Morphology-dependent CeO2 catalysis in acetylene semihydrogenation reaction

Abstract

We herein investigated the catalytic performances and reaction mechanisms of CeO2 cubes (denoted as c-CeO2), CeO2 rods calcined at 500 °C (denoted as r-CeO2-500), and CeO2 rods calcined at 700 °C (denoted as r-CeO2-700) in the acetylene semihydrogenation reaction. r-CeO2-500 is most catalytically active while r-CeO2-700 is least. The acetylene semihydrogenation reaction catalyzed by various CeO2 nanocrystals was observed to follow the same reaction mechanism. C2H2 adsorbs at both O and Ce sites on CeO2 surfaces to form C2H2(a) species that facilely hydrogenates stepwise to C2H3(a) and C2H4(a). The surface hydrogenation reactions proceed more facilely at the O site than at the Ce site. The desorption of C2H4(a) proceeds slowly and is the rate-limiting step. c-CeO2, r-CeO2-500 and r-CeO2-700 do not exhibit obvious differences among these elementary surface reactions, but show much different abilities to dissociate H2 and chemisorb C2H2. The C2H2(a) coverage follows an order of r-CeO2-500 ≫ c-CeO2 ≈ r-CeO2-700. The ability to dissociate H2 follows an order of c-CeO2 > r-CeO2-500 ≫ r-CeO2-700. The observed morphology-dependent behaviors of various CeO2 nanocrystals were correlated to their defects and exposed facets. These results successfully demonstrate morphology-dependent CeO2 catalysis in acetylene semihydrogenation reaction and elucidate the underlying mechanisms.