The multipulse titrations and temperature programmed desorption (TPD) experiments were performed in a home-built TAP reactor based on the TAP-2 approach developed by Gleaves et al. . Two independently working, piezoelectrically driven pulse valves generated pulses containing between 10 16 and 10 17 molecules per pulse (CO/Ar: 1.6•10 16 -5.6•10 16 , H2/CO2/Ar: 8.2•10 15 -3.7•10 16 , C 18 O2/Ar: 2.0•10 16 -5.9•10 16 , pulse length: 50 ms) in a highly reproducible way. Pulse sequences were admitted to the three-zone catalyst bed, which was fixed in a tubular quartz glass reactor (215 mm length, 4.0 mm i.d., 6.0 mm o.d.) by stainless steel sieves (Haver & Boecker OHG; aperture 25 μm, transmission 25%). It contained 25 mg Ru/Al2O3 diluted by -Al2O3 (4:1), (Sigma-Aldrich, Germany, 16.7 m 2 g -1 ) surrounded by layers of 25 mg (incoming pulse side) and 100 mg (effluent pulse side) inert SiO2, respectively. This resulted in a bed length of approx. 15 mm. The pulses leaving the micro-reactor, which contained non-consumed reactants as well as species formed upon reaction with the catalyst, entered the analysis chamber via a home-built differentially-pumped gate valve , where they were detected by a quadrupole mass spectrometer (QMG 700, Pfeiffer Vacuum). The background pressure in the micro-reactor was approx. 8•10 -9 mbar and increased to approx. 1•10 -7 mbar during pulsing or during TPD measurements. Ar was used as internal standard in the gas mixtures to enable a quantitative evaluation of the incoming pulse size and thus of the consumption of reactants.