Thermochemical analysis of desorption and adsorption of water on the surface of zirconium dioxide nanoparticles

Article abstract of DOI:10.1134/S1070427208040071

Desorption and adsorption of water on the surface of ZrO₂ particles obtained under hydrothermal conditions were considered. The heat effect of desorption was determined.

Full text of DOI:10.1134/S1070427208040071

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2008, Vol. 81, No. 4, pp. 609 613.
Pleiades Publishing, Ltd., 2008.
Original Russian Text
pp. 571 575.
O.V. Al’myasheva, V.L. Ugolkov, and V.V. Gusarov, 2008, published in Zhurnal Prikladnoi Khimii, 2008, Vol. 81, No. 4,
PHYSICOCHEMICAL STUDIES
OF SYSTEMS AND PROCESSES
Thermochemical Analysis of Desorption and Adsorption
of Water on the Surface
of Zirconium Dioxide Nanoparticles
O. V. Al’myasheva, V. L. Ugolkov, and V. V. Gusarov
Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, St. Petersburg, Russia
Received July 26, 2007
Abstract Desorption and adsorption of water on the surface of ZrO2 particles obtained under hydrothermal
conditions were considered. The heat effect of desorption was determined.
DOI: 10.1134/S1070427208040071
The large specific surface area of nanopowders
and the active state of functional groups on the surface
of nanoparticles [1 5] make promising their use in
development of highly efficient sorbents, catalysts,
and catalyst supports and in syntheses of substances
with a developed and active surface. ZrO2-containing
materials are known as sorbents [6 8] and are wide-
ly used as catalysts [9 13]. Because ZrO2 particles
have a hydrophilic surface and actively adsorb water
[14 17], data on the adsorption and desorption of
water on the surface on ZrO2-based nanocrystalline
powders are necessary for development and use of
these materials. It should be noted that data on these
processes for ZrO2 nanoparticles are also exceedingly
important for development of a technology for produc-
tion of ceramic materials from nanopowders, because
adsorption and desorption of water on the surface of
nanoparticles exert a decisive influence on the den-
sification of materials in their compaction and, par-
ticularly, sintering [18 24].
at a temperature of 250 C and pressure of 70 MPa in
the course of 4 h. The parameters of synthesis of ZrO2
nanocrystals were chosen in accordance with [25 27]
and corresponded to complete dehydration of zirconi-
um oxyhydroxide.
The structural state of ZrO2 nanoparticles was
determined by X-ray phase analysis (XPA). X-ray
diffraction patterns of samples were measured on
a DRON-3 diffractometer (CuK radiation).
The size of crystallites (coherent-scattering regions,
CSR) was found from the line broadening in X-ray
diffraction patterns by the Scherrer formula. In addi-
tion, the particle size was determined by transmission
electron microscopy (TEM) on an EM-125 electron
microscope with Uacc = 75 kV and from small-angle
scattering data obtained on the installation described
in [28].
The specific surface area of the nanopowder was
found by analyzing isotherms of butanol adsorption.
This study is devoted to a thermochemical ex-
amination of the adsorption and desorption of water
on the surface of nanocrystalline ZrO2 particles by
the method of comprehensive thermal analysis.
The desorption and adsorption of water on the sur-
face of nanoparticles was studied in a flow of air con-
taining H2O in an amount corresponding to a 70 80%
relative humidity of air and to a saturated vapor pres-
sure at atmospheric pressure and temperature of 25 C
EXPERIMENTAL
(PH O (sat) = 0.0312 atm [29]) by the method of com-
2
prehensive thermal analysis on a Netzsch STA-429
installation. The heat effects accompanying these pro-
cesses were found from data furnished by differential
scanning calorimetry. The composition of the gas
phase over a sample was monitored by mass spec-
trometry [QMS 403C Aecolos (Netzsch)].
Nanocrystalline zirconium dioxide was produced
by hydrothermal treatment of zirconium hydroxide
precipitated from a solution of ZrOCl2 8H2O (chem-
ically pure grade) with a concentrated solution of
NH4OH. The hydrothermal treatment was carried out
609

610
AL’MYASHEVA et al.
are single crystals, the amorphous phase is virtually
absent, and the nanoparticle size distribution is nar-
row. These characteristics of the zirconium dioxide
nanoparticles under study, produced in hydrothermal
conditions, is confirmed both by published data
[25 27, 30] and by the results of separate exper-
imental methods used in the study. For example, TEM
data (Fig. 1b) show that the nanocrystals have an iso-
metric shape and only slightly vary in size. It should
be noted that the most pronounced, even though only
falling within the experimental error, discrepancy
between the specific surface areas is observed for
the method in which Ssp is calculated from the SCR
size. This, presumably, occurs because the procedure
used to calculate the SCR size by the Scherrer formula
for the most part takes into account coarser crystals,
which results in that Ssp values are underestimated.
The results of a comprehensive thermal analysis of
ZrO2 nanocrystals (Fig. 3) indicate that two endo-
thermic effects associated with the desorption of water
from the surface of zirconium dioxide nanoparticles
and with the transition of the monoclinic modification
of ZrO2 to t-ZrO2, the equilibrium modification at
the given temperature. Four exothermic effects, ac-
companied by a noticeable loss of mass, are pre-
sumably due to a structural rearrangement of ZrO2
nanoparticles. A mass-spectrometric analysis of the
gas phase over a sample demonstrated that mainly
water is desorbed (Fig. 3b). The loss of mass by
the sample, attributed to the desorption process, is
virtually independent of the rate at which temperature
is raised, but depends on the partial pressure of water
of the sample in its isothermal keeping at 25 C.
The data obtained demonstrate that the loss of mass by
a nanopowder kept under saturated-vapor conditions
at 25 C is about 3.7 wt %, and that by samples kept
at room temperature in an atmosphere with a humidity
of 70 80% is 2.2 2.7 wt %. It is noteworthy that
the latter data are also confirmed by the results of
other studies of ZrO2 nanopowders obtained under
similar conditions [13, 14, 30].
Fig. 1. (a) X-ray diffraction pattern and (b) TEM micro-
graph of zirconium dioxide nanocrystals produced by
hydrothermal synthesis. (2 ) Bragg angle.
Fig. 2. Specific surface area of nanocrystalline ZrO ,
2
found by analyzing (1) small-angle X-ray scattering data,
(2) widths of X-ray diffraction lines, (3) TEM data, and
The area of the landing site for a water molecule
is = 0.102 nm2 [31]. With account of the values
obtained for the specific surface area of ZrO2 nano-
(4) data on butanol adsorption. (S ) Specific surface area
sp
of nanoparticles.
1
crystals [Ssp = 61 1 m2 g (Fig. 2)] and for
Comparison of the values obtained for the specific
surface area of nanoparticles by the adsorption de-
sorption method and from the nanoparticle sizes found
from X-ray diffraction line broadening (Fig. 1a) and
by TEM (Fig. 1b) shows that these values coincide
within the experimental error (Fig. 2). This coinci-
dence is only possible if the following conditions
are satisfied: nanoparticles have an isometric shape,
the amount of water adsorbed on their surface [ m =
2
2
(2.2 2.7) 10 or 3.7 10 g H2O per gram of
a sample], it can be concluded that, to within the error
in the available data, a water layer that approaches a bi-
molecular layer as the air humidity increases to satura-
tion is present in both cases on the surface of zirco-
nium dioxide nanoparticles. This conclusion is based
on the fact that the surface area that would be oc-
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THERMOCHEMICAL ANALYSIS OF DESORPTION AND ADSORPTION OF WATER
611
Fig. 3. Results of (a) differential-thermal, (b) mass-spectrometric, and (c) thermogravimetric analyses of nanocrystalline ZrO .
2
1
(T) Temperature and (I) ionic current. Heating rate (deg min ): (I) 1, (II) 5, (III) 10, and (IV) 20.
cupied by a monomolecular layer of water released in
desorption is Ssp. mono = 75 92 or 126 m2 (saturated
vapor), which exceeds by a factor of 1.2 1.5 or nearly
2 (for the saturated vapor) the real specific surface
area of ZrO2 nanoparticles.
the other hand, the fall of the desorption rate, ob-
served as temperature is raised above a certain value
may be due, in accordance with the desorption equa-
tion [32], both to a decrease in the amount of the ad-
sorbed substance in the course of desorption and to
a stronger binding of water molecules, desorbed at
higher temperatures, to the surface of ZrO2 nano-
particles.
Analysis of the thermogravimetric data shows that,
1
at a heating rate of 1 deg min , the curves that de-
scribe the loss of mass (TG and DTG) and charac-
terize the evaporation intensity monotonically decrease
as temperature becomes higher (Fig. 3c). At the same
It should be noted, when analyzing the case in
which a virtually bimolecular layer of adsorbed waster
is present on the surface of ZrO2 nanoparticles
(Fig. 4), that the desorption rate decreases if a pre-
dominantly monomolecular layer of H2O, known to
be more strongly bound to the hydrophilic surface
[33], remains on the nanoparticle surface. According
to [34], a virtually monomolecular layer remains on
hydrophilic surfaces when temperature is raised 65
70 C, in good agreement with the results obtained in
the present study (Fig. 3). The absence of a maximum
1
time, at heating rates of 5, 10, and 20 deg min ,
the TG curves remain monotonically decreasing,
whereas the DTG curves show a maximum that is
shifted, depending on the heating rate, from 60 to
80 C (Fig. 3c). The peak in the DTG curves is pre-
sumably due, on the one hand, to a kinetic hindrance
to the desorption process at low temperatures, which
becomes less pronounced as temperature is raised, in
accordance with the known kinetic equations [32]. On
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612
AL’MYASHEVA et al.
ACKNOWLEDGMENTS
The study was financially supported by the Rus-
sian Foundation for Basic Research (grant no. 07-03-
00750).
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