Hydrothermal synthesis attempts of dawsonite-type hydroxymetalocarbonate precursor compounds for...

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Hydrothermal synthesis attempts of dawsonite-type

hydroxymetalocarbonate precursor compounds for

catalytic Ho, Sm, and La oxides

Asma A. Ali a, Muhammad A. Hasan a, Mohamed I. Zaki b,*a Chemistry Department, Faculty of Science, Kuwait University, P.O. Box 5969, Safat 13060, Kuwait

b Chemistry Department, Faculty of Science, Minia University, El-Minia 61519, Egypt

Received 16 March 2007; received in revised form 7 July 2007; accepted 17 July 2007

Available online 22 July 2007

Abstract

Chemical interactions in mixed, aqueous solutions of NH4HCO3 and M(NO3)3�9H2O, where M stands for Ho, Sm, or La, were

facilitated under various hydrothermal treatment conditions (pH 8–12 and temperature = 75–135 8C). The solution chemistry

established did not make available necessary concentrations of soluble HCO3� and MO(OH)2

� species for the formation of

dawsonite-type ammonium hydroxymetalocarbonates, NH4M(CO3)(OH)2, but, alternatively, high concentrations of soluble

CO32�, and M(H2O)n

3+ or M(H2O)n�1(OH)2+ facilitating, respectively, precipitation of corresponding hydrated carbonate,

M2(CO3)2�2H2O, or carbonate hydroxide, MCO3(OH). X-ray powder diffractometry, infrared spectroscopy, and thermal analyses

proved alternative formation of Ho2(CO3)3�2H2O or LaCO3(OH) under the whole set of hydrothermal treatment conditions probed,

and Sm2(CO3)3�2H2O at pH < 10 or SmCO3(OH) at pH � 10, thus implying dependence of the composition of the product

carbonate compound on the hydrolysability of the initial M(H2O)n3+ species and, hence, the metal ionic size (La > Sm > Ho).

Calcination of the various hydrothermal treatment products at�600 8C resulted in the thermal genesis of the corresponding sesqui-

oxides (M2O3). Bulk and surface characterization studies of the product oxides, employing N2 sorptiometry and scanning electron

microscopy, in addition to the above analytical techniques, revealed overall strong crystallinity, large average crystallite size, and

well-defined particle morphology. They revealed, moreover, surfaces, though of limited accessibilities (�13 m2/g), exposing OH

groups of various coordination symmetries and, hence, acid–base properties, thus furnishing promising surface catalytic attributes.

# 2007 Elsevier Ltd. All rights reserved.

Keywords: A. Inorganic compounds; A. Oxides; B. Chemical synthesis; C. X-ray diffraction; D. Microstructure

1. Introduction

Though both less active and less selective, metal oxides are yet increasingly sought as more stable and abundant,

economic substitutes for noble metal catalysts [1]. Catalytic-grade, bulk metal oxides must fulfill, foremost, for high

purity, thermal stability, and specific surface area (�10 m2/g) [1,2]. These challenging objectives have been the goal of

a great deal of research endeavors focusing on the design of appropriate methods of preparation of catalytic metal

oxides [3]. Consequently, a variety of methods have been devised [3], ranging from pure chemical to pure physical

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Materials Research Bulletin 43 (2008) 16–29

* Corresponding author. Tel.: +20 86 2360833; fax: +20 86 2360833.

E-mail address: [email protected] (M.I. Zaki).

0025-5408/$ – see front matter # 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2007.07.026


methods. It is noteworthy, however, that the industrially applicable of these methods are yet to be rather the most

economic and manageable [4]. For instance, thermo-chemical methods of catalyst preparation have been widely

considered the most straightforward [4,5], and they can, moreover, be economically feasible when inexpensive

catalyst precursors are used.

In an earlier communication [6], we have reported results of successful attempts to synthesize dawsonite-like

ammonium hydroxymetalocarbonates of aluminum [NH4Al(CO3)(OH)2] and chromium [NH4Cr(CO3)(OH)2] atoms,

where dawsonite is a mineralogical nomenclature meant to specifically indicate naturally occurring sodium

hydroxyaluminocarbonate [7], NaAl(CO3)(OH)2. The preparation was undertaken via a hydrothermal method

optimized at 10 < pH < 12 and temperature�100 8C [6], and suggested to be critically dependent on the availability

of high concentrations of soluble MO(OH)2� and HCO3

� ionic species in the synthesis course [8]. The undesirable

precipitation of metal oxyhydroxide (MOOH) species was found to intercept the preparation course of the

hydroxymetalocarbonate of iron atoms [6]. Calcination at�600 8C of the dawsonite-like compounds resulted in pure,

high surface area g-Al2O3 (146 m2/g) and a-Cr2O3 (32 m2/g) powders, respectively. It is worth mentioning, that a very

recent communication [9] has described a novel in-line dispersion-precipitation (ILDP) whereby nano-particle

ammonium dawsonite-type compounds of Al, Fe, Co, or Mn atoms were prepared, whose thermal activation resulted

in oxide materials with superior particle dispersity, porosity, thermal stability, and improved catalytic performance.

In the present communication, we report and discuss results of synthesis attempts of ammonium dawsonite-like

compounds of Ho, Sm, and La atoms via a similar hydrothermal processing [6]. The yielding materials were identified

by X-ray powder diffractometry, infrared spectroscopy, thermogravimetry, and differential thermal analysis. Their

specific surface areas were determined by N2 sorptiometry, whereas their particle morphologies were visualized by

scanning electron microscopy. Subsequently, they were calcined to the onset of formation of the corresponding M2O3,

where M stands for the metal atoms used. The oxides thus produced were characterized similarly. The thrust behind

this work was the fact that the ultimate calcination products are typical basic oxides [10,11]. In the wake of current

increasing interest in adequate chemical compositions of metal oxide-based total oxidation (combustion) catalysts

[12,13], for obvious natural gas related economic feasibilities and environmental necessities [12], such metal oxides

may be considered promising ingredients [11,13].

2. Experimental

2.1. Synthesis method and reagents

Synthesis of ammonium dawsonite-like compounds (NH4M(CO3)(OH)2; denoted MDw), where M stands for Ho,

Sm, or La, was attempted following the hydrothermal method described by Wen et al. [14]. Accordingly, a 250 mL

aliquot of 0.1 M aqueous solution of M(NO3)3�9H2O (�99.0% pure Aldrich products) was added slowly to an equal

volume of 1.25 M aqueous solution of NH4HCO3 (99% pure Aldrich Product) while being continuously stirred at

room temperature (RT). The pH value of the mixture was maintained constant (�0.1) at either of the following values:

8, 9, 10, 11, and 12, using NH4OH solution (25% Aldrich product). The gel thus obtained was hydrothermally treated

at various temperatures (75–135 8C), for 24 h, inside a glass-lined, stainless steel autoclave (model 3870EP, Tuttnauer

Europe C.V.). Then, the slurry was removed, filtered, and washed several times with distilled water and ethanol. The

pink-, light pink-, and white-colored solid residues obtained using Ho-, Sm-, or La-nitrate, respectively, were dried at

100 8C for 24 h. For simplicity, the products are discerned below by the pH and temperature magnitudes applied. For

instance, MDw8-75 means the hydrothermal product at pH 8 and 75 8C, whereas MDw11-135 signifies that obtained

at pH 11 and 135 8C. The hydrothermal products were kept dry over self-indicating silica gel till further use.

2.2. Characterization methods and techniques

The bulk crystalline and noncrystalline structures were elucidated by X-ray powder diffractometry (XRD) and

infrared absorption spectroscopy (IR), respectively. The specific surface area and particle morphology were,

respectively, measured by N2 sorptiometry and scanning electron microscopy (SEM). The thermal stability was

probed by thermogravimetry (TG) and differential thermal analysis (DTA). Accordingly, the onset temperature of the

thermal genesis of the corresponding M(III) oxide was determined, and the oxides thus obtained were subjected to

similar bulk and surface characterization studies. The calcination products are indicated below by the chemical

A.A. Ali et al. / Materials Research Bulletin 43 (2008) 16–29 17


formula of the anticipated metal oxide (M2O3), suffixed by the magnitudes of the applied calcination temperature (in

8C) and duration (in hours). Hence, La2O3-2h900 indicates the calcination product of LaDw at 900 8C for 2 h.

XRD was performed at RT using a D5000 Siemens diffractometer equipped with a source of Ni-filtered Cu Ka

radiation (l = 0.15406 nm). The diffractometer was operated at 40 kV and 30 mA, the data being acquired stepwise

over the 2u range 10–808 at a step size of 0.028, a step time of 15 s, and a divergence slit of 18. The data were

manipulated using an on-line microcomputer. For crystalline phase identification purposes, an automatic JCPDS

library [15] search and match was conducted using a standard SEARCH and DIFFRAC software (Siemens Corp.),

whereas for crystallite sizing, X-ray line broadening technique [16] and Scherrer formula [17] were implemented.

IR spectra were measured for KBr supported test samples (<1 wt.%) over the frequency range 4000–400 cm�1 at a

resolution of 5.2 cm�1, using a model 2000 Perkin-Elmer FT-IR spectrometer.

TG and DTA analyses were carried out on heating, typically, a 8–10 mg portion of test samples, at 10 8C/min and

50 cm3 of air/min, using a Shimadzu thermal system, model TGA-50H, equipped with a workstation for data

acquisition and handling. Highly sintered a-Al2O3 (Shimadzu Corp.) was the thermally inert reference material for

DTA measurements.

N2 sorptiometry was performed (at �195 8C) with an automatic ASAP 2010 Micromeritics sorptiometer equipped

with a degassing platform and an on-line data acquisition and handling system powered with BET-based [18]

analytical software for the specific surface area (SBET, m2/g) determination. The N2 gas was a 99.999% pure product of

KOAC (Kuwait), and the test materials (500 � 2 mg) were pre-degassed at 110 8C and 10�5 Torr (1 Torr = 133.4 Pa)

for 3 h.

SEM was conducted using a JSM-630 JEOL scanning electron microscope, operated at 30 kV. Test samples, spread

in a thin layer over a double adhesive tape on a 10 mm aluminum stub, were sputter coated with gold prior to

examination.

3. Results and discussion

3.1. Thermal treatment products

Fig. 1 compares XRD diffractograms for HoDw products at pH 8 as a function of the hydrothermal treatment

temperature (75–135 8C). The product at 75 8C is shown to exhibit nothing but three diffraction peaks due, exclusively,

to the sample holder (Pt/Rh). Hence, it is largely noncrystalline. In contrast, the products at the higher temperatures

100–135 8C give rise to numerous additional diffraction peaks; most of them are quite intense and narrow. A thorough

inspection of the results helps discerning two diffraction patterns: pattern I, characterized by the three strongest peaks

(TSP) at 2u = 15.78, 22.88, and 11.38; pattern II, having the TSP at 19.38, 30.28, and 25.08. It is obvious from Fig. 1 that

pattern I is relatively the strongest for the product at 100 8C, and weakens gradually as the treatment temperature

increases. On the other hand, the pattern II is weakly displayed in the diffractogram of HoDw8-100, but intensifies

considerably upon increasing the treatment temperature to 120 8C. Whereas a further increase of temperature to

135 8C, retrogresses its strength to a level similar to that exhibited for HoDw8-100. Hence, the strength of pattern II

passes through a maximum corresponding to the product at 120 8C.

As a matter of fact neither pattern I nor II do perfectly match the patterns indexed for NH4Al(CO3)(OH)2 and like

ammonium dawsonite-type compounds [6,9]; e.g. in JCPDS 76-1923 (TSP at 15.38, 26.98, and 35.08 [19]). Moreover,

a JCPDS library search could not establish a satisfactory match of either pattern I or II with compiled XRD standard

data for Ho(OH)3 (JCPDS 83-2040; TSP at 16.38, 30.18, and 28.58 [20]), HoOOH (JCPDS 18-0610; TSP at 29.28,22.38, and 15.78 [21]), Ho2(OH)4CO3 (JCPDS 78-0901; TSP at 14.28, 27.68, and 20.58 [22]), Ho2O2CO3 (JCPDS

26-0741; TSP at 32.28, 27.48, and 47.28 [23]) or Ho(H2O)4(HCO3)3(H2O)2 (JCPDS 70-2055; TSP at 11.18, 14.48, and

17.28 [24]).

Consulting IR spectra taken of the above HoDw8 products (Fig. 2), one can readily realize the absence of

absorption bands due to nNH (at 3180, 3030, and 2850 cm�1) and dNH (at 1830 and 1730 cm�1) of NH4+ ions [25].

This confirms that neither of the HoDw8 products at 75–135 8C is constituted of ammonium compound(s). In fact, IR

spectra shown in Fig. 2 are generally similar in displaying bands assignable to bond vibrations of associated OH

groups (nOH at 3500–3200 cm�1 [26,27]), freely rotating H2O molecules (dOH at 1630 cm�1 [26]), CO32� groups

(nOCO at 1550–1060 cm�1; dOCO at 870–720 cm�1 [28,29]) and metal–oxygen linkages (nHo–O at <700 cm�1

[30,31]). However, spectrum taken of the noncrystalline HoDw8-75 (Fig. 2) is discerned from spectra of the other

A.A. Ali et al. / Materials Research Bulletin 43 (2008) 16–2918


high-temperature products by displaying a single frequency for each mode of vibration of CO32� groups; viz., nasOCO

at 1530 cm�1(n3), nsOCO at 1390 cm�1(n3), nCO3 at 1080 cm�1(n1), dCO3 at 840 cm�1(n2), and dOCO at

760 cm�1(n4) [28,29]. The sharpness of the tip of the absorption at 1390 cm�1 may imply a contribution from nNO3 of

ionic nitrate species, inherited from the synthesis course. Expansion of the frequency ranges 2000–1000 and 1000–

400 cm�1 could help resolving at least two frequency values for each mode of vibration of CO32� groups in the spectra

of the high-temperature products; viz., n1 at 1094 and 1066 cm�1; n2 at 865, 848, and 835 cm�1; n3 at 1552 and

1422 cm�1 and 1513 and 1463 cm�1; n4 at 765 and 726 cm�1. These results may reveal a uniform mode of carbonate

bonding in HoDw8-75, and a non-uniform one in the products at the higher temperatures of 100–135 8C. According to

Busca and Lorenzelli [29], a value of DnOCO (=nas � ns) �100 cm�1 is indicative of polydentate (or monodentate)-

bound carbonate species, whereas a Dn = >100–300 cm�1 accounts for a bidentate (chelate)-bound carbonate species.

Accordingly, the uniform mode of carbonate bonding prevailing in HoDw8-75, where DnOCO (=1530–1390 cm�1) is

>100 cm�1, is a bidentate (chelate) type of bonding. When correlated with the XRD results (Fig. 1), these IR findings

may help suggesting that the two diffraction patterns (I and II) observed for the high-temperature HoDw8 products are

due to two different crystalline modifications of Ho carbonates (phases I and II) assuming different carbonate bonding

modes. Phase I, which predominates in HoDw8-100 and HoDw8-135, is, according to IR results (Fig. 2), most

probably a polydentate carbonate compound. In contrast, phase II, which optimizes in HoDw8-120, is most likely the

IR characterized bidentate carbonate compound.

To characterize the pH impact of the hydrothermal treatment on the formation and subsequent crystallization of the

two crystalline phases (I and II) constituting HoDw8-120, the products at pH > 8, namely at pH 9, 10, 11, and 12, were

XRD analyzed. The results obtained showed that the products at pH < 10 exhibited very similar diffraction patterns,

and the same was found to apply to the products obtained at pH > 10. Thus, only the diffractograms recorded for the

products at pH 10 and 11 are compared to that of HoDw8-120 in Fig. 3. It is obvious from Fig. 3 that the product at pH

10 exhibits slightly stronger intensification of diffraction peaks of both phases I and II as compared to the peaks

exhibited for the product at pH 8. In contrast, the product at pH 11 gives rise to weaker peaks for both patterns than the

two products at pH 8 and 10. Recalling that the test samples were of almost the same amount, and that the

diffractograms were plotted in Fig. 3 to the same ordinate scale, the present results may imply that amounts of the two


Fig. 1. X-ray powder diffractograms for HoDw8 products obtained at the hydrothermal treatment temperatures indicated. The diffractograms are

plotted to the same ordinate scale, # points to peaks due to the sample holder (Pt/Rh), and the inset highlights 2u values corresponding to the three

strongest peaks (TSP) of the two patterns resolved (I and II).


crystalline carbonate compounds (phases I and II) constituting HoDw-120 increase with the pH to the value of 10, and

then decrease on further pH increase to �11. Hence, the pH of the hydrothermal treatment most probably influences

equilibria controlling formation of the carbonate compounds contained.

Similar attempts to prepare ammonium dawsonite-like compounds for Sm and La, under identical hydrothermal

pretreatment conditions (pH 8–12 and 75–135 8C), resulted in the formation of the following crystalline materials:

SmDw8-120, SmDw10-120, LaDw8-75, and LaDw8-120. XRD diffractograms obtained for these materials are

compared to that exhibited by HoDw10-120 in Fig. 4. The diffractogram of SmDw8-120 displays strong peaks (at

2u = 11.48, 15.48, 22.48, 23.08, 34.98, and 43.68) assignable, exclusively, to samarium carbonate dihydrate,

Sm2(CO3)3�2H2O (JCPDS 31-1201; TSP at 11.58, 22.58, and 23.08 [32]). This diffraction pattern corresponds

satisfactorily to pattern I of HoDw10-120 (Fig. 4), except for some minor mismatches of peak relative intensities. Due

to the absence of standard XRD data matching those of pattern I, one may, accordingly, consider the relevant

crystalline phase I of HoDw10-120 to be isomorphous to SmDw8-120, i.e. to crystalline Sm2(CO3)3�2H2O. The

display in the IR spectrum taken of HoDw10-120 of most of the absorptions exhibited by SmDw8-120 over the

frequency regions of OH and CO3 vibrations (Fig. 5) is a strong indication that phase I is not only structurally

isomorphous to Sm2(CO3)3�2H2O, but also chemically similar. Hence, one may strongly suggest that phase I of

HoDw10-120 may assume the chemical composition of Ho2(CO3)3�nH2O. Phase II, which co-exists with phase I in

HoDw10-120, does not present itself chemically as a distinct entity from phase I in the corresponding IR spectrum

(Fig. 5). Therefore, it may be considered to assume a similar hydrated carbonate composition but organized in a

different crystallographic modification than phase I.

The samarium-containing product at pH 10, i.e. SmDw10-120, is shown (Fig. 4) to be partially crystalline, with the

corresponding XRD diffractogram displaying weak, but narrow, peaks matching those filed for crystalline

SmCO3(OH) in JCPDS 41-0663 (TSP at 21.18, 24.58, and 31.28 [33]), and much weaker peaks similarity to those

exhibited by Sm2(CO3)3�2H2O (Fig. 4), i.e. SmDw8-120. Hence, SmDw10-120 contains in addition to the samarium

carbonate hydroxide, a minority proportion of poorly crystalline hydrated carbonate. Consistently, IR spectrum taken


Fig. 2. IR transmission spectra taken of HoDw8 products obtained at the hydrothermal treatment temperatures indicated.


of SmDw10-120 (Fig. 5) displays, in addition to bands assignable to associated OH groups (at 3500–3000 cm�1),

polydentate-bound CO32� groups (at 1600–1086 cm�1), and two strongly overlapping sharp bands at 3477 and

3452 cm�1, as well as the following distinct spectral modifications: (i) considerable intensification of bands due to n2

(at 848 and 838 cm�1) and n4 (at 725 and 701 cm�1) modes of dCO3 vibrations, (ii) emergence of two weak, but

distinct, absorptions at 480 and 430 cm�1 (not shown in Fig. 5) in the reported nSm–O lattice frequency region [30].

The two high-frequency bands at 3477 and 3452 cm�1are assignable to nOH vibrations of isolated basic hydroxide

groups [26], and, hence, the other spectral modifications are consistent with the co-existence of samarium carbonate

hydroxide species, SmCO3(OH).

LaDw8-120, i.e. the lanthanum-containing product at pH 8 and 120 8C, exhibits an XRD diffraction pattern (Fig. 4)

typical of LaCO3(OH), whose standard data are filed in JCPDS 26-0815 (TSP at 30.38, 24.38, and 43.98 [34]). The

strong, narrow peaks displayed account for formation of large crystallites of the carbonate hydroxide. The

corresponding IR spectrum (Fig. 5) displays two strong and sharp absorptions at 3616 and 3477 cm�1 due to stretching

vibrations of two different types of isolated OH groups, and absorptions at 870, 848, 776, and 705 cm�1 assignable to

different modes of bending vibrations of carbonate groups, which have been considered indicative of the formation of

carbonate hydroxide moieties dealing with SmDw10-120 (Fig. 5). Indeed, these spectral features are compatible with

the general IR band structure reported for metal carbonate hydroxide compounds [35].

Summing up, findings of the XRD and IR results presented and discussed above are compiled in Table 1. They

confirm that crystalline compounds formed in HoDw, SmDw, or LaDw are not ammonium compounds, but rather

hydrated metal carbonates, viz., Sm2(CO3)2�2H2O in SmDw8-120 or Ho2(CO3)3�nH2O in HoDw10-120, or metal

carbonate hydroxides, viz., SmCO3(OH) in SmDw10-120 or LaCO3(OH) in LaDw8-120. It is noteworthy, that

Ho2(CO3)3�nH2O is a suggested composition for the crystalline phase I of HoDw10-120, whereas phase II is

tentatively proposed to be another crystalline modification of a similar hydrated carbonate composition. The escape of

formation of ammonium hydroxymetalocarbonate (NH4MDw) for the present test metal atoms (Ho, Sm, or La),

despite the earlier success in formation of NH4AlDw (=NH4Al(CO3)(OH)2) and NH4CrDw (=NH4Cr(CO3)(OH)2) [6],


Fig. 3. X-ray powder diffractograms for HoDw-120 products obtained at the hydrothermal treatment pH values indicated. The diffractograms are

plotted to the same ordinate scale, and # points to peaks due to the sample holder (Pt/Rh).


may be justified on basis of what is established about the solution chemistry leading to the formation of NH4MDw in

general. Focusing on the formation of NH4AlDw, as for instance, availability of soluble AlO(OH)2� and HCO3

� ionic

species is necessary for the following formation reaction to take place [8]:

NH4þ þAlOðOHÞ2� þHCO3

� ! NH4AlðCO3ÞðOHÞ2þOH� (1)

The solution equilibria, HCO3� $ CO3

2� and AlO(OH)2� $ Al(OH)4

�, established following dissolution of the

parent NH4HCO3 and Al(NO3)3, are shifted to the left, i.e. in favor of HCO3� and AlO(OH)2

�, at pH < 12. Hence, the

formation of NH4AlDw and NH4CrDw was found to optimize at pH 11 and temperature <100 8C, this is in the sense

that at higher temperatures the HCO3� $ CO3

2� equilibrium is shifted to the right, and the consequent increase in the

[CO32�]/[HCO3

�] ratio could not help formation of NH4AlDw but rather resulted in the precipitation of the

intermediate g-AlOOH.

Considering the solution chemistry of the present lanthanide metal nitrates, taking Ho(NO3)3 as for example, the

equilibrium [Ho(H2O)n]3+$ [Ho(H2O)n�1(OH)]2+ is reported [36] to occur at pH > 6, due to the high

electropositivity of Ho3+ and like lanthanide metal cations [37]. It persists against a further deprotonation of

[Ho(H2O)n�1(OH)]2+ into [Ho(H2O)n�2(OH)2]1+ species until the pH reaches a magnitude near 12 [36]. On the other

hand, formation of soluble HoO(OH)2� species must follow formation and subsequent dissolution of the hydroxide

Ho(OH)3 into Ho(OH)4�, which do not occur unless the medium is rendered strongly basic (pH � 13); a condition to

fulfill requires the employment of a stronger base than the present NH4OH. Hence, it is practically impossible to make

available the soluble species, i.e. HCO3� and MO(OH)2

�, necessary for the formation reaction of NH4HoDw and like

compounds for Sm and La, at one and the same pH regime. This is in the sense that MoO(OH)2� predominates at

pH > 11, whereas HCO3� at pH < 11.


Fig. 4. X-ray powder diffractograms for the hydrothermal treatment products indicated. The diffractograms are plotted to the same ordinate scale, #points to peaks due to the sample holder (Pt/Rh), and the insets indicate the best matching standard diffraction data file and molecular formula.



Fig. 5. IR transmission spectra taken over the OH and CO3 bond vibration frequency regions of the hydrothermal treatment products indicated.

Table 1

Chemical composition, and XRD and IR characteristic features of hydrothermally prepared LaDw, SmDw and HoDw products at the indicated pH

and temperature magnitudes

Product Chemical composition XRD IR (cm�1)

JCPDS TSP (8) Proportion nOHa n2CO3a n4CO3

a

HoDw10-120 Ho2(CO3)3�nH2O –b 15.7c Medium 3394(b) 846(sp) 761(sp)

22.8 3300(b) 836(sp) 686(sp)

11.3

SmDw10-120 SmCO3(OH) 41-0663 21.1 Major 3477(sp) 848(b) 725(sp)

24.5 3452(sp) 838(b) 701(sp)

31.2

SmDw8-120 Sm2(CO3)3�2H2O 31-1201 11.5 Dominant 3394(b) 850(sp) 755(sp)

22.5 3300(b) 838(sp) 681(sp)

23.0

LaDw8-120 LaCO3(OH) 26-0815 30.3 Sole 3616(sp) 870(sp) 776(b)

24.4 3477(sp) 848(sp) 705(b)

43.9

a sp = sharp and b = broad.b Not available.c Data displayed in the pattern of phase I, whereas the data stacked above are those reported in the corresponding JCPDS cards.


In contrast, the high pH regime, at which formation of M(H2O)n�1(OH)2+ is likely, favors increasing the [CO32�]/

[HCO3�] ratio and, hence, formation of corresponding metal carbonate hydroxides:

MðH2OÞn�1ðOHÞ2þ þCO32� ! MCO3ðOHÞ þ ðn� 1ÞH2O (2)

Whereas, formation of corresponding hydrated carbonates may precede of the initial deprotonation:

2MðH2OÞn3þ þ 3CO32þ ! M2ðCO3Þ3�2nH2O (3)

Accordingly, it is plausible to relate the chemical composition of the present end products, i.e. whether it is of a

hydrate carbonate or a carbonate hydroxide, to the basicity of the lanthanide metal ion, which would define the

threshold value of the pH for shifting the equilibria: (i) M(H2O)n3+$M(H2O)n�1(OH)2+, and (ii) HCO3

� $ CO32�.

La3+ has a significantly larger ionic size (0.114 nm) than Sm3+ (0.092 nm) and Ho3+ (0.084 nm) [36], and that the

larger the size of the lanthanide metal ion, the lower the pH at which hydrolysis of the aquo-ion commences. This may

explain, in terms of reactions (2) and (3), the observed formation of LaCO3(OH) at pH 8, and SmCO3(OH) at the

higher pH of 10. Whereas, the formation of HoCO3(OH), which would, thus, occur at pH > 12, was preceded by the

precipitation at pH � 12 of the hydrated carbonate Ho2(CO3)3�nH2O.

In the following section, the thermal decomposition behaviors of LaDw8-120, as a representative of a pure

carbonate hydroxide composition (LaCO3(OH)), and SmDw8-120, as a representative of a dominant hydrated

carbonate composition (Sm2(CO3)3�2H2O), are described and compared to that observed for HoDw10-120, being of a

non-uniform chemical composition.

3.2. Thermal decomposition

Fig. 6 displays TG curves obtained (at 20 8C/min and 50 cm3 air/min) for LaDw8-120, SmDw8-120 and HoDw10-

120. TG curve of LaDw8-120 monitors two well-defined mass loss (ML) steps maximized at 520 8C (step I) and

786 8C (step II). A corresponding DTA curve revealed that both ML steps are endothermic in nature. The almost

identical, observed (24.4%) and calculated (24.5%), total ML% values may evidence the high purity of the suggested

composition (LaCO3(OH)) of the test material and the eventual solid product (La2O3) at�800 8C. Moreover, the close

agreement between the observed and calculated ML% values for the individual ML steps (I and II) is in support of the

suggested oxycarbonate intermediate (La2O2(CO3)) of the carbonate hydroxide decomposition into the oxide.

In contrast, three decomposition steps are monitored for SmDw8-120: step I, Tmax = 413 8C; step II, Tmax = 566 8C;

and step III, Tmax = 680 8C, which were DTA shown to be similarly endothermic in nature. The incomplete agreement

between the observed (31.4%) and calculated (32.5%) total ML% values may be consistent with the fact that the

composition (Sm2(CO3)3�2H2O) identified in the test material may form most of it, but not all of it (Table 1). The fact

that the most obvious disagreements between the observed and calculated ML% values are confined to ML steps I and

III (Fig. 6), may help assuming that decomposition of the minority unidentified species overlaps largely with these two

steps. Considering the predominant composition of the test material, i.e. Sm2(CO3)3�2H2O, the three ML steps are

suggested to involve (I) complete dehydration to give Sm2(CO3)3, (II) the decomposition into Sm2O2(CO3), and (III)

the eventual production of Sm2O3 at �800 8C.

The excellent agreement between the observed (30.6%) and calculated (30.8%) total ML percentage values for

HoDw10-120 (Fig. 6), though may be considered supportive to the compositions suggested for the test material

(Ho2(CO3)3�2H2O) and the eventual decomposition product (Ho2O3), seems to be apparent. The two decomposition

steps monitored in the corresponding TG curve (Fig. 6) are far from being straightforward. Step I (Tmax = 411 8C) is

neither steep, nor well resolved. Step II (Tmax = 661 8C), though is much steeper than step I, is intercepted towards

completion by a slow ML leading, eventually, to formation of Ho2O3 near 800 8C. A corresponding DTA revealed an

endothermic nature for the two decomposition steps observed, and displayed a strong negative deflection of the

baseline with what appears to be a strong endothermic effect maximized at 956 8C. These results may account for a

second course of decomposition occurring at a detectably slower rate than the principle one (steps I and II), which

involves, most likely, the decomposition of phase II of the test material. The strong overlap of the two decomposition

courses may imply that phase II is, most probably, a different crystalline modification of a similar composition to that

(Ho2(CO3)3�2H2O) of phase I.



3.3. Calcination products

Fig. 7 compares XRD patterns exhibited by high-temperature calcination products of HoDw10-120 at 900 8C for

5 h (denoted Ho2O3-5h900), SmDw8-120 at 600 8C for 5 h (Sm2O3-5h600), and LaDw8-120 at 900 8C for 2 h (La2O3-

2h900). The calcination temperatures applied were chosen in terms of the TG results (Fig. 6), whereas the calcination

duration was, in each case, the shortest, effective period. The diffraction peaks observed for Ho2O3-5h900 are almost

identical to the standard data filed for cubic-Ho2O3 (a = 1.0608 nm; SG = Ia3; Z = 16) in JCPDS 83-0932

(TSP = 29.28, 48.68, and 33.78 [38]). The obvious strength and narrowness of the peaks, account for strong

crystallinity and a large average crystallite size. Fig. 8B shows, consistently, large flake-like crystallites, which are

different from the parent needle-like crystallites of HoDw10-120 (Fig. 8A). The average crystallite size derived from

the peak width at half-maximum amounts to 55 nm, and the specific surface area (SBET) is as low as 12 m2/g (Table 2).

Table 2 also summarizes frequencies of IR absorptions exhibited by Ho2O3-5h900. IR absorptions observed are

confined, exclusively, to the nO–H and nHo–O bond vibration frequency regions, with no sign of absorptions due to the

parent carbonate species. The nHo–O frequencies (Table 2) are similar to those reported for Ho2O3 in the literature

[30]. Thus, the nO–H frequencies at 3440 and 3250 cm�1 are due to associated (hydrogen bonded) surface-OH groups,

whereas the tiny absorptions at 3770 and 3738 cm�1 are assignable to a few isolated surface-OH groups. Their high

frequency values may be reminiscent to their strongly basic character [26,27].

Similarly strong and narrow XRD diffraction peaks (Fig. 7), occurring at 2u values matching those filed in JCPDS

76-0153 for cubic-Sm2O3 (a = 1.085 nm; SG = 1213; Z = 16 [39]), are exhibited by the calcination product of

SmDw8-120, i.e. Sm2O3-5h600. An IR spectrum taken of the sample was similar to that of Ho2O3-5h900 in displaying

three absorptions in the nSm–O frequency region (Table 2) due reportedly to Sm2O3 lattice vibrations [30], two strong

absorptions (at 3300 and 3210 cm�1) due to associated surface-OH’s, and two tiny high-frequency absorptions (at


Fig. 6. TG curves recorded on heating (at 10 8C/min and 50 cm3 air/min) of the hydrothermal treatment products indicated. The insets indicate mass

loss steps displayed (I–III), and the corresponding observed (ob) and calculated (c) mass loss percentages, as well as suggested molecular

stoichiometry changes involved in each step.


3775 and 3588 cm�1) assignable to strongly basic, isolated surface-OH’s. The similarity of the IR features of Sm2O3-

5h600 and Ho2O3-5h900 owes to their similar cubic lattice structure, despite the relatively larger lattice parameter of

the former oxide (a = 1.085 nm) than the latter one (a = 1.060 nm). Also, the average crystallite size derived for

Sm2O3-5h600 (54 nm) is very close to that (55 nm) derived for Ho2O3-5h900, as well as the SBET values of the two

oxides (Table 2). Probably the obvious difference between the two oxides lies in the particle morphology, since

Sm2O3-5h600 is shown (Fig. 8D) to have leaf-like particle morphology, which is insignificantly different from that of

the parent SmDw8-120 (Fig. 8C).


Fig. 7. X-ray powder diffractograms for the indicated calcination yields of the test hydrothermal treatment products. Each corresponding JCPDS

card files matching standard diffraction data.

Table 2

Average crystallite size (D), specific surface area (SBET) and IR absorption frequencies of O–H and metal–oxygen (M–O) bond vibrations of

calcinations products

Oxide D (�2 nm) SBET (�1 m2/g) IR (cm�1)a

nO–H nM–O

Ho2O3-5h900 55 12 3770(vw, sp) 570(s)

3738(vw, sp) 500(m)

3440(s, b) 465(m, sh)

3250(m, sh) 417(w)

Sm2O3-5h600 54 13 3775(vw, sp) 550(s)

3588(vw, sp) 469(m, sh)

3300(s, b) 440(m, b)

3210(s, sh)

La2O3-2h900 25 8 3610(s, sp) 650(m, b)

3430(m, b) 520(s)

3250(w, sh) 437(vw)

a vw = very weak, w = weak, m = medium strong, s = strong, sh = shoulder, b = broad, sp = sharp.


Fig. 7 demonstrates a diffractogram for La2O3-2h900 displaying strong, narrow peaks composing a very similar

pattern to that filed in JCDPS 83-1344 for hexagonal-La2O3 (a = 0.3938 nm; c = 0.6136 nm; SG = P3m1; Z = 1 [40]).

Accordingly, IR absorptions exhibited by the sample over the nLa–O lattice vibrations (Table 2) are compatible with

the typical IR band structure reported for the a-modification of sesqui-oxides (M2O3) in general; viz., absorptions at

650–618, 569–510, 469–440, and 435–410 cm�1 [41]. On the other hand, La2O3-2h900 exhibited a unique IR band

structure over the nO–H frequency region (Table 2), accounting for a surface largely covered with isolated, basic OH

groups (band at 3610 cm�1), with minority bridging (band at 3430 cm�1) and associated OH groups (shoulder at

3250 cm�1) [26,27]. Moreover, weak absorptions over the carbonate stretching vibration region (1600–1400 cm�1)

were still detectable. Table 2 indicates that La2O3-2h900 has significantly smaller crystallites (25 nm) than the other

two oxides, which is visible in the corresponding SEM image (Fig. 8F). Nevertheless, the specific surface area

determined for La2O3-2h900 (8 m2/g) is, unexpectedly, smaller than those determined for the other two oxides

(Table 2). Tentatively speaking, this might be considered relevant to a higher tendency of La2O3 towards adsorption of

atmospheric CO2 than the other two oxides. Compatibly, IR spectra taken of the three oxides (vide supra) revealed a

higher persistence of absorptions (at 1400–1600 cm�1) assignable to surface carbonate species to high-temperature


Fig. 8. Scanning electron micrographs for the indicated hydrothermal treatment products (A, C, and E) and their corresponding calcination products

(B, D, and F), respectively.


calcination of La2O3 as compared to the oxides of Ho and Sm, which exhibited spectra almost completely void of

carbonate spectral features of any significance.

4. Conclusion

The solution chemistry of the starting materials NH4HCO3 and M(NO3)3�9H2O (where M = Ho, Sm, or La), at the

applied hydrothermal treatment conditions (pH 8–12 and temperature = 75–135 8C), though may facilitate

predominance of soluble HCO3� at pH � 11 and temperature �100 8C, it does not facilitate formation of soluble

MoO(OH)2� unless the pH exceeds the magnitude of 12. This is due to the high electropositivity typical of the

lanthanide M3+ ions used. Hence, formation of ammonium hydroxymetalocarbonates of the test metal ions, i.e.

NH4M(CO3)(OH)2, was not feasible. Alternatively, the large ionic size of La3+ (=0.114 nm) enhances hydrolysis of the

aquo-ion La(H2O)n3+ into the mononuclear La(H2O)n�1(OH)2+ at a pH value as low as 8, thus leading to the formation

of the corresponding carbonate hydroxide:

LaðH2OÞn�1ðOHÞ2þ þCO32� ! LaCO3ðOHÞ þ ðn� 1ÞH2O

On the other hand, the relatively much smaller ionic size of Ho3+ (0.084 nm) sustains the persistence of Ho(H2O)n3+

to hydrolysis until pH values exceeding 10, thus facilitating the precipitation of the corresponding hydrated carbonate

at pH < 10:

2HoðH2OÞn3þ þ 3CO32� ! Ho2ðCO3Þ3�2H2O þ ð2n� 2ÞH2O

The intermediate ionic size of Sm3+ (=0.092 nm) has, accordingly, facilitated formation of SmCO3(OH) only at

pH � 10, otherwise Sm2(CO3)3�2H2O was the major product yielded.

The calcination of the carbonate or carbonate hydroxide compounds, under thermal conditions specified by their

observed thermal decomposition behaviors, results in the formation of sesqui-oxides (M2O3) with high purity, strong

crystallinity, large average crystallite size, and well-defined particle morphology. The surfaces thereon exposed,

though of limited accessibility (SBET � 13 m2/g), exhibit surface OH groups of various coordination symmetries and,

hence, acid–base properties.

Acknowledgments

We acknowledge with appreciation the financial support facilitated by the College of Post-graduate Studies, and the

excellent technical assistance found at the Science Analytical Facility (SAF) and Electron Microscopy Unit (EMU) of

the Faculty of Science/Kuwait University.

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