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J . Phys. Chem. 1994, 98, 5104-51 10
Investigations of Graphitic Overlayers Formed from Methane Decomposition on Ru(0001) and Ru(1120) Catalysts with Scanning Tunneling Microscopy and High-Resolution Electron Energy Loss Spectroscopy Ming-Cheng Wu, Qiang Xu, and D. Wayne Goodman’ Department of Chemistry, Texas A&M University, College Station. Texas 77843- 3255 Received: December 13, 1993; In Final Form: February 18, 1994”
A graphitic phase is very unreactive toward hydrogenation and represents a deactivated surface. In the present study, the inactive form of carbon formed from methane decomposition on single-crystal Ru catalysts has been characterized using scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS). The results show that the carbonaceous species formed on Ru surfaces a t temperatures exceeding 800 K is graphitic. On a Ru(0001) surface, a graphitic species initially forms discrete clusters with dimensions 10-15 A in diameter and 2-3 A in apparent height. Only a t temperatures exceeding 1300 K is the hexagonal superstructure of continuous graphite monolayers observed. In contrast, the carbonaceous species from methane decomposition at 800 K nucleates to form large three-dimensional particles of graphite on Ru( 1 130).
I. Introduction The characterization of various carbonaceous deposits on supported transition-metal catalysts from either methane gas or synthesis gas has received much attention in catalytic research. This issue is critical to the understanding of mechanisms of various reactions, such as methane dimerization and Fischer-Tropsh synthesis. An important consideration, which apparently has been of little concern in the work to date, is the phase transition temperature and the associated conditions required for the onset of the “graphitic” carbon phase on metals, such as Nil and Ru.2 This graphitic phase is very unreactive toward hydrogenation The poisoned surface and represents a deactivated usually requires a rigorous oxidation and reduction cycle to restore the clean surface activity. The “carbidic” phase, on the other hand, is very reactive to hydrogenation and represents an active surface.I.2 Recently, we have initiated a study of methane dimerization at low temperatures on single-crystal Ru catalysts using elevated pressure kinetic measurements and surface analytical techniq~es.39~ The low-temperature, two-step approach, suggested by van Santen and co-w~rkers,~-* consists of the activation of methane on transition-metal catalysts to produce carbonaceous intermediates at temperatures between 400 and 700 K and rehydrogenation of these species to higher hydrocarbons at 373 K. This approach effectively circumvents the thermodynamical limitations of the direct conversion of methane to ethane. The objectives of our study are to identify various surface reaction intermediates formed from methane decomposition and to define theoptimum temperature for methane activation. The production of graphitic carbon must be avoided to assure that the reaction cycle is reversible. In a recent article,3 we have shown that methane decomposition on single-crystal Ru surfaces produces three distinct forms of active carbon intermediates in the 400-700 K reaction temperature range. The key precursor responsible for ethane production has been shown to be a vinylidene species.3~~ The present study explores the nature of the inactive carbonaceous species, i.e., graphitic carbon, using scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS). 11. Experimental Details These studies were carried out in a combined elevated-pressure reactor/ultrahirrh vacuum WHV) svstem. described elsewhere.’
* To whom correspondence should be addressed. @
Abstract published in Aduance ACS Abstracts, April 1, 1994.
equipped with HREELS, Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), temperature-programmed desorption (TPD), and facilities for sample heating and cooling. Following cleaning in the surface analytical chamber, the single-crystal Ru catalyst was transferred in-situ into the reaction chamber through a double-stage differentially pumped Teflon sliding seal. The details of crystal cleaning and handling can be found el~ewhere.~ Methane decomposition reactions were carried out in the reaction chamber with 10Torr of methane at a sample temperature of 800 K. For the gas pressure (10 Torr) employed in the present study, the gas temperature ( TcH~)in the vicinity of the heated Ru crystal is equal to the sample temperature. HREELS data then were collected in the surface analytical chamber following the reaction. STM measurements were carried out ex-situ; the Ru crystals were transferred to the sample holder of an STM instrument (Nanoscope 11) after completionof the reaction. STM images then were obtained in the height mode with a tunneling current of 1 nA and a bias voltage of 100 mV applied to the tip. To minimize possible contamination and slow oxidation, all STM measurements were carried out within 6 h after the freshly prepared sample was exposed to the air.
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111. Results
A. STM Results. Shown in Figure 1 and Figure 2 is a series of STM images obtained after the reaction of methane with Ru(0001) a t 800 K and then cooling to room temperature. The reaction conditions used here were PCH,= 10 Torr, T C H=~800 K, and ~ C = H 0.5 ~ h for Figure 1 and P C H= ~ 10 Torr, T C H=~800 K, and ~ C =H 5~h for Figure 2. It will be shown in the HREELS section (1II.B) that methane decomposition at temperatures exceeding 800 K produces no hydrocarbonaceous species; at this temperature all hydrogen has desorbed from the surface. Bright spots in the images thus correspond to carbon islands. The major difference between Figure 1 and Figure 2 then is that the size of the carbon particles appears to be smaller for low methane exposure than for high exposure. The stripes seen in the images arise from monolayer steps on the Ru(0001) surface. For comparison, a STM image and the corresponding three-dimensional surface contour from a clean Ru(0001) surface are shown in Figure 3, where the monolayer steps of the Ru(0001) surface appear somewhat narrower than those observed in Figure 1 and Figure 2. These steps with an average terrace width of -200 A correspond to a misorientation of -0.8’ from the (0001) plane of the Ru crystal.
0022-3654/94/2098-5 104%04.50/0 0 1994 American Chemical Society
Investigations of Graphitic Overlayers Using STM
The Journal of Physical Chemistry, Val. 98, No. 19. 1994 5105
Figure 1. STM images obtained after the reaction of methane with Ru(0001). The range of the gray scale is (a) 0-40,(b) 0-30, ( c ) 0-30, and (d) W20 A. The reaction conditions used werc PCH.= I O Torr, T c b = 800 K,and IC". = 0.5 h.
At lower resolution, such as that shown in Figures la+ and 2a+, one finds that the carbon islands attach to the step edges of the surface, covering a portion of the lower terrace. At higher resolution (Figures Id and 2d), one observes that these carbon islands actually contain manysmallclusters. Thesesmall clusters are 1&15 A in diameter and -2-3 A in apparent height and are evenly distributed inside the carbon patches. Displayed in Figure4isa 1500. X 1500-ASTMpicturewiththecorresponding surface contour. The apparent height of the carbon overlayers shown in Figure 4h does not exceed the height of the step edges of the Ru surface, demonstrating the two-dimensional growth character of carbon on Ru(OOO1). Figure Sa shows the corresponding low-energy electron diffraction (LEED) pattern obtained following a methane decomposition reaction at 800 K for 5 h. Besides the sharp, intense diffractionspots from the Ru(0001) substrate, extradiffuse spots are seen; the distance of these extra spots to the center of the LEED screen is slightly longer than that of the sharp spots. Considering the lattice constant of Ru(OOO1) (2.71 A) and the basal plane of graphite (2.46 A, the second-nearest-neighbor distance), theseextra spotsare attributed to thosefrom thecarbon overlayers exhibitinglatticecharacter resembling the basal plane ofgraphite. In the followingsection, theHREELS measurements
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further confirm that the carbonaceous overlayers formed under high-temperature (>SO0 K) conditions are graphitic. Upon annealing the above graphitic overlayers in UHV to higher temperatures,thediffraction spotsof theoverlayershecame sharper and, at 1200 K, evolved into fragmented arches surrounding the substrate diffraction spots. Finally, a sharp (1 1 X 1I ) LEED pattern appeared as the sample was annealed to 1300 K, as shown in Figure 5b. Shown in Figure 6 are two correspondingSTM images obtained after heating the crystal to 1300Kandcoolingtoroomtemperature. Theseimagesrepresent only a portion of the surface covered hy graphite. The carbon overlayersare seen to organize into ordered hexagonalstructures, covering theentireareaoftheterraces. Onenotices that thestep edges become rough compared to those shown in Figures 1 and 2. It should he noted that the hexagonal structure observed in these images has a periodicity of 30.0 f 0.3 A and thus does not represent the actual graphite lattice, but a moird superstructure pattern produced by a higher order commensurability of the graphiteand the Ru(0001) lattice. Thesuperstructure observed in the STM images represents the real-space lattice of the (1 1 X 11) LEED pattern. (See further discussion in section IV). In contrast to graphitic carbon on Ru(0001), methane decomposition on Ru(l120) produces large three-dimensional
5106 The Journal of Physical Chemistry, Vol. 98, No. 19, 1994
Wu et al
images obtained after the reaction of methane with Ru(0001). The reaction conditions used were PCH. = IO Torr. TCH,= 800 K, h. The range of the gray scale is (a) 0-40,(b) &30. ( c ) G30,and (d) G20 A.
Figure 2. STM
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~ C H
