15242
J. Phys. Chem. 1996, 100, 15242-15246
Catalytic Reduction of Nitrogen Oxides by Methane over Pd(110)
S. M. Vesecky,† J. Paul,‡ and D. W. Goodman*
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843-3255
ReceiVed: June 5, 1996X
The catalytic reduction of NO with CH4 has been studied over Pd(110) in the temperature range 700-800 K
at a total pressure of 10 Torr. The rate of NO reduction is at a maximum at a high CH4/NO stoichiometry
(8/1) and at a minimum at a low (1/4) CH4/NO ratio. Correspondingly, the activation energy for NO reduction
decreases with increasing CH4/NO ratio, and the reaction orders are -1.0 in NO and +1.5 in CH4. Unlike
NO, the rate of N2O reduction is at a maximum at low CH4 pressures, and the reaction orders are positive in
N2O and negative in CH4. In the presence of oxygen, NO is oxidized to NO2, which is in turn reduced back
to NO by CH4. The NO/NO2 oxidation/reduction cycle has the effect of delaying NO reduction to N2 (or
N2O) until all of the O2 is consumed.
1. Introduction
The impact of environmental catalysis is becoming increasingly more important due to stricter regulations concerning
automobile and flue gas emissions.1 The subfield of environmental catalysis concerned with air quality control involves the
reduction of NOx species and the oxidation of CO and volatile
organic compounds (VOC’s) produced in mobile and stationary
sources2 There are many stationary sources of environmental
gas phase pollutants. Methane is perhaps the largest pollutant
by volume, emitted from sources such as livestock, gas wells,
and landfills. Another serious source of pollution is coal-fired
power plants, which emit large quantities of NOx species that
subsequently react with HO2 and OH- to form HNO3 in acid
rain.3 Current technology for limiting NOx emissions involves
the selective catalytic reduction (SCR) or NOx with NH34
NOx + NH3 + O2 f N2 + H2O
(1)
Although this process is efficient, it involves the long-range
transport of NH3 to the stationary NOx sources. Transportation
of ammonia through pipelines poses serious safety concerns,
the most important being that corrosion of the pipes can lead
to catastrophic releases of ammonia in populated areas en route
to the NOx sources (namely power plants).
The use of methane (and higher hydrocarbons) to reduce NOx
species is a promising alternative to current SCR technology.5
The reaction of CH4 and NOx to form CO2, N2, and H2O
removes two serious greenhouse pollutants at once. Additionally, CH4 is cheaper and much safer to transport than NH3 and
is often found in close proximity to the stationary sources.
Furthermore, excess methane can be used directly to operate
many conventional power plants. In order to be useful in
practical applications, however, methane must be proven to have
reduction efficiencies similar to those of ammonia.
Typically, Pt/Rh and Pd catalysts dispersed on high surface
area supports have been used for the reduction of NO with
CO.6-8 In the case of the CH4 + NO reaction, however,
oxidation of methane either by the oxide support or by ambient
† Present address: Monsanto Company, 800 N. Lindbergh Blvd., St.
Louis, MO 63167.
‡ KTH/the Royal Institute of Technology, Physics III, 100 44 Stockholm,
Sweden
* To whom correspondence should be addressed.
X Abstract published in AdVance ACS Abstracts, August 15, 1996.
S0022-3654(96)01644-9 CCC: $12.00
oxygen is a serious concern. If too much methane is oxidized
to CO2, the efficiency of the NOx reduction process will suffer.
Several catalysts appear to have promising activities for CH4
+ NO conversion; among them are titania-supported Pd and
Pd/Cu particles.9 The goal of these studies is to deconvolute
the catalytic properties of Pd, Cu, and the oxide support.
Currently, there is very little spectroscopic data for NOx and
CH4 chemisorption on these catalysts. By characterizing the
catalyst structure, the chemisorption properties, and the catalytic
activities and selectivities, some insight may be gained on the
basic mechanism of the CH4 + NO reaction on Pd.
As a first step in understanding the CH4 + NO reaction over
palladium catalysts, this study addresses only the effects of the
Pd(110) surface in mediating the reaction kinetics. The single
crystal surface has the advantage over more complex catalysts
of allowing a fundamental understanding of the interactions of
Pd with CH4 and NO over a range of temperature and partial
pressure conditions. The ideal stoichiometry for this reaction
should be
CH4 + 4NO f CO2 + 2N2 + 2H2O
(2)
Under oxidizing conditions, the partial reduction of NO should
proceed as
CH4 + 6NO f CO + 3N2O + 2H2O
(3)
The secondary reduction of partially reduced NOx species should
then continue as
CH4 + 4N2O f CO2 + 4N2 + 2H2O
(4)
The key feature to all of these reactions is that they are highly
energetically favorable, with ∆H’s of reaction of -200 to -300
kcal/mol.10 Therefore, if a suitable catalyst can be found, the
reduction of NOx species by CH4 should proceed completely
to N2.
In this study, infrared reflection-absorption spectroscopy
(IRAS) is used to determine the surface coverages of CH4 and
NO at reaction conditions. IRAS is also used in conjunction
with transmission IR for acquiring kinetics data. The reaction
kinetics data presented show the utility of using gas phase IR
for studying relatively uncomplicated reactions in situ and in
real time.
2. Experimental Section
2.1. Apparatus. The experiments were performed in a
combined ultrahigh-vacuum (UHV) surface analysis chamber
© 1996 American Chemical Society
Catalytic Reduction of NO with CH4
and high-pressure reactor (e1 atm).11 The UHV surface
analysis chamber is equipped with an array of analytical
techniques for determining surface composition and cleanliness.
For this study, Auger electron spectroscopy (AES) was used to
monitor carbon, oxygen, and sulfur contamination. Since the
energy for the Auger transition of carbon overlaps one of the
main Auger signals from palladium, AES cannot be used to
verify the absence of carbon on Pd. Instead, temperatureprogrammed desorption (TPD) of O2 was used to monitor
residual carbon contamination. If CO and CO2 desorption
features are seen in the O2/Pd TPD spectrum, then the presence
of surface carbon is confirmed. The absence of any gas phase
CO or CO2 in the O2 TPD spectrum is the most reliable means
of confirming a carbon-free palladium surface.12
The high-pressure reactor cell is separated from the UHV
analysis chamber by a series of differentially pumped sliding
seals. The sealing surface is formed by the 1 in. diameter
sample probe compressing against a set of three 1 in. diameter
spring-loaded Teflon seals. This allows the analysis chamber
to be maintained at UHV while running reactions at pressures
up to 1 atm in the high-pressure cell. The reactor cell has a
volume of about 350 cm3 and is quickly pumped by a
turbomolecular pump from atmospheric pressures to pressures
of less than 10-8 Torr. The sample can be transferred from the
high-pressure cell to the UHV surface analysis chamber within
2 min after evacuation of the reactor, thus allowing postreaction
analysis of the surface at pressures of ∼10-9 Torr.
The high-pressure cell is coupled to a Fourier transform
infrared spectrometer, operating in the reflection-absorption
mode. This allows adsorbed molecules with dipoles perpendicular to the surface, such as NO and N2O, to be monitored
with infrared reflection-absorption spectroscopy (IRAS). Gas
phase infrared spectra can also be obtained in the reflectionabsorption geometry, thus allowing the kinetics of gas phase
product formation and reactant depletion to be followed with
IR.
2.2. Sample Preparation. The Pd(110) sample was polished
with 1 µm diamond paste and 0.05 µm alumina following
standard polishing procedures. The sample was then cleaned
in the UHV chamber by repeated cycles of oxidation at 900 K
(1 min at 10-6 Torr of O2) followed by flash annealing to 1200
K.13 After this procedure, the sample was found to be free of
bulk carbon contamination. O2 TPD, however, showed the
presence of a small amount of residual surface carbon, as seen
by the presence of CO and CO2 in the O2 TPD spectrum. This
residual carbon was easily removed by running the CO oxidation
reaction on the crystal at elevated pressures (g1 Torr) at a
temperature of about 600 K. The ratio of CO/O2 must be kept
at stoichiometric (2/1) or slightly oxidizing conditions to prevent
the deposition of additional carbon. TPD of O2/Pd following
this procedure indicates the removal of residual surface carbon
on all low index facets of Pd.
2.3. CH4 + NO Reaction. Reactions were performed in
the batch mode. The same results were obtained whether the
reactants were introduced into the high-pressure cell individually
or mixed in the manifold prior to introduction into the reactor
cell. For power law studies, the pressure of either CH4 or NO
was kept constant at 0.9 Torr while the pressure of the other
reactant was varied between 0.112 and 7.2 Torr. The reactant
stoichiometries (CH4/NO) ranged from 1/4 to 8/1, with the total
pressure kept constant at 10 Torr by filling with argon. The
reaction was studied over a temperature range of 700-800 K,
at 25 K increments. Extreme care was taken to eliminate
background reactions from the heater legs at these high
temperatures.
J. Phys. Chem., Vol. 100, No. 37, 1996 15243
Figure 1. Reaction profile for 0.9 Torr of NO + 0.9 Torr of CH4 (10
Torr total pressure, fill with Ar) at 725 K (batch mode).
Figure 2. Reaction profile for 0.9 Torr of NO + 7.2 Torr of CH4 (10
Torr total pressure, fill with Ar) at 725 K (batch mode).
The reaction kinetics were followed with infrared spectroscopy by monitoring the reduction in gas phase NO and CH4,
while simultaneously monitoring the evolution of gas phase N2O
and CO2. After all of the NO was consumed, the reaction of
N2O + CH4 could be observed similarly by monitoring the
reduction in gas phase N2O and CH4 and the evolution of gas
phase CO2. The IR peak areas of each measured species were
converted into the corresponding partial pressures using a set
of calibration curves. The calibration curves were obtained by
plotting the gas phase infrared intensity of a given molecule
versus the partial pressure of that species.
3. Results and Discussion
3.1. CH4 + NO on Pd(110). Figures 1 and 2 show the
profiles for the reaction of CH4 and NO at 725 K on Pd(110)
as a function of time. All reactions performed on model
catalysts in the high-pressure cell (350 cm3) were run in the
batch mode. As a result, the reactions are at equilibrium only
at low conversions (10-20%) where the effect of the change
in concentration of each reactant is negligible. The gas phase
concentrations of CH4, NO, and N2O were monitored simultaneously with infrared spectroscopy, and the IR intensities were
converted to the number of moles produced (or consumed) per
second by the formula
∆n/s ) (∆P/s)(V/RT)
(5)
∆P/s was determined by multiplying the slopes of the lines
shown in Figures 1 and 2 by the conversion factor (1 min/60
s). By sampling the gas phase infrared spectra at fixed time
15244 J. Phys. Chem., Vol. 100, No. 37, 1996
Vesecky et al.
intervals (every 30 s), we obtained values of ∆P/s for each
reactant and product species. V is the volume of the cell, 350
mL, R is the universal gas constant, 1.987 × 10-3 (Torr‚mL)/
(mol‚K), and T is the sample temperature, in kelvin. The rate
of the reaction is given as the turnover frequency (TOF), which
corresponds to the number of molecules produced (or consumed)
per surface site per second. In this case, the overall reaction
rate was determined from the TOF for NO reduction. This value
was calculated by normalizing the moles of NO molecules
consumed per second (∆n/s) to the total number of surface sites,
given by the equation
TOF ) (∆n/s)(6.02 × 1023)/(1.15 × 1015)
(6)
where 1.15 × 1015 is the total number of surface atoms on the
front face of the Pd(110) single crystal used in these experiments. For these studies, the Pd atoms on the back face of the
crystal were rendered inactive by depositing a multilayer of an
inert oxide such as silica or alumina onto the back face.
Under all conditions studied (including Figures 1 and 2), a
significant amount of N2O was formed, most likely because
reaction 3 was more favorable than reaction 2. IRAS data of
adsorbed NO at reaction conditions show that the coverage of
NO was relatively unaffected by the presence of CH4, even at
highly reducing conditions (CH4:NO > 8:1). The heat of
adsorption of NO/Pd(110) was determined isosterically to be
30 ( 1 kcal/mol. The heat of adsorption of CH4/Pd(110), on
the other hand, is only on the order of about 10 kcal/mol. These
values for the heats of adsorption, combined with the qualitative
IRAS data which show the surface coverage of NO to be only
slightly lowered at high CH4:NO ratios (>4:1), point to the fact
that reaction 3 should be favored over reaction 2 under all
conditions due to the much higher coverage of NO versus CH4.
Figures 1 and 2 support this idea, showing a selectivity for
reaction 3 over reaction 2 of about 90% and 70%, respectively.
Any CO produced in reaction 3 is quickly converted to CO2
by
CO + NO f CO2 + N2 + N2O (or)
(7)
CO + N2O f CO2 + N2
(8)
Each of these reactions is significantly faster than the CH4 +
NO reaction, thus accounting for the fact that no gas phase CO
is observed at these reaction conditions.14 In addition to the
CO oxidation reactions, (7) and (8), the water gas shift reaction
may remove some CO by
CO + H2O a CO2 + H2
(9)
As seen in Figures 1 and 2, the concentration of gas phase
N2O steadily increases while the concentration of gas phase NO
is decreasing. N2O is not reduced at a significant rate by CH4
(reaction 4) until after all the NO is reduced because N2O
adsorption on palladium is inhibited by NO(a), as seen by IRAS.
The surface coverage of N2O/Pd(110) is only measurable at low
temperatures (<200 K) and relatively high pressures (>10-3
Torr). Similar to the case of CH4, this low θN2O relative to θNO
along with a heat of adsorption for N2O of only about 10-15
kcal/mol favors reaction 3 over reaction 4.
The rate of N2O reduction is at a maximum at a 1:1 CH4:NO
stoichiometry (Figure 1). As the CH4:NO ratio increases, the
rate of NO reduction also increases, but the rate of N2O
reduction decreases (Figure 2). The increase in the rate of NO
reduction with increasing PCH4 can be rationalized by the more
competitive adsorption of CH4 with NO at reducing conditions
(seen qualitatively with IRAS). The decrease in the rate of N2O
Figure 3. Arrhenius plots for the rates of NO reduction as a function
of CH4:NO ratio.
Figure 4. Apparent activation energy for NO reduction as a function
of CH4:NO ratio.
reduction at reducing conditions, however, is most likely caused
by a higher θCH4 versus θN2O. Since both the heat of adsorption
and the surface coverage of N2O are lower than for NO, it should
be expected that the secondary reduction reaction 4 can more
easily be poisoned at reducing conditions by CH4 (Figure 2).
Figure 3 shows a series of Arrhenius plots for the rates of
NO reduction as a function of PCH4. It can readily be seen from
this figure that the activity for NO reduction increases with
increasing PCH4, whereas the apparent activation energy for NO
reduction decreases with increasing CH4:NO ratio. Figure 4
shows the apparent activation energy for the reaction over a
much broader range of CH4:NO pressure ratios. Again, it can
clearly be seen that the apparent activation energy for the
reaction is strongly dependent on the CH4:NO pressure ratio.
At higher CH4:NO ratios, the desorption of NO is facilitated
because CH4 is able to compete more effectively with NO for
surface sites. This, in turn, decreases the apparent activation
energy for the reaction and increases the rate of reaction because
the activation energy for NO desorption becomes less of a
limiting factor.
Figure 5 shows the power laws of the reactants for NO
reduction at 725 K. The negative order in NO strongly supports
the idea that θNO and the desorption of NO from the Pd(110)
surface limit the rate and in turn increase the apparent activation
energy for the reaction (Figures 3 and 4). The positive order
in CH4 corresponds to the IRAS data which show that θNO
decreases slightly with increasing PCH4. The fact that the power
law in CH4 is greater than 1 may imply that each CH4 is
displacing more than one NO and/or that the equilibrium shifts
from reaction 3 toward reaction 2 with increasing PCH4.
Catalytic Reduction of NO with CH4
J. Phys. Chem., Vol. 100, No. 37, 1996 15245
Figure 7. Reaction profile for 1 Torr of NO + 1 Torr of CH4 + 1/2
Torr of O2 (10 Torr total pressure, fill with Ar) at 800 K (batch mode).
Figure 5. Reaction power laws in NO and CH4 at 725 K. The pressure
of one component was held constant at 0.9 Torr while the pressure of
the other component was varied from 0.225 to 3.6 Torr. The total
pressure was 10 Torr.
enough concentrations, thus promoting the activity and lowering
the apparent activation energy for the reaction.
3.2. CH4 + NO + O2 on Pd(110). For practical applications, the reaction of CH4 + NO must be considered in the
presence of O2. The first reaction which occurs in the presence
of O2 is the direct gas phase oxidation of NO to NO2.15 Since
both NO and NO2 can act as oxidants for CH4, the reaction
profile for CH4 + NO + O2 becomes somewhat more
complicated (Figure 7). In the absence of O2, the reaction
profile shows only two regimes: one for NO reduction and one
for N2O reduction (Figures 1 and 2). In the presence of O2, an
additional reaction regime is formed by the reaction of NO2 +
CH4 (first region of Figure 7). Since NO2 is a better oxidant
than NO (and N2O), NO is reduced at a measurable rate only
after all of the NO2 and O2 have been consumed (Figure 7).
The stoichiometry of this additional reaction pathway should
ideally follow as
CH4 + 2NO2 f N2 + CO2 + 2H2O
Figure 6. NO:CH4 reaction rate versus initial CH4:NO concentration.
Although the microscopic effect on CH4 (a) on NO (a) is
difficult to determine, a slight shift in the selectivity toward
reaction 2 with increasing PCH4 can be seen by comparing the
maximum concentration of N2O in Figure 2 (0.36 Torr) versus
Figure 1 (0.41 Torr). Repeated experiments confirm that the
selectivity for N2O decreases with increasing PCH4.
The shift in the selectivity between the partial reduction (3)
and the optimum stoichiometry (2) can also be seen by plotting
the TOF for NO reduction versus the TOF for CH4 oxidation
as a function of the initial partial pressure conditions (Figure
6). At oxidizing conditions (low CH4:NO ratios), over eight
NO molecules are reduced (or at least partially reduced to N2O)
for every CH4 molecule oxidized. This, in fact, is a higher
stoichiometry than that predicted by eq 3. The additional NO
reduction may occur first by NO dissociation into N(a) and O(a)
followed by
NO(a) + N(a) f N2O(a) f N2O(g)
(10)
The O(a) may then react with either CH4 or CO to form CO2.
As the CH4 concentration increases, the consumption of NO
relative to CH4 decreases. At an 8:1 excess of CH4:NO, the
number of NO molecules reduced per CH4 oxidized approaches
the ideal stoichiometry (4:1 NO:CH4) of reaction 2. These
results again support the idea that CH4 can displace NO at high
(11)
Alternatively, NO2 may only be partially reduced, giving a
stoichiometry of
CH4 + 4NO2 f 4NO + CO2 + 2H2O
(12)
Since these reactions were run in the batch mode, NO produced
by reaction 12 was reoxidized to NO2, thus repeating a catalytic
cycle, with NO as the catalyst:
NO + 1/2O2 a NO2
(13)
The effect of this catalytic cycle in the presence of oxygen
is to prevent the reduction of NO by CH4. NO reduction readily
occurs at these conditions in the absence of oxygen (Figures 1
and 2), implying that either O(a) or NO2(a) inhibits the NO +
CH4 reaction. Once all of O2 was consumed, the reduction of
NO by CH4 proceeded as in the absence of O2 (region 2 of
Figure 7). The fall in the concentration of NO2 corresponds to
the consumption of O2 either by reaction 13 or by direct
oxidation of CH4 and CO.
Region 1 of Figure 7 shows that N2O production was
suppressed in the presence of oxygen. This selectivity for N2
versus N2O in the presence of O2 is encouraging, implying that
eq 11 is indeed viable. Unfortunately, at the conditions studied,
only a small fraction of the NO was converted to NO2. As
seen in region 2 of Figure 7, about 0.9 Torr of the original 1
Torr of NO remained after all of the NO2 had been consumed.
Still, the 0.1 Torr of NO reduced (indirectly) by eq 11 showed
complete selectivity for N2 versus N2O.
15246 J. Phys. Chem., Vol. 100, No. 37, 1996
Figure 8. NO reduction rates as a function of O2 partial pressure.
An attempt was made to optimize the NO2 concentration by
varying the partial pressure of oxygen. Figure 8 shows the
results of the measured NO reduction rates as a function of
oxygen partial pressure at the same temperature and pressure
conditions shown in Figure 7. For oxygen partial pressures
between 0 and 0.5 Torr, the primary effect seen for a batch
mode reaction is to delay the onset of the direct reduction of
NO by CH4. Even at partial pressures of O2 of more than 100
Torr, the equilibrium concentration of NO2 never exceeded more
than 10% of the concentration of NO. The most likely cause
of this low concentration of NO2 is the reverse reaction 13
caused by the thermal dissociation of NO2 back to NO.
At high enough oxygen pressures, all NO should eventually
be reduced to N2, indirectly, through reaction 11. As the partial
pressure of oxygen increases, however, the rate of NO2 reduction
decreases, perhaps due to increased surface poisoning by O(a).
At higher temperatures (>800 K), the reaction should run faster.
The equilibrium between NO2 and NO, however, increasingly
favors NO due to thermal dissociation. The power laws of NO2,
CH4, NO, and O2 for this system have yet to be determined.
For practical applications in the presence of oxygen, however,
the complete reduction of NO to N2 over Pd only catalysts does
not appear feasible.
4. Conclusions
Overall, these results show that the ideal stoichiometry of 4
NO + 1 CH4 predicted by reaction 2 is followed only at highly
reducing conditions due to the much higher heat of adsorption
of NO versus CH4. At an 8:1 excess of CH4:NO, the apparent
activation energy of the reaction is about 27 kcal/mol lower
than at the “ideal” stoichiometry of 1:4 CH4:NO (Figure 4).
This difference in the apparent activation energy for the reaction
at reducing versus oxidizing conditions corresponds very closely
to the measured heat of adsorption of NO/Pd(110) ≈ 30 kcal/
mol. Although an 8:1 excess of CH4:NO is optimum for NO
reduction (2), the secondary reduction of N2O (4) is inhibited
at highly reducing conditions due to displacement of N2O by
CH4.
In the presence of oxygen, the rate of NO (and N2O) reduction
by CH4 is negligible due to displacement of NO/Pd by either
NO2(a) or O(a). Oxygen catalyzes the formation of NO2, which
then either goes on to react (slowly) with CH4 or thermally
dissociates back into NO. When O2 and NO2 are present in
Vesecky et al.
the CH4 + NO system, the selectivity for NO reduction goes
completely to N2. Once all of the O2 has been depleted,
however, the NO + CH4 reaction follows the same activity and
selectivity pattern as if O2 had never been present.
To optimize this reaction, ideally all NO should be converted
to NO2. Unfortunately, the rate of the NO2 + CH4 reaction is
so slow that extremely high temperatures must be employed.
Not only do these high temperatures make this reaction
impractical and inefficient, they also lead to the thermal
dissociation of most NO2 back to NO. Although the NO +
CH4 reaction is more efficient, the selectivity for N2 versus N2O
was extremely poor under the conditions studied (10-30%).
Although Pd only catalysts do not appear practical for the
reduction of NO by CH4, the role of the Pd surface in a
bimetallic Pd/Cu or an oxide supported system may be much
different. The fundamental kinetics data obtained on the
Pd(110) surface provides a starting point for interpreting kinetics
results in more complicated systems.
Acknowledgment. The authors acknowledge, with pleasure,
the support of this work by the Department of Energy, Office
of Basic Energy Sciences, Division of Chemical Sciences.
S.M.V. gratefully acknowledges the support of an Eastman
Chemical graduate fellowship.
References and Notes
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(14) The rate of the CO + NO reaction to form CO2, N2, and N2O is
almost 2 orders of magnitude faster than the CH4 + NO reaction at the
most favorable (highly reducing) conditions studied. Similarly, the rate of
the CO + N2O reaction is about 1 order of magnitude faster than the CH4
+ NO reaction.
(15) Calvert, J. G.; Stockwell, D. R. In SO2, NO and NO2 Oxidation
Mechanisms: Atmospheric Considerations; Calvert, J. G., Ed.; Butterworth: Boston, 1984; p 1.
JP961644P