Chemisorption Properties Research Articles

The STM Images of Pt (111) (<img src="http://latex.codecogs.com/gif.latex?\sqrt{3}\times&space;\sqrt{3}" title="\sqrt{3}\times \sqrt{3}" />)R30°/CO Surface by DFT Calculations

In this work we have performed total-energy calculations of the chemisorption properties and STM images of Pt (111) ( × )R30°/CO Surface; STM Image; ChemisorptionR30°/CO surface by using the density functional theory (DFT) and the projector-augmented wave (PAW) method. The calculations show that carbon monoxide molecule (CO) adsorbs on FCC site in the Pt (111) ( × )R30°/ surface is energetically favored by the GGA-PBE XC-functional, this is in agreement with most of the theoretical calculations which is using different XC-functional at the most. However, these results strongly conflicted with the existing experiments. Actually the calculated work function for the FCC adsorption is quite different from the experiments while the atop one is in good agreement with experiments. We speculate that the atop adsorption for (CO is favorable for the adsorption case at the most. Furthermore, we have calculated the scanning tunneling microscopy (STM) images for both adsorption geometries and suggest that there should be existed remarkable differences in the STM images. The present work provides a faithful criterion accounting for the local surface geometry in Pt (111) ( × )R30°/CO surface from surface work functions and STM images instead of totalenergy calculations.

Quantum size effects on chemisorption properties: CO on ultrathin Cu films from first principles

We address, by means of ab-initio calculations, the origin of the correlation that has been observed experimentally between the chemisorption energy of CO on nanoscale Cu(001) supported films and quantum-size effects. The calculated chemisorption energy shows systematic oscillations, as a function of film thickness, with a periodicity corresponding to that of quantum-well states at the surface-Brillouin-zone center crossing the Fermi energy. We explain this trend based on the oscillations, with film thickness, of the decay length on the vacuum side of the quantum-well states at the Fermi energy. Contrary to previous suggestions, we find that the actual oscillations with film thickness of the density of states per atom of the film at the Fermi energy cannot account for the observed trend in the chemisorption energy.

XPS and TPD investigation of CO adsorption on mixed Rh–V layers supported by gamma-alumina

Alumina-supported mixed bimetallic Rh–V thin films, with the overall thickness of 0.8ML, were prepared under the ultrahigh vacuum (UHV) conditions and characterized with respect to their electronic and CO adsorption properties. X-ray photoelectron spectroscopy (XPS) was utilized to characterize electronic changes accompanying bimetallic Rh–V interaction and interaction between metal and polycrystalline γ-Al2O3 substrate. The chemisorption properties were probed by temperature-programmed desorption spectroscopy (TPD) of CO molecules. The electronic and chemisorption properties of the mixed layers were compared with pure Rh and V layers grown on the same γ-Al2O3 substrate and with a model bimetallic Rh–V system prepared by V deposition on a polycrystalline Rh foil. By varying the preparation conditions, we observed a strong dependence of the studied properties on the position of the V atoms. The presence of V atoms on the surface led to a fast deactivation, while vanadium presented under the surface resulted in a weakening of CO-metal surface bond, a change in the proportion of the adsorption side species, and an increase of CO dissociation.

Effect of Chlorine on the Chemisorptive Properties and Ammonia Synthesis Activity of Alumina-Supported Ru Catalysts

A series of Ru/Al2O3 catalysts were prepared to study the effect of the amount and the origin of residual chlorine on chemisorptive property and the ammonia synthesis activity. The catalysts were characterized by X-ray fluorescence, CO chemisorption, transmission Electron Microscopy, X-ray photoelectron spectroscopy, hydrogen temperature-programmed desorption, hydrogen temperature-programmed reduction. It is found that the presence of chlorine had a limited impact on Ru particle size. Residual chlorine originated from RuCl3 would not influence on Ru 3d5/2 binding energy, but chlorine from HCl solution significantly increased Ru 3d5/2 binding energy. Regardless of their source, the presence of chlorine severely reduced the amount of hydrogen species corresponding to the desorption peak at medium temperature. The inhibition effect of chlorine on hydrogen adsorption was more strong for Ru catalyst with residual chlorine from the RuCl3 precursor. With a similar amount of residual chlorine, the catalysts with chlorine originated from the RuCl3 precursor showed much lower catalytic activity than those prepared by impregnation of HCl. These results suggest that chlorine mainly affects the catalytic properties of alumina supported Ru catalysts for ammonia synthesis by selective site blocking. Chlorine affects on the sites at the metal/support interface for Ru/Al2O3 catalysts, and then hydrogen adsorption and ammonia synthesis both are suppressed.

Electrocatalytic Properties of TiO2-Embedded Pt Nanoparticles in Oxidation of Methanol: Particle Size Effect and Proton Spillover Effect

Size-controlled Pt nanoparticles embedded in TiO2 were prepared by simultaneous dual-gun sputtering from pure targets of Pt and TiO2. The mean diameter of the Pt nanoparticles, as confirmed by their transmission electron microscopic images, was varied from ∼2 to ∼4 nm by changing the RF power ratio of Pt and TiO2. The transmission electron diffraction and X-ray diffraction patterns of the Pt nanoparticles embedded in TiO2 confirmed that the Pt particles are polycrystalline, whereas the TiO2 matrix is amorphous. The electrocatalytic properties of Pt/TiO2 were strongly influenced by the particle size and the TiO2 support. The presence of the TiO2 support led to higher electronic density on Pt, changing its chemisorption properties, weakening the Pt–CO bonds, and increasing its CO oxidation activity. The high CO oxidation activity of the Pt nanoparticles embedded in TiO2 can be also attributed to the ability of TiO2 to provide highly reactive oxygen atoms. CO desorbed at higher onset potential with a decrease in the particle size, which is related to quantum-size effects in the Pt nanoparticles. The high activity of methanol oxidation on the Pt/TiO2 electrode resulted from the homogeneous dispersion and the miniaturization of Pt. In addition, we found that the enhanced catalytic activity in the Pt/TiO2 electrodes correlated to proton spillover phenomena in TiO2 and was measured by performing an in situ electrochromic test.

Surface Vibrational Spectroscopy Study of Benzene and 2,2,2-Trifluoroacetophenone on Pt(111)

High resolution electron energy loss spectroscopy (HREELS) data for C6H6 and C6D6 on Pt(111) were measured as a function of coverage. The spectra show five additional loss peaks with respect to earlier reports that were used in density functional theory (DFT) calculations of the properties of benzene on Pt(111). The spectra display a strong coverage dependence in the CH stretching region. HREELS and STM data for the adsorption of 2,2,2-trifluoroacetophenone (TFAP) on Pt(111) are used in the analysis of the benzene/Pt(111) system. The TFAP spectra display the same coverage dependence as benzene in the CH stretching region. An analysis of STM images of TFAP dimers shows that their azimuthal orientation is not consistent with the occupation of hcp0 sites. The data for benzene adsorption cannot be explained on the basis of existing density functional theory (DFT) calculations. In particular, the spectra are not consistent with either a C2v symmetry bridge adsorption or a combination of adsorption at C2v symmetry and hcp0 sites. The data provide a benchmark for evaluating the ability of emerging computational approaches to predict the chemisorption properties of strongly chemisorbed aromatics. The data for both benzene and TFAP are of relevance to the asymmetric hydrogenation of ketones on chirally modified Pt catalysts.

Activated organically doped silver: enhanced catalysis of methanol oxidation

Detailed study of the synthesis parameters of silver doped with the organic dye Congo-red (CR@Ag) have led to an understanding of the origins of the superior performance of this novel-type methanol-oxidation catalyst (compared to pure silver) as demonstrated by the significant lowering of the temperature needed to reach maximal conversion by more than 100 °C. The origins of the effect of the organic dopant on the catalytic properties of silver are suggested and discussed in terms of its effects on morphology, on oxygen chemisorption properties, on the surface area, on the thermal behavior and on the sinterability of the silver aggregated crystallites. For instance, the organic dopant affects the surface area dramatically, increasing it from 600–3000 cm2 g−1 for undoped silver to 46 000 cm2 g−1 for CR@Ag; and oxygen chemisorption, crucial for this catalytic process, increases from 32 cm2 g−1 for Ag to 893 cm2 g−1 for CR@Ag. Preliminary work with CR@copper provides a positive outlook for the general use of organic dopants to improve catalytic properties of other metals.

Effects of promotion of TiO2 with alkaline earth metals on the chemisorptive properties and water–gas shift activity of supported platinum catalysts

The effects of promotion of TiO2 with Group II alkaline earth metals on the chemisorptive properties and water–gas shift (WGS) activity of dispersed platinum have been investigated over Pt/TiO2(x%MO) catalysts of variable promoter type (M=Mg, Ca, Sr, Ba) and loading (x=0–4wt.%). DRIFTS experiments carried out using CO as a probe molecule showed that the presence of calcium does not affect appreciably adsorption on sites located on the surface of Pt crystallites, but results in the development of new, low-frequency bands in the ν(CO) region attributed to the creation and population of sites with increased electron density located at the metal–support interface. Results of H2-TPD experiments indicated a weakening of the adsorption strength of these sites toward hydrogen with increasing CaO content. Catalytic performance tests and kinetic measurements showed that activity of alkaline earth metal-promoted catalysts depends appreciably on the nature and loading of the promoter, as well as on the calcination temperature employed for the preparation of doped TiO2 supports. Optimal results were obtained for Pt catalyst supported on TiO2 promoted with 1–2wt.% CaO, the specific activity of which is 2–3 times higher, compared to that of the unpromoted catalyst. Optimized catalysts are characterized by increased activity and excellent stability under realistic reaction conditions and may be used efficiently in practical applications, provided that promoter loading, calcination temperature and operating conditions are properly selected.

Tuning the electrocatalytic activity of Pt nanoparticles on carbon nanotubes via surface functionalization

Polyelectrolytes with various characteristic functional groups as interlinkers to anchor Pt nanoparticles were used to functionalize carbon nanotubes (CNTs) as Pt electrocatalyst support. It was found that polyanions (poly(styrenesulfonic acid) (PSS), and poly(acrylic acid sodium) (PAA)) have a beneficial effect on methanol electrooxidation on Pt nanoparticles supported on carbon nanotubes via modifying their electronic structure through charge transfer from polyanions to Pt sites and supply of oxygen-containing species. The increased electron density around Pt sites by the charge transfer from polyanions would cause partial filling of Pt 5 d-bands, resulting in the downshift of d-band center and weaker chemisorption with oxygen-containing species (e.g. CO ad). The weakened chemisorption of CO on Pt nanoparticles would promote the methanol electrooxidation. On the contrary, polycations would have an opposite effect on the electronic structure and chemisorption properties of Pt nanoparticles.

Formation of the heteropolynuclear complex ([NH2(C4H9)2][Au{S2CN(C4H9)2}2][CdCl4]) n during the chemisorption of gold(III) with cadmium dibutyldithiocarbamate: Structure and thermal properties

The chemisorption properties of cadmium dibutyldithiocarbamate with respect to [AuCl4]− in 2M HCl solutions are studied. The heterogeneous reaction of gold(III) binding affords the heteropolynuclear compound ([NH{2(C4H9)2][Au{S2CN(C4H9)2}2][CdCl4])n (I). The molecular and crystal structures of compound I are determined from the X-ray diffraction data. Each of three ions is presented in the structure by two conformers. The irreversible decomposition of some dibutyldithiocarbamate groups is observed during the chemisorption of gold(III), which noticeably decreases the efficiency of gold binding with the sorbent studied and its sorption capacity. The study of the thermal properties of compound I shows that gold(III) is reduced to the metal during thermolysis.

A first principles study of water dissociation on small copper clusters

Water dissociation on copper is one of the rate-limiting steps in the water-gas-shift (WGS) reaction. Copper atoms dispersed evenly from freshly made catalyst segregate to form clusters under the WGS operating conditions. Using density functional theory, we have examined water adsorption and dissociation on the smallest stable 3-dimensional copper cluster, Cu(7). Water molecules are adsorbed on the cluster sequentially until full saturation at which no direct water-copper contact is sterically possible. The adsorption is driven mainly by the overlap between the p-orbital of O atom occupied by the lone pair and the 3d-orbitals of copper, from which a fractional charge is promoted to the 4s-orbital to accommodate the charge transfer from water. Water dissociation on the Cu(7) cluster was investigated at both low and high water coverage. It was found that water dissociation into OH and H is exothermic but is inherently a high temperature process at low coverage. At high coverage, the reaction becomes more exothermic with fast kinetics. In both cases, water can catalyze the reaction. It was found that direct dissociation of the OH species is endothermic with a significantly higher barrier at both low and high coverage. However, the OH species can readily react with another adjacent hydroxyl group to form an O adatom and water molecule. Our studies indicate that the basic chemical properties of water dissociative chemisorption may not change significantly with the size of small copper clusters. Similarities between water dissociation on copper clusters and on copper crystalline surfaces are discussed.

Electrochemical determination of nitrite using silver nanoparticles modified electrode

In this work, we report the fabrication and characterization of silver nanoelectrode ensembles (Ag-NEEs) on a glassy carbon electrode. For this purpose, Ag nanoparticles (NPs) were anchored to the mercaptopropyl functionalized MCM-41 type silica spheres utilizing the chemisorption property of Ag NPs by -SH groups. The successful anchoring of Ag NPs into the silica matrix is characterized by several techniques including UV-vis diffuse reflectance and X-ray powder diffraction methods. The surface morphology of the Ag-NEEs was assessed by scanning and transmission electron microscopy (SEM and TEM respectively). Further, nitrite (NO(2)(-)) is electrocatalytically oxidized at Ag-NEEs, which leads to a sensitive determination of NO(2)(-). The fabrication, characterization, and efficient sensing of NO(2)(-) at the Ag-NEEs are presented.

Stability and chemisorption properties of ultrathin TiOx/Pt(111) films and Au/TiOx/Pt(111) model catalysts in reactive atmospheres

The stability of three ultrathin TiO(x)/Pt(111) films with different stoichiometry and defectivity and the corresponding Au/TiO(x)/Pt(111) model catalysts in CO or a CO-O(2) (1 : 1) gas mixture up to a pressure of 100 mbar has been investigated. According to previous studies, the ultrathin films proved to be effective substrates to deposit in UHV Au nanoparticles with specific morphologies and lateral sizes ranging between 1 and 6 nm. The films have been characterized before and after the exposure using X-ray photoemission spectroscopy (XPS), low-energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). Additional in situ measurements of the CO chemisorption behavior were performed using polarization-modulation infrared reflection-absorption spectroscopy (PM-IRAS). A fully oxidized film is stable in CO and CO-O(2) (1 : 1) ambient, while the reduced films undergo an oxidative dewetting process at RT in the latter atmosphere. This process ultimately produces a nano-composite surface, where very tiny (from 0.5 to 3 nm lateral sizes) titania nanograins are mixed with open, uncovered areas of the Pt substrate. IRAS measurements on the corresponding Au/TiO(x)/Pt(111) model catalysts demonstrated that the CO chemisorption strongly depends on the Au nanoparticle size and morphology, while the actual Ti oxidation state of the oxide support does not seem to play a significant role.

Effects of alkali promotion of TiO 2 on the chemisorptive properties and water–gas shift activity of supported noble metal catalysts

The effects of alkali promotion of TiO 2 on the chemisorptive properties and water–gas shift (WGS) activity of dispersed noble metal catalysts (NM = Pt, Ru, Pd) have been investigated over NM/X–TiO 2 samples of variable promoter type (X = Li, Na, K, Cs) and loading (0.0 to 0.68 wt.%). Results of H 2-TPD experiments show that addition of alkalis does not affect appreciably the population and chemisorption strength of hydrogen adsorbed on the surface of dispersed metal crystallites indicating that these sites are not influenced by the presence of the promoter. In contrast, the desorption temperature of hydrogen adsorbed on sites located at the metal-support interface shifts monotonically toward lower temperatures with increasing alkali content. This has been attributed to alkali-induced reduction of Ti 4+ surface species and creation of a new type of sites at the perimeter of the dispersed metal crystallites, which are in contact with the support. These sites are of the form of NM–□ s–Ti 3+, where □ s denotes an oxygen defect vacancy, and provide the necessary dual-function sites required for the WGS reaction to proceed. Catalytic performance tests and kinetic measurements show that activity depends appreciably on the nature and loading of added alkali, whereas the apparent activation energy ( E a) of the reaction does not vary to a large extent. For Pt/X–TiO 2 catalysts, a volcano-type dependence of intrinsic reaction rate on the chemisorption strength of Pt–□ s–Ti 3+ sites toward hydrogen has been found to exist. Optimized catalysts are about three times more active than unpromoted Pt/TiO 2. Results are discussed with respect to the alkali-induced modifications of the physicochemical properties of the support on the population, chemisorptive properties, and catalytic activity of sites located at the metal-support interface.

Propane Dehydrogenation Over PtSn Catalysts Supported on ZnO-Modified MgAl2O4

Platinum and tin supported on ZnO-modified MgAl2O4 constitutes a new and efficient catalyst for the dehydrogenation of propane. The structure of the catalyst has been studied by XRD, TEM, BET, TG, H2-TPR and H2 chemisorption and catalytic properties for propane dehydrogenation have been tested on a microreactor. It has been shown that the catalyst is superior to conventional alumina-supported PtSn systems and PtSn/MgAl2O4 in terms of lifetime stability, activity and propene selectivity. The characterization results of catalysts revealed that ZnO-modified catalysts can increase the Pt dispersion and enhance the interaction between metal and support, resulting in the increase of catalytic activity for propane dehydrogenation.

Electrocatalytic activity of Pt nanoparticles deposited on porous TiO2 supports toward methanol oxidation

Porous TiO2 thin films were prepared on the Si substrate by hydrothermal method, and used as the Pt electrocatalyst support for methanol oxidation study. Well-dispersed Pt nanoparticles with a particle size of 5–7nm were pulse-electrodeposited on the porous TiO2 support, which was mainly composed of the anatase phase after an annealing at 600°C in vacuum. Cyclic voltammetry (CV) and CO stripping measurements showed that the Pt/TiO2 electrode had a high electrocatalytic activity toward methanol oxidation and an excellent CO tolerance. The excellent electrocatalytic performance of the electrode is ascribed to the synergistic effect of Pt nanoparticles and the porous TiO2 support on CO oxidation. The strong electronic interaction between Pt and the TiO2 support may modify CO chemisorption properties on Pt nanoparticles, thereby facilitating CO oxidation on Pt nanoparticles via the bifunctional mechanism and thus improving the electrocatalytic activity of the Pt catalyst toward methanol oxidation.

The effect of Fe on SiO 2-supported Pt catalysts: Structure, chemisorptive, and catalytic properties

SiO 2-supported PtFe catalysts with a wide range of Pt/Fe ratios were prepared from individual H 2PtCl 6 and Fe(NO 3) 3 precursors and characterized by high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR), and extended X-ray absorption fine structure (EXAFS) spectroscopies. Treatment with H 2 at 350 °C leads to the formation of metal particles with average sizes in the order of 2.6 nm, some of which are bimetallic in nature. The fraction of Pt–Fe bimetallic contributions in each sample, the nature and extent of electronic interactions between Pt and Fe, and the strength of the CO adsorption on Pt strongly depend on the Fe content. PtFe/SiO 2 samples thus prepared were found to be active catalysts for various reactions taking place in both oxidative and reducing environments, including the oxidation of CO in air, the dehydrogenation of cyclohexane, and the selective hydrogenation of citral. The results indicate that the catalytic behavior of Pt is significantly affected by the presence of Fe. The enhancement of the catalytic activity observed for the oxidation of CO in air correlates with the fraction and degree of electronic Pt–Fe interactions and the strength of CO adsorption on Pt. Similarly, Fe in small concentrations promotes the activity of Pt for the dehydrogenation of cyclohexane and the selective hydrogenation of citral, which can be attributed to either an electronic effect and/or the presence of bimetallic Pt–Fe sites. The close proximity between Pt and Fe in such sites leads to the reduction of Fe, which can thus become active for cyclohexane dehydrogenation. Furthermore, it is possible that Pt–Fe adsorption sites favor the di-σ CO mode of adsorption for α,β-unsaturated aldehydes, thus promoting the selective hydrogenation of the C O bond and the formation of α,β-unsaturated alcohols.

Surface Modification of Pd/α-Si3N4 Catalysts Through the Solvent Used During Synthesis. Implications on the CO Chemisorption Properties and Catalytic Performances

Palladium catalysts supported on α-Si3N4 were prepared by impregnation with Pd(II)-acetate dissolved either in toluene or in water. The mean metal particle size of ~0.5 wt% Pd catalysts was similar (~5 nm) and independent of the way of preparation. Nevertheless, the two catalysts present very different chemisorption behaviour chemisorptive and catalytic properties. Fourier transformed infrared (FTIR) spectra of adsorbed CO at different temperatures (ranging from room temperature to 300 °C) show a very different behaviour for both catalysts. While the CO adsorption states on the Pd/α-Si3N4 prepared in toluene are very similar to those generally measured for silica and/or alumina supported palladium catalysts, CO chemisorbs less strongly on Pd/α-Si3N4 prepared in water and on different adsorption sites. The Pd/α-Si3N4 catalyst obtained by aqueous impregnation is much less efficient for the methane total oxidation. It is less active and less stable: it deactivates strongly after 3 h on stream at 650 °C. The two catalysts present about the same activity for the 1,3-butadiene hydrogenation after stabilisation at 20 °C. But, the catalyst prepared in water shows a much better selectivity to butenes. The results are discussed in terms of the possible migration of silicon atoms from the silicon nitride support to the surface of the palladium particles, when the catalyst is prepared in water. This is not the case when prepared in an organic solvent.

Effect of heat-treated basalt fibre on the structure of chemisorption composite material made from it

Preliminary heat treatment of basalt fibre used for synthesis of phenol-formaldehyde cation-exchange fibre material increases the activity of the reinforcing system. Better chemisorption properties of cationexchange fibre material (CEFM) made from this fibre are the consequence of heat treatment.

Effects of alkali additives on the physicochemical characteristics and chemisorptive properties of Pt/TiO2 catalysts

The effects of alkali additives on the physicochemical and chemisorptive properties of 0.5% Pt/TiO2 have been investigated over catalysts promoted with variable amounts of Na (0–0.2 wt%) or Cs (0–0.68 wt%) with the use of diffuse reflectance infrared spectroscopy (DRIFTS) and temperature-programmed (TPD, TPR) techniques. It has been found that addition of alkalis does not affect adsorption of CO and H2 on the surface of Pt crystallites, indicating the absence of strong electronic-type interactions between these sites and the promoters. However, the presence of alkalis results in the creation and population of new sites with increased electron density, proposed to be located at perimetric sites of Pt crystallites, which are in contact with the support. The adsorption strength of these sites toward CO increases with increasing alkali content, which is evidenced by the development of new, low-frequency IR bands in the ν(CO) region. In contrast, addition of alkali results in weakening of hydrogen adsorption on sites located at the metal/support interface, which is reflected to a significant shift of the corresponding TPD peak toward lower temperatures. Results of CO-TPD experiments indicate that CO adsorbed on Pt interacts with hydroxyl groups associated with the support to yield formate, which decomposes during TPD to CO2 and H2. Thermal decomposition of formate is accomplished at lower temperatures in the presence of alkali. Finally, CO-TPR experiments indicate that the reducibility of TiO2 is enhanced in the presence of alkali, which can be related to the creation of the new sites at the metal/support interface.