Structural Properties of the Bimetallic Cluster Au 12 Ag 6 on the NO x (x=1,2,3) Adsorption Processes
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1217-Y08-32
Structural properties of the bimetallic cluster Au12Ag6 on the NOx (x = 1, 2, 3) adsorption processes B. Molina1, J. J. Castro1, 2, J. R Soto1, E. Yépez1, and A. Calles1 1 Dep. Física, Facultad de Ciencias, Universidad Nacional Autónoma de México, Apdo. Post. 70646, 04510 México D. F., México. 2 Departamento de Física, CINVESTAV del IPN, Apdo. Post. 14-740, 07000 México D. F., México.
ABSTRACT It has been shown that Au and Ag nanoparticles present catalytic properties of pollutant gases, product of the hydrocarbons combustion processes, whose efficiency is strongly dependant on their size, morphology and supporting material. The activity of Ag subnanometric particles supported on alumina in the NOx reduction mechanism, for the reaction HC-SCR (Hydrocarbon Selective Catalytic Reaction) is a good example of these catalytic properties. It is also well established that both Au and Ag nanoparticles are oxygen poisoning resistant and that in certain reactions the bimetallic combination may be more efficient than their isolated parts. In this work we present a DFT calculation of the NOx (x =1, 2, 3) adsorption energies on the Au12Ag6 cluster surface. It is also discussed the possible synergetic effect of the different configurations, forming small Ag islands composed of one to five atoms, on the surface of the bimetallic cluster. The analysis was done using DFT in the ZORA approximation.
INTRODUCTION The Nitrogen oxides (NOx) are an important pollutant component emitted into the atmosphere of large cities. The introduction of diesel and lean-burn engines for environmental and economic purposes has increased the NOx emission due to the inefficiency of the three-way catalysts to reduce this pollutant at high air-to-fuel rates. The selective catalytic reduction (SCR) of NOx is an alternative solution proposed to circumvent this problem, which basically consists in the NOx dissociation and subsequent oxygen removal by a reductant agent. Ag/γ-Al2O3 has been measured to have high catalytic activity in SCR of NOx using hydrocarbons as the reducing agents under lean-burn conditions [1, 2]. There are experimental evidence pointing to the predominant participation of subnanometric Ag clusters in the catalytic processes carried on the Ag/Al2O3 catalyst [3, 4]. From the theoretical point of view, Grönbeck and coworkers have studied from density functional theory (DFT) calculations the adsorption processes of NOx (x =1-3) on Agn (n=1-8) [5] and recently on α-alumina [6]. They found marked odd/even alternations on the adsorption energies in both cases, suggesting a possible high catalyst poisoning during the SCR process. In this work we present DFT calculations in the zeroth order regular approximation (ZORA) of the adsorption energies of the NOx (x= 1-3) on small islands composed by m silver atoms (m =1- 5) grouped on the bimetallic Au12Ag6 hollow cluster surface. We investigate possible synergetic effects [7,8] of this bimetallic configuration for the NOx
adsorption process, and show that they diminish the poisoning o
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