第5章-气固界面-1吸附等温式.ppt

Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,BEA Confidential.|,*,第五章,气固界面,李硕琦,山东科技大学材料学院,2015.4,8 The gas-solid interfaces:adsorption;catalysis,8.1 Introduction,引言,A solid surface in contact with gas usually attracts an adsorbed layer of gas molecules.When adsorption occurs the process is spontaneous,which means the free energy change,G,is negative.In most cases there is also a decrease in entropy(,熵,),S,because the gas loses degrees of freedom.Hence,to ensue that,G is negative,the enthalpy(,焓,),，,H,，,negative and large enough to outweigh the entropy term:,固体表面和气体接触时会吸附一层,气体分子，如果该过程是自发的，,即意味着,G,是负的。,而此时,S,下降，所以，,H,一定为负，,而且超过,S,项。,G=,H-T,S,8.1 Introduction,引言,The adsorption of gases plays an important role in many processer,in particular,heterogeneous catalysis,（异相催化）,.,气体的吸附在很多进程中其重要作用，尤其是异相催化。,8.1 Introduction,引言,It is the weakening or breaking of chemical bonds that occurs upon adsorption that makes some solid surfaces effective catalysts,（催化剂）,for gas phase reaction.,吸附的同时发生化学键的减弱或者加强，导致某些固体成为气相反应的催化剂。,8.2 Physical and chemical adsorption,The adsorption of a gas(often called the adsorbate,吸附质,)on a solid surface(often called the adsorbent,吸附剂,)may involve only physical interactions due to van der Waals force,or there may be a chemical interaction between the solid and the gas.,吸附质和吸附剂之间可能存在范德华力的物理相互作用，或者化学相互作用。,Activated carbon is used as an adsorbent,8.2 Physical and chemical adsorption,These two processes are referred to respectively as,physical adsorption,and,chemical adsorption,.The distinction between them is very important as the adsorption processes and the behavior of the adsorbed layers are quite different.,物理吸附,化学吸附,Activated carbon is used as an adsorbent,8.2 Physical and chemical adsorption,Physical adsorption,because involves only the forces of molecular interaction,is very similar to the condensation of a vapor to form a liquid.,物理吸附只涉及分子间力，所以和气体的,凝聚,类似。,8.2 Physical and chemical adsorption,The enthalpy(,焓变,)of the physical adsorption is roughly(20 kJ/mol)the same as the enthalpy of condensation and in many ways the adsorbed materials,especially when many layers have been adsorbed,behaves like a two dimensional liquid.,物理吸附的焓变大约是,20 kJ/mol,，和凝聚的焓变相同。,8.2 Physical and chemical adsorption,In chemisorption,on the other hand,a chemical bond is formed.Consequently the enthalpies of the adsorption are much greater(80kJ/mol)than for physical adsorption.There are occasional exceptions(e.g.for the chemisorption of hydrogen on glass the enthalpy is only about 12 kJ/mol).,化学吸附存在化学键的形成，,其焓变大得多,(80kJ/mol),。,不过也有例外。,8.2 Physical and chemical adsorption,Furthermore,the adsorbed atoms are localized at particular sites on the solid surface and only one layer of adsorbate may be chemisorbed.,化学吸附的位点是特定的，吸附的层数只能为一。,Fig.3 Optimized structure of Pt 4/-Al 2 O 3 system:(a)side view;(b)top view.,Ryota Ishimoto,Changho Jung,Hideyuki Tsuboi,Michihisa Koyama,Akira Endou,Momoji Kubo,Carlos A.Del Ca.,Periodic density functional and tight-binding quantum chemical molecular dynamics study of catalytic properties on-Al2O3 supported Pt catalysts,Applied Catalysis A:General,Volume 305,Issue 1,2006,64-69,dx.doi.org/10.1016/j.apcata.2006.02.049,氧化铝表面的,P,4,催化剂,Fig.4 Calculation models of NO adsorption on Pt 4/-Al 2 O 3:(a)on-top site,(b)bridge site,and(c)three-fold site.,Ryota Ishimoto,Changho Jung,Hideyuki Tsuboi,Michihisa Koyama,Akira Endou,Momoji Kubo,Carlos A.Del Ca.,Periodic density functional and tight-binding quantum chemical molecular dynamics study of catalytic properties on-Al2O3 supported Pt catalysts,Applied Catalysis A:General,Volume 305,Issue 1,2006,64-69,dx.doi.org/10.1016/j.apcata.2006.02.049,NO,在,P,4,上的三种化学吸附的形式,“,The calculations for the supported system clarify that the,-Al2O3 support increases NO activation for three-fold sites,while it decreases it for on-top and bridge sites.,”,8.2 Physical and chemical adsorptionComparing physical and chemical adsorption,In comparing the characteristic of physical and chemical adsorption it is useful to use the analogies with condensation and chemical reaction.It will be seen that no single characteristic can be used to distinguish these two types of adsorption as there are often exceptions,so several characteristics should be examined.,8.2 Physical and chemical adsorptionComparing physical and chemical adsorption,Enthalpy of adsorption,吸附焓变,The enthalpy of physical adsorption is usually no greater than 20 kJ mol,-1,whereas the enthalpy of chemisorption is usually much greater than and is rarely less than 80 kJ mol,-1,.,物理吸附的焓变不超过,20 kJ mol,-1,，但是化学吸附的焓变很少低于,80 kJ mol,-1,。,8.2 Physical and chemical adsorptionComparing physical and chemical adsorption,Specificity,特异性,As chemical adsorption involves a chemical between gas molecules and groups on the surface of the solid it is highly specific.Physical adsorption,on the other hand,，,and occurs with any gas-solid combination under the appropriate conditions of temperature and pressure.,化学吸附因为存在化学反应而有特异性，但物理吸附则只要有合适的温度和压强即可发生。,8.2 Physical and chemical adsorptionComparing physical and chemical adsorption,Reversibility,可逆性,Physical adsorption can be reversed by simple reducing the gas pressure and usually without raising of the temperature.,物理吸附在降压甚至不用升温的条件下即可逆转。,8.2 Physical and chemical adsorptionComparing physical and chemical adsorption,Thickness of the adsorbed layer,吸附层的厚度,Chemical adsorption is limited to one adsorbed layer,but in physical adsorption there are usually several layers adsorbed.,化学吸附只有单层分子，但是物理吸附可以是多层吸附。,Physical adsorption on top of a chemisorbed layer is possible if the conditions are appropriate.,化学吸附层上的物理吸附是可以的。,8.2 Physical and chemical adsorptionComparing physical and chemical adsorption,Conditions of adsorption,吸附的条件,The experimental conditions favoring physical adsorption and chemical adsorption are similar to those favouring condensation and chemical reaction,respectively:for physical adsorption,low temperature,and pressures close to those for liquefaction;for chemisorption,high temperatures and a wide range of pressures.,8.3 Measurement of gas adsorption,气体吸附量的测量,Gas adsorption is usually measured by one of two broad methods:observing the decrease in the amount of gaseous adsorbate(,吸附质,)present after exposure of the gas to the solid,or the use of a sensitive microbalance to measure the increase in weight of the solid after exposure to the gas.,两种测量吸附量的方法：测定吸附前后吸附质量的减少，或用微天平测定吸附后吸附剂质量的增加。,8.3 Measurement of gas adsorption,气体吸附量的测量,In each case,the amount of gas adsorbed is plotted as a function of equilibrium gas pressure as(,吸附等温线,).,将气体吸附量和气压做曲线，得到的就是,吸附等温线,。,8.3*Measurement of gas adsorption,Routine,automated measurement of gas adsorption is usually performed with the help of a flow system,where the adsorbate gas is mixed with a diluent and changes in concentration of the gas mixture occur as the gas is adsorbed on the surface.,An example commonly used is a mixture of nitrogen in helium(,测量氮气浓度的变化,).,直读动态流动法比表面积分析仪,基于直读动态流动法比表面积分析仪是美国康塔仪器公司（,Quantachrome,）为快速比表面测定设计的直读动态比表面积分析仪，可在,6,分钟内完成比表面测试,8.3*Measurement of gas adsorption,以,He,作为载气，,N,2,为被吸附气体。当外界温度降低到氮气沸点温度,-195.8,即氮气的相变温度时，氮分子能量降低，在范德华力作用下被固体表面吸附，达到动态平衡，形成近似于单分子层的状态。,当混气中氮气的分压在,B.E.T,公式要求的,0.050.35,范围内时，固体对氮分子的吸附量与其总比表面积成线性关系，即可以用被吸附氮气量来定量表征固体的总表面积。由于氮分子直径相对于固体的各种物理表面结构及物理空隙形态都足够小，其能充分的布满及进入固体的各种物理结构形态中，所以能准确而全面的反映固体,表面积大小,。,被吸附氮气量的确定由热导池检测器及数据采集处理系统来完成。热导池检测器可以通过检测混合气体热导率的变化而确定混气中气体比例的变化。,8.3*Measurement of gas adsorption,被吸附氮气量的确定由热导池检测器及数据采集处理系统来完成。热导池检测器可以通过检测混合气体热导率的变化而确定混气中气体比例的变化。,气体组份,H,2,He,N,2,热导系数,Cal/cm,sec,10,5,39.7,33.6,5.66,8.3 Measurement of gas adsorption,3H-2000,型全自动氮吸附比表面积测试仪,8.3 Measurement of gas adsorption,仪器型号：,ASAP 2020,型,购置日期：,2009.8.3,国别厂家：美国,Micromeritics,公司,金额：,403710,元,8.3 Measurement of gas adsorption,AUTOSORB-1C,化学物理吸附分析仪,8.3 Measurement of gas adsorption,A microbalance is just a very sensitive weighing device,usually comprising a modern form of the classic beam balance(,杠杆式天平,).,Microbalance can be sensitive to weight difference of nanograms.With such extreme sensitivity it is possible to measure the weight change caused by the adsorption of a single monolayer on a solid if the surface area is large.,IGA,智能重量分析仪,Intelligent Gravimetric Analysers,Hiden,公司生产的,IGA,系列全自动重量分析系统代表着目前世界重量法吸附仪的最高水准。,Hiden,率先将电子微量天平引入全自动的吸附科学研究系统，在设计上不采用任何折中办法就能适应较宽范围材料的表征。,BEA Confidential.|,32,8.4 Adsorption isotherms(,吸附等温线,),博鲁瑙尔,匈牙利犹太人,1921,年赴美读书,二战时候中校军衔,Upon his death,the American Chemical Society called him,undoubtedly one of the most highly respected surface scientists of our time,.,横坐标：压强,纵坐标：吸附量,8.4 Adsorption isotherms,Often the pressure is referenced(,参考,)to the saturation pressure,p0,at the selected temperature.The saturation pressure is the gas pressure at which condensation would commence:in other words,the equilibrium vapour pressure of the liquefied(,液化,)at the selected temperature.,在选定温度下，一般压强会和饱和压强做比。饱和压强是冷凝开始的压强，或者可以说，是选定温度下蒸气开始液化的压强。,8.4 Adsorption isotherms,It is common to use the relative pressure,which is the ratio of the actual gas pressure to the saturation pressure(p/p0),as the independent variable in adsorption isothersms.,通常使用的就是相对压强。,Fig.1 A representative nitrogen isotherm for the charred dolomite sorbents investigated(1 h at 800 C).,A.Duffy,G.M.Walker,S.J.Allen,Investigations on the adsorption of acidic gases using activated dolomite,Chemical Engineering Journal,Volume 117,Issue 3,2006,239-244,dx.doi.org/10.1016/j.cej.2005.11.016,氮气在白云石吸附剂,上的吸附等温线,8.4 Adsorption isotherms,Understanding of adsorption at the gas-solid interface was greatly enhanced when Brunauer(1940)realized that the numerous isotherms that had been reported to that time could be classified by their shapes into five groups.These shapes are termed Type I,Type IIType V.,博鲁瑙尔发现所有吸附等温线一共分为五类。,8.4 Adsorption isotherms(,吸附等温线,),Fig.1 The,IUPAC classification,for adsorption isotherms(Types IVI)with the,x,-axis representing the relative pressure of the fluid and the,y,-axis denoting amount of gas adsorbed.,Onuma Carmody,Ray Frost,Yunfei Xi,Serge Kokot,Surface characterisation of selected sorbent materials for common hydrocarbon fuels,Surface Science,Volume 601,Issue 9,2007,2066-2076,dx.doi.org/10.1016/j.susc.2007.03.004,8.4 Adsorption isotherms,The type I isotherms are formed when the gas is chemisorbed and limited to one layer of coverage.,I,型的吸附等温线对应的化学吸附的单层情况。,Fig.8 Adsorption isotherms of CO 2 on H-STI at various temperatures.,Jun Li,Jin Qiu,Yaojun Sun,Yingcai Long,龙英才,Studies on natural STI zeolite:modification,structure,adsorption and catalysis,Microporous and Mesoporous Materials,Volume 37,Issue 3,2000,365-378,dx.doi.org/10.1016/S1387-1811(99)00282-6,CO2,在,H-HSI,沸石上的,吸附等温线,8.4 Adsorption isotherms,The other four types are considered to derive from physical adsorption.,其他,4,种类型包含物理吸附。,8.4 Adsorption isotherms,Type II and IV show steep rises in adsorption at low gas pressures suggesting a high affinity between gas and solid,whereas Tyep III and V exhibit low adsorption at low gas pressures indicating relatively weak affinity between gas and solid.,II,型,IV,型在低压时上升很快，说明气体和固体之间吸附较强。,III,型,V,型在低压时吸附量低，说明气体和固体之间吸附较小。,Fig.1 Idealized adsorption isotherms.These five isotherm types are representative of those experimentally obtained for various gases and adsorbents and published in the literature.,V,ads is the amount of gas adsorbed at a given.,Gordon L Jendrasiak,Ralph L Smith,The effect of the choline head group on phospholipid hydration,Chemistry and Physics of Lipids,Volume 113,Issues 12,2001,55-66,dx.doi.org/10.1016/S0009-3084(01)00174-8,吸附强,吸附弱,8.4 Adsorption isotherms,At very low gas pressures all of the isotherms types approximate to a straight line.As this behavior is reminiscent(/reminisnt/,怀旧的,)of Henrys law(solubility of a gas is proportional to its pressure)this section of the isotherms is often called the Henrys law region.,亨利定律区域,This figure shows a series of isotherms for various gases at 25C.Except for CO,2,all the gases remain in the Henrys Law regime up to 13 bar.,8.4 Adsorption isotherms,对吸附等温线的研究的落脚点，是对催化剂（一般是多孔材料）对气体吸附性能和比表面积性质的研究，最终找到合适的、有效的催化剂。,Fig.2 Surface SEM images of PVDFPEO/,x,%ZSM-5 composite microporous membranes:(a),x,=5;(b),x,=10;(c),x,=20;(d),x,=30.,Solid State Ionics,Volume 177,Issues 78,2006,709-713,Fig.1 TEM images of(a)calcined SBA-15 recorded along the 0 0 1,zone axis and(b)calcined(3%)Fe/SBA-15 recorded along the 1 1 0 zone axis.,Microporous and Mesoporous Materials,Volume 86,Issue,s 13,2005,335-340,8.5 Adsorption equations,吸附等温式,There have been a number of attempts to derive an equation that will fit the experimental adsorption isotherms and several of these have been reasonably successful.However,the ability of an equation to fit the shape of an experimental isotherm does not provide a sensitive test for the model on which the equation was based.,（下一步的努力就是用数学公式为吸附等温线建模）,A more stringent(,严格的,)test is to compare the enthalpy of adsorption(,吸附焓变,)deduced from,the isotherms using the equation with the value measured calorimetrically.,（如果能用公式计算处吸附焓变，则更为严格。）,8.5 Adsorption equations,Single layer adsorption:Langmuir(1918),Multilayer adsorption:Brunauer,Emmett and Teller(1938)(,从右到左,),8.5 Adsorption equations The Langmuir equation,朗格缪尔吸附等温式,The Langmuir equation is based on a chemisorption model in that the adsorption is limited to a single adsorbed monolayer on the solid surface,（单层化学吸附层）,.The equation is:,Where n,is the amount adsorbed,n,m,is the amount required for complete monolayer coverage of the surface(the monolayer capacity),and,is defined by:,E1 enthalpy of desorption,解吸附焓变,8.5 Adsorption equations,The equation can be rearranged into a linear form:,Thus,by plotting experimental values of p/n,against p a straight line should be obtained from which the values of,and n,m,(,斜率,),the monolayer capacity,can be calculated.,通过,p/n,和,p,可做直线，从而可以得到,of,和,n,m,最终可以得到,单分子层的容量,。,8.5 Adsorption equations,Application of the Langmuir equation,Plots of the Langmuir equation are shown in Figure 8.5,It is clear that the equation generates curves that fit the Type I shape and that the adsorption reaches a limit when one complete monolayer has been adsorbed.,8.5 Adsorption equations,Increasing in the value of,and hence in the value of E1,show increasing steepness in the isotherms at low pressures reflecting increasing affinity between adsorbate and adsorbent.,8.5 Adsorption equations,For data that fit the linear form of the equation(8.3)values of the monolayer capacity can readily be obtained.If the size of the adsorbate molecules is known the value of n,m yields an estimate of the surface area of the solid.,利用单分子层容量和吸附质分子的体积计算固体的表面积。,BEA Confidential.|,54,BEA Confidential.|,55,BEA Confidential.|,56,SEM,图片,BEA Confidential.|,57,BEA Confidential.|,58,BEA Confidential.|,59,BEA Confidential.|,60,BEA Confidential.|,61,BEA Confidential.|,62,BEA Confidential.|,63,吸附等温线,BEA Confidential.|,64,直线形式,BEA Confidential.|,65,8.5 Adsorption equations,The BET equation,Th