Origin and reactivity of active and inactive carbon formed during DRM over Ni/Ce0.38Zr0.62O2-delta studied by transient isotopic techniques

摘要

The role of Ce0.38Zr0.62O2-delta redox support and that of reactants CH4 and CO2 in the carbon-path of dry reforming of methane (DRM) at 750 degrees C over a 3 wt% Ni/Ce0.38Zr0.62O2-delta catalyst were investigated. In particular, meticulously designed SSITKA and other transient isotopic experiments were conducted in an attempt to provide conclusive answers to important issues related to the improvement of CeZrO2-supported Ni DRM catalyst design, namely: (i) the extent of CO2 and CH4 contribution to inactive "carbon" deposition, (ii) the participation and to what extent of support lattice oxygen in the DRM, (iii) differences in the amount (mu mol g(-1)) and reactivity of the "carbon" formed between methane decomposition (CH4/He) and DRM reactions, (iv) the possible participation of CO2 and to what extent in the gasification of "carbon" to form CO and (v) the ability of hydrogen gas product towards gasification of the "carbon" formed during DRM. It was shown that a large reservoir of support's lattice oxygen (beyond the surface monolayer) participates in the carbon-path towards the formation of CO and which is considered largely responsible for the relatively low amount (0.3 wt%) of inactive "carbon" deposition after 20 h on stream (44.2 vol% CH4, CH4/CO2 = 1). It was found that the CH4 and CO2 activation routes contribute equally to the formation of inactive "carbon" after DRM at 750 degrees C (5% CH4, CH4/CO2 = 1) and the structural characteristics of "carbon" appear very similar according to temperature-programmed oxidation (TPO). The surface coverage of active carbon that truly participates in the formation of CO and which is associated with the CO2 activation route was found to be very small (theta(C) = 0.006 or 0.6%) and remains practically constant for up to 2 h on TOS. On the other hand, a pool of inactive reversibly adsorbed CO2 was measured, which was found to increase up to 2 h of TOS (theta(CO2) = 0.017 or 1.7%). The latter is suggested to be linked to catalyst's deactivation to a small extent only. It was also shown that the rate of inactive "carbon" accumulation seems to be influenced by the reaction of it with the hydrogen gas product.