H.H. Huang

The interaction of Ni and Fe with sulfur and molybdenum-sulfide surfaces: a TDS, XPS and hydrogen-chemisorption study

Abstract Sulfur multilayers, containing Sn species (n = 2, 4, and 8), are very reactive toward admetals like nickel and iron. Ni and Fe atoms supported on sulfur films at 200–300 K exhibit core-level binding energies and band structures very similar to those of nickel and iron sulfides. In contrast, Ni atoms supported on molybdenum-sulfide surfaces remain in a metallic state. NiMoS and FeMoS films can be generated by heating Ni/Sfilm/Mo(110) and Fe/Sfilm/Mo(110) systems to high temperature. The behavior of the Ni/Sfilm/Mo(110) and Fe/Sfilm/Mo(110) systems indicates that Ni and Fe promote Mo↔S interactions and the subsequent formation of molybdenum sulfides. On TM MoS x and TM/S/Mo(110) surfaces (TM = Ni or Fe), the slow step in the D2,gas + Ssolid → D2Sgas reaction is the dissociation of molecular hydrogen. Ni MoS x and Fe MoS x surfaces interact strongly with atomic hydrogen (D), sorbing this element and forming gaseous hydrogen sulfide. The sorption of D produces uniform changes in the electronic properties of the MoSx substrate, with positive binding energy shifts (0.3–0.4 eV) in the core levels of molybdenum and sulfur. Most of the sorbed hydrogen evolves into gas phase as D2 at temperatures between 350 and 500 K. Trends seen in the hydrodesulfurization activity of NiMoS and FeMoS catalysts are analyzed following our results for the sulfidation of Mo and the hydrogenation of S in NiMoS and FeMoS films.

Reaction of hydrogen with SMo(110) and MoSx films: formation of hydrogen sulfide

Abstract The reaction of hydrogen (H 2 , D 2 , or D) with sulfur multilayers, S Mo (110) , surfaces and MoS x films has been investigated at temperatures between 100 and 400 K. All the surfaces were unreactive toward molecular hydrogen under UHV conditions. However, these systems showed a large reactivity toward atomic hydrogen. As gas-phase hydrogen atoms impinged on the surfaces, gaseous hydrogen sulfide was formed. This reaction was very effective for the removal of sulfur atoms from sulfur multilayers and MoS x films. On MoS x films the 2D(gas) + S(solid) → D 2 S(gas) reaction was 3–4 times slower than on sulfur multilayers, and at least 6 times faster than on S Mo (110) surfaces. A good correlation was found between the rate of formation of gaseous hydrogen sulfide and the stability of the SS or SMo bonds in a surface. The bonding interactions between hydrogen sulfide and S 0.9−0.6 Mo (110) or MoS x were negligible at temperatures above 200 K. Rough MoS x films that exposed unsaturated molybdenum sites were more reactive toward hydrogen sulfide than the sulfur-basal plane of MoS 2 or S 0.9−0.6 Mo (110) surfaces. The behavior of molybdenum sulfide catalysts in hydrogenation and hydrodesulfurization processes is discussed in light of these results.

The adsorption of N2O on clean and chemically modified Ru(001) surfaces

Abstract The adsorption of N 2 O on clean, oxygen, deuterium and carbon monoxide modified Ru(001) surfaces was investigated. On the clean surface, three adsorption states have been observed, desorbing at 160–165, 145 and 116–123 K, respectively. On the oxygen modified surface, the presence of oxygen shifts the N 2 O desorption temperature from 129 K (on a clean surface) to a higher temperature of 173 K ( θ o = 0.12). The enhanced adsorption of N 2 O on partially O-covered Ru(001) was also observed. However, on D- and CO-precovered surfaces, the N 2 O adsorption is destabilized. These results can be accounted for by considering the “throughmetal” and the lateral interactions between coadsorbates.

Synthesis of sulfur films from S2 gas: spectroscopic evidence for the formation of Sn species

Abstract Thin films of molecular sulfur were prepared by the deposition of S 2 gas from an electrochemical closer on single crystal metal surfaces kept at temperatures below 250 K. These films were characterized by XPS, UPS, TDS and IRAS, and were found to be stable up to 390 K even under UHV conditions. Cyclooctasulfur S 8 was identified as one of the sulfur molecules present.

Chemical and electronic properties of silver atoms supported on sulfur and molybdenum sulfide surfaces

The chemical and electronic properties of a series of Ag/Smult/Mo(110) and AgMoSX systems have been investigated using X-ray photoemission, thermal desorption mass spectroscopy, hydrogen (H2, D2, or D) chemisorption and molecular orbital calculations. At 100 K, sulfur multilayers supported on Mo(110) react with silver to form sulfide compounds. Upon annealing to high temperature, the silver sulfides promote the sulfidation of the Mo support leading to the formation MoSX. Silver atoms deposited on molybdenum sulfide surfaces remain in a metallic state at temperatures below 300 K. The results of INDO/S and ab initio self-consistent-field calculations indicate that the AgMoS2 bond is best described as covalent with a small degree of ionic character. On MoS2 surfaces, Ag is a poor electron donor compared with Co and Ni. At temperatures above 400 K Ag diffuses into the bulk of molybdenum sulfide, forming AgMoSX compounds. These bimetallic sulfides decompose at high temperatures (> 800 K) with Ag desorbing and MoSX remaining solid. The AgSYMoSX and AgMoSX systems were unreactive towards molecular hydrogen under ultrahigh vacuum conditions. However, gas-phase atomic hydrogen reacted with the surfaces to form gaseous hydrogen sulfide and led to sorption of hydrogen by the AgSYMoSX and AgMoSX systems. Compared with other similar systems (MoSX, NiSYMoSX, CoSYMoSX, ZnSYMoSX), the AgSYMoSX systems show the lowest rate of hydrogenation of Mo-bonded S atoms. The Ag adatoms are very efficient for blocking D↔S interactions.

The alternative thermal decomposition mode of 2-oxetanone and 2-azetidinone: a DFT and PES study

Abstract We have found that, while 2-oxetanone undergoes thermal decarboxylation at 350°C to give CO 2 and ethene, 2-azetidinone proceeds in two different modes at 550°C, giving isocyanic acid, ethene, ketene, hydrogen cyanide and carbon monoxide among the pyrolysis products. B3LYP/6-31G ** calculations were performed to investigate the two alternative cycloreversion pathways for the two compounds. Theoretical thermodynamic data predicted that decarboxylation of 2-oxetanone has among the lowest activation barrier (159 kJ mol −1 ) and free energy (Δ G =−183 kJ mol −1 ). While the cycloreversion to ethene can be classified as a concerted asynchronous process, natural bond orbital (NBO) analysis suggested that the cycloreversion to ketene is more asynchronous for 2-azetidinone than for 2-oxetanone.

A theoretical study on the isomerization of cyclopropane to propene with ab initio and DFT methods

Abstract The isomerization of cyclopropane to propene had been studied by ab initio post-HF and DFT methods. Single-point energy calculations at UMP4, UCCSD(T), UQCISD(T) and UBecke3LYP levels were carried out on the UMP2/6-31G∗∗ fully optimized structures. The reaction heat and activation energies for the whole isomerization reaction and the structural isomerization of the C2-symmetry trimethylene intermediate were evaluated. The correlation methods employed release an accuracy order of UBecke3LYP > UQCISD(T) ≈ (UCCSD(T) > UMP4 > UMP2. UBecke3LYP is found to be the best method to reproduce the experimental results. The spin contamination problem can be solved by using projected MP energies.

Oxidative dehydrogenation of glycol to glyoxal on a P-modified electrolytic silver catalyst

A phosphorus-modified electrolytic silver catalyst was prepared and used as catalyst in the oxidative dehydrogenation of glycoto glyoxal. The yield of glyoxal was observed as high as 82% at 98% conversion for the Ag-P catalyst, while 62% at 89% conversion for the pure electrolytic silver. The formation of the surface compounds between the phosphorus additives and the silver surface was demonstrated by means of XPS and SEM. It caused the decrease of the surface concentration of atomic oxygen species, and restrained the decomposition and total oxidation of adsorbed glycol to C1 products.

Water Dissociation and KOH Formation on Potassium-Covered MgO/Ru(001)

The sequential adsorption and reaction of H2O with K on MgO thin films have been studied using TDS and XPS. Upon H2O adsorption on K/MgO at 100 K, two hydrated species of K+(H2O)n- and K+(H2O)n(OH)- are formed, resulting in corresponding H2O-TDS peaks at 200 and 270 K, respectively. The thermal conversion from hydrated K to hydrated KOH leads to H2 desorption at 190−220 K. The formation of KOH is evidenced by the O 1s bonding energies at 530.6−530.9 eV. Both additional K deposition and thermal activation favor H2O dissociation. Annealing the coadsorbed surface to 500−600 K results in desorption and decomposition of KOH. The coincident desorption of K2O, H2O, and K at θK > 0.3 ML strongly suggests the decomposition pathway 2KOH → K2O + H2O, which further produces K desorbing into the gaseous phase and oxygen being retained on the surface. The KH species formed during H2O dissociation is also evidenced by the coincident desorption of H2 and K at 470−490 K for θK ≥ 1 ML.

The oxidation of potassium on MgO(100)

Abstract The interaction of oxygen and potassium on MgO(100) has been studied using thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). The results show that several oxygen species are present on the surface. An atomic oxygen species is initially present at lower θK as shown by the evidence of O 1s BE at 528.5 eV, together with the consistent but not coincident desorption of potassium and oxygen at 610 and 525 K, respectively. The oxygen desorption at a low temperature of 425 K coupled with the O 1s BE at 532.1 eV indicates the associative molecular oxygen adsorption on oxygen vacancies in the substrate. The non-stoichiometric 3D KO clusters form on the surfaces for θK ≥ 1 ML. The desorption of K2O2 followed by K2O at ∼600 K was observed.