The role of zinc oxide in Cu/ZnO catalysts for methanol synthesis and the water–gas shift reaction

All commercial catalysts for methanol synthesis and for the water–gas shift reaction in the low temperature region contain zinc oxide in addition to the main active component, copper. The varied benefits of zinc oxide are analysed here. The formation of zincian malachite and other copper/zinc hydroxy carbonates is essential in the production of small, stable copper crystallites in the final catalyst. Further, the regular distribution of copper crystallites on the zinc oxide phase ensures long catalyst life. Zinc oxide also increases catalyst life in the water–gas shift process by absorbing sulphur poisons but it is not effective against chloride poisons. In methanol synthesis, zinc oxide (as a base) removes acidic sites on the alumina phase which would otherwise convert methanol to dimethyl ether. Although bulk reduction of zinc oxide to metallic zinc does not take place, reduction to copper–zinc alloy (brass) can occur, sometimes as a surface phase only. A new interpretation of conflicting measurements of adsorbed oxygen on the copper surfaces of methanol synthesis catalysts is based on the formation of Cu–O–Zn sites, in addition to oxygen adsorbed on copper alone. The possible role of zinc oxide as well as copper in the mechanisms of methanol synthesis is still the subject of controversy. It is proposed that, only under conditions of deficiency of adsorbed hydrogen on the copper phase, hydrogen dissociation on zinc oxide, followed by hydrogen spillover to copper, is significant. This revised version was published online in June 2006 with corrections to the Cover Date.     

The effect of SiO_2 particle size on iron based F–T synthesis catalysts

The effect of particle size of silica, as catalyst binder, on the chemical and mechanical properties of iron based FT catalyst was studied in this work. The samples were characterized using XRD, BET, TEM, FT-IR, and H2-TPR, respectively. The attrition resistance and the FT activity were tested. Si-8–Si-15 catalysts prepared with 8–15 nm silica sol show good attrition resistance(attrition loss b 4%), especially Si-13 with an attrition loss of 1.89%. Hematite appeared in XRD patterns when silica sol above 15 nm is used. TEM micrographs show that no obvious Si O_2 particles appear when silica sol particle with size less than 8 nm was used, but Si O_2 particles coated with small ferrihydrite particles appear when silica sol above 8 nm was used. Si–O–Si vibration peak in FT-IR spectra increases with increasing silica sol size. Samples prepared with silica sol show good stability of FT reactions, and the average molecular weight of FT products increases with the increase of Si O_2 particle.     

Pd/polyaniline as the catalysts for 2‐ethylanthraquinone hydrogenation. The effect of palladium dispersion

By doping of polyaniline (PANI) in PdCl₂ aqueous and ethanol solutions the catalysts containing crystalline and colloidal Pd particles of different sizes were prepared. The size of palladium particles present in Pd/PANI catalysts (characterised by SEM and XRD methods) influenced the course of 2‐ethylanthraquinone (eAQ) hydrogenation, a key step in the industrial production of H₂O₂. The presence of large palladium particles promotes reactions leading to the formation of the so‐termed “degradation products” not capable of hydrogen peroxide formation. This revised version was published online in July 2006 with corrections to the Cover Date.     

The function of zeolite on Pt autoreduction and dispersion in Pt/L and Pt/β catalysts

TPR, CO-FTIR and¹²⁹Xe NMR spectroscopic techniques were used to measure the distribution of platinum species after the calcination of Pt/L and Pt/ zeolites. Autoreduction which occurred in Pt/ zeolite was avoided in the channel of L zeolite. Pt particles dispersed well and exhibited excellent reactivity for the aromatization ofn-hexane in L zeolite.     

Characterization and oxidation states of Cu and Pd in Pd–CuO/ZnO/ZrO2 catalysts for hydrogen production by methanol partial oxidation

Copper and zinc oxide based catalysts prepared by coprecipitation were promoted with palladium and ZrO₂, and their activity and selectivity for methanol oxidative reforming was measured and characterized by N₂O decomposition, X-ray absorption spectroscopy, BET, X-ray photoelectron spectroscopy, X-ray diffraction, and temperature programmed reduction. Addition of ZrO₂ increased copper dispersion and surface area, with little effect on activity, while palladium promotion significantly enhanced activity with little change of the catalytic structure. A catalyst promoted with both ZrO₂ and palladium yielded hydrogen below 150 °C. EXAFS results under reaction conditions showed that the oxidation state of copper was influenced by palladium in the catalyst bulk. A palladium promoted catalyst contained 90% Cu⁰, while the copper in an unpromoted catalyst was 100% Cu¹⁺ at the same temperature. Palladium preferentially forms an unstable alloy with copper instead of zinc during reduction, which persists during reaction regardless of copper oxidation state. A 100-h time on stream activity measurement showed growth in copper crystallites and change in copper oxidation state resulting in decreasing activity and selectivity. A kinetic model of the reaction pathway showed that palladium and ZrO₂ promoters lower the activation energy of methanol combustion and steam reforming reactions.     

Correlation of activity and stability of CuO/ZnO/Al2O3 methanol steam reforming catalysts with Cu/Zn composition obtained by SEM–EDAX analysis

A series of CuO/ZnO/Al₂O₃ catalysts were prepared and characterized by TPR, surface area, metal area, XRD and SEM–EDAX analysis. These systems were evaluated in the development of a methanol steam reforming catalyst (MSR). A correlation of activity and stability of MSR catalysts with the Cu/Zn ratio derived by SEM–EDAX analysis is observed. The stable activity of these catalysts is also supported by the method of preparation, low temperature reducibility and the presence of reversibly oxidizable Cu species observed by TPR of fresh and used catalysts.     

The effect of copper valence on catalytic combustion of styrene over the copper based catalysts in the absence and presence of water vapor☆

Catalysts Cu Ox/γ-Al_2O_3-IH and Cu Ox/γ-Al_2O_3-IM were prepared, characterized, and tested for styrene combustion in the absence and presence of water vapor. The effect of copper valence of the catalysts on the catalytic activity for styrene combustion was discussed using the theory of hard soft acids and bases(HSAB).The results showed that the existence of water vapor in feed stream inhibited the catalytic activity for styrene combustion due to the competition adsorption of water molecule. HSAB theory confirmed that the local soft acidity of the catalyst Cu Ox/γ-Al_2O_3-IH was much stronger than that of the catalyst Cu Ox/γ-Al_2O_3-IM because of the higher content of soft acid Cu+on its surface, which increased the adsorption ability toward soft base of styrene and reduced the adsorption toward hard base of water vapor, and thus increased the catalytic activity for styrene combustion and weakened the negative influence of water vapor.     

The synthesis of new chiral salen complexes immobilized on MCM‐41 by grafting and their catalytic activity in the asymmetric borohydride reduction of ketones

The new chiral salen complexes were synthesized and supported on mesoporous MCM‐41 through the condensation of 3‐aminopropyltrimethoxysilane and 2,6‐diformyl‐4‐tert‐butylphenol by the multi‐grafting method. The immobilized optically active Co(II) salen complexes showed a very high enantioselectivity in the asymmetric borohydride reduction of aromatic ketones. The chiral salen Co(II) complexes immobilized over MCM‐41 were stable during the reaction and exhibited a relatively high enantioselectivity for the reduction of ketones as compared with the homogeneous salen catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of heat treatment conditions on the textural and catalytic properties of nickel–titania composite aerogel catalysts

Highly dispersed nickel–titania composite aerogel catalysts have been prepared by CO₂ supercritical drying of alcogels obtained by the sol–gel process. The effect of heat treatment conditions on the textural and structural properties of the resulting aerogels was investigated by nitrogen adsorption–desorption, TG/DTA, XRD and TEM. The nickel–titania composite aerogel calcined and reduced under carefully controlled conditions exhibited an excellent catalytic performance for the liquid-phase hydrogenation of benzophenone to benzhydrol.     

Study on Cu–Mn–Si catalysts for synthesis of cyclohexanone and 2-methylfuran through the coupling process

A series of Cu–Mn–Si catalysts for synthesis of cyclohexanone (CHN) and 2-methylfuran (2-MF) through the coupling of cyclohexanol (CHL) dehydrogenation and furfural (FFA) hydrogenation were prepared by co-precipitation method. The catalysts were characterized by using N₂ physisorption, X-ray diffraction (XRD), H₂ temperature programmed reduction (H₂-TPR), N₂O-chemisorption and ammonia temperature-programmed desorption (NH₃-TPD) methods. The results show that metal–silica/or metal–metal interactions are presented in the catalysts, and the contents of copper, manganese and silicon affect the BET surface area, acidity and copper dispersion. The results of the catalytic tests indicate that manganese increases the activity of CHL dehydrogenation and FFA hydrogenation as well as the selectivity of 2-MF. A Cu–Mn–Si catalyst including appropriate copper, manganese and silicon is found to be most active, selective and stable under reaction conditions.     

The effect of CuO–ZnO–Al2O3 catalyst structure on the ethanol synthesis from syngas

CuO–ZnO–Al₂O₃ catalysts were prepared by complete liquid phase technology with different addition sequences. The results indicated that the catalyst prepared by the addition of (C₃H₇O)₃Al to Cu(NO₃)₂ and Zn(NO₃)₂ solutions shows excellent ethanol selectivity at the initial stages of reaction, reaching approximately 40%. X-ray photoelectron spectroscopy results showed that ethanol synthesis requires a higher Cu⁺ content and higher Cu/Zn ratio on the catalyst surface. The temperature-programmed reduction test revealed strong interactions between Cu species and zinc or aluminum oxide. The increase in the difficulty of catalyst reduction indicated higher ethanol selectivity.