To identify the nature and the local structure of the surface of supported catalyst nanoparticles, we have performed a detailed comparative study of CO adsorption on two categories of oxide-supported Palladium catalysts:  (1) polycrystalline MgO and γ-Al2O3 supported Pd metal catalysts prepared by impregnation techniques and characterized by different degrees of regularity and perfection and (2) single-crystal based Pd model catalysts prepared under ultrahigh vacuum (UHV) conditions. The assignment of the CO vibrational frequencies to different types of sites on these systems has allowed a detailed structural characterization. For the Pd model catalyst, at low CO coverage, the infrared (IR) reflection absorption spectra closely resemble the expected behavior for terminations by a majority of (111) facets and a minority of (100) facets. The spectral features are indicative of defect sites such as particle steps and edges. Occupation of the defect sites can be affected by surface contaminations such as atomic carbon. Thus the CO spectra at high coverage can be used as both a structural and chemical probe under reaction conditions, provided that complementary information on the particle morphology is available. For the MgO and γ-Al2O3 supported Pd systems, two distinct narrow bands (ν ≅ 2070 and ≅ 1970 cm-1) have been assigned to linearly bonded and bridge-bonded CO species, on Pd (100)/(111) edges or facets, in agreement with the previous results obtained on model catalysts. The broad character of the 2070 cm-1 feature indicates the simultaneous presence of (100) and (111) faces, with edge and corner sites present at their intersection. The high intensity and the small half-width (fwhm) of the band at 1970 cm-1 on a Pd/MgO sample treated at high temperature, assigned to bridge-bonded CO species, suggests that the metal particles expose faces with a high level of regularity. Further spectroscopic features (ν ≅ 1920−1800 cm-1), are ascribed to the presence of different types of 3-fold hollow sites on (111) faces.

In this work we used several complementary techniques (TEM, TPR, CO chemisorption, EXAFS and FTIR spectroscopy) to understand the effects of the activation temperature and activation atmosphere (air or H 2 ) on the particle size distribution, the fraction, and the type of exposed surface sites of Pd nanoparticles supported on a high surface area SiO 2 -Al 2 O 3 (SA) support. Pd particle distribution has been carefully determined by a high statistic TEM study, from which the cuboctahedral-like shape of the metal particles is demonstrated. Assuming a model of perfect cuboctahedral particles, from the TEM particle size distribution we estimated the expected average Pd first shell coordination number. This value is slightly larger than that directly found by EXAFS owing to the fraction of very small Pd particles (d < 6-8 Å) that basically escape TEM detection. The same geometrical model allows prediction, from TEM particle size distribution, of the metal dispersion observed by CO chemisorption (S/V Chemi ). The S/V Chemi value drops significantly upon increasing the H 2 -reduction temperature. According to TEM, the sintering process can account only for a very small fraction of the S/V Chemi decrease, suggesting an important poisoning of the potentially available Pd surface. This hypothesis is supported by a parallel experiment of thermal decomposition at the same temperature (in absence of H 2 ), showing a S/V Chemi value almost unchanged. FTIR spectroscopy of adsorbed CO, probing the nature of the Pd surface available for adsorption, confirms the hypothesis.

The adsorption of H 2 on high surface area, sintered and smoke MgO samples fully characterized by HRTEM and AFM microscopies has been investigated in the 300-20 K temperature interval by FTIR spectroscopy On high surface area MgO, dissociative adsorption of H 2 has been observed with formation of reversible (absorbing at 3454 and 1325 cm -1 ) and irreversible (absorbing at 3712 and 1125 cm -1 ) OH and MgH species already reported in previous studies at 300 K. Cooling the MgO/H 2 system down to 20 K results in the irreversible formation at about 200 K of new OH (absorbing at 3576-3547 cm -1 ) and MgH (absorbing at 1430-1418 cm -1 ) surface groups never observed before. The spectra recorded at 20 K in H 2 atmosphere also show absorptions in the 4800-4000 cm -1 frequency interval undoubtedly due to molecularly adsorbed species. Decreasing the MgO surface area results in the disappearance of all of the spectroscopic manifestations due to the hydride and hydroxyl groups formed upon dissociative adsorption of hydrogen, whereas those due to H 2 adsorbed in molecular form are maintained (although with much reduced intensity). This behavior is the consequence of the reduction, revealed by HRTEM and AFM, of the concentration of surface defects (cationic and anionic sites located on edges, corners, steps, inverse edges and inverse corners). On the basis of the morphological characterization and of the IR spectroscopic studies, it is concluded that the sites responsible for the H 2 dissociative adsorption are mainly inverse steps "coupled" with edges and corners, whereas more usual "isolated" defects (edges, steps, and corners) adsorb hydrogen only in molecular form. The specific adsorption energy for the formation of molecular Mg nC 2+ ‚‚‚H 2 adducts on Mg 3C 2+ (corners; 7.5 kJ/mol), Mg 4C 2+ (edges; 4.6 kJ/mol), and Mg 5C 2+ (on (100) planes; 3.6 kJ/mol) coordinatively unsaturated sites has been also calculated from the temperature dependence of the intensity of the related IR bands (ν(HH) mode).

Using scanning tunneling microscopy and infrared reflection absorption spectroscopy we have observed that the alpha-Fe2O3(0001) surface exhibits ferryl (Fe=O) groups, which may coexist with domains of the Fe-terminated surface. We therefore fully support ab initio calculations recently reported in the literature [W. Bergmeyer, H. Schweiger, and E. Wimmer, Phys. Rev. B 69, 195409 (2004)]. The close similarity to the results on the (0001) surfaces of Cr2O3 and V2O3 strongly suggests that the M=O termination under certain oxygen pressure conditions is the most stable for the close-packed surfaces of transition metal oxides with the corundum structure.

The acid-catalyzed polymerization and resinification, in the 300-673 K interval, of furfuryl alcohol adsorbed in the framework of a protonic Y zeolite is studied by means of FTIR, Raman, and UV-vis spectroscopies. The idea is that restricted spaces can impose a constraint to the growth of the oligomeric chains, therefore moderating the formation of conjugated sequences responsible for the color of the products and allowing their observation by means of spectroscopic techniques. The detailed study of the evolution of UV-vis, FTIR, and Raman spectra upon dosed amount, contact time, and temperature has allowed the spectroscopic features of some of the single species, either neutral and positively charged (carbocationic intermediates), to be singled out and assigned to understand the mechanism of initiation. The vibrational assignments have been confirmed by computer simulations on model compounds and compared with the results of the mechanistic description of the reaction mechanism made in the past (Choura, et al. Macromolecules 1996, 29, 3839-3850). The spectroscopic methods have been applied in a large temperature range in order to follow also the formation of more complex products into the pores, associated with longer conjugated sequences, gradually filling the open spaces of the zeolite. For samples contacted with furfuryl alcohol at 673 K, this methodology gives information also on the incipient carbonization process, leading to the formation of a carbonaceous replica phase inside the internal porosity of the zeolite.