No abstract

No abstract

No abstract

Phosphorus K-edge X-ray absorption fine structure (XAFS) was explored as a means to distinguish between aqueous and solid phosphates and to detect changes in phosphate protonation state. Data were collected for H 3 PO 4 , KH 2 PO 4 , K 2 HPO 4 and K 3 PO 4 solids and solutions and for the more complex phosphates, hydroxylapatite (HAP) and struvite (MAP). The X-ray absorption near-edge structure (XANES) spectra for solid samples are distinguishable from those of solutions by a shoulder at ∼4.5 eV above the edge, caused by scattering from cation sites. For phosphate species, the intensity of the white line peak increased for solid and decreased for aqueous samples, respectively, with phosphate deprotonation. This was assigned to increasing charge delocalization in solid samples, and the effect of solvating water molecules on charge for aqueous samples. In the extended X-ray absorption fine structure (EXAFS), backscattering from first-shell O atoms dominated the (k) spectra. Multiple scattering (MS) via a four-legged P-O 1 -P-O 1 -P collinear path was localized in the lower k region at ∼3.5 Å -1 and contributed significantly to the beat pattern of the first oscillation. For EXAFS analysis, increasing Debye-Waller factors suggest more disorder in the P-O shell with addition of protons to the crystal structure due to the lengthening effects of P-OH bonds. This disorder produces splitting in the hybridized P 3p-O 2p band in the density of states. For aqueous samples, however, increased protonation reduced the structural disorder within this shell. This was linked to a change from kosmotropic to chaotropic behavior of the phosphate species, with reduced effects of H bonding on structural distortion. The intensity of MS is correlated to the degree of disorder in the P-O shell, with more ordered structures exhibiting enhanced MS. The observed trends in the XAFS data can be used to distinguish between phosphate species in both solid and aqueous samples. This is applicable to many chemical, geochemical and biological systems, and may be an important tool for determining the behavior of phosphate during the hydrothermal gasification of biomass.

A method is presented to prepare nanocrystalline α‐Zn2SiO4 with the smallest crystal size reported so far for this system. Our approach combines the advantages of organometallic single‐source precursor routes with aerosol processing techniques. The chemical design of the precursor enables the preferential formation of pure zinc silicates. Since gas‐phase synthesis reduces intermolecular processes, and keeps the particles small, zinc silicate was synthesized from the volatile organometallic precursor [{MeZnOSiMe3}4], possessing a Zn‐methyl‐ and O‐silyl‐substituted Zn4O4‐heterocubane framework (cubane), under oxidizing conditions, using the chemical vapor synthesis (CVS) method. The products obtained under different process conditions and their structural evolution after sintering were investigated by using various analytical techniques (powder X‐ray diffraction, transmission electron microscopy, EDX analysis, solid‐state NMR, IR, Raman, and UV/Vis spectroscopy). The deposited aerosol obtained first (processing temperature 750 °C) was amorphous, and contained agglomerates with primary particles of 12 nm in size. These primary particles can be described by a [Zn‐O‐Si] phase without long‐range order. The deposit obtained at 900 °C contained particles with embedded nanocrystallites (3–5 nm) of β‐Zn2SiO4, Zn1.7SiO4, and ZnO in an amorphous matrix. On further ageing, the as‐deposited particles obtained at 900 °C form α‐Zn2SiO4 imbedded in amorphous SiO2. The crystallite sizes and primary particle sizes in the formed α‐Zn2SiO4 were found to be below ∼50 nm and mainly spherical in morphology. A gas‐phase mechanism for the particle formation is proposed. In addition, the solid‐state reactions of the same precursor were studied in detail to investigate the fundamental differences between a gas‐phase and a solid‐state synthesis route.