Structure Sensitivity of Metal Catalysts Revealed by Interpretable Machine Learning and First-Principles Calculations.

The nature of the active sites and their structure sensitivity are the keys to rational design of efficient catalysts but have been debated for almost one century in heterogeneous catalysis. Though the Brønsted-Evans-Polanyi (BEP) relationship along with linear scaling relation has long been used to study the reactivity, explicit geometry, and composition properties are absent in this relationship, a fact that prevents its exploration in structure sensitivity of supported catalysts. In this work, based on interpretable multitask symbolic regression and a comprehensive first-principles data set, we discovered a structure descriptor, the topological under-coordinated number mediated by number of valence electrons and the lattice constant, to successfully address the structure sensitivity of metal catalysts. The database used for training, testing, and transferability investigation includes bond-breaking barriers of 20 distinct chemical bonds over 10 transition metals, two metal crystallographic phases, and 17 different facets. The resulting 2D descriptor composing the structure term and the reaction energy term shows great accuracy to predict the reaction barriers and generalizability over the data set with diverse chemical bonds in symmetry, bond order, and steric hindrance. The theory is physical and concise, providing a constructive strategy not only to understand the structure sensitivity but also to decipher the entangled geometric and electronic effects of metal catalysts. The insights revealed are valuable for the rational design of the site-specific metal catalysts.

Molecule Saturation Boosts Acetylene Semihydrogenation Activity and Selectivity on a Core-Shell Ruthenium@Palladium Catalyst.

Increasing selectivity without the expense of activity is desired but challenging in heterogeneous catalysis. By revealing the molecule saturation and adsorption sensitivity on overlayer thickness, strain, and coordination of Pd-based catalysts from first-principles calculations, we designed a stable Pd monolayer (ML) catalyst on a Ru terrace to boost both activity and selectivity of acetylene semihydrogenation. The least saturated molecule is most sensitive to the change in catalyst electronic and geometric properties. By simultaneously compressing the Pd ML and exposing the high coordination sites, the adsorption of more saturated ethylene is considerably weakened to facilitate the desorption for high selectivity. The even stronger weakening to the least saturated acetylene drives its hydrogenation such that it is more exothermic, thereby boosting the activity. Tailoring the molecule saturation and its sensitivity to structure and composition provides a tool for rational design of efficient catalysts.

Conjugated dual size effect of core-shell particles synergizes bimetallic catalysis.

Core-shell bimetallic nanocatalysts have attracted long-standing attention in heterogeneous catalysis. Tailoring both the core size and shell thickness to the dedicated geometrical and electronic properties for high catalytic reactivity is important but challenging. Here, taking Au@Pd core-shell catalysts as an example, we disclose by theory that a large size of Au core with a two monolayer of Pd shell is vital to eliminate undesired lattice contractions and ligand destabilizations for optimum benzyl alcohol adsorption. A set of Au@Pd/SiO2 catalysts with various core sizes and shell thicknesses are precisely fabricated. In the benzyl alcohol oxidation reaction, we find that the activity increases monotonically with the core size but varies nonmontonically with the shell thickness, where a record-high activity is achieved on a Au@Pd catalyst with a large core size of 6.8 nm and a shell thickness of ~2-3 monolayers. These findings highlight the conjugated dual particle size effect in bimetallic catalysis.

Tuning crystal-phase of bimetallic single-nanoparticle for catalytic hydrogenation.

Bimetallic nanoparticles afford geometric variation and electron redistribution via strong metal-metal interactions that substantially promote the activity and selectivity in catalysis. Quantitatively describing the atomic configuration of the catalytically active sites, however, is experimentally challenged by the averaging ensemble effect that is caused by the interplay between particle size and crystal-phase at elevated temperatures and under reactive gases. Here, we report that the intrinsic activity of the body-centered cubic PdCu nanoparticle, for acetylene hydrogenation, is one order of magnitude greater than that of the face-centered cubic one. This finding is based on precisely identifying the atomic structures of the active sites over the same-sized but crystal-phase-varied single-particles. The densely-populated Pd-Cu bond on the chemically ordered nanoparticle possesses isolated Pd site with a lower coordination number and a high-lying valence d-band center, and thus greatly expedites the dissociation of H2 over Pd atom and efficiently accommodates the activated H atoms on the particle top/subsurfaces.

Boosting Activity and Stability of Metal Single-Atom Catalysts via Regulation of Coordination Number and Local Composition.

Controlling the chemical environments of the active metal atom including both coordination number (CN) and local composition (LC) is vital to achieve active and stable single-atom catalysts (SACs), but remains challenging. Here we synthesized a series of supported Pt1 SACs by depositing Pt atoms onto the pretuned anchoring sites on nitrogen-doped carbon using atomic layer deposition. In hydrogenation of para-chloronitrobenzene, the Pt1 SAC with a higher CN about four but less pyridinic nitrogen (Npyri) content exhibits a remarkably high activity along with superior recyclability compared to those with lower CNs and more Npyri. Theoretical calculations reveal that the four-coordinated Pt1 atoms with about 1 eV lower formation energy are more resistant to agglomerations than the three-coordinated ones. Composition-wise decrease of the Pt-Npyri bond upshifts gradually the Pt-5d center, and minimal one Pt-Npyri bond features a high-lying Pt-5d state that largely facilitates H2 dissociation, boosting hydrogenation activity remarkably.

A simple base-mediated synthesis of diverse functionalized ring-fluorinated 4H-pyrans via double direct C-F substitutions.

A straightforward and efficient approach to structurally diverse and synthetically useful ring-fluorinated 4H-pyrans via a simple base-mediated cascade reaction of readily available trifluoromethylated alkenes with 1,3-dicarbonyl compounds was developed. The key events of this reaction involve two consecutive C-F substitutions under very mild conditions.