1 Introduction

Heterogeneous catalytic reactions usually involve complex surface reaction networks on solid catalysts, whose fundamental understanding has been mostly achieved by studying model catalysts with well-defined surface structures and surface species under well-controlled conditions [1,2,3]. The Fischer–Tropsch synthesis (FTS) is an important catalytic reaction transforming the syngas (CO + H2) into higher hydrocarbons, such as gasoline, diesel, and kerosene [4,5,6,7,8,9,10,11,12]. Cobalt catalysts exhibit low temperature, high activity, high chain growth probability, and high stability in the FTS process [13] This has motivated many fundamental studies of adsorption, surface reactions and desorption of reactants (CO and H2), products (hydrocarbons and H2O) and intermediates (carbon species, oxygen species, CHx and CxHy) experimentally on Co single crystal model catalysts and polycrystalline Co [14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34] and theoretically using density functional theory (DFT) calculations [35,36,37,38,39,40,41,42].

Acetylene and ethylene are among the major products of FTS. C2H4 was reported to dehydrogenate on polycrystalline cobalt at room temperature into acetylene which partially dissociated into CH-like species at elevated temperatures [30] but to completely decomposed on Co(0001) into C and H adatoms via acetylene intermediate below room temperature [31]. Later studies found that adsorption and surface reactions of C2H4 on Co(0001) depended on the available vacant surface sites, leading to formation of carbon adatoms, C2 clusters and graphitic carbon [18, 21]. Adsorption and surface reactions of C2H2 on Co single crystals were also studied [31,32,33,34]. Varri et al. [32] reported that C2H2 molecularly adsorbed on Co(0001) at room temperature and decomposed to form a likely vinylidene (CCH2) intermediate at 410 K and eventually graphitic and carbidic carbon species at elevated temperatures, whereas Ramsvik et al. [33, 34] proposed C2H or C2 fragments as surface intermediates of C2H2 decomposition on Co(0001) and (11–20) surfaces. Thus, arguments exist on adsorption and surface reactions of C2H4 and C2H2 on Co surfaces. In this paper we used temperature desorption spectrum (TDS), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) to study adsorption and surface reactions of acetylene and ethylene on Co(0001) and observed temperature and coverage dependent surface reactions of C2H2 and C2H4 on Co(0001).

2 Methods

All experiments were performed in a Leybold stainless-steel ultrahigh vacuum (UHV) chamber with a base pressure of 1.2 × 10–10 mbar [17] The UHV chamber was equipped with facilities for XPS, UPS, LEED, and differential-pumped TDS measurements, in which 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 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 could be detected by XPS. The annealing temperature of Co(0001) was kept always below 650 K to avoid the likely transition of from hcp Co(0001) to fcc structure.

C2H4 (> 99.9999%, Arkonic Gases & Chemicals Inc.), C2H2 (> 99.99%, Nanjing ShangYuan Industry Factory), 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 and C2H2 exposure was controlled to be below 5 × 10–10 torr, therefore, a line-of-sight stainless steel doser (diameter: 8 mm) positioned ~ 2 mm in front of the Co(0001) surface was used for relatively large C2H4 and C2H2 exposures. 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 L H2O at 130 K by back filling and by the doser [17]. The exposures of C2H4 reported herein were corrected with the enhancement effect of the doser. Other gases were dosed by back filling. 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 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) while the work function data was calculated from the He I UPS spectra (hν = 21.2 eV) by measuring the gap between the Fermi edge and the secondary electron edge.

3 Results and discussion

We previously reported C2H4 adsorption and surface reaction on Co(0001) at 130 K [18, 21] and thus herein reported C2H4 adsorption and surface reaction on Co(0001) at different temperatures. Figure 1 shows TDS spectra of C2H4 and H2 following saturating C2H4 exposure on Co(0001) at different temperatures. At 130 K, molecular desorption peaks of C2H4 appeared up to around 200 K, meanwhile, two H2 desorption peaks emerged at around 327 and 421 K, arising from the recombination of H adatoms and the dehydrogenation of molecularly-adsorbed C2H2 species (C2H2(a)) [18, 21], respectively. As the exposure temperature increased to 300 K, C2H4 desorption traces disappeared while only a broad H2 desorption peak arising from the dehydrogenation of adsorbed C2H2(a) emerged at around 405 K. When exposed at 400 K, only a small H2 desorption peak at around 500 K could be observed.

Fig. 1
figure 1

A C2H4 and B H2 TDS spectra following saturating C2H4 exposure on Co(0001) at indicated temperatures

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C 1s XPS spectra of saturating C2H4 exposure on Co(0001) at different temperatures on Co(0001) followed by heating to 630 K were shown in Fig. 2. Exposed at 130 K, two C 1s features with the binding energy at 283.6 and 284.4 eV, which could be assigned to adsorbed C2H2(a) and molecularly-adsorbed C2H4 species (C2H4(a)) on Co(0001) [18, 21], respectively. With an increasing of exposure temperature to 300 K, the C 1s feature of adsorbed C2H2(a) species at 283.6 eV dominated in the C 1s spectrum. As the exposure temperature further increased to 400 K, the C 1s feature of adsorbed C2H2(a) species disappeared, while two C1s features with the binding energy at 283.2 and 284.5 eV emerged and could be assigned to C2 clusters and graphitic carbon [22], respectively. Exposed at 500 K, the C 1s spectrum was dominated by the C1 adatoms with the binding energy at 283.0 eV [22]. Upon subsequent heating to 630 K, the C 1s spectrum of the C2H4/Co(0001) surface exposed at 130 K weakened with the disappearance of C2H4(a) and C2H2(a) features and the emergence of C2 clusters and graphitic carbon features; the intensity of C 1s spectrum of the C2H4/Co(0001) surface exposed at 300 K remained with the disappearance of C2H2(a) features and the emergence of C2 clusters and graphitic carbon features; the C 1s spectra of the C2H4/Co(0001) surface exposed at 400 and 500 K did not vary much.

Fig. 2
figure 2

C 1s XPS spectra of C2H4 exposure on Co(0001) at indicated temperatures (A) and then followed by subsequent heating to 630 K (B)

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Corresponding He II UPS spectra of saturating C2H4 exposure on Co(0001) at different temperatures on Co(0001) and then followed by heating to 630 K were shown in Fig. 3. Exposed at 130 K, four peaks at 6.5, 8.2, 9.3, 13.0 eV below EF respectively arising from the 1b2g, 3ag, 1b3u and 2au orbitals of adsorbed C2H4(a) species and three peaks at 5.4, 8.9 and 11.3 eV below EF respectively arising from the 1π, 3σg and 2σu orbitals of adsorbed C2H2(a) species [31] appeared in the UPS spectrum. Exposed at 300 K, the features of adsorbed C2H2(a) species dominated in the UPS spectrum. Exposed at 400 K, a major broad peak at around 8.7 eV and another small peak at 5.4 eV below EF appeared, which could be assigned to the orbitals from graphitic carbon and C2 clusters [18], respectively. Exposed at 500 K, only a peak at around 4.8 eV below EF was observed, arising from the orbital of C1 adatoms [18]. Upon subsequent heating to 630 K, the features of C2 clusters, graphitic carbon, and C1 adatoms dominated in the UPS spectra of the C2H4/Co(0001) surfaces exposed at 130, 300 and 400, and 500 K, respectively. These UPS results were consistent with the above XPS results. It is noteworthy that the feature of adsorbed H atoms resulting from C2H4 dissociation should appear in the UPS spectra of C2H4 adsorption at 130 and 300 K but not at 400 and 500 K.

Fig. 3
figure 3

He II UPS spectra of C2H4 exposure on Co(0001) at indicated temperatures (A) and then followed by subsequent heating to 630 K (B)

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The above TDS, XPS and UPS results demonstrated temperature-dependent adsorption and surface reactions of C2H4 on Co(0001). C2H4 majorly adsorbed molecularly and minorly dissociated into C2H2(a) at 130 K. Upon heating, C2H4(a) desorbed from the surface before 300 K and C2H2(a) further dehydrogenated before 450 K to form C2 clusters. C2H4 dissociated into C2H2(a) at 300 K, which, upon heating, dehydrogenated before 450 K to form dominant graphitic carbon. C2H4 dissociated directly into dominant graphitic carbon at 400 K while C1 adatoms at 500 K. These observations indicated that surface reactions of C2H2(a) on Co(0001) should depend on the coverage and temperature.

C2H2 adsorption on Co(0001) at 130 K was studied by TDS. Only H2 desorption traces were observed (Fig. 4) but no C2H2 desorption traces could be observed. In addition to a recombinative H2 desorption peak of H adatoms resulting from residual H2 dissociation at around 380 K, an exposure of 0.01 L C2H2 gave another H2 desorption peak at around 416 K (denoted as α), which could be assigned to the dehydrogenation of adsorbed C2H2(a) species. With an increase of C2H2 exposure to 0.1 L, the α peak obviously increased, and two minor peaks emerged at around 362 (denoted as β) and 395 K (denoted as γ), corresponding to new C2H2(a) dehydrogenation reactions. With the further increase of C2H2 exposure up to 10 L, both the β and γ peaks grew while the α peak did not vary much; meanwhile, the β peak shifted to lower temperatures, implying that the corresponding surface reaction kinetics should be larger than the first-order, but the γ peak did not shift, suggesting a first-order desorption kinetics.

Fig. 4
figure 4

H2 TDS spectra following indicated C2H2 exposures on Co(0001) at 130 K

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C 1s XPS spectra of various exposures of C2H2 on Co(0001) at 130 K (Fig. 5A) all showed a single peak at 283.6 eV arising from adsorbed C2H2(a) species. Thus, C2H2 exclusively adsorbed on Co(0001) molecularly at 130 K at any coverage. The annealing processes of Co(0001) exposed to 0.1 L and 10 L C2H2 at 130 K were studied using XPS and He II UPS. As shown in Fig. 5B, the C 1s feature of C2H2(a) following 0.1 L C2H2 exposure at 130 K did not change much upon subsequent annealing processes except that the binding energy slightly shifted to 283.5 eV above 400 K, suggesting that C2H2(a) exclusively decomposed to produce the H2 dominantly at around 416 K (the α H2 desorption peak) and form C2 clusters with a similar C 1s binding energy to C2H2(a). For an exposure of 10 L C2H2 (Fig. 5C 1 and 2), the C 1s feature of C2H2(a) remained stable up to 310 K and greatly weakened at 340 K, meanwhile, another C 1s feature at 284.4 eV emerged. This new C 1s feature grew and shifted to at 284.5 eV at the expense of the C 1s feature of C2H2(a) slightly at 370 K and greatly at 400 K. The C 1s feature of C2H2(a) disappeared at 430 K, while a major C 1s peak at 284.6 eV and a minor C 1s peak at 283.3 eV were present on the surface, corresponding to the graphitic carbon and C2 clusters, respectively. The C 1s XPS spectra remained unchanged upon annealing at higher temperatures up to 630 K. During the annealing processes, the total intensity of C 1s XPS spectrum did not decrease, suggesting that all C2H2(a) species underwent dehydrogenation reactions to major graphitic carbon and minor C2 clusters. In combination with corresponding H2 TDS spectrum, the major C2H2(a) underwent a two-step dehydrogenation process between 340–370 K and 370–430 K to produce the β and γ H2 desorption peaks, respectively, and the graphitic carbon, while the minor C2H2(a) underwent a one-step dehydrogenation process between 400 and 430 K to produce the α H2 desorption peak and the C2 clusters. It could be thus inferred that a stable surface intermediate with the C 1s binding energy at around 284.5 eV should exist during the two-step dehydrogenation of C2H2(a) into the graphitic carbon.

Fig. 5
figure 5

C 1s XPS spectra of A various exposures of C2H2 exposure on Co(0001) at 130 K, B 0.1 L C2H2 exposure on Co(0001) at 130 K followed by annealing at indicated temperatures and (C1) C 1s XPS spectra of 10 L C2H2 exposure on Co(0001) at 130 K followed by annealing at indicated temperatures, and (C2) derived evolutions of various surface species as a function of annealing temperature

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Figure 6A shows the He II UPS spectra of the annealing processes of Co(0001) exposed to 0.1 L C2H2 at 130 K. Adsorbed C2H2(a) on Co(0001) showed three features at 5.4, 8.7 and 11.2 eV below EF arising from its 1π, 3σg and 2σu orbitals, respectively. These features did not change up to 400 K, then the 2σu peak at 11.2 eV disappeared and the 3σg peak at 8.7 eV weakened upon annealing at 430 K, while the 1π peak at 5.4 eV remained unchanged. This indicated the occurrence of C2H2(a) dehydrogenation with the carbon–carbon preserved. Upon annealing at 630 K, only a broad peak at 5.4 eV arising from the C2 clusters could be observed. These UPS results were consistent with the above XPS results that C2H2(a) at low coverages dehydrogenated into C2 clusters. Figure 6B shows the He II UPS spectra of the annealing processes of Co(0001) exposed to 10 L C2H2 at 130 K. The C2H2(a) features remained unchanged upon annealing at 310 K, and then weakened at 340 K and almost disappeared at 370 K. The difference spectrum between the UPS spectra at 370 and 130 K clearly demonstrated an emergence of a new peak at 7.8 eV below EF at the expense of the C2H2(a) features. It was reported that the σCH bond orbital of adsorbed benzene on Co(0001) film at 300 K located at 7.9 eV below EF [43]. We then assigned the new feature at 7.8 eV to the σCH bond in alkyl fragments with carbon ring structures. The difference spectrum between the UPS spectra at 630 and 370 K showed the disappearance of the peak at 7.8 eV, and two features at 9.2 and 5.6 eV below EF remained in the UPS spectrum annealed at 630 K, which could be assigned to graphitic carbon and C2 clusters. These UPS results, together with the above XPS and TDS results, suggested the alkyl fragments with carbon ring structures as the surface intermediates for C2H2(a) dehydrocyclization into graphitic carbon on Co(0001). It could also be seen that a high C2H2(a) coverage suppressed the reaction pathway of C2H2(a) dehydrogenation into C2 clusters due to the limited available vacant surface sites but facilitated the reaction pathway of C2H2(a) dehydrocyclization into graphitic carbon.

Fig. 6
figure 6

He II UPS spectra of A 0.1 L and B 10 L C2H2 exposure on Co(0001) at 130 K followed by annealing at indicated temperatures

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In order to determine the average C:H stoichiometric of the alkyl fragment intermediates, we compared the H2 TDS spectra and C 1s XPS spectra of 10 L C2H2 exposures on Co(0001) at 130 and 370 K (Fig. 7). Exposed at 370 K, a single H2 desorption peak at around 450 K and a single C 1s feature at around 284.4 eV appeared in the H2 TDS spectrum and C1s XPS spectrum, respectively. The C2H2(a) dehydrocyclization into the alkyl fragment intermediates with carbon ring structures mainly occurred before 370 K while the subsequent dehydrogenation of alkyl fragment intermediates into graphitic carbon mainly occurred after 370 K, thus the surface species following C2H2 exposure at 370 K were dominantly the alkyl fragment intermediates with carbon ring structures. It was found that the C 1s peak intensity/H2 desorption peak area ratio for 10 L C2H2 exposure at 370 K was around half of that for 10 L C2H2 exposure at 130 K. Since 10 L C2H2 exposure at 130 K exclusively gave C2H2(a) with a C:H atomic ratio of 1, the average C:H atomic ratio of the alkyl fragment intermediates was around 0.5. Thus, the alkyl fragment intermediates could be (C2H)n species with carbon ring structures. It should be noted that the (C2H)n intermediates with carbon ring structures could be with different sizes.

Fig. 7
figure 7

A H2 TDS spectra and B C 1s XPS spectra of 10 L C2H2 exposure on Co(0001) at 130 and 370 K

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Therefore, similar to C2H4, C2H2 exhibited temperature-dependent adsorption and surface reactions on Co(0001). C2H2 molecularly adsorbed at 130 K. The resulting C2H2(a) species at low coverages remained stable up to 400 K and then exclusively dehydrogenated into C2 clusters, while the resulting C2H2(a) species at high coverages remained stable up to 310 K and then majorly dehydrocyclizated into (C2H)n intermediates with ring structures at 340 K which further dehydrogenated into graphitic carbon, and minorly dehydrogenated into C2 clusters. Exposed at 370 K, C2H2 dehydrocyclizated into (C2H)n intermediates with ring structures.

4 Conclusions

In summary, we have successfully demonstrated temperature and coverage dependent adsorption and surface reactions of C2H2 and C2H4 on Co(0001). C2H4 majorly adsorbed molecularly and minorly dissociated into C2H2(a) at 130 K. Upon heating, C2H4(a) desorbed from the surface before 300 K and C2H2(a) further dehydrogenated before 450 K to form C2 clusters. C2H4 dissociated into C2H2(a) at 300 K, which, upon heating, dehydrogenated before 450 K to form dominant graphitic carbon. C2H4 dissociated directly into dominant graphitic carbon at 400 K while C1 adatoms at 500 K. C2H2 molecularly adsorbed at 130 K but exclusively dehydrogenated upon heating. The resulting C2H2(a) species at low coverages remained stable up to 400 K and then exclusively dehydrogenated into C2 clusters, while the resulting C2H2(a) species at high coverages remained stable up to 310 K and then majorly dehydrocyclizated into (C2H)n intermediates with ring structures at 340 K which further dehydrogenated into graphitic carbon, and minorly dehydrogenated into C2 clusters. Exposed at 370 K, C2H2 dehydrocyclizated into (C2H)n intermediates with ring structures. These results greatly enrich our fundamental understanding of Co-catalyzed F-T synthesis reaction.