ZnO Supports Research Articles

Enhanced catalytic performance of Au/CuO–ZnO catalysts containing low CuO content for preferential oxidation of carbon monoxide in hydrogen-rich streams for PEMFC

In this study, effects of Au and CuO loadings in Au/CuO–ZnO nanocatalysts for preferential oxidation of carbon monoxide in H2-rich streams (PROX) are investigated. CuO–ZnO supports were synthesized by a co-precipitation method. Au was also incorporated into the catalysts by a deposition-precipitation procedure. The catalysts were characterized by XRD, BET surface area, FESEM, HRTEM, H2-TPR, FTIR, and CO-TPD. 2–10 nm Au nanoparticles are dispersed on CuO–ZnO support and significantly enhance the reducibility of CuO. The Au/CuO–ZnO catalysts containing low amount of CuO were found to be more active for PROX compared to the Au/ZnO catalyst. Moreover, as more CuO is added to Au/ZnO, the CO2 selectivity increases in the whole PROX temperature range. The catalyst containing 2 wt% Au and 1 wt% CuO on ZnO exhibited the highest activity and selectivity in the operating temperature range of PEM fuel cells. The activity of this catalyst also remained almost intact during 900 min of PROX time on stream at 80 °C.

High CO2 Selectivity of ZnO Powder Catalysts for Methanol Steam Reforming

To investigate the contribution of the ZnO supports in PdZn/ZnO catalysts used for methanol steam re-forming (MSR), we have investigated methanol steam re-forming on two different ZnO powder samples. The powder samples included a commercial powder with both polar and nonpolar surfaces exposed, and a plate-like sample preferentially exposing the polar ZnO(001)/ (001̅) surfaces. ZnO was found to be active and selective for MSR, with the polar surfaces being most active, but the activation energy for reaction (∼100 kJ/mol) was much larger than on unsupported PdZn(∼50 kJ/mol) or PdZn/ZnO. The experimental findings are supported by DFT calculations on key elementary steps and reaction intermediates on the ZnO(001) surface. We conclude that the contribution of the ZnO support to the MSR activity PdZn/ZnO catalyst system may not be very significant.

Synthesis of a Novel Nano-Sized Pt/ZnO Catalyst for Water Gas Shift Reaction in Medium Temperature Application

A novel nano-sized Pt/ZnO catalyst has been prepared by impregnating 1 wt% Pt on a nano-sized ZnO supports, with spherical and rod-type morphology and tested for the water gas shift reaction at a gas hourly space velocity of 9,583 h−1. The Pt/ZnO catalyst with spherical ZnO morphology exhibited an almost thermodynamic equilibrium CO conversion value of 70 % at 400 °C with 100 % CO2 selectivity. The high activity/stability of the catalyst is due to a good interfacial contact between Pt and ZnO. A novel nano-sized Pt/ZnO catalyst has been prepared by impregnating 1 wt% Pt on a nano-sized ZnO supports, with spherical and rod-type morphology and tested for the water gas shift reaction at a gas hourly space velocity of 9,583 h−1. The Pt/ZnO catalyst with spherical ZnO morphology exhibited an almost thermodynamic equilibrium CO conversion value of 70 % at 400 °C with 100 % CO2 selectivity. The high activity/stability of the catalyst is due to a good interfacial contact between Pt and ZnO.

Deposition of gold nanoparticles on ZnO and their catalytic activity for hydrogenation applications

Gold nanoparticles have been deposited on zinc oxide by deposition–precipitation methods. The selection of the support characteristics is critical to control gold nanoparticle sizes and surface properties. Two ZnO supports have been studied, one is commercially obtained and the second is lab prepared by chemical vapor deposition resulting in tetrapod morphologies. In the latter two gold loadings were incorporated. These catalysts were mainly characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Also the three samples were tested for the selective hydrogenation of cinnamaldehyde. We have shown that the use of AuZnO as catalyst permits higher selectivity in desired products than that observed using alternative catalysts. The selectivity values which vary among 58% and 99% depending on the surface structure of the gold nanoparticles induce steric constraints between the phenyl group of the substrate and the metal surface of large particles. These catalysts show also good stability without detectable sintering.

Catalytic performance of Au/ZnO nanocatalysts for CO oxidation

Au was loaded (1% wt.) on three different ZnO supports, by three different methods: photodeposition, double impregnation and ultrasonication. Two commercial ZnO supports were used, along with a ZnO sample consisting of nanosized single crystals prepared by chemical vapour deposition. The samples were extensively characterised by various techniques. CO oxidation was used as a test reaction to compare the catalytic activity. The best results were obtained with ZnO prepared by chemical vapour deposition with Au loaded by ultrasonication that showed an activity of 2 mol CO g Au - 1 h - 1 at room temperature. High-resolution electron microscopy images of the sample showed a number of gold nanocrystals epitaxially oriented on the ZnO rod. The unique catalyst–substrate interaction can be related to the increased catalytic activity of this sample, which was not observed for any of the others.

Co/ZnO and Ni/ZnO catalysts for hydrogen production by bioethanol steam reforming. Influence of ZnO support morphology on the catalytic properties of Co and Ni active phases

Renewable hydrogen production from steam reforming of bioethanol is an interesting approach to produce sustainable hydrogen. However, simultaneous competitive reactions can occur, decreasing the hydrogen production yield. To overcome this problem, modifications in the steam reforming catalysts are being studied. Ni and Co active phases supported over modified ZnO have been widely studied in hydrogen production from steam reforming of bioethanol. However, the influence of the morphology and particle size of ZnO supports on the catalytic behaviour of the supported Ni and Co has not been reported. In the present work, we show how the morphology, shape, and size of ZnO support particles can control the impregnation process of the metal active centres, which manages the properties of active metallic particles. It has been found that nanorod particles of ZnO, obtained by calcination of Zn acetate, favour the metal-support interactions, decreasing the metallic particle sizes and avoiding metal (Co or Ni) sinterization during the calcination of metal precursors. Small metallic particle sizes lead to high values of active metal exposure surface, increasing the bioethanol conversion and hydrogen production.

Selective Hydrogenolysis of Glycerol to Propylene Glycol on Cu–ZnO Composite Catalysts: Structural Requirements and Reaction Mechanism

Cu-ZnO catalysts were prepared by homogeneous coprecipitation with varying Cu/Zn atomic ratios (0.4-2:1). The catalysts were examined in selective hydrogenolysis of glycerol to propylene glycol. Although propylene glycol selectivities remained essentially constant (over 93%) on the different Cu-ZnO catalysts, the turnover frequencies changed markedly with the Cu/Zn ratio, and reached the greatest value at the ratio of 1:1. Such activity dependence on the Cu/Zn ratio was in parallel with the change in the interaction between Cu and ZnO and in the Cu microstrain, as a consequence of the effects on the crystalline phases of hydroxycarbonate precursors for the Cu-ZnO catalysts, reflecting the structural requirements for glycerol hydrogenolysis. The hydrogenolysis mechanism was also examined and apparently involves kinetically relevant glycerol dehydrogenation to glyceraldehyde on Cu-ZnO and subsequent glyceraldehyde dehydration and hydrogenation to propylene glycol. The mechanism is consistent with the observed superior activities of the more strained Cu particles, and the higher propylene glycol selectivities obtained at higher glycerol and hydrogen concentrations. These findings provide a rationale for the design of more effective Cu-based catalysts in selective hydrogenolysis of glycerol and other biomass-derived polyols, for example, by synthesis of highly strained Cu particles strongly interacting with ZnO or other oxide supports.

Lithium-promoted hydrogenation of carbon dioxide to formates by heterobimetallic hydridozinc alkoxideclusters

The remarkably distinct reactivity of hydridozinc heterobimetallic cubanes [(HZnOtBu)4-n(thf LiOtBu)n] 1a-1d towards CO2 is reported--the hydride transfer from Zn-H to CO2 is drastically accelerated in the presence of Li ions in 1b-1d which led to the respective metal formate hydrates; the systems are inspiring models for the selective conversion of water gas into formates on lithium-promoted ZnO supports.

Highly effective conversion of CO2 to methanol over supported and promoted copper-based catalysts: influence of support and promoter

In this study, gallium-promoted copper-based catalysts prepared by impregnation methods on silica and ZnO supports, were examined for the hydrogenation of CO2 to methanol. The surface characteristics of catalysts depended on the support and were related to their catalytic performance. Silica-supported catalysts tested at reaction temperatures between 523 and 543K were highly selective and stable. The selectivity to methanol was around 99%, the conversion to CO was very low and negligible amounts of hydrocarbons were formed. The use of hydrophobic silica enhanced the performance of the catalyst in terms of activity, selectivity and stability. The modification of properties of copper particles is related to the presence of very small particles of Ga2O3 on the surface.

Selective synthesis of alcohols from syngas and hydroformylation of ethylene over supported cluster-derived cobalt catalysts

Hydroformylation of ethylene and CO hydrogenation were studied over cobalt-based catalysts derived from reaction of Co2(CO)8 with ZnO, MgO and La2O3 supports. At 433 K a similar activity sequence was reached for both reactions: Co/ ZnO > Co/La2O3 > Co/MgO. This confirms the deep analogy between hydroformylation and CO hydrogenation into alcohols. In the CO hydrogenation the selectivity towards alcohol mixture (C1-C3) was found to be near 100% at 433 K for a conversion of 6% over the Co/ZnO catalyst; this catalyst showed oxo selectivity higher than 98% in the hydroformylation of ethylene. Magnetic experiments showed that no metallic cobalt particles were formed at 433 K. It is suggested that the active site for the step that is common to both reactions is related to the surface homonuclear Co2+/[Co(CO)4]− ion-pairing species.