Tailoring Surface Properties of TiO2 for Atomic Insights into Its Catalytic Reactions

Abstract

  In recent decades, acid catalysis in heterogeneous systems such as metal oxides and porous zeolites has played a crucial role in various chemical and petrochemical processes. The performance of acid-catalyzed reactions, including activity and selectivity, is closely linked to the surface acidic features of catalysts. These features include the type (Lewis vs. Brønsted), distribution (external vs. internal surface), strength (strong vs. weak), concentration (amount), and spatial interactions of acidic sites.

To comprehensively understand the relationship between catalyst's surface acidic features and catalytic activity, a careful study of these features is essential. However, traditional surface characterizations, such as XPS, Pyridine-IR, and NH₃-TPD, provide limited information on the chemical states of surface features. In contrast, probe-assisted solid-state ³¹P NMR has been reported to offer detailed surface information. In this thesis, anatase titanium dioxide (TiO₂) nanosheets with varying (001) / (101) ratios and different surface modifications were prepared and used for three typical acid-catalyzed reactions. By employing probe-assisted solid-state ³¹P NMR, a thorough differentiation of TiO₂ surface acidic features was achieved. This allowed for the establishment of a reliable structure-activity relationship in catalytic reactions, supported by a range of other characterization tools.

Specifically, we conducted a systematic investigation of three different catalytic reactions, including:

(1)(Dephosphorylation) Dephosphorylation that removes a phosphate group from substrates is an important reaction for living organisms and environmental protection. Although CeO₂ has been shown to catalyze this reaction, cerium is low in natural abundance and has a narrow global distribution (>90 % of these reserves are located within six countries). It is thus imperative to find another element/material with high worldwide abundance that can also efficiently extract the phosphate out of agricultural waste for phosphorus recycle. Using para-nitrophenyl phosphate (p-NPP) as a model compound, we demonstrate that TiO₂ with a F-modified (001) surface can activate p-NPP dephosphorylation at temperatures as low as 40 °C. By probe-assisted nuclear magnetic resonance (NMR), it was revealed that the strong electron-withdrawing effect of fluorine makes Ti atoms (the active sites) on the (001) surface very acidic. The bidentate adsorption of p-NPP on this surface further promotes its subsequent activation with a barrier ≈20 kJ mol⁻¹ lower than that of the pristine (001) and (101) surfaces, allowing the activation of this reaction near room temperature.

(2)(Amide hydrolysis) Amide hydrolysis holds significant importance in organic chemistry and biological systems. In recent decades, there has been extensive research on Lewis acid (LA) sites (metal ion) catalyzed amide hydrolysis. While the role of LA in this reaction is well understood, the impact of introducing Brønsted acid (BA) sites has been less investigated. Limited studies suggest that BA sites facilitate the formation of the leaving group (-NH₃⁺) and thereby promote amide bond hydrolysis. Using acetamide as substrate, we found that the pristine (001) surface of TiO₂, which only possesses LA sites with low strength, can activate acetamide hydrolysis at temperatures as low as 25°C. In contrast, the fluorine-modified (001) facet (F-(001)) with BA sites induced on the surface and LA sites with high acidity requires a higher temperature of 70°C. Control experiments and mechanistic studies revealed the adverse role of BA sites on the F-(001) surface in acetamide hydrolysis. This research is expected to provide insights for future catalyst design in amide hydrolysis.

(3)(Epoxidation) Despite extensive studies on the effect of LA sites in the activation of H₂O₂, which is crucial in epoxidation reactions using H₂O₂ as oxidants, the role of BA sites in this reaction has received limited attention. We found that the introduction of BA sites has a great influence in the activity and selectivity in epoxidation reaction. The induced BA sites on F-(001) surface co-activate H₂O₂ with LA sites with high strength. This synergistic effect leads to the formation of bidentate Ti-η₂-OOH species, which exhibit higher activity in transferring oxygen to cyclohexene compared to the Ti-η₁-OOH species formed on the pristine (001) surface containing only LA sites with low strength. In addition, these strong LA sites on F-(001) surface also promote the further conversion of epoxides. Our study systematically investigated the effect of surface acidic features of TiO₂ on epoxidation reactions, which could provide guidance for the future design of related catalysts.

Date of Award	11 Dec 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yung-kang PENG (Supervisor)