Formation of Hydrogen Cyanide over Cu/ZSM-5 -Catdlst Used for the Removal of Nitrogen Oxides from Exhausts of lean-Bum Engines FRANK RADTKE, RENE A. KOEPPEL, AND ALFONS BAIKER* Department of Chemical Engineering and Industrial Chemishy, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092Zurich, Switzerland
Introduction The removal of nitrogen oxides (NO,) from the exhaust stream of various combustion sources has become a major issue due to its detrimental impact on the environment. For stationary combustion sources such as power plants and for gasoline-fueledengines (I),technical solutions are available. However, for mobil diesel-fueled engines and lean-burn gasoline-fueled engines operating under net oxidizing conditions, the conventional three-way catalysts show little ability to promote NO, reduction (1). Consequently, there is a great deal of interest in finding suitable catalysts for the selective catalytic reduction of NO, under lean (net oxidizing) conditions. Several zeolites ion exchanged with various elements (2-17) as well as oxides with (15-19) and without transition metal additives (1522) were reported to be effective for this reaction. One of the most efficient materials evaluated to date is copperexchanged ZSM-5 zeolite (2-4). Recently IR evidence has been shown suggesting that hydrogen cyanide (HCN) formation may pose a severe problem for the application of Cu/ZSM-5(13)and alumina (22) in the selective catalytic reduction of NO, by olefins under "dry conditions", Le., in the absence of water in the simulated exhaust gas stream. More recently, Misono and co-workers (23,241also reported the formation of HCN in the selective catalytic reduction of NO, over Na/ZSM-5and Ce/ZSM-5,using gas chromatographyas an analytical tool. The role of water in the elimination of HCN in these exhausts has not been addressed so far. Consequently,we have used simulated exhausts containing 10%water as met with real lean-burn exhausts.
Experimental Section Catalyst, Ion-exchanged CulZSM-5 zeolite was prepared from NalZSM-5 (ChemieUetikon) as described elsewhere (13). The copper loading of the catalyst, as determined by atomic absorption spectroscopy, was 2.60 wt %. BET * To whom correspondence should be addressed.
0013-936)(/95/0929-2703$09.00/0
@ 1995
American Chemical Societv
TABLE 1
Simulated Exhaust Compositions (Feed Gas Mixtures) Employed in Catalytic Studies; Balance Nitrogen feed no.
NO (ppm) NO~(ppm) CZH4(ppm) C3H6 (ppm) 0 2
(%)
HzO (%)
I/(lwP
2/(2w)
3/(3w)
4/(4w)
0 0 970/(940) 980/(950) 1290/(1260) 0 1450/(1270) 0 910/(860) 0 910/(860) 0 2.0/(2.0) 2.0/(2.0) 1.9/(2.0) 1.9/(2.0) 0/(10) 0/(10) 0/(10) 0/(10)
980/(940) 920/(940) 0
0
a Values in parenthesis correspond to feeds, containing 10% water, which are denoted with w.
surface area measurements of the ion-exchanged catalyst amounted to 373 m2/g. Apparatus. The apparatus used for the catalytic measurements consisted essentiallyof a gas mixing system for the reactant gases, a U-tube quartz glass reactor (i.d.6 mm), and a FTIR spectrometer for gas analysis at the reactor inlet and outlet, respectively. The reactant gas feeds were mixed from pure components by means of mass flow controllers. The following gases and gas mixtures were supplied by Pan Gas: 0 2 (99.999%),NZ (99.995%),5.03% C2H4 (99.5%)in nitrogen, 10 % C3Hs (99.5%)in nitrogen, and 4.9%NO (99.0%)in nitrogen. Water was added into the nitrogen flow by means of a microstep pump through a capillary. The selection of either NOz or NO as the NO, component was accomplished by mixing at room temperature 5.0%NO/Nz with pure 0 2 before and after dilution with nitrogen, respectively. A FTIR spectrometer (Bruker IFS66) with heatable gas cell (100-mL volume; Infrared Analysis Inc.) and MCT detector was used for gas analysis. For calibration, absorbance FTIR spectra (resolution 0.5 cm-l, 50 scans/spectrum) of known concentrations were recorded for each component: NO, NOz, NzO, NH3,HCN, C2H4,C3H6, CO, COZ,and H20. Nitrogen was used for the background spectra. Concentrations of each component were determined by integrating the specific absorption frequencies. Procedure. Catalytic tests were carried out with 250 mg of Cu/ZSM-5 (42-80 mesh) at atmospheric pressure. Before measurements, the catalyst was pretreated at 873 K for 2 h with 5%oxygen in nitrogen (150 mL/min) and then cooled to 473 K. Subsequently,the reactant gas was passed through the catalyst bed with a flow rate of 150 mL/min. The compositions of the simulated exhausts used in this study are listed in Table 1. The temperature dependence of the catalytic behavior was measured by raising the temperature in steps of 50 K from 473 to 873 K. The concentration of nitrogen formed by the reduction of NO, has been calculated using a mass balance over allnitrogencontainingspecies (NO, NOZ,NzO, NH3,and HCN). Further information on apparatus, analysis, and catalytic tests is given in ref 13.
VOL. 29. NO. 10, 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
2703
I
40 1
A -0-0-
Fwd 1 Feed l w
-0-
Feed 3 Feed 3w
$ 20 30/ 0.
z‘
10
0
I * 0 0
.-5
e,
!. f 30
c
0 C
4-
0
u
20 0
10
0
-
1
500
.
1
600
p-0
.
700
800
730
900
temperature / K FIGURE L Temperature dependence of formationof hydrogencyanide over CUES#-5 with propene containing feeds (Table 1). (A) Concentration of HCN vs temperature for reduction of NO with without water (feed 1); (0)with water (feed lwl. (B) propene: (0) Concentration of HCN vs temperature for reduction of NO2 with without water (feed 3); ( 0 )with water (feed 3w). propene: (0)
725
720
715
2704 B ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 10,1995
700
FlGURE2. Relevant spectra, proofingformation of hydrogen cyanide. (A) Shows a part of the original spectrum from CuESM-5 at 520 K with feed 3w (Table 1). (B) Spectrum A after subtracting the background spectrumfor 10%water in nitrogen. (C) Reference spectrum of 36 ppm hydrogen cyanide in nitrogen. 40 I -0- Feed 2
Results and Discussion As can be seen from Figure 1, water has a significant influence on the formation of hydrogen cyanide for both NO and NO2reduction with propene (feed 1 and 2). With NO (Figure lA), maximum hydrogen cyanide formation shifts in the presence of water by ca. 100 K from 560 to 660 K. Simultaneously,the HCN concentration increases from 4 to 10 ppm. An even more pronounced influence of water is found for the reduction of NO2 (Figure 1B). In the absence of additional water, HCN formation peaked at 505 K (29 ppm) and then decreased substantially. By adding 10% water to the feed, the maximum HCN concentration increased to 38 ppm at 520 K, followed by a second maximum of 21 ppm at 660 K. Note that the hydrogen cyanide concentration of 38 ppm exceeds the ‘threshold limit value’ of 10 ppm (25) markedly. Beside HCN, minor amounts of nitrous oxide (
