ASTM_D_4641_-_12.pdf

Designation:D464112Standard 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 be begun at apoint on the isotherm near saturation 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 assumed to function independently ofeach other 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: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 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.Vp(i)=liquid volume contained in a group of poreshaving mean radius r p,mm3/g.vp=cumulative pore volume,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 May 1,2012.Published July 2012.Originally approvedin 1987.Last previous edition approved in 2006 as D464194(2006).DOI:10.1520/D4641-12.2For 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 States1 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 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 becomes exposed asdesorption proceeds.In principle,the computational procedurecan be applied to either the adsorption branch or desorptionbranch of the nitrogen isotherm.Unl