Light-active Metal-Organic Frameworks for Photocatalytic Hydrogen Evolution.

Stavroula Kampouri
EPFL, 2020
EPFL thesis

Abstract

The general scope of this thesis lies in the application of MOFs in photocatalysis. MOFs demonstrate inherent properties â such as high porosity, tunable optoelectronic and catalytic properties, which render them promising candidates for photocatalysis. The main research activity relative to this thesis aims at the advancement of MOF-based photocatalytic systems in terms of activity, cost and sustainability. This was achieved by employing different strategies that can lead to the favorable modification of the three major photocatalytic steps of light absorption, charge separation-migration and catalysis, which govern the overall performance of a photocatalytic system.

More specifically, Chapter 2 describes the impact of different co-catalysts on the photocatalytic activity of a MOF-based system, based on the well-known MIL-125-NH2. Variation of the co-catalysts can significantly influence the two major photocatalytic steps of charge separation and the catalytic reaction. All the metal oxide and phosphide co-catalysts investigated are found to significantly improve the activity of MIL-125-NH2, with the system using Ni2P nanoparticles (NPs) exhibiting a high H2 evolution rate (1230 ÎŒmol h-1 g-1) and an apparent quantum yield of 6.6% at 450 nm, which is comparable to the state of the art. Comparison of Ni2P with Pt showed that the noble-metal-free Ni2P/MIL-125-NH2 system significantly outperforms Pt/MIL-125-NH2. These results are attributed to the enhanced electronic interactions between MIL-125-NH2 and Ni2P, and prove that earth abundant co-catalysts can challenge the commonly used noble metals.

The low cost and high efficiency of Ni2P/MIL-125-NH2 prompted me to focus on another component used in photocatalytic systems, which is the electron donor. Chapter 3 shows that variation of the electron donors highly influences the activity of Ni2P/MIL-125-NH2, with triethylamine significantly boosting the H2 evolution rate. However, the utilization of electron donors is another factor hindering the industrialization of such systems, since these substances can be toxic and expensive. Inspired by this challenge, we then replaced the electron donor with rhodamine B (RhB) â a simulant organic pollutant â envisioning dual-functional photocatalysis for simultaneous H2 generation and organic pollutant degradation. This research project revealed the first example of a MOF-based dual-functional photocatalytic system able to generate H2 in a high rate and degrade RhB under visible-light.

Chapter 4 describes a strategy for enhancing the photocatalytic performance of MOF-based systems by favourably altering the photocatalytic steps of light harvesting and charge separation. This was achieved by developing MOF/MOF heterojunctions. The combination of MIL-125-NH2 with MIL-167 â a Ti-based MOF with complementary light absorption properties â leads to the formation of a type II heterojunction MIL-167/MIL-125-NH2, with enhanced optoelectronic properties. MIL-167/MIL-125-NH2 significantly outperforms its single components MIL-167 and MIL-125-NH2, in terms of photocatalytic H2 production. This strategy contributes to the discovery of novel MOF-based photocatalytic systems that can harvest the solar energy and exhibit high catalytic activities in the absence of co-catalysts.

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Stavroula Kampouri
EPFL, 2020
EPFL thesis

Abstract

Official source

https://infoscience.epfl.ch/record/279627?ln=en

About this result

Related concepts (20)

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Related publications

Related publications (36)

Factors influencing photocatalytic drinking water detoxification and disinfection by suspended and fixed TiO2

Damien Gumy

The work presented in this thesis was carried out in the framework of two European projects: SOLWATER, and AQUACAT. The global objective of those projects, including the present work, was to set an autonomous system for drinking water production in developing countries using solar energy. More specifically, the technology of drinking water decontamination by photocatalysis using inexpensive TiO2 catalyst and free of charge sun energy was investigated. In details, the research project dealt with the correlation of the TiO2 physicochemical characteristics with its photocatalytic activity on both organic and bacterial removal, the comparison of the photoactivity of suspended and fixed TiO2, and the development of a large scale prototype for fixed catalyst system. Finally, this work also included the creation of a mathematical model to predict the photo and photocatalytic bacterial inactivation. In photocatalytic experiments, phenolic compounds (phenol, 4-nitrophenol, 4-hydroxybenzoic acid and gallic acid) and benzoic acid were used as model organics, and E. coli as model bacterium. The titania physicochemical characteristics, crystalline structure, specific surface area (BET), particle size, aggregate size, and isoelectric point (IEP) of 13 different suspended TiO2 samples were determined and correlated with photocatalytic activity for chemicals and bacteria abatement. Organics degradation kinetics were found to be significantly affected by the TiO2 surface charge, i.e. the IEP. Indeed, the organic compounds degradation was greatly enhanced by negatively charged TiO2 samples. BET affected differently the primary compounds degradation and the total organic carbon mineralization. The primary degradation kinetics of strongly adsorbed pollutants was enhanced by large BET. Whereas, there was an optimum BET value for a total organic carbon removal, for all organic compounds studied. In addition, the mixed crystalline structure anatase-rutile, in the form of the commercially available Degussa P25, was more active for the purpose of organic degradation than pure anatase TiO2. Bacteria abatement was not influenced by BET specific surface area, particle and aggregate size of suspended TiO2. However, surface charge was crucial for the TiO2 and bacteria interaction. Indeed, negatively charged TiO2 were not efficient in inactivating E. coli, probably due to electrostatic repulsions with the negatively charged E. coli outer membrane. Depending on the nature of the contaminants, different catalyst characteristics have to be considered with regards to TiO2 efficiency on water detoxification and disinfection. Thus, it is not possible to draw a general scheme to determine which photocatalyst is the most suitable for the decontamination of particular water. Comparison of suspended and fixed catalyst revealed that the system with suspended TiO2 is more efficient for bacteria inactivation, yet the post-treatment catalyst filtration presents a serious disadvantage compared to fixed TiO2. Large scale experiments in compound parabolic collector (CPC) prototype reactor with fixed TiO2 revealed that a significant photocatalytic effect appeared only at a low bacterial initial concentration (≤ 103 CFU/ml). High organic compounds removal efficiency with fixed TiO2 was observed at laboratory and large scales, with kinetics similar to the ones obtained with suspended TiO2. A mathematical model was developed to predict bacterial photo-inactivation as a function of the light intensity and of the bacteria physiological state. The modelled curves fitted well with the experimental results for both exponential and stationary phases. Moreover, bacteria were more sensitive to light irradiation in the exponential than in the stationary state. Solar photocatalysis with fixed TiO2 was shown to be an efficient technology for organic pollutant removal and could be used as complement to other treatment process. Indeed, due to the slow bacteria inactivation kinetics observed, the technology needs further development to be used as a unique drinking-water treatment.

EPFL2007

Kinetic Assessment and Numerical Modeling of Photocatalytic Water Splitting toward Efficient Solar Hydrogen Production

Takashi Hisatomi

Photocatalytic water splitting has been studied extensively as a promising technology for scalable and cost-efficient hydrogen production using solar energy. Although overall water splitting has been achieved under visible light irradiation, significant progress in reaction efficiencies at longer wavelengths is needed to enable practical solar energy conversion. This article reports recent advancements in kinetic investigation and numerical modeling of photocatalytic water splitting to understand problems with existing photocatalysts and develop strategies to boost the reaction efficiency. Kinetic investigation based on cocatalyst loading, light intensity, hydrogen/deuterium isotopes, and reaction temperature suggests that the majority of photoexcited carriers recombine before contributing to the water splitting reaction on the surface in most cases, and that improvement in semiconducting properties of photocatalysts by decreasing defect and donor concentrations is beneficial to boost photocatalytic performance. It is demonstrated that an appropriate material design to suppress generation of donor species actually improves photocatalytic activity. Numerical modeling of steady state carrier concentrations suggests that an asymmetric built-in potential in photocatalyst particles enhances charge separation and thus extends the lifetimes of photoexcited carriers. On the basis of the recent findings, we suggest strategies to improve the reaction efficiency of photocatalytic water splitting.

2012

At the dawn of the 21st century, mankind is facing an important energy challenge. Future energy demand can only be met by large scale exploitation of renewable energy resources. Solar energy is abundant, with a sufficient capacity for the growing energy demand. However, it is necessary to implement renewable energy storage means. Hydrogen is considered as the energy carrier of the future. An energy economy based on hydrogen is viable only if hydrogen is produced from sustainable means. Water electrolysis is the most promising technique to produce hydrogen directly from water. Yet, the efficiency is limited and electrocatalysts are required to drive both the hydrogen evolution reaction and oxygen evolution reaction. Scarce and expensive materials are currently used to drive these reactions. It is, thus, imperative that efficient Earth-abundant catalysts are developed. Nanostructuring enhances the performance of inexpensive materials for water splitting and several catalysts were studied for this goal. Chapter 2 describes the application of nanostructuring technique to the archetypical catalysts that are nickel oxides for oxygen evolution. The prepared nanoparticles show superior activity towards oxygen evolution than that of their bulk counterpart. The ultrafine size of nanoparticles synthesized allowed the exposure of a higher number of active sites. The activity for oxygen evolution was, thus, significantly enhanced. We also detailed that short conditioning of the electrode improved the performance of the evaluated materials. In chapter 3 we carefully studied nanoparticles and nanowires of nickel phosphide. The catalysts is known to be really active for hydrogen evolution. Our group suspected that this material was, also, a potential oxygen evolution catalyst. We proved, for the first time, that nickel phosphide is a remarkable oxygen evolution catalyst. Under alkaline conditions, used for oxygen evolution, we observed an in-situ formation of a core-shell heterostructure, a typical nanostructure architecture. The surface oxidation of this material allowed high oxygen evolution capabilities. We concluded that careful oxidation of nanostructured materials, as observed for nickel phosphide, is essential for the preparation of future outstanding oxygen evolution catalysts. Chapter 4 details the fabrication of direct solar-to-fuel electrode using cobalt phosphide as hydrogen evolving catalyst. Instead of using the electrical energy provided by solar energy, solar irradiation on the developed assembly allows direct hydrogen production. The careful design of the light-sensitive electrode (photocathode) is detailed. Simple incorporation of cobalt phosphide by means of photodeposition alleviates the cost of the electrode fabrication. The assembled electrode is active for hydrogen evolution under visible light irradiation. In the chapter 5 we sought to fabricate a 3-dimensional hydrogen evolving catalyst. This nanostructure is based on polymer brushes on a flat conducting substrate. Once the brushes are grown on the substrate, a molybdenum sulfide catalyst is then incorporated by simple soaking technique. We were able to tune the loading of the catalyst and the corresponding activity by changing the polymer properties. The height of the polymer was modified as well as its packing density. The resulting activity proved superior to similar approaches and to previous reports on carefully engineered molybdenum sulfide catalysts.

EPFL2017

Factors influencing photocatalytic drinking water detoxification and disinfection by suspended and fixed TiO2

Damien Gumy

EPFL2007