Dissociative Chemisorption Of CO Research Articles

Inducing synergy in bimetallic RhNi catalysts for CO2 methanation by galvanic replacement

CO2 utilisation as a chemical feedstock could transform fuels production and help mitigate climate change. Direct CO2 reduction for energy production requires the development of active, stable, and low-cost catalysts selective for methane. A bimetallic Ni@Rh core–shell catalyst prepared by galvanic replacement (GR) exhibits a 3.5-fold rate enhancement for CO2 methanation relative to an analogue prepared by chemical reduction (CR) and is twice as active as monometallic Rh/Al2O3. Superior performance of RhNi/Al2O3 (GR) is attributed to Rh dispersion as an atomically thin RhOx shell encapsulating Ni nanoparticles, stabilised by a strong Rh-Ni interaction. Operando IR spectroscopy identifies reactively-formed CO from the dissociative chemisorption of CO2 over Rh as the key intermediate for methane production. Surface formate from the dissociative chemisorption of CO2 and subsequent hydrogenation (via spillover from Rh sites) over alumina is a catalytic spectator. This mechanistic insight paves the way to high activity nanostructured catalysts for CO2 methanation.

Multifunctional, defect-engineered metal-organic frameworks with ruthenium centers: sorption and catalytic properties.

A mixed-linker solid-solution approach was employed to modify the metal sites and introduce structural defects into the mixed-valence Ru(II/III) structural analogue of the well-known MOF family [M3(II,II)(btc)2] (M=Cu, Mo, Cr, Ni, Zn; btc=benzene-1,3,5-tricarboxylate), with partly missing carboxylate ligators at the Ru2 paddle-wheels. Incorporation of pyridine-3,5-dicarboxylate (pydc), which is the same size as btc but carries lower charge, as a second, defective linker has led to the mixed-linker isoreticular derivatives of Ru-MOF, which display characteristics unlike those of the defect-free framework. Along with the creation of additional coordinatively unsaturated sites, the incorporation of pydc induces the partial reduction of ruthenium. Accordingly, the modified Ru sites are responsible for the activity of the "defective" variants in the dissociative chemisorption of CO2, the enhanced performance in CO sorption, the formation of hydride species, and the catalytic hydrogenation of olefins.

Hydroxyapatite as a novel support for gold and ruthenium catalysts: Behaviour in the water gas shift reaction

Novel catalysts comprising gold or ruthenium supported on hydroxyapatite are prepared by a deposition–precipitation method and examined for their behaviour in the water gas shift reaction at various reaction temperatures and at atmospheric pressure. The physicochemical properties of the catalysts were characterized using BET-surface area (BET-SA), XRD, TPR, TPD of NH 3 and CO 2. High WGS activities were observed over Au/HAP compared with Ru/HAP catalyst. The Ru/HAP catalyst showed methanation activity at temperatures above 300 °C, caused by a tendency for the dissociative chemisorption of CO to occur, though this process can be suppressed to some extent by an increase in the partial pressure of water under reaction conditions.

A Comparative Study of Carbon Gasification with O2 and CO2 by Density Functional Theory Calculations

A comparative study of carbon gasification with O-2 and CO2 was conducted by using density functional theory calculations. It was found that the activation energy and the number of active sites in carbon gasification reactions are significantly affected by both the capacity and manner of gas chemisorption. O-2 has a strong adsorption capacity and the dissociative chemisorption of O-2 is thermodynamically favorable on either bare carbon surface or even isolated edge sites. As a result, a large number of semiquinone and o-quinone oxygen can be formed indicating a significant increase in the number of active sites. Moreover, the weaker o-quinone C-C bonds can also drive the reaction forward at (ca. 30%) lower activation energy. Epoxy oxygen forms under relatively high O-2 pressure, and it can only increase the number of active sites, not further reduce the activation energy. CO2 has a lower adsorption capacity. Dissociative chemisorption of CO2 can only occur on two consecutive edge sites and o-quinone oxygen formed from CO2 chemisorption is negligible, let alone epoxy oxygen. Therefore, CO2-carbon reaction needs (ca 30%) higher activation energy. Furthermore, the effective active sites are also reduced by the manner Of CO2 chemisorption. A combination of the higher activation energy and the fewer active sites leads to the much lower reaction rate Of CO2-carbon.

Surface reactions on inverse model catalysts: CO adsorption and CO hydrogenation on vanadia- and ceria-modified surfaces of rhodium and palladium

Reducible transition metal oxides are well-known promoters of the hydrogenation of CO on noble metal surfaces. In this study the promotional effect of vanadia and ceria adlayers on Rh and Pd surfaces was investigated with emphasis on the effect of the oxidation state on CO adsorption and catalytic activity. “Inverse supported catalysts” were prepared by UHV deposition of V and Ce on the noble metal surface (Rh(111), Pd(111) or Rh foil). After oxidation and specified reduction, the reaction kinetics on polycrystalline Rh was measured at atmospheric pressure, and the molecular and dissociative chemisorption of CO on Rh(111) and Pd(111) and the methanation kinetics on Rh(111) were investigated by molecular beam techniques. On Rh(111), the probability of CO dissociation and the reaction rate are enhanced by submonolayer VOx deposits. Local pressures between 10-2 and 1 mbar are sufficient to drive the methanation at 573 K with measurable amounts of products, accompanied by significant restructuring of the catalyst surface. Although the reaction on Rh is generally promoted by small quantities of vanadia and ceria, the reaction rates depend strongly on the extent and temperature of hydrogen reduction. The observed increase of the reaction rate by reduction up to 673 K can be correlated to concomitant changes of the structure and composition of the VOx deposits. If the reduction temperature is raised above 673 K, metallic V is partially dissolved in the bulk, and the resulting V/Rh subsurface alloy exhibits a particularly high activity. Contrary to vanadia, ceria islands on Rh promote the initial reaction only after a low-temperature reduction, but the activity decreases after reduction above 573 K.

Photochemical deposition of thin films from the metal hexacarbonyls

Metal films grown by photolysis of Cr, Mo, and W hexacarbonyls are known to contain large amounts of carbon and oxygen, suggesting incomplete removal of CO from the precursor as well as possible reactions with other sources of carbon and oxygen. In order to identify microscopic processes responsible for film composition, a systematic study of thin films photochemically deposited by continuous, low-power 257-nm light from Cr, Mo, and W hexacarbonyls has been carried out. Since photodissociation of the precursor can occur on the surface as well as in the gas phase, experimental conditions have been chosen such that surface reaction kinetics are rate limiting. The experiments show that background gases during deposition and exposure of newly deposited films to air both result in significant oxidation of the films through their entire thickness, as determined by scanning Auger microscopy and sputter depth profiling. The results are discussed in terms of studies of photolysis of the metal hexacarbonyls and dissociative chemisorption of CO on clean crystalline metal surfaces.

ChemInform Abstract: Selective Metal Oxidation by Dissociative Chemisorption of CO: Reactivity of Nd Overlayers and Nd/Cu Alloy Films on Cu(100).

AbstractThe initial dissociative chemisorption of CO on Nd and Nd/Cu films is accompanied by substantial structural rearrangements in the surface region and the formation of a disordered oxide which is oxygen deficient relative to Nd2O3.

Model studies of the SMSI phenomenon: I. Co and hydrogen chemistry at the RuTi interface

Ti films grow on Ru(0001) in a layer-by-layer mode, a (√91 × √91)R5.2 ° LEED pattern appearing at loadings close to monolayer completion. Such films are extremely active towards H 2 and CO chemisorption, although in both cases no aggregation of the Ti deposit occurs as a result. The uptake of β-CO (characteristic of clean Ru) is strongly suppressed by Ti dosing: this effect is markedly nonlinear and the results suggest that islands of Ti exert a significant long-range influence on the chemisorption of CO by Ru. Isotope data show that the presence of Ti leads to the formation of dissociatively adsorbed CO, while hydrogen chemisorption results in a surface hydride species of limiting stoichiometry TiH 3. This latter species decomposes at ~600 K with concomitant formation of an RuTi surface alloy; the TiH 3 surface hydride is itself very active in the dissociative chemisorption of CO. The relevance of these findings to aspects of the SMSI phenomenon and to K-promoted Ru is discussed.

Lack of translational energy activation of the dissociative chemisorption of CO on Ni(111)

The activation of the dissociative chemisorption of CO on Ni(111) by translational and vibrational energy is probed. Molecular beam techniques produce CO molecules with high kinetic energies and with some vibrational excitation. Thermal desorption and high resolution electron energy loss spectroscopy detect the product of the chemisorption event. The maximum translational and vibrational energies attainable in these experiments, 45 and 18 kcal/mol, respectively, are observed not to activate the dissociative chemisorption of CO. These experiments are sensitive to dissociation probabilities as small as 2×10−6 and 9×10−4 at the maximum values of translational and vibrational energy, respectively. It is concluded that translational energies greater than 45 kcal/mol do not contribute to the CO dissociation rate at high pressures. Rather, the potential energy surface of the CO–Ni(111) interaction likely requires vibrational excitation greater than the amount that can be achieved in this experiment for activation of the C 3/4 O bond.

CO hydrogenation on [formula omitted]: Selectivity under transient conditions

Transient response and isotopic tracer techniques were used to investigate the surface coverages of the reactants and the selectivity of ruthenium/alumina catalysts in CO hydrogenation. Two forms of carbon—active carbidic carbon, C α, and hydrogen-containing alkylic carbon, C β—are found to be present on the catalyst surface. Approximately 0.1 monolayer of C α is deposited on the catalyst through the dissociative chemisorption of CO. C β is formed only in the presence of hydrogen. Unlike C α, the amount of C β on the catalyst surface is found to increase continuously. Under varying partial pressures of CO and H 2 the surface coverage of C α changes much less than the overall rate of reaction. In addition to C α and C β, the catalyst surface also contained approximately 0.7 monolayer of CO and 0.1 to 0.2 monolayer of inactive carbon. Measurements of product distributions of C α and C β during the hydrogen titration transients show that for C α the Schultz-Flory distribution is obeyed. In contrast, C β products do not follow the same distribution. The relative selectivities of the two species of carbon during hydrogen titration transients were very different. C β titration produced normal and branched alkanes in comparable amounts. No alkene is produced from C β. C α titration produced primarily normal alkenes and alkanes. Our results also show that the chain propagation step is much faster than the other steps in hydrocarbon synthesis (Fischer-Tropsch synthesis) and that CO dissociation is not the rate-limiting step.

Highly dispersed rhodium on alumina catalysts: influence of the atmosphere on the state and dispersion of rhodium

The state and dispersion of a highly dispersed Rh/Al/sub 2/O/sub 3/ catalyst in various reactive atmospheres were investigated by low-temperature infrared spectroscopy of chemisorbed CO. Rh metal particles disintegrate in the presence of CO, and Rh/sup +/ species form as a result of oxidation by surface -OH groups at temperatures less than or equal to 300 K. The predominant oxidation process at higher temperatures is presumably related to the dissociative chemisorption of CO. The rates of these processes are strongly dependent on the initial reduction conditions, being less for the more severely reduced samples containing the larger Rh metal particles. A cyclic process of CO adsorption and H/sub 2/ reduction leads to particle growth and increasing resistance of the larger particles to disintegration and oxidation in CO. Simultaneous exposure of the samples to CO and H/sub 2/ does not prevent the oxidative conversion of some of the Rh/sup 0/ into Rh/sup +/ at temperatures greater than or equal to 473 K, but complete conversion of the Rh/sup 0/ is hindered because the CO ligands are removed by reaction with the H/sub 2/. Water vapor in the presence of CO assists the formation of Rh/sup +/ species.

Methanation on Rh surfaces at low pressure and low temperature: A pulsed-laser imaging atom-probe study

We have successfully detected methanation on Rh surfaces at low pressure (∼10 −8 Torr) and low temperature (150 K) using pulsed-laser imaging atom-probe. Thus it is possible to observe directly the reaction intermediates in a time-of-flight mass spectrum and study the atomic steps of such reaction. We find direct evidence of dissociative chemisorption of CO on Rh surfaces and show that under the conditions of our experiment, a dissociative mechanism is responsible for methanation on Rh surfaces. We have also observed an oscillation in signal intensity of CO + and that of methanation products with a time period of ∼ 8 s under the conditions of that experiment.

Dynamics of the dissociative adsorption of CO 2 on Ni(100)

The dynamics of the dissociative chemisorption of CO 2 on clean Ni(100) has been investigated using supersonic molecular beam and ultrahigh-vacuum techniques. The dissociative sticking probability, s 0, to form adsorbed CO and O, was found to be enhanced by three orders of magnitude by the addition of translational and vibrational energy to the incident CO 2 molecules. S 0 increased from 4 × 10 −4 to 0.15 as the component of translational energy normal to the surface was increased from 8 to 100 kJ/mol. Translational activation was also seen at a nozzle temperature of 1000 K, but s 0 was greater by a factor of two to ten at each translational energy, indicating the participation of vibrational excitation in activation of the molecule. The magnitude of the enhancement indicates that population of excited bending mode levels significantly increase the dissociative adsorption probability.

The interfacial kinetics of the reaction of CO2 with liquid nickel

Measurements of the rate of dissociation of CO2 on liquid nickel have been made by the14CO2-CO isotope exchange technique between 1490 and 1670 °C at CO2/CO ratios between 0.01 and 7. Apparent first order rate constants are given by the expression:ka = (1 + 2pCO2/pCO)−1exp(−12700/T - 0.65) mol cm−2 s−1 atm−1. It is shown that the results are consistent with blockage of the surface by oxygen which exhibits ideal Langmuirian adsorption over the conditions of the experiments. The adsorption coefficient of oxygen with respect to the infinitely dilute solution with 1 wt pct as the standard state is deduced to be given by the equation: logK′o = 11880/T - 4.6. It is deduced that the interfacial rate of oxidation of nickel by CO2 is given by the rate of dissociative chemisorption of CO2. Measurements of the rate of decarburization of liquid nickel are reexamined in the light of the present results.

The interfacial kinetics of the reaction of CO2 with nickel: Part II. the decarburization of liquid nickel

The kinetics of decarburization of liquid nickel in CO2-CO mixtures have been studied at 1400 and 1500°C, using the experimental arrangement of the impinging jet. At carbon concentrations above about 1 wt pct, pressures of CO2 ⪯ 0.1 atm, and for total gas flow-rates above about 40 l/min (STP) impinging on a metal surface of 2.08 cm2, it is concluded that the interfacial reaction step controls the rate. Comparison with isotope exchange studies indicates that dissociative chemisorption of CO2 is the rate determining step. Rate constants, based on the nominal surface area, are 1.2 ×10−3 and 1.4 × 10−3 mol/cm2 · s · atm at 1400 and 1500°C, respectively.

A NGR STUDY OF IRON CATALYSTS IN CO HYDROCONDENSATION REACTIONS

Fe/Al2O3, either alone or with K, Mo or Mn additive were studied by absorption NGR spectroscopy in situ. The reduction of the bimetallic systems did not induce alloying. Desorption of hydrogen resulted in oxidizing Feo into Fe2+, as an effect of a redox equilibrium involving properties of alumina. The decrease or growth of iron metal, carbides and ionic species during Fischer-Tropsch synthesis are consistent with a mechanism involving a dissociative chemisorption of CO. It is thought that bulk carbide is not an intermediate in the reaction.

Temperature programmed desorption of carbon monoxide adsorbed on supported platinum catalysts

Temperature programmed desorption (TPD) measurements have been made for CO from Pt/aerosil ( d Pt ≈ 2.5 nm), Pt γ-Al 2O 3 ( d Pt, 2.4 nm) and Pt-Au/aerosil ( d Pt, 1.5–2.0 nm) catalysts. CO was adsorbed at room temperature, with TPD at 35 K min −1. On Pt/aerosil and Pt/γ-Al 2O 3 the TPD profile showed a weak maximum at 388–398 K, with the rest of the profile continuining with very substantial strength out to about 800 K. Increasing the gold content caused an enhancement of the peak at around 390 K relative to the rest of the profile. Adsorbed CO was recovered quantitatively by 860–870 K, while on Pt/aerosil a second adsorption/TPD sequence without intervening catalyst clean-up gave an identical profile, confirming negligible dissociative chemisorption of CO on Pt under these conditions. A small amount of CO 2 was desorbed along with the CO (CO 2 ≈ 6–7% of CO on Pt/ and Pt-Au/aerosil; CO 2 ≈ 12% of CO on Pt γ-Al 2O 3 ). Evidence is given for believing that the CO 2 originated from a catalysed reaction between CO and traces of residual H 2O (the latter from the support) rather than from CO disproportionation. Monolayer uptake measurements with CO and H 2 gave values for the ratio CO( ads)/H( ads) : Pt/aerosil, 0.75; Pt γ-Al 2O 3 , 0.84; Pt76-Au24/aerosil, 0.98; Pt15-Au85/ aerosil, 1.3.1. Reasons for the effect of Au on CO adsorption on Pt-Au are discussed.

Tin oxide surfaces. Part 8.—Infrared study of the mechanism of formation of a surface isocyanate species on SnO2· 0·55 CuO during catalysis of the oxidation of carbon monoxide by nitric oxide

An experimental study has been carried out on the mechanism of formation of a surface isocyanate species on the mixed oxide catalyst SnO2· 0·55 CuO during the initial stages of catalysis of the CO + NO reaction at ≈470 K. Using infrared spectroscopy, the isotopic shifts of the 2189 cm–1 pseudo-antisymmetric stretching vibration have been measured for 13C, 15N and 18O substitution. The oxygen atom of the surface isocyanate has been shown to originate from NO rather than CO as was previously assumed. This observation has been interpreted in terms of a mechanism involving initial dissociative chemisorption of CO followed by the formation of a fulminate via reaction of NO with the surface carbon atom and subsequent rapid isomerisation to the isocyanate: [graphic omitted].

The thorium (100) crystal face: A study of its cleaning, surface structure, and interaction with oxygen and carbon monoxide

The cleaned Th(100) crystal face was studied by low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The various thorium Auger transitions have been identified and assigned. Chemisorption of oxygen and CO was monitored by LEED, AES, and work function change (WFC) measurements. The work function of thorium was found to decrease with the chemisorption of oxygen and CO. Continued oxygen adsorption leads to its dissolution in the bulk and subsequent oxide formation at the surface. The oxide may be ordered by heating up to 1000 K while heating to higher temperatures leads to dissolution of the oxide by oxygen diffusion into the bulk. The data suggest dissociative chemisorption of CO on the studied (100) surface.

Étude des interactions chimiques gaz-solide par la technique du faisceau moléculaire

Abstract We use the reflection of a noble gas (helium) molecular beam to study the superficial reactions of oxygen and carbon monoxide with polycrystalline tantalum. The fraction of the incident beam which is specularly reflected gives direct information on the formation of an oxygen chemisorbed layer and we observed only a single apparent binding state for oxygen chemisorbed on tantalum. The initial value of the sticking coefficient of oxygen on clean polycrystalline tantalum is 0.86, decreasing rapidly during the formation of a chemisorbed layer. The specularly reflected fraction of the incident beam is also modified by the chemisorption of carbon monoxide and this modification would confirm the dissociative character of the CO chemisorption on tantalum. The influence of the partial pressure of CO on the temperature at which the surface is completely covered by the products of the dissociative chemisorption of CO shows that the coverage becomes equal to unity at temperatures at which the solubility limit of CO is attained and tantalum carbide is formed.