Reaction of formaldehyde over birnessite catalyst: A combined XPS and ToF-SIMS study

Abstract : Birnessite with very high surface area (>180 m2 g−1) has been prepared by oxidation of Mn(NO3)2 with H2O2 in KOH solution. The catalytic performance of this free noble metal based material for the formaldehyde (HCHO) selective conversion into CO2 is excellent. Therefore this birnessite material has been selected for X-ray photoelectron spectroscopy (XPS) analysis in combination with time-of-flight secondary ion mass spectroscopy (ToF-SIMS) study to understand the mechanistic interaction between adsorbate and adsorbent. Thermo-desorption of formaldehyde saturated birnessite has been conducted under argon atmosphere, using a catalysis cell allowing the monitoring of the birnessite surface modification. XPS study shows (i) the partial oxidation of formaldehyde at room temperature through the formate species formation and manganese species reduction and (ii) the generation of carbonate species with temperature. ToF-SIMS analyses gave more insight in the kind of cations from birnessite interacting with adsorbed molecules: formate ions interact with manganese and potassium ions while carbonate ions interact only with potassium ions. Formate oxidation takes place on Mn ions to give COx(g) species while formate ions readily decompose on K+ sites at higher temperature.