Introduction Methanol is one of the most important products synthesized industrially. The catalyst applied for the synthesis of methanol is a ternary system containing copper (Cu), zinc oxide (ZnO), and alumina (Al 2 O 3 ) [1, 2]. Although this catalyst has been used during the last four decades and in spite of the importance of methanol synthesis, the nature of the active site and the mechanism of the methanol synthesis are still under debate. The active site is described e.g. as metallic copper, isolated Cu(I) ions in the ZnO matrix, Cu(I) species stabilized by Cu–Zn alloys, the Cu/ZnO interface, or strained copper particles [3–9]. Results presented by Muhler et al. [10–14] indicate that under conditions of methanol synthesis copper is completely reduced to a zero-valent state, the surface of the metallic copper particles is not covered by significant amounts of atomic oxygen and strong metal-support interactions (SMSI) exist between copper and zinc oxide. Askgaard et al. [15] presented a detailed kinetic model for the synthesis of methanol over copper catalysts based on surface science studies. Following their model methanol is synthesized from carbon dioxide only. Adsorbed carbon dioxide is hydrogenated to methanol and water via formate as stable intermediate. The model includes the water-gas shift reaction of carbon monoxide and water. One of the elementary reaction steps on the copper surface is the reaction of adsorbed carbon monoxide with adsorbed atomic oxygen. Hinrichsen et al. [16] studied the chemisorption of nitrous oxide (N 2 O) and hydrogen (H 2 ) for the determination of the free copper surface area of copper catalysts. The authors were able to show that N 2 O reactive frontal chromatography (N 2 O RFC) carried out under carefully chosen conditions resulted in the formation of a complete monolayer of chemisorbed atomic oxygen without bulk oxidation of copper. In the present contribution, the surface reaction of CO with atomic oxygen preadsorbed on the surface of supported copper particles is studied by adsorption microcalorimetry and transient kinetics. Experimental Samples The investigated samples are a ternary and a binary catalyst containing copper, zinc oxide and alumina: Cu 50 ZnAl and Cu 75 Al. Cu 50 ZnAl is a catalyst of industrial interest with a nominal content of 50% Cu. Cu 75 Al is a model catalyst with a Cu content of 75%. The samples are prepared by coprecipitation using solutions of nitrates and sodium carbonate, followed by aging, washing, drying and calcination. The catalysts are characterized by nitrogen physi- sorption measurements (BET surface area), temper- ature-programmed reduction (copper content), and N 2 O RFC (reduced copper surface area). The prepar- ation and the characterization of the samples are described in detail elsewhere [16–18]. Table 1 summarizes the main characteristics of the used samples. 1388–6150/$20.00 Akadémiai Kiadó, Budapest, Hungary © 2008 Akadémiai Kiadó, Budapest Springer, Dordrecht, The Netherlands Journal of Thermal Analysis and Calorimetry, Vol. 91 (2008) 1, 173–179 THE BACK-TITRATION OF CHEMISORBED ATOMIC OXYGEN ON COPPER BY CARBON MONOXIDE INVESTIGATED BY MICRO- CALORIMETRY AND TRANSIENT KINETICS R. Naumann d’Alnoncourt * , B. Graf, X. Xia and M. Muhler Laboratory of Industrial Chemistry, Ruhr-University Bochum, 44780 Bochum, Germany The back-titration of atomic oxygen chemisorbed on metallic copper using carbon monoxide is investigated by microcalorimetry. Results from simulations based on a microkinetic model of the back-titration are used for processing of microcalorimetric data. In addition, surface oxidation of copper by nitrous oxide is investigated by microcalorimetry. The results are compared with results obtained by nitrous oxide reactive frontal chromatography and by static oxygen adsorption studied by microcalorimetry. The heat of adsorption of nitrous oxide on copper amounts to 304 kJ mol –1 , and the heat of adsorption of carbon monoxide on surface- oxidized copper is in the range from 120 to 70 kJ mol –1 . Keywords: adsorption microcalorimetry, carbon monoxide, copper, methanol synthesis, nitrous oxide * Author for correspondence: raoul@techem.ruhr-uni-bochum.de