Tailoring MgO-based supported Rh catalysts for purification of gas streams from phenol

The present work is focused on the sol–gel synthesis, characterization and catalytic performance evaluation of novel Mg-Ce-Zr-O-based mixed oxides supported Rh catalysts for the purification of H2-rich gas streams from phenol through steam reforming in the 575–730 °C range. Phenol is used as a model compound of tars, the former being one of the main constituents of tars formed during steam gasification and pyrolysis of biomass. Novel Rh catalysts (0.1 wt%) supported on Mg-Ce-Zr-O-based mixed oxides modified by alkaline-earth, rare earth, and transition metal ions, having the general formula Mg-Ce-Zr-X-O (where X stands for La, Sr, Ba, Ca and Zn) were systematically studied. The physicochemical properties of the catalysts were evaluated using complementary bulk and surface characterization techniques, such as BET, XPS, in situ XRD, TEM/SAED and SEM/EDX. Transient techniques including H2-TPR, TPD-CO2 and TPD-NH3 were employed in order to characterize redox behavior, surface basicity, and surface acidity of support, respectively. Among the catalysts examined, a 0.1 wt% Rh/40Mg-20Ce-20Zr-20La-O was found to exhibit the highest hydrogen product yield, specific H2 production rate (μmol/m2 min), and the lowest CO/CO2 product ratio (575–730 °C), even when compared to a Ni-based commercial steam reforming of tars catalyst. At 655 °C, a H2 yield of ∼95% and a specific reaction rate of 100 μmol H2/m2 min were obtained for a feed containing 0.5 vol% phenol and 40 vol% H2O at a gas hourly space velocity of ∼80,000 h−1. This activity behavior is correlated with the largest concentration of basic sites and labile oxygen species (μmol/m2), and also with the largest acid/base site ratio present in the 40Mg-20Ce-20Zr-20La-O support composition. Steam was also found to be an efficient reagent for the removal of carbonaceous deposits (e.g., CxHy) formed on the catalyst surface during phenol steam reforming. The H/C atom compositional ratio of the carbonaceous deposits was found to depend on reaction temperature.

Effect of support chemical composition on the H2 yield (%) obtained over the 0.1 wt% Rh/40Mg-25Ce-25Zr-20X-O (X = La, Ba, Zn, Ca, Sr) catalysts in the 575–735 °C range after phenol steam reforming (0.5% C6H5OH/40% H2O/He; t = 30 min; GHSV = 80,000 h−1). Results are also shown for the 0.1 wt% Rh/50Mg-25Ce-25Zr-O and commercial 44 wt% NiO/γ-Al2O3 catalysts.Figure optionsDownload as PowerPoint slideHighlights
► La3+ addition in Rh/Mg-Ce-Zr-O largely enhances its phenol steam reforming activity.
► The beneficial role of La3+ is largely due to the enhanced concentration of active support labile O.
► Acid/basic surface properties of Mg-Ce-Zr-X-O (X = La3+, Sr2+, Ca2+, Ba2+, Zn2+) regulates phenol steam reforming activity.
► Steam is more effective than oxygen in removing “carbon” deposits formed after phenol steam reforming.