Catalyst Design Based on DFT Calculations: Metal Oxide Catalysts for

May 15, 2014 - A method based on theoretical DFT calculations to predict the reactivity of supported metal oxide for selective catalytic reduction (SC...
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Catalyst Design Based on DFT Calculations: Metal Oxide Catalysts for Gas Phase NO Reduction Xuesen Du, Xiang Gao,* Wenshuo Hu, Jinpin Yu, Zhongyang Luo, and Kefa Cen State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, People’s Republic of China S Supporting Information *

ABSTRACT: A method based on theoretical DFT calculations to predict the reactivity of supported metal oxide for selective catalytic reduction (SCR) of NO with NH3 was developed in this study. Surface acidity, oxidizing ability, and reoxidability of the reduced catalyst are crucial for a NOx reducing catalyst. In this method, LUMO, hydrogenation, and HOMO energy of the reduced state were used to evaluate the above three properties, respectively. A series of supported metal oxides were calculated and discussed. Based on the computational results, these metal oxides were separated into three categories that can possibly act as “active component”, “promoter”, and “inactive component”, respectively. Experiments were performed to testify our theoretical conclusions. The comparison between experimental and theoretical results has led to an excellent match, which has proven our method to be an effective way to evaluate the reactivity of supported metal oxide for SCR of NO with NH3.

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INTRODUCTION Heterogeneous catalysis has become crucial to many aspects of energy and environmental science. Thus far, essentially all heterogeneous catalysts are developed by trial-and-error experimentally. There is no doubt that to be able to rationally design catalysts is one of the most attractive goals. Density functional theory (DFT) is a computational quantum mechanical modeling method to investigate the electronic structure of many-body systems. With this theory, the properties of a many-electron system can be determined by using functionals of the electron density. In the last two decades, DFT calculations have been widely used to understand catalytic systems. However, rational designs of catalysts from DFT calculations are rare.1,2 In this work, a method based on DFT calculations to evaluate the reactivity of a metal oxide catalyst was proposed. This method provides a methodology that approaches the rational design of catalysts. NO reduction, one of the hottest topics in environmental science, was used to demonstrate this method. NO emitted from mobile and stationary resources is one of the main gas-phase pollutants. Selective catalytic reduction (SCR) with NH3 is regarded as the state-of-the-art technology for its abatement. Since the invention of SCR technology by U.S. researchers in 1961, the study of this catalysis has been tremendously active during the past half century. Numerous chemists and engineers have been focusing on the development of a catalyst with high activity and low cost. Though the commercial V2O5−WO3/TiO2 or V2O5−MoO3/TiO2 catalysts have been commonly used for over two decades, the research on the novel NO-SCR catalysts has never slowed down due to several drawbacks of the commercial catalysts including: the relatively narrow temperature window (300−400 °C); the toxicity of vanadium pentoxide; and the high conversion of SO2 © XXXX American Chemical Society

to SO3 with increasing vanadium amounts. A number of supported metal oxides, such as Fe2O3,3 CuO,4 CeO2,5 MnO2,6 and so on, were found to be suitable as promoters or main catalysts. Benefiting from the rapid development in catalyst characterization, the mechanism of SCR reaction has reached a fairly reliable level. The deep understanding of the reaction mechanism has promoted the rational catalyst design to be possible. While the development of SCR catalyst is still relying on experiments. In the last two decades, density functional theory (DFT) calculations have been well improved and widely used to understand this catalytic system. For SCR of NO (NOSCR) with NH3, researchers have employed DFT calculations to reveal the mechanism7 and the poisoning effect by chemical species8,9 theoretically. As published, theoretical DFT calculations can present the reaction pathway structurally and energetically at the atomic level, which have provided a promising supplement to the experimental study. However, rational design of catalysts from DFT calculations has not been reported in previous literatures. In this work, theoretical data from DFT calculations were related with the chemical properties of supported metal oxide molecules to determine the reactivity of these components for NO-SCR with NH3.

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THEORETICAL AND EXPERIMENTAL Theoretical calculations were performed with the Gaussian 0310 package using the gradient corrected Becke’s11,12 threeparameter hybrid exchange functional in conjunction with the correlation functional of Lee, Yang, and Parr13 (B3LYP). Throughout the calculation process, the O, H, Si, and Al atoms Received: February 27, 2014 Revised: May 8, 2014

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NH3 in N2. The catalytic reactions were carried out at temperatures from 150 to 450 °C with an interval of 50 °C under atmosphere pressure, with a total flow rate of 1200 mL/ min GHSV of 3.6 × 105 ml/(g·h). The NO concentrations before and after reaction were determined using a Thermo Fisher NOx analyzer. In excess of oxygen and with NH3/NO (molar ratio) ≥ 1, this process can be assumed to be first order with respect to NO and zero order with respect to NH3.14,15 Thus, the catalytic activity can be described by the reaction rate constant k, which was calculated according to the following rate expression:16,17

were treated by 6-31G** basis set. For other atoms in this study, except Ce, the Los Alamos set of double-ζ type basis set (LANL2DZ) was employed. Due to the complex electronic condition of the lanthanide elements, the Ce atoms were treated by the sdd basis set. The initial structures of the oxides were created as those shown in Figure S1. In the real environment of the metal oxides, the metallic O atoms are further bonded to outer atoms. In our cluster, hydrogen atoms were used to satisfy the periphery O atoms and maintain the charge neutrality. During geometry optimizations, H atoms were fixed in the M−O−H direction. All the structures were optimized to create the stationary models. For the structures of the hydrogenated oxides, one H atom (from NH3, as demonstrated in Figure 1) was added onto the optimized

k=−

F ln(1 − X ) W

where k is the reaction rate constant based on mass of samples (cm3 g−1 s−1), F is the gas flow rate (mL s−1), W is the weight of catalyst (g), and X was the fractional NO conversion.

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RESULTS AND DISCUSSION We started from the typical mechanism of the NO-SCR reaction over titania supported V2O5 catalyst. According to previous studies,7,18 NO-SCR with NH3 undergoes three important steps, including: (1) adsorption of NH3 on catalyst; (2) oxidation of intermediates by catalyst; and (3) reoxidation of the reduced catalyst. This mechanism can also be applied to the SCR reaction over other metal oxide catalysts, as illustrated in Figure 1. In step 1, the basic NH3 molecule tends to donate its electrons to share with acidic molecules. Step 2 requires a good oxidizing ability of the catalyst and the rate of step 3 is determined by the reoxidability of the reduced molecule. Among various computational theories, frontier molecular orbitals (FMO) theory has become one of the most popular theories to evaluate the interaction between two molecules and molecule reactivity since its invention by Kenichi Fukui.19 In our study, we intend to correlate the FMO theory with the reactivity of a supported metal oxide for SCR reaction. In FMO theory, lowest unoccupied molecular orbital (LUMO)/highest occupied molecular orbital (HOMO) implies the ability to accept/donate electrons of a molecule. The lower the LUMO energy is, the more favorable it is to adsorb electrons from basic NH3. Therefore, the NH3 adsorbing ability can be evaluated by LUMO. Step 2, as shown in Figure 1, is the capture of a hydrogen atom from adsorbed intermediate by the metal oxide. This procedure can be simulated by the hydrogenation of the oxide molecule. The hydrogenation energy can be used to evaluate the oxidizing ability. Step 2 has left the catalyst being reduced. Reoxidation of the reduced catalyst requires the reduced catalyst to easily donate electrons to oxidant, such as gas phase oxygen. The higher the HOMO energy is, the easier it is to donate electrons. Thus, the reoxidability can be evaluated by the HOMO energy of the reduced molecule. The relationship between SCR reactivity and theoretical properties is summarized in Figure 1. The method has been demonstrated above. In order to assess the reactivity of a molecule, the criterion should be set first. Since the purpose of this study is to evaluate the reactivity of metal oxides for NO-SCR with NH3, the reliable and widely used commercial V2O5−WO3/TiO2 catalyst can be made as the criterion. Monomeric pentavalent vanadia oxide (V5+Ox), hexavalent tungsten oxide (W6+Ox) and tetravalent titania oxide (Ti4+Ox) were modeled (Table S1) and computed. Their LUMO orbitals and hydrogenation energies are shown in Table 1.

Figure 1. Relationship between catalyst properties and theoretical parameters.

metal oxides. The hydrogenated structures have also been optimized. Their initial structures are given in Figure S2. The hydrogenation energy was calculated by Ehydrogenation = EMOx‑H − EMOx − EH, where EMOx‑H is the energy of the reduced structure, EMOx is the energy of metal oxide, EH is the energy of a gas phase H atom. The stationary reduced structures were achieved from the hydrogenated structures. If H2O molecules were formed after hydrogenation, the reduced structures were obtained by removing the H2O molecules. Otherwise, the reduced model is consistent with the hydrogenated model. The structures and HOMO snapshots are shown in Figure S3. The supported metal oxides were prepared using impregnation method. A commercial P25 TiO2 with a surface area of 55 m2/g was used as the support. NH4VO3, (NH4)10W12O41, Cu(NO3)2, Mn(NO3)2, Fe(NO3)3, C10H5NbO20, (NH4)6Mo7O24, Zr(NO3)4, Cr(NO3)3, Co(NO3)3, Al(NO3)3, Zn(NO3)2, GeCl4, C6H9O6Sb, SnC4H6O4, Ga(NO3)3, In(NO3)3, and Ce(NO3)3 were used as the precursors of the corresponding metal oxides. Aqueous solutions were used for all the impregnations, with the exception of GeCl4. Anhydrous alcohol solution was used in this case, due to the hydrolysis and decomposition of GeCl4 in water. After impregnation, the mixture was heated at 70 °C in a water bath for 4 h and then dried at 105 °C for 12 h. Afterward, the catalyst was calcined at 500 °C in air for 5 h. To well compare the performances of different supported metal oxides, the same loading rate (M/Ti = 1 mol %) was applied. The catalytic activity tests for the reduction of NO with NH3 were carried out in a fixed bed reactor for 0.2 g catalyst samples with particle size of 250−380 μm. The simulated gas for these tests contained 1000 ppm of NO, 5 vol % O2, and 1000 ppm of B

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among the LUMO energies of V5+Ox, W6+Ox, and Ti4+Ox. The high LUMO energy makes Ti4+Ox much unfavorable to accept electrons from another molecule. Thus, Ti4+Ox can hardly catalyze the SCR reaction. This makes it suitable to be the criterion for supports or nonactive components. The content above has been focusing on the acidity and oxidizing ability of the metal oxides. The reoxidability of the reduced active site is also crucial for NO-SCR reaction. Since V5+Ox is the criterion for main active sites, the HOMO energy of the reduced from of vanadia oxide (V4+Ox-H), as shown in Table S1, can be used to further evaluate the reactivity of a metal oxide after the initial screen using LUMO energies and hydrogenation energies. To model the supported metal oxides rationally and correctly, we have globally checked their oxidation states and existing forms on titania support at low loading rate. The supported metal oxides are mostly monomeric on titania at low loading rate. Based on the reported structures of the metal oxides, the structures in Figure S1 are applied for various metal oxides. For the calculation of the hydrogenation energies, one H atom was added onto the optimized metal oxides (Figure S2). The stationary structures of the oxidized and reduced forms (for HOMO calculations) are shown in Tables S2 and S3, respectively. All the structures have been optimized. A series of the experimentally well-studied oxides have been calculated and analyzed in this study. First, the acidities of these oxides were tested through calculating the LUMO energies. The result is shown in Figure 2. The LUMO energies of the oxides in gray (Al3+, Si4+, Ga3+, Ge4+, Zr4+, In3+, Sn4+ oxides) are higher than that of Ti4+Ox. The higher LUMO energy implies poorer acidity, which will lead to the poor NH3 adsorption ability. For the oxides in green (W6+, Zn2+, Nb5+, Mo6+, Sb5+ oxides), their LUMO energies are between that of Ti4+Ox and V5+Ox. Thus, similar to W6+Ox, these oxides can possibly raise the acidity and enhance the NH3 adsorption of the SCR catalyst. Among the oxides that had been calculated, Cr5+, Mn4+, Fe3+, Co2+, Cu2+, Ru4+, Rh4+, and Ce4+ oxides were found to possess lower LUMO energies than V5+Ox. As discussed above, lower LUMO energy implies stronger acidity. Since SCR catalyst requires strong acidity, the acidities of these oxides are sufficient for NH3 adsorption and conducting SCR reaction. Based on our initial hypothesis, the oxidizing ability of the oxide can be evaluated by hydrogenation energy. The larger

Table 1. LUMO Energies and Hydrogenation Energies of V5+Ox, W6+Ox, and Ti4+Ox and HOMO Energy of the Reduced V4+Ox-H

V5+Ox W6+Ox Ti4+Ox

LUMO energy (Hartree)

Ehydrogenation (kcal/mol)

HOMO energy of reduced state (Hartree)

−0.1018 −0.0538 −0.0284

−70.92 −30.02 −21.44

−0.2145

As reported,20,21 supported vanadium oxide has favorable acidity and a strong redox ability, which make it the main active component. In Table 1, V5+Ox has a LUMO orbital with a lowest energy of −0.1018 hartree and a largest hydrogenation energy of −70.92 kcal/mol. The low energy of LUMO will facilitate the attraction of electrons from NH3 and large hydrogenation energy can make the catalyst to easily oxidize the intermediate. Therefore, the basic NH3 can be easily adsorbed and intermediate can be readily oxidized on the vanadium sites. V5+Ox was made the criterion for main active sites in this study. According to Chen et al.,22 WO3/TiO2 is much less active than V2O5/TiO2 for SCR reaction. While supported tungsten oxide is an excellent promoter for V2O5/TiO2 catalyst, it will significantly increase the acidity, but not the oxidizing ability. Under the real condition of NO-SCR implemented in power plants, the harmful side reactions, such as NH3 direct oxidation and SO2 oxidation by O2, should be inhibited. This means the oxidizing ability of SCR catalyst cannot be too strong. The role of a promoter is to increase the surface acidity but not significantly increase the oxidizing ability. In our calculation, both the LUMO energy and hydrogenation energy of W6+Ox are between those of V5+Ox and Ti4+Ox. This implies that W6+Ox has an acidity stronger than Ti4+Ox, which can make the support to be more acidic. However, the smaller hydrogenation energy of W6+Ox than that of V5+Ox implies its poorer oxidizing ability. Thus, W6+Ox can be the criterion for promoters. Ti4+Ox is a superior support for the NO-SCR with NH3. But pure anatase TiO2 is barely active for the SCR reaction in typical temperature range (under 500 °C) because of its relatively poor redox ability when compared with active components and promoters. The small hydrogenation energy (−21.44 kcal/mol) can well explain the relatively poor oxidizing ability. The LUMO energy of Ti4+Ox is the highest

Figure 2. Theoretical LUMO energies of the oxides. C

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hydrogenation energy (more negative) means stronger oxidizing ability. Figure 3 shows the hydrogenation energies,

Figure 4. LUMO energies of the oxidized state and HOMO energies of the reduced state of the oxides in “Zone B” of Figure 3. Black (solid) symbol represents the LUMO energy of the oxidized state; blue (hollow) symbol represents the HOMO energy of the reduced state.

reduced V5+Ox, while, since its LUMO energy is much lower than that of V5+Ox, Cr5+Ox can still probably perform well. The reduced forms of Fe3+Ox and Co3+Ox are harder to be reoxidized and thus they possibly do not perform as well as V5+Ox. While the above discussion points out that the oxides in “Zone B” (Figure 3) can possibly be the promoters of SCR catalyst, another factor to consider is the interaction between these oxides and the main active components if they are simultaneously doped in the catalyst. As reported in the literature,23 the doped species with strong basicity will cause the deactivation of the main active components. Basic compounds tend to donate their electrons to the adjacent electronacceptors. The active sites are often lively electron-acceptors because of their strong oxidizing abilities. Once accepting the electrons from the doped compounds, the oxidizing ability of the main active components will drop and then the catalyst will be poisoned. As discussed above, the tendency of donating electrons can be evaluated by HOMO energy. The higher HOMO energy indicates the easier donation of electrons. Table 2 summarizes the HOMO energies of the oxide in “Zone B” of

Figure 3. Hydrogenation energies and their relationship with corresponding LUMO energies.

as well as the LUMO energies, to comprehensively evaluate the acidity and oxidizing ability of the oxides. It is interesting that the LUMO energies and hydrogenation energies show a well linear correlation. The lower LUMO energy corresponds to a larger hydrogenation energy. Essentially, this is derivable because the hydrogenation process is the donation of an electron from hydrogen to LUMO of the oxide. The lower LUMO can facilitate this approaching and donation, and thus, the hydrogenation energy will be greater. Consequently, it is potentially possible that the oxidizing ability can also be evaluated by the LUMO energies. Due to the linear relation, the hydrogenation result is in line with the LUMO conclusion. According to the V−W−Ti data, these oxides can be divided into three sections (Figure 3). The oxides in “Zone A” possess adequate acidity and oxidizing ability to be the main active components for SCR reaction. In “Zone B”, though the oxides are more acidic than the support, the oxidizing ability is not sufficient for SCR reaction. They can possibly be nice promoters, similar to tungsten oxide. The gray “Zone C” includes the oxides with poor acidity and oxidizing ability, which make these oxides barely active for SCR reaction. The exceptions in “Zone C” are those with well textured properties, which can make them to be proper support of the SCR catalyst. The reoxidability of the reduced oxide is crucial for the proceeding of the third step of the SCR reaction, as shown in Figure 1. HOMO energy can be used to evaluate the reoxidability. The HOMO energies of the oxides in “Zone A” of Figure 3 were calculated to further inspect their reactivity. LUMO energy of the oxidized state and HOMO energy of the reduced state are summarized in Figure 4. The data of vanadium oxide is set as the criterion. As can be seen from the HOMO data, the HOMO energies of the reduced Cu, Mn, Ru, and Rh oxides are close to that of the reduced V5+Ox. This indicates that these oxides can well process the reoxidation step as vanadium oxide does. However, because of their lower LUMO energies, these elements are possible to have better activities than vanadium oxide. The reduced Ce4+Ox can be much more easily reoxidized than the reduced V5+Ox. This might also make Ce4+Ox more active than V5+Ox. The HOMO energies of the reduced Cr5+Ox is slightly lower than that of

Table 2. HOMO Energies of the Oxides in Zone B of Figure 3 HOMO energy (Hartree) 6+

W Ox Zn2+Ox Nb5+Ox Mo6+Ox Sb5+Ox

−0.2973 −0.2525 −0.3097 −0.3065 −0.2863

Figure 3. The HOMO energies of Nb5+Ox and Mo6+Ox are lower than that of W6+Ox, which means these two components will donate less electrons to the main active sites than W6+Ox does. The HOMO energy of Sb5+Ox is slightly higher than that of W6+Ox and a much higher HOMO energy can be found for Zn2+Ox. This indicates that Zn2+Ox may noticeably donate electrons to main active components and cause the deactivation of the catalyst. Based on our theoretical calculations, the studied oxides have been classified into three categories, as shown in Figure 3. The oxides in Zones A−C can possibly be active, promotional, and inactive, respectively, for the NO reduction reaction. ExperiD

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Figure 5. (a) Rate constants of the SCR of NO with NH3 over the titania-supported metal oxides at 350 °C, the mark numbers’ colors are in accordance with the theoretical categories, “Zone A” in red, “Zone B” in green, and the “Zone C” in gray. (b) Influences of the “Zone B” metal oxides additions to the V2O5/TiO2 catalyst.

to be an effective way to evaluate the reactivity of metal oxide for SCR of NO with NH3.

ments have been carried out to testify our theoretical conclusions. As can be seen from Figure 5a, the TiO2supported metal oxides of “Zone A” are much more active than those belong to “Zone B” and “Zone C”. The sole supported “Zone B” metal oxides are barely active, as well as the “Zone C” species. In the experimental results of the “active site” metal oxides, it can be seen that apart from Fe and Co, other metal oxides are all more active than vanadium oxide at 350 °C. Fe and Co oxide are less active than vanadium oxide, and these can be attributed to the harder reoxidation of their corresponding reduced states, as shown in Figure 4 and discussed above. The possible promotional effect of the oxides in “Zone B” has also been testified. These metal oxides were doped on the TiO2 support before the main active component (vanadium oxide) was doped. Figure 5b shows the influence of these oxides to the NO conversion performance of the titania supported vanadium oxide. Tungsten, niobium, molybdenum, and antimony oxides are found to enhance the NO reduction, though an activity depression can be found at 450 °C for the antimony doped one, which has also been discussed in our previous research.24 However, zinc oxide was found to depress the NO conversion of the V2O5/TiO2 catalyst from 250 to 450 °C. This is in line with our theoretical speculation that zinc oxide tends to donate electrons to main active component and cause the deactivation of the catalyst. Thus, these experimental results perfectly support our theoretical conclusions.

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ASSOCIATED CONTENT

S Supporting Information *

Full description of the material. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-13505711887. Fax: +86-571-87951616. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 51076140), Development of China (863 Program; No. 2013AA065401), and Shanghai Tongji Gaotingyao Environmental Protection Science and Technology Development Foundation.

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CONCLUSIONS In summary, a method based on DFT calculations was developed in this study to evaluate the reactivity of metal oxide for NO SCR. The well studied SCR mechanism has pointed to the fact that acidity, oxidizing ability, and reoxidability will determine the reactivity of a metal oxide. In this work, LUMO energy and hydrogenation energy were used to measure the acidity and oxidizing ability, respectively. HOMO energy was applied to test the reoxidability of the reduced oxide. In order to verify this method, a series of metal oxides were calculated and analyzed. Based on our results, the metal oxides were separated into three categories (Zones A− C), which are potential “active components”, “promoters”, and “inactive components/support”, respectively. Experiments were conducted to compare with our theoretical result. The comparison has led to an excellent match between experiments and our theoretical calculations. Our method has been proven E

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