The state of zirconia supported platinum catalysts for CO₂/CH₄ reforming

In Pt/ZrO₂ catalysts used for CO₂/CH₄ reforming to syngas, not all accessible Pt contributes equally to the activity of the catalyst. The catalytic activity is determined by the accessibility of Pt on the Pt-ZrO₂ perimeter. This is explained in terms of the CO₂ activation via carbonate species on the support which must be in the proximity of the Pt particles to react with the methane activated on the metal. The significance of the support is also emphasized by the low activity of Pt black and Pt/SiO₂, which were incapable of forming carbonates on the support. The perimeter concentration of Pt/ZrO₂ can be changed by changing the Pt concentration in the catalyst or by increasing the calcination temperature which causes sintering of the Pt particles. In contrast to the calcination temperature, the reduction temperature did not influence the activity although the hydrogen chemisorption capacity was markedly decreased by increasing the reduction temperature. As sintering was excluded on basis of particle size determination from EXAFS results a SMSI state on Pt/ZrO₂ during high temperature reduction is concluded to exist. This state does not persist under reaction conditions due to the presence of adsorbed oxygen from CO₂ dissociation.