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

Abstract Carbon dioxide reforming of methane to synthesis gas in the 550–750 °C range over 5 wt% Ni/Ce 1− x M x O 2− δ (M = Zr 4+ , Pr 3+ ) 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 CH 4 and CO 2 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 13 CO and 13 CO 2 isotope gases were conducted. TPO following dry reforming (5% 13 CO 2 /5% 12 CH 4 /45%Ar/45%He) demonstrated that the relative amount of the various kinds of “carbon” formed via the CH 4 and CO 2 activation routes was strongly dependent on reaction temperature and support chemical composition. At 550 °C, the ratio of 12 CO 2 to 13 CO 2 of the 12 C-containing and 13 C-containing inactive “carbon” formed was 0.4, 0.27 and 0.19, whereas at 750 °C was 1.07, 1.06 and 0.29, respectively, for the 5 wt% Ni supported on Ce 0.8 Zr 0.2 O 2 , Ce 0.8 Pr 0.2 O 2 and Ce 0.5 Zr 0.5 O 2 carriers. The origin of “carbon” formation via the CO 2 activation route was illustrated to be the Boudouard reaction (2CO-s → CO 2 (g) + C-s + s) through a transient isotopic experiment with a feed gas containing 13 CO and 12 CH 4 . It was also found that CO-s derived from the direct dissociation of CO 2 and the CH 4 activation route can lead to a number of different kinds of “carbon” which depends on support chemical composition. The present 5 wt% Ni/Ce 0.8 Pr 0.2 O 2 catalytic system exhibited CO 2 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 °C (20% CH 4 , 20% CO 2 , 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.