Journal of Catalysis 247 (2007) 61–67 www.elsevier.com/locate/jcat Lithium ion induced surface reactivity changes on MgO nanoparticles Thomas Berger a , Johannes Schuh b , Martin Sterrer a,1 , Oliver Diwald a,∗ , Erich Knözinger a a Institute of Materials Chemistry, Vienna University of Technology, Veterinärplatz 1/GA, A-1210 Vienna, Austria b Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/164, A-1060 Vienna, Austria Received 21 June 2006; revised 24 October 2006; accepted 12 January 2007 Available online 14 February 2007 Abstract Aiming at an estimate of the number of chemically active surface defects on MgO nanocrystals, we used Li + doping in conjunction with subsequent thermal annealing. Changes in the surface reactivity of the Li + -doped nanocrystals were monitored by IR and electron paramagnetic resonance spectroscopy using chemisorbed hydrogen, surface trapped electrons, and surface complexed O − 2 as molecular probes. It was found that the admixture of 0.2 at% Li + not only significantly reduces the thermal stability, but also changes all surface spectroscopic features specific to pure MgO. On the basis of the Li + doping effect, the maximum concentration of active surface sites on undoped MgO nanocrystals corresponds to 3% of a nanocrystal monolayer.  2007 Elsevier Inc. All rights reserved. Keywords: Lithium segregation; MgO nanostructures; Surface doping; Oxide nanostructures; Surface defect concentration; Grain coarsening; Spectroscopic surface probes 1. Introduction Insights into electronic structure and chemical surface reac- tivity of polycrystalline materials are important for heteroge- neous catalysis, engineering of nanomaterials, and the defini- tion of new device concepts on the nanoscale. We have shown in previous work that MgO nanoparticles can be produced by chemical vapor deposition and that the choice of the produc- tion parameters sets the average size of the particles [1]. MgO nanoparticles appear to be outstanding building blocks for the construction of functional nanostructures and mesostructures and to serve as suitable model systems for the investigation of surface reactivity on oxides. Pure alkaline earth oxides (AEOs) have been intensively investigated in the fields of surface sci- ence and catalysis [2–11] due to their simple crystal structure and perfect ionicity. Furthermore, the MgO surface can be ef- fectively doped with selected impurities by making use of ther- mally induced impurity segregation into the surface [12,13]. As * Corresponding author. Fax: +43 1 25077 3890. E-mail address: odiwald@mail.zserv.tuwien.ac.at (O. Diwald). 1 Present address: Department of Chemical Physics, Fritz-Haber-Institute of the Max-Planck-Society, Faradayweg 4-6, D-14195 Berlin, Germany. demonstrated for mixed Ca x Mg 1−x O particles, this can lead to unexpected optical and chemical surface properties [14,15]. The motivation for the present investigation has been to elu- cidate how traces of Li impurities affect the surface proper- ties of the MgO nanoparticles. The close ionic radius of Li + (r Li + = 0.76 Å) compared with that of Mg 2+ (r Mg 2+ = 0.72 Å) allows for easy substitutional accommodation within the MgO lattice. After thermal treatment, Li + ions tend to localize in the surface and near-surface region of the MgO-based crys- tallites [16]. They induce defects that are important for het- erogeneous catalysis, where Li-promoted MgO is discussed as a nonredox catalyst for hydrocarbon conversion reactions [17–22]. Li + O − sites are proposed to abstract atomic hydrogen from the hydrocarbon-generating radicals, which then desorb to start a chain propagation reaction in the gas phase. The catalytic activity of the solid exhibits a strong correlation to the amount of removable oxygen that is apparently destabilized by Li + ions [21,23]. Along with the generation of [Li + O − ] centers, an alterna- tive mechanism that involves surface F-centers as active sites has been put forward [24,25]. A direct theoretical comparison of the associated energetics was made recently using electronic structure techniques based on DFT calculations. This provided 0021-9517/$ – see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2007.01.008