Hysteresis and oscillations in the selectivity during the NO-H2 reaction over Rh(533)

The NO-H₂ reaction over Rh(533) shows oscillatory behaviour at H₂-rich mixtures in the 10^–6 mbar pressure regime around 470 K. The selectivity changes periodically in time: the rate of N₂ formation is out of phase with the NH₃ and H₂O formation rates. Accumulation of atomic N plays a central role in the oscillating behaviour. A comparison will be made with the NO-H₂ reaction over Pt(100).     

Structure and Reactivity of Alkyl Ethers Adsorbed on CeO2(111) Model Catalysts

The effect of surface hydroxyls on the adsorption of ether on ceria was explored. Adsorption of dimethyl ether (DME) and diethyl ether (DEE) on oxidized and reduced CeO₂(111) films was studied and compared with Ru(0001) using RAIRS and sXPS within a UHV environment. On Ru(0001) the ethers adsorb weakly with the molecular plane close to parallel to the surface plane. On the ceria films, the adsorption of the ethers was stronger than on the metal surface, presumably due to stronger interaction of the ether oxygen lone pair electrons with a cerium cation. This interaction causes the ethers to tilt away from the surface plane compared to the Ru(0001) surface. No pronounced differences were found between oxidized (CeO₂) and reduced (CeOx) films. The adsorption of the ethers was found to be perturbed by the presence of OH groups on hydroxylated CeOx. In the case of DEE, the geometry of adsorption resembles that found on Ru, and in the case of dimethyl ether DME is in between that one found on clean CeOx and the metal surface. Decomposition of the DEE was observed on the OH/CeOx surface following high DEE exposure at 300 K and higher temperatures. Ethoxides and acetates were identified as adsorbed species on the surface by means of RAIRS and ethoxides and formates by s-XPS. No decomposition of dimethyl ether was observed on the OH/CeOx at these higher temperatures, implying that the dissociation of the C?CO bond from ethers requires the presence of ??-hydrogen.     

Formation and reactions of CH3 species over Mo2C/Mo(111) surface

The reaction pathways of adsorbed CH₃ on the Mo₂C/Mo(111) surface were investigated by means of temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS). CH₃ fragments were produced by the dissociation of the corresponding iodo-compound. CH₃I adsorbs molecularly on Mo₂C at 90 K and dissociates at and above 140 K. The main products of the reaction of adsorbed CH₃ are hydrogen, methane and ethylene. The coupling into ethane was not observed. The results are discussed in relevance to the conversion of methane into benzene on Mo₂C deposited on ZSM-5. This revised version was published online in August 2006 with corrections to the Cover Date.     

The effects of a potassium overlayer on the reaction pathway of CH2 and C2H5 on Rh(111)

The effect of potassium on the reaction pathways of adsorbed CH₂ and C₂H₅ species on Rh(111) was investigated by means of reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TDS). Hydrocarbon fragments were produced by thermal and photo-induced dissociation of the corresponding iodo compounds. Potassium adatoms markedly stabilized the adsorbed CH₂ and converted it into C₂H₄, the formation of which was not observed for K-free Rh(111). New routes of the surface reactions of C₂H₅ have been also opened in the presence of potassium, namely its transformation into butane and butene.     

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).     

The Surface Chemistry of 1-Iodopropane Adsorbed on Pt(111)

On Pt(111) at 110 K, 1-iodopropane, C₃H₇I, adsorbs molecularly, but for doses below 1.7 × 10¹⁴ molecules cm⁻², only H₂ and I appear in thermal desorption. C–I bond cleavage occurs between 160 and 220 K, forming adsorbed n-propyl, C_(a)H₂CH₂CH₃, and atomic iodine, based on temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), and X-ray photoelectron spectroscopy (XPS). n-Propyl undergoes β-hydride elimination forming propylene, with desorption peaks at 185 and 240 K. At 240 K, hydrogenation to propane also occurs. Some di-σ bonded propylene, C_(a)H₂C_(a)HCH₃, remains at 240 K and it rearranges to propylidyne near 300 K. Atomic H, bound to Pt, recombines and desorbs at ca. 260 K. Further desorption of H₂ is limited by C–H bond breaking and occurs over a broad temperature range with local maxima at ca. 280, 320, and 420 K, typical of propylidyne fragments on Pt. Atomic iodine desorbs in a broad feature at 825 K.     

Support effects in the dissociation of CO on Rh/CeO2(111)

The adsorption and reaction of CO on Rh particles supported on stoichiometric and partially reduced CeO₂(111) surfaces was studied using a combination of HREELS and TPD. A fraction of the CO adsorbed on the supported Rh particles was found to undergo dissociation to produce adsorbed C and O atoms. TPD results for isotopically labeled CO demonstrated that O atoms produced by CO dissociation rapidly exchange with the oxygen in the ceria lattice. The fraction of adsorbed CO which dissociated was found to increase significantly with the extent of reduction of the CeO₂(111) surface, suggesting that oxygen vacancies on the surface of the support play a direct role in the CO dissociation reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

The interfacial capacitance of Rh(111) in HCl solutions

The impedance spectra of Rh(111) in aqueous HCl solutions have been measured as function of potential. In the double layer region, just as for other platinum group metals, the equivalent circuit of the interface is a circuit of a R_ad-W-C_ad element combination parallely connected to a C_dl capacitance; whereas at the H⁺/Cl⁻ adsorbate exchange peak an additional parallel R_ad,2-C_ad,2 branch appears in the equivalent circuit. The (R_ad-W-C_ad) and the R_ad,2-C_ad,2 branches were demonstrated to be due to chloride and hydrogen adsorption, respectively. Due to the same anion appearing in the charging of C_ad and C_dl the two branches are coupled to each other.     

The role of Rh on Pt-based catalysts: structure sensitive NO + H2 reaction on Pt(110) and Pt(100) and structure insensitive reaction on Rh/Pt(110) and Rh/Pt(100)

The catalytic activity of the Pt(110) surface for the reaction of NO + H₂ was much less than that of the Pt(100) surface. However, the catalytic activity of the Rh deposited Pt(1l0) surface was almost equal to that of the Rh deposited Pt(100) surface. That is, the catalytic reaction of NO + H₂ on Pt(110) and Pt(100) surfaces is highly structure sensitive, but it changes to structure insensitive by the deposition of Rh atoms. These results are rationalized by formation of an active overlayer on the Pt(110) and Pt(100) surfaces, which is very analogous to the Rh-O/Pt-layer formed on Rh/Pt(100), Pt/Rh(100) and Pt-Rh(100) alloy surfaces during catalysis. The formation of the common overlayer of Rh-O/Pt-layer during catalysis is responsible for the structure insensitive catalysis of Rh deposited Pt-based catalysts, which is an important role of Rh in a three way catalyst.     

Photocatalytic reaction of H2O+CO2 over pure and doped Rh/TiO2

In the photocatalytic reduction of carbon dioxide to formic acid, formaldehyde and methanol in aqueous suspensions of TiO₂ and Rh/TiO₂, the effects of doping the TiO₂ with W⁶⁺ were investigated.This laboratory is a part of the Center for Catalysis, Surface and Material Science at the University of Szeged.     

HREELS study of photo-induced formation of CO2 anion radical on Rh(111) surface

High-resolution electron loss spectroscopy revealed, probably for the first time, that the illumination of adsorbed CO₂ on K-promoted Rh(111) induces or enhances formation of the CO₂ radical.This laboratory is a part of the Center for Catalysis, Surface and Material Science at the University of Szeged.     

Decomposition of methane and its reaction with CO2 over Rh/ZSM-5 catalyst

The decomposition of methane, its conversion into higher hydrocarbons and the reaction between CH₄ and CO₂ have been investigated on Rh/ZSM-5 in a fixed bed continuous-flow reactor. Independently of the temperature at 523–973 K, the decomposition of methane gave hydrogen, surface carbon and a small amount of ethane: ethylene and benzene were not detected. The reactivity of surface carbon formed at different temperatures has been examined toward H₂, O₂ and CO₂. The carbon exhibited less reactivity toward CO₂. The reaction between CH₄ and CO₂ occurred rapidly above 673 K to give CO and H₂ with a ratio of 1.3–1.6. Very little carbon was deposited during the reaction. It is concluded that the facile reactions between CH_x and CO₂ are responsible for the lack of carbon deposition. However, a significant amount of carbon deposition and the deactivation of the catalyst occurred when more than 4–5% of ethane was added to the reacting gas mixture. The extent of deactivation can be decreased by using a large excess of CO₂. This revised version was published online in June 2006 with corrections to the Cover Date.     

Investigation of h-BN/Rh(111) nanomesh interacting with water and atomic hydrogen

Recent STM experiments show that by exposing h-BN/Rh(111) nanomesh to water or atomic hydrogen interesting phenomena can be observed. We investigated by Density Functional Theory (DFT) the structure of bare nanomesh as well as in the presence of water clusters and atomic hydrogen. Our simulations allow the correct interpretation of the observed modifications of the STM topography under different tested conditions. For example, we could determine that the frequently observed three protrusions within the pore appearing in STM images obtained after dosing small amounts of water, are most likely determined by water hexamers. We also could confirm that the flattening of the h-BN overlayer after dosing atomic hydrogen is determined by the intercalation of the latter between BN and metal, which prevents the effective binding between N and Rh.     

Methanol synthesis by the hydrogenation of CO2 over Zn-deposited Cu(111) and Cu(110) surfaces

The hydrogenation of CO₂ over Zn-deposited Cu(111) and Cu(110) surfaces was performed at 523 K and 18 atm using a high pressure flow reactor combined with XPS apparatus. It was shown that the ZnO _x species formed on Cu(111) during reaction directly promoted the methanol synthesis. However, no such promotional effect of the Zn was observed for methanol formation on Cu(110). Thus, Zn on Cu(111) acts as a promoter, while Zn on Cu(110) acts as a poison. The activation energy and the turnover frequency are in fairly good agreement with those obtained for Cu/ZnO powder catalysts.     

Light Emission from M-Type Enantiomer of 2,13-bis(hydroxymethyl)[7]-thiaheterohelicene Molecules Adsorbed on Au(111) and C60/Au(111) Surfaces Investigated by STM-LE

Light emission from the M-type enantiomer of a helicene derivative (2,13-bis(hydroxymethyl)[7]-thiaheterohelicene) adsorbed on the clean Au(111) and the C₆₀-covered Au(111) surfaces were investigated by tunneling-current-induced light-emission technique. Plasmon-originated light emission was observed on the helicence/Au(111) surface and it was strongly suppressed on the area where the helicene molecules were adsorbed at the edges of the Au(111) terraces. To avoid luminescence quenching of excited helicene molecules and to suppress strong plasmon light emission from the Au(111) surface, C₆₀ layers were used as decoupling buffer layers between helicene molecules and the Au(111) surface. Helicene molecules were adsorbed preferentially on the Au(111) surface rather than on the C₆₀ buffer layers due to the small interaction of the molecules and C₆₀ islands. This fact motivated us to deposit a multilayer of helicene molecules onto the C₆₀ layers grown on the Au(111) surface, leading to the fact that the helicene/C₆₀ multilayer showed strong luminescence with the molecules character. We consider that such strong light emission from the multilayer of helicene molecules has a plasmon origin strongly modulated by the molecular electronic states of (M)-[7]TH-diol molecules.     

The NO-H2 reaction over Pt(100) oscillatory behaviour of activity and selectivity

The NO-H₂ reaction has been studied over a Pt(100) single crystal surface as a function of temperature and partial pressures of the reactants. The activity as well as the selectivity, shows oscillatory behaviour under isothermal conditions from 420 K to 520 K. The oscillations observed for the formation rates of N₂ and NH₃ are out of phase with those found for the formation rate of N₂O. These observations are in line with recently proposed mechanisms for the formation of N₂, NH₃ and N₂O.     

Long and short range effect of alkali promoters on metal surfaces: K on Rh(111)

Photoemission of physisorbed noble gases has been used to investigate the variation of the surface potential on a potassium-promoted rhodium (111) surface. The results confirm that the predominant effect of potassium is to lower the potential on nearest neighbour sites by 1–1.5 eV, but indicate also that the potential on next nearest neighbour sites is still 0.4–1 eV lower than on unpromoted rhodium, depending on the potassium coverage. Theoretical calculations show that the latter values are in qualitative agreement with the cumulative effect of potassium/rhodium dipoles which form an ordered array on the surface.     

Ethylene dimerization over a model Sn/Pt(111) catalyst

The dimerization reaction of ethylene was studied over Pt(111) and (3×3)R30°-Sn/ Pt(111) model catalysts at moderate pressures (20–100 Torr). The catalyst surfaces were prepared and characterized in a UHV surface analysis system and moderate pressure catalytic reactions were conducted with an attached batch reactor. The overall catalytic activity of the (3×3)R30°-Sn/Pt(111) surface alloy for C₄ products was slightly higher than that at Pt(111). In addition to the dimerization reaction, hydrogenolysis of ethylene to propane and methane was also observed, with the (3×3)R30°-Sn/Pt(111) surface alloy less active than Pt(111). Among the C₄ products, butenes andn-butane were the major components. Carbon buildup was observed to be significant above 500 K with the (3×3)R30°-Sn/Pt(111) surface alloy much more resistant than Pt(111). The dimerization of ethylene was not eliminated by the presence of surface carbonaceous deposits and even at significant surface coverages of carbon the model catalysts exhibited significant activities. The results are discussed in terms of the surface chemistry of ethylene and the previously reported catalytic reactions of acetylene trimerization andn-butane hydrogenolysis at these surfaces.