Ce-Based Catalysts for the Selective Catalytic ... - ACS Publications

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Ce-Based Catalysts for the Selective Catalytic Reduction of NOx in the Presence of Excess Oxygen and Simulated Diesel Engine Exhaust Conditions Thirupathi Boningari,† Arpad Somogyvari,‡ and Panagiotis G. Smirniotis*,† †

Chemical Engineering Program, Department of Biomedical, Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0012, United States ‡ Catalyst Elements, Cummins Emission Solutions, Columbus, Indiana 47202, United States S Supporting Information *

ABSTRACT: A family of various cerium oxide-based catalysts were synthesized by adopting flame aerosol (FSP), coprecipitation, wet impregnation, and hydrothermal synthesis techniques. The resulting catalysts were explored for the selective catalytic reduction (SCR) of NOx using NH3 as reductant. In our studies, both the preparation method and the Ce/W ratios were found to be critical variables for successful catalyst promotion. For the industrial realization, we have scaled up the SCR activity tests. The microreactor catalytic formulations at simulated diesel engine exhaust conditions revealed that the Ce−W (1:1 atomic ratio) and Ce−W/TiO2 catalysts showed high deNOx activity, while the other catalysts’ activity was found to be rather low. Of interest is the finding that the Ce−W/TiO2/cordierite and Ce−W (1:1 atomic ratio)/cordierite formulations show impressive deNOx performance and high N2 selectivity with respect to a commercial vanadia based reference currently used for mobile applications. To gain fundamental insights which may acquaint further improvements to the promoted Ce-based catalysts, X-ray photoelectron spectroscopy and other characterizations were executed to study the relationship between catalyst surface and NOx reduction activity. Our XRD results indicate smaller lattice parameters of prepared catalysts compared to that of CeO2 (0.540 nm). The crystal lattice contraction is attributed to the lesser ionic radius of relevant foreign metal ions (W6+ = 0.067 nm and Ti4+ = 0.074 nm) in relation to Ce4+ (0.092 nm) in the host lattice. This lattice shrinkage elucidates the formation of solid solutions. These results illustrate that the synthesis technique and various promoters could indeed influence the lattice structures and electronic state of the active components. The XPS results illustrate the higher atomic ratios of Ce3+/(Ce3+ + Ce4+) 0.30 and 0.29 in Ce−W/TiO2 and Ce−W (1:1) coprecipitation catalysts, respectively, compared to other samples. The higher surface Ce3+/Ce4+ ratio in Ce−W (1:1) coprecipitation and Ce− W/TiO2 samples indicate the enrichment in surface oxygen vacancies, which results in activation of reactive molecules and enhanced adsorption of oxygen species in SCR reaction. Interestingly, the surface atomic ratio of Ce3+/Ce4+ and Ce3+/Cen+ are interrelated to the SCR activity of the individual catalysts.

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includes metal exchanged zeolites and V2O5−WO3(MoO3)/ TiO2 in practical industrial applications.6,7 Nevertheless, most of the zeolite-based catalysts used in postcombustion removal of NOx are sensitive to sulfur poisoning and required to be desulfated at high temperatures, in which upstream platinum group metal (PGM) catalysts produce SO3 from the oxidation of SO2 which deactivates the catalyst more quickly than SO2. Conversely, existing V-based commercial catalytic systems suffer from the toxic nature of vanadium species, cold-start emissions, and the decrease in N2 selectivity at hightemperatures due to the unselective ammonia oxidation.8 Most of the researchers are focusing on the advancement of highly efficient catalysts for the SCR of NOx reaction to evade

INTRODUCTION Future legislations will impose very severe restrictions on nitrogen oxides emissions and initiate a renewed interest in low-temperature NOx reduction. Future low emissions standards such as the US EPA Tier-3 new LD vehicle emissions standard from 2017 and other emissions standards for HD are driving OEMs and catalyst suppliers to focus on reducing another 90% of NOx emissions. The current EURO-VI and EPA-2013 regulations enforce a large reduction in pollutants; present after-treatment systems should meet over 99% reduction in particulate matter and over 97% reduction in NOx.1,2 This requirement is especially demanding for the selective catalytic reduction (SCR) of nitrogen oxides using NH3 (NH3-SCR) under simulated conditions of diesel engine exhaust. Hence, the advancement of highly active, time stable, catalytic formulations for the SCR of NOx under diesel engine exhaust conditions is challenging for practical use.3−5 The postcombustion removal of NOx in mobile applications © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

January 6, 2017 April 24, 2017 April 27, 2017 April 27, 2017 DOI: 10.1021/acs.iecr.7b00045 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

to attain homogeneous material, followed by calcination in an oven at 500 °C for 5 h in air (150 mL min−1). 2.1.2. Synthesis of CeWOx Nanoparticles with 1:1 and 2:1 Atomic Ratio of Ce−W, Respectively, and CeTiOx Nanoparticles with 1:1 Atomic Ratio of Ce−Ti, Respectively, Using Flame Aerosol Technique. The CeWOx nanoparticles with 1:1 and 2:1 atomic ratio of Ce−W, respectively, and CeTiOx nanoparticles with 1:1 atomic ratio of Ce−Ti, respectively, were prepared by the flame spray pyrolysis (FSP) synthesis method. A predetermined amount of the corresponding metal precursors were dissolved in ortho-xylene (Sigma−Aldrich Reagent, 98%) for the respective metal oxides. The metal precursors (cerium(III) 2-ethylhexanoate 49% in 2-ethylhexanoic acid, 12.20% Ce, Strem Chemicals), (tungsten(V) ethoxide, 95%, Alfa Aesar), (titanium(IV) 2-ethylhexoxide, 84% Ti, Strem Chemicals) were used as the source of Ce, W, and Ti, respectively. In the course of the FSP synthesis, a 2 mL min−1 flow rate of liquid precursor was injected by using a Cole Parmer (series 74900) syringe pump. The 5 L min−1 oxygen gas flow (1.5 bar, 99.98%, Wright Brothers) was engaged as a dispersion gas to atomize the precursor solution. The premixed (1.0 L min−1 O2/0.85 L min−1 CH4) gas was used as the surrounding supporting flame for the combustion of dispersed droplets. A 150 mm in diameter glass fiber filter (Whatman GF/A) was used to collect the as-synthesized fine nanoparticles leaving the flame.17,20,24 The as-synthesized materials were collected from the flat glass fiber filter and directly used as the catalyst without any calcination or further treatment. The active metals in the as-synthesized aerosol nanoparticles are indicated as molar fractions [Ce−W-(1:1) FSP, Ce−W-(2:1) FSP, and Ce−Ti (1:1)-FSP]. 2.1.3. Preparation of Siliceous Mesocellular Foam (MCF) Support. The spherical mesocellular siliceous foam samples were prepared by altering the general MCF preparation procedure.23 During the synthesis of mesocellular siliceous foam (MCF) sample, 5.0 g of Pluronic P123 (MW = 5800, PEO20PPO70-PEO20, Sigma-Aldrich) was added to 75 mL of 1.6 M HCl aqueous solution at ambient temperature. A 10.0 g sample of trimethylbenzene TMB (Sigma-Aldrich) was added under vigorous stirring when the solution temperature was raised to 42 °C; 9.3 mL of tetraethylorthosilicate TEOS (98%, Sigma-Aldrich) was added after 2 h and the obtained solution was kept at 42 °C under continuous stirring for another 30 min. The hydrolyzed solution was then transferred to an airtight Teflon vessel and kept in an oven maintained at 42 °C. After 24 h, 100 mg of NH4F (99.99%, Alfa Aesar) was added to the contents of the Teflon bottle and then the aging process continued for 5 days at 140 °C. The as-made sample was filtered and washed with water and ethanol repeatedly to obtain white flaky solid composites of silica and the surfactant template. Removal of the template was implemented by increasing the temperature to 900 °C with a ramp rate of 2 °C per minute and maintaining the calcination temperature for 6 h. The mass ratio of the silica precursor to surfactant was 2.5 (TEOS/P123 = 2.5), while the mass ratio of organic pore expander to the surfactant was adjusted to 3 (TMB/P123 = 3). 2.1.4. Preparation of W-Promoted Siliceous Mesocellular Foam Supported Ce-Based Catalyst (Ce−W/MCF). Wpromoted Ce supported on the MCF sample was synthesized by a conventional wet-impregnation technique. The predetermined amount of cerium nitrate (Ce(NO3)2·6H2O, 99.99% metal basis Aldrich) and ammonium metatungstate ((NH4)6H2W12O40·XH2O and the siliceous mesocellular foam

all the difficulties related to the current industrial catalytic systems. It has been well established in the open literature that CeO2 can easily promote the catalytic activity of numerous reactions such as CO2 activation,19 CO oxidation,10 and CO/NO removal11−15 due to its exceptional redox properties (Ce3+ ↔ Ce4+) and to provide labile surface and bulk oxygen vacancies. Accordingly, ceria can improve the oxidization of NO to NO2 and thus promote the deNOx performance using ammonia as reductant. Hence, we have investigated the influence of ceria on NOx reduction over a family of catalyst formulations. Moreover, in our previous studies, we observed an enhancement in the SCR activity and stability of the catalyst by the addition of tungsten.16 The tungsten to vanadia (W/V) ratio = 0.66 has promoted the NO conversion and demonstrated ∼98% conversion in a broad temperature window.16 On the other hand, several authors reported the promotional effect of Ce, Fe, W, and Ti on the NH3-SCR reaction. In particular, Shan et al. developed the (CeO2/WO3−TiO2) catalysts by stepwise precipitation, and the CeWOx catalyst by homogeneous precipitation method, Wang et al. established a promoting impact of CeOx on FeOx−TiOx catalysts.17−19 The main objectives of this work are to elucidate correlations among catalyst synthesis methods, structural and surface characterizations with the SCR of NOx activity under process conditions similar to those found in active mobile diesel exhaust systems. In our earlier studies, we investigated as-synthesized aerosol nanoparticles (V/ZrO2, V-WOx(0.66)/ZrO2, V/CeO2− Al2O3, V/CeO2−ZrO2, and V/TiO2−Al2O3, etc.) at a relatively low GHSV (gas hourly space velocity) of 24 000 h−1 and low concentrations of oxygen in the reaction feed.16,20 Current studies are focused on improving the deNOx performance of our materials under industrially relevant diesel exhaust conditions. Flame spray pyrolysis (FSP) is well-known in both industry and academia for producing nanoparticles containing strong metal support interactions. In addition to a rapid one-step synthesis and yielding ready to use catalysts at high production rates, FSP produces highly dispersed active components on the functional support.21,22 By combining all these aspects, our investigation is concentrated on the progress of highly efficient catalyst formulations to be used in the SCR of NOx with NH3 under an oxygen-rich environment. Therefore, the current work illustrates the effect of synthesis method as well as promoters in Ce-based materials for the prospective SCR of NOx applications.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Catalysts. 2.1.1. Preparation of Ce−W/ TiO 2 (Hombikat) Catalysts Using Wet Impregnation Technique. Ce−W/TiO2 catalysts were synthesized by using the wet impregnation method. Titania (Hombikat, 100% anatase) was used as support to synthesize the corresponding catalysts. Cerium nitrate (Ce(NO3)2·6H2O, 99.99% metal basis Aldrich), and ammonium metatungstate ((NH4)6H2W12O40· XH2O were taken as the source of ceria and tungsten, respectively. The active metals in the catalyst are denoted as nominal weight fractions. The predetermined amounts of precursors were mixed in a 1000 mL beaker containing 5.0 g of support in 400 mL of deionized water. The additional water was then gradually evaporated on a hot plate with vigorous stirring at 70 °C. The obtained powders were oven-dried at 120 °C for 12 h. Later these dry powders were crushed and sieved B

DOI: 10.1021/acs.iecr.7b00045 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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total flow rate was 804 sccm (GHSV = 500 000 h−1) and the gas mixture consists of 500 ppm of NOx, NO2/NOx = 0.1, ammonia to NOx ratio (ANR) = 1.0, 8 vol % CO2, 7 vol % H2O, 10 vol % O2, and rest argon. The reaction temperature was ramped up from 100 to 550 °C then ramped down to 100 with 50 °C intervals. The NOx conversions and N2 selectivity were calculated as follows:

was added to 100 mL of deionized water. After several hours of heating at 70 °C, all the water had evaporated leaving behind light yellow solids. Calcination was performed in air at 500 °C during 5 h after ramping to temperature at 5 °C per minute. 2.1.5. Synthesis of CeWOx catalyst with 1:1 Atomic Ratio of Ce−W, Respectively, Using Coprecipitation Method. The Ce−W mixed oxide catalyst was synthesized using the coprecipitation technique. The cerium nitrate (Ce(NO3)3· 6H2O, ≥99.0%, Aldrich) and ammonium paratungstate hydrate (H42N10O42W12·xH2O, 99.99%, Aldrich) were used as metal precursors. An equal amount of H42N10O42W12·xH2O and oxalic acid dihydrate (C2H2O4·2H2O, ACS grade, Fisher Scientific) were mixed in deionized water and stirred for 30 min. After complete dissolution of H42N10O42W12·xH2O, the aqueous solution of Ce(NO3)3·6H2O was added with the requisite molar proportion of Ce−W = 1:1. Subsequently, an excess urea (CH4N2O, ACS grade, Fisher Scientific) aqueous solution with a urea/(Ce + W) molar ratio of 50:1 was added to the mixed solution. Afterward, the resulting solution was heated and kept at 90 °C for 12 h under continuous stirring until a pH of 8 was reached. The obtained precipitant was filtered and washed with deionized water, followed by drying at 110 °C overnight, followed by calcination at 500 °C for 5 h in air (150 mL/min). The obtained catalyst was denoted as Ce− W (1:1)-co-precipitation. 2.2. Characterizations. The X-ray diffraction profiles were measured on a Phillips Xpert diffractometer by employing nickel-filtered Cu Kα (wavelength 0.154056 nm) radiation. The scanning range was from 10° to 80° with a step time of 0.25 s and a step size of 0.025°. The specific surface area of catalysts was calculated by the BET method from the nitrogen adsorption isotherms obtained at 77 K using Micromeretics Gemini surface area apparatus. H2-TPR was carried out on AutoChem II 2910, Micromeritics instrument equipped with a thermal conductivity detector (TCD). The samples were preheated at 200 °C for 2 h in helium before each experiment. The samples were then cooled to 50 °C in a flow of UHP helium. Subsequently, the reduction of the samples was initiated from 50 to 900 °C. The 10% H2 in argon gas mixture was used with a flow rate of 20 mL min−1 with a ramp rate of 10 °C min−1 . Temperature-programmed desorption of ammonia (NH3-TPD) was carried out on a Micromeritics Autochem 2910. Before the analysis, each sample was preheated at 200 °C in helium for 2 h. Subsequently, the samples were cooled to 100 °C, and saturated with anhydrous NH3 (4% in He) for 1 h until adsorption equilibrium was reached followed by flushing with helium for 2 h at 100 °C to eliminate physisorbed (weakly bound) ammonia. Later the ammonia-saturated samples were heated to 900 °C. The X-ray photoelectron spectroscopy (XPS) analysis was performed on a Pyris-VG Thermoscientific X-ray photoelectron spectrometer system. 2.3. Catalytic Activity. 2.3.1. Microreactor Catalytic Experiments Procedures at Simulated Diesel Engine Exhaust Conditions. State-of-the-art selected Ce−W-based SCR powder catalysts were diluted into cordierite or SiC with a cat/M (M = cordierite, SiC) mass ratio of 1:12 to minimize bypassing and axial dispersion. A baseline was obtained using 8 vol % CO2, 7 vol % H2O, 10 vol % O2, balance Ar. Prior to the evaluation, 0.050 g of powder catalyst or 0.141 g of reference sample (Vbased benchmark catalyst) was physically mixed with the 0.450 g of cordierite or 0.600 g of SiC diluent to keep the catalyst bed about the same length, and loaded into the quartz reactor. The

NOx conversion (XNOx %) = (NOx in − NOx out)/NOx in × 100

(1)

N2 selectivity (S N2) = [N2 /total N containing species] (2)

In eq 1, NOxin and NOxout denote the concentrations of NOx inlet and NOx outlet, respectively. 2.3.2. SCR of NOx with Excess Oxygen. The SCR of NOx activity measurements were carried out in a fixed bed quartz reactor (i.d.: 6 mm at atmospheric pressure). A thermocouple was inserted into the catalyst bed and controlled by a temperature regulator (Omega CN 2041). The typical reaction mixture consists of NO = 900 ppm, NO2 = 100 ppm, NH3 = 1000 ppm, oxygen = 10 vol %, and helium in balance. The premixed gases, nitrogen oxides (NO = 1.8%, NO2 = 0.2% in helium), ammonia (4.0% in helium), and oxygen (20% in helium), were supplied from Wright Brothers Inc. The products and reactants were examined and recorded online by means of a quadrapole mass spectrometer (MKS PPT-RGA), and a chemiluminescence analyzer (Eco Physics CLD 70S) only after steady state was attained at each temperature. The SCR of NOx activity was calculated as in our earlier publications.2,3,9,10

3. RESULTS AND DISCUSSION 3.1. Crystallite and Surface Characterization of Ce−W Based Catalysts. 3.1.1. X-ray Diffraction. The X-ray diffraction profiles of the prepared catalysts are shown in Figure 1. In Figure 1, all the prepared catalysts exhibited the diffraction peaks which correlated to cerium oxide (111), (200), (220), (311), (222), (400), (331), (420) planes positioned at 2θ = 28.5°, 33.0°, 47.4°, 56.3°, 59.1°, 69.4°, 76.7°, 79.0°, and 88.4°, respectively. All the diffractogram patterns are corresponding to the FCC (face centered cubic) of CeO2 (JCPDS No. 34-0394), which can be ascribed to the fluorite structure ceria.25 Fluorite is the most stable structure of ceria, which illustrates various structural defects subject to the stress in the material.26 Oxygen vacancies and electrons localized on cerium cations (polarons) are of key components due to the useful range location of these defective sites to ceria.26 The enhanced diffusion rate of oxygen in the labile lattice oxygen also leads to enhancement in catalytic activity. All these factors make ceria-based materials as remarkable heterogeneous catalysts. For the Ce−W/TiO2−WI sample, the representative peaks of titania typically at d = 3.54, 1.90, and 2.40 Å can be observed. These diffraction peaks can be attributed to the anatase phase (JCPDS #71-1169). Interestingly, we have not observed any titania to rutile phase transformation in our XRD studies of Ce−W/TiO2−WI and Ce−Ti (1:1) FSP samples. For all the prepared catalysts, X-ray reflections belonging to the crystalline WOx are absent. This illustrates that either WOx is highly dispersed or tungsten ions had inserted into the ceria lattice. In addition, the diffraction peaks associated with the CeO2 (111) plane in Ce−W/TiO2 C


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3.1.2. Specific Surface Area (BET) Measurements. The specific surface area of the TiO2 (Hombikat, anatase) support was found to be 309 m2 g−1. After calcination, the surface area of the TiO2 sample decreased to 161 m2 g−1. The surface area of a family of Ce-based metal oxide formulations calcined at 500 °C and as-prepared catalysts are shown in Table 2. As shown in Table 2, the surface areas of 20Ce−10W/TiO2−WI (109 m2/g) and Ce−W (1:1) coprecipitation (118 m2/g) are lower than that of 20Ce−10W/MCF (350 m2/g). In Figure 2 and Table 2, pore size distributions were calculated from the N2 desorption isotherm by means of the BJH method (cylindrical pore model). As one can see from Figure 2 and Table 2, only Ce−W (1:1) coprecipitation and 20Ce−10W/TiO2 WI catalysts demonstrated small pores distribution with an average pore diameter of 5.6 and 6.7 nm, respectively. Because of inaccessibility and technical problems, we have not performed AAS/XRF analysis for these samples. 3.2. Selective Catalytic Reduction (SCR) of NOx with NH3 as Reducing Agent. 3.2.1. Development of Efficient SCR Catalysts. Our catalyst synthesis techniques afforded the opportunity to fine-tune low-temperature SCR catalyst, optimizing the composition that directly improves NOx conversion efficiency. Additionally by using the available physicochemical characterization methods we can better manage catalyst synthesis procedures that thus allow for dramatically enhanced low-temperature deNOx activity. The Ce-based catalysts were prepared using flame aerosol, coprecipitation, and wet impregnation techniques and examined for the selective catalytic reaction of NOx with NH3 at low temperatures. Both the preparation method and the promoters were found to be critical variables for successful catalyst promotion. The 20Ce−10W/TiO2 prepared by wet impregnation technique and Ce−W (1:1 atomic ratio) catalyst synthesized by using the coprecipitation method illustrated the most promising deNOx activity and broadening of the temperature window. These catalysts demonstrated high NOx conversion in the temperature regime of 200 to 350 °C (Figure 3). Conversely, the maximum pragmatic reaction rates (Table 1) are in the range related to kinetic control with no diffusion limitations rendering to the typical standards.27−32 The kinetic constant established the molar content of active component of the catalyst kac, can be determined as

Figure 1. X-ray diffraction patterns of (a) Ce−W (2:1) FSP, (b) Ce− W/TiO2 WI, (c) Ce−Ti (1:1) FSP, (d) Ce−W (1:1) coprecipitation, (e) Ce−W/MCF WI, and (f) Ce−W (1:1) FSP catalysts: #, cerianite CeO2; ◇, anatase TiO2; ¤, Ce2(WO4)3. Higher angle shift in the diffraction peak corresponding to the CeO2 (111) and CeO2 (200) planes.

WI, Ce−Ti (1:1) FSP, 20Ce-10W/MCF, Ce−W (1:1) coprecipitation, Ce−W (2:1) FSP, and Ce−W (1:1) FSP samples, respectively, shifted to higher angles compared to that of pure CeO2 (Figure 1 inset and Table 1). These results confirm the formation of binary and/or ternary oxide solid solutions by the addition of different cations into the CeO2 lattice. In the case of the lattice parameter, it is strongly influenced by the radius of the foreign metal ions as shown in Table 1. The calculated lattice parameters of Ce−W/TiO2 WI (0.529 nm), Ce−Ti (1:1) FSP (0.534 nm), 20Ce−10W/MCF (0.537 nm), Ce−W (1:1) coprecipitation (0.537 nm) catalysts are smaller than that of CeO2 (0.540 nm). The reduction or shrinkage of crystal lattice can be attributed to the smaller ionic radius of respective foreign metal ions (W6+ = 0.067 nm and Ti4+ = 0.074 nm) in relation to Ce4+ (0.092 nm) in the host lattice. This lattice shrink elucidates the formation of solid solutions.

kac (molNOx gac−1 Pa−1) =

o 1 ⎛ FNOx ⎞ 1 ⎟ ln ⎜ o pNOx ⎝ wm ⎠ 1 − X

(3)

Table 1. Summary of the Lattice Parameter a, Lattice Strain along CeO2 (111) Plane, Crystal Size, Reaction Rates, and Turnover Frequencies of All the Prepared Catalysts catalyst

crystal size (nm)

lattice spacing (Å)

lattice parameter a (nm)a

lattice strain along CeO2 (111) (×10−1)

reaction rate ( μmol g−1 s−1)b

reaction rate ( μmol m−2 s−1)c

TOF (×10−2 s−1)d

CeO2 Ce−W (1:1) FSP 20Ce−10W/MCF 20Ce−10W/TiO2 Ce−Ti (1:1) FSP Ce−W (2:1) FSP Ce−W (1:1) Co−P

11.0 30.0 2.4 7.7 8.0 21.5 7.1

3.12 3.12 3.10 3.05 3.08 3.12 3.10

0.541 0.540 0.537 0.529 0.534 0.540 0.537

0.017 0.164 0.166 0.067 0.027 0.094

5.9 7.0 10.2 7.2 8.1 8.0

0.212 0.020 0.094 0.069 0.208 0.068

2.33 8.24 10.4 1.37 2.49 3.30

Calculated from d111 spacing using a = d111√(h2 + k2 + l2) = d111√3. bReaction rate at 473 K reaction temperature. cReaction rate per specific surface area at 473 K reaction temperature. dTurnover frequency (TOF): number of NOx molecules converted per active component atom site per second. a

D


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Industrial & Engineering Chemistry Research Table 2. BET Surface Area, Pore Diameter, and Pore Volume Measurements catalyst TiO2 anatase MCFa Ce−W (1:1) FSPb Ce−W (2:1) FSPb Ce−W/TiO2 WIc Ce−Ti (1:1) FSPb Ce−W (1:1) Co−Pd Ce−W/MCF WIc a

XRD phases A SiO2 CeO2, Ce2 (WO4)3, A CeO2, Ce2 (WO4)3, A CeO2, Ce2 (WO4)3, A CeO2, A CeO2, Ce2(WO4)3 CeO2

SBET (m2/g)

micropore volume (cm3/g STP)

micropore area (m2/g)

external surface area (m2/g)

average pore diameter (nm)

pore volume (cm3/g) 0.37 2.8 0.17

309 482 28

3.5 × 10−3

9.4

19.1

4.5 43.8 24.5

39

4.1 × 10−3

10.9

27.9

19.1

0.18

109

3.1 × 10−4

3.8

105.7

6.7

0.18

104 118

4.4 × 10−3 5.2 × 10−4

14.5 5.3

89.4 113.2

16.6 5.6

0.43 0.18

350

2.7 × 10−2

75.0

275.6

18.7

1.63

b

Siliceous mesocellular foam. Flame spray pyrolysis method. cWet impregnation method. dCoprecipitation method.

Figure 3. Catalytic reduction of NOx over (a) Ce−W (1:1 atomic ratio) FSP, (b) Ce−W (2:1 atomic ratio) FSP, (c) 20 wt % Ce-10 wt % W/TiO2 WI, (d) Ce−Ti (1:1 atomic ratio) FSP, (e) Ce−W (1:1 atomic ratio) coprecipitation, and (f) 20 wt % Ce−10 wt % W/MCF WI catalysts at various temperatures in 50 °C increments at a gas hourly space velocity (GHSV) = 80 000 h−1. Feed: NO = 900 ppm, NO2 = 100 ppm, NH3 = 1000 ppm, O2 = 10 vol %, He carrier gas. Figure 2. BJH pore-size distribution curves of (a) Ce−W (1:1 atomic ratio) FSP, (b) Ce−W (2:1 atomic ratio) FSP, (c) 20 wt % Ce-10 wt % W/TiO2 WI, (d) Ce−Ti (1:1 atomic ratio) FSP, (e) Ce−W (1:1 atomic ratio) coprecipitation, and (f) 20 wt % Ce-10 wt % W/MCF WI catalysts.

cordierite or SiC to minimize bypassing and axial dispersion (Figures 4−7). The microreactor catalytic formulations under simulated conditions of diesel engine exhaust revealed that the nominal 20 wt % Ce−10 wt % W/TiO2 and Ce−W (1:1 atomic ratio) catalysts were eventually active for the reduction of NOx, while the other formulations activity was found to be rather low. This fact can be largely attributed to the effective and selective NH3 oxidation at high temperatures. Of interest is the finding that the 20 wt % Ce−10 wt % W/TiO2/cordierite and Ce−W (1:1 atomic ratio)/cordierite demonstrate impressive DeNOx performance and high N2 selectivity (Figure 4 and Figure 5). Conversely, as discussed above in BET section, the surface areas of Ce−W/TiO2−WI (109 m2/g) and Ce−W (1:1) coprecipitation (118 m2/g) are lower than that of Ce−W/MCF (350 m2/g) (Table 2), yet the SCR performance of Ce−W/ TiO2−WI and Ce−W (1:1) coprecipitation catalysts was found to be greater than that of Ce−W/MCF-WI. These results imply that there is a synergistic effect between ceria and tungsten. Such results tend to suggest that surface area of the catalyst alone does not correlate directly with the deNOx performance

where poNOx = inlet partial pressure of NOx, wm = molar mass of active components (mol gac−1 s−1), FoNOx = molar feed rate of NOx actual inlet NO molar flow rate, and X is the fractional NOx conversion in eq 3. The rate constant kac (molNOx gac−1 Pa−1) is measured by assuming a first-order-dependence of the reaction rate on the NOx partial pressure, zero order on the ammonia partial pressure, and the oxygen partial pressure.33−38 The catalytic activity (Figure 3) and pore size distribution (Figure 2) results illustrate that the presence of small pore active sites is also beneficial to achieve higher NOx conversions, which can be attributed to the effective contacts of the reactants for the SCR reaction.39 3.2.2. Development of Efficient SCR Catalysts at Simulated Diesel Engine Exhaust Conditions. Further, the activity test of the most promising catalysts was scaled up (GHSV = 500 000 h−1), using powder catalyst dilution into E


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Figure 4. Catalytic reduction of NOx over the selected formulations: GHSV = 500 000 h−1, 8% CO2, 7% H2O, 10% O2, 0.99 Ar balance; 500 ppm of NOx; NO2/NOx = 0.12; ammonia to NOx ratio ≈ 1.0. Total gas flow, 804 sccm; catalyst loading, 0.050 g.

Figure 5. N2 selectivity over Ce-based materials; GHSV = 500 000 h−1, 8% CO2, 7% H2O, 10% O2, 0.99 Ar balance; 500 ppm of NOx; NO2/NOx = 0.12; ammonia to NOx ratio ≈ 1.0. Total gas flow, 804 sccm; catalyst loading, 0.050 g.

Ce3+/Ce4+, surface oxygen vacancies, and small pore sizes although other factors may play a role. This work explores the fundamental aspects of promising catalysts with a view toward understanding the interaction among Ce, modifiers, and support materials in terms of catalytic performance under simulated diesel exhaust conditions. The temperature of the reaction was raised up from 100 to 550 °C then decreased to 100 with 50 °C intervals to ensure about the hysteresis effect. As we can see from Figure 4, the Ce−W/TiO2 and Ce−W (1:1) mixed with SiC or cordierite formulations demonstrated impressive performance under the simulated automotive diesel exhaust gas conditions. In particular, Ce−W (1:1) catalyst synthesized by the coprecipitation method exhibited preeminent deNOx activity (Figure 4) and N2 selectivity (Figure 5) compared to our reference benchmark catalyst in the temperature range from 300 to 550 °C. As we

of a catalyst. Rather, surface area of the support in conjunction with other critical parameters such as redox properties, synergetic effect between foreign metal ions, presence of active components, and average pore size, can be used to characterize effective catalyst materials. As can be seen from Figure 2 and Table 2, pore diameter and pore size distribution also seem to be the reason for high activity. The addition of tungsten cations could improve the oxygen vacancies, amount of active sites and Lewis acid sites in the catalyst, which is favorable to boost the SCR activity at low temperatures by enabling faster kinetics. In summary, our selective reduction of nitrogen oxides evaluation results reveal that the titania-supported W-promoted Ce-based catalysts and Ce−W (1:1)-(coprecipitation) formulations demonstrate high NOx conversions in the temperature regime of 200−350 °C. This fact can be largely attributed to the redox behavior of F


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Figure 6. N2O formation over the tested materials; GHSV = 500,000 h−1, 8% CO2, 7% H2O, 10% O2, 0.99 Ar balance, 500 ppm of NOx, NO2/NOx = 0.12, ammonia to NOx ratio ≈ 1.0. Total gas flow, 804 sccm; catalyst loading, 0.050 g.

Figure 7. NH3 conversion over the tested materials: GHSV = 500 000 h−1, 8% CO2, 7% H2O, 10% O2, 0.99 Ar balance; 500 ppm of NOx; NO2/ NOx = 0.12; ammonia to NOx ratio ≈ 1.0. Total gas flow, 804 sccm; catalyst loading, 0.050 g.

can see from Figure 5, N2 selectivity is very low up to 300 °C because we considered all the outlet N species including unreacted NH3 for industrial purposes. We can observe a significant (4−19%) difference between the powder catalysts diluted with SiC and cordierite (Ce−W (1:1)/SiC and Ce−W (1:1)/cordierite). The catalytic activities and N2 selectivity boosted immensely when the Ce−W (1:1) powder catalyst diluted in cordierite (Figure 4 and Figure 5). The N2O formation profiles over the tested materials also presented in Figure 6. The total concentration of the N2O is below 10 ppm (Figure 6) in the 100−550 °C temperature window due to high selectivity in NOx to N2 reduction and selective NH3 oxidation even in the presence of 8 vol % CO2, 7 vol % H2O vapors, and 10 vol % oxygen. Ammonia oxidation (Figure 7) patterns of Ce−W (1:1)-co-precipitation/Cordierite, Ce−W (1:1)-co-precipitation/SiC and Ce−W/TiO2−WI/Cordierite formulations

are similar to the reference (Benchmark catalyst/SiC) sample. According to the above results, it should be emphasized that the Ce−W (1:1)-co-precipitation and Ce−W/TiO2−WI catalysts would be promising SCR catalysts for practical application in diesel engine after treatments and power plants. 3.3. Effect of Synthesis Method on TemperatureProgrammed Reduction (H2-TPR) Profiles and Ammonia Desorption. We have carried out (H2-TPR) experiments for all the prepared catalysts to get substantial information on the reducibility of active metal components (Figure 8). It has been well established that the H2-TPR profile for bulk WOx reflect a distinctive stage reduction process from WO3 to WO2 through two nonstoichiometric WOx oxides with two reduction peaks and a shoulder at 650, 748, and 870 °C.40 The reduction patterns of our Ce−W (1:1) coprecipitation, Ce−W/TiO2, and Ce−Ti (1:1) FSP catalysts can be found in Figure 8. The G


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Figure 9. Deconvoluted Ce 3d (XPS) spectra of (a) Ce−Ti (1:1) FSP, (b) 20Ce-10W/MCF, (c) Ce−W (1:1) coprecipitation, (d) Ce−W (2:1) FSP, (e) 20Ce-10W/TiO2 WI, and (f) Ce−W (1:1) FSP catalysts.

Figure 8. Hydrogen temperature-programmed reduction (H2-TPR) patterns of tungsten-promoted titania-supported cerium oxide catalysts.

curves (v, v″, v‴, u, u″, and u‴) are attributed to Ce4+ atoms, while filled green curves (v0, v′, and uo, u′) are ascribed to Ce3+ species. The curves categorized as v and v″ can be ascribed to a mix of Ce 3d9 4f2 O 2p4 and Ce 3d9 4f1 O 2p5 initial electronic configuration associated with surface Ce4+ final states, and the curves categorized as v‴ exemplify the Ce 3d9 4f0 O 2p6 corresponding to the Ce4+ final state. Conversely, curves v′ and v0 are allocated to Ce 3d9 4f1 O 2p6 and Ce 3d9 4f2 O 2p5 electronic state of Ce3+. The similar designation can be pragmatic to the “u”, which corresponds to the Ce 3d3/2.42,43 The atomic ratios of Ce3+/Ce4+ of the samples were investigated by using the XPS peak area of the relevant oxidation states and eq 4.43 The values are listed in Table 3. I v0 + I v ′ + Iu0 + Iu ′ [Ce3 +/Ce 4 +] = Iv + Iv ″ + Iv ‴ + Iu + Iu ″ + Iu ‴ (4)

reduction peak maxima near 590 °C and another reduction peak near 480 °C with a shoulder peak at 330 °C are lower than the initial reduction temperatures of WO3 (650 °C) and CeO2 (520 °C including a shoulder peak at 377 °C) samples.40,41 These results indicate that the interface between the tungsten oxide and surface ceria facilitates the reduction of Ce4+ and/or W6+ to their substandard valence states. Conversely, Ce−W (1:1) FSP, Ce−W/MCF, and Ce−W (2:1) FSP catalysts showed different reduction (H2-TPR) patterns. The reduction peak of WO3 moved to higher temperatures and the intensity of CeO2 reduction peak drastically declined. The above results reveal that the reduction probability of W species increased due to the weak interaction between CeO2 and WOx. The acidic sites distribution of the selected samples (Ce−Ti (1:1) FSP, Ce−W (1:1) coprecipitation and Ce−W/TiO2 WI) catalysts was examined using NH3-TPD. Among all the catalysts tested, broad acid site distribution patterns were observed for the Ce−W (1:1) coprecipitation and Ce−W/TiO2 WI catalysts (see Figure S1 in Supporting Information) whereas, as-synthesized Ce−Ti (1:1) showed poor distribution of Lewis as well as Brönsted acid sites. These results suggest that the addition of tungsten ions into the CeO2 results in the enhancement of Lewis acid sites as well as Brönsted acid sites distribution. 3.4. X-ray Photoelectron Spectroscopy (XPS). We have investigated all the synthesized catalysts by XPS studies (Figure 9 and Table 3). The Ce 3d deconvoluted XPS spectra of Ce−W (1:1) FSP, 20Ce-10W/MCF, Ce−W (1:1) coprecipitation, Ce−W (2:1) FSP, 20Ce-10W/TiO2 WI, and Ce−Ti (1:1) FSP catalysts are shown in Figure 9a−f. The XPS spectrum of Ce 3d can be denoted as two sets which can be ascribed to the 3d3/2 and 3d5/2 spin−orbital multiplets. In Figure 9, the Ce 3d5/2 peaks denoted as “v” and Ce 3d3/2 peaks as “u”. The black

where I is the integrated peak area. The atomic ratios of Ce3+/(Ce3+ + Ce4+) and Ce3+/Ce4+ over the surface of the investigated samples are enriched by the addition of W6+ and Ti4+ into the CeO2 crystal lattice. These results imply that the replacement of Ce4+ cations (r = 0.092 nm) by W6+ (r = 0.067 nm) and Ti4+ (r = 0.074 nm) in the CeO2 lattice will result in tlattice shrinkage, whereas the alteration of Ce4+ (r = 0.092 nm) into the larger Ce3+ (r = 0.103 nm) ion can recompensate for this lattice reduction.44 The Ce3+/Cen+ atomic ratio (0.30 and 0.29) in Ce−W/TiO2 and Ce−W (1:1) coprecipitation catalysts, respectively, measured from the XPS spectra are relatively greater than that of other catalysts. Other catalysts prepared by flame aerosol technique and siliceous mesocellular foam supported Ce−W catalysts demonstrated relatively low Ce3+/Cen+ atomic ratio. These results illustrate that the synthesis technique and various promoters could indeed influence the electronic state of the H


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active component species. The higher surface Ce3+ ratio in Ce− W/TiO2 and Ce−W (1:1) coprecipitation catalysts indicate that the presence of enriched surface oxygen vacancies will enhance the activation of reactants in SCR of NOx and enable the adsorption of oxygen species. On the other hand, the XPS peak shift to high binding energies and widening can be ascribed to the reduction of Ce4+ to Ce3+. The higher binding energy shift can also be due to the high tendency of tungsten and titania to attract electrons compared to that of CeO2.45,46 As we can see from the results, the XPS peak shifts to much higher binding energies with the addition of tungsten and/or titania content. In another words, substitution of Ce4+ atoms (χCe = 1.12) by more electronegative W6+ atoms (χW = 2.36) and Ti4+ atoms (χTi = 1.54) increases the binding energy by decreasing the final-state relaxation. As reported earlier,47−49 after deconvolution, the O1s peak shows three distinctive peaks. The Oα′, Oα, and Oβ sub-bands are positioned at higher, middle, and lower binding energies. The subpeaks of O1s spectra in the Ce−W (1:1) FSP, 20Ce10W/MCF, Ce−W (1:1) coprecipitation, Ce−W (2:1) FSP, 20Ce−10W/TiO2 WI, and Ce−Ti (1:1) FSP samples are shown in Figure 10a−f. The deconvoluted peaks at 529.0−

Figure 10. Deconvoluted O 1s (XPS) spectra of (a) Ce−W (1:1) FSP, (b) Ce−W/MCF WI, (c) Ce−W (1:1) coprecipitation, (d) Ce−W (2:1) FSP, (e) 20Ce−10W/TiO2 WI, and (f) Ce−Ti (1:1) FSP catalysts.

530.0 eV are ascribed to the O2− lattice oxygen (labeled as Oβ). Two peaks at 531.1−531.7 eV and 532.7−533.2 eV are attributed to O22− and O− surface adsorbed oxygen (labeled as Oα and Oα′).50,51 Owing to greater mobility, Oα (surface labile oxygen) is highly active compared to bulk oxygen.52,53 Moreover, in the ceria fluorite crystal structure (Figure 1), all oxygen atoms positioned in one plane will permit fast diffusion with respect to the number of oxygen vacancies. When the oxygen vacancies rise, movement of the oxygen atoms improves, which permits CeO2 to react with the molecules and reduce or oxidize the reactant over its surface.53−56 In our studies, the Oα/(Oα + Oβ) ratios of Ce−W (1:1)-co-

a

Ti 2p1/2 464.8 Ti 2p3/2 459.4

W 4f5/2 36.3 W 4f5/2 37.54 W 4f5/2 37.8 W 4f5/2 37.1 W 4f5/2 37.6 Ti2p1/2 465.0 W 4f7/2 33.2 W 4f7/2 35.3 W 4f7/2 36.7 W 4f7/2 34.9 W 4f7/2 35.9 Ti2p3/2 459.7 901.8 901.0 901.0 900.8 900.8 900.4 885.5 884.8 885.1 884.8 885.0 884.2 530.4 530.5 530.6 530.4 530.6 530.2 Ce−W (1:1) FSP Ce−W/MCF WIc Ce−W (1:1) Co−Pd Ce−W (2:1) FSPe Ce−W/TiO2c Ce−Ti (1:1) FSPe

Ce 3d3/2 Ce 3d5/2 O 1s catalyst

Binding energy of various metals. bRelative amounts are according to the metal atomic ratio. cWet impregnation technique. dCoprecipitation method. eFlame spray pyrolysis method.

25 27 30 25 31 20 0.32 0.34 0.41 0.32 0.40 0.24 0.25 0.35 0.41 0.26 0.43 0.23 0.80 0.74 0.71 0.79 0.70 0.81 0.20 0.26 0.29 0.21 0.30 0.19 Si 2p1/2 103.7 Si 2p3/2 102.6

Oα (%)b Oα/Oα+Oβ Ce3+/Ce4+b Ce4+/Cen+b (Ce3+/Cen+)b M′a binding energy (eV) Ce 3d

Table 3. Binding Energy, Surface Atomic Ratio of Ce3+/Cen+, Ce4+/Cen+, Ce3+/Ce4+, Oα/Oα+Oβ, and Oα(%) for All the Prepared Catalysts Determined from Deconvoluted XPS Spectra


I


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Figure 11. Surface atomic concentration of Ce3+/Ce4+ and the surface atomic ratio of Ce3+/Cen+ with respect to the catalysts acquired from the XPS analysis: direct correlation of SCR activity of the particular formulations with the surface atomic concentrations.

elucidates the formation of solid solutions. Our studies illustrate that the synthesis technique and various promoters could indeed influence the structural and electronic state of the catalyst. The higher surface Ce3+/Ce4+ ratios in Ce−W (1:1) coprecipitation and Ce−W/TiO2 samples indicate enrichment in surface oxygen vacancies, which results in activation of reactive molecules and enhanced adsorption of oxygen species in a SCR reaction.

precipitation (41.0%) and Ce−W/TiO2 (40.0%), respectively, measured from O1s XPS spectra, were significantly greater than those of other samples. The surface atomic ratio of Ce3+/Ce4+ and Ce3+/Cen+ can be linked to the NOx conversions of the individual catalysts (Figure 11). Our XPS results illustrate that the introduction of tungsten ions into the crystal lattice of CeO2 and the synthesis technique significantly enriched the Ce3+/Ce4+ ratio as well as enhanced chemisorbed labile oxygen, which is the motive for enhanced SCR activity of Ce−W (1:1)co-precipitation and Ce−W/TiO2 (anatase) formulations under simulated diesel engine exhaust conditions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00045. NH3-TPD profiles of Ce−Ti (1:1)-FSP, Ce−W/TiO2− WI, and Ce−W (1:1) Co−P samples (PDF)

4. CONCLUSIONS In the current studies, we have successfully prepared CeO2 modified catalyst formulations by flame aerosol, wet-impregnation, coprecipitation synthesis techniques. The corresponding formulations were explored for the selective catalytic reduction of NOx using NH3 as reductant to examine the effect of synthesis method, different supports (siliceous mesocellular foam and TiO2) and the promoters with different atomic ratios. Our XRD studies indicated a fluorite crystal structure of ceria in all the prepared catalysts. The reduction profiles of Ce−W (1:1), Ce−W/TiO2 catalysts show the reduction peak maxima near 590 °C and another reduction peak near 480 °C are lower than the initial reduction temperatures of WO3 (650 °C) and CeO2 (520 °C) samples. Our temperature-programmed reduction results indicate that the surface ceria and tungsten oxide interface facilitates the reduction of Ce4+ and/or W6+ to their substandard valence states. XPS and other characterizations were executed to study the relationship between catalyst surface and NOx reduction activity. The Ce3+/Cen+ atomic ratios (0.30 and 0.29) in Ce−W/TiO2 and Ce−W (1:1) coprecipitation catalysts, respectively, measured from XPS results are relatively greater compared to that of the other samples. The calculated lattice parameters of Ce−W/TiO2 WI (0.529 nm), Ce−Ti (1:1) FSP (0.534 nm), 20Ce−10W/MCF (0.537 nm), and Ce−W (1:1) coprecipitation (0.537 nm) catalysts are smaller than that of CeO2 (0.540 nm). The lesser ionic radius of relevant foreign metal ions (W6+ = 0.067 nm and Ti4+ = 0.074 nm) in relation to Ce4+ (0.092 nm) in the host lattice leads to the crystal lattice contraction. This lattice shrink

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (513) 556-1474. Fax: (513) 556-3473. ORCID

Panagiotis G. Smirniotis: 0000-0002-5260-2412 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge financial support from the Cummins Emission Solutions, Cummins, Inc. (Grant No. SRS 007955).

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REFERENCES

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