Particle Size Effect on CH4 Oxidation Over Noble Metals: Comparison of Pt and Pd Catalysts

Influence of the drying mode of support on the properties of Pd/Al2O3-ZrO2 materials used for methane combustion

This work constitutes a new trial to enhance the properties of palladium supported on alumina modified with zirconium used as catalysts for methane combustion. The effect of the support drying mode is studied. For this aim, Al2O3-ZrO2 binary oxides with zirconium loading of 2 and 5% in weight were prepared using sol-gel process then dried under ordinary or supercritical conditions. Palladium with a loading of 0.5% was deposited on the support by wet impregnation. Several techniques have been used to investigate differences between the two types of the derived catalysts.

High-Temperature Behavior of Laser Electrodispersion-Prepared Pd/ZSM-5 Hydrocarbon Traps under CO Oxidation Conditions

Zeolites and metal-doped zeolites are now widely considered as low-temperature hydrocarbon traps to be a part of emission control systems in automobiles. However, due to the high temperature of exhaust gases, the thermal stability of such sorbent materials is of great concern. To avoid the thermal instability problem, in the present work, laser electrodispersion was used to deposit Pd particles on the surface of ZSM-5 zeolite grains (SiO₂/Al₂O₃ = 55 and SiO₂/Al₂O₃ = 30) to obtain Pd/ZSM-5 materials with a Pd loading as low as 0.03 wt.%. The thermal stability was evaluated in a prompt thermal aging regime involving thermal treatment at temperatures up to 1000 °C in a real reaction mixture (CO, hydrocarbons, NO, an excess of O₂, and balance N₂) and a model mixture of the same composition with the exception of hydrocarbons. Low-temperature nitrogen adsorption and X-ray diffraction analysis were used to examine the stability of the zeolite framework. Special attention was paid to the state of Pd after thermal aging at varied temperatures. By means of transmission electron microscopy, X-ray photoelectron spectroscopy, and diffuse reflectance UV-Vis spectroscopy, it was shown that palladium, having been initially located on the surface of zeolite, undergoes oxidation and migrates into the zeolite's channels. This enhances the trapping of hydrocarbons and their subsequent oxidation at lower temperatures.

Modulating the strong metal-support interaction of single-atom catalysts via vicinal structure decoration

Metal-support interaction predominately determines the electronic structure of metal atoms in single-atom catalysts (SACs), largely affecting their catalytic performance. However, directly tuning the metal-support interaction in oxide supported SACs remains challenging. Here, we report a new strategy to subtly regulate the strong covalent metal-support interaction (CMSI) of Pt/CoFe2O4 SACs by a simple water soaking treatment. Detailed studies reveal that the CMSI is weakened by the bonding of H+, generated from water dissociation, onto the interface of Pt-O-Fe, resulting in reduced charge transfer from metal to support and leading to an increase of C-H bond activation in CH4 combustion by more than 50 folds. This strategy is general and can be extended to other CMSI-existed metal-supported catalysts, providing a powerful tool to modulating the catalytic performance of SACs.

Morphology Effects on Structure-Activity Relationship of Pd/Y-ZrO2 Catalysts for Methane Oxidation

Pd/Y-ZrO2 catalysts were prepared by Y-ZrO2 with different morphologies (flower-like, spherical, reticulated, and bulk-specific morphology), which were prepared by hydrothermal synthesis. Activity evaluation and characterization results show that the morphology influences the microstructures of Y-ZrO2 and the chemical states of active Pd species, thus affecting the activity of methane oxidation. Bulk Pd/Y-ZrO2 exhibits the best CH4 oxidation activity and thermal stability due to the block shape exposed (101) surface, and the single tetragonal phase structure maintained after high-temperature aging. The relatively large-sized Pd particles and Pd0 jointly promote the catalytic oxidation of CH4.

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.

Microkinetic Modeling of the Oxidation of Methane Over PdO Catalysts—Towards a Better Understanding of the Water Inhibition Effect

Water, which is an intrinsic part of the exhaust gas of combustion engines, strongly inhibits the methane oxidation reaction over palladium oxide-based catalysts under lean conditions and leads to severe catalyst deactivation. In this combined experimental and modeling work, we approach this challenge with kinetic measurements in flow reactors and a microkinetic model, respectively. We propose a mechanism that takes the instantaneous impact of water on the noble metal particles into account. The dual site microkinetic model is based on the mean-field approximation and consists of 39 reversible surface reactions among 23 surface species, 15 related to Pd-sites, and eight associated with the oxide. A variable number of available catalytically active sites is used to describe light-off activity tests as well as spatially resolved concentration profiles. The total oxidation of methane is studied at atmospheric pressure, with space velocities of 160,000 h−1 in the temperature range of 500–800 K for mixtures of methane in the presence of excess oxygen and up to 15% water, which are typical conditions occurring in the exhaust of lean-operated natural gas engines. The new approach presented is also of interest for modeling catalytic reactors showing a dynamic behavior of the catalytically active particles in general.

Present and new frontiers in materials research by ambient pressure x-ray photoelectron spectroscopy

In this topical review we catagorise all ambient pressure x-ray photoelectron spectroscopy publications that have appeared between the 1970s and the end of 2018 according to their scientific field. We find that catalysis, surface science and materials science are predominant, while, for example, electrocatalysis and thin film growth are emerging. All catalysis publications that we could identify are cited, and selected case stories with increasing complexity in terms of surface structure or chemical reaction are discussed. For thin film growth we discuss recent examples from chemical vapour deposition and atomic layer deposition. Finally, we also discuss current frontiers of ambient pressure x-ray photoelectron spectroscopy research, indicating some directions of future development of the field.

Palladium dispersion effects on wet methane oxidation kinetics

The catalytic activity for dry and wet methane oxidation over a series of palladium–alumina catalysts with systematically varied palladium loadings and PdO dispersions was measured and compared with conceptual multiscale simulations.

A highly active Pd/H-ZSM-5 catalyst in lean methane combustion prepared via a sol–gel method and treated by reduction–oxidation

The reduction–oxidation treatment can reconstruct Pd nanoparticles, strengthen metal–support interactions and enhance catalytic performance of Pd/H-ZSM-5 in methane combustion.

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.

Surfactant/organic solvent free single-step engineering of hybrid graphene-Pt/TiO2 nanostructure: Efficient photocatalytic system for the treatment of wastewater coming from textile industries

In this study, hybrid graphene-Pt/TiO₂ nanostructure were synthesized by single-step, inexpensive and surfactant/organic solvent free route; hydrothermal technique. The physicochemical properties of hybrid graphene-Pt/TiO₂ nanostructure were carefully analyzed by multiple techniques, including X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). The synthesized hybrid nanostructures were utilized as photocatalyst for the degradation of methylene blue (MB) dye under natural environment at average ambient temperature and mean daily global solar radiation, of about 22–25 °C and 374.9 mWh/cm², respectively. The activity performance indicated considerable degradation of methylene blue (MB) dye and was in the following order Gr (13%), TiO₂ (60%) and hybrid graphene-Pt/TiO₂ nanostructure (90%) over 21 min under the natural light illumination. The physiochemical characterization suggests that, the tightly attached metalized TiO₂ nanoparticles (Pt-TiO₂) on the high surface area graphene sheets improved utilization of visible light and increased separation and transfer of photo-excited electron (ē) hole (h⁺) pairs. Notably, the hybrid graphene-Pt/TiO₂ nanostructure exhibited an excellent cyclic stability for methylene blue (MB) dye removal. Finally, the kinetic behavior indicated that the photocatalytic degradation reaction of the dye obeyed the pseudo-first order (Langmuir-Hinshelwood) kinetics model.

Methane combustion over Pd/CoAl2O4/Al2O3 catalysts prepared by galvanic deposition

Pd/CoAl₂O₄/Al₂O₃ methane combustion catalysts were synthesized using a galvanic deposition (GD) method (PdCoAl-GD).

Enhanced activity for methane combustion over a Pd/Co/Al2O3 catalyst prepared by a galvanic deposition method

A Pd/Co/Al₂O₃ catalyst prepared by a galvanic deposition method exhibited notable catalytic activity for methane combustion, due to the higher reducibility of PdO nanoparticles on CoO_x.