Single-atom catalysts are rewriting the rules of industrial hydrogenation

In the chemical industry, hydrogenation reactions - adding hydrogen atoms to molecules - are everywhere. They refine petroleum, synthesize pharmaceuticals, produce fine chemicals, and clean up environmental pollutants. But precision has always been the challenge. When a molecule has multiple sites where hydrogen could attach, conventional catalysts often lack the selectivity to target just one, leading to over-hydrogenation and unwanted byproducts that reduce yields and waste expensive starting materials.

Over the past decade, a class of catalytic materials has emerged that addresses this problem at the most fundamental level possible: the single atom. Single-atom catalysts (SACs) consist of individual metal atoms dispersed on a solid support material, each one isolated from its neighbors and serving as an independent active site. A comprehensive review published in the Chinese Journal of Catalysis maps the current state of this rapidly advancing field, documenting how SACs are transforming selective hydrogenation across multiple industrial domains.

One atom, one active site

The appeal of SACs begins with efficiency. In a conventional nanoparticle catalyst, most metal atoms are buried inside the particle where reactant molecules cannot reach them. Only surface atoms participate in catalysis, meaning that the bulk of the expensive metal - often platinum, palladium, or gold - serves no catalytic purpose. SACs achieve nearly 100% atomic utilization because every metal atom sits on the support surface, exposed and available for reaction.

But the real advantage is selectivity. A single metal atom on a support surface has a well-defined coordination environment - a specific arrangement of neighboring atoms that determines how it interacts with reactant molecules. Unlike a nanoparticle surface, which presents a heterogeneous landscape of terraces, edges, and corners each with different reactivity, a single-atom site offers a uniform catalytic environment. This uniformity translates into the ability to hydrogenate one functional group on a complex molecule while leaving others untouched.

Noble metals, earth-abundant alternatives, and hybrids

The review categorizes SACs by metal type. Noble metal systems - platinum, palladium, gold, ruthenium, iridium, and rhodium dispersed as single atoms - have been the most extensively studied and generally show the highest catalytic performance. But their cost limits industrial scalability.

Non-noble metal SACs using iron, cobalt, nickel, copper, and other earth-abundant elements represent a more economically viable path. Their catalytic performance is generally lower than noble metal counterparts, but recent advances in support engineering and coordination tuning have narrowed the gap considerably.

A third category - bimetallic and dual-single-atom catalysts - combines two different metal atoms on the same support, sometimes in adjacent positions that create synergistic effects. These systems can access reaction pathways that neither metal achieves alone, offering additional levers for tuning selectivity and activity.

Theory meets experiment

One of the review's key themes is the growing integration of computational modeling with experimental work. Density functional theory (DFT) calculations allow researchers to predict how a single metal atom on a specific support will interact with hydrogen and with target molecules before synthesizing the catalyst. Microkinetic modeling extends these predictions to reaction rates and selectivity under realistic conditions.

This computational-experimental feedback loop is accelerating catalyst design. Rather than screening hundreds of catalyst compositions experimentally - a slow and expensive process - researchers can use theory to identify the most promising candidates, then synthesize and test only those. The review documents numerous cases where computationally predicted catalysts performed as expected in the laboratory.

The stability problem

SACs face a fundamental challenge: isolated metal atoms are thermodynamically inclined to clump together into nanoparticles, especially under the high temperatures and reactive conditions of industrial catalysis. This sintering destroys the single-atom character and its associated selectivity advantages. Long-term stability under operating conditions remains the primary barrier to industrial deployment.

Strategies to combat sintering include anchoring metal atoms in defect sites on the support surface, using strong metal-support interactions to pin atoms in place, and designing support materials with built-in trapping sites. The review discusses these approaches in detail, noting that while progress has been significant, no universal solution has yet emerged.

Scalable synthesis is another concern. Many SAC preparation methods work well at the milligram or gram scale in academic laboratories but have not been validated for the kilogram or ton scales required by industry. Bridging that gap will require process engineering that the field has only recently begun to address.

A platform technology for greener chemistry

The broader significance of SACs extends beyond any single reaction. By minimizing metal usage while maximizing selectivity, they align with the principles of green chemistry - reducing waste, lowering energy consumption, and improving atom economy. In pharmaceutical manufacturing, where complex molecules often require multiple selective hydrogenation steps, the precision of SACs could reduce the number of processing steps and eliminate the need for protecting groups, simplifying synthesis and reducing costs.

The review, published in the Chinese Journal of Catalysis (DOI: 10.1016/S1872-2067(25)64906-0), serves as both a reference for researchers designing next-generation catalysts and a roadmap for translating fundamental discoveries into practical industrial technology.