Subscriber access provided by CMU Libraries - http://library.cmich.edu
Article
A Microscopic View of the Active Sites for Selective Dehydrogenation of Formic Acid on Cu(111) Matthew D Marcinkowski, Colin J Murphy, Melissa L. Liriano, Natalie A. Wasio, Felicia R. Lucci, and E. Charles H. Sykes ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01994 • Publication Date (Web): 30 Oct 2015 Downloaded from http://pubs.acs.org on November 4, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 29
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
A Microscopic View of the Active Sites for Selective Dehydrogenation of Formic Acid on Cu(111) Matthew D. Marcinkowski, Colin J. Murphy, Melissa L. Liriano, Natalie A. Wasio, Felicia R. Lucci and E. Charles H. Sykes*
Department of Chemistry, Tufts University, 62 Talbot Ave., Medford, Massachusetts 02155, United States.
[email protected] Abstract Formic acid is an important molecule due to its potential for hydrogen storage and the role of formate in methanol synthesis. Formic acid can decompose on metals and oxides via dehydrogenation or dehydration, although dehydrogenation is preferred for most applications. These two pathways are linked via the water-gas shift reaction (WGSR), making them hard to separate and debate over the mechanisms still exists. Cu catalysts are known to selectively decompose formic acid via dehydrogenation to produce CO2 and H2. Formic acid’s interaction with Cu(110) has been extensively studied, but despite the (111) facet being predominant in many nanoparticles, Cu(111) has received little attention. Using temperature programmed desorption/reaction (TPD/R) and scanning tunneling microscopy (STM), we have probed key steps in the decomposition of formic acid on Cu(111) at the atomic-scale, observing intact adsorption and surface intermediates, as well as the surface after product desorption. Our model system allows us to investigate the reaction under conditions where WGSR is inactive. We find that Cu(111) decomposes formic acid 100% selectively through dehydrogenation. At 85 K, formic acid adsorbs molecularly on Cu(111), forming hydrogen bonded chains in the β configuration. The acid loses a H atom by 160 K producing the formate intermediate and surface bound H atoms, both of which are visualized by STM. All molecules at surface step edges react to formate, but on the Cu(111) terraces desorption of formic acid competes with formate production which limits formate production to 0.05 monolayers. H atoms formed by deprotonation recombine to form H2 in a desorption rate limited process by 360 K. CO2 and H2 desorb from the surface in a reaction rate limited processes at 400 and 450 K due to formate decomposition on terraces and steps respectively.
ACS Paragon Plus Environment
ACS Catalysis
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Keywords: Formic acid, dehydrogenation, Cu catalysts, hydrogen storage molecules, temperature programmed reaction, scanning tunneling microscopy. Introduction Hydrogen is prevalent in both current and future energy applications since it shows great promise as an alternative to carbon based sources. Proton exchange membrane fuel cells (PEMFCs) provide electricity by reacting hydrogen cleanly with oxygen to form H2O. While hydrogen gas can be fed directly into PEMFCS, the storage and transportation of compressed hydrogen gas is a potential safety hazard.1–4 In order for hydrogen to be a practical source of energy, alternative strategies for hydrogen storage are necessary. Methanol has been used as a hydrogen source in direct methanol fuel cells, but exhibits significant membrane crossover with Nafion, the typical membrane used in PEMFCs.5–8 As a result, interest in formic acid as a potential hydrogen storage molecule is increasing. Formic acid can reformed to release hydrogen3,4,9, or the intact molecule can be fed into a direct formic acid fuel cell (DFAFC).4,5,10,11 While the hydrogen content of formic acid is less than that of methanol, it is less susceptible to diffusion across the permeable Nafion membrane and thus higher concentrations of formic acid can be used in a fuel cell.5,6,10 Formic acid can decompose on metal surfaces either through dehydrogenation to produce CO2 and H2 or dehydration to produce CO and H2O. As hydrogen is the product of interest for PEMFCs, high selectivity to dehydrogenation is preferred. In addition, since fuel cells operate at relatively low temperatures, the CO formed via dehydration can poison the catalyst.10–16 Typically, DFAFCs employ Pt or Pd as the decomposition catalyst,
ACS Paragon Plus Environment
Page 2 of 29
Page 3 of 29
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
and while these metals display high activity, they are expensive and can produce the undesired dehydration products.4,5,10–16 Low Miller index Cu surfaces are known to selectively decompose formic acid through dehydrogenation,17–41 producing the intermediate formate, which is also involved in important reactions such as methanol synthesis on Cu.42–45 A formate pathway is known to promote high selectivity to the dehydrogenation.15,46,47 Pt and Pd surfaces also primarily decompose formic acid to formate,48,49 but unlike Cu, are known to produce some reactively formed CO.12,14,15,50–52 While the decomposition of formic acid to CO2 and H2 has been thoroughly examined on the Cu(110) surface17–31, the interaction of formic acid on Cu(111) has received very little attention36–41 , despite the fact that the (111) facet is dominant in many nanoparticles. There are a few studies in the literature that focus on the formate intermediate on Cu(111), although formate is often synthesized by the hydrogenation of CO2 rather than formic acid decomposition, as this process is of interest for methanol synthesis.36,37,39 Intact formic acid and formate have been previously examined on Cu(111) using scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy, and infrared reflection adsorption spectroscopy, but to the best of our knowledge, no formic acid temperature programmed desorption/reaction (TPD/R) work has been performed on Cu(111).37,38,40,41 In this study we examine the adsorption and reaction sites of formic acid on Cu(111) over a range of temperatures. This allows us to elucidate the energetics of the elementary steps occurring during the decomposition of formic acid. Using high-resolution STM and TPD/R, each step of the reaction is observed including the intact adsorption of the formic acid, formation of
ACS Paragon Plus Environment
ACS Catalysis
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
the formate intermediate, and evolution of reactively formed CO2 and H2 at 400 K on terraces and 450 K on steps. We show that intact formic acid adsorbs on Cu(111) in the β-polymorphic configuration at 80 K, and dehydrogenates to formate by 160 K. Our TPR results demonstrate that defects in the form of step edges on the Cu surface are 100% saturated with formate while reaction to formate on the Cu(111) terraces is limited to 0.05 monolayers (ML) by competition with intact desorption of formic acid. While the surface step edges are more reactive than the terraces, counter intuitively, CO2 formation at steps actually occurs at a higher temperature than on terraces (450 K vs 400 K). This is because the initial dehydrogenation step to produce formate is more exothermic on step edges than terraces. Since formate is bound more strongly to step edges, the second dehydrogenation step to produce CO2 becomes more endothermic, leading to a higher reaction barrier on step edges.53 Therefore, in catalysis one would expect a tradeoff between the step edges being initially more reactive to intact HCOOH, but more susceptible to being blocked and unavailable for further reaction due to the higher decomposition temperature of the formate intermediates bound there. Importantly, no restructuring of the Cu surface itself is observed.
Experimental Methods Experiments were performed in two ultrahigh vacuum (UHV) instruments which have both been described previously.54 STM experiments were performed using an Omicron Nanotechnology LT-UHV STM. The base pressure in the chamber was
