Catalytic Activity and Stability over Nanorod-Like Ordered Mesoporous Phosphorus-Doped Alumina Supported Palladium Catalysts for Methane Combustion

Redox dynamics and surface structures of an active palladium catalyst during methane oxidation

Catalysts based on palladium are among the most effective in the complete oxidation of methane. Despite extensive studies and notable advances, the nature of their catalytically active species and conceivable structural dynamics remains only partially understood. Here, we combine operando transmission electron microscopy (TEM) with near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory (DFT) calculations to investigate the active state and catalytic function of Pd nanoparticles (NPs) under methane oxidation conditions. We show that the particle size, phase composition and dynamics respond appreciably to changes in the gas-phase chemical potential. In combination with mass spectrometry (MS) conducted simultaneously with in situ observations, we uncover that the catalytically active state exhibits phase coexistence and oscillatory phase transitions between Pd and PdO. Aided by DFT calculations, we provide a rationale for the observed redox dynamics and demonstrate that the emergence of catalytic activity is related to the dynamic interplay between coexisting phases, with the resulting strained PdO having more favorable energetics for methane oxidation.

Reversal Effect of Phosphorus on Catalytic Performances of Supported Nickel Catalysts in Reductive Amination of 1,6-Hexanediol

The reductive amination of 1,6-hexanediol with ammonia is one of the most promising green routes for synthesis of 1,6-hexanediamine. Herein, we developed a phosphorous modified Ni catalyst of Ni-P/Al2O3. It presented satisfactory improved selectivity to 1,6-hexanediamine in the reductive amination of 1,6-hexanediol compared to the Ni/Al2O3 catalyst. The phosphorous tended to interact with Al2O3 to form AlPOx species, induced Ni nanoparticle to be flatter, and the decrease of strong acid sites, the new-formed Ni-AlPOx-Al2O3 interface and the flatter Ni nanoparticle were the key to switch the dominating product from hexamethyleneimine to 1,6-hexanediamine. This work develops an efficient catalyst for production of 1,6-hexanediamine from the reductive amination of 1,6-hexanediol, and provides a point of view about designing selective non-noble metal catalysts for producing primary diamines via reductive amination of diols.

Support Engineering for the Stabilisation of Heterogeneous Pd3 P-Based Catalysts for Heck Coupling Reactions

Herein we report the use of a supported Pd3 P catalyst for Heck coupling reactions. For the stabilisation of Pd3 P and Pd, as reference system, the silica support material was modified via phosphorus doping (0.5 and 1 wt % P). Through this so-called support engineering approach, the catalytic activity of Pd3 P was clearly enhanced. Whereas an iodobenzene conversion of 79 % was witnessed for Pd3 P@SiO2 in the coupling of styrene and iodobenzene in 1 h, 90 % conversion could be achieved using Pd3 P@1P-SiO2 . This improved catalytic activity probably stems from an electronic modulation of the support surface via the introduction of phosphorus. Simultaneously, the recyclability was boosted and the Pd3 P@1P-SiO2 catalyst has shown to maintain its catalytic activity over several recovery tests. Hereby, metal leaching could almost be suppressed completely to 3 % by the use of a P-modified silica support.

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.

Pd-Nanoparticles@Layered Double Hydroxide/ Reduced Graphene Oxide (Pd NPs@LDH/rGO) Nanocomposite Catalyst for Highly Efficient Green Reduction of Aromatic Nitro Compounds

A facile chemical method is developed to fabricate well-dispersed and an approx. 5 nm sized Pd-nanoparticles (Pd-NPs) deposited ZnAl-LDH/rGO nanocomposite (Pd NPs@LDH/rGO) as a highly efficient and stable catalyst for...

Study on the Synthesis of High-Purity γ-Phase Mesoporous Alumina with Excellent CO2 Adsorption Performance via a Simple Method Using Industrial Aluminum Oxide as Raw Material

To mitigate the global greenhouse effect and the waste of carbon dioxide, a chemical raw material, high-purity γ-phase mesoporous alumina (MA) with excellent CO₂ adsorption performance was synthesized by the direct aging method and ammonium salt substitution method. With this process, not only can energy consumption and time be shortened to a large extent but the final waste can also be recycled to the mother liquor by adding calcium hydroxide. Reaction conditions, i.e., pH value, calcination temperature, and desodium agent, were investigated in detail with the aid of X-ray fluorescence spectrum (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) and Barret-Joyner-Hallender (BJH) methods, nonlocal density functional theory (NLDFT), transmission electron microscopy (TEM), temperature-programmed desorption of CO₂ (CO₂-TPD), and presented CO₂ adsorption measurement. The results of this study are summarized as follows: the impurity content of the MA synthesized under optimal conditions is less than 0.01%, and its total removal rate of impurities is 99.299%. It was found that the MA adsorbent has a large specific surface area of 377.8 m²/g, pore volume of 0.55 cm³/g, and its average pore diameter is 3.1 nm. Under the condition of a gas flow rate of 20 cm³/min, its CO₂ adsorption capacity is 1.58 mmol/g, and after 8 times of cyclic adsorption, the amount of CO₂ adsorption remained basically unchanged, both of which indicate that the material has excellent adsorption properties and can be widely used for the adsorption of carbon dioxide.

Construction and evolution of active palladium species on phase-regulated reducible TiO2 for methane combustion

Non-traditional amorphous Pd²⁺ species on the surface of Pd/TiO₂ catalysts facilitate CH₄ combustion, while formed Pd_xTi_1−xO₂ would be detrimental.

Nb2O5 as a radical modulator during oxidative dehydrogenation and as a Lewis acid promoter in CO2 assisted dehydrogenation of octane over confined 2D engineered NiO–Nb2O5–Al2O3

Ordered mesoporous 2D NiO–Nb₂O₅–Al₂O₃ nano-composites were used for CO₂ assisted dehydrogenation of n-octane; and the close proximity of Ni and Nb₂O₅ in the optimised catalyst promoted CO₂ dissociation and substantially prolonged alkane activation.

Engineering multicomponent metal-oxide units for efficient methane combustion over palladium-based catalysts

A composition modulation strategy was exploited to rationally design high-performance Mg-promoted Pd/Ce_xZr_1−xO₂–Al₂O₃ catalysts for methane combustion.

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.

A Hydrothermally Stable Irreducible Oxide-Modified Pd/MgAl2 O4 Catalyst for Methane Combustion

Catalytic combustion is promising in removing trace amounts of CH₄ to address serious environmental concerns. Supported Pd-based catalysts are most effective but often suffer from low stability in applications owing to the water-vapor-induced sintering. Herein, we develop a universal strategy to prepare irreducible-oxide-modified Pd/MgAl₂ O₄ catalysts which show high activity and excellent stability against both hydrothemal aging at elevated temperatures and deactivation in long-term reaction under wet conditions. The addition of irreducible oxides inhibited the deep oxidation of Pd in the oxygen-rich conditions, which preserved not only the epitaxial structure but also a suitable active phase of Pd-PdO_x on MgAl₂ O₄ , thus promoting both activity and stability. This work provides new insights into the effect of metal-oxide interaction on CH₄ combustion and offers an avenue to design hydrothermally stable and active combustion catalysts for industrial applications.

The Underlying Chemistry to the Formation of PO2 Radicals from Organophosphorus Compounds: A Missing Puzzle Piece in Flame Chemistry

Reactive species, such as ^. PO₂ and HOPO, are considered of upmost importance in flame inhibition and catalytic combustion processes of fuels. However, the underlying chemistry of their formation remains speculative due to the unavailability of suitable analytical techniques that can be used to identify the transient species which lead to their formation. This study elucidates the reaction mechanisms of the formation of phosphoryl species from dimethyl methyl phosphonate (DMMP) and dimethyl methyl phosphoramidate (DMPR) under well-defined oxidative conditions. Photoelectron photoion coincidence techniques that utilized vacuum ultraviolet synchrotron radiation were applied to isomer-selectively detect the elusive key intermediates and stable products. With the help of in situ recorded spectral fingerprints, different transient species, such as PO₂ and triplet O radicals, have been exclusively identified from their isomeric components, which has helped to piece together the formation mechanisms of phosphoryl species under various conditions. It was found that ^. PO₂ formation required oxidative conditions above 1070 K. The combined presence of O₂ and H₂ led to significant changes in the decomposition chemistry of both model phosphorus compounds, leading to the formation of ^. PO₂ . The reaction ^. PO+O₂ →^. PO₂ +O: was identified as the key step in the formation of ^. PO₂ . Interestingly, the presence of O₂ in DMPR thermolysis suppresses the formation of PN-containing species. In a previous study, PN species were identified as the major species formed during the pyrolysis of DMPR. Thus, the findings of this study has shed light onto the decomposition pathways of organophosphorus compounds, which are beneficial for their fuel additive and fire suppressant applications.

Facile construction of ultrastable alumina anchored palladium catalysts via a designed one pot strategy for enhanced methane oxidation

A highly stable Pd–Al₂O₃ catalyst with anchored palladium species was facilely prepared through a one pot strategy for efficient methane oxidation.

Oxygen exchange in Bi2MoO6 nanosheets with different thicknesses during oxidative dehydrogenation of 1-butene

A series of Bi₂MoO₆ nanosheet catalysts with different thicknesses were synthesized for oxidative dehydrogenation (ODH) of 1-butene and the oxygen exchange process during ODH was clarified.

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.

Dehydrogenation of 2-[(n-Methylcyclohexyl)Methyl]Piperidine over Mesoporous Pd-Al2O3 Catalysts Prepared by Solvent Deficient Precipitation: Influence of Calcination Conditions

A pair of 2-[(n-methylcyclohexyl)methyl]piperidine (H12-MBP) and its full dehydrogenation product (H0-MBP) has recently been considered as a potential liquid organic hydrogen carrier with 6.15 wt% H2 storage capacity. In the discovery of an active and stable catalyst for H2 discharge from H12-MBP at lower temperatures, a mesoporous Pd-Al2O3 catalyst (MPdA) was synthesized by a one-pot solvent deficient precipitation (SDP). In the present work, the sensitivity and effectiveness of the SDP method are examined by varying the calcination temperature and time in the preparation of the MPdA catalyst. The characterization revealed that the final properties of the MPdA catalyst greatly rely on both the calcination temperature and time. The MPdA catalyst showed better dehydrogenation activity for calcination at 600 °C than at other temperatures, because of Pd particles of smaller size with higher dispersion. Although the MPdA catalysts calcined at 600 °C for different periods of time have similar size and dispersion of Pd particles, the dehydrogenation efficiency was superior as the calcination time became shorter (e.g., 1 h), which originated from the better arrangement of Pd particles over a higher surface area. These MPdA catalysts, irrespective of the calcination time, displayed a remarkable stability in the dehydrogenation of H12-MBP owing to the protection of Pd particles by the Al2O3 layer.

Pentacoordinated Al3+ -Stabilized Active Pd Structures on Al2 O3 -Coated Palladium Catalysts for Methane Combustion

Supported Pd catalysts are active in catalyzing the highly exothermic methane combustion reaction but tend to be deactivated owing to local hyperthermal environments. Herein we report an effective approach to stabilize Pd/SiO₂ catalysts with porous Al₂ O₃ overlayers coated by atomic layer deposition (ALD). ²⁷ Al magic angle spinning NMR analysis showed that Al₂ O₃ overlayers on Pd particles coated by the ALD method are rich in pentacoordinated Al³⁺ sites capable of strongly interacting with adjacent surface PdO_x phases on supported Pd particles. Consequently, Al₂ O₃ -decorated Pd/SiO₂ catalysts exhibit active and stable PdO_x and Pd-PdO_x structures to efficiently catalyze methane combustion between 200 and 850 °C. These results reveal the unique structural characteristics of Al₂ O₃ overlayers on metal surfaces coated by the ALD method and provide a practical strategy to explore stable and efficient supported Pd catalysts for methane combustion.

Pd4S/SiO2: A Sulfur-Tolerant Palladium Catalyst for Catalytic Complete Oxidation of Methane

Sulfur species (e.g. H2S or SO2) are the natural enemies of most metal catalysts, especiallypalladium catalysts. The previously reported methods of improving sulfur-tolerance were toeffectively defer the deactivation of palladium catalysts, but could not prevent PdO and carrierinteraction between sulfur species. In this report, novel sulfur-tolerant SiO2 supported Pd4Scatalysts (5 wt. % Pd loading) were prepared by H2S–H2 aqueous bubble method and applied tocatalytic complete oxidation of methane. The catalysts were characterization by X-ray diffraction,Transmission electron microscopy, X-ray photoelectron Spectroscopy, temperature-programmedoxidation, and temperature-programmed desorption techniques under identical conditions. Thestructural characterization revealed that Pd4S and metallic Pd0 were found on the surface of freshlyprepared catalysts. However, Pd4S remained stable while most of metallic Pd0 was converted toPdO during the oxidation reaction. When coexisting with PdO, Pd4S not only protected PdO fromsulfur poisoning, but also determined the catalytic activity. Moreover, the content of Pd4S could beadjusted by changing H2S concentration of H2S–H2 mixture. When H2S concentration was 7 %, thePd4S/SiO2 catalyst was effective in converting 96% of methane at the 400 °C and also exhibitedlong-term stability in the presence of 200 ppm H2S. A Pd4S/SiO2 catalyst that possesses excellentsulfur-tolerance, oxidation stability, and catalytic activity has been developed for catalyticcomplete oxidation of methane.

Comparative Study of the Characteristics and Activities of Pd/γ-Al2O3 Catalysts Prepared by Vortex and Incipient Wetness Methods

5 wt% Pd/γ-Al2O3 catalysts were prepared by a modified Vortex Method (5-Pd-VM) and Incipient Wetness Method (5-Pd-IWM), and characterized by various techniques (Inductively coupled plasma atomic emission spectroscopy (ICP-AES), N2-physisorption, pulse CO chemisorption, temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and X-ray diffraction (XRD)) under identical conditions. Both catalysts had similar particle sizes and dispersions; the 5-Pd-VM catalyst had 0.5 wt% more Pd loading (4.6 wt%). The surfaces of both catalysts contained PdO and PdOx with about 7% more PdOx in 5-Pd-VM. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and scanning electron microscope (SEM) images indicated presence of PdO/PdOx nanocrystals (8–10 nm) on the surface of the support. Size distribution by STEM showed presence of smaller nanoparticles (2–5 nm) in 5-Pd-VM. This catalyst was more active in the lower temperature range of 275–325 °C and converted 90% methane at 325 °C. The 5-Pd-VM catalyst was also very stable after 72-hour stability test at 350 °C showing 100% methane conversion, and was relatively resistant to steam deactivation. Hydrogen TPR of 5-Pd-VM gave a reduction peak at 325 °C indicating weaker interactions of the oxidized Pd species with the support. It is hypothesized that smaller particle sizes, uniform particle distribution, and weaker PdO/PdOx interactions with the support may contribute to the higher activity in 5-Pd-VM.

The rational synthesis of aerosol-assisted alumina powders having uniform mesopores and highly accessible surfaces

Several important factors to fabricate high-quality aerosol-assisted mesoporous alumina powders are summarized by considering rational and reliable synthetic routes.

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.