PROMOTING EFFECT OF PT ON CO MORDENITE UPON THE REDUCIBILITY AND CATALYTIC BEHAVIOR OF CO2 HYDROGENATION

The reducibility of Co and Pt mordenite monometallic and bimetallic sa mples with different Co/Pt ratios was studied by temperature-programme d reduction. The species formed after reduction were characterized by X-ray diffraction (XRD) and XPS. Co mordenite samples presented a very low reducibility in the 273-823 K range, whereas the Pt monometallic samples were totally reduced in that range with a H-2/Pt ratio = 1. Tw o well-differentiated reduction zones in the 273-573 K and 573-823 K r anges were observed in these samples, suggesting a Pt2+ ions distribut ion at the exchange sites of the mordenites. A proportion of 70:30% fo r the two zones resulted, independent of the Pt loading. The incorpora tion of platinum notably increased the Co reducibility. The type and s tability of the species formed during the reduction depended on the Co /Pt ratio. In the sample with a lower content of Pt, a catalytic actio n of the latter prevailed, leading to reduced species formed by a fine dispersion of metallic Co and Pt particles. When the Pt content incre ased, the formation of PtCoxOy species during calcination seemed to be the precursor of the promoting effect. For high contents of Pt (5%), Co/Pt = 0.6, XPS results revealed an important Co-Pt interaction. Afte r sinterization, Pt3Co particles were clearly observed by XRD. The Pt- Co interaction modified the specific activity and selectivity to CH4 o f the monometallic species in the CO2 hydrogenation reaction. The samp les with a Co/Pt ratio = 5.9 presented a maximum in the methanating ac tivity, thus resulting in a turnover frequency for CH4 production, N-C H4 = 0.20 (s(-1)) at 623 K and 1 atm. This turnover frequency dropped to 0.015 for Co/Pt = 0.6. (C) 1996 Academic Press, Inc.