Showing papers on "Chemisorption published in 2014"

Quantification of Zinc Atoms in a Surface Alloy on Copper in an Industrial-Type Methanol Synthesis Catalyst

Abstract: Methanol has recently attracted renewed interest because of its potential importance as a solar fuel.1 Methanol is also an important bulk chemical that is most efficiently formed over the industrial Cu/ZnO/Al2O3 catalyst. The identity of the active site and, in particular, the role of ZnO as a promoter for this type of catalyst is still under intense debate.2 Structural changes that are strongly dependent on the pretreatment method have now been observed for an industrial-type methanol synthesis catalyst. A combination of chemisorption, reaction, and spectroscopic techniques provides a consistent picture of surface alloying between copper and zinc. This analysis enables a reinterpretation of the methods that have been used for the determination of the Cu surface area and provides an opportunity to independently quantify the specific Cu and Zn areas. This method may also be applied to other systems where metal–support interactions are important, and this work generally addresses the role of the carrier and the nature of the interactions between carrier and metal in heterogeneous catalysts.

Air stable p-doping of WSe2 by covalent functionalization.

Abstract: Covalent functionalization of transition metal dichalcogenides (TMDCs) is investigated for air-stable chemical doping. Specifically, p-doping of WSe2 via NOx chemisorption at 150 °C is explored, with the hole concentration tuned by reaction time. Synchrotron based soft X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) depict the formation of various WSe2–x–yOxNy species both on the surface and interface between layers upon chemisorption reaction. Ab initio simulations corroborate our spectroscopy results in identifying the energetically favorable complexes, and predicting WSe2:NO at the Se vacancy sites as the predominant dopant species. A maximum hole concentration of ∼1019 cm–3 is obtained from XPS and electrical measurements, which is found to be independent of WSe2 thickness. This degenerate doping level facilitates 5 orders of magnitude reduction in contact resistance between Pd, a common p-type contact metal, and WSe2. More generally, the work presents a platform for man...

A metal-organic framework-based material for electrochemical sensing of carbon dioxide.

Abstract: The free primary hydroxyl groups in the metal–organic framework of CDMOF-2, an extended cubic structure containing units of six γ-cyclodextrin tori linked together in cube-like fashion by rubidium ions, has been shown to react with gaseous CO2 to form alkyl carbonate functions. The dynamic covalent carbon–oxygen bond, associated with this chemisorption process, releases CO2 at low activation energies. As a result of this dynamic covalent chemistry going on inside a metal–organic framework, CO2 can be detected selectively in the atmosphere by electrochemical impedance spectroscopy. The “as-synthesized” CDMOF-2 which exhibits high proton conductivity in pore-filling methanolic media, displays a ∼550-fold decrease in its ionic conductivity on binding CO2. This fundamental property has been exploited to create a sensor capable of measuring CO2 concentrations quantitatively even in the presence of ambient oxygen.

Catalytic and adsorption studies for the hydrogenation of CO2 to methane

Abstract: CO2 methanation has been evaluated as a means of storing intermittent renewable energy in the form of synthetic natural gas. A range of process parameters suitable for the target application (4720 h−1 to 84,000 h−1 and from 160 °C to 320 °C) have been investigated at 1 bar and H2/CO2 = 4 over a 10% Ru/γ-Al2O3 catalyst. Thermodynamic equilibrium was reached at T ≈ 280 °C at a GHSV of 4720 h−1. Cyclic and thermal stability tests specific to a renewable energy storage application have also been conducted. The catalyst showed no sign of deactivation after 8 start-up/shut-down cycles (from 217 °C to RT) and for total time on stream of 72 h, respectively. In addition, TGA-DSC was employed to investigate adsorption of reactants and suggest implications on the mechanism of reaction. Cyclic TGA-DSC studies at 265 °C in CO2 and H2, being introduced consecutively, suggest a high degree of short term stability of the Ru catalyst, although it was found that CO2 chemisorption and hydrogenation activity was lowered by a magnitude of 40% after the first cycle. Stable performance was achieved for the following 19 cycles. The CO2 uptake after the first cycle was mostly restored when using a H2-pre-treatment at 320 °C between each cycle, which indicated that the previous drop in performance was not linked to an irreversible form of deactivation (sintering, permanent poisoning, etc.). CO chemisorption on powder Ru/γ-Al2O3 was used to identify metal sintering as a mechanism of deactivation at temperatures higher than 320 °C. A 10% Ru/γ-Al2O3//monolith has been investigated as a model for the design of a catalytic heat exchanger. Excellent selectivity to methane and CO2 conversions under low space-velocity conditions were achieved at low hydrogenation temperatures (T = 240 °C). The use of monoliths demonstrates the possibility for new reactor designs using wash-coated heat exchangers to manage the exotherm and prevent deactivation due to high temperatures.

Development of Pd-Cu/hematite catalyst for selective nitrate reduction.

Abstract: A new hematite-supported Pd-Cu bimetallic catalyst (Pd-Cu/hematite) was developed in order to actively and selectively reduce nitrate (NO3(-)) to nitrogen gas (N2). Four different iron-bearing soil minerals (hematite (H), goethite (G), maghemite (M), and lepidocrocite (L)) were transformed to hematite by calcination and used for synthesis of different Pd-Cu/hematite-H, G, M, and L catalysts. Their characteristics were identified using X-ray diffraction (XRD), specific surface area (BET), temperature programed reduction (TPR), transmission electron microscopy with energy dispersive X-ray (TEM-EDX), H2 pulse chemisorption, zeta-potential, and X-ray photoelectron spectroscopy (XPS). Pd-Cu/hematite-H exhibited the highest NO3(-) removal (96.4%) after 90 min, while a lower removal (90.9, 51.1, and 30.5%) was observed in Pd-Cu/hematite-G, M, and L, respectively. The results of TEM-EDX, and TPR analysis revealed that Pd-Cu/hematite-H possessed the closest contact distance between the Cu and Pd sites on the hematite surface among the different Pd-Cu/hematite catalysts. The high removal can be also attributed to the highly active metallic sites on its positively charged surface. The XPS analysis demonstrated that the amount of hydrogen molecules can have a pivotal function on NO3(-) removal and a ratio of nitrogen to hydrogen molecule (N:H) on the Pd sites can critically determine N2 selectivity.

NO Chemisorption on Cu/SSZ-13: a Comparative Study from Infrared Spectroscopy and DFT Calculations

Abstract: The locations and energies of Cu ions in a Cu/SSZ-13 zeolite catalyst were investigated by density functional theory (DFT) calculations. For “naked” Cu2+ ions (i.e., Cu2+ ions with no ligands in their coordination spheres other than zeolite lattice oxygen atoms), the more energetically favorable sites are within a 6-membered ring. However, with the presence of various adsorbates, the energy difference between 6- and 8-membered ring locations greatly diminishes. Specifically, Cu2+ ions are substantially stabilized by −OH ligands (as [CuII(OH)]+), making the extra-framework sites in an 8-membered ring energetically more favorable than 6-membered ring sites. Under fully dehydrated high vacuum conditions with different Si/Al and Cu/Al ratios, three chemisorbed NO species coexist upon exposure of NO to Cu/SSZ-13: NO+, Cu2+–NO, and Cu+–NO. The relative signal intensities for these bands vary greatly with Si/Al ratios. The vibrational frequency of chemisorbed NO was found to be very sensitive to the location of ...

Counting of Oxygen Defects versus Metal Surface Sites in Methanol Synthesis Catalysts by Different Probe Molecules

Abstract: Different surface sites of solid catalysts are usually quantified by dedicated chemisorption techniques from the adsorption capacity of probe molecules, assuming they specifically react with unique sites. In case of methanol synthesis catalysts, the Cu surface area is one of the crucial parameters in catalyst design and was for over 25 years commonly determined using diluted N2O. To disentangle the influence of the catalyst components, different model catalysts were prepared and characterized using N2O, temperature programmed desorption of H2, and kinetic experiments. The presence of ZnO dramatically influences the N2O measurements. This effect can be explained by the presence of oxygen defect sites that are generated at the Cu-ZnO interface and can be used to easily quantify the intensity of Cu-Zn interaction. N2O in fact probes the Cu surface plus the oxygen vacancies, whereas the exposed Cu surface area can be accurately determined by H2.

Selective hydrogenation of acetylene in excess ethylene using Ag- and Au–Pd/SiO2 bimetallic catalysts prepared by electroless deposition

Abstract: Selective hydrogenation of acetylene in excess ethylene has been investigated for a series of Ag- and Au–Pd/SiO2 bimetallic catalysts having incremental surface coverages of Ag and Au on the Pd surface. The catalysts were prepared by electroless deposition and characterized by atomic absorption spectroscopy and selective chemisorption. Both Ag- and Au–Pd/SiO2 catalysts showed increased selectivity of acetylene conversion to form ethylene, rather than ethane, at high coverages. The catalyst performance results suggest that at high coverages of Ag or Au on Pd, that result in small ensembles of Pd sites, acetylene is adsorbed as a π-bonded species that favors hydrogenation to ethylene. At low coverages, where ensemble sizes of contiguous Pd surface sites are much larger, acetylene is strongly adsorbed as a multi-σ-bonded species which preferentially forms ethane, lowering the selectivity to ethylene. Both kinetic analyses and the calculated turnover frequencies of acetylene conversion and ethane formation were consistent with the above explanation. The similar performance trends for Ag- and Au–Pd/SiO2 suggest that the bimetallic effect for these catalysts is geometric and not electronic in nature.

Hydrogen activation, diffusion and clustering on CeO2(111): a DFT+U study

Abstract: We present a comprehensive density functional theory+U study of the mechanisms underlying the dissociation of molecular hydrogen, and diffusion and clustering of the resulting atomic species on the CeO2(111) surface. Contrary to a widely held view based solely on a previous theoretical prediction, our results show conclusively that H2 dissociation is an activated process with a large energy barrier ∼1.0 eV that is not significantly affected by coverage or the presence of surface oxygen vacancies. The reaction proceeds through a local energy minimum – where the molecule is located close to one of the surface oxygen atoms and the H–H bond has been substantially weaken by the interaction with the substrate –, and a transition state where one H atom is attached to a surface O atom and the other H atom sits on-top of a Ce4+ ion. In addition, we have explored how several factors, including H coverage, the location of Ce3+ ions as well as the U value, may affect the chemisorption energy and the relative stability of isolated OH groups versus pair and trimer structures. The trimer stability at low H coverages and the larger upward relaxation of the surface O atoms within the OH groups are consistent with the assignment of the frequent experimental observation by non-contact atomic force and scanning tunneling microscopies of bright protrusions on three neighboring surface O atoms to a triple OH group. The diffusion path of isolated H atoms on the surface goes through the adsorption on-top of an oxygen in the third atomic layer with a large energy barrier of ∼1.8 eV. Overall, the large energy barriers for both, molecular dissociation and atomic diffusion, are consistent with the high activity and selectivity found recently in the partial hydrogenation of acetylene catalyzed by ceria at high H2/C2H2 ratios.

Removal of aluminium from aqueous solutions using PAN-based adsorbents: characterisation, kinetics, equilibrium and thermodynamic studies

Abstract: Economic adsorbents in bead form were fabricated and utilised for the adsorption of Al3+ from aqueous solutions. Polyacrylonitrile (PAN) beads, PAN powder and the thermally treated PAN beads (250 °C/48 h/Ar and 600 °C/48 h/Ar-H2) were characterised using different techniques including Fourier transform infrared spectroscopy, X-ray diffraction, specific surface analysis (Brunauer–Emmett–Teller), thermogravimetric analysis as well as scanning electron microscopy. Effects of pH, contact time, kinetics and adsorption isotherms at different temperatures were investigated in batch mode experiments. Aluminium kinetic data best fit the Lagergren pseudo-second-order adsorption model indicating a one-step, surface-only, adsorption process with chemisorption being the rate limiting step. Equilibrium adsorption data followed a Langmuir adsorption model with fairly low monolayer adsorption capacities suitable for freshwater clean-up only. Various constants including thermodynamic constants were evaluated from the experimental results obtained at 20, 40 and 60 °C. Positive values of ΔH° indicated that the adsorption of Al3+ onto all three adsorbents was endothermic with less energy input required for PAN powder compared to PAN beads and low-temperature thermally treated PAN. Negative ΔG° values indicated that the aluminium adsorption process was spontaneous for all adsorbents examined.

Departures from the Adsorption Energy Scaling Relations for Metal Carbide Catalysts

Abstract: The activity of heterogeneous catalysts is often limited by a strong correlation between the chemisorption energies of reaction intermediates described by the “scaling relations” among the transition metals. We present electronic structure calculations that suggest that metal carbides do not in general follow the transition-metal scaling relations and tend to exhibit a carbophobic departure relative to the transition metals, meaning they tend to bind carbon-bound species weakly compared to oxygen-bound species. This contrasts with the oxophobic departure exhibited by Pt and Pd. Relative to the parent metals, carbides tend to bind carbon and oxygen more weakly and hydrogen more strongly. The departures are rationalized with the adsorbate–surface valence configuration and the energy of the metal sp-states. We employ these general trends to aid in the understanding of various catalytic properties such as the high activity of iron carbides for Fischer–Tropsch synthesis and Pt-group catalysts for partial oxida...

Adsorption of Cr (VI) on synthetic hematite (α-Fe2O3) nanoparticles of different morphologies

Abstract: The adsorption of Cr (VI) from aqueous solution onto nanoparticles hematite (α-Fe2O3) of different morphologies synthesized by acid hydrolysis, transformation of ferrihydrite, sol gel methods has been investigated. The hematite particle sizes were in the range 15.69-85.84 nm and exhibiting different morphologies such as hexagonal, plate-like, nano-cubes, sub-rounded and spherical. The maximum adsorption capacity of Cr (VI) was found to be in the range 6.33–200 mgg−1 for all hematite samples. The kinetics of sorption was rapid, reaching equilibrium at 45–240 minutes. Sorption kinetics and equilibria followed pseudo-second order and Langmuir adsorption isotherm models. The rate constants were in the range 0.996–2.37×10−2 g/mg/min for all samples. The maximum adsorption was attained at pH 3.0, while adsorption decreased as the pH increased from pH 3.0 to 10.0. The study revealed that the hematite with plate-like morphology has the highest adsorption capacity. The sorption process has been found to be feasible following a chemisorption process, and adsorption of Cr (VI) onto hematite nanoparticles was by inner sphere surface complexation due to low desorption efficiency in the range 9.54–53.4%. However, the result of ionic strength revealed that the reaction was by outer sphere complexation. This study showed that morphologies play a vital role in the adsorption capacities of samples of hematite in the removal of Cr (VI) from aqueous solution.

Synthesis and Characterisation of a Highly Active Cu/ZnO:Al Catalyst

Abstract: We report the application of an optimised synthesis protocol of a Cu/ZnO:Al catalyst. The different stages of synthesis are all well-characterised by using various methods with regard to the (micro-)structural, textural, solid-state kinetic, defect and surface properties. The low amount of the Al promoter (3 %) influences but does not generally change the phase evolution known for binary Cu/ZnO catalysts. Its main function seems to be the introduction of defect sites in ZnO by doping. These sites as well as the large Cu surface area are responsible for the large N2O chemisorption capacity. Under reducing conditions, the Al promoter, just as Zn, is found enriched at the surface suggesting an active role in the strong metal–support interaction between Cu and ZnO:Al. The different stages of the synthesis are comprehensively analysed and found to be highly reproducible in the 100 g scale. The resulting catalyst is characterised by a uniform elemental distribution, small Cu particles (8 nm), a porous texture (pore size of approximately 25 nm), high specific surface area (approximately 120 m2 g−1), a high amount of defects in the Cu phase and synergetic Cu–ZnO interaction. A high and stable performance was found in methanol synthesis. We wish to establish this complex but well-studied material as a benchmark system for Cu-based catalysts.

Highly Dispersed Ru on Electride [Ca24Al28O64]4+(e–)4 as a Catalyst for Ammonia Synthesis

Abstract: A high surface area 12CaO·7Al2O3 electride (HT-C12A7:e–) was synthesized by the hydrothermal process and subsequent reduction with CaH2 as a reducing agent, and was then examined as a support for a Ru catalyst in ammonia synthesis. Electrons are successfully incorporated into the cages of C12A7 powder by reduction with CaH2 at 973 K. The concentration of encaged electrons increases with the dehydration temperature of the sample before the CaH2 treatment. The surface area of the resulting material (ca. 20 m2 g–1) is higher than that of the C12A7 electride prepared by conventional solid-phase reaction (SP-C12A7:e–, ca. 1 m2 g–1). The rate of ammonia synthesis over Ru/HT-C12A7:e– is twice that of Ru/SP-C12A7:e–, and there is no significant difference in the apparent activation energy between both catalysts under the optimal conditions. Transmission electron microscopy (TEM) and CO chemisorption measurements revealed that Ru nanoparticles are more highly dispersed on HT-C12A7:e– than that on SP-C12A7:e–, whic...

Influence of Cu doping on the structural, photoluminescence and formaldehyde sensing properties of SnO2 nanoparticles

Abstract: In this paper we report Cu doping induced modifications in the structural, photoluminescence and gas sensing behaviour of SnO2 nanoparticles. Our results show that crystallinity is reduced upon Cu doping. The PL emissions observed in the visible region are attributed to the defect levels arising due to oxygen vacancies. The 1.5 at% Cu-doped SnO2 shows the selective high response (∼80%) to 50 ppm concentration of formaldehyde over methanol, ethanol, propanol-2-ol, acetone and n-butylacetate at 200 °C. The sensing mechanism has been explained based on chemisorption of oxygen on the SnO2 surface and the subsequent reaction between the adsorbed oxygen species and the formaldehyde molecules.

Effect of Copper Oxide Concentration on the Formation and Persistency of Environmentally Persistent Free Radicals (EPFRs) in Particulates

Abstract: Environmentally persistent free radicals (EPFRs) are formed by the chemisorption of substituted aromatics on metal oxide surfaces in both combustion sources and superfund sites. The current study reports the dependency of EPFR yields and their persistency on metal loading in particles (0.25, 0.5, 0.75, 1, 2, and 5% CuO/silica). The EPFRs were generated through exposure of particles to three adsorbate vapors at 230 °C: phenol, 2-monochlorophenol (2-MCP), and dichlorobenzene (DCBz). Adsorption resulted in the formation of surface-bound phenoxyl- and semiquinoine-type radicals with characteristic EPR spectra displaying a g value ranging from ∼2.0037 to 2.006. The highest EPFR yield was observed for CuO concentrations between 1 and 3% in relation to MCP and phenol adsorption. However, radical density, which is expressed as the number of radicals per copper atom, was highest at 0.75–1% CuO loading. For 1,2-dichlorobenzene adsorption, radical concentration increased linearly with decreasing copper content. At t...

Efficient CO2 capture and photoreduction by amine-functionalized TiO2.

Abstract: Amine-functionalization of TiO2 nanoparticles, through a solvothermal approach, substantially increases the affinity of CO2 on TiO2 surfaces through chemisorption. This chemisorption allows for more effective activation of CO2 and charge transfer from excited TiO2 , and significantly enhances the photocatalytic rate of CO2 reduction into methane and CO.

Polyacrylonitrile-chalcogel hybrid sorbents for radioiodine capture.

Abstract: Powders of a Sn2S3 chalcogen-based aerogel (chalcogel) were combined with powdered polyacrylonitrile (PAN) in different mass ratios (SnS33, SnS50, and SnS70; # = mass% of chalcogel), dissolved in dimethyl sulfoxide, and added dropwise to deionized water to form pellets of a porous PAN-chalcogel hybrid material. These pellets, along with pure powdered (SnSp) and granular (SnSg) forms of the chalcogel, were then used to capture iodine gas under both dynamic (dilute) and static (concentrated) conditions. Both SnSp and SnSg chalcogels showed very high iodine loadings at 67.2 and 68.3 mass%, respectively. The SnS50 hybrid sorbent demonstrated a high, although slightly reduced, maximum iodine loading (53.5 mass%) with greatly improved mechanical rigidity. In all cases, X-ray diffraction results showed the formation of crystalline SnI4 and SnI4(S8)2, revealing that the iodine binding in these materials is mainly due to a chemisorption process, although a small amount of physisorption was observed.

Adsorption, chemisorption, and catalysis

Abstract: A short history of the relationships among adsorption, chemisorption, and catalysis with solid catalysts is reviewed. A special focus is on the development of quality and descriptions accuracy using computers, both for the modeling of elementary physical phenomena and adsorption, as well as for the solution of more complex problems like quantum chemical approach to chemisorption, kinetics over solid catalysts, and reactor systems. Modern approaches to the characterization of solid catalysts from the adsorption-desorption data based mainly on n-layer adsorption and non-linear three parameter BET isotherm regarding the volume of micropores as one of the parameters are demonstrated. Instrumentation techniques like infrared spectroscopy or NMR techniques for the analysis of the strength of component chemisorption are mentioned. As for the kinetics, a vague capability of the Langmuir-Hinshelwood-Hougen-Watson models to describe a reaction system in more complicated cases, e.g. bimolecular surface reactions, is discussed. In this context, the simplest model with a minimum number of parameters is advised. To estimate the most realistic values, intrinsic reaction kinetic and mass transport phenomena are taken into account. Usefulness of quantum mechanistic models for better understanding of the catalytic phenomena and more efficient design of catalysts are outlined.

Tunable electronic and magnetic properties of graphene-like ZnO monolayer upon doping and CO adsorption: a first-principles study

Abstract: Graphene-like zinc oxide monolayer (g-ZnO) is a new class of two-dimensional nanomaterials with unique new properties that is still largely unknown. This work studies the tunability of the electronic and magnetic properties of g-ZnO upon chemical doping (with B, N and C) and CO adsorption by using first-principles calculations. Both electronic and magnetic properties of g-ZnO exhibit strong dependency on its structural change and molecular adsorption. The g-ZnO with oxygen atom substituted by a C or N atom (one atom per supercell) are ferromagnetic (FM) half metal (HM), while that substituted by a B atom is an FM semiconductor. The doped g-ZnO shows strong chemisorption to CO molecule by forming A–CO bond (A = B, N or C), in contrast to the weak physisorption of the intrinsic g-ZnO. Furthermore, CO adsorption converts the N- and C-doped g-ZnO to n-type semiconductor with nonmagnetic (NM) ground states, while B-doped g-ZnO becomes a ferromagnetic half metal (FM-HM). The mechanism for property change has been investigated by analyzing their partial density of states (PDOS) upon different conditions. This study provides insights in the physical properties and chemical reactivity of g-ZnO, which could help in realizing their diverse potentials in electronic and magnetic devices.