Perspective pubs.acs.org/acscatalysis
The Surface Chemistry of Metal-Based Hydrogenation Catalysis Francisco Zaera* Department of Chemistry and UCR Center for Catalysis, University of California, Riverside, California 92521, United States ABSTRACT: The promotion of hydrogenation reactions by transitionmetal-based heterogeneous catalysts was established many decades ago but is still quite common in the chemical industry. Because of their importance, these processes have been studied in great detail from both fundamental and practical points of view, and much has been learned about them. However, some key questions remain unanswered, and solutions to specific industrial needs are still pending. In this Perspective, we discuss the state-of-the-art of our understanding of some of the fundamental issues associated with hydrogenation catalysis. From the mechanistic point of view, we use the example of olefin hydrogenation to assess the status of our knowledge on the adsorption of the organic reactants, the role that the strongly adsorbed carbonaceous deposits that form during reaction play in defining the catalytic kinetics, the mechanistic details of the hydrogen dissociative uptake and surface mobility during reaction, and the dynamic changes of the structure of the surface induced by the catalytic conditions. We then introduce the issue of selectivity in connection with the hydrogenation of alkynes, dienes, trienes, and aromatics; unsaturated aldehydes and imines; and cases where hydrogenation competes with other types of reactions such as dehydrogenations, skeletal rearrangements, cyclizations, and hydrogenolysis. Two general approaches to the control of selectivity are discussed: via the tuning of the structure of the catalytic surface, which can now be addressed by using nanoparticles with specific sizes and shapes, and by modifying the electronic properties of the metal, via the addition of a second element. Finally, we make reference to the interest in designing enantioselective hydrogenation processes using heterogeneous metal catalysts, and we briefly summarize the ideas that have developed from surface-science studies toward this goal and the advances made in understanding the most promising approach to date, which involves the addition of molecular chiral modifiers to the reaction mixtures. KEYWORDS: selectivity, surface chemistry, hydrogen activation, carbonaceous deposits, surface restructuring, olefins, unsaturated aldehydes, chiral catalysis
1. INTRODUCTION The hydrogenation of unsaturated bonds in organic molecules was one of the first processes to be promoted catalytically by solids,1,2 yet it is still used extensively in industry for a variety of applications.3−8 It is also one of the most studied systems in catalysis at both fundamental and applied levels.9−12 Much is understood about catalytic hydrogenations, which are typically promoted by solids based on late transition metals, but some issues remain unresolved. In this Perspective, we aim to provide a discussion of some of those pending questions from a surface-science point of view. We start by introducing key aspects of the basic mechanism of catalytic hydrogenation reactions, including the mode in which the reactants bind to the surface of the catalyst; the role of the carbonaceous deposits that form and are present on the surface during reaction; the mechanistic details of hydrogen adsorption, activation, and surface diffusion under catalytic conditions; and the dynamic nature of the structure of the active surface. We follow with a look at the selectivity of hydrogenation reactions in a number of important cases, namely, the limited conversion of carbon−carbon triple bonds to double bonds; making olefins rather than alkanes; the partial hydrogenation of dienes, trienes, and aromatics; the selective hydrogenation of specific unsaturated bonds in the presence of other types, as in the © XXXX American Chemical Society
case of unsaturated aldehydes and unsaturated imines; and the hydrogenation of unsaturated bonds selectively when in competition with other steps such as dehydrogenations, isomerizations, cyclizations, and hydrogenolysis. A discussion is provided on the main parameters considered to control such selectivity: the surface structure and the electronic properties of the metal. Special mention is made of the performance of single-atom-site catalysis as well as of the recent consideration of gold nanoparticles as catalysts for selective hydrogenations under mild conditions. Finally, we summarize the avenues being pursued to impart enantioselectivity to hydrogenation catalysts. We conclude with a brief overview of the state of the field and the promises for the future.
2. BASIC MECHANISTIC DETAILS The basic framework for explaining the mechanism of hydrogenation reactions on metal surfaces was advanced by Horiuti and Polanyi almost a century ago.13 The basic ideas are that the key function exerted by the metal is the facilitation of the dissociation of hydrogen molecules into adsorbed hydrogen Received: April 27, 2017 Revised: June 7, 2017
4947
DOI: 10.1021/acscatal.7b01368 ACS Catal. 2017, 7, 4947−4967
Perspective
ACS Catalysis
Figure 1. Left: Ethylene hydrogenation kinetics, in the form of turnover numbers (TON) versus time, for conversion on Pt(111) surfaces predosed with various precursor molecules. Shown are data from 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. Right: Summary of the turnover frequencies (TOFs), calculated via derivatization of kinetic data such as those in the left panel, for ethylene hydrogenation as a function of the nature of the carbonaceous layer on the initial Pt(111) surface. Shown are results from runs with propylidyne-presaturated Pt(111) annealed to the indicated temperatures to produce new surface species with various degrees of hydrogen content as well as with Pt(111) surfaces precovered with different surface species at room temperature (the experiments reported on the left). The rate of ethylene hydrogenation is seen to depend on the nature of the predosed species, in particular to the ease with which they can be removed from the surface. Reproduced with permission from reference 78. Copyright 2017 American Chemical Society.
hydrogenation catalysis does not occur on pristine metal surfaces but rather on surfaces covered with strongly adsorbed carbonaceous deposits, as will be discussed in more detail in the following section. Yet, the Horiuti−Polanyi mechanism is likely to require strong bonding to the surface in order to afford the first hydrogenation step, to form adsorbed alkyl moieties (and ultimately alkanes). Perhaps the initial π-bonded intermediate transitions to a di-σ-bonded state prior to the incorporation of the first hydrogen atom, but if so, no evidence for this pathway is available to date. Alternatively, the alkyl intermediate could form via the direct hydrogenation of the π-bonded adsorbate, in which case a transition would still be required to bring the extrinsic precursor closer to the surface. The picture has been further muddled by the recent identification of a second direct Eley−Rideal mechanism operative under certain circumstances where the olefin is directly converted into the alkane via the addition of two hydrogen surface atoms, possibly simultaneously.33−39 Clearly, the dynamics of the initial step in the hydrogenation of olefins (and other unsaturated reactants) is complex and not well understood. Quantum mechanic calculations could potentially help resolve this issue, but no attempts to do so have been published yet (to the best of our knowledge), possibly because the problem has not been framed in the way discussed here, that is, with a focus on the fact that the adsorption and conversion of the olefin takes place on surfaces partially covered with other strongly bonded species.40,41 Hopefully, our discussion will entice theoreticians to study the details of this dynamics more closely in the future. 2.2. Role of Carbonaceous Deposits. The second issue to be discussed here has to do with the cleanliness of the catalytic surface under operational conditions. It has been long known that clean metal surfaces are in general too reactive to adsorb unsaturated hydrocarbons intact at the temperatures used for catalysis. Instead, they promote partial dehydrogenation leading to the formation of new strongly bonded surface species.14,42,43 In the case of simple olefins, these new species have convincingly been shown to often be alkylidyne (PtC−
atoms and that the unsaturated bond to be hydrogenated, after adsorption on the surface, incorporates those hydrogen atoms in a stepwise manner, forming a half-hydrogenated intermediate (a surface-bound alkyl moiety in the case of olefins) along the way. The initial proposal was made in reference to the catalytic hydrogenation of olefins, but the basic idea is likely to be applicable to other types of reactants as well. This mechanistic outline is still believed to be valid in most cases.10,14,15 However, such deceivingly simple scheme hides a number of complications, as have been identified over the years. Some of those are discussed in the following sections. 2.1. Adsorption Mode of Reactants. It is clear that unsaturated hydrocarbons must adsorb on the surface of the catalyst before they can be hydrogenated, but there has been much discussion in the literature about the nature of the bond in those adsorbed reactants. The bulk of the work aimed to answer this question has been carried out with olefins, for which two limiting types of interactions with the surface have been identified: a strong di-σ arrangement, in which the CC double bond rehybridizes to form two metal−carbon covalent bonds, and a second, weaker π mode, which requires only minimum rearrangement of the molecular electronic density distribution.16−19 In reality, most adsorbed olefins exist in an intermediate state, with their extent of rehybridization varying depending on the nature of the substrate. In addition, the π intermediate may itself exist in two forms, namely, as an intrinsic precursor that ultimately leads to a stronger chemisorbed state on the clean surface of the metal, or as an extrinsic precursor on top of other adsorbates. The former was identified experimentally some time ago but only at very low temperatures,20−23 whereas evidence for the latter has been reported in more recent years, mainly from spectroscopic experiments carried out in situ under catalytic conditions.24−31 Most studies on the molecular details of hydrogenation catalysis have concluded that it is an extrinsic π-bonded species that intervenes directly in these reactions.24,32 The extrinsic nature of this intermediate is driven by the fact that 4948
DOI: 10.1021/acscatal.7b01368 ACS Catal. 2017, 7, 4947−4967
Perspective
ACS Catalysis R) moieties.44−46 Much work was dedicated in the past to pin down the structure of those species and the mechanism by which they form,44,47−60 and those issues have by and large been resolved.15 It has also been demonstrated that such adsorbates are present on the surface of the metal during catalytic conversions.24,46,61−71 What has not yet been fully settled is the role that they play in catalysis. It has been clearly proven that these alkylidyne surface species are not direct intermediates in the hydrogenation of olefins to alkanes because their hydrogenation and removal from the surface display rates several orders of magnitude slower than those for the catalytic production of alkanes from alkenes.46,61,63,72−74 It was early on suggested that the strongly bonded carbonaceous layers in hydrocarbon conversion catalysis 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,62,75,76 but that idea has been mostly discarded (even if some lingering uncertainty about the second option remains).15,77 On the other hand, it is clear that they temper the reactivity of the bare metal, inhibiting dehydrogenation reactions and favoring weak olefin adsorption and hydrogenation. It would appear that, once deposited, these strongly bonded species mainly block catalytic sites, but that the hydrogenation steps still take place on bare patches of the clean (but less reactive) metal. It is interesting to note that the saturation coverage of alkylidyne layers on (111) facets of late transition metals is 0.25 ML, that is, one alkylidyne moiety per four metal surface atoms.72 This coverage may seem low, but it is such that it completely blocks access of organic molecules to the surface. The adsorbed species need to either be partially removed or displaced to open up metal sites for catalysis. Recent experiments from our group support the general view of the role of carbonaceous deposits mentioned above but also point to other subtle effects. In particular, by independently controlling the nature of the carbonaceous layer, it was determined that their structure and reactivity do affect the performance of the catalyst.78 For one, ethylene hydrogenation on Pt(111) was found to be the fastest on a propylidyneprecovered surface and to exhibit rates that are lower by ∼20% with ethylidyne, by ∼35% with butylidyne, and by ∼40% with benzyl moieties (Figure 1). These changes are reversible: the surface regains the activity expected by starting with the clean substrate after one or two catalytic runs. Critically, in situ infrared absorption spectroscopy analysis of the surface shows 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, for instance). Much larger changes in catalytic activity are seen if the adsorbed layers are dehydrogenated at higher temperatures, at which point irreversible species are formed: the turnover frequency for ethylene hydrogenation is reduced by more than an order of magnitude upon the conversion of propylidyne to CnH(ads) species, via annealing at temperatures between 500 and 650 K, before the start of the catalytic reaction (Figure 1, right panel). The picture that emerges is one in which the strongly bonded carbonaceous layer that forms on the surface immediately upon exposure to the reaction mixture and that is present during catalysis blocks many but not all surface sites and may be somewhat mobile, allowing for the transient exposure of the Pt surface atom ensembles needed for the promotion of olefin hydrogenation. The dynamics appears to be dependent on the exact nature of the carbonaceous layer and
requires a certain degree of hydrogenation−dehydrogenation reversibility within those species, to help with their surface mobility. The extent to which the surface is poisoned by the carbonaceous layer is dependent on the reaction conditions. Because alkylidynes can be hydrogenated slowly and removed from the surface of metals,14,46,63,72,73,79 their steady-state coverage during catalysis is expected to vary with both temperature and the partial pressures of the reactants. Specifically, low steady-state coverages of alkylidyne should be reached with reaction mixtures with high hydrogen content.80 In addition, because the rate law for most catalytic hydrogenation reactions display a weak (or no) dependence on the pressure of the organic reactant (the olefin, for instance) in the gas phase, the argument can be made that reaction probabilities should increase with decreasing total pressure and should reach values close to unity in the μTorr to mTorr range (the so-called “pressure gap”).15,46 This prediction has been recently proven correct experimentally by us with the aid of high-flux molecular beams.46,80,81 Such increase in intrinsic catalytic activity was traced to the reduction of site blocking by the carbonaceous layer.80 2.3. Hydrogen Adsorption and Diffusion under Reaction Conditions. As mentioned in the Introduction of Section 2, the key function of the metal in hydrogenation catalysis is the facilitation of the dissociation of hydrogen molecules. Because of this, the kinetics of the dissociative adsorption and recombinative desorption of H2 on late transition metals has been studied extensively using many modern surface-sensitive techniques and determined to be fast even under mild conditions, namely, at low temperatures and pressures.82−89 However, most of that work has been carried out on clean surfaces and under ultrahigh vacuum (UHV) conditions; virtually nothing is known about the mechanistic details associated with the uptake of H2 under the conditions used for catalysis.90,91 We have recently addressed this issue by performing catalytic studies on Pt surfaces with C2H4 + H2 + D2 mixtures in order to follow the kinetics of HD formation, a proxy for the dissociative adsorption, surface diffusion, and recombination of hydrogen on the surface, simultaneously with the kinetics of the hydrogenation of ethylene to ethane and the isotope scrambling that takes place concurrently on the organic molecules.92,93 Perhaps the most striking observation deriving from that work is a sharp increase in the rate of hydrogen isotope scrambling seen after approximately 80% of conversion of the olefin, a change that is not accompanied by any significant changes in the rate of the olefin hydrogenation itself (Figure 2).92 Reflection−absorption infrared spectroscopy (RAIRS) data taken at different times during the course of the reaction (after quenching with carbon monoxide) show a decrease in ethylidyne coverage and a concurrent increase in the availability of Pt sites as the reaction progresses (Figure 2, top). A model was proposed on the basis of this information where a decrease in the size of the islands of the strongly adsorbed hydrocarbons leads, at a certain threshold value, to a sudden increase in atomic hydrogen surface mobility. It is interesting to also point out that, until reaching that threshold, the rate of HD production is comparable to that of ethane formation, a fact that suggests that hydrogen adsorption may be rate-limiting under catalytic conditions. In a related study, it was seen that the average stoichiometry of the final product retains the value expected from the gas mixture, that is, four H and two D atoms 4949
DOI: 10.1021/acscatal.7b01368 ACS Catal. 2017, 7, 4947−4967
Perspective
ACS Catalysis
hydrogen (deuterium) atoms upon impingement on the surface.33,34 Yet another pathway is available to hydrogenation reactions in the case of palladium because that metal has the ability to absorb hydrogen atoms into its bulk. The diffusion of hydrogen between the surface and the bulk has been recently corroborated, and its dynamics has been quantified via the analysis of a resonant 1H(15N,αγ)12C nuclear reaction.88 In olefin hydrogenation catalysis, the role of hydrogen dissolved in the Pd bulk was determined to predominantly be as a source for reactive surface H species.95 Carbon deposits on the surface were shown to help this bulk-to-surface hydrogen diffusion, presumably thanks to the flexible nature of low-coordinated surface sites in small metal clusters.96,97 The rate of olefin hydrogenation was also clearly shown to be enhanced by the addition of carbon deposits on the surface, leading to a steadystate regime not available on the clean metal.98 Curiously, this limitation does not apply to the isomerization of the CC bond in olefins, which can be carried out catalytically on both clean and carbon-covered palladium.99 Also, it should be noted that the turnover frequencies measured for olefin hydrogenation on Pd in these molecular-beam experiments are quite high, orders of magnitude higher than those seen with regular supported palladium catalysts. To date, this discrepancy has not been addressed, and an explanation is still lacking. 2.4. Hydrogen Spillover. As discussed above, the kinetics of molecular-hydrogen dissociative adsorption and surface mobility are central to hydrogenation reactions. On singlecomponent metal catalysts, H2 activation competes with any of the other steps that take place on the surface, including, as already mentioned, the formation and conversion of strongly bonded carbonaceous moieties. On the other hand, on multicomponent catalysts, there is an opportunity to activate H2 on one type of site and to carry out the catalytic hydrogenation on another. The hydrogen atoms in this scenario need to travel from the first site to the second. This phenomenon, known as spillover, was proposed many years ago and has been invoked to explain the behavior of a number of catalytic processes.100−102 However, direct evidence of such hydrogen transfer has been difficult to come by.103 A few recent surface-science and catalytic experiments have contributed toward this goal. A running controversy in the literature relates to the requirements on the oxide used in supported catalysts for the promotion of hydrogen spillover. The conventional wisdom has been that such oxide needs to be reducible because (1) hydrogen atoms, produced on the surface of the metal, do not have a strong driving force to migrate to the oxide unless a redox reaction takes place; and (2) the diffusion of hydrogen on oxide surfaces is believed to involve separate proton and electron particles rather than hydrogen radicals.103 However, new data suggest that this may not always be the case. For instance, Beaumont and co-workers reported catalysts consisting of mixtures of pure Pt and Co nanoparticles dispersed on a silicon oxide support capable of promoting CO2 methanation reactions.104 In that case, the successful promotion of the hydrogenation of CO2, which occurs on the Co phase, is aided by the prior dissociation of H2 on the Pt phase; the intermediate H atoms need to travel in between the two metals via diffusion on the oxide surface. In another study, with catalysts consisting of Pt encapsulated in aluminosilicate zeolites with controlled diffusional properties, the authors proposed (with the help of DFT calculations) that surface
Figure 2. Main frame: Kinetics of ethylene hydrogenation to ethane and of HD formation from the conversion of C2H4 + H2 + D2 mixtures on a Pt(111) single-crystal surface, in the form of TOFs versus time. Top: Coverages of ethylidyne and empty sites present on the Pt(111) surface versus reaction time, as determined from in situ infrared absorption spectroscopy measurements after quenching the reaction with carbon monoxide. By correlating the two sets of data, it can be seen that there is a threshold surface coverage of ethylidyne, reached after approximately 500 s of reaction (shaded area), where enough Pt empty sites open up for the HD production rate to abruptly increase, by approximately 1 order of magnitude. No concomitant changes are seen in the rate of ethane production, which continues virtually unaffected until the reactant is exhausted (after approximately 800 s of reaction). Adapted with permission from reference 92. Copyright 2016 The Royal Society of Chemistry.
per ethane molecule in the case of the experiments with C2H4 + D2.94 This means that, by and large, no surface hydrogen is removed as X2 (X = H or D). It was concluded that, under catalytic conditions, surface hydrogen atom recombination is much slower than ethylene hydrogenation and H−D exchange. The behavior of the metal surface toward hydrogen activation during catalysis is intimately related to the nature and coverage of the strongly bonded carbonaceous layer. In experiments using high-flux molecular beams under UHV, three kinetic regimes were identified as a function of the partial pressure (or flux) of ethylene in the reaction mixture.93 For reaction mixtures with more than 1% of ethylene, the ethylidyne surface layer that forms at the start of the reaction reaches steady-state coverages close to saturation and controls the HD and ethane formation kinetics via site blocking. This is the regime operational under most typical catalytic conditions, where ethylene hydrogenation takes place in a stepwise fashion, following the well-established Horiuti−Polanyi mechanism. Just below the 1% ethylene threshold, the surface is still covered with hydrocarbons (increasingly reversibly adsorbed ethylene instead of ethylidyne), but HD production is still relatively fast. The probability for ethane formation increases noticeably as the ethylene content in the reaction mixture is reduced, until reaching the third regime, seen for mixtures with less than 10 ppm of ethylene, where the steady-state production of ethane shows kinetics similar to those measured in UHV studies, with a rate law dependent linearly on both ethylene partial pressure and hydrogen atom coverage.32 In this latter regime, the surface is mainly covered by hydrogen, and a new mechanism opens up for ethane formation, via a “reverse” Eley−Rideal step where olefin molecules from the gas phase pick up two adsorbed 4950
DOI: 10.1021/acscatal.7b01368 ACS Catal. 2017, 7, 4947−4967
Perspective
ACS Catalysis
Figure 3. Experimental study of the hydrogen spillover effect. Top, left: Description of the spillover of hydrogen studied in this work. Platinum nanoparticles are used to dissociate the incoming H2 molecules, after which the resulting hydrogen atoms travel through the oxide support and reduce adjacent iron oxide nanoparticles. Top, right: Schematic representation of the samples prepared for this test, highlighting the varying interparticle distances (d) used. Bottom: results for samples based on aluminum oxide (left) and titanium oxide (right) thin-film supports, in the form of the extent of the reduction of the iron oxide nanoparticles observed as a function of d. Adapted with permission from reference 107. Copyright 2017 Macmillan Publishers Limited, part of Springer Nature.
surfaces. Curiously, similar spillover was not seen on Au(111).111,112,114 Their new single-atom-site design was also tested for hydrogenation reactions. Specifically, they showed that the hydrogenation of styrene can be promoted in temperature-programmed desorption (TPD) experiments via the addition of isolated Pd atoms to a Cu surface.115 Both the selective catalytic hydrogenation of butadiene in the presence of propylene116 and the catalytic hydrogenation of acetylene in the presence of carbon monoxide117 were also proven possible on supported Pt−Cu single-atom alloys. The use of this type of single-atom sites within alloys has gain some interest in connection with hydrogenation reactions in recent years, as will be discussed again later in this Perspective.118 2.5. Catalyst Structure and Reconstruction. A final issue associated with the mechanism of hydrogenation reaction is the role that the structure of the surface plays in determining rates and selectivity. Catalytic reactions have traditionally been divided into structure sensitive and structure insensitive, and the hydrogenation of unsaturated bonds in organic molecules has typically been considered to belong to the latter category.119−122 However, new work using well-defined nanoparticles has challenged that view.15,123−125 For instance, surface-science experiments with size-selective Ptn clusters (n = 8 to 15, d < 1 nm) supported on an MgO surface identified an onset for the hydrogenation of ethylene on nanoparticles with at least 10 Pt atoms and a maximum in yield for n = 13.126 The authors of that work also reported reactivity at T ∼ 150 K, a temperature much lower than that seen on Pt single crystals. These are results from TPD experiments under UHV, though, and do not reflect steady-state catalytic conversions. Additional research will be needed to explore the validity of extrapolating these conclusions to more realistic catalytic systems. In another direction of research, the exploration of the role of structure in catalytic processes has benefited recently from new
hydroxyls, presumably Brønsted acids, are crucial for hydrogen spillover in benzene hydrogenation catalysis.105 On the other hand, experiments based on the detection of hydrogen adsorption on gold using surface plasmon resonance measurements with surfaces containing both Au and Pt nanoparticles demonstrated that spillover occurs only on semiconducting oxides (anatase TiO2 and ZnO), not on silicon oxide.106 Perhaps the most elegant study to date on the viability of hydrogen spillover on oxide supports is that of Karim et al., who used nanolithography to prepare a series of model catalyst samples with variable distances between Pt nanoparticles, used for hydrogen activation, and iron oxide nanoparticles, where oxide reduction was employed as the probe for hydrogen spillover (Figure 3, top).107,108 Clear reduction was seen when titanium oxide was used as the support, indicating effective spillover to distances of up to 45 nm (the maximum distance tested), whereas virtually no spillover was detected on aluminum oxide films (Figure 3, bottom). The authors of that work also showed that, in the case of the titanium oxide support, the titanium ions are reduced during the process, from Ti4+ to Ti3+, an observation that strongly suggests that that spillover indeed requires the catalyst support to be reducible. Surface-science experiments have pointed to the possibility of hydrogen to spillover in other types of systems as well. For instance, the platinum tip of a scanning tunneling microscopy (STM) instrument has been used to dissociate H2 and facilitate the hydrogenation of carbonaceous species deposited on the surface being imaged.109 More recently, STM work by Sykes and co-workers on bimetallic surfaces have demonstrated that H2 can be dissociated on isolated sites of one type of metal to promote hydrogenation reactions on another. They tested this idea with several intermetallic combinations, and revealed that either Co nanoparticles110 or individual Pd111,112 or Pt113 atoms can promote H2 dissociation and spillover onto Cu(111) 4951
DOI: 10.1021/acscatal.7b01368 ACS Catal. 2017, 7, 4947−4967
Perspective
ACS Catalysis
Figure 4. Coordination numbers n (top; a,b) and interatomic distances r (bottom; c,d) in Pd/SiO2 (left; a,c) and Pt/SiO2 (right; b,d) catalysts as a function of the composition of the H2 + C2H4 reaction mixture used to test the hydrogenation catalysis. The filled symbols indicate the forward sequence from pure H2 to pure C2H4, whereas the open symbols depict the reverse sequence from C2H4 to H2. Significant changes in the metal− metal coordination numbers were seen with both metals in transitioning from hydrogen- to ethylene-rich atmospheres. In addition, with Pt/SiO2, the measured coordination numbers of low-Z (O) atoms around the Pt atoms (for which two types were identified) were similar in all atmospheres, but the Pt-low-Z long-bond contribution was only seen in the C2H4-rich atmospheres (no coordination to low-Z elements was detected with Pd). Reproduced with permission from reference 142. Copyright 2015 American Chemical Society.
tetrahedral and to multipode nanoparticles.138 This trend was rationalized in terms of poisoning because of the larger number of low-coordination sites in the first and second cases compared to the third. It can be seen that not all these results and conclusions are consistent; ultimately, the topic of the structure sensitivity of hydrogenation reactions is still unresolved and will require further studies. One issue with studies of structure sensitivity in catalysis is that the surface of solids, in particular in nanoparticle form, have been shown to be dynamic and to reconstruct in response to the environment to which they are exposed.139 Recently, Nuzzo and co-workers have combined the use of X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM) for the operando characterization of catalysts to address this issue.140−142 They have shown that exposure of catalysts consisting of metal nanoparticles dispersed on an oxide support to an atmosphere of hydrogen leads to a small reduction in the distance of the metal−metal bonds.143 In specific studies on the hydrogenation of ethylene using Pd/ SiO2 and Pt/SiO2, they reported noticeable changes in the atomic and electronic structures of the catalysts during reaction, including defined transitions between hydrogen- and hydrocarbon-covered surfaces, carbide-phase formation, hydrogen (de)intercalation, and particle coarsening.141,142 For instance, with the Pd/SiO2 catalyst, the coordination number was found
advances in nanotechnology that afford the making of metal nanoparticles with narrow size distributions and well-defined shapes, with different facets exposed.124,127−133 Studies on the catalysis of simple olefin hydrogenations using this approach are still limited, however, and have so far yielded mixed results. On the one hand, Somorjai and co-workers, using a series of catalysts based on Pt nanocubes and nanopolyhedra with tunable size from 5 to 9 nm, concluded that ethylene hydrogenation is structure insensitive.134 On the other hand, their results with cyclohexene were more nuanced: they identified two different temperature regimes, and found that the hydrogenation is structure insensitive at low temperatures but structure sensitive in a non-Arrhenius regime seen at higher temperatures.135 The authors attributed the latter particle-size dependent reactivity to a change in the coverage of reactive hydrogen. In a separate effort, we have looked in detail into the hydrogenation and isomerization of 2-butenes on tetrahedral, cubic, and spherical Pt nanoparticles, all approximately 5 nm in diameter, and we have observed that the (100) facets of the nanocubes are particularly active for CC bond hydrogenation.136 These results are consistent with the relative reactivity seen in TPD experiments by using Pt single crystals and UHV conditions.137 Finally, with Pd catalysts dispersed on a SiO2 support, it was shown that the rate of hydrogenation of cyclohexene increases in transitioning from spherical to 4952
DOI: 10.1021/acscatal.7b01368 ACS Catal. 2017, 7, 4947−4967
Perspective
ACS Catalysis
position and shape of the d band believed to be central to catalytic promotion.168 Again, new nanotechnologies afford studies of these effects in ways not previously available. For instance, colloidal nanoparticles can be made using two or more metals either fully mixed, in alloy form, or with core−shell profiles.169−171 In general, these new tools, when applied to catalysis, have the ability to open new doors to the tuning of catalytic selectivity.124,165,170,172 Next, we discuss some recent developments in this area. 3.1. Partial Hydrogenation: Carbon−Carbon Triple versus Double Bonds. One challenge when hydrogenating molecules with more than one unsaturation is to stop at a specific designated intermediate point, before reaching the full conversion to the saturated product. Perhaps the simplest case in this category is the selective hydrogenation of alkynes to alkenes rather that to alkanes, a task of relevance well beyond pure academic interest.173 For instance, in order to produce polymer-grade ethylene, with acetylene impurities below 5 ppm, the approximately 0.5−2 volume% of acetylene present in the ethylene collected from naphtha steam crackers needs to be selectively hydrogenated without promoting further hydrogenation to ethane or any other side reactions.174−176 This is typically carried out by using supported Pd or Pd−Ag catalysts, but those are expensive, and their performance still needs to be improved, especially in terms of stability and of the minimization of their deactivation. The selectivity conversion of acetylene to ethylene is thought to be aided by either thermodynamics, thanks to the stronger binding of acetylene relative to ethylene on metals, which may lead to the displacement of the latter from the surface of the catalyst by the former, or by kinetics, if ethylene desorbs before it can hydrogenate.177,178 The mechanistic details of this process, however, have been hard to tease out because the catalytic performance is affected by a wide range of parameters, including the buildup of carbonaceous layers on the surface during catalysis,179 the formation of metal hydrides and/or metal carbide phases,180,181 the dispersion and size distribution of the metal nanoparticles,182,183 the nature of the catalyst support,184 the composition of the feedstock,185 and the participation of promoters such as CO.186 These factors can influence the surface coverages of the reactants (acetylene and hydrogen) as well as those of any other hydrocarbon intermediates involved, a fact that makes difficult to extract mechanistic information from simple measurements of kinetic parameters such as activation energies and reaction orders with respect to the partial pressures of the reactants. Much recent understanding of the mechanism of the competitive hydrogenation of acetylene versus ethylene, specifically with palladium catalysts, comes from a combination of surface-science experiments,187−189 catalytic studies using well-shaped nanoparticles,188,190,191 and quantum mechanics calculations.178,192−194 Those have highlighted the fact that the palladium surface is, under catalytic conditions, covered with a number of carbonaceous species, which include ethylidyne and vinylidene species but perhaps vinyl and ethylidene as well.187 The coverages of all of those species have been shown to depend strongly on the pressure of hydrogen as well as on other reaction conditions and to alter the overall performance of the catalyst.187 They may be spectators, as discussed in Section 2.2, or may participate directly in the reaction mechanism. Some studies178,189,195 have suggested that the selective hydrogenation of acetylene to ethylene proceeds via the formation of a surface vinyl intermediate and that the
to change by as much as two units as the atmosphere is switched from hydrogen-rich to ethylene-rich atmospheres, and the Pd−Pd bond distance to increase in the hydrogen-rich atmospheres (Figure 4, left).142 The behavior with Pt/SiO2 is quite different: the Pt−Pt coordination number also goes down significantly when adding ethylene to the reaction mixture, but unlike with the Pd clusters, substantial contributions from the bonding of Pt to its low-Z neighbors (oxygen atoms) are seen, mainly in ethylene-rich atmospheres, and the Pt−Pt and Pt−O bond distances do not change much (Figure 4, right).142 Interestingly, only minimal impact was seen from these dynamic changes on reactivity, although the effect that they may exert on the kinetics of the catalytic reactions has not been explored in any detail to date: the techniques to follow the evolution of nanoparticle shapes in operando mode are quite recent, and there is no obvious way to control those shapes in a systematic way in order to be able to find correlations with catalytic performance. For now, what can be said is that the knowledge available on surface restructuring during catalysis brings into question the value of controlling nanoparticle shape to direct catalytic performance.
3. SELECTIVITY Most late-transition metals are quite effective at promoting hydrogenation reactions.3,10 The challenge often is to perform hydrogenation processes selectively.5,6,144 In some instances, it is desirable to only catalyze hydrogenation partially, as in the selective conversion of triple bonds to double bonds rather than to fully saturated molecules, or in the preferential hydrogenation of only one out of the several CC bonds in dienes, trienes, or aromatic compounds. In other cases, the target may be to hydrogenate one type of unsaturated bond in the presence of others, as with carbonyl moieties in unsaturated aldehydes. Alternatively, hydrogenation steps may compete with other types of reactions such as dehydrogenations and skeletal rearrangements. Finally, it may be desirable to carry out hydrogenations enantioselectively if new chiral centers are produced in the process. Selectivity in many catalytic processes with solids can be tuned by affecting one or both of the two main factors that define the catalytic site, namely, their structural details and/or their electronic properties.121,123,145,146 This is the case with hydrogenations as well. Traditionally, the structure of metal nanoparticles, dispersed on high-surface-area supports as is done in most heterogeneous catalysis, is controlled by varying the total metal loading and/or its dispersion, and can be affected by the preparation method as well.147−149 Because hydrogenation reactions have historically been considered structure-insensitive, however, limited research was done in the early days of catalysis to explore the effect of size or dispersion on catalytic hydrogenations.150−156 New efforts in this area have led to a challenge of the original view,157,158 though, and some research groups have retaken this issue, as we discuss below. New self-assembly synthetic methods and other nanotechnologies have afforded greater control of both the size and shape of nanoparticles and their electronic properties, and with that the preparation of better-defined catalysts.131,132,159 For instance, colloidal metal particles can now be synthesized not only with narrow size distributions but also with specific shapes, even going beyond simple geometrical structures such as cubes and octahedra.130,160−165 In terms of the electronic structure, the typical approach with metals is to mix two or more of them166,167 as a way to tune the 4953
DOI: 10.1021/acscatal.7b01368 ACS Catal. 2017, 7, 4947−4967
Perspective
ACS Catalysis unselective formation of ethane results from overhydrogenation of this vinyl intermediate to ethylidene. Ethylidene can also dehydrogenate to form ethylidyne, a surface species that, together with di-σ-ethylene, is not reactive but blocks adsorption sites and thus limits the availability of hydrogen on the surface, enhancing selectivity. An alternative view advocates that the selectivity of the acetylene hydrogenation is not so much dependent on the organic species that form on the surface, at least not directly, but rather controlled by the nature of the catalyst. This idea relies on the fact that the Pd metal is capable of forming new carbide and hydride phases: unselective hydrogenation has been suggested to proceed on hydrogen-saturated β-hydride Pd surfaces, whereas selective hydrogenation may be favored by reactions on the metallic surface.181,196 According to this explanation, the extent of the formation of the hydride and of its effect on ethylene adsorption strength can be modified by controlling particle shape and morphology, by the selection of the support, or by the use of organic or inorganic surface modifiers,191 and such manipulation of the hydride formation can presumably be then used to tune selectivity. These two explanations for the selective behavior of Pd catalysts, based on surface species or hydride/carbide formation respectively, are perhaps the most discussed, but others have been mentioned in the literature as well; they all present some contradicting proposals that still need to be settled.197 In terms of the role of surface structure on catalytic performance, it is important to separate the effects of nanoparticle shape from those due to nanoparticle size. This has been recognized in studies of other reactions, but becomes particularly critical in catalysis promoted by palladium because that metal is known to form a hydride phase easily via hydrogen diffusion into the octahedral lattice vacancies of the Pd crystallites. The relative number of such vacancies in reference to surface atoms is highly dependent on nanoparticle size,198 and, as discussed above, some researchers have identified the metal hydride phase as critical in defining the selectivity of the hydrogenation processes. Keeping this in mind, there are a few reports on the effect of Pd nanoparticle size and/or Pd nanoparticle shape on the hydrogenation of carbon−carbon triple bonds. Telkar et al. observed a variation in catalytic performance for the hydrogenation of 2-butyne-1,4-diol (an activated alkyne) between cubic and spherical nanoparticles,199 whereas Semagina and co-workers found that the hydrogenation of 2-methyl-3-butyn-2-ol (also activated) takes place with similar selectivity on nanohexagons, nanocubes, nanooctahedra, and nanospheres, but that the activity can be correlated to the number of Pd(111) atoms available.200,201 Those experiments were performed in solution and by using free Pd colloidal particles, but more recent studies on the gasphase hydrogenation of acetylene on supported Pd catalysts, using nanocubes, nanooctahedra, nanocubooctahedra, and nanospheres, led to the same conclusion (Figure 5).202,203 This correlation of activity with the total area of the (111) facets is also consistent with the fact that both the decomposition of the Pd hydride phase and the desorption of C2 hydrocarbons from Pd surfaces occur at lower temperatures with cubic Pd particles than with spherical particles. Although Pd catalysts are fairly active for alkyne hydrogenations, their selectivity is not sufficiently high for many applications. Luckily, it has been shown that such selectivity can be improved significantly by alloying Pd with a second element, typically Ag, even if that may decrease the overall rate.204,205
Figure 5. Illustration of the effect of nanoparticle shape on the activity and selectivity of acetylene hydrogenation catalysis. In this example, the turnover frequencies (TOF, squares and right scale) and reaction selectivity (bars and left scale) are reported for reactions promoted by catalysts made with Pd nanocubes (Pdcub), Pd nanocubooctahedra (PdCO), and Pd nanooctahedra (Pdoct) dispersed on carbon nanofibers. The reaction conditions were as follows: P(C2H2) = 1.4 kPa, P(H2) = 21 kPa, T = 393 K, acetylene conversion = 13%. It is clear that while the selectivity remains virtually the same in all cases, the activity increases with the fraction of (111) facets exposed. Adapted with permission from references 202 and 201. Copyright 2012 Pleiades Publishing, Ltd. and Copyright 2011 American Chemical Society, respectively .
Within the carbonaceous-layer model described above, Ag presumably promotes the selective hydrogenation of vinyl to ethylene and suppresses the unselective hydrogenation of vinyl to ethylidene and the dehydrogenation of ethylidene to ethylidyne.178,195 Alternatively, the ease with which the unselective Pd hydride phase forms may be hindered by the decrease in Pd ensemble sites in the alloys.191 In the end, the same uncertainties in the understanding of the reaction mechanism discussed above apply to the explanation of the effect of alloying on catalytic performance. There has been some interest recently in testing the use of the single-atom-site alloys discussed in Section 2.4 to control selectivity in acetylene hydrogenation. In this case, Pd is not viewed as the main phase, and the performance of the catalysts is not described in terms of the models described before but rather in a cooperative way where isolated Pd atoms act as H2 dissociation sites and the second metal as the hydrogenation catalyst. Excellent performance was reported in one recent case using this approach,206 even if spectroscopic evidence pointed to a more complex synergistic behavior not fully compatible with the single-atom picture. Moreover, the similar trends seen in another study with Ag−Pd versus Au−Pd bimetallic nanoparticles of different compositions suggested that the effect with these catalysts may be geometric and not electronic in nature.205 The reasons for the unique performance of singleatom site catalysts are also unresolved and need further research. 3.2. Partial Hydrogenation: Limited Carbon−Carbon Double Bond Hydrogenation in Dienes, Trienes, and Aromatics. Another family of reactions where the extent of hydrogenation may need to be controlled is with reactants with multiple carbon−carbon double bonds such as dienes, trienes, and aromatics. There are some analogies between these systems 4954
DOI: 10.1021/acscatal.7b01368 ACS Catal. 2017, 7, 4947−4967
Perspective
ACS Catalysis
Figure 6. Performance of supported Pt nanopolyhedra as catalysts for the hydrogenation of 1,3-butadiene as a function on nanoparticle size. Left: In situ sum-frequency generation (SFG) spectra of the species present on the surface during reaction. Right, top: Normalized SFG intensities for the methylene/methyl (CH2(a)/CH3(p,up), red) and methyl (CH3(s), blue) vibrational modes associated with the key surface intermediates identified by SFG, indicated in the accompanying sketches, as a function of nanoparticle size. Right, bottom: Selectivity in the 1,3-butadiene hydrogenation conversion, also as a function of nanoparticle size. Larger particles favor more 1-butene production, presumably because of the ease with which the species with the methyl groups pointing upward from the surface are formed. Adapted with permission from reference 133. Copyright 2015 Elsevier Inc.
and the cases discussed in the previous section.10 In terms of industrial interest, the production of high-purity butene streams for polymerization or copolymerization processes requires the hydrogenation of the butadiene impurities contained in the butene cuts from naphtha steam crackers, as with the acetylene impurities in the ethylene cuts. Palladium is also considered the best catalyst for this type of partial hydrogenation,153,207−209 again either because of differences in relative adsorption strengths,210,211 with the strongly adsorbing 1,3-butadiene displacing the more weakly adsorbing butenes, or thanks to its unique kinetics, by which the intermediate mono-olefin desorbs before more extensive hydrogenation can take place.212,213 Yet, the performance of palladium-only catalysts is not always satisfactory; efforts are under way to improve that, either by modifying the structure of the supported Pd nanoparticles or, more commonly, via alloying with a second metal. From a mechanistic point of view, a number of surface intermediates have been proposed on the basis of results from both experimental and theoretical studies, including the halfhydrogenated species expected from the Horiuti−Polanyi mechanism as well as the more strongly bonded carbonaceous layers expected in hydrogenation reactions, as discussed in Section 2.2.212−217 Much has been made of the difference in selectivity in processes with Pd versus Pt catalysts,218,219 which different quantum-mechanic studies have explained in terms of relative differences in adsorption energies for different intermediates. The Sautet group, for instance, established that radical species are more clearly stabilized on Pt than on Pd,216 and proposed that while 1,3-butadiene may adsorb in a di-σ bonding mode and with minimal distortion on Pd, it may adopt a 1,2,3,4-tetra-σ structure on Pt.220 These ideas are in general agreement with experiments.211 Mittendorfer et al., however,
emphasized the lower adsorption strength of the intermediate 1-butene on Pd instead, which prevents its further hydrogenation to butane.221 That idea is consistent with the fact that, generally, butenes are primary products of butadiene hydrogenation with palladium but not with platinum,218 and also with the observation than butenes, especially 2-butene, are difficult to hydrogenate on Pd nanoparticles.222 The relative importance of these two factors in explaining the uniqueness of the performance of Pd is still an open question. In addition, the partial hydrogenation of polyunsaturated compounds is often accompanied by isomerization, double bond migration, and/or cis−trans interconversion. Following the Horiuti−Polanyi mechanism, the first expected intermediate from the addition of one hydrogen atom to adsorbed 1,3-butadiene, for instance, is an allylic species, which is relatively stable as a π-adsorbed intermediate and therefore may switch bonding points via π/diσ bonding-mode transitions on the surface.223 This is an issue common to all compounds with conjugated double bonds that leads to the production of a mixture of olefins, to the detriment of selectivity.218,224 The structure sensitivity of the selectivity of 1,3-butadiene hydrogenation has been discussed in the literature for years now. It was reported decades ago that, contrary to the case of the hydrogenation of 1-butene, the hydrogenation of 1,3butadiene (and also of isoprene) is sensitive to the dispersion of the Pd metallic phase, an observation that was then explained by the ability of the reactants to form a strong complex on the small particles expected at high dispersions.225 More recently, in a molecular beam study of the selective hydrogenation of 1,3butadiene on a series of Pd/Al2O3/NiAl(110) model catalysts with Pd mean particle sizes ranging from 2 to 8 nm, it was found that the reaction on large (>4 nm) Pd nanoparticles exhibits a near zero-order kinetics with respect to butadiene but 4955
DOI: 10.1021/acscatal.7b01368 ACS Catal. 2017, 7, 4947−4967
Perspective
ACS Catalysis
remains on the effect that the surfactants used to make those particles exert on the performance of the catalysts because the cleaning of the metal nanoparticles once dispersed on the solid supports is not trivial and is often not carried out to completion.237−243 Extensive work has also been performed to test the changes in diene hydrogenation selectivity as Pd (and other metals) is alloyed with a second metal.175,212,244−246 It has been shown empirically that the addition of a more inert metal such as Au or Cu minimizes the conversion of alkenes to alkanes during the hydrogenation of dienes at high conversions, when Pd-pure catalysts lose their selectivity and also reduce the formation of the so-called green oil responsible for catalyst deactivation.247 The single-atom-site approach has been implemented for these reactions as well: alumina-supported single-atom alloy catalysts with
