Woo-Sung Ju

Quantum Chemical Calculations on the Structure and Adsorption Properties of NO and N2O on Ag+ and Cu+ Ion-Exchanged Zeolites

Ab initio cluster quantum chemical calculations at the Hartree–Fock (HF/Lanl2dz) and correlated second-order Moller–Plesset perturbation theory (MP2/Lanl2dz) levels were performed for NO and N2O interactions with Ag+ and Cu+ ion-exchanged zeolites. The interaction energies were estimated in a conventional way and also corrected for basis set superposition errors. It was shown that the highly dispersed Ag+ counterions establish twofold coordination to the lattice oxygens on the zeolite surface, similar to the case of Cu+ ions. However, both NO and N2O bind relatively strongly to the Cu active sites of Cu+ ion-exchanged zeolites than those of the Ag+ site of the Ag+ ion-exchanged zeolites. Based on the results of these calculations, the two different forms of adsorption for these molecules on the catalyst surface, the nature of their binding and characteristics of the adsorption properties have been discussed. Finally, some comparisons with the results obtained by a variety of density functional theory calculations on target systems have been presented.

The photocatalytic reduction of nitrous oxide with propane on lead(II) ion-exchanged ZSM-5 catalysts

UV irradiation of the Pb2+/ZSM-5 catalyst prepared by an ion-exchange method in the presence of N2O leads to the decomposition of N2O into N2. This reaction is found to be dramatically enhanced by the addition of propane to produce N2 and oxygen-containing compounds such as ethanol or acetone. UV light effective for the reaction lies in wavelength regions shorter than 250 nm where the absorption band of the Pb2+ ion ([Xe] 4f145d106s2 → [Xe] 4f145d106s16p1) exists, indicating that the excited state of the isolated Pb2+ ions plays a significant role in this decomposition of N2O both in the absence and the presence of propane, and the role of propane is found to be a capture of oxygen atoms formed by the decomposition of N2O.

Incorporation of silver (I) ions within zeolite cavities and their photocatalytic reactivity for the decomposition of N2O into N2 and O2

Ag + /ZSM-5 catalysts were prepared by an ion-exchange method. UV-irradiation of the Ag + /ZSM-5 catalysts in the presence of N 2 O led to the photocatalytic decomposition of N 2 O into N 2 and O 2 at 298 K. Investigations of the effective wavelength of the irradiated UV-light for the reaction as well as the in-situ characterization of the catalysts by means of UV-Vis, photoluminescence and FT-IR spectroscopies revealed that the photoexcitation of the Ag + − N 2 O complexes formed between gaseous N 2 O and the isolated Ag + ions exchanged within the zeolite cavities plays a significant role in the reaction.

Investigations on the local structure of Ag+/ZSM-5 catalysts and their photocatalytic reactivities for the decomposition of N2O at 298 K.

Ag+/ZSM-5 catalysts were prepared by an ion-exchange method and UV-irradiation of the catalysts in the presence of N2O led to the photocatalytic decomposition of N2O into N2 and O2 at 298 K. Investigations on the effective wavelength of irradiated UV-light for the reaction as well as the in-situ characterization of the catalysts by means of XAFS, UV-Vis, photoluminescence and FT-IR spectroscopies revealed that the photoexcitation of the Ag+-N2O complexes formed between gaseous N2O and the isolated Ag+ ions play a significant role in this reaction.

Local structure of highly dispersed lead containing zeolite. An ab initio and density functional theory study

Abstract Ab initio quantum chemical studies at the HF/Lanl2dz, B3PW91/Lanl2dz and B3LYP/Lanl2dz levels were carried out to investigate the local structures of the Pb 2+ active site of lead containing zeolites. It was shown that the coordination number which equals three is found to be the most favorable one for the Pb 2+ site within the zeolite lattice. The latter arises from the ground electronic state for the Pb 2+ within the zeolite lattice that involves lone pair of electrons at the 6s-AO. Thus, the Pb 2+ site would be in a strongly distorted tetrahedral environment. Due to the presence of the above lone pair of electrons, the adsorbate molecules would experience the repulsion from these electrons. The latter may explain some differences observed for the decomposition of pollutant molecules over the Cu + as well as Ag + ion-exchanged zeolites and the Pb 2+ ion-exchanged ones.

Local structure of Pb(II) ion catalysts anchored within zeolite cavities and their photocatalytic reactivity for the elimination of N2O.

The Pb2+/ZSM-5 catalyst was prepared by an ion-exchange method and its photocatalytic activity for the decomposition of N2O under UV irradiation was investigated. In-situ UV-Vis absorption spectroscopy and XAFS (XANES and FT-EXAFS) investigations revealed that the Pb2+ ions exist in a highly dispersed state within the pores of the zeolites. UV irradiation of the catalysts in the presence of N2O led to the photocatalytic decomposition of N2O into N2 at temperatures as low as 298K. The effective wavelength of the irradiated UV light indicated that the excited state of the Pb2+ ions included within the zeolite cavities plays a significant role in the photocatalytic decomposition of N2O molecules.

Local structure of highly dispersed lead species incorporated within zeolite: experimental and theoretical studies

XAFS (both XANES and FT-EXAFS) measurements revealed that the Pb2+ /ZSM-5 catalyst prepared from precursor H-ZSM-5 by a conventional ion-exchange method includes a highly dispersed 3-fold coordinated Pb2+ ion species within the zeolite framework. UV-irradiation of Pb2+ /ZSM-5 led to effective decomposition of NO and N2O producing N2. The photocatalytic decomposition of NO is found to be slightly preferable than that of N2O. The isolated Pb2+ ions play a significant role in the decomposition of pollutant NOx. Ab initio and DFT quantum chemical studies at the HF/Lanl2dz and B3PW91/Lanl2dz levels further shed light on local structures of the Pb2+ active site of lead-containing zeolites, as well as on their interactions with pollutant NO and N2O molecules. In agreement with experiments, 3-fold coordination was found to be the most favorable state for the Pb2+ site within the zeolite framework.

Photochemical properties of Ag ion clusters included within ZSM-5 zeolites prepared by an ion-exchange method

Photoluminescence investigations of the Ag ion-exchanged ZSM-5 (Ag+ /ZSM-5) zeolite revealed that a Ag ion cluster (Agnm+) exists in the pore structure of ZSM-5 exhibiting photoluminesm cence at 380 nm upon excitation at 332 nm. UV irradiation (λ = 285 nm) of Ag+ /ZSM-5 at 77 K leads to the transformation of Agnm+ into a different Ag ion cluster (Agm(n-1)+) which exhibits photoluminescence at 465 nm upon excitation at 315 nm. This photo-transformation of the Ag ion clusters was found to be thermally reversible under vacuum. It was demonstrated that an electron transfer from the photo-excited Al3+ -O2- to Agnm+ plays a significant role in this process. In the presence of oxygen, UV irradiation of Ag+ /ZSM-5 leads to the formation of O2- instead of an Ag ion cluster (Agm(n-1)+), suggesting that oxygen acts as an efficient electron scavenger, which interferes with the electron capture of Agnm+ under UV irradiation at 285 nm.