Selective Hydrogenation of Acetylene to Ethylene in the Presence of a

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Selective Hydrogenation of Acetylene to Ethylene in the Presence of a Carbonaceous Surface Layer on a Pd/Cu(111) Single-Atom Alloy Christopher M Kruppe, Joel D Krooswyk, and Michael Trenary ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02862 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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Selective Hydrogenation of Acetylene to Ethylene in the Presence of a Carbonaceous Surface Layer on a Pd/Cu(111) Single-Atom Alloy Christopher M. Kruppe, Joel D. Krooswyk, Michael Trenary* Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607 *[email protected] ABSTRACT Reflection absorption infrared spectroscopy (RAIRS) was used to simultaneously monitor gas phase and surface species in the presence of ambient pressures of acetylene and hydrogen over a single-atom alloy (SAA). The alloy consisted of isolated Pd atoms at surface coverages in the range 0.0028 to 0.085 ML in a Cu(111) surface. When C2H2(g) is present, but not H2(g), the RAIR spectra are similar for Cu(111) with and without Pd, independent of C2H2(g) pressure for temperatures between 180 and 500 K. The addition of H2(g) leads to different RAIR spectra depending on the presence of Pd. With a C2H2:H2 ratio of 1:100 and a SAA-Pd/Cu(111) surface with less than 1% Pd, C2H2(g) is converted to C2H4(g) at 400 K at total pressures up to 10 Torr. From the rate of change in the gas phase IR peaks, a range of initial turnover frequencies was estimated, which depend on which sites are assumed to be active for hydrogenation. Postreaction surface analysis with Auger electron spectroscopy (AES) showed a significant carbon 1 ACS Paragon Plus Environment

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coverage, which decreased with increasing Pd coverage. The combined RAIRS and AES results suggest that by increasing reactivity for ethylene formation, Pd also limits the amount of carbon that is deposited, while also changing the extent of oligomer formation. Keywords: Single-atom alloy; acetylene selective hydrogenation; Pd/Cu(111); reflection absorption infrared spectroscopy; turnover frequency

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1.

INTRODUCTION

The selective hydrogenation of alkynes to alkenes is an important area of heterogeneous catalysis. For example, conversion of acetylene to ethylene is used to remove small quantities of acetylene from the ethylene feedstocks used for the production of polyethylene.1-2 The challenge is to achieve a high reactivity for alkyne hydrogenation in the presence of a large excess of alkene without also hydrogenating the alkene to the alkane. This goal is usually achieved through the use of bimetallic catalysts that contain Pd, with the exact composition designed to retain high activity while increasing selectivity. Effective catalysts have even been reported for Pd combined with non-metals such as sulfur.3 Among the many metals used to form Pd-containing bimetallic catalysts, Cu is particularly well studied.2, 4 In general, in any catalyst it is desirable to minimize the use of expensive metals, such as Pd, that are needed to achieve a particular catalytic objective. An especially efficient use of Pd is in the form of single-atom alloys (SAAs), a term first used by Sykes and coworkers to describe Cu(111) surfaces in which the Pd is present as isolated atoms that replace Cu atoms in the topmost surface layer.5 Other studies have shown that the Pd atoms are not distributed evenly on the surface but are concentrated near the top of Cu{111} steps.6-8 It was found that the Pd promoted H2 dissociation with spillover of H atoms to nearby Cu sites.5 Using DFT calculations of various configurations of surface and subsurface Pd in Cu(111), Fu and Luo argued that the lowest activation energy for H2 dissociation was achieved when a Pd surface atom was located above a subsurface Pd atom.9 Kyriakou et al. used temperature programmed desorption (TPD) under ultrahigh vacuum conditions to show that a Cu(111) surface with 0.01 monolayer (ML) of Pd could selectively hydrogenate acetylene to ethylene.5 Ma et al. used DFT calculations to show that a Pd/Cu(111) SAA achieves selectivity


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in acetylene hydrogenation mainly by lowering the barrier to ethylene desorption.10 Supported Pd/Cu SAA catalysts have been shown to be effective for the partial hydrogenation of acetylene11-13 and propyne.14 It has also been found that the addition of CO to the feed gas can enhance the selectivity of Pd/Cu catalysts.15 Other studies have shown that the addition of CO can alter the structure of Cu catalysts that don’t contain Pd and thereby enhance alkene selectivity.16-17 Theoretical calculations of SAAs consisting of Pt, Pd, Ni, and Rh in a Cu host indicate that Pd/Cu and Pt/Cu should be stable as acetylene hydrogenation catalysts, whereas Ni/Cu and Rh/Cu are not.18 Here we have used reflection absorption infrared spectroscopy (RAIRS) to monitor acetylene hydrogenation over a Pd/Cu(111) SAA under ambient pressures of acetylene and hydrogen. The results reveal that while acetylene is selectively hydrogenated to ethylene over Pd/Cu(111) but not over Pd-free Cu(111), the elevated acetylene pressures lead to coupling products not observed with RAIRS under UHV conditions and that these products are different on the Cu(111) and Pd/Cu(111) surfaces. The elevated pressure conditions more closely mimic those of practical hydrogenation catalysis and add new insights into the surface species present under actual acetylene hydrogenation conditions. In addition to the reaction that is desired, namely hydrogenation of acetylene to ethylene, acetylene is known to undergo other reactions on both Cu and Pd surfaces. In particular, trimerization to form benzene is a structure sensitive reaction that occurs on several surfaces.19 On Cu(111), Kyriakou et al., used TPD to observe the desorption from Cu(111) of coupling products including benzene and cyclooctatetraene under UHV conditions.20 They observed similar products under atmospheric conditions for a catalyst consisting of Cu nanoparticles supported on alumina.20 In their experiments they only observed desorption products, although they inferred that non-desorbing oligomers also formed on the surface. It has been proposed that 4 ACS Paragon Plus Environment

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trimerization occurs via a C4H4 metallacyle intermediate.21 In addition to benzene formation on the Pd(111) surface, when hydrogen is present acetylene is readily hydrogenated to ethylene.22-24 Under UHV conditions, the acetylene isomerizes to vinylidene and can be hydrogenated to the spectator species ethylidyne, with the latter yielding a characteristic RAIR spectrum.25 Ethylidyne was readily detected on Pd(111) with RAIRS while benzene was being formed in the presence of 5 Torr of acetylene.26 The formation of oligomers over practical Pd-based acetylene hydrogenation catalysts is well-known.27 These oligomers are thought to be intermediates to the formation of so-called green oil, a product that is associated with the deactivation of the catalyst.28 As the coupling reactions to form carbonaceous surface species compete with acetylene hydrogenation, it is clearly desirable to perform studies over model catalysts in which the hydrogenation activity and the formation of carbonaceous species can be simultaneously monitored. In a recent study of acetylene hydrogenation over a Pt(111) surface, we used polarization-dependent RAIRS to monitor the conversion of gas-phase acetylene first to C2H4(g) and then to C2H6(g) while also monitoring the formation of various surface species, the most prominent of which was ethylidyne (CCH3).29 We use a similar approach here to observe that C2H2(g) is converted to C2H4(g) but not to C2H6(g) while also observing the formation of surface-bound carbonaceous species. Through comparisons with IR spectra in the literature, we conclude that whereas polyacetylene forms on Pd-free Cu(111) under hydrogenation conditions, the presence of Pd atoms in the Pd/Cu(111) SAA promotes the formation of a surface carbonaceous species similar in structure to green oil. 2.

EXPERIMENTAL SECTION


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All experiments were performed in a UHV system that houses an upper analysis chamber and a lower ambient pressure IR cell. The upper chamber is equipped with a LK Technologies RVL2000 Auger-LEED system, Pfeiffer Prisma quadrupole mass spectrometer, and a PerkinElmer ion sputter gun. The sample is transferred into the IR cell through spring-loaded Teflon seals that isolate the main chamber during the ambient pressure experiments. The upper analysis chamber is kept at 1×10-9 Torr during the ambient pressure experiments with an ion pump. The IR experiments were performed with a Bruker Vertex 70v FTIR. The polarization-dependent reflection absorption infrared spectroscopy (PD-RAIRS) setup allows for acquisition of surface and gas phase species through the use of a rotatable polarizer as described earlier.30 The Cu(111) crystal (99.9999%, Princeton Scientific) was cleaned by Ar ion sputtering (1 keV, 10 µA) and annealing (950 K). The surface cleanliness was confirmed with AES. Pd was deposited on the Cu(111) crystal at 380 K from a resistive evaporator consisting of a thin Pd wire (99.99%, Alpha Aesar) wrapped around a W wire. The deposition rate was ~0.006 ML/min allowing for low coverages of Pd to be prepared. Surface coverages of Pd have been determined by PD-RAIRS measurements of CO as described earlier.31 Surface Pd coverages determined this way were found to be lower than Pd coverages determined from AES by a factor of 0.28 due to the detection of both surface and subsurface Pd with AES whereas CO only adsorbs on surface Pd atoms. This relation was used to determine surface Pd coverages from post reaction AES measurements to avoid using CO. The Pd deposition rate was reproducible so that it was not necessary to verify the Pd coverage after each deposition with AES, which minimized carbon contamination from electron-beam induced dissociation. RAIRS experiments were performed with 1024 scans at 4 cm-1 resolution, unless otherwise noted. For most experiments the sample was held at a given temperature, exposed to 6 ACS Paragon Plus Environment

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the reactant gases, annealed to a specific temperature, and cooled to the starting temperature where a spectrum was obtained. For experiments performed at low temperatures, a UHV exposure of C2H2(g) in fixed Langmuir (L) units (1 L = 1×10-6 Torr s) was made followed by admitting a static pressure of H2(g) into the IR cell. The ambient pressure experiments starting at 300 K were done with gas mixtures of C2H2(g) and H2(g). The TOF experiment was done by holding the crystal temperature at 400 K, admitting the gas phase reactants into the IR cell, and taking IR scans as a function of time. Atomic absorption grade acetylene (99.6%) and hydrogen (99.999%) was purchased from Praxair. The acetylene was further purified by the freeze-pump thaw method and all gases were checked by mass spectrometry. The hydrogen was exposed through a liquid N2 trap to remove any residual contaminants. 3. RESULTS and DISCUSSION 3.1 Low coverage, low temperature C2H2 adsorption and hydrogenation on Cu(111) and Pd/Cu(111) Infrared spectra of 2 L of C2H2 exposed to Pd-free Cu(111) and to 0.085 ML Pd/Cu(111) at 150 and 180 K, respectively, are displayed in Figure 1. The spectra agree with earlier vibrational studies of C2H2 exposed to Cu(111) at low temperatures and we assume that the molecule bonds with the structure previously proposed.32-33 The identical peak positions in the two cases indicate that the Pd atoms do not affect the internal bonding of C2H2 interacting with the Cu(111) surface. This could be due to Pd atoms adsorbed near the step sites, while C2H2 is mainly bound to the Cu(111) terraces. There was no significant change in the spectra for the two surfaces after 2×10–2 Torr of H2(g) was added to the cell with the crystal at 180 K. However, annealing the surfaces to 280 K showed a significant decrease in the acetylene peaks for the 7 ACS Paragon Plus Environment

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Pd/Cu(111) surface whereas there was little change for the Cu(111) surface, even after annealing to 360 K. In contrast, for the Pd/Cu(111) surface, the acetylene peaks are essentially gone after annealing to 300 K. Other results, not shown, show little difference between Pd-free Cu(111) and Pd/Cu(111) for annealing to 280 K in the absence of H2(g). These results are readily explained

Figure 1. RAIR spectra obtained while monitoring C2H2 hydrogenation at low temperature. 2 L of C2H2 was exposed to Cu(111) at 150 K (A) and to 0.085 ML Pd/Cu(111) at 180 K (B). 2×10-2 Torr of H2 was then added to the IR cell and the samples annealed at the indicated temperatures for 30 s and a spectrum was taken upon cooling to 180 K.

by removal of adsorbed acetylene through hydrogenation, which occurs to a much greater extent in the presence of Pd atoms on the surface. When an ambient pressure of H2(g) is not present, Kyriakou et al. used temperature programmed reaction (TPR) to show that acetylene adsorbed under UHV conditions is removed from the surface by desorption and by coupling reactions to form benzene and cyclooctatetraene, along with some hydrogenation to ethylene and butadiene, all of which desorb in the range of 280 to 350 K.20 After the TPR sweeps, no carbon was detected by XPS.20 3.2 C2H2 coupling at 1×10-2 Torr


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The peaks observed in the RAIR spectra in Figure 1 following exposure to C2H2(g) in UHV are all assigned to adsorbed acetylene. As the surfaces were annealed, no new peaks were observed that would be indicative of the formation of additional surface species. Distinctly

Figure 2. Comparison of ambient pressure C2H2 coupling over clean Cu(111) (blue dotted line), 0.0028 ML Pd/Cu(111) (green), and 0.080 ML Pd/Cu(111) (red). Spectra were acquired in the presence of 1×10-2 Torr of C2H2 at 300 K then annealing to 480 K. Green numbers denote gas phase peaks.

different results were observed when the surfaces were annealed in the presence of gas phase C2H2(g). Figure 2 shows spectra for Pd-free Cu(111) as well as for 0.0028 and 0.080 ML Pd/Cu(111) at 300 K and after annealing to 380 and 480 K in the presence of 1×10-2 Torr of C2H2(g). At 300 K, the spectra for the three surfaces match almost perfectly and display the same peaks as observed in Figure 1 under UHV conditions. However, upon annealing to 380 K, new peaks appear with different intensities for the three surfaces. Gas phase acetylene peaks are also present (identified with green numbers) but do not change with annealing temperature and have 9 ACS Paragon Plus Environment

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the same intensity for the three surfaces. For the 380 K anneal, the most prominent new peak is at 1006 cm–1, which is a good match to polyacetylene.34 In particular, for polyacetylene prepared at 150° C, a peak at 1015 cm-1, assigned to an out-of-plane C–H bending mode, is by far the most intense peak in the IR spectrum.34 Also present in Figure 2 is a derivative-shaped peak with a negative lobe at 1044 cm-1. Such shapes are typically observed with RAIRS when a peak present in the background spectrum is shifted as more of the adsorbate responsible for the peak is added to the surface. Since a peak at 1044 cm–1 has previously been assigned to H atoms adsorbed in the three-fold hollow sites of Cu(111),35-36 the results here imply that there was already hydrogen inadvertently present when the background spectrum was taken. Whether hydrogen was initially present or not depends on the exact conditions used so there is considerable variability in this region from one experiment to the next. For example, this feature is even more prominent in Figure 3. The other peaks observed in the 380 K spectrum in Figure 2 are not assigned to polyacetylene for the following reasons. First, the C–H stretch peaks of polyacetylene all occur at or above 3000 cm–1, whereas C–H stretch peaks are observed at 2865 and 2929 cm–1 for the 480 K anneal. Second, these C–H stretch peaks increase in intensity in the 480 K spectra relative to the 380 K spectra, whereas the 1006 cm–1 peak decreases. Furthermore, the 1006 cm–1 peak behaves the same for all three surfaces, whereas the peaks at 2865 and 2929 cm–1 are most intense for the Pd-free Cu(111) surface, and least intense for the highest Pd coverage. These results indicate that the presence of Pd has little influence on the formation and decomposition of polyacetylene on Cu(111). However, the presence of Pd slightly suppresses formation of the second coupling product with characteristic peaks at 2929 and 2865 cm–1, which we assign to an oligomer. 3.3 C2H2 hydrogenation by SAA Pd/Cu(111) at elevated pressures 10 ACS Paragon Plus Environment

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Figure 3. A. Full RAIR spectra of C2H2 hydrogenation over 0.0055 ML Pd/Cu(111). B. RAIR spectra of the CH2 wag region for gas phase C2H4. Hydrogenation is performed by admitting 1×10-2 Torr of C2H2 followed by 1 Torr of H2 to the IR cell. The sample was then annealed to the listed temperatures for 30 s prior to cooling to 300 K where spectra are obtained. Green numbers denote peaks due to gas phase species.

Figure 3A shows the results of annealing a 0.0055 ML Pd/Cu(111) surface in the presence of 1× 0-2 Torr of C2H2(g) and 1 Torr of H2(g). Comparison with the results in Figure 2 shows that the addition of H2 had several effects. First, the gas phase acetylene peaks disappeared after the 480 K anneal, which was accompanied by the appearance of a gas-phase ethylene peak at 950 cm–1. To see this peak more clearly, Figure 3B shows expanded spectra from 850 to 1100 cm–1. As previous work has shown, the 950 cm–1 peak is the most intense one in the spectrum of gas phase ethylene.29 In contrast to our earlier study of acetylene hydrogenation over a Pt(111) surface, gas phase ethane is not observed.29 Figure 3 thus demonstrates that acetylene is selectively hydrogenated to ethylene over this surface. Second, after annealing to 480 K, the intensity of the oligomer peaks, such as the ones at 2929 and 2960 cm–1, are more intense by a factor of ~ 5 compared to the corresponding peaks in Figure 2. This result implies that hydrogen 11 ACS Paragon Plus Environment

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plays a significant role in the mechanism by which the oligomer forms. Third, the polyacetylene peak at 1006 cm–1 is almost entirely eliminated in the spectrum after annealing to 480 K, whereas it is still present in Figure 2 when no H2(g) is present. The behavior of the 3014 cm–1 peak is similar to the one at 1006 cm–1, suggesting that the former can be assigned to a C–H stretch of polyacetylene. Although the 3014 cm-1 peak is close to the frequency of the asymmetric C–H stretch of gas phase methane, this possibility is excluded as it is not observed when the experiment was performed with s-polarized light, which is sensitive only to gas-phase molecules. The 3014 cm-1 peak also remains after the IR cell is evacuated providing further

Table 1 Comparison of peak position observed in Figure 3 with literature values for the upper phase of green oil obtained from C2H2 hydrogenation over a Pd/Al2O3 catalyst. Assignment

Upper Phase Green Oil.a, [cm–1]

Oligomer Peaks from Fig. 3b, [cm–1]

CH3, asymm stretch

2960

2960

CH2, asymm stretch

2930

2929

CH2, symm stretch

2860

2857

CH2 def + CH3(as) def

1463

1463

CH3, symm def

1380

1378

Trans C–H wagging

980

969

Vinyl CH2 wagging

920

909

a. Ref. 37 b. this work


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evidence that it is not due to a gas phase molecule. Our assignment of the 3014 and 1006 cm–1 peaks to polyacetylene is consistent with Shirakawa et al., where the IR spectrum of polyacetylene produced at 150° C features only two prominent peaks, at 1015 and 3013 cm–1.34 To more clearly distinguish between gas-phase and surface species, the s-polarized spectra shown in Figure 4A were obtained under the same conditions as for Figure 3. With the 0.08 ML Pd/Cu(111) surface at 300 K, after adding 1×10–2 Torr of C2H2 and 1 Torr of H2 to the IR cell, gas phase acetylene peaks are seen at 1300, 1352, 3265, and 3312 cm–1. When the surface is annealed to 340 K, the decrease in intensity of the C2H2(g) peaks is accompanied by the appearance of the C2H4(g) peak at 950 cm–1. The C2H2(g) peaks are absent in the 360 and 380 K spectra and the disappearance of the 950 cm–1 peak upon evacuation of the IR cell is consistent with assigning it to a gas phase species. These results are in contrast to what we observed with spolarized spectra of acetylene hydrogenation over Pt(111) where the gas phase ethylene peak was completely replaced upon annealing to 370 K by a peak centered at ~ 2980 cm–1, which was broadened due to unresolved rotational structure of the IR active C–H stretches of ethane.29 There is some indication of IR absorption near 2980 cm–1 for the 360 and 380 K spectra in Figure 4A, but these features could be attributable entirely to a C–H stretch of ethylene. For this reason it is difficult to quantify the selectivity, but the results imply the amount of ethane produced is at most only a few percent of the amount of the ethylene and could be much less. Figure 4B shows the results of subtracting the s-polarized spectra from the p-polarized spectra taken under the same conditions, which clearly reveals peaks attributable only to surface species. The formation of oligomers during acetylene hydrogenation over Pd catalysts is well known in the literature. In some cases a liquid byproduct has been separated from the catalyst and characterized. This liquid has been termed “green-oil”, based on its color. As studied by Zhang et 13 ACS Paragon Plus Environment

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al., the consensus from various studies is that green oil is formed from acetylene through coupling of two C2H2 molecules to form 1,3-butadiene, followed by formation of an oligomer intermediate, from which the green oil forms.28 Sárkány et al. have observed that the green oil separates upon standing into two liquid phases and have presented IR spectra for both the upper and lower phases.37 As indicated by the comparison in Table 1, there is a reasonable match between their IR spectra of the upper green oil phase and the peaks of the product observed in Figure 3. Furthermore, the intensity patterns are also similar, with the C–H stretch peaks most intense in the spectrum of Sárkány et al. 37 Sárkány et al.37 also observed intense peaks at 1750 and 1720 cm–1 due to C=O stretches of carbonyl groups that are absent in our spectra. However, they also note that CO is often added to the gas stream to improve the selectivity of the hydrogenation reaction, and presumably they also added CO in their experiment. Although they don’t explicitly indicate this in their IR study, they mention CO addition in a companion publication.38 In another study, it was suggested that the

Figure 4. A. s-polarized spectra after 1×10–2 Torr of C2H2 and 1 Torr of H2 were added to the IR cell with the 0.08 ML Pd/Cu(111) crystal at 300 K followed by annealing the surface to the indicated temperatures for 30 s. B. Subtraction of the s-polarized spectra from the p-polarized spectra to yield spectra of the surface species.


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presence of ketone and carboxylic acids in NMR spectra of green oil indicated that it is formed through a reaction between C2H2 and CO.39 Given the generally high intensities of the C=O stretch peaks of carbonyls, the carbonyl-containing compounds in the green oil obtained by Sárkány et al. may constitute only a small amount in a mixture of compounds. Nevertheless, the strong similarity between our spectra and the remaining peaks in their IR spectra strongly suggests that the same coupling product is produced. What is not clear is whether green oil can be distinguished by infrared spectroscopy from its oligomer precursor. Therefore we will refer to the product with RAIRS peaks listed in Table 1 as the oligomer. The conclusion from Table 1 that a similar product is formed in the two cases demonstrates that the small amount of Pd in our Pd/Cu(111) SAA is able to produce similar surface chemistry to that of a Cu-free Pd/Al2O3 catalyst. The most prominent peaks of the oligomer in the C–H stretch region at 2929 and 2960 cm–1 are assigned to asymmetric stretches of CH2 and CH3 groups, respectively. It has been proposed that the intensity ratio of these peaks can be used to quantify the ratio of the number of CH2 groups to the number of CH3 groups Figure 5. Spectra after annealing to the indicated

and hence the chain length in the oligomers.40- temperatures a Pd-free Cu(111) surface (blue) and a 42

0.08 ML Pd/Cu(111) surface (red) in a background

Using the extinction coefficient ratio of 0.36 of 1×10-2 Torr of C2H2 and 1 Torr of H2. For the annealing experiments, the crystal was heated to the

established by McNab et al.,40-41 we calculate target temperatures, held there for 30 s, then cooled to 300 K where the spectra were obtained.

that the CH2:CH3 ratio of the oligomers in our 15 ACS Paragon Plus Environment

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samples is in the range of 8 to 11. However, with RAIRS there can be a definite orientation of the surface species with respect to the electric field of the infrared radiation, which is restricted to being perpendicular to the surface, whereas in a transmission IR experiment on high-surface area powders a random orientation of the molecules with respect to the electric field of the light can be assumed. Therefore, the extinction coefficient ratios determined by McNab et al.40-42 may not be directly applicable to our results. The role of Pd on the Cu(111) surface under acetylene hydrogenation conditions is highlighted by the results in Figure 5 in which spectra over Pd-free Cu(111) are directly compared with spectra obtained over a 0.08 ML Pd/Cu(111) surface. At 300 K, in the presence of 1×10-2 Torr of C2H2 and 1 Torr of H2, the spectra from the two surfaces are almost identical. After annealing to 320 K, the oligomer peaks become prominent over the Pd/Cu(111) surface and continue to develop as the surface is annealed to temperatures up to 380 K. The peaks at 1008 and 3011 cm–1 are clearly seen for both surfaces after annealing to 340 K and are again attributed to polyacetylene. However, once the surface is annealed to 360 and 380 K, the polyacetylene peaks are still quite prominent for the Cu(111) surface, but have disappeared from the spectra for the Pd/Cu(111) surface with the oligomer peaks increasing. This suggests that polyacetylene is hydrogenated to a green oil-like oligomer at these temperatures if and only if Pd is present on the surface. Annealing to 360 K also marks the disappearance of the gas phase acetylene peaks at 3265 and 3313 cm–1 for Pd/Cu(111) but they are still present in the Cu(111) spectra for the 360 and 380 K anneals. For Pd/Cu(111), the disappearance of the gas phase acetylene peaks is accompanied by the simultaneous appearance of the gas phase ethylene peak at 950 cm–1. It is only after annealing to 380 K that some evidence of the characteristic C–H stretch peaks associated with the oligomer are seen for the Cu(111) surface. The results of Figure 16 ACS Paragon Plus Environment

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5 indicate that the Pd promotes oligomer formation in the presence of H2. Since the most obvious role of Pd atoms in Cu(111) is to provide sites for H2 dissociation, the results suggest that the availably of atomic hydrogen not only allows C2H2 hydrogenation over the Pd/Cu(111) surface, but also promotes conversion of polyacetylene to the oligomer. In general, C-H stretch values can be correlated with the hybridization on the carbon atom and the sp2 carbon atoms of the (–HC=CH–) units of polyacetylene are consistent with C-H stretch values above 3000 cm–1. Similarly, the C–H stretch vibrations of the oligomer below 3000 cm–1 are consistent with sp3 hybridized carbon, as would be expected as C=C bonds are replaced by C-C bonds by hydrogenation. Although the contrasting spectra in Figure 5 are simply explained by assuming that the presence of Pd, in a background of H2(g), allows polyacetylene that forms on Cu(111) sites to be hydrogenated to the oligomer, the results of Figure 2 indicate that in the absence of H2(g), the oligomer can form on both Pd/Cu(111) and Pdfree Cu(111), but that it requires higher temperatures. At the higher temperatures of Figure 2, the influence of Pd is to actually decrease the amount of oligomer formed. 3.4 Trends with Pd coverage The hydrogenation activity for two different Pd coverages compared to the behavior of the Pd-free Cu(111) surface is clearly revealed by monitoring the C–H stretch region for gas phase acetylene as shown by the spectra in Figure 6. The black spectrum was obtained after adding 1×10-2 Torr of C2H2 and 1 Torr of H2 to the cell with the Cu(111) surface at 300 K. The surface was then annealed to 500 K, cooled back to 300 K and the orange spectrum obtained. This shows a slight decrease in the intensity of the peaks. A further decrease is observed for 0.0028 ML of Pd, while for 0.0055 ML of Pd, the gas phase acetylene peaks are below the noise level. The 17 ACS Paragon Plus Environment

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disappearance of the gas phase acetylene peaks in Figure 5 is accompanied by the appearance of a gas phase ethylene peak, confirming that the change seen in Figure 6 is associated with acetylene hydrogenation to ethylene.

Figure 6. RAIR spectra for the C2H2 gas phase region after annealing surfaces with different Pd coverages in a background of 1 × 10–2 Torr of C2H2 and 1 Torr of H2. Black, after adding C2H2 and H2 at 300 K to the Pd-free Cu(111) surface. Orange, after annealing the surface for the black spectrum to 500 K. Blue and green are for annealing a Cu(111) surface with 0.0028 and 0.055 ML of Pd to 480 K for 30 s.

An increased Pd coverage not only increases the extent of hydrogenation but also leads to a decrease in carbon deposited on the surface as determined with AES. Figure 7 shows the correlation between gas phase ethylene, oligomer, post-reaction carbon coverage, and Pd coverage. In panels A, B, and C, each color corresponds to the amount of Pd on the surface. The RAIR spectra shown for each surface were taken after gas phase C2H2 had disappeared from the spectra. The temperature where this occurred was lower for higher Pd coverages. The only changes with increasing annealing temperature are slight differences in the C–H stretch


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intensities. The results were obtained after annealing for 30 s to the temperatures (380-580 K) needed to eliminate the gas phase acetylene peaks in a background of 1×10–2 Torr C2H2 and 1 Torr of H2. In Figure 7A, RAIR spectra in the region of the gas phase ethylene peak are shown,

Figure 7. Summary of C2H2 hydrogenation results for increasing Pd coverages on Cu(111) after annealing with different Pd coverages in a background of 1×10–2 Torr of C2H2 and 1 Torr of H2. RAIR spectra in the CH2 wag region of gas phase ethylene (A), and in the C–H stretch region (B). Auger electron spectra (C), plots of RAIRS peak data, and carbon coverage from AES versus Pd coverage to show trends (D). In (A), (B), and (C), the colors correspond to surface Pd coverages of 0.0055 ML (blue), 0.08 ML (red), and 0.2 ML (black).


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with the blue spectrum corresponding to 0.0055 ML of Pd. In this case, the C2H4(g) peak at 950 cm–1 is accompanied by peaks at 909 and 969 cm–1 assigned to the oligomer. The ethylene peak increases with Pd coverage but the oligomer peaks are absent at the higher Pd coverages in Figure 7A. As shown in Figure 7B, the C–H stretch peaks assigned to the oligomer are most intense for the lowest Pd coverage, but don’t seem to decrease monotonically for the higher Pd coverages. The AES results in Figure 7C show a correlation between the amount of Pd and the post reaction carbon coverage, with the lowest Pd coverage giving the highest carbon signal. The results of Figure 7A-C are summarized in the plots in Figure 7D. For a Pd coverage of 0.08 ML, it was also found that first admitting 1×10-2 Torr of H2 to the cell before adding C2H2(g) caused an increase in the amount of C2H4(g) and a decrease in the amount of post-reaction carbon detected with AES. However, the C-H stretch peaks of the oligomer were largely unaffected by pre-exposure to hydrogen. This suggests that the pathway involved in converting C2H2 to C2H4 is distinct from the pathway that leads to oligomer formation and that C2H4 is formed from intermediates that can also decompose to deposit surface carbon. The first step of the hydrogenation reaction must be to form a C2H3 species, the most likely of which is vinyl, while the first step in the coupling reactions that lead to polyacetylene and the oligomer is presumably the formation of a C4H4 metallacycle. The hydrogenation reaction that we observe over the Pd/Cu(111) SAA converts a static volume of gas phase acetylene into ethylene. Figure 8 shows a plot of the areas (in integrated absorbance) of the C2H2(g) peaks in the 3200 to 3350 cm–1 region (blue) and the C2H4(g) peak at 950 cm–1 (green) as a function of time in the presence of a gas phase mixture of 0.1 Torr of C2H2 and 10 Torr of H2 for a 0.0055 ML Pd/Cu(111) surface at 400 K. The corresponding spectra were obtained with 128 scans with 4 cm–1 resolution, which required about 30 s per spectrum. 20 ACS Paragon Plus Environment

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There is a monotonic decrease in the acetylene until it becomes undetectable after about 1500 s. The initial signal decreases linearly with one slope until about 900 s, where there is a change in slope, followed by another linear decrease. The

time

dependence

of

the

ethylene

concentration mirrors that of the acetylene, offering further proof that acetylene disappears Figure 8. Time dependence of the IR peak areas

through hydrogenation to ethylene. Some of the for C2H2(g) and C2H4(g) for 0.0055 ML Pd/Cu(111) exposed to 0.1 Torr C2H2 and 10 Torr

acetylene is also lost through surface coupling H2 at 400 K. The peak areas were normalized to the largest peak area.

reactions, but the amount should be negligible compared to the total amount of gas phase acetylene present. From the volume of the reaction cell, the initial partial pressure of acetylene, the Pd coverage, and the size of the Cu(111) crystal, the time dependence of the acetylene disappearance can be used to estimate a turn over frequency (TOF), i.e., the number of acetylene molecules converted to ethylene per second per active surface atom. Division of the initial rate of acetylene decrease by the number of surface Pd atoms yielded a TOF of 95 s-1. This, of course, assumes that the reaction takes place only at the Pd sites. If we instead assume that all surface atoms, Cu and Pd, are active sites for hydrogenation, then the TOF would be only 0.52 s-1. As STM images show that spill over does not produce a uniform distribution of H atoms on the Pd/Cu(111) SAA,8 the TOF based on the actual active sites must be between the extremes of 95 and 0.52 s–1. The TOFs for acetylene hydrogenation to ethylene over Pd catalysts are generally in the range of ~0.03 – 35 s-1.43-51 When Pd concentrations are lower, such as with a bimetallic catalyst, TOFs tend to be higher.


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Experimental measurements such as these of TOFs for a well-characterized single crystal SAA provide the most direct way to compare results obtained on model catalysts to results for more practical catalysts. As Boudart has noted, TOFs from single crystals do not suffer from massand heat-transport limitations or from as much ambiguity as to the number of surface sites and therefore set a standard by which the quality of TOFs from supported catalysts can be judged.52 4.

CONCLUSIONS By simultaneously monitoring gas phase and surface species when a Pd/Cu(111) single-atom

alloy was exposed to an ambient pressure of acetylene and hydrogen, it was found that acetylene hydrogenation proceeds in the presence of a carbonaceous layer on the surface. The turn over frequency obtained is in the same range as reported for practical acetylene hydrogenation catalysts. Under the same conditions, hydrogenation of acetylene to ethylene does not occur on the Pd-free Cu(111) surface. The presence of Pd also alters the nature of the carbonaceous species by promoting the hydrogenation of the C=C bonds of polyacetylene that forms from acetylene coupling reactions on Cu(111). The RAIR spectrum of the oligomer that forms on the Pd/Cu(111) surface matches that of the green oil product formed during selective acetylene hydrogenation over supported Pd catalysts. The results demonstrate that elevated acetylene pressures over a model single-crystal Pd/Cu(111) SAA catalyst produce similar surface chemical reactions as observed for practical Pd catalysts. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]


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ORCID Michael Trenary: 0000-0003-1419-9252 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by a grant from the National Science Foundation (CHE-1464816).


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TOC Figure


Selective Hydrogenation of Acetylene to Ethylene in the Presence of a

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