DRIFTS Study of Ammonia Activation over CaO and Sulfated CaO for NO Reduction by NH₃

Abstract

CaO catalyzes NH(3) oxidation, while sulfated CaO catalyzes NO reduction by NH(3) in the presence of O(2), and the adsorption and transformation of ammonia over CaO and sulfated CaO has been investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to understand their catalytic mechanism. It has been found that ammonia is first adsorbed over Lewis or Brönsted acid sites, and later undergoes hydrogen abstraction giving rise to either NH(2) amide or NH imide intermediates. The intermediates react with NO or lattice O to produce N(2) or NO. Comparing the DRIFTS of NH(3) adsorption over CaO and sulfated CaO, it is obvious that ammonia adsorbed over CaO is activated mainly in NH form apt to react with surface oxygen to produce NO, while ammonia adsorbed over sulfated CaO is activated mainly in NH(2) form apt to reduce NO. The DRIFTS results agree with experimental data and explain the catalytic mechanisms of CaO and sulfated CaO.

In situ FT-IR study of highly dispersed MnO x /SAPO-34 catalyst for low-temperature selective catalytic reduction of NO x by NH 3

“…Additionlly, the bands at 1259 and 1150 cm −1 represent the adsorbed NH 3 over Lewis acid sites [37,43,50]. After the sample was purged by Ar for 30 min, the intensity of the bands was unchanged, indicating that the adsorbed NH 3 was relatively stable.…”

“…It is worth mentioning that the intensity of band at 1540 cm −1 ascribed to the amide species (-NH 2 ) weakened significantly, and the intensity of the bands at 1468 and 1405 cm −1 due to NH 4 + on Brønsted acid sites [43,45] decreased relatively slightly. During this process, the intensity of the bands at 1259 and 1150 cm −1 due to coordinate ammonia species on Lewis acid sites [37,43,50] and the intensity of bands at 3338 and 3228 cm −1 in the N H stretching region varied slightly, due to the compensation from the gaseous or weakly adsorbed NH 3 (964 cm −1 ). After the catalyst was purged with NO + O 2 for 10 min, the bands at 1060 cm −1 (linear nitrite) and 1610 cm −1 (v(ONO), bridged nitrate) could be observed and the intensity of the bands increased with time.…”

Bauxite-supported Transition Metal Oxides: Promising Low-temperature and SO2-tolerant Catalysts for Selective Catalytic Reduction of NOx

“…Mn/bauxite shows two bands at 1236 and 1329 cm −1 owing to the coordination of NH 3 on Lewis acid sites. Other two bands at 1489 and 1438 cm −1 in bauxite may be assigned to -NH 2 vibration and anti-symmetric bending vibration of NH 4 + ions on Brønsted acid sites, respectively 3334. In Mn/bauxite, the band at 1387 and 1498 cm −1 represent −NH 2 vibration on Brønsted acid sites.…”

Effects of water vapor, CO2 and SO2 on the NO reduction by NH3 over sulfated CaO

“…The in situ DRIFTS experiments of NH 3 adsorption over sulfated CaO in our previous study indicated that NH 3 adsorbed over both Lewis acid sites and Brønsted acid sites[30]. CO 2 and SO 2 , as the acidic gas may influence the NH 3 molecule adsorbed over Lewis acid sites, and thus influence the SCR reaction.…”