Anna Chiorino

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.

Preparation and characterization of SnO2 and MoOx–SnO2 nanosized powders for thick film gas sensors

Abstract This work gives results about the characterization of SnO 2 materials, prepared via the sol–gel route, pure and Mo 6+ -added. The materials were characterized as powders or thick films using a variety of techniques. The morphology of the powders was analyzed by XRD, SEM, TEM and HRTEM, their texture by volumetric measurements. The morphology of the thick films was analyzed by SEM. The goal of obtaining powders and films made by regularly shaped and nanosized (30÷50 nm) particles, even after thermal treatments at 850°C is attained. FT-IR spectroscopic and electrical measurements were employed on powders and films, respectively, to obtain information on the electronic effect due to the molybdenum addition. FT-IR results show that Mo lowers the intensity of the light scattered by free electrons and the intensity of a broad absorption, previously assigned to the photoionization of V O + [V O + + hν →V O 2+ +e − (c.b.)]. Accordingly, electrical data show that molybdenum markedly lowers (of about 2 orders of magnitude) the conductance of the films in air. Electrical measurements show that Mo lowers the response of tin oxide towards CO, but leaves almost unaltered or enhances its ability to sense NO 2 , depending on the thermal pretreatments. Both pure and Mo-added materials treated at 650°C show the same response to NO 2 . However, for the pure material treated at 850°C the response to NO 2 is halved, while it is almost unaffected by the thermal treatment on the Mo-added materials. The sensing temperature of maximum response is in any case 150°C. FT-IR spectroscopy was also employed to obtain information on the Mo species present on the surface of the materials after treatments in oxygen and on how they are affected in the presence of the different testing gases. Furthermore surface species formed by NO 2 interaction were carefully investigated.

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.

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.

Surface acidity of η-alumina. Part 1.—Pyridine chemisorption at room temperature

The room temperature adsorption of pyridine on η-Al2O3 dehydrated at several temperatures in the 25–700°C range has been studied by i.r. spectroscopy, u.v.-vis reflectance spectroscopy and microgravimetry.Several species are formed that can be identified on the basis of the spectroscopic features of the 8a ring mode, whereas the 8b mode, which is common to all species, yields the total amount adsorbed. Quantitative interpretation of the spectral data is carried out using a computer program.The adsorbed species are interpreted as follows: a liquid-like physisorbed species, a species H bonded to surface OH groups through the nitrogen lone pair and three Lewis coordinated species. The latter are assigned to purely octahedral, tetrahedral–octahedral and purely tetrahedral cationic sites, respectively.The reactions occurring upon Py desorption above 400°C are discussed and a different interpretation is given to that in the literature.

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.

Moisture effects on pure and Pd-doped SnO2 thick films analysed by FTIR spectroscopy and conductance measurements

Abstract Pure SnO2 and Pd/SnO2 (0.4 wt.%) pastes have been prepared starting from SnO2 powder obtained with the procedures generally used for gas-sensor materials. The pastes are printed on 96% alumina substrates for electrical measurements and the layers detached from the support are used for Fourier-transform infrared (FTIR) investigations. Conductance measurements in dry and wet air are presented together with the FTIR spectra for temperatures ranging from 100 to 450°C. By alternating wet and dry air, it has been observed that the conductance of pure SnO2 samples changes in a reversible way only for temperatures over approximately 200°C, while the samples catalytically treated with Pd do not present any significant irreversible component. The IR analysis on pure SnO2 samples treated in wet air at temperatures above 200°C shows the formation of a very broad absorption of electronic nature almost completely destroyed by a subsequent dry-air treatment. The absorption may be due to two families of donor levels at 01.5−0.50 eV, respectively, from the bottom of the conduction band. Changes of the electronic absorption shape, intensity and reversibility to dry-air contact are observed on the same samples treated in wet air below 200°C. These results are compared with those obtained for Pd catalytically modified materials. The conductance and impedance measurements in the presence of methane are presented for both the wet- and dry-air treated pure and Pd-doped thick films.

I.r. spectroscopic study of CO2 adsorption onto η-Al2O3

The adsorption of CO2 at room temperature onto η–Al2O3 dehydrated at various temperatures between 25 and 800°C was studied by i.r. and microgravimetric techniques. The following surface species are formed; three σ-coordinated linear structures, two bicarbonates that can be interconverted, several “organic” carbonates and minor amounts of mono- and bi-dentate carbonates. Relative concentrations of these species depend strictly on pretreatment conditions, while the nature of the species is primarily determined by the coordination of surface cations. The overall coverage is quite small, (θ⩽ 10 %) and nearly the same as in all transition aluminas. A cooperative mechanism for CO2 adsorption is thus invoked to account for this and other features. The interaction between CO2 adsorption and catalytic activity is considered, and the probable nature of the centres responsible for exchange and equilibration reactions is suggested.

Characterization of SnO2-based gas sensors. A spectroscopic and electrical study of thick films from commercial and laboratory-prepared samples

Abstract The aim of this work has been to obtain a better understanding of the influence of both morphology and palladium addition on the electrical and electronic properties of thick SnO2 films. Two different SnO2 powders, one commercial and one prepared in the laboratory, both pure and after Pd addition (0.36 Pd wt.%), have been studied. The morphology of the samples has been analyzed by transmission electron microscopy (TEM), the texture by volumetric measurements. Films made by commercial samples show particles with sharp borders and inhomogeneous in size (from 20 to 200 nm), while in the laboratory films particles with indented borders and very homogeneous in size (30 nm) are present. Fourier transform infrared (FT-IR) and UV–Vis spectroscopies together with impedance and resistivity measurements have been employed to provide information on the electronic and electrical properties of the four samples in wet air or in the presence of reducing gases. In particular, we have investigated the different responses to CH4 of the four films in the presence of wet air at 350 and 450°C. The morphological differences have been proposed to be at the origin of the different electronic phenomena showed by the commercial and laboratory powders. Palladium addition results in a resistivity increase on both commercial and laboratory samples, in wet air, but the effect is particularly enhanced for the laboratory sample. The response to 1000 ppm CH4 admission (measured by the resistivity decrease) becomes greater after palladium addition, but while commercial samples, both pure and with addition of Pd, show a higher response at 450°C, on the laboratory-prepared sample the Pd addition also lowers the temperatures of maximum sensibility from 450 down to 350°C.