Low-temperature conversion of phenol into CO, CO2 and H2 by steam reforming over La-containing supported Rh catalysts

Abstract

The steam reforming of phenol in the low-temperature range of 350–550°C was studied over 0.5wt% Rh supported on Ce0.15Zr0.85O2, Zr0.93La0.07O2, and Ce0.13Zr0.83La0.04O2 mixed metal oxides. The reforming specific activity (per gram of catalyst basis) was found to depend on (a) Rh dispersion, (b) the concentration (μmolg−1) of labile oxygen species of support, as determined by “oxygen storage capacity” measurements, and (c) the promotion of the water–gas shift (WGS) reaction rate, all of which largely influenced by the support chemical composition. The addition of 4atom% La3+ into the Ce0.15Zr0.85O2 crystal structure was found to largely promote the dispersion of Rh, to increase significantly (by a factor of 1.75) the concentration of surface basic sites (μmolm−2) of Ce0.13Zr0.83La0.04O2 compared to Ce0.15Zr0.85O2 support, to prevent to a large extent the inhibiting role of hydrogen in the rate of WGS reaction, and to influence significantly the concentration and structure of surface reaction intermediates of the WGS, namely: formate (HCOO–) and –OH groups formed on the support, and adsorbed CO formed on the Rh metal. The 0.5wt% Rh/Ce0.13Zr0.83La0.04O2 catalyst led to a significantly better performance towards steam reforming of phenol in terms of phenol conversion, H2-yield, and CO/CO2 product ratio in the 350–450°C range compared to a commercial Ni-based catalyst (44wt% Ni) used for steam reforming reactions. At 400°C, the 0.5wt% Rh/Ce0.13Zr0.83La0.04O2 catalyst exhibited approximately 54% and 50%, respectively, higher phenol conversion and hydrogen yield, compared to the Ni-based industrial catalyst. The significantly lower CO/CO2 product ratio obtained in the case of Rh/Ce0.13Zr0.83La0.04O2 compared to that obtained in the other supported-Rh catalysts is in agreement with the higher kinetic rate for the WGS reaction observed in the former catalyst.