Mesoporous Silica-Supported Nanostructured PdO/CeO2 Catalysts for Low-Temperature Methane Oxidation

Direct Catalytic Oxidation of Low-Concentration Methane to Methanol in One Step on Ni-Promoted BiOCl Catalysts

The direct oxidation of low-concentration methane (CH₄) to methanol (CH₃OH) is often regarded as the "holy grail". However, it still is very difficult and challenging to oxidize methane to methanol in one step. In this work, we present a new approach to directly oxidize CH₄ to generate CH₃OH in one step by doping non-noble metal Ni sites on bismuth oxychloride (BiOCl) equipped with high oxygen vacancies. Thereinto, the conversion rate of CH₃OH can reach 39.07 μmol/(g_cat·h) under 420 °C and flow conditions on the basis of O₂ and H₂O. The crystal morphology structure, physicochemical properties, metal dispersion, and surface adsorption capacity of Ni-BiOCl were explored, and the positive effect on the oxygen vacancy of the catalyst was proved, thus improving the catalytic performance. Furthermore, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was also performed to study the surface adsorption and reaction process of methane to methanol in one step. Results demonstrate that the key to keep good activity lies in the oxygen vacancies of unsaturated Bi atoms, which can adsorb and active CH₄ and to produce methyl groups and adsorbing hydroxyl groups in methane oxidation process. This study broadens the application of oxygen-deficient catalysts in the catalytic conversion of CH₄ to CH₃OH in one step, which provides a new perspective on the role of oxygen vacancies in improving the catalytic performance of methane oxidation.

Current Progress on Methods and Technologies for Catalytic Methane Activation at Low Temperatures

Methane (CH₄ ) is an attractive energy source and important greenhouse gas. Therefore, from the economic and environmental point of view, scientists are working hard to activate and convert CH₄ into various products or less harmful gas at low-temperature. Although the inert nature of CH bonds requires high dissociation energy at high temperatures, the efforts of researchers have demonstrated the feasibility of catalysts to activate CH₄ at low temperatures. In this review, the efficient catalysts designed to reduce the CH₄ oxidation temperature and improve conversion efficiencies are described. First, noble metals and transition metal-based catalysts are summarized for activating CH₄ in temperatures ranging from 50 to 500 °C. After that, the partial oxidation of CH₄ at relatively low temperatures, including thermocatalysis in the liquid phase, photocatalysis, electrocatalysis, and nonthermal plasma technologies, is briefly discussed. Finally, the challenges and perspectives are presented to provide a systematic guideline for designing and synthesizing the highly efficient catalysts in the complete/partial oxidation of CH₄ at low temperatures.

Oxidative Addition of Methane and Reductive Elimination of Ethane and Hydrogen on Surfaces: From Pure Metals to Single Atom Alloys

Oxidative addition of CH₄ to the catalyst surface produces CH₃ and H. If the CH₃ species generated on the surface couple with each other, reductive elimination of C₂H₆ may be achieved. Similarly, H's could couple to form H₂. This is the outline of nonoxidative coupling of methane (NOCM). It is difficult to achieve this reaction on a typical Pt catalyst surface. This is because methane is overoxidized and coking occurs. In this study, the authors approach this problem from a molecular aspect, relying on organometallic or complex chemistry concepts. Diagrams obtained by extending the concepts of the Walsh diagram to surface reactions are used extensively. C-H bond activation, i.e., oxidative addition, and C-C and H-H bond formation, i.e., reductive elimination, on metal catalyst surfaces are thoroughly discussed from the point of view of orbital theory. The density functional theory method for structural optimization and accurate energy calculations and the extended Hückel method for detailed analysis of crystal orbital changes and interactions play complementary roles. Limitations of monometallic catalysts are noted. Therefore, a rational design of single atom alloy (SAA) catalysts is attempted. As a result, the effectiveness of the Pt₁/Au(111) SAA catalyst for NOCM is theoretically proposed. On such an SAA surface, one would expect to find a single Pt monatomic site in a sea of inert Au atoms. This is desirable for both inhibiting overoxidation and promoting reductive elimination.

Optimizing the Microstructure of SnO2-CeO2 Binary Oxide Supported Palladium Catalysts for Efficient and Stable Methane Combustion

The preparation of palladium-based catalysts with both high catalytic activity and hydrothermal stability currently appears as a critical topic in methane combustion. Herein, we propose a facile strategy to boost the performance of SnO₂-CeO₂ binary oxide supported palladium catalysts by tuning the composition of supports. The coexistence of SnO₂ and CeO₂ phases in an appropriate ratio is favorable for the formation of both Pd_xCe_1-xO_2-δ and Pd_xSn_1-xO_2-δ solid solutions due to the reduced crystallite size. This unique microstructure could enhance the metal-support interaction to stabilize the active PdO phase and promote its reoxidation, meanwhile generating more oxygen vacancies to improve the reducibility of PdO. On account of the facilitated conversion of PdO ↔ Pd, coupled with the low-temperature dissociation of methane promoted by abundant active oxygen species, the Pd/5Sn5Ce catalyst exhibits a superior catalytic activity with a T₉₉ of ca. 360 °C, a robust stability under both dry and wet conditions, and an excellent thermal stability during heating-cooling light-off tests.

Nonoxidative Coupling of Methane over Ceria-Supported Single-Atom Pt Catalysts in DBD Plasma

Plasma-catalytic direct nonoxidative coupling of methane (NCM) into C₂ hydrocarbons was investigated over ceria-supported atomically dispersed Pt (Pt/CeO₂-SAC) and nanoparticle Pt (Pt/CeO₂-NP) catalysts in dielectric barrier discharge (DBD) plasma. Nonthermal plasma facilitated C-H bond dissociation in CH₄ at low temperatures (<150 °C) and atmospheric pressure. The presence of Pt/CeO₂ catalysts in plasma further enhanced CH₄ conversion and C₂ hydrocarbon selectivity by enabling the conversion of vibrationally excited methane species with high internal energy on active Pt sites. Noticeably, the Pt/CeO₂-SAC catalyst displayed a more remarkable performance, with a CH₄ conversion of 39% and a C₂ selectivity of 54% at 54 W. The enhanced CH₄ conversion was attributed to abundant coordinatively unsaturated Pt sites in Pt/CeO₂-SAC, which were more active for C-H bond scission. Meanwhile, isolated Pt atoms in Pt/CeO₂-SAC promoted C₂ hydrocarbon formation by hindering the unselective formation of coke from deep dehydrogenation of CH_x^• intermediates and higher hydrocarbons from oligomerization reactions.

Towards highly active heterogeneous catalysts via a sequential noncovalent bonding strategy

The synthesis strategy of Cu/CeO2 and Pd–Cu/CeO2 nanocatalysts.

Yb/Chitosan Catalyzed Synthesis of Highly Substituted Piperidine Derivatives for Potential Nuclease Activity and DNA Binding Study

BACKGROUND

Herein, a new chitosan-supported ytterbium nano-catalyst has been prepared and used in a mild, efficient, and expeditious method for the synthesis of substituted piperidine derivatives via threecomponent condensation of substituted anilines, formaldehyde and different cyclic/acyclic active methylene compounds at room temperature.

METHODS

The catalyst was characterized by FTIR, XRD, SEM, EDX, TEM, ICP-AES and the stability of the catalyst was evaluated by TG analysis. The synthesized compound 3,3,11,11-Tetramethyl-15-(phenyl)-15- azadispiro[5.1.5.3]hexadecane-1,5,9,13-tetrone (3a) was explored for pBR322 DNA cleavage activity and genotoxicity. Further, the interaction of 3a with CT-DNA was investigated through UV-vis, fluorescence and viscosity.

RESULTS

The preparation of Yb/chitosan nano-catalyst was verified and the catalyst was found effective towards substituted piperidine formations with the catalyst reusability. Compound 3a was successfully tested for DNA cleavage activity. In addition, fluorescence results revealed that compound 3a interacted with DNA with a binding affinity of 4.84 x 10⁴ M^-1.

CONCLUSION

Our findings suggest that compounds bearing spiro-piperidine scaffold, synthesized using reusable nano-catalyst, could be effective biological agents.

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.

Exceptionally stable sol-immobilization derived Pd/SBA-15 catalysts for methane combustion

Pd/SBA-15-SI catalysts are more efficient for methane combustion than Pd/SBA-15-IWI catalysts due to anti-sintering property of the catalysts.

Facile Strategy to Extend Stability of Simple Component-Alumina-Supported Palladium Catalysts for Efficient Methane Combustion

It is of practical importance to develop a stable and accessible methane combustion catalyst which could retain an excellent activity under drastic conditions. Herein, we introduce a facile approach to extend the stability of conventional Pd/Al₂O₃ catalysts through tailoring the pore size of mesoporous aluminas (MAs) and the interaction between Pd and Al. By modulating the addition of templates (deoxycholic acid and polyvinylpyrrolidone), a series of MAs with tunable and uniform pore size were obtained through a designed sol-gel method. Unexpectedly, Pd/MA-800-5 catalyst prepared with relatively large pore size (ca. 12 nm) MAs exhibited an efficient and sustained performance under a variety of operating conditions, while those prepared with small pore size (ca. 5-7 nm) MAs suffered from a significant loss of activity during high temperature cyclic reactions (280-850 °C) due to the decomposition of confined PdO. The enhancement could be attributed to the suitable particle size, higher crystallinity, generated active sites, improved reducibility, and thermal stability of PdO species. Moreover, the variation of pore size also resulted in a different reaction mechanism. Such a pore size promotion strategy effectively invoked a superior catalytic performance while keeping the catalyst components simple, which can be extended to prepare other high-performance metal oxide-supported catalysts for catalytic applications.

Alloying Effect of Silver on Zirconia Support Manipulated Palladium Catalyst for Methane Combustion

PdAg/ZrO2 alloy catalysts calcined at different temperatures were employed to elucidate the effect of support-metal interaction (SMI) on methane combustion. Combustion activity was depressed when the sample was calcined at an elevated temperature from 500 °C to 700 °C. However, calcination at 850 °C enhanced the beneficial SMI, which facilitated a more active phase for the oxidation reaction. The high-resolution transmission electron microscopy experiments show that a special micro-domain structure at the interface is formed during the reduction pretreatment. H2-TPR and O2-TPD measurements illustrate that the active phase would undergo reconstruction upon redox cycles. The active phase manipulated by the support is more suitable for combustion reaction in the course of temperature altering.

Metal-organic framework templated Pd/CeO2@N-doped carbon for low-temperature CO oxidation

A new Pd/Ce based metal-organic framework is designed and synthesized as a self-sacrificial template for fabrication of an efficient catalyst for CO oxidation. The catalyst obtained by thermal annealing at 700 °C (Pd/CeO₂@NC-700) is composed of N-doped carbon with embedded Pd and CeO₂ nanoparticles, which are highly dispersed and closely connected in the N-doped carbon; the high Pd loading (33.7 wt%) and the coupling between Pd and the CeO₂ phase synergistically boost the CO oxidation performance. The Pd/CeO₂@NC-700 catalyst exhibits a 100% conversion temperature of 89 °C and excellent long-term stability. By combining structural characterization with density functional theory calculations, two possible CO oxidation pathways of TPB and TOP are revealed, in which the adsorbed O₂ directly dissociates to O* atoms and activates CO* molecules. The transfer of O* between Pd and Ce (TPB) or Pd and Pd (TOP) facilitates the formation of intermediates and finally results in the production of CO₂. This work provides a new insight into the development of novel efficient catalysts for CO oxidation based on metal-organic frameworks.

MnOx dispersed on attapulgite derived Al-SBA-15: a promising catalyst for volatile organic compound combustion

To improve the catalytic activity when utilizing metal oxides for VOCs combustion, Mn/Al-SBA-15 catalysts have been synthesized through a wetness impregnation technique involving Mn(NO₃)₂ on Al-SBA-15, hydrothermally prepared from attapulgite.

Size effects of Pd nanoparticles supported over CeZrPAl for methane oxidation

Pd nanoparticles accompanied with distorted morphology result in considerable active sites and enhance the intrinsic activity for catalytic methane oxidation.

Cooperative Catalysis of Methane Oxidation through Modulating the Stabilization of PdO and Electronic Properties over Ti-Doped Alumina-Supported Palladium Catalysts

Poor low-temperature catalytic activity and durability are the main drawbacks of palladium-based catalysts for methane combustion. Herein, stable and active PdO particles are constructed by incorporating Ti into an alumina support, which makes the catalysts exhibit satisfactory methane combustion activity. The results of comprehensive characterization reveal that an appropriate amount of Ti doping induces the optimization of electron transfer and distribution, thus contributing to the construction and stabilization of active PdO lattices. The reactive oxygen mobility is improved and the optimal PdO/Pd⁰ combination is achieved, thanks to the amplified PdO-support interaction. In addition, the acid-base properties are regulated and Brønsted acid sites are generated by virtue of the adjustment of electronic properties, which facilitate stabilization of PdO as well. Hence, the Ti-containing catalyst exhibits superior activity for methane oxidation at low temperatures. Notably, the activity and cyclic performance of the catalyst can be further enhanced when undergoing long-term and isothermal heat treatment under the reactant stream and methane, and it demonstrates a high performance with 90% CH₄ conversion at 340 °C.

Rod-Like Nanoporous CeO2 Modified by PdO Nanoparticles for CO Oxidation and Methane Combustion with High Catalytic Activity and Water Resistance

A PdO/CeO2 composite with a rod-like nanoporous skeletal structure was prepared by combining the dealloying of Al-Ce-Pd alloy ribbons with calcination. For CO oxidation and CH4 combustion, the nanoporous PdO/CeO2 composite exhibits excellent catalytic activity, and the complete reaction temperatures of CO and CH4 are 80 °C and 380 °C, respectively. In addition, the composite possesses excellent cycle stability, CO2 toxicity, and water resistance, and the catalytic activity hardly decreases after 100 h of long-term stability testing in the presence of water vapour (2 × 105 ppm). The results of a series of characterizations indicate that the enhanced catalytic activity can be attributed to the good dispersion of the PdO nanoparticles, large specific surface area, strong redox capacity, interaction between PdO and CeO2, and more surface active oxygen on PdO. The results of the characterization and experiments also indicate that the PdO nanoparticles, prepared by combining dealloying and calcination, have a stronger catalytic activity than do Pd nanoparticles. Finally, a simple model is used to summarize the catalytic mechanism of the PdO/CeO2 composite. It is hoped that this work will provide insights into the development of high-activity catalysts.

Enhanced CH4 and CO Oxidation over Ce1- xFe xO2-δ Hybrid Catalysts by Tuning the Lattice Distortion and the State of Surface Iron Species

CeO₂-Fe₂O₃ mixed oxides are very attractive as catalysts for catalytic oxidation. Herein, we report the structural dependence of the Ce_{1- x}Fe _xO_2-δ catalysts for CH₄ combustion and CO oxidation via changing lattice distortion degrees, surface Fe₂O₃ states, and oxygen vacancy concentrations. The lattice distortion degree and oxygen vacancy concentration of Ce-Fe-O solid solution can be tuned by changing the contents of Fe and the precipitation temperatures in the preparation process. The precipitation at relatively high temperature (70 °C) promotes the lattice distortion, whereas a lower temperature (0 °C) helps the formation of surface oxygen vacancies. The in situ diffuse reflectance infrared/Raman experiments and the physicochemical characterization suggest that both the CO and CH₄ oxidations mainly follow a Mars-van Krevelen mechanism. Both the lattice distortion and the surface iron species play a crucial role in determining the catalytic activity by affecting the redox property of the catalysts. The surface iron species, combined with the oxygen vacancies, improve the catalytic performance by enhancing the adsorption capacity of reactants and reducibility of catalysts. The lattice distortion of CeO₂ contributes to the catalytic activity by tuning the oxygen mobility in the bulk, which promotes the re-oxidation rate of catalysts.

In situ DRIFT spectroscopy insights into the reaction mechanism of CO and toluene co-oxidation over Pt-based catalysts

The Pt–CeO₂ catalyst with adsorption sites and oxygen-rich vacancies exhibited outstanding activity towards CO and toluene co-oxidation.

Natural clay-supported palladium catalysts for methane oxidation reaction: effect of alloying

Bimetallic Pd-supported halloysite nanotubes revealed outstanding catalytic activity towards catalytic methane oxidation especially PdNi.

Synthesis of a Highly Stable Pd@CeO2 Catalyst for Methane Combustion with the Synergistic Effect of Urea and Citric Acid

Making use of synergy between urea and citric acid, a core-shell Pd@CeO₂ catalyst with spherical morphology was facilely synthesized by a hydrothermal method. The formation mechanism of the core-shell structure in the presence of citric acid and hydrogen peroxide was studied. Results showed that the Pd@CeO₂ catalyst exhibited high catalytic activity in methane oxidation. Pd nanoparticles were well stabilized by CeO₂ shell encapsulation, resulting in high stability of the catalyst. A high CH₄ conversion of 99% was retained after 50 h on-stream reaction at 500 °C. Additionally, many tiny pores on the CeO₂ shell surface were beneficial for the full contact between reactants and active components. Pd nanoparticles were highly dispersed inside the shell, improving the utilization efficiency of active components. The results also demonstrated that the Pd species in the catalyst existed in the form of oxidation state, mainly in PdO (ca. 66.6%), which played an essential part in methane combustion.

Crystal plane dependent dopant migration that boosts catalytic oxidation

CeO₂ rods with {110} facets and cubes with {100} facets were utilized as catalyst supports to probe the effect of crystallographic facets on the iron species and the structure-dependent catalytic performance.