Restructuring of Ag catalysts for methanol to formaldehyde conversion studied using in situ X-ray ptychography and electron microscopy

Abstract

Dynamic restructuring of silver catalysts during the industrial conversion of methanol to formaldehyde leads to surface faceting and pinhole formation. Subsequent sintering under reaction conditions, followed by increased pressure drop and decreased catalyst activity requires catalyst bed replacement after several months of operation. This necessitates a comprehensive understanding of the bulk catalyst restructuring under exposure to different gas environments. In this work, Ag restructuring was studied at elevated temperatures under different reactive and inert gas environments. Bubble formation within catalysts of 5–8 μm thickness was visualized in real-time using in situ X-ray ptychography. Stepwise heating up to 650 °C in combination with imaging was used to determine the effect of temperature on silver restructuring. Dynamic changes within the catalyst were further quantified in terms of relative changes in mass on selected regions at a constant temperature of 500 °C. Quantitative assessment of dynamic changes in the catalyst resulting from bubble growth and movement revealed the influence of temperature, time, and gas environment on the degree of restructuring. Post-mortem scanning electron microscopy with energy-dispersive X-ray spectroscopy mapping confirmed the redistribution of material as a consequence of bubble rupture and collapse. The formation of pores and cavities under different gas environments was additionally confirmed using a fixed bed reactor, and subsequent examination using focused-ion beam milling, providing detailed analysis of the surface structure. This study demonstrates the unique advantage of correlative hard X-ray and electron microscopy for quantitative morphological studies of industrial catalysts.