Effects of Gas- and Liquid-Phase Environments on the Sintering Behavior of Platinum Catalysts

Abstract

Platinum has been used extensively as a catalyst in the petroleum industry for hydrocarbon cracking. More recently, Pt has been considered for an electrocatalyst in fuel cells and for catalytic mufflers in the exhaust systems of automobiles. The performance of Pt as a catalyst in these heterogeneous catalysed systems is dependent upon the Pt activity and the real Pt surface area accessible to the reactant (in the absence of catalyst deactivation by poisoning and mass transfer limitations). The high temperature application of supported Pt (Pt/Al₂O₃) as a heterogeneous catalyst in a gasphase environment causes sintering and loss of Pt surface area (1–7). Unsupported Pt black sinters rapidly in a gas-phase environment (8, 9) but at a much lower temperature (< 200°C) than supported Pt. In contrast, unsupported pt black (10–13) and Pt supported on carbon (14, 15) sinter in liquid-phase environments in the same temperature range (100–200°C). Since a loss of Pt surface area due to sintering results in a decrease of the total catalyst activity (constant pt specific activity assumed) there is significant profit in maintaining high surface area Pt catalysts in diverse operational environments. Yet, despite the numerous publications presenting experimental data for the sintering of supported and unsupported metal catalysts, the mechanisms responsible for this phenomenon remain a subject of controversy.