Author: Maxim A. Nasalevich
PhD Thesis, Delft University of Technology The Netherlands, March 2016
for solar energy utilization
Proefschrift
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus Prof. ir. K. Ch. A. M. Luyben; voorzitter van het College voor Promoties,
in het openbaar te verdedigen op
24 Maart 2016 om 15:00 uur
door
Maxim A. NASALEVICH
Master of Science, Chemistry Novosibirsk State University geboren te Poykovskiy, USSR
Composition of the doctoral committee:
Rector Magnificus Chairperson
Prof. dr. F. Kapteijn Delft University of Technology, promotor Prof. dr. J. Gascon Delft University of Technology, promotor Independent members:
Prof. dr. B. Dam Delft University of Technology
Prof. dr. H. Garcia Universidad Politécnica de Valencia
Prof. dr. D. De Vos K. U. Leuven
Prof. dr. E. J. M. Hensen Eindhoven University of Technology
Prof. dr. G. Mul University of Twente
Prof. dr. ir. M. T. Kreutzer Delft University of Technology (reserve)
The research reported in this thesis was conducted at the Catalysis Engineering section of the Chemical Engineering Department, Faculty of Applied Science, Delft University of Technology (Julianalaan 136, 2628 BL, Delft, the Netherlands), with financial support received from The Dutch National Research School Combination Catalysis Controlled by Chemical Design (NRSC-Catalysis).
Proefschrift, Technische Universiteit Delft Met samenvatting in het Nederlands ISBN 978-94-6186-610-3
Copyright © 2016 by Maxim A. Nasalevich
All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in any form by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without the prior permission of the author.
Chapter 2. Organic linker defines the excited-state decay of
photocatalytic MIL-125(Ti)-type materials.
29
Chapter 3. Electronic origins of photocatalytic activity in
d
0metal organic frameworks.
55
Chapter 4. Enhancing optical absorption of Metal–Organic Frameworks
for improved visible light photocatalysis via post-synthetic
modification of NH2-MIL-125(Ti).
95
Chapter 5. Active site engineering in Metal-Organic Frameworks:
cobaloxime-derived MOF-based composite
for light-driven H2 production.
119
Summary and outlook
167
Samenvatting en vooruitzichten
175
About the author
183
List of publications
187
This chapter is based on the following publications: “M. A. Nasalevich, M. van der Veen, F. Kapteijn, J. Gascon, Metal–organic frameworks as heterogeneous photocatalysts: advantages and challenges, CrystEngComm, 2014,16, 4919-4926; F. Khodadadian, M. Nasalevich, F. Kapteijn, A. I. Stankiewicz, R. Lakerveld, J. Gascon, Photocatalysis: Past Achievements and Future Trends, Chapter in ‘Alternative Energy Sources for Green Chemistry’ ed. by A., Stankiewicz, G. Stefanidis, RSC, accepted.”
Abstract:
The use of metal organic frameworks (MOFs) as heterogeneous photocatalysts is critically reviewed and analogies are drawn with strategies applied to improve performance of classical semiconductor-based photocatalysts. Recent advances in materials development for both types of catalysts are discussed. At first the general assumption of MOFs behaving as semiconductors is revisited, demonstrating that such semiconducting behaviour only occurs in a very limited subset of materials. Further, the main approaches for efficient light harvesting and active site engineering in MOF-based photocatalysts are discussed. Finally, the main advantages of MOFs as photocatalysts and the challenges that need to be addressed in order to improve catalytic performance are evaluated.
2
1.1. INTRODUCTION
After the oil-based chemical technology revolutionized nearly every aspect of life of humanity in the 20-th century, the concept of sustainable chemistry emerged in the 90s as a rational and necessary alternative.1 Undoubtedly, the increasing demand of today for energy as well as for materials and raw chemicals can no longer be accounted for without appeal to innovative chemical processes utilizing renewable feedstock and preserving the fragile ecological balance on the planet. The production of chemicals and polymers yet majorly relies on oil and it can hardly be fully substituted by biomass-derived feedstock. A number of everyday-life's products already contain a certain percentage of bio-based polymers such as that of Coca-Cola, Danone and others, whose products are delivered to customers with the help of new bottles produced by Avantium technologies.2 The energy landscape has experienced even a more drastic change where the energy of wind and sun has a substantially greater impact on the overall distribution of various energy sources exploited and this contribution is expected to grow over the next few decades.3 Renewable technologies are indeed changing the world: wind turbines in the fields and waters of Europe alongside the sapphire glow of silicon photovoltaic (PV) panels on the roofs already became a typical landscape in developed countries. Whereas in the early years the use of PV-panels was only justified in space, today it is an established and often economically reasonable technology. However, when this technology is employed, the question of energy storage appears to be of vital importance.4 One has to deal with the intrinsic mismatch of the peak hours of energy production and consumption. Storing the overproduced electrical energy in batteries does not seem to be feasible considering the scale of this issue and the shortage of lithium as well as some other rare-earth elements, which is less spoken off yet, even more dramatic than in the case of oil and natural gas.5 The ideal solution is to store the energy in the form of chemical bonds of energy-dense molecules such as H2, CH3OH and others.6 Electrocatalytic pathways for obtaining these molecules at the minimum cost are being actively explored.7, 8 Nevertheless, even the theoretically attainable efficiency of such sequence of transformations is not very high: modern PV panels operate at the typical efficiency of around 10%9 while this efficiency still has to be multiplied by the efficiency of electrocatalysis. An ideal alternative would be a process in which the energy of the sun is directly converted to valuable chemicals,
3 would them be hydrogen or hydrocarbons, in one single step and in an efficient manner. Photocatalysis has the great potential to become the ultimate choice.
Since the discovery of photocatalysis in 1972 by Fujishima and Honda10, much effort has been devoted to improve the relatively low efficiency of photocatalytic transformations. In this pioneering work the researchers observed that when illuminated TiO2 is used as a photoanode for water splitting in combination with Pt as a cathode, the reaction occurs at a much lower potential than one would expect for the ordinary water electrolysis. Since this historical moment, a great number of scientists in catalysis attempted to enhance the so-called quantum efficiency of this process, defined as ratio between the number of molecules produced in a photocatalytic reaction and the number of incident photons. Unfortunately, despite the efforts, in the vast majority of reported cases the efficiency does not exceed a 10%.11, 12 After 40 years of intense research TiO2 remains the most investigated photocatalyst (ca. 50% of the publications in the field) and, to the best of our knowledge, the only one applied commercially. The constantly growing scientific attention to this problem is reflected in the impressive 4200 publications in 2014.13 The captivation of photocatalytic reactions can be attributed to the abundancy of sunlight as an energy source. However, titania is only capable of operating under illumination of light with a wavelength shorter than ca. 400 nm that corresponds to its band gap energy of 3.2 eV.14 The sun irradiation spectrum contains only 5% of photons with such desired energies.15 Therefore, one of the main pathways to obtain highly active titania photocatalysts lies in sensitization of the semiconductor towards visible light absorption. Applied synthetic strategies and a few representative examples will be briefly given in this chapter. Since photocatalysis proceeds at the surface of semiconductors, another generally accepted solution for increasing activity is employing TiO2 materials with high surface area. Addition of noble metals was proven to yield substantially higher photocatalytic rates, this applies to TiO2 and other semiconductors. Zinc oxide, CdS, WO3 and many other inorganic compounds were subjected to successful evaluation in a variety of photocatalytic transformations. Despite the progress achieved with these more traditional semiconductors, the absence of the ultimate photocatalyst triggers the endeavours in seeking for new alternative materials. Oxides like TiO2, ZnO and ZrO2 and many others suffer from undesirably large bandgap energies,16, 17 hematite (Fe2O3) is characterized by fast electron-hole recombination,18, 19 whereas sulphides such as CdS experience deactivation.20 The ultimate photocatalyst should have a narrow bandgap for capturing a large fraction of solar light, must be free of noble metals, be of heterogeneous nature and remain stable after
4
multiple cycles of photocatalytic reactions while performing at a high quantum efficiency. Many novel materials emerged as potential photocatalysts over the last decades including perovskites and heteropolyacids.
Another new class of advanced materials for photocatalysis, and the main topic of this PhD thesis, are Metal-Organic Frameworks (MOFs), crystalline porous solids of hybrid organic-inorganic nature. Although the first reports on photocatalytic activity of MOFs appeared in the early 2000s, these coordination polymers are receiving massive attention primarily due to their excellent tuneability as well as extremely high surface areas. The idea to apply MOFs in photocatalysis most likely originates from the similarity between Metal-Organic Frameworks based on a given metal and their corresponding metal oxides. However, this ‘at first sight’ analogy is false, as these solids possess properties very distinct from the ones found in classical semiconductors, as discussed in detail in this chapter.
1.2. PHOTOCATALYST DEVELOPMENT
1.2.1. GENERAL CONSI DERATIONS
The development of photocatalytic systems originates from using TiO2 as an electrode in photo-electrocatalytic water splitting in the early 70s. At that time, carrying out a photocatalytic reaction on a titania slurry was not a straightforward task.21 In one of the pioneering works published by Kawai et al. in 1980, the researchers tackled the problem of water splitting on a powdered RuO2/TiO2/Pt system by adding various organic compounds, such as methanol, to the reaction mixture. This resulted in a quantum efficiency of 44%.22 Already in this early work they speculated that most likely the reduction of protons occurs on the platinum surface whereas the oxidation counterpart takes place at the ruthenium oxide component of the composite. This system demonstrated an activity two orders of magnitude higher than the one of bare titania. Even from the early days of research in photocatalysis it was understood that pristine bulk titanium dioxide could not serve as an efficient catalyst unless modified or combined with other materials.
5
Figure 1.1. Schematic view of processes following photoexcitation in a semiconductor.
In a traditional sense, a photocatalytic process on a heterogeneous semiconducting catalyst proceeds as follows (Figure 1.1): at first the light of the energy exceeding the bandgap energy is absorbed by a semiconductor and the charge separation takes place. Electrons are promoted from the valence band of the semiconductor to the conduction band leaving behind positively charged holes. Further, the free charges can undergo several pathways as they are travelling within the semiconductor bands. They can recombine and loose the acquired energy in radiative (photoluminescence) or non-radiative (heat) ways. In photocatalysis this scenario is undesired and different strategies are applied in order to avoid a fast charge recombination. A few of them will be touched upon below. Another possibility for the photogenerated charge carriers is to migrate to the surface of the semiconductor crystallite and to react with various chemical species residing on the surface. It is worth noting that there is a driving force for the charge carriers to travel to the surface. This driving force is mainly due to the charge concentration gradient as the charge carriers are spent on the surface due to the redox chemical transformations. Another factor contributing to this concentration gradient is band-bending.23 Once the charge carriers reach the surface, holes and electrons carry out oxidation and reduction of substrates, respectively. For a long time it was generally accepted that oxidation and reduction take place at different, well defined, crystal facets24, 25 whereas recent investigations show that this phenomenon is more complex.26 Moreover, redox reactions on the surface can only occur if the thermodynamic potentials of the valence and conductance band of the semiconductor are suitable for a given substrate, i.e. the valence band maximum is more positive on the NHE potential scale (more negative in the absolute energy scale) than
6
the oxidation potential of a substrate. On the other hand, the conduction band minimum is more negative on the NHE potential scale (more positive in absolute energy scale) than the reduction potential of the same or another substrate. This very simplistic representation of the mechanism of photocatalysis is essential for understanding the basic concepts behind the development strategies of heterogeneous catalysts. The important aspects of this mechanism in the scope of this work can be split into the following: (1) absorption of light is the first vital step in initiating any photocatalytic reaction; (2) charge handling properties (mobility, recombination rate etc.) are of crucial importance; (3) photocatalysis is a phenomenon taking place on surfaces similarly to any other heterogeneous catalytic process; (4) valence band and conduction band positions on the absolute energy scale are the ones defining thermodynamic capabilities of a certain photocatalyst to oxidize and reduce given substrates.
1.2.2. IMPROVING T
i
O
2PERFORMANCE
Nearly 40% of all solar photons reaching the surface of our planet have energies falling into the visible region of electromagnetic radiation. Therefore the predictable trend in photocatalyst development was in tailoring absorption edges of catalysts towards visible light. Theoretically, the full water splitting can be accomplished by a catalyst with a bandgap of 1.23 eV.27 Proton reduction has a potential of 0 V vs. NHE while oxygen evolution happens at the potential of +1.23 V. In practice, an overpotential is often required and the more negative is the conduction band potential the greater is the thermodynamic force for the proton reduction. Depending on the application of choice one can select a photocatalyst with desired valence band and/or conduction band potentials. Applications such as water and air purification would favor semiconductors with largely positive valence band potentials while the reduction of CO2 and H2 evolution are in demand of negative conduction band potentials. The conduction band electrons can be of great reduction potential of +0.5 to -1.5 V vs. NHE while the valence band holes typically have oxidative power of +1 to +3.5 V vs. NHE.11, 28 The bandgap requirements are, however, more universal and do not depend on a specific application: the lower the bandgap, the more photons captured. Balancing the low bandgap requirement and the band potentials yields an appropriate catalyst. In this part we will briefly discuss the synthetic strategies for lowering the bandgap of TiO2.
Among the most exploited pathways for increasing the visible light absorption, doping is probably the most robust and has the greatest industrial potential. Titania can be doped with metals as well as non-metals. Chemical doping by metals is commonly carried out by adding
7 Fe3+,29, 30 Cr3+,31, 32 Ce4+,33 V5+ 34 and many other cations to the TiO2.12, 35 Such treatment leads to enhanced light absorption by the semiconductor, improved separation of charge carriers and often to an increase in the photocatalytic activity. In these materials the cations exist on the titania surface in the form of small metal-oxide isles. However, sometimes these dopants may serve as recombination sites reducing the photogenerated charge carriers lifetimes and thus photocatalytic activity.31 This drawback can be avoided by using ion-implantation technique (bombarding TiO2 with high energy ions) developed by Anpo and co-workers instead of chemical doping.36 Another useful strategy is to dope titania with light elements like N,37 C,38 B,39 F.40 In the case of anion chemical doping the dopants can be incorporated into the TiO2 lattice without disturbing the charge handling properties. Moreover, these materials have superior thermal stability with respect to the metal-doped titania. Sensitizing titanium dioxide with organic dyes was also documented in a number of reports.41, 42 This approach was successfully implemented by Grätzel and colleagues in their dye-sensitized solar cells,43 however it was not widely applied in powdered photocatalysis due to the instability of organic molecules under photocatalytic conditions. This particularly applies to photocatalytic oxidations in which the great oxidative power of TiO2 causes the destruction of the sensitizers. A special case of increasing the visible light absorption of TiO2 is achieved via surface plasmon resonance (SPR) of gold nanoparticles residing at the semiconductor surface.44, 45 The improved photocatalytic performance is not only due to the visible light absorption of the composite but also due to the suitable catalytic sites for hydrogen evolution at or close to the gold nanoparticles in Au/TiO2 material.46
In photocatalysis by semiconductors such as TiO2 the lifetime of photogenerated charge carriers is of crucial importance as well as the mobility of these species. Obviously, in a dense crystallite, charges can only react with substrates at the surface, meaning that charges generated in the bulk of the semiconductor have to reach this surface. The charge mobility within a TiO2 crystal depends on a number of parameters such as preparation method, crystallinity etc. It is worth noting that, on one hand high charge carrier mobility is beneficial as the charges can faster reach the surface but, on the other hand, the high mobility increases the chances of electron – hole recombination, unless one of the charge carriers is trapped. To date, a variety of methods for trapping charge carriers has been documented. Electrons may be commonly trapped by noble metal nanoparticles whereas alcohols are used for hole trapping. Instead of increasing the mobility (speed with which charge carriers propagate over the crystal), one could also think of decreasing the distance these charges have to travel. This
8
can be accomplished by employing TiO2 nanoparticles as well as particles of desired morphologies.47 At the same time lowering the size of titania particles leads to an increased surface area that is immediately reflected in the improved photocatalytic performance. Unfortunately, this positive influence is achieved at the cost of bandgap energy as this increases while decreasing the size due to the quantum size effects in semiconductors.48, 49 Moreover, synthesizing nanosized TiO2 retaining its high crystallinity is challenging since a high crystallinity is generally achieved via thermal treatment causing particle agglomeration.50 An alternative route of increasing specific surface areas of titania catalysts is to induce mesoporosity.51
In addition to the aforementioned functions of noble metal nanoparticles immobilized on TiO2 such as SPR and electron capture, another important role in the overall performance of M/TiO2 system is to provide appropriate catalytic surfaces for assembling and/or splitting of certain molecules. Noble metals like Pt and Pd are known to promote photocatalytic hydrogen evolution as well as a variety of other chemical transformations.52, 53 Fortunately, the use of additional catalytic sites is not restricted to precious metals only. Examples of employing cobalt-based molecular complexes56, 57 as well as enzymes58 for this purpose are documented by Reisner and co-workers.
1.2.3. METAL-ORGANIC FRAMEWORKS
Metal-Organic Frameworks (MOFs) emerged as potential photocatalysts in the early 2000s.59, 60
Most of the pioneering research was carried out on Zn-based coordination polymers55, 61 that offered a relatively high stability as compared to their predecessors.54 In the early beginning these solids were classified as semiconductors analogously to their corresponding metal oxides.61 The main argument supporting the semiconducting nature of MOFs was based on the UV-Vis studies that demonstrated band-like spectral features as well as bandgap energies similar to the ones found in inorganic semiconductors.62, 63 In spite of several differences between photocatalysis by MOFs and the one by semiconductors revealed by further studies, this first erroneous judgment triggered the interest of the scientific community.
The characteristic aspects of MOFs as photocatalysts opposed by TiO2 as a representative example of heterogeneous photocatalysis by semiconductors will be highlighted in this chapter as well as clear similarities of the strategies for the MOF-based catalyst development.
9
Figure 1.2. Structure of MOF-5 (also known as IRMOF-1) (left); bandgap energy of IRMOF series
frameworks as a function of linkers constituting the framework (right). Reprinted from [54] – left and [55] – right.
Metal Organic Frameworks (MOFs) are crystalline compounds consisting of infinite lattices built up of the inorganic secondary building unit (SBU, metal ions or clusters) and organic linkers, connected by coordination bonds of moderate strength. Distinct from traditional inorganic materials, MOFs can be synthesized from well-defined molecular building blocks thanks to both the reliability of molecular synthesis and the hierarchical organization via crystal engineering. MOFs can therefore be understood as molecules arranged in a crystalline lattice.64 High adsorption capacities and easy tunability have spurred applications in gas storage, separation and molecular sensing.65-69 Bio-compatible scaffolds hold promise for medical applications.70, 71 The easy compatibilization of MOFs with either organic or inorganic materials may result in composites with applications varying from (opto)electronic devices to food packaging materials and membrane separation.72, 73 Last but not least, their tunable adsorption properties, high dispersion of components and pore size and topology, along with their intrinsic hybrid nature, all point at applications in heterogeneous catalysis. 74-76
One of the first MOFs discovered to have photocatalytic properties was MOF-5. In this material Zn4O tetrahedra are interconnected by terephthalates as shown in Figure 1.2. Very shortly after the first photocatalytic reports, a number of scientists computed electronic properties of MOF-5 in order to assess the nature of electronic transitions possessed by the solid. The theory predicted a quantum dot-like behavior where Zn4O moieties can be considered as zinc oxide quantum dots spaced by organic antennas.60 Despite the fact that the calculated bandgap energy for MOF-5 was 5 eV which would determine this material to be an
10
insulator, the computational study revealed another important feature characteristic of this solid: the HOMO was mainly localized at the organic ligand (linker) whereas the LUMO was located at the inorganic SBU.77 This type of transition is referred to as ligand to metal charge transfer (LMCT). The authors also speculated that the HOMO-LUMO gap must be strongly influenced by the substituents of the aromatic rings of the linkers while variations in metal nodes (Zn, Cd, Be, Mg, Ca) do not affect the bandgap energy, as predicted by Widom et al.78 Indeed, in 2008 Gascon and co-workers experimentally proved this concept by altering the bandgap of MOF-5 using various aromatic dicarboxylic acids (Figure 1.2). They discovered that in contrast to classical semiconductors such as TiO2, CdS and ZnO, MOFs exhibit excellent optical tunability. The energy required to induce LMCT transitions depends on the level of conjugation of the aromatic system of the ligand. Recent theoretical studies by Han and co-workers revealed that bandgap energies of materials adopting MOF-5 topology can also be tuned by substituting the oxygen atoms within the Zn4O tetrahedra by S, Se and Te,79 however this strategy is not yet synthetically accessible.
Later on, the photocatalytically active MOF-5 being unstable upon exposure to water83 was replaced by more robust Metal-Organic Frameworks based on Ti,84 Zr,85 Fe86 and others. Nowadays the general strategy consists of using functionalized linkers like aminoterephthalic acid (ATA). For example, the amino-substituent, once introduced to the ligand, provides the lone pair of nitrogen for the interaction with the π*-orbitals of the benzene ring, donating electron density to the antibonding orbitals. This results in a new, higher HOMO level that brings absorption to the visible region.87 This concept was first realized by Garcia and coworkers for the case of Zr-based UiO-type materials.88 The use of ATA allowed sensitizing the originally deep-UV absorbing MOF (when synthesized with unsubstituted terephthalic acid) to the visible region. This red-shift in absorption resulted in an enhanced photocatalytic activity. The addition of a second amino-group in the linker was calculated to follow a similar trend: the absorption edge was found at 1.3 eV for the diaminated MIL-125(Ti) against 2.4 and 3.6 eV for the mono-aminated and amino-free framework, respectively.89 Similar theoretical90 and experimental studies on UiO-66-type frameworks additionally confirmed the influence of the substituents (NH2, NO2, Br) on the light absorption properties and the photocatalytic activity in As(III) oxidation.91 Interesting results were obtained when a combination of linkers was used for constructing the UiO-66-type MOFs. The authors found that by combining 2-fluoro-1,4-benzenedicarboxylic acid with 2-amino-1,4-benzenedicarboxylic acid the activity of the resulting mixed-linker-MOF in benzyl alcohol
11 photocatalytic oxidation increased by a factor of 3 as compared to the pure NH2 -UiO-66(Zr).92
In addition to the strategies altering the LMCT in MOFs, several examples of assembling a MOF of porphyrin-like linkers were documented.93, 94 In these solids the framework is built of a bi- or tetradentate porphyrinic entities having intrinsically high visible light absorption capacity. Such MOFs were also reported to demonstrate photocatalytic activity, although in this case the porphyrin core is likely to be responsible for it rather than the LMCT found in other coordination polymers as the activity is observed even for materials with redox-inert metals such as Al.95
However, introducing the desired functionalization in a framework of choice is not always synthetically feasible.96 For the MIL-125(Ti) topology this was achieved only when 10% of diaminoterephthalic acid was mixed with 90% of ATA; the attempts to obtain pure (NH2)2 -MIL-125(Ti) were unsuccessful. Therefore, the use of post-synthetic modifications (PSM)97 can certainly help introduce functionalities not achievable by direct synthesis. An example of such a PSM was recently reported by our group.80 In this instance amino groups of the ATA linkers were converted to dye-like molecular fragments after MOF synthesis.98 This transformation delivered a material exhibiting a significant red-shift in light absorption with respect to the parent NH2-MIL-125(Ti). The improved light absorption resulted in a higher activity of the framework in benzyl alcohol oxidation.
In contrast to the expected destruction of the dye-like moiety at the photocatalytic oxidation conditions, the catalyst remained stable upon recycling. This can probably be explained by the week redox power of the photogenerated holes within the MR-MIL-125(Ti). Generally speaking, photocatalytic oxidations by MOFs are often mild and much more selective towards intermediate oxidation products as compared to the classical inorganic semiconductors.99 At the same time we should point out that many MOFs were successfully applied for decolorization of various organic dyes.100
For the purposes of photocatalytic reductions, MOFs can be post-synthetically loaded with organic dyes. Particularly the UiO-66(Zr) MOF was sensitized with Erythrosine B101 and Rhodamine B102 that were applied in photocatalytic hydrogen evolution. Both of the systems afforded substantial amounts of H2 gas. However, the downside was that the dyes were dissolved in the catalyst slurry: the catalysts were not fully heterogeneous and the stability of the dyes was questionable. In 2015 Li and co-workers reported a very elegant post-synthetic
12
Figure 1.3. Post-synthetic sensitization of NH2-MIL-125(Ti) with dye-like molecular fragments
yielding methyl red-MIL-125(Ti) (MR-MIL-125(Ti)) (A); Post-synthetic metal-exchange in NH2
-UiO-66(Zr) and its influence on light absorption properties (B). Reprinted from [80, 81] – A and [82] – B.
approach of increasing the visible light absorption of NH2-UiO-66(Zr) by substituting one of the zirconium atoms within the Zr6O4(OH)4 inorganic nodes by titanium as displayed in Figure 1.3B.82 This route holds promise for future research considering the number of MOF structures and resembles the doping strategies of titanium dioxide discussed above.
In summary, optical absorption of MOFs can be easily tuned either by choosing an appropriate linker or post-synthetically. Alternatively, engineering the optical response by fine-tuning the cluster-forming metal or even by using mixed metal cluster (and/or ligand) might be in the future a powerful tool, but at this moment rational design is still rather challenging.96 Such manipulations with absorptive properties lead to improvements in the photocatalytic activity of the frameworks. However, this activity of so far reported MOF photocatalysts is very modest.
One of the intrinsic properties of MOFs that fascinates many researchers around the globe is their extremely high surface areas reaching up to 7000 m2/g BET.103 For the frameworks that are most frequently applied in photocatalysis these values are somehow more modest, ca. 1500 m2/g for NH2-MIL-125(Ti)104 and 800 m2/g for NH2-UiO-66(Zr),105 yet still more than sufficient for providing enough surface for catalysis. Therefore, the synthetic effort in the field of photocatalysis with MOFs is not directed toward obtaining materials with higher surface areas as is the case for TiO2. Moreover, the mechanism behind MOF photocatalysis is substantially different from that typical for inorganic semiconductors. As highlighted in the beginning of this chapter, one of the key reasons for reducing the particle size of TiO2 is to diminish the distance the photogenerated charge carriers have to travel before they reach the
A
13 surface and react with the substrates adsorbed on it. The necessity of having mobile charge carriers in MOFs is more questionable. It should be noted that due to the extremely porous nature of these coordination compounds, the substrates (reactants) can readily diffuse throughout the MOF crystallite. On the other hand, the rates of photocatalytic reaction can be different at the external surface and in the bulk of the MOF crystallite. This was pointed out by Garcia and colleagues as early as in 2006.106 They found that MOF-5 exhibited reverse shape selectivity in photocatalytic oxidation of phenols meaning that bulky derivatives of phenol oxidized faster than phenol itself. The diffusion of the bulkier molecules was hindered due to the size restrictions imposed by the MOF lattice. We also speculate that the diffusion of substrates and O2 is not the only factor influencing this peculiar behavior. Another explanation can be the limited light penetration depth in the absence of photogenerated charge mobility. From that point of view, a detailed study of the photocatalytic activity of MOFs as a function of particle size would shed the light on these processes.
As mentioned above, MOFs have been labelled as semiconductors based on their optical transitions and electrochemical and photochemical activity. Yet such activity does not necessarily imply semiconductivity. Inorganic semiconductors are characterized by a delocalized valence band and a conduction band through which the charge carriers are mobile. Organic semiconductors are typically characterized by delocalized orbitals via extended conjugated π-bonds, allowing for charge carrier mobility. Also in metal–organic frameworks a certain degree of delocalization is necessary to show a semiconductive behavior.108 Of course, only the measurement of the current through the material or the charge carrier mobility directly determines whether a material behaves as a (semi)conductor.
For a small subset of metal-organic frameworks their conductivity has been reported. The reported frameworks typically have small, but discernible conductivities of 10-9 – 10-3 S/cm.109 These frameworks are based on Cu(I) or Ag(I) ions,109 or dithiolene based frameworks.110 Another rare example of a 3-D porous metal-organic framework that shows conductivity, is based on triazole ligands. Out of an isoreticular series of different divalent metal ions, only the conductivity of the framework with Fe(II) ions has been reported.111 However, more recent works by Dinca and colleagues report an exceptional MOF exhibiting conductivities up to 40 S/cm as shown in Figure 1.4.107 The framework can be readily assembled from aqueous NH3 solution of Ni2+ and 2,3,6,7,10,11- hexaiminotriphenylene. This 2-D MOF even outperforms some of the organic conductors.
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Figure 1.4. A Ni-based MOF with exceptionally high electroconductivity, Ni3(HITP)2 =
Ni3(2,3,6,7,10,11- hexaiminotriphenylene)2, (A); TRMC photoconductivity studies for MIL-125(Ti) at
different temperatures: thermal-activated hopping (B). Adapted from [107] – A and [62, 80] - B.
At this point, it should be mentioned that electroconductivity is related to the ability of a given material to conduct an electric current. When it comes to photocatalysis and light harvesting, photo-conductivity, the mobility of electrons and holes generated upon electromagnetic radiation, is much more important. For a few MOFs the photo-induced time-resolved microwave conductivity (TRMC) is reported (see Figure 1.4). This technique probes the local charge mobility and is an essential tool when studying materials for photocatalysis. In such measurements the product of the charge carrier density and the charge carrier mobility is given.112 The signal is obtained with nanosecond time resolution after light absorption. The reported mobility values are in the range between 1·10-5 and 4·10-5 cm2/Vs. This is quite low compared to conjugated polymers where values of 10-3 cm2/Vs and higher are common.112, 113 These values have been reported for a MOF with stacked thiafulvalene ligands,114 a MOF that contains infinite Mn-S chains that should facilitate charge carrier mobility, 115 and MIL-125 a Ti(IV)116 containing structure that has also been studied for its photocatalytic behaviour.117 For the thiafulvalene, and Mn-S chains structures also the amount of photogenerated mobile charges was determined. The quantum yields are in the order of 10-4 – 10-3. This meant thus a high intrinsic charge mobility of 0.2 cm2/Vs for the thiafulvalene framework, and 0.02 cm2/Vs for the Mn-S chain framework. For the latter structure this corresponds to charge delocalization over 8-12 Mn-S units. Yet only a very small fraction (10-4-10-3) of the absorbed photons leads to charges that are mobile.
Taking MIL-125(Ti) as the most representative example of photo-active MOFs, it shows a poor photoconductance (mobility ~10-5 cm2V-1s-1 upon 340 nm illumination, see Figure 1.4B).116 This conductance is significantly suppressed upon lowering the temperature, in clear
A
15 contrast to pure TiO2, with mobilities in the range of 1 cm2V-1s-1 nearly independent of temperature.118, 119 This difference suggests thermally activated hopping as the main mechanism for the charge transport due to the isolation of the Ti clusters by the organic linkers in the MOF.120 Indeed, such clusters in MOFs are too far apart to fulfil the Mott transition conditions, being approximately 4 Bohr radii.121,122 Moreover, in most MOFs, the distance between linkers is too large as to allow efficient π-π stacking123
and there is hardly any orbital overlap, keeping the electrons preferentially in a localized state. This fact demonstrates, as recently rationalized by Lin and co-workers, that MOFs have to be understood as molecules arranged in a crystalline lattice.124 In case of photocatalysis, materials like MIL-125(Ti) should therefore be seen as an array of self-assembled molecular catalysts rather than as classical semiconductors. Therefore, optical absorption spectra should be considered as sets of individual discrete absorption bands, and the HOMO-LUMO gap terminology should be used in order to describe the discrete character of the light-induced transitions in these coordination compounds. For MOF-5, the most prominent MOF to which semiconductive behaviour has been ascribed due to electro- and photochemical behaviour, Walsh et al. calculated the electronic band structure. No band dispersion was observed, which is consistent with localized carriers and low levels of conductivity.108 These results are in line with experimental observations by our research group comparing photo-catalytic performance of isoreticular MOFs and their corresponding monodentate analogues. 125
Semiconductivity in metal-organic frameworks seems thus to occur only in a limited subset of materials, and is so far of relatively low magnitude. This notwithstanding, it has indeed been shown in literature that upon absorption of light electrons and holes can be generated in MOFs with reductive and oxidative power, respectively. Yet, for most MOFs these charges are not mobile. This has implications for photocatalysis. Photocatalysis consists of a reduction and an oxidation half reaction. When the photo-generated holes and electrons are not mobile, this implies that the oxidation and reduction sites need to be present in close vicinity to the location where the photo-excited charges are generated. In contrast to bulk solids, the crystalline nanoporous structure of metal-organic frameworks allows such a multi-modal construction. The spatial proximity of the photo-generated charge carriers though might favour charge recombination competing with the desired red-ox reactions. On the other hand, the porosity of MOFs facilitates diffusion of reactants and products throughout their crystals, which can compensate for that.
16
Figure 1.5. Different approaches to induce photocatalytic activity in a MOF scaffold: a) the organic
linker is used as antenna for light sensitizing and charge transfer to the inorganic cluster occurs; b) the MOF is used as a container for the encapsulation of a photocatalyst that directly absorbs light and c) charge transfer occurs between the MOF scaffold and the encapsulated catalyst.
Similarly to the TiO2-based photocatalysts, MOFs were subjected to Active Site Engineering (ASE).62 In the case of photocatalysis with MOFs methods of including additional catalytic sites, to the best of our knowledge, yielded the most promising improvements of photocatalytic performance. In contrast to classical semiconductors where ASE is normally restricted to the modification of surfaces by noble metal nanoparticles or, more rarely, transition metal complexes, the synthetic strategies employed for MOFs are much more diverse. The general pathways for using MOFs in photocatalysis are depicted in Figure 1.5: The first approach is to utilize MOFs as such and carrying out photocatalytic transformations by exploiting the LMCT: the MOF organic linkers can be considered as light-harvesting units transferring the energy of excited states to inorganic MexOy clusters consisting of only few metal atoms. Such approach usually results in the generation of free charges upon illumination at the appropriate wavelength and in moderate to low photocatalytic performances. A more elaborate approach to employ MOFs for photocatalysis is to use them as carriers for photocatalytically active species. This approach can be used for the encapsulation of a variety of active sites: from semiconductor nanoparticles127, to molecular
17
Figure 1.6. Doping of UiO-67(Zr) frameworks with molecular catalysts: schematic view (left), plots
of O2 evolving turnover number (O2-TON) vs. time for doped MOFs and the corresponding
homogeneous analogues (top right), recyclability studies (bottom right).126 Reproduced with permission from American Chemical Society.
catalysts based on transition metal complexes,128 they all have been successfully encapsulated in MOFs.129 In this case, the MOF can act either as mere container or participate in the charge transfer process (see Figure 1.5C where a ligand to metal charge transfer (LMCT) is indicated).
One of the main advantages of using metal-organic frameworks for supporting active species is that these moieties can be either covalently bonded to the framework or encapsulated in its cavities. This strategy was proven to prevent leaching of homogeneous catalysts, often consisting of precious metals and being soluble under given reaction conditions. In 2011 Lin and co-workers reported a series of UiO-67(Zr) materials doped with Ir-, Ru- and Re-complexes that were subsequently applied for water oxidation, aza-Henry transformations and CO2 reduction respectively.130 The solids exhibit outstanding photocatalytic performance, comparable to the one of the corresponding homogeneous analogues. In addition, the catalysts were confirmed to be recyclable, proving their heterogeneous nature. Another interesting catalyst was introduced in 2012: in this case, a framework with the UiO-67 topology was synthesized following a mixed-linker strategy. Biphenyl-4,4’-dicarboxylic acid being the primary linker was combined with [Ir(ppy)2(bpy)]Cl-derived dicarboxylic acid (see Figure 1.6).126
18
This yielded a crystalline porous solid with an iridium complex content of 2 wt %. In addition to the molecular catalyst, platinum nanoparticles (Pt NPs) were deposited within the cavities of the MOF by photodeposition (PD). This bi-functional catalyst, in which the charges generated by the Ir-complex are injected into Pt NPs, showed a remarkable activity in H2 evolution from H2O (3400 TONs), exceeding that of the homogeneous system. The enhanced activity was attributed to the more efficient electron transfer favoured in the confined space of the MOF cavities. Inspired by enzymes found in nature, Ott and co-workers coordinated a molecular diiron catalyst to the UiO-type MOF for catalysing H2 evolution.131 The light absorption was achieved by a Ru(bpy)3 photosensitizer. In 2013 Xu and colleagues reported an analogous system based on the redox-innocent MOF-253(Al). In their catalyst the visible light absorption as well as the desirable catalytic site were introduced into the MOF by coordinating PtCl2 to the bipyridin-motive of the organic linker.132 A similar way to introduce desirable active sites was implemented in the case of porphyrin-based MOFs. In these catalysts, the frameworks are built of porphyrin-like linkers responsible for photocatalysis. Rosseinsky and co-workers reported a MIL-60(Al) type framework containing meso-tetra(4-carboxyl- phenyl) porphyrin that was active in H2 evolution in combination with Pt.95 The solid was synthesized in a base-free form, conserving the possibility of tuning the active sites by changing the metal coordinated to the porphyrin rings. Recently, Al-based framework constructed by Cu-porphyrin building units and Zn-based Sn(IV)-porphyrin MOFs were proven to catalyse the reduction of carbon dioxide to methanol133 with very moderate production rates and the oxidation of phenols and sulphides,134 respectively.
The use of MOFs for the encapsulation of polyoxometalates (POMs) has been explored for several years for different catalytic applications.135-137 A Ln3+-based MOF containing [BW12O40]5- anions was recently applied for photocatalytic oxidation of thiophene with molecular oxygen.138 The UV light-driven transformation was speculated to be assisted by the charge separation within the Keggin anions. Another example of photocatalytically active POM-based metal-organic framework contained [Mo6O18(O3AsPh)2]4- polyoxoanions and Cu(I)-organic moieties. The solid behaves as a photocatalyst in methylene blue degradation.139
In all the examples above, the role of the MOF is limited to that of a container or ‘nano-reactor’ for species that are active in various photocatalytic transformations. This approach allows the controlled anchoring or heterogenization of active sites. However, it should be emphasized that following this approach, the outstanding ability of MOFs to separate charges
19
Figure 1.7. Schematic illustration of photocatalytic hydrogen production reaction over Pt-supported
NH2-MIL-125(Ti) on the basis of the LMCT mechanism. Reproduced from [104] with permission
from American Chemical Society.
upon light illumination is largely unutilised. One of the first materials reported which takes advantage of the MOF component was reported by Matsuoka and colleagues.140, 141 This catalyst is composed of Pt NPs photo-deposited on NH2-MIL-125(Ti). Platinum surfaces are known to be among the best platforms for H-H bond formation and in this particular instance Pt-nanoparticles introduce active sites required for efficient hydrogen evolution in combination with the light-absorbing unit, NH2-MIL-125(Ti). Mechanistic studies suggest that absorption of visible light by the organic linkers is followed by LMCT, yielding Ti3+ species. These surplus electrons are likely to be injected into Pt NPs that act as electron-‘reservoirs’ (see Figure 1.7).
The reaction takes place at the surface of these nano-particles. A similar effect was observed for Pt@NH2-UiO-66(Zr).142 Employing noble metal nanoparticles as additional catalytic sites is a common strategy for classical semiconductors as well as MOF-based systems.101, 102, 143, 144
However, as recently pointed out by Li et al., noble metals can influence the photocatalytic activity of MOFs differently.145 They found that Pt NPs substantially increased the activity of NH2-MIL-125(Ti) in photocatalytic CO2 reduction whereas gold had a negative effect on it. One of the latest examples of employing the full functional of MOFs was reported by our group. In this work a photocatalytically active NH2-MIL-125(Ti) was loaded with cobaloxime, a well-known electrocatalyst for H2 evolution as highlighted in Chapter 5 of the current thesis.146 The encapsulation resulted in a highly active, recyclable composite Co@MOF that is free of noble metals. Moreover, the experimental proof for the MOF-to-cobaloxime charge transfer was provided (Figure 1.8).
20
Figure 1.8. Mechanism of hydrogen evolution catalysed by Co@MOF composite.
1.3. CONCLUSIONS
In summary, the global synthetic strategies for catalyst development in the case of classical semiconductors such as TiO2 and novel photocatalysts, Metal-Organic Frameworks, are rather similar. Both of them include sensitization towards visible light absorption and the introduction of additional catalytic sites. However, while in the case of TiO2 charge recombination processes have to be minimized, the absence of intrinsic charge mobility in the case of MOFs imposes certain distinct differences. First of all, MOFs should be rather seen as infinite arrays of coordination complexes where the individual building units do not have a long-range interaction. As photoconductance is rare in MOFs, reduction and oxidation catalytic sites should be close to each other because the charge migration is unlikely in these materials. The proximity of the oppositely charged electrons and holes would always imply a greater chance for them to recombine, therfore the photogenerated charges should be stabilized by trapping. An interesting example of such trapping by forming Ti3+ in the case of NH2-MIL-125(Ti) is given in Chapter 3. In a number of MOFs the lowest in energy electronic
21 transition has ligand-to-metal charge transfer character. This means that by tuning the organic linkers (introducing additional substituents, using mixed linkers or even caping additional metal ions) one should be able to tune the position of HOMO (Chapter 4). This in turn affects the oxidative power of the resulting MOFs. The same analogy can be extected towards reductions: as metal orbitals in such MOFs define the position of LUMO, the reductive power can be altered by choosing metal ions possessing appropriate orbitals. Moreover, MOFs possess extremely large surface area and are highly tunable which greatly enhances the possibilities of employing additional catalytic sites as demonstrated in Chapter 5. A variety of additional catalytic sites for both oxidation (POM, transition metal complexes, porphyrins) and reduction (cobaloxims, hydrogenases porphyrins) can be employed by encapsulation and/or covalent bonding. Another key difference between semiconducting oxides and MOFs is the relatively poor stability of the former. Only MOFs possesing a reasonable stability must be selected. Thermal stability can potentially be an issue in future solar-driven applications. However at this stage the stability against water, free radicals (especially in the case of photocatalytic oxidations) and extreme pHs is of crucial importance. From that perspective, zirconium-based MOFs seem to hold the biggest promise yet, as explained in Chapter 3, their activity towards hydrogen evolution is very poor as compared to the one of NH2 -MIL-125(Ti). Recently proposed synthetic strategies of obtaining mixed Zr/Ti UiO-type MOFs appear to be an appealing solution.
At this stage semiconductors such as TiO2 often outperform the MOF-based systems but the growing attention to the photocatalysis with MOFs as well as nearly infinite possibilities of designing such systems evidence that this class of materials holds a promise for photocatalytic applications in the coming decades.
22
The main objective of the present thesis is to exploit the potential of Metal-Organic Frameworks in photocatalysis. This is achieved through understanding of the fundamentals behind photoexcitation in MOFs as well as by implementing this knowledge in fabricating active photocatalysts.
Chapter 1 introduces Metal-Organic Frameworks as potential photocatalysts and their characteristics. They are critically compared against more common photocatalysts such as TiO2.
Chapter 2. Here fundamentals of photoexcitation in NH2-MIL-125(Ti) are investigated with the help of ultrafast spectroscopy. It was found that the photogenerated holes localize on the NH2-group of aminoterephthalates. Kinetics of the photogenerated charge carriers in different solvents is described.
Chapter 3. In this chapter the role of d0 metal ion constituing frameworks of NH2-MIL-125 and NH2-UiO-66 is unraveled. It was found that the LUMO of NH2-MIL-125(Ti) is localized on titanium whereas the ones of NH2-UiO-66(Zr/Hf) are localized on the ligand. This results in a longer lifetime of photogenerated electrons and a significantly better performance in H2 evolution in the case of Ti-based MOF.
Chapter 4 introduces a framework with an extented conjugated aromatic system of the linker derived from NH2-MIL-125(Ti). The resulting MOF absorbs a larger fraction of visible light and thus has a superior performance in photocatalytic oxidation of benzyl alcohol.
Chapter 5 illustrates an example of Active Site Engineering in Metal-Organic Frameworks. A cobaloxime is encapsulated within the cavities of NH2-MIL-125(Ti). The obtained Co@MOF catalyst evolves hydrogen at a rate 20 times greater than a bare MOF does. With the help of EPR it was found that the MOF acts as a photosensitizer absorbing photons and injecting electrons in the cobalt complex.
All the chapters are based on published work as indicated in the footnotes at each title page. Therefore, partial repetition of information is unavoidable.
23
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