J. Phys. Chem. 1995,99, 6951-6956
6951
Reduction Mechanism of Nitrogen Monoxide with Propane over Palladium-Y Zeolite Catalyst Hiroshige Matsumoto" and Shuji Tanabe Department of Chemistry, Faculty of Liberal Arts, Nagasaki University, Bunkyomach 1-14 Nagasaki 852, Japan Received: September I , 1994; In Final Form: February 9, 1995@
In the reduction of nitrogen monoxide with propane, the Pd-Y zeolite catalyst with a high palladium content exhibited excellent activity and selectivity at low temperatures of 573-873 K. The palladium species in the zeolite crystals was characterized by an EXAFS examination and determined to be small clusters consisting of 10-17 metallic atoms under the operating conditions. It was concluded from the results of temperatureprogrammed and transient experiments that in the main pathway of reaction, propane was decomposed into hydrogen in the gas phase and carbonaceous species on the surface of reduced palladium clusters, the latter of which react with nitrogen monoxide in the gas phase to produce nitrogen and carbon dioxide.
Introduction In recent years, the emission of nitrogen oxides, especially NO, has become a serious problem to be solved in order to prevent air pollution and acid rain. As important catalytic processes, direct decomposition and reduction with NH3 or CO have been widely studied to remove nitrogen oxides. The exhaust gases from vehicles and boilers usually contain considerable portions of nitrogen oxides accompanied by residual hydrocarbons of fuels. During the past five years, especially in Japan, the catalytic reduction of nitrogen oxides with light hydrocarbons has taken the limelight and has been expected to have a promising future because it can remove both of the contaminants simultaneously and selectively. The catalyst first employed in this process was Cu-ZSM-5 zeolite reported by Held et al.' and independently by Iwamoto and co-workers.2 Afterward, numerous examinations of this novel process have been performed over various catalysts, such as some rare earth metals on Cu-ZSM-5?s4 Ga-zsM-5;~~H-ZSM-5:-9 various transition metals on A1203," H- and Pt-A1203,I1 Fe-Si02,I2 Cu-SAP0,I3 and VPO catalysts.I4 There have, however, been only a few papers of this process over Pd catalyst^.'^ The reaction mechanism of the NO reduction with light hydrocarbons has just started to be investigated and become a topic of debate. Hamada and co-workers" observed that over acid-type catalysts such as H-zeolite and alumina, the oxidation of NO with 0 2 into NO2 first occurred and then the latter was easily reduced into N2 with hydrocarbons, whereas on metallic sites of a Pt-A1203 catalyst, the initial step of the reaction was concluded to be the partial oxidation of hydrocarbons with 0 2 . The similar promoting effect of oxygen was reported in the reduction of NO with hydrocarbons over Cu-ZSM-5 ze01ite.'~~'' On the other hand, Ansell et al.Is proposed in their detailed study of the reaction mechanism that hydrocarbons converted initially into long-lived carbon species on the surface of CuZSM-5, whether 0 2 is present or not. They also stated that the key intermediates in this system are coke on the zeolite component and Nortype species on the Cu sites and that the role of oxygen is not relevant to the activation of hydrocarbons, but it is crucial to the formation of the Cu-NO2 species. Kikuchi and co-workers12 recognized the initial stage of the process to be the formation of carbonaceous deposits on the surface of Fe-silicate catalyst, which act as the active reductant for NO in the gas phase. @
Abstract published in Advance ACS Abstracts, April 15, 1995.
In the fundamental studies of catalytic processes, zeolites have been the subject of numerous studies owing not only to their high activity as catalysts but also to their excellent properties as catalyst supports, Le., in zeolites, active species tend to be more homogeneous in size and shape owing to the constraints of the uniform pore structures, which have molecular dimensions. From this point of view, zeolites are considered to be one of the most suitable materials for the fundamental investigations of catalytic mechanisms. The original form of the Y zeolite used in this study is expressed by a chemical composition of Na56(A102)56(Si02),36. The diameter of catalytically available pores (supercages) in this faujasite zeolite is determined to be 12-13 A. We have recently been studying the redox and catalytic behaviors of C U - Y ' ~ - and ~ ~ Pd-Y22-25 zeolites. In the present study, the structure of catalytic sites and reaction mechanism have been qualitatively investigated in the reduction of NO with C3Hg over the Pd-Y zeolite catalyst.
Experimental Section The zeolite with a high content of Pd ions (7 wt % Pd) used in the present work was derived from Linde LZY-52 molecular sieves. Ion exchange of the parent Na-Y zeolite was conducted at room temperature by dropwise addition of 0.05 mL of aqueous Pd(NH3)&12 solution, followed by stirring overnight in an ultrasonic reactor. The Pd-Y sample was then washed with deionized water, air-dried, pelletized, sieved, and stored over saturated ammonium chloride solution. Prior to catalytic tests, the fresh Pd-Y zeolite was oxidized with pure 0 2 at 575 K for 30 min and consecutively reduced with 10% H2 in He at 675 K for 30 min in the reactor. Reactant gas mixtures, typically 1000 ppm NO and 890 ppm C3H8 in He, were fed into the reactor from high-pressure cylinders through mass-flow controllers. The catalytic reactions in isothermal and temperatureprogrammed modes were performed under atmospheric pressure in a conventional flow system, as described e l ~ e w h e r e . ~It~ ~ * ~ consists of a gas-feeding manifold with mass-flow controllers (Ueshima, MKS-222B) and a quartz U-type reactor heated in a computer-interfaced infrared image fumace. The reaction temperature was measured by a platinum-rhodium thermocouple located at the center of the catalyst bed. Isothermal reactions were investigated in a temperature range of 373-873 K and at 50 cm3 min-' of the reactant flow rate over 200 mg of the catalyst. Temperature-programmed reduction and oxidation
0022-365419512099-6951$09.0010 0 1995 American Chemical Society
6952 J. Phys. Chem., Vol. 99, No. 18, 1995
Matsumoto and Tanabe photoelectron in the solid, A, is assumed to be independent of K. The EXAFS function is Fourier transformed by weighting to yield the radial distribution function, (P(R), as follows,
100-
8 .
-
2 0
80-
@(R)= (1/2)1’2~ K ~ X (eXp(-2mR) K) dK
60-
*E
s
40.
C
8
Curve-fitting procedures are employed to determine structural parameters, such as R and N . The main peak in the radial distribution function is inverse-transformed into K space, and least-squares calculations are made by using eq 1.
;$
C
0
20.
(2)
Results \ \
0. 2 3
\A
A
Steady-StateReactions. In the reduction of NO with C3H8, the Pd-Y zeolite showed an excellent catalytic activity at considerably low temperatures of 573-873 K. Under the reaction conditions employed in the present work, the system was allowed to attain steady-state conditions within 20 min after the reaction started. Figure 1 shows the results of the steadystate reactions, where the experiments were performed with a reactant mixture consisting of 1000 ppm NO and 890 ppm C3H8 in He at a flow rate of 50 cm3 min-’ over 200 mg of the zeolite. The major products of the reaction were N2 and C02, although an appreciable portion of the N20 formation was observed at a low temperature around 473 K. It should be noted that virtually no CO formation was recognized under the steady-state conditions tested. The NZand C02 formations increased with the increase of temperature in similar manners and attained 90% at 573 K. Thus, the reaction proceeds almost stoichiometrically over the Pd-Y catalyst above 573 K. A measurement of an EXAFS spectrum of the zeolite was carried out in order to investigate the structure of the Pd species in the zeolite under the operating conditions. The EXAFS spectrum was measured at room temperature after the treatment of the catalyst with the reactant mixture consisting of 1000 ppm NO and 890 ppm C3Hs at 773 K for 30 min in the in situ chamber of the EXAFS instrument. The observed spectrum and the derived oscillation are illustrated in Figure 2A. Fourier transformation of the oscillation into real space results in the radial distribution function, @(I?), as shown in Figure 2B. The filtered range for this transformation is from 3.0 to 13.6 k 1 in the EXAFS oscillation. In the latter figure, the peaks are slightly displaced from true interatomic distances because of the phase shift. A single predominant peak appears at 2.75 A in the radial distribution function after the correction of the phase shift. This distance agrees well with the Pd-Pd bond in infinite crystals of Pd metal. It should be noted that no appreciable peaks can be observed at the distances corresponding to the second and larger shells of Pd atoms. Probably because extremely diluted NO and C3H8 were used as the
A
A
7
1
T
T
T
373
473
573
673
773
873
973
temperature / K Figure 1. Major products in the reduction of NO with C3Hg over Pd-Y zeolite under the steady-state conditions, where the reactant gas mixture is 1000 ppm NO and 890 ppm C3H8 in He.
were carried out at a heating rate of 2 K min-’ under the same conditions of the reactant and the catalyst. Analyses of reactant/ product gas mixtures were carried out continuously by a computer-interfaced mass spectrometer (Ulvac, MSQ-150) with a multichannel programmer and periodically by a gas chromatograph with Polapak Q and 5A molecular sieve columns. The structure of the Pd species in the zeolite was investigated by a K-edge extended X-ray absorption fine structure (EXAFS) spectrometer. Basically it consists of a rotating anode X-ray generator (Rigaku, RU-300), a spectrometer with bent Ge(660) and LiF(200) crystals of Johanson cut, ion chambers, slits, and counting electronics by a computer through a CAMAC bus. The X-ray source with a gold target was operated at 50 kV and 280 mA. Prior to measurement at room temperature, the Pd-Y sample was pressed into a thin wafer and treated with the reactant gas mixture in an in situ chamber in a similar manner as in the catalytic tests. The analysis of EXAFS data was performed by a conventional m e t h ~ d . ~ ~A. single ~ ’ scattering model for EXAFS oscillations is expressed by
a,(dI (1) where K is the photoelectron wave vector, N, is the number of atoms in the jth shell, R, is the distance from the central Pd atoms, Fj is the scattering amplitude, a, is the Debye-Waller factor, and ajis the phase shift. The mean free path of the 1.5
1
A
~
I
~~~
24.0
24.5
25.0
energy I keV
25.5
RIA
Figure 2. (A) Normalized and extracted oscillations and (B) Fourier transforms of the EXAFS spectrum for Pd-Y zeolite under the operating condition.
J. Phys. Chem., Vol. 99,No. 18, 1995 6953
Reduction Mechanism of Nitrogen Monoxide with Propane I V W
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