Synthesis of a hybrid Pd0/Pd-carbide/carbon catalyst material with high selectivity for hydrogenation reactions
Graphical abstract
Palladium carbide species (PdCx) are formed on the upper layers of a Palladium carbon catalyst using a hydrothermal synthesis method. Its presence modifies the electronic state of surface Pd atoms resulting in a highly active and selective catalyst for hydrogenation of alkynes, dienes and heteroaromatic imines.
Introduction
Currently, selective hydrogenation of conjugated double carboncarbon bonds in dienes and trivalent carboncarbon bonds such as alkynes to achieve partial hydrogenation products is a highly desirable and challenging process in the field of chemical production [1], [2]. A variety of heterogeneous and homogeneous catalysts based on transition metals for the partial hydrogenation of dienes and alkynes have been reported, being Pd the most widely used. In fact, it is observed that Pd-based catalysts can selectively hydrogenate triple into double CC bonds. On this base, it is possible to remove acetylene from ethylene streams for the manufacture of polymer grade ethylene. The high selectivity of Pd to hydrogenate mixtures of alkyne and alkene hydrocarbons has been explained on the base of the weaker adsorption energy of the alkene with respect to the alkyne on Pd [3]. Recent spectroscopic studies based on operando X-ray absorption spectroscopy [4], [5] combined with X-ray powder diffraction and theoretical simulation of the X-ray absorption near-edge structure (XANES) spectra using a Monte-Carlo approach [6], as well as near ambient X-ray photoelectron spectroscopy [7], have shown the tendency of Pd to form carbides and hydrides under reaction conditions, which markedly affects the adsorption of olefins and alkynes, influencing accordingly the selectivity to the alkene. It has been experimentally and theoretically found [8] that bulk dissolved hydrogen in Pd, resulting in the formation of β-hydride phase, is considerably more energetic than the hydrogen adsorbed on the surface and can emerge from the bulk to the surface, enhancing total hydrogenation of acetylene [9]. On the other hand, when no subsurface hydrogen exists, the hydrogen on the surface of the metallic nanoparticles is more selective toward partial hydrogenation of acetylene to ethylene. By studying binding energy maps, Tescher et al. [10] proposed that the distribution of hydrogen in Pd is affected by the presence of dissolved carbon species within the metal nanoparticles, decreasing the population of hydrogen in the subsurface region. In this case the binding energy of hydrogen on the surface is strongly reduced and its population decreases leading to the selective hydrogenation of the alkyne into the corresponding alkene [11]. These carbon species are formed in situ during the reaction from the fragmentation of carbon containing feed molecules, being their amount strongly dependent on the reaction conditions such as reaction time, pressure and temperature [4], [5], [6], [11], [12]. The formation of metal-C species in the absence of carbon species in the feed have been also detected by dispersing small metal clusters onto a carbon support, resulting in a high degree of metal-carbon interaction, specifically at defects sites of the support [13] with the diffusion of carbonaceous species from the support into the Pd lattice [13], [14]. While the incorporation of carbon into the Pd lattice has been proven to be beneficial for catalyst selectivity, the controlled synthesis of palladium based catalysts containing interstitial carbon atoms has been scarcely reported. To the best of our knowledge, there are only few reports dealing with the efficient incorporation of C atoms in Pd catalysts. Between these studies, Okitsu et al. [15] reported a sonochemical reduction of a palladium salt in the presence of an organic additive. However, the radical nature of the synthesis results in important compositional changes of the final material depending on the synthesis conditions, hampering the scalability of the method. In another approach Guo et al. [16] described a method for the synthesis of palladium carbide (PdCx) nanocubes, starting from Pd nanocube seeds, glucose and oleylamine stirred at 200 °C for a certain time. The method requires the initial synthesis of Pd nanocubes using a surfactant mediated method and the use of amines, which are not desirable from an environmental perspective. However, it has been shown the possibility to control the C/Pd atomic ratio by simply adjusting the reaction time. On the other hand, Beltzung et al. [17] reported a KOH activated high temperature (900 °C) pyrolysis procedure of a polyacrylonitrile polymer containing Pd nanoparticles, resulting in partial carburization of the Pd nanoparticles. The method uses harsh conditions and has drawbacks of accessibility of the reactants to the active sites. On the basis of these outcomes, the possibility to employ a sustainable method using mild conditions for the synthesis of Pd catalysts containing dissolved carbon atoms would be very interesting from a point of view of eco-efficiency. On the other hand, the effect of carburization on the catalytic performance of Pd catalysts in other reactions than alkyne semihydrogenation has not been yet investigated, being matter of interest. Thus, the goal of our work is to state an efficient and facile approach of effective incorporation of carbon atoms in the Pd lattice and study their influence on the catalytic behaviour of highly demanding selective hydrogenation reactions.
The hydrothermal synthesis methodology has been successfully employed for the synthesis of carbon materials [18] and mono and bimetallic carbon coated catalysts, with applications in many catalytic processes [19], [20], [21], [22], however in none of these studies metal carburization has been reported. Recently, we demonstrated the formation of ruthenium carbide species stabilized in the upper surface layers of a ruthenium carbon catalyst, using a hydrothermal approach [23]. Inspired on this finding and based on the high affinity of Pd to carbon, we tried to use the same approach for the synthesis of Pd carbide containing catalysts. The herein employed hydrothermal synthesis procedure involves an aqueous solution of a palladium salt (i.e. Pd(NO3)2), an organic compound as carbon source (i.e. Na2EDTA) and a water-methanol mixture, heat-treated under autogenous pressure at 200 °C for 24 h. This synthesis method is very easy, reproducible and cost-efficient in comparison to the previous methods found in the literature for the controlled synthesis of palladium based catalysts containing interstitial carbon atoms. The method can also be applied using glucose instead of Na2EDTA as carbon source, and water as solvent, rendering as a “green” alternative to the actual synthesis procedure.
Section snippets
Synthesis of PdHT catalyst
The PdHT-EDTA catalyst was prepared through a hydrothermal process using Pd(NO3)2 as the metal precursor and Na2EDTA as organic compound. The pH of the synthesis gel was 13. In detail, 3.50 g Pd(NO3)2, 1.80 g Na2EDTA and 0.38 g NaOH were dissolved in 8.1 mL H2O and 4 mL MeOH, resulting in a black suspension. The solution was transferred into a 35 mL stainless steel autoclave and heated at 200 °C for 24 h. After cooling to room temperature, the solid was precipitated, filtered and washed with
Synthesis of Pd samples
The samples have been prepared under autogenous pressure in a stainless steel autoclave heated in an oven at 180–200 °C for 18–24 h. The synthesis gels contains a palladium salt, an organic compound and water and/or methanol solution. Details are given in the experimental section. The as prepared samples are labelled as PdHT-EDTA (when using Na2EDTA as carbon source) and PdHT-Glucose (when using glucose as carbon source). The metal loading in both samples is ~20 wt% Pd, according to ICP
Conclusions
We present a highly selective and active hydrogenation Pd catalyst composed by a palladium carbide phase stabilized on the upper surface layers of a Pd carbon catalyst. The catalyst synthesis method comprises a hydrothermal process at 200 °C, starting from a palladium precursor (i.e. Pd(NO3)2), an organic compound (i.e. Na2EDTA or glucose) and a water-methanol solution. The easy and mild conditions of this synthetic procedure contrast with those reported in the literature, where scalability and
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Funding: The research leading to these results has received funding from the Spanish Ministry of Science, Innovation and Universities through “Severo Ochoa” Excellence Programme (SEV-2016-0683) and the PGC2018-097277-B-100 project. The authors also thank the Microscopy Service of UPV for kind help on measurements. A. García-Ortiz thanks “Severo Ochoa” Programme (SEV-2016-0683) for a predoctoral fellowship. J. Cored thanks the Spanish Government (MINECO) for a “Severo Ochoa” grant (
Author contributions
A.C. conceived the project and contributed in the production of the manuscript. S.I, M.J.C and P.C. directed the study and wrote the manuscript. P.C. did the Raman and participated together with D.R., V.P.D and J.C. in the XPS measurements at ALBA Synchrotron. A.G.O. did the synthesis of the samples, the TEM analysis and the catalytic study. All authors participated in the discussion of the results.
Data and materials availability
All data are presented in the paper and/or supplementary materials.
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