Light-Driven Water Oxidation with Ligand-Engineered Prussian Blue Analogues

The elucidation of the ideal coordination environment of a catalytic site has been at the heart of catalytic applications. Herein, we show that the water oxidation activities of catalytic cobalt sites in a Prussian blue (PB) structure could be tuned systematically by decorating its coordination sphere with a combination of cyanide and bidentate pyridyl groups.  K0.1[Co(bpy)]2.9[Fe(CN)6]2 ([Cobpy–Fe]), K0.2[Co(phen)]2.8[Fe(CN)6]2 ([Cophen–Fe]), {[Co(bpy)2]3[Fe(CN)6]2}[Fe(CN)6]1/3 ([Cobpy2–Fe]), and {[Co(phen)2]3[Fe(CN)6]2}[Fe(CN)6]1/3 Cl0.11 ([Cophen2–Fe]) were prepared by introducing bidentate pyridyl groups (phen: 1,10-phenanthroline, bpy: 2,2′-bipyridine) to the common synthetic protocol of Co–Fe Prussian blue analogues. Characterization studies indicate that [Cobpy2–Fe] and [Cophen2–Fe] adopt a pentanuclear molecular structure, while [Cobpy–Fe] and [Cophen–Fe] could be described as cyanide-based coordination polymers with lower-dimensionality and less crystalline nature compared to the regular Co–Fe Prussian blue analogue (PBA), K0.1Co2.9[Fe(CN)6]2 ([Co–Fe]). Photocatalytic studies reveal that the activities of [Cobpy–Fe] and [Cophen–Fe] are significantly enhanced compared to those of [Co–Fe], while molecular [Cobpy2–Fe] and [Cophen2–Fe] are inactive toward water oxidation. [Cobpy–Fe] and [Cophen–Fe] exhibit upper-bound turnover frequencies (TOFs) of 1.3 and 0.7 s–1, respectively, which are ∼50 times higher than that of [Co–Fe] (1.8 × 10–2 s–1). The complete inactivity of [Cobpy2–Fe] and [Cophen2–Fe] confirms the critical role of aqua coordination to the catalytic cobalt sites for oxygen evolution reaction (OER). Computational studies show that bidentate pyridyl groups enhance the susceptibility of the rate-determining Co(IV)-oxo species to the nucleophilic water attack during the critical O–O bond formation. This study opens a new route toward increasing the intrinsic water oxidation activity of the catalytic sites in PB coordination polymers.


■ INTRODUCTION
Co−Fe Prussian blue analogues (PBAs) are three-dimensional (3D) coordination polymers with a general formula, A x Co y [Fe-(CN) 6 ]·zH 2 O, where A is an alkali or an alkaline metal ion positioned in the tetrahedral interstitial sites. It adopts a facecentered cubic (fcc) structure in the Fm3m space group. Iron and cobalt sites are connected through cyanide bridging ligands to afford an extended framework. 1 Co−Fe PBAs have been attractive compounds for heterogeneous water oxidation due to their facile synthesis procedures, stabilities, fast electron transfer between Fe to Co sites through the short cyanide bridge, and easily tunable metal sites. 2 Following the leading study by Galań-Mascaroś and co-workers in 2013, 3 several research endeavors have been devoted to elucidating the origin of the activity and to enhancing the catalytic performance of Co−Fe PBAs. 4−6 These studies indicate that the cobalt sites with accessible sites, which are on the surface or in the vacancies created to provide charge balance, are the catalytic active sites for water oxidation. Although the iron site does not serve as a catalytic site due to the lack of an accessible coordination site, it plays an important electronic role in enhancing the activity and the stability of cobalt sites.
Studies in this field could be divided into four main strategies. (i) Utilizing the PBA as a precatalyst: the majority of the studies focus on decomposing the cyanide network to their corresponding oxides or phosphides or selenides to form what is referred to as "PB derived catalyst". For instance, Zhang et al. converted Co−Fe PBA to (Fe−Co)Se 2 by a selenization procedure. The derived catalyst with high porosity and fast electron transfer exhibits superior activity and long-term stability. 7 (ii) Coupling PBAs with a light-absorbing component: a strategy commonly employed in this field is to couple PBA with proper semiconductor (SC) or photosensitizers. 8,9 This strategy aims to achieve a proper energy level matching between the valence band of SC and the highest occupied molecular orbital (HOMO) of the catalytic site to boost the oxygen evolution reaction (OER) activity and the stability of PB-based assembly. Several Co−Fe PBA catalyst/ visible-light-absorbing SC assemblies have been designed by our group, e.g., a Co−Fe PBA catalyst was coupled with layered double hydroxide (LDH), 10 BiVO 4 , 11 niobates, 12 and brown-TiO 2 9 in different studies, in which each of these assemblies displayed an excellent increase in activity and stability. In addition to our work, Shi et al. also designed an anisotropic PBA−TiO 2 Janus nanoreactor that displayed improved photocatalytic activity compared to ordinary PBA or TiO 2 . 13 Furthermore, Co−Fe PBA/CoS 2 hybrid prepared by Xu et al. gave rise to a remarkable increase in activity due to the efficient electron transfer between CoS 2 and Co−Fe PBA. 14 (iii) Changing the cyanoiron precursor: another adopted strategy is to chemically modify or tune the iron site, which consequently has an indirect electronic effect on the activity of the cobalt sites. 4 For instance, our group reported that replacing the hexacyanoferrate precursor with a polymerbound pentacyanoferrate results in the formation of an amorphous structure with an enhanced activity. 15 Pires et al. leveraged on comparative studies to understand the influence of ligands attached to the Fe sites of Co−Fe PBA on its photoactivity. 16 (iv) Changing the cobalt site: few studies on directly tuning the catalytic cobalt sites without decomposition in the cyanide network are focused on changing the cobalt metal entirely to different metal, partially substituting, or doping with a different metal. 17−19 Despite the growing research in this field and all of the efforts to increase the activity of cobalt sites, direct tuning of the coordination sphere of catalytic cobalt sites in Co−Fe PBAs still remains uncharted territory. Herein, we report a series of Co−Fe PBAs, which consist of cobalt sites surrounded by a combination of bidentate pyridyl ligands, cyanide groups, and water molecules. The facile synthetic method allows the tuning of the number and the type of bidentate pyridyl ligands coordinated to the cobalt site in a PB network structure, which provides an ideal platform to elucidate the structure and activity relationship in PBAs. We prepared and characterized a series of 3D ( Physical Measurements. The surface morphology of the catalyst was revealed from scanning electron microscopy (SEM) images (FEI QUANTA 200 FEG ESEM), the instrument is equipped with an Ametek EDAX energy-dispersive X-ray (EDX) system for elemental composition analysis. The powder X-ray diffraction (PXRD) patterns were obtained using a PANalytical X'pert PRO X-ray diffractometer using Cu Kα radiation (1.5406 Å). Infrared (IR) spectra were recorded on a Bruker α Platinum-ATR spectrometer within the wavenumber range of 400−4000 cm −1 for 64 scans. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo Fisher Scientific K-Alpha X-ray photoelectron spectrometer using an Al Kα microfocused monochromator as the X-ray source and a flood gun for charge neutralization. All of the peaks were shifted with reference to the C 1s peak position (284.8 eV). UV−vis absorption spectra of the cobalt precursors were obtained on Agilent Cary 5000 UV−vis−NIR spectrophotometer using a quartz cuvette with a path length of 1 cm. Thermogravimetric analysis (TGA) was carried out on a Q500 thermogravimetric analyzer within the temperature range of 30−800°C at 5°C/min under a nitrogen atmosphere. CHN elemental analysis was obtained on a Thermo Scientific FLASH 2000 Series CHNS/O elemental analyzer using BBOT as a standard and V 2 O 5 as a catalyst.
Photocatalytic OER Experiment. Photocatalytic experiments were conducted in a Pyrex flask totally sealed with a septum. Thirty milliliters of a 0.1 M potassium phosphate buffer solution (PBS) containing 10 mg of catalyst, 5 mM sodium persulfate (Na 2 S 2 O 8 ), and 1 mM ruthenium photosensitizer ([Ru(bpy) 3 ]Cl 2 ) was prepared. PBS was prepared by mixing aqueous solutions of KH 2 PO 4 (0.1 M) and K 2 HPO 4 (0.1 M). The Pyrex flask was covered with aluminum foil before adding the ruthenium complex to prevent an early lightinduced reaction. Initially, the mixture was purged with N 2 gas thoroughly for 25−30 min. The photocatalytic experiment was carried out for 1 h, and the amount of oxygen evolved was determined by injecting 100 μL of the headspace gas at a 15 min interval into a gas chromatograph (Agilent 7820A, a gas chromatograph equipped with a molecular sieve and a thermal conductivity detector (TCD), using argon as the carrier gas). The experiment was conducted at least twice for each catalyst to obtain a reproducible result.
Electrochemical Experiments. Electrochemical experiments were conducted on a Gamry Instruments Interface 1000 potentiostat/galvanostat at 25°C. Using the conventional three-electrode setup, with Pt mesh as the counter electrode, Ag/AgCl (3.5 M KCl) as the reference electrode, and fluorine-doped tin oxide (FTO) coated electrode (∼80% transmittance; 2 mm slides with 7 Ω·sq −1 surface resistivity and 1 cm × 2 cm size) as the substrate for the working electrode. Cyclic voltammetry experiments were performed in a PBS at pH 7 containing 1 M KNO 3 as the supporting electrolyte.
Working Electrode Preparation. A 1 cm × 2 cm FTO electrode was used as the working electrode, but only 1 cm × 1 cm of the conducting surface was coated with the catalyst. Prior to coating the FTO surface, the electrode was adequately cleaned by sonicating for 10 min in a basic soapy solution, deionized water, and isopropanol and then annealed at 350°C in the furnace. The surface of FTO was coated with the catalyst by following a previously reported two-step in situ method with slight changes. 15 Briefly, 1000 μL of a 0.05 M aqueous solution of hexacyanoferrate was spin-coated on FTO at 500 rpm for 150 s, air-dried, and dipped into a 0.075 M aqueous solution of Co 2+ precursor for 15 min. This procedure was repeated three times. Finally, the electrode was dried in an oven at 60°C for about 10 min, washed with deionized water, air-dried, and kept in the desiccator until used.

Synthesis and Characterization of Cobalt Precursors.
The cobalt complex precursors were prepared following the previously established protocols 21,22 by simply mixing a proper equivalent of bpy or phen ligands with cobalt ions to afford mononuclear cobalt bipyridyl complexes. One equivalent of bidentate pyridyl ligand was used to obtain CobpyCl 2 and CophenCl 2 , while 2 equiv were used to obtain Cobpy 2 Cl 2 and Cophen 2 Cl 2 . A color change was observed with naked eyes as the precursors were mixed. The UV−vis spectra of the complexes display an MLCT band (∼475 nm) in the visible region, which originates from the electron transfer from the d- Inorganic Chemistry pubs.acs.org/IC Article orbital of the Co to the π*-orbital of the ligands ( Figure S1). The remaining bands below 350 nm are attributed to π−π* and n−π* transitions of the ligands. 23 The similarity in the visible region of the absorption spectra ( Figure S1 inset) of CobpyCl 2 and CophenCl 2 proves that cobalt complex with one pyridyl ligand is successfully synthesized. A similar trend is obtained for Cobpy 2 Cl 2 and Cophen 2 Cl 2 . The Fourier transform infrared (FTIR) technique was also used to verify the formation of these complexes ( Figure S2). The bands that were obtained match well with the fingerprints of the bpy and phen ligands, with a slight shift indicating the formation of the cobalt complex. The sharp band at ∼1600 cm −1 , the one at around 1200 cm −1 , the broad bands at ∼750− 1000 cm −1 , and the weak bands at 2900−3100 cm −1 correspond to CN/CC stretches, C−C/C−N bending, aromatic C−H vibrations, and C−H stretching frequencies, respectively. 24 Synthesis and Characterization. images display agglomeration into clusters. The chemical formula of the compounds was estimated based on EDX elemental composition analysis, CHN elemental analysis, and thermogravimetric analysis (TGA) (Tables S1 and S2). A Co/Fe atomic ratio of 3:2 is obtained by EDX analysis. The number of water molecules present in each compound was ascertained from thermogravimetric analysis (TGA) ( Figure S4). The gradual decrease in TGA curves ranging from 30 to 150°C is attributed to the loss of coordinated and non-coordinated water molecules, while the steep decrease in temperature above 250°C corresponds to the decomposition/transformation of the cyanide network to an oxide. 25 FTIR technique is employed mainly to investigate the nature of the cyanide bond and the presence of pyridyl groups in these compounds. The presence of pyridyl bands in the fingerprint region of the FTIR spectra clearly distinguishes In all Co(pyridyl)−Fe compounds, the Co 3+ sites dominate over the Co 2+ ones since the low-spin d 6 state is preferred over the high-spin d 7 case. 28 This high oxidation state also favors the formation of highvalent Co(IV)-oxo species required for water oxidation. 29 The crystal structures of the synthesized compounds were analyzed using PXRD (Figure 1b Figure  S6a). 32 (Figure 2a). Similarly, the Co 2p signal (∼775−805 eV) is deconvoluted into Co 2p 3/2 and Co 2p 1/2 peaks, each of which is fitted into Co 2+ and Co 3+ peaks (Figure 2b). 34,35 A shake-up satellite peak is observed between the 3/2 and 1/2 spin states of both Fe 2p and Co 2p signals. In [Co−Fe], a higher atomic percentage of Co 2+ to Co 3+ ions is observed, suggesting that Fe 3+ −CN−Co 2+ is the dominant coordination mode. However, for all of the Co(pyridyl)−Fe compounds, a higher atomic percent of Co 3+ ions is obtained than Co 2+ ions. Therefore, Fe 2+ −CN−Co 3+ is the dominant coordination mode when cobalt sites are surrounded by electron-accepting bidentate pyridyl ligands. This result is also consistent with the cyanide stretches observed in the FTIR spectra. As displayed in Figure 2c (Figure 2d). A peak in the 529−530 eV region due to a possible metal oxide formation is not observed, which rules out the transformation of the structure to oxides during the synthesis. 11,36 The C 1s corelevel XPS signal at 284.8 eV is present as well in all of the samples ( Figure S7). Photocatalytic Studies. The photocatalytic water oxidation experiments were performed in a PBS solution, using [Ru(bpy) 3 (Figure 3). An approximately 30% increase in the catalytic activity could be attributed to the tuning of electron density of catalytic cobalt sites in [Cophen−Fe] and [Cobpy−Fe] with electronwithdrawing bidentate pyridyl groups, which will be discussed in the following sections in detail.
Furthermore, [Cobpy2−Fe] and [Cophen2−Fe] show no oxygen evolution due to the lack of catalytic sites since all of the cobalt sites in these pentanuclear molecular complexes are fully coordinated to six −NC groups, thereby preventing an aqua (water) coordination to the cobalt sites. The complete inactivity of these complexes, therefore, signifies that the metal sites in cyanide-based Co−Fe compounds do not release the pyridyl or cyanide groups under photocatalytic conditions. Postcatalytic Characterization. Postcatalytic FTIR and XPS studies were performed further to confirm the stability of the catalysts under photocatalytic conditions. No distinguishable differences between the O 1s XPS signals of the pristine and postcatalytic powder samples of [Cobpy−Fe] and [Cophen−Fe] are observed ( Figure S8b,d). Moreover, the absence of a lattice cobalt oxide peak in the 529−530 eV region of the postcatalytic samples rules out the possible transformation of the cyanide-based compounds to metal oxides. 11 The only present peak at ∼532.4 eV is assigned to the coordinated and non-coordinated water molecules in the compound. Similar Fe 2p and Co 2p XPS peaks are obtained in the pristine and postcatalytic XPS spectra (Figure 4a,b,d,e), suggesting that the metal centers are intact.
The postcatalytic FTIR analysis reveals an additional small band in the cyanide region (∼2100 cm −1 ) and a broadening around 900−1100 cm −1 (Figure 5a,b) compared to the spectra of the pristine samples. Both changes could be attributed to a phenomenon referred to as bipyridyl poisoning of the catalyst. 6 In a photocatalytic process in the presence of [Ru(bpy) 3 ] 2+ / S 2 O 8 2− couple, the ruthenium complex decomposes by releasing bipyridyl groups. The bpy groups could then coordinate to the catalytic sites and inactivate them for water oxidation. The appearance of broad peaks at around 900−1100 cm −1 indicates the coordination of bipyridyl ligands to the Co−Fe structures (Figure 5a), while the small band in the cyanide region (∼2100 cm −1 ) is ascribed to structural changes in the cyanide environment due to the coordination of bipyridyl groups to cobalt sites (Figure 5b). Almost a twofold increase in the atomic percent of the pyridyl-N ( N-py) peak at 398.95 eV in the N 1s signal of the postcatalytic XPS spectra (Figure 4c,f) also suggests the presence of an additional pyridyl group in the structure that could only arise from bipyridyl poisoning. A small Ru 3d core-level XPS peak is also observed at 280.5 eV in the postcatalytic XPS spectra, which confirms the degradation of [Ru(bpy) 3 ] 2+ complex ( Figure S8a,c). The saturation in the activity after 1 h of light exposure ( Figure S9), therefore, could be attributed to the decomposition of the ruthenium complex, followed by the inhibition of the catalytic cobalt sites.
Overall, postcatalytic studies show that cyanide-based Co− Fe systems are robust under photocatalytic conditions and their water oxidation activities are greatly influenced by the incorporation of pyridyl groups into the coordination environment of the catalytic sites.
Evaluation of TOF. We performed comprehensive characterization and electrochemical studies to elucidate the effect of pyridyl groups on morphological and electronic properties of the Co−Fe compounds. For this purpose, turnover frequencies (TOFs) of compounds were estimated with two different methods, which differ mainly in the estimation of the active cobalt sites (see eqs S1−S6 for details of calculations): lower-bound TOF (TOF lb ) and upper-bound TOF (TOF ub ). In the first approach, all of the cobalt sites, even the cobalt ions in the bulk of the PB network structure, are assumed to be active to estimate the number of moles of active sites. The TOF lb values attained in the first 15 min of photocatalysis for [Cophen−Fe], [Cobpy−Fe] , and [Co−Fe] were estimated as 4.35 × 10 −4 , 3.91 × 10 −4 , and 1.73 × 10 −4 s −1 , respectively, which are comparable to the previously reported ones for PBA-based catalysts. 6,10 This method reveals that the activity of cobalt sites increases at least two times when pyridyl groups are introduced to its coordination sphere.
In the second method, cyclic voltammetry experiments were performed at different scan rates (50−300 mV·s −1 ) to extract the surface concentration (Γ) of active cobalt sites from the electrochemical linear dependence between peak current (I) of the Co 3+ /Co 2+ reduction wave and the scan rate (ν). 3     Inorganic Chemistry pubs.acs.org/IC Article with one equivalent of stronger π-accepting phen and bpy ligands, the strong withdrawing ability increases the electrophilicity and susceptibility of the high-valent catalytic Co(IV)oxo species to the nucleophilic attack of water for O 2 formation. As shown in several studies on water oxidation mechanistic analysis, 41,42 the nucleophilic attack of water on the high-valent catalytic Co(IV)-oxo species is the ratedetermining step for water oxidation. Furthermore, a close competition between the activity of [Cophen−Fe] and [Cobpy−Fe] is observed although phen as a more π-accepting ligand than bpy, 43 should increase the electrophilicity of the high-valent Co(IV)-oxo species. A possible explanation could be because bpy is substantially more σ-donating than phen. 44 This σ-donating ability of bpy could increase the electron density on the cobalt site of [Cobpy−Fe], which favors the formation of a stable electron-deficient high-valent Co(IV)-oxo species. 45 Electronic Structure Calculations. The effect of bidentate pyridyl ligands on the Co center was further investigated with electronic structure calculations. The suggested mechanism in the literature along with our previous works 46−49 suggests that the oxidation of water would require a Co IV -oxo/Co III -oxyl moiety. PB-based water oxidation catalytic process presumably proceeds through proton-coupled electron transfer (PCET) steps to afford the Co(IV)-oxo/Co(III)-oxyl structure, i.e., Co II (OH 2 )→Co III (OH)→Co IV (O)/Co III (O). Incoming water then attacks the Co(IV)-oxo/Co(III)-oxyl moiety to yield the O−O bond formation (Figure 7a). Therefore, the structural and electronic features of the Co IVoxo/Co III -oxyl are the focus of our quantum mechanical calculations.
Electronic structures of the Co IV −oxo/Co III −oxyl moiety bears a local quartet Co−oxygen center for . The remaining d-orbitals are nonbonding, but oxygen p-orbitals form π-bonding interactions with the metal center. 50 Consequently, the xz and yz components of the dorbitals also form bonding and antibonding orbitals. An approximate local occupation pattern can be represented as Figure  7b). The local quartet assignment of the Co center is also verified with spin density analysis. It is important to note that no restrictions are imposed on the distribution of electrons in our quantum chemical calculations.
The critical O−O-bond-forming process can be readily understood by analyzing the electronic structure. Lone pairs of oxygen in water seek available orbitals for electron transfer, for which the LUMO is the best candidate. Following the MO description above, Co−oxygen center bears three singly occupied orbitals (Figure 7b and Table S3), and LUMO is composed of Co(d z 2 ) and O(p z ) orbitals. One of the singly occupied orbitals, σ[Co(d (x 2 −y 2 ) ) − ligand], resides mainly on the cobalt center. The remaining singly occupied orbitals are centered on the Co−O bonding axis and orthogonal to each other.
In this quartet electronic structure of the Co−oxygen center, the reported reactivities and energies of the LUMO should be correlated since oxygen lone pairs of water are proposed to fill in the LUMO on Co−oxygen center. In particular, lower LUMO energies should yield higher catalytic activities. 16,42,46,51 Furthermore, [Cobpy2−Fe] and [Cophen2− Fe] compounds suffer from no activity as there is no bound Co−oxygen structure in these complexes and, therefore, no available orbitals for the electron transfer. Note, however, that catalytic activity stems from numerous complex, molecular, and applicational features and our MO analyses here attempt to uncover the electronic structure causes of the activity.
Orbital distributions and energies of LUMO for the complexes are given in Figure 7c. In line with our MO arguments, LUMO is obtained at lower energies for [Cobpy− Fe] and [Cophen−Fe] (−3.31 and −3.29 eV relatively) compounds compared to [Co−Fe] (−3.07 eV). The electron affinity of the Co−oxygen center is hence increased when the aromatic ligands, bpy or phen, are coordinated to the Co center. Consequently, the attack of water becomes more facile in the presence of aromatic ligands.
Note that our quantum chemical calculations did not use the complete molecular structure of PB surface, but the electronic structure calculations and our experimental results were in   Muhammed Buyuktemiz − Department of Chemistry, Faculty