dissertation
Andraž Šuligoj (Author), Urška Lavrenčič Štangar (Mentor), Nataša Novak Tušar (Mentor)

Abstract

TiO2 - SiO2 composites were synthesized by low-temperature sol-gel impregnation method, using four different titania sources (P-25 from Degussa, PC500 from Millennium, CCA 100 AS and CCA 100 BS from Cinkarna, later denoted as AS and BS, respectively) and deposited on aluminium and glass carriers. Ordered and disordered mesoporous silicas were impregnated with ce{TiO2} in powder or suspension form in the Ti : Si molar ratio 1 : 1. Structure, size, band gap, chemical composition and specific surface area of nanoparticles were determined by X-ray diffraction (XRD), scanning (SEM) and transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DR-UV-vis), Fourier transform infrared spectroscopy (FT-IR) and ce{N2} physisorption. Additionally, quantity of surface hydroxyl groups, surface acidity and mechanical stability of the coatings were determined by temperature programmed Fourier transform infrared spectroscopy (TPD-IR) and Wolff-Wilborn method, respectively. The photocatalytic activity of TiO2 and TiO2 - SiO2 composites was evaluated in the photodegradation of toluene and formaldehyde, as model VOCs, under UVA light irradiation in lab-made photoreactor system with two different regimes; batch and plug-flow mode. These two VOCs are being considered as examples of two of the six major classes of indoor air contaminants. Adsorption properties of the samples with toluene, have shown that the addition of mesoporous silica was beneficial. The increase of the adsorption of the bare AS TiO2 (9.5 %) was higher in the case of ordered silica, SBA-15 (2.8 times for AS/SBA15 to 26.8 %) than disordered SiO2 KIL-2 (2.4 times for AS/KIL2 to 22.7 %) although it was significant in both types of mesoporous silica supports (over 20 %). Adsorption was found to be dependent mostly on the quantity of surface Si-OH groups. Regarding the photocatalytic activities towards toluene degradation, the results with pure TiO2 showed the fastest kinetics in case of sample AS followed by PC500 and P25. The observed behaviour was ascribed to smaller particle size, and consequently higher specific surface area. Grafting titania onto silica showed the importance of structural parameters. Most importantly, if the pore structure of bare TiO2 collapsed, this resulted in decreased activity. On the other hand, retainment of the pore structure improved the distribution of nanoparticles, crystallinity and optical properties, which resulted in improved photocatalytic activity. In the degradation of formaldehyde, it was found that adsorption and degradation abilities of the materials were much more dependent on the structural properties of the samples, i.e., the pore structure. This was explained as a consequence of the different degradation mechanisms of both pollutants. Toluene degradation is governed by the oxidation through photogenerated holes - direct oxidation, whereas in the degradation of formaldehyde, the major oxidative species are OH radicals. In addition, the same as in toluene oxidation, the activity was dependent mainly on the number of crystal defects and the band gap values, that is the oxidation and reduction potentials of the catalysts. The degradation efficiency was increased from 88 % in pure AS TiO2 to 97 % when this titania was grafted onto colloidal silica (7C). An important highlight of the thesis is synthesis of a novel photocatalyst, labelled AS7C, which comprises an acidic colloidal suspension AS as TiO2 source and colloidal silica, using a low-temperature sol-gel impregnation method. This sample used all the above mentioned properties that improve photocatalytic activity towards both pollutants. Mechanical stability of the samples was also tested. The tests showed that binder, in the form of colloidal SiO2 (in size of 25 nm) in combination with colloidal titania of appropriate size - 6 nm, produced the highest mechanical stability of the coatings, which also showed excellent photocatalytic activity. Stability of the coatings, using AS as titania source, was greatly improved. The Wolff-Wilborn test on AS coating showed no mechanical resistance, while with the optimal (in terms of photocatalytic performance) addition of 7C SiO2 binder stability was excellent (F, which is in the middle of 6B-6H hardness scale). This sample (AS7C) was also tested for release of aerosols, during operation of the reactor, which could be harmful for human health at longer exposure times. It was found that aerosols are formed, probably as a consequence of detachment of nanoparticles in the first period of photodegradation test. However, their formation in consecutive tests was greatly reduced. Last but not least, a pilot plug-flow reactor was constructed to test the photocatalyst's efficiency in one-pass degradation of toluene. Sample AS7C was able to degrade toluene at conditions applied (v= 400 mL/min, m(catalyst) = 1049 mg, C(0) = 1 ppmv), which means that the out-flow from reactor was clean of the pollutant and any possible intermediates, comprising only of humid air and CO2. Deactivation of the catalyst was found at higher air flow and higher initial concentration of the pollutant. However, the concentrations of pollutants in living conditions are few orders of magnitude lower, hence this is a promising result.

Keywords

Air remediation;TiO2 photocatalysis;Immobilization;Thin layers;TiO2/SiO2 composites;

Data

Language: English
Year of publishing:
Typology: 2.08 - Doctoral Dissertation
Organization: UNG FPŠ - Graduate School
Publisher: [A. Šuligoj]
UDC: 544.5/.6
COBISS: 3996667 Link will open in a new window
Views: 7663
Downloads: 183
Average score: 0 (0 votes)
Metadata: JSON JSON-RDF JSON-LD TURTLE N-TRIPLES XML RDFA MICRODATA DC-XML DC-RDF RDF

Other data

Secondary language: Slovenian
Secondary title: Razvoj naprednega TiO2/SiO2 fotokatalizatorja za čiščenje zraka notranjih prostorov
Secondary abstract: Sintetizirani so bili TiO2 − SiO2 kompoziti po nizko-temperaturnem sol-gel postopku impregniranja, z uporabo štirih komercialno dostopnih virov TiO2 (P-25 iz Degusse, PC500 iz Millenniuma ter CCA 100 AS in CCA 100 BS iz Cinkarne, kasneje označena kot AS in BS) ter njihova imobilizacija na aluminij in steklene nosilce. Urejeni in neurejeni mezoporozni SiO2 so bili impregnirani s TiO2 v obliki prahu ali v obliki suspenzije v Ti : Si molskem razmerju 1: 1. Določeni so bili struktura, velikost delcev, energijska vrzel kompozitov, njihova kemična sestava in specifična površina s praškovno rentgensko difrakcijo (XRD), vrstičnim elektronskim mikroskopom (SEM) in transmisijsko elektronsko mikroskopijo (TEM), UV-vis difuzno reflektanco (DR-UVvis), infrardečo spektroskopijo s Fourierjevo transformacijo (FT-IR) in N2 fizisorpcijo. Poleg tega je bila določena količina hidroksilnih skupin na površini, kislost na površini ter mehanske stabilnosti prevlek, določene z uporabo temperaturno programirane infrardeče spektroskopije s Fourierjevo transformacijo (TPD-IR) in Wolff-Wilborn testom trdnosti. Fotokatalitska aktivnost TiO2 in TiO2 − SiO2 kompozitov je bila ocenjena za fotorazgradnjo toluena in formaldehida kot modelnih VOC-ov pod UVA osvetlitvijo v laboratorijsko narejenem fotoreaktorskem sistemu v okviru dveh različnih režimov; krožnem in pretočnem. Ta dva VOC-a sta predstavnika dveh od šestih glavnih skupin onesnaževal notranjih prostorov. Absorpcijske lastnosti vzorcev, glede toluena kot onesnaževala, so pokazale, da je dodajanje mezoporoznega SiO2 koristno. Adsorpcija čistega TiO2 vira iz Cinkarne Celje (AS) (9.5 %) se je bolj povišala v primeru dodatka urejenega vira SiO2 – SBA-15 (2,8-krat za AS/SBA15 na 26.8 %) kot neurejenega vira SiO2 – KIL-2 (2,4-krat za AS/KIL2 na 22.7 %). Po drugi strani pa je bilo ugotovljeno, da je adsorpcija odvisna predvsem od količine Si−OH skupin na površini. Še več, impregnacija TiO2 z mezoporozno siliko adsorpcije ne izboljša vedno. Glede fotokatalitskih aktivnosti za razgradnjo toluena, so rezultati z uporabo ˇ cistega TiO2 pokazali najhitrejšo kinetiko v vzorcu AS, ki sta mu sledila PC500 in P25. Opaženo vedenje sem pripisal manjši velikosti delcev in s tem višji specifični površini. Vgradnja TiO2 v/na SiO2 je izpostavila pomen strukturnih parametrov. Kot najpomembnejša se je izkazala porušitev strukture por čistega TiO2, namreč to je povzročilo zmanjšano aktivnost. Po drugi strani pa je ohranitev strukture por pri impregnaciji izboljšala porazdelitev nanodelcev, kristaliničnost in optične lastnosti materiala, kar je povzročilo izboljšano fotokatalitsko aktivnost. Pri razgradnji formaldehida je bilo ugotovljeno, da so adsorpcijske in aktivnostne lastnosti materialov veliko bolj odvisne od strukturnih lastnosti vzorcev, to je strukture por. To si razlagamo kot posledico različnih mehanizmov razgradnje obeh onesnaževal. Razpad toluena poteka z oksidacijo preko fotovzbujenih lukenj - direktno oksidacijo, medtem ko so pri degradaciji formaldehida glavni oksidanti OH radikali. Poleg tega je enako kot pri oksidaciji toluena aktivnost odvisna predvsem od števila napak v kristalni strukturi in velikosti energijske vrzeli, torej redoks potenciala katalizatorjev. Sintetiziral sem nov fotokatalizator, označen z AS7C, ki izhaja iz kisle koloidne suspenzije AS kot vira TiO2 in koloidnega SiO2 kot veziva, z uporabo preproste, nizkotemperaturne sol-gel metode impregnacije. Ta vzorec izkorišča prednosti vseh zgoraj navedenih lastnosti, ki izboljšujejo fotokatalitsko aktivnost pri obeh onesnaževalih. Preizkusil sem tudi mehansko stabilnost vzorcev. Testi so pokazali, da je vezivo v obliki koloidnega SiO2, (velikost SiO2 delcev 25 nm), v kombinaciji s koloidno suspenzijo delcev titanovega dioksida primerne velikosti - 6 nm poveˇ calo mehansko obstojnost premazov. Prav tako so te plasti pokazale odlično fotokatalitsko aktivnost. WolffWilborn-ov test plasti iz čistega AS TiO2 ni pokazal nobene mehanske odpornosti, medtem ko sem z optimalno količino (v smislu fotokatalitske aktivnosti) dodanega 7C SiO2 veziva uspel dobiti plasti, ki so pokazale odlično trdnost (F, kar je na sredini 6b–6H trdnostne lestvice). Vzorec AS7C je bil testiran tudi za nastajanje aerosolov, med delovanjem reaktorja, ki so lahko pri daljših ˇ casih izpostavitve škodljivi za zdravje ljudi. Ugotovil sem, da aerosoli nastajajo, verjetno zaradi odstranjevanja nanodelcev iz plasti v prvem obdobju delovanja reaktorja. Tvorba aerosolov v sledečih testih pa je bila močno zmanjšana, kar kaže na dolgoročno ustreznost uporabe materiala za čiščenje zraka. Nenazadnje je bil izdelan pilotni pretočni reaktor za testiranje učinkovitosti fotokatalizatorja pri razgradnji enkratnega prehoda toluena. Vzorec AS7C je bil sposoben razgraditi začetno koncentracijo toluena, 1 ppmv (pretok zraka = 400 mL/min, m(katalizator) = 1049 mg), kar pomeni, da je bil zrak pri izhodu iz reaktorja čist ter brez vemsnih produktov. Pri višjih pretokih zraka in višji začetni koncentraciji onesnaževala sem zaznal deaktivacijo katalizatorja. Kljub temu pa so koncentracije VOC-ev v bivalnih prostorih nekaj redov velikosti nižje, zato je to obetajoč rezultat.
Secondary keywords: čiščenje zraka;TiO2 fotokataliza;imobilizacija;tanke plasti;TiO2/SiO2 kompoziti;disertacije;
URN: URN:SI:UNG
Type (COBISS): Dissertation
Thesis comment: Univ. v Novi Gorici, Fak. za podiplomski študij
Pages: XII, 155 str.
ID: 9063293