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Perturbation of reactivity with geometry-How far can we go? Randall J. Meyer, Qiang Zhang, Anna Kryczka, Carolina Gomez, and Ruzica Todorovic ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03228 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 1, 2017
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ACS Catalysis
Perturbation of reactivity with geometry-How far can we go? Randall J. Meyer1*, Qiang Zhang2, Anna Kryczka3, Carolina Gomez2 and Ruzica Todorovic2 1
ExxonMobil Research and Engineering, Annandale, NJ, USA
2 3
Department of Chemical Engineering, University of Illinois at Chicago, Chicago IL,USA
Illinois Math and Science Academy, Aurora, IL, USA
KEYWORDS: Heterogeneous catalyst, reaction ensembles, bimetallic catalysts, single atom alloys, intermetallics, selfassembled monolayers, surface modifier Interest in transition metal alloys as opposed to monometallic catalysts has grown in the years following the inspiring work of John Sinfelt. The intrinsic complexity of these systems allows us to better tailor reaction selectivity and to tune the reactivity of one metal by the presence of another. One of the early themes of the Sinfelt work was the invocation of reaction ensembles to explain differences in hydrogenolysis behavior between monometallic and bimetallic systems1. However, it was also recognized from the beginning that perturbation of reactivity through alloying involved both geometric effects involving creation of specific reaction ensembles (atomistic sites) and electronic effects due to changes in electron density and bonding due to charge transfer and hybridization effects in alloying. For the purposes of this perspective, the term electronic effect will refer to the strength of an adsorbate interaction with the metal surface while a geometric effect refers to the bonding configuration. This perspective will comment upon the interplay between the two effects and potential for exploiting geometric effects in alloying. Norskov and coworkers have established relationships with regard to the adsorption behavior of d-band transition metals based upon the energy of the d-band center with respect to the Fermi level2. One observation from Norskov and Hammer is that since all transition metals in principle have the same s occupancy (and p occupancy for that matter), then the s electron is not considered to be important in the bonding of the system as its effect is identical for all d-band transition metals. The perturbation of the d-band can be broken into two terms: a charge transfer effect where the filling of the d-band changes due to a charge transfer event between the metals and a hybridization effect due to the change in the overlap between the orbitals due to the change in nuclear distance and change in orbital extent when one metal is replaced for another3. For most d-band transition metal alloys the degree of charge transfer is insignificant and the alloy is dominated by changes in hybridization. The changes in hybridization can manifest themselves in different ways. For example, the Pd-M bond distance in Pd is 2.74 Å, but in the alloy Pd-Ni bonds are 2.63 Å. This contraction results in a shift of the d-band of Pd states away from the Fermi level from -2.01 eV in Pd to -2.48 eV in the alloy. Similarly, in PdAu, the Pd-Au bond expands to 2.86 Å, and the d-band shifts accordingly toward the Fermi level to -1.64 eV. Based on these examples, in a PdPt alloy, one would expect the Pd d-band center to shift toward the Fermi level due to the larger lattice constant of Pt compared to Pd. However, the d-band center shifts instead slightly away from the Fermi level to -2.23 eV due to the increased overlap between Pd and Pt from the larger orbital extent
of Pt (3.79 vs.3.48 Å)4, demonstrating that both orbital extent and equilibrium bond distance determine electronic structure. It should be noted the electronic structure of the alloying element also shifts in response to alloying event (in this case the d-band center of Pt shifts from -2.55 eV to -2.34 eV).Once the perturbation of the d-band due to alloying is known, the adsorption behavior of alloys (to a first approximation) can be described by average d-band filling of the metal atoms in contact with the adsorbate5. This simple model has been shown to provide effective guidance for understanding catalytic performance in a number systems as the adsorbate binding strength (and by extension the dband center) can be correlated with its activation on the metal surface6.
Figure 1. PDOS of d-orbitals of Pd, Pt and PdZn (see SI for calculation details). When a d-band transition metal is alloyed with a non-transition metal (hereafter described as sp-electron metals), charge transfer can be significant7. Charge transfer may result in filling of the dband for the d-band transition metal, stabilizing the d-orbitals and shifting them to lower energy, resulting in a depletion of d-orbital density near Fermi level. For example, in the case of PdZn, the dband center of Pd shifts from -2.01 eV to -2.62 eV when alloyed with zinc. Coincidentally the d-band center of Pt is -2.55 eV and therefore, if one naively applied the d-band center model in this case then a simple adsorbate like CO should bind with the same energy on a Pd atom in the PdZn(111) surface as it does on Pt(111). However, instead the adsorption energy of CO on a Pd atop site on PdZn(111) is -0.83 eV and the adsorption energy of CO on a Pt atop site on Pt(111) is -1.90 eV (CO prefers the atop
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site on both of these surfaces and in fact the presence of the PdZn alloy can be deduced from the loss of bridge sites that are preferred for CO adsorption on monometallic Pd)8 . As the d-band center model proposed by Norskov and co-workers relies upon the concept that d-orbitals extend up to the Fermi level, the d-band center is no longer a good descriptor for the adsorption event. Plotting the d-bands of Pd, Pd in PdZn and Pt, it is evident in Figure 1 that although Pt and PdZn may have similar d-band centers, the presence of Zn results in both a shift and a narrowing of the density of states with nearly complete depletion of d-electron density near the Fermi level demonstrating a large electronic modification from the alloying event. In addition, Bader charge analysis indicates a charge transfer of 0.41 e- from Zn into Pd. Other sp-electron metal combinations with d-band transition metals give similar results for d-band center shifts (for example, for Pd , the d-band centers of PdGa and PdGe are -2.73 eV and -3.08 eV respectively while the Bader charges show donation of 0.29 and 0.47 e- into Pd respectively) Recently Montemore and Medlin have shown that electronic effects of the s and p-electron in metals and their interactions with adsorbates can be accounted for by a more complete treatment of orbital overlap terms that describe their bonding interactions with the surface9. Critical deviations between scaling of the d-band center with OH and CH adsorption can be accounted for by evaluating the additional terms and new correlative relations can be developed. This approach outlined by Montemore and Medlin could potentially be extended to systems with metals with sp-electron valence with equal success, as now terms involving the s and p-character of the metal become quite significant. This method could aid in the screening of these alloys for catalytic applications. Although electronic modification of adsorbate binding strength has been demonstrated to impact catalysis positively10, a common problem is often evident in that the strength of all reactants and products are impacted in a similar manner which may limit the degree to which improvements can be made. For example, Norskov and co-workers have shown that the binding energies of CO and HCO are correlated in CO2 reduction which limits one to a single correlative relationship in reactivity11. Even moving to systems with chemical intermediates of a different nature such as HCN synthesis, Grabow and co-workers found that reactivity could be described by scaling relationships of N and C binding to the surface6c. All monometallic surfaces examined showed behavior clustered in a broad line which therefore limited the maximum selectivity that could be achieved. Similarly, for formic acid decomposition, selectivity to H2 formation can be correlated to the binding energy of CO vs. OH12. Although judicious choices in alloying can be made to maximize selectivity, there remains a fairly narrow range of behavior that can be accessed. So while these alloys may exhibit behavior that lies outside bounds set by their monometallic component surfaces, the behavior is still correlated directly to their electronic structure13. In order to break from these relationships, the bonding configuration of the adsorbate must be perturbed (in contrast to only changing the strength of adsorbate binding). At least three pathways for changing geometry have emerged in the literature over the course of the past few years: 1) Intermetallic compounds 2) single atom alloys (SAAs) and 3) co-adsorbed structural modifiers. Intermetallic compounds lend themselves well to changing adsorbate configuration. Unlike homogeneous alloys (fully miscible solid solutions, often called random alloys), the metals are located in distinct positions within a unit cell such that the spatial relationship between the metal atoms is fixed. Furthermore, these systems have an enthalpic driving force (often driven by charge transfer) to remain in their prescribed positions which prevents (or at least retards) surface segregation in response to adsorption
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events (which often plague homogenous alloys). Knowledge of the precise atomic locations also lends study of intermetallic surfaces to theoretical examination. A recent review highlights many examples of improved catalytic performance for intermetallic compounds when compared to their monometallic cousins14. Table 1. Adsorption Energies of Toluene and Ethylene on various low energy surfaces of PtSn alloys. All values are reported for low coverage (either 1/9 or 1/16 ML-See Figure S1 for details). Dispersion corrections are included. Surface
C2H4
C7H8
Adsorption Energy [eV]
Adsorption Configuration
Adsorption Energy [eV]
Pt (111)
- 1.64
di σ
-2.37
Pt3Sn(111)
-1.48
di σ
-1.06
Pt2Sn(111)
-1.07
di σ
-0.66
PtSn(110)
-0.75
π
-0.75
Pt2Sn3(110)
-0.73
π
-0.61
PtSn2(110)
-0.36
π
-0.60
PtSn is the primary example of an intermetallic system (multiple intermetallic compounds of PtSn exist with varying stoichiometry) who has found extensive use in the field. For example, PtSn alloys have found use commercially in hydrocarbon reforming15, and propane dehydrogenation16 while academics have demonstrated their success in selective hydrogenation17 and CO tolerant fuel cell electrodes18. While formation of an intermetallic phase is often speculated rather than proven in supported nanoparticle catalysis, the rich phase diagram allows, in principle, the tuning of the ensemble size to some degree to perturb adsorbate geometry. For example, Table 1 shows a group of PtSn surfaces that have been examined using density functional theory showing the switch from di-σ-bonded C2H4 to π-bonded C2H4 as the tin content increases. Similarly, only a very small amount of Sn, dramatically decreases the binding strength of toluene due to the disturbance of the proper ensemble for adsorption of the aromatic ring. Such changes in adsorbate configuration can have profound effects on what reactions may occur. For example, in propane dehydrogenation, a common unwanted side reaction is hydrogenolysis to create ethane and methane8. It has been shown by Iglesia and co-workers that hydrogenolysis often involves multiple dehydrogenation steps before C-C bond cleavage can occur19. Alloying the active metal with a sp-electron metal to alter the reactivity can have two positive effects. First, as shown in Figure 2 by changing the electronic structure, the binding energy of propene to the surface is weakened, resulting in desorption before further dehydrogenation can occur. Second, the activation barrier for C-C bond cleavage is higher due to the lack of the appropriate sites 3-fold hollow sites on the PdZn surface. Similar work on propane dehydrogentation on PtSn surfaces has been reported by Chen et al..although they chose to focus their interpretation on electronic effects (at least for the Pt rich surfaces) resulting from the d-band perturbation of the alloying event20. In this way, one can see that the intermetallic compound surface combines both electronic
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effects and geometric effects to perturb reactivity in a positive way for increased selectivity in propane dehydrogenation. One interesting aspect of intermetallic compounds is that by altering the composition slightly, different types of ensembles can be created.21 For example, Feng et al. have shown that moving from PdIn to Pd3In results in a change in the Pd ensemble size from single isolated Pd atoms to Pd trimers. This structural change results in a loss of selectivity from 92% to 21% C2H4 in acetylene semi-hydrogenation22. It is interesting to ponder what the critical ensemble size would be for a given adsorption or reaction event. Intermetallic compounds are actually quite common23 so the possible ensembles are boundless but to date, much of the focus has been on a few systems (PtSn, PdZn). Armbruster and co-workers have written several papers on the use of PdGa compounds and nanoparticles thereof for acetylene semi-hydrogenation.24 Similarly, Miller and co-workers have continued to test various intermetallics for alkane dehydrogenation,25 However, much work remains to be done in order to fully explore how systematic perturbation of ensembles may be exploited for catalyst selectivity, particularly for new reactions. It should be noted that the creation of intermetallic phases with specific compositions in the form of nanoparticles supported on high surface area carriers remains a considerable synthetic challenge26. In addition, while an enthalpic driving force exists to prevent surface segregation in response to some adsorbates, sp metals are subject to oxidation and therefore the surface of the intermetallic may deviate strongly from the bulk in the presence of oxygen.27
PdZn(111) Pd(111)
2.0 1.5
Energy [eV]
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Catalysis
C3H6(g)
1.0 0.5 0.0
C3H8(g) CH3CHCH(ads)
-0.5
C3H6(ads) -1.0
C3H8(ads)
CH3CHCH3(ads)
Figure 2. Potential Energy Surface Diagram of C3H8 dehydrogenation over Pd(111) and PdZn(111) . See SI for additional details.
An extreme example of site design second material pathway toward geometric perturbation is the use of single atom alloys (the smallest possible ensemble). To this end, Sykes and co-workers have demonstrated that Pd atoms embedded in a Cu(111) surface can act as unique sites for the activation of hydrogen and thereby boost activity and selectivity in selective hydrogenation28. In their combined temperature programmed desorption/scanning tunneling microscopy study, Kyriakou et al. deposited 0.01 ML (monolayer), and 0.1 ML of Pd on Cu (111) surface to form structures where individual Pd atoms were alloyed into Cu (111) surface28a. DFT calculations predicted that the barrier for hydrogen dissociation was reduced from 0.4 eV to 0.02 eV, but that the adsorption enthalpy was not strongly affected (-0.35 eV on the alloy but 0.20 eV on Cu(111)). This prediction indicates a break from Bronsted Evans Polanyi scaling rules that would dictate that the reaction enthalpy and reaction barrier are correlated29. Breaking these scaling rules through a geometric effect (changing the active
site) allows departure from a purely electronic structure description of catalysis. In agreement with this prediction, the desorption temperature (manifested as the barrier to association- the reverse of the dissociation process) dropped dramatically from 310 K on Cu(111) to about 220 K on 0.01 ML Pd/Cu(111) (and even lower to ~180 K on 0.1 ML Pd/Cu(111)). Selective hydrogenation of acetylene to ethylene and styrene to ethylbenzene were demonstrated successfully as to illustrate the advantageous catalytic properties of the Pd@Cu SSA. More recently, Boucher et al. extended this approach to nanoparticles by galvanic replacement of Cu with Pd demonstrating selective hydrogenation of phenylacetylene to styrene at a Cu:Pd ratio of 80:130. Similar work presented by Louis and co-workers which examined the selective hydrogenation of butadiene over AuPd catalysts31 and by Aich et al. which examined the selective hydrogenation of acrolein to allyl alcohol over PdAu catalysts32 also invoked the idea of single Pd atoms allowing for a much greater selectivity to the desired products whereas contiguous Pd atoms led to unselective hydrogenation. The following criteria probably apply for any successful SAA catalyst. First, the reaction temperature must be rather mild. High temperature reactions (e.g. above the Tamman temperature of the metal) may result in significant metal-metal diffusion leading to aggregation of the guest metal33. Second, one must avoid reactive environments that can result in segregation of the metal. For example, if one metal forms very strong metal-adsorbate bonds but the other does not then the former metal will migrate to the surface34. For oxidation reactions, for instance, the host metal probably should be gold since gold will not form a bulk oxide35. Finally the reaction chosen must be able to take advantage of the geometric site created by the single atom alloy. This probably implies that “structure sensitive” reactions are more likely to be successful36. It should be mentioned that another pathway toward steering both geometry and electronic structure has emerged through coadsorbates that act as structural modifiers. This approach extends back many years through the use of sulfur, chlorine and alkali metals to locally modify the electronic structure and block certain potential adsorption sites37. However, this idea has been expanded by Medlin and co-workers to tune selectivity in hydrogenation of functionalized hydrocarbons38. In many cases, the presence of the modifier can change the adsorption configuration of the reactant. For example, in the hydrogenation of furfural, the presence of C18 alkane thiols on palladium nanoparticles shifted the selectivity from furan to methyl furan and furfuryl alcohol by shutting down the pathways to decarbonylation while still maintaining the pathways for hydrodeoxygenation38b. The presence of the C18 thiol forced furfural to bond in an upright configuration whereas in the absence of the modifier, furfural can lie down on the surface. The role of the modifier can even be an active one whereby the reactant interacts favorably with the modifier such that it adopts a more favorable adsorption geometry. In the selective hydrogenation of cinnamaldehyde over coated Pt/Al2O3, the use of 3-phenyl1-propanethiol was found to give over 90% selectivity to cinnamyl alcohol whereas the analogous phenyl-butanethiol and phenylethanethiol modifiers gave less than 80% selectivity38d. This increase in selectivity was attributed to favorable aromatic stacking interactions. Depending upon the type of adsorbate employed, the functionality of the modifier can be potentially adjusted accordingly in an enzymatic inspired design to molecular specificity. The modifiers must be thermally stable well above the reaction temperature of interest and preferably stable in the presence of air/water so that they are sufficiently robust. In addition, the modifier coverage must be appropriately tuned such that the surface is
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adequately accessible to potential adsorbates. Obviously, if the surface coverage so dramatically reduces the measured dispersion, then the inefficient use of the metal may be detrimental to the application. Although the previous examples generally involve monometallic catalysts, the same idea could easily be applied to bimetallic systems where specific site configurations on the surface could be built through the use of modifiers. In addition, Medlin’s group has recently extended this concept to selective oxidation showing that the SAM may also help prolong catalyst life by shutting off deactivation pathways leading to carbonaceous residues39. In summary, both geometric effects and electronic effects have long been known to be present in alloy systems. As Reuter and Norskov have shown, limits emerge to the degree that electronic modification of a metal can change catalytic performance that stem from scaling relationships40. Therefore, exploitation of geometric effects is critical to paradigm shifting performance improvement in metal catalysts. Through the use of unique materials which control the manner in which substrates may adsorb, these advances can be realized. Opportunities still exist in the realm of intermetallic compounds, single atom alloys and surface modifiers for catalyst design.
AUTHOR INFORMATION Corresponding Author Email:
[email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENTS C.G.. gratefully acknowledge funding for this work from the National Science Foundation (CBET Grant Number 0747646). The authors gratefully acknowledge NCSA for allocation grant TGCHE080020N for computational time which was used to perform some of the work reported herein.
SUPPORTING INFORMATION Details of the DFT calculations from Figures 1 and 2 as well as Table 1 are provided along with additional data from these calculations. This material is available free of charge via the Internet at http://pubs.acs.org
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