Mark E. Bussell

Metal Phosphides: Preparation, Characterization and Catalytic Reactivity

The preparation, characterization, and catalytic activity of supported metal phosphides are reviewed. Reduction of metal compounds together with phosphate is a convenient method to prepare metal phosphides, but requires high temperature. Reduction with phosphite, hypophosphite, or phosphine and the plasma reduction of phosphate can be carried out at lower temperatures, which leads to smaller metal phosphide particles and more active catalysts. Organometallic routes allow the separate synthesis of metal phosphide nanoparticles, which have to be added to the support in a second step. LEED, STM, XPS, and DFT studies have shown that the surfaces of Ni2P reconstruct to P-rich surfaces. The investigation of metal phosphides as catalysts for hydrotreating reactions continues to be a topic of considerable research with recent advances realized in using bimetallic and noble metal phosphides to achieve high activities and tailored selectivities. Finally, hydrodeoxygenation catalysis over metal phosphides is a growing area of research given the need to develop catalysts for upgrading biomass to transportation fuels.Graphical Abstract

XPS study of the passive films formed on nitrogen-implanted austenitic stainless steels

Abstract Austenitic stainless steels (304-type) have been implanted with nitrogen ions in order to investigate the effects of implanted nitrogen on their electrochemical behaviour and on the nature of the passive film formed on the steels in acid (0.5M H 2 SO 4 ). Alloys with two nitrogen doses have been prepared (2.5×10 16 and 2×10 17 N atoms/cm 2 ). The implanted alloys have been characterized by 15 NNRA (nuclear reaction analysis) and XPS (X-ray photoelectron spectroscopy). Alloy surfaces with well-defined N concentrations were prepared, prior to the electrochemical measurements, by argon-ion sputtering of the implanted material for a fixed time in order to reach a well-defined point on the nitrogen depth profile. The samples were then transferred without exposure to air to an electrochemical cell mounted in an inert gas glove box. The implanted nitrogen modifies the electrochemical behaviour of the alloy. The anodic dissolution in the active state is enhanced, and the current density in the passive state is increased. Surface analysis of the alloys by XPS after passivation shows that implanted nitrogen is enriched on the surface during dissolution and passivation of the alloys. The process by which N is enriched on the surface is anodic segregation , which was first observed and characterized for S on Ni and NiFe alloys [J. Oudar and P. Marcus, Appl. Surf. Sci. 3 (1979) 48: P. Marcus, A. Teissier and J. Oudar, Corros. Sci. 24 (1984) 259]. The passive films formed on both the unimplanted and implanted alloys have a bilayer structure with an inner oxide layer and an outer hydroxide layer, but on the nitrogen-implanted alloy, a chromium nitride phase is formed at the expense of the chromium oxide. After passivation of the implanted alloys, three chemical states of nitrogen are detected in the N 1s spectrum. The high binding energy (399.4 eV) peak corresponds to a nitrogen species located on the surface of the passive film, which is produced by reaction of the implanted nitrogen with the solution. The peak at low binding energy (396.3 eV) is assigned to nitrogen bonded essentially to chromium, under the form of a nitride, which is incorporated in the passive film. The peak at intermediate binding energy (397.0 eV) corresponds to implanted nitrogen in the alloy, and is detected because the passive film is very thin. The XPS measurements and the sputter depth profiles indicate that during the anodic segregation of N, the chromium-rich nitride which is formed is incorporated as islands in the passive film.

Spongy chalcogels of non-platinum metals act as effective hydrodesulfurization catalysts

Aerogels are low-density porous materials, made mostly of air, for which hundreds of applications have been found in recent years. Inorganic oxide-based aerogels have been known for a long time, carbon aerogels were discovered in the early 1990s and sulfur- and selenium-based aerogels (chalcogels) are the most recent additions to this family. Here we present new aerogels made of Co(Ni)-Mo(W)-S networks with extremely large surface areas and porosity. These systems are formed by the coordinative reactions of (MoS(4))(2-) and (WS(4))(2-) with Co(2+) and Ni(2+) salts in non-aqueous solvents. We show that these low-density sponge-like networks can absorb conjugated organic molecules and mercury ions, and preferentially adsorb CO(2) over H(2), which illustrates their high potential as gas-separation media. The chalcogels are shown to be twice as active as the conventional sulfided Co-Mo/Al(2)O(3) catalyst for the hydrodesulfurization of thiophene.

Hydrodesulfurization over supported monometallic, bimetallic and promoted carbide and nitride catalysts

Abstract The preparation of alumina-supported β-Mo 2 C, MoC 1− x ( x ≈0.5), γ-Mo 2 N, Co–Mo 2 C, Ni 2 Mo 3 N, Co 3 Mo 3 N and Co 3 Mo 3 C catalysts is described and their hydrodesulfurization (HDS) catalytic properties are compared to conventional sulfide catalysts having similar metal loadings. Alumina-supported β-Mo 2 C and γ-Mo 2 N catalysts (Mo 2 C/Al 2 O 3 and Mo 2 N/Al 2 O 3 , respectively) are significantly more active than sulfided MoO 3 /Al 2 O 3 catalysts, and X-ray diffraction, pulsed chemisorption and flow reactor studies of the Mo 2 C/Al 2 O 3 catalysts indicate that they exhibit strong resistance to deep sulfidation. A model is presented for the active surface of Mo 2 C/Al 2 O 3 and Mo 2 N/Al 2 O 3 catalysts in which a thin layer of sulfided Mo exposing a high density of sites forms at the surface of the alumina-supported β-Mo 2 C and γ-Mo 2 N particles under HDS conditions. Cobalt promoted catalysts, Co–Mo 2 C/Al 2 O 3 , have been found to be substantially more active than conventional sulfided Co–MoO 3 /Al 2 O 3 catalysts, while requiring less Co to achieve optimal HDS activity than is observed for the sulfide catalysts. Alumina-supported bimetallic nitride and carbide catalysts (Ni 2 Mo 3 N/Al 2 O 3 , Co 3 Mo 3 N/Al 2 O 3 , Co 3 Mo 3 C/Al 2 O 3 ), while significantly more active for thiophene HDS than unpromoted Mo nitride and carbide catalysts, are less active than conventional sulfided Ni–Mo and Co–Mo catalysts prepared from the same oxidic precursors.

Catalytic hydrodesulfurization over the Mo(100) single crystal surface: II. The role of adsorbed sulfur and mechanism of the desulfurization step

The study of thiophene hydrodesulfurization (HDS) over initially clean Mo(100) surfaces has been extended to include sulfided surfaces. Low sulfur coverages (0 ≤ θs

A radiotracer (14C) and catalytic study of thiophene hydrodesulfurization on the clean and carbided Mo(100) single-crystal surface

0.67) inhibit HDS activity. Increasing the sulfur coverage in the range 0.67 ≤ θs ≤ 1.0 produces a surface with an HDS activity of about half that of the clean Mo(100) surface. Excessive exposure of the surface to a sulfur-containing environment results in the formation of a MoS2 layer which is, at least in part, responsible for complete catalytic deactivation. Radiotracer (35S) labeling techniques have been used to measure rates of hydrogenation of sulfur adsorbed on the Mo(100) surface. In ambient atmospheres of both hydrogen (1 atm) and the thiophene HDS reaction mixture (P(H2) = 1 atm, P(Th) = 2.5 Torr) the rate of hydrogenation of adsorbed sulfur is two orders of magnitude less than the HDS rate. This fact has been used to suggest that the desulfurization step of the reaction does not proceed via the formation of a tightly bound MoS species.

Thiophene hydrodesulfurization over bimetallic and promoted nitride catalysts

The role of adsorbed carbon overlayers and possible product poisoning (H/sub 2/S and cis-2-butene) in the thiophene hydrodesulfurization (HDS) reaction over Mo(100) single-crystal surfaces was investigated. Using the ..beta../sup -/ emitting /sup 14/C isotope, it was shown that adsorbed carbon remains on the Mo(100) surface after many thiophene turnovers (TN approx. = 10/sup 3/). HDS activities measured for carbided surfaces were found to be identical to that of initially clean Mo(100) single crystals, suggesting that the active catalyst surface is carbon covered. Deactivation of the single-crystal catalysts was caused by adsorption of H/sub 2/S onto the active HDS sites. Removal of the H/sub 2/S by evacuation of the reaction mixture readily regenerated the catalysts activity toward a fresh mixture of thiophene and hydrogen. The product cis-2-butene had no effect on the activity of the catalysts. 27 references.

Thiophene hydrodesulfurization over transition metal surfaces: structure insensitive over molybdenum and structure sensitive over rhenium

Alumina-supported bimetallic (Co3Mo3N/Al2O3” and promoted (Co–Mo2N/Al2O3) nitride catalysts have been prepared and characterized. Thiophene hydrodesulfurization (HDS) measurements show that the Co3Mo3N/Al2O3 and Co–Mo2N/Al2O3 catalysts are significantly more active than a Mo2N/Al2O3) catalyst with the same Mo loading. Furthermore, the Co–Mo2N/Al2O3 catalyst has a substantially higher HDS activity than a sulfided Co–MoO3/Al2O3 catalyst with an identical metal loading.

Mesoporous Matrix Encapsulation for the Synthesis of Monodisperse Pd5P2 Nanoparticle Hydrodesulfurization Catalysts

In this note the authors present results of thiophene hydrodesulfurization (HDS) experiments over molybdenum and rhenium single crystals. Previous work in their laboratory showed that a Mo(100) single crystal catalyzes thiophene HDS and that the product distribution is similar to the distribution obtained over an unsupported molybdenum disulfide (MoS/sub 2/) catalyst. The similarity of these results suggests that by determining the effects of surface structure and adsorbate overlayers (C, S) on thiophene HDS over molybdenum single crystals, one can gain insight about the nature of the active sites on MoS/sub 2/-based catalysts. This led the authors to expand studies to molybdenum single crystals of different surface orientations. They have also begun to investigate thiophene HDS over rhenium single crystals; rhenium disulfide (ReS/sub 2/) has been shown to be a more active HDS catalyst than MoS/sub 2/.

Thiophene hydrodesulfurization over transition metal foils: Comparison with metal sulfides

The synthesis of monodisperse 5-10 nm Pd5P2 catalytic particles by encapsulation in a mesoporous silica network, along with preliminary data on hydrodesulfurization (HDS) activity, is reported. Precursor Pd-P amorphous nanoparticles are prepared by solution-phase reaction of palladium(II) acetylacetonate with trioctylphosphine at temperatures up to 300 °C. Direct crystallization of Pd5P2 in solution by increasing the temperature to 360 °C leads to sintering, but particle size can be maintained during the transformation by encapsulation of the amorphous Pd-P particles in a mesoporous silica shell, followed by treatment of the solid at 500 °C under a reducing atmosphere, yielding Pd5P2@mSiO2. The resultant materials exhibit high BET surface areas (>1000 m(2)/g) and an average pore size of 3.7 nm. Access to the catalyst surface is demonstrated by dibenzodithiophene (DBT) HDS testing. Pd5P2@mSiO2 shows a consistent increase in HDS activity as a function of temperature, with DBT conversion approaching 60% at 402 °C. The ability to control particle size, phase, and sintering is expected to enable the fundamental catalytic attributes that underscore activity in Pd5P2 to be assessed.