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1.
The formation process of diamond from supercritical H2O–CO2 fluid was studied using 13C-graphitic carbon and oxalic acid dihydrate, (COOH)2·2H2O, as starting materials under a diamond stable high pressure–high temperature (HP–HT) condition of 7.7 GPa and 1600°C. The exchange reaction between 13C-graphitic carbon and 12CO2 in the supercritical H2O–CO2 fluid, which was first formed by the decomposition of oxalic acid dihydrate, occurred very rapidly and became nearly equilibrated after 6 h. At the same time, graphite was recrystallized and coexistent with the fluid until traces of diamond were first observed after 8 h. All graphite transformed into diamond after 17 h, showing that a considerably long induction time was present for the formation of diamond in this fluid system.  相似文献   

2.
Crystallization of diamond was studied in the CO2–C, CO2–H2O–C, H2O–C, and CH4–H2–C systems at 5.7 GPa and 1200–1420°C. Thermodynamic calculations show generation of CO2, CO2–H2O, H2O and CH4–H2 fluids in experiments with graphite and silver oxalate (Ag2C2O4), oxalic acid dihydrate (H2C2O4·2H2O), water (H2O), and anthracene (C14H10), respectively. Diamond nucleation and growth has been found in the CO2–C, CO2–H2O–C, and H2O–C systems at 1300–1420°C. At a temperature as low as 1200°C for 136 h there was spontaneous crystallization of diamond in the CO2–H2O–C system. For the CH4–H2–C system, at 1300–1420°C no diamond synthesis has been established, only insignificant growth on seeds was observed. Diamond octahedra form from the C–O–H fluids at all temperature ranges under investigation. Diamond formation from the fluids at 5.7 GPa and 1200–1420°C was accompanied with the active recrystallization of metastable graphite.  相似文献   

3.
Effect of additives, In2O3, SnO2, CoO, CuO and Ag, on the catalytic performance of Ga2O3–Al2O3 prepared by sol–gel method for the selective reduction of NO with propene in the presence of oxygen was studied. As for the reaction in the absence of H2O, CoO, CuO and Ag showed good additive effect. When H2O was added to the reaction gas, the activity of CoO-, CuO- and Ag-doped Ga2O3–Al2O3 was depressed considerably, while an intensifying effect of H2O was observed for In2O3- and SnO2-doped Ga2O3–Al2O3. Of several metal oxide additives, In2O3-doped Ga2O3–Al2O3 showed the highest activity for NO reduction by propene in the presence of H2O. Kinetic studies on NO reduction over In2O3–Ga2O3–Al2O3 revealed that the rate-determining step in the absence of H2O is the reaction of NO2 formed on Ga2O3–Al2O3 with C3H6-derived species, whereas that in the presence of H2O is the formation of C3H6-derived species. We presumed the reason for the promotional effect of H2O as follows: the rate for the formation of C3H6-derived species in the presence of H2O is sufficiently fast compared with that for the reaction of NO2 with C3H6-derived species in the absence of H2O. Although the retarding effect of SO2 on the activity was observed for all of the catalysts, SnO2–Ga2O3–Al2O3 showed still relatively high activity in the lower temperature region.  相似文献   

4.
The diamond deposition on a molybdenum substrate was examined in an Ar---H2 DC plasma jet adding C6H5Cl, C6H4Cl2 and C6H3Cl3 as the carbon sources, compared with adding C6H6 to to examine the effect of chlorine species on diamond deposition. Well-faceted high-quality diamond was formed from the Ar---C6H4Cl2---H2 plasma jet. A small amount of graphite and/or amorphous carbon were identified on the surface of the deposits from other plasma jets. Hydrogen abstraction of surface-adsorbed chlorine, chlorine abstraction of surface-adsorbed hydrogen and/or dehydrochlorination play a dominant role in providing higher diamond deposition rates and the removal of graphite and/or amorphous carbon. Excess amounts of chlorine, however, seem to foster the deposition of non-diamond carbon phase.  相似文献   

5.
A La–Sr–Cu–O–S system with K2NiF4 perovskite-type structure has been studied as a novel SOx-resistant combustion catalyst. The XRD result implied that sulfur is incorporated into the structure as non-sulfate-type cations. An introduction of sulfur with highly positive valence (S6+ or S4+) into the lattice requires the charge compensation by decreasing the oxidation number of Cu. This is accompanied by the creation of more reducible Cu species, which would achieve the light-off of catalytic C3H6 oxidation at lower temperatures. More important feature of sulfur-containing compounds is that the catalytic C3H6 oxidation was significantly accelerated by addition of SO2 to the gas feed. The catalytic performance for the oxidation of C3H6 and CO and the reduction of NO was finally evaluated in a simulated automotive exhaust in the presence of SO2.  相似文献   

6.
Selective catalytic reduction of NOx by C3H6 in the presence of H2 over Ag/Al2O3 was investigated using in situ DRIFTS and GC–MS measurements. The addition of H2 promoted the partial oxidation of C3H6 to enolic species, the formation of –NCO and the reactions of enolic species and –NCO with NOx on Ag/Al2O3 surface at low temperatures. Based on the results, we proposed reaction mechanism to explain the promotional effect of H2 on the SCR of NOx by C3H6 over Ag/Al2O3 catalyst.  相似文献   

7.
A series of CoOx/Al2O3 catalysts was prepared, characterized, and applied for the selective catalytic reduction (SCR) of NO by C3H8. The results of XRD, UV–vis, IR, Far-IR and ESR characterizations of the catalysts suggest that the predominant oxidation state of cobalt species is +2 for the catalysts with low cobalt loading (≤2 mol%) and for the catalysts with 4 mol% cobalt loading prepared by sol–gel and co-precipitation. Co3O4 crystallites or agglomerates are the predominant species in the catalysts with high cobalt loading prepared by incipient wetness impregnation and solid dispersion. An optimized CoOx/Al2O3 catalyst shows high activity in SCR of NO by C3H8 (100% conversion of NO at 723 K, GHSV: 10,000 h−1). The activity of the selective catalytic reduction of NO by C3H8 increases with the increase of cobalt–alumina interactions in the catalysts. The influences of cobalt loading and catalyst preparation method on the catalytic performance suggest that tiny CoAl2O4 crystallites highly dispersed on alumina are responsible for the efficient catalytic reduction of NO, whereas Co3O4 crystallites catalyze the combustion of C3H8 only.  相似文献   

8.
The effect of the addition of a second fuel such as CO, C3H8 or H2 on the catalytic combustion of methane was investigated over ceramic monoliths coated with LaMnO3/La-γAl2O3 catalyst. Results of autothermal ignition of different binary fuel mixtures characterised by the same overall heating value show that the presence of a more reactive compound reduces the minimum pre-heating temperature necessary to burn methane. The effect is more pronounced for the addition of CO and very similar for C3H8 and H2. Order of reactivity of the different fuels established in isothermal activity measurements was: CO>H2≥C3H8>CH4. Under autothermal conditions, nearly complete methane conversion is obtained with catalyst temperatures around 800 °C mainly through heterogeneous reactions, with about 60–70 ppm of unburned CH4 when pure methane or CO/CH4 mixtures are used. For H2/CH4 and C3H8/CH4 mixtures, emissions of unburned methane are lower, probably due to the proceeding of CH4 homogeneous oxidation promoted by H and OH radicals generated by propane and hydrogen pyrolysis at such relatively high temperatures.

Finally, a steady state multiplicity is found by decreasing the pre-heating temperature from the ignited state. This occurrence can be successfully employed to pilot the catalytic ignition of methane at temperatures close to compressor discharge or easily achieved in regenerative burners.  相似文献   


9.
The influences of calcination temperatures and additives for 10 wt.% Cu/γ-Al2O3 catalysts on the surface properties and reactivity for NO reduction by C3H6 in the presence of excess oxygen were investigated. The results of XRD and XPS show that the 10 wt.% Cu/γ-Al2O3 catalysts calcined below 973 K possess highly dispersed surface and bulk CuO phases. The 10 wt.% Cu/γ-Al2O3 and 10 wt.% Mn–10 wt.% Cu/γ-Al2O3 catalysts calcined at 1073 K possess a CuAl2O4 phase with a spinel-type structure. In addition, the 10 wt.% La–10 wt.% Cu/γ-Al2O3 catalyst calcined at 1073 K possesses a bulk CuO phase. The result of NO reduction by C3H6 shows that the CuAl2O4 is a more active phase than the highly dispersed and bulk CuO phase. However, the 10 wt.% Mn–10 wt.% Cu/γ-Al2O3 catalyst calcined at 1073 K possesses significantly lower reactivity for NO reduction than the 10 wt.% Cu/γ-Al2O3 catalyst calcined at 1073 K, although these catalysts possess the same CuAl2O4 phase. The low reactivity for NO reduction for 10 wt.% Mn–10 wt.% Cu/γ-Al2O3 catalyst calcined at 1073 K is attributed to the formation of less active CuAl2O4 phase with high aggregation and preferential promotion of C3H6 combustion to COx by MnO2. The engine dynamometer test for NO reduction shows that the C3H6 is a more effective reducing agent for NO reduction than the C2H5OH. The maximum reactivity for NO reduction by C3H6 is reached when the NO/C3H6 ratio is one.  相似文献   

10.
The effectiveness of Ag/Al2O3 catalyst depends greatly on the alumina source used for preparation. A series of alumina-supported catalysts derived from AlOOH, Al2O3, and Al(OH)3 was studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet–visible (UV–vis) spectroscopy, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, O2, NO + O2-temperature programmed desorption (TPD), H2-temperature programmed reduction (TPR), thermal gravimetric analysis (TGA) and activity test, with a focus on the correlation between their redox properties and catalytic behavior towards C3H6-selective catalytic reduction (SCR) of NO reaction. The best SCR activity along with a moderated C3H6 conversion was achieved over Ag/Al2O3 (I) employing AlOOH source. The high density of Ag–O–Al species in Ag/Al2O3 (I) is deemed to be crucial for NO selective reduction into N2. By contrast, a high C3H6 conversion simultaneously with a moderate N2 yield was observed over Ag/Al2O3 (II) prepared from a γ-Al2O3 source. The larger particles of AgmO (m > 2) crystallites were believed to facilitate the propene oxidation therefore leading to a scarcity of reductant for SCR of NO. An amorphous Ag/Al2O3 (III) was obtained via employing a Al(OH)3 source and 500 °C calcination exhibiting a poor SCR performance similar to that for Ag-free Al2O3 (I). A subsequent calcination of Ag/Al2O3 (III) at 800 °C led to the generation of Ag/Al2O3 (IV) catalyst yielding a significant enhancement in both N2 yield and C3H6 conversion, which was attributed to the appearance of γ-phase structure and an increase in surface area. Further thermo treatment at 950 °C for the preparation of Ag/Al2O3 (V) accelerated the sintering of Ag clusters resulting in a severe unselective combustion, which competes with SCR of NO reaction. In view of the transient studies, the redox properties of the prepared catalysts were investigated showing an oxidation capability of Ag/Al2O3 (II and V) > Ag/Al2O3 (IV) > Ag/Al2O3 (I) > Ag/Al2O3 (III) and Al2O3 (I). The formation of nitrate species is an important step for the deNOx process, which can be promoted by increasing O2 feed concentration as evidenced by NO + O2-TPD study for Ag/Al2O3 (I), achieving a better catalytic performance.  相似文献   

11.
The total oxidation of propane and its oxy-dehydrogenation to propene have been studied on spinel-type catalysts Mn3O4, Co3O4 and MgCr2O4 in a flow reactor and in an IR cell. Analogous studies have been performed on the oxy-dehydrogenation of n-butenes to 1,3-butadiene over MgFe2O4. Information on the reaction mechanism have been reached. The activation of the hydrocarbons is thought to occur by abstraction of a hydrogen from the weakest C–H bond, with a simultaneous reduction of a surface site and with the formation of a surface alkoxy-group.  相似文献   

12.
The SCR of NO and NO decomposition were investigated over a V–W–O/Ti(Sn)O2 catalyst on a Cr–Al steel monolith. The conversions of NO and NH3 over the reduced and oxidised catalysts were determined. The higher conversion of NO than of NH3 was observed in SCR over the reduced catalyst and very close conversions of both substrates were found over the oxidised one. The increase of the pre-reduction temperature was found to cause an increase in catalyst activity and its stability in direct NO decomposition. The surface tungsten cations substituted for vanadium ones in vanadia-like active species are considered to be responsible for the direct NO decomposition. The results of DFT calculations for the 10-pyramidal clusters: V10O31H12 (V–V) and V9WO31H12 (V–W) modelling (0 0 1) surfaces of vanadia and WO3–V2O5 solid solution (s.s.) active species, respectively, show that preferable conditions for NO adsorption exist on W sites of s.s. species and that reduction causes an increase in their ability for electron back donation to the adsorbed molecule. Electron back donation is believed to be responsible for the electron structure reorganisation in the adsorbed NO molecule resulting in its decomposition. The high selectivity of NO decomposition to dinitrogen was considered to be connected with the formation of the tungsten nitrosyl complexes solely via the W–N bond.  相似文献   

13.
The reaction mechanism of methane activation using non-equilibrium pulsed discharge was largely clarified from the emission spectroscopic study and experiments with higher hydrocarbons and some kinds of isotopes. The strong emission of atomic carbon and C2 swan band system was observed as well as H Balmer series emission. This indicates that methane was highly dissociated into C and H by electron impact, which is consistent with the result of high C2D2 composition in produced acetylene when the mixture of CH4 and D2 was fed into discharge region. High electron energy contributed to produce atomic carbon directly from methane, and high electron density promoted the dehydrogenation from CH3, CH2 and CH to produce atomic carbon consecutively. The reason for the high selectivity to C2H2 was the high concentration of CH or C2 formed from atomic carbon, and the repetition mechanism of decomposition and recombination among C, CH, C2 and C2H2.  相似文献   

14.
The effects of carbon dioxide on the dehydrogenation of C3H8 to produce C3H6 were investigated over several Cr2O3 catalysts supported on Al2O3, active carbon and SiO2. Carbon dioxide exerted promoting effects only on SiO2-supported Cr2O3 catalysts. The promoting effects of carbon dioxide over a Cr2O3/SiO2 catalyst were to enhance the yield of C3H6 and to suppress the catalyst deactivation.  相似文献   

15.
This paper deals with the activity of bimetallic potassium–copper and potassium–cobalt catalysts supported on alumina for the reduction of NOx with soot from simulated diesel engine exhaust. The effect of the reaction temperature, the soot/catalyst mass ratio and the presence of C3H6 has been studied. In addition, the behavior of two monometallic catalysts supported on zeolite beta (Co/beta and Cu/beta), previously used for NOx reduction with C3H6, as well as a highly active HC-SCR catalyst (Pt/beta) has been tested for comparison. The preliminary results obtained in the absence of C3H6 indicate that, at temperatures between 250 and 400 °C, the use of bimetallic potassium catalysts notably increases the rate of NOx reduction with soot evolving N2 and CO2 as main reaction products. At higher temperatures, the catalysts mainly favor the direct soot combustion with oxygen. In the presence of C3H6, an increase in the activity for NOx reduction has been observed for the catalyst with the highest metal content. At 450 °C, the copper-based catalysts (Cu/beta and KCu2/Al2O3) show the highest activity for both NOx reduction (to N2 and CO2) and soot consumption. The Pt/beta catalyst does not combine, at any temperature, a high NOx reduction with a high soot consumption rate.  相似文献   

16.
Characteristics of MnOy–ZrO2 and Pt–ZrO2–Al2O3 as reversible sorbents of NOx were investigated under dynamic changes in atmosphere. These sorbents can be used reversibly with a change of C3H8 concentration in the reaction gases. Catalytic reduction of NO occurred in the presence of propane, which was more pronounced on Pt–ZrO2–Al2O3 than on MnOy-ZrO2 due to high activity of Pt surface for this reaction on MnOy in MnOy–ZrO2. The sorption was observed as soon as the atmosphere changed from a reducing to an oxidizing one. This implies that a high equilibrium partial pressure of O2 is necessary for NO uptake since the sorbed NO3 species becomes stable. The beginning of NOx desorption atmospheres was somewhat dependent on the amount of stored NOx. The presence of propane in the gas phase strongly affected the characteristic sorption and desorption properties of MnOy–ZrO2 and Pt–ZrO2–Al2O3. The sorption and desorption properties are different for MnOy–ZrO2 and Pt–ZrO2–Al2O3, since the noble metal or metal oxide possesses unique activity for the NO reaction with C3H8 and the amount of oxygen available for oxidative sorption of NO.  相似文献   

17.
Alkali halide added transition metal oxides produced ethylene selectively in oxidative coupling of methane. The role of alkali halides has been investigated for LiCl-added NiO (LiCl/NiO). In the absence of LiCl the reaction over NiO produced only carbon oxides (CO2 + CO). However, addition of LiCl drastically improved the yield of C2 compounds (C2H6 + C2H4). One of the roles of LiCl is to inhibit the catalytic activity of the host NiO for deep oxidation of CH4. The reaction catalyzed by the LiCl/NiO proceeds stepwise from CH4 to C2H4 through C2H6 (2CH4 → C2H6 → C2H4). The study on the oxidation of C2H6 over the LiCl/NiO showed that the oxidative dehydrogenation of C2H6 to C2H4 occurs very selectively, which is the main reason why partial oxidation of CH4 over LiCl/NiO gives C2H4 quite selectively. The other role of LiCl is to prevent the host oxide (NiO) from being reduced by CH4. The catalyst model under working conditions was suggested to be the NiO covered with molten LiCl. XPS studies suggested that the catalytically active species on the LiCl/NiO is a surface compound oxide which has higher valent nickel cations (Ni(2+δ)+ or Ni3+). The catalyst was deactivated at the temperatures>973 K due to vaporization of LiCl and consumption of chlorine during reaction. The kinetic and CH4---CD4 exchange studies suggested that the rate-determining step of the reaction is the abstraction of H from the vibrationally excited methane by the molecular oxygen adsorbed on the surface compound oxide.  相似文献   

18.
The effect of pH during sol–gel synthesis on the structural and physicochemical properties of a Pd–Al2O3 three-way catalyst (TWC) prepared by the sol–gel method was investigated by using BET, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and solid state 27Al MAS NMR. The Pd–Al2O3 catalyst prepared at pH=10 (Pd–Al2O3–B) showed a high activity in three-way catalytic reaction, a high dispersion of Pd, and large surface area and pore volume. A basic condition (pH=10) in the sol–gel process was essential for the preparation of highly dispersed palladium clusters on alumina gel. The formation of highly stable palladium oxide species in Pd–Al2O3–B that were not completely reduced at 423 K was ascribed to the strong interaction between Pd and oxygen in alumina texture, resulting in the formation of –Al–O–Pd bond.  相似文献   

19.
The pulse corona plasma has been used as an activation method for reaction of methane and carbon dioxide, the product was C2 hydrocarbons and by-products were CO and H2. Methane conversion and the yield of C2 hydrocarbons were affected by the carbon dioxide concentration in the feed. The conversion of methane increased with increasing carbon dioxide concentration in the feed whereas the yield of C2 hydrocarbons decreased. The synergism of La2O3/γ-Al2O3 and plasma gave methane conversion of 24.9% and C2 hydrocarbons yield of 18.1% were obtained at the power input of plasma was 30 W. The distribution of C2 hydrocarbons changed by using Pd-La2O3/γ-Al2O3 catalyst, the major C2 product was ethylene.  相似文献   

20.
This work investigates performances of supported transition-metal oxide catalysts for the catalytic reduction of SO2 with C2H4 as a reducing agent. Experimental results indicate that the active species, the support, the feed ratio of C2H4/SO2, and pretreatment are all important factors affecting catalyst activity. Fe2O3/γ-Al2O3 was found to be the most active catalyst among six γ-Al2O3-supported metal oxide catalysts tested. With Fe2O3 as the active species, of the supports tested, CeO2 is the most suitable one. Using this Fe2O3/CeO2 catalyst, we found that the optimal Fe content is 10 wt.%, the optimal feed ratio of C2H4/SO2 is 1:1, and the catalyst presulfidized by H2+H2S exhibits a higher performance than those pretreated with H2 or He. Although the feed concentrations of C2H4:SO2 being 3000:3000 ppm provide a higher conversion of SO2, the sulfur yield decreases drastically at temperatures above 300 °C. With higher feed concentrations, maximum yield appears at higher temperatures. The C2H4 temperature-programmed desorption (C2H4-TPD) and SO2-TPD desorption patterns illustrate that Fe2O3/CeO2 can adsorb and desorb C2H4 and SO2 more easily than can Fe2O3/γ-Al2O3. Moreover, the SO2-TPD patterns further show that Fe2O3/γ-Al2O3 is more seriously inhibited by SO2. These findings may properly explain why Fe2O3/CeO2 has a higher activity for the reduction of SO2.  相似文献   

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