Gold nanoparticles (AuNPs) have emerged as highly effective catalysts due to their unique physicochemical properties, including high surface-to-volume ratio, tunable surface chemistry, and exceptional stability under reaction conditions. Their catalytic applications span oxidation reactions, carbon monoxide (CO) oxidation, and various organic transformations. The catalytic performance of AuNPs is strongly influenced by factors such as particle size, choice of support material, and surface modifications, which collectively determine their reactivity and selectivity.

One of the most studied catalytic applications of AuNPs is in oxidation reactions. Gold nanoparticles exhibit remarkable activity in the oxidation of alcohols, alkenes, and other organic substrates. For instance, AuNPs supported on metal oxides such as TiO2 or CeO2 catalyze the selective oxidation of alcohols to aldehydes or ketones under mild conditions. The catalytic efficiency is highly dependent on particle size, with smaller nanoparticles (2–5 nm) demonstrating superior activity due to the increased number of active edge and corner sites. The support material plays a crucial role by stabilizing the nanoparticles and facilitating electron transfer processes. For example, reducible oxides like CeO2 enhance catalytic activity by providing oxygen vacancies that participate in the reaction mechanism.

CO oxidation is another critical reaction where AuNPs demonstrate exceptional performance. This reaction is not only industrially relevant for air purification and fuel cell applications but also serves as a model reaction to understand the catalytic behavior of gold nanoparticles. Studies have shown that AuNPs smaller than 5 nm exhibit high activity for CO oxidation, even at sub-ambient temperatures. The support material significantly influences the reaction kinetics, with metal oxides such as Fe2O3, TiO2, and Al2O3 being widely used. The interface between AuNPs and the support is particularly important, as it often serves as the active site for oxygen activation. Surface chemistry modifications, such as the introduction of cationic gold species or the removal of stabilizing ligands, can further enhance catalytic performance by increasing the accessibility of active sites.

Organic transformations catalyzed by AuNPs include hydrogenation, coupling reactions, and selective reductions. For example, AuNPs supported on carbon or silica effectively catalyze the hydrogenation of nitroarenes to anilines, a reaction of significant importance in pharmaceutical and agrochemical industries. The selectivity of these reactions can be tuned by controlling the nanoparticle size and surface functionalization. Smaller AuNPs tend to favor hydrogenation pathways, while larger particles may promote alternative reaction routes. Additionally, the use of bimetallic systems, where gold is combined with metals like palladium or platinum, can further enhance catalytic activity and selectivity by modifying the electronic structure of the nanoparticles.

The role of support materials in AuNP catalysis cannot be overstated. Supports not only prevent nanoparticle aggregation but also participate in the catalytic cycle by providing reactive oxygen species or acting as electron donors/acceptors. Metal oxides with high oxygen mobility, such as CeO2 and TiO2, are particularly effective for oxidation reactions due to their ability to replenish oxygen vacancies during the reaction. In contrast, inert supports like carbon or silica are preferred for reactions requiring minimal support interaction, such as hydrogenation or coupling reactions. The choice of support also affects the stability of AuNPs under reaction conditions, with some materials offering superior resistance to sintering or leaching.

Surface chemistry plays a pivotal role in determining the catalytic properties of AuNPs. Ligands or capping agents used during synthesis can influence nanoparticle dispersion, stability, and accessibility of active sites. For instance, citrate-stabilized AuNPs exhibit different catalytic behavior compared to those stabilized by thiols or polymers. The removal of capping agents through thermal or chemical treatments often leads to increased catalytic activity by exposing clean gold surfaces. However, excessive removal can result in nanoparticle aggregation, highlighting the need for precise control over surface modifications.

The shape and morphology of AuNPs also contribute to their catalytic performance. Anisotropic structures such as nanorods, nanocubes, or nanostars expose specific crystal facets that exhibit varying reactivity. For example, AuNPs with exposed (100) facets are more active for CO oxidation than those with (111) facets due to differences in oxygen adsorption energies. Synthetic methods that allow precise control over nanoparticle shape, such as seed-mediated growth or templated synthesis, enable the design of catalysts with tailored activity and selectivity.

In summary, gold nanoparticles serve as versatile catalysts for a wide range of reactions, including oxidations, CO oxidation, and organic transformations. Their catalytic performance is intricately linked to factors such as particle size, support material, and surface chemistry, which collectively dictate reactivity and selectivity. Advances in synthetic techniques and characterization methods continue to refine our understanding of these relationships, paving the way for the development of more efficient and sustainable catalytic systems. The ability to tailor AuNP properties for specific reactions underscores their potential in both industrial and academic applications, making them a cornerstone of modern nanocatalysis.