A perspective on catalytic oxidative processes for sustainable water

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Kinetics, Catalysis, and Reaction Engineering th

110 Anniversary: A perspective on catalytic oxidative processes for sustainable water remediation Mayfair C Kung, Junqing Ye, and Harold H Kung Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b04581 • Publication Date (Web): 25 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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Industrial & Engineering Chemistry Research

110th Anniversary: A perspective on catalytic oxidative processes for sustainable water remediation Mayfair C. Kung1*, Junqing Ye1,2 and Harold H. Kung1* 1

Chemical and Biological Engineering Department, Northwestern University, Evanston, IL 60208, USA. 2

College of Science, China University of Petroleum, Beijing, China.

*[email protected] and *[email protected] Abstract Catalytic oxidation of organic pollutants is an attractive and sustainable method of water purification. This paper focuses on discussion of catalytic activity, selectivity, and stability in Catalytic Wet Air Oxidation and Advanced Oxidation Processes. The emphasis is on exploring the potential of applying relevant catalytic knowledge and information outside the field of water remediation, such as in aqueous phase biomass processing and catalyst design, toward meeting the challenges in these processes. One example is to explore utilizing the interfacial perimeter sites of a supported metal catalyst to modify the catalytic properties. Another example is to improve catalyst stability from metal leaching by using overcoats of oxide or carbon and by understanding the phase transformation of the supported oxide. This paper also examines the prospect of harvesting the chemical energy stored in the organic contaminants in polluted water to catalytically generate H2O2 in situ for the Advanced Oxidation Process, thereby eliminating the use of costly oxidant. Keywords: catalytic wet air oxidation, advanced oxidation process, CWAO, AOP, activity and stability, biomass, interfacial site catalysis and in-situ H2O2 generation.

I.

Introduction

Water is the essence of life on earth, and is at the core of sustainable development.1 The United Nations has recognized that water resources, and the range of services they provide, underpin poverty reduction, economic growth and environmental sustainability. From food and energy security to human and environmental health, water contributes to improvements in social wellbeing and inclusive growth, affecting the livelihoods of billions.1 Yet according to World Water Development Report,2 two thirds of the world’s population currently live in areas that experience water scarcity for at least one month a year, and about 500 million people live in areas where water consumption exceeds the locally renewable water resources by a factor of two. It is likely that the situation will worsen as the world population continues to increase. This United Nations Report advocates more effective use of waste water as a source of water for human consumption. Furthermore, reducing the volume of discharge of untreated sewage, agricultural runoff and inadequately treated industrial wastewater will help mitigate degradation of water quality around the world, particularly in resource-poor countries and in dry areas, 1 ACS Paragon Plus Environment

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improve human health and ecosystems, lessen water scarcity, and help toward attaining the goal of sustainable economic development.2 Many water treatment technologies have been deployed in the developed countries. The UN estimates that treatment of municipal and industrial wastewater in the developed countries is around 70% but is as low as 8% in the poorly developed countries. Globally, over 80% of all wastewater is discharged without treatment.2 From these statistics, it is clear that low-cost, effective, and robust waste water treatment technology is necessary for meaningful contributions to solving the global water scarcity problem. Catalytic water treatment technology has the potential to be a viable solution. There has been extensive effort expanded in this area of research as attested by the numerous published reviews.3-8 The volume of work already done makes it difficult to provide a comprehensive review of the current advances. Thus, this paper is structured to focus on some critical issues involved in oxidative catalytic water treatment and exploration of potential solutions to these issues with an eye towards integrating knowledge not confined in the field of water remediation. Two oxidative modes in catalytic water remediation will be covered: catalytic wet air oxidation (CWAO) usually applied to treat wastewater with higher pollutant contents and advanced oxidation processes (AOP) for water with lower pollutant contents. Typically, the latter incurs substantially higher expenses because of the cost of reactive oxidants needed for this process. In general, CWAO operates in the presence of a catalyst at moderate temperatures (125-320 oC) and with O2 pressure between 5-20 bars,9 whereas by utilizing reactive oxidants to mineralize the pollutants, AOP is able to operate much closer to ambient conditions than CWAO. For both processes, the critical catalytic issues involve catalytic activity, selectivity, and stability. A more active and selective catalyst in CWAO could result in milder reaction conditions for remediation, whereas a more selective catalyst for the AOP process could result in more efficient utilization of the expensive oxidant. Catalyst deactivation adds to operating costs, and is often a result of carbonaceous deposit, metal leaching, as well as sintering of metal particles. Instead of summarizing existing data on these issues, this paper focuses on perspectives of catalyst designs to improve catalyst activity, selectivity, and stability, drawing from examples not only from this area of study but also from other well-studied relevant areas of catalysis that had not been applied to water remediation. In addition to these catalytic issues, this paper also examines the perspective of harvesting the chemical energy stored in the organic contaminants in polluted water by using them to catalytically generate oxidants in situ for the AOP process, thereby eliminating the need for costly oxidants. This approach differs from the more developed electrocatalytic technology or the widely practiced photocatalytic technology in that it has the potential to be self-sustaining, i.e. potentially not requiring energy or material input but utilizing the energy released in the reactions of the organic contaminant to overcome the activation barriers of the degradation reactions. It can be practiced using very simple reactor configurations, and there is tremendous knowledge in the operation, design, control, and maintenance of this type of systems from the fuels and chemicals productions industry, although adaptation to an aqueous environment and low concentrations of reactants are needed. 2 ACS Paragon Plus Environment

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II.

Catalyst activity and selectivity

II.a CWAO catalysts CWAO is a technology for treating waste streams with relatively high contents of organic pollutants that are common in industrial settings.10 Supported metal catalysts are often used. Published literature focuses mainly on relating the catalytic activity and selectivity to the chemical nature and composition of the metal particles and the metal particle size and size distribution. It is common to observe that, for a certain metal, there is an optimal crystallite size for high turnover frequencies in the mineralization of acetic acid.11 The oxidation state of the metal also affects the activity. Tran, et al. found that a Au/CeO2 pretreated to result in a higher fraction of metallic Au was more active in the CWAO of model carboxylic acids.12 Use of bimetallic catalysts was reported to be advantageous compared with monometallic catalysts in some instances, especially when the pollutant contains heteroatoms such as nitrogen. Barbier, et al. conducted a detailed study of the mineralization of aniline, a nitrogenous pollutant.9 Although Ru/CeO2 was active and selective for the conversion of aniline to CO2, its activity was poor in the conversion of NH4+, an intermediate in the mineralization reaction, to the desired N2 product. The activity and selectivity to convert NH4+ to N2 improved by forming a bimetallic Pd50Ru50/CeO2 catalyst with the same total metal weight loading as Ru/CeO2. Other bimetallic systems showed a similar behavior. Song et al. found that an Al2O3–ZrO2 supported Pt-Ru catalyst was more active and selective than the monometallic Pt or Ru catalyst in the mineralization of methylamine.13 The enhancement in activity and selectivity was attributed to the fact that the interaction of the two metals stabilized the metallic state for both components against oxidation and increased the metal particle dispersion. In addition, Pt also was found to be inactive in C-H bond breaking whereas Ru was active. Conversely, Ru was not active in NH4+ oxidation whereas Pt was able to catalyze this reaction. There are other strategies that have yet to be explored to modify the catalytic property of a metal on a support beyond manipulating the metal composition and particle size. One strategy of interest is to utilize the exposed sites at the interface between a metal and a metal oxide in contact with each other (also known as interfacial perimeter sites, which for simplicity are referred to as interfacial sites) as catalytic active sites. Recent advances in catalytic research have shown that such interfacial sites are important in many catalytic reactions.14 For example, the interface between Cu metal and ZnO is important in CO2 hydrogenation,15 the Pt-FexOy interface catalyzes CO oxidation,16 the Pt-CeO217 and the Ni-TiO2 interfaces18 play significant roles in effecting the water gas shift reaction, and the Au-TiO2 interface is active for acetic acid oxidation 19 and in isopropanol decomposition.20 There have been very few reports that applied the interfacial sites for water remediation reactions. One interesting observation in this area was that Ru supported on high surface area ceria (CeO2-160) was found to be a very active catalyst for the total oxidation of acetic acid, despite the large Ru metal particle size present.21 Lafaye, et al.7 suggested that the high activity of this sample was because the large Ru particles were surrounded by small nanoclusters of ceria, reminiscent of the inverse ceria-metal catalyst systems (i.e. ceria nanoclusters deposited on metal particles, a catalytic structure we will elaborate on later) that had demonstrated high activities in several other reactions.22 They 3 ACS Paragon Plus Environment


indicated that their proposal7 was in accordance with the observation of Hosokawa et al.23 who proposed that the active site for the catalytic oxidation of acetic acid was the interfacial Ru-O-Ce site. Based on extensive characterization results, Posada et al. proposed that the metal-oxide interfacial sites of Cu/CeO2 catalysts were the catalytically relevant sites in the CWAO of substituted phenols.24 The few available examples cited above suggest the opportunity of applying and manipulating interfacial active sites for water remediation. Insights from the study of catalysis with interfacial active sites may help understand the reaction mechanism and improve catalyst design. For example, in the oxidation of acetic acid over Au/TiO2 catalysts, Green, et al.19, 25 proposed that acetate is adsorbed on TiO2 and C-H bond scission is facilitated by oxygen atom generated at the Au-Ti dual site. Subsequent to the C−H bond scission, the Au−C bonds start to form, which in turn activates the C−O bond of the intermediate coordinated to the Ti cationic sites leading to the formation of a gold ketenylidene (Au2=C=C=O) intermediate (Figure 1). This proposed intermediate had been verified by FTIR and isotope labeling studies. Such an intermediate may have bearings in CWAO of acids as carboxylic acids are difficulty to mineralize but are often intermediates formed from organic compound degradation.26

Figure 1. Schematic showing formation of Au ketenylidene intermediate at the dual perimeter site of Au/TiO2 catalyst during aerobic oxidation of acetic acid.19 Knowledge from interfacial site catalytic studies can be used to help design better CWAO catalysts. In conventional supported metal catalysts, the quantity of interfacial sites is usually low because large metal particles have small amount of interfacial sites and the density of very small metal nanoparticles is small although the fraction of their atoms at the interface is large.27 Synthesis of a high density of very small nanoparticles and maintaining the high metal dispersion are difficult for most systems. One strategy to access catalysts with high densities of metal/metal oxide interfacial sites is to use inverse catalysts, which is the approach alluded to in the CWAO of acetic acid using Ru-CeO2-160 catalyst, vida supra. The common supported metal catalysts are fabricated with the metal particles residing on an oxide support. In an inverse catalyst, metal oxide nanoclusters of catalytic relevance decorate metal nanoparticles residing on an inert support (Figure 2)28 generating high densities of interfacial sites whose properties can be easily modified. An example of the potential of tuning interfacial properties can be deduced from studies of metal supported on ceria. It is well established that the catalytic properties of metal supported on ceria are strongly influenced by the crystal shape29-31 and size32 of ceria. This strong dependence on support morphology has been ascribed to factors such as different redox behavior of different crystal planes, relaxation of the lattice with decreasing oxide size, changes in electrostatic force affecting the relative stability of the cations at different oxidation states and 4 ACS Paragon Plus Environment

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consequently vacancy densities. Thus, it is likely changing the oxide cluster size, morphology, and crystallinity would affect the properties of the interfacial sites, offering the system a unique set of tunability. There are many synthetic approaches to form a variety of small metal oxo clusters,28, 33-35 making inverse catalyst preparation a versatile method for creating catalysts possessing high densities of interfacial active sites Knowledge accumulated from studies in inverse catalyst preparation are highly useful. For example, an active interfacial site must be accessible to the aqueous medium to be useful. Thus, an optimal inverse catalyst would have a partial surface coverage of the metal by the decorating oxide nanoclusters, or the metal oxide overlayer needs to have sufficient porosity for rapid diffusion of the reactants. This is exemplified in one published work, in which Mullins and coworkers decorated a Au(111) surface with Fe2O3 clusters.36 They observed that both a clean surface and one covered with a complete layer of Fe2O3 were inactive for CO oxidation, but a partially covered surface was active, with the maximum activity observed at roughly 0.5 monolayer equivalent coverage.

Figure 2. A schematic diagram depicting metal oxide clusters (MOX)-decorated Au particles on a support in an inverse catalyst. Reactant A is activated on the metal atoms, while B is activated by a MOx cluster. Reaction proceeds at the interface to form C.

IIb. AOP catalysts AOP is usually applied to treat water with relatively low chemical oxygen demand (COD < 5g/l) due to the high cost of the oxidants or the energy needed to generate the oxidants.37 Much of the AOP literature focuses on maximizing the removal of the pollutants with very little discussion on the efficiency of the oxidant or the energy needed to generate them – two major contributors to the operating costs. Here, we focus on the issue of effective use of an oxidant in a catalytic AOP, which can be viewed in terms of the selectivity of a catalyst for the oxidation of a pollutant using the oxidant instead of other oxidation reactions or decomposition of the oxidant. We will discuss some of the strategies the community of water remediation researchers is using to resolve this issue and give examples on how knowledge in other fields of catalysis studies could be applied to improve catalytic performance. IIb.1 Generation of Reactive Oxygen Species (ROS) via Photocatalysis and Sonocatalysis

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Both sonocatalytic and photocatalytic processes utilize input energy to generate ROS. For sonocatalysis, the input energy is ultrasonic energy and for photocatalysis, the input energy is light. Between the two, photocatalysis is much more heavily studied and broadly applied. There are quite a few review articles on photocatalytic treatment of wastewater.38-41 Here, we focus on the generation of different ROS by photolysis that are important in photocatalysis. Often many of these ROS are important in other AOP as well. We will discuss the chemical reactivity of the different ROS and the roles they play in water remediation. ROS generated with light radiation can be in the form of OH∙ radical or singlet oxygen (1O2). In photocatalysis, photons with energy larger or equal to the band gap is absorbed by the catalyst to generate an electron-hole pair (Figure 3). Although these photoelectrons and holes can react directly with the pollutants, they are more likely to react with water and adsorbed oxygen on the solid that are present in much higher concentrations than the pollutants. The photo-holes can react with water to form OH∙ radicals and the photoelectrons can react with adsorbed O2 to form superoxide anion radical. The latter radical through successive protonation and reductive steps can be converted to H2O2 (Fig. 3).42 It can also be oxidized by a photo-generated hole to form a singlet oxygen 1 O2.43 The efficiency of forming 1O2 depends on the O2 adsorption capability of the photocatalyst. With wide bandgap photocatalysts such as TiO2 and ZnO, organic photosensitizers embedded on solid supports can be used to enhance visible light absorption to generate 1O2.38

Figure 3. Generation of different ROS under photo excitation of photocatalyst.

OH∙ and 1O2 have different reactivities. OH∙ is a very powerful oxidant (2.8 V versus standard hydrogen electrode44) and often can effect total mineralization of the organics. Its high reactivity also makes it non-specific and react with a broad spectrum of pollutants. Singlet oxygen on the other hand is more selective. Brame et al.45 compared the efficiency of OH∙ and 1O2 in the degradation of several model pollutants in water with background constituents that are often present in water that requires treatment. Background constituents reduce the amount of ROS by adsorption of the incoming light and can act as radical scavengers. Natural organic matters (NOM) is a heterogeneous mixture of organic matter in water and its composition depends on the source of water. The aromatic fraction of NOM absorbs UV light, thus impeding formation of electron-hole pairs in the photocatalyst and generation of OH∙ radicals, thereby affecting the amount of OH∙ available for oxidation. Brame et al.45 also found that NOM and phosphate react with OH∙ preferentially to 1O2. Thus, depending on the nature of wastewater, 1O2 or OH∙ radicals


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may be preferable and there are many studies on the preferential generation of these species with different photocatalysts.46 Due to the limited penetration depth of light energy, an issue that is especially severe for murky water, ultrasound excitation has also been explored for ROS generation. Absorption of acoustic energy can induce cavities formation in liquid. When the cavitation bubbles reach their critical size they implode, resulting in extreme temperature and pressure within the cavity that is sufficient to break bonds, i.e. pyrolysis.47 Thus, in an aqueous medium, acoustic cavitation can result in the generation of reactive radicals such as OH∙48 as shown in reaction 1. H2O + sonic energy → H∙ +OH∙

(1)

The H∙ atoms can subsequently combine with O2 to form HOO∙ and eventually with another H∙ to form H2O2. The use of OH∙ and H2O2 in the mineralization of pollutants will be discussed in detail later. When the pollutant concentration is high, ultrasound alone is insufficient for complete mineralization,49 and sonocatalysis is employed. There are different manners the catalyst can contribute to the enhancement of abatement activity. In one mechanism, the presence of solid surfaces provides nucleation sites for the formation of cavitation bubbles50 and increases OH∙ radical formation by homolytic scission of water. In another mechanism, the enhancement of OH∙ generation is due to thermal or sonoluminescence triggered formation of electron hole pairs on the solid catalyst surface subsequently leading to formation of different radicals by reaction with water and oxygen as is shown in Figure 4.51 Different solids have different efficiencies for OH∙ generation, and TiO2 is one of the best oxides. Stabilization of holes on TiO2 surface can be achieved by doping with Fe 3+.52 In the third mechanism, the decomposition of water into radicals is facilitated by a metal surface. Thus, Au/TiO2 catalyst was a much more efficient catalyst than TiO2 alone for the degradation of azo dyes.52 Similarly, a large enhancement in the rate of mineralization of oxalic acid was observed when 3% Pt was added to TiO2.48 The mechanism of catalytic homolytic splitting of water by Au or Pt is not yet clear. Thus, deeper understanding of the properties of metal that could enhance this reaction would be very useful.

Figure 4. Schematic diagram showing sonocatlytic process. Implosion of cavitation bubbles released sonoluminescence and heat, triggering formation of electron hole pairs, which eventually leads to the formation of radicals. 7 ACS Paragon Plus Environment


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IIb.2 Activation of H2O2 and Peroxymonosulfate to form ROS Instead of energy input to generate ROS as discussed above, reactive radical species can be generated through catalytic decomposition of powerful oxidants added to the wastewater. Two of the frequently used oxidants are H2O2 and peroxymonosulfate and the ROS generated from these oxidants catalytically are discussed below. Catalytic wet peroxide oxidation (CWPO) is a process using H2O2 to generate reactive OH∙ radicals to degrade organic contaminants. Often an iron-based catalyst is used involving Fenton chemistry. Iron-based catalysts are ideal for water remediation because iron is the fourth most abundant element in the earth crust (after oxygen, Si and Al) and is relatively non-toxic. The nature of the oxidants formed from H2O2 reaction with Fe(II) depends strongly on pH and other reaction conditions. He, et al.53 noted that although the general formulation of the Fenton reaction (reaction 2) implies an outer-sphere electron transfer reaction between H2O2 and Fe(II), thermodynamic considerations suggest that it occurs via an inner-sphere water exchange mechanism as denoted by reactions 3 and 4. Reaction 3 shows the formation of iron-peroxide which may dissociate to form the OH∙ radical as depicted in reaction 4 or form the less powerful oxidizing ferryl (Fe[IV]) species (reaction 5). The partitioning between reactions (4) and (5) is pH dependent. The Fe (III) formed consequent of the Fenton reaction further reacts with H2O2 to regenerate Fe2+ via a two-step process which are depicted in reactions 6a and 6b54-55 with the latter being the rate limiting step of the Fenton cycle.56 OH∙ reacts with organics by hydrogen abstraction and is not substrate specific and removes a very broad spectrum of pollutants due to its high reactivity. Problems associated with this process are sludge formation, narrow pH range of operation and the difficulty in catalyst recovery. Thus heterogeneous Fenton process is currently actively pursued57 and various supported Fe catalysts had been investigated.58-59 In addition to Fe-based materials other supported catalysts were also investigated.60-62 Fe2+ +H2O2 → OH∙ + OH- +Fe3+

(2)

[Fe(OH)(H2O)5]++H2O2 = [Fe(OH)(H2O2)(H2O)4]++H2O

(3)

[Fe(OH)(H2O2)(H2O)4]+ → [Fe(OH)(H2O)4]2++OH·+OH-

(4)

[Fe(OH)(H2O2)(H2O)4]+ →[Fe(OH)(H2O)4]3++2OH-

(5)

Fe3+ + H2O2=Fe-OOH +H+

(6a)

Fe-OOH→ Fe2+ +OOH∙

(6b)

Fe3+ + HO2∙→ Fe2+ + O2 +H+

(7)


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For heterogeneous CWPO, two areas have been identified for improvement of the catalyst: faster regeneration of the active center in the catalytic cycle and more efficient utilization of H2O2. Reaction 6, is the rate limiting step, and is orders of magnitude slower than reaction II.63 Georgi et al.64 used a two-catalyst system: a very low concentration of Fe2+ for the Fenton reaction and a Pd/Al2O3 catalyst together with H2 for the regeneration of Fe2+. Ma et al. found that sodium dodecyl sulfate (SDS) immobilized on FeCo2O4 accelerated the reduction of Fe(III) to Fe(II)65 resulting in remarkable enhancement in the rate of degradation of methylene blue. For CWPO to be cost effective, H2O2 needs to be utilized efficiently. However, until recently, most of the literature focuses on the total amount of pollutants degraded with little or no discussion of H2O2 utilization efficiency. H2O2 is the source of OH∙ radical and faster radical formation improves its utilization. This was observed with isomorphous substitution of Cr (III), Mn (II) and Co (II) into magnetite (Fe3O4).66 In the same report, however, the authors found that OH∙ production rate dropped with substitution of Ni(II) and Ti(IV) into (Fe3O4).66 Dai et al.67 found that CuFeO2 nanocubes with surface facets were four times less active than nanoplates with surface facets (Figure 5) in the degradation of the pollutant ofloxacin. Computational modeling simulations showed that the O−O and H-Osurface bond lengths of H2O2 chemisorbed on facets favored OH∙ formation, whereas these bond lengths of H2O2

chemisorbed on facets favored formation of −OH groups that lead to passivation of the

facets. Au nanoparticles deposited on Fenton-treated diamond nanoparticles were particularly efficient in dissociating H2O2 into OH∙.62 Furthermore, because of the low affinity of the OH∙ for the Au or the diamond surface, these radicals were present mostly as free radicals in solution, resulting in high utilization efficiencies.



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Figure 5. Comparison of CuFeO2 with facets (left) and facets (right). Top: SEM images, middle: HRTEM images and bottom: H2O2 chemisorbed on optimized surface structure of CuFeO2.67

H2O2 utilization can also be improved by decreasing the rate of its non-productive catalytic decomposition into O2 and H2O (reaction 8). This reaction has been studied rather extensively outside the context of water remediation, but those findings and understanding are applicable here. For example, the decomposition rate of H2O2 on Pt was found to increase with the surface roughness of Pt consequent of increase in the surface area of Pt.68 There is also an observed dependence on Pt particle size,69 which may arise because the larger Pt particles have lower work functions and hence favor chemisorption of O onto Pt to form Pt(O). To delineate the contribution due to chemisorbed oxygen and particle size effect in H2O2 decomposition SerraMaia et al.70 combined the structural and chemical properties of the reaction catalysts with multivariate linear analysis of H2O2 decomposition rates to arrive at the conclusion that Pt particle size was not the causal effect; instead Pt catalysts with a higher density of chemisorbed O were more active independent of the particle size. Yook et al. found for AuPd/carbon catalysts, a higher ratio of micropores to mesopores of the carbon support favored H2O2 decomposition consequent of mass transfer limitations of the microporous carbon.71 Therefore, a catalyst design for heterogeneous CWPO should reach a compromise between rapid OH· formation and low H2O2 decomposition through reaction 8. H2O2 → H2O + 0.5O2

(8)

The other two common oxidants peroxydisulfate (S2O8 2−) or peroxymonosulfate (PMS, HSO5-) are much cheaper than H2O2, easier to store and transport.72 Using catalysts based on MnO2 as an example, PMS reacted with MnO2 to form sulfate and OH∙ radicals (reactions 9 and 10)73-74 which were detected using the spin trapping agent 5,5-dimethyl-1-pyrroline (DMPO).75 The catalytic cycle was completed by oxidation of Mn (III) with PMS to regenerate Mn(IV) as depicted by reaction 11.73 Mn(IV) + HSO5- → Mn(III) + SO4∙− + OH−

(9)

Mn(IV) + HSO5−→ Mn(III) + SO42− +OH∙

(10)

Mn(III) + HSO5− → Mn(IV) + SO5∙−+H+

(11)

2SO5∙− + 2OH-

(12)

→2SO4= +2 OH∙ +O2

Evidence points to the fact that the sulfate radicals are more reactive than the OH∙ radicals in oxidation reaction. In the presence of phenol, DMPO-SO4∙− adduct decayed more rapidly than DMPO-OH∙ adduct and 80% of the degradation of phenol over a MnO2 catalyst in the first 10 min was due to reaction with sulfate radicals.73 The presence of radical quenchers ethanol and tert-butyl alcohol affected the reactivity of sulfate and OH∙ radicals differently.73-75 Whereas 10 ACS Paragon Plus Environment

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OH∙ radicals reacted readily with both alcohols, sulfate radicals were quenched more effectively by ethanol than tert-butyl alcohol, a difference attributed to the absence of an α-hydrogen in the latter.76 The addition of tert-butyl alcohol instead of ethanol appeared to have less negative impact on the rate of phenol75 and methylene blue74 degradation, attesting to the more dominant role the more reactive sulfate radicals played in pollutant remediation. For Mn based catalyst, the crystal morphology and the preferential facets were found to be important in influencing key catalytic parameters such as activation energy in the activation of PMS.75 Efficiency in radical generation from PMS has started to gain attention of researchers in the field. Zhang et al.77 determined that the sulfate radical yield from PMS over magnetically separable spinel CuFe2O4 catalyst was close to unity (1 mole PMS consumed per mole oxalate degraded). The high efficiency in radical production was ascribed to efficient redox cycling between Cu(III) and Cu(II) on the catalysts surface. Such a high efficiency suggests opportunities for further investigation and development. Thus far, we have focused the discussion of reactive radical generation on heterogeneous catalytic systems. There are exciting advances in homogeneous catalytic systems as well. For example, Zhou, et al.78 observed that benzoquinone can catalytically activate PMS and produce 1 O2 which can degrade sulfamethoxazole, an antibiotic frequently detected in the environment. 1 O2 is less susceptible than SO4∙− and OH∙ to NOM and phosphates often present in wastewaters.45 Dai et al.79 observed that iron tetracarboxyphthalocyanine (FeTCPc) was better than Co (II) in the activation of PMS for degradation of Acid Orange 7 (AO7), a common azo dye contaminant. They determined that HO2∙/ O2∙− and 1O2 instead of SO4∙− and OH∙ radicals were the primary active oxidants in the catalytic oxidation cycle. In addition, the presence of HCO3−, an ubiquitous ion in industrial wastewater, accelerated instead of negatively impacted the AO7 degradation as in the Co(II)/PMS system. In spite of their catalytic promises, recovery of homogeneous systems is much more difficult than heterogeneous systems. Thus, another fruitful direction of research is to heterogenize promising homogeneous catalysts. III.

Catalyst Stability

Stability is just as important as activity of a catalyst for industrial applications. For metal catalysts, two major sources of deactivation are metal leaching and metal sintering. Noble metal catalysts are less prone to leaching than other transition metals such as iron, cobalt, copper, and manganese. For example, no detectable metal leaching was observed when Au/Al2O3 was used for CWPO of dyes in wastewaters,80 and no leaching of Au or Ti was detected for 300h in the CWPO of phenol using Au/TiO2.81 For base metal catalysts, leaching is a problem and is observed independent of the specific oxidation process (including CWAO, CWPO and PMSinitiated processes). Copper oxide-based catalysts, although very active in CWAO, are prone to leach. In fact, the leached Cu in solution was proposed to be responsible for the high activity of phenol oxidation.82 The need to capture leached toxic Cu introduced complexity to the purification process. Furthermore, some intermediates of phenol in the initial stages of oxidation are substantially more toxic than the parent phenol and complexation of these intermediates with leached Cu ions further enhances their toxicity.83 Leaching from supported Fe catalysts used in CWPO remains an unresolved problem84 and when leaching is extensive, the contribution due to 11 ACS Paragon Plus Environment


homogeneous Fenton reaction can be considerable.85 In PMS-initiated oxidation, Co leaching occurred in many highly active catalysts.86-88 At high enough concentrations, cobalt is a health hazard.89 Thus, the less active Mn based catalysts are actively investigated for PMS initiated water remediation instead. It has been found that the stability of some catalysts can be improved by changing the catalyst architecture and support composition. At present, the underlying reasons are not well understood. For example, Cu leaching was substantially suppressed for CuO-CeO2 catalysts prepared with the sol-gel method compared with those prepared by co-precipitation, and negligible Cu leaching was observed when Cu(BDC), a porous copper metal–organic framework, was used in wet peroxide oxidation of simulated phenol wastewater.90 Yan et al. found that the leaching rate was slower when Fe was an integral part of a zeolitic ZSM-5 framework than present as extra framework Fe3+.84 Prucek et al.85 found that amorphous Fe2O3 were much more prone to leaching than crystalline Fe2O3. The rate of leaching is highly dependent on the forms of cobalt oxide with leaching rate far higher for CoO than Co3O4.91 Yang et al. observed that although cobalt leaching was suppressed consequent of the strong FeCo interactions in CoFe2O4, complete elimination of leaching was not achieved.92 Other fields of catalysis may offer solutions to the leaching problems. Alonso-Fagúndez et al. found that, for biomass valorization, pre-conditioning the catalysts decreased leaching during the catalytic runs.93 Such a procedure can be readily adapted for water remediation. The group of Dumesic94 found that an overcoat of Al2O3 deposited on Cu/Al2O3 prevented Cu leaching and proposed that although undercoordinated copper atoms are more prone to leaching, their selective interactions with the alumina overcoat deterred them from leaching. Similar in concept to this approach, construction of yolk shell Co@C-N reactor (Figure 6) prevented Co leaching because the Co nanoparticles were stabilized through interaction with C-N nanosheet within the confines of the core.95

Figure 6. a: Pictorial representation of hollow yolk−shell Co@C−N nanoreactors, b: TEM and c: EDS mapping.95


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Sintering is the other most common cause of catalyst degradation. Structural degradation of both metal and metal oxides may take place in hot water, especially if the pH deviates significantly from neutrality. The literature on improving hydrothermal stability of catalyst in the field of water remediation is sparse. In the recent years there are some interesting works emerging from bio-mass processing that may be relevant to water remediation as the former process often takes place in aqueous environment in the presence of acids or bases. Abdelrahman et al.96 found the particle growth of Ru on supported catalysts in aqueous water was related to the bulk electronegativity of the support, with sintering rate decreasing in the order of SiO2>C≈TiO2>γ Al2O3. This dependence arose because Ru-O-M (M= support cation) interaction is influenced by the electron density on the oxygen which in turn is affected by the electronegativity of the support cation. Zhan et al.97 isolated Au nanoparticles on the support by forming a carbonaceous shell around them which deterred them against secondary crystallite growth. The group of Dumesic et al. found that an alumina overcoated Cu/Al2O3 catalyst reduced copper sintering and attributed this to the interaction of undercoordinated Cu with the overcoat which stabilized the metal particles.94 Recent work in biomass processing in aqueous system at elevated temperatures also shed light on the origin of degradation of various supports under harsh conditions. Design features based on these understandings have enhanced catalyst stability. Ravenelle et al.98 noted that γ- Al2O3 had a tendency to rehydrate and convert to boehmite with an accompanying loss in surface area, in the presence of water at 150 oC. The rate of transformation to the boehmite phase was found to decrease in the presence of metal particles (Ni or Pt). Thus, the authors proposed that the metal particles deterred the hydration of the support and hindered the boehmite phase formation. Prolonged (10 h) exposure at 200 ºC to aqueous water under autogenous pressure also led to metal sintering which was attributed to the erosion of the support around the metal particles. Thus, the authors proposed that capping the surface hydroxyls to deter the initial boehmite crystal nucleation may be a way of stabilizing alumina supports. Tang et al.99 fabricated r-Ru– NH2–γ-Al2O3 catalyst using alumina with surface hydroxyls partially capped by 3-aminopropyl functionalities. The r-Ru–NH2–γ-Al2O3 catalyst was significantly more stable in water compared with a Ru/γ-Al2O3 catalysts prepared with unmodified γ-Al2O3. The Si-O-Al bonds present in the modified samples hindered the rehydration of γ-Al2O3 and the –NH2 group enhanced the uniformity and dispersion of Ru particles. The group of Resasco100 systematically evaluated the contribution of the density of Brφnsted acid sites, the framework type, the density of silanol defects and extraframework Al towards the instability of the zeolite in hot liquid water. They found significant enhancement in maintaining zeolite crystallinity after capping the silanol defects with organosilanes. Thus, they concluded that the crucial factor that determined zeolite stability in hot liquid water was the density of silanol defects. Hahn et al.101 found that amorphous silica alumina prepared by deposition of aluminum nitrate onto silica was more stable in hot liquid water than samples synthesized by co-gelation, and attributed it to the fact that the presence of an alumina shell suppressed the hydrolytic decay of the solid beneath it. Finally, Datye, et al.102 observed enhanced hydrothermal stability by carbon over-coating SBA-15. These observations, although gathered from experiments designed for other purposes, were with aqueous systems in which the catalysts were often subjected to conditions harsher than those 13 ACS Paragon Plus Environment


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used in water remediation. Thus, approaches that are proven to be effective in enhancing catalyst stability should be directly applicable here. Importantly, understandings of the deactivation phenomenon and the prevention measures can also apply, offering the opportunity to pick and choose, and fine tune the most appropriate strategy for water remediation, in terms of cost, complexity, and effectiveness. IV. In situ production of H2O2 H2O2 is a powerful and environmentally benign oxidant and is currently produced primarily using the anthraquinone process.103 There are however difficulties associated with its storage and transportation and in situ production of this powerful oxidants at its site of application would ameliorate many of these problems. Direct oxidation of H2 with O2 has been heavily explored as a means of generating H2O2 in situ for subsequent usage in reactions such as the low temperature oxidation of sulfides104 and the epoxidation of alkenes.105 Besides the added precautions necessary in the handling of H2 and O2 gas mixture, the two gases have very low solubility in water and as a result, hydrogen surrogates such as formic acid are necessary for reactions in aqueous medium.106-107 Direct oxidation of H2O with O2 is the most attractive option but this reaction is thermodynamically uphill (ΔG⁰ = 116.7 kJ/mol). Thus, this reaction needs to be coupled with a free-energy downhill reaction such that the overall reaction becomes thermodynamically favorable. Typically, this is achieved by using a sacrificial reductant, which adds cost. The most ideal situation would use the pollutants already present in the waste stream as the sacrificial reductants for the production of H2O2 from water and air. To our knowledge, this has not been explored. In this section, we begin with examples of zero valent iron as the sacrificial reductant for the in-situ generation of H2O2 and then proceeds to discuss using organic sacrificial reductant, eventually leading to exploring the possibility of using pollutants in the waste stream. IVa. Zero valent iron as sacrificial reductant One actively researched area using heterogeneous Fenton chemistry is the use of zero valent iron (Feº, ZVI), especially nano zerovalent iron (nZVI), as the sacrificial reductant to produce H2O2 with O2. Stoichiometric reaction of ZVI with H2O2 results in formation of Fe2+ which can initiate Fenton reactions. An interesting aspect associated with nZVI is its reaction with O2 and H2O to produce H2O2 (reaction 13). The group of Waite53 demonstrated that when nZVI was placed in oxic water, H2O2 was formed and the solid nZVI was subsequently transformed to ferrihydrite and then to lepidocrocite (Figure 7). By comparing filtered and non-filtered reaction solutions (Figure 7a and 7b), the authors claimed that most of the H2O2 formed was adsorbed on the surface of the solid particles. Concomitant with the transformation of nZVI, high concentrations of Fe2+ ions were detected, but like H2O2, most of the Fe(II) were surface-associated. nZVI under aerobic condition was able to mediate the oxidation of formate with CO2 as the primary end product. Fe0 + O2 + 2H+→ Fe2+ + H2O2

(13)


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Figure 7. Events associated with the exposure of nZVI to oxygen containing H2O at circumneutral pH. a: H2O2 concentration in the aqueous phase, b: total H2O2, and c: Fe speciation.53

The low oxidant yield of nZVI reaction with O2 hampers the practical applicability of this technology. Intensive research effort has been expanded to address this question. One approach is to add ligands to enhance the reaction rates to favor H2O2 production and usage and to minimize Fe(III) precipitation at high pH.108 Another approach is to modify the property of nZVI by adding another metal such as Ni. Lee et al.109 observed that bimetallic nickel-iron nanoparticles were more efficient oxidant producer than nZVI and produced higher yields of formaldehyde in the oxidation of methanol. The enhanced oxidant yield for the bimetallic over the monometallic arose because of the low reactivity of the former in decomposing H2O2, thus increased the lifetime of adsorbed H2O2, which in turn facilitated H2O2 release and favored oxidant production via the Fenton reaction. At neutral pH, the bimetallic surface also promoted O2 reaction with Fe(II) to produce oxidants. Drawback of using nZVI is the non-recovery of sacrificial reductant Fe0 and the leaching of Fe2+ and Fe3+ into the water. Attempts have been made to solve this problem. Ambika et al.110 synthesized a CMCFe2+ (-nZVI) hybrid structure in which Fe2+ present on the surface of nZVI was encapsulated with a carboxymethyl cellulose (CMC) shell. The -OH, –C=O and –COO functional groups present on CMC interacted with Fe2+ and deterred its leaching into water. This hybrid CMCFe2+ (-nZVI) structure was tested for phenol oxidation and the iron hydroxide sludge production and iron leaching were reduced by 99.3% and 98.6%, respectively relative to the bare nZVI without the CMC. 15 ACS Paragon Plus Environment


IV B. Organic sacrificial reductants and implication of leveraging pollutants in wastewater for in-situ production of H2O2 AOP technology is usually reserved for water with low pollutant load because of the high cost associated with the production of the oxidant. However, strategic utilization of the organics present in high strength polluted water as sacrificial reductants might be a method to produce H2O2 in situ. This strategy has the potential of achieving oxidative degradation of contaminants without any input of energy or reactive chemicals. The system would be completely passive and self-contained in removing organics in water. To the best of our knowledge this has not been actively pursued in the field of wastewater treatment. To explore this possibility, we will first present data that oxidation of organics in an aqueous environment can produce H2O2. Then we will examine different types of high pollutant load waste water and evaluate the potential of using this strategy for in-situ production for H2O2 to be used in AOP. There are literature examples of coupling chemical reactions to achieve production of H2O2 from H2O or organic hydroperoxides from alcohol. One example used CO as the sacrificial reductant, and the reaction of CO oxidation to CO2 is coupled with oxidation of H2O to form H2O2 using a Au/C catalyst 111-112 or NiP(B)/Al2O3 catalyst.113 H2O2 formation was also observed during glycerol oxidation over Au/C catalysts.111-112 Choudhary et al. found that H2O2 can be formed at high yield at neutral conditions via oxidation of hydroxylamine with oxygen over Pd/Al2O3 catalyst.114 In our laboratory, we found that oxidation of benzyl alcohol, benzaldehyde, or hydroxyacetone all generated H2O2, albeit with very different kinetic profiles (Figure 8). It is known that catalytic alcohol oxidation is accelerated at alkaline pH due to the favorable deprotonation of the alcohol by solution and adsorbed OH-.115 Thus, these reactions were carried out at a high pH, and the H2O2 production rate decreased rapidly with decreasing pH of the solution. Although the conversion of the oxygenates increased with reaction time, the H2O2 concentration in solution first increased rapidly and then slowly declined after reaching a maximum. This profile indicated that decomposition or reaction of H2O2 was significant after it had built up to a certain concentration. Since the deprotonated form of an alcohol is essential in the mechanism, the H2O2 production rate depends strongly on the pKa of the alcohol. This can be used to distinguish different oxygenates for their participation in the reaction. For example, the relative ratios of the deprotonated vs. the protonated form of the alcohol at pH=13 for hydroxyacetone with a pKa of 13.14 and benzyl alcohol with a pKa of 15.02 are 0.72 and 0.01, respectively. Therefore, at that pH, hydroxyacetone is much more reactive than benzyl alcohol in forming H2O2. On the other hand, reaction of aldehyde does not involve deprotonation and is much less pH dependent. Thus, when the reactive group of the molecule is an aldehyde functionality, milder pH conditions can be tolerated. For example, Biella et al.54 observed in the competitive reaction of propanol and butanal over Au/C catalyst at pH 8, there was significant reaction of the aldehyde but the alcohol exhibited no reaction. Based on this observation, the authors proposed that in the reaction using glucose, the aldehyde group but not the alcohol groups of the molecule contributed to its reactivity at relatively mild pH. 16 ACS Paragon Plus Environment

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Figure 8. H2O2 production (left panel) and hydrocarbon conversion (right panel) catalyzed by 15 mg 0.6% Au/C, 1M NaOH, T=35℃, stirrer speed=300 rpm, O2 flow rate = 20mL/min at ambient pressure. ∆: benzyl alcohol, 0.063M; O: hydroxyacetone, 0.062M; □: benzaldehyde, 0.009M due to limited solubility in water. H2O2 was determined by colorimetric method using H2SO4 acidified titanyl sulfate solution112 and hydrocarbon conversion was determined by HPLC.

Having established that oxidation of organics can generate H2O2, we next survey different types of wastewater stream to see whether they contain pollutants suitable for H2O2 generation. In pyrolysis of biomass, a fraction of the biomass pyrolysis carbon (between 12 g kg−1 and up to 315 g kg−1 of organic carbon for a highly aqueous catalytic fast pyrolysis and the fast pyrolysis aqueous streams, respectively) is not converted to target fuels or chemicals and would be released as waste aqueous streams.116 Although the rich carbon contents make it attractive to valorize the waste streams to recover valuable products,117 the mixture of chemicals demands extensive purification processes. An alternate scheme is to use the high but diverse carbon contents to generate H2O2 in situ. Some examples of compounds detected in high concentrations from pilot plants are listed in Table 1. The various sugars and aldehydes should be appropriate for H2O2 generation. Waste streams from breweries and wineries118 are also high in sugar and alcohol contents and are also ideal for use to generate H2O2. Lin et al.119 found that the wastewater from petrochemical companies that produce secondary butyl alcohol are very alkaline and have high chemical oxygen demand (COD) around 42,000 mgL-1. This brief survey suggests that many different types of waste streams contain pollutants that are suitable sacrificial reductants for the production of H2O2. One advantage of this approach is that H2O2 production is relatively tolerant to a diverse class of organic compounds, which makes it suitable to treat typical waste streams that may contain a large variety of compounds each present at low concentrations. For example, a total of around 16 g L-1 of sugar and polysaccharides was present in the soluble fraction of a starch plant waste stream after two decantation processes.120 Since 17 ACS Paragon Plus Environment


these molecules all contain alcohol groups and some also contain aldehyde groups, they could be utilized to effect the oxidation of H2O to produce H2O2.

Table 1. Selected high-concentration compounds detected in aqueous stream from biomass fast pyrolysis and catalytic fast pyrolysis presented in g kg-1 on a wet basis.116 Compound Fast PyrolysisNRELa,b Catalytic Fast PyrolysisRTI-pine a,c D-xylose 7.48 ND D-glucose 1.90 ND D-galactose 1.43 ND 1,6-anhdro-β-D-glucopyranose 45.39 ND 1,4:3,6-dianhydro-α-D8.12 ND glucopyranose 1,6-anhydro- β-D-cellobiose 4.92 ND methanol T 20.75 2-methoxyphenol 0.20 0.39 2-hydroxyacetaldehyde 56.1 2.72 furan-2-carbaldehyde 1.92 6.11 Cyclopent-2-en-1-one 0.37 4.15 2(5H)-furanone 3.92 0.89 a: T = trace amounts detected, < 0.01 g kg−1; ND = not detected in the sample, below the limit of detection. b: Fast Pyrolysis aqueous sample at National Renewable Energy Laboratory(NREL)was collected from pilot plant.116. c: Catalytic Fast Pyrolysis sample collected at Research Triangle Institute International.116

Conclusions and Future Perspectives Devising sustainable processes for water remediation is one of the grand challenges for scientists and engineers in the twenty first century. Catalytic water purification is a potential avenue to meet this challenge and appears to offer many opportunities for catalytic scientists. However, catalyst activity and stability in an aqueous medium remain to be crucial hurdles that impede achieving water treatment with minimal or no energy demand or other environmental impact, in spite extensive effort to solve these issues. This article suggests that to make transformative advances, besides building on the foundation of current knowledge, it will be useful to integrate information and insights harnessed from other branches of catalysis beyond water remediation. An example is the recognition in recent years that interfacial sites in supported metal catalysts are important in various catalytic reactions. It suggests that designing and fabricating catalysts to achieve a high density of metal/metal oxide interface may be a way to enhance activity in the next generation of water purification catalysts. Catalyst stability is of utmost importance. Recently, there are substantial advances in both knowledge and insight in improving catalyst stability in aqueous phase bio-mass processing, which is often conducted at elevated 18 ACS Paragon Plus Environment

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temperatures in the presence of acids or bases. Many of those approaches are directly applicable to catalytic wet oxidation. Finally, a new area that could be the final answer to sustainable water remediation is in-situ production of H2O2 using organic containments present in the waste stream. If these organic contaminants can be utilized effectively as sacrificial reductants to generate the versatile H2O2 oxidant that can degrade dissolved organics as well as disinfect, one can achieve water purification without any input of energy or reactive chemicals. Currently, this is very under-explored, but the potential impact can be very large.

Acknowledgement: The work is supported in part by Northwestern University through a generous gift from Dr. Warren Haug. H.H.K. and M.C.K. acknowledges partial support from the Advanced Materials for Energy-Water Systems (AMEWS) Energy Frontier Research Center, which is supported by the U.S. Department of Energy (FWP 30208-1). J.Y. acknowledges support from China Scholarship Council.

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Valorization via Enhanced Microbial Toxicity Tolerance. Energy & Environmental Science 2018, 11 (6), 1625-1638. (118) Welz, P. J.; le Roes-Hill, M.; Holtman, G.; Haldenwang, R., Characterisation of Winery Wastewater from Continuous Flow Settling Basins and Waste Stabilisation Ponds over the Course of 1 Year: Implications for Biological Wastewater Treatment and Land Application. Water Sci. Technol. 2016, 74 (9), 2036-2050. (119) Lin, S. H.; Ho, S. J., Treatment of High-Strength Industrial Wastewater by Wet Air Oxidation. A Case Study. Waste Management (Oxford) 1997, 17 (1), 71-78. (120) Souza Filho, P. F.; Zamani, A.; Taherzadeh, M. J., Edible Protein Production by Filamentous Fungi using Starch Plant Wastewater. Waste and Biomass Valorization 2018, 10 (12), 2487-2496.



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A perspective on catalytic oxidative processes for sustainable water

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