A study of the reactions of ethanol on CeO2 and Pd/CeO2 by steady state reactions, temperature programmed desorption, and in situ FT-IR

The reaction of ethanol on unreduced and H-2-reduced CeO2 and 1 wt% Pd/CeO2 has been investigated by steady state reactions, temperature programmed de sorption (TPD), and in situ Fourier transform infrared (FT-IR) spectroscopy . Steady state reactions have shown a zero reaction order dependency for di atomic oxygen at and above 20%, whilst the addition of Pd to CeO2 decreases the apparent activation energy of the reaction from 75 kJ mol(-1) on CeO2 alone to 40 kJ mol(-1) (Pd/CeO2). TPD experiments following ethanol adsorpt ion on both CeO2 and Pd/CeO2 have shown desorption profiles corresponding t o unreacted ethanol and various reaction and decomposition products (acetal dehyde, acetone, CO, CO2, and methane). Ethanol conversion to reaction prod ucts was increased by the addition of Pd, from 15 and 30% on CeO2 and H-2-r educed CeO2, to 71 and 63% on Pd/CeO2 and H-2-reduced Pd/CeO2, respectively . Acetaldehyde desorbed in two temperature domains on CeO2, and desorbed as one peak only on Hz-reduced CeO2(555 K), Pd/CeO2 (395 K), and H-2-reduced Pd/CeO2 (410 K). Desorption of acetone was observed on all surfaces; howeve r, the desorption temperatures were considerably lower on Pd/CeO2 than on C eO2, suggesting that the formation of acetone on Pd/CeO2 occurs from a diff erent reaction pathway from that of CeO2. Benzene formation was detected on ly on Pd/CeO2 catalysts, with the H-2-reduced Pd/CeO2 catalyst decreasing b enzene formation to almost negligible amounts. FT-IR results have shown tha t ethanol adsorbs dissociatively at room temperature to form adsorbed ethox ide species on all surfaces studied. Acetate species (nu(as)(OCO) 1572 cm(- 1) and nu(s)(OCO) 1424 cm(-1)) were detected on the unreduced CeO2 surface at room temperature. In contrast, H-2-reduced CeO2, as well as "as prepared '' Pd/CeO2, did not show evidence of acetates at room temperature. The decr ease of the XPS O(1s)/Ce(3d) ratio in the case of Pd/CeO2 (1.76) together w ith the absence of acetate formation may indicate partial reduction of the CeO2 support upon the addition of Pd. Adsorbed acetaldehyde was detected on Pd/CeO2 (1711 cm(-1)) and H-2-reduced Pd/CeO2 (1704 cm(-1)) upon heating t o 373 and 423 K, respectively. Adsorbed crotonaldehyde (nu(CO) ca. 1651 and nu(C=C) ca. 1634 cm(-1)), from the beta-aldolisation of two acetaldehyde m olecules, was observed on both unreduced and high temperature reduced Pd/Ce O2 at 473 K. Carbonates were the remaining species at 625 K and above. (C) 1999 Academic Press.