Insights into dynamic surface processes occurring in Rh supported on Zr-grafted gamma-Al2O3 during dry reforming of methane

FERNÁNDEZ, C.; MIRANDA, N.; GARCÍA, X.; ELOY, P.; RUIZ, P.; GORDON, A.; JIMÉNEZ, R.:
Applied Catalysis B: Environmental 156-157 (2014) 202-212.

DOI: 10.1016/j.apcatb.2014.03.023

Abstract

γ-Al2O3 support was modified with zirconia by the grafting method, using zirconium(IV) n-propoxide as precursor. Rh catalysts were prepared by wet impregnation of pure γ-Al2O3 (Rh/A), Zr modified γ-Al2O3(Rh/100Z-A), and pure ZrO2 (Rh/Z) supports. Rh/A presents a good activity in dry reforming of methane and exhibits a high amount of surface oxidized rhodium (92% of surface Rh atoms) after the reaction. Rh/Z is poorly active and 45% of Rh surface atoms remain reduced during the reaction. The catalytic activity significantly increased when using Rh/100Z-A. Tetragonal ZrO2 crystallites are formed on the surface of modified alumina. It was obtained that, after Rh deposition, the grafted catalyst is composed by Rh/A and Rh/Z, as separated catalytic particles in good contact. To further understand the high activity of the grafted catalyst, mechanical mixtures of Rh/A and Rh/Z catalysts were synthesized. Compared with the single catalysts, an important synergistic effect in methane conversion and yields to CO and H2 was observed when high amounts of Rh/A were mixed with Rh/Z catalyst, along with a promotion of rhodium oxidation in Rh/Z, which leads to a high surface fraction of Rhx (88%) in the mixture. The formation of mixed phases between the supports and the migration of Rh atoms from one phase to the other were discarded. It is suggested that a catalytic cooperation occurs due to the presence and mobility of oxygen species generated by the dissociation of CO2 on Rh/A, which migrate to the Rh/Z catalyst promoting the oxidation of rhodium at its surface, and the catalytic activity. This cooperation mechanism is also expected to operate in the Rh/100Z-A catalyst, between the Rh/A and Rh/Z particles present in the surface. The importance of oxidized Rh for dry reforming of methane is evident from the high catalytic activity achieved when highly oxidized rhodium atoms are present together with the metallic Rh sites where methane activation occurs.

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