Vacuum Ultraviolet Photoionization Studies of PtCH2 and H‐Pt‐CH3: A Potential Energy Surface for the Pt+CH4 Reaction

DOI: 10.1002/anie.200((will be filled in by the editorial staff)) Methane Activation Vacuum Ultraviolet Photoionization Studies of PtCH 2 and H-Pt-CH 3 : A Potential Energy Surface for the Pt + CH 4 Reaction Manori Perera, Ricardo B. Metz,* Oleg Kostko and Musahid Ahmed Activation of methane is a long-standing goal of catalysis, due to its potential industrial utility as an inexpensive hydrocarbon feedstock, and is of interest as the most fundamental C-H bond activation reaction. [1] The observation that several gas-phase third-row transition metal cations M + efficiently dehydrogenate methane at room temperature [2] M + + CH 4 ! MCH 2+ + H 2 has led to extensive gas-phase studies of reactions of metal atoms, ions and clusters with methane.[See [1-3] and references therein] These studies provide valuable information regarding reaction mechanisms, the energetics and structures of intermediates and of critical barriers. These results inform the choice of metal, cluster size, support and ligand properties in a potential catalyst. In addition, they serve as a benchmark against which the accuracy of computational studies can be assessed. Due to its importance as an industrial catalyst, reactions of Pt and Pt + with methane have received particular attention. At thermal energies, Pt + reacts at ~50% of the collision rate, producing PtCH 2+ . [2, 3c, 4] Guided ion beam (GIB) experiments have measured the overall exothermicity of the reaction, as well as the stability of the [H-Pt-CH 3 ] + insertion intermediate. [5] Neutral metal atoms are much less reactive than the cations. This has been attributed to the absence of long-range ion- induced dipole attraction, and the lack of low-lying reactive (low- spin d n or d n-1 s) electronic states. Carroll et al. find that platinum is unique in activating methane at thermal energies. [6] Campbell confirms the reactivity of Pt, at ~3% of the collision rate. [7] These kinetics studies monitor Pt via laser-induced fluorescence, but do not observe products. As a result, it is difficult to determine to what Prof. R.B. Metz Dept. of Chemistry University of Massachusetts Amherst Amherst, MA 01003 USA E-mail: rbmetz@chem.umass.edu Prof. M. Perera Dept. of Chemistry Illinois Wesleyan University Bloomington, IL 61702 USA Dr. O. Kostko, Dr. M. Ahmed Chemical Sciences Division Lawrence Berkeley National Laboratory Berkeley, CA 94720 USA MA and OK gratefully acknowledge support from the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy under contract No. DE-AC02- 05CH11231. MP and RBM gratefully acknowledge the support of the National Science Foundation under Award CHE-0911225. Supporting information for this article is available on the WWW under http://www.angewandte.org. extent loss of Pt is due to true bimolecular reaction, producing PtCH 2 + H 2 , and to what extent it is due to termolecular formation of the H-Pt-CH 3 insertion intermediate, which is stabilized by collisions with the buffer gas. These kinetics studies do not determine the reaction mechanism or provide much information on the potential energy surface. A number of questions remain outstanding. How endothermic is dehydrogenation of methane by atomic platinum? How stable is the insertion intermediate, and is there a barrier to its formation? This study addresses these questions, determining the energetics of H-Pt-CH 3 and PtCH 2 by coupling photoionization measurements of their ionization energy with guided ion beam (GIB) measurements of the thermodynamics of the corresponding cations. [5] In addition, spin-orbit DFT (SODFT) calculations are used to characterize other intermediates and the barriers that separate them. Comparison of the potential energy surfaces for the reactions of Pt and Pt + with CH 4 shows that charge has surprisingly little effect on the potential energy surface for this most fundamental C-H activation reaction. The laser ablation experiments are performed using tunable VUV light accessed at a synchrotron (Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory). The molecules of interest are produced using a pulsed laser ablation source [8] and ionized using tunable VUV light. The resulting ions are extracted into a reflectron time-of-flight mass spectrometer and tallied using a fast counter. Photoionization efficiency curves (PIE) are measured by integrating the area under the photo-ion of interest and normalizing to ALS current and VUV flux. Further details are available in the Supporting Information. Figure 1 shows the PIE of PtCH 2 and H-Pt-CH 3 near the ionization onset. The figures also show simulated PIE spectra. They are calculated by computing the geometries and vibrations of the neutral and cation, and calculating Franck-Condon factors for each vibration. The products of the Franck-Condon factors are then integrated and convolved with the 25 meV VUV Gaussian linewidth to generate the PIE spectrum. [9] The electronic structure calculations are described in more detail below. For these two molecules, there is little geometry change upon ionization, leading to a sharp onset in the PIE. The ionization energy is determined by shifting the energy origin of the simulation until it provides the best match to the PIE spectrum. This procedure determines the ionization energy of PtCH 2 to be 8.78±0.03 eV. The IE(H-Pt-CH 3 ) is slightly higher: 8.89±0.03 eV. These values are very slightly lower than IE(Pt)=8.959 eV. The PIE spectrum clearly shows that the [Pt,C,H 4 ] + ion is formed by photoionization of the H-Pt-CH 3 insertion intermediate and is not due to a Pt(CH 4 ) entrance channel van der Waals complex. First, in an entrance channel complex, CH 4 would interact much more strongly with Pt + than it does with neutral Pt, due to the strong ion-induced dipole force. This would lead to an ionization energy of Pt(CH 4 ) at least 0.5 eV lower than that of Pt. In addition, the large change in geometry between the neutral and cation entrance channel complexes would lead to a very gradual onset in the PIE. Furthermore, the calculations predict that there is no barrier to Pt