Mixed Conducting Perovskite Materials as Superior Catalysts for Fast

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Mixed Conducting Perovskite Materials as Superior Catalysts for Fast Aqueous-Phase Advanced Oxidation: A Mechanistic Study Chao Su, Xiaoguang Duan, Jie Miao, Yijun Zhong, Wei Zhou, Shaobin Wang, and Zongping Shao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02303 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 1, 2016

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Mixed Conducting Perovskite Materials as Superior Catalysts for Fast Aqueous-Phase Advanced Oxidation: A Mechanistic Study Chao Su,#,† Xiaoguang Duan,#,† Jie Miao,‡ Yijun Zhong,‡ Wei Zhou,*,‡ Shaobin Wang,*,† and Zongping Shao†,‡ †

Department of Chemical Engineering, Curtin University, Perth, WA 6845, Australia

‡

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), State Key

Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, No.5 Xin Mofan Road, Nanjing 210009, P.R. China

ABSTRACT: A mixed ionic-electronic conducting (MIEC) double perovskite, PrBaCo2O5+δ (PBC), was synthesized and evaluated as the heterogeneous catalyst to generate radicals from peroxymonosulfate (PMS) for the oxidative degradation of organic wastes in aqueous solution. A superior catalytic activity was obtained for PBC, which was much higher than that of the most popular Co3O4 nanocatalyst. More importantly, a detailed mechanism of PMS activation on the MIEC perovskite was proposed. Electron paramagnetic resonance (EPR) and radical competitive reactions suggested that both sulfate radicals (SO4•-) and hydroxyl radicals (•OH) participated in and played important roles in the catalytic oxidation processes. Oxygen temperatureprogrammed desorption (O2-TPD) demonstrated that the PBC perovskite oxide is capable of

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facilitating easier valence-state change of the B-site cation (cobalt-ions) to mediate a redox process. Additionally, the oxygen vacancies could facilitate the bonding with PMS molecules and promote the reactivity of cobalt-ions for PMS activation. Electrochemical impedance spectroscopy (EIS) was also carried out to evidence much faster charge transfer and surface reaction rates of PBC catalyst than that of Co3O4. Additionally, the suppressed cobalt leaching was also achieved through tailoring the pH value of the reaction solution. This study provides insight of MIEC perovskites in catalytic reaction and application.

KEYWORDS: perovskite, heterogeneous catalysis, sulfate radicals, peroxymonosulfate, phenol

INTRODUCTION Energy shortage and environmental deterioration are the most important issues that seriously hinder the development of a sustainable future. Innovations in materials science and engineering play a significant role in clean energy supply and environmental protection. Perovskites or double perovskites (the derivative of perovskite) with a nominal composition of ABX3 or A2BB’X6, where A and B are typically cations with different sizes and X is an anion, have been demonstrated great potentials in the energy conversion and environmental catalysis because of their unique physico-chemical properties.1-13 For example, organic-inorganic hybrid perovskite materials such as CH3NH3PbI3 demonstrate a very high separation efficiency of electron-hole pairs, and the corresponding perovskite solar cells have reached a power conversion efficiency as high as ~20%.2,3 Many perovskite oxides show superior activities for oxygen reduction reaction (ORR) at elevated temperature and oxygen evolution reaction (OER) in alkaline solution at room temperature, and they have been intensively exploited as cathode materials for high-temperature


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solid oxide fuel cells (SOFCs) and electrocatalysts for water oxidation.4-7 Additionally, perovskite oxides as catalysts for the reduction of toxic gases such as NOx and automobile exhaust, the combustion of volatile organic compounds and methane have been extensively exploited.8-11 For the environment, conversion of contaminated wastewater to harmless and readily disposable water is of vital importance because of the fast expansion of fresh water consumption worldwide. Organic pollutants in wastewater can be introduced from a wide range of sources, including natural processes, human activities and industrial discharge, some of which are very reluctant to be digested and degraded by natural processes.14 Currently, advanced oxidation processes (AOPs) have become a promising technology to completely eliminate the hazardous organics in aqueous media. Most of AOPs are based on the generation of reactive species (e.g. O2•-, •OH and SO4•-) with high redox potentials from various superoxides, such as ozone (O3), hydrogen peroxide (H2O2), peroxydisulfate (PS), and peroxymonosulfate (PMS) for organic abatement.15-19 Although sulfate radicals (SRs) can be easily and effectively produced from PMS under the activation by cobalt ions (Co2+) in a homogeneous system, the use of cobalt in solution would inevitably introduce secondary contamination due to the toxic nature of cobalt to human beings and the eco-system, leading to several health issues such as asthma, pneumonia and other lung problems.20,21 Instead, a heterogeneous system has been demonstrated to be a promising alternative pathway, in which charge (electron) transfer occurs at the interface of the metal/oxide-based catalysts and H2O2/PMS to generate radicals such as •OH and SO4•-,22-25 and the catalyst can be recycled for multi-use. However, most of the heterogeneous catalysts showed poorer catalytic activity than Co2+ for PMS activation. An increase in specific surface area by reducing the particle size of Co3O4 nanocrystals did promote the catalytic activity;21,26 however,


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it results in the difficulty for catalyst recovery and enhanced cobalt leaching. Therefore, a novel and high-performance cobalt-based Fenton-like catalyst is urgently needed. A very recent report has pioneered the research of perovskite-like catalysts LaBO3 (B = Cu, Fe, Mn, Co, Ni) for wet oxidation of phenol with H2O2, and LaCuO3 and LaFeO3 were discovered to exhibit a modest catalytic activity for phenol oxidation due to the redox couples of Cu2+/Cu+ and Fe3+/Fe2+ in the perovskite.19 However, the heavy metal-leaching and poor stability of the catalysts in acidic solution are still needed to be improved. In this study, we reported for the first time that mixed ionic-electronic conducting (MIEC) double perovskite PrBaCo2O5+δ (PBC) that exhibited an excellent activity for ORR at elevated temperature or OER at room temperature in alkaline solution, are also turned out to be outstanding catalysts for the activation of PMS towards organic oxidation in a wide pH range. The perovskite oxides in a coarse size presented a much higher catalytic activity for catalytic phenol oxidation than the conventional nano-crystalline cobalt-based heterogeneous catalysts. In addition, the cobalt leaching problem can be greatly mediated through managing the pH value of the reaction solution at neutral and basic conditions. More importantly, we facilitate the first insight into the mechanism of PMS activation on MIEC. The oxygen vacancies were discovered to play a crucial role in activation of PMS for mediating reactive radicals. The current study opens up a new avenue for the wastewater treatment based on MIEC oxide catalysts, expanding the application of MIEC oxides from typical electrochemical energy storage and conversion to environmental remediation. EXPERIMENTAL SECTION Materials. PrBaCo2O5+δ (PBC) powder was synthesized by a combined EDTA-citric acid complexing sol-gel process. Pr(NO3)3·6H2O, Ba(NO3)2 and Co(NO3)2·6H2O (Sigma-Aldrich,


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99.9%) were used as the metal sources. Stoichiometric amounts of the metal nitrates based on the nominal composition of PBC were mixed in deionized water and heated at 100 °C under stirring. The molar ratio of total metal nitrates, EDTA and citric acid in the solution was 1:1:2. The pH value of the solution was adjusted to 6 by adding NH3·H2O to ensure complete complexation. A transparent gel attained after the evaporation of water was pre-treated at 250 °C in an oven to form a black solid precursor, and then calcined at 1000 °C for 5 h in air, resulting in the desired perovskite oxide. Commercial Co3O4 was purchased from Sigma-Aldrich Company. Two wetchemical techniques were used to synthesize nano-size Co3O4, i.e., hydrothermal synthesis and liquid-precipitation. Regarding the hydrothermal synthesis, firstly 0.93 g of Co(C2H3O2)2·4H2O was dissolved in 70 mL of deionized water, and 10 mL of NH3·H2O was added dropwise to the solution. Then the mixture was transferred to a 120 mL Teflon-lined stainless-steel autoclave. After heating at 180 °C for 12 h, the product was filtered and washed with ethanol and deionized water for several times, and then dried at 60 °C overnight for obtaining Co3O4 (H-Co3O4). As to the liquid-precipitation method, 30 mL of 4 M cobalt nitrate solution was added to 30 mL of 2 M ammonium hydrogen carbonate solution drop by drop under stirring. The temperature was maintained at 30 °C for 3 h. The resulting precipitate was then filtered, washed with deionized water for several times, and dried at 110 °C overnight. The dried sample was then calcined at 400 °C for 4 h in air for obtaining Co3O4 (P-Co3O4). Characterizations. The crystal structure of samples was determined by X-ray diffractometer (XRD, Bruker D8 Advance). The data were collected in a step-scan mode within the range of 10-90° (2θ) with intervals of 0.02°. Rietveld refinement on the XRD patterns was carried out using the GSAS-EXPGUI software. The morphology and particle size of catalysts were observed by scanning electron microscopy (SEM, Zeiss Neon 40EsB) and high-resolution


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transmission electron microscopy (HRTEM, Titan G2 80-200 TEM/STEM). The BrunauerEmmett-Teller (BET) specific surface areas of the samples were determined by N2 adsorption /desorption isotherm at the temperature of liquid nitrogen using a Micromeritics TriStar II instrument. Prior to the measurement, the samples were degassed at 200 °C for 5 h under vacuum to remove the surface adsorbed species. X-ray photoelectron spectra (XPS) were recorded using a PHI5000 VersaProbe spectrometer equipped with an Al Kα X-ray source. The oxygen temperature-programmed desorption (O2-TPD) test was performed on a homemade apparatus equipped with a mass spectrometer (MS, Hiden QIC-20). Approximately 150 mg sample was pre-treated under flowing argon with a flow rate of 15 mL min-1 at 200 °C for 1 h to remove the potential contaminants on the surface of the sample and then cooled down to room temperature. After that the temperature was increased from room temperature to 930 °C at a rate of 10 °C min-1. The effluent gases were analyzed on-line using the MS. The hydrogen temperature-programmed reduction (H2-TPR) test was carried out on a BELCAT-A apparatus (Japan). Approximately 30 mg sample was placed in a quartz reactor and outgassed at 400 °C for 1 h under a pure argon atmosphere with a flow rate of 30 ml min-1, and then cooled down to room temperature. The gas was switched to 10 vol.% H2/Ar gas mixture, and then the reactor was heated to 930 °C at a rate of 10 °C min-1. The consumption of hydrogen was monitored by an on-line TCD detector. Thermogravimetric analysis-differential scanning calorimetry (TGADSC) was performed on a Perkin Elmer thermal analyzer at the temperature range of 25 to 1000 °C under argon atmosphere with a heating rate of 10 °C min-1. Electron paramagnetic resonance (EPR) was applied to detect the free radicals during the activation of PMS, which was performed on a Bruker EMX-E spectrometer (Germany) using 5,5-dimethyl-1-pyrroline (DMPO, >99.0%) as a spin-trapping agent. The quantitative information about free radicals was gained from the


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Spin Fitting from Bruker Xenon Software Package. The concentration of cobalt-ions in filtered reaction solution was analyzed by an inductively coupled plasma-atomic emission spectrometer (ICP-AES, Optima 7000 DV, PerkinElmer, USA). The total organic carbon (TOC) was determined by a TOC analyzer (Multi N/C 3100, Germany). Electrochemical impedance spectroscopy (EIS) was conducted on the CHI 760E bipotentiostat within the frequency range from 100 kHz to 0.1 Hz using an AC voltage with 5 mV amplitude and recorded at -0.3 V vs. Ag/AgCl. Catalytic Activity Tests. The catalytic activity of catalysts was evaluated for activation of peroxymonosulfate (PMS) toward methylene blue or phenol degradation. A typical reaction was performed in a batch reactor containing 200 mL of 10 ppm methylene blue (20 ppm phenol), 0.05 g L-1 catalyst (or 0.1 g L-1 for phenol degradation) and 0.75 g L-1 PMS (or 2.0 g L-1 for phenol degradation) with stirring at 25 °C. At certain time intervals, 1 mL of aqueous solution was withdrawn using a syringe and filtered through a 0.45 µm Millipore film into a vial that was filled with 0.5 mL of methanol as a quenching agent. Then the obtained solution was analyzed for phenol by ultrahigh-performance liquid chromatography (UHPLC, Varian) with a C-18 column for separating the organics with a UV detector set at a wavelength of 270 nm. For the stability test, the spent catalyst after each run was collected by filtration, washing with deionized water and ethanol for several times and drying at 60 °C overnight for reuse. RESULTS AND DISCUSSION The PBC double perovskite, which was reported to exhibit outstanding activities for ORR at intermediate temperature and OER at room temperature in alkaline solution,27,28 was selected as a potential catalyst for heterogeneous activation of PMS for wastewater treatment in our present study. Comparative studies were also conducted by applying a commercial micron-size Co3O4


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(M-Co3O4) with a low specific surface area (SSA) of 0.65 m2 g-1, and two nano-size Co3O4 (HCo3O4 and P-Co3O4) with high SSAs of 35.3 and 40.8 m2 g-1, respectively. The XRD patterns of the different materials confirmed the formation of pure-phase double perovskite and cubic spinel structure for PBC and nano-size Co3O4, respectively (Figure S1 in Supporting Information, SI). To obtain more structural information of PBC, its XRD pattern was further analyzed via Rietveld refinement (Figure S2 in SI). All diffraction peaks can be indexed based on an orthorhombic crystal symmetry with space group P4/mmm, and no additional diffraction peaks were observed. The lattice parameters were turned out to be a = b = 3.903(6) Å and c = 7.644(6) Å, and α = β = γ = 90° with the reliability factors of Rwp = 6.00%, Rp = 4.67% and χ2 = 1.780. These data are in good agreement with the literature results.28,29 The SSA of PBC was found to be 0.46 m2 g-1, which is similar to that of M-Co3O4. The particulate morphology was then examined by SEM/TEM with the typical images shown in Figure S3 (SI). The PBC had a large grain size of 210 µm, while M-Co3O4, H-Co3O4 and P-Co3O4 showed a particle size of 1-3 µm, ~50 nm and 2050 nm, respectively.

Figure 1. Catalytic degradation of phenol on various catalysts.


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The catalytic performance of PBC was first investigated for the oxidative degradation of methylene blue (MB), which is a typical organic waste from dyeing industry. As shown in Figure S4 (SI), only ~15 min was required for achieving 100% MB degradation using PBC as the catalyst, although it is in a coarse size with a low SSA. As to M-Co3O4 that has a similar SSA to that of PBC, there was still more than 40% of MB in the solution not successfully decomposed after the continuous reaction for more than 40 min. To further explore the capability of MIEC oxides for removing other organics, phenol, a toxic and reluctant organic with a stable molecular structure, was also tested for the catalytic oxidation. As shown in Figure 1, PBC provided less than 5% of phenol adsorption, indicating negligible phenol removal by adsorption. Without any catalyst, PMS itself could hardly decompose the phenol. Only 5% of phenol was removed after continuous operation for 2 h. In contrast, it only took approximately 20 min to completely remove 20 ppm phenol by PBC in solution. M-Co3O4 could not accelerate the degradation of phenol during the continuous reaction of 2 h. The decrease of the particle size is usually utilized as an effective strategy for promoting the catalytic activity of cobalt oxides owing to the increased surface area and more exposed active sites.21,26 However, it still took more than 2 h to realize complete phenol degradation by H-Co3O4 and P-Co3O4 under the same reaction conditions. It further demonstrates the superior activity of MIEC perovskite oxides for the AOPs in water remediation. The intrinsic activity, namely, the turnover frequency (TOF) per active site, is an important factor for dominating the performance of the catalysts, which was estimated based on the assumption that every cobalt atom is catalytically active. The PBC catalyzed the phenol oxidation with a TOF ~196-fold higher than the H-Co3O4 catalyst, indicating that the PBC had intrinsically much better activity than the classical nano-sized Co3O4 catalyst for the degradation of phenol. Homogenous Co2+ activator shows the highest reported activity for the


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degradation of organic wastes up to now,30 and most of the metal cobalt-ions would take part in the catalytic reactions. As a comparison, given very limited cobalt-ions locating on the surface of bulk PBC as the active sites for activation of PMS, PBC catalyst should possess much higher intrinsic activity in catalytic PMS activation for organic degradation. In the homogeneous catalytic reaction, the effect of mass transfer is mostly negligible because reactants, products and catalyst are in the same phase. In contrast, mass transfer or diffusion limitations play a key role in the heterogeneous catalytic system. In order to reveal the practical catalytic activity of PBC, internal and external mass transfer limitations were examined by estimating PBC catalyst in different sizes and adjusting stirring speed of reaction solutions. As shown in Figure S5 (SI), there was almost no influence on the phenol oxidative efficiency regardless of the catalyst size or mixability of solid catalyst and reactants, implying no significant mass transfer limitations in the PBC/PMS system. To estimate the activity of perovskite oxides for advanced oxidation, a comparative study about the reaction kinetics of phenol degradation over the PBC, H-Co3O4 and P-Co3O4 catalysts was conducted. The responses of ln(C/C0) to the reaction time were fit based on the pseudo-firstorder kinetics:

ln = −

Eq (1)

where C0 and C are the concentrations of phenol at the beginning and time t, respectively, and k is the apparent reaction rate constant. As shown in Figure S6 (SI), almost linear responses of ln(C/C0) to the reaction time were observed for phenol degradation with high values of regression coefficients (R2). It implies the phenol degradation over these catalysts followed the first-order reaction kinetics. For the phenol degradation, the rate constants of PBC, H-Co3O4 and


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P-Co3O4 are 0.144, 0.016 and 0.010 min-1, respectively, indicating the reaction rate over PBC catalyst was 9 times and 14 times higher than that over H-Co3O4 and P-Co3O4, respectively. In addition to PBC, many other MIEC perovskite oxides, such as Ba0.5Sr0.5Co0.8Fe0.2O3-δ, SrCo0.9Ti0.1O3-δ, SrNb0.1Co0.7Fe0.2O3-δ and BaCo0.7Fe0.2Sn0.1O3-δ, also demonstrated outstanding activities for catalytic phenol oxidation (Figure S7 in SI). It is worthwhile noting that these perovskite oxides also showed high activities for ORR and OER.31-34 Since the crucial step of PMS (HO-OSO3-) activation and oxygen reduction lies in the activation and cleavage of O-O bond, it may imply the ultimate relevance between PMS activation and oxygen activation over the perovskite oxides. It is generally recognized that the PMS activation and formation of SO4•- and/or •OH radicals play a crucial role in effective oxidation of organic wastes in an aqueous solution, as the generated reactive species possess high capability for the deep oxidation of organic contaminants into harmless mineralized products, carbon dioxide and water.35,36 In order to identify the reactive species generated during the PMS activation, the in situ EPR test was conducted to capture the free radicals in PBC/PMS system. As shown in Figure 2a, both SO4•- and •OH were detected in the presence of PBC catalyst, while they were hardly monitored in PMS only. It strongly suggested the high catalytic activity of PBC for PMS activation. Radical competitive reactions were also applied to probe the dominant reactive species of SO4•- and •OH radicals in PBC/PMS system. Ethanol (EtOH) and tert-butyl alcohol (TBA) were chosen as quenching agents to screen SO4•- and •OH, respectively, as EtOH (with α-H) is able to react with both SO4•and •OH readily, while TBA (without α-H) mainly reacts with OH• and is inert to SO4•-.37,38 As shown in Figure 2b, the reaction rate remarkably decreased by 45% when 0.2 M EtOH was added in the original reaction solution. With the addition of 0.2 M TBA, the decrease of less than


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12% in reaction rate was observed. It suggests both SO4•- and •OH radicals were generated in the PMS activation process and SO4•- was the principal specie. It further demonstrates that the MIEC perovskite oxides can mediate PMS to produce both SO4•- and •OH radicals.

Figure 2. a) EPR spectra of PMS activation with PBC (●: DMPO-OH, ♦: DMPO-SO4, ♥: oxidized impurities); b) Effect of quenching agents on phenol degradation in PBC/PMS system. Inset: the changes of k with and without quenching agents of EtOH (0.2 M) and TBA (0.2 M). Reaction conditions: [phenol]0 = 0.02 g L-1, catalyst loading = 0.1 g L-1, PMS loading = 2 g L-1, temperature = 25 °C. In previous studies, the mechanism for organics oxidation by free radicals from heterogeneously cobalt-mediated decomposition of PMS was proposed as follows.39-41 → •

Eq (2)

→ •

Eq (3)

• → !… #$ %& … ' →

Eq (4)

According to above reaction mechanism, PMS tends to bond with the cobalt sites and the variation in oxidation state of the cobalt ions (Co2+/Co3+) is experienced during the activation of


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PMS. In order to achieve a high activity for PMS activation, the energy gap between Co2+ and Co3+ might be as small as possible to allow the facile transition in oxidation state of the cobalt ions to facilitate a redox cycle with PMS molecules to produce reactive radicals. Herein, the PMS activation on the Co3O4 crystal is more favored at edges/corners with a higher chemical activity and {001} facets that possess only Co2+ cations.42 The main difference between MIEC perovskite oxides and spinel Co3O4 is that the MIEC perovskite oxides contain a high concentration of oxygen vacancies and possess a high oxygen diffusivity and electronic conductivity. The electronic conductivity of perovskites is originated from the electron hoping between the B-site cation and oxygen-ion.43 The MIEC perovskite oxides are capable of facilitating easier valence-state change of the B-site cation(s) without phase transition. The excellent redox property of the MIEC perovskite oxides contributes to the superb catalytic activity for the electron-transfer between PMS and PBC catalyst.

Figure 3. O2-TPD profiles of PBC and Co3O4 catalysts. The high redox property of cobalt-ions and oxygen mobility in PBC were supported by O2-TPD study. With the programmed increase of temperature, cobalt-ions underwent a thermal-


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reduction process, accompanying by the release of O2. The starting temperature for O2 release is an important parameter to evaluate the easiness of oxidation state change of cobalt-ions in an oxide. As shown in Figure 3, the O2 release appeared at ~800 °C for Co3O4, implying the Co3+ in Co3O4 could be thermally reduced to Co2+ mainly at a temperature higher than 800 °C. As to PBC, the O2 release started at a temperature as low as 300 °C, approximately 500 °C lower than that of Co3O4, indicating the high valence state cobalt-ions in PBC was much easier to be reduced to a lower oxidation state than that in Co3O4. The excellent redox potential of cobaltions in PBC was further demonstrated by H2-TPR result (Figure S8 in SI). The easy transition in oxidation state of the cobalt-ions in PBC accounts for its good performance for the activation of PMS. It is well known that PBC is an oxide with extraordinary high oxygen surface exchange kinetics.29 The easy transition in oxidation state of cobalt ions in PBC may be closely related to the high oxygen surface exchange kinetics of PBC oxide, which was determined from EIS measurements (see a detailed discussion below).

Figure 4. XRD patterns of a) Co3O4 and b) PBC before and after the O2-TPD process.


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To ensure the easy variation of oxidation state of cobalt-ions, the phase transition should be avoided because it usually requires a high activation energy and results in an inferior catalytic activity. As a homogeneous catalyst, phase transition and deactivation is not applicable for Co2+/Co3+-based system in solution. To further get information about the phase transition related to the change in oxidation states of cobalt in Co3O4 and PBC, both samples after the O2-TPD experiments were subjected to XRD characterization. Co3O4 was almost fully transferred from spinel-type lattice structure to cubic CoO with a space group of Fm-3m after the O2-TPD test (Figure 4a), while there was no phase transition occurred in PBC during the test (Figure 4b). The results were also confirmed by TGA-DSC (Figure S9 in SI). The XRD patterns of PBC after the O2-TPD process can be refined well based on space group P4/mmm without any secondary phase with low reliability factors (Figure S10 in SI). The lattice parameters were a = b = 3.911(2) Å and c = 7.630(2) Å. As compared to the PBC before the O2-TPD experiment (the lattice parameters were a = b = 3.903(6) Å and c = 7.644(6) Å), the only difference was that a slight lattice expansion was experienced after the release of lattice oxygen from the bulk of PBC. Therefore, PBC possesses a robust structure and is free from phase transition in even harsh conditions, ensuring the superb catalytic performance for PMS activation without severe passivation from structure transition.


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Figure 5. EIS of PBC and Co3O4 catalysts recorded at -0.3 V vs Ag/AgCl under the influence of an AC voltage of 5 mV in PMS and Na2SO4 solution. [Na2SO4] = 0.1 M, [PMS] = 6.5 mM (2 g L-1). Representative frequencies in each curve are marked with solid points, and the unit of frequency is Hz. Inset: magnified EIS at high frequency. Another big difference between PBC perovskite oxide and Co3O4 spinel is that the former is a good oxygen-ion conductor, in which the transition in oxidation state of surface cobalt may be quickly recovered through the diffusion of electron and oxygen-ions from the oxide bulk. However, such oxygen-ion conductivity from the inner core of PBC could only be predominant at high temperatures, and the migration of oxygen-ions may be insignificant at room temperature in aqueous solution. Hence, only oxygen-ions and electrons in bulk layers near PBC surface would take part in the alteration of cobalt valence state in activation of PMS, which may partially contribute to the catalytic activity of PBC to some extents. However, Co3O4 possesses a negligible oxygen-ion conductivity,44 in which only the surface would be involved in the reaction. Such a unique property of PBC further enhanced its catalytic activity for PMS activation. During the heterogeneous activation of PMS, PMS was first adsorbed over the


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catalyst surface, then charge transfer occurs between the active sites and PMS to generate sulfate and hydroxyl radicals, which are subsequently desorbed from the surface into aqueous solution for attacking and oxidizing the organics. Meanwhile, the elevated metal-site may undergo a redox cycle (Mn+1/Mn+) to produce persulfate radicals (Eq 3) to retain the active centers. Therefore, the bonding capability and charge transport at the interfaces of PMS and catalyst are two factors for an efficient catalyst toward the PMS activation. EIS technique was used to provide insight into the kinetics of PMS activation. Usually, the arcs of EIS at high and low frequencies can be regarded as the results of charge transfer and surface process, respectively.45 As shown in Figure 5, the diameter of the semicircle from PBC was much smaller than that of Co3O4, suggesting much faster charge transfer and surface reaction rate for PBC catalyst.46 Sabatier’s principle qualitatively describes that the high catalytic activity of the catalyst can be obtained when the binding strength between the adsorbate and the catalyst is neither too strong nor too weak.47 For the activation of oxygen in alkaline media, it was pointed out that the binding strength between the perovskite catalyst and oxygen is closely related to the σ* orbital occupancy. The eg occupancy of surface cation close to unity could result in the best activity for OER and ORR.28,30,48,49 The same indicator should also be applicable for the activation of PMS. It was reported the eg filling of cobalt ion is ~1.25 in PBC,27 while the theoretical eg value was calculated to be 1.33 for Co3O4. The more proper σ* orbital occupancy for the cobalt ion in PBC may further contribute to its better activity for wastewater treatment than that of Co3O4.


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Figure 6. XPS spectra of a) overall survey, b) Co 2p, c) Co 3p and d) O 1s on the PBC before and after the catalytic degradation of phenol. In order to further comprehend the activation mechanism of PMS on PBC catalyst, XPS analysis was then performed to measure the oxidation states of surface cobalt and the composition of surface oxygen species. The overall XPS survey spectra verify the co-existence of Pr, Ba, Co and O elements in the fresh and used PBC catalysts (Figure 6a). Pr3+ was the predominant oxidation state in both the samples (Figure S11 in SI). Figure 6b shows the highresolution XPS spectra of the Co 2p core-levels on the PBC surface before and after the


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oxidation process. Because of strong overlap between Co 2p and Ba 3d XPS peaks, it is difficult to identify the valence state of surface Co by fitting Co 2p peaks. The shoulder peaks at binding energies (BEs) of ~778 and ~793 eV which possibly come from the interrelation between Ba cations and oxygen-ions would also interfere the analysis of Co 2p.50,51 However, the existence of a small amount of Co2+ in fresh PBC could be inferred from the shake-up satellite peak at the BE of 803.8 eV, which is diagnostic signal of Co2+.52 After the catalytic reaction, the positions of characteristic peaks shifted to higher BEs due to the partial oxidation of surface Co2+ on the PBC surface, suggesting Co2+ is the active site for PMS activation.53 It has been demonstrated that there are various Co valence states including +2, +3 and +4 in LnBaCo2O5+δ (Ln = rare earth; 0 ≤ δ ≤ 1) double perovskites depending on the value of δ.54,55 The oxygen content of the fresh PBC was determined by an iodometric titration technique, which was found to be δ = 0.79, and thus the average valence of cobalt was calculated to be +3.29. It signifies that Co was mainly present in +3 oxidation state, together with +4 and +2 states in the PBC, which was further supported by XPS spectra of the Co 3p as shown in Figure 6c. A main line at the BE of 61 eV was observed for both the samples, indicating the presence of Co3+ in the PBC before and after the PMS activation. As to the fresh PBC, the main peak broadened toward higher BE, implying the partial oxidation of surface Co3+ to Co4+,56 while the main peak in Co 3p for the used PBC was more symmetrical compared to the fresh PBC due to the reduced Co4+ in the spent PBC. Based on the aforementioned analysis, we tentatively proposed that the redox processes of Co2+‒Co3+‒Co2+ and Co4+‒Co3+ were involved during the PMS oxidation. Figure 6d shows the XPS spectra of O 1s core levels of fresh and used PBC. Two features were present at BEs of ~528.5 and ~531.5 eV, respectively, which can be ascribed to the lattice oxygen species (O2-) and less electron-rich oxygen species correspondingly. The peak at a higher BE can be deconvoluted to three


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characteristic peaks located at ~529.6, ~531.2 and ~532.9 eV, which were assigned to highly oxidative oxygen species in the form of O22-/O-, hydroxyl groups (-OH) or the surface adsorbed oxygen, and adsorbed molecular water or carbonates, respectively.57,58 The relative concentrations of various oxygen species were estimated from the relative area of fitting peaks are listed in Table S1 (SI). It has been reported that hydroxyl groups are important active oxygen species on the surface of metal oxides which would facilitate the chemical binding with PMS via M-O-H—H-O-SO3 and facilitate the electron transfer from M2+ to PMS via inner-sphere interacition.18 The relative contents of O22-/O- and -OH species decreased after the catalytic reaction, indicating that hydroxyl radicals might be involved in the catalytic reaction accompanying with the redox of cobalt-ions. The catalytic activity of PBC double perovskite was further enhanced due to the active oxygen sites. An increased amount of adsorbed molecular water or carbonates may be derived from the adsorption of the organic intermediates and mineralized products in the aqueous solution.

Figure 7. Proposed mechanism of PMS activation and phenol oxidation on PBC catalyst. Based on above analysis, a mechanism for the high activity of PBC for wastewater treatment is proposed as follows (Eq (5)-(8)) and is also schematically shown in Figure 7. By


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applying PBC mixed conducting oxide, once the surface cobalt-ions were reduced or oxidized, they could be fast regenerated by the charge transfer from the bulk of the oxide due to its mixed conductivity properties. Besides, the abundant oxygen vacancies of double perovskite would further influence the redox processes for the interaction between PBC and PMS. The oxygen vacancies on the PBC surface can work as defective sites to facilitate the chemical bonding with PMS molecules and fulfil the redox cycle of Co3+ to Co2+ (Eq 7). More importantly, the existence of oxygen vacancies would pronouncedly impact the electron asymmetry chemical states of cobalt atoms, leading to enhanced catalytic sites toward PMS activation. Therefore, high oxygen surface exchange kinetics and electronic conductivity, more active cobalt sites, and robust phase stability during the oxidation state transition of cobalt-ions synergistically contribute to the high activity of PBC for the promoted PMS activation and consequently leading to the deep oxidation of organics. ()•• → • )×

Eq (5)

()•• → • 2 )×

Eq (6)

)× → • ()••

Eq (7)

• ( • - • ) / → %#%-% →

Eq (8)

where ()•• notes double charged oxygen vacancy in bulk and/or surface of perovskite; )× is oxygen ion in a normal oxygen site. Firstly, SO4•- and •OH were generated simultaneously by the activation of PMS in the presence of PBC catalyst. Most produced SO4•- and •OH reacted with phenol rapidly due to their high redox potentials (SO4•-: E0 = 2.5-3.1 V, •OH: E0 = 1.8-2.7 V).59,60 Moreover, SO5•- with a lower redox potential (E0 = 1.1 V),60 may also contribute to the oxidation of phenol and the mineralized intermediates.


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Figure 8. Phenol degradation on PBC at different runs after repeated uses. For practical application, a good catalytic stability and easy regeneration of the catalyst is highly desired for industrial applications. The rapidly oxidative degradation of organic contaminants in wastewater is highly expected under complex conditions. Figure 8 shows the stability tests of PBC with a lower loading (0.05 g L-1) for the phenol degradation in three runs. As can be seen, the PBC still delivered an excellent activity although the catalyst loading decreased by half, which completely removed phenol within 35 min. The excellent phenol removal efficiency was also achieved in the second run without much deactivation. In the third run, the catalytic activity decreased somewhat, but it still presented a favorable catalytic activity, allowing 100% oxidation of phenol in 45 min, compared with 30 min in the first run. Similar catalytic stability was also demonstrated in MB degradation using PBC catalyst (Figure S12 in SI). Surface contamination and coverage/change of the active sites in catalytic reactions would give rise to the decreased catalytic performance.61,62 The regeneration of the catalyst can be realized by simply re-calcining the used PBC catalyst at 1000 °C in air to burning out any possible surface contaminated organic species, and to restore the double perovskite structure


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after the wastewater treatment. It should be mentioned that, in principle, the re-calcination may also be applied to the regeneration of Co3O4 catalyst, the Co3O4 nanoparticles would be sintered to lose their high specific surface area during the high-temperature calcination as a trade-off. This is not the case for PBC, because it has already been a sintered oxide with highly thermal stability. Cobalt leaching is an important concern for the cobalt-based heterogeneous catalysts in water treatment. After the first run, the cobalt-ion concentration in the reaction solution was 9.8 mg L-1 according to ICP measurement, suggesting cobalt leaching occurred on the PBC perovskite oxide. PBC is basic in nature, while it was found that the solution is quickly acidized, possibly due to the decomposition of PMS and oxidation of phenol into mineralized by-products and small molecular acids via a series of H-abstraction, addition, and ring-opening reactions. We then investigated the cobalt leaching impact on PBC catalysis in the reaction solutions with different initial acid/basic conditions tailored by adding H2SO4 or KOH (Note: buffered solution was not applied here due to the radical-scavenger nature of the buffer agents). The initial pH range of reaction solutions stretched from 2 to 9.

Figure 9. Catalytic degradation of phenol on PBC catalyst in different reaction solutions with a) different initial pH values and b) constant pH values. Both of insets: the concentration of cobalt-


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ion in the different reaction solutions after the tests. Reaction conditions: [phenol]0 = 0.02 g L-1, catalyst loading = 0.1 g L-1, PMS loading = 2 g L-1, temperature = 25 °C. As shown in Figure 9a, the reaction rate of phenol oxidation increased with the increment of pH value, particularly the efficiency was greatly enhanced under basic condition. Complete phenol removal was achieved within 10 min. After the catalytic oxidation of 2 h, the total organic removal was attained at 39.3%, 63.1%, 72.7% and 82.0% under initial pH values of 2, 4, 6 and 9, respectively. It suggests that most of phenol decomposed into mineralized intermediates and harmless small molecules (e.g. CO2, H2O) in the neutral and basic solutions. As it is listed in the inset of Figure 9a, the cobalt leaching is significantly mediated from acid conditions (pH = 2 ~ 6) by initially tailoring the solution to a basic environment (pHinitial = 9). The losing of cobalt-ions would change the surface composition of perovskite, giving rise to descending activity of the catalyst. A relatively basic condition contributes to maintaining the surficial chemistry of PBC with a more robust framework. Besides, the mild base activation of PMS (Figure S13 in SI) and formation of phenoate species may also influence the phenol decomposition. Since the solution tended to be acidized during the reaction proceeding, we controlled the pH value of reaction solution at a neutral condition of 7 or 6 during the whole phenol oxidation process. As shown in Figure 9b, no notable effect occurred on the catalytic activity of PBC while the concentration of leached cobalt-ions dramatically decreased to 1.7 mg L-1 in the reaction solution (pHconstant = 7) compared to the uncontrolled ambience (9.8 mg L-1). Besides, a high TOC removal of 88.6% was achieved. It suggests that, by stabilizing the solution pH at a neutral condition, high catalytic activity of PBC and suppressed toxic metal leaching can be reached simultaneously by applying coarse-type cobalt-based MIEC perovskite oxides as the catalyst for facile and green remediation of contaminated wastewater. Compared with other Fenton-like reaction systems, PBC/PMS


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exhibits the advantages in treatment of a wide range of wastewaters, particularly for strong basic solutions without acidification, metal leaching and low efficiency. CONCLUSIONS In summary, PBC double perovskite oxide, a well-known composite with mixed conductivity, was successfully applied as an effective catalyst to activate PMS for MB and phenol degradation with a superior catalytic activity. Organic waste (MB or phenol) removal at 100% was achieved within only around 15 min, which outperformed the popular Co3O4 catalyst. The excellent activity of PBC for organics degradation attributes to superior oxygen surface exchange kinetics, abundant oxygen vacancies, high electronic conductivity, more active cobalt sites, and robust phase stability during the oxidation state transition of cobalt-ions. Both of SO4•- and OH• reactive radicals were produced during the activation process, and SO4•- was the major radical for promoting the degradation of organic pollutants. Our study broadens the application of MIEC perovskite oxides to the field of environmental remediation, meanwhile develops a new type of catalysts with outstanding performances for persulfate activation and catalytic oxidation of organics. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. XRD patterns of PBC, H-Co3O4 and P-Co3O4; Rietveld refinement of XRD patterns for fresh PBC and after the O2-TPD test; SEM images of PBC and M-Co3O4; TEM images of H-Co3O4 and P-Co3O4; H2-TPR profile of PBC; TGA-DSC curves of Co3O4 and PBC; XPS spectra of Pr 3d on PBC before and after phenol degradation test; the relative concentrations of surface oxygen species; iodometric titration technique; methylene blue and


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phenol oxidation on various catalysts; first order kinetic model of phenol oxidation on PBC, HCo3O4 and P-Co3O4; the stability test of PBC for methylene blue degradation; the degradation of phenol without catalyst or PMS (the initial pH value of reaction solution was 9); the calculations of turnover frequency. AUTHOR INFORMATION Corresponding Author *W.Z.: [email protected] *S.W.: [email protected] Author Contributions #

These authors contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work was supported by the Australian Research Council Discovery Project grants DP150104365 and DP160104835. Dr. Chao Su acknowledges Curtin University for a postdoctoral fellowship. The authors would like to thank the facilities, scientific and technical assistance of the Curtin University Electron Microscope Facility and X-ray Laboratory, both of which are partially funded by the University, State and Commonwealth Governments. REFERENCES (1) Royer S.; Duprez D.; Can F.; Courtois X.; Batiot-Dupeyrat C.; Laassiri S.; Alamdari H. Chem. Rev. 2014, 114, 10292-10368.


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Mixed Conducting Perovskite Materials as Superior Catalysts for Fast

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