CO Disproportionation on a Nanosized Iron Cluster Giorgio Lanzani, † Albert G. Nasibulin, ‡ Kari Laasonen,* ,† and Esko I. Kauppinen* ,‡ Department of Chemistry, UniVersity of Oulu, P.O. Box 3000, FIN-90014 Finland, and Department of Applied Physics and Center for New Materials, Helsinki UniVersity of Technology, P.O. Box 5100, FIN-02150 Espoo, Finland ReceiVed: May 5, 2009; ReVised Manuscript ReceiVed: June 16, 2009 First-principles electronic structure calculations, fully incorporating the effects of spin polarization and noncollinear magnetic moments, have been used to investigate CO disproportionation on an isolated Fe cluster. After CO dissociation, which occurs on a vertex between the facets, O atoms remain on the surface while C atoms move into the cluster as the initial step toward carbide formation. The lowest CO dissociation barrier found (0.77 eV) is lower than that on most of the studied Fe surfaces. Several possible paths for CO 2 formation were identified. The lowest reaction barrier was 1.08 eV. The carbon nanotubes (CNTs) are of great interest since they exhibit unique and useful chemical and physical properties related to toughness, electrical/thermal conductivity, and mag- netism. 1 The chemical vapor deposition (CVD) methods are widely used as a CNTs synthesis method since they open a way to highly controlled and continuous CNT production. 2-5 In these processes, metal nanoparticles are produced in a mixed flow of carbon precursors and other gases (e.g., hydrogen), and the growth process is driven by cleaving the carbon atoms from the precursors, and these atoms will form carbon structures on the nanoparticles’ surface. All properties, like diameter and chirality, of the nanotube are determined by the metal particle. In addition to the CNT synthesis, the metal nanoclusters with a size of less than 10 nm have attracted a great deal of attention due to their applications in magnetism, 6 electronics, 7 and catalysts. 8 The metal nanoparticles are widely used in several real-world catalytic applications, including the car exhaust catalysts, where reactions happen on 3-8 nm size Pt group metal particles. Often, the good catalytic activity can be related to catalytic sites, like atomic size steps, on the cluster. Due to the high curvature of the clusters, the special site density and distribution is much higher than that on almost flat surfaces. Furthermore, the real nanoclusters have several unique active sites like facets and vertexes between the facets, which can have catalytic properties that differ drastically from the ones of almost flat surfaces. The research related to the active sites is mainly limited to atomic steps, and the nanosized clusters have received much less attention. 9 Experimentally, many investigators have studied metal nanostructures using a wide range of surface science techniques, but these studies were done with rather a arbitrary size of clusters because it is very difficult to prepare fixed size clusters. 9 If we want to understand the active sites on clusters, we have to know which cluster we are studying. Even then, each cluster has several different active sites, and it is very difficult to know which of them is the most active one. For this reason, the computational approach, where precise sites can be studied, is very attractive. The present study is addressing the CO disproportionation CO (g) + CO (g) h CO 2(g) + C (s) on an iron nanocluster during the CVD method for the synthesis of single-walled carbon nanotubes (SWCNTs). 2,10,11 The chemistry on the surface of SWCNT catalyst transition-metal nanoparticles is largely un- known, but it is believed that the adsorbed CO first dissociates, CO (s) f C (s) + O (s) , and the O will react with an undissociated CO (s) to form surface CO 2(s) (Figure 1). 12,13 In order to understand the role of the cluster in this reaction, we have used first-principle calculations to study the steps of this reaction on a 55 atom nanocluster. There are only a few ab initio studies of nanosized clusters, 14,15 but these works do not address any chemical reactions. The computational work is combined with experimental investigations of the same reaction on larger nanoclusters. A gas-phase process of SWCNTs formation, based on thermal decomposition of ferrocene in the presence of carbon monoxide (CO), was investigated in ambient pressure laminar flow reactors in the temperature range of 600-1300 °C. 10 In situ sampling carried out at 1000 °C showed that the SWCNT’s growth occurred from individual metal particles in the heating section of the furnace in the temperature range of 891-928 °C, in which the growth rate was estimated to exceed 2 μm/s. 16 Kinetic investigations of the CO disproportionation reaction were performed in a horizontal quartz tube at a heating rate of 5 °C/ min from room temperature. A silica substrate with deposited 15 nm sized iron particles was placed inside of the tube. Investigations show appreciable reaction rates in the temperature interval from 470 to 820 °C, with a maximum rate at about 625 °C. The region of the CO 2 concentration increase from about 325 to about 600 °C is the kinetic region where the rate of the CO disproportionation reaction can be measured. By plotting the kinetic region in the coordinates of ln X CO2 versus 1/T, one gets an Arrhenius dependence, X CO2 ) k 0 exp(-E a /RT), where X CO2 is the carbon dioxide mole fraction, k 0 ) 7.16091, and E a * To whom correspondence should be addressed. E-mail: Kari.Laasonen@ oulu.fi (K.L); Esko.Kauppinen@hut.fi (E.I.K.). † University of Oulu. ‡ Helsinki University of Technology. 12939 10.1021/jp904200e CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009 2009, 113, 12939–12942 Downloaded by AALTO UNIV on August 3, 2009 Published on July 1, 2009 on http://pubs.acs.org | doi: 10.1021/jp904200e