A study of ethanol reactions over Pt/CeO2 by temperature-programmed desorption and in situ FT-IR spectroscopy: Evidence of benzene formation

The reaction of ethanol over unreduced and H-2-reduced 1 wt% Pt/CeO2 hits b een investigated primarily by temperature-programmed desorption (TPD) and i n situ Fourier transform infrared spectroscopy (FT-IR). Steady state reacti ons have been conducted to pro, ide information regarding the kinetics of t he above reaction. Characterisation of the catalyst has been achieved throu gh the use of XPS and titration of surface metal sites with CO. XPS studies have shown that the addition of Pt partially reduces the CeO2 surface, as indicated by the decrease in the O/Ce ratio from the stoichiometric value t o 1.59. FT-IR studies have shown, that ethanol adsorbs dissociatively to fo rm two types of ethoxides (monodentate and bidentate). The presence of Pt r esulted in a Delta v(C-O)= ca. -20 cm(-1), for both species, with respect t o observed ethoxide band positions on CeO2 alone. This shift may indicate t hat these species are perturbed due to the presence of Pt2+ clusters and th e surface oxygen vacancies associated with them. The oxidative dehydrogenat ion of these ethoxide species produces acetaldehyde on both the unreduced a nd H-2-reduced Pt/CeO2 surfaces. Adsorbed crotonaldehyde species were obser ved and characterised by bands at 1657 and 1632 cm(-1) corresponding to v(C =O) and v(C=C), respectively. TPD experiments have shown that between unrea cted ethanol and acetaldehyde, benzene is produced in appreciable amounts f rom both the unreduced and H-2-reduced surfaces of Pt/CeO2. The formation o f benzene has been postulated to proceed via reaction of surface-bound crot onaldehyde and acetaldehyde species. Additional studies involving the adsor ption of CO over H-2-reduced CeO2 and Pt/CeO2 have been conducted. FT-W res ults show that CO does not adsorb appreciably on CeO2. In addition to linea r and bridged CO species, a tilted CO species was identified, by a broad ba nd at ca. 1704 cm(-1) on Pt/CeO2 following adsorption of CO. Carbonite spec ies (1325, 1297, 1190, and 1082 cm(-1)) were also detected on Pt/CeO2 follo wing adsorption of CO and subsequent heating. TPD following CO adsorption o n both CeO2 and Pt/CeO2 have shown that the predominant desorption product is CO2. CO2 desorbs from both CeO2 and Pt/CeO2 in three temperature domains . The lower desorption temperatures, 400 and 525 K, correspond to oxidation of CO to CO2 and decomposition of formate species, respectively. The high- temperature (700 K, CeO2 and 578 K, Pt/CeO2) CO2 desorptions arise from the decomposition of hydrogenocarbonates and carbonate species. (C) 2000 Acade mic Press.