Real‐time XAFS analysis of Rh/alumina catalyst

Abstract

Reduction rates of rhodium supported on alumina were investigated with in situ time-resolved X-ray absorption fine structure (XAFS). The catalytic sample was prepared with 0.5-wt% rhodium on the alumina powder by a usual impregnation method. Two types of XAFS techniques were applied for the experiments: energy-dispersive XAFS and usual step-scanning XAFS. The reduction rates of the rhodium supported on alumina after oxidizing treatment at 500• C were measured with dispersive XAFS. The reducing condition was with 3% H 2 in He at various temperatures from 100 to 250 o C. The spectra of Rh K-edge X-ray absorption near edge structure (XANES) were acquired every 0.2 s. The oxidation state of the rhodium was evaluated by the energy shift of Rh K-edge. The reduction rate of the rhodium was on a line in an Arrhenius plot. Reduction of the rhodium on the alumina in 3% H 2 in He at 900• C after high temperature oxidization at 900 and 1000• C were measured with step-scan XAFS. Though a portion of the rhodium was reduced in a few minutes, the residual rhodium was hardly reduced. The hardly reducible rhodium was considered to be reacted with the alumina support and lost catalytic activities. The ratio of the deactivated rhodium has been estimated as a function of the oxidization temperature and period.

Lab Scale Fixed-Bed Reactor for Operando X-Ray Absorption Spectroscopy for Structure Activity Studies of Supported Metal Oxide Catalysts

“…In marked contrast, most of Rh in the aged catalyst is not sensitive to reduction even up to 450°C as evidenced by the negligible decrease in intensity of the spectra taken at 30°C (initial spectrum, black) and 450°C (green). This indicates that only a small fraction of Rh is reducible in the catalyst, in line with literature [23][24][25]. For example, it was shown that the reducibility of Rh in 0.5 wt% Rh/Al 2 O 3 decreases with increasing aging temperature by H 2 -TPR.…”

“…And, only 14% Rh is reducible in the catalyst that was aged at 702°C for 1 h [25]. However, such a behaviour of Rh is not yet completely understood and different mechanisms were proposed [23,26]. Our recent findings by UltraSTEM suggest that the diffusion of Rh into the alumina lattice is responsible for highly reduction resistant Rh oxide formation [27].…”

Inhibition of Rh sintering and improved reducibility of Rh on ZrO2 nanocomposite with an Al2O3 diffusion barrier

“…Studies have found that Rh sintering is promoted by the reduction of the specific surface area (SSA) of the oxide support due to poor thermal stability [5][6][7][8][9]. Most of these studies investigated Rh/Al 2 O 3 catalysts after high-temperature aging under oxidizing conditions [10][11][12]. The results showed that supported Rh strongly interacted with Al 2 O 3 , and irreducible Rh species were formed.…”

High three-way catalytic activity of rhodium particles on a Y-stabilized La-containing ZrO2 support: the effect of Y on the enhanced reducibility of rhodium and self-regeneration

“…[1][2][3][4][5][6][7][8][9][10][11] Since emission regulations are constantly becoming more stringent worldwide, 12,13 there has been consistent focus on improving three-way catalytic activity and durability. [1][2][3][4][5][6][7][8][9][10][11] Since emission regulations are constantly becoming more stringent worldwide, 12,13 there has been consistent focus on improving three-way catalytic activity and durability.…”

“…This new operation causes highly transient and fluctuating conditions, including high oxygen concentrations that deactivate three-way catalysts. 3,4 In the case of alumina supports, rhodium readily reacts with the support to form different phases at high temperatures. 2 Rhodium is a key catalytic component in three-way catalysts for the effective conversion of CO, hydrocarbons and NO x to H 2 O, CO 2 and N 2 .…”