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Key Engineering Materials
ISSN: 1662-9795, Vol. 757, pp 131-137
doi:10.4028/www.scientific.net/KEM.757.131
© 2017 Trans Tech Publications, Switzerland
Submitted: 2017-03-20
Revised: 2017-07-09
Accepted: 2017-07-18
Online: 2017-10-27
Effect of Sulfuric Acid Treatment and Calcination
on Commercial Zirconia Nanopowder
Maisari Utami, Karna Wijaya* and Wega Trisunaryanti
Department of Chemistry, Faculty of Mathematics and Natural Sciences,
Universitas Gadjah Mada, Indonesia
*e-mail: karnawijaya@ugm.ac.id
Keywords: Sulfated zirconia, Sulfuric acid, Calcination
Abstract. The modification of commercial zirconia nanopowder by sulfuric acid and heat treatment
was conducted. The aim of this present research was to obtain a stable modified zirconia
nanopowder chemically and thermally by studying the effect of sulfuric acid treatment and
calcination temperature on commercial zirconia nanopowder. The material was prepared by
dispersing the commercial zirconia nanopowder into 0.2, 0.5 and 0.8 M sulfuric acid solutions,
followed by calcination at varied temperatures, i.e. 600, 700, 800 and 900 °C. The so called sulfated
zirconias then were characterized their physicochemical properties using FT-IR, XRD and SEMEDX analysis methods. The optimized condition for that modification was obtained by using
sulfuric acid of 0.8 M and calcination temperature of 600 °C. The characterization results also
revealed that using ammonia adsorption method, the acidity of the catalyst was found to be
1.06 mmol/g.
Introduction
The modification of zirconia using sulfate anions to form solid acid catalysts are known to
have numerous important reactions [1]. The addition of sulfate will increase thermal stabilization,
lower reaction temperatures and stabilize the structure of zirconia [2]. Reactivity of solid acid
catalysts depends on the nature of the active sites (Brønsted and Lewis acid) and other parameters,
such as the nature of the starting materials and procedures applied for the thermal treatment [3]. A
slight variation of the preparative procedures can greatly affect the acid properties of the resulting
oxide surface [4].
The catalytic performance of sulfated zirconia depends on the temperature of calcination and
the concentration of the sulfating agent [5]. Busto et al. [6] reported that the calcination temperature
is the variable that affects the acidity distribution. The optimum temperature corresponded to the
production of the highest concentration of acid sites and activity level. The sulfated zirconia under
the optimum conditions enhanced the acidity amount has been found to be an efficient catalyst [7].
The use of zirconyl nitrate hydrate, zirconium oxychloride octahydrate, zirconium hydroxide
as the precursor of sulfated zirconia preparation by the wet impregnation has been already discussed
[8-14]. This present study is focused on the synthesis of sulfated zirconia prepared using
commercial zirconia nanopowder as a precursor and study the effect of sulfuric acid concentration
and calcination temperature on the crystalline structure and acidic properties of sulfated zirconia.
The optimization will be used to obtain a stable modified commercial zirconia nanopowder
chemically and thermally.
Experimental
Materials. The commercial zirconia nanopowder was industrial grade and supplied by China,
Jiaozou Huasu Chemical Co., Ltd (ZrO2 60-70 nm, purity 99%). The sulfuric acid and ammonia
were analytical grade and obtained from Merck. All chemical used without further treatment.
Instrumentations. The functional groups of the materials were identified by Fourier
transform infrared spectroscopy (FT-IR, Shimadzu Prestige-21) equipped with data station in the
range of 400-4000 cm-1 with a KBr disc technique. The crystalline structures were characterized by
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Green Convergence on Materials Frontiers
an X-ray diffraction (XRD, Rigaku Multiflex, λ = 1.54 Å, 60 kV, 30 A) with source of CuKα
radiation (Ni filtered), patterns were recorded from 4-70° (2θ). Particle size was estimated using
Scherrer equation:  = ⁄ ; where D is the mean crystallite diameter (nm), K is Scherrer
constant (0.89), λ is the X-ray wavelength, β is the observed angular width at half maximum
intensity of the peak and θ is Bragg’s angle. The morphology and elemental analysis of the test
samples were estimated by Scanning electron microscopy with energy dispersive X-ray
spectroscopy (SEM-EDX, JEOL JED-2300 Analysis Station, 20 kV).
Procedures. A series of sulfated zirconia (SZ) samples having different concentrations of
sulfate (0.2; 0.5 and 0.8 M) were synthesized by wetness impregnation of ZrO2 nanopowder with
H2SO4 solution that were stirred at room temperature for 24 h. The samples were dried in an oven at
100 °C for 24 h and calcined at various calcination temperatures (600, 700, 800 and 900 °C). The
acidity of the SZ catalysts was determined by studying the ammonia adsorption using the
gravimetric method.
Result and Discussion
Fig. 1. XRD patterns of the commercial ZrO2 nanopowder, 0.2 M SZ, 0.5 M SZ and 0.8 M SZ
calcined at 600, 700, 800 and 900 °C
XRD Characterizations. The XRD patterns of commercial ZrO2 nanopowder and 0.2, 0.5
and 0.8 M SZ calcined at 600, 700, 800 and 900 °C are presented in Fig. 1. The diffraction peaks
appearing at 2θ = 28.3°, 31.6° and 50.2° refer to the monoclinic phase. However, the appearance of
a minor diffraction peak at 2θ of 30° in the patterns of 0.5 M SZ-600 and 0.8 M SZ-600 catalysts,
indicates the phase transformed from monoclinic to metastable tetragonal. Basically three
Key Engineering Materials Vol. 757
133
crystalline modifications of ZrO2 are known: the monoclinic which is stable up to 1200 °C, the
tetragonal which is stable up to 1900 °C and the cubic which is stable above 1900 °C. In addition,
above three phases, a metastable tetragonal phase is stable up to 650 °C [15]. Transformation of the
metastable tetragonal form is due to the pretreatments of the catalysts that are exposed after a
further increase of sulfate amount and calcined at 600 °C. The phase of SZ depends on the
crystallinity properties of zirconia precursor [16]. The crystallinity of SZ significantly decreases
with increasing sulfate loading. It indicates that the relative amounts of Brønsted and Lewis acid
sites on the surface concentration of sulfates are increased. The crystallite sizes of sulfated and nonsulfated zirconia samples are cited in Table 1. The overall results demonstrate that crystallite sizes
increase with the addition of sulfate.
Table 1. Crystallite sizes (D) of commercial ZrO2 nanopowder (31.54 nm) and SZ
Sulfuric acid
D (nm)
concentration (M) 600 °C 700 °C 800 °C 900 °C
0.2
32.17
33.63
32.68
34.48
0.5
33.09
32.16
31.06
34.48
0.8
34.06
32.29
31.17
32.81
Fig. 2. FT-IR spectra of the commercial ZrO2 nanopowder, 0.2 M SZ, 0.5 M SZ and 0.8 M SZ
calcined at 600, 700, 800 and 900 °C
Infrared Analysis. Fig. 2 depicts the FT-IR spectra of commercial ZrO2 nanopowder and SZ
with 0.2, 0.5 and 0.8 M sulfuric acid solutions calcined at 600, 700, 800 and 900 °C in the 4004000 cm-1 region. The FT-IR spectra of ZrO2 and SZ show the bands assigned at 424-741 cm-1
which are corresponded to Zr-O-Zr bond [6]. The bands around 3449 and 1636 cm-1 are attributed
134
Green Convergence on Materials Frontiers
to stretching and bending frequencies of O-H from adsorbed water, respectively [17, 18]. The FTIR spectra of SZ show bands at 995-1227 cm-1 are corresponded to asymmetric stretching modes for
sulfate groups of ionized S=O bonds and S-O bonds with zirconia [19]. It indicates that sulfate has
modified ZrO2 well.
All possible peaks in the FT-IR spectra appear with varying intensities. However, the highest
intensity of sulfate is obviously revealed in the obtained spectrum for 0.8 M SZ-600. Thus, the
sulfated sample containing 0.8 M sulfuric acid solution and calcined at 600 °C reflects the high
dispersion of SO42- in commercial ZrO2 nanopowder. The catalytic activity of SZ is found to depend
on the calcination temperature as well as sulfate concentration. Yadav and Nair [1] proposed that
the calcination temperature upon sulfation is 650 °C. The calcination temperature beyond 650 °C
leads to a reduction in superacidity. The vibration bands of sulfate groups at 995-1227 cm-1
decrease by increasing the calcination temperature. As a general rule, the increase in the calcination
temperature provokes a lowering of sulfate [5] and the extremely high temperature treatment of SZ
can reduce the acidity and reactivity of catalyst. Moreover, it is seen that the higher sulfate
concentration up to certain maximum, leads to contents of sulfate that are certainly higher. The
combination of calcination temperature and concentration of sulfuric acid can be used to obtain a
better catalyst [1].
Acid Properties. Table 2 contains values of the strength of acid sites of commercial ZrO2
nanopowder and SZ that are obtained by ammonia adsorption. Based on the table, total acidity is
increased as the sulfuric acid concentration is increased. This phenomena is explained by Föttinger
et al. [20] that the relative amount of Brønsted and Lewis acid sites depend on the surface
concentration of sulfate. On the other hand, Busto et al. [7] suggested that total acidity is decreased
as the calcination temperature is increased. This is due both to the dehydration of protonic sites and
the loss of surface sulfate groups.
Table 2. Acidity of commercial ZrO2 nanopowder (0.18 mmol/g) and SZ
Acidity (mmol/g)
Sulfuric acid
concentration (M)
600 °C
700 °C 800 °C 900 °C
0.2
0.39
0.32
0.29
0.26
0.5
0.83
0.33
0.30
0.28
0.8
1.06
0.56
0.53
0.33
Fig. 3. Shows the FT-IR spectra of adsorbed ammonia. Ammonia adsorbed over the samples
on Brønsted acid sites forms ammonium ion with vibration bands at 1404 cm-1 while the
coordination bond of ammonia on Lewis acid sites yields characteristic bands at 1119 cm-1 [21, 22].
At higher sulfuric acid concentration of SZ calcined at 600 °C, the amount of Brønsted and Lewis
acid sites is increased. It can also be seen that the treatment of the samples calcined at higher
temperatures decreases the presence of Brønsted and Lewis acid sites. Total acidity is decreased as
the calcination temperature is increased, in coincidence with a decrease of the amount strong acid
sites and sulfur surface density of the support that has been pointed out as the main cause of acidity
decrease [7].
Microscopic Studies. Fig. 4 shows the SEM images of commercial ZrO2 nanopowder (a) and
0.8 M SZ-600 (b) samples. SEM was used to probe the change in morphological feature after the
addition of sulfate and calcination process. As the addition of sulfate, it can be observed that the
presence of particle agglomeration. The surface morphology of both samples exhibit disordered
structure. Table 3 shows the content of Zr, O and S using EDX. EDX is able to identify the element
content of the sample. The content of S and O in the ZrO2 are increased after the sulfation process.
It indicates that sulfate ion has been successfully impregnated to ZrO2.
Table 3. Elements analysis of commercial ZrO2 nanopowder and 0.8 M SZ-600
Sample
Mass (%)
Zr
O
S
ZrO2 nanopowder
64.08
35.46
0.46
0.8 M SZ-600
60.51
37.42
2.07
Key Engineering Materials Vol. 757
135
Fig. 3. FT-IR spectra of ammonia adsorbed over commercial ZrO2 nanopowder, 0.2 M SZ, 0.5 M
SZ and 0.8 M SZ calcined at 600, 700, 800 and 900 °C
Fig. 4. SEM images of (a) commercial ZrO2 nanopowder and (b) 0.8 M SZ-600
Conclusions
A further increase of sulfate amount significantly increased the acidity and decreased the
crystallinity of SZ. It indicated that the relative amounts of active sites were increased. The
enhancement of the calcination temperature caused a lowering of sulfate. The treatment of 0.8 M
sulfuric acid on commercial ZrO2 nanopowder and calcined at 600 °C produced the optimum
conditions in the preparation of SZ. This optimum conditions coincided with the production of the
highest acidity that was found to be 1.06 mmol/g.
136
Green Convergence on Materials Frontiers
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