ELECTRON SPECTROSCOPIC STUDIES OF CH3OH CHEMISORPTION ON CU2O AND ZNOSINGLE-CRYSTAL SURFACES - METHOXIDE BONDING AND REACTIVITY RELATED TOMETHANOL SYNTHESIS
Pm. Jones et al., ELECTRON SPECTROSCOPIC STUDIES OF CH3OH CHEMISORPTION ON CU2O AND ZNOSINGLE-CRYSTAL SURFACES - METHOXIDE BONDING AND REACTIVITY RELATED TOMETHANOL SYNTHESIS, Journal of the American Chemical Society, 120(7), 1998, pp. 1506-1516
Adsorption of CH3OH on Cu2O(111), ZnO(0001), and ZnO(10 (1) under bar
0) has been investigated with XPS, NEXAFS, variable-energy photoelectr
on spectroscopy (PES), and SCF-X alpha Scattered Wave (SW) molecular o
rbital 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. Un
der these conditions the Cls XPS peak positions are 289.5, 290.2. and
290.2 eV below the vacuum level, respectively. Annealing the CH3O-/Cu2
O(111) surface complex to 523 It produces no other surface intermediat
e. 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 su
rface 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 lengt
h of 1.41 Angstrom, a 0.02 Angstrom contraction from the gas-phase met
hanol value. Methoxide chemisorbed on the ZnO(0001) surface is found t
o have a NEXAFS determined C-O bond length of 1.39 Angstrom. These bon
d length contractions of the chemisorbed methoxide are due to the grea
ter polarization of the C-O bond upon deprotonation and surface bondin
g. 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 indic
ate 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) l
evels of 0.4e. As a consequence of deprotonation and a donation the ca
rbon atom in CH3O- is calculated to be 0.085e more positive than gas-p
hase methanol. The variable-energy PES of CH3O- on ZnO(0001) also exhi
bits four methoxide peaks, at 21.0 (5a'), 16.7 (2a '', 5a'), 13.6 (sig
ma(CO)), and 9.8 eV (pi(0), sigma(0)). However, the a donation is calc
ulated 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, consis
tent with the relative binding energies of the Cls peaks and the great
er 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 el
ectronic and geometric origins of the differences in bonding and react
ivity among the Cu(I) and Zn(LI) sites are addressed and provide insig
ht into the molecular mechanism of the methanol synthesis reaction.