Temperature-Programmed Reduction Study on Calcination of Nano-Palladium

Effects of calcination treatment on samples of mono-metallic Pd and Pd77-Ag23 alloy with primary particles around 8 nm were studied with the temperature-programmed reduction technique. Temperature profiles of hydrogen consumption for calcined samples from a stream of 10% H2 in N2 were monitored by a thermal conductivity detector. Two distinct peaks, i.e., a consumption of hydrogen for PdO reduction and a subsequent desorption of hydrogen from bulk palladium hydride, were observed. The extent of palladium oxidation upon calcinations increased with the temperature of calcination (T o): i.e., chemisorption of oxygen on particle surface upon calcination at T o < 373 K, reconstruction into a surface PdO structure at 473 K and incorporation into sublayers to form bulk PdO structure at high T o. Minimum temperature (T r) required for reduction of oxidized palladium by the hydrogen stream was generally low (150 K < T r < 320 K) and increased with the extent of oxidation. A quantitative measurement of hydrogen desorbed from reduced samples suggested a formation of alloy phase in freshly prepared Ag77−Pd23 primary particles. The freshly prepared alloy was inhomogeneous in composition but became homogeneous upon calcination at T o > 673 K.