Simon R. Kelemen

Direct determination and quantification of sulphur forms in heavy petroleum and coals: 1. The X-ray photoelectron spectroscopy (XPS) approach

X-ray photoelectron spectroscopy (XPS) was applied to the problem of speciating and quantifying organically bound forms of sulphur in non-volatile and solid hydrocarbons. XPS results from model compounds were used to interpret sulphur 2p signals from coals and heavy petroleum samples in terms of the amounts of alkyl sulphide and thiophen-like forms present. The determination of organic sulphur forms in Illinois No. 6 coal was accomplished by carefully monitoring the contributions due to iron sulphides and sulphates and application of the curve resolution method used with heavy petroleum samples. XPS data indicate that thiophenic sulphur forms the majority of the organic sulphur species in unoxidized Illinois No. 6 and Rasa coals.

Model experiments on the poisoning of Pt catalysts by sulfur

Surface reactions have been studied in high vacuum on the (100) surface of platinum that was clean or covered with various amounts of sulfur. This system was chosen for its simplicity in order to observe the mechanisms by which sulfur poisons catalytic activity. Three different poisoning mechanisms were identified: (i) When the surface is covered with one S atom per two surface Pt atoms, it is chemically inert. (ii) At lower coverages, the strong chemical bond to sulfur modifies the chemical properties of the platinum surface and weakens its interaction with adsorbates. (iii) When the sulfur coverage is one S per four Pt, a regular sulfur overlayer is established; molecules can adsorb on the surface but are prevented by the sulfur structure from participating in Langmuir--Hinshelwood reactions. The reactions on which these observations were made are the dissociation of H/sub 2/S, the adsorption and desorption of CO, the reduction of NO by CO, the dissociation and desorption of NO, and the adsorption and dehydrogenation of benzene and acetylene. 7 figures.

Direct determination and quantification of sulphur forms in heavy petroleum and coals: 2. The sulphur K edge X-ray absorption spectroscopy approach

Abstract A sulphur K edge X-ray absorption spectroscopic method has been developed for the direct determination and quantification of the sulphidic and thiophenic forms of organically bound sulphur in non-volatile petroleum and coal samples. XANES spectra were taken of a number of model compounds, mixtures of model compounds, heavy petroleum and coal samples. A third-derivative analysis of these spectra allowed approximate quantification of the sulphidic and thiophenic components of the model mixtures and the heavy petroleum and coal samples.

Adsorption of acetylene and benzene on the Pt(100) surface

Abstract Chemisorption of acetylene and benzene on Pt(100) has been studied by Auger electron spectroscopy, ultraviolet photoemission spectroscopy, flash dehydrogenation and LEED. Both hydrocarbons follow Langmuir adsorption kinetics. Adsorption of benzene and acetylene cause a decrease in the work function proportional to coverage and corresponding to a dipole moment of 1.5 D for benzene and 0.5 D for acetylene. UPS indicates π bonding for benzene. Acetylene shows a broadening and chemical shift of the π orbital and an increased splitting of the σ orbital energies. These data and flash dehydrogenation rule out σ-bonding of a vertical, partially dehydrogenated (C2H) molecule. They also favor π-bonding over bridging disigma bonding without completely ruling out the latter. Our results are compared with hydrocarbon adsorption on other transition metal surfaces and with bonding of acetylenic molecules in transition metal complexes.

The binding energy of CO on clean and sulfur covered platinum surfaces

Abstract The desorption of CO from clean Pt(111) and (100), and from the same surfaces with partial overlayers of sulfur, was studied by Thermal Desorption Spectroscopy. The method of desorption rate isotherms was employed for data analysis. The desorption of CO from the (111) surface and both surfaces with ordered sulfur overlayers can be described as a first order process with coverage dependent activation energies. The desorption of CO from the clean Pt(100) surface is complicated by the dynamic interaction of the molecule with a thermally activated change of platinum surface structure. On both platinum faces surface sulfur decreases the initial binding energy of CO. As the CO concentration increases, its binding energy decreases very rapidly. This is due to a repulsive interaction which exists between co-adsorbed species.

Interaction of H2S with the Ru(001) surface

The interaction of sulfur and H2S with Ru(001) was studied between 350 and 1500 K with LEED, UPS, AES and Thermal Desorption Spectroscopy. Exposure to H2S at 350 K results first in the formation of a (2 × 2) LEED pattern followed by a (√3 × √3)R30°. Further exposure results in a diffuse c(4 × 2) pattern which sharpens upon heating to 500 K. Quantitative AES indicates that these structures correspond to14,13,12 sulfur atom per surface Ru atom, respectively. UPS indicates that H2S dissociates upon adsorption at 350 K over the entire coverage range. Work function and Thermal Desorption Measurements show that below 13 (S/Ru) at 350 K only sulfur is adsorbed, indicating that hydrogen from H2S recombines and desorbs during the adsorption of sulfur. At higher sulfur coverages hydrogen remains trapped on the surface, but UPS measurements show that the H2S is dissociated. AES and Thermal Desorption Measurements yield an initial heat of desorption for sulfur of 105 kcal/mole. At coverages approaching saturation, the heat of desorption decreases rapidly to 50 kcal/mole.

O2 oxidation studies of the edge surface of graphite

We have studied the reactive adsorption of O2 on the edge surface of graphite. At 300°C the efficiency of oxygen uptake showed a strong coverage-dependent reactive adsorption coefficient. In general, the efficiencies were low (< 10−9) over the majority of the coverage range. In contrast, the uptake of oxygen from O2 and H2O on sputter-damaged graphite was far more rapid. Sputter-damaged carbon surfaces exhibit greatly enhanced reactivity and are poor models of edge carbon activity. Thermal stability studies on the resultant oxidized edge graphite surfaces provide information about the energetics of product formation in gasification reactions. CO was the dominant product. A fraction of the oxygen on the surface is very tightly bound with energies greater than 85 kcal/mole. The energy decreases to 70 kcal/mole over a wide coverage range. At the highest attainable coverages representing a small fractional population, the energy decreases further down to 58 kcal/mole. Our results show that increasing the amount of oxygen surface coverage decreases the energy barrier for gaseous CO formation but increases the barrier for O2 dissociation.

Thermal reactivity of sulphur forms in coal

X-Ray absorption near edge structure spectroscopy (XANES), X-ray photoelectron spectroscopy (XPS) and temperature programmed decomposition (TPD) have been used to follow the chemical transformation of sulphur that takes place during the low temperature pyrolysis of coals of varying rank. This chemistry takes place under milder pyrolysis conditions than those required for the production of almost all of the other organic volatile matter of coal. The results provide evidence for the conversion of aliphatic sulphides to aromatic forms below 400 °C in addition to the elimination of some of the aliphatic sulphur forms as H2S. Results between temperatures of 400 and 750 °C indicate that a considerable amount of the aromatic sulphur forms identified by XPS and XANES in the chars made at 400 °C subsequently react (by 750 °C) to produce H2S. For some low rank coals, a direct relationship cannot be made between the initial aliphatic sulphur content and the total amount of H2S produced below 750 °C. A comparison of the TPD pattern of H2S evolution for high rank coal with those of chars made from lower rank coals at 400 °C shows that they are remarkably similar and suggests that the thermal reactions induced in sulphur species in laboratory pyrolysis experiments could be related to those that occur as a result of thermal reactions during coal metamorphism.

Chemistry of organically bound sulphur forms during the mild oxidation of coal

Abstract X-ray absorption near edge structure (XANES) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to study the chemistry of organically bound sulphur species in coal during mild coal oxidation. Spectra of preserved and oxidized samples were obtained and compared. Both techniques indicate that sulphide sulphur forms are converted to oxidized forms while the thiophenic forms remain largely untouched under these experimental conditions.

Characterization and reactivity of organically bound sulfur and nitrogen fossil fuels

Advances in X-ray instrumentation over the last decade have allowed the determination and quantification of organically bound sulfur and nitrogen forms in fossil fuels, which led to deeper understanding of their reactivities. This paper reviews recent technical advances in this area, highlights achievements of significant progress in chemical understanding and areas where further advances will likely occur.