Intermolecular migration of methyl groups at a Cu(211) surface

The exposure of preadsorbed oxygen to monomethylamine or the coadsorption of a 2:1 monomethylamine (CH₃NH₂)/dioxygen mixture at a Cu(211) surface at room temperature results in the formation of a surface species characterised by C(1s) and N(1s) binding energies of 285.2 and 398.2 eV, respectively, with a calculated carbon to nitrogen ratio of 2:1. This species, which we assign to a chemisorbed dimethylamine ((CH₃)₂NH_x(a)), is the only adsorbed product of the reaction and its formation must involve the breaking of a carbon–nitrogen bond and the intermolecular transfer of a methyl group. This revised version was published online in November 2006 with corrections to the Cover Date.     

A reactive oxygen state at a barium promoted Au (100) surface: the oxidation of ethene at cryogenic temperatures

Although Au (100) does not adsorb oxygen at either 295 K or 80 K, a barium modified Au(100) surface is active in oxygen dissociation resulting, through surface diffusion of oxygen adatoms, in the formation of a chemisorbed oxygen adlayer. This oxygen species is inactive for ethene oxidation, as is the oxygen species pre-adsorbed at an Au(100)–Ba surface at 80 K, and the clean Au(100)–Ba surface. However, when molecularly adsorbed ethene present at a Au(100)–Ba surface at 80 K is exposed to dioxygen and warmed to 140 K, surface carbonate is observed. We conclude that a transient oxygen species is the oxidant.     

Evidence for the instability of surface oxygen at the Zn(0001)-O-Cu interface from core-level and X-ray induced Auger spectroscopies

By monitoring the O(1s) and Zn(LMM) Auger spectra it has been shown that the deposition of copper atoms at a Zn(0001)-O overlayer results in the desorption of oxygen with simultaneous reduction of Zn²⁺ to Zn⁰ at 300 K. The surface concentrations of oxygen and copper adatoms are calculated from the O(1s) and Cu(2p) intensities while the X-ray induced Auger Zn(LMM) transition provides evidence for Zn²⁺ and its reduction to Zn⁰. The driving force for oxygen desorption is suggested to arise from the formation of a copper-zinc intermetallic brass-like overlayer which has little affinity for oxygen.     

Structural aspects of chemisorption at Cu(110) revealed at the atomic level

We illustrate the impact that scanning tunnelling microscopy (STM) has made on our understanding of chemisorption and catalysis at metal surfaces at the atomic level by considering four examples where information from surface sensitive techniques was also available. The advantages of STM and the limitations of some of the other experimental methods are discussed. (1) Through a combination of STM and X-ray photoelectron spectroscopy (XPS) we have established that a number of distinct oxygen chemisorbed states can exist at a Cu(110) surface. These are metastable and temperature dependent. Furthermore, the presence of chemisorbed sulphur is shown to promote a specific oxygen state – isolated oxygen strings – which are likely to be more chemically reactive than the oxygen overlayer present at Cu(110). In this sense the sulphur is a structural promoter. (2) The oxidation of ammonia under ammonia-rich conditions results in the growth of imide (NH) strings at a Cu(110) surface and this has been followed quantitatively by STM. The reactive surface oxygen state participates in an oxydehydrogenation reaction generating NH-radical species which undergo surface diffusion and result in string growth. (3) Nitric oxide dissociates at Cu(110) to generate a two-phase system of chemisorbed nitrogen and oxygen adatoms. The oxygen is present in a well ordered (2 × 1) structure and the nitrogen in a (3 × 2) structure. The limitations of an earlier LEED study are discussed. (4) Structural aspects of chemisorbed sulphur generated by the dissociative chemisorption of hydrogen sulphide and methyl mercaptan are discussed. In the latter case carbon–sulphur bond cleavage results in the formation of a sulphide overlayer at 450 K with complete removal of carbon as desorbed hydrocarbons. Various sulphur structures have been delineated over a wide temperature range. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Development of a New Concept: The Role of Oxygen Transients, Defect and Precursor States in Surface Reactions

Surface sensitive spectroscopies, and more recently scanning tunnelling microscopy, have provided a new insight into the dynamics of oxygen chemisorption at metal surfaces. Studies at low temperatures, using probe molecules, provided the first evidence for the role of metastable hot oxygen transients and molecular complexes in opening up low-energy pathways in oxidation catalysis. Progress over the past 15 years is considered, emphasizing the limitations of the classical approach in discussions of the mechanisms of surface reactions and how concepts developed through surface spectroscopies could be sustained at the atom-resolved level.     

Chemical reactivity of CO and CO2 at a Cu(110)-Cs surface

Carbon dioxide and carbon monoxide undergo reactive chemisorption with cesium modified Cu(110) and Cu(110)-O surfaces and via the anionic intermediate CO ₂ ^– (a) form a surface carbonate. The CO ₂ ^– (a) species was characterised by VEELS and XPS at low temperature (80 K) and the surface carbonate at 295 K. For cesium modified Cu(110) surfaces chemisorption of carbon monoxide gives rise to electron energy loss peaks (v _co) as low as 1450 cm^–1 at 295 K.     

The mechanism of the poisoning of ammonia synthesis catalysts by oxygenates O2, CO and H2O: an in situ method for active surface determination

The effect of adding an oxygenated poison (O₂, CO or H₂O) to a hydrogen/nitrogen stream producing ammonia over a triply promoted (K₂O, CaO, Al₂O₃) commercial catalyst is not unsurprisingly rapidly to poison the catalyst. However, immediately the oxygenated poison reacts with the catalyst and before total poisoning has occurred, which in these experiments took 10 min, there was an explosive release of ammonia producing concentrations in the gas phase in excess of the equilibrium value. This is thought to be due to a convulsive reorganisation of the surface of the catalyst in forming regions of an oxide overlayer, resulting in the expulsion of the standing surface nitrogen atom coverage as ammonia. However, in contradistinction to the observation of complete poisoning of the triply promoted catalyst shortly after switching the water (2.9%) into the hydrogen/nitrogen stream, when polycrystalline iron was used as the catalyst, after the initial pulse of ammonia was observed, the small quantity of water (2.9%) in the hydrogen/nitrogen stream resulted in an increased rate ( ×3) of ammonia synthesis which declined only slightly over the twenty minute duration of the experiment. The difference in behaviour between the triply promoted catalyst and the polycrystalline iron is thought to be due to the relative ease of reduction of the latter, so that submonolayer quantities of oxide can be stabilised on the surface of the polycrystalline iron. The promoting effect of this oxide overlayer is either structural or electronic; no distinction can be made from these experiments. The technique of injecting either O₂ or CO into a hydrogen/nitrogen stream which is producing ammonia over promoted catalysts in quantities insufficient to cause complete poisoning and measuring the oxygen coverage of the catalyst to a measured decrease in the ammonia synthesis rate, appears to be a ready, in situ method for the determination of the active catalyst area.     

Atomistic insight into the minimum wear depth of Cu(111) surface

In the present work, we investigate the minimum wear depth of single crystalline Cu(111) under single asperity friction by means of molecular dynamics simulations. The atomistic mechanisms governing the incipient plasticity are elucidated by characterizing specific defect structures and are correlated to the observed mechanical and frictional responses of the material. Furthermore, the effect of probe radius on the friction process is studied. Our simulations indicate that the formation of wear impression is closely associated with defect nucleation and the minimum wear depth is equivalent to the critical penetration depth at which plasticity initiates. It is found that the probe radius has a strong influence on the formation of defect structures and the observed mechanical responses.     

Oxygen sites active in H-abstraction at a Cu(110)-O surface: Comparison of a Monte Carlo simulation with imide formation studied by XPS and VEELS

A Monte Carlo simulation of the development of the topographic structure of the Cu(110)-O overlayer has provided quantitative data for the distribution of different oxygen sites during formation of the overlayer. Those oxygen atoms present at the ends of copper-oxygen chains are shown to be able to account for the observed activity of the overlayer in imide formation through H-abstraction from ammonia. The Monte Carlo simulation also mimics closely features of the Cu(110)-O system observed by scanning tunnelling microscopy.     

A new approach to the design of a sequence with the highest affinity for a molecular surface

We describe an algorithm to design the primary structures forpeptides which must have the strongest binding to a given molecularsurface. This problem cannot be solved by a direct combinatorialsorting, because of an enormous number of possible primary andspatial structures. The approach to solve this problem is todescribe a state of each residue by two variables: (i) aminoacid type and (ii) 3-D coordinate, and to minimize binding energyover all these variables simultaneously. For short chains whichhave no long-range interactions within themselves, this minimizationcan be done easily and efficiently by dynamic programming. Wealso discuss the problem of how to estimate specificity of bindingand how to deduce a sequence with maximal specificity for agiven surface. We show that this sequence can be deduced bythe same algorithm after some modification of energetic parameters.     

Catalytic Epoxidation of Styrene by Molecular Oxygen over a Novel Catalyst of Copper Hydroxyphosphate Cu2(OH)PO4

Catalytic epoxidation of styrene by molecular oxygen over a novel copper hydroxyphosphate catalyst, Cu₂(OH)PO₄, was studied. Catalytic data show that the catalyst Cu₂(OH)PO₄ is very active, and the main products are benzaldehyde and styrene epoxide. Some important factors associated with the catalytic activity and selectivity have been investigated extensively.     

In situ observation of methoxide hydrogenation on the Ni(111) surface

The surface chemistry of methoxide (CH₃O-) on the Ni(111) surface has been studied in the presence of hydrogen pressures up to 2 Torr. During heating in vacuum methoxide decomposes to H₂ and CO, which desorb at 380 and 445 K, respectively. The CH₃O-decomposition process is rate limited by CH bond breaking and exhibits a strong deuterium kinetic isotope effect in CD₃O-. In the presence of ambient hydrogen pressures of 0.02–2.0 Torr both CH₃O- and CD₃O-are hydrogenated directly to methanol at 310 K. Methoxide is hydrogenated by adsorbed hydrogen, which nearly saturates the surface at these pressures and temperatures.     

Study on reconstruction mechanism at the surface of a glassy polymer

The introduction of hydrophilic functional groups at various depths in the functionalized interfacial region of poly(4-methylstyrene) (P4MS) provided a system for studying the surface reconstruction mechanism of this glassy polymer. The degree of surface reconstruction (RD) and the rate of surface reconstruction (1/t_1/2) were employed to compare the surface reconstruction behavior for various samples. The results showed that 1/t_1/2 decreased with increase of the depth in the functionalized interface region at a temperature below T_g of poly(4-methylstyrene). By studying the relation of water contact angles and the surface free energy of the samples with temperature, it was found that surface reconstruction of surface-oxidized P4MS samples took place in two steps when the samples were heated in air. The first step took place below 80 °C, in which polar side groups turned into the bulk, leaving a relatively nonpolar backbone projecting out of the surface to form a “hydrophobic conformation”. The second step occurred above the P4MS T_g (110 °C), in which P4MS molecular chains migrated to the surface in order to minimize the interfacial free energy between surface-oxidized P4MS film and air, since oxidized P4MS molecules containing carbonyl groups have higher surface free energy than the unmodified P4MS molecules. As the depth of the functionalized interfacial region increases, a longer time is needed for the polar side groups to reorient (the first step) and for unoxidized P4MS molecules to migrate to the surface (the second step), which resulted in the sample with deeper functionalized region having lower reconstruction rate and RD using the same treatment condition.     

A new approach to the statistical optimisation of catalyst preparation

The statistical design of experiments is not widely used due to the apparent complexity of the procedures involved. A simplified approach, based on the methods of Taguchi, is described and is illustrated by applying it to the optimisation of the preparation of a metal-carbon catalyst. The surface area was increased from 8 to 250 m² g⁻¹. It is shown that the method gives five significant advantages. It enables the amount of experimentation to be significantly reduced; it provides a means for assessing the significance of synergistic effects (interactions) between variables (factors) and it can yield evidence for the existence of previously unsuspected interactions. The method has the advantage of being applicable to existing production processes by assessing the consequences of variations in operating parameters found within normal run conditions. Finally, the experimental design process is simplified and the analysis of results is facilitated by the ready availability of the required statistical techniques in commercial software for microcomputers.     

From the nucleation of wiggling Au nanostructures to the dome-shaped Au droplets on GaAs (111)A, (110), (100), and (111)B

In this paper, the systematic evolution process of self-assembled Au droplets is successfully demonstrated on GaAs (111)A, (110), (100), and (111)B. On various GaAs substrates, self-assembled Au clusters begin to nucleate at around 300°C, and then, they develop into wiggly Au nanostructures at 350°C. Between 400°C and 550°C, the self-assembled dome-shaped Au droplets with fine uniformity are fabricated with various sizes and densities based on the Volmer-Weber growth mode. Depending on the annealing temperature, the size including the average height and lateral diameter and the density of Au droplets show the opposite trend of increased size with correspondingly decreased density as a function of the annealing temperature due to the difference in the diffusion length of adatoms at varied activation energy. Under an identical growth condition, depending on the surface index, the size and density of Au droplets show a clear distinction, observed throughout the temperature range. The results are systematically analyzed and discussed in terms of atomic force microscopy (AFM) images, cross-sectional line profiles, and Fourier filter transform (FFT) power spectra as well as the summary plots of the size and density.     

A Study of the Activated Decomposition of CO2 on the Cu Component of a Cu/ZnO/Al2O3 Catalyst

The decomposition of CO₂ over the Cu component of two ZnO/Al₂O₃ supported Cu catalysts, having different Cu areas, has been studied over the temperature range 393–513 K. The time dependence of the evolution of CO from a CO₂/He stream (10% CO₂, 101 kPa) which was dosed continuously over the catalyst showed two peak maxima, the first of which moved to shorter times on raising the temperature. The activation energy for the decomposition of CO₂ on the ZnO/Al₂O₃ supported polycrystalline copper was obtained from a plot of the logarithm of the time to the peak maximum of the first peak against the reciprocal of the dosing temperature. The value so obtained was 83±10 kJ mol^-1 (catalyst A) and 86±10 kJ mol^-1 (catalyst B) for fresh catalysts reduced in H₂ at 513 K. This value fell to 49 ±4 kJ mol^-1 (catalyst A) and 55±5 kJ mol^-1 (catalyst B) after CO reduction at 473 K of the Cu which had been oxidised by the decomposition of the CO₂. This lowering of the activation energy for the second CO₂ decomposition is considered to be due to the original morphology of the Cu not being restored by reduction in CO after the oxygen-driven reconstruction of the Cu deriving from the decomposition of the CO₂.     

Pd deposited on Cu(110): a highly performant catalyst for the 1,3-butadiene hydrogenation reaction

The catalytic properties, with respect to the 1,3-butadiene hydrogenation reaction, of strained Pd films on Cu(110) (lattice mismatch 8%) has been probed as a function of the film thickness. The characterization of the adlayer has been made by the combined use of STM with LEED and AES. For deposits below 10¹⁵ Pd/cm² (i.e., about 1 ML) the catalytic activity is near zero. This is the consequence of the formation of a Pd–Cu surface alloy with tendency for Cu to migrate/segregate to the surface. The catalytic activity suddenly increases to reach a maximum value for about 3 ML; the activity is then one order of magnitude higher than that of the pure Pd(110) surface. This is the consequence of the presence of a strained Pd overlayer, with Pd surface atoms having very unusual geometry, and hence very peculiar electronic and chemical properties. The catalytic activity then decreases as the Pd coverage is increased, and tends to values near that of the pure Pd(110). Gradual relaxation of the film geometry towards that of the normal fcc Pd structure probably exists.     

A new technique for online visualization of the electrode surface under electrochemical corrosion processes

A new method for the online visualization of corrosion processes has been developed. It relates two techniques, electrochemical studies (potentiodynamic, potentiostatic, chronoamperometric etc.) and image processing. The main advantage of this method is that it is possible to relate the morphological changes of the electrode surfaces to the electrochemical signal measured without disturbing the electrochemical system. The online visualization technique is based on a horizontal electrochemical cell. The cell allows observation of the surface of the electrodes in a horizontal position by means of a triocular microscope–stereoscope assembled to an image acquisition system. The methodology was applied to the study and understanding of the surface changes of a copper and an AISI 316L electrode, respectively, when their potentiodynamic curves were taken in a lithium bromide solution. The corroded area of an AISI 316L electrode was determined by image processing in order to calculate the local current density in a potentiostatic test. The visualization method proposed can be used to gain a better understanding of electrochemical corrosion processes.     

The influence of soil properties on the effectiveness of phenylphosphorodiamidate (PPD) in reducing ammonia volatilization from surface applied urea

Urea can be an inefficient N source due to rapid hydrolysis by soil urease leading to NH₃ volatilization. The current study investigated the effect of the urease inhibitor phenylphosphorodiamidate (PPD) incorporated at two concentrations (0.5% and 1% w/w) within the fertilizer granule on NH₃ volatilization from surface applied urea. The daily rates of NH₃ loss from 20 soils of widely differing properties from Northern Ireland were measured over 14 days using ventilated enclosures under simulated spring conditions. Cumulative loss rates were calculated and fitted to a logistic model from which total NH₃ loss (Amax) and the time to maximum rate of loss (Tmax) were determined. Stepwise multiple linear regression analysis related the effectiveness of PPD in reducing NH₃ volatilization from urea to soil properties.The total cumulative loss of ammonia from unamended urea varied from 0.37 to 29.2% depending on soil type. Ammonia volatilization appeared to be greatest on a soil with a high pH (R² = 0.65), a low titratable acidity (TA) (R² = 0.63) and a soil that was drying out (R² = 0.50). Soil pH was negatively correlated with TA (r = –0.826, P < 0.001) suggesting that soils with a low TA may have received recent lime. Including cation exchange capacity (CEC) and % N as well as pH-KCl in the multiple linear regression equation explained 86% of the variance.The effectiveness of PPD in reducing Amax varied between 0% to 91% depending on soil type, the average over all 20 soils being 30 and 36% for 0.5% and 1% PPD respectively. The most important soil properties influencing the effectiveness of the urease inhibitor were soil pH-H₂O and TA accounting for 33% and 29% of the variance respectively. PPD was less effective on a soil with a high pH and low TA. These were the soil conditions that led to high NH₃ volatilization from unamended urea and may explain why PPD had limited success in reducing ammonia loss on these soils. Multiple linear regression analysis indicated that 75% of the variation in the % inhibition of NH₃ loss by PPD could be significantly accounted for by pH-H₂O, initial soil NO ₃ ^- -N concentration, % moisture content and % moisture loss.The delay in Tmax by PPD ranged from 0.19 to 7.93 days, the average over all 20 soils being 2.5 and 2.8 days for 0.5% and 1% PPD respectively. TA, % moisture content, urease activity and CEC were soil properties that significantly explained 83% of the variation in the % delay in Tmax by PPD in multiple linear regression analysis. However, none of these soil properties were significant on their own. As urea hydrolysis occurs rapidly in soil, delaying Tmax under field conditions would increase the chance of rain falling to move the urea below the soil surface and reduce NH₃ volatilization. A urease inhibitor should be more effective than PPD on soils with a high pH and low TA to be successful in reducing high NH₃ losses.