Maela Manzoli

FTIR study of low-temperature water-gas shift reaction on gold/ceria catalyst

Chemisorption and reactivity of the molecules involved in the water-gas shift (WGS) reaction on gold/ceria catalyst have been studied at 90 and 300 K by FTIR spectroscopy. Forward and reverse WGS reaction at 300 K and up to 573 K have been investigated, too. The FTIR results show that gold causes a strong modification of the surface properties of the support. The nanosized metallic gold particles in close contact with defective ceria play an essential role for the genesis of high catalytic activity in WGS reaction at low temperature and appear to be of crucial importance in explaining the remarkably high stability of this catalytic system. An electronic interaction between small gold metallic nanoparticles and ceria has been evidenced.

Gold, silver and copper catalysts supported on TiO2 for pure hydrogen production

A catalytic study of the hydrogen production by CO water gas shift reaction (WGSR) on gold, silver and copper particles supported on TiO2 has been carried out. A deep characterisation of the catalysts by TPR and FTIR has been performed. Silver catalyst exhibits no catalytic activity, copper and gold catalysts show intermediate and very high performances, respectively. These strong differences have been interpreted on the basis of FTIR data of CO adsorption at 90 K and on the effect of coadsorbed species. Gold and copper catalysts, either oxidised or reduced, are able to adsorb CO. Reduced silver catalyst does not adsorb CO at all, while oxidised silver catalyst does quite strongly.

Au/TiO2 nanostructured catalyst: effects of gold particle sizes on CO oxidation at 90 K

Abstract An FTIR study of CO adsorption and oxidation at 90 K on gold–titania catalysts is presented, concerning three nanostructured Au/TiO2 catalysts, with the same gold loading, calcined at three different temperatures and with different gold particle mean sizes of 2.4, 2.5 and 10.6 nm. On all the samples, the CO adsorption and different CO–18O2 interactions were examined. From the experimental results, it can be deduced that: (i) at 90 K, carbon monoxide and oxygen are molecularly and competitively adsorbed on gold step sites; (ii) the reaction of C16O18O formation occurs if carbon monoxide is pre-adsorbed on the calcined small particles, while it is almost completely inhibited if oxygen is pre-adsorbed; and (iii) on the sample with size 10.6 nm, the reaction does not occur at all at 90 K. The role of the concentration of step sites on the mechanism of the reaction is discussed.

FTIR study of methanol decomposition on gold catalyst for fuel cells

The interaction of methanol (m), methanol–water (mw) and methanol–water–oxygen (mwo) on Au/TiO2 catalyst has been investigated by in situ infrared spectroscopy (FTIR) and quadrupole mass spectrometry (QMS) at different temperatures. The aim of the work is to elucidate the nature and the abundance of the surface intermediates formed in different experimental conditions and to understand the mechanisms of methanol decomposition, of steam reforming and of combined reforming reactions. FTIR spectra run at room temperature in the different reaction mixtures show that differently coordinated methoxy species, that is on top species adsorbed on oxygen vacancy sites, on top species on uncoordinated Ti 4þ sites and bridged species on two Ti 4þ ions, are produced in all the mixtures. Quite strong formaldehyde and formate species adsorbed on gold are produced already at 403 K only in the combined reforming reaction mixture. At 473 K, on top species on uncoordinated Ti 4þ sites and methoxy species adsorbed on oxygen vacancy sites reduce their intensity and, at the same time, some formate species adsorbed on the support are produced in the steam reforming and combined reforming mixtures. At 523 K, on both methanol and methanol–water reaction mixtures, no more definite surface species are evidenced by FTIR on the catalysts, while in the methanol–water–oxygen mixture some residual methoxy and formate species are still present. Moreover, methanol is no more detected by QMS in the gas phase. A role of oxygen adsorbed on gold particles near oxygen vacancies of the support in the oxidative dehydrogenation of methanol is proposed. # 2003 Elsevier Science B.V. All rights reserved.

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.

Highly Dispersed Gold on Zirconia: Characterization and Activity in Low‐Temperature Water Gas Shift Tests

Gold-loaded zirconia and sulfated zirconia catalysts were tested in the low-temperature water gas shift reaction. The samples were characterized by N2 adsorption analysis, temperature-programmed reduction, X-ray diffraction, pulse-flow CO chemisorption, FTIR spectroscopy, and high-resolution transmission electron microscopy. A reference catalyst, Au/TiO2, provided by the World Gold Council was investigated for comparison. CO chemisorption and FTIR data indicate the presence of only highly dispersed gold clusters on the sulfated sample and both small clusters and small particles on the non-sulfated sample. Both gold-zirconia catalysts are much more active than the Au/TiO2 reference sample over all the temperature range investigated. The sample prepared on sulfated zirconia exhibits higher stability than the catalyst on unmodified zirconia. The prominent role in the water gas shift reaction of gold clusters in close contact with the support was deduced.

Au/TiO2 nanostructured catalyst: pressure and temperature effects on the FTIR spectra of CO adsorbed at 90 K

Abstract The effects of the equilibrium pressure and of the temperature on the FTIR spectra of CO adsorbed on a nanostructured Au/TiO2 catalyst are discussed. At 90 K, at full coverage, in addition to strong bands assigned to CO adsorbed on Ti4+ cations at usual frequencies, a band related to CO adsorbed on gold sites, significantly red shifted in respect to the usual position, is detected. By reducing the CO pressure, a large fraction of CO adsorbed on titania is desorbed and the band ascribed to CO adsorbed on gold sites blue shifts up to the usual frequency. The possibility that ligand effects of CO coverage on the support play a role is proposed. Starting from an incomplete monolayer, an intensity transfer from the band assigned to CO adsorbed on Ti4+ to that of CO adsorbed on the gold sites is observed. A blue shift is also detected. It appears that the CO molecules weakly adsorbed on the support cations act as precursors for CO chemisorbed on gold sites.

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.

Rationalising the role of solid-acid sites in the design of versatile single-site heterogeneous catalysts for targeted acid-catalysed transformations

A versatile design strategy for rationalising the role of well-defined and isolated multifunctional solid-acid active centres, employing Mg(II)Si(IV)AlPO-5 nanoporous architectures has been demonstrated, with a view to affording structure–property correlations compared to its corresponding mono-substituted analogues (Mg(II)AlPO-5 and Si(IV)AlPO-5). The simultaneous incorporation of Mg(II) and Si(IV) ions, as isomorphous replacements for Al(III) and P(V) ions in the microporous architecture, plays an important role in modulating the nature and strength of the solid-acid active sites in the industrially-important, vapour-phase Beckmann rearrangement of cyclohexanone oxime to produce e-caprolactam (the precursor for renewable nylon-6) and in the isopropylation of benzene to cumene. The structural integrity, coordination geometry and local environment of the active (Bronsted-acid) sites could be rationalised at the molecular level, using in situ spectroscopic techniques, for tailoring the catalytic synergy by adroit design of the framework architecture.