Catalytic activity of nanosized Au/CeO2 catalyst towards H2O2 decomposition and the role of cationic/metallic ratio in its activity European Journal of Chemistry 10 (4) (2019) 317-322 European Journal of Chemistry View Journal Online View Article Online Catalytic activity of nanosized Au/CeO2 catalyst towards H2O2 decomposition and the role of cationic/metallic ratio in its activity Ayman Abd El-Moemen * Chemistry Department, Faculty of Science, Suez Canal University, Ismailia, 41522, Egypt * Corresponding author at: Chemistry Department, Faculty of Science, Suez Canal University, Ismailia, 41522, Egypt. Tel: +20.109.9403540 Fax: +20.64.3230416 e-mail: ayman.moemen@science.suez.edu.eg (A.A. El-Moemen). 10.5155/eurjchem.10.4.317-322.1895 Received: 20 May 2019 Received in revised form: 12 July 2019 Accepted: 13 July 2019 Published online: 31 December 2019 Printed: 31 December 2019 The catalytic decomposition of H2O2 on differently pre-treated Au/CeO2 catalyst was studied by kinetic measurements at 20-50 °C. The prepared catalyst was subjected to pre-treatment by heating either in oxidative (10% O2/N2) or inert (pure N2) atmosphere at 400 °C. The different oxidation states of gold were determined by X-ray photoelectron spectroscopy measurements. The Au/CeO2 catalyst exhibited an excellent catalytic activity towards H2O2 decomposition. The catalytic activity of oxygen pre-treated sample was about twice higher than that measured for nitrogen pre-treated sample. This finding ran parallel to the amount of Aun+ as determined by XPS, indicating the role played by Aun+ species as the most active catalyst’s constituent. However, one cannot overlook the role of metallic gold in catalyzing the H2O2, decomposition showing small activity compared to that of cationic gold. The average crystallites size of metallic gold particles was found to be 7±0.5 nm independent of the pre-treatment conditions. The apparent activation energy of the catalyzed reaction was found to be 46.5 and 47.8 kJ/mol for oxygen and nitrogen pre-treatment, respectively. Au/CeO2 catalyst Catalytic activity H2O2 decomposition Catalyst pre-treatment X-ray diffraction (XRD) X-ray photoelectron spectroscopy (XPS) Cite this: Eur. J. Chem. 2019, 10(4), 317-322 Journal website: www.eurjchem.com 1. Introduction H2O2 decomposition reaction has been extensively studied [1-10]. This is mainly due to the following reasons: (i) H2O2 decomposition is a useful model test reaction for studying the catalytic activity of different materials, (ii) H2O2 might be an efficient oxygen source, if highly active and selective catalysts are used. (iii) H2O2 used as an oxidant (clean oxidizing agent or ideal liquid phase green oxidation processes) in the presence of a good and selective catalyst, for organic synthesis, and water treatment technologies as it emits only H2O by product with high atom efficiency. Various organic water and soil pollutants can be successfully oxidized and degraded by hydrogen peroxide promoted by iron oxides [11-13]. Recently as a development of fuel cell technology, both water gas shift reaction and CO oxidation reaction are important for the removal of CO from the fuel cell [14,15]. On the other hand, one of the most important problems in Proton Exchange Membrane Fuel Cell (PEMFC) technology, is a production of H2O2 electrochemically or chemically during operation at the cathode as a by-product, and therefore may deteriorate materials in the membrane-electrode [14,16]. Using a highly active and selective catalyst in the PEMFC may be working in more than one way simultaneously, decompo- sition of H2O2 and producing of pure oxygen which is sufficient for CO oxidation. In the last few years, gold catalysts supported on metal oxides have been reported to be highly active for several reactions, including the water gas shift (WGS) reaction [17- 20], the low temperature CO oxidation by oxygen [21,22], and H2O2 decomposition [3]. The catalytic activity of these catalysts is strongly affected by gold particle size, synthesis method, pre-treatment conditions, oxidation state of gold and the nature of the support material [22,23]. It has been reported that, the H2O2-Au/support catalytic system has been proven to be effective in removing low-level hazardous organic compounds including formaldehyde, acetone, and phenol from waste water [24]. It is also found that, the catalytic activity of Au/TiO2 towards the H2O2 decomposition strongly depends on the gold particle size, and also exhibits a high activity for the chemoselective oxidation of cinnamyl alcohol to cinnamaldehyde, based on “liquid-phase green oxidation processes” using an H2O2-Au/metal oxide system [3,25]. Catalysts based on cerium oxide (Ceria, CeO2) are promising for these applications. Ceria is widely used as an oxygen storage component in the automobile three-way catalyst. It gives up oxygen under rich engine operating conditions and accepts oxygen under lean conditions. ABSTRACT RESEARCH ARTICLE KEYWORDS European Journal of Chemistry ISSN 2153-2249 (Print) / ISSN 2153-2257 (Online) – Copyright © 2019 The Authors – Atlanta Publishing House LLC – Printed in the USA. This work is published and licensed by Atlanta Publishing House LLC – CC BY NC – Some Rights Reserved. http://dx.doi.org/10.5155/eurjchem.10.4.317-322.1895 http://dx.doi.org/10.5155/eurjchem.10.4.317-322.1895 https://crossmark.crossref.org/dialog/?doi=10.5155/eurjchem.10.4.317-322.1895&domain=pdf&date_stamp=2019-12-31 http://www.eurjchem.com/ http://dx.doi.org/10.5155/eurjchem.10.4.317-322.1895 mailto:ayman.moemen@science.suez.edu.eg http://www.eurjchem.com/ https://crossmark.crossref.org/dialog/?doi=10.5155/eurjchem.10.4.317-322.1895&domain=pdf&date_stamp=2019-12-31� 318 Ayman Abd El-Moemen / European Journal of Chemistry 10 (4) (2019) 317-322 Table 1. XPS data of gold, and ceria species measured after different pre-treatments. Catalyst pre-treatment Au0 (%) Au1+ (%) Au3+ (%) Aun+/Au0 Ce3+ (%) IAu(4f)/ICe(3d) Oxygen 35 45 20 1.86 21 0.08 Nitrogen 54 30 16 0.85 23 0.07 Thus, ceria is good for oxygen-storage capacity (OSC). Luo et al. found a direct relationship between the WGS activity and the OSC [26]. Different models have been proposed to explain the unique catalytic activity of the CeO2 supported Au catalysts towards either CO oxidation or WGS reaction. Nano-sized Au/CeO2 catalyst consisted of metallic and cationic gold species. The role of these species in its activity towards WGS reaction had been investigated by several authors [27-29]. Fu et al. reported that non-metallic (cationic) gold species strongly associated with surface cerium-oxygen groups are the most catalytically active constituent [27] while, Kim and Thompson claimed an opposite trend [28]. However, Karpenko et al. have reported that the catalytic activity of Au/CeO2 catalyst towards WGS reaction depends on the contribution of both metallic and cationic gold species present on the catalyst surface [29]. The aim of the present work is to study the effect of synthesis and characterization of the nano- sized Au/CeO2 system on its activity using H2O2 decom- position, and the role of cationic/metallic gold species on the activity of the prepared catalysts. The extent of both metallic and cationic gold in the prepared catalyst was effectively modified by pre-treatment with oxygen or nitrogen at 400 °C. 2. Experimental 2.1. Materials The nanosized Au/CeO2 catalyst was prepared by a deposition-precipitation procedure. CeO2 support material was pre-calcined in air at 400 °C for 4 h and re-dispersed in water at 60 °C. The gold precursor (HAuCl4·3H2O) was added dropwise, while adjusting the pH value at 5-6 by adding Na2CO3 solution [29]. The Au metal loading was determined via inductively coupled plasma atomic emission spectroscopy (ICP-AES). All measurements were performed with catalysts of 4.5 wt% Au loading. The obtained sample was subjected to heat in a current of 10% O2/N2 or pure N2 flowing at a rate of 20 mL/min at 400 °C for 30 min. 2.2. Techniques 2.2.1. X-ray diffraction measurements The X-ray investigations of the obtained pre-treated catalysts were carried out using a Bruker diffractometer (Bruker D8 advance target). The scanning rate was fixed for phase identification at 0.018° in 2θ/min for line broadening profile analysis. The patterns were run with CuKα1 with secondly monochromator (λ = 0.15405 nm) at 40 kV and 35 mA. The crystallite size of each phase present in different solids was calculated, using the line broadening profile analysis by direct application of the Scherrer’s equation. 2.2.2. XPS measurements The chemical composition of the catalyst surface and the oxidation states of both gold and cerium species were characterized by XPS (PHI 5800 ESCA system), using monochromatized Al-Kα radiation. The survey spectra were measured in the range between 0 and 1400 eV binding energy (BE). Detail spectra of gold (Au(4f)) and ceria (Ce(3d)) were measured in the range of 75-100 eV and 875-925 eV (0.125 eV and 20 ms per step), respectively. 2.2.3. Specific surface area The specific surface area of the prepared sample was measured by nitrogen adsorption isotherm measured at -196 °C using Porotec Sorptomatic 1990 systems. The specific surface area of the prepared catalyst measured 188 m2/g. 2.2.4. Activity measurements For kinetic measurements, the decomposition of hydrogen peroxide in aqueous medium was studied. The kinetics of reaction was followed up gasometrically according to the technique described by Deren et al. [30], using 0.2 M H2O2 solution. The catalytic reaction was monitored by measuring the volume of oxygen liberated at different time intervals. 3. Results and discussion 3.1. XPS investigation of different pre-treated catalyst samples The chemical composition of the catalyst after the oxygen or nitrogen pre-treatments at 400 °C was characterized via the Au(4f) and Ce(3d) signals. For quantitative evaluation, the XPS spectra of the Au(4f) signals were fitted by three different states with BEs of 84.0, 84.6 and 85.9 eV, respectively [28,31- 33]. The first and the last peaks are assigned to Au0 and Au3+ species [32,34,35], while the peak at 84.6 eV was previously attributed to Au1+ [28,31]. The XPS spectra of the two pre-treated samples were carried out and the obtained spectra concerning Au(4f) are graphically illustrated in Figure 1. Analysis of XPS spectra enables an accurate determination of Au0, Au1+, and Au3+ present in different pre-treated samples. The computed values of relative abundance of each gold species are given in Table 1. It is clear that Aun+/Au0 greatly increase with oxygen pre- treatment. An important portion of metallic gold interacted with O2 yielding trivalent and monovalent gold species. The ratio of the Au and Ce related intensities, IAu(4f)/ICe(3d), contains a qualitative information on the Au particle size. Similar values of 0.07±0.01 were obtained (Table 1), indicating similar Au particle sizes after the two pre-treatments. This can be confirmed by the XRD measurements. 3.2. XRD investigation of the pre-treated solids Figure 2 shows the XRD patterns of oxygen and nitrogen pre-treated samples. The average crystallite size of metallic gold particles was determined, using line broadening profile analysis of the main diffraction peak of the gold (111) by direct application of the Scherrer’s equation. The computed values were 7±0.5 nm for both pre-treated catalysts, indicating that the pre-treatment conditions have no effect on the particle size of gold. 3.3. Catalytic properties of different pre-treated solids Preliminary experiments showed that CeO2 support material exhibited no measurable catalytic activity at 20-50 °C. Therefore, CeO2 acts as a catalyst support, while the Au species are the catalytically active constituent in the catalyst under investigation. 2019 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.10.4.317-322.1895 Ayman Abd El-Moemen / European Journal of Chemistry 10 (4) (2019) 317-322 319 Figure 1. XPS spectra of the Au(4f) region (left panel) and Ce(3d) region (right panel) of different investigated catalysts: (a) Oxygen pre-treated, (b) Nitrogen pre-treated. 30 35 40 45 50 55 60 (b) In te ns ity / cp s 2 Theta (o) (a) Figure 2. XRD pattern of the Au/CeO2 catalysts: (a) Oxygen pre-treated, (b) Nitrogen pre-treated. Figure 3. Relationship between the reaction temperature and rate constant for H2O2 decomposition over oxygen and nitrogen pre-treated catalyst. The catalytic reaction was followed by measuring the volume of O2 gas liberated at different time intervals until no further oxygen evolved. The catalytic reaction was carried out at 20 to 50 °C. First order kinetics (not given) were observed in all cases. The slopes of these plots directly determine the values of a reaction rate constant (k) at a given temperature. Figure 3 shows the relation between the calculated reaction rate constant (k) per gram catalyst and the reaction temperature. The effect of the mass of catalyst on the rate of decompo- sition of H2O2 was investigated at 30 °C. Figure 4 shows the variation of a reaction rate constant measured at 30 °C (k30°C) as a function of mass of oxygen and nitrogen pre-treated catalysts. A good linear relationship was obtained, suggesting the absence of any diffusion phenomenon. So, the mass of the catalyst sample taken for each kinetic experiment varied between 10-50 mg, depending on the catalytic reaction temperature. 2019 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.10.4.317-322.1895 320 Ayman Abd El-Moemen / European Journal of Chemistry 10 (4) (2019) 317-322 Table 2. The values of a first order reaction rate constant measured at different temperatures over oxygen and nitrogen pre-treated Au/CeO2 catalysts. Reaction temperature (°C) Oxygen pre-treated rate constant (min-1. g-1) Nitrogen pre-treated rate constant (min-1. g-1) 20 4.0 2.2 30 7.7 4.2 40 13.1 7.5 50 24.0 13.7 10 20 30 40 50 0.0 0.1 0.2 0.3 0.4 Oxygen pre-treated Nitrogen pre-treated k 3 0° C (m in -1 ) Mass of catalyst (mg) Figure 4. Relationship between the mass of catalyst and rate constant for H2O2 decomposition carried out at 30 °C over oxygen and nitrogen pre-treated catalyst. 3.1 3.2 3.3 3.4 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Oxygen pre-treated Nitrogen pre-treated T-1×103 / K-1 ln k Figure 5. Relation between ln k and 1/T for H2O2 decomposition carried out over oxygen and nitrogen pre-treated catalyst samples. The computed values of reaction rate constants (per unit mass) for the reaction carried out at 20 to 50 °C over various catalysts are given in Table 2. The comparison between the activity of oxygen and nitrogen pre-treated solids are better investigated by comparing the k values for reaction carried out at different temperatures. The computed k values for the oxygen pre-treated sample at any reaction temperatures is about twice the value measured for nitrogen pre-treated sample. As we have shown, the Aun+/Au0 ratio for oxygen pre-treated sample was about twice for that of nitrogen pre-treated catalyst. So, the Aun+ species acted as the most active catalyst’s constituent in the investigated system. However, one cannot overlook the role of metallic gold in catalyzing the H2O2, decomposition, showing small activity compared to that of cationic gold. Similar results have been reported for 4.5 wt% Au/CeO2 using WGS reaction and CO oxidation by oxygen, where the oxygen pre-treated sample showed higher activity than the nitrogen pre-treated one [36-39]. The apparent activation energies (Ea) of H2O2 decompo- sition were determined for both pre-treated samples, from the data of k measured at 20-50 °C. Figure 5 depicts the Arrhenius plot of ln k as a function of 1/T for both catalysts. The computed Ea values were found to be 46.5 and 47.8 kJ/mol for oxygen pre-treated and nitrogen pre-treated catalysts, respectively. This indicates that the different pre- treatments of the Au/CeO2 system do not change the mechanism of the catalyzed reaction, but leads to a change in the concentration of active sites involved in the catalytic reaction. It also supports the assumption that the change in activity between both catalysts referred to the change in Aun+ concentration. The standard enthalpies of activation (∆H‡) were calculated from the linear plot of the Eyring equation (Figure 6). ‡ ‡ ln ln Bkk H S T RT h R ∆ ∆  = − + +        (1) where k is the reaction rate constant; T is the absolute reaction temperature; ∆H‡ is the enthalpy of activation; R is the gas constant; kB is the Boltzmann constant; h is the Planck constant; and ΔS‡ is the standard entropy of activation. The calculated values of activation energies and thermodynamics parameters for activations are given in Table 3. The negative values for ΔS‡ indicate that entropy decreases on forming the activated complex during the catalytic decomposition of H2O2 over the given catalyst. It often indicates an associative mechanism in which two reaction partners (hydroxyl (•OH) and hydroperoxyl (•OOH) radicals) form a single activated complex, followed by decomposition to give a final product. 2019 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.10.4.317-322.1895 Ayman Abd El-Moemen / European Journal of Chemistry 10 (4) (2019) 317-322 321 Table 3. Arrhenius activation energies (Ea), standard enthalpies of activation (∆H‡), and standard entropy of activation (ΔS‡) for the decomposition of H2O2 Over different pre-treated Au/CeO2 catalysts. Catalyst pre-treatment Ea (kJ/mol) ∆H‡ (kJ/mol) ΔS‡ (J/mol.K) Oxygen 46.5 44.3 ‒ 82.2 Nitrogen 47.8 45.5 ‒ 83.0 0.0031 0.0032 0.0033 0.0034 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 Oxygen pre-treated Nitrogen pre-treated ln ( k / T ) 1/T (K-1) Figure 6. Eyring plots for the determination of the enthalpy and entropy of activation for the catalytic decomposition of H2O2 over different pre-treated Au/CeO2 catalysts. The proposed mechanisms for the catalytic decomposition of H2O2 into hydroxyl and hydroperoxyl radicals in the presence of nanosiezd Au/CeO2 by a redox cycle [40], can be discussed as follows: H2O2 + Au0 → •OH + OH− + Au+ (2) H2O2 + Au+ → •OOH + H+ + Au0 (3) OH− + H + → H2O (4) •OH + •OOH → H2O + O2 (5) ______________________________________________ The net reaction is: 2 H2O2 → H2O + O2 (6) Another suggested mechanism can be interpreted as follows [41]; H2O2 + Auo → •OH + OH− + Au+ (7) H2O2 + OH− → HO2 − + H2O (8) HO2 − + Au+ → •OOH + Auo (9) •OH + •OOH → H2O + O2 (10) ______________________________________________ The net reaction is: 2 H2O2 → H2O + O2 (11) He et al. [7] demonstrated that Au nanoparticles exhibited intrinsic catalytic activity, resulting in the generation of hydroxyl radicals and oxygen as well as scavenging superoxide. In the presence of H2O2, Au nanoparticles (NPs) can elicit the generation of hydroxyl radicals in an acidic environment and the production of oxygen under alkaline conditions. A third suggested mechanism can be illustrated as follows [42]; 1st step: Au + H2O2 → H2O + Au(O) (12) 2nd step: Au(O) + H2O2 → Au + O2 + H2O (13) ______________________________________________ Overall: 2 H2O2 → 2H2O + O2 (14) where Au(O) is the chemisorbed oxygen onto gold catalyst. Comparing the obtained results with those previously reported for CuO/CeO2 system containing 8.5 wt% CuO [4,5], it is clear that the activity of Au/CeO2 catalyst in H2O2 decomposition carried out at 30 °C is 8-fold greater than that measured for CuO/CeO2 system. 4. Conclusions Nanosized Au/CeO2 catalyst containing 4.5 wt% gold prepared by deposition-precipitation procedure, shows an excellent catalytic activity towards H2O2 decomposition. The calculated crystallites size of the gold particles is 7.0±0.5 nm for oxygen and nitrogen pre-treated samples. Heating at oxidative atmosphere had a great impact on the presence of cationic gold (Aun+) as determined by (XPS). The Aun+/Au0 ratio was much higher in oxygen pre-treated sample than that of nitrogen pre-treated sample. The catalytic activity of oxygen pre-treated sample was about twice higher than that measured for nitrogen pre-treated one. This finding ran parallel to the extent of Aun+ present in the oxygen and nitrogen pre-treated catalysts. Acknowledgements I am grateful to Institute of Surface Chemistry and Catalysis, Ulm University, Germany, for XPS measurements. ORCID Ayman Abd El-Moemen http://orcid.org/0000-0001-7037-6490 References [1]. Lousada, C. M.; Jonsson, M. J. Phys. 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This is an open access article distributed under the terms and conditions of the CC BY NC License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited without any further permission from Atlanta Publishing House LLC (European Journal of Chemistry). No use, distribution or reproduction is permitted which does not comply with these terms. Permissions for commercial use of this work beyond the scope of the License (http://www.eurjchem.com/index.php/eurjchem/pages/view/terms) are administered by Atlanta Publishing House LLC (European Journal of Chemistry). 2019 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.10.4.317-322.1895 http://www.eurjchem.com/index.php/eurjchem/pages/view/terms http://creativecommons.org/licenses/by-nc/4.0 http://www.eurjchem.com/index.php/eurjchem/pages/view/terms 1. Introduction 2. Experimental 2.1. Materials 2.2. Techniques 2.2.1. X-ray diffraction measurements 2.2.2. XPS measurements 2.2.3. Specific surface area 2.2.4. Activity measurements 3. Results and discussion 3.1. XPS investigation of different pre-treated catalyst samples 3.2. XRD investigation of the pre-treated solids 3.3. Catalytic properties of different pre-treated solids 4. Conclusions Acknowledgements ORCID References PrintField10: PrintField11: PrintField12: PrintField13: PrintField14: PrintField15: PrintField20: PrintField21: PrintField22: PrintField23: PrintField24: PrintField25: