Morphology and Activity of Electrolytic Silver Catalyst for Partial Oxidation of Methanol to Formaldehyde Under Different Exposures and Oxidation Reactions
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Electrolytic silver particles were studied in relation to its morphology changes under different reactive and non-reactive atmospheres, and its catalytic activity in oxidation of methanol to formaldehyde (MTF), carbon monoxide to carbon dioxide, and hydrogen to water. Scanning electron microscopy and X-ray diffraction (XRD) were applied to analyze structural changes in the silver catalyst after exposure or interaction with nitrogen, oxygen, methanol/water, carbon monoxide and hydrogen, applied either individually or in selected combinations, at temperatures approaching 700 °C. The as-received Ag catalyst consists of agglomerated, faceted, polycrystalline particles. These undergo massive morphological changes during MTF reaction conditions. It was found that Ag catalysts exposed to oxygen-free atmospheres (N2, H2/N2 and CH3OH/H2O/N2) at 650 °C exhibit minimal changes in surface morphology compared to the fresh catalyst, while severe restructuring occurs on the mesoscopic scale under oxygen containing atmospheres (O2/N2, H2/O2/N2 and CO/O2/N2) at elevated temperature. This restructuring renders a smoothened surface with refacetted areas and many pinholes, while a small primary crystallite size (~ 40 nm, XRD) is maintained. Such pinholes are commonly described as a result of sub-surface oxygen/hydrogen/hydroxyl interactions. Here, they are present in all samples exposed to oxygen, indicating that presence of hydrogen is not prerequisite. For the CO and H2 oxidation sub-systems, the initial activity was comparable. But, while the conversion of H2 is preserved during 70 h time on stream, the CO conversion gradually reduces from 70 to 10%. This suggests that the restructuring associated with dissolution of O at high temperature inhibits the CO to CO2 pathway.