Effect of support composition on the origin and reactivity of carbon formed during dry reforming of methane over 5 wt% Ni/Ce1-xMxO2-delta (M = Zr4+, Pr3+) catalysts

摘要

Carbon dioxide reforming of methane to synthesis gas in the 550-750 degrees C range over 5 wt% Ni/Ce1-xMxO2-delta (M = Zr4+, Pr3+) solids has been investigated with respect to the effects of support chemical composition and reaction temperature on the amount, reactivity (towards H-2 and O-2) and relative contribution of CH4 and CO2 activation routes towards "carbon" formation. For these "carbon" characterisation studies, various transient isothermal and temperature-programmed oxidation (TPO) and hydrogenation (TPH) experiments coupled with the use of (CO)-C-13 and (CO2)-C-13 isotope gases were conducted. TPO following dry reforming (5%(CO2)-C-13/5%(CH4)-C-12/45%Ar/45%He) demonstrated that the relative amount of the various kinds of "carbon" formed via the CH4 and CO2 activation routes was strongly dependent on reaction temperature and support chemical composition. At 550 degrees C, the ratio of (CO2)-C-12 to (CO2)-C-13 of the C-12-containing and C-13-containing inactive "carbon" formed was 0.4, 0.27 and 0.19, whereas at 750 degrees C was 1.07, 1.06 and 0.29, respectively, for the 5 wt% Ni supported on Ce0.8Zr0.2O2, Ce-0.8 Pr0.2O2 and Ce0.5Zr0.5O2 carriers. The origin of "carbon" formation via the CO2 activation route was illustrated to be the Boudouard reaction (2CO-s -> CO2(g) + C-s + s) through a transient isotopic experiment with a feed gas containing (CO)-C-13 and (CH4)-C-12. It was also found that CO-s derived from the direct dissociation of CO2 and the CH4 activation route can lead to a number of different kinds of "carbon" which depends on support chemical composition. The present 5 wt% Ni/Ce0.8Pr0.2O2 catalytic system exhibited CO2 conversion of 84%, H-2-yield of 48%, and H-2/CO ratio of 1.04 after 50 h of dry reforming of methane at 750 degrees C (20% CH4, 20% CO2, He; GHSV = 30,000 h(-1)) with a relatively low amount (17.5 mg C/g(cat) or 1.75 wt%) of accumulated inactive "carbon". The support chemical composition was found to influence the nickel particle size, which in turn influenced the origin, kinetics and the reactivity of "carbon" deposition under dry reforming reaction conditions. (C) 2015 Elsevier B.V. All rights reserved.