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Exciting progress has been made in the area of solar fuel generation by CO 2 reduction. New photocatalytic materials containing well-defined surface catalytic sites have emerged in recent years, including heterogenized molecular catalysts and single atom catalysts. This Feature Article summarizes our recent research in this area, together with brief discussions of relevant literature. In our effort to obtain heterogenized molecular catalysts, a diimine-tricarbonyl Re( i ) complex and a tetraaza macrocyclic Co( iii ) compound were covalently attached to different surfaces, and the effects of ligand derivatization and surface characteristics on their structures and photocatalytic activities were investigated. Single atom catalysts combine the advantages of homogeneous and heterogeneous catalysis. A single-site cobalt catalyst was prepared on graphitic carbon nitride, which demonstrated excellent activity in selective CO 2 reduction under visible-light irradiation. Doping carbon nitride with carbon was found to have profound effects on the structure and activity of the single-site cobalt catalyst. Our research achievements are presented to emphasize how spectroscopic techniques, including infrared, UV-visible, electron paramagnetic resonance, and X-ray absorption spectroscopies, could be combined with catalyst synthesis and computation modeling to understand the structures and properties of well-defined surface catalytic sites at the molecular level. This article also highlights challenges and opportunities in the broad context of solar CO 2 reduction. 

“Single-atom” catalysts (SACs) have demonstrated excellent activity and selectivity in challenging chemical transformations such as photocatalytic CO 2 reduction. For heterogeneous photocatalytic SAC systems, it is essential to obtain sufficient information of their structure at the atomic level in order to understand reaction mechanisms. In this work, a SAC was prepared by grafting a molecular cobalt catalyst on a light-absorbing carbon nitride surface. Due to the sensitivity of the X-ray absorption near edge structure (XANES) spectra to subtle variances in the Co SAC structure in reaction conditions, different machine learning (ML) methods, including principal component analysis, K-means clustering, and neural network (NN), were utilized for in situ Co XANES data analysis. As a result, we obtained quantitative structural information of the SAC nearest atomic environment, thereby extending the NN-XANES approach previously demonstrated for nanoparticles and size-selective clusters. 

Abstract Single‐atom catalysts have demonstrated interesting activity in a variety of applications. In this study, we prepared single Co²⁺sites on graphitic carbon nitride (C₃N₄), which was doped with carbon for enhanced activity in visible‐light CO₂reduction. The synthesized materials were characterized with a variety of techniques, including microscopy, X‐ray powder diffraction, UV‐vis spectroscopy, infrared spectroscopy, photoluminescence spectroscopy, and X‐ray absorption spectroscopy. Doping C₃N₄with carbon was found to have profound effect on the photocatalytic activity of the single Co²⁺sites. At relatively low levels, carbon doping enhanced the photoresponse of C₃N₄in the visible region and improved charge separation upon photoactivation, thereby enhancing the photocatalytic activity. High levels of carbon doping were found to be detrimental to the photocatalytic activity of the single Co²⁺sites by altering the structure of C₃N₄and generating defect sites responsible for charge recombination.