An infrared spectroscopic study of the adsorption of BF3 and BCl3 on silica has been carried out. The silica samples were dehydrated under vacuum at various temperatures from 400-1020°C before admitting the adsorbate to the reaction cell and the nature of surface species produced depended on this pretreatment temperature. Upon adsorbing BF3 on a silica sample that had been previously evacuated at temperatures below 1000°C, a set of bands were produced which were replaced by a new set of bands on evacuation. Similar results were observed with BCl3 treated silica. However, during this evacuation, a growth of the 3749 cm-1 Si-O-H band was observed after BF 3 adsorption but a corresponding growth of the 3700 cm-1 B-OH band was observed after BCl3 adsorption. Further, the initial set of bands, produced on adding BF3 to silica samples which had been previously dehydrated at temperatures above 1000°C were extremely stable to evacuation, whereas the corresponding bands with BCl 3 were again replaced on evacuation. A mechanism is proposed that would explain the observed results. Some of the features of this mechanism are: (1) BX3 (X=F, Cl) initially reacts with the surface hydroxyls to give Si-O-BX2 type species. (2) On evacuation, these groups further react to give the Si-O2 -BX type species. (3) A small amount of 'H2O' is available to the surface during evacuation and this 'H2O' reacts with the BX3 treated surface to refurnish Si-O-H groups. (4) However, the Si-O-BCl2 and the Si-O2-BCl groups further react with this 'H2O' to give Si-O-B-(OH)2 and Si-O2 -B(OH). It has been shown that both BF3 and BCl3 are chemically adsorbed on silica evacuated at very high temperatures. It has been proposed that on these surfaces, various other active sites, for example Si-O• radicals and Si-O-Si bridges are possibly present. An attempt has been made to study the bands due to BO2 symmetrical stretching modes in the 1200 cm-1 region by adsorbing BX3 on various other oxides, e.g. Al2O3, TiO2, MgO and ZnO. It has been shown that the surface silanols exchange with H2O 18 at elevated temperatures and a maximum 70% exchange has been obtained at 400°C. A new band present at 3738 cm-1 is assigned to the OH stretching mode of Si-O18-H. Using the O18 exchange technique it has been possible to assign surface adsorbate stretching modes arising from species of the type Si-O-BX2 .
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/7170 |
| Date | January 1971 |
| Creators | Paul, Ashoka |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Detected Language | English |
| Type | Thesis |
Page generated in 0.0016 seconds