Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Superior co-catalytic activity of Pd(core)@Au(shell) nanocatalyst imparted to TiO 2 for the selective hydrogenation under solar radiations Aadil Bathla, Bonamali Pal ⁎ School of Chemistry and Biochemistry, Thapar Institute of Engineering & Technology, Patiala 147004, India ARTICLE INFO Keywords: Pd@Au core–shell nanostructure Cinnamaldehyde hydrogenation Pd@Au-TiO 2 co-catalysis Sunlight photocatalysis Galvanic replacement reaction ABSTRACT The bimetallic core–shell nanostructures of galvanic metals have gained considerable scientific interest in im- proving the TiO 2 photocatalysis under solar radiations over the monometallic analogues. In the present research work, Pd@Au core–shell supported TiO 2 nanostructures were synthesized via galvanic replacement reaction and were examined for their catalytic/ photocatalytic hydrogenation. Three different types of bimetallic Pd@Au nanostructure were synthesized by varying Pd:Au weight ratio i.e. (1:1), (1:2) and (1:3). DLS measurements revealed that with increasing Au weight ratio, the hydrodynamic size increases from 126 to 157 nm. The optical studies showed a considerable blue shift in the absorption band of Au nanoparticles from 529 to 518 nm in the case of Pd@Au (1:1). The co-existence of absorption characteristic of Pd and Au suggests the successful synthesis of bimetallic nanostructure. STEM and EDS mapping further confirmed the formation of Pd@Au nanostructure with inner Pd core and outer Au shell. Bimetallic Pd@Au nanocatalyst displayed superior activity and selectivity towards hydrogenation of cinnamaldehyde in comparison to monometallic analogues. However, when Pd@Au nanostructures were impregnated on the surface of TiO 2, a significant improvement in the hydrogenation re- action was observed under solar radiations relative to catalytic conditions. The photocatalytic performance of Pd@Au-TiO 2 was found to be varied as a function of shell thickness and the optimized APT-2 (Pd 1 @Au 2 -TiO 2 ) photocatalyst exhibited higher rate constant (2.3 × 10 -1 h -1) for cinnamaldehyde hydrogenation. Hence, the plasmonic Pd@Au-TiO 2 hetero-junction could be a promising greener photocatalyst for selective hydrogenation of unsaturated carbonyls for large scale industrial applications. 1. Introduction Bimetallic nanostructure of galvanic metals (Cu, Zn, Au, Pd, Ni, etc.) with different morphological appearance frequently shows superior catalytic/photocatalytic activities compared to their monometallic analogues (Bathla and Pal, 2018; Gupta et al., 2019; Olmos et al., 2019; Pieta et al., 2019; Qin et al., 2019; Seemala et al., 2019). The tre- mendous performance of these bimetallic nanoparticles credited to their diverse arrangement/combination and synergistic interaction be- tween two different metal atoms. Among the different forms of bime- tallic nanoparticles, core@shell nano-arrangement constructed via co- reduction method (based on galvanic interaction) found numerous applications in different research fields like catalysis, photocatalysis, sensing, and drug delivery (Li et al., 2019; Ma et al., 2019; Miyamura et al., 2019; Qadir et al., 2019; Sivamaruthi et al., 2019; Tsai et al., 2019; Wu et al., 2019a). The electrochemical potential difference be- tween the two metals plays a crucial role in galvanic replacement re- action and also in finalizing the structural design (Gawande et al., 2015; Gilroy et al., 2016). Wu et al. (2019b) reported the synthesis of bime- tallic Cu@Pt nanostructure by galvanic replacement reaction and ob- served that Cu@Pt display superior performance for oxygen reduction reaction relative to monometallic counterparts. Bathla and Pal (2019) reported the formation of Cu@Zn/TiO 2 core–shell nanostructure via galvanic interaction method for the selective quinoline hydrogenation under visible light irradiations. Similarly, Yang et al. demonstrated the formation of Ag@Au core–shell nanocubes via galvanic replacement reaction between AgNO 3 and HAuCl 4 and studied for its enhanced SERS activity (Yang et al., 2014). The selective hydrogenation of α-β unsaturated aldehydes to their corresponding alcohols is of great interest from the scientifically and industrial point of view. The hydrogenated product of cinnamaldehyde (CAL) i.e. cinnamyl alcohol (COL) is an essential component for the synthesis of fine chemicals, perfumes, and cosmetics (Plessers et al., 2016; Rong et al., 2015; Vriamont et al., 2015; Zhao et al., 2014). The selective hydrogenation of CAL to COL is a challenging task as it in- volves both C]C and C]O bonds. From the previous studies (Kołodziej https://doi.org/10.1016/j.solener.2020.05.038 Received 18 November 2019; Received in revised form 11 May 2020; Accepted 14 May 2020 ⁎ Corresponding author. E-mail address: bpal@thapar.edu (B. Pal). Solar Energy 205 (2020) 292–301 Available online 29 May 2020 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. T