Ammonia Formation in the Catalytic Reduction of Nitric Oxide. 111. The Role of Water Gas Shift, Reduction by Hydrocarbons, and Steam Reforming M. Shelef* and H. S. Gandhi fuel Sciences Department, Scientific Research Staff, Ford Motor Company, Dearborn, Michigan 48 12 1
Ammonia formation in the catalytic reduction of NO by molecular hydrogen, under dry conditions, has been studied earlier over supported base metal oxides and noble metals (Shelef and Gandhi, 1972a,b). The present work examines ammonia formation under conditions where the primary sources of hydrogen are (a) hydrogen formed on the surface by the water gas shift, (b) hydrogen present in hydrocarbon molecules (paraffinic and olefinic), and (c) hydrogen formed on the surface by steam reforming of the hydrocarbons. On base metal oxide catalysts, the extent of NH3 formation by hydrogen from the shift depends mainly on the activity of the catalyst in the water gas shift reaction. On Ru catalysts, the presence of CO increases NH3 formation. Thus, NH3 formation by the hydrogen from the shift is higher than that observed in NO-H2 system but less than in the NO-CO-H* system, where substantial amounts of CO are present, which makes pairing of N atoms a less probable event. Ammonia formation in the reduction of nitric oxide with saturated hydrocarbons is minimal or absent over a variety of catalysts under dry conditions. In the reactions between NO and olefins, ammonia formation begins at much higher temperature than in the NO-H2 reaction, and the thermodynamic instability of the NH3 molecule at these high temperatures limits NH3 formation to a narrow temperature range. The addition of steam to NO-hydrocarbon systems enhances ammonia formation on catalysts which are relatively good steam-reforming catalysts. Even so, the production of ammonia in reactions between NO and hydrocarbons in the presence of steam is very small due to low saturation of catalyst surface by hydrogen, and therefore hydrocarbons can be disregarded as an important source of hydrogen for ammonia formation.
Introduction In the previous communications in this series (Shelef and Gandhi, 1972a,b), the formation of ammonia in the reduction of NO by molecular hydrogen was investigated on base metal oxides and noble metal catalysts. In gas streams, such as combustion gases, which contain large amounts of water vapor the molecular hydrogen may not be the important source of hydrogen for the formation of ammonia. All the catalysts employed for nitric oxide reduction are active to some degree in the water gas shift CO H2O C02 H2 by which hydrogen (and carbon dioxide) is formed from water and carbon monoxide (Klimisch and Barnes, 1972; Jones, et al., 1971). This hydrogen, generated at the surface in dissociated form before desorption, may well behave differently in nitric oxide reduction from molecular hydrogen in the gas phase, which must adsorb dissociatively before reacting. The relative importance of these two sources of hydrogen will depend primarily on the activity of a given catalyst both for the water gas shift and for NO reduction. Another possible source of hydrogen in combustion gases is in the unburned hydrocarbons. These either can directly react catalytically with NO, producing "3, or can be steam reformed, analogously to carbon monoxide during the shift reaction, to carbon dioxide and hydrogen. For paraffins these processes can be expressed by the general equation
+
+
C ~ H Z ( ~+ + I2nHz0 ,
2 nCOz
+ (3n + I)&
Similar equations can be written for olefins and aromatic hydrocarbons by appropriate balancing of the stoichiometry. The hydrogen generated by these reactions is also present in the dissociated form before desorption. Hence, it is possible that, depending on the activity of a given catalyst in the steam-reforming reactions and on the nature of the hydrocarbon, the extent of NH3 formation in the reduction of NO by hydrocarbons will be influenced to a lesser or greater degree by the presence of water. The thermodynamics of the steam-hydrocarbon reaction lead80
Ind. Eng. Chem., Prod. Res. Develop., Vol. 13,No. 1, 1974
ing to C02 and Hz are given for the more common hydrocarbons by Gross, et al. (1968). For methane this reaction becomes thermodynamically favorable at >600"C; for ethane and hexane, above the 300-400°C range. Olefinic, acetylenic, and aromatic hydrocarbons can be converted by water to COz and H2 at temperatures lower than those required by paraffins. This temperature range overlaps the range of NHJ stability (Shelef and Gandhi, 1971a) and, therefore, hydrogen formed from hydrocarbons by the action of water vapor should be considered as a potential source for NH3 formation if the rates of the surface reactions are sufficiently fast. It was the objective of the present study (a) to compare the NH3 formation on representative base metal oxides and noble metal catalysts by molecular hydrogen and by hydrogen from the water gas shift, (b) to observe the extent of NH3 formation in the reaction between representative hydrocarbons and NO under dry conditions, and (c) to follow the effect of the addition of water to the hydrocarbon-NO systems. This then is the third (and last) communication in this series and concludes the scope of the work delineated in the last paragraph of the first communication (Shelef and Gandhi, 1972a).
Experimental Section The description of the catalysts is given in Table I. Out of the ten catalysts employed in this study, three were base metal oxides, five were noble metals, and two were mixed base metal oxide-noble metal catalysts. Nickel oxide, copper chromite, MOD, Engelhard 0.5% Ru, and platinum catalysts were also used in earlier studies (Shelef and Gandhi, 1972a,b). The catalyst designated as MOD (Editor's Note: To protect the interest of the inventors, the composition of this catalyst cannot be disclosed, regretfully. The publication of the results is nevertheless deemed appropriate, its aim being to stimulate further research on catalysts for NO reduction which will possess the desirable selectivity to produce nitrogen and not am-
Table I. Description of Catalysts
Catalyst description
Catalyst designation
Active ingredient
Nickel oxide
NiO
Ni -10%
Copper-chromite MOD Monolithic ruthenium
Cu-Cr MOD Ru(M)
Cu 6%, Cr 6 % Unknown Ru 0.2%
Pelleted ruthenium
Support Aeroban extrudates, 95% A1203,5% SiOz Alumina extrudates Alumina extrudates Corning monolithic support, L = 1 . 5 in., d = 1 . 0 in., r-A1203washcoat -10% Engelhard alumina spheres,
Ru 0.2%
d =
Ru 0.5%
Pelleted Engelhard ruthenium Ruthenium-platinum Platinum Copper-nickelruthenium Copper-chromiteruthenium
Pt Cu-Ni-Ru
Cu-Cr-Ru
Pt Cu 3.7%, Ni 6 . 3 % , Ru 0.01% Cu 6%, CP 6%, R u 0.02%
monia.) is that referred to as "modified" by Meguerian and Lang (1971). Its composition is proprietary, and it was included in this study because it minimizes NH3 formation in actual vehicle tests (Figure 16, Meguerian and Lang, 1971). A monolithic ruthenium catalyst was prepared by depositing 10% -y-A1203 washcoat from a slurry of Dispal-M (colloidal alumina made by Continental Oil Corp.) on a Corning monolithic support. The support was dried at 120" overnight and calcined at 600" for 6 hr. Ruthenium was impregnated on this monolith and Engelhard %-in. alumina spheres from a solution containing ruthenium trichloride so as to give 0.2% Ru by weight. A ruthenium-platinum catalyst (Ru-Pt) was prepared by impregnating American Cyanamid alumina pellets in a solution containing ruthenium trichloride and chloroplatinic acid so as to give 0.2% Ru and 0.1% Pt on alumina pellets. A copper chromite-ruthenium catalyst was prepared by impregnating a copper chromite catalyst with a solution of ruthenium trichloride so as to give 0.02% Ru by weight on copper chromite catalyst. Copper-nickel-ruthenium catalyst was prepared by impregnation of Kaiser alumina (KA-201) spheres in a solution of copper nitrate and nickel nitrate so as to give 3.7% copper and 6.3% nickel by weight. These alumina spheres were then dried and calcined at 600" for 6 hr and subsequently impregnated with ruthenium trichloride solution to give 0.01% ruthenium by weight. All the catalysts were reduced in hydrogen at 450" for 4 hr before use. Experimental apparatus, procedures, and analytical methods for analyzing the gas stream were identical with those described earlier (Shelef and Gandhi, 1972a,b). Two methods were used for the ammonia measurements. The wet chemical method which was used previously is described in earlier publications (Shelef and Gandhi, 1972a). In addition, a new method, developed for the analysis of NO2 and NHs (Breitenbach and Shelef, 1973) which uses metal-doped carbon converters in conjunction with an NO measuring instrument, such as an NO optical detector (NOOD) (Stedman, et al., 1972) or NDIR, was used to determine the ammonia in the gas stream. This method is based on quantitative oxidation of ammonia to nitric oxide. Nitric oxide alone is measured by bypassing the the gas stream to catalytic converters. To determine "3, which a known volume of 0 2 is added is passed over a copper-doped carbon converter and the increment over the previous reading gives the ammonia in the gas stream. Results obtained by both of the methods are in good agreement.
97 Unknown 10
137
in.
Alumina pellets, d = in., L = I/ 8 in. American Cyanamid alumina pellets, d = in., L = in. Alumina spheres, d = 1/8 in. Kaiser alumina spheres, KA-201, d = l/* in. Alumina extrudates $,Iaq
R u 0 . 2 % , Pt 0 . 1 %
Surface area, m2/g 212
98
'
Not measd Unknown 200 97
Results a n d Discussion Extent of Water Gas Shift on Various Catalysts. To compare the extent of NH3 formation with hydrogen from the shift reaction with the formation by molecular Hz it is necessary to assess the extent of the shift reaction on the various catalysts. Figure l'gives the extent of CO conversion by the shift reaction for four representative catalysts when starting from a gas composition containing 10% H20 and 1.4% CO. The equilibrium conversion is given by the top line in Figure 1. The solid part of this line is computed, and the broken part is drawn by extrapolation. The measured conversion is given as a band, instead of a line, due to the following uncertainty. The upper limit of CO conversion on Figure 1 is obtained by assuming that all NO that is converted to Nz is by reduction with hydrogen formed on the catalyst surface by the shift reaction or, perhaps, by NH3 decomposition. Therefore, water gas shift accounts for all the CO converted. On the other hand, the lower limit of CO conversion is obtained by assuming that all the Nz is formed by direct reduction of NO with CO. In this case, the corresponding amount of CO used in direct reduction of NO to Nz is subtracted from the total CO converted, and the remaining amount of CO is considered as undergoing the water gas shift. The bandwidths on Figure 1 represent this uncertainty; therefore, in Table 11, where the extent of the shift at three representative temperatures is summarized for all the employed catalysts, averaged conversions are given. From Figure 1 and Table I1 it is clear that the pure Ru catalyst and the MOD catalyst are outstanding water gas shift catalysts. The Cu-Ni-Ru catalyst is a moderate one, and the other catalysts are relatively poor shift catalysts. Comparison of NH3 Formation by Hydrogen Derived from Water Gas Shift a n d by Molecular Hydrogen. The comparison is made between the formation of NH3 in a gas stream containing 1.4% Hz, 1.4% CO, and varying amounts of nitric oxide at the inlet and a gas stream in which the 1.4% of HZ has been replaced by 10% H20. As will be seen below, the presence of CO has a pronounced effect on the formation of KH3. This was noticed before in dry systems (Shelef and Gandhi, 1972a,b) and is apparently due to the competitive adsorption of NO and CO. Base Metal Oxide Catalysts. The formation of NH3 in the systems NO-CO-H20 and NO-CO-Hz on three representative base metal oxide catalysts, MOD, copper chromite, and nickel oxide, is compared in Figure 2. From Figure 2 and Table I1 it becomes immediately apparent Ind.
Eng. Chem., Prod. Res. Develop., Vol. 13, No. 1, 1974
81
1600
1400 -
1200
- 100 - 00 - %
NH3 as % o f NO Converted 100 200 300 400 500 600 Temperature, O C +
"0
Figure 1. Conversion of CO by the water gas shift reaction as a function of temperature. Inlet concentrations: NO, 1000 ppm; CO, 1.4%; H20, 10% MOD A A
(NO+CO+H20) (NO+CO+H2)
.
W r (NO+CO+H~O)
0
(NO+CO+Hz)
0
I n l e t NO Concentration, ppm +
Figure 3. Ammonia formation in the NO-CO-Hz system (at 305") and in the NO-CO-HzO system (at 515") as a function of * (NO+CO+H~O) NO inlet concentration on a copper chromite catalyst. Inlet concentrations: CO, 1.4%; HP,1.470,or HzO, 10% O(NO+CO+H2) Ni 0
2
300
=
200
n
fll 0
100 Temperature, 'C
+
Figure 2. Ammonia formation in the NO-CO-Hz and NO-COHzO systems on base metal oxide catalysts as a function of temperature. Inlet concentrations: NO, 1000 ppm; CO, 1.4%; Hz, 1.4%. or HzO, 1070 Table 11. Extent of CO Conversion by Water Gas Shift Reaction over Differeht Catalysts % CO converted
0
Catalyst
At 200'
At 350'
At 500'
MOD Cu-Cr Ni o R u /M ) Cu-Cr-Ru Cu-Ni-Ru
28.5
77 $ 0
