A publication of CHEMICAL ENGINEERING TRANSACTIONS VOL. 34, 2013 The Italian Association of Chemical Engineering Online at: www.aidic.it/cet Guest Editors: Neven Duić, Petar Sabev Varbanov Copyright © 2013, AIDIC Servizi S.r.l., I SBN 978-88-95608-25-9; I SSN 1974-9791 DOI: 10.3303/CET1334004 Please cite this article as: Andoni A., van Santen R. A., 2013, Propenal hydrogenation on silver surface-a theoretical approach, Chemical Engineering Transactions, 34, 19-24 DOI:10.3303/CET1334004 19 Propenal Hydrogenation on Silver Surface - A Theoretical Approach Adelaida Andoni* ,a , Rutger A. van Santen b a Department of Chemistry, Faculty of Natural Sciences, University of Tirana, Blv. “Zog I”, 1001, Tirana, Albania b Department of Inorganic Chemistry and Catalysis, Schuit Institute of Catalysis, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands adelaida.andoni@unitir.edu.al Catalytic reactions perform best if the interaction between the adsorbates and the surface is not too strong and not too weak. This relates to the d-band occupancy of the metals. In this respect, hydrogenation of propenal or acrolein on silver surface rises up the question whether H 2 proceeds in the mechanism of the reaction as molecule or dissociates on silver. Silver having an electronic configuration 4d 10 5s 1 cannot dissociate the strong bond of H 2 . However, it is known that even on gold provided the gold particles are very small molecules such as O 2 can dissociatively chemisorb. Aiming elucidation of the reaction mechanism, small clusters of silver atoms are considered (mimicking small particles) by checking first their stability by means of theoretical calculations. Next, adsorption of hydrogen was allowed in order to estimate whether the adsorption energy is exothermic or not. In this context, computational calculations carried out by means of Density Functional Theory for small silver clusters indicated that rhombic structure was the most stable configuration. Adsorption of hydrogen on rhombic silver cluster yielded negative value of the adsorption energy suggesting that hydrogen can dissociatively be adsorbed on small particle and step surfaces as well. 1. Introduction Catalysts may make significant difference in reaction rate. For example, a hydrogen/oxygen mixture may be stable for years at 25 °C. Nevertheless, if one introduces a platinum wire into the mixture, the mixture explodes. Another introductory example is the key reaction in the cleaning automotive exhaust, the catalytic oxidation of CO to CO 2 on the surface of noble metals such as Pt, Pd, Rh and Au-Cu (Liu et al. 2011) as well as over FeOx-supported Pd catalyst (Liu et al. 2012). Reduction of CO 2 , on the other hand, remains challenging as recently reported (Siitonen and Ahtila, 2009). Selective catalytic hydrogenation of organic α-β unsaturated aldehydes (for example, propenal or acrolein) is a reaction with important applications in the industrial field (Bron et al. 2007). It gives the following types of products (Figure 1). The most important product from an industrial point of view is the unsaturated alcohol, which is also the most difficult to obtain since it is known that the hydrogenation of the C=C group in α-β unsaturated aldehydes is more favourable than the hydrogenation of C=O group (Bron et al. 2007). To improve the selectivity towards the unsaturated alcohol several attempts are made to develop a catalytic system suitable for this reaction (Bron et al. 2007). It is known that monometallic silver is able to hydrogenate selectively the C=O group of α-β unsaturated aldehydes in the gas phase (silver having a low value of the heat of adsorption for hydrogen can preferentially hydrogenate the C=O group because strongly adsorbed hydrogen reacts more readily with the C=C group than weakly adsorbed hydrogen). It is assumed that the first step of the reaction is the dissociation of hydrogen on silver and the second one is the hydrogenation of double bonds C=C and C=O (Bron et al. 2007). Nevertheless from catalysis point of view, as we go from the left to the right in a row of the periodic table the tendency in the reactivity of metals changes as shown in Figure 2 (Chokendorf and Niemantsverdriet, 2003).