Semi-Empirical Calculations on the Stability and Reactivity of NH x Species on Metal Surfaces

The semi-empirical method of interacting bonds was used to elucidate the mechanism of oscillation phenomena in the NO + H₂ reaction on metal surfaces. Basic single-crystal planes of Pt, Rh, Ir, Fe, Ru, and Re were examined with respect to the stability of adsorbed NH _n species (n = 0, 1, 2, 3); to the reactivity of NH _n (n = 0, 1, 2) species toward adsorbed hydrogen atoms; and to the possibility of combination reactions between two NH or two NH₂ species resulting in the formation of gaseous N₂ molecules. All studied surfaces were found to form readily stable NH species. The principal difference between Pt, Rh, and Ir single-crystal planes exhibiting reaction rate oscillations, and Fe, Ru, and Re surfaces, which do not show an oscillatory behavior, is that the combination reaction of NH species can easily proceed in the former case, whereas this reaction is not allowed thermodynamically in the latter. This result is consistent with an earlier suggested model that attributes the oscillatory surface wave propagation to the intermediate formation of NH species.Stable NH₂ species can be formed on Ru, Re, and Fe surfaces, whereas the noble metal surfaces of Pt, Rh, and Ir can only form weakly stable NH₂ species at the very edge of their existence region. The combination reaction between two NH₂ species is endothermic in all cases.     

Imaging and probing catalytic surface reactions on the nanoscale: Field Ion Microscopy and atom-probe studies of O2–H2/Rh and NO–H2/Pt

We present dynamic studies of surface reactions using video-Field Ion Microscopy (FIM) along with Pulsed Field Desorption Mass Spectrometry (PFDMS). Catalytic water formation is followed using rhodium and platinum 3D field emitter crystals for the oxidation of hydrogen with either oxygen (Rh) or NO (Pt). Strongly non-linear dynamics are observed with nanoscale spacial resolution. For both reactions quasi-oscillatory behaviour exists under certain conditions of temperatures and partial pressures. An influence of the probing electric field is observed and possibly essential in establishing oscillatory behaviour. Local chemical probing of selected surface areas with up to 400 atomic surface sites proves catalytic water formation to take place. Since water ions (H₂O⁺/H₃O⁺) cause image formation of the O₂–H₂ reaction on Rh, respective videos provide space-time resolved information on the catalytically active sites. Atom-probe data also reveal that the surface of the Rh sample reversibly switches from a metallic to an oxidized state during oscillations. As to the NO–H₂ reaction on Pt, fast ignition phenomena are observed to precede wave fronts. After catalytic water formation, NO molecules diffuse into emptied areas and cause high image brightness. Depending on the size of the Pt crystal, the reaction may ignite in planes or kinked ledges along the <100> zone lines. Thus FIM provides clear experimental evidence that kinks are more reactive than steps in the catalytic NO + H₂ reaction. Pt surface oxidation occurs and has probably been underestimated in previous FIM studies.     

Gas inclusion crystal between 1-D coordination polymer and nitrous oxide: occurrence of crystal phase transition induced by a slight amount of fluid guests inside host

A novel inclusion crystal, {[Rh(II)₂(bza)₄(pyz)]_n · 3n(N₂O)} (1), was prepared and characterized by single-crystal X-ray diffraction analysis; linear alignment (N₂O)_n coherently generates in the channels of host rhodium benzoate pyrazine and there is a high possibility of the existence of an intermediate phase induced by a slight amount of adsorbed N₂O inside the crystal even at room temperature.     

Formation and extraction of Rh–Sn–Cl complexes with an alkylated 8-hydroxyquinoline

The work presented here focuses on the solution pretreatment and extraction stages of a solvent extraction system for rhodium from aqueous chloride solutions. The feed solution pretreatment stage involves a complexation reaction between the aqueous rhodium chloride complexes, [RhCl_6-x(H₂O)_x]^(3-x)− and stannous chloride. Depending on the amount of stannous chloride used, at least two different Rh–Sn complexes are formed, either [Rh(SnCl₃)₅)]⁴⁻ or [RhCl₃(SnCl₃)₃]³⁻. Both of these respond well to extraction with Kelex 100, the extractant investigated in this work. The extraction stage was found to be quantitative for rhodium and it was also found to be very rapid, with contact times of less than 5 min sufficient for rhodium extraction. The extraction mechanism was determined to be ion-pair formation with the protonated Kelex 100 molecules at a stoichiometry such that the overall charge in the organic phase is neutral, i.e. three Kelex 100 molecules for [RhCl₃(SnCl₃)₃]³⁻ and four for [Rh(SnCl₃)₅]⁴⁻. © 1998 SCI     

Preparation of a rhodium catalyst from rhodium trichloride on a flat,conducting alumina support studied with static secondary ion mass spectrometry and monochromatic X-ray photoelectron spectroscopy

A rhodium catalyst has been prepared by electrostatic adsorption of RhCl₃-derived species in aqueous solution on a model support, consisting of a 4–5 nm thick layer of aluminum oxide on an aluminum foil. The conversion of the rhodium precursor species into metallic rhodium has been studied by monochromatic XPS and static SIMS. Freshly prepared catalysts contain adsorbed Rh-complexes with only one chloro ligand; this is explained by a mechanism in which chloro ligands of the initially adsorbed complex, of the form [RhCl _n (OH)_4-n (H₂O)₂]^–, are displaced by surface OH groups. Analysis of molecular secondary cluster ions of the type RhCl^– shows that the Rh-Cl species decompose at reduction temperatures below 200 °C, whereas reduction temperatures well in excess of 200 °C are needed to remove chlorine from the alumina support.     

Rhodium supported in basic zeolite Y: A selective and stable catalyst for CO hydrogenation

Catalysts prepared by a condensation reaction of Rh(CO)₂(acac) within the supercages of zeolite Y made basic by treatment with NaN₃ are active for CO hydrogenation and selective for low-molecular-weight olefins and methanol. High partial pressures of CO (or CO + H₂) stabilize the catalyst. The predominant species in the catalyst are suggested to be rhodium carbonyl clusters trapped in the zeolite cages.     

Adsorbed NHx species on Pt(111) and Pt(100) surfaces studied by the semi‐empirical method of interacting bonds

The properties of the adsorbed NH_x species (x=0,1,2,3) on platinum(111) and (100)‐(1×1) single‐crystal planes are studied by the semi‐empirical method of interacting bonds. Both surfaces reveal similar features. The adsorbed species NH and NH₂ are stable on the surface, and stable NH_3(ads) species cannot form. The NH_2(ads) species is favourable in adsorbed hydrogen excess, but lack of the latter results in NH_ads becoming dominant. Both NH and NH₂ species are expected to diffuse easily over the surface due to the small difference between their bond strengths to various adsorption sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Role of chiral amine cocatalysts in the helix-sense-selective polymerization of a phenylacetylene using a catalytic system

We have examined the role of the chiral amine cocatalyst in the recently discovered helix-sense-selective polymerization of a phenylacetylene using a catalytic system consisting of an achiral rhodium complex ([Rh(norbornadiene)Cl]₂) as the catalyst and a chiral amine as the cocatalyst. Several chiral amines were effective for the polymerization reaction and their effectiveness depended on their bulkiness and coordination ability to rhodium. The sense selectivity of the polymerization increased with the concentration of the chiral amines. To discuss the structure of the true active species in the catalytic system, we have synthesized a new chiral rhodium complex having two chiral amines as ligands ([Rh(norbornadiene)(chiral amine)₂]⁺BF₄⁻). The isolated chiral complex also catalyzed the helix-sense-selective polymerization. These findings suggest that a chiral rhodium complex having two chiral amines may be the true active species when using the catalytic system consisting of [Rh(norbornadiene)Cl]₂ and a chiral amine.     

Organometallic Rhodium Complexes Encapsulated in Mesoporous Molecular Sieves

Rh(III) complexes both dimmer [Cp*RhCl]₂(μ-Cl)₂ and monomer ([RhCp*(S)₃]²⁺) were encapsulated into MCM-41 channels. All silica MCM-41 molecular sieve and aminosililated MCM-41 matrix were used for rhodium complexes accommodation. Reactivity of Cp* rhodium complexes encapsulated in meso structure was estimated on the grounds of their susceptibility to interaction with CO molecules resulting in the formation of carbonyl complexes. Formation of Cp*Rh carbonyls was recorded by means of FTIR spectra. It was found that accommodation of Rh(III) complexes in MCM-41 molecular sieves activated the complex and led to the formation of Rh(III)Cp* carbonyls as a result of contact with CO. Contact of rhodium (III) complexes encapsulated in MCM-41 matrix with CO did not result in rhodium (III) reduction, whereas in the presence of amine groups in aminosililated MCM-41 the reduction of Rh(III) to Rh(I) occurred relatively easily and formation of Cp*Rh(CO)₂ complex containing Rh(I) was noted. Encapsulated rhodium complexes showed some activity in methanol carbonylation reaction carried out under heterogeneous conditions. For the most active catalyst the amount of methyl acetate reached about 8 mol.%, however, deactivation of catalyst occurred and after 2 h on stream methyl acetate was not found in the product.     

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).     

Comparison of separation behavior of Ir(IV) and Rh(III) between tin(II) chloride and ascorbic acid as a reducing agent in the extraction with Cyanex 921 and Cyanex 301

In order to compare the separation of Ir(IV) and Rh(III) between SnCl₂ and ascorbic acid as a reducing agent, solvent extraction with Cyanex 921 and Cyanex 301 was investigated in the HCl concentration range from 1 M to 9 M. Addition of both SnCl₂ and ascorbic acid led to the selective extraction of rhodium by the two extractants, leaving Ir(III) in the raffinate. Since tin was selectively extracted over Rh(I) in the presence of SnCl₂, it is necessary to separate Rh(I) and tin by selective stripping from the organic phase. In the presence of ascorbic acid, the extraction percentage of rhodium by Cyanex 921 was much smaller than that in the presence of SnCl₂. UV spectra was analyzed to verify the reduction reaction of both metal ions. FT-IR was analyzed between fresh and loaded organic solution. The reduction of Ir(IV) and Rh(III) in the presence of ascorbic acid was explained. Selective stripping of Rh(I) over tin from the loaded Cyanex 921 was obtained by the mixture of HCl and (NH₂)₂CS.     

CO dissociation and oxidation on small supported rhodium particles: SSIMS and TPR study

The dissociation and oxidation of carbon monoxide on small rhodium particles prepared by vapour deposition of Rh on either MgO or alumina substrate has been investigated by means of static secondary ion mass spectrometry (SSIMS), and temperature programmed reaction (TPR). The intensity ratios Rh _n C⁺/Rh⁺ _n measured by SSIMS, have been used to monitor the build-up of surface carbon concentration. It was shown that a part of the CO molecularly adsorbed on clean particles undergoes dissociation during heating. The dissociation is more important for smaller particles. This behaviour is explained in terms of increase of CO dissociation probability in the case of CO adsorption near structural irregularities of a surface (edges, corners, steps). During the reaction of CO oxidation the intermediate carbon formation, which is more pronounced for smaller particles, is observed. The temperature dependent carbon concentration exhibits a maximum resulting from a counterbalance between CO dissociation first and carbon reaction with adsorbed oxygen consequently.     

Electrochemically Induced Oscillations of C2H4 Oxidation Over Thin Sputtered Rh Catalyst Films

The electrochemically promoted induction of self-sustained catalytic rate and potential oscillations during C₂H₄ oxidation was studied over sputtered Rh thin (40 nm catalyst films interfaced with ZrO₂ (8 mol% Y₂O₃). The reaction rate oscillates simultaneously with the catalyst potential, and always in the opposite direction. The electrochemically induced oscillatory rate is typically 60 times larger than the open-circuit catalytic rate and 1000 times larger than the electrochemical rate of O²⁻ supply to the catalyst. The phenomenon is completely reversible and only observed under anodic polarization where the rate oscillates between the values corresponding to metallic Rh and surface Rh₂O₃. The oscillations are caused by the electrochemically controlled backspillover of O²⁻ to the catalyst surface and the concomitant, via repulsive lateral interactions, decomposition of surface rhodium oxide followed by surface reoxidation to Rh₂O₃ by gaseous O₂.     

Development of catalysts for hydrogen generation from hydride compounds

Catalytic hydrolysis of NaBH₄ and NH₃BH₃ has been studied. It was shown that the nature of the support and the active component of the catalyst affect the H₂ generation rate. Despite similar sizes of rhodium particles formed on the surface of different supports (γ-Al₂O₃, TiO₂, carbon), their reactivity is different. Rh/TiO₂ with low rhodium concentration (1 wt.%) is the most active catalyst both in the hydrolysis of NaBH₄ and NH₃BH₃. The obtained results show that the rhodium chloride interaction with titania determines the reactivity of rhodium particles formed under action of NaBH₄ medium. TEM, DRS UV–vis and XPS were used to characterize the catalysts.     

Carbonylation of dimethyl ether on solid Rh-promoted Cs-salt of Keggin 12-H3PW12O40: A solid-state NMR study of the reaction mechanism

The carbonylation of dimethyl ether (DME) with carbon monoxide on Rh-promoted cesium salt of 12-tungstophosphoric acid, Rh/Cs₂HPW₁₂O₄₀ (HPA), has been studied with ¹³C solid-state NMR. The bi-functional character of Rh/Cs₂HPW₁₂O₄₀ catalyst in halide-free carbonylation of DME has been directly demonstrated. The activation of the C–O bond of DME proceeds on Brønsted acid sites of HPA with the formation of the methyl group attached to the surface of HPA (methoxy species), whereas the role of rhodium consists in trapping carbon monoxide from gaseous phase and a transfer of CO to the center of DME activation, acidic OH-group of the catalyst, in the form of rhodium carbonyls. The lattice of Cs₂HPW₁₂O₄₀ makes it possible to locate these two different active centers in close proximity to each other, e.g., on two adjacent oxygen atoms, terminal and bridging, of one Keggin anion, thus facilitating the insertion of carbon monoxide from rhodium carbonyl into the C–O bond of methoxy-group to produce the acetate group bound to the Keggin anion. The latter offers finally methyl acetate under the interaction with DME, the intermediate surface methoxy-groups being restored.     

Effect of Rh loading on the performance of Rh/Al2O3 for methane partial oxidation to synthesis gas

Catalytic performance for partial oxidation of methane (POM) to synthesis gas was studied over the Rh/Al₂O₃ catalysts with Rh loadings between 0.1 and 3 wt%. It was found that the ignition temperature of POM reaction increased with the decreasing of the Rh loadings in the catalysts. For the POM reaction over the catalysts with high (≥1 wt%) Rh loadings, steady-state reactivity was observed. For the reaction over the catalysts with low (≤0.25 wt%) Rh loadings, however, oscillations in CH₄ and reaction products (CO, H₂, and CO₂) were observed. Comparative studies using H₂-TPR, O₂-TPD and high temperature in situ Raman spectroscopy techniques were carried out in order to elucidate the relation between the redox property of the Rh species in the Rh/Al₂O₃ with different Rh loadings and the performance of the catalysts for the reaction. Three kinds of oxidized rhodium species, i.e. the rhodium oxide species insignificantly affected by the support (RhO_x), that intimately interacting with the Al₂O₃ surface (RhⁱO_x) and the Rh(AlO₂)_y species formed by diffusion of rhodium oxides in to sublayers of Al₂O₃ [C.P. Hwang, C.T. Yeh, Q.M. Zhu, Catal. Today, 51 (1999) 93.], were identified by H₂-TPR and O₂-TPD experiments. Among them, the first two species can be easily reduced by H₂ at temperature below 350 °C, while the last one can only be reduced by H₂ at temperature above 500 °C. The ignition temperatures of POM reaction over the catalysts are closely related to the temperature at which most of the RhO_x and RhⁱO_x species can be reduced by CH₄ in the reaction mixture. Compared to the Rh/Al₂O₃ with high Rh loadings, the catalysts with low Rh loadings contain more RhⁱO_x species which possess stronger RhO bond strength and are more difficult to be reduced than RhO_x by the reaction mixture. Higher temperature is therefore required to ignite the POM reaction over the catalysts with lower Rh loadings. The oscillation during the POM reaction over the Rh/Al₂O₃ with low Rh loadings can be related to the behaviour of Rh(AlO₂)_y species in the catalyst switching cyclically from the oxidized state to the reduced state during the reaction.     

Hydroformylation of mixed octenes catalyzed by supported rhodium-based catalyst

In order to solve the difficult separation between catalyst and products in homogeneous system, the activated carbon (AC)-supported rhodium-based catalyst (Rh/AC) was prepared. Hydroformylation of mixed octenes catalyzed by Rh/AC was studied, and compared to that catalyzed by RhCl(CO)(TPPTS)₂ [TPPTS: trisodium salt of tris(m-sulphonylphenyl) phosphine], and [Rh(CH₃COO)₂]₂-Ph₃PO (Ph₃PO: triphenyl phosphine oxide). The performance test of the catalysts showed the Rh/AC presented higher catalytic activity, selectivity and air-stability. During the recycle experiments Rh/AC could be used 4 times without significant loss of rhodium. The effects of the supports, rhodium loading and reaction conditions on the catalytic performance of Rh/AC were investigated. The results showed petroleum coke-based activated carbon with higher surface area and more basic groups was advantageous to the formation of aldehydes. The heterogeneous Rh/AC catalyst displayed higher catalytic activity and reusability.     

Long-Chain-Amine Metal Complexes as Hydrogenation Catalysts. Heterogenisation on Activated Carbon

The catalytic activity of [PdCl₂(NH₂(CH₂)₁₂CH₃)₂] (named [Pd(TDA)]) and [RhCl(NH₂(CH₂)₁₂CH₃)₃] (named [Rh(TDA)]) complexes for the hydrogenation of cyclohexene has been analysed both in homogeneous phase and heterogenised on activated carbon. The [Rh(TDA)] complex has been found to be more active than the [Pd(TDA)], both homogeneous and heterogenised. Experimental and modelled results indicate that these complexes follow a similar reaction mechanism, but with different rates. A clear positive effect of the carbon support has been found in the case of the complex [Rh(TDA)], which has been related to the anchorage of the aliphatic chains of the amine ligands on the activated carbon pores. Experiments in consecutive catalytic runs show that the heterogenised complexes can be used several times giving an acceptable conversion level.     

Correlation between metal oxidation state and catalytic activity: hydrogenation of crotonaldehyde over Rh catalysts

The effects of the rhodium (oxidation) state on the activity and selectivity for the crotonaldehyde hydrogenation reaction over Rh/Al₂O₃ and Rh/SiO₂ catalysts were examined using the techniques of temperature-programmed reduction, hydrogen chemisorption and X-ray absorption near-edge structure (XANES). In the alumina-supported system, the active phase-support interaction is shown to affect the chemical behavior of rhodium under the influence of a reductive atmosphere by stabilizing Rh³⁺ species. This behavior is not observed (as expected) for Rh/SiO₂ catalysts. The structural and electronic bases of the active phase-support interaction and the effect of the latter phenomenon on the hydrogenation of crotonaldehyde are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Investigation of rhodium complexes in micro- and mesoporous materials by computer modeling, FTIR, and 31P MAS NMR

Tetracarbonyldichlororhodium(I) [Rh(CO)₂(μ -Cl)]₂ entrapped in faujasite type zeolites reacted with phosphines PMe_{3- x}Ph_x at 393-463 K to a different extent. Although according to computational studies phosphines with x<2 should be small enough to enter the micropore system, the reaction was in no case complete and led to a mixture of products as observed by IR spectroscopy. Furthermore surface-bonded rhodium carbonyl complexes were synthesized in mesoporous aluminium-containing MCM-41 material by reacting acetylacetonatodicarbonylrhodium(I) [Rh(acac)(CO)₂] with Brønsted-acidic centers under formation of a chemically bonded [Rh(CO)₂]- species and acetylacetone according to IR spectroscopy. The corresponding surface-bonded phosphine complex [(O_s)_x-Rh(chiraphos)] was synthesized and identified with IR and ³¹P MAS NMR spectroscopy.