XANES Studies of the Reduction Behavior of (Ce1-y Zry )O2 and Rh / (Ce1-y Zry )O2
Catalysis Letters, 51 (1998), 133
The oxidation and reduction of small ceria particles at
atmospheric pressures were studied by x-ray absorption spectroscopy (XAS) at the
Ce LIII edge on Beamline X19a at the National
Synchrotron Light Source. Particles
with surface areas ~ 100 m2/g were synthesized using sol-gel
techniques and hypercritical drying. In
addition to pure CeO2, mixed oxides were prepared with Zr forming CeXZr1-XO2
particles. Some of these oxide were
impregnated with 0.5 - 2% Rh, by weight. The
oxides were reduced in 4% H2/He and were oxidized in zero gas air.
The degree of reduction can be determined from several features in the Ce LIII XAS. As shown at right, the relative amount of Ce+3 was proportional to the intensity of peak B0 while the amount of Ce+4 was proportional to peaks B1 and C. Each sample was fully oxidized in air and then measured at 300 K to establish a reference spectra for pure Ce+4. Subsequent spectra were measured in flowing H2 / He at the temperatures indicated. Difference spectra were created by subtracting the reference spectrum of pure CeO2. The Ce+3 content was determined from the peak to peak intensity of (B0 - C) in the difference spectra. CeF3 and reagent grade CeO2 were used as standards for Ce+3 and Ce+4 spectra, respectively. The Ce+3 intensity was normalized to the (CeF3 - CeO2) difference spectrum.
The
greatest enhancement in the reducibility of CeO2 was
achieved when both Rh and ZrO2 were
added to the samples. lThe
addition of Rh lowered the onset of reduction to < 400 K in all samples
while the addition of ZrO2 raised
the degree of reduction achieved at a given temperature.
