Supported noble metal catalysts make the best utilization of noble metal atoms. Recent advances in nanotechnology have brought many attentions into the rational design of catalysts in the nanometer and subnanometer region. Recent studies showed that catalysts in the subnanometer regime could have extraordinary activity and selectivity. However, the structural performance relationships behind their unique catalytic performances are still unclear. To understand the effect of particle size and shape of noble metals, it is essential to understand the fundamental reaction mechanism. Single atoms catalysts and subnanometer clusters provide a unique opportunity for designing heterogeneous catalysts because of their unique geometric and electronic properties.

CO oxidation is one of the important probe reactions. However, the reaction mechanism of noble single atoms is still unclear. Additionally, there is no agreement on whether the activity of supported single atoms is higher or lower than supported nanoparticles. In this study, we applied different operando techniques including x-ray absorption fine structure (XAFS), diffuse reflectance infrared spectroscopy (DRIFTS), with other characterization techniques including calorimetry and high-resolution scanning transmission electron microscopy (STEM) to investigate the active and stable structure of Ir/MgAl2O4 and Pt/CeO2 single-atom catalysts during CO oxidation. With all these characterization techniques, we also performed a kinetic study and first principle calculations to understand the reaction mechanism of single atoms for CO oxidation. For Ir single atoms catalysts, our results indicate that instead of poisoning by CO on Ir nanoparticles, Ir single atoms could adsorb more than one ligand, and the Ir(CO)(O) structure was identified as the most stable structure under reaction condition. Though one CO was strongly adsorbed during the entire reaction cycle, another CO could react with the surface adsorbed O* through an Eley-Rideal reaction mechanism. Ir single atoms also provide an interfacial site for the facile O2 activation between Ir and Al with a low barrier, and therefore O2 activation step is feasible even at room temperature. For Pt single-atom catalysts, our results showed that Pt(O)3(CO) structure is stable in O2 and N2 at 150 °C. However, when dosing CO at 150 °C, one surface O* in Pt(O)3(CO) could react with CO to form CO2, and the reacted O* can be refilled when flowing O2 again at 150 °C. This suggests that an adsorbed CO is present in the entire reaction cycle as a ligand, and another gas phase CO could react with surface O* to form CO2 during low-temperature CO oxidation.

Supported single atoms synthesized with conventional methods usually consist of a mixture of single atoms and nanoparticles. It is important to quantify the surface site fraction of single atoms and nanoparticles when studying catalytic performances. Because of the unique reaction mechanism of Ir single atoms and Ir nanoparticles, we showed that kinetic measurements could be applied as a simple and direct method of quantifying surface site fractions. Our kinetic methods could also potentially be applied to quantifying other surface species when their kinetic behaviors are significantly different. We also benchmarked other in-situ and ex-situ methods of quantifying surface site fraction of single atoms and nanoparticles.

To bridge the gap between single atoms and nanoparticles and have a better understanding of the effect of nuclearity on CO oxidation, we also studied supported Ir subnanometer clusters with the average size less than 0.7 nm (< 13 atoms) prepared by both inorganic precursor and organometallic complex Ir4(CO)12. Low-temperature CO adsorption indicates that CO and O2/O could co-adsorb on Ir subnanometer clusters, however on larger nanoparticle the particle surface is covered by CO only. Additional co-adsorption of CO and O2 was studied by CO and O2 calorimetry at room temperature. CO oxidation results showed that Ir subnanometer clusters are more active than Ir single atoms and Ir nanoparticles at all conditions, and this could be explained by the competitive adsorption of CO and O2 on subnanometer clusters.