The effect of phosphate treatment on nickel dispersion on MCM-41 mesoporous material

The incorporation of nickel into mesoporous molecular sieves MCM-41 was carried out. Ni-PO/MCM41 and Ni-Cl/MCM41 were prepared by using Ni(II) acetate solution adjusted to pH=2.5 with phosphoric acid and hydrochloric acid, respectively, by the incipient wetness method. Photoacoustic spectroscopy (PAS) was used to study the local environments of Ni(II) incorporated into mesopores. The PAS of as-prepared Ni/MCM41, Ni-Cl/MCM41, and Ni-PO/MCM41 with Ni(II) acetate solution exhibits two bands of lambda(max) around 400 nm and 750 nm, which could be assigned to the (3)A(2g) --> T-1g(F) and the (3)A(2g) --> 3T(1g)(P) transition of octahedral Ni(II) species. After calcination, Ni ion within Ni-PO/MGM41 dispersed atomically onto the Surface of MCM-41 channel as an octahedral species without the transformation to nickel oxide phase, while Ni ion within Ni-Cl/MCM41 and Ni/MCM41 transformed to nickel oxide phase. It may be attributed to the fact that each Ni ion is separated by the phosphate grafted to surface silanol group. The effects of phosphoric acid on the dispersity of Ni ion within MCM-41 have been investigated using P-31 and Si-21 MAS NMR spectroscopy and X-ray photoelectron spectroscopy.