Promoting Effect of Pt on Co Mordenite upon the Reducibility and Catalytic Behavior of CO2Hydrogenation

Abstract

The reducibility of Co and Pt mordenite monometallic and bimetallic samples with different Co/Pt ratios was studied by temperature-programmed 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 H2/Pt ratio=1. Two well-differentiated reduction zones in the 273–573 K and 573–823 K ranges were observed in these samples, suggesting a Pt2+ions distribution at the exchange sites of the mordenites. A proportion of 70 : 30% for the two zones resulted, independent of the Pt loading. The incorporation of platinum notably increased the Co reducibility. The type and stability of the species formed during the reduction depended on the Co/Pt ratio. In the sample with a lower content of Pt, a catalytic action of the latter prevailed, leading to reduced species formed by a fine dispersion of metallic Co and Pt particles. When the Pt content increased, the formation of PtCoxOyspecies 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. After sinterization, Pt3Co particles were clearly observed by XRD. The Pt–Co interaction modified the specific activity and selectivity to CH4of the monometallic species in the CO2hydrogenation reaction. The samples with a Co/Pt ratio=5.9 presented a maximum in the methanating activity, thus resulting in a turnover frequency for CH4production, NCH4=0.20 (s−1) at 623 K and 1 atm. This turnover frequency dropped to 0.015 for Co/Pt=0.6.