Study of Charge Transfer in Photocatalytic Reduction of C1 OXYGENATED COMPOUNDS

Study of Charge Transfer in Photocatalytic Reduction of C1 OXYGENATED COMPOUNDS. PhD thesis, King Fahd University of Petroleum and Minerals.

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Arabic Abstract

تهدف هذه الرسالة إلى دراسة الاختزال الضوئي لمركبات C1 المؤكسجة. على وجه الخصوص ، سنقوم بإعداد هياكل فلزية عضوية نشطة ضوئيًا (MOFs) للتحقيق في آلية نقل الشحنة وزيادة ضبط خصائصها من خلال التطعيم باستخدام ذرات معدنية داخل وحدات البناء الثانوية (SBUs) . سيسمح ذلك بإجراء دراسة منهجية للتأثير الذي يحدثه التطعيم بالمعادن على النشاط التحفيزي الضوئي لهذه MOFs. على الرغم من التقدم المحرز في السنوات القليلة الماضية في دراسة التحفيزات الضوئية للأطر الضوئية ، هناك بعض الأسئلة التي تحتاج إلى مزيد من البحث ، مثل العوامل التي تؤثر على نقل ناقلات الشحنة في المادة المؤثرة ضوئيًا ، ودور المنشطات المعدنية على مستويات الطاقة ذات فجوة الحزمة ، والتعديلات في حواف الحزمة من حفاز ضوئي. في الواقع ، سيمكننا فهم أفضل لآلية نقل الشحنات والعوامل المؤثرة عليها من إعداد محفزات ضوئية أكثر نشاطًا قد تستخدم جزءًا أكبر من الطيف الشمسي في إنتاج الوقود الشمسي. في هذا العمل ، سوف نبحث في MOFs التحفيزية المحضرة للهيكل باستخدام تقنيات التوصيف مثل حيود الأشعة السينية (XRD) ، والمجهر الإلكتروني لانبعاث الانبعاثات (FESEM) .سنقوم بدراسة نقل الشحنة في نموذج ضوئي ضوئي باستخدام MOFs المعدة وتأثير تغيير الأيونات المعدنية في وحدات SBU على نشاط التحفيز الضوئي باستخدام تقنيات مثل مطيافية الامتصاص العابر الفمتوثاني (FTA) وطيف الرنين الإلكتروني (EPR) . سيتم تطوير نموذج حاسوبي رقمي للعينات المعدة لحساب فجوة الشريط ودراسة تأثير التبادل المعدني على نشاط التحفيز الضوئي باستخدام نظرية الكثافة الوظيفية (DFT). نأمل أن تمكننا هذه الدراسات من تطوير فهم شامل لكيفية تحسين الحفز الضوئي كوسيلة لدراسة تخزين الطاقة الشمسية في النظم الكيميائية.

English Abstract

Despite progress in the last few years in the study of photocatalytic MOFs, there are questions in need of further investigation, such as the factors affecting the transfer of the charge carriers in the photoactive material, the role of metal doping on the bandgap energy levels, and modifications in the band edges of the photocatalyst. Indeed, a better understanding of the charge transfer mechanism and the factors affecting it will enable the research community to prepare more active photocatalysts that might make use of a greater part of the solar spectrum in producing solar fuels in the future. This thesis targets the investigation of photocatalytic reduction of CO2 as an example of oxygenated C1 compounds. In particular, we have prepared photocatalytically-active metal-organic frameworks (MOFs) to investigate their charge transfer mechanism, and further to tune their properties through doping with metal atoms within the secondary building units (SBUs). This allowed for the systematic study of the impact that doping has on the photocatalytic activity of these MOFs. In this work, we investigated the prepared photocatalytic MOFs for structure and morphology using characterization techniques such as X-ray diffraction and field emission scanning electron microscopy. We studied the charge transfer in a model photoreaction using the prepared MOFs and the effect of changing the metal ions in the SBUs on the photocatalytic activity using different techniques such as femtosecond transient absorption spectroscopy and electron paramagnetic resonance spectroscopy. A computational model for the prepared samples was developed to calculate the band gap and study the effect of the metal exchange on the photocatalytic activity using density functional theory. For the UiO-66 system, we studied different incorporation methods of titanium metal into UiO-66-NH2 based frameworks via post-synthetic as well as in situ metal exchange. Each method successfully produced crystalline material with enhancement in photophysical and photocatalytic properties. The origin of the enhanced photophysical activity was investigated with electron paramagnetic resonance and ultraviolet-visible diffuse reflectance spectroscopy measurements. A detailed investigation of CO2 reduction was then carried out with the different UiO-66 catalysts. A remarkable enhancement of the photocatalytic activity was observed by using the UiO-66-NH2 based frameworks compared to the pristine UiO-66. A nearly doubled photocatalytic activity was obtained by the Ti-Zr mixed MOF materials regardless of the method or the ratio of incorporated titanium metal loading. For the MIL-125 system, We studied the incorporation of zirconium metal into MIL-125-NH2 based framework via in situ method. The photocatalytic activity investigation of the MIL-125-NH2 confirms the effect of the amine linker toward better electron transfer from the organic linker to the central metal as verified by electron paramagnetic resonance spectroscopy. The observed decrease in the electronic bandgap of the MIL-125-NH2 and its Zr-exchange is confirmed and attributed to the effect of NH2-functionalization that boosts the reactivity of different MOFs. On the other hand, a detailed study of CO2 reduction was carried out with the pristine and the modified MOF catalysts to investigate the effect of NH2 functionalization and Zr-exchange on the photocatalytic CO2 reduction performance of MIL-125. DFT calculations were used to study the band structure of the UiO-66 systems, the CO2 interaction with the MOF surface, and to prepare a computational model of the charge transfer. The observed decrease in the electronic band gaps of the modified UiO-66-based MOFs is supported by density functional theory (DFT)-electronic structure calculations. The DFT-simulations also confirmed the synergistic effect between -NH2 functionalization and titanium metal incorporation that boosts the reactivity of the respective MOFs. A computational model for the charge transfer was developed describing the light interaction with the MOF. The light is absorbed by the organic linker producing electrons. The produced electrons transfer to the metal cluster, reducing the metal centers. In the presence of the TEOA, the electron transfers from the linker to the TEOA, where the CO2 reduction takes place. The adsorption of the CO2 on the catalyst was studied. It was found that the produced formate blocks the active catalyst sites and prevents the reaction from converting CO2 to higher products.

Item Type: Thesis (PhD)
Subjects: Physics
Divisions: College Of Sciences > Physics Dept
Committee Advisor: Yamani, Zain
Committee Members: Yaghi, Omar and Morsy, Mohammed and Gondal, Mohammad and Harrabi, Khalil
Depositing User: MOSTAFA ZEAMA (g201310350)
Date Deposited: 10 May 2020 15:38
Last Modified: 10 May 2020 15:38
URI: http://eprints.kfupm.edu.sa/id/eprint/141524