ASTM_D_4641_-_17.pdf

Designation:D464117Standard Practice forCalculation of Pore Size Distributions of Catalysts andCatalyst Carriers from Nitrogen Desorption Isotherms1This standard is issued under the fixed designation D4641;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon()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 and 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 when 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 begin at a pointon the isotherm near saturation preferably in a region near P/Po=0.99,establishing an upper limit on the pore size distributionrange to be studied.Simplifications are necessary regardingpore shape.A cylindrical pore model is assumed,and themethod treats the pores as non-intersecting,open-ended capil-laries which are assumed to function independently of eachother during the adsorption or desorption of nitrogen.NOTE1This 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-sions.1.2 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.1.3 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.Referenced Documents2.1 ASTM Standards:2D3766 Terminology Relating to Catalysts and CatalysisD4222 Test Method for Determination of Nitrogen Adsorp-tion and Desorption Isotherms of Catalysts and CatalystCarriers by Static Volumetric Measurements3.Terminology3.1 DefinitionsConsult Terminology D3766.3.2 Symbols:i=numerical index representing each successivedata point,with i=1,2 n.P4(i)=pressure after equilibration during desorption,torr.P0(i)=liquid nitrogen vapor pressure,torr.Vde=Quantity of gas desorbed(cm3 STP/g);see12.4.10 and 12.5 in Test Method D4222.rk(i)=radius of inner core calculated from Kelvinequation,.T=boiling point of nitrogen,K.VL=liquid nitrogen molar volume at T,cm3/mole.=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(r p/r k)2.VT(i)=decrease in the amount of nitrogen adsorbedcaused by a lowering in relative pressure,mm3/g.Vf(i)=volume of liquid nitrogen desorbed from porewalls during thinning of the film,mm3/g.Vk(i)=liquid volume of the inner core in which capillarycondensation of the nitrogen occurs,mm3/g.1This practice is under the jurisdiction of ASTM Committee D32 on Catalystsand is the direct responsibility of Subcommittee D32.01 on Physical-ChemicalProperties.Current edition approved Feb.1,2017.Published February 2017.Originallyapproved in 1987.Last previous edition approved in 2012 as D464112.DOI:10.1520/D4641-17.2For referenced ASTM standards,visit the ASTM website,www.astm.org,orcontact ASTM Customer Service at serviceastm.org.For Annual Book of ASTMStandards volume information,refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards,Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade(TBT)Committee.1 Vp(i)=liquid volume contained in a group of poreshaving mean radius r p,mm3/g.vp=cumulative pore volume,mm3/g.Sp(i)=area of the pore walls of a cylinder having volume Vp,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 film ofnitrogen on the pore walls and by capillary condensation of thenitrogen in the“inner core”regions of the pores.The relativepressure at which filling of the core occurs for a given pore sizeby capillary condensation is predicted from the Kelvin equa-tion(1).3During desorpt