An attempt has been made to analyze the effect of surface site on the spin state for the interaction of NO with Pd 2, Rh 2 and PdRh nanoparticles that supported at regular and defective MgO(001) surfaces. The adsorption properties of NO on homonuclear, Pd 2, Rh 2, and heteronuclear transition metal dimers, PdRh, that deposited on MgO(001) surface have been studied by means of hybrid density functional theory calculations and embedded cluster model. The most stable NO chemisorption geometry is in a bridge position on Pd 2 and a top configuration of Rh 2 and PdRh with N-down oriented. NO prefers binding to Rh site when both Rh and Pd atoms co-exist in the PdRh. The natural bond orbital analysis (NBO) reveals that the electronic structure of the adsorbed metal represents a qualitative change with respect to that of the free metal. The adsorption properties of NO have been analyzed with reference to the NBO, charge transfer, band gaps, pairwise and non-pairwise additivity. The binding of NO precursor is dominated by the E (i) Mx-NO pairwise additive components and the role of the support was not restricted to supporting the metal. The adsorbed dimers on the MgO surface lose most of the metal-metal interaction due to the relatively strong bond with the substrate. Spin polarized calculations were performed and the results concern the systems in their more stable spin states. Spin quenching occurs for Rh atom, Pd 2, Rh 2 and PdRh complexes at the terrace and defective surfaces. The adsorption energies of the low spin states of spin quenched complexes are always greater than those of the high spin states. The metal-support and dimer-support interactions stabilize the low spin states of the adsorbed metals with respect to the isolated metals and dimers. Although the interaction of Pd, Rh, Pd 2, Rh 2 and PdRh particles with Fs sites is much stronger than the regular sites O 2-, the adsorption of NO is stronger when the particular dimers are supported on an anionic site than on an Fs site of the MgO(001). The encountered variations in magnetic properties of the adsorbed species at MgO(001) surface are correlated with the energy gaps of the frontier orbitals. The results show that the spin state of adsorbed metal atoms on oxide supports and the role of precursor molecules on the magnetic and binding properties of complexes need to be explicitly taken into account.
Fundamental understanding of the electronic structure and activity of transition metal atoms and nanoclusters supported on metal-oxide surfaces is of great interest due to their broad applications in catalysis, coating for thermal applications, corrosion protection, and other technologically important fields [
The strength of interaction between metal and substrate is due to metal-substrate covalent bonding that implies a polarization of the metal orbitals or redistribution of the atomic orbital population. The metal s-orbital combines with the oxygen p-orbital perpendicular to the surface of an oxide material resulting in a bonding (occupied) and antibonding (unoccupied) combinations. This leads to a decrease in the atomic population of the metal atom [
On the basis of the performance of different density functionals, Markovits et al. [
Sousa et al. [
Bimetallic nanoparticles may create a synergistic catalytic effect that involves the change in local electronic properties of pure metal nanoparticles to modify the strength of the surface adsorption for oxygen reduction reactions [
It is frequently observed that a transition metal atom doped in a small cluster of other metal can strongly change the properties of the host cluster [
The intriguing heterogeneous processes associated with nitric oxide, NO, observed at transition metal and metal-oxide surfaces, are a continuous topic for research. The molecule, which is one of the simplest and most stable radicals, is spontaneously formed in combustion processes at elevated temperatures. Being a major environmental hazard, it is of vital importance to remove NO from the exhaust gases. The reduction of NO by CO on palladium is of practical interest and experimental investigations show that nanosized palladium clusters have significant capacity to catalyze the CO + NO reaction at low temperatures [
Hybrid density functional theory and embedded cluster models have been extensively employed in the description of the electronic and geometrical structures of Pd2, Rh2 and PdRh particles nucleated on regular and defect sites on the MgO(001) surface [
To represent the substrate, the ionic clusters Mg9O14 and Mg9O13 Fs have been embedded in arrays of point charges. This was done by following an embedding procedure previously reported for alkaline earth oxides [
The density functional theory calculations were performed by using Becke’s three parameter exchange functional B3 with LYP correlation functional [
The Stevens, Basch and Krauss Compact Effective Potential (CEP) basis sets [
The defect free surfaces exhibit very small relaxations only and therefore they have been kept fixed when studying deposition of metal atoms. A minimal energy search on a defect free surface does not usually include surface relaxation since this is experimentally very small, less than 5% [
The binding energy, Ea, of the Pd2, Rh2 and PdRh dimers at various sites of the metal oxide surface can be calculated as follows:
Positive values of the binding energies mean that the formed dimers are stable.
The high to low spin transition energies were calculated from the relation
where Ecomplex is the total electronic energy of the complex.
The nucleation energy (Enucl), dimer formation energy, is an important parameter to study the atom-by-atom growth of a particle from atoms in the gas phase. It is defined as the energy associated with the formation of homonuclear dimers Rh2,Pd2, and heteronuclear dimers, PdRh, when an atom of the gaseous phase, Rh or Pd bonds with a pre-adsorbed metallic particle, Rh/MgO_site or Pd/MgO_site [
where MgO_site indicates the nucleation site. These two quantities, Ea (M2) and Enucl., measure the binding energy of gas phase Pd2, Rh2 and PdRh to a given MgO site [
The dimer binding energy, Eb, measures the stability of the adsorbed dimer with respect to Pd and Rh adatoms, where one of which is bound on a five coordinated terrace anion, O5c. Eb is simply the difference between the adsorption energy of transition metal, TM, atom to the supported TM/MgO and the binding energy of the atom to the metal oxide terrace.
The trapping energy, Et, measures the energy gain when Pd and Rh atoms move from a terrace site to a strongly binding site, anion vacancy. The trapping energy is the difference in Ea between a regular and a defect site. Et can be quite large on some specific defects, indicating their strong tendency to capture metal atoms. Thus, metal atoms have a high probability to find a defect in the diffusion process and to stick to this defect [
It was well established that small metal particles adsorb preferentially on sites where negative charge accumulates [
In addition, it is not a trivial task to conclude a priory which one of the 4F (4d8s1) and 2D (d9) states of Rh determines the ground state energy of the unit cell of MgO(001) surface with the adsorbed Rh atom. Therefore, the effect of the substrate on the electronic states of the adsorbate and the energy required to switch from high-spin to low-spin state are analyzed. By using the B3LYP calculation, high- to low-spin transition energies of Rh atoms free,
Nevertheless, the important point here is the trend of the adsorbed atoms from one site to another. The analysis of these results clearly show that, there is a change in the transition energy required to switch from high spin to low spin,
Pd/MgO (O2−) site | Pd/MgO (Fs) site | Rh/MgO (O2−) site | Rh/MgO (Fs) site | |
---|---|---|---|---|
0.904 | −0.034 | |||
0.919 | 1.933 | 1.034 | 1.812 | |
dH(M-S) | 2.39 | 1.82 | 2.54 | 2.06 |
1.234 | 2.156 | 0.232 | 1.417 | |
0.2 | 0.28 | 0.48 | 0.44 | |
Electronic configuration | 5s0.284d9.745p0.016p0.02 | 5s0.794d9.865p0.145d0.016p0.02 | 5s 0.43 4d 8.6 5p 0.03 5d 0.01 6p 0.01 | 5s 0.83 4d 8.815p 0.13 5d 0.026p 0.02 |
Ne | 10.05 | 10.82 | 9.05 | 9.81 |
There are some differences in the adsorption heights between the lowest spin state and the highest spin state of the adsorbed Rh and Pd atoms on both the O5c and oxygen vacancy sites. In particular, for the perfect surface, the adsorption height of 2.54Åand 2.39 Å for the quartet Rh and triplet Pd that is larger by 0.48 Å and 0.2Å than for the doublet Rh and singlet Pd, respectively. The similar phenomena for the vacancy surface are also observed. This observation may result from the larger overlaps between the highest occupied molecular orbital, HOMO, of MgO cluster and the lowest unoccupied molecular orbital, LUMO, of the adsorbed metal atoms in the doublet and singlet state than those in the quartet and triplet states of the adsorbed Rh and Pd atoms that have anti-bonding character as shown from
The increase in the adsorption heights of supported Pd and Rh can contribute to the Pauli repulsion of the valence s orbital of the Pd and Rh that is almost empty with those in the p orbitals of the surface oxygen atoms. As well as, the HOMO for the oxygen vacancy has a large s-like character, which would also lead to repulsive interaction with the metal s orbital; this orbital is occupied by ~−0.8e due to the charge transfer. The adsorption heights of supported Pd and Rh increased at oxygen anion and oxygen vacancy sites also the energy gain of 1.90 eV and 2.0 eV due to the electron occupying this bonding orbital of the Pd and Rh atom, where the binding energy is calculated to be 3.192 eV and 3.263 eV for the adsorption complexes Pd/Mg9O13Fs and Rh/Mg9O13 Fs, respectively.
It is interesting to explore the electronic configuration for the interaction of Pd and Rh atoms with the regular site at MgO(001) surface. The only appreciable change with respect to the free atom is the hybridization between 5s and 4d orbitals with negligible contribution of the 5p subshell, 5s0.284d9.745p0.016p0.02 and 5s 0.43 4d 8.6 5p 0.03 5d 0.01 6p 0.01 for the supported Pd and Rh atoms and no appreciable charge transfer, −0.054e and −0.07e, respectively, These results are consistent with [
As it has been shown later, the interaction of NO with supported Pd and Rh depends strongly on the metal- oxide interaction and it is essential to dispose of an adequate substrate model for the subsequent NO adsorption [
To underscore the most stable configuration of Rh2, Pd2 and Pd-Rh dimer on the MgO(001) surface, two confi-
gurations, parallel and perpendicular to the surface plane have been considered. The best optimized geometries of Rh2, Pd2 and PdRh supported particles anchored on terrace sites of MgO(001) are with the molecular axis almost parallel to the surface and the supported two atoms of the dimer nearly on the top of two O5c anions,
Because of the spin polarization, the corresponding values of binding, nucleation, trapping and charges transfer for the deposition of the Rh2, Pd2 and PdRh particles on the regular oxygen site and Fs center have been summarized in
From these results, it is observed that the interaction of Rh2, Pd2 and PdRh on Fs site is characterized by stronger binding energy with shorter equilibrium adsorption distance than on the surface O2− site. Although the binding energy is noticeably affected by the support, the nucleation energy is weakly affected; for both sites the values of Enucl are less significantly changed (0.001 - 0.246 eV). This can mean that (a) the regular metal oxide surface is always an appropriated place to form Rh2, Pd2 and Pd-Rh(b) the dimers formation is independent of the adsorption site (regular or diamagnetic Fs site). However the dimer formation will be favored on the Fs center due to the trapping energy, consistently with [
On a terrace site, the addition of second TM atom leads to a nucleation energy Enucl. = 1.716 eV, 2.465 eV, and 2.354 eV that are 0.466 eV, 1.165 eV and 1.053 eV higher than the adsorption energies of the TM atom on a terrace site. Consequently, the dimer formation of Rh2, Pd2 and PdRh are preferred with respect to two isolated atoms adsorbed on O2− anions (
The elongation of the Pd-Pd, Rh-Rh and Pd-Rh distances with respect to the gas-phase is explained by the fact that the dimer is oriented towards two nearest neighbor O2− anions on the surface to maximize the bonding with the O2− anions. The Pd-Pd bond length becomes close to 2.98Å, is only 0.22 Å longer than in the free molecule, in agreement with [
Concerning the bimetallic PdRh particle, the ground state geometry of the bimetallic is significantly modified
MgO (O2−) site | MgO (Fs) site | ||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Pd2 | Rh2 | PdRh | Pd2 | Rh2 | PdRh | ||||||||||||||
M = 1 | M = 3 | M = 5 | M = 1 | M = 3 | M = 5 | M = 2 | M = 4 | M = 6 | M = 1 | M = 3 | M = 5 | M = 1 | M = 3 | M = 5 | M = 2 | M = 4 | M = 6 | ||
Ea (M2) | 3.681 | 2.828 | 2.252 | 3.116 | 2.647 | 1.767 | 3.038 | 2.164 | 1.894 | 5.616 | 4.155 | 3.644 | 4.875 | 4.399 | 2.795 | 4.728 | 3.688 | 3.585 | |
Enucl | 1.716 | 1.412 | -0.894 | 2.465 | 2.653 | 2.502 | 2.354 | 2.139 | 4.647 | 1.717 | 0.804 | -1.438 | 2.263 | 2.443 | 1.567 | 2.108 | 1.694 | 4.403 | |
Eb | 0.466 | 0.161 | -2.145 | 1.165 | 1.352 | 1.201 | 1.053 | 1.071 | 2.119 | 0.466 | -0.447 | -2.688 | 0.962 | 1.142 | 0.267 | 0.807 | 0.659 | -1.183 | |
Etrap. | - | - | - | - | - | - | - | - | - | 2.489 | 1.328 | 1.392 | 3.116 | 2.647 | 1.767 | 3.038 | 2.164 | 1.894 | |
q(M1) | −0.069 | 0.03 | −0.006 | −0.064 | −0.05 | 0.081 | −0.01 | 0.008 | −0.082 | 0.022 | 0.028 | 0.173 | 0.008 | 0.261 | 0.053 | −0.001 | −0.782 | −0.842 | |
q(M2) | −0.029 | 0.069 | 0.233 | −0.097 | 0.087 | 0.112 | 0.001 | 0.107 | 0.334 | −0.78 | −0.727 | −0.674 | −0.999 | −1.005 | −0.751 | −0.803 | 0.09 | 0.312 | |
q(M2) | −0.098 | 0.099 | 0.227 | −0.161 | 0.037 | 0.193 | −0.009 | 0.115 | 0.252 | −0.758 | −0.699 | −0.501 | −0.991 | −0.744 | −0.698 | −0.804 | −0.692 | −0.530 | |
d(M1-S) | 2.14 | 2.26 | 2.36 | 2.02 | 2.14 | 2.26 | 2.19 | 2.19 | 2.25 | 2.18 | 2.24 | 2.34 | 1.98 | 2.02 | 2.18 | 2.1 | 2.24 | 2.24 | |
d(M2-S) | 2.23 | 2.28 | 2.41 | 2.06 | 2.1 | 2.12 | 2.1 | 2.19 | 2.23 | 1.54 | 1.62 | 1.76 | 1.58 | 1.62 | 1.62 | 1.57 | 1.59 | 1.67 | |
q(M1), q(M2): atomic charges at each metal of the dimer. d(M1-S), d(M2-S): optimal distances between adsorbed metals of the dimer and surface site of MgO.
after deposition. The electronic density of states analysis reveals that after deposition, Pd-Rh favors doublet spin multiplicity as the lowest energy configuration,
In this section, the stability trends of the Pd2, Rh2 and PdRh dimers for ground state structures are analyzed in terms of the energy gaps between HOMO and LUMO. A large HOMO-LUMO gap has been considered as an important requisite for the chemical stability of transition metal clusters [
Indeed, the molecular orbital, MO, interaction is controlled by the level of the frontier orbitals. Therefore, the relations between spin quenching of supported Rh, Pd, Pd2, Rh2 and PdRh dimers at MgO surface and energy gaps between frontier orbitals are established. In
Pd2 | Rh2 | PdRh | ||||
---|---|---|---|---|---|---|
Pd2/MgO (O2−) site | Pd2/MgO (Fs) site | Rh2/MgO (O2−) site | Rh2/MgO (Fs) site | PdRh/MgO (O2−) site | PdRh/MgO (Fs) site | |
( | 2.611 | 3.155 | -0.036 | 0.695 | 2.223 | 2.222 |
( | 2.306 | 2.242 | 0.151 | 0.875 | 2.009 | 1.842 |
( | 0.22 | 0.16 | 0.24 | 0.2 | 0.06 | 0.14 |
( | 0.1 | 0.1 | 0.12 | 0.16 | 0.00 | 0.00 |
( | 0.18 | 0.22 | 0.06 | 0.04 | 0.13 | 0.1 |
( | 0.13 | 0.14 | 0.02 | 0.00 | 0.04 | 0.08 |
( | 0.325 | 0.257 | 0.354 | 0.293 | 0.261 | 0.274 |
( | 0.13 | 0.19 | 0.156 | 0.046 | 0.137 | 0.162 |
*a: High spin multiplicity is (5) for Pd2 Rh2 and (6) for PdRh; *b: High spin multiplicity is (3) for Pd2 Rh2 and (4) for PdRh; Where low spin state is (1) for Pd2 Rh2 and (2) for PdRh
The binding energy for the interaction of NO molecule on different spin states of Pd, Rh, Rh2, Pd2 and PdRh that supported on MgO can be calculated as Ea(NO) = −[E(NO/Mx/MgO_site) − E(Mx/MgO_site) − E(NO)] where x = 1 or 2 [
In Tables 5-7, the adsorption energies, optimal distances and charge transfer for the interaction of NO molecule on different spin states of Rh2, Pd2 and PdRh that supported at MgO (O2−) and MgO (Fs) through its N atom at various spin states have been calculated. Again, the NO molecule adsorbs much more strongly at the Pd2, with low spin state, Rh2 and PdRh with different two spin state that deposited on the MgO (O2−) site than on the MgO (Fs),
At the high spin states a dramatic change is found when the NO which bind to the supported Pd and Pd2 where there is an increase in the binding energies at the MgO (Fs) site than on the MgO (O2−). The different behavior of Pd and Pd2 with high spin states that adsorbed on Fs centers towards NO can be explained as follows. At the low spin states of supported Pd and Pd2 the delocalization of the trapped electrons into the 5 s level leads to an increased Pauli repulsion with the NO molecule and in a strong weakening of the bond. On contrary, at the supported Pd and Pd2 with high spin states this effect is smaller because of the presence of an incomplete d shell.
Since the high occupied molecular orbital of NO is π* anti-bonding orbital with an unpaired electron therefore, the charge transferred from Pd2/MgO, Rh2/MgO and PdRh/MgO to the NO will occupy the π* orbital and weak the NO bond strength. The NO bond length is elongated after the adsorption of NO on the particular dimers, this
NO/Pd | NO/Rh | ||||||||
---|---|---|---|---|---|---|---|---|---|
MgO (O2−) site | MgO (Fs) site | MgO (O2−) site | MgO (Fs) site | ||||||
Spin multiplicity | 2 | 4 | 2 | 4 | 1 | 3 | 1 | 3 | |
Spin state | L | H | L | H | L | H | L | H | |
Ea (NO) | 1.499 | 1.038 | 0.702 | 1.261 | 3.048 | 2.115 | 1.449 | 1.026 | |
Ea (MNO) | 1.516 | 1.327 | 2.655 | 2.563 | 1.771 | 1.582 | 2.134 | 2.455 | |
1.695 | 1.598 | 0.934 | 0.423 | ||||||
qMNO | −0.137 | −0.126 | −0.816 | −0.936 | −0.182 | −0.063 | −0.819 | −0.742 | |
d(M-S) | 2.13 | 2.21 | 1.60 | 1.56 | 1.98 | 2.18 | 1.76 | 1.80 | |
d(N-M) | 1.90 | 1.96 | 2.06 | 2.02 | 1.74 | 1.82 | 1.78 | 1.96 | |
d(N-O) | 1.228 | 1.288 | 1.208 | 1.248 | 1.207 | 1.207 | 1.207 | 1.207 | |
q(M): atomic charges at the adatom. q(NO): molecular charge at NO molecule. d(M-S): optimal distances between adatom and surface site of MgO. d (N-O): Equilibrium N-O distances. d(N-M): optimal distances between adatom and nitrogen atom.
MgO (O2−) site | MgO (Fs) site | |||
---|---|---|---|---|
Spin multiplicity | 2 | 4 | 2 | 4 |
Spin state | L | H | L | H |
Ea (NO) | 1.808 | 1.052 | 1.231 | 1.342 |
Ea (Pd2NO) | 1.887 | 2.352 | 3.245 | 3.969 |
Total q(Pd2) | 0.415 | 0.525 | -0.493 | -0.049 |
q(N) | −0.248 | −0.473 | −0.071 | −0.460 |
q(O) | −0.27 | −0.225 | −0.199 | −0.247 |
q(NO) | −0.518 | −0.697 | −0.27 | −0.707 |
q(Pd2NO) | −0.103 | −0.172 | −0.763 | −0.756 |
Red spheres: O2−; yellow spheres: Mg2+; light blue spheres: Pd atom; dark blue sphere: N atom.
Spin multiplicity | 2 | 4 | 2 | 4 |
---|---|---|---|---|
Spin state | L | H | L | H |
MgO (O2−) site | MgO (Fs) site | |||
Ea (NO) | 2.909 | 2.506 | 2.971 | 2.097 |
Ea (Rh2NO) | 1.786 | 2.542 | 3.607 | 3.885 |
q(Rh2) | −0.164 | 0.01 | −0.912 | −0.639 |
q(N) | 0.22 | 0.093 | 0.219 | 0.070 |
q(O) | −0.185 | −0.222 | −0.188 | −0.231 |
q(NO) | 0.035 | −0.129 | 0.031 | −0.161 |
q(Rh2NO) | −0.129 | −0.119 | −0.881 | −0.799 |
Red spheres: O2−; yellow spheres: Mg2+; blue spheres: Rh atom; dark blue sphere: N atom.
Spin multiplicity | 1 | 3 | 1 | 3 | |
---|---|---|---|---|---|
Spin state | L | H | L | H | |
MgO (O2−) site | MgO (Fs) site | ||||
Ea (NO) | 2.666 | 1.315 | 2.872 | 1.329 | |
Ea (PdRhNO) | 2.057 | 2.319 | 3.953 | 3.856 | |
q(PdRh) | −0.201 | 0.539 | −0.901 | −0.109 | |
q(N) | 0.22 | −0.413 | 0.206 | −0.359 | |
q(O) | −0.204 | −0.248 | −0.205 | −0.236 | |
q(NO) | 0.016 | −0.661 | 0.001 | −0.595 | |
q(PdRhNO) | −0.184 | −0.122 | −0.901 | −0.704 | |
Red spheres: O2−; yellow spheres: Mg2+; light blue spheres: Pd atom; blue spheres: Rh atom; dark blue sphere: N atom.
is consistent with the electron transfer direction [
The adsorption energy of NO on Rh2/MgO is larger than on Pd2/MgO (2.909 vs. 1.808 eV), possibly due the decrease of the d electrons on the Rh which can lead to a decrease of the σ-σ repulsion. The metal-nitrogen bond is shorter for Rh than Pd (1.74 vs. 2.01 Å), which also indicates a strong bonding between NO and Rhx/MgO. The larger M-N-O angle for Rh2 than Pd2 (180˚ vs. 132.2˚) indicates that the 5σ orbital is much more involved in the adsorption at Rh than Pd. Although, the charge of Rh2NO and Pd2NO supported on the (O2−) and (Fs) site is practically the same than that of the supported Rh2 and Pd2 at the same sites, the MgO(Fs) site acquires a much more significant negative charge, Tables 5-7. These results are confirmed by
Indeed, the different behavior of rhodium and palladium supported at MgO(001) towards NO can be explained as follows. On palladium the delocalization of the trapped electrons into the 5s level leads to an increased Pauli repulsion with the NO molecule and in a strong weakening of the bond, whereas on Rh this effect is smaller because of the presence of an incomplete d shell and the easier mixing of the 5s with the 4d orbitals to form new hybrid orbitals [
The optimized geometry of admolecule NO with the N-end to the Rh atom of bimetallic PdRh and the molecular axis of NO normal to the surface plane is presented in
Interestingly, the interaction is assumed to mainly be a HOMO-LUMO type [
Recently, several authors were interested in studying the CO-induced modification of the metal-MgO interaction [
The spin transition energies,
NO/Pd2/MgO (O2−) site | NO/ Pd2/MgO (Fs) site | NO/Rh2/MgO (O2−) site | NO/ Rh2/MgO (Fs) site | NO/PdRh/MgO (O2−) site | NO/PdRh/MgO (Fs) site | |
---|---|---|---|---|---|---|
( | 1.061 | 0.801 | 0.216 | 0.694 | 1.565 | 1.923 |
( | 0.14 | 0.27 | 0.06 | 0.04 | 0.427 | 0.35 |
( | 0.021 | 0.041 | 0 | 0.02 | 0.06 | 0.02 |
( | -0.069 | 0.007 | 0.01 | 0.082 | 0.063 | 0.197 |
where low spin multiplicity is (2) for NOPd2, NORh2 and (1) for NOPdRh high spin multiplicity is (4) for NOPd2 , NORh2 and (3) for NOPdRh.
and ON∙RhPd∙MgO (Fs) shows that the transition energy exhibits the largest increase, the interaction of ON∙Pd2∙MgO (Fs)exhibits the largest decrease. Notice that, as expected, there is an inverse correlation between adsorption energy and equilibrium distance, the larger the former the shorter the later, Tables 5-8. In these cases, it is clear that the low-spin state is more favored because of the formation of a direct bond between the adsorbed transition metal dimer and the electronic levels corresponding to the oxygen vacancy electrons. The results show that the magnetic-spin states of transition metals atoms and clusters supported at metal oxide surface and the role of a precursor molecule on the considered magnetic and binding properties need to be explicitly taken into account.
The concept of pairwise and non-pairwise additivity has been studied for atom clusters and insulators [
where every energy term on the right-hand side of Equation (5) is calculated using geometrical parameters corresponding to the equilibrium geometry of S-Mx-NO systems. The left-hand side represents the energy required to separate the three subsystems without altering any change in their geometrical parameters. Such energy can be divided into contributions from three-pairwise components and a non-additive term, εnadd, as follows:
where εnadd is a measure of cooperative interactions among the three subsystems [
The total interaction energies, the pairwise energy components to the S-Mx-NO systems, and the non-additive energy term, εnadd, are presented in
The non additivity term, εnadd, is a measure of cooperative interaction among the subsystems, decreases with surface defect-formation at ON∙Pd∙MgO, ON∙Rh∙MgO, ON∙Pd2∙MgO and ON∙Rh2∙MgO. Except at ON∙PdRh∙MgO complex, εnadd increases with surface defect-formation,
An attempt has been made to understand the effect of surface site on the spin state for the interaction of NO with Pd2, Rh2 and PdRh nanoparticles that supported at regular and defective MgO (001) Surfaces. A spin-polarized treatment is considered to properly describe the ground-state electronic structure, adsorption energies and the low- to high-spin energy transition. The calculated results are compared with experimental data and previous theoretical studies as possible. The geometrical optimizations have been considered to represent the most stable structures for the adsorption of NO at the supported Pd2, Rh2 and PdRh and to investigate the changes induced by the oxide substrate in the chemisorption properties of the adsorbed particles.
Upon interaction with O2− and Fs surface sites, the high to low spin transition energies of Pd atom is positive indicating that the spin states are preserved, and the low spin states are favored. Hence, the number of unpaired electrons in the adatom tends to be the same as in the gas phase and the ground state of Pd-MgO is spin singlet. However, the main contributions to the Rh atom, Pd2, Rh2 and PdRh at MgO are the polarization of the metal electrons induced by the ionic substrate and the small mixing between the s and d orbitals of the transition metal with the 2p orbitals of the surface oxygen. Consequently, the interaction of Rh atom, Pd2, Rh2 and PdRh dimers at MgO (001) surface induces a quenching of the magnetic moment, which results in a doublet ground state for Rh atom and PdRh as well as a singlet ground state for Pd2 and Rh2 at MgO (001) surface. As a consequence, the formation of the dimer in its singlet state, Rh2 and Pd2, and doublet state, Pd-Rh deposited at MgO (001), is
Complex | εnadd | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
O2− | F | O2− | F | O2− | F | O2− | F | O2− | F | |
ON∙Pd∙MgO | −2.749 | −3.888 | −1.238 | −3.188 | −0.061 | −0.092 | −1.229 | −1.128 | −0.221 | 0.519 |
ON∙Rh∙MgO | −4.892 | −4.713 | −1.294 | −2.938 | −0.086 | −0.080 | −2.570 | −2.537 | −0.942 | 0.842 |
ON∙Pd2∙MgO | −5.489 | −6.847 | −3.624 | −5.614 | 0.035 | −0.068 | −3.529 | −2.659 | 1.629 | 1.494 |
ON∙Rh2∙MgO | −5.989 | −7.846 | −3.058 | −4.870 | −0.063 | −0.120 | −2.528 | −2.528 | −0.341 | −0.328 |
ON∙PdRh∙MgO | −5.704 | −7.600 | −3.039 | −4.727 | −0.062 | −0.117 | −2.638 | −2.619 | 0.035 | −0.136 |
favored with respect to the presence of two isolated atoms on the surface. Notice that, as expected, there is an inverse correlation between adsorption energy and equilibrium distance, the larger the former the shorter the later. In any case, the extent of metal-metal bonding in supported dimer has been increased compared with the gas-phase unit. This leads to a considerable elongation of the metal-metal bond to maximize the metal-O interaction. Notice that the dimer as a unit adsorbs much more strongly on the MgO (Fs) site than on the MgO (O2−) site. Moreover, the large enhancement in the activity of supported dimers is due mainly to the electron transfer from the cavity to the supported dimers. Theoretical calculations indicate that the formation of Rh2, Pd2 and PdRh dimer on an Fs center is favored by 0.466, 0.962, and 0.807 eV respectively with respect to a TM atom bound at the Fs center and other TM atom on a terrace site. The dimers deposited interact relatively strongly with the substrate oxide forming predominantly covalent bonds with the adsorbed sites. The interaction is not accompanied by a significant charge transfer at the interface. The PdRh bimetallic has larger HOMO-LUMO gap and is relatively more chemically stable than the Pd2 and Rh2 monometallic that deposited on the MgO (O2−). The transition energy,
In summary, it seems that NO prefers to bound with Rh atoms when both Rh and Pd site co-exist in the Pd- Rh bimetallic. The electronic structures and N-O bond lengths of the chemisorbed systems are similar for NO∙Rh2∙MgO and NO∙PdRh∙MgO with the top geometries but show significant differences from bridge geometries, NO/Pd2/MgO. Bridge-site adsorption causes the N-O bond to lengthen and soften due essentially to increase an electrostatic repulsion between both N and O atoms. In addition, the NO adsorbs much more strongly at the PdRh that is deposited on the MgO (Fs) than on the MgO (O2−) site.
The transfer of electron charge density from such a defect to a dimer reinforces the metal-metal bonds. Therefore, color centers at the MgO surface not only reduce the diffusion of metal atoms and dimers, but also act as stabilizing agents for the whole structure. This point could be particularly important in the context of identifying methods to stabilize the support particles on an oxide substrate under chemical reaction conditions.
To summarize, the larger interaction of NO at bimetallic PdRh at oxygen anions and oxygen vacancies induces an enhancement of the energy required to switch from high spin to low spin 1.565 eV and 1.923 eV respectively. These results show that the spin state of adsorbed PdRh dimer on oxide supports tends to preserve the number of unpaired electrons as found in the case of the regular terrace sites.
My gratitude and deep thanks to Prof. Dr. A.S. Shalabi for his interest, and useful discussions.
S. AbdelAal, (2016) Effect of Surface Site on the Spin State for the Interaction of NO with Pd2, Rh2 and PdRh Nanoparticles Supported at Regular and Defective MgO(001) Surfaces. Open Journal of Physical Chemistry,06,1-20. doi: 10.4236/ojpc.2016.61001