Chemistry of NO2 on Mo(110):: decomposition reactions and formation of MoO2

Synchrotron-based high-resolution photoemission and ab initio self-consistent field (SCF) calculations have been used to study the chemistry of nitrogen dioxide on Mo(110). The Mo surface reacts readily with nitrogen dioxide, decomposing the molecule into adsorbed NO and O adatoms at temperatures between 100 and 150 K. The majority of the NO produced further dissociates (NOads-->N-ads+O-ads) between 250 and 350 K. Dosing of NO2 at 300 K leads to full decomposition of the molecule and in addition O is deposited on the Mod(110) surface by the reaction: NO2,gas-->NOgas+O-ads. Two important factors make easy the dissociation of NO2 on Mo(110). The first one is a transfer of electrons from the metal into the 6a(1) orbital of NO2 that weakens the bonds within the molecule. The second one is the very large stability of the N and O atoms on Mo(110). The ab initio SCF calculations predict that on Mo(110) nitrogen dioxide probably adopts eta(2)-O,O or eta(2)-N,O bonding configurations as precursors for dissociation of the molecule, Molybdenum has a larger ability to cleavage N-O bonds than late transition metals (Rh, Pt, Pd, Ag, Au) but a lower ability than W. At high temperatures (similar to 1000 K), the reaction of NO2 with Mo(110) produces pure films (thickness > 20 Angstrom) of polycrystalline MoO2. Nitrogen dioxide is a much better oxidizing agent than molecular oxygen. The MoO2 films are good models for studying the chemical properties of molybdenum oxide surfaces, and can be used in the synthesis of Al2O3 (2Al + 1.5MoO(2)-->Al2O3 + 1.5Mo) or mixed-metal oxide systems. (C) 2000 Elsevier Science B.V. All rights reserved.