an excellent stability at high conversion.     

Influence of the Alkali Promoter on the Activity and Stability of Transition Metal (Cu, Co, Fe) Based Structured Catalysts for the Simultaneous Removal of Soot and NOx

Structured catalysts prepared by means of coating cordierite monoliths with alumina-based suspensions containing transition metals such as Cu, Co and Fe and alkali/alkali-earth promoters such as K and Ba. Textural and structural features of these catalysts were analyzed by means of N₂ adsorption and SEM. Their activity in the simultaneous removal of soot and NO_x was assayed in a lab-scale installation, using a carbon black as diesel surrogate. Catalysts exhibited significant activity in deNO_x and soot oxidation. K and Ba enhanced both NO_x adsorption and soot–catalyst contact. However Ba contributed to a greater extent to the adsorption of N-species, which moreover presented higher thermal stability than on K-catalysts, and K showed higher mobility than Ba. Thus, Ba-containing catalysts showed increased activity towards NO_x reduction but shifted to higher temperatures in comparison to K-catalysts, which on the other hand resulted more active towards soot oxidation than Ba-ones. Fe-based catalyst turned out to be less active both in soot oxidation and NO_x reduction than Co and Cu-based ones. Intensive calcination of the catalysts at 800 °C for 5 h resulted in substantial loss of K and Ba. Loss of promoter depends, however, on the metal contained in the catalyst. In this sense Fe-containing catalysts showed higher stability. Calcination has a substantial effect on catalytic activity. Catalyst significantly lost their NO_x adsorption capacity and showed similar activity than a catalyst prepared in absence of promoter, pointing to a substantial change in reaction mechanism and reaction predominantly occurring on metallic sites upon the loss of alkali/alkali-earth compound.     

Effect of surface hydroxyl groups of pure TiO2 and modified TiO2 on the photocatalytic oxidation of aqueous cyanide

It is still debatable whether the photocatalytic oxidation of cyanide proceeds via hydroxyl radicals or by photogenerated holes. We synthesized pure TiO₂ catalysts via sol-gel process. In order to elucidate the oxidation pathway of cyanide, we used hydroxyl radical scavengers and controlled the concentration of surface hydroxyl group on the catalysts adopting fluoride-exchange. The degree of fluoride-exchange of TiO₂ catalysts was independent of the pH of suspension. We also adopted a polyoxometalate, tungstophosphoric acid (TPA, H₃PW₁₂O₄₀) which is well known for high charge transfer ability and hydrolytic stability. TPA-modified TiO₂ catalysts were prepared with sol-gel technique to overcome the high solubility of TPA in water. As another attempt for the insoluble TPA, proton of TPA supported on TiO₂ catalysts was replaced by cesium ion to form Cs-TPA/TiO₂ catalysts. Both attempts were successful in immobilizing TPA on TiO₂ catalysts. Commercially available TiO₂ catalysts such as P25 from Degussa AG were also used as catalysts. XRD analysis revealed that pure TiO₂ and TPA-modified TiO₂ catalysts prepared by sol-gel process were composed of well-developed anatase crystalline structure. In the presence of hydroxyl radical scavengers, the photoactivity of TPA-modified TiO₂ catalysts was retarded much less than that of pure TiO₂ catalysts. The concentration of surface hydroxyl group was effectively suppressed by the fluoride-exchange causing the decrease of the activity of the catalysts. In the case of fluoride-exchanged catalysts, the drop in activity was obvious for the pure TiO₂ catalysts in the presence of iodide as a hydroxyl radical scavenger suggesting that indirect oxidation via hydroxyl radicals was the preferential reaction pathway. For the TPA-modified TiO₂ catalysts, meanwhile, the diminution was such a small extent suggesting that direct oxidation by photogenerated holes was the main reaction pathway. The activity arising from TPA in the catalysts was due to the Keggin structured anion (PW₁₂O ₄₀ ^3- ) which acted as an electron relay with the aid of dissolved oxygen in the reaction system. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Highly active MgO-supported TiCl4 catalyst for the ethylene polymerization

Summary The MgO-supported TiCl₄ catalysts prepared by heating MgO with TiCl₄ showed a high activity for the ethylene polymerization in combination with Et₃Al or i-Bu₃Al. In these highly active catalysts, it has been shown that MgCl₂ is formed in the MgO-TiCl₄ reaction and is considered to contribute to the enhancement of the activity of the catalysts.     

Effects of oxidation and redox-properties on the selectivity of heat-treated Ag/Al2O3 catalysts for HC-SCR of NOx

Ag/Al₂O₃ catalysts (2 wt% Ag) have been prepared and calcined at different temperatures to render catalysts with different silver particle size or silver configuration. The differences in activity and selectivity of these catalysts are related to the activity for oxidation of NO and hydrocarbons, the NO_x storage properties, and to the oxidation state of silver.     

Effect of Aging Time and Calcination on the Preferential Oxidation of CO Over Au Supported on Doped Ceria

Au/CeLaO_x mixed oxide catalysts containing 0.6–1.0 wt% Au were prepared by co-precipitation with Na₂CO₃. BET surface areas ranged from 15 to 45 m²/g depending on aging time (precipitation time) and calcination conditions. The differences in the activity of the catalysts for preferential oxidation (PROX) of CO are ascribed to the differences in the metal loading, Ce/La ratio and support crystallinity, chloride content, and the resultant effect on the reduction properties of the catalysts. The catalysts did not require activation in H₂ prior to reaction. The temperature at which the catalysts exhibit significant activity correlates with the temperature of reduction, indicating that reduction of the metal and support is important for high activity.     

Effect of additives on the WGS activity of combustion synthesized CuO/CeO2 catalysts

This paper presents a study on the role of additives (samaria, lanthana, zirconia, and zinc oxide) to ceria as a support of copper catalysts for low-temperature WGSR. A single-step urea-combustion procedure was used for preparation of highly active catalysts. The results revealed the beneficial role of Sm₂O₃ and ZnO doping in increasing BET surface area and total pore volume of the catalysts and decreasing the degree of crystallinity of ceria. TPR profiles evidenced enhanced reducibility of modified catalysts. The WGS activity of these catalysts correlates well with BET, XRD, and TPR data. By comparing the catalysts with a commercial CuO–ZnO–Al₂O₃ sample we point out the feasibility of using CuO/doped-ceria catalysts for low-temperature WGSR.     

An investigation into the effect of Cs promotion on the catalytic activity of NiO in the direct decomposition of N2O

A series of Cs promoted NiO catalysts have been prepared and tested for direct decomposition of N₂O. These catalysts are characterized by BET surface area, X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption of N₂O (TPD-N₂O) and X-ray photo electron spectroscopy (XPS). The Cs promoted NiO catalysts exhibit higher activity for the decomposition of N₂O compared to bulk NiO. The catalyst with Cs/Ni ratio of 0.1 showed highest activity. The enhancement in catalytic activity of the Cs promoted catalysts is attributed to the change in the electronic properties of NiO. The characterization techniques suggest weakening of Ni–O bond thereby the desorption of oxygen becomes more facile during the reaction. The Cs promoted NiO catalyst is effective at low reaction temperature and also in the presence of oxygen and steam in the feed stream. IICT Communication No: 070523.     

Unsupported NiMo Sulfide Catalysts Obtained from Nickel/Ammonium and Nickel/Tetraalkylammonium Thiomolybdates: Synthesis and Application in the Hydrodesulfurization of Dibenzothiophene

Ammonium and tetraalkylammonium tetrathiomolybdates impregnated with nickel nitrate were used as precursors of unsupported NiMo sulfide catalysts. The precursors were decomposed either in situ during the course of a dibenzothiophene (DBT) hydrodesulfurization (HDS) test or ex situ through sulfidation by H₂S/H₂ (15% v/v H₂S). The catalysts were characterized by thermogravimetric analysis, N₂ adsorption, scanning electron microscopy (SEM), and X-ray diffraction. Textural and catalytic properties of these NiMo catalysts were strongly influenced both by the nature of the precursor and the activation procedure. For ex-situ activated NiMo catalysts, the use of carbon-containing tetraalkylammonium thiosalts as precursors did not lead to a significant improvement in HDS activity. For in situ activated NiMo catalysts, the role of carbon is more complex. The use of tetramethyl- or tetrapropylammonium tetrathiomolybdate salts led to a poor final HDS activity while using tetrabutylammonium tetrathiomolybdate, a net increase in HDS activity was observed compared to the use of the non-carbon containing ammonium tetrathiomolybdate. This was related to the development of a mesoporous structure and to a high increase in surface area. This result is in agreement with those found previously for CoMo catalysts and confirms that tetraalkylammonium tetrathiomolybdate salts with long alkyl chains lead to Co- or Ni-promoted MoS₂-based catalysts with enhanced HDS activity if in situ activated.     

Influence of the Formation of Ternary Oxide of Pd and Na Supported on Alumina Catalysts for the Room Temperature Hydrodechlorination of Chlorobenzene

A series of Pd/Na modified Al₂O₃ catalysts was prepared by deposition–precipitation method. These catalysts were characterized by X-ray diffraction, temperature programmed reduction, X-ray photoelectron spectroscopy, N₂ adsorption, CO pulse chemisorption and SEM-EDAX. The characterization data reveal the formation of a ternary oxide of Pd with Na for the catalysts at an optimum Pd to Na ratio. The activity of the catalysts studied for the room temperature hydrodechlorination of chlorobenzene, is also found to be maximum at this ratio.     

A basic study of catalyst aging in the H-coal process

Samples of CoMo/Al₂O₃ catalysts used in an H-coal process demonstration run were studied to determine causes of catalyst deactivation. Physical and surface properties of the aged and regenerated catalysts were examined. Model compounds were used to assess four catalyst activity functions, viz., hydrodesulfurization (HDS), hydrogenation, cracking and hydrodeoxygenation (HDO). Other tests were performed to study the effects of coke and metals separately on the four catalyst activity functions.Catalyst coke content and metals deposits first increased rapidly, then more gradually with exposure time in the process run. Surface area and pure volume markedly decreased with exposure time. Catalyst activities of aged catalysts showed a rapid decline with exposure time. One-day exposure to coal resulted in significant losses in HDS and hydrogenation activities and nearly complete loss in cracking and HDO activities. Although metal deposits caused some permanent catalyst deactivation, coke had a much greater effect. Regenerated catalysts showed less recovery of catalytic activity as processing time increased. These results agreed well with product inspections from the process run.Oxygen chemisorption on aged—regenerated catalysts decreased with catalyst exposure time, indicating a significant loss of active sites. However, ESCA results showed no evidence of extensive sintering of the active MoS₂ phase. Permanent deactivation of the longer-time exposed catalysts can be ascribed, at least partly, to lateral growth of the active molybdenum sulfide phase. In addition, some loss in cobalt promotion occurred early in the process, which may account for the rapid loss in HDS and HDO activity in regenerated catalysts.     

Combustion of a trace amount of CH4 in the presence of water vapor over ZrO2-supported Pd catalysts

Combustion of a trace amount of CH₄ over Pd catalysts supported on calcined ZrO₂ was examined under nearly exhaust gas conditions where the temperature is not so high and water vapor coexists. High catalytic activity was obtained with ZrO₂ support calcined at 1073 and 1273 K. The durability test at 673 K for 100 h revealed that the activity of these catalysts hardly decreased, while that of the Pd catalysts supported on calcined Al₂O₃ were much decreased in the course of time. These results demonstrated the advantages of ZrO₂ as a support for Pd catalysts in the present reaction. This revised version was published online in November 2006 with corrections to the Cover Date.     

Carbon dioxide reforming of methane over Co-X/ZrO2 catalysts (X = La,Ce, Mn,Mg, K)

Dry reforming of methane has been studied over Co/ZrO₂ catalysts promoted with different metal additives (La, Ce, Mn, Mg, K) aiming to improve the performance of the catalysts and increase their resistance to coking. Scanning electron microscopy studies and different activity levels of the catalysts clearly show that the type of the promoter significantly affected the metal dispersion properties and catalytic performances of Co/ZrO₂ catalysts. La-modified catalyst exhibited high stability, but moderate activity. It showed no severe coke deposition. Ce-doped Co/ZrO₂ displayed the highest activity among all the catalysts prepared and had a very limited activity loss.     

Support effect on the properties of iron–molybdenum hydrodesulfurization catalysts

Two series of Mo and Fe containing catalysts have been prepared over alumina and titania supports using H₃PMo₁₂O₄₀ heteropolyacid (HPMo) and Fe salt of HPMo. Catalysts have been characterized by BET, SEM, IR, TPR, XPS methods and by their HDS activity in the reaction of thiophene conversion. The TiO₂-supported catalysts with low Mo concentration (6 wt%) show higher HDS activity than the catalyst with 12 wt% Mo. Iron promoting effect (Fe/Mo ~ 0.1) is observed with both, the alumina- and titania-supported catalysts. Iron supported over alumina increases Mo reducibility and decreases it on TiO₂-supported catalysts. Compared to alumina-supported catalysts, the TiO₂-supported catalysts show higher surface concentration of Mo⁶⁺ and Mo⁵⁺ in octahedral coordination – Mo(Oh). Iron increases the Mo(Oh) concentration even more. After sulfidation the Fe-containing catalysts show formation of different Mo valence states (Mo⁴⁺, Mo⁵⁺, Mo⁶⁺), Fe–P, Mo–P and/or Fe–Mo–P bonds, which affect the HDS catalytic activity.     

Influence of porosity and surface groups on the catalytic activity of carbon materials for the microwave-assisted CO2 reforming of CH4

In this work, various carbon materials were studied as catalysts/microwave receptors for the CO₂ reforming of the CH₄ reaction. Carbon materials with a different textural development (metallurgical coke, activated carbons, re-activated carbon) were selected as catalysts in order to determine the role of porosity and pore size in dry reforming. Microporosity was found to be necessary for a good performance of the carbon catalysts. An activated carbon and an oxidized activated carbon were compared in order to evaluate the influence of oxygen surface groups on the catalytic activity of carbons for the dry reforming reaction. Oxidized carbons were found to be bad catalysts, especially under microwave heating. The importance of CO₂ reactivity for carbon materials to be able to act as acceptable catalysts was also established.     

How oxide carriers affect the reactivity of V2O5 catalysts in the oxidative dehydrogenation of propane

The catalytic pattern of several oxide carriers (MgO, Al₂O₃, ZrO₂, TiO₂, SiO₂, HY zeolite) and supported V₂O₅ (4.7–5.3 wt%) catalysts in the oxidative dehydrogenation of propane to propylene (PODH) has been comparatively investigated. The fundamental role of the oxide support on both reducibility and reactivity of vanadia catalysts has been assessed. A direct relationship between the specific surface activity of oxide carriers and that of vanadia catalysts is discussed. The inverse relationship between the specific activity and the onset temperature of reduction marks the prevailing redox behaviour of V₂O₅ catalysts in the PODH reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Organometallics derived (Pd + Ln)/SiO2 catalysts for the reactions of synthesis gas conversion

C₃H₆ hydroformylation and CH₃OH synthesis on organometallics derived (Pd + Ln)/ SiO₂ and Pd/SiO₂ catalysts have been studied. The activity and selectivity towards methanol in CO + H₂ reaction were observed to increase for all the modified catalysts while both the hydroformylation activity and selectivity towards oxygenates in C₃H₆ hydroformylation decreased for the catalysts in comparison to those of Pd/SiO₂. The FTIR, TPD data and characteristic catalytic properties of the catalysts studied allow to suggest that C₃H₆ hydroformylation on (Pd + Ln)/SiO₂ catalysts occurs on monometallic Pd clusters without participation of mixed active sites and CO complexes activated thereon.     

Recyclable Solid Ruthenium Catalysts Supported on Metal Oxides for the Addition of Carboxylic Acids to Terminal Alkynes

Ceria‐supported ruthenium catalysts (Ru/CeO₂) were found to be quite effective for the addition of various carboxylic acids to terminal alkynes, which gave the corresponding enol esters in moderate to high yields. The major products of the reaction were E‐isomers of anti‐Markovnikov adducts. Among the ceria‐supported ruthenium catalysts examined, those prepared using ruthenium precursors with chloride ligands showed high activities. The zirconia‐supported ruthenium catalyst (Ru/ZrO₂) showed activity comparable to that of the ceria‐supported catalyst. These catalysts were recyclable without a significant loss of activity, and the leaching of ruthenium species into the liquid phase was negligible after cooling the reaction mixture, which indicates marked superiority of the present solid oxide catalysts to conventional homogeneous catalysts.