Our experiments are designed to answer a variety of important questions regarding metal catalysts:
- What is the geometric structure of the transition state?
- How do atoms move during reaction?
- What is the molecular-level mechanism for reaction?
- Is the surface a static template or an active participant?
- How quickly can energy flow within the reaction complex?
- Can we exploit energy flow to control reactivity?
- How accurate are theoretical predictions of reactivity?
Our Chambers
We use ultra-high vacuum (UHV) chambers equipped with powerful surface science instrumentation to help us elucidate these molecular-level details. Though the two chambers in our lab are similar, each chamber is outfitted with unique equipment allowing us to work on multiple projects at once.
Chamber 1
Initially designed with the idea of state-resolved molecular beam experiments, Chamber 1 sought to understand the transition state geometry and vibrational efficacy towards the reactivity of methane on nickel surfaces. Establishing that vibrations have a profound impact on reactivity, the precise control over the impinging methane molecules also allowed for subtle surface temperature effects to appear. Studies focusing largely on the flat surface of a Ni(111) single-crystal in recent years, making it a simplified system and the most common surface site on industrial nanoparticles.
Though the (111) surface is of great interest, it neglects the quantity of defect sites present in the nickel nanoparticles. Recently, the experiments have been pointed towards defected surface sites by using a lightly-stepped Ni(997) surface in order to differentiate the effects of steps and terraces. Additionally, steam reforming contains a nontrivial percentage of other hydrocarbons besides methane. Ethane in particular is the next step up in complexity of the alkanes. Due to the complexity of the degrees of freedom present in larger hydrocarbons, it is of interest whether they still undergo comparable reaction rates as smaller ones. Our most recent studies on Chamber 1 have investigated this question.
Ultimately, these experiments will provide insight into the experimental limits of state-resolved chemical reactivity studies while also furthering our understanding of the industrially significant reaction of activated C-H bond cleavage.
Chamber 2
Chamber 2, the newer (and smaller!) of our two chambers, has shifted gears towards studying methane reactivity on an interesting new class of catalyst called single-atom alloys (SAAs). In combination with reflection-absorption infrared spectroscopy (RAIRS), we can perform gas-surface reaction experiments and collect data regarding what is happening at the gas-surface interface, both in real time. This allows us to collect reactivity measurements at the same time as well as examine what is happening at various sites across the surface.
The current and future of experiments on Chamber 2 include studying reactions on different SAA surfaces to better understand various industrially important catalytic processes. Of interest as well is studying larger and more complicated molecules. Our extensive studies of methane provide a great foundation for advancing our knowledge of simple hydrocarbons and molecules on single-crystal metal surfaces.
Main focuses:
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Mode- and Bond-Selective Chemistry
Bond Selectivity
By pre-exciting the C-H stretch and measuring the product yield for CHD3 dissociation on Ni(111), C-H bond cleavage products yielded nearly 100-fold greater. The vibrational mode excited in the molecule dictates product identity.
Mode- and Bond-selective chemistry occurs because there is too little time during the molecule-surface encounter (ca. 200 fs or less) for Evib to be redistributed.
What’s Next?
- FT-IR allows us to study other, larger molecules.
- Can we exert bond selective control over chemically distinct bonds (e.g. C-H and O-H in CH3OH)?
- Could bond-selective chemistry be a new synthetic tool for mechanistic studies of reaction intermediates?
Vibrational Mode Selectivity
Vibrational states differ in their ability to promote reaction, such as the C-H bond stretch being particularly important for transition state (TS) access. A molecule’s vibrational state influences the reactivity (sticking probability, S0). This is called mode selectivity. There are two requirements for mode selectivity: initial Evib persists long enough to enhance TS access, and the vibration has to enhance access to distorted TS geometry.
What’s Next?
- Does mode selectivity appear in larger molecules where energy redistribution processes may be faster?
- Ethane (C2H6) reactivity studies on Ni(111) and Ni(997).
- Methanol (CH3OH).
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Surface Temperature Effects
Commercial steam reforming reactors operate at temperatures of 1000K or higher, and methane dissociation on the Ni catalyst is believed to be the rate-limiting step in this process. Despite the commercial importance of this reaction, nearly all studies probing the dynamics of methane dissociation have focused on surface temperatures of 600K or lower. We use energy and vibrationally state-selected methane molecules in a supersonic molecular beam to quantify the impact of surface temperature on methane activation over a wide range of surface temperatures, including those approaching industrial levels. Our use of methane molecules with a precisely defined energy provides a clear view of how surface temperature impacts reactivity.
Vibrationally state-resolved reactivity measurements reveal details of fundamental processes that impact reactivity in the field of heterogeneous catalysis. Non-statistical, mode-specific, and bond-selective enhancements observed for methane and its isotopologues on transition metal surfaces provide insights into energy flow during reactions. Reactive gas molecules with strictly defined energy in well-defined energetic coordinates used in state-selective experiments have also proven to be valuable probes of how surface atom motion affects overall reactivity.
Vibrationally state-resolved data was collected via infrared (IR) laser excitation of various vibrational modes of supersonically expanded methane (CH4) gas molecules impinging on a various low-index nickel surfaces. Measurements on the single-crystal were investigated over a broad range of surface temperatures (82 K ≤ TS ≤ 1000 K) while utilizing varying incident energies (Ei = 20 kJ/mol to >140 kJ/mol). See the figure below for more information.
Carbon Dissolution
Additionally, a distinctive characteristic of nickel is the ability for carbon to diffuse into the subsurface and bulk layers of the crystal. The interaction of carbon with metal catalysts is of significant interest. In methane steam reforming, the build-up of carbon in the nickel subsurface leads to a gradual reduction in reactivity on the surface and ultimately results in deactivation of the metal catalyst. Additionally, the initial dissolution and subsequent reemergence of carbon from the subsurface are key steps in the growth of well-ordered graphene on nickel substrates via chemical vapor deposition (CVD).
Researchers have previously used Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) to investigate the dynamics of carbon dissolution into nickel surfaces. We instead employ beam reflectivity measurements to monitor the process of carbon diffusion into the nickel subsurface in real-time.
We observe significant changes in the reaction profile by increasing surface temperature as the rate of dissolution approaches the reactive flux of the high energy gas molecules. We use these results to further develop kinetic models for methane reactivity as a function of surface coverage as well as carbon diffusion into the nickel crystal. The major parameters from these models include the site-blocking coverage and its subsequent dependence on surface temperature as well as an updated measure of the barrier to diffusion for the C/Ni system.
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Reaction Pathways
Section coming soon
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Reaction Energetics
Studies exploring how vibrational energy (Evib) promotes chemical reactivity most often focus on molecular reagents, leaving the role of substrate atom motion in heterogeneous interfacial chemistry underexplored. Using a combination of theoretical and experimental studies on methane dissociation in Ni(111), it can be shown that lattice atom motion modulates the reaction barrier height during each surface atom’s vibrational period, which leads to a strong variation of the reaction probability (S0) with surface temperature. The results show the first direct measurement in a beam-surface scattering experiment that reveal a sharp energetic threshold that permits a direct comparison with density functional theory (DFT) calculations for barrier height.
Accurately simulating heterogeneously catalyzed reactions requires reliable barriers for molecules reacting at defect sites, such as steps. Unfortunately, theory has struggled to compute the barriers in this case. Predictions of reaction rates and dynamics are contingent on the accuracy of potential energy surface projections. However, dissociative chemisorption of polyatomic reagents on metal is more complicated, relying on the electronic structure of the metal, van der Waals interactions, lattice motion effects, and the participation of numerous degrees of freedom.
By combining state-resolved molecular beam experiments with theory, specifically the ab initio molecular dynamics (AIMD) using specific reaction parameter density functional theory (SRP-DFT), reliable molecule-metal interactions can be determined. The experimental and theoretical initial state-selected sticking measurements are within 4 kJ/mol of each other. The accuracy on a flat (111) surface can also be extended to stepped (211) surfaces to emulate defects, which bridges a gap between fundamental surface science studies and real-life heterogeneous catalysts that often proceed over defected metal nanoparticles.
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Single-Atom Alloy Catalysis
Individual atoms of precious metals dispersed on a more inert metal host form a single-atom alloy (SAA) that exhibits unique catalytic selectivity and activity. SAAs are a new generation of heterogeneous catalysts (HC) which show promising potential to catalytic insights, due not only to their high activity and selectivity but their cost- and resource-effectiveness. However, key mechanistic details of gas-surface interactions on SAAs, which would reveal insights into molecular mechanics and energy flow dynamics, remain largely experimentally unexplored. Our lab aims to fill this gap by using state-resolved molecular beam experiments to extract precise information with regard to these atomic-sized kinetic details. We pair our RAIRS-equipped FT-IR spectrometer alongside our supersonic molecular beams to allow for real-time studies of reactions at specific catalytic sites.
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Support
This material is based upon work supported by the National Science Foundation under Grant Numbers 1465230 and 1800266. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.