The steam reforming of phenol over natural calcite materials

Abstract

Five natural calcite materials of different geographical origin were studied towards the steam reforming of phenol reaction and the activity results obtained were correlated with some of their physico-chemical properties. It was found that the specific catalytic activity (μmol/(m2 s)) of the calcite material following calcination at 850 °C was not correlated with the BET area and the primary crystal size of calcium oxide (CaO). The metal impurity levels of the natural calcite materials investigated were found to be very low as evidenced by EDX and XPS measurements, leading to the conclusion that their effect on the catalytic chemistry of phenol steam reforming and surface adsorption properties of CaO was minor. These results suggest that the reaction at hand depends on the heterogeneity and site reactivity present in each CaO surface examined. On the other hand, based on diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) CO2 chemisorption and TPD-CO2 mass spectrometry studies, it was found that the catalytic activity can be correlated with the amount of CO2 adsorbed at 25 °C in the unidentate form. The latter species was found to be significantly more thermally stable than the bicarbonate adsorbed species also populated on the CaO surface after CO2 chemisorption (15 Torr) at 25 °C. The least concentration of unidentate carbonate species accumulated on the catalyst surface the higher the catalytic activity of CaO to be expected. It was found that the presence of hydrogen in the feed stream results in a significant decrease of steam reforming of phenol activity over CaO. This result was probed to be likely related to the reduction in the rate of water dissociation to form -OH species, as revealed by the significant decrease in the concentration of adsorbed bicarbonate and -OH species on the surface of CaO according to in situ DRIFTS-CO2 adsorption experiments in the presence of water and of varying hydrogen concentration in the feed. The inhibiting role of H2 during steam reforming of phenol over CaO is discussed. © 2008 Elsevier B.V. All rights reserved.