Effect of Surface Oxygen on the Activation of Methane on Palladium and Platinum Surfaces

Mechanisms of CH4 activation over oxygen-preadsorbed transition metals by ReaxFF and AIMD simulations

The chemisorbed oxygen usually promotes the CH bond activation over less active metals like IB group metals but has no effect or even an inhibition effect over more active metals like Pd based on the static electronic structure study. However, the understanding in terms of dynamics knowledge is far from complete. In the present work, methane dissociation on the oxygen-preadsorbed transition metals including Au, Cu, Ni, Pt, and Pd is systemically studied by reactive force field (ReaxFF). The ReaxFF simulation results indicate that CH4 molecules mainly undergo the direct dissociation on Ni, Pt, and Pd surfaces, while undergo the oxygen-assisted dissociation on Au and Cu surfaces. Additionally, the ab initio molecular dynamics (AIMD) simulations with the umbrella sampling are employed to study the free-energy changes of CH4 dissociation, and the results further support the CH4 dissociation pathway during the ReaxFF simulations. The present results based on ReaxFF and AIMD will provide a deeper dynamic understanding of the effects of pre-adsorbed oxygen species on the CH bond activation compared to that of static DFT.

Investigation of the evolution of Pd-Pt supported on ceria for dry and wet methane oxidation

Efficiently treating methane emissions in transportation remains a challenge. Here, we investigate palladium and platinum mono- and bimetallic ceria-supported catalysts synthesized by mechanical milling and by traditional impregnation for methane total oxidation under dry and wet conditions, reproducing those present in the exhaust of natural gas vehicles. By applying a toolkit of in situ synchrotron techniques (X-ray diffraction, X-ray absorption and ambient pressure photoelectron spectroscopies), together with transmission electron microscopy, we show that the synthesis method greatly influences the interaction and structure at the nanoscale. Our results reveal that the components of milled catalysts have a higher ability to transform metallic Pd into Pd oxide species strongly interacting with the support, and achieve a modulated PdO/Pd ratio than traditionally-synthesized catalysts. We demonstrate that the unique structures attained by milling are key for the catalytic activity and correlate with higher methane conversion and longer stability in the wet feed.

Investigation on the Role of Pd, Pt, Rh in Methane Abatement for Heavy Duty Applications

Methane abatement remains a challenge in aftertreatment systems of natural gas engines, currently under discussion in combination with synthetic methane. In this study, Pt/Rh and Pd/Rh-based three-way catalysts are investigated under various transient conditions because transients between O2 excess (lean) and O2-poor (rich) conditions can significantly enhance methane abatement. At mid to high temperatures, transitions from rich to lean feed yield higher rates of methane direct oxidation under lean conditions with the Pt/Rh catalyst, compared to the Pd/Rh catalyst. Both catalysts are able to trigger methane steam reforming (SR) after transitions from lean to rich feed. The SR reaction leads to increased H2 and NH3 formation. However, SR deactivates much faster in the Pt/Rh catalyst. At low temperature, the Pt/Rh catalyst is more active for SR. Results from an additional Pd-only catalyst confirm that Rh is essential for NOx conversion and high N2 selectivity. The distinct characteristics of Pt, Pd and Rh demonstrate the benefits obtained from the combination of the three platinum group metals. The potential of the Pt/Pd/Rh catalyst is proved to be significant throughout the complete engine map. Under optimized lean/rich oscillatory conditions, the Pt/Pd/Rh catalyst yields more than 95% methane conversion under almost all conditions while maintaining efficient abatement of all other pollutants.

Catalysis by platinum and palladium species confined in the bulk of glass fibre materials

The results of studies on the application of silicate glass fibre materials in catalysis are summarized and analyzed. Despite the very low noble metal content, catalysts based on these materials showed exceptionally high activities and selectivities in some catalytic reactions. This is due to specificity of the glassy state, which makes it possible, first, to confine highly dispersed palladium and platinum species in the bulk of glass fibres and, second, selectively absorb polar molecules, thus excluding the undesirable reactions involving non-polar molecules. The size dependences of the complete oxidation of propane and selective hydrogenation of acetylene, the nature of the structure sensitivity of these reactions and the reaction mechanisms are discussed. Ways for improving glass fibre catalyst performance are proposed and examples of the successful application of Pt/glass fibre catalysts for purification of industrial gases from volatile organic compounds are given.

The bibliography includes 175 references.

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.

Highly Active and Stable CH4 Oxidation by Substitution of Ce4+ by Two Pd2+ Ions in CeO2(111)

Methane (CH₄) combustion is an increasingly important reaction for environmental protection, for which Pd/CeO₂ has emerged as the preferred catalyst. There is a lack of understanding of the nature of the active site in these catalysts. Here, we use density functional theory to understand the role of doping of Pd in the ceria surface for generating sites highly active toward the C-H bonds in CH₄. Specifically, we demonstrate that two Pd²⁺ ions can substitute one Ce⁴⁺ ion, resulting in a very stable structure containing a highly coordinated unsaturated Pd cation that can strongly adsorb CH₄ and dissociate the first C-H bond with a low energy barrier. An important aspect of the high activity of the stabilized isolated Pd cation is its ability to form a strong σ-complex with CH₄, which leads to effective activation of CH₄. We show that also other transition metals like Pt, Rh, and Ni can give rise to similar structures with high activity toward C-H bond dissociation. These insights provide us with a novel structural view of solid solutions of transition metals such as Pt, Pd, Ni, and Rh in CeO₂, known to exhibit high activity in CH₄ combustion.

First principles prediction of CH4 reactivities with Co3O4 nanocatalysts of different morphologies

Co₃O₄ nanocatalysts have been experimentally shown to have excellent performance in catalyzing CH₄ combustion. These nanocatalysts of different morphologies, such as nanoparticle/nanocube, nanorod/nanobelt, and nanoplate/nanosheet, were previously synthesized and characterized to mainly expose the (001), (011), and (112) surfaces, respectively, with distinct reactivities. In this study, rigorous first principles calculations were performed to investigate CH₄ reactivities of the above Co₃O₄ surfaces of different terminations. CH₄ dissociation was predicted to occur at the Co-O pair site on these surfaces. For each surface, the most reactive Co-O pair site was identified based on calculated energy barriers of the different active sites, which should contribute most significantly to the reactivity of that surface. The lowest energy barriers for the (001), (011), and (112) surfaces were predicted to be 0.96, 0.90, and 0.79 eV, respectively, suggesting CH₄ reactivity to increase in that order for the different Co₃O₄ surfaces, consistent with the trend found experimentally for Co₃O₄ nanocatalysts of different morphologies. Direct comparison between the estimated and experimental CH₄ reaction rates per gram of the nanocatalysts at 325 °C further indicate that their relative ratios were well reproduced by considering three main factors: the effective energy barrier for CH₄ dissociation, the surface area of the nanocatalyst, and the number of independent active sites per unit surface area. The important influence of surface area on CH₄ reactivity is also demonstrated by the significant difference in the reactivities of the nanocatalysts when exposing the same facet but with distinct surface areas.

Surface-Engineered Polydopamine Particles as an Efficient Support for Catalytic Applications

Mussel-inspired polydopamine (PDA) particles with the size of ∼270 nm are used as a support of palladium (Pd) nanoparticles (NPs) for catalyst preparation. The surface morphology of the PDA particle has been modified via corrosion of CF₃COOH. Surface chemistry of the obtained PDA particle has been engineered by the formation of a carboxylic acid-terminated alkanethiol monolayer. The obtained self-assembled monolayer-modified PDA (SAM-PDA) particles are used to load Pd NPs by simply adding H₂PdCl₄ solution to a suspension of SAM-PDA particles at room temperature. Transmission electron microscopy, energy-dispersive X-ray mapping, dynamic light scattering, X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis, and Fourier transform infrared are used to characterize the catalyst and to investigate the process. Uniform Pd NPs (2-3 nm) have been well-dispersed on the SAM-PDA particles via controllable surface engineering. Surface charges and interactions with a metal ion are regulated by the monolayer of carboxylic acids. The surface chemistry of PDA particles has been finely engineered for efficient loading of noble metal NPs. The obtained Pd/SAM-PDA catalyst has shown greatly increased activity and good reusability compared with Pd/PDA in the reduction of 4-nitrophenol (4-NP) by sodium borohydride or H₂. The kinetic data of 4-NP hydrogenation catalyzed by Pd/SAM-PDA are fitted to a Langmuir-Hinshelwood (L-H) model, and the calculated apparent activation energy of this process is 40.77 kJ mol^-1.