Understanding The Structure And Photocatalytic Mechanism Of TiO₂ Photocatalysts With Solid-State NMR Spectroscopy

With increasing energy demand and environmental crisis,photocatalytic hydrogen production from water splitting via TiO2 semiconductor photocatalysts has become a promising approach to produce environmentally friendly hydrogen energy by utilizing sustainable solar energy.However,TiO2 photocatalysts are suffering from low solar to hydrogen(STH)efficiency,which severely limits the further development of photocatalysis.In the past few decades,facets engineering and elemental doping are widely adopted to modify the TiO2 photocatalysts to improve their photocatalytic activity.Unfortunately,the STH of these modified TiO2 photocatalysts is usually less than 0.03%.To resolve these challenging issues,the mechanism of photocatalytic water splitting must be clarified.In this dissertation,the micro-structure and dynamics of H2O,and the water splitting mechanism on TiO2 photocatalyst were investigated by using solid-state nuclear magnetic resonance(SSNMR)and electron paramagnetic resonance(ESR)techniques,and a new strategy for photolysis of pure water was proposed.In addition,the structure-activity relationship of Aldoped TiO2 with {001} facets dominantly exposed(Al-TiO2-xFx{001})was investigated,which is crucial for understanding the roles of F and Al in the photocatalysts.(1)The photocatalytic water splitting mechanism on Pt/TiO2 catalyst is studied by SSNMR and ESR spectroscopy.It is well-known that the oxidization of photoinduced holes(h+)is the rate determining step in the photocatalytic water splitting.The h+oxidation mechanism on the TiO2 surface usually involves two processes:(1)the photoinduced h+interacts with surface active sites to form active paramagnetic intermediates,and then(2)the active intermediates oxidize surface-adsorbed molecules.However,there is still few report on how and where the active paramagnetic intermediates(Ti-O·-、Ti-O2·-和 Pt-O2-)react with the surface-adsorbed molecules.Herein,we use methanol as a probe to follow the reaction process of active paramagnetic intermediates on TiO2 photocatalysts.Combined with the microstructure of the adsorbed methanol as revealed by the two-dimensional(2D)SSNMR techniques,the photocatalytic water splitting mechanism is proposed.Furthermore,it is found that the photocatalytic pure water splitting can proceed efficiently by controlling the H2O loading on TiO2,and the overall solar energy conversion efficiency(STH)can reach as high as 0.46%.(2)The interaction between H2O and metal oxide is of fundamental importance to various fields of catalysis and biochemistry,which,however,is still ambiguous due to the lack of the atomic-level structure of H2O/oxide interface.Herein,we choose the {101}dominated anatase TiO2 as a model photocatalyst to study metal oxide-water interactions.The high-resolution structures and the quantitative evolution of surface oxygen sites interacted with different adsorbed H2O molecules are for the first time acquired by 17O solid-state NMR techniques.Combined with theoretical calculations,we are able to map the atomic-level interaction of surface oxygen sites and the adsorbed H2O,and clarify the relative stability of the molecular and dissociative adsorption of H2O on {101} facet of TiO2.According to experimental and theoretical results,it is found that the dissociation of H2O,the initial step of the photocatalytic H2O splitting,can spontaneously occur only within certain range of H2O loading(0.3-0.5 mmol/gcat.)at room temperature.(3)A series of Al-doped TiO2 with exposed {001} facets were synthesized by hydrothermal method,and 0.5%Al-TiO2-xFx{001} sample shows the highest photocatalytic performance for RhB degradation in aqueous suspensions.19F and 27Al MAS NMR experiments demonstrate that A1 doping can introduce F-Ti2Al structural unit in Al-TiO2xFx{001},which can facilitate the separation and transfer of photogenerated carriers,and is considered as the active site in photocatalysts.However,when excess Al is doped into the TiO2 crystals,oxygen vacancies emerge as the recombination centers for photogenerated carriers.Based on the experimental results,we propose the photocatalytic mechanism on AlTiO2-xFx{001} catalysts.