Relating adatom emission to improved durability of Pt–Pd diesel oxidation catalysts

Surface-Diffusion-Induced Amorphization of Pt Nanoparticles over Ru Oxide Boost Acidic Oxygen Evolution

Phase transformation offers an alternative strategy for the synthesis of nanomaterials with unconventional phases, allowing us to further explore their unique properties and promising applications. Herein, we first observed the amorphization of Pt nanoparticles on the RuO2 surface by in situ scanning transmission electron microscopy. Density functional theory calculations demonstrate the low energy barrier and thermodynamic driving force for Pt atoms transferring from the Pt cluster to the RuO2 surface to form amorphous Pt. Remarkably, the as-synthesized amorphous Pt/RuO2 exhibits 14.2 times enhanced mass activity compared to commercial RuO2 catalysts for the oxygen evolution reaction (OER). Water electrolyzer with amorphous Pt/RuO2 achieves 1.0 A cm-2 at 1.70 V and remains stable at 200 mA cm-2 for over 80 h. The amorphous Pt layer not only optimized the *O binding but also enhanced the antioxidation ability of amorphous Pt/RuO2, thereby boosting the activity and stability for the OER.

Study on the Catalytic Characteristics of Precious Metal Catalysts with Different Pt/Pd Ratios for Soot Combustion

Soot particles in engine exhaust seriously pollute the atmosphere and endanger human health. For soot oxidation, Pt and Pd precious metal catalysts are widely used and are effective. In this paper, the catalytic characteristics of catalysts with different Pt/Pd mass ratios for soot combustion were studied through X-ray diffraction, X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller analysis, scanning electron microscopy, transmission electron microscopy, the temperature-programmed oxidation reaction, and thermogravimetry. Besides, the adsorption characteristics of soot and O₂ on the catalyst surface were explored by density functional theory (DFT) calculations. The research results showed that the activity of catalysts for soot oxidation from strong to weak is Pt/Pd = 10:1, Pt/Pd = 5:1, Pt/Pd = 1:0, and Pt/Pd = 1:1. XPS results showed that the concentration of oxygen vacancies in the catalyst is the highest when the Pt/Pd ratio is 10:1. The specific surface area of the catalyst increases first and then decreases with the increase of Pd content. When the Pt/Pd ratio is 10:1, the specific surface area and pore volume of the catalyst reach the maximum. The following are the DFT calculation results. With the increase of Pd content, the adsorption energy of particles on the catalyst surface decreases first and then increases. When the Pt/Pd ratio is 10:1, the adsorption of C on the catalyst surface is the strongest, and the adsorption of O₂ is also strong. In addition, this surface has a strong ability to donate electrons. The theoretical simulation results are consistent with the activity test results. The research results have a guiding significance for optimizing the Pt/Pd ratio and improving the soot oxidation performance of the catalyst.

Progress on metal-support interactions in Pd-based catalysts for automobile emission control

The interactions between metals and oxide supports, so-called metal-support interactions (MSI), are of great importance in heterogeneous catalysis. Pd-based automotive exhaust control catalysts, especially Pd-based three-way catalysts (TWCs), have received considerable research attention owing to its prominent oxidation activity of HCs/CO, as well as excellent thermal stability. For Pd-based TWCs, the dispersion, chemical state and thermal stability of Pd species, which are crucial to the catalytic performance, are closely associated with interactions between metal nanoparticles and their supporting matrix. Progress on the research about MSI and utilization of MSI in advanced Pd-based three-way catalysts are reviewed here. Along with the development of advanced synthesis approaches and engine control technology, the study on MSI would play a notable role in further development of catalysts for automobile exhaust control.

Solid-State Reaction Synthesis of Nanoscale Materials: Strategies and Applications

Nanomaterials (NMs) with unique structures and compositions can give rise to exotic physicochemical properties and applications. Despite the advancement in solution-based methods, scalable access to a wide range of crystal phases and intricate compositions is still challenging. Solid-state reaction (SSR) syntheses have high potential owing to their flexibility toward multielemental phases under feasibly high temperatures and solvent-free conditions as well as their scalability and simplicity. Controlling the nanoscale features through SSRs demands a strategic nanospace-confinement approach due to the risk of heat-induced reshaping and sintering. Here, we describe advanced SSR strategies for NM synthesis, focusing on mechanistic insights, novel nanoscale phenomena, and underlying principles using a series of examples under different categories. After introducing the history of classical SSRs, key theories, and definitions central to the topic, we categorize various modern SSR strategies based on the surrounding solid-state media used for nanostructure growth, conversion, and migration under nanospace or dimensional confinement. This comprehensive review will advance the quest for new materials design, synthesis, and applications.

Electronic metal-support interaction constructed for preparing sinter-resistant nano-platinum catalyst with redox property

Generally, support materials with particular structural properties could effectively anchor metal nanoparticles and provide lower activation barriers in heterogeneous catalysis. To tailor the structure of stable iron oxide, NiFe₂O₄ of inverse spinel structure was obtained by combining nickel with iron element under an alkaline environment and high-temperature calcination. The p-type conductivity of NiFe₂O₄ provides the possibility of constructing electronic interfacial interaction with Pt nanoparticles by electron transfer. The constructed metal-support interaction could effectively stabilize Pt nanoparticles and be further enhanced during long-term harsh calcination (700 °C for 48 h) even under an O₂ atmosphere. Meanwhile, the abundant structural defects of NiFe₂O₄ are beneficial for constructing low-temperature redox centers with the aid of Pt nanoparticles. Pt/NiFe₂O₄ exhibited not only excellent activity in room-temperature oxidation (CO and HCHO) and reduction reactions (chemo-selective hydrogenation of nitroarenes), but also high stability even after storage for more than 6 months. A self-adjusting mechanism triggered by structural defects is disclosed by in situ characterization and systematic reaction results. This work demonstrates an alternative concept to construct sinter-resistant and highly-effective nano-platinum catalysts robust for oxidation and reduction reactions.

Atomic Structure of Pd-, Pt-, and PdPt-Based Catalysts of Total Oxidation of Methane: In Situ EXAFS Study

In this study, 3%Pd/Al2O3, 3%Pt/Al2O3 and bimetallic (1%Pd + 2%Pt)/Al2O3 catalysts were examined in the total oxidation of methane in a temperature range of 150–400 °C. The evolution of the active component under the reaction conditions was studied by transmission electron microscopy and in situ extended X-ray absorption fine structure (EXAFS) spectroscopy. It was found that the platinum and bimetallic palladium-platinum catalysts are more stable against sintering than the palladium catalysts. For all the catalysts, the active component forms a “core-shell” structure in which the metallic core is covered by an oxide shell. The “core-shell” structure for the platinum and bimetallic palladium-platinum catalysts is stable in the temperature range of 150–400 °C. However, in the case of the palladium catalysts the metallic core undergoes the reversible oxidation at temperatures above 300 °C and reduced to the metallic state with the decrease in the reaction temperature. The scheme of the active component evolution during the oxidation of methane is proposed and discussed.

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.

Anomalous metal vaporization from Pt/Pd/Al2O3 under redox conditions

Al2O3-supported Pt/Pd bimetallic catalysts were studied using in situ atmospheric pressure and ex situ transmission electron microscopy. Real-time observation during separate oxidation and reduction processes provides nanometer-scale structural details - both morphology and chemistry - of supported Pt/Pd particles at intermediate states not observable through typical ex situ experiments. Significant metal vaporization was observed at temperatures above 600 °C, both in pure oxygen and in air. This behavior implies that material transport through the vapor during typical catalyst aging processes for oxidation can play a more significant role in catalyst structural evolution than previously thought. Concomitantly, Pd diffusion away from metallic nanoparticles on the surface of Al2O3 can also contribute to the disappearance of metal particles. Electron micrographs from in situ oxidation experiments were mined for data, including particle number, size, and aspect ratio using machine learning image segmentation. Under oxidizing conditions, we observe not only a decrease in the number of metal particles but also a decrease in the surface area to volume ratio. Some of the metal that diffuses away from particles on the oxide support can be regenerated and reappears back on the catalyst support surface under reducing conditions. These observations provide insight on how rapid cycling between oxidative and reductive catalytic operating conditions affects catalyst structure.

Effects of Hydrothermal Aging on CO and NO Oxidation Activity over Monometallic and Bimetallic Pt-Pd Catalysts

By combining scanning transmission electron microscopy, CO chemisorption, and energy dispersive X-ray spectroscopy with CO and NO oxidation light-off measurements we investigated deactivation phenomena of Pt/Al2O3, Pd/Al2O3, and Pt-Pd/Al2O3 model diesel oxidation catalysts during stepwise hydrothermal aging. Aging induces significant particle sintering that results in a decline of the catalytic activity for all catalyst formulations. While the initial aging step caused the most pronounced deactivation and sintering due to Ostwald ripening, the deactivation rates decline during further aging and the catalyst stabilizes at a low level of activity. Most importantly, we observed pronounced morphological changes for the bimetallic catalyst sample: hydrothermal aging at 750 °C causes a stepwise transformation of the Pt-Pd alloy via core-shell structures into inhomogeneous agglomerates of palladium and platinum. Our study shines a light on the aging behavior of noble metal catalysts under industrially relevant conditions and particularly underscores the highly complex transformation of bimetallic Pt-Pd diesel oxidation catalysts during hydrothermal treatment.

Boosting the sintering resistance of platinum–alumina catalyst via a morphology-confined phosphate-doping strategy

The size-dependent metal–support interaction, high surface area, and, above all, the support morphology-confined effect contribute to a good sintering-resistant catalyst.

Atomically dispersed platinum on low index and stepped ceria surfaces: phase diagrams and stability analysis

Through the combination of density functional theory calculations and ab initio atomistic thermodynamics modeling, we demonstrate that atomically dispersed platinum species on ceria can adopt a range of local coordination configurations and oxidation states that depend on the surface structure and environmental conditions. Unsaturated oxygen atoms on ceria surfaces play the leading role in stabilization of PtOx species. Any mono-dispersed Pt0 species are thermodynamically unstable compared to bulk platinum, and oxidation of Pt0 to Pt2+ or Pt4+ is necessary to stabilize mono-dispersed platinum atoms. Reduction to Pt0 leads to sintering. Both Pt2+ and Pt4+ prefer to form the square-planar [PtO4] configuration. The two most stable Pt2+ species on the (223) and (112) surfaces are thermodynamically favorable between 300 and 1200 K. The most stable Pt4+ species on the (100) surface tends to desorb from the surface as gas phase above 950 K. The resulting phase diagrams of the atomically dispersed platinum in PtOx clusters on various ceria surfaces under a range of experimentally relevant conditions can be used to predict dynamic restructuring of atomically dispersed platinum catalysts and design new catalysts with engineered properties.

Emission reduction characteristics of a catalyzed continuously regenerating trap after-treatment system and its durability performance

The primary purpose of this study was to investigate the effect of a catalyzed continuously regenerating trap (CCRT) system composed of a diesel oxidation catalyst (DOC) and a catalyzed diesel particulate filter (CDPF) on the main gaseous and particulate emissions from an urban diesel bus, as well as the durability performance of the CCRT system. Experiments were conducted based on a heavy chassis dynamometer, and a laboratory activity test as well as X-ray photoelectron spectroscopy (XPS) test were applied to evaluate the changes of the aged CCRT catalyst. Results showed that the CCRT could reduce the CO by 71.5% and the total hydrocarbons (THC) by 88.9%, and meanwhile promote the oxidation of NO. However, the conversion rates for CO and THC dropped to 25.1% and 55.1%, respectively, after the CCRT was used for one year (~60,000 km), and the NO oxidation was also weakened. For particulate emissions, the CCRT could reduce 97.4% of the particle mass (PM) and almost 100% of the particle number (PN). The aging of the CCRT resulted in a reduced PM trapping efficiency but had no observable effect on the PN; however, it increased the proportion of nucleation mode particles. The activity test results indicated that the deterioration of the CCRT was directly relevant to the increase in the light-off temperatures of the catalyst for CO, C₃H₈ and NO₂. In addition, the decreased concentrations of the active components Pt²⁺ and Pt⁴⁺ in the catalyst are also important factors in the CCRT deterioration.

Supercritical flow synthesis of PtPdFe alloyed nanoparticles with enhanced low-temperature activity and thermal stability for propene oxidation under lean exhaust gas conditions

Supercritical flow technology was used for the one step production of PtPd and PtPdFe nanoparticles supported on high surface area γ-Al₂O₃.

Palladium Intercalated into the Walls of Mesoporous Silica as Robust and Regenerable Catalysts for Hydrodeoxygenation of Phenolic Compounds

Nanostructured noble-metal catalysts traditionally suffer from sintering under high operating temperatures, leading to durability issues and process limitations. The encapsulation of nanostructured catalysts to prevent loss of activity through thermal sintering, while maintaining accessibility of active sites, remains a great challenge in the catalysis community. Here, we report a robust and regenerable palladium-based catalyst, wherein palladium particles are intercalated into the three-dimensional framework of SBA-15-type mesoporous silica. The encapsulated Pd active sites remain catalytically active as demonstrated in high-temperature/pressure phenol hydrodeoxygenation reactions. The confinement of Pd particles in the walls of SBA-15 prevents particle sintering at high temperatures. Moreover, a partially deactivated catalyst containing intercalated particles is regenerated almost completely even after several reaction cycles. In contrast, Pd particles, which are not encapsulated within the SBA-15 framework, sinter and do not recover prior activity after a regeneration procedure.

A Linear Scaling Relation for CO Oxidation on CeO2-Supported Pd

Resolving the structure and composition of supported nanoparticles under reaction conditions remains a challenge in heterogeneous catalysis. Advanced configurational sampling methods at the density functional theory level are used to identify stable structures of a Pd8 cluster on ceria (CeO2) in the absence and presence of O2. A Monte Carlo method in the Gibbs ensemble predicts Pd-oxide particles to be stable on CeO2 during CO oxidation. Computed potential energy diagrams for CO oxidation reaction cycles are used as input for microkinetics simulations. Pd-oxide exhibits a much higher CO oxidation activity than metallic Pd on CeO2. This work presents for the first time a scaling relation for a CeO2-supported metal nanoparticle catalyst in CO oxidation: a higher oxidation degree of the Pd cluster weakens CO binding and facilitates the rate-determining CO oxidation step with a ceria O atom. Our approach provides a new strategy to model supported nanoparticle catalysts.

Theoretical Study of Ripening Mechanisms of Pd Clusters on Ceria

We carried out density functional theory calculations to investigate the ripening of Pd clusters on CeO₂(111). Starting from stable Pd _n clusters (n = 1-21), we compared how these clusters can grow through Ostwald ripening and coalescence. As Pd atoms have mobility higher than that of Pd _n clusters on the CeO₂(111) surface, Ostwald ripening is predicted to be the dominant sintering mechanism. Particle coalescence is possible only for clusters with less than 5 Pd atoms. These ripening mechanisms are facilitated by adsorbed CO through lowering barriers for the cluster diffusion, detachment of a Pd atom from clusters, and transformation of initial planar clusters.

Key role of surface oxidation and reduction processes in the coarsening of Pt nanoparticles

Particle coarsening is the main cause for thermal deactivation and lifetime reduction of supported Pt nanocatalysts. Here, Atomic Layer Deposition (ALD) was used to prepare a model system of Pt nanoparticles with high control over the metal loading and the nanoparticle size and coverage. A series of samples with distinct as-deposited size and interparticle spacing was annealed under different oxygen environments while Grazing Incidence Small Angle X-ray Scattering (GISAXS) was employed as in situ tool for monitoring the change in average nanoparticle size. The obtained results revealed three morphological stages during the thermal treatment, which can be explained by (I) the formation of a PtO₂ shell on stable Pt nanoparticles at low temperature (below 300 °C), (II) the reduction of the PtO₂ shell at moderate temperature (300 to 600 °C), creating mobile species that trigger particle coarsening until a steady morphological state is reached, and (III) the evaporation of PtO₂ at high temperature (above 650 °C), causing particle instability and coarsening reactivation. The onset temperatures for stages (II) and (III) were found to depend on the initial particle size and spacing as well as on the O₂ partial pressure during annealing, and could be summarized in a morphological stability diagram for Pt nanoparticles. The coarsening model indicates an important role for the reduction of the PtO₂ shell in inducing particle coarsening. The key role of the reduction process was corroborated through isothermal experiments under decreasing O₂ partial pressure and through forced reduction experiments near room temperature via H₂ exposure.

Thermally Stable and Regenerable Platinum-Tin Clusters for Propane Dehydrogenation Prepared by Atom Trapping on Ceria

Ceria (CeO₂) supports are unique in their ability to trap ionic platinum (Pt), providing exceptional stability for isolated single atoms of Pt. The reactivity and stability of single‐atom Pt species was explored for the industrially important light alkane dehydrogenation reaction. The single‐atom Pt/CeO₂ catalysts are stable during propane dehydrogenation, but are not selective for propylene. DFT calculations show strong adsorption of the olefin produced, leading to further unwanted reactions. In contrast, when tin (Sn) is added to CeO₂, the single‐atom Pt catalyst undergoes an activation phase where it transforms into Pt–Sn clusters under reaction conditions. Formation of small Pt–Sn clusters allows the catalyst to achieve high selectivity towards propylene because of facile desorption of the product. The CeO₂‐supported Pt–Sn clusters are very stable, even during extended reaction at 680 °C. Coke formation is almost completely suppressed by adding water vapor to the feed. Furthermore, upon oxidation the Pt–Sn clusters readily revert to the atomically dispersed species on CeO₂, making Pt–Sn/CeO₂ a fully regenerable catalyst.

Catalysts of self-assembled Pt@CeO2-δ-rich core-shell nanoparticles on 3D ordered macroporous Ce1-xZrxO2 for soot oxidation: nanostructure-dependent catalytic activity

The catalytic performance in heterogeneous catalytic reactions consisting of solid reactants is strongly dependent on the nanostructure of the catalysts. Metal-oxides core-shell (MOCS) nanostructures have potential to enhance the catalytic activity for soot oxidation reactions as a result of optimizing the density of active sites located at the metal-oxide interface. Here, we report a facile strategy for fabricating nanocatalysts with self-assembled Pt@CeO_2-δ-rich core-shell nanoparticles (NPs) supported on three-dimensionally ordered macroporous (3DOM) Ce_1-xZr_xO₂via the in situ colloidal crystal template (CCT) method. The nanostructure-dependent activity of the catalysts for soot oxidation were investigated by means of SEM, TEM, H₂-TPR, XPS, O₂-isothermal chemisorption, soot-TPO and so on. A CeO_2-δ-rich shell on a Pt core is preferentially separated from Ce_1-xZr_xO₂ precursors and could self-assemble to form MOCS nanostructures. 3DOM structures can enhance the contact efficiency between catalysts and solid reactants (soot). Pt@CeO_2-δ-rich core-shell nanostructures can optimize the density of oxygen vacancies (O_v) as active sites located at the interface of Pt-Ce_1-xZr_xO₂. Remarkably, 3DOM Pt@CeO_2-δ-rich/Ce_1-xZr_xO₂ catalysts show super catalytic performance and strongly nanostructure-dependent activity for soot oxidation in the absence of NO and NO₂. For example, the T₅₀ of the 3DOM Pt@CeO_2-δ-rich/Ce_0.8Zr_0.2O₂ catalyst is lowered down to 408 °C, and the reaction rate of the 3DOM Pt@CeO_2-δ-rich/Ce_0.2Zr_0.8O₂ catalyst (0.12 μmol g^-1 s^-1) at 300 °C is 4 times that of the 3DOM Pt/Ce_0.2Zr_0.8O₂ catalyst (0.03 μmol g^-1 s^-1). The structures of 3DOM Ce_1-xZr_xO₂-supported Pt@CeO_2-δ-rich core-shell NPs are decent systems for deep oxidation of solid reactants or macromolecules, and this facile technique for synthesizing catalysts has potential to be applied to other element compositions.

Ostwald ripening versus single atom trapping: towards understanding platinum particle sintering

Stepped edge of the CeO₂(111) surface effectively traps PtO₂ mobile species, generating atomically dispersed catalysts with square-planar [PtO₄] structure.

Challenges and breakthroughs in post-combustion catalysis: how to match future stringent regulations

This short overview briefly summarizes the prominent evolutions and scientific breakthroughs in the development of end-of-pipe technologies with respect to the standard regulations of atmospheric pollutant emissions from automotive exhaust.

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.