Revealing the Synergistic Effects of Rh and Substituted La₂B₂O₇ (B = Zr or Ti) for Preserving the Reactivity of Catalyst in Dry Reforming of Methane

Abstract

Rh substituted-La2B2O7 (B = Zr or Ti) composite oxides were synthesized and applied in the dry reforming of methane (DRM) reaction. The characterizations of XRD, Raman, UV–vis diffuse reflectance, HRTEM, HAADF-STEM, in situ DRIFTS, XAFS XPS, TPR and the tests of DRM reaction have shown that the degree of substitution and the catalytic performance depend on the composition of La2B2O7 (B = Zr or Ti). It is found that almost all Rh species substituted Zr over the compact Rh-LZO while a part of Rh substituted Ti, and the rest existed in the form of Rh2O3 on the surface over the loose Rh-LTO. CH4 was prone to dissociate on Rh–La2Zr2O7 but hard to continue owing to the quickly depositing of intermediate carbon, which would not be resolved unless enough active O* was furnished. On the other hand, titanium-doped La2Ti2O7 conferred unique structural and charge effects to supported Rh through the metal–support interface, leading to the coexistence of Rh0 and Rhδ+ which performed synergistically during DRM at 800 °C. We proposed that CH4 would be activated on Rh sites continuously if generated carbon could be promptly oxidized by active O* species that originated from CO2 dissociation. The coexistence of Rh0 and Rhδ+ in Rh–La2Ti2O7 facilitated the electron transfer and thus accelerated the mobility of active oxygen species, which could be proved by the variations of binding energy in Ti, O, and Rh.