Adsorbed CO Probe Research Articles

Nature of Sintering‐Resistant, Single‐Atom Ru Species Dispersed on Zirconia‐Based Catalysts: A DFT and FTIR Study of CO Adsorption

AbstractRecent studies on the upgrade of cellulosic biomass to renewable chemicals and fuels based on Ru/ZrO2 catalysts have shown that the catalyst contains atomically dispersed, highly stable Ru atoms. FTIR spectra after CO dosage reveal a complex manifold of bands resulting from Ru‐CO carbonyl species. The nature of the atomically dispersed Ru species on monoclinic zirconia, and the origin of their thermal stability are the object of this investigation. Combining density functional theory (DFT) calculations on model systems of both tetragonal and monoclinic ZrO2, with novel experimental data, we provide a basis for the identification of the monoatomic Ru species. Various candidates are explored based on DFT calculations of their intrinsic stability and of the vibrational properties of adsorbed CO probe molecules. The results allow us to discard a number of possible structures, and restrict the analysis to a few potential candidates. Most likely, the atomically dispersed sites result from the interaction of Ru atoms with one OH group of the surface with elimination of H2 by condensation, leaving RuO species strongly bound to the zirconia surface. CO binds strongly to the RuO units forming (RuO)(CO)2 geminal complexes with characteristic signatures in terms of CO vibrational frequency. Despite the formal positive oxidation state of the Ru species, large negative shifts are found in the CO stretching frequency compared with the free CO molecule.

Can the state of platinum species be unambiguously determined by the stretching frequency of an adsorbed CO probe molecule?

The paper addresses possible ambiguities in the determination of the state of platinum species by the stretching frequency of a CO probe, which is a common technique for characterization of platinum-containing catalytic systems. We present a comprehensive comparison of the available experimental data with our theoretical modeling (density functional) results of pertinent systems - platinum surfaces, nanoparticles and clusters as well as reduced or oxidized platinum moieties on a ceria support. Our results for CO adsorbed on-top on metallic Pt(0), with C-O vibrational frequencies in the region 2018-2077 cm(-1), suggest that a decrease of the coordination number of the platinum atom, to which CO is bound, by one lowers the CO frequency by about 7 cm(-1). This trend corroborates the Kappers-van der Maas correlation derived from the analysis of the experimental stretching frequency of CO adsorbed on platinum-containing samples on different supports. We also analyzed the effect of the charge of platinum species on the CO frequency. Based on the calculated vibrational frequencies of CO in various model systems, we concluded that the actual state of the platinum species may be mistaken based only on the measured value of the C-O vibrational frequency due to overlapping regions of frequencies corresponding to different types of species. In order to identify the actual state of platinum species one has to combine this powerful technique with other approaches.

Controlling the Surface Environment of Heterogeneous Catalysts Using Self-Assembled Monolayers

Rationally designing and producing suitable catalysts to promote specific reaction pathways remains a major objective in heterogeneous catalysis. One approach involves using traditional catalytic materials modified with self-assembled monolayers (SAMs) to create a more favorable surface environment for specific product formation. A major advantage of SAM-based modifiers is their tendency to form consistent, highly ordered assembly structures on metal surfaces. In addition, both the attachment chemistry and tail structures can easily be tuned to facilitate specific interactions between reactants and the catalyst. In this Account, we summarize our recent modification approaches for tuning monolayer structure to improve catalytic performance for hydrogenation reactions on palladium and platinum catalysts. Each approach serves to direct selectivity by tuning a particular aspect of the system including the availability of specific active sites (active-site selection), intermolecular interactions between the reactants and modifiers (molecular recognition), and general steric or crowding effects. We have demonstrated that the tail moiety can be tuned to control the density of SAM modifiers on the surface. Infrared spectra of adsorbed CO probe molecules reveal that increasing the density of the thiols restricts the availability of contiguous active sites on catalyst terraces while maintaining accessibility to sites located at particle edges and steps. This technique was utilized to direct selectivity for the hydrogenation of furfural. Results obtained from SAM coatings with different surface densities indicated that, for this reaction, formation of the desirable products occurs primarily at particle edges and steps, whereas the undesired pathway occurs on particle terrace sites. As an alternative approach, the tail structure of the SAM precursor can be tuned to promote specific intermolecular interactions between the modifier and reactant in order to position reactant molecules in a desired orientation. This technique was utilized for the hydrogenation of cinnamaldehyde, which contains an aromatic phenyl moiety. By using a phenyl-containing SAM modifier with an appropriate tether length, > 90% selectivity toward reaction of the aldehyde group was achieved. In contrast, employing a modifier where the phenyl moiety was closer to the catalyst surface biased selectivity toward the hydrogenation of the C═C bond due to reorienting the molecule to a more "lying down" conformation. In addition to approaches that target specific interactions between the reactant and modified catalyst, we have demonstrated the use of SAMs to impose a steric or blocking effect, for example, during the hydrogenation of polyunsaturated fatty acids. The SAMs facilitated hydrogenation of polyunsaturated to monounsaturated fatty acids but inhibited further hydrogenation to the completely saturated species due to the sterically hindered, single "kink" shape of the monounsaturated product. The recent contributions discussed in this Account demonstrate the significant potential for this approach to design improved catalysts and to develop a deeper understanding of mechanistic effects due to the near surface environment.

Unravelling the structure and reactivity of supported Ni particles in Ni-CeZrO2 catalysts

We report about the synthesis and characterisation of two NiCeZrO2 catalysts prepared by one pot-co-precipitation method. The two samples are characterised by similar chemical composition (15wt% Ni, Ce/Zr=80/20 atomic ratio) and properties of the CeZrO2 support, which is a cubic phase solid solution with nanometer sized (5–6nm) crystalline hexagonal/roundish particles. The main differences between the samples are in the NiO particles dispersion and surface area, which were found to be affected by the synthesis pH. Both samples were characterised about their surface properties by employing FTIR spectroscopy of adsorbed CO probe, and tested about their activity and stability in methane steam reforming (MSR) reaction at relatively low temperature (793K). The results show a good activity in MSR irrespective of the NiO size and morphology, with negligible CO formation, but a strong dependence of the catalysts stability upon these parameters. Only the catalyst with smaller (5–10nm) NiO particles was found stable for 250h in MSR at relatively high steam to carbon ratio (S/C=2). This performance could be related to a positive effect of the CeZrO2 support, allowing fast and easy oxygen transfer to and from the NiO/Ni0 active phase, most likely thanks to the occurrence of a strong metal–support interaction.

Surface features of TiO2 nanoparticles: combination modes of adsorbed CO probe the stepping of (101) facets

Integrated studies of CO adsorption on TiO2 materials of different morphology and surface complexity identify, for the first time, frustrated translational CO modes by detecting their combination with the CO stretching mode (νCO). All the considered materials exhibit IR spectra with low-intensity bands in the 2235-2205 cm(-1) range, a region where components due to strong Lewis acid Ti(4+) sites may be present as well. These observations lead to a powerful method for associating high-wavenumber bands to TiO2 surface features and interpreting IR spectra of drastically complex/defective TiO2 materials. The proposed band assignment is based on vibrational analyses of CO-TiO2 theoretical models, indicating that the frustrated translational mode with frequency in the 30-50 cm(-1) range involved in the observed combination bands is perpendicular to the Ti(4+) rows. Our results reveal that this low-energy CO mode is much more sensitive than νCO in probing the TiO2 surface topography, and that its higher-energy components can be specifically associated with the presence of steps on the (101) faces. In a broader perspective, the frustrated translational CO mode surface sensitivity could become a key tool for detecting specific defective sites on TiO2 surfaces.

Influence of the temperature of calcination of bayerite-containing aluminum hydroxide pellets on the water vapor adsorption capacity and acid-base properties of alumina

The paper considers the influence of the temperature of calcination (350–550°C) of alumina-based water adsorbents prepared from bayerite-containing aluminum hydroxide on the static water sorption capacity at high and low relative air humidities and on the acid-base properties of the surface. The acid-base properties of aluminas have been determined by means of IR spectroscopy of adsorbed CO probe molecule (Lewis sites) and by the spin probe method, which was used to determine the concentrations of electron acceptor and electron donor sites. It has been demonstrated that, at a high relative air humidity (60%), there is no correlation between the static capacity of the drier and its acid-base properties, while at low relative humidities (1–1.5%), an increase in the concentration of electron donor sites and a decrease in the concentration of electron donor and Lewis acid sites enhance the water sorption capacity of alumina.

Infrared Spectroscopic Study of the Potential Change at Cocatalyst Particles on Oxynitride Photocatalysts for Water Splitting by Visible Light Irradiation

Visible light-induced changes in Pt/(Ga1–xZnx)(N1–xOx), Pt/TaON, and Pt/LaTiO2N powder photocatalysts were investigated using infrared spectroscopy with adsorbed CO probe molecules on Pt cocatalysts. Among these photocatalyst materials, only the (Ga1–xZnx)(N1–xOx) photocatalyst has achieved overall water splitting with visible-light irradiation, although all of these materials have suitable band gaps for the reaction. For Pt/(Ga1–xZnx)(N1–xOx) and Pt/TaON, the C–O stretching vibrations of adsorbed CO on Pt shifted to a lower frequency under visible light irradiation. However, no frequency shift was observed for Pt/LaTiO2N. This lower-frequency shift indicates a negative electronic potential shift of the Pt cocatalysts, which results in hydrogen evolution. Differences in the light intensity dependence of the potential shifts of these photocatalysts were observed, and the involvement of a different recombination mechanism of photoexcited carriers in these photocatalysts will be discussed. These results clarify some of the reasons for the activities of various photocatalytic materials for overall water splitting.

Mechanical Activation of α-AlF3: Changes in Structure and Reactivity

α-AlF3, the thermodynamically stable rhombohedral phase of aluminum fluoride, was used as starting material to produce nanostructured powders by high-energy ball milling. Both the polycrystalline and the nanostructured powders were studied by XRD, TEM, and 27Al and 19F MAS NMR. Thermally programmed desorption of NH3 and IR spectroscopy of adsorbed CO probe molecules unambiguously demonstrate the formation of Lewis acid and Brønsted acid sites as a consequence of the mechanical impact. Catalytic test reactions using milled α-AlF3 as solid catalyst proved experimentally the theoretically suggested catalytic activity of nanosized AlF3 particles as a result of high structural distortion brought into the solid by milling.

Determination of the Acidity of High Surface AlF3 by IR Spectroscopy of Adsorbed CO Probe Molecules

The surface of highly Lewis acidic high surface AlF3, HS-AlF3, prepared in different ways was investigated by FTIR spectroscopy of adsorbed CO and compared to the previously known and well-characterized β-AlF3. The majority of adsorbed CO shows large blue shifts up to more than 80 cm-1 of the band of the CO stretching frequency, corresponding to very strong Lewis acidic adsorption sites. In contrast to that behavior, most of the CO adsorbed on β-AlF3 shows a smaller blue shift of around 40 cm-1. These results can explain the very high acidity of HS-AlF3 and the differences in catalytic activity between both samples. The influence of water is also investigated. Adsorption of water at room temperature blocks the Lewis acid sites and generates Bronsted acidity. Heating the sample under a certain partial pressure of water followed by degassing results in a significant loss of acidity. Both phenomena were confirmed by IR spectroscopy.

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.