The formation of PdZn bimetallic alloys on ZnO, TiO and AlO supports was investigated, together with the effect of alloy formation on the CO hydrogenation reaction. The chemical vapour impregnation (CVI) method produced PdZn nanoparticles with diameters of 3-6 nm. X-ray photoelectron spectroscopy and X-ray diffraction revealed the changes in the structure of the PdZn alloy that help stabilise formate intermediates during methanol synthesis. PdZn supported on TiO exhibits high methanol productivity of 1730 mmol kg h that is associated with the high dispersion of the supported PdZn alloy.
The effect of sodium species on the physical and catalytic properties of Cu/ZnO catalysts derived from zincian georgeite has been investigated. Catalysts prepared with <100 ppm to 2.1 wt% Na, using a supercritical CO antisolvent technique, were characterised and tested for the low temperature water-gas shift reaction and also CO hydrogenation to methanol. It was found that zincian georgeite catalyst precursor stability was dependent on the Na concentration, with the 2.1 wt% Na-containing sample uncontrollably ageing to malachite and sodium zinc carbonate. Samples with lower Na contents (<100-2500 ppm) remained as the amorphous zincian georgeite phase, which on calcination and reduction resulted in similar CuO/Cu particle sizes and Cu surface areas. The aged 2.1 wt% Na containing sample, after calcination and reduction, was found to comprise of larger CuO crystallites and a lower Cu surface area. However, calcination of the high Na sample immediately after precipitation (before ageing) resulted in a comparable CuO/Cu particle size to the lower (<100-2500 ppm) Na containing samples, but with a lower Cu surface area, which indicates that Na species block Cu sites. Activity of the catalysts for the water-gas shift reaction and methanol yields in the methanol synthesis reaction correlated with Na content, suggesting that Na directly poisons the catalyst. In situ XRD analysis showed that the ZnO crystallite size and consequently Cu crystallite size increased dramatically in the presence of water in a syn-gas reaction mixture, showing that stabilisation of nanocrystalline ZnO is required. Sodium species have a moderate effect on ZnO and Cu crystallite growth rate, with lower Na content resulting in slightly reduced rates of growth under reaction conditions.
The rise in atmospheric CO 2 concentration and the concomitant rise in global surface temperature have prompted massive research effort in designing catalytic routes to utilize CO 2 as a feedstock. Prime among these is the hydrogenation of CO 2 to make methanol, which is a key commodity chemical intermediate, a hydrogen storage molecule, and a possible future fuel for transport sectors that cannot be electrified. Pd/ZnO has been identified as an effective candidate as a catalyst for this reaction, yet there has been no attempt to gain a fundamental understanding of how this catalyst works and more importantly to establish specific design criteria for CO 2 hydrogenation catalysts. Here, we show that Pd/ZnO catalysts have the same metal particle composition, irrespective of the different synthesis procedures and types of ZnO used here. We demonstrate that all of these Pd/ZnO catalysts exhibit the same activity trend. In all cases, the β-PdZn 1:1 alloy is produced and dictates the catalysis. This conclusion is further supported by the relationship between conversion and selectivity and their small variation with ZnO surface area in the range 6–80 m 2 g –1 . Without alloying with Zn, Pd is a reverse water-gas shift catalyst and when supported on alumina and silica is much less active for CO 2 conversion to methanol than on ZnO. Our approach is applicable to the discovery and design of improved catalysts for CO 2 hydrogenation and will aid future catalyst discovery.
Articles you may be interested inCO2 hydrogenation to methanol over Cu/CeO2 and Cu/ZrO2 catalysts: Tuning methanol selectivity via metal-support interaction Journal of Energy Chemistry 40, 22 (2020);In situ synthesis of biomass-derived Ni/C catalyst by self-reduction for the hydrogenation of levulinic acid to γ-valerolactone a b s t r a c t A novel pH gradient methodology was used to synthesise a series of Cu-ZrO 2 catalysts containing different quantities of Cu and Zr. All of the catalysts were highly selective to the desired product, γvalerolactone, and are considerably more stable than Cu-ZrO 2 catalysts prepared by other co-precipitation methods for this reaction. Characterisation and further investigation of these catalysts by XRD, BET, SEM and XPS provided insight into the nature of the catalytic active site and the physicochemical properties that lead to catalyst stability. We consider the active site to be the interface between Cu/CuO x and ZrO x and that lattice Cu species assist with the dispersion of surface Cu through the promotion of a strong metal support interaction. This enhanced understanding of the active site and roles of lattice and surface Cu will assist with future catalyst design. As such, we conclude that the activity of Cu-ZrO 2 catalysts in this reaction is dictated by the quantity of Cu-Zr interface sites.
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