ABSTRACT
The formation and atom distribution in two-dimensional Pd(x)Ag(1-x)/Pd(111) monolayer surface alloys were studied by high resolution scanning tunnelling microscopy (STM) with chemical contrast. From short-range order (SRO) parameters, we calculate preferences for like or unlike nearest neighbours to elucidate the mixing behaviour of the two components for various sub monolayer Ag surface contents. In the regime of low Ag surface contents (<40% Ag), the system shows a weak tendency towards phase separation, high Ag coverages (>60% Ag) result in a disperse distribution of the atoms in the surface. Effective pair interactions (EPIs) were derived by comparing the measured distribution with distributions obtained using Monte Carlo (MC) simulations. From the EPIs, we derived a function for the mixing energy, which can describe the change from clustering to a disperse distribution. The effects of the resulting surface atom distributions and of the Ag coverage dependent surface mixing/demixing on catalytic reactions are discussed.
ABSTRACT
We investigated the electrochemical oxidation and reduction processes on ultrahigh vacuum prepared, smooth and structurally well-characterized Ru(0001) electrodes in a CO-saturated and, for comparison, in a CO-free flowing perchloric acid electrolyte by electrochemical methods and by comparison with previous structural data. Structure and reactivity of the adsorbed layers are largely governed by a critical potential of E = 0.57 V, which determines the onset of O(ad) formation on the CO(ad) saturated surface in the positive-going scan and of O(ad) reduction in the negative-going scan. O(ad) formation proceeds via nucleation and 2D growth of high-coverage O(ad) islands in a surrounding CO(ad) phase, and it is connected with CO(ad) oxidation at the interface between the two phases. In the negative-going scan, mixed (CO(ad) + O(ad)) phases, most likely a (2 × 2)-(CO + 2O) and a (2×2)-(2CO + O), are proposed to form at E < 0.57 V by reduction of the O(ad)-rich islands and CO adsorption into the resulting lower-density O(ad) structures. CO bulk oxidation rates in the potential range E > 0.57 V are low, but significantly higher than those observed during oxidation of pre-adsorbed CO in the CO-free electrolyte. We relate this to high local CO(ad) coverages due to CO adsorption in the CO-saturated electrolyte, which lowers the CO adsorption energy and thus the barrier for CO(ad) oxidation during CO bulk oxidation.
ABSTRACT
The formation of PtRu surface alloys by deposition of submonolayer Pt films on a Ru(0001) substrate and subsequent annealing to about 1350 K and the distribution of the Pt atoms in the surface layer were investigated by scanning tunneling microscopy. Quantitative statistical analysis reveals (i) negligible losses of Pt into subsurface regions up to coverages close below 1 monolayer, (ii) a homogeneous distribution of the Pt atoms over the surface, and (iii) the absence of a distinct long-range or short-range order in the surface layer. In addition, the density of specific adsorption ensembles is analyzed as a function of Pt surface content. Possible conclusions on the process for surface alloy formation are discussed. The results are compared with the properties of PtRu bulk alloys and the findings in previous adsorption studies on similar surface alloys (H. Rauscher, T. Hager, T. Diemant, H. Hoster, F. Bautier de Mongeot and R. J. Behm, Surf. Sci., 2007, 601, 4608; T. Diemant, H Rauscher and R. J. Behm, J. Phys. Chem. C, in press).
Subject(s)
Alloys/chemistry , Platinum/chemistry , Ruthenium/chemistry , Adsorption , Alloys/chemical synthesis , Carbon Monoxide/chemistry , Computer Simulation , Deuterium/chemistry , Hot Temperature , Microscopy, Energy-Filtering Transmission Electron , Microscopy, Scanning Tunneling , Models, Molecular , Monte Carlo Method , Surface Properties , ThermodynamicsABSTRACT
The energetics determining the distinct short-range order in two-dimensional (2D) monolayer Cu(x)Pd(1-x) surface alloys on a Ru(0001) substrate were investigated by Monte Carlo simulations and density functional theory calculations. Using a 2D lattice gas Hamiltonian based on effective pair interaction (EPI) parameters, the EPIs were derived for different Cu concentrations with Monte Carlo (MC) simulations by comparing with the atomic distributions obtained from atomic resolution STM images and the related Warren-Cowley short-range order parameters (Hoster et al., Phys. Rev. B, 2006, 73 165413). The ground state structures and mixing energies at 0 K derived from these EPIs agree well with mixing energies determined from DFT calculations of different ordered surface alloys. Additional MC simulations yield rather low transition temperatures which explain the absence of ordered 2D phases in the experiments. The consequences of our findings for the use of alloy surfaces and surface alloys as model systems for adsorption and catalytic reaction studies are discussed.
