Electrochemical Methods to Assess Kinetic Factors in CO2 Reduction

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Electrochemical Methods to Assess Kinetic Factors in CO2 Reduction to Formate: Implications for Improving Electrocatalyst Design Atefeh Taheri, Cody R Carr, and Louise A Berben ACS Catal., Just Accepted Manuscript • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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Electrochemical Methods to Assess Kinetic Factors in CO2 Reduction to Formate: Implications for Improving Electrocatalyst Design Atefeh Taheri, Cody R. Carr, and Louise A. Berben* Department of Chemistry, University of California, Davis, California 95616, United States ABSTRACT: Thermochemical insights are often employed in the rationalization of reactivity and in the design of electrocatalysts for CO2 reduction reactions targeting C-H bond-containing products. This work identifies experimental methods to assess kinetic aspects of reactivity. These methods are illustrated using [Fe4N(CO)12]- which produces formate from CO2 at -1.2 V versus SCE in either MeCN/H2O (95:5) or pH 6.5 buffered water. Elementary rates for each reaction step are identified along with the rate determining step (RDS), as C-H bond formation. Transition state kinetics were determined from an Eyring analysis for the rate determining C-H bond formation step using temperature dependent electrochemical measurements. Lower measured ΔG‡ (298K, 12.3 ± 0.1 kcalmol-1) in pH 6.5 aqueous solution, compared with ΔG‡(298K) = 15.0 ± 0.1 kcal mol-1 in MeCN/H2O (95:5), correlates with faster observed reaction rates and provides a kinetic rationalization for the solvent-dependent chemistry. Taken together the experimentally determined kinetic insights highlight that enhancement of local concentration of CO2 at catalyst-hydride sites should be a primary focus of ongoing catalyst design. This will both enhance reaction rates and increase selectivity for C-H bond formation over competing H-H bond formation, since that step is fast in H2 evolution reactions.

INTRODUCTION Iterative electrocatalyst design – an interplay between new catalyst synthesis and mechanistic studies – in H2 evolution chemistry, has resulted in molecular catalysts with rates and stability that rival hydrogenase enzymes,1 and efforts continue to guide homogeneous and heterogeneous photo- and electro-catalyst 2 development. Detailed mechanistic information on electrocatalytic C-H bond formation with CO2 is barely available because, until recently, there have been no ,4, suitable catalysts to study.3 5

This report presents electrochemical methods to obtain information on the RDS and transition state kinetics, and the methods could be generally applied to other catalyst systems. We apply the electrochemical analysis to [Fe4N(CO)12] , (1 ) which catalyzes the conversion of CO2 selectively into formate in either MeCN/H2O (95:5) or in buffered 11 water (pH 5 – 9). This iron cluster remains one of the most selective catalysts for electrocatalytic CO2 reduction to formate. We identify hydride transfer to CO2 as the RDS, along with insights on the transition state structure of the RDS determined by Eyring analysis. The results suggest that new catalysts with enhanced CO2 transport to, and/or enhanced local concentration of CO2 at the active site will yield large gains in reaction rate. We outline the experimental approach to assessing kinetic factors in catalyst design which will complement prior work highlighting thermochemical considerations.

Information on elementary reaction steps is necessary 6 for accurate benchmarking of catalysts, and to understand the factors which should be improved upon to derive faster catalysts from existing state-ofthe-art. Reduction of CO2 to formate using a metal hydride, either via thermal hydrogenation or electrochemical means, requires hydride transfer to CO2. Most often C-H bond formation is assumed to be RESULTS AND DISCUSSION the rate determining step (RDS) in formate production For a transition-metal complex to serve as a C-H bond from CO2, but analysis of the individual rates has not forming electrocatalyst for CO2 there are two main been presented. In addition, discussions of reaction selectivity and catalyst design primarily focus on stages in the reaction: formation of metal hydride, 7,8,9 thermochemical insights, which includes electron transfer and proton transfer or theoretical 5 (ET-PT); and secondly donation of hydride from metal approaches, while experimental probes of reaction hydride to CO2. To figure out which step is the RDS we kinetics and transition state structures are needed. While hydricity shows the inherent hydride donating first measured rate constants for each elementary step ability of a metal hydride, it does not account for the in formate formation using cyclic voltammetry 12 experiments (Scheme 1). mechanism of hydride transfer, and depending on the mechanism, hydricity may not be an accurate predictor of metal hydride reactivity.10 ACS Paragon Plus Environment

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We also assessed the ET rate constant using the ButlerVolmer approach in which the dependence of both the anodic and cathodic peak currents, Epa and Epc, on scan 14 rate are considered according to equations 2 and 3: 0

Epc = E ' =

0

Epa = E ' = Scheme 1. Elementary step, ET-PT and hydride transfer, in + the electrocatalytic reduction of CO2 and H to formate.

Formation of Metal Hydride in MeCN (ET-PT). The heterogeneous ET rate constant (ks) for step 1 was determined using CV experiments in a 0.1 M Bu4NBF4 MeCN solution (Scheme 1). Using the method developed by Nicholson,13 we employed a plot of (∆Ep – 1.16 0.5 59) vs. υ to determine ks according to equation 1: 1.16

1/2

(∆Ep – 59) /40.045 =(1/ks)(πnFυDO/RT) (DO/DR)

α/2

(1)

where ∆Ep is the potential difference between Ep and Ea, n is the number of electrons, Do and DR are diffusion coefficients of oxidized and reduced species determined using the Randles-Sevcik equation (Calculation S1 and Figure S1), F, R, and T are Faraday’s -1 -1 -1 constant (Cmol ), the ideal gas constant (J K mol ), and temperature (K), and the symmetry of the energy barrier to ET is described by the unitless parameter α, which we found to be 0.8 for the cathodic ET event (Calculation S3). The supporting information contains the derivation of equation 1 and other details 1.16 (Calculation S2). Based on the slope of the (∆Ep – 59) 0.5 vs. υ plot, ks was determined as 0.004 cm/s (Figure 1 left).

. α

+

.   α

. α

+

.   √ log 

α  αυ

log

√  √αυ

(2)

(3)

where E°' is the formal reduction potential of the electron transfer, and other symbols have been defined earlier in the text. The symmetry of the energy barrier to ET is described by α, which we found to be 0.8 for the cathodic ET event. (Figure 1 right, Calculation S3). Using the intercept of the best fit line from plots of Epa -1 and Epc vs. log(υ), over the range 0.1 - 0.6 Vs , afforded -1 14 ks as 0.005 cms . Albertin and coworkers have measured ks for two oxidation and two reduction events of [{Fe(CO)2[P(OEt)3]2}2{μ-4,4-N2C6H4-C6H4N2}](BPh4)2 -5 -6 and these were very slow: between 1.2 × 10 and 5 × 10 + 15 cm/s. Ferrocene (Fc/Fc ) has ks = 0.25 cm/s and is considered fast.16 For various organometallic III/II complexes, Co couples are reported with ks = 0.003 II/I 0.051 cm/s, Co couples with ks = 0.012 - 0.11 cm/s, and I/0 2b,c a Co couple as 0.12 cm/s. ,17 All ks values measured for cobalt are considered fast and not rate determining compared to the PT steps following electron transfer (> 7 -1 -1 10 M s ). These comparisons allow us to say that ET is likely not RDS in the formation of (H-1) . To determine the PT rate constant we employed 12b, observations specific to the EC-type mechanism. 18 The shift in peak position (Ep) upon variation of scan 12 rate in the presence of acid is given by equation 2: o

(Ep – E )(F/RT) = -0.78 + 0.5ln(RT/F) + 0.5ln(kobs,2) – 0.5ln(υ)

(4)

where, Ep is the new peak position after addition of acid, kobs,2 is the pseudo-first order PT rate constant for step 2, and is equal to k2[HA]. k2 is the second order -1 -1 rate constant for reaction with acid (M s ), [HA] has units M, and other symbols were defined earlier. In addition, Ep must be independent of [1 ] (Figure 2 left). -

Figure 1. (left) CVs of 0.1 mM 1 under N2 in 0.1 M Bu4NPF6 1.16 MeCN solution at 0.1 - 5 V/s, Inset: Plot of (∆Ep – 59) vs. 0.5 υ ; and (right) Plots of Epa and Epc vs. log(υ). Inset: Fit to the -1 data over the range 0.1 - 0.6 Vs . Red lines are linear fits. GC working electrode.

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), and other symbols were defined earlier. This equation applies when both of the ET steps (step 1 and 4) are fast, and we know that step 1 is fast (ks = 0.01 cm -1 s ). We know that Step 4 is fast at -1.2 V since this is 9 more than 1.5 V of overpotential for Step 4. Additionally, eqn 3 can be simplified to eqn 6, when step 3 is the RDS and significantly slower than step 2; ie. when k2[HA] >> k3[CO2]: 1/2

jcat,max = 2 F [Cat] D (kobs) Figure 2. CVs in 0.1 M Bu4NBF4 MeCN/H2O (95:5) under N2 of (left) 0.051, red; 0.17, green; and 0.20 mM, blue, 1 at 100 mV/s. Inset: Ep vs. [1 ] at varied scan rates: 0.1 (squares), 1.5 -1 (circles), and 3 V/s (triangles): (right) 0.1 mM 1 at 0.1 – 1 Vs . 0 Inset: (Ep – E )(F/RT) vs. ln(υ). Red line is linear fit. GC working electrode.

The experimental data display a distinct curvature at -1 scan rate greater than 1 Vs , consistent with kinetic competition between ET (step 1) and PT (step 2), and so that data was not used in this analysis. With a series -1 of CV experiments collected between 0.1 and 1.0 Vs in 0 MeCN/H2O (95:5), a fit to the plot of (Ep – E )(F/RT) 9 -1 vs. ln(υ), with slope of -0.5, gave kobs,2 = 1.5 × 10 s (Figure 2 right), and using [H2O] = 2.78 M, k2 is 5.6 × 8 -1 -1 10 M s . A comparison of kobs,2 with kobs is given below. Reaction of Metal Hydride with CO2 in MeCN/H2O (95:5). We cannot directly measure the rate constant (k3) of hydride transfer from (H-1) to CO2 since (H-1) 11 cannot be isolated. Therefore, we measured the overall rate of CO2 reduction, kobs, to compare this with ks and kobs,2. To determine kobs we first determined an appropriate concentration of 1 for the experiment to ensure that [CO2] is not limiting. We performed two experiments where [CO2] was varied, using [1 ] = 0.5 or 0.2 mM. Experiments with 0.5 mM 1 reveal ½ dependence on [CO2] , consistent with a mechanism that is first-order in CO2, but experiments performed with 0.2 mM 1 showed a pseudo zero order dependence on [CO2]. Therefore, to determine kobs we use concentrations of 1 that do not exceed 0.2 mM. Savéant and coworkers have shown that the catalytic current density in an ECCE mechanism can be described in terms of catalyst and substrate concentration, and the rate constants for the two chemical steps which we are calling step 2 (k2) and step 12 3 (k3): ,  2 Cat" #/

%& '(" *& +," 0 ) *& +," *- ./& " 01 *- ./& "

(5)

In equation 5, jcat,max is the scan rate independent -2 current density of the catalytic process (A cm , Figure S2), [Cat] is the bulk concentration of catalyst (molcm

where TOF = kobs =

1/2

(6)

%& '(" * +," ) & *- ./& "

-1

with units, s .

0 *& +," 01 *- ./& "

In eqn 6, kobs and TOF are rates for the reduction of -1 CO2 to formate (s ), and other symbols were defined earlier. -

The slope of a linear fit of jcat,max vs. [1 ] was determined -1 according to equation 4, and gave TOF = kobs = 0.6 s (Figure 3 left, Calculation S4).

Figure 3. (left) CVs under 1 atm CO2 of (left) 0.21 (black), 0.15 (red), 0.10 (orange), 0.069 (green) and 0.035 (blue) mM 1 in o 22 C 0.1 M Bu4NBF4 MeCN/H2O (95:5); and (right) 0.060 (black), 0.053 (red) and 0.050 (orange), 0.042 (green), 0.033 o (blue) mM 1 in 25 C 0.1 M pH 6.5 bicarbonate buffer. Insets: Plot of jcat,max versus [1 ] determined at -1.25 and -1.22 V vs. SCE, respectively.

The value for kobs is much smaller than kobs,2, and this is consistent with step 3 as the RDS in the reduction of CO2 into formate, and therefore TOF can be defined in terms of the rate constant for step 3 (equation 7): TOF = kobs = kcat[CO2] = k3[CO2]

(7)

When [CO2] in MeCN/H2O (95:5) is 0.24 M (under 1 -1 -1 atm of CO2),19 k3 = 2 M s . We further confirmed that a bond-making or breaking event associated with an intermediate hydride, (H-1) is involved in the RDS by measurement of the H/D kinetic isotope effect. From measurements of kobs performed in MeCN/H2O (95:5) and in MeCN/D2O (95:5), we found KIE = 2.8 ± 0.1 (Figure 4,

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Calculation S5). This isotope effect could be consistent with a RDS where hydride is transferred from (H-1) to CO2. The relatively small magnitude of the KIE might arise from a non-linear transition state involving the 2 µ -H functional group. Alternatively, the small KIE could also be consistent with hydride migration on the cluster, such as an isomerization from bridging hydride to terminal hydride, prior to a faster step in which hydride is transferred to CO2.20 Various reasons describing effects on the magnitude of KIE have been proposed and discussed elsewhere.21

-

Figure 4. (left) CVs of 1 under 1 atm CO2, in 0.1 M Bu4NBF4 MeCN/H2O (95:5), and in 0.1 M Bu4NBF4 MeCN/D2O (95:5) shifted anodically by 0.4 V for clarity. (right) Plots of jcat,max vs. [1 ] determined at -1.25 vs. SCE for kH, red; and kD, blue. -1 Overlaid lines are linear fits. Scan rate: 100 mVs . GC working electrode.

Determination of RDS in pH 6.5 Buffered Water. We cannot measure ks or kobs,2 in water because o protons are always present and E cannot be obtained for use in equations 1 - 3. However, the absence of a reversible CV signal between pH 5 – 12 tells us that ks is faster than protonation (kobs,2). In addition, we know that Ep for electrocatalytic reduction of protons to H2 does not change between pH 5 and 13,22 and so neither ks (step 1) or k2 (step 2) are rate determining in the H2 evolution reaction. Step 4 is not the RDS for the same reason as was outlined earlier for MeCN/H2O (95:5). This verifies that Step 3, protonation of (H-1) , must be the RDS. We also measured kobs in water using data fit -1 to equation 3, and we found kobs = 210 s (Figure 3 right, Calculation S6). Since kobs for H2 evolution in -1 18 water (659 s ) is faster than kobs for formate -1 production (210 s ), we infer that hydride transfer to CO2 in water (step 3) is the RDS. Using kobs, knowledge that step 3 is RDS, and [CO2] = 33 mM in water in 3 -1 -1 equation 7,23 we determined k3 as 6.4 × 10 M s . Reaction of Metal Hydride with CO2: Eyring Analysis. We have previously provided evidence that thermochemistry may determine the observed increased selectivity of 1 for formate production in 8d,11 water vs. MeCN/H2O (95:5), and thermochemical arguments are regularly employed to rationalize the behavior of other H2- and formate-evolving electrocatalysts. To better understand the origin of

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increased selectivity and rates of hydride transfer in water, we performed an Eyring analysis to probe transition state solvent dependence. An Eyring analysis using variable temperature kobs (k3) data will provide information about the structure of a transition state for the RDS in a catalytic cycle according to equation 8:24 ‡

‡

ln(k3/T) = (-∆H /RT) + ln(kBT/h) + (∆S /R) (8) 2

-1

where h is Planks constant (m kgs ), kB is the 2 -2 -1 ‡ ‡ Boltzman constant (m kgs K ), ∆H and ∆S are the -1 transition state enthalpy (kcalmol ) and entropy -1 (calmol ), respectively, and other symbols have been defined already. kobs values were first obtained at several temperatures using equation 6 (Figure S3). To determine k3 from kobs at each temperature we used equation 7. Values for [CO2] at each temperature were estimated using Henry’s constants (Calculation S7, , Table S1).25 26 The Stokes-Einstein relationship was used to determine the diffusion coefficient, D, at each temperature (Calculation S1, S8, Table S1, Figure S1),27,28 and radii used in the Stokes-Einstein relationship were assumed to remain constant with temperature. The temperature dependence of kobs for CO2 reduction o in MeCN/H2O (95:5) by 1 was investigated from -5 C o to 22 C (Figure 5, left). In 0.1 M KHCO3 aqueous o solution (pH 6.5), kobs was determined between 5 C o and 40 C (Figure 5, right). At each temperature, kobs (and subsequently k3) were determined from the plot of jcat,max versus [1 ] (Calculations S7, S8, Tables S1, S2, Figure S3). Eyring plots, ln(k3/T) vs. 1/T, are linear for both solvent systems (Figure 5 insets, Table S2).

-

Figure 5. (left) CVs of 0.095 mM of 1 in 0.1 M Bu4NBF4 o o MeCN/H2O (95:5) under 1 atm CO2, 22 C (orange), 10 C o o o (green), 5 C (blue), 0 C (red), and -5 C (black). (right) CVs of 0.06 mM of 1 in 0.1 M KHCO3 water under 1 atm CO2 at 40 o o o o C (green), 25 C (red), 15 C (blue), and 5 C (black). Insets: Plots of ln(k3/T) versus (1/T). Overlaid lines are linear fits. -1 Scan rate: 100 mVs . GC working electrode. ‡

-1

In MeCN/H2O (95:5), ΔH = 10.6 ± 1.0 kcalmol , and ‡ -1 -1 ΔS = -15.0 ± 3.8 cal mol K . In pH 6.5 aqueous ‡ -1 ‡ solution, ΔH = 8.5 ± 1.7 kcal mol , and ΔS = -12.6 ±

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-1

‡

6.2 calmol K . At 298 K, ΔG = 15.0 ± 0.1 and 12.3 ± 0.1 -1 kcal mol for MeCN/H2O (95:5) and pH 6.5 aqueous ‡ solutions, respectively. Values of ∆S from the Eyring analyses suggest ordered transition states, and the -1 ‡ difference of 2.7 kcal mol for ΔG implies that transition state structures could be different in the two media. A formato intermediate is most likely and could be formed by CO2 insertion with (H-1) , or hydride transfer from (H-1) to CO2, in either solvent system ‡ (Scheme 2). The more negative value of ΔS = -15.0 cal -1 -1 ‡ mol K in MeCN/H2O (95:5) compared with ΔS = -1 -1 12.6 calmol K in pH 6.5 water might suggest a charge separated transition state where formate is more closely associated with 1 in the less polar solvent system. This observation indirectly supports a hydride transfer mechanism (Scheme 2A) in both solvents, where greater variation in transition state structure could be obtained with solvation effects.

Scheme 2. Possible transition states for C-H bond formation.

Precedent for an ordered transition state in hydride transfer from a terminal hydride is seen in work on homogeneous hydride transfer to CO2.29 For example, Meyer and Sullivan studied the reaction of CO2 with fac-[Re(bpy)(CO)3H], bpy = 2,2΄-bipyridine, and found ‡ -1 ‡ -1 -1 30 ΔH = 13 kcal mol and ΔS = -33 calmol K in THF. Solvent effects observed in that study were attributed to charge separation in the transition state, and an increase in rate constant with higher dielectric constant solvents was also reported. We also note that bridging hydrides such as (H-1) are less reactive and are less commonly observed to perform C-H bond formation or CO2 insertion but precedent does exist. For example, Eisenberg and coworkers reported + [Rh2(μ-H)(CO)2(dpm)2] (dpm = bis(diphenylphosphino)methane) and its reaction with 31 CO2, and CO2 insertion into [(μ-H)Ru3(CO)11] affords - 32 structurally characterized [(μ -HCO2)Ru3(CO)10] . Photoreaction of CO2 saturated THF solutions with 2 33 (Cp*Ru)2(μ-H)4 yields (Cp*Ru)2(μ-H)3(μ−κ -OCHO). ‡

The relative values of ΔG observed in MeCN/H2O (95:5) and pH 6.5 aqueous solution, 15.0 vs. 12.3 -1 kcalmol , correlate with our observation that formate production is faster in water, and with our previously reported thermochemical studies showing that free o energies for hydride transfer to CO2 (ΔG H trans) are 5 -1 and -8.6 kcalmol in MeCN/H2O (95:5) and pH 6.5 aqueous buffer, respectively. The lower transition state ‡ barrier (ΔG ) supported by aqueous media corresponds

to the more favorable reaction in aqueous solution o (more negative ΔG H trans). Solvent Effects and Benchmarking. A Tafel-style plot is often used to evaluate which catalyst is most suitable for a certain potential regime. In the semilogarithmic plot, TOF increases linearly until it reaches a plateau value at elevated η, which corresponds to kobs (TOFmax). The intersection with the vertical line at zero o overpotential corresponds to the TOF value, which has been proposed as a metric to determine the intrinsic performance of a catalyst.34 The rate of hydride transfer to CO2, k3, which is the RDS, is two orders of magnitude faster in water than in MeCN/H2O (95:5). The lower concentration of CO2 in water results in kobs values that are similar in both solvents. Future catalyst design must address CO2 transport to the first coordination sphere, and/or enhancement of local [CO2]. Either of these goals might be achieved by synthetic modification of the secondary coordination sphere. Tafel-style plots further illustrate effects of [CO2] and the importance of improving CO2 transport or local [CO2] at active sites to improve rate and selectivity of formate formation. Plots of log(kobs) vs. η, and log(k3) vs. η, viewed side by side, draw attention to the effects of low [CO2] in water (Figure 6). Catalysts 1 and PCP-Ir, are reported in Cy Bn + 4,5 water and MeCN, and [CpCo(P 2N 2)I] in DMF.

-

Figure 6. (left) log(kobs) vs. η, and (right) log(k3) vs. η. 1 , Cy Bn + blue and teal; PCP-Ir, black and green; [CpCo(P 2N 2)I] , red. See Calculation S9 and Table S3 for details.

The hydride formation step is most often the RDS in 2 the electrocatalytic reduction of protons to H2. The identification of C-H bond formation as the RDS in formate production further highlights the challenge in avoiding H2 evolution, since the equivalent H-H bond formation step rarely limits the H2 evolution reaction rate. It also reinforces the notion that increasing local CO2 concentration near hydride intermediates is a promising path to improve rates. CONCLUSIONS In conclusion, we have presented a general approach to assess the reaction kinetics of electrocatalytic C-H bond formation with CO2. C-H bond formation is the RDS in electrocatalytic formate production with 1 , and this result stands in contrast to H2 evolving

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electrocatalysts which most often have hydride formation as the RDS. Using an Eyring analysis, transition state free energy barriers were determined to be lower in water than in MeCN/H2O (95:5), and this could explain the faster rates of formate formation observed in water. Previous work in the field has attributed these solvent effects primarily to solvent7-11 dependent thermochemistry for hydride transfer. More generally, previous design rationale for C-H bond formation with CO2 has focused on thermochemical considerations and insights derived from theory. The experimental approach described here for assessing kinetic factors in reactivity, points to further efforts in synthetic inorganic chemistry needed to enhance local CO2 concentration, and CO2 transport near the active sites of next-generation catalysts. Enhancement of local CO2 concentration is expected to increase rates of operation and increase selectivity for CO2 over protons, as substrate. Moreover, well designed CO2binding active sites have the potential to turn H2 evolution catalysts into selective C-H bond formation catalysts. Supporting Information. This material is available free

of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

* [email protected] Author Contributions

All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT We are grateful to the Department of Energy Office of Science for support from award number DE-SC0016395. We thank Ms. N. D. Loewen for assistance with Tafel calculations.

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