No abstract

Changes in a Au/TiO(2) catalyst during the activation process from an as-prepared state, consisting of supported AuO(x)(OH)(4-2x)(-) species, were monitored with X-ray absorption spectroscopy and FTIR spectroscopy, complemented with XPS, microcalorimetry, and TEM characterization. When the catalyst was activated with H(2) pulses at 298 K, there was an induction period when little changes were detected. This was followed by a period of increasing rate of reduction of Au(3+) to Au(0), before the reduction rate decreased until the sample was fully reduced. A similar trend in the activation process was observed if CO pulses at 273 K or a steady flow of CO at about 240 K was used to activate the sample. With both activation procedures, the CO oxidation activity of the catalyst at 195 K increased with the degree of reduction up to 70% reduction, and decreased slightly beyond 80% reduction. The results were consistent with metallic Au being necessary for catalytic activity.

The adsorption of CO and its reaction with oxygen were investigated using a combination of in situ Fourier transform infrared spectroscopy, step response measurements in a microreactor, (18)O isotopic labeling, and X-ray absorption near edge structure spectroscopy. An as-prepared sample in which Au is present as a surface oxyhydroxy complex does not adsorb CO. On an activated sample in which only metallic Au is detected, 0.18 +/- 0.03 mol CO/(mol Au) are adsorbed on Au at -60 degrees C, which shows an IR band at 2090 cm(-1). When oxygen is present in the gas phase, this species reacts with a turnover rate of 1.4 +/- 0.2 mol CO(mol Au min)(-1), which is close to the steady-state turnover rate. In contrast, there is a very small quantity of adsorbed oxygen on Au. A small IR peak at 1242 cm(-1) appears when an activated sample is exposed to CO. It reacts rapidly with oxygen and is shifted to 1236 cm(-1) if (18)O is used. It is assigned to the possible intermediate hydroxycarbonyl.

The influence of reaction parameters such as temperature, contact time, and O 2 , CO, H 2 , CO 2 , and H 2 O partial pressures on the catalytic performances of 4 wt % CuO/CeO 2 in the CO-PROX process (the preferential oxidation of CO) has been investigated in a fixed-bed reactor. Catalyst appears very active (light-off at 70°C) and selective; reaction kinetics is weakly depressed by CO 2 and H 2 O and not significantly increased with increasing O 2 concentration, while the reaction order in CO is variable with temperature ranging from values <1 up to 110°C and >1 at higher temperatures. A power-law rate equation fitted the experimental data under typical CO-PROX conditions in order to evidence the lower activation energy of the catalytic oxidation of CO with respect to H 2 . A Langmuir-Hinshelwood type reaction rate is able to describe the catalytic behavior in the whole field of experimental conditions explored.