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Ce0.75Zr0.25O2(111) surface. A DFT+U study
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Oxygen vacancy formation on the Ni/Ce0.75Zr0.25O2(111) surface. A DFTDU study Delfina Garcı´a Pintos a, Alfredo Juan b, Beatriz Irigoyen a,* a
Departamento de Ingenierı´a Quı´mica, Facultad de Ingenierı´a, Universidad de Buenos Aires, Pabello´n de Industrias, Ciudad Universitaria, 1428 Capital Federal, Argentina b Instituto de Fı´sica del Sur, Universidad Nacional del Sur. Avda. Alem 1253, 8000 Bahı´a Blanca, Argentina
article info
abstract
Article history:
Ni/CeO2eZrO2 solid solutions are well known by their promissory catalytic role in hydrogen
Received 6 September 2011
production processes such as reforming and partial oxidation of hydrocarbons. CeO2eZrO2
Accepted 12 December 2011
support can effectively disperse Ni particles and donate oxygen promoting Ni active phase
Available online 2 January 2012
resistance to poisoning by carbon deposition. Thus, in this work we study the influence of Ni on oxygen vacancy formation on the Ce0.75Zr0.25O2(111) surface, by using the density
Keywords:
functional theory (DFT) approach with the Hubbard’s correction (U) for Ce(4f) electrons. We
DFTþU
performed DFTþU energetic calculations on Ni/Ce0.75Zr0.25O2 (Ni is adsorbed over an OeO
Ni/CeO2eZrO2
bridge site) and NieCe0.75Zr0.25O2 (Ni is inserted into the slab) systems, with an O-vacancy.
Ce0.75Zr0.25O2
Moreover, we analyzed the electronic structures of these solids by drawing their spin
O-vacancy
polarized density of states (DOS) plots. Our results indicate that Ni insertion between two O subsurface layers facilitates Ce0.75Zr0.25O2 mixed oxide reduction and enhances its ionic mobility and oxygen donation properties. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen has attracted great attention due to its potential for sustainable energy generation, especially because of its high energy content per unit mass and low level of environmental pollutants emission. Currently, H2 is mainly generated from fossil fuels by catalytic steam reforming and partial oxidation processes [1], which imply a strong dependence on non-renewable resources and the discharge of contaminants that harm the environment. These disadvantages promote the use of alternative fuels for H2 production. The biomass as renewable resource with neutral emission of CO2 has emerged as an interesting option [2]. Aqueous fractions of bio-fuels have also
gained attention for their catalytic steam reforming to produce H2 [3]. The catalysts most widely used in these processes contain Ni or noble metals in their formulation. The solids based on Ni are preferably chosen for their low cost, although they easily deactivate by metal sintering and coking. This poor achievement can be improved by using reducible CeO2-based supports. CeO2 can itself offer active sites for molecules interaction during catalytic reactions [4]. This characteristic is originated in the Ce cations reduction facility, which is mainly related to the formation of Ce4þ/Ce3þ couples and oxygen release/storage capacity (OSC) [5]. CeO2 performance is enhanced by the addition of transition, alkaline earth or rareearth metals [6,7]. Particularly, the doping with Zr results in
* Corresponding author. Tel.: þ54 11 45763241; fax: þ54 11 45763240. E-mail address: [email protected] (B. Irigoyen). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.12.079
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better redox properties, oxygen storage capacity, thermal resistance and catalytic activity [8e13]. Ni/CeO2eZrO2 solid solutions are promising catalysts for H2 generation during hydrocarbon reforming and partial oxidation processes [14,15]. Among the benefits of using CeO2eZrO2 mixed oxides as support we can mention their ability to effectively disperse Ni, which results in a better resistance to poisoning by carbon deposition [16]. The presence of Zr4þ cations substantially diminishes the energy necessary for Ce4þ to Ce3þ reduction and facilitates the formation of anionic defects in CeO2 [7,17,18]. In spite of the importance of surface Ni/CeO2eZrO2 catalysts redox behaviour, the influence of Ni on surface oxygen vacancies formation, which is related to Ce4þ/Ce3þ couples, is not yet thoroughly understood. Thus, in this work we have studied the effect of Ni interactions on/in Ce0.75Zr0.25O2 mixed oxide on its capacity to donate oxygen atoms. The knowledge of this effect at fundamental level would help to explain the reactivity of Ni/CeO2eZrO2 solid solutions as promissory catalysts for H2 production. We performed DFTþU periodic calculations by using the density functional approach (DFT) with the Hubbard correction (U) for Ce(4f) electrons. Then, we calculated the energy necessary for a surface oxygen vacancy formation on the bare Ce0.75Zr0.25O2(111) and compared its value to that on Ni/ Ce0.75Zr0.25O2(111) (Ni is adsorbed on a bridge OeO site) and NieCe0.75Zr0.25O2(111) (Ni is inserted between oxygen subsurface layers) systems. Also, we evaluated the changes in their electronic structures by means of the density of states (DOS) plots.
2.
Computational methods
2.1.
Solid and surface models
Cerium oxide (CeO2) is an insulating, non-magnetic rare-earth oxide [19]. It has a cubic fluorite structure, which is a facecentred cubic structure (FCC), with an experimental lattice ˚ [20]. This structure has four Ce and parameter value of 5.41 A eight O atoms per unit cell, in which Ce cations are located on the FCC lattice sites and O anions on the tetrahedral holes. In CeO2eZrO2 mixed oxides, Zr is easily inserted without generating excessive tensions in the lattice, as both oxides have similar structures [21]. In this work, CeO2eZrO2 mixed oxides has been represented with a 25% Zr-doped CeO2 model [18]. The crystal structure of the Ce0.75Zr0.25O2 mixed oxide is shown in Fig. 1(a). In order to model the most stable Ce0.75Zr0.25O2 catalytic surface [18,19,22], an extra O layer has been added to the (111) plane. This surface was modelled as a stack of OeCeeO sandwiches to prevent the appearance of a nonzero dipole moment normal to the surface [23]. In a previous work we have found that the most stable Ni interactions on the Ce0.75Zr0.25O2(111) surface corresponds to its deposition over OeO bridge sites [24]. These preferential Ni adsorption sites, named “W” and “X”, are shown in Fig. 1(b). As it can be seen, these sites are located between two O anions surrounded by Ce and Zr cations. The W site has a Zr as the nearest neighbour metal cation, while the X site has a Ce as nearest neighbour cation.
2.2.
Calculations
In this work, we have performed periodic energy calculations on the Ni/Ce0.75Zr0.25O2 system. We used the functional density theory (DFT) approach, implemented with the Viena Ab-initio Simulation Package (VASP) code [25,26]. We used the generalized gradient approximation with Perdew-BurkeErnzerhof (PBE) functional [27], to solve the KohneSham equations. The core electrons have been treated with the projected augmented waves (PAW) approximation [28,29]. The valence electrons are: Ce (5s), (5p), (5d), (4f) and (6s), O (2s) and (2p), and Zr (5s), (4d) and (5p). The KohneSham orbitals have been expanded in plane waves with a cut-off energy value of 400 eV. For the integration of the Brillouin zone a 4 4 1 k-points grid has been employed, following the MonkhorstePack scheme [30]. Within all this, we found ˚. a Ce0.75Zr0.25O2 lattice parameter value of 5.41 A The standard DFT formulation usually fails to describe strongly correlated electrons behaviour. This limitation can be corrected using the DFTþU method, which introduces a Hubbard parameter “U” for the description of the on-site interactions of those electrons [31,32]. Thus, in this work we used the DFTþU methodology with an U value of 5.0 eV to describe the interactions of Ce(4f) electrons [5,19].
3.
Results and discussion
In a previous work, we have optimized the atomic positions of the three different systems: bare Ce0.75Zr0.25O2, Ni/ Ce0.75Zr0.25O2 (Ni is adsorbed over the OeO bridge sites), and NieCe0.75Zr0.25O2 (Ni is inserted between two subsurface O layers) [24]. For that purpose, we employed a superlattice consisting on 12 layers of Ce, Zr y O atoms and a vacuum space ˚ . The atomic coordinates of Ni, Ce, Zr of approximately 13 A and O atoms located in the first six layers of the slab has been relaxed to find the optimum geometric structure of each system. The atomic positions were optimized until the ˚ . For the resulting force on each atom was less than 0.02 eV/A system total energy convergence, a tolerance of 104 eV was employed.
3.1.
Ni interactions with the Ce0.75Zr0.25O2 mixed oxide
The Ni adsorption (DENi,ads) and insertion (DENi,ins) energies have been evaluated with the following expressions [24]: DENi;ads ¼ E½Ni=Ce0:75 Zr0:25 O2 ð111Þ E½Ce0:75 Zr0:25 O2 ð111Þ E½Ni DENi;ins ¼ E½Ni Ce0:75 Zr0:25 O2 ð111Þ E½Ce0:75 Zr0:25 O2 ð111Þ E½Ni where: E[Ni/Ce0.75Zr0.25O2(111)] and E[NieCe0.75Zr0.25O2(111)] are the energy values of the Ce0.75Zr0.25O2(111) slab with Ni adsorbed and inserted, respectively. E[Ce0.75Zr0.25O2(111)] is the energy of the bare slab. E[Ni] is the energy corresponding to the lowest triplet Ni atom state: E[Ni(d9,s1)] ¼ 0.52 eV [33]. With these expressions, negative values of DENi,ads and DENi,ins indicate Ni favourable interactions.
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Fig. 1 e Ce0.75Zr0.25O2 mixed oxide. (a) Fluorite-type structure with indication of the (111) plane (as a shadowed area). (b) Ni preferential adsorption OeO bridge sites: W (the O nearest neighbour metal cation is a Zr), and X (the O nearest neighbour metal cation is a Ce).
Our previous calculations showed that Ni deposition on top of Ce cations of the Ce0.75Zr0.25O2 (111) surface is not favourable, evidencing the repulsive character of NieCe interactions [34]. On the other hand, Ni interactions on the Zr and O ions and over the OeO bridge sites are favourable. Ni is preferably adsorbed on the different sites in the following order: on top O < on top Zr < over the OeO bridge site X < over the OeO bridge site W [24]. The calculated energy values for Ni adsorption over the OeO bridge site W (Ni/Ce0.75Zr0.25O2 system) and Ni insertion between two subsurface O layers (NieCe0.75Zr0.25O2 system) are shown in Table 1. The most stable Ni interaction results from its location over an OeO bridge site, near to the Zr-dopant. When Ni is adsorbed over the OeO bridge site W,
˚ and 1.92 A ˚ , respecthe two NieO bond distances are 1.86 A tively. The Zr cation located near Ni is moved down to an O of ˚ , and the O bonded to Zr is relaxed the third layer by 0.18 A ˚ (see Fig. 2(a)). outward the surface by 0.3 A Ni insertion energy is comparable to that of Ni deposition over the W site (see Table 1, DENi,abs ¼ 3.88 eV). Ni is bonded to the oxygen atoms of the upper and lower O layers, with ˚ and 2.07 A ˚ , and NieOthird layer NieOsecond layer distances of 1.96 A ˚ . Note that these distances are similar to distances of 2.03 A ˚ ). In the optimized geombonds length in nickel oxide (2.08 A etry, the separation between the O layers into which Ni is ˚ (1.5 A ˚ in the bare CeeZr slab). As inserted increases to 2.18 A a result, surface Ce, Zr and O atoms are displaced from their original positions. Particularly dramatic results the anionic
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Table 1 e Energy values for Ni preferential interactions on/in the Ce0.75Zr0.25O2 (111) slab. Preferential interaction sites
DENi ads/ins (eV)
Ni adsorption over the oxygen bridge site “W” Ni insertion between two subsurface oxygen layers
4.21 3.88
relaxations, with surface and subsurface oxygens being moved ˚ as can be seen in Fig. 2(b). up to 1.14 A
3.2. Formation of surface oxygen vacancies in the Ce0.75Zr0.25O2, Ni/Ce0.75Zr0.25O2 and NieCe0.75Zr0.25O2 systems In this section, we have studied the oxygen donation ability of the systems resulting from Ni interactions with CeO2eZrO2 mixed oxides. Thus, we calculated the energy necessary for releasing oxygen O1 from the surface (see Fig. 3 for O1 vacancy location). This oxygen has been selected because of its location near to a Zr cation, which is known for its anion vacancy formation promoter effect [7]. The calculated energy values were compared with that obtained for the formation of a similar O-vacancy on the bare Ce0.75Zr0.25O2(111) surface. We introduced the O defect on the topmost O layer of Ce0.75Zr0.25O2(111), Ni/Ce0.75Zr0.25O2(111) and NieCe0.75Zr0.25 O2(111) systems and then optimized the ions atomic positions keeping fixed the lattice parameter. The surface O-vacancy formation energy on these systems was calculated in accordance with the following expressions: DEOvac:;CeO2 ¼ E½Ce0:75 Zr0:25 O2 ð111Þ x E½Ce0:75 Zr0:25 O2x ð111Þ E½O2 2 DEOvac:;Ni W ¼ E½Ni=Ce0:75 Zr0:25 O2 ð111Þ x E½Ni=Ce0:75 Zr0:25 O2x ð111Þ E½O2 2
DEOvac:;Ni ins ¼ E½Ni Ce0:75 Zr0:25 O2 ð111Þ E½Ni x Ce0:75 Zr0:25 O2x ð111Þ E½O2 2 where: E[Ce0.75Zr0.25O2(111)], E[Ni/Ce0.75Zr0.25O2 (111)] and E [NieCe0.75Zr0.25O2(111)], indicate the energy values of bare Ce0.75Zr0.25O2 mixed oxide, and that for Ni adsorbed on the W site and Ni inserted on/in the Ce0.75Zr0.25O2 slab. E E[Ni/Ce0.75Zr0.25O2x(111)] and E [Ce0.75Zr0.25O2x(111)], [NieCe0.75Zr0.25O2x(111)] are the energy values of the same systems with a surface O-vacancy near to a Zr-doping centre. E[O2] is the energy value for the O molecule in the vacuum. In Table 2, we show the calculated energy values for the formation of a surface O-vacancy in these systems. The easy of O1 vacancy formation on the different studied systems, follows the order: NieCe0.75Zr0.25O2 > Ce0.75Zr0.25O2 > Ni/ Ce0.75Zr0.25O2. As it can be seen, the formation of the O1 vacancy is more favourable on the NieCe0.75Zr0.25O2(111) surface. The O atoms located in the three first anionic layers show important movements outward the surface. Particularly, one of the ˚. subsurface O anions is moved up to the anion hole by 1.57 A The O anions displacements are shown in Fig. 4(a). The creation of oxygen O1 vacancy has similar relaxation effects on neighbour Ce and Zr cations, with surface cerium being dis˚ as it is shown in Fig. 4(b). placed up to 0.74 A
3.3. Electronic structure of the stoichiometric and reduced Ce0.75Zr0.25O2, Ni/Ce0.75Zr0.25O2 and NieCe0.75Zr0.25O systems CeO2, an insulating and non-magnetic rare-earth oxide, shows a characteristic gap between the valence and the conduction bands [19]. From the electronic point of view, every time an O-vacancy is formed on this oxide, Ce(4f) states are occupied in two neighbouring Ce cations [4,18,35]. Consequently, these localized Ce(4f) states appear in the band gap; which gives the solid the character of semiconductor [7]. The CeeZr mixed oxide total density of states (DOS) calculated
Fig. 2 e Ni interactions on/in the Ce0.75Zr0.25O2 (111) slab. (a) Ni/Ce0.75Zr0.25O2 system (Ni is adsorbed over the OeO bridge site W ). (b) NieCe0.75Zr0.25O2 system (Ni is inserted between two subsurface O layers).
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Fig. 3 e Ni/Ce0.75Zr0.25O2 system top view. Ni is adsorbed on the OeO bridge site W. The striped circle indicates the O1 vacancy (note that Ce1 and Ce2 cations become 6-fold O-coordinated, while Ce3 cation remains 7-fold O-coordinated).
in this work exhibits these characteristics; which emerge from the comparison of the spin polarized DOS plots of Ce0.75Zr0.25O2 systems in oxidized (see Fig. 5) and reduced (see Fig. 6(a)) states. In CeO2eZrO2 solids, the O vacancies formation and the ionic mobility are significantly enhanced due to CeO2 lattice distortion by Zr doping [17]. Besides, the analysis of the atomic structure modifications, caused by Ni adsorption and insertion on/in the Ce0.75Zr0.25O2 mixed oxide, have shown important relaxations of O belonging to inner layers. These O anions were moved up to the surface empty sites originated by Ni interactions. Their displacements, in agreement with reported diffusive movement of O atoms towards the CeO2 surface [36], can also contribute to electron localization on Ce(4f) orbitals. Thus, we draw the total DOS plots and performed a Bader charge analysis [37], to evaluate the electronic structure of Ni/ Ce0.75Zr0.25O2 and NieCe0.75Zr0.25O2 systems. Our results indicate that Ce3þ/Ce4þ redox couples formation was promoted by the important O atoms relaxations resulting from Ni adsorption and insertion on/in the CeeZr slab. On the other hand, our DFTþU calculations show that Ni deposition disfavours in 0.46 eV the creation of surface anion vacancies on Ce0.75Zr0.25O2 mixed oxides. However, Ni insertion into the slab promotes surface O defects formation, and the
Table 2 e Calculated energy values for an anionic vacancy formation on the (111) surface of Ce0.75Zr0.25O2, Ni/ Ce0.75Zr0.25O2a and NieCe0.75Zr0.25O2b systems. Systems Ce0.75Zr0.25O2 Ni/Ce0.75Zr0.25O2 NieCe0.75Zr0.25O2 a Ni is adsorbed on a bridge OeO site. b Ni is inserted between oxygen subsurface layers.
DEO-vac. (eV) 2.21 2.67 1.34
required energy is 0.87 eV lower than that on the bare Ce0.75Zr0.25O2(111) face (see NieCe0.75Zr0.25O2(111) in Table 2). Looking at the electronic structure, we realized that the DOS plots corresponding to Ni/Ce0.75Zr0.25O2 (see Fig. 6(b)) and NieCe0.75Zr0.25O2 (see Fig. 6(c)) systems with a surface Ovacancy, show peaks localized in the gap between valence and conduction band. This is a distinctive feature of the DOS plot for reduced Ce0.75Zr0.25O2 mixed oxide as it can be seen in Fig. 6(a). The DOS plot for Ni/Ce0.75Zr0.25O2 system with Ni adsorbed over the OeO bidge site W and a vacancy of oxygen O1, shows three Ce(4f) peaks localized in the gap. These peaks correspond to Ce1, Ce2 and Ce3 cations (see Fig. 7, for Ce1eCe3 locations). Note that Ce1(4f) peak, localized at 0.23 eV was originated by Ni deposition, while Ce2(4f) and Ce3(4f) peaks (localized at 0.01 and 0.3 eV, respectively), appears after O1 vacancy formation. On the other hand, the DOS curve for NieCe0.75Zr0.25O2 system with Ni inserted into the slab and a defect of oxygen O1, shows four peaks Ce(4f) localized in the gap. These peaks were formed by contributions of surface and inner Ce cations. Ce3(4f) and Ce4(4f) peaks (localized at 0.61 and 0.09 eV, respectively), were originated by Ni insertion into the slab. Ce2(4f) and Ce5(4f) peaks (localized at 0.07 and 0.01 eV, respectively), appears due to the surface O1 vacancy creation. In addition, is important to note that the lower energy peak that appears in the gap between valence and conduction band in the DOS plots for the three studied systems (see Fig. 6(aec)) is formed by Ce(4f) orbitals of the heptahedral Ce3 cation. This is in agreement with reported results indicating that electrons left behind upon oxygen removal prefer to be localized on surface Ce(4f) orbitals of Ce atoms with higher coordination number (7-fold coordinated) [35].
3.4.
Discussion
Our results indicate that Ni adsorption and insertion on/in Ce0.75Zr0.25O2 mixed oxide cause important relaxations of
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Fig. 4 e NieCe0.75Zr0.25O2 slab side view with indication of Ce, Zr and O relaxations due to a surface O-vacancy creation. (a) Anions movements. (b) Cations movements.
surface and inner O atoms. NieO interactions provoke the displacement of these O, by moving them away from bonded Ce cations, and thus generate localized Ce(4f) states in the DOS band gap of Ni/Ce0.75Zr0.25O2 and NieCe0.75Zr0.25O2 systems. On the other hand, the energy values calculated for Ovacancy formation on the Ce0.75Zr0.25O2(111) surface show that Ni adsorption disfavours the oxygen release. The energy necessary for an O defect creation in Ni/Ce0.75Zr0.25O2 is 0.46 eV higher than that on the bare mixed oxide. However, Ni insertion into the slab significantly facilitates the formation of surface oxygen vacancies (the required energy is 0.87 eV lower than that on the bare Ce0.75Zr0.25O2(111) face). The atomic and electronic structures of CeO2eZrO2 mixed oxides are intimately related to their catalytic behaviour. Besides, the performance of Ni/CeO2eZrO2 solid solutions is connected with their reducibility and oxygen mobility properties. The results discussed in this work indicate that Ni diffusion into the slab could enhance anionic mobility, Ce4þ/ Ce3þ couples formation and oxygen donation in CeO2eZrO2 mixed oxides. Moreover, the promoter effect of inserted Ni could be related to the important relaxations of subsurface O atoms observed in NieCe0.75Zr0.25O2 system, where those oxygens experience dramatic movements upward the surface
Fig. 5 e Spin polarized density of states (DOS) plot for the bare Ce0.75Zr0.25O2 system.
Fig. 6 e Spin polarized density of states (DOS) plot for the different studied systems with a surface oxygen vacancy. (a) Ce0.75Zr0.25O2. (b) Ni/Ce0.75Zr0.25O2. (c) NieCe0.75Zr0.25O2 systems. The localized Ce(4f) states corresponding to surface Ce 6-fold and 7-fold coordinated, and inner Ce3D cations are indicated.
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references
Fig. 7 e NieCe0.75Zr0.25O2 system side view. Surface Ce1, Ce2 and Ce3 cations, and inner Ce4 and Ce5 cations are shown. The topmost atom is the oxygen atom O1, which is the one removed in order to create the surface O-vacancy.
O hole. Thus, our findings are in agreement with the reported enhanced catalytic activity of Ni/CeO2eZrO2 materials because of the strong Ni interactions with CeO2eZrO2 mixed oxides [10].
4.
Conclusions
In this work, we have performed DFTþU calculations to better understand the behaviour of Ni/CeO2eZrO2 catalysts in hydrogen production during hydrocarbon reforming and partial oxidation processes. Our results show that Ni interactions with Ce0.75Zr0.25O2 mixed oxides provoke important crystal lattice distortions, which promote Ce4þ/Ce3þ couples formation. Ni adsorbs on Ce0.75Zr0.25O2(111) face, forming strong NieO bonds and disfavouring the creation of surface anion vacancies. On the other hand, Ni insertion into the slab promotes the formation of surface O defects, requiring 0.87 eV less than that on the bare Ce0.75Zr0.25O2(111) face. Thus, the calculations performed in this work indicate that Ni incorporation into the Ce0.75Zr0.25O2 slab facilitates the donation of surface oxygens; which could be related to the good catalytic performance and resistance to poisoning by carbon deposition of the Ni/CeO2eZrO2 solid solutions, during hydrogen production processes.
Acknowledgement The authors gratefully acknowledge the funding from Universidad de Buenos Aires-UBACyT No. 20020090200157, Universidad Nacional del Sur, CONICET-PIP No. 11220090100785, and ANPCyT PICT 560 and PICTR 656.
[1] Pefia MA, Gomez JP, Fierro JLG. New catalytic routes for syngas and hydrogen production. Appl Catal A Gen 1996;144: 7e57. [2] Vagia ECh, Lemonidou AA. Hydrogen production via steam reforming of bio-oil components over calcium aluminate supported nickel and noble metal catalysts. Appl Catal A Gen 2008;351:111e21. [3] Sutton D, Kelleher B, Ross JRH. Review of literature on the catalysts for biomass gasification. Fuel Process Technol 2001; 73:155e73. [4] Nolan M, Parker SC, Watson GW. The electric structure of oxygen vacancy defects at the low index surface of ceria. Surf Sci 2005;595:223e32. [5] Wang H, Gong X, Guo Y, Guo Y, Lu GZ, Hu P. A model to understand the oxygen vacancy formation in Zr-doped CeO2: electrostatic interaction and structural relaxation. J Phys Chem C 2009;113:10229e32. [6] Trovarelli A. Catalysis by ceria and related materials. Catalytic science series. England: Imperial College Press; 2002. [7] Yang Z, Wei Y, Fu Z, Lu Z, Hermansson K. Facilitated vacancy formation at Zr-doped ceria(111) surfaces. Surf Sci 2008;602: 1199e206. [8] Laosiripojana N, Assabumrungrat S. Methane steam reforming over Ni/CeeZrO2 catalyst: influences of CeeZrO2 support on reactivity, resistance toward carbon formation, intrinsic reaction kinetics. Appl Catal A 2005;290:200e11. [9] Dong WS, Roh H, Jun K, Park S, Oh Y. Methane reforming over Ni/CeeZrO2 catalysts: effect of nickel content. Appl Catal A 2002;226:63e72. [10] Roh HS, Potdar HS, Jun K. Carbon dioxide reforming of methane over co-precipitated NieCeO2, NieZrO2 and NieCeeZrO2 catalysts. Catal Today 2004;93e95:39e44. [11] Gonza´lez-Velasco J, Gutie´rrez-Ortiz M, Marc J, Botas J, Gonza´lez-Marcos M, Blanchard G. Contribution of cerium/ zirconium mixed oxides to the activity of a new generation of TWC. Appl Catal B Environ 1999;22:167e78. [12] Hori C, Permana H, Brenner A, More K, Rahmoeller K, Belton D. Thermal stability of oxygen storage properties in a mixed CeO2eZrO2 system. Appl Catal B Environ 1998;16: 105e17. [13] Ka spar J, Di Monte R, Fornasiero P, Graziani M, Bradshaw H, Norman C. Dependency of the oxygen storage capacity in zirconiaeceria solid solutions upon textural properties. Top Catal 2001;16:83e7. [14] Marquevich M, Czernik S, Chornet E, Montane D. Hydrogen from biomass: steam reforming of model compounds of fastpyrolysis oil. Energy & Fuels 1999;13:1160e5. [15] Galdamez JR, Garcia L, Bilbao R. Hydrogen production by steam reforming of bio-oil using coprecipitated NieAl catalysts. Acetic acid as a model compound. Energy & Fuels 2005;19:1133e42. [16] Jalowiecki-Duhamel L, Zarrou H, D’Huysser A. Low temperature hydrogen production from methane on cerium nickel- and zirconium-based oxyhydrides. Catal Today 2008; 138:124e9. [17] Yang Z, Woo TK, Hermansson K. Effects of Zr-doping on stoichiometric and reduced ceria: a first-principles study. J Chem Phys 2006;124:224704-1. [18] Conesa JC. Computer modeling of surfaces and defects on cerium dioxide. Surf Sci 1995;339:337e52. [19] Nolan M, Grigoleit S, Sayle DC, Parker SC, Watson GW. Density functional theory studies of the structure and electronic structure of pure and defective low index surfaces of ceria. Surf Sci 2005;576:217e29.
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[20] Eyring L. In: Gschneider KA, Eyring L, editors. Handbook on the physics and chemistry of rare earths; 1979. NorthHolland, Amsterdam. [21] Lascalea G, Lamas D, Pe´rez L, Cabanillas E, Walso¨e de Reca N. Synthesis of ZrO2e15 mol% CeO2 nanopowders by a pHcontrolled nitrateeglycine process. Mat Lett 2004;58: 2456e60. [22] Skorodumova NV, Baudin M, Hermansson K. Properties of CeO2 from first principles. Phys Rev B 2004;69. 0754011e075401-8. [23] Yang Z, Woo TK, Baudin M, Hermansson K. Atomic and electronic structure of unreduced and reduced CeO2 surfaces: a first-principles study. J Chem Phys 2004;120: 7741e9. [24] Cova F, Garcia Pintos D, Juan A, Irigoyen B. A first-principles modeling of Ni interactions on CeO2eZrO2 mixed oxide solid solutions. J Phys Chem C 2011;115:7456e65. [25] Kresse G, Furthmuller J. Efficiency of ab initio total energy calculations for metals and semiconductors using a plane wave basis set. Comp Mat Sci 1996;6:15e50. [26] Kresse G, Hafner J. Ab initio molecular dynamics for liquid metals. Phys Rev B Cond Matt Mat Phys 1993;47:558e61. [27] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865e8. [28] Kresse G, Joubert D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys Rev B Cond Matt Mat Phys 1999;59:1758e75.
[29] Blo¨chl PE. Projector augmented-wave method. Phys Rev B Cond Matt Mat Phys 1994;50:17953e79. [30] Monkhorst HJ, Pack JD. Special points for Brillouin-zone integrations. Phys Rev B 1976;13:5188e92. [31] Dudarev SL, Botton GA, Savrasov SY, Humphreys CJ, Sutton AP. Electron-energy-loss spectra and the structural stability of nickel oxide: an LSDAþU study. Phys Rev B Cond Matt Mat Phys 1998;57:1505e9. [32] Anisimov V, Zaanen J, Andersen O. Band theory and Mott insulators: Hubbard U instead of Stoner. J Phys Rev B Cond Matt Mat Phys 1991;44:943e54. [33] Markovits A, Skalli MK, Minot C, Pacchioni G, Lopez N, Illas F. The competition between chemical bonding and magnetism in the adsorption of atomic Ni on MgO(100). J Chem Phys 2001;115:8172e7. [34] Chafi Z, Keghouche N, Minot C. DFT study of NieCeO2 interaction: adsorption and insertion. Surf Sci 2007;601: 2323e9. [35] Conesa JC. Surface anion vacancies on ceria: quantum modelling of mutual interactions and oxygen adsorption. Catal Today 2009;143:315e25. [36] Skorodumova N, Simak S, Lundqvist B, Abrikosov I, Johansson B. Quantum origin of the oxygen storage capability of ceria. Phys Rev Lett 2002;89:166601e4. [37] Henkelman G, Arnaldsson A, Jonsson H. A fast and robust algorithm for Bader decomposition of charge density. Comput Mater Sci 2006;36:354e60.
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Oxygen vacancy formation on the Ni/Ce0.75Zr0.25O2(111) surface. A DFTDU study Delfina Garcı´a Pintos a, Alfredo Juan b, Beatriz Irigoyen a,* a
Departamento de Ingenierı´a Quı´mica, Facultad de Ingenierı´a, Universidad de Buenos Aires, Pabello´n de Industrias, Ciudad Universitaria, 1428 Capital Federal, Argentina b Instituto de Fı´sica del Sur, Universidad Nacional del Sur. Avda. Alem 1253, 8000 Bahı´a Blanca, Argentina
article info
abstract
Article history:
Ni/CeO2eZrO2 solid solutions are well known by their promissory catalytic role in hydrogen
Received 6 September 2011
production processes such as reforming and partial oxidation of hydrocarbons. CeO2eZrO2
Accepted 12 December 2011
support can effectively disperse Ni particles and donate oxygen promoting Ni active phase
Available online 2 January 2012
resistance to poisoning by carbon deposition. Thus, in this work we study the influence of Ni on oxygen vacancy formation on the Ce0.75Zr0.25O2(111) surface, by using the density
Keywords:
functional theory (DFT) approach with the Hubbard’s correction (U) for Ce(4f) electrons. We
DFTþU
performed DFTþU energetic calculations on Ni/Ce0.75Zr0.25O2 (Ni is adsorbed over an OeO
Ni/CeO2eZrO2
bridge site) and NieCe0.75Zr0.25O2 (Ni is inserted into the slab) systems, with an O-vacancy.
Ce0.75Zr0.25O2
Moreover, we analyzed the electronic structures of these solids by drawing their spin
O-vacancy
polarized density of states (DOS) plots. Our results indicate that Ni insertion between two O subsurface layers facilitates Ce0.75Zr0.25O2 mixed oxide reduction and enhances its ionic mobility and oxygen donation properties. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Hydrogen has attracted great attention due to its potential for sustainable energy generation, especially because of its high energy content per unit mass and low level of environmental pollutants emission. Currently, H2 is mainly generated from fossil fuels by catalytic steam reforming and partial oxidation processes [1], which imply a strong dependence on non-renewable resources and the discharge of contaminants that harm the environment. These disadvantages promote the use of alternative fuels for H2 production. The biomass as renewable resource with neutral emission of CO2 has emerged as an interesting option [2]. Aqueous fractions of bio-fuels have also
gained attention for their catalytic steam reforming to produce H2 [3]. The catalysts most widely used in these processes contain Ni or noble metals in their formulation. The solids based on Ni are preferably chosen for their low cost, although they easily deactivate by metal sintering and coking. This poor achievement can be improved by using reducible CeO2-based supports. CeO2 can itself offer active sites for molecules interaction during catalytic reactions [4]. This characteristic is originated in the Ce cations reduction facility, which is mainly related to the formation of Ce4þ/Ce3þ couples and oxygen release/storage capacity (OSC) [5]. CeO2 performance is enhanced by the addition of transition, alkaline earth or rareearth metals [6,7]. Particularly, the doping with Zr results in
* Corresponding author. Tel.: þ54 11 45763241; fax: þ54 11 45763240. E-mail address: [email protected] (B. Irigoyen). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.12.079
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better redox properties, oxygen storage capacity, thermal resistance and catalytic activity [8e13]. Ni/CeO2eZrO2 solid solutions are promising catalysts for H2 generation during hydrocarbon reforming and partial oxidation processes [14,15]. Among the benefits of using CeO2eZrO2 mixed oxides as support we can mention their ability to effectively disperse Ni, which results in a better resistance to poisoning by carbon deposition [16]. The presence of Zr4þ cations substantially diminishes the energy necessary for Ce4þ to Ce3þ reduction and facilitates the formation of anionic defects in CeO2 [7,17,18]. In spite of the importance of surface Ni/CeO2eZrO2 catalysts redox behaviour, the influence of Ni on surface oxygen vacancies formation, which is related to Ce4þ/Ce3þ couples, is not yet thoroughly understood. Thus, in this work we have studied the effect of Ni interactions on/in Ce0.75Zr0.25O2 mixed oxide on its capacity to donate oxygen atoms. The knowledge of this effect at fundamental level would help to explain the reactivity of Ni/CeO2eZrO2 solid solutions as promissory catalysts for H2 production. We performed DFTþU periodic calculations by using the density functional approach (DFT) with the Hubbard correction (U) for Ce(4f) electrons. Then, we calculated the energy necessary for a surface oxygen vacancy formation on the bare Ce0.75Zr0.25O2(111) and compared its value to that on Ni/ Ce0.75Zr0.25O2(111) (Ni is adsorbed on a bridge OeO site) and NieCe0.75Zr0.25O2(111) (Ni is inserted between oxygen subsurface layers) systems. Also, we evaluated the changes in their electronic structures by means of the density of states (DOS) plots.
2.
Computational methods
2.1.
Solid and surface models
Cerium oxide (CeO2) is an insulating, non-magnetic rare-earth oxide [19]. It has a cubic fluorite structure, which is a facecentred cubic structure (FCC), with an experimental lattice ˚ [20]. This structure has four Ce and parameter value of 5.41 A eight O atoms per unit cell, in which Ce cations are located on the FCC lattice sites and O anions on the tetrahedral holes. In CeO2eZrO2 mixed oxides, Zr is easily inserted without generating excessive tensions in the lattice, as both oxides have similar structures [21]. In this work, CeO2eZrO2 mixed oxides has been represented with a 25% Zr-doped CeO2 model [18]. The crystal structure of the Ce0.75Zr0.25O2 mixed oxide is shown in Fig. 1(a). In order to model the most stable Ce0.75Zr0.25O2 catalytic surface [18,19,22], an extra O layer has been added to the (111) plane. This surface was modelled as a stack of OeCeeO sandwiches to prevent the appearance of a nonzero dipole moment normal to the surface [23]. In a previous work we have found that the most stable Ni interactions on the Ce0.75Zr0.25O2(111) surface corresponds to its deposition over OeO bridge sites [24]. These preferential Ni adsorption sites, named “W” and “X”, are shown in Fig. 1(b). As it can be seen, these sites are located between two O anions surrounded by Ce and Zr cations. The W site has a Zr as the nearest neighbour metal cation, while the X site has a Ce as nearest neighbour cation.
2.2.
Calculations
In this work, we have performed periodic energy calculations on the Ni/Ce0.75Zr0.25O2 system. We used the functional density theory (DFT) approach, implemented with the Viena Ab-initio Simulation Package (VASP) code [25,26]. We used the generalized gradient approximation with Perdew-BurkeErnzerhof (PBE) functional [27], to solve the KohneSham equations. The core electrons have been treated with the projected augmented waves (PAW) approximation [28,29]. The valence electrons are: Ce (5s), (5p), (5d), (4f) and (6s), O (2s) and (2p), and Zr (5s), (4d) and (5p). The KohneSham orbitals have been expanded in plane waves with a cut-off energy value of 400 eV. For the integration of the Brillouin zone a 4 4 1 k-points grid has been employed, following the MonkhorstePack scheme [30]. Within all this, we found ˚. a Ce0.75Zr0.25O2 lattice parameter value of 5.41 A The standard DFT formulation usually fails to describe strongly correlated electrons behaviour. This limitation can be corrected using the DFTþU method, which introduces a Hubbard parameter “U” for the description of the on-site interactions of those electrons [31,32]. Thus, in this work we used the DFTþU methodology with an U value of 5.0 eV to describe the interactions of Ce(4f) electrons [5,19].
3.
Results and discussion
In a previous work, we have optimized the atomic positions of the three different systems: bare Ce0.75Zr0.25O2, Ni/ Ce0.75Zr0.25O2 (Ni is adsorbed over the OeO bridge sites), and NieCe0.75Zr0.25O2 (Ni is inserted between two subsurface O layers) [24]. For that purpose, we employed a superlattice consisting on 12 layers of Ce, Zr y O atoms and a vacuum space ˚ . The atomic coordinates of Ni, Ce, Zr of approximately 13 A and O atoms located in the first six layers of the slab has been relaxed to find the optimum geometric structure of each system. The atomic positions were optimized until the ˚ . For the resulting force on each atom was less than 0.02 eV/A system total energy convergence, a tolerance of 104 eV was employed.
3.1.
Ni interactions with the Ce0.75Zr0.25O2 mixed oxide
The Ni adsorption (DENi,ads) and insertion (DENi,ins) energies have been evaluated with the following expressions [24]: DENi;ads ¼ E½Ni=Ce0:75 Zr0:25 O2 ð111Þ E½Ce0:75 Zr0:25 O2 ð111Þ E½Ni DENi;ins ¼ E½Ni Ce0:75 Zr0:25 O2 ð111Þ E½Ce0:75 Zr0:25 O2 ð111Þ E½Ni where: E[Ni/Ce0.75Zr0.25O2(111)] and E[NieCe0.75Zr0.25O2(111)] are the energy values of the Ce0.75Zr0.25O2(111) slab with Ni adsorbed and inserted, respectively. E[Ce0.75Zr0.25O2(111)] is the energy of the bare slab. E[Ni] is the energy corresponding to the lowest triplet Ni atom state: E[Ni(d9,s1)] ¼ 0.52 eV [33]. With these expressions, negative values of DENi,ads and DENi,ins indicate Ni favourable interactions.
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Fig. 1 e Ce0.75Zr0.25O2 mixed oxide. (a) Fluorite-type structure with indication of the (111) plane (as a shadowed area). (b) Ni preferential adsorption OeO bridge sites: W (the O nearest neighbour metal cation is a Zr), and X (the O nearest neighbour metal cation is a Ce).
Our previous calculations showed that Ni deposition on top of Ce cations of the Ce0.75Zr0.25O2 (111) surface is not favourable, evidencing the repulsive character of NieCe interactions [34]. On the other hand, Ni interactions on the Zr and O ions and over the OeO bridge sites are favourable. Ni is preferably adsorbed on the different sites in the following order: on top O < on top Zr < over the OeO bridge site X < over the OeO bridge site W [24]. The calculated energy values for Ni adsorption over the OeO bridge site W (Ni/Ce0.75Zr0.25O2 system) and Ni insertion between two subsurface O layers (NieCe0.75Zr0.25O2 system) are shown in Table 1. The most stable Ni interaction results from its location over an OeO bridge site, near to the Zr-dopant. When Ni is adsorbed over the OeO bridge site W,
˚ and 1.92 A ˚ , respecthe two NieO bond distances are 1.86 A tively. The Zr cation located near Ni is moved down to an O of ˚ , and the O bonded to Zr is relaxed the third layer by 0.18 A ˚ (see Fig. 2(a)). outward the surface by 0.3 A Ni insertion energy is comparable to that of Ni deposition over the W site (see Table 1, DENi,abs ¼ 3.88 eV). Ni is bonded to the oxygen atoms of the upper and lower O layers, with ˚ and 2.07 A ˚ , and NieOthird layer NieOsecond layer distances of 1.96 A ˚ . Note that these distances are similar to distances of 2.03 A ˚ ). In the optimized geombonds length in nickel oxide (2.08 A etry, the separation between the O layers into which Ni is ˚ (1.5 A ˚ in the bare CeeZr slab). As inserted increases to 2.18 A a result, surface Ce, Zr and O atoms are displaced from their original positions. Particularly dramatic results the anionic
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Table 1 e Energy values for Ni preferential interactions on/in the Ce0.75Zr0.25O2 (111) slab. Preferential interaction sites
DENi ads/ins (eV)
Ni adsorption over the oxygen bridge site “W” Ni insertion between two subsurface oxygen layers
4.21 3.88
relaxations, with surface and subsurface oxygens being moved ˚ as can be seen in Fig. 2(b). up to 1.14 A
3.2. Formation of surface oxygen vacancies in the Ce0.75Zr0.25O2, Ni/Ce0.75Zr0.25O2 and NieCe0.75Zr0.25O2 systems In this section, we have studied the oxygen donation ability of the systems resulting from Ni interactions with CeO2eZrO2 mixed oxides. Thus, we calculated the energy necessary for releasing oxygen O1 from the surface (see Fig. 3 for O1 vacancy location). This oxygen has been selected because of its location near to a Zr cation, which is known for its anion vacancy formation promoter effect [7]. The calculated energy values were compared with that obtained for the formation of a similar O-vacancy on the bare Ce0.75Zr0.25O2(111) surface. We introduced the O defect on the topmost O layer of Ce0.75Zr0.25O2(111), Ni/Ce0.75Zr0.25O2(111) and NieCe0.75Zr0.25 O2(111) systems and then optimized the ions atomic positions keeping fixed the lattice parameter. The surface O-vacancy formation energy on these systems was calculated in accordance with the following expressions: DEOvac:;CeO2 ¼ E½Ce0:75 Zr0:25 O2 ð111Þ x E½Ce0:75 Zr0:25 O2x ð111Þ E½O2 2 DEOvac:;Ni W ¼ E½Ni=Ce0:75 Zr0:25 O2 ð111Þ x E½Ni=Ce0:75 Zr0:25 O2x ð111Þ E½O2 2
DEOvac:;Ni ins ¼ E½Ni Ce0:75 Zr0:25 O2 ð111Þ E½Ni x Ce0:75 Zr0:25 O2x ð111Þ E½O2 2 where: E[Ce0.75Zr0.25O2(111)], E[Ni/Ce0.75Zr0.25O2 (111)] and E [NieCe0.75Zr0.25O2(111)], indicate the energy values of bare Ce0.75Zr0.25O2 mixed oxide, and that for Ni adsorbed on the W site and Ni inserted on/in the Ce0.75Zr0.25O2 slab. E E[Ni/Ce0.75Zr0.25O2x(111)] and E [Ce0.75Zr0.25O2x(111)], [NieCe0.75Zr0.25O2x(111)] are the energy values of the same systems with a surface O-vacancy near to a Zr-doping centre. E[O2] is the energy value for the O molecule in the vacuum. In Table 2, we show the calculated energy values for the formation of a surface O-vacancy in these systems. The easy of O1 vacancy formation on the different studied systems, follows the order: NieCe0.75Zr0.25O2 > Ce0.75Zr0.25O2 > Ni/ Ce0.75Zr0.25O2. As it can be seen, the formation of the O1 vacancy is more favourable on the NieCe0.75Zr0.25O2(111) surface. The O atoms located in the three first anionic layers show important movements outward the surface. Particularly, one of the ˚. subsurface O anions is moved up to the anion hole by 1.57 A The O anions displacements are shown in Fig. 4(a). The creation of oxygen O1 vacancy has similar relaxation effects on neighbour Ce and Zr cations, with surface cerium being dis˚ as it is shown in Fig. 4(b). placed up to 0.74 A
3.3. Electronic structure of the stoichiometric and reduced Ce0.75Zr0.25O2, Ni/Ce0.75Zr0.25O2 and NieCe0.75Zr0.25O systems CeO2, an insulating and non-magnetic rare-earth oxide, shows a characteristic gap between the valence and the conduction bands [19]. From the electronic point of view, every time an O-vacancy is formed on this oxide, Ce(4f) states are occupied in two neighbouring Ce cations [4,18,35]. Consequently, these localized Ce(4f) states appear in the band gap; which gives the solid the character of semiconductor [7]. The CeeZr mixed oxide total density of states (DOS) calculated
Fig. 2 e Ni interactions on/in the Ce0.75Zr0.25O2 (111) slab. (a) Ni/Ce0.75Zr0.25O2 system (Ni is adsorbed over the OeO bridge site W ). (b) NieCe0.75Zr0.25O2 system (Ni is inserted between two subsurface O layers).
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Fig. 3 e Ni/Ce0.75Zr0.25O2 system top view. Ni is adsorbed on the OeO bridge site W. The striped circle indicates the O1 vacancy (note that Ce1 and Ce2 cations become 6-fold O-coordinated, while Ce3 cation remains 7-fold O-coordinated).
in this work exhibits these characteristics; which emerge from the comparison of the spin polarized DOS plots of Ce0.75Zr0.25O2 systems in oxidized (see Fig. 5) and reduced (see Fig. 6(a)) states. In CeO2eZrO2 solids, the O vacancies formation and the ionic mobility are significantly enhanced due to CeO2 lattice distortion by Zr doping [17]. Besides, the analysis of the atomic structure modifications, caused by Ni adsorption and insertion on/in the Ce0.75Zr0.25O2 mixed oxide, have shown important relaxations of O belonging to inner layers. These O anions were moved up to the surface empty sites originated by Ni interactions. Their displacements, in agreement with reported diffusive movement of O atoms towards the CeO2 surface [36], can also contribute to electron localization on Ce(4f) orbitals. Thus, we draw the total DOS plots and performed a Bader charge analysis [37], to evaluate the electronic structure of Ni/ Ce0.75Zr0.25O2 and NieCe0.75Zr0.25O2 systems. Our results indicate that Ce3þ/Ce4þ redox couples formation was promoted by the important O atoms relaxations resulting from Ni adsorption and insertion on/in the CeeZr slab. On the other hand, our DFTþU calculations show that Ni deposition disfavours in 0.46 eV the creation of surface anion vacancies on Ce0.75Zr0.25O2 mixed oxides. However, Ni insertion into the slab promotes surface O defects formation, and the
Table 2 e Calculated energy values for an anionic vacancy formation on the (111) surface of Ce0.75Zr0.25O2, Ni/ Ce0.75Zr0.25O2a and NieCe0.75Zr0.25O2b systems. Systems Ce0.75Zr0.25O2 Ni/Ce0.75Zr0.25O2 NieCe0.75Zr0.25O2 a Ni is adsorbed on a bridge OeO site. b Ni is inserted between oxygen subsurface layers.
DEO-vac. (eV) 2.21 2.67 1.34
required energy is 0.87 eV lower than that on the bare Ce0.75Zr0.25O2(111) face (see NieCe0.75Zr0.25O2(111) in Table 2). Looking at the electronic structure, we realized that the DOS plots corresponding to Ni/Ce0.75Zr0.25O2 (see Fig. 6(b)) and NieCe0.75Zr0.25O2 (see Fig. 6(c)) systems with a surface Ovacancy, show peaks localized in the gap between valence and conduction band. This is a distinctive feature of the DOS plot for reduced Ce0.75Zr0.25O2 mixed oxide as it can be seen in Fig. 6(a). The DOS plot for Ni/Ce0.75Zr0.25O2 system with Ni adsorbed over the OeO bidge site W and a vacancy of oxygen O1, shows three Ce(4f) peaks localized in the gap. These peaks correspond to Ce1, Ce2 and Ce3 cations (see Fig. 7, for Ce1eCe3 locations). Note that Ce1(4f) peak, localized at 0.23 eV was originated by Ni deposition, while Ce2(4f) and Ce3(4f) peaks (localized at 0.01 and 0.3 eV, respectively), appears after O1 vacancy formation. On the other hand, the DOS curve for NieCe0.75Zr0.25O2 system with Ni inserted into the slab and a defect of oxygen O1, shows four peaks Ce(4f) localized in the gap. These peaks were formed by contributions of surface and inner Ce cations. Ce3(4f) and Ce4(4f) peaks (localized at 0.61 and 0.09 eV, respectively), were originated by Ni insertion into the slab. Ce2(4f) and Ce5(4f) peaks (localized at 0.07 and 0.01 eV, respectively), appears due to the surface O1 vacancy creation. In addition, is important to note that the lower energy peak that appears in the gap between valence and conduction band in the DOS plots for the three studied systems (see Fig. 6(aec)) is formed by Ce(4f) orbitals of the heptahedral Ce3 cation. This is in agreement with reported results indicating that electrons left behind upon oxygen removal prefer to be localized on surface Ce(4f) orbitals of Ce atoms with higher coordination number (7-fold coordinated) [35].
3.4.
Discussion
Our results indicate that Ni adsorption and insertion on/in Ce0.75Zr0.25O2 mixed oxide cause important relaxations of
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Fig. 4 e NieCe0.75Zr0.25O2 slab side view with indication of Ce, Zr and O relaxations due to a surface O-vacancy creation. (a) Anions movements. (b) Cations movements.
surface and inner O atoms. NieO interactions provoke the displacement of these O, by moving them away from bonded Ce cations, and thus generate localized Ce(4f) states in the DOS band gap of Ni/Ce0.75Zr0.25O2 and NieCe0.75Zr0.25O2 systems. On the other hand, the energy values calculated for Ovacancy formation on the Ce0.75Zr0.25O2(111) surface show that Ni adsorption disfavours the oxygen release. The energy necessary for an O defect creation in Ni/Ce0.75Zr0.25O2 is 0.46 eV higher than that on the bare mixed oxide. However, Ni insertion into the slab significantly facilitates the formation of surface oxygen vacancies (the required energy is 0.87 eV lower than that on the bare Ce0.75Zr0.25O2(111) face). The atomic and electronic structures of CeO2eZrO2 mixed oxides are intimately related to their catalytic behaviour. Besides, the performance of Ni/CeO2eZrO2 solid solutions is connected with their reducibility and oxygen mobility properties. The results discussed in this work indicate that Ni diffusion into the slab could enhance anionic mobility, Ce4þ/ Ce3þ couples formation and oxygen donation in CeO2eZrO2 mixed oxides. Moreover, the promoter effect of inserted Ni could be related to the important relaxations of subsurface O atoms observed in NieCe0.75Zr0.25O2 system, where those oxygens experience dramatic movements upward the surface
Fig. 5 e Spin polarized density of states (DOS) plot for the bare Ce0.75Zr0.25O2 system.
Fig. 6 e Spin polarized density of states (DOS) plot for the different studied systems with a surface oxygen vacancy. (a) Ce0.75Zr0.25O2. (b) Ni/Ce0.75Zr0.25O2. (c) NieCe0.75Zr0.25O2 systems. The localized Ce(4f) states corresponding to surface Ce 6-fold and 7-fold coordinated, and inner Ce3D cations are indicated.
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references
Fig. 7 e NieCe0.75Zr0.25O2 system side view. Surface Ce1, Ce2 and Ce3 cations, and inner Ce4 and Ce5 cations are shown. The topmost atom is the oxygen atom O1, which is the one removed in order to create the surface O-vacancy.
O hole. Thus, our findings are in agreement with the reported enhanced catalytic activity of Ni/CeO2eZrO2 materials because of the strong Ni interactions with CeO2eZrO2 mixed oxides [10].
4.
Conclusions
In this work, we have performed DFTþU calculations to better understand the behaviour of Ni/CeO2eZrO2 catalysts in hydrogen production during hydrocarbon reforming and partial oxidation processes. Our results show that Ni interactions with Ce0.75Zr0.25O2 mixed oxides provoke important crystal lattice distortions, which promote Ce4þ/Ce3þ couples formation. Ni adsorbs on Ce0.75Zr0.25O2(111) face, forming strong NieO bonds and disfavouring the creation of surface anion vacancies. On the other hand, Ni insertion into the slab promotes the formation of surface O defects, requiring 0.87 eV less than that on the bare Ce0.75Zr0.25O2(111) face. Thus, the calculations performed in this work indicate that Ni incorporation into the Ce0.75Zr0.25O2 slab facilitates the donation of surface oxygens; which could be related to the good catalytic performance and resistance to poisoning by carbon deposition of the Ni/CeO2eZrO2 solid solutions, during hydrogen production processes.
Acknowledgement The authors gratefully acknowledge the funding from Universidad de Buenos Aires-UBACyT No. 20020090200157, Universidad Nacional del Sur, CONICET-PIP No. 11220090100785, and ANPCyT PICT 560 and PICTR 656.
[1] Pefia MA, Gomez JP, Fierro JLG. New catalytic routes for syngas and hydrogen production. Appl Catal A Gen 1996;144: 7e57. [2] Vagia ECh, Lemonidou AA. Hydrogen production via steam reforming of bio-oil components over calcium aluminate supported nickel and noble metal catalysts. Appl Catal A Gen 2008;351:111e21. [3] Sutton D, Kelleher B, Ross JRH. Review of literature on the catalysts for biomass gasification. Fuel Process Technol 2001; 73:155e73. [4] Nolan M, Parker SC, Watson GW. The electric structure of oxygen vacancy defects at the low index surface of ceria. Surf Sci 2005;595:223e32. [5] Wang H, Gong X, Guo Y, Guo Y, Lu GZ, Hu P. A model to understand the oxygen vacancy formation in Zr-doped CeO2: electrostatic interaction and structural relaxation. J Phys Chem C 2009;113:10229e32. [6] Trovarelli A. Catalysis by ceria and related materials. Catalytic science series. England: Imperial College Press; 2002. [7] Yang Z, Wei Y, Fu Z, Lu Z, Hermansson K. Facilitated vacancy formation at Zr-doped ceria(111) surfaces. Surf Sci 2008;602: 1199e206. [8] Laosiripojana N, Assabumrungrat S. Methane steam reforming over Ni/CeeZrO2 catalyst: influences of CeeZrO2 support on reactivity, resistance toward carbon formation, intrinsic reaction kinetics. Appl Catal A 2005;290:200e11. [9] Dong WS, Roh H, Jun K, Park S, Oh Y. Methane reforming over Ni/CeeZrO2 catalysts: effect of nickel content. Appl Catal A 2002;226:63e72. [10] Roh HS, Potdar HS, Jun K. Carbon dioxide reforming of methane over co-precipitated NieCeO2, NieZrO2 and NieCeeZrO2 catalysts. Catal Today 2004;93e95:39e44. [11] Gonza´lez-Velasco J, Gutie´rrez-Ortiz M, Marc J, Botas J, Gonza´lez-Marcos M, Blanchard G. Contribution of cerium/ zirconium mixed oxides to the activity of a new generation of TWC. Appl Catal B Environ 1999;22:167e78. [12] Hori C, Permana H, Brenner A, More K, Rahmoeller K, Belton D. Thermal stability of oxygen storage properties in a mixed CeO2eZrO2 system. Appl Catal B Environ 1998;16: 105e17. [13] Ka spar J, Di Monte R, Fornasiero P, Graziani M, Bradshaw H, Norman C. Dependency of the oxygen storage capacity in zirconiaeceria solid solutions upon textural properties. Top Catal 2001;16:83e7. [14] Marquevich M, Czernik S, Chornet E, Montane D. Hydrogen from biomass: steam reforming of model compounds of fastpyrolysis oil. Energy & Fuels 1999;13:1160e5. [15] Galdamez JR, Garcia L, Bilbao R. Hydrogen production by steam reforming of bio-oil using coprecipitated NieAl catalysts. Acetic acid as a model compound. Energy & Fuels 2005;19:1133e42. [16] Jalowiecki-Duhamel L, Zarrou H, D’Huysser A. Low temperature hydrogen production from methane on cerium nickel- and zirconium-based oxyhydrides. Catal Today 2008; 138:124e9. [17] Yang Z, Woo TK, Hermansson K. Effects of Zr-doping on stoichiometric and reduced ceria: a first-principles study. J Chem Phys 2006;124:224704-1. [18] Conesa JC. Computer modeling of surfaces and defects on cerium dioxide. Surf Sci 1995;339:337e52. [19] Nolan M, Grigoleit S, Sayle DC, Parker SC, Watson GW. Density functional theory studies of the structure and electronic structure of pure and defective low index surfaces of ceria. Surf Sci 2005;576:217e29.
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[20] Eyring L. In: Gschneider KA, Eyring L, editors. Handbook on the physics and chemistry of rare earths; 1979. NorthHolland, Amsterdam. [21] Lascalea G, Lamas D, Pe´rez L, Cabanillas E, Walso¨e de Reca N. Synthesis of ZrO2e15 mol% CeO2 nanopowders by a pHcontrolled nitrateeglycine process. Mat Lett 2004;58: 2456e60. [22] Skorodumova NV, Baudin M, Hermansson K. Properties of CeO2 from first principles. Phys Rev B 2004;69. 0754011e075401-8. [23] Yang Z, Woo TK, Baudin M, Hermansson K. Atomic and electronic structure of unreduced and reduced CeO2 surfaces: a first-principles study. J Chem Phys 2004;120: 7741e9. [24] Cova F, Garcia Pintos D, Juan A, Irigoyen B. A first-principles modeling of Ni interactions on CeO2eZrO2 mixed oxide solid solutions. J Phys Chem C 2011;115:7456e65. [25] Kresse G, Furthmuller J. Efficiency of ab initio total energy calculations for metals and semiconductors using a plane wave basis set. Comp Mat Sci 1996;6:15e50. [26] Kresse G, Hafner J. Ab initio molecular dynamics for liquid metals. Phys Rev B Cond Matt Mat Phys 1993;47:558e61. [27] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865e8. [28] Kresse G, Joubert D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys Rev B Cond Matt Mat Phys 1999;59:1758e75.
[29] Blo¨chl PE. Projector augmented-wave method. Phys Rev B Cond Matt Mat Phys 1994;50:17953e79. [30] Monkhorst HJ, Pack JD. Special points for Brillouin-zone integrations. Phys Rev B 1976;13:5188e92. [31] Dudarev SL, Botton GA, Savrasov SY, Humphreys CJ, Sutton AP. Electron-energy-loss spectra and the structural stability of nickel oxide: an LSDAþU study. Phys Rev B Cond Matt Mat Phys 1998;57:1505e9. [32] Anisimov V, Zaanen J, Andersen O. Band theory and Mott insulators: Hubbard U instead of Stoner. J Phys Rev B Cond Matt Mat Phys 1991;44:943e54. [33] Markovits A, Skalli MK, Minot C, Pacchioni G, Lopez N, Illas F. The competition between chemical bonding and magnetism in the adsorption of atomic Ni on MgO(100). J Chem Phys 2001;115:8172e7. [34] Chafi Z, Keghouche N, Minot C. DFT study of NieCeO2 interaction: adsorption and insertion. Surf Sci 2007;601: 2323e9. [35] Conesa JC. Surface anion vacancies on ceria: quantum modelling of mutual interactions and oxygen adsorption. Catal Today 2009;143:315e25. [36] Skorodumova N, Simak S, Lundqvist B, Abrikosov I, Johansson B. Quantum origin of the oxygen storage capability of ceria. Phys Rev Lett 2002;89:166601e4. [37] Henkelman G, Arnaldsson A, Jonsson H. A fast and robust algorithm for Bader decomposition of charge density. Comput Mater Sci 2006;36:354e60.