Relating methanol oxidation to the structure of ceria-supported vanadia monolayer catalysts

摘要

Vanadia "monolayer"-type catalysts supported on reducible oxides such as ceria previously have shown high activity for the selective oxidation of alcohols. Here, a model system consisting of vanadia particles deposited on well-ordered CeO_2(1 1 1) thin films has been employed. Scanning tunneling microscopy (STM), photoelectron spectroscopy (PES), and infrared reflection absorption spectroscopy (IRAS) were used to characterize the VO _x/CeO_2 surface as a function of vanadia loading. The formation of isolated monomeric species as well as two-dimensional vanadia islands that wet the ceria support was directly observed by STM. The vanadia species exhibit V in a +5 oxidation state and expose vanadyl (VO) groups with stretching vibrations that blue-shift from ～1005 cm~(-1) to ～1040 cm~(-1) with increasing coverage. Temperature programmed desorption (TPD) of methanol revealed three peaks for formaldehyde production. One is correlated with reactivity on the ceria support (565-590 K). Another is correlated with reactivity on large vanadia particles (475-505 K) similar to that previously observed on vanadia/silica and vanadia/alumina model systems. A low temperature reaction pathway (～370 K) is observed at low coverage, which is assigned to the reactivity of isolated vanadia species surrounded by a reduced ceria surface. It is concluded that strong support effects reported in the literature for the real catalysts are likely related to the stabilization of small vanadia clusters by reducible oxide supports.