Influence of catalyst morphology on the performance of electrolytic silver catalysts for the partial oxidation of methanol to formaldehyde Original Research Article

Scanning electron microscopy (SEM), specific surface area measurements, in situ Raman spectroscopy and steady-state microreactor experiments were used to explore the relationship between silver catalyst morphology and performance in the titled reaction. Two electrolytic silver catalysts, obtained from different commercial suppliers, were examined in relation to their physico-chemical and catalytic properties. The work focused on the activation behaviour of the catalysts under conditions of industrial CH2O synthesis (ca. 923–953 K, 1 atm). The results show that the silver catalysts, denoted A and B, differed enormously in their surface morphology, which in turn impacted on their oxygen chemisorption behaviour and initial catalytic properties. Silver catalyst A possessed a high grain boundary density in its surface, and exhibited enhanced initial activity and selectivity to CH2O during CH3OH oxidation at 598–973 K compared to catalyst B, which possessed significantly lower surface area and roughness. Performance differences correlated strongly with the speciation of oxygen on the catalysts during CH3OH oxidation, which is discussed in terms of a framework comprising three chemically distinct atomic oxygen species: two surface species (denoted Oα and Oγ) and a bulk-dissolved species (denoted Oβ). The population of each state was found to be both temperature and structure-sensitive. The weakly bound Oα species was dominant at low temperatures and opened reaction pathways towards CH2O, HCOOCH3, CO2 and HCOOH. At temperatures of industrial CH2O manufacture (ca. 923 K), the strongly bound Oγ species was the dominant oxygen state and selectively activated the catalysts for the oxidative-dehydrogenation of CH3OH to CH2O+H2O. The dissociative chemisorption of O2 and the surface segregation of Oβ served to replenish the Oα and Oγ consumed by reaction. The CH2O yield on both catalysts increased with temperature up to 923 K, reflecting accompanying increases in the surface Oγ/Oα ratio. Grain boundaries facilitated the formation of Oγ and Oβ, which explained why catalyst A exhibited the higher initial activity and selectivity to CH2O. The performance of both catalysts, in particular catalyst B, improved with time-on-stream at 923 K before stabilising after approximately 48 h of operation. These improvements were due to reaction-induced changes in catalyst morphology, which created structures that promoted Oβ and Oγ formation. Results were used to develop reaction schemes for CH3OH oxidation on fresh and aged silver catalysts under industrial conditions. The critical role of defects in the silver catalyst structure, such as grain boundaries in the efficient ‘start up’ operation of electrolytic silver catalysts for CH2O production is demonstrated.