No reduction on Cu-ZSM-5 studied by EXAFS

Huang, Yu Chun; Wang, H. Paul; Lee, Jyh Fu

doi:10.1016/s0045-6535(99)00037-5

Cited by 8 publications

(9 citation statements)

References 25 publications

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“…Calculated distances between the Cu–Cu pair are 2.594 and 3.064 Å respectively for 1T and 2T models. This is in line with the experimentally reported Cu–Cu distances, which fall in the range of 2.47–3.13 Å. ,,, …”

Section: Methodssupporting

confidence: 91%

Role of Acidic Proton in the Decomposition of NO over Dimeric Cu(I) Active Sites in Cu-ZSM-5 Catalyst: A QM/MM Study

2014

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The influence of protons on the mechanism of the direct decomposition of NO over adjacent dimeric Cu(I) active sites in zeolite is theoretically investigated by using ONIOM (QM/MM) calculations with two dicopper model systems 1T and 2T, where the Cu(I) atoms are separated by one and two SiO 4 tetrahedra, respectively. The reaction proceeds through the formation of N 2 O as a reaction intermediate and further its decomposition into oxygen and nitrogen. The present study shows that the presence of proton plays an important role in the production of N 2 O from two NO molecules. In the proton-free mechanism, this process requires a large activation barrier of 56.3 and 55.3 kcal/mol on 1T and 2T, respectively, while the inclusion of protons reduces it to 31.4 and 17.3 kcal/mol. The significant decrease in the activation barrier is due to the strengthening of the N−N bond of the formed NO dimer upon protonation, which facilitates the formation of N 2 O. On the other hand, the presence of protons disfavors the decomposition of N 2 O and needs an activation barrier of 6−9 kcal/mol higher than that of the corresponding reaction in the absence of protons. The stable intermediate Cu− OH + −Cu formed in the proton-assisted mechanism is responsible for the larger activation energy for N 2 O decomposition. The proton-assisted NO decomposition mechanism is in agreement with the experimental observation that the decomposition of N 2 O as well as O 2 desorption are the governing reaction steps in the decomposition of NO. The present study explains the role of the Cu−O−Cu species in the NO decomposition reaction. The results disclosed herein will also pave a way to understanding the mechanism of the reductive N−N coupling of NO molecules catalyzed by metalloenzymes and transition-metal catalysts.

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Section: Methodssupporting

confidence: 91%

Role of Acidic Proton in the Decomposition of NO over Dimeric Cu(I) Active Sites in Cu-ZSM-5 Catalyst: A QM/MM Study

2014

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“…Here we note in passing that in the ZSM‐5 samples investigated by all these groups the number of Cu ions per unit cell was two at most; this fact renders the occurrence of Cu I pairs rather unlikely. 4) EXAFS measurements on Cu‐ZSM‐5 with various Cu loadings and Si/Al ratios indicate a Cu–O distance in the range between 1.93 and 2.00 Å 9, 11, 22, 23. The average coordination number of Cu I ions in Cu‐ZSM‐5 prepared by gas‐phase reaction of H‐ZSM‐5 with CuCl is 2.5 11.…”

Section: Introductionmentioning

confidence: 96%

“…The average coordination number of Cu I ions in Cu‐ZSM‐5 prepared by gas‐phase reaction of H‐ZSM‐5 with CuCl is 2.5 11. For zeolites pretreated with hydrogen, Cu–Cu distances between 2.95 and 3.00 Å as well as very short Cu–Cu distances in the range of 2.53–2.57 Å have been reported 9, 23, 24. The longer Cu–Cu distances have been attributed to oxygen‐bridged Cu pairs, while the closer contacts were interpreted as evidence for the presence of metal clusters 9…”

Section: Introductionmentioning

confidence: 99%

Time‐Dependent Density Functional Theory Molecular Dynamics Simulations of Liquid Water Radiolysis

et al. 2008

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The early stages of the Coulomb explosion of a doubly ionized water molecule immersed in liquid water are investigated with time-dependent density functional theory molecular dynamics (TD-DFT MD) simulations. Our aim is to verify that the double ionization of one target water molecule leads to the formation of atomic oxygen as a direct consequence of the Coulomb explosion of the molecule. To that end, we used TD-DFT MD simulations in which effective molecular orbitals are propagated in time. These molecular orbitals are constructed as a unitary transformation of maximally localized Wannier orbitals, and the ionization process was obtained by removing two electrons from the molecular orbitals with symmetry 1B(1), 3A(1), 1B(2) and 2A(1) in turn. We show that the doubly charged H(2)O(2+) molecule explodes into its three atomic fragments in less than 4 fs, which leads to the formation of one isolated oxygen atom whatever the ionized molecular orbital. This process is followed by the ultrafast transfer of an electron to the ionized molecule in the first femtosecond. A faster dissociation pattern can be observed when the electrons are removed from the molecular orbitals of the innermost shell. A Bader analysis of the charges carried by the molecules during the dissociation trajectories is also reported.

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“…4) EXAFS measurements on Cu-ZSM-5 with various Cu loadings and Si/ Al ratios indicate a Cu ± O distance in the range between 1.93 and 2.00 ä. [9,11,22,23] The average coordination number of Cu I ions in Cu-ZSM-5 prepared by gas-phase reaction of H-ZSM-5 with CuCl is 2.5. [11] For zeolites pretreated with hydrogen, Cu ± Cu distances between 2.95 and 3.00 ä as well as very short Cu ± Cu distances in the range of 2.53 ± 2.57 ä have been reported.…”

Section: Introductionmentioning

confidence: 99%

“…[11] For zeolites pretreated with hydrogen, Cu ± Cu distances between 2.95 and 3.00 ä as well as very short Cu ± Cu distances in the range of 2.53 ± 2.57 ä have been reported. [9,23,24] The longer Cu ± Cu distances have been attributed to oxygen-bridged Cu pairs, while the closer contacts were interpreted as evidence for the presence of metal clusters. [9] Until now neither experimental technique applied to metalion-exchanged ZSM-5 has led to an unequivocal identification of the local environment of active sites.…”

Section: Introductionmentioning

confidence: 99%