Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Poisonous Species in Complete Ethanol Oxidation Reaction on Palladium Catalysts Zhi-Peng Wu,*,†,‡,§ Bei Miao,†,‡ Emma Hopkins,§ Keonwoo Park,§ Yifei Chen,†,‡ Haoxi Jiang,†,‡ Minhua Zhang,†,‡ Chuan-Jian Zhong,*,§ and Lichang Wang*,†,‡,∥
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Key Laboratory of Ministry of Education for Green Chemical Technology and the R & D Center for Petrochemical Technology, Tianjin University, Tianjin 300072, China ‡ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China § Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902, United States ∥ Department of Chemistry and Biochemistry and the Materials Technology Center, Southern Illinois University, Carbondale, Illinois 62901, United States S Supporting Information *
ABSTRACT: Direct ethanol fuel cell technology suffers from a lack of effective anode catalysts for complete ethanol oxidation reaction (EOR). Pd and Pd-based catalysts showed some promise, but only a trace amount of CO2 was detected as the product. The difficulty of C−C bond cleavage and the formation of acetic acid are commonly believed to be great obstacles toward complete EOR. The limited formation of CO2 also suggests that acetic acid may not be the only dead-end product that prevents complete EOR. A careful study on the reaction pathway leading to complete EOR is needed to better understand and design effective EOR catalysts. As such, we studied 17 key elementary reactions on Pd surfaces using density functional theory (DFT) and designed experiments to confirm some of the DFT findings. The results show that, in addition to the acetic acid formation, other poisonous species, C, CH, CCO, or dimerization of acetaldehyde, are also largely responsible for the limited formation of CO2 on Pd catalysts due to their strong adsorptions to the catalysts which block the active sites. The ethanol oxidation shows totally different reaction pathways in neutral and alkaline media. The DFT calculation result provided important insights into the catalysis of complete ethanol oxidation. The experiment result showed that EOR on PdCu alloy nanoparticle catalyst has higher catalytic activity than that on Pd nanoparticle catalyst, suggesting fast kinetics of initial dehydrogenation on the alloy catalyst.
1. INTRODUCTION Direct ethanol fuel cell (DEFC) has attracted great attention due to its high theoretical mass energy density (8.0 kWh/kg) comparable to that of gasoline, its nontoxicity property compared to methanol, its convenience for storage compared to hydrogen, and the expanding production of bioethanol feedback from biomass resources.1−3 However, there are no commercially available DEFCs due to the lack of anode catalysts that can lead to complete ethanol oxidation. Pt-based catalysts, which are the most efficient catalysts up to now, tend to oxidize ethanol to acetate.4−6 A major limiting factor is the difficulty in breaking the C−C bond to completely oxidize ethanol to CO2.6−12 Partial oxidation of ethanol to acetate is also the main reason that limits the commercialization of DEFC, as this only releases 4 electrons and acetate is difficult to be eliminated and then accumulates in the fuel cell. Furthermore, expensive Pt is easily poisoned by surface intermediates, such as COads. The Pd/C catalysts were studied for ethanol oxidation reaction (EOR) and they exhibit slightly © XXXX American Chemical Society
better performance on C−C bond cleavage of ethanol in strong alkaline media in comparison to Pt/C catalysts.13−15 In addition, a higher abundance of Pd in earth’s crust, compared to Pt, provides further motivation to exploit Pd-based catalysts for EOR.16 As such, Pd and Pd-based catalysts have received increased attention.6−10,17−24 Pd catalysts were reported to be inert for EOR in acidic media, but showed high catalytic performance in alkaline media, which was validated by both experimental and computational investigations.6,8,15,18,19,25−29 One key is that the adsorbed hydroxyl species on the catalysts in alkaline media promotes the elimination of the adsorbed intermediates, hence facilitating the overall reaction. Hibbitts et al. suggested that the surface-adsorbed OH moieties act as Brønsted bases which easily digest protons from adsorbed or solution-phase Received: May 4, 2019 Revised: July 12, 2019
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DOI: 10.1021/acs.jpcc.9b04229 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C ethanol.30 Further investigation elucidates that adsorbed hydroxyl on the Pd electrode is the active center for EOR.31 The selectivity of ethanol to CO2 was only 2.5% and most of the ethanol was incompletely oxidized to acetate as characterized by in situ Fourier transform infrared spectroelectrochemical (FTIR).15 Another in situ FTIR study demonstrated that the best performance of DEFC was observed in 1 M NaOH solution, and that C−C bond breaking of ethanol to form CO2 usually took place at a pH under 13.27 The cyclic voltammetry study illustrated that the kinetics of EOR was affected by both the adsorbed OH− group and the formation of an inactive oxide layer on the Pd electrode at higher potentials.25 Recently, the investigation combining density functional theory (DFT) and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) studied the reaction mechanism of ethanol electro-oxidation. The results showed that it is possible to break C−C bond to form CO2 on Pd/C in alkaline media at a lower potential. The selectivity of CO2 was poor, however, on account of the competitive formation of other side products. Pivotal intermediate acetyl can either be oxidized to CH3COOH or cleave C−C bond undergoing a C1 pathway. CH3COOH was also identified as a poisonous intermediate in the reaction route since it was very hard to be further oxidized to form C1 species.32 Furthermore, this work showed clearly that the pathway to CH3COOH through the gem-diol must be avoided. The difficulty described in this work was based on the DFT results of the C−C cleavage in CH3CX species with X = O, OH, (OH)2, HOH, and H(OH)2. Other DFT studies, however, showed that the C−C bond cleavage does not occur in CH3CO species.33,34 Additionally, our recent studies are trying to seek applicable catalysts for EOR by preventing the formation of acetic acid while facilitating breaking the C−C bond. A systematic computational screening of potential EOR catalysts was carried out. The activation energies of C−C bond breaking and C−O bond coupling in the EOR process showed opposite trends. Ni and Ir were identified to be the most promising catalyst candidates for EOR.35,36 The role of the surface Ni in PdNi bimetallic catalysts was studied. The results revealed that the Ni on the surface has the abilities of preventing the C−O bond coupling while facilitating the C−C bond breaking.37 Not surprisingly, nanostructured Pd alloy catalysts are attracting increasing interest and are favored over Pd catalysts for their greater catalytic performances.6,8,38−44 Despite the extensive efforts on the EOR on Pd catalysts, both theoretically and experimentally, there are still critical questions to be answered for a better understanding of EOR catalysis under Pd and the future design of better EOR catalysts. The most important question is what species will be formed if sufficient acetaldehyde, CH3CHO, is formed that can lead to the completion of the C1 reaction pathway as the experiment demonstrated, even though only a small fraction of CO2 was formed. Additionally, the effect of OH in the C1 pathway is less extensively studied, though it plays an important role.31 As such, we selected 17 reactions starting from CH3CHO to study using DFT calculations and designed experiments to understand the roles of CH3CHO and CH3COOH toward complete EOR. Specifically, we studied 17 reactions starting from acetaldehyde, CH3CHO, on the Pd(100) surface. Pd(111) is the dominant facet while the Pd(100) surface is more reactive and remains sufficiently abundant.32,34 One example is that Pd
nanoflowers enriched with (100) facets show much higher EOR activity and stability than Pd nanoflowers dominated by (111) facets and other Pd nanocatalysts.45 Acetaldehyde is a main byproduct and vital intermediate in EOR, which has been verified by many experimental and theoretical works.17,19,28,31,33,46 Furthermore, Monyoncho and co-workers32 validated that C−C bond cleavage is very difficult when splitting before +2 oxidation state, i.e., the oxidation intermediate of ethanol releasing less than 2 electrons and protons, typically for acetaldehyde. We carried out four bond breaking reactions starting from acetaldehyde, five reactions starting from acetyl, CH3CO, four reactions starting from CH2CO, three reactions starting from CH2CO, and one from CCO. We also performed electrochemical experiments to investigate the role of intermediates, e.g., acetic acid and acetaldehyde, during the EOR process to verify our DFT results and further unravel our understanding of the C1 pathway to CO2 formation. Finally, PdCu catalysts were synthesized and examined in EOR to explore a better understanding of this bimetallic catalyst and the roles of the two elements.
2. COMPUTATIONAL AND EXPERIMENTAL SECTIONS 2.1. Computational Details. All of the DFT calculations stated in this work were performed using Dmol3 package,47−49 coupled with the generalized gradient approximation (GGA) of Perdew−Burke−Ernzerhof (PBE) for the exchangecorrection potential.50,51 Spin unrestricted DFT calculations were used and a double-numerical basis set with polarization functions (DNP) was performed. For the Pd(100) surface, a four-layer slab model with the bottom two layers fixed was used to carry out the DFT calculations. A smaller p(3 × 3) surface unit cell was selected to conduct reactions without a hydroxyl group, while a bigger p(4 × 4) unit cell was employed to calculate elementary steps in the presence of hydroxyl species. A 15 Å vacuum gap along the z-direction was utilized in the models to eliminate slab− slab interactions. The convergence tolerance values of 2 × 10−5 Ha, 0.004 Ha/Å, and 0.005 Å for energy, max. force, and max. displacement, respectively, together with the self-consistentfield (SCF) density convergence threshold value of 1 × 10−5 Ha, were specified. We note here more strict convergence tolerance values of 1 × 10−5 Ha, 0.002 Ha/Å, and 0.005 Å for energy, max. force, and max. displacement, respectively, and the SCF threshold value of 1 × 10−6 Ha was tested, and subtle differences were shown in terms of our system. After careful calculation (Figure S1), a 3 × 3 × 1 Monkhorst−Pack k-point mesh was used for all geometry optimization calculations to sample the surface Brillouin zone. The C−C bond cleavage reactions of CH3CHO, CH3CO, CH2CO, CHCO, and CCO, C−H bond cleavage reactions of CH3CHO, CH3CO, CH2CO and CHCO, and the C−O bond breaking step of CH3CHO were performed. The effect of alkaline media was investigated via introducing hydroxyl species onto the Pd(100) surface. In addition, the formation of acetate in the presence of hydroxyl was also studied. As noted, all the adsorption species provided here were fully relaxed. Owing to the impact of the fixed bottom atoms, some imaginary frequencies (
