and Carbon Monoxide-Covered Co(0001) Surfaces - ACS Publications

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Surface Chemistry of CHI on Clean, Hydrogenand Carbon Monoxide-Covered Co(0001) Surfaces Hong Xu, Yuekang Jin, Lingshun Xu, Zhengming Wang, Guanghui Sun, Peng Chai, Feng Xiong, and Weixin Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06010 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 20, 2018

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The Journal of Physical Chemistry

Surface Chemistry of CH2I2 on Clean, Hydrogenand Carbon Monoxide-Covered Co(0001) Surfaces Hong Xu,1 Yuekang Jin,1 Lingshun Xu,2 Zhengming Wang,1 Guanghui Sun,1 Peng Chai,1 Feng Xiong,1 and Weixin Huang1* 1

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 2

Tsinghua University Hefei Institute for Public Safety Research, Hefei 230601, China

ACS Paragon Plus Environment

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ABSTRACT: Fundamental understanding of complex FT synthesis is of great interest. We have employed x-ray photoelectron spectroscopy and temperature programmed desorption to comparatively investigate CH2I2 adsorption and reactions on clean, hydrogen- and COcovered Co(0001) surfaces. Surface chemistry of CH2I2 was demonstrated to sensitively depend on available vacant surface sites on Co(0001). Upon adsorption on clean Co(0001) surface at 110 K, CH2I2 undergoes stepwise decomposition reactions to produce carbon adatoms, CH(a) and CH2(a) species at small coverages, and chemisorbs both dissociatively and molecularly at large coverages. Upon heating, CH2(a) species facilely undergoes surface reactions to produce CH4, C3H6 and C2H4 in gas phase and CH(a) species on the surface at low temperatures. CH(a) species undergoes surface reactions to produce CH4 in gas phase and C2H2(a) species on the surface at higher temperatures, and both CH(a) and C2H2(a) species undergo further surface reactions to produce H2 in gas phase and carbon species on the surface. Co-adsorbed H adatoms and CO molecules were found to strongly affect surface chemistry of CH2I2 and resulting CHx species on Co(0001) via suppressing the decomposition reactions and promoting the carbon-carbon bond coupling reactions. These results add novel insights in fundamental understanding of complex FT synthesis.


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1. Introduction Surface chemistry studies of single crystal-based model catalysts have been demonstrated as an effective approach to acquire fundamental understanding of heterogeneous catalysis,1 and efforts are now be devoted to approximating complexities of heterogeneous catalysis as closely as possible. Complexities of powder catalysts are considered by extending model catalysts from single crystal-based model catalysts to single crystal thin films-based model catalysts2-5 and nanocrystal-based model catalysts.6-10 Complexities of catalytic reactions are considered by establishing networks of elementary surface reactions involving all likely adsorbates and intermediates on catalyst surfaces.11-12 Fischer-Tropsch (FT) synthesis that converts CO and H2 to high-molecular-weight hydrocarbons is a catalytic reaction of great importance but meanwhile great complexity.13 Although extensively studied, the reaction mechanisms of FT synthesis are still debated due to the complexity.14-16 CHx species are generally accepted as the key surface intermediates responsible for the chain propagation surface reactions in FT synthesis,17-18 thus surface chemistry of CHx species on catalyst surfaces mainly prepared by thermal dissociation or photo-dissociation of halohydrocarbons have been extensively studied.19-42 Adsorbed methylene fragment(CH2(a)) belongs to one of the most extensively studied CHx species. On reactive metal single crystal surfaces such as Ni(110), Pd(100)、Rh(111) and Ru(001),24-28 CH2(a) undergoes dehydrogenation reactions to produce surface carbon and H2, hydrogenation reactions to produce CH4, and insertion reactions to produce heavier hydrocarbons including ethene, ethane, propene, propane, and butane. On coinage metal single crystal surfaces, CH2(a) selectively undergoes insertion reactions.23,33-35,38-42


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Co-based catalysts are used for commercial FT synthesis catalyzing steam-reforming of alcohols

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and are also active in

. DFT calculations were carried out to study C-C

coupling reactions on Co surfaces,46-49 in which CH3(a) + C(a) and CH2(a) + CH2(a) coupling at steps were proposed as the major coupling pathways on Co(0001) but CH2(a) was demonstrated not stable. However, surface chemistry of CHx species on Co single crystal surfaces have been seldom explored experimentally. We have recently reported a series of work on surface chemistry of CO, H2, H2O, various carbon species, various oxygen species on clean and modified Co(0001) surfaces with an attempt to understand elementary surface reactions of FT synthesis.50-55 Herein, we report surface chemistry of CH2(a) on Co(0001) prepared by thermal dissociation of CH2I2. Clean, hydrogen- and CO-covered Co(0001) surfaces were comparatively studied in order to address the reaction complexity of FT synthesis. Coverage-dependent surface chemistry of CH2(a) on clean Co(0001) surface and strong influences of pre-adsorbed H(a) and CO(a) on Co(0001) are successfully identified. 2. Experimental All experiments were performed in a Leybold stainless-steel ultrahigh-vacuum (UHV) chamber with a base pressure of 1.2×10-10 mbar. The UHV chamber was equipped with facilities for XPS, UPS, LEED, and differential-pumped TDS measurements. 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 110 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 Co(0001) sample was cleaned by repeated cycles of Ar ion sputtering and annealing until LEED gave a sharp (1×1) diffraction pattern and no contaminants


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could be detected by XPS. The annealing temperature of Co(0001) was kept always below 650 K to avoid the likely transition of Co(0001) from hcp to fcc structure. CH2I2 (purity > 99%, Acros Organics) was purified by repeated freeze-pump-thaw cycles and exposed to the sample surface through a doser which was positioned ~2 cm in front of the sample surface. CO and H2 were purchased from Nanjing ShangYuan Industry Factory and used as received. The purity of all reactants was checked by QMS prior to experiments, and the purity of CH2I2 was further examined by NMR to confirm the absence of CH3I impurity. All exposures were reported in Langmuir (1 L = 1.0 × 10-6 Torr⋅s) without corrections for the gauge sensitivity. During the TDS experiments, the sample was positioned 1 mm away from a collecting tube of the differentially pumped QMS, and the heating rate was 3 K/S. XPS spectra were recorded using Al Kα radiation (hν = 1486.6 eV) with a pass energy of 20 eV. The C 1s XPS spectra were peak-fitted by minimum numbers of peaks with the XPSPEAK software (Version 4.1), in which a line shape of 30% Gaussian and 70% Lorentzian and a Shirley-type background and a fullwidth at half-maximum (FWHM) of 1.8 eV were used. 3. Results and Discussion Figure 1 shows TDS spectra following various CH2I2 exposures on clean Co(0001) at 110 K. Desorption products include H2 (m/z=2), CH4 (m/z=16), C2H4 (m/z=27), C3H6 (m/z=42), CH3I (m/z=142 & 141) and CH2I2 (m/z=141). Adsorption and surface reactions of CH2I2 on clean Co(0001) sensitively depend on the coverage. For an exposure of 0.001 L CH2I2, H2 is the major product in the TDS spectra and exhibits a symmetric desorption peak at ∼404 K that can be attributed to the recombinative desorption of H(a) adatoms on Co(0001), and C2H4 and C3H6 desorption peaks also appear at ∼130 K. With an increase of CH2I2 exposure to 0.01 L, the H2


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desorption trace gives two peaks at ∼413 and ∼295 K. The peak at ∼413 K is stronger than that at ∼404 K following a 0.001 L CH2I2 exposure but appears at higher temperatures. Thus it does not result from the second-order recombinative desorption of H(a) adatoms on Co(0001), instead, it is a reaction-controlled H2 desorption process. The peak at ∼295 K can be attributed to the recombinative desorption of H(a) adatoms on Co(0001). In addition to the C2H4 and C3H6 desorption peaks at ∼130 K, a strong C2H4 desorption peak and a shoulder C3H6 desorption peak arise at arises at ∼185 and ∼140 K, respectively. Meanwhile, a strong and another weak symmetric CH4 desorption peaks were observed at ∼145 and ∼272 K, respectively. With a further increase of CH2I2 exposure to 0.02 L, obvious desorption traces of CH3I and CH2I2 molecules appear. The CH3I desorption trace consists of a major peak at ∼140 K and a shoulder peak at ∼170 K while the CH2I2 desorption trace give a desorption peak at ∼185 K. The formation of CH3I upon CH2I2 exposures on other metal surfaces were previously reported 25, 2728

and attributed to the hydrogenation reaction of CH2I fragment formed by the rupture of one C-

I bond of CH2I2. The H2 desorption feature at ∼413 K does not change much while another broad feature with two local maxima at ∼265 and ∼310 K, both of which are weaker than the H2 desorption peak at ∼295 K following a 0.01 L CH2I2 exposure. The H2 desorption peak at ∼265 K can be attributed to the recombinative desorption of H(a) adatoms on Co(0001) affected by coadsorbates54 while that at ∼310 K should be a reaction-controlled H2 desorption process. The CH4 desorption trace consists of a narrow and symmetric feature at ∼145 K and a broad and strong feature at ∼240 K. The C2H4 desorption trace shows a broad and asymmetric feature at ∼163 K and an additional tiny feature at ∼310 K. In the C3H6 desorption spectrum, the feature at ∼140 K grows at the expense of that at ∼130 K, and an additional tiny feature appears at ∼310 K.


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The H2, CH4, C2H4, C3H6 desorption traces do not vary much with an increase of CH2I2 exposure from 0.02 L to 0.03 L while the CH3I desorption feature at ∼140 K grows slightly and the CH2I2 desorption feature increases greatly and shifts to higher temperatures, indicating the likely formation of physisorbed CH2I2 layers in addition to chemisorbed CH2I2(a). Thus, CH2I2 prefers molecular adsorption and desorption at large exposures. Figure 2 A and B shows C 1s XPS spectra with peak-fitting results and I 3d5/2 XPS spectra as a function of CH2I2 exposure at 110 K. At an exposure of 0.001 L CH2I2, the C 1s and I 3d5/2 XPS spectra respectively exhibit a single component at 283.1 and 619.6 eV. The C 1s XPS assignments of various CHx species on different metal surfaces are still debated. However, according to previous work of CH2I2 on transitional metal surfaces 23-27 and our work of carbon species on Co(0001) 54,55, the C 1s component at 283.1 eV and the I 3d5/2 component at 619.6 eV can be assigned to C and I adatoms, respectively. This suggests a complete decomposition of CH2I2 into C, H and I adatoms on Co(0001) at 110 K. With the CH2I2 exposure increasing up to 0.005 L, the C 1s component at 283.1 eV shifts to 283.4 eV and an additional C 1s component at 284.8 eV appears at an exposure of 0.002 L CH2I2 and grows, and the I 3d5/2 component at 619.6 eV grows. With the CH2I2 exposure further increasing up to 0.01 L and above, a third C 1s component appears at 285.4 eV and simultaneously an additional I 3d5/2 component appears at 620.4 eV, and both components grow and eventually dominate the XPS spectra. The C 1s component at 285.4 eV and the I 3d5/2 component at 620.4 eV can be assigned to CH2I2(a) and CH2I(a).23-27 The C 1s components at 284.8 eV that appears with the I adatoms on the surface could only be assigned to CHx species formed via CH2I2 dissociation. By decomposition of CH2Cl2 and CH3Cl on a polycrystalline Co foil, C adatoms, CH(a), CH2(a) and CH3(a) species were reported to exhibit the C 1s binding energy at 283.3, 283.8, 284.9 and 285.8 eV,


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respectively.56 However, it was also reported that CH3(a) on Co(0001) formed by CH3I decomposition gave the C 1s binding energy as low as 283.5 eV.57 Since the C 1s binding energy of C adatoms on Co(0001) is identified at 283.1 eV in our case, we assigned the C 1s component at 284.8 eV to CH2(a) species on Co(0001). Meanwhile, the C 1s component at 283.4 eV could be taken as an implication for the formation of CH(a) species that can not be distinguished from C adatoms with employed XPS facility. Figure 2C shows integrated XPS peak intensities of observed surface species as a function of incremental CH2I2 exposure at 110 K. The I 3d5/2 XPS peak intensity varies linearly with the total C 1s XPS peak intensity, which is reasonable. The observation variations of various surface species clearly demonstrate coverage-dependent surface chemistry of CH2I2 on Co(0001) at 110 K. CH2I2 undergoes stepwise decomposition reactions to produce carbon adatoms, CH(a) and CH2(a) species up to 0.005 L CH2I2 and chemisorbs both dissociatively and molecularly at large exposures. It can be seen that the total coverage of C adatoms and CH(a) species does not vary with CH2I2 exposures. This suggests that the coverage of C adatoms should decrease with the CH2I2 exposure since CH2I2 decomposes to exclusively produce carbon adatoms at a 0.001 L exposure. The coverages of CH2(a) and CH2I2(a)/CH2I(a) grow with CH2I2 exposure after their appearances. CH2I2(a) or CH2I(a) species are formed after 0.01 L CH2I2 exposure at 110 K, but few CH3I and CH2I2 desorption trace were observed in the corresponding TDS spectra. These suggest the occurrence of full C-I bond rupture of CH2I2(a)/CH2I(a) during the heating process of TDS experiments. The coverage of CH2I2(a)/CH2I(a) is almost doubled with the CH2I2 exposure increasing from 0.02 L to 0.03 L. This is consistent with the corresponding TDS results, indicating the likely formation of physisorbed CH2I2 layers in addition to chemisorbed CH2I2(a). We also roughly estimated the coverages of total and individual carbon species by comparing the


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integrated C 1s XPS peak intensity following various CH2I2 exposures at 110 k with that of 0.33 ML CO adsorbed on Co(0001) (see below). Fig. 2D displays the estimated coverages of various carbon species as a function of total carbon coverage. The saturating coverage of C adatoms and CH(a) species formed by CH2I2 decomposition on Co(0001) at 110 K is about 0.11 ML while a 0.03 L CH2I2 exposure gives a total carbon coverage of 0.64 ML consisting of 0.33 ML CH2I2(a)/CH2I(a), 0.20 ML CH2(a) and 0.11 ML C adatoms/CH(a). In order to examine evolutions of various surface species during the heating process, the annealing experiment of a Co(0001) surface individually exposed to 0.03 L CH2I2 at 110 K was performed. Figure 3 shows C 1s XPS spectra with peak-fitting results, I 3d5/2 XPS spectra after an exposure of 0.03 L CH2I2 on Co(0001) at 110 K followed by annealing at elevated temperatures, estimated coverages of observed different carbon species and integrated I 3d5/2 XPS peak intensities as a function of annealing temperature and the corresponding TDS spectra. As shown in the XPS results, an individual 0.03 L CH2I2 at 110 K forms 0.06 ML C adatoms/CH(a), 0.23 ML CH2(a), 0.37 ML CH2I2(a)/CH2I(a), and H and I adatoms species. It is noteworthy and reasonable that the surface speciation following an individual 0.03 L CH2I2 exposure at 110 K is different from that following an incremental CH2I2 exposure up to 0.03 L (Fig. 2). Upon annealing at 170 K, the I 3d5/2 component (620.5 eV) and C 1s component (285.6 eV) of CH2I2(a)/CH2I(a) and the C 1s component (284.8 eV) of CH2(a) weaken while the C 1s component (283.4 eV) of CH(a)/C grows. In the corresponding TDS spectra, desorption traces of CH3I, CH4, C3H6 and C2H4 occur prior to 170 K. These observations demonstrate the occurrence of CH2I(a) hydrogenation reaction into CH3I, CH2(a) hydrogenation reaction into CH4, CH2(a) migratory coupling reactions into C2H4 and C3H6, and CH2(a) decomposition reaction into CH(a), C and H adatoms. Upon annealing at 200 K, the I 3d5/2 component and C 1s component of


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CH2I2(a)/CH2I(a) almost varnish, and the I 3d5/2 component of I(a) adatoms at 619.7 eV dominates in the I 3d5/2 XPS spectrum. These correspond to the molecular desorption of CH2I2(a) from the surface observed in the TDS spectra. The C 1s component of CH2(a) further weakens while the C 1s component at 283.4 eV grows and shifts to 283.7 eV. This suggests the decomposition of CH2(a) into CH(a) and H adatoms because few desorption traces of carboncontaining products other than CH2I2 were observed between 170 and 200 K in the TDS spectrum. Upon annealing at 250 K, both C 1s components at 284.8 and 285.6 eV disappear while a new C 1s component at 285.4 eV emerges and the C 1s component at 283.7 eV further shifts to 283.9 eV but its intensity remains unchanged; meanwhile, the I 3d5/2 XPS peak intensity slightly decreases, in consistence with the disappearance of the C 1s components at 285.6 eV. Thus only the carbon species with C 1s binding energy at 285.4 and 283.9 eV remain on the surface after annealing at 250 K. Upon further annealing up to 270, 360 and 550 K, the C 1s component at 285.4 eV grows at the expense of that at 283.9 eV, moreover, the C 1s component at 283.9 eV resumes to 283.4 eV after annealing at 550 K. The corresponding TDS spectrum displays a hightemperature CH4 desorption peak between ∼180 and ∼330 K and centering at ∼240 K, C2H4 and C3H6 desorption peaks at ∼310 K, and H2 desorption peaks at ∼310 and ∼413 K. The remaining C 1s components at 283.4 and 285.4 eV after annealing at 550 K should correspond to surface species without H atoms and thus respectively assigned to C adatoms and graphitic carbon on Co(0001) with co-adsorbed I adatoms. Co-adsorbed I adatoms likely weaken the charge transfer from Co(0001) to carbon species, resulting in a positive C1 s binding energy shift comparing with those on clean Co(0001) surface 54-55. Since hydrogenation of CH2(a) to CH4 occurs below 170 K and CH2(a) is barely present on Co(0001) at 250 K, the high-temperature CH4 desorption


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peak should result from the hydrogenation of CH(a) species. It is noteworthy that the C 1s component at 283.7 eV was observed to shift to 283.9 eV without variation in its intensity when the surface annealed at 200 K was further annealed at 250 K. On one hand, this suggests that the consumption of CH(a) species in the hydrogenation reaction should be compensated by its formation from the preceding CH2(a) and of CH2I(a) decompositions at lower temperatures. On the other hand, this indicates the formation of additional surface species responsible for the positive shift in the C 1s binding energy. A likely surface species is C2H2(a) species formed by the self-coupling reaction of two CH(a) species. The C 1s binding energy of C2H2(a) on Co(0001) was reported to vary between 283.3-284.0 eV.51,54,57 and the self-coupling reaction of two CH(a) species into C2H2(a) species was observed to occur below 250 K on Co(0001) and Ni(111) surfaces.57,58 Thus surface reactions of CH(a) and C2H2(a) species produce H2, C2H4 and C3H6 at ∼310 K and H2 at ∼413 K in the gas phase and graphitic carbon on the surface. We propose that C2H2(a) species undergoes major dehydrocyclization reactions and minor hydrogenation reactions at ∼310 K respectively to produce graphitic carbon on the surface and H2, C2H4 and C3H6 while CH(a) species undergoes dehydrocyclization reactions at ∼413 K to produce graphitic carbon on the surface and H2 in the gas phase. Meanwhile, the direct decomposition of CH(a) and C2H2(a) can also occur to produce H2 and respectively C1 adatoms and C2 adatoms on the surface. The C1 adatoms and C2 adatoms were reported to exhibit similar C 1s binding energies on Co(0001) surface.54,55 A key surface intermediate of surface reactions of CH(a) species, CCH(a) species, was previously identified on Pt(111) surface with high-resolution electron energy loss spectroscopy.59 The above TDS and XPS results suggest vacant surface sites-dependent surface chemistry of CH2I2 on Co(0001). CH2I2 molecules preferentially dissociate on Co(0001) at 110 K, and the


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required number of vacant surface sites increases with the sequential decomposition reactions into CH2I(a)+I(a), CH2(a)+2I(a), CH(a)+H(a)+2I(a), and C(a)+2H(a)+2I(a). Thus, at small exposures and plenty of vacant surface sites on Co(0001), CH2I2 undergoes a full decomposition into C, H, and I adatoms. With the exposure increasing and available vacant surface sites decreasing, CH2I2 undergoes partial decomposition into CH2(a) and CH(a) species and I adatoms, and CH2I(a) species and I adatoms, and eventually undergoes molecular chemisorption and physisorption. When the surfaces are heated, surface reactions and desorption processes will release additional vacant surface sites that induce new decomposition reactions. Such behaviors lead to the complex coverage- and temperature- dependent evolutions of surface species formed by CH2I2 adsorption on Co(0001). Under the experimental condition shown in Figure 3, the dominant CHx species is CH2(a) at 110 K, but CH2(a) is not stable and facilely undergoes hydrogenation reaction into CH4, coupling reactions into C3H6 and C2H4, and decomposition reactions to CH(a) at low temperatures. CH(a) species is more stable than CH2(a) and is the dominant CHx species at temperatures above 170 K. CH(a) can undergo hydrogenation reaction to produce CH4 and self-coupling reaction to form C2H2(a) species, and major CH(a) and C2H2(a) species undergo dehydrocyclization or decomposition reactions above 300 K to produce carbon species on the surface and H2 in gas phase. Stability of various CHx species and their hydrogenation and coupling reactions were studied on flat and stepped Co(0001) surfaces by DFT calculations.46-49 The stability of various CHx species follows an order of C adatom > CH(a) > CH2(a) > CH3(a) and CH2(a) species facilely decomposes into CH(a) species on both flat and stepped Co(0001) surfaces with barriers respectively of 0.28 and 0.29 eV while the decomposition of CH(a) into C adatom is difficult on flat Co(0001) surface with a barrier of 1.36 eV but greatly facilitated on stepped Co(0001)


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surface with a barrier of 0.76 eV. Our experimental observations of higher stability of CH(a) than CH2(a) are consistent with these previous DFT calculations results, but our experimental results also show that CH2I2 can decompose on Co(0001) to exclusively produce C adatoms upon a 0.001 L exposure on Co(0001) at 110 K and that CH2(a) is the dominant surface species at 110 K at large CH2I2 exposures. We propose that the formation of C adatoms should occur at defective sites on Co(0001) and that the surface site blocking by co-adsorbed I and H adatoms should suppress the decomposition reaction of CH2(a) that requires available vacant surface sites, which can enhance the stability of CH2(a). Such an effect was observed by DFT calculations of CH2(a) dissociation on Ru(0001) with adsorbed H adatoms.60 Our results also demonstrated that CH2(a) facilely decomposes to CH(a) at low temperatures as long as vacant surface sites are available, consistent with our arguments. CO and H2 are the reactants of FT synthesis, thus H adatoms and adsorbed CO are unavoidable on Co catalyst surfaces. It is of interest to study effects of pre-adsorbed H adatoms and CO molecules on surface chemistry of CH2I2 on Co(0001). Figure 4A shows H2 TDS spectra following various H2 exposures on Co(0001) surface at 110 K. The H2 desorption feature resulting from recombinative desorption of H adatoms (H(a)) displays a typical second-order desorption kinetics and saturates at an exposure of 10 L H2. The H(a) coverages (θH) were calculated by integrating the H2 desorption peaks and assuming the saturating coverage of H(a) on Co(0001) as 0.5 monolayer (ML) 61. Figure 4B shows CO TDS spectra following various CO exposures on Co(0001) surface at 110 K. With CO exposure increasing, three CO desorption peaks develop sequentially at ∼410, ∼310, and ∼233 K, the first two and the third of which respectively arise from CO adsorbed at on-top sites and both on-top and bridge/hollow sites according to previous results.62 The CO(a) coverages (θCO) were derived by integrating the CO


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desorption peaks and assuming the coverage of saturating CO(a) exclusively at on-top sites as 0.33 ML 62. Figure 4C and 4D respectively show C 1s and O 1s XPS spectra of saturating CO adsorption on Co(0001) at 110 K followed by annealing at indicated temperatures. It can be seen that CO molecules adsorbed at on-top sites give C 1s binding energy at 286.2 eV and O 1s binding energy at 532.2 eV while CO molecules adsorbed at bridge/hollow sites give C 1s binding energy at 285.5 eV and O 1s binding energy at 531.0 eV. Figure 5A compares TDS spectra of 0.001 L CH2I2 adsorption on clean Co(0001) surface and Co(0001) surfaces exposed to 0.26 and 0.5 ML H(a) at 110 K. An exposure of 0.001 L CH2I2 on clean Co(0001) surface leads to desorption traces of C2H4 and C3H6 at ∼130 K arising from CH2 coupling reactions in addition to H2 desorption trace. On 0.26 ML H(a) pre-covered Co(0001) surface, desorption traces of C2H4 and C3H6 at ∼130 K grow, and CH4 desorption peaks at ∼135 and ∼242 K arise and can be attributed respectively to CH2(a) and CH(a) hydrogenation reactions. On 0.5 ML H(a) pre-covered Co(0001) surface, C3H6 desorption peak at ∼130 K keeps growing while C2H4 desorption peak at ∼130 K does not change but an additional C2H4 desorption peak appears at ∼225 K and should be formed by CH(a) or C2H2(a)initiated surface reactions; and CH4 desorption mainly occurs at ∼175 K, demonstrating that the presence of plenty of H adatoms promotes CH(a) hydrogenation reactions. These observations suggest that the presence of H adatoms decreases available vacant surface sites and suppresses the C-H rupture of CH2I2 at small coverages, facilitating the formation of CH2(a) and CH(a) species. When the CH2I2 exposure was increased to 0.03 L (Figure 5B), the presence of H adatoms increases the desorption of CH2I2 and decreases the desorption of C2H4 and C3H6 at ∼130 K; meanwhile, an additional CH4 desorption trace resulting from CH(a) hydrogenation reactions appears at ∼250 K and grows with the pre-adsorbed H adatom coverage. These


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observations suggest that the presence of H adatoms suppresses the C-I rupture of CH2I2 at large coverages, stabilizing CH2I(a) and CH2I2(a) species. Figure 5C and 5D further compare C 1s and I 3d5/2 XPS spectra following 0.001 L CH2I2 adsorption on clean Co(0001) surface and Co(0001) surfaces exposed to 0.26 ML and 0.5 ML H(a) , respectively. It can be seen that the formed surface species change from C adatoms on clean Co(0001) surface to C adatoms, CH2(a) and CH2I(a)/CH2I2(a) species on 0.26 ML H(a) pre-covered Co(0001) surface and to CH2(a) and CH2I(a)/CH2I2(a) species on 0.5 ML H(a) pre-covered Co(0001) surface, agreeing with the above TDS results. Previous results reported that pre-covered H adatoms on Ni (110) surface facilitated hydrogenation reactions into methane and suppressed coupling reactions into longchain hydrocarbons.27 Our results demonstrate that influences of H adatoms on Co(0001) on surface chemistry of CH2I2 depend on the coverages of both H adatoms and CH2I2, which is reasonable because of vacant surface sites-dependent surface chemistry of CH2I2 on Co(0001). Figure 6A and 6B compares TDS spectra of 0.001 L CH2I2 adsorption on clean Co(0001) surface and Co(0001) surfaces exposed to 0.33 ML and 0.67 ML CO(a) at 110 K. On 0.33 ML CO(a) pre-covered Co(0001) surface, C3H6 desorption peak at ∼130 K grows while C2H4 desorption peak at ∼130 K does not change but an additional C2H4 desorption peak appears at ∼210 K, and a new CH4 desorption peak emerges at ∼170 K. Such TDS results are similar to those of 0.001 L CH2I2 adsorption on 0.5 ML H(a) pre-covered Co(0001) surface, demonstrating the presence of adsorbed CO(a) decreases available vacant surface sites and suppresses the C-H rupture of CH2I2 at small coverages, facilitating the formation of CH2(a) and CH(a) species. Meanwhile, the H2 TDS spectrum exhibit two desorption features at ∼310 and ∼415 K respectively arising from the recombinative desorption of H adatoms and the dehydrogenation reaction of CH(a) species. The H adatoms likely interact repulsively with co-adsorbed CO(a) and


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CH(a) species and thus desorbs at much lower temperatures although the peak intensity is similar to that on clean Co(0001) surface. Meanwhile, adsorbed CO(a) molecules also interact with coadsorbed CH(a) species and their TDS spectrum is different from that on clean Co(0001) surface. On 0.67 ML CO(a) pre-covered Co(0001) surface, few desorption of CH4, C2H4, C3H6 and H2 occur while desorption traces of CH3I and CH2I2 appear, suggesting that the presence of adsorbed CO(a) molecules suppresses the C-I rupture of CH2I2 even at an exposure of 0.001 L, stabilizing CH2I(a) and CH2I2(a) species. These results indicate that adsorbed CO(a) molecules on Co(0001) exert a stronger suppressing effect on CH2I2 decomposition surface than H adatoms. When the CH2I2 exposure was increased to 0.03 L (Figure 6C and 6D), the presence of 0.33 ML adsorbed CO(a) molecules increases the desorption of CH2I2 and decreases the desorption of CH4, C2H4, C3H6, CH3I and H2, and the presence of 0.67 ML adsorbed CO(a) molecules increases the desorption of CH2I2 and CH3I and almost completely suppress the desorption of CH4, C2H4, C3H6 and H2. These results demonstrate that influences of adsorbed CO(a) molecules on Co(0001) on surface chemistry of CH2I2 depend on the coverages of both CO and CH2I2, similar to influences of H adatoms. Figure 7 compares TDS spectra of 0.33 ML CO(a)-precoveded Co(0001) surfaces exposed to 0.001 L CH2I2 and then to 0.33 ML or 0.67 ML CO(a) at 110 K. CH4 and C3H6 desorption features and C2H4 desorption feature at ∼210 K varnish upon additional CO exposures, and C2H4 desorption feature at ∼130 K weakens with the CO exposure while a new and broad C2H4 desorption feature emerges at ∼160 K and grows with the CO exposure; meanwhile, H2 desorption peaks weaken and CH3I and CH2I2 desorption peaks appear. These results suggest that CH4 and C3H6 formations should involve the same species, likely CH(a) species and further demonstrate that surface chemistry of CHx is sensitively affected by co-adsorbates.


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The above comprehensive results of CH2I2 adsorption and reactions on clean, hydrogenand CO-covered Co(0001) surfaces demonstrate vacant surface sites-dependent surface chemistry of CH2I2 and resulting CHx species and highlight the roles of co-adsorbed surface species. In can be thus inferred that comprehensive information on the surface speciation of Co catalysts in FT synthesis, instead of information on the key CHx species, are needed for fundamental understanding of their catalytic performance. 4. Conclusions Via comparative studies of CH2I2 adsorption and reactions on clean, hydrogen- and COcovered Co(0001) surfaces, we have successfully demonstrate vacant surface sites-dependent surface chemistry of CH2I2 and resulting CHx species. At 110 K, CH2I2 undergoes stepwise dehydrogenates to form C, H and I adatoms and CH(a) and CH2(a) species and at low exposures and chemisorbs both dissociatively and molecularly at large exposures. Upon heating, CH2(a) facilely undergoes hydrogenation reaction into CH4, coupling reactions into C3H6 and C2H4, and decomposition reactions to CH(a) at low temperatures. CH(a) species is more stable than CH2(a) and is the dominant CHx species at temperatures above 170 K. CH(a) can undergo hydrogenation reaction to produce CH4 and self-coupling reaction to form C2H2(a) species, and major CH(a) and C2H2(a) species undergo dehydrocyclization or decomposition reactions above 300 K to produce carbon species on the surface and H2 in gas phase. Co-adsorbed H adatoms and CO(a) molecules strongly affects surface chemistry of CH2I2 and resulting CHx species on Co(0001) via suppressing the decomposition reactions and promoting the carbon-carbon bond coupling reactions. These results add novel insights in fundamental understanding of complex FT synthesis.


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AUTHOR INFORMATION Corresponding Author *Tel.: 008655163600435. Email: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was financially supported by the National Key R & D Program of Ministry of Science and Technology of China (2017YFB0602205), the National Natural Science Foundation of China (21525313, 91745202, 21703117), the Changjiang Scholars Program of Ministry of Education of China, the Fundamental Research Funds for the Central Universities of Ministry of Education of China (WK2060030017) and Collaborative Innovation Center of Suzhou Nano Science and Technology.


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Figure captions Figure 1. TDS spectra of (A) m/z=141 (CH2I2 & CH3I), (B) m/z=142 (CH3I), (C) m/z = 2 (H2), (D) m/z = 16 (CH4), (E) m/z = 27 (C2H4) and (F) m/z = 42 (C3H6) following various CH2I2 exposures on Co(0001) at 110 K. Figure 2. (A) C 1s and (B) I 3d5/2 XPS spectra after various exposures of CH2I2 in an incremental exposure manner on Co(0001) at 110 K. Scattered data and solid lines represent experimental data and fitted curves, respectively. (C) Integrated XPS peak intensities of observed surface species as a function of CH2I2 exposure at 110 K. (D) Estimated coverages of observed different carbon species as a function of total carbon coverage following various CH2I2 exposures at 110 K. Figure 3. (A) C 1s and (B) I 3d5/2 XPS spectra after an individual 0.03 L CH2I2 exposure (0.66 ML) on clean Co(0001) surface at 110 K followed by annealing at indicated temperatures. Scattered data and solid lines represent experimental data and fitted curves, respectively. (C) Estimated coverages of observed different carbon species and integrated I 3d5/2 XPS peak


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intensities as a function of annealing temperature. (D) TDS spectra following 0.03 L CH2I2 exposure on clean Co(0001) surface at 110 K. Figure 4. (A) H2 TDS spectra following various H(a) coverages on Co(0001) surface at 110 K. (B) CO TDS spectra following various CO coverages on Co(0001) surface at 110 K. (C) C 1s and (D) O 1s XPS spectra of saturating CO adsorption on Co(0001) at 110 K followed by annealing at indicated temperatures. Figure 5. (A) TDS spectra after 0.001 L CH2I2 exposure on 0.26 ML H(a) (0.26 ML H(a) + 0.001 L CH2I2) or 0.5 ML H(a) (0.5 ML H(a) + 0.001 L CH2I2) pre-covered Co(0001) surfaces at 110 K. (B) TDS spectra after 0.03 L CH2I2 exposure on 0.26 ML H(a) (0.26 ML H(a) + 0.03 L CH2I2) or 0.5 ML H(a) (0.5 ML H(a) + 0.03 L CH2I2) pre-covered Co(0001) surfaces at 110 K. (C) C 1s and (D) I 3d5/2 XPS spectra of (a) clean Co(0001) surface, (b) Co(0001) surface exposed to 0.001 L CH2I2 at 110 K, (c) 0.26 ML H(a) pre-covered Co(0001) surface exposed to 0.001 L CH2I2 at 110 K, (d) 0.5 ML H(a) pre-covered Co(0001) surface exposed to 0.001 L CH2I2 at 110 K. Figure 6. (A & B) TDS spectra after 0.001 L CH2I2 exposure on 0.33 ML CO (0.33 ML CO + 0.001 L CH2I2) or 0.67 ML CO (0.67 ML CO + 0.001 L CH2I2) pre-covered Co(0001) surfaces at 110 K. (C & D) TDS spectra after 0.03 L CH2I2 exposure on 0.33 ML CO (0.33 ML CO + 0.03 L CH2I2) or 0.67 ML CO (0.67 ML CO + 0.03 L CH2I2) pre-covered Co(0001) surfaces at 110 K. Figure 7. TDS spectra after 0.33 ML CO pre-covered Co(0001) surface exposed to 0.001 L CH2I2 (0.33 ML CO + 0.001 L CH2I2) and then to 0.33 ML CO (0.33 ML CO + 0.001 L CH2I2 + 0.33 ML CO) or 0.67 ML CO (0.33 ML CO + 0.001 L CH2I2 + 0.67 ML CO) at 110 K.


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Figure 1. TDS spectra of (A) m/z=141 (CH2I2 & CH3I), (B) m/z=142 (CH3I), (C) m/z = 2 (H2), (D) m/z = 16 (CH4), (E) m/z = 27 (C2H4) and (F) m/z = 42 (C3H6) following various CH2I2 exposures on Co(0001) at 110 K.


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Figure 2. (A) C 1s and (B) I 3d5/2 XPS spectra after various exposures of CH2I2 in an incremental exposure manner on Co(0001) at 110 K. Scattered data and solid lines represent experimental data and fitted curves, respectively. (C) Integrated XPS peak intensities of observed


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surface species as a function of CH2I2 exposure at 110 K. (D) Estimated coverages of observed different carbon species as a function of total carbon coverage following various CH2I2 exposures at 110 K.

Figure 3. (A) C 1s and (B) I 3d5/2 XPS spectra after an individual 0.03 L CH2I2 exposure (0.66 ML) on clean Co(0001) surface at 110 K followed by annealing at indicated temperatures.


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Scattered data and solid lines represent experimental data and fitted curves, respectively. (C) Estimated coverages of observed different carbon species and integrated I 3d5/2 XPS peak intensities as a function of annealing temperature. (D) TDS spectra following 0.03 L CH2I2 exposure on clean Co(0001) surface at 110 K.

Figure 4. (A) H2 TDS spectra following various H(a) coverages on Co(0001) surface at 110 K. (B) CO TDS spectra following various CO coverages on Co(0001) surface at 110 K. (C) C 1s and (D) O 1s XPS spectra of saturating CO adsorption on Co(0001) at 110 K followed by annealing at indicated temperatures.


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Figure 5. (A) TDS spectra after 0.001 L CH2I2 exposure on 0.26 ML H(a) (0.26 ML H(a) + 0.001 L CH2I2) or 0.5 ML H(a) (0.5 ML H(a) + 0.001 L CH2I2) pre-covered Co(0001) surfaces at 110 K. (B) TDS spectra after 0.03 L CH2I2 exposure on 0.26 ML H(a) (0.26 ML H(a) + 0.03 L CH2I2) or 0.5 ML H(a) (0.5 ML H(a) + 0.03 L CH2I2) pre-covered Co(0001) surfaces at 110 K. (C) C 1s and (D) I 3d5/2 XPS spectra of (a) clean Co(0001) surface, (b) Co(0001) surface exposed to 0.001 L CH2I2 at 110 K, (c) 0.26 ML H(a) pre-covered Co(0001) surface exposed to


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0.001 L CH2I2 at 110 K, (d) 0.5 ML H(a) pre-covered Co(0001) surface exposed to 0.001 L CH2I2 at 110 K.

Figure 6. (A & B) TDS spectra after 0.001 L CH2I2 exposure on 0.33 ML CO (0.33 ML CO + 0.001 L CH2I2) or 0.67 ML CO (0.67 ML CO + 0.001 L CH2I2) pre-covered Co(0001) surfaces at 110 K. (C & D) TDS spectra after 0.03 L CH2I2 exposure on 0.33 ML CO (0.33 ML CO +


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0.03 L CH2I2) or 0.67 ML CO (0.67 ML CO + 0.03 L CH2I2) pre-covered Co(0001) surfaces at 110 K.

Figure 7. TDS spectra after 0.33 ML CO pre-covered Co(0001) surface exposed to 0.001 L CH2I2 (0.33 ML CO + 0.001 L CH2I2) and then to 0.33 ML CO (0.33 ML CO + 0.001 L CH2I2 + 0.33 ML CO) or 0.67 ML CO (0.33 ML CO + 0.001 L CH2I2 + 0.67 ML CO) at 110 K.


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and Carbon Monoxide-Covered Co(0001) Surfaces - ACS Publications

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