The average Pd oxidation state in Pd/SiO2 quantified by L3-edge XANES analysis and its effects on catalytic activity for CO oxidation

Abstract

The palladium oxidation state of an SiO 2 -supported palladium catalyst was quantitatively determined by Pd L 3 -edge XANES (X-ray absorption near-edge structure) analysis. By changing the time of CO-reduction pre-treatment at 673 K, a series of 5 wt% Pd-loaded SiO 2 catalysts (PdO x/2 /SiO 2 ) containing different amounts of the Pd metal and PdO phases were prepared, and the average oxidation number (x) was estimated from the number of CO 2 molecules formed during the CO-reduction treatment. L 3 -edge XANES spectra of these samples and a reference sample (Pd powder) were recorded, and the white line area of the spectra was evaluated. A good linear relationship was confirmed between the white line area intensity and the average oxidation number (x), indicating that the oxidation state of Pd in structurally unknown Pd samples could be quantitatively determined by a simple XANES analysis. To demonstrate the utility of this method in a catalytic study, the effect of the oxidation number (x) on the CO oxidation activity of PdO x/2 /SiO 2 was also examined, and metallic Pd 0 sites in PdO x/2 /SiO 2 were shown to be active species.

An in situ sample environment reaction cell for spatially resolved x-ray absorption spectroscopy studies of powders and small structured reactors

“…The spectral quality is clearly su cient for proper XANES analysis of both the Pd and Ag based samples. The spectral features of the L III edges are in line with previously reported data for both Pd catalysts [27] and Ag oxides [28]. Although these measurements were car- ried out under stationary air conditions at room temperature feeding of a reaction mixture through the reaction cell is not expected to significantly influence the quality of the spectra and thus these results demonstrate that relevant in situ experiments are possible also in fluorescence mode.…”

Effect of Pd dispersion on the catalytic activity of Pd/Al2O3 for C3H6 and CO oxidation

“…Metallic Pd 0 sites has been reported to act as catalytically active site for CO oxidation [10][11][12][13][14][15]. On the other hand, as mentioned above, metallic Pd 0 sites, which was formed by H 2 reduction as a pretreatment, supported on Al 2 O 3 is partly oxidized to PdO x during the oxidation reaction, leading to the different Pd 0 /PdO x ratio depending on the Pd dispersion.…”

“…For example, Satsuma et al [10,11] evaluated the reaction rate for CO oxidation over Pd/SiO 2 having various average oxidation number of PdO x , and observed that the CO oxidation activity of PdO x /SiO 2 was steeply decreased with the increase in the x value in PdO x . They concluded that the metallic Pd 0 is the active species for CO oxidation over Pd/SiO 2 .…”

Templating metastable Pd2 carboxylate aggregates

“…S11†). 43 Based on comparison of the edge energy for Pd 3 (OAc) 6 , Pd 3 btei, PdZn(OAc) 4 , and Pd metal, the edge energy is inconsistent with the presence of Pd(0) in the exchanged samples.…”