The Active Phase of Palladium during Methane Oxidation

Hellman, Anders; Resta, Andrea; Martin, Natalia; Gustafson, Johan; Trinchero, Adriana; Carlsson, Per-Anders; Balmès, Olivier; Felici, Roberto; Rijn, Richard M. van; Frenken, J.W.M.; Andersen, Jesper N; Lundgren, Edvin; Grönbeck, Henrik

doi:10.1021/jz300069s

Cited by 202 publications

(244 citation statements)

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“…It was found that the PdO(101) facet develops preferentially during the oxidation of Pd(100) 17,18 and that the formation of PdO(101) coincides with increased rates of methane oxidation during reaction at millibar pressures. 19 In addition to these studies of PdO nanoparticles or surfaces, the gas-phase C-H bond activation of CH 4 by various single transition-metal-oxide ions and bare metal cations has been experimentally and theoretically investigated by a large number of groups. [16][17][18][19][20][21][22][23] Schwarz's group has systematically examined the efficiency and product branching ratio of the gas phase reactions of various transition metal oxide ions with methane.…”

Section: Introductionmentioning

confidence: 99%

“…19 In addition to these studies of PdO nanoparticles or surfaces, the gas-phase C-H bond activation of CH 4 by various single transition-metal-oxide ions and bare metal cations has been experimentally and theoretically investigated by a large number of groups. [16][17][18][19][20][21][22][23] Schwarz's group has systematically examined the efficiency and product branching ratio of the gas phase reactions of various transition metal oxide ions with methane. It was found that transition metal oxide ions ScO + , TiO + , VO + and CoO + do not react, while other transition metal oxide ions, MnO + , FeO + , NiO + , PtO + and OsO + , react with methane to give methanol.…”

Section: Introductionmentioning

confidence: 99%

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Oxidation of Methane to Methanol over Single Site Palladium Oxide Species on Silica: A Mechanistic view from DFT

et al. 2017

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Abstract:A theoretical analysis was carried out on the mechanism of methane oxidation to methanol occurring on single site palladium oxide species [PdO] 2+ supported on a model of Al-MCM-41 silica. Both 6 and 8-membered ring structures were considered to represent the support.The energy profile for each elementary reaction was determined from density functional theory calculations with the OPBE functional. The calculated overall activation energies are close to the experimental values. Our calculations confirm that spin inversion can play a significant role in decreasing the barrier heights for the pathways. Indeed, in this type of reactions we could show a crossing between singlet and triplet reaction paths. We showed that the mechanism for the C-H bond cleavage and for the formation of methanol has a radical nature. According to our results, the [PdO] 2+ species located on a 8-membered ring of silica is more active than that deposited on a 6-membered ring. The calculated activation energies to cleave the methane C-H bond are 35 and 84 kJ/mol for the radical and ionic pathways, respectively. The activation barrier and the transition state geometry of this H-abstraction step are directly correlated with the optimal angle at which the substrate should approach the [Pd=O] 2+ moiety, with the elongation of the Pd-O oxo bond and finally with the energy of the π* acceptor orbital.2

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Oxidation of Methane to Methanol over Single Site Palladium Oxide Species on Silica: A Mechanistic view from DFT

et al. 2017

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“…[30][31][32][33][34][35][36][37][38][39][40] In the case of Pd(100), it has been shown experimentally that the higher activity of this surface towards CO oxidation at these conditions coincides with the presence of the √ 5 surface oxide. 30,32,35,38,39,41 Kinetic Monte-Carlo simulations support the view that a surface oxide on Pd(100) could be responsible for increased reactivity.…”

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Reversed Hysteresis during CO Oxidation over Pd₇₅Ag₂₅(100)

et al. 2016

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CO oxidation over Pd(100) and Pd 75 Ag 25 (100) has been investigated by a combination of near-ambient pressure X-ray photoelectron spectroscopy, quadrupole mass spectrometry, density functional theory calculations and micro-kinetic modeling. For both surfaces, hysteresis is observed in the CO 2 formation during heating and cooling cycles. Whereas normal hysteresis with higher light-off temperature than extinction temperature is present for Pd(100), reversed hysteresis is observed for Pd 75 Ag 25 (100).The reversed hysteresis can be explained from dynamic changes in the surface composition. At the beginning of the heating ramp, the surface is rich in palladium which gives a CO coverage that poisons the surface until the desorption rate becomes sufficiently high. The thermodynamic preference for an Ag rich surface in the absence of adsorbates promotes diffusion of Ag from the bulk to the surface as CO desorbs.During the cooling ramp, an appreciable surface coverage is reached at temperatures too low for efficient diffusion of Ag back into the bulk. The high concentration of Ag in the surface leads to a high extinction temperature and, consequently, the reversed hysteresis.

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“…Pd-O sites associated with Pd/CeO2 surfaces appear to have the highest activity for CH4 oxidation [21,22]. Mechanisms and kinetics of CH4 oxidation over Pd/PdO catalysts have elicited continued debate in the literature [6,13,14,23], for which data interpretation is complicated by the transitions that Pd catalysts undergo during thermal pre-treatment and reaction [24]. Furthermore, the high concentration of H2O in the NGV exhaust and the typically transient reaction conditions that result from cycling between oxidizing and reducing conditions in the NG engine [6,11] are known to significantly impact catalyst activity and stability.…”

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Deactivation of Pd Catalysts by Water during Low Temperature Methane Oxidation Relevant to Natural Gas Vehicle Converters

2015

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Abstract:Effects of H2O on the activity and deactivation of Pd catalysts used for the oxidation of unburned CH4 present in the exhaust gas of natural-gas vehicles (NGVs) are reviewed. CH4 oxidation in a catalytic converter is limited by low exhaust gas temperatures (500-550 °C) and low concentrations of CH4 (400-1500 ppmv) that must be reacted in the presence of large quantities of H2O (10-15%) and CO2 (15%), under transient exhaust gas flows, temperatures, and compositions. Although Pd catalysts have the highest known activity for CH4 oxidation, water-induced sintering and reaction inhibition by H2O deactivate these catalysts. Recent studies have shown the reversible inhibition by H2O adsorption causes a significant drop in catalyst activity at lower reaction temperatures (below 450 °C), but its effect decreases (water adsorption becomes more reversible) with increasing reaction temperature. Thus above 500 °C H2O inhibition is negligible, while Pd sintering and occlusion by support species become more important. H2O inhibition is postulated to occur by either formation of relatively stable Pd(OH)2 and/or partial blocking by OH groups of the O exchange between the support and Pd active sites thereby suppressing catalytic activity. Evidence from FTIR and isotopic labeling favors the latter route. Pd catalyst design, including incorporation of a second noble metal (Rh or Pt) and supports high O mobility (e.g., CeO2) are known to improve catalyst activity and stability. Kinetic studies of CH4 oxidation at conditions relevant to natural gas vehicles have OPEN ACCESSCatalysts 2015, 5 562 quantified the thermodynamics and kinetics of competitive H2O adsorption and Pd(OH)2 formation, but none have addressed effects of H2O on O mobility.

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The Active Phase of Palladium during Methane Oxidation

Cited by 202 publications

References 35 publications

Oxidation of Methane to Methanol over Single Site Palladium Oxide Species on Silica: A Mechanistic view from DFT

Oxidation of Methane to Methanol over Single Site Palladium Oxide Species on Silica: A Mechanistic view from DFT

Reversed Hysteresis during CO Oxidation over Pd₇₅Ag₂₅(100)

Deactivation of Pd Catalysts by Water during Low Temperature Methane Oxidation Relevant to Natural Gas Vehicle Converters

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The Active Phase of Palladium during Methane Oxidation

Cited by 202 publications

References 35 publications

Oxidation of Methane to Methanol over Single Site Palladium Oxide Species on Silica: A Mechanistic view from DFT

Oxidation of Methane to Methanol over Single Site Palladium Oxide Species on Silica: A Mechanistic view from DFT

Reversed Hysteresis during CO Oxidation over Pd75Ag25(100)

Deactivation of Pd Catalysts by Water during Low Temperature Methane Oxidation Relevant to Natural Gas Vehicle Converters

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Reversed Hysteresis during CO Oxidation over Pd₇₅Ag₂₅(100)