Active-Site Modulation in an Fe-Porphyrin-Based Metal–Organic Framework through Ligand Axial Coordination: Accelerating Electrocatalysis and Charge-Transport Kinetics

The construction of artificial solar fuel generating systems requires the heterogenization of large quantities of catalytically active sites on electrodes. In that sense, metal–organic frameworks (MOFs) have been utilized to assemble unpreceded concentration of electrochemically active molecular catalysts to drive energy-conversion electrocatalytic reactions. However, despite recent advances in MOF-based electrocatalysis, so far no attempt has been made to exploit their unique chemical modularity in order to tailor the electrocatalytic function of MOF-anchored active sites at the molecular level. Here, we show that the axial coordination of electron-donating ligands to active MOF-installed Fe-porphyrins dramatically alters their electronic properties, accelerating the rates of both redox-based MOF conductivity and the electrocatalytic oxygen reduction reaction (ORR). Additionally, electrochemical characterizations show that in multiple proton-coupled electron transfer reactions MOF-based redox hopping is not the only factor that limits the overall electrocatalytic rate. Hence, future efforts to enhance the efficiency of electrocatalytic MOFs should also consider other important kinetic parameters such as the rate of proton-associated chemical steps as well as mass-transport rates of counterions, protons, and reactants toward catalytically active sites.


■ INTRODUCTION
Metal organic frameworks (MOFs), 1,2 a crystalline, porous subclass of coordination polymers, have stimulated significant interest in the scientific community over the last two decades due to their exceptional properties as high surface area and porosity, well-defined crystallinity, and unique chemical and physical modularity. 1,3−7 Therefore, MOFs are being used in a wide variety of applications as gas storage/separation, 8,9 chemical catalysis, 10−12 sensing, 13 and photocatalysis. 14,15 Over the last few years, attention has also been given to develop new schemes for electrocatalysis 16−23 and artificial photosynthesis 24−30 which are based on MOFs. In principle, much like catalytic enzymes, electrocatalytic MOFs could potentially incorporate all of the functional elements needed for efficient catalysis: (1) the ability to immobilize an unparalleled number of catalytic sites; 31 (2) the inclusion of mass-transport channels in contrast to dense, bulk inorganic, or polymeric films, where only a small fraction of the material is solution-accessible (and thus catalytically active); an MOF creates an ordered, porous heterogeneous network that allows the electrolyte species and catalytic substrates to permeate the interior of the film; 15,18,32,33 (3) separation between chargetransporting and catalytic moieties by the addition of shuttles to deliver redox equivalents to and from the MOF-tethered catalytic sites; and (4) modulation of the catalyst secondary chemical environment. 31 Indeed, over the last years several examples of successful heterogenization of a high concentration of molecular catalysts in MOFs were presented, 18,20 showing the ability to drive important electrocatalytic reactions as hydrogen evolution, CO 2 reduction, 34,35 water oxidation, and oxygen reduction. For these electrocatalytic MOFs, it has been shown that redox conductivity constitutes a kinetic bottleneck impeding the overall catalytic performance of the system. As a consequence, a number of different approaches were suggested to overcome this obstacle, including (a) π-stackings of MOF-based linkers and incorporated conductive guests; 36−38 (b) the delocalization of the charge in 2D MOFs; 39 (c) charge transport through MOF-installed conductive polymers; 40 and (d) conduction through a redox hopping mechanism between spatially isolated redox-active moieties (MOF nodes or linkers). 41−45 Yet, so far to the best of our knowledge, no attempt has been made to exploit the exceptional chemical modularity of MOFs for electrocatalysis in order to precisely tune and manipulate the catalytically active site's properties. In that sense, for biological enzymatic systems it is well known that molecular moieties residing in the surroundings of the active site can greatly influence the catalytic activity. For instance, in naturally occurring oxygen utilization reactions, the axial coordination of electron-donating ligands as imidazole and histidine is used to control the function of the Fe-porphyrin catalytic sites. 46 In this work, we demonstrate that the electronic properties of MOF-anchored Fe-porphyrins could be systematically tailored via axial coordination of an electron-donating ligand, 2-methylimidazole (MeIM), to the active metal site. Due to the chemical and structural robustness of Zr 6 -oxo-based MOFs, as a model system we have chosen to explore an Feporphyrin (Hemin)-modified UIO-66 (termed UIO-66@ Hemin) for an electrocatalytic oxygen reduction reaction (ORR). We found that the effect of MeIM axial coordination is 2-fold: (i) it serves as a powerful tool to boost the ORR catalytic rates, increasing catalytic currents by a factor of 2.9 while anodically shifting the onset potential by 124 mV and (ii) it enhances the kinetics of charge hopping between adjacent redox-active Fe-porphyrin sites. Moreover, electrocatalytic analysis conducted under hydrodynamic convection conditions revealed that the rate of a single redox hopping event (extracted under noncatalytic conditions) is not the sole parameter that dictates the overall electrocatalytic rate of the MOF-based system. In complex proton-coupled electron transfer reactions (such as ORR), the completion of a catalytic cycle requires multiple hopping events/chemical steps while the effect of mass-transport kinetics (counterions, protons, and catalytic reactants in both the bulk electrolyte and inside the MOF's pores) should also be taken into account.

■ RESULTS AND DISCUSSION
In order to obtain UIO-66@Hemin, first a UIO-66 MOF having a high density of defect sites (missing linkers) was synthesized according to a previously reported procedure. 47,48 Briefly, 1 mL of hydrochloric acid was included in the reaction solution to accelerate the rate of MOF crystallization, hence forming a large concentration of undercoordinated Zr 6 -oxo MOF nodes (defect sites). In turn, these formed defect sites serve as tethering spots for Hemin immobilization via postsynthetic solvent-assisted ligand-incorporation (SALI), 49 as can be seen in Scheme 1a.
Powder X-ray diffraction (PXRD) confirms the successful synthesis of UIO-66 as well as the preservation of MOF crystallinity upon Hemin anchoring ( Figure S1). 48,50,51 In addition, scanning electron microscopy (SEM) images show the retention of crystal size and morphology upon Hemin modification ( Figure S2). Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) measurements show that for UIO-66@Hemin the Zr 6 -oxo node to Hemin ratio is 4:1 (Table S1). N2 physisorption measurements show similar surface areas for UIO-66 and UIO-66@Hemin (1557 and 1594 m 2 /gr, respectively), in accordance with previous reports on the defective UIO-66 surface area ( Figure S3a). 48,52 As shown in Figure S3b, pore size distribution analysis reveals a slight decrease in average pore width for UIO-66@Hemin compared to UIO-66 due to the installation of the catalyst in the MOF pores. 48 In addition, to calculate the defect-site density in UIO-66 and UIO-66@Hemin, we combined nuclear magnetic resonance ( 1 H NMR) 52 and ICP-AES ( Figure S4). First, 1 H NMR measurements were taken in order to quantify the number of BDC linkers in UIO-66 and UIO-66@Hemin MOFs. In addition, ICP-AES measurements were conducted to analyze the concentration of Zr 6 -based nodes in the MOFs.
Thus, by combining both results, we were able to extract the Zr 6 -to-BDC ratio and hence quantitively detect the density of defect sites (missing linkers). For both UIO-66 and UIO-66@ Hemin, we obtained ∼9 BDC linkers per node ( Figure S4b), meaning that our MOFs indeed contain significant numbers of defects. (Fully saturated UIO-66 contains 12 BDC linkers per node.) Confirmation of Hemin tethering to the Zr 6 -oxo MOF node was obtained through Raman spectroscopy ( Figure S5). For UIO-66 two peaks appear at 1432 and 1447 cm −1 , attributed to in-phase carboxylate (OCO) symmetric stretching of the BDC linkers. For UIO-66@Hemin, the relative intensity of both bands is significantly attenuated due to Hemin-based OCO stretching, indicating successful Hemin anchoring to the MOF's node. 53 In order to monitor the MeIM axial coordination to UIO-66@Hemin (Scheme 1b), we have thoroughly characterized our samples using Raman spectroscopy, electron paramagnetic resonance (EPR), and X-ray photon electron spectroscopy (XPS). Raman spectroscopy is known to be sensitive to the electronic and structural properties of porphyrins through the ν 2 -and ν 4 -symmetric pyrrole stretching bands. 54 As can be seen in Figure S6, MeIM axial coordination to UIO-66@ Hemin can be followed by comparing the changes in ν 2 and ν 4 bands. Specifically, upon axial ligation, we observe a shift of ν 2 toward higher frequencies (from 1369 to 1372 cm −1 ), indicating the conversion of low-spin Fe 3+ -Hemin to a highspin species ( Figure S6b). Additionally, ν 4 bands of both highand low-spin states are observed (1568 and 1554 cm −1 , respectively), but upon MeIM coordination, the relative intensity of the high-spin band is enhanced ( Figure S6c). Further confirmation of MeIM coordination to UIO-66@ Hemin is obtained via EPR analysis ( Figure S7), showing a significant shift in the g x signal (g x = 6.57) in comparison to that of noncoordinated UIO-66@Hemin (g x = 6.15) due to a change in Hemin's electronic configuration. 55 Moreover, the N 1s XPS of MeIM-coordinated UIO-66@Hemin reveals the  Figure S8). 56 Thereafter, the effect of MeIM axial coordination on the electrochemical activity of UIO-66@Hemin was examined. Thin films of UIO-66@Hemin were deposited on a conductive carbon cloth electrode using an ink drop-casting method. (See the Supporting Information for experimental details.) Cyclic voltammetry (CV) measurements were conducted with a standard three-electrode setup containing a Pt counter and a Ag-based quasi-reference electrode (referenced against the Fc +/0 redox couple) in 0.1 M LiClO 4 acetonitrile (MeCN) electrolyte (with varying concentrations of MeIM) under an inert Ar atmosphere. As can be seen in Figure 1a, regardless of the presence of MeIM in solution, all UIO-66@Hemin samples exhibit quasi-reversible redox activity corresponding to the Hemin-based Fe 3+/2+ couple, yet upon MeIM axial coordination to Hemin, a clear cathodic shift in the formal redox potential is observed (coupled with a decrease in peak separation), thus indicating the modulation of Hemin's electronic properties by the axial coordination. The extent of the cathodic shift can be easily seen through monitoring the change in the cell's open-circuit potential (Figure 1b). Up to an MeIM concentration of 0.5 mM, a cathodic shift of more than 120 mV is observed, while at higher concentration there is no further change in Hemin's redox potential, possibly due to axial coordination to all electroactive Hemins. Typically, upon axial ligation of MeIM, the redox potential will shift according to the difference in the MeIM binding constant to Fe 2+ and Fe 3+ . Thus, generally for Fe-porphyrins (where Fe 2+ binds more strongly to MeIM) one should expect an anodic shift in the redox potential. However, it is well known that in the case of a porphyrin system that has steric hindrance toward MeIM binding (such as our MOF-based system which contains an anchored Hemin within small cavities of about 1 nm), a cathodic shift is often observed. 57 The electrochemical activity of MOF-immobilized Hemins (as opposed to the case of freely diffusing homogeneous Feporphyrins) is largely affected by the propagation of charge through hopping between neighboring redox-active sites. Consequently, we were interested in understanding how the observed electronic tuning of MOF-tethered Hemins (through MeIM axial coordination) alters the kinetics of charge transport in our UIO-66@Hemin films. To do so, potentialstep chronoamperometry measurements were recorded to determine the rates of charge hopping between neighboring Hemin-based ligands in the MOF films. We stepped the potential from 0. 15   where r is defined as the distance (cm) between two adjacent redox-active Hemin moieties (estimated to be 2.9 nm, see Figure S10). Figure 1c and Table S2 show the obtained k hopping as a function of MeIM concentration. Interestingly, k hopping indeed exhibits a clear dependency on the extent of MeIM Journal of the American Chemical Society pubs.acs.org/JACS Article axial coordination to the MOF-anchored Hemins. In fact, k hopping could be systematically enhanced from 2513 s −1 for 0 mM MeIM to 3858 s −1 for 2 mM MeIM (a 53% increase in redox hopping rates). Next, we were interested in disclosing the effect of MeIM axial coordination on UIO-66@Hemin's electrocatalytic properties. Homogeneous Fe-porphyrin is a well-known active molecular catalyst for the electrocatalytic oxygen reduction reaction (ORR). 59 Moreover, for these homogeneous catalysts, the axial coordination of electron-donating ligands increases the rate of ORR via the "push" mechanism. 60 •− ). 62 The affinity of O 2 for the Fe II (porphyrin) species could be tuned by manipulating the metal's electronic properties. A promising route to do so is by the axial coordination of electron-donating ligands (such as MeIM) to the active metal site. In that manner, one can increase the electron density at the Fe site, thus lowering the barrier for electron transfer to form the catalyst-bound superoxide (O 2 •− ) and thus boost the overall ORR performance.
Hence, we set out to examine the ORR activity of our system. To do so, thin films of UIO-66@Hemin were dropcast on a glassy carbon rotating ring-disk electrode (RRDE). redox couple. Remarkably, by varying the concentrations of MeIM one could systematically tune the resulting ORR catalytic rates. As summarized in Figure 2b, catalytic currents are enhanced by ∼290%, from −6.44 mA/cm 2 up to −18.59 mA/cm 2 for 0 and 2 mM MeIM, respectively. (At higher MeIM concentrations, catalytic currents essentially remain constant.) Additionally, as can be seen in Figure 2c,d, the ORR onset potential (termed the potential needed to drive a catalytic current of 1 mA/cm 2 ) is also tuned by the extent of axial coordination to the Hemin. By increasing the MeIM concentration, a gradual anodic shift in the onset potential is observed, reaching a maximum of 124 mV at 2 mM MeIM. On the contrary, a control bare UIO-66 film (without installed Hemin catalysts) shows significantly diminished activity (in terms of both catalytic currents as well as the onset potential), and as expected, no apparent effect is observed in the presence of MeIM in electrolyte ( Figure S11). Furthermore, the electrocatalytic ORR activity of homogeneous Hemin was characterized in an O 2 -saturated 0.1 M TBAPF 6 DMF electrolyte containing 10% H 2 O ( Figure S12). As expected, homogeneous Hemin exhibits clear ORR activity at potentials corresponding to the Fe 3+/2+ couple. Additionally, in the presence of a large excess of MeIM (8 mM), a similar enhancement in ORR activity is observed (increased catalytic current coupled to the anodic shift of the reaction's onset potential), albeit to a lesser extent. Hence, these results confirm that Hemin is responsible for the measured ORR activity in our MOF.
In order to evaluate the ORR product selectivity of UIO-66@Hemin as a function of MeIM concentration, an RRDE electrochemical analysis was performed. As can be seen in Figure S13, linear sweep voltammetry (LSV) measurements were obtained for all samples by scanning the potential of a UIO-66@Hemin disc electrode cathodically (in order to reduce oxygen), while the Pt-based ring electrode was held constant at a potential of 0.75 V vs NHE to oxidize the ORRformed H 2 O 2 product (RRDE was rotated at 1600 rpm to avoid any limitation of H 2 O 2 mass transport toward the ring electrode). As a result of this characterization, for all applied potentials, we were able to extract both the portion of H 2 O 2 produced during ORR and the average number of transferred electrons per catalytic cycle ( Figure S14). All samples produce only trace amounts of H 2 O 2 (less 2%) and exhibit essentially four transferred electrons, thus implying the high product selectivity toward the H 2 O of UIO-66@Hemin regardless of MeIM axial coordination. As shown in Figures S15 and S16, the ORR selectivity of a bare UIO-66 film was also analyzed, showing significantly lower product selectivity (up to 30% H 2 O 2 production).
The stability of the best-performing system (UIO-66@ Hemin with 2 mM MeIM) was monitored through a 7 h electrolysis test conducted at −0.45 V vs NHE ( Figure S17a). Due to the low O 2 solubility in MeCN, during prolonged catalytic conditions O 2 was depleted in the vicinity of the catalytic sites, and thus a decline in catalytic currents was observed. As a result, to ensure that the electrolyte remains O 2saturated, every 30 min of electrolysis was followed by a 10 min O 2 -purging period. Notably, it is clearly seen that UIO-66@Hemin preserves its ORR activity for about 4 h. Nevertheless, for a longer electrolysis duration, one can observe a slow decay in catalytic performance. As seen in Figure S17b, Raman spectroscopy of an UIO-66@Hemin film before and after electrolysis reveals at least a partial Journal of the American Chemical Society pubs.acs.org/JACS Article preservation of the Hemin-based active site (Hemin-based pyrrole stretching bands remained essentially unchanged). In addition, SEM images show the retention of the MOF's crystal morphology during electrolysis ( Figure S17c,d).
By now we know that in UIO-66@Hemin systems axial coordination of MeIM to the active MOF-installed Hemins modulates its electronic nature and consequently boosts the rates of both charge hopping and overall ORR electrocatalysis.
In an effort to further understand the operation mechanisms governing electrocatalysis in our MOF-based system, we set out to determine the ORR kinetic rate constants by carrying out rotating disk electrode (RDE) voltammetry experiments at varying rotation speeds. When measuring LSVs under convection conditions, catalytic currents follow the Koutecky−Levich relation 63 where C b is the bulk concentration of the catalytic substrate in solution (1.5 mM for O 2 at saturation), Γ is the catalyst's total surface coverage (5 × 10 −8 mol/cm 2 ), ν is the solution's kinematic viscosity, ω is the rate of rotation (rad/s), F is the Faraday constant, A is the surface area of the electrode, n is the number of electrons transfered in one catalytic event (four for ORR in our system), D is the diffusion constant, and k is the first-order catalytic rate constant (in M −1 s −1 ). Pseudo 0-order rate constant k cat (s −1 ) could be obtained by multiplying k by C b . Looking at the Koutecky−Levich equation, the a parameter corresponds to the maximum rate at which O 2 is converted to H 2 O within the MOF, while the b parameter signals the rate of O 2 arrival at the outer boundary of the MOF film. Thus, plotting 1/I lim vs 1/ω 1/2 and extrapolating the intercept (infinite rotation rate, eliminating mass-transport effects) allows us to distinguish between the MOF-electrode related rate-limiting steps and solution-based diffusion limitation and extract the catalytic rate constant, k cat . Consequently, to determine the effect of MeIM axial coordination on the intrinsic ORR activity of UIO-66@ Hemin, we have calculated k cat as a function of applied potential for two samples: 0 and 2 mM MeIM ( Figure 3). To do so, for each sample a series of LSVs were recorded at varying rotation speeds (100−3000 rpm). For each applied potential, intercepts of the catalytic current were extrapolated and used to calculate k cat (Tables S3). Figure 3 highlights several important insights. First, throughout the entire potential range, the addition of 2 mM MeIM enhances k cat by a factor of about 1.5, thus indicating the direct effect of axial coordination on the intrinsic activity of MOF-installed Hemins. Second, both samples possess similar linear slopes, shifted from one another by more than 100 mV, comparable to the obtained shift in catalytic onset potentials. Third, for both samples (regardless of the MeIM axial coordination) k cat reaches saturation at a certain potential threshold, meaning that at these potentials the overall ORR rate is limited by factors other than the intrinsic catalysis rate at the Hemin active site.
In that regard, it is often assumed that redox-based charge hopping is the sole rate-limiting step in MOF-based electrocatalytic systems. Yet, for UIO-66@Hemin, potential-stepdetermined k hopping values are about 3 orders of magnitude higher than the maximum value of k cat (Table 1), thus hinting at the fact that under electrocatalytic operation this general convention is oversimplified and other important factors should also be taken into account. Scheme 2 illustrates several key processes that in principle could impact the kinetics of MOF-based electrocatalysis: (1) Mass transport of bulk-solution reactants (for ORR: Li + , H + , and O 2 ) toward the MOF film. As discussed earlier, in the case of RDE-based Koutecky−Levich analysis, one can effectively eliminate this factor. (2) Mass transport of reactants (Li + , H + , and O 2 ) within the MOF's pores. As opposed to bulk solution mass transport, diffusion through the confined space in the interior of the MOF should impose a kinetic limitation even under infinite convection conditions. (3) The charge hopping rate between adjacent redox-active moieties. (4) The intrinsic catalytic rate at each MOF-anchored active site. In addition, it is important to realize that electrocatalytic proton-coupled

■ CONCLUSIONS
In this work, we demonstrate that by using axial coordinative ligation one can manipulate the electronic properties of catalytically active Fe-porphyrin (Hemin) sites installed within a UIO-66 MOF film. Specifically, it was found that MeIM coordination to MOF-installed Hemins systematically tunes their electrochemical redox activity, altering the porphyrinbased Fe 3+/2+ formal potential while accelerating the rate of charge hopping between neighboring Hemin sites. Moreover, MeIM coordination also boosts the MOF's activity toward electrocatalytic ORR, significantly enhancing catalytic currents while lowering the reaction's onset potential. Furthermore, extracted catalytic ORR rates (k cat ) were found to be 3 orders of magnitude larger than the rate of redox hopping between adjacent MOF-installed Hemins (k hopping ). In other words, as opposed to the conventional assumption in the field, k hopping is not the only parameter that limits the overall electrocatalytic rate of MOF-based films. Hence, future efforts to develop efficient MOF-based electrocatalytic systems will have to consider additional key kinetic factors such as ion/proton/ reactant mass transport in bulk electrolyte as well as inside the MOF's pores.