Initial oxidation of the Rh(110) surface: ordered adsorption and surface oxide structures

The initial oxidation of the Rh(110) surface was studied by scanning tunneling microscopy, core level spectroscopy, and density functional theory. The experiments were carried out exposing the Rh(110) surface to molecular or atomic oxygen at temperatures in the 500-700 K range. In molecular oxygen ambient, the oxidation terminates at oxygen coverage close to a monolayer with the formation of alternating islands of the (10x2) one-dimensional surface oxide and (2x1)p2mg adsorption phases. The use of atomic oxygen facilitates further oxidation until a structure with a c(2x4) periodicity develops. The experimental and theoretical results reveal that the c(2x4) structure is a "surface oxide" very similar to the hexagonal O-Rh-O trilayer structures formed on the Rh(111) and Rh(100) substrates. Some of the experimentally found adsorption phases appear unstable in the phase diagram predicted by thermodynamics, which might reflect kinetic hindrance. The structural details, core level spectra, and stability of the surface oxides formed on the three basal planes are compared with those of the bulk RhO2 and Rh2O3.     

Strategies to stabilize new members of the (A3A'BO6)alpha (A3B3O0)beta homologous series in the Sr-Rh-O system: structure of the one-dimensional (alpha = 3, beta = 2) [Sr10(Sr0.5Rh1.5)TP(Rh6)Oh])24 oxide

The (alpha =3, beta =2) member of the (A3ABO6) (A3B3O9) homologous series has been stabilised in the Sr-Rh-O system for a [Sr10(Sr0.5Rh1.5)TP(Rh6)Oh]O24 composition. The structural characterisation has been performed by powder X-ray and electron diffraction measurements and high-resolution electron microscopy. In this structure, three face-sharing [RhO6] octahedra linked by one [Rh/SrO6] trigonal prism comprise the infinite one-dimensional chain that runs parallel to the c axis of a trigonal unit cell (Pc1), with parameters a=9.6411(1) and c=21.2440(4) A.     

Growth of Sr6Rh5O15 single crystals from high-temperature solutions: structure determination using the traditional 3-D and the 4-D superspace group methods and magnetic measurements on oriented single crystals

Single crystals of Sr6Rh5O15 were grown from a molten potassium carbonate flux. The structure was solved by both the traditional 3-D crystallographic approach and the 4-D superspace group approach using JANA2000. Both methods produced an equivalent structure determination, thereby confirming the 4-D superspace group approach as an effective structure solution method for 3-D commensurate composite structures. Sr6Rh5O15 corresponds to the n = 1, m = 1 member of the A3n+3mA'nB3m+nO9m+6n family of 2H hexagonal perovskite-related oxides. This compound is characterized by pseudo-one-dimensional polyhedral chains of four face-sharing RhO6 octahedra followed by one RhO6 trigonal prism. These chains in turn are separated by [Sr](infinity) chains. Magnetic measurements were carried out on oriented single crystals, and a very large magnetic anisotropy in the magnetic susceptibility was observed.     

Initial oxidation of a Rh(110) surface using atomic or molecular oxygen and reduction of the surface oxide by hydrogen

The formation conditions, morphology, and reactivity of thin oxide films, grown on a Rh(110) surface in the ambient of atomic or molecular oxygen, have been studied by means of laterally resolved core level spectroscopy, scanning tunneling microscopy and low energy electron diffraction. Exposures of Rh(110) to atomic oxygen lead to subsurface incorporation of oxygen even at room temperature and facile formation of an ordered, laterally uniform surface oxide at approximately 520 K, with a quasi-hexagonal structure and stoichiometry close to that of RhO(2). In the intermediate oxidation stages, the surface oxide coexists with areas of high coverage adsorption phases. After a long induction period, the reduction of the Rh oxide film with H(2) is very rapid and independent of the coexisting adsorption phases. The growth of the oxide film by exposure of a Rh(110) surface to molecular oxygen requires higher pressures and temperatures. The important role of the O(2) dissociation step in the oxidation process is reflected by the complex morphology of the oxide films grown in O(2) ambient, consisting of microscopic patches of different Rh and oxygen atomic density.     

Crystal growth and structure determination of barium rhodates: stepping stones toward 2H-BaRhO3

Single crystals of two new barium rhodates were grown from a molten potassium carbonate flux. The new rhodates, Ba(11)Rh(10)O(30) and Ba(32)Rh(29)O(87), are structurally related to the 2H-hexagonal perovskite structure and are characterized by pseudo one-dimensional chains of alternating face-sharing trigonal prisms and octahedra. The structures of Ba(11)Rh(10)O(30) and Ba(32)Rh(29)O(87) were solved using the 4D superspace group approach in Jana2000. Ba(11)Rh(10)O(30), with a repeat of nine RhO(6) octahedra followed by one RhO(6) trigonal prism, contains the longest chain sequence of face-sharing octahedra known for this 2H-perovskite related family of oxides. A structural analysis of these two compounds revealed clear trends in metal-metal distances and octahedral heights not previously identified for this family of oxides. The application of these trends toward the structure of the all-octahedra-containing end member of the structural series, the hypothetical 2H-BaRhO(3), enabled a prediction of its rhodium-rhodium distance, octahedral height, and lattice parameters.     

Undulating layers in a new rhodate network: structure of Bi(1.4)CuORh5O10

A new rhodate, Bi(1.4)CuRh(5)O(11), with an hitherto unknown channel structure containing undulating layers of RhO(6) octahedra sharing corners and edges has been discovered and its structure refined from single crystal X-ray diffraction data. The channels contain Bi(3+), Cu(2+), and some O strongly bound to Cu. The Cu coordination is distorted square planar. Mixed Rh(3+)/Rh(4+) valency leads to significant electrical conductivity.     

Ln18Li8Rh5O39 (Ln = La, Pr): a mixed-metal oxide with a charge-ordered arrangement of Rh3+ and Rh4+

Polycrystalline samples of Ln18Li8Rh5O39 (Ln = La, Pr) have been synthesized by the ceramic method and characterized by X-ray and neutron diffraction. The compounds crystallize in the cubic space group Pmn, with a0 approximately 12.1 Angstroms. The unit cell contains four intersecting 111 chains, each comprised of an alternating sequence of face-sharing RhO6 octahedra and LiO6 trigonal prisms. The octahedra located at the points of intersection contain Rh4+, whereas the remainder contain Rh3+; the compounds thus contain a charge-ordered arrangement of the two cations. The polyhedral chains are enclosed in tunnels formed by the Ln-O sublattice. The magnetic properties of the two new compounds are discussed briefly: both are paramagnetic over the temperature range 5 < TK < 300.     

Chemical Reconstruction of Pt-Rh(100) Alloy and Pt/Rh(100), Rh/Pt(100) Bimetallic Surfaces

When Cu(110), Ni(l 10), Ag(110) surfaces are exposed to O₂ at room temperature, one dimensional metal-oxygen strings grow in the < 001 > direction of the (110) surfaces. A similar phenomenon occurs in the adsorption of H₂ on Ni( 110) surface at room temperature, where the one dimensional strings grow along the < 110 > direction. These phenomena are undoubtedly different from the adsorption induced reconstruction but are explained by the chemical reconstruction involving the formation of quasi-compounds and their self-ordering on the metal surfaces. The chemical reconstruction is indispensablly important to understand the structure and catalysis of alloy and bimetallic surfaces. Pt_0.25Rh_0.75(100) alloy surface being active for the reaction of NO with H₂ is an interesting example. When the Pt-Rh(100) alloy surface is exposed to NO or O₂ at arround 500 K, a p(3 × 1) ordered Rh-O over-layer is obtained on a Pt-enriched 2nd layer by the chemical reconstruction. Ordering of Rh-0 in the p(3 × 1) structure on the Pt(100) surface was reproduced by heating a Rh/Pt(100) bimetallic surface in O₂, and the chemical reconstruction making the p(3 × 1) Rh-O overlayer on a Pt enriched 2nd layer was also proved by heating a Pt/Rh(100) bimetallic surface in O₂ or NO. The activation mechanism of the Pt-Rh alloy and the Pt/Rh bimetallic surfaces by the chemical reconstruction was evidently shown by using a Pt deposited Rh(100), Pt/Rh(100), surface. That is, the Pt/Rh(100) is not so active for the reaction of NO with H₂, but the reconstructed p(3 × 1)Rh-O/Pt-layer/Rh(100) surface is very active for the reaction. Therefore, it was concluded that the chemical reconstruction of the Pt-Rh catalyst makes the active surface which is composed of Rh-O and a Pt layer.     

Traveling interface modulations in the NH3 + O2 reaction on a Rh(110) surface

A new type of traveling interface modulation has been observed in the NH(3) + O(2) reaction on a Rh(110) surface. A model is set up which reproduces the effect, which is attributed to diffusional mixing of two spatially separated adsorbates causing an excitability which is strictly localized to the vicinity of the interface of the adsorbate domains.     

Ab initio thermodynamics examination of sulfur species present on Rh, Ni, and binary Rh-Ni surfaces under steam reforming reaction conditions

The stable form of adsorbed sulfur species and their coverage were investigated on Rh, Ni, and Rh-Ni binary metal surfaces using density functional theory calculations and the ab initio thermodynamics framework. S adsorption, SO(x) (x = 1-4) adsorption, and metal sulfide formation were examined on Rh(111) and Ni(111) pure metals. Both Rh and Ni metals showed a preference for S surface adsorption rather than SO(x) adsorption under steam reforming conditions. The transition temperature from a clean surface (<(1)/(9) ML) to S adsorption was identified on Rh(111), Ni(111), Rh(1)Ni(2)(111), and Rh(2)Ni(1)(111) metals at various P(H(2))/P(H(2)S) ratios. Bimetallic Rh-Ni metals transition to a clean surface at lower temperatures than does the pure Rh metal. Whereas Rh is covered with (1)/(3) ML of sulfur under the reforming conditions of 4-100 ppm S and 800 °C, Rh(1)Ni(2) is covered with (1)/(9) ML of sulfur at the lower end of this range (4-33 ppm S). The possibility of sulfate formation on Rh catalysts was examined by considering higher oxygen pressures, a Rh(221) stepped surface, and the interface between a Rh(4) cluster and CeO(2)(111) surface. SO(x) surface species are stable only at high oxygen pressure or low temperatures outside those relevant to the steam reforming of hydrocarbons.     

Rh/SiO~2和Rh/NaY催化剂上合成气反应的高压原位 红外光谱研究

采用高压原位FT-IR技术,对比研究了CO加H~2反应条件下Rh/SiO~2和Rh/NaY催化剂表面反应中间物种。在Rh/SiO~2表面上,无论在常压还是在1.0MPa合成气中,只观察到线式和桥式吸附CO。而在常压合成气中,Rh/NaY上不仅存在上述CO吸附物种,而且还有孪生型的Rh(Ⅰ)(CO)~2和少量Rh~6(CO)~1~6;当合成气压力升至1.0MPa后,Rh(Ⅰ)(CO)~2迅速转化成Rh~6(CO)~1~6和在2042cm^-^1产生吸收的单核羰基Rh物种,与此同时催化剂表面还生成了单齿和双齿乙酸根物种;这些在高压下生成的物种在合成气压力重新降回到常压时依然稳定存在。研究Rh/NaY上合成气反应表面物种与H~2的反应行为表明单齿乙酸根很可能是反应的活性中间物。这些结果说明Rh/NaY催化剂在高压合成气中的重构是诱发选择生成乙酸反应的基础。     

A reflection absorption infrared spectroscopy and density-functional theory investigation of methanol dehydrogenation on Rh(111)V alloy surfaces

The dehydrogenation reaction of methanol on a Rh(111) surface, a Rh(111)V subsurface alloy, and on a Rh(111)V islands surface has been studied by thermal-desorption spectroscopy, reflection absorption infrared spectroscopy, and density-functional theory calculations. The full monolayer of methanol forms a structure with a special geometry with methanol rows, where two neighboring molecules have different oxygen-rhodium distances. They are close enough to form a H-bonded bilayer structure, with such a configuration, where every second methanol C-O bond is perpendicular to the surface on both Rh(111) and on the Rh(111)V subsurface alloy. The Rh(111)V subsurface alloy is slightly more reactive than the Rh(111) surface which is due to the changes in the electronic structure of the surface leading to slightly different methanol species on the surface. The Rh(111)V islands surface is the most reactive surface which is due to a new reaction mechanism that involves a methanol species stabilized up to about 245 K, partial opening of the methanol C-O bond, and dissociation of the product carbon monoxide. The latter two reactions also lead to a deactivation of the Rh(111)V islands surface.     

Single-crystal structure of the 2H-related perovskites (A(3-x)Nax)NaBO6 (A = La, Pr, Nd; B = Rh, Pt)

Single crystals of La(2.47)Na(1.53)RhO6, Pr(2.45)Na(1.55)RhO6, Nd(2.45)Na(1.55)RhO6, La2Na2PtO6, and Nd2Na2PtO6 were grown from carbonate and "wet" hydroxide fluxes. All were found to crystallize in the trigonal space group R3c and adopt the K4CdCl6 structure.     

Spin state, structure, and reactivity of terminal oxo and dioxygen complexes of the (PNP)Rh moiety

[Rh(III)H{(tBu(2)PCH(2)SiMe(2)NSiMe(2)CH(2)PtBu{CMe(2)CH(2)})}], ([RhH(PNP*)]), reacts with O(2) in the time taken to mix the reagents to form a 1:1 eta(2)-O(2) adduct, for which O--O bond length is discussed with reference to the reducing power of [RhH(PNP*)]; DFT calculations faithfully replicate the observed O-O distance, and are used to understand the oxidation state of this coordinated O(2). The reactivity of [Rh(O(2))(PNP)] towards H(2), CO, N(2), and O(2) is tested and compared to the associated DFT reaction energies. Three different reagents effect single oxygen atom transfer to [RhH(PNP*)]. The resulting [RhO(PNP)], characterized at and above -60 degrees C and by DFT calculations, is a ground-state triplet, is nonplanar, and reacts, above about +15 degrees C, with its own tBu C--H bond, to cleanly form a diamagnetic complex, [Rh(OH){N(SiMe(2)CH(2)PtBu(2))(SiMe(2)CH(2)PtBu{CMe(2)CH(2)})}].     

CO oxidation on Pt-modified Rh(111) electrodes.

The CO electro-oxidation reaction was studied on platinum-modified Rh(111) electrodes in 0.5 M H2SO4 using cyclic voltammetry and chronoamperometry. The Pt-Rh(111) electrodes were generated during voltammetric cycles at 50 mV s(-1) in a 30 microM H2PtCl6 and 0.5 M H2SO4 solution. Surfaces generated by n deposition cycles were investigated (Ptn-Rh(111) with n=2, 4, 6, 8, 10, and 16). The blank cyclic voltammograms of these surfaces are characterized by a pronounced sharpening of the hydrogen/(bi)sulfate adsorption/desorption peaks, typical for Rh(111), and the appearance of contributions between 0.1 and 0.4 V, which were ascribed to hydrogen/(bi)sulfate adsorption/desorption on the deposited platinum. At higher potentials, the surface oxidation of Rh(111) is enhanced by the presence of platinum. The structure of the Pt-modified electrodes was investigated by STM imaging. At low Pt coverages (Pt2-Rh(111)), monoatomically high islands are formed, which grow three dimensionally as the number of deposition cycles increases. After eight cycles, the monolayer islands have grown in diameter and range from mono- to multiatomic height. At even higher Pt coverage (Pt16-Rh(111)), the islands grow to particles of approx. 10 nm in diameter, which are 5-6 atoms high. The CO stripping voltammetry on these surfaces is characterized by two peaks: A low-potential, structure-insensitive peak, ascribed to CO reacting at the platinum monolayer islands, whose onset is shifted 150, 250, and 100 mV negatively with respect to pure Rh(111), Pt(111), and polycrystalline Pt, respectively, indicating the enhanced CO electro-oxidation properties of the Pt overlayer system. A peak at higher potentials displays strong structure sensitivity (particle-size effect) and was ascribed to CO reacting on the islands of multiatomic height. Current-time transients recorded on the surface with the highest amount of monolayer islands (Pt4-Rh(111)) also indicate enhanced CO-oxidation kinetics. Comparison of the Pt4-Rh(111) current-time transients recorded at 0.635, 0.675, and 0.750 V versus RHE (reversible hydrogen electrode) with those of pure Rh(111) and Pt(111) shows greatly reduced reaction times. A Cottrellian decay at long times indicates surface-diffusion-limited CO oxidation on the bare Rh(111) surface, while the peak visible at short times is indicative of CO reacting at the monolayer platinum islands. The results presented here show that, as indicated by density functional theory (DFT) calculations, the CO-adlayer oxidation for this system is enhanced compared to both pure Rh and Pt.     

K-stabilized high-oxygen-coverage states on Rh(110): a low-pressure pathway to formation of surface oxide

Using scanning tunneling microscopy, low-energy electron diffraction, and X-ray photoelectron spectroscopy, we studied the evolution of the structure and chemical state of a Rh(110) surface, modified by K adlayers and exposed to high O2 doses at elevated temperatures. We find that oxygen coadsorption on the K-covered Rh(110) leads to massive reconstruction of the Rh(110) surface. Stable reconstructed (10 x 2) and (8 x 2) segmented phases with a local coverage of more than two oxygen atoms per surface Rh atom were observed. Formation of surface oxide, which coexists with the (10 x 2) and (8 x 2) segmented adsorption phases, is evidenced at the highest O2 doses. The development of strongly reconstructed adsorption phases with oxide-like stoichiometry and surface oxide under UHV conditions is explained in terms of the stabilization of the (1 x 2) reconstruction and promotion of O2 dissociation by the K adatoms.     

Transition from the layered Sr2RhO4 to the monodimensional Sr4RhO6 phase

Study of the structural changes occurring during the reduction process of the Sr2RhO4+delta, (214), n=1 term of the Ruddlesden and Popper series, shows that for delta <0.02 values, this material dissociates into the Sr4RhO6 (416) monodimensional phase, alpha = infinity, beta = 0 compound of the (A3B2O6)alpha-(A3B3O9)beta family, and Rh metal. During the first stage, this process occurs by the formation of an intergrowth between the (214) and (416) materials which can be only detected by high resolution electron microscopy and is easily interpreted on the basis of the structural relationship established between them. Further reduction allows the segregation of both phases as separated entities, which coexist with Rh metal. The dissociation process is reversible and, under oxidizing conditions, a layered material with anionic composition delta =0.06 is always obtained. This behaviour seems to be a general way of accommodating the compositional changes in layered A2BO4 phases where the B cation is always in a octahedral environment. The structural mechanism of this transformation is proposed, and the structural relationship between these two low-dimensional oxides is established.     

Structural isomers and reactivity for Rh6 and Rh6+

The structure, energetics, and interconversion of isomers of Rh(6) and Rh(6)(+) are studied by using density functional theory with Gaussian basis sets, using guess structures derived from basin-hopping simulations, and obtained by using the Sutton-Chen potential. A large range of spin multiplicities is considered for each isomer. Our calculations suggest two low-lying structures as possible structural isomers: a square bipyramid and a trigonal prism. The reactivity of these two candidate structural isomers with respect to adsorption of nitric oxide is studied via location of reaction transition states and calculation of reaction barriers. Similarities and differences with surface reaction studies are highlighted. These data provide powerful evidence that structural isomerism, and not different spin states, is responsible for the observed biexponential reaction kinetics.     

Electrochemical behavior of Pd–Rh alloys

Pd–Rh alloys were prepared by electrochemical codeposition. Bulk compositions of the alloys were determined by the energy dispersive X-ray analysis method, while surface compositions were determined from the potential of the surface oxide reduction peak. Cyclic voltammograms, recorded in 0.5 M H₂SO₄ for Pd–Rh alloys of different bulk and surface compositions, are intermediate between curves characteristic of Pd and Rh. The influence of potential cycling on electrochemical properties and surface morphologies of the alloys was studied. Due to electrochemical dissolution of metals, both alloy surface and bulk become enriched with Pd. Carbon oxides were adsorbed at a constant potential from the range of hydrogen adsorption. The presence of adsorbed CO₂ causes remarkable diminution of hydrogen adsorption but it does not significantly influence hydrogen insertion into the alloy bulk. On the other hand, in the presence of adsorbed CO, both hydrogen absorption and adsorption are strongly suppressed. Oxidative removal of the adsorbates results in a characteristic voltammetric peak, whose potential increases with the decrease in Rh surface content. Electron per site (eps) values calculated for the oxidation of the adsorbates change with alloy surface composition, more for CO₂ than CO adsorption, indicating the variation of the structure and composition of CO₂ and CO adsorption products. The course of the dependence of eps values on surface composition suggests that the products of CO₂ and CO adsorption on Pd–Rh alloys are similar but not totally identical.     

Water production reaction on Rh110

By means of scanning tunneling microscopy and density functional theory calculations, we studied the water formation reaction on the Rh(110) surface when exposing the (2 x 1)p2mg-O structure to molecular hydrogen, characterizing each of the structures that form on the surface during the reaction. First the reaction propagates on the surface as a wave front, removing half of the initial oxygen atoms. The remaining 0.5 monolayers of O atoms rearrange in pairs, forming a c(2 x 4) structure. Second, as the reaction proceeds, areas of an intermediate structure with c(2 x 2) symmetry appear and grow at the expense of the c(2 x 4) phase, involving all the oxygen atoms present on the surface. Afterward, the c(2 x 2) islands shrink, indicating that complete hydrogenation occurs at their edges, leaving behind a clean rhodium substrate. Two possible models for the c(2 x 2) structure, where not only the arrangement but also the chemical identity is different, are given. The first one is a mixed H + O structure, while the second one resembles the half-dissociated water layer already proposed on other metal surfaces. In both models, the high local oxygen coverage is achieved by the formation of a hexagonal network of hydrogen bonds.