The effect of the support on the infrared spectra of carbon monoxide adsorbed on nickel oxides

Abstract

The influence of the support on the surface properties of nickel oxide has been studied experimentally by carbon monoxide adsorption characteristics. The latter were determined by the infrared spectra of the adsorbed species. The first type of nickel oxide studied was prepared by adding an excess of oxygen, at room temperature, to supported nickel. The excess oxygen was removed, and then carbon monoxide added at various pressures. This procedure was repeated for all the supports used: alumina, silica and titania. On the silica support, the oxide was also made by adding a series of small doses of oxygen to the nickel. Evidence for carbonate complexes was obtained for alumina supported nickel oxide, but not with titania or silica. On titania, CO 2 − groups were found which may have been formed by the interaction between catalytically formed carbon dioxide and the titania. On silica, neither CO 3 or CO 2 − complexes were found on adding CO to nickel oxide. Nevertheless, CO 2 − complexes have been observed when small amounts of oxygen were added to silica-supported reduced nickel, on which CO had previously been adsorbed. With alumina and silica, the nickel oxide was “reduced,” to some extent, by CO. The sites left vacant by “reduction” then adsorbed CO in a manner similar to one of the species observed on supported nickel. Bulk nickel oxide was prepared by heating supported nickel in oxygen at 300°C. In most cases the oxygen was removed from the sample at 300°C. In some other preparations, the sample was cooled in oxygen and then evacuated at room temperature. The spectra obtained on adding CO to these two types of bulk oxide were very similar. As in the nickel oxide on nickel experiments, on adding CO, CO 3 groups were found using alumina, and CO 2 − groups using titania. However, large differences were found in the number of CO molecules absorbed on nickel sites formed by “reduction”. In every case, those nickel oxide on nickel systems which were “reduced” by CO (the alumina and silica supports) were not “reduced” when the same supports were used with bulk oxides. Conversely, the titania-supported nickel oxide on nickel was only very slightly “reduced” on adding CO, while the bulk oxide on the support was “reduced” to quite a marked extent.