The Influence of Hydrogen Treatment and Catalyst Morphology on the Interaction of Oxygen with a Silver Catalyst

The interaction of an unsupported silver catalyst which had been pretreated by hydrogen at various temperatures with oxygen at 210°C has been studied using Temperature Programmed Reduction (TPR) over a temperature range up to 900°C. Hydrogen treatment at 500°C or above before the oxidation step causes the formation of extra species, thought to be OH groups in the sub-surface of the sample. A peak in the spectra attributable to oxygen strongly bound in the vicinity of surface defects is found to be dependent on the surface roughness and grain size of the silver sample used; hydrogen pretreatment causes the strongly' bound oxygen in the vicinity of surface.defects to be converted to subsurface OH. It is also shown that the TPR measurements themselves influence the morphology of the sample and that these changes are comparable with structural changes which occur during the use of the catalysts for oxidative dehydrogenation of methanol. It is suggested that these structural changes are caused by the interaction of the sub-surface of the silver with both oxygen and hydrogen. INTRODUCTION Silver is widely used as catalyst for the production of formaldehyde by oxidative dehydrogenation of methanol. Under industrial conditions, 600°C and atmospheric pressure, almost total conversion of methanol is achieved with a selectivity of 90% towards formaldehyde, as reported by Sperber [I]. We have recently shown that interaction of oxygen with the silver is also of importance in methanol oxidation; the influences of temperature and reactant concentrations on the conversion to the different products (H2, CO, C02, CH20, H20) can be explained with the suggestion that total oxidation takes place at sites associated with weakly bound atomic surface oxygen whereas the selective reaction involves surface oxygen which is more strongly bound in the vicinity of surface defects [2]. Evidence for the existence of these species was obtained with help of temperature programned reduction (TPR) and temperature programmed desorption (TPD); the activity of the strongly bound oxygen species for the dehydrogenation of methanol was shown by using temperature programmed reaction spectroscopy (TPRS) with methanol vapour C31. It was also shown [3] that hydrogen treatment at 900°C strongly influences the interaction with oxygen: a new 0166-9834/87/$03.50