Aijuan Han

Influence of facets and heterojunctions in photoactive bismuth oxyiodide

The role of facetted crystalline nanosheets and heterojunctions in bismuth oxyiodide composites for the photodegradation of phenolic compounds is investigated. Single-crystalline nanosheets with either {001} or {110} planes as the dominant facets were formed by tuning the pH during synthesis. The {001}-facetted BiOI consisted of large crystallites with low surface areas in contrast to the thinner and smaller {110}-facetted platelets. The specific activity per m2 for the photodegradation of p-cresol was ∼5 times higher over the {001} than over the {110}-facetted BiOI which can be correlated to the stronger adsorption of the substrate at the oxygen-rich {001} surface. However, the low surface area of <1 m2 g−1 is a drawback for practical use. Instead, by simple calcination of {110}-facetted BiOI at 350 °C, composites of different BixOyIz phases with closely related crystalline structures were formed. The intimate contact between the phases results in the formation of heterojunctions which greatly improve the photoactivity, even over that of {001}-facetted BiOI. The most active photocatalyst was a composite of Bi7O9I3/α-Bi5O7I which maintained its activity in the presence of chloride and nitrate anions and was recyclable. Scavenger studies revealed that holes were active species for the degradation of p-cresol.

Efficient photodegradation of chlorophenols by BiOBr/NaBiO3 heterojunctioned composites under visible light

Forming heterojunctioned composites is an effective way to develop visible-light-driven photocatalysts. A series of BiOBr/NaBiO3 composites were synthesized by surface transformation of NaBiO3 with hydrobromic acid. Commensurate planes of BiOBr and NaBiO3 enabled the formation of a closely bound interface. Composites with <20wt.% BiOBr exhibited excellent photocatalytic activity towards the degradation of chlorophenols under low intensity visible light (λ>400nm). The best photocatalyst was 9% BiOBr/NaBiO3 with a quantum yield of 0.365. No photocorrosion was observed after three cycles. Using radical scavengers and inert atmosphere, holes, superoxide and hydroxyl radicals were found to be involved in the photoactivity of the BiOBr/NaBiO3 composite. Hydroxylated and open-ring diacid molecules were identified as intermediates in the mineralization of 4-chlorophenol.

Chemoselective transfer hydrogenation of α,β-unsaturated carbonyl compounds using potassium formate over amine-grafted Ru/AlO(OH) catalysts

Grafting of 3-(2-aminoethylamino)propyltrimethoxysilane onto Ru/AlO(OH) resulted in an active and highly chemoselective heterogeneous catalyst for the transfer hydrogenation of α,β-unsaturated carbonyl compounds to the corresponding allylic alcohols. Potassium formate was used as a sustainable hydrogen donor. A range of substrates including cinnamaldehyde, α-amylcinnamaldehyde, citral, 3-methyl-2-butenal, trans-2-pentenal, and trans-hexenal were selectively hydrogenated at the CO moiety with >96% selectivity. In comparison, the unmodified 1 wt% Ru/AlO(OH) catalyzed hydrogenation of cinnamaldehyde at the CC bond, yielding 3-phenylpropanal as the product. Higher loaded samples with 2–10 wt% Ru exhibited 20–25% selectivity to cinnamyl alcohol. The results show that low coordination sites were more selective to hydrogenation of the internal CC than the terminal CO bond. Immobilization of the amine via chemical bonding with hydroxyl groups of the AlO(OH) support blocks adjacent exposed metal sites, increasing the chemoselective reduction of CO. Optimum results were achieved at an amine/Ru ratio of 6. The catalyst maintained high activity and chemoselectivity even after five cycles.

Enhanced p-cresol photodegradation over BiOBr/Bi2O3 in the presence of rhodamine B

Bismuth oxide is a visible-light activated photocatalyst that is adversely affected by a high rate of electron–hole recombination. To mitigate this, BiOBr/Bi2O3 composites were synthesized where BiOBr formed submicron thick platelets at the surface of the Bi2O3 particles. XRD measurements show the preferential formation of a (110)-facetted BiOBr overlayer which can be attributed to the commensurate structure of this plane with the (120) plane of Bi2O3. The photodegradation of p-cresol and RhB was studied as representative of an organic pollutant and a dye, respectively. The composite with 85% BiOBr/Bi2O3 exhibited the highest photoactivity for both molecules. Its higher activity compared to that of either Bi2O3 or BiOBr alone, or a mechanical mixture with the same composition, supports the hypothesis that the formation of an hetero-epitactic interface between BiOBr and Bi2O3 is instrumental in reducing electron–hole pair recombination. Interestingly, in mixtures of p-cresol and RhB, the rate of p-cresol photodegradation was enhanced but that for RhB was decreased compared to the pure solutions. This is not caused by competitive adsorption of the molecules but rather by excitation transfer from RhB to the co-adsorbed p-cresol. Therefore, the RhB degradation by deethylation, which is a surface reaction, is suppressed and only the reaction channel through attack of the OH· radical at the aromatic chromophore remains open.