The Effect of Gamma-Al2O3Support on the NO Adsorption on Pd4Cluster

Letícia M Prates,Julio C G Correia,Alexandre N M Carauta,José W M Carneiro,Maurício T M Cruz,Wagner B De Almeida,Glaucio B Ferreira

doi:10.5935/0103-5053.20160096

Abstract

The effect of γ-Al2O3 support on the NO adsorption on Pd4 clusters was investigated by means of density functional theory (DFT) calculations. Pd4 adsorbed on γ-Al2O3 (represented by a Al14O24H6 cluster) changes its preferential geometry from tetrahedral to a distorted planar structure. The alumina support promotes a higher dispersion in the palladium catalyst and reduces the NO adsorption energy to -25.6 kcal mol-1 (computed at B3LYP/LANL2DZ/6-311+G(d)), in close agreement with the experimental value of -27.2 kcal mol-1. On the bare planar Pd4 cluster the NO molecule adsorbs in a bridge arrangement, with adsorption energy of -41.2 kcal mol-1. Adsorption on the tetrahedral Pd4 cluster occur preferentially in an atop mode, with adsorption energy of -30.6 kcal mol-1. Charge density analysis show that the electron flux between the NO molecule and Pd4 depends on the adsorption form, with back-donation being stronger in the bridge adsorption mode.

Highlights

Palladium catalysts have found wide appllication in several areas of chemistry.[1,2,3] One of the most common and established use of palladium catalyst is to control automobile exhaust gas emission.[4,5,6] Use of catalytic converter in automobile exhaust is a requirement to reach the recommended limits for emission of toxic byproducts
In a study employing density functional theory (DFT) to compute the adsorption of formaldehyde on palladium supported on γ-Al2O3 we showed that the Pd–Al2O3 interactions result in charge transfer from octahedral aluminum atoms to the deposited Pd cluster.[33]
We computed the adsorption of nitric oxide (NO) on three different Pd4 clusters, planar (Figure 1b), tetrahedral (Figure 1c) and the distorted cluster obtained after optimization of Pd4 over the γ-alumina surface (Figures 1f and 1g)

Summary

Introduction

Palladium catalysts have found wide appllication in several areas of chemistry.[1,2,3] One of the most common and established use of palladium catalyst is to control automobile exhaust gas emission.[4,5,6] Use of catalytic converter in automobile exhaust is a requirement to reach the recommended limits for emission of toxic byproducts. In the vast literature on theoretical studies of NO adsorption on metal surfaces, the catalyst is frequently represented as a single crystal[29] or as a small atomic cluster,[21,22,23,24,27,28] evidence shows that the support. In a study employing density functional theory (DFT) to compute the adsorption of formaldehyde on palladium supported on γ-Al2O3 we showed that the Pd–Al2O3 interactions result in charge transfer from octahedral aluminum atoms to the deposited Pd cluster.[33] The tetrahedral aluminum site has high acididity, acting as electron acceptors, while octahedral aluminum sites act as electron donors.[33] it was possible to establish the electron flux between the γ-Al2O3 support and the metal cluster. The goal was to quantify geometric, electronic and energetic parameters involved in the adsorption of NO on the palladium clusters and evaluate the effect of the support γ-Al2O3 on the adsorption process

Methodology

Results and Discussion

Conclusions