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1.
NF3 decomposition over transition metal oxides coated MgO reagents in the absence of water is investigated. The results show that NF3 can be decomposed completely over pure MgO but the time of NF3 steady full conversion kept as short as 80 min, while the reactivities of coated MgO reagents were remarkably enhanced by transition metal oxides, for example the time of NF3 complete conversion over 12%Fe/MgO extended to 380 min. It is suggested that not only an increase in surface area but also a significant enhancement in the fluorination of MgO substrate caused by the surface transition metal oxides result in an improved reactivity of coated MgO reagents for NF3 decomposition. 相似文献
2.
The preparation of synthesis gas from carbon dioxide reforming of methane (CDR) has attracted increasing attention. The present
review mainly focuses on CDR to produce synthesis gas over Ni/MO x/Al 2O 3 (X = La, Mg, Ca) catalysts. From the examination of various supported nickel catalysts, the promotional effects of La 2O 3, MgO, and CaO have been found. The addition of promoters to Al 2O 3-supported nickel catalysts enhances the catalytic activity as well as stability. The catalytic performance is strongly dependent
on the loading amount of promoters. For example, the highest CH 4 and CO 2 conversion were obtained when the ratios of metal M to Al were in the range of 0.04–0.06. In the case of Ni/La 2O 3/Al 2O 3 catalyst, the highest CH 4 conversion (96%) and CO 2 conversion (97%) was achieved with the catalyst (La/Al = 0.05 (atom/atom)). For Ni/CaO/Al 2O 3 catalyst, the catalyst with Ca/Al = 0.04 (atom/atom) exhibited the highest CH 4 conversion (91%) and CO 2 conversion (92%) among the catalysts with various CaO content. Also, Ni/MgO/Al 2O 3 catalyst with Mg/Al = 0.06 (atom/atom) showed the highest CH 4 conversion (89%) and CO 2 conversion (90%) among the catalysts with various Mg/Al ratios. Thus it is most likely that the optimal ratios of M to Al
for the highest activities of the catalysts are related to the highly dispersed metal species. In addition, the improved catalytic
performance of Al 2O 3-supported nickel catalysts promoted with metal oxides is due to the strong interaction between Ni and metal oxide, the stabilization
of metal oxide on Al 2O 3 and the basic property of metal oxide to prevent carbon formation. 相似文献
3.
Nanometer MgO samples with high surface area, small crystal size and mesoporous texture were synthesized by thermal decomposition
of MgC 2O 4 · 2H 2O prepared from solid-state chemical reaction between H 2C 2O 4 · 2H 2O and Mg (CH 3COO) 2 · 4H 2O. Steam produced during the decomposition process accelerated the sintering of MgO, and MgO with surface area as high as
412 m 2 · g −1 was obtained through calcining its precursor in flowing dry nitrogen at 520°C for 4 h. The samples were characterized by
X-ray diffraction, N 2 adsorption, transmission electron microscopy, thermogravimetry, and differential thermal analysis. The as-prepared MgO was
composed of nanocrystals with a size of about 4–5 nm and formed a wormhole-like porous structure. The MgO also had good thermal
stability, and its surface areas remained at 357 and 153 m 2·g −1 after calcination at 600 and 800°C for 2 h, respectively. Compared with the MgO sample prepared by the precipitation method,
MgO prepared by solid-state chemical reaction has uniform pore size distribution, surface area, and crystal size. The solid-state
chemical method has the advantages of low cost, low pollution, and high yield, therefore it appears to be a promising method
in the industrial manufacture of nanometer MgO.
Translated from Chinese Journal of Catalysis, 2006, 27(9): 793–798 (in Chinese) 相似文献
4.
A model has been developed to describe the chemistry which occurs in CF 4 plasmas and the etching of Si both in the plasma and downstream. One very important feature of this model is that for discharge residence times which vary by more than an order of magnitude, the amount of CF 4 consumed is low and relatively constant. This is because the gas-phase combination reactions between F and both CF 3 and CF 2 lead to the rapid reforming of CF 4. The model predicts that CF 2 is a major species in the gas phase and that the [F] detected as a sample point downstream is a very sensitive function of [CF 2]/[F] in the discharge. Even though the calculations show that [F] in the discharge varies only slightly over the wide range of experimental conditions considered, large variations in [F] at the sample point occur because the [CF 2]/[F] ratio in the discharge changes. The concentrations of C 2F 6 and SiF 4 are predicted to within a factor of 2 over a very wide range of experimental conditions. This confirms the importance of gas-phase free radical reactions in the etching of Si. 相似文献
5.
The rate constants of the hydrogen abstraction reactions of CF 3CHFCF 3 + H (R1) and CF 3CF 2CHF 2 + H (R2) have been calculated by means of the dual-level direct dynamics method. Optimized geometries and frequencies of stationary points and extra points along the minimum-energy path (MEP) are obtained at the MPW1K/6-311+G(d,p) level, and the classical energetic information is further corrected with the interpolated single-point energy (ISPE) approach by the G3(MP2) level of theory. Using the canonical variational transition state theory (CVT) with small-curvature tunneling corrections (SCT), the rate constants are evaluated over a wide temperature range of 200-2000 K. The calculated CVT/SCT rate constants are in good agreement with available experimental values. It is found that the variational effect is very small and almost negligible over the whole temperature region. However, the small-curvature tunneling correction plays an important role in the lower temperature range. Furthermore, the heats of formation of species CF 3CF 2CHF 2 (SC 1 or SC 2) and CF 3CF 2CF 2 are studied using isodesmic reactions to further elucidate the thermodynamic properties. 相似文献
6.
The reaction of methyl hypofluorite (MeOF) with certain fluoroolefins, such as CF 2CF 2, CF 2CFCF 3, CF 2CFOCF 3, CF 2CFOMe, CF 2CClF, CF 2CHF, CF 2CH 2, CHFCH 2, CF 2CFCFCF 2, occurred in CD 3CN or in the presence of NaF. Using neat MeOF in the presence of NaF was a novel method and gave good results. We observed that when more than three fluorine atoms are bonded to the CC double bond, the addition products were obtained in mostly good yields. 相似文献
7.
Reaction rate coefficients have been measured at 295 K for both CF 3 and CF 2 with atomic and molecular fluorine. The reaction between CF 3 and F was studied over a gas number density range of (2.4–23)×10 16 cm –3 with helium as the bath gas. The measured rate coefficient increased from (1.1–1.7)×10 –11 cm 3 s –1 as the gas number density increased over this range. In contrast to this relatively small change in rate coefficient with gas number density, the rate coefficient for CF 2+F increased from (0.4–2.3)×10 –12 cm 3 s –1 as the helium gas number density increased from (3.4–28.4)×10 16 cm –3. Even for the highest bath gas number density employed, the rate coefficient was still more than an order of magnitude lower than earlier measurements of this coefficient performed at comparable gas number densities.Both these association reactions are examined from the standpoint of the Gorin model for association of radicals and use is made of unimolecular dissociation theory to examine the expected dependence on gas number density. The calculations reveal that CF 3+F can be explained satisfactorily in these terms but CF 2+F is not well described by the simple Gorin model for association.CF 3 was found to react with molecular fluorine with a rate coefficient of (7±2)×10 –14 cm 3 s –1 whereas only an upper limit of 2×10 –15 cm 3 s –1 could be placed on the rate coefficient for the reaction between CF 2 and F 2. The values obtained for this set of reactions mean that the reaction between CF 3 and F will play an important role in plasmas containing CF 4. The high rate coefficient will mean that, under certain conditions, this particular reaction will control the amount of CF 4 consumed. On the other hand, the much lower rate coefficient for reactions between CF 2 and F means that CF 2 will attain much higher concentrations than CF 3 in plasmas where these combination reactions are dominant. 相似文献
8.
The reaction mechanism that takes place in ZrSiO 4-Mg Ca(CO 3) 2 mixtures was studied in air up to 1300°C by collecting neutron diffraction patterns during the heating ramp. Neutron diffraction intensities were used to monitor and establish the mechanism of reaction that occurs in successive stages. (a) MgCa(CO 3) 2 decomposition yielding MgO and CaCO 3; (b) CaCO 3 decomposition; (c) reactions between CaO, MgO, and ZrSiO 4 involving the formation of phases such as: tetragonal-ZrO 2, α-Ca 2SiO 4, and Ca 3MgSi 2O 8, some of them acting as transitory phases; and (d) formation of CaZrO 3. The results obtained by this technique agree with data obtained by differential thermal analysis and thermogravimetry. The final product has a porous structure, due to the release of CO 2, with a very narrow pore size distribution (≈1 μm). This open-porosity can be controlled by tailoring the reaction sintering process. 相似文献
9.
The solid–solid interactions in pure and MoO 3-doped
CuO/MgO system were investigated using TG, DTA and XRD. The composition of
pure mixed solids were 0.1CuO/MgO, 0.2CuO/MgO and 0.3CuO/MgO and the concentrations
of MoO 3 were 2.5 and 5 mol%. These solids were prepared
by wet impregnation of finely powdered basic magnesium carbonate with solutions
containing calculated amounts of copper nitrate and ammonium molybdate followed
by heating at 400–1000°C.
The results revealed that
ammonium molybdate doping of the system investigated enhanced the thermal
decomposition of copper nitrate and magnesium hydroxide which decomposed at
temperatures lower than those observed in case of the undoped mixed solids
by 70 and 100°C, respectively. A portion of CuO present dissolved in the
lattice of MgO forming CuO–MgO solid solution with subsequent limited
increase in its lattice parameter. The other portion interacted readily with
a portion of MoO 3 at temperatures starting from 400°C
yielding CuMoO 4 which remained stable up to 1000°C.
The other portion of MoO 3 interacted with MgO producing
MgMoO 4 at temperatures starting from 400°C and
remained also stable at 1000°C. The diffraction peaks of Cu 2MgO 3
phase were detected in the diffractograms of pure and MoO 3-doped
0.3CuO/MgO precalcined at 1000°C. The formation of this phase was accompanied
by an endothermic peak at 930°C. 相似文献
10.
This paper investigates the synthetic mechanism of trifluoroiodomethane (CF 3I) in the reaction of trifluoromethane and iodine via vapor-phase catalytic reaction. It is suggested that CF 2 carbene is the key intermediate and is formed in the pyrolysis process of CHF 3 at high temperature. However, in pyrolysis of CHF 3 under activated charcoal (AC) existing conditions, no C 2F 4 was detected. H 2 and 2-methyl-2-butene could not trap the CF 2 carbene. When treating the remained compounds on the used AC with H 2, CH 4 is formed on the process. It is proposed that CF 2 carbene combines with AC strongly and transfers into CF 3 radical on heat. In addition, it is found that the AC is not only the catalyst supporter to form CF 3I, but also a co-catalyst to promote the formation of CF 2 carbene and CF 3 radical. 相似文献
11.
A self-consistent, one-dimensional simulator for the physics and chemistry of radio frequency (rf) plasmas was developed and applied for CH 4 and CF 4. The simulator consists of a fluid model for the discharge physics, a commercial Boltzmann equation solver for calculations of electron energy distribution fuction (EEDF), a generalized plasma chemistry code, and an interface module among the three models. The CH 4 and CF 4 discharges are compared and contrasted: CH 4 plasmas are electropositive, with negative ion densities one order of magnitude less than those of electrons, whereas CF 4 plasmas are electronegative, with ten times more negative ions than electrons. The high-energy tail of tire EEDF in CH 4, lies below both the Druyvensteyn and Maxwell distributions, whereas tire EEDF high-energy tail in CF 4 lies between the two. For CH 4, the chemistry model was applied for four species, namely, CH 4 CH 3 CH 2, and H, whereas for CF 4, five species were examined namely CF 4, CF 3, CF 2, CF, and F The predicted densities and profiles compare favorably with experimental data. Finally, the chemistry results were fedback into the physics model until convergence was obtained. 相似文献
12.
A solid ternary mixture consisting of NaF,silicon and one metal oxide such as La2O3,CeO2,Pr6O11,Nd2O3,and Y2O3 was prepared and usedas de-fluorinated reagent for CF4 decomposition.The results show that 90% conversion of CF4 can be reached initially over NaF-Si-La2O3,NaF-Si-CeO2,NaF-Si-Nd2O3,and NaF-Si-Y2O3 at 850 C.The fresh and used reagents were characterized using XRD and XPS techniques.It was found that the active components of NaF and metal oxides in NaF-Si-CeO2,NaF-Si-Pr6O11,NaF-Si-Nd2O3,and NaF-Si-Y2O3 weretransformed into inert phases of mixed metal fluorides and silicates,respectively,resulting in an ineffective utilization of these de-fluorinatedreagents,whereas no inert phases from NaF and La2O3 can be observed in the used NaF-Si-La2O3,indicating the NaF-Si-La2O3 reagent couldbe utilized more efficiently than the other reagents in CF4 decomposition. 相似文献
13.
Fluorocarbon (CF +
x), fluorine (F +), and carbon (C +) ion beams with highcurrent density (50 i<800 A/cm2) were irradiated to Si and SiO2surfaces to investigate the influence of the ion species on the etchingefficiency. The ion beams were extracted from magnetized helicon-wave CF4plasmas operated in pulsed modes. The CF+
3 beam had the largest etchingefficiency for Si at the same beam energy. When the same data weresummarized as a function of the momentum of the incident ion beam, thedifference in the etching efficiency became small, although the CF+
3 beamstill had a slightly larger etching efficiency. On the other hand, theetching efficiency for SiO2 by the CF+
3 beam was larger than that by theother ion beams in the low-momentum region. In addition, in the low-momentumregion, the etching efficiency for SiO2 by CF+
3 was larger than that forSi. These results suggest the high chemical reactivity of CF+
3 with SiO2,leading to the high etching selectivity of SiO2 over underlying Si in thefabrication of semiconductor devices. 相似文献
14.
The G3 and CBS-QB3 theoretical methods are employed to study the decomposition of CF 3OH into FCFO and HF by water, water dimmer, and ammonia. The decomposition of CF 3OH into FCFO and HF is unlikely to occur in the atmosphere due to the high activated energy of 88.7 kJ/mol at the G3 level of theory. However, the computed results predict that the barrier for unimolecular decomposition of CF 3OH is decreased to 25.1 kJ/mol from 188.7 kJ/mol with the aid of NH 3 at the G3 level of theory, which shows that the ammonia play a strong catalytic effect on the split of CF 3OH. In addition, the calculated rate constants show that the decomposition of CF 3OH by NH 3 is faster than those of H 2O and the water dimmer by 109 and 105 times respectively. The rate constants combined with the corresponding concentrations of these species demonstrate that the reaction CF 3OH with NH 3 via TS4 is of great importance for the decomposition of CF 3OH in the atmosphere. 相似文献
15.
The thermal decomposition mechanisms and the intermediate morphology of MgCl 2·6H 2O and MgCl 2·H 2O were studied using integrated thermal analysis, X-ray diffraction, scanning electron microscope and chemical analysis. The results showed that there were six steps in the thermal decomposition of MgCl 2·6H 2O: producing MgCl 2·4H 2O at 69 °C, MgCl 2·2H 2O at 129 °C, MgCl 2· nH 2O (1 ≤ n ≤ 2) and MgOHCl at 167 °C, the conversion of MgCl 2· nH 2O (1 ≤ n ≤ 2) to Mg(OH)Cl·0.3H 2O by simultaneous dehydration and hydrolysis at 203 °C, the dehydration of Mg(OH)Cl·0.3H 2O to MgOHCl at 235 °C, and finally the direct conversion of MgOHCl to the cylindrical particles of MgO at 415 °C. To restrain the sample hydrolysis and to obtain MgCl 2·H 2O, MgCl 2·6H 2O was first calcined in HCl atmosphere until 203 °C when MgCl 2·H 2O was obtained; HCl gas was then turned off and the calcination process continued, producing Mg 3Cl 2(OH) 4·2H 2O calcined at 203 °C, Mg 3(OH) 4Cl 2 at 220 °C and MgO at 360 °C. The temperature of producing MgO from calcination of MgCl 2·H 2O was lower (360 °C) than that from MgCl 2·6H 2O (415 °C) because of its more reactive intermediate products: the irregular shape and tiny needle-like Mg 3Cl 2(OH) 4·2H 2O particles and the uneven surface porous Mg 3(OH) 4Cl 2 particles. The MgO particles obtained at 360 °C had a flake structure. 相似文献
16.
Conversion of CF4 was experimentally evaluated with three systems including catalytic hydrolysis, NTP-alone and plasma catalysis. Firstly, activity of γ-Al2O3 for CF4 conversion was tested via catalytic hydrolysis. Experimental results indicate that the highest CF4 conversion (72%) could be achieved with γ-Al2O3 in the presence of 45% H2O(g) with the operating temperature of 900 °C and the apparent activation energy is calculated as 85 kJ/mol using power rate law model. For plasma-based systems, results indicate that CF4 conversions achieved with plasma catalysis remain 100% with the applied voltage ranging from 12 to 23 kV, while the highest CF4 conversion achieved with NTP-alone is 86%. For the effects of various parameters on plasma-based systems, the results indicate that plasma catalysis also has better resistivity for higher total flow rate, CF4 concentration and O2 contents. Especially, CF4 conversion could maintain at 100% with the operating applied of 23 kV as CF4 concentration is increased to 10,000 ppm. On the other hand, CF4 conversion achieved with NTP-alone is 18% at the same conditions. In addition, negative effect of O2 on plasma catalysis could be reduced by the addition of Ar due to its good discharge properties and high excitation threshold energy (13 eV). The kinetics of plasma catalysis is investigated for CF4 conversion by a simplified model, and the results indicate that overall energy constant achieved with plasma catalysis reaches 0.015 mg J?1. Overall, plasma catalysis is demonstrated with good potential for the reduction of CF4 emission. 相似文献
17.
The solid-solid interactions between manganese and magnesium oxides in absence and in presence of small amounts of Li 2O have been investigated. The molar ratios between manganese and magnesium oxides in the form of Mn 2O 3 and MgO were varied between 0.05:1 to 0.5:1. The mixed solids were calcined in air at 400-1000°C. The techniques employed were DTA, XRD and H 2O 2 decomposition at 20-40°C.The results obtained revealed that solid-solid interactions took place between the reacting solids at 600-1000°C yielding magnesium manganates (Mg 2MnO 4, Mg 6MnO 8, MgMnO 4 besides unreacted portions of MgO, Mn 2O 3 and Mn 3O 4). Li 2O-doping (0.75-6 mol%) of the investigated system followed by calcination at 600 and 800°C decreased progressively the intensity of the diffraction lines of Mn 2O 3 (Bixbyite) with subsequent increase in the lattice parameter ' a' of MgO to an extent proportional to the amount of Li 2O added. This finding might suggest that the doping process enhanced the dissolution of Mn 2O 3 in MgO forming solid solution. This treatment led also to the formation of Li 2MnO 3. Furthermore, the doping with 3 and 6 mol% Li 2O conducted at 800°C resulted in the conversion of Mn 2O 3 into Mn 3O 4, a process that took place at 1000°C in absence of Li 2O. The produced Li 2MnO 3 phase remained stable by heating at up to 1000°C. Furthermore, Li 2O doping of the investigated system at 400-1000°C resulted in a progressive measurable increase in the particle size of MgO.The catalytic activity measurements showed that the increase in the molar ratio of Mn 2O 3 in the samples precalcined at 400-800°C was accompanied by a significant increase in the catalytic activity of the treated solids. The maximum increase in the catalytic activity expressed as reaction rate constant measured at 20°C ( k
20°C) attained 3.14, 2.67 and 3.25-fold for the solids precalcined at 400, 600 and 800°C, respectively. Li 2O-doping of the samples having the formula 0.1 Mn 2O 3/MgO conducted at 400-600°C brought a progressive significant increase in its catalytic activity. The maximum increase in the value of k
20°C due to Li 2O attained 1.93 and 2.75-fold for the samples preheated at 400 and 600°C, respectively and opposite effect was found for the doped samples preheated at 800°C.This revised version was published online in November 2005 with corrections to the Cover Date. 相似文献
18.
Natural gas resources, stimulate the method of catalytic methane decomposition. Hydrogen is a superb energy carrier and integral component of the present energy systems, while carbon nanotubes exhibit remarkable chemical and physical properties. The reaction was run at 700 °C in a fixed bed reactor. Catalyst calcination and reduction were done at 500 °C. MgO, TiO 2 and Al 2O 3 supported catalysts were prepared using a co‐precipitation method. Catalysts of different iron loadings were characterized with BET, TGA, XRD, H 2‐TPR and TEM. The catalyst characterization revealed the formation of multi‐walled nanotubes. Alternatively, time on stream tests of supported catalyst at 700 °C revealed the relative profiles of methane conversions increased as the %Fe loading was increased. Higher %Fe loadings decreased surface area of the catalyst. Iron catalyst supported with Al 2O 3 exhibited somewhat higher catalytic activity compared with MgO and TiO 2 supported catalysts when above 35% Fe loading was used. CH 4 conversion of 69% was obtained utilizing 60% Fe/Al 2O 3 catalyst. Alternatively, Fe/MgO catalysts gave the highest initial conversions when iron loading below 30% was employed. Indeed, catalysts with 15% Fe/MgO gave 63% conversion and good stability for 1 h time on stream. Inappropriateness of Fe/TiO 2 catalysts in the catalytic methane decomposition was observed. 相似文献
19.
We have analyzed decay kinetics of CF 2 radicals in the afterglow of low-pressure, high-density C 4F 8 plasmas. The decay curve of CF 2 density has been approximated by the combination of first- and second-order kinetics. The surface loss probability evaluated from the frequency of the first-order decay process has been on the order of 10 –4. This small surface loss probability has enabled us to observe the second-order decay process. The mechanism of the second-order decay is self-association reaction between CF 2 radicals (CF 2+CF 2C 2F 4). The rate coefficient for this reaction has been evaluated as (2.6–5.3)×10 –14 cm 3/s under gas pressures of 2 to 100 mTorr. The rate coefficient was found to be almost independent of the gas pressure and has been in close agreement with known values, which are determined in high gas pressures above 1 Torr. 相似文献
20.
A novel method for the preparation of o-, m-, p-SF 5CF 2CFYC 6H 4X (Y = Br, F and X = m-Br, p-Br, Cl, CH 3, CF 3, NO 2, o-NO 2, F, CF 3, CH(CH 3) 2) derivatives was devised by a two-step reaction: SF 5Br-addition to o-, m-, p-CF 2CFC 6H 4X followed by reaction of AgBF 4 with the o-, m-, p-SF 5CF 2CFBrC 6H 4X adducts. Additional studies have been carried out with several derivatives and includes the preparation of SF 5CF 2C(O)C 6H 5, p-CF 3CFBrC 6H 4NO 2, SF 5CF 2CF 2C 6H 3(NO 2) 2, SF 5CF 2CF 2C 6H 3(NH 2) 2, and an SF 5CF 2CF 2-containing polyimide and dye. The complete characterization (IR, NMR, and MS) of these compounds is given. 相似文献
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