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1.
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/Al 2O 3 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 (H 2/CO = 1–3). The results include data on the activity and selectivity of CoRe/Al 2O 3 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. 相似文献
2.
A novel regenerable Fe/activated coke (AC) desulfurizer prepared by impregnation of Fe(NO 3) 3 on an activated coke was investigated. Experiment results showed that at 200 °C the SO 2 adsorption capacity of the Fe/AC was higher than that of AC or Fe 2O 3. Temperature-programmed desorption (TPD) revealed that H 2SO 4 and Fe 2(SO 4) 3 were generated on the desulfurizer upon adsorption of SO 2. Effect of desulfurization temperature was also investigated which revealed that with increasing temperature from 150 to 250 °C, the SO 2 removal ability gradually increases. The used Fe/AC can be regenerated by NH 3 at 350 °C to directly form solid ammonium-sulfate salts. 相似文献
3.
A detailed temperature-programmed desorption (TPD) study on NO and O 2 saturated Cu-ZSM-5 at different temperatures (300–723 K) has been performed. In the temperature range 373–723 K, the evolution of O 2 and NO 2 accompanying the desorption of NO from NO saturated Cu-ZSM-5 suggested the formation of nitrite/nitrate species. The amount of O 2 absorbed was very much lower than that of NO. The desorption profile of O 2 after contacting Cu-ZSM-5 with O 2 at 623 K showed a low temperature peak (369K) confirming the spontaneous ability of O 2 desorption from copper zeolite. Moreover, successive saturation cycles of NO followed by O 2 and vice versa have been performed at various temperatures (298–623 K) to understand the modifications which the adsorption sites undergo when the two molecules NO and O 2 are available together for adsorption on the catalyst sites. After each saturation cycle, a TPD profile was recorded following the evolution of NO, O 2 and other NO x species. The competitive adsorption experiments revealed that, at 623 K, NO was not able to successfully compete with O 2 for the adsorption sites, therefore the adsorption of NO at 623 K on O 2 saturated catalyst was not completely restored. On the basis of the experimental work, an overall adsorption reaction scheme of NO on Cu-ZSM-5 was proposed 相似文献
4.
The role of La 2O 3 loading in Pd/Al 2O 3-La 2O 3 prepared by sol–gel on the catalytic properties in the NO reduction with H 2 was studied. The catalysts were characterized by N 2 physisorption, temperature-programmed reduction, differential thermal analysis, temperature-programmed oxidation and temperature-programmed desorption of NO. The physicochemical properties of Pd catalysts as well as the catalytic activity and selectivity are modified by La2O3 inclusion. The selectivity depends on the NO/H2 molar ratio (GHSV = 72,000 h−1) and the extent of interaction between Pd and La2O3. At NO/H2 = 0.5, the catalysts show high N2 selectivity (60–75%) at temperatures lower than 250 °C. For NO/H2 = 1, the N2 selectivity is almost 100% mainly for high temperatures, and even in the presence of 10% H2O vapor. The high N2 selectivity indicates a high capability of the catalysts to dissociate NO upon adsorption. This property is attributed to the creation of new adsorption sites through the formation of a surface PdOx phase interacting with La2O3. The formation of this phase is favored by the spreading of PdO promoted by La2O3. DTA shows that the phase transformation takes place at temperatures of 280–350 °C, while TPO indicates that this phase transformation is related to the oxidation process of PdO: in the case of Pd/Al2O3 the O2 uptake is consistent with the oxidation of PdO to PdO2, and when La2O3 is present the O2 uptake exceeds that amount (1.5 times). La2O3 in Pd catalysts promotes also the oxidation of Pd and dissociative adsorption of NO mainly at low temperatures (<250 °C) favoring the formation of N2. 相似文献
5.
We report results on the adsorption and desorption of H 2S on polycrystalline UO 2 at 100 and 300 K, using ultrahigh vacuum X-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), and temperature programmed desorption (TPD). Our work is motivated by the potential for using the large stockpiles of depleted uranium in industrial applications, e.g., in catalytic processes, such as hydrodesulfurization (HDS) of petroleum. H 2S is found to adsorb molecularly at 100 K on the polycrystalline surface, and desorption of molecular H 2S occurs at a peak temperature of 140 K in TPD. Adsorption rates of sulfur as a function of H 2S exposure are measured using XPS at 100 K; the S 2p intensity and lineshapes demonstrate that the saturation coverage of S-containing species is 1 monolayer (ML) at 100 K, and is 0.3–0.4 ML of dissociation fragments at 300 K. LEIS measurements of adsorption rates agree with XPS measurements. Atomic S is found to be stable to >500 K on the oxide surface, and desorbs at 580 K. Evidence for a recombination reaction of dissociative S species is also observed. We suggest that O-vacancies, defects, and surface termination atoms in the oxide surface are of importance in the adsorption and decomposition of S-containing molecules. 相似文献
6.
In the preparation of 1% Au/TiO 2 catalysts supported on either Degussa P-25 or anatase (90 m 2 g −1) by deposition–precipitation, the gold content passes through a maximum at about the isoelectric point (pH 6), but maximum specific rates occur at pH 8–9 because the Au particle size becomes smaller as the pH is further increased. The gold uptake increases with the surface area of the support (anatase, rutile, P-25) and is complete above 200 m 2 g −1; adsorption of the gold precursor at pH 9 is shown to be equilibrium-limited. Highest activities are found with supports of 50 m 2 g −1. Catalysts made with high-area anatase (240 or 305 m 2 g −1) are least active but show least deactivation.With Au/SnO 2 catalysts, gold uptake does not depend on the area of the support, and is highest at pH 7–8; very active catalysts ( T50 = 230–238 K) are obtained using SnO 2 of 47 m 2 g −1. Storing a catalyst at 258 K for 1 week dramatically improves its stability. Results for Au/CeO 2 and Au/ZrO 2 catalysts confirm that moderate support areas give the most active catalysts, and suggest that surface area is often more important than chemical composition. 相似文献
7.
This study aimed to investigate the removal of a reactive dye from aqueous solution by adsorption. Activated carbon prepared from olive stone, an agricultural solid by-product, was used as adsorbent. Different amounts of activating agent (ZnCl 2) and adsorbent particle size were studied to optimise adsorbent surface area. The adsorption experiments were conducted at different process parameters such as adsorbent dose, temperature, equilibrium time and pH. The experimental results showed that at equilibrium time 120 min, optimum pH ranged between 3 and 4, and adsorbent dosage was 2.0 g 200 ml −1. While the kinetic data support pseudo-second order, a pseudo-first order model shows very poor fit. Adsorption isotherms were obtained at three different temperatures (288, 298 and 308 K). The fitness of adsorption data to the Langmuir and Freundlich isotherms was investigated. In addition, the thermodynamic parameters such as isosteric enthalpy of adsorption (Δ H ads) y , isosteric entropy of adsorption (Δ S ads) y and free energy of adsorption Δ G 0ads were calculated. BET surface area measurements were made to reveal the adsorptive characteristics of the produced active carbon. The surface area of the activated carbon produced with 20% w/w ZnCl 2 solution was 790.25 m 2 g −1. 相似文献
8.
This work investigates the effects of Au 3+/Au 0 ratio or distribution of gold oxidation states in Au/ZrO 2 catalysts of different gold loadings (0.01–0.76% Au) on CO oxidation and 1,3-butadiene hydrogenation by regulating the temperature of catalyst calcination (393–673 K) and pre-reduction with hydrogen (473–523 K). The catalysts were prepared by deposition–precipitation and were characterized with elemental analysis, nitrogen adsorption/desorption, TEM, XPS and TPR. The catalytic data showed that the exposed metallic Au 0 atoms at the surface of Au particles were not the only catalytic sites for the two reactions, isolated Au 3+ ions at the surface of ZrO 2, such as those in the catalysts containing no more than 0.08% Au were more active by TOF. For 0.76% Au/ZrO 2 catalysts having coexisting Au 3+ and Au 0, the catalytic activity changed differently with varying the Au 3+/Au 0 ratio in the two reactions. The highest activity for the CO oxidation reaction was observed over the catalyst of Au 3+/Au 0 = 0.33. However, catalyst with a higher Au 3+/Au 0 ratio showed always a higher activity for the hydrogenation reaction; co-existance of Au 0 with Au 3+ ions lowered the catalyst activity. Moreover, the coexisting Au particles changed the product selectivity of 1,3-butadiene hydrogenation to favor the formation of more trans-2-butene and butane. It is thus suggested that for better control of the catalytic performance of Au catalyst the effect of Au 3+/Au 0 ratio on catalytic reactions should be investigated in combination with the particle size effect of Au. 相似文献
9.
In this study, we examine the interaction of N 2O with TiO 2(1 1 0) in an effort to better understand the conversion of NO x species to N 2 over TiO 2-based catalysts. The TiO 2(1 1 0) surface was chosen as a model system because this material is commonly used as a support and because oxygen vacancies on this surface are perhaps the best available models for the role of electronic defects in catalysis. Annealing TiO 2(1 1 0) in vacuum at high temperature (above about 800 K) generates oxygen vacancy sites that are associated with reduced surface cations (Ti 3+ sites) and that are easily quantified using temperature programmed desorption (TPD) of water. Using TPD, X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), we found that the majority of N 2O molecules adsorbed at 90 K on TiO 2(1 1 0) are weakly held and desorb from the surface at 130 K. However, a small fraction of the N 2O molecules exposed to TiO 2(1 1 0) at 90 K decompose to N 2 via one of two channels, both of which are vacancy-mediated. One channel occurs at 90 K, and results in N 2 ejection from the surface and vacancy oxidation. We propose that this channel involves N 2O molecules bound at vacancies with the O-end of the molecule in the vacancy. The second channel results from an adsorbed state of N 2O that decomposes at 170 K to liberate N 2 in the gas phase and deposit oxygen adatoms at non-defect Ti 4+ sites. The presence of these O adatoms is clearly evident in subsequent water TPD measurements. We propose that this channel involves N 2O molecules that are bound at vacancies with the N-end of the molecule in the vacancy, which permits the O-end of the molecule to interact with an adjacent Ti 4+ site. The partitioning between these two channels is roughly 1:1 for adsorption at 90 K, but neither is observed to occur for moderate N 2O exposures at temperatures above 200 K. EELS data indicate that vacancies readily transfer charge to N 2O at 90 K, and this charge transfer facilitates N 2O decomposition. Based on these results, it appears that the decomposition of N 2O to N 2 requires trapping of the molecule at vacancies and that the lifetime of the N 2O–vacancy interaction may be key to the conversion of N 2O to N 2. 相似文献
10.
An evaluative investigation of the Fischer–Tropsch performance of two catalysts (20%Co/Al 2O 3 and 10%Co:10%Mo/Al 2O 3) has been carried out in a slurry reactor at 2 MPa and 220–260 °C. The addition of Mo to the Co-catalyst significantly increased the acid-site strength suggesting strong electron withdrawing character in the Co-Mo catalyst. Analysis of steady-state rate data however, indicates that the FT reaction proceeds via a similar mechanism on both catalysts (carbide mechanism with hydrogenation of surface precursors as the rate-determining step). Although chain growth, , on both catalysts were comparable ( 0.6), stronger CH 2 adsorption on the Co-Mo catalyst and lower surface concentration of hydrogen adatoms as a result of increased acid-site strength was responsible for the lower individual hydrocarbons production rate compared to the Co catalyst. The activation energy, E, for Co (96.6 kJ mol −1), is also smaller than the estimate for the Co-Mo catalyst (112 kJ mol −1). Transient hydrocarbon rate profiles on each catalyst are indicative of first-order processes, however the associated surface time constants are higher for alkanes than alkenes on individual catalysts. Even so, for each homologous class, surface time constants for paraffins are greater for Co-Mo than Co, indicative that the adsorption of CH 2 species on the Co-Mo surface is stronger than on the monometallic Co catalyst. 相似文献
11.
The primary reason why the RuO 2(1 1 0) surface is much more active in the oxidation of CO than the corresponding metal Ru(0 0 0 1) surface is correlated with the weaker oxygen bonding on RuO 2(1 1 0) compared to chemisorbed oxygen on Ru(0 0 0 1). The RuO 2(1 1 0) surface stabilizes at least two potentially active oxygen species, i.e., bridging O and on-top O atoms. Together with various adsorption sites for CO during the reaction, the CO oxidation reaction over RuO 2(1 1 0) becomes quite complex. Using the techniques of temperature programmed reaction and desorption in combination with state-of-the-art density functional theory calculation we studied the CO oxidation reaction over RuO 2(1 1 0) in the temperature range of 300–400 K. We show that the CO oxidation on RuO 2(1 1 0) surface is not dominated by the recombination of CO with on-top O, although the binding energy of the on-top O is 1.4 eV lower than that of the bridging O atom. 相似文献
12.
Mesoporous ZrO 2 with high surface area and uniform pore size distribution, synthesized by surfactant templating through a neutral [C 13(EO) 6–Zr(OC 3H 7) 4] assembly pathway, was used as a support of gold catalysts prepared by deposition–precipitation method. The supports and the catalysts were characterized by powder X-ray diffraction, scanning and transmission electron microscopy, N 2 adsorption analysis, temperature programmed reduction and desorption. The catalytic activity of gold supported on mesoporous zirconia was evaluated in water–gas shift (WGS) reaction at wide temperature range (140–300 °C) and at different space velocities and H 2O/CO ratios. The catalytic behaviour and the reasons for а reversible deactivation of Au/mesoporous zirconia catalysts were studied. The influence of gold content and particle size on the catalytic performance was investigated. The WGS activity of the new Au/mesoporous zirconia catalyst was compared to the reference Au/TiO 2 type A (World Gold Council), revealing significantly higher catalytic activity of Au/mesoporous zirconia catalyst. It is found that the mesoporous zirconia is a very efficient support of gold-based catalyst for the WGS reaction. 相似文献
13.
Supported Au catalysts Au-Au +-Cl x/Fe(OH) y ( x < 4, y ≤ 3) and Au-Cl x/Fe 2O 3 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 Au 0 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(NO 3) 3 and HAuCl 4 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 O 2 were more easily adsorbed on Au + and Au 0, but was very difficultly absorbed on Au −. It also indicated that when Cl − was adsorbed on Au 0, 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. 相似文献
14.
In order to improve a “Three Function Catalysts Model”, the present paper deals with alumina based catalysts containing cobalt and palladium for the NO reduction by methane. The deNOx temperature window was estimated by adsorption and subsequent desorption of NO in lean conditions. Two NOx desorption peaks were detected for both catalysts. For Pd(0.63)Co(0.58)/Al2O3, the two desorption peaks appeared at 205 and 423 °C, whereas for Pd(0.14)Co(0.57)/Al2O3, the maxima desorption temperature peaks were at 205 and 487 °C. In addition, NO oxidation was also studied to evaluate the catalyst first function. It was found that, the oxidation begins on Co–Pd/Al2O3 around 250 °C. On Pd(0.63)Co(0.58)/Al2O3, 8% of deNOx were found in the range of the second NOx desorption peak temperature (410 °C). During TPSR, CxHyOz species such as formaldehyde were detected. These oxygenate species are the reactive intermediate for deNOx by methane. 相似文献
15.
Oxygen adsorption on silica-supported gold catalyst from NO 2 and O 2 exposures were investigated by temperature-programmed desorption spectroscopy under a vacuum condition. NO 2 and O 2 exposures of the surface of the catalyst at room temperature gave adsorbed oxygen in atomic state. Adsorbed oxygen penetrated beneath the gold with lower activation energy for NO 2 exposure than for O 2 exposure. Adsorbed oxygen in oxidic state which was desorbed above 600 K altered the surface properties of gold and resulted in the decrease of activation energy for oxygen to penetrate beneath the gold surface. 相似文献
16.
Scanning tunneling microscopy (STM) is demonstrated to be a powerful tool to characterize adsorption and reaction on oxide surfaces by imaging molecular adsorbates and reactive intermediates. The molecules were used to probe surface structure and to study surface reactivity spatially at the atomic level. Results for three systems are presented: alcohol adsorption on WO 3(0 0 1), carboxylates on the anatase polymorph of TiO 2, and propene adsorption on a PdO monolayer on Pd(1 0 0). When the alcohols were exposed to the WO 3(0 0 1)-c(2×2) surface at room temperature the molecules could not be imaged. Heating the surface to temperatures above a water desorption peak associated with alcohol deprotonation, however, allowed 1-propoxide to be imaged. The images reveal that the alkoxide has no preference for defects, rather it binds to W 6+ ions exposed on the fully oxidized c(2×2) surface. Temperature-programmed desorption revealed that alkoxides at these sites undergo only dehydration reactions. To probe the structure of the unusual (1×4) reconstruction on anatase (0 0 1), formic and acetic acid adsorption were used. Following dissociative adsorption, both formate and acetate adsorb solely centered atop the bright rows that define the surface reconstruction, and the molecules are always at least two lattice constants apart. This result may be attributed to carboxylates bridge-bonded to Ti atoms at the center of the bright rows. This finding eliminates several suggested models of the reconstruction and suggests that a recently proposed ad-molecule model is a good representation of the surface structure. Propene was observed to initially randomly adsorb on the PdO monolayer. At higher coverages, however, the adsorbates cluster, disrupting the surface structure and causing the adsorption rate adjacent to the clusters to increase. Temperature-programmed reaction revealed that once propene adsorbs, the oxide monolayer catalyzes its oxidation at lower temperatures than metallic Pd, but that the propene sticking coefficient on the ordered oxide layer is a factor of 5 lower. 相似文献
17.
Porous carbon materials have been widely used for the removal of SO 2 from flue gas. The main objective of this work is to clarify the effects of adsorption temperature on SO 2 adsorption and desorption energy consumption. Coal-based porous powdered activated coke (PPAC) prepared in the drop-tube reactor was used in this study. The N 2 adsorption measurements and Fourier transform infrared spectrometer analysis show that PPAC exhibits a developed pore structure and rich functional groups. The experimental results show that with a decrease in adsorption temperature in the range of 50–150?℃, the adsorption capacity of SO 2 increases linearly; meanwhile, the adsorption capacity of H 2O increases, resulting in the increase in desorption energy consumption per unit mass of adsorbent. The processes of SO 2 and H 2O desorption were determined by the temperature-programmed desorption test, and the desorption energies for each species were calculated. Considering the energy consumption per unit of desorption and the total amount of adsorbent, the optimal adsorption temperature yielding the minimum total energy consumption of regeneration is calculated. This study systematically demonstrates the effect of adsorption temperature on the adsorption–desorption process, providing a basis for energy saving and emission reduction in desulfurization system design. 相似文献
18.
低挥发性有机酸不仅自身污染环境而且显著促进颗粒污染物形成,对其吸脱附性能的研究有助于这类物质的控制。采用程序升温脱附(TPD)技术对乙二酸、苯甲酸在活性炭(AC)上的脱附行为进行了研究。结果表明,吸附主要发生在粗微孔(0.7~2 nm)、细微孔(<0.7 nm)中,对应TPD曲线中的吸附位Ⅰ、Ⅱ。粗微孔对乙二酸、苯甲酸的脱附活化能为101.63、112.43 kJ·mol -1,吸附量均大于总吸附量的91%。细微孔对乙二酸、苯甲酸的脱附活化能为118.01、130.87 kJ·mol -1,吸附量均小于总吸附量的9%。细微孔吸附强度高于粗微孔,但吸附量远低于粗微孔,因为细微孔对吸附质的迁移阻力较大,仅少量吸附质能进入细微孔中。苯甲酸在迁移中受到阻力较乙二酸大,在细微孔中吸附量更小,表现为分子筛分作用。 相似文献
19.
利用物理溶剂环丁砜替代部分水,采用气液搅拌实验装置和真实热流量热法测定了环丁砜对乙醇胺(MEA)溶液吸收和解吸二氧化碳(CO 2)过程的影响,考察了CO 2循环负载、吸收速率、吸收热和解吸热等性质变化。研究表明:环丁砜对MEA溶液负载CO 2的吸收热影响较小,但对吸收速率、循环吸收容量和解吸过程影响较大。环丁砜可降低MEA溶液对CO 2的表观吸收速率,且随CO 2负载量的增大,降幅也逐渐变大。环丁砜有利于富液解吸过程,加快解吸速率,增大CO 2解吸程度,同时单位热流负荷、单位冷流负荷和单位能耗均有不同程度的降低。在燃煤电厂烟气条件下,20% MEA+20% sulfolane体系相对20% MEA体系,其表观吸收速率平均降低约10%,CO 2循环吸收容量增加24%,单位CO 2解吸能耗降低18%。 相似文献
20.
This paper is a report of angle-resolved product desorption measurements in the course of catalyzed NO and N 2O reduction on Pd(1 1 0). Surface-nitrogen removal processes show different angular distributions, i.e. normally directed N 2 desorption takes place in process (i) 2N(a) → N 2(g). Highly inclined N 2 desorption towards the [0 0 1] direction is induced in process (ii) N 2O(a) → N 2(g) + O(a). N 2O or NH 3 desorption follows the cosine distribution characterizing the desorption after the thermalization in process (iii) N 2O(a) → N 2O(g) or (iv) N(a) + 3H(a) → NH 3(a) → NH 3(g). Thus, a combination of the angular and velocity distributions provides the analysis of most of surface-nitrogen removal processes in the course of catalyzed NO reduction. At temperatures below 600 K, processes (ii) and (iii) dominate and process (iv) is enhanced at H2 pressures higher than NO. Process (i) contributes significantly above 600 K. Only three processes except for NH3 formation are operative when CO is used. Only process (ii) was observed in a steady-state N2O + CO (or H2) reaction. 相似文献
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