M. Valden

Structure sensitivity of CO oxidation over model Au/TiO22 catalysts

Model catalysts of Au clusters supported on TiO2 thin films were prepared under ultra-high vacuum (UHV) conditions with average metal cluster sizes that varied from ~2.5 to ~6.0 nm. The reactivities of these Au/TiO2 catalysts were measured for CO oxidation at a total pressure of 40 Torr in a reactor contiguous to the surface analysis chamber. Catalyst structure and composition were monitored with Auger electron spectroscopy (AES) and scanning tunneling microscopy and spectroscopy (STM/STS). The apparent activation energy for the reaction between 350 and 450 K varied from 1.7 to 5 kcal/mol as the Au coverage was increased from 0.25 to 5 monolayers, corresponding to average cluster diameters of 2.5–6.0 nm. The specific rates of reaction ((product molecules) × (surface site)-1 × s-1 were dependent on the Au cluster size with a maximum occurring at 3.2 nm suggesting that CO oxidation over Au/TiO2(001)/Mo(100) is structure sensitive.

Scanning tunneling microscopy studies of metal clusters supported on TiO2 (110): Morphology and electronic structure

Abstract A brief review of our laboratorys recent scanning tunneling microscopy (STM) studies on nanoclusters supported on TiO2(110) is presented. Particular emphasis is placed on the system Au TiO 2 (110) . The nucleation and growth of the clusters, which were vapor-deposited on TiO2(110) under ultra high vacuum (UHV) conditions, were investigated using STM. It was found that Au, Pd, and Ag clusters all grow in a three-dimensional (3D) (Volmer-Weber) fashion on TiO2(110), but that at low coverages, quasi-two dimensional (quasi-2D) Au and Pd clusters were observed. These quasi-2D clusters are characterized by heights of 1–2 atomic layers. Annealing studies show that Au and Pd clusters form large microcrystals with well-defined hexagonal shapes. Al clusters, which have a strong interaction with the substrate, are oxidized upon deposition, “wetting” the surface and forming small clusters. In addition to the topographic studies, the local electronic properties of these clusters have been studied using scanning tunneling spectroscopy (STS) to measure the cluster band gaps. The electronic structure was found to be cluster size-dependent, as seen by the appearance of a band gap as the cluster size decreased. More specifically, the onset of cluster metallic properties correlates with the transition from quasi-2D to 3D cluster growth.

Influence of preadsorbed oxygen on activated chemisorption of methane on Pd(110)

Dissociative chemisorption of methane on clean and oxygen modified Pd(110) has been studied by using molecular beam surface scattering. The absolute dissociation probability of CH4 (Stot) is found to increase exponentially with the incident normal energy (En) of CH4 and with surface temperature (TS) on clean Pd(110). The kinetic isotope effect is also found; namely, Stot of CD4 is 4 to 5 times smaller than Stot of CH4 throughout the entire range of En studied. These results are consistent with a direct dissociation mechanism. Measurements on preadsorbed oxygen on Pd(110) show that Stot of CH4 decreases linearly, as oxygen coverage is increased from 0 to 0.4 ML in good agreement with the first-order Langmuir kinetics when approximately two active sites are blocked by one oxygen atom. No influence of the oxygen induced surface reconstructions on the dissociative adsorption kinetics of CH4 is observed.

Adsorption of Benzotriazole on the Surface of Copper Alloys Studied by SECM and XPS

The formation of inhibitive benzotriazole films on copper and copper alloyed with silver has been studied with scanning electrochemical microscopy SECM and X-ray photoelectron spectroscopy XPS. The SECM measurements showed that the film grows rapidly on oxygen-free dehydrated copper OF-HC and that the rate depends on the substrate potential. The benzotriazole films were also observed on silver alloyed copper; however, electropolishing prevents film growth by enriching the surface with silver. XPS results show that the film consisted of CuI–BTA on both materials. No evidence of AgI–BTA was found.

Dissociative chemisorption of methane on clean and oxygen precovered Pt(111)

Effects of adsorbed oxygen on dissociative chemisorption of methane molecules on a platinum (111) surface have been studied by using molecular beam surface scattering. It has been found that for an oxygen precoverage of 0.42 ML on Pt(111), and for the clean surface, an increase in translational energy (En) of CH4 and in surface temperature (Ts) of the sample strongly enhance the methane activation in the range of parameters studied, 60 < En < 125 kJ/mol and 300 K < Ts < 750 K. However, the absolute dissociation probability of CH4 is always much smaller for the oxygen precovered surface than for the clean surface. Clearly, oxygen poisons the CH activation. This poisoning of the surface implies that oxygen sterically blocks the active surface sites for dissociative chemisorption of CH4 or it inhibits the dissociation electronically. Despite of surface poisoning, the CH4 activation mechanism is probably the same for the clean and oxygen covered Pt(111) surfaces.

Oxygen adsorption-induced nanostructures and island formation on Cu{100}: Bridging the gap between the formation of surface confined oxygen chemisorption layer and oxide formation

Surface oxidation of Cu(100) has been investigated by variable temperature scanning tunneling microscopy and quantitative x-ray photoelectron spectroscopy as a function of O(2) pressure (8.0x10(-7) and 3.7x10(-2) mbar) at 373 K. Three distinct phases in the initial oxidation of Cu(100) have been observed: (1) the formation of the mixed oxygen chemisorption layer consisting of Cu(100)-c(2x2)-O and Cu(100)-(2sqrt[2]xsqrt[2])R45 degrees -O domains, (2) the growth of well-ordered (2sqrt[2]xsqrt[2])R45 degrees-O islands, and (3) the onset of subsurface oxide formation leading to the growth of disordered Cu(2)O. We demonstrate that the (2sqrt[2]xsqrt[2])R45 degrees-O reconstruction is relatively inert in the low pressure regime. The nucleation and growth of well-ordered two-dimensional Cu-O islands between two (2sqrt[2]xsqrt[2])R45 degrees-O domains is revealed by time-resolved scanning tunneling microscopy experiments up to 0.5 ML of oxygen. The formation of these islands and their nanostructure appear to be critical to the onset of further migration of oxygen atoms deeper into copper and subsequent Cu(2)O formation in the high pressure regime. The reactivity of each phase is correlated with the surface morphology and the role of the various island structures in the oxide growth is discussed.

Nanoscale oxidation of Cu(100): Oxide morphology and surface reactivity

Surface oxidation of Cu(100) in O(2) has been investigated in situ by x-ray photoelectron spectroscopy, x-ray induced Auger electron spectroscopy (XAES), and scanning tunneling microscopy (STM) as a function of surface temperature (T(S)=303-423 K) and O(2) pressure (p(O(2) )=3.7 x 10(-2)-213 mbars). Morphology of the oxide on the surface and in the near surface layers is characterized by utilizing STM and the inelastic electron background of the XAES O KLL signal. Analysis of the peak shape of the XAES Cu LMM facilitates the quantification of Cu, Cu(2)O, and CuO surface concentrations. The authors conclude that the surface oxidation of Cu(100) proceeds in three distinct steps: (1) Dissociative adsorption of O(2) and the onset of Cu-(2 square root 2 x square root 2)R45 degrees -O (theta(O)=0.5 ML) surface reconstruction, (2) initial formation of Cu(2)O and the appearance of 1.8 A high elongated islands that also adopt the Cu-(2 square root 2 x square root 2)R45 degrees -O structure, and (3) formation of highly corrugated Cu-O islands which together with the surface reconstruction strongly enhance the reactivity of the surface towards further oxide formation. Both Cu(2)O and CuO formations are enhanced by increased surface temperature, but no pressure dependence can be seen.

Instrumentation and analytical methods of an x-ray photoelectron spectroscopy–scanning tunneling microscopy surface analysis system for studying nanostructured materials

The design and performance of an x-ray photoelectron spectroscopy (XPS)–scanning tunneling microscopy (STM) surface analysis system for studying nanostructured materials are described. The analysis system features electron spectroscopy methods (XPS and Auger electron spectroscopy) in addition to a variable temperature STM. With the analytical methods of the system, surface chemical analysis as well as surface morphology down to atomic resolution can be obtained. The system also provides facilities for sample cleaning, annealing, gas dosing, depth profiling, and surface modifications by sputtering and evaporation. Controlled gas exposures from ultrahigh vacuum to atmospheric pressures in the adjustable temperature range of 120–1100K can be carried out in different chambers. A fast entry air lock allows the transfer of samples and STM tips into the system without air exposures. The surface analysis system uses a common sample holder in all five chambers which are independently pumped and separated from each...

Activated adsorption of methane on clean and oxygen-modified Pt{111} and Pd{110}

Abstract Activated adsorption of CH 4 on clean and oxygen modified Pt{111} and Pd{110} has been studied using molecular beam surface scattering. The absolute dissociation probability of CH 4 was measured as a function of the incident normal energy ( E ) and the surface temperature ( T s ). The results from clean Pt{111} and Pd{110} are consistent with a direct dissociation mechanism. The dissociative chemisorption dynamics of CH 4 is addressed by using quantum mechanical and statistical models. The influence of adsorbed oxygen on the dissociative adsorption of CH 4 on both Pt{111} and Pd{110} shows that the dissociation probability decreases linearly with increasing oxygen coverage.

Molecular beam studies of CO oxidation and CO-NO reactions on a supported Pd catalyst

Abstract The surface reactivity of highly dispersed (85%) palladium cluster catalysts supported on γ-alumina (1.5 wt% loading) had been probed using the molecular beam scattering technique. This work illustrates the utility of molecular beam sources to study reaction kinetics on highly dispersed catalyst surfaces under UHV conditions. While CO adsorbs and desorbs molecularly throughout the coverage range, the Pd particles have been found to be extremely active with respect to NO dissociation forming a “mixed” layer of molecularly bound NO and atomic nitrogen and oxygen. Reaction kinetics of CO oxidation has been studied as a function of surface temperature by reaction of a pre-adsorbed oxygen layer with a CO molecular beam. A maximum reaction rate and percentage conversion to CO 2 was found to occur between 570 and 620 K. In contrast to previous studies of model Pd cluster catalysts, the highly dispersed Pd clusters have been found to be active for the CO/NO reaction probed by reaction of a CO beam with a NO pre-adsorbed layer.