Investigation of the Redispersion of Pt Nanoparticles on Polyhedral Ceria Nanoparticles

Theoretical study of structure sensitivity on ceria-supported single platinum atoms and its influence on carbon monoxide adsorption

Density functional theory (DFT) calculations explore the stability of a single platinum atom on various flat, stepped, and defective ceria surfaces, in the context of single-atom catalysts (SACs) for the water-gas shift (WGS) reaction. The adsorption properties and diffusion kinetics of the metal strongly depend on the support termination with large stability on metastable and stepped CeO2(100) and (210) surfaces where the diffusion of the platinum atom is hindered. At the opposite, the more stable CeO2(111) and (110) terminations weakly bind the platinum atom and can promote the growth of metallic clusters thanks to fast diffusion kinetics. The adsorption of carbon monoxide on the single platinum atom supported on the various ceria terminations is also sensitive to the surface structure. Carbon monoxide weakly binds to the single platinum atom supported on reduced CeO2(111) and (211) terminations. The desorption of the CO2 formed during the WGS reaction is thus facilitated on the latter terminations. A vibrational analysis underlines the significant changes in the calculated scaled anharmonic CO stretching frequency on these catalysts.

In Situ Visualization and Mechanistic Understandings on Facet-Dependent Atomic Redispersion of Platinum on CeO2

Redispersion is an effective method for regeneration of sintered metal-supported catalysts. However, the ambiguous mechanistic understanding hinders the delicate controlling of active metals at the atomic level. Herein, the redispersion mechanism of atomically dispersed Pt on CeO2 is revealed and manipulated by in situ techniques combining well-designed model catalysts. Pt nanoparticles (NPs) sintered on CeO2 nano-octahedra under reduction and oxidation conditions, while redispersed on CeO2 nanocubes above ∼500 °C in an oxidizing atmosphere. The dynamic shrinkage and disappearance of Pt NPs on CeO2 (100) facets was directly visualized by in situ TEM. The generated atomically dispersed Pt with the square-planar [PtO4]2+ structure on CeO2 (100) facets was also confirmed by combining Cs-corrected STEM and spectroscopy techniques. The redispersion and atomic control were ascribed to the high mobility of PtO2 at high temperatures and its strong binding with square-planar O4 sites over CeO2 (100). These understandings are important for the regulation of atomically dispersed platinum catalysts.

Local environmental engineering for highly stable single-atom Pt1/CeO2catalysts: first-principles insights

Single-atom Pt1/CeO2catalysts may cope with the high cost and durability issues of fuel cell electrocatalysts. In the present study, the stability and underlying interaction mechanisms of the Pt1/CeO2system are systematically investigated using first-principles calculations. The Pt adsorption energy on CeO2surfaces can be divided into chemical interaction and surface deformation parts. The interaction energy, mainly associated with the local chemical environment, i.e. the number of Pt-O bonds, plays a major role in Pt1/CeO2stability. When forming a Pt-4O configuration, the catalytic system has the highest stability and Pt is oxidized to Pt2+. An electronic metal-support interaction mechanism is proposed for understanding Pt1/CeO2stability. In addition, our calculations show that the Pt1/CeO2(100) system is dynamically stable, and the external O environment can promote the further oxidation of Pt to Ptn+(2 ≤n< 4). The present study provides useful guidance for the experimental development of highly stable and efficient electrocatalysts for fuel cell applications.

Distinct Role of Surface Hydroxyls in Single-Atom Pt1/CeO2 Catalyst for Room-Temperature Formaldehyde Oxidation: Acid-Base Versus Redox

The development of highly efficient catalysts for room-temperature formaldehyde (HCHO) oxidation is of great interest for indoor air purification. In this work, it was found that the single-atom Pt₁/CeO₂ catalyst exhibits a remarkable activity with complete removal of HCHO even at 288 K. Combining density functional theory calculations and in situ DRIFTS experiments, it was revealed that the active O_latticeH site generated on CeO₂ in the vicinity of Pt²⁺ via steam treatment plays a key role in the oxidation of HCHO to formate and its further oxidation to CO₂. Such involvement of hydroxyls is fundamentally different from that of cofeeding water which dissociates on metal oxide and catalyzes the acid-base-related chemistry. This study provides an important implication for the design and synthesis of supported Pt catalysts with atom efficiency for a very important practical application-room-temperature HCHO oxidation.

Keys Unlocking Redispersion of Reactive PdOx Nanoclusters on Ce-Functionalized Perovskite Oxides for Methane Activation

Nowadays, trace CH₄ emitted from vehicle exhausts severely threaten the balance of the ecology system of our earth. Thereby, the development of active and stable catalysts capable of methane conversion under mild conditions is critical. Here, we present a convenient method to redisperse catalytically inert PdO nanoparticles (NPs) (>10 nm) into reactive PdO_x nanoclusters (∼2 nm) anchored on a Ce-doped LaFeO₃ parent. Isothermally activated in an N₂ flow, the redispersed catalyst achieved a CH₄ conversion of 90% at 400 °C, which is significantly higher than the fresh and H₂- and O₂-treated counterparts (625, 616, and 641 °C, respectively), indicating the importance of the gas atmosphere in the redispersion of PdO NPs. In addition, the comprehensive catalyst characterizations demonstrated that the isolated Ce ions in the perovskite lattice play an irreplaceable role in the redispersion of reactive sites and the reduction of the energy barrier for C-H scission. More importantly, the Ce additive helps to stabilize the PdO_x species by reducing overoxidation, resulting in significant lifetime extension. Through a thorough understanding of structural manipulation, this study sheds light on the design of highly performing supported catalysts for methane oxidation.

The facet effect of ceria nanoparticles on platinum dispersion and catalytic activity of methanol partial oxidation

The effect of platinum-supported nano-shaped ceria catalysts on methanol partial oxidation and methyl formate product selectivity has been investigated. A Pt-supported CeO2 nanocube catalyst had a higher turnover frequency than nanosphere catalysts; however, nanosphere catalysts showed higher selectivity towards methyl formate. The observed ceria shape effect in catalysis was associated with the shape-dependent Pt dispersion and its oxidation states. Furthermore, in situ studies revealed that the reduced platinum and mono-dentate methoxy group were responsible for the higher turnover frequency.

Development of a thermally stable Pt catalyst by redispersion between CeO2 and Al2O3

For catalytic systems consisting of Pt as the active component and CeO2-Al2O3 as the support material, the metal-support interaction between the Pt and CeO2 components is widely applied to inhibit aggregation of Pt species and thus enhance the thermal stability of the catalyst. In this work, a highly thermostable Pt catalyst was prepared by modifying the synthesis procedure for conventional Pt/CeO2/Al2O3 (Pt/Ce/Al) catalyst, that is, the CeO2 component was introduced after deposition of Pt on Al2O3. The obtained CeO2/Pt/Al2O3 (Ce/Pt/Al) catalyst exhibits significantly different aging behavior. During the hydrothermal aging process, redispersion of Pt species from the surface of Al2O3 to the surface of CeO2 occurs, resulting in a stronger metal-support interaction between Pt and CeO2. Thus, the formed Pt-O-Ce bond could act as an anchor to retard aggregation of Pt species and help Pt species stay at a more oxidative state. Consequently, excellent reduction capability and superior three-way catalytic performance are acquired by Ce/Pt/Al-a after hydrothermal aging treatment.

High-Temperature Pulse Method for Nanoparticle Redispersion

Nanoparticles suffer from aggregation and poisoning issues (e.g., oxidation) that severely hinder their long-term applications. However, current redispersion approaches, such as continuous heating in oxidizing and reducing environments, face challenges including grain growth effects induced by long heating times as well as complex procedures. Herein, we report a facile and efficient redispersion process that enables us to directly transform large aggregated particles into nanoscale materials. In this method, a piece of carbon nanofiber film was used as a heater and high treatment temperature (∼1500-2000 K) is rapidly elevated and maintained for a very short period of time (100 ms), followed by fast quenching back to room temperature at a cooling rate of 10⁵ K/s to inhibit sintering. With these conditions we demonstrate the redispersion of large aggregated metal oxide particles into metallic nanoparticles just ∼10 nm in size, uniformly distributed on the substrate. Furthermore, the metallic states of the nanoparticles are renewed during the heat treatment through reduction. The redispersion process removes impurities and poisoning elements, yet is able to maintain the integrity of the substrate because of the ultrashort heating pulse time. This method is also significantly faster (ca. milliseconds) compared to conventional redispersion treatments (ca. hours), providing a pragmatic strategy to redisperse degraded particles for a variety of applications.

Interfacial Pd-O-Ce Linkage Enhancement Boosting Formic Acid Electrooxidation

Metal-support interaction enhancement is critical in the fuel cell catalyst design and fabrication. Herein, taking the Pd@CeO₂ system as an example, we revealed the substrate morphology coupling effect and the thermal annealing-induced Pd-O-Ce linkage enhancement in the improved catalytic capability for formic acid electrooxidation. Three well-defined CeO₂ nanocrystals were employed to support Pd nanoparticles, and the best catalytic performance for formic acid oxidation and anti-CO poisoning ability was found on CeO₂ plates because of the high oxygen vacancy, Ce³⁺, and more Pd-O-Ce linkages resulting from the more edge/corner defects. This interaction of Pd-O-Ce linkages could be largely enhanced by thermal annealing in the N₂ atmosphere, as confirmed by a series of crystal structures, surface chemical state, and Raman analysis because the oxygen vacancies and lattice oxygen resulting from the oxygen atoms leaching from the CeO₂ lattice would trap the mobile Pd nanocrystals by forming strengthened Pd-O-Ce linkages. Due to the high oxygen vacancy and strong Pd-O-Ce linkages, largely increased catalytic activity and stability, catalytic kinetics, and rapid charge transfer were found for all the thermal annealed Pd@CeO₂ catalysts. A nearly 1.93-fold enhancement in the mass activity was achieved on the Pd@CeO₂-plate catalysts demonstrating the significance of Pd-O-Ce linkage enhancement. The formation mechanism of Pd-O-Ce linkage was also probed, and a valid Pd-O-Ce linkage can only be formed in the inert atmosphere because of the reaction between metallic Pd and CeO₂. This finding sheds some light on the more efficient catalyst interface construction and understanding for the fuel cell catalysis via metal-support interaction enhancement.

Boosting the Removal of Diesel Soot Particles by the Optimal Exposed Crystal Facet of CeO2 in Au/CeO2 Catalysts

Optimized surface facet of the catalysts is an efficient strategy to boost catalytic purification of diesel soot as important components of atmospheric fine particles. Herein, we have elaborately constructed the nanocatalysts of Au nanoparticles supported on the well-defined CeO₂ (rod, cube, and polyhedron) with predominantly exposed facets of {110}, {100}, and {111}, respectively. The strong interaction between Au and CeO₂ with the optimal crystal facet is crucial to adjust the active site density for activated O₂, and the synergy effect of Au and the CeO₂{110} facet possesses the largest density of active sites compared with other crystal facets of {100} and {111}. The catalytic activity for soot combustion was tuned by exposed crystal facets of CeO₂. The Au/CeO₂-rod catalyst exhibits the highest catalytic activity (T₅₀ = 350 °C, TOF = 0.18 h^-1) and the lowest apparent activation energy (72 kJ mol^-1) during soot combustion. Based on the results of in situ Raman spectra, the formation and stability of oxygen vacancy located at the interface of the Au-O-Ce bond, boosting the key step of NO oxidation to NO₂, are dependent on the exposed crystal facets of CeO₂. It highlights a new strategy for the fabrication of high-efficient CeO₂-based catalysts for the removal of soot particles or other pollution.

Dynamic charge and oxidation state of Pt/CeO2 single-atom catalysts

The catalytic activity of metals supported on oxides depends on their charge and oxidation state. Yet, the determination of the degree of charge transfer at the interface remains elusive. Here, by combining density functional theory and first-principles molecular dynamics on Pt single atoms deposited on the CeO₂ (100) surface, we show that the common representation of a static metal charge is oversimplified. Instead, we identify several well-defined charge states that are dynamically interconnected and thus coexist. The origin of this new class of strong metal-support interactions is the relative position of the Ce(4f) levels with respect to those of the noble metal, allowing electron injection to (or recovery from) the support. This process is phonon-assisted, as the Ce(4f) levels adjust by surface atom displacement, and appears for other metals (Ni) and supports (TiO₂). Our dynamic model explains the unique reactivity found for activated single Pt atoms on ceria able to perform CO oxidation, meeting the Department of Energy 150 °C challenge for emissions.

Pt-Embedded CuO x-CeO2 Multicore-Shell Composites: Interfacial Redox Reaction-Directed Synthesis and Composition-Dependent Performance for CO Oxidation

Exploring the state-of-the-art heterogeneous catalysts has been a general concern for sustainable and clean energy. Here, Pt-embedded CuO _x-CeO₂ multicore-shell (Pt/CuO _x-CeO₂ MS) composites are fabricated at room temperature via a one-pot and template-free procedure for catalyzing CO oxidation, a classical probe reaction, showing a volcano-shaped relationship between the composition and catalytic activity. We experimentally unravel that the Pt/CuO _x-CeO₂ MS composites are derived from an interfacial autoredox process, where Pt nanoparticles (NPs) are in situ encapsulated by self-assembled ceria nanospheres with CuO _x clusters adhered through deposition/precipitation-calcination process. Only Cu-O and Pt-Pt coordination structures are determined for CuO _x clusters and Pt NPs in Pt/CuO _x-CeO₂ MS, respectively. Importantly, the close vicinity between Pt and CeO₂ benefits to more oxygen vacancies in CeO₂ counterparts and results in thin oxide layers on Pt NPs. Meanwhile, the introduction of CuO _x clusters is crucial for triggering synergistic catalysis, which leads to high resistance to aggregation of Pt NPs and improvement of catalytic performance. In CO oxidation reaction, both Pt^δ+-CO and Cu⁺-CO can act as active sites during CO adsorption and activation. Nonetheless, redundant content of Pt or Cu will induce a strongly bound Pt-O-Ce or Cu-[O _x]-Ce structures in air-calcinated Pt/CuO _x-CeO₂ MS composites, respectively, which are both deleterious to catalytic reactivity. As a result, the composition-dependent catalytic activity and superior durability of Pt/CuO _x-CeO₂ MS composites toward CO oxidation reaction are achieved. This work should be instructive for fabricating desirable multicomponent catalysts composed of noble metal and bimetallic oxide composites for diverse heterogeneous catalysis.

Decomposition behavior of platinum clusters supported on ceria and γ-alumina in the presence of carbon monoxide

Carbon monoxide facilitates decomposition of platinum clusters supported on ceria, while carbonylated platinum clusters are stable on gamma alumina.

Transient structural and catalytic behaviour of Pt-particles probed by operando spectroscopy during a realistic driving cycle

Pt catalysts investigated with operando XAS under rapid transient temperature automotive driving cycle conditions.

Oxide-based nanomaterials for fuel cell catalysis: the interplay between supported single Pt atoms and particles

Nanomaterials coated with atomically dispersed platinum on ceria are structurally dynamic and show high potential for applications in fuel cells.

Thermally stable single-atom platinum-on-ceria catalysts via atom trapping

Catalysts based on single atoms of scarce precious metals can lead to more efficient use through enhanced reactivity and selectivity. However, single atoms on catalyst supports can be mobile and aggregate into nanoparticles when heated at elevated temperatures. High temperatures are detrimental to catalyst performance unless these mobile atoms can be trapped. We used ceria powders having similar surface areas but different exposed surface facets. When mixed with a platinum/aluminum oxide catalyst and aged in air at 800°C, the platinum transferred to the ceria and was trapped. Polyhedral ceria and nanorods were more effective than ceria cubes at anchoring the platinum. Performing synthesis at high temperatures ensures that only the most stable binding sites are occupied, yielding a sinter-resistant, atomically dispersed catalyst.

The redispersion behaviour of Pt on the surface of Fe2O3

Pt could be redispersed on the surface of Fe₂O₃ with alternating treatment under oxidative and reductive atmospheres.

Towards stable single-atom catalysts: strong binding of atomically dispersed transition metals on the surface of nanostructured ceria

Surface oxygen sites on CeO₂ nanostructures are able to bind atoms of various transition metals strong enough to prevent their sintering. This finding opens a knowledge-driven way to prepare stable single-atom catalysts with maximum metal efficiency.

Reactivity of atomically dispersed Pt2+ species towards H2: model Pt–CeO2 fuel cell catalyst

Formation of at least two oxygen vacancies triggers the reduction of one Pt²⁺ species.

Aerobic oxidation of 5-hydroxymethylfurfural (HMF) effectively catalyzed by a Ce0.8Bi0.2O2−δ supported Pt catalyst at room temperature

98% yield of FDCA was obtained within 30 min at room temperature when HMF oxidation was catalyzed by Pt/Ce_0.8Bi_0.2O_2−δ.