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1.
In CO 2 reforming of methane over a calcium hydroxyapatite-supported nickel catalyst, the carbon deposition occurred more severely
with increase of the methane partial pressure and at temperatures below about 1,000 K. The effects of ceria that was added
as a promoter to the nickel catalyst were investigated. It was observed that the ceria not only enhanced the catalyst stability
but also increased the activity, and this is considered owing to the oxygen storage capacity of ceria. The TGA analysis demonstrated
that the ceria promoted the removal of the deposited carbon. The optimum Ce/Ni mole ratio was ca. 0.3/2.5. The deposited carbon
could easily be removed by oxygen treatment at 1,023 K and the catalytic activity could be restored. 相似文献
2.
CO 2 reforming of methane was investigated with regard to carbon deposition on 4.5 wt% NiO/SiO 2 catalyst at 1023 K, 1 atm and a CH 4/CO 2 ratio of 1.0 employing micro-fluidised- and fixed-bed reactors. A higher catalytic activity (by 20%) was observed in the initial stage (0.5 h) of the fluidised-bed reforming which may be attributed to lesser deactivation of the catalyst compared to fixed-bed operation. Only a limited amount of carbon was deposited in a period of 11 h on stream. In the case of the fixed-bed reactor, a much larger amount of carbon was found on the spent catalyst, particularly, when sampled from the bottom of the bed. TPO results suggest that carbon deposits on the catalyst samples from the fluidised-bed as well as the top of the fixed-bed are rather small and of similar nature. The carbon deposited at the bottom of the fixed-bed reactor contained two distinct species according to XPS results (corresponding to C–O and C–C bonds). 相似文献
3.
Temperature-programmed reaction techniques and Raman spectroscopy were used to characterize coke species deposited on a 5% Ni/MgAl 2O 4 catalyst for dry reforming of methane. The CH 4 temperature-programmed decomposition profiles showed that the ignited decomposition temperatures of CH 4 increased from 273 to 378 °C when MgAl 2O 4 replaced the catalyst support γ-Al 2O 3. The temperature-programmed oxidation, temperature-programmed hydrogenation and temperature-programmed CO 2 reaction profiles showed that there were three carbon species (i.e. C α, C β and C γ) on the catalyst surface. Raman spectroscopy showed that C γ was graphite-like carbon species, which was responsible for catalyst deactivation. The C γ species was the most inactive species toward H 2 and O 2, while it was unexpectedly more active toward CO 2. The unique reactivity of CO 2 with different coke specie could be ascribed to the carbonate, bidentate and formate species formation on MgAl 2O 4 surface. These surface species enhanced the oxidation of C γ species and thus contributed to the high stability of Ni/MgAl 2O 4 catalyst. 相似文献
4.
The CO 2 reforming of methane over reduced NiO/MgO solid solution catalysts was studied at 800°C by a novel transient method, which
couples a broadened pulse of CH 4/CO 2 with a step change to the carrier gas and/or with a sharp isotopic pulse of either 18O 4, CO 18
2 or 13CO 16
2. The response curves indicated that two kinds of oxygen were formed over the catalysts during reaction: adsorbed oxygen which
reacts fast with C species and lattice oxygen which reacts more slowly with C species. One concludes that a redox cycle of
lattice oxygen formation through the oxidation of Ni and its reaction with C species takes place on the catalyst surface.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
5.
Hydrogen production from ethanol reforming was investigated on bimetallic PtNi catalysts supported on CeO 2/Al 2O 3. Pt content was varied from 0.5 to 2.5 %. Physico-chemical characterization of the as-prepared and H 2-reduced catalysts by TPR, XRD and XPS showed that Pt phase interacted with the Ni and Ce species present at the surface of the catalysts. This interaction leads to an enhancement of the reducibility of both Ni and Ce species. Loadings of Pt higher than 1.0 wt% improved the activity and stability of the Ni/CeO 2–Al 2O 3 catalyst in ethanol steam reforming, in terms of lower formation of coke, C 2 secondary products and a constant production of CO 2 and H 2. The amount and type of carbon deposited on the catalyst was analyzed by TG–TPO while the changes in crystalline phases after reaction were studied by XRD. It was found that for Pt contents higher than 1 wt% in the catalysts, a better contact between Pt and Ce species is achieved. This Pt–Ce interaction facilitates the dispersion of small particles of Pt and thereby improves the reducibility of both Ce and Ni components at low temperatures. In this type of catalysts, the cooperative effect between Pt 0, Ni 0 and reduced Ce phases leads to an improvement in the stability of the catalysts: Ni provides activity in C–C bond breakage, Pt particles enhance the hydrogenation of coke precursors (C xH y) formed in the reaction, and Ce increases the availability of oxygen at the surface and thereby further enhances the gasification of carbon precursors. 相似文献
6.
Ni/Al 2O 3 promoted catalysts with alkaline earth metal oxides (MgO, CaO, and BaO) were prepared and employed in dry reforming of methane (DRM). The catalysts were prepared by impregnation method and characterized by XRD, BET, TPR, TPO, and SEM techniques. The obtained results showed that the addition of MgO, CaO, and BaO as promoter decreased the surface area of catalysts (S BET). The catalysis results exhibited that adding alkaline earth promoters (MgO, CaO, and BaO) enhanced the catalytic activity and the highest activity was observed for the MgO promoted catalyst. TPR analysis showed that addition of MgO increased the reducibility of nickel catalyst and decreased the reduction temperature of NiO species. The TPO analysis revealed that addition of promoters decreased the amount of deposited coke; and among the studied promoters, MgO has the most promotional effect for suppressing the carbon formation. SEM analysis confirmed the formation of whisker type carbon over the spent catalysts. 相似文献
7.
Nanostructured γ-Al 2O 3 with high surface area and mesoporous structure was synthesized by sol-gel method and employed as catalyst support for nickel catalysts in methane reforming with carbon dioxide. The prepared samples were characterized by XRD, BET, TPR, TPH, SEM and TPO techniques. The BET analysis showed a high surface area of 204m 2g ?1 and a narrow pore-size distribution centered at a diameter of 5.5 nm for catalyst support. The results revealed that an increase in nickel loading from 5 to 15 wt% decreased the surface area of catalyst from 182 to 160 m 2g ?1. In addition, the catalytic results showed an increase in methane conversion with increase in nickel content. TPO analysis revealed that the coke deposition increased with increasing in nickel loading, and the catalyst with 15 wt% of nickel showed the highest degree of carbon formation. SEM and TPH analyses confirmed the formation of whisker type carbon over the spent catalysts. Increasing CO 2/CH 4 ratio increased the methane conversion. The BET analysis of spent catalysts indicated that the mesoporous structure of catalysts still remained after reaction. 相似文献
8.
The reforming of methane with carbon dioxide over rhodium dispersed on silica, Rh/SiO 2, and vanadia-promoted silica, Rh/VO x/SiO 2, was studied by kinetic test reactions under differential conditions in a temperature range from 723 to 773 K. Transmission
infrared spectroscopy was applied to observe the interaction of CO 2 with the catalysts and the formation of surface intermediates during the CO 2–CH 4 reforming reaction. To analyze carbon deposition XP spectroscopy and TPO was carried out. It has been shown that the promotion
of Rh/SiO 2 catalysts with vanadium oxide enhances the catalytic activity for CO 2 reforming of methane and decreases the deactivation by carbon deposition. This is attributed to the formation of a partial
VO x overlayer on the Rh surface, which reduces the size of accessible ensembles of Rh atoms required for coke formation and creates
new sites at the Rh–VO x interfacial region that are considered to be active sites for the activation/dissociation of carbon dioxide.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
9.
Ni/Al 2O 3 aerogel catalysts were synthesized by a sol-gel method combined with a supercritical drying route. The catalytic performances of the catalysts in methane reforming with CO 2 were investigated in a quartz micro-reactor. The results indicated that the aerogel catalyst showed higher specific surface area and higher dispersivity of nickel species than those of impregnation catalyst. The excellent catalytic performances and stabilities were achieved over the aerogel catalysts in the fluidized bed reactor. Comprehensive characterization with TG, XRD and FESEM revealed that the aerogel catalyst in the fluidized bed had much lower carbon deposition than that in the fixed bed. The fluidization of the aerogel catalyst greatly improved the contact efficiency of gas-solid phase, which accelerated the gasification of the deposited carbon. In contrast, the deactivation of the aerogel catalyst was caused by the carbon deposition due to the catalyst without moving in the fixed bed. Moreover, decreasing activity of the impregnation catalyst in the fluidized bed resulted from the poor fluidization state of catalyst particles and low effective active sites on surface of catalyst. 相似文献
10.
The nature of surface sites responsible for methane activation and CO x formation on Rh catalysts for the partial oxidation of methane to syngas was investigated. The interaction of H 4 with Rh-black after oxidative and reductive pretreatments was studied applying (a) pulse experiments at reduced total pressure (10 –4 Pa) and 1013 K in the temporal-analysis-of-product (TAP) reactor and (b) in situ DRIFTS at 973 K. The saturation of the metal surface sites with oxygen was found to inhibit methane dissociation. Direct methane oxidation to CO 2 on the oxidized surface sites proposed earlier was excluded. Methane is first dissociated on reduced surface sites; the carbon species formed, then, react with surface oxygen to CO 2. Rh sites responsible for methane activation are neither related to the formation of the Rh 2O 3 nor Rh 0. Probably the partially oxidized species (Rh +) or highly dispersed Rh 3+ entities act as active surface centers for the dissociation of methane. For supported catalyst, such sites are stabilized by the support, which on the other side acts as a source of active oxygen involved in the oxidation of surface carbon and hydrogen. 相似文献
11.
Argon glow discharge plasma was applied for drying the impregnated Ni/SiO 2 catalyst instead of the drying thermally. Such plasma treated catalyst significantly inhibits the coke formation from methane decomposition. The methane-derived carbon shows an improved reactivity against CO 2. This can result in a better balance of coke formation and gasification by CO 2 when the plasma-treated Ni/SiO 2 catalyst is used for the CO 2 reforming of methane. 相似文献
12.
Ni/Ce–ZrO 2 showed good methane steam reforming performance in term of stability toward the deactivation by carbon deposition. It was first observed that the catalyst with Ce/Zr ratio of 3/1 showed the best activity among Ni/Ce–ZrO 2 samples with the Ce/Zr ratios of 1/0, 1/1, 1/3, and 3/1. Temperature-programmed oxidation (TPO) experiments indicated the excellent resistance toward carbon formation for this catalyst, compared to conventional Ni/Al 2O 3; the requirement of inlet H 2O/CH 4 to operate without the formation of carbon species is much lower. These benefits are related to the high oxygen storage capacity (OSC) of Ce–ZrO 2. During the steam reforming process, in addition to the reactions on Ni surface (*), the redox reactions between the gaseous components present in the system and the lattice oxygen (O x) on Ce–ZrO 2 surface also take place. Among these reactions, the redox reactions between the high carbon formation potential compounds (CH 4, CH x-* n and CO) and the lattice oxygen (O x) can prevent the formation of carbon species from the methane decomposition and Boudard reactions, even at low inlet H 2O/CH 4 ratio (1.0/1.0). Regarding the intrinsic kinetic studies in the present work, the reaction order in methane over Ni/Ce–ZrO2 was observed to be approximately 1.0 in all conditions. The dependence of steam on the rate was non-monotonic, whereas addition of oxygen as an autothermal reforming promoted the rate but reduced CO and H2 production selectivities. The addition of a small amount of hydrogen increased the conversion of methane, however, this positive effect became less pronounced and the methane conversion was eventually inhibited when high hydrogen concentration was added. Ni/Ce–ZrO2 showed significantly stronger negative impact of hydrogen than Ni/Al2O3. The redox mechanism on ceria proposed by Otsuka et al. [K. Otsuka, T. Ushiyama, I. Yamanaka, Chem. Lett. (1993) 1517; K. Otsuka, M. Hatano, A. Morikawa, J. Catal. 79 (1983) 493; K. Otsuka, M. Hatano, A. Morikawa, Inorg. Chim. Acta 109 (1985) 193] can explain this high inhibition. 相似文献
13.
The catalytic behavior of bi-metallic Co–Ni/TiO 2 catalysts for CO 2 reforming of CH 4 to synthesis gas was investigated under atmospheric pressure with a particular attention to carbon deposition. The catalysts with optimized Co/Ni ratios showed high catalytic stability towards the reaction with very little amount of deposited carbon at a wide range of reaction temperature (773–1123 K). The results suggest that adjusting of composition of the active metals (Co and Ni) can kinetically control the elementary steps (formation of carbon species and its removal by oxygen species) of CH 4/CO 2 reaction. 相似文献
14.
The catalytic properties of Ni/Al 2O 3 composites supported on ceramic cordierite honeycomb monoliths in oxidative methane reforming are reported. The prereduced
catalyst has been tested in a flow reactor using reaction mixtures of the following compositions: in methane oxidation, 2–6%
CH 4, 2–9% O 2, Ar; in carbon dioxide and oxidative carbon dioxide reforming of methane, 2–6% CH 4, 6–12% CO 2, and 0–4% O 2, and Ar. Physicochemical studies include the monitoring of the formation and oxidation of carbon, the strength of the Ni-O
bond, and the phase composition of the catalyst. The structured Ni-Al 2O 3 catalysts are much more productive in the carbon dioxide reforming of methane than conventional granular catalysts. The catalysts
performance is made more stable by regulating the acid-base properties of their surface via the introduction of alkali metal
(Na, K) oxides to retard the coking of the surface. Rare-earth metal oxides with a low redox potential (La 2O 3, CeO 2) enhance the activity and stability of Ni-Al 2O 3/cordierite catalysts in the deep and partial oxidation and carbon dioxide reforming of methane. The carbon dioxide reforming
of methane on the (NiO + La 2O 3 + Al 2O 3)/cordierite catalyst can be intensified by adding oxygen to the gas feed. This reduces the temperature necessary to reach
a high methane conversion and does not exert any significant effect on the selectivity with respect to H 2. 相似文献
15.
Ni catalysts promoted with 0.5 and 1.0 wt% boron were synthesized, characterized and tested during steam methane reforming, to evaluate the effect of boron on the deactivation behavior. Boron adsorbs on the γ-Al 2O 3 support and on the Ni particles and 1.0 wt% boron is found to enhance the stability without compromising the activity. Catalytic studies at 800 °C, 1 atm, a stoichiometric methane to steam ratio, and space velocities of 330,000 cm 3/(h g cat) show that promotion with 1.0 wt% boron reduces the rate of deactivation by a factor of 3 and increases the initial methane conversion from 56% for the unpromoted catalyst to 61%. Temperature-programmed oxidation (TPO) and scanning electron microscopy (SEM) studies confirm the formation of carbonaceous deposits and illustrate that 1.0 wt% boron reduces the amount of deposited carbon by 80%. 相似文献
16.
Mesoporous Ni-CaO-ZrO 2 nanocomposites with high thermal stability were designed and employed in the CO 2/CH 4 reforming. The nanocomposites with appropriate Ni/Ca/Zr molar ratios exhibited excellent activity and prominent coking resistivity. The Ni crystallites were effectively controlled under the critical size for coke formation in such nanocomposites. It was found that low Ni content resulted in high metal dispersion and good catalytic performance. Moreover, the basicity of the matrices improved the chemisorption of CO 2 and promoted the gasification of deposited coke on the catalyst. 相似文献
17.
The reaction of methane with surface oxygen as well as the interaction of methane/oxygen mixtures with a Rh(1 wt%)/-Al 2O 3 catalyst was studied by applying the temporal-analysisof-product (TAP) reactor. The product distribution was strongly affected by the degree of surface reduction. CO 2 is formed as a primary product via a redox mechanism with the participation of surface oxygen. The dehydrogenation of methane yielding carbon deposits on the surface occurs on reduced surface sites. The formation of CO proceeds with high selectivity (up to 96%)at 1013 K via fast reaction of surface carbon species with CO 2. 相似文献
18.
Mesoporous nanocrystalline MgSiO 3 with high surface area was synthesized by a hydrothermal method and employed as support in dry and steam reforming of methane. Ni/MgSiO 3 catalysts were prepared by an impregnation method and characterized by different techniques. N 2 adsorption analysis indicated that addition of nickel shifted the pore size distributions to smaller sizes. Temperature‐programmed reduction analysis revealed that a higher nickel loading enhanced the reducibility of the catalyst. The catalytic performance was improved with increasing the nickel content. The Ni/MgSiO 3 catalyst exhibited high stability in dry reforming but methane conversion declined with time‐on‐stream in the steam reforming reaction. Temperature‐programmed oxidation profiles of spent catalysts indicated that the high amount of carbon deposited on the catalyst surface in dry and steam reforming was assigned to whisker‐type carbon. 相似文献
19.
Ethanol reforming and partial oxidation were studied on Cu/Nb 2O 5 and Ni/Al 2O 3 catalysts. Compared to the Ni/Al 2O 3 catalyst, the Cu/Nb 2O 5 catalyst presents conversion as high as Ni/Al 2O 3 catalyst, however, for the same level of formation of hydrogen it occurs at much lower temperature on the Cu/Nb 2O 5 catalyst, 200 °C lower than for the Ni/Al 2O 3 catalyst, with remarkable little formation of CO, which can be attributed to the strong interaction between copper and niobia. Temperature-programmed desorption (TPD-ethanol) and surface reactions (TPSR) of partial oxidation of ethanol showed formation of ethylene, acetaldehyde, ethane and mainly H 2 and CO 2 besides little methane. DRIFTS results are in accordance with TPD analysis and the formation of acetate species at room temperature suggests reactivity of the surface and its oxidative dehydrogenation capacity. The adsorption of ethanol gives rise to ethoxide species, which form acetate and acetaldehyde that can be oxidized to CO 2 via carbonate. A comparison with reported results for Cu/Al 2O 3 this catalyst is promising, yielding high level of H 2 with little CO production during reforming and partial oxidation reaction. The maximum H 2 formation for the partial oxidation of ethanol was 41% at ratio (O 2/Et) 0.8, increasing to 50% at ratio 1.5. The H 2/CO is around 10 for the partial oxidation and 7 for steam reforming, which is excellent, compared to the Ni/Al 2O 3 catalyst with a factor 4–8 lower. 相似文献
20.
The kinetic behavior of the Ni/La 2O 3 catalyst in the reforming reaction of methane with carbon dioxide was investigated as a function of temperature and partial pressures of CH 4 and CO 2. The apparent activation energy of the reforming reaction was estimated to be 13.2 kcal/mol. It was also found that increase of the H 2 partial pressure leads to a continuous enhancement of the rate of CO formation, due to the simultaneous occurrence of the water-gas shift reaction. The mechanism of the CH 4/CO 2 reaction has been investigated using steady-state isotopic tracing and transient experiments, as well as FTIR, XRD, XPS and HR-TEM techniques. Based on the mechanistic results, a kinetic model was developed, which was found to predict satisfactorily the kinetic measurements. Methane cracking and the surface reaction between C and oxycarbonate species, are suggested to be the rate determining steps of the CH 4/CO 2 reaction over the Ni/La 2O 3 catalyst. 相似文献
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