Carbidisation of Pd Nanoparticles by Ethene Decomposition with Methane Production

Abstract

In the presence of oxygenated organic molecules pure Pd, which is widely used in chemicals processing and the pharmaceutical industry, tends to defunctionalise and dehydrogenate such molecules to H2, CO and surface/bulk carbon, in the form of a palladium carbide. We have investigated the formation of this carbide by ethene adsorption using a variety of techniques, including pulsed flow reaction measurements, XAS and DFT calculations of the lattice expansion during carbidisation. These experiments show that two main reactions take place above 500 K, that is, both total dehydrogenation, but also disproportionation to methane and the carbide, after which the activity of the Pd is completely lost. We estimate the value of x in PdCx to be 0.28 (±0.03), and show by computer modelling that this fits the lattice expansion observed by XAFS, and that there is charge transfer to C from Pd of around 0.2–0.4 e.

Dehydrogenation of Ethylene on Supported Palladium Nanoparticles: A Double View from Metal and Hydrocarbon Sides

“…The observed decomposition of ethylene with the formation of carbide correlates with a number of previous reports [ 29 , 30 , 31 , 33 , 36 , 37 , 38 , 39 , 40 ]. The lattice expansion at 50 °C is, however, significantly reduced, in terms of both the kinetics and saturated values compared to the studies performed at higher temperatures [ 40 ].…”

“…A similar result was observed for the Pd (100) surface, at which ethylene dehydrogenates to vinyl without the formation of ethylidyne [ 63 ]. However, the main reason for the difference is the experimental conditions (atmospheric pressure and 50 °C temperature), under which the dehydrogenation and subsequent decomposition of ethylene to Pd carbide is the dominant reaction pathway, which is in correlation with a recent study by Jones et al [ 29 ]. It should be noted that although having similar size distribution, the NPs proved by surface sensitive DRIFTS and bulk sensitive XANES/EXAFS/XRD techniques had two different types of supports—carbon and alumina, respectively—due to the features of the experimental techniques employed.…”

“…In the recent works of Bowker [ 29 , 30 , 31 ], it was highlighted that under industrially relevant pressures on real catalysts, the processes are different from that on ideal surfaces and high-vacuum conditions, which are typical for surface science studies. In addition, palladium can easily form carbide phase in the presence of hydrocarbons [ 29 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 ], which affects its catalytic properties. For this reason, the application of bulk-sensitive structural methods in addition to surface-sensitive ones is important to understand the structural evolution of the metal.…”

A DFT and KMC based study on the mechanism of the water gas shift reaction on the Pd(100) surface

“…58 Additionally, the inclusion of this elementary step could lead to another step i.e., C srf " C bulk , as shown by us in our previous studies on palladium carbides, which may greatly affect the mechanism of the water gas shift reaction. 59,60 Fig. 1 A mechanism consisting of seven core elementary steps involving both redox and carboxyl mechanisms for water gas shift reaction.…”

Interstitial and substitutional light elements in transition metals for heterogeneous catalysis

“…FTIR spectroscopy is used to probe the variations in vibrational frequencies of metal–adsorbate bonds. Bowker et al 43 compared the CO absorption on doped and undoped Pd NPs using diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS). For the reduced undoped Pd NPs, a band located at ∼1990 cm −1 was observed when exposed to CO, which was assigned to the bridge-bonded CO on Pd.…”