MoO_3负载在新型Ti O_2/γ-Al_2O_3载体上的表面状态与性质

SURFACE STATE AND PROPERTIES OF MoO_3 SUPPORTED ONTO THE SURFACE OF TiO_2/γ-Al_2O_3 CARRIER

MoO_3担载在TiO_2或γ-Al_2O_3载体上是一类重要的工业催比剂,其表面状态和表面性质国内外已有过许多报导,本文报导MoO_3负载在我们所研制的新型TiO_2/γ-Al_2O_3载体(指以γ-Al_2O_3为骨架,表面上铺以TiO_2物种的载体)上的表面状态与性质。

In this paper, surface state and properties of MoO_3 supported onto the surface ofTiO_2/γ-Al_2O_3 carrier (i. e. TiO_2 supported onto the γ-Al_2O_3) are reported. Carrier weselected is S-20 of which TiO_2 loading is 0.20g/g γ-Al_2O_3. The physico-chemical pro-perties of the carrier was described in reference [6]. MoO_3 supported on carrier wasprepared in two ways: one is general impregnation method and another is that theMoO_3 and S-20 powders were mixed throughly and then the mixture was calcined at450℃ for 24 hrs. The surface state of MoO_3/TiO_2/γ-Al_2O_3 was characterized by usingXRD, XPS, LRS and ammonia leaching techniques. It shows that the surface propertiesof it are quite different from MoO_3 supported on TiO_2 or γ-Al_2O_3. This results areof benefit to developing new catalysts. The experimental results of XRD quantitative analysis and XPS intensity ratioand LRS quantitative analysis show that the utmost dispersion capacity of MoO_3 on thesurface of S-20 approaches 0.32g MoO_3/g S-20 which is much bigger than the valuecalculated by close-packed model. However, the utmost monolayer dispersion capacityof MoO_3 supported on TiO_2 or γ-Al_2O_3 meets the calculated value very well. It illus-trates that the new carrier we made has an ability to increasing the MoO_3 dispersioncapacity to 23% more. The results of LRS and ammonia leaching and XRD experiments reveal the che-mical state of MoO_3 supported on S-20 carrier. When MoO_3 loadining is less than0.140g/g S-20, a part of dispersed state MoO_3 is dissolved into ammonia water andthe ratio of dissolved part and undissolved part is 7:3. This is in line with the re-sults of ammonia leaching of MoO_3/TiO-2 and MoO_3/γ-Al_2O_3 system. However, whenMoO_3 loading is bigger than 0.140g/S-20, the increment of MoO_3 is dissolved into theammonia water completly. The results of LRS show that Mo species dissolved intoammonia water resides in octahedrally coordinated sites Mo(O) and Mo species undis-solved resides in the tetrahedrally coordinated sites Mo(T). The results of LRS quan-titative analysis shows that MoO_3 surface species Mo(O) increases with the increaseof MoO_3 loading in the range of lower MoO_3 loading and MoO_3 surface species Mo(O) is no longer increase with the increase of MoO_3 loading when MoO_3 loading isbigger than 0.25 g/g S-20. It follows that the increment of MoO_3 in the range of0.25-0.31g MoO_3/g S-20 might be a new type of MoO_3 species. Furthermore the LRS results indicate that in the range of 0.25-0.31 g MoO_3/g S-20, the increment ofMoO_3 shows 1002cm^(-1) Raman peak which some author assigned it to Al_2(MoO_4)_3. Inour case, we believe that it is small cluster of three dimensional polymolebdate be-cause it also is dissolved into ammonia water. The distribution of the species ofMoO_3 supported onto the surface of S-20 carrier is described in the following figure. In addition, S-20 carrier has a behaviour to resist the formationor Al_2 (MoO_4)_3. MoO_3/S-20 system even calcined at 700℃, no crystal-line Al_2(MoO_4)_3 was detected by XRD.