The project realized in years 2012-2015 was supported by the Foundation for Polish Science “TEAM Programme” co-financed by the European Regional Development Fund (Innovative Economy Operational Programme).
Activation of small molecules attracts a particular attention of researchers working in the fields of chemistry, medicine, biology and environmental protection. Reactivity of activated oxygen, water, nitric oxide plays an indispensable role in biology and medicine. Although reactive oxygen species formed in the processes of O2 and H2O activation are toxic for living cells, they can be used in various therapeutic strategies
(e.g. photodynamic therapy). Singlet oxygen may be considered as a reagent in synthesis of selected organic peroxides, polyoxygenated natural products, etc. Syntheses involving carbon monoxide and carbon dioxide are even more intriguing – in the context of “green chemistry” CO2appears as a prospective reagent for production of fuels and valuable chemicals. Another small molecule remaining in the focus of research is dinitrogen. Activation of N2 under mild conditions followed by its reduction to ammonia might appear an alternative to Haber-Bosch process developed a century ago.
The methods of small molecules activation involve various strategies – they are based on (i) coordination to metal centres of inorganic and organometallic complexes; (ii) adsorption at catalytic surfaces; (iii) photochemical or photocatalytic activation. This project focuses on the third type of activation, however the photochemical activation is preceded by adsorption/chemisorption process at the surface of a heterogeneous photocatalyst. Since the planned studies will involve both “raw” surfaces of metal oxides/sulphides (semiconductor photocatalysts) enabling adsorption of small molecules, as well as photocatalysts modified with metal complexes, also the first and the second type of activation will play a role.
The goal of this project was to elaborate various types of heterogeneous (semiconductor-based) photocatalytic systems, preferably active upon visible light irradiation, which would enable activation of small molecules used then for useful chemical transformations. The project assumed design and synthesis of new photocatalysts in the form of colloids, suspensions or films, tests of their photoactivity and detailed studies of mechanisms of processes taking place at the solid-liquid or solid-gas interfaces.
Main types of small molecules (sm) activation: (i) coordination to metal centre; (ii) chemisorption at the surface of catalyst; (iii) photochemical/photocatalytic activation preceded by coordination (A) or adsorption (B). In the latter case coordinated or adsorbed molecule (sm’) is involved in energy or electron transfer process (ET) leading to generation of another form of activated molecule (sm*).
Research planned within this project will be grouped in the following sub-projects:
The goal of this sub-project is to design and study new photocatalysts suitable for generation of reactive oxygen species under mild (mainly in sense of photon energy) conditions. Metal oxides (TiO2, ZnO, WO3) are the best candidates, offering proper energies of valence and conduction band edges enabling oxygen reduction and water oxidation, however their bandgap energies usually enforce application of ultraviolet light. Development of new photocatalysts will be directed into construction of hybrid inorganic materials composed of the solid oxide matrix (support) and coordination compounds playing a double role of a photosensitizer and/or coordination centre for the small molecule (O2, H2O). The electronic complex-support interaction should result in the photosensitization effect, i.e. visible light-induced electron transfer between the complex and the oxide. Excited states should offer sufficient lifetimes and redox properties enabling reduction or oxidation of the coordinated or adsorbed small molecules.
Photosensitizers (usually organic dyes) responsible for singlet oxygen generation are typically used in homogeneous systems (solutions). Generation of singlet oxygen in the presence of heterogeneous photosensitizers is much less common, although it was reported for silicon nanocrystals, zinc oxide and titanium dioxide. The energy difference between excited (1O2) and the ground state of oxygen (3O2) amounts 0.98 or 1.63 eV. An efficient energy transfer process requires matching of the excited state energies of the photosensitizer (semiconductor particle) and O2. However, generation of 1O2 at some semiconducting photosensitizers may result from the redox reaction of photogenerated superoxide with holes:
O2•– + h+ → 1O2
Within this project a development of new semiconductor-based photosensitizers of oxygen is planned, as well as elucidation of mechanisms governing singlet oxygen generation.
Activation of carbon-containing molecules is considered as an important goal of researchers working in the field of catalysis and photocatalysis. Efforts on photocatalytic activation of carbon dioxide are directed either onto CO2 reduction to simple C1 molecules (e.g. methanol, methane) or its utilization as a substrate in more sophisticated organic syntheses. Until now photocatalytic systems do not offer satisfactory efficiencies of such transformations due to several reasons, like high CO2 activation energy, low CO2 reduction potential, reoxidation of the products or problems with sufficient yields of multielectron reduction. Heterogeneous systems described in literature are based on TiO2, ZrO2, ZnS, ZnO, CuO and many others. Activation of methane would constitute a primary step in synthesis of chemicals from this compound. Photocatalytic activation of CH4 and its subsequent oxidation to carbon dioxide can be considered wherever CH4 emission to atmosphere should be diminished. Recently Ga2O3 photocatalysts have been reported to photocatalyze steam reforming of methane (2H2O(g)+CH4→4H2 + CO2) at room temperature. Finally, activation of carbon monoxide, followed by its oxidation to CO2, can also be considered as a process of CO removal from air.
The goal of this sub-project encompasses development of heterogeneous photocatalysts enabling activation of carbon-containing small molecules leading finally to reduction of CO2 or oxidation of CH4 and CO.
Together with PhD students undergraduate students will work on the sub-projects 1-3, however some of them will also test a possibility of other molecules photoactivation (e.g. NOx or H2O2) in the presence of developed photomaterials. In the case of especially interesting results that would appear worthy of a deeper investigation, selected systems could be further elaborated within the projects 1-3 and 5-6. This sub-project opens a possibility to realize new ideas of the team members.
The photocatalytic systems developed within the projects 1-4 will be subjected to application tests.
A close cooperation between the PhD students and the post-doc fellow #1 will be crucial to design the optimal systems for various applications. The application tests and optimization of the performance will be directed towards:
- oxidation of organic pollutants of water, air and surfaces (photodetoxification)
- removal of microorganisms and biofilm (photodisinfection);
- tests of photodynamic activity against selected cells (e.g. mouse melanoma cells);
- synthesis of chemicals involving singlet oxygen as a reagent;
- tests of other applications (e.g. related to water splitting or systems developed within the project 4).
This part of the project will be dedicated to modelling of the surface of developed photocatalysts and their interactions with adsorbed or coordinated small molecules. In the case of the surface modified solids the influence of the modifier (surface complex) on the electronic properties of the support will be considered. These studies should enable elucidation of the photosensitization effect and influence of the surface species on changes in density of states (DOS).
Detailed analysis of primary chemical reactions taking place at the surface of photocatalysts will be supported by modelling the reaction paths. Determination of indispensable terms for an efficient energy transfer process would be an important achievement of these studies.