Article pubs.acs.org/EF
Study on the Deactivation of V2O5−WO3/TiO2 Selective Catalytic Reduction Catalysts through Transient Kinetics Xinhua Xie,† Jidong Lu,*,†,‡ Erich Hums,§ Qiuxiong Huang,‡ and Zhimin Lu‡ †
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ‡ School of Electric Power, South China University of Technology, Guangzhou, Guangdong 510640, People’s Republic of China § Consulting Environmental Catalysis, Post Office Box 1848, 91008 Erlangen, Germany S Supporting Information *
ABSTRACT: The mechanism underlying the deactivation of a commercial V2O5−WO3/TiO2 catalyst for NH3 selective catalytic reduction (SCR) of NOx through exposure to the flue gas of a coal-fired power plant was investigated by a transient kinetic analysis that focused on the distinction between the deactivation behaviors of adsorption sites and redox sites. The results showed that alkali dopants preferentially poison the active sites associated with vanadium (V5+−OH and/or V5+O) rather than the sites associated with titania and tungsten. Obvious changes in the activation energies for NH3 desorption, oxidation, and SCR surface reaction over the used catalyst were observed. Kinetic variations showed that three other factors that are involved in the elementary surface steps are responsible for the catalyst deactivation rather than simply the decline of the NH3 adsorption capacity. Finally, the effects of these factors on the catalyst activity were analyzed at different temperatures.
1. INTRODUCTION Because of its harmful impacts on human health and the environment, NOx emission regulations have been tightened for both stationary and mobile sources. To meet the increasingly stringent regulations, selective catalytic reduction (SCR) of NOx is considered the most efficient and widely used technology because of its efficiency, selectivity, and favorable economics.1−4 The choice of catalyst is crucial to the success of SCR in NOx removal technologies.5 A wide variety of catalytic materials have been tested for the SCR of NOx, with NH3, urea, hydrocarbons, CO, and even H2 all being used as reducing agents.3−11 However, many of these are of only academic interest and have not been proven suitable for industrial applications in the presence of H2O and SO2, as found in the exhaust gas of power plants and diesel engines.3,4,8−12 In contrast, SCR with NH3 over a V2O5−WO3 (MoO3)/TiO2 catalyst is a well-established technique for NOx abatement on the industrial scale.3−6,12−14 Deactivation phenomena are still a relevant challenge in SCR technology, particularly concerning its commercial aspects. Thus, studying catalyst deactivation is important for optimizing both the catalyst composition and the SCR reactor operation to reduce the running costs. This is particularly needed for the widely used V2O5−WO3/TiO2 catalyst. The deactivation mechanisms of V2O5/TiO2-based SCR catalysts have been extensively studied.15−25 However, previous investigations with V2O5/TiO2 and V2O5−WO3/TiO2 SCR catalysts generally focused on the properties of the active sites that are associated with vanadium alone. Many researchers are convinced that alkali metals are the most serious poison and have thus focused their studies on the poisoning effects of alkali metals using wet impregnation methods.15−22 They have investigated the catalyst activity and surface properties using activity measurements and catalyst characterization, respec© 2015 American Chemical Society
tively. Poisoning by alkaline earth metals (Ca and Mg) has also been studied, although the effects were found to be smaller than the deactivation caused by sodium and potassium.22−25 In these studies, catalyst deactivation was attributed to the loss of vanadium-linked Brønsted and/or Lewis sites and to the decline of NH3 adsorption capacity. This may be a reasonable explanation for deactivation in binary V2O5/TiO2 catalysts, but it is less likely for ternary V2O5−WO3/TiO2 catalysts. Therefore, in view of the complexity of the reactive sites in the ternary V2O5−WO3/TiO2 system, more work is required. NH3 may be coordinatively adsorbed by Ti-, W-, and V-linked Lewis acidic sites and in its protonated form at Brønsted acidic sites (V−OH and W−OH) in the ternary V2O5−WO3/TiO2 system.1,26 However, the properties of the individual acidic sites likely differ to a certain degree. Most research has suggested that Lewis sites or Brønsted sites that involve vanadium or dual sites that are composed of V−OH and an adjacent VO species are the active sites responsible for the SCR-DeNOx with NH3 on vanadium-based SCR catalysts.15,27−29 Aside from vanadium sites, Lietti et al.30,31 observed that Ti- and W-bound ammonia species acted as a “reservoir” in the SCR reaction on ternary V2O5−WO3/TiO2 catalysts. Accordingly, further investigation into the deactivation of ternary V2O5−WO3/ TiO2 catalysts should distinguish between the deactivation behaviors of adsorption sites and redox sites. Previous studies on the deactivation of V2O5−WO3/TiO2 catalysts focused primarily on the variations in surface acidity and NH3 adsorption capacity with decreasing DeNOx activity.17−23 Few reports have been devoted to the distinction between the Received: February 11, 2015 Revised: May 25, 2015 Published: May 26, 2015 3890
DOI: 10.1021/acs.energyfuels.5b01034 Energy Fuels 2015, 29, 3890−3896
Article
Energy & Fuels
reaction.38−40 The gases used in the experiments were 99.999% pure N2, 99.999% pure O2, 1.0% NH3 in N2, and 1.0% NOx (NO/NOx = 95%, corresponding to flue gas of the coal-fired power plant) in N2. A 165 mg sample of catalyst (approximately 0.2 mL) with particle diameters of 106−150 μm (100−150 mesh) was used in each run to minimize the diffusional intrusions and temperature fluctuation during the experiments. The space velocity used in the experiments was 750 000 h−1 (STP). The absences of both interphase and intraparticle concentration gradients were verified according to common diagnostic criteria.41 In a typical experiment, a step feed of NH3 (0−400 ppm) was admitted to the reactor for 1200 s at a constant catalyst temperature. The total flow rate for each run was 2.5 NL/min, including 5% O2. The NH3-TAD, NH3-DeNOx, and corresponding blank experiments followed the protocols described by Lietti et al.30 The NH3-TAD experiments were performed at 493−653 K. The transient NH3-DeNOx experiments were performed at 553−653 K. Blank experiments confirmed that dead time and the response broadening because of axial dispersion in the experimental system are limited.
deactivation behaviors of adsorption sites and redox sites on these catalysts. Because distinctions between the deactivation behaviors at adsorption sites and redox sites cannot necessarily be drawn from conventional analyses, a kinetic analysis of the deactivation phenomenon is expected to bridge this gap. Transient-response experiments and kinetic analyses were employed to do this in the present work because of its effectiveness in studying the catalytic mechanism of fresh V2O5/TiO2-based SCR catalysts.30,31 The motivation for this work was to elucidate the deactivation behaviors of adsorption sites and redox sites on commercial V2O5−WO3/TiO2 catalysts after exposure to the flue gas of a coal-fired power plant. Several reports have confirmed that the SCR catalyst is principally deactivated by exposure to aerosol particles in cases when a catalyst bed is located downstream of a combustion boiler, biomass gasifier, or diesel engine.32−36 Larsson et al.37 noted that methods that involve impregnating catalyst samples are not applicable to the study of the deactivation of SCR catalysts in commercial plants. In this context, we present a study of a commercial V2O5−WO3/TiO2 catalyst before and after exposure to flue gas of a coal-fired power plant. The variations in adsorption and the catalytic characteristics and corresponding kinetic parameters were analyzed to distinguish the deactivation behavior of NH3 adsorption sites and redox sites. The effects of these deactivation factors on the elementary steps that occur on the catalyst surface in the SCR-DeNOx process were investigated to obtain more detailed information about the deactivation mechanism.
3. RESULTS AND DISCUSSION 3.1. Textural Properties and Surface Acid Sites. SEM and XRD analyses indicated that the USED-CM68 catalyst is slightly sintered and that the TiO2 anatase content is reduced (see S_Figure 1 and S_Figure 2 of the Supporting Information). XRF measurements showed that the loss of active material is minimal, although clear increases in sulfur and alkali metals were observed in the USED-CM68 catalyst. Accordingly, the BET surface area and pore volume of the USED-CM68 catalyst are reduced by 9.34 and 1.26%, respectively (see S_Table 1 of the Supporting Information). FTIR analysis showed clear reductions in the quantities of Brønsted and Lewis acid sites on the USED-CM68 catalyst. 3.2. Adsorption−Desorption Characteristics of NH3. Variations in the NH3 adsorption and desorption characteristics were studied by NH3-TAD experiments. The NH3-TAD results collected at 493 and 623 K over fresh and used CM68 catalysts are presented in Figure 1. In comparison to the blank experiment (short-dashed line), the increases in the NH3 concentration (solid line) were postponed by strong NH3 adsorption on the catalyst surface. The difference in the steady-state NH3 concentrations between the NH3-TAD and blank experiments is evidence of direct NH3 oxidation.42 Figure 1 shows that the ascending NH3 concentration curves are nearly identical for the fresh and used CM68 catalysts after the NH3 step addition (t = 0 s) at a lower catalyst temperature (T = 493 K). However, the NH3 desorption peak with the USEDCM68 catalyst is shifted to a slightly lower temperature (approximately 9 K). Moreover, the peak value and the area under its curve are reduced. Upon increasing the catalyst temperature, the time delay for the increase in the NH3 concentration was reduced. The peak value and the area of the ammonia desorption peak decreased with increasing catalyst temperature. In contrast, NH3 oxidation was observed at higher catalyst temperatures (T = 623 and 653 K). Nova et al.43 found no NH3 oxidation over commercial V2O5−WO3/ TiO2 catalysts, even at 673 K, when the feed gas contained water vapor. In that case, the effects of catalyst deactivation on NH3 oxidation could not be detected. From this point of view, it is advised to exclude H2O from the feed gas when studying the effects of catalyst deactivation on the adsorption and catalytic characteristics of V2O5−WO3/TiO2 catalysts. It was found that the difference in the ascending NH3 concentration curves measured for the fresh and used CM68 catalysts increased gradually with increasing adsorption time because of
2. EXPERIMENTAL SECTION 2.1. Catalysts. A high-dust commercial V2O5−WO3/TiO2 SCR catalyst (type CM68) was employed in this study. CM68 [V2O5 = 1.15% (w/w), and WO3 = 2.84% (w/w)] is a honeycomb catalyst from CORMETECH with a Brunauer−Emmett−Teller (BET) surface area of 46.2 m2/g. The studied CM68 catalyst was removed from the inlet of the second layer of a SCR reactor at a 330 MW coal-fired power plant after 20 000 h of use. The combusted fuel was Jinbei bituminous coal that contained 21.09% ash and 0.63% sulfur. The space velocity, temperature, ash loading, and SO2 content in flue gas under boiler maximum continuous rating (BMCR) conditions were 5321 h−1 [standard temperature and pressure (STP)], 638 K, 23.44 g/Nm3, and 1520 mg/Nm3, respectively. The fresh and used catalyst samples were denoted as NEW-CM68 and USED-CM68, respectively. The blocked channels and floating dust in the used CM68 monolith catalyst were first cleaned with compressed air. Both fresh and used monolith catalysts were cut into plates for BET and scanning electron microscopy (SEM) analyses. The monolith catalysts were also ground (particle diameters < 150 μm) for X-ray diffraction (XRD), X-ray fluorescence (XRF), and Fourier transform infrared (FTIR) analyses as well as transient-response experiments. 2.2. Transient Kinetic Experiments. Transient NH3 adsorption− desorption (NH3-TAD) and NH3-DeNOx experiments were performed in a microreactor system. The reactor consisted of a quartz tube (14 mm outer diameter and 10 mm inner diameter) with a blowoff branch (6 mm inner diameter) directly connected to a FTIR gas analyzer (ProtIR 204M) that was used to measure the concentrations of NH3, NO, NO2, N2O, H2O, and O2. The limits of detection were 0.1 mg/m3 NH3, 0.9 mg/m3 NO, 0.3 mg/m3 NO2, and 0.2 mg/m3 N2O. The reactor was inserted into an electric furnace that was driven by a proportional−integral−derivative (PID) controller (YuDian AI517). The catalyst temperature was measured using a K-type thermocouple, which was directly inserted into the catalyst bed. To simplify the problem and focus on the impacts of catalyst deactivation on adsorption sites and redox sites, H2O was not included in the feed gases because it would have affected the NH3 adsorption and SCR 3891
DOI: 10.1021/acs.energyfuels.5b01034 Energy Fuels 2015, 29, 3890−3896
Article
Energy & Fuels
ature; and the NH3 oxidation is enhanced. There was very little formation of N2O ( 1.44 A minimal amount of N2O formation (
