Research

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The dissociative chemisorption of methane on nickel, CH4 → CH3(ad) + H(ad), is the rate limiting step in industrial steam reforming. It also happens to be one of our lab’s model catalytic systems. In general, heterogeneous catalysts reduce a reaction’s activation energy (Ea) by binding to and stabilizing the transition state (TS). Lowering the Ea thus increases the reaction rate and may allow a reaction to proceed under less harsh (lower T, P) conditions. By selectively reducing the Ea for one of several reaction paths, the catalyst may increase selectivity toward a desired product and subsequently permit milder reaction conditions. At the industrial level, this could save millions of dollars per year.

Our experiments are designed to answer several 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 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 beams, Chamber 1 sought to understand the transition state geometry and vibrational efficacy towards reactivity of methane on nickel surfaces. Establishing that vibrations have a profound impact on reactivity, the precise control over the impinging methane molecules allowed for subtle surface temperature effects to appear. With studies focusing largely on Ni(111) in recent years, the (111) surface is flat which makes it a simplified system and the most common site on industrial nanoparticles. Although the (111) is of great interest, it neglects the quantity of defect sites present 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. Ultimately, these experiments will provide insight into the experimental limits of state-resolved studies while also furthering our understanding of the industrially significant reaction of activated C-H bond cleavage.

Chamber 2

Currently on Chamber 2, we are working on setting up a Reflection Absorption Infrared Spectroscopy (RAIRS) system to study single-atom alloys (SAAs). The RAIRS will allow us to take measurements in real time of gas-surface reactions as well as determine at which sites the reaction is occurring.

Experiments on Chamber 2 include studying reactions on different surfaces to get a better handle on different industrial catalytic processes as well as studying larger and more complicated molecules than methane as has been previously studied in our group.

Main focuses:

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?
  • New FTIR will allow 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 stretching the C-H bond being particularly important for TS access. Molecule’s vibrational state influences S0 (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?
  • Will mode selectivity appear in larger molecules where energy redistribution processes may be faster?
  • Ethane (C2H6) on Ni(111) and Ni(997)
  • Methanol (CH3OH)

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 industrially important 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. Our use of methane molecules with a precisely defined energy highlights 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).

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 and X-ray photoelectron spectroscopy 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.

Reaction Pathways

Section coming soon

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 modulated 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.

J. Phys. Chem. Lett. 2016, 7, 13, 2402-2406

Accurately simulating heterogeneously catalyzed reactions require 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.

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J. Phys. Chem. Lett. 2016, 7, 13, 2402-2406

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.

J. Phys. Chem. Lett. 2017, 8, 17, 4177-4182.

Single Atom 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 FTIR spectrometer alongside our molecular beams to allow for real-time studies of reactions at specific catalytic sites.

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.