Electronic Origin of Catalytic Nitric Oxide Reduction upon Small Rhodium and Copper Clusters

Abstract

Both rhodium and copper show a catalytic activity for nitric oxide (NO) reduction; however, the reaction mechanisms can be different. Herein, we elucidate the difference in the NO reduction mechanisms between Rh and Cu clusters regarding the electronic structures using DFT computations and small cluster models involving four metal atoms. The computational results show that the dissociative adsorption proceeds on the Rh cluster with the reaction barrier of 33 kcal mol⁻¹. The calculated heat of the reaction is almost zero. On the Cu cluster, the calculated reaction barrier reaches to 78 kcal mol⁻¹ indicating that the dissociative adsorption hardly occurs. Instead of the dissociative adsorption, dimerization of NO initiates the catalytic NO reduction on Cu cluster. The calculated energy barrier for the dimerization is 8 kcal mol⁻¹. The adsorbed NO dimer has a similar stability to co-adsorbed two NO molecules. In contrast, the dimerization hardly occurs on the Rh cluster; the reaction pathway is remarkably endothermic, and a stable adsorbed product is not found. The adsorption structures of NO can explain such differences. On Cu cluster, NO takes bent-nitrosyl conformation that acts as an electron acceptor. On Rh cluster, NO acts as an electron donor having linear-nitrosyl conformation.

Figures

Figure 1.

 The potential energy curves for N-O dissociative adsorption on Rh₄ (open circles) and Cu₄ (filled circles) clusters with representative molecular structures (blue; nitrogen, red: oxygen).

Figure 2.

 The potential energy curves for NO dimerization on Rh₄ (open circles) and Cu₄ clusters (filled circles) with representative molecular structures.

Figure 3.

 The isosurface plots (isovalue = 0.04) of several NBOs for bonding between metal and NO in Cu₄(NO)₂ (a, b) and Rh₄(NO)₂ (d, e, f) at the reactant structure and for N-N bond in Cu₄(NO)₂ (c) at the transition state structure.

Tables

Table 1  . The structural parameters (Å) and bond indices for Rh₄NO and Cu₄NO in reactant, product, and transition state (TS) structures.

	Reactant	TS	Product
Structure
N-O	1.258	1.925	3.000
N-Rh	1.969	1.768	1.794
N-Rh	1.970	1.893	1.812
O-Rh	2.243	2.002	1.835
O-Rh	2.704	2.096	1.941
Bond indices
N-O	1.44	0.55	0.10
N-Rh	0.77	1.35	1.23
N-Rh	0.76	0.90	1.21
O-Rh	0.24	0.48	0.93
O-Rh	0.22	0.45	0.61
q(NO)a	−0.443	−0.872	−1.00
Structure
N-O	1.242	2.300	3.549
N-Cu	1.928	1.908	1.786b
N-Cu	2.483	1.920
O-Cu	2.015	1.769	1.769b
Bond indices
N-O	1.49	0.35	0.08
N-Cu	0.48	0.54	0.63b
N-Cu	0.13	0.58
O-Cu	0.23	0.68	0.63b
q(NO)a	−0.518	−0.988	−2.565

^aNBO gross charge on NO moiety (in e). ^bAveraged value.

Table 2 . The structural parameters (Å and degrees) and bond indices for Rh₄(NO)₂ and Cu₄(NO)₂ in reactant, product, and transition state (TS) or intermediate (Int) structures.

	Reactant	TS/Int	Product
Structure
N-N	3.655	2.000	1.400
N-Rh	1.787	1.938	2.121
N-Rh	1.787	1.938	1.918
N-O	1.158	1.179	1.223
N-O	1.158	1.179	1.226
Rh-N-O	163.6	123.0	122.7
Rh-N-O	163.6	123.0	139.9
Bond indices
N-N	0.06	0.296	1.07
N-Rh	1.24	0.80	0.37
N-Rh	1.24	0.80	0.69
N-O	1.84	1.77	1.55
N-O	1.84	1.77	1.49
q(ONNO)a	0.011	−0.270	−0.397
Structure
N-N	5.000	2.100	1.459
N-Cu	1.844	1.926	1.973
N-Cu	1.906	1.984	1.982
N-O	1.170	1.175	1.216
N-O	1.174	1.174	1.213
Cu-N-O	143.7	131.6	133.6
Cu-N-O	135.4	126.9	137.4
Bond indices
N-N	0.01	0.34	1.04
N-Cu	0.59	0.42	0.34
N-Cu	0.48	0.39	0.30
N-O	1.85	1.82	1.60
N-O	1.84	1.79	1.61
q(ONNO) a	−0.341	−0.419	−0.534

^aNBO gross charge on (NO)₂ moiety (in e).

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