Effect of surface vacancies on the adsorption of Pd and Pb on MgO(100)

Effect of surface vacancies on the adsorption of Pd and Pb on MgO(100) Theoretical quantum mechanical calculations have been carried out to establish the effect of surface vacancies on the adsorption of Pd and Pb atoms on the defective MgO(100) surface. The investigated defects included neutral, singly and n? n- doubly charged O and Mg vacancies on the (100) surface of MgO. These vacancies played the role of F and V (n = 0, s s 1, 2) adsorption centers for a single Pd or Pb atom. From the results of calculations, it is clear that the Pd- and Pb-atom n? n- 2- 2? adsorption at the F and V centers shows different characteristics than at the regular O and Mg centers. Drastic s s n- n- 0 ? changes in geometric, energetic, and electronic parameters are evident in Pd/V and Pb/V . The effect of F and F , s s s s which in practice are the most important vacancies, is smaller, yet the adsorption of Pd and Pb at these centers is more 2- 0 ? energetically favorable than at the regular O center. Of the two metals studied, the atom of Pd is bound by the F and F s s centers with higher adsorption energies. Graphical abstract Keywords Quantum chemical calculations  Metals  Surface  Heterogeneous catalysis  Defects Introduction coupling their Pd part with another metallic element [4, 5]. In the resulting bimetallic Pd-M/oxide catalysts, Pd is Palladium supported on oxides has found numerous usually combined with a typical metal or a half metal applications in heterogeneous catalysis [1–3]. The catalytic (M=Al, Si, Zn, Ga, Ge, In, Sn, Sb, Te, Tl, Pb, or Bi) [5]. performance of Pd/oxide systems can be improved by Recently, it has been reported that bimetallic Pd–Pb/MgO catalysts are more effective than monometallic Pd/MgO catalysts in performing aerobic oxidations of amines [6] Electronic supplementary material The online version of this article and oxidative esterification of methacrolein with methanol (https://doi.org/10.1007/s00706-018-2159-1) contains supplementary material, which is available to authorized users. [7, 8]. Understanding the enhanced catalytic performance of these bimetallic catalysts requires a detailed knowledge & Piotr Matczak of several fundamental aspects of their metal-oxide inter- piotr.matczak@chemia.uni.lodz.pl; p.a.matczak@gmail.com faces. These aspects include, in particular, geometric and electronic features of interfaces and the strength of metal- Department of Physical Chemistry, Faculty of Chemistry, ´ ´ University of Łodz, Pomorska 163/165, 90-236 Lodz, Poland 123 1010 P. Matczak n- oxide interaction. Ideally, the first step of such a charac- and V centers has been calculated to characterize the terization should concern small clusters of Pd and Pb, or fundamental aspects of Pd- and Pb-atom adsorption on even better single Pd and Pb atoms, at individual adsorp- defective MgO(100). Due to the lack of any previous the- tion sites on a well-defined single-crystal MgO surface, oretical studies for Pb/MgO(100), it is vitally important to such as the MgO(100) one. This surface is often regarded provide an insight into the effect of surface vacancies on as a prototypical oxide surface in studies of metal Pb-atom adsorption at atomic level. adsorption, because it has a simple structure and well-de- fined stoichiometry [9]. Additionally, it is relatively easy to form defects on this surface [10]. Results and discussion Various experimental techniques can yield information on the structure and energetics of metal nanoparticles and The calculated values of three essential parameters (height films deposited on oxides [11] and many experimental from the surface h, adsorption energy E and electron ads efforts have indeed been undertaken to characterize both charge q) characterizing a single Pd atom adsorbed at the n? n- Pd/MgO(100) [12–17] and Pb/MgO(100) [18–22]. On the F and V centers on the defective MgO(100) surface s s other hand, properties of single atoms adsorbed on oxide are listed in Table 1. It is evident that the Pd-atom n? surfaces are available mostly from theoretical investiga- adsorption at the F centers is far different from that at the n- tions based on computational quantum mechanical V centers. The heights of the adsorbed Pd atom from the n? n- approaches [23]. Of Pd and Pb on MgO(100), only the F centers are much greater than the h values of Pd/V s s former has become a subject for a large number of theo- structures. The Pd atom is almost inserted into the vacancy 0 - retical studies of single metal atom adsorption so far cavity for Pd/V and Pd/V in their high-spin (HS) states. s s n- [15, 24–37]. To the best of our knowledge, no theoretical The smaller h values of Pd/V are accompanied by the quantum mechanical investigations of Pb/MgO(100) have highly exothermic E energies. The comparison of E ads ads been reported until now. for the low-spin (LS) and HS states reveals that, with the 2- n? This work is aimed at providing a theoretical quantum possible exception of Pd/V , all remaining Pd/F and s s n- mechanical description for the adsorption of Pd and Pb on Pd/V structures prefer their LS state. In other words, the the MgO(100) surface with various point defects. Both adsorbed Pd atom tends to conserve its spin state (the n? n- oxygen (F ) and magnesium (V ) vacancies in three ground state of free Pd atom exhibits a singlet multiplicity). s s charge states (n = 0, 1, 2) have been taken into account. The preference of LS state is particularly noticeable for Pd/ From experimental studies [38, 39], it is known that such F because its HS state does not lead to any energetic 2- defects may be formed on MgO(100), but with a significant stabilization. In the case of Pd/V , its LH and HS states differentiation in their concentrations. What is particularly lie very close to each other and it is difficult to appoint the important is that various defects occurring on the ground state with absolute certainty. The E energy shows ads MgO(100) surface can act as anchoring sites for metal a clear dependence on the formal charge (n) of vacancies. n? nanoparticles [38, 40], and additionally, they can modify The Pd-atom adsorption at F becomes less and less 0 ? 2? the properties of deposited metal nanoparticles [38, 41]. exothermic in the order F [ F [ F . The same s s s n- Here, a set of essential geometric, energetic and electronic sequence can be observed for Pd/V . The values of q ac- n? parameters for a single Pd or Pb atom adsorbed at the F quired by the adsorbed Pd atom indicate that it behaves as Table 1 Essential parameters characterizing the adsorption of a single Pd atom at various centers on the defective MgO(100) surface LS HS LS HS LS HS ˚ ˚ Center h /A h /A E /eV E /eV q /e q /e ads ads F 1.540 (1.539) 1.708 (1.726) 3.85 (3.78) 1.65 (1.57) - 1.521 (- 1.525) - 1.536 (- 1.522) F 1.504 (1.503) 2.53 (2.45) - 0.871 (- 0.873) 2? F 1.509 (1.505) 1.706 (1.713) 1.37 (1.27) 0.29 (0.26) - 0.261 (- 0.264) - 0.239 (- 0.234) V 0.383 (0.382) 0.160 (0.160) 7.58 (7.59) 7.29 (7.31) 0.795 (0.810) 0.888 (0.894) V 0.601 (0.589) 0.157 (0.133) 5.42 (5.40) 4.63 (4.98) 0.437 (0.448) 0.880 (0.893) 2- V 0.366 (0.331) 0.584 (0.538) 4.39 (4.70) 4.52 (4.66) 0.799 (0.782) 0.427 (0.433) 2- O 2.165 (2.148) 2.351 1.34 (1.30) 0.28 - 0.231 (- 0.233) - 0.193 2? Mg 2.633 (2.636) 0.47 (0.39) - 0.075 (- 0.082) Results obtained from calculations in which the Pd atom was described by the LANL08(f) basis set are listed without parentheses, whereas the results from calculations utilizing the def2-TZVP basis set for Pd are in parentheses Results for centers with the unbound Pd atom (E \ 0) are not presented ads 123 Effect of surface vacancies on the adsorption of Pd and Pb on MgO(100) 1011 n? an electron acceptor when it sits at the F centers. The Pd/ experimental estimation of adsorption energy for Pd on F structure demonstrates the greatest charge transfer to the MgO(100) is ca. 1.2 eV [14]. From an experimental mea- metal atom. This is because the isolated F center possesses surement, a value of 2.22 A was also deduced to be the 2- two extra electrons that are largely localized in its cavity height of an adsorbed Pd atom from the O center [26]. LS LS 2- [42] and a significant amount of this electron charge can be Our E and h values for Pd/O are very close to these ads n? easily transferred to an adsorbed atom [34]. Unlike Pd/F , experimental estimations. Similarly to metal adsorption on n- the Pd/V structures show the opposite direction of charge the defect-free MgO(100) surface, metal atoms on transfer. Our calculations predict that the charge transfer MgO(100) with defects also adsorb preferentially at centers n- from the Pd atom to the V centers never exceeds 0.9 e where negative charge accumulates [33, 44]. More n- 0 even if the Pd atom is almost inserted into the V cavity. specifically, the F centers play the key role in the s s This charge transfer and the small h heights lead to a sig- adsorption of Pd atoms [15, 16]. This is because these nificant electrostatic stabilization between the ionized Pd centers are the main part of vacancies formed on n- atom and the V centers. MgO(100), which was confirmed both experimentally [10] 0 ? It is instructive to compare the Pd-atom adsorption at the and theoretically [45, 46]. Besides the F centers, the F s s vacancies with that occurring at non-defective sites. centers can also occur, but they are less likely due to their Results describing the adsorption of a single Pd atom at the large formation energy [42]. Even larger formation energy 2- 2? 2? regular anionic O and cationic Mg centers of defect- was determined for the F center [42]. Previous compu- free MgO(100) surface are appended to Table 1. As evi- tational studies have shown that the Pd/F interaction is 2- 2? 0 denced by the E values of Pd/O and Pd/Mg , the Pd weaker than the Pd/F interaction but stronger than that of ads s 2- 2- atom binds preferentially to the O center in the LS state. Pd/O [15, 33, 36]. Apart from rendering this trend cor- A small charge transfer to the metal atom appears for Pd/ rectly, our h and E values also reproduce quantitatively ads 2- O , while the Pd atom remains essentially neutral at the other theoretical results [15, 35, 36]. It has also been 2? 2- n- Mg center. The Pd-atom adsorption at O is less reported that the interaction between Pd and V centers is 0 ? 2- energetically favorable than at F and F . The Pd/O extremely strong [24]. According to an experimental study s s structure also demonstrates a larger h value compared to [10], the concentration of surface Mg vacancies seems, 0 ? 2- 0 ? those of Pd/F and Pd/F . On the other hand, the Pd/O however, to be much lower than that of F and F . Again, s s s s 2? n- and Pd/F structures are formed with very similar E this is in line with large formation energies of V s ads s energies, although the former exhibits a much larger vacancies [42, 47]. h value. This review of existing results for Pd/MgO clearly An inspection of the results in Table 1 also reveals that indicates that the computational methodology applied in the kind of the basis set assigned to the Pd atom most often this work leads to the correct description of Pd-atom has a rather minor effect on the calculated values of h, E adsorption on MgO(100) with surface vacancies. Thus, one ads and q. A discrepancy in the interpretation of the results can expect that the parameters characterizing the adsorp- n? n- obtained from LANL08(f) and def2-TZVP appears for Pd/ tion of Pb atom at the F and V centers should also be s s 2- 2- V and Pd/O . The calculations employing the two basis predicted reliably. n? sets designate different spin states as the energetically Essential parameters for the Pb atom adsorbed at the F 2- 2- n- preferred state of Pd/V . In the case of Pd/O in the HS and V centers are collected in Table 2. A careful s s state, the calculations involving the LANL08(f) basis set inspection of these results reveals that there are several predict an exothermic adsorption, in contrast to those car- similarities between the Pd-atom adsorption and its Pb HS n- ried out with def2-TZVP. However, the E value obtained counterpart. The formation of Pb/V structures is asso- ads s from LANL08(f) is actually quite close to zero, and ciated with extremely exothermic E values, many times ads n? therefore, the significance of this discrepancy should not be greater than those calculated for the Pb/F structures. For n? overemphasized. Pb/F , their E energies decrease regularly with the s ads n? Our findings made for the Pd-atom adsorption are growing formal charge of F center. The adsorption of Pb n- essentially in good agreement with conclusions reported in at V leads to a significant charge transfer from Pb to the n- previous experimental [14–16] and theoretical V centers, while the reverse direction of charge transfer 0 ? [15, 24, 26–28, 33, 35, 36] studies of Pd/MgO(100). It is is observed for Pb/F and Pb/F . A strong correlation s s well-known that the defect-free MgO(100) surface is gen- between E and the magnitude of charge transfer can be ads n? n? erally rather unreactive toward the adsorption of metal found for both the Pd/F and Pb/F structures. On the s s 2? atoms [43]. The Mg centers exhibit particularly low other hand, the Pb-atom adsorption turns out to be different reactivity toward metal atoms [25]. In consequence, Pd in certain aspects from the Pd-atom adsorption. First, the 2- atoms preferably occupy the O centers [28], with no large atomic radius of Pb causes this atom not to replace n- significant charge transfer from or to the surface [27]. An the missing Mg atom at the V cavity. The h values of Pb/ 123 1012 P. Matczak Table 2 Essential parameters characterizing the adsorption of a single Pb atom at various centers on the defective MgO(100) surface LS HS LS HS LS HS ˚ ˚ Center h /A h /A E /eV E /eV q /e q /e ads ads F 2.395 (2.331) 2.368 (2.340) 1.45 (1.66) 2.16 (2.33) - 1.361 (- 1.363) - 1.365 (- 1.376) F 2.464 (2.422) 2.300 (2.282) 1.19 (1.32) 1.24 (1.37) - 0.610 (- 0.630) - 0.640 (- 0.660) 2? F 2.671 (2.612) 2.797 (2.737) 0.65 (0.73) 0.81 (0.86) 0.172 (0.134) 0.162 (0.129) V 1.028 (1.105) 0.606 (0.728) 9.55 (9.24) 5.85 (5.38) 1.226 (1.211) 1.215 (1.429) V 1.005 (1.056) 0.571 (0.675) 7.04 (6.82) 4.01 (3.53) 1.215 (1.185) 1.544 (1.429) 2- V 0.986 (1.026) 0.980 (1.009) 6.46 (6.22) 6.75 (6.58) 1.192 (1.156) 1.188 (1.158) 2- O 2.520 (2.562) 2.547 (2.576) 0.29 (0.33) 1.07 (1.11) - 0.117 (- 0.140) - 0.122 (- 0.133) 2? Mg 3.570 (3.464) 0.07 (0.09) - 0.028 (0.003) Results obtained from calculations in which the Pb atom was described by the LANL08d basis set are listed without parentheses, whereas the results from calculations utilizing the def2-TZVP basis set for Pb are in parentheses Results for centers with the unbound Pb atom (E \ 0) are not presented ads n- V clearly indicate that the Pb atom sits higher above the center on the defect-free MgO(100) surface. Similarly to n- n- n? 2- V centers than it has been detected for Pd/V . Second, Pb/F , the Pb/O structure tends to conserve the triplet s s s n- HS the Pb atom easily becomes ionized, if adsorbed at the V multiplicity of Pb and its E energy becomes more s ads LS HS centers, and the resulting charge transfer from Pb to these exothermic than E . On the other hand, the E value for ads ads 2- 0 ? centers far exceeds one electron. The ionization potential Pb/O is smaller than those of Pb/F and Pb/F . It proves s s of Pb is lower than that of Pd (7.42 eV [48] versus 8.34 eV that Pb atoms adsorb preferentially at the F centers on the [49]), thus the enhanced tendency of the former to donate defective MgO(100) surface. It worth reminding here that, n- n? n- 0 electron charge to the V centers. The same direction of of the considered F and V vacancies, the F centers are s s s s charge transfer yet much smaller in magnitude occurs for most abundant on the defective MgO(100) surface. 2? n? Pb/F , whereas a negatively charged metal atom was For the Pb/F structures, their propensity to change the s s 2? found for Pd/F . Third, the HS state is preferred for the spin state from HS to LS can be evaluated by calculating n? HS LS Pb/F structures, which is a consequence of the triplet the difference between their E and E energies. The s ads ads multiplicity of free Pb atom in its ground state. However, resulting HS ? LS transition energies adopt smaller val- 0 - the extremely high E values of Pb/V and Pb/V are ues than the excitation energy of a free Pb atom from its ads s s sufficient for spin paring, and therefore, these structures ground state to the lowest singlet state (0.95 and 0.89 eV at 2- favor the LS state. In the case of Pb/V , the difference the B3LYP/LANL08d and B3LYP/def2-TZVP levels, LS HS between its E and E energies is too small for spin respectively). Moreover, these transition energies are ads ads 2- quenching. smaller than the HS ? LS transition energy of Pb/O .It n? The kind of basis set assigned to metal atom affects the implies that the F centers facilitate the HS ? LS tran- parameters of Pb-atom adsorption to a greater extent than sition in the adsorbed Pb atom. the results for the Pd-atom adsorption. The greater dis- An experimental study concerning the growth of Pb film crepancies in the parameters obtained using LANL08d and on well-defined oxide surfaces [18] reported a calorimet- def2-TZVP result from an inherent difference in the rically measured initial heat of adsorption of 1.07 eV for treatment of Pb atom with the two basis sets. These basis Pb/MgO(100) at 300 K. This value was an average of the sets differ not only in the number of basis functions in their bonding of Pb atoms to MgO(100) and Pb–Pb bonding valence parts, but also in the size of their core parts treated within small Pb nanoparticles formed on MgO(100). For with pseudopotentials. LANL08d is expected to yield less such nanoparticles, their Pb–MgO(100) bond strength was accurate results because (1) its quality is formally inferior roughly estimated to be either 0.33 or 0.16 eV, depending to that of def2-TZVP and (2) a previous benchmark study on the kind of Pb nanoparticles adsorbed (whether two- or confirmed its poorer performance [50]. Notwithstanding three-dimensional Pb nanoparticles). In a more recent study this difference, the application of either basis sets provides based on atomic beam/surface scattering measurements a qualitatively consistent picture of Pb-atom adsorption at [22], a range from 0.72 to 0.81 eV was proposed to be the n? n- the F and V centers. heat of Pb adsorption at terrace sites on MgO(100). Our s s HS 2- To establish the effect of surface vacancies on the Pb- E energy of Pb/O exceeds by ca. 0.3 eV the upper ads atom adsorption, Table 2 also shows the h, E , and q pa- limit of this range. ads 2- 2? rameters calculated for Pb/O and Pb/Mg . It is clear It is also interesting to examine how the surface 2- that the Pb-atom adsorption is possible only at the O vacancies affect the highest occupied molecular orbital 123 Effect of surface vacancies on the adsorption of Pd and Pb on MgO(100) 1013 Fig. 1 Plots of HOMO contours 2- 0 for Pd/O and Pd/F in their 2- LS state and for Pb/O and Pb/ F in their HS state. These contours are plotted with an isovalue of 0.01 a.u. Magnesium, oxygen, palladium and lead are colored yellow, red, blue, and gray, respectively (color figure online) n? n? (HOMO) for the Pd/F and Pb/F structures. The structure (see Fig. S2 in Electronic Supplementary s s HOMO determines, to a certain extent, the reactivity of Material). adsorbed metal atoms in catalytic processes. The contours 0 0 of HOMO for Pd/F and Pb/F are plotted in Fig. 1. For s s 2- 2- comparison, the HOMO contours of Pd/O and Pb/O Conclusion are also depicted. It can be seen that the presence of vacancy noticeably influences the shape of HOMO for Pd/ The results reported in this work point out that the presence 0 2- n? F . The HOMO of Pd/O consists of the dominant con- of vacancies on the MgO(100) surface, such as F and s s n- tribution from Pd orbitals and several minor contributions V , has an important influence on the geometric, ener- of p-type orbitals belonging to the surface O atoms of getic, and electronic parameters characterizing the 0 ? adsorption center. The part of HOMO around Pd has the adsorption of Pd and Pb atoms. The F and F vacancies, s s 2 n? n- characteristics of an s-d hybridized orbital. For the HOMO which are most likely among the F and V defects on z s s 0 2 of Pd/F , the share of Pd-atom d orbital is reduced and an MgO(100), constitute the centers at which the adsorption of s z s-like contribution from the extra electrons of F predom- single Pd or Pb atoms is more exothermic than at the 2- 0 2- 0 ? inates. In contrast to the O and F centers occupied with regular O centers. The E values of Pd/F and Pd/F in s ads s s 2- 0 the Pd atom, the Pb/O and Pb/F structures exhibit very their preferred spin states are at least 1 eV larger than the 0 ? similar shapes of their HOMOs. These HOMOs are singly corresponding energies of Pb/F and Pb/F . In that regard, s s 0 ? occupied orbitals, because the HS state is preferred for Pb/ the presence of F and F on MgO(100) does not change s s 2- 0 O and Pb/F . The HOMOs show the dominant contri- the energetic preference of Pd-atom adsorption over Pb- bution from the Pb-atom p orbital with its lobes parallel to atom adsorption. Such preference was previously detected the surface. The lack of any significant change in the shape experimentally and is confirmed here computationally. The 2- 0 0 ? of HOMO for Pb/O and Pb/F is associated with the Pd/F and Pd/F structures favor the spin state with the s s s 2- 0 0 ? geometries of Pb/O and Pb/F . For these structures, their maximum spin pairing, whereas Pb/F and Pb/F are most s s s h height of Pb from the surface adopts large values that stable in their HS states. Due to its large atomic radius, the 0 ? additionally are quite close to one another. The shape of Pb atom at the F and F centers is adsorbed at only s s ? 0 2- HOMO for Pb/F resembles that observed for the Pb/F slightly smaller height than at the O center. This s s 123 1014 P. Matczak contrasts with the large reduction of h in the Pd/F and Pd/ for Pd and LANL08d for Pb. The def2-TZVP basis set [61] ? 2- F structures, if compared to the h value of Pd/O .This was the second kind of basis set assigned to the metal reduction leads to larger increases in E and in the amount atoms. Two low-lying electronic states with different spin ads of electron charge transferred to the metal atom, as well as multiplicities were studied for the Pd- and Pb-atom 0 ? to a change in the shape of HOMO for Pd/F and Pd/F . adsorption. The low-spin state (LS) was characterized by s s n- The Pd- and Pb-atom adsorption at the V vacancies, the singlet multiplicity of the center with a Pd or Pb atom which are less abundant on MgO(100), is highly exother- adsorbed, whereas the high-spin state (HS) assumed a tri- n? mic, far exceeding the E energies obtained for Pd/F plet for each adsorbed metal atom. ads s n? 0 and Pb/F . In particular, the formation of Pb/V and Pb/ To calculate the adsorption energy (E ), the total s s ads V structures is associated with extremely high E energy of an adsorption center occupied with a metal atom s ads energies, which turn out to be sufficient to stabilize the LS was subtracted from a sum of the total energies of free state of these structures. metal atom in its ground state and the isolated surface The presented quantum mechanical study of the surface center in its relaxed geometry. According to this definition, vacancy effect is a tentative step in elucidating the prop- adsorption with a positive E value is an energetically ads erties of Pd–Pb/MgO catalysts. The findings made for Pb/ favorable (exothermic) process. The electron charge n? n- F and Pb/V may be of particular importance, because (q) acquired by an adsorbed Pd or Pb atom was estimated s s the Pb-atom adsorption on the defective MgO(100) surface by the partial charge of the atom. This partial charge was has not been investigated theoretically so far. determined according to the Bader charge analysis [62]. All calculations except the Bader charge analysis were carried out using the GAUSSIAN 09 D.01 program [63]. Methods The Bader charge analysis was done with the Multiwfn 3.4 program [64]. n? n- The structures of F and V centers with an adsorbed Pd s s Acknowledgements This work was partially supported by PL-Grid or Pb atom were determined using a theoretical quantum Infrastructure. mechanical approach based on the B3LYP computational method [51–53] and the embedded cluster model of surface Open Access This article is distributed under the terms of the [54]. These structures are denoted in this work by the Creative Commons Attribution 4.0 International License (http://crea tivecommons.org/licenses/by/4.0/), which permits unrestricted use, abbreviation ‘metal atom/adsorption center’. 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Effect of surface vacancies on the adsorption of Pd and Pb on MgO(100)

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Chemistry; Chemistry/Food Science, general; Organic Chemistry; Inorganic Chemistry; Analytical Chemistry; Physical Chemistry; Theoretical and Computational Chemistry
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Abstract

Theoretical quantum mechanical calculations have been carried out to establish the effect of surface vacancies on the adsorption of Pd and Pb atoms on the defective MgO(100) surface. The investigated defects included neutral, singly and n? n- doubly charged O and Mg vacancies on the (100) surface of MgO. These vacancies played the role of F and V (n = 0, s s 1, 2) adsorption centers for a single Pd or Pb atom. From the results of calculations, it is clear that the Pd- and Pb-atom n? n- 2- 2? adsorption at the F and V centers shows different characteristics than at the regular O and Mg centers. Drastic s s n- n- 0 ? changes in geometric, energetic, and electronic parameters are evident in Pd/V and Pb/V . The effect of F and F , s s s s which in practice are the most important vacancies, is smaller, yet the adsorption of Pd and Pb at these centers is more 2- 0 ? energetically favorable than at the regular O center. Of the two metals studied, the atom of Pd is bound by the F and F s s centers with higher adsorption energies. Graphical abstract Keywords Quantum chemical calculations  Metals  Surface  Heterogeneous catalysis  Defects Introduction coupling their Pd part with another metallic element [4, 5]. In the resulting bimetallic Pd-M/oxide catalysts, Pd is Palladium supported on oxides has found numerous usually combined with a typical metal or a half metal applications in heterogeneous catalysis [1–3]. The catalytic (M=Al, Si, Zn, Ga, Ge, In, Sn, Sb, Te, Tl, Pb, or Bi) [5]. performance of Pd/oxide systems can be improved by Recently, it has been reported that bimetallic Pd–Pb/MgO catalysts are more effective than monometallic Pd/MgO catalysts in performing aerobic oxidations of amines [6] Electronic supplementary material The online version of this article and oxidative esterification of methacrolein with methanol (https://doi.org/10.1007/s00706-018-2159-1) contains supplementary material, which is available to authorized users. [7, 8]. Understanding the enhanced catalytic performance of these bimetallic catalysts requires a detailed knowledge & Piotr Matczak of several fundamental aspects of their metal-oxide inter- piotr.matczak@chemia.uni.lodz.pl; p.a.matczak@gmail.com faces. These aspects include, in particular, geometric and electronic features of interfaces and the strength of metal- Department of Physical Chemistry, Faculty of Chemistry, ´ ´ University of Łodz, Pomorska 163/165, 90-236 Lodz, Poland 123 1010 P. Matczak n- oxide interaction. Ideally, the first step of such a charac- and V centers has been calculated to characterize the terization should concern small clusters of Pd and Pb, or fundamental aspects of Pd- and Pb-atom adsorption on even better single Pd and Pb atoms, at individual adsorp- defective MgO(100). Due to the lack of any previous the- tion sites on a well-defined single-crystal MgO surface, oretical studies for Pb/MgO(100), it is vitally important to such as the MgO(100) one. This surface is often regarded provide an insight into the effect of surface vacancies on as a prototypical oxide surface in studies of metal Pb-atom adsorption at atomic level. adsorption, because it has a simple structure and well-de- fined stoichiometry [9]. Additionally, it is relatively easy to form defects on this surface [10]. Results and discussion Various experimental techniques can yield information on the structure and energetics of metal nanoparticles and The calculated values of three essential parameters (height films deposited on oxides [11] and many experimental from the surface h, adsorption energy E and electron ads efforts have indeed been undertaken to characterize both charge q) characterizing a single Pd atom adsorbed at the n? n- Pd/MgO(100) [12–17] and Pb/MgO(100) [18–22]. On the F and V centers on the defective MgO(100) surface s s other hand, properties of single atoms adsorbed on oxide are listed in Table 1. It is evident that the Pd-atom n? surfaces are available mostly from theoretical investiga- adsorption at the F centers is far different from that at the n- tions based on computational quantum mechanical V centers. The heights of the adsorbed Pd atom from the n? n- approaches [23]. Of Pd and Pb on MgO(100), only the F centers are much greater than the h values of Pd/V s s former has become a subject for a large number of theo- structures. The Pd atom is almost inserted into the vacancy 0 - retical studies of single metal atom adsorption so far cavity for Pd/V and Pd/V in their high-spin (HS) states. s s n- [15, 24–37]. To the best of our knowledge, no theoretical The smaller h values of Pd/V are accompanied by the quantum mechanical investigations of Pb/MgO(100) have highly exothermic E energies. The comparison of E ads ads been reported until now. for the low-spin (LS) and HS states reveals that, with the 2- n? This work is aimed at providing a theoretical quantum possible exception of Pd/V , all remaining Pd/F and s s n- mechanical description for the adsorption of Pd and Pb on Pd/V structures prefer their LS state. In other words, the the MgO(100) surface with various point defects. Both adsorbed Pd atom tends to conserve its spin state (the n? n- oxygen (F ) and magnesium (V ) vacancies in three ground state of free Pd atom exhibits a singlet multiplicity). s s charge states (n = 0, 1, 2) have been taken into account. The preference of LS state is particularly noticeable for Pd/ From experimental studies [38, 39], it is known that such F because its HS state does not lead to any energetic 2- defects may be formed on MgO(100), but with a significant stabilization. In the case of Pd/V , its LH and HS states differentiation in their concentrations. What is particularly lie very close to each other and it is difficult to appoint the important is that various defects occurring on the ground state with absolute certainty. The E energy shows ads MgO(100) surface can act as anchoring sites for metal a clear dependence on the formal charge (n) of vacancies. n? nanoparticles [38, 40], and additionally, they can modify The Pd-atom adsorption at F becomes less and less 0 ? 2? the properties of deposited metal nanoparticles [38, 41]. exothermic in the order F [ F [ F . The same s s s n- Here, a set of essential geometric, energetic and electronic sequence can be observed for Pd/V . The values of q ac- n? parameters for a single Pd or Pb atom adsorbed at the F quired by the adsorbed Pd atom indicate that it behaves as Table 1 Essential parameters characterizing the adsorption of a single Pd atom at various centers on the defective MgO(100) surface LS HS LS HS LS HS ˚ ˚ Center h /A h /A E /eV E /eV q /e q /e ads ads F 1.540 (1.539) 1.708 (1.726) 3.85 (3.78) 1.65 (1.57) - 1.521 (- 1.525) - 1.536 (- 1.522) F 1.504 (1.503) 2.53 (2.45) - 0.871 (- 0.873) 2? F 1.509 (1.505) 1.706 (1.713) 1.37 (1.27) 0.29 (0.26) - 0.261 (- 0.264) - 0.239 (- 0.234) V 0.383 (0.382) 0.160 (0.160) 7.58 (7.59) 7.29 (7.31) 0.795 (0.810) 0.888 (0.894) V 0.601 (0.589) 0.157 (0.133) 5.42 (5.40) 4.63 (4.98) 0.437 (0.448) 0.880 (0.893) 2- V 0.366 (0.331) 0.584 (0.538) 4.39 (4.70) 4.52 (4.66) 0.799 (0.782) 0.427 (0.433) 2- O 2.165 (2.148) 2.351 1.34 (1.30) 0.28 - 0.231 (- 0.233) - 0.193 2? Mg 2.633 (2.636) 0.47 (0.39) - 0.075 (- 0.082) Results obtained from calculations in which the Pd atom was described by the LANL08(f) basis set are listed without parentheses, whereas the results from calculations utilizing the def2-TZVP basis set for Pd are in parentheses Results for centers with the unbound Pd atom (E \ 0) are not presented ads 123 Effect of surface vacancies on the adsorption of Pd and Pb on MgO(100) 1011 n? an electron acceptor when it sits at the F centers. The Pd/ experimental estimation of adsorption energy for Pd on F structure demonstrates the greatest charge transfer to the MgO(100) is ca. 1.2 eV [14]. From an experimental mea- metal atom. This is because the isolated F center possesses surement, a value of 2.22 A was also deduced to be the 2- two extra electrons that are largely localized in its cavity height of an adsorbed Pd atom from the O center [26]. LS LS 2- [42] and a significant amount of this electron charge can be Our E and h values for Pd/O are very close to these ads n? easily transferred to an adsorbed atom [34]. Unlike Pd/F , experimental estimations. Similarly to metal adsorption on n- the Pd/V structures show the opposite direction of charge the defect-free MgO(100) surface, metal atoms on transfer. Our calculations predict that the charge transfer MgO(100) with defects also adsorb preferentially at centers n- from the Pd atom to the V centers never exceeds 0.9 e where negative charge accumulates [33, 44]. More n- 0 even if the Pd atom is almost inserted into the V cavity. specifically, the F centers play the key role in the s s This charge transfer and the small h heights lead to a sig- adsorption of Pd atoms [15, 16]. This is because these nificant electrostatic stabilization between the ionized Pd centers are the main part of vacancies formed on n- atom and the V centers. MgO(100), which was confirmed both experimentally [10] 0 ? It is instructive to compare the Pd-atom adsorption at the and theoretically [45, 46]. Besides the F centers, the F s s vacancies with that occurring at non-defective sites. centers can also occur, but they are less likely due to their Results describing the adsorption of a single Pd atom at the large formation energy [42]. Even larger formation energy 2- 2? 2? regular anionic O and cationic Mg centers of defect- was determined for the F center [42]. Previous compu- free MgO(100) surface are appended to Table 1. As evi- tational studies have shown that the Pd/F interaction is 2- 2? 0 denced by the E values of Pd/O and Pd/Mg , the Pd weaker than the Pd/F interaction but stronger than that of ads s 2- 2- atom binds preferentially to the O center in the LS state. Pd/O [15, 33, 36]. Apart from rendering this trend cor- A small charge transfer to the metal atom appears for Pd/ rectly, our h and E values also reproduce quantitatively ads 2- O , while the Pd atom remains essentially neutral at the other theoretical results [15, 35, 36]. It has also been 2? 2- n- Mg center. The Pd-atom adsorption at O is less reported that the interaction between Pd and V centers is 0 ? 2- energetically favorable than at F and F . The Pd/O extremely strong [24]. According to an experimental study s s structure also demonstrates a larger h value compared to [10], the concentration of surface Mg vacancies seems, 0 ? 2- 0 ? those of Pd/F and Pd/F . On the other hand, the Pd/O however, to be much lower than that of F and F . Again, s s s s 2? n- and Pd/F structures are formed with very similar E this is in line with large formation energies of V s ads s energies, although the former exhibits a much larger vacancies [42, 47]. h value. This review of existing results for Pd/MgO clearly An inspection of the results in Table 1 also reveals that indicates that the computational methodology applied in the kind of the basis set assigned to the Pd atom most often this work leads to the correct description of Pd-atom has a rather minor effect on the calculated values of h, E adsorption on MgO(100) with surface vacancies. Thus, one ads and q. A discrepancy in the interpretation of the results can expect that the parameters characterizing the adsorp- n? n- obtained from LANL08(f) and def2-TZVP appears for Pd/ tion of Pb atom at the F and V centers should also be s s 2- 2- V and Pd/O . The calculations employing the two basis predicted reliably. n? sets designate different spin states as the energetically Essential parameters for the Pb atom adsorbed at the F 2- 2- n- preferred state of Pd/V . In the case of Pd/O in the HS and V centers are collected in Table 2. A careful s s state, the calculations involving the LANL08(f) basis set inspection of these results reveals that there are several predict an exothermic adsorption, in contrast to those car- similarities between the Pd-atom adsorption and its Pb HS n- ried out with def2-TZVP. However, the E value obtained counterpart. The formation of Pb/V structures is asso- ads s from LANL08(f) is actually quite close to zero, and ciated with extremely exothermic E values, many times ads n? therefore, the significance of this discrepancy should not be greater than those calculated for the Pb/F structures. For n? overemphasized. Pb/F , their E energies decrease regularly with the s ads n? Our findings made for the Pd-atom adsorption are growing formal charge of F center. The adsorption of Pb n- essentially in good agreement with conclusions reported in at V leads to a significant charge transfer from Pb to the n- previous experimental [14–16] and theoretical V centers, while the reverse direction of charge transfer 0 ? [15, 24, 26–28, 33, 35, 36] studies of Pd/MgO(100). It is is observed for Pb/F and Pb/F . A strong correlation s s well-known that the defect-free MgO(100) surface is gen- between E and the magnitude of charge transfer can be ads n? n? erally rather unreactive toward the adsorption of metal found for both the Pd/F and Pb/F structures. On the s s 2? atoms [43]. The Mg centers exhibit particularly low other hand, the Pb-atom adsorption turns out to be different reactivity toward metal atoms [25]. In consequence, Pd in certain aspects from the Pd-atom adsorption. First, the 2- atoms preferably occupy the O centers [28], with no large atomic radius of Pb causes this atom not to replace n- significant charge transfer from or to the surface [27]. An the missing Mg atom at the V cavity. The h values of Pb/ 123 1012 P. Matczak Table 2 Essential parameters characterizing the adsorption of a single Pb atom at various centers on the defective MgO(100) surface LS HS LS HS LS HS ˚ ˚ Center h /A h /A E /eV E /eV q /e q /e ads ads F 2.395 (2.331) 2.368 (2.340) 1.45 (1.66) 2.16 (2.33) - 1.361 (- 1.363) - 1.365 (- 1.376) F 2.464 (2.422) 2.300 (2.282) 1.19 (1.32) 1.24 (1.37) - 0.610 (- 0.630) - 0.640 (- 0.660) 2? F 2.671 (2.612) 2.797 (2.737) 0.65 (0.73) 0.81 (0.86) 0.172 (0.134) 0.162 (0.129) V 1.028 (1.105) 0.606 (0.728) 9.55 (9.24) 5.85 (5.38) 1.226 (1.211) 1.215 (1.429) V 1.005 (1.056) 0.571 (0.675) 7.04 (6.82) 4.01 (3.53) 1.215 (1.185) 1.544 (1.429) 2- V 0.986 (1.026) 0.980 (1.009) 6.46 (6.22) 6.75 (6.58) 1.192 (1.156) 1.188 (1.158) 2- O 2.520 (2.562) 2.547 (2.576) 0.29 (0.33) 1.07 (1.11) - 0.117 (- 0.140) - 0.122 (- 0.133) 2? Mg 3.570 (3.464) 0.07 (0.09) - 0.028 (0.003) Results obtained from calculations in which the Pb atom was described by the LANL08d basis set are listed without parentheses, whereas the results from calculations utilizing the def2-TZVP basis set for Pb are in parentheses Results for centers with the unbound Pb atom (E \ 0) are not presented ads n- V clearly indicate that the Pb atom sits higher above the center on the defect-free MgO(100) surface. Similarly to n- n- n? 2- V centers than it has been detected for Pd/V . Second, Pb/F , the Pb/O structure tends to conserve the triplet s s s n- HS the Pb atom easily becomes ionized, if adsorbed at the V multiplicity of Pb and its E energy becomes more s ads LS HS centers, and the resulting charge transfer from Pb to these exothermic than E . On the other hand, the E value for ads ads 2- 0 ? centers far exceeds one electron. The ionization potential Pb/O is smaller than those of Pb/F and Pb/F . It proves s s of Pb is lower than that of Pd (7.42 eV [48] versus 8.34 eV that Pb atoms adsorb preferentially at the F centers on the [49]), thus the enhanced tendency of the former to donate defective MgO(100) surface. It worth reminding here that, n- n? n- 0 electron charge to the V centers. The same direction of of the considered F and V vacancies, the F centers are s s s s charge transfer yet much smaller in magnitude occurs for most abundant on the defective MgO(100) surface. 2? n? Pb/F , whereas a negatively charged metal atom was For the Pb/F structures, their propensity to change the s s 2? found for Pd/F . Third, the HS state is preferred for the spin state from HS to LS can be evaluated by calculating n? HS LS Pb/F structures, which is a consequence of the triplet the difference between their E and E energies. The s ads ads multiplicity of free Pb atom in its ground state. However, resulting HS ? LS transition energies adopt smaller val- 0 - the extremely high E values of Pb/V and Pb/V are ues than the excitation energy of a free Pb atom from its ads s s sufficient for spin paring, and therefore, these structures ground state to the lowest singlet state (0.95 and 0.89 eV at 2- favor the LS state. In the case of Pb/V , the difference the B3LYP/LANL08d and B3LYP/def2-TZVP levels, LS HS between its E and E energies is too small for spin respectively). Moreover, these transition energies are ads ads 2- quenching. smaller than the HS ? LS transition energy of Pb/O .It n? The kind of basis set assigned to metal atom affects the implies that the F centers facilitate the HS ? LS tran- parameters of Pb-atom adsorption to a greater extent than sition in the adsorbed Pb atom. the results for the Pd-atom adsorption. The greater dis- An experimental study concerning the growth of Pb film crepancies in the parameters obtained using LANL08d and on well-defined oxide surfaces [18] reported a calorimet- def2-TZVP result from an inherent difference in the rically measured initial heat of adsorption of 1.07 eV for treatment of Pb atom with the two basis sets. These basis Pb/MgO(100) at 300 K. This value was an average of the sets differ not only in the number of basis functions in their bonding of Pb atoms to MgO(100) and Pb–Pb bonding valence parts, but also in the size of their core parts treated within small Pb nanoparticles formed on MgO(100). For with pseudopotentials. LANL08d is expected to yield less such nanoparticles, their Pb–MgO(100) bond strength was accurate results because (1) its quality is formally inferior roughly estimated to be either 0.33 or 0.16 eV, depending to that of def2-TZVP and (2) a previous benchmark study on the kind of Pb nanoparticles adsorbed (whether two- or confirmed its poorer performance [50]. Notwithstanding three-dimensional Pb nanoparticles). In a more recent study this difference, the application of either basis sets provides based on atomic beam/surface scattering measurements a qualitatively consistent picture of Pb-atom adsorption at [22], a range from 0.72 to 0.81 eV was proposed to be the n? n- the F and V centers. heat of Pb adsorption at terrace sites on MgO(100). Our s s HS 2- To establish the effect of surface vacancies on the Pb- E energy of Pb/O exceeds by ca. 0.3 eV the upper ads atom adsorption, Table 2 also shows the h, E , and q pa- limit of this range. ads 2- 2? rameters calculated for Pb/O and Pb/Mg . It is clear It is also interesting to examine how the surface 2- that the Pb-atom adsorption is possible only at the O vacancies affect the highest occupied molecular orbital 123 Effect of surface vacancies on the adsorption of Pd and Pb on MgO(100) 1013 Fig. 1 Plots of HOMO contours 2- 0 for Pd/O and Pd/F in their 2- LS state and for Pb/O and Pb/ F in their HS state. These contours are plotted with an isovalue of 0.01 a.u. Magnesium, oxygen, palladium and lead are colored yellow, red, blue, and gray, respectively (color figure online) n? n? (HOMO) for the Pd/F and Pb/F structures. The structure (see Fig. S2 in Electronic Supplementary s s HOMO determines, to a certain extent, the reactivity of Material). adsorbed metal atoms in catalytic processes. The contours 0 0 of HOMO for Pd/F and Pb/F are plotted in Fig. 1. For s s 2- 2- comparison, the HOMO contours of Pd/O and Pb/O Conclusion are also depicted. It can be seen that the presence of vacancy noticeably influences the shape of HOMO for Pd/ The results reported in this work point out that the presence 0 2- n? F . The HOMO of Pd/O consists of the dominant con- of vacancies on the MgO(100) surface, such as F and s s n- tribution from Pd orbitals and several minor contributions V , has an important influence on the geometric, ener- of p-type orbitals belonging to the surface O atoms of getic, and electronic parameters characterizing the 0 ? adsorption center. The part of HOMO around Pd has the adsorption of Pd and Pb atoms. The F and F vacancies, s s 2 n? n- characteristics of an s-d hybridized orbital. For the HOMO which are most likely among the F and V defects on z s s 0 2 of Pd/F , the share of Pd-atom d orbital is reduced and an MgO(100), constitute the centers at which the adsorption of s z s-like contribution from the extra electrons of F predom- single Pd or Pb atoms is more exothermic than at the 2- 0 2- 0 ? inates. In contrast to the O and F centers occupied with regular O centers. The E values of Pd/F and Pd/F in s ads s s 2- 0 the Pd atom, the Pb/O and Pb/F structures exhibit very their preferred spin states are at least 1 eV larger than the 0 ? similar shapes of their HOMOs. These HOMOs are singly corresponding energies of Pb/F and Pb/F . In that regard, s s 0 ? occupied orbitals, because the HS state is preferred for Pb/ the presence of F and F on MgO(100) does not change s s 2- 0 O and Pb/F . The HOMOs show the dominant contri- the energetic preference of Pd-atom adsorption over Pb- bution from the Pb-atom p orbital with its lobes parallel to atom adsorption. Such preference was previously detected the surface. The lack of any significant change in the shape experimentally and is confirmed here computationally. The 2- 0 0 ? of HOMO for Pb/O and Pb/F is associated with the Pd/F and Pd/F structures favor the spin state with the s s s 2- 0 0 ? geometries of Pb/O and Pb/F . For these structures, their maximum spin pairing, whereas Pb/F and Pb/F are most s s s h height of Pb from the surface adopts large values that stable in their HS states. Due to its large atomic radius, the 0 ? additionally are quite close to one another. The shape of Pb atom at the F and F centers is adsorbed at only s s ? 0 2- HOMO for Pb/F resembles that observed for the Pb/F slightly smaller height than at the O center. This s s 123 1014 P. Matczak contrasts with the large reduction of h in the Pd/F and Pd/ for Pd and LANL08d for Pb. The def2-TZVP basis set [61] ? 2- F structures, if compared to the h value of Pd/O .This was the second kind of basis set assigned to the metal reduction leads to larger increases in E and in the amount atoms. Two low-lying electronic states with different spin ads of electron charge transferred to the metal atom, as well as multiplicities were studied for the Pd- and Pb-atom 0 ? to a change in the shape of HOMO for Pd/F and Pd/F . adsorption. The low-spin state (LS) was characterized by s s n- The Pd- and Pb-atom adsorption at the V vacancies, the singlet multiplicity of the center with a Pd or Pb atom which are less abundant on MgO(100), is highly exother- adsorbed, whereas the high-spin state (HS) assumed a tri- n? mic, far exceeding the E energies obtained for Pd/F plet for each adsorbed metal atom. ads s n? 0 and Pb/F . In particular, the formation of Pb/V and Pb/ To calculate the adsorption energy (E ), the total s s ads V structures is associated with extremely high E energy of an adsorption center occupied with a metal atom s ads energies, which turn out to be sufficient to stabilize the LS was subtracted from a sum of the total energies of free state of these structures. metal atom in its ground state and the isolated surface The presented quantum mechanical study of the surface center in its relaxed geometry. According to this definition, vacancy effect is a tentative step in elucidating the prop- adsorption with a positive E value is an energetically ads erties of Pd–Pb/MgO catalysts. The findings made for Pb/ favorable (exothermic) process. The electron charge n? n- F and Pb/V may be of particular importance, because (q) acquired by an adsorbed Pd or Pb atom was estimated s s the Pb-atom adsorption on the defective MgO(100) surface by the partial charge of the atom. This partial charge was has not been investigated theoretically so far. determined according to the Bader charge analysis [62]. All calculations except the Bader charge analysis were carried out using the GAUSSIAN 09 D.01 program [63]. Methods The Bader charge analysis was done with the Multiwfn 3.4 program [64]. n? n- The structures of F and V centers with an adsorbed Pd s s Acknowledgements This work was partially supported by PL-Grid or Pb atom were determined using a theoretical quantum Infrastructure. mechanical approach based on the B3LYP computational method [51–53] and the embedded cluster model of surface Open Access This article is distributed under the terms of the [54]. These structures are denoted in this work by the Creative Commons Attribution 4.0 International License (http://crea tivecommons.org/licenses/by/4.0/), which permits unrestricted use, abbreviation ‘metal atom/adsorption center’. 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Monatshefte für Chemie - Chemical MonthlySpringer Journals

Published: Feb 13, 2018

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