Interaction of H2 with strained rings at the silica surface from ab initio calculations

Importance of the embedding environment on the strain within small rings in siliceous materials

“…Calculating the energy of the planar ring and a relaxed puckered OH-terminated four-ring cluster at exactly the same level of theory reported in Ref. 17 verifies that the planar ring is an energetically higher-lying minima. We also note that the reported O-Si-O angles of 138°in Ref.…”

“…In contrast, the study in Ref. 17, using the B3LYP functional and DFT cluster subtractive calculations, gives a relatively low two-ring strain energy of 1.42 eV. In these calculations it is assumed that a symmetric planar OH-terminated four-ring ͑Si 4 O 4 ͒ is a suitable U system.…”

“…Using an elegant subtractive technique Uchino et al 16 employed four different Si 9 O 25 H 14 clusters to give Hartree-Fock ͑HF͒ calculated estimates of the strain energies of two-rings and three-rings of 1.85 and 0.26 eV, respectively. In another subtractive cluster calculation, using DFT and based directly on OH-terminated small rings of sizes ͑SiO͒ N N =2-4, 17 two-and three-ring strain energies were estimated to be 1.42 and 0.24 eV. The two-ring angular strain energy quoted in Ref.…”

“…The two-ring angular strain energy quoted in Ref. 17 of 0.71 eV is that of an OH-terminated two-ring normalized to its two constitutive O-Si͑OH͒ 2 units. The total energy per two-ring is thus twice this value, giving 1.42 eV per two-ring.…”

Fully Coordinated Silica Nanoclusters:(SiO2)NMolecular Rings

“…The strain energy of a relaxed two-ring has been estimated by numerous high-level calculations in the literature, all giving similar values. Cluster DF calculations using hydrogen-terminated two-rings yield relatively high estimates (0:72 eV=SiO 2 [16]) but neglect the stabilizing effect of an extended silica bonding environment [17]. Periodic DF calculations on two-rings in a relaxed amorphous silica surface give a value of 0:69 eV=SiO 2 [6], while similar calculations comparing the energy of silica-w to that of -quartz, yield 0:62 eV=SiO 2 [18].…”