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: firstname.lastname@example.org 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 . The addition of sulfate will increase thermal stabilization, lower reaction temperatures and stabilize the structure of zirconia . 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 . A slight variation of the preparative procedures can greatly affect the acid properties of the resulting oxide surface . The catalytic performance of sulfated zirconia depends on the temperature of calcination and the concentration of the sulfating agent . Busto et al.  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 . 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 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.scientific.net. (#103391264, Swinburne University, Hawthorn, Australia-12/11/17,18:48:07) 132 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 . 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 . 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 . 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 . 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  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  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 . 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.  that the relative amount of Brønsted and Lewis acid sites depend on the surface concentration of sulfate. On the other hand, Busto et al.  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 . 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. 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