Pd-Containing Nanostructures for Electrochemical CO2 Reduction

Jan 5, 2018 - College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, China. ACS Catal. , 2018, 8 (2), pp 1510–1519...
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Pd-Containing Nanostructures for Electrochemical CO2 Reduction Reaction Dunfeng Gao,† Hu Zhou,‡ Fan Cai,† Jianguo Wang,‡ Guoxiong Wang,*,† and Xinhe Bao*,† †

ACS Catal. 2018.8:1510-1519. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/02/18. For personal use only.

State Key Laboratory of Catalysis, CAS Center for Excellence in Nanoscience, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China ‡ College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, China ABSTRACT: The electrochemical CO2 reduction reaction (CO2RR), with water as a hydrogen source, has been attracting great attention due to its promising applications for carbon recycle utilization and renewable electricity storage. In order to drive the process economically, highly efficient catalysts are urgently needed to overcome the constraints of high overpotential, low Faradaic efficiency, and current density of CO2RR. This Perspective summarizes the performance of Pd-containing nanostructures toward CO2RR and related reaction mechanisms. The product selectivity of the Pd catalysts strongly depends on the structure and composition, and the dynamic evolution of active phases induced by the applied potential and reaction intermediate of CO2RR. Introducing a second metal can effectively suppress the decay in the catalytic performance of a Pd catalyst and further improve the activity and selectivity of CO2RR. The electrochemical promotion of catalysis effect drastically improves the production rate of formate over Pd nanoparticles, which demonstrates the advantage of the coupled thermo- and electrocatalytic CO2 reduction. The challenges and tentative strategies for the further application of Pd-containing catalysts in CO2RR are also discussed. KEYWORDS: electrochemical CO2 reduction reaction, Pd-containing nanostructures, active site, active phase transformation, bimetallic catalysts, electrochemical promotion of catalysis, coupled thermo- and electrocatalysis, density functional theory calculations

1. INTRODUCTION The conversion and utilization of CO2 is becoming very urgent in realizing energy and environmental sustainability, because the accumulation of CO2 in the atmosphere, mainly caused by the combustion of fossil fuels, raises serious environmental concerns.1 Electrochemically converting CO2 to fuels and chemicals, with water as a hydrogen source, has been attracting great attention as a promising approach for simultaneously achieving carbon recycle utilization and renewable electricity storage.2−4 However, the CO2 molecule is much more thermodynamically and kinetically stable than the H2O molecule; therefore, electrochemical CO2 reduction reaction (CO2RR) usually suffers from high overpotential, low Faradaic efficiency (FE), and limited current density due to the competitive hydrogen evolution reaction (HER).5−10 In order to achieve efficient CO2RR, a variety of catalysts and nanostructures have been developed in the past decades.11−14 Early studies of CO2RR have mainly focused on polycrystalline bulk transition metal catalysts, which can be divided into several subgroups based on the primary product of CO2RR: formateselective metals (Pb, Sn, In, etc.), CO-selective metals (Au, Ag, Zn, etc.), H2-selective metals (Ni, Fe, Pt, etc.), and hydrocarbon-selective Cu.11 Recently, the activity and selectivity of CO2RR have been vastly improved via nanostructuring bulk metals by a series of physical and chemical synthesis strategies.13 Among them, Pd and Pd-based nanostructures widely used in many electrochemical reactions,15−19 show © 2018 American Chemical Society

superior CO2RR activity and selectivity toward CO (Table 1) and formate (Table 2) production compared with other catalysts. The exclusive capability for both formate and CO production with high FEs (up to 90%) on these Pd-based nanostructures attracts significant interest from the researchers on the structure evolution of the catalysts during the CO2RR and the reaction mechanisms behind the superior catalytic performance. This Perspective focuses on the rational design of nanostructured Pd and Pd-based bimetallic catalysts for highly efficient CO2RR and the reaction mechanism, revealed by advanced in situ spectroscopic characterizations and density functional theory (DFT) calculations, with emphasis on the dynamic evolution of active phases during the CO2RR. The advantage of the coupled thermo- and electrocatalytic CO2 reduction is also discussed and demonstrated by the electrochemical promotion of catalysis (EPOC) effect over Pd nanoparticles (NPs) for CO2 reduction to formate. The challenges and tentative strategies for the further application of Pd-containing catalysts in CO2RR are also discussed.

2. PD CATALYSTS Formate, CO, and H2 are the reported major products of CO2RR on Pd catalysts, but their FEs strongly depend on the Received: October 23, 2017 Revised: December 10, 2017 Published: January 5, 2018 1510

DOI: 10.1021/acscatal.7b03612 ACS Catal. 2018, 8, 1510−1519

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Table 1. Overview of Faradaic Efficiencies (FE) and Partial Current Densities of CO on Various Catalysts in Aqueous Electrolytes catalysts 20

Pd foil Pd NPs21 Pd NPs22 Pd/C23 mesoporous PdCu24 PdCu NPs25 Au NPs26 Au nanowire27 oxide-derived Au28 nanoporous Ag29 Ag NPs30 Zn dendrite31 Cu hollow fiber32 CuIn33 CuSn34 rGO-PEI-MoSx35 Ni−N−C36 graphene foam37 a

electrolyte

potential (V vs RHE)

FECO (%)

jCO (mA cm−2)

jCO (mA mg−1)

0.1 M KHCO3 1 M KHCO3 0.1 M KHCO3 0.5 M NaHCO3 0.1 M KHCO3 0.1 M KHCO3 0.5 M KHCO3 0.5 M KHCO3 0.5 M KHCO3 0.5 M KHCO3 0.5 M KHCO3 0.5 M NaHCO3 0.3 M KHCO3 0.1 M KHCO3 0.1 M KHCO3 0.5 M NaHCO3 0.1 M KHCO3 0.1 M KHCO3

−0.8 −0.7 −0.89 −0.6 −0.8 −0.89 −0.67 −0.35 −0.4 −0.6 −0.75 −1.1 −0.4 −0.6 −0.6 −0.65 −0.75 −0.58

28.3 93.4 91.2 ∼40 80 86 90 94 >98 ∼92 84.4 79 72 85 ∼90 85.1 ∼85 ∼85

1.4 22.9 8.9 0.3 ∼0.8 6.9 N.R. 4.2 10 18 3.5 ∼17 ∼10 ∼0.6 ∼1 4.8 ∼10 ∼1.8

N.R.a 57.5 23.9 N.R. N.R. 24.5 ∼8 1.84 N.R. N.R. ∼40 N.R. N.R. N.R. N.R. 21.7 ∼9 N.R.

N.R.: not reported.

Table 2. Overview of Faradaic Efficiencies (FE) and Partial Current Densities of Formate on Various Catalysts in Aqueous Electrolytes electrolyte

potentiala,b

FEformate (%)

jformate (mA cm−2)

jformate (mA mg−1)

0.1 M KHCO3 1 M KHCO3 0.5 M KHCO3 0.5 M KHCO3 0.1 M K2HPO4/0.1 M KH2PO4 0.1 M KHCO3 0.1 M Na2SO4 0.1 M Na2SO4 0.1 M KHCO3 0.5 M NaHCO3 0.5 M KHCO3 0.1 M KHCO3 0.1 M KHCO3

−0.8 −0.1a −0.15a −0.43a −0.4a −0.15a −0.85b −0.88b −0.45a −0.7a −0.7a −1.8b −1.8b

2.8 97.8 >90 99.3% 88 83 90.1 64.3 23 95 40 86.2 87

0.14 15.4 ∼2.5 ∼2.5 ∼5 ∼4 10.59 0.68 2.2 ∼0.3 1.8 13.1 9.5

N.R.c 40.5 40 N.R. 44 N.R. N.R. N.R. N.R. N.R. N.R. N.R. N.R.

catalyst 20

Pd foil Pd NPs21 Pd/C38 PdSn/C39 PdPt NPs40 Pd-PAN/CNT41 partially oxized atomic Co layer9 Co3O4 layer42 Cu foam43 oxide-derived Pb44 tin/tin oxide film45 SnOx/graphene46 PEI-coated NCNT47 a

a

V vs RHE. bV vs SCE. cN.R.: not reported.

ance is probably accompanied with the variation of the active phase of Pd NPs.53 The drastic fluctuations of the product selectivity over Pd NPs were systematically investigated by combined electrochemical measurements, in situ spectroscopic characterizations, and theoretical calculations in our recent work.21 The major product, either formate in region I (0.05 ∼ −0.25 V vs RHE) or CO in region III (−0.45∼−0.90 V vs RHE), can be produced with high FE (up to ∼90%) over Pd NPs with a mean particle size of 3.7 nm in CO2-saturated 1 M KHCO3 aqueous solution (pH 7.8), while the competitive HER predominates in region II (−0.25∼−0.45 V vs RHE) as shown in Figure 1a. In situ Pd K edge and L3 edge X-ray absorption spectroscopy (XAS) measurements indicate the phase transformation from the mixture of α+β Pd hydride (PdHx) to β PdHx with negatively shifting the applied potentials (Figure 2a,b). In situ synchrotron radiation X-ray diffraction (XRD) by Sheng et al.23 and our recent quasi in situ XRD measurements54 also confirm the formation of β PdHx under the reducing conditions (Figure 2e). Both XAS and XRD characterizations demonstrate very

structure of the catalyst and applied potential (Figure 1).20−22,38 The crystalline Pd foil electrode in CO2-saturated 0.1 M KHCO3 aqueous solution (pH 6.8) was reported by Hori et al.20 to show ∼28% CO FE and ∼3% formate FE at around −0.8 V (vs reversible hydrogen electrode, RHE) with a total FE of ∼60% due to the significant hydrogen absorption into the bulk Pd,48,49 while Pd NPs can efficiently reduce CO2 to CO with up to ∼90% FE at −0.7 ∼ −1.0 V vs RHE,22 although the overpotential is higher compared with the Au NPs which is generally considered the most active catalyst for CO production from CO2RR.26 The improved catalytic performance on the nanostructured metallic catalysts is attributed to tunable surface structures and electronic properties at the nanometer scale.50 On the other hand, formate is also selectively produced over the nanostructured Pd catalysts with >90% FE at less-negative potentials,38,51 even near the theoretical equilibrium potential.52 Apart from the higher FEs, higher partial current densities for CO (Table 1) and formate (Table 2) production were also reported over Pd NPs21 than those on Pd foils.20 The potential-dependent CO2RR perform1511

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Figure 1. (a) Applied potential dependence of FE for formate, H2, and CO production in a CO2-saturated 1 M KHCO3 solution (top), and tafel plots for formate and CO production (bottom). Reprinted with permission from ref 21. Copyright 2017 Springer. (b) Applied potential dependence of CO FE over Pd NPs with different sizes (top), and size dependence of turnover frequency (TOF) for CO production on Pd NPs at various potentials (bottom). Reprinted with permission from ref 22. Copyright 2015 American Chemical Society.

over Pd NPs. The product-selectivity fluctuations observed over the Pd NPs are attributed to the thermodynamic and kinetic properties of different active phases, as well as the adsorption strength of the reactants, intermediates, and products.21 Interestingly, our previous work22 shows a significant particle size effect on CO2 electroreduction to CO over Pd NPs at more-negative potentials (Figure 1b). Decreasing the NP size can simultaneously increase the FE and partial current density for CO production, ascribed to the increased number of corner and edge sites (coordinatively unsaturated sites) on small NPs. The remarkably promoting effect of coordinatively unsaturated sites on CO2RR is also observed on other transition metals, such as Au,26,27 Ag,29,30 Cu,58 Zn.31,59 The Gibbs free energy diagrams show that CO2 adsorption and COOH* formation are much easier to occur on the edge and corner sites compared with those on the terrace sites of Pd NPs.22 The turnover frequency for CO production shows a volcano-like curve with the particle size of Pd NPs, suggesting that the changing ratio of the corner, edge and terrace sites on differently sized Pd NPs tunes CO2 adsorption, COOH* formation, and CO* removal during the CO2RR. The catalytic performance of the Pd/C catalyst also shows a strong dependence on the pH of the electrolyte, and the CO FE at −1.19 V vs RHE is increased from 3.2% to 93.2% in a narrow pH window between 1.5 and 4.2 by significantly inhibiting the competitive HER at high pH.60 Surface strain can also regulate the catalytic properties of heterogeneous catalysts.61,62 Huang et al.63 designed a model platform based on Pd octahedra and icosahedra (with an identical exposed facet and similar sizes, except for the different surface strains) to explore the strain effect on CO2RR. The Pd icosahedra/C catalyst shows a maximum FE for CO production of 91.1% at −0.8 V vs RHE, 1.7 times that over Pd octahedra/C catalyst at −0.7 V vs RHE. The improved selectivity is

well the phase transformation in the bulk of Pd NPs.21,23,54 However, the surface of Pd NPs shows a different trend in its composition with negatively shifting the applied potentials. The first principle thermodynamic diagram (Figure 2c,d) suggests a decreased surface H coverage from a pure PdHx surface (above −0.2 V vs RHE) to an almost bare metallic Pd surface (below −0.5 V vs RHE). The surface H coverage is not only controlled by the electrode potential but also significantly affected by the intermediate of CO2RR at the applied potential. The minor CO* intermediate derived from CO2RR at less-negative potentials gradually destroys the PdHx surface, deactivating the formation of formate as a poisoning species. At morenegative potentials, the CO* intermediate significantly occupies the surface of Pd NPs, forming a metallic Pd surface rather than a PdHx surface.55 Therefore, two different active phases, α+β PdHx@PdHx above −0.2 V vs RHE and β PdHx@Pd below −0.5 V vs RHE, are proposed. By further combining Gibbs free energy calculations, it is demonstrated that the α+β PdHx@ PdHx phase selectively reduces CO2 to formate via the HCOO* intermediate56 and the β PdHx@Pd phase selectively reduces CO2 to CO via the COOH* intermediate (Figure 2f). An active-phase transition from α+β PdHx@PdHx to β PdHx@ Pd exists at the middle potentials (−0.25∼−0.45 V vs RHE), where most of the surface PdHx is quickly destroyed by the CO* intermediate while the CO is still difficult to desorb from the surface due to the high CO binding energy.21,38 As a result, a fast decay in the current density occurs, and afterward HER becomes predominated. On the other hand, the increasing coverage of the CO* intermediate on the surface, gradually accumulated with negatively shifting the applied potentials, makes the formation of the bulk PdHx phase energetically favorable21,57 and weakens CO adsorption on the surface,21 which well explains the high overpotential for CO production 1512

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Figure 2. (a) Average Pd−Pd interatomic distance from the K edge EXAFS fitting and (b) In situ L3 edge XANES spectra of 3.7 nm Pd in a CO2saturated 1 M KHCO3 solution; (c) Surface free energy diagram of Pd(211) with different hydrogen coverage formed by proton reduction; (d) Current densities of 3.7 nm Pd in an Ar-saturated 1 M KHCO3 solution as a function of potential; (e) Contour map of in situ synchrotron radiation XRD patterns of Pd/C catalyst under linear sweep voltammetry from 0.68 V to −0.56 V vs RHE at 1 mV s−1; (f) Active phase transformation of Pd nanoparticles with negatively shifting the applied potential. (a−d,f): Reprinted with permission from ref 21. Copyright 2017 Springer. (e): Reprinted with permission from ref 23. Copyright 2017 Royal Society of Chemistry.

desirably, the deactivation problem can be intrinsically solved by designing a catalyst that would stabilize the HCOO* intermediate for formate production and inhibit the COOH* intermediate for CO production. DFT calculations on the Pd(111), Pd(110), Pd(100), and Pd(211) surfaces and a Pd19 cluster by Klinkova et al.64 show that the formation of CO* on the Pd surface is less favored with increasing the index, while the formation of HCOO* becomes more favored (Figure 4). Following the theoretical predictions, they developed highindex-faceted Pd catalysts with the improved catalytic activity and stability for formate production. Zhou et al.65 prepared Pd nanostructured films by optimizing the substrates, electrodeposition process and post treatments, and these films show stable formate production from CO2RR at low overpotentials

attributed to the tensile strain on the surface of the icosahedra, which shifts up the d-band center and thus promotes COOH* adsorption during the CO2RR (Figure 3). At less-negative potentials, CO2 can be reduced to formate due to the presence of an electrochemically generated PdHx surface.21,38 Although formate is the major product of CO2RR, a trace amount of CO can be also produced as a minor product. Because the CO binding on Pd is strong, the generated CO would block the surface of Pd NPs and destroy the surface PdHx, leading to a fast decay for formate production.21 The deactivated Pd catalysts caused by the CO poisoning could be efficiently recovered in situ by brief oxidative treatments (e.g., purging the electrode with oxygen/air21,38,64 and applying a highly positive potential65 to remove the generated CO). More 1513

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Figure 3. (a) TEM images of the Pd octahedral (top) and icosahedra (bottom); (b) Surface strain fields of a Pd octahedron and icosahedron, and projected d-density of states (PDOS) of surface atoms on Pd (111) surfaces with different surface strains; (c) Reduction potential dependence on FEs (top) and mass activities (bottom) over the Pd octahedra/C and Pd icosahedra/C catalysts. Reprinted with permission from ref 63. Copyright 2017 Wiley.

Figure 4. (a) Free energy diagrams for CO2 electroreduction to formic acid on Pd(111), Pd(100), Pd(110), Pd(211), and Pd19 cluster; (b) Surfacemorphology-dependent performance on Pd NPs. Reprinted with permision from ref 64. Copyright 2016 American Chemical Society.

competitive HER and facilitating CO2 electroreduction to formic acid and methanol in acidic solution.

by suppressing the CO formation. The Pd needles reported by Liu et al.66 exhibit a stable geometric current density of ∼10 mA cm−2 at −0.2 V vs RHE for formate production with >91% FE in CO2-saturated 0.5 M KHCO3 solution. The enhanced CO2-to-formate conversion is attributed to the field-induced reagent concentration. The CO2RR performance of Pd catalysts can be further tuned by various carbon supports. Perez-Rodriguez et al.67 reported that the nanostructured carbon support in Pd/C catalyst played an important role in the CO2RR due to the variation on hydrogen adsorption/absorption, evolution, and oxidation. Zhao et al.41 synthesized Pd NPs loaded on polyaniline-covered carbon nanotubes (Pd-PANI/CNT) with an in situ method by reducing H2PdCl4 in a mixture of CNTs and aniline with sodium citrate as a stabilizer. The composite catalyst efficiently reduces CO2 to formate with the highest FE of 83% at −0.8 V vs saturated calomel electrode (SCE). Similarly, Zheng et al.68 reported that polyaniline supported Pd NPs displayed high overpotential for HER, suppressing the

3. PD-BASED BIMETALLIC CATALYSTS The product selectivity strongly depends on the binding energies of the intermediates of CO2RR (e.g., COOH*, HCOO*, CO*, CHO*, COH*) and the H* intermediate of the competitive HER.69,70 Apart from engineering the nanostructures in the pure Pd, introducing a second metal also provides the feasibility to tune the binding energies of the intermediates on Pd and achieve highly efficient production of desired products, even high-value added hydrocarbons and oxygenates. Hansen et al.71 predicts theoretically some possible bimetallic systems by analyzing the binding energies of the intermediates through the models based on DFT calculations. Experimentally, PdCu, 24,25,72−79 PdAu,80−83 PdPt,40,84,85 PdSn,39 PdRu,86 PdTe,87 and PdIn88 have been identified to show improved activity and selectivity of CO2RR compared to the pure Pd. 1514

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Figure 5. (a) Product distribution of bimetallic PdCu catalysts with different mixing patterns: ordered, disordered, and phase-separated. Reprinted with from ref 72. Copyright 2017 American Chemical Society. (b) Formic acid FE on PdxPt(100−x)/C NPs. Reprinted with permission from ref 40. Copyright 2015 American Chemical Society.

NPs from 1 to 10 nm. The decrease in the lattice stain increases the CO coverage at the Pd shell and thus leads to the production of methane and ethane, other than CO and H2. Kortlever et al.82 prepared a PdAu catalyst by electrodepositing Pd that binds CO strongly on an Au substrate that binds CO weakly. The noncopper-containing catalyst can produce C1−C5 hydrocarbons (∼3% FE at −1.4 V vs RHE), probably due to the moderate CO binding strength on the Pd-rich surface of the PdAu alloy. They also electrodeposited Pd on a polycrystalline Pt substrate, and the thin-layer Pd is active for both CO2 electroreduction to formic acid and formic acid electrooxidation, thus showing the reversible catalytic behavior.84 The effect of the PdPt composition on CO2RR is further studied on carbon supported PdPt NPs, and the Pd70Pt30/C catalyst shows the highest FE for formic acid production (Figure 5b).40 Our recent work demonstrates the remarkable effect of the metal deposition sequence on CO2 electroreduction to formate on the PdPt/C catalysts, and the deposition of Pt NPs prior to Pd NPs on the carbon support shows the highest formate mass activity with high FE.85 Bai et al. reported a Pd−Sn alloy catalyst for the exclusive conversion of CO2 into formic acid.39 The formation of the key reaction intermediate HCOO* as well as the formic acid product are considered to be the most favorable over the PdSn alloy catalyst surface with an atomic composition of PdSnO2. Designing bimetallic Pd-based catalysts also provides a strategy to weaken the CO binding affinity on the pure Pd surface, thus suppressing the fast decay for formate production at less-negative potentials.21,38 Takashima et al.77 reported that Cu layers on the surface of Pd NPs by the under-potential deposition technique induced the charge transfer from Pd to Cu and a downward shift of the average d-band center of the catalysts relative to the Fermi level. As a result, the Cu-modified Pd catalyst shows improved CO tolerance without sacrificing the CO2RR performance. Similarly, a gas diffusion electrode with the PdRu alloy catalyst shows a formate FE of 90% at 80 mA cm−2, and no CO formation is observed.86 The PdPt bimetallic NPs also show higher stability for formate production than the pure Pd NPs, which strongly depends on the geometrical and electronic properties of the PdPt catalysts.40,85

Pd−Cu arrangement mainly produces CO24,25 and also favors the production of methane over C2 products, while the phaseseparated PdCu catalyst with neighboring Cu atoms favors the production of C2 products.72 Geometric effects, namely atomic ordering transformation,92 rather than electronic effects, seem to be key in determining the selectivity of the bimetallic PdCu catalysts by tuning the probability of the dimerization of the C1 intermediates. The tetrahexahedral Pd nanocrystals modified with Cu overlayers exhibit high FEs toward alcohols rather than formate over Pd alone, and the selectivity for ethanol or methanol production can be readily tuned by changing the Cu coverage.74 Interestingly, the Pd-decorated Cu electrode significantly improves the stability for CO2 electroreduction to hydrocarbons in comparison to the polycrystalline Cu electrode through a restructuring-induced self-cleaning mechanism.76 The predeposited Pd atoms continuously induce restructuring on the morphology and composition of the Cu surface, and refresh the catalyst surface. Plana et al.80 and Humphrey et al.81 reported that the effective lattice strain of Pd shell was decreased from 3.5% to less than 1% with increasing the Pd shell thickness of Au@Pd

4. COUPLED THERMO- AND ELECTROCATALYTIC REDUCTION OF CO2 ON PD CATALYSTS Although the high FEs for formate and CO production from CO2RR have been achieved as discussed above, this process is still limited by low reaction rates.93−95 Coupling CO2RR with the thermocatalytic CO2 hydrogenation is an efficient way to increase the reaction rate of CO2 reduction. Our recent work54 shows an example that the reaction rate for CO2 hydrogenation to formate is significantly improved on Pd NPs by the EPOC effect, which was discovered by Vayenas96 and can achieve a significant enhancement in the performance in many heterogeneous catalytic reactions.96−98 An early study demonstrates that Pd/C catalysts could reduce bicarbonate in an aqueous solution to formate in the presence of 1 atm H2 at room temperature.99 The reaction rate could be further improved in a CO2-saturated KHCO3 solution due to the rapid equilibrium between the bicarbonate and the dissolved CO2 molecule.100−102 In our work,54 the production rate of formate at −0.1 ∼ −0.4 V vs RHE in H2/CO2-saturated 1 M KHCO3 solution is significantly improved compared to that at an open-circuit voltage, with a rate enhancement ratio ranging

Due to the unique capability of Cu for producing hydrocarbons and oxygenates in considerable amounts,89−91 complex product distributions are also observed on bimetallic PdCu catalysts. Ma et al.72 demonstrate that mixing patterns of Pd and Cu greatly affect the activity and selectivity of CO2RR (Figure 5a). The ordered PdCu catalyst with an alternating

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scaled application of Pd catalysts for CO2RR to CO or formate is severely limited by the high price of Pd as a noble metal. On the other hand, more reduced products beyond formate and CO, such as multicarbon hydrocarbons and oxygenates, are highly desirable due to their high energy density.3 In order to overcome these limitations and further expand the application of Pd-containing nanostructures in CO2RR, we propose several tentative strategies: (i) Designing clusters, even single-site/ atom catalysts, could achieve high atom utilization efficiency of metals, which would significantly decrease the Pd loading in the catalysts. The Pd atoms/clusters coordinating with the other metallic or nonmetallic atoms are expected to tune the binding energy of C1 intermediates, which would determine the pathway of CO2RR.69,70,103 Nanoclusters56,104 as well as the single metal sites/atoms stabilized by carbon materials36,105 and metal organic frameworks106 of other metals has been explored for CO2RR, while the preparation technology of single Pd site/ atom catalysts has also been well developed.107−110 However, the overall catalytic performance of the single Pd site/atom catalysts would be limited in general by the low metal loading in the catalysts. Therefore, developing single site/atom catalysts with reasonably high Pd loading dispersed in carbon matrix111 or other non-noble metals such as Cu107 would be a focused area for the further application of the Pd catalysts in the CO2RR. (ii) Another option could be the deposition of an atom-thick Pd layer onto a cheap substrate (such as metal, metal oxide, heteroatom-doped graphene) by a facile preparation technology, such as atomic layer deposition.110 (iii) Pd-containing composite nanostructures such as PdAu by finely tuning the binding energy of CO on the metal surface, provide the possibility of producing hydrocarbons on a nonCu-containing catalyst, but the FEs are very low.82 To selectively produce multicarbon products, systematical screening on the optimal binding energy of C1 intermediates on the PdAu as well as other non-noble metals should be conducted with the aid of theoretical calculations and high-throughput methods.112 The structure evolution of the pure Pd nanostructures and related reaction mechanisms of the CO2RR have been well understood using Pd (211) as the representative model with the aid of in situ spectroscopic characterizations and theoretical calculations at the atomic level. However, tracking the dynamic evolution of the active phases, oxidation states and the intermediates during the CO2RR is still lacking to understand the reaction mechanisms over Pd-containing binary/multiple nanostructures, especially for the PdCu catalyst selectively producing hydrocarbons and alcohols.72 Furthermore, the combination of in situ/operando spectroscopy and microscopy techniques, such as XAS,21,113 XRD,23 scanning electrochemical cell microscopy,114 scanning tunneling microscopy,115−118 ambient-pressure X-ray photoelectron spectroscopy,119−121 FT-IR spectroscopy21,100,102 could simultaneously measure the electronic properties, structure and chemical state of the catalysts, as well as detect the surface adsorbates and reaction intermediates on the catalysts during the CO2RR. Therefore, developing compatible in situ/operando spectroscopy and microscopy techniques with high sensitivity and high temporal and spatial resolution would provide new insights into the reaction mechanisms of CO2RR and rational design of highly efficient Pd-containing catalysts. Since CO2RR is urgently needed for energy and environment issues,1−3 special attention should be paid to its potential industrial application, apart from the fundamental research on

from 10 to 143 (Figure 6). Besides, the addition of H2 into the reaction system can stabilize the PdHx active phase and thus

Figure 6. (a) Schematic of thermocatalytic and electrocatalytic reduction of CO2 over Pd NPs conducted in a CO2 + D2 atmosphere; (b) Rate enhancement ratios for formate production at different negative potentials compared to those at an open circuit voltage over differently sized Pd NPs in 20% D2/CO2-saturated 1 M KHCO3 solution. Reprinted with permission from ref 54. Copyright 2017 Royal Society of Chemistry.

promote the CO2RR.54 It is proposed that the thermocatalytic and electrocatalytic reduction of CO2 share the same reaction intermediate HCOO* over Pd NPs, which promotes the formate production by the EPOC effect. Based on the understanding of the EPOC effect, a composite electrode containing Pd/C and Pt/C catalysts on the two sides of a piece of carbon paper is constructed for catalyzing the CO2 reduction without directly feeding H2. The H2 in situ generated over Pt NPs promotes the thermocatalytic and electrocatalytic reduction of CO2 to formate over Pd NPs.

5. CONCLUSIONS AND OUTLOOK The CO2RR performance of the nanostructured Pd catalysts strongly depends on the structure and composition of the catalysts as well as the applied potential and reaction intermediate. Pd NPs can efficiently reduce CO2 to CO or formate with high FEs (up to 90%) by applying appropriate potentials. The α+β PdHx@PdHx phase formed above −0.2 V vs RHE selectively produces formate via the HCOO* intermediate, whereas the β PdHx@Pd phase formed below −0.5 V vs RHE selectively produces CO via the COOH* intermediate. The CO production is significantly affected by the size effect and strain effect of the Pd NPs. At less-negative potentials, the production of formate suffers from a fast decay due to the CO poisoning, which can be suppressed by oxidative treatments or by rationally designing new nanostructures inactive for CO formation and/or inefficient for CO adsorption. As an example of the coupled thermo- and electrocatalytic CO2 reduction, the EPOC effect drastically improves the production rate of formate over Pd NPs. Although these Pd-containing nanostructures discussed above have shown vastly improved CO2RR performance, the 1516

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the catalysts. The activity and selectivity toward desired products as well as the energy efficiency and durability of Pdcontaining nanostructures should be evaluated in an industrially relevant continuous-flow CO2RR reactor with gas diffusion electrode configuration,5,122−124 which would eliminate the mass-transport limitations in an H-cell due to the low solubility of CO2 in aqueous electrolytes.94 The scale-up synthesis of Pdcontaining catalysts, configuration design of electrode and electrolyzer, optimization of operation parameters such as electrolyte, temperature, pressure,125 are essential to maximize the CO2RR performance of Pd-containing nanostructures in a continuous-flow CO2RR reactor for evaluating the feasibility of their industrial applications.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for G.W.: [email protected]. *E-mail for X.B.: [email protected]. ORCID

Dunfeng Gao: 0000-0002-2472-7349 Jianguo Wang: 0000-0003-2391-4529 Guoxiong Wang: 0000-0001-6042-1171 Xinhe Bao: 0000-0001-9404-6429 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We gratefully acknowledge the financial support from the Ministry of Science and Technology of China (Grants 2016YFB0600901), the National Natural Science Foundation of China (Grants 21573222 and 91545202), Dalian Institute of Chemical Physics (Grant DICP DMTO201702), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB17020200). G.X. Wang thanks the financial support from the CAS Youth Innovation Promotion (Grant No. 2015145).

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