Methanol oxidation on platinum-rare-earth metal oxide activated catalysts
Date
2023-07-17
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Abstract
Kütuseelemendid on seadmed rohelise energia tootmiseks. Roheline vesinik on juba üpris levinud. Rohelist metanooli toodetakse kinnipüütud CO2-st taastuvenergia abil.
Vesinik-kütuseelementides tekib jäägina vesi. Metanooli puhul lisaks ka CO2 - tegu on sama CO2-ga, millest toodeti roheline metanool. See on kui null-summa mäng. Mis kasu on null-summa mängust? Sest nulliring on vaid CO2 puhul. Kasu tekib üleliigse taastuvenergia salvestamisest metanoolina, mida saab kasutada rohelise energia tootmiseks, kui energiat on puudu.
Kütuseelemendid ei tekita ohtlikke nanoosakesi ega ühendeid, mis põhjustavad sudu ja happevihmu. Metanool-kütuseelemendi väljundist saab CO2 taassiduda.
Kütuseelemendis on vaja Pt katalüsaatori nanoosakesi, mis kiirendavad reaktsioone. Mida väiksem on osake, seda suurem on pind reaktsioonide toimumiseks. Töö üks eesmärke oli Pt nanokatalüsaatorite süntees. On erinevaid mooduseid, kuidas neid valmistada: näiteks etüleenglükooli ning mikrolaineahju abil saab sadestada sobivaid Pt nanoosakesi, samuti plaatina ühendite kuumutamisel vesinikus. Nanoosakesed sadestati süsinikkandjale, mis juhib elektrit ja võimaldab reagentide liikumist kütuseelemendis. Tartu Ülikoolis valmistatud süsinikmaterjalidel on hea elektrijuhtivus, stabiilsus ning suur eripind.
Pt on hea katalüsaator metanooli oksüdeerimiseks, kuid Pt hoiab mõnd vaheühendit oma pinnal liiga tugevasti kinni. Need ühendid tuleb pinnalt puhastada. Üks kallis variant selleks on ruteeniumi kasutamine. Selles töös uuriti odavamate haruldaste muldmetallide oksiidide kasutusvõimalusi. Tseeriumi või praseodüümi oksiidide sadestamiseks süsinikule koos Pt nanoosakestega uuriti erinevaid meetodeid. Parimad tulemused saavutati kuumutades praseodüümhüdroksiidi inertgaasis 1100 kraadini. Pt sadestati vesinikuga redutseerides madalamal temperatuuril. Katalüsaator koosnes väikestest hästi jaotunud Pt nanoosakestest ning eri vormides olevatest praseodüümoksiidi nanovarrastest.
Need katalüsaatorid kiirendavad efektiivselt metanooli oksüdeerimist, kuna on tagatud Pt pinna puhastumine. Haruldase muldmetalli oksiidi pinnale adsorbeeruvad hapnikku sisaldavad ühendid, mis difundeeruvad Pt pinnale ja puhastavad selle. Tekib CO2, mis lahkub pinnalt, ning roheline tsükkel jätkub.
Fuel cells are promising devices for the production of green energy. While green hydrogen is quite common, green methanol can be produced using renewable energy and captured CO2. Hydrogen fuel cells emit only H2O. Methanol fuel cells also emit CO2, which is the same CO2 captured to produce green methanol, so it is essentially a zero-sum game. What is the benefit of a zero-sum game? This only applies to CO2. The benefit comes from storing excess cheap renewable energy in methanol, and using it to produce green electricity, when electricity is scarce and expensive. Fuel cells do not produce dangerous nanoparticles, or NOx or SOx, which cause smog and acid rain. The CO2 from the exhaust of a methanol fuel cell can be recaptured. However, fuel cells depend on Pt catalyst nanoparticles to increase the reaction rate. The smaller the catalyst particles, the higher the surface area on which reactions occur. One of the aims of this work was to make smaller catalyst particles. Various methods exist; for example, using ethylene glycol and a microwave oven to synthesise Pt nanoparticles proved suitable, as well as heating a Pt precursor in hydrogen. Nanoparticles are deposited onto a carbon support material. It plays an essential role in fuel cells: it conducts electricity and allows reagents to flow. High surface area materials developed at the University of Tartu showed good conductivity and durability. While Pt is a good catalyst for methanol oxidation, it holds on to some of the intermediate compounds produced on its surface too strongly. These need to be cleaned from the surface. Ruthenium is one costly option. In this work, cheaper alternatives, such as rare-earth metal oxides, were used. Various methods for synthesising cerium and praseodymium oxide along with Pt were studied. The best results were obtained by heating praseodymium hydroxide to 1100 °C in an inert gas and depositing Pt by heating the material impregnated with Pt precursor in hydrogen at a lower temperature. This material had small, well-dispersed Pt nanoparticles and praseodymium oxide nanorods containing several forms of Pr oxide. These oxides aid methanol oxidation by cleaning the Pt surface. They provide oxygen-containing compounds adsorbed on their surface, which diffuse onto the Pt particle. CO2 forms, leaves the surface, and the green cycle continues.
Fuel cells are promising devices for the production of green energy. While green hydrogen is quite common, green methanol can be produced using renewable energy and captured CO2. Hydrogen fuel cells emit only H2O. Methanol fuel cells also emit CO2, which is the same CO2 captured to produce green methanol, so it is essentially a zero-sum game. What is the benefit of a zero-sum game? This only applies to CO2. The benefit comes from storing excess cheap renewable energy in methanol, and using it to produce green electricity, when electricity is scarce and expensive. Fuel cells do not produce dangerous nanoparticles, or NOx or SOx, which cause smog and acid rain. The CO2 from the exhaust of a methanol fuel cell can be recaptured. However, fuel cells depend on Pt catalyst nanoparticles to increase the reaction rate. The smaller the catalyst particles, the higher the surface area on which reactions occur. One of the aims of this work was to make smaller catalyst particles. Various methods exist; for example, using ethylene glycol and a microwave oven to synthesise Pt nanoparticles proved suitable, as well as heating a Pt precursor in hydrogen. Nanoparticles are deposited onto a carbon support material. It plays an essential role in fuel cells: it conducts electricity and allows reagents to flow. High surface area materials developed at the University of Tartu showed good conductivity and durability. While Pt is a good catalyst for methanol oxidation, it holds on to some of the intermediate compounds produced on its surface too strongly. These need to be cleaned from the surface. Ruthenium is one costly option. In this work, cheaper alternatives, such as rare-earth metal oxides, were used. Various methods for synthesising cerium and praseodymium oxide along with Pt were studied. The best results were obtained by heating praseodymium hydroxide to 1100 °C in an inert gas and depositing Pt by heating the material impregnated with Pt precursor in hydrogen at a lower temperature. This material had small, well-dispersed Pt nanoparticles and praseodymium oxide nanorods containing several forms of Pr oxide. These oxides aid methanol oxidation by cleaning the Pt surface. They provide oxygen-containing compounds adsorbed on their surface, which diffuse onto the Pt particle. CO2 forms, leaves the surface, and the green cycle continues.
Description
Väitekirja elektrooniline versioon ei sisalda publikatsioone
Keywords
methanol, oxidation, catalysts, rare earth metals, nanoparticles, electrochemistry