CO Probe Molecules Research Articles

Interaction of Ag, Rh, and Pd Atoms with MgO Thin Films Studied by the CO Probe Molecule

The interaction of carbon monoxide with Ag, Rh, and Pd atoms deposited on MgO thin films has been studied by thermal desorption and Fourier transform infrared spectroscopies. To obtain an atomistic view of the surface complexes, we performed density functional calculations on cluster models of the MgO(100) surface. The combined experimental−theoretical information leads to the following picture. Ag atoms interact weakly with the surface and do not form stable complexes with CO. Pd atoms become trapped at oxygen vacancies (F centers) already at low temperature; at these sites Pd(CO)2 complexes form after exposure to CO. The CO molecules desorb from these sites at T = 230 K. At higher temperature, 370 K, we observe formation of small Pd aggregates with bridge-bonded CO molecules, indicating diffusion of Pd atoms or Pd(CO) units. Rh atoms bind quite strongly to the oxide anions at steps where they form relatively stable Rh(CO)2 or even Rh(CO)3 complexes; a minority of the Rh atoms populates the F centers. A ...

Redox properties of Re2O7/Al2O3 as investigated by FTIR spectroscopy of adsorbed CO

This paper reports the first use of CO probe molecules in an IR investigation of the redox behaviour of alumina supported rhenium. After O2 treatment the rhenium surface species are coordinatively saturated and do not allow the adsorption of CO. Treatment with H2 at 823 K leads to the complete reduction of all rhenium species to rhenium metal. However, partially reduced rhenium Rex+ (where 0<x<7) species can be generated through treatment with CO at 333 K, or similarly by exposure to isobutene at the same temperature. These results provide further evidence that partially reduced rhenium species are involved in the metathesis of alkenes.

Evolution of Extraframework Iron Species in Fe Silicalite: 1. Effect of Fe Content, Activation Temperature, and Interaction with Redox Agents

A study was made of the iron species dislodged from the tetrahedral lattice position by thermal treatments in four Fe silicalite samples with Si/Fe ratios of 25, 50, 90, and 150. The last two samples have a sufficiently low iron concentration to be considered true catalysts (i.e., having Fe dilutions as low as that used in the industrial conversion of benzene to phenol (Si/Fe>80)) in partial oxidation reactions. A systematic spectroscopic study is presented in order to obtain a global picture of a system whose complexity can be understood by assuming the formation of a wide variety of transient coordinatively unsaturated iron species, with different oxidation states, which ultimately lead to grafted or anchored isolated and/or clustered species by interaction with residual tetrahedral Si(OH)Fe, defective SiOH groups, or strained SiOSi bridges of the hosting matrix. The structure characterization was obtained by means of the parallel use of IR, X-ray absorption near-edge spectroscopy, and electron paramagnetic resonance techniques, before, during, and after interaction with CO, N2O, and NO probe molecules. This approach was applied to samples characterized by different iron content, activation temperature, and redox treatments. This allowed us to throw light onto structure, oxidation state, and mutual interconversion of a very complex family of extraframework iron species. Oxidation with O2 or N2O led to the formation of strongly bonded oxygen species. The relation between these species and the so-called α-oxygen species, claimed as the active sites for the hydroxylation of benzene to phenol by N2O, is discussed.

Interaction between probe molecules and zeolites.

An important problem when studying the interaction between a CO probe molecule and a Na4Ca4A type zeolite is the estimation of the central repulsive coefficients versus the internuclear distance of CO. In particular, this dependence cannot be estimated in the case of the unstable linear “framework oxygen–CO molecule” pair due to the electrostatic repulsive interaction. Hence, we discuss the application of two approximate forms of this dependence either allowing or disregarding the repulsive contribution in the interval wherein the vibrational CO probability distribution cannot be neglected. The consequences of these approximations are compared through calculation of the interaction energy and band shift of CO adsorbed inside Na4Ca4A. The CO spatial parameters (semi-axes) are estimated by fitting both the band shift, corresponding to two different positions of CO relative to the zeolite, and the interaction energy values to the experimental data obtained at small coverage.

Catalytic combustion of methane on Co/MgO: characterisation of active cobalt sites

A series of Co/MgO catalysts with 3–12wt.% Co were prepared by impregnation and calcined at 1073K for 10h. The catalytic behaviour of these samples toward CH4 combustion was found to increase with cobalt loading, though a plateau was reached at ca. 9wt.% Co content. Bulk characterisation was carried out using XRD, TPR and Raman spectroscopy, and showed that the solids were made up of a CoO–MgO solid solution and a MgO phase. A detailed examination of their surfaces was achieved through FTIR spectroscopy of adsorbed CO probe molecules, which indicated that at low cobalt loadings only a small proportion of the Co going into the solid solution was present on exposed faces as either Co2+ oxo-species or pentacoordinated Co2+. However, as the cobalt content of the samples increased, a larger amount was exposed on the surface. This effect levelled off at 9wt.% Co, after which the increase in exposed Co2+ sites was countered by the masking effect of islands of MgO. In addition, at high cobalt loadings (9 and 12wt.%) Co formed small clusters which showed bulk CoO-like behaviour. Consequently, the benefit of having surface Co2+ species was balanced by the clustering effect of these species and the presence of MgO islands, negating their contribution to the overall catalytic activity of the samples.

Enhanced surface acidity in mixed alumina–silicas: a low-temperature FTIR study

The investigation of the surface acidity in commercially available alumina–silicas through FTIR spectroscopy of adsorbed CO probe molecules has been reported. The modification of γ-alumina with silica led to the creation of both highly acidic Lewis and Brönsted acid sites (BASs); the former through isomorphous substitution of Si 4+ ions by Al 3+ ions at tetrahedral lattice sites; and the latter through formation of bridged hydroxy groups, similar to those found in zeolites. The relative strength and quantity of these sites reached a maximum with 40 wt.% silica content, above which the surface of the samples became silica coated and exhibited acidity approximating that of pure silica. This paper highlights (i) the suitability of CO as a probe molecule for the investigation of surface acidity, capable of differentiating between sites of very similar acid strength and/or coordination, and (ii) the enhancement of surface acidity in alumina through silica addition and subsequent formation of a mixed alumina–silica phase.

Modification of structural and chemisorption properties of small palladium clusters supported on the MgO(0 0 1) surface from density functional calculations

We have performed gradient corrected density functional calculations on Pd tetramers and pentamers deposited on the regular MgO(0 0 1) surface as well as on the chemisorption of CO molecules on such Pd clusters. These systems can be considered as models of supported metal catalysts on a fully dehydroxylated MgO surface. The oxide surface has been represented by MgO clusters embedded in total ion model potentials and an array of point charges. The geometrical structures of Pd, Pd 4, and Pd 5 supported on the MgO substrate have been optimized partially and the corresponding electronic properties have been analyzed. Both two- and three-dimensional Pd clusters have been considered. Changes in the electronic structure of the clusters induced by interaction with the substrate have been analyzed by adsorbing a CO probe molecule on free and supported Pd clusters. Perturbation of the metal cluster due to interaction with the support, although not large, has two important consequences: a weakening of the metal–metal bonds which results in an elongation of the Pd–Pd distances within the cluster, and a change in the preferred adsorption site. On the other hand, the geometrical and vibrational properties of the adsorption complexes CO–Pd/MgO and CO–Pd do not differ significantly.

Reactivity of oxide surfaces modified by palladium atoms

We report CO adsorption studies on modified surfaces of Al 2O 3 and MgO. The oxides were bulk crystals (α-Al 2O 3 and MgO) or thin films (γ-Al 2O 3); they were sown by palladium atoms. By using static secondary ion mass spectroscopy (SSIMS), the surfaces were in situ characterized with respect to metal coverage. We found that upon deposition at room temperature, 3D palladium clusters or chains are formed on alumina depending on the oxide stoichiometry. On MgO cleavage, 2D islands grow. We show that the work function of metal deposit varies in the 8–6 eV range during the early stages of deposition. The adsorption of CO probe molecules on ultrathin Pd deposits was studied, results on their chemical reactivity are discussed with respect to the morphology of the metal clusters.

Small Platinum Clusters in Zeolites: A Density Functional Study of CO Adsorption on Electronically Modified Models

Very small transition metal particles can be stabilized inside zeolite cavities. Both electron-enriched and electron-deficient encapsulated metal species have been proposed on the basis of experimental data. In this work, structure and adsorption properties of the cluster Pt4, in both neutral and electronically modified forms, have been studied computationally with the help of a scalar-relativistic density functional method. The species Pt4+ has been chosen to represent the case of a metal particle interacting with an electron attracting zeolite host; likewise, Pt4- has been taken to mimic the effect of an electron-donating host. Adsorption of CO probe molecules at on-top, bridge, and 3-fold hollow sites of the moieties Pt4, Pt4+, and Pt4- has been investigated to determine a relationship between the cluster charge and the C−O vibrational frequency shift Δω(CO). The chemical effect of electron-donor and electron-acceptor species on the electronic structure of the Pt4 clusters and on the properties of adso...

Acidic properties of [Al], [Ga] and [Fe] isomorphously substituted zeolites. Density functional model cluster study of the complexes with a probe CO molecule

The Brønsted acid strength and related characteristics of bridging hydroxyl groups in [Al]-, [Ga]- and [Fe]-framework-substituted zeolites have been studied using a model cluster density functional approach based on a gradient-corrected exchange-correlation energy functional. The acidity is found to decrease in the order Al(OH)Si > Ga(OH)Si > Fe(OH)Si in agreement with existing experimental and theoretical results. The present quantification is based on the calculated deprotonation energy of H 3Si(OH)TH 3, on the adsorption energy of a CO probe molecule as well as on the changes of the vibrational frequencies and absolute IR intensities of the OH and of the CO modes induced by CO adsorption. The vibrational parameters of the carbonyl adsorption complex in [Fe]-zeolites are predicted.

Surface properties of the Ni-Cu-Cr oxide catalyst for dehydrogenative hydrolysis of amines

Catalysts for dehydrogenative hydrolysis of amines, prepared by sorption of copper ions from a solution on a preformed Ni-Cr oxide system, were studied by Auger electron spectroscopy, X-ray photoelectron spectroscopy, and IR spectroscopy of adsorbed CO and NH3 probe molecules. It was shown that on adsorption copper blocked the chromium ions in the Ni-Cr catalyst with concomitant stabilization as Cu+. The incorporation of copper into the Ni-Cr system increased the ability of nickel to reduce water with the formation of oxygen-containing complexes.

Study of the nature and properties of surface centers of a Ni-Cr oxide catalyst according to ir spectra of adsorbed CO and NH3 probe molecules

The modification of a Ni-Cr oxide catalyst during reduction was studied by IR spectroscopy of adsorbed CO and NH3 probe molecules. It was determined that in the starting oxidized sample the nickel cations were in the form of oxide clusters distributed over the surface of chromium oxide. As a result of reduction, they were depleted in oxygen, and metal clusters were formed and enlarged. Effective oxidation, by water vapor, of part of the surface of the metallic-nickel clusters that formed during reduction was observed.

Characterization of supported NiICO(TAP)n(n &lt; 3) complexes by the CO stretching vibration. Electron-donating effect of trialkylphosphine ligands

The i.r. characterization of supported NiI trialkylphosphine (TAP) complexes NiI(TAP)n(n < 3) is performed using a CO probe molecule. The affinities of NiII and NiI ions for CO are compared. It is evidenced that TAP is a stronger ligand than CO: NiI(TAP)n(n < 3) complexes cannot coordinate more than one CO ligand at low CO pressure, whereas phosphine-free NiI complexes can coordinate two CO molecules. The increase of electron density on the metal ion causes a shift downwards of the νco frequency, proportional to the number and to the basicity of TAP ligands. It also produce a weakening of the M—CO bond, resulting in the easier removal of CO.

Infrared spectroscopic study of the adsorption of CO, CO2 and NO on fluorinated alumina and supported Molybdenum–Nickel catalysts

Adsorption of CO, CO2, NO and O2 probe molecules has been used to study the surface of fluorinated alumina and of supported Mo–Ni catalysts. Addition of F to alumina drastically inhibited formation of carbonates from CO (at 673 K) and CO2(at room temperature) adsorption and also inhibited the formation of nitrates from NO adsorption at room temperature. Reduction with H2(773 K) followed by O2 and NO adsorption at low temperatures suggest that the active-phase Mo–Ni is more highly dispersed in the catalyst with higher F content. These data are in agreement with spectroscopic results for NO adsorption. This effect of F on the degree of metallic dispersion is probably associated with the anchoring of the polymeric ion Mo7O246– to the support, which is highly unfavoured by the partial substitution of hydroxyl groups by F–.