Coprecipitated iron-containing catalysts (Fe-Al2O3, Fe-Co-Al2O3, Fe-Ni-Al2O3) for methane decomposition at moderate temperatures: Part II. Evolution of the catalysts in reaction

Coprecipitated Fe-Al₂O₃, Fe-Co-Al₂O₃ and Fe-Ni-Al₂O₃ catalysts is shown to be very efficient in carbon deposition during methane decomposition at moderate temperatures (600–650 °C). The carbon capacity of the most efficient bimetallic catalysts containing 50–65 wt.% Fe, 5–10 wt.% Co (or Ni) and 25–40 wt.% Al₂O₃ is found to reach 145 g/g_cat. Most likely, their high efficiency is due to specific crystal structures of the metal particles and formation of optimum particle size distribution. According to the TEM data, catalytic filamentous carbon (CFC) is formed on them as multiwall carbon nanotubes (MWNTs). The phase composition of the catalysts during methane decomposition is studied using a complex of physicochemical methods (XRD, REDD, Mössbauer spectroscopy and EXAFS). Possible mechanisms of the catalyst deactivation are discussed.     

Oxidative gas phase carbonylation of methanol to dimethyl carbonate over chloride-free Cu-impregnated zeolite Y catalysts at elevated pressure

Incipient wetness impregnation of zeolite Y with copper(II) nitrate solution and inert activation at 650 °C led to active catalysts for the oxidative carbonylation of methanol to dimethyl carbonate in the gas phase. Activities were measured under elevated pressure (0.4–1.6 MPa) with feed compositions of CO/MeOH/O₂ = 40/20/6–1.5 vol.% (balanced by N₂) over zeolite Y loaded with 10–17 wt.% copper. It could be shown that inert activation at 650 °C enhanced the activity, and that Cu loading of 14–17 wt.% gave the best performance. By combined XRD, TEM, TPR and DRIFT characterization it was found that the inert activation initiated dispersion of crystalline CuO, auto-reduction of Cu²⁺ to Cu⁺ and redistribution of copper ions with enrichment inside the supercages of the zeolite. The O₂ content of the feed was found to control the selectivity to dimethyl carbonate. Dimethyl carbonate selectivities of 70–75% were achieved within the temperature range of 140–170 °C at an O₂ content of 1.5 vol.%. This allowed space-time yields of dimethyl carbonate up to 632 g l_cat⁻¹ h⁻¹ at methanol conversions of 5–12%. Formation of the main side product, dimethoxymethane, was surprisingly affected by CO, which is not in line with suggested reaction pathways. A mechanism is proposed including formation of surface carbonate structures as common intermediate.     

Hydrotreating of gas oil on SBA-15 supported NiMo catalysts

The siliceous and the metal substituted (B or Al)-SBA-15 molecular sieves were used as a support for NiMo hydrotreating catalysts (12 wt.% Mo and 2.4 wt.% Ni). The supports were characterized by X-ray diffraction (XRD), scanning electron microscopy and N₂ adsorption–desorption isotherms. The SBA-15 supported NiMo catalysts in oxide state were characterized by BET surface area analysis and XRD. The sulfided NiMo/SBA-15 catalysts were examined by DRIFT of CO adsorption and TPD of NH₃. The HDN and HDS activities with bitumen derived light gas oil at industrial conditions showed that Al substituted SBA-15 (Al-SBA-15) is the best among the supports studied for NiMo catalyst. A series of NiMo catalysts containing 7–22 wt.% Mo with Ni/Mo weight ratio of 0.2 was prepared using Al-SBA-15 support and characterized by BET surface area analysis, XRD and temperature programmed reduction and DRIFT spectroscopy of adsorbed CO. The DRIFT spectra of adsorbed CO showed the presence of both unpromoted and Ni promoted MoS₂ sites in all the catalysts, and maximum “NiMoS” sites concentration with 17 wt.% of Mo loading. The HDN and HDS activities of NiMo/Al-SBA-15 catalysts were studied using light gas oil at temperature, pressure and WHSV of 370 °C, 1300 psig and 4.5 h⁻¹, respectively. The NiMo/Al-SBA-15 catalyst with 17 wt.% Mo and 3.4 wt.% of Ni is found to be the best catalyst. The HDN and HDS activities of this catalyst are comparable with the conventional Al₂O₃ supported NiMo catalyst in real feed at industrial conditions.     

The influence of zeolite acidity for the coupled hydrogenation and ring opening of 1-methylnaphthalene on Pt/USY catalysts

The influence of framework and extraframework composition of USY zeolite on the catalytic performance of bifunctional Pt/USY (1 wt.% Pt) catalysts for the coupled hydrogenation and ring opening of 1-methylnaphthalene (1-MN) has been studied on a continuous fixed bed high pressure reactor. All Pt/USY catalysts showed very high methylnaphthalene (MN) conversions under the reaction conditions studied (T=300–375 °C, P=4.0 MPa, WHSV=2 h⁻¹, H₂/1-MN=30 mol/mol). Product yields and selectivities were mainly determined by the zeolite composition (i.e. acidity). Selectivity to products with the same number of carbon atoms than the feed (C11) increased, at constant temperature, with decreasing the Brönsted acidity of the USY zeolite, that is, with decreasing the concentration of framework Al (FAL) and increasing extraframework Al (EFAL). Selectivity to high cetane ring opening products (ROP=C11-alkylbenzenes (C11AB) and C11-alkylcycloalkanes) within the C11 fraction was higher for the less acidic catalysts. A maximum yield of ROP of ca. 15 wt.% at a C11 yield of ca. 73 wt.% was obtained at 350 °C (P=4.0 MPa, WHSV=2 h⁻¹, H₂/1-MN=30 mol/mol) for a USY zeolite with an intermediate degree of dealumination (a₀=24.33 Å) and containing all the EFAL (bulk Si/Al ratio of 2.6). For this catalyst, a slight increase in ROP yield (ca. 17 wt.%) at similar C11 yield (ca. 74 wt.%) was obtained by working at lower temperature (300 °C) and lower space velocity. Increasing the reaction pressure above 4.0 MPa had only a marginal influence on product yields and selectivities.     

Fischer–Tropsch synthesis using monolithic catalysts

Square channel cordierite monoliths have been loaded with alumina washcoat layers of various thicknesses (20–110 μm) and loaded with rhenium and cobalt resulting in a 0.1 wt.% Re/17 wt.% Co/Al₂O₃ catalyst. These monolithic catalysts have been tested in the Fischer–Tropsch synthesis in a temperature window (180–225 °C) under synthesis gas compositions ranging from stoichiometrically excess carbon monoxide to excess hydrogen (H₂/CO = 1–3). The results include data on the activity and selectivity of CoRe/Al₂O₃ monolithic catalysts for FTS under these process conditions. Washcoat layers thicker than about 50 μm appear to lead to internal diffusion limitations. Thinner washcoat layers yield, depending on the conditions, to larger amounts of -olefins than alkanes for chain lengths below 10 carbon atoms. ASF and non-ASF chain length distributions are obtained for thin washcoats, whereby the chain growth probability increases from 0.83 to 0.93. Under certain conditions the amounts of alkanes even increase with chain length. These experimental results with different diffusion lengths have been used to analyze the effects of secondary reactions on FTS selectivity.     

Ceria-based oxides as supports for LaCoO3 perovskite; catalysts for total oxidation of VOC

Supported LaCoO₃ perovskites with 10 and 20 wt.% loading were obtained by wet impregnation of different Ce_1−xZr_xO₂ (x = 0–0.3) supports with a solution prepared from La and Co nitrates, and citric acid. Supports were also prepared using the “citrate method”. All materials were calcined at 700 °C for 6 h and investigated by N₂ adsorption at −196 °C, XRD and XPS. XRD patterns and XPS measurements evidenced the formation of a pure perovskite phase, preferentially accumulated at the outer surface. These materials were comparatively tested in benzene and toluene total oxidation in the temperature range 100–500 °C. All catalysts showed a lower T₅₀ than the corresponding Ce_1−xZr_xO₂ supports. Twenty weight percent LaCoO₃ catalysts presented lower T₅₀ than bulk LaCoO₃. In terms of reaction rates per mass unit of perovskite calculated at 300 °C, two facts should be noted (i) the activity order is more than 10 times higher for toluene and (ii) the reverse variation with the loading as a function of the reactant, a better activity being observed for low loadings in the case of benzene. For the same loading, the support composition influences drastically the oxidative abilities of LaCoO₃ by the surface area and the oxygen mobility.     

Dechlorination of chlorinated hydrocarbons by catalytic steam reforming

Past research in this laboratory on catalytic steam reforming of chlorinated hydrocarbons demonstrated extremely high levels of destruction (0.99999+) at 600–750 °C, with GHSVs as high as 2.5 × 10⁵ h⁻¹. Feasible operation was demonstrated with chlorinated alkanes, alkenes, aromatics and PCBs using Pt/γ-Al₂O₃ catalysts. The major mechanism for deactivation with trichloroethylene was sintering of the γ-Al₂O₃ support and encapsulation of Pt crystallites.

Evidence is presented here that ZrO₂ is a superior support for steam reforming of trichloroethylene (TCE), due to its low acidity and ability to store oxygen. Formulations of 0.8 wt.% Pt/ZrO₂ tested at a GHSV of 20,000 h⁻¹ and a H₂O/C ratio of 20 operated for 42 days at 750 °C, with only slight carbon deposits in the first 15% of the catalyst bed. No pyrolysis was found, and the product CO/CO₂ ratio was at equilibrium, indicating high water gas shift activity with very low CO concentrations. Kinetic measurements revealed a pseudo-first order rate equation, sintering of the support and Pt was much less than with γ-Al₂O₃ supports, and no encapsulation was detected. Slow deactivation occurred due to deposition of catalytic carbon. This carbon was removed by combustion with air, and the rate of deactivation indicated the 42-day run would have lasted seven months.     

Impact of support oxide and Ba loading on the NOx storage properties of Pt/Ba/support catalysts: CO2 and H2O effects

A series of 1 wt.%Pt/_xBa/Support (Support = Al₂O₃, SiO₂, Al₂O₃-5.5 wt.%SiO₂ and Ce_0.7Zr_0.3O₂, x = 5–30 wt.% BaO) catalysts was investigated regarding the influence of the support oxide on Ba properties for the rapid NO_x trapping (100 s). Catalysts were treated at 700 °C under wet oxidizing atmosphere. The nature of the support oxide and the Ba loading influenced the Pt–Ba proximity, the Ba dispersion and then the surface basicity of the catalysts estimated by CO₂-TPD. At high temperature (400 °C) in the absence of CO₂ and H₂O, the NO_x storage capacity increased with the catalyst basicity: Pt/20Ba/Si < Pt/20Ba/Al5.5Si < Pt/10Ba/Al < Pt/5Ba/CeZr < Pt/30Ba/Al5.5Si < Pt/20Ba/Al < Pt/10BaCeZr. Addition of CO₂ decreased catalyst performances. The inhibiting effect of CO₂ on the NO_x uptake increased generally with both the catalyst basicity and the storage temperature. Water negatively affected the NO_x storage capacity, this effect being higher on alumina containing catalysts than on ceria–zirconia samples. When both CO₂ and H₂O were present in the inlet gas, a cumulative effect was observed at low temperatures (200 °C and 300 °C) whereas mainly CO₂ was responsible for the loss of NO_x storage capacity at 400 °C. Finally, under realistic conditions (H₂O and CO₂) the Pt/20Ba/Al5.5Si catalyst showed the best performances for the rapid NO_x uptake in the 200–400 °C temperature range. It resulted mainly from: (i) enhanced dispersions of platinum and barium on the alumina–silica support, (ii) a high Pt–Ba proximity and (iii) a low basicity of the catalyst which limits the CO₂ competition for the storage sites.     

Reactivity of Ru-based catalysts in the oxidation of propene and carbon black

The relationship between the state of Ru on different supports and catalytic activity in the oxidation of propene and carbon black was investigated for catalysts prepared by different impregnation methods. It is demonstrated that the addition of ruthenium to ceria (CeO₂), alumina (Al₂O₃) and ceria–alumina significantly improves the reactivity: the temperature of carbon black oxidation decreases by 100–140 °C. It is also shown that the addition of Ru to the different supports is very beneficial for the total oxidation of propene. Temperature programmed reduction (TPR) experiments of the catalysts showed that the oxygen species of ruthenium oxides are reduced at low temperatures which is the main reason of its high reactivity in oxidation reactions.     

Effects of electrodeposited Co and Co–P catalysts on the hydrogen generation properties from hydrolysis of alkaline sodium borohydride solution

Co and Co–P catalysts electroplated on Cu in sulfate based solution without or with an addition of H₂PO₂⁻ ions were developed for hydrogen generation from alkaline NaBH₄ solution. The microstructures of the Co and Co–P catalysts and their hydrogen generation properties were analyzed as a function of cathodic current density and plating time during the electrodeposition. An amorphous Co–P electrodeposit with micro-cracks was formed by electroplating in the sulfate based solution containing H₂PO₂⁻ ions. It was found that the amorphous Co–P catalyst formed at 0.01 A/cm² exhibited 18 times higher catalytic activity for hydrolysis of NaBH₄ than did the polycrystalline Co catalyst. The catalytic activity of the electrodeposited Co–P catalyst for hydrolysis of NaBH₄ was found to be a function of both cathodic current density and plating time, that is, parameters determining the concentration of P in the Co–P catalyst. Especially, Co–13 at.% P catalyst electroplated on Cu in the Co–P bath at a cathodic current density of 0.01 A/cm² for 1080 s showed the best hydrogen generation rate of 954 ml/min g-catalyst in 1 wt.% NaOH + 10 wt.% NaBH₄ solution at 30 °C.     

Formation of active state in vanadium–titanium oxide system regarding to reaction of oxidation of β-picoline to nicotinic acid

Binary vanadia–titania catalysts comprising 5–75 wt.% of V₂O₅ and 95–25 wt.% of TiO₂, pretreated at the temperature ranging between 300 and 700°C, were studied as heterogeneous catalysts for oxidation of β-picoline at 250°C, and inlet concentrations of the following components (vol.%): 1% of 3-picoline, 20% of oxygen, 30% of steam. Nicotinic acid, 3-pyridinecarbaldehyde and CO₂ were the reaction products. The most active state for oxidation of 3-picoline into nicotinic acid was shown to result from formation of coherent interface between V₂O₅ and TiO₂ (anatase) crystallites. This state was generated at the temperature particular for each composition and persists below the temperature of the anatase to rutile transition.     

Effect of Re loading on the structure, activity and selectivity of Re/C catalysts in hydrodenitrogenation and hydrodesulphurisation of gas oil

A series of Re-containing catalysts supported on activated carbon, with Re loading between 0.74 and 11.44 wt.% Re₂O₇, was prepared by wet impregnation and tested in the simultaneous hydrodesulphurisation (HDS) and hydrodenitrogenation (HDN) of a commercial gas oil. Textural analysis, XRD, X-ray photoelectron spectroscopy (XPS) and surface acidity techniques were used for physicochemical characterisation of the catalysts. Increase in the Re concentration resulted in a rise in the HDS and HDN activity due to the formation of a monolayer structure of Re and the higher surface acidity. At Re concentrations >2.47 wt.% Re₂O₇ (0.076 Re atoms nm⁻²) the reduction in the catalytic activity was related to the loss in specific surface area (BET) due to reduction in the microporosity of the carbon support. The magnitude of the catalytic effect was different for HDS and HDN, and depended strongly on the Re content and reaction temperature. The apparent activation energies were about 116–156 kJ mol⁻¹ for HDS and 24–30 kJ mol⁻¹ for HDN. This led to a marked increase in the HDN/HDS selectivity with decreasing temperature (values >3 at 325 °C), due to the large differences in the apparent activation energies of HDS and HDN found for all catalysts. A gradual increase in the HDN/HDS selectivity with increased Re loading was also found and related to the observed increase of catalyst acidity. The results are compared with those obtained for a series of Re/γ-Al₂O₃ catalysts.     

Isomerization of n-hexane over mono- and bimetallic Pd–Pt catalysts supported on ZrO2–Al2O3–WOx prepared by sol–gel

The catalytic performance of mono- and bimetallic Pd (0.6, 1.0 wt.%)–Pt (0.3 wt.%) catalysts supported on ZrO₂ (70, 85 wt.%)–Al₂O₃ (15, 0 wt.%)–WO_x (15 wt.%) prepared by sol–gel was studied in the hydroisomerization of n-hexane. The catalysts were characterized by N₂ physisorption, XRD, TPR, XPS, Raman, NMR, and FT-IR of adsorbed pyridine. The preparation of ZrW and ZrAlW mixed oxides by sol–gel favored the high dispersion of WO_x and the stabilization of zirconia in the tetragonal phase. The Al incorporation avoided the formation of monoclinic-WO₃ bulk phase. The catalysts increased their S_BET for about 15% promoted by Al₂O₃ addition. Various oxidation states of WO_x species coexist on the surface of the catalysts after calcination. The structure of the highly dispersed surface WO_x species is constituted mainly of isolated monotungstate and two-dimensional mono-oxotungstate species in tetrahedral coordination. The activity of Pd/ZrW catalysts in the hydroisomerization of n-hexane is promoted both with the addition of Al to the ZrW mixed oxide and the addition of Pt to Pd/ZrAlW catalysts. The improvement in the activity of Pd/ZrAlW catalysts is ascribed to a moderated acid strength and acidity, which can be correlated to the coexistence of W⁶⁺ and reduced-state WO_x species (either W⁴⁺ or W⁰). The addition of Pt to the Pd/ZrAlW catalyst does not modify significantly its acidic character. Selectivity results showed that the catalyst produced 2MP, 3MP and the high octane 2,3-dimethylbutane (2,3-DMB) and 2,2-dimethylbutane (2,2-DMB) isomers.     

Thermal stability and catalytic activity of flame-made silica–vanadia–tungsten oxide–titania

Vanadia (0.9 or 2 wt.%) and silica (0–5 wt.%) doping of flame-made tungsten oxide–titania nanostructured catalyst powders (anatase, 100 m²/g, 10 wt.% WO₃) is investigated. The effect of dopants on structural and chemical properties of these powders were analyzed by nitrogen adsorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), transmission electron microscopy (TEM) and Raman spectroscopy. After calcination for 20 h at 700 °C in air, the thermally most stable composite powder conserved its specific surface area (SSA) to 90 m²/g and its anatase content to 96 wt.%. Tungsten oxide and vanadia form thin polymeric layers (1 nm) on the surface of the titania support. Adding silica improves the thermal and crystal stability of the catalysts even at higher reactor temperatures. As a result both NO conversion and the rate of selective catalytic reduction (SCR) with NH₃ were increased.     

HDS and HDN activity of molybdenum and nickel–molybdenum catalysts supported on alumina–titania carriers

Alumina–titania supports containing 5–50 wt.% of TiO₂ were prepared by coprecipitation method using inorganic precursors (sodium aluminate and titanium chloride). DTA-TGA, XRD, SEM, TPD_NH3, and IR spectroscopy were used to characterise these materials. The study shows that the promoting effect of nickel on the HDS activity of molybdenum catalysts supported on Al₂O₃TiO₂ is significantly lower than that for molybdenum catalyst supported on Al₂O₃, and depends on the TiO₂ content. The SEM results show that in the case of rich Al support (20 wt.% of TiO₂) molybdenum was aggregated on the external surface of the catalyst, whereas it was uniformly dispersed on the external surface of alumina. Results also show that molybdenum is preferably supported on aluminum oxide. Application of Al₂O₃TiO₂ oxides enhances the HDN activity of nickel–molybdenum catalysts. The highest HDN efficiency was obtained for the NiMo/Al₂O₃TiO₂ catalyst containing 50 wt.% of TiO₂. HDN activity was found to depend on protonic acidity and anatase content.     

Potential rare-earth modified CeO2 catalysts for soot oxidation: Part III. Effect of dopant loading and calcination temperature on catalytic activity with O2 and NO + O2

CeO₂ and CeReO_x_y catalysts are prepared by the calcination at different temperatures (y = 500–1000 °C) and having a different composition (Re = La³⁺ or Pr^3+/4+_, 0–90 wt.%). The catalysts are characterised by XRD, H₂-TPR, Raman, and BET surface area. The soot oxidation is studied with O₂ and NO + O₂ in the tight and loose contact conditions, respectively. CeO₂ sinters between 800–900 °C due to a grain growth, leading to an increased crystallite size and a decreased BET surface area. La³⁺ or Pr^3+/4+ hinders the grain growth of CeO₂ and, thereby, improving the surface catalytic properties. Using O₂ as an oxidant, an improved soot oxidation is observed over CeLaO_x_y and CePrO_x_y in the whole dopant weight loading and calcination temperature range studied, compared with CeO₂. Using NO + O₂, the soot conversion decreased over CeLaO_x_y catalysts calcined below 800 °C compared with the soot oxidation over CeO₂_y. CePrO_x_y, on the other hand, showed a superior soot oxidation activity in the whole composition and calcination temperature range using NO + O₂. The improvement in the soot oxidation activity over the various catalysts with O₂ can be explained based on an improvement in the external surface area. The superior soot oxidation activity of CePrO_x_y with NO + O₂ is explained by the changes in the redox properties of the catalyst as well as surface area. CePrO_x_y, having 50 wt.% of dopant, is found to be the best catalyst due to synergism between cerium and praseodymium compared to pure components. NO into NO₂ oxidation activity, that determines soot oxidation activity, is improved over all CePrO_x catalysts.     

Effect of LaCoO3 perovskite deposition on ceria-based supports on total oxidation of VOC

Supported LaCoO₃ perovskites with 10 wt.% loading were prepared by impregnation of different supports containing ceria with a solution of La and Co nitrates and citric acid. All precursors were calcined at 700 °C for 5 h. XRD investigations indicated the perovskite formation via “citrate” precursor only on ceria support. All catalysts were tested for toluene total oxidation in the temperature range 100–600 °C. In spite of a large surface area, alumina-supported perovskites showed a lower global activity. It appears then the necessity of the presence of a perovskite phase for good oxidative activity. In terms of reaction rates higher reaction rates per perovskite weight were observed for all supported catalysts when compared to bulk LaCoO₃.     

Effect of solvo-thermal treatment temperature on the properties of sol–gel ZrO2–TiO2 mixed oxides as HDS catalyst supports

ZrO₂–TiO₂ mixed oxide (30–70 mol/mol) was prepared by low-temperature sol–gel followed by solvo-thermal treatment (1 day) at various temperatures (40, 80, 120, 160 and 200 °C). Selected samples of the corresponding single oxides were also prepared. Materials characterization was carried out by N₂ physisorption, XRD, thermal analysis (TG-DTA) and UV–vis DRS, infra-red and Laser-Raman spectroscopies. Binary solids of enhanced pore volume and pore size diameter were obtained by increasing the post-treatment severity. Anatase TiO₂ micro-segregation was evidenced by Raman spectroscopy for the mixed oxide solvo-treated at the highest temperature. This solid also showed the highest crystallization temperature to ZrTiO₄ (702 °C). Mo impregnated (2.8 atom nm⁻²) on various mixed oxides was sulfided under H₂S/H₂ (400 °C, 1 h), the catalysts being tested in the dibenzothiophene hydrodesulfurization (HDS, T = 320 °C, P = 5.59 MPa). By increasing the severity of the solvo-treatment improved supports for MoS₂ phase were obtained. The HDS activity of the catalyst with carrier post-treated at 200 °C was 40% higher (in per total mass basis) than that of sulfided Mo supported on the binary oxide solvo-treated at 80 °C. The ZrO₂–TiO₂-supported catalysts showed higher selectivity to products from the hydrogenation route than their counterparts supported on either single oxide.     

Simultaneous production of hydrogen and carbon nanostructures by decomposition of propane and cyclohexane over alumina supported binary catalysts

Nano-scale, binary, 4.5 wt.% Fe–0.5 wt.% M (M = Pd, Mo or Ni) catalysts supported on alumina have been shown to be very effective for the decomposition of lower alkanes to produce hydrogen and carbon nanofibers or nanotubes. After pre-reduction at 700 °C, all three binary catalysts exhibited significantly lower propane decomposition temperatures and longer time-on-stream performances than either the non-metallic alumina support or 5 wt.% Fe/Al₂O₃. Catalytic decomposition of propane using all three catalysts yielded only hydrogen, methane, unreacted propane, and carbon nanotubes. Above 475 °C, hydrogen and methane were the only gaseous products. Catalytic decomposition of cyclohexane using the (4.5 wt.% Fe–0.5 wt.% Pd)/Al₂O₃ catalyst produced primarily hydrogen, benzene, and unreacted cyclohexane below 450 °C, but only hydrogen, methane, and carbon nanotubes above 500 °C. The carbon nanotubes exhibited two distinct forms depending on the reaction temperature. Above 600 °C, they were predominantly in form of multi-walled nanotubes with parallel walls in the form of concentric graphene sheets. At or below 500 °C, carbon nanofibers with capped and truncated stacked-cone structure were produced. At 625 °C, decomposition of cyclohexane produced a mixture of the two types of carbon nanostructures.     

Effective Au-Au-Clx/Fe(OH)y catalysts containing Cl for selective CO oxidations at lower temperatures

Supported Au catalysts Au-Au⁺-Cl_x/Fe(OH)_y (x < 4, y ≤ 3) and Au-Cl_x/Fe₂O₃ prepared with co-precipitation without any washing to remove Cl⁻ and without calcining or calcined at 400 °C were studied. It was found that the presence of Cl⁻ had little impact on the activity over the unwashed and uncalcined catalysts; however, the activity for CO oxidation would be greatly reduced only after Au-Au⁺-Cl_x/Fe(OH)_y was further calcined at elevated temperatures, such as 400 °C. XPS investigation showed that Au in catalyst without calcining was composed of Au and Au⁺, while after calcined at 400 °C it reduced to Au⁰ completely. It also showed that catalysts precipitated at 70 °C could form more Au⁺ species than that precipitated at room temperatures. Results of XRD and TEM characterizations indicated that without calcining not only the Au nano-particles but also the supports were highly dispersed, while calcined at 400 °C, the Au nano-particles aggregated and the supports changed to lump sinter. Results of UV–vis observation showed that the Fe(NO₃)₃ and HAuCl₄ hydrolyzed partially to form Fe(OH)₃ and [AuCl_x(OH)_4−x]⁻ (x = 1–3), respectively, at 70 °C, and such pre-partially hydrolyzed iron and gold species and the possible interaction between them during the hydrolysis may be favorable for the formation of more active precursor and to avoid the formation of Au–Cl bonds. Results of computer simulation showed that the reaction molecular of CO or O₂ were more easily adsorbed on Au⁺ and Au⁰, but was very difficultly absorbed on Au⁻. It also indicated that when Cl⁻ was adsorbed on Au⁰, the Au atom would mostly take a negative electric charge, which would restrain the adsorption of the reaction molecular severely and restrain the subsequent reactions while when Cl⁻ was adsorbed on Au⁺ there only a little of the Au atom take negative electric charge, which resulting a little impact on the activity.