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1.
Crystallization of diamond was studied in the CO 2–C, CO 2–H 2O–C, H 2O–C, and CH 4–H 2–C systems at 5.7 GPa and 1200–1420°C. Thermodynamic calculations show generation of CO 2, CO 2–H 2O, H 2O and CH 4–H 2 fluids in experiments with graphite and silver oxalate (Ag 2C 2O 4), oxalic acid dihydrate (H 2C 2O 4·2H 2O), water (H 2O), and anthracene (C 14H 10), respectively. Diamond nucleation and growth has been found in the CO 2–C, CO 2–H 2O–C, and H 2O–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 CO 2–H 2O–C system. For the CH 4–H 2–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
2.
The synthesis of a novel 3D aluminophosphate is described. The thermal properties of the material were investigated, and the existence of three high-temperature variants was revealed. The crystal structures of the as-synthesized material (UiO-26-as) and the material existing around 250°C (UiO-26-250) were solved from powder X-ray diffraction data. UiO-26-as with the composition [Al 4O(PO 4) 4(H 2O)] 2−[NH 3(CH 2) 3NH 3] 2+ crystallizes in the monoclinic space group P2 1/c (no. 14) with a=19.1912(5), b=9.3470(2), c=9.6375(2) Å and β=92.709(2)°. It exhibits a 3D open framework consisting of connections by PO 4 tetrahedra with AlO 4 tetrahedra, AlO 5 trigonal bipyramids and AlO 5(H 2O) octahedra forming two types of layers stacked along [1 0 0] and connected by Al–O–P bondings. The structure possesses a 1D 10-ring channel system running along [0 0 1], in which doubly protonated 1,3-diaminopropane molecules are located. UiO-26-250 with the composition [Al 4O(PO 4) 4] 2−[NH 3(CH 2) 3NH 3] 2+ crystallizes in the monoclinic space group P2 1/c with a=19.2491(4), b=9.27497(20), c=9.70189(20) Å and β=93.7929(17)°. The transformation to UiO-26-250 involves removal of the water molecule which originally is coordinated to aluminum. The rest of the structure remains virtually unchanged. The crystal structures of the two other variants existing around 400 (UiO-26-400) and 600°C (UiO-26-600) remain unknown. 相似文献
3.
During the reactions related to oxidative steam reforming and combustion of methane over -alumina-supported Ni catalysts, the temperature profiles of the catalyst bed were studied using an infrared (IR) thermograph. IR thermographical images revealed an interesting result: that the temperature at the catalyst bed inlet is much higher under CH 4/H 2O/O 2/Ar = 20/10/20/50 than under CH 4/H 2O/O 2/Ar = 10/0/20/70; the former temperature is comparable to that over noble metal catalysts such as Pt and Pd. Based on the temperature-programmed reduction and oxidation measurements over fresh and used catalysts, the metallic Ni is recognized at the catalyst bed inlet under CH 4/H 2O/O 2/Ar = 20/10/20/50, although it is mainly oxidized to NiAl 2O 4 under CH 4/H 2O/O 2/Ar = 10/0/20/70. This result indicates that the addition of reforming gas (CH 4/H 2O = 10/10) to the combustion gas (CH 4/O 2 = 10/20) can stabilize Ni species in the metallic state even under the presence of oxygen in the gas phase. This would account for its extremely high combustion activity. 相似文献
4.
A kinetic study on CH 4 combustion over a PdO/ZrO 2 (10%, w/w) catalyst has been performed in a temperature range between 400 and 550 °C by means of an annular catalytic microreactor. The role of mass transfer phenomena including diffusion in the catalyst pore, gas–solid diffusion and axial diffusion in the gas phase, has been preliminary addressed by means of mathematical modeling. Simulation results have pointed out the key role of internal diffusion showing that thicknesses of the active catalyst layer as thin as 10–15 μm are required to minimize the impact of mass transfer limitations. The thermal behavior of the reactor has been also addressed by means of catalytic combustion tests with CH4 and CO–H2 mixtures as fuels. The results have demonstrated the possibility to obtain nearly isothermal temperature profiles under severe conditions (up to 3% of CH4) thanks to effective dissipation of reaction heat by radiation from the catalyst outer skin. Finally the effect of reactants (CH4 and O2) and products (H2O and CO2) on CH4 combustion rate has been addressed. The results have shown that both H2O and CO2 markedly inhibit the reaction up to 550 °C. The data have been fitted by the following simple power law expression r=krPCH4PH2O−0.32PCO2−0.25 with an apparent activation energy of 108 kJ/mol. Evidences have been found and discussed indicating a key role of the support on the extent of such inhibition effects. 相似文献
5.
The inhibition effect of H 2O on V 2O 5/AC catalyst for NO reduction with NH 3 is studied at temperatures up to 250 °C through TPD, elemental analyses, temperature-programmed surface reaction (TPSR) and FT-IR analyses. The results show that H 2O does not reduce NO and NH 3 adsorption on V 2O 5/AC catalyst surface, but promotes NH 3 adsorption due to increases in Brønsted acid sites. Many kinds of NH 3 forms present on the catalyst surface, but only NH 4+ on Brønsted acid sites and a small portion of NH 3 on Lewis acid sites are reactive with NO at 250 °C or below, and most of the NH 3 on Lewis acid sites does not react with NO, regardless the presence of H 2O in the feed gas. H 2O inhibits the SCR reaction between the NH 3 on the Lewis acid sites and NO, and the inhibition effect increases with increasing H 2O content. The inhibition effect is reversible and H 2O does not poison the V 2O 5/AC catalyst. 相似文献
6.
LiNiVO 4 was prepared from Li 2CO 3, Ni(CH 3COO) 2·4H 2O and NH 4VO 3 using tartaric acid as a complexing agent with 1:1–1:4 mole ratios of metals:tartaric acid and subsequent calcination at 350–700 °C for 6–12 h. Inverse spinel LiNiVO 4 was detected using XRD. FTIR and Raman analyses revealed the presence of stretching band of VO 4 tetrahedrons. Only Ni, V and O were detected by EDX. The 1:4 mole ratio for the product with 450 °C calcination for 6 h analyzed by SEM, TEM and electron diffraction (ED) composes of LiNiVO 4 nano-powder with 10–30 nm in diameter. 相似文献
7.
In this work, we investigated the activity and stability of Ag–alumina catalysts for the SCR of NO with methane in gas streams with a high concentration of SO 2, typical of coal-fired power plant flue gases. Ag–alumina catalysts were prepared by coprecipitation–gelation, and dilute nitric-acid solutions were used to remove weakly bound silver species from the surface of the as prepared catalysts after calcination. SO 2 has a severe inhibitory effect, essentially quenching the CH 4-SCR reaction on this type catalysts at temperatures <600 °C. SO 2 adsorbs strongly on the surface forming aluminum and silver sulfates that are not active for CH 4-SCR of NO x. Above 600 °C, however, the reaction takes place without catalyst deactivation even in the presence of 1000 ppm SO 2. The reaction light-off coincides with the onset of silver sulfate decomposition, indicating the critical role of silver in the reaction mechanism. SO 2 is reversibly adsorbed on silver above 600 °C. While alumina sites remain sulfated, this does not hinder the reaction. Sulfation of alumina only decreases the extent of adsoption of NO x, but adsorption of NO x is not the limiting step. Methane activation is the limiting step, hence the presence of sulfur-free Ag–O–Al species is a requirement for the reaction. Strong adsorption of SO 2 on Ag–alumina decreases the rates of the reaction, and increases the activation energies of both the reduction of NO to N 2 and the oxidation of CH 4, the latter more than the former. Our results indicate partial contribution of gas phase reactions to the formation of N 2 above 600 °C. H 2O does not inhibit the reaction at 625 °C, and the effect of co-addition of H 2O and SO 2 is totally reversible. 相似文献
8.
An experimental study is presented concerning solubility and primary nucleation kinetics of monohydrate citric acid from pure and selectively impure (KH 2PO 4, MgSO 4 · 7H 2O and FeSO 4 · 7H 2O) aqueous solutions. The metastable range of pure solutions is wide, ranging from 6 to 14°C, and the order of primary nucleation, equal to 2.51, is quite low. The addition of the tested impurities, which are usual fermentation aids, at concentrations lower than wt. 1%, produces negligible effects on the solubility and slightly enhances the primary nucleation rate. However, the presence of these compounds modifies the crystal habit at birth: in particular, the addition of MgSO 4 · 7H 2O and FeSO 4 · 7H 2O makes these crystals more elongated, but this effect diminishes as the crystals grow. 相似文献
9.
The formation of ettringite (3CaO · Al 2O 3 · 3CaSO 4 · 32H 2O) and monosulfate (3CaO · Al 2O 3 · CaSO 4 · 12H 2O) from tricalcium aluminate (3CaO · Al 2O 3) and gypsum (CaSO 4 · 2H 2O) in sodium hydroxide (NaOH) solutions was investigated by isothermal calorimetry and X-ray diffraction analyses. Tricalcium aluminate/gypsum mixtures with a molar aluminate-to-sulfate ratio of 1:3 were hydrated at constant temperatures from 40 to 80°C in deionized water and 200 and 500 mM of NaOH solutions. Ettringite was the only crystalline phase ultimately formed between 40 and 80°C, regardless of whether hydration was carried out in deionized water or sodium hydroxide solutions. The rates of ettringite formation were retarded in sodium hydroxide solutions at all temperatures when compared to hydration in deionized water. The apparent activation energy for the conversion of the tricalcium aluminate/gypsum mixture to ettringite was observed to depend on the concentration of sodium hydroxide. 相似文献
10.
This work investigates the effect of treatments under different CH 4-containing atmospheres on the reactivity of fresh and S-poisoned 2% w/w Pd/Al 2O 3/CeO 2 catalysts for methane combustion. Over the fresh catalyst the decomposition/reformation processes of PdO occurring during cycles of CH4-reducing/lean combustion pulses allowed the complete recovery of activity losses possibly associated with H2O poisoning which were observed during prolonged exposure under lean combustion conditions. The presence of CeO2 markedly enhances both the activity losses under lean combustion conditions and the rate of PdO reoxidation/reactivation upon Pd redox cycle. Under lean combustion conditions, regeneration of catalyst deactivated by exposure to SO2-containing atmosphere required very high temperatures (above 750 °C) in order to decompose stable sulphate species adsorbed on the support. Treatments consisting of alternate CH4-reducing/lean combustion pulses allowed a complete recovery of activity at much lower temperatures (550–600 °C) due to the reduction of sulphates by CH4 activated on the surface of Pd metal. A protecting role of CeO2 on Pd poisoning due either to exposure to SO2-containing atmosphere or to spill-back of support sulphates species was also evidenced. 相似文献
11.
Spontaneous precipitation in the aqueous system Mg +2-Na +-SO 3-2-SO 4-2 affected by mixing solutions of MgSO 4 and Na 2SO 3, together was studied for temperature varying from 40 to 80°C and for pH from 5,5 to 9. The initial composition of a precipitating system was 0·67 and 1·17 mol of MgSO 3 and Na 2SO 4 per liter and 0·82 mol of MgSO 3 and Na 2SO 4 together with 0·83 mol of MgSO 4 per liter, respectively. Depending on the prevailing reaction conditions, solid phase consisting of MgSO 3 6H 2O, MgSO 3 3H 2O, Mg 2NaOH(SO 3) 2 H 2O or their mixture is formed. Each solid phase forms crystals of typical size and shape. The precipitation diagrams drawn in the temperature and pH coordinates for three different initial composition of the studied system are presented. 相似文献
12.
Fine powders of submicron-sized crystallites of BaTiO 3 were prepared at 85–130°C by the hydrothermal method, starting from TiO 2.ξH 2O gel and Ba(OH) 2 solution. The products obtained below 110°C incorporated considerable amounts of H 2O and OH − in the lattice. As-prepared BaTiO 3 is cubic and converts to the tetragonal phase after heat treatment at 1200°C, accompanied by the loss of residual OH − ions. Hydrothermal reaction of SnO 2.ξH 2O gel with Ba(OH) 2 at 150–260°C gives rise to the hydrated phase, BaSn(OH) 6.3H 2O, due to the amphoteric nature of SnO 2.ξH 2O which stabilises Sn(OH) 62− anions in basic media. On heating in air or releasing the pressure in situ at 260°C, BaSn(OH) 6.3H 2O converts to BaSnO 3 through an intermediate, BaSnO(OH) 4. Solid solutions of Ba(Ti,Sn)O 3 are directly formed from (TiO 2 + SnO 2)..ξH 2O gel up to 35 mol% SnO 2. At higher Sn contents, the hydrothermal products are mixtures of BaSn(OH) 6.3H 2O and BaTiO 3, which on annealing at 1000°C result in monophasic Ba(Ti,Sn)O 3. The sintering characteristics and the dielectric properties of the ceramics prepared out of these fine powders are presented. The dielectric properties of fine-grained Ba(Ti,Sn)O 3 ceramics are explained on the basis of the prevailing diffuse phase transition behaviour. 相似文献
13.
NO removal using CH 4 as a reductant in a dual-bed system has been investigated with Co-NaX and Ag-NaX catalysts, which were prepared by Co 2+-, Ag +-ion exchange into zeolite NaX, respectively, and activation for 5 h at 500 °C. The experimental result has been compared with that of a Co-NaX-CO catalyst, additionally pre-treated under CO flow for the Co-NaX catalyst. The cobalt crystal structure of a Co-NaX-CO catalyst is Co 3O 4, which promotes NO oxidation to NO 2 by excess O 2 at a low temperature (523 K). The mechanical mixture of Co-NaX-CO and Ag-NaX catalysts shows a synergy effect on NO reduction to N 2 by CH 4 in the presence of excess O 2 and H 2O, but the NO reduction decreases quickly as time passes. However, the NO reduction to N 2 in a deNO bed at 523 K and a deNO 2 bed at 423 K, which are relatively lower than the reaction temperatures for common SCR systems, still remained at 67% even in a H 2O 10% gas mixture after 160 min. 相似文献
14.
The homogeneous nucleation rates for H 2SO 4---H 2O solution droplets at 25, 0 and −55°C are calculated and plotted as a function of H 2SO 4 concentration and H 2O concentration. The plotted contours are very useful for quick estimates of the nucleation rates of H 2SO 4---H 2O aerosols in smog chambers, polluted atmospheres and the stratosphere under different environmental conditions. The effect of the uncertainty in the saturated vapor pressure of H 2SO 4 on the nucleation rate is also discussed. 相似文献
15.
Mono- and multi-metallic (bi- and tri-) Pt, Pd and Rh supported on cerium-promoted alumina (La Roche, SAS-1/16) catalysts were tested for activity as TWC, both fresh [G.C. Koltsakis, and A.M. Stamatelos, Progr. Energy Combust. Sci. 23 (1997) 1] and after accelerated aging. Aging consisted of a treatment at 900°C for 5 h during which an oxidizing (2.5% O 2, 10% H 2O, in N 2) and a reducing (5.0% CO, 10% H 2O, in N 2) feedstream were cycled at 0.017 Hz through the catalyst. Activity tests were carried out by increasing temperature from 100 to 600°C at 3°C min −1, while two oxidizing and reducing (±0.5 A/ F) feedstreams were alternately (1 Hz) fed through the reactor at 125 000 h −1 (STP). Conversion was continuously analyzed. Light-off temperature, T50, conversion at 500°C (normal running temperature), X500, and the stoichiometric window ( A/ F from 14.13 to 15.13) for stationary feedstreams, were determined. 相似文献
16.
Activity and stability of an industrial Cr-free iron-based catalyst (NBC-1) for high-temperature water gas shift (WGS) reaction were studied in a fixed-bed reactor under 350 °C, 1 atm, H 2O:gas = 1:1 and 3000 h −1 (dry-gas basis). Physical properties of the NBC-1 catalyst before and after the WGS reaction, the desorption behavior of H 2O, CO, CO 2 and H 2, and surface reaction over the catalyst were characterized by BET, X-ray diffraction (XRD), Mössbauer emission spectroscopy (MES), temperature programmed desorption (TPD) and temperature programmed surface reaction (TPSR). The NBC-1 catalyst is active and has excellent thermo-stability even after pretreatment at a high temperature of 530 °C. Its activity and thermo-stability are comparable to those of an UCI commercial Fe-Cr catalyst, C12-4. XRD and MES studies show that iron in the fresh NBC-1 catalyst is present as γ-Fe 2O 3, most of which is converted to Fe 3O 4 during reduction and reaction. Results of TPD demonstrate that adsorbed CO 2 and CO cannot exist on the NBC-1 surface beyond the temperature of 300 °C while higher temperatures (>400 °C) are required to completely desorb H 2O. A redox mechanism of WGS on the NBC-1 surface is proposed based on the TPD and TPSR observations. 相似文献
17.
Sharp NO and O 2 desorption peaks, which were caused by the decomposition of nitro and nitrate species over Fe species, were observed in the range of 520–673 K in temperature-programmed desorption (TPD) from Fe-MFI after H 2 treatment at 773 K or high-temperature (HT) treatment at 1073 K followed by N 2O treatment. The amounts of O 2 and NO desorption were dependent on the pretreatment pressure of N 2O in the H 2 and N 2O treatment. The adsorbed species could be regenerated by the H 2 and N 2O treatment after TPD, and might be considered to be active oxygen species in selective catalytic reduction (SCR) of N 2O with CH 4. However, the reaction rate of CH 4 activation by the adsorbed species formed after the H 2 and N 2O or the HT and N 2O treatment was not so high as that of the CH 4 + N 2O reaction over the catalyst after O 2 treatment. The simultaneous presence of CH 4 and N 2O is essential for the high activity of the reaction, which suggests that nascent oxygen species formed by N 2O dissociation can activate CH 4 in the SCR of N 2O with CH 4. 相似文献
18.
This paper presents an investigation into the complex interactions between catalytic combustion and CH 4 steam reforming in a co-flow heat exchanger where the surface combustion drives the endothermic steam reforming on opposite sides of separating plates in alternating channel flows. To this end, a simplified transient model was established to assess the stability of a system combining H 2 or CH 4 combustion over a supported Pd catalyst and CH 4 steam reforming over a supported Rh catalyst. The model uses previously reported detailed surface chemistry mechanisms, and results compared favorably with experiments using a flat-plate reactor with simultaneous H 2 combustion over a γ-Al 2O 3-supported Pd catalyst and CH 4 steam reforming over a γ-Al 2O 3-supported Rh catalyst. Results indicate that stable reactor operation is achievable at relatively low inlet temperatures (400 °C) with H 2 combustion. Model results for a reactor with CH 4 combustion indicated that stable reactor operation with reforming fuel conversion to H 2 requires higher inlet temperatures. The results indicate that slow transient decay of conversion, on the order of minutes, can arise due to loss of combustion activity from high-temperature reduction of the Pd catalyst near the reactor entrance. However, model results also show that under preferred conditions, the endothermic reforming can be sustained with adequate conversion to maintain combustion catalyst temperatures within the range where activity is high. A parametric study of combustion inlet stoichiometry, temperature, and velocity reveals that higher combustion fuel/air ratios are preferred with lower inlet temperatures (≤500 °C) while lower fuel/air ratios are necessary at higher inlet temperatures (600 °C). 相似文献
19.
Development in highly active catalysts for the reforming of methane with H 2O, CO 2, and H 2O+CO 2, and partial oxidation of methane was conducted to produce hydrogen with high reaction rates. A Ni-based three-component catalyst such as Ni---La 2O 3---Ru or Ni---Ce 2O 3---Pt supported on alumina wash-coated ceramic fiber in a plate shape was very suitable for both reactions. The catalyst composition was set at 10 wt.-% Ni, 5.6 wt.-% La 20 3, and 0.57 wt.-% Ru for example, or molar ratios of these components were 1:0.2:0.03. Even with such a low concentration, the precious metal enhanced the reaction rate markedly, and this synergistic effect was ascribed to the hydrogen spillover effect through the part of precious metal and it resulted in a more reduced surface of the main catalyst component. In particular, a marked enhancement in the reaction rate of CO 2-reforming of methane was observed by the modification of a low concentration Rh to the Ni---Ce 20 3---Pt catalyst. Very high space-time yields of H 2 (i.e., 8300 mol/1 h in partial oxidation of methane at 600°C with a methane conversion of 37.5%, and 3585 mol/1 h in CO 2reforming of methane at 600°C with a methane conversion of 58%) were realized in those reactions. By combining the catalytic combustion reaction, methane conversion to syngas was markedly enhanced, and even with a very short contact time (10 ms) the conversion of methane increased more than that at 50 ms. The space-time yield of hydrogen amounted to 2,780 mol/1 h with a methane conversion of 90% at 700°C. Furthermore, in a reaction of CH 4---CO 2---H 2O---O 2 on the four components catalyst, an extraordinarily high space-time yield of hydrogen, 12 190 mol/1 h, could be realized under the conditions of very high space velocity (5 ms). 相似文献
20.
The wettability of nanocrystalline CVD diamond films grown in a microwave plasma using Ar/CH 4/H 2 mixtures with tin melt (250–850 °C) and water was studied by the sessile-drop method. The films showed the highest contact angles θ of 168 ± 3° for tin among all carbon materials. The surface hydrogenation and oxidation allow tailoring of the θ value for water from 106 ± 3° (comparable to polymers) to 5° in a much wider range compared to microcrystalline diamond films. Doping with nitrogen by adding N 2 in plasma strongly affects the wetting presumably due to an increase of sp 2-carbon fraction in the films and formation of C–N radicals. 相似文献
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