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4242 Chem. Mater., Vol. 15, No. 22, 2003 Deshpande et al.Figure 3. (a) Pore size distribution plot for the 0.4 M Pd-Coxerogel indicative of its microporous nature. The plot has beenmade using the Saito-Foley extended Horvath Kawazoemodel. (b) BJH plot for the 0.4 M Pd-Co aerogel showing thepresence of mesopores.obtained on the application of the Brunauer, Emmett,and Teller 13 (BET) model to the adsorption data for the0.4 M Pd-Co xerogel. In systems with narrow microporosity,the diffusion of N 2 molecules in narrowmicropores at -196 °C is kinetically restricted. No suchbarrier exists for CO 2 molecules, as the adsorption takesplaces at 0 °C. 21 Hence, in microporous materials, theapplication of the DR equation to the CO 2 adsorptiondata leads to a larger pore volume vis-à-vis thatobtained from the N 2 adsorption data. Both theseobservationssthe curved adsorption isotherms in thelow relative pressure regime and the difference in themicropore volumes obtained from the N 2 and CO 2adsorption datasclearly indicate that the pores in thexerogel are exceedingly narrow, i.e., the xerogel possessesa narrow microporosity. Transmission electronmicroscope imaging of the Pd-Co xerogels, Figure 5,leads to the conclusion that the average particle size inthe xerogels is ∼10 nm.(21) Garrido, J.; Linares-Solano, A.; Martin-Martinez, J. M.; Molina-Sabio, M.; Rodriguez-Reinoso, F.; Torregrosa, R. Langmuir 1987, 3,76.Figure 4. (a) Close-up of the N 2 adsorption isotherm in thelow partial pressure region illustrating the curved isothermsindicative of narrow microporosity for both the 0.4 M aeroandxerogel. (b) CO 2 adsorption isotherms obtained at 273 Kfor both the 0.4 M aero- and xerogel in the low partial pressureregion. The curvature of the isotherms is characteristic ofnarrow microporosity in these gels.Dual Porosity of the Aerogels. Unlike the predominantlymicroporous xerogel, the aerogel possesses bothmicro- and mesoporosity. Figure 3(b) displays the PSDplot for the 0.4 M Pd-Co aerogel obtained by theapplication of the BJH model to the desorption branchof the nitrogen isotherm on the aerogel. It indicates thatthe pore diameters in the aerogels range from 3 to 40nm implying that the Pd-Co aerogel is primarilymesoporous. A pronounced uptake in the low relativepressure regionsas Figure 4(a) and (b) showssimpliesthe existence of micropores in the aerogel. The curvednature of the isotherms in this pressure region, togetherwith the difference in the pore volume values calculatedfrom the DR equation applied to the N 2 and CO 2adsorption data, as shown in Table 1, point to theexistence of narrow microporosity within the aerogel.Table 1 lists the adsorption data for the 0.4 M Pd-Coaerogel primarily because the adsorption data for otheraerogels studied in this paper are similar. Additionally,in our earlier studies, 6 it has been observed that
Morphology of Palladium-Cobalt-Based Cyanogels Chem. Mater., Vol. 15, No. 22, 2003 4243sampleTable 1. Surface Areas and Micropore Volumes of the 0.4 M Pd-Co CyanogelsN 2-S BETa(m 2 /g)N 2-S DRb(m 2 /g)CO 2-S DRb(m 2 /g)N 2-V pc(cm 3 /g)N 2-V DRd(cm 3 /g)CO 2-V DRd(cm 3 /g)0.4 M aerogel 730.71 ( 3.29 450.62 ( 3.17 822.82 ( 5.14 2.37 ( 0.17 0.23 ( 0.05 0.26 ( 0.050.4 M xerogel 439.36 ( 4.17 491.78 ( 3.84 705.11 ( 4.93 0.22 ( 0.07 0.19 ( 0.04 0.28 ( 0.06a N 2-S BET. Total surface area determined by applying the Brunauer-Emmett-Teller (BET) equation to the nitrogen adsorption data.b N 2-S DR and CO 2-S DR. Surface area of the micropore region determined by applying the Dubinin-Radushkevich equation to the nitrogenand carbon dioxide adsorption data. c N 2-V p. Total pore volume determined by measuring the volume of nitrogen absorbed at the saturationpressure of N 2. d N 2-V DR and CO 2-V DR. Micropore pore volume determined by applying the Dubinin-Radushkevich equation to thenitrogen and carbon dioxide adsorption data.Figure 5. Transmission electron microscope image of a 0.4MPd-Co xerogel. The particle size averages ∼10 nm.Figure 6. t-Plot for the 0.4 M Pd-Co aerogel showing boththe mesoporosity and the microporosity (inset). The pronouncedcurvature in the low-t region (inset) is indicative ofmicroporosity in the aerogel.although the mean pore radius in the Pd-Co cyanogelsis highly sensitive to the gel concentration, the porosityis very weakly dependent on gel concentration: it variesby less than 5% when the concentration is varied by afactor of 5. The final evidence for microporosity in Pd-Co aerogels is provided by the t-plot analysis of theaerogel isotherm. Figure 6 shows the t-plot for the 0.4MPd-Co aerogel. The curvature in the t-plot in theregion of low t values clearly attests to the presence ofmicroporosity in the Pd-Co aerogels.Unlike the N 2 adsorption isotherm for the xerogel,which is of Type I, the N 2 adsorption isotherm for the0.4MPd-Co aerogel is a type IV isotherm, 15 characteristicof a mesoporous solid (cf. Figure 2(b)). Theisotherm displays hysteresis: the H1 hysteresis loop isFigure 7. Scanning electron micrograph of a 0.4 M Pd-Coaerogel illustrating the “packed cotton ball” model. The“packed cotton ball” refers to the micropore-containing aerogelparticles that, on average, are ∼40 nm in diameter. Theinterstitial voids that arise from packing of these aerogelparticles constitute the mesopores.Table 2. Comparison of Sorption Data, Beam BendingData, and Calculated Estimatestype ofmeasurementperformedon aerogelpermeabilitymeasurementnitrogen sorptionexperimentcalculatedvalues due to N 2sorption shrinkagedensity ofaerogel(g/cm 3 )mean poreradius awithinaerogel (nm)total porevolume ofaerogel(cm 3 /g)0.10 ( 0.02 17.1 ( 1.8 9.36 ( 1.836.48 ( 1.28 2.37 ( 0.460.28 ( 0.05 5.98 ( 1.17 3.08 ( 0.55a Mean pore radius found from the nitrogen sorption experimentwas determined by (2 × volume of N 2 adsorbed)/(BET surfacearea).characteristic of capillary condensation in mesopores.Such a loop is often obtained with agglomerates orcompacts of spherical particles of fairly uniform size andis typical of aerogels. 15Scanning electron microscope images of the 0.4 Maerogel, Figure 7, indicate that it is composed ofspherical particles of 40 ( 3 nm in diameter. The meanpore radius calculated from the three-point beam bendingmeasurements 6 is 17.1 ( 1.8 nm. This value is insharp contradiction with the mean pore radius calculatedfrom the N 2 desorption which yields a significantlysmaller pore radius of 6.48 ( 1.28 nm. Table 2 lists thevalues of mean pore radius and pore volume obtainedthrough different techniques. The discrepancy betweenthe pore values obtained from the N 2 adsorption andfrom beam-bending measurements is not surprisingbecause gas adsorption is expected to significantlydeform the shape and size of pores in these compliantaerogels. As the liquid adsorbate begins to desorb fromthe pores, high capillary pressure gradients 22 that are
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