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Routine Analysis of the Pore Structure of Mesoporous and Macroporous Solids by Gas Adsorption, Mercury Porosimetry and Densitometry
K.K. Unger et al. (Editors), Characterization of Porous Solids 1988 Elsevier Science Publishers B.\'.. Amsterdam - Printed in The Netherlands
491
ROUTINE ANALYSIS OF THE PORE STRUCTURE OF MESOPOROUS AND MACROPOROUS SOLIDS BY
GAS ADSORPTION, MERCURY POROSIMETRY AND DENSITOMETRY
M Day, R Fletcher ICI Chemicals and Polymers Group Research and Technology Department Wilton Cleveland UK
ABSTRACT This paper describes a study of a range of porous solids using nitrogen adsorption/desorption at 77°K, mercury intrusion/extrusion porosimetry and densitometry in helium and mercury. Qualitative evidence from the techniques used indicated that the materials were representative of classes of pore structures of differing complexity across the spectrum of meso-macropore structures. Since commercial automated instruments and procedures are now widely used routinely in pore structural characterization, the materials offer a means of evaluating the validity 6f the analytical techniques as a function of the complexity of the pore structure. It appears that only materials consisting exclusively of mesopores can sustain quantitative analysis by these methods. When macropores are present the analyses are less reliable and less consistent.
INTRODUCTION The advent of commercial automated instruments for measuring nitrogen adsorption/desorption isotherms and mercury intrusion/extrusion, with dedicated computer faCilities for transforming data into pore size distribution, means that quantitative descriptions of the pore structures of solids are easily generated, but must be used with caution. The follOWing recommendations are contained in the IUPAC guidelines (1) recently issued on the assessment of pore structure from nitrogen adsorption/desorption data. Assess microporosity via t or o<.s methods (2, 3). 2
Assess the likely formal shape of the pores in the structure via inspection and classification of isotherm shape and hysteresis loop: check for loop closure at p/Po~O.4.
3
Apply pore size distribution transforms only in cases where there are no micropores, where pores are rigid in well-defined, mesopore r3nges.
Mercury porosimetr) is an important complementary technique, but formally agreed procedures for its use and interpretation are less developed.
492
This paper sets out to investigate the problems associated with the derivation of descriptions of pore structures from nitrogen and mercury techniques using a series of porour solids which could represent classes in a spectrum of increasing structural complexity. 2
THEORY The theories of nitrogen adsorption/desorption and mercury porosimetry, and the models and algebra for deriving pore size distributions are well known (4, 5). In the case of nitrogen data, the derivation will be suspect if: a)
The pores are not rigid, eg located between small plates.
b)
Isotherm shape is affected by the presence of micropores: closure of hysteresis loop at p/Po-",0.4 is indicative.
c)
The pore sizes cover a wide enough range to render the Kelvin equation imprecise.
On these criteria the most reliable derivation will emerge from solids with type IV isotherms, HI hysteresis loops, representing sharp mono-modal mesoporous structures with no micropores. Qualifications concerning the interpretation of mercury intrusion data are as follows:a)
Mercury must not react or interact strongly with any part of the pore structure.
b)
The solid must not break under the pressures employed.
c)
The shape of the pores eg ink bottle shapes, can affect the interpretation of intrusion and extrusion.
3
EXPERIMENTAL
3.1
APPARATUS Nitrogen adsorption/desorption measurements were carried out wi~h a Micromeritics Inc 'Digisorb 2500' uSing 5-point BET plots and 43 point adsorption/desorption isotherms. 0~4gassing was c:rried out o;-the instrument under moderate vacuum 10 torr, at 140 C for 4 hours. The isotherm data were output to a plotter for inspection and classification and were transformed into o(s plots against an alumina standard. Pore size distributions can be derived from the software incorporated into the instrument, uSing the method of Robert (6). Mercury intrusion was carried out using a Micromeritics Inc 'Autopore 9200' operating from 2 to 60,000 psia and hence in theory able to penetrate pores as small as 36A o in diameter. Outgaasing on !~e instrument is limited to evacuation at room temperature to 10 torr. He-Hg densitometry was used as a measure of total pore volume against which pore volumes derived from N2 and mercury could be compared.
3.2
MATERIALS These were stable alumina-based materials derived from our laboratory, and represent a larger body of materials consisting of six from sets 1, 3 and three from set 2. The range of their micromeritic properties was as follows:
493
SAMPLE SET
PORE VOLUME (He/Hg) ml/g
SURFACE AREA ~GE REPRESENTED m / g
0.5 - 0.8
100 - 250
2
0.2 - 0.5
50 - 150
3
0.3 - 0.6
100 - 350
All were pellets or granules not less than lmm in size, and had been extensively dried at 110°C in air before introduction into instrumental outgassing facilities.
4
RESULTS AND DISCUSSION Figs 1-3 A, B compare the nitrogen adsorption/desorption isotherms and mercury intrusion/extrusion data for sets 1, 2, 3. The data have been normalized because they are representative of numerous individual sets of data. Nitrogen adsorption isotherms were transformed into s plots against an alumina standard; no detectable microporosity was observed in them. The software contained in each instrument can be used to derive pore size distributions from nitrogen adsorption/desorption and mercury intrusion. These derivations and the agreement between N2 and mercury-derived pore sizes and volumes are reported in Table 1. In addition,_the retention of mercury after 1st cycle intrusion and extrusion, which may be specific to the mode of operation of our porosimeter, was noted. The isotherms of set 1 are assigned type IV with HI hysteresis loops according to IUPAC recommendations. Isotherms in set 2 are more complex, clearly bimodal and are assigned as the superposition of type IV isotherms, but there is a slight upturn as p/Po -->1, indicating that pore structures are more complex than indicated by the type IV assignment. Comparison of Figs 1-3 indicates some qualitative parallelism between hysteresis in nitrogen adsorption/desorption and mercury intrusion/ extrusion. Qualitatively the sets can be assigned to the following three classes:a)
Sharp monomodal - mesoporous
b)
Narrow range bimodal - meso/macroporous
c)
Wide range bimodal - meso/macroporous
For set 1, nitrogen and mercury-derived pore size distributions agree well in both pore size and pore volume (typically to within 10%). Pore size distribution derived from nitrogen and mercury data for set 2 do not agree well, despite their qualitative similarities. In the larger set of pores agreement in derivation of pore size and volume is poor (no better thari 50%), and is similarly poor in the derivation of mesopore volume. However, agreement in derivation of mesopore size is bette, but variable (15-25%).
494
Fig 1
N2 Adsorption - Desorption
Set 1
"'C Q)
N
0.9
(1J
E ... 0
2:
o
0.8 0.7
Q. --I-
0.6
...J
0.5
CIJ
0.4
CIJ
2
C
ex
CIJ
0.3
0
0.2
2:
>
0.1
0.1
0.5
0.3
0.9
0.7
P/PO
Mercury Intrusion - Extrusion: Set 1 "'C Q)
0.9
co
..E
0.8
0
0.7
l:
0.6
til
;:,
0.5
w
0.4
.!!1
2: 0
.
....x l:
.S!til
..
;:,
....l:
0.3 0.2 0.1 0
1.4
1.8
2.2
2.6
3
o
Log 10 (Pore Diameter) A
3.4
3.8
495
Fig 2
N2 Adsorption - Desorption "'C OJ N
co
...0E
2
C1
c..
'<,
l-
Set 2
0.9 0.8 0.7
0.6
(/)
s
...J
0.5
(/)
0.4
Q
c:(
(/)
0.3
0
0.2
2
>
0.1 0.1
0.5
0.3
0.9
0.7
P/PO
Mercury Intrusion - Extrusion: Set 2 "'C OJ
.!!! co
E ...
0.9 0.8
0
0.7
c
0.6
2
0
Vl
.......
0.5
w
0.4
c:
0.3
Vl
0.2
:::l )(
0
.......c:
:::l
0.1 0 1.4
1.8
2.2
2.6
3
o
Log 10 (Pore Diameter) A
34
3.8
496
Fig 3
N2 Adsorption - Desorption
Set 3
"tl
Ql
N
0.9
It!
...0E
Z
o
0.8 0.7
'<,
0-
I-
0.6
C/)
...J
~ C/)
0
0.5 0.4
C/)
0.3
0
0.2
Z
>
0.1
0.1
0.7
0.5
0.3
0.9
P/PO
Mercury Intrusion - Extrusion: Set 3 "tl
Ql
0.9
ell
0.8
0
0.7
c
0.6
......><
0.5
w
0.4
r:::
0.3
...c...
0.2
.!!!
E ... Z
.2 II)
:::l
.2 II)
:::l
0.1 0 1.4
1.8
2.2
2.6
3
o
Log 10 (Pore Diameter) A
34
3.8
497 TABLE 1 DERIVATION OF PORE PARAMETERS FROM N2 ADSORPTION, MERCURY INSTRUSION AND DENSITOMETRY
DERIVATION OF MEAN PORE SIZE AO
SET
MESOPORES N2 Hg
2
3
MACROPORES NZ Hg
PORE VOLUME ml/g MESOPORES Hg N2
MACROPORES N2 Hg
TOTAL DENSITOMETRY
95
100
- NA -
0.65
0.65
- NA -
0.65
160
176
- NA -
0.59
0.55
- NA -
0.55
55
65
200
600
0.12
0.10
0.05'f 0.15
0.22
38
50
300
600
0.10
0.05
0.20
0.32
0.38
50
55
ND
3000
0.30
0.15
ND
0.11
0.32
320
250
ND
500060000
0.10
0.06
ND
0.04
0.10
T The
values of macropore volume were derived from NZ adsorption/desorption data using the procedure (6) contained in the analytical software on the commercial instrument (Micromeritics Digisorb 2500)
The pore size distributions of samples in set 3 derived from nitrogen adsorption/desorption data indicate monomoda1 sets of mesopores, but fail to detect the presence of macropores (well separated from the mesopores) which mercury intrusion data indicates. The slight upturn in the adsorption isotherm as p/Po -->1 is an indication that the sample has some macroporous structure, but insufficient to be detected by the algebraic derivation of pore size distribution. Derivation of mesopore size from nitrogen and mercury data agree to <10% at best, more typically 15-25%. Non-detection of macropores is probably a failure in precision and applicability of the Kelvin equation. In all cases, entrapment of mercury on 1st cycle extrusion is of the order of 20-30% of total intrusion. In sample sets I, 3 there is a good degree of parallelism between intrusion and extrusion branches of the mercury data (width 200-300AO) in the mesopore region (log d basis) and drainage of mercury from mesopores of set 3 is similar to that from sample 1. It appears that in these sets (where mesopores in a bimodal system are well separated from macropores in size) the mesopores can behave independently with regard to nitrogen and mercury and hence exhibit similarities to systems containing only mesopores.
498 The parallelism in intrusion/extrusion is not observed in set 2, and the width of the enclosed loop increases as pore diameter increases in the mesopore regIon (log d DasIs). Set 2 represents interesting bimodal systems in which the two pore systems are close enough in size to cause an overlap, for instance, in the extrusion of mercury, and in which the macropores are small enough to be qualitatively detected by the Kelvin-based procedure and yet sufficiently separate in size to be distinguishable from the true mesopores. It is possible that mercury drains from both mesopores and small macropores. A number of workers have compared the derivation of surface areas or pore size distributions from nitrogen adsorption/desorption and mercury porosimetry data for a range of porous oxides (7), compacts (8), and crystalline polymeric filters (9), have found varying degrees of agreement between the two techniques, and advise that they shall be used to provide complementary data (10). The importance of using complementary techniques of nitrogen adsorption/ desorption and mercury intrusion is obvious in the cases of sets 2 and 3. Only in case 1 can reliance on the derivation of pore size distribution from nitrogen data be justified, because it is well supported by mercury data. In set 3 reliance solely on nitrogen data would even result in incorrect qualitative description of the pore structure. In set 2 both techniques provide broadly similar qualitative descriptions. This could be indicative of the influence of the closeness of the distributions, and probably of ill-defined small mesoporosity, on the algebraic derivation of pore size distribution. In all cases there is an interesting qualitative parallelism between hysteresis loops for nitrogen and for mercury processes.
5
CONCLUSIONS A wide range of non-microporous materials can be classified according to the relative size and abundance of meso and macropores. It appears that only class 1 materials (mesoporous only) can sustain quantitative analysis by methods now routinely accessible on modern commercial instruments, due to the complementarity of nitrogen adsorption and mercury intrusion data. The importance of using complementary techniques is emphasised by class 3, where nitrogen data tends to fail to detect macroporosity in pores_lOOOAo diameter. In class 2 both techniques have sufficient precision to generate similar qualitative views of complex bimodal pore structures but substantially different quantitative assessments. Considerable care is required in analysing this class. In all cases, qualitative assessment can be surprisingly informative. There is some evidence from Table 1 that the derivation of meso pore size may be reliable in all three classes.
499 REFERENCES IUPAC, Pure and Appl. Chern. 1985, 57, 603 2
de Boer J.H., J. Catal 1964, 3, 32
3
Sing K.S.W., Payne D.A.
4
Barrett E.P., Joyner L.G., Halenda P.H. 373
5
Drake L.C., Ritter H.C.
6
Robert B.F.,
7
de Wit L.A., Scholten J.J.F.,
8
Astier M.P., Sing K.S.W. "Studies in Surface Science and Catalysis" 1982, 10, 269, Publ. Elsevier
9
Ternan M., Fuller O.M.
10
Lecloux A.J., 'Catalysis Science and Technology' 1981, 2, 171, Publ. Springer - Verlag
Chern. and Ind. 1969, 918 J. Amer. Chern. Soc. 1951, 73,
Ind. Eng. Chern. 1945, 17, 787
J. Colloid Interface Sci. 196/, 23, 266 J. Catal 1975, 36, 36
Can. J. Chern. Eng. 1979, 57, 750
491
ROUTINE ANALYSIS OF THE PORE STRUCTURE OF MESOPOROUS AND MACROPOROUS SOLIDS BY
GAS ADSORPTION, MERCURY POROSIMETRY AND DENSITOMETRY
M Day, R Fletcher ICI Chemicals and Polymers Group Research and Technology Department Wilton Cleveland UK
ABSTRACT This paper describes a study of a range of porous solids using nitrogen adsorption/desorption at 77°K, mercury intrusion/extrusion porosimetry and densitometry in helium and mercury. Qualitative evidence from the techniques used indicated that the materials were representative of classes of pore structures of differing complexity across the spectrum of meso-macropore structures. Since commercial automated instruments and procedures are now widely used routinely in pore structural characterization, the materials offer a means of evaluating the validity 6f the analytical techniques as a function of the complexity of the pore structure. It appears that only materials consisting exclusively of mesopores can sustain quantitative analysis by these methods. When macropores are present the analyses are less reliable and less consistent.
INTRODUCTION The advent of commercial automated instruments for measuring nitrogen adsorption/desorption isotherms and mercury intrusion/extrusion, with dedicated computer faCilities for transforming data into pore size distribution, means that quantitative descriptions of the pore structures of solids are easily generated, but must be used with caution. The follOWing recommendations are contained in the IUPAC guidelines (1) recently issued on the assessment of pore structure from nitrogen adsorption/desorption data. Assess microporosity via t or o<.s methods (2, 3). 2
Assess the likely formal shape of the pores in the structure via inspection and classification of isotherm shape and hysteresis loop: check for loop closure at p/Po~O.4.
3
Apply pore size distribution transforms only in cases where there are no micropores, where pores are rigid in well-defined, mesopore r3nges.
Mercury porosimetr) is an important complementary technique, but formally agreed procedures for its use and interpretation are less developed.
492
This paper sets out to investigate the problems associated with the derivation of descriptions of pore structures from nitrogen and mercury techniques using a series of porour solids which could represent classes in a spectrum of increasing structural complexity. 2
THEORY The theories of nitrogen adsorption/desorption and mercury porosimetry, and the models and algebra for deriving pore size distributions are well known (4, 5). In the case of nitrogen data, the derivation will be suspect if: a)
The pores are not rigid, eg located between small plates.
b)
Isotherm shape is affected by the presence of micropores: closure of hysteresis loop at p/Po-",0.4 is indicative.
c)
The pore sizes cover a wide enough range to render the Kelvin equation imprecise.
On these criteria the most reliable derivation will emerge from solids with type IV isotherms, HI hysteresis loops, representing sharp mono-modal mesoporous structures with no micropores. Qualifications concerning the interpretation of mercury intrusion data are as follows:a)
Mercury must not react or interact strongly with any part of the pore structure.
b)
The solid must not break under the pressures employed.
c)
The shape of the pores eg ink bottle shapes, can affect the interpretation of intrusion and extrusion.
3
EXPERIMENTAL
3.1
APPARATUS Nitrogen adsorption/desorption measurements were carried out wi~h a Micromeritics Inc 'Digisorb 2500' uSing 5-point BET plots and 43 point adsorption/desorption isotherms. 0~4gassing was c:rried out o;-the instrument under moderate vacuum 10 torr, at 140 C for 4 hours. The isotherm data were output to a plotter for inspection and classification and were transformed into o(s plots against an alumina standard. Pore size distributions can be derived from the software incorporated into the instrument, uSing the method of Robert (6). Mercury intrusion was carried out using a Micromeritics Inc 'Autopore 9200' operating from 2 to 60,000 psia and hence in theory able to penetrate pores as small as 36A o in diameter. Outgaasing on !~e instrument is limited to evacuation at room temperature to 10 torr. He-Hg densitometry was used as a measure of total pore volume against which pore volumes derived from N2 and mercury could be compared.
3.2
MATERIALS These were stable alumina-based materials derived from our laboratory, and represent a larger body of materials consisting of six from sets 1, 3 and three from set 2. The range of their micromeritic properties was as follows:
493
SAMPLE SET
PORE VOLUME (He/Hg) ml/g
SURFACE AREA ~GE REPRESENTED m / g
0.5 - 0.8
100 - 250
2
0.2 - 0.5
50 - 150
3
0.3 - 0.6
100 - 350
All were pellets or granules not less than lmm in size, and had been extensively dried at 110°C in air before introduction into instrumental outgassing facilities.
4
RESULTS AND DISCUSSION Figs 1-3 A, B compare the nitrogen adsorption/desorption isotherms and mercury intrusion/extrusion data for sets 1, 2, 3. The data have been normalized because they are representative of numerous individual sets of data. Nitrogen adsorption isotherms were transformed into s plots against an alumina standard; no detectable microporosity was observed in them. The software contained in each instrument can be used to derive pore size distributions from nitrogen adsorption/desorption and mercury intrusion. These derivations and the agreement between N2 and mercury-derived pore sizes and volumes are reported in Table 1. In addition,_the retention of mercury after 1st cycle intrusion and extrusion, which may be specific to the mode of operation of our porosimeter, was noted. The isotherms of set 1 are assigned type IV with HI hysteresis loops according to IUPAC recommendations. Isotherms in set 2 are more complex, clearly bimodal and are assigned as the superposition of type IV isotherms, but there is a slight upturn as p/Po -->1, indicating that pore structures are more complex than indicated by the type IV assignment. Comparison of Figs 1-3 indicates some qualitative parallelism between hysteresis in nitrogen adsorption/desorption and mercury intrusion/ extrusion. Qualitatively the sets can be assigned to the following three classes:a)
Sharp monomodal - mesoporous
b)
Narrow range bimodal - meso/macroporous
c)
Wide range bimodal - meso/macroporous
For set 1, nitrogen and mercury-derived pore size distributions agree well in both pore size and pore volume (typically to within 10%). Pore size distribution derived from nitrogen and mercury data for set 2 do not agree well, despite their qualitative similarities. In the larger set of pores agreement in derivation of pore size and volume is poor (no better thari 50%), and is similarly poor in the derivation of mesopore volume. However, agreement in derivation of mesopore size is bette, but variable (15-25%).
494
Fig 1
N2 Adsorption - Desorption
Set 1
"'C Q)
N
0.9
(1J
E ... 0
2:
o
0.8 0.7
Q. --I-
0.6
...J
0.5
CIJ
0.4
CIJ
2
C
ex
CIJ
0.3
0
0.2
2:
>
0.1
0.1
0.5
0.3
0.9
0.7
P/PO
Mercury Intrusion - Extrusion: Set 1 "'C Q)
0.9
co
..E
0.8
0
0.7
l:
0.6
til
;:,
0.5
w
0.4
.!!1
2: 0
.
....x l:
.S!til
..
;:,
....l:
0.3 0.2 0.1 0
1.4
1.8
2.2
2.6
3
o
Log 10 (Pore Diameter) A
3.4
3.8
495
Fig 2
N2 Adsorption - Desorption "'C OJ N
co
...0E
2
C1
c..
'<,
l-
Set 2
0.9 0.8 0.7
0.6
(/)
s
...J
0.5
(/)
0.4
Q
c:(
(/)
0.3
0
0.2
2
>
0.1 0.1
0.5
0.3
0.9
0.7
P/PO
Mercury Intrusion - Extrusion: Set 2 "'C OJ
.!!! co
E ...
0.9 0.8
0
0.7
c
0.6
2
0
Vl
.......
0.5
w
0.4
c:
0.3
Vl
0.2
:::l )(
0
.......c:
:::l
0.1 0 1.4
1.8
2.2
2.6
3
o
Log 10 (Pore Diameter) A
34
3.8
496
Fig 3
N2 Adsorption - Desorption
Set 3
"tl
Ql
N
0.9
It!
...0E
Z
o
0.8 0.7
'<,
0-
I-
0.6
C/)
...J
~ C/)
0
0.5 0.4
C/)
0.3
0
0.2
Z
>
0.1
0.1
0.7
0.5
0.3
0.9
P/PO
Mercury Intrusion - Extrusion: Set 3 "tl
Ql
0.9
ell
0.8
0
0.7
c
0.6
......><
0.5
w
0.4
r:::
0.3
...c...
0.2
.!!!
E ... Z
.2 II)
:::l
.2 II)
:::l
0.1 0 1.4
1.8
2.2
2.6
3
o
Log 10 (Pore Diameter) A
34
3.8
497 TABLE 1 DERIVATION OF PORE PARAMETERS FROM N2 ADSORPTION, MERCURY INSTRUSION AND DENSITOMETRY
DERIVATION OF MEAN PORE SIZE AO
SET
MESOPORES N2 Hg
2
3
MACROPORES NZ Hg
PORE VOLUME ml/g MESOPORES Hg N2
MACROPORES N2 Hg
TOTAL DENSITOMETRY
95
100
- NA -
0.65
0.65
- NA -
0.65
160
176
- NA -
0.59
0.55
- NA -
0.55
55
65
200
600
0.12
0.10
0.05'f 0.15
0.22
38
50
300
600
0.10
0.05
0.20
0.32
0.38
50
55
ND
3000
0.30
0.15
ND
0.11
0.32
320
250
ND
500060000
0.10
0.06
ND
0.04
0.10
T The
values of macropore volume were derived from NZ adsorption/desorption data using the procedure (6) contained in the analytical software on the commercial instrument (Micromeritics Digisorb 2500)
The pore size distributions of samples in set 3 derived from nitrogen adsorption/desorption data indicate monomoda1 sets of mesopores, but fail to detect the presence of macropores (well separated from the mesopores) which mercury intrusion data indicates. The slight upturn in the adsorption isotherm as p/Po -->1 is an indication that the sample has some macroporous structure, but insufficient to be detected by the algebraic derivation of pore size distribution. Derivation of mesopore size from nitrogen and mercury data agree to <10% at best, more typically 15-25%. Non-detection of macropores is probably a failure in precision and applicability of the Kelvin equation. In all cases, entrapment of mercury on 1st cycle extrusion is of the order of 20-30% of total intrusion. In sample sets I, 3 there is a good degree of parallelism between intrusion and extrusion branches of the mercury data (width 200-300AO) in the mesopore region (log d basis) and drainage of mercury from mesopores of set 3 is similar to that from sample 1. It appears that in these sets (where mesopores in a bimodal system are well separated from macropores in size) the mesopores can behave independently with regard to nitrogen and mercury and hence exhibit similarities to systems containing only mesopores.
498 The parallelism in intrusion/extrusion is not observed in set 2, and the width of the enclosed loop increases as pore diameter increases in the mesopore regIon (log d DasIs). Set 2 represents interesting bimodal systems in which the two pore systems are close enough in size to cause an overlap, for instance, in the extrusion of mercury, and in which the macropores are small enough to be qualitatively detected by the Kelvin-based procedure and yet sufficiently separate in size to be distinguishable from the true mesopores. It is possible that mercury drains from both mesopores and small macropores. A number of workers have compared the derivation of surface areas or pore size distributions from nitrogen adsorption/desorption and mercury porosimetry data for a range of porous oxides (7), compacts (8), and crystalline polymeric filters (9), have found varying degrees of agreement between the two techniques, and advise that they shall be used to provide complementary data (10). The importance of using complementary techniques of nitrogen adsorption/ desorption and mercury intrusion is obvious in the cases of sets 2 and 3. Only in case 1 can reliance on the derivation of pore size distribution from nitrogen data be justified, because it is well supported by mercury data. In set 3 reliance solely on nitrogen data would even result in incorrect qualitative description of the pore structure. In set 2 both techniques provide broadly similar qualitative descriptions. This could be indicative of the influence of the closeness of the distributions, and probably of ill-defined small mesoporosity, on the algebraic derivation of pore size distribution. In all cases there is an interesting qualitative parallelism between hysteresis loops for nitrogen and for mercury processes.
5
CONCLUSIONS A wide range of non-microporous materials can be classified according to the relative size and abundance of meso and macropores. It appears that only class 1 materials (mesoporous only) can sustain quantitative analysis by methods now routinely accessible on modern commercial instruments, due to the complementarity of nitrogen adsorption and mercury intrusion data. The importance of using complementary techniques is emphasised by class 3, where nitrogen data tends to fail to detect macroporosity in pores_lOOOAo diameter. In class 2 both techniques have sufficient precision to generate similar qualitative views of complex bimodal pore structures but substantially different quantitative assessments. Considerable care is required in analysing this class. In all cases, qualitative assessment can be surprisingly informative. There is some evidence from Table 1 that the derivation of meso pore size may be reliable in all three classes.
499 REFERENCES IUPAC, Pure and Appl. Chern. 1985, 57, 603 2
de Boer J.H., J. Catal 1964, 3, 32
3
Sing K.S.W., Payne D.A.
4
Barrett E.P., Joyner L.G., Halenda P.H. 373
5
Drake L.C., Ritter H.C.
6
Robert B.F.,
7
de Wit L.A., Scholten J.J.F.,
8
Astier M.P., Sing K.S.W. "Studies in Surface Science and Catalysis" 1982, 10, 269, Publ. Elsevier
9
Ternan M., Fuller O.M.
10
Lecloux A.J., 'Catalysis Science and Technology' 1981, 2, 171, Publ. Springer - Verlag
Chern. and Ind. 1969, 918 J. Amer. Chern. Soc. 1951, 73,
Ind. Eng. Chern. 1945, 17, 787
J. Colloid Interface Sci. 196/, 23, 266 J. Catal 1975, 36, 36
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