Négrier Fabien, Marceau Eric, Che Michel, Giraudon Jean-Marc, Gengembre Léon, Löfberg Axel
Laboratoire de Réactivité de Surface (UMR 7609 CNRS), Université Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France.
J Phys Chem B. 2005 Feb 24;109(7):2836-45. doi: 10.1021/jp0403745.
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.
采用初湿浸渍法，使用[Ni(diamine)x(H2O)(6 - 2x)]Y2前驱体（二胺 = 1,2 - 乙二胺（en）和反式 - 1,2 - 环己二胺（tc）；x = 0、1和2；Y = NO3-和Cl-）制备了1.5 Ni wt%/Al2O3催化剂，以避免在煅烧过程中形成难以还原的镍铝酸盐。选择N2进行热处理，以帮助揭示并利用Ni2+、配体、抗衡离子和载体之间发生的反应。以[Ni(en)2(H2O)2]Y2盐作为前驱体时，原位紫外可见光谱和漫反射红外傅里叶变换光谱表明，在230℃处理后，Ni(II)离子通过两个O - Al键接枝到氧化铝上，并且二胺配体仍以单齿方式与接枝的镍离子配位，将阳离子与氧化铝表面桥连。当Y = Cl-时，氯化物抗衡离子以氯化氢形式解吸，并且如XPS所证明的，en配体分解时释放的氢能够将一部分镍离子还原为金属。相反，当Y = NO3-时，在热处理过程中会形成诸如CO或NO的化合物，表明硝酸根离子会燃烧en配体。在500℃热处理后，形成了含有Ni(II)离子的表面相，通过XPS和紫外可见光谱对其进行了表征。程序升温还原表明，与从[Ni(H2O)6]2+制备的铝酸盐相比，这些离子在500℃时可以定量还原为金属态，后者在950℃时仅部分还原。另一方面，无论Y抗衡离子如何，通过富含氢的配体如tc释放的氢进行热处理时，镍配合物会发生完全自还原，形成2 - 5纳米的金属颗粒。由于氧化铝上存在的化学组分之间发生的反应，配体和抗衡离子的适当选择使得在N2中处理后能够选择性地获得Ni(II)离子或分散的还原镍相。
