3.1.3.2. XPS analysis. The XPS spectra of Ni/Al2O3 and Ni-based
catalysts prepared by different impregnation sequences are displayed
in Fig. 6, and the binding energies of Ni 2p3/2 core-level and
surface atomic ratios obtained from XPS spectra are summarized in
Table 2. Generally, the binding energy of Ni 2p3/2 in pure NiO was
about 854.4 eV and in NiAl2O4 spinel was about 856.2 eV (Sanchez-
Sanchez et al., 2007; Iriondo et al., 2008). Catalyst Ni/Al2O3 showed
the binding energy of Ni 2p3/2 at 856.1 eV, closer to the binding
energy of NiAl2O4 spinel, indicating that the two Ni species coexisted
on the surface of Ni/Al2O3 catalyst and the interaction between
Ni species and the Al2O3 support was very strong (Sanchez-
Sanchez et al., 2007). The Ce-promoted catalysts showed the
binding energy of Ni 2p3/2 between 855.6 eV and 855.8 eV, which
were lower than 856.1 eV of Ni/Al2O3, and catalyst NieCe/Al2O3
prepared by coimpregnation of Ni and Ce showed the Ni 2p3/2
binding energy of 855.5 eV, whichwas the lowest binding energy of
all the catalysts. The results indicated that the introduction of Ce
weaken the interaction between Ni species and the support, which
facilitated to generate more active sites.