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 ͑UPS͒, thermal desorption mass spectroscopy ͑TDS͒, and first-principles density functional ͑DF͒ generalized-gradient-corrected calculations were used to study the adsorption of CO and NO on MgO͑100͒, Ni 0.06 Mg 0.94 O͑100͒, and Cr 0.07 Mg 0.93 O͑100͒ 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 ϳ1.5 eV above the occupied ͕O 2pϩMg 3s͖ band. A similar phenomenon is found after adding Cr, but now the dopant levels are ϳ3 eV above the ͕O 2pϩMg 3s͖ band. In CO-and NO-TDS experiments, the reactivity of the oxide surfaces increases in the sequence: MgO͑100͒ϽNi 0.06 Mg 0.94 O͑100͒ ϽCr 0.07 Mg 0.93 O͑100͒. Cr-bonded molecules exhibit adsorption energies as large as 15 ͑CO͒ and 20 kcal/mol ͑NO͒. For CO and NO on MgO͑100͒, 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 Cr 0.07 Mg 0.93 O͑100͒ 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 TM 0.06 Mg 0.94 O͑100͒ systems (TMϭZn, Ni, Fe, or Cr͒ 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.

Catalytic dissociation of N2O on pure and Ni-doped MgO surfaces

“…All the Ni d orbitals lie above the oxide valence band edge, from 0.3 to 1.7 eV, with a computed band gap for pure MgO of 5.5 eV. These results are in agreement with the density of states obtained with plane-waves calculations by Rodriguez et al [27] The cluster has been reoptimized after substituting Mg with Ni, and the structure with Ni at the surface is 0.26 eV more stable than the sub-surface one. On this Nidoped MgO surface we have considered the adsorption of the N 2 O molecule and compared the results for reactions (1)-(3) with those of pure MgO.…”

“…First of all, N 2 O does not bind to the surface Ni 2+ ion; both N-down and O-down orientations have been considered, but the result is the same, virtually no bonding. This is a completely different behavior from that of Cr-, Fe-or Mn-doped MgO on which N 2 O is found to directly dissociate with no reaction barrier by forming a Cr-, Fe-or Mn-O bond and a Mg-N 2 bond, but more similar to Zn-doped MgO [27]. N 2 O reacts with Ni impurities through an activation barrier.…”

Density functional study of CO and NO adsorption on Ni-doped MgO(100)

“…1,2 Both metal and metal oxide surfaces are important for heterogeneous catalysis. Experimentally, the use of a variety of methods for the preparation and cleaning of metal oxide surfaces ͑epitaxial metal oxide film grown on a metal surface, [20][21][22] surfaces obtained by cleaving single crystals under ultra high vacuum conditions, 14 metal oxide powder, 7 or sintered polycrystalline metal oxide solid solutions͒ 11 yields substrates with diverse compositions and different numbers and types of defects; and the adsorbate-substrate bonding energy, especially when weak, is very sensitive to these variables. [3][4][5][6] However, when the interaction involves metal oxide surfaces, the situation is less clear and obtaining the binding energy of even the simplest neutral molecules such as NH 3 , CO, or NO with these surfaces constitutes a complex problem both experimentally and theoretically.…”

The treatment of the spin coupling in the bonding of NO to the Ni-doped MgO (100) surface

“…A spin polarized gradient corrected DFT periodic calculation based on plane waves has given an adsorption geometry and a binding energy close to the expeorimental values for NO/NiO, r(Ni-N) = 1.85A and D, = 0.74eV [43], suggesting a correct description of the interaction. Unfortunately, no analysis of the spin distribution has been reported.…”