Multi-Purpose Heterogeneous Catalyst Material from an Amorphous Cobalt Metal–Organic Framework†

Sustainable technologies rely on the development of universal catalyst materials. While a lot of the attention has been given to improving the performance of one single catalyst material for one specific application, there is still a need to find ways to develop catalysts that can simultaneously be utilized for several chemo- and electrocatalytic processes. In this work, we have surveyed a series of novel, cobalt-based catalyst materials derived from an amorphous MOF in an array of diverged applications. Specifically, we have focused on organic transformations such as oxidative of alkylarenes and benzylic homocoupling reactions as well as several electrocatalytic processes, which directly relate to energy conversion and storage devices, such as oxygen reduction (ORR), oxygen evolution (OER) and hydrogen evolution (HER) reactions. We have observed that only one material, TAL–2–900, delivered the optimal solution. The stability and recyclability of this unique multifunctional material has been examined.


Introduction
The notion of M-N-C catalysts, where M is typically Fe, Co, Mn or Ni, is often reserved for the catalysts developed for applications in energy conversion and storage devices. [1][2][3] However, recent reports have also emphasized the potential of this type of materials in facilitating heterogeneous organic transformations. [4][5][6] There is an accountable divergence in the ways that the M-N-C materials are prepared, and they depend on the ultimate application prerogative. [1][2][3][4][5][6][7][8][9][10][11][12] When used in the electrochemical settings, the underlying materials are typically acid-etched to make sure that the additional influence of related nanoparticles (metal and/or that of metal oxides, carbides, nitrides, sulfides and phosphides) deposited directly at the surface of the catalyst is eliminated. In heterogeneous catalysis applications, however, the contribution of these species to the overall chemical reactivity -while being well acknowledged and often relied upon -is not readily distinguished from the instances of single-atom catalysis 9,10 and the role that nanocrystals protected by graphitic carbon layers might play.
Metal-organic frameworks (MOFs) are one of the most attractive building blocks for the formation of active M-N-C materials. 12 Whereas only a handful of crystalline MOFs can directly be used as electrocatalysts, [13][14][15] a vast majority of MOFs still require an additional carbonization step to allow for increased stability and conductivity as well as to introduce a variety of catalytically active species (M-N x ). 12 During this process, additional doping with heteroatoms and metals 7,8 or the use of alternative supports 16,17 is often advantageous.
While crystalline MOFs are widely used for carbonization, 7-9 their innate crystallinity may not always be a critical factor to achieve high catalytic activities (aside from contributing to the topology). All the catalyst materials reported to date are based on monometallic (e.g. MIL-101, ZIF-67, MOF-74-Zn, Prussian blue analogues) 11 and mixed ligand/mixed metal (e.g. the MIL-101/ZIF-67 composite 18 and ZnCo-MOFs 19 ) MOFs are highly crystalline. During the pyrolysis, however, MOFs undergo drastic molecular rearrangements; hence, amorphous MOFs can be used as the precursors. 20,21 To this end, we recently assessed iron-based catalysts derived from polycrystalline metal-organic framework precursors (TAL-1-900, TAL-6-900, TAL-7-900), 22,23 which were based on carbon-rich 24 organic linkers. Even the slightest variations around the benzimidazole core have led to alterations in the performance of the materials as bifunctional oxygen electrocatalysts 22 and in heterogeneous catalysis. 23 Herein, we report a unique amorphous cobalt-based MOFderived multifunctional material, which acts as an efficient trifunctional electrocatalyst 25,26 and as a robust heterogeneous catalyst.

Morphological and physical characterizations
We prepared a series of catalysts from an amorphous metalorganic framework precursor TAL-2 by carbonizing it at different temperatures (Fig. 1). Importantly, we have treated these carbonized materials with 0.5 M HNO 3 to remove the traces of cobalt(0) and cobalt oxides from the surface of the materials. Regardless of this treatment, the final porous M-N-C materials still contained cobalt(0) nanoparticles, which were protected by the graphitic layers as is evident from the high resolution transmission electron microscopy (HRTEM; Fig. 1B-1D, S1 and S2) micrographs. Increasing the temperature of carbonization did not lead to a substantial change in the N 2 adsorption/desorption isotherms or the distribution of pore sizes, which effectively remained in the 3-4 nm range ( Fig. 1E and 1F). However, as is seen in the powder X-ray diffraction (PXRD; Fig. 1G) patterns and the microwave plasma-atomic emission spectroscopy (MP-AES; Table S1) datasets, the graphitic carbon and cobalt(0) content was significantly higher in the samples that have been treated at 900 and 1000 o C. The X-ray photoelectron spectroscopy (XPS) data indicated that carbonization at higher temperatures both lowered the nitrogen content and altered the distribution of nitrogen species at the surface ( Fig. S3; Tables S1-S5).

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Interestingly, TAL-2-900 was a more robust catalyst for converting toluene to benzoic acid, while TAL-2-1000 was better at oxidizing diphenylmethane to benzophenone. This indicates that small differences in the active sites (e.g. the total content of pyridinic nitrogen) at the surface of the catalyst play an important role in catalyst stability. A further set of substrates was subjected to oxidation (Fig. 2C). Addition of acetic acid slightly improved the yields in the cases where polymethylated arenes were converted to the corresponding carboxylic acids.

Benzylic homocoupling
Knowing that radical pathways were involved in oxidative transformation reactions, has encouraged us to extend the use of TAL-2-900 as a heterogeneous catalyst for carbon-carbon bond forming reactions via activation at the benzylic positions.   We were interested to see whether the M-N-C catalysts could be compatible with organometallic reagents; hence, we explored C(sp 3 )-C(sp 3 ) homocoupling reaction of benzyl bromides in the presence of a Grignard reagent. This reaction traditionally requires stoichiometric metallic lithium, 44 magnesium, 45 copper 46 or nickel. 47 Recently, a rhodiumcatalyzed version employing dimethylzinc was demonstrated, 48 and several photocatalyzed conditions were developed. 49,50 Gratifyingly, we observed the desired homocoupling products with cobalt based catalyst TAL-2-900 but not the iron version (TAL-1-900) (Scheme 1). The reactions were carried out using methylmagnesium bromide at ambient temperature to give corresponding bibenzyls in good to excellent yields, while aryliodides and bromides were also well tolerated. After standard aqueous workup, the spent catalyst had a significantly reduced activity. Specifically, on the next cycle, the isolated yield of homocoupled product has dropped from 93% to 38%. Alternatively, the THF solution containing the product was removed by syringe under anhydrous conditions and the reaction vessel was recharged with THF, benzyl bromide and MeMgBr. This modification gave the desired product with an improved yield on the second run (93%).

Electrocatalytic transformations
After having identified the most promising catalyst for the organic transformations, we wished to screen the TAL-2 catalyst series as trifunctional electrocatalyst materials across oxygen reduction (ORR), oxygen evolution (OER) and hydrogen evolution (HER) reactions (Fig. 3). Potentially, these datasets should give insights into whether there is a link between the performance of the individual catalysts across electrocatalytic and chemocatalytic interconversions. 51,52 Out of the three catalysts, TAL-2-900, once again, showed a superior activity profiles in all of the electrocatalytic reactions.
To learn the effect of different carbonization temperatures on electrocatalytic ORR performance, TAL-2 based catalysts were tested by rotating disc electrode (RDE) and rotating ringdisc electrode (RRDE) techniques in 0.1 M KOH electrolyte. Oxygen reduction polarization curves (Fig. 3A) confirmed that TAL-2 based catalyst carbonized at 900 o C had the highest ORR electrocatalytic activity with an onset potential (E on ) of 1.00 V, a half-wave potential (E 1/2 ) of 0.85 V and a diffusion-limiting current density of -5.78 mA cm -2 . The kinetic parameters of TAL-2-900 were on par with the ones obtained for commercial Pt/C catalyst (E on = 1.01 V; E 1/2 = 0.86; j d = -6.16 mA cm -2 ) and other promising non-noble metal ORR catalysts (Table S9). Despite the fact that BET surface area of TAL-2-900 (421 m 2 g -1 ) is lower than that obtained for TAL-2-800 (589 m 2 g -1 ) (Table  S11), the superior ORR performance of TAL-2-900 indicates higher density of electrochemically accessible active sites for ORR.
To shed light on the enhancement of the ORR activity, electrochemically active surface area (ECSA) of TAL-2 derived catalysts was estimated by collecting the electrochemical double-layer capacitance (C dl ) from CV curves ( Table S10). The ECSA values have decreased for the samples that were obtained by increasing the carbonization temperatures. Hence, the superior electrocatalytic activity of the TAL-2-900 sample may be explained by a higher amount of ORR active sites, specifically in the form of cobalt(0) nanoparticles.
The Koutecky-Levich (K-L) plots were constructed using data derived from RDE (Fig. S4) and the calculated number of electrons transferred per oxygen molecule n was in all the cases approximately four. RRDE studies confirmed that oxygen reduction proceeds via a two-step 2×2 H 2 O 2 mechanism ( Fig.  S5B and S5C). The Tafel plot analysis demonstrated that the TAL-2-900 material had the highest slope value (-76 mV decade −1 ) against all the other samples that were surveyed in this study (Fig. 3B). Similarly to Pt/C, this means that the rate determining step for ORR is the first electron transfer step, whereby O 2(ads) is reduced to O 2 -(ads) . A continuous potential cycling in the range between 0.6 and 1.0 V was used to assess the long-term stability of TAL-2-900, with E 1/2 shifting only by 20 mV after 5,000 cycles (Fig. 3C). The RDE studies have confirmed that TAL-2-900 was a highly stable active ORR electrocatalyst in 0.1 M KOH.
The OER performance of the TAL-2 derived catalysts was assessed and iR-corrected OER polarization curves are shown in Fig. 3D. The benchmark current density of 10 mA cm −2 was achieved at 1.60 V for TAL-2-900 material, which is superior to that of ruthenium oxide (1.69 V). The Tafel slopes obtained for the OER electrocatalytic activity among the series, confirm that TAL-2-900 functions similarly to the RuO 2 systems. Under the long-term OER operation, this catalyst material is stable (Fig.  3F). The overall oxygen bifunctional electroactivity (ΔE) value for TAL-2-900 is 0.75 V, which makes it much lower than the rest of the M-N-C samples, whilst haven been screened at the similar loadings (Table S10). The electrocatalytic HER activity of the best performing catalyst was compared with the commercial Pt/C in 1 M KOH ( Fig. 3G and 3H) at the current density of -10 mA cm −2 . Under these conditions, the overpotential with TAL-2-900 as a catalyst was achieved at -264 mV vs. RHE with a low Tafel slope value of 115 mV dec -1 . The HER overpotential values η for TAL-2-800 and TAL-2-1000 were 417 and 411 mV, respectively, indicating that the carbonization step at 900 o C was the optimal path to improve the electroactivity toward HER. In acidic media, the overpotential (η) was the smallest for TAL-2-900, while its overpotential was comparable to the values obtained for other Pt-free HER catalysts (Fig. 3I, Table S9).
Finally, the performance of TAL-2-900 as catalyst material was evaluated in alkaline electrolyte membrane fuel cell and zinc-air battery. Alkaline membrane fuel cell test yielded current density up to 1100 mA cm -2 (Fig. 3J). Loading of the catalyst was kept at 2 mg cm -2 on GDL membrane modified with direct suspension pipetting. HMT-PMBI (hydroxide conducting membrane) was used as polymer electrolyte in both tests for Pt/C and TAL-2-900. Under similar conditions, TAL-2-900 showed 20% better activity than Pt/C. The value of power density for TAL-2-900 is > 400 mW cm -2 leaving behind Pt/C 350 mW cm -2 . Similar results were recently reported using the same anion exchange membrane and an alternative Co-N-C material as a cathode catalyst. 53 Durability tests of poly(vinyl alcohol)-based solid-state zinc-air battery assembled using TAL-2-900 as an electrode material were performed by cycling at constant current density of 5.0 mA cm -2 (Fig. 3K). As expected from the ORR/OER data, TAL-2-900 catalyst-driven Zn-air battery delivered long cycle life over 8 h (Fig. 3L).

Conclusions
In conclusion, we have introduced a new materials preparation strategy by involving amorphous not crystalline MOFs, which were obtained from dihydroxybenzimidazole as a carbonaceous linker. Upon carbonization, it delivered a unique Co-N-C material, which simultaneously served as a heterogeneous catalyst for several organic transformations (incl. oxidation and homocoupling reactions) as well as an electrocatalyst material for ORR, OER and HER processes. This is the first example of an M-N-C catalyst being used for C(sp 3 )-C(sp 3 ) carbon-carbon bond formation, and it was carried out in the presence of an organometallic reagent.