Cool and photocatalytic yellow ceramic pigments; from lead-tin to Cr doped scheelite pigments

Abstract

The historical lead based pigments (XVII-XVIII century), the modern Ni,Sb-rutile (1939) and Pr-zircon (1961) yellow pigments as well as the novel yellow pigments based on Cr-scheelite and Mo-yttrium cerate pigments were synthesized and compared as semiconductors of high NIR reflectance (cool pigments) and with photocatalytic activity. All powders are wide band gap semiconductors (Eg=2.1–3.1 eV) with high NIR reflectance, ranging from 80% for Cr-Scheelite to 86% of Naples yellow, except Ni,Sb-rutile pigment that shows 55%. The low color performance of lead based pigments in alkyd paint and lead-free glazes along with its toxicity, discard these pigments as cool eco-friendly pigments. The Pr-zircon, Cr-scheelite and Mo-yttrium cerate pigments exhibit high NIR reflectance (RNIR = 80–83%) and yellow coloration (b* = 57.6–45.3) but only Pr-zircon and Cr-scheelite show color stability in alkyd paint and lead free glaze applications. Regarding to results of Orange II photodegradation test, lead based and Ni,Sb-rutile pigments inhibits the photolysis of the UV irradiation, acting as protecting agents from irradiation. The Mo-cerate shows a slight photocatalytic activity (t1/2 = 301 min) and both Cr-scheelite and Pr-zircon pigments show a moderate activity (t1/2 = 198 and 184 min respectively). Looking at a possible industrial application, the leaching test shows that the fluorides released by Pr-zircon (16.8 mg/g) as well as the Cr(VI) of Cr-scheelite (6 mg/g) should be treated and controlled. The Cr-scheelite solid solution Ca(CrxW1-x)O4 is stable to x = 0.2 but its pigmenting capacity saturates at x = 0.1.