Three Types of Adsorptions of Nitric Oxide on the MgO Surface

Abstract

FT-IR spectroscopic studies of the adsorption of natural and 18 O-enriched NO gases on thermally and cleaned MgO powders were carried out. The temperature dependence of the IR spectra was investigated to characterize the adsorbate species. Three types [1, 2, and 3] of NO adsorptions were obtained in the wavenumber region of 1500-1000 cm -1 . Those types were examined using density functional theory [(U)B3-LYP/6-31G*] vibrational analyses of (MgO) n -NO cluster calculations. Types 1, 2, and 3 were assigned to monodentate, asymmetric, and symmetric bidentate geometries, respectively. A type 4, a "tridentate" model which involves a NO 3 -like species, was calculated and was found to give wavenumbers of less than 1000 cm -1 . The splitting of IR bands of type 3 due to the 15 N 18 O adsorbate was ascribed to the oxygen exchange, 15 N 18 O f 15 N 16 O, via the reaction 16 O-15 N-18 O f 16 O-15 N-16 O on MgO surfaces.

First-Principles Characterization of NOx Adsorption on MgO

“…To treat the neutral and charged adsorbates even-handedly and to facilitate analysis, we employ cluster models of the MgO(001) surface. These models have been used successfully in the past to study NO x adsorption on alkaline earth oxide surfaces. ,, As will be discussed, they provide adsorption geometries and energetics comparable to published supercell calculations of NO x adsorption on MgO . While an embedding field of point charges is sometimes used to simulate the electrostatic potential of the extended ionic lattice (e.g., see ref ), such embedding was not found here to be necessary to obtain reliable results.…”

“…One solution being explored is NO x storage catalysts or “traps” that hold, release, and reduce NO x through operation in alternating fuel-lean (NO x -oxidizing) and fuel-rich (NO x -reducing) modes . The trapping mechanism is thought to involve NO oxidation to NO 2 on a precious metal constituent and subsequent adsorption on an alkaline earth oxide constituent as nitrate (NO 3 - ). − However, the precise roles of both the precious metal and the metal oxide in the formation of nitrate are not well understood. To help unravel the questions behind this complex adsorption behavior, NO, NO 2 , and NO 3 have been separately characterized on several representative alkaline earth metal oxide surfaces using both experiment and theory.…”

“…Temperature-programmed desorption (TPD) experiments indicate that NO desorbs in two peaks from alkaline earth oxide surfaces, − which have been attributed to two types of surface adsorbates . TPD experiments with isotopically labeled oxygen show exchange between NO and MgO, CaO, and SrO surfaces prior to desorption, which has been proposed to occur via nitrite (NO 2 - ) or nitrate surface intermediates. ,, Similarly, several in situ Raman and FTIR studies of NO + O 2 reaction on alkaline earth oxides propose metal−nitro, nitrite, or monodentate and bidentate nitrate intermediates or a combination of these as precursors to NO decomposition. ,− …”

“…12 TPD experiments with isotopically labeled oxygen show exchange between NO and MgO, CaO, and SrO surfaces prior to desorption, which has been proposed to occur via nitrite (NO 2 -) or nitrate surface intermediates. 11,12,14 Similarly, several in situ Raman and FTIR studies of NO + O 2 reaction on alkaline earth oxides propose metal-nitro, nitrite, or monodentate and bidentate nitrate intermediates or a combination of these as precursors to NO decomposition. 4,[15][16][17][18][19] Computational studies suggest that a single NO molecule weakly adsorbs on the (001) terraces of MgO and CaO, forming neither nitrite nor nitrate.…”

Formation of radical anions on the adsorption of NO + O2 and NO2 + O2 mixtures on ZrO2 according to EPR data

“…The dependence of the O 2 concentration on T v and P NO allows us to conclude that Lewis acid sites are formed during the dehydration of the ZrO 2 surface. [10][11][12] We estimated the concentration of coordinatively unsaturated Zr 4+ cations from the EPR spectra of (NO-Zr 4+ ) complexes 13 formed on the adsorption of NO (1×10 2 Pa, 77 K) to be 2×10 19 g -1 . This value is consistent with the concentration of O 2 radicals 6×10 18 g -1 .…”