Effect of Ni/Co ratio on Ce-Sc-ZrO2 catalysts for selective H2 production via methane partial oxidation

Abstract

Partial oxidation of methane (POM) is a promising route for hydrogen production, and achieving a high H₂ yield with an H₂/CO ratio >3 is highly appealing. Optimization of Ni/Co ratios over Ce-Sc-ZrO₂ (CSZ) is investigated for POM reaction and characterized by X-ray diffraction, Raman spectroscopy, temperature-programmed reduction/oxidation/desorption, and transmission electron microscopy. The active site derived from the reduction of “strongly interacted NiO” is responsible for the dissociation of C–H (of CH₄), resulting high activity towards POM. 5Ni/CSZ has the highest amount of such active sites and attains the highest activity. 5Co/CSZ catalyst has cobalt-based active sites, and there is an inert carbon deposit during the reaction, causing the least activity. 3.75 wt% Ni and 1.25 wt% Co combination over CSZ support surges the highest density of basicity, oxide vacancy, and adequate amount of active sites derived from “strongly interacted NiO”. The active sites with enhanced metal-support interaction are further grown under exposure to oxidizing gas (O₂) and reducing gas (H₂) during the POM reaction. The highest density of basicity and oxide vacancy involves more CO₂ and H₂O in the sequential oxidation of CH₄ under indirect pathways. The exclusive involvement of indirect pathways of POM and inhibition of hydrogen consuming reaction (like reverse water gas shift reaction) over 3.75Ni1.25Co/CSZ results into 48 % H₂ yield and 3.26 H₂/CO ratio up to 24 h time on stream at 600 °C. The H₂ yield doubles to ∼97 %, and the H₂/CO ratio comes close to 2 over 3.75Ni1.25Co/CSZ catalyst at 900 °C.