Dual-Functional Photocatalysis for Simultaneous Hydrogen

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Review

Dual-Functional Photocatalysis for Simultaneous Hydrogen Production and Oxidation of Organic Substances Stavroula Kampouri, and Kyriakos C. Stylianou ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00332 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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Dual-Functional Photocatalysis for Simultaneous Hydrogen Production and Oxidation of Organic Substances Stavroula Kampouri, Kyriakos C. Stylianou* Laboratory of Molecular Simulation (LSMO), Institute of Chemical Sciences and Engineering (ISIC), Ecole Polytechnique Fédérale de Lausanne (EPFL Valais), Rue de l’industrie 17, 1951 Sion, Switzerland.

Abstract The present review showcases the scientific progress in the field of dual-functional photocatalysis for hydrogen evolution coupled with the oxidation of chemical substances. Considering that hydrogen is a promising alternative to fossil fuels, photocatalytic water splitting represents an approach to produce hydrogen using abundant solar energy. However, industrialization of this process has not occurred yet, mainly due to limitations related with either the high cost, toxicity, poor stability or low efficiency of the majority of the photocatalytic systems employed for water reduction. An approach to tackle these limitations is by taking advantage the oxidative energy of the holes and by replacing the expensive and often toxic sacrificial electron donors with either organic pollutants (targeting their degradation) or organic substances that can be oxidized to added-value products. Following this two-fold strategy, the production of sustainable hydrogen is accompanied by either the oxidative degradation of pollutants or the valorization of organic processes, in a single process. Herein, a detailed overview of the advancements in this dual-purpose field is offered, along with a discussion of the basic principles, the differences between similar fields and how these can be distinguished. Although in its infancy, this dual-purpose technology has received radically increasing scientific

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interest over the last few years; this is expected as the utilization of this approach can overcome multiple major environmental challenges, such as the global warming, water scarcity, and pollution.

Keywords: dual-functional photocatalysis, hydrogen evolution, organic oxidation, solar energy conversion, water remediation

1. Introduction Extensive environmental pollution, abnormal climate change, abused natural resources, and excessive deforestation is humanity’s heritage from the 20th century. The increasing growth of the world’s population, the rise of industrial actions, and the fossil fuel-based economy have led to the release of large amounts of hazardous substances into the air and/or water, causing innumerable issues.1

It is, therefore, a virtual necessity to develop sustainable and

environmentally friendly technologies to eliminate the above challenges. A promising approach lies on the use of energy from light to excite a material, which can enable different redox reactions. This broad field is known as photocatalysis. Each potential redox reaction that can occur represents an individual photocatalytic field. More specifically, the major photocatalytic fields that have been extensively studied include the water splitting (reduction, H2 production and oxidation, O2 production), carbon dioxide, CO2 reduction, synthesis of molecules, oxidation of organic pollutants and metal reduction (Figure 1).2-5


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Figure 1: Major Photocatalytic fields.

Photocatalytic water splitting into H2 and O2 constitutes a field of significant importance since H2 is regarded as an ideal replacement of fossil fuels, allowing for an energy cycle free of greenhouse gasses.6 Another major field in photocatalysis includes the light-driven oxidation of organic substances. Light-induced oxidation can be used to either achieve full degradation of hazardous organic contaminants or to synthesize useful organic molecules. Increasing amounts of wastewater and the high toxicity of several organic substances in streams have prompted extensive research in this field.7-8 The fundamental principles in both fields will be thoroughly described in the following sections of this review (see sections 3 and 4). Photocatalytic reduction of metal ions is another efficient approach to alleviate water pollution induced by inorganic pollutants.9-10 Among the pollutants frequently found in wastewater streams, heavy metals, such as lead (PbII), chromium (CrVI) and mercury (HgII) are very hazardous for public health. Unfortunately, exposure to these substances is still increasing in some parts of the world.11 Since different oxidation states of a given metal can have a profound impact on its toxicity, it is crucial to either transform such metals to a more environmentally benign form (e.g. CrVI to CrIII) or to reduce it to its elemental form and remove it for safe disposal.


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Since CO2 represents the most common greenhouse gas contributing to global warming, the photocatalytic reduction of CO2 into useful chemicals (e.g. methane, methanol, formaldehyde, formic acid, and others) is regarded as a promising technology in order to avoid further increase in the concentration of CO2 in the atmosphere.12-16 In principle, this photocatalytic process involves the reduction of CO2 along with the oxidation of water, which leads to the production of solar fuels.17 In addition, converting CO2 into hydrocarbons through the utilization of H2 produced from photocatalytic water splitting in a single process has recently attracted considerable research attention.18 The simultaneous photocatalytic H2 production and CO2 hydrogenation allows for the direct transformation of H2 into renewable fuels. Dual-functional photocatalysis constitutes a hybrid field where different photocatalytic fields are combined for a two-fold purpose. In recent years, integrating different photocatalytic fields for dual-functional photocatalysis has gained great attention. So far, a significant fraction of research studies in this field have focused on the simultaneous photolysis of organic pollutants and metal reduction, with CrVI representing the paradigm of such an application.19-23 Furthermore, dual-purpose photocatalysis for concurrent generation of H2 and oxidation of organic substances has received increasing scientific interest. A review article summarizing the different photo(electro)catalytic processes (i.e. H2 or H2O2 production, heavy-metal recovery, denitrification, fuel cell) that can be combined with organic oxidation reactions was very recently published, and the readers are encouraged to read it.24 Our review will focus on the field of dual-functional photocatalysis for the production of H2 coupled with the oxidation of organic molecules, which can be divided in two subcategories depending on the nature and target of the oxidation process. Both of these categories involve the photocatalytic generation of H2 from water (as the main source) coupled with either the


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degradation of organic pollutants or the transformation of organic substances into value added products. While in its infancy, the field of photocatalytic H2 production coupled with oxidation of organic substances has received considerably increasing scientific efforts, reaching its apogee over the last few years with numerous scientific studies reported in the literature. Thus, the aim of our review is to give a detailed overview of the progress in this dual-functional photocatalytic field. The focus of this review is to provide information regarding the basic principles and the mechanistic aspects of the field, sharing personal evaluation of different examples in the literature and identifying the challenges that should be addressed.

2. Fundamental Principles of Photocatalysis The first photocatalyst, reported in 1972 by Fujishima and Honda, was titanium dioxide (TiO2) and was explored for light-induced water splitting.25-26 Since then, immense research in photocatalysis has focused on TiO2 and other semiconducting materials.27-28 In contrast with conductors where the valence and conduction bands (VB and CB, respectively) overlap, a given semiconducting material is characterized by a VB and CB that are separated by a band gap (Figure 2a). In this case, the band gap is significantly smaller than that of insulating materials and thus, when sufficient light energy is employed, electron-hole pairs can be generated. The main requirement for this process to occur is that the photons energy should be equal to or higher than the band gap energy of a given semiconductor. Subsequently, the photogenerated charge carriers will be transferred to the surface of the photocatalyst where redox reactions can be initiated. Conclusively, the major steps in every photocatalytic reaction involve the light harvesting by the photocatalyst and generation of electron–hole pairs, then the charge separation and transportation to the surface of the photocatalyst, followed by the


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last redox reactions (Figure 2b). The overall equilibrium of the kinetics and thermodynamics of these three distinct steps defines the photocatalytic activity (performance) of a light-active material.

Figure 2: a) Band gap diagram of conductors, semiconductors and insulators. b) Schematic illustration of major photocatalytic steps. Step 1: Light absorption, Step 2: Generation of electron-hole pairs and migration to the surface of the photocatalyst and Step 3: Surface redox reactions.

Regarding the first step of light absorbance, it is highly important not only for the material to exhibit enhanced light absorbance (high value of absorbance coefficient), but also to harvest light with specific energy. The ultimate goal in the development of new technologies is to utilize energy from renewable sources such as the sun, wind and wave power. Thus, in the field of photocatalysis it is of crucial importance to harvest the abundant solar energy. The power of the solar energy incident on the Earth’s surface is around 1000 W m-2. Should all this energy be exploited, it could completely cover the global energy demand.29 The solar spectra at sea level is comprised of around 43% visible and 53% infrared light. UV light, on the other hand, accounts for only ~4% of the solar radiation. While it is known that the most stable semiconductors are characterized by wide band gaps, materials with narrow band gaps, which are capable of absorbing light in the visible or infrared region, are highly desirable. Numerous research studies have been devoted in the development of strategies in order to enhance or extend the absorbance of materials (photocatalysts) in the visible region of the 6 ACS Paragon Plus Environment

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solar spectrum. Approaches to achieve this include the utilization of dyes as photosensitizers,30-32 doping a given semiconductor with metal or non-metal ions,33-36 coupling with narrow band gap semiconductors for the formation of heterojunctions,37-39 surface plasmon resonance,40-43 and changing the elemental combinations or introducing electron donating functional groups (-NH2, -OH) in the cases of organic or semi-organic hybrid photocatalysts (e.g. covalent-organic frameworks COFs, metal-organic frameworks MOFs, Figure 3).44-46

Figure 3: Influence of R-terephthalic acid ligand on the band gap of MIL-125. The bars represent the band gap energy of the MIL-125 with different linkers: terephthalic acid (black), 2-methylterephthalic acid (purple), 2-chloroterephthalic acid (white), 2-aminoterephthalic acid (blue), mixed linkers of 10% 2,5-diaminoterephthalic acid and 90% 2-aminoterephthalic acid (red) and 2,5-diaminoterephthalic acid (green). Reprinted with permission from Ref 46. Copyright © 2013, American Chemical Society.

As mentioned above, the second step in a photocatalytic reaction involves separation and migration of the photogenerated charge carriers, which is of significant importance. A photocatalyst with a large visible-light absorbance does not guarantee photocatalytic efficiency. The excited state lifetime of the photocatalyst should be long enough for the photogenerated charge carriers to be transported to the surface of the material and, subsequently, to initiate a chemical reaction. High electron–hole recombination rates within the photocatalysts can explain why many photocatalytic reactions offer low efficiencies. Improvement of this second step has attracted intense research interest, with numerous 7 ACS Paragon Plus Environment

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successful strategies being reported, including the design of high crystalline photocatalysts with less grain boundaries or other defects,47-49 the formation of heterojunctions,50-51 or incorporation of co-catalysts to inhibit the undesired electron–hole recombination, by withdrawing the generated charges.5, 52-56 Moreover, modification of the optical and electronic properties of a photocatalyst by following the aforementioned approaches could also result in enhanced charge separation efficiency, in addition to improving the first step of light harvesting.57 The third step of the photocatalytic reaction has been rather neglected in terms of scientific attention, as compared to the previous two steps. This final step can be significantly promoted by inclusion of catalytic active sites, such as the co-catalysts. Co-catalysts are frequently incorporated in photocatalytic systems, since they play a key role in inhibiting the undesired electron hole recombination and reducing the activation energy of a given reaction.13 An appropriate co-catalyst should exhibit thermodynamic compatibility with the photocatalyst, high intrinsic catalytic activity for a given reaction, and affinity with the reactants.

3. Photocatalytic Hydrogen Evolution Reaction Continuously growing energy consumption, along with severe environmental problems caused by the utilization of fossil fuels, have induced immense research efforts directed toward the development of an alternative energy “path”, free of pollutants or greenhouse gases, such as N2O and CO2.58-59 H2 is considered as a clean fuel and a sustainable energy carrier,60-61 since it can be produced from natural sources (e.g. water, biomass), it stores high mass-specific energy density and it only produces water as result of its combustion.62-64 Currently, most of the industrially produced H2 is derived from fuel-based processes, more specifically steam methane reformation and coal gasification. Therefore, these H2 production methods contribute to the global warming since CO2 is also generated as a by-product in high quantities.65 8 ACS Paragon Plus Environment

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Interestingly, 5% of the industrially produced H2 is generated through water electrolysis, which can be a sustainable process when the electricity used is generated by renewable resources (e.g. solar radiation, wind, hydropower). However, so far, the high cost of this technique has hindered its broad industrialization.66 As mentioned above, H2 can also be directly generated through photochemical water splitting. Over the last few decades, photocatalytic H2 evolution has been widely studied, and, solardriven photocatalysis is regarded as an ideal method to sustainably produce H2.54, 67-68 Pure water splitting involves two half reactions of H2 and O2 generation. For this process to occur, it is necessary that the highest level of a material’s VB is more positive than the water oxidation level (1.23 eV vs NHE, normal hydrogen electrode), while the lowest level of the CB should be more negative than the H2 evolution potential (0 eV vs NHE). Therefore, the minimum band gap for a suitable water splitting photocatalyst should be 1.23 eV. It should be mentioned that the water oxidation half reaction involves the transfer of four electrons and is more energetically demanding than the reduction process (minimum potential of 1.23 V per electron transfer).69 In addition, since the Gibbs energy for overall water splitting is positive (ΔG>0), it is challenging to avoid the back reaction.70 Consequently, the oxidation half-reaction represents the bottleneck of overall water splitting. Thus, sacrificial electron donors are usually employed, omitting the O2 evolution reaction, to promote the H2 generation reaction.71-72 In addition to the photocatalyst, the following auxiliary components can assist a photocatalytic H2 evolution system (Figure 4): i)

Co-catalysts are employed in most cases to promote the photocatalytic performance.

The co-catalysts can significantly increase the H2 evolution reaction yield, since they serve a two-fold purpose, inhibiting the undesired recombination of the photo-generated charge carries


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and reducing the activation energy for the H2 evolution reaction.73 There are innumerable examples of noble metal-based (e.g. Pt, Pd, Au),74-75 transition-metal-based (e.g. Co, Ni, MoS2, Ni2P, CoP, CuO),76-81 nanocarbon-based (e.g. graphene, carbon nanotubes),82-83 and molecular-based (e.g. cobaloximes, Mo3S132-),84-86 co-catalysts. Among them, Pt is considered the paradigm of efficient co-catalysts for H2 production due to its low overpotential for this reaction and the suitable Fermi level for withdrawing the photoexcited electrons.87 ii)

Photosensitizers can also be utilized to extend or enhance the light absorbance of the

photocatalyst in the visible region, which accounts for 44% of the solar energy. Typical examples of photosensitizers are organic dyes, such as eosin Y,88-89 [Ru(bpy)3]2+,90-91 or porphyrins.91-92 However, it should be mentioned that the commonly used photosensitizers can be expensive as well as toxic. iii)

Sacrificial reagents such as electron acceptors (e.g. methyl viologen dication, MV2+)93

or electron donors (triethylamine, triethanolamine, ethanol, methanol) can be used to either mediate the transfer of the photogenerated electrons or balance the H2 evolution half reaction, respectively. In particular, electron donors are regularly used since they can remarkably enhance the photocatalytic performance. However, the majority of these substances is highly toxic, expensive, while they could be another direct source of energy by themselves.

Figure 4: Schematic illustration of the possible constituents of a photocatalytic H2 evolution system.


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Summarizing, the incorporation of toxic or expensive substituents in the photocatalytic H2 evolution systems has constrained the potential of widespread industrialization of this technology. To overcome this challenge, several issues need to be tackled. Firstly, photocatalysts that exhibit strong light absorbance – especially in the visible region – must be developed to avoid the use of photosensitizers. Secondly, abundant photocatalysts that exhibit high electron–hole separation efficiencies or noble-metal free co-catalysts are highly desired materials and, hence, intense research efforts should focus on this aspect. Finally, the incorporation of the highly toxic or expensive sacrificial electron donors can be avoided by focusing on the oxidation of targeted compounds (e.g. water, organic contaminants). However, as mentioned before, pursuing overall water splitting (both H2 and O2 evolution) can have a profound impact on the H2 evolution reaction yield, while in some cases the photocatalysts are incapable of generating H2 in the presence of pure water. Therefore, the alternative of dualfunctional photocatalysis for H2 evolution coupled with the oxidation of organic compounds can be very beneficial and will further be discussed in the following chapters. It is worth noting that examples of H2 evolution photocatalytic systems are beyond the scope of this review, and the readers are advised to read other reviews which are extensively covering this area.94-97

4. Photocatalytic Oxidation of Organic Compounds Although industrial development has engendered countless advancements in technology, amenities in lifestyle and rise in the economy, it has also inflicted extended deterioration of the environment. One of the most critical environmental problems faced over the last century is water pollution, mainly caused by human activities. Textile or food processing industries, pharmaceutical companies, agricultural activities, and power plants are some of the main sources of hazardous contaminants. Wastewater contains a variety of organic pollutants, such as organic dyes, phenols, fertilizers and hydrocarbons, which can cause incontrovertible 11 ACS Paragon Plus Environment

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environmental problems.98 A large fraction of these contaminants is mutagenic, carcinogenic, or toxic and their presence poses significant risks for public health.99 In addition, the vast majority of these organic substances can damage aquatic ecosystems and inhibit the exchange of O2 in water.100 Moreover, severe water scarcity is projected to impact a quarter of the world’s population within the next few years.101 Thus, increasing generation of wastewater along with the water shortage projected in the near future has provoked the urgent development of methodologies that can offer effective decontamination of water so as to enable an efficient recycling of wastewater. So far, conventional water treatment methods (e.g. coagulation, sedimentation, filtration, adsorption) have been characterized by high operating costs, as well as the fact that they can generate other undesired hazardous by-products, or they cannot fully degrade the targeted organic pollutants.102 Another option to decompose these organic substances from water is through advanced oxidation processes (AOPs), which are based on the generation of strongly oxidizing radical species (e.g. •OH) that can efficiently and non-selectively destruct even the most resistant organic species.7 The AOPs include a range of techniques, such as wet, electrochemical or super critical water oxidation103-105, ozonation,106-107 hydrogen peroxidebased methods,108-109 and photolysis. In particular, heterogeneous photocatalytic oxidation is a promising approach that can lead to complete degradation of a wide range of hazardous organic substances into easily biodegradable compounds or less toxic molecules by utilizing light. It is widely known that i. the holes (h+), ii. the hydroxyl radicals (•OH) and iii. the superoxide radicals (O2•-) are the three main active species involved in photocatalytic oxidation processes (Figure 5).110-112 As mentioned above, upon irradiation of a photocatalyst, generation of excited high-energy states of electron and hole pairs occurs. The positive hole in the VB is a powerful 12 ACS Paragon Plus Environment

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oxidizing agent that can react either directly with organic molecules and successfully oxidize them to less hazardous products such as CO2 and H2O or it can react with water to generate highly reactive hydroxyl radicals (•OH). Moreover, the promoted electron in the CB (e-) can initiate oxygen ionosorption by reacting with O2 to form an anion superoxide radical (O2•-). The latter can further react with hydrogen cations (H+) and be protonated to the hydroperoxyl radical (HOO•), which acts as an e- scavenger, being reduced to HOO−, elongating the lifetime of the excited state and subsequently being protonated to hydrogen peroxide (H2O2).7

Figure 5: Illustration of the photoexcitation of a photocatalyst in the presence of organic pollutants and the mechanism for the generation of anion superoxide and hydroxyl radicals, and the organic pollutants oxidation.

TiO2 has also been considered as a very promising photocatalyst for the mineralization of wastewater and, thus it has been extensively studied.113-114 However, its incapability of operating under visible light restricts its utilization in practical applications and thus immense research endeavours have focused on extending its absorption into the visible light region.115 Among these research studies, recently Palmisano and co-workers explored the possibility of doping Pt‐TiO2 with tungsten and nitrogen, in order to extend its light absorbance in the visible region and increase its charge separation efficiency.116 The Pt‐TiO2‐rutile based materials


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doped with N or W species were tested for H2 production through photoreforming of a glucose aqueous solution. In contrast with photocatalytic H2 evolution systems, photocatalytic organic degradation systems are usually less complicated as they usually consist of less components. In the majority of examples reported in the literature, only a photocatalyst is used to harvest the light. It should be mentioned that the photocatalytic oxidation of organic substances is commonly performed under aerated conditions, where O2 acts as an electron scavenger, capturing the photogenerated electrons and being transformed to superoxide radicals, which can also promote the degradation process. Furthermore, additional electron scavengers are often included, such as H2O2, persulfate (PS), and peroxymonosulfate (PMS), to further suppress the electron–hole pair recombination, thus enhancing the photodegradation efficiency.117-118 In contrast to the photocatalytic H2 evolution field, co-catalysts are not as frequently used for this application.110, 119-120 In the field of photocatalytic degradation of organic substances, parameters such as the pH of the solution and the initial concentration of the pollutant can strongly influence the photocatalytic efficiency.121 In particular, variations in the pH can have a profound impact on the photocatalytic degradation rate, since it is a critical parameter for the adsorption of a targeted compound on the surface of the photocatalyst. This is of paramount importance, especially when the photocatalyst is amphoteric (i.e. TiO2) and consequently, the charge of its surface is pH-dependent.122-123 In addition, the relationship between the initial concentration of a given organic compound and the photocatalytic degradation should, in principle, follow a linear trend, indicative of a Langmuir–Hinshelwood mechanism that confirms the heterogeneous catalytic character of the system.124-125 Therefore, it is crucial that these parameters are varied and optimized to obtain legitimate and comparable results. 14 ACS Paragon Plus Environment

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5. Concurrent Hydrogen Evolution and Oxidation of Organic Compounds Dual-functional photocatalysis for H2 evolution coupled with the oxidation of organic molecules in a single step is a relatively new field that has attracted radically increasing research interest over the last few years. Such dual-purpose activity can also be achieved by means of electrochemistry

105, 126-127

or photoelectrochemistry.128-134 However, in this review, we will

discuss the recent advances concerning dual-functional photocatalytic systems. Prior to a thorough description of different examples in the literature, we will highlight the differences between dual-functional photocatalysis and other comparable photocatalytic fields. The utilization of organic substrates for H2 evolution through photocatalysis was introduced in the late ‘70s and early ‘80s,135-137 when the conversion of carbohydrates to H2 was explored, commonly known as photoreforming.138-139 Similar to dual-functional photocatalysis, photoreforming focuses on the production of H2 by utilizing organic substrates. However, in photoreforming, the sacrificial organic molecules to be oxidized are not necessarily hazardous substances and can be useful carbohydrates, such as methane and ethanol.140-143 In addition, in the field of dual-purpose photocatalysis, the H2 should be mainly derived from the water reduction reaction. For example, earlier this year, Zhang et al. reported the photocatalytic dehydrogenation of benzyl alcohol for the simultaneous H2 and benzaldehyde production using a Ni-decorated Zn0.5Cd0.5S photocatalytic system.144 However, in this work, the authors attributed the H2 generation to its extraction from benzyl alcohol and thus, it cannot be considered a paradigm of dual-functional photocatalytic activity. Consequently, although these two fields can overlap, only a fraction of studies in the photoreforming field can be considered dual-functional photocatalysis, which is when either the targeted organic molecules are pollutants or when the aim of the oxidation is the synthesis of value-added products such as aldehydes, furoic acid or imine. Although there is a wealth number of photoreforming studies 15 ACS Paragon Plus Environment

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in the literature,116, 145-156 this research area is beyond the scope of this review. For more details about photoreforming, there are several interesting reviews about that field in the literature, which the readers are advised to read.157-160 Even though H2 evolution and organic oxidation have been studied separately in photocatalytic systems,161-162 their integration in a single process is relatively new. The basic principles of the individual photocatalytic fields are combined, generating new mechanistic aspects and criteria for the selection of appropriate dual-functional photocatalysts. A suitable dual-functional photocatalyst should be capable of generating electrons, which should then be transferred to protons or water molecules for the generation of H2 and oxidizing the organic substances with holes as the dominant active species. Therefore, the majority of studies in this field are conducted under anoxic conditions to avoid potential electron transfer to O2, which is a very strong electron scavenger. Examples of appropriate photocatalysts in the literature for this twofold purpose are divided into two categories based on the target of the oxidation reaction (degradation of pollutants or valorization of organic compounds) and will be critically reviewed in the following sections.

5.1 Photocatalytic Hydrogen Evolution Coupled with the Degradation of Pollutants The first subcategory of dual-functional photocatalysis involves the production of H2 integrated with the oxidation of hazardous organic molecules found in wastewater, which results in their decomposition or transformation to less harmful molecules. The general stoichiometric equation describing the photocatalytic organic oxidation in aqueous media is the following: CxHyOz + (x + 0.25y ― 0.5z)O2→Intermediate products→xCO2 +0.5yH2O

(equation 1)

One of the earliest studies in this field was conducted by Li and coworkers in the early 2000’s. 163-164

This group investigated the photocatalytic performance of commercial TiO2 (P25 16 ACS Paragon Plus Environment

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Degussa) with Pt as a co-catalyst, for H2 evolution using organic pollutants as a replacement for expensive electron donors (Table 1, Entry 2). Oxalic acid, formic acid and formaldehyde were individually explored as electron donors under UV irradiation. The incorporation of these electron donors significantly enhanced the photocatalytic performance of the system, with oxalic acid boosting the H2 production by nearly a factor of three, as compared to the other organic substances tested. Through in situ attenuated total reflection infrared spectroscopy (ATRIR), the authors suggested that the photocatalytic performance of the different electron donor systems is correlated with the nature of the interaction between the electron donors and TiO2 surface sites; the strong adsorption of the electron donor on TiO2 facilitates the electron transfer, promoting the H2 evolution. Then, the photocatalytic performance in mixture systems of electron donors was investigated in order to simulate real wastewater. The H2 evolution of an equimolar binary mixture system of oxalic and formic acid (with the same concentration) was comparable to that when solely oxalic acid was used. This is attributed to the decomposition of solely the oxalic acid, which is strongly linked to TiO2 inhibiting the adsorption of the other electron donors. Similar results were obtained when a mixture system of formaldehyde and oxalic acid was incorporated. However, when the concentration of oxalic acid decreased while the formic acid concentration was kept constant, it was apparent that formic acid also contributed to the H2 evolution. These results highlight that the concentration as well as the competitive adsorption of electron donors on TiO2 highly determines the overall performance of binary systems. Their argument was further verified by the infrared spectra of adsorbed species on TiO2 from mixed electron donor solutions (Figure 6). A few years later, the same group investigated the possibility of using different chloroacetic acids as electron donors for the H2 evolution reaction, again using Pt/TiO2 (Table 1, Entry 3).165 17 ACS Paragon Plus Environment

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Interestingly, the authors showed that such possibility is feasible, nevertheless, it cannot be generalized for all chloroacetic acids since monochloroacetic and dichloroacetic acid enhanced the photocatalytic H2 generation, but trichloroacetic acid did not. Therefore, although this group has significantly contributed in the field, investigating a wide variety of organic pollutants as sacrificial electron donors, the complete generalization of this concept could not be achieved for all organic substances employed.

Figure 6: Infrared spectra of adsorbed species on TiO2 from (a) 1.0 x 10-2 M oxalic acid and 1.0 x 10-2 M formic acid solution, (b) 1.0 x 10-2 M oxalic acid and 1.0 x 10-2 M formaldehyde solution, (c) 1.0 x 10-3 M oxalic acid and 1.0 x 10-2 M formic acid solution and (d) 1.0 x 10-3 M oxalic acid and 1.0 x 10-2 M formaldehyde solution. Solid line: the spectra of 1.0 x 10-2 M oxalic acid (a and b) and that of 1.0 x 10-3 M oxalic acid (c and d); broken line: the spectra of the mixtures. Reference spectrum was of TiO2 film in contact with water. Inset: models of formic acid (a) and oxalic acid (b) adsorbed on TiO2. Reprinted with permission from Ref 164. Copyright 2003 Elsevier.

Later during the same year, Patsoura et al. enquired into the performance of a Pt/TiO2-based photocatalytic system in the presence of a different type of organic pollutants as the electron donor.166 The selected pollutants were the following three different azo-dyes; Acid Orange 7, 18 ACS Paragon Plus Environment

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Basic Blue 41 and Basic Red 46, the employment of which resulted in enhanced photocatalytic H2 production rates (Table 1, Entry 4). In all cases, the H2 evolution rate went through a maximum and then dropped, reaching values similar to those acquired in the presence of pure water. In particular, the system with Acid Orange 7 demonstrated the highest rate maximum. These results stimulated the group to examine the impact of operational variables (dye concentration, pH of the solution and temperature) on the photocatalytic performance of the system implementing Acid Orange 7. It was found that an increase in the initial dye concentration introduced a shift of the rate maximum toward longer irradiation times (Figure 7a). The amount of H2 evolved was comparable in all cases, apart from the system with the highest concentration of 50 mg L-1, which produced significantly less H2. The reverse effect was observed when the pH values of the solution were varied. In this case, increasing pH induced faster acquisiition of the maximum rate (Figure 7b). On the other hand, smaller differences in the shift of the rate maximum were noticed when the temperature increased from 40 to 80 °C, with higher temperatures resulting in greater H2 evolution rates (Figure 7c).

Figure 7: Effect of (a) solution pH, (b) Acid Orange 7 concentration and (c) solution temperature on the rate of H2 production over Pt/ TiO2. Reprinted with permission from Ref 166. Copyright 2006 Elsevier.

Although, these research studies from both Li’s and Verikios’ groups can now be found to be barren of detailed characterization techniques, materials and range of metrics for the evaluation of the photocatalysts, they represent milestones, as they constitute very early research studies in the dualfunctional photocatalysis field. 19 ACS Paragon Plus Environment

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In the same context of TiO2 dual-functional photocatalysis, Choi’s group investigated the photocatalytic degradation of 4-chlorophenol and bisphenol A with the concurrent generation of H2 over different TiO2-based systems with and without Pt and/or fluoride surface modification (Table 1, Entry 6).167 The titania sample with both surface fluorination and platinization (Pt/FTiO2) demonstrated higher H2 evolution rates in both the presence of 4-chlorophenol and bisphenol A, attributed to a synergistic effect. However, the time-dependent total organic carbon removal (TOC) collected in the solution with 4-chlorophenol showed that complete mineralization is unlikely to occur under deaerated conditions, evincing intermediates which were detected by liquid chromatographic analysis. Therefore, air was introduced into the reactor after 5 h of irradiation, which led to rapid TOC removal. Furthermore, the photonic efficiency for H2 production was explored through ferrioxalate actinometry, which is a metric of significant importance that had been rather neglected by that time in this field. The obtained values at a wavelength within the range of 300 -500 nm were 1.16 10-3 and 3.34 10-3 for 4-chlorophenol and bisphenol A, respectively. Moreover, the possibility of using natural sunlight for this dual-functional photocatalytic application was investigated outdoors. As shown in Figure 8, the photocatalytic degradation of the examined organic substrates (4-chlorophenol and bisphenol A) coupled with H2 production successfully occurred under solar irradiation. Even though this group also focused on the most studied photocatalytic system of TiO2 with Pt, this study was circumstantial and ahead of its time, since the authors covered the important aspect of solar light harvesting and utilized proper metrics for the evaluation of the photocatalytic activity; including both rate and photonic efficiency. However, an important drawback of this work lies on the fact that photocatalyst cannot mineralize the organic substances completely under deaerated conditions. 20 ACS Paragon Plus Environment

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Figure 8: Concurrent degradation of: (a) 4-chlorophenol for three consecutive days and (b) bisphenol A for one day, over Pt/TiO2-F. Red squares: production of H2, filled circles: organic substrate, empty circles: chlorides, purple solid line: solar light intensity, green triangles: direct solar photolysis without catalyst. Reproduced from Ref 167. Copyright 2010 with permission from the Royal Society of Chemistry.

The photocatalytic degradation of 4-chlorophenol coupled with H2 generation was further explored using a different photocatalytic system based on the UV-active SrTiO3 decorated with Rh nanoparticles covered with a thin Cr2O3 barrier layer (Cr2O3/Rh/SrTiO3) (Table 1, Entry 12).168 However, in this study progressive mineralization (i.e., TOC removal) of the solution could occur even in deaerated conditions, which was associated with a different reaction mechanism induced by the addition of the Cr2O3 thin layer. More specifically, the authors suggested that the Cr2O3 layer over Rh could inhibit the back reaction of H2O formation, which leads to the observed presence of photogenerated O2 in the suspension. This indicates that the holes react with both the organic substrate and water molecules. The in-situ evolved O2


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allows for the subsequent degradation of 4-chlorophenol through its consumption. The mineralization process was expressed with the following reaction: C6H5OCl (6 Corg) + 6.5 O2 ⇒ 6 CO2 + 2 H2O + HCl. In addition, Cr2O3/Rh/SrTiO3 exhibited greater photocatalytic performance towards the H2 evolution reaction, with an apparent photonic efficiency of 0.7% at a wavelength of 330 ± 10 nm. Subsequently, the photocatalytic mechanism was further elucidated by examining the effect of dissolved O2, as well as t-butyl alcohol as a •OH radical scavenger and EDTA (Ethylenediaminetetraacetic acid) as a hole scavenger. The results highlighted the key role of the photogenerated holes, since the degradation of 4-chlorophenol was significantly suppressed in the presence of EDTA. Nevertheless, the addition of dissolved O2 or t-butyl alcohol did not influence the light-induced degradation of the organic substrate, demonstrating the insignificant electron transfer from the semiconductor to O2 and the minor contribution of •OH radicals in the degradation process (Figure 9).


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Figure 9: Schematic illustration of the photocatalytic reaction mechanisms occurring on the surface of Cr2O3/Rh/SrTiO3. Reproduced from Ref 168. Copyright 2016 permission from the Royal Society of Chemistry.

While one of the main advantages of that study was the ability of the photocatalytic system to progressively mineralize the solution even under deaerated conditions, the utilization of the noble Rh is a hindering factor for large-scale applications. Numerous research studies in the field are based on the utilization of noble metals (e.g. Pt, Ru, and Ag), but with dwindling supplies of precious metals, the development of low-cost, abundant alternatives is inevitable for a sustainable future. Considering the importance of visualizing real-life application of this dual-functional photocatalytic field, there are few literature reports exploring the utilization of wastewater169-172 and secondary effluents from municipal wastewater treatment plants.173-175 Among them, Badawy et al. investigated the simultaneous photocatalytic H2 production and mineralization of olive mill wastewater, over the nanostructured mesoporous TiO2, which was synthesized using the sol-gel method (Table 1, Entry 8).171 The initial chemical oxygen demand (COD) and total organic carbon of the olive mill wastewater examined was 8410 mg(O2) L-1 and


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3150 mg(C) L-1, respectively. The influence of different parameters (i.e. pH value, photocatalyst amount) on the H2 evolution was explored and found that at pH 3 and by using 2 g L-1 of the photocatalyst, maximum amount of H2 was generated (36 mmol), while the COD removal was 87%. Although this study has a great potential for real-life implementation of the dual-functional photocatalysis field, it exhibits the disadvantage of utilizing solely UV light in the range of 100-280 nm. Such light energy cannot be sufficiently provided by the sun and thus non-renewable sources will be needed for illumination. Recently, Wang and co-workers brought the field one-step closer to practical applications by reporting a twin reactor where the H2 produced by the dual-functional photocatalytic system was separated, avoiding further purification.176 The selected organic pollutant for degradation was phenol, an industrial compound that can be lethal to marine life even at relatively low concentrations and is highly toxic to aquatic biota and human health.177-178 In that study, two visible-light active photocatalysts were used; WO3 for the phenol degradation and Rh-doped SrTiO3 with Pt (Pt/STO:Rh) as the H2 evolution photocatalyst (Table 1, Entry 19). The latter was synthesized through a solid-state fusion and a photo-deposition method and its visiblelight absorption was ascribed to the Rh doping of SrTiO3. In addition to the photocatalysts, the system involved Fe3+/Fe2+ pairs as electron mediators. The half-reactions of H2 evolution and phenol degradation were optimized separately and then the dual-function reaction of H2 evolution and simultaneous phenol degradation was explored in a single batch reactor and then in a twin reactor. When the twin reactor was utilized, the two kinds of photocatalysts were set apart by a Nafion membrane, allowing the permeation of only FeIII ions, which were the electron mediators (Figure 10). Under the solar simulating irradiation that was used, H2 was generated by the electrons on Pt/STO:Rh, while on the other side, phenol was oxidized by the holes on WO3, 24 ACS Paragon Plus Environment

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producing CO2 and carbon monoxide (CO). The residual holes on Pt/STO:Rh and the electrons on WO3 were to be scavenged by FeII and FeIII, respectively, achieving charge balance of the photocatalysts by the counter diffusion of FeIII and FeII toward the opposite compartment, through the Fe ion exchanged Nafion membrane. The authors suggested that the rate of the photocatalytic reactions was not slowed down by the inclusion of the membrane in the twin reactor, since the transfer rate of the charge mediators was notably higher. The photocatalytic performance of this hybrid system in the twin reactor was better than in the single batch reactor (H2 production rate of 1.90 μmol g−1 h−1 and 22% phenol degradation within 6 h); this is thought to be attributed to the direct separation of H2 from the other products, suppressing the backward reaction. Although this is an innovative approach of dual-functional photocatalysis, the authors acknowledged the downsides of their study, lying into the low photo quantum efficiency, the inadequate hydrogen evolution rates and the fact that the details of the kinetics of the mechanism in the twin reactor remain elusive.

Figure 10: Illustration of dual-function reaction conducted in a twin-reactor. Reprinted with permission from Ref 176. Copyright 2018 Elsevier.

Considering the importance of photocatalytic systems in harvesting visible-light, earlier this year Jiang et al. designed a heterojunction between CdS quantum dots (QDs) and graphitic carbon nitride.179 The photocatalyst composites exhibited visible-light harvesting properties 25 ACS Paragon Plus Environment

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and adequate photocatalytic activity, especially when combined with carbon QDs (CDs/CdS/ GCN, Table 1, Entry 20). More specifically, the group investigated the concurrent photocatalytic H2 production with the decomposition of typical pollutants found mainly in printing/dyeing or pharmaceutical wastewater (e.g p-chlorophenol, bisphenol A and tetracycline) under visible-light illumination. The photocatalytic system was optimized through variation of the CdS QDs loading amount, the CDs adding amount and the initial concentration of organic pollutants. The optimized 3% CDs/10%CdS/GCN composite catalyst generated less H2 when p-chlorophenol was introduced in the system, compared to that when the photocatalytic solution was comprised of pure water. However, incorporation of either bisphenol A or tetracycline led to a noticeable increase in the H2 evolution rate. As well as with the H2 evolution rates, the photodegradation rates differed between the systems and thus it was elucidated through density functional theory (DFT) calculations. The authors attributed the highest degradation rate of p-chlorophenol to the smaller size of this molecule. The inferior photodegradation rate of bisphenol A compared to tetracycline was associated with the higher charge recombination rate within the bisphenol A molecules due to the overlapping electron clouds in the LUMO (lowest unoccupied molecular orbital) and HOMO (highest occupied molecular orbital) of these molecules. Moreover, the dissimilarity of the H2 evolution rates of the systems comprising different organic pollutants was explored by means of liquid chromatography mass spectrometry (LCMS). Based on the results obtained, during the degradation process of p-chlorophenol, the formation of intermediate products that consumed the photogenerated electrons for the reduction reactions and thus depopulate the electrons transferred to protons/water molecules. This explanation could justify the significantly lower H2 evolution rate induced by the addition of p-chlorophenol. The photocatalytic mechanism of the examined system can be also described in Figure 11. The apparent quantum 26 ACS Paragon Plus Environment

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efficiency of the system was not reported. However, considering the low-cost and great visiblelight harvesting properties of the photocatalyst, the apparent quantum yield would also be a very good indicator of the potential of these photocatalytic systems for large scale application.

Figure 11: Schematic representation of the photocatalytic mechanism for the dual-functional activity of the CDs/CdS/GCN catalyst under visible light irradiation. Reprinted with permission from Ref 179. Copyright © 2018, American Chemical Society.

Moving toward a newer class of materials, a MOF was recently reported as a dual-functional photocatalyst for simultaneous H2 production and degradation of an organic dye.180 Based on a strict definition MOFs cannot be considered as semiconductors, but they can demonstrate a semiconductor like behaviour upon illumination, stemming from the optically active ligands within the structure and their electronic interactions with the metal nodes. In the case of lightresponsive MOFs, upon irradiation, an electron will be excited from the highest occupied crystalline orbitals (HOCOs) to the lowest unoccupied crystalline orbitals (LUCOs), analogous to the VB and CB in the band theory of solids, or the bonding and antibonding orbitals in the molecular orbital theory. MOFs are a new type of photocatalysts, studied toward the H2 evolution reaction almost a decade ago.181-182 Since then, immense research studies have


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focused on their application in photocatalysis, due to their structural versatility and topologies, high porosity and tunable optical and electronic properties.183-193 In the study reported recently by Kampouri et al.,180 the well-known MIL-125-NH2 (MIL stands for Materials from Institute Lavoisier) was physically mixed with Ni2P nanoparticles as the co-catalyst and tested for concurrent H2 production and degradation of the dye Rhodamine B (RhB), under visible light irradiation (Table 1, Entry 21). Prior to the dual-functional photocatalytic tests, the MIL-125NH2-based system was optimized for the H2 evolution half-reaction, through the use of different abundant co-catalysts. MIL-125-NH2 exhibited higher H2 evolution rates when combined with both NiO and Ni2P co-catalysts, with the latter demonstrating enhanced synergy with the MOF, as manifested by photoluminescence measurements. Subsequently, the impact of different electron donors on the stability of the MIL-125-NH2 based photocatalytic system and the H2 generation rate was explored, and found that triethylamine (TEA) fulfils both requirements. Based on these insights, the integration of photocatalytic H2 evolution and degradation of RhB in a single process was investigated using the Ni2P/MIL-125-NH2 system. As a first step, the initial dye concentration was varied, in order to find the most favourable concentration for H2 production (Figure 12). The relationship between the RhB concentration and H2 production followed a volcano type trend. Initially, increasing the RhB concentration promoted H2 evolution, ascribed to the availability of more dye molecules to scavenge the photoexcited holes. Further increase in the RhB concentration induced a decrease in the amount of H2 generated, attributed to the prevention of a fraction of the incident light from being absorbed by the MIL-125-NH2 due to the excess amount of RhB. Almost complete degradation of the dye was achieved for the photocatalytic solution with lower initial RhB concentration. In the case of the photocatalytic solution that offered the highest H2 evolution rate (1.2 ppm RhB), the dye concentration was substantially decreased after the photocatalytic test. Different 28 ACS Paragon Plus Environment

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control experiments confirmed that the dye was not adsorbed by MIL-125-NH2 and that no selfphotolysis of RhB occurred. Trapping experiments of active species (with tert-Butanol as the •OH radical scavenger or triethanolamine, TEOA, as the hole scavenger) showed that the holes were the dominant species involved in the degradation process with the assistance of •OH radicals, reaffirming that RhB acts as a sacrificial electron donor.

Figure 12: (a) Impact of the initial Rhodamine concentration on the H2 evolution rate over Ni2P/MIL-125NH2 under visible light irradiation for 8 h. Inset: a photograph showing the difference in the color of the 1.2 ppm solution before and after the photocatalytic test. (b) UV-Vis spectra of reference samples and supernatants after the photocatalytic RhB decolorization process. Reprinted with permission from Ref 180. Copyright © 2018, John Wiley and Sons.

Moving to a different type of dual-functional photocatalytic activity, which involves the simultaneous production of H2 and the oxidation of inorganic compounds such as As. However, examples of such a dual-purpose approach have been rarely reported. In this context, Suib and co-workers recently reported the coupling of H2 evolution with the oxidation of AsIII over a heterostructured CdS/Sr2(Nb17/18Zn1/18)2O7−δ (CSNZO) composite (Table 1, Entry 29).194 As a first step, the CdS amount loaded was optimized for the H2 evolution reaction. Then, the effect of the pH value on the photocatalytic oxidation of AsIII was studied over the optimized 30% 29 ACS Paragon Plus Environment

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CdS/Sr2(Nb17/18Zn1/18)2O7−δ. It was found that the photocatalytic oxidation of AsIII over the optimized CSNZO was more efficient under alkaline conditions (pH 9−11). Subsequently, simultaneous photocatalytic H2 production and As ion oxidation was explored in a solution containing various amounts of AsIII (1, 5, and 10 mg L-1) and Na2S/Na2SO3 as a sacrificial reagent, at pH 9. In contrast to the photocatalytic systems integrating H2 production with organic oxidation, in this case the experiments were carried out in the presence of O2, which apparently did not influence the H2 evolution rate. Nevertheless, the oxidation of AsIII could not occur under anoxic conditions. The introduction of AsIII reduced the H2 evolution rate, with increasing amounts of AsIII causing further decrease in the production of H2. At the same time, increasing H2 evolution rates were accompanied with decreasing AsIII to AsV oxidation rates, indicating that these reactions are competitive in the presence of O2. The group attempted to elucidate the photocatalytic mechanism through trapping experiments, using TEOA (h+ scavenger), p-benzoquinone (•O2− scavenger) and isopropyl alcohol (•OH radical scavenger). Based on the results, the authors suggested the following mechanism (Figure 13): upon illumination, the photogenerated electrons flow from the CB of CdS to that of Sr2(Nb17/18Zn1/18)2O7−δ, whereas the holes follow the opposite direction, migrating from the VB of Sr2(Nb17/18Zn1/18)2O7−δ to that of CdS. The holes are scavenged by the Na2S/Na2SO3 electron donor, while the electrons can either interact with protons or with O2 to generate superoxide radicals, that oxidize AsIII to AsV. Although pioneering, in such a dual-purpose approach the utilization of electron donors is not avoided, inevitably resulting in the oxidation energy of the holes not to be utilized. Furthermore the two reactions of H2 generation and As oxidation are not promoting each other but instead competing.


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Figure 13: Photocatalytic mechanistic scheme of 30% CdS/Sr2(Nb17/18Zn1/18)2O7−δ under simulated sunlight irradiation. Reprinted with permission from Ref 194. Copyright © 2015, American Chemical Society

5.2 Photocatalytic Hydrogen Evolution Coupled with the Organic Oxidation to ValueAdded Products Chemical industry processes can not only be expensive but they can also have a deleterious impact on the environment and thus, energy sustainable and environmental friendly organic transformations are highly desired.195 A promising approach to address these challenges is by employing photocatalysis to carry out different transformation of organic molecules.196-197 Moreover, biomass carbohydrates are considered as one of the most abundant renewable sources and are predicted to be the backbone of the sustainable chemistry of the future.198 Their transformation to value-added chemicals, clean solvents, or high-energy density biofuels has been the subject of intense research efforts during the past decade, with a wide variety of materials being applied for this application (e.g. metals, carbon materials, MOFs).199-202 This is expected, bearing in mind that valorization of biomass into fuels or useful chemicals can lead


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into an economy that is less dependent on fossil fuels. This technology can be further beneficial by using waste and by-product streams as resources for valorization.203-205 For more information about the field of photocatalytic conversion of biomass to value-added products, the readers are recommended to read the recent review by Bahnemann and co-workers.206 The second subcategory of dual-functional photocatalysis for H2 production is coupled with the transformation of organic substances to useful chemicals. In this context, Jiang and colleagues used a MOF named as PCN-777 and they combined it with Pt for the photocatalytic generation of H2 and the simultaneous oxidation of benzylamine to N-benzylbenzaldimine (Table 1, Entry 24, Figure 14).55 PCN-777 is a mainly UV light-active, highly stable, mesoporous MOF, based on Zr-oxo clusters and 4,4’,4’’-(1,3,5-triazine-2,4,6-triyl)tribenzoic acid.207 The group explored the photocatalytic activity of Pt/PCN-777 for the H2 evolution half-reaction. In the presence of TEOA as an electron donor, the Pt/PCN-777 system reached a H2 evolution rate of 586 μmol h-1 g-1. Replacing TEOA with benzylamine led to a decrease of the H2 evolution (332 μmol h-1 g-1), which is thought to be associated with a weaker electron donating ability of benzylamine. When Pt/PCN-777 was irradiated in the absence of any electron donor, the H2 evolution rate was significantly lower. On the other hand, the other half-reaction of the benzylamine oxidation to N-benzylbenzaldimine, occurred at a rate of 1512 μmol h-1 g-1 and with more than 99% selectivity, in the presence of AgNO3 as an electron scavenger. While the coupling of H2 evolution with benzylamine oxidation had a negative impact on the oxidation rate as well (486 μmol h-1 g-1), the selectivity was not influenced, remaining over 99%. The group tried to prove general applicability through coupling the photocatalytic H2 generation with different photo-oxidation reactions over PCN-777. Particularly, they examined dibenzylamine oxidative dehydrogenation and cinnamyl alcohol oxidation, which successfully occurred with the simultaneous generation of H2. Furthermore, MOF-808 was also explored 32 ACS Paragon Plus Environment

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for the concurrent proton photo-reduction and benzylamine photo-oxidation. MOF-808 is another Zr-based MOF, consisting of a shorter ligand (trimesic acid) and it exhibits the same structural topology and comparable UV light absorbance to that of PCN-777.205 In contrast to Pt/PCN-777, Pt/MOF-808 demonstrated unsatisfactory photocatalytic activity for both the H2 evolution and benzylamine oxidation half reactions. The notably better photocatalytic performance of PCN-777 was attributed to its elongated and highly conjugated ligand, which extends the light absorption and greatly improves charge separation as manifested by photocurrent,

open

circuit

photovoltaic,

electrochemical

impedance

spectroscopy

measurements, and DFT calculations. In particular, DFT calculations revealed that the highest unoccupied and the lowest unoccupied crystalline orbitals (HOCOs and LUCOs, respectively) within the PCN-777 are spatially spaced, resulting in longer excited state lifetime and better electron-hole separation. However, this is not the case for MOF-808, where there is spatial proximity between the HOCOs and LUCOs, which results in very short lifetimes. Although apparent quantum yields were not calculated and the MOFs examined were predominantly active in the UV region of the solar spectrum, the group conducted a very innovative and consistent study, exploring materials for this application, with extensive exploration of different organic substrates.


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Figure 34: Schematic representation of simultaneous photocatalytic proton reduction and selective benzylamine oxidation over Pt/PCN-777. Reprinted with permission from Ref 55. Copyright © 2018, John Wiley and Sons.

Similar to the previous strategy, Li et al. reported the photocatalytic activity of a twodimensional conjugated polymer combined with a ruthenium molecular catalyst for the selective oxidation of different benzyl alcohols and concomitant H2 evolution (Table 1, Entry 25).208 However, in this case the Pt modified graphitic carbon nitride photocatalytic system (Ptg-C3N4) favorably operated under visible-light irradiation. More specifically, the Pt/g-C3N4 system demonstrated better conversion and H2 yields when alcohols bearing electron-donating and electron-withdrawing functional groups were employed, compared to the unsubstituted benzyl alcohol. However, the selectivities for aldehydes were inadequate. The same phenomenon was observed when cyclohexanol or cinnamyl alcohol were incorporated into the system. Based on a detailed analysis of the reaction intermediates and the carbon balance over 20 h, the authors ascribed the poor selectivity to the strong oxidative nature of the photogenerated holes. To tackle this issue, a well-studied catalyst [Ru(tpa)(H2O)2]2+ (RuCat, tpa = tris(2pyridylmethyl)amine) for alcohol to aldehyde conversion was combined with the Pt/g-C3N4 system. As it was expected from the redox potentials of g-C3N4 and the Ru-based molecular


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ACS Catalysis

complex, the photogenerated holes were transferred from g-C3N4 to the Ru-based catalyst, forming very catalytically active high-valent Ru(IV) = O species. The selectivity of 4methoxybenzyl alcohol oxidation to 4-methoxybenzyl aldehyde was notably improved over the hybrid RuCat/Pt/g-C3N4 system, while there was no significant change in the H2 evolution rate. The high adsorption of RuCat on Pt/g-C3N4 was associated with the enhanced electrostatic interactions between the positively charged RuCat and the negatively charged surface of Pt/gC3N4, as evidenced by Zeta potential measurements. Despite the adequate photocatalytic activity and great visible-light absorbance of the photocatalytic system studied, during longterm irradiation (20 h), a slightly decreased selectivity (91%) was observed, which was related to the partial decomposition of the molecular catalyst. Such phenomenon is not unexpected, since molecular complexes can often have a very high catalytic activity, but their stability over time can be questioned.86 That was not the case for the Pt/g-C3N4, whose stability and reusability was retained for at least ﬁve consecutive runs. In addition, the group calculated the apparent quantum yields of the RuCat/Pt/g-C3N4 with variant concentrations of 4methoxybenzyl alcohol at 400 nm. Interestingly, the apparent quantum yields of the 4methoxybenzyl alcohol oxidation demonstrated first order with respect to the concentration of the substrate. Considering that plastic waste is now ubiquitous in the environment, with more than eight million tons of plastic entering the oceans annually,209-210 producing H2 either with the simultaneous decomposition of waste plastic or from plastic waste as feedstock are considered as very attractive strategies. Such approaches could be used to supplement conventional recycling techniques, striving to handle the enormous amount of plastic waste.211 In this context, Reisner and co-workers recently proposed an ambient-temperature plastic waste photoreforming strategy, implementing CdS/CdOx QDs as the photocatalyst in alkaline media 35 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 66

under simulated solar light (Table 1, Entry 26).208 According to their definition of photoreforming, the photoexcited holes oxidized the substrate to smaller organic molecules that remained in solution, while the excited electrons reduced water to H2 (Figure 15). Therefore, that study belongs to the fraction of photoreforming studies that can be viewed as dual-functional photocatalysis, since both the criteria of water being the main source of H2 and using waste/pollutants as substrates, valorizing them to organic products, are fulfilled.

Figure 15: Diagram of the polymer photoreforming process with CdS/CdOx QDs as photocatalyst in alkaline aqueous solution. Reproduced from Ref 208. Copyright 2018 with permission from the Royal Society of Chemistry.

To be more specific, the group examined a variety of polymers, including polyvinyl pyrrolidone, polyethylene glycol, polyethylene, polyvinyl chloride, polymethyl methacrylate, polylactic acid, polyethylene

terephthalate

and

polyurethane.

Among

them,

the

CdS/CdOx-based

photocatalytic systems with polylactic acid (PLA), polyethylene terephthalate (PET), and polyurethane (PUR) generated higher quantities of H2 and thus, they were further studied. The most favorable conditions for the H2 evolution reaction were identified through optimization of the photocatalyst, NaOH and substrate concentrations. Under optimized conditions, all of the


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ACS Catalysis

photocatalytic systems with the three different polymers exhibited high H2 evolution rates for long periods of time (64.3–0.85 mmol h-1 g-1) and high external quantum yields (15–0.14 %) at λ=430 nm. Comparison experiments with 5% Pt/TiO2 and PLA or PET as substrates under identical conditions highlighted the advantage of using the noble-metal free CdS/CdOx QDs system, since the photocatalytic performance of the titania-based systems were significantly inferior (0.011-0.074 mmol h-1 g-1). These results were not surprising, considering the fact that TiO2 is a wider-band-gap semiconductor when compared to CdS/CdOx. In addition, mass spectrometry in deuterated and non-deuterated solvent proved that the evolved H2 originated from water rather than the substrate, confirming the dual-functional nature of the system. The group tried to further enhance the activity by employing a pre-treatment protocol, initiating alkaline hydrolysis of the polymers. The pre-treatment procedure involved the stirring of the substrate in the photocatalytic solution (10 M NaOH aqueous solution) at 40 °C for 24 h in the dark, followed by centrifugation and use of only the supernatant as the photocatalysis substrate. As expected, this technique promoted the photocatalytic activity for PET and PUR by a factor of four since releasing monomers into solution and removing undissolved polymer by centrifugation reduces the absorbance and scattering of the solution and accelerates the photoreforming process. In the case of PLA, the pre-treatment did not affect the activity because this polymer already dissolves readily in NaOH. Since the generation of value-added products instead of CO2 can be achieved by controlling the oxidation half-reaction, the group explored the hydrolyzed and photoreformed intermediates and products by means of proton nuclear magnetic resonance experiments (1H-NMR). The detected hydrolysis products included terephthalate, ethylene glycol and isophthalate for PET, sodium lactate for PLA and aromatic (2,6-diaminotoluene) or aliphatic (propylene glycol) components for PUR. Most of these hydrolysis intermediates can be photo-oxidized to other products, including pyruvate or 37 ACS Paragon Plus Environment

ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 66

pyruvate-based compounds, formate, glycolate, ethanol, acetate and lactate (more information can be found in Table 1, Entry 26). Finally, the group tried to give proof of real-world applicability of the system by employing a PET water bottle as the substrate. Interestingly, continuous H2 evolution was observed over the course of 6 days, with an adequate rate (~4.13 mmol h-1 g-1), sufficient external quantum yield (~2.17%) and conversion efficiency of ~5.15% (Figure 16). This systematic study covered most of the important photocatalytic aspects, offering detailed descriptions and consistent assessment of the photocatalytic performance, highlighting the high potential of this dual-functional photocatalytic field in real life applications.

Figure 16: Long-term photocatalytic performance towards H2 evolution over CdS/CdOx QDs, using a PET bottle under simulated sunlight (6 days, AM 1.5G, 100 mW cm-2, 25 °C). Reproduced from Ref 208. Copyright 2018 with permission from the Royal Society of Chemistry.

Recently, Ye et al. attempted to split pure water using P-doped ZnxCd1−xS solid solutions and after, they aimed to replace the water oxidation half-reaction with the oxidative valorization of biomass-derived 5-hydroxymethylfurfural (HMF).212 The target oxidation product 2,5diformylfuran (DFF) has attracted a lot of scientific interest because of its versatility as a monomer of furan-based biopolymers and intermediate of ligands, nematocides, antifungal 38 ACS Paragon Plus Environment

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ACS Catalysis

agents, and pharmaceuticals.213-215 The pure water splitting experiments, which showed that the undoped ZnxCd1−xS samples did not produce H2, highlighted the advantage of using interstitial P doping in ZnxCd1−xS solid solutions with rich S vacancies. The better performance of the P-doped ZnxCd1−xS was attributed to the elongated lifetime of charge carriers and the high separation efficiency of photogenerated electrons-holes. According to the results, the sample with the optimum Zn content, P-doped Zn0.5Cd0.5S, outperformed its counterparts ZnSP and CdS-P by 72 and 7.5 times, respectively. Considering the low efficiency of overall water splitting, the group then continued with replacing the oxidation half-reaction with the more useful and thermodynamically favorable HMF oxidation reaction. Employing HMF in the system boosted the H2 evolution rate to almost a two-times higher value (786 μmol h−1 g−1). After 8 h of visible-light irradiation, the conversion of HMF was approximately 40%, and the selectivity of HMF to the useful product DFF was estimated to be around 65%. The authors ascribed the production of DFF to the photocatalytic oxidation of HMF, however they did not further elucidate the mechanism through other experiments (e.g. trapping experiments of active species). Table 1: Dual-functional photocatalytic systems for H2 evolution coupled with oxidation of organic substances. The data shown in this table are presented based on IUPAC recommendations.216-217

Entry

Photocatalytic

no.

Conditions

Activity

system

Re f.

H2 Generation Coupled with Degradation of organic pollutants 1.

Pt/TiO2, P25

Solvent: Aqueous

Targeted Compound:

HER: 1160 μmol h-1 g-1 *

Degussa Amount:

solution Volume: 100

Oxalic acid

Efficiency/Yield: N/A

50 mg

mL

Initial C: 4.9×10−3 M

Stability: > 5 h

Light: 250 W high

pH: 2.9

SBET:

55±15 m g (TiO2) 2

−1

163

pressure Hg lamp


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Av Size: 30 nm

(UV-Vis)

Cocat: 0.5 wt.% Pt

Anoxic conditions

Page 40 of 66

Cocat Size: 1-2 nm 2.

Pt/TiO2 P25

Solvent: Aqueous

Targeted Compound:

HER: 2850 μmol h-1 g-1 *

Degussa

solution

Oxalic acid


Amount: 20 mg

Volume: 60 mL

Initial C: 0.01 M

Stability: > 5 h

SBET: 55±15 m g

Light: 250 W high

pH: 2.0

(TiO2)

pressure Hg lamp

Targeted Compound:

HER: 1150 μmol h-1 g-1 *

Av Size: 30 nm

(UV-Vis)

Formic acid


Cocat: 0.5 wt.% Pt

Anoxic conditions

Initial C: 0.01 M

Stability: > 5 h

2

−1

Cocat Size: 1-2 nm

164

pH: 2.9 Targeted Compound:

HER: 550 μmol h-1 g-1 *

Formadehyde


Initial C: 0.01 M

Stability: > 5 h

pH: 5.5 Targeted Compound:

HER: ~2600 μmol h-1 g-1 *

i.

Oxalic acid


ii.

Formic acid

Stability: > 5 h

Initial C: i.

0.01 M

ii.

0.01 M

pH: N/A 3.

Pt/TiO2 P25

Solvent: Aqueous

Targeted Compound:

HER: 374 μmol h-1 g-1 *

Degussa

solution

Chloroacetic acid

PY: 2.33%

Amount: 40 mg

Volume: 100 mL

(ClCH2COOH)

Stability: > 5 h

SBET: 50 m g **

Light: 250 W high

Initial C: 0.001 M

(TiO2)

pressure Hg lamp

pH: 3.6

Av Size: 30 nm**

(UV-Vis)

Targeted Compound:

HER: 190.5 μmol h-1 g-1 *

Cocat: 0.5 wt.% Pt

Anoxic conditions

Dichloroacetic acid

PY: 0.62%

(Cl2ClCOOH)

Stability: > 5 h

2

−1

Cocat Size: N/A

165

Initial C: 0.001 M pH: 3.3 4.

Pt/TiO2 P25

Solvent: Aqueous

Targeted Compound:

HER: 502.5 μmol h-1 g-1 *

Degussa

solution

Acid Orange 7


Amount: 80 mg

Volume: 60 mL

Initial C: 12 mg L-1

Stability: > 50 h

SBET: 50 m g

Light: 450 W Hg(Xe) arc

pH: 10

(TiO2) Av Size: 30

lamp (UV-Vis)

nm Cocat: 0.5 wt.%

Anoxic conditions

2

−1

166

Pt Cocat Size: N/A 5.

Pt/TiO2 P25

Solvent: Aqueous

Targeted Compound:

HER: 292.5 μmol h-1 g-1 *

Degussa

solution

Acetic acid


Amount: 80 mg

Volume: 60 mL

Initial C: 0.87 mM

Stability: > 22.5 h

SBET: 41 m g

Light: 280 W Xe-arc

pH: N/A

(Pt/TiO2)

lamp (UV-Vis)

Targeted Compound:

HER: 1275 μmol h-1 g-1 *

Av Size: N/A

Anoxic conditions

2

−1

Formic acid


Cocat: 0.5 wt.% Pt

Initial C: 1.3 mM

Stability: > 22.5 h

Cocat Size: N/A

pH: N/A

218


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ACS Catalysis

6.

7.

Pt/F-TiO2

Solvent: Perchloric acid

Targeted Compound:

HER: ~160 μmol h-1 g-1*

(Surface fluorinated

aqueous solution

4-chlorophenol

PE: 1.16 10-3

and platinized TiO2)

Volume: 60 mL

Initial C: 300 μM

Stability: > 50 h

Amount: 15 mg

Light: 300-W Xe arc

pH: 3

SBET: N/A

lamp

Targeted Compound:

HER: ~467 μmol h-1 g-1 *

Av Size: N/A

500 nm >  > 300 nm

Bisphenol A

PE: 3.34 10-3

Cocat: 3.7 wt.% Pt

Anoxic conditions

Initial C: 300 μM

Stability: > 50 h

Cocat Size: N/A Pt/CdS/TiO2 Amount: 80 mg SBET: 63 m2 g−1 (CdS/TiO2) Av Size: Crystallite size of 25 nm for TiO2 and 5 nm for

167

pH: 3 Solvent: Aqueous

Target: S2- / SO32-

HER: ~703 μmol h-1 g-1 *

solution

Initial C: 4.8 mM for


Volume: 60 mL

Na2S and 7.0 mM for

Stability: > 20 h

Light: Solar light

Na2SO3

simulator (450 W)

pH: N/A

219

Anoxic conditions

CdS Cocat: - Pt Cocat Size: N/A

CdS content: 50 mol% 8.

TiO2 nanostructure

Solvent: Aqueous

Target:

HER: N/A

mesoporous

solution

Olive mill wastewater

(H2 evolved: 38 10 μmol

Amount: 2 g L-1

Volume: N/A

COD: 8410 mg(O2) L-1

in 2h)

SBET: 59 m g

Light: 150 W Hg lamp

TOC: 3150 mg(C) L


Av Size: ~19.5 nm

(UV, λ = 100-280 nm)

pH: 3

Stability: > 4 h

Cocat: -

Anoxic conditions

Pt/F-TiO2

Solvent: HClO4 aqueous

Targeted Compound:

HER: ~467 μmol h-1 g-1 *

2

9.

10.

−1

-1

171 3

(Surface fluorinated

solution (pH=3)

4-chlorophenol



Volume: 30 mL

Initial C: 300 μM

Stability: > 20 h

Amount: 15 mg

Light: 300-W Xe arc

pH: 3

SBET: N/A

lamp

Targeted Compound:

HER: ~367 μmol h-1 g-1 *

Av Size: N/A

500 nm >  > 320 nm

Urea


Cocat: 1 wt.% Pt

Anoxic conditions

Initial C: 0.33 M

Stability: > 20 h

Cocat Size: 2-4 nm

pH: 3

Pt/ P–TiO2

Targeted Compound:

HER: ~333 μmol h-1 g-1 *

(surface phosphated

4-chlorophenol



Initial C: 300 μM

Stability: > 20 h

Amount: 15 mg

pH: 3

SBET: N/A

Targeted Compound:

HER: ~267 μmol h-1 g-1 *

Av Size: N/A

Urea


Cocat: 1 wt.% Pt

Initial C: 0.33 M

Stability: > 20 h

Cocat Size: 2-4 nm

pH: 3

Pt/GO/TiO2–F

Solvent: NaOH aqueous

Targeted Compound:

HER: ~230 μmol h-1 g-1 *

TiO2 modified with

solution

4-chlorophenol

Efficiency/Yield: N/A Stability: > 20 h

graphene oxide,

Volume: N/A

Initial C: 300 μM

GO, along with Pt

Light: 300 W Xe arc

pH: 3

and fluoride)

lamp

Amount: 0.5 g L-1

 > 320 nm

SBET: N/A

Anoxic conditions

220

221

Av Size: N/A Cocat: 0.1 wt.% Pt


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 66

Cocat Size: N/A

GO content: 2 wt.% NaF content: 1mM 11.

12.

Pt/Cd0.5Zn0.5S

Solvent: 1 M NaOH

Targeted Compound:

HER: 590 μmol h-1 g-1 *

Amount: 50 mg

aqueous solution

Glycerol


SBET: 95.3 m2 g−1

Volume: 100 mL

Initial C: 1.368 M

Stability: > 15 h

(Cd0.5Zn0.5S)

Light: 250 W high

pH: 14

Av Size: 2 nm

pressure Hg lamp

Cocat: 0.5 wt.% Pt

 ≥ 420 nm

Cocat Size: N/A

Anoxic conditions

Cr2O3/Rh/SrTiO3

Solvent: Aqueous

Targeted Compound:

HER: 800 μmol h-1 g-1 *

Amount: 15 mg

solution

4-chlorophenol

APE: 0.7%, λ= 330 ± 10 nm

SBET: N/A

Volume: 30 mL

Initial C: 300 μM

Stability: > 8 h

Av Size: 2 nm

Light: 300-W Xe arc

pH: 7

Cocat: 0.5 wt.% Rh,

lamp λ ≥ 320 nm, IR

0.75 wt.% Cr2O3

filter

(core-shell

Anoxic conditions

Cr2O3/Rh,)

222

168

Cocat

Size: 2–4 nm (Rh) 13.

Ag/Co3O4/Cu2O

Solvent: Aqueous

Targeted Compound:

HER: 97.17 μmol h-1 g-1 *

Amount: 100 mg

solution

Rhodamine B


SBET: N/A

Volume: 100 mL

Initial C: 0.02 mM

Stability: > 25 h

Av Size: 750 nm*

Light: Xe lamp (AM

pH: N/A

(Cu2O)

1.5 G, 100 mW cm-2)

Cocat: 0.5 wt.% Ag, 0.5 wt.% Co3O4

Anoxic conditions

223

Cocat Size: N/A 14.

MoS2QDs@ZnIn2S4

Solvent: Aqueous

Targeted Compound:

HER: 37.8 μmol h-1 g-1 *

@RGO

solution

Rhodamine B

AQE: 0.35%,λ= 420 nm

(MoS2 QD-

Volume: 80 mL


Stability: > 30 h

decorated

Light: 300 W xenon arc

pH: N/A

hierarchical

(AM 1.5 G, 100 mW cm-

Targeted Compound:

HER: ~18.4 μmol h-1 g-1 *

assembly of

2

Eosin Y

AQE: 0.27%, λ= 420 nm

ZnIn2S4 on reduced

780 nm ≥ λ ≥ 320 nm


Stability: > 30 h

graphene oxide)

Anoxic conditions

pH: N/A

)

Amount: 100 mg SBET: 132.27 m g 2

-1

(ZnIn2S4@RGO)

Targeted Compound:

HER: ~4.1 μmol h-1 g-1 *

Fulvic acid

AQE: 0.07%,λ= 420 nm


Stability: > 30 h

224

pH: N/A

Av Size: 1 μm (ZnIn2S4) Cocat: 5 wt.% MoS2QD RGO content: 1 wt.%

Targeted Compound:

HER: 0 μmol h-1 g-1

Methylene blue

AQE: 0%, λ= 420 nm


Stability: > 30 h

pH: N/A

Cocat Size: N/A

Targeted Compound:

HER: 4.2 μmol h-1 g-1 *

p-Nitrophenol

AQE: 0.06%, λ= 420 nm

Initial C: 20 mg L

-1

Stability: > 30 h

pH: N/A 15.

ZnO0.6S0.4 Amount: 50 mg SBET: 107.3 m g 2

−1

Solvent: Aqueous

Targeted Compound:

HER: 49.2 μmol h-1 g-1

solution

Reactive violet 5


Volume: 50 mL

Initial C: 20 mg L

-1

225

Stability: > 3 h


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ACS Catalysis

Av Size: 10-20 nm

Light: 300 W Xe lamp

Na2S and Na2SO3

Cocat: -

λ ≥ 400 nm

(S22−/SO32−) were used

Anoxic conditions

as sacrificial reagents pH: 12.3

16.

Carbon-doped TiO2

Solvent: Aqueous

Targeted Compound:

HER: 373.8 μmol h-1 g-1

Amount: 60 mg

solution

Lactic acid


SBET: N/A

Volume: 200 mL

Initial C: 0.5 M

Stability: > 6 h

Av Size:


pH: ~2

0.6 ± 1.2 nm

690 nm ≥ λ ≥ 400 nm

Targeted Compound:

HER: 311.5 μmol h-1 g-1

Cocat: -

Anoxic conditions

Acetic acid


Initial C: 0.5 M

Stability: > 6 h

226

pH: ~2 Targeted Compound:


Butyric acid


Initial C: 0.5 M

Stability: > 6 h

pH: ~2 Targeted Compound:


Propionic acid


Initial C: 0.5 M

Stability: > 6 h

pH: ~2 Targeted Compound: i. ii.

HER: ~666.67 μmol h-1 g-1 *

Lactic acid


and

Stability: > 6 h

Acetic acid

Initial C: i.

0.83 M

ii.

1.25 M

pH: ~2 17.

Pt/TiO2

Solvent: Aqueous

Targeted Compound:

HER: 101 μmol h-1 g-1 *

Amount: 10 mg

solution

Oxalic acid

PE: 0.126%

SBET: 120.4 m2 g−1

Volume: 215 mL

Initial C: 1.11 mM

Stability: > 4 h

(Pt/TiO2)

Light: 450 W Xe arc

pH: 3

Av Size: 10 nm

lamp (UV-Vis)

Cocat: 0.25 wt.% Pt

Anoxic conditions

227

Cocat Size: 1-2 nm 18.

TiO2 (mixed TiO2

Solvent: Aqueous

Targeted Compound:

HER: 19.4 μmol h-1 g-1

nanospheres and

solution

Glycerol


nanosheets)

mL

Initial C: 10 vol%

Stability: > 5 h

Amount: 100 mg

Light: 300 W Xe arc

pH: N/A

Volume: 120

SBET: 65.9 m2 g−1 for

lamp (UV-Vis)

TiO2 nanospheres

Anoxic conditions

228

and 98.3 m2 g−1 for TiO2 nanosheets Av Size: 21.5 nm for TiO2 nanospheres and 30.0–60.0 nm for TiO2 nanosheets Cocat: -


ACS Catalysis 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

19.

i.

Page 44 of 66

Solvent: 2 mM FeCl2

Targeted Compound:

HER: 1.9 μmol g−1 h−1

Pt/STO:Rh (Rh-

aqueous solution

Phenol

PQE: 0.014%

doped SrTiO3 with

Volume:

Initial C: 200 μM

Stability: > 5 h

Pt ) and

Light: 300 W Xe

*Fe3+/Fe2+ pairs were

ii.

WO3

Amount:

lamp (AM1.5 G)

added as electron

Anoxic conditions

transfer mediators

176

pH: 2.4

i. 250 mg (Pt/STO:Rh) ii. 100 mg (WO3)

*In a twin reactor SBET: i. 4.01 m2 g−1 (Pt/STO:Rh) ii. 4.1 m2 g−1 (WO3) Av Size: i. 196 nm (Pt/STO:Rh) ii. N/A (WO3) Cocat: 0.8 wt.% Pt Cocat Size: 3-5 nm 20.

CDs/CdS/ GCN

Solvent: Aqueous

Targeted Compound:

HER: 5 μmol g−1 h−1

(3%CDs/10%CdS/

solution Volume: 80 mL

p-Chlorophenol


GCN,


Initial C: 10 mg L

carbon quantum

λ > 420 nm

pH: N/A

dots/CdS quantum

Anoxic conditions

Targeted Compound:


dots/g-C3N4)

Bisphenol A


Amount: 50 mg

Initial C: 10 mg L

SBET: N/A

pH: N/A

Av Size: 3-8 nm

Targeted Compound:


(CDs and CdS)

Tetracycline


Cocat: -

Initial C: 10 mg L

-1

-1

-1

179

Stability: > 20 h

Stability: > 20 h

Stability: > 20 h

pH: N/A 21.

Ni2P/MIL-125-NH2

Solvent: CH3CN-based

Targeted Compound:


Amount: 17 mg

aqueous solution

Rhodamine B


(MIL-125-NH2

Volume: 17 mL

Initial C: 1.2 ppm (~2.5

Stability: > 36 h

formula:


μM)

[Ti8O8(OH)4-(O2C-

λ > 420 nm

pH: N/A

C6H3NH2-CO2)6])

Anoxic conditions

229

180

SBET: ~1200 m2 g1

for MIL-125-NH2

and 27 m2 g-1 for Ni2P

Av Size:

~1 μm (MIL-125NH2) Cocat: 9.2 wt.% Ni2P Cocat Size: 10-20 nm

H2 Generation Coupled with Transformation of Organic Substances to Value-added Oxidation Products


Page 45 of 66 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

22.

NiP/ NCNCNx

Solvent: 0.02 M

Targeted Compound:


(Ni(II)

phosphate (buffer)

4-methylbenzyl alcohol

Products: 4-

bis(diphosphine)/

aqueous solution

Initial C: 0.01 M*

Methylbenzaldehyde

Cyanamide

Volume: 3 mL

pH: 4.5

Products Stoichiometry: 1:1

functionalized

Light: Solar light

(H2 : 4-Methylbenzaldehyde)

carbon nitride)

simulator (AM 1.5G, 100

Amount: 0.005 g

mW cm )

EQE: 15.2 ± 1.5%, λ = 360 ± 10 nm

(

Anoxic conditions

NCN

2

CNx)

230

Stability: > 25 h

SBET: N/A Av Size: N/A Cocat: 50 nmol Cocat Size: N/A 23.

Ni/CdS nanosheets

Solvent: Aqueous

Targeted Compound:

HER: ~56.25 mL h-1 g-1 *

Amount: 10 mg

solution

Furfural alcohol

(in 8h)

SBET: 63 m2 g-1 for

mL

Initial C: 10 mM

Products: Furoic acid

CdS and 64 m g

Light: blue 8 W LED

pH: N/A


2

1

-

for Ni/CdS

Av Size: ~ 1.1 nm

Volume: 10

λ=440-460 nm

Anoxic conditions

231

Stability: > 96 h Targeted Compound:

HER: ~6.25 mL h-1 g-1 *

thickness

5-

(in 8h)

Cocat: 2.66 wt.% Ni

hydroxymethylfurfural

Products:

(Ni2+

Initial C: 10 mM

2,5-Furandicarboxylic acid

oxide/hydroxide and

pH: N/A


metallic Ni, 1:1 ratio)

Stability: > 96 h

Cocat Size: N/A 24.

Pt/PCN-777

Solvent: DMF-based

Targeted Compound:


Amount: 10 mg

solution with H2O

Benzylamine

Products: >99% N-

(PCN-777 formula:

Volume: 5.1 mL

Initial C: 0.99 vol%*

Benzylbenzaldimine

pH: N/A


[Zr6(O)4(OH)10(H2O)6


(TATB)2]

(UV-Vis)

TATB: 4,4’,4’’-s-

Anoxic conditions

55

Stability: > 6 h

triazine-2,4,6-triyltribenzoate)207 SBET: 1700 m2 g-1 (PCN-777) Av Size: N/A Cocat: 2.3 wt.% Pt Cocat Size: 2.8 nm 25.

Pt/g-C3N4

Solvent: Aqueous

Targeted Compound:

HER: 810 μmol h-1 g-1*

alcohol

solution

4-methoxybenzyl

Products:

dehydrogenation

mL

alcohol

4-methoxybenzyl aldehyde

Amount: 10 mg


Initial C: 10 mM

AQY: N/A

SBET: N/A

 > 400nm

pH: N/A

Stability: > 100 h

Av Size: N/A

Anoxic conditions

Volume: 10

232

Cocat: 1 wt.% Pt Cocat Size: N/A Pt/Ru(IV)/g-C3N4

HER: 450 μmol h-1 g-1 *

alcohol

Products:

dehydrogenation

4-methoxybenzyl aldehyde


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Amount: 10 mg

AQY: 0.28%, λ= 400 nm at

SBET: N/A

0.1 M concentration of

Av Size: N/A

substrate

Cocat: 1 wt.% Pt

Stability: > 20 h

and 0.08 mM RuCat Cocat Size: N/A 26.

CdS/CdOx quantum

Solvent: NaOH aqueous

Targeted Compound:

HER: 64.3 103 μmol h−1 g−1

dots (QS)

solution

Polylactic acid

(± 14.7 103 μmol h−1 g−1)

Amount: 0.21 mg

Volume: 2mL

Initial C: 50 mg mL

SBET: N/A

Light: Solar light

pH: N/A

Av Size: ~5nm (CdS

simulator (AM 1.5G, 100

EQY: 15 ± 0.7%, λ= 430 ±

QS)

mW cm2)

1.5 nm (pre-treated solution)

Cocat: -

Anoxic conditions

Amount: 0.24 mg

-1

208

Products: Pyruvate and pyruvate-based compound

Stability: > 144 h Targeted Compound:

HER: 3.42 103 μmol h−1 g−1

Polyethylene

(± 0.87 103 μmol h−1 g−1)

terephthalate

Products: Formate,

Initial C: 25 mg mL

-1

pH: N/A

glycolate, ethanol, acetate and lactate EQY: 3.74 ± 0.34%,λ= 430 ± 1.5 nm (pre-treated solution) Stability: > 144 h

Amount: 0.21 mg

Targeted Compound:

HER: 0.89 103 μmol h−1 g−1

Polyurethane

(± 0.12 103 μmol h−1 g−1)

Initial C: 25 mg mL-1

Products: Formate, acetate,

pH: N/A

pyruvate and lactate EQY: 0.14 ± 0.03%,λ= 430 ± 1.5 nm (pre-treated solution) Stability: > 144 h

27.

Ni/CdS

Solvent: CH3CN-based

Targeted Compound:

HER: 21.4 103 μmol h−1 g−1

Amount: 10 mg

aqueous solution

Benzylamine

Products: 97% Imine

(CdS)

Volume: 3.15 mL

Initial C: 0.158 mmol

Products Stoichiometry: 1:2

SBET: N/A

Light: 1.85 W Xe lamp

mL-1

(H2 : Imine)

 > 420nm

pH: N/A

AQY: 11.2%,λ= 450 nm

SBET: 24.2 m g 2

-1

Anoxic conditions

(CdS)

233

Stability: > 6 h

Av Size: 40-100 nm * Cocat: 3.5 wt.% Ni Cocat Size: N/A 28.

P-doped Zn0.5Cd0.5S

Solvent: Aqueous

Targeted Compound:

HER: 786 μmol h−1 g−1

Amount: 1 mg

solution Volume: 5 mL

5-

Products: 2,5-diformylfuran

SBET: N/A

Light: White LED light

hydroxymethylfurfural

Av Size: 20–60 nm

(30 × 3 W)

Initial C: 2 mg mL

a

Cocat: -

Anoxic conditions

pH: N/A

organic substrate)

-1

212

AQYa: 0.12%, λ= 450 nm

in pure water (absence of

Stability: > 8 h

H2 Generation Coupled with Oxidation of Inorganic Compounds 29.

CdS/Sr2(Nb17/18Zn1/18

Solvent: Aqueous

Targeted Compound:

HER: 1237.6 μmol h-1 g-1

)2O7−δ

solution with Na2S/

AsIII


Na2SO3 as sacrificial

Initial C: 1 mg L

-1

194

Stability: > 1.25 h


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Amount: 20 mg

reagents (0.35 M Na2S;

SBET: 28.5 m g

0.25 M Na2SO3)

2

-1

Av Size: N/A

Volume: 80 mL

Cocat: -


30% (atom %)CdS

780 nm ≥ λ ≥ 320 nm

Sr2(Nb17/18Zn1/18)2O7−

Oxic conditions

pH: 9

δ

*Data calculated based on the reported results. ** Data based on the literature. SBET: Brunauer– Emmett–Teller surface area, Av: Average, Cocat: Co-catalyst, C: Concentration, HER: H2 evolution rate, COD: Chemical oxygen demand, TOC: Total organic carbon, AQY: Apparent quantum yield, PY: Photonic Yield, APE: Apparent photonic efficiency, AQE: Apparent quantum efficiency, PQE: Photo quantum efficiency, EQE: External Quantum Efficiency, EQY: External quantum yield.

5.3 Apparent quantum yields and quantum efficiencies Metrics based on the incident or absorbed radiation and photon flux (e.g. apparent or not, quantum yield or efficiency, photonic yield or efficiency) are very important for the evaluation of photocatalytic systems. However, there is a general confusion in the terminology and how these values are reported in the literature.234 Although, in table 1, the terminology used is based on each scientific report, in this section, prior to comparison of the values, each metric will be defined and differentiated from the others based on recommendations by the International Union of Pure Applied Chemistry (IUPAC) report.217 Generally, the terms that contain the word yield (e.g. quantum or apparent quantum or photonic yield) are based on monochromatic irradiation, while the terms that include the word efficiency (e.g. quantum or external quantum or photonic efficiency) are based on polychromatic irradiation. Li et al. used the metric of photonic yield (PY) to report the photocatalytic activity of the Pt/TiO2 system, toward the H2 evolution reaction in the presence of chloroacetic acids (table 1, entry 3).165 The PY was estimated to be 2.33% and 0.62% when chloroacetic and dichloroacetic acid were used, respectively. In principle, PY is defined as the ratio of the photoreaction rate measured for a specified time interval to the rate of incident photons within a defined 47 ACS Paragon Plus Environment

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wavelength interval. However, the authors did not describe in detail the process they followed to determine the PY value and reported briefly that their calculations were based on the apparent intensity of incident light as determined through ferrioxalate actinometry. Kim et al. reported the photonic efficiency (PE) of a similar system (surface fluorinated and platinized TiO2 system) to be 1.16 10-3 % and 3.34 10-3 % for the H2 half reaction with the simultaneous degradation of 4-chlorophenol and bisphenol A, respectively (table 1, entry 6).167 The group defined PE to be equal to double the H2 molecules generated divided by the number of incident photons within 300 nm - 500 nm for 5 h. Cho et al. reported the metric apparent photonic efficiency (APE), as previously defined by Kim and co-workers (APE = 2 X [number of H2 molecules generated]/[number of incident photons]), however in this case the group used monochromatic irradiation at λ = 330 ± 10 nm (table 1, entry 12).168 The value obtained was 0.7% for the Cr2O3/Rh/SrTiO3 photocatalytic system in the presence of chlorophenol. Furthermore, several groups reported the apparent quantum yield (AQY) of their system which is consistently and correctly defined as the number of reacted electrons divided by the incident photons of specific monochromatic energy. More specifically, Liu and co-workers estimated the AQY for the photocatalytic system of MoS2 QD-decorated ZnIn2S4 on reduced graphene oxide to be 0.35% at 420 nm, for their best performing system with RhB (table 1, entry 14).224 Slightly lower was the AQY value reported for the system containing molecular Ru catalyst with Pt modified g-C3N4, reported by Sun and co-workers (0.28%) (table 1, entry 25).232 In this case the H2 evolution reaction was coupled with the selective oxidation of 4-methoxybenzyl alcohol to 4-methoxybenzyl aldehyde. The values for the AQY calculation were collected at λ= 400 nm, which is barely the threshold of the visible spectra, despite the extensive visible-light harvesting properties of the photocatalytic system. The AQY values in the visible region at λ= 450 nm were reported by Zhao’s and Chen’s groups for Ni/CdS and P-doped Zn0.5Cd0.5S (table 48 ACS Paragon Plus Environment

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1, entries 27, 28).212, 233 However, the value obtained for the Ni/CdS system was almost two orders of magnitude higher than that for the P-doped Zn0.5Cd0.5S (11.2 % versus 0.12%). It should be mentioned that the targeted oxidation reactions were based on the conversion of benzylamine to imine and 5-hydroxymethylfurfural to 2,5-diformylfuran for the Ni/CdS and the P-doped Zn0.5Cd0.5S, respectively. However the AQY value for the P-doped Zn0.5Cd0.5S was calculated based on the irradiation of the photocatalyst in the absence of organic substrate (pure water). Furthermore, the Ni/CdS photocatalytic system demonstrated higher selectivity and conversion rate compared to the P-doped Zn0.5Cd0.5S (100% conversion and 97% selectivity for imine versus 40% conversion and 65% selectivity for 2,5-diformylfuran). In addition, two very innovative studies recently used the terms external quantum efficiency and yield (EQE, EQY) to evaluate the photocatalytic performance of the Ni(II) bis(diphosphine)/Cyanamide functionalized carbon nitride (NiP/NCNCNx) and CdS/CdOx QS, respectively (table 1, entries 22, 26).208 The NiP/NCNCNx was used for the production of H2 coupled with the simultaneous conversion of 4-methylbenzyl alcohol to methylbenzaldehyde, while CdS/CdOx QS were used for H2 evolution with the simultaneous degradation of plastics (please see previous section for more information). Nevertheless, EQE is generally defined as the photocurrent collected per incident photon flux, as a function of illumination wavelength.234 Although the groups used different terms, they reported the same metric defining it as

2 𝑛𝐻2

𝑁𝐴 ℎ 𝑐

𝑡𝑖𝑟𝑟 𝜆 𝐼 𝐴

100 (where nH2 is the moles of photogenerated H2, NA is Avogadro’s constant, h is the Planck constant, c is the speed of light, tirr is the irradiation time, and A is the cross-sectional area of irradiation).The obtained values were 15.2% at λ = 360 ± 10 nm for NiP/NCNCNx and 15% at λ = 430 ± 1.5 nm for CdS/CdOx QS, when polylactic acid was employed.


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5.4 Implementation of dual-functional photocatalysis in real-life Complete mineralization of organic pollutants during tertiary wastewater treatment through photocatalytic oxidation can be challenging, since during this process several heavier organic molecules (in terms of molecular weight) are transformed into intermediate smaller molecules, instead of CO2 and H2O. However, these intermediate organic products are more suitable to be used as electron donors for photocatalytic H2 evolution.173 Thus, an approach to implement the dual-functional photocatalysis can be similar to the biological wastewater treatment, during which sequential anaerobic and aerobic phases are employed.167 More specifically, the secondary effluents could be treated through photocatalytic oxidation in aerobic conditions, generating smaller organic molecules as intermediates. Subsequently, the effluents could be further processed through dual-functional photocatalysis in anoxic conditions, where the intermediate organic molecules can work as electron donors for the production of H2 which will lead to their gradual degradation. More cycles of sequential photocatalysis in aerobic and anoxic conditions, for organic molecule oxidation of larger organic molecules and dual-purpose photocatalysis for H2 production and organic oxidation of smaller molecules, respectively, can be employed until complete mineralization of the effluents occurs. Regarding the reactors that can be employed for such process, the use of single batch photocatalytic reaction systems that are frequently employed is not an optimum solution. This is attributed to the fact that during the dual-purpose photocatalytic process, not only H2 but also other gaseous products such as CO and CO2 can be present at the top region of the reactor, due to the degradation of organic molecules. Thus, H2 should be separated from the gaseous mixture prior to capture for utilization, which increases both the operation cost and the technological difficulty. A strategy to avoid the limiting step of H2 purification is by using


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twin reactors, as suggested by Wang and co-workers.176

Following this strategy, H2 is

separated instantly when produced on the one side of the twin reactor. Employment of dual-purpose photocatalysis, serving to recover energy from wastewater is not only a sustainable approach, but could also decrease the overall cost of wastewater treatment. Although visualizing practical application of this field has been achieved through intense research efforts during the last few years, there are still challenging issues that should be addressed, associated with the photocatalytic activity of the materials. Currently, there are very few examples of dual-purpose photocatalytic systems in the literature that exhibit high AQY or EQY and H2 evolution rates of more than few mmol h-1 g-1.163-164,

208

Thus, future research

efforts should focus on developing efficient and robust photocatalysts, exhibiting intense and wide light harvesting properties, and low charge carrier recombination rates, as to surpass a quantum efficiency of 10%, which is the lower limit for commercial applications to be feasible.29, 235-236

6. Conclusion and Perspectives Our perspective summarizes the innovative advances in the relatively new field of dualfunctional photocatalysis for the simultaneous H2 generation and oxidation of organic substances. In this review, the differences between this field and the comparable photoreforming field are identified, along with the criteria for a photoreforming study to be acknowledged as a dual-functional photocatalytic study as well. While overlapping, only a small part of the photoreforming studies can be regarded as dual-purpose photocatalysis, which to our point of view involves either the oxidation of pollutants or the transformation of organic substances to high-value chemicals, coupled with H2 generation from water, as the


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main source. The studies in this scientific area were divided into the following categories based on the nature and target of the oxidation reaction; oxidative valorization or degradation. It is worth noting that the former subcategory has attracted more scientific interest over the last few years. The description of the basic principles and mechanistic aspects of the two photocatalytic fields comprising this new hybrid process (water reduction and organic substances oxidation), allows for better comprehension of the criteria for the selection of appropriate dual-functional photocatalysts. Desired characteristics of a dual-purpose photocatalytic system include those applied in general photocatalysis, comprising low cost, enhanced light absorbance in the visible region, prolonged excited state lifetime and high electron-hole separation efficiency. In addition, a dual-purpose photocatalyst for H2 production coupled with organic oxidation should have VB and CB with suitable energy and be capable of oxidizing the organic molecules with holes as the dominant active species and with the electrons being transferred to water molecules or protons. Since O2 is a good electron scavenger, almost every study in the field is carried out under anoxic conditions. In order to bring this field a step closer to real-world applications, future research endeavors should be directed toward the generation of abundant, chemically stable and visible-light responsive materials that can efficiently harvest the solar irradiation. It is apparent that the majority of the initial scientific efforts were conducted over wide-bandgap semiconductors (i.e. TiO2), restricting the profitable irradiation to only 4% of the solar spectrum that constitutes the UV region. In addition, most of the early studies in this field were very specific in terms of the targeted oxidation compounds, representing only proofs-ofconcept. Nevertheless, during the last few years there has been a rapid development in this field, with the research being gradually shifted toward visible-light responsive materials 52 ACS Paragon Plus Environment

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including MOFs, quantum dots and other, that can take advantage of a wider region from the solar spectrum. Furthermore, some of the latest research endeavors have escaped the proof-

of-concept window and are focused on the general applicability of this technology. This rapid progress demonstrates that real-life application of this dual-functional field cannot be considered utopia anymore. Practical widespread application of this dual-purpose photocatalytic approach using abundant materials and solar energy can be revolutionary in both the energy and the environmental areas.

Acknowledgements SK and KCS thank Swiss National Science Foundation (SNSF) for funding under the Ambizione Energy Grant n.PZENP2_166888. KCS is thankful to Materials’ Revolution: Computational Design and Discovery of Novel Materials (MARVEL), of the Swiss National Science Foundation (SNSF) for funding – DD4.5. The authors thank Dr. C. P. Ireland for valuable discussions.

AUTHOR INFORMATION

Corresponding Author *[email protected]

References 53 ACS Paragon Plus Environment

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Dual-Functional Photocatalysis for Simultaneous Hydrogen

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