Methanol Partial Oxidation on Ag(1 1 1) from First Principles

Aljama, Hassan; Yoo, Jong Suk; Nørskov, Jens K.; Abild‐Pedersen, Frank; Studt, Felix

doi:10.1002/cctc.201601053

Cited by 16 publications

(9 citation statements)

References 31 publications

(66 reference statements)

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“…Given the dearth of successful examples in which noble metal catalysts convert methane to useful products in the literature, such a finding may seem unexpected17 . However, this discrepancy can likely be attributed to the low oxygen coverages that exist at most relevant oxygen chemical potentials on noble metals due to their unfavorable formation energies18 . It should also be noted that a more stable radical TS does not imply the final state will include a methyl radical; even on Ag and Au, CH 3 * is approximately 0.8 eV more stable than a gas-phase radical.The near-zero slope of the surface-stabilized O-promoted metals can be explained by a bond-order conservation argument.…”

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confidence: 99%

Mechanistic insights into heterogeneous methane activation

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Aljama

Kakekhani

et al. 2017

Phys. Chem. Chem. Phys.

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While natural gas is an abundant chemical fuel, its low volumetric energy density has prompted a search for catalysts able to transform methane into more useful chemicals. This search has often been aided through the use of transition state (TS) scaling relationships, which estimate methane activation TS energies as a linear function of a more easily calculated descriptor, such as final state energy, thus avoiding tedious TS energy calculations. It has been shown that methane can be activated via a radical or surface-stabilized pathway, both of which possess a unique TS scaling relationship. Herein, we present a simple model to aid in the prediction of methane activation barriers on heterogeneous catalysts. Analogous to the universal radical TS scaling relationship introduced in a previous publication, we show that a universal TS scaling relationship that transcends catalysts classes also seems to exist for surface-stabilized methane activation if the relevant final state energy is used. We demonstrate that this scaling relationship holds for several reducible and irreducible oxides, promoted metals, and sulfides. By combining the universal scaling relationships for both radical and surface-stabilized methane activation pathways, we show that catalyst reactivity must be considered in addition to catalyst geometry to obtain an accurate estimation for the TS energy. This model can yield fast and accurate predictions of methane activation barriers on a wide range of catalysts, thus accelerating the discovery of more active catalysts for methane conversion.

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confidence: 99%

Mechanistic insights into heterogeneous methane activation

Latimer

Aljama

Kakekhani

et al. 2017

Phys. Chem. Chem. Phys.

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110

153

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“…It is instructive to consider our observations in light of previous microkinetic models and discuss how near-surface gas-phase diagnostics could improve such models in the future. In their theoretical work, Aljama et al 57 predicted that CH 3 (s) (the superscript "(s)" indicates surface-bound species), CH 2 O (s) , CH 3 O (s) , and CO (s) are the dominant carboncontaining surface species during methanol oxidation on Ag(111), with CH 3 (s) having the highest surface coverage of these species at all temperatures from 200 to 700 °C. In addition to expected desorption of CH 2 O, desorption of CH 3 is included in their model, but to our knowledge has not been previously observed, likely because it is challenging to detect free radicals, which decay quickly upon entering the gas phase, as Figure 9d shows.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Near-Surface Imaging of the Multicomponent Gas Phase above a Silver Catalyst during Partial Oxidation of Methanol

et al. 2020

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Fundamental chemistry in heterogeneous catalysis is increasingly explored using operando techniques in order to address the pressure gap between ultrahigh vacuum studies and practical operating pressures. Because most operando experiments focus on the surface and surface-bound species, there is a knowledge gap of the near-surface gas phase and the fundamental information the properties of this region convey about catalytic mechanisms. We demonstrate in situ visualization and measurement of gas-phase species and temperature distributions in operando catalysis experiments using complementary near-surface optical and mass spectrometry techniques. The partial oxidation of methanol over a silver catalyst demonstrates the value of these diagnostic techniques at 600 Torr (800 mbar) pressure and temperatures from 150 to 410 °C. Planar laser-induced fluorescence provides two-dimensional images of the formaldehyde product distribution that show the development of the boundary layer above the catalyst under different flow conditions. Raman scattering imaging provides measurements of a wide range of major species, such as methanol, oxygen, nitrogen, formaldehyde, and water vapor. Near-surface molecular beam mass spectrometry enables simultaneous detection of all species using a gas sampling probe. Detection of gas-phase free radicals, such as CH3 and CH3O, and of minor products, such as acetaldehyde, dimethyl ether, and methyl formate, provides insights into catalytic mechanisms of the partial oxidation of methanol. The combination of these techniques provides a detailed picture of the coupling between the gas phase and surface in heterogeneous catalysis and enables parametric studies under different operating conditions, which will enhance our ability to constrain microkinetic models of heterogeneous catalysis.

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“…43 A recent computational study also showed that a low oxygen surface coverage (<0.2 ML, ML = monolayer) was needed for high formaldehyde selectivity in methanol oxidation. 6 These studies suggest that the precise control of the low surface coverage oxygen species on the catalyst surface is essential in the SCO of methanol.…”

Section: ■ Introductionmentioning

confidence: 99%

“…For example, ethylene oxide is used to produce pharmaceuticals, detergents, and plastics, while formaldehyde is heavily consumed to make a variety of chemicals for the construction, automotive, and furniture sectors. − Recent reports indicate that the global annual consumption of ethylene oxide and formaldehyde is over 35 million tons, each. ,, The annual global production of these chemicals is estimated to grow monotonically due to the increasing demand by the chemical industry. Thus, the SCO continues to be an active field of not only applied but also fundamental research. − …”

Section: Introductionmentioning

confidence: 99%

“…In contrast, formaldehyde production in the industry is typically carried out under methanol-rich conditions (e.g., CH 3 OH/O 2 = 2.0–2.5). , A former in situ Raman study conducted at atmospheric pressure and elevated temperatures showed that the polycrystalline silver existed in a reduced (i.e., metallic) state during methanol oxidation . A recent computational study also showed that a low oxygen surface coverage (<0.2 ML, ML = monolayer) was needed for high formaldehyde selectivity in methanol oxidation . These studies suggest that the precise control of the low surface coverage oxygen species on the catalyst surface is essential in the SCO of methanol.…”

Section: Introductionmentioning

confidence: 99%

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Formaldehyde Selectivity in Methanol Partial Oxidation on Silver: Effect of Reactive Oxygen Species, Surface Reconstruction, and Stability of Intermediates

et al. 2021

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Selective oxidation reactions on heterogeneous silver catalysts are essential for the mass production of numerous industrial commodity chemicals. However, the nature of active oxygen species in such reactions is still debated. To shed light on the role of different oxygen species, we studied the methanol oxidation reaction on Ag(111) single-crystal model catalyst surfaces containing two dissimilar types of oxygen (electrophilic, O e and nucleophilic, O n ). X-ray photoelectron spectroscopy and low energy electron diffraction experiments suggested that the atomic structure of the Ag(111) surface remained mostly unchanged after accumulating low O e coverage at 140 K. Temperature-programmed reaction spectroscopic investigation of low coverages of O e on Ag(111) revealed that O e was active for methanol oxidation on Ag(111) with a high selectivity toward formaldehyde (CH 2 O) production. High surface oxygen coverages, on the other hand, triggered a reconstruction of the Ag(111) surface, yielding Ag oxide domains, which catalyzes methanol total oxidation to CO 2 and decreases the formaldehyde selectivity. This important finding indicates a trade-off between CH 2 O selectivity and methanol conversion, where 93% CH 2 O selectivity can be achieved for an oxygen surface coverage of θ O = 0.08 ML (ML = monolayer) with moderate methanol conversion, while methanol conversion could be boosted by a factor of ∼4 for θ O = 0.26 ML with a suppression of CH 2 O selectivity to 50%. Infrared reflection absorption spectroscopy results and density functional theory calculations indicated that Ag oxide contains dissimilar adsorption sites for methoxy intermediates, which are also energetically less stable than that of the unreconstructed Ag(111). The current findings provide important molecular-level insights regarding the surface structure of the oxidized Ag(111) model catalyst directly governing the competition between different reaction pathways in methanol oxidation reaction, ultimately dictating the reactant conversion and product selectivity.

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Methanol Partial Oxidation on Ag(1 1 1) from First Principles

Cited by 16 publications

References 31 publications

Mechanistic insights into heterogeneous methane activation

Mechanistic insights into heterogeneous methane activation

Near-Surface Imaging of the Multicomponent Gas Phase above a Silver Catalyst during Partial Oxidation of Methanol

Formaldehyde Selectivity in Methanol Partial Oxidation on Silver: Effect of Reactive Oxygen Species, Surface Reconstruction, and Stability of Intermediates

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