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Microstructure of Ex-Hydroxide Magnesium Oxide & Products of Rehydration
F. Rodriguez-Reinosoet al. (Editors), Churacterizatwn of Porous Solids 11 0 1991 Elsevier SciencePublishers B.V., Amsterdam
635
MICROSTRUCTURE OF EX-HYDROXIDE MAGNESIUM OXIDE & PRODUCTS OF REHMRATION
M.M.L. Ribeiro Carrott, P.J.M.
Carrott, M.M. Brotas de Carvalho
Departamento de Quimica, Faculdade de Cibncias, Universidade de Lisboa, Rua da Escola PolitBcnica, 58, 1294 Lisboa Codex, Portugal. K.S.W.
Sing
Department of Chemistry, Brunel University, Uxbridge, UBS 3PH, UK.
ABSTRACT The thermal decomposition of magnesium hydroxide under carefully controlled conditions, followed by subsequent rehydration, has been studied by means of nitrogen and neopentane adsorption isotherm measurements. Up to about 85% decomposition a uniform layered particle structure, consisting of plate-like microcrystallites intercalated by slit-shaped pores of width ca. 0 . 9 3 run, gradually spreads from the outside towards the centre of each crystal. At this stage each particle consists of MgO of normal crystallographic structure, with exposed faces partially covered by chemisorbed water. The presence of this water stabilizes the structure and, when it is removed at higher temperatures, a restructuring of the microcrystallites occurs leading to an increase in the mean pore width. Below 85% decomposition, narrow constrictions, near points of contact of the microcrystallites, are present. As a result of this, approximately half of the total micropore volume is inaccessible to nitrogen. On the other hand, the smaller water molecule appears to be able to penetrate the entire micropore structure. The restructuring of the microcrystallites at higher temperatures appears to lead to complete closure of some micropores, with the result that rehydration of the closed porosity now involves a bulk reaction and therefore occurs at a considerably slower rate.
INTRODUCTION
Although the thermal decomposition of magnesium hydroxide has been extensively studied in the literature the precise mechanism of decomposition is still not fully understood. Furthermore, there remains considerable controversy over the structure of the decomposition products. The bulk of the published work has involved either crystallographic studies of the structural transformations involved (reviewed in [1,21) or spectroscopic investigations of the nature of the active sites generated (reviewed in [ 3 , 4 1 ) . Rather less use has been made of detailed adsorption isotherm measurements to elucidate the textural and surface chemical properties of either Mg(0H) itself, the products of its thermal decomposition or their rehydration products (a guide to recent work can be found in 15-91).
636 At the outset of our investigation the state of knowledge ( i . e . the features which appeared to be generally accepted at the time, and which our results in no way subsequently contradicted) was as follows. The decomposition appears to proceed by the advance of a two-dimensional reaction interface from the edges of the crystal towards the centre [10,111. A definite orientation relationship is maintained between the crystal planes of the parent hydroxide and those of the product oxide, the major relationship deriving from the conversion of the hydroxide (0001) plane to a (111) plane of the oxide [2,101. A theoretical 54% contraction of the lattice is involved which, in the case of large crystals or when the decomposition is carried out too rapidly, leads t o the development of cracks and, ultimately, to the particle morphology being destroyed [ l l . Under conditions of slow thermal decomposition, on the other hand, the morphology of the parent hydroxide is preserved and a well defined micropore structure is generated [8,9l.Recent work from three independent laboratories has confirmed that approximately half of the total theoretical microporosity is inaccessible to nitrogen [6,8,91(although not necessarily to the smaller H20 molecule). Disagreement in
the
current
literature
centres mainly
around
the
structure o f the decomposition products, for which essentially two models have been proposed. In one [2,6,101it is argued that the product oxide consists of an array of MgO microcrystallites which are cubic in shape. The cubes are thought to be regular in size, with edge length ca. 2nm, and possibly in spatial distribution. The alternative model is based on an exfoliation of oxide into layers parallel to the basal faces of the crystals [8,9,121. One attempt has been made to try to combine both models by proposing that exfoliation into buckled layers of interpenetrating cubic microcrystallites occurs [lo]. Some of our experimental adsorption data has already been analysed in detail, and from a rather different point of view, in a preceeding paper [91. As
the reversibility of the decomposition might be expected to throw some
additional light on the structure of the decomposition products, additional results concerning the effect of rehydration were also obtained and are presented here. The structure of the decomposition products and the mechanism of decomposition are discussed in the light of these new results.
EXPERIMENTAL Details of the preparation of the two magnesium hydroxide samples, HID1 and HIDZ, and the controlled conditions used to carry out the decomposition have been given previously [91. Rehydration at room temperature was carried
637
out using freshly prepared samples, which had not been exposed to air, by equilibrating with water vapour at its saturated vapour pressure for 2 days. The
decomposed
“precursor/T1/Rh/T
‘I,
and
rehydrated
samples
are
designated
by
where the precursor is HIDl or HID2 and Rh indicates
that the sample was rehydrated after decomposition. TI and T are the final outgassing temperatures (15OoC if not stated) before and after rehydration respectively. Thus HID2/850/Rh refers to a sample of HID2 which was outgassed while gradually increasing the temperature in the manner previously described [91 over a period of about 2 weeks until a final outgassing temperature of
85OoC, then rehydrated, followed by outgassing firstly at room temperature for 15 hours and then at 150’C. Nitrogen adsorption isotherms at 77 K were determined using a Carlo-Erba Sorptomatic. Neopentane (ca. 273
adsorption isotherms at
ice melting
temperature
K) were determined using a CI Robal microbalance with pressure
measurement by means of a calibrated CEC strain gauge pressure transducer.
RESULTS Nitrogen isotherms were determined on samples of HIDl and HID2 at successive stages of decomposition and analysed by means of comparison plots using the corresponding undecomposed material as the reference adsorbent in each case [91. Each comparison plot had two linear regions. One at low pressures passed through the origin thereby enabling the total surface area, As,
of each sample to be estimated from the slope
-
excellent agreement with
the BET area was obtained in all cases. The second linear region, in the
multilayer, was of slope equal to unity, thereby indicating that the external surface area,
*ext
of the samples was constant, and that the hexagonal
particle morphology and aggregate structure of both materials were not significantly altered during decomposition. From the intercept of the linear multilayer region micropore volumes, vs, were estimated. Assuming slit-shaped pores [8,91,hydraulic pore widths were estimated from d = 2 vS / (As - Aext)
(Eq. 1)
P
For both starting materials, HIDl and HID2, the variation of vs and dP with %
decomposition are shown in Figure 1 (a) and (b), respectively.
It is evident from Figure 1 that, up to 85% decomposition, the micropore volume gradually increases, while the micropore width remains almost constant and the same for both materials. At decomposition levels greater than 90%. on the other hand, the micropore volume tends to decrease, while the pore width increases significantly. This alteration
in the micropore
structure is
638 1.8-
t
C
1.6-
41
E,
\
mal.L-
i
'0
'5
.
1.2-
: a
$
._
1.0-
E
OB
rehydrated
0.8-
I
20 Fig.1
Yo decomposition
80
I
1
I
1
% decomposition
Evolution of micropore volume and pore width for HID1 (on) and HID2 (0.) during decomposition (0.1 and after rehydration (om).
0.2 Fig.2
60
LO
-
0
0.L
P/PO
0.6
0.8
Nitrogen (a) and neopentane ( 0 ) isotherms corresponding to the two stages of decomposition.
accompanied by a characteristic change in the shape of the isotherms of nitrogen, but not those of neopentane, as can be seen in Figure 2, where representative isotherms for both stages are shown. It should be noted that this figure is a particularly good example of how the mechanism of adsorption, and hence the corresponding isotherm shape, depends on the ratio of pore width to molecular diameter and not just on pore width [131.
639
0.20
0.20
H I D1/300
--?
c
cn 0.15. 0
0
-
015
-
._ i+
U 3
._ I
i=i E u 0.10
m
E
u 0.10
\
\ U ul
U ul
P
p”
I
>
I1 1 original0 (3)rehydrated. 15%1
0.05
I
0.2
1
I
0.L
06
0.05
1
1
005
0.8
P/PO
Fig. 3
Nitrogen isotherms and corresponding comparison plots for HIDl (01 HID1/300 (01 HID1/300/Rh (01 and HID1/300/Rh/250 (m).
r
I
0.2
Fig.4
I
0.10
v,,.~,~HID1/150) / cd(liquid)g-’
I
04
I
P/P”
0.6
I
I
08
Nitrogen isotherms and corresponding comparison plots for HID2 (01 HID2/850 (01 and HID2/850/Rh (01.
Representative nitrogen isotherms and corresponding comparison plots for the decomposed/rehydrated samples, below 85% and above 90% decomposition, are shown in Figure 3 for HIDl and Figure 4 for HID2. As with the decomposed samples, the comparison plots are linear in the multilayer region. For each sample, the micropore volume, v , was estimated from the intercept of the linear multilayer region. The results, given in Table 1 and Fi.gure 1 (a),
640
TABLE 1 Characterisation of HIDl and HID2 samples. %
decomposition, total surface area, As and micropore volume, vs.
Values refer to unit weight of stoichiometric Mg(OHI2.
sample
HIDl HID1/270 HID1/300 HID1/300/Rh HID1/300/Rh/250
HID2 HID2/300 HID2/300/Rh HID2/850 HID2/850/Rh
%
*s
0
78.6 85.6 4.5 82.2
0 86.6 8.3 100.0 40.8
V
8
m'8-l
cm3g-'
99 239 268
0 0.064 0.080 0
96 256
37 272 42 140 57
0.079
0 0.105 0.002 0.081 0.008
indicate that, for the same % decomposition, the micropore volumes of the rehydrated samples are somewhat less than those of the decomposed samples. In contrast to the decomposed samples, the low pressure region of each comparison plot gives rise to a negative intercept, indicating that the nature of the surface of the samples is modified after rehydration. Furthermore, a change in the shape of the capillary condensation hysteresis loop occurs after rehydration, thereby indicating that the aggregate structure is slightly modified (but without a modification of the external surface area or hexagonal particle morphology). For this reason the values of total surface area given in Table 1 were calculated from the BET equation rather than from the comparison plots. The hydraulic pore widths, indicated on Figure 1 fb), tend to be somewhat lower than those of the decomposed samples. Transmission electron microscopy confirmed that the hexagonal particle morphology is retained during decomposition and rehydration. Powder X-ray diffraction demonstrated
that during decomposition the lines of Mg(OHI2
gradually decrease, disappearing completely at 85% decomposition, while those of MgO gradually increase in intensity. During rehydration the MgO lines decrease in intensity while the Mg(0Hl2 lines reappear. Under the conditions used, the MgO lines never completely disappear indicating that rehydration is not complete.
641 DISCUSSION
A detailed analysis [91 of the nitrogen adsorption data on the decomposed samples has confirmed that decomposition starts at the periphery of each crystal. The maximum observed in the micropore volumes of
the samples
(Figure 1 (a)), taken together with TEM, XRD and thermogravimetric results, indicates that the reaction reaches the centre at about 85% decomposition. At this stage the particles consist entirely of MgO of normal crystal structure, with the residual 15% of H 0 being present, not as Mg(OHI2, but as chemisorbed water [91. As neither the aggregate structure nor the morphology of the particles changes during decomposition, the theoretical micropore volume, based on the crystallographic densities of Mg(OHI2 and MgO, is 0.23 cm3g-' (with respect to
It is evident therefore that less than
unit weight of stoichiometric Mg(OH)2).
50% of the total micropore volume is accessible to nitrogen molecules and, hence, that the difference in the maximum micropore volumes for HIDl and HID2 arises due to a difference in the ratio of accessible to
inaccessible
porosity. The constancy of the hydraulic pore width (Figure 1 (b)), and the fact that it is the same for both samples, is consistent with a uniform particle structure which probably consists of parallel layers of magnesium oxide rnicrocrystallites intercalated with micropores of mean width 0.93nm,
which
can be formally considered as 4 missing (111) MgO planes [ 9 1 . At higher levels of decomposition there is an increase in the micropore width. Although this can be seen from the neopentane isotherms (by a gradual roundening of the knee of the isotherm), it has a much more dramatic effect on the nitrogen isotherms. In this case, the micropore width is clearly in a critical region where the mechanism of adsorption of nitrogen has just changed from
primary
micropore
filling
to
secondary
micropore
filling
[141.
Furthermore, it is apparent that the two stages of the secondary process only become distinguishable when the pore width is greater than about three molecular diameters. The absence of a step in the neopentane isotherms is thus readily explained, as the pore width never exceeds three neopentane molecular diameters. In the case of HID1, the enlargement of micropore width is accompanied by a significant decrease in micropore volume, whereas, with HID2, the micropore volume only decreases to a relatively small degree. These results suggest that, after the decomposition reaction has reached the centre of the crystals, a
restructuring
of
microcrystallites
join
the
MgO
microcrystallites
together with
the
loss
of
occurs.
Adjacent
one micropore
and a
concomitant increase in the width of another. In the case of HIDl it is
642
evident that this process leads to a large increase in the amount of inaccessible porosity. Turning now to the results of rehydration, it is evident that the processes of decomposition and rehydration are not completely reversible. Thus, the aggregate structure and the nature of the surface are slightly modified (Figures 3 and 4) and, for the same % decomposition, the hydraulic pore width and micropore volume are decreased (Figure 1). In addition, the results in the Table, and the XFlD results, indicate that none of the samples was completely rehydrated. Actually, this conclusion is to be expected. The decomposition was carried out in a very controlled manner, allowing the reaction to proceed from the outside towards the centre of each crystal. Upon exposure of the samples to water vapour, the accessible micropores would initially have been completely filled by physisorbed water in a liquid-like state, and subsequent rehydration would therefore have occured simultaneously at all points throughout the crystal. The fact that the samples decomposed at 3OO0C are almost completely rehydrated indicates that most of the micropores, including those inaccessible to nitrogen, are accessible to the smaller water molecule. The molecular sieving of nitrogen is therefore probably associated with constrictions occurring near points of contact of adjacent microcrystallites, rather than with very narrow micropores p e r se. The presence of constrictions is supported by gas chromatographic measurements of isosteric heats of adsorption of n-alkanes at infinite dilution [151.
At
higher
levels
of
decomposition,
on
the
other
hand,
complete
rehydration is significantly more difficult to achieve, which suggests that, after restructuring of the microcrystallites has occurred, some of the micropore entrances become genuinely closed. Rehydration of the closed pores will therefore involve a bulk reaction which occurs at a significantly slower rate than rehydration of exposed surfaces [71. Because the micropore volume
of
the rehydrated samples is spread
throughout the particle, rather than concentrated at the peripheries as with the partially decomposed samples, it follows that, for a given pore volume, the mean pore width will be less (Figure 1). In addition, the pore walls will be fully, rather than partially, hydroxylated. As a result of these two factors, strong physisorption of molecular water can occur in the micropores, and an outgassing temperature of 15OoC may not be sufficient to desorb this water
[131. The
presence of
this molecularly adsorbed water, and
the
difference in the degree of hydroxylation of the exposed surfaces are consistent with the negative intercepts of the low pressure regions of the comparison plots in Figures 3 and 4.
643
CONCLUSIONS The results indicate that thermal decomposition of magnesium hydroxide occurs in two stages. The first stage, which involves conversion of Mg(0H)
to
microcrystallites of MgO, is complete at 85% decomposition. It appears that the residual water, which is chemisorbed on the exposed MgO faces, stabilizes the structure. The second stage of decomposition involves removal of the residual water, and leads to a restructuring of the microcrystallites. During this process the mean pore width is increased and some of the micropore entrances become closed. During the first stage the decomposition can (almost) be reversed by exposing the sample to water vapour. This reversibility, taken in conjunction with the constancy of the external surface area and the high degree of topotaxy of the reaction, provide very strong evidence in favour of the exfoliative mechanism of decomposition and indicate that the existence of cube-shaped MgO microcrystallites is extremely unlikely.
ACKNOWLEDGEMENTS The authors are grateful to Junta Nacional de Investiga@o Tecnol6gica
(Portugal) and
the
Commission of
the
Cientifica e
European Communities
(Belgium) for financial support and to The British Council for the award of travel grants under the Anglo-Portuguese Treaty of Windsor accord.
REFERENCES J. Green, J.Mat.Sci., 18 (1983)637. 1 2 M.G. Kim, U. Dahmen & A. W. Searcy, J. Am. Ceram.Soc. , 70 (1987)146. 3 A. Zecchina, S. Coluccia & C. Morterra, Appl.Spectr.Rev., 21 (1985) 259. 4 S. Coluccia, S. Lavagnino & L. Marchese, Mater.Chem.Phys., 18 (1988)445. M.M. Brotas de Carvalho, M.M. Lopes Ribeiro, M.R. Sales Grade & 5 A. Ruiz Paniego, Anal.Quim., 84 (1988)300. 6 D. Beruto, R. Botter & A.W. Searcy, J.Am.Ceram.Soc., 70 (1987) 155. 7 Y. Kuroda, E. Yasugi, H. Aoi, K. Miura and T.Morimoto, J.Chem.Soc., Faraday Trans. I , 84 (1988)2421. 8 H. Naono, Colloids G Surfaces, 37 (1989) 55. 9 M.M.L. Ribeiro Carrott, P.J.M. Carrott, M. Brotas de Carvalho & K. S.W. Sing, J. Chem.Soc., Faraday Trans., in press. A.F. Moodie & C.E. Warble, J.Crysta1 Growth, 74 (1986) 89. 10 11 P. J. Anderson & R.F. Horlock, Trans.Faraday Soc., 58 (1962) 1993. 12 J.C. Niepce, G. Watelle & N.H. Brett, J.Chem.Soc.,Faraday Trans. I , 74 (1978) 1530. 13 K.S.W. Sing, D.H. Everett, R.A.W. Haul, L.Moscou, R.A. Pierotti, J. Rouquerol & T. Siemieniewska, Pure 6 Appl.Chem., 57 (1985) 603. 14 P.J.M. Carrott & K.S.W. Sing, in K.K. Unger, J. Rouquerol, K.S.W. Sing & H. Kral (Eds.1, Characterization of Porous Solids, Elsevier, Amsterdam, (19881,pp. 77-87. 15 M.M.L. Ribeiro Carrott, Ph. D. Thesis, University of Lisbon, 1990.
635
MICROSTRUCTURE OF EX-HYDROXIDE MAGNESIUM OXIDE & PRODUCTS OF REHMRATION
M.M.L. Ribeiro Carrott, P.J.M.
Carrott, M.M. Brotas de Carvalho
Departamento de Quimica, Faculdade de Cibncias, Universidade de Lisboa, Rua da Escola PolitBcnica, 58, 1294 Lisboa Codex, Portugal. K.S.W.
Sing
Department of Chemistry, Brunel University, Uxbridge, UBS 3PH, UK.
ABSTRACT The thermal decomposition of magnesium hydroxide under carefully controlled conditions, followed by subsequent rehydration, has been studied by means of nitrogen and neopentane adsorption isotherm measurements. Up to about 85% decomposition a uniform layered particle structure, consisting of plate-like microcrystallites intercalated by slit-shaped pores of width ca. 0 . 9 3 run, gradually spreads from the outside towards the centre of each crystal. At this stage each particle consists of MgO of normal crystallographic structure, with exposed faces partially covered by chemisorbed water. The presence of this water stabilizes the structure and, when it is removed at higher temperatures, a restructuring of the microcrystallites occurs leading to an increase in the mean pore width. Below 85% decomposition, narrow constrictions, near points of contact of the microcrystallites, are present. As a result of this, approximately half of the total micropore volume is inaccessible to nitrogen. On the other hand, the smaller water molecule appears to be able to penetrate the entire micropore structure. The restructuring of the microcrystallites at higher temperatures appears to lead to complete closure of some micropores, with the result that rehydration of the closed porosity now involves a bulk reaction and therefore occurs at a considerably slower rate.
INTRODUCTION
Although the thermal decomposition of magnesium hydroxide has been extensively studied in the literature the precise mechanism of decomposition is still not fully understood. Furthermore, there remains considerable controversy over the structure of the decomposition products. The bulk of the published work has involved either crystallographic studies of the structural transformations involved (reviewed in [1,21) or spectroscopic investigations of the nature of the active sites generated (reviewed in [ 3 , 4 1 ) . Rather less use has been made of detailed adsorption isotherm measurements to elucidate the textural and surface chemical properties of either Mg(0H) itself, the products of its thermal decomposition or their rehydration products (a guide to recent work can be found in 15-91).
636 At the outset of our investigation the state of knowledge ( i . e . the features which appeared to be generally accepted at the time, and which our results in no way subsequently contradicted) was as follows. The decomposition appears to proceed by the advance of a two-dimensional reaction interface from the edges of the crystal towards the centre [10,111. A definite orientation relationship is maintained between the crystal planes of the parent hydroxide and those of the product oxide, the major relationship deriving from the conversion of the hydroxide (0001) plane to a (111) plane of the oxide [2,101. A theoretical 54% contraction of the lattice is involved which, in the case of large crystals or when the decomposition is carried out too rapidly, leads t o the development of cracks and, ultimately, to the particle morphology being destroyed [ l l . Under conditions of slow thermal decomposition, on the other hand, the morphology of the parent hydroxide is preserved and a well defined micropore structure is generated [8,9l.Recent work from three independent laboratories has confirmed that approximately half of the total theoretical microporosity is inaccessible to nitrogen [6,8,91(although not necessarily to the smaller H20 molecule). Disagreement in
the
current
literature
centres mainly
around
the
structure o f the decomposition products, for which essentially two models have been proposed. In one [2,6,101it is argued that the product oxide consists of an array of MgO microcrystallites which are cubic in shape. The cubes are thought to be regular in size, with edge length ca. 2nm, and possibly in spatial distribution. The alternative model is based on an exfoliation of oxide into layers parallel to the basal faces of the crystals [8,9,121. One attempt has been made to try to combine both models by proposing that exfoliation into buckled layers of interpenetrating cubic microcrystallites occurs [lo]. Some of our experimental adsorption data has already been analysed in detail, and from a rather different point of view, in a preceeding paper [91. As
the reversibility of the decomposition might be expected to throw some
additional light on the structure of the decomposition products, additional results concerning the effect of rehydration were also obtained and are presented here. The structure of the decomposition products and the mechanism of decomposition are discussed in the light of these new results.
EXPERIMENTAL Details of the preparation of the two magnesium hydroxide samples, HID1 and HIDZ, and the controlled conditions used to carry out the decomposition have been given previously [91. Rehydration at room temperature was carried
637
out using freshly prepared samples, which had not been exposed to air, by equilibrating with water vapour at its saturated vapour pressure for 2 days. The
decomposed
“precursor/T1/Rh/T
‘I,
and
rehydrated
samples
are
designated
by
where the precursor is HIDl or HID2 and Rh indicates
that the sample was rehydrated after decomposition. TI and T are the final outgassing temperatures (15OoC if not stated) before and after rehydration respectively. Thus HID2/850/Rh refers to a sample of HID2 which was outgassed while gradually increasing the temperature in the manner previously described [91 over a period of about 2 weeks until a final outgassing temperature of
85OoC, then rehydrated, followed by outgassing firstly at room temperature for 15 hours and then at 150’C. Nitrogen adsorption isotherms at 77 K were determined using a Carlo-Erba Sorptomatic. Neopentane (ca. 273
adsorption isotherms at
ice melting
temperature
K) were determined using a CI Robal microbalance with pressure
measurement by means of a calibrated CEC strain gauge pressure transducer.
RESULTS Nitrogen isotherms were determined on samples of HIDl and HID2 at successive stages of decomposition and analysed by means of comparison plots using the corresponding undecomposed material as the reference adsorbent in each case [91. Each comparison plot had two linear regions. One at low pressures passed through the origin thereby enabling the total surface area, As,
of each sample to be estimated from the slope
-
excellent agreement with
the BET area was obtained in all cases. The second linear region, in the
multilayer, was of slope equal to unity, thereby indicating that the external surface area,
*ext
of the samples was constant, and that the hexagonal
particle morphology and aggregate structure of both materials were not significantly altered during decomposition. From the intercept of the linear multilayer region micropore volumes, vs, were estimated. Assuming slit-shaped pores [8,91,hydraulic pore widths were estimated from d = 2 vS / (As - Aext)
(Eq. 1)
P
For both starting materials, HIDl and HID2, the variation of vs and dP with %
decomposition are shown in Figure 1 (a) and (b), respectively.
It is evident from Figure 1 that, up to 85% decomposition, the micropore volume gradually increases, while the micropore width remains almost constant and the same for both materials. At decomposition levels greater than 90%. on the other hand, the micropore volume tends to decrease, while the pore width increases significantly. This alteration
in the micropore
structure is
638 1.8-
t
C
1.6-
41
E,
\
mal.L-
i
'0
'5
.
1.2-
: a
$
._
1.0-
E
OB
rehydrated
0.8-
I
20 Fig.1
Yo decomposition
80
I
1
I
1
% decomposition
Evolution of micropore volume and pore width for HID1 (on) and HID2 (0.) during decomposition (0.1 and after rehydration (om).
0.2 Fig.2
60
LO
-
0
0.L
P/PO
0.6
0.8
Nitrogen (a) and neopentane ( 0 ) isotherms corresponding to the two stages of decomposition.
accompanied by a characteristic change in the shape of the isotherms of nitrogen, but not those of neopentane, as can be seen in Figure 2, where representative isotherms for both stages are shown. It should be noted that this figure is a particularly good example of how the mechanism of adsorption, and hence the corresponding isotherm shape, depends on the ratio of pore width to molecular diameter and not just on pore width [131.
639
0.20
0.20
H I D1/300
--?
c
cn 0.15. 0
0
-
015
-
._ i+
U 3
._ I
i=i E u 0.10
m
E
u 0.10
\
\ U ul
U ul
P
p”
I
>
I1 1 original0 (3)rehydrated. 15%1
0.05
I
0.2
1
I
0.L
06
0.05
1
1
005
0.8
P/PO
Fig. 3
Nitrogen isotherms and corresponding comparison plots for HIDl (01 HID1/300 (01 HID1/300/Rh (01 and HID1/300/Rh/250 (m).
r
I
0.2
Fig.4
I
0.10
v,,.~,~HID1/150) / cd(liquid)g-’
I
04
I
P/P”
0.6
I
I
08
Nitrogen isotherms and corresponding comparison plots for HID2 (01 HID2/850 (01 and HID2/850/Rh (01.
Representative nitrogen isotherms and corresponding comparison plots for the decomposed/rehydrated samples, below 85% and above 90% decomposition, are shown in Figure 3 for HIDl and Figure 4 for HID2. As with the decomposed samples, the comparison plots are linear in the multilayer region. For each sample, the micropore volume, v , was estimated from the intercept of the linear multilayer region. The results, given in Table 1 and Fi.gure 1 (a),
640
TABLE 1 Characterisation of HIDl and HID2 samples. %
decomposition, total surface area, As and micropore volume, vs.
Values refer to unit weight of stoichiometric Mg(OHI2.
sample
HIDl HID1/270 HID1/300 HID1/300/Rh HID1/300/Rh/250
HID2 HID2/300 HID2/300/Rh HID2/850 HID2/850/Rh
%
*s
0
78.6 85.6 4.5 82.2
0 86.6 8.3 100.0 40.8
V
8
m'8-l
cm3g-'
99 239 268
0 0.064 0.080 0
96 256
37 272 42 140 57
0.079
0 0.105 0.002 0.081 0.008
indicate that, for the same % decomposition, the micropore volumes of the rehydrated samples are somewhat less than those of the decomposed samples. In contrast to the decomposed samples, the low pressure region of each comparison plot gives rise to a negative intercept, indicating that the nature of the surface of the samples is modified after rehydration. Furthermore, a change in the shape of the capillary condensation hysteresis loop occurs after rehydration, thereby indicating that the aggregate structure is slightly modified (but without a modification of the external surface area or hexagonal particle morphology). For this reason the values of total surface area given in Table 1 were calculated from the BET equation rather than from the comparison plots. The hydraulic pore widths, indicated on Figure 1 fb), tend to be somewhat lower than those of the decomposed samples. Transmission electron microscopy confirmed that the hexagonal particle morphology is retained during decomposition and rehydration. Powder X-ray diffraction demonstrated
that during decomposition the lines of Mg(OHI2
gradually decrease, disappearing completely at 85% decomposition, while those of MgO gradually increase in intensity. During rehydration the MgO lines decrease in intensity while the Mg(0Hl2 lines reappear. Under the conditions used, the MgO lines never completely disappear indicating that rehydration is not complete.
641 DISCUSSION
A detailed analysis [91 of the nitrogen adsorption data on the decomposed samples has confirmed that decomposition starts at the periphery of each crystal. The maximum observed in the micropore volumes of
the samples
(Figure 1 (a)), taken together with TEM, XRD and thermogravimetric results, indicates that the reaction reaches the centre at about 85% decomposition. At this stage the particles consist entirely of MgO of normal crystal structure, with the residual 15% of H 0 being present, not as Mg(OHI2, but as chemisorbed water [91. As neither the aggregate structure nor the morphology of the particles changes during decomposition, the theoretical micropore volume, based on the crystallographic densities of Mg(OHI2 and MgO, is 0.23 cm3g-' (with respect to
It is evident therefore that less than
unit weight of stoichiometric Mg(OH)2).
50% of the total micropore volume is accessible to nitrogen molecules and, hence, that the difference in the maximum micropore volumes for HIDl and HID2 arises due to a difference in the ratio of accessible to
inaccessible
porosity. The constancy of the hydraulic pore width (Figure 1 (b)), and the fact that it is the same for both samples, is consistent with a uniform particle structure which probably consists of parallel layers of magnesium oxide rnicrocrystallites intercalated with micropores of mean width 0.93nm,
which
can be formally considered as 4 missing (111) MgO planes [ 9 1 . At higher levels of decomposition there is an increase in the micropore width. Although this can be seen from the neopentane isotherms (by a gradual roundening of the knee of the isotherm), it has a much more dramatic effect on the nitrogen isotherms. In this case, the micropore width is clearly in a critical region where the mechanism of adsorption of nitrogen has just changed from
primary
micropore
filling
to
secondary
micropore
filling
[141.
Furthermore, it is apparent that the two stages of the secondary process only become distinguishable when the pore width is greater than about three molecular diameters. The absence of a step in the neopentane isotherms is thus readily explained, as the pore width never exceeds three neopentane molecular diameters. In the case of HID1, the enlargement of micropore width is accompanied by a significant decrease in micropore volume, whereas, with HID2, the micropore volume only decreases to a relatively small degree. These results suggest that, after the decomposition reaction has reached the centre of the crystals, a
restructuring
of
microcrystallites
join
the
MgO
microcrystallites
together with
the
loss
of
occurs.
Adjacent
one micropore
and a
concomitant increase in the width of another. In the case of HIDl it is
642
evident that this process leads to a large increase in the amount of inaccessible porosity. Turning now to the results of rehydration, it is evident that the processes of decomposition and rehydration are not completely reversible. Thus, the aggregate structure and the nature of the surface are slightly modified (Figures 3 and 4) and, for the same % decomposition, the hydraulic pore width and micropore volume are decreased (Figure 1). In addition, the results in the Table, and the XFlD results, indicate that none of the samples was completely rehydrated. Actually, this conclusion is to be expected. The decomposition was carried out in a very controlled manner, allowing the reaction to proceed from the outside towards the centre of each crystal. Upon exposure of the samples to water vapour, the accessible micropores would initially have been completely filled by physisorbed water in a liquid-like state, and subsequent rehydration would therefore have occured simultaneously at all points throughout the crystal. The fact that the samples decomposed at 3OO0C are almost completely rehydrated indicates that most of the micropores, including those inaccessible to nitrogen, are accessible to the smaller water molecule. The molecular sieving of nitrogen is therefore probably associated with constrictions occurring near points of contact of adjacent microcrystallites, rather than with very narrow micropores p e r se. The presence of constrictions is supported by gas chromatographic measurements of isosteric heats of adsorption of n-alkanes at infinite dilution [151.
At
higher
levels
of
decomposition,
on
the
other
hand,
complete
rehydration is significantly more difficult to achieve, which suggests that, after restructuring of the microcrystallites has occurred, some of the micropore entrances become genuinely closed. Rehydration of the closed pores will therefore involve a bulk reaction which occurs at a significantly slower rate than rehydration of exposed surfaces [71. Because the micropore volume
of
the rehydrated samples is spread
throughout the particle, rather than concentrated at the peripheries as with the partially decomposed samples, it follows that, for a given pore volume, the mean pore width will be less (Figure 1). In addition, the pore walls will be fully, rather than partially, hydroxylated. As a result of these two factors, strong physisorption of molecular water can occur in the micropores, and an outgassing temperature of 15OoC may not be sufficient to desorb this water
[131. The
presence of
this molecularly adsorbed water, and
the
difference in the degree of hydroxylation of the exposed surfaces are consistent with the negative intercepts of the low pressure regions of the comparison plots in Figures 3 and 4.
643
CONCLUSIONS The results indicate that thermal decomposition of magnesium hydroxide occurs in two stages. The first stage, which involves conversion of Mg(0H)
to
microcrystallites of MgO, is complete at 85% decomposition. It appears that the residual water, which is chemisorbed on the exposed MgO faces, stabilizes the structure. The second stage of decomposition involves removal of the residual water, and leads to a restructuring of the microcrystallites. During this process the mean pore width is increased and some of the micropore entrances become closed. During the first stage the decomposition can (almost) be reversed by exposing the sample to water vapour. This reversibility, taken in conjunction with the constancy of the external surface area and the high degree of topotaxy of the reaction, provide very strong evidence in favour of the exfoliative mechanism of decomposition and indicate that the existence of cube-shaped MgO microcrystallites is extremely unlikely.
ACKNOWLEDGEMENTS The authors are grateful to Junta Nacional de Investiga@o Tecnol6gica
(Portugal) and
the
Commission of
the
Cientifica e
European Communities
(Belgium) for financial support and to The British Council for the award of travel grants under the Anglo-Portuguese Treaty of Windsor accord.
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