Dry reforming of methane over 5 wt% Ni/Ce1-xPrxO2-δ catalysts: Performance and characterisation of active and inactive carbon by transient isotopic techniques

Abstract

The effects of Ce1-xPrxO2-δ support composition (x=0.0–0.8) and reaction temperature (550–750°C) on the catalytic activity and selectivity and important features of the mechanism of the dry reforming of methane (DRM) over supported Ni (5wt%) were investigated. Of particular interest were the effects on the concentration of active and inactive carbon formed, the relative contribution of CH4 and CO2 activation routes towards carbon formation and the structure and morphology of the inactive carbon which was formed. For these carbon characterization studies, steady-state isotopic transient kinetic analysis (13CO2-SSITKA), temperature-programmed oxidation (TPO) following 13CO2/12CH4/He dry reforming, thermal gravimetric analysis coupled with TPO (TGA-TPO), scanning electron miscroscopy (SEM-EDX) and transmission electron microscopy with atomic resolution (HRTEM) and powder X-ray diffraction (XRD) were employed. The relative amount of inactive carbon formed via the CH4 and CO2 activation routes was found to strongly depend on reaction temperature and Pr-dopant support composition. At 550°C, the contribution of the CO2 activation route to the inactive carbon formation (besides that of the CH4 activation route) was 65.7 and 60.1%, respectively, for Ni supported on CeO2 and Ce0.2Pr0.8O2 carriers, whereas at 750°C the respective values were 54.0 and 50.9%. Filamentous carbon and thin layers of graphitic carbon were identified as the main morphologies of inactive carbon. The surface coverage of active carbon that truly participates in the formation of CO was found to depend on support composition and reaction temperature (θC=0.03–0.15at 550°C and 0.07–3.4at 750°C). A pool of inactive reversibly adsorbed CO2 was measured (θ=1.5–4.1) for the first time, which was dependent on support composition and reaction T. The introduction of 20 atom-% Pr in the ceria lattice caused a significant reduction in the rate of inactive carbon formation with marginal decrease in catalyst’s activity and stability after 25h on stream. Further introduction of Pr-dopant (80 atom-%) caused drastic reduction in the deposited carbon after 25h on the reaction stream at 750°C (0.07wt%C) but with appreciable decrease in CH4- and CO2-conversions and H2/CO gas-product ratio (a drop by a factor of 1.85, 1.45 and 1.47, respectively). The latter decrease in catalyst’s performance is correlated with the increase in the pool of inactive adsorbed CO2 in the form of carbonate-like species.