H₂ Cracking at SiO₂ Defect Centers

Abstract

The interaction of H 2 with the defect sites of the SiO 2 surface has been studied by means of gradient-corrected density functional theory calculations on cluster models. The mechanism of hydrogen dissociation, the energy of reactants and products, and the corresponding activation energies and transition states have been determined for the following defect sites: Si singly occupied sp 3 dangling bonds (E′ centers), tSi • ; nonbridging oxygen centers (NBO), tSi-O • ; divalent Si, dSi:; and neutral oxygen vacancies, tSi-Sit. H 2 cracking on the NBO sites is exothermic by ∼0.4 eV and has an energy barrier of ∼0.1 eV (or less considering nonadiabatic effects) which suggest the occurrence of the process even at low temperature. On Si dangling bonds the formation of tSi-H and neutral H atom is endothermic and occurs with an activation energy of less than 0.5 eV; the reaction can occur at room temperature. The interaction of molecular hydrogen with the diamagnetic oxygen deficient centers, dSi: and tSi-Sit, leads to the formation of stable tSi-H groups with exothermic processes and relatively high activation energies of about 2 eV. Thus, H 2 cracking is predicted to occur at room temperature on paramagnetic defects and only at high temperatures on the diamagnetic centers.

The Effect of Ge Incorporation on the Brønsted Acidity of ZSM‐5

“…Of the 12 distinct tetrahedral sites present in this structure, the T 12 position was chosen as the site of Al substitution, consistent with previous studies, [8][9][10] regarding the acidity of ZSM-5. To saturate the dangling oxygen bonds of the cluster, hydrogen atoms were placed at an appropriate bond length of 0.98 [28] in the direction of the missing OÀSi bonds from the periodic structure. This method of cluster termination has been used widely and successfully in the modelling of silicas and zeolites, [25,29] and owes much of its utility to the electronegativity of hydrogen, which lies between that of silicon and oxygen.…”

Defects in oxide glasses

“…Since interstitial H and H 2 are mobile in silica above ca. 80 and 200 K, respectively, and NBOHC's can crack H 2 molecules [12,53], at room temperature the defects are rapidly annealed:…”

Maximizing the catalytic function of hydrogen spillover in platinum-encapsulated aluminosilicates with controlled nanostructures

“…It was pointed out that hydrogen spillover onto these non-reducible oxides such as SiO 2 , Al 2 O 3 and zeolite would be energetically improbable if there are no defect sites that can stabilize atomic hydrogen 5 . Defect sites generated from Brønsted acid sites (BAS, Si-OH-Al) 11 or other surface hydroxyls [12][13][14][15][16] may stabilize the atomic hydrogen, but their possible contributions have not been systematically investigated. Another important concern is the external surface area of the support matrix.…”