CHARACTERIZATION BY PULSED OXIDATION OF COKE ON PLATINUM ALUMINA

Pt/Al2O3 samples of different metal loads and reduced at different temperatures were used as catalysts in n-hexane conversion. The "coke" overlayer formed during 24 h was characterized by pulsed oxidation, keeping the temperature constant during a given oxygen pulse but usually increasing it between pulses in steps from 30-degrees-C to 550-degrees-C. Carbon-to-metal (C/M) and H/C ratios of the coke were determined for defined regimes of the coke oxidation process. It was found that platinum is partially oxidized before carbon dioxide is formed; platinum oxidation, either to PtO or to a surface compound of Pt2+ with Al2O3, is complete in an early stage of coke oxidation. Coke oxidation to T < 300-degrees-C is catalyzed by PtO or, though less efficiently, by the ternary Pt-Al-O compound; carbon dioxide evolution starts at lower temperature for the high platinum load catalysts. Dramatic differences, both in catalytic and coke oxidation parameters are found between samples originally reduced at 350-degrees-C or 600-degrees-C respectively; e.g. the C/M ratios are significantly lower for samples reduced at 600-degrees-C. Independent data show that chemical interaction takes place between platinum and Al2O3 at 600-degrees-C. It follows that coke formation is an ensemble-specific process. For the high platinum load sample which was originally reduced at 350-degrees-C the coke oxidation-vs.-temperature profile shows two characteristic "spikes" where coke oxidation is extremely fast.