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Inside the Interaction of Methanol Selective Oxidation Intermediates with Au- or/and Pd-containing Monometallic and Bimetallic Core@Shell Catalysts Kamil Czelej, Karol Cwieka, Juan Carlos Colmenares, and Krzysztof J. Kurzydlowski Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b01906 • Publication Date (Web): 03 Jul 2016 Downloaded from http://pubs.acs.org on July 5, 2016

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Inside the Interaction of Methanol Selective Oxidation Intermediates with Au- or/and Pdcontaining Monometallic and Bimetallic Core@Shell Catalysts Kamil Czelej*,†, Karol Cwieka†, Juan Carlos Colmenares‡, Krzysztof J. Kurzydlowski† †

Faculty of Materials Science and Engineering, Warsaw University of Technology, 141 Woloska Str., 02-507 Warsaw, Poland

‡

Institute of Physical Chemistry, Polish Academy of Science, 44/52 Kasprzaka Str., 01-224 Warsaw, Poland

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ABSTRACT: Using density functional theory (DFT), the interaction of crucial molecules involved in the selective partial oxidation of methanol to methyl formate (MF), with monometallic Au, Pd and bimetallic Au/Pd, Pd/Au core@shell catalysts is systematically investigated. The core@shell structures modeled in this study consist of Au(111), Pd(111) cores covered by monolayer of Pd, Au, respectively. Our results indicate that the adsorption strength of the molecules examined as a function of catalytic surface decreases in the order of Au/Pd(111) > Pd(111) > Au(111) > Pd/Au(111) and correlate well with the d-band center model. The preadsorbtion of oxygen is found to have a positive impact on the selective partial oxidation reaction due to the stabilization of CH3OH and HCHO on the catalyst surface and the simultaneous intensification of MF desorption. Based on a dynamical matrix approach combined with statistical thermodynamic we propose a simple route for evaluating the Gibbs free energy of adsorption as a function of temperature. This method allows us to anticipate the relative temperature stability of molecules involved in the selective partial oxidation of methanol to MF in terms of catalytic surface.


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INTRODUCTION Selective partial oxidation of methanol to methyl formate (MF) over heterogeneous catalysts is an attractive and environmentally benign route for methanol conversion in industrial chemistry. The efficient production of methanol downstream products remains a challenging problem of technological importance; therefore, the design of highly efficient heterogeneous catalysts with enhanced selectivity and activity for partial oxidation of methanol is of paramount significance in academia. Currently, noble metals and their alloys13

, transition metal oxides in the form of nanoparticles4, 5, or metal-supported TiO26 are

commonly used to conduct methanol transformation to MF. However, severe limitations such as low yield and the production of a large amount of undesirable byproducts, mainly CO2 have been encountered7. To overcome these limitations worldwide attention has been focused on developing novel, highly efficient core@shell catalysts with tunable chemical composition8-18. Bimetallic nanoparticles exhibit distinctively different physico-chemical properties in comparison to their monometallic counterparts. Specifically, gold-palladium (Au-Pd) catalysts have displayed significantly increased activity in a number of chemical reactions, such as the oxidation of CO19, 20, the acetoxylation of ethylene to vinyl acetate21, 22, the direct synthesis of hydrogen peroxide from H2 and O223,

24

, the epoxidation of alkene25, and the

selective oxidation of alcohol to aldehyde26-28. The morphology of bimetallic nanoparticles depends on the synthesis technique, post-synthesis heat treatment, and the solubility of both metals. Due to unlimited solubility in Au-Pd system the atomic distribution within nanoparticles may range from homogeneous alloys to so-called core@shell structures. Commonly used synthesis techniques such as under potential deposition (UPD) enables the fabrication of a uniform shell on the core (from monolayer up to multilayer in thickness) via careful potential control29-33. Amongst a variety of Au/Pd core@shell structures Silva et al.10 3 ACS Paragon Plus Environment

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have recently shown that Au cores with a monolayer of Pd exhibit the highest catalytic activity in selective oxidation of benzyl alcohol. By means of combined experimental and theoretical approaches they observed the volcano-like behavior in terms of a variable Au:Pd molar ratio. An increased catalytic activity was attributed to the balance between the number of active sites and the ease of product desorption. The superiority of core@monolayer-shell catalysts over uniform catalysts was also demonstrated by Zhang and Henkelman in their recent study11. Based solely on adsorbates binding energies computed within density functional theory and the linear correlation between the binding of adsorbates to the shell vs alloy-core composition, they proposed a simple approach to the design of a catalyst for a particular reaction. In the current study, we analyzed the crucial molecules involved in the selective partial oxidation of methanol to MF, interacting with Au(111), Pd(111), Au/Pd(111) and Pd/Au(111) surfaces, by means of ab initio calculations. Interesting trends in adsorption energies for various molecules as a function of catalytic surface have been found, and subsequently discussed in term of the electronic properties of the surfaces. Further, our study demonstrates the impact of preadsorbed oxygen on the binding of adsorbates to the surfaces. Finally, using the dynamical matrix approach combined with statistical thermodynamic, we developed a simple route to evaluate the Gibbs free energy of adsorption, and hence the temperature stability of adsorbate species.

THEORETICAL METHODS In the slab-supercell approximation, the adsorption energy of molecule on a solid surface can be simply expressed as follows: = + +

(1)

The first term: 4 ACS Paragon Plus Environment

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= / − +

(2)

denotes the electronic contribution to the adsorption energy, computed as a difference between the total energy of slab with adsorbed molecule and the total energy of bare slab plus the total energy of isolated molecule in a gas phase. The second term is the dispersion energy correction, which is computed using a pairwise potential defined as:

' = − ∑' ) ∑( ∑&

where

-.)(

represents

the

)(,& *+ , !" ,$ %

interatomic

(3)

(dipole-dipole)

dispersion

coefficient,

, )(,& = |, ), − , (,& |, where *+ , )(,& is a damping function, used to cancel spurious interaction at short distances within the overlapping electron densities regime. The summation is over all atoms 0 and all translations of the unit cell 1 = 2 , 2 , 23 , under assumption that

4 ≠ 6 for 1 = 0. A detailed explanation of the dispersion energy correction and its evaluation can be found elsewhere34. The last term of the equation (1) is the zero-point energy, which

can be calculated in the harmonic approximation:

= ∑; = + ?B -@ CD

(5)

where the heat capacity in the harmonic approximation can be expanded in series: 5 ACS Paragon Plus Environment

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∗

?B -@ CD = ∑; − DP

(10)

and hence the Gibbs free energy of adsorption: ∆h = h − h:

j

(11)

j denotes the Gibbs free energy of an ideal gas, which is reasonable estimation where h:

within low pressure regime.

COMPUTATIONAL DETAILS


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Electronic structure calculations were performed in the framework of density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP)36. The exchange-correlation energy was computed using Perdew-Burke-Ernzerhof (PBE) general gradient approximation (GGA) functional37. The projector augmented wave (PAW) method38 was employed to describe valence-core interaction. The valence electron configuration for each element was: (1s1) H, (2s22p2) C, (2s22p4) O, (5d106s1) Au and (4d95s1) Pd. An impact of nonlocal long-range interactions between adsorbed molecules and metal surfaces on the adsorption energy was verified a posteriori according to the vdW correction scheme proposed by Tkatchenko and Scheffler (DFT-TS)39. Dipole correction was considered for all calculations40. In order to simulate individual CH3OH, CH3O, CH2OH, CH3OOCH2, HCHO and CH3COOH molecules on Au(111), Pd(111), Au/Pd(111) and Pd/Au(111) surfaces, p(3 x 3) slab geometries, corresponding to a 1/9 ML coverage, containing 4 metal layers were constructed from the optimized bulk unit cell of gold and palladium. In this work Au/Pd(111) and Pd/Au(111) denote an Au(111) surface with a monolayer of Pd shell and a Pd(111) surface with a monolayer of Au shell, respectively. Our model represents the most stable (111) facets of large core@shell nanoparticles41, 42. The slab approach was widely used to model physicochemical properties of nanoparticles, such as binding energy of adsorbates, surface reactivity as a function of chemical composition, concentration and geometry43-47. In comparison to cluster approach which is computationally demanding and tractable only for relatively small nanoparticles, the slab representation gives the possibility of studying variety of structures in a systematic way. In addition, there are few studies published that compare adsorption energies calculated using cluster and slab approaches. For instance, the adsorption energy of NO on Pd79 nanoclusters turned out to be 9% higher than on Pd(111) slab48. Another investigations report the adsorption energies of C atoms on Pd79 nanocluster to be 7 ACS Paragon Plus Environment

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only 4% different than on Pd(111) slab49, 50. Since the typical size of metallic nanoparticles is tens of nanometers the difference in the adsorption energy ought to be even smaller; therefore, the use of slab approach is justified. A vacuum level of ~15 Å was large enough to screen the spurious interaction between periodic images of the surface. Convergence of the total energy for the slab-molecule system was assured by setting a 450 eV cutoff energy, and the Brillouin zone sampling in a 5x5x1 Monkhorst-Pack k-point grid51. For geometry optimization, the top 2 layers of metal atoms together with the molecule were allowed to relax towards a minimum of the total energy, while the bottom 2 layers were kept frozen in their lattice positions. After all forces acting on atoms dropped below 0.01 eV/Å and energy convergence criterion of 1e5 eV was reached, the relaxations were terminated. Vibrational modes and frequencies were provided by diagonalization of the Hessian matrix using the dynamical matrix method. To quantify charge transfer from the (111) surfaces to the molecules, a Bader analysis was carried out for all conformations using the code developed by Henkelman et al.52. Gas-phase structures of the molecules and their vibrational properties were calculated in a simulation box (20x20x20 Å) applying the gamma k-point sampling. The procedure presented in Section 2 was implemented in Fortran 95 environment and subsequently used to determine Gibbs free energies of adsorption.

RESULTS AND DISCUSSION Analysis of Surface Species The adsorption geometries and energies of crucial intermediates involved in the selective partial oxidation of methanol to MF on Au(111), Pd(111), Au/Pd(111) and Pd/Au(111) are investigated in this section. The most stable conformations are displayed in Figure 1 and the adsorption energies together with geometrical parameters are listed in the Supporting Information (see Table S1). Apart from alkoxy hemiacetal (CH3OOCH2), the most stable 8 ACS Paragon Plus Environment

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adsorption site for a given molecule and the corresponding molecular orientation are roughly the same for each surface. The bond lengths and adsorption energies, however, depend strongly upon the chemical composition of the surface.

Figure 1. Optimized geometries of the most stable adsorption species involved in the selective partial oxidation of methanol to MF on Au(111), Pd(111) and core@shell Au/Pd(111), Pd/Au(111) catalysts. Table 1. Bader charges for possible intermediates of methanol selective partial oxidation to MF adsorbed on the Au(111), Pd(111), Au/Pd(111) and Pd/Au(111). The values in parentheses denote oxygen preadsorbed surfaces.

Molecule CH3OH CH2OH CH3O HCHO CH3OOCH2 CH3COOH

Pd/Au(111) 0.014 (0.104) 0.144 (0.288) -0.423 (-0.351) 0.003 (0.043) -0.411 (-0.379) -0.005 (0.011)

Bader charge of the moiety on the Au(111) Pt(111) 0.026 (0.108) 0.050 (0.081) 0.135 (0.282) 0.099 (0.174) -0.429 (-0.410) -0.372 (-0.359) 0.013 (0.044) 0.021 (0.021) -0.438 (-0.409) -0.390 (-0.368) -0.005 (-0.003) -0.006 (-0.002)

Au/Pd(111) 0.040 (0.079) 0.082 (0.170) -0.398 (-0.383) 0.010 (0.031) -0.413 (-0.418) -0.001 (-0.003)


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CH3OH. Relaxation of the gas-phase methanol leads to equilibrium bond lengths equal to 1.10 Å for C–H, 1.43 Å for O–C, and 0.97 Å for O–H, which is in good agreement with the experimental values of 1.09 Å, 1.43 Å and 0.95 Å, respectively53. According to previous investigations54-56, the CH3OH adsorption on metallic surface is realized by donation of the lone-pair electrons from oxygen to the surface atoms, forming a weakly bound moiety, unstable upon increasing temperature of system. Slightly positive Bader charges of CH3OH molecule adsorbed on the examined surfaces, presented in Table 1, support the donation nature of adsorption. Our calculations reveal that methanol is bound to the surface top site through its oxygen atom, with a decreasing M-O distance of 2.95 Å, 2.77 Å, 2.37 Å and 2.35 Å for Pd/Au(111), Au(111), Pd(111) and Au/Pd(111), respectively. The corresponding adsorption energies of -0.30 eV, -0.36 eV, -0.48 eV, and -0.53 eV follow the same trend (see Figure 2 and Supporting Information) and agree well with available experimental data of 0.42 eV for Au(111)57 and -0.38÷-058 eV for Pd(111)58. Previous theoretical works report the adsorption energy of CH3OH on Au(111) to be -0.15 eV59 and -0.17 eV3. The significant underestimation may be explained by negligence of long-range van der Waals interactions, which have a remarkable, over 60%, contribution to the binding energy.

CH2OH. A hydroxymethyl radical is the intermediate formed by the β-H elimination of the adsorbed methanol. It has a nonplanar structure with the most stable conformation at a top site, bound to the surface through the carbon atom. Due to the open-shell electronic configuration of CH2OH, its adsorption energy is relatively high, -1.66 eV, -1.81 eV, 2.40 eV, and -2.42 eV for Pd/Au(111), Au(111), Pd(111) and Au/Pd(111), respectively. The corresponding M-C distances are computed to be 2.19 Å, 2.17 Å, 2.06 Å and 2.04 Å. Positive Bader charges, quantitatively larger for CH2OH than CH3OH, indicate that the delocalized


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radical electron in a π*orbital can be effectively transferred from the molecule to the metal atoms.

CH3O. A methoxy radical is formed by the α-H elimination of the adsorbed methanol molecule. The CH3O prefers to adsorb at the three-fold fcc site through its oxygen atom, with the C–O axis nearly perpendicular to the surface. The chemical bond formed between molecule and surface incorporates 3 metal atoms adjacent to the hollow site. As opposed to CH2OH, methoxy acts as an electron acceptor (see Table 1), with the computed Bader charges of ~ -0.4 eV. The adsorption energies of -1.29 eV, -1.86 eV, -2.35 eV, and -2.57 eV for Pd/Au(111), Au(111), Pd(111) and Au/Pd(111), respectively and small van der Waals energy contributions indicate rather strong chemisorption.

HCHO. There are two possibilities of formaldehyde (HCHO) formation during the selective partial oxidation of methanol to MF: first is the α-H elimination of the adsorbed CH2OH or second is the β-H elimination of the adsorbed CH3O. Our results reveal a weak interaction between HCHO and the examined surfaces, consistently with previous theoretical predictions1, 3. In case of Au-terminated surfaces the adsorption has predominantly physical nature, whereas for Pd-terminated surfaces the contribution of dispersive and electronic energy is comparable. Formaldehyde preferentially binds to the bridge site through oxygen and hydrogen atoms, and its molecular plane is perpendicular to the surface. The calculated adsorption energies are -0.23 eV, -0.27 eV, -0.36 eV, and -0.42 eV for Pd/Au(111), Au(111), Pd(111) and Au/Pd(111), respectively. The distance between molecule and the nearest surface atom decreases significantly from ~3.00 Å (Au-termination) to ~2.27 Å (Pd-termination).


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CH3OOCH2. An alkoxy hemiacetal intermediate is formed as a result of the coupling reaction between electron-rich oxygen in methoxy and electron-deficient carbon in formaldehyde. Two distinguishable adsorption sites, dependent on the surface termination have been found. For Au-terminated surfaces, CH3OOCH2 binds to the bridge site through two of its oxygen atoms, whereas for Pd-terminated surfaces, the most stable adsorption site is the three-fold fcc hollow with only one oxygen involved. Negative Bader charges indicate a charge transfer from surface to the molecule. The calculated adsorption energies of 1.56 eV, -1.75 eV, -2.31 eV, and -2.53 eV for Pd/Au(111), Au(111), Pd(111) and Au/Pd(111), respectively suggest a strong chemisorption.

CH3COOH. The final product, methyl formate, is created by abstraction of a hydrogen atom from CH3OOCH2. As can be seen in Figure 2 the interaction between MF and the examined surfaces is very weak and almost entirely governed by the van der Waals forces. The most stable conformation was found at the bridge site through oxygen and hydrogen atoms, with large (3.29–3.90 Å) distance between molecule and the nearest surface atom. This conformation affords an adsorption energies of -0.24 eV, -0.27 eV, -0.29 eV, and -0.36 eV for Pd/Au(111), Au(111), Pd(111) and Au/Pd(111), respectively.


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Figure 2. Adsorption energies of molecules involved in the selective partial oxidation of methanol to MF on clean and preadsorbed oxygen Au(111), Pd(111), Au/Pd(111) and Pd/Au(111) surfaces. Impact of vdW and ZPE Corrections on the Adsorption Energy In order to predict the adsorption energy of molecules on solid surfaces at 0 K more accurately, one has to take into account both, the nonlocal van der Waals interactions and the zero point energy contribution. As shown in our previous work60 the incorporation of van der Waals energy may even determine stability of molecular adsorption by changing its character from endothermic to exothermic. The electronic, vdW and ZPE contributions to the adsorption energy of crucial intermediates involved in the selective partial oxidation of methanol to MF on Au(111), Pd(111), Au/Pd(111) and Pd/Au(111) are juxtaposed together in Figure 2. In the case of weakly bound species (CH3OH, HCHO, CH3COOH) the conventional 13 ACS Paragon Plus Environment

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DFT values of the adsorption energy are significantly underestimated; therefore, the vdW+ZPE term has to be taken into account to reflect experimental measurements. By contrast, strongly bound intermediates (CH2OH, CH3O, CH3OOCH2) are only slightly affected by the vdW+ZPE contribution. Their adsorption energy can be predicted fairy well using the conventional DFT approach exclusively.

Binding Energy Trends vs Intrinsic Properties of Surfaces Trends in the binding energy of six crucial molecules involved in the selective partial oxidation of methanol to MF were disclosed as a function of surface chemical composition. As can be seen in Figure 2, for a given molecule the adsorption energy with respect to the catalyst surface decreases as follows: Au/Pd(111) > Pd(111) > Au(111) > Pd/Au(111). In order to clarify relative binding properties of the surfaces examined, the d-projected density of states (d-DOS) together with the d-band center were depicted in Figure 3. The presented trends correlate well with the increasing d-band center of the top surface layer, defined as:

k =

? l ? l

(12)

where m is the d-DOS. For the core@shell catalysts there are two major factors affecting the d-band center: strain induced within the shell, derived from the core-shell mismatch, and charge redistribution11. The higher the difference in atomic radius of core and shell atoms the larger mismatch, and therefore the impact of strain on the d-band center shift is more pronounced. In turn, charge redistribution is a function of electronegativity difference between elements in the core and shell. The variation of Bader charges computed for the surface atoms (CX/SX vs CY/SX where X and Y are elements and C and S denotes core and shell, respectively) are negligibly small (0.01 - 0.04 e). The small charge transfer results from similar electronegativity of Pd and Au. Based on this observation we anticipate that the d-


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band center shift is dominated by the strain effect. Similar conclusions were drawn for PdAu@Pt catalyst by Zhang and Henkelman11.

Figure 3. The d-projected density of states (d-DOS) together with the d-band center calculated for the top layer of Au(111), Pd(111), Au/Pd(111) and Pd/Au(111) bare surfaces. Total charges per atom of first and second layer of each slab are shown on the right side.

Effect of Preadsorbed Oxygen on Adsorption It is commonly known that preadsorbed oxygen on noble metal surfaces significantly influences both the reaction barriers and selectivity in catalytic oxidation of alcohols1, 3, 61. For this reason, an impact of preadsorbed oxygen on the adsorption of molecules involved in the selective partial oxidation of methanol to MF has to be highlighted. According to previous 15 ACS Paragon Plus Environment

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theoretical data, atomic oxygen prefers binding at the fcc sites with very high adsorption energy. Our calculations provide oxygen adsorption energies of -2.74 eV, -3.47 eV, -4.68 eV and -4.73 eV for Pd/Au(111), Au(111), Pd(111) and Au/Pd(111), respectively, confirming very strong, exothermic chemisorption. Adsorption energies of CH3OH, CH3O, CH2OH, CH3OOCH2, HCHO and CH3COOH on the oxygen precovered Pd/Au(111), Au(111), Pd(111) and Au/Pd(111) surfaces and corresponding geometries are presented in Figure 2 and Figure S1 (Supporting Information). Depends on molecule, the presence of atomic oxygen on catalyst surface may either improve or deteriorate the adsorption strength. A remarkable enhancement of binding energy is observed for the weakly bound CH3OH and HCHO. In the case of methanol, the reported adsorption energy gain is ~0.2 eV for Au-terminated surfaces and ~0.1 eV for Pd-terminated surfaces, whereas in the case of formaldehyde the values are ~0.15 eV and ~0.1 eV, respectively. In both cases, the creation of a hydrogen bond is responsible for additional stabilization effect manifesting itself in energy gain. From the point of view of catalytic performance, the additional preadsorbed oxygen stabilization effect will simultaneously intensify the CH3OH adsorption and hamper the HCHO desorption from the catalyst surface. In particular, the latter phenomenon plays an essential role in selectivity to MF, since the intense desorption of HCHO suppresses its coupling reaction with CH3O, which in turn, leads to the formation of CH3OOCH2 and subsequently MF. Furthermore, the adsorbed oxygen species act as an electron reservoir, and as can be seen in Bader analysis results (see Table 1), promote the charge transfer from moiety to the surface, increasing activation of chemical bonds in the molecule. This may explain the significant reduction of elemental reaction energy barriers, as observed for the selective partial oxidation of CH3OH using oxygen precovered conditions. The strongly chemisorbed species, such as CH2OH, CH3O or CH3OOCH2 are however only slightly affected by the presence of preadsorbed oxygen atoms. Basically, their binding energy is slightly lower in comparison to counterparts 16 ACS Paragon Plus Environment

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on clean surfaces. A positive effect of preadsorbed oxygen on catalytic performance in the selective partial oxidation of methanol to MF is also visible for CH3COOH. Due to the reduction in adsorption energies the ease of product (MF) desorption should be accordingly improved.

Thermodynamic Stability of Adsorbates In the previous sections we discussed the adsorption of CH3OH, CH3O, CH2OH, CH3OOCH2, HCHO and CH3COOH on Pd/Au(111), Au(111), Pd(111) and Au/Pd(111), taking into account electronic, dispersive and ZPE energy contributions. In principle, we assumed that molecules have in the gas phase 100% of their standard entropy, whereas adsorbates possess no entropy at all. These assumptions are meaningful within a low temperature regime but may fail remarkably as the temperature rises. To provide insight into the thermodynamic stability of adsorbates involved in the selective partial oxidation of methanol to MF, we calculated the Gibbs free energy changes as a function of temperature, consistent with the procedure described in Section 2. The results are presented in Figure 4, with dotted vertical lines indicating the temperature at which desorption occurs. The temperature stability of methanol, formaldehyde, and MF adsorbed on the surfaces examined are in reasonable agreement with available experimental data57, 62-64; nevertheless, there is one problem with the TPD results which needs to be highlighted. To be meaningful, weakly bound CH3OH, HCHO, and CH3COOH have to be detectable using the TPD experiment. In the case of adsorbate high-coverages the TPD peaks are high and sharp; therefore, they can be easily attributed to specific temperatures. However, as the coverage decreases well below 1 ML, the TPD peaks become low and smeared, making it difficult to unambiguously interpret. Since our calculations concern very low (~0.11 ML) coverage, the comparison 17 ACS Paragon Plus Environment

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between the computed and the TPD data is qualitative rather than quantitative. The predicted temperature stability of strongly chemisorbed species on a given surface decreases as follow: CH3OOCH2 > CH2OH > CH3O. This order was determined under the assumption that only adsorption and desorption processes exist. In reality, however, decomposition of big molecules such as CH3OOCH2 may occur at elevated temperatures, changing the relative stability.

Figure 4. The Gibbs free energies of adsorption as a function of temperature calculated for molecules involved in the selective partial oxidation of methanol to MF on Au(111), Pd(111) and core@shell Au/Pd(111), Pd/Au(111) catalysts. Thermodynamic results for oxygen preadsorbed conditions are presented in Figure 5, in a similar manner. In comparison to the adsorption on bare surfaces, desorption temperature for weakly bound CH3OH and HCHO is shifted to higher values due to the additional stabilization effect via hydrogen bond. As we anticipated in previous paragraph, the preadsorbed oxygen intensify MF desorption by lowering desorption temperature. It is worth 18 ACS Paragon Plus Environment

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mentioning that relative stability of molecular species as a function of catalytic surface changes with respect to temperature for the oxygen preadsorbed conditions. The main factor causing the changes arises from the entropy contribution, which in turn depends on the ground state geometry of adsorbate. As can be seen in the Supporting Information (see Figure S1), the ground state adsorption geometry for a given molecule can differ remarkably when the atomic oxygen is preadsorbed on the catalytic surface.

Figure 5. The Gibbs free energies of adsorption as a function of temperature calculated for molecules involved in the selective partial oxidation of methanol to MF on oxygen preadsorbed Au(111), Pd(111) and core@shell Au/Pd(111), Pd/Au(111) catalysts.

CONCLUSIONS Systematic DFT calculations have been employed to investigate the interaction of crucial molecules involved in the selective partial oxidation of methanol to MF, with monometallic 19 ACS Paragon Plus Environment

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Au, Pd and bimetallic Au/Pd, Pd/Au core@shell catalysts. We have found that for a given molecule the adsorption energy with respect to the catalyst surface decreases in the order of Au/Pd(111) > Pd(111) > Au(111) > Pd/Au(111) and correlate well with the d-band center of the top surface layer. The oxygen preadsorbed on the surfaces examined can either stabilize weakly bound molecules (CH3OH and HCHO) by forming the hydrogen bonds, slightly decrease the adsorption strength of strongly chemisorbed species (CH3O, CH2OH and CH3OOCH2) or destabilize the weakly adsorbed CH3COOH, promoting its desorption. These results partially explain the huge impact of oxygen species on the selectivity and reaction yield observed previously for gold catalysts. Furthermore, we proposed simple yet robust route to evaluate the Gibbs free energy of adsorption, which may be useful in the interpretation of TPD data in particular for very low surface coverage. Test calculations carried out for the surfaces examined showed reasonable agreement with available experimental TPD results. Further investigation of the reaction barriers and effect of tuning the chemical composition of core and shell will be interesting in terms of maximizing the catalytic activity towards methanol selective oxidation to MF.

AUTHOR INFORMATION Corresponding Author * E-mail address: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS 20 ACS Paragon Plus Environment

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Computing resources were provided by High Performance Computing facilities of the Interdisciplinary Centre for Mathematical and Computational Modeling (ICM) of the University of Warsaw under Grant No. G61-4 and Poznań Supercomputing and Networking Center (PSNC) under Grant No. 275. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via Internet at http://pubs.acs.org. Detailed geometries and adsorption energies Ead of stable adsorption species involved in the selective partial oxidation of methanol to MF on clean and oxygen precovered Au(111), Pd(111), Au/Pd(111) and Pd/Au(111).

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TOC GRAPHIC


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Figure 1. Optimized geometries of the most stable adsorption species involved in the selective partial oxidation of methanol to MF on Au(111), Pd(111) and core@shell Au/Pd(111), Pd/Au(111) catalysts. 190x204mm (300 x 300 DPI)

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Figure 2. Adsorption energies of molecules involved in the selective partial oxidation of methanol to MF on clean and preadsorbed oxygen Au(111), Pd(111), Au/Pd(111) and Pd/Au(111) surfaces. 140x193mm (300 x 300 DPI)


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Figure 3. The d-projected density of states (d-DOS) together with the d-band center calculated for the top layer of Au(111), Pd(111), Au/Pd(111) and Pd/Au(111) bare surfaces. Total charges per atom of first and second layer of each slab are shown on the right side. 190x196mm (300 x 300 DPI)


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Figure 4. The Gibbs free energies of adsorption as a function of temperature calculated for molecules involved in the selective partial oxidation of methanol to MF on Au(111), Pd(111) and core@shell Au/Pd(111), Pd/Au(111) catalysts. 189x129mm (300 x 300 DPI)


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Figure 5. The Gibbs free energies of adsorption as a function of temperature calculated for molecules involved in the selective partial oxidation of methanol to MF on oxygen preadsorbed Au(111), Pd(111) and core@shell Au/Pd(111), Pd/Au(111) catalysts. 189x132mm (300 x 300 DPI)


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TOC Graphic 47x26mm (600 x 600 DPI)


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Insight on the Interaction of Methanol-Selective ... - ACS Publications

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