RESUMO
The rise in atmospheric CO2 concentration and the concomitant rise in global surface temperature have prompted massive research effort in designing catalytic routes to utilize CO2 as a feedstock. Prime among these is the hydrogenation of CO2 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 CO2 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 ß-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 m2g-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 CO2 conversion to methanol than on ZnO. Our approach is applicable to the discovery and design of improved catalysts for CO2 hydrogenation and will aid future catalyst discovery.
RESUMO
The influence of Fe speciation on the decomposition rates of N2O over Fe-ZSM-5 catalysts prepared by Chemical Vapour Impregnation were investigated. Various weight loadings of Fe-ZSM-5 catalysts were prepared from the parent zeolite H-ZSM-5 with a Si:Al ratio of 23 or 30. The effect of Si:Al ratio and Fe weight loading was initially investigated before focussing on a single weight loading and the effects of acid washing on catalyst activity and iron speciation. UV/Vis spectroscopy, surface area analysis, XPS and ICP-OES of the acid washed catalysts indicated a reduction of ca. 60% of Fe loading when compared to the parent catalyst with a 0.4 wt% Fe loading. The TOF of N2O decomposition at 600 °C improved to 3.99 × 103 s-1 over the acid washed catalyst which had a weight loading of 0.16%, in contrast, the parent catalyst had a TOF of 1.60 × 103 s-1. Propane was added to the gas stream to act as a reductant and remove any inhibiting oxygen species that remain on the surface of the catalyst. Comparison of catalysts with relatively high and low Fe loadings achieved comparable levels of N2O decomposition when propane is present. When only N2O is present, low metal loading Fe-ZSM-5 catalysts are not capable of achieving high conversions due to the low proximity of active framework Fe3+ ions and extra-framework É-Fe species, which limits oxygen desorption. Acid washing extracts Fe from these active sites and deposits it on the surface of the catalyst as FexOy, leading to a drop in activity. The Fe species present in the catalyst were identified using UV/Vis spectroscopy and speculate on the active species. We consider high loadings of Fe do not lead to an active catalyst when propane is present due to the formation of FexOy nanoparticles and clusters during catalyst preparation. These are inactive species which lead to a decrease in overall efficiency of the Fe ions and consequentially a lower TOF.
