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Decomposition of Supported Pd Hydride Nanoparticles for the Synthesis of Highly Dispersed Metallic Catalyst Yage Zhou, Walid Baaziz, Ovidiu Ersen, Pavel A. Kots, Evgeny I. Vovk, Xiaohong Zhou, Yong Yang, and Vitaly V. Ordomsky Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02192 • Publication Date (Web): 02 Nov 2018 Downloaded from http://pubs.acs.org on November 3, 2018
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Chemistry of Materials
Decomposition of Supported Pd Hydride Nanoparticles for the Synthesis of Highly Dispersed Metallic Catalyst Yage Zhou[a,b] , Walid Baaziz[c], Ovidiu Ersen[c], Pavel A. Kots[d], Evgeny I. Vovk[e], Xiaohong Zhou[e], Yong Yang[e], Vitaly V. Ordomsky[b]* State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East China University of Science and Technology, 130 Mei long Road, Shanghai 200237, China [b] Eco-Efficient Products and Processes Laboratory (E2P2L), UMI 3464 CNRS-Solvay, Shanghai, P.R. China, E-mail:
[email protected] [c] Department of Surfaces and Interfaces (DSI), 23, rue du Loess BP 43, F-67034 Strasbourg, France [d] Chemistry Department of Lomonosov Moscow State University 119991, Leninskye Gory, 1, bld, 3, Moscow, Russia [e] School of Physical Science and Technology, Shanghai Tech University, Shanghai 201210, People’s Republic of China [a]
ABSTRACT: Palladium (Pd) is one of the most popular metal catalysts in chemical industry. Increase of the metal dispersion is an important challenging task which would decrease cost of the processes. Herein we report significant increase of the dispersion of Pd in Pd/Al2O3 catalyst after contact of Pd hydride nanoparticles with hydrogen peroxide. The effect has been ascribed to fast heating of Pd hydride nanoparticles by exothermic reaction with their explosive decomposition. The prepared material demonstrated superior performance in hydrogenation reactions to that of parent catalyst.
Pd based materials (Pd/Al2O3, Pd/C and Pd/SiO2) are among the most popular catalysts in the modern chemical industry. They are widely used in the reactions of hydrogenation, dehydrogenation, oxidation, isomerization and hydrocracking. The activity of such catalysts is often the function of available metallic surface area. The conventional methods of catalyst preparation are based on impregnation of Pd salts over traditional supports like silica, alumina, titanium dioxide or carbon with subsequent calcination in air or inert atmosphere to form palladium oxide and followed by reduction in hydrogen to metallic palladium. However, the size of the metal nanoparticles prepared by this way is in the range 5-10 nm with relatively low dispersion [1]. Otherwise, there are several aspects which are important for preparation of highly dispersed catalysts. First of all, it is necessary to create high amount of nucleation centers by application of high surface area supports like mesoporous materials (MCM-41 and SBA15) [2], ion exchange [3] or direct reduction of soluble metal salts using solvent as reducing agent and microwave heating [4,5,6]. Another factor is uniform distribution of metal nanoparticles in the catalyst which might be attained using sonochemistry [7,8]. Use of stabilizers, for example, polymers and ligands lead to stabilization and isolation of small size metal nanoparticles from their segregation [9,10,11,12]. However, presence of organic stabilizers after calcination leads to deactivation of the catalyst. Nevertheless, the main disadvantage of the most part of these routes is application of quite complex procedures using expensive equipment and
difficulties for upscaling of catalyst preparation. It would be of great interest development of simple method to increase the dispersion of existing catalyst for more efficient industrial application.
Figure 1. Scheme of the synthesis of decomposed Pd hydride nanoparticles
The Pd as a metal is widely used for its application as hydrogen storage materials due to the formation of Pd hydride which can accumulate up to 900 times its own volume of hydrogen [13]. The process of equilibrated absorption and desorption might be repeated many times without loss of capacity and morphology of Pd crystals. The disintegration of Pd particles has been observed by Pielaszek [14] by multiple hydride formation decomposition cycling in H2 and He flows. Pielaszek using XRD diffraction method has detected that
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hydride formation-decomposition cycles resulted in decrease of the Pd particles sizes over alumina from 16 to 7.5 nm after two cycles [14]. This effect earlier has been observed by the same group for nickel hydride formation-decomposition cycles [15]. This phenomenon has been explained by the formation and interaction dislocations during hydride formation and decomposition [14]. The effect was much more pronounced for palladium supported on gamma alumina. Decomposition of hydride in Pd single crystals results in crack formation [16]. It has been shown recently that non-equilibrated fast electric heating of Pd hydride films results in explosive decomposition of metal and deformation of the structure [17]. It led us to the new approach for the synthesis of highly dispersed Pd catalyst. The method is based on hydrogen absorption on Pd supported catalyst to form Pd hydride. Afterwards, the metal hydride nanoparticles have been contacted with hydrogen peroxide solution which results in highly exothermic
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about 3 nm (Figure 2, Figure S1, SI). Standard hydrogen pulse adsorption has been used for analysis of the metallic surface area which was 24 % and corresponds to the average size of metal nanoparticles. The hydrogenation of the catalyst has been performed in a batch reactor under 20 bar of hydrogen at 100 °C during 1 h. The amount of hydrogen and the state of Pd hydride has been studied by hydrogen TPD. Figure 3 shows that sample desorbs hydrogen with two main peaks at 80 and 220 °C. Low temperature peak might be assigned to desorption of hydrogen close to the surface where the structure of hydride is defected [19]. The high temperature peak corresponds to the decomposition temperature of β hydride. The amount of hydrogen corresponds to the formation of the composition PdH0.76 [19].
Figure 3. TPD of Pd/Al2O3 after absorption at 100 °C and 20 bar H2 (a), PdH-dec/Al2O3 after decomposition (b) and PdHdec/Al2O3 after absorption at 100 °C and 20 bar H2 (c)
Figure 2. Comparison of HAADF-TEM images before and after decomposition of Pd hydride
reaction of hydrogen peroxide decomposition over metal surface [18]: 2H2O2=2H2O+O2 , ∆H= -98.2 kJ/mol. The reaction proceeds locally over metal nanoparticles with explosive decomposition of Pd hydride and significant increase of the dispersion of metal in the catalyst. The commercial Pd/Al2O3 (JM) catalyst with 5 wt. % of Pd over γ-Al2O3 has been used in this work. The size of metal nanoparticles studied by HAADF-TEM technique in the parent catalyst is in the range 1-7 nm with an average diameter
The decomposition of Pd hydride catalyst after absorption has been performed by fast addition of the powder to hydrogen peroxide (30 wt. %) solution with intensive stirring. The intensive boiling starts for several seconds due to highly exothermic reaction. The catalyst has been filtered, washed with ethanol afterwards and dried at 50 °C in the vacuum oven. First of all, the fact of decomposition of Pd hydride in PdHdec/Al2O3 sample has been studied by TPD analysis. Figure 3 demonstrates absence of hydrogen peaks which indicates that Pd hydride totally decomposed during the contact with hydrogen peroxide. The size of metal nanoparticles after reaction has been studied by HAADF-TEM. Figure 2 and Figure S2, SI shows the presence of metal nanoparticles decomposed after contact with hydrogen peroxide in form of NPs but also clusters of about 1 nm. The sample was analyzed by STEM-EDX in order to confirm the presence of Pd in the form of dispersed clusters (Figure S3, SI). It is interesting to note that according to TEM only part of Pd nanoparticles have been affected by this procedure. Figure S1, SI shows the histogram of distribution of the metal nanoparticle in decomposed sample in comparison with initial one. The results clearly show significant increase of the contribution of small size nanoparticles and Pd clusters in the catalyst after decomposition procedure in the presence of large nanoparticles. The analysis of the metal surface area by hydrogen pulses at r.t. shows almost twice higher surface area
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Chemistry of Materials in comparison with initial catalyst which correlates with the Table 1. Characterization of the catalysts
Catalyst Pd/Al2O3 PdH-dec/Al2O3 Pd-dec/Al2O3
Dispersio n, %a 24 51 (40c) 21
H/Pd, at.b 0.76 0.28 -
XPS Pd 3d5/2, eV 335.1 335.4 -
- defined by H2 pulse adsorption at r.t. - defined by TPD after H2 absorption (100 °C, 20 bar H2) c- defined by H pulse adsorption at 60 °C 2 a
b
presence of the new Pd clusters observed by TEM (Table 1). The high consumption of hydrogen could be also explained by weakly chemisorbed and absorbed hydrogen. However, hydrogen pulse test at higher temperature (60 °C) to avoid these factors has demonstrated similar dispersion (Table 1). For exploring the possible electronic effect of the morphology and structure features of Pd particles, the surface information was further evaluated using an XPS technique (Figure 4a). The binding energy (BE) of Pd 3d5/2 for the Pd/Al2O3 sample appears at 335.4 eV, which corresponds to metallic Pd [20]. After in-situ reduction in hydrogen at 100 °C, the shift (0.3 eV) to lower BE is observed. It might be assigned to the presence of PdHx which is not totally decomposed at 100 °C in vacuum. PdH-dec/Al2O3 sample demonstrates higher Pd 3d5/2 BE (336.0 eV) which corresponds to PdOx, x
