Single-atom catalysts (SACs) have garnered significant and sustained interest in the field of heterogeneous catalysis. These catalysts are formed by placing individual metal atoms onto suitable substrates [[1], [2], [3]]. Heterogeneous catalysis refers to a surface reaction mechanism wherein just the surface atoms of the catalyst are utilized as the active sites for the reaction [[4], [5], [6]]. Therefore, the catalytic activity can be improved by reducing the catalyst size and exposing more active sites. The catalytic activity of a catalyst is shown to rise as the size of the metal particles decreases. Therefore, the highest level of catalytic activity may be achieved when the metal is spread in the form of individual atoms on the carrier material [7]. Based on this, SACs show superior catalytic performance and 100% atomic utilization compared with the conventional metal nanoparticle catalysts.

In addition, SACs have the following advantages over conventional nanoparticle catalysts. (1) There are no metal-metal bonds in the metal catalyst, and this allows the SACs to have a particular coordination structure with a distinct electronic and physicochemical structure [[8], [9], [10], [11]]. Furthermore, the presence of robust contact at the interface between the metal and the carrier material serves to hinder the catalytic deactivation that may arise from the aggregation of individual metal atoms [1,12]. (2) Owing the tiny metal usage and strong chemical bonding, the SACs present low catalyst cost and reduce the metal leakage risk [13]. Therefore, the catalytic efficiency, stability and safety of SACs are superior to conventional nano-scale catalysts.

In 2011, Qiao et al. [14] conducted a study in which they synthesized Pt-based catalysts with great dispersion using FeOx as the carrier. Notably, they introduced the novel notion of single-atom catalysts, marking the first instance of its proposal. The Pt-based SACs exhibited three times higher activity than the corresponding Pt-based nano-scale catalysts for CO oxidation. Since then, SACs have become a research spotlight for heterogeneous catalysis reactions owing to their superior catalytic capacity as well as the preferable stability. Previously various metal SACs, (e.g., Fe [15], Co [16], Ni [17], Pd [18], Ru [19], Pt [20]), have been developed by immobilizing the metals on diverse metal oxide carriers (e.g., SiO2 [21], TiO2 [22], FeOx [14], Co3O4 [23], CeO2 [24], ZnO [25] etc.). For example, Wang et al. [26] used an impregnation method to disperse Rh on Co3O4 nanorods, and Rh was reduced on the carbon carrier surface and subsequently bonded to oxygen atoms. Characterization using extended X-ray absorption fine structure (EXAFS) showed that a single dispersed Rh atom formed an active site with the oxygen atom on the surface and was uniformly distributed on the carrier. The acquired catalyst showed a significant activity degree in nitric oxide reduction using hydrogen. The selectivity to produce N2 at a temperature of 220 °C was 87%, and this selectivity increased to 97% at a temperature of 300 °C.

Around 2018, carbon-based materials had begun to attract interest as SACs carriers owing to their defect-rich and high surface-to-volume ratio compared to metal oxide [27]. For example, Zhang et al. [28] synthesized iron-based SACs (FeSA/CNT) for hydrazine oxidation reaction (HzOR) by pyrolyzing a mixture of carbon nanotubes and metal precursors at 900 °C under an inert atmosphere. The Fe-SACs composites presented a peak current density of 17.2 mA·cm−2, while the scanning rate employed was 20 mA·cm−2. This value was found to be 5.3 times more than that seen from catalysts composed of transition Fe nanoparticles. The key to preparing SACs using carbon materials lies in manufacturing defects on the carrier surface [29]. Robledo et al. [30] showed that metal atoms and carbon materials with defects had high binding energies between 1.25 and 3.1 eV, which was higher than the metal binding energy in nanoparticles (around 1.25 eV) [31], indicating that metal atoms are more likely to form stable structures with defective carbon. Moreover, the introduction of nitrogen doping into carbon materials with defects may significantly augment the binding energy of metal atoms [32].

Biomass feedstock is a green carbon material owing to its renewability and low pollution [[33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43]]. With the introduction of concepts about carbon neutrality and the peak carbon dioxide emissions, biomass feedstocks have become a promising alternative to replace fossil resources for fuels, chemicals, and materials production, which can effectively alleviate energy pressure [[44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54]]. Therefore, SACs prepared from biomass are gaining attention. Previous studies have shown that metal-biomass composite can be formed by interacting the metal precursors with the functional groups in the biomass-derived carbon materials (e.g., -OH, -NH2), which is critical for the production of single-atom metal [55]. To date, lignin, chitosan, alginate, protein, cellulose, and biomass wastes have been used as biomass feedstock precursors for carbon-based SACs preparation. Currently, the main methods for the synthesis of SACs includes atomic layer deposition method [56], organometallic complexes approach [57], metal leaching method [58] and wet-chemistry method [59]. Among them, wet chemical synthesis is considered a suitable method for the preparation of biomass derived SACs because it is operationally convenient, easy to create defects, and can confine the metal active site to a narrow catalytic space [60].

A chronology of significant recent advancements in SACs is provided (Fig. 1). The range of support materials has been expanded from metal oxide to carbon-based compounds derived from lignin, cellulose, and other biomass sources since 2018. Based on current understanding, while there have been notable reviews highlighting the synthesis and catalytic advantages of carbon-based SACs, there is a dearth of literature addressing the fundamental principles pertaining to the synthesis strategy and structure-property relationship of carbon-based SACs derived from biomass. This review focuses on enumerating the different biomass feedstocks for preparing SACs, as well as the applicability of biomass-derived SACs in different catalytic reactions, with an endeavor to elucidate the fundamental determinants of catalyst performance. Finally, we classify these catalysts in different fields of catalytic applications and provide a prospective outlook on this swiftly growing research domain.