Research Article pubs.acs.org/acscatalysis
Understanding the Low-Overpotential Production of CH4 from CO2 on Mo2C Catalysts Seok Ki Kim,† Yin-Jia Zhang,‡ Helen Bergstrom,† Ronald Michalsky,† and Andrew Peterson*,† †
School of Engineering and ‡Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States S Supporting Information *
ABSTRACT: While Cu is the only electrocatalyst that converts CO2 into meaningful quantities of CH4 fuel, it requires significant overpotentials (onset potential of ∼−0.80 V vs RHE), decreasing energy conversion efficiencies. We report that Mo2C is capable of catalyzing CO2 into CH4 at low potentials (onset potential of ∼−0.55 V vs RHE), where Cu electrocatalysts do not convert CO2. This lowoverpotential catalyst was first identified as a candidate by electronic structure calculations, which indicated the free energetics of CO hydrogenation to be more favorable than that on conventional transition metals such as Cu. Despite the low onset potential for CH4, the CH4 has a steep Tafel slope (∼−280 mV/dec), resulting in most of the current passing through the Mo2C electrocatalysts being utilized for the competitive hydrogen evolution reaction. We conducted a detailed theoretical analysis on the basis of density functional theory calculations, microkinetic analysis, and simulated Pourbaix diagrams to suggest the reasons for these characteristics. These analyses suggest that the potential-limiting step in CH4 evolution is the clearing of OH from the surface, while the rate-limiting step is the nonelectrochemical C−O bond scission, resulting in a high OH coverage and a high Tafel slope. Our calculations suggest that this high coverage weakens H binding, causing enhancement of the H2 evolution reaction in comparison to that under CO2-free conditions. This analysis shows that the detailed interaction of theory and experiment can be used to design and analyze operational electrocatalysts for CO2 reduction and other complicated electrocatalytic reactions. KEYWORDS: CO2 reduction, coverage effect, electrode kinetics, molybdenum carbide, Pourbaix diagram
1. INTRODUCTION Due to volatilities in fossil resource supplies and growing concerns about greenhouse gas emissions, it is imperative to develop an efficient and economically feasible process for converting atmospheric CO2 into energetically valuable compounds. Electrochemical reduction of CO2 into hydrocarbons over metal catalysts has attracted considerable attention as a promising process because of its straightforward operation in comparison to high-temperature reactors.1 However, only Cu has been repeatedly shown to catalyze CO2 into significant amounts of hydrocarbons, but it requires a large overpotential to drive the reaction, ultimately lowering its thermodynamic efficiency.2−5 Many experimental6−18 and theoretical16,19−24 studies have focused on understanding the distinctive characteristics of Cu in catalyzing CO2. In particular, recent studies14,18−21,24 have concluded that the most difficult elementary step is not the activation of CO2 itself but the subsequent conversion of adsorbed CO into the next hydrogenated species, such as CHO or COH. The poor thermodynamics of this transformation is suggested to determine the onset potential and in practice forces the use of large overpotentials with Cu catalysts. Upon determination of scaling relations between the binding energy of CO and CHOspecifically a strong correlation across transition metals in the binding energy of these two species, with a slope © XXXX American Chemical Society
near 1poor thermodynamics were shown to apply across other transition metals.23 Cu’s slightly better thermodynamics and the surface interaction between CO2 reduction and H2 evolution25−27 suggest the reason no materials have shown performance better than that of Cu. Interestingly, when the early transition metals, such as Mo, W, Ta, Fe, and Ti, are converted to carbides, the above scaling relations need to be modified. These transition-metal carbides were found to weaken the binding energy of CO “selectively” (relative to CHO) due to a more carbophobic and oxophilic nature in comparison to their parent metals.28,29 This finding suggests that the free energy difference between adsorbed CO and CHO can be lowered by carburizing transition metals, resulting in a decreased overpotential requirement for CH4 formation. In the present study, we present an in-depth experimental and theoretical analysis of Mo2C electrocatalysts, which previous work28 suggest would have a more favorable CO → CHO thermodynamic transformation in comparison to that for the transition metals. We prepared two different types of Mo2C Received: October 27, 2015 Revised: January 29, 2016
2003
DOI: 10.1021/acscatal.5b02424 ACS Catal. 2016, 6, 2003−2013
Research Article
ACS Catalysis
a reversible hydrogen electrode scale (VRHE) scale by adding an offset of 0.202 V and a proton-activity correction of 0.059 V per pH unit. The working electrode underwent 10 cycles of cyclic voltammetry (CV) between −0.4 and +0.4 VRHE at 50 mV/s to remove any surface contaminants. Subsequently, another three CV cycles were taken between −1.4 and +0.8 VRHE at 50 mV/s; the second cycle at steady state is shown in the present study. For the production of hydrocarbons from the electroreduction of CO2, a series of potentiostatic measurements was performed with 0.05 V steps from −0.5 to −1.1 VRHE. To avoid confusion with the cathode/anode reaction, we will refer to the absolute value of the overpotential as the absolute overpotential, which we defined as |η| ≡ |V − Veq|, where V is the working electrode potential and Veq is the equilibrium potentials of CO2 + 8(H+ + e−) → CH4 + 2H2O (+0.17 VRHE). After 20 min of electrolysis at each voltage step, gas products were taken from the continuous gas flow with an in-line injector and analyzed with a gas chromatograph (GC, 7890A, Agilent) equipped with three different types of columns (HP-PLOT/Q+PT, HP-PLOT/Q, MoleSieve 5A, Agilent), a thermal conductivity detector (TCD), and a flame ionization detector (FID), which allowed separation and detection of H2, O2, N2, CO, and hydrocarbons from C1 to C4 species. The detection limits of hydrocarbons were about 50 ppb. The Faradaic efficiency (FE) of a gas product k was given by
catalysts and compared their catalytic activities with product analysis for CO2 reduction into CH4 with those of Cu. To understand the different catalytic behavior between Cu and Mo2C, we combined density functional theory (DFT) calculations with microkinetic analyses. Using DFT, we present reaction pathways over Mo2C with free energy diagrams, considering all feasible elementary reactions, including coverage effects of major intermediates. We couple a microkinetic analysis with experimental Tafel slopes to provide an in-depth understanding of reaction mechanisms, such as the rate-limiting steps and surface coverage of major reaction intermediates.
2. METHODS 2.1. Catalyst Preparation. Mo2C catalysts were prepared in two different forms for the present study: as a Mo2C sheet and as a Mo2C bulk powder. The Mo2C sheet was synthesized by carburization of a Mo sheet (99.98%; 0.4 mm × 10 mm × 40 mm, ESPI Metals). Prior to the carburization, the Mo sheet was abrasively polished with emery paper and diamond paste and then was washed with deionized water and acetone. The prepared Mo sheet was heated in a tube furnace with a flowing CH4/H2 gas mixture (CH4:H2 = 20 sccm:40 sccm) at 2.5 °C/min to 450 °C, followed by 5 °C/min to 850 °C, where it was held for 1 h. The Mo2C bulk powder was purchased from SigmaAldrich (particle size
