The phenol steam reforming reaction towards H2 production on natural calcite

Abstract

The steam reforming of phenol towards H2 production was studied in the 650-800 °C range over a natural pre-calcined (air, 850 °C) calcite material. The effects of reaction temperature, water, hydrogen, and carbon dioxide feed concentrations, and gas hourly space velocity (GHSV, h-1) were investigated. The increase of reaction temperature in the 650-800 °C range and water feed concentration in the 40-50 vol% range were found to be beneficial for catalyst activity and H2-yield. A similar result was also obtained in the case of decreasing the GHSV from 85,000 to 30,000 h-1. The effect of concentration of carbon dioxide and hydrogen in the phenol/water feed stream was found to significantly decrease the rate of phenol steam reforming reaction. The latter was probed to be related to the reduction in the rate of water dissociation as evidenced by the significant decrease in the concentration of adsorbed bicarbonate and {single bond}OH species on the surface of CaO according to in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)-CO2 adsorption experiments in the presence of water and hydrogen in the feed stream. Details of the CO2 adsorption on the CaO surface at different reaction temperatures and gas atmospheres using in situ DRIFTS and transient isothermal adsorption experiments with mass spectrometry were obtained. Bridged, bicarbonate and unidentate carbonate species were formed under CO2/H2O/He gas mixtures at 600 °C with the latter being the most populated. A substantial decrease in the surface concentration of bicarbonate and {single bond}OH species was observed when the CaO surface was exposed to CO2/H2O/H2/He gas mixtures at 600 °C, result that probes for the inhibiting effect of H2 on the phenol steam reforming activity. Phenol steam reforming reaction followed by isothermal oxygen titration allowed the measurement of accumulated "carbonaceous" species formed during phenol steam reforming as a function of reaction temperature and short time on stream. An increase in the amount of "carbonaceous" species with reaction time (650-800 °C range) was evidenced, in particular at 800 °C (4.7 vs. 6.7 mg C/g solid after 5 and 20 min on stream, respectively). © 2009 Elsevier B.V. All rights reserved.