Ce-, Zr-containing oxide layers formed by plasma electrolytic oxidation on titanium as catalysts for oxidative desulfurization

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Mixed cerium-zirconum-titanum oxide layers: CeOx + TiO2, CeOx + ZrO2 + TiO2, ZrO2 + TiO2 - on titanium were formed by plasma electrolytic oxidation technique in the electrolytes with different ratio of Ce(SO4)3: Zr(SO4)2 and investigated by the complex of physical-chemical methods. Their activity was studied in the peroxide oxidation of sulfur compounds (thiophene, thioanizole and dibenzothiophene) and the desulfurization of diesel fuel. Element and phase composition of the PEO-coated samples affect their catalytic activity. ZrO2 + TiO2 + CeOx systems with the largest content of zirconium showed the highest catalytic activity in oil desulfurization. Their catalytic activity correlates with the presence of the double oxides ZrTiO4 and Ti2ZrO6 in the coating composition. The decrease in the activity of catalysts when they are used in consecutive cycles can be accounted for by the etching of the coatings in the area of the pores. This drawback one can overcome by the addition of an ionic liquid namely 4-(3′-ethylimidazolium)-butanesulfonate, which increases not only the activity, but also the stability of the catalysts. Under these conditions, the structure and activity of the catalyst did not change significantly over several reaction cycles. The combination of PEO treatment and deposition of the ionic liquid allowed one not only to oxidize model substrates, but also carry out the desulfurization of diesel fuel to a residual sulfur content of less than 10 ppm.