Facile Redox Synthesis of Novel Bimetallic Crn+/Pd0 Nanoparticles Supported on SiO2 and TiO2 for Catalytic Selective Hydrogenation with Molecular Hydrogen

Olga A Kirichenko,Marina S Chernova,Anastasiya A Shesterkina,Leonid M Kustov,Gennady I Kapustin,Elena A Redina

doi:10.3390/catal11050583

Abstract

The bimetallic Crn+/Pd0 nanoparticles have been synthesized for the first time by a two-step redox method. The method includes the deposition of Pd0 nanoparticles on the surface of SiO2 and TiO2 carriers followed by the deposition of Crn+ on the surface of Pd0 nanoparticles using the redox procedures, which are based on the catalytic reduction of Crn+ with H2 in aqueous suspensions at ambient conditions. Transmission (TEM) and scanning (SEM) electron microscopy, X-ray photoelectron spectroscopy (XPS), Fourie-transformed infrared spectroscopy of adsorbed CO (FTIR-CO), and CO chemisorption studies were performed to characterize the morphology, nanoparticle size, element, and particle distribution, as well as the electronic state of deposited metals in the obtained catalysts. A decrease in nanoparticle size from 22 nm (Pd/SiO2) to 2–6 nm (Pd/TiO2) makes possible deposition of up to 1.1 wt.% Cr most likely as Cr3+. The deposition of CrOx species on the surface of Pd nanoparticles was confirmed using FTIR of adsorbed CO and the method of temperature-programmed reduction with hydrogen (TPR-H2). The intensive hydrogen consumption in the temperature ranges from −50 °C to 40 °C (Cr/Pd/SiO2) and from −90 °C to −40 °C (Cr/Pd/TiO2) was first observed for the supported Pd catalysts. The decrease in the temperature of β-PdHx decomposition indicates the strong interaction between the deposited Crn+ species and Pd0 nanoparticle after reduction with H2 at 500 °C. The novel Crn+/Pd/TiO2 catalysts demonstrated a considerably higher activity in selective hydrogenation of phenylacetylene than the Pd/TiO2 catalyst at ambient conditions.

Highlights

IntroductionThere is no single interpretation to explain the effect of second metal on the performance of Pd catalysts
The deposition of a second metal on the surface of a noble metal as a three-dimensional deposit is possible by the method of a redox reaction with adsorbed species (RRA) [24]
The highly dispersed bimetallic nanoparticles on TiO2 support that were synthesized via the catalytic reduction by gaseous hydrogen (RC procedure) are considerably more active than the initial Pd catalyst in the reaction of the selective liquid-phase hydrogenation of phenylacetylene

Summary

Introduction

There is no single interpretation to explain the effect of second metal on the performance of Pd catalysts. Depending on the nature of both the co-metal and the reaction, the beneficial presence of a co-metal was interpreted in terms of geometric effects, electronic effects, and mixed sites. In the case of partly reduced second metal Pd–Mn+ , the promoting effect was attributed to a positively charged cationic species Mn+ activating the functional groups of a substrate, which becomes hydrogenated. Electronic modifications upon alloying Pd were used to interpret the better selectivity observed in the selective hydrogenation of alkadienes and alkynes to alkenes. The presence of the second metal at the surface or in the bulk was shown to change the relative adsorption strength of the alkynes, alkadienes, and alkenes, which resulted in an increased reactivity for alkynes and a decreased reactivity for alkenes.

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Results

Conclusion