Photon Factory Activity Report 2005 #23 Part B (2006)
Chemistry

7C, NW10A/2005G200

Study of Local Atomic Environment of Metal Ions in Combustion Synthesized
M/CeO2-TiO2 (M = Pd, Pt) Catalysts by X-ray Absorption Fine Structure (XAFS)
Prabhakar Ravaji Sarode1, Kaustubh Ravikar Priolkar1, Gosuke Tateno2 and Kiyotaka Asakura2
1
Goa University, India
2
Catalysis Research Centre, Hokkaido University, Sapporo 001-0021,Japan
In recent years much attention has been focused onCeO2 supported noble metal catalysts for theirapplications in the automotive exhausts, oxygen storage
capacity and stabilization of dispersed metals. Recently it
has been found that the oxygen storage capacity of theseceria supported catalysts can be greatly enhanced bysubstitution of Ti ions to form Ce1-xTixO2 solid solutions[1].
We have synthesized M/CeO2-TiO2 type catalysts with
M = Pd and Pt using novel solution combustion technique.
The particular samples of interest were 2% and 5%
oPd/Ce0.75Ti0.25O2 as prepared and heat treated at 650 C and2% Pt/ Ce0.75Ti0.25O2. The aim of this study was toinvestigate the local environment of the metal ions in thisceria-titania matrix and correlate it to the observed
catalytic properties. The EXAFS spectra were recorded atthe Pd K and Pt LIII edge in these catalysts and modelcompounds on BL7C and at PF using Si(311)monochromator.
The XANES spectra at Pd K and Pt LIII edges clearly
indicate that both Pd and Pt are ionically dispersed in theceria-titania matrix. In fact the XANES spectra of Pd K
edge in Pd/CeO2-TiO2 is similar to that in PdO. The
presence of white line in Pt L3 XANES in Pt/CeO2-TiO2
also points to the presence of ionic Pt in the catalyst
samples.
The fourier transforms (FT) of the Pd K-edge EXAFS
for 2% as prepared and heat treated samples are shown inFigure 1. The FT spectra of both these samples show
three peaks at 1.6Å, 2.6 Å and 3.1 Å. The first peak at 1.6
Å corresponds to Pd-O correlation indicating thereby thatthe first nearest neighbours of Pd are oxygen atoms. The
second and third peaks are likely due to Pd-Pd
interactions but the possibility of the Pd-Ce correlation
cannot be ruled out. In fact, in a similar study on
Pd/CeO2 catalysts we have shown Pd-O-Ce linkages
which are responsible for enhanced catalytic activity [2].
In figure 2, the FT spectra of 2% Pt/Ce0.75Ti0.25O2 as
prepared sample is presented. The spectrum shows astrong peak at about 1.4A most likely corresponding to
Pt-O correlation. In addition there is a broad peakcomprising of at least two correlations in the region 2.2 to3.2 Å. The deconvolution of this broad peak gives twocorrelations at 2.8 Å and 3.0 Å respectively. The firstcorrelation matches well with Pt-Pt interaction in Pt metaland hence could be ascribed to such a interaction even inthe catalyst. The second correlation at 3.0A does notmatch with any correlation either in PtO or PtO2. This

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could be possibly due to Pt-Ce interaction that results due
to substitution of Pt ions in the ceria-titania matrix. Sucha interaction has been observed by us in EXAFS study ofPt L3 edge in Pt/CeO2 catalysts [3]. However a more
careful analysis of EXAFS data is required before
arriving at any conclusion.

Figure 1 Fourier Transform (FT) spectra at Pd K-edge
EXAFS in Pd/CeO2-TiO2 catalysts

Figure 2 Fourier Transform (FT) spectra at Pt L3-edge
EXAFS in Pt/CeO2-TiO2 catalysts
References[1] G. Dutta et al Chem. Mater. (2006) accepted.
[2] K. R. Priolkar et al, Chem. Mater. 14, 2120 (2002)[3] P. Bera et al, Chem Mater. 15, 2049 (2003)