Pd Segregation on the Surface of Bimetallic PdAu Nanoparticles

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Pd Segregation on the Surface of Bimetallic PdAu Nanoparticles Induced by Low Coverage of Adsorbed CO Mikhail Mamatkulov, Ilya V. Yudanov, Andrey V. Bukhtiyarov, Igor P. Prosvirin, Valerii I. Bukhtiyarov, and Konstantin M. Neyman J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07402 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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The Journal of Physical Chemistry

Pd Segregation on the Surface of Bimetallic PdAu Nanoparticles Induced by Low Coverage of Adsorbed CO Mikhail Mamatkulov,† Ilya V. Yudanov,*,†,‡ Andrey V. Bukhtiyarov,† Igor P. Prosvirin,† Valerii I. Bukhtiyarov,† and Konstantin M. Neyman*,§,ǁ

†

Boreskov Institute of Catalysis of the Siberian Branch of Russian Academy of Sciences (SB

RAS), 630090 Novosibirsk, Russia, ‡

Institute of Chemistry and Chemical Technology SB RAS, Federal Research Center

“Krasnoyarsk Scientific Center SB RAS”, 660036 Krasnoyarsk, Russia, §

Departament de Ciència de Materials i Química Física and Institut de Quimica Teòrica i

Computacional, Universitat de Barcelona, c/Martí i Franquès 1, 08028 Barcelona, Spain, ǁ ICREA

(Institució Catalana de Recerca i Estudis Avançats), Pg. Lluís Companys 23, 08010

Barcelona, Spain CORRESPONDING AUTHOR FOOTNOTE: * (I.Y.) E-mail: [email protected]. * (K.N.) E-mail: [email protected]. ABSTRACT: For bimetallic Pd-Au/HOPG model catalysts the reversible enrichment of surface by Pd under CO oxidation conditions was found using NAP XPS technique. Density functional calculations combined with calculations using topological energy expression method (TOPmethod) were applied to reveal the mechanism of this phenomenon and to quantify the stability of different arrangements of metal atoms in bimetallic PdAu nanoparticles in the presence of CO adsorbate. According to results of this computational approach, adsorption of CO already at a rather moderate coverage is sufficient to make energetically feasible segregation of Pd at terraces of PdAu nanoparticles similar in size with experimentally studied ones.

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1. INTRODUCTION Bimetallic nanoparticles (NPs) are of high interest for heterogeneous catalysis, because addition of the second metal to supported monometallic particles can drastically improve their catalytic performance and reduce catalyst’s cost. 1 The interaction of two metals in bimetallic nanosystems may create a variety of atomic arrangements with quite distinct and different structures, where metal components are either mixed (as in alloyed nanoparticles) or segregated (as e.g. in core-shell structures). Different arrangements may occur even in systems with the same stoichiometries depending on the preparation conditions. Indeed, not only the ratio of the introduced metals, but also temperature of calcination, among other factors, will influence the surface composition. Moreover, reaction mixture to which a catalyst is exposed may alter the distribution of components within a particle and on its surface. Thus, the structure of a bimetallic catalytic particle becomes a dynamic characteristic. A remarkable example of the dynamical restructuring at the nanoscale was reported in a recent in situ transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) study of methanol oxidation over gold-silver alloy catalysts, where a direct dependence of the oxidation activity on surface silver concentration changing under the influence of reaction conditions was demonstrated. 2 Recently, some of us observed dynamic behavior of surface composition and concomitant catalytic activity changes for nanoalloy PdAu model catalysts during CO oxidation. 3 Since the chemical functionality of bimetalic NPs is intimately linked to their surface, a detailed understanding of nanoparticle architecture and its eventual evolution under reaction conditions is crucial for the rational design of nano-sized materials with enhanced catalytic properties. Application of model catalysts is a very useful approach to reveal correlations of electronic properties and morphology of bimetallic particles under reaction conditions with their catalytic performance. 4- 8 One of the key advantages of the model catalyst approach is the possibility to obtain atomic-level information. 9,10 Moreover, the most profound understanding of the actions and properties of catalysts may be achieved combining the experimental study of model catalyst with accurate quantum chemical calculations (modeling) of model NPs. 11,12 A modeling approach based on density functional (DF) calculations of highly symmetric particles was developed 13- 15 to simulate the properties of nanosized model metal catalysts with well-ordered structure studied by experimental techniques. 16,17 This approach proved to be efficient in a number of applications11,18- 20 including studies of bimetallic NPs. 21 Recently


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supplemented by Monte-Carlo search for thermodynamically the most stable distribution of bimetallic components within a given NP structure (TOP method) this approach became very powerful also to simulate the properties of bimetallic NPs. 22- 24 In the following we present results of a combined experimental and theoretical study of the reorganization of PdAu NPs under CO oxidation conditions. The manuscript is organized as follows: First, we address Near Ambient Pressure (NAP) XPS results on the structural evolution of a model PdAu catalyst supported on highly oriented pyrolytic graphite (HOPG) and its catalytic activity for CO oxidation. Then we proceed with theoretical analysis of structural characteristics of PdAu NPs and investigate how the stability of initial (adsorbatefree) NP structure can be altered by CO adsorption. 2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. Preparation and characterization of PdAu/HOPG model catalyst. Preparation of the sample and primary characterization was done in a SPECS photoelectron spectrometer (Germany) equipped with the hemispherical analyzer PHOIBOS-150-MCD-9, the ellipsoidal monochromator FOCUS 500, and the X-ray source XR 50M with double Al/Ag anode. Commercially available HOPG (7×7 mm, approx. 1 mm thickness, SPI supplies, Grade SPI-2) was used as a support for bimetallic Pd-Au catalysts. The bimetallic Pd-Au/HOPG model catalyst was prepared by the physical vapor deposition of Pd on Au/HOPG matrix followed by annealing at ultra-high vacuum (UHV) to 400°C to form Pd-Au alloyed particles. The details of preparation of bimetallic Pd-Au NPs supported on HOPG and of their characterization by STM and XPS were presented elsewhere. 25 STM measurements of the Pd-Au/HOPG samples were performed utilizing an UHV 7000 VT microscope (RHK Technology, USA) operating in the constant current mode. Cut Pt–Ir tips were employed in the STM experiments. Si(111) single crystal with the 7×7 surface reconstruction and clean HOPG were used as standard samples for scanner calibration. 2.2. NAP XPS measurements. NAP XPS measurements were performed using a photoelectron spectrometer at the ISISS beamline at BESSY II/HZB (Berlin, Germany). 26 The Pd-Au/HOPG sample was placed between a stainless steel backplate and lid (with 6 mm hole) and mounted onto a sapphire sample holder. The samples were heated up from the back side using the infrared laser and the temperature was measured with a K-type thermocouple. The CO/O2 = 2:1 mixture was introduced into the high-pressure cell up to 0.25 mbar by the mass-


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flow controllers. All samples were annealed to 300°C at the UHV before the testing under CO oxidation reaction in order to desorb the contaminations from the surface. The experiments under CO oxidation reaction were performed as follows: the CO+O2 flow (CO:O2 = 2:1; P=0.25 mbar) at room temperature (RT) was filed into the gas cell; then the sample was heated step by step to 250°C (RT-100°C-150°C-200°C-230°C-250°C) and subsequently cooled down to room temperature. XPS spectra for Au4f, Pd3d and C1s core levels at different kinetic energy were acquired at each temperature. For the purpose of quantitative analysis all XPS lines of Au, Pd and C were measured at different excitation energies in order to provide the same kinetic energy of 300 eV. To determine the positions of the peaks in the Au4f and Pd3d spectra, XPS peaks were calibrated against the C1s spectra taken at the same primary excitation energies as Au4f and Pd3d. The base pressure of the spectrometer did not exceed 1×10−9 mbar. Spectral analysis and data processing were performed with XPS Peak 4.1 program. 27 For the quantitative analysis the integral intensities of Au4f, Pd3d and C1s lines were corrected using ionization cross-section data taken from Yeh and Lindau 28 and normalized with respect to current and photon flux. For fitting of the peak Pd3d, the Au4d5/2 peak overlapping with the latter was subtracted from the spectra. The shape and intensity of the Au4d5/2 peak was calculated from the less intensive Au4d3/2 peak. 2.3. Computational details. DF calculations were performed with the VASP code, 29,30 employing PAW pseudopotentials 31,32 and a gradient-corrected exchange-correlation functional by Perdew, Becke and Ernzerhof (PBE). 33,34 For the determination of energetic descriptors from the topological expression (see Section 3.2) cutoff energy of 250.9 eV was used with the unit cell size 25×25×25 Å3 leading to a distance of ≥8 Å between periodically repeated bimetallic NPs composed of locally relaxed 201 atoms. Usage of such somewhat less precise than commonly computational parameters in series of DF calculations of metal NPs required for the determination of the TOP energetic descriptors provides sufficient accuracy of the latter, as shown elsewhere.22,23 For the DF calculations of CO adsorption on metal NPs cutoff energy was set to 415 eV, the unit cell size was 30×30×30 Å3 and structural relaxations were performed until forces on atoms were less than 0.01 eV. In all calculations of NPs only Γ-point was used for the Brillouin zone sampling. Energies of CO adsorption were calculated as follows: Eads ( CO = )

( E ( NP ∗ CO ) − E ( NP ) − m × E (CO ) ) / m , m

(1)


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where E ( NP ∗ COm ) , E ( NP ) and E ( CO ) are, respectively, DF total energies of adsorption complex NP*COm (NP with m adsorbed CO molecules), of bare NP and of gas-phase CO molecule. In calculations of bulk metals and of the Pd(111) slab 15×15×15 and 4×4×1 k-point grids were used, respectively. Vacuum spacing of 15 Å between slabs was created to calculate CO adsorption on Pd(111). As models of bimetallic NPs in the catalysts we use truncated octahedron particles terminated by six square (001) facets and eight regular hexagonal (111) facets. Monometallic NPs of this shape exhibit Oh symmetry. For the bulk-terminated geometry the ratio of distances from the NP center to the (111) and (001) facets is r111 3 = , r001 2

which satisfies the Wulff construction: r111 γ 111 = r001 γ 100

with the surface energies γ 111 and γ 001 calculated in the bond-terminated approximation. This implies higher stability of truncated octahedrons compared, for instance, to octahedrons and cuboctahedrons. The nanoparticles NPn with n= 38, 201, 586, 1289, 2406, 4033, etc. belong to this series; the NP201 and NP1289 were studied in the present work. No symmetry restrictions were imposed in the calculations of bimetallic NPs. Truncated octahedral NP models have been successfully used to simulate properties of wellordered model metal catalysts on various supports.11-15,18-24 Under conditions of CO adsorption and oxidation PdAu NPs may undergo significant rearrangements leading to structures different from those of the close-packed well-ordered models. Thus, neglecting the high-index surfaces and non-crystallographic packings our present theoretical consideration represents in part a simplified and idealized picture, which, nevertheless is helpful for understanding the basic mechanism of the model catalyst functionality. 3. RESULTS AND DISCUSSION 3.1. Evolution of Pd-Au/HOPG model catalyst during CO oxidation according to NAP XPS. We characterized the catalysts Pd-Au/HOPG prepared and investigated in the present work. Table 1 shows Au/C, Pd/C, and Pd/Au atomic ratios calculated from the XPS spectra taken at


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each step of the preparation procedure. At the first step, monometallic Au/HOPG samples with the Au/C = 0.009 were prepared using the procedure developed for monometallic gold and silver particles supported on defective HOPG surface. 35,36 These samples served as a matrix for the deposition of the second metal Pd leading to the formation of bimetallic particles with a “Au core - Pd shell” structure on the HOPG surface.25,37 Annealing of the core-shell samples up to 400°C at UHV conditions resulted in the formation of bimetallic particles with an alloyed structure. Indeed, derived from the XPS data Au/Pd atomic ratio increase to 1.35 after annealing indicates a redistribution of the components – particle surface enrichment by Au alloyed with Pd, in line with our previous results.25 STM data show the presence of threedimensional (3D) particles with a rather narrow size distribution characterized by the mean particle size of 7.1 nm (see Figure 1). The alloyed Pd-Au/HOPG catalyst was investigated in CO oxidation (CO:O2 = 2:1, P = 0.25 mbar) using NAP XPS at ISISS end station at the Berlin synchrotron radiation source BESSY II.26 Figure 2 shows CO2 mass-spectrometric signal (m/e = 44), as well as the Au/Pd atomic ratios calculated from Au4f and Pd3d spectra, measured during the sample heating followed by cooling (from RT to 250°C and then back to RT). All dependences are in good agreement with the published data.35 The catalytic activity was observed at temperatures above 150°C. One can notice redistribution of Au and Pd components on the surface depending on the reaction conditions. By means of NAP XPS different Pd and Au surface species can be identified, whose fractions change under the influence of reaction mixture depending on temperature. At low temperatures (T ≤ 150°C) the reaction mixture partly destroys the alloy structure due to surface segregation of Pd relocating most of surface Au atoms in subsurface layers. Evidently, the driving force of the Pd segregation is stronger adsorption of CO on Pd sites than on Au ones and formation of Pd-CO bonds. The formation of Pd-CO surface species deactivates the alloyed particles against CO oxidation – no CO2 formation is observed in this temperature range. Heating the sample above 150°C causes CO desorption. Pd atoms move back to the alloy structure, and the sample becomes again active in CO oxidation. A part of Pd atoms does not incorporate into the alloy structure and exists as Pd metal probably due to an excess of Pd in the Pd-Au samples under study. Cooling back to RT causes the catalytic activity loss and rearrangement of bimetallic particles, which again basically consist of gold layers covered with Pd-CO species.


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In this work we focused experimentally on destroying of PdAu alloyed structure by COinduced Pd segregation. According to the XPS data in Fig. 2, alloy destruction occurs first after exposing the initial sample to the CO+O2 reaction gas mixture and then during cooling down from 250°C to RT under reaction mixture flow. In both cases the drop of Au/Pd atomic ratio was considered as an indicator of Pd segregation, i.e. as a measure of alloy destruction. Fig. 3 shows Pd3d5/2 spectra with their deconvolution in several components and fractions of different Pd states taken for a PdAu sample and that under the reaction mixture depending on the temperature. Different Pd species were identified based on the data of our previous work.35 Namely, the Pd3d5/2 peaks at binding energies (BE) ~ 335.0 eV and ~ 335.6 eV were attributed to Pd in PdAu alloy3,38,39 and metallic Pd,38,40,41 respectively. The Pd3d5/2 species with BE of about 336 eV was attributed at temperatures below 200ºC to the generated by CO-induced segregation surface Pd components with adsorbed CO, 42,43 whereas at higher temperatures the XPS signal with the BE = 336.0 eV was assigned to oxidized Pd-O species.42 These results suggest that the surface segregation of Pd observed in the present work at temperatures up to 150ºC is mainly triggered by interactions with CO rather than with dissociated oxygen, which would also promote surface segregation of Pd in PdAu alloys. Finally, the Pd3d5/2 peak at 337.2 eV was assigned to small Pd clusters attached to some defective sites on the HOPG surface.36,44 For the pristine sample at UHV conditions (before exposure to the CO+O2 reaction mixture) mainly the signal of Pd in the alloy (BE = 335.1 eV) is present in Pd3d5/2 spectra implying that PdAu alloy dominates on the surface. Concentration of the Pd-CO state (BE = 336.0 eV) increases on the surface under reaction mixture flow at RT, but decreases at 250°C, when the sample becomes active in CO oxidation. It is remarkable, that together with the formation of Pd-CO species metallic Pd0 is also formed. Cooling the sample back to RT brings the intensity of these signals to the initial level obtained at RT under CO+O2 gas mixture. Thus, reversible CO-induced surface segregation of Pd takes place at RT for PdAu alloyed particles supported on HOPG. 3.2. Distribution of Pd atoms in bare bimetallic NP according to DF and TOP approaches We begin our theoretical investigation with considering, how palladium and gold atoms are distributed in bare bimetallic NPs in the absence of adsorbates. As a model for computations we have chosen a NP201 consisting of 201 metal atoms with the stoichiometry Pd56Au145, roughly corresponding to the Pd/Au ratio 1:3. The main question to be answered here is what is the most favorable energetically geometric arrangement of Pd and Au atoms in this NP. The


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principal challenge is that for a bimetallic NP of a given structure, containing n1 and n2 atoms of types 1 and 2, respectively, the number of different atomic arrangements (homotops) equals ((n1+n2)!/n1!n2!), which is generally huge - for the stoichiometry Pd56Au145 it is 2.77×1050. Performing DF (or any other) calculations for all these homotops is impossible. To address this challenge we used a recently developed TOP method.22,23 The principal idea of the method is to define the energy of each homotop in a bimetallic crystalline nanoparticle under consideration in the form of a topological expression, e.g. as follows: Au Au Au Au − Pd Au − Pd Au Au Au ETOP = E0 + ε BOND N BOND + ε CORNER NCORNER + ε EDGE N EDGE + ε TERRACE NTERRACE

(2),

Au − Pd where E0 is a constant, N BOND is the number of Au-Pd bonds (nearest-neighbor contacts) in Au Au Au , N EDGE , NTERRACE are the numbers of Au atoms located in corner, edge the homotop; N CORNER

and facet terrace positions on the surface of the homotop, respectively. The coefficients ε YX are energy descriptors, representing energy contributions of either one Au-Pd bond or an Au atom located in the corresponding surface position of the NP to the total energy of the homotop, ETOP. Applications of the method to various bimetallic NPs, including Pd-Au ones of different sizes, shapes and compositions,22,23 revealed that ETOP expressions obtained in the simple form of Eq. (2) by fitting to several dozen of DF energies for distinct homotops allow to very efficiently determining geometry of the energetically most stable homotops, in full agreement with DF data.22-24,45- 47 As detailed elsewhere,22 to determine the descriptors values in Eq. (2), one calculates DF energies for a series of selected homotops of the bimetallic NP under study. Then, ETOP energies closely matching the DF energies for these homotops are obtained by means of a multilinear regression, providing all ε YX and E0 values and thus defining Eq. (2). Using the latter in Monte-Carlo (MC) simulations for permuting locations of atoms of two elements in the NP allows to minimize ETOP and to define the energies for various homotops, including the most stable one. To examine quality of the obtained descriptor values, the following two tests are performed: precision δ of the topological expression is defined as twice the residual standard deviation of the energy differences EDF and ETOP for several low-energy homotops obtained via MC simulations and not used in the DF fitting data; TOP energy difference ΔE is calculated for the most stable homotops obtained at the TOP and DF levels.


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To determine the TOP descriptors for the chosen NP of the stoichiometry Pd56Au145 we performed DF calculations for 44 homotops. The obtained descriptors are presented in Table 2 along with the fitting procedure precision data. For comparison, we also present results from a previous TOP study for a smaller truncated-octahedral NP Pd140-xAux with varying Pd/Au atomic ratios.22 Low-lying homotops of Pd56Au145 NP exhibit the following features: (i) Au atoms show strong preference to surface locations; (ii) the number of Pd-Pd bonds tends to be minimized; (iii) the number of Au-Pd bonds tends to be maximized. All these trends are easily understood from the descriptor values and agree with our results for smaller PdAu NPs.22,23 Some lowlying homotops of Pd56Au145 contain several (six or eight) surface Pd atoms located on (111) nano-terraces. In such homotops the energy effect of maximized number of Au-Pd bonds compensates the energy required for substituting several Au atoms in the surface shell by Pd atoms. The most stable homotop with all Pd atoms located inside the NP Pd56Au145 is by ca. 0.3 eV higher in energy than the lowest-energy homotop. We take the homotop without surface Pd atoms as a reference to estimate the energy required to put all 56 Pd atoms on the surface. Energy needed for segregation of all Pd atoms to the (111) facets of the NP to occur has been estimated from two DF calculations (see Fig. 4): 1) for the homotop b, where all 56 Pd atoms are located on (111) facets of the NP and 2) for the low-energy homotop a, where all the surface, corner and edge sites of the NP are occupied by Au atoms. According to our DF data, the homotop b is 17.9 eV less stable than the homotop a. We also estimated the energy difference for these two homotops from the topological expression (see Table 2), giving 13.5 eV. Notably, the TOP method designed for accurate description of DF energy differences mainly between low-energy homotops22 also provides reasonable relative energy estimate for the high-energy homotop b with all Pd atoms located on (111) facets. For comparison we also added in Table 2 energy differences between the homotops a and b obtained with descriptors calculated for PdAu NP140 with different Pd/Au ratious.22 One may notice that for Pd35Au105 with the Pd/Au ratio 0.33, which is not very different from the ratio 0.39 for Pd56Au145, ΔEseg = 13.7 eV, is strikingly close to ΔEseg = 13.5 eV for the Pd56Au145. 3.3. DF screening of CO adsorption energy on terraces of Pd56Au145 model nanoalloy In the presence of adsorbates like CO molecules, surface segregation of palladium in Pd-Au alloys may become favored due to stronger interaction of Pd with adsorbed species. 48 -53 The


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central question for the present study is what coverage of adsorbed CO is necessary to stabilize palladium at the surface of a PdAu nanoalloy. In this section we present an overview of CO adsorption on different sites of PdAu nanoalloys. With the intention to study the strength of CO interaction with surface groups consisting of several Pd atoms (Pd multiplets) in comparison to Au surface moieties we constructed a model particle with the same stoichiometry as in previous section, Pd56Au145, however, with a specific distribution of Pd atoms in the outer shell and in the core: 26 Pd atoms are located in the outer shell (on the surface) as Pd islands, Pd1, Pd2, Pd3 and Pd7 (a pair of islands of each type), surrounded by Au atoms (Fig. 5). Since the adsorption energy may notably depend on the nature of subsurface layer (either Pd or Au), the core of the model NP is composed of two compact blocks of Pd (30 atoms) and Au (49 atoms). This is different from the most stable arrangement discussed in Section 3.2, where Pd and Au components are intermixed to maximize the number of Pd-Au bonds. These core blocks of Pd and Au are arranged in such a way that each type of surface moiety exists in two variants: either with gold or with palladium in the subsurface layer. For instance, Pd7 multiplet (in a (111) terrace fully filled by Pd) is present on the surface both with Pd (Fig. 5a) and Au (Fig. 5b) sublayers, respectively. Adsorption energies of CO on the sites of interest (Fig. 5) were calculated probing each adsorption site by a single CO molecule. Since the absorption on three-fold hollows is known to be the most favorable for CO on Pd(111) at low coverage, 54,55 we considered only this adsorption mode on Pd multiplets of the model NP (Fig. 5). The results of the calculations are presented in Table 3. Evidently, the principal factor for strengthening the interaction of bimetallic NP with CO is the availability of Pd multiplets on the surface of NP. The (111) terrace filled by Pd (Pd7 septet) and Pd3 triplet with Au in sublayer bind CO equally strongly. The nature of the sublayer seems to be not particularly important here: Pd sublayer only slightly strengthens binding with CO on Pd7 septet, by 0.2 eV, and has essentially no effect on the CO binding to Pd3 triplets. Also, Pd sublayer has only minor effect on CO adsorption strength of Au terrace sites. Overall, very low CO adsorption energies are calculated on Au sites. As seen from Table 3, even on Pd triplets embedded into Au environment (3f-Pd3/Au) CO adsorption is by 1.6 eV more favorable than on the most strongly adsorbing CO Au on-top sites. Already from these tests we can derive some estimates of CO coverage required to stabilize segregation of Pd on the surface of a bimetallic PdAu NP. Given that the energy difference between two homotops of Pd56Au145 considered in Section 3.2, with Pd distributed in core of NP (Pdin-core ACS Paragon Plus Environment

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homotop) and Pd filling the (111) terraces of NP (Pdterr), is about 18 eV we may expect that adsorption of 12 CO molecules (two per (111) facet) on such NP should be sufficient to stabilize the Pdterr homotop. In a general case, this is less than one CO molecules per four surface Pd atoms. In the following we will develop these estimates on a more systematic basis. 3.4. Determination of threshold CO coverage to stabilize Pd segregated on facets of model NP Pd56Au145 In the previous section a rough estimate of the quantity of CO molecules needed to pull all Pd atoms to the surface of Au145Pd56 nanoalloy was given. To verify this estimate we explicitly calculated CO adsorption on two homotops Pdin-core and Pdterr of Pd56Au145 gradually increasing the coverage of adsorbate: one, two and, finally, three CO molecules per (111) facet, which corresponds in total to 8, 16 and 24 adsorbed CO molecules per NP (Fig. 6). Based on the analysis of CO binding presented in Section 3.3, in the cases of Pdterr and Pdin-core structures CO molecules were adsorbed at 3-fold and on-top positions, respectively. For each of the considered CO coverage the energy difference between two homotops, ΔEseg, was calculated. Without adsorbates ΔEseg is about 18 eV (see Fig. 4). Adsorption of just one CO molecule per facet decreases the energy difference between the Pdterr and Pdin-core homotops to 4 eV. With two and three adsorbed CO per facet ΔEseg is calculated to be about -9 eV and -20 eV, respectively, with the negative sign implying that surface segregation of Pd became energetically favorable. The dependence of ΔEseg on CO coverage is plotted in Fig. 7, where the coverage θCO is introduced with respect to the number of terrace metal atoms. For instance, θCO = 1 ML corresponds to seven CO molecules per terrace of seven metal atoms located on (111) facet of model NP. The calculated dependence is well approximated linearly (Fig. 7), with ΔEseg = 0 at θCO = 0.195 ML, being a threshold above which surface segregation of Pd on the considered model NP Pd56Au145 becomes energetically stabilized. Quantifying the CO coverage, which stabilizes Pd surface segregation, we should point to the different level of accuracy provided by DF functionals for metal-metal and metal-adsorbate bond strength. The PBE functional reasonably well describes the metal-metal interaction in the bulk for both metals, Pd and Au, where the calculated cohesive energies, Ecoh, are 3.70 eV (Pd) and 2.98 eV (Au) compared to experimental values of 3.91 and 2.98 eV, respectively. 56 One can be quite confident that the binding of Pd and Au in NPs is described by this functional with similar accuracy, since Ecoh in NPs scales linearly with reciprocal particle size or average


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coordination number, Nc ~ 1/D, and the corresponding relations streaming to the bulk limit value are well-known from DF studies of Pd and Au NPs. 57-59 On the other hand, the interaction of CO with Pd(111) surface is somewhat overestimated by the PBE approach: for CO adsorption on three-fold sites of Pd(111) (5-layer slab, θCO = 1/9 ML) we calculated -Eads = 2.06 eV to be compared with the experimental heat of CO adsorption on the Pd(111) single crystal at low coverage, 1.54±0.04 eV.12 Thus, estimates of the threshold CO coverage based on PBE data can be corrected accordingly. Nevertheless, our computations indicate that stabilization of Pd segregated on surface terraces may occur at CO coverage far from the saturation. Now, we need to find the way for examining and generalizing this result for larger particles with sizes comparable to the sizes of NPs treated by experimental methods. Adsorption energy on NPs is known to be size-dependent.12,18,60,61 However, for terrace sites of NPs containing over 200 metal atoms only minor changes of CO binding occur, mainly due to lattice contraction effects in NPs caused by surface tension.18,60,61 The size effects on Eads (by at most 0.1-0.15 eV for NPs in the site range 200-1000 metal atoms even for such strongly binding metals as Pd and Pt18,60) are not decisive for our estimates and can be neglected here. 3.5. Prediction of chemical ordering and palladium surface segregation in larger particles PdAu NPs of model catalysts studied experimentally are 4-12 nm large with the average size value around 7 nm (Fig. 1). The theoretical model of 201 atoms has diameter, D, of 1.7 nm (here we take the diameter of a sphere, which is built on vertex atoms of the truncated octahedron NP201). To approach the size region of particles treated experimentally, we applied the TOP method to a PdAu NP1289 consisting of 1289 atoms with the same shape as NP201 and D = 3.5 nm (Fig. 8). To keep the same Pd/Au ratio of about 2:5 as that of the NP201 (Pd56Au145) homotops discussed before, we take the composition of 359 Pd and 930 Au atoms. Note, that the surface/core ratio for the NP1289 is 0.6 (482 surface atoms and 807 inner atoms) while for NP201 the ratio was 1.5 (122 and 79 atoms, respectively). Thus, the core of 3.5 nm large NPs is rather bulky and contains more atoms than are exposed to surface. At variance to NP201, it is exceedingly demanding for our DF setup to calculate particles as big as NP1289. Therefore, we used the topological energy descriptors obtained above for the NP201 (Pd56Au145) to calculate the chemical ordering in the NP1289 (Pd359Au930). Such approach is justified by the observations22 that values of energy descriptors for a nanoalloy only slightly depend on the particle size, differently from often stronger variation with the particle composition.


12

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The structure of a homotop Pd359Au930 obtained by Monte-Carlo minimization of ETOP (Eq. 2 with the descriptors, which we obtained for Pd56Au145, Table 2) is presented in Fig. 8a. There, whole palladium is dissolved in the volume of the NP1289, whose surface is completely covered by gold (the gold outer shell is typical for PdAu NPs at low Pd contents as shown by previous theoretical simulations23,62). We denote this homotop as Pdin-core by analogy to similar homotops of NP201 discussed in previous sections. To evaluate energy required to pull all Pd atoms to the surface we performed Monte-Carlo energy minimization also for the homotops Pdterr (Fig. 8b) with terraces of all (111) facets completely occupied by Pd, similarly to the analogous homotop of NP201. In this computation locations of all atoms forming the surface shell of Pdterr were fixed and the minimization procedure was confined to searching for the most stable locations of Pd and Au atoms in the core of the NP1289. Note, that 296 Pd atoms of the NP Pdterr are located at the surface to fill the (111) terraces and the remaining 63 Pd atoms are distributed in the core; it is slightly different from Pdterr of NP201, where the total number of Pd atoms was equal to the number of terrace positions. As in Section 3.2, the energy difference between homotops Pdin-core and Pdterr defines the segregation energy ΔEseg. The subtraction of ETOP energies of these two homotops calculated according Eq. 2 yields ΔEseg = 90.7 eV which has to be compensated by CO adsorption to stabilize the Pdterr homotop. Here, one should keep in mind that the TOP method tends to underestimate ΔEseg compared to DF: for NP201 ΔEseg(TOP) was about 30% smaller than the DF value (see Section 3.2). This underestimation may partly originate from the fact that the energy descriptors were obtained based primarily on low-lying homotops of NP201 (Pd56Au145), while the homotop Pdterr is obviously far from the energy minimum and may fall out somewhat from the trends established in the vicinity of the energy minimum. Assuming the same level of underestimation of ΔEseg, about 30%, by TOP approach for NP1289, one can conservatively estimate the DF value of ΔEseg to be about 120 eV. Since CO adsorption on Pd terraces of Pdterr is by ca. 1.5 eV stronger than on Au shell of Pdin-core homotop, adsorption of about 80 CO molecules should suffice to make the Pdterr homotop equally energetically stable to Pdin-core one. This corresponds to the coverage of 0.27 ML (80 adsorbed CO per 296 surface Pd atoms), which is somewhat higher than calculated at DF level for NP201, however, still in the range of the low CO coverage.


13


Page 14 of 29

4. CONCLUSION The redistribution of Au and Pd on the surface depending on the reaction conditions has been demonstrated using NAP XPS, namely, fast reconstruction of NP structure under CO oxidation conditions. Apparently, CO adsorption induces the Pd segregation on the surface: Pd migrates to the surface forming Pd-shell to replace the initial Au-shell/PdAu-alloyed-core structure. Such surface segregation occurs at temperature below 150°C. Heating of the sample above 150°C under reaction conditions leads to decomposition of Pd-CO species due to catalytic oxidation and desorption of CO. Simultaneously, Pd-Au alloy formation on the surface takes place. Cooling down back to RT results in reversible Pd segregation due to Pd-CO formation and the sample becomes inactive for oxidation at this temperature. The driving force for such structural reconstruction is a strong interaction of surface Pd sites with adsorbed CO molecules, which changes surface energy making surface segregation of Pd with formation of Pdx-CO moieties energetically favorable. Indeed, our DF and TOP-method calculations showed that adsorption of CO already at coverage below 0.3 per surface Pd atom is sufficient to thermodynamically stabilize the structure with (111) terraces of particles completely occupied by Pd. The kinetic mechanism is not touched in the present work, however, the fast exchange of metal components between the surface shell and the core of bimetallic NP would involve more open structures than the close-packed (111) facets. Involvement of more open active sites (on high-index or defect-rich surfaces) stronger interacting with CO when composed of Pd may reduce the energy of Pd segregation compared to close-packed structures considered in the present study. Also, the presence of atomic oxygen, which is adsorbed significantly stronger than CO, may further facilitate surface segregation of Pd. ACKNOWLEDGMENTS. K.N. acknowledges a Spanish grant PRX17/00348 that enabled his research stay at BIC as well as grants CTQ2015-64618-R and 2017SGR13. A.V.B., P.R.S. and V.I.B. would like to thank Russian Science Foundation (grant #14-23-00146) for the financial support of the preparation and characterization of model bimetallic Pd-Au/HOPG catalyst as well as for the supporting of NAP XPS study. We also thank Helmholtz-Zentrum Berlin (HZB) for the allocation of synchrotron radiation beamtime at the ISISS beamline. I.Y. acknowledges the extensive support by his brother Kirill Yudanov. The extensive use of computational resources of the Red Española de Supercomputación, Novosibirsk Siberian Supercomputing Center and Lomonosov Moscow State University 63 is gratefully acknowledged. ACS Paragon Plus Environment

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Supporting Information Available: Structural and energetic data of calculated PdAu nanoparticles and their adsorption complexes with CO along with relevant data for Pd, Au and CO systems (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.


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Table 1. Atomic ratios of elements calculated from XPS data. Sample

Au deposition

Heating at UHV

Annealing at 400°C

Pd deposition

(Alloy formation)

Pd-Au/HOPG

Au/C

Au/C

Pd/C

Au/Pd

Au/Pd

0.009

0.005

0.004

1.28

1.35

Table 2. Energy descriptors in the topological energy expression, Eq. (2), obtained for NP140 with different Pd/Au ratios (ref. 22) and for NP201 (present work, Pd56Au145). NFIT is the number of DF calculations performed to define Eq. (2), δ is the precision of the energy descriptor values, vide supra. For each set of descriptors determined for NP140 and NP201 the segregation energy of all 56 Pd atoms on the surface of Pd56Au145 is estimated as the energy difference of two homotops a and b displayed in Fig. 4, ΔEseg = ETOP(b) - ETOP(a). All energies are in eV. Pd56Au145

Pd70Au70

Pd35Au105

Pd30Au110

Pd11Au129

-0.026

-0.013

-0.032

-0.030

-0.030

-0.370

-0.404

-0.291

-0.390

-0.372

-0.336

-0.301

-0.382

-0.296

-0.212

-0.192

-0.200

-0.185

-0.202

-0.228

NFIT

44

32

24

22

30

δ

0.094

0.096

0.148

0.060

0.170

ΔEseg

13.5

12.6

13.7

14.4

15.9

ε

BOND

ε

CORNER

ε

EDGE

ε

TERRACE

Au − Pd

Au

Au

Au


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Table 3. Energy of of CO adsorption, Eads, on various surface sites of the model NP Pd56Au145 shown in Fig. 5. Site a

Eads, eV

3f-Pd7/Pd

-2.10

3f-Pd3/Pd

-1.93

t1-Au/Pd

-0.25

t2-Au/Pd

-0.33

3f- Au/Pd

-0.11

br-Au/Pd

-0.14

3f-Pd7/Au

-1.91

3f-Pd3/Au

-1.93

t1-Au/Au

-0.13

t2-Au/Au

-0.25

3f-Au/Au

-0.15

br-Au/Au

-0.10

a

The designation of the adsorption sites is composed of three parts: (i) the type of site by CO

coordination: 3f – three-fold hollow site; br – bridge site; t1 and t2 - on-top sites; (ii) the sort of surface atomic group (Pd7 and Pd3 multiplets or Au terrace); (iii) the type of sublayer – either Pd or Au. For instance, 3f-Pd3/Au implies a 3f site on a Pd3 multiplet with Au in the sublayer.


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Figure 1. STM image (200×200 nm) and the particle size distribution of Pd-Au/HOPG. Tunneling parameters: 0.47 nA, -1490 mV.

Au/Pd 1.40E-012

T (°C)

1.3 250

CO2 signal (QMS) (A)

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


1.2

CO+O2

UHV, RT

1.30E-012

200 1.1 150

1.0

1.20E-012

1.10E-012

0.9

100

0.8

50

0.7 0

2

4

6

0 8

Time (hours)

Figure 2. Mass spectrometry (MS) signal of CO2 in gas phase and Au/Pd atomic ratios (calculated from Au4f and Pd3d spectra acquired for kinetic energy of 300 eV) on the surface of a fresh Pd-Au sample and the sample in CO+O2 gas mixture depending on temperature.


23


336.0

Pd-CO Pd clusters

335.1

Pd-Alloy Pd0 Pd-CO

Pd alloy

Pd 0

100 90

back to RT

CO+O2

UHV Conditions

80

reaction mixture 70 60 50

250°C 40 30

RT

Pd states fractions (%)

Pd3d5/2

20 10

UHV 334

Binding energy (eV)

332

R T

336

25 0

338

R T

0 340

R T

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

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Temperature (°C)

Figure 3. Pd3d5/2 XPS spectra and fraction of different Pd states with respect to (Pd-Alloy + Pd0 + Pd-CO) for the Pd-Au/HOPG sample not exposed to CO+O2 and then after exposing it to the reaction mixture at different temperature (XPS data obtained at kinetic energy 300 eV).


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Figure 4. Two homotops of the NP Pd56Au145: a – with all Pd atoms (blue spheres) located inside the particle (approximating the lowest-energy homotop); b – with all Pd atoms in surface positions on (111) terraces (representing the atomic arrangement with completely surface segregated Pd).


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Figure 5. Adsorption activity of various sites on the surface of a model Pd56Au145 NP with respect to CO probe: (a) view from the side, where surface Pd multiplets are located above a Pd block of the particle core; (b) view of the same model from the side, where surface Pd multiplets are located above an Au block of the particle core. The energy of CO adsorption bond, -Eads, is given in eV. For classification of sites see Table 3. This specially constructed Pd56Au145 homotop is by 8.9 eV higher in energy than the homotop in Fig. 4a.


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Figure 6. Structures of NP201 (Pd56Au145): Pdin-core (top row) and Pdterr (bottom row) homotops with CO adsorbate on (111) terraces (0, 8, 16 and 24 CO molecules from left to right, respectively). The corresponding segregation energy, ΔEseg, is given in eV.

Figure 7. ΔEseg as function of CO coverage calculated for NP201 (Pd56Au145). θCO = 1 ML corresponds to seven CO molecules per terrace of seven metal atoms located on (111) facet of NP. Linear approximation (solid line) of DF-calculated points (triangles) yields ΔEseg = 0 at θCO0 = 0.195 ML, a threshold above which surface segregation of Pd on (111) facets of model NP becomes energetically favorable due to CO adsorption.


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Figure 8. Surface (left) and cross-section (right) views of atomic structures of NP1289 (Pd359Au930): Pdin-core (a) and Pdterr (b) homotops with Pd distribution in core of NP obtained by TOP-method.


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Pd Segregation on the Surface of Bimetallic PdAu Nanoparticles

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