Abstract
Au nanoparticles supported on ZrO2 enhance its surface acidic/basic properties to produce a high yield of methanol via the hydrogenation of CO2. Amorphous ZrO2-supported 0.5–1 wt.% Au catalysts were synthesized by two methods, namely deposition precipitation (DP) and impregnation (IMP), characterized by a variety of techniques, and evaluated in the process of CO2 hydrogenation to methanol. The DP-method catalysts were highly advantageous over the IMP-method catalyst. The DP method delivered samples with a large surface area, along with the control of the Au particle size. The strength and number of acidic and basic sites was enhanced on the catalyst surface. These surface changes attributed to the DP method greatly improved the catalytic activity when compared to the IMP method. The variations in the surface sites due to different preparation methods exhibited a huge impact on the formation of important intermediates (formate, dioxymethylene and methoxy) and their rapid hydrogenation to methanol via the formate route, as revealed by means of in situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) analysis. Finally, the rate of formation of methanol was enhanced by the increased synergy between the metal and the support.
Highlights
The production of carbon-based pollutants through the utilization of fossil fuels has increased enormously since the industrial revolution
Both catalysts synthesized by the deposition precipitation (DP) method showed a high conversion and selectivity of methanol compared to the catalyst synthesized by the IMP method
The rate of methanol formation and the space–time yield (STY) of methanol are reported in Figure 14; it can be observed that the values of both parameters are appreciably higher for the 0.5 Au/ZrO2 DP catalyst compared to the 0.5 Au/ZrO2 IMP solid, which is in agreement with the above discussion
Summary
The production of carbon-based pollutants through the utilization of fossil fuels has increased enormously since the industrial revolution. Another study based on different supported oxides with Au shows the importance of the selection of a support to promote the reaction [38]. Mechanistic studies in the presence of Au-based catalysts were conducted using different techniques, such as in situ DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) and near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS), as well as DFT analyses; it was reported that the CO2 hydrogenation to methanol reaction occurs through the formation of surface intermediates, namely formate and methoxy species [9,39,40,41,42]. The formation of a formate intermediate occurs rapidly over reducible supports [41], but further hydrogenation steps depend on the above-stated factors, such as the size of active metal ensembles and their acidic/basic properties.
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