Oxidation of o-Xylene to Phthalic Anhydride on Sb-V/ZrO2 Catalysts

Zirconia-supported and bulk-mixed vanadiumantimonium oxide catalysts were used for the oxidation of o-xylene to phthalic anhydride. X-ray diffraction, Raman spectroscopy and photoelectron spectroscopy were used for characterization. It was found that vanadium promotes the transition of tetragonal to monoclinic zirconia. The simultaneous presence of Sb and V on zirconia at low coverage led to a preferential interaction of individual V and Sb oxides with the zirconia surface rather than the formation of a binary Sb-V oxide, while at higher Sb-V contents the formation of SbVO₄ took place. Sb-V/ZrO₂ catalysts showed high activity for o-xylene conversion and better selectivity to phthalic anhydride as compared to V/ZrO₂ catalysts. However, their selectivity to phthalic anhydride was poor in comparison to a V/TiO₂ commercial catalyst. The improved selectivity of the Sb-containing catalysts is attributed to the blocking of non-effective surface sites of ZrO₂, the decrease of the total amount of acid sites and the formation of surface V-O-Sb-O-V structures.     

Highly dispersed sol–gel synthesized Cu–ZrO2 materials as catalysts for oxidative steam reforming of methanol

New nanodispersed Cu–ZrO₂ systems (containing 0 and 8.3 mol% Cu) were synthesized by sol–gel method. Homogeneous gels were prepared starting from Zr propoxide and Cu(NO₃)₂·2.5H₂O. The materials were characterized by XRD, TG/DTA, N₂ adsorption, TPR techniques and N₂O surface oxidation. In the Cu-containing material part of Cu²⁺ ions were incorporated into the zirconia lattice and strongly influenced the crystallisation behaviour of zirconia matrix. After treatment at 450–600 °C, the materials contained ZrO₂ nanocrystals of the tetragonal polymorph. The samples heat treated up to 450 °C showed high surface areas in the range 140–180 m²/g. Copper oxide species with different reducibility were detected by TPR measurements. The H₂ reduction treatments gave rise to metallic copper with very high dispersion. The catalysts showed high activity for the oxidative steam reforming of methanol. A noticeable activity was observed also with the not pre-reduced catalyst, although a previous reduction in H₂ led to a higher selectivity and H₂ production.     

Influences of preparation methods of ZrO2 support and treatment conditions of Cu/ZrO2 catalysts on synthesis of methanol via CO hydrogenation

ZrO₂ supports were prepared by different methods (conventional precipitation method, shortened as “CP”, and alcogel/thermal treated with nitrogen method, shortened as “AN”), and Cu/ZrO₂ catalysts were prepared by impregnation method. The supports and catalysts were characterized by BET, XRD, TEM and TPR. The effects of the preparation methods of ZrO₂ supports and the treatment conditions (calcination and reduction temperatures) of the catalyst precursors on the texture structures of the supports and catalysts as well as on the catalytic performances of Cu/ZrO₂ in CO hydrogenation were investigated. The results showed that the support ZrO₂-AN had larger BET specific surface area, cumulative pore volume and average pore size than the support ZrO₂-CP. Cu/ZrO₂-AN catalysts showed higher CO hydrogenation activity and selectivity of oxygenates (C₁–C₄ alcohols and dimethyl ether) than Cu/ZrO₂-CP catalysts. Calcination and reduction temperatures of supports and catalyst precursors affected the catalytic performance of Cu/ZrO₂. The conversion of CO and the STY of oxygenates were 12.7% and 229 g/kg h, respectively, over Cu/ZrO₂-AN-550 at the conditions of 300 °C, 6 MPa.     

Low Temperature Selective CO Oxidation in Excess of H2 over Pt/Ce—ZrO2 Catalysts

Pt/Ce—ZrO₂ catalysts have been designed and applied to selective CO oxidation at low temperature. Both tetragonal and cubic phase Ce—ZrO₂ supports were prepared by co-precipitation method to get high surface area materials after calcination at 500 °C for 6 h in air. Selective CO oxidation was conducted using stoichiometric amounts of O₂. Cubic Ce—ZrO₂ supported Pt catalyst exhibited 78% CO conversion and 96% CO₂ selectivity even at 60 °C, while Pt/Al₂O₃ catalyst showed less than l% CO conversion at the same condition. The higher CO conversion and CO₂ selectivity (to CO₂ as opposed to H₂O) of Pt/Ce—ZrO₂ catalyst is mainly due to the high oxygen storage capacity of Ce—ZrO₂ and nano-crystalline nature of cubic Ce_0.8Zr_0.2O₂.     

Influence of oxygen and promoting effect of barium on the reduction of NOx by ethanol on Pd/ZrO2 catalyst

The NO reduction by ethanol over barium promoted Pd/ZrO₂ catalyst and the effect of the oxygen on the selectivity were studied. The catalysts were prepared by incipient wetness impregnation with 14.3% of Ba over zirconia and 1% of palladium. The specific surface areas were 58 and 47 m²/g and the dispersions of Pd were 37% and 30% for the Pd/ZrO₂ and Pd–Ba/ZrO₂ catalysts, respectively. The X-ray diffraction patterns indicate the presence of monoclinic zirconia phase on the support and BaCO₃, which is decomposed at 715 and 815 °C. Temperature programmed desorption profiles of NO on Pd/ZrO₂ and Pd–Ba/ZrO₂ catalyst showed a huge amount N₂ formation for the promoted Ba catalyst. Catalytic results showed high NO conversion even at low temperature, in accordance with the TPD results and an increasing selectivity to N₂ when compared with Pd/ZrO₂. The effect of O₂ in the NO_x reduction with ethanol provoked less NO dissociation and lower selectivity to methane.     

Influences of particle size and crystallinity of highly loaded CuO/ZrO2 on CO2 hydrogenation to methanol

In this study, highly loaded CuO (65.2 wt%) on ZrO₂ support was prepared by flame spray pyrolysis, and the effects of their particle sizes and ZrO₂ crystallinity on CO₂ hydrogenation to methanol were investigated. By varying the precursor feed rate (1–10 mL min⁻¹), the crystallite size (3–7 nm) and the crystallinity of tetragonal ZrO₂ were controlled. After H₂ reduction at 300°C, the Cu species in the catalysts, prepared at the feed rate = 2–10 mL min⁻¹, were converted to Cu particles (approximately 10–20 nm); however, the size and crystallinity of ZrO₂ remained the same. The activity and selectivity of the catalysts prepared at the feed rate = 2–3 mL min⁻¹ were higher than those of the catalysts prepared at the feed rates = 5–10 mL min⁻¹, because the smaller ZrO₂ particles in the former provided more surface to stabilize small Cu particles and from interfacial Cu-ZrO₂ sites (active sites).     

Influence of SBA-15 support on CeO2–ZrO2 catalyst for the dehydrogenation of ethylbenzene to styrene with CO2

Pure oxides of ceria (CeO₂) and zirconia (ZrO₂) were prepared by precipitation method and a catalyst comprising of 25 mol% of CeO₂ and 75 mol% of ZrO₂ (25CZ) mixed metal oxide was prepared by co-precipitation method and also a catalyst with 25 wt% of 25CZ (25 mol% of CeO₂ and 75 mol% of ZrO₂) and 75 wt% SBA-15(25/25CZS) was prepared by precipitation–deposition method. Aqueous NH₃ solution was used as a hydrolyzing agent for all the precipitation reactions. These catalysts were characterized by X-ray diffraction and nitrogen adsorption–desorption techniques for the confirmation of SBA-15 structural intactness. All these catalysts were found to be effective for the oxidative dehydrogenation of ethylbenzene (ODHEB) to styrene in the presence of CO₂ and also it was observed that there was a sequential enhancement in the catalytic activity from individual oxides to mixed oxides followed by supported mixed oxide catalysts. Of the catalysts studied in this work, the supported 25/25CZS catalyst exhibited the superior activity, which was about 10–20 times higher than the activity of bulk single oxides in terms of turn over frequency.     

Influence of zirconia crystal phase on the catalytic performance of Au/ZrO2 catalysts for low-temperature water gas shift reaction

The influence of crystal phase of zirconia on the performance of Au/ZrO₂ catalysts for low temperature water gas shift reaction was investigated. Au/ZrO₂ catalysts with pure tetragonal and monoclinic phases of ZrO₂ were prepared by the deposition-precipitation method with similar gold loading and dispersion. It was found that the Au/m-ZrO₂ catalyst showed much higher activity than that of the Au/t-ZrO₂ catalyst, which could be attributed to the higher CO adsorption capacity of the Au/m-ZrO₂ catalyst. The chemical state of gold that was strongly related to the pretreatment atmosphere also played an essential role in determining the catalytic activity for water gas shift reaction. Hydrogen and/or helium pretreated samples only contained Au⁰ species and exhibited higher activity than that of the sample pretreated with oxygen-containing atmosphere which resulted in the co-existence of Au⁰ and Au⁺ species. FTIR study further revealed that the formate species formed by the reaction of the adsorbed CO on gold nanoparticle with the hydroxyl groups on the surface of m-ZrO₂ acted as the most important intermediates for the water gas shift reaction.     

WGS activity of a novel Cu–ZrO2 catalyst prepared by a reflux method. Comparison with a conventional impregnation method

The activity in the WGS reaction of Cu/ZrO₂ catalysts prepared by a method of refluxing in an aqueous NH₄OH solution is studied. It is shown that at 3% Cu load the methods of impregnation over monoclinic or tetragonal ZrO₂ do not produce active catalysts for the WGS reaction. However, the method of refluxing generates highly active catalysts with Cu loads of 3% (w/w) or higher. The activity of the catalysts prepared by refluxing is associated with the formation of small Cu clusters, which would allow the regrouping of the H atoms to generate molecular H₂ in the presence of the crystalline tetragonal ZrO₂.     

Influence of acidic and basic properties of ZrO2 based catalysts on isosynthesis

Zirconium dioxide based catalysts were synthesized by mechanical mixing method and evaluated for the direct synthesis of isobutene and isobutane (i-C₄) from CO hydrogenation. The experiments of temperature-programmed desorption of ammonia and carbon dioxide indicated that both the amount of acid–base sites and the strength of acid and base of the catalysts varied with the incorporation of promoters including various calcium salts, Al₂O₃ and KOH in ZrO₂. The test results indicated that the activity and selectivity of the zirconia based catalysts depended on the acid–base properties of the catalysts. An appropriate amount of moderately strong acid–base sites and ratio of basic to acidic sites on the catalysts are significant for an active catalyst to selectively produce isobutene and isobutane from CO hydrogenation.     

Influence of the crystalline structure of ZrO2 on the activity of Cu/ZrO2 catalysts on the water gas shift reaction

The structure sensitivity of copper catalysts supported on monoclinic and tetragonal ZrO₂ was studied on the water gas shift (WGS) reaction. The results show that the activity of copper supported on tetragonal ZrO₂ is much higher than when supported on monoclinic ZrO₂. The obtained results favour an associative mechanism, which considers the formation of formate species on the surface of ZrO₂. The differences on the observed WGS activity are attributed to the stability of adsorbed formates on the different ZrO₂ polymorphs. It was found that the active Cu species for the WGS reaction is the bulk CuO.     

CO2 hydrogenation to methanol over Cu/ZnO/ZrO2 catalysts prepared via a route of solid-state reaction

Cu/ZnO/ZrO₂ catalysts were prepared by a route of solid-state reaction and tested for the synthesis of methanol from CO₂ hydrogenation. The effects of calcination temperature on the physicochemical properties of as-prepared catalysts were investigated by N₂ adsorption, XRD, TEM, N₂O titration and H₂-TPR techniques. The results show that the dispersion of copper species decreases with the increase in calcination temperature. Meanwhile, the phase transformation of zirconia from tetragonal to monoclinic was observed. The highest activity was achieved over the catalyst calcined at 400 °C. This method is a promising alternative for the preparation of highly efficient Cu/ZnO/ZrO₂ catalysts.     

Activity and Selectivity of a Nanostructured CuO/ZrO2 Catalyst in the Steam Reforming of Methanol

Steam reforming of methanol for production of hydrogen can be carried out over copper based catalyst. In the work presented here, the catalytic properties of a CuO/ZrO₂ catalyst (8.5wt%) synthesised by a templating technique were investigated with respect to activity, long term stability, CO formation, and response to oxygen addition to the feed. The results were obtained using a fixed bed reactor and compared to a commercial methanol synthesis catalyst CuO/ZnO/Al₂O₃. It is shown that, depending on the time on stream, the temporary addition of oxygen to the feed has a beneficial effect on the activity of the CuO/ZrO₂ catalyst. After activation, the CuO/ZrO₂ catalyst is found to be more active (per copper mass) than the CuO/ZnO/Al₂O₃ system, more stable during time on stream (measured up to 250h), and to produce less CO. Structural characterisation by means of X-ray powder diffraction (XRD) and X-ray absorption spectroscopy (XAS) reveals that the catalyst (as prepared) consists of crystalline, tetragonal zirconia with small domain sizes (about 60Å) and small/disordered crystallites of CuO.     

Partial Oxidation of Ethanol to Hydrogen over Ni–Fe Catalysts

A study has been conducted to identify the influence of zirconia phase and copper to zirconia surface area on the activity of Cu/ZrO₂ catalysts for the synthesis of methanol from either CO/H₂ or CO₂/H₂. To determine the effects of zirconia phase, a pair of Cu/ZrO₂ catalysts was prepared on tetragonal (t-) and monoclinic (m-) zirconia. The zirconia surface area and the Cu dispersion were essentially identical for these two catalysts. At 548 K, 0.65 MPa, and H₂/CO_x= 3 (x = 1, 2), the catalyst prepared on m-ZrO₂ was 4.5 times more active for methanol synthesis from CO₂/H₂ than that prepared on t-ZrO₂, and 7.5 times more active when CO/H₂ was used as the feed. Increasing the surface area of m-ZrO₂ and the ratio of Cu to ZrO₂ surface areas further increased the methanol synthesis activity. In situ infrared spectroscopy and transient-response experiments indicate that the higher rate of methanol synthesis from CO₂/H₂ over Cu/m-ZrO₂ is due solely to the higher concentration of active intermediates. By contrast, the higher rate of methanol synthesis from CO/H₂ is due to both a higher concentration of surface intermediates and the more rapid dynamics of their transformation over Cu/ZrO₂.     

Synthesis of the ZrO2–SiO2 microspheres as a mesoporous candidate material

A series of mesoporous ZrO₂?CSiO₂ microspheres with different amounts of silica were synthesized by a polymerization-induced colloid aggregation process, using zirconyl chloride and commercial SiO₂ colloids as the raw materials. The microspheres were characterized by scanning electron microscopy (SEM), N₂ adsorption?Cdesorption isotherms, X-ray powder diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). According to the SEM results, the ZrO₂ and ZrO₂?CSiO₂ microspheres had spherical morphologies and the sizes of the ZrO₂?CSiO₂ microspheres increased with increasing SiO₂/ZrO₂ weight ratios. The XRD spectrum of the pure ZrO₂ microspheres contained characteristic peaks for a monoclinic crystalline zirconia structure. The XRD spectrum of the ZrO₂?CSiO₂ microspheres showed a tetragonal crystalline structure. The specific surface areas of the ZrO₂?CSiO₂ microspheres increased with increasing SiO₂/ZrO₂ weight ratios, and the pore volumes also increased. The average pore size of the ZrO₂?CSiO₂ microspheres was 3?C5?nm. The FT-IR spectrum of the ZrO₂?CSiO₂ microspheres confirmed the formation of Zr-O-Si bonds. The Zr-O-Si bonds make the metastable tetragonal zirconia stable at room temperature.     

Performance of CuO/Al2O3 Catalysts Promoted by SnO2 and ZrO2 in the Selective Oxidation of Carbon Monoxide

The effect of added SnO₂ and ZrO₂ to CuO/Al₂O₃ catalysts was investigated with reference to the oxygen spillover phenomena in the selective oxidation of carbon monoxide. The TPR and TPO analyses indicated that SnO₂ and ZrO₂ addition caused oxygen migration and induced the formation of high concentrations of active oxygen species on the SnO₂ and ZrO₂ surface. The catalytic activities of SnO₂ and ZrO₂ supported CuO/Al₂O₃ catalysts were superior to that for CuO/Al₂O₃ catalysts in the selective oxidation of carbon monoxide. Oxygen, when absorbed to the SnO₂ and ZrO₂ surface can spill over to the CuO phase and easily react with carbon monoxide. Consequently, the addition of SnO₂ and ZrO₂ led to significantly improved activities. This can be attributed to the enhanced migration of oxygen to the catalyst surface.     

Catalyst deactivation and regeneration in low temperature ethanol steam reforming with Rh/CeO2–ZrO2 catalysts

Rh/CeO₂–ZrO₂ catalysts with various CeO₂/ZrO₂ ratios have been applied to H₂ production from ethanol steam reforming at low temperatures. The catalysts all deactivated with time on stream (TOS) at 350 °C. The addition of 0.5% K has a beneficial effect on catalyst stability, while 5% K has a negative effect on catalytic activity. The catalyst could be regenerated considerably even at ambient temperature and could recover its initial activity after regeneration above 200 °C with 1% O₂. The results are most consistent with catalyst deactivation due to carbonaceous deposition on the catalyst.     

Surface phase transformation of ZrO2 in VOx/ZrO2 catalysts for boosting propane nonoxidative dehydrogenation

The nanocatalysts of VO_x deposited on ZrO₂ supports with single monoclinic (ZrO₂-M), tetragonal (ZrO₂-T), and binary monoclinic-tetragonal (ZrO₂-MT) phase were synthesized. VO_x/ZrO₂-MT catalysts exhibit better performance during propane nonoxidative dehydrogenation than VO_x/ZrO₂-M and VO_x/ZrO₂-T catalysts. Among VO_x/ZrO₂-MT catalysts, the conversion and deactivation rate constant of VO_x/ZrO₂-M₃₁T₆₉ catalyst is 35.2% and 0.22 h⁻¹, respectively. The promoting role of ZrO₂-MT is revealed by experiments and theoretical calculations. The MT-mixed phase structure in VO_x/ZrO₂-MT catalyst improves the structural properties and dispersion of VO_x. The tetragonal-monoclinic transformation on the ZrO₂-MT surface facilitates VO_x reduction and produces additional V³⁺ active sites. The highly dispersed V³⁺ sites on the ZrO₂-MT surface accelerate C H bond breaking and boost the desorption of propylene, which is the key reason for enhancing activity and stability during the reaction, respectively. Insight into the role of surface phase transformation of ZrO₂-MT is expected to obtain high-efficient catalysts further.     

n-Butane Isomerization Over SO4 =/NiO/Al2O3/ZrO2 Catalysts. Effect of the Reaction Pressure and Metal Loading

The effect of alumina and nickel in sulfated ZrO₂ as a catalyst for n-butane isomerization was investigated. Samples were synthesized by supporting nickel sulfated zirconia on boehmite and then calcining the material. The crystalline structure of ZrO₂ was studied by X-ray powder diffraction and refined by the Rietveld method. Surface areas were determined by N₂ adsorption and BET analysis, while the acid properties were studied by NH₃ adsorption. The chemical reaction was carried out in a fixed-bed microreactor at 338 K under atmospheric (78 kPa) or 245 kPa total pressure. Results showed that either nickel or alumina improved the catalytic activity, but a synergic effect was observed when both components assisted. The catalytic activity was related to the relative content of tetragonal zirconia and acid site density. Alumina stabilized tetragonal zirconia increased the acid site density and presumably led to a better dispersion of nickel oxide. The catalytic activity could be related to both oxidation and acid sites produced by nickel. A bimolecular reaction mechanism helps explain the observed trends. The increase in the reaction rate would be explained by the increase in the rate of the initial step of dehydrogenation either caused by a better dispersion of nickel or higher operating pressure.     

Carbon monoxide hydrogenation on Fe2O3/ZrO2 catalysts

Fe₂O₃/ZrO₂ catalysts prepared by impregnation and coprecipitation methods were used for catalytic hydrogenation of CO. It was shown that the structure, reduction behavior of iron species, and catalytic properties of the catalysts were obviously affected by the preparation methods. For the Fe₂O₃/ZrO₂ catalyst prepared by the impregnation method, the Fischer-Tropsch catalytic activity and the selectivity to light olefins were much higher than those of the corresponding catalyst prepared by the coprecipitation method, the formation of methane was suppressed and the selectivity to light olefins was enhanced. Various intermediates formed during the successive steps of reduction of the catalysts were studied by using temperature-programmed reduction combined with in situ Mössbauer spectroscopy. The role of zirconia in the catalysts was discussed.