Article pubs.acs.org/est
Design Strategies for P‑Containing Fuels Adaptable CeO2−MoO3 Catalysts for DeNOx: Significance of Phosphorus Resistance and N2 Selectivity Huazhen Chang,† Min Tze Jong,† Chizhong Wang,† Ruiyang Qu,†,‡ Yu Du,† Junhua Li,*,†,§ and Jiming Hao† †
State Key Joint Laboratory of Environment Simulation and Pollution Control (SKLESPC), School of Environment, Tsinghua University, Beijing, 100084, China ‡ State Key Laboratory of Clean Energy Utilization, Department of Energy Engineering, Zhejiang University, Hangzhou, 310027, China § State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Tsinghua University, Beijing, 100084, China S Supporting Information *
ABSTRACT: Phosphorus compounds from flue gas have a significant deactivation effect on selective catalytic reduction (SCR) DeNOx catalysts. In this work, the effects of phosphorus over three catalysts (CeO2, CeO2−MoO3, and V2O5−MoO3/ TiO2) for NH3−SCR were studied, and characterizations were performed aiming at a better understanding of the behavior and poisoning mechanism of phosphorus over SCR catalysts. The CeO2−MoO3 catalyst showed much better catalytic behavior with respect to resistance to phosphorus and N2 selectivity compared with V2O5−MoO3/TiO2 catalyst. With addition of 1.3 wt % P, the SCR activity of V2O5−MoO3/TiO2 decreased dramatically at low temperature due to the impairment of redox property for NO oxidation; meanwhile, the activity over CeO2 and CeO2−MoO3 catalysts was improved. The superior NO oxidation activity contributes to the activity over P-poisoned CeO2 catalyst. The increased surface area and abundant acidity sites contribute to excellent activity over CeO2−MoO3 catalyst. As the content of P increased to 3.9 wt %, the redox cycle over CeO2 catalyst (2CeO2 ↔ Ce2O3 + O*) was destroyed as phosphate accumulated, leading to the decline of SCR activity; whereas, more than 80% NOx conversion and superior N2 selectivity were obtained over CeO2−MoO3 at T > 300 °C. The effect of phosphorus was correlated with the redox properties of SCR catalyst for NH3 and NO oxidation. A spillover effect that phosphate transfers from Ce to Mo in calcination was proposed.
1. INTRODUCTION Phosphorus exists in fossil fuel and especially biomass fuel, such as biological residue and sewage sludge.1−3 Co-combustion of coal and biomass fuel increases the content of phosphorus in produced fine particles and flue gas. The presence and behavior of phosphorus compounds in flue gas has gained increased attention due to the co-combustion of fuels with high content of volatile phosphorus compounds.4 The deactivation effect of phosphorus compounds to the selective catalytic reduction (SCR) DeNOx catalysts from flue gas has been identified previously.5 A correlation of the phosphorus content and the loss of activity of the SCR catalysts had been established. Formation of vanadyl phosphate, pore blocking by phosphoric acid, or phosphorus(V) oxide are supposed as the reasons for deactivation of V-based catalyst.6 Over the lifetime of a heavyduty diesel vehicle equipped with an SCR system, phosphorus compounds deposited on the surface of the V2O5/WO3−TiO2 catalyst also affect its activity and selectivity.7 Up to now, there © 2013 American Chemical Society
has been no effective way to avoid P-poisoning of V-based catalyst.8 MoO3 is an important additive in traditional catalysts (i.e., V2O5−MoO3/TiO2) for NH3−SCR in industrial application, which acts as “structural” and “chemical” promoters in SCR reaction.9 Compared with WO3 in commercial SCR catalyst, MoO3 are even more competitive in tolerance to As.10,11 Even though the Mo-containing catalyst exhibits lower N2 selectivity and NO conversion than V2O5−WO3/TiO2 at high temperatures (which is possibly associated with their higher reactivity in the ammonia oxidation reactions), SCR catalyst producers began to use MoO3 instead of WO3 due to the rise in price of tungsten in the past decade.10,11 Received: Revised: Accepted: Published: 11692
May 16, 2013 August 8, 2013 September 11, 2013 September 11, 2013 dx.doi.org/10.1021/es4022014 | Environ. Sci. Technol. 2013, 47, 11692−11699
Environmental Science & Technology
Article
rate of 100 mL min−1, GHSV = 7.0 × 104 h−1 (STP). The oxidation of NH3 and NO and temperature-programmed desorption of NH3 (NH3-TPD) were also performed on the same system. The concentrations of NO, NO2 (NOx = NO + NO2), and other components were measured with a FT-IR gas analyzer (Gasmet Dx-4000). The NOx/NH3 conversion was calculated according to the inlet and outlet concentrations of NOx/NH3 at which the catalysts were kept on stream at each temperature for more than 1 h. The N2 selectivity in SCR reaction was calculated from
CeO2, which possesses high oxygen storage capacity and excellent redox property, has attracted much attention for use as an NH3−SCR catalyst.12−14 CeO2 itself showed certain SCR activity at 250−350 °C,15 whereas the performance would be enhanced significantly after sulfation in SO2+O2.14,16 The CeO2/TiO2 catalyst exhibited good catalytic performance in the presence of H2O, CO2, and C3H6.17 Shan et al.13,18,19 reported that doping of tungsten to CeO2 could induce a synergistic effect between Ce and W and enhance the SCR activity, as well as the resistance to high space velocity. Li et al.20 reported that CeO2 catalyst with addition of phosphorus showed excellent NH3−SCR activity in a wide temperature range. However, other researchers found that incorporation of P caused a significant decline in the concentration of labile active (OSC) and total (OSCC) oxygen species, and resulted in deactivation of Ce-based catalyst.21 Further investigations are needed in the effect of phosphorus on CeO2 catalyst. The structure of CeO2−MoO3 catalyst was previously studied by Raman spectroscopy.22−24 Detailed investigations on the performance of CeO2−MoO3 in NH3−SCR reaction is still scarce in the literature. In this study, the SCR activity and influence of phosphorus on CeO2−MoO3 and V2O5−MoO3/ TiO2 catalysts were studied. It indicated that the resistance to phosphorus and N2 selectivity was improved on CeO2−MoO3 catalyst. An extensive investigation has been undertaken in our laboratories, aiming at a better understanding of the Ppoisoning effect and chemico−physical properties of the Ce− Mo−O catalytic system.
⎛ ⎞ 2C N2O(out ) ⎟ SΝ2 = ⎜⎜1 − C NOx (in) + C NH3(in) − C NOx (out ) − C NH3(out ) ⎟⎠ ⎝ × 100%
The N2 selectivity in NH3 oxidation was calculated from ⎛ 2C N2O(out ) + C NOx (out ) ⎞ ⎟ × 100% S′N2 = ⎜⎜1 − C ΝH3(in) − C ΝH3(out ) ⎟⎠ ⎝
The normalized reaction rate in NO/NH3 oxidation was calculated from Rs =
Vstd × Cconv 22.4 × SBET × m
where Rs stands for normalized reaction rate (mol s−1 m−2), Vstd stands for flow rate at standard conditions (L s−1), Cconv stands for volume ratio of converted reactant to total flow, SBET stands for specific surface area (m2 g−1), and m stands for the weight of catalysts (g). 2.3. Catalyst Characterization. The BET specific surface area was measured by N2 physisorption at 77 K using a Quantachrome Nova automated gas sorption system. The samples were degassed at 300 °C for 4 h before the N2 physisorption. During a typical experiment of NH3-TPD, a gas flow rate of 300 mL min−1 and a sample mass of 0.15 g were used. The sample underwent experimental procedures as follows: (1) degasification in a N2 flow at 500 °C for 1 h; (2) cooling to 100 °C for 1 h adsorption in a NH3 + N2 flow; (3) removal of physically adsorbed NH3 in a N2 flow for 1 h; and (4) temperature-programmed desorption at a heating rate of 10 °C min−1 in a N2 flow. In-situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) was performed on a Nicolet Nexus spectrometer equipped with a liquid-nitrogencooled MCT detector. The catalyst sample was preheated in a flow of N2 at 350 °C for 1 h and then cooled and kept for 10 min at the desired temperature prior to background scanning.
2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and P-Poisoning. The Ce− Mo−O composite oxide catalysts were prepared by a coprecipitation method, where Ce(NO3)3 and (NH4)6Mo7O24 were used as the catalyst precursors and (NH4)2CO3 was used as precipitator. The mash obtained from coprecipitation was filtered, followed by subsequent washing with DI water, drying overnight at 110 °C, and calcination at 500 °C for 4 h. The samples are expressed as Ce−Mo(x)−O, where x denotes the molar ratio of Mo/Ce, e.g. Ce−Mo(0.5)− O. The reference CeO2 catalyst was prepared by the same method. The MoO3 catalyst was obtained from calcination of (NH4)6Mo7O24 directly at 500 °C for 4 h. The reference V2O5−MoO3/TiO2 (denoted as VMo/Ti) catalyst with nominal V2O5 content of 1 wt % and MoO3 content of 5 wt % was prepared by the incipient wetness method. TiO2 was impregnated in aqueous solution of NH 4 VO 3 and (NH4)6Mo7O24 (oxalic acid as cosolvent), followed by rotary evaporation. The resulting precursor was dried at 110 °C overnight and calcined at 500 °C for 4 h. The Mo loading has been selected to roughly correspond to that of commercial catalysts. The P-poisoned catalysts (P content of 1.3 wt %, if not specified) were prepared by impregnating of samples in aqueous solution of corresponding amount of NH4H2PO4, followed by drying and calcination at 500 °C for 4 h. These contaminant loadings are within the range detected in aged commercial catalyst.21 The poisoned samples were denoted with P as prefix, such as 1.3P/Ce−Mo(0.5)−O. 2.2. Catalytic Performance. The steady-state NH3−SCR activity of catalysts was estimated in a fixed-bed quartz reactor. The reaction conditions were as follows: 0.15 g of catalyst, 500 ppm NO, 500 ppm NH3, 3% O2, 100 ppm SO2 (when used), 7% H2O (when used), and balance in N2 with a total gas flow
3. RESULTS 3.1. Comparison of CeO2−MoO3 and VMo/Ti Catalysts. 3.1.1. SCR Activity. Our recent study indicated that the SCR activity of CeO2 catalyst was remarkably enhanced after addition of Mo. The catalyst with a molar ratio of Mo/Ce = 0.5 exhibits the best SCR activity and N2 selectivity (Supporting Information (SI) Figure S1). The NOx conversion as function of reaction temperatures over Ce−Mo(0.5)−O and VMo/Ti catalysts is showed in Figure 1a. In the test temperature range, the Ce−Mo(0.5)−O catalyst exhibited nearly the same NOx conversion compared with VMo/Ti catalyst. More than 99% NOx conversion was obtained over both catalysts at 250−350 °C. However, the N2 selectivity varied. At T < 300 °C, more 11693
dx.doi.org/10.1021/es4022014 | Environ. Sci. Technol. 2013, 47, 11692−11699
Environmental Science & Technology
Article
lower over VMo/Ti catalyst, especially at temperature higher than 400 °C; only 47% of N2 selectivity was obtained at 500 °C. Larger amounts of N2O and NO formed over VMo/Ti catalyst if compared with the Ce−Mo(0.5)−O catalyst. This was consistent with the trend of N2 selectivity in SCR reaction over two catalysts. It should be noted that the amounts of N2O observed during the SCR reaction were higher if compared to that detected in NH3 oxidation reaction, suggesting that both NH3−SCR and NH3−SCO (selective catalytic oxidation of NH3) reactions contributed to the formation of nitrous oxide. As already suggested in previous investigations, Mo may contribute to the N2O formation over VMo/Ti catalyst.9 It was reported that the electron-donating effect of cerium would reduce the activity for NH3 oxidation, in combination with the enhanced NH3 adsorption capacity by Cen+ as additional Lewis acid sites, endowed the Ce-doped catalyst a higher N2 selectivity.25 Hence, with the synergistic effect of Mo and Ce, higher N2 selectivity was obtained over the Ce−Mo(0.5)−O catalyst than over the VMo/Ti catalyst. 3.2. Effect of Phosphorus on CeO2, VMo/Ti, and CeO2−MoO3 Catalysts. 3.2.1. SCR Activity. The effect of phosphorus on SCR activity over CeO2, VMo/Ti, and Ce− Mo(0.5)−O catalysts was tested and the results are shown in Figure 2. After addition of 1.3 wt % P, nearly 100% NOx Figure 1. (a) NOx conversion and N2 selectivity vs temperature over the Ce−Mo(0.5)−O and VMo/Ti catalysts. Reaction conditions: 0.15 g of samples, 500 ppm NO, 500 ppm NH3, 3% O2, N2 balance, GHSV = 7.0 × 104 h−1. (b) NH3 oxidation activity over the same catalysts. Reaction conditions: 0.15 g of samples, 500 ppm NH3, 3% O2, N2 balance, GHSV = 7.0 × 104 h−1.
than 98% of N2 selectivity was achieved over both catalysts. As temperature increased, the N2 selectivity decreased rapidly over the VMo/Ti catalyst. The N2 selectivity of the Ce−Mo(0.5)−O catalyst is higher than that over the VMo/Ti catalyst at 350− 400 °C. The decrease of the N2 selectivity at high temperatures has already been evidenced over VMo/Ti catalyst. 9 The synergistic effect between Ce and Mo may contribute to the restraint of N2O formation, and the N2 selectivity was enhanced in SCR reaction. These two catalysts were further characterized to discuss the relationship between the SCR activity and chemico−physical properties of two catalysts. 3.1.2. NH3 Oxidation Correlation with SCR Activity. The BET specific surface area of Ce−Mo(0.5)−O and VMo/Ti catalysts were 63.8 and 58.0 m2 g−1, respectively (see SI Table S1). It seems that surface area was not the main reason for the difference of N2 selectivity over two catalysts. In a previous study, the decrease in the NOx conversion and N2 selectivity that is observed at high temperatures under SCR conditions was attributed to the occurrence of the NH3 oxidation.9 To elucidate the superior N2 selectivity over Ce−Mo(0.5)−O catalyst, NH3 oxidation reactions were performed over Ce− Mo(0.5)−O and VMo/Ti catalysts. As shown in Figure 1b, both catalysts showed low NH3 conversion at 250 °C. From 300 to 400 °C, the NH3 conversion increased rapidly to >95% over VMo/Ti catalyst. The Ce−Mo(0.5)−O catalyst exhibited similar behavior, even though the NH3 conversion was a little lower compared with VMo/Ti catalyst; whereas, the N2 selectivity was kept at >91% below 400 °C over Ce− Mo(0.5)−O catalyst. Then it decreased at high temperatures due to the formation of N2O. The N2 selectivity was much
Figure 2. Effect of 1.3 wt % P on the SCR activity of Ce−Mo(0.5)−O, VMo/Ti, and CeO2 catalysts (a), normalized NO reaction rates over the fresh and P-poisoned catalysts (inset); and effect of 3.9 wt % P on the SCR activity of the Ce−Mo(0.5)−O and CeO2 catalysts (b). Reaction conditions: 0.15 g of samples, 500 ppm NO, 500 ppm NH3, 3% O2, N2 balance, GHSV = 7.0 × 104 h−1.
conversion was obtained over CeO2 catalyst at 250 °C. Over the 1.3P/VMo/Ti catalyst, the NOx conversion decreased dramatically at
