Lecture 5 Solid Catalysts - NPTEL
NPTEL – Chemical Engineering – Catalyst Science and TechnologyLecture 5Solid catalystsCatalyst componentsA solid catalyst consists of mainly three components :1. Catalytic agent2. Support /carrier3. Promoters and InhibitorsCatalytic agent:These are the catalytically active component in the catalyst. These components generatethe active sites that participate in the chemical reaction. Activity of any catalyst isproportional to the concentration of these active sites. Though concentration of the activesites depends on the amount of catalytically active component, however, it is not alwaysdirectly proportional. Availability of active sites depends mainly on the dispersion ofcatalytic agent. The dispersion is defined as ratio of total number of exposedatoms/molecules of catalytic agent available for reaction to total number ofatoms/molecules of catalytic agent present in the catalyst sample.Catalytic agents may be broadly divided in the following categories:i. Metallic conductors ( e.g Fe, Pt, Ag, etc.)ii. Semiconductors (e.g. NiO, ZnO,etc.)iii. Insulators (e.g. Al2O3, SiO2,MgO etc.)Metallic conductors: The metals that have strong electronic interaction with theadsorbates are included in this category.The metals are used in various catalyticreactions such as methanol synthesis, oxidation , hydrogenation and dehydrogenationprocesses.Joint initiative of IITs and IISc – Funded by MHRDPage 1 of 254
NPTEL – Chemical Engineering – Catalyst Science and TechnologyExamples of metal catalysts :Cu for water gas shift reaction and methanol synthesis ; Ag for oxidation of ethylene toethylene oxide,Au for oxidation of methanol to formaldehyde; Fefor ammoniasynthesis; Pd and Pt for hydrogenation of olefins, dienes, aniline or nitriles as well asdehydrogenation of alkanes, alcohols, cyclohexanes, cyclohexanols etc.Semiconductors :The oxides and sulfides of transition metals that have catalytic activity are included inthis category. Similar to conducting metals, they are also capable of electronic interactionwith adsorbed species and catalyze the same type of reactions. Usually the lower valenceband electrons participate in bonding. The upper conduction band separated by band gapenergy is empty unless electrons are promoted by heat or radiation. Semiconductorcharacteristics may be intrinsic or induced by addition of foreign ion, creating cationic oranionic vacancies. Common transition oxides and sulfides such as CuO, AgO, NiO CoO,Fe2O3 , MnO, Cr2O3, FeS, V2O5 show conductivity. These materials participate incatalytic reactions and reaction occurs through acceptation or donation of electronsbetween the reactant material and catalysts. Few applications of semiconductor catalystsare : CuO for oxidation of nitric oxides, NiO for dehydrogenation of alkanes, MnO2 foroxidation of alcohols, and V2O5 for oxidation of hydrocarbons.Insulators : Catalytic functions of insulators are different from that of conductor andsemi conductor materials. Insulators have large values of band gap energy and very lowconcentration of impurity levels. The electrons remain localized in valence bonds andredox type reactions involving electronic interaction as observed for metal orsemiconductor catalysts does not occur. However, insulators have sites that generateprotons, thereby, promote carbonium ion based reactions such as cracking, isomerizationor polymerization. Al2O3, SiO2, SiO2-Al2O3, zeolites, MgO, CaO, MgAl2O4, SiO-MgOare few examples of the insulators used as catalysts.Joint initiative of IITs and IISc – Funded by MHRDPage 2 of 254
NPTEL – Chemical Engineering – Catalyst Science and TechnologySupport or carrierSupport or carrier provides large surface area for dispersion of small amount ofcatalytically active agent. This is particularly important when expensive metals, such asplatinum, ruthenium, palladium or silver are used as the active agent. Supports give thecatalysts its physical form, texture, mechanical resistance and certain activity particularlyfor bifunctional catalysts. Area of the support can range from 1 - 1000 m2/gm. Commonsupports are alumina, silica, silica-alumina, molecular sieves etc. The surface area of α alumina is in the range 1-10 m2/gm whereas the surface area for γ or η - alumina can bein the range 100 – 300 m2/gm.Support may be inert or interact with the active component. This interaction may result inchange in surface structure of the active agent and thereby affect the catalyst activity andselectivity. The support may also exhibit ability to adsorb reactant and contribute to thereaction process.Promoters :Promoters are generally defined as substances added during preparation of catalysts thatimprove the activity or selectivity or stabilize the catalytic agents. The promoter ispresent in a small amount and by itself has little or no activity.Promoters are termed as physical or chemical promoter depending on the manner theyimprove the catalyst performance.The additives that maintain physical integrity of the support and/or deposited catalyticagents are termed as physical promoters. For example, addition of small quantities ofalumina to an iron catalyst employed in ammonia synthesis prevents sintering of the ironcrystallites. Thus, for this catalyst, alumina is a physical promoter. The addition of K2Oto the same catalyst increases the intrinsic activity of the iron crystallites and thereforeacts as a chemical promoter. The promoter can be added during catalyst preparation orduring reaction.Joint initiative of IITs and IISc – Funded by MHRDPage 3 of 254
NPTEL – Chemical Engineering – Catalyst Science and TechnologyNegative promoters or inhibitors: Inhibitors act opposite to promoters. When added insmall amounts, these can reduce catalyst activity, selectivity or stability. Inhibitor isparticularly useful for reducing the activity of a catalyst for undesirable side reactions. Inoxidation of ethylene, ethylene dichloride is added to inhibit CO2 formation thus acting asan inhibitor.Industrial catalystsIndustrial catalysts can be broadly grouped into three categories:1.Bulk catalysts : When the entire catalyst consists of the catalytically activesubstance ,then the solid catalyst is called a bulk catalyst. Examples include silicaalumina catalysts for catalytic cracking; iron- molybdate for oxidation of methanolto formaldehyde; iron doped with alumina and potassium oxide for the synthesis ofammonia.2.Supported catalysts: In supported catalysts, the catalytically active materials aredispersed over the high surface area support material. For example,hydrodesulphurization is carried out over molybdenum oxide supported onalumina.3.Mixed agglomerates : These catalysts are agglomerated mixture of active substanceand support. These type of catalysts are used less frequently.Joint initiative of IITs and IISc – Funded by MHRDPage 4 of 254
NPTEL – Chemical Engineering – Catalyst Science and TechnologyPrecipitation or gel formation from starting materialsDecantation/ filtrationWashingDryingCrushing & grindingFormingCalcinationImpregnationFinal ActivationFig. 1. Basic unit operations in solid catalyst preparationJoint initiative of IITs and IISc – Funded by MHRDPage 5 of 254
NPTEL – Chemical Engineering – Catalyst Science and TechnologyPreparation of solid catalystThe catalyst preparation methods can broadly categorized as follows :1.Bulk preparation process:Bulk catalysts and supports are prepared by this method. Bulk preparation ismainly done by the following methods :a. Precipitation processb.2.Sol gel processImpregnation process:Supports are first prepared by bulk preparation methods and then impregnated withthe catalytically active material. The active materials can be deposited on thesupports by various methods. Most of the methods involve aqueous solutions andliquid solid interface. In some cases, deposition is done from the gas phase andinvolves gas- solid interface.3.Physical mixing :Mixed agglomerated catalysts are prepared by this method. These catalysts areprepared by physically mixing the active substances with a powdered support orprecursors of support in ball mill. The final mixture is then agglomerated andactivated.Basic unit operations involved in preparation of solid catalyst is shown in Fig 1. Eachstep is discussed in detail in the following sections.Book References : J.J. Carberry , Chemical and catalytic reaction engineering, Dover Publications,2001 G. Ertl, H. Knozinger & J. Weitkamp, Handbook of Heterogeneous Catalysis, Vol1, Wiley – VCH, 1997 R. J. Farrauto & C. H. Bartholomew, Fundamentals of Industrial Catalytic Processes,Blackie Academic & Professional, 1997 J.T. Richardson, Principle of catalysts development, Plenum Press, 1989Joint initiative of IITs and IISc – Funded by MHRDPage 6 of 254
NPTEL – Chemical Engineering – Catalyst Science and TechnologyLecture 6Precipitation and co-precipitationIn this process, the desired component is precipitated from the solution. Co precipitationis used for simultaneous precipitation of more than one component. Catalysts based onmore than one component can be prepared easily by co-precipitation. The precipitationprocess is used for preparation of bulk catalysts and support material such as Al2O3, SiO2,TiO2, ZrO2 etc.ProcessIn general, the metal hydroxides are precipitated from their precursor salt solutionbecause of their low solubility. The precipitation of hydroxides can be performed eitherby starting from an alkaline solution which is acidified or from acidic solution by raisingthe pH. However, most hydroxides for technical application are precipitated from anacidic solution by the addition of an alkaline precipitating agent. Usually, ammonia orsodium bicarbonate is used as the precipitating agent. Highly soluble inorganic salts suchas nitrates, carbonates or chlorides are generally used as metal precursors. For example,preparation of alumina is done by precipitating aluminium hydroxide from aluminiumnitrate solution by addition of ammonium hydroxide.Al ( NO3 )3 NH 4 OH Al(OH)3 NH 4 NO3During precipitation, several processes occur and the major steps are :1. liquid mixing / supersaturation2. nucleation3. crystal growth to form primary products4. aggregation of the primary particlesJoint initiative of IITs and IISc – Funded by MHRDPage 7 of 254
NPTEL – Chemical Engineering – Catalyst Science and TechnologyInitial mixing or interdispersing of components in the solution has a significant effect onthe precipitation. Good mixing result in a more homogeneous product particularly in caseof co- precipitation. Rate of stirring primarily affects the nucleation whereas growth rateis much less influenced by this factor. Stirring rate also affect the aggregation. Aggregatesize can be influenced by changing the stirring rate and the manner of mixing.Fig 1. Parameters affecting supersaturationFor nucleation to occur the solution must be super saturated with respect to thecomponents which is to be precipitated. Parameters affecting supersaturation is shown inFig. 1. In supersaturated region the system is unstable and precipitation occurs with anysmall disturbance. The supersaturaton region is approached either by increasing theconcentration through evaporation, lowering the temperature or by increasing pH. Thesolubility of a component increases with temperature as shown in Fig. 1. The solubilitycurve is also function of pH. As pH increases solubility decrease and curve shift from 1to position 2. Then the point which was initially in solution region becomes insupersatured region. The increase in pH is the most convenient method for precipitation.The reaction during precipitation, M n nOH M(OH)n , is controlled by increasing thepH through addition of a basic solution. Hence by raising the pH value of a solution byaddition of alkaline or ammonium hydroxide the corresponding metallic hydroxidecompounds can be made insoluble and precipitated from solution. Commonly usedJoint initiative of IITs and IISc – Funded by MHRDPage 8 of 254
NPTEL – Chemical Engineering – Catalyst Science and Technologyreagents are NaOH, KOH, NH4OH, carbonates and bicarbonates. Particles withinsupersaturated region develop in two steps : nucleation and growth.Nucleation may proceed spontaneously through the formation of M(OH)n entities or beinitiated with seed materials such as dust , particle fragments, roughness of vesselssurface. Addition of seed material enhances rate of nucleation. The nucleus is defined asthe smallest solid phase aggregate of atoms, molecule or ions which is formed duringprecipitation and which is capable of spontaneous growth. In super saturated solutionwhen the concentration exceeds a critical threshold value, a nucleus will form and theprecipitation will begin. As long as the concentration of the species stays above thenucleation threshold, new particles are formed. Nucleation starts with the formation ofclusters which are capable of spontaneous growth by the subsequent addition ofmonomers until a critical size is reached. Clusters, smaller than the critical size, tend tore–dissolve, while larger clusters continue to grow. As soon as the concentration fallsbelow the critical concentration due to consumption of the precursors by nucleation or bythe growth process, only growth of existing particles continues. Growth proceed throughadsorption of ions on surface of seeded particle. This growth is a function ofconcentration, temperature and pH. Rates of nucleation and growth can be independentlycontrolled. If nucleation is faster than growth, the system produces a narrow distributionof small particles. Fast growth results in narrow distribution of large particles.Several equations are proposed for nucleation rate and the most commonly used is : 16πσ 3ν 2 dN β exp 32dt 3 ( kT ) ln s where β is the pre-exponential term, σ is solid –fluid interfacial energy, υ is solidmolecular volume and T is the temperature. The super saturation ‘s’ is defined as the ratioof actual concentration to solubility; s The equation can be simplified asactual concentrationsolubilitydN A β exp 2 dt ln s Joint initiative of IITs and IISc – Funded by MHRDA 16πσ 3ν 23 ( kT )3Page 9 of 254
NPTEL – Chemical Engineering – Catalyst Science and TechnologyThus, nucleation strongly depends on the concentration as well as temperature. There is acritical super saturation concentration below which nucleation is very slow and abovewhich nucleation is very fast.There are several mechanisms of crystal growth and most of these lead to the simpleequation of growth rate, G k ( c ceq ) , where ‘k’ is the kinetic coefficient, ‘c’ is thenactual concentration and‘ceq’ is the equilibrium concentration. The value of exponent‘n’ lies in the range of 1 to 2 and often close to 1.Hence, the dependency of the crystallite growth rate on concentration is closer to a linearfunction while nucleation rate increases exponentially with concentration. Therefore,high super-saturation level promotes nucleation rather than crystal growth and favor theprecipitation of highly dispersed materials. In contrast, precipitation from a more dilutesolution tends to produce fewer but larger crystals.Apart from nucleation and crystal growth,aggregation is also an important step.Aggregation leads to fewer and larger but yet porous particles. It is the formation ofclusters of nano-scale primary particles into micrometer scale secondary particles.Physical and chemical forces can hold these particles together. Porosity is thendetermined by how the particles are stacked and the pores are considered as void spacesbetween the primary particles. Because of very high super-saturation during theprecipitation of most base metal hydroxides or carbonates , nucleation is spontaneous.Process variationPrecipitation process can be carried out in different ways. The process can be carried outeither in batch mode or in continuous mode. The other process variation that affects theprecipitate properties is the sequence of addition of the starting materials.In a batch process, the salt solution from which the metal hydroxide is to beprecipitated is taken in a vessel and the precipitating agent is added. The advantage ofthis method is its simplicity.However, variation of batch composition duringprecipitation process is a major limitation. This can lead to differences in the propertiesof the precipitate formed in the initial and final stages. The continuous process involvesJoint initiative of IITs and IISc – Funded by MHRDPage 10 of 254
NPTEL – Chemical Engineering – Catalyst Science and Technologycontinual simultaneous addition of salt solution and precipitating agent to a vessel withsimultaneous withdrawal of precipitate. This process has a higher demand on processcontrol. All the parameters (pH, temperature, concentration, residence time) can becontrolled as desired.The order of addition of starting materials also affects the final properties of theprecipitated catalysts. Different schemes of addition of starting materials in precipitationprocess is shown in Fig. 2.When metal solution is added to the precipitating agent, theproduct tends to be homogeneous since the precipitating agent is present in large excess.This process is particularly important in co-precipitation as it give more homogeneousproduct than the process where the precipitating agent is added to a mixed metal solution.In the latter case, the hydroxide with lower solubility tends to precipitate first, resulting information of non-homogeneous product. Simultaneous addition of both reagents to abuffer solution of constant pH results in better homogeneity and process control. In thisprocess, ratio of metal salt and precipitating agent can be controlled. However, product atthe start and at the end may vary due to change in concentration of other ions that are notprecipitated. These counter ions tend to occlude in larger extent in final products. Agingis also longer for final products. Aging represent time of formation of coprecipitated andits separation from solution. Aging results in change in structure and properties ofhydroxide network. Aging leads to more crosslinked network.Buffer solutionFig. 2. Different schemes of addition of starting materials in precipitation processJoint initiative of IITs and IISc – Funded by MHRDPage 11 of 254
NPTEL – Chemical Engineering – Catalyst Science and TechnologyAdvantages and disadvantages: The main advantage of the precipitation process is thepossibility of creating pure and homogenous material. However, the major disadvantagesinclude necessity of product separation after precipitation and generation of the largevolume of salt containing solutions. There is also difficulty in maintaining a constantproduct quality throughout the whole precipitation process if the precipitation is carriedout discontinuously.Process parametersIn addition to the process variations discussed above there are many other parameters thataffect the final product properties as shown in Fig.3. The properties of the final productthat are affected include phase formation, chemical composition, purity, particle size,surface area, pore size and pore volume. It is necessary to optimize the parameters toproduce the desired product.TemperatureRaw materialsAdditivespHPrecipitate properties(phase, composition, purity,particle size, surface area,pore sizeSolventCompositionConcentrationFig. 3. Parameters affecting the properties of the precipitateEffect of raw materials : The precursors are usually chosen with counter ions that caneasily be decomposed to volatile products during heat treatment steps. Nitrates andcarbonate salts are preferably used as metal precursors whereas ammonia or sodiumcarbonate as the precipitating agent. Chloride and sulphate ions act as poisons in manycatalytic processes. Such ions should be avoided in the precipitation process. However, ifthe precipitation is needed to be carried out in the presence of these ions then rep
n entities or be initiated with seed materials such as dust , particle fragments, roughness of vessels surface. Addition of seed material enhances rate of nucleation. The nucleus is defined as the smallest solid phase aggregate of atoms, molecule or ions which is formed during precipit
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cases the catalyst is used in a filter or in an air purification system of some type. The CARULITE 200 granular catalysts are used to convert ozone emitted from water off-gas operations, corona treaters or digital printers, back to oxygen. The CARULITE 300 catalysts granular catalysts are used to convert poisonous carbon monoxide
