Wetting and CO Oxidation on Transition Metal and Bimetallic Surfaces

The oxidation of carbon monoxide has been investigated in two different systems, Au alloyed with Pd(111) at room temperature and Cu(110) at 90 to 170 K. All experiments were conducted under UHV conditions using mass spectrometry and temperature-programmed desorption to follow the reaction. For the Au/Pd(111) system the intention was to improve the catalytic activity of the pure Pd(111) crystal by alloying with Au, as bimetallic alloys often display enhanced catalytic activities compared to that of the pure metal due to a combination of both ensemble and ligand effects. The ensemble effects being the arrangement of the active atoms and the ligand effects the electronic modifications or charge transfer associated with these active atoms. This was carried out by vapour deposition of various amounts Au on the Pd(111) crystal and annealing to form a surface alloy. Enhanced catalytic activities of up to 27 % was achieved by alloying with Au coverages of 0.2 0.4 ML on the Pd(111) substrate followed by annealing to temperatures 650 – 750 K compared to that of the pure Pd(111) surface. For the Cu(110) system we aimed to assess the catalytic activity of Cu, the objective being to determine the exact kinetic mechanism of the oxidation reaction at 90 to 170 K. This was done initially by pre-adsorbing O2 on the Cu(110) substrate surface at various coverage, followed by CO and monitoring for CO2 production. The experiment was then reversed with CO pre-dosed followed by O2. The catalytic oxidation reaction was found to proceed by a hot-precursor mechanism when O2 was dosed on a CO pre-covered surface above 130 K. Below this temperature any O2 dosed prior to or after the CO uptake would tend to physisorb molecularly and only reacted during dissociation while the O atom remained in a reactive transient. Here, the atomic O reacted directly with adsorbed molecule CO transiently prior to adsorption to the Cu substrate a catalytic reaction akin to that of an Eley-Rideal mechanism. We have studied the wetting properties, autocatalytic dissociative adsorption and the reactivity of the hydroxyl species originating from the co-adsorption of water and oxygen on Cu(110) at 200 to 300 K. Experiments were conducted by two separate methods; using molecular beam uptakes and by varying the static H2O pressures from 1.0 x 10 11 up to 5.0 x 10 7 mbar in conjunction with LEED and TPD. It has been hypothesized that the strong attractive interaction between mixed OH and H2O slows down the desorption kinetics of H2O sufficiently to produce H2O-OH structures that are stable up to RT. As a result the main focal point of the research was to gain some insight into the bonding mechanisms at elevated water pressures and ambient temperatures, whilst assessing the stability of the H2O-OH species. In addition to the 1H2O:1OH chains structures and dimers observed previous at these temperatures we also observed an additional stable structure during the TPD experiments. This structure appeared particularly when H2O was adsorbed with low (0.06 ML) atomic O coverages and became increasingly prominent when the uptake temperature was increased from 200 up to 240 K. The origin of this structure is discussed in the context of the structure proposed by earlier investigators