Electron spectroscopic studies of CH3OH chemisorption on Cu2O and ZnO single-crystal surfaces:: Methoxide bonding and reactivity related to methanol synthesis

Adsorption of CH3OH on Cu2O(111), ZnO(0001), and ZnO(10 (1) under bar 0) has been investigated with XPS, NEXAFS, variable-energy photoelectron spectroscopy (PES), and SCF-X alpha Scattered Wave (SW) molecular orbital calculations. At high coverage (greater than or equal to 25.0L), CH3OH is adsorbed as molecular multilayers on all three surfaces. At low temperatures (140 K) and coverage (0-0.6L), CH3OH is deprotonated to form chemisorbed methoxide on all of the surfaces investigated. Under these conditions the Cls XPS peak positions are 289.5, 290.2. and 290.2 eV below the vacuum level, respectively. Annealing the CH3O-/Cu2O(111) surface complex to 523 It produces no other surface intermediate. Alternatively, at temperatures above 220 K on the ZnO(0001) surface methoxide decomposes to produce a formate intermediate that is stable at the methanol synthesis reaction temperature (523 K). No formate surface intermediate is observed on the ZnO(10 (1) under bar 0) surface. The NEXAFS spectrum of chemisorbed methoxide on the Cu2O(111) surface exhibits a sigma* shape resonance at 294.8 eV giving a C-O bond length of 1.41 Angstrom, a 0.02 Angstrom contraction from the gas-phase methanol value. Methoxide chemisorbed on the ZnO(0001) surface is found to have a NEXAFS determined C-O bond length of 1.39 Angstrom. These bond length contractions of the chemisorbed methoxide are due to the greater polarization of the C-O bond upon deprotonation and surface bonding. Variable-energy PES of the chemisorbed methoxide on Cu2O(111) gives a four peak valence band spectrum, with features at 20.0 (3a'), 15.6 (3a ", 5a "), 14.0 (sigma(CO)), and 10.0 eV (pi(0), sigma(0)), below the vacuum level. SCF-X alpha-SW molecular orbital calculations indicate that the bonding between the Cu(I) site and the CH3O- is dominated by the pi(0), sigma(0), and sigma(CO) levels, with a calculated sigma charge donation from these levels into the empty Cu 4s and Cu 4p(z) levels of 0.4e. As a consequence of deprotonation and a donation the carbon atom in CH3O- is calculated to be 0.085e more positive than gas-phase methanol. The variable-energy PES of CH3O- on ZnO(0001) also exhibits four methoxide peaks, at 21.0 (5a'), 16.7 (2a ", 5a'), 13.6 (sigma(CO)), and 9.8 eV (pi(0), sigma(0)). However, the a donation is calculated to be less than half that found for CH3O- on the Cu(I) site (0.12e) and with 0.015 greater positive charge on the carbon atom, consistent with the relative binding energies of the Cls peaks and the greater C-O bond contraction. These results show that methoxide bonding to both Cu(I) and Zn(LI) surface sites is dominated by a donation. The electronic and geometric origins of the differences in bonding and reactivity among the Cu(I) and Zn(LI) sites are addressed and provide insight into the molecular mechanism of the methanol synthesis reaction.