Effect of Pt:Pd ratio on CO and hydrocarbon oxidation

Kang, Sung Bong; Hazlett, Melanie J.; Balakotaiah, Vemuri; Kalamaras, Christos M.; Epling, William S.

doi:10.1016/j.apcatb.2017.05.057

Cited by 51 publications

(31 citation statements)

References 36 publications

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“…On Pd and the similar Pt, acetylene adsorbs to form di-σ and π bonds on a total of three sites. ,,,,, This tribonded species is the most stable form of adsorbed acetylene and has been well documented in the literature. ,,,,,, A metastable species π-bonded to a single active site has also been reported. , Depending on the initial state of the catalyst, adsorbed acetylene has been observed to undergo hydrogenation, decomposition, and/or oxidation. ,− ,,− ,,, On clean surfaces and in the absence of oxygen, acetylene decomposes to form hydrogen and surface carbon; adsorbed vinylidene species (CCH 2 ) form upon heating, ,,,, which may lead to benzene . For a surface covered by H, acetylene hydrogenates to adsorbed ethylidyne (CH 3 C), which decomposes upon heating. ,− ,,, On an O covered surface, acetylene decomposes (e.g., to vinylidene) and oxidizes to CO, CO 2 , and H 2 O via a CH intermediate; hydrogen is first removed and surface carbon is then oxidized. ,,,, CO formed as a product has been observed to oxidize instantaneously. , Species detectable by FTIR, including ethylene, methane, ethane, formaldehyde, acetone, propylene, and propane were not observed during acetylene oxidation experiments in this study; trace amounts of CO were detected but at very low concentrations (<10 ppm).…”

Section: Discussionmentioning

confidence: 94%

“…Studies have shown that reaction order may switch from positive to negative with respect to ethylene and negative to positive with respect to oxygen depending on partial pressures of reactants. ,,,, The incomplete conversion of ethylene at higher temperatures (Figure ) could also be due to kinetic limitations. A slow-reacting form of ethylene (e.g., di-σ-bonded) could exist in this regime as more vacancies are created with oxidation of fast-reacting π-bonded species. ,, Carbonaceous species and deposits would further decrease the rate of oxidation by blocking active sites. ,,,,,, Kang et al attributed the lower slope of ethylene light-off curves on Pt, Pd, and Pt:Pd alloy diesel oxidation catalysts (operated under lean conditions with 5% O 2 ) to the formation and oxidation of carbonaceous deposits; furthermore, ethylene (150 ppm oxidized alone or in mixtures with CO, other hydrocarbons, and NO) is not completely converted on Pd/Al 2 O 3 at temperatures as high as 350 °C . The lower slopes observed in this study also correlate to a relatively low activation energy, a measure of the reaction sensitivity to temperature.…”

Section: Discussionmentioning

confidence: 99%

“…Like acetylene, ethylene is relatively abundant in engine exhaust, accounting for up to 25% of hydrocarbon species. ,,, In this study, ethylene was additionally chosen as a model hydrocarbon to complement acetylene (also a C2 hydrocarbon) and propylene (another alkene). Trends for ethylene oxidation in the presence of CO and other hydrocarbons have likewise toggled inhibiting and enhancing regimes. , Surface science and mechanistic studies have suggested that, similar to acetylene, ethylene can form CO and CH intermediates. − ,− Different reaction pathways (decomposition, oxidation, hydrogenation) that adsorbed acetylene and ethylene can undertake on Pt, Pd, and Rh catalysts have also been proposed. − ,,,− …”

Section: Introductionmentioning

confidence: 99%

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Rate Inhibition and Enhancement on Ceria-Promoted Pd Monolith Catalysts: Oxidation of Acetylene, Ethylene, and Propylene and Their Mixtures

Lang

Harold

2019

Ind. Eng. Chem. Res.

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The catalytic oxidations of acetylene, ethylene, and propylene under transient and steady-state conditions were studied using model Pd/Al2O3 and Pd/CeO2–ZrO2 monolith catalysts. Kinetics and light-off measurements highlight the differences between the oxidation of each hydrocarbon. Acetylene is strongly self-inhibiting and −1 order while the reaction order with respect to ethylene is +1, both at temperatures below light-off. Ethylene oxidation is moderately inhibited by oxygen at low temperatures and ethylene at high temperatures. Mixture light-off experiments reveal acetylene inhibits the oxidations of ethylene, propylene, and CO. Light-off of the other species follow acetylene oxidation, which is nearly unaffected by the other species. The Pd/CeO2–ZrO2 catalyst gives consistently higher conversion compared with Pd/Al2O3 at the same Pd loading for each of the individual oxidation reactions, although the negative-order kinetics of acetylene is sustained. Oxidation of acetylene and mixtures containing acetylene are enhanced with Pd/CeO2–ZrO2 compared to Pd/Al2O3. The effects of oxidative or reductive pretreatment of the catalysts on species oxidations are also evaluated. Mechanistic implications for the reactions are described to guide kinetic model development. The findings highlight and clarify inhibition effects during single and co-oxidation and offer insights into optimizing the catalyst composition and its operation.

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Section: Discussionmentioning

confidence: 94%

Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Rate Inhibition and Enhancement on Ceria-Promoted Pd Monolith Catalysts: Oxidation of Acetylene, Ethylene, and Propylene and Their Mixtures

Lang

Harold

2019

Ind. Eng. Chem. Res.

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“…In heterogeneous catalytic reactions, photocatalytic and thermocatalytic conditions are usually regarded as two independent catalytic conditions, each with high expectations. However, over the past few decades, researchers have found that regardless of the type of catalytic system, there are technical shortcomings in the practical application of both systems [1][2][3]. Therefore, by combining thermocatalysis and photocatalysis, an effective and feasible new catalytic pathway, which maximizes the synergistic effects of both systems to overcome these limitations, has been designed.…”

Section: Introductionmentioning

confidence: 99%

Thermo-driven photocatalytic CO reduction and H2 oxidation over ZnO via regulation of reactant gas adsorption electron transfer behavior

Wang

et al. 2021

Chinese Journal of Catalysis

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Photothermal catalysis is a widely researched field in which the reaction mechanism is usually investigated based on the photochemical behavior of the catalytic material. Considering that the adsorption of reactants is essential for catalytic reactions to occur, in this study, the synergistic effect of photothermal catalysis is innovatively elucidated in terms of the electron transfer behavior of reactant adsorption. For the H2 + O2 or CO + H2 reaction systems over a ZnO catalyst, UV irradiation at 25 °C or heat without UV irradiation did not cause H2 oxidation or CO reduction; only photothermal conditions (100 or 150 °C + UV light) initiated the two reactions. This result is related to the electron transfer behavior associated with the adsorption of CO or H2 on ZnO, in which H2 or CO that lost an electron could be oxidized by O2 or hydroxyls. However, the electron-accepting CO could be reduced by the electron-donating H2 into CH4 under photothermal conditions. Based on the in-situ characterization and theoretical calculation results, it was established that the synergistic effect of the photothermal conditions acted on the (002) crystal surface of ZnO to stimulate the growth of zinc vacancies, which resulted in the formation of defect energy levels, adsorption sites, and an adjusted Fermi level. As a result, the electron transfer behavior between adsorbed CO or H2 and the crystal surface varied, which further affected the photocatalytic behavior. The results show that the effect of photothermal synergy may not only produce the expected kinetic energy, but more importantly, produce energy that can change the activation mode of the reactant gas. This study provides a new understanding of the CO catalytic oxidation and reduction processes over semiconductor materials.

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“…6,[11][12][13] Additional catalyst optimization emerges in the study of bimetallic Pt-Pd catalysts with different studies reporting various optimal Pt:Pd ratios. [14][15][16] Even for a nominal alloy catalyst composition, single particle nanostructure could vary between random alloys, core-shell and phase segregated structures, and intermetallic structures, each with their own catalytic reactivity. The development of structure-property relationships for emissions control materials will help facilitate the rapid design of improved catalysts.…”

Section: Introductionmentioning

confidence: 99%

Palladium oxidation leads to methane combustion activity: Effects of particle size and alloying with platinum

Goodman

Aitbekova

et al. 2019

The Journal of Chemical Physics

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Pd-and Pt-based catalysts are highly studied materials due to their widespread use in emissions control catalysis. However, claims continue to vary regarding the active phase and oxidation state of the metals. Different conclusions have likely been reached due to the heterogeneous nature of such materials containing various metal nanoparticle sizes and compositions, which may each possess unique redox features. In this work, using uniform nanocrystal catalysts, we study the effect of particle size and alloying on redox properties of Pd-based catalysts and show their contribution to methane combustion activity using operando quick extended x-ray absorption fine structure measurements. Results demonstrate that for all studied Pd sizes (3 nm-16 nm), Pd oxidation directly precedes CH 4 combustion to CO 2 , suggesting Pd oxidation as a prerequisite step to methane combustion, and an oxidation pretreatment shows equal or better catalysis than a reduction pretreatment. Results are then extended to uniform alloyed PtxPd 1−x nanoparticles, where oxidative pretreatments are shown to enhance low-temperature combustion. In these uniform alloys, we observe a composition-dependent effect with Pt-rich alloys showing the maximum difference between oxidative and reductive pretreatments. In Pt-rich alloys, we initially observe that the presence of Pt maintains Pd in a lower-activity reduced state. However, with time on stream, PdO eventually segregates under oxidizing combustion conditions, leading to a slowly increasing activity. Overall, across particle sizes and alloy compositions, we relate increased catalytic activity to Pd oxidation, thus shedding light on previous contrasting results related to the methane combustion activity of these catalysts.

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Effect of Pt:Pd ratio on CO and hydrocarbon oxidation

Cited by 51 publications

References 36 publications

Rate Inhibition and Enhancement on Ceria-Promoted Pd Monolith Catalysts: Oxidation of Acetylene, Ethylene, and Propylene and Their Mixtures

Rate Inhibition and Enhancement on Ceria-Promoted Pd Monolith Catalysts: Oxidation of Acetylene, Ethylene, and Propylene and Their Mixtures

Thermo-driven photocatalytic CO reduction and H2 oxidation over ZnO via regulation of reactant gas adsorption electron transfer behavior

Palladium oxidation leads to methane combustion activity: Effects of particle size and alloying with platinum

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