The Crucial Role of the Support in the Transformations of Bimetallic Nanoparticles and Catalytic Performance

Atomic-Scale Engineering of CuOx-Au Interfaces over AuCu Single-Nanoparticles

A face-centered tetragonal (fct) AuCu particle with a size of 7.1 nm and an Au/Cu molar ratio of 1/1 was coated by a silica shell of 6 nm thickness. Segregation of Cu atoms from the metal particle under an oxidative atmosphere precisely mediated the CuO_x-Au interfacial structure by simply varying the temperature. As raising the temperature from 473 to 773 K, more Cu atoms emigrated from the AuCu particle and were oxidized into CuO_x layers that grew up to 0.8 nm in thickness. Simultaneously, the size of the Au-rich particle lowered moderately while the crystalline structure transformed from the fct phase into the face-centered cubic (fcc) phase. The CuO_x-Au interface shifted from the CuO_x monolayer bound to Au single-atoms to Au@CuO_x core-shell geometry, while the catalytic activity for CO oxidation at 433 K decreased dramatically. Moreover, a sharp loss in activity was observed as the crystal-phase transition occurred. This change in catalytic performance was ascribed to the geometrical configuration at the interfacial sites: the synergetic effect between the fct-AuCu particle and CuO_x monolayer contributed to the much higher activity, whereas the fcc-AuCu/Au particle weakened its interaction with the thicker CuO_x layer and thus decreased the activity.

Cooperative Catalysis of Vibrationally Excited CO2 and Alloy Catalyst Breaks the Thermodynamic Equilibrium Limitation

Using nonthermal plasma (NTP) to promote CO₂ hydrogenation is one of the most promising approaches that overcome the limitations of conventional thermal catalysis. However, the catalytic surface reaction dynamics of NTP-activated species are still under debate. The NTP-activated CO₂ hydrogenation was investigated in Pd₂Ga/SiO₂ alloy catalysts and compared to thermal conditions. Although both thermal and NTP conditions showed close to 100% CO selectivity, it is worth emphasizing that when activated by NTP, CO₂ conversion not only improves more than 2-fold under thermal conditions but also breaks the thermodynamic equilibrium limitation. Mechanistic insights into NTP-activated species and alloy catalyst surface were investigated by using in situ transmission infrared spectroscopy, where catalyst surface species were identified during NTP irradiation. Moreover, in in situ X-ray absorption fine-structure analysis under reaction conditions, the catalyst under NTP conditions not only did not undergo restructuring affecting CO₂ hydrogenation but also could clearly rule out catalyst activation by heating. In situ characterizations of the catalysts during CO₂ hydrogenation depict that vibrationally excited CO₂ significantly enhances the catalytic reaction. The agreement of approaches combining experimental studies and density functional theory (DFT) calculations substantiates that vibrationally excited CO₂ reacts directly with hydrogen adsorbed on Pd sites while accelerating formate formation due to neighboring Ga sites. Moreover, DFT analysis deduces the key reaction pathway that the decomposition of monodentate formate is promoted by plasma-activated hydrogen species. This work enables the high designability of CO₂ hydrogenation catalysts toward value-added chemicals based on the electrification of chemical processes via NTP.

Atmosphere-Induced Transient Structural Transformations of Pd-Cu and Pt-Cu Alloy Nanocrystals

We have investigated the transformations of colloidal Pd-Cu and Pt-Cu bimetallic alloy nanocrystals (NCs) supported on γ-Al₂O₃ when exposed to a sequence of oxidizing and then reducing atmospheres, in both cases at high temperature (350 °C). A combination of in situ diffuse reflectance infrared Fourier transform spectroscopy and X-ray absorption spectroscopy was employed to probe the NC surface chemistry and structural/compositional variations in response to the different test conditions. Depending on the type of noble metal in the bimetallic NCs (whether Pd or Pt), different outcomes were observed. The oxidizing treatment on Pd-Cu NCs led to the formation of a PdCuO mixed oxide and PdO along with a minor fraction of CuO _x species on the support. The same treatment on Pt-Cu NCs caused a complete dealloying between Pt and Cu, forming separate Pt NCs with a minor fraction of PtO NCs and CuO _x species, the latter finely dispersed on the support. The reducing treatment that followed the oxidizing treatment largely restored the Pd-Cu alloy NCs, although with a residual fraction of CuO _x species remaining. Similarly, Pt-Cu NCs were partially restored but with a large fraction of CuO _x species still located on the support. Our results indicate that the noble metal present in the bimetallic Cu-based alloy NCs has a strong influence on the dealloying/migrations/realloying processes occurring under typical heterogeneous catalytic reactions, elucidating the structural/compositional variations of these NCs depending on the atmospheres to which they are exposed.

Influence of Annealing Atmospheres on Photoelectrochemical Activity of TiO2 Nanotubes Modified with AuCu Nanoparticles

In this article, we studied the annealing process of AuCu layers deposited on TiO2 nanotubes (NTs) conducted in various atmospheres such as air, vacuum, argon, and hydrogen in order to obtain materials active in both visible and UV-vis ranges. The material fabrication route covers the electrochemical anodization of a Ti plate, followed by thin AuCu film magnetron sputtering and further thermal treatment. Scanning electron microscopy images confirmed the presence of spherical nanoparticles (NPs) formed on the external and internal walls of NTs. The optical and structural properties were characterized using UV-vis, X-ray diffraction, and X-ray photoelectron spectroscopies. It was proved that thermal processing under the argon atmosphere leads to the formation of a CuAuTi alloy in contrast to materials fabricated in air, vacuum, and hydrogen. The electrochemical measurements were carried out in NaOH using cyclic voltammetry, linear voltammetry, and chronoamperometry. The highest photoactivity was achieved for materials thermally treated in the argon atmosphere. In addition, the Mott-Schottky analysis was performed for bare TiO2 NTs and TiO2 NTs modified with gold copper NPs indicating a shift in the flatband potential. Overall, thermal processing resulted in changes in optical and structural properties as well as electrochemical and photoelectrochemical activities.

Designing Nanoparticles and Nanoalloys for Gas-Phase Catalysis with Controlled Surface Reactivity Using Colloidal Synthesis and Atomic Layer Deposition

Supported nanoparticles are commonly applied in heterogeneous catalysis. The catalytic performance of these solid catalysts is, for a given support, dependent on the nanoparticle size, shape, and composition, thus necessitating synthesis techniques that allow for preparing these materials with fine control over those properties. Such control can be exploited to deconvolute their effects on the catalyst's performance, which is the basis for knowledge-driven catalyst design. In this regard, bottom-up synthesis procedures based on colloidal chemistry or atomic layer deposition (ALD) have proven successful in achieving the desired level of control for a variety of fundamental studies. This review aims to give an account of recent progress made in the two aforementioned synthesis techniques for the application of controlled catalytic materials in gas-phase catalysis. For each technique, the focus goes to mono- and bimetallic materials, as well as to recent efforts in enhancing their performance by embedding colloidal templates in porous oxide phases or by the deposition of oxide overlayers via ALD. As a recent extension to the latter, the concept of area-selective ALD for advanced atomic-scale catalyst design is discussed.

Cobalt-Nickel Nanoparticles Supported on Reducible Oxides as Fischer-Tropsch Catalysts

Efficient and more sustainable production of transportation fuels is key to fulfill the ever-increasing global demand. In order to achieve this, progress in the development of highly active and selective catalysts is fundamental. The combination of bimetallic nanoparticles and reactive support materials offers unique and complex interactions that can be exploited for improved catalyst performance. Here, we report on cobalt-nickel nanoparticles on reducible metal oxides as support material for enhanced performance in the Fischer-Tropsch synthesis. For this, different cobalt to nickel ratios (Ni/(Ni + Co): 0.0, 0.25, 0.50, 0.75, or 1.0 atom/atom) supported on reducible (TiO₂ and Nb₂O₅) or nonreducible (α-Al₂O₃) oxides were studied. At 1 bar, Co-Ni nanoparticles supported on TiO₂ and Nb₂O₅ showed stable catalytic performance, high activities and remarkably high selectivities for long-chain hydrocarbons (C₅₊, ∼80 wt %). In contrast, catalysts supported on α-Al₂O₃ independently of the metal composition showed lower activities, high methane production, and considerable deactivation throughout the experiment. At 20 bar, the combination of cobalt and nickel supported on reducible oxides allowed for 25-50% cobalt substitution by nickel with increased Fischer-Tropsch activity and without sacrificing much C₅₊ selectivity. STEM-EDX and IR of adsorbed CO pointed to a cobalt enrichment of the nanoparticle's surface and a weaker adsorption of CO in Co-Ni supported on TiO₂ and Nb₂O₅ and not on α-Al₂O₃, modifying the rate-determining step and the catalytic performance. Overall, we show the strong effect and potential of reducible metal oxides as support materials for bimetallic nanoparticles for enhanced catalytic performance.

Development of Simultaneous Measurement System for X-ray Absorption Spectra at Two Absorption Edges

Dispersive X-ray absorption fine structure (DXAFS) spectroscopy is a versatile measurement technique for analyzing chemical reactions in real time. We have developed a novel time-resolved DXAFS instrument based on two polychromators and a wide-range position-sensitive detector that permits direct observation of the synergistic effects of two elements. This system enables simultaneous acquisition of the X-ray absorption spectra for two different elements without any mechanical movement of the X-ray optics. The developed system was successfully applied to monitor both the synthesis of a Ni-Cu bimetallic catalyst, which revealed that the reduction of Ni occurred at a higher temperature than that of Cu, and the charge-discharge processes of a Li_xNi_0.5Mn_1.5O₄-based lithium-ion battery, which demonstrated that the redox reactions of Ni and Mn occurred sequentially at specific electrode potentials.

Single and Dual Metal Oxides as Promising Supports for Carbon Monoxide Removal from an Actual Syngas: The Crucial Role of Support on the Selectivity of the Au–Cu System

A catalytic screening was performed to determine the effect of the support on the performance of an Au–Cu based system for the removal of CO from an actual syngas. First, a syngas was obtained from reforming of ethanol. Then, the reformer outlet was connected to a second reactor, where Au–Cu catalysts supported on several single and dual metal oxides (i.e., CeO2, SiO2, ZrO2, Al2O3, La2O3, Fe2O3, CeO2-SiO2, CeO2-ZrO2, and CeO2-Al2O3) were evaluated. AuCu/CeO2 was the most active catalyst due to an elevated oxygen mobility over the surface, promoting CO2 formation from adsorption of C–O* and OH− intermediates on Au0 and CuO species. However, its lower capacity to release the surface oxygen contributes to the generation of stable carbon deposits, which lead to its rapid deactivation. On the other hand, AuCu/CeO2-SiO2 was more stable due to its high surface area and lower formation of formate and carbonate intermediates, mitigating carbon deposits. Therefore, use of dual supports could be a promising strategy to overcome the low stability of AuCu/CeO2. The results of this research are a contribution to integrated production and purification of H2 in a compact system.

Moving Frontiers in Transition Metal Catalysis: Synthesis, Characterization and Modeling

Nanosized transition metal particles are important materials in catalysis with a key role not only in academic research but also in many processes with industrial and societal relevance. Although small improvements in catalytic properties can lead to significant economic and environmental impacts, it is only now that knowledge-based design of such materials is emerging, partly because the understanding of catalytic mechanisms on nanoparticle surfaces is increasingly improving. A knowledge-based design requires bottom-up synthesis of well-defined model catalysts, an understanding of the catalytic nanomaterials "at work" (operando), and both a detailed understanding and a prediction by theoretical methods. This article reports on progress in colloidal synthesis of transition metal nanoparticles for preparation of model catalysts to close the materials gap between the discoveries of fundamental surface science and industrial application. The transition metal particles, however, often undergo extensive transformations when applied to the catalytic process and much progress has recently been achieved operando characterization techniques under relevant reaction conditions. They allow better understanding of size/structure-activity correlations in these systems. Moreover, the growth of computing power and the improvement of theoretical methods uncover mechanisms on nanoparticles and have recently predicted highly active particles for CO/CO₂ hydrogenation or direct H₂ O₂ synthesis.

In Situ Observation of Atomic Redistribution in Alloying Gold-Silver Nanorods

The catalytic performance and optical properties of bimetallic nanoparticles critically depend on the atomic distribution of the two metals in the nanoparticles. However, at elevated temperatures, during light-induced heating, or during catalysis, atomic redistribution can occur. Measuring such metal redistribution in situ is challenging, and a single experimental technique does not suffice. Furthermore, the availability of a well-defined nanoparticle system has been an obstacle for a systematic investigation of the key factors governing the atomic redistribution. In this study, we follow metal redistribution in precisely tunable, single-crystalline Au-core, Ag-shell nanorods in situ, both at a single particle and an ensemble-averaged level, by combining in situ transmission electron spectroscopy with in situ extended X-ray absorption fine structure validated by ex situ measurements. We show that the kinetics of atomic redistribution in Au-Ag nanoparticles depend on the metal composition and particle volume, such that a higher Ag content or a larger particle size led to significantly slower metal redistribution. We developed a simple theoretical model based on Fick's first law that can correctly predict the composition- and size-dependent alloying behavior in Au-Ag nanoparticles, as observed experimentally.