Langmuir 2007, 23, 8891-8898
8891
The Chemistry of Trimethylamine on Ru(001) and O/Ru(001) B. F. Hallac and M. Asscher* Department of Physical Chemistry and the Farkas Center for Light Induced Processes, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel ReceiVed March 27, 2007. In Final Form: May 20, 2007 The interaction and reactivity of trimethylamine (TMA) has been studied over clean and oxygen-covered Ru(001) under UHV conditions, as a model for the chemistry of high-density hydrocarbons on a catalytic surface. The molecule adsorbs intact at surface temperature below 100 K with the nitrogen end directed toward the surface, as indicated from work function change measurements. At coverage less than 0.05 ML (relative to the Ru substrate atoms), TMA fully dissociates upon surface heating, with hydrogen as the only evolving molecule following temperature-programmed reaction/desorption (TPR/TPD). At higher coverage, the parent molecule desorbs, and its desorption peak shifts down from 270 K to 115 K upon completion of the first monolayer, indicating a strong repulsion among neighbor molecules. The dipole moment of an adsorbed TMA molecule has been estimated from work function study to be 1.4 D. Oxygen precoverage on the ruthenium surface has shown efficient reactivity with TMA. It shifts the surface chemistry toward the production of various oxygen-containing stable molecules such as H2CO, CO2, and CO that desorb between 200 and 600 K, respectively. TMA at a coverage of 0.5 ML practically cleans off the surface from its oxygen atoms as a result of TPR up to 1650 K, in contrast to CO oxidation on the O/Ru(001) surface. The overall reactivity of TMA on the oxidized ruthenium surface has been described as a multistep reaction mechanism.
1. Introduction The initial stages of hydrocarbon chemistry on catalytic metal surfaces often involves C-X (X ) C, H, O, or halides) bond scission, resulting in methyl radical or longer-chain fragments residing on the surface. Upon substrate heating, these subsequently react among neighboring fragments to form new molecules or further decompose. In an attempt to better understand the elementary steps of this chemistry, various methyl halides were studied where the methyl radical became the initial adsorbate as a result of parent molecule thermal or photochemical fragmentation.1-8 The chemistry of trimethylamine (TMA-(CH3)3N) on Mo(100) was studied by means of LEED, XPS, AES, and TPD under UHV conditions.9-11 The advantage in using TMA as the source of the methyl fragments is the particularly high density of initial methyl adsorbates on the surface, following parent molecule dissociation. In these studies of TMA, its chemical reactivity was discussed in terms of Lewis acids and Lewis bases on different metal surfaces.9-11 Dissociative adsorption of TMA molecules resulting in its respective fragments was recorded on a clean Mo surface by careful examination of the carbon Auger peak shapes. When the surface became partially passivated by the dissociation products, parent molecule desorption could be detected. A (1) Chinta, S.; Choudhary, T. V.; Daemen, L. L.; Eckert, J.; Goodman, D. W. Angew. Chem., Int. Ed. 2002, 41 (1), 144-146. (2) Dickens, K. A.; Stair, P. C. Langmuir 1998, 14 (6), 1444-1450. (3) Harris, J. J. W.; Fiorin, V.; Campbell, C. T.; King, D. A. J. Phys. Chem. B 2005, 109 (9), 4069-4075. (4) Kim, S. H.; Stair, P. C. J. Phys. Chem. B 2000, 104 (14), 3035-3043. (5) Kis, A.; Kiss, J.; Olasz, D.; Solymosi, F. J. Phys. Chem. B 2002, 106 (20), 5221-5229. (6) Parker, B. R.; Jenkins, J. F.; Stair, P. C. Surf. Sci. 1997, 372 (1-3), 185192. (7) Peng, X. D.; Viswanathan, R.; Smudde, G. H.; Stair, P. C. ReV. Sci. Instrum. 1992, 63 (8), 3930-3935. (8) Smudde, G. H.; Yu, M.; Stair, P. C. J. Am. Chem. Soc. 1993, 115 (5), 1988-1993. (9) Henry, R. M.; Walker, B. W.; Stair, P. C. Surf. Sci. 1985, 155 (2-3), 732-750. (10) Walker, B. W.; Stair, P. C. Surf. Sci. 1980, 91 (2-3), L40-L44. (11) Walker, B. W.; Stair, P. C. Surf. Sci. 1981, 103 (2-3), 315-337.
significant change in reactivity of molybdenum surfaces was noted when the surface was pretreated with oxygen. The surface became less reactive so that TMA is adsorbed in a molecular form and does not decompose anymore upon surface heating during the TPD experiment. In summary, the Mo surface could be significantly passivated by the decomposition products or following incorporation of 0.8 ML or more of oxygen atoms into the surface region. Upon heating the TMA-covered Mo sample, different products were obtained, divided into two classes: (1) diatomic molecules H2, N2, and CO; and (2) the polyatomic molecules CH4 and HCN. The combination of surface spectroscopy techniques with isotopic labeling has led these authors to suggest a sequential dehydrogenation mechanism to describe the reactivity of TMA on the Mo surface.9 The rate-limiting step in the decomposition of TMA was the C-N bond scission. Intact methyl groups adsorbed on the surface were suggested to accumulate as a result of the TMA dissociation. The combination of previous work12,13 and C-Auger line-shape analysis has demonstrated that despite the presence of all the constituent atoms on the surface neither methane nor hydrogen cyanide were observed in TPD. Similar results were obtained from studies of the chemistry of methylamine on Ni(111) using HREELS and TPD.14 The chemistry of methyl bromide on Ru(001) and Cu/Ru(001) has been studied earlier15,16 utilizing ∆p-TPD and ∆ΦTPD techniques, attempting to study the chemistry of methyl on clean and modified Ru(001). Methyl bromide species underwent decomposition, similar to TMA, producing coadsorbed bromine atoms and methyl at coverage below 0.6 ML. The methyl fragments further dissociated at higher crystal temperature. The reactivity at the submonolayer coverage has been monitored by work function change measurements at temperature prior to any desorption. Similar behavior was reported previously in the chemistry of methyl iodide on Ru(001).15,40 (12) Han, H. R.; Schmidt, L. D. J. Phys. Chem. 1971, 75 (2), 227-&. (13) Ignatiev, A.; Jona, F.; Jepsen, D. W.; Marcus, P. M. Surf. Sci. 1975, 49 (1), 189-200. (14) Gardin, D. E.; Somorjai, G. A. J. Phys. Chem. 1992, 96 (23), 9424-9431. (15) Livneh, T.; Asscher, M. J. Phys. Chem. B 1997, 101 (38), 7505-7519. (16) Livneh, T.; Asscher, M. J. Phys. Chem. B 1999, 103 (27), 5665-5674.
10.1021/la700895r CCC: $37.00 © 2007 American Chemical Society Published on Web 07/18/2007
8892 Langmuir, Vol. 23, No. 17, 2007
The original motivation for this study has been the dynamics of energetic colliders with multilayers of TMA-covered Ru(001).17 Initially, however, exploration of the chemistry of the first layers on the ruthenium substrate is necessary. In this paper, therefore, we present the chemistry of the first layers of TMA on the Ru(001) single-crystal surface, studied by a combination of ∆p-TPD and ∆Φ-TPD techniques. TMA was found to adsorb intact on clean Ru(001) at low temperatures of
