CO adsorption on supported and promoted Ag epoxidation catalysts

CO was used to probe the nature of adsorption sites on Ag/α-Al₂O₃ epoxidation catalysts and to investigate the effect of Cs and Cl promoters by employing diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and chemisorption measurements. In contrast to previous studies, IR absorption bands for CO chemisorbed on reduced, supported Ag crystallites were observed; however, CO adsorption occurred on only 3–7% of the total Ag surface at 300 K and coverage depended on both the pretreatment and CO pressure utilized. No irreversible CO adsorption occurred on the alumina, whereas linearly bonded CO was the dominant species on the metallic Ag sites. After a 30-min purge, the bands due to these chemisorbed forms of CO decreased in intensity while a band due to bridge-bonded CO increased in intensity, which implies that CO reoriented as the surface concentration of CO decreased. In the presence of Cs, similar behavior was observed and the band intensity of the bridge-bonded CO increased. After reduction at 673 K, cesium suboxides appeared to be formed based on the formation of carbonyl complexes at 2028, 1950, and 1869 cm⁻¹. On reduced Ag catalysts, electronic effects of Cs and Cl were observed and adsorbed CO gave a lower frequency, i.e., 2018 and 2009 cm⁻¹ for Cs-promoted samples reduced at 473 and 673 K, respectively, due to an increase in the electron density on surface Ag atoms, while this band occurred at a higher frequency of 2129 cm⁻¹ with a CsCl-promoted Ag catalyst due to a net decrease in the electron density on surface Ag atoms. After CO adsorption on O-covered Cs-promoted and CsCl-promoted catalysts, a band between 1520 and 1491 cm⁻¹ existed which was assigned to a COO⁻ stretching mode in a carbonate species formed on composite AgCs_xO_y sites. These studies with CO provide evidence that reduction at 673 K following a calcination step can lead to redistribution of Cs atoms.     

Components for PEM fuel cell systems using hydrogen and CO containing fuels

Proton exchange membrane fuel cells (PEMFC) show a significant performance drop in CO containing hydrogen as fuel gas in comparison to pure hydrogen. The lower performance is due to CO adsorption at the anode thus poisoning the hydrogen oxidation reaction. Two approaches to improve the cell performance are discussed. First, the use of improved electrocatalysts for the anode, such as PtRu alloys, can significantly enhance the CO tolerance. On the other hand, CO poisoning of the anode could be avoided by the use of non-electrochemical methods. For example, the addition of liquid hydrogen peroxide to the humidification water of the cell leads to the formation of active oxygen by decomposition of H₂O₂ and the oxidation of CO. In such a way a complete recovery of the CO free cell performance is achieved for H₂/100 ppm CO.     

Role of promoters on Rh/SiO2 in CO hydrogenation: A comparison using DRIFTS

La, V, Zn, Cu, Fe, Li and Ag promoted Rh/SiO₂ catalysts were investigated for the synthesis of ethanol during CO hydrogenation at 230 °C and 1.8 atm. As is well known, the activity and selectivity depend heavily on the choice of promoter. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to probe the effects of La, V, Zn and Cu on CO adsorption and hydrogenation. From the IR study, it was found that the behavior of CO adsorbed on the differently promoted catalysts was very different. While La enhanced total CO adsorption, the addition of V, Zn and Cu suppressed CO adsorption to different extents. The doubly promoted Rh-La/V/SiO₂ showed only moderate CO adsorption. Results from DRIFTS suggest that the higher catalytic activity (compared to the non-promoted catalyst) observed for the La singly promoted Rh/SiO₂ catalyst may primarily be caused by an increase in the concentration of the adsorbed CO species in the presence of H₂, possibly due to the formation of new active sites at the LaO_x-Rh interface. The higher catalytic activity of the V singly promoted Rh/SiO₂ catalyst could be ascribed to an increased desorption rate/reactivity of the adsorbed CO species. The La and V doubly promoted catalyst showed both new adsorbed CO species and increased desorption rate/reactivity of the adsorbed species during CO hydrogenation due to a synergistic promoting effect of La and V. The addition of Zn or Cu promoters significantly reduced the desorption rate/reactivity of the adsorbed CO species on Rh/SiO₂, leading apparently to the much reduced activities for CO hydrogenation observed.     

Theoretical study on interaction between CO2 and carbonyl compounds: Influence of CO2 on infrared spectroscopy and activity of CO

The interactions between CO₂ and carbonyl compounds at different CO₂ pressures have been studied both experimentally and theoretically. In situ high-pressure FTIR on carbonyl compounds, i.e., acetaldehyde, acetone, and crotonaldehyde, in supercritical CO₂ have been measured at various CO₂ pressures varying from 6 to 22 MPa. In order to get insights into the mechanism, theoretical study has been conducted concerning the effect of CO₂ on frequency shift of CO in acetaldehyde, acetone, benzaldehyde, crotonaldehyde and cinnamaldehyde at different CO₂ pressures. It has been shown that the experimental frequency shifts can be well simulated by the theoretical model calculations using particular structures, in which a carbonyl compound interacts with a few CO₂ molecules, depending on the carbonyl compounds examined, except for acetaldehyde.The interaction energies between CO₂ and those carbonyl compounds are also given. In addition, the effect of CO₂ on hydrogenation of crotonaldehyde and benzaldehyde has been discussed by means of the local softness (s⁺) calculated at CO₂ pressures of 0-22 MPa, which can explain the reactivity difference in the crotonaldehyde and benzaldehyde hydrogenations in supercritical CO₂.     

Adsorption and reactivity during low temperature oxidation of carbon monoxide on Au/TiO2 catalysts studied by rapid response mass spectrometry

The adsorption and reaction of CO, CO₂ and O₂ on TiO₂ and Au/TiO₂ have been studied using a mass spectrometric method which can detect processes occurring on a time scale of seconds. Adsorption of CO on TiO₂ at 300 K is rapidly reversible and less on reduced samples than oxidised ones indicating that the adsorption sites are oxide ions. The amount adsorbed reversibly on reduced Au/TiO₂ is less still, consistent with enhanced reduction, but additional amounts adsorb irreversibly at a slower rate. The amount of CO₂ adsorbed under similar conditions is also greater on TiO₂ than reduced Au/TiO₂ and approximately one order of magnitude greater than that of CO. However, adsorption of O₂ is undetectable on the time scale of the measurement. Exposure of Au/TiO₂ to mixtures of CO and O₂ results in near instantaneous generation of CO₂ although its appearance is attenuated by adsorption. Adsorption of CO occurs concurrently in a way similar to that seen with CO alone except that the amount of the more slowly adsorbed form seems less. This suggests that it is the form utilised in catalysis. Oxygen uptake beyond that generating CO₂ is appreciable during the initial stages of exposure to reaction mixtures and this capacity is enhanced if one or other reactant is removed and then reintroduced, possibly due to the generation of reducible interface sites. It is concluded that the remarkable activity of Au/TiO₂ for CO oxidation at ambient temperature resides in a very high turnover frequency on sites at the interface between the metal and oxide. This revised version was published online in June 2006 with corrections to the Cover Date.     

Electrochemical quartz crystal microbalance study on carbon oxides adsorption in the presence of electrosorbed hydrogen on Pd alloys with Pt and Rh

CO₂ and CO adsorption on Pd-Pt and Pd-Rh alloys has been studied by cyclic voltammetry (CV) and the electrochemical quartz crystal microbalance (EQCM). Adsorbed CO₂ inhibits partially hydrogen adsorption on Pt and Rh surface atoms but does not block significantly hydrogen absorption into alloy bulk. In the presence of adsorbed CO both hydrogen adsorption and absorption are strongly suppressed. On electrodes covered with adsorbed CO the oxidation of previously absorbed hydrogen is significantly shifted into higher potentials. The EQCM response in CO₂/CO adsorption experiments is affected by both the effects connected with the changes in mass attached to the resonator and the non-mass effects including changes in metal-solution interactions and variation of solution density and viscosity in the vicinity of the electrode. Differences in the EQCM behavior suggest that the products of CO₂ and CO adsorption on the alloys studied are not totally identical.     

A kinetic and DRIFTS study of low-temperature carbon monoxide oxidation over Au-TiO2 catalysts

Titania-supported gold catalysts are extremely active for room temperature CO oxidation; however, deactivation is observed over long periods of time under our reaction conditions Impregnated AuTiO₂ is most active after a sequential pretreatment consisting of high temperature reduction at 773 K, calcination at 673 K and low temperature reduction at 473 K (HTR/C/LTR); the activity after either only low temperature reduction or calcination is much lower. A catalyst prepared by coprecipitation had much smaller Au particles than impregnated AuTiO₂ and was active at 273 K after either an HTR/C/LTR or a calcination pretreatment. Deposition of TiO_x overlayers onto an inactive Au powder produced high activity; this argues against an electronic effect in small Au particles as the major factor contributing to the activity of AuTiO₂ catalysts and argues for the formation of active sites at the AuTiO_x interface produced by the mobility of TiO_x species. DRIFTS (diffuse reflectance FTIR) spectra of impregnated AuTiO₂ reveal the presence of weak reversible CO adsorption on the Au surface but not on the TiO₂; however, a band for adsorbed CO is observed on the pure TiO₂. Kinetic studies with a 1.0 wt.-% impregnated AuTiO₂ sample showed a near half-order rate dependence on CO and a near zero-order rate dependence on O₂ between 273 and 313 K with an activation energy near 7 kcal/mol. A two-site model, with CO adsorbing on Au and O₂ adsorbing on TiO₂, is consistent with Langmuir-Hinselwood kinetics for noncompetitive adsorption, fits partial pressure data well and shows consistent enthalpies and entropies of adsorption. The formation of carbonate and car☐ylate species on the titania surface was detected but it appears that these are spectator species. DRIFTS experiments under reaction conditions also show the presence of weak, reversible adsorption of CO₂ (near 2340 cm⁻¹) which may be competing with CO for adsorption sites.     

IR study of the interaction of hydroxyl groups of silica gel with Cr species of Phillips' type catalysts

Spectroscopic evidence for the interaction of hydroxyl groups and chromium ions was obtained using a catalyst prepared from chromyl chloride. A new OH peak, observed at 3705 cm^–1 after pumping away CO gas, is attributed to the direct interaction of OH with the low-valent chromium. This peak shifts to 3590 cm^–1 on contact with O₂ at room temperature and it is assigned to a hydroxyl interacting with the oxidized chromium. New assignments are also proposed for IR bands of CO presorbed on the catalyst. The peak due to CO at 2188 cm^–1 decreases as the OH intensity at 3705 cm^–1 increases, suggesting that the former peak arises from adsorption on Cr(II) species to which two oxygen atoms are attached.     

Steam reforming of methanol using Cu-ZnO catalysts supported on nanoparticle alumina

Methanol steam reforming was studied over several catalysts made by deposition of copper and zinc precursors onto nanoparticle alumina. The results were compared to those of a commercially available copper, zinc oxide and alumina catalyst. Temperature programmed reduction, BET surface area measurements, and N₂O decomposition were used to characterize the catalyst surfaces. XRD was used to study the bulk structure of the catalysts, and XPS was used to determine the chemical states of the surface species. The nanoparticle-supported catalysts achieved similar conversions as the commercial reference catalyst but at slightly higher temperatures. However, the nanoparticle-supported catalysts also exhibited a significantly lower CO selectivity at a given temperature and space time than the reference catalyst. Furthermore, the turnover frequencies of the nanoparticle-supported catalysts were higher than that of the commercial catalyst, which means that the activity of the surface copper is higher. It was determined that high alumina concentrations ultimately decrease catalytic activity as well as promote undesirable CH₂O formation. The lower catalytic activity may be due to strong Cu-Al₂O₃ interactions, which result in Cu species which are not easily reduced. Furthermore, the acidity of the alumina support appears to promote CH₂O formation, which at low Cu concentrations is not reformed to CO₂ and H₂. The CO levels present in this study are above what can be explained by the reverse water-gas-shift (WGS) reaction. While coking is not a significant deactivation pathway, migration of ZnO to the surface of the catalyst (or of Cu to the bulk of the catalyst) does explain the permanent loss of catalytic activity. Cu₂O is present on the spent nanoparticle catalysts and it is likely that the Cu⁺/Cu⁰ ratio is of importance both for the catalytic activity and the CO selectivity.     

The effect of impregnation of activated carbon with SnCl2.2H2O on its porosity, surface composition and CO gas adsorption

Activated carbon was impregnated with different concentrations of SnCl₂.2H₂O. Unimpregnated and impregnated activated carbons were analysed by means of physical adsorption and XPS and were tested for CO gas adsorption in a PSA system. The adsorption isotherms of N₂ at 77 K were measured and showed a Type I isotherm indicating microporous carbon for all the samples. The surface area, pore volume and pore size distribution were reduced with impregnation. XPS analysis showed an increase in the intensity of Sn3d peak with impregnation. The impregnated activated carbon showed a very good adsorption ability of CO gas compared to the unimpregnated sample. The adsorptive species responsible for CO gas adsorption was confirmed to be SnO₂ instead of SnO due to the former’s comparative thermodynamic stability.     

N2O Decomposition and Formation of NO x Species on Fe–ferrierite. Effect of NO and CO Addition on the Decomposition and the Role of Surface Species

The adsorption of CO and CO₂ on Pt supported on ZrO₂ and Ce/La-promoted ZrO₂ was studied using DRIFTS. The presence of both La and Ce resulted in a decrease in the adsorption of CO at room temperature after reduction at 350 °C. The reduction in the CO adsorption is ascribed to an increase in the support reducibility when La and Ce are both present. Reduction at 350 °C leads to the formation of oxygen defects in the dual promoted support which have been probed using DRIFTS to monitor CO₂ dissociation. Hydrogen assisted dissociation is demonstrated on the ZrO₂, CeZrO₂, and LaZrO₂ supports. In the absence of hydrogen, the presence of oxygen vacancies is shown to be necessary for CO₂ dissociation.     

Hydrogenation of phenol with supported Rh catalysts in the presence of compressed CO2: Its effects on reaction rate, product selectivity and catalyst life

Hydrogenation of phenol to cyclohexanone and cyclohexanol in/under compressed CO₂ was examined using commercial Rh/C and Rh/Al₂O₃ catalysts to investigate the effects of CO₂ pressurization on the total conversion and the product selectivity. Although the total rate of phenol hydrogenation with Rh/C was lowered by the presence of CO₂, the selectivity to cyclohexanone was improved at high conversion levels >70%. On the other hand, the activity of Rh/Al₂O₃ was completely lost in an early stage of reaction. The features of these multiphase catalytic hydrogenation reactions using compressed CO₂ were studied in detail by phase behavior and solubility measurements, in situ high-pressure FTIR for molecular interactions of CO₂ with reacting species and formation/adsorption of CO on the catalysts, and simulation of reaction kinetics using a simple model. The CO₂ pressurization was shown to suppress the hydrogenation of cyclohexanone to cyclohexanol, improving the selectivity to cyclohexanone. The formation and adsorption of CO were observed for the two catalysts at high CO₂ pressures in the presence of H₂, which was one of important factors retarding the rate of hydrogenation in the presence of CO₂. It was further indicated that the adsorption of CO on Rh/Al₂O₃ was strong and caused the complete loss of its activity.     

Influence of Mg and Ce addition to ruthenium based catalysts used in the selective hydrogenation of α,β-unsaturated aldehydes

Ce and Mg were used as promoters in two series of Ru based catalysts supported on alumina (Al₂O₃) and activated carbon (AC). The catalysts were characterized by H₂ chemisorption and temperature-programmed reduction (TPR), and studied in the crotonaldehyde (gas phase) and the citral (liquid phase) hydrogenations. Addition of MgO and CeO₂ decreased the catalytic activity in crotonaldehyde and citral hydrogenations. With regard to the selectivity towards unsaturated alcohols, similar trends were observed for the two reactions. MgO did not influence the selectivity, but CeO₂ increased the selectivity to unsaturated alcohols, especially on carbon supported catalyst. Bulk CeO₂ and Ce/AC catalyst showed low activity but very high selectivity (93 and 100%, respectively) to the unsaturated alcohols. Based on these results and the calorimetric experiments of CO adsorption it was suggested that defect sites on the surface of the promoter are the active and highly selective sites for unsaturated aldehydes due to their influence on the CO bond activation.     

Influence of temperature and relative humidity on performance and CO tolerance of PEM fuel cells with Nafion-Teflon-Zr(HPO4)2 higher temperature composite membranes

The carbon monoxide (CO) poisoning effect on carbon supported catalysts (Pt-Ru/C and Pt/C) in polymer electrolyte membrane (PEM) fuel cells has been investigated at higher temperatures (T > 100 °C) under different relative humidity (RH) conditions. To reduce the IR losses in higher temperature/lower relative humidity, Nafion^®-Teflon^®-Zr(HPO₄)₂ composite membranes were applied as the cell electrolytes. Fuel cell polarization investigation as well as CO stripping voltammetry measurements was carried out at three cell temperatures (80, 105 and 120 °C), with various inlet anode relative humidity (35%, 58% and 100%). CO concentrations in hydrogen varied from 10 ppm to 2%. The fuel cell performance loss due to CO poisoning was significantly alleviated at higher temperature/lower RH due to the lower CO adsorption coverage on the catalytic sites, in spite that the anode catalyst utilization was lower at such conditions due to higher ionic resistance in the electrode. Increasing the anode inlet relative humidity at the higher temperature also alleviated the fuel cell performance losses, which could be attributed to the combination effects of suppressing CO adsorption, increasing anode catalyst utilization and favoring OH_ads group generation for easier CO oxidation.     

The influence of MSI (metal–support interactions) on activity and selectivity in the hydrogenation of aldehydes and ketones

The vapor–phase hydrogenation of a group of aldehydes and ketones which contain unsaturated C=C bonds has been studied over a family of supported Pt catalysts. Turnover frequencies, activation energies and reaction orders for crotonaldehyde, benzaldehyde, phenylacetaldehyde and acetophenone are provided and compared to those for butyraldehyde, benzyl alcohol, 1–phenylethanol, acetylcyclohexane and acetone. Metal–support interactions (MSI) induced in Pt/TiO₂ not only enhance specific activity but also markedly shift selectivity because hydrogenation of the carbonyl bond is favored. The retention of high selectivity to the intermediate unsaturated alcohols shows that adsorption properties, as well as kinetic parameters, are altered. A model is discussed which describes the sites responsible for this behavior. This revised version was published online in June 2006 with corrections to the Cover Date.     

CO2 and CO adsorption equilibrium on ZSM-5 for different SiO2/Al2O3 ratios

CO₂ and CO adsorption on MFI type zeolites with different SiO₂/Al₂O₃ ratios (ZSM-5(30), ZSM-5(50), ZSM-5(280), and silicalite) were investigated in this study by a static gravimetric analyzer for pure isotherms at 30°C, 65°C, 100°C, and 135°C over the pressure range of 0–10 atm. Adsorption capacity of CO increases with decreasing SiO₂/Al₂O₃ ratios within ZSM-5. The adsorption of CO₂ for decreasing SiO₂/Al₂O₃ ratios, showed stronger adsorption at lower pressures and at higher pressures, the highest capacity varied from ZSM-5(50) to ZSM-5(30). ZSM-5(280) was found to have the highest selectivity for CO₂ within the widest range of pressures and temperatures tested.     

CO adsorption kinetics and adlayer build-up studied by combined ATR-FTIR spectroscopy and on-line DEMS under continuous flow conditions

Using a novel combined spectro-electrochemical DEMS/ATR-FTIRS technique, the CO adsorption kinetics on a Pt film electrode were studied, performing transient CO adsorption experiments at different constant potentials (0.06-0.6 V). CO adsorption rate and CO_ad coverage were determined continuously from the CO consumption measured by on-line differential electrochemical mass spectrometry (DEMS). Simultaneously measured FTIR spectra, recorded in situ in an attenuated total reflection (ATR) configuration, allow a direct correlation of the IR band intensity and frequency with CO_ad surface coverage at different constant potentials. The data show that (i) the CO adsorption kinetics are independent of the adsorption potential up to 0.5 V, (ii) a significant potential dependence of the ratio between CO_L and CO_M for the same coverage, (iii) in the regime of very high CO_ad coverages there is no proportional relation between CO_ad coverage and CO_L,M intensity, and (iv) a distinct tendency for CO_ad island formation at E_ads < 0.2 V and > 0.4 V, most likely due to coadsorption of H-upd at the lower potentials and (bi-)sulfate at higher potentials. Finally, at 0.6 V, CO_ad oxidation follows a Langmuir-Hinshelwood mechanism with the highest CO₂ formation rate at a relative CO_ad coverage of ∼0.4.     

CO adsorption energy on planar Au/TiO2 model catalysts under catalytically relevant conditions

The adsorption of CO on planar Au/TiO₂ model catalysts was studied by polarization-modulation infrared reflection–absorption spectroscopy (PM-IRAS) under catalytically relevant pressure (10–50 mbar) and temperature (30–120 °C) conditions, both in pure CO and in CO/O₂ reaction gas mixtures. The adsorption energy of CO on the Au particles was determined by a quantitative analysis of the temperature dependence of the CO absorption intensity in adsorption isobars. The data reveal considerable effects of the Au particle size when pure CO is used; the initial adsorption energy decreases from 74 kJ mol⁻¹ (2 nm mean Au particle diameter) to 62 kJ mol⁻¹ (4 nm). For CO/O₂ gas mixtures, the initial CO adsorption energy is, irrespective of the Au particle size, constant at 63 kJ mol⁻¹ (i.e., the CO adsorption energy is reduced for smaller Au particles), but this effect vanishes for larger Au particles.     

Spectroscopic evidence for adsorption sites located at Cu/ZnO interfaces

FTIR spectra are reported of CO and formic acid adsorption on a series of Cu/ZnO/SiO₂ catalysts. Peaks due to linear CO adsorbed on copper diminished in intensity as the loading of ZnO was increased. This behaviour was explained in terms of ZnO island growth on the copper surface. Similarly, reduction of the copper concentration while maintaining a constant ZnO loading also resulted in further attenuation in bands ascribed to CO chemisorbed on copper. Formic acid exposure to a Cu/SiO₂ sample produced a formate species displaying a _as(COO) mode at 1585 cm^–1. Addition of a small quantity of ZnO to the catalyst resulted in substantial promotion of formate growth, which was accompanied by a shift (and broadening) of the _as(COO) vibration to 1660–1600 cm^–1. Since further ZnO incorporation poisoned formate creation it was concluded that formate species bonded to Cu and Zn sites located at interfacial positions had been formed. The role of such species in methanol synthesis is discussed.