The dynamic restructuring of electrolytic silver during the formaldehyde synthesis reaction

The partial oxidation of methanol to formaldehyde was studied over an industrial electrolytic silver catalyst. Special attention was paid to the influence of reaction-induced restructuring of the silver surface and bulk on the activity for the partial oxidation of methanol to formaldehyde. Drastic differences are observed in the performance exhibited by fresh samples and those which have been previously treated at high temperature (T > 873 K) reaction conditions. The temperature dependence of the conversion for a fresh catalyst shows a pronounced hysteresis which can be separated into three temperature regimes. These regimes are strongly correlated with various morphological changes induced in the silver during reaction. SEM: images reveal that heating silver in an excess of methanol to 773 K results in the destruction of grain-boundary defects. Comparison of the various hysteresis profiles with TDS analysis of silver pretreated at high temperature in oxygen shows that elimination of these defects results in the inhibition of oxygen diffusion from the bulk to the surface in the temperature region between 673 K and 873 K, The formation of holes resulting from the reaction of bulk-dissolved hydrogen and oxygen indicates that diffusion along these defect structures supplies bulk oxygen for the catalytic reaction. Hole formation above 923 K is no longer restricted to grain boundaries, indicating that volume (interstitialcy) diffusion replaces grain-boundary diffusion at elevated temperatures. This incorporation of oxygen into the silver lattice results in an increased conversion of methanol with a higher conversion to formaldehyde. Consequently, no hysteresis is observed subsequent to having treated the sample in the reaction mixture at temperatures in excess of 873 K. Reaction-induced restructuring of the catalyst on the time scale of a typical run occurs. This leads to an improvement in the catalytic performance of silver for this extremely structure-sensitive reaction