J . Phys. Chem. 1984, 88. 2465-2469 various isomers, knowledge of which is important in establishing the number of species which might be present under matrix isolation conditions. The theoretical calculations and the matrix isolation work both indicate that there are stable C, and C2, forms of LiC02, with the latter being more stable, and that N a C 0 2 possesses only a C2, form. We also find that there are both C, and C2, forms of Li2C02and Na2C02,with the C, forms being much more stable. This is consistent with the fact that only the former is observed in the matrix isolation work.3 The more stable C2, species are found to have M2+C02-structures, while the charge distribution and geometries of the C, species are more consistent with (M+)2C022-structures. The C, structures can also be viewed as having M 2 0 . C 0 structures. A C2, potential energy minimum is characterized for Li-C204. Since C20; was previously found to have both DZhand C, (C02-.C02) forms, LiC204is also expected to possess a minimum of C, symmetry with the structure.
/"\'0--6 0' \ Li/
0
I
I
6
In fact, Kafafi et al. have found that the initially formed C, species rearranges to a LiC02.C02 structure. Two different DZhstructure
2465
are predicted for Li2C204and Na2C204. In one case one alkali atom is found to be bound to each C 0 2 group, and in the other the alkali atoms bridge the oxygen atoms from different C 0 2 groups. The greater stability of the latter follows from simple electrostatic considerations. The results of the present study underscore the valuable interplay between theory and experiment in characterizing species formed under matrix isolation conditions. Until quite recently, complete (Le., including all nuclear degrees of freedom) geometry optimizations and frequency calculations were limited to small molecules. With the development of gradient techniques, such calculations on moderate size (up to 6-10 heavy atoms) molecules are now possible using minicomputers. This opens up for theoretical study a larger number of unusual and interesting species detected in low-temperature matrices.
Acknowledgment. This research was supported by the National Science Foundation. The calculations were performed on the Chemistry Department's Harris 800 minicomputer, funded in part by the National Science Foundation. It is a pleasure to acknowledge several helpful discussions with Drs. R. Hauge and Z. Kafafi. Registry No. LiCO,, 80480-95-1; NaCO,, 80480-96-2; Li,C02, 85355-11-9; Na2C02,88057-34-5;LiC204,85355-12-0; Li2Cz04,55391-3.
Application of Positron Annihilation to Study the Surface Properties of Porous Resins K. Venkateswaran, K. L. Cheng,* and Y. C. Jean* Departments of Chemistry and Physics, University of Missouri, Kansas City, Missouri 641 10 (Received: November 18, 1983)
Positron lifetime measurements in various XAD resins with different pore sizes and surface areas have been made at room temperature. Three lifetimes were observed in these resins, -0.4, -4, and >30 ns, due to positron annihilation in the bulk, 0-Ps in the bulk and on the surface, and 0-Ps in the pores. A correlation between the intensity of the long-lived 0-Ps and the effective surface area is found. The correlation provides an alternative method to determine the effective surface area, especially for those with high surface area. The 0-Ps lifetime is characterized by the pore size as well as by the chemical constituents of the resins. These results demonstrate that positron annihilation spectroscopy is a sensitive technique to probe the microstructure and the chemical properties of internal surfaces in porous environments and for studying catalytic materials.
Introduction Macromolecular adsorbents have found immense applications in industry as catalysts, ion exchangers, and chromatographic agents. Of particular interest are the XAD resins having discrete physical properties, such as pore size and internal surface area, and polarities. These resins have been widely used as adsorbents in various analytical applications, such as preconcentration, trace element analysis, water treatment, and pharmaceutical analysis.' These applications are highly dependent on the surface structure and chemical constituents of the resins. Various analytical methods2 have been employed to investigate the surface properties of such porous materials, such as the BET (Brunauer, Emmett, Teller) technique, electron microscopy, X-ray and neutron diffraction, secondary ion mass spectroscopy (SIMS), light scattering, etc. Each has a different degree of perturbation due to the interactions between the probes and the system. The interpretations of the measured results are usually complicated by the employed probes when they approach the interested surface (1) "Amberlite XAD Macroreticular Adsorbents", Technical Manual, Rohm and Hass Co., Philadelphia, PA, 1972. (2) For example, see G. A. Somorjai, "Chemistry of Two-Dimensional Surfaces", Cornel1 University Press, Ithaca, NY, 1982.
0022-3654/84/2088-2465$01 .50/0
externally. Therefore, these methods are generally cataloged as ex situ methods. One of the most interesting areas of research in surface science is the search for an in situ surface technique where the probes approach surfaces internally. The most significant advantage of this is that it closely resembles practical systems, such as catalysts in fine powder forms. Positron annihilation spectroscopy3 has been known for a few decades in materials science applications but its analytical applications are yet to be exploited. The characterization of surface properties by positron probes has been a new mainstream in the field of positron research since the discovery that positrons are preferentially either localized on surfaces or emitted into v a c ~ u m . ~ This is due to the fact that positrons have a negative work function in many solid surfaces. Therefore, it is a new hope that positron annihilation spectroscopy may be utilized as an in situ surface technique for routine analysis and applications related to catalysts. When a positron enters a condensed medium, it may annihilate directly with the surrounding electrons, or it may pick up an electron by forming a bound state, the so-called positronium (Ps) atom. This Ps formation is easily observed experimentally since (3) For example, see, H. Hautojarvi, Ed., 'Positrons in Solids", Springer-Verlag, West Berlin. 1979.
0 1984 American Chemical Society
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The Journal of Physical Chemistry, Vol. 88, No. 12, 1984
Venkateswaran et al.
TABLE I: Properties of XAD Resins Employed in This Study chemical sample functionalitv XAD-2 styrene-DVBn XAD-4 styrene-DVB” XAD-7 acrylic ester XAD-8
skeletal resin mean He porosity, density, density,d surface area, void concn,b av pore void lo1*g-l diameter, A cm2/g g/cm3 g/cm3 polarity fraction m2/e: 1.2 90 0.693 1.081 300 0.62 non polar 0.41 10 50 0.998 1.085 784 0.52 non polar 0.5 1 450 0.53 intermediately 0.55 1.8 90 1.080 1.251 polar 235 0.822 1.259 140 0.08 0.62 intermediately 0.52 polar 69 0.018 352 0.616 1.209 strongly polar 0.69 0.41 22 0.0004 1300 0.787 1.169 strongly polar 0.61 0.45 I
acrylic ester
XAD- 11 amide XAD- 12 very polar with N-0 group
-
“DVB = divinylbenzene. bThe void concentrations were calculated from the surface area and the pore diameter data assuming the voids are spherical and of narrow diameter. ‘The average pore diameters were the calculated values as reported in ref 1. No special information was reported on its distribution. dThe values of resin density were calculated from the values of porosity and skeletal density. TABLE II: Positron Lifetime Results for Various XAD Resins lifetime, ns system XAD-2 XAD-4 XAD-7 XAD-8 XAD- 1 1 XAD- 12‘
intensity, %
71n
72a
73“
0.428 0.415 0.468 0.383 0.378 0.324
4.56 5.91 5.30 3.57 3.51 4.73
95.0 78.2 105.9 100.8 86.0
Izb 10.7 4.2 5.9 13.9 11.9
I, 75.9 68.6 76.8 78.3 83.1 97.9
2.1
13b
I2 + 1 3
13.4 27.2 17.3 7.8 5.O
24.1 31.4 23.2 21.7 16.9 2.1
“The standard deviations for rl, r2, and r3 are about *0.03, *0.04, and h l . 0 ns, respectively. bThe standard deviations for I,,12, and I3 are about il.O%. cFor the XAD-12 resin, I, is the only 0-Ps component where strong chemical reacting functions on the surfaces have effectively quenched the long 0-Ps components. 7,for the XAD-12 resin is the mean value of the two shortest lifetimes at 0.166 (35.3%) and 0.424 (62.6%) ns, respectively, from a three-component computer analysis. the lifetime of one of the Ps states is distinctly long in such porous media. Of the two possible ground states of Ps atom, the 0-Ps (triplet spin state) has an intrinsic lifetime of 1.4 X lO-’s while s. This the p-Ps (singlet spin state) has a lifetime of 1.2 X long 0-Ps lifetime can be observed only under vacuum or in media with high surface areas while most 0-Ps lifetimes in the bulk of s due to a process called solids are on the order of a few pick-off. Very long-lived 0-Ps ( > l o X lo4 s) states in porous media have been observed in various systems, such as highly dispersed powders: silica gels,5 and zeolites.6 The nature of the pore, and of the gas filling the pore, has a significant effect on the annihilation rate and mechanism of Ps formation. This opens a new possibility for the study of the local properties of porous surfaces and processes occurring internally, by the so-called in situ surface technique. We therefore report here a systematic study by the positron lifetime technique on well-known macroreticular copolymer systems, where the pore size and surface area can be well-defined. Experimental Section The XAD resins were obtained from Rohm and Haas Cos1 (Philadelphia, Pa). They are copolymers with different functional groups. Their properties are listed in Table I. All the XAD resins were subjected to a Soxhlet extraction procedure with dichloromethane, methanol, and HzOto remove nonpolar and polar organic contaminants from the surfaces, respectively. The resins were then allowed to dry in an inert atmosphere to avoid any further contamination. The pore structure of XAD resins is stable under cycling treatment by this method and the observed lifetime results are reproducible at each cycling treatment. About 1.0 g of XAD resin was then packed in a sample cell with a 2zNa source at the center. A 5 MCi 22Nasource (obtained from New England Nuclear, Boston, MA) was sealed between two Mylar films 0.2 mil in thickness. The prepared cell and sample were kept under (4) R. Paulin and G. Ambrosio, J . Phys. (Orsay, Fr,), 29, 263 (1968). (5) S. Y.Chuang and S.J. Tao, J. Phys., 5 2 , 7 4 9 (1970); S. Y . Chuang and S.J. Tao, Can. J . Phys., 51, 823 (1973). ( 6 ) H. Nakanishi and Y . Ujihira, J . Phys. Chem., 86, 4446 (1982).
a vacuum of torr during the measurement. Each data point took about 3 h. Each measurement was performed in triplicate and the results were found reproducible. After each measurement, the sample was removed from the chamber and was monitored for its radioactivity and we found no contamination of 22Na in the sample of the cell. Positron Lifetime Measurements The positron lifetime measurements were carried out by a conventional fast-fast coincident method, which monitors the starting signal (1.28-MeV y-rays) from the positron decay in 22Na isotopes and the stopping signal (0.51-MeV y-rays) from the positron annihilation in the material studied. The resolution of the spectrometer was found to be 380 ps by measuring the coincident photons from a 6oCosource. The time scale of the system was fied at 0.88 ns/channel. The obtained lifetime spectra were resolved into a function with a sum of multinegative exponential terms by a Computer program-POSITRONFIT EXTENDED’ with two Gaussian resolution functions. The lifetimes were fitted into three components with a source correction of 5% in Mylars, which support 22Na sources. The 5% source correction was determined from a lifetime result in a sample of nitrobenzene in which the solution completely quenches Ps, thus permitting us to determine the proportion of positrons annihilated in Mylar. All the measurements were made in a sample chamber and under continuous vacuum (10” torr) pumping so that the effect of oxygen in the annihilation spectra is negligible except a few experiments in air as specified. Results and Discussions We have measured the positron lifetimes for six kinds of XAD resins. These are macroreticular resins with various hard surface areas, pore diameters, and polarities as listed in Table I. We observed three positron lifetimes, -0.4, -4.5 i 1.0, and a long lifetime -30 ns, in all these resins. The results are tabulated in Table 11. We have tried to fit the measured lifetime spectra into ~~
~
(7) P. Kirkegaard, and M. Eldrup, Compur. Phys. Commun., 1, 401 (1974).
The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 2467
Surface Properties of Porous Resins
1
GSSation Curve etermination of Surface Area
40
-
0
I 60
30
-
~~
~~
0-Ps lifetime versus Pore radius.
XAD resins Graphite SlOZ
- silica gel
/-
"I 10
T
/
./ 200
0
-
I
- Silica gel
xAD resins
u - Zeolites 0
* - Porous Vycor glass 120
-
0
-
Alumina gel
. 400
600
800
Surface Area (rn'g')
Figure 1. Correlationsbetween the observed long-lived 0-Ps intensity I,
and surface area.
I
four components in the computer program and found that the results are not as reliable as threecomponent fits due to the outputs with large standard deviations. Therefore, we interpret the results in terms of three lifetime components. When positrons are emitted from a 22Na source, they are thermalized in a few picosecond^.^ These thermalized positrons can either directly annihilate with surrounding electrons or pick up an electron from the media and form a neutral Ps atom. In a porous medium with high surface area, positrons and Ps atoms can diffuse out from the bulk which then either are trapped on the surface or escape further into the vacuum spaces. Fates of these positron and positronium states in a porous medium can be classified as follows: (1) positrons in the bulk, (2) trapped positrons on the surface of the pores, (3) Ps formation in the bulk, (4) trapped Ps on the surface of the pores, ( 5 ) Ps formation in the pores or interfacial spaces. Each positron or Ps state of the above annihilates with a different electronic environment which results in a different observed lifetime. The observed three lifetimes are attributed to different contributions of these positrons and Ps atoms that annihilate in the media. Here we assign three positron lifetime components as the following: (1) the shortest-lived component, -0.4 ns, is due to the free positrons in the bulk and due to p-Ps formation; (2) the second, -4.5 f 1.0 ns, is the 0-Ps state due to Ps pick-off in the bulk, and on the surface; and (3) the long-lived component, -30 ns, is due to 0-Ps formation in the pores or interfacial spaces of the resins. It is the second and third components, which are well characterized as 0-Ps states, that will be utilized here to characterize the chemical and physical properties of these resins. It is obviously seen from Table I1 that the 0-Ps intensity ( I 3 ) is proportional to the surface area of the resins. We therefore plot Z3vs. the surface area of resins in Figure 1 . We found a good linear relationship between them but independent of resin functionality. We therefore also include the known results of different high surface area systems, oxide^,^ graphites? and silica gel,5 in this correlation curve. We found that a linear curve fits fairly well for all systems except at the very low surface area (70 m2/g) (
