Stepped Single Crystal Surfaces Research Articles

Deconvoluting the Role of Electrolyte pH, Structural Sensitivity, and Electric Field Effects on the Electrochemical Hydrogenation of Phenol

Traditional thermochemical hydrogenation (TCH) methods that are used in the U.S. chemical and petroleum industries to produce value-added fuels are highly energy intensive. To achieve hydrogenation, they require high temperature and pressure, along with an external source of hydrogen that usually comes from steam methane reformation. Electrochemical hydrogenation (ECH) is an attractive alternative that replaces the thermal driving force with an applied electrical potential, allowing ECH to operate near ambient conditions. The participating protons come from the aqueous electrolyte solution rather than externally produced hydrogen. These features of ECH lower its energy demand compared to TCH and result in less greenhouse gas emissions. ECH is also promising for the processing of biomass feedstocks and could be powered directly by renewable energy, further minimizing the environmental impact.The addition of an aqueous electrolyte and electric field means that insights from TCH should not be directly translated to ECH systems, however, this is often the case in the literature. Fundamental studies on ECH of aromatic hydrocarbons (AHs) are necessary to address current limitations including: competition with the hydrogen evolution reaction (HER) resulting in lower faradaic efficiency, low rates compared to TCH, and little control over selectivity and degree of hydrogenation. For instance, the role of electrolyte pH on phenol ECH remains poorly understood. While some groups have reported negligible phenol turnover rates in electrolytes of pH 10 and above on Pt/C and Pd/C catalysts [1], Song et al. have shown that turnover rates and efficiency are highest on Rh/C at pH 10 [2]. Yet, the idea that low pH (3-5) is best has persisted and the source of the pH dependence remains unknown.In this work we use phenol as a simple representative AH for lignocellulosic bio-oil. We have shown with rotating disk electrode voltammetry that the presence of phenol in electrolytes of pH ~10 enhances HER rates on Pt, Pd, and Au catalysts. Considering that the pKa of phenol is also 10.0, we propose that interfacial phenol molecules channel protons to the catalyst surface by way of facile proton dissociation/reassociation, lowering the barrier for adsorbed H formation in alkaline electrolytes. While this may explain, in part, the low phenol turnover rates observed at higher pH, we further investigate why the opposite is observed on Rh/C catalysts. The role of atomic surface structure is also critical to the adsorption geometry and coverage of AHs. We use well-defined, low-index Pt single crystals to identify the true structural sensitivity for the ECH of AHs. We show with select stepped single crystal surfaces that there is a synergetic role between terrace width/step density that defines activity, balancing coverage of AH molecules, formation of surface adsorbed H, and transfer of that H to AH reactants. We use a low-volume, membrane-separated flow cell for chrono-amperometry followed by product analysis to correlate pH, structural sensitivity, and applied potential to phenol ECH rates and faradaic efficiency. With an understanding of the role of atomic surface structure and the true source of pH dependence, we propose strategies for enhancing phenol turnover rates at lower overpotentials, improving faradaic efficiency, and controlling selectivity.

Adsorption and Reaction of SO2 on Graphene-Supported Pt Nanoclusters

Atomic sulfur and its oxides are common catalyst poisons and intriguing research subjects. Recently, graphene-supported nanoclusters were introduced as suitable model catalysts. We investigated the adsorption and reaction of SO2 on graphene-supported Pt nanocluster arrays with high-resolution X-ray photoelectron spectroscopy. SO2 adsorbs in two geometries—perpendicular and parallel to the surface—on both cluster facets and steps. Further insight is gained from the comparison of our results to previous data of SO2 on Pt(111) and two stepped single crystal surfaces—Pt(322) and Pt(355)—with (100) and (111) steps, respectively. We find a remarkable similarity to the adsorption situation on Pt(322). However, thermal evolution experiments revealed several similarities to both Pt(322) and Pt(355), showing that the Pt nanoclusters exhibit a mixture of (100) and (111) steps.

Elucidating the activity of stepped Pt single crystals for oxygen reduction

The unexpectedly high measured activity of Pt[n(111) × (111)] and Pt[n(111) × (100)] stepped single crystal surfaces towards the oxygen reduction reaction (ORR) is explained utilizing the hydroxyl binding energy as the activity descriptor. Using this descriptor (estimated using experimental data obtained by different groups), a well-defined Sabatier-type volcano is observed for the activities measured for the Pt[n(111) × (111)] and Pt[n(111) × (100)] stepped single crystals, in remarkable agreement with earlier theoretical studies. We propose that the observed destabilisation of *OH species at these surfaces is due to the decreased solvation of the adsorbed hydroxyl intermediates on adjacent terrace sites.

In Situ Surface and Reaction Probe Studies with Model Nanoparticle Catalysts

This review paper discusses the in situ surface characterization and catalytic measurements of colloidally synthesized model metal nanoparticle (NP) catalysts studied in the Somorjai lab. Sum Frequency Generation (SFG) vibrational spectroscopy technique revealed the vibrational signatures of binding geometry and surface orientation of adsorbate molecules by probing the immediate surface structure during the catalytic reactions. Metal surfaces were studied by Synchrotron-based spectroscopic techniques at the Advanced Light Source in the Lawrence Berkeley National Laboratory. Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS) was employed to measure chemical and elemental structure of bimetallic NP catalysts under the catalytically relevant pressures in the Torr range. Surface chemical structure (i.e., oxidation states) of metals was obtained by X-ray Absorption Fine Structure Spectroscopy by constructing a gas flow cell that operates under atmospheric pressures as the reaction occurs. Environmental Transmission Electron Microscopy (E-TEM) supplemented the bimetallic structure that was obtained by X-ray spectroscopies. The morphology and chemistry induced by gas reactants on the stepped single crystal surfaces as determined by high-pressure in situ Scanning Tunneling Microscopy (HPSTM) and APXPS were also described.

Tailoring the Selectivity and Stability of Chemically Modified Platinum Nanocatalysts To Design Highly Durable Anodes for PEM Fuel Cells

Big and small: Chemically modifying platinum with calix[4]arene yields a highly stable anode catalyst that effectively suppresses the oxidation reduction reaction without altering the maximum activity for the hydrogen oxidation reaction (see picture, Pt blue, C gray, O red, S yellow). This behavior extends from long-range-ordered stepped single-crystal surfaces to nanocatalysts.

Reaction of water with MgO(100) surfaces. Part II:: Synchrotron photoemission studies of defective surfaces

Synchrotron-based photoemission spectroscopy (PES), low energy electron diffraction, X-ray reflectivity, and optical and atomic force microscopy were used to study the reaction of water vapor [at pressures p(H 2O)≤2.0×10 −5 Torr for 3 min] with UHV cleaved MgO(100) surfaces with different levels of surface defects. The surfaces studied included flat and stepped single-crystal surfaces, wavy polycrystalline surfaces, and Ar +-sputtered flat single-crystal surfaces. AFM and optical microscopy studies revealed that these surfaces have very different surface step densities. The threshold p(H 2O) value for hydroxylation of the flat MgO(100) surface is ≈10 −4 Torr (independent of exposure time), and reactions below this threshold occur dominantly at surface defect sites, which provides a means of estimating surface defect densities. Our results show that water dissociatively chemisorbs at defect sites on MgO(100) surfaces at the residual gas pressure of the UHV chamber (<1×10 −10 Torr). At p(H 2O)≤2×10 −5 Torr the coverage of hydroxyls is estimated to be approximately 5%, 20%, 20%, and 35% of a monolayer for the flat, stepped, wavy, and Ar +-sputtered MgO(100) surfaces, respectively (here we define one monolayer as one hydroxyl per surface site, or one water molecule, or two hydroxyls, per surface unit cell); these hydroxyl coverage values are used as a measure of surface defect densities. The defects are dominantly steps on cleaved surfaces, except for the Ar +-sputtered surface which should contain a significant number of point defects as well. Thermodynamic calculations were carried out to estimate the lower limit p(H 2O) for hydroxylation of surface defect sites, and the calculated p(H 2O) for dissociative chemisorption of water on defect sites is <10 −11 Torr, which is consistent with experimental observations.

Adsorption of Chiral Hydrocarbons on Chiral Platinum Surfaces

Binding energies and configurations of chiral hydrocarbons adsorbed on stepped Pt surfaces are calculated using Monte Carlo simulations. McFadden et al. have recently shown that many stepped single-crystal surfaces have chiral characteristics and that chiral molecules adsorbed on these surfaces should display enantiospecific properties [McFadden, C. F.; Cremer, P. S.; Gellman, A. J. Langmuir 1996, 12, 2483]. The results presented here provide the first theoretical predictions of this phenomenon by measuring enantiomeric shifts in the binding energies of chiral hydrocarbons on chiral Pt surfaces. Multiple examples of enantiomeric shifts that should be readily detectable using temperature-programmed desorption experiments are identified. The role of adsorbate size and surface structure on the magnitude of enantiomeric shifts in binding energies is discussed.

Scanning tunneling microscopy of stepped single crystal surfaces of gold

STM has been used to examine the high index (11,1,1) surface of gold which is vicinal to the (100) oriented surface and makes an angle of 7° with it. The STM images exhibited a distribution of terrace widths ranging typically from 50 to 80 Å with an occasional width of 120 Å. For an ideal Au(11,1,1) surface, the minimum terrace width should involve six atoms with the expected terrace width of 6 × 2.9 = 17.4 A ̊ . The width of the smallest terrace examined on this surface was two to three times larger and the step similarly larger. This may be due to the restructuring of the surface during the flame annealing. The STM results for the Au(755) and Au(322) surfaces also show terraces which vary in length and show evidence of kinks — particularly the Au (755) surface. The various factors which may control the terrace widths are discussed in the paper.

Structural effects in electrocatalysis : oxidation of formic acid on single crystal platinum electrodes and evidence for oscillatory behaviour

Kinetics of the oxidation of formic acid depends strongly on the crystallographic orientation of the electrode surface. Not only the rate of reaction is structure dependent but also the reaction mechanism appears to be dependent on the symmetry of the surface atoms. The terrace orientation, the step orientation and the step density affect the reaction kinetics as inferred from the study of reaction on stepped single crystal surfaces. Based on the isotopic effect it was concluded that the H-C bond is broken first in the rate determining step. Oscillatory behaviour was found for Pt(100) and vicinals Pt(11,1,1) and Pt(610) faces. The current oscillations disappear with increased step density and do not occur for Pt(311) and Pt(310).

Decomposition of methanol over Rh and Ru

The reaction of methanol with stepped single crystal surfaces of Ru and Rh has been studied by pulsed field desorption mass spectrometry (PFDMS) in order to provide insight into the reaction mechanisms of the decomposition reaction. CH3OH dissociates easily on Ru and Rh and initially forms adsorbed OCH3 at temperatures T=300–700 K. At temperatures T>460 K, on Ru(001) the surface coverage of COad is limited by thermal desorption during the reaction, time tR. CH3OH readily decomposes at, empty sites and, consequently, the amount of detected reaction intermediates is small. For T<460 K high intensities of CH3+ (OCH3+) are observed, indicating that the decomposition of adsorbed methoxy, OCH3, is inhibited by the product species, COad. Thus, here the thermal desorption of COad is the rate determining step. This inhibition can be suppressed by field desorption of COad. In this way we observe that the decomposition reaction is faster than the shortest measurable time tR=100 μs, even at T=300 K. The influence of the electrical field on the decomposition reaction over Rh has been studied by applying a steady field, FR, during the reaction interval between the pulses. At FR≈4 V/nm the intensities of COH+, COH2+, and CH3+ increase steeply. This increase is most pronounced for COH2+ and coincides with a decrease of the CO+ and Hn+ intensities. Thus, the steady electrical field reduces the reaction rate and stabilizes the intermediate stages.

Stepped single-crystal surfaces as models for small catalyst particles

Infrared spectroscopy is used to study the vibration of CO adsorbed on small, silica-supported platinum particles. Different samples have average Pt-particle sizes varying from 11 to 105 A in diameter. The infrared spectrum shows that all CO molecules are linearly bound to single Pt atoms; however, three bands can be detected on each of the different samples. The bands appear at 2081, 2070 and 2063 cm −1 and shift to higher wavenumbers (by about 6 cm −1) as the coverage increases. The intensities of the three bands vary with particle size in the same manner as the relative number of face, corner and edge atoms in our samples vary with particle size. This correlation leads to the assignment of the three bands to CO, linearly bound to face, corner and edge atoms in order of decreasing wavenumber. Some confirmation of these assignments are obtained from infrared reflection-absorption measurements of CO adsorbed on three extended-crystal surfaces of Pt: a (111) crystal, a crystal with (111) terraces and (100) steps, and a crystal with (111) terraces and kinked steps. These surfaces are described as (111), (533) and (432). The stepped crystals give bands for CO molecules associated with terrace sites appearing at about 2085 cm −1 and with step and kink sites appearing at about 2065 cm −1. Within ± 2 cm −1 there is no difference between the frequencies of CO adsorbed on step and on kink sites.

Stepped single-crystal surfaces as models for small catalyst particles

Stepped single-crystal surfaces as models for small catalyst particles

The adsorption of CO, O 2, and H 2 on Pt : I. Thermal desorption spectroscopy studies

Thermal desorption spectroscopy (TDS) has been used to study the chemisorption of CO, O 2, and h 2 on Pt. It has been found that TDS is quite sensitive to local surface structure. Three single crystal and two polycrystalline Pt surfaces were studied. One single crystal was cut to expose the smooth, hexagonally close-packed plane of the fee Pt crystal (the (111) surface). The other two single crystals were cut to expose stepped surfaces consisting of smooth, hexagonally close-packed terraces six atoms wide separated by one atom high steps (the 6(111) × (100) and 6(111) × (111) surfaces). Only one predominant desorption state was observed for CO and H adsorbed on the smooth (111) single crystal surface, while two predominant desorption states were observed for these gases adsorbed on the stepped single crystal surfaces. The low temperature desorption states on the stepped surfaces are attributed to desorption from the terraces, while the high temperature desorption states are attributed to desorption from the steps. TDS of CO from the polycrystalline foils exhibited some desorption states which were similar to those observed on the stepped single crystal surfaces, indicating the presence of adsorption sites on the polycrystalline foils that were similar to the terrace and step sites on the stepped single crystals. In general, these results suggest a high density of defect sites on the polycrystalline foils which can not be attributed simply to adsorption at grain boundaries. Oxygen was found to adsorb well on the stepped single crystals and on the polycrystalline foils, but not on the smooth (111) single crystal, under the conditions of these experiments. This is attributed to a higher sticking probability for dissociative O 2 adsorption at steps or defects than on terraces.