AVS 67 Session HC-Contributed On Demand: Fundamental Discoveries in Heterogenous Catalysis Contributed On Demand Session

Selectivity of multi-pathway surface reactions depends on subtle differences in the activation barriers of competing reactive processes, which is difficult to control. One of the most promising strategies to overcome this problem is to introduce a specific selective interaction between the reactant and the catalytically active site, directing the chemical transformations towards the desired route. This interaction can be imposed via functionalization of a solid catalyst with ligands, promoting the desired pathway via steric constrain and/or electronic effects. The microscopic-level understanding of the underlying surface processes is an important prerequisite for rational design of this new class of ligand-functionalized catalytic materials.

In this contribution, we present a mechanistic study on formation and dynamic changes of a ligand-based heterogeneous Pd catalyst for chemoselective hydrogenation of a,b-unsaturated aldehyde acrolein. Deposition of allyl cyanide as a precursor of a ligand layer renders Pd highly active and nearly 100 % selective toward propenol formation by promoting acrolein adsorption in a desired configuration via the C=O end. Employing a combination of real space microscopic (STM) and in operando spectroscopic (IRAS) surface sensitive techniques, we show that an ordered active ligand layer is formed under operational conditions, consisting of stable butylimin species. In a competing process, unstable amine species evolve on the surface, which desorb in the course of the reaction. Obtained atomistic-level insights into the formation and dynamic evolution of the active ligand layer under operational conditions provide important input required for controlling chemoselectivity by purposeful surface functionalization.

The electrochemical conversion of carbon monoxide (CO) has attracted attention for its use in renewable energy sources. In general, noble metals have been commonly used as effective catalysts for the carbon monoxide reduction (COR) reactions. However, noble metals have many problems including high overpotential and high cost. Therefore, noble-metal-free catalysts with high COR reaction activity are required. Recently, cobalt phthalocyanine (CoPc) has been confirmed to exhibit a high activity in the COR reaction [1].On the other hand, it has been found that the catalytic activity of the iron phthalocyanine (FePc) molecule for oxygen reduction reaction (ORR) is significantly improved by its derivatization [2].

In this study, we elucidate the change in catalytic activity with derivatization of CoPc for the reduction process of CO using first-principles calculations. The catalytic activity for the COR reactions was evaluated using the computational hydrogen electrode model proposed by Nørskov et al. [3] For derivative molecules of CoPc, we considered CoAzPc-4N and CoAzPc-8N. It has been confirmed that such an introduction of N to Pc results in the catalytic activity improvement for ORR [2]. The CoAzPc-4N and CoAzPc-8N molecules have four pyridine and pyrimidine rings, respectively, instead of the four benzene rings of CoPc. In order to evaluate catalytic activities of CoPc and its derivatives for COR reaction, we calculated free energies for various intermediates. Methane and methanol were assumed as final products.

It has been confirmed that (1) the reaction proceeds as CO→*CO→*CHO→*CHOH→*CH₂OH→CH₃OH, (2) methanol is the most stable final product, and (3) the reaction determining step is *CHO→*CHOH for CoPc and its derivatives. The overpotentials of CoPc, CoAzPc-4N, and CoAzPc-8N at the reaction determining step are estimated to be 1.21 V, 1.18 V, and 1.14 V, respectively; such derivatization of CoPc improves the catalytic activity of the COR reaction as well as the ORR of FePc. It has been concluded that the substitution by the electron-withdrawing species leads to the higher catalytic activity of CoPc for the COR reaction.

References

[1] Y. Wu, Z. Jiang, X. Lu, Y. Liang, H. Wang, Nature 575, 639 (2019)

[2] H. Abe, Y. Hirai, S. Ikeda, Y. Matsuo, H. Matsuyama, J. Nakamura, T. Matsue, H. Yabu, NPG Asia Materials 11, 57 (2019)

[3] J. K. Nørskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. Phys. Chem. B 108, 17886 (2004)

Many efforts have been devoted to efficiently utilize methane, the primary component of abundant low-cost natural gas and a major contributor to global warming. Therefore, the conversion of methane to other value-added chemicals has been one of the intensive studies in catalysis in the past several decades. The key to converting methane to such chemicals is to activate highly stable C-H bonds in methane and control the first C-H bond dissociation known as the rate limiting step in the direct methane conversion process. The aim of this project is to develop an efficient catalyst which can reduce the energy barrier for the first C-H bond dissociation and activate methane at low temperatures. Herein, we introduce a novel SnO_x/Cu₂O/Cu(111) inverse model catalyst. The surface structure and the chemical state of SnO_x nanoclusters on Cu₂O/Cu(111) were investigated by Scanning Tunneling Microscopy (STM) and Ambient Pressure X-ray Photoemission Spectroscopy (AP-XPS). Our results show that this novel catalyst activates methane at low temperature and will provide new guidance to design new efficient catalysts for the methane conversion.

Metal nanoparticle supported on metal oxide is the most widely used heterogeneous catalyst. In supported metal catalyst, the interface between metal and support modify their coordination and electronic structure due to strong metal-support interaction (SMSI) which result in altering catalytic performance. Thus, controlling the interface of metal and support is a key strategy for enhancing catalytic activity. However, a fundamental understanding of SMSI is not fully unveiled due to difficulties in characterization in actual catalytic reaction conditions. Herein, using Cu₂O nanocubes and octahedral clusters as model catalysts, we investigated the catalytic performances of Pt/Cu₂O by using ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and diffuse reflectance FT-IR spectroscopy (DRIFT) as in-situ characterization technique. The catalytic measurement as well as in-situ surface characterization results indicates that the facet-dependent interfacial site affects the catalytic activity of CO oxidation significantly, implying the tunability of the catalytic activity by controlling the crystal plane of Cu₂O.

Biofuels can be used to reduce global dependence on fossil fuels while contributing to a carbon neutral cycle. Biobutanol has low volatility and multiple transportation options making it an attractive alternative fuel. To better understand how butanol breaks down in heterogeneous catalytic processes, temperature programmed desorption (TPD) is used to investigate its reaction on TiO₂/Au(111). Inverse model catalysts of interest were formed by depositing TiO₂ nanoparticles onto Au(111) using physical vapor deposition. Low temperature desorption features help to understand how the molecule adorbs to the surface while the high temperature peaks are used to understand chemical reactivity and selectivity. Low temperature peaks indicate different molecular packing of 1- and 2-butanol. The major high temperature products from the reaction of 2-butanol on TiO₂/Au(111) are 2-butanone and butene, observed at ~500 K. The selectivity of the reaction was not altered during successive desorption experiments, indicating that the model catalyst was stable without reoxidation between experiments. Preliminary studies of the reaction of 1-butanol indicate that both reduced and oxidized products are formed, but need to be further studied to identify the species and stability. Atomic force microscopy (AFM) images show that the inverse model catalyst has ~0.16 ML of TiO₂ dispersed across the Au(111) surface in predominantly 1D nanoparticles. Early studies of butanol on TiO₂/Au(111) suggest that the structure affects the reactivity and stability of butanol at high temperatures.

Particle size, catalyst support, and in situ structural transformation can all impact the electrocatalytic activity of metal and oxide catalysts. Here we used a combination of ultrahigh vacuum catalyst synthesis, surface science characterizations including scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), electrochemical measurements, and density functional theory (DFT) modeling to study the electrocatalytic oxygen evolution reaction (OER) at NiFeO_x catalysts deposited on Au(111) and highly oriented pyrolytic graphite (HOPG) electrodes. The formation of well-defined catalysts with precise loadings and atomic compositions allowed us to accurately track their composition and loading dependent electrocatalytic activity, monitor structural transformations that occurred during OER, and create realistic computational models. DFT calculations predicted Fe atoms residing at the edges of NiFeO_x catalysts to be the most favorable OER reaction site, and soft X-ray absorption spectroscopy data suggested a higher population of undercoordinated Fe sites in small, as-synthesized NiFeO_x catalyst particles. However, Au(111) substrates with low catalyst loadings experienced severe surface restructuring during OER that inhibited the activity of very small NiFeO_x particles. At sufficiently high catalyst coverage the Au surface restructuring was suppressed and the NiFeO_x catalysts transformed from their initial structure into an aggregated collection of smaller nanoparticles. The dual effects of Au surface restructuring and catalysts transformation created an unexpected loading-dependent activity trend for Au-supported catalysts. The detrimental impacts of surface restructuring were not found with HOPG substrates, and HOPG-supported NiFeO_x catalysts demonstrated a more expected loading-dependent activity trend. Our study reveals some key fundamental insights into the NiFeO_x catalyst system and points to the importance of catalyst transformations and substrate choice for understanding and optimizing OER performance.

It is well known that VOCs being recognized as major responsible for the increase in global air pollution. Catalytic combustion is an efficient technology for the abatement of VOC, which are oxidized over a catalyst at temperatures much lower than those of the thermal process. Specifically, gold supported catalysts on CeO have shown a great performance in the oxidation of CO, methanol, toluene, etc. Besides, it is important to clarify the role of the support in such reaction. Ceria has the key property of high oxygen storage capacity which originates in its ability to rapidly switch from Ce⁺³ to Ce⁺⁴ oxidation states as the environment changes from reducing to oxidizing and vice versa. Its redox behaviour is influenced by the substituent lattice groups that could be incorporated during different catalyst pretreatments and could affect the oxidation of VOC. This could be understood as the influence of oxygen vacancies and/or absorbed or coadsorbed H on the activation of oxygen molecules. The latter leads to the formation of superoxide and peroxide molecules on the surface, which could in principle be highly reactive towards oxidation of organic molecules.

In this context, we study, by IR spectroscopy (DRIFTS) and mass spectrometry (MS), the interaction of O₂ with the modified CeO₂ based material, by creating vacancies following different treatments in reducing environments. The possible role of the vacancies and/or presence of H atoms in the electron transfer from the surface to the oxygen molecule is discussed. Using AP-XPS we are able to prove that the surface/near surface of CeOx presents a charging effect which could be due to accumulated charge/electrons which then transfer to O₂. Preliminary results showing reactivity of these activated molecular oxygen species (super- and peroxides) are presented regarding the catalytic oxidation of CO.

Despite its high cost, rhodium is a widely applied catalyst primarily used in nanoparticle form for converting toxic gases in automobiles. It is also utilized in organometallic complexes, such as the Wilkinson catalyst, for the hydrogenation of olefins and for converting alkenes to aldehydes through a process known as hydroformylation. So-called “single-atom” catalysis offers an opportunity to reduce the amount of Rh required for traditional heterogeneous catalysis, and a path to heterogenize homogeneous reactions, with the advantage of easy separation of catalyst and product.

Using scanning tunneling microscopy (STM), non-contact atomic force microscopy (nc-AFM) and x-ray photoemission spectroscopy (XPS) we compare the stability of Rh adatoms on two different model supports: α-Fe₂O₃(1-102) and TiO₂(110), both after metal deposition in UHV and in a 2 × 10⁻⁸ mbar water background. We show that the Rh adatoms on α-Fe₂O₃(1-102) sinter in UHV but are stabilized by water up to 150 °C through coordination to 2 – 3 OH ligands. In contrast, Rh adatoms on TiO₂(110) could not be stabilized above room temperature in either environment.

The experimental reflection absorption infrared spectrum of 1-propanol at 180 K is remarkably simple with usually sharp peaks with FWHM (full-width half maxima) of 1.1, 2.1, 1.6, and 4.0 cm^‑1 for the peaks at 1015, 1051, 1455, and 2948 cm^-1, respectively. This suggests that 1-propanol adsorbs as a single conformer, despite the fact that five different conformers of nearly the same energy are known for the gas-phase molecule. The stability of the different 1-propanol conformers on Ag(111) was investigated with DFT using a hybrid density functional (B3LYP) with additional empirical dispersion corrections (B3LYP-D3) using a Ag₁₉ cluster model of the Ag(111) surface. The five conformers (Gg, Gg’, Gt, Tg, Tt) are named with a two-letter code. The first letter (G for gauche, T for trans) corresponds to the structure relative to the C-C-C-O dihedral angle, and the second letter (g for gauche, t for trans) refers to the C-C-O-H dihedral angle. In the gas phase, full geometry optimization gives zero-point corrected relative energies of 0.0, 0.06, 0.08, 0.21, and 0.28 kcal/mol respectively for Tt, Tg, Gt, Gg, and Gg’ conformers at the B3LYP/6311++G(2d,2p) level. The global minimum of 1-propanol at this level is for the Tt conformer. On the silver cluster at the B3LYP-D3/6-311++G(2d,2p) level, the adsorption energies with dispersion correction are 17.84, 17.41, 17.01, 16.86, and 16.84 kcal/mol, respectively for Tt, Gg’, Tg, Gg, and Gt conformers. The Tt conformer is the most stable conformer at this level of theory.The theoretical infrared spectra of all conformers are compared to the experimental spectrum. The calculated Tt spectrum best matches the experimental spectrum at 180 K. This reveals that adsorption on the Ag(111) preferentially stabilizes only one of the five possible conformers.

Recently, restructuring of surfaces that occur under applicable operating conditions has aroused great interest. The structural changes have been correlated with changes in selectivity and linked to bond scission in heterogeneous catalysis. Metal single-crystals make excellent model systems to study restructuring by adsorbates since they are well defined and can be atomically prepared in ultra-high vacuum. The most popular adsorbates that cause well-defined surface restructuring are atomic oxygen and carbon monoxide. Restructuring due to CO has been seen on Cu(111) before¹, and CO is known to induce segregation in bimetallic materials. Furthermore, CO plays multiple roles including reactant, poison, product, or additive in catalytic processes such as CO hydrogenation, CO oxidation, methanol synthesis, and the water gas-shift (WGS) reaction. We have used reflection absorption infrared spectroscopy (RAIRS) to study the ability of CO to restructure Cu(111) and its subsequent impact on reactivity in the WGS reaction under ambient conditions. With polarized infrared radiation, the gas phase and surface species are easily distinguished. In the presence of ambient pressures of CO at 300 K, peaks at 1800-2100 cm^-1 were readily observed and the spectral changes were monitored for different time intervals. The results demonstrate that the Cu surfaces have restructured, and new CO binding sites were created. Upon evacuation of the high-pressure CO, a new peak related to strongly bound CO at 2005-2023 cm^-1 is reported that remains on the surface in UHV at room temperature.

References

1. Science 2016, 351 (6272), 475-478.

Many important catalysts and electrocatalysts for energy and environmental technologies involve late transition metal atoms and nanoparticles dispersed across the surface of some powdered oxide support material.The long-term stability of these materials depends strongly on the strength of bonding of the metal atom and nanoparticles to the support surface. This talk will review recent studies were we have measured these bond strengths to clean and well-defined surfaces using metal vapor adsorption calorimetry.This includes planar single crystal surfaces, as well as the clean surfaces of high-surface-area powdered support materials of the type used industrially. Specifically, we measured the heat of adsorption versus coverage of Ni on MgO(100) and CeO₂(111) surfaces, from which we extracted the adhesion energy (E_adh) at these Ni/oxide interfaces. The results proved the predictive ability of our earlier linear correlation of E_adh with metal oxophilicity based on earlier measurements with metals that are less oxophilic than Ni. We also describe measurements of the adsorption energies of Ag atoms and the adhesion energies of Ag nanoparticles to drop-cast powders of anatase TiO₂ and to the (001) surface of calcium niobate nanosheets deposited in thin films by Langmuir-Blodgett (LB) techniques. This second application shows that LB deposition of multilayer films of such perovskite nanosheets, followed by their annealing in ultrahigh vacuum, provides a powerful new approach for studying surface of mixed oxides that are as clean and well-ordered as typical surfaces of single crystal oxides studied in surface science. The new calorimeter used for the study of these high-area support materials offers the potential for rapid screening of technical support materials for the strength with which they bond metal atoms and nanoparticles, which may aid in discovering better catalyst support materials.

Work supported by DOE-OBES Chemical Sciences Division.

Using time-lapsed ambient-pressure X-ray photoelectron spectroscopy, we investigate the thermal oxidation of single-crystalline Ir(100) films toward rutile IrO₂(110) in situ [1]. We initially observe the formation of a carbon-free surface covered with a complete monolayer of oxygen, based on the binding energies of the Ir 4f and O 1s core level peaks. During a rather long induction period with nearly constant oxygen coverage, the work function of the surface changes continuously as sensed by the gas phase O 1s signal. The sudden and rapid formation of the IrO₂ rutile phase with a thickness above 3 nm, manifested by distinct binding energy changes and substantiated by quantitative XPS analysis, provides direct evidence that the oxide film is formed via an autocatalytic growth mechanism that was previously proposed for PbO and RuO₂.

[1] Z. Novotny, B. Tobler, L. Artiglia, M. Fischer, M. Schreck, J. Raabe, J. Osterwalder, J. Phys. Chem. Lett.2020, 11, 9, 3601–3607.

Transition metals are commonly employed as heterogeneous catalysts for the functionalization of molecules, as well as the synthesis of many bulk materials and useful commodities. One well-studied catalytic application involves the use of an oxygen rich silver surface to induce partial oxidation of ethylene to ethylene oxide. While this reaction is required as a precursor to sterilization procedures and the synthesis of ethylene glycol, the structure of the active catalyst, specifically how oxygen is adsorbed to the silver surface, has not yet been fully elucidated. Past studies suggest that atomic oxygen adsorbs to the surface as well as the region below the surface, the subsurface. To investigate the formation of subsurface oxygen at different oxygen temperatures and surface coverages, we have theoretically studied the adsorption of atomic oxygen to the surface and subsurface of Ag(111) using a combination of density functional theory (DFT), a pairwise-additive site-adsorption model, and Monte Carlo simulations. Results show that oxygen accumulates in the subsurface at surface temperatures greater than ~500 K and oxygen coverages greater than ~1/3 ML, strongly suggesting that subsurface oxygen participates in industrial oxidative catalysis. Future studies will explore the role of subsurface oxygen in surface reconstruction and catalytic mechanisms of Ag(111), as well as the formation of subsurface oxygen on Ag(110) and Ag(100) surfaces.

The adhesion energy is also the key factor that determines how strongly metal nanoparticles bind to different catalyst support materials.² We present SCAC measurements of the heat of adsorption of azulene to Pt(111) and show that it binds ~100 kJ/mol more strongly than naphthalene to Pt(111). Azulene has been shown to have very similar electronic character to the pentagon-heptagon (5-7)-type defects in graphene, while naphthalene resembles perfect graphene. This SCAC result therefore implies that Pt(111) binds ~100 kJ/mol more strongly to (5-7) type defects in graphene than to perfect regions of graphene.From this we estimate that the adhesion energy of Pt nanoparticles to such (5-7)-type defects in graphene will be ~0.63 J/m² higher (locally) than to perfect graphene surfaces.This will greatly stabilize Pt nanoparticles at such defects on graphene-like and graphite-like catalyst support materials.For example, all the Pt atoms in a 1 nm particle would be ~30 kJ/mol more stable, and have an activation energy for sintering that is ~30 kJ/mol higher, when attached to such a defect.

.

Supported by the National Science Foundation.

1. ACS Catalysis9 (2019) 8116.

1. 2.Charles T. Campbell and Zhongtian Mao, ACS Catalysis7 (2017) 8460 (2017).

Due to the recent prevalence of small hydrocarbons, there has been renewed interest in direct dehydrogenation of small alkanes to the corresponding alkenes. One of the major issues in this reaction is the deactivation of catalysts due to coke formation. Here, we report a new RhCu single-atom alloy which displays considerable activity for C-H activation without coke formation. First, using a combination of scanning tunneling microscopy, temperature programmed desorption, and infrared spectroscopy, we characterize the model catalyst surface. We find that Rh atoms exist as isolated sites in the Cu host. We correlate this structure with the binding energy and vibrational frequency of CO on the isolated Rh sites. With knowledge of the atomic-scale structure, we then examine how the isolated Rh sites promote C-H activation. Using methyl iodide as a reporter on C-H activation, we find the isolated Rh sites promote C-H activation at a significantly lower temperature than Cu(111). We observe the formation of methane (from the hydrogenation of methyl groups) in addition to the formation of ethene (via coupling of CH₂ to CH₃ followed by beta-dehydrogenation). This is in strong contrast to extended Rh ensembles where coke formation is apparent. Together, these results indicate that RhCu single-atom alloys offer significant opportunities for efficient and coke-free C-H activation.

Dissociative chemisorption of methane on a Ni catalyst via C-H bond cleavage is generally believed to be the rate-limiting step in the industrial steam reforming reaction. The commercial importance of this reaction, and methane's role as a model system for unravelling the energetics and dynamics of dissociative chemisorption, has resulted in extensive experimental and theoretical study including efforts to improve the absolute accuracy of DFT-based calculations of catalytically important reactions. While methane is the majority component of natural gas feedstock, ethane (C₂H₆) can also represent a significant fraction.

Previously, we used energy and vibrationally state-selected methane molecules in a supersonic molecular beam to quantify reaction probability over a wide range of energies and energetic configurations (i.e. the distribution of energy among translational, vibrational, rotational, and surface degrees of freedom). Here, we report on the extension of that approach to ethane molecules prepared with similar energy configurations. Our goal is to understand how reactivity patterns observed in the smaller methane molecule may extend to larger molecules as well as to provide accurate benchmark data for computational studies of increasingly larger and more complex molecule-surface reaction systems.

Single-atom catalysts (SACs) consisting of late-transition metals on oxide supports have recently gained significant world-wide attention due to their well-defined sites that enable more selective chemistry and the fact that they use precious metals at the ultimate limit of atom efficiency. SACs involving reducible oxide supports are good catalysts for oxidation reactions, but they tend to deactivate by sintering under reducing conditions. On the other hand, single-atom alloys (SAAs) are robust towards sintering and exhibit exceptional catalytic performance for a wide range of reactions under reducing conditions, but some SAAs deactivate under oxidizing environments. This project is aimed at systematically exploring catalytic reactivity between the SAC and SAA oxidation state extremes. Historically, bimetallic RhCu and supported Rh systems have been investigated for several reactions including CO oxidation, NO_x reduction, and the hydrogenation of hydrocarbons. Recent studies conducted on RhCu SAAs have shown that while the Rh sites bind CO strongly, CO desorbs reversibly. In contrast, model Rh/CuO_x SACs perform CO oxidation just above room temperature. However, there is a lack of understanding of structure-function relationships between these two extremes when the surface is partially oxidized. We report a study of CO oxidation on partially oxidized RhCu alloys using temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and low energy electron diffraction (LEED). The TPD results demonstrate that CO oxidation occurs around 456 K on an oxidized RhCu SAA. Increasing the Rh coverage led to gradual decrease of the CO oxidation temperature to 362 K at 25% of a monolayer of Rh. Isotopic TPD experiments provided evidence of a Mars van Krevelen (MvK) CO oxidation mechanism. Furthermore, more fully oxidized RhCu SAAs were inactive for CO oxidation meaning that intermediate oxidation states of the SAA are most efficient for CO oxidation. LEED and IR results provide evidence for how the incorporation of Rh in the system affects its structure and the accessibility of the Rh sites to reactants. Together, these results begin to shed light on the effect of surface oxidation on the structure and reactivity of Rh sites in Cu-based SACs.

Fourier-transform Infrared (FTIR) spectroscopy is a powerful technique for identification of small molecules adsorbed to metal surfaces. We have added FTIR to an ultra-high vacuum (UHV) chamber as a non-destructive and highly sensitive surface analysis technique. Because IR measurements can be performed in UHV conditions, interference from atmospheric species are avoided, while enabling investigation of catalytic systems, like carbon monoxide (CO) to carbon dioxide (CO₂) on Rh(111). To determine the reactivity of the various oxide phases, the oxidation reaction of CO to CO₂on oxidized Rhodium (Rh) will be utilized as a probe reaction. We will be able to determine the chemical significance of various oxygen phases on different Rh surfaces, and the CO coverage and binding sites on the different oxygenaceous phases. Studying CO oxidation on different Rh surfaces will provide atomic level information regarding oxidation reactions, progressing the understanding of various surface phases relevant to many Rh catalyzed processes. Past exposure conditions determined that at low temperatures, CO was observed to adsorb along (2x1)-O and RhO2 domain boundaries, and O_sub replenished the reacted oxygen at these boundaries at higher temperatures. When CO was prolonged exposure it consumed all O_sub and reacted with oxides at the defects. In recent studies, it was determined that there are multiple reaction pathways available for CO oxidation, but at temperatures at or below 350K reaction sites are not regenerate. Via FTIR, these and other reaction sites of CO oxidation will be investigated to determine reaction pathways or mechanisms.Methods developed for Rh can also be applied to other metal surfaces and small molecules of interest.

Acetic acid decomposition on metal surfaces and the effects of water on the decomposition are good model systems for solvent effects on small oxygenates with applications in biomass conversion. Numerous studies have found that solvents influence the selectivity and rate of heterogeneous catalytic reactions. Fundamental understanding of how water affects acetic decomposition on metal surfaces gives us valuable insight into how water changes the selectivity of decomposition reactions on different metals, further enabling bottom up design of effective catalyst and catalyst system. Here we present a combined density functional theory (DFT) calculations, microkinetic analysis and AP-XPS study of the effects of co-adsorbed water on acetic acid decomposition over Pd (111). The combination of theory and experiments is used to improve the AP-XPS analysis as well as providing atomistic insights into the mechanism. AP-XPS data show an increase in surface coverage of both chemisorbed acetic acid and acetate, while coverage of CO decreases in the presence of co-adsorbed water. MS-RGA data collected concurrently during the exposure also show an increase in the ratio of CO2(g)/CO(g) up to 80% for exposure of acetic acid with water. This supports the microkinetic analysis, where we show that in the absence of co-adsorbed water, the decarboxylation pathway (CO2) is more favorable than the decarbonylation pathway (CO) but in the presence of co-adsorbed water, the decarboxylation pathway is more favored. On Pd (111), the shift in selectivity is mostly due to changes in the OC-O bond cleavage. Water has a similar effect on the selectivity for this reaction on Pt(111) but the water has a larger effect on OC-OH bond cleavage on Pt than Pd.

Understanding the interaction of oxygen with transition metal surfaces is important in many areas including corrosion and catalysis. Of interest to us is the formation and chemistry of subsurface oxygen (O_sub); oxygen atoms dissolved in the near-surface region of catalytically active metals. The goal of these studies is to understand how incorporation of O_subinto the selvedge alters the surface structure and chemistry. The oxygen – Ag system, in particular, has been studied extensively both experimentally and theoretically because of its role in two important heterogeneously catalyzed industrial reactions: the epoxidation of ethylene to produce ethylene oxide and the partial oxidation of methanol to produce formaldehyde. In addition, the O/Rh and O/Ag systems serve as models for the dissociative chemisorption of diatomic molecules on close packed metal surfaces. Despite extensive research, there remain questions about the fundamental chemistry of the O/Ag system. Rh is also used in partial oxidation reactions, and its response to adsorbed oxygen provides an interesting complement to Ag. Where Ag extensively reconstructs, Rh does not. In particular, the structure of the catalytically active surface remains poorly understood under conditions of high oxygen coverages or subsurface oxygen. To improve our understanding of this system, we use ultra-high vacuum (UHV) surface science techniques to characterize Ag and Rh surfaces after exposure to atomic oxygen (AO) to obtain O coverages in excess of 1 ML. AO is generated by thermally cracking molecular O₂. We then use low-energy electron diffraction (LEED) and UHV Scanning Tunneling Microscopy (UHV-STM) to further characterize the various oxygenaceous structures produced, and quantify the amount of oxygen with temperature programmed desorption (TPD). We have found that the surface temperature during deposition is an important factor for the formation of O_sub and the consequent surface structures. Finally, we have recently found that Rh surfaces are significantly more reactive towards CO oxidation when O_sub is present. This enhanced reactivity is located at the interface between the less reactive RhO₂ oxide and O-covered metallic Rh. These results reveal the conditions under which O_sub is formed and stable, and show that O_sub also leads to enhanced reactivity of oxidized metal surfaces.