Impact of trace noble metals on the catalytic performance of LaNiMgO towards methane dry reforming

CO₂ reforming of methane was investigated using trace noble metal anchored MgO-promoted Ni/La₂O₃ catalyst (LaNiMgO-X), where X = Ir, Pt, Ru, Pd, and Rh. The incorporation of noble metals enhanced Ni dispersion on the support and facilitated the Ni reducibility due to the H₂ spillover effect. At 650 °C and in the early stages of the reaction, all catalysts exhibited enhancement in the catalytic activity. However, at longer times, the control and Pd-embedded catalysts revealed sharp decay in the catalytic activity compared to counterparts, which was assigned to the extensive coke deposition originated by methane cracking methane and Boudouard reactions. Whereas, other noble metals embedded catalysts, in particular, Rh- and Ir-anchored catalysts exhibited high chemical durability with minor decay in the catalytic activity. For instance, after 25h, LaNiMgO-Pd exhibited 22.7 % and 33.2 % decay of its maximum CO₂ and CH₄ conversion, respectively, while LaNiMgO lost 19.7 % and 27.2 % of its maximum CO₂ and CH₄ conversion, respectively. On the other hand, at the same time-on-stream LaNiMgO-Rh lost 0.3 and 0.7 % of its maximum CO₂ and CH₄ conversions, respectively. In durability measurements, and after 140h LaNiMgO-Rh exhibited slight decay in the CO₂ conversion from 77.6 % to 70.5%, the CH₄ conversion decreased from 72.3% to 64.2%. On the other hand, LaNiMgO-Pd depicted rapid decay in the CO₂ conversion from 68.3 % to 49.3 % and the CH₄ conversion felled from 66.5 % to 41.5 % after 50h. In addition, the reactor was blocked in case of LaNiMgO-Pd after about 54h. Excluding Pd, the enhancement of noble metals was assigned to the formation of noble metal-rich Ni surfaces, which suppressed the oxidation of nickel and enhanced the gasification of deposited coke. In addition, the presence of noble metals at the Ni surface afforded a high density of adsorbed oxygen species, which enhanced coke gasification and consequently decreased the extent of deactivation of the catalysts by coke formation. While Pd-anchored sample showed extensive carbon deposition, in particular, recalcitrant γ-carbon, which is hardly gasified, encapsulating the active Ni catalyst, which causes catalyst deactivation.