Gas-phase approach to the studies of elementary steps of heterogeneous photocatalysis

The use of solar light in photocatalytic process of water splitting for hydrogen production is a key point for hydrogen energetics. Other fields of current and perspective applications of photocatalysis are water and air-purification from organic impurities, self-cleaning and sterilization of surfaces, "green chemistry" and others. All these applications dictate very big interest of the researches to the problems of photocatalysis. Titanium dioxide is the most effective known photocatalyst. But it has one disadvantage due to bad overlapping with the spectrum of solar radiation. For shifting of its absorption spectrum into the visible, the coverage by dyes is used as well as doping by different admixtures. On the way to creating the future efficient TiO2-based photocatalysits the main problem is the lack of knowledge about mechanism of photocatalytic process, including mechanism of reactive intermediates photogeneration responsible for photocatalytic behavior of titanium dioxide. Efficient catalytic activity and better fitting of absorption to solar radiation spectrum is provided by other superconductor tungsten trioxide WO₃. For creation of task-oriented photocatalysts based on WO3 it is also necessary to know mechanism of oxygen-involved photocatalytic processes on its surface. It is known that the surface centers play a key role in heterogeneous catalysis. Moreover there is a perspective direction in photocatalysis where isolated centers of titanium oxide are used which are named single site catalysts. This approach allows one to provide many unusual photocatalytic processes where bulky photocatalysts are less effective. Mechanism of these sites action is not clear. Approach suggested in this project which is based on the use of small cluster (TiO₂)_nas the model should allow us to investigate the mechanism of reactive intermediates photogeneration on these centers. Suggested approach based on the use of small clusters (TiO₂)_n as a model of photocatalyst allows us to study mechanism of reactive intermediates photogeneration on these centers. According to the data of quantum-chemical calculations of structure and electronic properties of small clusters of tungsten trioxide even small clusters (WO₃)_n (n=2-4) can be considered as an adequate model for the study of photocatalytic processes in bulky photocatalyst. Generation of complexes of these clusters with oxygen in molecular beam allows us to use powerful technique of identification and investigation of elementary photoprocesses based on the velocity map imaging of photofragments appearing after photoexcitation of complexes. For investigation of these processes new approach is suggested. This approach is based on the use of complexes of small clusters of TiO₂ and WO₃ with oxygen as the model for the study of the processes proceeding on the surface of photocatalyst. This approach seems to be very promising because possibility to generate these complexes in molecular beam allows one to study elementary steps of heterogeneous photochemistry with the use of very powerful techniques like velocity map imaging which were elaborated for gas phase studies. Within our research, it is supposed to identify and investigate elementary photoprocesses, which govern photocatalytic activity of titanium dioxide and tungsten trioxide, to establish structure and energetic characteristics of photocatalyst-oxygen complexes, to study their photophysics and photochemistry, and to study supramolecular photoprocesses, analogous to those revealed by us in weakly bound complexes of oxygen. It is also supposed to study the effect of water molecules inclusion in the complex.

We have already discovered the process of formation of singlet oxygen from superoxide anion on the surface of TiO₂ passing through the formation of superoxide anion as a result of electron photodetachment. We have shown that singlet oxygen reacts with titanium dioxide to form titanium peroxide without an activation barrier. We think that similar fast processes also take place on the surface of the photocatalyst particles. In connection with the important role of titanium and tungsten peroxides, their study is of great interest. But for this it is necessary to learn how to get them and we intend to investigate this question. Assessing the relevance of the planned research, it should also be noted that the importance of studying the mechanism of photogeneration of singlet oxygen and the implementation of a direct method for measuring the kinetics of processes involving superoxide anion is not limited to problems of photocatalysis. The fact is that singlet oxygen is an active agent in many biological systems, and is also a key agent in photodynamic therapy, widely used in the treatment of oncology. Therefore, new knowledge about the mechanisms of generation of singlet oxygen can be useful in many applications. The same applies to superoxide anion, which plays an important role in many biological processes. Reactions involving this particle are important from the point of view of such phenomena as oxygen toxicity, the formation and development of malignant tumors, the aging process. Superoxide anion plays a very important role in oxidative stress. Therefore, we believe that the results of the planned project aimed at studying the elementary physicochemical processes of photocatalysis will be of great importance in the application to the study of the above phenomena.

Publications

Tungsten Isotope-Specific UV-Photodecomposition of W(CO)₆ at 266 nm

The Journal of Physical Chemistry A

UV photodissociation of tungsten hexacarbonyl W(CO)₆ has been studied in the molecular beam conditions using time-of-flight mass spectrometry and velocity map imaging. Irradiation of W(CO)₆ by pulsed laser radiation at 266 nm results in the appearance of singly and doubly charged tungsten ions. The isotope composition of these ions deviates essentially from natural abundance with deviation being pulse energy-dependent. The velocity map images of the tungsten ions indicate proceeding of several, more than two, parallel channels (sequences of the one-photon processes) of photodissociation, giving rise to tungsten atoms. Isotope effect is assigned to appear in a one-photon bound–bound transition in W(CO) intermediate followed by its predissociation. In the model suggested, the final state of this transition is a vibronic state with excited vibrational mode of W–C stretching vibration. This vibrational excitation is responsible for isotopic shift in the location of the final state. The suggested model fits the observed isotopic composition quantitatively.

Singlet Oxygen ¹O₂ in Photocatalysis on TiO₂. Where Does It Come from?

The Journal of Physical Chemistry C

Features of the near-infrared phosphorescence of singlet oxygen ¹O₂ photosensitized by TiO₂ in solutions have been investigated. The short-lived phosphorescence of ¹O₂ with spectral maximum at approximately 1300 nm is observed to follow excitation by pulsed laser radiation at 355 nm of TiO₂ suspended in CCl₄ and water H₂O. The shorter lifetime and spectral shift of this phosphorescence, as compared with phosphorescence of ¹O₂ sensitized by phenalenone in these solvents, are attributed to the adsorbed state of emitting singlet oxygen. The observed laser pulse energy dependence indicates the two-photon and three-photon nature of this phosphorescence initiation in CCl₄ and H₂O, respectively. The formation of ¹O₂ in CCl₄ is assumed to be provided by the sequence of one-photon excitation of TiO₂, giving rise to superoxide anion O₂^– and one-photon photodetachment of electron from O₂^–. The photodetachment of electron from O₂^– in water H₂O is a two-photon process because of higher solvation energy of superoxide anion in water. The processes that govern the rise and decay of singlet oxygen phosphorescence sensitized by TiO₂ are discussed. The factors affecting the yield of singlet oxygen provided by electron photodetachment are also considered.

Reply to Comments on “Singlet Oxygen ¹O₂ in Photocatalysis on TiO₂. Where Does It Come from?”

The Journal of Physical Chemistry C

In the Comment the authors Y. Nosaka and A. Y. Nosaka present arguments against the mechanism of singlet oxygen production which was suggested in the earlier published paper (Demyanenko et al., J. Phys. Chem. C 2019, 123, 2175) to explain the experimental data for the Near IR (NIR)-luminescence observed to appear after laser excitation at 355 nm of TiO₂ suspension in air-saturated solvents CCl₄ and H₂O. The main arguments of the Comment are directed against Assignment of the NIR Emission and Electron Photodetachment Mechanism suggested by Demyanenko et al.. Particularly the Comment claims that the red shift (near 30 nm) and bandwidth (50-60 nm) of the NIR luminescence observed in paper by Demyanenko et al. are too large to be assigned to the luminescence of singlet oxygen ¹O₂. In the Reply it is shown that several arguments of the Comment are based on the incorrect statements. The current interpretation of the nature of the red shift and bandwidth of the luminescence of singlet oxygen governed by interaction with the environment is considered. It is shown that the characteristics of NIR luminescence (rather large red shift and bandwidth as compared with the literature data for the luminescence of ¹O₂ in solutions) observed in experiments by Demyanenko et al. are in a qualitative agreement with expected tendencies. All the arguments of the Comment are considered in detail and extended reply is given.

An Automated Apparatus for Measuring Spectral Dependences of the Mass Spectra and Velocity Map Images of Photofragments

Instruments and Experimental Techniques

Decomposition Pathways of Titanium Isopropoxide Ti(OⁱPr)₄: New Insights from UV-Photodissociation Experiments and Quantum Chemical Calculations

The Journal of Physical Chemistry A

The UV-photodissociation at 266 nm of a widely used TiO₂ precursor, titanium tetraisopropoxide (Ti(OⁱPr)₄, TTIP), was studied under molecular-beam conditions. Using the MS-TOF technique, atomic titanium and titanium(II) oxide (TiO) were detected among the most abundant photofragments. Experimental results were rationalized with the aid of quantum chemical calculations (DLPNO–CCSD(T) and DFT). Contrary to the existing data in the literature, the new four-centered acetone-elimination reaction was found to be the primary decomposition process of TTIP. According to computational results, the effective activation barrier of this channel was ∼49 kcal/mol, which was ∼13 kcal/mol lower than that of the competing propylene elimination. The former process, followed by the dissociative loss of an H atom, was a dominating channel of TTIP unimolecular decay. The sequential loss of isopropoxy moieties via these two-step processes was supposed to produce the experimentally observed titanium atoms. In turn, the combination of these reactions with propylene elimination can lead to another detected species, TiO. These results indicate that the existing mechanisms of TTIP thermal and photoinitiated decomposition in the chemical-vapor deposition (CVD) of titanium dioxide should be reconsidered.

Binding of oxygen with titanium dioxide on singlet potential energy surface: An ab initio investigation

Chemical Physics Letters

Gas-phase approach to the studies of elementary steps of heterogeneous photocatalysis

Publications

Tungsten Isotope-Specific UV-Photodecomposition of W(CO)6 at 266 nm

Singlet Oxygen 1O2 in Photocatalysis on TiO2. Where Does It Come from?