Shin-ichi Tanaka

Simultaneous removal of N2O and CH4 as the strong greenhouse‐effect gases over Fe‐BEA zeolite in the presence of excess O2

Simultaneous catalytic removal of N2O and CH4 as the strong greenhouse‐effect gases was found to be possible over an Fe‐ion‐exchanged BEA zeolite (Fe‐BEA) by the selective catalytic reduction (SCR) of N2O with CH4. The direct decomposition of N2O (2N2O → 2N2 + O2) and the oxidation of CH4 (CH4 + 2O2 → CO2 + 2H2O) over Fe‐BEA zeolite required high temperature above 400 and 450 °C, respectively. Nevertheless, the catalytic reduction of N2O by adding CH4 over Fe‐BEA zeolite readily occurred at much lower temperatures (ca. 250–350 °C) whether in the presence of O2 or not. No oxidation of CH4 by O2 took place at these temperatures. On the basis of these results and the kinetic studies, it was concluded that CH4 reacted selectively with N2O to produce N2, CO2 and H2O over Fe‐BEA zeolite even in the presence of excess O2. Overall stoichiometry of the SCR of N2O with CH4 was determined as follows: 4N2O + CH4 → 4N2 + CO2 + 2H2O.

Reaction between N2O and CH4 over Fe ion-exchanged BEA zeolite catalyst: A possible role of nascent oxygen transients from N2O

The reaction between N2O and CH4 over an Fe ion-exchanged BEA zeolite (Fe-BEA) catalyst was studied by using a pulse reaction technique, temperature-programmed desorption (TPD) and infrared (IR) spectroscopy. N2O readily reacted with CH4 in the presence of an N2O + CH4 mixture above 200 °C, while both the O2 + CH4 reaction and the catalytic decomposition of N2O over the Fe-BEA catalyst required higher temperatures (above 400 °C). In the O2-TPD studies, a desorption peak of O2 was observed above 600 °C after O2 treatment at 250 °C, while a new O2 desorption peak appeared at the lower temperatures after N2O treatment at 250 °C. However, the new O(a) species resulting from the N2O treatment hardly reacted with CH4 even at 350 °C, which was confirmed by the CH4-pulsed experiments. On the other hand, a new IR band at 3683 cm−1, which can be assigned to the OH group on Fe ion species, was observed after O2 or N2O treatment. The peak intensity at 3683 cm−1 was not changed in the exposure of CH4 only, but decreased in the exposure of N2O + CH4 mixture above 150 °C. At the same time, the CHxOy(a) species such as Fe–OCH3 were formed, which were observed by IR measurements. The adsorbed surface species showed a high reactivity with N2O even at low temperatures (∼200 °C). A possible mechanism is discussed in terms of active oxygen species such as nascent oxygen transients (O*(a)), which are formed in the exposure of N2O + CH4 mixture, and may play an important role in the activation/oxidation of CH4 at initial steps to form CHxOy(a) species.

Mechanism of N2O decomposition over a Rh black catalyst studied by a tracer method the reaction of N2O with 18O(a)

Abstract N2O decomposition on an unsupported Rh catalyst has been studied using tracer technique in order to reveal the reaction mechanism. N216O was pulsed onto 18O/oxidized Rh catalyst at 220°C and desorbed O2 molecules (m/e=32,34,36) were monitored by means of mass spectrometer. The 18O fraction in the desorbed dioxygen was the same value as that on the surface oxygen. The result shows that the O2 molecules desorb via Langmuir–Hinshelwood mechanism, i.e., the desorption of dioxygen through the recombination of adsorbed oxygen. On the other hand, TPD measurements in He showed that desorption of oxygen from the Rh black catalyst occurred at the higher temperatures. Therefore, reaction-assisted desorption of oxygen during N2O decomposition reaction at the low temperature was proposed.

Isotopic Study of N2O Decomposition on an Ion-Exchanged Fe-Zeolite Catalyst: Mechanism of O2 Formation

N2O decomposition on an ion-exchanged Fe-MFI catalyst has been studied using an 18O-tracer technique in order to reveal the reaction mechanism. N216O was pulsed onto an 18O2-treated Fe-MFI catalyst at 693 K, and the O2 molecules produced were monitored by means of mass spectrometry. The 18O fraction in the produced oxygen had almost half the value of that on the surface oxygen, and 18O18O was not detected. The result shows that O2 formation proceeds via the Eley–Rideal mechanism (N216O + 18O(a) → N2 + 16O18O).

Selective catalytic reduction of N2O with C3H6 over Fe-ZSM5 catalyst in the presence of excess O2: The correlation between the induction period and the surface species produced

The catalytic activity of N2O reduction over Fe-ZSM5 prepared by an ion exchange method was significantly increased by adding C3H6, even in the presence of excess O2, i.e., selective catalytic reduction (SCR) of N2O occurred with C3H6. We found that there was an induction period for the increase in activity after adding C3H6 to the N2O–O2 flow. Another induction period was also observed for the decrease in activity after removing C3H6 or O2–C3H6 from the N2O–O2–C3H6 mixture. The length of the induction period was influenced by the partial pressure of C3H6, the composition of the reaction gases and the reaction temperature. During the latter induction period, CO2, H2O and N2 were detected as products. These results suggest that adsorbed surface species such as CxHy(a) and/or CxHyOz(a), produced by the reaction in the N2O–O2–C3H6 mixture, reacted with N2O or N2O–O2 to produce CO2, H2O and N2. In situ DRIFT studies showed the formation of CxHy(a) and/or CxHyOz(a) species during the N2O–O2–C3H6 reaction, the behavior of which (formation and reactivity) correlated well with the induction period phenomenon.

Enhancement of C2H6 Oxidation by O2 in the Presence of N2O over Fe Ion-Exchanged BEA Zeolite Catalyst

Selective catalytic reduction (SCR) of N2O with C2H6 took place effectively over Fe ion-exchanged BEA zeolite catalyst (Fe-BEA) even in the presence of excess oxygen. The mechanism in the SCR of N2O with C2H6 over Fe-BEA catalyst was studied by a transient response experiment and an in situ DRIFT spectroscopy. No oxidation of C2H6 by O2 took place below 350 °C (in C2H6/O2). In the N2O/C2H6/O2 system, however, it was found that the reaction of C2H6 with O2 was drastically enhanced by the presence of N2O even at low temperatures (200-300 °C). Therefore, it was concluded that N2O played an important role in the oxidation of C2H6 (i.e., activation of C2H6 at an initial step). On the basis of these findings, the mechanism in the SCR of N2O with C2H6 is discussed.

Isotopic study of nitrous oxide decomposition on an oxidized Rh catalyst: mechanism of oxygen desorption

N2O decomposition on an oxidized Rh catalyst (unsupported) has been studied using a tracer technique in order to reveal the reaction mechanism. N216O was pulsed onto an 18O/oxidized Rh catalyst at 493 K and desorbed O2 molecules were monitored. The 18O fraction in the desorbed oxygen had the same value as that on the surface oxygen. The result shows that the oxygen molecules do not desorb via the Eley–Rideal mechanism, but via the Langmuir–Hinshelwood mechanism. On the other hand, desorption of oxygen from Rh surfaces (in vacuum or in He) occurs at higher temperatures, which suggests reaction-assisted desorption of oxygen during the N2O decomposition reaction at low temperature.

Selective catalytic reduction of N2O with methane in the presence of excess oxygen over Fe-BEA zeolite

An Fe ion-exchanged BEA zeolite (Fe-BEA), which effectively performs selective catalytic reduction of N2O with methane in the presence of excess oxygen, is much more active than Fe-MFI zeolite reported in the literature.