AVS 70 Session SS-ThP: Surface Science Poster Session

Epoxidation reactions are some of the highest value processes in the chemical industry; however, despite decades of research, the mechanism of ethylene epoxidation is still under debate. A central question in the field is the state of the industrial Ag catalyst under reaction conditions. Controversy remains about whether the active site for selective oxidation is metallic or oxidized and several in situ studies have reported the presence of both phases. We utilize near ambient pressure XPS under both purely oxidizing conditions and ethylene: oxygen ratios at a temperature that yields the same reactant chemical potentials as a typical flow reactor. Our XPS results reveal that electrophilic and nucleophilic oxygen formed on Ag(111) after exposure to pure oxygen at 433 K, with a O : surface Ag ratio of 0.11. However, when ethylene is introduced to the oxygen atmosphere at an industrially relevant 5:2 ratio, the nucleophilic oxygen is reacted away immediately, and the electrophilic oxygen coverage slightly increases which we attribute to the formation of carbonate which has the same binding energy as electrophilic oxygen. Most significantly, the O : surface Ag ratio decreases to 0.05, almost half of which is carbonate. These results indicate that the Ag(111) surface is mostly metallic under simulated reactor conditions meaning that Ag/oxide interfaces, at which the proposed oxametallacycle (OMC) intermediate is formed, must be considered in mechanistic epoxidation models.

Transition metal diborides are known to either have a metal-terminated or boron-terminated surface. While group-V MB₂ has boron-terminated surfaces, group-IV MB₂ has metal-terminated surfaces. ZrB_2, a group-IV metal-terminated diboride, is an extremely hard material with a high melting point of 3246 °C and can be grown conformally via chemical vapor deposition (CVD) using zirconium borohydride Zr-(BH₄)₄ as a precursor. Using reflection absorption infrared spectroscopy (RAIRS), we investigated the surface chemistry of ZrB₂ growth from Zr-(BH₄)₄ on a ZrB₂(0001) surface. Using low-energy electron diffraction and X-ray photoelectron spectroscopy (XPS), we demonstrate that ZrB₂ can have a boron-terminated surface in boron-rich conditions. The RAIR spectrum obtained after exposing the surface at 90 K to Zr-(BH₄)₄(g) matched that of the pure compound, indicating adsorption without decomposition. However, new surface intermediates were formed upon heating to 280 K, as shown by the presence of vB-Ht stretch (2569 cm^-1) and δH-B-H (1228 & 1057 cm^-1) peaks in the RAIR spectra. Decomposition of Zr(BH₄)₄ on ZrB₂(0001) surface at 1173 K revealed a √3×√3 boron terminated surface, with a stoichiometry of ZrB_2.60. In contrast to the zirconium-terminated surface, the boron-terminated surface is resistant to oxidation.

Selective hydrogenation of 1,3-butadiene (BD) to 1-butene (1-B) is critical in refining alkene streams for high-quality polymer production. Typically, Pd and Pt are employed in hydrogenation reactions due to their nearly negligible barrier for H₂ activation. However, these catalysts are prone to coking and their high activity often reduces selectivity. Single-atom alloy (SAAs) catalysts are being developed to achieve high selectivity while retaining high activity. In an SAA, small amounts of an active metal, such as Pd or Pt, are doped into a less active host metal such as Cu. Previous work has shown that a Pd(111) surface exhibits superior selectivity for BD hydrogenation to 1-B compared to Pt(111),^1-4 suggesting that Pd/Cu(111) could be a suitable SAA model catalyst for this reaction. In this study, we investigated the adsorption and hydrogenation of 1,3-butadiene (BD) to 1-butene (1-B) over a Pd/Cu (111) SAA under ultrahigh vacuum (UHV) and ambient pressure conditions using reflection absorption infrared spectroscopy (RAIRS). Temperature programmed reaction spectroscopy (TPRS) in UHV showed that monolayer BD desorbs at 217 K, while 2nd-layer and multilayer BD desorb between 112 to 180 K. RAIRS detected gas-phase 1-B formation and BD consumption. In ambient pressure conditions, this reaction was found to be first-order (1.12±0.03) in H₂ and zero-order (-0.12±0.01) in BD, correlating to a turnover frequency of 36 s^-1 at 380 K. The activation energy was calculated to be 63.2 ± 2.8 kJ/mol from an Arrhenius plot of the temperature dependence of the rate constant. Complete conversion of BD was found with 84% selectivity towards 1-B, without butane production. No surface species were detected during the reaction. Post-reaction analysis using Auger electron spectroscopy (AES) revealed carbon deposition, indicating some dissociation during hydrogenation.

Thiolate self-assembled monolayers (SAMs) are widely used for their ability to tune the interfacial properties of Au surfaces for applications such as nano-fabrication and bio-functionalization. This molecular control over interfacial properties can be further enhanced by introducing a second molecular species, resulting in a binary SAM. Binary SAMs offer an appealing route to control the molecular scale density of specific functional groups. One approach to forming binary SAMs is through the displacement of one molecule with another through solution phase deposition. The fabrication of molecularly precise binary SAMs requires precise control of the molecular interactions and substrate properties, such as surface roughness.

In this work, using ultra-high vacuum scanning tunneling microscopy (UHV-STM) and solution-phase preparation methods, the relationship between chain length, functional group, and substrate roughness on the displacement of 3-Mercaptoproprionic Acid (MPA) by 1-Decanethiol (DT) was investigated. First, three categories of MPA substrates were prepared: MPA-1) well-ordered smooth surface formed during a 3 hr MPA incubation at 35 °C, MPA-2) disordered smooth surface formed during a 3 hr MPA incubation at 25 °C, and MPA-3) well-ordered rough surface formed during a 24 hr MPA incubation at 25 °C. Then, following the formation of MPA-1, -2, and -3, the substrates were sequentially placed in a 2 μM DT solution for 20 min, 60, min, 3 hr, and 24 hr. UHV-STM analysis of the MPA substrates showed that MPA-1 and MPA-2 had faster rates of displacement than MPA-3 and resulted in a uniform high-density DT film across the surface. MPA-3, which started with a uniform MPA SAM and nanoscale roughness due to the formation of Au nano-islands across the substrate related to the longer MPA incubation time, had the slowest rate of displacement. Interestingly, the DT SAM that formed following displacement of the MPA-3 sample was comprised of the low-coverage, lying down phase (β) and 2-D gas phase (α) of DT. Ongoing work is focused on understanding the relationship between substrate roughness and MPA ordering on the slower displacement rate and low-density DT phases observed for MPA-3.

Click chemistry is a well-known and established reaction scheme in organic chemistry for the synthesis of well-defined organic molecular structures. The direct application of click chemistry reactions to selectively functionalized silicon surfaces could thus open the route to synthesizing new organic molecular architectures on these substrates, e.g., with tailored optical or physicochemical properties (“more than Moore”). However, click reactions such as the most prominent alkyne-azide coupling are performed in the presence of a catalyst dissolved in an adequate solvent. On the other hand, highly reactive surfaces such as the technologically most important Si(001) surface, are typically prepared and stored under UHV conditions in order to guarantee a high level of cleanliness and structural perfection. The direct application of solution-based click chemistry schemes to such vacuum processed surfaces thus seems as an experimental contradiction.

Here we show two different approaches to solve this problem: First, we demonstrate how to combine surface functionalization performed under UHV conditions with a solution-based alkyne-azide click reaction in order to build organic molecular architectures on functionalized semiconductor surfaces. The UHV-based functionalization of the Si(001) surface was realized via chemoselective adsorption of ethinyl cyclopropyl cyclooctyne (ECCO) from the gas phase [1]. The samples were directly transferred from UHV into the azide solution without contact to ambient conditions. The second organic layer was then coupled in acetonitrile solution via the copper-catalyzed alkyne-azide click reaction. Each reaction step was monitored by means of X-ray photoelectron spectroscopy in UHV; the N 1s spectra clearly indicated the click reaction of the azide group of the two test molecules employed, i.e., methyl-substituted benzylazide and azide substituted pyrene. Using optimized copper (I) catalysts, effective reaction yields of up to 75 % were obtained [2].

Second, a carefully tuned enolether/tetrazine cycloaddition was shown to be applicable even under UHV conditions and without catalyst [3]. We employed this reaction for coupling a tetrazine molecule to an enol ether group which was covalently attached on a Si(001) surface via cyclooctyne as a linker.

[1] C. Länger, J. Heep, P. Nikodemiak, T. Bohamud, P. Kirsten, U. Höfer, U. Koert, and M. Dürr, J. Phys.: Condens. Matter 31, 34001 (2019).

[2] T. Glaser, J. Meinecke, C. Länger, J. Heep, U. Koert, and M. Dürr, J. Phys. Chem. C 125, 4021 (2021).

[3] T. Glaser, J. Meinecke, L. Freund, C. Länger, J.-N. Luy, R. Tonner, U. Koert, and M. Dürr, Chem. Eur. J. 27, 8082 (2021).

This study investigates the surface modification of ZnO nanopowder using gas-phase fluorinated β-diketones, namely hexafluoro acetylacetone (hfacH) and trifluoro acetylacetone (tfacH), to elucidate their attachment chemistry.

Surface modification was investigated through in-situ infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), and solid-state NMR analysis (ssNMR) to determine the dominant adsorbate on nanopowder surfaces. To supplement the experimental findings, density functional theory was employed to identify stable surface species of ZnO nanopowder.

This study examines how the binding behavior of β-diketones on the ZnO surface varies depending on the specific nature of the β-diketone molecule. The gas phase for both diketones investigated consisted of the mixture of enol and ketone forms, with enol being most dominant for hfacH. Despite this observation, surface adsorption is dominated by the tetra-σ-bonded diketonate, which is very different from the commonly accepted adsorption model for β-diketones on oxide surfaces. In the case of tfacH, the adsorption is indeed dominated by the dissociated enolate form. These differences are apparently governed by the amphoteric nature of ZnO. When a more basic oxide material, MgO, is used, hfacH does form enolate as a dominant adsorbate, as confirmed by ssNMR.

This work expands our understanding of the β-diketone adsorption on oxide materials that is used in a variety of applications, from surface sensitization to heterogeneous catalysis, and from material growth to etching.

Carbon dioxide (CO₂) is the main gas responsible for the greenhouse effect in Earth's atmosphere, leading to higher global temperatures and climate change. To limit global warming to 1.5ºC and reach net zero carbon dioxide emissions by 2050, it is necessary to advance in industrial processes that facilitate the generation of clean fuels from CO₂. One of the most promising strategies in this regard is the utilization of CO₂ and its transformation into valuable chemicals.

This study examines the effectiveness of two catalyst types, RuCeO₂ and RuCeO₂-TiO₂ systems, for converting CO₂ into methane. Results demonstrate that despite lower Ru content, TiO₂-containing systems exhibit significantly enhanced catalytic activity for CO₂ conversion to methane. To understand this fact, in situ X-ray absorption measurements have been carried out on the Ru K-edge and Ce L₃-edge analyzing their behavior under H₂, CO₂ and H₂+CO₂ between 30^oC to 250^oC. The Ru K-edge results indicate that the RuCeO₂ systems display a remarkable oxidation-reduction capacity, as evidenced by the reduction of the metal center in the presence of both H₂ and CO₂/H₂, along with its oxidation in CO₂ atmosphere. However, this behavior undergoes a drastic change in the TiO₂-containing catalyst, as a consequence of strong CeO₂-TiO₂ interactions, where the presence of hydrogen at 250°C leads to irreversible reduction of Ru.

The analysis of the Ce L₃-edge revealed that the RuCeO₂ systems predominantly exhibit Ce⁴⁺, which was observed to undergo partial reduction under both H₂ and H₂/CO₂ atmospheres, followed by its re-oxidation under CO₂. However, the RuCeO₂-TiO₂ catalysts demonstrate a substantially higher concentration of Ce³⁺ compared to the RuCeO₂ sample under all conditions (H₂, CO₂, H₂/CO₂), with the amount of Ce³⁺ further increasing under atmospheres with H₂ and H₂/CO₂.

Therefore, the XAS findings indicate that the presence of TiO₂ in the catalysts stabilizes the metallic state of Ru, which remains in this state during the methanation reaction. Moreover, TiO₂ promotes the formation of Ce³⁺, enhancing the catalysts' reactivity. This effect is attributed to TiO₂ facilitating an electronic transfer at the interface and perturbing the regular fluorite geometry of ceria, thus promoting the presence of Ce³⁺. A trend that is in agreement with previous studies for CeO₂/TiO₂(110) model systems. The presence of Ce³⁺ significantly impacts the catalytic properties of the sample, aiding in the oxidation-reduction of Ce and stabilizing Ru. Consequently, the presence of reduced cerium plays a crucial role in determining the surface chemistry of the catalyst, crucial for efficiently converting CO₂ into methane.

1. Cobalt Sulfide Sheets on Au(111) [1]
   Transition metal dichalcogenides (TMDCs) are a type of two-dimensional (2D) material that has been widely investigated by both experimentalists and theoreticians because of their unique properties. In the case of cobalt sulfide, density functional theory (DFT) calculations on free-standing S−Co−S sheets suggest there are no stable 2D cobalt sulfide polymorphs, whereas experimental observations clearly show TMDC-like structures on Au(111). In this study, we resolve this disagreement by using a combination of experimental techniques and DFT calculations, considering the substrate explicitly. We find a 2D CoS(0001)-like sheet on Au(111) that delivers excellent agreement between theory and experiment. Uniquely this sheet exhibits a metallic character, contrary to most TMDCs, and exists due to the stabilizing interactions with the Au(111) substrate.
2. Initial Oxidation of Pt(111) [2]
   In situ scanning tunneling microscopy experiments on the initial oxidation of Pt(111) found complex intermediary platinum surface oxides, consisting of spoke wheel and stripe structures [3]. The structure of the spoke wheels is poorly understood because of their size and complexity. Here we employ atomistic thermodynamics based on an established reactive force field to investigate the structure and stability of spoke wheels at the elevated temperature (>530 K) and pressure (1−4 bar) conditions of the in situ experiments. At those conditions, the thermodynamic stability of the structural model for the spoke wheel is similar to that of the stripes, while the degree of surface oxidation is much lower. The spoke wheel is found to be much more stable than partially formed stripes with a similar degree of oxidation. These results are consistent with experimental findings, where the spoke wheel is observed first, at slightly lower oxygen pressures. They thus provide a better understanding of the oxidation pathway for Pt(111)-based catalysts in the context of oxidative catalysis.

1. M. K. Prabhu, D. Boden, M. J. Rost, J. Meyer, and I. M. N. Groot, J. Phys. Chem. Lett. 11, 9038 (2020).
2. D. Boden, I. M. N. Groot, and J. Meyer, J. Phys. Chem. C 126, 20020 (2022).
3. M. A. van Spronsen, J. W. M. Frenken, and I. M. N. Groot, Nat. Commun. 8, 1 (2017).

Alkaline water electrolysis represents one of the simplest methods employed to perform water splitting reaction [1]. This process is one of the most efficient ways of producing H₂ and O₂ at low cost and high purity [2]. The bottleneck of this reaction stems from the sluggish kinetics of the anodic reaction, i.e., the oxygen evolution reaction (OER), which consists of a four-electron transfer process [3]. The extraordinary performance of NiFe as electrocatalysts for the OER [4] is still a subject of debate. The changes that occur on the electrode surface during electrochemical reactions add another dimension of complexity, which hinders the rational design of electrodes for water splitting. Particularly for binary alloy electrodes, there are various phenomena ranging from the formation of oxides, (oxy)hydroxides and the associated segregation of metal atoms. In this work, we study various NiFe electrodes as model systems for the OER. We have developed the procedure for the quantification of chemical depth-profiling by XPS/HAXPES measurement, showing a marked Fe segregation and dissolution. The results explain the electrochemical performance of NiFe electrodes for OER. All the electrodes studied suffer from segregation of iron and subsequent formation of FeO_x on the surface, with only minor influence from morphology, porosity and total Fe content.

Surface modification of titanium dioxide (TiO2) nanoparticles with trimethylolpropane (TMP) and dimethylolpropionic acid (DMPA) holds significant promise for advancing various technological applications. This research explores the functionalization of TiO2 nanomaterials and compares with pigmentary TiO2 surface coatings technologies used at industrial scale. Through a combination of experimental techniques including FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations, the interactions between surface modifiers and TiO2 are elucidated, revealing the formation of specific surface adsorbates. The results obtained from these studies demonstrated the displacement of surface impurities by adsorption of functionalized small molecules and evaluated the displacement processes among these surface modifiers. Hexafluoroacetylacetone (Hfac) was used as a test compound in displacement investigations utilizing fluorine as a spectroscopic label to precisely trace the displacement chemistry with XPS. This work provides fundamental understanding and offers novel strategies and valuable insights for enhancing the performance of TiO2-based materials.

Single-atom alloy (SAA) catalysts have become a prominent way to enhance selectivity in a number of catalytic processes.The mechanism by which these alloys attain their improved performance is still being debated, however, especially when considering the effect of the reaction environment on their structure and chemical composition.We have embarked on a project to emulate those catalysts using model systems where the metals are dispersed as nanoparticles (NPs) onto a flat oxide surface under controlled ultra-high vacuum (UHV) conditions, characterized using a combination of surface-sensitive techniques, and tested for their catalytic behavior using a so-called high-pressure cell. This work focuses on the characterization of the nature of surfaces of SAA catalysts made out of Pt (minority) and Cu (majority) metals and their performance for the promotion of the hydrogenation of unsaturated aldehydes. Cu/Ta_xO_y/Ta surfaces have been prepared via the oxidation of a Ta crystal followed by the vapor deposition of controlled amounts of Cu and characterized using reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) together with carbon monoxide (CO) as a probe molecule. A series of changes in the RAIRS data has been observed as a function of the Cu deposition time, an indication of the development of the Cu NPs from small clusters of atoms with multiple low-coordination sites to larger NPs with more basal planes exposed. These Cu NPs were found to form stable in the temperature range from 300 K to 600 K, but to change beyond 600 K. Like Cu, Pt and Cu-Pt NPs can be grown and characterized via CO titrations using RAIRS and TPD, following a similar approach. Kinetic measurements under catalytic conditions can then be performed to test the efficacy of the Cu, Pt, or Cu-Pt NPs catalysts supported on the tantalum oxide surface and to identify the appropriate structure-reactivity correlations.

Achieving precise control over surface roughness at the sub-nanometer scale is paramount for enhancing the physical properties of semiconductor materials, particularly their electrical transport characteristics. Current methodologies, predominantly scanning probe microscopy, facilitate quantitative roughness assessment but face challenges when applied to surfaces with intricate structures such as vertical and facet faces of three-dimensional (3D) structures, owing to steric hindrance. Our prior research effectively utilized reflection high-energy electron diffraction (RHEED) to observe atomic ordering on various structure surfaces in different directions, overcoming the challenge of steric hindrance [1-3]. In this study, we explore the potential of RHEED as an alternative avenue for evaluating surface roughness. Kinematic diffraction theory introducing diffraction spots from atomic positions, we explore the correlation between RHEED spot profile and surface roughness. Our approach involves systematically analyzing RHEED intensity profiles alongside surface roughness measurements of Highly Ordered Pyrolytic Graphite (HOPG), serving as a prototype sample.

HOPG samples with varying degrees of surface roughness were prepared via Ar sputtering, and their roughness was assessed using atomic force microscopy (AFM). Figure 1(a) shows a typical RHEED pattern for the HOPG whose roughness was 0.163 nm. Through the analysis of RHEED spot profiles, we discerned distinct intensity components, and , corresponding to sharpened and broadened spot intensities, respectively (Fig 1(b)). Our findings reveal an excellent correlation between increased surface roughness and the ratio of broadened spot intensity to total intensity (Fig 1(c)), indicating RHEED's adaptability for quantitative evaluation of surface roughness. This study not only shows how effective RHEED can be in measuring surface roughness but also demonstrates its usefulness in analyzing complex 3D surfaces in small-scale research and making semiconductor devices. The details will be discussed in the presentation.

References:

[1] A. N. Hattori, K. Hattori, et al., Surf. Sci. 644 (2016) 86.

[2] A. N. Hattori, K. Hattori, et al., Appl. Phys. Exp. 9 (2016) 085501.

Polymer electrolyte membrane fuel cells (PEMFCs) offer a promising avenue for sustainable electricity production with minimal environmental impact. However, their widespread adoption faces challenges due to durability concerns and the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode. Consequently, to advance PEMFCs, there is a pressing need for the advancement of catalyst technologies to overcome these limitations. Previous works has looked at the synthesis of extended-surface platinum-nickel (PtNi) nanowires (NWs) via atomic layer deposition (ALD) and their durability. Here we show how the composition and chemical states of the nanowires evolve after a series of post-synthetic modifications designed to maximize the longevity of these nanowires as catalysts. By combining complimentary surface analytical techniques (X-ray photoelectron spectroscopy, hard X-ray photoelectron spectroscopy, and nanoscale Auger electron spectroscopy) we develop a more complete model of the complex chemical nature of these catalysts than any single technique could by itself, accelerating the development of more durable and efficient PEMFC catalysts.

Secondary ion mass spectrometry (SIMS) is an ideal technique to provide indium (In) implant depth profiles in Si. However, this approach faces a critical challenge for overcoming the Si molecular mass interferences (MIs). These MIs can create high backgrounds during profiling and create an ambiguous determination of true In depth distributions. This phenomenon is especially true for SIMS analysis of lower dosed In implants <1E15 (at/cm²). The challenge is that the combination of three Si isotopes creates significant MIs with a high mass resolution (MR) > ~45370 needed for separation from In. This is far beyond the magnetic sector dynamic SIMS mass separation capability <10000MR. A special SIMS analysis method of energy filtering by applying an offset voltage has been often used to minimize or remove the Si MIs for In implant analysis. However, applying the different voltage offsets produces different In tailing profiles and background levels. Fig. 1 shows seven SIMS depth profiles acquired on In30KeV7.85E13 implanted in Si, acquired by applying seven voltage offsets from -10 to -30eV, which create different In background levels and tailing profile shapes below 1E18 (at/cm³). Increasing the voltage offset values from -10 to -30eV reduces the In background levels and shrinks the tailing profiles (Fig. 2, as determined at 100nm depth scale). Some previous SIMS studies used the background subtraction rather than applied voltage offset to characterize and obtain the precise In implant profiles, but determination of the specific subtraction value might be an issue since different subtractions would also affect the measured In implant doses and shapes. Fig. 3 shows the same SIMS In implant depth profiles with no subtraction and no voltage offset in green, and light and heavy subtractions in red and blue. To obtain more accurate In implant profiles in Si to overcome random background subtraction issues, we have used SIMS analysis combined with TCAD simulation. This combined method uses an appropriate voltage offset to obtain the SIMS profiles and then it is compared with the TCAD simulation, as shown in Fig. 4. Although several different voltage offsets have to be used to obtain the SIMS depth profiles to match the TCAD simulations to determine the applied voltage offsets at the beginning, once the SIMS analysis protocol is established, the analysis recipes would be used for analyzing the similar In implants in Si. In addition, we present the first study of using secondary ion energy distribution to determine the voltage offsets for analyzing In implants (Fig. 5) and that data will be shown in a full paper.

Carbon-carbon (C-C) bond formation is paramount for a large variety of man-made chemicals such as commodity chemicals, synthetic materials, and pharmaceuticals. Monitoring the dynamics of the C-C bond formation in real time at surfaces and how this is influenced by the surface properties could be a game changer in improving the efficiency of various heterogeneous reactions.To study the dynamics of C-C bond formation, an experimental technique is employed that combines time-of-flight mass spectrometry with pump-probe spectroscopy and fast surface preparation with molecules. CH₃I and CO are used as precursor molecules. These molecules are dosed on metal oxides such as titanium dioxide and cerium oxide, on which the reactions are monitored.

The reaction is triggered by the pump laser pulse at a central wavelength of 266 nm which excites CH₃I into the dissociative A-band, which leads to the formation of CH₃ and I fragments. Subsequently, the probe laser pulse in ultraviolet spectral domain will ionize the reaction intermediates and final products, which are immediately removed from the surface by a static electric field and detected by the time-of-flight mass spectrometer. By varying the pump-probe time delay, the formation dynamics of CH₃ intermediate, as well as the reaction of CH₃ with CO, which leads to the formation CH₃CO (acetyl) and finally to CH₃COCH₃ (acetone) will be monitored. Details on CO and CH₃I partial pressure dependence as well as the influence of the surface composition and temperature will be presented.

The adsorption and dissociation of carbon monoxide (CO) on metal surfaces is a fundamental reaction with many applications in the field of surface chemistry. Particularly, this research studied the reaction between CO and a Ru (0001) single crystal. The adsorption and dissociation of the gas were investigated through the use of techniques such as infrared spectroscopy (IR) and mass spectroscopy (MS). We also employed a pulsing valve system, where controlled pulses of CO (and other reactant gases) influence the surface reaction. The effects of temperature and pressure on the adsorption kinetics and subsequent dissociation were explored in detail, to reach a broad understanding in the transient kinetics of the reaction. By modifying various parameters, the intention is to optimize the dynamic reaction conditions. Through this comprehensive approach, we sought to contribute valuable insights into the mechanisms driving CO adsorption and dissociation, with implications for catalysis and surface science applications.

Hydrogen (H₂) is one of the most promising clean and renewable energy sources. Nevertheless, the storage of hydrogen shows poor performance due to the low gravimetric and volumetric densities. Nitrogen-doped graphene (Gr) has been identified as a potential material for H₂ storage. Here, we study the growth of Gr on a Ru(0001) surface by chemical vapor deposition (CVD) of pyridine and N-doping through N₂⁺ beam irradiation using scanning tunneling microscopy (STM) and x-ray photoelectron spectroscopy (XPS). A high-quality Gr film with low N densities was obtained by pyridine CVD on Ru(0001) at 1063 K. Higher concentrations of N-dopants were introduced on the Gr/Ru(0001) through low-energy N₂⁺ irradiation at 100 eV.Nitrogen can be embedded in the Gr lattice preferentially in two configurations, namely graphitic N (N substituted in the C lattice) and pyridinic N (substitutional N next to a C vacancy). Atomically resolved STM images of graphitic and pyridinic-N defects demonstrate their preferential locations within the Gr Moiré. XPS shows that coverage of up to 3.9% of pyridinic-N and 2.3% of graphitic N can be embedded into the high-quality Gr film using N₂⁺irradiation at room temperature, indicating a preferential formation of pyridinic N over graphitic N. Only graphitic N was observed upon annealing the ion-irradiated Gr/Ru(0001) to 1063 K, revealing higher thermal stability of graphitic N over pyridinic N. Our current efforts center on the adsorption studies of atomic hydrogen, its interactions with N dopants, and thermally induced diffusion.

The study of N-methylaniline (NMA) coverage dependent adsorption and reaction on the Pt(111) surface has gained attention because of the alkyl and aromatic amine interaction with transition metal surface.In this work, we use Density functional theory (DFT)simulations and experiment to study the structural and electronic properties of coverage dependent adsorption of NMA on Pt(111) surface. Firstly, we found that the molecule adheres to the surface by forming bonds with both the phenyl ring and the nitrogen (N) atom at the lower coverage. However, as coverage increases, a fascinating phenomenon unfolds: the phenyl ring undergoes a distinctive tilting motion away from the surface. This tilting is accompanied by a change in the incline angle, transitioning from a mere 2^o at low coverage (approximately 0.02 ML) to a substantial 33^o at higher coverage (around 0.17 ML). This structural transformation also brings about a variation in the adsorption energy, shifting from -2.9 eV at low coverage to -1.5 eV at high coverage. Secondly, Bader charge analysis provided further insights into these interactions. At low coverage, the charge is sharing between the N atom and the surface Pt atom and from the surface to the carbon (C) atoms with in the phenyl ring. In contrast, at higher coverage, charge sharing primarily takes place through the N atom to the surface. This shift towards a more robust and bi-centered bonding with the surface at lower coverage indicates an increased propensity for molecular dissociation under these conditions. Finally, the analysis of the projected density of states shed more light on revealing the hybridization of the N p_z orbital with the Pt d_z2 orbital at higher coverage. Conversely, at low coverage,hybridization of the carbon p_z orbital with the Pt d_z2 orbital was observed, alongside Pt-N hybridization. These observations regarding molecular adsorption at different coverages indicated that, at high coverage, the molecule easily lifts off intact from the surface, while at low coverage, it tends to break into smaller fragments, consistent with experimental findings.

The adsorption of carbon monoxide (CO) on platinum (Pt) has been widely studied in the literature. Pt is one of the most frequently used active metals in catalysis, with CO playing an important role in many reactions, including hydrogenation, oxidation, and car emission controls. Here, we present our preliminary progress on the study of the effect of an electric field on CO adsorption on Pt. To do this, we designed a device that can apply a direct electric field to the surface of our Pt catalyst. A layer of porous alumina was synthesized on a Pt film, to allow CO molecules to diffuse through, making the underlying Pt accessible. A layer of graphene is added on top of the porous layer to complete the device. The bottom Pt and the top layer of graphene are biased to apply an electric field that directly interacts with the Pt surface where the CO is being adsorbed. Using infrared reflective adsorption spectroscopy (IRRAS) we study the effect of such fields on CO adsorption.

Epoxidation of olefins on catalytic surfaces is considered to play a major role in chemical industries where Ag-based catalysts are prominently used due to the surface chemistry of the alkene/silver system. However, the fundamentals of olefin oxidation on metal surfaces are still not well understood. Therefore, in this research, we use controlled ultra-high vacuum conditions to understand the fundamentals of the styrene oxidation on an Ag(111) surface.

For this purpose, we initially formed an atomic oxygen layer on the Ag(111) surface by dosing 10% NO₂ seeded in He at 510 K surface temperature using a supersonic molecular beam. Subsequently, we dosed styrene onto the oxidized silver surface. Desorbing products from the surface were ionized using focused laser radiation and ionized molecules were detected using an ion imaging system equipped with micro-channel plates, a phosphor screen, and a CMOS camera.

We systemically form nucleophilic and electrophilic oxygen at the silver surface using two different preparation methods. When the surface was sputtered with Ar⁺ ions and annealed to 700 K we predominantly formed nucleophilic oxygen, which led to the complete combustion of the styrene. When we only annealed the surface to 700 K, the surface predominantly formed electrophilic oxygen, which led to the formation of styrene oxide. We identified electrophilic oxygen being accompanied by the formation of subsurface oxygen, which seems to amplify the production of styrene oxide.

Keywords: Epoxidation, Ion imaging, Molecular beam, Silver, Styrene

Although H₂O adsorption on SiO₂ surfaces has been studied extensively, there are still many questions about the nature of the H₂O adsorbed layer. In this work, the H₂O layer thickness on flat hydroxylated SiO₂ surfaces was examined in a vacuum environment using in situ spectroscopic ellipsometry (SE). The H₂O water layer thickness was measured versus H₂O pressure at various temperatures. Complementary Fourier transform infrared (FTIR) analysis was also performed on SiO₂ powders.

Flat SiO₂ surfaces were hydroxylated using H₂O₂ plasma exposure to produce a hydrophilic surface with a water contact angle of < 10°. The in situ SE measurements were then conducted in a warm-wall vacuum chamber designed with a temperature-controlled sample stage. The H₂O layer thickness was measured versus pressure at various temperatures (Figure 1). The H₂O pressures were varied up to the saturation H₂O vapor pressure corresponding to the sample temperature. The H₂O layer thickness versus relative humidity was consistent with general expectations from the BET adsorption isotherm model.

The SE measurements showed that there were two distinct types of H₂O layers: a weakly adsorbed layer and a strongly adsorbed layer (Figure 2). The weakly adsorbed layer could be added or subtracted by increasing or removing the H₂O pressure. The strongly adsorbed layer was not lost by removing the H₂O pressure. However, the strongly adsorbed layer could be desorbed by heating the sample stage to 120°C. The SE measurements characterized the layer thicknesses for the weakly and strongly adsorbed layers versus H₂O pressure at various sample temperatures.

Using repeating H₂O exposures, the strongly adsorbed layer reached an approximate plateau at ~1 Å at various temperatures. In contrast, the weakly adsorbed layer obtained higher thicknesses at larger H₂O pressures. For example, the weakly adsorbed layer thickness was 7 Å at 92% relative humidity at 30.4ºC (30 Torr). The FTIR investigations on SiO₂ powders were in qualitative agreement with the SE studies. These studies confirm the existence of a strongly adsorbed vicinal layer and a weakly adsorbed layer explained by the BET model on hydroxylated SiO₂.