Mono and bifunctional pathways of CO₂/CH₄ reforming over Pt and Rh based catalysts

Despite the high thermodynamic driving force to form coke under the reaction conditions applied Pt/ZrO₂ and Rh supported on γ-Al₂O₃ and ZrO₂ are active and stable catalysts for CO₂/CH₄ reforming. Using steady state, transient kinetic measurements and physico-chemical characterization techniques have shown that the catalyst activity is determined by the available Pt-ZrO₂ perimeter. Methane is decomposed on the metal to CH_x (average value of x = 2) and H₂. The main route to CO₂ reduction occurs via initial formation of carbonate close to the metal-support boundary. Carbon on the metal reduces that carbonate to formate by forming CO. The formate decomposes rapidly to CO and a surface hydroxyl group. Hydroxyl groups recombine and form water or react further with methane to CO and hydrogen (steam reforming). When the rate of methane decomposition and carbonate reduction are in balance, the catalytic activity remains stable. In contrast, the activity of Rh is mainly determined by the concentration of accessible surface atoms and a concerted metal catalyzed mechanism of methane decomposition and subsequent CO₂ reduction dominates. The support plays a minimal role in that chemistry.