The Reaction of Poisonous Alkali Oxides with Vanadia SCR Catalyst

Article pubs.acs.org/JPCC

The Reaction of Poisonous Alkali Oxides with Vanadia SCR Catalyst and the Afterward Influence: A DFT and Experimental Study Xuesen Du,†,‡ Xiang Gao,*,‡ Kunzan Qiu,‡ Zhongyang Luo,‡ and Kefa Cen‡ †

Key Laboratory of Low-grade Energy Utilization Technologies & Systems of Ministry of Education of China, College of Power Engineering, Chongqing University, Chongqing 400044, China ‡ State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Brønsted acid sites and reducibility are crucial for a vandaia catalyst in selective catalytic reduction of NO reaction. Alkali oxides are poisonous to vanadia catalyst. Theoretical chemistry computations and experiments were performed to study the reaction of alkali oxides with the catalyst and the afterward influence left on the catalyst. Both NH3 adsorption and neutralization by alkali oxides proceed on Brønsted acid sites. Theoretical computation with density functional theory shows that neutralization by alkali oxide is more exothermic than NH3 adsorption. The easy consumption of Brønsted acid sites by alkali oxides results in the transformation of surface nature and the decrease of acidity, which is experimentally testified by the NH3 sorption results. The replacement of hydrogen ion by the alkali ion after neutralization will cause the reduction of vanadium atom, as indicated by computed Mulliken charges. This has led to the lower reducibility of the catalyst, which is verified by the theoretical hydrogenation process and experimental hydrogen reduction profile. the 1980s, Shikada and Fujimoto12 noticed the poisoning effect of potassium. Chen and Yang13 found that the poisoning strengths of metal oxides depend on their basicity, which means higher basicity will cause heavier deactivation. Several authors14−16 correlated the acidity with the activity of the SCR catalyst because their experimental results showed an excellent connection between the decrease in surface acidity and the loss of activity after poisoning by alkali metal. Krocher and Nicosia et al.17,18 published that basic elements like K or Ca drastically affect the acidity of the catalysts. Their detailed DRIFT spectroscopy experiments revealed that these poisoning agents mostly interact with the Brønsted acid sites of the V2O5 active phase. After XPS study, they also pointed out that the V5+O sites are much less reactive on the poisoned catalysts. Theoretical researchers19−21 tried to discover the poisoning mechanism of the SCR catalyst using DFT calculations. The results show that the potassium ion was responsible for both the decrease in the V5+O stretching frequency and the

1. INTRODUCTION The selective catalytic reduction (SCR) of nitric oxides with ammonia is the most widely used technology for the abatement of nitric oxides (NOx). Vanadia catalyst, typically V2O5−WO3− TiO2 or V2O5−MoO3−TiO2, is commonly used as the commercial catalyst for stationary sources.1 The SCR mechanism over vanadia-based catalyst has been studied by many researchers by experimental and theoretical means since 1980s.2−6 Most of them have proposed that the acidity of catalyst is crucial for the adsorption and activation of NH3 and reducibility of VO is important for the formation of H2NNO, which is the key intermediate of the SCR reaction.7,8 Therefore, the amount of acid sites and the reducibility of VO sites are crucial for a vanadia SCR catalyst. During the practical application of V2O5 based catalysts, catalyst poisoning has drawn great interest because it is thought to be one of the main reasons for catalyst deactivation.9,10 Alkali metals in flue gas are a major concern for catalysts used at coalfired power plants and municipal solid waste incineration plants. For diesel vehicles, lubrication oil additives and urea solutions contain alkali metals as well.11 These alkali metals containing substances are much poisonous to SCR catalysts. In © 2015 American Chemical Society

Received: November 16, 2014 Revised: January 7, 2015 Published: January 8, 2015 1905

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Figure 1. Adsorption of NH3 and reaction between catalyst and the poisoning agents.

stretch of V5+O double bond, which indicates the weakening of VO bond. Although lots of effort has been made to study the influence on catalyst after poisoning by alkali metals, why and how these substances would react with the vanadia sites have been rarely reported. In this work, DFT calculations were carried out to study the interaction of the poisoning elements with the catalyst. Why these components are poisonous and how do they react with the catalyst were discussed. The afterward influence caused by these reactions were also characterized by theoretical computations and experiments.

instrument manufactured by Micromeritics Corporation. NH3 desorption was detected by a QIC20 mass spectrum instrument provided by Hiden Corporation. Approximately 0.2 g of powder catalyst was first pretreated in a stream of He at 20 mL/ min while heating up to 500 °C at 50 °C/min. After a hold time of 30 min, the sample was cooled down to 100 °C and saturated with a 20 mL/min gas mixture of 5% NH3 in He for 30 min. At the end of the saturation process, the sample was flushed with pure He at 50 mL/min until the TCD signal was stabilized. The sample was then heated up to 700 °C (ramp 10 °C/min) and the mass spectrum signal of NH3 desorbed from the catalyst surface was collected. Hydrogen temperature-programmed reduction (H2-TPR) was performed in a quartz tube reactor. A H2/Ar mixture (10 vol % H2) with a flow rate of 20 mL min−1 was used as the reductant. One tenth of a gram of each sample was placed in the reactor and pretreated at 500 °C in helium for 30 min. Subsequently, the sample was cooled to room temperature in air and the temperature was then increased to 800 °C at a rate of 10 K/min under 10 vol % H2/Ar.. The consumption of H2 was detected by a thermal conductivity detector (TCD). The catalytic activity tests for the reduction of NO by NH3 were carried out in a fixed bed microreactor with catalyst samples of 0.125 g. The simulated gas for these tests contained 500 ppm of NO, 3 vol % O2, and 500 ppm of NH3 in N2. The catalytic reactions were carried out at 350 °C and 0.1 MPa with a total flow rate 0.5 L min−1 and GHSV = 1.2 × 105 h−1 (based on the total bed volume of catalyst samples). The NO concentrations before and after reaction were determined with infrared methods by using an NGA2000 analyzer manufactured by the Rosemount Corporation in Germany. In excess of oxygen and with NH3/NO (molar ratio) ≥1, this process can be supposed to be first order with respect to NO and zero order with respect to NH3.27,28 Thus, the catalytic activity can be described by the reaction rate constant k, which was calculated according to the rate expression below29,30

2. COMPUTATIONAL AND EXPERIMENTAL DETAILS All density functional theory (DFT) calculations were performed with Gaussian 0322 code using the gradient corrected Becke’s23,24 three-parameter hybrid exchange functional in conjunction with the correlation functional of Lee, Yang, and Parr25 (B3LYP). Each of the models were treated by 6-31G(d,p) basis set. Each stationary structure has been confirmed as a minimum-energy structure from the calculated vibrational frequencies. The zero point correction and vibrational frequencies obtained by frequency calculations were scaled by 0.9607.26 All experiments were carried out on a V2O5/TiO2 catalyst with V2O5 loading of 1 wt % (1 V/Ti) prepared using the impregnation method. An ammonium metavanadate (NH4VO3) precursor was used to impregnate the TiO2 support (anatase). 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. The catalyst prepared has a BET surface area of 73.75 m2/g measured by a Quantachrome Autosorb-1 instrument. The K- and Na-containing samples were prepared by impregnating the 1 V/Ti catalyst with a respective aqueous acetate solution (KCOOH, NaCOOH). The catalyst doping procedure is as follows: impregnated with aqueous acetate solution, heated at 70 °C in a water bath for 4 h, dried at 105 °C for 12 h, and calcined at 500 °C in air for 5 h. To compare the poisoning strength of different metal elements, the same doping concentration (50 mol % based on vanadium; K, Na/V = 0.5) was used. High doping amount of alkali was applied in order to clearly detect the chemical change of catalyst brought by alkali species. The temperature-programmed desorption of ammonia (NH3-TPD) was carried out on an AutoChem II 2920

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 catalyst weight (g), and X is the fractional NO conversion. 1906

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Figure 2. Frontier molecular orbitals of VOx, NH3, Na2O, K2O, NaOH, and KOH.

Figure 3. (a) The adsoption/reaction energies between NH3/poisoning agents and the catalyst. (b) The correlation between “LUMOCAT.− HOMOREACTANT” and “exothermic energy of the corresponding reaction”.

The above figure shows the reaction process of NH3 adsorption and alkali metal interaction. NH3 will need Brønsted acid to adsorb on while alkali oxides tend to consume these sites. These two competitive processes require a comparison of the reactions and evaluations of the reactants. As discussed in our previous work,31 the acidity can be evaluated by the energy of lowest unoccupied molecular orbital (LUMO). Surface acidity is crucial for SCR catalyst. The basic NH3 molecule tends to donate its electrons to share with acidic sites on the catalyst surface. Frontier molecular orbitals (FMO) theory is one of the most popular theories to evaluate the interaction between two molecules. In FMO theory, LUMO energy implies the ability to accept 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. On the other side, NH3 and alkali species donate electrons from highest occupied molecular orbitals (HOMO) in the adsorption/poisoning process. The higher the HOMO energy is, the easier it is to react with the catalyst sites. Figure 2 shows the FMO profile of the catalyst, ammonia, and the poisoning agents. The LUMO of the vanadia molecule centralizes on the vanadium atom and is mainly composed by the V 3d orbital. The donation of electrons to the bonded oxygens has made the vanadium atom much reducible and favorable to adsorb electrons. The HOMO of NH3 molecule majorly locates on the N 2d orbital. For the alkali compounds,

3. RESULTS AND DISCUSSION 3.1. The Reaction. Surface acidity is crucial for a SCR catalyst. In a supported vanadia catalyst, Brønsted acid is generated by the hydration of the V−O−M structure. The produced Brønsted acid site is shown in Figure 1, as well as the adsorption of NH3. The vanadia catalyst was represented by the mono VO4H3 cluster in this study. The structure convergence is discussed in the Supporting Information. Alkali metals are well-known for the neutralization of the Brønsted acid. Figure 1a also reports the reaction between alkali metals and the catalyst. In this study, alkali metal oxide was chosen to represent the alkali metal-containing components in flue gas. The reaction between K2O/Na2O and catalyst is simulated as the reactions shown in Figure 1(b1, b2, c1, c2). The initial interaction will be the bonding of the H+ ion of the catalyst with O2− of the alkali oxide. A potassium ion will separate from the alkali oxide and bonding with the vanadia oxygen atoms. An alkali hydroxide is produced after this process. These alkali hydroxides are also possibly active for the neutralization of Brønsted acid. Figure 1(b2, c2) show the reactions between KOH/NaOH and the catalyst. The OH− of the alkali hydroxide capture the H+ from the catalyst and the potassium ion is left on the vanadia oxo sites. All the theoretical structures and energy profiles listed in Figure 1 are presented in the Supporting Information. 1907

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and sodium bond to two oxygen atoms. The structure parameters, computed infrared profile and Mulliken charges are listed in Table 1. The bond length of VO are prolonged from 0.156 to 0.164 and 0.163 nm when doped with sodium and potassium atom, respectively. LUMO configuration was found to be significantly changed by the doping of alkali metals. The LUMO of the vanadia molecule shifts from the vanadium atom to the 4s orbital of the alkali atom. The LUMO energy also increases from −0.0919 to −0.0555 and −0.0644 hartree when doped with Na and K atoms, respectively. The LUMO change caused by the neutralization reaction indicates the great decrease of the acidity. Frequency calculation results show a VO stretching frequency shift from 1161 to 984 for Na-doping and 1001 cm−1 for K-doping. The red shift of vibration frequency is consistent with the extension of VO bond length caused by alkali metals. Mulliken charge calculation was performed to analyze the electron interactions. The Mulliken charge of K/Na ion (+0.724/+0.861) is higher than the hydrogen ion (+0.351) of the Brønsted acid. Thus, the replacement of hydrogen ion by alkali ion has imposed electron transformation on the vanadia molecule. Higher Mulliken charge of the alkali metal implies that more electrons are transferred to the vanadia molecule than hydrogen atom. Vanadium atom is reduced with the replacement of hydrogen ion by K/Na ion. The high oxidation state of vanadium atom can create a LUMO with low energy, which is the reason for the high acidity of vanadia site. The reduction of vanadium atom has raised the LUMO energy of the molecule, which further lowers the acidity. For the charge and LUMO profiles, potassium is more influential than sodium. The acidity drop concluded from the theoretical computations was verified by the temperature-programmed desorption of ammonia (NH3 TPD). The titania-supported vanadia catalyst was prepared as the vanadia catalyst sample in this study. Figure 5a shows the original NH3 TPD curves of the catalyst samples in the range of 100−600 °C. All curves exhibit multiple peaks. To better explain the NH3 adsorption profile, deconvolutions were performed. The results are shown in Figure 5b−d. The peaks 1, 2, and 3 belong to the desorption of physisorbed, weakly chemisorbed, and strongly chemisorbed of ammnonia. As published before,15,18 titania oxide has only Lewis acid sites on Ti4+ ions. Loading of vanadia will generate Brønsted acid by the hydration of V−O−M (M could be V or Ti) species. The produced Brønsted acid (V−OH) is stronger than the Ti4+ lewis acid. Thus, the peaks 2 and 3 can be attributed to the desorption of NH3 from the Ti4+ lewis acid and Brønsted acid (V−OH) respectively. The comparison of the fresh, Na-doped and K-doped samples shows that the amount of physisorbed and weakly chemisorbed NH3 did not change with the doping of alkali metal. This is reasonable given that physisorption is independent to the chemical component but related to the physical structure of the sample. Similarly, both the peak temperature and integrated area of peak 2 did not change because of alkali, which means alkali ions do not influence the Ti4+ Lewis acid sites. However, peak 3, that is, the amount of NH3 adsorbed on the Brønsted acid sites greatly dropped by alkali ions. This is consistent with the theoretical computation that Brønsted acid sites can be easily neutralized by the alkali oxide. 3.2.2. Reducibility and Charge State. Another crucial property of the vanadia catalyst for an SCR reaction is the reduction profile VO. The reduction of V5+=O to V4+−OH is thought to be vital in the SCR reaction.7,8 In the theoretical

the HOMOs mostly focus on the O 2p orbitals. The interaction between LUMO of the vanadia site and HOMO of NH3 results in the adsorption of NH3 on catalyst. For this process, the energy difference between LUMO of vanadia (green dotted line) and HOMO of NH3 indicates the adsorption ability of NH3 on catalyst. As can be seen from Figure 2, the HOMO energies of alkali compounds are much higher than that of NH3. The energy difference between vanadia LUMO and NH3 HOMO is 0.161 hartree, while the energy differences between vanadia LUMO and HOMO of Na2O, K2O, NaOH and KOH are 0.030, 0.007, 0.073, and 0.047 hartree, respectively. The much smaller gap between LUMO vanadia site and HOMO of alkali species implies that the neutralization by alkali compounds is more favorable than adsorption of NH3. The neutralization of Brønsted acid will lead to the deactivation of this acid site. Among these four agents, alkali oxides are more reactive than hydroxides, which means the stronger poisoning ability. Potassium oxide and hydroxide are also more poisonous than sodium oxide and hydroxide. The FMO energies have revealed the reaction tendencies. Figure 3a shows the energy profile of the reactions. The adsorption of NH3 means the approach of electrons from HOMO of NH3 to the acid catalyst sites. Similarly, the neutralization of acid catalyst sites by alkali species undergoes the donation of electrons from HOMO of alkali to LUMO of catalyst. The hartree gaps between HOMO (NH3 and alkali species) and LUMO (catalyst) imply the possibility of these reactions. The larger the gap is, the harder it is to adsorb or to neutralize. The adsorption of NH3 on the vanadia site is exothermic by 69.03 kJ/mol. In accordance with the HOMO energies, the neutralization reactions are much more exothermic than NH3 adsorption. Alkali oxides, which possess the highest HOMO energies, are also most exothermic while participating in the neutralization reaction. Figure 3b summarizes the energy gaps (energy differences from the LUMO of vanadia cluster) of the reactants and the exothermic energies of the respective reaction. The smaller energy gap (higher HOMO energy) produces the more exothermic energy, which means the more favorable reaction. These energies also indicate that alkali compounds are more reactive than NH3. Consumption of Brønsted acid can happen more readily than adsorption of NH3 on the catalyst. 3.2. After Reaction. 3.2.1. Structure and Acidity. As discussed in the above section, the alkali metal atom will replace H+ to form the molecules shown in Figure 4. LUMO pictures of the molecules before and after doped with alkali metals are also illustrated. As can be seen from this figure, both potassium

Figure 4. Structures and LUMO snapshots of the fresh and alkali metal-doped catalyst. 1908

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The Journal of Physical Chemistry C Table 1. Model Characters before and after Being Doped with Sodium and Potassium catalyst model

stretching frequency (VO)

Mulliken charge (V)

Mulliken charge (alkali metal)

LUMO energy

undoped Na-doped K-doped

1115 945 962

+1.306 +1.167 +1.147

+0.724 +0.861

−0.0919 −0.0555 −0.0644

Figure 5. NH3 TPD curves of the fresh, Na-doped, and K-doped catalysts (a) and the deconvolution results of fresh (b), Na-doped (b), and Kdoped (c) catalysts.

Figure 6. Theoretical reduction of V5+O by hydrogenation (A), the experimental reduction of titania-supported vanadia catalyst by hydrogen (B) and the relationship between theoretical hydrogenation energy and experimental hydrogen consumption temperature (C).

energy can be calculated as E(hydrogenation) = E(VO4H4) − E(VO4H3) − E(H). More exothermic energy for this reaction indicates the higher reducibility of the molecule.10,31 The calculation results of the fresh, Na-doped, and K-doped models

computation, the reduction of VO can be simulated by the hydrogenation process. A hydrogen atom was added to VO and the system was optimized to a stationary structure (Supporting Information Figure S2). The hydrogenation 1909

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and V3+ species, which is consistent with the result that reduction of V5+ is prohibited as revealed by the above theoretical hydrogenation and experimental H2 reduction. The new peak centered at binding energy between V5+ and V4+ can be assigned to the formation of V−O−K structure. As indicated by Mulliken charges, V5+ species will be reduced by alkali metals. This has led to the shift of V5+ peak to lower binding energy. 3.3. NO Reduction. NO reduction experiments were carried out to identify the loss of SCR activity of the catalyst caused by alkali oxide. Table 2 shows the experiment results. As

are shown in Figure 6A. The fresh model possesses a high exothermic energy of 2.48 eV, which is the reason for the easy reduction of vanadia species and high activity for SCR reaction. Sodium and potassium will both decrease the exothermic energy of the hydrogenation. The reduction of the Na-doped molecule is 0.14 eV less exothermic than that of the undoped molecule. The K-doped model has the least exothermic energy, 2.24 eV, in the process of hydrogenation. The hydrogenation energy profile indicates that the reducibility decreases in the order of fresh > Na-doped > K-doped. This can be explained by the above theoretical Mulliken charge computation. Potassium and sodium atoms have reduced the vanadium atom and thus caused the decrease of oxidative ability of the molecule. Experimental temperature-programmed reduction of hydrogen (H2 TPR) was conducted to verify the calculation results. In Figure 7B, the fresh catalyst sample shows a high H2

Table 2. NO Reduction Activities and k) of fresh, Na-Doped, and K-Doped Catalysts at 350 °C NO conversion (%) k/(cm3 g−1 s−1)

fresh

Na-doped

K-doped

82.5 116.19

55.2 53.53

49.0 44.89

expected, the fresh catalyst shows the highest NO conversion, 82.5% at GHSV = 1.2 × 105 h−1. The doping of sodium and potassium oxides decreases the NO conversion to 55.2 and 49.0%, respectively. Rate constants were calculated to reveal the instinct catalyst performance. Sodium doping greatly diminishes the rate constant to about half of the fresh catalyst. Potassium exhibits even more influence and the rate constant decreases by 61% as compared to that of the fresh catalyst. These results support the above findings on the influence of alkali oxides. 3.4. Conclusions. Both NH3 adsorption and alkali metal poisoning happen on the acid sites. On the vanadia catalyst, the V−OH Brønsted acid produced by hydration is highly active for NH3 adsorption and activation. As indicated by the energetic profile, the neutralization of vanadia Brønsted acid by sodium and potassium oxides are more exothermic than the NH3 adsorption. Thus, the active V−OH Brønsted acid will be consumed by doping of alkali oxides. NH3 sorption experiments have verified the loss of acidity. On the other side, the Mulliken charge of K/Na ion in V−O−K/Na is higher than the H ion in V−OH, which results in the reduction of the adjacent vanadium atom. This reduction has made the hydrogenation of the catalyst more difficult. That means the reducibility of the catalyst is weakened by doping of alkali ions. The experimental reduction of catalyst with hydrogen has confirmed the drop of reducibility of the catalyst. The loss of Brønsted acid sites and drop of reducibility caused by alkali oxide have led to the decline of SCR reactivity, which was supported by the NO reduction experiment.

Figure 7. XPS patterns of the fresh, Na2O-doped, and K2O-doped V2O5/TiO2 catalyst.

consumption peak at 467 °C. As predicted from the theoretical results, both sodium and potassium decrease the reducibility of the V2O5 based catalyst. In the TPR result, Na-doped sample shows a much small peak at a higher temperature than the fresh sample. The K-doped sample possesses a smallest peak at a highest temperature. The experimental result declares that both sodium and potassium oxide will decrease the reducibility of the catalyst and potassium is more influential than sodium. The theoretical hydrogenation energies and experimental hydrogen consumption temperatures of the catalyst are summarized in Figure 7C to correlate the theoretical and experimental factors. The solid squares indicate the experimental hydrogen consumption temperatures detected by H2 TPR, and the hollow squares indicate the theoretical hydrogenation energies. These two curves fit well, and both lead to the conclusion that the reducibility decreases in the order of undoped > Na-doped > K-doped. X-ray photoelectron spectra were collected to analyze the change of chemical state of the vanadium atoms. Figure 7 shows the XPS spectra of V 2p3/2. In literatures,32 the binding energies at 516.9−517.3, 515.7−516.3, and 515.0−515.6 eV can be assigned to V5+, V4+, and V3+ in V2O5, VO2, and V2O3 species. The fresh sample shows a wide peak containing vanadium atoms with pentavalent, tetravalent, and trivalent states. This indicates that V5+ will be partially reduced during the preparation process. With the doping of alkali oxide, the XPS peak shrinks to sharp peak at a medium binding energy. Doping with alkali oxides has diminished the amount of V4+

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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

*E-mail: [email protected]. Tel: +86-13505711887. Fax: +86571-87951616. Notes

The authors declare no competing financial interest. 1910

DOI: 10.1021/jp511475b J. Phys. Chem. C 2015, 119, 1905−1912

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 51125025), Development of China (863 Program) (No. 2013AA065401).

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DOI: 10.1021/jp511475b J. Phys. Chem. C 2015, 119, 1905−1912

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DOI: 10.1021/jp511475b J. Phys. Chem. C 2015, 119, 1905−1912