Charles T. Campbell

Ultrathin metal films and particles on oxide surfaces: structural, electronic and chemisorptive properties

Abstract Ultrathin metal films on clean and well-defined oxide surfaces have been prepared by several groups using vapor deposition techniques in ultrahigh vacuum, and the structural, electronic and chemisorptive properties of these films have been characterized using a variety of surface science techniques. Those studies will be reviewed here, and trends in these properties will be identified. While films of mid- or late-transition metals can sometimes be grown in a quasi-layer-by-layer fashion at very low temperatures where kinetics prevail, heating these usually leads to thickening into the thermodynamically preferred structure: three-dimensional metal particles that cover only a fraction of the oxide surface. This occurs in two stages: individual island thickening, then Ostwald ripening. The kinetics of growth, nucleation and thickening will be examined. In some cases, this thermodynamic preference can be shifted to favor complete spreading of the metal film by adding gas molecules. This is driven by the higher adsorption energy of the molecule on the metal sites. Early transition metals which have very stable oxides can partially reduce the substrate oxide, and themselves become oxidized. The chemisorption properties of metal overlayers correlate well with their structural and electronic properties. Sites formed by neutral metal adatoms in these films on oxide surfaces often have chemisorption properties resembling that of some bulk metal crystalline plane. Surprisingly, this is even true for metal islands that are only one atom thick, in which case they resemble a very open or coordinatively unsaturated plane. However, the sites on and in the nearby oxide can alter the final surface chemistry, because adsorbates, especially hydrogen adatoms, can diffuse rapidly from the metal particles to these sites (spillover). While some real advances have already been made, many fundamental questions which are of great importance in oxide-supported metal catalysis and other fields remain to be addressed with this approach.

A molecular beam study of the catalytic oxidation of CO on a Pt(111) surface

The oxidation of carbon monoxide catalyzed by Pt(111) was studied in ultrahigh vacuum using reactive molecular beam–surface scattering. Under all conditions studied, the reaction follows a Langmuir–Hinshelwood mechanism: the combination of a chemisorbed CO molecule and an oxygen adatom. When both reactants are at low coverage, the reaction proceeds with an activation energy E*LH =24.1 kcal/mole and a pre‐exponential υ4 =0.11 cm2 particles−1 sec−1. At very high oxygen coverage, E*LH decreases to about 11.7 kcal/mole and υ4 to about 2×10−6 cm2 particles−1 sec−1. This is largely attributed to the corresponding increase in the energy of the adsorbed reactants. When a CO molecule incident from the gas phase strikes the surface presaturated with oxygen, it enters a weakly held precursor state to chemisorption. Desorption from this state causes a decrease in chemisorption probability with temperature. Once chemisorbed, the CO molecule then has almost unit probability of reacting to produce CO2 below 540 K. The C...

Ceria Maintains Smaller Metal Catalyst Particles by Strong Metal-Support Bonding

Keeping Nanoparticles Small Heterogeneous catalysts that consist of small metal nanoparticles absorbed on oxide supports can deactivate over time through a process called sintering. Elevated temperatures increase the rate of diffusion of metal atoms over the support, and larger, less reactive particles grow at the expense of smaller ones. Some supports that contain reducible metals, such as cerium oxide, tend to resist sintering better than oxides such as alumina. Farmer and Campbell (p. 933) present an analysis of previous calorimetry data for silver nanoparticles on magnesium oxides and cerium oxide surfaces and show that nanoparticles smaller than 1000 atoms are bound much more strongly to reduced cerium oxide. The energetic driving force for creating larger particles is quite low on these surfaces and increases the lifetime of smaller particles. The stability of cerium oxide surfaces allows deposited silver nanoparticles to maintain a small size distribution. The energies of silver (Ag) atoms in Ag nanoparticles supported on different cerium and magnesium oxide surfaces, determined from previous calorimetric measurements of metal adsorption energies, were analyzed with respect to particle size. Their stability was found to increase with particle size below 5000 atoms per particle. Silver nanoparticles of any given size below 1000 atoms had much higher stability (30 to 70 kilojoules per mole of silver atoms) on reduced CeO2(111) than on MgO(100). This effect is the result of the very large adhesion energy (~2.3 joules per square meter) of Ag nanoparticles to reduced CeO2(111), which we found to be a result of strong bonding to both defects and CeO2(111) terraces, apparently localized by lattice strain. These results explain the unusual sinter resistance of late transition metal catalysts when supported on ceria.

A molecular beam study of the adsorption and desorption of oxygen from a Pt(111) surface

Abstract The adsorption and desorption of O 2 on a Pt(111) surface have been studied using molecular beam/surface scattering techniques, in combination with AES and LEED for surface characterization. Dissociative adsorption occurs with an initial sticking probability which decreases from 0.06 at 300 K to 0.025 at 600 K. These results indicate that adsorption occurs through a weakly-held state, which is also supported by a diffuse fraction seen in the angular distribution of scattered O 2 flux. Predominately specular scattering, however, indicates that failure to stick is largely related to failure to accommodate in the molecular adsorption state. Thermal desorption results can be fit by a desorption rate constant with pre-exponential ν d = 2.4 × 10 −2 cm 2 s −1 and activation energy E D which decreases from 51 to 42 kcal/mole −1 with increasing coverage. A forward peaking of the angular distribution of desorbing O 2 flux suggests that part of the adsorbed oxygen atoms combine and are ejected from the surface without fully accomodating in the molecular adsorption state. A slight dependance of the dissociative sticking probability upon the angle of beam incidence further supports this contention.

Atomic and molecular oxygen adsorption on Ag(111)

Abstract The adsorption of O 2 on Ag(111) between 150 and 650 K has been studied with thermal desorption spectroscopy, Auger and photoelectron spectroscopies, and low-energy electron diffraction. A molecularly adsorbed O 2 species is populated with extremely low sticking probability (~ 5 X 10 −6 ) at 150 K. This species desorbs, with little dissociation, at 217 K. An atomically adsorbed species, with an O(1s) BE of 528.2 eV, is populated at 490 K with a sticking probability near 10 −6 . This species exists in islands of local coverage θ O ≅ 0.41, displaying a p(4 X 4)-O LEED pattern. It associatively desorbs at 579 K as O 2 , and can be titrated at room temperature with CO to produce CO 2 . There is also evidence for a subsurface oxygen species which reactivates below 600 K. Surface carbonate (CO 3,a ) can be produced from O a and CO 2 gas. These results are compared with similar species on Ag(110). A kinetic model is developed which describes the interaction of O 2 with these surfaces over a broad range of temperatures, and provides energetic values for the O 2 /Ag interaction potential.

The interaction of H2O with a TiO2(110) surface

Abstract The interaction of H2O with rutile TiO2(110) surfaces with different defect densities (oxygen vacancies) was studied with TPD, work function measurements and XPS. On the nearly perfect surface, a thermal desorption peak is observed at 250–300 K which is attributed to molecularly adsorbed H2O at Ti4+ sites based on its O(1s) peak position and work function change. The heat of adsorption of water in this state is estimated to be 71-9θ kJ/mol. The coverage of water in this state is estimated from O(1s) signals to be about one per unit cell, or one for every Ti4+ site. A tail of this peak which extends to 375 K is attributed to disproportionation of surface hydroxyl groups present in lower concentrations. Higher coverages of water give rise to a TPD peak at 170 K, which we attribute to water bound to bridging oxygen anion sites. Finally, multilayer water is populated, which desorbs in a peak at 160 K. Surface hydroxyls bound to thermally-induced oxygen vacancies of ∼ 1% concentration disproportionate to give a water TPD peak at ∼ 500 K.

The interactions of O2, CO and CO2 with Ag(110)

The interactions and reactions of O2, CO and CO2 with Ag(110) have been studied with AES, XPS, TDS and LEED. Three states of adsorbed oxygen, molecularly adsorbed, atomically adsorbed, and an unreactive (probably subsurface) form, are characterized by O(1s)-XPS peaks at 529.3, 528.1 and 528.5 eV BE, respectively. By dosing at 50 Torr O2 and 485 K and transferring rapidly (∼ 17 s) back into UHV, a coverage θO=0.67 of atomically adsorbed oxygen could be achieved. This gave a new c(6 × 2)-O overlayer structure in LEED and a new, lower-temperature thermal desorption peak of O2 at 565 K. The reaction of CO gas with this state proceeded with a reaction probability of about 0.05 at room temperature to produce CO2gas, until the coverage reached θO=0.5. Below this coverage, the well-known p(2 × 1)-O LEED pattern appeared and the reaction probability for CO dropped suddenly by a factor of about five. Gaseous CO2 reacted in 1-to-1 stoichiometry with oxygen adatoms to produce surface carbonate, CO3,a, which was characterized by a C(1s) peak at 287.7 eV and a broadened O(1s) peak at 529.9 eV BE. The XPS data for O2,a and CO3,a are in agreement with structures for these species which were suggested by earlier vibrational analysis.

A molecular beam investigation of the interactions of CO with a Pt(111) surface

Abstract Carbon monoxide adsorbs on clean Pt(111) with an angular and temperature-independent sticking coefficient of 0.84 ± 0.05. The coverage-dependence of the adsorption rate can be fitted by a precursor state model, where a weak adsorption state for CO existing in the presence of pre-adsorbed CO plays the major role. Thermal desorption shows two peaks, one due to CO adsorbed on terraces and a high temperature shoulder due to CO on step or defect sites. The desorption rate varies as cos γr, where γr is the angle from the surface normal. Modulated molecular beam measurements sample solely the lifetime of CO adsorbed on the step/defect sites due to the high mobility of CO across the surface. Desorption from these sites occurs with frequency factor, νd = 1.25 × 1015s−1, and activation energy, Ed = 34.9 kcal/mole.

A kinetic model of the water gas shift reaction

A kinetic model of the water gas shift reaction based on a description of its elementary steps at the atomic level is presented. Input data for elementary steps are taken from available single crystal studies. The model is successfully tested against kinetic data for a working Cu-based catalyst. Expressions are derived for the activation energy and reaction orders.

Catalyst–support interactions: Electronic perturbations

Oxide materials typically used as supports for the active metal nanoparticles of heterogeneous catalysts are known to influence catalytic activity through strong metal–support interactions. Researchers have now revealed electronic interactions between platinum and ceria that go well beyond known effects and lead to excellent catalytic activity.