Tetrapirol türevlerinin sentezi ve enerji alanındaki uygulamaları
Synthesis of tetrapyrole derivatives and applications in the field of energy
Yazar: EKREM KAPLAN
Danışman: PROF. DR. ESİN HAMURYUDAN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Kimya Ana Bilim Dalı
Derece: Doktora, 2023
Konu: Kimya = Chemistry

Özet:
Günlük yaşantımızın vazgeçilmezlerinden biri de çoğu faaliyetin gerçekleşmesi hususunda gerekli olan enerjidir. Artan enerji talebi ve fosil yakıtların tükenme durumuna gelmesi insanları, alternatif enerji kayanağı bulmak konusunda motive etmektedir.
Doğada serbest halde bulunmayan hidrojen, bitkilerin yapısında ve özellikle suda, metanol, metan gibi bileşikleri halinde bulunur. Farklı enerji kaynaklarından elde edilebilen yakıtlara örnek olarak H2 verilebilir. Hidrojenin depolanabilen ve taşınabilen bir kaynak olması, onu çok önemli bir malzeme konumuna getirmektedir. Pozitif özelliklerine rağmen, hidrojenin bazı üretim yöntemlerinin sera gazı etkisine sahip olduğu da bilinmektedir. Bu etkiden kurtulmak için çevreye zararlı olmayan alternatif üretim yöntemlerinin keşfi çok önem taşımaktadır. En çok bilinen yöntem olan suyun elektrolizinin maliyet, verimlilik ve gerilim bakımından çeşitli olumsuzlukları olduğu bilinmektedir.
Dünya şu anda birbirine çok yakından bağlı iki sorunla karşı karşıya bulunmaktadır. Bunlar atmosferdeki yüksek CO2 seviyelerine bağlı iklim değişiklikleri ve artan küresel enerji talebidir. Bu zorlukların üstesinden gelmek için, düşük karbonlu bir ekonomiyi sürdürürken yenilenebilir enerjileri entegre etmek ve enerji sistemini yerelleştirmek çok önemlidir. Bu nedenle, CO2'i hammadde olarak kullanan ve elektrik enerjisini diğer yararlı enerji biçimlerine dönüştüren teknolojilerin geliştirilmesi gerekmektedir. CO2'nin elektrokimyasal indirgenmesi, bu gereklilikleri karşılama ve atmosferdeki CO2 miktarının azaltılmasına aktif olarak katkıda bulunma potansiyeline sahiptir.
Bu tez kapsamında yapılan çalışmalar ftalosiyanin ve porfirin bazlı elektrot üretimi olmak üzere iki ana gruba ayrılabilir. Bunlardan biri karboksilik asit ile fonksiyonlandırılmış bakır-ftalosiyaninlerin rGO ile birlikte hibrit nanokompozitlerinin hazırlanması, karakterizasyonu, GCE yüzeyine kaplanması ve HER için elektrokatalizör olarak kullanımlarının araştırılmasıdır. Porfirin tabanlı çalışmaların biri, iki tienil ve iki fenil grubu içeren yeni metalsiz ve kobalt(II) porfirin sentezi, karakterizasyonu, CO2 indirgeme katalizörleri olarak kullanılmalarının araştırılması şeklinde planlanmıştır. Diğeri ise, 3 fenil ve 1 tiyofenli elektropolimerleşebilen grup içeren asimetrik porfirin sentezi, karakterizasyonu ve enerji alanında elektrokatalizör olarak kullanım verimliliğinin araştırılması üzerinedir.
Bu amaçla çalışmalardan birinde; hidrojenin elektrokatalitik üretimindeki katalizör etkinliğinin ölçülmesi amacıyla çevresel ve çevresel olmayan pozisyonlarda tetra ve okta sübstitüye olmak üzere üç ftalosiyanin sentezlenip indirgenmiş grafen oksit ile nanokompozit elde edilmiştir. Elde edilen nanokompozit yapılarının ileri karakterizasyon yöntemleri (SEM, XRD, Raman, XPS, FT-IR, UV-Vis ve ICP-MS) kullanılarak yapısal karakterizasyonları yapılmıştır. Alınan elektrokimyasal ölçümler sonucunda çevresel pozisyonlardan sübstitüye edilmiş ftalosiyaninler ile elde edilen nanokompozitlerin daha iyi sonuç verdiği görülmüştür. Sentezlenen üç yapının sonuçları mukayese edildiğinde en iyi sonucun okta sübstitüye ftalosiyaninden elde edilmiş nanokompozitte olduğu, ikinci sırada çevresel pozisyonda sübstitüye olmuş ftalosiyanin ile elde edilen nanokompozitin olduğu ve son sırada ise çevresel olmayan pozisyonda sübstitüye olmuş ftalosiyanin ile elde edilen nanokompozitin olduğu görülmüştür.
Tez içerisinde gerçekleştirilen diğer çalışmada elektropolimerleşebilen grup içeren bir porfirin sentezlenip, kalem ucu elektrot üzerine elektropolimerleşme yöntemi ile kaplanmasıyla oluşturulan elektrotların karbondioksitin elektrokimyasal indirgenmesinde ki katalizör olarak kullanımlarının verimliliği incelenmiştir. Sentezlenen maddelerin karakterizasyonları FT-IR, UV-Vis, Raman ve SEM yöntemleri kullanılarak yapılmıştır. Farklı döngü sayılarında (10, 30, 50, 100) yapılan kaplamalar ile elde edilen metalsiz ve kobalt metali içeren elektrotların yapılan elektrokimyasal ölçümler sonucunda alınan verilere göre verimlilikleri sıralandığında, 100 döngüde kaplanan kobalt porfirin içeren elektrotun en verimli sonuç verdiği görülmüştür.
Bir diğer porfirin içeren çalışmada ise, tiyofen grubu içeren bir asimetrik porfirin sentezlenerek elektropolimerleştirme yöntemiyle elektrot oluşturulup enerji alanlarındaki elektrokatalizör verimlilikleri incelenmesi planlanmıştır.
Bu tez kapsamında elektrokimyasal hidrojen üretiminde ve karbondioksitin elektrokimyasal indirgenmesinde elektrot olarak kullanılmaya aday yeni tetrapirol türevi mazemelerin sentezi ve karakterizasyonu gerçekleştirilmiş, ayrıca uygulamaya dair özellikleri incelenerek literatüre katkı sağlanması amaçlanmıştır.

Abstract:
In every part of life, energy plays a crucial role. Energy is required for industrial production as well as for the survival of living creatures. Almost everywhere in the world, there is a growing demand for energy. In the state that technology is in now, life would cease to exist in the absence of energy sources. In order to find ways to acquire energy using the possibilities offered by nature, several research have been conducted and are still being conducted.
Energy not only affects a nation's economic progress, but it has also caused security issues. Because of this, many nations are concerned with managing both their own energy resources and other global energy sources. Countries throughout the world continue to engage in military and financial projects to safeguard energy resources.
As a nation's economic strength in the global economy, energy has come to have strategic relevance. Due to the supremacy of fossil fuels, the energy age persisted until the 1960s. However, the realization that fossil fuels have their limits and the ensuing oil crises have led to a mistrust of the world's energy supplies, hastening the hunt for new, renewable sources of energy apart. The USA, Europe, Japan, and many other nations have started doing extensive research on alternate energy sources.
Furthermore, it is impossible to overlook the detrimental consequences of fossil fuels, which provide 85% of the world's energy demands, on the environment and human health, the harmful chemicals they discharge into the environment, and their scarcity. The quest for energy resources by various nations, their military and financial investments, and their detrimental impacts on living things lead to wars between nations and the necessity for an alternative energy source to fossil fuels. Due to its versatility and ease of transportation, hydrogen energy has emerged as a leading alternative energy source. It also has minimal adverse impacts on the environment and human health, and it is also cost-effective. Compared to fossil fuels, renewable energy sources are more plentiful and have a developed infrastructure. One of the energy sources that might one day take its rightful position among clean and sustainable energy sources is hydrogen.
There are a finite number of fossil fuels. On the other hand, water is said to be limitless, and the best approach is to produce hydrogen gas by the electrolysis of water. However, the production of hydrogen gas loses its economic level and is highly expensive owing to anodic and cathodic reactions in the electrolysis of water and excessive voltages caused by the resistance created in the electrolysis cell. In recent years, research has been moving forward quickly to develop the instruments and circumstances necessary for the synthesis of hydrogen gas by lowering the voltage of the electrode reactions in the creation of electrocatalytic hydrogen gas. A technologically significant reaction occurs when water is electrolyzed and produces hydrogen gas. In order to demonstrate the proper extremely low voltage of the electrodes utilized, solutions containing acids or bases are often used.
The manufacture of anode or cathode materials for the electrolysis-based generation of hydrogen gas involves the use of various metals or their alloys. However, the majority of the utilized metals do not exhibit the necessary catalytic effect. The best electrodes for catalyzing the creation of hydrogen gas are made of noble metals like Pt, Pd, Ru, and Ir because of their high electrocatalytic activity, low overvoltages, and excellent stability. The limited availability of these metals in nature and their high cost prevent their broad usage in actual applications. By expanding the electrode's real surface area, the efficiency of the electrodes may be improved. For this aim, nanoscale particles have also produced encouraging outcomes.
The concentration of CO2 in the atmosphere has grown due to greenhouse gas emissions from anthropogenic sources, and this trend is expected to continue until significant CO2 emission reductions are made. Allowing CO2 levels to continue growing is a significant test of our comprehension of the global environment since it interrupts the regular carbon cycle and increases atmospheric CO2 concentrations.
The capture or sequestration of CO2 has been the subject of several research projects. Over the past ten years, there has been much research on CO2 conversion and usage. This entails a variety of techniques, such as biochemical, radiolysis, thermocatalysis, photocatalysis, electrocatalysis, and photoelectrochemical.
A synthetic way to produce industrial chemicals or fuels that are normally generated from petroleum using electrochemical reduction of CO2 and renewable energy sources is possible. However, due to CO2's thermodynamic stability, activation is challenging and electrical energy must be supplied to power the CO2 reduction process. By creating a potential difference between the anode and cathode electrodes in an electrolyzer, CO2 may be reduced electrochemically at the cathode. In this respect, solid oxide electrolysis cells can be employed at high temperatures (750-900°C) and high current densities, however under these process conditions, only CO can be generated. However, low temperature CO2 electrolysis (25–100°C) could provide a practical approach for economically manufacturing a variety of hydrocarbons, alcohols, formic acid, etc. from CO2 reduction.
The research done for this thesis may be separated into two primary categories: the synthesis of porphyrin- and phthalocyanine-based electrodes. One of these involves creating and characterizing carboxylic acid-functionalized copper-phthalocyanines with rGO hybrid nanocomposites, coating them on the GCE surface, and researching their potential usage as HER electrocatalysts. The synthesis and characterisation of novel metal-free and cobalt(II) porphyrins with two thienyl and two phenyl groups, as well as their usage as CO2 reduction catalysts, were proposed as one of the porphyrin- based investigations. Also, Asymmetric porphyrins with three phenyl and one electropolymerizable thiophene group are synthesized, characterized, and their effectiveness as an electrocatalyst in the energy sector are examined.
In this thesis, three phthalocyanines, tetra and octa substituted at peripheral and non- peripheral locations, were synthesized to create a nanocomposite of reduced graphene oxide, which was used to test the catalyst's effectiveness in the electrocatalytic generation of hydrogen. Raman, XRD, and SEM investigations were used to look into the surface morphology and structure of the generated electrodes. Electrochemical tests reveal that peripherally carboxylic acid functionalized rGO/CuPc(3) has higher electrocatalytic activity and efficiency toward HER, displaying the lowest Tafel slope, Rct, and greatest Cad values when compared to its competitors. It is discovered that the tafel slope of rGO/CuPc(3) is 173 mV/dec, indicating that the Volmer mechanism is predominate in the hydrogen evolution reaction on rGO/CuPcs. Due to reduced steric barrier than non-peripherally substituted ones, peripherally substituted phthalocyanines interact better with the rGO structure. Furthermore, according to the spectroscopic and morphological findings, the hetero-structure of rGO/CuPc nanocomposites has a homogenous structure that is mostly caused by strong intermolecular contacts between graphene sheets and phthalocyanine. Additionally, this research shows that the kind of substituents, particular locations, and orientations of substituents (peripheral and non-peripheral) are crucial for the nanocomposite's electrocatalytic activity. The potential for these nanocomposite catalysts to be utilised in prospective electrochemical energy applications is quite high.
The effectiveness of using electrodes created by synthesizing an electropolymerizable group containing a porphyrin and coating the pencil graphite electrode with the electropolymerization method as a catalyst in the electrochemical reduction of carbon dioxide was examined in another study conducted for the thesis. New thiophenyl- porphyrin-based CO2-reduction catalysts have been presented. Through electropolymerization, varying number of cycles of poly-H2Por and poly-CoPor deposition on a PGE surface were effectively accomplished. In order to assess the surface morphology of the synthesized electrodes, Raman and SEM analyses were carried out. It has been established that the CO2RR mechanism is influenced by the central metal and the quantity of electropolymerization cycles. Compared to poly- H2Por (-1.04 V) and bare PGE (-1.35 V), poly-CoPor electropolymerized at 100 cycles primarily lowered the overpotential of CO2RR to -0.63 V. The outcomes demonstrated the potential of these porphyrin-coated PGE electrodes for electrochemical CO2 reduction.
In a different porphyrin-based investigation, an asymmetric porphyrin with a thiophene group will be created, electrodes will be made using the electropolymerization process, and the effectiveness of electrocatalysts in energy fields will be assessed.
The goal of this thesis was to contribute to the literature by synthesizing and characterizing new tetrapyrrole-derived materials, which are candidates to be used as electrodes in electrochemical hydrogen evolution and electrochemical reduction of carbon dioxide.