Single atom detachment from Cu clusters, and diffusion and trapping on CeO2(111): implications in Ostwald ripening and atomic redispersion

In-depth understanding the synergisms of Cu atomic clusters on Cu single atoms for highly effective electrocatalytic oxygen reduction reaction and Zn-Air battery

In this paper, a novel, simple and mild soft template assisted strategy and further carbonization approach has been constructed to the size-tunable preparation of porous Cu-N-C/Surfactant catalysts successfully. Note that the pluronic F127 has a significant influence on the synthesis of porous Cu-N-C/F127 with the atomically dispersed Cu-N4 and adjacent Cu atomic clusters (ACs) than other surfactants owing to their particular non-ionic structure. By combining a series of experimental analysis and density functional theory (DFT) calculations, the synergistic effects between the adjacent Cu ACs and atomically dispersed Cu-N4 are favorable for manipulating the binding energy of O2 adsorption and intermediates desorption at the atomic interface of catalysts, resulting in an excellent electrocatalytic ORR performance with a faster kinetics for Cu-N-C/F127 than those of the Cu-N-C, Cu-N-C/CTAB, Cu-N-C/SDS, and comparable with the commercial Pt/C catalyst. This method not only provides a novel approach for synthesizing highly effective copper based single atom catalysts toward ORR, but also offers an in-depth understanding of the synergisms of adjacent ACs on the Cu single atoms (SAs) for highly effective electrocatalytic ORR and Zn-air Battery.

Understanding and controlling the formation of single-atom site from supported Cu10 cluster by tuning CeO2 reducibility: Theoretical insight into the Gd-doping effect on electronic metal-support interaction

Controlling the formation of single-atom (SA) sites from supported metal clusters is an important and interesting issue to effectively improve the catalytic performance of heterogeneous catalysts. For extensively studied CO oxidation over metal/CeO2 systems, the SA formation and stabilization under reaction conditions is generally attributed to CO adsorption, however, the pivotal role played by the reducible CeO2 support and the underlying electronic metal-support interaction (EMSI) are not yet fully understood. Based on a ceria-supported Cu10 catalyst model, we performed density functional theory calculations to investigate the intrinsic SA formation mechanism and discussed the synergistic effect of Gd-doped CeO2 and CO adsorption on the SA formation. The CeO2 reducibility is tuned with doped Gd content ranging from 12.5 % ∼ 25 %. Based on ab initio thermodynamic and ab initio molecular dynamics, the critical condition for SA formation was identified as 21.875 % Gd-doped CeO2 with CO-saturated adsorption on Cu10. Electronic analysis revealed that the open-shell lattice Oδ- (δ < 2) generated by Gd doping facilitates the charge transfer from the bottom-corner Cu (Cubc) to CeO2. The CO-saturated adsorption further promotes this charge transfer process and enhances the EMSI between Cubc and CeO2, leading to the disintegration of Cubc from Cu10 and subsequent formation of the active SA site.

CO-Induced Dimer Decay Responsible for Gem-Dicarbonyl Formation on a Model Single-Atom Catalyst

The ability to coordinate multiple reactants at the same active site is important for the wide-spread applicability of single-atom catalysis. Model catalysts are ideal to investigate the link between active site geometry and reactant binding, because the structure of single-crystal surfaces can be precisely determined, the adsorbates imaged by scanning tunneling microscopy (STM), and direct comparisons made to density functional theory. In this study, we follow the evolution of Rh1 adatoms and minority Rh2 dimers on Fe3O4(001) during exposure to CO using time-lapse STM at room temperature. CO adsorption at Rh1 sites results exclusively in stable Rh1CO monocarbonyls, because the Rh atom adapts its coordination to create a stable pseudo-square planar environment. Rh1(CO)2 gem-dicarbonyl species are also observed, but these form exclusively through the breakup of Rh2 dimers via an unstable Rh2(CO)3 intermediate. Overall, our results illustrate how minority species invisible to area-averaging spectra can play an important role in catalytic systems, and show that the decomposition of dimers or small clusters can be an avenue to produce reactive, metastable configurations in single-atom catalysis.

Dealuminated Beta zeolite reverses Ostwald ripening for durable copper nanoparticle catalysts

Copper nanoparticle-based catalysts have been extensively applied in industry, but the nanoparticles tend to sinter into larger ones in the chemical atmospheres, which is detrimental to catalyst performance. In this work, we used dealuminated Beta zeolite to support copper nanoparticles (Cu/Beta-deAl) and showed that these particles become smaller in methanol vapor at 200°C, decreasing from ~5.6 to ~2.4 nanometers in diameter, which is opposite to the general sintering phenomenon. A reverse ripening process was discovered, whereby migratable copper sites activated by methanol were trapped by silanol nests and the copper species in the nests acted as new nucleation sites for the formation of small nanoparticles. This feature reversed the general sintering channel, resulting in robust catalysts for dimethyl oxalate hydrogenation performed with supported copper nanoparticles for use in industry.

Hydride Generation on the Cu-Doped CeO2(111) Surface and Its Role in CO2 Hydrogenation Reactions

Ceria-based catalysts exhibit great activity in catalyzing selective hydrogenation of CO2 to methanol. However, the underlying mechanism of this reaction, especially the generation of active H species, remains unclear. In this work, we performed extensive density functional theory calculations corrected by on-site Coulomb interaction (DFT + U) to investigate the H2 dissociation and the reaction between the active H species and CO2 on the pristine and Cu-doped CeO2(111) (denoted as Cu/CeO2(111)) surfaces. Our calculations evidenced that the heterolytic H2 dissociation for hydride generation can more readily occur on the Cu/CeO2(111) surface than on the pristine CeO2(111) surface. We also found that the Cu dopant can facilitate the formation of surface oxygen vacancies, further promoting the generation of hydride species. Moreover, the adsorption of CO2 and the hydrogenation of CO2 to HCOO* can be greatly promoted on the Cu/CeO2(111) surface with hydride species, which can lead to the high activity and selectivity toward CO2 hydrogenation to methanol.

Thermally-stable single-atom catalysts and beyond: A perspective

Single-atom catalysis is a research Frontier and has attracted extensive interests in catalysis. Significant progresses have been carried out in the synthesis and characterization of metal single-atom catalysts (SACs). However, the stability and catalytic reactivity of metal SAC at elevated temperatures are not well documented because single atoms sinter at elevated temperatures. Therefore, the development of stable and reactive SAC at high temperatures remains a formidable challenge. In this perspective, we summarize recent efforts on the preparation of the thermally-stable SACs synthesized at elevated temperature via the reverse-Ostwald ripening mechanism, including the approaches of atom trapping and vapor-phase self-assembly. The reducibility of lattice oxygen, the loading upper limit and the location of the metal single atom are discussed, combining experiments with simulations. In addition, we demonstrate that the coordination structure of the metal single atom can be tailored to address the relationship of structure and performances of the metal SAC in reactions. We expect that this perspective can provide some insights to guide the study for the rational design of thermally-stable and active single atom catalysts, which are especially suitable for high-temperature reactions.

Effects of Subsurface Oxide on Cu1/CeO2 Single-Atom Catalysts for CO Oxidation: A Theoretical Investigation

Supported atomic dispersion metals are of great interest, and the interfacial effect between isolated metal atoms and supports is crucial in heterogeneous catalysis. Herein, the behavior of single-atom Cu catalysts dispersed on CeO₂ (100), (110), and (111) surfaces has been studied by DFT + U calculations. The interactions between ceria crystal planes and isolated Cu atoms together with their corresponding catalytic activities for CO oxidation are investigated. The CeO₂ (100) and (111) surfaces can stabilize active Cu⁺ species, while Cu exists as Cu²⁺ on the (110) surface. Cu⁺ is certified as the most active site for CO adsorption, which can promote the formation of the reaction intermediates and reduce reaction energy barriers. For the CeO₂ (100) surface, the interaction between CO and Cu is weak and the CO adsorbate is more likely to activate the subsurface oxygen. The catalytic performance is closely related to the binding strength of CO to the active Cu single atoms on the different subsurfaces. These results bring a significant insight into the rational design of single metal atoms on ceria and other reducible oxides.

Cu/O Frustrated Lewis Pairs on Cu Doped CeO2(111) for Acetylene Hydrogenation: A First-Principles Study

In this work, the H2 dissociation and acetylene hydrogenation on Cu doped CeO2(111) were studied using density functional theory calculations. The results indicated that Cu doping promotes the formation of oxygen vacancy (Ov) which creates Cu/O and Ce/O frustrated Lewis pairs (FLPs). With the help of Cu/O FLP, H2 dissociation can firstly proceed via a heterolytic mechanism to produce Cu-H and O-H by overcoming a barrier of 0.40 eV. The H on Cu can facilely migrate to a nearby oxygen to form another O-H species with a barrier of 0.43 eV. The rate-determining barrier is lower than that for homolytic dissociation of H2 which produces two O-H species. C2H2 hydrogenation can proceed with a rate-determining barrier of 1.00 eV at the presence of Cu-H and O-H species., While C2H2 can be catalyzed by two O-H groups with a rate-determining barrier of 1.06 eV, which is significantly lower than that (2.86 eV) of C2H2 hydrogenated by O-H groups on the bare CeO2(111), showing the high activity of Cu doped CeO2(111) for acetylene hydrogenation. In addition, the rate-determining barrier of C2H4 further hydrogenated by two O-H groups is 1.53 eV, much higher than its desorption energy (0.72 eV), suggesting the high selectivity of Cu doped CeO2(111) for C2H2 partial hydrogenation. This provides new insights to develop effective hydrogenation catalysts based on metal oxide.

Sabatier principle of metal-support interaction for design of ultrastable metal nanocatalysts

[Figure: see text].

Adsorbate-assisted migration of the metal atom in atomically dispersed catalysts: An ab initio molecular dynamics study

We present a phenomenological study of dynamical evolution of the active site in atomically dispersed catalysts in the presence of reaction intermediates associated with CO oxidation and low-temperature water-gas shift reaction. Using picosecond ab initio molecular dynamics, we probe the initiation of adsorbate-induced diffusion of atomically dispersed platinum on rutile TiO₂(110). NVT trajectories spanning 5 ps at 500 K reveal that the dynamical stability of the metal atom is governed by its local coordination to the support and adsorbate. Adsorbates that bind the strongest to Pt typically also lead to the fastest diffusion of the metal atom, and all adsorbates weaken Pt-support interactions, resulting in higher diffusion coefficients compared to bare Pt. We note, however, the absence of quantitative correlations between adsorption characteristics (Pt Bader charge, adsorbate binding energy) and ensemble-averaged quantities (diffusion coefficients). A recurring structural motif identified in several trajectories is a near-linear coordination between support oxygen, Pt, and specific adsorbates. These geometries, on account of enhanced metal support interactions, stabilize Pt and inhibit migration over picosecond timescales. We also identify hydrogen bonding events between the adsorbate and support for OH-containing groups. In the case of OH-bound Pt, for instance, we believe that short-lived H-bonds between OH and support promote Pt migration in the beginning of the NVT trajectory, while the subsequent formation of a near-linear geometry stabilizes the Pt atom despite the continued formation of short-lived hydrogen bonds. These observations are consistent with prior studies that report stabilization of isolated metal atoms in the presence of hydroxyl groups.

Nanocluster and single-atom catalysts for thermocatalytic conversion of CO and CO2

We highlight different aspects of single-atom and nanocluster catalysts for CO₂ reduction and CO oxidation, including synthesis, dynamic restructuring, and trends in activity and selectivity.

Design of a High-Performance Electrocatalyst for N2 Conversion to NH3 by Trapping Single Metal Atoms on Stepped CeO2

Single-atom catalysts (SACs) have recently been shown to have high performance in catalyzing the synthesis of NH₃ from N₂. Here, we systematically investigated a series of single transition metal atoms anchored on stepped CeO₂ (CeO₂-S) to screen the potential electrocatalysts for a N₂ reduction reaction (NRR) via density functional theory computations. We first demonstrated that these SACs are stable via large calculated binding energies. Second, we evaluated the adsorption of *N₂ over CeO₂-S-supported single atoms. Here, those systems that can activate N₂ molecules were selected as candidates. We then showed that CeO₂-S-supported single Mo and Ru atoms have high catalytic activity for NRR via low limiting potentials of -0.52 and -0.35 V, respectively. Meanwhile, the competitive hydrogen evolution reaction is highly suppressed over these two SACs because the adsorption of *N₂ is prior to *H. Finally, the origin of the NRR activity over these SACs was investigated. This work offers useful insights into designing high-performance CeO₂-based electrocatalysts for NRR.

CeO2(111) electronic reducibility tuned by ultra-small supported bimetallic Pt-Cu clusters

Controlling Ce4+ to Ce3+ electronic reducibility in a rare-earth binary oxide such as CeO2 has enormous applications in heterogeneous catalysis, where a profound understanding of reactivity and selectivity at the atomic level is yet to be reached. Thus, in this work we report an extensive DFT-based Basin Hopping global optimization study to find the most stable bimetallic Pt-Cu clusters supported on the CeO2(111) oxide surface, involving up to 5 atoms in size for all compositions. Our PBE+U global optimization calculations indicate a preference for Pt-Cu clusters to adopt 2D planar geometries parallel to the oxide surface, due to the formation of strong metal bonds to oxygen surface sites and charge transfer effects. The calculated adsorption energy values (Eads) for both mono- and bimetallic systems are of the order of 1.79 up to 4.07 eV, implying a strong metal cluster interaction with the oxide surface. Our calculations indicate that at such sub-nanometer sizes, the number of Ce4+ surface atoms reduced to Ce3+ cations is mediated by the amount of Cu atoms within the cluster, reaching a maximum of three Ce3+ for a supported Cu5 cluster. Our computational results have critical implications on the continuous understanding of the strong metal-support interactions over reducible oxides such as CeO2, as well as the advancement of frontier research areas such as heterogeneous single-atom catalysts (SAC) and single-cluster catalysts (SCC).

First-principles study of rocksalt early transition-metal carbides as potential catalysts for Li-O2 batteries

A series of early transition-metal carbides (TMCs) in the NaCl structure have been constructed to compare the catalytic activity in Li-O2 batteries by first-principles calculations. The reasonable interfacial models of LixO2 (x = 4, 2, and 1) molecules adsorbed on early TMCs surfaces were used to simulate oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes. Taking overpotentials as a merit parameter of catalytic activity, more relationships between material properties relative to the adsorption/desorption behavior of active molecules and catalytic activity are constructed for early TMCs. The equilibrium and charging potentials used to calculate the OER overpotentials of early TMCs are inversely proportional to the adsorption energies of (Li2O)2 and LiO2, respectively. The ORR overpotentials are inversely proportional to the adsorption energies of (Li2O)2 and LiO2 for early TMCs, but the relationship between OER overpotentials and the adsorption energies of reactive intermediates is unclear. Additionally, the overpotentials of early TMCs for ORR and OER are proportional to the desorption energies of Li+ and O2, respectively. In general, both the adsorption energy of (Li2O)2/LiO2 and desorption energy of Li+/O2 are effective characterization parameters of catalytic activity. By providing the comprehensive valuable parameters on electrochemical performance to compare the catalytic activity of early TMCs and establishing more correlations between material properties relative to the adsorption/desorption behavior of active molecules with their catalytic activity, our investigation is helpful for knowing more about the catalytic process and beneficial to screen and design novel highly active catalysts for Li-O2 batteries.