Poisoning of Heterogeneous, Late Transition Metal Dehydrocoupling Catalysts by Boranes and Other Group 13 Hydrides

Abstract

Borane reagents are widely used as reductants for the generation of colloidal metals. When treated with a variety of heterogeneous catalysts such as colloidal Rh, Rh/Al2O3, and Rh(0) black, BH3.THF (THF = tetrahydrofuran) was found to generate H2 gas with the concomitant formation of a passivating boron layer on the surface of the Rh metal, thereby acting as a poison and rendering the catalyst inactive toward the dehydrocoupling of Me2NH.BH3. Analogous poisoning effects were also detected for (i) colloidal Rh treated with other species containing B-H bonds such as [HB-NH]3, or Ga-H bonds such as those present in GaH3.OEt2, (ii) colloidal Rh that was generated from Rh(I) and Rh(III) salts using borane or borohydrides as reductants, and (iii) for other metals such as Ru and Pd. In contrast, analogous poisoning effects were not detected for the catalytic hydrogenation of cyclohexene using Rh/Al2O3 or the Pd-catalyzed Suzuki cross-coupling of PhB(OH)2 and PhI. These results suggest that although this poisoning behavior is not a universal phenomenon, the observation that such boron layers are formed and surface passivation may exist needs to be carefully considered when borane reagents are used for the generation of metal colloids for catalytic or materials science applications.

Hydrogen sorption in MgH2-based composites: The role of Ni and LiBH4 additives

“…A previous investigation on the catalytic effect of Ni, Mg 2 Ni and Mg 2 NiH 4 on the hydrogen desorption kinetics confirms our interpretation [26]. It was found that Mg 2 NiH 4 was a much efficient catalyst than Ni or Mg 2 Ni compound to favor the hydrogen desorption from M [30]. Although the MgNi 3 B 2 phase has been also reported to possess a catalytic effect on the hydrogen releasing [17], it is not surprising that its catalytic efficiency could be inferior to that attributed to Mg 2 NiH 4 and/or Mg 2 Ni.…”

“…The co-milling of M with Ni conduces to the lowest hydrogen desorption temperature, obtaining a single peak at 267 and 283 C for M2N and MN, respectively. It is consistent with the known catalytic effect of Ni during hydrogen desorption, which is favored by the increasing of the Ni loading [30]. On the other hand, simultaneous milling of M, 2Ni and LB (M2NLB) induces the appearance of two peaks during M decomposition, with the main peak at the highest hydrogen desorption temperature (339 C).…”

“…Two possible different roles could be operating in relation to diborane and boron formation. On one hand, diborane and other borane species have been reported as catalysts poisons via the formation of a passivating boron/ boride layer [30]. When LB decomposes during the first hydrogen desorption in the MNLB and (M2N)LB composites (Fig.…”

Cobalt–nickel–phosphorus supported on Pd-activated TiO2 (Co–Ni–P/Pd-TiO2) as cost-effective and reusable catalyst for hydrogen generation from hydrolysis of alkaline sodium borohydride solution

“…The slight decrease in catalytic activity in subsequent runs may be attributed to the passivation of the catalyst surface by increasing the concentration of boron products, e.g. metaborate, which decreases the accessibility of active sites [58,59]. Despite this activity loss, it can be concluded that the Co-Ni-P catalyst is isolable, redispersible and yet catalytically active.…”

Rhodium(0) nanoparticles supported on hydroxyapatite nanospheres and further stabilized by dihydrogen phosphate ion: A highly active catalyst in hydrogen generation from the methanolysis of ammonia borane

“…S5 in the Supplementary Materials). The decrease in catalytic activity in successive runs is likely caused by the increase in viscosity of the solution during methanolysis and probably due to the passivation of nanoparticles surface by increasing amount of methoxyborate, which might decrease the accessibility of active sites [57,58].…”