XANES reflects coordination change and underlying surface disorder of zinc adsorbed to silica

Abstract

Zinc K-edge X-ray absorption near-edge structure (XANES) spectroscopy of Zn adsorbed to silica and Zn-bearing minerals, salts and solutions was conducted to explore how XANES spectra reflect coordination environment and disorder in the surface to which a metal ion is sorbed. Specifically, XANES spectra for five distinct Zn adsorption complexes (Znads) on quartz and amorphous silica [SiO2(am)] are presented from the Zn–water–silica surface system: outer-sphere octahedral Znads on quartz, inner-sphere octahedral Znads on quartz, inner-sphere tetrahedral Znads on quartz, inner-sphere octahedral Znads on SiO2(am) and inner-sphere tetrahedral Znads on SiO2(am). XANES spectral analysis of these complexes on quartz versus SiO2(am) reveals that normalized peak absorbance and K-edge energy position generally decrease with increasing surface disorder and decreasing Zn–O coordination. On quartz, the absorption-edge energy of Znads ranges from 9663.0 to 9664.1 eV for samples dominated by tetrahedrally versus octahedrally coordinated species, respectively. On SiO2(am), the absorption-edge energy of Znads ranges from 9662.3 to 9663.4 eV for samples dominated by tetrahedrally versus octahedrally coordinated species, respectively. On both silica substrates, octahedral Znads presents a single K-edge peak feature, whereas tetrahedral Znads presents two absorbance features. The energy space between the two absorbance peak features of the XANES K-edge of tetrahedral Znads is 2.4 eV for Zn on quartz and 3.2 eV for Zn on SiO2(am). Linear combination fitting of samples with a mixture of Znads complex types demonstrates that the XANES spectra of octahedral and tetrahedral Znads on silica are distinct enough for quantitative identification. These results suggest caution when deciphering Zn speciation in natural samples via linear combination approaches using a single Znads standard to represent sorption on a particular mineral surface. Correlation between XANES spectral features and prior extended X-ray absorption fine structure (EXAFS) derived coordination environments for these Znads on silica samples provides insight into Zn speciation in natural systems with XANES compatible Zn concentrations too low for EXAFS analysis.

Structure Sensitivity of ZnZrOx Catalysts in CO2 Hydrogenation to Methanol: Significance of Surface Oxygen Content and Synthesis Strategy

“…According to the Zn K-edge X-ray absorption near-edge structure (XANES), Zn in CP std -ZnZrO x is in the approximate 2+ state (Figure S2a). Comparing the K-edge energy position ( E 0 ) and normalized edge absorbance of tetrahedral and octahedral Zn 2+ compounds (Figure S2b), we found that tetrahedral Zn 2+ species generally have lower E 0 and absorbance of edge than octahedral Zn 2+ species. Notably, Zn in CP std -ZnZrO x shows an intermediate value of E 0 and edge absorbance (9663 eV and 1.65, respectively) compared to other standard Zn compounds, indicating that the chemical state of Zn is intermediate between tetrahedral and octahedral.…”

“…According to the Zn K-edge X-ray absorption near-edge structure (XANES), Zn in CPstd-ZnZrO x is in the approximate 2+ state (Figure S2a). Comparing the K-edge energy position (E 0 ) and normalized edge absorbance of tetrahedral and octahedral Zn 2+ compounds (Figure S2b), 45 we found that tetrahedral Zn 2+ species generally have lower E 0 and absorbance of edge than octahedral Zn CO 2 -to-MeOH Performance of ZnZrO x Catalysts. The CO 2 hydrogenation performance of ZnZrO x catalysts was measured at 320 °C, 5 MPa, CO 2 /H 2 /Ar = 24/72/4, and 24,000 cm 3 g cat.…”

“…D 2 -adsorbed spectra were measured at 200 °C under 100 kPa D 2 flow (40 cm 3 min −1 ). To obtain the time-dependent spectra, the IR signals were recorded every 1 min and the spectra at 0, 1, 2, 5, 10, 25,30,35,40,45,50,55, and 60 min time-on-stream of D 2 flow are demonstrated. In situ DRIFTS during CO 2 hydrogenation were carried out after sequential gas treatment.…”

Interplay Between Element-Specific Distortions and Electrocatalytic Oxygen Evolution for Cobalt-Iron Hydroxides

“…23,24 Local bonding geometry affects electronic structure, which affects the shape and location of K-edge spectra without altering oxidation states. [33][34][35][36][37][38][39] For example, the Zn(II) K-edge is shifted and distorted by local geometry when ions adsorbed to silica and quartz, 33 changes in coordination symmetry for Cu(II) ions distort the rising edge portion of spectra, 37 and even the fluorine K-edge spectra show variations with changes in local symmetry in the highly-ionic alkali and alkali earth salts. 38 Simple analysis of edge location cannot differentiate between the two possibilities.…”

Biogenic-to-lithogenic handoff of particulate Zn affects the Zn cycle in the Southern Ocean

“…The XANES spectra were collected from ~10 or more regions (Zn-containing particles) of sample filters to evaluate variations in Zn speciation, encompassing particles with variable Zn concentrations and the diversity of pZn species found in the filter samples. Because Zn-XANES spectra are highly sensitive to Zn speciation, we compiled more than 50 Zn reference spectra of standard compounds, whose Zn coordination is well understood ( 25 – 29 ) (Fig. 2).…”