Sulfur Speciation in Biochars by Very High Resolution Benchtop Kα X-ray Emission Spectroscopy.

The analytical chemistry of sulfur-containing materials poses substantial technical challenges, especially due to the limitations of 33S NMR and the time-intensive preparations required for wet-chemistry analyses. A number of prior studies have found that synchrotron-based X-ray absorption near edge structure (XANES) measurements can give detailed speciation of sulfur chemistry in such cases. However, due to the obvious access limitations, synchrotron XANES of sulfur cannot be part of routine analytical practice across the chemical sciences community. Here, in a study of the sulfur chemistry in biochars, we compare and contrast the chemical inferences available from synchrotron XANES with that given by benchtop, extremely high resolution wavelength-dispersive X-ray fluorescence (WD-XRF) spectroscopy, also often called X-ray emission spectroscopy (XES). While the XANES spectra have higher total information content, often giving differentiation between different moieties having the same oxidation state, the lower sensitivity of the S Kα XES to coordination and local structure provides pragmatic benefit for the more limited goal of quantifying the S oxidation state distribution. Within that constrained metric, we find good agreement between the two methods. As the sulfur concentrations were as low as 150 ppm, these measurements provide proof-of-principle for characterization of the sulfur chemistry of biochars and potential applications to other areas such as soils, batteries, catalysts, and fossil fuels and their combustion products.

Speciation of sulfur in biochar produced from pyrolysis and gasification of oak and corn stover.

The effects of feedstock type and biomass conversion conditions on the speciation of sulfur in biochars are not well-known. In this study, the sulfur content and speciation in biochars generated from pyrolysis and gasification of oak and corn stover were determined. We found the primary determinant of the total sulfur content of biomass to be the feedstock from which the biochar is generated, with oak and corn stover biochars containing 160 and 600-800 ppm sulfur, respectively. In contrast, for sulfur speciation, we found the primary determinant to be the temperature combined with the thermochemical conversion method. The speciation of sulfur in biochars was determined using X-ray absorption near-edge structure (XANES), ASTM method D2492, and scanning electron microscopy-energy-dispersive spectroscopy (SEM-EDS). Biochars produced under pyrolysis conditions at 500-600 °C contain sulfate, organosulfur, and sulfide. In some cases, the sulfate contents are up to 77-100%. Biochars produced in gasification conditions at 850 °C contain 73-100% organosulfur. The increase of the organosulfur content as the temperature of biochar production increases suggests a similar sulfur transformation mechanism as that in coal, where inorganic sulfur reacts with hydrocarbon and/or H2 to form organosulfur when the coal is heated. EDS mapping of a biochar produced from corn stover pyrolysis shows individual sulfur-containing mineral particles in addition to the sulfur that is distributed throughout the organic matrix.

Initial reduction of the NiO(100) surface in hydrogen.

The reduction of NiO in hydrogen, a reaction with many industrial applications, has not received sufficient attention from theoretical standpoint because the complexity of the material properties and the process present considerable computational challenges. We report here the results of a systematic study on the hydrogen reduction of an ideal NiO(100) surface that produces a water molecule and an NiO(100) surface with an oxygen vacancy, using the Hubbard U corrected density functional theory method, with some of the key results verified by the hybrid density functional method. The major findings are: (1) the O vacancy in the NiO(100) surface slab is stabilized in the subsurface layer, although the vacancy is likely to remain on the outermost surface layer because the barrier for O vacancy migration from the surface to the second layer is as high as 3.02 eV; (2) regarding the energetics of hydrogen interaction with the ideal NiO(100) surface, water formation, and concomitant reduction of NiO is favored at higher H coverage even though surface hydrogenation is energetically more favorable than water formation at lower H coverage; (3) kinetically, the pull-off of the surface oxygen atom and simultaneous activation of the nearby Ni atoms play key roles in hydrogen reduction of NiO(100); and (4) a dual role of hydrogen is revealed as both a reactant and a mediator, which reduces the maximum kinetic barrier from 2.41 eV to 1.86 eV.