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Quantitative Coordination-Activity Relations for the Design of Enhanced Pt catalysts for CO Electro-Oxidation Federico Calle-Vallejo, Marcus D. Pohl, and Aliaksandr S Bandarenka ACS Catal., Just Accepted Manuscript • Publication Date (Web): 16 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017
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ACS Catalysis
Quantitative Coordination-Activity Relations for the Design of Enhanced Pt Catalysts for CO Electro-Oxidation
Federico Calle-Vallejo,1,* Marcus D. Pohl,2 Aliaksandr S. Bandarenka2,3,*
1
Leiden Institute of Chemistry, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands. 2
Physik-Department ECS,Technische Universität München, James-Franck-Str. 1, D-85748 Garching, Germany.
3
Nanosystems Initiative Munich (NIM), Schellingstraße 4, 80799 Munich, Germany.
* Corresponding authors:
[email protected],
[email protected] Abstract: Efficient redox transformations of CO are vital for remediating the imbalance of the biogeochemical cycle of carbon and also for the development of next-generation energy technologies such as fuel cells. For instance, CO oxidation is ubiquitous in hydrocarbon-based fuel cells and determines to a large extent their efficiency. As Pt is used in these cells, minimal catalyst loadings that do not compromise the activity are needed. In view of that, we present here a “coordination-activity plot” and electrochemical experiments on electro-oxidation of adsorbed CO to CO2 to establish quantitative structure-activity relations for various Pt electrodes. We predict theoretically and verify experimentally that catalytic CO oxidation is enhanced by creating surface defects with optimal coordination, without alloying. Both largely overcoordinated and undercoordinated defects on Pt (at rough surfaces with cavities and metal adatoms, respectively) hinder the reaction, so that the maximal activity is reached on sites with a generalized coordination of
___
CN = 5.4 .
Importantly, this moderate coordination is found at defects
such as step edges of electrodes with relatively short terrace lengths of 3-4 metal atoms.
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Keywords: CO oxidation • CO stripping • platinum • coordination-activity plot • generalized coordination number • structure sensitivity • scaling relations
Small organic molecules such as methanol, ethanol or dimethyl ether can be electrochemically oxidized under relatively mild conditions.1-4 This is the operational principle of certain lowtemperature fuel cells targeted for portable applications that generate electric energy by transforming those molecules into CO2 and water.5-7 Remarkably, there is a ubiquitous reaction at the anodes of all cells that make use of those fuels, namely the catalytic oxidation of CO:
(
*CO + H2O(l ) → * + CO2 + 2 H + + e −
)
(1)
Its high degree of surface sensitivity8-11 and the fact that only two proton-electron pairs are transferred make CO oxidation an excellent model reaction. In comparison, CO reduction to CH4 and C2H4, which are other reactions of importance in electrocatalysis for energy applications,12-13 require six and eight proton-electron transfers and have proven difficult to optimize. These practical considerations and the fact that CO oxidation controls the efficiency of the Pt electrodes used in the aforementioned fuel cells, call for the identification of quantitative structure-activity relations that ultimately lead to designing superior catalyst materials. Here we show that it is possible to enhance Pt active sites for CO electro-oxidation based on simple and systematic coordination considerations.14-15 Using the latter, one can engineer catalysts that outperform most of electrodes containing low-index facets and alloy catalysts. The design methodology used, namely “coordination-activity plots”,16-17 is not only in agreement with previous reports on the structure sensitivity of CO oxidation8-11,
18
but also takes a step
forward by providing clear quantitative guidelines for the design of enhanced CO oxidation catalysts made of Pt. Figure 1a shows the reaction energies involved in the following widely-accepted mechanism of CO oxidation to CO2: 8, 18 * + CO( g ) → *CO
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*CO + * + H2O(l ) → *CO + *OH + H + + e −
(3)
*CO + *OH → 2 * +CO2 (g ) + H + + e −
(4)
where * denotes a free surface site. The first step is the adsorption of CO, followed by the formation of *OH from water and the recombination of *CO and *OH to produce CO2. Figure 1a contains data for active sites on various Pt facets, as indicated on the upper x-axis. The data are plotted as a function of the generalized coordination numbers of the active sites,14-17 which capture smoothly and in a linear fashion the adsorption-energy trends. While conventional coordination numbers ( cn ) are a count of the first nearest neighbors of the active sites “i”, the ___
generalized ones ( CN ) weight every neighbor “j” by their respective coordination numbers ( cn( j ) ). In order for
___
CN
to span the same range as , a normalization factor is used ( cnmax ) that
corresponds to the total number of first nearest neighbors in the bulk. This normalization allows ___
to be defined for all types of sites, namely atop, bridge and hollow sites, on fcc,14-17 hcp19 and
CN
bcc crystals. For atop adsorbates such as *CO and *OH on Pt,
___
cnmax = 12 . CN
is easily calculated
arithmetically for a given surface site “i” as: ___
ni
cn( j ) j =1 cnmax
CN (i ) = ∑
The way of calculating
(5) ___
CN
for the sites in Figure 1a, which are flat terraces (Pt(111)),
cavities on Pt(111), stepped surfaces (Pt(110), (221) and (331)) and rough surfaces with Pt adatoms (2AD@111 and 4AD@100), is described in the Supporting Information (SI). Note that if
cnmax = 12
for all ni neighbors, then
___
CN (i ) = cn( i ) ,
meaning that conventional coordination is a
special case of the generalized one in which all neighbors are identical and possess the bulk coordination. The overpotential for CO oxidation of a given active site on Figure 1a can be assessed from the most positive value among the energetics of Equations 2-4:
η = max( ∆G2, ∆G3, ∆G4 ) − E 0 ,
where E0
is the standard equilibrium potential of Equation 1, namely -0.105 V. Figure 1b contains the coordination-activity plot in which the DFT-calculated overpotentials of the active sites are
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linked to
___
CN .
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A volcano-shaped curve is formed in which strong-binding sites on the left of the
volcano curve are limited by the recombination of *OH and *CO. For weak-binding sites on the right side of the curve, the potential-determining step is the formation of *OH from water. The optimum catalyst, which is found at the crossing point between the green and orange lines in Figure 1b, possesses adsorption energies of
∆GCOopt = −1.1 eV
and
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∆GOH opt = 0.4 eV
.
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Figure 1. (a) Energetics of the three reaction steps considered for CO oxidation (see Equations 2-4) as a function of the ___
generalized coordination numbers ( CN ) of the active sites, which appear on the upper x-axis (see Figure S1). The adsorption energies of *OH and *CO are separated by ~1.42 eV. (b) Coordination-activity plot for CO oxidation on Pt. Optimal sites possess the step edges of Pt(331) and Pt(221). The overpotential is calculated as
___ CN opt = 5.4
, close to the coordination of
η = max( ∆G2 , ∆G3 , ∆G4 ) − E 0
.
These two pieces of information are as much as conventional volcano plots based on adsorption energies can provide. On top of that, the coordination-activity plot provides insight on the geometric structure of the optimal active sites.16-17 In this case, the optimal catalyst possesses ___
CN opt = 5.4 .
Thus, we conclude that the optimal active sites should be located at convex defects 5 ACS Paragon Plus Environment
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similar to the (on-top) step sites on Pt(331) or (221), where the coordination is relatively low and strong adsorption energies are observed compared to Pt(111). However, rough surfaces such as Pt(110) and surfaces with Pt adatoms bind the intermediates too strongly preventing CO2 formation and desorption, which is also deleterious for the catalytic activity. Note that the use of
___
CN
is advantageous in this case as it allows distinguishing between sites
with identical but different catalytic activities. This is the case of the step-edge sites on Pt(110), (221) and (331), which have which have
cn = 9 .
cn = 7 ,
and also the case of cavity and pristine (111) sites,
With respect to Pt(331), the most active surface, the optimal catalyst binds
*CO ~0.1 eV more weakly and *OH ~0.05 eV more strongly.
Figure 2. Anodic parts of the potentiodynamic voltammograms characterizing the adsorbed CO stripping in 0.1 M HClO4 for various Pt electrodes: Pt(111), Pt(775), Pt(331), Pt(221), Pt(110), and Pt(111) with concave surface defects (Pt(111), cavities (from16) and CuPt(111) surface alloy (from18). dE/dt = 50 mV s-1.
Now, it is important to verify the predictive power of the coordination-activity plot in Figure 1b. Figure 2 shows that the experimentally measured activities for CO stripping are drastically 6 ACS Paragon Plus Environment
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different for the studied model surfaces. For the experiments, the electrodes were pre-saturated with CO, and after that adsorbed CO was oxidized in Ar-saturated electrolytes, according to standard procedures. In particular, it is important to analyze the potentials of the main CO oxidation peaks. First of all and as predicted by the coordination-activity plot, Pt(331) shows the least positive CO oxidation peak, followed by Pt(221) and Pt(110), which is nearly as active as a Cu-Pt(111) surface alloy reported recently.18 The trends continue with Pt(775), pristine Pt(111) and Pt(111) with cavities, all of which have relatively high CO oxidation potentials. This catalytic sequence shows that it is possible to enhance the CO oxidation activity of Pt sites using only coordination-activity relations, producing catalysts (with active sites similar to those at Pt(331) and Pt(221) surfaces) that outperform highly active alloys. The activity ordering for Pt(775), Pt(221), Pt(331), and Pt(110) which can also be denoted Pt[7(111)×(111)], Pt[4(111)×(111)] and Pt[3(111)×(111)], Pt[2(111)×(111)] is revealing because these surfaces possess the same step type, namely a (111) step (see Figure S1), and differ only in their terrace lengths, which are 7-, 4- and 3- and 2-atom long. Therefore, the activity increases as the terrace length decreases, peaking at 3-atom long terraces, and decreasing for Pt(110), confirming the predictions of the coordination-activity plot in Figure 1. Figure 3 contains cyclic voltammograms (CVs) obtained in Ar-saturated aqueous 0.1 M HClO4 electrolytes that also provide a strong evidence for the theoretical prediction that the active catalytic centers for CO oxidation are likely located at step sites. The CV of Pt surfaces has three well-known regions: between 0.05 and ~0.4 V one can distinguish broad cathodic and anodic peaks mainly associated with adsorption and desorption of hydrogen species. In addition, the introduction of steps allows for the adsorption/desorption of OH species in this region.20 Between ~0.4 and ~0.6 V, there is a “double-layer region”, where the measured current is associated with the charge/discharge of the electric double layer. Finally, the peaks between ~0.6 and ~0.85 V are attributed to *OH adsorption at terraces. Electrodes in which surface defects are present display some characteristic features in the CVs that correlate with the type and amount of those defects. For instance, Figure 3 shows that the introduction of defects causes the appearance of peaks at more negative potentials than those of the double-layer region related to *OH adsorption (see the OH(step) labels and reference20-21), which is an important intermediate in CO oxidation (see Equations 3 and 4 and Figure 1). 7 ACS Paragon Plus Environment
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Conversely, terrace sites bind *OH significantly more weakly than Pt(111), and display peaks at potentials more positive than those of the double layer region (see OH(terrace) labels in Figure 3).
Figure 3. Cyclic voltammograms of different Pt electrodes in Ar-saturated 0.1 M HClO4 as indicated in the figure: Pt(111), Pt(775), (331), (221) and (110), Pt(111) with concave surface defects (Pt(111), cavities, from16) and PtCu(111) surface alloy (from18). dE/dt = 50 mV s-1.
Therefore, Figure 3 indicates that the CO stripping protocol in Figure 2 on stepped Pt surfaces starts at electrode potentials in which *OH can be mostly, if not only, adsorbed at steps. This suggests that those sites are the active ones for CO oxidation, in line with previous reports.811
In contrast, for Pt(111) the onset of CO oxidation correlates with that of *OH adsorption at 8 ACS Paragon Plus Environment
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terraces. Although *OH is present at the defective Pt(111) with cavities already at ~0.15 V, the cavities (see Figures S1g and S1h) are the least active sites towards CO oxidation among all under study. Similarly to pristine Pt(111), the onset of this reaction correlates with the formation of *OH inside the cavities. This suggests two things: first, the cavities are narrow enough to have sites with larger
___
CN
compared to Pt(111), as recently shown based on oxygen reduction reaction
activities.16-17 Second, the surface diffusion of *OH from the edges into the cavities is kinetically hindered, see Figures 4a and b. This is confirmed by our calculations, which predict a diffusion barrier for *OH of at least 0.52 eV from the upper to the lower terrace of Pt(221) (see Figure S2). Furthermore, the adsorption energy trends of *OH in Figure 1, which are in agreement with the experimental ones in Figure 3, show that highly coordinated sites on Pt surfaces adsorb *OH more weakly compared to lower-coordinated sites and thus, *OH will form at earlier potentials on the latter compared to the former. In this order of ideas, CO oxidation on stepped surfaces proceeds at undercoordinated sites where *OH is more easily formed. If *CO from terraces were oxidized by *OH at steps, the predicted optimal coordination would be the same, as the limiting potentials of the coordination-activity plot are related to *OH formation or reactivity. Note that coordination-activity plots exploit structure-energy relationships,14-17 unlike the state of the art which is dominated by energy-energy relationships called “scaling relations”.22-24 This is important because in Figure 1 there is an approximately constant energetic separation of 1.4-1.5 eV between the trends for
∆GCO
and
∆GOH
(red and green lines in Figure 1). As energy-
energy relationships are believed to come from the combination of similar structure-energy relationships,23-24 such constant separation must as well appear in the scaling relation between those two sets of adsorption energies. This is what we observe in Figure 4c, where *CO and *OH are seen to scale linearly with a slope ~1 and an offset of ~1.4 eV. The energies of adsorption of the optimal Pt catalyst with
___
CN opt = 5.4 are
also provided for comparison together with those of the
ideal CO oxidation catalyst (see the SI, section S2). Clearly, the ideal catalyst possesses very extreme adsorption energies ( ∆GCO = 0 eV and
∆GOH = −0.105 eV
) that none of the pure Pt sites under
study exhibits and would require the breaking of the scaling relation between *CO and *OH. Before closing the discussion, we would like to mention that some authors have proposed that CO oxidation proceeds via *CO + *OH coupling to form *COOH, which is subsequently dehydrogenated to finally produce CO2.9 We have analyzed the energetics of such alternative 9 ACS Paragon Plus Environment
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reaction pathway and confirmed that
___
CN opt = 5.4 ,
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see the results in section S4 in the SI, where a
discussion on reaction kinetics is also provided. To summarize and conclude, we have shown that
___
CN
is useful to capture the well-known
structure sensitivity of CO oxidation on Pt in quantitative terms. This allows for the design of enhanced catalysts for CO oxidation without alloying by means of coordination-activity plots. As these plots are 2D but provide geometric and energetic design principles, they go beyond the state of the art, in which 3D or contour plots are used and only energetic design principles are obtained.18 More precisely, coordination-activity plots predicted high activities for Pt(331) and Pt(221) and defined the optimal catalyst for *CO oxidation on Pt as one that possesses
___
CN opt = 5.4
and binds *CO ~0.1 eV more weakly and *OH ~0.05 eV more strongly than the step-edge sites on Pt(331). Such predictions were verified experimentally and we showed that Pt(331) and Pt(221) outperform active “bifunctional” catalysts such as Pt-Cu surface and near-surface alloys.18 We also provided experimental evidence to support the predictions that 1) steps are the active sites for CO oxidation, and 2) CO oxidation is generally limited by *OH formation. Last but not least, we identified a scaling relationship between the adsorption energies of *CO and *OH on Pt facets that allows for the activities to be optimized to some extent but makes it difficult to find pure Pt sites that oxidize CO reversibly.
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Figure 4. Schematic representation of the differences in CO oxidation at (a) convex and (b) concave sites on Pt. On convex defects such as step edges *OH formation is relatively fast and readily followed by recombination with *CO. Conversely, on concave defects such as cavities *OH formation requires large potentials and the diffusion towards the cavity is kinetically difficult (see Figure S2). (c) Adsorption-energy scaling relation between *CO and *OH. The optimal Pt catalyst (magenta) and the ideal (blue) catalyst are provided for comparison.
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Experimental Section
Computational Methods. The DFT calculations in this study were performed using the VASP code.25 Full details of the calculations and computational models can be found in the SI, sections S1, S2 and S4.
Experimental protocols and procedures. A full account of the experiments and the used experimental setup, including cleaning procedures, working and reference electrodes, etc. can be found in section S3 in the SI.
Supporting Information. The Supporting Information contains a detailed description of the DFT computational methods, further explanations regarding the active catalytic sites, reaction energies and generalized coordination numbers, as well as modelling of an alternative reaction pathway via *COOH. This material is available free of charge via the Internet at http://pubs.acs.org
Acknowledgements We thank Prof. Juan Feliù (University of Alicante, Spain) for providing high-quality Pt(221) and Pt(775) single-crystal electrodes. We acknowledge funding from NWO, Veni project number 722.014.009, SFB 749 and the cluster of excellence Nanosystems Initiative Munich (NIM). The use of supercomputing facilities at SURFsara was sponsored by NWO Physical Sciences, with financial support by NWO.
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