High Catalytic Activities of RENi_5–xAl_x (RE = La, Er) and Low Activity of Mg₂Ni Following Hydrogen Uptake: The Role of Absorbed Hydrogen

Abstract

Catalysis by intermetallic compounds (IMCs) has recently received significant attention, and IMCs that are also capable of storing hydrogen, including RENi 5−x Al x (RE = La, Er) and Mg 2 Ni, have been employed as catalysts for the hydrogenation of ethylene. RENi 5−x Al x with absorbed hydrogen exhibits improved catalytic activity and high levels of ethylene conversion. The reaction rate for RENi 5−x Al x with absorbed hydrogen is higher than those for pure Ni and Cu and close to that for Pd. Pulsed gas tests have demonstrated that the presence of absorbed hydrogen is essential for high catalytic activity. In this work, the role of absorbed hydrogen in catalytic reactions was assessed by performing the trial using deuterium. It was found that absorbed deuterium in ErNi 3.75 Al 1.25 D n undergoes very little reaction with ethylene. A novel activation mechanism which is different from the reaction of absorbed hydrogen directly with C 2 H 4 was proposed. This mechanism was investigated based on adsorption experiments and density functional theory calculations of electronic states. It was concluded that weakly adsorbed ethylene and hydrogen are more reactive on the hydride surface than on the matrix surface with no absorbed hydrogen, leading to an increase in catalytic activity. In contrast, the hydride of Mg 2 Ni, Mg 2 NiH 4 , showed decreased activity toward the hydrogenation of ethylene, representing the direct opposite behavior to that for RENi 5−x Al x alloys. We considered the reason for decline in catalytic activity of Mg 2 NiH 4 from the electronic states. The reason can be the low ability to break multiple bonds in hydrocarbons or single bonds in hydrogen molecules due to a lack of an electron supply from the non-metallic Mg 2 NiH 4 .

Beyond Hydrogen Storage: Metal Hydrides for Catalysis

“…50 In contrast, as shown in Figure 3c, hydrogen absorption of Mg 2 Ni into Mg 2 NiH 4 resulted in lower activity for hydrogenation of C 2 H 4 . 48 The metal hydride complex CaPdH 2 is also reported to selectively catalyze C 2 H 2 hydrogenation to C 2 H 4 due to the modification of C 2 H 2 and C 2 H 4 adsorption mode and H 2 activation barriers, lattice H species are calculated to be involved in the hydrogenation reactions but the calculated equilibrium conversion is only 0.004% at 120 °C. 51 The studies of the catalytic olefin hydrogenation by MHs lay the foundation of MHs heterogeneous catalysis.…”

“…(e) Density of states (DOS) of Ni d-states in ErNi 4 Al and ErNi 4 AlH 3 . [Reprinted from ref . Copyright 2021, American Chemical Society, Washington, DC.…”

“…Recently, Kameoka et al studied catalytic hydrogenation of alkyne and C 2 H 4 over RENi 5– x M x (RE = La, Ce, Er, and M = Al and Ga)-type hydrogen storage intermetallic. − They also found that the hydride form of RENi 5– x M x showed much higher activity than their corresponding intermetallic form as well as Ni- and Cu-catalysts, as shown in Figure b. However, based on the isotope labeling experiments, the authors concluded that only a low percentage of the product originated from a direct reaction of C 2 H 4 with bulk H (Figure d).…”

Catalytic Hydrogenation of C₂H₂ over Amorphous CeNi₂H<i>_x</i> and Crystalline CeNi₂: Effects of Hydrogen-Induced Amorphization and Oxidation

“…7) Soga et al used LaNi 5 , one of the most well-known hydrogen storage alloys, to hydrogenate ethylene (C 2 H 4 ) and found that the hydrogen contained in LaNi 5 H n is used for hydrogenation without the need for hydrogen in gas phase. 5) On the other hand, recent research by our own group with RENi 5 (RE = rare earth element)-based hydrogen storage alloys 10,11) showed that absorbed hydrogen in these compounds exhibited minimal activity, but changes in the electronic states of these materials due to hydrogen absorption increased their ability of the hydrogenation of acetylene (C 2 H 2 ), 10) propyne (C 3 H 4 ) 10) and C 2 H 4 . 11) Furthermore, recent studies have demonstrated that the catalytic activity of some IMCs for the hydrogenation of hydrocarbons is reduced following hydrogen absorption.…”

“…11) Furthermore, recent studies have demonstrated that the catalytic activity of some IMCs for the hydrogenation of hydrocarbons is reduced following hydrogen absorption. 9,11,13) As an example, the catalytic activity for the hydrogenation of C 2 H 2 and C 2 H 4 over Mg 2 Ni is reduced by hydrogen absorption and formation of Mg 2 NiH 4 . 9,11) BaGa 2 H 2 also shows lower activity for the hydrogenation of phenylacetylene than Zintl phase BaGa 2 .…”