NOx Abatement by HC-Assisted SCR over Combustion Synthesized

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NOx Abatement by HC-Assisted SCR over Combustion Synthesized-Supported Ag Catalysts Murid Hussain,†,‡ Nunzio Russo,*,† and Guido Saracco† † ‡

Department of Materials Science and Chemical Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy Department of Chemical Engineering, COMSATS Institute of Information Technology, M.A. Jinnah Building, Defence Road, Off Raiwind Road, Lahore-54000, Pakistan ABSTRACT: In this work, catalysts containing 2% by weight of silver particles dispersed on different oxide supports (CeO2, La2O3, ZrO2, Al2O3), and oxides without silver particles were prepared by means of the combustion synthesis method. This cheap and quick technique has been developed to produce very fine, homogeneous, crystalline powders without the intermediate decomposition and/or calcining steps which other conventional synthesis routes require. These features are also interesting for the cosynthesis of supported Ag catalysts; it has in fact been possible in one-shot to achieve values of the Ag clusters (estimated size 5
10 nm) that are comparable with those obtained by means of the more complex methods reported in literature. The catalysts were characterized via XRD, BET, TEM, FESEM-EDS, and XPS analysis. The catalytic performances of the prepared catalysts were evaluated in a TPR apparatus for the selective reduction of NO with benzene in the presence of oxygen. A noticeable enhancement in activity was achieved when the selected oxides were doped with Ag. Lantana and ceria-supported Ag catalysts with good hydrothermal stability were found to perform comparatively well, as a significant NOx to N2 conversion was obtained at temperatures slightly above 250 °C. As far as the results concerning the catalysts are concerned, a synergic effect of the supports with the Ag clusters has been highlighted. Some conclusions may be drawn concerning the reaction mechanism.

1. INTRODUCTION A reduction in nitrogen oxide emissions has become one of the greatest challenges in environment protection. In the coming years, the US, European, and Japanese governments will intensify legislation to reduce the quantity of emitted nitrogen oxides (NOx). NOx are serious pollutants that cause not only the formation of acid rain but also photochemical smog1
3 and hazardous effects on health.4,5 They can generate secondary contaminants by interacting with other primary pollutants (like carbonyl corresponding molecules, alcohol radicals, etc.), which also result from the combustion of fossil fuels in stationary sources, such as industrial boilers, power plants, waste incinerators, gas turbines, and diesel and lean-burn gasoline engines. The diesel engine offers the advantage of a lower consumption of fuel and lower CO/HC emissions than the conventional sparkignition engine, but it has the disadvantage of emitting a large amount of NOx in the presence of excess oxygen. NOx abatement is particularly difficult for engines operating under lean conditions. Among the various requirements, a successful automotive catalyst for lean NOx reduction must be able to withstand both high temperatures and water concentrations. Among the technical approaches so far developed, the direct decomposition of NOx and its reduction with hydrocarbons (HC) would appear to be the most interesting. Ag has been found to be even more selective than Pt or Rh in NOx removal by SCR with hydrocarbons and more stable to hydrothermal aging than conventional V-based catalysts for the SCR reaction with NH3.6
13 This paper concerns the preparation of some metal oxides, supporting 2% by weight of silver, and the investigation of their r 2011 American Chemical Society

catalytic activity toward nitrogen oxide selective reduction to nitrogen.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation and Characterization. A series of catalyst supports (CeO2, La2O3, ZrO2, Al2O3) was prepared via a highly exothermic and self-sustaining reaction, the so-called “combustion synthesis” method.14,15 This technique is particularly suitable for the production of a spongy foam that crumbles easily, and which is characterized by rather high specific volume and surface area. A concentrated aqueous solution of various precursors (metal nitrates and urea) was placed, inside a crucible, in an oven at 600 °C for a few minutes, in order to ignite the following very fast reactions which lead to the formation of La2O3, ZrO2, CeO2, and Al2O3, respectively:

2LaðNO3 Þ3 3 6H2 O þ 5COðNH2 Þ2 f La2 O3 þ 5CO2 þ 8N2 þ 22H2 O 3ZrOðNO3 Þ2 þ 5COðNH2 Þ2 f 3ZrO2 þ 5CO2 þ 8N2 þ 10H2 O Special Issue: Russo Issue Received: May 5, 2011 Accepted: September 21, 2011 Revised: September 19, 2011 Published: September 21, 2011 7467

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Figure 1. XRD results of the synthesized materials: (a) Al2O3 and Al2O3-2%Ag; (b) ZrO2 and ZrO2-2%Ag; (c) La2O3 and La2O3-2%Ag; and (d) CeO2 and CeO2-2%Ag, showing the crystalline phases.

6CeðNO3 Þ3 3 6H2 O þ 14COðNH2 Þ2 f 6CeO2 þ 14CO2 þ 23N2 þ 64H2 O 2AlðNO3 Þ3 3 9H2 O þ 5COðNH2 Þ2 f Al2 O3 þ 5CO2 þ 8N2 þ 28H2 O The overall set of reactions is markedly exothermic, which, within the reacting solid mixture, entails a thermal peak that exceeds 1000 °C for a few seconds. Under these conditions, the nucleation of the metal oxide crystals is induced, their growth is limited, and nanosized grains can be obtained. The Ag-based catalysts (CeO2-2%Ag, La2O3-2%Ag, ZrO2-2% Ag, Al2O3-2%Ag) were prepared according to the same method described above but also adding AgNO3 to the precursors solution in order to obtain a catalyst with 2% in weight of silver. All the catalysts were then ground in a ball mill at room temperature and characterized. X-ray diffraction analysis (PW1710 Philips diffractometer equipped with a monochromator for the Cu Kα radiation) was used on all the fresh catalysts to examine whether the desired structure was actually achieved. A Field Emission Scanning Electron Microscope (FESEM
Hitachi S4700-ICV) was employed to analyze the microstructure of the crystal aggregates of the prepared catalysts. Direct observation of the obtained nanosized Ag clusters was performed by means of Transmission Electron Microscopy (TEM - Philips CM 30 T). The BET specific surface areas of the prepared catalysts were evaluated from the linear parts of the BET plot of the N2 isotherms, using a Micromeritics ASAP 2010 analyzer. The specific surface area of the bulk and nonporous catalysts, such as the oxide ones, can be directly related to the average crystal size.

The XPS spectra were recorded using a PHI 5000 Versa Probe (USA) with a scanning ESCA microscope fitted with an Al monochromatic X-ray source (1486.6 eV, 25.6 W), a beam diameter of 100 μm, a neutralizer at 1.4 eV 20 mA, and a FAT analyzer mode. All the binding energies were referenced to the C1s peak at 284 eV of the surface adventious carbon. The individual components were obtained by curve fitting after proper subtraction of the baseline. 2.2. Catalytic Activity Tests. The activity of the prepared catalysts was analyzed by temperature programmed reaction (TPR), according to the following standard operating procedures: a gas mixture (650 ppm NO; 650 ppm C6H6, 7 vol% O2, 0 or 3 vol% H2O vapors, He = balance) was fed at a constant rate of 100 mL 3 min
1, via a set of mass flow controllers, to the catalytic fixed-bed reactor enclosed in a quartz tube placed in an electric oven. Benzene was selected as being a representative of the HC class and as the NOx reducing agent and also because considering that it is detrimental for effective HC-SCR over Ag/catalysts, the activity results can be considered conservative.8 The tubular quartz reactor was loaded with 500 mg of pelletized catalyst. The space velocity of the gases, through the catalyst bed, was about 30,000 h
1. The reaction temperature was controlled using a PID-regulated oven and was varied from 100 to 500 °C at a 5 °C 3 min
1 rate. The outlet gas composition was monitored using a NOx/N2O NDIR analyzer (URAS 14 by Hartmann&Braun) and a NO/ NO2 chemiluminescence analyzer (CLD 700 RE hT by ECO PHYSICS) as well as through a quadrupole detector (Baltzer Quadstar 422), N2 by mass spectrometer, as a function of the bed temperature.

3. RESULTS AND DISCUSSION The effectiveness of the Ag/Al2O3 catalyst depends to a great extent on the Ag loading, the best catalytic performance being 7468

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Table 1. Collection of Results of Catalyst Characterization Tests Concerning Catalytic Activity and BET Specific Surface Area Ag content catalyst

Figure 2. Electron microscopy results concerning the CeO2-2%Ag catalyst: (a) TEM micrograph of the catalyst crystals and (b) FESEM view of the CeO2-2%Ag catalyst microstructure.

achieved for intermediate loadings (2
3%); higher silver loadings were found to produce higher hydrocarbon oxidation rates with O2, at the expense of its reaction with NO.16
21 Consequently, all the silver based catalysts were prepared with an Ag content of 2% in weight by means of the combustion synthesis technique. This technique enabled the supported Ag catalysts to be synthesized in one easy step. The XRD patterns of the synthesized metal oxides as well as with 2% Ag-co-synthesized by combustion synthesis technique were recorded and are shown in Figure 1. The characteristic peaks of the synthesized Al2O2, ZrO2, La2O3, and CeO2 were matched with XRD database of the corresponding material (Al2O2: pdf# 79-1558, tetragonal-ZrO2: pdf# 80-0965, monoclinic-ZrO2: pdf# 86-1451, La2O3: pdf# 05-0602, CeO2: pdf# 810792). These corresponding peaks of Al2O2, ZrO2, La2O3, and CeO2 were also similar as found in the literature.22
25 Therefore, the crystalline phases of the synthesized metal oxides are confirmed. However, no Ag peaks were observed in the XRD patterns due to low loading, which shows the uniform distribution and good dispersion of silver oxides with preserved structure. Figure 2a illustrates a TEM image of the CeO2-2%Ag catalyst produced via combustion synthesis. As pointed out later on, it regards one of the two catalysts that showed the highest activity among those prepared. However, it is representative of all the

Tp (°C) η (%) T50 (°C) BET (m2/g) (% in weight)

Al2O3

>536

19.7

500

82.51

-

Al2O3-2%Ag

455

28.1

337

54.22

2

ZrO2

313

7.8

510

19.15

-

ZrO2-2%Ag La2O3

400 209

32.9 11.5

300 480

17.23 10.70

2 -

La2O3-2%Ag

329

73.4

270

6.49

2

CeO2

338

27.2

300

13.88

-

CeO2-2%Ag

312

73.3

235

11.88

2

crystal sizes of the prepared catalysts, with the only exception of Al2O3 based catalysts whose crystals are slightly smaller than those of the other oxides. Employing this direct observation technique, values of the Ag cluster size of 5
10 nm could be estimated. These values are comparable with those that can be obtained by means of more complex and expensive deposition methods, i.e. by coprecipitation or by impregnation.26,27 As far as the microstructure of the catalyst crystal agglomerates is concerned, Figure 2b shows how it is rather foamy. This is a consequence of the sudden release of a large amount of gas during the combustion synthesis, because of the decomposition/ combustion of the reacting precursors. Catalysts obtained with this technique have a useful open pore structure, but the same technique, used for the deposition process of the catalyst onto a support, could involve adhesion problems. All the data regarding the catalysts are listed in Table 1. The peak temperature of the maximum NO to N2 conversion is reported as Tp, η is the maximum conversion of NO to N2, and T50 refers to the temperature of the half conversion of benzene. The presence of Ag was always found to lead to a decrease in the BET specific surface area values of the pure support oxides. Figure 3 compares the reduction in NO with C6H6 over Al2O3 and Al2O3-2%Ag. The reduction in NO becomes appreciable from 400 °C onward with the maximum conversion rate occurring over 550 °C for the Al2O3 and at 455 °C for the Al2O3-2%Ag catalysts, respectively. The maximum NO reduction only increases from 19.7% to 28.1% when 2% Ag is added. The maximum C6H6 oxidation also improves in the presence of Ag and leads to an almost total conversion above 400 °C. Similar performances were reported by Iglesias-Juez and his coworkers.28 Analogous, unsatisfactory results were obtained for the ZrO2 support (see Figure 4).29 It is rather interesting to notice how, despite the superior specific surface area, the alumina based catalysts offered the worst performance for both investigated reactions. Figures 5 and 6 show the catalytic performances of lantana and ceria-based catalysts, respectively. The other two supports, however, were both able to ignite benzene combustion and NO reduction at temperatures close to 250 °C with a higher maximum NO conversion than 70% at temperatures slightly above 300 °C, which can be considered well inside the range of normal operating temperatures of the diesel oxidation catalysts that are routinely employed in the USA and in Europe.30,31 It is worth noticing that, if benzene combustion occurs at any temperature above the ignition one, NO reduction always takes 7469

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Figure 3. TPR plots of the Al2O3 (black line) and Al2O3-2%Ag (dotted line) catalysts. ( Conversion of NO to N2; 0 conversion of benzene to CO2. The gray line refers to NOx thermodynamic equilibrium.

Figure 4. TPR plots of the ZrO2 (black line) and ZrO2-2%Ag (dotted line) catalysts. ( Conversion of NO to N2; 0 conversion of benzene to CO2. The gray line refers to NOx thermodynamic equilibrium.

place in a limited temperature window, at low temperatures, via reaction kinetics and, at high temperatures, via NO-NO2 thermodynamic equilibrium. As far as this mechanism is concerned, it could be suggested that the reduction in NO over Ag/MxOy catalysts proceeds through the oxidation of NO to NO2 over Ag followed by the reduction of NO2 to N2 over the oxide support.32 The temperature range for both the selected CeO2-2%Ag and La2O3-2%Ag catalysts is 200
500 °C, whereas it moves to higher temperatures (300
500 °C) for the less active ZrO22%Ag and Al2O3-2%Ag catalysts. Comparing the results concerning all the investigated catalysts, it is also possible to state that the catalytic activity may not result exclusively from Ag, as the support itself may be involved in

the reaction mechanism. Some authors have suggested that Al2O3 directly participates in the reduction of NO.33,34 This hypothesis is plausible also for the other supports, if we take into consideration the dramatic synergic effect of the ceria and lantana when Ag is present. As far as the selectivity of NOx to N2 reduction is concerned, very good behavior was observed for all the catalysts, since the maximum N2O production was always under 20 ppm. The automotive catalysts are required to be stable hydrothermally and active at relatively low temperature region in the presence of water vapor. Zeolite-based catalysts, which are the majority of the de-NOx catalysts reported, are unlikely to be suitable as an automotive catalyst due to its instability under hydrothermal conditions.35,36 In addition, the catalysts are required to effectively 7470

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Figure 5. TPR plots of the La2O3 (black line) and La2O3-2%Ag (dotted line) catalysts. ( Conversion of NO to N2; 0 conversion of benzene to CO2. The gray line refers to NOx thermodynamic equilibrium.

Figure 6. TPR plots of the CeO2 (black line) and CeO2-2%Ag (dotted line) catalysts. ( Conversion of NO to N2; 0 conversion of benzene to CO2. The gray line refers to NOx thermodynamic equilibrium.

use higher hydrocarbons, which are already present in diesel fuel and emissions. However, most of the studies on HCSCR have been conducted using lower hydrocarbons as the reductant. Figures 7 and 8 show the effect of water vapors during the feed stream on the catalytic activity of La2O3-2%Ag and CeO22%Ag, respectively. No suppression in the activity of the catalysts was observed in any case. It is due to the higher hydrothermal stability of La2O3 and CeO2.37 These results also agree with the work of Shimizu et al.35 in which the catalytic activity and water tolerance of Ag
Al2O3 was markedly promoted by using higher alkanes as the reductant, where the presence of water vapor does not suppress but enhances the NO reduction activity especially in the temperature range below 400 °C.

Figure 9 shows the XPS peaks of Ag 3d for the fresh Ag-based catalysts (Al2O3-2%Ag, ZrO2-2%Ag, La2O3-2%Ag, CeO2-2%Ag). The original peaks (solid lines) regarding the Ag 3d5/2 have been resolved into a couple of peaks (broken line). The binding energies for these two peaks are 368.1 and 369.1 eV for the Al2O3-2%Ag and ZrO2-2%Ag samples and 367.9 and 369.4 eV for the La2O3-2%Ag and CeO2-2%Ag samples, respectively. This suggests that there are two different states of metallic silver on the surface, where the difference in binding energy between the two states can be explained by the difference of the charge effect.38 The one at the lower binding energy was attributed to an aggregated state of silver metal (large particles) because of its small charge effect, while the other at higher binding energy was ascribed to a highly dispersed metallic silver because of its big charge effect.38
40 The area values of the peaks at higher 7471

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Figure 7. TPR plots of the comparison of effect of 3 vol% of water in feed stream (wet) and without water (dry) on catalytic activity of La2O3-2%Ag catalysts.

Figure 8. TPR plots of the comparison of effect of 3 vol% of water in feed stream (wet) and without water (dry) on catalytic activity of CeO2-2%Ag catalysts.

binding energy are about 3.7% for the Al2O3-2%Ag and ZrO2-2% Ag samples and about 8% for the La2O3-2%Ag and CeO2-2%Ag samples which showed the best catalytic performances due to the higher amount of highly dispersed metallic silver.

3. CONCLUSIONS Supported silver catalysts have been developed for a simultaneous NO-reduction and HC-combustion under lean conditions. The combustion synthesis technique has been successfully adopted as it has been possible to produce, in an easy and lowcost “one shot” way, catalysts as active as those synthesized by more expensive and more burdensome modus operandi. The

success of these catalysts could lead to the substitution of the highly expensive platinum group catalysts that are generally adopted in automotive catalytic converters. In particular, CeO2-2%Ag and La2O3-2%Ag catalysts were already found to ignite HC combustion and promote NO selective reduction at 250 °C, a temperature routinely reached in the exhaust line of lean operating engines. Studies are now in progress to establish the role of the Agsupport interaction on the achieved catalytic activity as well as to optimize the catalyst preparation methods in order to minimize the size of the deposited Ag clusters. In this way, further improvements in catalytic activity are expected to be achieved. 7472

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Figure 9. XPS analysis of Ag-based catalysts: (A) Ag/Al2O3; (B) Ag/ ZrO2; (C) Ag/La2O3; and (D) Ag/CeO2.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +39-011-0904710. Fax: +39-011-0904699. E-mail: [email protected].

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