Preparation of Ce-Zn mixed oxides as supports for the study of the methanol steam reforming reaction

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

Cancún, Quintana Roo, México

Congreso; XVIII International Material Research Congress; 2009

Mexican Materials Research Society (MRS-MEXICO), Materials Research Society (MRS), NACE International Section Mexico

The Pd/ZnO-CeO2 system has proved to be more stable than the traditional Cu/ZnO catalyst for the steam reforming of methanol. In this study, we report two rational methods to prepare mesoporous Ce-Zn-O support materials: (i) coprecipitation of mixed oxalates in a microemulsion of n-hexanol in water (H2O/n-hexanol= 2.5; dimethyloxalate concentration= 4% w/v; T= 333 K, 2h), coded OC, and (ii) coprecipitation of carbonates using ammonium carbonate (final pH= 7.1), coded CC. The theoretical Zn/Ce atomic ratio was varied between 0 and 100%. The washed precipitates were dried at 323 K under vacuum during 12h. The decomposition of the dried solids was studied under flowing 20%O2/N2 by TG, DSC, FTIR, XRD and MS. Basically, the dried OC materials were identified as either ZnC2O4.2H2O, Ce2(C2O4)3.6H2O, or a mixture of both oxalates, which decomposed in two steps by releasing: first, the whole hydration water (up to aprox. 433 K) and, then, CO and CO2 (up to 723 K). During the oxalate decompositions, O2 was consumed only on the materials with cerium in their formulations, which confirmed the oxidation of Ce(III) to Ce(IV). The dried CC materials were either Zn5(OH)6(CO3)2, Ce2(CO3)3.6H2O, or a mixture of both carbonates. The Zn(II) hydroxycarbonate decomposed to ZnO in a single step (ca. 500 K), but cerium(III) hydrate carbonate released water up to 440 K and then CO2 and CO (at 550 K) to yield CeO2. ZnO2 and CeO2 or a mixture of both oxides were the final calcined materials after heating at 723K (4h) under flowing air (W/F= 10-3 g h cc-1). For the complete set of oxide materials, the BET surface areas (SBET) were in the range of 25-90 m2 g-1, and increased with the cerium content. Conversely, the mean pore diameter ranged between 30-40 nm, for pure ZnO, and 4 nm for pure CeO2.