CO oxidation on colloidal Au(0.80)Pd(0.20)-Fe(x)O(y) dumbbell nanocrystals.

We report a colloidal synthesis of Au(0.80)Pd(0.20)-Fe(x)O(y) dumbbell nanocrystals (NCs) derived from Au(0.75)Pd(0.25) NCs by metal oxide overgrowth. We compared the catalytic activity of the two types of NCs in the CO oxidation reaction (CO + 1/2O(2) → CO(2)), after they had been dispersed on an alumina nanopowder support. In both cases, the surface active sites were identified by means of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The enhanced catalytic performance of the dumbbell NCs (Au(0.80)Pd(0.20)-Fe(x)O(y)) catalyst over that of the initial Au(0.75)Pd(0.25) NCs could be correlated to the presence of the epitaxial connection between the Fe(x)O(y) and the Au(0.80)Pd(0.20) domains (as the main factor). Such connection should result in an electron flow from the metal oxide (Fe(x)O(y)) domain to the noble metal (Au(0.80)Pd(0.20)) domain and appears to influence favorably the nature and composition of the catalytically active surface sites of the dumbbells. Our experiments indicate indeed that, when the metal alloy domain is attached to the metal oxide domain (that is, in the dumbbell), surface Pd species are more active than in the case of the initial Au(0.75)Pd(0.25) NCs and also Au(δ-) sites are formed that were not present on the initial Au(0.75)Pd(0.25) NCs.

A reexamination of the adsorption of CO and nitriles on alkali-metal mordenites: characterization of multiple interactions.

The low temperature adsorption of CO and the room temperature adsorption of propionitrile and ortho-toluonitrile on LiMOR, NaMOR, KMOR and CsMOR zeolites have been investigated by FT-IR spectroscopy. Two different CO species, both most probably located in the main channels coordinated on Na ions at IV and VI sites, have been observed. They are associated to a shift of the CO stretching to higher frequencies, as usual. However, together, more strongly bonded species associated to a slight shift of the CO stretching to a lower frequency are also observed. Similar species, with the CN stretching shifted upwards (weaker adsorption) and with the CN stretching shifted downwards (stronger adsorption) are also observed in the case of the interaction of propionitrile (PrN), a molecule that should enter the main channels, and in the case of the interaction of ortho-toluonitrile (o-TN), whose access to the main channels should be highly hindered. The data show that the species characterized by a stronger adsorption but a lower stretching frequency may form both in the main channels and at the external surface. Their formation is easier with the larger cations. These species are identified as "multiply bonded", possibly to two cations. The evidence for this new interaction, stronger than the usual one site-one molecule species, may change considerably the view of the adsorption chemistry of cationic zeolites, from localized simple sites to cooperative complex interactions.

Surface modification of H-ferrierite by reaction with triethoxysilane.

The interaction of triethoxysilane (TES) with H-ferrierite (H-FER) and its effects on acidity have been investigated by infrared spectroscopy. TES adsorbs only on the external surface of H-FER and allows the almost complete disappearance of the external silanol groups. New SiH groups are formed which appear to be inactive in acid-base interactions. The adsorption of propionitrile, which diffuses into the zeolitic channels, provides evidence for the lack of substantial perturbation of the strongly acidic internal bridging OH groups. On the contrary, the adsorption of the hindered basic probe molecule o-toluonitrile, which cannot penetrate the FER channels, shows that not only terminal silanols but also Al3+ Lewis acid sites present on the external surface of H-FER almost totally disappear after TES treatment. Treatment with TES seems to allow virtually the total deactivation of the H-FER external surface.

Fourier transform infrared spectroscopic study of the adsorption of CO and nitriles on Na-mordenite: evidence of a new interaction.

The low temperature adsorption of CO and the room temperature adsorption of acetonitrile, propionitrile, isobutyronitrile, pivalonitrile, benzonitrile, and o-toluonitrile on Na-mordenite (NaMOR) have been investigated by Fourier transform infrared (FT-IR) spectroscopy. The results have been compared with analogous experiments performed on H-mordenite, Na-X zeolite, and Na-silica-alumina. The Na distribution in NaMOR has also been investigated by X-ray diffraction and far-IR spectroscopy. The conclusions are that Na+ ion distribution is essentially random and that, together with the well-known interaction of the probes with Na+ ions in the side pockets and the main channels, a stronger additional interaction occurs in all cases. This new interaction is likely multiple, involving either more Na+ ions or Na and oxygen species. This interaction is more pronounced with the hindered nitriles, whose access at the main channels is likely forbidden. This suggests that this interaction, which is also observed on Na-X zeolites but not with Na-silica-alumina, occurs at the external mouths of the mordenite channels.