Studies on the behavior of mixed-metal oxides: Adsorption of CO and NO on MgO(100), NixMg1-xO(100), and CrxMg1-xO(100)

Ultraviolet photoelectron spectroscopy (UPS), thermal desorption mass spect roscopy (TDS), and first-principles density functional (DF) generalized-gra dient-corrected calculations were used to study the adsorption of CO and NO on MgO(100), Ni0.06Mg0.94O(100), and Cr0.07Mg0.93O(100) surfaces. UPS spec tra and DF calculations show clear differences in the electronic properties of these oxides. After doping MgO with nickel, states with Ni 3d character appear similar to1.5 eV above the occupied {O 2p+Mg 3s} band. A similar ph enomenon is found after adding Cr, but now the dopant levels are similar to 3 eV above the {O 2p+Mg 3s} band. In CO- and NO-TDS experiments, the reacti vity of the oxide surfaces increases in the sequence: MgO(100)< Ni0.06Mg0.9 4O(100)< Cr0.07Mg0.93O(100). Cr-bonded molecules exhibit adsorption energie s as large as 15 (CO) and 20 kcal/mol (NO). For CO and NO on MgO(100), the mixing between the frontier orbitals of the adsorbate and the bands of the surface is poor, and the low adsorption energy is mainly due to weak MgO <- ---> CO or MgO <----> NO electrostatic interactions. On the other hand, the Cr 3d levels in Cr0.07Mg0.93O(100) are energetically well positioned for r esponding to the presence of adsorbates, leading to substantial binding of CO and NO. DF results for a series of TM0.06Mg0.94O(100) systems (TM=Zn, Ni , Fe, or Cr) show a correlation between their electronic and chemical prope rties: the less stable the occupied levels of a mixed-metal oxide, the high er its chemical reactivity. An important parameter to consider when designi ng a mixed-metal oxide catalyst is the final energy position of the occupie d states provided by the second metal or dopant agent. (C) 2001 American In stitute of Physics.