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

Abstract

Ultraviolet photoelectron spectroscopy (UPS), thermal desorption mass spectroscopy (TDS), and first-principles density functional (DF) generalized-gradient-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 spectra and DF calculations show clear differences in the electronic properties of these oxides. After doping MgO with nickel, states with Ni 3d character appear ∼1.5 eV above the occupied {O 2p+Mg 3s} band. A similar phenomenon is found after adding Cr, but now the dopant levels are ∼3 eV above the {O 2p+Mg 3s} band. In CO- and NO-TDS experiments, the reactivity of the oxide surfaces increases in the sequence: MgO(100)<Ni0.06Mg0.94O(100)<Cr0.07Mg0.93O(100). Cr-bonded molecules exhibit adsorption energies 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 responding 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 properties: the less stable the occupied levels of a mixed-metal oxide, the higher its chemical reactivity. An important parameter to consider when designing a mixed-metal oxide catalyst is the final energy position of the occupied states provided by the second metal or dopant agent.