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

Abstract

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.

Interaction of Zinc Oxide Clusters with Molecules Related to the Sulfur Vulcanization of Polyolefins (“Rubber”)

“…This result is in agreement with spectroscopic studies on the chemisorption of MeOH on ZnO single-crystal surfaces (0001 and 1010), which at low coverage also show exclusively dissociative adsorption with deprotonation, even at 140 K. [20] No experimental binding energy was reported. In contrast to MeOH, the addition of methanethiol to the ZnO cluster gave two products: molecular adduct 10 and mixed hydroxo mercapto complex 10 a resulting from dissociative adsorption [Eqs.…”

“…Expectedly, the tetrahapto form is more stable but only slightly so [Eqs. (20) and (21)], since the bonding geometry at the two five-coordinate zinc atoms of 16 a is far from optimum ( Figure 5). …”

The surface chemistry of ZnO nanoparticles applied as heterogeneous catalysts in methanol synthesis

“…Differences in reactivity of the ZnO surfaces for the adsorption and decomposition of methanol [86][87][88][89][90] as well as formic acid [86,[91][92][93][94] on the polar and the non-polar surfaces have already been known for a long time. It was proposed that the interaction of methanol with any of the common ZnO surfaces readily led to the formation of methoxy (methoxide) species (CH 3 O) due to a heterolytic splitting of the O-H bond [87,90].…”

“…It was proposed that the interaction of methanol with any of the common ZnO surfaces readily led to the formation of methoxy (methoxide) species (CH 3 O) due to a heterolytic splitting of the O-H bond [87,90]. Only at low temperatures and high coverages the formation of molecular multilayers was observed using different electron spectroscopic techniques [90].…”

Fabrication of coaxial p-Cu2O/n-ZnO nanowire photodiodes

“…Using metalorganic chemical vapor deposition (MOCVD), Jeong et al reported the formation of 2D p-Cu 2 O/n-ZnO films with rectifying behavior recently [7]. It is known that Cu 2 O is a natural ptype direct-gap semiconductor with a cubic crystal structure and a room temperature bandgap energy of 2.1 eV [8]. It has also been predicated that Cu 2 O is promising for photovoltaic applications with theoretical energy conversion efficiency of 20% [9].…”