Kinetics of Low-Temperature Methane Oxidation over SiO₂-Encapsulated Bimetallic Pd–Pt Nanoparticles

Abstract

A kinetic study of lean methane combustion on a silica-encapsulated bimetallic Pd–Pt (1:1 molar ratio) catalyst at varying methane concentrations and temperatures and in the absence/presence of added water is presented. With dry feed, the kinetic behavior of the bimetallic catalyst is correlated using a previously reported rate expression that is first order in methane and negative one order in water. The model does not adequately correlate the conversion of wet lean CH4 combustion in the temperature range of 550 to 750 K. For wet conditions, an alternative mechanism is suggested that is based on the previous experimental observations of the prevailing chemical state of Pd in wet feed, the ability of Pt to activate methane in oxygen-deficient atmospheres, and the inhibitory effect of water on the support-mediated oxygen exchange. The corresponding rate expression successfully predicts the activity of the silica-encapsulated Pd–Pt catalyst with wet feed (5 vol % water) in the temperature range of 550 to 750 K. The study also evaluates the internal mass transfer across the silica shell. It is shown that for the catalysts used here, the diffusion resistance across the shell is negligibly small.

Chemical and Structural Dynamics of Nanostructures in Bimetallic Pt–Pd Catalysts, Their Inhomogeneity, and Their Roles in Methane Oxidation

“…CH 4 turnover rates ( r CH 4 ,M , per exposed metal site) during steady-state CH 4 –O 2 reactions on bimetallic Pt–Pd and monometallic Pt and Pd catalysts are first order in CH 4 (0.1–2 kPa) and zero order in O 2 (10–50 kPa) between 733 and 803 K, as established in previous studies. ,, The mechanistic reason for these rate dependencies is obvious, as the first C–H activation in methane limits the catalytic turnovers, occurring on monometallic Pt 0 cluster surfaces saturated with chemisorbed oxygen adatoms or on monometallic PdO crystallites without any lattice oxygen vacancies, expected at these high oxygen chemical potentials relevant to the treatment of exhaust gas under fuel-lean conditions. Under these conditions, the first-order rate constant ( k CH 4 , M , where M denotes metal identity; M = Pt+Pd, Pt, Pd) is where r CH 4 ,M is the methane turnover rate (per exposed metal site), P CH 4 is the methane pressure, and A M and E C–H,M are the pre-exponential factor and activation energy, respectively.…”

“…CH 4 turnover rates were determined using the CO 2 concentrations in the reactor effluent stream after achieving a stable reactivity, together with the number of exposed metal atoms measured from oxygen chemisorption experiments at 313 K. between 733 and 803 K, as established in previous studies. 3,4,42 The mechanistic reason for these rate dependencies is obvious, as the first C−H activation in methane limits the catalytic turnovers, occurring on monometallic Pt 0 cluster surfaces saturated with chemisorbed oxygen adatoms 3 or on monometallic PdO crystallites without any lattice oxygen vacancies, 4 expected at these high oxygen chemical potentials relevant to the treatment of exhaust gas under fuel-lean conditions. Under these conditions, the firstorder rate constant (k CH 4 , M , where M denotes metal identity;…”

Site-Oriented Design of High-Performance Halloysite-Supported Palladium Catalysts for Methane Combustion

“…However, because of their huge surface energies, small nanoparticles are prone to agglomerate or sinter during reactions, especially at elevated temperatures. , Thereby, in order to improve the catalytic performance, many efforts have been devoted to finding an approach to keep the active components in small particle sizes and simultaneously prevent their coalescence into larger particles, migration, and sintering during reaction. One of the important strategies is to confine active particles inside pore channels of supports and anchor them on some specific sites. , Hence, it is of great significance to further explore carriers with specific structure to achieve the effectively stabilized active components. − …”

“…One of the important strategies is to confine active particles inside pore channels of supports and anchor them on some specific sites. 17,18 Hence, it is of great significance to further explore carriers with specific structure to achieve the effectively stabilized active components. 19−21 Aluminosilicate has gathered widespread attention in catalysis science, thanks to its versatile characteristics of high surface area, narrow pore size distribution, large pore volume, and good surface acid−base property, 22 which enable this nanomaterial to be adopted as an attractive support for fabricating nanocatalysts toward CH 4 combustion.…”

Controlled Synthesis of Sintering-Resistant Pd@CeO2 Core–Shell Nanotube Catalysts for CO Oxidation

“…Physical confinement is one of the most commonly employed sinter-resistance strategies for maintenance of thermal stability, ,, especially for the core–shell configuration because the shell was designed as a physically protective layer to sterically confine the noble-metal cores from aggregation. Compared with the inert protection layers such as carbon, zirconia, and silica, , whose sole function is spatial stabilization, ceria features with an additional advantage of strong metal-support interaction (SMSI). − Such an interaction was proposed to be able to anchor and stabilize precious noble atoms and combine with the unique redox property of ceria synergistically to promote the catalytic performance . Pd@CeO 2 catalysts with different configurations were fabricated to improve their thermal stability and catalytic behavior.…”