Insight into the effects of different ageing protocols on Rh/Al2O3 catalyst

Abstract

Scheme of the Rh/Al 2 O 3 catalysts aged under different protocols. For the N 2 and the H 2 aged samples, the Rh species mainly exist in large metallic Rh particles on the surface of Al 2 O 3 in comparison to the fresh sample. For the air aged and cyclic aged samples, the RhO x particles with much larger size partially diffuse into the sub-layers of the Al 2 O 3 supports and strongly interact with the support to form inactive Rh(AlO 2 ) y . • The N 2 aged and H 2 aged samples have better activity than the fresh, the air aged and the cyclic aged samples. • The N 2 and the H 2 aged samples have more surface Rh content and better reducibility than the air aged and the cyclic aged samples. • The air ageing and the cyclic ageing protocols result in strong interaction between Rh species and the Al 2 O 3 support. In this work, a catalyst of Rh loaded on Al 2 O 3 was prepared by impregnating method with rhodium nitrate aqueous solution as the Rh precursor. The catalyst was aged under different protocols (lean, rich, inert and cyclic) to obtain several aged samples. All the Rh/Al 2 O 3 samples were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) method, CO-chemisorption, H 2 -temperature programmed reduction (H 2 -TPR), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). It was found that a specific ageing treatment could strongly affect the catalytic activity. The N 2 aged and the H 2 aged samples had a better catalytic activity for CO + NO reaction than the fresh sample while the air aged and the cyclic aged samples exhibited much worse activity. More surface Rh content and better reducibility were obtained in the N 2 and the H 2 aged samples and the Rh particles existed with an appropriate size, which were all favorable to the catalytic reaction. However, the air and the cyclic ageing protocols induced a strong interaction between Rh species and the Al 2 O 3 support, which resulted in a severe sintering of particles of Rh species and the loss of active sites. The structure evolution scheme of the catalysts aged in different protocols was also established in this paper.