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 VO _x 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 VO _x 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.     

Metal–support interactions on rhodium model catalysts

Metal–support interactions on supported rhodium catalysts were studied by using specially prepared Rh/TiO₂/Mo model systems. For their characterization and the analysis of modifications due to various heat treatments several surface analytical methods were applied: low-energy ion scattering, X-ray photoemission spectroscopy and thermal desorption spectroscopy. Heating in ultrahigh vacuum to 670 K leads to Rh agglomeration followed (above 720 K) by encapsulation including the formation of reduced titanium oxide species. These morphological and chemisorption changes are reversible upon reoxidation and low-temperature reduction and thus exhibit the characteristic features of strong metal–support interactions. For the effective mechanism a reaction is suggested that involves oxygen chemisorption on the Rh clusters and partial reduction of the surrounding support oxide.     

Structure sensitivity for NO dissociation on palladium and rhodium surfaces

We present a comparative density-functional theory study of the chemisorption and the dissociation of the NO molecule on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium. The energetic and kinetic properties of the reaction pathways are reported. The structure sensitivity is correlated to the catalytic activity, which can be linked to the calculated dissociation rate constants at 300 K: Rh (100)⩾terrace Rh (511)>step Rh (511)>step Pd (511)>Rh (111)>Pd (100)⩾terrace Pd (511)>Pd (111). The effect of the steps on the activity is found to be clearly favorable for the Pd (511) surface and unfavorable for the Rh (511) surface. The reaction barriers are correlated to the stability of the final states and the geometries of the molecular precursor states.     

Electrochemical promotion of Rh catalyst in gas-phase reduction of NO by propylene

The concept of non-faradaic electrochemical modification of catalytic activity (NEMCA) has been applied for the in situ control of catalytic activity of a rhodium film deposited on YSZ (yttria stabilized zirconia) solid electrolyte towards reduction of 1000 ppm NO by 1000 ppm C₃H₆ in presence of excess (5000 ppm) O₂ at 300 °C. A temporary heating at this feed composition results in a long-lasting deactivation of the catalyst under open circuit conditions due to partial oxidation of the rhodium surface. Positive current application (5 A) over both the active and the deactivated catalysts gives rise to an enhancement of N₂ and CO₂ production, the latter exceeding several hundred times the faradaic rate. While active rhodium exhibits a reversible behaviour, electrochemical promotion on the deactivated catalyst is composed of a reversible and an irreversible part. The reversible promotion results from the steady-state accumulation of current-generated active species at the gas exposed catalyst surface whereas the irreversible effect is due to the progressive reduction of the catalyst resulting in an increased recovery rate of lost catalytic activity. The results are encouraging with respect to application of rhodium for the catalytic removal of NO from auto-exhaust gases under lean-burn conditions.     

Electrochemical behavior of rhodium(III) in 1-butyl-3-methylimidazolium chloride ionic liquid

Electrochemical behavior of rhodium(III) chloride in 1-butyl-3-methylimidazolium chloride was investigated by various electrochemical transient techniques at glassy carbon working electrode at different temperatures (343-373 K). Cyclic voltammogram of rhodium(III) in bmimCl consisted of a surge in reduction current occurring at a potential of −0.48 V (vs. Pd) is due to the reduction of Rh(III) to metallic rhodium and a very small oxidation wave occurring at −0.1 V. Increase of scan rate increases the peak current and remarkably shifts the cathodic peak potential () in negative direction indicating the irreversibility of electroreduction of rhodium(III). The diffusion coefficient of rhodium(III) in bmimCl (∼10⁻⁹ cm²/s) was determined and the energy of activation (∼25 kJ/mol) was deduced from cyclic voltammograms at various temperatures. The cathodic (τ_r) and anodic (τ_o) transition times were measured from chronopotential transients and the ratio τ_o/τ_r was found to be 1:7. Electrowinning of rhodium from bmimCl medium results in a deposition of metallic rhodium with lower (20-25%) Faradaic efficiency. A separation factor of rhodium from co-existing noble metal fission product palladium in bmimCl was determined during electrodeposition.     

Exafs evidence for direct Rh-Tan+ bonding and coverage of the metal particles in a Rh/Ta2O5 SMSI catalyst

An EXAFS investigation showed that the rhodium particles in a Rh/Ta₂O₅ catalyst were fully reduced and in the normal state after reduction in H₂ at 523 K. After reduction at 858 K, in the SMSI state, tantalum ions could be detected in the reduced supporting oxide directly underneath the rhodium metal particles and in tantalum oxide covering the rhodium metal particles. Neither alloy formation, nor the formation of raftlike structures was observed.     

Deactivation and Regeneration of Spent Three-Way Automotive Exhaust Gas Catalysts (TWC)

The effect of oxidation, oxy-chlorination and reduction treatments at elevated temperatures on the dispersion of palladium (Pd) and rhodium (Rh) for commercially aged three-way automotive exhaust gas catalysts (TWC) has been investigated. The catalytic activity of treated samples was compared with a reference sample, which was taken from the corresponding aged TWC and tested using a mini-cuts reactor simulating real driving conditions. In the case of oxygen, the improvement of the noble metal dispersion on the catalysts was dependent on the noble metal loading and the degree of metal sintering. Adding chlorine to the oxygen atmosphere facilitates the restructuring of the metals with an improved increase in the noble metal dispersion. The temperature and the composition of the gas used during these thermal treatments proved to be of importance not only to increase the metal dispersion, but also to prevent possible losses of noble metals, in the form of volatile MO _x Cl _y compounds. TEM-EDS techniques indicated changes in the size of the largest noble metal agglomerates of up to 100 nm in size after thermal gas treatment. BET porosity and XRD analyses were employed to investigate restructuring of the washcoat and showed a decrease in pore size distribution and an increase in surface area.     

Comparative Study on the Mechanisms of Partial Oxidation of Methane to Syngas Over Rhodium Supported on SiO2 and γ-Al2O3

A comparative study on the mechanisms of the partial oxidation of methane (POM) to syngas over SiO₂- and -Al₂O₃-supported Rh catalysts was carried out using in situ time-resolved FTIR spectroscopy to follow the primary products of POM reaction over the catalysts. Experiments of catalytic performance evaluation and temperature-programmed reduction (TPR) characterization of the catalysts, as well as the in situ FTIR spectroscopic study using CO to probe the oxidation state of Rh species over the catalysts were performed. It was found that the direct oxidation of CH₄ to syngas is the main pathway of the POM reaction over Rh/SiO₂ catalysts, while the combustion--reforming mechanism is the dominant pathway of syngas formation over Rh/-Al₂O₃ catalysts. The results of TPR characterization indicate that Rh supported on -Al₂O₃ is more difficult to reduce than Rh supported on SiO₂. The IR experiments of CO adsorption over Rh/SiO₂ and Rh/-Al₂O₃ after the POM reaction reveal that the surface of the Rh/-Al₂O₃ catalyst contains more partially oxidized rhodium (Rh⁺) species as compared to the Rh/SiO₂ catalyst. These results suggest that the significant difference in the mechanisms of the POM reaction over Rh/SiO₂ and Rh/-Al₂O₃ catalysts can be related to the difference in the surface concentration of O^2- species over the catalysts under the reaction conditions mainly due to the difference in oxygen affinity of the Rh species on the two supports.     

Surface grafting of polymers onto ultrafine silica: cationic polymerization initiated by benzylium perchlorate groups introduced onto ultrafine silica surface

Summary The cationic graft polymerization initiated by benzylium perchlorate groups introduced onto ultrafine silica surface was investigated. The introduction of benzylium perchlorate groups onto the surface was achieved by the reaction of silver perchlorate with surface benzyl chloride groups, which were introduced by the treatment of silica with 4-(chloromethyl)phenyltrimethoxysilane. The cationic graft polymerization of styrene and cationic ring-opening polymerization of -caprolactone were found to be initiated by the surface benzylium perchlorate groups and the corresponding polymers were grafted onto the surface. The percentage of grafting onto silica surface decreased with increasing polymerization temperature, because chain transfer reaction of growing polymer cation is accelerated with increasing polymerization temperature.     

Stripping of Rh–Sn–Cl‐loaded alkylated 8‐hydroxyquiniline with Na2SO3–HCl

Acidic rhodium(III) chloride solutions pretreated with SnCl₂ at a ratio Sn(II) : Rh(III) = 3 are contacted with an alkylated 8‐hydroxyquinoline (Kelex 100) to prepare loaded organics which subsequently are subjected to stripping with Na₂SO₃–HCl solutions. Rh(III) was found to be stripped selectively involving the following reaction: where H₂Q is the protonated form of kelex 100. Direct characterization of the stripped complex and slope analysis confirmed this stripping reaction. Equilibrium was reached within 20 min. A kinetic analysis found the stripping process to be irreversible and to be controlled most probably by the slow release of SnCl₃^? ligand from the intermediate Rh–Q–SO₃–SnCl₃ organic complex. Copyright © 2004 Society of Chemical Industry     

Structure Sensitivity of CO Dissociation on Rh Surfaces

Using periodic self-consistent density functional calculations it is shown that the barrier for CO dissociation is 120 kJ/mol lower on the stepped Rh(211) surface than on the close-packed Rh(111) surface. The stepped surface binds molecular CO and the dissociation products more strongly than the flat surface, but the effect is considerably weaker than the effect of surface structure on the dissociation barrier. Our findings are compared with available experimental data, and the consequences for CO activation in methanation and Fischer–Tropsch reactions are discussed.     

Screening of Soluble Rhodium Nanoparticles as Precursor for Highly Active Hydrogenation Catalysts: The Effect of the Stabilizing Agents

We report the preparation of rhodium nanoparticles (NPs) stabilized by 1-octadecanethiol (ODT), polyvinyl alcohol (PVA), and tetraoctylammonium bromide (TOAB), and their application for hydrogenation catalysis. The three metal–ligand systems correspond to different mechanism of NPs stabilization via strong covalent linkage, chemisorbed atoms and electrostatic interactions, respectively. We found a strong effect of the interaction between the stabilizer and the surface of the metal nanoparticle on the catalytic activity. The Rh NPs were studied as soluble nanoparticle catalysts and as precursors for the synthesis of supported catalysts. All catalysts were tested in the hydrogenation of cyclohexene under similar conditions as a model reaction. Generally, Rh–ODT NPs were inactive, Rh–PVA NPs exhibited distinct activities in solution (aqueous biphasic catalysis) and as a supported catalyst, and Rh–TOAB NPs exhibited similar activities in solution and after immobilization. This last result opens the opportunity for the preparation of highly active Rh NP catalysts both in solution and as a heterogeneous catalyst. Additionally, the stability of the nanoparticles depends on the choice of ligand and on the functionalization of the support surface before immobilization. By optimizing the catalyst synthesis and reaction conditions, turnover frequencies as high as 700,000 h^?1 where observed for stable and recyclable catalyst.     

The non-linear behaviour of the NO-H2 reaction over Rh surfaces

In this paper we describe the non-linearity of the NO-H₂ reaction over Rh surfaces. Rate oscillations have been observed over a stepped (111) surface with (100) steps, (Rh(533) at low pressures (10^?4 Pa) below 500 K, while no oscillations could be observed under these conditions over a Rh(100) surface and a stepped (100) surface with (111) steps, Rh(711). The thermal stability of the N atoms formed during the reaction explains the observed structure sensitivity. Moreover, the results suggest that diffusion of N atoms is needed to synchronise the rate oscillations, a process that is absent on Rh(100) and Rh(711).     

Mechanism of anodic oxidation of chlorate to perchlorate on platinum electrodes

The kinetics and mechanism of the anodic oxidation of chlorate to perchlorate on platinum electrodes have been investigated. The current efficiency for perchlorate formation and the electrode potential have been determined as a function of current density for various solution compositions, flow rates and pHs at 50°C. The results have been compared with theoretical relations between the ratio of the current efficiencies for perchlorate and oxygen formation, the electrode potential, the concentration of chlorate at the electrode surface and the current density for various possible mechanisms. It is concluded that the formation of an adsorbed hydroxyl radical is the first step in the overall electrode reaction. The mechanism proposed for the C10₄ perchlorate and oxygen formation is:

Impedance spectroscopic investigation of a Rh/YSZ catalyst under polarization

Electrochemical impedance spectra at 450–600 °C and kPa of a rhodium catalyst interfaced with yttria-stabilized-zirconia (Rh/YSZ) were compared with a model based on the mechanism of electrochemical promotion. In the proposed equivalent electric circuit, existence of an “effective” double layer at the gas-exposed catalyst surface and its potential-controlled modification via diffusion of oxygen ions between the O²⁻ conducting solid electrolyte support (YSZ) and the catalyst are represented by two additional elements: adsorption capacitance and Warburg impedance. Under positive polarization, the adsorption capacitance increases dramatically indicating reinforcement of the “effective” double layer at the catalyst/gas interface, in agreement with the observation known from electrochemical promotion practice that positive polarization of a rhodium electrode leads to rhodium oxide reduction, hence, to dramatic increase in catalytic reaction rate.     

Surface intermediates in the catalytic reaction of NO+H2 on Rh studied by method of interacting bonds calculations

The semi-empirical method of interacting bonds was used to clarify the mechanism of oscillatory behavior of the catalytic system (NO+H₂)/Rh. Various rhodium planes and surface defect regions were characterized by the strength of the nitrogen bond to the surface, the stability of the adsorbed NH_n species (n=0, 1, 2, 3), and the reactivity of NH_n species towards hydrogen. Calculations admit the earlier suggested reaction mechanism, which attributes the surface wave propagation to the intermediate formation of NH_a species. The activity of the rhodium surface in oscillations is expected to increase in a row of planes: (100)<(111)<(335). The activity of Rh(335) single crystal in the reaction rate oscillations is probably governed by the presence (in contrast to ideal terraces) of gradient and broad range of the reaction intermediate properties. This revised version was published online in July 2006 with corrections to the Cover Date.     

Adsorption and reaction of CO on vanadium oxide–Pd(111) “inverse” model catalysts: an HREELS study

The room temperature adsorption and reaction of CO on Pd(111) surfaces decorated with submonolayer coverages of vanadium oxide – so-called inverse model catalysts – have been studied by high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). The HREELS surface phonon spectra of the V oxide phases have been measured and used to monitor the changes in the oxide as a result of the interaction with CO. The intramolecular C–O stretching frequency of CO adsorbed on the V-oxide/Pd(111) surfaces displays two vibrational loss components as a function of CO coverage as it has been observed on the clean Pd(111) surface. The relative intensities of the two vibrational features as a function of V oxide coverage however suggest that the balance of CO adsorption sites is modified as compared to clean Pd(111) by the presence of the V oxide–Pd phase boundary. Preferential population of high coordination adsorption sites by CO in the vicinity of the oxide–metal interface is proposed. The analysis of the V oxide phonon spectra indicates that adsorbed CO partially reduces the V oxide at the boundaries of the oxide islands to the Pd metal. The reduction of V oxide by CO is dependent on the oxygen content of the V oxide phase. The reduction of V oxide is confirmed by the XPS V 2p core level shifts.     

Simulation of kinetics of nitrogen desorption from Rh(111)

Associative desorption of N atoms from the Rh(111) surface is simulated in the framework of the lattice-gas model. The Arrhenius parameters and nearest-neighbour lateral interaction employed to describe the measured thermal desorption spectra are as follows:v=10¹³ s^–1,E _d=40 kcal/mol, and ₁=1.7 kcal/mol. The results obtained are used to clarify the role of nitrogen desorption in the NO + CO reaction on Rh(111) atT=400–700 K andP _NOP _CO0.01 atm.     

Rh/C catalysts for methanol hydrocarbonylation: I. Catalyst characterisation

Commercially available activated carbon supports, and carbon supported rhodium catalysts were characterised by BET, XPS, TPD, SEM, TEM and chemisorption measurements to elucidate the effect of the pore structure and chemical nature of the carbons on the Rh/C catalysts. During impregnation, both of the parameters had a significant effect on the Rh/C catalysts. First, the meso- and macropores were important for the mass transfer of the metal precursor within the support particle; the larger the pores the better the distribution of rhodium within the support particle. Second, the chemical composition of the carbon surface determined the amount of interaction of the rhodium species and the carbon surface; the pH influenced the attraction of the species, and the oxygen containing surface groups acted as adsorption sites for rhodium. During reduction, the thermal decomposition of the oxygen containing surface groups was essential for dispersion. The thermally stable (CO evolving, weakly acidic, neutral or basic) surface groups remained intact, whereas the thermally unstable (CO₂ evolving, acidic) surface groups decomposed inducing agglomeration of rhodium. Thus, it is not only the amount of oxygen containing adsorption sites that affected dispersion but also their nature and stability. Evidently, the degree of agglomeration depends strongly both on the type of carbon and on the reduction conditions. Accordingly, TEM provides a good measure for the particle size since it also accounts for the hydrogen induced agglomeration, whereas hydrogen chemisorption only affords a less informative average value.     

The stability of a polymer-supported rhodium complex in the batch hydroformylation of 1-hexene. II. Effect of reaction conditions

The stability of a polymer-supported rhodium complex has been studied in a batch process for hydroformylation of 1-hexene, using [Rh(CO)₂Cl]₂ as the catalytic precursor and poly(vinylbenzyl diethylenetriamine) as the polymeric ligand. It has been found that the reaction conditions, temperature, pressure, solvent and reaction time etc., have considerable influence on the degree of rhodium elution. Elution of rhodium is serious under normal conditions but low in certain blank experiments. These results indicate that rhodium elution is probably related to the substitution of a low molecular ligand for the polymeric ligand in the catalytic cycle. Several possible mechanisms of rhodium elution based on these results are proposed.