Full paper
Journal of the Ceramic Society of Japan 124 [2] 155-159 2016
Characterization and catalytic behavior of cerium oxide doped into aluminosilicophosphate glasses Hyun Joon PARK and Bong Ki RYU³ Department of Materials Science and Engineering, Pusan National University, Busan 609–735, Korea
Cerium oxide is commonly used as a catalyst for the oxidation of carbon monoxide (CO) in vehicle exhaust gases, and it is used as a petroleum-cracking catalyst in oil refineries. In this study, cerium aluminosilicophosphate glasses were synthesized and various physical and catalytic properties were measured, including optical properties and decomposition temperatures, by thermogravimetric analysis (TGA) and Fourier transform infrared (FT-IR) analysis. Ce3+/Ce4+ ratios and structural changes in the glass attributable to doping with ³25 mol % CeO2 were investigated by FT-IR spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis. Changes in the catalytic properties of the glasses as a function of CeO2 content were confirmed by changes in the decomposition starting temperatures. These temperatures decreased with increasing CeO2 content of the glasses. We also discuss the role of CeO2 in terms of the catalytic properties of the glass structure. ©2016 The Ceramic Society of Japan. All rights reserved.
Key-words : Phosphate glasses, CeO2, X-ray photoelectron spectroscopy, Thermogravimetric analysis, Catalyst [Received October 2, 2015; Accepted December 18, 2015]
1.
Introduction
Cerium oxide is a catalyst with a high oxygen-storage capacity, high surface reactivity, and phase-change desorption capability. Also, cerium has a unique electronic structure, in which the unfilled 4f orbitals are shielded from interactions with the surrounding environment by a full octet of electrons in the 5s2p6 outer shell. Because of this unique electronic structure, cerium oxide has been used in non-stoichiometric1),2) and oxygen storage applications3)5) and metalceria interactions.6),7) It has also been used to enhance hydrophobicity.8) In general, CeO2 becomes many amount is added to the phosphate system in the glass than the addition has been silicate-based glass in the formula oxide in the glass, CeO2 phosphate-based glass that has been the addition is, the majority crystallization it will form a CePO4 crystal.9) Moreover, it now serves to reduce the thus formed CePO4 crystal catalytic properties of the glass in CeO210) and is used as the glass of the catalyst from various quarters to be limited. Therefore, for enhancing the catalytic properties of the glass, it is important to design a composition that can suppress the crystallization of phosphate glass to which CeO2 is added. Such glasses can be used as catalysts, in self-cleaning household ovens11),12) and catalyzed diesel particulate filter.13),14) These studies are of interest from a commercial and academic viewpoint. Further, Ce is a typical oxidation catalyst material, Pt, Pd, Ag, the various CeO2, Al2O3, Fe2O3, MnO2, Co3O4, NiO, CuO, the synthesis of the supporter,15)18) such as ZnO, TiO2, SiO2, ZrO2 activating moves the CO reduction catalysis lower temperature through. For example, in the case of Au, an increased concentration of Au3+ ions helps to activate the catalytic reaction. The addition of CeO2 combined with a combustion technique helps to increase the levels of Au3+ ions and decrease CO levels.19) In this study, CeO2 was added to glass and the catalytic function of this system was analyzed. CeO2 in the glass should ³
Corresponding author: B. K. Ryu; E-mail:
[email protected]
©2016 The Ceramic Society of Japan DOI http://dx.doi.org/10.2109/jcersj2.15223
be present as Ce2O3 or CeO2.20) However, when CeO2 crystals precipitate, the ratio between these two species changes. If precipitation occurs, the effectiveness of the catalyst is reduced owing to the deformation of the molecular structure. The relationship between precipitation and the degree of ionic bonding and electronegativity was studied in terms of the catalytic reaction. Therefore, cerium oxide-doped aluminosilicophosphate (SAP) glasses were synthesized and characterized. The addition of CeO2 was expected to cross-link the glass network and increase the catalytic properties for the fatty acid (stearic acid)
2.1
2. Experimental procedure Preparation of glass samples
Glasses with compositions of 15 SiO215 Al2O370 P2O5 (0 + x) CeO2 (x = 025 mol %) were prepared (see Table 1). The glasses were prepared from reagent grade SiO2, Al2O3, NH4H2PO4, and CeO2. Well-mixed batches calculated to yield 50 g of glass were melted in a clay crucible for 0.5 h at 1500°C. The glass melt was quenched by pouring it on a stainless steel plate. The obtained glasses were annealed at their glass-transition temperatures (Tg) for 2 h and then cut and mechanically polished to obtain samples for thermal and chemical analysis. We also created pellets made of SAP and SAPC glass frit (20 g). The pellets were fired at 1000°C for 5 min. To record the infrared (IR) absorption spectra of the glass samples, the KBr pellet technique was employed. The glass samples were ground to a fine powder Table 1. Analyzed compositions of SAP and SAPC glasses
Glass SAP SAPC1 SAPC2 SAPC3 SAPC4 SAPC5
Composition (mol %) SiO2
Al2O3
P2O5
CeO2
15.00 15.00 15.00 15.00 15.00 15.00
15.00 15.00 15.00 15.00 15.00 15.00
70.00 70.00 70.00 70.00 70.00 70.00
0.00 5.00 10.00 15.00 20.00 25.00
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Park et al.: Characterization and catalytic behavior of cerium oxide doped into aluminosilicophosphate glasses
and a weighed quantity (0.001 g) of the powder was thoroughly mixed with desiccated, highly purified (99.99%) KBr powder (0.1 g). The mixtures were pressed into thin pellets to record the spectra.
2.2
Measurements
The thermal behavior of the glasses was examined with differential thermal analysis (DTA) (TA, Q600) operating in the temperature range 2981473 K using a heating rate of 10 K/min. The structures of the glasses were analyzed by Fourier transform infrared (FT-IR) spectroscopy (Spectrum GX). We performed thermogravimetric analysis (TGA) (TA, Q600) measurements using 0.01 « 0.005 mg of the glass powders and 0.01 « 0.005 mg of a fatty acid (stearic acid) under an atmosphere of N2 gas and using heating rates of 10°C/min up to 500°C. We also checked the type of the upper analyzed by X-ray diffraction (XRD) (Ringaku X-ray diffractometer, Cu K¡, 30 kV, 20 mA). X-ray photoelectron spectroscopy (XPS) measurements were taken using the ESCALAB 250 XPS system and Theta Probe XPS system using monochromatic Al K¡ (h¯ = 1486.6 eV) radiation conditions. An analysis area of 400 ¯m in diameter was used and it was corrected to reference C1s (284.6 eV).
3. 3.1
Results and discussion
DTA & XRD analysis
Among the studies related to the crystallization of cerium phosphate glasses, precipitation of CePO4 by heat treating the cerium phosphate glass has been demonstrated.21) Therefore, we checked for the formation CePO4 in the SAP and SAPC glasses. The DTA and XRD results are shown in Fig. 1. The DTA curves do not show an onset crystallization temperature (Tc), and peak crystallization temperatures (Tp) and appear broad peak in XRD data. Thus, we considered there was no crystallization of CePO4 on the SAP and SAPC glasses.
3.2
These points can be guessed by the following reaction mechanism. At decomposition temperature the Ce atoms of glass frit interface are mostly covered by COads. These COads do not dispose of any atomic oxygen on glass frit but they are able to pick up the “oxide” oxygen atoms provided by the interfacial ceria as already demonstrated in work. Thus, we think that the first stage of the reaction consists of the formation of CO2 via the reduction of the interfacial ceria by carbon monoxide adsorbed on glass frit. The CO2 desorption leads to vacant site on the interfacial glass frit and makes possible the dissociative adsorption of stearic acid. Ceria is then regenerated by a spill-over phenomenon of Oads toward Ce2O3 and re-oxidation of the interface into CeO2. This interpretation is described by the following three steps: COads þ 2CeO2 ! CO2 þ þ Ce2 O3 1=2O2 þ ! Oads Oads þ Ce2 O3 ! 2CeO2 þ
ð1Þ ð2Þ ð3Þ
where the asterisk ( ) stands for an adsorption site on glass frit and “ads” indicates an adsorbed species. From this mechanism, the very high activity observed for the pre-reduced glassCeO2 catalyst would be the possibility of initiating the CO oxidation without any vacant sites for the dissociation of oxygen. According to Yusaku et al., isobutene is adsorbed onto the lattice O2¹ and Ce4+ to form isobuthylcarbenium ions and H¹. Isobutene is released from buthylcarbenium, leaving one proton. The electron of H migrates to Ce4+ to form Ce3+ and H radical. The elimination of a water molecule from the surface results in the formation of OH and H radicals to form a pair of Ce3+ ions,
TGA analysis
We compared the performance of the glass catalyst with that of CeO2, which has been reported to be a very good oxidation catalyst. The TGA curve for the mixture of the glass frit and stearic acid in a weight ratio of 1:1 are shown in Fig. 2. For SAP glass frit and SAPC glass frit + stearic acid, the starting decomposition temperatures were 233270°C and the closing decomposition temperatures were 285322°C, as shown in Table 2.
TGA curves for SAP and SAPC glass catalysts in stearic acid mixed in weight ratios of approximately 1:1.
Fig. 2.
Table 2. Starting and closing decomposition temperatures for SAP and SAPC glass catalysts mixed with stearic acid
DTA curves of glasses from the SiO2Al2O3P2O5CeO2 system. The inset shows the XRD curve of SAPC5 glass at 1000°C for 30 min. Fig. 1.
156
SAP SAPC1 SAPC2 SAPC3 SAPC4 SAPC5
Starting Stearic Acid Decomposition Temperature (°C)
Closing Stearic Acid Decomposition Temperature (°C)
270.19 249.23 244.25 240.25 236.01 233.02
322.16 301.12 296.23 292.22 287.99 284.99
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Journal of the Ceramic Society of Japan 124 [2] 155-159 2016
DSC curves for glass samples mixed approximately 1:1 (wt %) with stearic acid obtained at ¡ = 10 K/min
Fig. 5.
Fig. 3.
Fig. 6.
Fig. 4.
Optical absorption of SiO2Al2O3P2O5CeO2 glasses.
which is then oxidized by molecular oxygen to Ce4+, i.e., its original state.22) That is why SAPC glasses performed well in comparison to another developmental catalyst. Catalytic property of glass samples was further analyzed by DSC. Through Fig. 3, it is confirmed that change of the endothermic peak of glass samples is similar to the results of TGA. The explanation of these results was confirmed through a structural analysis.
3.3
UV–vis spectroscopy
Figure 4 shows the optical absorption spectra of the SAP and SAPC glasses. It was observed that the band edge of the glasses progressively shifted toward longer wavelengths in the region from 320 to 420 nm. It shifted more from approximately 325 to 345 nm. The influence of irradiation is explained by the following reaction: Ce3þ þ HC ! Ce4þ =Ce4þ þ EC ! Ce3þ ; where HC is the hole center captured by the cations and EC is the electron center captured by the anions.23)25) The continuous shifting of the band edge in these glasses confirms that the Ce3+ + HC ¼ Ce4+ reaction is the predominant reaction. It was also observed that the irradiation-induced absorption band from 320 to 420 nm extended to longer wavelengths, which is due to the origin of Ce4+ ions.
Optical band gap of SiO2Al2O3P2O5CeO2 glasses.
FT-IR spectra of SiO2Al2O3P2O5CeO2 glasses.
The optical band gap of the samples after irradiation was obtained by using the plot between (¡h¯)1/2 and energy (h¯), where ¡, h, and ¯ are the absorption coefficient, Planck’s constant, and frequency, respectively.26) The values of the optical energy gap were obtained from the line on the plot shown in Fig. 5. The optical band-gap energy shifts to a lower energy with the change in composition. This is due to the formation of more tetrahedral [PO4] groups and Ce4+ ions in the glasses. It is observed that with the addition of CeO2, a large number of oxygen ions are available in the glass network by breaking up the POP linkages in the Q2 unit, creating non-bridging oxygen (NBO) atoms by forming ionic bonds. As reported in the literature, forming NBOs in glass networks affects the structural and optical behavior of the glass. The oxygen atoms are utilized by cerium oxide for converting the Ce3+ groups to Ce4+ groups. These factors shift the absorption edge to the lower energy, which leads to a significant compaction in the band gap (shown in Fig. 5). This change in band gap shows that the CeO2 acts as a network modifier.
3.4
FT-IR analysis
The FT-IR transmission spectra recorded for the SAP and SAPC glasses is shown in Fig. 6. We examined the FT-IR spectra to determine the influence of non-crosslinked oxygen on the catalytic properties of the glasses. In the glasses fabricated in the current study, the introduction of aluminum is generally considered to play the role of a former 157
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Park et al.: Characterization and catalytic behavior of cerium oxide doped into aluminosilicophosphate glasses
rather than a modifier. Aluminum and silicon have similar masses and ionic rations, which favor coupling of their vibrations. The band at approximately 540 cm¹1 is attributed to the bending vibration of OPO and the next peak located at approximately 680 cm¹1 is attributed to the symmetric stretching of the POP mode vibration in the long-chain phosphate groups (Q2 unit). The peak at 740 cm¹1 is attributed to the symmetric stretching of the POP mode (Q0 unit). Also, the band at approximately 1150 cm¹1 is attributed to the symmetric stretching mode of the OPO non-bridging oxygen, indicating the formation of a Q2 phosphate tetrahedral.27)32) With the increase of CeO2 content, the intensity of the OPO bending vibration increases. Higher CeO2 content leads to bonds that are shorter than the phosphate ionic bonds. At the same time, CeO2 ions enter the glass network by breaking up the POP linkages in Q2 units, creating NBOs by forming ionic bonds, which decreases the Q2 unit. When the glasses were doped with 5 mol % of CeO2, the intensity of the band due to [PO2] units was observed to decrease, and correspondence to this a new band is rise at approximately 450 cm¹1 band. This band is attributed to the presence of a CeO stretching vibration.33) In some cases, it refers to the formation of cerium units combined with the network.34),35) The change of band intensity at approximately 450 cm¹1 with increasing CeO2 content leads one to consider that this band is due more to vibrations of the modifier than to those of the network former CeO2 unit.35) Furthermore, rare-earth ions were found to play the role of a modifier in aluminophosphate glasses and cation coordination numbers were determined.36) Also, Du et al. reported that the CeP5+ first-peak intensity is stronger for Ce4+ than for Ce3+. This is consistent with the higher field strength of Ce4+ ions, which would be accompanied by a higher incidence of nonbridging oxygen ions in the first coordination shell.37) Hence, cerium plays the role of a modifier in phosphate networks. As the CeO2 amount increases, the number of Ce4+ ions increases.
3.5
XPS analysis
In XPS, the chemical state of an element is manifest in chemical shifts of core-level photoelectron peaks. Here, the ability to distinguish between Ce3+ and Ce4+ is of primary interest, and in this case, photoelectron peaks associated with core-level transitions in the 3d spectra of Ce possess the relevant information. However, the multitude of overlapping 3d photoelectron transitions for Ce creates a set of peaks whose quantitative analysis requires curve-fitting. We followed the work and notation of Romeo and Pfau, who identified ten peaks (five pairs) in the 3d region, as shown in Fig. 7.38),39) Those five peaks, designated as v peaks, belong to the 3d5/2 spinorbit split doublet, and the five analogous peaks in the 3d3/2 spinorbit split doublet are designated as u peaks.38)42) Of these ten peaks, two pairs are assigned to Ce3+ (®0, ®0*, ®1, ®1*), whereas the remaining three pairs are assigned to Ce4+ (V0, V0*, V1, V1*, V2, V2*).38)41) These ten peaks overlap into three adjacent regions: the first encompasses four peaks from the 3d5/2 spinorbit split doublet (®0, ®1, V0, V1), the second encompasses the V1 peak and four peaks from the 3d3/2 spinorbit split doublet (®0*, ®1*, V0*, V1*, V2, V2*), and the V2* peak stands alone. The abundance of these species is reported in Table 3. It was observed that at low Ce content, the preferred oxidation state is Ce3+; however, with increasing Ce content in these glasses, the presence of the Ce4+ species gradually increases, a phenomenon also observed by other research groups.42)44) Pestryakov et al. reported that the addition of CexOy and ZrxOy to Au clusters stabilizes the Au¤+ (0 < ¤ < 1) oxidation state due to electron transfer from gold to the metal oxide.45) 158
Ce 3d core-level peaks for the (a) x = 5, (b) x = 15, and (c) x = 20 glass composition curve fitted with ten individual peaks corresponding to contributions from Ce(III) ions (u line) and Ce(IV) ions (v line).
Fig. 7.
Table 3. Ce3+ and Ce4+ ratios in SAPC glasses
x
Ce(III) ratio
Ce(IV) ratio
5 10 15 20 25
52.34% 50.43% 49.02% 47.13% 46.37%
47.66% 49.57% 50.98% 52.87% 53.63%
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Journal of the Ceramic Society of Japan 124 [2] 155-159 2016
The reduction of Ce4+ to Ce3+ after deposition of Au on CeO2 (1 1 1) was observed by Skoda et al.46)
4.
Conclusion
We investigated the effect of CeO2 content on the catalytic behavior of SiO2Al2O3P2O5CeO2 glasses (SAPC system) by open-crucible melting. Their catalytic properties increased with increasing CeO2 content. The reasons for the change in the catalytic properties of the SAPC glass samples were investigated through the use of UVvis/FT-IR/XPS measurements. It was confirmed that CeO2 acts as a network modifier in aluminosilicophosphate glasses. Also, CeO2 is present as Ce3+ and Ce4+ in the glass, depending on the percentage of Ce3+/Ce4+, and varying catalytic properties by varying the energy band gap. And show a tendency to increase the greater the amount of Ce4+ when the catalytic properties, such Ce4+ ions were able to see trends formed by network modifier in the glass. Therefore, in order to understand the catalytic properties in glass, more catalytic materials need to be studied in terms of changes in the structure of glass. Additional fundamental and applied research needs to be performed in order to design improved or new active glass-based systems for different catalytic applications. Acknowledgments This research was financially supported by the Ministry of Education (MOE) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation (No. 2015028537).
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