ArticleFebruary 27, 2026

The Effect of Exogenous Acid Identity on Iron Tetraphenylporphyrin-Catalyzed CO₂ Reduction
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Kaeden Teindl
Kaeden Teindl
Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
More by Kaeden Teindl
https://orcid.org/0009-0005-3367-6845
Eva M. Nichols*
Eva M. Nichols
Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
*Email: [email protected]
More by Eva M. Nichols
https://orcid.org/0000-0002-3718-7273

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Inorganic Chemistry

Cite this: Inorg. Chem. 2026, 65, 10, 5339–5347

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https://doi.org/10.1021/acs.inorgchem.5c05122

Published February 27, 2026

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Abstract

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Iron tetraphenylporphyrin (FeTPP) is a privileged electrocatalyst for the 2e^–/2H⁺ reduction of CO₂ to CO. FeTPP-catalyzed CO₂ reduction typically employs phenol as an exogenous acid to promote the rate-limiting proton-coupled electron transfer. Beyond the observation that catalytic rates increase with decreasing pK_a, the effects of acid identity on reaction kinetics are largely unexplored. Herein, we report rates of FeTPP-catalyzed CO₂ reduction with structurally diverse O–H, N–H, and C–H acids. While many of these acids follow the expected Brønsted relationship, there are several notable exceptions: the fluorinated alcohols hexafluoroisopropanol (log(k_cat) = 4.54) and 2,2,2-trifluoroethanol (log(k_cat) = 3.55)─and the N–H acid imidazole (log(k_cat) = 4.41)─display catalytic rates that are several times greater than rates obtained with similarly acidic phenols. Amides with pK_as < 19 (in dimethyl sulfoxide) display similar activity as comparably acidic O–H acids, while rates obtained with less acidic amides are ∼2 orders of magnitude slower than O–H donors of similar pK_a. Each C–H acid affords poor activity. An Eyring analysis suggests that acids enforcing less ordered transition states afford superior kinetics. This study reveals that acid pK_a is only one relevant parameter for altering catalytic rates, and judicious selection of the acid is crucial for enhancing catalytic rates.

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Subjects

1. Introduction

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Electrochemical CO₂ reduction using sustainable energy inputs is a promising approach for the valorization of this abundant and stable C₁ feedstock. However, significant kinetic barriers must be overcome to reduce CO₂ rapidly and selectively. (1−4) While several families of molecular catalysts are active toward electrochemical CO₂ reduction, (5−19) iron tetraphenylporphyrin (FeTPP) is a privileged homogeneous electrocatalyst that displays favorable activity toward the 2e^–/2H⁺ reduction of CO₂ to CO in organic electrolyte with mild acids, most frequently phenol. (20−26) Efforts to improve the activity of FeTPP by modifications to the primary coordination sphere (i.e., changing the electron donor ability of porphyrin substituents) are often restricted by electronic scaling relationships. These relationships reveal that more negative reduction potentials─larger electron transfer driving forces─are required to accelerate the rate-limiting proton coupled electron transfer (PCET) step associated with C–O cleavage. (20,25,27,28) As expected, a similar trade-off is observed with proton transfer driving forces, where more acidic exogenous phenols correlate with faster reaction kinetics. (21,29) However, the slopes of the rate vs pK_a relationships (Brønsted α) are quite shallow, and large changes in pK_a typically yield modest rate enhancements. These scaling relationships significantly constrain the activity of FeTPP-catalyzed CO₂ reduction.

Modification of the secondary coordination sphere (SCS) can allow these thermodynamic scaling relationships to be circumvented. Functional groups positioned in the catalyst periphery can lower reaction barriers by stabilizing otherwise high-energy intermediates and transition states, or by relaying protons from an exogenous acid to the catalytic active site. Accordingly, a diverse range of SCS design strategies have been investigated, including altering the positioning (20) and number (24,30−32) of protic SCS donors, tuning proton transfer driving forces, (21,33−36) templating exogenous acids for optimal proton delivery, (37) and incorporating polycationic functional groups. (27,38,39) Although these SCS strategies can significantly improve the kinetics of FeTPP catalysts, many such modifications are time-consuming and synthetically challenging. It is therefore of interest to establish more practical methods to enhance CO₂ reduction kinetics.

We envisioned that a complementary approach to SCS modification would instead focus on optimizing the design of the exogenous acid (Figure 1). Few reports have investigated how properties of the exogenous acid itself─aside from acid pK_a─might alter the activity of FeTPP-catalyzed CO₂ reduction. Notably, Savéant and co-workers have investigated a small series of nonphenolic O–H acids (2,2,2-trifluoroethanol, H₂O, and acetic acid), yet they reported the existence of a linear Brønsted relationship. (28) However, examples reported with other electrocatalyst platforms do suggest that performance is governed by additional parameters beyond pK_a, thereby implying that exogenous acid identity can be strategically modified to improve catalytic activity. Nichols et al. reported the CO₂ reduction activity of nickel cyclam with a limited series of N–H acids and found that catalytic rates were not restricted by a simple Brønsted relationship. (40) Altering the structure of exogenous amines and ammonium acids has also been shown to improve the selectivity and activity toward formate production with several different CO₂ reduction electrocatalysts. (41−43) Investigations of electrochemical O₂ and N₂ reduction systems have also revealed that various additives and solvents yield unique Brønsted relationships. (44−47)

Figure 1. Comparison of strategies to improve catalytic activity, where the catalyst is synthetically modified to include a secondary sphere proton relay (left) or the identity of the exogenous acid is altered (right).

Against this backdrop, we sought to investigate whether simply diversifying the exogenous acid might alter the activity of FeTPP-catalyzed CO₂ reduction. Accordingly, we evaluated a series of structurally diverse O–H, N–H, and C–H acids. Most of the O–H acids approximately follow a Brønsted relationship, but aliphatic fluorinated alcohols consistently display improved reaction rates compared to acids of similar pK_a, and hexafluoroisopropanol (HFIP) is found to be the most active proton source. Moreover, we find that amides with pK_a(DMSO) less than ∼19 show similar kinetics to comparably acidic O–H acids, but amides with higher pK_as show notably slower kinetics than the O–H acids of similar pK_as. Imidazole performs exceedingly well and some of the fastest rates are obtained with this exogenous acid. C–H acids generally show comparatively slow reaction kinetics, consistent with their poor ability to engage in hydrogen bonding. Altogether, this work demonstrates that the kinetics of FeTPP catalyzed CO₂ reduction are not always constrained by linear Brønsted relationships, and that judicious selection of the exogenous acid is a crucial design consideration for optimizing catalytic activity.

2. Results and Discussion

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2.1. Evaluating the Activity of Structurally Diverse Exogenous Acids by Cyclic Voltammetry

To assess the electrocatalytic activity of FeTPP with a range of exogenous acids (Figure 2A), we performed cyclic voltammetry (CV) experiments in N,N-dimethylformamide (DMF) and 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF₆) electrolyte under 1 atm CO₂ with 50 mM exogenous acid (Figure 2B–D). Due to the poor selectivity and stability displayed by FeTPP in the presence of highly acidic donors, (28,43) we primarily targeted acids with pK_as(DMSO) greater than ∼15 (we report pK_a values in dimethyl sulfoxide, DMSO, because they are widely available in this solvent; pK_a values in DMF are nearly identical (48)). We further note that the trace water present in anhydrous DMF is expected to have a very minor effect on catalytic activity in this pK_a range, as shown by Savéant and co-workers. (28)

Figure 2. A) Structure of FeTPP and each of the structurally diverse exogenous acids. Values in parentheses indicate pK_a values reported in DMSO. The cyclic voltammograms obtained with each (B) O–H, (C) N–H, and (D) C–H exogenous acid (CV conditions: 0.5 mM catalyst in dry DMF, 0.1 M TBAPF₆, 50 mM acid, and 0.23 M CO₂; scan rate = 0.1 V s^–1). E) The k_cat values of each exogenous acid as a function of pK_a; k_cat values are averaged over at least four different acid concentrations. Black line and symbols correspond to a Brønsted plot constructed from a series of phenols of tunable acidity reported in ref (21). Colors and symbols correspond to the exogenous acids displayed in panel A.

We begin with the structurally diverse set of O–H acids. We first evaluated catalytic activity with both 1- and 2-naphthol (pK_as of 16.2 and 17.1 in DMSO, respectively (49)) to investigate how an expanded π-system might affect the kinetics of rate-limiting PCET. This question was of interest because previous work from our group has shown that pyrene─a planar aromatic additive─can enhance the kinetics of FeTPP-catalyzed CO₂ reduction by mitigating solution-phase aggregation. (22) Additionally, 2,6-di-tert-butyl phenol (pK_a = 17.3 (49)) was studied to ascertain the influence of steric bulk near the locus of proton transfer. Both hexafluoroisopropanol (HFIP; pK_a = 17.9 (50)) and 2,2,2-trifluoroethanol (TFE; pK_a = 23.5 (51)) were investigated as a pair of nonaromatic O–H donors. We have previously shown that exogenous bicarbonate acts as a competent proton donor in organic electrolyte, (37) so we further compare the catalytic activity measured with tetraethylammonium bicarbonate ([NEt₄][HCO₃]; pK_a = 20.8 (37)) to the activity observed with the acids surveyed here (Caution! Tetraethylammonium salts are recognized as channel blockers with effects on the neuromuscular junction; therefore, they should be handled with appropriate personal protective equipment to avoid exposure).

Cyclic voltammograms recorded with each O–H acid show a large current increase at potentials near the catalytically relevant formal Fe^I/0 couple (E_1/2(Fe^I/0)) (Figure S1), consistent with electrocatalytic CO₂ reduction. Furthermore, we find an approximate correlation between acid pK_a and catalytic activity, wherein a larger i/i_p⁰ value (catalytic current normalized to the current in absence of substrate) and a more positive catalytic onset is generally observed with more acidic donors (Figure 2B). One notable deviation from this trend, however, is HFIP, which provides the largest i/i_p⁰, despite being less acidic than both naphthols, suggesting that catalytic activity is not solely determined by exogenous acid pK_a.

Building on these results, we next investigated a series of N–H acids. In our previous reports, we demonstrated that amides can significantly enhance CO₂ reduction activity when installed in the SCS of FeTPP catalysts; (20,21,52) however, their role as exogenous acids has not been explored. Herein, we include a series of tunable aryl acetamide exogenous acids. No experimental pK_a(DMSO) value was available for pentafluorophenyl acetamide, but we estimate a value of ∼15 based on a reported pK_a(MeCN) and using the empirically determined pK_a relationship between DMSO and acetonitrile. (53) The pK_as of 3,5-bis(trifluoromethyl)phenyl acetamide (17.7 ± 0.2) and (4-trifluoromethyl)phenyl acetamide (19.2 ± 0.1) were not previously reported, so we measured these values using UV–visible spectrophotometric titrations with a colorimetric indicator of known pK_a (Figures S2–S4; see the Supporting Information for details). The pK_a of the least acidic acetamide (acetanilide) is reported to be 21.5. (54) Benzanilide (pK_a = 18.8) was selected to probe the effects of an expanded π-system with an N–H acid. Lastly, imidazole (pK_a = 18.6 (51)) was investigated to further increase the structural diversity of the series of N–H donors.

Much like the O–H acids, the voltammetric responses observed with the acetamides qualitatively follow a Brønsted relationship, wherein more acidic donors promote increasingly large catalytic peak currents (Figure 2C). Noteworthy exceptions to this trend include benzanilide, which appears to have no effect on catalytic activity, and imidazole, which displays an i/i_p⁰ that is considerably larger than many of the comparably acidic N–H acids surveyed above.

As a final category, we measured electrocatalytic activity in the presence of exogenous C–H acids (Figure 2D). The nonacidic character of most C–H bonds limits our analysis to a pair of reductively stable acids: fluorene (pK_a = 22.6 (55)) and acetylacetone (pK_a = 13.3 (56)); we note that the potential tautomerization of acetylacetone to its enol form complicates its exclusive assignment as a C–H acid. CV responses under CO₂ are comparable in the absence or presence of fluorene, meaning that this donor has a negligeable effect on catalytic activity. By contrast, a minor increase in i/i_p⁰ is observed with the substantially more acidic acetylacetone, suggesting this acid does enhance CO₂ reduction activity. Qualitatively, these results indicate that properties of the exogenous acid─aside from pK_a─are important determinants of the kinetics of FeTPP-catalyzed CO₂ reduction. A more detailed analysis of product selectivity and reaction kinetics is described below.

2.2. Controlled Potential Electrolysis to Evaluate Selectivity and Stability for Each Exogenous Acid Family

We next evaluated reaction selectivity and catalyst stability in the presence of each class of exogenous acid using controlled potential electrolysis (CPE) and headspace gas chromatography. In these experiments, FeTPP (0.5 mM) was electrolyzed at potentials near the Fe^I/0 couple under a saturated CO₂ atmosphere with 50 mM of exogenous acid. FeTPP catalysts are well-documented to show favorable stability and high Faradaic efficiency (FE) for CO in the presence of mild O–H acids; (20,21,27,28,37) for this reason we have chosen to evaluate reaction selectivity using the less well-studied C–H and N–H acids (Table S1). Accordingly, in the presence of the C–H acid acetylacetone, CO is produced at a low FE of 25 ± 8%. As a representative N–H acid, we examined 3,5-bis(trifluoromethyl)phenyl acetamide and found that it displays a slightly higher FE of 47 ± 2% for CO. In both cases, FeTPP is selective for CO, and no other gaseous products are detected, suggesting that the remaining electron equivalents are likely directed toward catalyst decomposition. The precipitous current decreases in the current-versus-time traces similarly support the conclusion of porphyrin degradation (Figures S8 and S9). Thus, in addition to altering catalytic activity, the structure of the exogenous acid appears to play a key role in determining the stability of FeTPP under electrocatalytic conditions.

2.3. Effects of Structurally Diverse Exogenous Acids on CO₂ Reduction Kinetics

After demonstrating high CO selectivity, kinetic parameters were extracted from catalytic voltammograms using foot-of-the-wave analysis (FOWA), which has been extensively detailed by Savéant and co-workers. (23) Herein, observed rate constants (k_obs, s^–1) were determined by FOWA for each of the exogenous acids described above. Associated catalytic rate laws were obtained by measuring k_obs under pseudo first order conditions at varying concentrations of exogenous acid (∼5–50 mM) and varying CO₂ partial pressures. From these rate laws, the intrinsic catalytic rate constant (k_cat) could be obtained for each acid, as reported in Table 1 and Figure 2E (k_cat values are averaged over several acid concentrations and uncertainties are reported to one standard deviation). Lastly, we compare the k_cat of each exogenous acid surveyed herein to our previously reported Brønsted plot that was generated with four phenol acids (21) (black line in Figure 2E).

Table 1. Summary of Rate Laws and Rate Constants for Each Exogenous Acid

exogenous acid	catalytic rate law (k_obs =)	log (k_cat)
4-trifluoromethyl phenola	k_cat[CO₂][acid]	4.74
3-fluorophenola		4.38
phenola		3.84
4-methoxy phenola		3.58
1-naphtholb	k_cat[CO₂][acid]	4.04 ± 0.03
2-naphtholb	k_cat[CO₂][acid]	3.70 ± 0.06
2,6-di-tert-butyl phenolb	k_cat[CO₂][acid]^1/2	2.28 ± 0.03
[NEt₄][HCO₃]b	k_cat[CO₂][acid]	3.38 ± 0.08
HFIPb	k_cat[CO₂][acid]	4.54 ± 0.08
TFEb	k_cat[CO₂][acid]	3.55 ± 0.08
pentafluorophenyl acetamideb	k_cat[CO₂][acid]	4.40 ± 0.04
3,5-bis(trifluoromethyl)phenyl acetamideb	k_cat[CO₂][acid]	3.39 ± 0.04
4-(trifluoromethyl)phenyl acetamideb	k_cat[CO₂][acid]^1/2	1.98 ± 0.03
acetanilideb	k_cat[CO₂][acid]^1/2	1.79 ± 0.04
benzanilideb^,c		1.63 ± 0.02
imidazoleb	k_cat[CO₂][acid]	4.41 ± 0.05
fluoreneb^,c		1.65 ± 0.02
acetylacetoneb	k_cat[CO₂][acid]^1/2	2.52 ± 0.03
no acidc^,d		1.64 ± 0.03

^{^a}

Kinetic parameters for each exogenous phenol used to construct the Brønsted relationship are obtained from ref (21).

^{^b}

k_cat values are averages of at least four different acid concentrations.

^{^c}

The proposed second-order dependence on CO₂ is based on the rate law reported in ref (58), in which FeTPP CO₂ reduction is performed under aprotic conditions; we presume this reaction order holds under conditions wherein a zeroth-order dependence on acid is observed.

^{^d}

k_cat value measured for the case of “no acid” is an average of three separate experiments, rather than different acid concentrations. All reported uncertainties represent one standard deviation.

We begin the analysis of catalytic rate laws with the series of O–H exogenous acids. FeTPP typically displays a first-order dependence on CO₂ in the presence of O–H donors; (20,21,28,57) we presume that this trend holds with each O–H acid surveyed below. Both 1- and 2-naphthol show a first-order dependence on acid (Figures S10 and S11), consistent with previously reported exogenous phenols in this concentration range. (20,21,25,28) Nevertheless, we find that k_cat for each naphthol is ca., three times lower relative to k_cat for a similarly acidic phenol, suggesting that the expanded π system has a moderately detrimental effect on reaction kinetics. By comparison, the k_cat for 2,6-di-tert-butyl phenol lies nearly 2 orders of magnitude below the phenolic Brønsted relationship line, confirming that bulky acids substantially diminish reaction kinetics. Notably, a more complex half-order dependence on acid is measured with this proton donor (Figure S12). The nonaromatic O–H acids HFIP and TFE both show a first-order dependence on acid concentration (Figures S13 and S14). We note that the first-order dependence on TFE contrasts with the second-order dependence reported by Savéant and co-workers; (57) however, in our hands at the concentration range specified above, we consistently observe a first-order dependence with this acid. HFIP enhances k_cat by approximately 5-fold─and TFE roughly 10-fold─relative to the phenolic Brønsted line, affording comparable kinetics to phenols that are ∼4 pK_a units more acidic. HCO₃^– affords a k_cat that lies close to the phenolic Brønsted plot, which is surprising given its anionic charge (Figure S15). Comparing across all O–H acids examined, the aliphatic fluorinated alcohols provide the largest kinetic benefit for CO₂ reduction.

Next, we evaluated rate laws and catalytic rate constants with the series of N–H acids. For the two most acidic aryl acetamides─pentafluoro phenyl acetamide and 3,5-bis(trifluoromethyl)phenyl acetamide─a first-order dependence on acid is observed (Figures S16 and S17). We also measure a first-order dependence on CO₂ with 3,5-bis(trifluoromethyl)phenyl acetamide and assume that this reaction order holds for the other structurally analogous acetamides. The most acidic acetamide displays the same k_cat value as a comparably acidic phenol, while 3,5-bis(trifluoromethyl)phenyl acetamide falls slightly below the phenolic scaling plot. Surprisingly, the two least acidic acetamides─4-(trifluoromethyl)phenyl acetamide and acetanilide─display a half-order dependence on acid (Figures S18 and S19), which is potentially indicative of a more complex mechanism. These less acidic acetamides also display poor kinetics compared to similarly acidic O–H acids. Finally, despite benzanilide having a pK_a of 18.8 (similar to phenol), the rate law shows a zeroth-order dependence on the concentration of this amide (Figure S20). We presume that the rate-limiting step of catalysis in the presence of benzanilide therefore involves disproportion of two CO₂ equivalents rather than a PCET, as detailed by Savéant and co-workers. (58,59) This is surprising, since we have previously shown that structurally comparable amides can significantly enhance the kinetics of CO₂ reduction when installed in the SCS of FeTPP catalysts. (20,21) Nevertheless, our results show that more acidic acetamides (pK_a < 19) have comparable activity to O–H acids of similar pK_a, while less acidic amides exhibit diminished CO₂ reduction kinetics. We postulate that this decrease in activity may be due to amide self-association and the formation of less active (or in the case of benzanilide, completely inactive) acid dimers or trimers, or strong association with the DMF solvent. (60−62) Lastly in this series, imidazole increases catalytic rates 5-fold compared to a similarly acidic phenol and achieves some of the highest rate constants observed in this study that that are on-par with HFIP. A first-order dependence on both acid and CO₂ concentration is observed with imidazole (Figure S21).

Since imidazole is commonly used as a ligand, we sought to investigate whether axial coordination of imidazole to the iron center might contribute to the enhanced rate of CO₂ reduction. Accordingly, we measured catalytic rate constants with 50 mM imidazole and increasing concentrations of N-methyl imidazole (i.e., an aprotic imidazole that may still coordinate to a metal center). Under these conditions, we observe a slight decrease in catalytic activity with increasing concentrations of N-methyl imidazole (Figure S22), suggesting it is unlikely that the favorable activity afforded by imidazole is the result of an axial ligand effect. However, there is precedent for imidazolate (the conjugate base of imidazole) to participate in reactive CO₂ capture to form a carbamate (assigned on the basis of ¹³C NMR spectroscopy and DFT calculations). (63−66) It is possible that this carbamate product could establish a slightly higher equilibrium concentration of CO₂ in solution, contributing to the positive deviation from the Brønsted relationship.

As a final class of additives, we measured kinetics for two exogenous C–H acids with comparable pK_as to the O–H and N–H acids described above. We find that CO₂ reduction rates do not depend on fluorene concentration (Figure S23), suggesting that─as with benzanilide─the rate-limiting step involves CO₂ disproportionation rather than a PCET. (58) By contrast, the more acidic acetylacetone does provide a minor rate enhancement, as the rate law displays a half-order dependence on acetylacetone concentration, along with a ca., first-order dependence on CO₂ concentration (Figure S24). The CO₂ reduction kinetics measured with both fluorene and acetylacetone fall significantly below the linear fit in the phenolic log(k_cat) vs pK_a plot (Figure 2E), confirming that these C–H acids are ineffective proton donors.

Based on the results above, it is possible to establish several trends between CO₂ reduction kinetics and properties of the exogenous acids. Broadly, the data support the standard expectation that k_cat increases as exogenous acid pK_a decreases; however, there are several exceptions that we wish to highlight, particularly in comparison to the popular class of phenol acids. Starting with the trends associated with diminished catalytic activity, it appears that C–H acids perform worse in comparison to both N–H and O–H donors. This may reflect the inability of C–H donors to prealign their proton transfer coordinate via hydrogen bonding interactions, which has been shown to significantly diminish PCET rates. (67−70) It might also reflect the apparent differences in mechanism associated with the two C–H acids evaluated here. Sterically bulky acids, such as 2,6-di-tert-butyl phenol, predictably diminish the kinetics of PCET. A perhaps less obvious trend is that acids with expanded π-systems appear to correlate with slower catalytic rates, and the inhibitory effect is the most significant for the amides surveyed above. We suggest this trend may result from cofacial π–π stacking between the exogenous acid and the porphyrin that sterically encumbers the catalytic active site and leads to slower kinetics. An additional unexpected trend is observed with respect to N–H acid acidity: at higher pK_as (>19), the amides surveyed here afford slow kinetics when compared to many similarly acidic O–H acids and further display an unexpected half order─and in the case of benzanilide a zeroth order─dependence on acid; however, at lower pK_as, N–H and O–H donors generally afford similar kinetics and display the expected first-order dependence on acid.

We further elucidate some preliminary trends that are associated with enhancements in catalytic activity. First, the two nonaromatic fluorinated O–H acids, TFE and HFIP, confer a significant kinetic advantage when compared to similarly acidic donors. Notably, Abraham’s hydrogen bond acidity scale indicates that these fluorinated alcohols are stronger hydrogen bond donors than para-substituted phenols of similar pK_a. (71) Previous studies have shown that favorable hydrogen bonding interactions between the proton donor and acceptor can lower the barrier of PCET reactions, (67−69) which may explain, in part, why this class of acid affords faster rates than expected based on pK_a considerations alone. These results suggest that nonaromatic fluorinated alcohols may represent a privileged class of exogenous acid. The N–H acid imidazole also shows improved kinetics relative to acids of comparable pK_a. While imidazole is more commonly employed as a ligand─often to alter substrate binding during small molecule activation (72,73)─it is rarely employed as an acid, yet the results presented here suggest it has a beneficial effect on rates of PCET. This indicates that azole-type additives may also represent a preferred acid class that warrant future investigation. Collectively, these results demonstrate that the rates of FeTPP catalyzed CO₂ reduction are not necessarily constrained by the pK_a of the proton source and that the structure of the exogenous acid can significantly alter the barrier to rate-limiting PCET.

To better understand how exogenous acid identity influences the rate-limiting transition state, we performed an Eyring analysis and obtained activation parameters with three acids of similar pK_a (∼18) but having different CO₂ reduction kinetics: HFIP, 3,5-bis(trifluoromethyl)phenyl acetamide, and phenol (Figure 3, Table 2). Variable temperature (263–313 K) CV experiments were performed with 50 mM of acid and k_obs was determined at each temperature using FOWA (Figures S25–S27; see the Supporting Information for details). All three acids display considerably negative activation entropies (ΔS^‡) that are consistent with a highly ordered, bimolecular transition state. We observe that the acid with the fastest kinetics (HFIP) has the least negative ΔS^‡ (least ordered transition state), while the two less-active exogenous acids afford much more negative ΔS^‡ values (more ordered transition states). Notably, this trend is the opposite of what could be expected based on hydrogen bond acidity alone, as the donor forming the strongest hydrogen bond (HFIP) ought to display the most ordered transition state, and the weakest donors the least ordered. This may indicate that the DMF solvent─a strong hydrogen bond acceptor (74)─partially disrupts the putative hydrogen bonding interactions between the acid and catalyst-bound CO₂, as previously suggested by Warren and co-workers. (26) It might also reflect a difference in additional noncovalent interactions (i.e., dipole–dipole) with solvent molecules at the polar transition state, which can be substantial in highly polar solvents like DMF. (75) Nevertheless, these results are consistent with our previous report describing the CO₂ reduction activity of FeTPP-based catalysts with amide groups in the SCS, wherein the fastest catalysts displayed the least negative ΔS^‡. (21) The enthalpies of activation (ΔH^‡) for each acid are similar within error. Together, these results suggest that entropic effects largely drive the kinetic differences observed with these three exogenous acids, meaning acids that enforce a less ordered transition state may provide the largest kinetic advantage.

Figure 3. Eyring plots for HFIP, phenol, and 3,5-*bis*(trifluoromethyl)phenyl acetamide. Colors and symbols correspond to the acids shown on the left. Kinetics were obtained from variable temperature CV experiments (T = 313, 298, 273, and 263 K) in dry DMF with 0.5 mM catalyst, 0.1 M TBAPF₆, 50 mM acid, and 1 atm CO₂; scan rate = 0.1 V s^–1. Each data point is the average of two separate replicates.

Table 2. Summary of Activation Parameters for Three Acids of Similar pK_a and Different Activitya

exogenous acid	ΔS^‡ (e.u.)	ΔH^‡ (kcal mol^–1)
HFIP	–18 ± 8	6 ± 2
phenol	–35 ± 3	2 ± 1
3,5-bis(trifluoromethyl) phenyl acetamide	–31 ± 4	4 ± 1

^{^a}

Uncertainties for each activation parameter are reported to two standard deviations.

2.4. Implications for Electronic Scaling Relationships

Rates of FeTPP-catalyzed CO₂ reduction typically follow electronic scaling relationships, wherein a more negative E_1/2(Fe^I/0) correlates with faster catalytic turnover. (20,25,27) The black circles and best-fit line in Figure 4 show such a scaling relationship for three iron porphyrins with different electronics with phenol as the exogenous acid (see CVs and rate constants in Figure S28 and Table S2). Deviations from these linear relationships indicate that additional factors beyond catalyst electronics alter the barrier for CO₂ reduction; modifications to the catalyst or the operating conditions that move toward the golden region in Figure 4 indicate faster turnover can be achieved at less negative potentials.

Figure 4. Electronic scaling plot (black symbols) showing the correlation between log(k_cat) and E_1/2(Fe^I/0) for the Fe complexes of *meso*-*tetra*(4-trifluoromethoxyphenyl) porphyrin, tetraphenyl porphyrin, and *meso*-*tetra*(4-methoxyphenyl) porphyrin (left to right). Colored symbols indicate the rate constants obtained for FeTPP with HFIP (red star) and 3,5-*bis*(trifluoromethyl)phenyl acetamide (blue triangle). log(k_cat) values are averaged over several acid concentrations. The yellow region (top left) indicates region of fast kinetics and mild reduction potentials, while the purple region indicates the opposite (lower right). Kinetic parameters were obtained in dry DMF with 0.5 mM catalyst, 0.1 M TBAPF₆, 50 mM acid, and 1 atm CO₂; scan rate = 0.1 V s^–1

Adding to this plot the two data points for FeTPP with exogenous HFIP (red star) and 3,5-bis(trifluoromethyl)phenyl acetamide (blue triangle), it is clear that changing the exogenous acid results in vertical displacement, or “breaking” of the aforementioned scaling relationship. We emphasize that the pK_a values for these three acids differ only by 0.3 units, meaning that the proton transfer driving force is nearly identical. Notably, the log(k_cat) associated with FeTPP in the presence of HFIP lies well above the electronic scaling plot obtained with phenol. Achieving an equivalent log(k_cat) with phenol would require a ∼100 mV more negative applied potential, meaning that the change in exogenous acid results in substantial energy savings. While the ability to break scaling relationships has been observed with SCS modified catalysts, (20,27,30) these data confirm that simply altering the exogenous acid can provide a similar effect.

3. Conclusions

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Herein, we report the kinetics of FeTPP-catalyzed CO₂ reduction with a series of structurally diverse exogenous acids. We find that the nonaromatic fluorinated alcohols hexafluoroisopropanol and 2,2,2-trifluoroethanol offer significantly faster catalytic rates when compared to acids of similar pK_a. They additionally enhance reaction rates by a factor of ∼5 relative to a comparably acidic phenol─one of the most ubiquitous exogenous acids. Imidazole performs nearly as well as these fluorinated alcohols and affords the fastest kinetics of any N–H acid, regardless of pK_a. For pK_as > 19, most O–H and N–H acids of comparable acidity afford similar kinetics; however, less acidic amides (pK_a > 19) perform significantly worse than comparably acidic O–H donors. C–H acids generally offer slower kinetics than any other class of exogenous acid. Catalytic rate laws were determined for each exogenous acid and reveal significant differences in acid behavior. An Eyring analysis further suggests that more active acids generally enforce a less ordered transition state during the rate-limiting PCET. Altogether, these results show that the activity of FeTPP-catalyzed CO₂ reduction is not inherently tied to the pK_a of the exogenous acid, as is often assumed. This study demonstrates that careful selection of the proton source is a key design consideration for optimizing the activity of FeTPP catalysts. We hope that these efforts will inspire further explorations of exogenous acid speciation and catalyst/acid interactions that either promote or inhibit catalytic performance.

4. Experimental Section

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4.1. General Methods for Electrochemistry

All electrochemistry experiments were conducted using either CHI620E or CHI650E potentiostats from CH instruments. All CV experiments were performed under argon (Ar) or CO₂ in 0.1 M TBAPF₆ electrolyte and anhydrous DMF. TBAPF₆ was crystallized three times from ethanol and dried under vacuum prior to its use. The working electrode used for CV was a 3.0 mm diameter glassy carbon disk (Bioanalytical Systems, Inc.). The counter electrode was a platinum wire. A silver wire in a glass tube sealed with a porous Vycor tip and filled with 0.1 M TBAPF₆ in DMF was used as a pseudoreference electrode. Between every CV scan the electrode was polished with a water-alumina slurry (particle size = 0.05 μM) on a felt pad, rinsed with distilled water, and dried with house air. To ensure no oxygen was present in the working solution, the electrolyte was vigorously sparged with either Ar or CO₂ for ∼5 min prior to every CV scan. At the end of each electrochemistry experiment, potentials were referenced against the ferrocene/ferrocenium (Fc/Fc⁺) couple as an external standard. iR compensation for all experiments was performed using the onboard compensation on the CH instruments potentiostats.

4.2. Controlled Potential Electrolysis Experiments

CPE experiments were conducted in a gastight PEEK electrolysis cell that we have previously reported. (20,21) The CPE cell has working and counter electrode compartments that are separated by an ultrafine glass frit. For each experiment, a 7 mL solution of 0.1 M TBAPF₆/DMF electrolyte containing 0.5 mM catalyst and 50 mM of exogenous acid was prepared and loaded into the working electrode compartment. The counter electrode compartment was filled with a 3 mL solution of 0.1 M tetrabutylammonium acetate (TBAOAc). TBAOAc was used as a sacrificial oxidant to generate CO₂ and ethane by the Kolbe reaction and mitigate the detection of solvent oxidation byproducts. Using mass flow controllers, the working compartment was sparged with a mixture of 95% CO₂ and 5% He (internal standard) for ∼30 min. The cell was then sealed, and a CV was collected to determine the potential needed for the CPE experiment. The solution was then stirred at 1150 rpm with a 1 cm stir bar and the CPE experiment was commenced. At the end of each experiment, the headspace was directly evacuated into the sampling loop of a gas chromatograph (SRI-GC Multiple Gas Analyzer #5) equipped with 6′ Hayesep D and 13X Molecular Sieve columns, as well as a second Hayesep D guard column to trap solvent. An in-line TCD was employed for He and H₂ detection, and an FID outfitted with a methanizer was used for detection of CO. Analytes were quantified by comparing the ratio of analyte/He peak integrals to a calibration curve with known quantities of analyte (Figures S6 and S7).

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.5c05122.

pK_a determination, electrochemical data, gas chromatography, and Eyring analysis (PDF)

ic5c05122_si_001.pdf (3.68 MB)

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Author Information

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Corresponding Author
- Eva M. Nichols - Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada; https://orcid.org/0000-0002-3718-7273; Email: [email protected]
Author
- Kaeden Teindl - Department of Chemistry, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada; https://orcid.org/0009-0005-3367-6845
Author Contributions
K.T. and E.M.N. conceived the project and wrote the manuscript. K.T. performed all experimental work and data analysis.
Notes
The authors declare no competing financial interest.
A previous version of this manuscript has been submitted to ChemRxiv. (76)

Acknowledgments

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This work was supported by the University of British Columbia, the Natural Sciences and Engineering Research Council of Canada (RGPIN-2021-03691 and DGECR-2021-00427), and the Research Corporation for Science Advancement (27752). K.T. acknowledges support from a British Columbia Graduate Fellowship, as well as UBC chemistry for a Head’s departmental scholarship. E.M.N. gratefully acknowledges NSERC for support as a Canada Research Chair and CIFAR for support as an Azrieli Global Scholar.

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Abstract
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Figure 1
Figure 1. Comparison of strategies to improve catalytic activity, where the catalyst is synthetically modified to include a secondary sphere proton relay (left) or the identity of the exogenous acid is altered (right).
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Figure 2
Figure 2. A) Structure of FeTPP and each of the structurally diverse exogenous acids. Values in parentheses indicate pK_a values reported in DMSO. The cyclic voltammograms obtained with each (B) O–H, (C) N–H, and (D) C–H exogenous acid (CV conditions: 0.5 mM catalyst in dry DMF, 0.1 M TBAPF₆, 50 mM acid, and 0.23 M CO₂; scan rate = 0.1 V s^–1). E) The k_cat values of each exogenous acid as a function of pK_a; k_cat values are averaged over at least four different acid concentrations. Black line and symbols correspond to a Brønsted plot constructed from a series of phenols of tunable acidity reported in ref (21). Colors and symbols correspond to the exogenous acids displayed in panel A.
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Figure 3
Figure 3. Eyring plots for HFIP, phenol, and 3,5-bis(trifluoromethyl)phenyl acetamide. Colors and symbols correspond to the acids shown on the left. Kinetics were obtained from variable temperature CV experiments (T = 313, 298, 273, and 263 K) in dry DMF with 0.5 mM catalyst, 0.1 M TBAPF₆, 50 mM acid, and 1 atm CO₂; scan rate = 0.1 V s^–1. Each data point is the average of two separate replicates.
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Figure 4
Figure 4. Electronic scaling plot (black symbols) showing the correlation between log(k_cat) and E_1/2(Fe^I/0) for the Fe complexes of meso-tetra(4-trifluoromethoxyphenyl) porphyrin, tetraphenyl porphyrin, and meso-tetra(4-methoxyphenyl) porphyrin (left to right). Colored symbols indicate the rate constants obtained for FeTPP with HFIP (red star) and 3,5-bis(trifluoromethyl)phenyl acetamide (blue triangle). log(k_cat) values are averaged over several acid concentrations. The yellow region (top left) indicates region of fast kinetics and mild reduction potentials, while the purple region indicates the opposite (lower right). Kinetic parameters were obtained in dry DMF with 0.5 mM catalyst, 0.1 M TBAPF₆, 50 mM acid, and 1 atm CO₂; scan rate = 0.1 V s^–1
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Supporting Information
Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.5c05122.
- pK_a determination, electrochemical data, gas chromatography, and Eyring analysis (PDF)
- ic5c05122_si_001.pdf (3.68 MB)
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The Effect of Exogenous Acid Identity on Iron Tetraphenylporphyrin-Catalyzed CO₂ Reduction
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Inorganic Chemistry

Publication History

Abstract

Subjects

1. Introduction

Figure 1

2. Results and Discussion

2.1. Evaluating the Activity of Structurally Diverse Exogenous Acids by Cyclic Voltammetry

Figure 2

2.2. Controlled Potential Electrolysis to Evaluate Selectivity and Stability for Each Exogenous Acid Family

2.3. Effects of Structurally Diverse Exogenous Acids on CO₂ Reduction Kinetics

Figure 3

2.4. Implications for Electronic Scaling Relationships

Figure 4

3. Conclusions

4. Experimental Section

4.1. General Methods for Electrochemistry

4.2. Controlled Potential Electrolysis Experiments

Supporting Information

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Author Information

Acknowledgments

References

Cited By

Inorganic Chemistry

Publication History

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Citations

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Abstract

Figure 1

Figure 2

Figure 3

Figure 4

References

Supporting Information

Supporting Information

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The Effect of Exogenous Acid Identity on Iron Tetraphenylporphyrin-Catalyzed CO2 ReductionClick to copy article linkArticle link copied!

Inorganic Chemistry

Abstract

1. Introduction

Figure 1

2. Results and Discussion

2.1. Evaluating the Activity of Structurally Diverse Exogenous Acids by Cyclic Voltammetry

Figure 2

2.2. Controlled Potential Electrolysis to Evaluate Selectivity and Stability for Each Exogenous Acid Family

2.3. Effects of Structurally Diverse Exogenous Acids on CO2 Reduction Kinetics

Figure 3

2.4. Implications for Electronic Scaling Relationships

Figure 4

3. Conclusions

4. Experimental Section

4.1. General Methods for Electrochemistry

4.2. Controlled Potential Electrolysis Experiments

Supporting Information

Terms & Conditions

Author Information

Acknowledgments

References

Cited By

Inorganic Chemistry

Article Views

Altmetric

Citations

Recommended Articles

Abstract

Figure 1

Figure 2

Figure 3

Figure 4

References

Supporting Information

Supporting Information

Terms & Conditions

The Effect of Exogenous Acid Identity on Iron Tetraphenylporphyrin-Catalyzed CO₂ Reduction
Click to copy article linkArticle link copied!

2.3. Effects of Structurally Diverse Exogenous Acids on CO₂ Reduction Kinetics