Interaction of methane with surface of alumina studied by FT-IR spectroscopy

Methane adsorption on alumina was investigated by FT-IR spectroscopy at 173 K. Adsorbed methane gives four distinct IR bands at 3008, 3000, 2900 and 1305 cm^–1 which are attributed to v₁ (2900 cm^–1), v₃ (3008, 3000 cm^–1), and v₄ (1305 cm^–1) modes of methane respectively. The appearance of the v₁ mode indicates that the T_d symmetry of methane is distorted by the adsorption. The intensities of these bands increase significantly with outgassing temperatures of alumina, reach their maxima at an outgassing temperature near 773 K, and then decrease with further higher outgassing temperatures. Two hydroxyls with IR bands at 3750 and 3665 cm^–1 are perturbed evidently by the adsorbed CH₄ thereby resulting in two redshifted bands at 3707 and 3640 cm^–1. Coadsorbed CO slightly affects the adsorbed CH₄ indicating the very weak interaction between CH₄ and surface cations of alumina. It is proposed that the adsorbed CH₄ on alumina is formed mainly via the interaction of CH₄ with both surface hydroxyl and c.u.s. oxygen anion.     

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.     

Infrared spectroscopy and microcalorimetry studies of CO adsorption on sulfated zirconia supported platinum catalysts

Carbon monoxide adsorption has been investigated on Pt particles supported on a high surface area zirconia and sulfated zirconias. The accessibility of the Pt surface determined from the comparison of H₂ chemisorption and transmission electron microscopy depends on two parameters: the temperature of treatment in air used to dehydroxylate sulfated zirconia, and the temperature of reduction. An oxidative pretreatment at 823 K yields a poor accessibility of Pt (0.03 < H/Pt < 0.05) whatever the temperature of reduction, whereas a Pt dispersion of 0.6 can be obtained by oxidation at 673 K followed by a mild reduction at 473 K. FTIR spectroscopy of adsorbed CO on Pt/ZrO₂ shows besides the normal linear species at 2065 cm^–1, a band at 1650 cm^–1 which is attributed to CO bridged between Pt and Zr atoms. On Pt/ZrO₂-SO ₄ ^2– , all bridged species tend to disappear, as well as the dipole-dipole coupling andv _CO is shifted by 57 cm^–1 to higher frequencies. These results are attributed to sulfur adsorption on Pt which decreases the electron back-donation from Pt to the 2 ^* antibonding orbital of CO. The lower initial heat of CO adsorption observed on Pt/ZrO₂-SO^4/2– supports this proposal.     

Surface-Enhanced Vibrational Spectroscopy: SERS and SEIRA

With respect to the origin of single-molecule sensitivity in surface-enhanced Raman scattering, elastic scattering and emission spectra were investigated for Ag particles adsorbed with dye. The scattering peak observed at 600–650 nm was extinguished during the inactivation process of an enormous SERS signal, whereas localized surface plasmon (LSP) peaks located at 520 nm and 730 nm did not change significantly. The scattering peak at 600–650 nm arises from increased electromagnetic coupling between the LSP of adjacent Ag particles through dye molecules. In addition, distinct emission peaks were observed at 550–600 nm and 600–750 nm for hot Ag particles with adsorbates. These bands were attributed to emissive relaxation of metal electrons and fluorescence of molecules, respectively. Furthermore, the shorter wavelength peak showed invariant Stokes shift irrespective of excitation wavelengths, most probably arising from inelastic scattering of excited electrons by adsorbed molecules. The adsorbed state of CO and related species on the Pt film electrode was investigated using attenuated total reflection—surface-enhanced infrared absorption spectroscopy. Intermediate species were found on the bare Pt surface in 1 mM CH₃OH + HClO₄ solutions at +0.2 V ≤ E ≤ +0.6 V that give absorption peaks at 1405 cm⁻¹ and 1300 cm⁻¹. These bands can be attributed to carbonate species or -COH. Water molecules located at the hydrophobic interfaces between CO and electrolyte solutions were evidenced by a quite high OH stretch absorption at 3664–3646 cm⁻¹, as well as a lower broad peak at ca. 3480 cm⁻¹.     

Cobalt species in promoted cobalt alumina-supported Fischer–Tropsch catalysts

The structure of cobalt species at different stages of the genesis of monometallic and Pt-promoted cobalt alumina-supported Fischer–Tropsch catalysts was studied using X-ray diffraction, UV–visible spectroscopy, in situ X-ray absorption, in situ magnetic method, X-ray photoelectron spectroscopy, and DSC–TGA thermal analysis. The catalysts were prepared by incipient wetness impregnation using solutions of cobalt nitrate and dihydrogen hexachloroplatinate. Both variation of catalyst calcination temperature between 473 and 773 K and promotion with 0.1 wt% of Pt had no significant affect on the size of supported Co₃O₄ crystallites. The size of cobalt oxide particles in the calcined catalysts seems to be influenced primarily by the pore diameter of the support. Cobalt reducibility was relatively low in monometallic cobalt alumina-supported catalysts and decreased as a function of catalyst calcination temperature. The effect was probably due to the formation of mixed surface compounds between Co₃O₄ and Al₂O₃ at higher calcination temperatures, which hinder cobalt reduction. Promotion with platinum spectacularly increased the rate of cobalt reduction; the promotion seemed to reduce the activation energy of the formation of cobalt metallic phases. Analysis of the magnetization data suggests that the presence of Pt led to the reduction of smaller cobalt oxide particles, which could not be reduced at the same conditions in the cobalt monometallic catalysts. Promotion of cobalt alumina-supported catalysts with small amounts of Pt resulted in a significant increase in Fischer–Tropsch cobalt time yield. The efficient control of cobalt reducibility through catalyst calcination and promotion seems to be one of the key issues in the design of efficient cobalt alumina-supported Fischer–Tropsch catalysts.     

Chemisorption of C3 hydrocarbons on cobalt silica supported Fischer–Tropsch catalysts

Chemisorption of propene and propane was studied in a pulse reactor over a series of cobalt silica-supported Fischer–Tropsch catalysts. It was shown that interaction of propene with cobalt metal particles resulted in its rapid autohydrogenation. The reaction consists in a part of the propene being dehydrogenated to surface carbon and CH_x chemisorbed species; hydrogen atoms released in the course of propene dehydrogenation are then involved in hydrogenation of remaining propene molecules to propane at 323–423 K or in propene hydrogenolysis to methane and ethane at temperatures higher than 423 K. The catalyst characterization suggests that propene chemisorption over cobalt catalysts is primarily a function of the density of cobalt surface metal sites. A correlation between propene chemisorption and Fischer–Tropsch reaction rate was observed over a series of cobalt silica-supported catalysts. No propane chemisorption was observed at 323–373 K over cobalt silica-supported catalysts. Propane autohydrogenolysis was found to proceed at higher temperatures, with methane being the major product of this reaction over cobalt catalysts. Hydrogen for propane autohydrogenolysis is probably provided by adsorbed CH_x species formed via propane dehydrogenation. Propene and propane chemisorption is dramatically reduced upon the catalyst exposure to synthesis gas (H₂/CO = 2) at 323–473 K. Our results suggest that cobalt metal particles are probably completely covered by carbon monoxide molecules under the conditions similar to Fischer–Tropsch synthesis and thus, most of cobalt surface sites are not available for propene and propane chemisorption.     

DRIFT Studies of Adsorbed CO Over Sulfided K–Rh–Mo/Al2O3 Catalysts: Detection of Rh–Mo–S Phase

Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy was used to study the nature of active species in K–Rh–Co–MoS₂/Al₂O₃ catalyst by means of probing with CO molecule. The effects of K addition to Rh and interaction between Mo and Rh were studied with varying K and Mo loadings over 1 wt% Rh/Al₂O₃ catalyst. In sulfided Rh–Mo/Al₂O₃, the formation of Rh–Mo–S phase was evidenced first time by a band at 2,095 cm^?1. The introduction of Co to K–Rh–MoS₂/Al₂O₃ catalyst showed the existence of both Rh and Co promoted MoS₂ sites, but the CO absorption frequencies in DRIFT spectra are significantly at lower side compared to Co free Rh–Mo catalyst. The stabilities of CO band from Rh and Co promoted and unpromoted MoS₂ sites are studied at different temperatures. When activated carbon used as support, bands for both promoted and unpromoted MoS₂ sites were appeared, but the intensity of these bands were decreased largely compared to alumina based catalyst, resulted from the coverage of added K not only on the support surface but also on the active metal components due to the neutral nature of activated carbon.     

Study of the deactivation mechanism of Al2O3-supported cobalt Fischer-Tropsch catalysts

The influence of water on alumina-supported cobalt catalysts has been studied. The deactivation of supported Co catalysts was studied in a fixed-bed reactor using synthesis gas feeds containing different concentrations of water vapour. Supporting model studies were carried out using H₂O/H₂ feeds in conjunction with XPS and gravimetry. Rapid deactivation occurs on Re-promoted CO/Al₂O₃ catalysts when H₂/CO/H₂O feeds are used, whereas unpromoted CO/Al₂O₃ shows more stable activity. The results from the gravimetric studies suggest that only a small fraction of the bulk cobalt metal initially present reoxidizes to cobalt oxide during reaction. However, the XPS results indicate significant reoxidation of surface cobalt atoms or highly dispersed cobalt phases, which is likely to be the cause of the observed deactivation. Rhenium is shown to have a marked effect on the extent of reoxidation of alumina-supported cobalt catalysts.     

Effects of sintering-resistance and large metal–support interface of alumina nanorod-stabilized Pt nanoparticle catalysts on the improved high temperature water gas shift reaction activity

Pt nanoparticles stabilized with alumina nanorods (Pt@Al₂O₃) were synthesized by a simple preparation method using a mixture of Pt nanoparticles, an alumina precursor, an organic surfactant, and a reducing agent. Because the alumina nanorods stabilized Pt nanoparticles, the sintering of Pt nanoparticles was significantly suppressed during a high temperature water gas shift reaction, demonstrating 1.7–13.6 times higher CO conversions or 1.0–8.0 times higher TOF compared to other alumina-supported Pt catalysts. In addition, the increased metal–support interface for Pt@Al₂O₃ significantly improved the water gas shift reaction activity.     

The Effect of Preparation Variables on the CO Hydrogenation Activity of Coprecipitated Co/Al2O3 Catalysts

Studies have been conducted on the effect of preparation variables on the activity of coprecipitated cobalt–alumina catalysts to be used for the production of C₁–C₄ hydrocarbons by CO hydrogenation. The preparation parameters considered were the precipitation pH, the precipitation agent, the metal loading, the reduction temperature and the reduction period. It was found that changing pH precipitation with a constant final pH between 11·0 and 12·5 and the use of NaOH together with nitrates as precursors yielded better catalysts with maximal metal surface area. The optimum cobalt loading for the selective production of C₁–C₄ hydrocarbons is around 35 wt%. Optimum activity and selectivity are obtained by applying an 8-h reduction scheme at 648 K under 100 cm³ min⁻¹ hydrogen. Calcination prior to reduction has a detrimental effect on metal surface area and hence on catalytic activity. © 1997 SCI.     

Formation of Ni(CO)4 during the Interaction between CO and Silica-Supported Nickel Catalyst: an FTIR Spectroscopic Study

The formation of Ni(CO)₄ during interaction of CO with silica-supported highly dispersed nickel metal (d _av4 nm) was investigated by FTIR spectroscopy. At temperatures below 145 K, in addition to linear and bridged nickel carbonyls, CO adsorption on Ni⁰/SiO₂ leads to the formation of Ni(CO) _x (x=2, 3) subcarbonyls (band at ca. 2090 cm^–1) and negligible amounts of Ni(CO)₄ adsorbed on SiO₂ (band at 2048 cm^–1). Up to this temperature CO causes no detectable erosion of the metal surface. Above 145 K the rate of interaction between CO and the nickel particles significantly increases. Until 235 K Ni(CO)₄ mainly remains in the adsorbed state, while at still higher temperatures the equilibrium between adsorbed and gaseous Ni(CO)₄ (band at 2058 cm^–1) is shifted towards the latter. It is assumed that subcarbonyls formed on defect sites of the metal surface are precursors of the nickel tetracarbonyl. Successive adsorption–evacuation cycles of CO at room temperature result in a decrease in the amount of the Ni(CO)₄ formed, probably due to a reduction of the number of defect metal sites. On the basis of ¹²CO and ¹³CO coadsorption, an alternative interpretation of the band at 2048 cm^–1 to species containing isolated Ni(CO)₃ groups is proposed.     

In situ carburization of metallic molybdenum during catalytic reactions of carbon-containing gases

Supported molybdenum clusters were prepared by sublimation of Mo(CO)₆ onto dehy-droxylated alumina followed by decomposition in flowing dihydrogen at 970 K. These alumina-supported molybdenum clusters were found by XAFS to transform into Mo₂C if heated in a 20% methane/H₂ mixture at 950 K. For the hydrogenolysis ofn-butane at 510 K and CO-H₂ reactions at 570 K, both at atmospheric pressure, molybdenum and carburized molybdenum showed similar, but different for each reaction, turnover rates. The product distribution was the same for each reaction on Mo and Mo₂C. In both reactions, in situ XAFS data for fresh and used catalysts indicated that Mo clusters progressively transformed into Mo₂C under the reaction conditions     

Redox chemistry in excessively ion-exchanged Cu/Na-ZSM-5

TPR/TPD and FTIR are used to characterize excessively ion-exchanged Cu/Na-ZSM-5. After calcination in O₂ at 773 K at least two copper-oxygen species are present in addition to Cu²⁺ ions; these have been identified as CuO and [Cu-O-Cu]²⁺. Reduction in H₂ transforms all these into Cu⁰ below 773 K. [Cu-O-Cu]²⁺ is autoreduced to Cu+ during outgassing. Reoxidation of Cu⁰ by zeolite protons to Cu⁺ is observed above 723 K in He or Ar; in the presence of CO this process is considerably enhanced and observed at much lower temperature, because CO is strongly adsorbed on Cu⁺. At 293 K CO adsorption causes reversible changes in the FTIR spectra.On leave from: Center for Catalysis, Surface and Material Science, Department of Organic Chemistry, József Attila University, Dóm tér 8, Szeged, H-6720 Hungary.     

FTIR study of the rearrangement of adsorbed CO species on Al2O3-supported rhodium catalysts

The adsorption of CO at low temperatures (130–293 K) has been investigated on Rh/Al₂O₃ catalysts of low (0.001–1 wt%) Rh loadings by means of Fourier transform infrared spectroscopy. The surface structure of Rh produced at different reduction temperatures (573 and 1173 K) was shock-cooled to 130 K, where the addition of CO caused the appearance of the band due to bridge-bonded CO ((Rh⁰)₂–CO) on all samples. The appearance of the bands due to gem-dicarbonyl (Rh⁺(CO)₂) and linearly bonded CO (Rh_x–CO) depended on the Rh content and the reduction temperature of the catalysts. The positions and the integrated absorbances of the symmetric and asymmetric stretchings of the Rh⁺(CO)₂ changed with temperature. On the basis of the above findings the rearrangement of the adsorbed CO species (indirectly that of surface Rh) is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation of CHO during interaction of CO and H2 on alumina-supported Pd catalysts

The reaction of CO and H₂ on alumina-supported Pd catalysts (2, 5 and 10 wt% Pd) has been studied by a transient pulse technique in vacuum (Temporal-Analysis-of-Products (TAP) reactor). CHO species were formed on a hydrogen-treated catalyst surface at temperatures between 423 and 530 K by the interaction of hydrogen solved in the palladium metal and CO (adsorbed at 300 K after reduction). The CHO species were released into the gas phase when hydrogen was pulsed over the catalysts. CHO is assumed to be an intermediate in the hydrogenation of CO to CH₄ starting at 560 K. No methanol was observed under the experimental conditions applied.     

Infrared study of crotonaldehyde and CO adsorption on a Pt/TiO2 catalyst

A Pt/TiO₂ catalyst has been subjected to reduction in hydrogen at 473, 573 and 773 K and the various degrees of metal-support interaction (SMSI) confirmed by means of CO and H₂ chemisorption, FTIR of CO and the hydrogenation of crotonaldehyde. Coadsorption of CO and crotonaldehyde were performed to identify the preferred adsorption site and mode of adsorption of the unsaturated aldehyde. Results which appear to suggest shifts to lower frequencies of bands due to adsorbed carbonyls are not due to electronic effects induced by coadsorption, but rather indicate displacement of CO from the weaker bonding sites which eliminate dipole coupling effects between different carbonyl clusters, and consequently removes intensity transfer phenomena leading to enhancement in intensity at lower frequencies. This revised version was published online in August 2006 with corrections to the Cover Date.     

Siting of Co2+ Ions in Cobalt-Modified High-Silica Zeolites Probed by Low-Temperature Molecular Hydrogen Adsorption

Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy was used to characterize the effects of introducing cobalt into the zeolites ZSM-5 and mordenite. Aqueous impregnation of the hydrogen form of the zeolite and subsequent in vacuo treatment at temperatures up to 920 K results in partial exchange of protons in bridging hydroxyl groups by Co²⁺ cations. These changes are evidenced by a decrease in intensity of the bands at 3606-3609 cm^-1 characteristic of Brønsted acid sites. DRIFT spectra of hydrogen adsorbed at 77 K also confirm the exchange of protons for Co²⁺ cations, as evidenced by a decrease in the intensity of the band at 4106 cm^-1 for H₂ adsorbed on protons. By contrast, the bands at 3905, 3965, and 4010 cm^-1 for H₂ adsorbed on the Co²⁺ cations increase in intensity. With increasing Si/Al ratio at a constant Co loading of 1 wt% the intensity of the band at 3905 cm^-1 for H-ZSM-5 strongly increases in intensity relative to the other bands. This feature is attributed to Co²⁺ cations interacting with two adjacent cation-exchange sites located in a 10-membered ring. It is hypothesized that the Lewis acidity of Co²⁺ cations in such environments is higher than that of Co²⁺ cations associated with oxygen atoms in individual five- or six-membered rings containing two Al atoms, because the Co²⁺ cation can interact with only two of the four basic oxygen anions located in the ring. It is proposed that Co²⁺ cations in the latter type of sites are identified by the bands at 3965 cm^-1 and 4010 cm^-1 for adsorbed H₂.     

The active phase of Au–Pd/Al2O3 for CO oxidation

Au–Pd/Al₂O₃ catalyst was prepared by modified impregnation method. It was found that the catalyst calcined in air at 473 K showed higher CO oxidation activity in comparison with the catalysts treated at other temperature. Nitrogen adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure spectroscopy (XANES) techniques were employed to study the relationship between the surface/bulk structures of these catalysts and their catalytic performance. The results indicated the higher activity was attributed to the smaller pore volume and co-existence of PdO and Au⁰ in their surface. The formation of Au_xPd_y alloy was unfavorable for the catalytic reaction.     

Effects of sodium hydroxide treatment on coordination changes of cobalt ions in CoNaY by means of UV/VIS diffuse reflectance spectroscopy

The coordination changes of Co²⁺ in CoNaY/NaOHs, prepared by cobalt ion-exchange and treatment with NaOH, during dehydration at various temperatures have been investigated by UV/VIS diffuse reflectance spectroscopy. After dehydration of CoNaY/NaOHs at 473 K, the DRS bands show the formation of mixed cobalt oxide species interacted with lattice oxygen. This mixed cobalt oxide species interacted with lattice oxygen decompose during further dehydration at 673 K. The strong intensities of the bands at 660, 575 and 530 nm after dehydration of CoNaY/NaOHs at 873 K indicate that a considerable amount of the cobalt ions remains in the sodalite cage with tetrahedral coordination even after dehydration at 873 K.     

Synthesis of nanocrystalline alumina by thermal decomposition of aluminum isopropoxide in 1-butanol and their applications as cobalt catalyst support

Nanocrystalline alumina powders were prepared by thermal decomposition of aluminum isopropoxide (AIP) in 1-butanol at 300 °C for 2 h and employed as cobalt catalyst supports. The crystallization of alumina was found to be influenced by the concentration of AIP in the solution. At low AIP content, wrinkled sheets-link structure of γ-Al₂O₃ was formed, while at high AIP concentrations, fine spherical particles of χ-Al₂O₃ were obtained. It was found that using these fine particles alumina as cobalt catalyst supports resulted in much higher amounts of cobalt active sites measured by H₂ chemisorption and higher CO hydrogenation activities.