A Systematic Study of the Interactions between Chemical Partners (Metal, Ligands, Counterions, and Support) Involved in the Design of Al₂O₃-Supported Nickel Catalysts from Diamine−Ni(II) Chelates

Abstract

1.5 Ni wt %/Al2O3 catalysts have been prepared by incipient wetness impregnation using [Ni(diamine)x(H2O)(6-2x)]Y2 precursors (diamine = 1,2-ethanediamine (en) and trans-1,2-cyclohexanediamine (tc); x = 0, 1, and 2; Y = NO3- and Cl-), to avoid the formation, during calcination, of difficult-to-reduce nickel aluminate. N2 was chosen for thermal treatment to help reveal and take advantage of the reactions occurring between Ni2+, ligands, counterions, and support. In the case of [Ni(en)2(H2O)2]Y2 salts used as precursors, in situ UV-vis and DRIFT spectroscopies show that after treatment at 230 degrees C Ni(II) ions are grafted to alumina via two OAl bonds and that the diamine ligands still remain coordinated to grafted nickel ions but in a monodentate way, bridging the cation with the alumina surface. With Y = Cl-, the chloride counterions desorb as hydrogen chloride, and hydrogen released upon decomposition of the en ligands is able to reduce a fraction of nickel ions into metal as evidenced by XPS. In contrast, with Y = NO3-, compounds such as CO or NO are formed during thermal treatment, indicating that nitrate ions burn the en ligands. After thermal treatment at 500 degrees C, a surface phase containing Ni(II) ions forms, characterized by XPS and UV-vis spectroscopy. Temperature-programmed reduction shows that these ions can be quantitatively reduced to the metallic state at 500 degrees C, in contrast with the aluminate obtained when the preparation is carried out from [Ni(H2O)6]2+, which is reduced only partly at 950 degrees C. On the other hand, a total self-reduction of nickel complexes leading to 2-5-nm metal particles is obtained upon thermal treatment via the hydrogen released by a hydrogen-rich ligand such as tc, whatever the Y counterion. An appropriate choice of the ligand and the counterion allows then to obtain selectively Ni(II) ions or a dispersed reduced nickel phase after treatment in N2, as a result of the reactions occurring between the chemical partners present on alumina.

Alumina‐Supported Ni–Au: Surface Synergistic Effects in Catalytic Hydrodechlorination

“…675 K) reduction of NiO to Ni 0 , as noted elsewhere. [6,42] The TPR behavior of alumina-supported nickel has been well studied [43][44][45][46][47][48] and the effect of sample pretreatment, use of different alumina substrates, and variations in Ni loading and Ni precursor have been considered. Medina et al [44] found that NiO/g-Al 2 O 3 required a higher reduction temperature compared with bulk NiO and NiO/aAl 2 O 3 , which implied a stronger interaction between NiO and g-Al 2 O 3 .…”

How nitric oxide affects the decomposition of supported nickel nitrate to arrive at highly dispersed catalysts

“…The dried impregnate is heat treated in air, called calcination, to convert the precursor into the desired metal oxide, optionally followed by a high temperature reduction treatment to obtain the metal in question. State-of-the-art studies at low Ni loadings and strong precursor-support interactions have revealed much insight into the interplay between metal, ligands, counterions and supports, and their impact on the activity, selectivity and stability of the final catalyst [13,16,17,19]. Moreover, these studies have yielded fundamental insight into the importance of liquid-solid interactions between precursor solution and support on the formation of poorly reducible mixed Ni phases like aluminates or phyllosilicates.…”

Effect of the Nickel Precursor on the Impregnation and Drying of γ-Al2O3 Catalyst Bodies: A UV−vis and IR Microspectroscopic Study

“…19,21,24 In particular, the effect of en on Ni catalysts have been studied in detail by Che et al using support powders. 18,19,[25][26][27][28] However, very few studies have been done on the effect of chelating ligands in the preparation of catalyst bodies. 15c,29 In this paper, the influence of the number of en ligands on the macrodistribution and molecular speciation of Ni 2+ in γ-Al 2 O 3 catalyst bodies during impregnation and drying has been investigated.…”