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Reaction Pathways and Kinetics for SCR of Acidic NO Emissions from Power Plants with NH x
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Minghui Zhu, Jun-Kun Lai, Uma Tumuluri, Michael E. Ford, Zili Wu, and Israel E. Wachs ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03149 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 3, 2017
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Reaction Pathways and Kinetics for SCR of Acidic NOx Emissions from Power Plants with NH3 1
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Minghui Zhu , Jun-Kun Lai , Uma Tumuluri2, Michael E. Ford1, Zili Wu2 and Israel E. Wachs1* 1
Operando Molecular Spectroscopy & Catalysis Laboratory, Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015 USA 2 Chemical Science Division and Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA *to whom correspondence is to be addressed (
[email protected])
Abstract Selective Catalytic Reduction (SCR) of NOx with NH3 by supported vanadium oxide catalysts is an important technology for reducing acidic NOx emissions from stationary sources and mobile diesel vehicles. Rational design of improved catalysts, however, is still hampered by lack of consensus about reaction pathways and kinetics of this critical technology. The SCR fundamentals were resolved by applying multiple time-resolved in situ spectroscopies (UV-vis, Raman and temperature programmed surface reaction (TPSR)) and isotopically labeled molecules (18O2, H218O, 15N18O, ND3). This unique series of experiments directly revealed that the SCR reaction occurs at surface V5+O4 sites that are maintained in the oxidized state by O2 and the rate-determiningstep involves reduction of V5+O4 sites by NO and NH3, specifically breaking of N-H bond during the course of formation or decomposition of the NO-NH3 intermediate. Keywords: SCR, NO, NH3, Vanadium oxide, Spectroscopy, UV-vis, TPSR.
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The selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia by supported VOxWOx/TiO2 catalysts is an important NOx emission control technology for stationary sources (power plants and industrial boilers) and mobile diesel vehicles.1–5 Depending on the composition of nitrogen oxides, two types of SCR reactions can take place with the “standard SCR” reaction being dominant: Standard SCR: 4NH3 + 4NO + O2 → 4N2 + 6H2O (1) Fast SCR: 4NH3 + 2NO + 2NO2 → 4N2 + 6H2O (2) The reaction mechanism of the “standard SCR” has been extensively investigated and discussed. It is generally agreed that the first step of the reaction is ammonia adsorption on the catalytic active sites (surface vanadium oxide). The adsorbed ammonia then either reacts with an adsorbed NO or gas phase NO to form a surface reaction intermediate that decomposes to N2 and H2O and also reduces the surface vanadium oxide site. The reduction of the surface vanadium oxide site takes place because of an excess H atom in the overall molecular reaction NO + NH3 → N2 + H2O + Hads. (3) The reduced surface vanadium oxide site is subsequently re-oxidized by gas phase molecular O2 that is generally considered as rate-determining step at temperatures below 300°C. 6–11 In the present study, new insights into the reaction pathway and rate-determining-step (rds) of the “standard SCR” by titania-supported vanadium oxide catalysts are provided from in situ time-resolved spectroscopy (UVvis, Raman and temperature programmed surface reaction (TPSR)) with isotopically labeled reactants (ND3, 18O2, 15 18 N O, H218O). The catalyst employed comprised 5 wt. % V2O5 supported on TiO2 (Degussa P-25) corresponding to almost monolayer surface VOx coverage (~8 V/nm2) with Raman spectroscopy (Figure S1) confirming that only surface vanadia species were present.12 After an initial calcination in 5% O2/He at 400oC to yield 100% V+5, the catalyst was sequentially exposed to i) NH3, ii) NH3+NO, iii) NH3+NO+O2 and iv) O2. The in situ UV-vis spectra were collected every 2 minutes and the experiments were performed at 5 different temperatures ranging from 100°C to 300°C. The oxidation state of vanadium oxide was monitored by the UV-vis d-d transition band at 799 nm as the fingerprint of V4+, which was calibrated by measuring both the signal for fully oxidized catalyst (100% V5+) in flowing 5% O2/He at 400°C and after complete reduction to V4+ (V3+ was not formed; see Figure S2) in flowing 10% CO/Ar at 400°C for 5 hrs.13,14 The calibration allowed quantifying the percentage of reduced V5+ during different environmental treatments and the findings are shown in Figure 1. 40
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Time (min) Figure 1. Reduction of the surface V5+ sites for the supported 5% V2O5/TiO2 catalyst as a function of environmental conditions. The catalyst was initially calcined in 5% O2/He at 400oC to yield 100% V+5 and the studies were performed at 100, 150, 200, 250 and 300°C: i) NH3 (35 ml/min; 2000 ppm NH3/He), ii) NH3 (35
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ml/min; 2000 ppm NH3/He) + NO (35 ml/min; 2000 ppm NO/He), iii) NH3 (35 ml/min; 2000 ppm NH3/He) + NO (35 ml/min; 2000 ppm NO/He) + O2 (5 ml/min; 5% O2/He), iv) O2 (30 ml/min; 5% O2/He). Exposure to NH3 barely reduces the surface V5+ sites at low temperatures (100, 150 and 200°C). At higher temperatures NH3 modestly reduces the surface V5+ (4% at 250°C and 16% at 300°C after 30min) indicating that NH3 is not a strong reducing agent for this catalyst. The percentage of reduced sites, however, increases upon exposure to NH3/NO at all temperatures. Upon addition of O2 to the NH3/NO/He feed, the surface V4+ sites partly re-oxidize to V5+ under the steady-state SCR reaction conditions. Under fully oxidizing O2/He, almost all the surface V4+ sites re-oxidize to V5+. The residual ~5% of reduced V5+ sites after 1 hour of exposure to oxygen is thought to be related to trapping of V4+ sites in the bulk lattice of TiO2(rutile) during the SCR reaction and can only be oxidized by an oxidation treatment at 400oC (Figure S4).15 These findings directly demonstrate that reduction of the surface V5+ to V4+ sites primarily takes place by the NO+NH3 reaction step and that oxidation of the surface V4+ to V5+ sites is performed by O2. Table 1. Rates for reduction of V5+ and re-oxidation of V4+ sites as a function of environmental conditions. (Reduction: 1000 ppm NH3 + 1000 ppm NO; re-oxidation: 5 vol.% O2/He) Reduction Re-oxidation Temperature TOFreduction TOFoxidation Rate V5+ (%) Rate V4+ (%) (°C) (×10-2 min-1) (×10-2 min-1) (%/min) (%/min) 100 0.60 ± 0.1 100 0.60 ± 0.1 0.10 ± 0.0 4 2.5 ± 0.0 150 1.2 ± 0.1 100 1.2 ± 0.1 0.63 ± 0.1 9 7.0 ± 0.1 200 2.5 ± 0.3 99 2.5 ± 0.3 1.4 ± 0.1 14 10 ± 0.1 250 2.5 ± 0.3 96 2.6 ± 0.3 1.9 ± 0.2 13 15 ± 0.2 300 1.7 ± 0.2 84 2.0 ± 0.2 2.1 ± 0.2 13 16 ± 0.2 The rates for reduction of V5+ by NH3/NO (experiments ii) and re-oxidation of V4+ by O2 (experiments iv) can be quantified from the initial slopes of the curves in Figure 1 and are presented in Table 1. The reduction and re-oxidation rates can further be normalized by the initial number of V5+ and V4+ sites, respectively, which allows for the direct comparison for the specific reduction rates of V5+ (TOFreduction) and re-oxidation rates of V4+ (TOFoxidation) per VOx site (see SI for calculation details). For number of V4+ sites, the sites in the TiO2 lattice were not considered. The TOF for reduction and re-oxidation both increase with temperature, as would be expected, except for the reduction of V5+ at 300°C that is related to the larger amount of initially reduced V4+ (16%) from NH3 reduction. Most importantly, the TOF for V4+ re-oxidation is ~4-8x higher than V5+ reduction over the wide temperature range (100-300°C). These findings clearly demonstrate that the re-oxidation of V4+ sites by O2 is a faster kinetic process than the reduction of V5+ sites by NO+NH3, with reduction being the ratedetermining-step for the “standard SCR” below 300°C. Additional fundamental insights about the rate-determining step of the SCR reaction were provided by temperature-programmed surface reaction (TPSR) with the aid of isotopically labeled reactants. Hereby, a more conventional supported 1%V2O5-5%WO3/TiO2 catalyst was employed. In situ Raman spectroscopic analysis revealed that both the vanadium oxide and tungsten oxide components were completely dispersed as surface VOx and WOx species on the titania support (Figure S1). The catalyst was dehydrated in flowing 5% O2/He (30 sccm) at 500°C and then cooled down to 50°C. The flowing gas was subsequently switched to NH3(2000 ppm; 35 sccm)/NO(2000 ppm; 35 sccm)/O2(5 vol. %; 5 sccm)/He and the temperature was then ramped to 500°C (10°C /min) with the reactants and products monitored by an online quadrupole mass spectrometer (Dycor Dymaxion DME200MS). A series of isotopically labeled TPSR experiments were performed: 1) 14N16O/14NH3/16O2 as reference; 2) 14N16O/14ND3/16O2 to evaluate N-H bond breaking; 3) 14N16O/14NH3/18O2 to evaluate O-O bond breaking; 4) 15N18O/14NH3/16O2 to evaluate N-O bond breaking; 5) 14N16O/14NH3/16O2 after H218O pre-treatment to evaluate V-O bond breaking with a H218O pre-treatment to exchange all surface 16O sites with 18O (V18O4, W18O5 and Ti18OH).16,17 ACS Paragon Plus Environment
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For the unlabeled reactants (14N16O, 14NH3 and 16O2), the SCR reaction initiates at ~150°C during TPSR as indicated by the simultaneously consumption of NO, NH3, O2 and evolution of N2, H2O as shown in Figure 2(a). The evolution of H2O is slightly delayed compared to N2 because of the longer residence time of moisture in the catalyst bed. The evolution of the 14N2, 15N2 and 14N15N isotopomers during the 15N18O/14NH3/16O2-TPSR experiment are shown in Figure 2(b). The only nitrogen product formed is 14N15N indicating that N2 formation always involves reaction between one NH3 molecule and one NO molecule.18,19 7x10-9
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Figure 2. SCR-TPSR spectra: (a) MS signals for NO, NH3, O2, N2 and H2O during 14N16O/14NH3/16O2 -TPSR; (b) MS signals for 14N2, 15N2 and 14N15N during 15N18O/14NH3/16O2 -TPSR and (c) NO conversion during TPSR with isotopically labelled reactants as well as catalyst pre-treated with H218O. The influence of the isotopically labeled molecules (15N18O, 14ND3, 18O2 as well as labelling the surface oxygen sites with 18O by exchanging with H218O) upon the TPSR for transient NO conversion are presented in Figure 2(c). The transient conversion of NO with the different isotopic labels follows quite similar reactivity trends except for ND3 that decreases the reactivity demonstrating a kinetic isotope effect (KIE) involving the NH/N-D bond breaking step. The retardation of the SCR reaction by switching from NH3 to ND3 is also observed during steady-state isotopic transient analysis (Figures S5-S6) that further indicates the KIE involving N-H/N-D bonds during SCR. The transient NO conversion at different temperatures also allows determining the influence of the isotopomers upon the apparent activation energy for NO conversion within a linear 225-250°C range (Arrhenius plot in Figure S7) and the Ea values are presented in Table S1. In the absence of any isotopic labels, the Ea is ~51 kJ/mol and is consistent with prior reports.16,20,21 In the presence of 18O2, 15N18O and surface 18O, the Ea values vary from ~42-52 kJ/mol within experimental error. The corresponding KIE values at 225°C were determined to be ~0.9-1 that are slightly lower within experimental error than the predicted KIE values of ~1.11.2 for breaking O-O, N-O and V-O bonds with labeled oxygen (16O/18O). Thus, it appears that there is no supporting evidence that breaking of O-O, N-O and V-O bonds is involved in the rate-determining-step of the
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SCR reaction. In the presence of ND3, however, the activation energy increases by ~40% to ~68 kJ/mol and the KIE (kNH3/kND3) at 225°C is ~2. Both the higher Ea and lower conversion of NO when ND3 is substituted for NH3 indicate that N-H bond breaking is involved in the rate-determining-step of the SCR reaction of NO with NH3. Surprisingly, the KIE for kNH3/kND3 was never examined in prior SCR studies with titania-supported vanadiatungsta catalysts. 1.2x10-8
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Figure 3. MS signals for H216O and H218O from a) 14N16O/14NH3/16O2-TPSR 15 18 N O/14NH3/16O2-TPSR and c) 14N16O/14NH3/18O2-TPSR.
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The TPSR spectra for the isotopic products of H218O and H216O during a) 14N16O/14NH3/16O2-TPSR after H218O pretreatment; b) 15N18O/14NH3/16O2-TPSR and c) 14N16O/14NH3/18O2-TPSR are presented in Figures 3. For the H218O-pretreated catalyst with the surface saturated by 18O-containing species (Figure 3a), the evolution of H218O slightly precedes H216O formation during the SCR-TPSR. The H218O signal decreases right after it reaches the maximum because of the initial finite amount of surface 18O and increasing amount of surface 16O as the SCR reaction proceeds. During the 15N18O/14NH3/16O2-TPSR (figure 3b), an opposite trend is observed: H218O formation is slightly delayed relative to H216O. The delay between evolution of H216O and H218O is more pronounced for 14N16O/14NH3/18O2-TPSR (Figure 3c) because catalyst re-oxidation involving 18O2 only occurs after the reduction process that produces H216O. For all three isotopically labeled experiments, the initial H2O isotope corresponds to the initial oxygen isotope present on the catalyst surface phase and may also be influenced by oxygen exchange between gas phase water and the surface oxygen. These findings directly demonstrate that the SCR reaction of NO with NH3 by titania-supported vanadia catalysts follows a Mars-van Krevelen mechanism where the NH3/NO react with surface oxygen associated with surface vanadia sites to produce H2O and N2. In situ Raman spectra during SCR confirmed that the V=16O band persisted during the early stages of
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SCR in the presence of gas phase 18O2 (Figure S8). The reduced surface vanadia site is subsequently re-oxidized by gas phase molecular O2. In conclusion, application of multiple in situ time-resolved spectroscopies with isotopically labeled molecules (18O2, H218O, 15N18O, ND3) finally allowed direct demonstration of many new fundamental insights about the industrially important SCR of NO with NH3 reaction. The “standard SCR reaction” by supported vanadia catalysts was shown to proceeds by (1) a Mars-van Krevelen reaction mechanism employing oxygen from surface V5+O4 sites, (2) the surface V5+O4 sites are reduced by NH3/NO to surface V4+O3, (3) the ratedetermining-step involves breaking of an N-H bond during the course of formation or decomposition of the NONH3 intermediate complex, and (4) the role of gas phase molecular O2 is to rapidly re-oxidize and maintain the surface vanadia sites in the fully oxidized state of V5+. The re-oxidation rate of the surface V4+O3 sites by molecular O2 is faster than the reduction rate of surface V5+O4 by NO/NH3 at temperatures below 300°C. The formation of N2 involves reaction between one molecule of NO and one molecule of NH3. These new molecular level insights have the potential to guide the rational design of improved catalysts for the reduction of toxic acidic NOx emissions from power plants. Supporting Information Catalyst preparation; Temperature-Programmed Surface Reaction (TPSR) Spectroscopy; Example of Calculating TOF (250°C); in situ Raman (Figure S1, S8); Time-resolved in situ UV-vis spectra (Figure S2-4); MS signals during isotope switch (Figure S5-6); Arrhenius plots (Figure S7); Kinetic data with different isotopes (Table S1). Acknowledgements The authors acknowledge financial support from the Center for Understanding & Control of Acid GasInduced Evolution of Materials for Energy (UNCAGE-ME), an Energy Frontier Research Center funded by DOE, Office of Science, and Office of Basic Energy Sciences under grant DE-SC0012577. Part of the work was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. The authors would like to express their appreciation to Dr. Chris Keturakis of Cummins Emission Solutions for many valuable comments.
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