Evolution of nitrogen functionalities in carbonaceous materials during pyrolysis

Electronic structure intrinsically controls the light absorbance, redox potential, charge-carrier mobility, and consequently, photoreactivity of semiconductor photocatalysts. The conventional approach of modifying the electronic structure of a semiconductor photocatalyst for a wider absorption range by anion doping operates at the cost of reduced redox potentials and/or charge-carrier mobility, so that its photoreactivity is usually limited and some important reactions may not occur at all. Here, we report sulfur-doped graphitic C(3)N(4) (C(3)N(4-x)S(x)) with a unique electronic structure that displays an increased valence bandwidth in combination with an elevated conduction band minimum and a slightly reduced absorbance. The C(3)N(4-x)S(x) shows a photoreactivity of H(2) evolution 7.2 and 8.0 times higher than C(3)N(4) under lambda > 300 and 420 nm, respectively. More strikingly, the complete oxidation process of phenol under lambda > 400 nm can occur for sulfur-doped C(3)N(4), which is impossible for C(3)N(4) even under lambda > 300 nm. The homogeneous substitution of sulfur for lattice nitrogen and a concomitant quantum confinement effect are identified as the cause of this unique electronic structure and, consequently, the excellent photoreactivity of C(3)N(4-x)S(x). The results acquired may shed light on general doping strategies for designing potentially efficient photocatalysts.

Unique Electronic Structure Induced High Photoreactivity of Sulfur-Doped Graphitic C3N4

Microporous activated carbon originating from coconut shell, as received or oxidized with nitric acid, is treated with melamine and urea and heated to 950 °C in an inert atmosphere to modify the carbon surface with nitrogen- and oxygen-containing groups for a systematic investigation of their combined effect on electrochemical performance in 1 M H2SO4 supercapacitors. The chemistry of the samples is characterized using elemental analysis, Boehm titration, potentiometric titration, and X-ray photoelectron spectroscopy. Sorption of nitrogen and carbon dioxide is used to determine the textural properties. The results show that the surface chemistry is affected by the type of nitrogen precursor and the specific groups present on the surface before the treatment leading to the incorporation of nitrogen. Analysis of the electrochemical behavior of urea- and melamine-treated samples reveal pseudocapacitance from both the oxygen and the nitrogen containing functional groups located in the pores larger than 10 A. On the other hand, pores between 5 A and 6 A are most effective in a double-layer formation, which correlates well with the size of hydrated ions. Although the quaternary and pyridinic-N-oxides nitrogen groups have enhancing effects on capacitance due to the positive charge, and thus an improved electron transfer at high current loads, the most important functional groups affecting energy storage performance are pyrrolic and pyridinic nitrogen along with quinone oxygen. (C)2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Combined Effect of Nitrogen- and Oxygen-Containing Functional Groups of Microporous Activated Carbon on its Electrochemical Performance in Supercapacitors

The characterization of activated carbons with oxygen and nitrogen surface groups

Roles of water for chemical reactions in high-temperature water

Sulfur K edge X-ray adsorption near edge structure spectroscopy (XANES) and X-ray photoelectron spectroscopy (XPS) have been developed for the direct determination and quantification of the forms of organically bound sulfur in nonvolatile petroleum and coal samples. Both techniques were used to characterize organically bound sulfur forms in coals from the Argonne Premium Coal Sample Program. XANES and XPS spectra were taken of a number of model compounds, mixtures of model compounds, and coal samples. A third-derivative analysis of the XANES spectra involving curve reconstruction was required to account for the present of pyrite and aromatic sulfur. Curve resolution of the XPS spectra allowed approximate quantification of the surface aliphatic and aromatic components. XPS spectra showed little evidence of oxidized sulfur forms despite the presence of iron oxides and hydroxyoxides in the iron 2p spectra. Pyritic iron was identified only in Illinois No. 6 coal. The measured quantity of pyritic iron was used to interpret the sulfur 2p XPS spectrum. Both XANES and XPS show a monotonic increase of aromatic sulfur, defined as sulfur bound to two sp 2 -hybridized carbons, with increasing rank in the Argonne premium samples

Direct determination and quantification of sulfur forms in coals from the Argonne Premium Sample Program

A combination of XPS and solid-state 13C NMR techniques have been used to characterize organic oxygen species and carbon chemical/structural features in peats, pyrolyzed peats, lignites, and other coals. Both the 13C NMR and XPS results show the same ordering for the amount of aromatic carbon, higher ranking coals > lignites > peats. In general the value for H/C decreases as the percent of aromatic carbon increases. For pyrolyzed peats, the H/C level is higher than lignites and other coals of comparable levels of aromatic carbon. This is likely due to significant differences in the carbon structural framework of these materials. A van Krevelen plot, based on elemental H/C data and XPS derived O/C data, shows the well-established pattern for peats, lignites, and other coals. In general, O/C decreases as the percent of aromatic carbon increases, with the expected magnitude ordering, peats > lignites > higher ranking coals. Most of the H/C values of pyrolyzed peats are higher than coals at comparable O/C. A ...

Characterization of Organically Bound Oxygen Forms in Lignites, Peats, and Pyrolyzed Peats by X-ray Photoelectron Spectroscopy (XPS) and Solid-State 13C NMR Methods

X-ray photoelectron spectroscopy (XPS) was used to identify and quantify the changes in organically bound nitrogen forms present in the tars and chars of coals after pyrolysis. For fresh coal, pyrrolic nitrogen is the most abundant form of organically bound nitrogen, followed by pyridinic, quaternary, and amino types. Some of the quaternary nitrogen species initially present in coal are lost upon mild pyrolysis, prior to hydrocarbon devolatilization. These quaternary species are attributed to pyridinic or basic nitrogen species associated with hydroxyl groups from carboxylic acids or phenols. A portion of the quaternary nitrogen species is lost at the very earliest stage of pyrolysis. Upon devolatilization, the resultant tar and char contain mostly pyrrolic and pyridinic forms; however, a portion of the quaternary nitrogen initially present in the coal appears in the coal char and tar. The relatively strong bonding interactions associated with these quaternary species suggests that there may be other quat...

Nitrogen Transformations in Coal during Pyrolysis

A slurry hydroprocessing process for upgrading a hydrocarbon feedstock containing nitrogen and sulfur using bulk multimetallic catalyst comprised of at least one Group VIII non-noble metal and at least two Group VIB metals wherein the ratio of Group VIB metal to Group VIII metal is about 10:1 to about 1:10.

Slurry hydroprocessing using bulk multimetallic catalysts

The chemical pathways for nitrogen and sulfur transformations during coalification are elucidated by comparing the chemical forms of unaltered peats, lignites, and coals and pyrolyzed peats. Nitrogen forms are characterized by a combination of X-ray photoelectron spectroscopy (XPS) and 15N nuclear magnetic resonance (NMR). In unaltered peats, the 15N NMR and XPS nitrogen (1s) spectra are consistent with the presence of amide nitrogen. When peat is pyrolyzed, the main peak in the 15N NMR spectrum broadens and shifts from −260 ppm to −245 ppm, which is consistent with the loss of some amide nitrogen and the appearance of pyrrolic nitrogen forms. The pyrolyzed peat shows a new XPS peak that appears at 398.6 eV, which is characteristic of pyridinic nitrogen. These results indicate that a thermal transformation of amide nitrogen into pyrrolic and pyridinic forms occurs after thermal stress that is roughly equivalent to lignitification. High total nitrogen levels are found in pyrolyzed peats relative to lignite...

Martin L. Gorbaty

Papers

Direct determination and quantification of sulfur forms in coals from the Argonne Premium Sample Program

Characterization of Organically Bound Oxygen Forms in Lignites, Peats, and Pyrolyzed Peats by X-ray Photoelectron Spectroscopy (XPS) and Solid-State 13C NMR Methods

Nitrogen Transformations in Coal during Pyrolysis

Slurry hydroprocessing using bulk multimetallic catalysts

Thermal transformations of nitrogen and sulfur forms in peat related to coalification