Adsorption and reaction of CO on (Pd–)Al2O3 and (Pd–)ZrO2: vibrational spectroscopy of carbonate formation

γ-Alumina is widely used as an oxide support in catalysis, and palladium nanoparticles supported by alumina represent one of the most frequently used dispersed metals. The surface sites of the catalysts are often probed via FTIR spectroscopy upon CO adsorption, which may result in the formation of surface carbonate species. We have examined this process in detail utilizing FTIR to monitor carbonate formation on γ-alumina and zirconia upon exposure to isotopically labelled and unlabelled CO and CO2. The same was carried out for well-defined Pd nanoparticles supported on Al2O3 or ZrO2. A water gas shift reaction of CO with surface hydroxyls was detected, which requires surface defect sites and adjacent OH groups. Furthermore, we have studied the effect of Cl synthesis residues, leading to strongly reduced carbonate formation and changes in the OH region (isolated OH groups were partly replaced or were even absent). To corroborate this finding, samples were deliberately poisoned with Cl to an extent comparable to that of synthesis residues, as confirmed by Auger electron spectroscopy. For catalysts prepared from Cl-containing precursors a new CO band at 2164 cm−1 was observed in the carbonyl region, which was ascribed to Pd interacting with Cl. Finally, the FTIR measurements were complemented by quantification of the amount of carbonates formed via chemisorption, which provides a tool to determine the concentration of reactive defect sites on the alumina surface

Theoretical Study of the Methanol Dehydrogenation on Platinum Nanocluster

Методом функционала плотности изучена реакция дегидрирования метанола по механизму разрыва O-H-связи на нанокластере платины Pt79, проведено сравнение с идеальной поверхностью Pt(111). Найдено, что наиболее устойчивые комплексы образуются при адсорбции COНх (x = 1-4) частиц на низкокоординированных атомах нанокластера Pt79, при этом такой предпочтительности для атомов Н не обнаружено. Абсолютные значения энергии адсорбции на вершинах и ребрах нанокластера Pt79 выше на 0,2–0,7 эВ, чем на высококоординированных центрах регулярной поверхности Pt(111). Стабильность адсорбционных комплексов на поверхности нанокластера уменьшается от вершин к ребрам и затем к центру граней (111) нанокластера. Анализ энергетического профиля реакции показывает, что тепловой эффект образования ключевого интермедиата CH3O на кластере Pt79 становится нулевым в отличие от эндотермического (0,5 эВ) на регулярной поверхности Pt(111). Экзотермический эффект всех остальных реакционных стадий, за исключением десорбции СО, на нанокластере увеличивается на ~0,2-0,5 эВThe methanol dehydrogenation through the initial breaking of the O-H bond at Pt79 nanoparticle was studied with the DFT method. The comparison with an ideal surface of Pt (111) was carried out. The most stable complexes were found for COНх (x = 1-4) species adsorbed at low-coordinated atoms of nanocluster Pt79, whereas no preference for adsorption at corners and edges for Н atoms was found. The absolute adsorption energies of COНх species at corner and edge sites of platinum nanocluster increased by 0.2–0.7 eV in comparison with high-coordinated sites of the regular Pt(111) surface. The stabilization effect of adsorption at the nanoparticle decreases from corners to edges and then to the center of (111) facet. According to the reaction energy profile, the thermal effect of the formation of CH3O at the nanocluster becomes close to zero, in contrast to the endothermic effect (0.5 eV) on the regular Pt(111) surface. The exothermic effects for other reaction stages at the platinum nanocluster, excluding CO desorption, increase by ~0.2-0.5 e