The interaction of methanol vapour with copper surfaces
| Dates: | 12 January 2022 |
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| Times: | 14:00 - 15:00 |
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| What is it: | Seminar |
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| Organiser: | Photon Science Institute |
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| Who is it for: | University staff, Current University students |
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| Speaker: | Dr. Baran Eren |
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Join us for this PSI seminar with guest speaker Dr. Baran Eren. Methanol-to-hydrogen conversion reactions are at the heart of onboard generation of hydrogen in vehicles powered by reformed methanol fuel cells. They include dry dehydrogenation, partial oxidation, steam reforming, and autothermal reforming. Cu-based materials are currently our best option as catalysts for all the mentioned reactions, which makes the interface between Cu and methanol vapour exceedingly important; yet, only a handful of spectroscopic and microscopic studies are available in the literature.
In the past few years, Dr Eren scrutinized the interface between various faces of Cu with methanol vapour using a variety of techniques including ambient pressure scanning tunneling microscopy, ambient pressure x-ray photoelectron spectroscopy (APXPS), and polarization modulation infrared-reflection absorption spectroscopy PM-IRRAS. In this talk, I will present a summary of all of these results. In brief, methanol adsorbs dissociately as methoxy on both Cu(111), Cu(100), and Cu(110) surfaces when present in the mbar range at room temperature and above, as evidenced by all three techniques. Formate formation, as often observed with APXPS by us and other authors originate from impurities in the measurements chambers. IR spectroscopy provides a more detailed information on the adsorption behavior as both C-O and C-H stretching frequencies are accessible. Dr. Eren believes that the initially formed methoxy adlayer is not in equilibrium; initial methanol adsorption forms a hydrogen-bonded assembly (similar to those observed at cryogenic temperatures, which I will also briefly go over) with a high coverage that leaves a high coverage of methoxy behind as it dehydrogenates. Methoxy adlayer gradually reaches a lower equilibrium coverage by further dehydrogenation into formaldehyde and carbon monoxide, the latter observed as a relatively short-lived species with IR spectroscopy. Kinetics analysis based on the shifts in the C-O stretching frequency of methoxy suggests that the structural transformation is faster on the Cu(110) surface compared to Cu(111) and Cu(100) surfaces. Further kinetics analysis based on the C?O stretching frequency suggests that the rate constants for methoxy-to-CO conversion and CO desorption are comparable to each other. We also think that APXPS, the most commonly used technique in the field, overestimates coverage of dissociated species because of enhanced surface activity due to secondary electron generation.
The observed coverage evolution kinetics and our proposed model for methanol dissociative adsorption on Cu surfaces are exceptional and fundamentally different from the classical kinetic models of chemisorption. Commonly used models, such as first- or second-order kinetics and Langmuir kinetic model for dissociative adsorption, predict that the surface coverage monotonically increases with time towards equilibrium. The new model presented here for methanol chemisorption suggests an opposite functional behavior of the coverage versus time. This behavior is explained by a precursor-mediated adsorption, that is, methanol forms a hydrogen-bonded network before it is dehydrogenated on the surface. This newly proposed adsorption mechanism could be more common than one would expect; for instance, molecules that are strongly attracted to each other through hydrogen bonding (e.g., water, larger alcohols, etc.) could also behave similarly to methanol.
This event will be taking place online and furthers details about the event and how to join will be made available shortly.
Speaker
Dr. Baran Eren
Organisation: Weizmann Institute of Science
Biography: Julia Weinstein is a Professor of Physical Chemistry at the Department of Chemistry, the University of Sheffield. Julia obtained her Diploma in Chemistry (with honours) from Moscow Lomonosov State University in 1990, followed by a PhD from the same institution in 1994, on "The role of exciplexes in the mechanism of electron transfer in Marcus's kinetic region". She has been a member of the academic staff at the MSU since 1994. In 2000, she became a Royal Society/NATO Postdoctoral Fellow at the University of Nottingham, which was followed by a temporary lectureship at the same institution. Julia was awarded an EPSRC Advanced Research Fellowship in 2004 on "Light-Switchable Molecular Devices based on Metal Chromophores". She joined the Department in Sheffield in 2005.
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