A Photoemission Study of Ethylene Decomposition on a Co(0001

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A Photoemission Study of Ethylene Decomposition on a Co(0001) Surface: Formation of Different Types of Carbon Species Lingshun Xu,†,‡,§ Yunsheng Ma,§ Zongfang Wu,†,‡,§ Bohao Chen,†,‡,§ Qing Yuan,†,‡,§ and Weixin Huang*,†,‡,§ †

Hefei National Laboratory for Physical Sciences at the Microscale, ‡CAS Key Laboratory of Materials for Energy Conversion, and Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, China

§

ABSTRACT: We have studied the interaction of ethylene on Co(0001) in detail by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). At 130 K, the chemisorption and decomposition of C2H4 were found to depend on the vacant sites available on Co(0001). C2H4 chemisorbs dissociatively at low exposures but both dissociatively and molecularly at large exposures. Upon heating, some C2H4(a) desorbs molecularly from the surface, releasing vacant surface sites that result in the simultaneous decomposition of other C2H4(a). C2H2(a) is the surface intermediate for the decomposition of C2H4(a) on Co(0001). At elevated temperatures, C2H2(a) simultaneously undergoes the direct dehydrogenation and the cyclopolymerization−dehydrogenation to form surface C2 cluster and graphitic carbon on Co(0001). At 500 K, C2H4 directly decomposes on Co(0001), forming surface atomic carbon. These results provide novel information on the chemisorption and decomposition of C2H4 on Co(0001) and the nature of resulted carbon species, greatly deepening our fundamental understanding of the relevant heterogeneous catalytic reactions catalyzed by Co-based catalysts and the growth of graphene on Co surfaces by chemical vapor deposition.

1. INTRODUCTION The interaction of ethylene (C2H4) with the model surface of transition metals has attracted extensive studies in the past decades for the fundamental understanding of important heterogeneous catalytic reactions such as F-T synthesis and methane conversion.1−25 On transition metal surfaces, C2H4 can adsorb molecularly with either di-σ-bonded or π-bonded structures at temperatures as low as 80 K. Upon heating, in addition to the molecular desorption, chemisorbed ethylene dissociates via different reaction pathways that depend both on the type of the metal and on the surface structure of the metal. Ethylidyne (CCH3) was generally observed as the surface intermediate for the decomposition of C2H4(a) on the closed packed surface of each 4d and 5d groups 8−10 metal including Pt(111),7,20 Rh(111),21 Pd(111),22 and Ir(111).16 More deeply dehydrogenated surface intermediates (acetylene and acetylide) were formed when C2H4(a) decomposed on other surfaces of transition metals, for example, the decomposition of C2H4(a) formed acetylide (CCH) on Ru(001)14 and Rh(100)13 and acetylene (CHCH) on W(100),8 Ni(111),23,24 Fe(110),18 and Pt(100).25 Ethylene decomposition eventually forms carbon deposits on the transition metal surface whose type and structure have been demonstrated to modify its catalytic performance.26−28 Recently, graphene growth on transition metal surfaces by means of chemical vapor deposition (CVD) has greatly reinspired the interest in the ethylene−transition metal interaction since C2H4 has been often used as the precursors in the chemical vapor deposition (CVD) method.29−33 It was shown that carbon nanoislands forming in the © 2012 American Chemical Society

initial stages of graphene growth possess an exclusive size of seven honeycomb carbon units (7C6) via temperatureprogrammed reaction of C2H4 on Rh(111).30 Thermal dehydrogenation following saturation exposure of ethylene on Pt(111) was reported to form well-defined carbon clusters.31 The growth temperature and the C2H4 exposure were observed to control the proportion of different rotational graphene domains and the quality of graphene on Pt(111).32 Cobalt-based catalysts are widely used in F-T synthesis34 and methane conversion,35 in which hydrocarbons including ethylene are products. Co(0001) has a hcp structure, and its surface lattice matches that of graphene very well (99.9999%, Arkonic Gases & Chemicals Inc.), O2 (>99.99%, Nanjing ShangYuan Industry Factory), and H2 (>99.999%, Nanjing ShangYuan Industry Factory) were used as received, and their purity was further checked by quadrupole mass spectrometer (QMS) prior to experiments. The base pressure of the chamber during the course of C2H4 exposure was controlled to be below 5 × 10−10 Torr; therefore, a line-ofsight stainless steel doser (diameter: 8 mm) positioned ∼2 mm in front of the Co(0001) surface was used for relatively large C2H4 exposures (herein ≥0.1 langmuir). The doser could be retracted 50 mm after the exposure. The enhancement factor of the doser was calibrated to be 920 by comparing the H2O desorption peak areas of H2O TDS spectra followed exposures of 0.005 langmuir of H2O at 130 K by backfilling and by the doser.39 The exposures of C2H4 reported herein were corrected with the enhancement effect of the doser. Other gases were dosed by backfilling. All exposures were reported in langmuirs (1 langmuir = 1.0 × 10−6 Torr·s) without corrections for the gauge sensitivity. During the TDS experiments, the Co(0001) surface was positioned ∼1 mm away from the collecting tube of a differential-pumped QMS and heated to 630 K with a heating rate of 3 K/s. The signals with m/e = 2 (H2), 18 (H2O), 25 (C2H2 and C2H4), 26 (C2H2 and C2H4), 27 (C2H4), 28 (C2H4 and CO), 30 (C2H6), and 44 (CO2) were monitored. XPS spectra were recorded using Al Kα radiation (hν = 1486.6 eV) with a pass energy of 20 eV. UPS spectra were obtained with a pass energy of 5 eV using He II radiation (hν = 40.8 eV), but the work function data were calculated from the He I UPS spectra (hν = 21.2 eV) by measuring the gap between the Fermi edge and the secondary electron edge.

investigated the thermal decomposition behavior of ethylene chemisorbed on Co(0001). On the basis of X-ray photoelectron spectroscopy (XPS) and UPS results, it was proposed that ethylene decomposed completely to C and H via acetylene intermediate below room temperature. Therefore, the authors proposed that C2H4(a) might occupy the on-top site in which each of the hydrogen atoms interacted with a unique adjacent surface cobalt atom, favoring simultaneous scissions of the four C−H bonds in C2H4(a). In our previous paper,37 we have reported the interaction of ethylene with Co(0001) mainly by means of thermal desorption spectroscopy (TDS), in which the chemisorption and decomposition behaviors of C2H4 were found to depend largely on the vacant sites available on Co(0001) at 130 K. C2H4 chemisorbs dissociatively to produce C2H2(a) and H(a) at low exposures but both molecularly and dissociatively at larger exposures. Upon heating, some C2H4(a) desorb molecularly, releasing vacant surface sites that result in the simultaneous decomposition of other C2H4(a). The dehydrogenation reaction completes prior to 500 K to form carbon deposits on the surface. In the present paper, we have performed a detailed XPS and UPS investigation of C2H4 on Co(0001). The surface intermediates of C2H4(a) decomposition have been clearly indentified, and the nature of formed carbon deposits has also been clarified. It was also found that different types of carbon deposits on Co(0001) could be prepared by ethylene decomposition on Co(0001) at different temperatures. These results deepen our fundamental understanding of hydrocarbon-involved heterogeneous catalytic reactions catalyzed by Co-based catalysts and graphene growth on Co(0001) employing C2H4 as the carbon source.

2. EXPERIMENTAL SECTION All experiments were performed in a Leybold stainless-steel ultrahigh-vacuum (UHV) chamber with a base pressure of 1.2 × 10−10 mbar.38 The UHV chamber was equipped with facilities for XPS, UPS, LEED, and differential-pumped TDS measurements, in which a new hemispherical energy analyzer (PHBIOS 100 MCD, SPECS GmbH), X-ray source (XR 50, SPECS GmbH), and UV source (UVS 10/35, SPECS GmbH) were recently installed. A Co(0001) single crystal purchased from MaTeck was mounted on the sample holder by two Ta wires spot-welded to the back side of the sample. The sample temperature could be controlled between 120 and 1473 K and was measured by a chromel−alumel thermocouple spot-welded to the back side of the sample. Prior to the experiments, the

3. RESULTS AND DISCUSSION Figure 1 shows C2H4 and H2 TDS profiles after various exposures of C2H4 on clean Co(0001) at 130 K. In agreement with our previous results,37 after a low C2H4 exposure of 0.0005 langmuir, H2 is the only desorption product and gives rise to two adjacent desorption peaks centering at ∼380 and ∼414 K. With the increase of the exposure to 0.01 langmuir of C2H4, a very weak C2H4 molecular desorption feature appears at ∼200 4168

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Figure 2. LEED patterns of Co(0001) surface exposed to various exposures of C2H4 at 130 K and Co(0001) surface exposed to 1.8 langmuirs of C2H4 at 130 K followed by annealing at different temperatures. Ep = 85 eV.

The chemisorption and surface reaction of C2H4 on Co(0001) were then investigated in detail by XPS. Figure 3

K and the H2 desorption features grow. The H2 desorption peak at ∼414 K does not shift but that at lower temperature shifts downward. After the saturated C2H4 exposure (1.8 langmuirs), two C2H4 desorption peaks were observed at ∼200 and ∼335 K, and the H2 desorption peaks further grow. These results can be well explained by the vacant-site-dependent chemisorption of C2H4 on Co(0001).37 C2H4 chemisorbs dissociatively to produce C2H2(a) and H(a) at low exposures but both molecularly and dissociatively at larger exposures. Upon heating, some C2H4(a) desorb molecularly at ∼200 K, releasing vacant surface sites that result in the simultaneous decomposition of other C2H4(a). The C2H4 desorption peaks at ∼335 K come from the hydrogenation of likely C2H3(a) species. H(a) recombinatively desorbs, giving the H 2 desorption feature at low temperatures, and C2H2(a) further dehydrogenates to carbon deposits, giving the H2 desorption feature at ∼414 K. The adsorption of C2H4 on Co(0001) at 130 K does not form any ordered structures, as revealed by LEED observations (Figure 2). However, annealing Co(0001) exposed to 1.8 langmuirs of C2H4 at 130 to 230 K induces the appearance of p(2 × 2) superstructure in the LEED pattern that arises from the formation of ordered hydrogen layer on Co(0001).40 This indicates that the simultaneous decomposition of some C2H4(a) on vacant surface sites released by molecular desorption of other C2H4(a) forms an ordered hydrogen layer on Co(0001). The ordered superstructure disappears in LEED after annealed at 350 K, which corresponds well to the recombinative H2 desorption peak of H(a). After annealed at 630 K, a diffuse ringlike superstructure is visible in addition to the substrate spots in the LEED pattern, implying the formation of adsorbate structure ordered in short-range but disordered in long-range. The lattice constant mismatch between graphitic carbon deposit and the metal substrate usually results in the formation of such a ring LEED pattern,41 and the ring LEED pattern observed after annealing C2H4(a)covered Pt(111) to 1000 K was revealed to arise from the formation of statistical distributed graphitic islands by STM.32 Therefore, the ring LEED pattern observed in our case arises from the carbon deposit formed on Co(0001) by the complete dehydrogenation of C2H2(a) surface intermediate.

Figure 3. C 1s XPS spectra of clean Co(0001) surface (a), Co(0001) surfaces exposed to 0.0005 (b), 0.01 (c), and 1.8 langmuirs of C2H4 (d) at 130 K and Co(0001) surface exposed to 1 langmuir of C2H2 at 130 K (e).

shows C 1s XPS spectra after various exposures of C2H4 to Co(0001) at 130 K. Only a single C 1s feature peak with its binding energy at 283.6 eV was observed after an exposure of 0.0005 langmuir of C2H4 at 130 K. For comparison, we also measured the C 1s XPS spectrum after Co(0001) was exposed to C2H2 at 130 K, giving a single C 1s peak at 283.5 eV (curve e in Figure 3), in consistence with previous study.42 Therefore, the C 1s feature at 283.6 eV after 0.0005 langmuir C2H4 exposure on Co(0001) at 130 K could be assigned to C2H2(a), indicating the decomposition of C2H4 into C2H2(a) and H(a) at low exposures. In addition to the C2H2(a) feature, another shoulder appears with its C 1s binding energy at 284.6 eV after the C2H4 exposure was increased to 0.01 langmuir. At this exposure, molecular C2H4 desorption peak was observed in the TDS spectrum. Thus, the C 1s feature at 284.6 eV could be reasonably assigned to molecularly chemisorbed C2H4(a) on 4169

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Figure 4. (A) C 1s XPS spectra after the exposures of 0.0005 langmuir of C2H4 on Co(0001) at 130 K followed by annealing at different temperatures. Scatter points and solid lines represent the original data and peak-fitting results, respectively. (B) The integrated XPS peak area of various surface species as a function of annealing temperature. Noting that the square filled with black and red colors represents the integrated XPS peak area of both C2H2(a) and C2 cluster at 400 K.

Figure 5. (A) C 1s XPS spectra after the exposures of 1.8 langmuirs of C2H4 on Co(0001) at 130 K followed by annealing at different temperatures. Scatter points and solid lines represent the original data and peak-fitting results, respectively. (B) The integrated XPS peak area of various surface species as a function of annealing temperature. Noting that the circles filled with red and blue colors represent the integrated XPS peak area of both C2H2(a) and C2 cluster at 400 and 430 K.

by C2H4 chemisorption at 130 K was estimated to be ∼0.3 ML, in which C2H2(a) amounts to ∼0.1 ML. Figure 4A displays the C 1s XPS spectra after Co(0001) was exposed to 0.0005 langmuir of C2H4 at 130 K and subsequently annealed at higher temperatures. A single C 1s feature peak with its binding energy at 283.6 eV appears in the C 1s XPS spectrum after an exposure of 0.0005 langmuir of C2H4 at 130 K, corresponding to C2H2(a) on Co(0001). The C 1s peak intensity (Figure 4B) does not change upon annealing up to 370 K, suggesting that C2H2(a) on Co(0001) is stable on the surface up to this temperature. This supports our argument that the H2 desorption peak at low temperatures in the H2-TDS

Co(0001). This is also in consistence of C 1s binding energy of C2H4(a) on other transition metals.43−45 The C2H4(a) feature dominates the C 1s XPS spectrum after the saturating C2H4 exposure of 1.8 langmuirs at 130 K. These XPS results support the vacant-site-dependent chemisorption of C 2 H 4 on Co(0001):37 C2H4 chemisorbs dissociatively to produce C2H2(a) and H(a) at low exposures but both molecularly and dissociatively at larger exposures. By comparing the C 1s XPS peak intensity after the saturating C2H4 exposure at 130 K with that after the saturating CO exposure at 130 K and assuming the saturating CO(a) coverage to be 0.58 ML,46 the saturated C2H4(a) + C2H2(a) coverage on Co(0001) prepared 4170

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spectra arise from the recombinative desorption of H(a). The C 1s peak position initially does not shift and then shifts slightly downward by ∼0.2 eV after annealing at 370 K, which could be associated with the desorption of coadsorbed H(a) from the surface. After annealed at 400 K, the C 1s XPS peak broadens and can be well fitted with two components with the C 1s binding energy at 283.2 and 284.3 eV. The H2-TDS spectrum (Figure 1) shows that the dehydrogenation reaction of C2H2(a) occurs at this temperature. With the further increasing of annealing temperature, the C 1s component at 283.2 eV slightly shifts downward to 283 eV while that at 284.3 eV shifts upward to 284.8 eV, but their peak intensities do not change much and the peak intensity ratio between the feature at 283 eV and that at 284.8 eV is about 3 (Figure 4B). Since C2H2(a) undergoes complete dehydrogenation reaction below 500 K (Figure 1), these two C 1s features indicates that two types of carbon species are formed by the dehydrogenation reaction of C2H2(a) on Co(0001), one with the C 1s binding energy at 283 eV and the other at 284.8 eV. Inferred from the H2-TDS result and the C 1s XPS spectra, we considered that the component at 283.2 eV appeared in the C 1s XPS spectrum after annealed at 400 K should arise from both the carbon species at 283 eV and the remaining C2H2(a) on the surface which can not be resolved by our XPS. The annealing process of Co(0001) exposed to 1.8 langmuirs of C2H4 at 130 K was also studied by XPS (Figure 5A). After the saturating exposure of C2H4 at 130 K, the C 1s XPS spectrum consists of two components at 284.5 and 283.5 eV, corresponding to C2H4(a) and C2H2(a) on Co(0001), respectively. As shown in Figure 5B, the C2H4(a) feature continuously attenuates upon heating and disappears after annealed at 220 K; meanwhile, the C2H2(a) feature keeps growing, but the total peak area keeps decreasing. These observation again supports the vacant-site-dependent chemisorption of C2H4 on Co(0001):37 upon heating, some C2H4(a) on Co(0001) molecularly desorb from the surface, releasing vacant surface sites that result in the simultaneous decomposition of other C2H4(a). The surface after annealed at 220 K is covered with C2H2(a) and H(a), and thus evolution of C 1s spectrum in the following annealing process at higher temperatures is similar to the case of low C2H4 exposure (Figure 4). The dehydrogenation reaction of C2H2(a) occurs around 400 K, giving two C 1s components with the binding energy at 283.4 and 284.5 eV; and after annealing at 630 K, two types of carbon species remain on the surface with the C 1s binding energy at 283 and 284.7 eV. Inferred from the H2-TDS result and the C 1s XPS spectra, we considered that the component at 283.4 eV appeared in the C 1s XPS spectrum after annealing at 400 and 430 K might consist of both the carbon species at 283 eV and the remaining C2H2(a) on the surface whose C 1s binding energies could not be resolved by our XPS. The shift of C 1s binding energy to 283 eV after annealing at higher temperatures indicates the complete dehydrogenation of C2H2(a). Compared to the total C 1s peak intensity of the spectrum after the C2H4(a) exposure at 130 K, it was estimated that about 42% of C2H4(a) on Co(0001) eventually decomposes to carbon deposits. The annealing process of Co(0001) exposed to 1.8 langmuirs of C2H4 at 130 K was also studied by He II UPS (Figure 6). The clean Co(0001) surface displays a narrow d-band feature until 4 eV below Fermi level (EF). After the exposure of 1.8 langmuirs of C2H4 at 130 K, several new features appear in the UPS spectrum which could be divided into two groups

Figure 6. He II UPS spectra after the exposure of 1.8 langmuirs of C2H4 on Co(0001) at 130 K followed by annealing at different temperatures.

according to their evolution upon the subsequent heating process. One group includes three main peaks at 6.5, 8.2, and 13.0 eV below EF and a shoulder at 9.3 eV below EF. These four features attenuate upon heating and disappear after annealing at 220 K. The other group includes three minor features at 5.4, 8.9, and 11.3 eV below EF that grow upon the heating up to 220 K. According to above XPS results, the four peaks at 6.5, 8.2, 9.3, and 13.0 eV below EF arise from C2H4(a) on Co(0001) and can be assigned to the 1b2g, 3ag, 1b3u, and 2au orbitals of C2H4(a), respectively.10 The three features at 5.4, 8.9, and 11.3 eV below EF arise from C2H2(a) and correspond to the 1π, 3σg, and 2σu orbitals of C2H2(a) on Co(0001), respectively.10 After annealing at 220 K, the Co(0001) surface is covered with C2H2(a) and H(a). The UPS spectrum does not change much up to annealing at 370 K. Upon further heating up to at 430 K, the dehydrogenation reaction of C2H2(a) on Co(0001) occurs, leading to the attenuation and disappearance of C2H2(a)related features at 8.9 and 11.2 eV below EF in the UPS spectra, but interestingly, the feature at 5.4 eV below EF remains and slightly shifts to 5.3 eV in the UPS spectra, and a weak and broad peak centering at 6.9 eV emerges. These two features are still well resolved in the UPS spectrum even after the surface was annealed at 630 K and thus can only to assigned to carbon deposits on Co(0001) formed by the dehydrogenation of C2H2(a). Compared to above XPS results, these two features should correspond to two types of carbon species with the C 1s binding energy at 283.0 and 284.7 eV. The change of work function was also monitored when Co(0001) was exposed to 1.8 langmuirs of C2H4 at 130 K followed by annealing at higher temperatures (Figure 7). The chemisorption of C2H4 after the saturating exposure decreases the work function of Co(0001) by ∼0.52 eV, consistent with the general observation that ethylene adsorption on metal surfaces usually decreases their work function.10 Annealing the surface up to 220 K results in the simultaneous molecular desorption and decomposition of C2H4(a), leading to a continuous increase of work function. The work function change reaches 0.2 eV. This is similar to the case of C2H4(a) on Ni(111).47 The work function remains unchanged for annealing between 220 and 370 K, indicating that the recombinative desorption of H(a) from the surface does not 4171

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suggest that C2H2(a) on Co(0001) might undergo different dehydrogenation routes to form different types of carbon species. Graphitic carbon on transition metal surfaces usually exhibit the C 1s feature at 284.6 eV48,49 and also exhibits a broad peak between 6.5 and 9 eV below EF in the valence band spectrum.50 Therefore, the carbon species with its C 1s binding energy at 284.7 eV and its valence band at 6.9 eV below EF can be surely assigned to graphitic carbon on Co(0001). Therefore, C2H2(a) on Co(0001) can dehydrogenate to form graphitic carbon between 400 and 450 K. We consider the likely route to be the cyclopolymerization−dehydrogenation of C 2 H 2 (a) on Co(0001). It has been observed that C2H2(a) can undergo the cyclotrimerization reaction to form C6H6(a) on various metal single crystal surfaces.51−54 It could be seen from Figures 4B and 5B that a large C2H2(a) coverage on Co(0001) leads to the formation of a large fraction of graphitic carbon, supporting the involvement of cyclopolymerization reaction during the formation of graphitic carbon from C2H2(a). Several types of carbon species on transition metal surfaces could exhibit its C 1s binding energy at ∼283 eV, including carbidic carbon (bulk and surface), surface atomic carbon, and surface C2 cluster.9,49,50,55−62 However, it was reported that the formation of bulk cobalt carbide usually occurs above 1000 K,55,56 which is far beyond our experimental conditions. Surface carbide formed on polycrystalline cobalt was reported to exhibit a broad valence band centering at ∼4 eV in the UPS spectrum,9 whereas the carbon species on Co(0001) in our case exhibit its valence band at ∼5.3 eV below EF; moreover, the formation of surface carbide on Co surfaces was reported to enhance its work function by ∼0.3 eV,57 whereas the work function of Co(0001) decreases by ∼0.1 eV with the formation of carbon species. Therefore, carbidic carbon, either bulk carbide or surface carbide, is not likely to be the carbon species with its C 1s binding energy at ∼283 eV formed on Co(0001) in our case. Surface atomic carbon whose formation needs break the C− C bond of C2H2(a) is also not likely to be the carbon species with its C 1s binding energy at ∼283 eV and valence band at ∼5.4 eV below EF formed on Co(0001) in our case. Surface

Figure 7. Work function change as a function of annealing temperature after the exposure of 1.8 langmuirs of C2H4 on Co(0001) at 130 K followed by annealing at different temperatures. The work function of clean Co(0001) was taken as the reference.

change the work function. The work function increases again by 0.2 eV when the surface was annealed in the temperature range between 370 and 430 K, corresponding to the dehydrogenation reaction of C2H2(a) to carbon deposits on Co(0001). Then the work function is almost constant, being ∼0.1 eV lower than the clean Co(0001) surface caused by the carbon deposits. Above XPS and UPS results adequately support the vacantsite-dependent chemisorption and decomposition behaviors of C2H4 on Co(0001) that we previously proposed mainly from TDS results.37 Moreover, the current XPS and UPS results shed light on the dehydrogenation reaction of C2H2(a) and the nature of formed carbon deposits on Co(0001). Very interestingly, two types of carbon species are formed by the dehydrogenation reaction of C2H2(a) on Co(0001), giving two C 1s features with the binding energy at 283 and 284.7 eV in the C 1s XPS spectrum and two valence band features at 5.3 and 6.9 eV below EF in the UPS spectrum. These results

Figure 8. (A) LEED; (B) He II UPS spectra; (C) C 1s XPS spectra of carbon-covered Co(0001) prepared by the saturation exposure of C2H4 at 130 K followed by annealing to 630 K (a) and by the saturation exposure of C2H4 at 500 K (b). 4172

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nature of resulted carbon species. The chemisorption and decomposition of C2H4 on Co(0001) at 130 K depend on the available vacant surface sites. C2H4 dehydrogenates to form C2H2(a) and H(a) at low exposures and chemisorbs both molecularly and dissociatively into C2H2(a) and H(a) at large exposures. Upon heating, some C2H4(a) molecularly desorbs, releasing vacant surface sites that result in the simultaneous decomposition of other C2H4(a) into C2H2(a). At elevated temperatures, C2H2(a) simultaneously undergoes the direct dehydrogenation and the cyclopolymerization−dehydrogenation to form surface C2 cluster and graphitic carbon on Co(0001). At 500 K, C2H4 directly decomposes on Co(0001), forming surface atomic carbon. These results reveal temperature-dependent formation of different carbon species by C2H4 decomposition on Co(0001), greatly deepening our fundamental understanding of relevant heterogeneous catalytic reactions catalyzed by Co-based catalysts and the growth of graphene on Co surfaces by CVD methods.

atomic carbon was reported to form on polycrystalline cobalt by heating CH3(a)-covered surface to 570 K, giving a valence band at ∼4.8 eV below EF that is correlated with the px and py states of surface atomic carbon.58 Surface atomic carbon on other transition metal surfaces gives its valence band of px and py states at ∼4.2 eV below EF.59,60 Actually, we found that surface atomic carbon could be prepared on Co(0001) by the saturation exposure of C2H4 at 500 K. C2H4 decomposes into carbon deposits on Co(0001) at 500 K, giving a p(4 × 4) superstructure LEED pattern (Figure 8A); meanwhile, the formed carbon deposit on Co(0001) exhibits its valence band at 4.6 eV below EF (Figure 8B) and its C 1s binding energy at 283.2 eV (Figure 8C). On one hand, these observations demonstrate that decomposition of C2H4 on Co(0001) at 500 K can break the C−C bond and form surface atomic carbon; on the other hand, these observations also prove that the carbon species with its C 1s binding energy at ∼283 eV and valence band at ∼5.4 eV below EF formed by heating C2H2(a)-covered Co(0001) up to 630 K is not surface atomic carbon. It was recently observed by means of RAIRS and TDS that exposure of C2H2 on Pt(111) at 750 K leads to the formation of carbon deposits primarily in the form of C2 clusters with smaller amounts of surface atomic carbon.62 It was also shown that C 2p in the −C−C− structures on Pd surfaces molecules exhibits a higher binding energy (3.8 eV below EF) than that in surface atomic carbon (3.4 eV).50 We thus tend to assign the carbon species with its C 1s binding energy at ∼283 eV and valence band at ∼5.4 eV below EF to surface C2 clusters formed by the direct dehydrogenation of C2H2(a) without the breaking of C−C bond. And the valence band at ∼5.4 eV below EF arises from C 2p in surface C2 clusters. Therefore, our results show that different types of carbon species on Co(0001) form upon the chemisorption and decomposition of C2H4 at different conditions. Heating the Co(0001) surface exposed to C2H4 at 130 K above 500 K leads to the simultaneous formation of surface C2 cluster and graphitic carbon on the surface that respectively result from the direct dehydrogenation and the cyclopolymerization−dehydrogenation of C2H2(a) surface intermediate, and the direct decomposition of C2H4 on Co(0001) at 500 K leads to the formation of surface atomic carbon. These results provide novel information about the carbon species present on Co(0001) by C2H4 decomposition under various conditions. Different types of carbon deposits on the transition metal surface formed by C2H4 decomposition have been demonstrated to exert different influences on its catalytic performance.26−28 Different types of carbon deposits on the transition metal surface formed by C2H4 decomposition could also affect the growth of graphene by CVD methods employing C2H4 as precursor. It was observed that the growth temperature and the C2H4 exposure were observed to control the proportion of different rotational graphene domains and the quality of graphene on Pt(111).32 A recent DFT theoretical calculation study have predicted the different energetics and kinetics in the initial stages of epitaxial graphene growth on transition metal surfaces from C1 (atomic carbon) and C2 species.63 The modification of atomic carbon and C2 species on the reactivity of Co(0001) and the transformation of atomic carbon and C2 species to graphene islands on Co(0001) are under investigation in our lab.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (Grant 20773113), National Basic Research Program of China (2010CB923301), the Fundamental Research Funds for the Central Universities, and the MPG-CAS partner group program.

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REFERENCES

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4. CONCLUSIONS We have successfully elucidated the chemisorption and decomposition mechanisms of C2H4 on Co(0001) and the 4173

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