The Role of Carbonaceous Deposits in Hydrogenation Catalysis

Article pubs.acs.org/JPCC

The Role of Carbonaceous Deposits in Hydrogenation Catalysis Revisited Juan Simonovis, Aashani Tillekaratne, and Francisco Zaera* Department of Chemistry and UCR Center for Catalysis, University of California, Riverside, California 92521, United States ABSTRACT: The effect of carbonaceous deposits on the performance of Pt(111) surfaces as catalysts for the hydrogenation of ethylene was tested by decoupling their preparation, which was done beforehand in an ultrahigh vacuum (UHV) environment, from the catalytic runs, which were carried out in an enclosed “high pressure” cell. The time evolution of the gas composition during reaction was followed continuously by mass spectrometry, and the nature of the surface species was determined by in situ reflection−absorption infrared absorption spectroscopy (RAIRS). Reaction rates were seen to vary by up to approximately 40% depending on the type of molecules preadsorbed at room temperature, with the maximum activity seen with a propylidyne layer, the result of propylene adsorption. The rate with ethylidyne-covered Pt(111) is reduced by ∼20%, with butylidyne by ∼35%, and with benzyl moieties by ∼40%. These changes are reversible: the surface regains the activity expected when starting with the clean substrate after one or two catalytic runs to full conversion. RAIRS data show that this is because the initial species are slowly replaced by a new layer of the adsorbate that forms with the olefin in the reaction mixture (ethylidyne for ethylene hydrogenation). More significant changes are seen if the adsorbed layers are dehydrogenated at higher temperatures: the turnover frequencies for ethylene hydrogenation are reduced by more than an order of magnitude upon the conversion of propylidyne to CnH(ads) species, by annealing at temperatures between 500 and 650 K. Some activity is regained upon annealing to even higher temperatures, presumably because of the formation of graphitic layers, which have a smaller footprint on the surface. It was concluded that although the main role of carbonaceous deposits in catalytic hydrogenations is to block surface sites, their influence is also affected by the reversibility of their bonding to the surface in a hydrogenation atmosphere and by the structure of their carbon skeleton.

1. INTRODUCTION The promotion of the hydrogenation of unsaturated hydrocarbons and other organic molecules with transition-metal catalysts is one of the oldest and most widely used processes in the chemical industry.1−6 This heterogeneous catalysis has been studied extensively over the years at both basic and practical levels.7−16 In terms of a molecular description of the chemistry involved, the main mechanistic elements were put forward by Horiuti and Polanyi many decades ago.17 In their proposal, the formation of alkanes from alkenes, for instance, takes place via sequential and reversible half-hydrogenation steps. Alternative ideas have been discussed in the literature18 and we have recently reported evidence for a second mechanism based on a direct Eley−Rideal reaction of gas-phase olefins with adsorbed hydrogen atoms,19,20 but to date it is the Horiuti−Polanyi scheme that is used the most to explain the experimental observations.21,22 Nevertheless, a few details remain unsettled still, among those the role of carbonaceous deposits in defining the performance of these catalysts. The fact that the surfaces of metal catalysts are covered with strongly bonded hydrocarbon fragments during hydrogenation catalysis was realized long ago,23 and modern surface-science experiments combining in situ spectroscopies with model surfaces and the use of ultrahigh vacuum (UHV)-catalytic reactor tandem apparatus have led to the conclusion that, in the case of the hydrogenation of simple olefins, such fragments are © XXXX American Chemical Society

alkylidyne moieties (PtC−R). However, their contribution to the catalytic process has not yet been fully understood.24−32 A combination of isotope labeling and spectroscopic characterization of the surface using model systems, together with kinetic measurements, has been used to categorically rule out the participation of these alkylidyne surface species as direct intermediates during the conversion of adsorbed olefins to alkanes.25,33,34 Instead, it has been suggested that these strongly bonded carbonaceous deposits may act as a source of hydrogen atoms, either directly or by shuttling them from the surface to a second layer where the weakly adsorbed reactants reside during catalysis.35−37 The first possibility has been discarded on the basis of in situ measurements of the rate of H−D exchange on alkylidyne layers during catalytic hydrogenation, which have been shown to be too slow to facilitate the olefin hydrogenation process.25,38,39 Hydrogen transferring from the surface to the olefins via alkylidyne moieties, on the other hand, is much harder to test, and it is still a controversial idea.16,40 Finally, it is quite possible that the carbonaceous deposits act only as spectator species.27,30,41,42 The last possibility, that the carbonaceous deposits are spectator species during the catalytic hydrogenation of unsaturated hydrocarbons promoted by transition metals, is, Received: December 12, 2016 Revised: January 4, 2017

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DOI: 10.1021/acs.jpcc.6b12517 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

reaction mixture and to follow its evolution in real time during the catalytic process. The so-called high-pressure cell also has NaCl windows to allow for the in situ characterization of the adsorbed species using reflection−absorption infrared spectroscopy (RAIRS), which was performed by using a Bruker Equinox 55 Fourier-transform infrared (FTIR) spectrometer. In a typical experiment, the Pt surface is first cleaned by sputtering-annealing and oxygen treatments, and then transferred to the side chamber, where background RAIRS data are acquired with both s- and p-polarized light. The Pt is then exposed to the desired gas to prepare the surface for catalysis, as discussed below, a new set of RAIRS acquired, the highpressure cell closed, and yet another pair of RAIRS traces recorded. The reaction mixture, premixed in a side gas manifold and typically consisting of 2 Torr of the olefin and 50 Torr of hydrogen, is let into the reaction volume, after which the timer for the kinetic measurements and the mass-spectrometer data acquisition are started. In the initial experiments (Figures 1−5),

to date, the most widely accepted. In fact, based on recent studies using a combination of high-flux molecular beams and in situ infrared absorption spectroscopy with model platinum surfaces, we have shown that the intrinsic reaction probability of these reactions may depend on the availability of bare metal sites and can be increased to values close to unity by choosing the appropriate reaction conditions needed to minimize the buildup of the poisoning carbonaceous layer.31,43−45 Nevertheless, there are still secondary effects that these deposits are likely to exert on the performance of the catalyst. For one, they clearly temper the reactivity of the bare metal, inhibiting dehydrogenation reactions and favoring weak olefin adsorption and hydrogenation. The clean transition-metal surfaces favor strong adsorption and dehydrogenation steps (to alkylidynes), whereas the surfaces covered with alkylidyne layers promote the adsorption of the olefins in the π-bonding state believed to be required for reaction.46,47 Other subtle factors may be in play in these systems as well. In general, these have been difficult to probe, since the formation of the carbonaceous layers is caused by exposure of the clean metal surfaces to the reaction mixture, to the unsaturated hydrocarbons in particular, and occurs within the first seconds of the catalytic runs. The nature of these layers is defined by the reactants and conditions used, and cannot be easily decoupled from other effects that such conditions may have on the kinetics of the hydrogenation conversions. It is this limitation that the work reported here was designed to address. By starting with clean platinum single-crystal surfaces under UHV conditions, we have been able to condition them at will before exposing them to the reaction mixture. Different types of carbonaceous layers were first prepared by dosing a variety of hydrocarbons and by tuning the adsorption temperature, and the resulting surfaces were then transferred to the reactor for kinetic measurements of the olefin hydrogenation reactions while simultaneously monitoring the surface species using infrared absorption spectroscopy. Several trends were identified this way, as discussed in more detail in the following sections.

Figure 1. Main Frame: Accumulation of ethane (in turnover numbers, TON = ethane molecules per Pt surface atoms) versus time during the hydrogenation of 2 Torr of ethylene with 50 Torr of H2 catalyzed by a Pt(111) single-crystal surface at 300 K. Three curves are reported, for three consecutive kinetic runs starting with a propylidyne-presaturated Pt(111) surface (prepared in the ultrahigh vacuum chamber beforehand by exposing the clean surface to 40 L of propylene at 300 K). Inset: Turnover frequencies (TOF, in units of TON/s) calculated by numerical derivatization of the TON versus time data. A slight decrease in conversion rate is seen between the first and second runs, as the initial surface species are replaced by ethylidyne moieties.

2. EXPERIMENTAL DETAILS The experiments reported here were all carried out in a UHV chamber equipped with several techniques for the cleaning and characterization of the surface as well as with a catalytic reactor where kinetic measurements can be carried out. The details of this instrumentation and the procedures used for the experiments are discussed in detail in previous publications.31,48 Briefly, a platinum single crystal cut in the (111) orientation was mounted on a manipulator used for sample transfer between the two stages of the instrument and for surface alignment, and also for controlled cooling or heating to any temperature between approximately 100 and 1100 K (as measured using a chromel−alumel thermocouple spotwelded to the side). Most of the initial surface cleaning and preparation of this crystal was carried out before each experiment in the main UHV chamber, which is equipped with an ion gun for surface sputtering, a mass spectrometer for the analysis of the gas composition, and a set of controlled leak valves for gas dosing. After proper preparation, which in many instances involved the adsorption of specific gases on the clean substrate, the crystal was transferred to the second stage and enclosed within a small “high-pressure” cell used as the catalytic reactor. This volume was pressurized to atmospheric pressures once sealed, and displayed a small fixed leak to the main chamber, which was used to analyze and quantify the composition of the

the neat reaction mixture was used to fill the sealed reaction cell, without any stirring, whereas for the measurements in Figure 6 an outside loop equipped with a bellows circulation pump was added to improve on the gas mixing (and Ar was added as ballast to facilitate the gas recirculation). This change, which led to an increase in total reaction mixture volume, affording better signal-to-noise ratios in the data, was done to check that there are no mass-transport limitations in our system; changes in gas composition could be seen in all cases within a few seconds of starting the catalytic reactions, and similar reaction rates were measured with both arrangements. The gas composition is monitored continuously until the reaction reaches completion, after which the volume of the reactor is pumped with a turbomolecular pump and the cell retracted to expose the crystal back to the UHV environment. RAIRS spectra are taken during the course of the catalytic B


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The Journal of Physical Chemistry C conversion as well as immediately before and right after opening the cell. All gases (hydrogen, H2, 99.995% purity, from Liquid Carbonic; ethylene, C2H4, 99.5% purity, from Matheson; propylene, C3H6, 99% purity, from Matheson; 1-butene, 1C4H8, > 99% purity, from Matheson; Ar, 99.995% purity, from Matheson) were used as supplied. The liquid (toluene, > 99.9% purity, from Aldrich-Sigma) was freeze-pumped-thaw in situ in the manifold and vaporized into the leak valve used for dosing. Exposures are expressed in Langmuirs (1 L = 1 × 10−6 Torr·s), uncorrected for ion gauge sensitivities. The reported RAIRS traces are averages of 2000 scans taken at a resolution of 4 cm−1, a process that takes about 4 min per experiment, and were ratioed against spectra from the clean sample. The raw kinetic data, which is acquired as mass-spectrometer signal versus time for a number of chosen amus, was deconvoluted and calibrated using a protocol developed in our laboratory to convert them into turnover numbers (TON, in units of number of olefin molecules converted per platinum surface area).48,49 Turnover frequencies (TOF) were then calculated from those data by numerical derivatization, and reported in units of TON/s.48 The reported error bars were estimated from the standard deviations around the averages of several runs carried out under identical conditions.

Figure 2. Reflection−absorption infrared spectra (RAIRS) for the initial propylidyne-dosed Pt(111) surface (bottom) and after each of the three kinetic runs reported in Figure 1. A stepwise change is observed as the initial propylidyne moieties, identified mainly by the 2961 cm−1 peak due to the C−H asymmetric stretching mode, is replaced by the features characteristic of ethylidyne, at 1120, 1341, and 2882 cm−1. The additional peaks seen in the 1400−1800 cm−1 range originate from water vapor due to incomplete purge of the path of the IR beam, and are not relevant to the chemistry studied here.

3. RESULTS Typical kinetic data for the conversion of ethylene in the presence of hydrogen obtained with our experimental setup are reported in Figure 1. In this case, the platinum surface was first saturated with a propylidyne layer, made by dosing 40 L of propylene at 300 K under vacuum, and then exposed three times (consecutively) to mixtures made out of 2 Torr of ethylene and 50 Torr of hydrogen. The conversion of ethylene to ethane is evident from the data, to completion after approximately 200 to 300 s of reaction (after which the TON signal levels off). After 500 s of reaction the gases were pumped and a new fresh reaction mixture added to the reactor without any other treatment of the surface in between. The first thing to notice from Figure 1 is that the accumulation of the product in each reaction run is approximately linear with time. This indicates zero-order kinetics in ethylene, as we have reported before (although there are some subtle changes at intermediate conversions);45 given that we here use a large excess of hydrogen, the changes in pressure of that reactant are minimal and do not affect the kinetics in these measurements.31,48 The linearity of the accumulation curves makes the estimation of the reaction rates easier, as they can be extracted from an average over a wide time range; the resulting TOFs for the three runs reported in Figure 1 are reported in the inset. The most important observation from the data in Figure 1 is that the reaction rate decreases with the number of catalytic runs performed, by approximately 10% between the first and third runs. This decrease in rate is explained by a concurrent change in the nature of the species adsorbed on the surface, as evidenced by the RAIRS data in Figure 2. What transpires from these spectroscopic results is that the initial propylidyne layer, deposited under UHV before the first catalytic run, is slowly displaced by the reaction mixture, and replaced by a layer of ethylidyne: the peak for the asymmetric C−H stretching mode in the methyl group of propylidyne, which is seen at 2961 cm−1,50,51 decreases in intensity as the features at 2882 (symmetric CH3 stretching), 1341 (CH3 umbrella deforma-

tion), and 1120 cm−1 (C−C stretching), all associated with ethylidyne,52−55 grow. The slow removal of alkylidynes from the surface of the catalyst under a hydrogen atmosphere was reported several years ago already,25,33,38 and the displacement of propylidyne by ethylidyne during ethylene conversion was also seen recently.31 What is significant here is that this change causes a reduction in reaction rate. These results indicate that the nature of the carbonaceous layer present on the surface does influence the performance of the catalyst. The changes seen in Figures 1 and 2 are mostly reversible, as evidenced by the set of RAIRS spectra reported in Figure 3. In this case a total of nine kinetic runs were carried out

Figure 3. RAIRS traces obtained after nine consecutive kinetic runs starting with a propylidyne-presaturated Pt(111) surface. Experiments with ethylene (first and last sets, of three runs each) were alternated with similar measurements with propylene (fourth, fifth, and sixth runs) to check on the reversibility of the formation of alkylidynes during reaction. All hydrogenation conversions were carried out with mixtures of 2 Torr of the olefin and 8 Torr of H2 at 300 K. C


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not capable of removing the initial propylidyne layer from the surface (bottom trace). On the other hand, half or more of the propylidyne surface moieties are displaced in catalytic runs with a mixture of ethylene and hydrogen, according to the decrease in signal intensity seen for the RAIRS peak at 2961 cm−1. The amount of propylidyne removed is approximately the same regardless of the partial pressure of hydrogen used, in the 10 to 50 Torr range. On the other hand, the amount of ethylidyne deposited to replace that initial layer increases with increasing hydrogen pressure, as indicated by the trend seen for the intensity of the 1338 cm−1 peak. Some interference from peaks from water vapor in the IR beam path are seen in the 1400−1800 cm−1 range of some of the traces, but the evidence for ethylidyne formation is only minimally disturbed by that, and additional information about the ethylidyne surface coverage can be extracted from the weaker peak at 1120 cm−1 (which is clearly seen for the experiments with hydrogen pressures above 30 Torr but not below). The trend deduced from the data in Figure 4 may seem counterintuitive at first sight, since it could be argued that higher H2 pressures should accelerate the removal of ethylidyne (as well as of propylidyne). We believe that the explanation may lie on the fact that, once some propylidyne is removed from the surface and empty Pt sites open up, there is room for the promotion of the decomposition of the remaining surface deposits, to form more dehydrogenated and less reversibly adsorbed fragments; those new species may poison the substrate and block further ethylidyne formation. In this interpretation, one role for adsorbed hydrogen is to preserve the clean surface and to block alkylidyne decomposition (or to favor its immediate reversible regeneration) long enough for new olefin molecules to adsorb and form new alkylidyne species. To further test the theory that the dehydrogenation of alkylidyne surface species may lead to the formation of new carbonaceous deposits that are more difficult to remove and that block surface sites for hydrogenation more effectively, the catalytic activity of propylidyne-predosed Pt(111) for ethylene hydrogenation was measured as a function of the temperature to which the carbonaceous layer was annealed before the start of the catalysis. For reference, the left-hand panel of Figure 5 shows the H2 temperature-programmed desorption (TPD) spectrum obtained from propylene adsorbed on Pt(111): these data can be used to map out the sequence of dehydrogenation steps that leads to the stepwise conversion of propylene to carbon on the surface. These reactions have been well documented in the literature already and interpreted on the basis of the stoichiometries implied by the relative areas of the different H2 TPD peaks and additional spectroscopic characterization.51,56−58 In general terms, the first peaks, below 300 K, are associated with the dehydrogenation of propylene to propylidyne, the second set of desorption features, between approximately 400 and 600 K, with a number of stepwise dehydrogenation reactions to form CnH(ads) surface species such as alkenyl and alkynyl moieties, and the residual hydrogen desorption observed upon heating above ∼650 K with complete dehydrogenation to graphitic carbon. The decomposition products first appear as small clusters, but further annealing to temperatures above 800 K results in the growth of graphite islands.59 The right-hand side of Figure 5 reports the kinetic data for the conversion of 2 Torr of ethylene with 50 Torr of H2 on propylidyne-Pt(111) surfaces annealed to the indicated temperatures. Six different annealing temperatures are reported,

sequentially, to completion, without cleaning the surface in between. The experiments were started on a propylidynesaturated substrate (bottom spectrum), which was used to hydrogenate 2 Torr of ethylene with 8 Torr of H2 three times in a row (second, third, and fourth traces from the bottom). In this case the removal of the propylidyne species seems to be almost over by the end of the first run, since the peak at 2961 cm−1 is no longer detectable by then; the feature at 1338 cm−1 for ethylidyne is evident instead. After the first three conversions of C2H4 + H2 mixtures, another three hydrogenation runs were performed, but now with a mixture of propylene (instead of ethylene) and hydrogen. The RAIRS data recorded after those runs (fifth to seventh traces from the bottom) show the reappearance of the 2961 cm−1 signature for propylidyne, and the disappearance of the 1338 cm−1 identifier of surface ethylidyne. Three more hydrogenation runs, now back with ethylene, regenerate the ethylidyne species (top three traces). Notice, however, that the propylidyne is not fully removed in this case. It is possible that the alkylidyne layers become more compressed and harder to displace over time, or that the surface becomes slowly contaminated with other RAIRS-silent species. Nevertheless, it is clear that, by and large, the alkylidyne layers that are present during the roomtemperature catalytic hydrogenation of olefins can be slowly removed and replaced by the species expected to form from adsorption of the reactant. The replacement of one type of alkylidyne by another could take place directly, via the reaction of the incoming olefin with the surface species, but it is more likely to occur via the intermediate hydrogenation of the initial adsorbed species with hydrogen and their removal from the surface before the adsorption and conversion of the new reactants. It is certainly known that alkylidynes can be removed from the surface with hydrogen alone, in the absence of any gas-phase olefins.31,33 In order to explore this process in more detail, RAIRS data were acquired for propylidyne-predosed Pt(111) surfaces after the hydrogenation of 2 Torr of ethylene with varying pressures of hydrogen, from 0 to 50 Torr (Figure 4). It was concluded that, in the absence of hydrogen in the gas mixture, ethylene alone is

Figure 4. RAIRS data acquired after individual runs for ethylene hydrogenation on propylidyne-presaturated Pt(111) surfaces with varying pressures of H2, from 0 (no hydrogen, bottom trace) to 50 Torr (top). The removal of approximately half of the initial propylidyne layer is seen in all cases, but only with high H2 pressures is the newly exposed area fully covered with new ethylidyne moieties. D


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and a more compact structure. It would appear that such graphitic compression leads to the opening of some bare Pt sites, making them available for hydrogenation catalysis. It may be possible to poison the surface further by depositing a higher coverage of graphitic carbon, by exposing the surface to a source of carbon at higher temperatures; this is an approach worth pursuing in the future to test and quantify the poisoning ability of the surface graphitic layer further. Finally, the activity of Pt toward ethylene hydrogenation was contrasted as a function of the initial species deposited on the surface. Figure 6 displays the kinetic data resulting from

Figure 5. Left panel: H2 temperature-programmed desorption (TPD) data acquired from a Pt(111) surface dosed with 5 L of propylene at 100 K. The several features in the spectra attest to the stepwise nature of the dehydrogenation of the adsorbed olefin, starting with the formation of propylidyne by approximately 300 K and followed by transitions to CnH(ads) and graphitic carbon. Right panel: Kinetic data for the hydrogenation of ethylene, in the form of TON versus time, on Pt(111) surfaces presaturated with propylidyne and annealed to the indicated temperatures. It is seen that the conversion of such propylidyne to other more dehydrogenated species leads to significant poisoning of the surface.

but the data can be easily grouped into three types: (a) surfaces annealed at 300 K, which preserves the original propylidyne saturated layer; (b) surfaces annealed at temperatures between 500 and 620 K, at which point the substrate is likely to be covered with CnH(ads) partially hydrogenated moieties; and (c) surfaces annealed at temperatures above 750 K, high enough to produce surface graphitic carbon. It is clearly seen from these data that even partial dehydrogenation of the propylidyne layer leads to a significant poisoning of the catalyst, decreasing its activity by approximately 1 order of magnitude. One important change associated with the transition from alkylidynes to CnH(ads) surface species is that the latter are much more strongly bonded to the surface and are almost impossible to remove by hydrogenation treatments.33,58 It would appear that only alkylidynes are capable to undergo reversible hydrogenation steps on the surface of the metal, and that this quality sets them apart in terms of the promotion of olefin hydrogenation reactions. This conclusion suggests that such carbonaceous layer may indeed act as a hydrogen shuttle between the surface and the π-bonded olefins during catalysis, as suggested in the past.16 A secondary curious change is seen in catalytic activity in Figure 5 upon annealing the propylidyne layer to temperatures above 750 K, at which point the surface is believed to be covered with graphitic carbon: the activity of those catalysts toward ethylene hydrogenation is twice to three times higher than that of the surfaces covered with the intermediate CnH(ads) fragments, the same within experimental error for the three runs performed after annealing the surface above 750 K. Since graphitic carbon is much harder to hydrogenate, the opposite would have been expected. On the other hand, the formation of graphitic layers upon annealing of adsorbed hydrocarbon molecules comes with a compression of their footprint, since the graphitic rings display shorter C−C bonds

Figure 6. Ethylene hydrogenation kinetics, in the form of TON versus time, for Pt(111) surfaces predosed with various precursor molecules at room temperature. Shown are data for experiments starting with the clean metal as well as with surfaces presaturated with ethylidyne, propylidyne, butylidyne, and benzyl species, made by dosing ethylene, propylene, 1-butene, and toluene, respectively. The rate of ethylene hydrogenation is seen to vary by up to 40% depending on the nature of the predosed species.

measurements with Pt(111) saturated at room temperature with ethylene, propylene, 1-butene, and toluene (the first three producing the corresponding alkylidyne layers). The data for the clean surface is also included for reference. Although all surfaces display high catalytic activities, they do differ within a range of approximately 40% of the maximum, and the trends are not obvious. The TOFs calculated from these experiments are reported in the right panel of Figure 7 (the left panel summarizes the equivalent data for the experiments in Figure 5). Particularly puzzling is the fact that the highest activity is seen with the propylidyne-covered surface (as also observed in Figure 1), in spite of the larger number of carbon atoms per moiety when compared with ethylidyne. Larger hydrocarbon chains may be expected to lead to more surface site blocking, as is in fact the case with butylidyne. It may be that propylidyne packs on the surface less efficiently than the other alkylidynes, leaving a few more Pt sites available for hydrogenation promotion. This could be associated with the odd number of carbons in propylidyne, which leaves the terminal methyl groups at an angle from the surface normal (they are perpendicular to it with ethylidyne and butylidyne); that hypothesis needs to be tested in the future by using alkylidyne with even larger carbon chains (although those have not been well characterized, so their structural information is not available). The last case, with toluene, may be easier to E


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pressures and high hydrogen-to-olefin ratios, and can even reach values close to unity.20,31,44,49 We believe, based on limited spectroscopic evidence, that these are also the conditions required to keep the coverage of the carbonaceous deposits low, a fact that suggests that the increase in olefin conversion efficiency may be related to the fraction of the metal surface that remains uncovered and available for adsorption. Site blocking must therefore be a major effect exerted by the carbonaceous deposits on the performance of hydrogenation catalysts. However, this cannot be the whole story. For one, clean metals tend to dehydrogenate, rather than hydrogenate, unsaturated hydrocarbons. It is likely that that layer of strongly adsorbed organic fragments present during catalysis partially neutralizes the high dehydrogenation activity of the metal, and helps promote hydrogenation steps instead. In conjunction with this idea, it should be mentioned that olefins (and other unsaturated organic molecules) can adsorb on metals in two modalities, via strong di-σ bonds or via a weaker π interaction,82−86 and that, from those, it is the π state that is believed to directly participate in the olefin hydrogenation process.43 It has been shown that adsorption via π bonding is enhanced by the presence of strongly bonded hydrocarbon fragments on the surface.28,47,51 Other subtle effects are likely to contribute to the modification of the hydrogenation catalysis by carbonaceous deposits, but those are more difficult to identify. Because these deposits form in the early stages of the reactions, as an intrinsic part of the process, it is not easy to isolate their role in catalysis from other effects that may take place concurrently. It is this problem that we set to address in the experiments reported here. By developing a methodology to prepare and characterize hydrocarbon-covered Pt(111) surfaces prior to the initiation of the catalytic reactions, we have been able to decouple the formation of the carbonaceous deposits from the hydrogenation kinetics and control the nature of the surface of the catalyst. Two types of surface treatments were explored: the deposition of different surface fragments at room temperature, by using various hydrocarbons, and the modification of those deposits via annealing of the surface at a number of increasing temperatures, to promote the partial dehydrogenation of the initial adsorbates. Several interesting trends were identified in terms of olefin hydrogenation rates from these experiments. The main conclusion from the work reported here is that the specific nature of the hydrocarbon fragments present on the surface during catalysis does indeed affect the kinetics of the hydrogenation processes. This is clearly illustrated by the summary of our TOF data reported in Figure 7. In terms of the specific structure of the adsorbates prepared at room temperature, ethylene hydrogenation TOFs vary from 4.0 s−1 on propylidyne-covered Pt(111) to 2.5 s−1 with surfaces covered with benzyl moieties (under the reaction conditions used in our experiments). There are also modest but measurable rate changes induced by changes in the length of the hydrocarbon chains in alkylidyne layers. Interestingly, these do not follow a monotonic trend but rather seem to fluctuate between odd and even numbers of carbon atoms (an idea that requires further testing). All these differences may be related to the different ability of the surface species to block surface sites, perhaps because of their relative efficiency in forming wellpacked layers. In general, though, the overall rates are all within the same order of magnitude. In addition, these layers can evolve in response to the composition of the reaction mixture,

Figure 7. Summary of the TOFs calculated via derivatization of the kinetic data in Figures 5 and 6 as a function of the nature of the carbonaceous layer on the initial Pt(111) surface. Left panel: Data from runs with propylidyne-presaturated Pt(111) annealed to the indicated temperatures to produce new surface species with various degrees of hydrogen content. Right panel: TOFs for Pt(111) surfaces precovered with different surface species at room temperature.

interpret: that surface shows the lowest activity of all the cases tried here, perhaps because toluene may resemble more closely the structure of graphite, which was shown above to effectively block catalytic sites. This explanation should be qualified, though, as high-coverage layers of aromatic compounds may stack at an angle on some transition metal surfaces.60−64 Toluene in particular is believed to form a tilted benzyl surface species upon adsorption at room temperature.65,66

4. DISCUSSION The promotion of hydrogenations of unsaturated organic molecules by supported transition-metal nanoparticles is ubiquotous in chemical synthesis, yet some mechanistic details of those processes are not yet fully understood. Central to the description of that chemistry is the knowledge of the nature of the active surface under catalytic conditions. It is well established that pristine metal surfaces are quite reactive and tend to decompose most organic molecules upon adsorption, even at low temperatures.67,68 In the case of small olefins, extensive work has gone into identifying the final products as adsorbed alkylidyne moieties.24,28,32,53,69−80 There is also ample evidence to show that these strongly bonded species are present in high coverages on the surface during catalytic hydrogenation processes.27−32 The consensus in the literature is that these deposits mainly block platinum sites from being accessible for catalysis. Yet, catalytic hydrogenation reactions still proceed readily under mild conditions. Olefin hydrogenations, for one, exhibit turnover frequencies (TOFs) in the 1−100 s−1 range even at room temperature;4,11,36,81 clearly, the metal catalysts are active in spite of being covered with a carbonaceous layer. On the other hand, the olefin conversions are not particularly efficient. Although the TOFs are high when compared to other catalytic processes, they are equivalent to the formation of one product molecule per million collisions of reactants or more.16 By using a combination of high-flux molecular beams and in situ infrared absorption spectroscopy, we have recently shown that the steady-state reaction probability for this catalysis can be greatly increased by selecting appropriate conditions, typically low total F


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alkylidyne with the species expected from the olefin in the gas phase. Importantly, these changes are reversible, even if they occur in a time scale about an order of magnitude slower that that of the catalytic hydrogenation of olefins itself. Benzyl fragments, prepared via the adsorption of toluene on the clean Pt(111) surface at room temperature, also slow down the subsequent ethylene hydrogenation reaction, by about 30− 40% from the TOF seen with propylidyne. Much more significant changes are seen when propylidyne is dehydrogenated to either CnH(ads) or graphitic layers, at which point the catalytic activity is reduced by an order of magnitude or more. It is argued that it is the irreversibility of the formation of those new fragments and their inability to incorporate hydrogen atoms that renders them more effective at poisoning the catalytic surface. In conclusion, it is clear that carbonaceous layers modify hydrogenation catalysis primarily by blocking surface sites, but do contribute in other ways to the mechanism of the reaction.

and can be slowly but reversibly replaced, as indicated by the data in Figures 2 and 3. The changes induced by annealing the propylidyne layers to different temperatures (Figure 7, right panel) are more significant. Specifically, the TOF transitions from 4.0 s−1 with the unmodified propylidyne layer to 0.3 s−1 if the surface is annealed at temperatures between approximately 500 and 620 K prior to the kinetic runs. As already discussed above, such treatment leads to the dehydrogenation of adsorbed propylidyne into CnH(ads) species. The significance of this result is that the new carbonaceous layers are more strongly bonded to the surface, and therefore harder to displace. Such reduction in the rate of carbonaceous layer removal leads to a higher steady-state coverage of strongly bonded deposits during catalysis, and to the blocking of a larger fraction of the metal catalytic sites. Ease of removal of the carbonaceous deposits from the surface may certainly be a consideration when explaining the change in catalytic activity reported in Figure 7, but the magnitude of the TOF changes indicate that other factors may be at play as well; recall that the steady-state coverage of alkylidynes on Pt(111) is already close to saturation under most reaction conditions, regardless of the fact that those can be hydrogenated away. Another element to consider here is the fact that the hydrogenation steps that aid in the removal of the alkylidynes from the surface also facilitate their mobility. In connection with this idea, some indirect evidence indicates that alkylidynes (PtC−R) can undergo rapid and reversible interconversion with alkylidenes (PtCH−R) or vinyl (Pt− CHR[−H]) species,39,71−73,79,87−91 which presumably adsorb on different surface ensembles (in bridge or atop, rather that 3-fold hollow, sites). The mobility of the carbonaceous layer could provide the means for the opening of appropriate reaction sites for hydrogenation on the surface, and the dehydrogenation of alkylidynes to CnH(ads) deposits could shut off this mechanism. The alkylidyne-alkylidene interconversion also offers a viable path for hydrogen shuttling to the olefins to facilitate their hydrogenation. A final observation from the data in Figure 7 is that annealing of the CnH(ads) deposits to higher temperatures, to promote their complete dehydrogenation and induce the formation of graphitic carbon, restores some of the catalytic activity of the Pt surface: ethylene hydrogenation TOFs more than double after this transition. Graphitic layers are not expected to be mobile on the surface or to facilitate any hydrogen transfer steps, but do have a smaller footprint than the partially hydrogenated residues. The increase in activity in this case may be the result of a simple geometrical effect, by which more bare platinum sites become available when the graphitic layer is formed.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Francisco Zaera: 0000-0002-0128-7221 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funds for this project were provided by the US National Science Foundation, Division of Chemistry, under Contract No. CHE-1359668.

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REFERENCES

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5. CONCLUSIONS The effect of carbonaceous deposits on the activity of Pt(111) as a catalyst for olefin hydrogenations was tested by decoupling their formation from the kinetics of conversion during catalysis. Clear changes in reaction rates were observed depending on the nature of the layer present on the surface, as summarized by the TOF data reported in Figure 7. Variations in the length of the chain of the alkylidyne species that form upon adsorption of terminal olefins on the clean metal lead to fluctuations in ethylene hydrogenation rates on the order of approximately 10−40%, with an apparent alternation between odd and even number of carbon atoms in the alkyl chain. Those changes are temporary, as the reaction mixture slowly replaces the original G


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The Role of Carbonaceous Deposits in Hydrogenation Catalysis

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