汽车三效催化剂失活研究的进展

随着对汽车三效催化剂耐久性能要求的日益提高,因而对催化剂抗老化性能提出了更高的挑战,本文主要介绍了汽车三效催化剂失活的主要类型,包括:机械失活、化学失活及热失活。阐明了目前汽车三效催化剂所面临的主要的失活问题就是涂层基体γ-Al2O3烧结和贵金属的老化,同时论述了目前对γ-Al2O3和贵金属的主要改性方法,指出开发新型的耐高温稳定的涂层材料是目前所要研究的重点。     

NOx storage/reduction over lean-burn automotive catalysts

This paper shows the behavior of a Pt/Ba/γ–Al₂O₃ automotive catalyst in a fixed bed reactor during cyclic operation at lean and rich gas phase conditions at short (seconds) and long (hours) cycling times at different temperatures. Reactor exit gas phase concentrations have been measured and catalyst properties have been determined before and after selective cycling experiments. The experimental results indicate that: (i) Upon 9 h lean and 15 h rich cycling, the NO oxidation efficiency of the catalyst decreases with time while incomplete regeneration is seen, even after 15 h rich exposure with H₂. The cyclic steady state is reached after 3 lean/rich cycles, at which only 60% of the available barium is involved in the NO_x storage/reduction. (ii) The BET surface area, pore volume, and Pt dispersion decrease by approximately 40%, which may be a result of masking of Pt sites or blocking of pores of the barium clusters as BaCO₃ becomes Ba(NO₃)₂. Experiments with catalyst pellet sizes of 180 and 280 μm along with XPS measurements show that blocking of catalyst pellet pores is not taking place. (iii) When applying lean/rich cycling in the order of seconds, it appears that catalyst history and lean/rich timing affect the number of cycles required to arrive at a closed N balance. XRD results after lean exposure confirm the formation of barium nitrate in the bulk of the barium cluster.     

Review on the deactivation of FCC catalysts by cyclic propylene steaming

The deactivation of FCC catalysts in small-scale units in the presence of contaminant metals has been the industry workhorse for a number of years due to the robustness and simplicity of such methods.

A major advance in the deactivation of metallated catalysts on a small-scale that better simulated the performance of commercial FCC catalyst, was the introduction of the cyclic propylene steaming (CPS) method. In the CPS protocol, FCC catalyst is impregnated with vanadium and nickel prior to deactivation in reduction–oxidation cycles and the latter provides significant advantages over traditional methods such as the Mitchell approach. In certain situations, however, the contribution of vanadium to catalyst deactivation is over-emphasized in the CPS method, and therefore this method has been developed further.

This paper describes a number of modifications that have been made to the CPS method to further attenuate the destructive effects of vanadium on the FCC catalyst during deactivation. This includes exposing the metallated catalyst to pre-stabilisation steps with reduction-oxidation cycles and changing the ratio of the time the catalyst spends in reducing and oxidising environments during the CPS cycles. The comparison of activity and selectivity data obtained from FCC catalysts having age distribution with those obtained from CPS showed that both scenarios gave the same catalyst ranking.

These investigations have also shown that the use of reduction–oxidation cycles better simulates the deactivation of FCC catalysts in the absence of contaminant metals. These investigations have also shown that the use of reduction–oxidation cycles better simulates the deactivation of FCC catalysts in the absence of contaminant metals “the part” than traditional deactivations in a 100% steam atmosphere.     

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.     

Deactivation of catalysts for fossil coal and biomass conversion

Increasing interest in the use of resources of organic raw materials alternative to petroleum stimulates researchers to study the catalytic intensification of fossil coal and vegetable biomass conversion. The industrial use of the developed technologies is restricted by the problem of enhanced catalyst deactivation. The main reasons for catalyst deactivation in solid organic raw materials conversion are considered in the review. The possible ways of the successful solution of the problem of catalyst deactivation in the processes of coal and biomass liquefaction and gasification and wood delignification and hydrolysis are exemplified by the studies accomplished over the last decade. In the case of technologies using the synthetic catalysts, the methods of their regeneration developed for the catalytic processing of heavy petroleum residues can be applied. The catalyst regeneration is not demanded for the processes accomplished in a fluidized catalytic bed under the conditions of catalyst loss as a result of its attrition and in the case of the application of melted catalysts, cheap iron-containing catalysts, catalytically active slag materials, and natural minerals. The substitution of dissolved catalytic systems for solid catalysts allows for the diminishing of their deactivation in the processes of wood delignification and hydrolysis.     

Asymmetric α‐Amination of Aldehydes Catalyzed by PS‐Diphenylprolinol Silyl Ethers: Remediation of Catalyst Deactivation for Continuous Flow Operation

Polystyrene (PS)‐supported diphenylprolinol silyl ethers have been developed as highly active catalysts for the enantioselective α‐amination of aldehydes. Understanding the mechanism of catalyst deactivation has led to the development of reaction conditions notably extending catalyst life in repeated recycling (10 cycles; accumulated TON of 480) and has allowed the implementation of a continuous flow α‐amination process (6 min residence time, 8 h operation).     

Investigations of sulphur deactivation of NOx storage catalysts: influence of sulphur carrier and exposure conditions

The influence of SO₂, H₂S and COS in low concentrations on the deactivation of Pt/Rh/BaO/Al₂O₃ NO_x storage catalysts was investigated. Different samples of the catalyst were exposed to synthetic gas mixtures mimicking lean/rich engine cycling in a mixed lean application at 400 °C. The lean gas mixture contained 8 vol.% O₂, 500 vol-ppm C₃H₆ and 400 vol-ppm NO balanced to 100 vol.% with Ar. The rich excursions were performed by switching off the oxygen supply. Sulphur, 25 vol-ppm of either SO₂, H₂S or COS, was added to the gas flow either during the lean, the rich or both periods. This procedure aimed at investigating the influence of the exposure conditions and therefore the lean and rich periods were kept equally long (5 min). In addition, thermodynamical calculations for the prevailing conditions were performed.

It was concluded that all sulphur compounds investigated, i.e. SO₂, H₂S and COS, had similar, negative impact on the NO_x storage ability of the catalyst and that they all showed increased deactivation rates during rich exposure compared to lean. During lean exposure, all sulphur carriers showed similar behaviour, while H₂S and COS caused severe loss of noble metal activity during rich exposure.     

移动床甲醇制丙烯技术的工艺与工程

提出了一种新型的移动床甲醇制丙烯工艺（MMTP）并进行工程化分析。首先,根据反应器必须与催化剂的反应失活速率相匹配的原则,认为移动床是与失活速率处于中等水平的ZSM-5催化剂相匹配的最佳反应器。因此,MMTP工艺在充分发挥ZSM-5催化剂多产丙烯优势的基础上,结合移动床的中等循环速率进行反应-再生,从而实现催化剂高产丙烯的连续化与稳定化;其次,针对MTP反应的强放热特点,对比了流化床工艺的一步法操作与固定床工艺的两步法操作,发现MMTP工艺采用两步法操作既能合理地分配反应热,又能灵活地调控产品分布;最后,通过换热网络的优化设计与醇油共炼的工艺集成,使得MMTP工艺在节水节能的同时仍具有高丙烯收率。目前,在现有的MMTP工艺技术基础上,已经完成1万吨丙烯/年的工艺包设计,技术和经济的竞争优势十分明显,市场推广应用前景广阔。     

Effect of Aging Atmosphere on Thermal Sintering of Modern Commercial TWCs

The effect of aging atmosphere on the sintering behavior of commercial Pd and Rh catalysts as well as the TWC performance thereof has been investigated under straight oxidizing, reducing and periodic cycling aging conditions, in search of useful guidance in the optimum design of thermally durable TWCs for advanced gasoline engines equipped with the deceleration fuel cutoff technology. Pd and Rh catalysts individually exhibit an opposite trend in the thermal sintering behavior with respect to the aging atmosphere. Under oxidizing conditions, the Pd catalyst becomes more resistant to sintering in higher O₂ concentrations, whereas the Rh catalyst reveals the opposite behavior, regardless of the aging temperature. Physicochemical characterizations by using TGA and CO chemisorption have indicated that the state of Pd (PdO vs. Pd⁰) and Rh (Rh₂O₃ vs. Rh⁰) on the catalyst surface formed during the thermal aging plays an important role for determining the trend of thermal sintering; the positive effect of the O₂ concentration on Pd sintering is attributable primarily to the formation of PdO, while the main cause for severe deactivation of the Rh catalyst under oxidizing conditions is the diffusion of Rh₂O₃ into the support along with the agglomeration of Rh particles.     

Coking behavior and catalyst deactivation for catalytic pyrolysis of heavy oil

For the catalytic pyrolysis of heavy oil on catalyst CEP-1, coking behavior was investigated in a confined fluidized bed reactor. Coke content on the spent catalyst decreases with the increase of H/C mol ratio of feeds and catalyst-to-oil weight ratio, while it increases with the enhancement of reaction temperature. An empirical model is proposed to predict the coke content based on feed properties and operating conditions. The predicted coke content is close to the experimental data. The relationship between micro-activity index of catalysts and coke content is studied. A coking deactivation model for pyrolyzing catalysts is established, and then model parameters are determined by the least square regression analysis. According to the deactivation model, the variations of relative activity of catalysts with both residence time of catalysts and catalyst-to-oil weight ratio are predicted.     

MFI zeolite as a support for automotive catalysts with reduced Pt sintering

The noble metals used as catalysts in automotive exhaust systems are subject to sintering at extreme temperatures, leading to deterioration of catalytic activity. Zeolite with the MFI (ZSM5) structure is examined as a support for Pt particulate catalysts. The MFI structure is composed of agglomerates of single-crystal zeolite with interstitial mesoporosity. Pt fixed within these mesopores is shown through high-temperature aging tests in air to be highly resistant to sintering due to the mechanical constraints on particle size imparted by the mesoporous structure. The deterioration of catalytic activity after aging is significantly lower than that for comparable γ-alumina supported catalyst.     

Elaboration of an Accelerated Oven CNG Heavy Duty Vehicles Catalyst Ageing for Road Ageing Simulation

The study of automotive catalyst deactivation is associated with technical, economical and environmental problems. The deactivation phenomenon can be divided into three main groups: thermal, chemical and mechanical. Both fresh and road aged commercial metallic honeycombs were used to determine an ageing impact on catalytic activity. Accelerated catalyst ageing procedures were performed to simulate thermal ageing process observed during the real conditions (30,000 km of mileage). Different water quantities and times ageing were used to simulate accelerated ageing process. SEM, DRX, N₂ adsorption and TPR analyses were carried out to explain catalyst deactivation.     

Thermal sintering studies of an autothermal reforming catalyst

This paper describes a novel approach to life studies on catalysts used in non-isothermal reactors, using a single long-term experiment. Temperature dependence of catalyst aging is determined by comparing the activity reduction of portions of the catalyst from different sections of the reactor, subjected to different temperatures. Time dependence is determined by fitting the drift in catalyst temperatures to a time-dependent reaction rate via a thermodynamic reactor model. Experimentally, a monolithic autothermal reforming catalyst was subjected to thermally accelerated aging under reforming conditions in an adiabatic laboratory mini-flow reactor for 1000 h. Methane was used as the fuel. The axial temperature profile of the catalyst was monitored using thermocouples placed at various locations along the catalyst. A gradual change in temperature profile, with increasing temperatures due to decreasing steam-reforming activity, was observed. The aged monolith was cut up into short pieces centered on the thermocouple locations. The pieces, each aged at a different temperature due to its location, were tested individually for activity. The reduced activities were correlated with the aging temperature to obtain the temperature dependence of thermal sintering rates. A generalized power-law equation (GPLE) model for sintering was fit to the activity data. A plug flow reactor (PFR) model describing the reaction was built and the sintering kinetics were incorporated. The PFR model was used to predict changes in catalyst performance due to sintering under normal operating conditions. Thermal sintering deactivation for this catalyst was found to be within acceptable limits for commercial applications.     

Catalyst pore plugging effects on hydrocracking reactions in an Ebullated bed reactor operation

This paper analyzes the deactivation effects of NiMo/Al₂O₃ catalyst during the operation in an Ebullated bed reactor for Heavy residue hydrocracking. The spent catalysts were characterized by chemical analysis, ¹³C NMR, ESM, DRX, and by using thermal programmed oxidation and diffusion studies in a shallow bed micro-reactor. The deactivations were performed in a 5 l continuously stirred tank reactor, while the spent catalysts were tested in a 0.05 l micro-reactor. The study focused on determining the properties of the external layer of the catalyst and on evaluating the internal coke and metal deposition. The results indicated that initial deactivation is mainly due to coke depositions, while its impact on mass transfer reaction control depends on temperature. In long-term deactivation, the metal deposition plays a more important role in blocking the internal micro- and meso-structures and in building up the external layer of the pellets.     

Rapid Aging as a Key to Understand Deactivation of Ni/Al2O3 Catalysts Applied for the CO2 Methanation

Catalysis Letters - We developed a rapid aging method for Ni/Al2O3 methanation catalysts mimicking the real aging in the actual application. The method is based on hydrothermal deactivation of the...     

Simulation of transient processes of the catalytic synthesis of carbon nanotubes in a fluidized bed

A transient model of the catalytic synthesis of multilayer carbon nanotubes in a fluidized bed under the condition of a sharp increase in its volume during synthesis has been developed. The model is based on experimental kinetic data of the synthesis of multiwalled carbon nanotubes on metallic catalysts and takes into account the induction period of the catalyst activation, its deactivation during the synthesis, changes in the bed’s height, and the mass transfer of the gas phase components. Various modes of the periodic discharge of the product and loading of the catalyst into a reactor have been considered and, under these conditions, the dynamics of changes in the process characteristics over time have been studied. Optimal controlling parameters that allow one to achieve maximum yields from the synthesis, a high conversion level of the gaseous carbon source, and high purity of the product have been determined for cyclic operation of the reactor.     

Experimental comparison of biomass chars with other catalysts for tar reduction

In this paper the potential of using biomass char as a catalyst for tar reduction is discussed. Biomass char is compared with other known catalysts used for tar conversion. Model tar compounds, phenol and naphthalene, were used to test char and other catalysts. Tests were carried out in a fixed bed tubular reactor at a temperature range of 700–900 °C under atmospheric pressure and a gas residence time in the empty catalyst bed of 0.3 s. Biomass chars are compared with calcined dolomite, olivine, used fluid catalytic cracking (FCC) catalyst, biomass ash and commercial nickel catalyst. The conversion of naphthalene and phenol over these catalysts was carried out in the atmosphere of CO₂ and steam. At 900 °C, the conversion of phenol was dominated by thermal cracking whereas naphthalene conversion was dominated by catalytic conversion. Biomass chars gave the highest naphthalene conversion among the low cost catalysts used for tar removal. Further, biomass char is produced continuously during the gasification process, while the other catalysts undergo deactivation. A simple first order kinetic model is used to describe the naphthalene conversion with biomass char.     

Hysteresis in activity of microporous lean NOx catalysts in the presence of water vapor

Increasingly stringent ambient air quality standards coupled with the need to improve fuel economy has drawn significant attention to the search for emission control systems for lean burn engine vehicles. Much of the focus has been on zeolite-based catalysts for the conversion of NO_x to N₂ for automotive exhaust emission control. Under certain conditions, these catalysts are highly active catalysts for the reduction of NO_x using hydrocarbons as the reductant. However, many of these catalysts suffer from a variety of deactivation processes such as irreversible poisoning by SO_x or hydrothermal dealumination.

In addition to these deactivation processes, a recent focus of our research has been on the influence of water vapor on the activity of zeolite-based catalysts at low operating temperatures. We observe a hysteresis in catalytic activity of lean NO_x reduction (NO feed concentrations <100 ppm) upon increasing and decreasing temperature ramps at the low end of the operating window, that being from 100 to 300 °C using hydrocarbons as reductants. We describe these reversible influences of water vapor and the implications for this hysteresis in catalytic activity for the application of zeolite-based catalysts in lean NO_x catalysis, and compare these results to the instance of using ammonia as reductant.     

REACTOR MODELING OF AN INNOVATIVE LIQUID-SOLID CIRCULATING MOVING BED FOR THE SYNTHESIS OF LINEAR ALKYL BENZENES

The circulating moving bed is an innovative coupled reactor for producing linear alkyl benzenes. It can be used with shorter life span catalysts compared to traditional fixed bed reactors. A model is developed in which the effect of catalyst deactivation on the reaction is simulated using infinitesimal balance equations. The results of step changes between steady states and unsteady states have been calculated with the model.     

REACTOR MODELING OF AN INNOVATIVE LIQUID-SOLID CIRCULATING MOVING BED FOR THE SYNTHESIS OF LINEAR ALKYL BENZENES

The circulating moving bed is an innovative coupled reactor for producing linear alkyl benzenes. It can be used with shorter life span catalysts compared to traditional fixed bed reactors. A model is developed in which the effect of catalyst deactivation on the reaction is simulated using infinitesimal balance equations. The results of step changes between steady states and unsteady states have been calculated with the model.