¹H Magic Angle Spinning NMR Evidence for Dissociative Adsorption of Hydrogen on Ag⁺-Exchanged A- and Y-Zeolites

Abstract

1H magic angle spinning NMR gave firm evidence for formation of acidic protons (4.0 ppm) and hydride species (−1.8 ppm), where the former exists as OH groups (acidic protons) and the latter has an interaction with three equivalent Ag atoms, existing as Ag3H when Ag+-exchanged A-zeolite is reduced with hydrogen. The formations of Ag3H and of acidic protons by the heterolytic dissociation of H2 over Ag3 + are reversible. The heterolytic dissociation of H2 was also observed over Ag-Y-zeolite, the silver hydride species being observed at −0.1 ppm.

Propene Production from Ethene and Methane Using Silver- and Proton-exchanged Zeolite Catalysts

“…The reversible change of acidic protons and Ag3 _ H was confirmed by 1 H MAS NMR measurements of Ag _ A in the presence and absence of H2 38) . After evacuation of the hydrogen-reduced Ag _ A at 313 K, the 1 H MAS NMR peaks due to acidic protons and Ag3 _ H ( Fig.…”

“…The partial reduction Ag3PW12O40 thus has a particular effect on catalytic activity in the presence of hydrogen due to the behavior of acidic protons. Figure 3 shows the 1 H MAS NMR spectrum for Ag _ A under hydrogen after hydrogen reduction at 313 K 38) . Two signals are apparent, at 4.0 ppm and -1.8 ppm.…”

“…As acidic pro tons have been observed at 3.9-4.4 ppm in various zeo lites 40) , the peak at 4.0 ppm can be confidently attributed to the acidic protons generated by the reduction of Ag ＋ under hydrogen. The signal at -1.8 ppm, consisting of four peaks with peak intensity ratios of 1 : 3 : 3 : 1 and a coupling constant of (131±1) Hz, is considered to represent protons interacting with three equivalent silver atoms and/or ions 38) . As metallic silver does not adsorb hydrogen, the cationic silver must be the chemi sorption center for the hydrogen molecules.…”

“…(7)). The follow ing scheme for heterolytic cleavage of the C _ H bond of CH4 is proposed as a plausible mechanism for this reac tion 38) :…”

Investigation on the catalytic roles of silver species in the selective catalytic reduction of NO with methane

“…As has been shown by many kinetic studies that the rate determining step in the CH 4 /NO/O 2 reaction is the breaking of C-H bond [38,39]. According to mechanisms proposed by Xu et al [40] in CH 4 dehydroaromatization and by Baba et al [41] in CH 4 oxidative coupling, we proposed the following mechanism for the activation of CH 4 by silver ions in the zeolite cation sites:…”