Cu-Bound Formates are Main Reaction Intermediates during CO2 Hydrogenation to Methanol over Cu/ZrO2.

Cu/ZrO2 is a promising catalyst for the hydrogenation of CO2 to methanol. Reaction pathways involving formates or hydroxycarbonyls have been proposed. We show here that three different types of formates can be observed under reaction conditions at 220 °C and 3 bar, one being located on metallic Cu and two others being bound to ZrO2 . The surface concentrations of formates were determined through calibration curves and their reactivity measured during chemical transient experiments. The Cu-bound formate represented only about 7 % of surface formates, but exhibited a higher reactivity and was found to be the only formate that could account for all the production of methanol. Copper is thus not there only to activate H2 , but also bears other crucial intermediates. This work reemphasizes that fully quantitative IR analyses and transient methods are required to unravel the role of surface species.

Synergy between Metallic and Oxidized Pt Sites Unravelled during Room Temperature CO†Oxidation on Pt/Ceria.

Pt-based materials are widely used as heterogeneous catalysts, in particular for pollutant removal applications. The state of Pt has often been proposed to differ depending on experimental conditions, for example, metallic Pt poisoned with CO being present at lower temperature before light-off, while an oxidized Pt surface prevails above light-off temperature. In stark contrast to all previous reports, we show herein that both metallic and oxidized Pt are present in similar proportions under reaction conditions at the surface of ca. 1 nm nanoparticles showing high activity at 30 °C. The simultaneous presence of metallic and oxidized Pt enables a synergy between these phases. The main role of the metallic Pt phase is to provide strong adsorption sites for CO, while that of oxidized Pt supposedly supplies reactive oxygen. Our results emphasize the complex dual oxidic-metallic nature of supported Pt catalysts and platinum's evolving nature under reaction conditions.

Unravelling Platinum-Zirconia Interfacial Sites Using CO Adsorption.

Understanding platinum (Pt) speciation on catalysts is crucial for the design of atom-efficient materials and optimized formulations. The adsorption of carbon monoxide (CO) as a probe molecule is widely used to reveal Pt dispersion and structures, yet the assignment of IR bands is not straightforward, hindering determination of the nature of the surface sites or ensemble involved. CO adsorption was studied here over a zirconia-supported Pt catalyst. Specific sites at the interface between Pt and the support were highlighted, giving rise to an unusual band around 1660 cm-1 that could be confidently assigned to a Pt2-CO bridging carbonyl interacting head-on with a support surface hydroxyl. This adduct was yet unstable in the present conditions and was converted into a linear and bridged carbonyl bound only to Pt. Such sites are potentially important for bifunctional reactions requiring both metal and acid/base properties, particularly those occurring at the metal-support perimeter. Such adducts have probably been mistaken for carbonate-type species in many past contributions and could potentially represent crucial reaction intermediates for CO oxidation and the water-gas shift reaction.

Effective bulk and surface temperatures of the catalyst bed of FT-IR cells used for in situ and operando studies.

The temperature prevailing in the catalyst bed of three different IR spectroscopic reaction cells was assessed by means of thermocouples, an optical pyrometer and reaction rate measurements. One of the cells was a custom-made transmission FT-IR cell for use with thin wafers and the two others were commercial Harrick and Spectra-Tech diffuse reflectance FT-IR (DRIFTS) cells used for the analysis of powdered samples. The rate of CO methanation measured over a 16 wt% Ni/alumina catalyst was used as a means to derive the effective temperature prevailing in the IR cells from that existing in a traditional (non-spectroscopic) reactor having a well-controlled temperature. The sample bed of these three IR cells exhibited a significantly lower temperature than that of the corresponding measure thermocouple, which was yet located in or close to the sample bed. The comparison of Arrhenius plots enabled us to determine a temperature correction valid over a large temperature range. The use of an optical pyrometer was assessed with a view to determining the temperature of the surface of the powdered beds and that at the centre of the wafer. The optical pyrometer proved useful in the case of the catalyst powder, which behaved as a black non-reflecting body. In contrast, the temperature reading was inaccurate in the case of the pressed wafer, probably due to the shiny surface and minute thickness of the wafer, which led to a significant portion of the IR radiation of the surroundings being reflected by and transmitted through the wafer. The optical pyrometer data showed that the temperature of the surface of the powdered beds was significantly lower than that of the bulk of the bed, and that the total flow rate and composition did not affect this value. This work emphasises that the effective bed temperature in spectroscopic cells can be significantly different from that given by measure thermocouples, even when located in the vicinity of the sample, but that the calibration curves derived from rate measurements can be used to overcome this problem.

An operando DRIFTS investigation into the resistance against CO2 poisoning of a Rh/alumina catalyst during toluene hydrogenation.

CO(2) is a major contaminant of renewable H(2) derived from biomass fermentation. The effect of the presence of CO(2) on the activity of alumina-supported Pt and Rh catalysts used for the hydrogenation of toluene at 348 K was investigated. The use of operando diffuse reflectance spectroscopy (DRIFTS) was crucial in unravelling the changes in the nature and abundance of species adsorbed at the sample surface and relating those to the changes of catalytic activity. Rhodium supported on alumina was only partly deactivated by the introduction of CO(2) during the hydrogenation of toluene, contrary to the case of Pt/alumina. Rh was only partially covered by carbonyl species derived from CO(2) and it was shown that toluene could successfully compete with some of the linearly adsorbed carbonyls for adsorption. The alumina support stored many CO(2)-derived adsorbates (carbonates, hydrogenocarbonates, carboxylates) that could spill over to the metal and form carbonyl species even after the removal of CO(2) from the feed.

The design and testing of kinetically-appropriate operando spectroscopic cells for investigating heterogeneous catalytic reactions.

This tutorial review discusses some of the current designs of reaction cells used for operando spectroscopy (X-ray absorption, UV-vis, Raman, transmission FTIR and DRIFTS) and the relation to the apparent reaction kinetics. Beam effects, the control of the catalyst bed temperature and bed by-pass are some of the potential problems that may lead to flawed activity measurements. Finally, four examples are given in which a good agreement was obtained between the activity of a powdered catalyst measured in a conventional reactor and in a spectroscopic cell. It is proposed that such comparison between reactors should become a standard procedure, to ensure the correctness of the data collected over typically non-ideal spectroscopic reaction cells.

Selective catalytic reduction of O2 with excess H2 in the presence of C2H4 or C3H6.

The selective reduction of molecular oxygen with excess H(2) in the presence of alkenes was achieved successfully for the first time: silver supported on alumina catalysts exhibited full conversion of O(2) at temperature as low as 50 degrees C, while the conversion of ethene or propene remained essentially zero up to 250 degrees C.

New insights into the reaction mechanism and the rate-determining step of n-butane hydroisomerisation over reduced MoO3 catalysts.

The comparison of the proportions of the minor products formed during the hydroisomerisation of n-butane over reduced MoO3 with thermodynamic data demonstrates that a bifunctional mechanism operates and that the rate-determining step is the isomerisation of the linear butene intermediates to iso-butene.

On the importance of steady-state isotopic techniques for the investigation of the mechanism of the reverse water-gas-shift reaction.

The formation and reactivity of surface intermediates in the reverse water-gas-shift reaction on a Pt/CeO2 catalyst are critically dependent on the reaction conditions so that conclusions regarding the reaction mechanism cannot be inferred using ex operando conditions.

Bridging the gap between surface science and industrial catalysis.

Understanding the structure-activity relationships of catalytic solids has always been hampered by the complexity and nonuniform structures of catalyst particles. Materials based on well-defined colloidal metal particles are ideal model solids to investigate such structure-activity relationships. A new paper by Tsang et al. in this issue indicates that highly selective alpha,beta-unsaturated aldehyde hydrogenation Pt-based catalysts can be obtained following the decoration of Pt nanocrystals with a second metal. The effects of the particle size, substrate shape, and electronic modifications were related to the sample activity. The possibility of depositing these tailor-made colloidal particles onto conventional supports opens exciting new routes toward rational catalyst design and ultraselective catalytic processes.

Quantitative analysis of adsorbate concentrations by diffuse reflectance FT-IR.

Fully quantitative analyses of DRIFTS data are required when the surface concentrations and the specific rate constants of reaction (or desorption) of adsorbates are needed to validate microkinetic models. The relationship between the surface coverage of adsorbates and various functions derived from the signal collected by DRIFTS is discussed here. The Kubelka-Munk and pseudoabsorbance (noted here as absorbance, for the sake of brevity) transformations were considered, since those are the most commonly used functions when data collected by DRIFTS are reported. Theoretical calculations and experimental evidence based on the study of CO adsorption on Pt/SiO2 and formate species adsorbed on Pt/CeO2 showed that the absorbance (i.e., = log 1/R', with R' = relative reflectance) is the most appropriate, yet imperfect, function to give a linear representation of the adsorbate surface concentration in the examples treated here, for which the relative reflectance R' is typically > 60%. When the adsorbates lead to a strong signal absorption (e.g., R' < 60%), the Kubelka-Munk function is actually more appropriate. The absorbance allows a simple correction of baseline drifts, which often occur during time-resolved data collection over catalytic materials. Baseline corrections are markedly more complex in the case of the other mathematical transforms, including the function proposed by Matyshak and Krylov (Catal. Today 1995, 25, 1-87), which has been proposed as an appropriate representation of surface concentrations in DRIFTS spectroscopy.