Enhanced photocatalytic reduction of Cr(vi) to Cr(iii) over g-C3N4 catalysts with Ag nanoclusters in conjunction with Cr(iii) quantification based on operando low-field NMR relaxometry

Abstract

The design of efficient semiconductor photocatalysts to promote solar-to-chemical conversion remains a tremendous challenge. The present study addresses this issue by loading Ag₉(H₂MSA)₇ and Ag₃₂(MPG)₁₉ nanoclusters (NCs) onto graphitic carbon nitride (g-CN) using a simple impregnation method to form Ag₉NCs/g-CN and Ag₃₂NCs/g-CN hybrid nanocomposites. The performance of the prepared photocatalyst samples investigated for the photocatalytic reduction of aqueous Cr(VI) to Cr(III) under visible-light irradiation is systematically investigated. The proposed nanocomposites exhibit excellent optical absorption properties and efficient separation and migration of photogenerated electron–hole pairs. As a result, the Ag₉NCs/g-CN and Ag₃₂NCs/g-CN nanocomposites provide improved photocatalytic performances relative to that of pristine g-CN photocatalysts with reduction rate constants that are greater than that of pristine g-CN by factors of 2.3 and 2.0, respectively. Furthermore, we for the first time demonstrate the viability of employing operando low-field NMR relaxometry for monitoring the photocatalytic Cr(VI) reduction performance by quantifying the concentration of paramagnetic Cr(III) ions in solution using the transverse relaxation times determined from the Carr–Purcell–Meiboom–Gill pulse sequence. Our study provides new insights for the rational design of high efficiency visible-light photocatalysts, and develops a more efficient method for quantifying the photocatalytic performance of catalysts for transforming heavy metal ions.