INFRARED STUDIES OF THE ADSORPTION-DESORPTION OF …

INFRARED STUDIES OF THE ADSORPTION-DESORPTION OF WATER ON MONOCLINIC AND TETRAGONAL Zr0 2 AS
A FUNCTION OF TEMPERATURE
JAAftTO
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ORIL-5001 VC-k — Cheaistry
Contract Ho. V-7li05-eng-26
MDROCUHIC AMD TETKAGCRAL ZrO, AS
A FUBCTCCB OF TEMPEBATDHE
P. A. Agron, E. L. « ta l l er , R. F. Holaes
SEPTEMKK 1974
C«ami«iMi. mr any * f JMr <
MBJM9 any witnnfy. •xpwsv • * M y M t •
«r hi
Mr av •»
operated by URIC* CARBIDE CORPORATION
for the U.S . ATOMIC EHERGY COMMISSIOK *>m
i i i
ZrCfe-G: ADSORPTIOH-DESORPTIOll STUDIES 7
Figure 1 . Transmission Spectra, 2U00-U000 ca" » at UO and 100°C 7
Figure 2 . Transmission Spectra, 2U00-U000 ca" , at 1»0, 200, and 300°C 9
Figure 3 . Transmission Spectra, 2*iC>J«O00 cm" 1, at liOO and 500°C 9
Figure U. Absorbance Spectra, 3100-1*000 cm" 1, at 500, 300 and 100°C 12
Figure 5. Transmission Spectra, 3200-UO00 cm' 1 , at 500, 30C, 100 and UO'C 12
Figure 6. Transmission Spectra, IfeO-DaO Exchange, 2lK»-liOOO cm" 1, at U0oC 15
Figure 7 • IfeO and DaO Desorption-Adsorption Spectra a t 300°C 15
Figure 8 . Bending Frequency Region for ffeO Molecule, 1U50-1800 cm"1, at ltO"C 18
Figure 9. Bending Freqneucy Region for IfeO Molecule, 1U5O-180O cm"1, at 300 and 500*C 20
ZrOa-C: ADSORFTION-DESORPTIOH STUDIES 23
Figure 10. Transmission Spectra, 2I4OO-U00O cm" 1, a t 100 and 20TC 23
Figure 11. Transmission Spectra, 2UOO-UOOO cm" 1, at 300°C 23
Figure 12. Transmission Spectra as a Function of Temperature 26
Figure 13. ;.:,»3rbance St ectra, 3000-VXX) cm" 1, 1»00, 500 and 300°C (1st cycl*} .'. 26
Figure lU. Transmissior Spectra, 3300-UOOO cm"1 (temperature cycling) 29
BLANK PAGE
I T
Figure 15. Transmission Spectra of BaO-D O Species, 2M0-JKJG0 at" 1 3?
Figure l o . TransBissios and Absorbance Spectra of D4O Exchange at 300*C 32
Figure 17 Transmission Spectra, 300*C (Before and
Aiter DjO Exchange) 32
SUiUuTf Uk
REFERENCES U7
MOMOCLHUC AMD TgTBAGOMAL ZrOa AS
A FJHCTIOM OF TEMPERATURE
P . A . A g r o n , E . L . F u l l e r , H. F- Holmes
ABSTRACT
The adsorption-desorption cycling studies at elevated temperatures
of subaonolayer water coverages on the surface of tvo zirconium oxide poly­
morphs leads to a parti&l resolution of the vibrational frequencies due to
chemtisorbed and phy s i sorted species . The narrow stretching frequency band,
u ^ - H ) for the monoclinic r>hase ZrOa-G, at 3760 car 1 i s assigned t o
cbeaisorbed OH groups bound t o a single cation at crystal corners and
edges. This s ingly coordinated at-en i s here designated as a "free" hy-
droxyl anion. This orientation enables the anion to accept an H-bonding
donor water molecule. The polarizing ef fect of the H-b-»d lovers the
stretch frequency of t h i s hydroxy1 anion. The re la t ive ly narrow band
u(0,-H) M at 366O est' 1 i s assigned t o chemisorbed hydroxyls coordinated
to s»re than a s ingle cation 00 oxide l a t t i c e s i t e s of low index faces .
In th i s instance the oxygen atom of the l a t t e r anion i s s t er i ca l ly unable
to accept an H-bonding donor water molecule.
The part ia l ly resolved high frequency bands u ^ - H ) and u ^ p - H )
at yr^O and 3720 o r 1 , respectively, are attributed t o chemically bound
"free" hydroxyl groups of the tetragonal oxide, ZrOg-C, at two non-equiva­
lent oxygen l a t t i c e s i t e s . The stretching frequencies of these two species
are likewise lowered by H-bonded water. Hydroxyls coordinated to acre
than one cat ion, at two different l a t t i c e spacing!, are assigned to
1
of the "free" hydroxyl is ~70 cm" and ~95 cm for the hydroxyls coordi­
nated to Multiple cations.
The partially resolved broader bands, of several hundred wave numbers,
observed at the lower frequencies on extensive desorption from both poly­
morphs are attributed to water molecules strongly bound to the oxide sur­
face (Table i, 2). The energy and extent of the H-bonded network of water
molecules held on the surface or in micropores of these two polymorphic
oxides may be judged from the position and intensity of the bending fre­
quency, respectively, for the water molecule.
The resolution of the HaO sub-bands is accomplished by exchange reac­
tions with DaO. The corresponding u(O-D) stretch bands are observed. The
ratios of the assigned u(o-H)/u(0-D) stretch frequeccles show satisfactory
agreement with the theoretical ratio. The rapid exchange of isotopic spe­
cies indicates that the exchange takes place on the solid cirfac^s. The
extent of DaO exchange is governed by the nimbcr of successive adsorption-
desorption cycles.
IBTRODUCTIOH
The physical and chemical adsorption cf water vapor on high surface
(3) (1 2^ area zirconium oxide has received emphasis here1 ' because of its
importance as a ceramic and catalytic oxide. De Boer and bis associates
utilised gas adsorption isotherms in earlier studi?s to characterize the
nature of zirconia prepared under varying conditions. An extensive re­
port on the preparatior, physical properties and catalytic behavior of (h) this oxide appeared recently
Early x-ray studies ' ' identified the existence of the tetragonal
and monoclinic phases of Z*^ • Accjrate structural determinations of
monoclinic single crystals of ZrQa have been reported. ' ' ' The structure
of the primitive tetragonal cel l was derived from a high temperature x-ray
diff-^tion *tr.dy.(9'
under lKaline conditions, indicates that a crystallization to the meta-
stable tetragonal form takes place at U05°C. Ref!uxing the precipitate (h) in neutral or acid media yields the monoclinic oxide. The latter i s
transformed to the tetragonal phase above 1200°C. On cooling tbe inversion
to the stable monoclinic form takes place at a transition point ~150T:
lower. This hysteresis has been observed by several investigators. ' '
Various proposed mechanisms have been made for the sluggish transforna- (7 &) tiotx * ; and for the large metastable region observed for the tetragonal
phase at the lower temperatures. '
Recent calorimetric measurements of the beats of i-maersion ir»
water of outgassed samples of ZrQ» have shown that the initial irteraction
of water with the oxide surface is chemical in nature. The character of
the subsequent slow heat release appears to be complex. The degree of
chemi sorption and slo* heat release has varied with the method of prepa-
(2)
(1 2) rati onv ' ' and with the specific polymorphic phaise studied. Gas adsorp­
tion isotherms of one of the samples used in this vtudy has been reported.
The structural nature of water plays a dominant role in i t s chemical
and physical properties. A comprehensive treatise on the physics and
physical chemistry of water has appeared recently. ' Coulson pro­
posed that the oipole moment of the water molecule i s due to the lone-pair
electrons occypjing the sp* orbital*. Hydrogen bonds directed toward
these lone pairs lead to tetrahedrally coordinated polymers of water in
the condensed phase. X-ray^ ^' and neutron^"" diffraction, infrared
and Raman^ spectroscopic studies of water have been instructive in
testing the proposed configurations which best fit the observed experi­
mental data. These studies yield a time average measure of the possible
conformations in the liquid phase.
Infrared* ' and Raman studies of HaO-DaO mixtures have led to
the suggestion that the broad stretching 0-H water frequency (3200-3600
cm'1) is composed of '+ gaussian contour bands and is attributed to dif­
ferent polymeric, H-bonded species. The lowering of the water stretch
frequency on hydrogen bonding is a reflection of a lowered 0-H force con- (17) stant. Hydrogen bonding here also leads to a large increase in the
absorption coefficient of the stretching frequency. ' On the other hand,
the bending mode of the water molecule appears in the 1600 to 1650 cm"1
region. For the latter mode, shifts to the higher frequency occur when
H-bonded networks form, but the absorption coefficient is not altered to
any large extent. Thus, this bendiiig mode also serves as an additional
indication of the extent of H-bonded molecular water adsorbed on the oxide.
Several investigators* ' ' ' propose that the broadened u(O-H)
for the water stretch band is due to the superposition of contributions (17) from H-bonded multimeric species. Sheppard suggests' ' that this comes
about from the anharmonic coupling of u(O-H) ± nu'(0-H..-0) where u'
(0-H...0) is of tne order of 100 to 200 cm"1. Fermi, resonance between
u(0-H) and combination bands which are of similar frequency and the same
symmetry class as the fundamental may also contribute to this effect.
However, the latter appears to be less likely in the frozen matrix study
or when water is adsorbed on high surface area solids.
Matrix freezing of hydrogen bonded molecules at varying dilutions of (19) the inert matrix media has been used for analyzing infrared spectra. "
The study of water molecules isolated in low temperature matrices were
reported for the stretching and bending frequencies for the monomer, dimer
and multimer species of K-bonded water molecules. An infrared study of
the matrix isolated molecules of HgO, D^O and HDO has appeared more
recently. The vibrational frequencies of the 1^0 species observed
as e function of matrix dilution are in good agreement in both investiga­
tions. The values observed for the bending frequencies^ range from
1597 cm"1 to 1633 cm"1. The lower value is assigned to the HgO monomer,
and the higher frequencies are attributed to H-bonded polymers. The
analogous bending frequencies of water adsorbed on the zirconium oxide
surfaces are pertinent to this study and will be taken up again in the
discussion.
EXPERIMENTAL
A double-beam Beckman IR-12 filter-grating spectrophotometer, modi- (12) fiedv to encompass a high temperature sample cell mounted in a "stretch"
compartment and incorporated into the optical bench, was used for this
study. A PbS photoconducting detector, at liquid nitrogen temperature,
served to cover the range from 21*00 to !4000 cm"1 with a reproducibly high
sensitivity. The lower frequency spectral range utilized a thermocouple
with the appropriate electronics.
Three hundred milligrams of each of the two zirconium oxide powders (°l) were pressed in a 13/l6-in.-diam die with a force of U0,800 psi. '
This results in a packing density of 22-5 mg/cm8. The pressed discs were
6
stored in co\ered petri dishes over an extended period prior to tnis
study.
All of the reported spectra were obtained at indicated equilibrium
sample temperaturis ranging I*Tin '-0 +,J 500°.'. In this investigation the
exposure so water vaprv was limite-i to a aaxijsum jf U.6 torr in the attempt
to resolve the spectral bands observed in a^sor^tion studies of water on
similar cxicics. The scans reported here we-e bXiiuired for each set
of condition: after r^uilibrating at respective temperatures for periods
of at leaot 16 hours. Outgassing pressures were in the ran^c r,f 10" 5 to
10" s torr. Most spectra were acquired at scanning rates of 3-2 to 8
aE-1/min. Faster scans were taken to observe the rate of change and to
delineate the approach to steady state. The spectral data are presented
\L\ chronological sequence to aid in the correlation and interpretation of
the observed effects. A fixed scale expansion,achieved by reference beam
attenuation, was used throughout each of the two studies.
The infrared technique was selected to follow the nature of the
adsorption-desorption of water on a sample of both the monoclinic and
tetragonal preparations with a previous history of analysis. The Zr03-G
sfjjple, primarily monoclinic phase, has been characterized by nitrogen
adsorption * ' to have a surface area of 23-7 nfYg. I"06 he?tf* of imaier-
sion* ' have also been reported for the selected tetragonal preparation,
7.?0?-C, with a surface area of 52.7 nrVg. It was hoped that the infrared
study of repeated adsorption-desorption cycling of water at selected
tempmatures would point up the differerces in surface chemistry for the
two polymorphic forms of Zr0 2.
The regions of 2UO0-U00O cm"x and the "»l*00-l300 cm"1 spectral scans
were examined extensively for the ZrOa-G wafer to delineate the vibrational
stretching and tending : xles, respectively, of the adsorbed water molecule.
The lower region \*as omitted for the ZrOa-C wafer because, in this instance,
it showed a great de<L of opacity. A 200-sg ZrOa-C pressed wafer showed
that the transmission in the bending region was analogous to that in the
ZrQa-C- disc and was omitted from this study. It is generally ectaowl edged
that the vibrational frequency contributions of molecular bound water on (22) high surface area solids lie in a broad band from 2^00 to 36OO cm - 1.
The unresolved broad band of adsorbed water on oxide surfaces has been (22 23) associated with H-bonded polynuclear water molecules.' ' In desorption
studies, narrow bands appearing at higher frequencies have been assigned
to the stretching vibration frequency of isolated OH groups bound to one (22 23') or more cations. ' '
ZrOa-G: ADSORPTION-DESOBPTION STUIIES
Figure 1. Transmission Spectra, 2U00-U000 cm"1, at UO and 1Q0°C
The initial spectra of the Zrt^-G wa^cr gave a bread unresolved trans­
mission band in the stretching rrequency region of molecular water. On
t<ctensive outgassing at kO°C the trace (la) indicates the initial removal
of weakly bound water molecules in the lower frequency region and at the
same time yields a narr:* sideband at 3760 cm"1. Several cycles of out-
gassing and exposure at 1+0°C to U.6 torr of water vapor results in trace
(lb); wherein, the frequency peak at 3760 cm ' ^s no longer resolved. A sub
sequent outgassing of the disc at 100°C is shown in (lc). On a four-fold
expanded frequency scale, from 36OO to ' 00 cm"1, the T.irtially resolved
• r_-»^<w«5s~»^J i!'Wi«^'^-**-'* i'*"1 *33t£&
1 1 • 1 ' 1 •
ORNLOWG. 73-8203R 1.00 1 1 1 ' ! •
Zr02-G TRACE rcc) P
c'.c too VACUUM C
H
> St
0.20
28 24 40 39 38 37 36 40 35 32 WAVE NUMBER (cnT'x 10~ 2)
Fig. 1. Transnission Spectra, 2U00-U000 cm"
Region, Adsorption-Desorption of HJ? on ZrO ?-G, a t UO and
100°C.
l a I n i t i a l outgassing of sample at >40°C.
Same, on a four-fold expanded frequency s c a l e .
Adsorption of H20 a t U.6 t o r r an1 hO"C-
Outgassed sample a t 100° C aftei several repeated
cycles of ( la) and (lb)
l c ' Same, on a four-fold expanded s ca l e , 360O-I+OOO cm"
l a '
9
shoulder of the 100°C trace (lc') shows a lower concentration of the 3760
<W* specie than that observed for the initial outgassiog (la').
Figure 2. Transmission Spectra, 2U00-UQ00 atrx, at UP, 200, and 300°C
The trace, (2b), obtained on exposure to U.6 torr HgO at 200°C indi­
cates a lower concentration of molecular bound water compared to that
observed at the same pressure in (2a) at U0°C. In the high frequency
region a poo:-ly resolved shoulder in the region of 3760 cm"1 is shown on
the expanded scale in (2b7) for The 200°C isotherm. The transmission
spectra in (2c) and in the expanded scale (2c') are obtained on outgassing
at 200°C. Here, the increased resolution of the high frequency shoulder
is accompanied by a depopulation of the specie4? contributing to the lower
frequency region. The spectral peaks in the 2860 to 3000 cm"x region are
due to C-H stretch frequency bands. The adsorption of traces of organic
species occurs from back diffusion aid degradation products from stopcock
grease.* A subsequent exposure of the sample to 1 atm of oxygen at 300°C
for an extended period results in a large reduction of this band. A mare
effective removal of this organic deposit is accomplished at higher tem- (2k) perature treatments. ' The reduction in the spectral intensities at
the lower frequencies under vacuum at 300°r (2d) indicates the further
removal of the less firmly bound water. At this point the faint resolution
of an absorbance peak at 3660 cm"1 emerges in (2d) and 2d').
Figure 3. Transmission Spectra, 2UOO-UOOO cm"1, at UOQ and 500°C
The resulting spectra on exposure to U.6 torr HgO at UO0°C is given
in (3*)- Outgassjng at U00°C and 500°C results in the traces shown in
(3b) and 3c). respectively. Here the monotonic reduction in absorbance
*Apiezon N Stopcock grease used for stopcocks of vacuum manifold.
- ' * * T " * 5 * "•*& =•»"•*•*"«>»•"£•""*— . * « * • -
^ j b'.b 200 4.6TbrrH20 / f ' ~ y , c'.c 200 VACUUM / i
—T • 1 » r ZrOz-G
0.20
38 37 36 40 36 32 WAVE NUMBER (cm"1* O" 2 )
-1 Fig. 2 . Transmission Spectra, 2U00-fc00O cm
Region, Adsorption-Desorption of H_0 on ZrO_-G at ^0, 200
and 300°C
Jutgassed sample at 200°C
Same, on a four-fold expanded frequency s c a l e , 360O-
1*000 cm' 1
Same, on a four-fold expanded scale, 3600-1*000 cm'
-1
u
ZrOfc-G TRACE TPC) P
400 4.6 Kirr \ H2O
b,b' 400 VACUUM c,c' 500 VACUUM d' 500 4.6 Tgrr H&
» . I 1
0RNL-0O& 73-820SR
-0 .6O
37 36 40 36 32 WAVE NUMBER (cm"'» 10 ' 2 )
Fig. 3 . Transmission Spectra, 2fcOO-UO0O c»~
Region, Adsorption-Desorption of HgO on ZrOp-G at IiOO and 500'C.
3a Adsorption of H.O at U.6 torr and UOO'C
3b Outgassed saaple at UOO'C
3b' Sane, on a four-fold expanded scale, 3600-1*000 ca
3c Outgassed s&aple at 500°C
3c* Saae, on a four-fold expanded scale, 360O-UOOO as" 1
3d' Adsorption of HgO at U.6 torr and 500°J
:2
of the lower spectral frequencies on desorption is accompanied by a pro­
gressively shan>er resolution of the band at 3660 ca'1 - The decreases in
the populations of the two high-frequency stretch vibrations, of the sample
outgasseC at MX) and 500°C, are shown in (3'o') and (3c'), respectively.
The oxide has been equilibrated with U.6 torr HQO at 500°C in (3d'), to*
comparative spectra.
Figure h. Absorbance Spectra, 3100-t*000 cm'1, at 500, 300 and 100°C
The absorbance spectrum on outgassing at 500°C is shown in (ha.) over
the frequency region from 3100 to *»000 cm'1. A two-fold shift of the
absorbance scale is inserted at (Ua'). The spectra obtain^ on subsequent
cooling of the system under vacuum to 300°C, (lib) and (Ub'), and then to
100 C, (Uc) and (he'), indicated a progressive improvement in the resol­
ution of several bands. The emergence of the 3760 car'" baud is accompanied
by an improved resolution of the 3660 cm*1 band.
Figure 5- Transmission Spectra, 32QO-UO00 cm"1, at 500, 300, 100 and U0°C
Figure 5 shows the trcnsmisslon spectra on a four-fold expanded
frequency scale at the fame desorption conditions observed in Fig. h. On
reducing the temperature of the outgassed sample (5a) from 500°C to 300°C,
a coincidence occurs in the intensity of tbe band at 3660 cm'1. The trace
at 300°C (5b) is therefore displaced for a better visual comparison with
the spectra of (5a). Cooling further under vacuum to 100 and U0°C re­
sulted in almost identical spectra (5c), for these two temperatures, over
the measured range of frequency. The latter trace shows better resolution
of the bands at 3760 cm'1 and 3660 cm'1 with additional improvements in
the resolution of the much broader sub-bands at 3580 cm'1 and 3W30 cm'1.
The assignments are listed in Table 1.
f
13
1 ' A
f 1.1U
» V 1 TRACE T PC) V o'.o 500 v _ \ \ b'.b 300 \ 0.80
- \ \ c'.c 100 \
\ UI u
• I \ s \ I \ 1 \
R
- 1 \ \ ! > v 0.40 5 1 * \ ! \ \ 1 \ \ K 1 \ \ ! \ \ 1 1 \ \ < f
- 5/ ' ' \
0.20
r » r ' \ / i \ / » i > * / i \ - 1 / ' * / i \ 1 \ ^ J J \ J \ * * - < i . \ i i ^ - < ! i .» i « ^ ~ < ! 1 .1 1
40 36 32 40 WAVE
36 NUMBER
of HgO on Zr02-G at 500, 300 and 100°C
U* Outgassed sample at 500 *C
ka.' Same, on 2-fold expanded absorbance scale
kb Sauple cooled under vacuum to 300*C
itb' Same, on 2-fold expanded absorbance scale
Uc Sample cooled under vacuum to 100"C
Uc* Same, on 2-fold expanded absorbance scale .
14
OftNL-MG 73-M07K
38 3? 36 35 34 WAVE NUMBER (cm'x 1 0 2 )
-I
Fig. 5. Transmission Spectra, ~>>00-<»000 cm"
(on a four-fold expanded frequency scale) at 5CC, 300, 100 and 1*>°C
5a Out«r.i«?«ea sample at 500°C
5b Sample cooled u^der vacuus to 300*C
5c Sample cooled under vacuum to 100*C
15
Figure G. Transmission Spectre. H O-D O Exchange. 2*00-^000 cmT*, at fr)*C
After outgassing the sample at 500*C and cooling under vacuus to *iO#Cf
the transmission spectra* resulted in (6a). This trace shows no marked
differer.ce from that obtained on a previous cooling cycle from 500'C under
vacuo*. The extensive outgassing here represents an unsaturated state
where additional water species can be itumii ally as well as physically
adsorbed. An in i t ia l exposure of th» system at *tO*C to 200 u. 1*0 drops
rapidly tr a pressure of 1*5 n on adsorption. The resulting transmission
spectrja i s shown in (6b). Outgassing the sample at 40"C results in the
trace shown in (6c). Here the frequency peak at 2770 cm'1 i s the 0-D ana­
log of the 0-H stretch at 3760 cm' 1 , and likewise, the peak at 2695 cm"1
i s the equivalent vibrational mode observed for the 0-H mode at 3660 cm*1.
Additional exposure to k torr 1*0 leads to the unresolved broad H»0 and
QaO bands shown in (6d).
Figure 7. HgO and PaO Desorptioo-Adsorption Spectra at 300*C
The sample equilibrated with k torr D O (6d) was outgassed at 300°C.
The resulting spectrx* i s shown in (7a). Here the resolution of the 366O
car 1 band ana the related band at 26^5 cm - 1 i s increased because of the
reduction in population of the associated lower stretching frequency
species. On further exposure to k torr JfeO at 300°C (7b), the resulting
increase in the population of the lew frequency species i s not sufficient
to obliterate the 366O cm'1 and 2693 cm"1 bands. A repeated outs»esing
of the sample leads to the spectrum shown in (7c). Each of the above
traces i s displaced to more clearly show the development of the absorption
bands of both the bound H3O and D O regions. The successive exchange of
a VACUUM b 2 0 C u OgO
32 28 24 4 0 36 WftVE NUMBER ( C M * 1 ! I0~2)
32 28
Zr02-G, 2kO-li000 CM"1 Region at bO#C-
6a Saaple reduced to UO*C in vacuus, following extended
desorptlon of HJ> at 500#C
6b
6c
6d
Outgassed saaple at 10 torr
Exchange at U torr DJO
17
1.00
OBOh
t i l
O VACUUM 4TorrD20
VACUUM 36 32 28 WAVE NUMBER (cnr ' i tO' 2 )
f ig . 7. Adsorption-Desorption of H-O-D-O Exchange Species
on Zi02-G at 300*C
7a Desorption of HgO-D 0 species at 300°C
7b Exctaange of U tor r D20 at 300°C
7c Repeated desorption of HgO-DgO species at 300°C
13
VaO at 30C°C, followed by outgassing, adds effectively to the further
delineation of the sub-bands of the water stretching frequencies at 3580
and 3U8O a T 1 - A correlation of the corresponding stretch frequencies of
the water sub-bands and of the 0-H and 0-D species i s given in Table 1.
Figure 8. Bending Region for IfeO Molecule, IU5O-I6OO cm"1, at U0°C
The frequency region of I6OO-X650 cm"1 has been associated with the
bending frequency of aolecular water. The lower wave rmaber has been
attributed to mono-molecular water and the higher values have been assigned
to "polymeric" H-bouded water.* 1 9- Figure 8 presents the effect of step­
wise adsorption of water at U0°C on exposure to a dehydrated sample. The
transBdssion spectrum of (8a) was obtained after the oxide had been cnt-
gassed at 500°C and allowed to cool under vacuum to 1*0°C. The spectrum of
this extensively desorbed sample suggests that the partially resolved peak
at 1595 cm"1 i s due to the bending motion of uni-molecular water. ^' As
the surface i s exposed to a larger partial pressure of water, the resulting
band broadens, increases in the integrated absorbance and shifts toward
the higher frequencies of 1620 cm"1 at 100 \t. IfeO (8b), and to 161*0 cm - 1 at
2.7 torr JfeO (8c). The peak at 151*0 cm"1 does not change markedly with
observed experimental conditions and i s assumed to represent a vibration
contribution from the zirconium oxide latt ice . The spectrum (8d) i s ob­
tained on exposure to water at k.6 torr. The increased absorbance of the
peak at 161*0 cm"1 i s indicative of a further increase in the concentration
of multimolecular water bound to the oxide. Steps in the desorption of
watf.T at 1+0°C are shown in the following traces. The successive out-
gassing of the oxide at 5 x 10"8 and 10"6 torr (8e, 8f) causes the
19
TRACE 0 b c d e f
P VACUUM
5O/*H20 VACUUM
0.80
18 16 14 18 16 WAVE NUMBER (cnT'x 10 ' 2 )
14
Pig. 8. Transmission Spectra of the Bending
Region of HpO Adsorbed on Zr02-G, 1U50-1800 cm"1 Region, at <*C*C
9a Temperature reduced to l0°C, under vacuum, after outgasjing
at 500°C
Adsorption of molecular water at 2.6 torr H2O
Adsorption of molecular water at U .6 torr H ?0
Spectral trace on - eduction of water pressure to 50 u.
8b
8c
8d
8e
20
^ending frequency band t o s h i f t tov&r-i 1600 cm" 1 , ind ica t ing a decrease in
the population of H-bonded molecular water spec ies .
Figure 9- Sending F r t ^ a n c y Region for H3O, IU5O-I8OO cm" 1 , a t 300 and 500°C
Traces in F ig . 9 ^ n o w the spec t ra l sfcii'i accompanying desorption from
the oxide a f t e r -.;quilibroting with k.b t o r r HgO a t 300'C ( 9 a ) . The p ro ­
gressive out tass in* of the sample a t 1 u. and 10~ 5 t o r r a t 300°C, followej
by 10~ 5 a t 500°C, i s displayed in (9b) , ( 9 c ) t and (9d) , r e spec t ive ly . The
in t ens i t y of t h e vater spectra can be judged r e l a t i v e t o the ZrOa l a t t i c e
vibra t ion a t 15'*0 cm" . Here each of the spec t ra l t r aces has been shif ted
by 0.15 uni te in the transmittance scale t o cive a be t t e r v isual est imate
of the Hecrea.se in i r t e n s i t y and the displacement of the bending frequency
mode as a function of the desorption condi t ions . Here again the progressive
outgassing a t 300°C reduces the broad psak a t 1600 cm - 1 t o a narrower band
a t 1590 cm - 1 and to an unresolved shoulder of the 15^0 u s " 1 band a t 500'1C.
I t should be noted tha t the t r ace of (9c) was reproducible before and a f t e r
exposure t o U.6 toi :* of water.
Zr0 2 -G TRACE r(°C) P
o 300 4.6 Torr H 2 0- i b 300 l / t H 2 0 -
300 VACUUM 500 VACUUM
Fig. 9. Transmission Spectra of the Bend-i g Region
of HgO Adsorbed on ZrOg-G, IU5O-I8OO cm"1 Region, at 300 and 500°C.
9a Adsorption of molecular water at k torr HgO at 30'J°C
9b Outgassed sample at 1 n and 300°C
9c Outgassed sample at "-0"5 torr and 300°C
o«i Desorbed sample at 10 torr and 500°C
22
Monoclinic ZrOs-G
^ ( 0 , - H ) ^ uCQa-K)^ 3660 2695
Ga Bound HaO 3580 2#*0
G3 Bound HaO 3^0 2560
(a) Frequency contribution of surface 0-H groups bound
to single cation ("free" hydroxyl group) of the monoclinic phase.
* 'Stretch frequency of bound 0-H groups coordinated to more than a single cation.
23
Figure 10. Transmission Spectra, 2UOO-UOOO en - 1, at and 2O0"C
The transmission spectrum of the expanded 36OO-UOOO cm - 1 region (10a')
again reveals a sharply resolved band at 3760 cm-1 on the initial out-
gassing of the sample at 100°C. Exposure to U.6 torr IfeO at this tempera­
ture gives rise to a broad absorption band, (10b), eliminating the resolu­
tion of the 3 760 cm-x band. Subsequent outgassing at lOCC results in
(10c), a trace similar to the initial outgassing. The difference Is ob­
served in (10c') where the intensity of the resolved 3760 cm" band is
reduced somewhat. A further outgassing at 20C°C results in trace (lOd)
and (10d'). The buildup of the band due to organic matter in the 2&50-
3000 cm'1 region is again encountered and was removed at a later stage by
exposure to Oa at 300 and U00°C.
Figure 11. Transmission Spectra, 2UOO-UOOO an"1, at 300°C
Outgassing the sample at 300°C resulted in trace (lla) and (lla'}.
The expanded trace (lla') indicates a broad snoulder ertending from 3720
to 3760 cm"1. Exposure to U.6 torr IfeO, (lib), results in an increased
absorbance in the lower frequency region and a reduced resolution of the
high frequency shoulder as noted in (lib) and (lib'). A subsequent out­
gassing depopulates the low frequency region (lie) fnd also changes the
resolution and contribution of the high frequency species (lie').
2U
omn-ass. 79 9a» y 1 1 n—i f .OO — I ' 1 • T
ZrOg-C TRACE rt«C) P
a' WO VACUUM b 100 4.6 Tvr HgO /
c'.c fOO VACUUM ; d'.d 200 VACUUM
40
-0.60
0.20
Ui >
37 36 40 36 32 WAVE NUMBER (cm~'i 1 0 * )
Fig. 10. Transmission Spectra, 2b00-lt000 cu"
Region, of ZrO -C at 100 and 200°C
10a' Initial outgassing of temple at 10OOC, 36OO-V0OO en" 1
region, on U-fold expanded frequency scale.
Adsorption of water at U.6 torr, 2l*0G-*O0O as" 1 , at 100*C
Desorption of sa«ple at 100*C, 2l*00-l<XV, cr" 1
10b
10c
10c'
region. Sate, on a l*-fold expanded frequency scale, 3600-kXX)
-1
lOd Desorption of water at 200'C, 2U00-l»OO0 as " region.
10d' Sane, on a U-fold expanded frequency scale, 3600-1*000 csf *
25
WAVE NUMBER (cm* 1i KT* )
Fig. 11. Transmission Spectra, 2fc00-l»00 cm~x
Region, of ZrOg-C, at 300°C.
11a Outgassed sample.
11a' Sane., on a U-fold expanded frequency scale, 36OO-I4OOO cm"
region,
l ib Adsorption of HgO at k.6 torr, 2*100-1*000 cat"1 region,
l ib ' Sane, on a It-fold expanded frequency scale, 36OO-UOOO cm"'
region,
l i e Desorption of water, 2U00-U000 CM"1 region.
l i e ' Sane, on a U-fold expanded frequency scale, 36OO-UOOO cm"
region.
26
Figure 12. Transmission Spectra as a Function of Temperature
Trace (W) and (v') represent the spectra obtained in an earlier
exposure to U.6 torr H»0 at 100°C and gives some measure of the extent of
desorption as a function of temperature. On outgassing at UOO'C a large
depopulation in the low frequency region is observed in (12a). A short
expc—ixe of the sample to 100 torr 0» folic-ed by outgassing at U00°C,
(12b), removes the organic band (G) and at the same time raises the trans*
mittance in the 3700 to 1*000 est"1 region by a factor of about 2.
The expanded trace (12b') indicates that partial resolution of the
high frequency band has shifted toward 3723 cm"1 and a band at 3660 cm'1
becomes visible. Spectral traces (12c) and (12c') are obtained on reducing
the temperature to 300" under vacuus. Here an improved resolution of
broad sub-bands appear at the 3593 en'1 and at the 3t*kO as"1 wavenumbers.
The band at 3725 as'1 shows better resolution in (12d) and (12d') on
proceeding to an outgassing temperature of 500°C.
Figure 13. Absorbance Spectra, 3000-1*000 C M ' 1 . UOO, 500 and 300°C (1st
cycle)
The absorbar.ee spectrua obtained at an outgassing temperature of kOO'c
is given in (13a), showing the partial resolution of bands at 3725, 3660,
and 3Ut0 en"1. Outgassing at 500°C followed by cooling to 300"C results
in traces (13b) and (13c), respectively. An estimate of the half widths
of the two higher frequency bands shows a half-width of 70 en"1 and 90
cm'1 at 300°C versus a value of 75 cm"1 and 120 cm"x at 500aC. The dif­
ference in the half-width bands at 3725 cm'1 cannot be considered as
significant because of the band overlap with the strong absorbance at
366O cm'1 at these elevated temperatures. Exposing the sample to U.6 torr
of Zr02-C, as a Function of Teaperature.
12v Spectral trace of water adsorbed at U.6 torr at 100*C,
2U0O-UO00 ca" 1 region.
12w' Saae, on a U-fold expanded frequency scale, 3600-UOOO
ca* region. 12a Outgassed saaple at UOO'C, 2UO0-U00O en ' 1 region. 12b Exposure of sanple to 100 torr oxygen at UOO'C, 2U00-
UOOO ca" 1 region.
12b' Saae, on a U-fold expanded frequency scale, 36OO-UOOO ca region.
12c Saaple tasperature reduced, in vacuua, to 300'C, 2UO0- UOOO ca region.
12c' Saae, on a U-fold expanded frequency scale, 36OO-UOOO ca" 1 region.
12d Saaple outgassed at 500"C, 2UO0-U000 ca" 1 region.
12d' Sane, on a U-fold expanded frequency scale, 36OO-UOOO c a _ 1 region.
2b
1.00
WAVE NUMBER t cm"' * fO^i
Fig. 13. Absorbance Spectra, 3000-1*000 cm"* Region,
of Zr02-C at UOO, 500 and 300°C (f irst cycle).
13a Ab&orbance spectra of saisple desorbed at U00°C at. 6 x 10
torr.
13b Desorption temperature raised to 500°C at 3 x 10"^ torr.
-6
13c Desorption temperature reduced to 300°C at 2.3 x 10 torr.
29
water at 100'C, a second cycle of outgassing at 500°C and cooling stepwise
to 300 and 100°C under vacuus gave results in agreement with the absorb-
ance spectra observed in he previous cycle.
Figure lU. Transmission Spectra, 33O0-1«000 ca"1 (temperature cycling)
Figure lU shows traces as a function of temperature en a fourfold
expansion of the frequency scale. The absorption and desorption of small
partial pressures of HaO at temperatures up to 500*C appear to have re­
solved the high frequency stretch vibration of 0-H species and several
sub-bands of the stretch frequency of K-bonded water. The very first
outgassing of the untreated discs at 100*C indicates that the high freq­
uency band in (lUa) occurs at 3760 cm'1 with no further resolution of
bands at lower frequencies. This band is similar to that observed for the
aonodinic sample, ZrOa-G. However, after several water adsorption and
desorption cycles to 500°C (Figs. IP. and ij) and cooling to 300°C in vacuum,
the spectrum of (lUb) indicates that the broad high frequency band has taken
a marked downward shift from tne value in (lUa) to an unresolved doublet
with a maximum absorbance value at 3725 cm"1. An unresolved doublet like­
wise appears at 3o62 cm - 1 and 36^5 cm"1. The broader sub-bands al. 3595
cm"1 and 3V«0 cm"1 are also observed here. On further cooling tc 100"C
the trace (lUc) indicates that the high frequency band divides into two
partially resolved bands at 37^0 and 3720 cm"1 and the broadened band at 3660
cm"1 is also evident. This trace (I've) has been lowered by 0.03 units in
transmittance to allow for spectral comparison viewing. After a second
adsorption-rtesorption cycle up to 500°C, the spectrum of (lUd) at U0°C
shows a further decrease in absorbance but with a somewhat better resolu­
tion of the absorption bands. A splitting of the band at 3662 cm"1
30
33
Pig. ll». Transmission Spectra of ZrOg-C as a
Function of Temperature Cycling, 3300-liOOG a t ' 1 Region (U~fold
expansion of frequency cycle).
lk> Desorbed sample at 300° after first desorption cycle of
Pig- 13-
lUc Desorption temperature reduced to 100°C
l*4d Desorption temperature reduced from 500"C to 1»0*C after
a second adsorption-desorption cycle.
31
indicates a poorly resolved shoulder at ~36**5 CB" 1 • The broad bands at
3595 at"1 and 3UJ0 CM" 1 are also resolved to a greater extent as a result
of the teBperature cycling. The above band assignments are listed in
Table 2.
Tetragonal ZrOa-C
Z r - ( 0 l b - H ) ( a ) «(°Xb-H)ia) 3720 zruo
£::(o.-H)<b> "(0.b-H)f }
Ca Bound H3O ^ V ) 25U0
v 'Frequency contribution of surface 0-H groups bound to single cation at two different spacings ("free" hydroxy 1 groups) of the tetragonal phase.
* 'Stretch frequency of bound 0-H groups each coordinated to •ore than a single cation, at two alternate spacings.
32
Figure 15. Transmission Spectra of FfeO-D O Species, 2U00-it000 cm'1 at l60°C
Spectrun (15a) of the outgassed water treated sample shows the resolu­
tion of the 3660, 3595 and the broader 3W*0 car1 bands. The pattern
obtained after 2 hours of exchange at 800 p, EfeO pressure is shown in (15b) •
The low concentration of D^O results in a partial resolution of its sub-
bands. Further exchange with h torr 0^0 produces the trace (15c) where
broad bands are observed for both the IfeO and DjO vibration frequencies.
Subsequent outgassing leads to a narked resolution of the sub-bands of
both species, (I5d).
Figure 16. Transnission and Absorbance Spectra of D^O Exchange at 300°C
An exposure of 800 •„. IfeO at 300°C for 18 hours produced the spectral
trace of (16a). The continued resolution of both sets of structure bands
and also the reduced overlap between the JfeO and D^O stretch frequencies
is observed. On outgassing, the spectrum of the lower frequency is re­
duced, resulting in a higher resolution of the spectral sub-bands (l6b).
An absorbance trace at a two-fold reduced scale factor of the outgassed
sample is shown in (l6c).
Figure 17- Transmission Spectra, 3C0oC (before and after D^O Exchange).
A four-fold expanded frequency trace of the outgassed oxide prior to
exchange with DjjO is given in (17a). Following the exchange at 800 ^
Dj»0 (16c) and the subsequent outgassing, the spectrum obtained in (17b)
showed a great deal of substructure for the 0-H stretch frequencies.
The corresponding substructure observed in the 0-D stretch frequency
region is shown in the inset of (17c) on a two-fold reduced intensity
scale. The correlation of the corresponding stretch frequencis.; for
the 0-H and 0-D species is given in Table 2.
33
o VACUUM b eOOpDzO c 4 Torr 020 d VACUUM
-aeo
1.00
0.20
32 28 24 40 36 32 WAVE NUMBER (cm* ' i<0 ' 2 )
Fig. 15. Transmission Spectra of F-20-D?o Species on
Zr0 2-C, 2liOO-l4000 a T 1 Region, at l 6 0 ° C
15a Outgassed saaple at 160°C
15b Exchange at 600 p DgO pressure
15c Further exchange at U torr DpO
15d Outgassed saqsle a t l60 e C
3^
1.00
0.80
~ I
C K ? N L - D » 0 . 73 -82 I9R T
ft
\ X
ZrOg-COOO'C) TRACE P o 800 p. C 20 b VACUUM c VACUUM
J i
WAVE NUMBER (cnr'x lO"2)
Fig. 16. Trajstti ssion and Absouance Spectra of
D#J Exch.ar.ge on Zr0 2-C, 2toO-l»000 cm"1 Region, at J00°C.
16a Transmission spectra of sample expo: ed to 800 ^ DoO
at 30L'°C
16c Absorbar.ce spectral trace of (l6b) , using a 2-fold reduced
scale factor
WAVE NUMBER (cm-^K)" 2 ) 28 27 26 25 24
1 • 1 • 1
~ » \ 1 \ » 1 1 1 * \ 1 1 \ 1
\ 1
~ » \ 1 \ » 1 1 1 * \ 1 1 \ 1
\ 1 — \ 1 V I V 1
X 1 1 1
TRACE P \ \ a VACUUM (PRIOR 0 ^ ) \
b.c VACUUM (POST D 2 0) ', t *
/ H 2 0 ** TRACE P \ \ a VACUUM (PRIOR 0 ^ ) \
b.c VACUUM (POST D 2 0) ', t * .-"'""* "
l . l . l 1 1 1 1 1 I . I .
1.00
-0.60
UJ > <
40 39 ' 9 37 36 35 34 WAVE NUMBER ( cm"' K 10"2 J
33
Fig. 17. Transmission Spectra Before and After
D9C Ex.ihange on Zr02-C, 2U00-1»000 cm"1 Region, at 300 °C
(It-fold expansion of frequency scale).
17a Outgaosed sarple before D O exchange, 3200-1*000 cm region
Transmission spectra of water stretch region of the out-
gassed saiqple following the D^ exchange shown in Pig. 16.
Transmission spectra of analogous water bands for DgO,
2toX)-2800 cm" region (2-fold reduced intensity scale).
17b
Jjc
36
DISCUSSION
Monoclinic ZrOa-G Spectra
The broad unresolved absorption band observed in the 2U00 to 3^00
cm"1 spectral region is generally attributed to the vibrational frequency
contributions of hydrogen bonded polynuclear water molecules adsorbed on (22) oxiie surfaces. In HaO desorption studies, relatively narrow bands
at high frequencies have been assigned to the stretching frequency vibra- (21 22 23) tions of hydroxyl groups bound to the cation. ' ' The asymmetric
distribution of forces of debydroxylated surface oxides produce active
sites which are capable of dissociating water molecules. The heat of ad­
sorption derived from heats of immersion for these strongly desorbed zir­
conium oxide surfaces are of the order of 30 kcal/mole IfeO. The chemi-
uorption of each water molecule satisfies the active surface by forming
2(0H) group3 on oxygen lattice sites.
The trausmittance spectra of the 2U00 to 14000 cm"1 traces in Figs. 1,
2, and 3 illustrate the effects on the stretching frequency region of
successive adsorption and desorption at U.6 torr of water at increasing
temperatures. The 3760 cm"1 band appears as a shoulder in the adsorption-
desorption cycles at the higher temperatures. In the same series of
spectra the desorption &•; increasing temperatures leads to a successive
depopulation of the lower frequency species; thus bringing about the reso­
lution of the band at 3660 cm"1 and the much broader band at 3Wk> cm"1.
The resolution of the latter bands suggests that an increasing desorption
of the more weakly bound molecular water occurs. The progressive improve­
ment in the resolution of the observed bands is marked on cooling the
desorbed sairple from 500°C to 100'C in vacuum [Fig. k). This improvement
37
i s undoubtedly due t o a decrease in to r s iona l not ion of the bound species
and to smaller thermal cont r ibut ions of the oxide l a t t i c e . Reproducible
absorbance t r a c e s were obtained on a repeated adsorpt ion-desorpt ion cycle
through t h i s temperature range. The transmission s r e c t r a of F ig . 5 on an
expanded frequency scale fur ther de l inea tes the spec t ra l bands on reducing
the temperature of the desorbed sample. The peak values a re observed a t
3760, 3660, 3580 and 3 ^ 0 cm" 1 .
Successive exchanges with up t o k t o r r DaO on the outgassed oxide a t
UO and 300°C (F igs . 6 and 7) take p lace r ap id ly , ind ica t ing t h a t *Jae ex­
change takes place on the surface - The repeated exchange of DaO a t 300*C
(Fig. 7), followed by outgassing, adds e f fec t ive ly t o a further increase in
resolut ion of the s t r e tch ing frequency bands a t 366O, 3580 and 3W0 cm" 1 .
The addi t iona l exchange . i t h DaO d i l u t e s the bound ffeO species and thus
leads t o the improved uncoupling of the water sub-bands. At low concen­
t r a t i o n s of EbO the absorbance bands of the DaO species likewise show
b e t t e r r e so lu t ion . A co r re la t ion of the corresponding s t r e t ch f.-fquendes
for the 0-H and 0-D species i s given i n Table 1 .
The absorption band at 1600 t o I65O cm"x i s associated with the
bending vibration of the water molecule bound to the oxide surface. The
observed spectra in th i s frequency region (Figs. 8 and 9) indicate tfcat
the band observed at 1590 t o 1600 cm"1 may be attributed t o the presence
of bound aoncaeric water in analogy t o the assignment given in matrix
isolated water s t u d i e s / 1 9 ' The increases in the adsorption of water
leads to an increase in the bending frequency and the absorbance of t h i s
band.
38
adcropores are present in this xircociuB oxide sasple. The contributions
to the bending frequency peak at values greater than 1600 cm"1 nay be
attributed to aultlmers of H-bonded water ao lecu le s* 1 9 , 2 0 * retained in
these snail pores even at elevated tenperatures (Fig. 9) • The presence of
H-bonded aultimers of water molecules i s also evident by the appearance of
the broad absorbance band in the stretch frequency region (Figs. 5 ajd 1U)
below 3600 cm" 1.
The bending frequency of the bydrasyl group i s expected* ' -to l i e in
the 1000 cmTx region. However, strong absorption band?, attributed to the
oxide la t t i ce , nasked this spectral region.
Tetragonal ZrOa-C Spectra
The transsdttaitce spectra of Figs. 10 and 11 show a remarkable simi­
larity to that observed for the monocllnic zirconium oxide sample in these
in i t ia l adsorption-desorption cycles. Desorption cycling at the tempera­
tures from 300 C to 500'C (Fig. 12) develops high frequency bands that
differ scmewhat froa the values observed for the aoooclinlc oxide. A
noticeable shift and splitting of the two highest frequency peaks develop
on repeated adsorption-desorption cycling.
Absorbance spectra on further thermal cycling in vacuo between 500 and
100'C (Fig. 13) «nov Improved resolution on cooling. Here the maxima ab-
sorbance of the high frequency band i s resolved nX 3725 cm"1 and differs
aarkedly fro*, the value of 3760 cm"1 obtained in the in i t ia l treatments of
this wafer. The estimated half widths of the observed absorbance bands
here also indicate that the thermal aotlon of the absorbate and absorbent
39
influences the respective band widths of the two high frequency stretch
bands (Fig. 13).
The transmission spectra of Fig. 1U taken on an expanded frequency
scale and at & slower scanning rate show improved resolution of several
bands as the desorption temperature is lowered from 500 to ltO°C in vacuo.
Soae splitting of the two highest frequency peaks is evident and will be
taken up after further discussion of the molecular structure cf these
oxides.
The exchange with DaO results in an improved uncoupling of the broad
water bands. The exposure of the desorbed disc to varying pressures of
DaO at 160 and 3O0°C (Figs. 15, 16) resulted in rapid exchange. Tie
corresponding sub-bands of the 0-H and 0-D species show improved reso­
lution of the expanded frequency scale of Fig. 17- The correlation* between
the observed 0-H and 0-D species are listed in Table 2. The ratio of the
corresponding frequencies is in good agreement with the expected value.
The additional complexity in the behavior of the ZrOa-C sample led to (25) an x-ray diffraction investigationv " of both the original oxide and the
pressed disc. The original powder geve the diffraction pattern for the
tetragonal phase of ZrOa - On the other hand, x-ray diffraction lines for
both the tetragonal and monoclinic phases vrere indicated on examining the
pressed disc. Further study showed that this transformation was also a (25) function of the severity of grinding/ " The diffraction pattern of the
original ZrOa-G powder corresponds to the monoclinic phase along with a
trace of the strongest reflection (111) of the tetragonal zirconium oxide
structure. The latter reflection was barely detectable for the ZrOa-G
pressed disc.
ko
The in i t ia l spectra obtained ua aitgassing the ZrOg-C sample at 100*C
(Fig. 10) showed a resolved high frequency band at 3760 cm"1, similar to
the value obtained for the monoclinic ZrOg-G disc. However, on adsorption
and thermal desorption cycling with water, this band shifted significantly
to a lower range. The explanation advanced i s that the pressure-induced
strain to the individual particles of the disc i s relieved by the cycling
treatment only on the oxide surface. The observed spectra would support
the proposal that the surface structure of the ZrOa-C disc reverts back to
the tetragonal phase in the region cf the bound absorbate Molecules on
r'-peated water cycling treatment. The strained •oncclinij structure say
retain to s u e extent below the surface of the individual particles; thus
accounting for th» observation that the x-ray pattern showed the precczce
of the monoclinic phase even after thermal cycling of water on the ZrOa-C
disc.
The reported crystal structurev ' ' (P2,/C) of the •onoclinic oxioe
indicates that zirconium has a coordination to 7 oxygen atoms in a some­
what distorted, pseudo fluorite type structure. The spacings occur at
the average distances of Zr-kO-t at 2.21 X,and Zr-30_ at 2.09 A. The
v^viog of the tetragonal form of ZrO» can also be represented by a dis­
torted Cafa-type arrangement. The zirconium atom i s coordinated to 8
oxygen atoms consisting of two interpenetrating tetrahedra, with two dif­
ferent spaclngs, namely, at the distances of Zr-UO.- at 2.1*3 A and Zr-U0_
at 2.0U A. The spectra observed in the water adsorption ctudies are
undoubtedly influenced by the differences in the zirconium-oxygen coordi­
nation and spacing*. Proposed models for the interaction of these two
phases with water will be discussed in a separate report.
An ant-iysis of the infrared spectra may be made by examining the
similarities and differences in the distorted fluorite packing Arrange­
ments for the two polymorphs. It is suggested that the stable configu­
ration on the surface of the zir^onia polymorphs consists mainly of
exposed oxygen and hydroxy anions u the proper ratio to satisfy the cation
charge and the nore stable coordination of 8 anions to each zirconium atom.
The concentration and statistical distributions of hydroxyl anions bound to
the surface are governed largely by t>& environmental and thermal exposure
conditions.
The average particle size of the ZrOa-G and ZrOa-C sample is 210 A*
and 129 A*, respectively. Thus we may assume that planar surfaces
with periodic low index steps are the major contributors to the absorbate
interface. In this instance the corners, edges and dislocations are con­
sidered to furnish the more active sites but statistically fewer contribu­
tors for hydroxyl formatior. The highest observed frequency bands are
considered to be due to chemisorbed hydroxyl anions and are present at
lower concentrations for both of these polymorphs. These bands are assigned
as "free" hydroxyls bound to a single zirconium, [Zr-(O-H)], on these more
active sites. Furthermore, it is geometrically feasible for sa'ter mole­
cules to form H-bonds to the oxygen a;om of the "free" fcydroxyl. We
designate this schematically as [H-0 — IfcO]. The donor water, H-bonded
to the oxygen atew of the "fre*" hydroxyl, polarizes the hydrox^l group
and thus reduces its vibration frequency. This assignment is based on the
infrared » and structure^28'29' data for the anhydrous and hydrated
compounds, LiOH and LiOH-HgO. A neutron structure determination^' of
the monohydrate shows that the hydrogen atom* of two water molecules are
kZ
H-bonded to each of the oxygen atoas of the hydroxyl groups without appre­
ciable distortion of the t-rtrahedral conforaation of the water snlecule.
The infrared data ' indicate that the aonohydrate reduced the hydroxyl
stretch frequency by about 100 ca" 1.
The sites available for hydroxyl binding on the surfaces of the low
index planes are statistically aore favorable. The frequency band at
3660 cm'1 eaerges with the largest total absorbance when the "".ess strongly
bound water aolecules are desorbed. Hater aolecules H-bonded to the "free"
hydroxyl groups are considered to be responsible for the increased overlap
of its stretch frequency with the 3660 ca"1 band.
These observations lead us to assign the 366O ca"1 band to surface
nydroxyls on planar surfaces. Each of these hydroxyl groups occupy an
oxygen site on a low index face and art thus coordinated to tvo or three Zr zirconiufi a teas, rfe represent this scheaatically as: _ XO4-H). Thus
in the (100) plane a hydroxyl group aay be foraed on the surface coordinated
to 2 cations and in the (111) plane to 3 cations. It is assuaed that the
3660 ca'1 band is broad enough to include the stretch vibration frequency
of the hydroxyl group shared by 2 or 3 zirconiua atoas. Steric restrictions
prevent an analogous hydrogen bonding of the water solecule to the oxygen
atca of this hydroxyl group, as appears for the "free" hydroxyl.
The cuch broader lower frequency bands that eaerge in the desorption
studies are attributed to strongly bound water species but are aore uncer­
tain in assignment as to specific binding types and sites. The frequencies
of the broad sub-bands, Qa and G 3, are sinilar to aultl-aolecular H-bonded
water bands derived froa infrared' 5 ' and Raaan* ' studies of HaO-D^O
±1
mixtures. Differences in assigned values can be attributed to the influ­
ence of the solid surface interaction with the absarbate.
The apparent splitting of the high frequency peaks observed for the
ZrOa-C (Figs. Ik and 17) mev be attributed to the structure of the tetrago­
nal phase. For this structure, the two interpenetrating tetrabedra of
anions about each zirconium at two different spacicgs offers two alter­
nate sites for the distribution of oxygen and hydroxy 1 species. Thus,
band splitting is observed for these "free" feydroxyls with two different
force constants. These are designated in Table 2 and are represented
schematically as Zr-(0 -H) and 2r-(0 -H). Surface bydroxylc likewise l a l b
coordinated at two alternate spacings to more than a single zircotium atoa
are assumed to produce the observed bond splitting at 3660 cm*1. The
broad sub-bands, Ca and Ca, are assigned to bound water species on the
surface but perhaps even more strongly held in the micropores of these
o x i d e s / 2 '
The DaO exchange study of Figs. 16, 17 and IS demonstrates that a
resolution of th-? sub-bands is enhanced. The observed ratios of y(0-K)/
u(O-D) are likewise in good agreement with theory (Table 2 ) .
The I.R- spectra of pressed disc* zt the monoclinic and tetragonal
zirconium oxides* ' were observed at 120 and ^30 C by Erfcelens. Tbe mono-
clinic oxide preheated at l*00*C shoved tbe presence of high frequency bands
at 3726 cm~x (weak) and 376O cm - 1 (strong) and a very broad band below
3500 cm' 1. The tetragonal sample rrcheated at 600*C gave a strong band
at 3728 cm"1 and a broal band bei-w 36OO cm"1. A reported* ' weak band
at 3815 cm" appears to be -spurious. Tbe powders were stated to brve been
pressed with 10 tons but no parade sizes were indicated. It i s likely
kk
that the latter saaple consisted 80111117 of the tetragonal phase. At any
rate i t i s our experience that the development of details in the sub-band
spectra would require thermal cycling with water.
9MUK¥
Thermal desorption spectra of low concentrations of water on large
surface area oxides appear to be analogous to Matrix freezing studies of
adsorbed gases. '* Although low concentrations of species are affected
in both instances, the thermal contributions from the absorbate and ab­
sorbent in this study result in such broader spectral bands.
The stretch frequency bands resulting from the dissociative adsorp­
tion (rehydroxylation) of water molecules on the LNO polymorphic forms
of zirconium oxide indicate that differences exist in the surface forces
of the respective oxides. The spectral traces result!*^ from desorption
cycling at temperatures up to 300*C show a large reduction of the broad
stretch frequency bands due to molecular water and impi*>ved resolution
of the spectral bands of chemisorbed species.
The high frequency bands, with relatively narrow half widti*«. are
assigned to the stretch frequency of chemisorbed hydroxy 1 anions. Toe
highest spectral bands are assigned to the "free" hydroxyls bound directly
to a single cation at a lattice site. Toe band width and intensity due to
this species are affected on exposure to relatively low pressures of
water vapor. This is attributed to the geometric accommodation of a donor
water molecule to H-bond to the available unsharM electrons of the 'free"
hydroxy1 anion.
The surface bydroxyl anions coordinated t o nore than a s ingle cation
are positioned at available oxygen l a t t i c e s i t e s on a given face and are
thus incapable of accepting a proton fi-om a donor water Molecule because
of s ter ic factors. Apparently the surface hydroxyl anions are present i c
larger concentrations as noted frost their greater spectral absorbance in
both of these zirconia preparations.
The resolution of the broader spectra attributed t o bound water Mole­
cules at the lower frequencies are enhanced by the repeated adsorption
and iesorption cycling. The part ia l ly resolved shoulders at 3580 CM"1
and 3595 CM' 1 for the respective polyMorphic oxides amy be tentat ive ly
assigned to the dipole attraction of the water Molecule, to exposed cat ­
ions , analogous to that observed in hydrate crystal structures.
H-bonded Molecular water, with i t s high extinction CDefficient, when
adsorbed on these oxide polymorphs results in a bro&d unresolved frequency
band (Fig. 1 ) . Successive desorption cycles at increasing teaperatures
produce a Mocotonic decrease in the intensity of the lower frequency
region of th i s oand due t o lover -«ater concentrations. The decrease in
intens i ty and the lowering of th* bending frequency band of water (Fig.
8 , 9) i s likewise a Measure or the desorption of bound Molecular water.
The part ia l ly resolved frequency bands, Mentioned above, eMerge in the
spectnm as a result of the lowered contribution of H-bonded Molecular
water. In addition to the assig-aent of these higher frequency bazds,
a More uncertain assignment i s proposed for the very broad sub-bands that
are part ia l ly resolved at 3U8O CM' 1 and 3^*0 CM' 1 Table 1, 2) on the
desorbed nonoclinic and tetragonal zirconiuM oxides, respectively. Molecu­
lar water i s retained in desorption Heasureaents, as evident by the observed
I*
bending frequency ibove 1600 est*1. The stretch frequency of bound water
probably contribute to tb»se broad sub-bands. Several types of bound water
species aay contribute to these broad frequency bands. On extensive de-
sorption, •olecular vater trapped in Micropores of these oxides are likely
contributors to these eaerging absorbance bands. The dipole interaction
of water •olecules with donor surface hydroxy1 anions is considered to be
weak; and thus, is an unlikely contributor to these bands on the
extensively desorbed oxides • However, this weak interaction aay be
effective in binding water on exposures of the hydroxylated oxide to smll
partial pressures of water. ?M»-ther spectroscopic studies of "titrations"
of hydroxylated surfaces with other molecular species aay shed addxtional
understanding and identification of the various active surface forces or
these oxides. The latter knowledge would be useful in evaluating the
catalytic capability of these tetravalent high surface area oxides.
47
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