Kinetics and mechanism of methanol decomposition over zinc oxide

Tawarah, Khalid M.; Hansen, Robert S.

doi:10.1016/0021-9517(84)90191-x

Cited by 32 publications

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References 67 publications

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“…Kung et al have used the single-crystal surfaces of ZnO to probe the decomposition of CH 3 OH; they showed that the ZnO(0001) surface is the dominant surface for this reaction where the main decomposition product was CO under the conditions of their experiment. The mechanism of methanol decomposition on the binary catalyst and ZnO has been extensively studied. − A formate surface species is thought to be a decomposition intermediate on both the binary catalyst and ZnO. Its relevance to the MSR relates to the CO 2 hydrogenation reaction , and its possible role in the water-gas shift reaction, where the binary catalyst is extremely effective in this reaction.…”

Section: Introductionmentioning

confidence: 99%

Electron Spectroscopic Studies of CH₃OH Chemisorption on Cu₂O and ZnO Single-Crystal Surfaces: Methoxide Bonding and Reactivity Related to Methanol Synthesis

et al. 1998

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Adsorption of CH 3 OH on Cu 2 O(111), ZnO(0001), and ZnO(1010) has been investigated with XPS, NEXAFS, variable-energy photoelectron spectroscopy (PES), and SCF-XR Scattered Wave (SW) molecular orbital calculations. At high coverage (g25.0L), CH 3 OH is adsorbed as molecular multilayers on all three surfaces. At low temperatures (140 K) and coverage (0-0.6L), CH 3 OH is deprotonated to form chemisorbed methoxide on all of the surfaces investigated. Under these conditions the C1s XPS peak positions are 289.5, 290.2, and 290.2 eV below the vacuum level, respectively. Annealing the CH 3 O -/Cu 2 O(111) surface complex to 523 K 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 (1010) surface. The NEXAFS spectrum of chemisorbed methoxide on the Cu 2 O(111) surface exhibits a σ* shape resonance at 294.8 eV giving a C-O bond length of 1.41 Å, a 0.02 Å 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 Å. 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 Cu 2 O(111) gives a four peak valence band spectrum, with features at 20.0 (3a′), 15.6 (3a′′, 5a′′), 14.0 (σ CO ), and 10.0 eV (π O , σ O ), below the vacuum level. SCF-XR-SW molecular orbital calculations indicate that the bonding between the Cu(I) site and the CH 3 O -is dominated by the π O , σ O , and σ CO levels, with a calculated σ charge donation from these levels into the empty Cu 4s and Cu 4p z levels of 0.4e. As a consequence of deprotonation and σ donation the carbon atom in CH 3 O -is calculated to be 0.085e more positive than gas-phase methanol. The variable-energy PES of CH 3 O -on ZnO(0001) also exhibits four methoxide peaks, at 21.0 (5a′), 16.7 (2a′′, 5a′), 13.6 (σ CO ), and 9.8 eV (π O , σ O ). However, the σ donation is calculated to be less than half that found for CH 3 O -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 C1s peaks and the greater C-O bond contraction. These results show that methoxide bonding to both Cu(I) and Zn(II) surface sites is dominated by σ donation. The electronic and geometric origins of the differences in bonding and reactivity among the Cu(I) and Zn(II) sites are addressed and provide insight into the molecular mechanism of the methanol synthesis reaction.

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