Pd–Ag/SiO₂ bimetallic catalysts prepared by galvanic displacement for selective hydrogenation of acetylene in excess ethylene

Abstract

8A series of bimetallic Pd-Ag/SiO 2 catalysts were prepared by galvanic displacement with 9 increasing loadings of Pd on Ag. The catalysts were characterized by atomic absorption 10 spectroscopy, Fourier-transform infrared spectroscopy of CO adsorption and X-ray photoelectron 11 spectroscopy. An actual Pd deposition beyond the theoretical limit for galvanic displacement 12 suggested that the large difference in surface free energy for Pd and Ag resulted in Pd diffusion 13 into the bulk of Ag particles, or Ag diffusion to the surface to provide fresh Ag atoms for further 14 galvanic displacement. Characterization results indicated that on this series of catalysts the Pd 15 atoms are distributed in very small ensembles or possibly even atomically on the Ag surface, and 16 there was a transfer of electrons from Pd to Ag at all Pd loadings. For comparison, the catalysts 17 were also evaluated for the selective hydrogenation of acetylene in excess ethylene at the 18 conditions used in our previous study of the reverse Ag-Pd/SiO 2 catalysts. The selectivities for 19 C 2 H 4 formation remained high and constant due to the geometric effects that Pd atom existed as 20 small ensembles. However, the electronic effects resulted in lower selectivities for C 2 H 4 21 formation than those from the catalysts with high coverage of Ag on Pd. 22 the base material. This method is different from ED, in that GD does not require chemical 1 reducing agents since the base metal itself already serves as the reducing agent. However, this 2 redox reaction is usually limited by the accessibility of the base metal, resulting in passivation of 3 the reaction when the entire surface of the base metal is covered. 14 4 Over the past decade, GD has been widely used for the development of metal 5 nanostructures with highly active surfaces for a variety of applications such as 6 electrocatalysts 13,16-22 , biomedicine 15 , functional coatings 23,24 , and deposition of metals on 7 semiconductors 14,25 . Xia et al. 15,26-28 have thoroughly studied the mechanism of GD for tuning 8 the properties of metal nanostructures through adjustment of composition, size, shape and 9 morphology. Complex hollow nanostructures of Pd-Ir, Au-Ag, Pd-Ag, and Pt-Ag have been 10 generated with potential for many applications. Yan et al. 13,20-22 also developed electrocatalysts 11 with Pd-Au, Pt-Cu, Pt-Pd nanotubes or nanowires prepared by GD for the oxygen reduction and 12 methanol oxidation reactions. Lee et al. 17-19 have investigated the performance of different forms 13 of hollow and porous Pd-Ag or Pt-Ag nanomaterials prepared by GD as electrocatalysts for the 14 oxygen reduction reaction. Maboudian and Carraro 14,23-25 have used GD to coat Au, Pt, Ag and 15 Cu onto Si in thin film or nanoparticle forms for surface-enhanced Raman spectroscopy, and for 16 improved interfacial behavior of semiconductors. 17 Although GD has been widely used in synthesizing electrocatalysts, it has had little 18 application for the development of catalysts used in hydrogenation reactions....

Advances in the preparation of highly selective nanocatalysts for the semi-hydrogenation of alkynes using colloidal approaches

“…Ranges of 20-40% of C 4+ selectivity have been reported for acetylene hydrogenation, thus evidencing the relevance of this reaction path. [23][24][25][26] For the hydrogenation of acetylene, several intermediates have been postulated based on surface characterization techniques combined, in some cases, with kinetic studies. 18,27,28 The reaction starts by H 2 dissociation and the adsorption of acetylene on the surface; both steps are exothermic.…”

“…When the same catalytic process was studied using Pd-Ag/SiO 2 bimetallic catalysts prepared by galvanic displacement during which controlled loadings of Pd were added to the Ag/SiO 2 pre-catalyst, the resulting bimetallic nanoparticles provided lower selectivity to ethene than in the previous case. 26 Experimental and computational studies were reported on the effect of the coverage of the palladium phase with a second metal, in the semi-hydrogenation of acetylene. Zhang and co-workers reported that the optimal Pd@Ag and Pd@Au catalysts (in terms of TOF and selectivity) were those with a coverage of 0.92, obtained using a molar ratio of ca.…”

Direct Synthesis of Hydrogen Peroxide Over Au–Pd Catalysts Prepared by Electroless Deposition

“…A more promising methodology to attain stable, isolated Pd sites in a Au matrix for this reaction may involve the inverse of this synthesis technique-that is, deposition of small amounts of Pd onto preformed Au particles. Notably, Zhang et al [40] have demonstrated successful isolation of Pd sites in pre-formed Ag particles through galvanic displacement of Ag by Pd 2? salts.…”

Fine-Tuning Synthesis and Characterization of Mono-Sized H-Beta Zeolite-Supported Palladium-Iridium Nanoparticles and Application in the Selective Hydrogenation of Acetylene

“…They demonstrate very high ethylene selectivity but require elevated temperatures to provide high acetylene conversion and are therefore strongly limited for commercial use. On the contrary, in spite of the high activity, selectivity, and stability of supported monometallic palladium catalysts, they are not sufficient, especially at high acetylene conversions, so the second metal (e.g., Ag, Cu) is introduced to improve catalytic performance [33,57]. It hinders palladium hydride formation and decreases the number of adjacent Pd sites responsible for multiplet adsorption leading to over-hydrogenation of acetylene to ethane and its oligomerization to higher hydrocarbons (green oil) [57,58].…”

“…On the contrary, in spite of the high activity, selectivity, and stability of supported monometallic palladium catalysts, they are not sufficient, especially at high acetylene conversions, so the second metal (e.g., Ag, Cu) is introduced to improve catalytic performance [33,57]. It hinders palladium hydride formation and decreases the number of adjacent Pd sites responsible for multiplet adsorption leading to over-hydrogenation of acetylene to ethane and its oligomerization to higher hydrocarbons (green oil) [57,58]. The Pd-Ag bimetallic catalysts have been found to be most efficient and are successfully employed on an industrial scale [59].…”