F. Boccuzzi

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.

Infrared study of ZnO surface properties: I. Hydrogen and deuterium chemisorption at room temperature

Abstract Hydride and hydroxyl groups formed by reversible hydrogen chemisorption at room temperature give rise to bands which have escaped previous observation. Close examination of the coverage dependence of the ZnH and OH band frequencies reveals appreciable interaction between surface groups, which are regarded as confined in densely populated patches. Novel bands due to irreversibly adsorbed hydrogen, which are associated with bridged OH…O and ZnHZn structures, are also described. A dissociative chemisorption process is proposed.

Chemical and Physical Properties of Copper-Based Catalysts for CO Shift Reaction and Methanol Synthesis

Abstract The modern low-pressure methanol synthesis catalysts are based on copper-containing systems such as Cu/ZnO/Al2O3 and Cu/ZnO/Cr2O3 with various compositions. These catalysts are also highly active for the low-temperature CO shift reaction. For both reactions the nature of the active sites is still an open question.

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.

Infrared study of the adsorption of pyridine on α-Al2O3

Abstract The adsorption and desorption of pyridine has been studied on two preparations of α-Al2O3, obtained by thermal treatment of γ-Al2O3 for different times. Although the X-ray diffraction spectra of the two are indistinguishable and correspond to a pure α-Al2O3 phase, remarkable differences are observed in the surface behavior. These differences are ascribed to the existence at the surface of small and variable amounts of Al ions still possessing a tetrahedral or quasitetrahedral coordination. These centers bring about the formation of two Lewis coordinated pyridine species, both with the 8a mode at \ gn > 1610 cm−1, on the material heat-treated for the shorter time, and of one such species on the oxide treated for the longer time. On both preparations another Lewis coordinated Py species is predominant, with the 8a mode at \ gn ≅ 1596 cm−1, i.e., at the same frequency as pyridine H-bonded to surface OH groups via the N lone pair and assigned to centers involving octahedrally coordinated Al ions. The overall pyridine coverage is fairly low on both materials (⩽20% at a pressure of ~8 Torr), while the concentration of defective centers able to coordinate pyridine is found to be between 0.5 and 1.0/nm2.

Infrared study of ZnO surface properties: II. H2CO interaction at room temperature

Abstract Carbon monoxide adsorption on hydrogen-covered surfaces gives rise to well-defined mixed CO-hydrogen structures in the adsorbed state. The centers adsorbing hydrogen and CO are formed by a triplet of exposed zinc ions and at least one reactive oxygen ion. These active groupings of ions are mutually interacting and grouped to form a bidimensional array which is similar to a patchy reconstructed 0001 face.

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.

An IR study of CO-sensing mechanism on Au/ZnO

Abstract An FTIR and quadrupole mass study of the adsorption and oxidation of CO on an Au/ZnO sample with 16 O 2 and 18 O 2 is presented. The results indicate that CO adsorbed on gold sites at the perimeter interface with ZnO reacts with surface lattice oxygens at the oxide surface to form carbonate-like intermediates. This reaction and the decomposition of the intermediates can be assumed to be mainly responsible for the conductivity changes of the semiconductive metal oxide caused by CO in air. Oxygen, present in the mixture and adsorbed on gold vicinal sites, makes the nucleophilic attack of surface oxygens on the CO easier as a consequence of an electron-withdrawing effect. This effect produces a positivization of the carbon of adsorbed CO by an inductive effect and activates the reactivity of support lattice oxygens on the CO.

Surface chemistry and electronic effects of H2 (D2) on two different microcrystalline ZnO powders

A FT-IR spectroscopic study has been made on two ZnO powders differently prepared: one obtained by thermal decomposition of ZnCO 3 (ex-C), the other by zinc metal combustion (Kadox). The morphological differences between the samples have been proposed to be at the origin of the different reactivity and electronic phenomena. In particular, on the more perfect, lower surface area Kadox microcrystals, H 2 reacts at RT to give hydroxyls and hydrides mainly reversibly adsorbed (type I; the hydrogen irreversibly adsorbed (type II) is mainly due to an IR inactive species formed by a reductive mechanism releasing electrons to the conduction band