Methanol steam reforming reaction on highly stable, ceria supported Pd/Zn-based catalysts

CELINA E. BARRIOS, MIGUEL A. BALTANÁS, ADRIAN L. BONIVARDI

Münich

Congreso; 15th International Congress on Catalysis,; 2012

Pd-ZnO based catalysts have been recently under intense scrutiny for the steam reforming of methanol (SRM), which is one of the most promising processes for the production of H2 with low amounts of CO for operating polymer electrolyte fuel-cells. Very high selectivity is required, though, which so far has been achieved using Pd/ZnO, a support with extremely low surface area. In this work we prepared a series of ZnO-CeO2 supports onto which palladium acetate was impregnated by incipient wetness. The metal (2% wt. Pd) was supported on pure ZnO and CeO2, and two ZnO-CeO2 composites, that is, a co-precipitated ZnO+CeO2 (Zn:Ce = 0.5 molar ratio) and an impregnated ZnO/CeO2 material (theoretical ZnO coverage = 0.4). After calcinations at 723 K, a very high dispersity of amorphous ZnO onto the ceria was achieved, as revealed by XRD. Under SRM conditions, the decomposition of methanol was the main reaction on Pd/CeO2, because CO was the prevalent carbonaceous product and the conversion of water was low (Fig. 1). Conversely, the presence of ZnO was critical to obtain high selectivity to CO2 and high production of H2 (not shown). In addition, higher conversions at lower temperatures were achieved using the ZnO and CeO2 combined supports (Fig. 1), together with an improvement of the catalytic stability of those mixed oxides. Therefore, the CO adsorption on Pd was followed by in situ infrared spectroscopy (IR) over the set of Pd/support materials, reduced between 423 and 623 K. Exposure time and CO partial pressure were also systematically studied together with the desorption and re-adsorption of CO, our probe molecule. The impact of CO2 pre-adsorption on the Pd/supports reduced at 623 K was also evaluated. By following the di(tri)- (B or H) and mono-coordinated CO (L) bands, adsorbed CO was able to detect the progressive formation of PdZn bimetallic surfaces at increasing temperature of reduction, as only the latter bands remained then (Fig. 2a). After exposing the bimetallic surfaces to CO, multi-coordinated CO species developed with increasing contact time (Fig. 2b). This last restructuring process was partially reversible, as revealed by evacuation (multi-coordinated bands disappeared) followed by further re-adsorption of CO (which lead to reappearance of the B+H bands). The pre-adsorption of CO2 over the reduced catalysts at 623 K, had almost no effect over the restructuring of the PdZn bimetallic surfaces. Because the partial pressure of CO used in the IR CO adsorption experiment is close to the one obtained under reaction conditions, the surface restructuring of the bimetallic PdZn supported particles seems to be indicating that the “isolation” of Pd sites is not a requirement to explain the high performance/selectivity of the Pd-Zn based catalyst for the SRM reaction but, instead, an intimacy between the metal and the zinc oxide rather than between Pd and ceria. The latter operates, merely as a high surface area provider onto which well disperse, amorphous ZnO is available, in intimate contact with nano-sized restructuring prone zinc-palladium crystallites under reaction conditions.