Pd‐Ag/α‐Al2O3 Catalyst Deactivation in Acetylene Selective Hydrogenation Process

The selective hydrogenation of acetylene to ethylene over Pd‐Ag/α‐Al₂O₃ catalysts prepared by different impregnation/reduction methods was studied. The best catalytic performance was achieved with the sample prepared by sequential impregnation. A kinetic model based on first order in acetylene and 0.5th order in hydrogen for the main reaction and second‐order independent decay law for catalyst deactivation was used to fit the conversion time data and to obtain quantitative assessment of catalyst performances. Fair fits were observed from which the reaction and deactivation rate constants were evaluated. Coke deposition amounts showed a good correlation with catalyst deactivation rate constants, indicating that coke formation should be the main cause of catalyst deactivation.     

乙炔氢氯化制氯乙烯Pt/C催化剂的性能

以PtCl₄和PtCl₂为活性组分,活性炭为载体,采用浸渍法制备Pt/C催化剂,考察Pt/C催化剂用于乙炔氢氯化制取氯乙烯的催化性能,并采用热重分析、N₂等温吸附-脱附和H₂-TPR等方法对Pt/C催化剂进行表征。结果表明, 加入少量Pt(Ⅱ)（质量分数20%）有利于提高催化剂的初始活性,但加剧了催化剂失活。而活性组分被还原是催化剂失活的主要原因,反应过程中积炭堵塞微孔结构也是导致催化剂失活的原因。     

Carbonaceous materials as catalysts for decomposition of methane

Thermo-catalytic decomposition of methane over different carbonaceous materials was studied by monitoring the mass gain with time. The initial decomposition rates as well as the long-term behaviour of the catalyst (i.e. the carbon mass that the catalyst can accumulate before deactivation occurs) were determined for a wide range of carbon blacks (CB) with different textural properties and surface chemistry, and for a commercial activated carbon (AC). The commercial carbon black BP2000 showed the highest amount of carbon deposited, 6.13 g C_dep/g C_o before deactivation while the higher initial carbon formation rate (r_o) among the different samples tested was obtained for the activated carbon CG Norit (85.9 mg C_dep/g C_o min). The relationship between the characteristics of the carbonaceous materials and their efficiency as catalysts were also evaluated. The amount of carbon deposited until deactivation shows a linear relationship with the total pore volume of the fresh catalysts. A good correlation is also found between the initial reaction rate and the concentration of oxygenated groups desorbed as CO after a temperature-programmed desorption (TPD) experiment.     

The ligand coordination approach for improving the stability of low-mercury catalyst in the hydrochlorination of acetylene

Mercuric chloride supported on activated carbon (HgCl2/AC) is used as an industrial catalyst for the hydrochlorination of acetylene.Loss of HgCl2 by sublimating from the surface of activated carbon causes the irreversible deactivation of mercury catalyst and environmental pollution.In this work,a ligand coordination approach based on the Principle of Hard and Soft Acids and Bases (HSAB) was employed to design more stable lowmercury catalyst.The low-mercury catalysts (4％ HgCl2 loading) were prepared by using HgCl2 and potassium halides (KX,X =Cl,I) as precursors.The HgCl2-4KI/AC catalyst showed best catalytic stability than HgCl2/AC and HgCl2-4KCl/AC in the hydrochloriantion of acetylene.HgCl2 could form more stable complex with KI,K2HgI4 as the main active component of the HgCl2-4KI/AC catalyst.The characterizations of XRD and EDX analysis illustrated that the active component of HgCl2-4KI/AC was highly dispersed on the surface of activated carbon.The sublimation rates of HgCl2 from the catalysts verified that the active component with larger stability constant had better thermal stability.Using Hg(Ⅱ) complexes with high stability constant as the active component may be the research direction of developing highly stable low-mercury catalyst for the hydrochlorination of acetylene.     

Stable Hydrogen Production from Ethanol through Steam Reforming Reaction over Nickel-Containing Smectite-Derived Catalyst

Hydrogen production through steam reforming of ethanol was investigated with conventional supported nickel catalysts and a Ni-containing smectite-derived catalyst. The former is initially active, but significant catalyst deactivation occurs during the reaction due to carbon deposition. Side reactions of the decomposition of CO and CH₄ are the main reason for the catalyst deactivation, and these reactions can relatively be suppressed by the use of the Ni-containing smectite. The Ni-containing smectite-derived catalyst contains, after H₂ reduction, stable and active Ni nanocrystallites, and as a result, it shows a stable and high catalytic performance for the steam reforming of ethanol, producing H₂.     

Roles of interaction between components in CZZA/HZSM-5 catalyst for dimethyl ether synthesis via CO2 hydrogenation

The roles of interaction between two catalyst components in CuO–ZnO–ZrO₂–Al₂O₃ (CZZA)/HZSM-5 bifunctional catalyst for dimethyl ether (DME) synthesis via carbon dioxide hydrogenation were investigated. It was found that CZZA catalyst showed excellent stability during methanol (MeOH) synthesis for 100 h, while there was a severe loss of catalytic activity in the bifunctional catalyst for DME synthesis. Hence, the effects of different degrees of intimacy of two catalyst components were studied for DME synthesis, including mixed and separated modes. For the mixed mode, the particle size of catalysts and the amount of reaction intermediates were proven to influence the catalyst deactivation. For the separated mode, the catalysts showed rapid deactivation within a short time. Various characterizations indicated that the remarkable deactivation of separated mode was mainly caused by the decrease of copper active centers (e.g., sintering and oxidation) and blockage of acid sites via increased coke deposition on HZSM-5.     

A basic study of catalyst aging in the H-coal process

Samples of CoMo/Al₂O₃ catalysts used in an H-coal process demonstration run were studied to determine causes of catalyst deactivation. Physical and surface properties of the aged and regenerated catalysts were examined. Model compounds were used to assess four catalyst activity functions, viz., hydrodesulfurization (HDS), hydrogenation, cracking and hydrodeoxygenation (HDO). Other tests were performed to study the effects of coke and metals separately on the four catalyst activity functions.Catalyst coke content and metals deposits first increased rapidly, then more gradually with exposure time in the process run. Surface area and pure volume markedly decreased with exposure time. Catalyst activities of aged catalysts showed a rapid decline with exposure time. One-day exposure to coal resulted in significant losses in HDS and hydrogenation activities and nearly complete loss in cracking and HDO activities. Although metal deposits caused some permanent catalyst deactivation, coke had a much greater effect. Regenerated catalysts showed less recovery of catalytic activity as processing time increased. These results agreed well with product inspections from the process run.Oxygen chemisorption on aged—regenerated catalysts decreased with catalyst exposure time, indicating a significant loss of active sites. However, ESCA results showed no evidence of extensive sintering of the active MoS₂ phase. Permanent deactivation of the longer-time exposed catalysts can be ascribed, at least partly, to lateral growth of the active molybdenum sulfide phase. In addition, some loss in cobalt promotion occurred early in the process, which may account for the rapid loss in HDS and HDO activity in regenerated catalysts.     

EFFECTS OF CARBON DEPOSITS ON THE SPECIFIC ACTIVITY OF NICKEL AND NICKEL BIMETALLIC CATALYSTS

The effects of carbon formation on methanation activity of nickel and nickel bimetallic catalysts were investigated. Carbon was deposited on these catalysts at 675-700 K, 1 atm, H₂/CO = 2 and space velocities of 80,000 to 200,000 h^?1 over a period of 6-24 hours. Specific methanation activities were measured before and after carbon depositing treatments at 500-575 K, 1 atm H₂/CO = 4 and space velocities of 100,000 h^?1. The results show that Ni/Al₂O₃ loses 20-60% of its initial activity within 10-15 hours of treatment. Platinum and cobalt promoted nickel are significantly more resistant to deactivation by carbon. However, Ni-MoO₂ is highly susceptible to deactivation, losing essentially all of its activity within a few hours. Data showing the effects of reaction conditions, metal concentration and catalyst composition on the extent of deactivation and the effects of deactivation on catalyst strength are presented and discussed     

Deactivation Studies of Rh/Ce0.8Zr0.2O2 Catalysts in Low Temperature Ethanol Steam Reforming

Rapid deactivation of Rh/Ce_0.8Zr_0.2O₂ catalysts during low temperature ethanol steam reforming was studied. A significant build-up of reaction intermediates, instead of carbon deposit, was observed at low reaction temperatures. This appears to be the cause of rapid catalyst deactivation. Co-feed experiments indicated that possible intermediate products acetone and ethylene caused more severe catalyst deactivation than other oxygenates such as acetic acid and acetaldehyde.     

低汞催化剂上氯化亚汞含量分析

采用电感耦合等离子体发射光谱、X射线衍射和X射线光电子能谱对电石乙炔法制备氯乙烯工艺中使用的氯化汞工业催化剂样品A、B和自制的催化剂样品进行表征。结果表明,工业催化剂样品为低汞催化剂;自制催化剂样品中的汞含量随着浸渍次数的增加而增加;低汞催化剂上有氯化亚汞存在。通过归一法得到催化剂表面不同价态汞元素的相对含量,发现催化剂样品中氯化亚汞质量分数约50%,表明负载于活性炭上的氯化汞有一半转化为氯化亚汞,催化剂活性降低。     

Kinetic study of carbon nanotube synthesis over Mo/Co/MgO catalysts

The kinetics of carbon nanotube (CNT) synthesis by decomposition of CH₄ over Mo/Co/MgO and Co/MgO catalysts was studied to clarify the role of catalyst component. In the absence of the Mo component, Co/MgO catalysts are active in the synthesis of thick CNT (outer diameter of 7-27 nm) at lower reaction temperatures, 823-923 K, but no CNTs of thin outer diameter are produced. Co/MgO catalysts are significantly deactivated by carbon deposition at temperatures above 923 K. For Mo-including catalysts (Mo/Co/MgO), thin CNT (2-5 walls) formation starts at above 1000 K without deactivation. The significant effects of the addition of Mo are ascribed to the reduction in catalytic activity for dissociation of CH₄, as well as to the formation of Mo₂C during CNT synthesis at high temperatures. On both Co/MgO and Mo/Co/MgO catalysts, the rate of CNT synthesis is proportional to the CH₄ pressure, indicating that the dissociation of CH₄ is the rate-determining step for a catalyst working without deactivation. The deactivation of catalysts by carbon deposition takes place kinetically when the formation rate of the graphene network is smaller than the carbon deposition rate by decomposition of CH₄.     

Monitoring Aging and Deactivation of Emission Abatement Catalysts for Selective Catalytic Reduction of NOx

Titania/vanadia, zeolite, and noble metal catalysts are utilized for selective catalytic reduction (SCR) of NO _x using ammonia as the reductant in different temperature ranges. Studies of aging have been carried out to probe deactivation rates and mechanisms. Periodic laboratory testing of samples of NO _x reduction catalysts from multi-layer reactors, such as those utilized at electric power plants, allows prediction of catalyst life-times. Testing has been carried out under protocol conditions with monolith, plate-type, and pelleted catalysts so that relative NO reduction rates can be compared, with or without the presence of SO₂. The catalysts were analyzed by surface analysis techniques, including electron microscopy and X-ray photoelectron spectroscopy, to probe surface morphology, loss of active components, presence of poisons, and blocking of pores and active sites by ammonium bisulfate to determine the dominant mode(s) of gradual deactivation.     

Stability of ruthenium catalysts supported on TiO2 or ZrO2 in catalytic wet air oxidation

The stability of ruthenium catalysts supported on TiO₂ and ZrO₂ were studied in the wet air oxidation of aqueous solution of succinic and p-hydroxybenzoic acids taken as model effluent and on real effluents from the paper-pulp industry. Catalyst recycling experiments were conducted in batch reactor and long-term stability tests were conducted in trickle-bed reactor. In all experiments, ruthenium and support materials were perfectly stable to leaching, sintering and fouling. Ruthenium catalysts experienced a weak deactivation as they were exposed to air, e.g., in recycling experiments however the loss of activity occurred only after the first exposure and was completely reversible upon catalyst reduction. The deactivation was attributed to an over-oxidation of the catalyst surface particularly noticeable in the case of very small Ru-clusters (1 nm).     

Deactivation of NaCl/B2O3/Fe2O3 catalysts and their improvement for the oxidative coupling of methane

In the present work, the deactivation of the NaCl/B₂O₃/Fe₂O₃ catalysts was studied for the oxidative coupling of methane. Several techniques, such as XPS, XRD, SEM, H₂-TPR, and flow-reaction, were employed to examine the function of each catalyst component, and its change during the catalytic reaction. NaCl and B₂O₃ show a synergistic effect on the Fe₂O₃ surface. B₂O₃ modifies the oxidative activity of Fe₂O₃ and makes the first reduction peak of Fe₂O₃ shift from 490 to 750°C. The NaCl modified B₂O₃/Fe₂O₃ catalyst has high reduction rate, high activity and selectivity at about 750°C. It is demonstrated that the deactivation of the NaCl/B₂O₃/Fe₂O₃ catalysts is a complicated process, consisting of chloride loss, sodium change, B₂O₃ loss, silica deposition and catalyst sintering. The chloride loss enhances the surface basicity, which causes the silica deposition and sodium change, and aggravates the catalyst sintering. The silica deposition and catalyst sintering cause permanent deactivation. The B₂O₃ loss is not a direct reason for catalyst deactivation. NaCl crystal diluted NaCl/B₂O₃/Fe₂O₃ catalysts have a better stability. The deactivated catalyst has a more stable structure. When it is regenerated by impregnating with NaCl again, a more stable catalyst can be obtained.     

CH4-CO2 reforming over Ni/Al2O3 aerogel catalysts in a fluidized bed reactor

Ni/Al₂O₃ 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 CO₂ 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.     

甲醇制芳烃催化剂失活特性研究进展

在系统分析甲醇制芳烃催化反应特性基础上,重点对影响催化剂长周期使用性能的失活原因及应对方案进行综述。催化剂失活是多因素共同影响的结果,主要包括可再生的积炭失活、可部分再生的分子筛骨架铝流失失活、及不可再生的活性金属迁移/聚并失活和杂质毒化失活。积炭导致的催化剂失活可通过调变ZSM-5的B酸强度、合成小晶粒ZSM-5分子筛及采用碱液预处理ZSM-5分子筛等方式减缓; ZSM-5分子筛骨架铝流失导致的催化剂失活可通过采用磷改性等方式降低;活性金属迁移导致的催化剂失活可通过引进稳定助剂、降低催化剂生产过程中铝的引入等方式解决;而杂质毒化导致的催化剂失活需要通过净化原料和反应体系的措施来避免。     

Deactivation mechanism of activated carbon supported copper oxide SCR catalysts in C2H4 reductant

Current state‐of‐the‐art NH₃‐SCR technology based on vanadium catalysts suffers problems associated with NH₃ slip and poisoning of the catalyst and blockage of heat recovery steam generators (HRSG). If environmentally‐friendly catalysts capable of efficient operation at lower temperatures could be developed that used a reductant other than NH₃, the issues with current state‐of‐the‐art SCR could be significantly lessened. Hence, in this study, activated carbon (AC) supported copper oxide‐based catalysts for SCR while using C₂H₄ as a reductant was discussed. Reaction testing of catalysts demonstrated high initial NO conversion with steeply declining activity over 2 h of testing when C₂H₄ was used as the reductant; in comparison, with the same catalyst and NH₃ as the reductant, stable, long‐term NO conversion was achieved, but at a lower rate than the initial reactivity with C₂H₄. As a consequence, catalyst characterization studies were performed to assess deactivation mechanisms when C₂H₄ was the reductant. These studies included x‐ray diffraction, BET surface area and porosity, temperature programmed reduction, scanning electron microscopy, Raman spectroscopy and x‐ray photoelectron spectroscopy of both fresh and deactivated catalysts. The analytical results showed the surface area and porosity of the catalyst remained unchanged and the initially highly‐dispersed Cu species became agglomerated and more crystalline during reaction testing. Furthermore, carbon black was also detected on the catalyst surface after testing, presumably formed during the decomposition of C₂H₄. Both agglomeration of the active Cu species and blockage by carbon deposits would decrease the availability of active sites and lead to decreased catalytic activity.     

Oxygen and nitrogen-doped metal-free carbon catalysts for hydrochlorination of acetylene

Activated carbon was tested as metal-free catalyst for hydrochlorination of acetylene in order to circumvent the problem of environment pollution caused by mercury and high cost by noble metals. Oxygen-doped and nitrogen-doped activated carbons were prepared and characterized by XPS, TPD and N2 physisorption methods.The influences of the surface functional groups on the catalytic performance were discussed base on these results.Among all the samples tested, a nitrogen-doped sample, AC-n-U500, exhibited the best performance, the acetylene conversion being 92% and vinyl chloride selectivity above 99% at 240 °C and C2H2 hourly space velocity30 h-1. Moreover, the AC-n-U500 catalyst exhibited a stable performance during a 200 h test with a conversion of acetylene higher than 76% at 210 °C at a C2H2 hourly space velocity 50 h-1. In contrary, oxygen-doped catalyst had lower catalytic activities. A linear relationship between the amount of pyrrolic-N and quaternary-N species and the catalytic activity was observed, indicating that these nitrogen-doped species might be the active sites and the key in tuning the catalytic performance. It is also found that the introduction of nitrogen species into the sample could significantly increase the adsorption amount of acetylene. The deactivation of nitrogendoped activated carbon might be caused by the decrease of the accessibility to or the total amount of active sites.     

Conversion of methane and carbon dioxide into synthesis gas over alumina-supported nickel catalysts. Effect of Ni-Al2O3 interactions

A series of alumina-supported nickel catalysts were prepared by calcination of the catalyst precursors in air at different temperatures. The increase in the intensity of Ni-Al₂O₃ interactions with the calcination temperature was found to be unfavourable to the reduction of the catalyst, and thus caused a decrease in activity for the low temperature reaction between methane and carbon dioxide. However, the catalyst with strong Ni-Al₂O₃ interactions suppressed carbon deposition effectively, which can be attributed to the formation of spinel, NiAl₂O₄, after calcination. When the reaction was carried out at 1023 K, all the catalysts tended to exhibit the same activity. At the same time, only filamentous carbon with a hollow inner channel was observed and there were nickel particles on the tip of this filamentous carbon.     

Methane decomposition over ceria modified iron catalysts

The catalytic behaviour of ceria supported iron catalysts (Fe–CeO₂) was investigated for methane decomposition. The Fe–CeO₂ catalysts were found to be more active than catalysts based on iron alone. A catalyst composed of 60 wt.% Fe₂O₃ and 40 wt.% CeO₂ gave optimal catalytic activity, and the highest iron metal surface area. The well-dispersed Fe state helped to maintain the active surface area for the reaction. Methane conversion increased when the reaction temperature was increased from 600 to 650 °C. Continuous formation of trace amounts of carbon monoxide was observed during the reaction due to the oxidation of carbonaceous species by high mobility lattice oxygen in the solid solution formed within the catalyst. This could minimise catalyst deactivation caused by carbon deposits and maintain catalyst activity over a longer period of time. The catalyst also produced filamentous carbon that helped to extend the catalyst life.