The irreversible formation of palladium carbide during hydrogenation of 1-pentyne over silica-supported palladium nanoparticles: in situ Pd K and L3 edge XAS

Abstract

The catalytically active phase of silica-supported palladium catalysts in the selective and non-selective hydrogenation of 1-pentyne was determined using in situ X-ray absorption spectroscopy at the Pd K and L(3) edges. Upon exposure to alkyne, a palladium carbide-like phase rapidly forms, which prevents hydrogen to diffuse into the bulk of the nano-sized particles. Both selective and non-selective hydrogenation occur over carbided particles. The palladium carbide-like phase is stable under reaction conditions and only partially decomposes under high hydrogen partial pressure. Non-selective hydrogenation to pentane is not indicative of hydride formation. The palladium carbide phase was detected in the EXAFS analysis and the K edge XANES showed representative features.

Shape-Dependence of Pd Nanocrystal Carburization during Acetylene Hydrogenation

“…Among the wealth of publications revolving around the formation of PdC x during reactions, there is only a handful dealing with its characterization under technologically relevant conditions, especially over well-defined nanocrystals. 19,28 Even in those cases, little information was given concerning the morphology of the active phase. This probably arises from the fact that only recent advances in nanotechnology has allowed to control the size and shape of metallic nanocrystals and to apply this knowledge to the service of catalysis.…”

“…21 Alternatively, the formation of a Pd carbide phase (PdC x ) in the early stages of the reaction has been widely documented. 16,19,20,22,23 Recent studies performed with in situ (mostly under ultrahigh vacuum, conditions) techniques such as XPS, 16 and DFT calculations 14,21,25 have shown that PdC x is formed homogeneously throughout the entire Pd nanoparticle starting from the surface, 28 and then propagates into the bulk. The experimental work performed either on single crystals or with model catalysts, as well as the DFT calculations revealed that the formation of PdC x is itself a structure sensitive phenomenon.…”

Future Challenges in Heterogeneous Catalysis: Understanding Catalysts under Dynamic Reaction Conditions

“…Another example is the intermetallic compound Pd 2 Ga for selective hydrogenations for which surface decomposition induced by oxygen impurities yields a Ga‐depleted Pd phase and Ga 2 O 3 as active phases of the catalyst 53. The phase change of a catalyst, for example, by oxidation54, 55 or by formation of carbides56 (Figure 4 f) is a frequently observed phenomenon in electro‐ and heterogeneous catalysis. Finally, the loss of active material by dissolution of metal particles (Figure 4 g) is for example, encountered in electrocatalysis, in particular under non‐stationary conditions 16, 41…”

Vapor-Deposited Pt and Pd-Pt Catalysts for Solid Acid Fuel Cells: Short Range Structure and Interactions with the CsH2PO4Electrolyte

“…8 This hypothesis also explains the lack of activity observed in more Pt-rich compositions, since the solubility of H in Pt-Pd alloys is negligible above 30 at% Pt, 33 and we similarly find limited activity enhancement above 30 at% Pt. 12 The break-in phenomenon can be explained by the desorption of carbon from the octahedral interstitial sites in Pd, which are shared by H and C, 34,35 and necessary for H transport. Deactivation of this scheme can be explained by chemical reactivity of CDP with Pd or Pd oxides to form a passivating layer comprised of CsPO 3 and Pd phosphates and pyrophosphates with low proton conductivity that prevents proton traversal into the Pd bulk.…”