Letter pubs.acs.org/JPCL
CO2 Activation and Methanol Synthesis on Novel Au/TiC and Cu/TiC Catalysts Alba B. Vidal,†,‡ Leticia Feria,§ Jaime Evans,∥ Yoshiro Takahashi,⊥ Ping Liu,† Kenichi Nakamura,⊥ Francesc Illas,§ and José A. Rodriguez*,† †
Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States Centro de Química, Instituto Venezolano de Investigaciones Cientificas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela § Departament de Química Física & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, C/Martí i Franquès 1, 08028 barcelona, Spain ∥ Facultad de Ciencias, Universidad Central de Venezuela, Apartado 20513, Caracas 1020-A, Venezuela ⊥ Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan ‡
ABSTRACT: Small Cu and Au particles in contact with a TiC(001) surface undergo a charge polarization that makes them very active for CO2 activation and the catalytic synthesis of methanol. The binding energy of CO2 on these systems is in the range of 0.6 to 1.1 eV, much larger than those observed on surfaces or nanoparticles of Cu and Au. Thus, in spite of the poor CO2 hydrogenation performance of Cu(111) and Au(111), the Cu/TiC(001) and Au/TiC(001) systems display a catalytic activity for methanol synthesis substantially higher than that of conventional Cu/ZnO catalysts. The turnover frequencies for methanol production on Cu/TiC(001) are 170−500 times much larger than on Cu(111). The present study moves away from the typical approach of using metal/oxide catalysts for the synthesis of methanol via CO2 hydrogenation. This work shows that metal carbides can be excellent supports for enhancing the ability of noble metals to bond and activate CO2. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis
A
synthesis. Metal carbides have unique chemical properties.18 In previous studies, we have found that the deposition of Au and Cu on TiC produces a charge polarization on the admetals that substantially enhances their chemical reactivity toward molecules such as SO219 and O2.20 The bonding and activation of CO2 is a more highly demanding chemical process.5,9 Nevertheless, as we will see below, small Au and Cu clusters in contact with TiC(001) bind CO2 much better than extended surfaces of the metals and produce catalysts that are more efficient for the synthesis of methanol than a standard Cu/ZnO catalyst. First, we will examine the activity of a clean TiC(001) surface for the synthesis of methanol. The clean TiC(001) sample was prepared following a methodology used in previous studies.18,19 Its catalytic activity was tested in a system that combines an ultrahigh-vacuum (UHV) chamber for surface characterization with a reaction cell for catalytic studies.9,21 The catalytic tests were done under the same conditions used in our study of methanol synthesis on Cu/ZnO(0001̅) catalysts {PCO2= 0.049 MPa (0.5 atm); PH2= 0.441 MPa (4.5 atm); T = 500, 525, 550, 575, and 600 K}.9,22 In the bottom part of Figure 1, one can see
mong all greenhouse gases in the atmosphere, carbon dioxide plays a special role due to the magnitude of the emissions generated by human activities involving the combustion of carbonaceous fuels, principally wood, coal, oil, and natural gas.1 CO2 chemistry has become a very attractive area of research, not only because environmental concerns, but also due to the potential use of CO2 as an alternative and economical feedstock.2−5 The recovery of CO2 for its hydrogenation to alcohols or other hydrocarbon compounds is an important approach to recycle the released carbon dioxide.3−5 This is a difficult task due to the challenges associated with the chemical activation of CO2.5 In the area of heterogeneous catalysis, a lot of attention has been focused on the synthesis of methanol through the hydrogenation of carbon dioxide (CO2 + 3H2 → CH3OH + H2O) on metal and metal/ oxide catalysts.6−17 Commercially, methanol is synthesized from syngas (CO−CO2−H2) over Cu-ZnO/Al2O3 catalysts at 493−573 K and 5−10 MPa.3,13 It has been shown that CO2 is the predominant carbon source for methanol under industrial conditions by means of isotope labeling experiments.16 To improve the methanol yield, extensive experimental and theoretical efforts have been devoted to understand the behavior of Cu−ZnO catalysts.3,7−9,13−17 In the continuous search for new catalysts, one may wonder whether metals deposited on supports other than oxides may provide alternative catalysts with increased activity for methanol © 2012 American Chemical Society
Received: July 19, 2012 Accepted: August 2, 2012 Published: August 2, 2012 2275
dx.doi.org/10.1021/jz300989e | J. Phys. Chem. Lett. 2012, 3, 2275−2280
The Journal of Physical Chemistry Letters
Letter
Figure 2. Calculated adsorption geometry for CO2 on TiC(001). Color code: Red (oxygen), light gray (carbon), and blue (titanium).
carbide→CO2 charge transfer.29 The chemisorption of CO2 leads to an elongation of the C−O bonds from 1.17 Å in gas phase to 1.29 Å in the adsorbed molecule. The calculated adsorption energy of CO2 on the TiC(001) surface was −0.62 eV. This value is not large, but in magnitude is still bigger than binding energies (
