Sm-CeO2 supported gold nanoparticle catalyst for benzyl alcohol oxidation using molecular O2
Graphical abstract
Highlights
► Design of nanocrystalline samarium doped ceria catalyst. ► Use of samarium doped ceria catalyst as a support of gold nanocluster. ► Change in redox property of the ceria support after gold deposition. ► Evaluation of catalytic activity for alcohol oxidation reaction using molecular O2. ► Role of oxygen vacancies toward the catalytic activity.
Introduction
Sustainable development is an important theme in the 21st century. The so-called E factor (defined as the weight of by-products, divided by weight of the desired product) is in the range of 5–10. Stoichiometric reactions need to be replaced by simple catalytic reactions that require the minimum of energy [1]. Development of catalyst that can demonstrate mild oxidation reaction using molecular O2 as an oxidant is of immense interest from the green chemistry point of view. Benzaldehyde is an important intermediate to a wide variety of fine chemicals (pharmaceuticals, perfumery and agrochemicals). Thus selective liquid phase aerobic oxidation of benzyl alcohol to benzaldehyde under mild condition is a shared desire from economics and environmental perspective [2], [3].
A key technology which is bringing a breakthrough for alcohol oxidation is catalysis by gold nanoparticles and clusters [4]. After the discovery of catalytic activity of supported gold nanoparticle for its activity toward low temperature CO oxidation [5] and propylene epoxidation reaction [6], the worldwide research spurred to find its activity for not only the oxidation reaction but also other reaction such as hydrogenation [7] dehydrogenation reaction [8] and several other reactions [9]. Though it was claimed initially that gold nanoparticle size of 2–5 nm is responsible for low temperature CO oxidation but very recently Haruta and co-workers have reported that gold nanoclusters below 2 nm size containing 300 atoms, is responsible for direct propylene epoxidation using only molecular O2 in presence of moisture [10]. However major challenges to design and develop the gold catalyst are to maintain homogeneous dispersion of the nanoparticles over the oxide support surface and strong interaction between gold and oxide support.
Nanocrystalline CeO2 has drawn its attention as the physicochemical property of its surface changes distinctly from its bulk counterpart. Several studies have demonstrated that doping of CeO2 with elements of different ionic radii or oxidation state improves the exchange of oxygen in the oxide network by decreasing the energy barrier for oxygen migration [11]. The synergy between the ceria and the modifier can be achieved throughout different mechanisms, such as creation of oxide vacancy, the formation of solid solution, [12], the integration between segregated oxides, and the deposition of metallic particles over the surface. Several doped-ceria systems have been investigated for catalytic applications such as total oxidation of CO [13], combustion of diesel soot [14], [15] preferential oxidation of CO in the presence of hydrogen (Prox) [16], total oxidation of propene [17] and water–gas shift reactions [18].
The nature of the gold–oxygen vacancy interactions on CeO2 surfaces has been studied by density functional theory calculations (DFT) [19], [20]. These works revealed that Au particles nucleate mainly on the oxygen vacancies in a reduced CeOx surface, allowing the growth of small and well-dispersed clusters, which facilitate the reactivity of the catalyst. As expected, the Au–oxygen vacancy interaction promotes changes in the electronic environment of CeO2 supported gold nanoparticles show its superior performance for CO oxidation reaction [11].
In this study we assumed that creation of defect structure in CeO2 lattice will facilitate gold–support interaction more strong by incorporating gold nanoclusters in the oxide vacancy. Though several reports are there in the literature for samarium doped ceria (as both are having close ionic radii) catalyst for other oxidation reaction but no study was carried to see the effect of oxide ion vacancy of the catalyst for benzyl alcohol oxidation. Three different catalysts e.g., CeO2, Sm-CeO2 (Sm/Ce = 4/100), Au/Sm-CeO2 (Sm/Ce = 4/100) were prepared and has been characterized by XRD, RAMAN, XPS, TPR, HRTEM, EXAFS and XANES techniques. Interesting correlations were found from the characterization results and catalytic activity of benzyl alcohol oxidation emphasizing the design of efficient gold nanoparticle catalyst.
Section snippets
Preparation of Sm-CeO2 mixed oxide support
The support material i.e., samarium doped cerium oxide was prepared by the non-hydrothermal sol–gel method described in our previous publication [12]. In this procedure the solution of Ce(NO3)3·6H2O was added to triethanolamine under stirring condition at room temperature. After complete addition of cerium nitrate solution, samarium nitrate hexahydrate in solid form was added to it with constant stirring. This total content was stirred for 10 min after that tetraethylammonium hydroxide was added
X-ray diffraction
The X-ray diffraction patterns of CeO2, and samarium doped CeO2 are presented in Fig. 1. The peaks of all the samples could be indexed as (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0) planes of cubic fluorite structure (Space Group: Fm3m, JCPDS 78-0694) of CeO2. However, mixed oxides of Ce and Sm, SmxCe1−xO2, are reported [21] to have the same crystal structure, Fm3mj, and very similar unit cell dimensions to pure CeO2. Therefore, the existence of some SmxCe1−xO2 could not be ignored based on the
Conclusion
In conclusion we have successfully synthesized samarium incorporated nanocrystalline cerium oxide by non-hydrothermal sol–gel method using triethanolamine/water mixture as a solvent. The XRD study reveals the decrease in crystallite size of cerium oxide after samarium incorporation which is mainly due to surface restructuring of the hydroxyl groups. The Raman study depicts the formation of oxygen vacancy due to samarium doping. The HRTEM investigation shows the 2–5 nm gold nanoparticles are
Acknowledgements
BC would like to acknowledge DST, Govt of India for funding under Fast Track Young scientist scheme (project no. SR (FTP)/ETA-16/2007-2008). BC also acknowledges DST and JSPS for funding under India-Japan exploratory exchange programme to carry out XAFS measurement in the KEK photon factory. The XAFS experiments were conducted under approval of PF-PAC (Project no. 2009G695) and Spring8 Priority Program for Disaster-Affected Quantum Beam Facilities (Project no. 2011A1968). SM acknowledges Indian
References (51)
- et al.
J. Catal.
(2011) - et al.
J. Mater. Chem.
(2012) - et al.
J. Phys. Chem. C
(2009) - et al.
Catal. Today
(2006) - et al.
J. Nat. Gas Chem.
(2008) - et al.
Appl. Catal. A: Gen.
(2008) - et al.
J. Rare Earths
(2008) - et al.
J. Phys. Chem. C
(2009) - et al.
J. Appl. Phys.
(1994) - et al.
J. Am. Chem. Soc.
(2003)