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Designing bimetallic zeolitic imidazolate frameworks (ZIFs) for aqueous catalysis: Co/Zn-ZIF-8 as a cyclic-durable catalyst for hydrogen peroxide oxidative decomposition of organic dyes in water

ZIF-8 is well known hybrid material that is self-assembled from inorganic and organic moieties. It has several potential applications due to its unique structure. One of these potential applications is in advanced oxidation processes (AOP) via a heterogeneous catalysis system. The use of modified ZIF-8/H2O2 for the destruction of the azo dye methyl orange (MO) is presented in this work to explore its efficacy. This work presents the bimetallic Co/Zn-ZIF-8 as an efficient catalyst to promote H2O2 oxidation of the MO dye. Co/Zn-ZIF-8 was synthesized through a hydrothermal process, and the pristine structure was confirmed using XRD, FTIR, and XPS. The Co/Zn-ZIF-8/H2O2 system successfully decolorized MO at the selected pH 6.5. It was found that more than 90% of MO (10 ppm) was degraded within only about 50 minutes. Proposed radical and redox mechanisms are presented for H2O2 decomposition where the redox mechanism is suggested to predominate via a Co(ii)/Co(iii) redox consecutive cyclic process.


Introduction
Zeolitic imidazolate frameworks (ZIFs) represent a particular class of coordination polymers that are built from metal ion nodes and methyl-imidazolate (MIM) linkers. ZIFs are crystalline solids possessing structures analogous to aluminosilicate zeolites with an intrinsic permanent porosity and considerable thermal and chemical stability. 1,2 Due to the many desirable characteristics of this class of microporous solids, ZIFs were heavily explored within the last decade for applications such as gas separation, adsorption (gas and liquid phases), sensing 3 electronic devices, drug delivery, and catalysis. 4,5 Indeed, applying ZIFs for heterogeneous catalysis has been systematically investigated and has covered several approaches such as degradation of some dyes/organics (e.g., methylene blue and indigo carmine) via photocatalysis 6 catalytic synthesis of some organic derivatives (styrene carbonates), 7 hydrogen generation, 8 CO 2 conversion, 9 Knoevenagel condensation, 10 Friedel-Cras acylation, 11 trans-esterication, 12 and monoglyceride synthesis. 13 Although reductive catalytic degradation of some dyes was previously investigated by the ZIF-8/NaBH 4 catalysis system, 14 or adsorption by ZSM-5 zeolite 15 utilization of ZIFs for the counter-redox direction, the oxidative catalytic degradation of organic contaminants (heterogeneous catalysis employing the oxidant H 2 O 2 ), to best of our knowledge has not previously been reported. This scarcity of in-depth investigations into the catalytic activity of ZIFs for oxidative degradation of organic dyes did not allow for critical evaluation of ZIFs in many environmental applications, one of which is the efficient oxidative remediation of wastewater.
In this work, the known bimetallic Co/Zn-ZIF-8 has been selected for oxidative catalysis degradation primarily due to its signicant hydrolytic stability in aqueous solutions, a decisive factor for any material to utilize in aqua-degradation systems. 6 Synthesis and characterization of Co/Zn-ZIF-8 are presented, especially detailed XPS analysis. The rate and kinetic-modelling of oxidative catalytic degradation of the stable dye methyl orange by Co/Zn-ZIF-8/H 2 O 2 system are presented as well. To the best of our knowledge, this is the rst report presenting the activity of a bimetallic ZIF-8 for degrading a synthetic dye via catalytic H 2 O 2 decomposition.
In addition, two ZIF-8 control samples were also prepared, each was based on a single metal ion, Zn-ZIF-8 and Co-ZIF-8. It was found that while Zn-ZIF-8 was hydro-stable, Co-ZIF-8 readily degraded in hydrogen peroxide solution.

Co/Zn-ZIF-8 characterization
The chemical structure of Co/Zn-ZIF-8 was explored via FTIR analysis (KBr pellet method using Jasco FT/IR 4100). For its crystalline nature, powder X-ray diffraction (PXRD, Shimadzu XD-l) was applied. SEM/EDX analysis (Zeiss EVO-10 microscopy) was utilized to study the surface morphology and to acquire more information about elemental composition. The X-ray photoelectron spectrum of Co/Zn-ZIF-8 (XPS-Thermo Fisher Scientic, USA) was acquired with a monochromatic Al-K a (1486.7 eV). BET analysis (N 2 adsorption, NOVA Station A) was also performed to determine surface area.

MO degradation
Three MO solutions (250 ml each with adjusted initial pH at 6.5) of different initial concentrations, 2, 5 and 10 ppm, were used to examine MO degradation rate by adding Co/Zn-ZIF-8 (0.05 g) and H 2 O 2 (1 ml (45% H 2 O 2 , 0.5 mM)) for each. With no agitation or shacking, the three samples were le in dark at ambient temperature (z27 C). The darkness condition was applied to avoid the probable photocatalysis that may happen due to lab stray lights. Periodically, every 10 minutes, about 3 ml from each sample were withdrawn to measure the absorbance of MO remnants using UV-Vis spectrophotometer (Jasco Model 7850 at l ¼ 591 nm). The dosage effect was also considered by using different amounts of Co/Zn-ZIF-8, 0.01, 0.025, 0.05, and 0.1 g, keeping other conditions the same as previous.
It is important to mention that the selection of pH 6.5, near neutral, was decided to avoid probable metal ion leaching from Co/Zn-ZIF-8 structure by acidity and to avoid possible hydrolysis by alkalinity. 16 In addition, such selection is appropriate to circumvent Co/Zn-ZIF-8 surface interaction with acidity or alkalinity, because of the probable buffering MIM moieties, which might affect H 2 O 2 -produced oxidative species. 17,18 In addition, the decomposition rate of H 2 O 2 by Co/Zn-ZIF-8 was monitored and recorded by the traditional triiodide-Ghormley method. 19,20 3. Results and discussion 3.1 Co/Zn-ZIF-8 characterization before and aer MO degradation Fig. 1(A) shows the SEM image of Co/Zn-ZIF-8 crystals before and aer 5 cycles of catalysis. The crystals morphology can be sought as of dodecahedron type of similar average sizes, about 0.4 mm. This shape is conventional to most of the ZIF-8 samples. 3 Some deterioration in morphology has been noticed aer 5 cycles. This may be due to the harsh condition by using H 2 O 2 . Fig. 1(B) shows the FTIR spectra of two Co/Zn-ZIF-8 samples: the before degradation sample (BE-S) and the aer 5 degradation-cycles sample (AF-S). For both spectra, the vibrations recorded in the range 760-1422 cm À1 are attributed to the 2-MIM ring. Also, for both spectra, the characteristic band at 1585 cm À1 arose from C]N stretching vibration while the peaks at 2800-3100 cm À1 correspond to the aliphatic and aromatic stretching C-H of 2-MIM, respectively. The broad peak around 3400 cm À1 could be assigned to adsorbed moisture from the atmosphere into the ZIF crystals, with spikes that can be associated with the N-H stretching vibration of the residual 2-MIM. The very similarity of these spectra indicates the chemical stability of Co/Zn-ZIF-8 against H 2 O 2 , especially the organic part.
XRD patterns of BE-S, AF-S, and the simulation are shown in Fig. 1(C). These patterns are almost identical which veries the successful preparation of Co/Zn-ZIF-8 as a ZIF-8 material with a body-centred cubic crystal lattice structure. 21 Specically, the similarity of BE-S and AF-S patterns validates its crystalline robustness against H 2 O 2 . Hence, it can be suggested that the incorporation of cobalt does not alter the topology of the framework structure and the original topology was retained due to: (i) the similarity of the ionic size of cobalt and zinc (Co 2+ 0.79Å and Zn 2+ 0.74Å) and (ii) the similarity of coordination number for the Co(II) and Zn(II) ions in this framework structure. 22 Fig. 1(D) presents the N 2 -adsorption isotherms of BE-S and AF-S and the accordingly determined BET surface areas are 1210 and 255 m 2 g À1 respectively. The obvious reduction in the surface area could be referred to as the adsorption of degradation products of MO which does not affect the successive degradation cycles as will be shown later. Fig. 2 presents the XPS survey spectra of Co/Zn-ZIF-8 samples, BE-S (A) and AF-S (B). The presence of the principal Co 2p, Zn 2p, N 1s, C 1s, and O 1s core levels before and aer degradation with relatively similar abundances proposes the signicant chemical stability of Co/Zn-ZIF-8 during H 2 O 2 decomposition process. Table 1 indicates that pristine Co/Zn-ZIF-8 suffered a slight degradation due to the 5-cycles of degradation where a signicant increase in oxygen abundance was recorded. Moreover, the observed similarities of atomic ratios of Co/Zn and C/N before and aer H 2 O 2 treatment conrms the successive H 2 O 2 -decomposition without any signicant change in the chemical composition of Co/Zn-ZIF-8.
Further, in-depth analysis concerning the XPS results was carried out to investigate the possible effects of H 2 O 2 treatment on Co/Zn-ZIF-8 structure and to assist the understanding of the decomposition mechanism. Fig. 3(A and B) presents Co 2p high-resolution XPS spectra of BE-S and AF-S. Two main peaks are present for both samples. These peaks are indexed to a spin-orbit splitting of Co 2p 3/2 and 2p 1/2 at 781.16 and 797.08 eV respectively for BE-S and 781.28 and 797.27 eV for AF-S respectively. The two peaks of BE-S are very similar to those of AF-S which indicate the chemical stability of Co/Zn-ZIF-8. The splitting energies of the 2p doublet (D ¼ 2p 1/2 À 2p 3/2 ) for both samples were found to be close and equal to about 16 eV, which is quite similar to the literature. 23,24 Deconvoluting cobalt Co 2p 3/2 and Co 2p 1/2 spectra present two main peaks under each one along with their signicant corresponding satellites peaks. The tted peak at 780.96 eV (BE-S) is very similar to the peak at 781.04 eV (AF-S) and the tted peak at 783.16 eV (BE-S) is also very similar to the peak at 783.3 eV (AF-S). These peaks should be attributed mainly to Co 2p 3/2 of Co 2+ /Co 3+ mixed valency. [24][25][26][27][28][29] Further, the observed peaks around 797.03 eV (BE-S) and 797.1 eV (AF-S) and 799.19 (BE-S) and 800.82 eV (AF-S) could be also attributed to Co 2p 1/2 of Co 2+ /Co 3+ mixed structure. [30][31][32][33] Accordingly, both oxidation states, Co 2+ and Co 3+ , are always present within Co/Zn-ZIF-8 structure before and aer H 2 O 2 treatment which also indicates the reasonable stability of the chemical structure of the pristine sample.
Co 2p   Also, Co 2p 1/2 satellite peaks are observed at 805.1 (BE-S) and 804.69 eV (AF-S) in addition to another one at 802.82 (AF-S) which also supports the possibility of having a stable Co 2+ / Co 3+ mixed structure before and aer H 2 O 2 treatment. 27,[36][37][38] The disappearance of the satellite peaks at 802.82 eV aer H 2 O 2 treatment may indicate the possibility of having a slight difference within the electronic environment and charge transfer around Co 2+ /Co 3+ aer H 2 O 2 decomposition. [39][40][41] Recalling that cobalt precursor was divalent, however, it seems that some Co 2+ ions have been oxidized to Co 3+ during synthesis. 34,42,43 This depiction conrms that cobalt ions should have a different coordination environment, 44-46 which subsequently allows consecutive redox electron transfer. 45,47,48 In addition, the presence of a mixed metal system, such as the present case (Zn/Co), rises the metal-heterogeneity mode of the mixed valence system that should enhance the redox activity a character that is essential for catalytic reactions. 49,50 Fig. 3(C and D) presents the 2p 3/2 and 2p 1/2 lines for Zn 2+ for both BE-S and AF-S which are assigned at 1022.02 and 1045.03 eV for BE-S and 1022.09 and 1045.1 eV for BE-S and AF-S, respectively. The binding-energy difference between these lines is about 23.0 eV and they are free from multiple splitting and other complicating effects. 51-53 A distinct weak peak located at 1039.09 eV (BE-S) and 1039.2 eV (AF-S) is observed, which may be due to the modulation of binding energies arising from some ligand dissimilarities of Zn 2+ ions. [53][54][55] The existence of some Co 3+ nods, in addition to Zn 2+ and Co 2+ nods, throughout Co/ Zn-ZIF-8 structure must be balanced with the negative anion (COO À ) available within the reaction medium. This phenomenon may induce some local dipole moments at Co 3+ sites inuencing spots of higher surface energies. 53,56,57 Fig. 3(E and F) presents N 1s core-level spectra of the Co/Zn-ZIF-8 for both BE-S and AF-S. They are resolved into multiple peaks for different nitrogen environments. The de-convolutions were decomposed into three (BE-S) and four (AF-S) peaks respectively. The most intense N 1s peak at 398.8 eV (BE-S) and 398.91 eV (AF-S) could be attributed to the pyridinic nitrogen atoms. [58][59][60][61][62] Because of the little difference between the binding energy of the metal-coordinated N and pyridinic N, 59,63 this peak indicates the successful coordination through nitrogen-sp 2 bonding with metal nods. 59,63,64 In addition, it also designates that most reacted methylimidazole should exist as methyl imidazolate. 65,66 Here are some important observations to comment on which reveals some side effect of H 2 O 2 on Co/Zn-ZIF-8 structure. The pyrrolic peak of nitrogen is observed at 399.75 eV (BE-S) and at  399.83 eV (AF-S). 58,59,61,62 The peak intensity increased aer H 2 O 2 treatment which could imply some partial consumption of the pyridinic species. Also, the intensity of N-graphitic 58,59,62,67 protonated pyrrolic 68 peak at 400.5 eV (BE-S) has been diminished, at 401.11 eV (AF-S), due to repetitive oxidation. On the other hand, a weak peak around 402.95 eV representing the oxidized N (pyridine-N-oxide) species 67-69 appears for (AF-S). This new peak is of higher energy concerning Npyridinic due to relatively stronger electron localization associated with poorer conjugation at sp 3 -bonded sites. Hence, lower charge mobility is expected to lead to only a minor decline in degradation performance. 70,71 These results propose a minor oxidative deterioration of the Co/Zn-ZIF-8 aer ve successive oxidation cycles with a survivable and stable structure. The presence C]O species, even with low abundance, conrms the possibility of incorporation of acetate anion within the structure as it was mentioned previously. It is obvious that there are no signicant changes within the deconvoluted C 1s due to H 2 O 2 treatment, and this suggests that Co/Zn-ZIF-8 chemical structure withstands H 2 O 2 oxidative medium to an acceptable extent. However, the cautious analysis showed a slight relative decrease in C]N abundance with respect to that of C-N aer H 2 O 2 treatment. This observation agrees with the decrease of pyridinic-N with respect to pyrrolic-N aer H 2 O 2 treatment. Furthermore, C]O peak showed a diminutive increase aer H 2 O 2 treatment, at 290.11 eV, which was expected due to possible minor oxidation of trace pyridinic units to form pyridone species integrated within Co/ Zn-ZIF-8 structure. 81,82 Fig. 3(I and J) presents the de-convoluted O 1s spectra. Although the relative abundance of oxygen is low with respect to other elemental compositions, specically C and N, the O 1s core-level de-convoluted seems to be very interesting. The O 1s core-level spectra of BE-S and AF-S have similarities and have a difference. There are two main observable peaks at 531.35 eV (BE-S) and at 531.39 eV (AF-S) and at 532.48 eV (BE-S) and at 532.61 eV (AF-S) which shows similarities and could be assigned for (C]O) acetate/quinone/pyridone [83][84][85] and (O-C) acetate, 83,84,[86][87][88] respectively. In addition, the possibility of having trapped or intercalated hydroxyl anions within the structure could be considered. 60,89,90 The difference is that there are two more peaks observed for AF-S, which appear at 530.1 and 533.9 eV. These peaks could be attributed to some adsorbed/intercalated oxygen 89,91 and N-O species. 88,92 Hence it should be assumed that, during H 2 O 2 decomposition process, there is a likelihood of oxidation of some pyridinic nitrogen leading to the formation of the corresponding N-oxides. [93][94][95] This hypothesis supports the previous discussion about N 1s peak considering oxidized N (pyridine-N-oxide). Fig. 4(A) shows the dark-condition decomposition rate of H 2 O 2 (C 0 ¼ 0.5 mM) by Co/Zn-ZIF-8. A gradual yet rapid decrease of H 2 O 2 concentration is quite observable which signies the likely catalytic affinity of Co/Zn-ZIF-8 towards H 2 O 2 decomposition. It is important to mention that without Co/Zn-ZIF-8, the dark-condition decomposition rate of H 2 O 2 is limited. The heterogeneous catalytic decomposition of H 2 O 2 usually obeys a pseudo-rst-order kinetic model with respect to H 2 O 2 regarding in-excess catalyst. 96 The known integrated form of decomposition rate equation is given as follow:   or less) should be always sufficient for continual degradation of the applied dye concentration. The adjustment of this ratio inuences the efficiency and economy of the catalytic decomposition process of dyes and should be taken into consideration, i.e., adjusting applied conditions to have k dec value properly higher than k deg . It is important to also mention that in an investigatory preliminary experiment, mixing of MO solution and Co/Zn-ZIF-8 did not present any signicant decrease in MO concentration, for about three hours, which indicates no degradation without H 2 O 2 and even no observable adsorption. Hence k deg can be sought catalysis-intrinsic behaviour rather than apparent and that a pure chemical process is responsible for degradation.

Decomposition of H 2 O 2 and degradation of MO
Another imperative observation, it has been elucidated previously that the decomposition rate of H 2 O 2 without Co/Zn-ZIF-8 is almost neglected. However, MO degradation rate using H 2 O 2 without Co/Zn-ZIF-8 is measurable as given in Table 3, k deg ¼ 0.007 min À1 on average. This suggests the importance of radical consumption, due to dye degradation, to draw further H 2 O 2 decomposition. Fig. 6(D) shows pseudo-rst-order modelling of MO degradation for the different applied amounts of Co/Zn-ZIF-8 and Table 4 presents the corresponding k deg values according to tting. From the gure and the table, the amount of Co/Zn-ZIF-8 causes a steady increase of k deg value. This signies the inherent fervent of Co/Zn-ZIF-8 towards H 2 O 2 decomposition. In addition, though, R 2 value constantly decreases with applied catalyst amount which points to a deviation from pseudo-rst-order modelling obedience, and this may indicate the contribution of more than one mechanism as Co/Zn-ZIF-8 amount increases and/or the need to control the dose of Co/Zn-ZIF-8 with respect to applied H 2 O 2 concentration. Table 5 gives a comparison for degradation rate constants given by Co/Zn-ZIF-8/H 2 O 2 and some other selected catalysts/H 2 O 2 systems which indicate Co/ Zn-ZIF-8 as a useful catalyst.

Radical scavenging experiments and reusability
In general, cOH and HO 2 c radicals are known as the key active type in the catalytic oxidation process. cOH, radical is a strong oxidant for many organic molecules. 2-Propanol and 1,4-benzoquinone (BQ) are known as cOH and HO 2 c scavengers respectively. [103][104][105] These scavengers were introduced into the reaction medium to capture cOH and HO 2 c during the MO degradation process. The results are illustrated in Fig. 7 and point out that the addition of scavengers causes a sharp decrease in the degradation of MO from 96% to 3.5, and 6% in the presence of 2-propanol, and BQ respectively. According to the above results, it is furthermore established that the degradation of MO is mainly dependent on the availability of cOH and HO 2 c radical species.
The reusability of a catalyst is important to consider of any heterogeneously catalyzed reaction. However, only limited experiments are there considering the reusability of powder photocatalysts in the powder form. This may be due to the difficulty in the separation of the catalyst powder aer degradation processes. In the present case, it is simple to separate Co/ Zn-ZIF-8 catalyst from the degradation solutions either by centrifugation or even by simple ltration. Regeneration of the catalyst was done aer each dye-removal experiment, by centrifugation at 6000 rpm for 30 min, washing with water, drying at 120 C, for 2 hours. The catalytic activity of Co/Zn-ZIF-8 remains efficient even up to 6 consecutive experiments, as depicted in Fig. 7(B), under the selected applied conditions. The remarkable stability of Co/Zn-ZIF-8 in water, as shown in the XRD patterns, sustains the chance for its reusability.

Proposed mechanisms of H 2 O 2 decomposition
For the present work, it is important to mention the following: (i) Zn-based ZIF-8 has also been synthesized and has been applied for decomposing H 2 O 2 , as a supporting auxiliary experiment, however, no measurable H 2 O 2 -decomposition was recorded and accordingly, it can be concluded that the sole presence of Zn(II) in the nodes does not intrinsically contribute in H 2 O 2 catalytic decomposition by either radical mechanism or redox mechanism, (ii) the quenching experiments revealed that    Fig. 8 shows the EPR patterns (range: 310-370 mT) for BE-S and AF-S. Comparison of AF-S pattern with respect to that of BE-S, conrms the presence of free radicals aer H 2 O 2 treatment as it shows six characteristic peaks corresponding to the Zn 2+ and Co 2+ catalytic centers within Co/Zn-ZIF-8 to produce free radicals via electron transfer upon treating the Co/ Zn-ZIF-8 with hydrogen peroxide. In some previous works, EPR/ ESR measurements revealed the presence of cOH and HO 2 c species on the surface of different oxides catalysts. These species are normally short-lived and reactive, however, they become long-lived because of the enhanced stabilization due to their adsorption onto the oxide surface. 106 Decomposition of hydrogen peroxide on a solid surface is a spontaneous process at temperatures that range from room temperature to 286 C. In addition, it was revealed that energy barrier for reactions catalyzed by surface sites where the metal atoms are of under-coordination should be lower than the energy barrier for equivalent reactions catalyzed by nondefected surfaces. 106 Two possible mechanisms can be given as follows: (1) The rst (radical mechanism) is based on the complexation of H 2    In this mechanism, Co(III) nodes may contribute to the decomposition process likewise Co(II).
(2) The second (redox mechanism) is analogous to the Haber-Weiss mechanism albeit with a metal ion belongs to solid surface: 109 Monitoring of pH during decomposition did not give a signicant change which suggests similar rates of both the eqn (2) and (4) where a balance occurs for the formation of OH À and H + . Hence, the catalytic overall decomposition reaction, via Co(II)/Co(III) redox consecutive cyclic process, should be: Herein, Co(III) nodes essentially contribute to the decomposition process by forming a redox pair with Co(II), i.e. Co(II)/ Co(III) redox system.
Both stability of pH and the previously discussed XPS analysis, which revealed that cobalt ions of different coordination environments and different oxidation states permit the consecutive redox reaction suggesting a redox mechanism to predominate the decomposition process (Fig. 9).

Conclusions
The successful synthesis of Co/Zn-ZIF-8 in an aqueous medium points to the importance of zinc nodes to stabilize the material against hydro-chemical stress. Detailed study of XPS patterns of a hybrid material before and aer catalysis is a strong tool to disclose some verities about the changes that happened within the material structure and the function(s) of its compositions. For Co/Zn-ZIF-8, XPS detailed analysis made known the importance of the mixed oxidation of cobalt for H 2 O 2 redox decomposition. For Co/Zn-ZIF-8, H 2 O 2 -decomposition rate constant, k dec , and the average MO-degradation rate constant, k deg , were found of the same order, where k deg /k dec z 0.65. Hence, regulating degradation conditions, specically H 2 O 2 and dye concentration and catalyst amount, is important for the appropriate decomposition/degradation process.

Conflicts of interest
There are no conicts to declare.