Floriana Vindigni

Sol immobilization technique: a delicate balance between activity, selectivity and stability of gold catalysts

Sol immobilization is a widely used method to prepare gold catalysts. The presence of the protective layer can have a significant influence on catalyst properties by mediating metal–support and reactant–metal interactions. This paper details the effect of a polyvinyl alcohol (PVA) protecting group on the activity of a supported gold catalyst as well as its selectivity towards glycerol oxidation.

CO-free hydrogen production for fuel cell applications over Au/CeO2 catalysts: FTIR insight into the role of dopant.

The impact of ceria doping by Zn (atomic ratio Zn/(Zn + Ce) = 0.05) on the structural and catalytic properties of Au/CeO(2) catalyst was studied. The ceria modification influenced the catalytic activity toward purification of hydrogen via water-gas shift (WGS) and preferential CO oxidation (PROX) reactions in a different way: it diminished the WGS activity and improved the PROX performance. A characterization by FTIR spectroscopy was conducted to explain differences in the catalytic performance. The nature of gold active species after different pretreatments, under different atmospheres (H(2), D(2)), and after admission of CO and its subsequent interaction with (18)O(2) was investigated. Evidence has been found of the dissociation of hydrogen at room temperature on gold, producing on the oxidized sample a broad absorption assigned to Au-OH vibrations, whereas on the reduced one, bands at 3200 and 1800 cm(-1) ascribed, respectively, to Au-OH and Au-H species have been detected. For the first time, the formation of Au-hydride on supported heterogeneous catalysts was proposed. These features were stronger on the Au/CeO(2) sample than on the Au/Zn-CeO(2) sample. The availability of highly dispersed gold clusters in contact with oxygen vacancies on the ceria surface could contribute to higher WGS activity, whereas the steps of small gold particles are the active sites for both CO and oxygen activation during the PROX reaction.

Catalytically active gold sites: nanoparticles, borderline sites, clusters, cations, anions? FTIR spectra analysis of12CO and of12CO-13CO isotopic mixtures

The analysis of FTIR absorption spectra of CO and of12CO-13CO isotopic mixtures is reported for gold catalysts supported on a number of oxides. Relationships between the nanostructure and the CO adsorbing properties of the different gold catalysts are discussed. In particular, on titania supported gold, where particles of metallic nature are present, two kinds of surface sites do adsorb CO, the 6-coordinated corners and the 7- coordinated edges, while the terrace sites, 8 coordinated, are not able to adsorb even at 90 K. Moreover, the adsorbing sites are not isolated, but mutually interacting. On zirconia supported gold, the adsorbing sites, mutually interacting, are exposed at the surface of non metallic gold nanoclusters. Almost isolated and negatively charged gold adsorbing nanoclusters have been highlighted on the sample supported on reduced ceria, and, finally, isolated cationic sites on oxidized gold supported on a mixed ceria-titania oxide have been detected.

Tailoring the selectivity of glycerol oxidation by tuning the acid–base properties of Au catalysts

Supported gold nanoparticles are very effective catalysts for the selective oxidation of glycerol which represents an important bio-derived feedstock. In this paper we report that the acid/base properties, especially the acid site density, of these catalysts are the key factor in tuning the selectivity. A range of supported AuPt catalysts have been prepared by sol immobilization using acidic (H-mordenite, SiO2, MCM-41, and sulfated ZrO2) and basic (NiO and MgO) oxides as supports. In particular, using MCM-41 as the support, a high selectivity to glyceraldehyde, an important labile intermediate, was found.

Effect of ceria structural properties on the catalytic activity of Au-CeO2 catalysts for WGS reaction.

Two gold based catalysts supported on ceria prepared by different methods (urea gelation coprecipitation, UGC, and coprecipitation, CP) have been synthesized and tested in the WGS reaction, showing quite different catalytic behaviors. Interestingly, the two catalysts have the same gold loading (3 wt% Au was inserted by deposition-precipitation) and the FTIR spectroscopy of the adsorbed CO revealed the same amount of gold exposed sites. With the aim to elucidate how the preparation method affects the properties of the support, a morphological, structural and textural characterization has been performed by HRTEM, XRD, BET and Raman analyses, as well as FTIR spectroscopy to probe both the Au and the support exposed sites. It was found that the UGC method gave rise to an enhancement of the defectivity of ceria and to an increase of the reactivity under reductive treatment. Further FTIR measurements of adsorbed acetone demonstrated the presence of two kinds of Ce(4+) sites with different coordination, (CUS) Ce(4+) A and (CUS) Ce(4+) B, on the UGC sample. Such sites can influence the catalytic activity, possibly favoring the water dissociation, making ceria prepared by UGC a better support for Au catalysts than the CP-prepared one.

Structure–reactivity relationship in Co3O4 promoted Au/CeO2 catalysts for the CH3OH oxidation reaction revealed by in situ FTIR and operando EXAFS studies

A strong influence of the amount of the Co3O4 promoter on the catalytic performance in methanol oxidation of different gold catalysts supported on ceria was observed. The activity followed the order: Au/10 wt% Co3O4-doped CeO2 > Au/5 wt% Co3O4-doped CeO2 > Au/15 wt% Co3O4-doped CeO2 > Au/CeO2 ≫ Au/Co3O4. FTIR measurements of adsorbed CO indicate that oxidized gold sites are initially present on the activated samples and that such species are involved in the methanol reaction. Methanol oxidation performed under static conditions gave rise at 75 °C to mainly formate species on Au/CeO2 and to a large variety of different carbonate species on Au/10 wt% Co3O4-doped CeO2. FTIR and EXAFS analyses revealed that the active sites present on the best performing Au/CeO2 catalyst added with 10 wt% Co3O4 are oxidized gold species, close to Co sites, at the interface with the support, which are reduced under reaction conditions. These species are able to activate and to react with oxygen giving rise to formate and carbonate species.

Aerobic oxidation of alcohols on Au/TiO2 catalyst: new insights on the role of active sites in the oxidation of primary and secondary alcohols

A study of secondary, benzylic, and primary alcohols oxidation on a reference Au/TiO2 catalyst is reported. Reactions gave rise to ketones, aldehydes, or esters in good yields. The FT-IR results and catalytic tests data provide to elucidate the reaction mechanism and the specific role of the active sites on titania.Graphical abstract