MICROCALORIMETRIC STUDY OF THE ADSORPTION AND REACTION OF CO, O2, CO+O2, AND CO2 ON NAX ZEOLITE, PT/NAX, AND PLATINUM METAL - EFFECT OF OXIDIZING AND REDUCING PRETREATMENT

Pt/NaX and Pt powder showed very similar behaviour in both chemisorption and heat release (Q) when CO, O2, CO + O2, or CO2 were dosed in the temperature range 300-500 K. However, an oxidizing or a reducing treatment given to a sample had a different effect on the Q values. Thus, CO adsorption on H2-pretreated Pt/NaX and Pt powder gave constant Q values of about 130 and 140 kJ mol−1, respectively, at all the temperatures studied. In the case of an oxygen pretreatment, the Q values were about 167 kJ mol−1 for both samples, with the exception of CO dosed to Pt/NaX at 305 K when a lower Q value of 83 kJ mol−1 was observed. The heat evolved when CO + O2 were dosed was higher by about 40-60 kJ mol−1 compared with that observed when CO was dosed under identical conditions. The metal-free zeolite showed negligible adsorption and heat release during exposure to both CO and CO + O2 at 300-500 K. The results provide evidence for exclusive participation of metal sites in the CO oxidation over Pt/NaX catalyst, the diffusion of reactants or products through zeolite pores playing an insignificant role. The chemisorption properties of the metal were nevertheless modified by the support. Both reactant species (CO and oxygen) are chemisorbed simultaneously and compete for metal sites. The results suggest that the three reactions, namely, 2CO(ad) → C + CO2(g), CO(g) + O(ad) → CO2(g), and CO(ad) + O(ad) → CO2(g) are equally probable over the platinum surface, and the preponderance of one of them may be decided by the chemical nature of the surface at the time of reaction. The study supports the view that the CO and O interact at a Pt site in a cyclic order, and the sequence in which these species are chemisorbed may have a significant effect on the catalytic activity. © 1993 Academic Press, Inc.