Article pubs.acs.org/JPCC
CH2 Stabilized at Steps on Ru(0001) by Coadsorbates Harald Kirsch,† Xunhua Zhao,† Zefeng Ren,‡ Sergey V. Levchenko,† and R. Kramer Campen*,† †
Fritz Haber Institute of the Max Planck Society, 4-6 Faradayweg, 14195 Berlin, Germany International Center for Quantum Materials and School of Physics, Peking University, No. 209 Chengfu Road, 100871 Beijing, China
‡
S Supporting Information *
ABSTRACT: Insight into the controls of stability of onecarbon moieties adsorbed on transition-metal surfaces is important in the optimization of such industrially important processes such as the Fischer−Tropsch (FT) synthesis, that is, metal
(2n + 1)H 2 + nCO XoooooY CnH(2n + 2) + nH 2O. W h i l e t h e broad steps of FT synthesis are clear, CO dissociatively adsorbs on steps on transition-metal surfaces, this carbon is hydrogenated, one-carbon groups are coupled, and the resulting larger molecule desorbs, a deeper description of the mechanism has proven challenging. In particular, while experiment and calculation of barriers for coupling reactions suggest that step-bound CH2 should be the chain growth one-carbon species, calculations of CH2/CH relative stability at low surface coverages suggest CH2 dissociation is too rapid to allow such a pathway and thus CH is the likely candidate. In this study, we characterize the dissociation of CH2 adsorbed at steps on the Ru(0001) surface in ultrahigh vacuum using laser-based technique vibrational sum frequency (VSF) spectroscopy and electronic structure calculation. Experimentally, we find the barrier for CH2 dissociation to be 0.47 eV, 3× larger than the calculated barrier for dissociation of an isolated CH2 on a terrace on the Ru(0001) surface. However, both our experiment and real application steps are likely saturated by adsorbed carbon species. For such a system, we show that both the barrier for CH2 dissociation and that for the diffusion of CH away from the steps each increase 2−3×. This result highlights the large influence of coadsorbates on step-bound one-carbon moieties and provides a means of reconciling previous apparently contradictory results on the FT synthesis.
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INTRODUCTION Understanding relative stabilities of one-carbon moieties adsorbed on transition-metal surfaces is important because such species are intermediates in the conversion of hydrocarbons to many useful products. For example, the steam
often created by dosing single-crystal surfaces with methane and characterized by ultra high vacuum (UHV).1,2,8−17 Single crystals of ruthenium, because of their high reactivity with methane and relatively smaller reactivity with other types of, adventitous, carbon, have been particularly popular model systems.4,14,18 Using this single crystal in UHV model system approach to understand FT synthesis on Ru(0001) is challenging. While both experiment and theory clarify that the first step in this process, the dissociative adsorption of CO, likely occurs at steps (on Ru(0001) the CO dissociative adsorption barrier at steps is 1/2 that on terraces4), the mechanism of chain growth is substantially less clear. Early work, which prepared surfaces with adsorbed CH2 by dosing Ru(0001) with CH2N2, suggested CH2 was the fundamental building block19 for chain growth. The results of computational studies of barriers for possible coupling reactions are consistent with this conclusion: in general, reactions involving CH2 as the chain building moiety in FT synthesis are kinetically favored over those involving CH.4,20−22 Calculations of the relative stability
metal
reforming reaction (CH4 + H 2O XoooooY CO + 3H 2 ) is a useful means of generating molecular hydrogen from CH4, and the Fischer−Tropsch (FT) synthesis, that is, metal
(2n + 1)H 2 + nCO XoooooY CnH(2n + 2) + nH 2O, is an important pathway in the controlled creation of long-chain hydrocarbons.1−5 Yields of these and other similar reactions are typically empirically optimized in industrial processes employing high pressure/temperature/surface area fixed-bed reactors.6,7 Because high temperatures tend to obscure transient intermediates and the catalytic surfaces in such systems are highly heterogeneous, gaining molecular level insight into the processes happening in such reactors is challenging both experimentally and theoretically. Driven by a desire to understand the mechanisms of this important chemistry, much work over the last several decades has focused on the stability and reactivity of surface bound one-carbon moieties, © 2016 American Chemical Society
Received: July 15, 2016 Revised: September 30, 2016 Published: October 11, 2016 24724
DOI: 10.1021/acs.jpcc.6b07088 J. Phys. Chem. C 2016, 120, 24724−24733
Article
The Journal of Physical Chemistry C
He, suggesting other compounds exist in sufficiently small quantities in the chamber to not influence the results. For all experiments, the surface was cleaned using an Ar+ sputtering, high temperature annealing in oxygen, high temperature annealing in vacuum procedure previously demonstrated to produce a carbon- and defect-free Ru(0001) surface.28 The effectiveness of this procedure, and cleanliness of the sample, was checked by Auger electron spectroscopy (AES) and by low energy electron diffraction (LEED). For characterization of the sample, we employ the laser-based technique vibrational sum frequency (VSF) spectroscopy. In our VSF measurements, the outputs of pulsed infrared and 800 nm lasers are overlapped spatially and temporally at an interface, and the emission at the sum of the frequencies of the two incident fields is monitored. This sum frequency emission is useful because it is, by its symmetry selection rules, interface specific and because it is a spectroscopy (if the frequency of the incident IR field is in resonance with an interfacial vibrational transition, VSF emission increases by several orders of magnitude). For background to the technique, see prior work.29−34 All VSF spectra were acquired at the substrate temperature of 110 K. Control experiments indicate that at this temperature all molecular species were stable over more than 24 h. To quantify the VSF spectral response, we follow prior workers and model the VSF signal as a coherent superposition of a nonresonant background and several homogeneously broadened resonances:28
of CH2 and CH in the limit of low surface coverage, however, have been interpreted to be inconsistent with this picture: CH2 should rapidly dehydrogenate to form CH, and thus CH must be the chain growth moiety.4,15 We have previously shown, in a combined computational and experimental study, that while the reaction Ru(0001)
CH 2 ⎯⎯⎯⎯⎯⎯⎯⎯→ CH + H on terraces has a barrier of 0.12 eV in isolation, with high surface coverages of hydrogen this barrier is dramatically higher, 0.67 eV, due to a surface site blocking effect.23 Because industrial applications of FT synthesis occur at high CO and H2 partial pressures on surfaces with high densities of under-coordinated metal atoms, it seems likely that understanding surface coverage effects on the relative stabilities of CH2 and CH at steps could shed significant light on the mechanism of the FT synthesis. Here, then, we extend the combined experimental/theoretical approach of our prior study to consider the stability of surface bound CH2 on steps of the Ru(0001) and the influence of high carbon coverages on this stability. We find experimentally that the conversion of CH2 to CH on steps has an activation energy (Ea) of 0.47 eV: ∼3× higher than the Ea of this reaction for isolated CH2 groups on terraces. By conducting density-functional theory calculations in parallel, we clarify that the origin of this enhanced stability is a coadsorbate effect: CH2 at steps is strongly stabilized in the presence of coadsorbed CH. This relative stability of step-bound CH2 on Ru(0001) suggests its potential role as the building block in the FT process, at least on Ru at low total carbon coverages, should be revisited. Taken together, this and our prior work describing CH2 adsorbed on Ru(0001) terraces23 both highlight the importance of quantitatively considering all coadsorbates, whether hydrogen, carbon, or some other species, in understanding one-carbon compound reactivity on transition-metal surfaces. For the case of CH2 on Ru(0001), such effects, at coadsorbate coverages common in application, can change reaction barrier heights by more than 5×.
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(2) 2 (2) IVSF ∝ |χeff | = |χNR + χR(2) |2
= |ANR | e
iϕNR
+
∑ q
2
Aq ωIR − ωq + iΓq
(1)
(2) in which χNR is the so-called nonresonant second-order nonlinear optical susceptibility and is, for the Ru surface, dominated by the response of the metal, and χ(2) R is the resonant second-order nonlinear optical susceptibility and contains all contributions from adsorbates. Aq, ωq, and 2Γq are the complex amplitude, center frequency, and line width of the qth resonance, and |ANR| the amplitude and ϕNR the phase, of the nonresonant contribution. χ(2) R is proportional to interfacial population. For technical details of the laser setup, details of the line-shape analysis, and a discussion of the benefits of VSF spectroscopy in analysis of these systems relative to electron energy loss (EELS) and reflection absorption infrared spectroscopy (RAIRS), see the Supporting Information and our prior work.23,33 We quantified surface coverage, after VSF spectra had been collected on a particular sample, by performing a temperatureprogrammed oxidation (TPO) measurement following prior workers.14,35−37 In a TPO measurement, we first adsorbed molecular oxygen at room temperature on a carbon covered sample. After oxygen dosing, we applied a linear heating ramp and measured the desorbing CO, using a shielded quadrupole mass spectrometer (QMS), as a function of sample temperature. The detected CO signal is related to % of a monolayer of carbon by calibration with the known density of carbon in a monolayer of CO on Ru(0001) (see the Supporting Information and our prior study for details and an example data set23).
METHODS
Experimental Methods. The experimental setup and details of the data analysis have been described in detail recently, 23 and thus we here mention only the key methodological points (relevant additional details are also reproduced in the Supporting Information). All experiments were performed in a UHV chamber with a base pressure of 1.5 × 10−10 mbar. In bulb experiments characterizing the dissociative adsorption of methane on Ru(0001), a strongly surface temperature-dependent sticking coefficient has been observed.14 Employing a supersonic molecular beam source to dose CH4 avoids this experimental limitation (dosing in this manner CH4’s sticking coefficient depends strongly on nozzle, but only very weakly on, sample temperature24,25). Motivated by these previous efforts in this study, we dosed CH4 using supersonic molecular beam source (MBS) with three-stage differential pumping modeled after prior workers.26,27 The gas mixture for the MBS contained 5% CH4 seeded in helium (99.9999% purity, Westphalen AG). The temperature of the nozzle was set to 860 K for all measurements, corresponding to a kinetic energy of Ekin ≈ 0.6 eV for each CH4.23 We found empirically that under these conditions CH4 dissociates in the nozzle: the nozzle became blocked over months of use. However, residual gas analysis in our sample chamber by quadrupole mass spectrometer (QMS) detected only CH4 and 24725
DOI: 10.1021/acs.jpcc.6b07088 J. Phys. Chem. C 2016, 120, 24724−24733
Article
The Journal of Physical Chemistry C vib Er = E FS − E IS + ΔEZP
Theoretical Methods. All DFT calculations were performed using the FHI-aims electronic structure package,38 with settings similar to our previous work23 and described below. The monoatomically stepped Ru(0001) surface is modeled by a periodic 8-layer slab with a (2×5) hexagonal surface supercell and two rows of topmost Ru atoms along one of the in-plane lattice vectors removed (see Figure 1). The top three Ru layers
where EIS, ETS, and EFS are total energies of initial state, transition state, and final state, respectively, and ΔEvib ZP is the difference between zero-point vibrational energies (ZPE) of the corresponding states calculated using the harmonic approximation. For transition states, the mode with an imaginary frequency does not contribute to the ZPE. In the vibrational calculations, all Ru atoms are kept fixed, while all adsorbate atoms are allowed to move. At finite temperature, the vibrational contribution to the free energy in the harmonic approximation is Nvib
F vib(T ) =
vib = EZP +
Figure 1. (2×5) periodic model for a stepped Ru(0001) surface. The topmost ruthenium atoms are shown in dark green, while all other ruthenium atoms are in blue. The gray lines indicate the periodic boundary.
⎡ ℏω
⎛ ⎛ ℏω ⎞⎞⎤ + kBT ln⎜⎜1 − exp⎜ − i ⎟⎟⎟⎥ ⎢⎣ 2 ⎝ kBT ⎠⎠⎥⎦ ⎝
∑⎢ i=0
i
Nvib
⎛
i=0
⎝
⎛ ℏωi ⎞⎞ ⎟⎟⎟ ⎝ kBT ⎠⎠
∑ kBT ln⎜⎜1 − exp⎜−
(4)
where Nvib is the number of contributing vibrational modes.
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RESULTS AND DISCUSSION CH2 Adsorption at Steps. Egeberg et al. have previously reported that steps on Ru(0001) do not control the activity of the surface with respect to methane dissociation14(in contrast, for example, to reactions like N2 or CO dissociation). However, their temperature-programmed oxidation (TPO) data clearly indicate that short time exposure of methane to a sample initially saturates a step bound carbon species, a point conclusively illustrated by conducting TPO measurements of a clean Ru(0001) surface dosed with CH4 and an Ru(0001) surface on which steps have been poisoned by Au despoition before methane dosing. While these experiments do not clarify the particular CHx species present at Ru(0001) steps, they suggest that with short methane dosing times it should be possible to prepare an Ru(0001) surface where step sites are, nearly, saturated by adsorbed CHx species while total adsorbed carbon is low. Dosing Ru(0001) for 5 min with a TRu = 350 K and a nozzle temperature of 860 K results in a sample with a single clear resonance in the CH stretch frequency range centered at 2925 cm−1 (see Figure 2b) and a surface coverage of 3−5% of a monolayer (ML) of carbon (see the Supporting Information for
are relaxed, while the remaining five are fixed at bulk geometry. The bulk lattice constants were obtained with a primitive cell, and are 2.717, 2.717, 4.296 Å. We have shown previously that a slab model of at least seven layers is important for getting converged reaction energies of C1 moieties in the Ru(0001) system.23 For the perfect Ru(0001) surface, a (2×2) hexagonal surface supercell is used. For both models, a large vacuum distance (>35 Å) is used to avoid interaction between neighboring slabs. A k-mesh of (9×4×1) is used for the stepped Ru(0001), and (9×9×1) is used for the perfect surface. The PBE functional39 and default tight numeric grids are used for all DFT calculations in this work. van der Waals interaction is taken into account by using the first-principles vdWsurf scheme.40 The string method41 is employed to calculate the transition states and minimum-energy paths. Seven or more images are used for each minimum-energy path calculation. The reaction barrier is defined as vib Ea = E TS − E IS + ΔEZP
(3)
(2)
and the reaction energy is defined as
Figure 2. VSF spectra of dissociated CH4 on Ru(0001) dosed for 5 min at (a) 250 K and (b) 350 K surface temperature. The peaks have their origin in the νs of CH2 at steps. 24726
DOI: 10.1021/acs.jpcc.6b07088 J. Phys. Chem. C 2016, 120, 24724−24733
Article
The Journal of Physical Chemistry C
at 350 K has the advantage that there should be no coadsorbed H43,44 and relatively little coadsorbed adventitious CO,28,45,46 thus minimizing any role of coadsorbates in stabilizing C1 surface bound moieties. Regardless of the details of preparation, this step bound CH2 can be converted to CH with heating to 400 K. As noted above, from an experimental point of view, this transformation is irreversible: cooling the sample back to 350 K does not cause the reappearance of a narrow resonance centered at 2925 cm−1. This implies that by heating samples of step bound CH2 prepared at TRu = 350 K, we quantify the barrier for the dissociation reaction of step-bound CH2 in the limit of low surface densities of coadsorbed H or CO. It is worth noting before continuing that similar insight is not possible by quantifying CH ingrowth as a function of temperature. We expect, as discussed further below, in the Supporting Information, and in previous work,23 that this difficulty may be a combination of the generated CH diffusing off of steps, the CH stretch of step-bound CH groups being indistinguishable from that of terrace-bound CH, or the enhanced sensitivity of CH stretch frequency to coadsorption of adventitious CO. Thermal Stability of CH2 at Steps. To characterize the thermal stability of CH2, and quantify the barrier for CH2 dissociation, we performed the following five-step experiment: (i) Dose the sample at TRu = 350 and 860 K nozzle temperature for 5 min. (ii) Cool the sample to 100 K and collect a VSF spectrum. (iii) Heat the sample to an elevated temperature for 5 min (between 330 and 380 K). (iv) Cool the sample to 100 K and collect a second VSF spectrum. (v) Quantify the surface coverage of, and remove carbon via, a TPO method (note that, while carbon coverage determination was necessary to assign the observed CH stretch feature to step bound CH2, and is important to ensure the correct functioning of our MBS, it is not necessary for the thermal stability analysis described in what follows.) Spectra before and after a heating step to 355 K are given in Figure 3. Clearly the intensity of the CH2 resonance feature decreases after the heating. Because, as discussed above and in the Supporting Information, VSF intensity is proportional to the square of the number density n of adsorbed CH2, comparing the spectra before and after heating allows the extraction of the decrease in population of surface bound CH2 due to the thermal treatment. The five-step procedure describes above thus allows us to quantify the relative population change at one annealing temperature. Performing this procedure at different annealing temperatures in step 3 (between 330 and 380 K and always with freshly prepared samples) allows us to quantify how the relative population change of adsorbed CH2 depends on temperature. If we assume the loss of CH2 is due to dissociation (i.e., CH2 → CH + H), and that the rate of this reaction is first order in CH2
carbon coverage determination details). Dosing for longer times, that is, carbon coverages higher than 3−5% of a monolayer, leads to no increase in the amplitude of the resonance centered at 2925 cm−1 but to the appearance of an additional spectral feature at 3010 cm−1 (data not shown). If we dose a sample at TRu = 350 K, shut off the dosing, and then heat it for 5 min to 355 K, the 2925 cm−1 feature partially disappears and a feature at 3010 cm−1 grows in (see Figure 3).
Figure 3. VSF spectra of dissociated CH4 on Ru(0001), dosed for 5 min at 350 K, before and after heating to 355 K for 5 min. Fits of the CH resonance are included in the Supporting Information.
We have previously shown that dosing Ru(0001) with CH4 seeded in He using our molecular beam source with TRu = 250 K for 30 min gives a surface with 15−20% of a ML of carbon surface coverage and a single narrow spectral feature centered at 2940 cm−1.23 Heating a sample prepared in this manner to 300 K leads to a suppression of the 2940 feature and an ingrowth of a narrow resonance at 3010 cm−1 indistinguishable from that shown in Figure 3. On the basis of comparison to earlier work42 that directly prepared surface bound CH2 on Ru(0001) by dosing of CH2N2 and theoretical studies, we assigned the 2940 cm−1 feature to surface bound CH223 and the 3010 cm−1 feature to surface bound CH, both adsorbed on Ru(0001) terraces.1,2,8,9 Given the study of Egeberg et al.,14 one possible assignment for the feature we observe at 2925 cm−1 dosing at the elevated temperatures and shorter times in this study (TRu = 350 K and 5 min vs 250 K and 30 min in our previous work23) is that we have populated steps on the Ru(0001) surface with adsorbed CH2. If this is true, and assuming step densities are
