Showing papers on "Chemisorption published in 2007"

Scaling properties of adsorption energies for hydrogen-containing molecules on transition-metal surfaces.

Abstract: Density functional theory calculations are presented for CHx, x=0,1,2,3, NHx, x=0,1,2, OHx, x=0,1, and SHx, x=0,1 adsorption on a range of close-packed and stepped transition-metal surfaces. We find that the adsorption energy of any of the molecules considered scales approximately with the adsorption energy of the central, C, N, O, or S atom, the scaling constant depending only on x. A model is proposed to understand this behavior. The scaling model is developed into a general framework for estimating the reaction energies for hydrogenation and dehydrogenation reactions.

Surface characterization studies of TiO2 supported manganese oxide catalysts for low temperature SCR of NO with NH3

Abstract: A series of TiO 2 supported manganese oxide catalysts were prepared by wet-impregnation method for the low temperature selective catalytic reduction (SCR) of NO with ammonia as a reductant. A combination of various physico-chemical techniques such as N 2 physisorption, O 2 chemisorption, TPR, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman were used to characterize the chemical environment of these catalysts. O 2 chemisorption and XRD results suggest that Mn exist in a well-dispersed state at below 16.7 wt.% of Mn on TiO 2 anatase (Hombikat), 7.5 wt.% on TiO 2 rutile (Kemira) and P-25 (80% anatase + 20% rutile), and in microcrystalline phase above these loading levels on respective support materials. These results also reveal that Mn interacts very well with pure anatase phase compared to rutile. XPS results of Mn/TiO 2 anatase (Hombikat) catalysts illustrated the presence of MnO 2 as a major phase (peak at 642.0 eV) along with Mn 2 O 3 as the minor phase at lower loadings. The presence of Mn 2 O 3 disappears at higher loadings. The characterization results indicated that the manganese oxide exists as an isolated species at very low loadings, highly dispersed state probably as two dimensional monolayer species at intermediate loadings, polymeric or microcrystalline form of manganese oxide at higher (above monolayer capacity) loadings was envisaged. The catalytic performance of various amounts of Mn loaded on different TiO 2 supported catalysts for low temperature SCR reaction at catalyst bed temperature 175 °C under power plant conditions using GHSV = 50,000 h −1 was studied. The catalyst with 16.7 wt.% Mn/TiO 2 anatase (Hombikat) was found to be highly active and selective catalyst for this reaction. The Raman studies acted as complimentary tool to XPS in order to characterize the manganese oxides (MnO, Mn 2 O 3 , Mn 3 O 4 , MnO 2 ). Raman data show that there is a strong interaction between the Mn oxides and the support, which is responsible for the impressive catalytic performance in comparison with other systems we investigated.

Structure and activity of nanosized iron-doped anatase TiO2 catalysts for phenol photocatalytic degradation

Abstract: A series of nanosized iron-doped anatase TiO 2 catalysts with different iron content (between 0.4 and 5.1 wt.%) has been prepared by a microemulsion method and examined with respect to their behaviour for UV photocatalytic degradation of aqueous phenol. The activity results have been correlated with structural, electronic and surface examinations of the catalysts done with XRD, Raman, UV–vis, EPR, N 2 physisorption and NH 3 chemisorption. An enhancement of the photocatalytic activity is observed for doping levels up to ca. 1 wt.% which is attributable to the effective introduction of Fe 3+ cations into the anatase structure along with associated modifications of the surface acid/base properties. Achievement of relatively high levels of surface segregation of oxidic iron-containing amorphous phases for higher doping levels results however detrimental to the photoactivity.

Hydrogen storage on platinum nanoparticles doped on superactivated carbon

Abstract: The equilibrium and kinetics of hydrogen storage on Pt nanoparticles doped on AX-21 superactivated carbon were studied. The Pt/AX-21 sample was prepared by ultrasound-assisted impregnation of H2PtCl6 in a solution of acetone. The dispersion of platinum was determined by CO/H2 chemisorption and high-resolution transmission electron microscopy (HRTEM). Hydrogen adsorption isotherms and kinetics were measured at 298−348 K and up to 10 MPa. HRTEM images showed that Pt was well dispersed with a uniform particle size ∼2 nm. CO chemisorption results indicated that the dispersion was 58%. By doping 5.6 wt % Pt, the hydrogen storage capacity of AX-21 was enhanced to 1.2 wt % at 298 K and 10 MPa. Furthermore, the isotherm was totally reversible and rechargeable at 298 K. The overall isosteric heats of adsorption (−23 to −15 kJ/mol) and the apparent activation energy for surface diffusion (7.6 kJ/mol) were determined from the temperature dependence of, respectively, equilibrium isotherm and diffusion time constant. ...

Factors Controlling the Interaction of CO2 with Transition Metal Surfaces

Abstract: On the basis of density functional theory calculations, the chemisorption of CO2 on the transition metal surfaces was investigated to find out the key factors controlling its adsorption strength and activation degree. The interaction mechanism of CO2 with the metal surfaces was discussed by analyzing the density of states. The adsorption strength of CO2 is controlled by the d-band center of the metal surfaces and also affected by the charge transfer from the metal surfaces to the chemisorbed CO2. The degree of CO bond activation depends on the transferred charge. Therefore, both d-band center of the metal surfaces and the charge transfer should control the chemisorption of CO2.

Mesoporous carbons with KOH activated framework and their hydrogen adsorption

Abstract: The effects of KOH activation on pore structure of ordered mesoporous carbons were analyzed by transmission electron microscopy, powder X-ray diffraction and argon adsorption. The activation led to remarkable increases in micropore volume and BET surface area up to 1.0 mL g−1 and 2700 m2 g−1, at the expense of the mesostructural order. The resultant carbons with various microporosity and mesoporosity were tested for room-temperature adsorption of hydrogen under high pressure. The adsorption data were analyzed in correlation with the varied carbon pore structures. The results showed that the hydrogen adsorption capacity increased approximately linearly with respect to micropore volume, or BET surface area, reaching a 2.5-times higher value when fully activated. However, the adsorption capacity at 100 atm (0.75 wt%) was still far below the US DOE target of 6.5 wt%. The extrapolation of our results to the carbon structure with the highest possible surface area could lead to no more than 2.5 wt%. This result suggests that chemisorption or other chemical storage methods should be combined with physisorption if carbon materials are considered for hydrogen storage.

Hydrogen chemisorption on supported platinum, gold, and platinum-gold-alloy catalysts.

Abstract: Alloyed catalysts receive considerable attention, because of their unique catalytic properties; they often show higher selectivity, activity, and stability compared to the pure metal particles. To provide insights in the origins of these features, we report the structure and the interaction of hydrogen with each of the metals in an intimately mixed platinum–gold catalyst and compare these characteristics to those in the single metal particles. X-Ray absorption spectroscopy (XAS) and electron microscopy analysis showed that the structure of the mixed particle differed from the single metal particles. The interaction of platinum with hydrogen is stronger than the H–Au interaction and the adsorption sites were different. EXAFS analysis showed that the structure of the platinum clusters changes with increasing hydrogen coverage, observed as a relaxation of the contracted Pt–Pt distance and an increase in the Pt–Pt coordination number. No such changes were observed for gold clusters. Well-mixed PtAu-alloy clusters, with a bulk Au-to-Pt ratio of two, supported on SiO2, adsorb hydrogen on both platinum and gold atoms, which indicates that gold cannot be regarded as an inert metal. The heat of adsorption on the platinum ensembles does not decrease upon alloying; the weakening of the overall hydrogen adsorption strength when alloying platinum with gold is an ensemble-size effect.

Preparation and catalytic properties of a bimetallic SnPt complex in the supercages of NaY zeolite by use of surface organometallic chemistry

Abstract: The grafting reaction of SnMe4 on the surface of Pt/NaY zeolite was investigated in a 1.6 × 104 Pa hydrogen atmosphere. The chemical compositions, structure and properties of the resulting solid were characterized by in situ FTIR, ICP, XRD, XPS, temperature programmed decomposition and nitrogen adsorption. The results show that Pt atoms of Pt/NaY zeolite react with tetramethyltin to form a bimetallic Me3SnPt surface species in the presence of hydrogen at 353 K. The reaction does not destroy the zeolite framework, while its surface properties are changed. The BET surface area and the pore volume of the zeolite decrease significantly. The CO chemisorption results and the XPS results show that the electronic properties of the modified Pt atoms on the surface of NaY zeolite are altered remarkably by the inductive effect of electron between Pt and methyl groups. The grafted product has excellent selective hydrogenation of furfural to furfuryl alcohol. Copyright © 2007 John Wiley & Sons, Ltd.

Adsorption and Surface Reactivity on Single-Walled Boron Nitride Nanotubes Containing Stone−Wales Defects

Abstract: Adsorption of chemical species H, O, CO, H2, O2, H2O, and NH3 on the sidewall of zigzag (8,0) and armchair (5, 5) boron nitride nanotubes (BNNTs) is studied using density-functional theory. Particular attention is paid to searching for the most-stable configuration of the adsorbates at a perfect site (PS) and near a Stone−Wales (SW) defect, and the surface reactivity at the PS and near the SW defect. Reactivity near the SW defect is generally higher than that at the PS because of the formation of frustrated B−B and N−N bonds and the local strain caused by pentagonal and heptagonal pairs. O2 is prone to dissociative chemisorption near SW defects, but is more likely physisorbed at the PS. The adsorption of H2O and NH3 on the sidewall of BNNT can be described as molecular chemisorption due to modest interaction between HOMO of H2O or NH3 with LUMO of BNNT. Adsorption of NH3 can affect electronic properties of BNNT through lifting the Fermi level. As such, NH3 can be viewed as an n-type impurity. CO and H2 ca...

Influence of the Iron Oxide Acid−Base Properties on the Chemisorption of Model Epoxy Compounds Studied by XPS

Abstract: In this work, the influence of the iron oxide acid−base properties on the adsorption of model epoxy compounds was examined. To study this, iron oxide layers with a different surface hydroxyl fractions were prepared in controllable and reproducible conditions. The surfaces were characterized by X-ray photoelectron spectroscopy (XPS). An expression for the hydroxyl fraction of the oxide films was deduced, starting from the measured XPS intensities. Two model epoxy compounds characteristic of an epoxy/amide system were adsorbed on the oxides: N,N‘-dimethylsuccinamide and N-methyldiethanolamine. Additionally, an amine molecule without alcohol groups, N,N‘-diethylmethylamine, was adsorbed to investigate the role of alcohol functionalities on the amine adsorption mechanism. The interaction between the oxide layers and the nitrogenous model compounds were studied by examination of the O 1s and N 1s XPS photopeaks. The data showed that the amine and amide nitrogen adsorbed via two different bonding modes: via L...

ATR-IR spectroscopic study of antimonate adsorption to iron oxide.

Abstract: Antimonate ions adsorb to iron oxides in mining contexts, but the nature of the adsorbed antimonate species has not frequently been investigated. In this study, ATR-IR spectroscopy was used to reveal that the adsorption of Sb(OH)6- ion from aqueous solutions onto an amorphous iron oxide particle film is accompanied by changes in the Sb(OH)6- spectrum and the loss of OH stretching absorptions from iron oxide surface hydroxyl groups. These spectral changes upon adsorption imply an inner-sphere surface interaction with the formation of Sb−O−Fe bonds as well as some outer-sphere adsorption. The corresponding results from solutions of antimonate in D2O confirm that chemisorption occurs. The dependence of antimonate adsorption on pH in the range from 8 to 3 follows that expected for anions on iron oxide considering its pH-dependent surface charge, with the greatest amount of adsorbed antimonate at pH 3. The study of adsorption/desorption kinetics showed a more rapid desorption of adsorbed antimonate under alkal...

Characterization and Reactivity of Alumina-Supported Pd Catalysts for the Room-Temperature Hydrodechlorination of Chlorobenzene

Abstract: A series of alumina-supported Pd catalysts were prepared by varying the metal loading between 0.5 and 5 wt. %, adopting the deposition precipitation (DP) method. These catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR), pulse CO chemisorption, Brunauer−Emmett−Teller specific surface area measurement, and transmission electron microscopy. The catalytic properties of the catalysts were studied for the hydrodechlorination of chlorobenzene. The reaction was carried out in a continuous mode passing liquid chlorobenzene and gaseous hydrogen over a fixed bed of catalyst taken in a reactor operating at room temperature and atmospheric pressure. The catalysts with metal loading up to 2 wt. % demonstrated high dispersion, homogeneous distribution of active species with predominance of metal−support interaction and without any formation of β-PdH. Beyond 2 wt. % loading, the agglomeration of PdO took place forming bulk particles that reduc...

Dissociative Adsorption of Water at Vacancy Defects in Graphite

Abstract: We have performed density-functional calculations to investigate the adsorption of H2O on perfect and defected graphite (0001) represented by a single graphene sheet. On the perfect surface, the water physisorbs, as expected, with no significant preference for the adsorption site. At a vacancy site, the interaction is much more significant, with a computed binding energy of ∼210 meV in a weak chemisorption/strong physisorption state. The H2O sits with one H pointing down to a carbon atom, which is pulled out of the plane by ∼0.55 A. From this physisorption state, dissociative chemisorption will occur after overcoming a barrier of 0.8−0.9 eV (∼0.6−0.7 eV relative to the gas-phase). The lowest dissociation barrier obtained is ∼0.47 eV along a path largely avoiding the physisorption well. The dissociation paths have an intermediate step, in which the molecule partially dissociates to H and OH. Subsequently, the chemisorbed OH stretches, breaking into O and H atoms chemisorbed on separate C atoms on the vacan...

The use of hydrogen chemisorption for the determination of Ru dispersion in Ru/γ-alumina catalysts

Abstract: Ru nanoparticles supported on γ-Al2O3, prepared by reduction of RuCl3 in ethylene glycol with using microwave irradiation, were thoroughly characterized by ICP-AES, BET, XRD, TEM, XPS and H2 chemisorption. Structure and chemisorptive properties of the supported colloidal Ru nanoparticles were compared with those of the Ru/γ-Al2O3 catalyst prepared by the incipient wetness impregnation method, using the same RuCl3 precursor. Results obtained by volumetric H2 chemisorption performed at different temperatures 20–200 °C, showed that the irreversible H2 adsorption onto colloidal Ru/γ-Al2O3 catalyst is a nonactivated process. For this catalyst saturation of the ruthenium surface with hydrogen was achieved already at room temperature. Ruthenium dispersion and particle sizes obtained from H2 measurements agreed well with the results of TEM and XRD methods. In contrast, activated chemisorption behavior has been observed on traditionally prepared Ru/γ-Al2O3 catalyst. Even at 100 °C, irreversible H2 uptake was lower then expected as evidenced by the large discrepancies between the mean particle sizes obtained from H2 chemisorption and TEM. Superior chemisorptive properties of the Ru nanoparticles supported on γ-Al2O3 are assigned to the higher dispersion of the smaller ruthenium particles and also their weaker interaction with the support. The colloidal catalyst, in contrast to traditionally prepared one, is free of chlorine contamination. Also, contamination of Ru by aluminum ions, possible during the catalyst preparation by conventional impregnation with acidic solution, was avoided.

Site-Specific vs Specific Adsorption of Anions on Pt and Pt-Based Alloys

Abstract: X-ray absorption spectroscopy (XAS) is utilized in situ to gain new insights into the electronic and chemical interactions of anions specifically adsorbed on Pt/C. A novel difference methodology was utilized, along with full-multiple scattering calculations using the FEFF8 code, to interpret the X-ray absorption near edge structure (XANES). Significant direct contact (“specific”) anion adsorption occurs in 1 M H2SO4 and 6 M TFMSA, while it does not in 1 M HClO4 and 1 M TFMSA. This specific anion adsorption significantly hinders O(H) chemisorption, particularly formation of subsurface O, causes the Pt nanoparticle to become more round, and weakens the Pt−Pt bonding at the surface. The specific anion adsorption becomes site-specific only after lateral interactions from other chemisorbed species such as OH on the surface force the anions to adsorb into specific sites. Alloying has a profound effect on the strength of the anion adsorption and whether site-specific or just specific adsorption occurs.

Rhodium and iridium nanoparticles entrapped in aluminum oxyhydroxide nanofibers: Catalysts for hydrogenations of arenes and ketones at room temperature with hydrogen balloon

Abstract: The recyclable metal nanoparticle catalysts, rhodium in aluminum oxyhydroxide [Rh/ AIO(OH)] and iridium in aluminum oxyhydroxide [Ir/AlO(OH)], were simply prepared from readily available reagents. The catalysts showed high activities in the hydrogenation of various arenes and ketones under mild conditions. Selective hydrogenation was possible for bicyclic and tricyclic arenes in high yields. The catalysts were active at room temperature even with a hydrogen balloon. Also, the catalysts showed high turnover frequency (TOF) values under solventless conditions at 75°C under 4 atm hydrogen pressure: ca. 1700 h -1 in the hydrogenation of benzene. Furthermore, Rh/AlO(OH) can be reused for at least 10 times without activity loss. The catalysts were characterized by the transmission electron microscopy (TEM), powder X-ray diffraction (XRD), inductively coupled plasma (ICP), energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption and hydrogen chemisorption experiments. The sizes of rhodium and iridium particles were estimated to be 3-4 nm and 2-3 nm, respectively. Aluminum oxyhydroxide nanofibers of these catalysts have surface areas of 500-600 m 2 g -1 .