Cobalt precursor role in the photocatalytic activity of the zeolite-supported TiO2-based photocatalysts under visible light: A promising tool toward zeolite-based core–shell photocatalysis

A new class of supported photocatalysts is introduced recently with high activity under visible light for water splitting purposes. The composition comprises silicoaluminates (zeolite) as the support, titanium dioxide (TiO2) as the semiconductor, cobalt ions (Co2+) as the hydrogen evolution sites and heteropolyacid (HPA) as the multifunctional solid acid with visible light activity. From photocatalyst preparation point of view, the synthesis consists of three impregnation steps of TiO2, Co2+, and HPA, respectively, followed by a specific thermal treatment for each stage. The focus of this study is on the cobalt impregnation step. The objective is to investigate the impact of employing two commercially available cobalt precursors on the photocatalytic activity of the synthesized photocatalysts, especially hydrogen production rates. Nitrate and chloride compounds of cobalt were examined on two classes of the zeolites namely Na–Y and Na–Mordenite which have emerged as suitable supports for hydrogen evolution application. Although nitrate-based photocatalysts exhibited an improvement in the UV–VIS absorbance spectra toward visible light, they caused an almost 30% lower H2 production rate in comparison to the chloride salts. The favorable shift toward visible light is possibly due to the incorporation of nitrogen (N) anions in the photocatalyst structure. However, their lower hydrogen production rate is mainly attributed to the competitive photo-reduction reactions of remnant nitrate anions, suggesting chloride (Cl−) species as a remedy of this so-called parasitic phenomenon. In addition, overshadowing the poisoning and parasitic effects of Cl− ions on the photo-oxidation sites of the zeolite-supported composition was another notable outcome of this study. This emulates core–shell photocatalysis concept insofar with providing a reasonable distance between redox sites.

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► Basic zeolite, TiO2, heteropolyacid and cobalt together are active under visible light.
► Cobalt precursor can have a determining role on photocatalytic hydrogen production.
► Nitrate residues suppress the hydrogen production via competitive photo-reduction.
► The composition removes the poisonous and parasitic impact of surface chlorides.
► The synthesized photocatalyst emulates core–shell concept for separate evolution.