A Study of the Reactions of Ethanol on CeO2 and Pd/CeO2 by Steady State Reactions, Temperature Programmed Desorption, and In Situ FT-IR

Abstract

The reaction of ethanol on unreduced and H2-reduced CeO2 and 1 wt% Pd/CeO2 has been investigated by steady state reactions, temperature programmed desorption (TPD), and in situ Fourier transform infrared (FT-IR) spectroscopy. Steady state reactions have shown a zero reaction order dependency for diatomic 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 adsorption on both CeO2 and Pd/CeO2 have shown desorption profiles corresponding to unreacted ethanol and various reaction and decomposition products (acetaldehyde, acetone, CO, CO2, and methane). Ethanol conversion to reaction products was increased by the addition of Pd, from 15 and 30% on CeO2 and H2-reduced CeO2, to 71 and 63% on Pd/CeO2 and H2-reduced Pd/CeO2, respectively. Acetaldehyde desorbed in two temperature domains on CeO2, and desorbed as one peak only on H2-reduced CeO2 (555 K), Pd/CeO2 (395 K), and H2-reduced Pd/CeO2 (410 K). Desorption of acetone was observed on all surfaces; however, the desorption temperatures were considerably lower on Pd/CeO2 than on CeO2, suggesting that the formation of acetone on Pd/CeO2 occurs from a different reaction pathway from that of CeO2. Benzene formation was detected only on Pd/CeO2 catalysts, with the H2-reduced Pd/CeO2 catalyst decreasing benzene formation to almost negligible amounts. FT-IR results have shown that ethanol adsorbs dissociatively at room temperature to form adsorbed ethoxide species on all surfaces studied. Acetate species (νas(OCO) 1572 cm−1 and νs(OCO) 1424 cm−1) were detected on the unreduced CeO2 surface at room temperature. In contrast, H2-reduced CeO2, as well as “as prepared” Pd/CeO2, did not show evidence of acetates at room temperature. The decrease of the XPS O(1s)/Ce(3d) ratio in the case of Pd/CeO2 (1.76) together with 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 H2-reduced Pd/CeO2 (1704 cm−1) upon heating to 373 and 423 K, respectively. Adsorbed crotonaldehyde (ν(CO) ca. 1651 and ν(C=C) ca. 1634 cm−1), from the β-aldolisation of two acetaldehyde molecules, was observed on both unreduced and high temperature reduced Pd/CeO2 at 473 K. Carbonates were the remaining species at 625 K and above.