Surface oxygenation of multicomponent nanoparticles toward active and stable oxidation catalysts

Preformed Pt Nanoparticles Supported on Nanoshaped CeO2 for Total Propane Oxidation

Pt-based catalysts have been widely used for the removal of short-chain volatile organic compounds (VOCs), such as propane. In this study, we synthesized Pt nanoparticles with a size of ca. 2.4 nm and loaded them on various fine-shaped CeO2 with different facets to investigate the effect of CeO2 morphology on the complete oxidation of propane. The Pt/CeO2-o catalyst with {111} facets exhibited superior catalytic activity compared to the Pt/CeO2-r catalyst with {110} and {100} facets. Specifically, the turnover frequency (TOF) value of Pt/CeO2-o was 1.8 times higher than that of Pt/CeO2-r. Moreover, Pt/CeO2-o showed outstanding long-term stability during 50 h. X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed that the excellent performance of Pt/CeO2-o is due to the prevalence of metallic Pt species, which promotes C-C bond cleavage and facilitates the rapid removal of surface formate species. In contrast, a stronger metal-support interaction in Pt/CeO2-r leads to easier oxidation of Pt species and the accumulation of intermediates, which is detrimental to the catalytic activity. Our work provides insight into the oxidation of propane on different nanoshaped Pt/CeO2 catalysts.

In Situ Studies of Single-Nanoparticle-Level Nickel Thermal Oxidation: From Early Oxide Nucleation to Diffusion-Balanced Oxide Thickening

High-temperature oxidation mechanisms of metallic nanoparticles have been extensively investigated; however, it is challenging to determine whether the kinetic modeling is applicable at the nanoscale and how the differences in nanoparticle size influence the oxidation mechanisms. In this work, we study thermal oxidation of pristine Ni nanoparticles ranging from 4 to 50 nm in 1 bar 1%O₂/N₂ at 600 °C using in situ gas-cell environmental transmission electron microscopy. Real-space in situ oxidation videos revealed an unexpected nanoparticle surface refacetting before oxidation and a strong Ni nanoparticle size dependence, leading to distinct structural development during the oxidation and different final NiO morphology. By quantifying the NiO thickness/volume change in real space, individual nanoparticle-level oxidation kinetics was established and directly correlated with nanoparticle microstructural evolution with specified fast and slow oxidation directions. Thus, for the size-dependent Ni nanoparticle oxidation, we propose a unified oxidation theory with a two-stage oxidation process: stage 1: dominated by the early NiO nucleation (Avrami-Erofeev model) and stage 2: the Wagner diffusion-balanced NiO shell thickening (Wanger model). In particular, to what extent the oxidation would proceed into stage 2 dictates the final NiO morphology, which depends on the Ni starting radius with respect to the critical thickness under given oxidation conditions. The overall oxidation duration is controlled by both the diffusivity of Ni²⁺ in NiO and the Ni in Ni self-diffusion. We also compare the single-particle kinetic curve with the collective one and discuss the effects of nanoparticle size differences on kinetic model analysis.

Lattice Strain and Surface Activity of Ternary Nanoalloys under the Propane Oxidation Condition

The ability to harness the catalytic oxidation of hydrocarbons is critical for both clean energy production and air pollutant elimination, which requires a detailed understanding of the dynamic role of the nanophase structure and surface reactivity under the reaction conditions. We report here findings of an in situ/operando study of such details of a ternary nanoalloy under the propane oxidation condition using high-energy synchrotron X-ray diffraction coupled to atomic pair distribution function (HE-XRD/PDF) analysis and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The catalysts are derived by alloying Pt with different combinations of second (Pd) and third (Ni) transition metals, showing a strong dependence of the catalytic activity on the Ni content. The evolution of the phase structure of the nanoalloy is characterized by HE-XRD/PDF probing of the lattice strain, whereas the surface activity is monitored by DRIFTS detection of the surface intermediate formation during the oxidation of propane by oxygen. The results reveal the dominance of the surface intermediate species featuring a lower degree of oxygenation upon the first C-C bond cleavage on the lower-Ni-content nanoalloy and a higher degree of oxygenation upon the second C-C bond cleavage on the higher-Ni-content nanoalloy. The face-centered-cubic-type phase structures of the nanoalloys under the oxidation condition are shown to exhibit Ni-content-dependent changes of lattice strains, featuring the strongest strain with little variation for the higher-Ni-content nanoalloy, in contrast to the weaker strains with oscillatory variation for the lower-Ni-content nanoalloys. This process is also accompanied by oxygenation of the metal components in the nanoalloy, showing a higher degree of oxygenation for the higher-Ni-content nanoalloy. These subtle differences in phase structure and surface activity changes correlate with the Ni-composition-dependent catalytic activity of the nanoalloys, which sheds a fresh light on the correlation between the dynamic change of atomic strains and the surface reactivity and has significant implications for the design of oxidation catalysts with enhanced activities.

Regulating the lattice strain of platinum–copper catalysts for enhancing collaborative electrocatalysis

PtnCu100−n alloy nanostellates showed the high catalytic activity for both the oxygen reduction and alcohol oxidation reactions.

In Situ Raman Observation of Oxygen Activation and Reaction at Platinum-Ceria Interfaces during CO Oxidation

Understanding the fundamental insights of oxygen activation and reaction at metal-oxide interfaces is of significant importance yet remains a major challenge due to the difficulty in in situ characterization of active oxygen species. Herein, the activation and reaction of molecular oxygen during CO oxidation at platinum-ceria interfaces has been in situ explored using surface-enhanced Raman spectroscopy (SERS) via a borrowing strategy, and different active oxygen species and their evolution during CO oxidation at platinum-ceria interfaces have been directly observed. In situ Raman spectroscopic evidence with isotopic exchange experiments demonstrate that oxygen is efficiently dissociated to chemisorbed O on Pt and lattice Ce-O species simultaneously at interfacial Ce³⁺ defect sites under CO oxidation, leading to a much higher activity at platinum-ceria interfaces compared to that at Pt alone. Further in situ time-resolved SERS studies and density functional theory simulations reveal a more efficient molecular pathway through the reaction between adsorbed CO and chemisorbed Pt-O species transferred from the interfaces. This work deepens the fundamental understandings on oxygen activation and CO oxidation at metal-oxide interfaces and offers a sensitive technique for the in situ characterization of oxygen species under working conditions.

Hierarchical CoP Nanostructures on Nickel Foam as Efficient Bifunctional Catalysts for Water Splitting

A highly active and cheap catalyst is also key to hydrogen production by water splitting. However, most of the high-efficiency catalysts reported to date only are catalytically active for either the hydrogen evolution reaction (HER) or the oxygen evolution reaction (OER), which makes the development of multifunctional catalysts more meaningful. Here, for the first time, Co(CO₃ )_0.5 OH^. 0.11 H₂ O (CHCH) as precursor with different microstructures on the surface of nickel foam (NF) was obtained using a facile hydrothermal method. The CoP/NF catalyst was obtained after thermal phosphating that retained the microhierarchical structure of the precursor and greatly improved the catalytic performance, with a highly efficiency performance as HER and OER dual-functional catalyst. Density functional theory (DFT) calculations showed that the possible reason for the excellent performance of the CoP/NF layered structure is an increase in the number of of surface defects and an increased active surface area. The results reported in this paper show that CoP/NF, a layered bifunctional electrocatalyst, is a cost-effective and efficient water-splitting electrode. This finding can offer the opportunity for the commercial use of excess electric energy for large-scale water splitting hydrogen production.

Alloying-realloying enabled high durability for Pt-Pd-3d-transition metal nanoparticle fuel cell catalysts

Alloying noble metals with non-noble metals enables high activity while reducing the cost of electrocatalysts in fuel cells. However, under fuel cell operating conditions, state-of-the-art oxygen reduction reaction alloy catalysts either feature high atomic percentages of noble metals (>70%) with limited durability or show poor durability when lower percentages of noble metals (<50%) are used. Here, we demonstrate a highly-durable alloy catalyst derived by alloying PtPd (<50%) with 3d-transition metals (Cu, Ni or Co) in ternary compositions. The origin of the high durability is probed by in-situ/operando high-energy synchrotron X-ray diffraction coupled with pair distribution function analysis of atomic phase structures and strains, revealing an important role of realloying in the compressively-strained single-phase alloy state despite the occurrence of dealloying. The implication of the finding, a striking departure from previous perceptions of phase-segregated noble metal skin or complete dealloying of non-noble metals, is the fulfilling of the promise of alloy catalysts for mass commercialization of fuel cells.

Copper-alloy catalysts: structural characterization and catalytic synergies

Recent progress in the development of copper-alloy catalysts is highlighted, focusing on the structural and mechanistic characterizations of the catalysts in different catalytic reactions, and challenges and opportunities in future research.

Dynamic Core-Shell and Alloy Structures of Multimetallic Nanomaterials and Their Catalytic Synergies

ConspectusMultimetallic nanomaterials containing noble metals (NM) and non-noble 3d-transition metals (3d-TMs) exhibit unique catalytic properties as a result of the synergistic combination of NMs and 3d-TMs in the nanostructure. The exploration of such a synergy depends heavily on the understanding of the atomic-scale structural details of NMs and 3d-TMs in the nanomaterials. This has attracted a great deal of recent interest in the field of catalysis science, especially concerning the core-shell and alloy nanostructures. A rarely asked question of fundamental significance is how the core-shell and alloy structural arrangements of atoms in the multimetallic nanomaterials dynamically change under reaction conditions, including reaction temperature, surface adsorbate, chemical environment, applied electrochemical potential, etc. The dynamic evolution of the core-shell/alloy structures under the reaction conditions plays a crucial role in the catalytic performance of the multimetallic nanocatalysts.This Account focuses on the dynamic structure characteristics for several different types of composition-tunable alloy and core-shell nanomaterials, including phase-segregated, elemental-enriched, dynamically evolved, and structurally different core-shell structures. In addition to outlining core-shell/alloy structure formation via processes such as seed-mediated growth, thermochemical calcination, adsorbate-induced evolution, chemical dealloying, underpotential deposition/galvanic displacement, etc., this Account will highlight the progress in understanding the dynamic core-shell/alloy structures under chemical or catalytic reaction conditions, which has become an important focal point of the research fronts in catalysis and electrocatalysis. The employment of advanced techniques, especially in situ/operando synchrotron high-energy X-ray diffraction and pair distribution function analyses, has provided significant insights into the dynamic evolution processes of NM/3d-TM nanocatalysts under electrocatalytic or fuel cell operating conditions. Examples will highlight Pt- or Pd-based nanoparticles and nanowires alloyed with various 3d-TMs with a focus on their structural evolution under reaction conditions. While the dynamic process is complex, the ability to gain an insight into the evolution of core-shell and alloy structures under the catalytic reaction condition is essential for advancing the design of multimetallic nanocatalysts. This Account serves as a springboard from fundamental understanding of the core-shell and alloy structural dynamics to the various applications of nanostructured catalysts/electrocatalysts, especially in the fronts of energy and environmental sustainability.

Synthesis of Quasi-Bilayer Subnano Metal-Oxide Interfacial Cluster Catalysts for Advanced Catalysis

Planar metal clusters possess high metal utilization, distinct electronic properties, and catalytic functions from their 3D counterparts. However, synthesis of these materials is challenging due to much elevated surface free energies. Here it is reported that silica supported planar bilayer Pt-CoO_x subnano clusters, consisting of approximately one atomic layer of Pt and one CoO_x layer on top, can be achieved by employing strong-electrostatic interactions during impregnation and precisely-controlled CoO_x coating using atomic layer deposition. Such bilayer structure is unambiguously confirmed by electron microscopy and in situ X-ray absorption fine spectroscopy which is never reported before. This synthetic approach can be extended to another eight permutations of planar metal-oxide subnano clusters. The resulting bilayer catalysts, owing to unique electronic properties and the abundant metal-oxide interfaces created, exhibit excellent catalytic performances in the reactions of preferential oxidation of CO in H₂ and selective hydrogenation of acetylene, by showing much higher selectivity and intrinsic activities at least 8 and 48 times greater than those conventional oxide coated 3D metal clusters/nanoparticles, highlighting the advances of bilayer interfacial structure. These findings open a new avenue to design abundant and highly active metal-oxide interfaces for advanced metal catalysis.