Low-temperature purification of gas streams from phenol by steam reforming over novel supported-Rh catalysts

The purification of gas streams from phenol at low-temperatures (350–550 °C) was investigated by its reaction with steam over novel catalytic systems, namely 0.5 wt% Rh supported on Ce0.15Zr0.85O2, Ce0.15Zr0.83Mg0.02O2 and Ce0.14Zr0.81Mg0.05O2 mixed metal oxides. It was found that the rate of reforming reaction was largely influenced by the support chemical composition, where catalytic activity and selectivity towards H2 formation were increased with increasing support Mg content (0–5 atom%). The 0.5 wt% Rh/Ce0.14Zr0.81Mg0.05O2 catalyst showed the best activity in terms of phenol conversion and H2-yield, and the lowest CO/CO2 product ratio in the 350–550 °C range. In particular, at 450 °C a phenol conversion of about 80% and a H2-yield of about 85% were obtained for a feed containing 0.6 vol.% phenol and 40 vol.% H2O at a gas hourly space velocity of 54,000 h−1. The latter catalyst composition exhibited significantly better catalytic performance in the 350–450 °C range when compared to a commercial Ni-based catalyst used for tar steam reforming. In particular, at 450 °C the H2-yield was increased by 75% and the CO/CO2 product ratio decreased by a factor of eight compared to the commercial Ni-based catalyst. It was found that the catalytic behaviour of 0.5 wt% Rh/Ce0.14Zr0.81Mg0.05O2 correlates with the presence of a larger concentration of basic sites, a larger concentration of labile oxygen species, and of very small mean Rh particle size (∼1.3 nm) compared to the other supported-Rh catalysts investigated. The significantly lower CO/CO2 product ratio obtained in the Rh/Ce0.14Zr0.81Mg0.05O2 compared to the other supported-Rh catalysts was related to the presence of a large surface concentration of Rhn+ cationic sites which may promote the water–gas shift (WGS) reaction. The activity towards the WGS reaction was found to be promoted by the presence of Mg2+ in the support composition. In situ DRIFTS–WGS reaction studies revealed that the kinds and surface concentration of formate species formed, likely active reaction intermediates in the WGS reaction, depend on the chemical support composition. The Ce0.14Zr0.81Mg0.05O2 support when used to deposit the same amount of Rh metal (0.5 wt%) resulted in a larger concentration of formate species than the Ce0.15Zr0.85O2 support. This result could partly explain the significantly larger phenol steam reforming activity towards H2 and CO2 observed on the Rh/Ce0.14Zr0.81Mg0.05O2 than Rh/Ce0.15Zr0.85O2 catalyst.