Facilitating Hydrogen Dissociation over Dilute Nanoporous Ti-Cu Catalysts

Precisely Tailoring the Second Coordination Sphere of a Cobalt Single-Atom Catalyst for Selective Hydrogenation of Halogenated Nitroarenes

The development of highly efficient and cost-effective nonprecious metal catalysts for the selective hydrogenation of halogenated nitroarenes is very appealing yet challenging. Here, we demonstrate that the hydrogenation activity and selectivity of Co single-atom catalyst (SAC) can be tuned by tailoring the structure of second coordination sphere via P doping. As revealed by synchrotron radiation-based X-ray absorption spectroscopy characterizations, such a P doping on N-coordinated Co SAC results in the unsymmetric Co-N4P1 coordination structure. With a combination of experimental characterizations and theoretical simulations, we find that tailoring the second coordination sphere can greatly improve H2 dissociation and product desorption. As a result, the Co-N4P1 SAC exhibits superior activity, selectivity and stability for the hydrogenation of halogenated nitroarenes to corresponding amines (20 examples, >99 % yields) at 80 °C under 0.5 MPa H2 pressure, significantly outperforming most heterogeneous catalysts reported in the literature. We expect that this work opens a new avenue for the design of highly efficient nonprecious metal SACs for important hydrogenation reactions.

Advances in simulating dilute alloy nanoparticles for catalysis

Dilute alloy (DA) catalysts, including single-atom alloys (SAAs), which are comprised of trace amounts of an active promoter metal dispersed on the surface of a selective host metal, offer exceptional activity and selectivity while utilizing precious metals more efficiently. Although most SAA and DA applications have focused on partial hydrogenation and oxidation reactions, their use has steadily expanded into more complex thermo-, photo-, and electro-catalytic processes. This progress has been largely driven by mechanistic insights derived from computational chemistry and is expected to accelerate with the advancement of artificial intelligence. This minireview discusses novel advances in simulating SAAs and DAs for catalysis applications, including ab initio calculations, multiscale modeling, and machine learning. Emphasis is placed on the impact of reaction conditions, promoter ensembles, and nanoparticle morphology on the stability and catalytic performance of SAAs and DAs. Finally, a perspective is offered on potential future directions of SAA and DA simulations and their extension to other systems with distinct, well-defined active sites.

Solvent-free selective hydrogenation of nitroaromatics to azoxy compounds over Co single atoms decorated on Nb2O5 nanomeshes

The solvent-free selective hydrogenation of nitroaromatics to azoxy compounds is highly important, yet challenging. Herein, we report an efficient strategy to construct individually dispersed Co atoms decorated on niobium pentaoxide nanomeshes with unique geometric and electronic properties. The use of this supported Co single atom catalysts in the selective hydrogenation of nitrobenzene to azoxybenzene results in high catalytic activity and selectivity, with 99% selectivity and 99% conversion within 0.5 h. Remarkably, it delivers an exceptionally high turnover frequency of 40377 h-1, which is amongst similar state-of-the-art catalysts. In addition, it demonstrates remarkable recyclability, reaction scalability, and wide substrate scope. Density functional theory calculations reveal that the catalytic activity and selectivity are significantly promoted by the unique electronic properties and strong electronic metal-support interaction in Co1/Nb2O5. The absence of precious metals, toxic solvents, and reagents makes this catalyst more appealing for synthesizing azoxy compounds from nitroaromatics. Our findings suggest the great potential of this strategy to access single atom catalysts with boosted activity and selectivity, thus offering blueprints for the design of nanomaterials for organocatalysis.

Spectra-Based Machine Learning for Predicting the Statistical Interaction Properties of CO Adsorbates on Surface

Theoretical analyses of small-molecule adsorption on heterogeneous catalyst surfaces often rely on simplified models of molecular adsorption with the most favorable configuration. Given that real-world experimental tests frequently entail multiple molecules interacting with the surface, there is a pressing need for a comprehensive multimolecule adsorption model to bridge the gap between theory and experiment. Using machine learning, we predict the average values of important adsorption properties from conformationally averaged, calculated infrared and Raman spectra and compare these values to those theoretically derived from the conformationally averaged ensemble. Remarkably, our approach yields excellent predictions even when faced with large and indeterminate numbers of surface molecules. These quantitative spectra-averaged property relationships provide a theoretical framework for extracting key interaction properties from the spectra of real chemical environments.

Tuning the Hydrogenation Selectivity of an Unsaturated Aldehyde via Single-Atom Alloy Catalysts

Selective hydrogenation of α,β-unsaturated aldehydes to produce unsaturated alcohols remains a challenge in catalysis. Here, we explore, on the basis of first-principles simulations, single-atom alloy (SAA) catalysts on copper as a class of catalytic materials to enhance the selectivity for C═O bond hydrogenation in unsaturated aldehydes by controlling the binding strength of the C═C and C═O bonds. We show that on SAA of early transition metals such as Ti, Zr, and Hf, the C═O binding mode of acrolein is favored but the strong binding renders subsequent hydrogenation and desorption impossible. On SAA of late-transition metals, on the other hand, the C═C binding mode is favored and C═C bond hydrogenation follows, resulting in the production of undesired saturated aldehydes. Mid-transition metals (Cr and Mn) in Cu(111) appear as the optimal systems, since they favor acrolein adsorption via the C═O bond but with a moderate binding strength, compatible with catalysis. Additionally, acrolein migration from the C═O to the C═C binding mode, which would open the low energy path for C═C bond hydrogenation, is prevented by a large barrier for this process. SAA of Cr in Cu appears as an optimal candidate, and kinetic simulations show that the selectivity for propenol formation is controlled by preventing the acrolein migration from the more stable C═O to the less stable C═C binding mode and subsequent H-migration and by the formation of the O-H bond from the monohydrogenated intermediate. Dilute alloy catalysts therefore enable tuning the binding strength of intermediates and transition states, opening control of catalytic activity and selectivity.

Accessing Ta/Cu Architectures via Metal-Metal Salt Metatheses: Heterobimetallic C-H Bond Activation Affords μ-Hydrides

We employ a metal-metal salt metathesis strategy to access low-valent tantalum-copper heterometallic architectures (Ta-μ2 -H2 -Cu and Ta-μ3 -H2 -Cu3 ) that emulate structural elements proposed for surface alloyed nanomaterials. Whereas cluster assembly with carbonylmetalates is well precedented, the use of the corresponding polyarene transition metal anions is underexplored, despite recognition of these highly reactive fragments as storable sources of atomic Mn- . Our application of this strategy provides structurally unique early-late bimetallic species. These complexes incorporate bridging hydride ligands during their syntheses, the origin of which is elucidated via detailed isotopic labelling studies. Modification of ancillary ligand sterics and electronics alters the mechanism of bimetallic assembly; a trinuclear complex resulting from dinuclear C-H activation is demonstrated as an intermediate en route to formation of the bimetallic. Further validating the promise of this rational, bottom-up approach, a unique tetranuclear species was synthesized, featuring a Ta centre bearing three Ta-Cu interactions.

Geometric and Electronic Effects in the Binding Affinity of Imidazole-Based N-Heterocyclic Carbenes to Cu(100)- and Ag(100)-Based Pd and Pt Single-Atom Alloy Surfaces

We have conducted nonlocal periodic density functional theory (DFT) calculations of N-heterocyclic carbenes (NHCs) adsorbed to Pd/Cu(100), Pt/Cu(100), Pd/Ag(100), and Pt/Ag(100) single atom alloys (SAAs) utilizing the nonlocal optPBE-vdW functional. NHCs with electron donating groups (EDGs) are predicted to bind more strongly to the SAA surface compared to NHCs functionalized with electron withdrawing groups (EWGs). Our calculations show that NHCs typically bind to SAA geometries containing a small space between the heteroatom sites for the SAAs considered. Generally, this pattern is predicted to persist for a single NHCs or for a pair of NHCs bound to the SAA surfaces. Approximate linear relationships between NMR-based parameters and NHC-SAA binding energies are uncovered. We predict that the binding of NHCs to SAA surfaces is composition-dependent and heteroatom geometry dependent.

The Atomic Observation of the Structural Change Process in Pt Networks in Air Using Environmental Cell Scanning Transmission Electron Microscopy

The structural change in Pt networks composed of multiple chain connections among grains was observed in air at 1 atm using atomic-resolution environmental cell scanning transmission electron microscopy. An aberration-corrected incident electron probe with a wide convergence angle made it possible to increase the depth resolution that contributes to enhancing the signal-to-noise ratio of Pt network samples in air in an environmental cell, resulting in the achievement of atomic-resolution imaging. The exposure of the Pt networks to gas molecules under Brownian motion, stimulated by electron beams in the air, increases the collision probability between gas molecules and Pt networks, and the Pt networks are more intensely stressed from all directions than in a situation without electron irradiation. By increasing the electron beam dose rate, the structural change of the Pt networks became significant. Dynamic observation on an atomic scale suggested that the structural change of the networks was not attributed to the surface atomic-diffusion-induced step motion but mainly caused by the movement and deformation of unstable grains and grain boundaries. The oxidized surface layers may be one of the factors hindering the surface atomic step motion, mitigating the change in the size of the grains and grain boundaries.