Synthesis and properties of zinc-aluminium and zinc-chromium layered double hydroxides and their derived mixed oxides

Plastoviti dvojni hidroksidi (angl. layered double hydroxides – LDH) so strukturno podobni alumosilikatom, ki so glavna sestavina glin. Sestavljeni so iz pozitivno nabitih plasti, v katerih se nahajajo kationi vsaj dveh različnih kovin. Pozitivni naboj plasti kompenzirajo anioni, ki se skupaj z vodo nahajajo v sloju med plastmi. LDH je enostavno sintetizirati z veliko različnimi kationi in medplastnimi anioni. Pri njihovi toplotni obdelavi nastanejo mešani oksidi. Lastnosti kot sta velika specifična površina in ionsko izmenjevalna kapaciteta, omogočajo raznoliko uporabo LDH in njihovih mešanih oksidov. Med drugim se uporabljajo kot ionski izmenjevalci, antacidi, adsorbenti v odpadnih vodah, katalizatorji in nosilci katalizatorjev. Namen mojega diplomskega dela je bila sinteza in karakterizacija cink-aluminijevih in cink-kromovih plastovitih dvojnih hidroksidov in njihovih mešanih oksidov. Primerjali smo stopnjo kristaliničnosti ter potek termičnega razpada Zn-Al LDH in Zn-Cr LDH, vpliv hidrotermalne obdelave na kristaliničnost dobljenih produktov ter anionsko izmenjavalno kapaciteto in fotokatalitsko aktivnost produktov, toplotno obdelanih pri različnih temperaturah. Cink-aluminijeve in cink-kromove plastovite dvojne hidrokside sem sintetizirala z metodo soobarjanja pri konstantnem pH. Raztopini prekurzorskih kationov (Zn2+, Al3+ oz. Zn2+, Cr3+) sem pripravila tako, da je bil delež trivalentnega kationa (x) enak 0,25. S tem sem upoštevala pogoj za pripravo enofaznih LDH (0,2≤x≤0,33). Pri obeh sintezah sem kot bazo uporabila raztopino NH3 (c = 2,0 M). Polovica sintetiziranih LDH je bila hidrotermalno obdelana pri 100 °C. Prav tako sem pri delu pripravljenih produktov izvedla zamenjavo kloridnih ionov s karbonatnimi. Vsak dobljen vzorec je bil nato še toplotno obdelan pri 300 °C, 400 °C, 500 °C in 600 °C. Na koncu sem vzorce karakterizirala z rentgensko praškovno difrakcijo (XRD), termogravimetrijo (TG) in Uv-Vis spektrometrijo. S slednjo sem preko razgradje azo barvila Plasmocorinth B merila adsorpcijsko sposobnost in fotokatalitsko aktivnost vzorcev. Pri svojem delu sem uspešno sintetizirala Zn-Al in Zn-Cr LDH. Zamenjava kloridnih ionov s karbonatnimi je potekla, kar smo dokazali z XRD in TG analizo. Ugotovila sem, da se kristaliničnost produkta zmanjša ob zamenjavi Al3+ ionov s Cr3+ ioni, medtem ko se ob zamenjavi Cl- ionov s CO32- ioni kristaliničnost poveča pri Zn-Al LDH in zmanjša pri Zn-Cr LDH. Na potek termičnega razpada precej bolj vpliva zamenjava kloridnih ionov s karbonatnimi med plastmi, kot pa zamenjava aluminijevih kationov s kromovimi v plasteh. Izkaže se, da so Zn-Cr LDH boljši adsorbenti od Zn-Al LDH in da so dobri fotokatalizatorji hidrotermalno obdelani vzorci s kloridnimi ioni, toplotno obdelani pri 600 °C.Layered double hydroxides (LDHs) are structurally similar to aluminosilicates, which are the main constituents of clays. They consist of positively charged layers containing cations of at least two different metals. The positive charge of the layer is compensated by anions, which are located in the interlayer together with water molecules. LDHs are easily synthesized with many different cations and interlayer anions and during thermal treatment form mixed oxides. Properties such as a large specific surface area and high capacity for ion exchange allow for a diverse use of LDHs and their mixed oxides, such as ion exchangers, antacids, wastewater adsorbents, catalysts and catalyst supports. The purpose of this diploma work was the synthesis and characterization of zinc-aluminium and zinc-chromium layered double hydroxides and their mixed oxides. Crystallinity and the course of thermal decomposition of Zn-Al LDHs and Zn-Cr LDHs, the influence of hydrothermal treatment on the crystallinity of the obtained products, the anion exchange capacity and photocatalytic activity of products heat treated at different temperatures, were examined. Zinc-aluminium and zinc-chromium layered double hydroxides were synthesized by the coprecipitation method at a constant pH level. The solution of precursor cations (Zn2+, Al3+ or Zn2+, Cr3+) was prepared so that the mole fraction of the trivalent cation (x) was equal to 0,25 so the condition for the preparation of pure LDHs (0,2≤x≤0,33) was fulfilled. In both syntheses a NH3 solution (c = 2.0 M) was used as the base. Half of the synthesized LDHs were hydrothermally treated at 100 °C. For half of the products, the chloride ions were replaced with carbonate ions. Each sample was then further heat treated at 300 °C, 400 °C, 500 °C and 600 °C. The samples were characterised by X-ray powder diffraction (XRD), thermogravimetry (TG) and UV-Vis spectrometry. The latter was used to measure the adsorption capacity and photocatalytic activity of the samples through the degradation of the azo dye Plasmocorinth B. Results show that Zn-Al and Zn-Cr LDHs were successfully synthesised. The replacement of chloride ions with carbonate ones was successful, which was proven by XRD and TG analysis. The crystallinity of the product was found to decrease upon replacement of Al3+ ions with Cr3+ ions, while replacing Cl- ions with CO32- ions increased the crystallization degree of Zn-Al LDHs, the opposite was observed for Zn-Cr LDHs. The replacement of chloride ions with carbonate ones between the layers has a much greater effect on the curve of thermal decomposition than a replacement of aluminium cations with chromium ones in the layers. It turns out that Zn-Cr LDHs are better adsorbents than Zn-Al LDHs and that good photocatalysts are both hydrothermally treated samples with chloride ions, heat treated at 600 °C