Research Article pubs.acs.org/acscatalysis
Selective Formic Acid Dehydrogenation on Pt-Cu Single-Atom Alloys Matthew D. Marcinkowski,† Jilei Liu,‡ Colin J. Murphy,† Melissa L. Liriano,† Natalie A. Wasio,† Felicia R. Lucci,† Maria Flytzani-Stephanopoulos,‡ and E. Charles H. Sykes*,† †
Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford, Massachusetts 02155, United States Department of Chemical and Biological Engineering, Tufts University, 4 Colby Street, Medford, Massachusetts 02155, United States
‡
ACS Catal. 2017.7:413-420. Downloaded from pubs.acs.org by UNIV OF SOUTHERN INDIANA on 04/12/19. For personal use only.
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ABSTRACT: Formic acid is a potential hydrogen storage molecule which dehydrogenates to form CO2 and H2 on metal surfaces. However, it can also decompose via a competing dehydration reaction that forms CO and H2O, reducing the amount of H2 produced and poisoning the catalyst with CO. Formic acid re-formation to hydrogen is typically performed by Pt and Pd catalysts, which while highly active for dehydrogenation also catalyze dehydration. Cu is typically not utilized, as it requires prohibitively high temperatures, although Cu surfaces are very selective toward dehydrogenation. We studied the reaction of formic acid on single-atom alloys (SAAs), consisting of single Pt atoms substituted into a Cu lattice. Surface science studies allowed us to relate alloy structure to reactivity and selectivity and visualize reaction intermediates. These experiments revealed that SAAs are able to selectively dehydrogenate formic acid with a 6-fold increase in yield in comparison to Cu. This increase in conversion is due to a more facile dehydrogenation of formic acid to formate on the SAA surface (120 K vs 160 K on Cu(111)). We acquired quantitative desorption and molecular scale imaging data showing spillover of formate from Pt sites to Cu. Increasing the Pt concentration beyond the SAA regime resulted in loss of selectivity. These results prompted us to test SAA nanoparticle (NP) catalysts under realistic conditions. However, only a slight increase in conversion was observed between pure Cu and Pt-Cu SAA NPs. In our surface science studies, dehydrogenation of formate to CO2 and H2 did not occur until above 400 K on both the SAA and pure Cu surfaces, indicating that Pt sites do not catalyze this rate-limiting step. While SAAs do not offer increased reactivity for formic acid dehydrogenation, they do offer significantly lower barriers for O−H bond breaking, which holds promise for other dehydrogenation reactions. KEYWORDS: formic acid, hydrogen storage, single-atom alloys, Pt-Cu alloys, temperature-programmed desorption, scanning tunneling microscopy, microreactor, nanoparticles
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the carboxyl intermediate (Figure 1A).8−10 CO binds very strongly to both Pd and Pt,22,23 and as fuel cells ideally operate at low temperature, this makes the catalyst susceptible to poisoning and deactivation over time.20,21 Even very small amounts of CO in the ppm range can be detrimental, and thus selectivity to dehydrogenation over dehydration should be extremely high.20,24 In order to achieve this high selectivity, alternative catalysts to Pt or Pd catalysts are needed, and a variety of strategies have been utilized to overcome this obstacle. Typically these strategies have focused on utilization of less reactive noble metals such as Ag and Au alongside Pd and Pt.24−28 A wellknown study by Tedsree et al. utilized Ag@Pd core−shell nanoparticles (NPs) to achieve high activity and selectivity to formic acid dehydrogenation at room temperature.24 A similar system was tested with Au@Pd core−shell catalysts by Huang et al., but at an elevated temperature in comparison to Tedsree’s study.25 Another strategy is the fabrication of alloys to maintain high activity but gain high selectivity. Wang et al.
INTRODUCTION Hydrogen is considered a promising alternative energy source; however, as pure hydrogen is a flammable gas at room temperature, safety issues involving its storage and transportation limit its use as a fuel.1−4 Formic acid is a liquid at room temperature and has attracted attention as a means of hydrogen storage.5−7 Formic acid can be dehydrogenated to H2 and CO2, through either a formate or carboxyl intermediate8−10 (Figure 1A), and the resulting hydrogen is fed into a proton exchange membrane fuel cell (PEMFC).5,11,12 Alternatively, formic acid itself can be fed into a direct formic acid fuel cell (DFAFC) similar to a direct methanol fuel cell.6,13,14 Although methanol has both a higher volumetric capacity of hydrogen and a higher weight percent hydrogen, formic acid is far less permeable to Nafion, the membrane typically used in PEMFCs.13,15,16 As a result, a high concentration of formic acid can be fed into a fuel cell in comparison to methanol. Regardless of whether formic acid is to be re-formed or fed directly into a DFAFC, Pd or Pt is typically used to catalytically dehydrogenate formic acid to CO2 and H2.13,14,17−19 In addition to being expensive, catalysts based on these reactive metals also perform an undesired dehydration reaction and form CO and H2O.20,21 This reaction proceeds solely through © 2016 American Chemical Society
Received: September 28, 2016 Revised: November 22, 2016 Published: November 29, 2016 413
DOI: 10.1021/acscatal.6b02772 ACS Catal. 2017, 7, 413−420
Research Article
ACS Catalysis
Figure 1. Decomposition of formic acid on metal surfaces. (A) Schematic showing that the decomposition of formic acid via dehydrogenation can go through carboxyl or formate intermediates, while dehydration proceeds solely through a carboxyl intermediate. (B) High-resolution STM image of a 0.01 ML Pt-Cu(111) SAA surface (scale bar 2 nm). (C) TPR spectra showing desorption products resulting from the reaction of multilayers of formic acid on Cu(111) (black) and from a monolayer of formic acid on 0.01 ML Pt-Cu(111) (red). The D peak results from H that is formed during the dehydrogenation of formic acid to formate. The T and S peaks result from the dehydrogenation of formate to CO2 and H2 on the terraces and steps, respectively. Formic acid is abbreviated as FA in the figure legend.
catalysts containing isolated Pt atoms in Cu. However, we find these NPs only exhibit a marginal increase in conversion in comparison to pure Cu NPs. Our surface studies explain this by parsing out the mechanism, revealing that single Pt atoms are able to lower the barrier to dehydrogenation of formic acid to formate, which occurs at low temperature, but they do not catalyze the decomposition of formate to CO2 and H2, which is the rate-limiting step of the reaction for the NP catalysts.
synthesized NiAuPd nanocatalysts that exhibit high activity and selectivity for formic acid decomposition at room temperature.26 Similarly, Zhou et al. showed that Pd-Au and Pd-Ag NPs achieved high selectivity to H2 at 365 K.27 Decreasing the size of Pd or Pt ensembles can help to increase selectivity.29−31 Taking this approach, a recent study by Bulushev et al. fabricated catalysts consisting of single Pt atoms supported on N-doped carbon nanofibers which exhibit extremely high selectivity and are stable up to 573 K.28 This was similar to a study by Yi and co-workers, who found atomically dispersed Au on CeO2 rods and cubes to dehydrogenate formic acid with ∼100% selectivity.32 Furthermore, Yi showed that these Au atoms were resistant to CO poisoning. With these results in mind, this study focuses on the use of Pt-Cu alloys where the Pt atoms exist as single isolated species in the Cu lattice. Single-atom alloys (SAAs) typically contain small amounts of a more reactive, expensive metal substituted into the lattice of a cheaper, less reactive metal.30,33−37 The reactive metal, typically Pt or Pd, exists as single isolated atoms in the lattice of the less reactive metal, typically Cu, Ag, or Au.33−36 Even with the smallest possible ensemble of Pt or Pd, these alloys are capable of performing reactions that metals such as Cu would normally be unable to, while maintaining high selectivity to the desired products. For example, SAAs of Pd-Cu and Pt-Cu perform highly selective hydrogenation reactions of styrene, acetylene, and butadiene.30,37 In this study, we examine the dehydrogenation of formic acid on a SAAs of Pt in a Cu(111) surface. Cu surfaces are known to be highly selective to the dehydrogenation of formic acid.8,9,38−42 Pt surfaces also primarily dehydrogenate formic acid, but dehydration products can be formed, which causes the selectivity of these catalysts to suffer.8−10,20,43,44 However, pure Cu-based catalysts would be impractical due to the high barrier to dehydrogenation on Cu surfaces in comparison to Pt.8,9 In our model studies we find that SAAs of Pt-Cu drastically improve the conversion of formic acid to formate, while maintaining the high selectivity of Cu. Importantly, we also demonstrate the spillover of formate species from Pt sites to Cu. We have also examined NP
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EXPERIMENTAL METHODS
Ultra-high Vacuum Experiments. Two different ultrahigh vacuum (UHV) instruments were utilized in these experiments. The first is an Omicron Nanotechnology lowtemperature scanning tunneling microscope (LT-STM) with a base pressure of
