Rare-earth - copper alloys as catalyst precursors in the synthesis of methanol
A wide range of surface sensitive techniques (AES, LEED, A^, XPS, UPS and TDS) have been employed to study ultralhin films of copper with neodymium and the intimate interaction of neodymium oxide with a copper surface. This has involved the evaporation of the rare earth onto well defined surfaces of single crystal copper - Cu(100) and Cu(lll) -under ultrahigh vacuum conditions. Such systems may serve as a model for the fundamental processes occurring at the surface of methanol synthesis catalysts derived from intermetallic compounds of copper and rare earth metals. The work aims to characterise the films with regard to structure, electronic interactions, chemisorption properties and reactivity. It is shown that the deposition conditions determine the morphology and properties of the thin films produced; more specifically, at 300 K the systems initially evolved by Frank-van de Merwe (layer-by-layer) growth whereas at elevated temperatures, a stable surface alloy is formed. The effect of exposure to various gases related to the catalytic chemistry of the bulk alloys (H2, CO, O2, CO2, H2O) is discussed and the properties of the oxidized and unoxidized systems compared. The activation of bulk Ce-Cu and Nd-Cu precursors for methanol synthesis catalysts has been investigated by in situ XRD observations, with concurrent measurements of methanol activity, and using a new high pressure microreactor system. The formation of certain intermediate hydride phases is crucial to the eventual production of highly active catalysts and the methanol activity does not correlate with Cu surface area. The role of CO2 as a catalyst poison and the implications of the experimental observations with respect to the reaction mechanism of these novel catalysts are discussed and compared with the properties of conventional Cu/ZnO/Al2/O3 catalysts.