Transformation of amorphous glassy metal/zirconium alloys into CO2 hydrogenation catalysts: an XPS, SIMS, and SNMS investigation

Abstract

Active catalysts for hydrogenation reactions starting from CO2 have been obtained from amorphous Cu70Zr30, Au25Zr75, Pd25Zr75 and Ni64Zr36 alloys. During in situ activation of these metallic glasses, oxidation and segregation phenomena are occurring, which lead to dispersed metal/zirconia catalysts. A combination of XPS and SNMS depth profiling is applied to examine changes in the oxidation state, as well as in the relative concentrations of the metal and of the zirconium components. From the chemical shifts of the metal 3d/Zr3p doublets analyzed as a function of sputtering time, the removal of an inactive surface ZrO2 layer by Ar ion bombardment, and the concomitant appearance of metallic Zr are demonstrated. Experiments on model substrates with defined layer structures show that the interpretation of depth profiles is most straightforward from the SNMS data. For Au25Zr75 and Pd25Zr75, the ZrO/Zr ratio at sputtering equilibrium increases significantly during transformation of the precursors into the respective metal/zirconia catalysts. The Ni64Zr36 alloy is not fully activated under identical conditions of pretreatment, such that the composition of the original bulk alloy is reached after prolonged sputtering. In situ activation experiments with Cu70Zr30 reveal a pronounced surface enrichment of metallic copper during exposure to CO2 hydrogenation conditions.