The structure of zinc and copper oxide species hosted in porous siliceous matrices

Abstract

Microporous and mesoporous siliceous phases (MFI, FAU, MCM-48, MCM-41, and SBA-15) were modified with zinc via aqueous and organometallic routes and characterized with techniques that reflect the structure of the matrix, the distribution of zinc over the matrix, and the structure of the zinc oxide species (XRD, nitrogen physisorption, IR spectroscopy, XPS, X-ray absorption fine structure (XAFS), UV-Vis spectroscopy). For comparison, MFI and MCM-48 were modified with copper alone and characterised by XPS/X-ray induced Auger electron spectroscopy and XAFS. From the results, it can be concluded that zinc interacts strongly with the siliceous surfaces, which prevents the formation of ZnO aggregates even when the coverage exceeds the monolayer limit. While zinc could be well distributed over FAU and mesoporous matrices by treatment with diethyl zinc, it remained on the external surface of MFI, possibly due to pore entrance narrowing by adsorbed species. For MCM-48 and MCM-41, the formation of a thin, probably monolayer surface zinc silicate coating was concluded from the data. With an aqueous impregnation technique, strong changes in the pore system of MCM-48 were noted, which indicate rupture of pore walls and partial structural damage. Different from zinc, copper forms small oxide aggregates upon introduction into micro and mesoporous siliceous matrices

Cu/ZnO aggregates in siliceous mesoporous matrices: Development of a new model methanol synthesis catalyst

“…For comparison, a preparation with an MCM-48 matrix is reported as well. The synthesis of this matrix has been described previously [23] where characterization data are given as well [XRD-d spacing of (211) …”

“…However, this approach carries the risk of interference by the pore walls of the matrix. Indeed, a strong interaction was previously observed between the silicate wall and zinc species introduced by aqueous techniques or by interaction with dimethyl zinc, which was stable during calcination and rendered the zinc in a completely disordered state, probably as a zinc silicate layer covering the wall instead of nanoparticulate ZnO [22,23]. In redox experiments in which the response of the IR reflectivity on changes in the gas atmosphere was followed, these zinc species turned out to be redox-inactive [23], unlike bulk ZnO.…”

“…After considerable effort aimed at achieving this goal with seemingly simple preparations starting from aqueous copper and zinc acetate solutions [22,23], a promising catalyst was obtained via a route involving the deposition of zinc into a mesoporous silicate via thermolysis of a liquid organometallic precursor [16]. Conventional impregnation of such ZnO-modified mesoporous silicate with copper nitrate resulted in a methanol synthesis catalyst with an activity of the same order of magnitude as a catalyst of technical relevance and clear indications of zinc redox activity as demonstrated by XAFS.…”

“…Table 1 summarizes the samples and gives their metal content and the labels used to describe them throughout this paper. The compositions were determined by ICP analysis or assessments on the basis of the XAFS step heights, which have given comparable results over a wide range of materials studied [22,23].…”

Nanostructured lanthanum cobaltate: oxidation and coordination states of Co atoms

“…After encapsulating the LaCo oxide species, the first signal was strongly decreased and the weaker signals have disappeared. While this might indicate serious structural damage of the regular MCM-41 pore system, such attenuation of the XRD signals of ordered silica matrices may be also resulted from the statistical distribution of heavy matter over the pore system because of the modified scattering contrast between pore walls and pore spaces filled with an amorphous phase [10][11][12]. The ESR spectra of bulk cobalt oxide Co 3 O 4 and LaCoO 3 perovskite are shown in Figure 2 (spectra a and b, respectively).…”