Surfactant-Mediated One-Pot Method To Prepare Pd–CeO₂ Colloidal Assembled Spheres and Their Enhanced Catalytic Performance for CO Oxidation

A simple, one-pot method to fabricate ordered, monodispersed Pd–CeO₂ colloidal assembled spheres (CASs) was developed using the surfactant-mediated solvothermal approach, which involves a tunable self-assembled process by carefully controlling different chemical reactions. The evolution process and formation mechanism of the CASs were thoroughly investigated by time-controlled and component-controlled experiments. For CO oxidation, this CAS nanocatalyst exhibited much higher catalytic activity and thermal stability than Pd/CeO₂ prepared by an impregnation method, and its complete CO conversion temperature is ∼120 °C. The enhanced catalytic performance for CO oxidation could be attributed to the synergistic effect of highly dispersed PdO species and Pd²⁺ ions incorporated into the CeO₂ lattice. For this CAS catalyst, each sphere can be viewed as a single reactor, and its catalytic performance can be further improved after being supported on alumina, which is obviously higher than results previously reported. Furthermore, this method was used to successfully prepare M–CeO₂ CASs (M = Pt, Cu, Mn, Co), showing further that this is a new and ideal approach for fabricating active and stable ceria-based materials

Effect of One-Pot Rehydration Process on Surface Basicity and Catalytic Activity of Mg_<i>y</i>Al_1‑aREE_<i>a</i>O_<i>x</i> Catalyst for Aldol Condensation of Citral and Acetone

The liquid phase synthesis of pseudoionones (PS) by the cross-aldol condensation of citral and acetone was investigated over MgAl mixed oxides containing rare earth elements (REE = Y, La, Eu), which were obtained from corresponding REE-modified hydrotalcite materials after calcination. The results showed that the unmodified and La­(Eu)-modified MgAl mixed oxide catalysts showed relatively low activity, and Y-modified MgAl mixed oxides presented an unexpected high catalytic activity. PS selectivity of ∼85% and citral conversion of 100% were achieved at 60 °C for 3 h. On the basis of the characterizations of the structural, textural, and basic properties, it was found that Mg₃Al_1‑aY_aO_<i>x</i> catalysts exhibited relatively well-developed small flake morphology with high surface area and pore volume, resulting in exposure of more basic sites on the catalyst surface. The formation of PS over Mg₃Al_1‑aY_aO_<i>x</i> may be accompanied by gradual modification of the catalyst surface to form re-Mg₃Al_1‑aY_aO_<i>x</i> through a rehydration process with produced water, which reconverts the O^2– basic sites to OH^– basic groups. Unlike La and Eu elements, the presence of Y could promote this “one-pot” or <i>in situ</i> rehydration process of MgAl mixed oxides during the aldol reaction. This Y-modified MgAl mixed oxides after a one-pot rehydration process with active Brønsted basic sites is responsible for the high activity in the cross-aldol condensation of citral and acetone

A Highly Effective Catalyst of Sm-MnO_<i>x</i> for the NH₃‑SCR of NO_<i>x</i> at Low Temperature: Promotional Role of Sm and Its Catalytic Performance

Sm-Mn mixed oxide catalysts prepared by the coprecipitation method were developed, and their catalytic activities were tested for the selective catalytic reduction (SCR) of NO with ammonia at low temperature. The results showed that the amount of Sm markedly influenced the activity of the MnO_<i>x</i> catalyst for SCR, that the activity of the Sm-Mn mixed oxide catalyst exhibited a volcano-type tendency with an increase in the Sm content, and that the appropriate mole ratio of Sm to Mn in the catalyst was 0.1. In addition, the presence of Sm in the MnO_<i>x</i> catalyst can obviously enhance both water and sulfur dioxide resistances. The effect of Sm on the physiochemical properties of the Sm-MnO_<i>x</i> catalyst were investigated by XRD, low-temperature N₂ adsorption, XPS, and FE-SEM techniques. The results showed that the presence of Sm in the Sm-MnO_<i>x</i> catalyst can restrain the crystallization of MnO_<i>x</i> and increase its surface area and the relative content of both Mn⁴⁺ and surface oxygen (O_S) on the surface of the Sm-MnO_<i>x</i> catalyst. NH₃-TPD, NO-TPD, and in situ DRIFT techniques were used to investigate the absorption of NH₃ and NO on the Sm-MnO_<i>x</i> catalyst and their surface reactions. The results revealed that the presence of Sm in the Sm_0.1-MnO_<i>x</i> catalyst can increase the absorption amount of NH₃ and NO on the catalyst and does not vary the SCR reaction mechanism over the MnO_<i>x</i> catalyst: that is, the coexistence of Eley–Rideal and Langmuir–Hinshelwood mechanisms (bidentate nitrate is the active intermediate), in which the Eley–Rideal mechanism is predominant

Effect of Ceria Crystal Plane on the Physicochemical and Catalytic Properties of Pd/Ceria for CO and Propane Oxidation

Ceria nanocrystallites with different morphologies and crystal planes were hydrothermally prepared, and the effects of ceria supports on the physicochemical and catalytic properties of Pd/CeO₂ for the CO and propane oxidation were examined. The results showed that the structure and chemical state of Pd on ceria were affected by ceria crystal planes. The Pd species on CeO₂-R (rods) and CeO₂-C (cubes) mainly formed Pd_<i>x</i>Ce_1–<i>x</i>O_2−σ solid solution with −Pd²⁺–O^2––Ce⁴⁺– linkage. In addition, the PdO_<i>x</i> nanoparticles were dominated on the surface of Pd/CeO₂-O (octahedrons). For the CO oxidation, the Pd/CeO₂-R catalyst showed the highest catalytic activity among three catalysts, its reaction rate reached 2.07 × 10^–4 mol g_Pd^–1 s^–1 at 50 °C, in which CeO₂-R mainly exposed the (110) and (100) facets with low oxygen vacancy formation energy, strong reducibility, and high surface oxygen mobility. TOF of Pd/CeO₂-R (3.78 × 10^–2 s^–1) was much higher than that of Pd/CeO₂-C (6.40 × 10^–3 s^–1) and Pd/CeO₂-O (1.24 × 10^–3 s^–1) at 50 °C, and its activation energy (<i>E</i>_a) was 40.4 kJ/mol. For propane oxidation, the highest reaction rate (8.08 × 10^–5 mol g_Pd^–1 s^–1 at 300 °C) was obtained over the Pd/CeO₂-O catalyst, in which CeO₂-O mainly exposed the (111) facet. There are strong surface Ce–O bonds on the ceria (111) facet, which favors the existence of PdO particles and propane activation. The turnover frequency (TOF) of the Pd/CeO₂-O catalyst was highest (3.52 × 10^–2 s^–1) at 300 °C and its <i>E</i>_a value was 49.1 kJ/mol. These results demonstrate the inverse facet sensitivity of ceria for the CO and propane oxidation over Pd/ceria

Total Oxidation of Propane over a Ru/CeO₂ Catalyst at Low Temperature

Ruthenium (Ru) nanoparticles (∼3 nm) with mass loading ranging from 1.5 to 3.2 wt % are supported on a reducible substrate, cerium dioxide (CeO₂, the resultant sample is called Ru/CeO₂), for application in the catalytic combustion of propane. Because of the unique electronic configuration of CeO₂, a strong metal–support interaction is generated between the Ru nanoparticles and CeO₂ to stabilize Ru nanoparticles for oxidation reactions well. In addition, the CeO₂ host with high oxygen storage capacity can provide an abundance of active oxygen for redox reactions and thus greatly increases the rates of oxidation reactions or even modifies the redox steps. As a result of such advantages, a remarkably high performance in the total oxidation of propane at low temperature is achieved on Ru/CeO₂. This work exemplifies a promising strategy for developing robust supported catalysts for short-chain volatile organic compound removal

Highly Efficient Oxidation of Propane at Low Temperature over a Pt-Based Catalyst by Optimization Support

Pt-based catalysts have attracted widespread attention in environmental protection applications, especially in the catalytic destruction of light alkane pollutants. However, developing a satisfying platinum catalyst with high activity, excellent water-resistance, and practical suitability for hydrocarbon combustion at low temperature is challenging. In this study, the Pt catalyst supported on the selected Nb2O5 oxide exhibited an efficient catalytic activity in propane oxidation and exceeded that of most catalysts reported in the literature. More importantly, the Pt/Nb2O5 catalyst maintained excellent activity and durability even after high-temperature aging at 700 °C and under harsh working conditions, such as a certain degree of moisture, high space velocity, and composite pollutants. The excellent performance of the Pt/Nb2O5 catalyst was attributed to the abundant metallic Pt species stabilized on the surface of Nb2O5, which prompted the C–H bond dissociation ability as the rate-determining step. Furthermore, propane was initially activated via oxidehydrogenation and followed the acrylate species path as a more efficient propane oxidation path on the Pt/Nb2O5 surface. Overall, Pt/Nb2O5 can be considered a promising catalyst for the catalytic oxidation of alkanes from industrial sources and could provide inspiration for designing superb catalysts for the oxidation of light alkanes

Incorporating Rich Mesoporosity into a Ceria-Based Catalyst via Mechanochemistry

Ceria-based materials possessing mesoporous structures afford higher activity than the corresponding bulk materials in CO oxidation and other catalytic applications, because of the wide pore channel and high surface area. The development of a direct, template-free, and scalable technology for directing porosity inside ceria-based materials is highly welcome. Herein, a family of mesoporous transition-metal-doped ceria catalysts with specific surface areas up to 122 m² g^–1 is constructed by mechanochemical grinding. No templates, additives, or solvents are needed in this process, while the mechanochemistry-mediated restructuring and the decomposing of the organic group led to plentiful mesopores. Interestingly, the copper species are evenly dispersed in the ceria matrix at the atomic scale, as observed in high resolution scanning transmission electron microscopy in high angle annular dark field. The copper-doped ceria materials show good activity in the CO oxidation

Low-Temperature Methane Combustion over Pd/H-ZSM-5: Active Pd Sites with Specific Electronic Properties Modulated by Acidic Sites of H‑ZSM‑5

Pd/H-ZSM-5 catalysts could completely catalyze CH₄ to CO₂ at as low as 320 °C, while there is no detectable catalytic activity for pure H-ZSM-5 at 320 °C and only a conversion of 40% could be obtained at 500 °C over pure H-ZSM-5. Both the theoretical and experimental results prove that surface acidic sites could facilitate the formation of active metal species as the anchoring sites, which could further modify the electronic and coordination structure of metal species. PdO_<i>x</i> interacting with the surface Brönsted acid sites of H-ZSM-5 could exhibit Lewis acidity and lower oxidation states, as proven by the XPS, XPS valence band, CO-DRIFTS, pyridine FT-IR, and NH₃-TPD data. Density functional theory calculations suggest PdO_<i>x</i> groups to be the active sites for methane combustion, in the form of [AlO₂]­Pd­(OH)-ZSM-5. The stronger Lewis acidity of coordinatively unsaturated Pd and the stronger basicity of oxygen from anchored PdO_<i>x</i> species are two key characteristics of the active sites ([AlO₂]­Pd­(OH)-ZSM-5) for methane combustion. As a result, the PdO_<i>x</i> species anchored by Brønsted acid sites of H-ZSM-5 exhibit high performance for catalytic combustion of CH₄ over Pd/H-ZSM-5 catalysts

Revealing the Size Effect of Ceria Nanocube-Supported Platinum Nanoparticles in Complete Propane Oxidation

The elimination of propane is one of the key tasks in reducing volatile organic compounds (VOCs) and automotive exhaust emissions. The platinum nanoparticle (NP) is a promising catalyst for propane oxidation, while the study of its structural characteristics and functionality remains in its infancy. In this work, we synthesized the nanocubes CeO2 with a well-defined (100) facet supporting Pt NPs with various sizes, from 1.3 to 7 nm, and systematically investigated the effect of the Pt size on complete propane oxidation efficiency. In particular, CeO2(100) supported Pt NPs smaller than 4 nm promote the formation of positively charged Pt sites, which hinder the adsorption and activation of propane and reduce the intrinsic activity for propane oxidation. Consequently, within this size range, the catalytic performance is primarily influenced by the electronic state of the Pt species, with metallic Pt being identified as the active site for the reaction. Conversely, as the particle size exceeds 4 nm, metallic Pt particles become dominant and the geometric structure starts to influence the activity as well. Such entanglement of electronic and geometric factors gives rise to a volcano relationship between reaction rates and Pt particle sizes ranging from 1.3 to 7 nm, while an increased correlation can be observed between the turnover frequencies and the particle sizes in this range. This knowledge can guide the synthesis of highly active catalysts, enabling the efficient oxidation of VOCs with reduced precious metal loadings

Shape-Controlled CeO₂ Nanoparticles: Stability and Activity in the Catalyzed HCl Oxidation Reaction

CeO₂ is a promising catalyst for the HCl oxidation (Deacon process) in order to recover Cl₂. Employing shape-controlled CeO₂ nanoparticles (cubes, octahedrons, rods) with facets of preferential orientations ((100), (111), (110)), we studied the activity and stability under two reaction conditions (harsh: Ar:HCl:O₂ = 6:2:2 and mild: Ar:HCl:O₂ = 7:1:2). It turns out that both activity and stability are structure-sensitive. In terms of space time yield (STY), the rods are the most active particles, followed by the cubes and finally the octahedrons. This very same trend is reconciled with the complete oxygen storage capacity (OSCc), indicating a correlation between the observed activity STY and the OSCc. The apparent activation energies are about 50 kJ/mol for cubes and rods, while the octahedrons reveal an apparent activation energy of 65 kJ/mol. The reaction order in O₂ is positive (0.26–0.32). Under mild reaction conditions, all three morphologies are stable, consistent with corresponding studies of CeO₂ powders and CeO₂ nanofibers. Under harsh reaction conditions, however, cubes and octahedrons are both instable, forming hydrated CeCl₃, while rods are still stable. The present stability and activity experiments in the catalytic HCl oxidation reaction over shape-controlled CeO₂ nanoparticles may serve as benchmarks for future ab initio studies of the catalyzed HCl oxidation reaction over well-defined CeO₂ surfaces