Oxidation reactions over metal surfaces by two different approaches

The presentation will cover two different endeavours into catalytic oxidation; methanol partial oxidation to formaldehyde (MTF) over (pure) silver and CO oxidation over different Pd75%Ag25% surface terminations. About half of the world’s production of formaldehyde (>25 Mt/year) proceeds via methanol oxidation over silver in the form of a shallow bed of Ag particles. Short residence time and product temperature quench facilitate yields up to 90%, but 8-10% of the feedstock is non-selectively oxidized to CO2. The catalysis is difficult to study at industrially relevant conditions. This is due to the high temperature (600-700°C), very fast reactions, homogeneous contributions, challenging product stability and product quantification, especially with steam co-feed (“water ballast process”), and interplay with a dynamic Ag catalyst surface and bulk morphology that involves dissolution of elemental O in the solid matrix. We have therefore proceeded along several directions, i.e., investigation of sub-reaction systems, detailed characterization (e.g., FIB/TEM), and adaptation of the annular reactor configuration that has been pioneered in the Polimi catalysis laboratory [1]. This has enabled new insight to the role of oxygen dissolution in Ag and the roles of CO/H2 oxidation, as well as reaction data at high temperature and low conversion without impact of mass transfer limitations or reactions in the gas phase [2,3]. Segregation and adsorption phenomena in Pd75%Ag25% single crystals as well as polycrystalline thin films have been studied to understand the dynamic interaction between the reaction environment and the surface structure and composition, with CO oxidation as the probe reaction. A combination of in-house characterization and performance testing and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) at the MAXIV HIPPIE Beamline (Lund, Sweden) has been used. The presentation will cover how alloying and segregation affects the reactivity of Pd75%Ag25% surfaces as compared to their pure Pd counterparts as well as the surface termination, (100) [4] vs. (111) [5]. This is done by combining the CO2 formation profiles (QMS) with overall changes to the XPS core level spectra, acquired in situ during temperature cycling according to specific protocols.