C. Potvin

NOx storage on barium-containing three-way catalyst in the presence of CO2

NOx trapping capability of “NOx storage–reduction” commercial catalysts (4–9 wt% Ba-containing three-way catalysts) was compared to that of bulk barium carbonate and alumina-supported barium carbonate from Rhodia (9 wt% Ba). These samples were characterized by infrared spectroscopy, X-ray diffraction, HRTEM and EDX. It was shown that bulk barium carbonate was partially converted to barium nitrate in flowing NO/O2 mixture without CO2. Thermodynamic calculation showed that bulk barium nitrate could not form in the presence of CO2-containing gas exhausts. Using HRTEM and EDX, it was evidenced that barium was engaged as either large barium carbonate crystals or highly dispersed barium species on the alumina support. NOx storage experiments using gas mixtures containing or not O2 or CO2, confirmed firstly that NO was stored on barium trap only via NO2 and secondly that NO2 and CO2 are competing for the same barium trapping sites. The fact that no significant amount of stored NOx could be evidenced in the bulk barium carbonate, suggested that over the catalytic surface, the well dispersed barium phase can play an important role in the NOx trapping properties of these catalysts.

Heterobimetallic aggregates of copper(I) with thiotungstates and thiomolybdates. Synthesis and characterization of polymeric (NEt4)3[Cu4(NCS)5WS4], (NEt4)2[(CuNCS)3WS4] and (NEt4)2[(CuNCS)4WS4], M = Mo, W

Abstract Reaction of (NEt 4 ) 2 MS 4 (M = Mo, W) with CuCl and KSCN (or NH 4 SCN) in acetone or acetonitrile affords a new set of mixed metal–sulfur compounds: infinite anionic chains Cu 4 (NCS) 5 MS 4 3− ( 1,2 ), (CuNCS) 3 WS 4 2− ( 3 ) and two dimensional polymeric dianions (CuNCS) 4 MS 4 2− ( 4,5 ). Crystal of 1 (M = W) and 3 are triclinic, space group P 1( 1 : a = 10.356(2), b = 15.039(1), c = 17.356(2)A, α = 78.27(1)°, β = 88.89(2)° and γ = 88.60(1)°, Z = 2, R = 0.04 for 3915 independent data; 3 : a = 8.449(2), b = 14.622(4), c = 15.809(8)A, α = 61.84(3)°, β = 73.67(3)° and γ = 78.23(2)°, Z = 2, R = 0.029 for 6585 independent data). Crystals of 4 (M = W) and 5 (M = Mo) are monoclinic, space group P 2 1 / m , Z = 2 ( 4 : a = 12.296(4), b = 14.794(4), c = 10.260(3)Aand β = 101.88(3)°, R = 0.034 for 4450 independent data; 5 : a = 12.306(2), b = 14.809(3), c = 10.257(2)Aand β = 101.99(3)°, R = 0.043 for 3078 independent data). The crystal structure determinations of 4 and 5 show that four edges of the tetrahedral MS 4 2− core are coordinated by copper atoms forming WS 4 Cu 4 aggregates linked by eight-membered Cu(NCS) 2 Cu rings. A two-dimensional network is thus formed in the diagonal (101) plane. The space between the anionic two-dimensional networks is filled with the NEt 4 + cations. Additional NCS groups lead to the [Cu 4 (NCS) 5 WS 4 ] 3− ( 1 ) trianion connected by NCS bridges forming pseudo-dimers. These latter are held together by weak Cu S(NCS) interactions giving rise to infinite chains along a direction parallel to [100]. In contrast complex 3 develops infinite chains from WS 4 Cu 3 aggregates with the same Cu(NCS) 2 Cu bridges as in 4 and 5 . These chains are running along a direction parallel to [010]. The structural data of the different types of polymeric compounds containing MS 4 2− and CuNCS have been used to interpret vibrational spectroscopic data of the thiocyanate groups.

Heterobimetallic aggregates derived from the double cubane-like structure (NMe4)4[MS4(CuCl)5Cl2] (MMo, W)

Abstract Nucleophilic substitutions in the double cubane-like structures [MS4(CuCl)5Cl2]4− (M=Mo, W) are complicated by elimination reactions. With NCS− an edge-elimination process gave the [MS4(CuNCS)4]2− anion whereas a face-elimination by PPh3 led to the monocubane structure [MS4(CuPPh3)4Cl]. Substitution by bipyridine produced the lacunary [MS4Cu3.75Cl1.75bipy2]. The different species were characterized by X-ray diffraction studies.

Synthesis and Characterization of Highly Dispersed Molybdenum Carbides in Mesoporous Silica

Highly dispersed molybdenum carbides in MCM-41 mesoporous silica are synthesized by temperature-programmed carburization and are characterized. Two methods of preparation are examined: (i) insertion of molybdenum during the synthesis of the MCM-41 silica and (ii) postsynthesis incorporation into a MCM-41 silica matrix by the incipient wetness method. Propene transformation (hydrogenation and metathesis) was used as a probe reaction; the observed catalytic behavior can be explained as a result of the preparation method, i.e., of the relative strength of interaction between the molybdenum oxide precursor and the support.

Novel phosphorus-doped alumina-supported molybdenum and tungsten carbides: synthesis, characterization and hydrogenation properties

The effect of atomically dispersed phosphorus on Mo2C- and WC-supported γ-Al2O3 has been studied. Phosphorus was introduced via molybdenum or tungsten heteropolyanions. Mo- and W-based heteropolyanions were used to synthesize supported materials. Propene and tetralin hydrogenation were used as molecular probe reactions to test the activity and selectivity of the alumina-supported molybdenum or tungsten carbides. The effect of phosphorus on the hydrogenation activity of materials was also considered. Catalysts were characterized by X-ray diffraction, transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS) and CO chemisorption. Phosphorus was found to increase significantly the activity of molybdenum and tungsten carbides. Supported molybdenum carbides are highly dispersed. Tungsten carbide particles are well dispersed with sizes ranging from 2 to 4 nm. To check the stability of carbides they were characterized after catalytic runs.

Preparation of zeolite Y and ZSM-5 coatings for cracking fuel in a cooling system for hypersonic vehicles

Abstract The preparation of catalysts for a cooling system adapted to hypersonic vehicles has been studied. Synthesis procedures have been developed for preparing zeolite Y and ZSM-5 coatings on stainless steel. Optimization of the synthesis parameters leads to coatings with a suitable thickness (2–3 μm) and a strong adhesion to the substrate. After activation, the Lewis and Bronsted acid sites of the two zeolite coatings are quantified by an adsorption-desorption method with iso -propylamine.

Derivatives of tetrathiometallates. Synthesis of [MS4M′(bpy)2] (M = Mo, W; M′ = Co, Ni, Mn, Ru) complexes. Structural characterization of [Ni(bpy)3] [(WS4)2Pd]

Abstract Reaction of M′Cl 2 (bpy) 2 (M′ = Co, Ni, Mn) with (PPh 4 ) 2 MS 4 (M = Mo, W) affords MS 4 M′(bpy) 2 . Crystals of WS 4 Co(bpy) 2 , 1 2 dmf are monoclinic space group P 2 1 / n ( a = 12.458, b = 21.862, c = 10.164 A and β = 100.0°, Z = 4, R = 0.039 for 5414 independent data). The coordination geometry around the cobalt atom is octahedrally distorted with two bipyridyl groups and two sulfur atoms being cis to each other. Reaction of M′Cl 2 (bpy) 2 with [NEt 4 ] 2 [(MS 4 ) 2 M″] (M″ = Pd, Pt) gives [M′(bpy) 3 ][(MS 4 ) 2 M″]. Crystals of [Ni(bpy) 3 ][(WS 4 ) 2 Pd] are triclinic, space group P 1 ( a = 12.771, b = 14.838, c = 12.077 A, α = 72.9, β = 116.9 and γ = 96.1°, Z = 2, R = 0.042 for 3842 independent data). In the anionic species, the central palladium atom is coordinated by two chelating WS 4 2− anions leading to a distorted PdS 4 square plane moiety, the W… Pd…W core being nearly linear (173.72(6)°). Synthesis of new ruthenium complexes [WS 4 Ru(bpy) 2 ] and [WS 4 (Ru(bpy) 2 ) 2 ][PF 6 ] 2 are also reported.

N-dodecane thermal and catalytic cracking under supercritical conditions

Abstract For an airbreathing vehicle, the generated heat fluxes increase with the flight velocity. Above Mach 6, the heat fluxes to be evacuated are so high that a cooling system adapted to hypersonic speed becomes indispensable. The idea is to use a hydrocarbonated liquid fuel, called “endothermic fuel.” The heat is absorbed by fuel circulation behind a double wall, where the fuel undergoes thermal or catalytic cracking (endothermic reactions). In the cooling system, the fuel is under high pressure and temperature, corresponding to supercritical conditions. Thermal and catalytic cracking of hydrocarbons under these conditions are mainly unknown. To catalyze the hydrocarbon cracking reactions, two zeolites, the zeolites Y and ZSM-5, are selected. This article presents the results of a parametric study of the thermal and catalytic cracking reactions of a model fuel (n-dodecane) under supercritical conditions. The tests are carried out in a stirred batch reactor heated up to 425 °C and under pressure up to 150 bar. After cooling down, the cracking products are characterized by several analytical techniques in order to determine the influence of the operating conditions on the cracking reaction mechanisms. The efficiency of catalytic cracking with the zeolites Y and ZSM-5 are compared with the thermal one. With the increase of temperature, a logical raise of the parent hydrocarbon conversion and the number of products are observed. The product distributions are significantly different between the thermal and catalytic cracking. This is explained by the cracking mechanisms (free radical and carbocations) and the supercritical conditions.

Characterization andn-heptane cracking/isomerization catalysis of tungsten oxynitrides on EMT zeolite

Catalysts made of two tungsten precursors (one of them containing phosphorus) impregnated on EMT zeolite were nitridated in flowing pure ammonia and passivated by O2 (1 vol%) in an Ar stream leading to tungsten oxynitrides/EMT (W2N/EMT) and phosphotungsten oxynitrides/EMT (PW2N/EMT). The catalysts were examined by X-ray diffraction, NMR spectroscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy. The materials were tested for the cracking/isomerization ofn-heptane at atmospheric pressure and 623 K. Cracking (C3 + C4) was the major (about 90%)n-heptane reaction for all the catalysts. The influence of EMT supercages upon selectivity was observed. The yield ofn-heptane isomers was significantly enhanced for W2N/EMT and principally for PW2N/EMT showing a higher hydrogenation function.