Single-crystal Model Catalysts Research Articles

Hydrogen Peroxide Generation and Hydrogen Oxidation Reaction on Pt/Co/Pt(111) and Pt/Co/Pt(100) Single-Crystal Model Catalyst Surface

Hydrogen Peroxide Generation and Hydrogen Oxidation Reaction on Pt/Co/Pt(111) and Pt/Co/Pt(100) Single-Crystal Model Catalyst Surface

Pt-surface stabilization by high-entropy alloys for enhancing oxygen reduction reaction property: Single-crystal model catalyst study

In this work, we study the oxygen reduction reaction (ORR) properties of Pt-containing 3d transition-metal high-entropy alloy (Pt-HEA) surfaces, focusing on the constituent alloying elements. The surface Pt and underlying Cr-Mn-Co-Ni(111) (Pt/Cr-Mn-Co-Ni(111)) stacked lattice layers, which are synthesized through the vacuum deposition of the underlying alloy and surface Pt stacking layers on Pt(111) substrate, exhibit high pristine ORR activity and structural stability under potential-cycle loading, compared to Pt/Cr-Co-Ni, Pt/Mn-Co-Ni, and Pt/Co-Ni(111) surfaces. The outperformed ORR properties are attributed to the effective suppression of the surface segregation of Cr. This study demonstrates that not only the “high-entropy” effect induced by increasing the numbers of constituent elements but also the “chemical affinity” of Pt and the individual HEA constituent elements determine the ORR performances of Pt-HEA.

Enhanced Oxygen Evolution Activity and Stability of RuO2(110) Surface by Ti Doping

Introduction Developing highly active and durable electrocatalysts for the oxygen evolution reaction (OER) is essential for efficient hydrogen production through polymer electrolyte membrane water electrolysis (PEMWE). Although RuO2 exhibits the highest OER activity among pure transition metal oxide catalysts, its low durability hampers its practical application as an OER catalyst for PEMWE. Recent studies have shown that the incorporation of dissimilar metal elements, such as Ni, Mn, and Cu, effectively enhances the OER activity and stability of RuO2 in acidic electrolytes (1-3). In this study, we focus on Ti, a corrosion-resistant element in the PEMWE environment, and investigate the effects of doping amounts on the OER activity and stability of a RuO2(110) single crystal model catalyst surface. Experimental Non-doped rutile-type TiO2(110) single crystals were used as sample substrates. Ru and Ti, with a total thickness of 40 nm, were deposited onto the substrate at 400 ℃ under 0.5 Pa-O2 partial pressure using the arc-plasma deposition (APD) method. Subsequently, the sample was post-annealed at 400℃ for 1 hour in air using a tube furnace. Hereafter, the samples are denoted as RuO2-Tix (x represents the atomic percentage of doped Ti).Oxygen evolution activity was measured by linear sweep voltammetry (LSV) at a scan rate of 20 mV s-1 in N2-purged 0.1 M HClO4 at room temperature. Chronopotentiometry (CP) was conducted at 1 mA cm-2 for 120 min to evaluate the durability. The electrochemical impedance spectroscopy was conducted at 1.6 V vs. reversible hydrogen electrode (RHE) to measure the solution resistance. All the LSV and CP data were iR-corrected using the Rs values. Out-of-plane X-ray diffraction (XRD) analysis was carried out to evaluate the crystal structure. The dissolution amounts of Ru into the electrolyte after CP were evaluated by inductively coupled plasma mass spectrometry (ICP-MS). Results and Discussions Fig. 1(a) presents the out-of-plane XRD patterns of Ti-doped and non-doped RuO2(110) samples. The results indicate that (110)-oriented Ti-doped and non-doped RuO2 thin films were successfully obtained on TiO2 substrates. However, the peak position of RuO2(110) gradually shifts to the lower angle side, and the peak becomes less distinct with increasing Ti-doping amounts, suggesting that doped Ti is dissolved in the RuO2 crystal lattice and the crystallinity decreases with increasing doping amounts. Initial LSV curves for OER are shown in Fig. 1(b). The current density of RuO2-Ti4, 5, and 7 samples significantly increases at higher potential regions above 1.6 V vs. RHE, while RuO2-Ti2 and 10 exhibit decreased activity. The CP curves presented in (c) demonstrate that Ti doping, especially at 5 and 7 at.%, effectively suppresses the overpotential increase during CP. The dissolution amounts of Ru after 120 min of CP are summarized in (d). The dissolution amounts tend to decrease with increasing Ti-doping amounts. These results clearly show that an appropriate amount of Ti-doping effectively enhances both the OER activity and electrochemical stability of the RuO2(110) surface.AcknowledgmentsThis study was partly supported by JSPS KAKENHI Grant Number 21H01661, 22K19078 and Toyota Mobility Foundation Hydrogen Initiative. References Z. Y. Wu, F. Y. Chen, B. Li, S. W. Yu, Y. Z. Finfrock, D. M. Meira, Q. Q. Yan, P. Zhu, M. X. Chen, T. W. Song, Z. Yin, H. W. Liang, S. Zhang, G. Wang and H. Wang, Nat. Mater., 22, 100 (2023). S. Chen, H. Huang, P. Jiang, K. Yang, J. Diao, S. Gong, S. Liu, M. Huang, H. Wang and Q. Chen, ACS Catal., 10, 1152 (2020). J. Su, R. Ge, K. Jiang, Y. Dong, F. Hao, Z. Tian, G. Chen and L. Chen, Adv Mater, 30, 1801351 (2018). Figure 1

Uniform Catalytic Nanocrystals: From Model Catalysts to Efficient Catalysts

ConspectusSolid catalysts play a key role in the chemical industry, energy, and the environment. Fundamental understanding of heterogeneous catalysis exerted by solid surfaces is useful for structural designs of efficient catalysts but is challenging due to the complexity. Emerging uniform catalytic nanocrystals (NCs) with controlled and well-defined surface structures have brought new opportunities for fundamental understanding and directed explorations of efficient catalysts. In a previous Account ( Acc. Chem. Res. 2016, 49, 520−527), I postulated and exemplified a concept of oxide nanocrystal model catalysts for the fundamental investigations of oxide catalysis without the “materials gap” and “pressure gap” which are often encountered using the traditional single crystal model catalysts. In this Account, I summarize our effort in fabricating efficient uniform nanocrystal catalysts based on the fundamental understanding of structure–activity relations and reaction mechanisms acquired by oxide nanocrystal model catalyst studies. Directed by the fundamental understanding that the Cu2O{110} facet is active in catalyzing propylene epoxidation with O2 and the fine Cu2O nanocube exposes high densities of Cu2O{110} edge sites, we successfully explore fine Cu2O nanocubes as a highly selective catalyst for propylene epoxidation with O2. Directed by the fundamental understanding that the Cu{100} facet is the active Cu facet in catalyzing the water–gas shift (WGS) reaction, we successfully fabricate a highly active ZnO/fine Cu nanocube WGS catalyst with enhanced ZnO-CuCu{100} active site density. Directed by the fundamental understanding of TiO2 facet effects on the surface band bending and adsorbate–surface interfacial energy level alignment, and consequent photocatalytic performance, we successfully fabricate highly active TiO2{001} NC-based photocatalysts for photocatalytic CH4 conversions. These results adequately exemplify the concept of fundamental understanding-directed explorations of efficient catalysts following a strategy of “identification of active site structure + maximization of active site density”, which, together with the advancement of controlled-synthesis methods, is expected to greatly accelerate the explorations of novel and efficient catalysts in future.

Water Formation Kinetics on Co(0001) at Low and Near-Ambient Hydrogen Pressures in the Context of Fischer–Tropsch Synthesis

Understanding the kinetics of oxygen removal from catalytically active metal surfaces by hydrogen is important for several catalytic reactions such as Fischer–Tropsch synthesis, methanation of CO or CO2, and the reverse-water–gas-shift reaction. Motivated by FTS, a Co(0001) single crystal model catalyst was used to study the kinetics of oxygen removal through reaction with hydrogen. Kinetic studies in the 10–7–10–4 mbar H2 pressure regime show that water formation is first order in the surface hydrogen concentration while the order in oxygen concentration changes from one at low oxygen coverage to zero at high oxygen coverage. In situ XPS shows that the hydrogen surface concentration saturates around 10–1 mbar and on this basis the typical temperature of 450 K needed for water formation in this pressure regime can be considered as typical for high pressures as well. The absence of OH buildup during the reaction points to O + H as the rate-limiting step, with a barrier of ∼120 kJ mol–1. Such a high barrier shows that slow removal of adsorbed oxygen from the surface of reactive metal catalysts such as cobalt may be rate-limiting for the overall reaction.

(Digital Presentation) Oxygen Evolution Activity of RuO2/MO2/TiO2(110) (M = Ir, Sn) Surfaces in Acidic Electrolyte

Introduction Developing highly active and durable electrocatalysts for oxygen evolution reaction (OER) are required for efficient hydrogen production by polymer electrolyte membrane water electrolysis (PEMWE). While RuO2 shows the highest OER activity among pure transition metal oxides catalysts, the low durability hinders its practical application as OER catalyst for PEMWE. Recent studies demonstrated that RuO2 mixed with other metal oxides, such as IrO2 and SnO2 etc. showed higher activity and durability than pure RuO2 (1-3). When the RuO2 deposited on the other metal oxides, interfacial resistance and lattice strain which derives from the lattice mismatch should influence on the OER properties. In this study, to clarify the influence of interface between different rutile-type oxides, we fabricated RuO2/MO2/TiO2(110) (M = Ir or Sn) single crystal model catalyst and evaluated the oxygen evolution activity and durability. Experimental Nb-doped TiO2(110) (Nb: 1 wt%) single crystal substrate was etched in 20 % HF solution for 10 min and annealed at 1000 ℃ in air. After the sample transfer to the APD chamber, 2 nm-thick Ir or Sn was deposited onto the substrate at room temperature under 0.5 Pa-O2 partial pressure and annealed at 350 ℃ for 30 min under the same O2 pressure. After that, 4 nm-thick Ru was deposited on the MO2/TiO2(110) and was annealed under the same condition of the MO2 layer. Hereafter, the sample was denoted as RuO2/MO2/TiO2.Oxygen evolution activity was measured by linear sweep voltammetry (LSV) at a scan rate of 1 mV/s in N2-purged 0.1 M HClO4 at room temperature. The electrochemical impedance spectroscopy was conducted at 1.5 V vs. reversible hydrogen electrode (RHE) to measure the resistance of solution (Rs), electrodes (Re), and charge transfer for OER (Rct). The LSV curves were corrected using the measured Rs. Chronopotentiometry was conducted at 0.5 mA/cm2 for 120 min to evaluate the durability. In-plane XRD analysis was performed for estimating lattice strain in the surface RuO2 layer. Results and Discussions XRD analysis showed that both of the MO2 interlayer and RuO2 catalyst layer grow with (110) orientation on the TiO2(110) substrate. Figure 1(a) shows initial LSV curves for OER. The onset potential of RuO2/SnO2/TiO2 samples clearly shifted to lower potential side relative to the RuO2/IrO2/TiO2 and RuO2/TiO2 samples. The overpotential at 0.1 mA cm-2 of the RuO2/SnO2/TiO2 (b) was ca. 25 mV lower than the other two samples. The chronopotentiometric curves (c) showed the overpotential of RuO2/TiO2 increased by 250 mV after 120 min. On the other hand, change in the overpotential of the RuO2/SnO2/TiO2 was only 25 mV, indicating the high durability.Cole-cole plots at 1.5 V (d) revealed that both of the Re and Rct were significantly reduced owing to SnO2 interlayer, while the IrO2 layer slightly increased the Re and decreased the Rct. In-plane XRD results showed the compressive lattice strain was induced in [001] direction in RuO2 layer on TiO2 substrate and decreased by insertion of IrO2 and SnO2 layer. However, the lattice spacing in [-110] direction was almost the same for all the samples. Figure 1 (e) shows the relation between Rct and the lattice strain in [001] direction. The Rct tended to decrease with decreasing the compressive strains, suggesting the OER activity of RuO2(110) surface is related to the anisotropic strains in [001] direction. The results demonstrated that the activity and durability of RuO2/TiO2 catalysts can be improved by insertion of SnO2 layer at the catalyst/substrate interface.AcknowledgmentsThis study was partly supported by JSPS KAKENHI Grant Number 21H01661 and Toyota Mobility Foundation Hydrogen Initiative. References R. V. Genova-Koleva, F. Alcaide, G. Álvarez, P. L. Cabot, H.-J. Grande, M. V. Martínez-Huerta and O. Miguel, Journal of Energy Chemistry, 34, 227 (2019).J. Li, X. Qin, G. Chen and G. Chen, Journal of the Taiwan Institute of Chemical Engineers, 125, 186 (2021).R. Samanta, P. Panda, R. Mishra and S. Barman, Energy & Fuels, 36, 1015 (2022). Figure 1

Cu2O Nanocrystal Model Catalysts

Comprehensive SummaryModel catalysts approach is widely adopted in fundamental studies of complex heterogeneous catalysis. Traditional model catalysts are single crystal‐based model catalysts, whose results, however, sometimes suffer from the “material gap” and “pressure gap” when applied to working catalysts. Recently, uniform catalytic nanocrystals with well‐defined structures have emerged, and we have put forward a concept of nanocrystal‐based model catalysts for fundamental studies of complex heterogeneous catalysis under conditions approaching working catalysts as closely as possible. In this perspective, we summarize our research progresses on Cu2O‐based nanocrystal model catalysts. Following a brief introduction, we summarize controlled synthesis of uniform Cu2O nanocrystals with various morphologies, morphology‐dependent surface compositions and structures and surface chemistry of uniform Cu2O nanocrystals, and heterogeneous catalysis of Cu2O‐based nanocrystals. Finally, we conclude the concept of nanocrystal‐based model catalysts and outlook that uniform nanocrystals can act not only as model catalysts but also as efficient catalysts. What is the most favorite and original chemistry developed in your research group?By far, the most favorite and original chemistry developed in my research group is the concept of “nanocrystal model catalysts”, which overcomes the so‐called “materials gap” and “pressure gap” encountered by the research field of surface chemistry for heterogeneous catalysis using traditional single crystal model catalysts.How do you get into this specific field? Could you please share some experiences with our readers?Surface chemistry studies of single crystal model catalysts had been considered to be able to achieve the fundamental understanding of complex heterogeneous catalysis before the “materials gap” and “pressure gap” were found to exist between single crystal model catalysts and working powder catalysts. Since then, to overcome the “materials gap” and “pressure gap” has been a main goal of the research field of surface chemistry for heterogeneous catalysis. I established my research group in 2004 when catalytic nanocrystals with uniform and well‐defined surface structures emerged quite a lot, and then had the idea of using nanocrystals as a novel type of model catalysts for fundamental studies of complex heterogeneous catalysis under conditions approaching workingcatalysts as closely as possible. Our work has successfully validated the concept of “nanocrystal model catalysts”. Therefore, my experience is to bear the important problems of the research field in mind and keep learning new knowledge from related research fields.What is the most important personality for scientific research?In my personal opinion, intuitivism, persistence, and self‐motivation are the most important personalities for scientific research.What is your favorite journal(s)?I prefer to publish my papers in journals that have large readerships so that the reported new knowledge can be widely shared.What are your hobbies? What's your favorite book(s)?My hobbies are the things that do not need thinking and special attention. These things make me truly relaxed. My favorite books are history of science with comments written by famous scientists (translated) in Chinese.If you have anything else to tell our readers, please feel free to do so.Scientific research is full of inter‐transformed frustration and happiness, but allows one to demonstrate the intelligence and capability freely and independently.

Investigation of Active Catalysts at Work.

ConspectusEven after being in business for at least the last 100 years, research into the field of (heterogeneous) catalysis is still vibrant, both in academia and in industry. One of the reasons for this is that around 90% of all chemicals and materials used in everyday life are produced employing catalysis. In 2020, the global catalyst market size reached $35 billion, and it is still steadily increasing every year. Additionally, catalysts will be the driving force behind the transition toward sustainable energy. However, even after having been investigated for 100 years, we still have not reached the holy grail of developing catalysts from rational design instead of from trial-and-error. There are two main reasons for this, indicated by the two so-called “gaps” between (academic) research and actual catalysis. The first one is the “pressure gap”, indicating the 13 orders of magnitude difference in pressure between the ultrahigh vacuum lab conditions and the atmospheric pressures (and higher) of industrial catalysis. The second one is the “materials gap”, indicating the difference in complexity between single-crystal model catalysts of academic research and the real catalysts, consisting of metallic nanoparticles on supports, promoters, fillers, and binders. Although over the past decades significant efforts have been made in closing these gaps, many steps still have to be taken. In this Account, I will discuss the steps we have taken at Leiden University to further our fundamental understanding of heterogeneous catalysis at the (near-)atomic scale. I will focus on bridging the pressure gap, though we are also working on closing the materials gap. Over the past years, we developed state-of-the-art equipment that is able to investigate the (near-)atomic-scale structure of the catalyst surface during the chemical reaction using several surface-science-based techniques such as scanning tunneling microscopy, atomic force microscopy, optical microscopy, and X-ray-based techniques (surface X-ray diffraction, grazing-incidence small-angle X-ray scattering, and X-ray reflectivity, in collaboration with ESRF). Simultaneously with imaging the surface, we can investigate the catalyst’s performance via mass spectrometry, enabling us to link changes in the catalyst structure to its activity, selectivity, or stability. Although we are currently investigating many industrially relevant catalytic systems, I will here focus the discussion on the oxidation of platinum during, for example, CO and NO oxidation, the NO reduction reaction on platinum, and the growth of graphene on liquid (molten) copper. I will show that to be able to obtain the full picture of heterogeneous catalysis, the ability to investigate the catalyst at the (near-)atomic scale during the chemical reaction is a must.

Formaldehyde Selectivity in Methanol Partial Oxidation on Silver: Effect of Reactive Oxygen Species, Surface Reconstruction, and Stability of Intermediates

Selective oxidation reactions on heterogeneous silver catalysts are essential for the mass production of numerous industrial commodity chemicals. However, the nature of active oxygen species in such reactions is still debated. To shed light on the role of different oxygen species, we studied the methanol oxidation reaction on Ag(111) single-crystal model catalyst surfaces containing two dissimilar types of oxygen (electrophilic, Oe and nucleophilic, On). X-ray photoelectron spectroscopy and low energy electron diffraction experiments suggested that the atomic structure of the Ag(111) surface remained mostly unchanged after accumulating low Oe coverage at 140 K. Temperature-programmed reaction spectroscopic investigation of low coverages of Oe on Ag(111) revealed that Oe was active for methanol oxidation on Ag(111) with a high selectivity toward formaldehyde (CH2O) production. High surface oxygen coverages, on the other hand, triggered a reconstruction of the Ag(111) surface, yielding Ag oxide domains, which catalyzes methanol total oxidation to CO2 and decreases the formaldehyde selectivity. This important finding indicates a trade-off between CH2O selectivity and methanol conversion, where 93% CH2O selectivity can be achieved for an oxygen surface coverage of θO = 0.08 ML (ML = monolayer) with moderate methanol conversion, while methanol conversion could be boosted by a factor of ∼4 for θO = 0.26 ML with a suppression of CH2O selectivity to 50%. Infrared reflection absorption spectroscopy results and density functional theory calculations indicated that Ag oxide contains dissimilar adsorption sites for methoxy intermediates, which are also energetically less stable than that of the unreconstructed Ag(111). The current findings provide important molecular-level insights regarding the surface structure of the oxidized Ag(111) model catalyst directly governing the competition between different reaction pathways in methanol oxidation reaction, ultimately dictating the reactant conversion and product selectivity.

Significance of the Mn-Oxidation State in Catalytic and Noncatalytic Promotional Effects of MnOx Domains in Formic Acid Dehydrogenation on Pd/MnOx Interfaces

Influence of MnOx overlayers/nanoclusters deposited on the Pd(111) single crystal model catalyst surface on the catalytic dehydrogenation of double-deuterated formic acid (FA, DCOOD) was studied un...

Enhancement of Formic Acid Dehydrogenation Selectivity of Pd(111) Single Crystal Model Catalyst Surface via Brønsted Bases

The influence of ammonia (NH3) on the doubly deuterated formic acid (DCOOD, FA) dehydrogenation selectivity for a Pd(111) single crystal model catalyst surface was investigated under ultrahigh vacuum conditions using temperature-programmed desorption and temperature-programmed reaction spectroscopy techniques. NH3 adsorption on Pd(111) revealed reversible, molecular desorption without any significant decomposition products, while DCOOD adsorption on Pd(111) yielded D2, D2O, CO, and CO2 as a result of dehydration and dehydrogenation pathways. Functionalizing the Pd(111) surface with ammonia suppressed the FA dehydration and enhanced the dehydrogenation pathway. The boost in the FA dehydrogenation of Pd(111) in the presence of NH3 can be linked to the ease of FA deprotonation as well as the stabilization of the decomposition intermediate (i.e., formate) due to the presence of ammonium counterions on the surface. In addition, the presence of a H-bonded ammonia network on the Pd(111) surface increased the hydrogen atom mobility and decreased the activation energy for molecular hydrogen desorption. In the presence of NH3, catalytic FA decomposition on Pd(111) also yielded amidation reactions, which further suppressed CO liberation and prevented poisoning of the Pd(111) active sites due to strongly bound CO species.

Combining Planar Laser-Induced Fluorescence with Stagnation Point Flows for Small Single-Crystal Model Catalysts: CO Oxidation on a Pd(100)

A stagnation flow reactor has been designed and characterized for both experimental and modeling studies of single-crystal model catalysts in heterogeneous catalysis. Using CO oxidation over a Pd(100) single crystal as a showcase, we have employed planar laser-induced fluorescence (PLIF) to visualize the CO2 distribution over the catalyst under reaction conditions and subsequently used the 2D spatially resolved gas phase data to characterize the stagnation flow reactor. From a comparison of the experimental data and the stagnation flow model, it was found that characteristic stagnation flow can be achieved with the reactor. Furthermore, the combined stagnation flow/PLIF/modeling approach makes it possible to estimate the turnover frequency (TOF) of the catalytic surface from the measured CO2 concentration profiles above the surface and to predict the CO2, CO and O2 concentrations at the surface under reaction conditions.

Molecular beam/infrared reflection-absorption spectroscopy apparatus for probing heterogeneously catalyzed reactions on functionalized and nanostructured model surfaces.

A new custom-designed ultrahigh vacuum (UHV) apparatus combining molecular beam techniques and in situ surface spectroscopy for reactivity measurements on complex nanostructured model surfaces is described. It has been specifically designed to study the mechanisms, kinetics, and dynamics of heterogeneously catalyzed reactions over well-defined model catalysts consisting of metal nanoparticles supported on thin oxide films epitaxially grown on metal single crystals. The reactivity studies can be performed in a broad pressure range starting from UHV up to the ambient pressure conditions. The UHV system includes (i) a preparation chamber providing the experimental techniques required for the preparation and structural characterization of single-crystal based model catalysts such as oxide supported metal particles or ordered oxide surfaces and (ii) the reaction chamber containing three molecular beams-two effusive and one supersonic, which are crossed at the same point on the sample surface, infrared reflection-absorption spectroscopy for the detection of surface-adsorbed species, and quadrupole mass spectrometry for gas phase analysis. The supersonic beam is generated in a pulsed supersonic expansion and can be modulated via a variable duty-cycle chopper. The effusive beams are produced by newly developed compact differentially pumped sources based on multichannel glass capillary arrays. Both effusive sources can be modulated by a vacuum-motor driven chopper and are capable of providing high flux and high purity beams. The apparatus contains an ambient pressure cell, which is connected to the preparation chamber via an in situ sample transfer system and provides an experimental possibility to study the reactivity of well-defined nanostructured model catalysts in a broad range of pressure conditions-up to ambient pressure-with the gas phase analysis based on gas chromatography. Additionally, a dedicated deposition chamber is connected to the preparation chamber, which is employed for the in situ functionalization of model surfaces with large organic molecules serving as promoters or modifiers of chemical reactions. We present a general overview of the apparatus as well as a description of the individual components and their interplay. The results of the test measurements involving the most important components are presented and discussed.

CO as a Promoting Spectator Species of CxHy Conversions Relevant for Fischer–Tropsch Chain Growth on Cobalt: Evidence from Temperature-Programmed Reaction and Reflection Absorption Infrared Spectroscopy

Cobalt-catalyzed low temperature Fischer–Tropsch synthesis is a prime example of an industrially relevant reaction in which CxHy intermediates involved in chain growth react in the presence of a large quantity of COad. In this study, we use a Co(0001) single-crystal model catalyst to investigate how CO, adsorbed alongside CxHy adsorbates affects their reactivity. Temperature-programmed reaction spectroscopy was used to determine the hydrogen content of the CxHy intermediates formed at different temperatures, and infrared absorption spectroscopy was used to obtain more specific information on the chemical identity of the various reaction intermediates formed. Ethene, propene, and but-1-ene precursors decompose below 200 K. The 1-alkyne adsorbate is identified as a major product, and some alkylidyne species form as well when the initial alkene coverage is high. The surface hydrogen atoms produced in the low temperature decomposition step start leaving the surface >300 K. When an alkyne/Had-covered surface i...

Selective Catalytic Ammonia Oxidation to Nitrogen by Atomic Oxygen Species on Ag(111)

Ammonia-selective catalytic oxidation was studied on the planar Ag(111) single-crystal model catalyst surface under ultra-high-vacuum (UHV) conditions. A variety of oxygen species were prepared via ozone decomposition on pristine Ag(111). Surface coverages of oxygen species were quantified by temperature-programmed desorption (TPD) and X-ray photoemission spectroscopy techniques. Exposure of ozone on Ag(111) at 140 K led to a surface atomic oxygen (Oa) overlayer. Low-energy electron diffraction experiments revealed that annealing of this atomic oxygen-covered Ag(111) surface at 473 K in UHV resulted in the formation of ordered oxide surfaces (Oox) with p(5×1) or c(4×8) surface structures. Ammonia interactions with O/Ag(111) surfaces monitored by temperature-programmed reaction spectroscopy indicated that disordered surface atomic oxygen selectively catalyzed N–H bond cleavage, yielding mostly N2 along with minor amounts of NO and N2O. Higher coverage O/Ag(111) surfaces, whose structure was tentatively ass...

Heterogeneous catalysis: More than skimming the surface.

The high temperatures and pressures used in heterogeneous catalysis make it difficult to observe catalysts using conventional techniques. Now, adsorbed product molecules on the surface of a single-crystal model catalyst have been observed during catalysis using a custom-built scanning tunnelling microscope that can work in situ.

Shape-dependence of the thermal and photochemical reactions of methanol on nanocrystalline anatase TiO2

Structure-activity relationships and the influence of particle size and shape on the partial- and photo-oxidation of methanol on nanocrystalline anatase TiO2 were investigated using temperature-programmed desorption. The study employed two distinct nanoparticle morphologies: truncated bipyramids exposing primarily {101} facets, and flatter platelets exposing primarily {001} surfaces, whose nominal sizes ranged from 10 to 25nm. The platelets were found to be more active for thermally-driven reactions, such as coupling of methoxide groups to produce dimethyl ether, and deoxygenation to produce methane. A dependence of the reactivity of {001} facets for the coupling of methoxide groups to produce dimethyl ether on facet size was also observed. In contrast to the thermally-driven reactions, the bipyramidal nanoparticles were observed to be more active for a range of photochemical reactions, including oxidation and coupling to produce methyl formate, and photo-decomposition of surface methoxide species. This study also shows how well-defined nanocrystals can be used to help bridge the materials gap between studies of single crystal model catalysts and their high surface area industrial analogs.

Synergistic Inhibition of Oxide Formation in Oxidation Catalysis: A First-Principles Kinetic Monte Carlo Study of NO + CO Oxidation at Pd(100)

Oxide formation under oxygen-rich reaction conditions has independently been reported for both CO oxidation and NO oxidation with Pd single-crystal model catalysts. We present a first-principles kinetic Monte Carlo study addressing the simultaneous occurrence of both reactions at Pd(100) exposed to CO- and NO-containing feeds. Even in most oxygen-rich feeds, very small amounts of NO are found to reduce the surface oxygen coverage well below the level required to induce oxide formation. Even though NO and CO compete for the same surface sites and surface oxygen, the ongoing NO oxidation reactions furthermore lead to a partially strong enhancement of the CO oxidation activity. This highlights synergistic effects of multicomponent gas feeds on both surface composition and catalytic activity that cannot be captured, nor extrapolated from prevalent studies focusing on individual reactions.

Diatomic Steps in Pt(997) Surfaces Are Better Catalysts than Monatomic Steps for the CO Oxidation Reaction near Atmospheric Pressure

The oxidation of CO to CO2 has been investigated on Pt(997) single-crystal model catalysts with a flow reactor near ambient pressure with X-ray diffraction. Depending on the temperature and gas composition, the surface may consist of monatomic steps separated by 9 atom wide terraces or diatomic steps separated by 18 atom wide terraces. The diatomic stepped surface has been found to be significantly more active for CO2 production. The kinetics of the single- to double-step transition under reaction conditions has been investigated, as well as the dependence of the transition temperature with the gas composition.

Structure and Dynamics of Reactant Coadsorption on Single Crystal Model Catalysts by HP-STM and AP-XPS: A Mini Review

Understanding the reaction mechanism of various heterogeneous catalytic reactions is of fundamental importance in catalysis science. In the past, scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) have proved to be powerful surface-sensitive techniques to characterize surface reactions on model catalysts under UHV conditions. The recent development of high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) has largely extended the application of these two excellent surface-sensitive imaging and electron spectroscopy techniques to a variety of catalytic systems under realistic conditions. In this mini review, we will review a series of catalytic systems studied by HP-STM and AP-XPS, including reactant coadsorption systems, coadsorption + reaction systems, and poisoned reaction systems. We will also illustrate one of the main difficulties in the practical execution of experiments where the initial surface cleanliness is easily compromised by the adsorption of adventitious contaminants. All of these examples will demonstrate that the combined use of HP-STM and AP-XPS can provide a deeper understanding of the structure and dynamics of reactant coadsorption on model catalysts, although great care has to been taken to maintain the cleanness of the in situ instrumentation.