Methane methods: IR reveals catalytic details

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  • Published: Jan 8, 2018
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Methane methods: IR reveals catalytic details

Spectroscopic model

The Pt(211) surface has three-atom-wide terraces and one-atom-high steps. The researchers labeled the row of atoms on the step edge as

Infrared spectroscopy techniques and a quantum theory model have been used by an international team to investigate the nature of methane dissociation reactions on platinum. The results could offer a route map to novel catalysts for converting natural gas into higher chemicals.

In experiments conducted at EPFL (École polytechnique fédérale de Lausanne) in Switzerland and with calculations undertaken at the University of Massachusetts Amherst in the USA, an international team has looked at how transition metal catalysts, such as nickel and cobalt, widely used in industry to convert methane into other useful compounds might function at the atomic level. Commonly transformation of natural gas into other compounds, including hydrogen gas, involves the process of steam reforming, wherein methane is heated with water steam over a metal catalyst to generate hydrogen gas and carbon monoxide, which can then be further reacted to more complicated molecules.

On transition

Transition metals are the catalysts of choice for a wide range of industrial chemical reactions. It is known that the most significant reactions occur at the surface of such catalysts. So far, however, the quest for more effective and efficient catalysts has been almost entirely based on trial and error rather than a systematic search. The assumption has always been that catalytic reactions take place on step edges and other atomic defect sites of the metal crystals. At the atomic scale, the surface of a catalyst consists of steps, terraces, and other defects that are important sites in the catalytic process. So testing novel catalysts one after the other was essentially the most worthwhile approach to finding new catalysts without detailed knowledge of how those defects affect catalysis.

Obtaining minute detail about how methane molecules interact with a catalytic surface might open up a new approach and this is what researchers from Switzerland, the Netherlands, and the USA have aimed at. Now, for the first time, the team has demonstrated exactly where the most significant reactions occur on the surface of a metal catalyst. In their work they used platinum (Pt) as the catalyst for the breakdown of methane to study. However, the catalytic model that they have constructed from their results and calculations might just as easily be applied to other transition metal catalysts, such as nickel. The team reports details of the work in the Journal of Chemical Physics.

Steps and terraces

“A tested predictive theory with chemical accuracy could change the way one searches for new catalysts and make the search more efficient and cheaper,” explains team member Rainer Beck of EPFL.

The team used infrared laser pumping to excite methane molecules into selected rotational and vibrational quantum states and then used reflection-absorption infrared spectroscopy (RAIRS) to observe how the methane dissociates on the various sites of the Pt(211) crystal. RAIRS, the team points out, is a non-intrusive technique that allowed them to monitor the chemical processes in real time. In this case, the deposition of methane molecules on to the platinum surface. It records the site-specific uptake curves for chemisorbed methyl species on the catalyst's steps and terraces. With this spectroscopic data in hand, the team could then determine the reactivity levels of methane on each of the sites.

The researchers also used the Reaction Path Hamiltonian model, a quantum theory framework, to calculate the potential energy surface and explore the dynamics during the catalytic processes. Their findings revealed that dissociation reactions are at least two orders of magnitude more efficient on the steps than on the terraces. However, they also found that no reaction took place at all on a third type of surface site located between steps and the terrace referred to as “corner atoms”.

“We demonstrated that it is possible to use RAIRS detection for state- and surface-site specific measurements of methane reactivity and to compare the effect of vibrational excitation on reactivity on the steps and terraces of a catalyst surface,” Beck adds. “This new area of study provides another level of detail in detecting methane’s dissociation products.”

Related Links

J Chem Phys 2018, 148, 014701: "Methane dissociation on the steps and terraces of Pt(211) resolved by quantum state and impact site"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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