Probing the Limits of d-Band Center Theory - ACS Publications

Jul 17, 2015 - metal surface can be correlated to the d-band center of gravity of that metal. ... band center for Pt-core−Pd-shell nanoparticles wer...
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Probing the Limits of d‑Band Center Theory: Electronic and Electrocatalytic Properties of Pd-Shell−Pt-Core Nanoparticles Maciej T. Gorzkowski and Adam Lewera* Department of Chemistry, Biological and Chemical Research Centre, University of Warsaw, ul. Ż wirki i Wigury 101, 02-089 Warsaw, Poland ABSTRACT: Theoretical DFT calculations suggest that chemisorption energy, activation barrier, and energy of dissociation of small molecules on metal surface can be correlated to the d-band center of gravity of that metal. This holds true for many systems and reactions, but there are also reports where significant discrepancies were found. Here we present the critical assessment of applicability of the d-band center theory to nonuniform catalytic systems, such as core−shell nanoparticles. For Pt-core−Pd-shell nanoparticles we found a significant enhancement of catalytic activity toward formic acid oxidation, which was assigned to observed changes of density of states close to the Fermi level, in general in agreement with d-band center theory. However, at the same time the changes in dband center for Pt-core−Pd-shell nanoparticles were contrary to those predicted by theory due to incorporation of Pt valence electrons to the overall band structure, which shifted the d-band center in the direction opposite to that predicted by the theory. Our data stress the role of experimental determination of electronic properties of catalytic systems when explaining the observed catalytic activity, as real systems can be more complicated than the one used for theoretical calculations. As a result the changes in d-band center energy must be used with care for explanation of the observed changes in catalytic activity. We show that for nonuniform systems density of states close to the Fermi level is a better predictor of chemisorption strength and catalytic activity than the d-band center.

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INTRODUCTION Establishing the relationship between surface electronic structure and its catalytic activity would revolutionize many branches of science and technology. Among many fundamental studies contributing to better understanding how the electronic properties influence the catalytic activity, one of the most spectacular is the d-band center theory, developed by Norskov and co-workers, correlating the energy of the d-band center of gravity (εd) of the metal catalyst to the adsorption energy, activation energy, and dissociation energy of small molecules.1−3 According to that theory the electron density of states (DOS) close to the Fermi level is correlated to the adsorbate− substrate adsorption energy due to interactions between electrons occupying d-type orbitals of the metal (d-band) and those of the adsorbate. As a result modifying the d-band DOS (d-DOS) should influence the adsorption strength and catalytic activity of the metal surface. To reflect the electronic properties of the d-band (d-DOS) εd was proposed. According to d-band center theory, εd can be altered, i.e., as a result of lattice strain. Changing the lattice parameters influences the degree of overlapping between d orbitals forming a d-band in metal and as a result causing narrowing or widening of the d-band and shifting εd toward or away from the Fermi level.1−3 There have been significant efforts to verify the d-band center theory. The effect of lattice strain on the d-band can be computed using DFT calculations, and numerous works reports the correlation between calculated electronic properties and catalytic activity.4,5 However, there are not many studies © XXXX American Chemical Society

addressing the relationship between experimentally determined d-band structure and catalytic activity, despite the fact that the d-band structure can be observed directly using X-ray and UV photoelectron spectroscopies (XPS and UPS)1,6 and some information regarding d-band structure can be obtained indirectly using XPS from core-level binding energy (BE) shifts, which are very sensitive to d-band structure.1,7,8 It should be noted here that using those methods d-band center theory was also confirmed experimentally for a couple of systems,6,9−11 but for numerous other systems disagreement was found. In general it can be easily understood that for more complex systems other factors, not included in the d-band center theory, can play a significant role. One of such cases is the bifunctional mechanism, where the presence of two metals allows for an additional reaction to occur, which increase the catalytic activity. An exemplary reaction is methanol oxidation on Pt− Ru, where Pt becomes poisoned by adsorbed CO, but the adsorbed OH groups present on Ru facilitate Pt depoisoning.12 Thus, the presence of Ru significantly enhances the catalytic activity of Pt, while electronic factors play a minor role here.13,14 However, even for model surfaces, where chemical factors, such as bifunctional mechanism, are eliminated, there are reports where authors found discrepancies between d-band center and catalytic activity.15,16 For instance, Hyman et al.16 found that d-band center position cannot predict the catalytic Received: June 3, 2015 Revised: July 16, 2015

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DOI: 10.1021/acs.jpcc.5b05302 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

left at open-circuit potential (OCP). Immediately following was the addition of deaerated solution of PdCl2 + HCl to supporting electrolyte in such an amount so the final Pd concentration of 1 mM was obtained. As the solution was constantly stirred by argon bubbling, the uniform concentration of PdCl2 in the whole electrochemical cell was obtained immediately. Directly following the addition of PdCl2 the process of UPD of palladium was started to avoid creation of platinum surface oxides. Namely the Pt nanoparticle sample was subjected to a CV scan from OCP (+0.923 V vs RHE) toward the palladium reduction potential (+0.74 V) at 0.1 mV· s−1. The scan was terminated when total charge reached the value equal to the double value of the charge of hydrogen adsorption on Pt nanoparticles in pure 0.5 M H2SO4 plus the value of double-layer charge extrapolated from the double-layer region to the potential 0.74 V vs RHE. This resulted in Pt nanoparticles covered by almost a full monolayer of Pd (PtPdML). From XPS analysis, finally the PtPdML sample contained 31% at. of Pd, which corresponds to 0.95 ML of Pd. As a reference we used the same Pt nanoparticles which were used as a substrate for PtPdML, but without the Pd layer, and also ultrapure Pd nanoparticles of the same (4 nm) size. For electrochemical experiments all solutions were deaerated using high purity argon (N5.0 from Air Products). A doublebridged mercury(I) sulfate reference electrode was used, but all potentials here are reported versus RHE. The potential of the mercury(I) sulfate reference electrode was measured vs RHE prior to and after the measurements. The stability of the potential was better than 4 mV. Large area platinum foil was used as a counter electrode. The working electrode consisted of disposable gold foil, on which investigated nanoparticles were deposited. No binding agent, such as Nafion, was used. The method of pure deposition of nanoparticles on gold substrate has been described elsewhere.20 The active surface area was determined based on surface oxygen reduction charge, assuming 424 mC·cm−2, and confirmed using hydrogen adsorption/desorption charge. For CO stripping experiments the electrode potential was set as +0.34 V vs RHE, and then gaseous CO (N4.8, Air Products) was introduced to the solution for 7 min, followed by 45 min of Ar purging to remove the dissolved CO from the solution. Electrochemical experiments were performed using PAR 263A and PAR 4000 potentiostats. For X-ray photoelectron spectroscopy (XPS) experiments a PHI5700 spectrometer, made by Physical Electronics, was employed. Monochromatized X-ray Al Kα radiation of energy equal to 1486 eV was used. Survey spectra were recorded with pass energy equal to 187.85 and 1 eV step, and core-level spectra were recorded with pass energy equal to 29.35 eV for Pd 3d, O 1s, and C 1s regions and 5.85 eV for Pt 4f, Au 4f and UPS He II and 2.95 eV for He I, with 0.100 eV step and 100 ms dwell time. XPS VB spectra were collected with 0.8 eV resolution due to the low cross section of VB electrons to Al Kα photons. The total energy resolution was about 0.35 eV. The measurements were performed in a vacuum of 1.3 × 10−7 Pa. Peak fitting was performed with CasaXPS 2.3.15 software. All spectra were corrected on the BE scale using the Au 4f signal from the underlying Au foil, assumed as 84.00 eV. The Au 4f7/2 BE varied by not more than 40 meV among all samples.

properties of bimetallic surfaces toward H2S adsorption. It is thus imperative to critically verify the relation between electronic structure and reactivity. One of the model systems allowing for studying the relation between lattice parameter, d-band center, and catalytic activity of transition metals is pseudomorphic layers. Such a system consists of one metal (substrate) on which another metal is deposited, forming a thin layer, usually a monolayer. It has been discovered that in those systems the top atom layer mimics the crystallographic structure and lattice parameter of the substrate.9,17,18 In the case of pseudomorphic palladium layers on platinum, palladium undergoes lattice expansion, and as a result d-band width should decrease and εd should move up. If the overlayer completely covers the substrate, the nonelectronic factors potentially influencing the catalytic activity are eliminated. Here we present the thorough experimental investigation of correlation between catalytic and electronic properties of model nanoparticle catalysts, such as Pt, Pd, and Pd monolayers on Pt nanoparticles (PtPdML). Electronic properties were measured using UPS and XPS and correlated to electrocatalytic properties toward oxidation of adsorbed carbon monoxide and formic acid. The comparison between PtPdML and Pd nanoparticles allowed for experimental elucidation of the role of electronic factors in the observed catalytic activity, as the chemical constituents to catalytic activity (such as surface morphology and bifunctional mechanism) are the same for Pd and PtPdML nanoparticles. Our results suggest that qualitatively the changes in adsorption and catalytic activity of this system are generally as predicted by d-band center theory, but at the same time a significant disagreement with theory for experimentally determined εd shift was found. In particular εd shift has completely opposite sign and significantly different magnitude as compared to the sign and magnitude predicted by d-band center theory. As such εd cannot be correlated to and used to explain the observed changes in catalytic activity for PtPdML nanoparticles. We explained the observed increase in catalytic activity as a result of increased electron density close to the Fermi level, which is in general in agreement with the assumptions of the d-band center theory, but we prove here that for nonuniform systems no straightforward correlation between εd and catalytic activity exists.

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METHODS All solutions were prepared using triple distilled water, subsequently deionized using a Milli-Q system, and finally purified by subboiling distillation. Sulfuric(VI) acid (p.a., POCh, Gliwice, Poland) was purified by triple subboiling distillation before being used. Ethanol (p.a., POCh, Gliwice, Poland) and formic acid (Sigma-Aldrich, ACS grade) were used without further purification. Ultrapure platinum nanoparticles with 4 nm diameter, subsequently used as a substrate for Pd monolayer deposition, were prepared using the polyol method, as described elsewhere.19 In preparation of thin Pd layer underpotential deposition (UPD) of Pd on Pt was used. First palladium was reduced on Pt nanoparticles from 1 mM PdCl2 + 2−5 mM HCl + 0.5 M H2SO4 solution to determine the onset of normal palladium reduction on platinum, which was between +0.74 and +0.77 V vs RHE. Then a new set of platinum nanoparticles were used. Those Pt nanoparticles (substrate) were cycled in deaerated 0.5 M H2SO4. The CV scan was ended at the potential of the double layer (+0.4 V vs RHE), and the cell was B

DOI: 10.1021/acs.jpcc.5b05302 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION Electrochemical Properties. Pt, Pd, and PtPdML nanoparticles were characterized using cyclic voltammetry (CV) in 0.5 M H2SO4 solution (Figure 1) to check the durability of the

Figure 2. Oxidation of the adsorbed CO monolayer on Pt, Pd, and PtPdML nanoparticles. 0.5 M H2SO4, v = 5 mV·s−1. The shift of CO stripping peak potential for PtPdML, as compared to Pd nanoparticles, suggests an increase in CO adsorption strength on PtPd ML nanoparticles. Figure 1. Electrochemical characterization of PtPdML nanoparticles and comparison to pure Pt and pure Pd nanoparticles in 0.5 M H2SO4, v = 50 mV·s−1.

small prepeak can be observed on the CO stripping peak on PtPdML. Shifting of the CO stripping peak suggests changes in CO chemisorption strength. Judging from CO chemisorption energy on Pt and Pd, the CO chemisorption energy on PtPdML can be estimated. Namely, the reported chemisorption energy of CO on Pt(111) and Pd(111) is −1.50 and −1.47 eV, respectively.21,22 Assuming that CO stripping peaks scale linearly with CO chemisorption strength, from CO stripping potential on PtPdML it can be estimated that CO adsorption strength on PtPdML is close to −1.48 eV, slightly higher than CO chemisorption on Pd. It must be remembered though that electrooxidation of CO requires adsorbed OH groups, thus CO electrooxidation potential and CO chemisorption strength may not be directly correlated. An ethanol electrooxidation experiment was used to determine if the palladium overlayer completely covers the Pt core, as suggested by XPS results (see Methods section). We used ethanol oxidation reaction, as ethanol can be oxidized on Pt but not on Pd. For PtPdML only a small increase in current, as compared to pure Pd, can be observed (Figure 3), which suggests that Pt is almost completely covered by the Pd overlayer, and the small increase in anodic currents for PtPdML is most probably caused by the inevitable minor discontinuity of PdML.

palladium monolayer (PdML) and to determine the overall characteristic of the surface, as CV is very sensitive to surface morphology. It was observed that the electrochemical response of the PtPdML sample differs from those of Pt and Pd as evidenced by the hydrogen adsorption/desorption region. In particular the hydrogen adsorption/desorption region is similar to Pd, with the hydrogen desorption peak shifted toward more negative potential (Figure 1). Concerning the durability of all the samples, in the case of Pt and Pd it was possible to cycle the nanoparticles a few times to +1.4 V without observable changes in CV (not shown). For the PtPdML small subsequent changes in the CV were observed when nanoparticles were cycled up to +1.3 V. In general up to 5 CV cycles did not change the surface significantly, but if more than 10 cycles were applied, the CV in the hydrogen region started to resemble those of pure Pt. Namely, the second hydrogen adsorption/desorption peaks pair appeared at ca. 150 mV, which is characteristic for Pt but does not exist for pure Pd (cf. Figure 1). We have also determined that the highest potential which could be applied without altering the Pd monolayer was 0.85 V (the onset of surface oxidation). Changes in CV suggest that the PdML is not completely stable when potential higher than 0.85 V is applied, and during prolonged cycling in the oxide region (above 0.85 V) the PdML can be partially removed from the Pt core. In the following experiments as a rule more than three cycles were never applied to avoid disturbance of the PdML, and when high potentials were not needed, the upper potential limit was decreased. For Pd-containing samples we also increased the cathodic potential limit to avoid strong signal of hydrogen absorption at ca. 0 V. The electrooxidation of adsorbed carbon monoxide (CO stripping experiment) is presented in Figure 2. The upper potential limit was decreased for PtPdML samples to avoid changes in surface composition and disturbance of the Pd overlayer. CO stripping potential (Figure 2) is the smallest for Pt (723 mV), has intermediate value (840 mV) for PtPdML, and is the highest for Pd (881 mV). It is worth noting that the onset potential and potential of the highest CO oxidation current (CO stripping potential) for PtPdML are shifted toward more negative potentials as compared to pure Pd. Additionally a

Figure 3. Ethanol electrooxidation on Pt, Pd, and PtPdML nanoparticles. 0.5 M H2SO4 + 0.5 M ethanol, v = 50 mV·s−1. Only small anodic currents can be observed for PtPdML, which suggests that the Pd layer almost completely covers the Pt core. C

DOI: 10.1021/acs.jpcc.5b05302 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Palladium is known to catalyze oxidation of formic acid, and this reaction at the potentials lower than 600 mV to a large extent occurs via a simple route, leading directly to CO2, without the adsorbed CO step, thus eliminating the role of the so-called bifunctional mechanism.23−25 Thus, using a formic acid oxidation reaction it should be possible to investigate how the altered electronic properties expected in the expanded Pd monolayer influence the electrocatalytic properties. A significant enhancement in catalytic activity toward formic acid oxidation for PtPdML as compared to pure Pd was observed. In particular the potential of formic acid oxidation, measured at 2 mA·cm−2 current density on PtPdML, is lower by 75 mV than those on pure Pd (Figure 4), a decrease from 270

Figure 5. Normalized UPS valence band spectra for Pt, Pd, and PtPdML. A significant increase in d-DOS close to the Fermi level (0−2 eV region) was observed for PtPdML as compared to Pt and Pd.

2 eV range) it can be assumed that the observation is indeed caused by increased electron DOS close to the Fermi level. To determine the possible constituents to the valence band UP spectra (Figure 5) from the gold substrate, platinum, and palladium and unavoidable carbon and oxygen contamination, binding energies and cross sections of all possible element/ transition combinations, considering the low kinetic energy of UV excited photoelectrons, were compared, based on the work of Yeh and Lindau.27 As can be seen in Table 1 the observed UP spectra reflect dband DOS (d-DOS), as Pt and Au 6s levels have negligible

Figure 4. Formic acid oxidation on Pt, Pd, and PtPdML nanoparticles. 0.5 M H2SO4 + 0.5 M HCOOH, v = 50 mV·s−1. Arrows mark the direction of CV registration. A significant decrease in formic acid oxidation potential is observed for PtPdML nanoparticles as compared to Pd nanoparticles.

Table 1. Binding Energy and Photoionization Cross Section Values for Electron States Possibly Contributing to Valence Band Spectraa

mV for Pd to 195 mV for PtPdML. The former value is significantly closer to the reversible (thermodynamic) potential for formic acid oxidation to CO2, which is −270 mV,26 but still a significant overpotential exist. At the same time the maximum formic acid oxidation current density (the peak current density observed in the 0.2−0.5 V range) is lower for PtPdML as compared to Pd nanoparticles. In particular in the anodic scan the peak current density of formic acid oxidation on Pd was equal to 5.6 mA·cm−2 where the peak current density of formic acid oxidation on PtPdML was equal to 3.5 mA·cm−2. A similar dependence was also observed in the cathodic scan, where peak current densities of formic acid oxidation were equal to 6.4 and 4.6 mA·cm−2 for Pd and PtPdML, respectively. It is also interesting to note that the anodic currents in the cathodic scan start at the same potential (ca. 850 mV) for Pt and PtPdML but for Pd anodic currents can be observed at significantly lower potentials (ca. 720 mV). The exact reason for this observation will be studied in the future. Electronic Properties. Electronic properties of Pt, Pd, and PtPdML were determined using UPS and XPS (Figure 5). UPS data suggest a significant increase in DOS close to the Fermi level for PtPdML as compared to Pt and Pd (cf. the 0−2 eV region in Figure 5). As the UPS spectra can be strongly affected by empty states above the Fermi level, we used He I and He II lines (UV photon energies equal to 21.22 and 40.81 eV, respectively) to eliminate the possible changes due to final state effects. As the increase in DOS is observed regardless of the excitation energy used (Figure 5, thin and thick continuous orange line in the 0−

element/transition

BE/eV

cross section/MB for 40 eV kinetic energy

C 2p O 2p Pd 4d Pt 5d Au 5d Pt 6s Au 6s

8.98 14.16 7.98 11.39 12.51 6.39 6.50

10 10 20 40 40 0.1 0.03

a

Based on photoionization cross-section tables by Yeh and Lindau.27

cross sections. The contribution from gold support (Au 5d) to observed spectra can be excluded, based on the fact that the gold 4f signal observed on XPS survey scans is in the order of a few percent and is actually the smallest for PtPdML(not shown for clarity). Another possible constituent to the observed UP spectra is contributions from carbon and oxygen contaminations. Those elements are unavoidable in XPS/UPS experiments and usually are removed using ion bombardment. For the PtPdML nanoparticles such cleaning is impossible as it would destroy the Pd monolayer. However, as can be seen in Table 1 the C 2p and O 2p BEs are well above the Fermi level (9.0 and 14.16 eV, respectively), which does not interfere with the DOS below 9.0 eV, and additionally in our case the amount of carbon was the lowest for PtPdML which was confirmed by XPS survey scans (not shown). All the above proves that for the PtPdML sample the observed increase in electron density close to the Fermi level cannot be attributed to contamination or signal from gold support. D

DOI: 10.1021/acs.jpcc.5b05302 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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comparison of the determined valence band parameters and comparison to literature data, as well as CO chemisorption energy and CO stripping potential, is presented in Table 2. As discussed in the Introduction, it is expected that in PtPdML Pd forms a pseudomorphic layer on Pt, which means that the Pd lattice parameter mimics the Pt lattice parameter, resulting in Pd lattice expansion (tensile strain). As predicted by d-band center theory, the tensile strain in the Pd pseudomorphic monolayer on Pt should cause a decrease of the d-band width, and εd should move toward the Fermi level, resulting in stronger adsorption on the PtPdML surface. It should be noted here that indeed an increase in adsorption strength was observed by us on PtPdML as evidenced by CO stripping and formic acid oxidation experiments (see Table 1), but the εd moved away from the Fermi level, in disagreement with the d-band center theory (see Table 1). The explanation of the disagreement between predicted and observed εd value is given below. Let us first quickly compare our results to literature data. Ruban et al. predicted, based on DFT calculations, that change in εd for the Pd monolayer deposited on Pt should be 0.02 eV.2 We observed an opposite effect: for the atom-thick Pd shell on the Pt core εd moved (down) by −0.59 eV as compared to Pd (Figure 6). At the same time d-band width increased for PdPtML as compared to Pd, which is consistently contrary to prediction. First it is important to determine why the εd shifted in the opposite direction (by −0.59 eV) than predicted by theory (+0.02 eV). To gain better insight on the possible causes and to determine the possible changes in DOS in PtPdML, the valence band of PtPdML was simulated from the XP spectra obtained for pure Pt and pure Pd nanoparticles, weighted using Pt and Pd content in PtPdML determined using XPS, and finally the simulated XP spectrum was compared to registered the XP spectrum (after 0−1 normalization). Results are plotted in Figure 7. It can be noted that the simulated and registered spectra match well, but small difference can be found for low BE values (0−2 eV), where the registered spectrum exhibits more electron density than the simulated one. Consequently at the high BE side (from ca. 5 eV), a decrease in DOS can be found. The changes are subtle but qualitatively correlated to UPS spectra, where also increased electron density in the 0−2 eV range was found (Figure 5). Additionally the overall good agreement between the linear combination of valence band spectra for pure metals and the spectrum registered for PtPdML suggests that the d-band of PtPdML contains electron levels of both metals constituting PtPdML, obviously as predicted by

Complementary information regarding valence band structure can be obtained using XPS. The kinetic energy of the excited photoelectrons in the case of XPS is high enough to avoid any resonances from the unoccupied electron states, thus the registered XPS spectrum better reflects the actual d-DOS. As a result d-band width can be estimated. Additionally the registered spectra are much less surface sensitive; for the corresponding photoelectron kinetic energy the expected inelastic mean free path is ca. 2 nm,28 thus the potential contributions from adsorbed contamination are much less pronounced in comparison to UPS. Valence band spectra obtained using XPS are presented in Figure 6. εd was determined by integrating the spectra from −2 to 10 eV after Shirley background correction.

Figure 6. Normalized valence band spectra of Pt, Pd, and PtPdML nanoparticles registered using XPS. PtPdML valence band is narrower than the valence band of Pt but significantly wider than the valence band of Pd.

It can be observed that the valence band spectrum of PtPdML is narrower than those of Pt but significantly wider than those of Pd. The registered valence band spectra (Figure 6) were used to determine εd values, giving −2.80 eV for Pt, −2.61 eV for PtPdML, and −2.02 eV for Pd. Comparing literature εd values obtained experimentally for bulk Pt and Pd (−2.94 and −2.09 eV, respectively29) versus our data obtained for Pt and Pd nanoparticles (−2.80 and −2.02 eV, respectively) a good agreement was obtained. Judging from it, it can be deducted that electronic properties of nanoparticles are quite similar to bulk systems, except a small decrease in εd values, which may be correlated to size effect.6 In general a good agreement between parameters determined for pure Pt and Pd and literature data confirms the validity of the method used. A detailed

Table 2. Comparison of Experimental and Calculated Values of εd, CO Chemisorption Energy, and CO Stripping Potentiala Pt εd experimental/eV

εd calculated/eV CO chemisorption energy/eV CO stripping potential/mV

−2.80b −2.9429 −2.751 −3.37 to −3.7729 −1.521 723b

PtPdML

Pd

−2.61b

−2.02b −2.0929 −2.49 to −2.646 −2.161 −2.30 to −2.4429 −1.4722 881b

−2.141,2 −1.48c 840b

Literature data for εd are given for bulk systems. To our knowledge, no similar data for Pt nanoparticles were ever published. bThis work. cThis work, estimated based on interpolation of the CO chemisorption energy on Pt and Pd,21,22 and determined CO stripping potential (this work, Figure 1). See Results for more details.

a

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CONCLUSIONS We have investigated the electronic and electrocatalytic properties of the Pd monolayer deposited on Pt nanoparticles (PtPdML). Such a system exhibits a significant increase in catalytic activity toward formic acid oxidation as compared to pure Pd nanoparticles. Our results suggest that the deposition of the Pd monolayer on Pt increases the electron density close to the Fermi level, which is responsible for changes in catalytic activitya direct link between electronic properties of the surface and its catalytic activity. For the first time we present measured d-band center values for ultrapure, small (4 nm), and unsupported Pt, Pd, and PtPdML nanoparticles. The measured d-band center values are well correlated to the values determined based on theoretical calculations for pure Pt and Pd nanoparticles, but a significant discrepancy between measured and calculated d-band center exists for PtPdML. The difference between the measured and calculated d-band center energy was attributed to incorporation of d-states of the nanoparticle core (Pt) to the band structure. Our results confirm a strong link between electronic and electrocatalytic properties but suggest that the relation between d-band center and electrocatalytic activity is more complex than direct correlation, even for systems where chemical factors (such as bifunctional mechanism) were eliminated. In particular electron density of states close to the Fermi level, which is easily observable, seems to be responsible for the observed difference in catalytic activity, but in general, depending on the overall electron distribution in the valence band, it might or might not be reflected in changes of the d-band center.

Figure 7. Linear, normalized combination of valence band spectra of Pt and Pd (PtPdML simulated), compared to registered spectra of PtPdML, Pt, and Pd.

band theory of solids, with the perturbations caused by lattice strain. The contradiction in expected and observed εd values for PtPdML can thus be explained based on the valence band spectra registered using UPS and XPS (Figures 5−7). In particular, the increase in electron density close to the Fermi level has been observed in UP and XP spectra (Figures 5−7) most probably as the result of tensile strain in the Pd overlayer, which is in agreement with d-band center theory. This effect should shift εd toward the Fermi level. At the same time significant broadening of the valence band occurred due to incorporation of Pt valence states at BE values higher than εd (Figure 6) to the overall valence band structure (compare the valence band width of pure Pt and Pd in Figure 6). This effect is not accounted for in d-band center theory and in our case shifts εd away from the Fermi level. Both effects change the dband center in opposite directions, and as the magnitude of the factor causing the εd decrease (band broadening due to incorporation of Pt states) is higher than the factor causing the εd increase (increase in d-DOS close to the Fermi level), the resulting change in d-band center has opposite sign and completely different magnitude to what was predicted. Concerning the contradiction between changes of εd and the observed changes in chemisorption strength and catalytic activity of PtPdML it is now clear that the factors influencing εd also impact the catalytic activity, but to a different degree. It is the d-DOS change in the close proximity of the Fermi level which is responsible for an increase in adsorption strength and increase in catalytic activity for PtPdML where the d-DOS away from the Fermi level has minimal impact on adsorption strength. In general our data suggest that, depending on the actual electron distribution and sample morphology, the increase in dDOS close to the Fermi level might or might not be correlated to shifting the d-band center toward the Fermi level. We have shown that a strong link between the experimentally determined electronic and electrocatalytic properties indeed does exist. However, our data suggest that the measured electron density of states close to the Fermi level is a better predictor of catalytic activity than the measured εd. This is especially important for nonuniform systems where εd can be influenced by more than one factor. Overall our data stress the need to actually measure the electronic structure to better understand the factors responsible for the changes in catalytic activity.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +48 22 552 6550. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was funded from Polish National Science Centre budget based on decision number DEC-2013/09/B/ST4/ 00099. We gratefully acknowledge Dr. M. Kulpa and Dr E. Rówiński from Silesian University, Department of Physics, Katowice, Poland, for the XPS measurements. The study was carried out at the Biological and Chemical Research Centre, University of Warsaw, established within the project cofinanced by European Union from the European Regional Development Fund under the Operational Programme Innovative Economy, 2007−2013.

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b05302 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.5b05302 J. Phys. Chem. C XXXX, XXX, XXX−XXX