Catalytic Active Site, Mechanistic and Kinetic Studies of Dry (CO2) Reforming of Methane over Lanthanum Zirconate (La2Zr2O7) Pyrochlores

Abstract

Dry (CO2) reforming of CH4 (DRM) produces commercially important synthesis gas (H2 and CO) with H2/CO ≤ 1, which can be used for synthesis of higher alkanes and oxygenates. DRM is highly endothermic and requires temperatures as high as 800°C-1000°C to attain high equilibrium conversions. A major problem associated with DRM is catalyst deactivation due to carbon deposition. Thus it is imperative that the catalyst used for DRM must resist deactivation due to sintering and carbon deposition. DRM is well studied in the literature over various catalysts, however, there is no literature, except the Ashcroft (1993) article, for DRM over pyrochlores. Pyrochlores are metal oxides (A2B2O7), with larger rare earth metal occupying the A-site and smaller alkali earth or transition metal occupying the B-site. Ashcroft. et al. studied pyrochlore catalysts composed of rare-earth metals at A-site and catalytically active transition metals like Ru and Ir at B-site (e.g., Nd2Ru2O7, Eu2Ir2O7, and Gd2Ru2O7). These pyrochlores lost their structure under CH4 and CO2 above 340°C. Unlike their work, we use La on A-site and Zr on B-site and only partially substitute the B-site with catalytically active Rh, Ru, or Pt. This La-Zr framework provides high thermal stability to the catalysts used in our study as compared to that by Ashcroft. The inherent lattice oxygen reactivity of pyrochlores helps to resist deactivation due to carbon formation. In this work, Rh, Ru, or Pt substituted lanthanum zirconate pyrochlore catalysts were synthesized, characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS). The catalytic active sites and the mechanistic steps of DRM reaction were studied by means of kinetic rate modeling, isotopic labeling, in-situ Fourier transform infrared spectroscopy (FTIR), in-situ XPS and transient pulsing of CH4/CO2. The rate limiting step in the DRM mechanism over pyrochlores was determined by studying the (CH4/CD4) deuterium kinetic isotope effect. A sequence of intermediate reaction steps was proposed based on these experimental results and kinetic rate modeling, to most closely depict the mechanism of DRM over pyrochlore catalysts.