Graphitic carbon nitride-based nanostructures as emergent catalysts for carbon monoxide (CO) oxidation

Pristine graphitic carbon nitride-based catalysts (g-C3N4) are promising in multidisciplinary applications due to their unique physicochemical properties, metal-free nature, optical properties and ease of preparation from green, inexpensive, and earth-abundant resources, which meet sustainability requirements. However, the low surface area, sluggish electron transfer, inadequate adsorption/activation/dissociation of gases (i.e., CO and O-2), poor electrical conductivity, and inferior visible-light absorption properties of bare g-C3N4 nanostructures are critical issues that hamper their utilization in practical CO oxidation applications. These barriers can be overcome via the incorporation of heteroatoms, metals/metal oxides, and single atoms into g-C3N4-based nanostructures to act as active sites for facilitating the adsorption and activation/dissociation of reactants and tolerate the adsorption of intermediates during CO oxidation. Also, g-C3N4 nanostructures can be coupled with other semiconductors to form heterojunction structures (i.e., Z-scheme, S-scheme, and Type-II) to enable photocatalytic CO oxidation. Also, electron donation from the g-C3N4 support enhances the surface charges on metal or non-metal atoms, which enhance O-2 adsorption and activation during CO oxidation. Notably, reviews related to g-C3N4-based catalysts for CO oxidation have not previously been published, to the best of our knowledge. Thereby, this review emphasizes the rational design of g-C3N4-supported metal/metal oxides, single atoms, and hetero-atoms for CO oxidation reactions (i.e., thermally, electrochemically, and photoelectrochemically), starting from green preparation methods to fundamental experimental and theoretical issues, in addition to life cycle assessments. Scientific questions related to g-C3N4-based catalysts for CO oxidation, including their role (i.e., support or promoter) and the effect of strong metal-support interactions (SMSIs), are also addressed. Eventually, a brief synopsis of the current challenges and invigorating perspectives to scale up g-C3N4-based materials for large-scale CO oxidation are proposed.