ASTM D4641-1994(2006) Standard Practice for Calculation of Pore Size Distributions of Catalysts from Nitrogen Desorption Isotherms《由氮解吸等温线计算催化剂孔隙尺寸分布的标准实施规程》.pdf
《ASTM D4641-1994(2006) Standard Practice for Calculation of Pore Size Distributions of Catalysts from Nitrogen Desorption Isotherms《由氮解吸等温线计算催化剂孔隙尺寸分布的标准实施规程》.pdf》由会员分享,可在线阅读,更多相关《ASTM D4641-1994(2006) Standard Practice for Calculation of Pore Size Distributions of Catalysts from Nitrogen Desorption Isotherms《由氮解吸等温线计算催化剂孔隙尺寸分布的标准实施规程》.pdf(6页珍藏版)》请在麦多课文档分享上搜索。
1、Designation: D 4641 94 (Reapproved 2006)Standard Practice forCalculation of Pore Size Distributions of Catalysts fromNitrogen Desorption Isotherms1This standard is issued under the fixed designation D 4641; the number immediately following the designation indicates the year oforiginal adoption or, i
2、n the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers the calculation of pore size distri-butions for catalysts an
3、d catalyst carriers from nitrogen des-orption isotherms. The computational procedure is particularlyuseful for determining how the pore volume is distributed incatalyst samples containing pores whose sizes range fromapproximately 1.5 to 100 nm (15 to 1000 ) in radius. It shouldbe used with caution w
4、hen applied to isotherms for samplescontaining pores both within this size range and pores largerthan 100 nm (1000 ) in radius. In such instances theisotherms rise steeply near P/Po= 1 and the total pore volumecannot be well defined. The calculations should be begun at apoint on the isotherm near sa
5、turation preferably in a regionnear P/Po= 0.99, establishing an upper limit on the pore sizedistribution range to be studied. Simplifications are necessaryregarding pore shape.Acylindrical pore model is assumed, andthe method treats the pores as non-intersecting, open-endedcapillaries which are assu
6、med to function independently ofeach other during the adsorption or desorption of nitrogen.NOTE 1This practice is designed primarily for manual computationand a few simplifications have been made for this purpose. For computercomputation, the simplified expressions may be replaced by exact expres-si
7、ons.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenc
8、ed Documents2.1 ASTM Standards:2D 3766 Terminology Relating to Catalysts and CatalysisD 4222 Test Method for Determination of NitrogenAdsorp-tion and Desorption Isotherms of Catalysts and CatalystCarriers by Static Volumetric Measurements3. Terminology3.1 DefinitionsConsult Terminology D 3766.3.2 Sy
9、mbols:P4(i) = pressure after equilibration during desorption,torr.P0(i) = liquid nitrogen vapor pressure, torr.Vde= see 12.4.10 and 12.5 in Test Method D 4222.rk(i) = radius of inner core calculated from Kelvinequation, .T = boiling point of nitrogen, K.VL= liquid nitrogen molar volume at T,cm3/mole
10、.g = liquid nitrogen surface tension at T, mN/m.T(i) = average thickness of the nitrogen film adsorbedon the pore walls, .rp(i) = radius of cylindrical pore given by rk(i)+ t (i),.Q = volume correction factor defined as ( rp/rk)2.DVT(i) = decrease in the amount of nitrogen adsorbedcaused by a loweri
11、ng in relative pressure,mm3/g.DVf(i) = volume of liquid nitrogen desorbed from porewalls during thinning of the film, mm3/g.DVk(i) = liquid volume of the inner core in which capil-lary condensation of the nitrogen occurs, mm3/g.DVp(i) = liquid volume contained in a group of poreshaving mean radius r
12、p,mm3/g.(Dvp= cumulative pore volume, mm3/g.DSp(i ) = area of the pore walls of a cylinder havingvolume DVp,m2/g.4. Summary of Practice4.1 The pore size distribution is determined by analyzingthe desorption data of the nitrogen isotherm. The nitrogenuptake is caused by the multilayer adsorption of a
13、 film ofnitrogen on the pore walls and by capillary condensation of thenitrogen in the “inner core” regions of the pores. The relative1This practice is under the jurisdiction of ASTM Committee D32 on Catalystsand is the direct responsibility of Subcommittee D32.01 on Physical-ChemicalProperties.Curr
14、ent edition approved Oct. 1, 2006. Published October 2006. Originallyapproved in 1987. Last previous edition approved in 1999 as D 464194(1999)e1.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards
15、 volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.pressure at which filling of the core occurs for a given pore sizeby capillary condensation is predi
16、cted from the Kelvin equa-tion (1).3During desorption, thinning of the multilayer filmadsorbed on the pore walls occurs in pores which havepreviously lost their capillary condensate. Corrections for filmthinning are determined by a procedure involving the surfacearea and radius of the film which bec
17、omes exposed asdesorption proceeds. In principle, the computational procedurecan be applied to either the adsorption branch or desorptionbranch of the nitrogen isotherm. Unless the presence ofink-bottle shaped pores is suggested by an abrupt closure of thedesorption branch on the adsorption branch,
18、the distributioncurve derived from the desorption data is preferred, and isdescribed in this procedure. The computational method isessentially the procedure developed by Barrett, Joyner, andHalenda (2) except for the incorporation of a few simplifica-tions.NOTE 2In cases where it has been establishe
19、d that the adsorptionbranch of the nitrogen isotherm is to be analyzed, the procedure proposedby Cranston and Inkley (3) is recommended.5. Significance and Use5.1 Pore volume distribution curves obtained from nitrogensorption isotherms provide one of the best means of character-izing the pore struct
20、ure in porous catalysts, provided that thelimitations of the method are kept in mind. Used in conjunctionwith the BET treatment for surface area determination (4),these methods provide an indispensable means for studying thestructure associated with pores usually important in catalysts.This practice
21、 is particularly useful in studying changes in aseries of closely related samples caused by treatments, such asheat, compression, or extrusion often used in catalyst manu-facturing. Pore volume distribution curves can often providevaluable information during mechanistic studies dealing withcatalyst
22、deactivation.6. Computational Procedure6.1 This procedure requires the use of a series of experi-mentally measured relative pressures P4(i)/P0(i) and thecorresponding quantities of nitrogen gas adsorbed Vde ex-pressed in units of cm3STP/g. The experimental data requiredin the use of this procedure c
23、an be measured by following thesteps outlined in Test Method D 4222. Inspect the nitrogensorption isotherm in the region above P/Po= 0.95. If the solidcontains no pores larger than 100 nm (1000 ) radius, theisotherm remains nearly horizontal over a range of P/Poapproaching unity and it is a simple m
24、atter to select a startingrelative pressure within this region, establishing an upper limiton the pore size range to be studied. If pores larger than 100 nm(1000 ) are present however, the isotherm rises rapidly nearP/PO= 1 and the total pore volume cannot be well defined.This limiting adsorption ca
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