Thermal analysis of the as-synthesized aerogel (Figure E.1) revealed weight-loss steps. The first step, over the temperature range 150–200◦C is assigned to desorption of water adsorbed on the aerogel surface. The second step, with peak weight loss occurring at∼280 ◦C and accounting for∼5 wt%, is taken to be due to the burn-off of the residual organic chelating agent, acetyl acetone. Exothermic peaks accompany both weight loss steps.
0 200 400 600 800 1000
70 80 90 100 110 120
-14 -12 -10 -8 -6 -4 -2 0
0 200 400 600 800 1000
-0.4 -0.3 -0.2 -0.1 0.0 0.1
organics
DSC(mW/mg)
Mass (%)
Temperature ( o
C) H
2 O
DSC
DTG
TG
DifferentialMass(%/
o C)
Figure E.1: Thermalgravimetric and differential scanning calorimetry curves of as-synthesized CeO2 aerogel obtained under flowing O2 at a heating rate of 10◦C/min.
The presence of residual organics and hydroxyl ions/water, in the as-synthesized gel is confirmed by the FTIR spectrum. Figure E.2, curve (a), shows a strong C=O stretching peak at ∼1600 cm−1, several C-H bands, and a weak O-H stretching band at∼3400 cm−1. After heat-treatment at 300 ◦C, curve (b), the intensities of the peaks due to the organics are substantially reduced, while treatment at 600◦C eliminates all peaks except that due to Ce-O stretching at∼500 cm−1, curve (c). These observations are in general agreement with the thermal analysis. The high intensity at 300 ◦C of the IR peak corresponding to O-H stretching is attributed to the rapid adsorption of atmospheric H2O onto the ceria aerogel surface after removal of the chelating organic groups.
As-synthesized, the CeO2 aerogel shows very broad and weak diffraction peaks that can be attributed to the cubic fluorite phase of ceria, Figure E.3. Upon annealing, these peaks sharpen considerably, as would be expected for the growth in the crystallite size with heat
4000 3500 3000 2500 2000 1500 1000 500 C-H bending
C=O stretching C-H stretching
O-H stretching
(c) annealed 600 o
C/2hr
(b) annealed 300 o
C/2hr
(a) as-synthesized
%transmission
wavelength cm -1
Figure E.2: FTIR spectra collected from ceria aerogel, heat treated as indicated.
treatment.
10 20 30 40 50 60 70 80
420
331 400 222 311 220
200 111
(b) 300C/2hrs
(a) as-prepared (c) 600C/2hrs
Intensity (arb.units)
2 ()
Figure E.3: X-ray diffraction patterns of CeO2 aerogel, heat-treated as indicated.
The morphology of the as-synthesized aerogel is highly porous and extremely uniform, as evidenced both by electron microscopy (Figure E.4), and nitrogen adsorption studies (Figures E.5 and E.6) and Table E.1. The aerogel exhibits an adsorption isotherm of type IV (IUPAC classification) with a marked hysteresis loop of H1 type (Figure E.5a).
Type IV isotherms are characteristic of mesoporous materials.110 The hysteresis loop of type H1 is usually associated with agglomerates or compacts of uniform spheres, which have a narrow pore size distribution and open tubular pores with circular or polygonal sections. While hysteresis between the adsorption and desorption branches is evident, its magnitude is sufficiently small as to confirm that nitrogen adsorption equilibration has been reached. Furthermore, the adsorption reaches a clear saturation value, particularly for the annealed samples, indicating corrections for compliancy need not be applied. In any case,
should equilibration and compliancy influence the results, the pore volumes reported here correspond to a lower bound.
(a) (b) (c)
Figure E.4: Pore size distribution in (a) as-prepared, (b) 300◦C/2hr annealed and (c) 600◦C/2hr annealed CeO2 aerogel.
0.0 0.2 0.4 0.6 0.8 1.0
0 250 500 750 1000
Relative pressure (p/p 0
) Adsorption
Desorption
c b a
(a) as-synthesized aerogel
(b) annealed at 300 o
C/2hr
(c) annealed at 600 o
C/2hr
Amountadsorbed(cm
3 /g)
Figure E.5: Nitrogen adsorption isotherms at 77 K of (a) as-prepared, (b) 300◦C/2hr annealed and (c) 600
◦C/2hr annealed CeO2 aerogel.
The narrowness of the pore diameter distribution, which has a maximum at 21.2 nm, is further evident in Figure E.6a. The as-prepared aerogel has a large surface area of 349 m2/g and, the large pore size (>10 nm) is well suited for facile mass transport. Moreover, the pores are randomly connected in a three-dimensional network (Figure E.5), which is further anticipated to promote gas diffusion through the aerogel structure.
Heat treatment reduces the porosity and increases the average grain size (Table E.1).
0 10 20 30 40 50 0.0
0.1 0.2 0.3
0.00 0.02 0.04 0.06 0.08
(a) as-prepared
(b) annealed at 300 o
C/2hr
(c) annealed at 600 o
C/2hr
c b
a
Pore Size (nm)
PoreSizeDistribution(cm
3 /gnm)
Figure E.6: Pore size distribution in (a) as-prepared, (b) 300◦C/2hr annealed and (c) 600◦C/2hr annealed CeO2 aerogel.
Table E.1: Microstructural properties of ceria aerogel
CeO2 X-ray BET Ave. pore Most freq. Pore Porosity
Aerogel grain size surf. area diameter pore diameter vol.
nm m2/g nm nm cm3/g %
As-prepared <2 349 14.3 21.2 1.33 90
300◦C/2hr 4.8 155 11.5 13.4 0.40 80
600◦C/2hr 15.9 49 10.4 11.0 0.15 51
However, the overall morphology of the aerogel remains unchanged, consisting of nano- cale grains with interconnected, mesoscale porosity (Figure E.5). Furthermore, the type of isotherm and hysteresis are also unchanged by the heat treatment; the pore size remains large and the distribution remains extremely narrow. The overall reduction in porosity is evident from the sharp decrease in the amount of nitrogen adsorbed (Figure E.5). Quanti- tatively, the specific surface area decreases from 349 to 49 m2/g, the porosity from 90% to 51% and the average pore diameter from 14.3 to 10.4 nm upon annealing at 600 ◦C, while the maximum in the pore size distribution shifts from 21.2 to 11.0 nm. The average grain or particle size, which is related to the aerogel wall thickness, increases from a value that cannot be reliably determined by X-ray diffraction, smaller than 2 nm to almost 16 nm upon heat treatment.
Implementation of aerogels in high temperature catalysis requires that high surface area and porosity be maintained for long time periods. Thermally stable structures can be pursued by introduction of zirconia via incorporation of zirconium precursor compounds in
the gel solution, which has been shown to mitigate loss of porosity.114