Self-Protection Mechanism and Mass Transport Governing O2 Tolerance in an Iron Porphyrin Homogeneous Catalyst for CO2 Electroreduction

Catherine Harvey, Sylvie Chardon-Noblat, Cyrille Costentin

► To cite this version:

Catherine Harvey, Sylvie Chardon-Noblat, Cyrille Costentin. Self-Protection Mechanism and Mass Transport Governing O2 Tolerance in an Iron Porphyrin Homogeneous Catalyst for CO2 Electroreduction. Journal of the American Chemical Society, 2025, 147 (27), pp.24171-24178. 10.1021/jacs.5c09840 . hal-05379490

HAL Id: hal-05379490

https://hal.science/hal-05379490v1

Submitted on 24 Nov 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Self-Protection Mechanism and Mass Transport Governing O₂ Tolerance in an Iron Porphyrin Homogeneous Catalyst for CO₂ Electroreduction.

Catherine M. Harvey, Sylvie Chardon-Noblat and Cyrille Costentin*

Univ Grenoble Alpes, DCM, CNRS, 38000 Grenoble, France.

## Supporting Information

**Keywords:** CO₂ reduction, O₂-tolerant catalyst, Electroreduction, Mass transport

**ABSTRACT** The electrochemical reduction of CO₂, coupled with renewable energy, offers a promising approach to convert CO₂ into valuable products. However, the presence of O₂ in operating environments presents challenges such as catalyst degradation. Transition metal complexes, like iron tetraphenylporphyrin (TPPFe), are molecular electrocatalysts with tunable structures and redox properties that make them attractive for CO₂ reduction. A critical challenge for practical application is achieving O₂ tolerance—the ability of the catalyst to sustain CO₂ reduction without degradation in the presence of O₂. This study highlights the self-protection mechanism of TPPFe in homogeneous electrocatalysis against O₂ and reactive oxygen species (ROS). Using rotating disk voltammetry, constant potential electrolysis, and spectro-electrochemistry, we demonstrate that lesser reduced TPPFe states selectively reduce O₂, form a protective layer that shields the active catalyst for CO₂ reduction. Furthermore, we reveal that mass transport significantly influences the efficiency of CO₂-to-CO conversion in O₂-containing environments, with stirring rates during electrolysis directly affecting CO faradaic yields. This self-protection mechanism, applicable to other catalysts with multiple redox states and adaptable to molecular catalysts immobilized in thick films, underscores the importance of optimizing mass transport conditions and catalyst design to achieve O₂-tolerant CO₂ reduction.

## Introduction

Innovative strategies to transform CO₂ into valuable products such as fuels and chemicals are currently sought-after. Among the proposed solutions, the electrochemical reduction of CO₂ stands out as a possible technology due to its potential to achieve sustainable, renewable energy cycles when coupled with green electricity sources.¹ However, among the various issues, a significant obstacle in the practical implementation of CO₂ electroreduction is the presence of dioxygen (O₂) in the operating environment, leading to challenges such as competitive reactions and catalyst degradation.² Molecular electrocatalysts, which employ well-defined active sites within organic or organometallic frameworks, offer specific advantages for CO₂ reduction. Their tunable structure and redox properties allow precise control over activity, selectivity, and mechanistic pathways.³ However, many conventional molecular catalysts are highly susceptible to oxidative degradation in the presence of O₂. This sensitivity arises because O₂ itself can deactivate the active form of the catalyst or because O₂ being easier to reduce than CO₂, the generated reduced oxygen species (ROS) can degrade the catalyst.⁴ Therefore, designing O₂-tolerant molecular electrocatalysts represents a critical area of research, as such catalysts can ensure long-term performance under realistic operating conditions. Importantly, we define O₂-tolerance of a molecular catalyst as its ability to durably reduce CO₂ without degradation in the presence of O₂. Consideration of efficiency figures of merit such as faradaic efficiency is obviously of interest and will be used to ascertain the effects of mass transport on the overall CO production though will not be used as a criterion that defines an O₂-tolerant catalyst. In other words, it may be acquiesced to for the reduction of the

O₂ present provided that the CO₂ reduction catalyst is still operating.

To achieve O₂ tolerance, various strategies have been proposed and investigated.⁵ This includes defensive strategies which rely on the use of O₂ shields/scavengers to remove O₂ and protect the catalyst from exposure to O₂ or ROS.⁶,⁷,⁸ Alternatively, offensive strategies involve either incorporating a co-catalyst to scavenge ROS before they can damage the CO₂ reducing catalyst⁹,¹⁰ or designing a catalyst capable of sustainably reducing both O₂ and CO₂ without degradation as developed for O₂-tolerant acid reducing catalysts.¹¹ Notably, a more challenging offensive strategy would be to develop a catalyst that can selectively reduce CO₂ at a potential positive enough so that O₂ is not directly reduce by the conductive support.

Iron porphyrins are well-studied molecular electrocatalysts for both O₂¹² and CO₂¹³ reduction and they are thus considered potential O₂-tolerant catalysts that can achieve this characteristic by reducing the O₂ and while also reducing CO₂ without degradation. However, because the O₂ reduction and CO₂ reduction are carried out at different reduced states of the iron porphyrin, an important question arises: what may be the actual mechanism behind the O₂-tolerance of iron porphyrins? Is it because the more reduced state, which is active toward CO₂ reduction, is inherently O₂-tolerant as currently proposed?⁴ Or is there an alternative mechanism governing their O₂ tolerance?

In this study, we investigate the mechanism underlying O₂-tolerance of the tetraphenylporphyrin (TPPFe), a homogeneous molecular electrocatalyst for CO₂-to-CO conversion. We demonstrate that a self-protection mechanism is responsible for imbuing the catalyst with an O₂-tolerance and that mass
transport plays a critical role in controlling the faradaic yield for CO production. This mechanism is not unique to TPPFe but is broadly applicable to any homogeneous molecular catalyst capable of reducing both O₂ and CO₂ at different redox states – a common characteristic of transition metal complexes. To provide context for the experimental results with TPPFe, we first outline the fundamental principle of self-protection in homogeneous electrocatalysis against contaminants.

## Results and Discussion

### Principle of Self-Protection in Homogeneous Electrocatalysis against Contaminants.

To illustrate the principle of self-protection of a homogeneous electrocatalyst toward a contaminant, we consider the simple reaction scheme depicted in Scheme 1. A is the molecule of interest whose reduction is catalyzed by a homogeneous molecular catalyst via the redox couple Q/R with a characteristic standard potential $E_2^0$. This molecular catalyst has another redox state P corresponding to the redox couple P/Q with a standard potential $E_1^0$ more positive than $E_2^0$. Hence, as indicated in Scheme 1, the homogeneous comproportionation between R and P is thermodynamically favorable and can thus be assumed to be diffusion controlled. In the presence of a contaminant, C, the active form of the catalyst toward A reduction, i.e., R, may be irreversibly deactivated to form I. However, C can be efficiently reduced by Q to create D. We will show that the existence of the lesser reduced Q form of the catalyst can set a self-protection mechanism for A conversion to B catalyzed by R, in the framework of a constant potential electrolysis (CPE) at $E \ll E_2^0$ for a bulk solution where P has been introduced as the homogeneous catalyst ($C_{cat}^0$), A introduced in large excess ($C_A^0/C_{cat}^0 \gg 1$) and C being a contaminant which concentration ($C_C^0$) is small compared to the A concentration but is maintained constant in the bulk of the solution. The hydrodynamic regime of the CPE is determined by the stirring of the solution, which induces a planar, linear diffusion process. This process can be effectively analyzed using the Nernst layer approximation.¹⁴ The diffusion layer, denoted as $\delta$, has a fixed thickness governed by the diffusion coefficient (assumed to be the same for all species), the kinematic viscosity, and the stirring rate of the solution. Within this conceptual framework, the behavior of the CPE can be understood by drawing an analogy to rotating disk voltammetry (RDV) at a planar electrode, where the steady-state hydrodynamic regime is controlled by electrode rotation rather than solution stirring.¹⁵ To emphasize the principle of self-protection, we introduce the following additional simplifications and assumptions. Both catalytic processes (reduction of A to B and reduction of C to D) are under pure kinetics conditions and the sizes of the correspond diffusion-reaction layers are much smaller than the size of the diffusion layer $\delta$.¹⁶

**Scheme 1.** Simple reaction mechanism of molecular catalysis (by Q/R redox couple) of the electrochemical reduction of a substrate (A) in the presence of a contaminant C, which can inhibit R state, but which is reduced by a lower redox state of the catalyst (P/Q redox couple).

<figure><img src="image_2.png" /></figure>

P, Q, R = different redox states of the catalyst

A = substrate of interest

C = contaminant

B, D = products of A and C reduction

I = deactivated catalyst

Furthermore, in the case of C, the catalytic process operates under a total catalysis regime.¹⁷ This indicates that C is completely consumed within the corresponding diffusion-reaction layer, whose thickness is assumed to be sufficiently small to be negligible compared to $\delta$. The substrate A is in large excess so that its concentration is constant throughout the diffusion layer and equal to its bulk value $C_A^0$. All interfacial electron transfers are fast and at equilibrium. All the species are assumed to have the same diffusion coefficient D. Within this framework, a step-by-step approach will help uncover the concept of self-protection and the impact of mass transport on a catalyst's tolerance to the contaminant C. In the absence of contaminant, the rotating disk voltammogram would exhibit two waves (Figure 1a), a first monoelectronic wave corresponding to P/Q couple centered at $E_1^0$ with a plateau current equal to $i_{pl,cat} = FSDC_{cat}^0/\delta$ and a second catalytic wave in which the plateau current is $i_{pl,A} = FSDC_{cat}^0/\sqrt{Dk_A C_A^0}$ (see Supporting Information (SI) for details). A representation of the concentration profiles at the catalytic plateau is shown in Figure 1b, clearly illustrating that the catalytic process occurs in a thin diffusion reaction layer close to the electrode surface; its width is roughly two times $\mu_A = \sqrt{D/k_A C_A^0}$. The contaminant C would have to penetrate this diffusion-reaction layer to deactivate the catalyst R. We now consider the opposite situation: absence of the substrate A but presence of the contaminant C. The total catalysis regime for the reduction of C by Q leads to a catalytic wave in RDV located at

$$E_{1/2,C} = E_1^0 + \frac{RT}{F} \ln \left( \frac{C_{cat}^0}{C_C^0} \delta \sqrt{\frac{2k_C C_C^0}{D}} \right) \quad \text{and current plateau of}$$

$i_{pl,C} = FSDC_C^0/\delta$ (Figure 1c and SI for details). It is then followed by the one electron reduction wave of the catalyst (P/Q couple) located at $E_1^0$, as expected from the total catalysis behavior and finally, followed by the one electron wave corresponding to the second reduction of the catalyst (Q/R couple, located at $E_2^0$). At a potential more negative than $E_2^0$, the concentration profiles reveal that the diffusion layer can be divided in three zones: a first layer adjacent to the electrode
which size is $\mu_1 = \delta C_{\text{cat}}^0 / (2C_{\text{cat}}^0 + C_C^0)$ (see SI for details)
where R and Q diffuse and the concentration of C is nil; then
a second layer of identical size where Q and P diffuse and still
free from C (Figure 1d). At the plane corresponding to
$\mu_2 = 2\mu_1$, C is indeed efficiently reduced by Q. This reduction
actually takes place in a very thin layer which size $\mu_C$ can
actually be considered as negligible (see SI for details). Then,
in the third layer, from $\mu_2$ to $\delta$, C and D simply diffuse
whereas P is the only state of the catalyst present. The key re-
sult is that R, the catalytically active species for A reduction,
is only present in the first layer and is therefore protected from
C. This sets the principle of self-protection because the protec-
tion is enabled by the efficient reduction of C by Q at the $\mu_2$
boundary. It is further confirmed considering now the full
mechanism of Scheme 1, i.e., presence of both the substrate A
and the contaminant C. The rotating disk voltammogram (Fig-
ure 1e) shows three successive waves. The first two waves are
identical to the ones described above in the case of C only pre-
sent. The third wave is now a catalytic wave corresponding to
A reduction into B catalyzed by R. Its intensity is equal to
$i_{\text{pl},A} = \text{FSDC}_{\text{cat}}^0 \sqrt{Dk_A C_A^0}$. As seen in Figure 1f showing the
concentration profiles at a potential corresponding to the cata-
lytic plateau ($E \ll E_2^0$), the catalysis of A to B conversion is
squeezed in the diffusion-reaction layer adjacent to the elec-
trode which size is approximate by $\mu_A = \sqrt{D/k_A C_A^0}$. This
layer is depleted from contaminant C because C is reduced cat-
alytically by Q at the outer boundary of the second layer (at
position $\mu_3$ in Figure 1f) therefore preventing any interference
of the contaminant into the catalysis. Hence, the catalyst is tol-
erant to the contaminant C by virtue of a self-protection mech-
anism. Additionally, we note that the layer of size

$\mu_3 - \mu_A = (\delta - \mu_A) \frac{C_{\text{cat}}^0}{C_{\text{cat}}^0 + C_C^0}$, in which P and Q diffuse, is also

depleted in C and acts as a protection layer for the active form
of the catalyst R.

Although the homogeneous catalyst has a self-protection
mechanism, it comes at the cost of reduced faradaic yield, as
electrons are consumed to reduce contaminant C. As seen in
Figure 1f showing the concentration profiles at a potential
$E \ll E_2^0$ in the presence of both the substrate A and the con-
taminant C, the steady-state flux of electrons at the electrode
surface (i) is converted at the outer boundary of the diffusion
layer (at $x = \delta$) into steady-state fluxes of B and D which then
accumulate in the bulk solution. As shown in the SI, this leads
to a faradaic yield in product B, $FY(B)$, expressed as:

$$FY(B) = 100 \times \frac{n_B}{Q/F} \approx 100 \times \left( 1 - \frac{\sqrt{D / k_A C_A^0}}{\delta} \left( 1 + \frac{C_C^0}{C_{\text{cat}}^0} \right) \right) \quad (1)$$

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Simulated RDV voltammograms for the mechanism in Scheme 1 and of the concentration profiles at $E \ll E_2^0$. Simulation parameters: $E_1^0 = 0.1$ V, $E_2^0 = -0.2$ V, $C_{\text{cat}}^0 = 1$ mM, $k_A = 10^3$ M$^{-1}$s$^{-1}$, $k_C = 10^{10}$ M$^{-1}$s$^{-1}$, $k_{\text{dif}} = 10^{10}$ M$^{-1}$s$^{-1}$, $\mu_1 = 10^7$ M$^{-1}$s$^{-1}$, $T = 298$ K, $\delta = 10^{-3}$ cm, $D = 10^{-5}$ cm$^2$s$^{-1}$ (a) $C_A^0 = 5$ M, $C_C^0 = 0$ mM (b) P (blue), Q (black), R (red), B (dark blue), $\mu_A = \sqrt{D / k_A C_A^0}$ (c) $C_A^0 = 0$ mM, $C_C^0 = 5$ mM (d) P (blue), Q (black), R (red), C (orange), D (green), $\mu_1 = \delta C_{\text{cat}}^0 / (2C_{\text{cat}}^0 + C_C^0)$, $\mu_2 = 2\mu_1$ (e) $C_A^0 = 5$ M, $C_C^0 = 5$ mM (f) P (blue), Q (black), R (red), C (orange), D (green), B (dark blue), $\mu_3 = \frac{\delta C_{\text{cat}}^0 - \mu_A C_C^0}{C_{\text{cat}}^0 + C_C^0}$.

In all cases the concentration of I remains equal to zero.</figcaption></figure>

As $C_A^0$ decreases and/or $C_C^0$ increases, $FY(B)$ is expected to
decrease. Importantly, $FY(B)$ is dependent on the mass
transport regime through the value of $\delta$ and therefore it is not
an intrinsic characteristic of the catalyst (see below, Fig. 4).
Two limiting cases can be delineated. At low stirring rate,
$FY(B)$ can approach 100% whereas, at fast stirring rate, $FY(B)$ can be severely decreased.

**CO₂ Electroreduction Catalyzed by Iron Tetraphenyl Porphyrin in the presence of O₂, an Illustration of a Self-protection Mechanism.**

We have shown that a homogeneous molecular catalyst can protect itself from a contaminant provided that the contaminant is efficiently reduced by a lesser reduced state of the catalyst (less reduced than its active form toward the desired substrate). This self-protection mechanism occurs at the expense of a decrease of the faradaic yield which is critically dependent on mass transport of the contaminant. While the mechanism used to establish these principles is oversimplified for a quantitative representation of an experimental system, it is remains valuable for rationalizing the behavior of CO₂ electro-reduction catalyzed by the iron tetraphenylporphyrin in the presence of the O₂ contaminant. As already mentioned, the iron tetraphenylporphyrin (either noted TPPFe or TPPFe(Cl) throughout the paper) is well-known to catalyzed the reduction reaction of O₂ (ORR)¹² and the reduction of CO₂ (CO2RR).¹³ In the first case, the catalytic mechanism of ORR has been studied in details and it was shown to process at the level of the TPPFe<sup>III</sup>/TPPFe<sup>II</sup> wave in the presence of a strong enough acid (e.g. protonated DMF ($pK_a=0$), p-toluenesulfonic acid ($pK_a = 2.2$ in DMF) or trifluoro acetic acid ($pK_a=6.1$ in DMF)).¹⁸,¹⁹,²⁰ Phenol is a much weaker acid ($pK_a=18.8$ in DMF) so that, after binding of O₂ to TPPFe<sup>II</sup>, the adduct is presumably further reduced to trigger catalysis as sketched in Scheme 2. A four electron-reduction of O₂ to H₂O is considered in Scheme 2 based on the observations made from rotating disk voltammetry described below. The direct reduction of O₂ on the RDV glassy carbon (GC) electrode gives a one-electron wave in the absence of acid (dashed blue line in Figure 2a). With phenol as a proton donor, the RDV wave for O₂ direct reduction becomes bi-electronic indicating that H₂O₂ is the major product (blue line in Figure 2a). Considering now TPPFe(Cl), as expected, it exhibits three successive mono-electronic diffusion waves in RDV (black dashed line in Figure 2a and c and see also Figure S1).²¹ With O₂ and PhOH (black line in Figure 2a), the reduction of O₂ in the presence of TPPFe as a catalyst occurs at less negative potential than its direct reduction, indicating a catalytic process due to TPPFe. However, as opposed to the behavior with strong enough acids,¹⁸ the catalytic wave is more negative than the TPPFe<sup>III</sup>/TPPFe<sup>II</sup> standard potential, indicating a process where the adduct TPPFe<sup>II</sup>(O₂) needs to be reduced (Scheme 2) to trigger catalysis. Additionally, the catalytic current is ca. twice the current obtained in the absence of catalyst thus justifying the consideration of a four electrons reduction of O₂ to H₂O. We note that in our conditions, using rotating disk voltammetry to quantify the selectivity of O₂ reduction would not be reliable because of the generation of phenolate which oxidation would passivate the ring electrode and because of the sluggish oxidation of H₂O₂ in organic solvents.²² Finally, as shown in Figure 2b, the RDV waves are almost proportional to the square root of the rotation rate, $\omega$, indicating that, in the present conditions (saturation gas composition of 95% CO₂ / 5% O₂, i.e., 0.155 mM of O₂ in the bulk solution²³), the reduction of O₂ catalyzed by TPPFe<sup>I</sup> in the presence of PhOH is

under total catalysis, i.e., controlled by O₂ diffusion. Importantly, the reduction of O₂ is catalyzed by a less reduced state of TPPFe than the redox state required to reduce CO₂, as the reduction of CO₂ by TPPFe is well-known to be triggered by the formation of the formal TPPFe⁰ redox state at -2.05 V vs. Ag⁺/Ag. Therefore, TPPFe should be an O₂-tolerant catalyst for CO₂ reduction due to a self-protection mechanism according to the principle described in the previous section.

**Scheme 2.** Schematic mechanism for the reduction of O₂ to H₂O and CO₂ to CO catalyzed by TPPFe in the presence of PhOH as a proton donor.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** RDV voltammograms in DMF + 0.1 M n-Bu₄NPF₆ at 10 mV/s on a rotating 3 mm diameter GC disk electrode. (a) at $\omega = 400$ rpm: under 95% Ar / 5% O₂ atmosphere (dashed blue); under 95% Ar / 5% O₂ atmosphere + 0.5 M PhOH (blue); TPPFeCl (1 mM) under Ar (dashed black); TPPFeCl (1 mM) + 0.5 M PhOH under 95% Ar / 5% O₂ atmosphere (full black) (b) TPPFeCl (1 mM) with 0.5 M PhOH under 95% Ar / 5% O₂ atmosphere at $\omega = 400$ (black), 900 (red), 1600 (green), 2500 (blue) rpm (c) TPPFeCl (1 mM) at $\omega = 400$ rpm: under Ar (black dashed line), with 0.5 M PhOH under 95% CO₂ / 5% Ar atmosphere (black full line), with 0.5 M PhOH</figcaption></figure>
under 95% CO₂ / 5% O₂ atmosphere (red full line).

In the presence of CO₂ and no O₂ (saturation gas composition 95% CO₂ and 5% Ar), a catalytic wave develops at the level of TPPFe⁰/TPPFe¹ wave via a well-established mechanism (Figure 2c black line). Replacing the 5% Ar by 5% O₂, the RDV features (Figure 2c red line) are indeed aligned with the self-protection mechanism previously described and illustrated in Figure 1. First, a mono-electronic wave corresponding to the TPPFe¹¹/TPPFe¹¹ redox couple is observed, followed by a diffusion-controlled total catalytic wave for O₂ reduction. This is succeeded by the TPPFe¹¹/TPPFe¹¹ wave (-1.5 V vs. Ag⁺/Ag), indicating the depletion of O₂. As the potential approaches the TPPFe⁰/TPPFe¹ standard potential (-2.05 V vs. Ag⁺/Ag), a second catalytic wave emerges attributed to CO₂ reduction. As described in the previous section, the maximum current of this wave corresponds the catalytic current in the absence of O₂ owing to the self-protection mechanism. However, this flux of electrons at the electrode surface (i.e. the current) is converted into a flux of both CO₂ and O₂ reduction products at the outer boundary of the diffusion layer and therefore a decrease of the faradaic yield for CO is expected in the presence of O₂. It is confirmed by the results of constant potential electrolyses conducted at -2.2 V vs. Ag⁺/Ag on a GC plate in the presence of 1 mM TPPFe as a catalyst, 0.1 M PhOH as a proton donor and various amount of CO₂ and O₂. Table 1 summarizes the CO faradaic efficiency *FY*(CO) obtained for variable percentage of CO₂ and O₂ for constant potential electrolyses (ca. 20 C under stirring of a 12 mL catalytic solution at 350 rpm, see SI for experimental conditions details). Importantly, as described in SI, the hydrodynamic regime during electrolysis is controlled and it is equivalent to a steady-state planar diffusion regime with a diffusion layer size (Figure S10) being controlled by the stirring rate *ω* of the bulk solution (Figure S11). In all cases, quantification of O₂ in the gas phase taking carefully into account contamination from air, allows to show that the amount of O₂ consumed corresponds to the consumption of 4 electrons per O₂ (see SI for details), in agreement with RDV experiments with a total faradaic yield thus being approximately 100%. This indicates that all transferred electrons are utilized either for CO₂ conversion to CO or O₂ conversion to H₂O. We note that a quantification of water would not be reliable because (i) the residual amount of water in the electrolyte after saturation with CO₂ has been determined to be 13 mM, even after taking maximum precautions and drying the electrolyte and gas, while the produced amount of water would be only 1 mM and (ii) water can react with CO₂ (see SI for details). The current remains stable throughout electrolyses, demonstrating that the catalyst is stable even when *FY*(CO) is small, i.e. when O₂ reduction dominates and generates highly reactive intermediate species that could potentially damage TPPFe⁰ (Figure S8). Additionally, CV recorded before and after electrolysis indicate no loss of integrity of the catalyst (Figure S8f). This strongly support the hypothesis that, as proposed by the self-protection mechanism, O₂ reduction and CO₂ reduction occur in spatially distinct regions.

<table><caption>**Table 1.** CPE at -2.2 V vs. Ag⁺/Ag at a 350 rpm stirring rate.ª</caption><thead><tr><td>Saturating gas mixture</td><td>Total charge</td><td>Current</td><td>FY(CO) <sup>c</sup></td></tr></thead></table>

<table><thead><tr><td>(v/v)<br>CO₂/O₂/Ar</td><td>(C)</td><td>(mA) <sup>b</sup></td><td></td></tr></thead><tbody><tr><td>100/0/0</td><td>22.6</td><td>2.5</td><td>100</td></tr><tr><td>50/0/50</td><td>19.3</td><td>2.0</td><td>95</td></tr><tr><td>95/5/0</td><td>23.4</td><td>3.5</td><td>75</td></tr><tr><td>90/10/0</td><td>23.5</td><td>2.5</td><td>34</td></tr><tr><td>75/25/0</td><td>29.7</td><td>5.5</td><td>2.5</td></tr><tr><td>50/50/0</td><td>22.7</td><td>8.0</td><td>&lt; 1</td></tr></tbody></table>

ª 1 mM TPPFe + 0.1 M PhOH in DMF + 0.1 M *n*-Bu₄NPF₆, on a GC plate working electrode, volume of the cathodic solution: 12 mL. ᵇ The surface area of the GC plate working electrode is not always the same in these experiments. ᶜ Faradaic yield for CO after passage of ca. 20 C. See SI for details on the calculation and experiments (Figures S2-S9).

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Faradaic yield for CO vs. % O₂ (in CO₂/O₂ mixture) for CPE in the presence of 1 mM TPPFe, 0.1 M PhOH in DMF + 0.1 M *n*-Bu₄NPF₆ at -2.2 V vs. Ag⁺/Ag, on a GC plate working electrode at 350 rpm (diffusion layer size: ca. 60 µm).</figcaption></figure>

A significant drop in *FY*(CO) is observed as the O₂ percentage is increased in the saturating gas composition, with CO₂ reduction being nearly completely suppressed at 25% O₂ (Figure 3). This behavior qualitatively aligns with the principle of the self-protection mechanism. However, equation (1) is not directly applicable in this case both because all the assumptions of the simple model (pure kinetics conditions, total catalysis for O₂ reduction) may not be fully valid and because stoichiometric factors would have to be considered.
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Faradaic yield for CO vs. the charge passed at different hydrodynamic regimes for electrolysis in the presence of 1 mM TPPFe, 0.1 M PhOH in DMF + 0.1 M *n*-Bu₄NPF₆ under 95% CO₂ / 5% O₂ atmosphere at -2.2 V vs. Ag⁺/Ag, on a GC plate working electrode. Open circle: 350 rpm (diffusion layer size: ca. 60 µm) and Full circle: 1000 rpm (diffusion layer size: ca. 35 µm).</figcaption></figure>

To provide further evidence that the proposed self-protection mechanism explains the O₂ tolerance of TPPFe, CPE experiments were conducted at -2.2 V vs. Ag⁺/Ag on a GC plate at a different hydrodynamic regime. The setup included 1 mM TPPFe as a catalyst, 0.1 M PhOH as a proton donor, a saturation gas mixture of 95% CO₂ / 5% O₂ and an increased stirring rate of the bulk solution to alter the O₂ diffusion rate. Under these conditions, O₂ reduction operates under total catalysis (vide supra). A substantial decrease in CO faradaic yield was observed, dropping from 75% at 350 rpm (diffusion layer size: ca. 60 µm) to 20% at 1000 rpm (diffusion layer size: ca. 35 µm) (Figure 4). Notably, in both cases, the current is stable over time (see SI for details), indicating no catalyst degradation and confirming TPPFe's O₂ tolerance. Additionally, the total faradaic efficiency was 100%, considering a four-electron reduction of O₂ (see SI for details).

As predicted by the self-protection mechanism, the resting state of the TPPFe catalyst in the bulk of the solution differs depending on the presence of O₂. For CO₂-to-CO conversion in the absence of O₂, the resting state is TPPFe¹.¹⁶ However, in the presence of O₂, the resting state is expected to be a less reduced form. UV-visible spectroscopy of the bulk of the solution during a CPE at -2.2 V vs. Ag⁺/Ag was conducted in the presence of 0.5 mM TPPFe as catalyst, 0.1 M PhOH as proton donor, and under saturation gas mixture of 95% CO₂ / 5% O₂. The analysis revealed that the initial TPPFe<sup>III</sup>(Cl) is converted into TPPFe<sup>III</sup>(OPh) (PhO⁻ = phenolate anion) after the passage of approximately one electron per Fe<sup>III</sup> center (1.5 C, or 23 min of electrolysis, Figures 5a, 5c, 5d). The UV-vis Q-bands characteristic of TPPFe<sup>III</sup>(Cl) (509, 570, 690 nm) are indeed converted into the characteristic Q-bands of TPPFe<sup>III</sup>(OPh) (555, 610, 647 nm) (Figure 5a and Figure S13).²⁴ TPPFe<sup>III</sup>(OPh) remains the resting state during electrolysis. (Figures 5b and c, see SI for more details). This observation confirms that the resting state for CO₂-to-CO reduction close to the electrode surface (i.e., TPPFe¹) is converted into a TPPFe<sup>III</sup> species in the “protecting layer” due to the catalytic activity of TPPFe¹ and

TPPFe<sup>II</sup> in reducing O₂. Since both O₂-to-H₂O and CO₂-to-CO reduction with PhOH as a proton donor produces phenolate as a co-product, which accumulates in the solution, the resting state in the bulk of the solution is TPPFe<sup>III</sup>(OPh). It is worth noting that phenolate, stabilizes TPPFe<sup>III</sup>(OPh) and makes it more difficult to reduce than TPPFe(Cl). As a result, the first reduction of TPPFe<sup>III</sup>(OPh) occurs at a potential where the O₂ reduction process of begins. This explains why the resting state is not TPPFe<sup>II</sup> or TPPFe<sup>II</sup>(O₂), as might otherwise be anticipated from RDV experiments conducted in the absence of phenolate (Figure 2 and Scheme 2).

When CPE is performed under a pure atmosphere of O₂ (no CO₂), with TPPFe as catalyst and PhOH as proton donor, at -1.25 V vs. Ag⁺/Ag, the initial resting state in the bulk solution is TPPFe<sup>III</sup>(OPh). Over time, as more O₂ is reduced and hydroxide is produced, TPPFe<sup>III</sup>(OPh) is converted to TPPFe<sup>III</sup>(OH) (Figure S12; see SI for details).²⁵ This conversion likely results from hydroxide generation via water and phenolate production (Scheme 3), as hydroxide might be a more coordinating ligand for TPPFe<sup>III</sup> than phenolate. In the presence of CO₂ (Figure 5), hydroxide is similarly produced due to O₂ reduction as part of the self-protective mechanism. However, OH⁻ presumably reacts with CO₂ to form bicarbonate, preventing it from phenolate substitution as ligand for the TPPFe<sup>III</sup> resting state (Scheme 3). Overall, these spectroelectrochemical observations align well with the proposed self-protection mechanism.

<figure><img src="image_7.png" /><figcaption>**Figure 5.** CPE of 0.5 mM TPPFeCl + 0.1 M PhOH in DMF + 0.1 M *n*-Bu₄NPF₆ under 95% CO₂ / 5% O₂ atmosphere at -2.2 V vs. Ag⁺/Ag, on a GC plate working electrode, stirring rate: 350 rpm, volume of the solution: 31 mL. (a) Time evolution of UV-vis spectra (*l* = 1mm, 1 scan / 2 min), from *t* = 0 (initial spectrum in blue) to 23 min (spectrum in red). (b) Time evolution of UV-vis spectra (*l* = 1mm, 1 scan / 30 s), from *t* = 23 to 113 min (spectra in red); the blue spectrum corresponds to the</figcaption></figure>
initial spectrum (t = 0). (c) Time evolution of the absorbance at 610 nm. (d) Time evolution of the charge passed.

**Scheme 3.** CO₂ and O₂ electrochemical reduction reactions catalyzed by TPPFe(Cl) in the presence of PhOH as a proton donor.

$$ \begin{array}{l} \text{In the diffusion-reaction layers} \\ \\ \mathrm{O_2 + 4e + 4PhOH} \xrightarrow{\mathrm{TPPFe(Cl)}} \mathrm{2H_2O + 4PhO^{-}} \\ \\ \mathrm{CO_2 + 2e + 2PhOH} \xrightarrow{\mathrm{TPPFe(Cl)}} \mathrm{CO + 2PhO^{-}} \end{array} $$

$$ \begin{array}{l} \text{In the bulk of the solution} \\ \\ \mathrm{H_2O + PhO^{-}} \rightleftharpoons \mathrm{OH^{-} + PhOH} \\ \\ \mathrm{CO_2 + OH^{-}} \rightleftharpoons \mathrm{HCO_3^{-}} \\ \\ \mathrm{TPPFe^{III}(Cl) + PhO^{-}} \rightleftharpoons \mathrm{TPPFe^{III}(OPh) + Cl^{-}} \\ \\ \mathrm{TPPFe^{III}(OPh) + OH^{-}} \rightleftharpoons \mathrm{TPPFe^{III}(OH) + PhO^{-}} \end{array} $$

Finally, we emphasize that although this study focuses on homogeneous catalysts, the findings could be extended to immobilized molecular catalysts in a thick film (tens to hundreds of micrometers). In such films, electron transport occurs via hopping, being equivalent to a diffusion process, as the diffusion of the catalyst in solution.²⁶ As a result, diffusion-reaction layers would develop in a similar manner to the present case, and a comparable self-protection mechanism would operate. Specifically, the O₂ reduction by TPPFe would remain spatially separated from CO₂, preserving the catalyst's functionality under these conditions.

## Conclusion

We have shown that homogeneous molecular catalysts can protect themselves from contaminants, provided the contaminant is efficiently reduced by a less reduced state of the catalyst (less reduced than its active form for reduction of the targeted substrate). This self-protection mechanism arises from the spatial separation between the layer where the contaminant is reduced and the layer where the active form of the catalyst is generated. As a result, the active form is produced in a contaminant-free environment. However, this protective mechanism comes at the cost of a reduction in faradaic yield for the reaction of interest. Importantly, this decrease in faradaic yield

can be mitigated by controlling (i.e., slowing down by lower stirring rate) the mass transport rate. These findings highlight that, in the presence of a contaminant, the faradaic efficiency is not an intrinsic property of the catalyst but instead depends on the operating conditions. These physical principles provide a foundation for understanding the O₂-tolerance of iron tetraphenyl porphyrin, a homogeneous molecular catalyst for CO₂ reduction to CO. This has been experimentally demonstrated using a combination of rotating disk voltammetry, constant-potential electrolysis under controlled hydrodynamic conditions, and spectro-electrochemistry. Notably, the physical principles established in the context of homogeneous molecular catalysis can be applied to heterogenized molecular catalysts in thick films, highlighting their potential as viable catalysts in electrolyzers fed with a substrate containing contaminants such as CO₂ with O₂ as a contaminant.

## ASSOCIATED CONTENT

### Supporting Information

Methods and materials. Determination of the faradaic yield for constant potential electrolysis. Additional data. Kinetic analysis of the self-protection mechanism. Digital simulations.

## ACKNOWLEDGMENT

This work was supported by the Labex ARCANE (CBH-EUR-GS, ANR-17-EURE-0003) and the French National Research Agency in the framework of the "Investissements d'avenir" program, allocated to the CDP DefiCO₂ (ANR-15-IDEX-02) for CMH post-doctoral fellowship. The authors thank Florian Molton for his technical assistance in gas chromatography analysis. Laure Lavernot (UGA, CNRS, Grenoble INP - LEPMI - UMR5279) is thanked for her technical assistance for K-F titration.

## AUTHOR INFORMATION

### Corresponding Author

cyrille costentin@univ-grenoble-alpes.fr

### Notes

The author declares no competing financial interest.

## REFERENCES

1. O'Brien, C. P.; Miao, R. K.; Zeraati, A. S.; Lee, G.; Sargent, E. H.; Sinton, D. CO₂ Electrolyzers. *Chem. Rev.* **2024**, *124*, 3648-3693.

2. Lin, Z.; Han, C.; O'Connell, G. E. P.; Lu, X. Recent Progress on electrode Design for Efficient Electrochemical Valorisation of CO₂, O₂, and N₂. *Angew. Chem. Int. Ed.* **2013**, *38*, e202301435.

3. Costentin, C.; Robert, M.; Savéant, J-M. Catalysis of the electrochemical reduction of carbon dioxide. *Chem. Soc. Rev.* **2013**, *42*, 2423-2436.

4. Mondal, B.; Sen, P.; Rana, A.; Saha, D.; Das, P.; Dey, A. Reduction of CO₂ to CO by an Iron Porphyrin Catalyst in the Presence of Oxygen. *ACS Catal.* **2019**, *9*, 3895-3899.

5. Wakerley, D. W.; Reisner, E. Oxygen-tolerant proton reduction catalysis: much O₂ about nothing? *Energy Environ. Sci.* **2015**, *8*, 2283-2295.

6. Lu, X.; Jiang, Z.; Yuan, X.; Wu, Y.; Malpass-Evans, R.; Zhong, Y.; Liang, Y.; McKeown, N. B.; Wang, H. A bio-inspired O₂-tolerant catalytic CO₂ reduction electrode. *Sci. Bull.* **2019**, *64*, 1890-1895.

7. Li, P.; Lu, X.; Wu, Z.; Wu, Y.; Malpass-Evans, R.; Zhong, Y.; McKeown, N. B.; Sun, X.; Wang, H. Acid-Base Interaction Enhancing Oxygen Tolerance in Electro-catalytic Carbon Dioxide Reduction. *Angew. Chem. Int. Ed.* **2020**, *59*, 10918-10923.

8. Pimlott, D. J. D.; Jewlal, A.; Kim, Y.; Berlinguette, C. P. Oxygen-Resistant CO₂ Reduction Enabled by Electrolysis of Liquid Feedstocks. *J. Am. Chem. Soc.* **2023**, *145*, 25933-25937.

9. Zhu, H.-J.; Si, D.-H.; Guo, H.; Chen, Z.; Cao, R.; Huang, Y.-B. Oxygen-tolerant CO₂ electroreduction over covalent organic frameworks via photoswitching control oxygen passivation strategy. *Nat. Commun.* **2024**, *15*, 1479.

10. Guo, H.; Si, D.-H.; Zhu, H.-J.; Chen, Z.; Cao, R.; Huang, Y.-B. Boosting CO₂ Electroreduction over Covalent Organic Framework in the Presence of Oxygen. *Angew. Chem. Int. Ed.* **2024**, *63*, e202319472.

11. Mondal, B.; Dey, A. Development of Air-stable Hydrogen Evolution Catalysts. *Chem. Commun.* **2017**, *53*, 7707-7715.

12. Martin, D. J.; Wise, C. F.; Pegis, M. L.; Mayer, J. M. Developing Scaling Relationships for Molecular Electrocatalysis through Studies of Fe-Porphyrin-Catalyzed O₂ Reduction. *Acc. Chem. Res.* **2020**, *53*, 1056-1065.
13. Costentin, C.; Robert, M.; Savéant, J-M. Current Issues in Molecular Catalysis Illustrated by Iron Porphyrins as Catalysts of the CO₂-to-CO Electrochemical Conversion. *Acc. Chem. Res.* **2015**, *48*, 2996-3006.

14. Lee, K. J.; Gruninger, C. T.; Lodaya, K. M.; Qadeer, S.; Griffith, B. E.; Dempsey, J. L. Analysis of multi-electron, multi-step homogeneous catalysis by rotating disc electrode voltammetry: theory, application, and obstacles. *Analyst* **2020**, *145*, 1258-1278.

15. Bard, A. J. Faulkner, L. R. *Electrochemical Methods: Fundamentals and Applications*, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2nd Ed., **2001**, p339.

16. Deeba, R.; Collard, A.; Rollin, C.; Molton, F.; Chardon-Noblat, S.; Costentin, C. Controlled Potential Electrolysis: Transition from Fast to Slow Regimes in Homogeneous Molecular Catalysis. Application to the Electroreduction of CO₂ Catalyzed by Iron Porphyrin. *ChemElectroChem* **2023**, e202300350.

17. Savéant, J-M.; Costentin, C. *Elements of Molecular and Biomolecular Electrochemistry*, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2nd Ed., **2019**, p 116.

18. Pegis, M. L.; McKeown, B. A.; Kumar, N.; Lang, J. K.; Wasylenko, D. J.; Zhang, W. P.; Raugei, S.; Mayer, J. M. Homogeneous Electrocatalytic Oxygen Reduction Rates Correlate with Reaction Overpotential in Acidic Organic Solutions. *ACS Cent. Sci.* **2016**, *2*, 850-856.

19. Pegis, M. L.; Wise, C. F.; Koronkiewicz, B.; Mayer, J. M. Identifying and Breaking Scaling Relations in Molecular Catalysis of Electrochemical Reactions. *J. Am. Chem. Soc.* **2017**, *139*, 11000-11003.

20. Pegis, M. L.; Martin, D. J.; Wise, C. F.; Brezny, A. C.; Johnson, S. I.; Johnson, L. E.; Kumar, N.; Raugei, S.; Mayer, J. M. Mechanism of Catalytic O₂ Reduction by Iron Tetraphenylporphyrin. *J. Am. Chem. Soc.* **2019**, *141*, 8315-8326.

21. Lexa, D.; Momenteau, M.; Mispelter, J. Characterization of the reduction steps of Fe(III) porphyrins. *Biochim. Biophys. Acta* **1974**, *338*, 151-163.

22. (a) Harraz, D. M.; Weng, S.; Surendranath, Y. Electrochemically Quantifying Oxygen Reduction Selectivity in Nonaqueous Electrolytes. *ACS Catal.* **2023**, *13*, 1462-1467.
(b) Fino, D.; Carlesi Jara, C.; Saracco, G.; Specchia, V.; Spinelli, P. Deactivation and regeneration of Pt anodes for the electro-oxidation of phenol. *J. Appl. Electrochem.* **2005**, *35*, 405-411.

23. James, H. J.; Broman, R. F. Modified Winkler determination of oxygen in dimethylformamide: oxygen solubility as a function of partial pressure. *Anal. Chim. Acta* **1969**, *48*, 411-417.

24. Goff, H. M.; Shimomura, E. T.; Lee, Y. J.; Scheidt, W. R. Synthesis, molecular structure, and solution properties of a phenolate-bridged (tetraarylporphinato)iron(III) dimer. *Inorg. Chem.* **1984**, *23*, 315-321.

25. Chartier, C.; Chardon-Noblat, S.; Costentin, C. Redox Behavior and Kinetics of Hydroxo Ligand Exchange on Iron Tetraphenylporphyrin: Comparison with Chloro Exchange and Consequences for its Role in Self-Modulation of Molecular Catalysis of Electrochemical Reactions. *Inorg. Chem.* **2024**, *63*, 7541-7548.

26. Costentin, C.; Savéant, J-M. Cyclic Voltammetry of Electro catalytic Films: Fast Catalysis Regimes. *ChemElectroChem* **2015**, *2*, 1774-1784.
TOC

<figure><img src="image_8.png" /></figure>