Chemical and Structural Dynamics of Nanostructures in Bimetallic Pt–Pd Catalysts, Their Inhomogeneity, and Their Roles in Methane Oxidation

Towards Low Temperature Operation of Catalytic Gas Sensors: Mesoporous Co3O4-Supported Au-Pd Nanoparticles as Functional Material

It is shown that the operating temperature of pellistors for the detection of methane can be reduced to 300 °C by using Au-Pd nanoparticles on mesoporous cobalt oxide (Au-Pd@meso-Co3O4). The aim is to reduce possible catalyst poisoning that occurs during the high-temperature operation of conventional Pd-based pellistors, which are usually operated at 450 °C or higher. The individual role of Au-Pd as well as Co3O4 in terms of their catalytic activity has been investigated. Above 300 °C, Au-Pd bimetallic particles are mainly responsible for the catalytic combustion of methane. However, below 300 °C, only the Co3O4 has a catalytic effect. In contrast to methane, the sensor response and the temperature increase of the sensor under propane exposure is much larger than for methane due to the larger heat of combustion of propane. Due to its lower activation energy requirement, propane exhibits a higher propensity for oxidation compared to methane. As a result, the detection of propane can be achieved at even lower temperatures due to its enhanced reactivity.

Bimetallic Sites for Catalysis: From Binuclear Metal Sites to Bimetallic Nanoclusters and Nanoparticles

Heterogeneous bimetallic catalysts have broad applications in industrial processes, but achieving a fundamental understanding on the nature of the active sites in bimetallic catalysts at the atomic and molecular level is very challenging due to the structural complexity of the bimetallic catalysts. Comparing the structural features and the catalytic performances of different bimetallic entities will favor the formation of a unified understanding of the structure-reactivity relationships in heterogeneous bimetallic catalysts and thereby facilitate the upgrading of the current bimetallic catalysts. In this review, we will discuss the geometric and electronic structures of three representative types of bimetallic catalysts (bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles) and then summarize the synthesis methodologies and characterization techniques for different bimetallic entities, with emphasis on the recent progress made in the past decade. The catalytic applications of supported bimetallic binuclear sites, bimetallic nanoclusters, and nanoparticles for a series of important reactions are discussed. Finally, we will discuss the future research directions of catalysis based on supported bimetallic catalysts and, more generally, the prospective developments of heterogeneous catalysis in both fundamental research and practical applications.

Novel Insights on the Metal-Support Interactions of Pd/ZrO2 Catalysts on CH4 Oxidation

With the environmental harm of unburnt CH₄ in natural gas vehicle exhaust, oxidizing CH₄ to CO₂ over catalysts at low temperatures becomes an exigent issue. Supported Pd catalysts possess higher CH₄ activity than other noble metal catalysts. A series of Pd/ZrO₂ catalysts were synthesized to research the potential relationship among Pd particle morphology, electron transfer, CH₄ oxidation mechanism, and catalytic activity. Characterizations show that the ratio of PdO_x facets to edge/corner sites on four catalysts increases in the order of PZ85 ≈ PZ40 < PZ55 < PZ70 because of the difference in content of surface -OH groups, and this order turns out to be the same as that of electron transfer intensity, revealing the degree of metal-support interactions. This kind of metal-support interaction in PZ70 can be helpful to accelerate CH₄ combustion via promoting the break of the C-H bond and dissociation of CO₃* according to density functional theory studies. T₉₀ of the PZ70 catalyst with optimum catalytic activity reaches 331 °C.

Cu12@Au30Ag12: a magnetic pentakis icosidodecahedron molecule with core-shell configuration

A 54-atom trimetallic core-shell Cu₁₂@Au₃₀Ag₁₂ molecule has been identified by using first-principles calculations. This molecule with I_h symmetry consists of an icosahedral 12-atom Cu core (Cu₁₂) coated by a pentakis icosidodecahedral 42-atom Au-Ag shell (Au₃₀Ag₁₂) composed of an icosidodecahedral Au₃₀ and an icosahedral Ag₁₂. Both the molecular dynamics simulations and vibrational frequency analysis demonstrate the high stability of Cu₁₂@Au₃₀Ag₁₂. The analyses for electronic properties indicate that there exists spd hybridization which is crucial to maintain the geometrical configuration of Cu₁₂@Au₃₀Ag₁₂. Moreover, a total spin magnetic moment of Cu₁₂@Au₃₀Ag₁₂ is found to be 4 µ_B, which chiefly arises from the 6 s states of Au atoms. A new type of sugar gourd-like [Cu₁₂@Au₃₀Ag₁₁]_n nanowire is also proposed; the results demonstrate that the nanowire exhibits metallic and magnetic behaviors. The magnetic Cu₁₂@Au₃₀Ag₁₂ molecule and [Cu₁₂@Au₃₀Ag₁₁]_n nanowire can be promising candidates of novel magnetic nanomaterials and nanodevices.

Effect of oxygen species, catalyst structure and their performance to methane activation over Pd-Pt catalyst

This paper investigates C-H bond activation in methane over monometallic Pd, Pt and bimetallic Pd-Pt catalysts via a differential reactor, chemisorption system, HAADF-STEM, TPR and XPS methods. The results show...

Elucidating the surface compositions of Pd@PtnL core-shell nanocrystals through catalytic reactions and spectroscopy probes

The catalytic behaviors or properties of bimetallic catalysts are highly dependent on the surface composition, but it has been a grand challenge to acquire such information. In this work, we employ Pd@Pt_nL core-shell nanocrystals with an octahedral shape and tunable Pt shell thickness as a model system to elucidate their surface compositions using catalytic reactions based upon the selective hydrogenation of butadiene and acetylene. Our results indicate that the surface of the core-shell nanocrystals changed from Pt-rich to Pd-rich when they were subjected to calcination under oxygen, a critical step involved in the preparation of many industrial catalysts. The inside-out migration can be attributed to both atomic interdiffusion and the oxidation of Pd atoms during the calcination process. The changes in surface composition were further confirmed using infrared and X-ray photoelectron spectroscopy. This work offers insightful guidance for the development and optimization of bimetallic catalysts toward various reactions.

Structural Evolution of Bimetallic PtPd/CeO2 Methane Oxidation Catalysts Prepared by Dry Milling

Bimetallic Pt-Pd catalysts supported on ceria have been prepared by mechanochemical synthesis and tested for lean methane oxidation in dry and wet atmosphere. Results show that the addition of platinum has a negative effect on transient light-off activity, but for Pd/Pt molar ratios between 1:1 and 8:1 an improvement during time-on-stream experiments in wet conditions is observed. The bimetallic samples undergo a complex restructuring during operation, starting from the alloying of Pt and Pd and resulting in the formation of unprecedented "mushroom-like" structures consisting of PdO bases with Pt heads as revealed by high-resolution transmission electron microscopy (HRTEM) analysis. On milled samples, these structures are well-defined and observed at the interface between palladium and ceria, whereas those on the impregnated catalyst appear less ordered and are located randomly on the surface of ceria and of large PdPt clusters. The milled catalyst prepared by first milling Pd metal and ceria followed by the addition of Pt shows better performances compared to a conventional impregnated sample and also to a sample obtained by inverting the Pd-Pt milling order. This has been ascribed to the intimate contact between Pd and CeO₂ generated at the nanoscale during the milling process.

Single-Site vs. Cluster Catalysis in High Temperature Oxidations

The behavior of single Pt atoms and small Pt clusters was investigated for high-temperature oxidations. The high stability of these molecular sites in CHA is a key to intrinsic structure-performance descriptions of elemental steps such as O₂ dissociation, and subsequent oxidation catalysis. Subtle changes in the atomic structure of Pt are responsible for drastic changes in performance driven by specific gas/metal/support interactions. Whereas single Pt atoms and Pt clusters (> ca. 1 nm) are unable to activate, scramble, and desorb two O₂ molecules at moderate T (200 °C), clusters <1 nm do so catalytically, but undergo oxidative fragmentation. Oxidation of alkanes at high T is attributed to stable single Pt atoms, and the C-H cleavage is inferred to be rate-determining and less sensitive to changes in metal nuclearity compared to its effect on O₂ scrambling. In contrast, when combustion involves CO, catalysis is dominated by metal clusters, not single Pt atoms.

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.

Palladium oxidation leads to methane combustion activity: Effects of particle size and alloying with platinum

Pd- and Pt-based catalysts are highly studied materials due to their widespread use in emissions control catalysis. However, claims continue to vary regarding the active phase and oxidation state of the metals. Different conclusions have likely been reached due to the heterogeneous nature of such materials containing various metal nanoparticle sizes and compositions, which may each possess unique redox features. In this work, using uniform nanocrystal catalysts, we study the effect of particle size and alloying on redox properties of Pd-based catalysts and show their contribution to methane combustion activity using operando quick extended x-ray absorption fine structure measurements. Results demonstrate that for all studied Pd sizes (3 nm-16 nm), Pd oxidation directly precedes CH₄ combustion to CO₂, suggesting Pd oxidation as a prerequisite step to methane combustion, and an oxidation pretreatment shows equal or better catalysis than a reduction pretreatment. Results are then extended to uniform alloyed Pt_xPd_1-x nanoparticles, where oxidative pretreatments are shown to enhance low-temperature combustion. In these uniform alloys, we observe a composition-dependent effect with Pt-rich alloys showing the maximum difference between oxidative and reductive pretreatments. In Pt-rich alloys, we initially observe that the presence of Pt maintains Pd in a lower-activity reduced state. However, with time on stream, PdO eventually segregates under oxidizing combustion conditions, leading to a slowly increasing activity. Overall, across particle sizes and alloy compositions, we relate increased catalytic activity to Pd oxidation, thus shedding light on previous contrasting results related to the methane combustion activity of these catalysts.