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Modificiran cinkov oksid kot katalizator za pridobivanje kisika in vodika
Hajduk, Špela (Author), Marinšek, Marjan (Mentor) More about this mentor... This link opens in a new window, Bele, Marjan (Co-mentor)

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Abstract
V doktorski nalogi predstavljamo dve področji uporabe katalizatorjev na osnovi ZnO. Prvo področje zajema pripravo modificiranih materialov na osnovi ZnO, ki so se izkazali kot učinkoviti fotokatalizatorji, saj imajo fotoelektrokemijsko sposobnost razklopa H2O. Drugi del doktorske naloge zajema pripravo modificiranih Cu-Zn oksidnih katalitskih materialov za reakcijo razklopa NH3. Lastnosti sintetiziranih fotokatalizatorjev in katalizatorjev smoraziskovali s spektroskopskimi in mikroskopskimi metodami, fotokatalitske oz. katalitske aktivnosti pa smo nato preverili s fotokatalitskim oz. katalitskim testom. Zanimala nas je predvsem razlika v aktivnostih med nemodificiranimi ter modificiranimi fotokatalizatorji oz. katalizatorji ter stabilnosti pripravljenih fotokatalitskih oz. katalitskih materialov. Prvi del raziskovalnega dela se nanaša na pripravo modificiranih fotokatalitskih materialov ZnO z grafitiziranimi ogljikovimi strukturami, ki vključujejo dušik (v nadaljevanju g-CxNy). Pripravljeni fotokatalizatorji so aktivni za proizvodnjo O2 s fotoelektrokemijskim razklopom H2O. Fotoelektrokemijske celice oz. fotokatalizatorji, modificirani z metodo potapljanja v vodne disperzije prekurzorja CM-kompleksa (CM – cianurna kislina–melamin), kažejo najvišje fotoelektrokemijske aktivnosti za proizvodnjo O2. Tako pripravljeni materiali ohranjajo porozno strukturo, ki med fotoelektrokemijskim testiranjem omogoča penetracijo elektrolita ter dotok simulirane sončne svetlobe do aktivnih mest na površini fotokatalizatorjev. Vzorec, sintetiziran z metodo potresanja CM-kompleksa, kaže najnižjo fotoelektrokemijsko aktivnost, ki je precej nižja od fotoelektrokemijske aktivnosti nemodificiranega ZnO, ki ima morfologijo nanožičk. Slednje je posledica dejstva, da prekomerna plast g-CxNy blokira možnost vstopa elektrolita ter simulirane sončne svetlobe v medprostore med nanožičkami ZnO. S podrobno fotoelektrokemijsko karakterizacijo je bilo ugotovljeno, da ima sloj g-CxNy sposobnost učinkovite ekstrakcije elektronskih vrzeli, ki so dejansko aktivna mesta za proces oksidacije H2O. Posledično to vodi do velike izboljšave v fotoelektrokemijski aktivnosti oksidacije H2O. Relativno na fotokatalizator ZnO brez nanosa g-CxNyima modificirani vzorec ZnO z optimalnim nanosom sloja g-CxNy, ki je sintetiziran z metodo potapljanja, kar 3,5-krat večjo gostoto toka pri potencialu 1,23 V, merjenem glede na reverzibilno vodikovo elektrodo (RHE; angl. Reversible Hydrogen Electrode). S tem se izboljša tudi stabilnost fotokatalizatorja v močno alkalni raztopini. V drugem delu raziskave se osredotočamo na pripravo Cu-Zn oksidnih katalizatorjev, sintetiziranih po sol-gel postopku. Katalizatorji so nadalje modificirani z Al2O3 na dva načina (s sol-gel metodo ter z metodo mokre impregnacije). Tovrstni katalizatorji so bili testirani za proces razklopa NH3 ter proizvodnjo H2 visoke čistote z namenom priprave goriva za gorivne celice. Katalizatorja, pripravljena s tehniko mokre impregnacije, ki vsebujeta nosilec γ-Al2O3, imata na površini boljšo disperzijo baker vsebujočih zvrsti v primerjavi s katalizatorji brez nosilca Al2O3. Analiza slik z vrstično elektronsko mikroskopijo (SEM) na omenjenih katalizatorjih kaže na enakomerno porazdelitev delcev zvrsti Cu in ZnO ter podobno morfologijo med katalizatorji ne glede na razmerje Cu/Zn. Katalizatorja, pripravljena z metodo mokre impregnacije, imata boljši katalitski aktivnosti v primerjavi s katalizatorjema, pripravljenima z modificirano sol-gel metodo. Slednje lahko pripišemo vsebnosti nanodelcev Cuᵒ na površini katalizatorjev ter sinergijskemu vplivu mezoporoznega nosilca γ-Al2O3. Ta pomaga pri porazdelitvi aktivnih mest na površini katalizatorjev ter znatno poveča njihovo specifično površino.

Language:Slovenian
Keywords:cinkov oksid, katalizator, proizvodnja kisika, proizvodnja vodika, aktivnost
Work type:Doctoral dissertation (mb31)
Tipology:2.08 - Doctoral Dissertation
Organization:FKKT - Faculty of Chemistry and Chemical Technology
Year:2019
COBISS.SI-ID:302853632 Link is opened in a new window
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Downloads:165
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Secondary language

Language:English
Title:Modified zinc oxide as catalyst for production of oxygen and hydrogen
Abstract:
In the doctoral thesis, two areas of the use of ZnO-based catalysts are presented. The first area involves the preparation of modified ZnO-based materials that have proven to be rather effective photocatalysts, with the ability to photoelectrochemically (PEC) split H2O. The second part of the doctoral thesis involves the preparation of the modified Cu-Zn oxide catalysts for ammonia decomposition reaction. Properties of photocatalytic and catalytic materials were investigated using spectroscopic and microscopic methods. Photocatalytic and catalytic activities were then tested with photocatalytic and catalytic measurements. The main focus was to differentiate between the pristine and modified photocatalyst/catalysts regarding their photocatalytic/catalytic activities, and to check the stability of the prepared photocatalytic/catalytic materials. The first part of the research work describes the preparation of modified photocatalytic materials ZnO with graphitic carbon structures containing nitrogen (hereinafter referred to as g-CxNy). The prepared photocatalysts were efficiently involved in the production of O2 with PEC water splitting. Photocatalysts (also referred to as PEC cells), which were modified by the dip-coating method and prepared by the immersion of the electrode into the aqueous dispersions of the precursor CM-complex (CM – cyanuric acid–melamine), showed the highest PEC activity. Namely, the materials retained their porous structure which, during PEC testing, allowed penetration of the electrolyte and simulated solar light to the active sites on the surface of the photocatalysts. The sample synthesised by the powder deposition method, showed the lowest PEC activity, in fact even significantly lower than the PEC activity of the pristine ZnO, which had the morphology of nanowires. This could be interpreted by assuming that the excessive g-CxNy layer blocked the electrolyte from entering the volume between the ZnO nanowires. With a detailed PEC characterisation, it was shown that the g-CxNy layer has the ability to effectively extract electronic holes, which are the active sites in the process of H2O oxidation. Consequently, this leads to a major improvement in PEC activity - with respect to the photocatalyst ZnO. The optimal sample, which is synthesised by the dip-coating method (when compared to a ZnO photocatalyst without a g-CxNy layer), shows a 3.5-times higher photocurrent density at a potential of 1.23 V, measured relative to the reversible hydrogen electrode (RHE). Good stability is also confirmed in a strongly alkaline solution. In the second part of the study, the main focus is on the preparation of Cu-Zn oxide catalysts synthesised by a modified citrate method. The catalysts are further modified with Al2O3 with two synthetic methods, i.e. sol-gel and wet-impregnation techniques. The catalysts were tested for NH3 decomposition reaction and the production of high purity H2 for fuel cells. The catalyst prepared using a wet-impregnation technique, having γ-Al2O3 as a catalyst support, show an improved dispersion of copper-containing species on the catalyst surface, when compared to the catalysts without catalyst support. The analysis of the mentioned catalysts using scanning electron microscopy (SEM) shows a uniform distribution of Cu-species and ZnO particles, and a similar morphology, regardless of the Cu/Zn ratio. Catalysts prepared by the wet-impregnation method, show a superior catalytic activity, compared to catalysts prepared by the modified sol-gel method. This could be attributed to the content of Cuᵒ nanoparticles on the catalyst surface and the synergistic effect of the γ-Al2O3 mesoporous support. The γ-Al2O3 support helps in distributing the active sites on the surface of catalysts, and significantly increases their specific surface area.

Keywords:zinc oxide, catalyst, oxigen production, hydrogen production, activity

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