Carbon deposition during carbon dioxide reforming of methane - Comparison between Pt/Al2O3 and Pt/ZrO2

The relation between coke formation and the deactivation of supported Pt catalysts during CO2 reforming of methane at temperatures above 1070 K such as used in the commercial process was studied. Temperature-programmed oxidation and temperature-programmed reaction with CO2 were applied to Pt catalysts (Pt/Al2O3 and Pt/ZrO2) which were exposed to CH4/CO2 (reforming reaction conditions) or CH4/He (facile coke formation) to identify the carbon species. The activity decrease for Pt/Al2O3 was rather slow and minor at high temperature (greater than or equal to 1070 K), while it was fast and almost complete during a comparative experiment at low temperature (875 K), Coke deposited on the supported Pt particles was oxidized by CO2, but coke on the Al2O3 support was not removed at 1070 K. At this temperature the decay in activity with time on stream corresponded solely to the amount of coke accumulated on Pt particles. This indicates the main reaction between CO2 and CH4 on all Pt atoms without significant participation of the support. The activity is concluded to decrease gradually due to coverage of Pt by coke induced by CH4 decomposition (initial phase of deactivation observed at high temperature). After a while only the perimeter of Pt particles remains as site of activity. There, the activity is speculated to be stable because of the higher reactivity of CO2 at the metal-support boundary. Gradually, the coke on the support (Al2O3) increases to an extent that it blocks the reaction also at that location. In contrast to the situation with Pt/Al2O3, coke was not observed on Pt/ZrO2 even after exposure to reforming gas (CH4/CO2) for 12 h. The combination of three factors is concluded to cause the high catalytic stability of Pt/ZrO2 in CO2/methane reforming: (i) coke on Pt (supported on ZrO2) is more reactive toward CO2 than coke on Pt (supported on Al2O3) under reforming reaction conditions; (ii) methane decomposition is slower on Pt/ZrO2 than on Pt/Al2O3; and (iii) coke is hardly formed on the ZrO2 Support (C) 2001 Academic Press.