The effect of Pt on the carbon pathways in the dry reforming of methane over Ni-Pt/Ce0.8Pr0.2O2-δ catalyst

Abstract

The present work extensively discusses the carbon pathways in the dry reforming of methane with carbon dioxide (DRM) at 750 °C towards syngas (CO/H2 ˜ 1) over 5 wt% Ni, 0.5 wt% Pt and their bimetallic alloy (5 wt% Ni-0.5 wt% Pt) supported on the reducible Ce0.8Pr0.2O2-δ carrier for the first time. Various transient and isotopic experiments (use of 13CO2 and 18O2) were designed and performed aiming at providing important information about the effect of insertion of Pt in the Ni/Ce0.8Pr0.2O2-δ solid (one pot wet impregnation) on the amounts (mg gcat−1) and transient rates (μmol gcat−1 s−1) of “carbon” formation via CH4/He (methane decomposition), CO/He (reverse Boudouard reaction) and the combination of the two (CH4/CO/He). Also, the effect of Pt on the amount and transient rate of “carbon” removal via the participation of support’s mobile active oxygen species during DRM was probed. Hydrogen reduction of the Ce0.8Pr0.2O2-δ support alone at 750 °C followed by transient isothermal CO2/He treatment (750 °C) probed the existence of an alternative path of CO2 activation in the presence of support’s oxygen vacant sites. The quantification of the origin of “carbon” in the DRM (CH4 vs CO2 activation route) was performed after using isotopically labelled 13CO2 in the feed gas stream. It was found that “carbon” deposition on supported Ni and Ni-Pt alloy was largely due to the CH4 activation route on the metal surface, whereas for the supported Pt, both activation routes applied. Temperature-programmed oxidation (TPO) was used to estimate the amount and reactivity of the “carbon” deposited as a function of time-on-stream in CH4/He, CO/He, CH4/CO/He and DRM reactions. It was found that even though supported bimetallic Ni-Pt provided a small drop in CH4-conversion compared to the supported monometallic Ni catalyst, a remarkable decrease in the rate of “carbon” accumulation was obtained for the former catalyst. The latter was ∼32 times lower after 12 h in DRM (20%CH4, CO2/CH4 = 1) at 750 °C. This is largely due to the reduced rate of “carbon” deposition (via CH4 decomposition) and the enhanced rate of its gasification by lattice oxygen. The supported bimetallic Ni-Pt catalyst presented overall an excellent performance and stability after 50 h in DRM at 750 °C with a low amount of accumulated “carbon” (0.38 wt%), which is considered, to our knowledge, as one of the lowest values reported so far for bimetallic Ni-based supported catalysts.