Characterization of gold catalysts for methanol synthesis
The activity (per mass of catalyst) of supported gold catalysts across a range of reduction and oxidation reactions is significantly affected by the average crystallite size of the gold crystallites in the catalyst. Supported gold catalysts are most commonly characterized for particle size using TEM and XRD. Both of these methods can have a large degree of inaccuracy associated with them at low metal loadings and for catalysts containing small gold crystallites. In this study oxygen chemisorption was used as an additional method to characterize supported gold catalysts to complement electron microscope techniques. Agreement between the results from these different methods was obtained only with regard to the order of magnitude range of crystallite size. The oxygen chemisorption can be used to estimate the mass fraction of gold present as nano-crystallites (typically less than 1%) implying a large room for improvement in catalyst preparation technique. In this study a range of supported gold catalysts were prepared by ion exchange, varying a range of preparation variables, including gold concentration in the precursor solution, washing procedure using an aqueous ammonia solution, as well as drying and calcination procedures. The washing procedure and in particular the concentration of ammonia and the duration affected the final metal loading of the catalyst. TEM analyses show crystallite size distributions between 2-5nm for all catalysts excepting those which were not washed using an aqueous ammonia solution and which did not show any small crystallites. Only the total omission of the ammonia wash resulted in a significant change in the gold crystallite sizes observed on TEM-images. Further characterization with SEM showed that catalysts that appeared identical on the TEM-images also contained large 50-500nm crystallites. This additional method of characterisation using SEM allowed for the identification of significant differences between catalysts upon varying the preparation method. Catalyst drying was also shown to be a crucial step in the catalyst preparation method, with SEM images displaying only small well-distributed gold crystallites for catalysts dried in the rotary evaporator. Two of the catalysts were then tested for their activity and selectivity in the hydrogenation of CO or CO 2. Although it has been shown that the production of methanol from CO (and CO 2) can be catalysed by gold particles with crystallite sizes below 5nm (Haruta, 1997), this reaction has received comparatively little attention compared to the more extensively studied CO oxidation reaction. Testing was done over a range of temperatures (200 - 350Â°C) at a pressure of 30bar. The obtained methanol yields and selectivities are comparable to reported values in literature. The hydrogenation of CO 2 was shown to have higher yields and selectivities to methanol than the hydrogenation of CO over the same catalyst. The preparation of the catalyst was shown to have an effect on the activity and selectivity, with the catalyst dried in the rotary evaporator having a higher yield and selectivity to methanol, while also forming a larger variety of products than the catalyst dried in the oven.
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