Bruce E. Koel

Study of high coverages of atomic oxygen on the Pt(111) surface

Atomic oxygen coverages of up to 0.75 ML may be adsorbed cleanly on Pt(111) surfaces under UHV conditions by exposure to NO2 at 400 K. We have studied this adsorbed oxygen layer by using AES, LEED, UPS, HREELS, TPD, and work function (ΔΦ) measurements. The (2 × 2)-O structure formed at θO = 0.25 ML is still apparent at θO = 0.60 ML and a faint (2 × 2) pattern persists even up to θO = 0.75 ML. AES and ΔΦ measurements show no evidence for chemically distinct species in the adlayer as a function of oxygen coverage. HREELS spectra clearly rule out the presence of molecular oxygen and oxide species over the range of oxygen coverage studied. UPS also shows no shift in binding energy of the oxygen-derived peak as the coverage is increased. These spectroscopic probes indicate that all oxygen is present as atomic oxygen with no indication of oxide formation or molecular oxygen at any coverage. Multiple O2 desorption peaks observed in TPD are interpreted as arising largely from kinetic effects rather than a result of multiple, distinctly different chemical species, even though large changes in the Pt-O bond energy are determined from the TPD data. The activation energy for O2 desorption reflects the sum of the heat of dissociative adsorption of O2 and the activation energy for O2 dissociation. The structure in the O2 TPD spectrum is due to large changes in the activation energy for O2 desorption resulting from increases in the barrier to dissociative O2 chemisorption and decreases in the Pt-O bond energy. These barriers arise from strong repulsive interactions between adsorbed oxygen adatoms that cause sharp reductions in the Pt-O bond strength at these coverages. Finally, we note that our spectroscopic probes are quite insensitive to the changes in the Pt-O bond strength over the entire range of oxygen coverage studied.

X-Ray photoelectron study of the reaction of oxygen with cerium☆

Abstract The interaction of D2 O with a polycrystalline cerium surface, successfully cleaned by heavy Ar+ bombardment and annealing, was studied at 120 were observed at BE 530.3 (Ce2O3) and 532.7eV (adsorbed OD). When clean Ce at 120 K was exposed to D2 O, the O(1s) spectra were initial eV (adsorbed D2 O). For exposures greater than 10 Langmuir (L), a multilayer of ice grows and the O(1s) spectra become dominated by a peak at 5 The results of interaction with D2 O are compared with oxidation by O2. The significant differences are: (1) the absence of Ce(IV) when oxidati relatively small extent of oxidation that occurs when Ce is exposed to D2 O at 120 K, and (3) the larger chemical-shift of the Ce(III)-derived spec The XPS studies of the interaction of D2 O with Ce reported here may be summarized as follows: (1) Exposure at 300 K gives rise to O(1s) features characteristic of oxide and hydroxide, while the Ce(3d) spectra indicate Ce(III), but no CE( (2) Exposure at 120 K gives O(1s) features characteristic of adsorbed OD, chemisorbed D2 O, a multilayer of ice, and a small amount of oxide. T are characteristic of clean Ce except for slight broadening. (3) Exposure at 120 K followed by warming to 240 and 300 K gives spectra characteristic of hydroxide and oxide surface-species. Between 240 and 300 K, O(1s) intensity. (4) At 300 K, a relatively thick layer of oxide forms, and after an exposure of 50 L the features characteristic of metallic Ce are no longer observabl (5) As compared to the case for O2, exposure to D2 O gives rise to different satellite-splittings in the Ce(3d) spectra, suggesting that di formed in the two cases. (6) The spectra observed for Ce exposed to D2 O are in excellent accord with those found for the heavier lanthanides [4].

Improving Electrocatalysts for O2 Reduction by Fine-Tuning the Pt−Support Interaction: Pt Monolayer on the Surfaces of a Pd3Fe(111) Single-Crystal Alloy

We improved the effectiveness of Pt monolayer electrocatalysts for the oxygen-reduction reaction (ORR) using a novel approach to fine-tuning the Pt monolayer interaction with its support, exemplified by an annealed Pd(3)Fe(111) single-crystal alloy support having a segregated Pd layer. Low-energy ion scattering and low-energy electron diffraction studies revealed that a segregated Pd layer, with the same structure as Pd (111), is formed on the surface of high-temperature-annealed Pd(3)Fe(111). This Pd layer is considerably more active than Pd(111); its ORR kinetics is comparable to that of a Pt(111) surface. The enhanced catalytic activity of the segregated Pd layer compared to that of bulk Pd apparently reflects the modification of Pd surfaces electronic properties by underlying Fe. The Pd(3)Fe(111) suffers a large loss in ORR activity when the subsurface Fe is depleted by potential cycling (i.e., repeated excursions to high potentials in acid solutions). The Pd(3)Fe(111) surface is an excellent substrate for a Pt monolayer ORR catalyst, as verified by its enhanced ORR kinetics on PT(ML)/Pd/Pd(3)Fe(111). Our density functional theory studies suggest that the observed enhancement of ORR activity originates mainly from the destabilization of OH binding and the decreased Pt-OH coverage on the Pt/Pd/Pd(3)Fe(111) surface. The activity of Pt(ML)/Pd(111) and Pt(111) is limited by OH removal, whereas the activity of Pt(ML)/Pd/Pd(3)Fe(111) is limited by the O-O bond scission, which places these two surfaces on the two sides of the volcano plot.

Iron nanoparticles for environmental clean-up: recent developments and future outlook

Nanoscale zero-valent iron (nZVI) is one of the most extensively applied nanomaterials for groundwater and hazardous waste treatment. In the past fifteen years, progress made in several key areas has deepened our understanding of the merits and uncertainties of nZVI-based remediation applications. These areas include the materials chemistry of nZVI in its simple and modified forms, the nZVI reactivity with a wide spectrum of contaminants in addition to the well-documented chlorinated solvents, methods to enhance the colloidal stability and transport properties of nZVI in porous media, and the effects of nZVI amendment on the biogeochemical environment. This review aims to provide an up-to-date account of advancement in these areas as well as insights gained through field experience.

Determination of the Oxide Layer Thickness in Core−Shell Zerovalent Iron Nanoparticles

Zerovalent iron (nZVI) nanoparticles have long been used in the electronic and chemical industries due to their magnetic and catalytic properties. Increasingly, applications of nZVI have also been reported in environmental engineering because of their ability to degrade a wide variety of toxic pollutants in soil and water. It is generally assumed that nZVI has a core-shell morphology with zerovalent iron as the core and iron oxide/hydroxide in the shell. This study presents a detailed characterization of the nZVI shell thickness using three independent methods. High-resolution transmission electron microscopy analysis provides direct evidence of the core-shell structure and indicates that the shell thickness of fresh nZVI was predominantly in the range of 2-4 nm. The shell thickness was also determined from high-resolution X-ray photoelectron spectroscopy (HR-XPS) analysis through comparison of the relative integrated intensities of metallic and oxidized iron with a geometric correction applied to account for the curved overlayer. The XPS analysis yielded an average shell thickness in the range of 2.3-2.8 nm. Finally, complete oxidation reaction of the nZVI particles by Cu(II) was used as an indication of the zerovalent iron content of the particles, and these observations further correlate the chemical reactivity of the particles and their shell thicknesses. The three methods yielded remarkably similar results, providing a reliable determination of the shell thickness, which fills an essential gap in our knowledge about the nZVI structure. The methods presented in this work can also be applied to the study of the aging process of nZVI and may also prove useful for the measurement and characterization of other metallic nanoparticles.

Nanoparticle manipulation by mechanical pushing: underlying phenomena and real-time monitoring

Experimental results that provide new insights into nanomanipulation phenomena are presented. Reliable and accurate positioning of colloidal nanoparticles on a surface is achieved by pushing them with the tip of an atomic force microscope under control of software that compensates for instrument errors. Mechanical pushing operations can be monitored in real time by acquiring simultaneously the cantilever deflection and the feedback signal (cantilever non-contact vibration amplitude). Understanding of the underlying phenomena and real-time monitoring of the operations are important for the design of strategies and control software to manipulate nanoparticles automatically. Manipulation by pushing can be accomplished in a variety of environments and materials. The resulting patterns of nanoparticles have many potential applications, from high-density data storage to single-electron electronics, and prototyping and fabrication of nanoelectromechanical systems.

Interaction of oxygen with Pd(111): High effective O2 pressure conditions by using nitrogen dioxide

Abstract High effective O 2 pressures can be simulated by using NO 2 as a source of atomic oxygen on Pd(111) at exposure temperatures at or above 530 K. Large oxygen concentrations ( θ O =0.0–3.1 ML) have been studied using TPD, AES, HREELS, and XPS. TPD results show four O 2 desorption peaks at 625, 715, 765, and 800 K. The low temperature desorption states have never been reported in previous work concerning oxygen adsorption on Pd(111). Upon heating an atomic oxygen adlayer, diffusion of oxygen into the near surface region is competitive with O 2 desorption for θ O =0.76–1.4 ML. For θ O > 1.4 ML, spectroscopic evidence shows the presence of an oxide on or near the surface. Our results for the interaction of atomic oxygen with Pd(111) are compared with the behavior of atomic hydrogen on Pd(111). Finally, a model for the chemical state and desorption behavior of oxygen for these coverages is proposed.

Hydrogenation and H, D exchange studies of ethylidyne (CCH3) on Rh(111) crystal surfaces at 1 ATM pressure using high resolution electron energy loss spectroscopy

Abstract We have combined high resolution electron energy loss spectroscopy (HREELS) with a high-pressure/low-pressure (HPLP) system to study the behavior of the monolayer structure of stable hydrocarbon species that form on single-crystal metal surfaces during catalytic reactions at atmospheric pressure. We find that a monolayer of adsorbed ethylidyne (CCH 3 ) on Rh(111) at 310 K does not hydrogenate to ethylene or ethane in one atmosphere of static D 2 . The methyl group hydrogens exchange with deuterium is a slow process. The amount of exchange depends strongly on the amount of uncovered, bare-metal surface, but little on the hydrogen pressure. A mechanism for H, D exchange involving ethylidene (CHCH 3 ) as an intermediate is proposed.

The molecular adsorption of nitrogen dioxide on Pt(111) studied by temperature programmed desorption and vibrational spectroscopy

The adsorption of nitrogen dioxide (NO 2 ) on Pt(111) has been investigated using temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). At 100 K, NO 2 is adsorbed molecularly at all coverages to form a Pt(111) μ-N,O-nitrito surface complex. This puts NO 2 in an upright bridge-bonded configuration with C s symmetry. Our assignment of this surface species is supported by the infrared spectrum of a μ-N,O-nitritoplatinate complex. The saturation coverage of chemisorbed NO 2 is about 0.5 monolayers (ML) at 100 K. At low coverages, θ NO 2 2 dissociates completely by 285 K to O atoms and NO. However, at coverages greater than θ NO 2 =0.25 ML, the adsorption is partially reversible and NO 2 desorbs molecularly with first-order kinetics with E a =19 kcal/mol. About 20% of the NO 2 adsorbs reversibly at saturation coverage. With higher exposures of NO 2 , a condensed multilayer of N 2 O 4 can be formed.

Adsorption of nitrogen dioxide and nitric oxide on Pd(lll)

Abstract The adsorption of NO 2 on Pd(111) was studied by using HREELS and TPD. NO 2 adsorbs molecularly at 110 K in a μ-N,O nitrito bonding geometry. The assignment of this bonding geometry is made by comparison of the vibrational data obtained in this study with IR measurements of the NO 2 symmetric and asymmetric stretching frequencies of a bridge-bonded nitro palladium complex. Chemisorbed NO 2 dissociates upon heating to 180 K (at low coverages) into NO(a) and O(a). Below θ NO 2 = 0.25 ML, NO 2 desorption is not detected; NO 2 adsorption is irreversible. Above θ NO 2 = 0.25 ML, reversible NO 2 adsorption occurs with adsorptio energies of 14 and 16 kcal mol −1 as determined by desorption activation energies. NO adsorption on Pd(111) was also studied by HREELS and TPD, in order to further understand NO 2 dissociation. The NO TPD data agree well with other studies of NO on Pd(111) with desorption activation energies of 38, 21, and 20 kcal mol −1 . For saturation coverages of NO at 100 K, two NO species are observed with N-O stretching modes at 1575 and 1735 cm −1 . These species are assigned as NO bonded at bridge and atop sites, respectively. We also note the lack of measureable adsorption of N 2 O on Pd(111) at 100 K.