A comparative study of the steam reforming of phenol towards H2 production over natural calcite, dolomite and olivine materials

Pre-calcined (air, 850 °C) naturally occurring calcite, dolomite and olivine materials were studied for phenol steam reforming towards H2 production. The effect of reaction temperature, feed volume flow rate, and hydrogen concentration in the feed stream on phenol conversion, H2 product concentration and selectivity were investigated. The increase of reaction temperature in the 650–800 °C range led to the increase of phenol conversion and H2-selectivity for the calcite and dolomite materials, while the opposite behaviour was observed for the olivine material. An increase of phenol conversion was obtained with decreasing Gas Hourly Space Velocity, GHSV in the 40,000–80,000 h−1 range for all three natural materials studied. The lower activity (per gram basis) of pre-calcined olivine compared to that of pre-calcined calcite and dolomite at high reaction temperatures (750–800 °C) is suggested to be linked to the increased extent of reducibility of α-Fe2O3 and Fe3O4, and of FexMg1−xOy and Ca2Fe2O5 phases likely present (X-ray diffraction measurements) in the pre-calcined olivine into less reactive FeIIO and Fe0 under reaction conditions. X-ray photoelectron spectroscopy studies performed over the raw olivine revealed also the presence of FeIIO. Phenol steam reforming reaction followed by transient isothermal oxidation allowed the measurement of “carbonaceous” species that react towards CO and CO2 and which accumulate during phenol steam reforming, as a function of reaction temperature and time on stream. The lower amount of “carbonaceous” species formed on the surface of calcined olivine under steam reforming of phenol at 650 °C, as well as its higher site reactivity compared to calcite and dolomite are considered likely reasons for the higher activity and H2-yield exhibited by olivine at 650 °C compared to calcite and dolomite. Comparative studies regarding the CO2 adsorption characteristics over pre-calcined calcite, dolomite and olivine using in situ CO2-DRIFTS and CO2-TPDs were conducted. It was found that olivine accommodates significantly lower amounts of CO2 compared to calcite and dolomite. The effect of hydrogen concentration in the feed stream towards phenol steam reforming activity on dolomite was found to be negative. In situ DRIFTS studies using H2/H2O/Ar and H2/H2O/CO2/He gas atmospheres suggested that this effect is related to the substantial decrease in the rate of water dissociation to form –OH active species caused by the simultaneous reversible interaction of H2 with the MgO/CaO surfaces.