Coadsorption studies of CO and H2 on ZnO

Abstract

We have studied the adsorption of pure CO and CO:H2 mixtures on powdered ZnO using the combined techniques of transmission infrared spectroscopy and temperature programmed desorption (TPD). When CO is adsorbed alone, the vibrational frequency ωCO decreases from 2192 to 2178 cm−1 with increasing CO coverage, and a repulsive CO–CO interaction is observed in the TPD spectrum. When CO is adsorbed on an H2-covered surface, ωZnH decreases from 1709 to 1653 cm−1, ωOH increases from 3490 to 3523 cm−1, and the zero-coverage limit of ωCO increases from 2191 to 2196 cm−1. There is also an increase in the CO adsorption energy due to an attractive CO–H2 interaction. Analysis of TPD spectra for CO yields an expression for the CO adsorption energy as a function of CO and H2 coverage: ΔHCOads (kcal/mol) = 12.2−0.16 nCO+0.08 nH2, where nCO and nH2 are the coverages in μmol/gm. We attribute the coverage dependence of the CO adsorption energy, as well as the observed IR frequency shifts, to both electrostatic and chemical interactions between adsorbates. The former arise from dipole interactions between neighboring species, while the latter are due to ’’through-substrate’’ inductive effects associated with the electron donating/withdrawing properties of each adsorbate.