Determination of the electronic and geometric structure of Cu sites during methane conversion over Cu-MOR with X-ray absorption spectroscopy

Alayon, Evalyn Mae C.; Nachtegaal, Maarten; Kleymenov, Evgeny; Bokhoven, Jeroen A. van

doi:10.1016/j.micromeso.2012.04.054

Cited by 76 publications

(119 citation statements)

References 29 publications

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“…In this case spectra of Cu-SAPO-34 measured during temperature-programmed reduction by H 2 were taken as Cu 2+ and Cu + references in the same way as in ref 37. The results shown in Figure 8a evidence an overall partial reduction of the catalyst which is in line with the data obtained during the SCR of NO over Cu-SSZ-13.…”

Section: Resultssupporting

confidence: 79%

Operando Spatially- and Time-Resolved XAS Study on Zeolite Catalysts for Selective Catalytic Reduction of NO_x by NH₃

et al. 2014

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The structure of iron and copper sites during the selective catalytic reduction (SCR) of NO x by NH 3 and related reactions (NH 3 adsorption/oxidation, NO oxidation) has been elucidated by spatially-and time-resolved X-ray absorption spectroscopy (XAS) along the catalyst bed over Fe-containing BEA and ZSM-5 zeolites as well as Cu-SAPO-34 silicoaluminophosphate. Strong gradients of the Fe and Cu oxidation state are present along the catalyst bed for the processes involving NH 3 and NO x (SCR) and less pronounced for NH 3 oxidation, whereas the catalyst state in the NO x containing feed resembles that of a catalyst exposed to air. The variation in the oxidation state is strongly correlated to the concentration of NH 3 and is more pronounced in the presence of NO x . For temperatures higher than 250°C the Fe and Cu sites at the beginning of the catalyst bed stay in partially reduced state, whereas they are more oxidized in the later zones where NH 3 and NO concentrations decrease. For temperatures lower than 250°C a reverse effect is seen for Fe-zeolites where the beginning of the catalyst bed contained more oxidized iron species than at the end of the catalyst bed. This is attributed to NH 3 inhibition. The obtained data allow to conclude that both NH 3 and NO x are involved in a reaction over the corresponding transition metal site, and its reoxidation is a rate-limiting step of the NH 3 -SCR for these catalysts.

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Section: Resultssupporting

confidence: 79%

Operando Spatially- and Time-Resolved XAS Study on Zeolite Catalysts for Selective Catalytic Reduction of NO_x by NH₃

et al. 2014

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“…[54] Note that this amount is by far more than that of active dicopper species determined traditionally by methanol extraction after methane reaction. This number is noteworthy and agrees with a recent extended X-ray absorption fine-structure spectroscopy (EXAFS) study of Van Bokhoven et al, who suggested that 50 % of the total Cu content changed oxidation state during the partial oxidation of methane.…”

Section: Epr Signalmentioning

confidence: 89%

“…Because of the increasing interest in CuMOR, [20,54] intense research on highly loaded copper MOR is a requisite for a more fundamental understanding. Because of the increasing interest in CuMOR, [20,54] intense research on highly loaded copper MOR is a requisite for a more fundamental understanding.…”

Section: Introductionmentioning

confidence: 99%

Spectroscopy and Redox Chemistry of Copper in Mordenite

et al. 2014

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Copper-containing zeolites, such as mordenite (MOR), have recently gained increased attention as a consequence of their catalytic potential. While the preferred copper loadings in these catalytic studies are generally high, the literature lacks appropriate spectroscopic and structural information on such Cu-rich zeolite samples. Higher copper loadings increase the complexity of the copper identity and their location in the zeolite host, but they also provide the opportunity to create novel Cu sites, which are perhaps energetically less favorable, but possibly more reactive and more suitable for catalysis. In order to address the different role of each Cu site in catalysis, we here report a combined electron paramagnetic resonance (EPR), UV/Vis-NIR and temperature-programmed reduction (TPR) study on highly copper-loaded MOR. Highly resolved diffuse reflectance (DR) spectra of the CuMOR samples were obtained due to the increased copper loading, allowing the differentiation of two isolated mononuclear Cu(2+) sites and the unambiguous correlation with extensively reported features in the EPR spectrum. Ligand field theory is applied together with earlier suggested theoretical calculations to determine their coordination chemistry and location within the zeolite matrix, and the theoretical analysis further allowed us to define factors governing their redox behavior. In addition to monomeric species, an EPR-silent, possibly dimeric, copper site is present in accordance with its charge transfer absorption feature at 22200 cm(-1), and quantified with TPR. Its full description and true location in MOR is currently being investigated.

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“…NMR and mass spectroscopy (MS) experiments using 13 C- and 18 O-labeled molecules confirmed that the intermediate species for carbonylation was a methoxy species localized on a Brønsted acid site. In addition, ultraviolet– visible (UV–vis) spectroscopic studies on Cu-MOR showed that the bent mono-( μ -oxo) dicupric species typically associated with the oxidation-active sites of Cu-Na-ZSM-5 5b–d,7c,12 and Cu-Na-MOR 8,13 was not active for carbonylation. For samples prepared from the acid form of the zeolite and with Cu/Al < 0.2, the characterization data suggest the presence of an alternative oxidation site in Cu-H-MOR that is responsible for methane oxidation and subsequent methoxy migration.…”

Section: Introductionmentioning

confidence: 99%

Methane to Acetic Acid over Cu-Exchanged Zeolites: Mechanistic Insights from a Site-Specific Carbonylation Reaction

et al. 2015

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The selective low temperature oxidation of methane is an attractive yet challenging pathway to convert abundant natural gas into value added chemicals. Copper-exchanged ZSM-5 and mordenite (MOR) zeolites have received attention due to their ability to oxidize methane into methanol using molecular oxygen. In this work, the conversion of methane into acetic acid is demonstrated using Cu-MOR by coupling oxidation with carbonylation reactions. The carbonylation reaction, known to occur predominantly in the 8-membered ring (8MR) pockets of MOR, is used as a site-specific probe to gain insight into important mechanistic differences existing between Cu-MOR and Cu-ZSM-5 during methane oxidation. For the tandem reaction sequence, Cu-MOR generated drastically higher amounts of acetic acid when compared to Cu-ZSM-5 (22 vs 4 μmol/g). Preferential titration with sodium showed a direct correlation between the number of acid sites in the 8MR pockets in MOR and acetic acid yield, indicating that methoxy species present in the MOR side pockets undergo carbonylation. Coupled spectroscopic and reactivity measurements were used to identify the genesis of the oxidation sites and to validate the migration of methoxy species from the oxidation site to the carbonylation site. Our results indicate that the CuII–O–CuII sites previously associated with methane oxidation in both Cu-MOR and Cu-ZSM-5 are oxidation active but carbonylation inactive. In turn, combined UV–vis and EPR spectroscopic studies showed that a novel Cu2+ site is formed at Cu/Al <0.2 in MOR. These sites oxidize methane and promote the migration of the product to a Brønsted acid site in the 8MR to undergo carbonylation.

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Determination of the electronic and geometric structure of Cu sites during methane conversion over Cu-MOR with X-ray absorption spectroscopy

Cited by 76 publications

References 29 publications

Operando Spatially- and Time-Resolved XAS Study on Zeolite Catalysts for Selective Catalytic Reduction of NO_x by NH₃

Operando Spatially- and Time-Resolved XAS Study on Zeolite Catalysts for Selective Catalytic Reduction of NO_x by NH₃

Spectroscopy and Redox Chemistry of Copper in Mordenite

Methane to Acetic Acid over Cu-Exchanged Zeolites: Mechanistic Insights from a Site-Specific Carbonylation Reaction

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Determination of the electronic and geometric structure of Cu sites during methane conversion over Cu-MOR with X-ray absorption spectroscopy

Cited by 76 publications

References 29 publications

Operando Spatially- and Time-Resolved XAS Study on Zeolite Catalysts for Selective Catalytic Reduction of NOx by NH3

Operando Spatially- and Time-Resolved XAS Study on Zeolite Catalysts for Selective Catalytic Reduction of NOx by NH3

Spectroscopy and Redox Chemistry of Copper in Mordenite

Methane to Acetic Acid over Cu-Exchanged Zeolites: Mechanistic Insights from a Site-Specific Carbonylation Reaction

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Product

Resources

About

Operando Spatially- and Time-Resolved XAS Study on Zeolite Catalysts for Selective Catalytic Reduction of NO_x by NH₃

Operando Spatially- and Time-Resolved XAS Study on Zeolite Catalysts for Selective Catalytic Reduction of NO_x by NH₃