Real-Time Simulation of the Reaction Kinetics of Supported Metal Nanoparticles.

A common issue with supported metal catalysts is the sintering of metal nanoparticles, resulting in catalyst deactivation. In this study, we propose a theoretical framework for realizing a real-time simulation of the reactivity of supported metal nanoparticles during the sintering process, combining density functional theory calculations, microkinetic modeling, Wulff-Kaichew construction, and sintering kinetic simulations. To validate our approach, we demonstrate its feasibility on α-Al2O3(0001)-supported Ag nanoparticles, where the simulated sintering behavior and ethylene epoxidation reaction rate as a function of time show qualitative agreement with experimental observation. Our proposed theoretical approach can be employed to screen out the promising microstructure feature of α-Al2O3 for stable supported Ag NPs, including the surface orientation and promoter species modified on it. The outlined approach of this work may be applied to a range of different thermocatalytic reactions other than ethylene epoxidation and provide guidance for the development of supported metal catalysts with long-term stability.

Rational Regulation of the Defect Density in Platinum Nanocrystals for Highly Efficient Hydrogen Evolution Reaction.

Constructing structural defects is a promising way to enhance the catalytic activity toward the hydrogen evolution reaction (HER). However, the relationship between defect density and HER activity has rarely been discussed. In this study, a series of Pt/WOx nanocrystals are fabricated with controlled morphologies and structural defect densities using a facile one-step wet chemical method. Remarkably, compared with polygonal and star structures, the dendritic Pt/WOx (d-Pt/WOx) exhibited a richer structural defect density, including stepped surfaces and atomic defects. Notably, the d-Pt/WOx catalyst required 4 and 16 mV to reach 10 mA cm-2, and its turnover frequency (TOF) values are 11.6 and 22.8 times higher than that of Pt/C under acidic and alkaline conditions, respectively. In addition, d-Pt/WOx//IrO2 displayed a mass activity of 5158 mA mgPt -1 at 2.0 V in proton exchange membrane water electrolyzers (PEMWEs), which is significantly higher than that of the commercial Pt/C//IrO2 system. Further mechanistic studies suggested that the d-Pt/WOx exhibited reduced number of antibonding bands and the lowest dz2-band center, contributing to hydrogen adsorption and release in acidic solution. The highest dz2-band center of d-Pt/WOx facilitated the adsorption of hydrogen from water molecules and water dissociation in alkaline medium. This work emphasizes the key role of the defect density in improving the HER activity of electrocatalysts.

Boosting Acidic Hydrogen Evolution Kinetics Induced by Weak Strain Effect in PdPt Alloy for Proton Exchange Membrane Water Electrolyzers.

Strain engineering is an effective strategy for manipulating the electronic structure of active sites and altering the binding strength toward adsorbates during the hydrogen evolution reaction (HER). However, the effects of weak and strong strain engineering on the HER catalytic activity have not been fully explored. Herein, the core-shell PdPt alloys with two-layer Pt shells (PdPt2L) and multi-layer Pt shells (PdPtML) is constructed, which exhibit distinct lattice strains. Notably, PdPt2L with weak strain effect just requires a low overpotential of 18 mV to reach 10 mA cm-2 for the HER and shows the superior long-term stability for 510 h with negligible activity degradation in 0.5 M H2SO4. The intrinsic activity of PdPt2L is 6.2 and 24.5 times higher than that of PdPtML and commercial Pt/C, respectively. Furthermore, PdPt2L||IrO2 exhibits superior activity over Pt/C||IrO2 in proton exchange membrane water electrolyzers and maintains stable operation for 100 h at large current density of 500 mA cm-2. In situ/operando measurements verify that PdPt2L exhibits lower apparent activation energy and accelerated ad-/desorption kinetics, benefiting from the weak strain effect. Density functional theory calculations also reveal that PdPt2L displays weaker H* adsorption energy compared to PdPtML, favoring for H* desorption and promoting H2 generation.

Discovery of Alloy Catalysts Beyond Pd for Selective Hydrogenation of Reformate via First-Principle Screening with Consideration of H-Coverage.

The highly selective hydrogenation to remove olefins is a significant refining approach for the reformate. Herein, a library of transition metal for reformate hydrogenation is tested experimentally to validate the predictive level of catalytic activity from our theoretical framework, which combines ab initio calculations and microkinetic modeling, with consideration of surface H-coverage effect on hydrogenation kinetics. The favorable H coverage of specific alloy surface under relevant hydrogenation condition, is found to be determined by its corresponding alloy composition. Besides, olefin hydrogenation rate is determined as a function of two descriptors, i.e. H coverage and binding energies of atomic hydrogen, paving the way to computationally screen on metal component in the periodic table. Evaluation of 172 bimetallic alloys based on the activity volcano map, as well as benzene hydrogenation rate, identifies prospective superior candidates and experimentally confirms that Zn3Ir1 outperforms pure Pd catalysts for the selective hydrogenation refining of reformate. The insights into H-coverage-related microkinetic modelling have enabled us to both theoretically understand experimental findings and identify novel catalysts, thus, bridging the gap between first-principle simulations and industrial applications. This work provides useful guidance for experimental catalyst design, which can be easily extended to other hydrogenation reaction.

Highly Selective Activation of C-H Bond and Inhibition of C-C Bond Cleavage by Tuning Strong Oxidative Pd Sites.

Improving the product selectivity meanwhile restraining deep oxidation still remains a great challenge over the supported Pd-based catalysts. Herein, we demonstrate a universal strategy where the surface strong oxidative Pd sites are partially covered by the transition metal (e. g., Cu, Co, Ni, and Mn) oxide through thermal treatment of alloys. It could effectively inhibit the deep oxidation of isopropanol and achieve the ultrahigh selectivity (>98%) to the target product acetone in a wide temperature range of 50-200 °C, even at 150-200 °C with almost 100% isopropanol conversion over PdCu1.2/Al2O3, while an obvious decline in acetone selectivity is observed from 150 °C over Pd/Al2O3. Furthermore, it greatly improves the low-temperature catalytic activity (acetone formation rate at 110 °C over PdCu1.2/Al2O3, 34.1 times higher than that over Pd/Al2O3). The decrease of surface Pd site exposure weakens the cleavage for the C-C bond, while the introduction of proper CuO shifts the d-band center (εd) of Pd upward and strengthens the adsorption and activation of reactants, providing more reactive oxygen species, especially the key super oxygen species (O2-) for selective oxidation, and significantly reducing the barrier of O-H and ß-C-H bond scission. The molecular-level understanding of the C-H and C-C bond scission mechanism will guide the regulation of strong oxidative noble metal sites with relatively inert metal oxide for the other selective catalytic oxidation reactions.

Interfacial Engineering of Copper-Nickel Selenide Nanodendrites for Enhanced Overall Water Splitting in Alkali Condition.

Fabricating heterogeneous interfaces is an effective approach to improve the intrinsic activity of noble-metal-free catalysts for water splitting. Herein, 3D copper-nickel selenide (CuNi@NiSe) nanodendrites with abundant heterointerfaces are constructed by a precise multi-step wet chemistry method. Notably, CuNi@NiSe only needs 293 and 41 mV at 10 mA cm-2 for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. Moreover, the assembled CuNi@NiSe system just requires 2.2 V at 1000 mA cm-2 in anion exchange membrane (AEM) electrolyzer, which is 2.0 times better than Pt/C//IrO2 . Mechanism studies reveal Cu defects on the Cu2-x Se surface boost the electron transfer between Cu atoms and Se atoms of Ni3 Se4 via Cu2-x Se/Ni3 Se4 interface, largely lowering the reaction barrier of rate-determining step for HER. Besides, the intrinsic activity of Ni atoms for in situ generated NiOOH is largely enhanced during OER because of the electron-modulating effect of Se atoms at Ni3 Se4 /NiOOH interface. The unique 3D structure also promotes the mass transfer during catalysis process. This work emphasizes the essential role of interfacial engineering for practical water splitting.

Isoprene selective hydrogenation using AgCu-promoted Pd nanoalloys.

Alloying is an effective approach to improve the catalysis performance of Pd-based catalysts for the selective hydrogenation of diolefins towards monoolefines. Herein, PdAgCu ternary nanoalloy catalysts were synthesised by a stepwise impregnation method for isoprene selective hydrogenation. The addition of a moderate amount of Ag and Cu to Pd significantly enhances the isoamylene selectivity in the isoprene hydrogenation, and decreases the non-desired over-hydrogenation. In addition, the loading molar ratio of PdAgCu with 3 : 2 : 3 as the optimal ternary nanoalloy composition maximizes the isoprene conversion (98%) and the monoolefins yield (92%). The surface structure of the catalyst was probed using H2-TPR, TEM, XRD, and XPS characterization methods, and it was confirmed that the surface Pd composition ratio between the metallic and oxidized states shows significant effects on the monoolefines yield. This work demonstrates the advantages of PdAgCu ternary nanoalloy catalysts for isoprene selective hydrogenation, which also provides guidelines for the development of other Pd-based ternary nanoalloys for diolefins selective hydrogenation.

One-Step Approach for Constructing High-Density Single-Atom Catalysts toward Overall Water Splitting at Industrial Current Densities.

The construction of highly active, durable, and cost-effective catalysts is urgently needed for green hydrogen production. Herein, catalysts consisting of high-density Pt (24 atoms nm-2 ) and Ir (32 atoms nm-2 ) single atoms anchored on Co(OH)2 were constructed by a facile one-step approach. Remarkably, Pt1 /Co(OH)2 and Ir1 /Co(OH)2 only required 4 and 178 mV at 10 mA cm-2 for hydrogen evolution reaction and oxygen evolution reaction, respectively. Moreover, the assembled Pt1 /Co(OH)2 //Ir1 /Co(OH)2 system showed mass activity of 4.9 A mgnoble metal -1 at 2.0 V in an alkaline water electrolyzer, which is 316.1 times higher than that of Pt/C//IrO2 . Mechanistic studies revealed that reconstructed Ir-O6 single atoms and remodeled Pt triple-atom sites enhanced the occupancy of Ir-O bonding orbitals and improved the occupation of Pt-H antibonding orbital, respectively, contributing to the formation of the O-O bond and the desorption of hydrogen. This one-step approach was also generalized to fabricate other 20 single-atom catalysts.

Solid-State Synthesis of Highly Dispersed Nitrogen-Coordinated Single Iron Atom Electrocatalysts for Proton Exchange Membrane Fuel Cells.

Fe-N-C with atomically dispersed Fe single atoms is the most promising candidate to replace platinum for the oxygen reduction reaction (ORR) in fuel cells. However, the conventional synthesis procedures require quantities solvents and metal precursors, sluggish adsorption process, and tedious washing, resulting in limited metal doping and uneconomical for large-scale production. For the first time, Fe2O3 is adopted as the Fe precursor to derive abundant single Fe atoms dispersed on carbon surfaces. The Fe-N-C catalyst synthesized by this simple method shows an excellent ORR activity with half-wave potentials of 0.82 and 0.90 V in acidic and alkaline solutions, respectively. A single fuel cell with an optimized Fe-N-C cathode shows a high peak power density of 0.84 W cm-2. The solid-state transformation synthesis method developed in this study may shed light on mass production of single-atom-based catalysts.

Synergy in Au-CuO Janus Structure for Catalytic Isopropanol Oxidative Dehydrogenation to Acetone.

The controlled oxidation of alcohols to the corresponding ketones or aldehydes via selective cleavage of the ß-C-H bond of alcohols under mild conditions still remains a significant challenge. Although the metal/oxide interface is highly active and selective, the interfacial sites fall far behind the demand, due to the large and thick support. Herein, we successfully develop a unique Au-CuO Janus structure (average particle size=3.8 nm) with an ultrathin CuO layer (0.5 nm thickness) via a bimetal in situ activation and separation strategy. The resulting Au-CuO interfacial sites prominently enhance isopropanol adsorption and decrease the energy barrier of ß-C-H bond scission from 1.44 to 0.01 eV due to the strong affinity between the O atom of CuO and the H atom of isopropanol, compared with Au sites alone, thereby achieving ultrahigh acetone selectivity (99.3 %) over 1.1 wt % AuCu0.75 /Al2 O3 at 100 °C and atmospheric pressure with 97.5 % isopropanol conversion. Furthermore, Au-CuO Janus structures supported on SiO2 , TiO2 or CeO2 exhibit remarkable catalytic performance, and great promotion in activity and acetone selectivity is achieved as well for other reducible oxides derived from Fe, Co, Ni and Mn. This study should help to develop strategies for maximized interfacial site construction and structure optimization for efficient ß-C-H bond activation.

Construction of Dual-Site Atomically Dispersed Electrocatalysts with Ru-C5 Single Atoms and Ru-O4 Nanoclusters for Accelerated Alkali Hydrogen Evolution.

Rationally integrating multi-active sites into one ideal catalyst is an effective approach to accelerate multistep reactions by synergic catalysis. Herein, a universal and facile room temperature impregnation strategy is developed to construct Ru atomically dispersed catalyst (Ru ADC) with Ru-C5 single atoms and Ru oxide nanoclusters (≈1.5 nm), which can also be extended to prepare Ir, Rh, Pt, Au, and Mo atomically dispersed catalysts (ADCs). It is found that the obtained Ru ADC largely boosts alkali hydrogen evolution by concerted catalysis between single atoms and sub-nanoclusters, which only needs an overpotential of 18 mV at 10 mA cm-2 . Further mechanistic studies reveal that Ru-C5 single atoms and Ru oxide nanoclusters with Ru-O4 configuration in one catalyst can synergically boost water molecule capture, water dissociation, and hydrogen release. This study opens up a simple method to synthesize dual-site metal ADCs for synergic catalysis of typical multistep reactions.

Unveiling the high-activity origin of single-atom iron catalysts for oxygen reduction reaction.

It is still a grand challenge to develop a highly efficient nonprecious-metal electrocatalyst to replace the Pt-based catalysts for oxygen reduction reaction (ORR). Here, we propose a surfactant-assisted method to synthesize single-atom iron catalysts (SA-Fe/NG). The half-wave potential of SA-Fe/NG is only 30 mV less than 20% Pt/C in acidic medium, while it is 30 mV superior to 20% Pt/C in alkaline medium. Moreover, SA-Fe/NG shows extremely high stability with only 12 mV and 15 mV negative shifts after 5,000 cycles in acidic and alkaline media, respectively. Impressively, the SA-Fe/NG-based acidic proton exchange membrane fuel cell (PEMFC) exhibits a high power density of 823 mW cm-2 Combining experimental results and density-functional theory (DFT) calculations, we further reveal that the origin of high-ORR activity of SA-Fe/NG is from the Fe-pyrrolic-N species, because such molecular incorporation is the key, leading to the active site increase in an order of magnitude which successfully clarifies the bottleneck puzzle of why a small amount of iron in the SA-Fe catalysts can exhibit extremely superior ORR activity.

Growth of Highly Active Amorphous RuCu Nanosheets on Cu Nanotubes for the Hydrogen Evolution Reaction in Wide pH Values.

Rational design of low-cost, highly efficient, and stable electrocatalysts for the hydrogen evolution reaction (HER) has attracted wide attention. Herein, 3D RuCu nanocrystals (NCs) are successfully synthesized by a facile wet chemistry method, in which amorphous RuCu nanosheets are directly grown on crystalline Cu nanotubes (NTs). Importantly, the obtained 3D RuCu NCs only need 18 and 73 mV to deliver the current density of 10 mA cm-2 for HER in alkaline and neutral media, respectively. Density functional theory calculations and experiments reveal that the Ru sites on the surface of amorphous nanosheets are the highly active centers for HER. Moreover, this catalyst can expose more surface area for water splitting compared to pure nanosheets because the unique 3D structure can effectively prevent the aggregation of nanosheets. Meanwhile, the interface between amorphous nanosheets and crystalline NTs is essential to boost the HER performance because the amorphous phase with many unsaturated bonds can facilitate adsorption of reactants and crystalline Cu with superior conductivity can promote the transfer of electrons. This work provides a facile method to prepare an original 3D Ru-based electrocatalyst with highly active HER performance in wide pH values.

Design of High-Performance Co-Based Alloy Nanocatalysts for the Oxygen Reduction Reaction.

Co-based nanoalloys show potential applications as nanocatalysts for the oxygen reduction reaction (ORR), but improving their activity is still a great challenge. In this paper, a strategy is proposed to design efficient Co-M (M=Au, Ag, Pd, Pt, Ir, and Rh) nanoalloys as ORR catalysts by using density functional theory (DFT) calculations. Through the Sabatier analysis, the overpotential as a function of ΔGOH * is identified as a quantitative descriptor for analyzing the effect of dopants and atomic structures on the activity of the Co-based nanoalloys. By adopting the suitable dopants and atomic structures, ΔGOH * accompanied by overpotential could be adjusted to the optimal range to enhance the activity of the Co-based nanoalloys. With this strategy, the core-shell structured Ag42 Co13 nanoalloy is predicted to have the highest catalytic activity for ORR among these Co-based nanoalloys. To give a deeper insight into the properties of Ag-Co nanoalloys, the structure, thermal stability, and reaction mechanism of Ag-Co nanoalloys with different compositions are also studied by using molecular simulations and DFT calculations. It is found that core-shell Ag42 Co13 exhibits the highest structural and thermal stability among these Ag-Co nanoalloys. In addition, the core-shell Ag42 Co13 shows the lowest ORR reaction energy barriers among these Ag-Co nanoalloys. It is expected that this kind of strategy could provide a viable way to design highly efficient heterogeneous catalysts in extensive applications.

Phase diagram and segregation of Ag-Co nanoalloys: insights from theory and simulation.

Understanding the phase diagram is the first step to identifying the structure-performance relationship of a material at the nanoscale. In this work, a modified nanothermodynamical model has been developed to predict the phase diagrams of Ag-Co nanoalloys with the size of 1 âˆ¼ 100 nm, which also overcomes the difference in the predicted results between theory and simulation for the first time. Based on this modified model, the phase diagrams of Ag-Co nanoalloys with various polyhedral morphologies (tetrahedron, cube, octahedron, decahedron, dodecahedron, rhombic dodecahedron, truncated octahedron, cuboctahedron, and icosahedron) have been predicted, showing good agreement with molecular dynamics simulations at the nanoscale of 1 âˆ¼ 4 nm. In addition, the surface segregation of Ag-Co nanoalloys has been predicted with a Co-rich core/Ag-rich surface, which is also consistent with the simulation results. Our results highlight a useful roadmap for bridging the difference between theory and simulation in the prediction of the phase diagram at the nanoscale, which will help both theorists and experimentalists.

PdCu alloy nanoparticle-decorated copper nanotubes as enhanced electrocatalysts: DFT prediction validated by experiment.

In order to combine the advantages of both 0D and 1D nanostructured materials into a single catalyst, density functional theory (DFT) calculations have been used to study the PdCu alloy NP-decorated Cu nanotubes (PdCu@CuNTs). These present a significant improvement of the electrocatalytic activity of formic acid oxidation (FAO). Motivated by our theoretical work, we adopted the seed-mediated growth method to successfully synthesize the nanostructured PdCu@CuNTs. The new catalysts triple the catalytic activity for FAO, compared with commercial Pd/C. In summary, our work provides a new strategy for the DFT prediction and experimental synthesis of novel metal NP-decorated 1D nanostructures as electrocatalysts for fuel cells.

Enhanced near-infrared shielding ability of (Li,K)-codoped WO3 for smart windows: DFT prediction validated by experiment.

By means of hybrid density functional theory (DFT) computations, we found that (Li,K)-codoped WO3 shows a significantly enhanced near-infrared (NIR) absorption ability for smart windows, and investigated the influence of doping through the analysis of the electronic structures of pure and doped hexagonal WO3. Furthermore, this codoped material, with a hexagonal tungsten bronze nanostructure, was successfully prepared via a simple one-step hydrothermal reaction for the first time. Transmission electron microscopy images showed that the as-prepared products possessed a nanorod-like morphology with diameters of about 5-10 nm. It was demonstrated that (Li,K)-codoped WO3 presents a better NIR absorption ability than pure, Li-monodoped or K-monodoped WO3, which is in good agreement with our theoretical predictions. The experiment and simulation results reveal that this enhanced optical property in NIR can be explained by the existence of high free electrons existing in (Li,K)-codoped WO3.

Bandgap engineering of Magnéli phase Ti(n)O(2n-1): Electron-hole self-compensation.

An electron-hole self-compensation effect is revealed and confirmed in nitrogen doped Magnéli phase Ti(n)O(2n-1) (n = 7, 8, and 9) by using hybrid density functional theory calculations. We found that the self-compensation effect between the free electrons in Magnéli phase Ti(n)O(2n-1) (n = 7, 8, and 9) and the holes induced by p-type nitrogen doping could not only prevent the recombination of photo-generated electron-hole pairs, but also lead to an effective bandgap reduction. This novel electron-hole self-compensation effect may provide a new approach for bandgap engineering of Magnéli phase metal suboxides.

Enhancing oxygen reduction reaction activity of Pt-shelled catalysts via subsurface alloying.

Despite remarkable efforts have been put into the field of Pt-shelled catalysts containing an atomically thin Pt surface layer for the oxygen reduction reaction (ORR) in the last decade, further development of new Pt-shelled catalysts is still necessary. Here, a new set of Pt-shelled catalysts by subsurface alloying with early transition metals such as Mn and Fe is predicted to be a good candidate for the ORR by using density functional theory (DFT) calculations. Trends in oxygen reduction activity of Pt-alloy catalysts are determined with calculations of oxygen binding by using the slab and cluster models. It is found that the subsurface alloys by the incorporation of submonolayer M (M = Mn and Fe) into Pt(111) in the slab model result in the enhancement of ORR activity, compared with the well-known Pt(111)-skin-M, pure Pt, and Pt3M alloy catalysts. For the cluster model, the Pt12Mn and Pt12Fe clusters are also found to be the optimal catalysts for the ORR. It is expected that this work can open up new opportunities for enhancing the ORR activity of Pt-alloy catalysts by subsurface alloying.