Combustion and Flame 157 (2010) 421–435

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Combustion and Flame

journal homepage: www.elsevier .com/locate /combustflame

Methane ignition catalyzed by in situ generated palladium nanoparticles

T. Shimizu a, A.D. Abid a, G. Poskrebyshev a, H. Wang a,*, J. Nabity b, J. Engel b, J. Yu b, D. Wickham c,B. Van Devener d, S.L. Anderson d, S. Williams e

a Department of Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089, United Statesb TDA Research, Inc., 12345 W. 52nd Ave, Wheat Ridge, CO 80033, United Statesc Reaction Systems, LLC, 19039 E. Plaza Drive, Suite 290, Parker, CO 80134, United Statesd Department of Chemistry, University of Utah, Salt Lake City, UT 84112, United Statese Air Force Research Laboratory, Mail Stop RZA, 1950 Fifth Street, WPAFB, OH 45433, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 9 May 2009Received in revised form 11 July 2009Accepted 13 July 2009Available online 10 August 2009

Keywords:Catalytic combustionIgnitionGas-surface reactionKinetic modelingMicroscopyFlow reactorParticle size distribution

0010-2180/$ - see front matter � 2009 The Combustdoi:10.1016/j.combustflame.2009.07.012

* Corresponding author. Address: Aerospace and Mversity of Southern California, Los Angeles, CA 90089, U8071.

E-mail address: haiw@usc.edu (H. Wang).

Catalytic ignition of methane over the surfaces of freely-suspended and in situ generated palladiumnanoparticles was investigated experimentally and numerically. The experiments were conducted in alaminar flow reactor. The palladium precursor was a compound (Pd(THD)2, THD: 2,2,6,6-tetramethyl-3,5-heptanedione) dissolved in toluene and injected into the flow reactor as a fine aerosol, along witha methane–oxygen–nitrogen mixture. For experimental conditions chosen in this study, non-catalytic,homogeneous ignition was observed at a furnace temperature of �1123 K, whereas ignition of the samemixture with the precursor was found to be �973 K. In situ production of Pd/PdO nanoparticles was con-firmed by scanning mobility, transmission electron microscopy and X-ray photoelectron spectroscopyanalyses of particles collected at the reactor exit. The catalyst particle size distribution was log-normal.Depending on the precursor loading, the median diameter ranged from 10 to 30 nm. The mechanismbehind catalytic ignition was examined using a combined gas-phase and gas-surface reaction model.Simulation results match the experiments closely and suggest that palladium nanocatalyst significantlyshortens the ignition delay times of methane–air mixtures over a wide range of conditions.

� 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction

Supersonic combustion has drawn attention in recent decadeswith possible applications to hypersonic propulsion systems.Hydrogen has long been used as a fuel for hypersonic systemsdue to its short ignition delay and fast kinetics. However, its lowfuel density results in poor vehicle packaging. Conversely, liquidkerosene based jet fuels package well but have additional compli-cations with regard to fuel injection and droplet vaporization lead-ing to reduced combustion performance. Furthermore, liquid jetfuels tend to produce significant amounts of coke during endother-mic cooling cycles. Thus, methane has gained interest for its poten-tial to overcome most of these problems. It has greater heat sinkcapacity and specific energy content than jet fuels and a muchhigher energy density, if stored as a cryogenic liquid, than hydro-gen [1]. However, the use of methane could lead to poor ignitionperformance. The current work explores the question whetherignition of methane can be enhanced catalytically by freely-sus-pended palladium (Pd) nanoparticles generated in situ from a suit-

ion Institute. Published by Elsevier

echanical Engineering, Uni-nited States. Fax: +1 213 740

able fuel-soluble Pd precursor. Entrained in the mixture, palladiumparticles are expected to provide catalytic surfaces to initiate gas-surface reactions, leading to fuel ignition at lower temperaturesand/or with drastically reduced ignition delay times.

Catalytic oxidation of hydrogen and small hydrocarbons overpalladium has been studied quite intensely in recent years [2]. Itis known that the palladium exhibits complex hysteresis behaviorwith respect to the rate of methane oxidation over its surface andthat the catalytic activity is influenced by the presence of water va-por [3] and can be sensitive to OH poisoning [4]. In general the cat-alytic reaction rate increases with an increase in temperature untilabout 900 K, above which the reaction rate decreases. Above1050 K, the catalytic reaction rate increases again [3]. This complexbehavior appears to be related to the variation of the bulk compo-sition of palladium to be discussed below.

In an oxidizing environment, palladium oxide (PdO) is stablethermodynamically for temperatures of relevance to catalytic oxi-dation. The equilibrium constant Kp of

2PdO ðsÞ ¼ 2Pd ðsÞ þ O2 ðgÞ

has been reported to be 2.2 � 10�3 atm at 909 K and 0.64 atm at1124 K [5]. Hence the preferred thermodynamic state is palladiumoxide in air below �900 K, but it dissociates into Pd(s) and O2(g)

Inc. All rights reserved.

422 T. Shimizu et al. / Combustion and Flame 157 (2010) 421–435

at temperatures above 1100 K. Kinetically, oxidation of the palla-dium surface can lead to penetration of oxygen atoms into the sur-face, creating local microstructures such as surface islands andpeninsulas [6]. Studies of oxygen adsorption and desorption on pal-ladium nanoparticles supported on Fe3O4 show quite complexbehaviors with respect to reversible chemisorption and surfaceoxide formation [7]. Hysteresis has also been observed for oxidationand reduction of Pd supported on Al2O3 [8]. Computationally it wasshown that the hysteresis behavior is caused, at least in part, by thecompetition between adsorption and desorption of oxygen from thesurface, coupled with the diffusion of oxygen into and from theatomic layers below the Pd surface [9]. In general, the studies dis-cussed above were conducted at temperatures near or below1000 K. Only a few studies have been reported for Pd catalytickinetics above 1000 K. Notably, Rosen and coworkers [10,11] exam-ined the reaction mechanism of hydrogen oxidation over a palla-dium surface in a stagnation flow reactor.

An added complexity is that the catalytic activity and mecha-nism on the surface of nanosized Pd particles can be different froma perfect Pd(1 1 1) surface [12]. These complexities indeed posesome significant challenges for initiating methane ignition within situ generated Pd/PdO nanoparticles, especially when one con-siders the limitation of catalyst loading. The fundamental difficultyto achieve fuel ignition below 900 K is that the catalytic oxidationof the fuel must be designed in such a way that the reaction pro-cess transitions from a catalytic process with mild oxidation ratesat low temperatures to a flame process driven by gas-phase free-radical chain branching above about 1400 K. Heat release fromthe catalytic process must occur, at least locally, to allow the gasto heat up and reach the cross-over temperature above which rad-ical chain branching becomes possible. At the same time, the cata-lytic reaction and heat release cannot be too fast, since the flame, ifignited, must have sufficient fuel left to sustain and propagateitself.

Meanwhile, the Pd/PdO nanoparticles require a finite time tonucleate out of its precursor. This time must be shorter than thetransit time through the combustor to allow the catalytic processto occur. On the other hand, if the nucleation time is too short com-pared to the transit time, particle–particle coagulation and coales-cence would lead to a reduced surface area per unit mass ofcatalyst injected and a reduced catalytic activity. Additionally asthe Pd/PdO particles grow in size and mass, they must passthrough a cluster size regime where the catalytic mechanism ispoorly understood.

Previously methane oxidation over a Pd surface has been stud-ied in flow reactors [13,14]. In general, these studies used wall-coated Pd to accomplish low temperature oxidation (up to1100 K) with the purpose of minimizing NOx formation. No studies

Fuel/Oxidizer/ Catalyst Solution

Pd(v) Pd2Particle Size Distributions/ Species Profiles

NucleationVaporization

Pdi

Flow direction and reaction time

Fig. 1. Conceptual drawing of in situ generation of nanoparticles follow

have been reported on fuel ignition on Pd nanoparticles suspendedin unburned fuel-oxidizer mixtures.

The objective of this study is to understand the catalytic ignitionbehavior of methane with Pd nanoparticles generated in situ. Thestudy combines flow reactor experiments, aerosol and surface sci-ences with kinetic modeling using detailed gas-phase and gas-sur-face reaction kinetics to explore the underlying mechanisms ofmethane ignition over Pd nanoparticles. A combined gas-phaseand gas-surface reaction model is proposed on the basis of previ-ous studies [15,16]. Combined with microscopy, particle mobilitysizing and other chemical analyses of the Pd particles, the modelwas used to explain the observations made in the flow reactor. Thisis not meant to be a quantitative kinetic modeling study. The use ofa laminar flow reactor indeed limits our ability to account for localreaction conditions in a precise manner. Nonetheless the resultsdescribed here are useful for designing a catalytic reaction processsuited for high-speed combustion.

2. Experimental

Generation of palladium particles in situ and the subsequentignition procedure is drawn in Fig. 1 schematically. The sequenceof the nanoparticle formation process is described as follows. Atthe reactor inlet section, a Pd precursor solution diluted in tolueneis introduced in the form of micron sized aerosol carried by nitro-gen stream. This flow was mixed with a stream of premixed CH4/O2/N2 mixture. Due to the high temperature and small droplet size,the aerosol quickly evaporates in the inlet section of the reactor.Dissociation of the precursor follows due to heating, releasing Pdatoms or clusters. At high enough atom or cluster concentrations,Pd nucleates into small clusters which grow in size and mass bycoagulation and surface condensation. The clusters or the surfacesof the nanoparticles become catalytically active at some point. Thegas mixture is then expected to undergo catalytic oxidation andeventually ignite in the flow reactor. Here, we follow the concen-trations of CH4 and CO2 to monitor fuel ignition.

2.1. Setup

A schematic flow diagram of the experimental setup is pre-sented in Fig. 2. A flow tube reactor was built in conjunction witha detector for measurement of CH4 and CO2 concentrations at theexit of the reactor. The particles are also sampled at the reactorexit, enabling us to simultaneously measure the particle size distri-bution functions and catalytic activity during experiments.

The flow reactor is made of a quartz tube (1.9 cm OD, 1.7 cm ID,and 94 cm in length). Although by visual inspection the tubes wereusually clean after each experiment, they were replaced after each

~ 20 nm

CH4 CO2

Pd nanoparticles

x, t

Furnace

Exhaust

Coagulation/Agglomeration

Ignition

ed by catalytic ignition of a fuel-oxidizer mixture in a flow reactor.

1.9cm OD×1.7cm ID×94cm L

Quartz Tube

TC-3

Vent

N2/ O2/ CH4

Temperature Controlled Furnace

LiquidPump

TC-1

N2

NDIR CO2/CH4Analyzer

CatalystSolution

N2

TC-2

Liquid Atomizer

PI-1

Vent

Vent

TSI 3080 EC

Kr 85 n-DMA

TSI 3025A UCPCΔP

SMPS System

TEM Grid

From liquid pump

To reactor

Liquid Pool

N2

Tf-1

Tf-2

Zone II

Zone I

L = 76cm

1.9cm OD×1.7cm ID×94cm L

Quartz Tube

TC-3TC-3

Vent

N2/ O2/ CH4

Temperature Controlled Furnace

LiquidPump

TC-1TC-1

N2

NDIR CO2/CH4Analyzer

NDIR CO2/CH4Analyzer

CatalystSolutionCatalystSolution

N2

TC-2TC-2

Liquid Atomizer

PI-1PI-1

Vent

Vent

TSI 3080 EC

Kr 85 n-DMA

Kr 85Kr 85 n-DMAn-DMA

TSI 3025A UCPCΔPΔPΔP

SMPS System

TEM Grid

From liquid pump

To reactor

Liquid Pool

N2

Tf-1

Tf-2

Zone II

Zone I

L = 76cm

Fig. 2. Schematic of the experimental apparatus.

T. Shimizu et al. / Combustion and Flame 157 (2010) 421–435 423

experiment to minimize the influence of wall deposits. Aninsulated furnace (Mellen SV12) with the test section length (L)76 cm was used for heating. The furnace has two separate zones.The temperature in each zone was measured using a thermocouple(OMEGA K-type). The measured temperatures are denoted as Tf-1

(TC-1, Zone I) and Tf-2 (TC-2, Zone II), respectively. Thesetemperature values were acquired by the LabVIEWTM programand used to control the furnace temperature. The temperatureprofiles inside the reactor were measured using 122 cm longK-type thermocouples (OMEGA) inserted from the reactor exit(TC-3).

The gas mixtures were prepared by mixing CH4, N2, and O2 fromcylinders. The flow rates were controlled by mass flow controllers(Porter Model 201) calibrated for each of the gases. The mixturecomposition tested and the total gas velocity U0 selected are sum-marized in Table 1. The residence time in the reactor was about 1 sat the temperature of reactor operation. The reactor exhaust wasdiluted eight times by a cold stream of N2. The flow rate was con-trolled by a mass flow controller (Porter Model 202). The concen-trations of CH4 and CO2 at the exit of the reactor werecontinuously monitored using an NDIR gas analyzer (CaliforniaAnalytical Instruments Model 200), which withdraws 2 L/min ofthe diluted exhaust gas.

Table 1Summary of flow and mixture conditions.

No. Total gas velocity (STP), U0 (cm/s) Mole fraction

C7H8 CH4 O2

F-0 52.9 – – –F-1 52.9 – 0.083 0.F-2 52.5 0.007 0.084 0.F-3 52.5 0.007 – 0.

a Calculation of the equivalence ratio includes toluene.

The precursor atomizer was designed to produce micron sizeddroplets suspended in a N2 flow. The atomizer utilized a dual, con-centric nozzle design, as shown schematically in Fig. 2, whereby acompressed N2 flow passes through the inner nozzle (2 mm IDglass tubing with a tip diameter of 0.5 mm). Under this condition,the Venturi effect draws the precursor solution from a liquid poolthrough the outer nozzle (4 mm ID glass tubing with a tip diameterof 1 mm), forming a stream of aerosol.

The atomizer outlet used 6.53 mm ID tubing and an elbow toform a series of impactors, thus eliminating the passage of largedroplets into the reactor. The large droplets collected onto the imp-actors were returned to the liquid pool. The aerosol production ratedepends on the liquid pool level, the flow rate of N2, and the posi-tion of the impactors. The N2 flow rate through the atomizer wascontrolled manually with a needle valve and an upstream pressureindicator (PI-1). The liquid pool was replenished with a single pis-ton syringe pump (Chrom Tech ISO-1000), resulting in steady oper-ation of the atomizer. The atomizer was cooled and insulated toavoid evaporation of the droplets collected on the inner wall,which would precipitate and crystallize the precursor onto thewall.

Pd(THD)2 (THD: 2,2,6,6-tetramethyl-3,5-heptanedione) (molec-ular weight 475 g/mol, sublimation temperature 150 �C at

Equivalence ratio, /a Purpose

N2

1.000 0.00 Background temperature417 0.500 0.40 Non-catalytic ignition420 0.490 0.55 Catalytic ignition420 0.573 0.15 TEM analysis

424 T. Shimizu et al. / Combustion and Flame 157 (2010) 421–435

0.01 Torr and melting point 239 �C [17]) was synthesized at TDAand used as the Pd precursor. Pd(THD)2 is a coordination complex,and as such the binding energy between Pd and THD is small. Uponheating, it dissociates readily, releasing a Pd atom. The precursorwas dissolved in toluene (J.T. Baker 99.7% purity). The concentra-tion of Pd(THD)2 in toluene solution was less than 1.9 wt% in theexperiments.

The size distributions of Pd nanoparticles were measured at thereactor exit using a scanning mobility particle sizer (TSI SMPS3090). A nominal flow of 0.3 L/min of the diluted exhaust was sam-pled into the SMPS. The residence time from the reactor exit to theSMPS inlet was 0.6 s, which is short enough to prevent coagulationof particles. The SMPS is composed of an electrostatic classifier(Model 3080) and a particle counter (Model 3025A). The classifierwas equipped with a nano differential mobility analyzer (Model3085). The diameter of the impactor nozzle was 0.0457 cm. Theparticle size distribution was scanned over the range of 4.5–160 nm in diameter. Each scan time took 55 s.

The particles were also collected on grids (Electron MicroscopyScience HC200-CU) and analyzed by Transmission Electron Micros-copy (TEM, Philips EM420). Each grid, affixed on top of an alumi-num rod (0.6 cm in diameter) with aluminum tape, was exposedto a diluted exhaust stream flow parallel to the grid surface. Thelinear flow rate of the aerosol was 13 m/s over the grid. Under thiscondition, the flow is turbulent and the principal mechanism ofparticle collection is diffusion of the particles across the laminarboundary layer near the surface, which may bias against large par-ticles. High-resolution TEM, STEM/EDX, and X-ray photoelectronspectroscopy (XPS) analyses of particle structure and chemicalcomposition were carried out for particles from selected experi-ments. The details of the particle characterization are describedelsewhere [18].

2.2. Procedure

Two different types of tests were performed as shown in Table2. In the first type (Runs 3 and 4), the concentration of Pd(THD)2

was kept constant at 1.92 wt% in the atomizer, corresponding to�4.5 � 10�4 moles Pd per mole CH4 in the main reactor flow. Thefurnace temperature was transient and increased at a constant rateof 20 K/min. This allowed us to determine the furnace temperatureat the point of ignition with a specified catalyst condition. In thesecond type the furnace temperature was held constant, whilethe Pd(THD)2 concentration was transient and raised gradually,from 0 to 1.92 wt% until ignition was observed (Run 7). This al-lowed for determination of the minimum catalyst concentration

Table 2Summary of experimental condition and key results.

No. Experimentaltype

Flow conditionof Table 1

Furnace temperature

Tf-1 (K) Tf-2 (K)

Transient furnace temperature at +20 K/min heating rate, constant precursor injection rateRun 1 Non-catalytic F-1 1149 1077Run 2 Non-catalytic F-1 1142 1073Run 3 Catalytic F-2 998 970Run 4 Catalytic F-2 954 916Run 5 Non-catalyticb F-1 1179 1117

Constant furnace temperature, increased precursor loading over timeRun 6 Non-catalytic F-1 1113 1113Run 7 Catalytic F-2 973 973

Controlled particle formation for TEM analysisRun 8 No ignition F-3 773 773Run 9 No ignition F-3 973 973

a Measured at the reactor exit.b The reactor tube used was contaminated by wall coating.c Molar loading, particle diameter and surface area at the point of ignition.

for ignition at a specified furnace temperature. Additional experi-ments were carried out to measure baseline methane ignition un-der non-catalytic conditions (Runs 1, 2 and 6).

A key experimental challenge was to minimize the effect of walldeposits. The effect of the Pd deposited on the reactor wall wascarefully investigated in Run 5. The test was carried out withoutthe use of a Pd precursor but with a quartz tube repetitively usedin prior catalytic experiments.

In addition, Runs 8 and 9 were designed to produce Pd/PdOnanoparticles for TEM sampling. To avoid fuel ignition, which re-sults in large changes in gas temperature and concentration, meth-ane was omitted from the O2/N2 flows, and their combined flowrate was equal to those of catalytic experiments. The mass loadingof the Pd(THD)2 in toluene was 1.2 wt%, which is about the same asthat in Run 7.

3. Detailed kinetic modeling

3.1. Mathematical descriptions

The underlying reacting flow problem was simulated as an ini-tial value problem. SURFACE CHEMKIN [19] was utilized to solvethe system of equations and to calculate the chemical reactionrates for both gas-phase and gas-surface processes. The model iscapable of simulating the evolution of temperature and speciesalong the flow axis. Simulations were performed at 1 atm pressurewith and without a catalyst. Convective time t was obtained fromthe local linear gas velocity u and the axial position x of the flowreactor by the relationship, t ¼

R x0 dx=u. Since the flow reactor

operates under the laminar flow condition, the flow is inherentlytwo-dimensional; and the velocity profile is parabolic. Nonethe-less, the current analysis assumes this to be a plug flow problemand neglects the radial transport – a simplification in comparisonto the actual experiments. Sensitivity analyses, experimentallyand computationally, were performed to examine this modelassumption.

The gas-surface reaction rate constant is described as

k ¼ c

ffiffiffiffiffiffiffiffiffiffiffi8kBTpmi

spr2

d ð1Þ

where c is the reaction probability or sticking coefficient, kB theBoltzmann constant, mi the molecular mass of ith gas-phase species,and rd the particle radius. The species conservation is written as

dYi

dt¼ ð _xg;i þ f _xs;iÞ

Mw;i

qð2Þ

Moles Pd/mole CH4 Median diameter, a

hDpi (nm)Surface-to-volumeratio,a f (cm�1)

–– –

4.3 � 10�4 24 0.0234.7 � 10�4 28 0.027

– –

– –2.1 � 10�4,c 19c 0.013c

26 0.01623 0.016

T. Shimizu et al. / Combustion and Flame 157 (2010) 421–435 425

where Y is the mass fraction, Mw the molecular weight, q the gasmass density, f the surface area-to-gas volume ratio of the catalyst,_xg and _xs are the gas-phase and gas-surface molar production

rates, respectively.The rate of temperature change in the reactor is obtained by

considering the heat release due to chemical reactions in additionto the convective heating/cooling

dTdt¼ dTb

dt� 1

qcp

Xi

ð _xg;i þ f _xs;iÞhiMw;i ð3Þ

where h is the specific enthalpy, cp the specific heat capacity of thegas mixture. The first term on the right-hand side of the equationexpresses the experimental temperature gradient obtained from adirect measurement for a non-reacting flow. As will be shown later,the particles formed are substantially smaller than the mean freepath of the gas. Hence the particles are treated as being in free mol-ecule regime. The time evolution of the site fraction hi for the ithsurface is given as,

dhi

dt¼

_xs;i

Cð4Þ

where C is the catalytic surface site density.

3.2. Chemical kinetic models

The gas-phase reaction model employs USC Mech II [16]. Thereaction model was developed for H2/CO/C1–C4 hydrocarbon com-bustion through a series of kinetic modeling studies over the lastdecade [16,20–29]. The model is composed of 784 reactions and111 species, and has been validated over a variety of experimentsincluding ignition delays, laminar flame speeds, and species pro-files under the conditions relevant to the present flow reactorexperiments.

The surface reaction mechanism was largely derived from thework of Jackson and coworkers [9,30,31]. The resulting model isshown in Table 3. It consists of five reversible and seven irrevers-ible reactions with nine surface species. The total surface site den-sity C available for the catalytic reactions on a Pd particle was set

Table 3Surface reaction model.

No. Reactionb Rate parameters

A

1f O2 + 2Pd(S) ? 2O(S) 0.8c

1b 2O(S) ? O2 + 2Pd(S) 5.13 � 1021d

2f H2O + Pd(S) ? H2O(S) 0.75c

2b H2O(S) ? H2O + Pd(S) 1.00 � 1013

3f H(S) + O(S) ? OH(S) + Pd(S) 5.13 � 1021d

3b OH(S) + Pd(S) ? H(S) + O(S) 5.13 � 1021d

4f H(S) + OH(S) ? H2O(S) + Pd(S) 5.13 � 1021d

4b H2O(S) + Pd(S) ? H(S) + OH(S) 5.13 � 1021d

5f 2OH(S) ? H2O(S) + O(S) 5.13 � 1021d

5b H2O(S) + O(S) ? 2OH(S) 5.13 � 1021d

6 CO2(S) ? CO2 + Pd(S) 5.00 � 1010

7 CO(S) + O(S) ? CO2 + Pd(S) 5.13 � 1021d

8 C(S) + O(S) ? CO(S) + Pd(S) 5.13 � 1021d

9 CH4 + 2Pd(S) ? CH3(S) + H(S) 4.00 � 105c

10 CH3(S) + 3Pd(S) ? C(S) + 3H(S)f 5.13 � 1021d

11 CH4 + Pd(S) + O(S) ? CH3(S) + OH(S) 4.20 � 10�2c

12 CH3(S) + 3O(S) ? C(S) + 3OH(S)h 5.13 � 1021d

a Arrhenius parameters for the rate constants written in k = ATb exp (�E/RT). The unitb The surface coverage is specified as the site fraction h. Total site density of Pd is C =c Sticking coefficient.d The frequency factor was estimated from vibration frequency (1013 s�1) and scalede The activation energies are based on the DFT results of Cao and Chen [34].f The reaction order is 1 for Pd(S).g The sticking coefficient was taken from Sidwell et al. [15]. The activation energy wah The reaction order is 1 for O(s).

to C = 1.95 � 10�9 mol/cm2, i.e., the average of the literature valuesfor the Pd/PdO phases [9,15,30–32].

The sticking coefficient of O2 (R1f) and the surface coverage (hO)dependent energy of desorption (R1b) are based on the tempera-ture-programmed desorption (TPD) experiments [33]. These rateparameters values were used also by Wolf et al. [9] and Jacksonand coworkers [30,31]. The rate parameters of H2O adsorptionand desorption (R2f and R2b) were adopted from a reaction modelby Deutschmann et al. [32], who studied catalytic ignition ofhydrogen over a Pd catalyst in a stagnation flow burner. Cao andChen [34] reported a Density Functional Theory (DFT) calculationfor water adsorption on Pd(1 1 1). The critical energies adoptedin our model were taken from their DFT study.

The reactions and their rate parameters of R6–12 were takenmainly from Sidwell et al. [15]. The activation energy of dissocia-tive adsorption of methane on oxide surface sites (R11) was re-duced from 38 to 20 kJ/mol. This downward revision issupported by the theoretical result of Broclawik et al. [35,36].Although a thermodynamically consistent approach [37] may beemployed for assigning the surface reaction rate parameters, thiswas not attempted here because the exact mechanism for the sur-face reaction of CH4 remains quite uncertain.

4. Results and discussion

4.1. Reactor temperature profiles

Under the current experimental condition the flow inside of thereactor is laminar (568 P Re P 236). The resulting parabolic veloc-ity profile creates a radial gradient in the gas temperature. In theaxial direction, a temperature gradient also exists due to the finitetime needed to equilibrate the gas flow with the wall temperature.For this reason, the gas temperature was measured both at the cen-terline Tb,center and adjacent to the wall Tb,wall, in a pure N2 flow at arate equal to the ignition tests (see, mixture F-0 of Table 1). The re-sults are presented in Figs. 3 and 4, respectively. The uncertaintyfor the temperature measurement is estimated to be ±15 K. Asseen, the gas in the reactor undergoes heating in the first two-

a Reference/comments

b E

�0.5 [9,30,31,33]230–115hO [9,30,31,33]

[32]44 [32]94.5 e

113.8 e

31.1 e

88.8 e

14.5 e

32.8 e

29 [15]76 [15]62.8 [15]196c [15]85.1 [15]20c g

25.1 [15]

s of A are given in terms of mol, cm3/s. E is in kJ/mol.1.95 � 10�9 mol/cm2.

by the total site density C.

s deduced from Broclawik et al. [35,36].

400

600

800

1000

1200

0 10 20 30 40 50 60 70

Cen

terl

ine

tem

per

atu

re, T

b,c

ente

r (K

)

Distance from reactor inlet, x (cm)

Tf =673 K

773 K

873 K

973 K

1073 K

1173 K

1273 K

Fig. 3. Background, centerline temperature Tb,center measured in an N2 flow (F-0 inTable 1) at several furnace temperatures Tf. Symbols are experimental data andsolid lines are fitted to data. The dashed lines were obtained from extrapolating thedata from the measured data points for Tf = 1073 K and 1273 K.

400

600

800

1000

1200

0 10 20 30 40 50 60 70

Wal

l tem

per

atu

re, T

b,w

all(K

)

Distance from reactor inlet, x (cm)

Tf =673 K

773 K

873 K

973 K

1073 K

1173 K

1273 K

Fig. 4. Background, wall temperature profiles Tb,wall measured in a N2 flow (F-0 inTable 1) at several furnace temperatures Tf. Symbols are data and solid lines arefitted to data. The dashed lines were obtained from extrapolating the data frommeasured data points for Tf = 1173 K and 1273 K.

0

50

100

150

600 700 800 900 1000 1100 1200 1300

T fT g

,max

(K

)

Furnace temperature, Tf (K)

Centerline

Reactor Wall

Fig. 5. Difference between the furnace temperature (Tf) and the peak gas temper-ature Tg,max as a function of the furnace temperature Tf.

0.00

0.02

0.04

0.06

0.08

0.10

900 1000 1100

Mo

le f

ract

ion

Furnace temperature, Tf-1 (K)

Non-Catalytic Run 1

Catalytic Run 3

CatalyticRun 4

CH4

CO2

Fig. 6. CH4 and CO2 mole fractions observed for Runs 1 and 3 as a function of thefurnace temperature of Zone I, Tf-1.

426 T. Shimizu et al. / Combustion and Flame 157 (2010) 421–435

thirds of the reactor and cooling in the remaining section. As ex-pected, the centerline and wall temperature profiles can differfrom each other quite significantly, by as much as 200 K. Alongthe centerline the temperature rises more gradually than that closeto the wall; and it reaches its peak value further downstream thanthe temperature adjacent to the wall. The delay in temperature riseis certainly caused by the finite heating rate under the laminar flowcondition. In comparison, Fig. 4 shows that heating along the innerwall is more rapid and reaches the highest temperatures closer tothe inlet than along the centerline. The difference between the fur-nace temperature Tf and the peak gas temperature Tg,max is plottedas a function of Tf in Fig. 5. The maximum centerline temperatureTcenter,max never reached the actual furnace temperature because ofcooling towards the end of the reactor.

The inhomogeneous temperature makes it difficult to interpretthe data quantitatively. A realistic modeling approach would haveto simulate the problem as a two-dimensional, laminar reacting

flow problem. Because of the computational difficulties involved,this was not attempted in this study. Rather we used both the cen-terline and near-wall temperatures as the two limiting cases forone-dimensional numerical simulation.

4.2. Catalytic ignition

We first examine the test cases in which the furnace tempera-ture increased at 20 K/min (Runs 1–4). The furnace temperatures,Tf-1 and Tf-2, at the point of ignition are recorded and shown in Ta-ble 2. The experiments were found to be reproducible (cf. Runs 1and 2). The variations of CH4 and CO2 concentrations are presentedas a function of the furnace temperature in Fig. 6. It is seen that atthe point of ignition the CH4 mole fraction drops quite abruptlywhile that of CO2 rises accordingly. For the catalytic case, the riseof the CO2 mole fraction prior to the disappearance of CH4 wascaused by oxidation of toluene. This oxidation process is seen tobe gradual and non-explosive. Experimentally, we found that un-der noncatalytic conditions toluene itself did not affect methaneignition.

T. Shimizu et al. / Combustion and Flame 157 (2010) 421–435 427

The results demonstrate that methane ignites noncatalyticallyat around 1100 K furnace temperature, corresponding to a maxi-mum gas temperature of roughly 950 K along the centerline ofthe reactor. The use of catalyst generated in situ leads to a reduc-tion of the furnace temperature by about 150 K at the point of igni-tion, using a Pd loading of �0.025 cm�1 (catalyst surface area tototal gas volume), as shown in Table 2. Under this condition, theatomic Pd to CH4 ratio is around 4.5 � 10�4, corresponding toabout 3 mg Pd/(g CH4).

Runs 3 and 4 were designed to test the operation of the atom-izer. In Run 3 the atomizer was covered by ice packs, reducing itstemperature to �278 K. For Run 4 the atomizer was operated ata somewhat higher temperature (283 K). This small difference intemperature was sufficient to change the loading of aerosol by10%. The impact was captured by the experiment. With a highercatalyst loading (by about 10%), the furnace temperature at igni-tion was reduced by about 50 K, as shown in Table 2 and Fig. 6.

4.3. Particle size distribution

The particle size distribution functions (PSDFs) were deter-mined at the exit of the reactor. Fig. 7 presents several PSDFs ob-tained during Run 3. These samples were collected with adilution ratio of �10 and an aerosol transmission time of 0.6 s.Since the number density of the particles is of the order of108 cm�3 at the exit of the reactor, this density drops to 107 cm�3

after dilution. Assuming the coagulation rate constant to be thecollision limit (�10�9 cm3/s), we find the characteristic coagulationtime to be 1/(107 � 10�9) � 100 s. Hence, little to no coagulationlosses are expected for the aerosol sampled.

As seen the median diameter of the particles falls between 20and 30 nm. Without injecting the Pd(THD)2 precursor solution asan aerosol, the same measurement yields only a trace of particleswith a number density similar to the ambient background.Although the particle diameters are mobility diameters, theyshould be very close to the true diameters for particles in the ob-served size range [38–40]. Over the period of 20 min during which

0

1x109

2x109

10

dN/d

log(

Dp)

(#/c

m3)

Particle Diameter, Dp (nm)

Fig. 7. Selected particle size distributions observe

the furnace temperature was increased, the PSDFs remained al-most identical. This indicates that the precursor injection ratewas steady, and that the processes of aerosol evaporation and pre-cursor decomposition followed by particle nucleation and growthwere independent of the temperature for Tf > 800 K. It also sug-gests that these processes were complete or almost complete nearthe inlet region of the reactor and at temperatures substantiallybelow the peak temperature of the gas.

All of the PSDFs observed are roughly log-normal with mediandiameter as expected for particle size growth dominated by parti-cle–particle coagulation. Additionally, the tail towards the smallsize end of the distribution was also expected, and is indicativeof persistent particle nucleation, perhaps due to the residue Pd va-por in the flow tube. The small shoulder towards the large particlesize is probably indicative of the presence of aggregates. Thisbehavior in PSDFs was commonly seen in homogeneous sootnucleation and growth [41–48], and is not indicative of particlesproduced from aerosol drying or breakup. Given the mass fractionof Pd(THD)2 in toluene and the initial aerosol droplet size, the par-ticles would be about 100 nm in diameter if they were producedpurely from toluene evaporation. If the particles were producedfrom a combination of aerosol drying and breakup, the size distri-bution would be significantly broader than observed.

The homogeneous nucleation mechanism for particle formationis also supported by second order of coagulation kinetics. If westart with Pd(v) as a monomer at a concentration ofN0 = 2 � 1014 cm�3 , uniformly dispersed in the gaseous reactantmixture, (calculated from the initial Pd(THD)2 loading for Runs 3and 4) and assuming a unity sticking probability and a bimolecularcoagulation rate constant of b = 1 � 10�9 cm3/s, we estimate thatafter 1 s, the number density of Pd particle is

N � N0

1þ bN0t� 109 cm�3 ð5Þ

This is consistent with the observation shown in Fig. 8. In addi-tion, the particle diameter may be estimated from the number ofatoms in each particle (�2 � 105) and an assumed mass density

100800

850

900

950

1000

1050

Furn

ace

tem

pera

ture

, Tf-1

(K)

d for Run 3 at several furnace temperatures.

107

108

109

1010

00101

Growth Mode

N2 = 7.5x108 (cm-3)

σ2 = 1.53

<Dp>2 = 23 nm

Nucleation mode

N1 = 5.8x109 (cm-3)

σ1 = 1.34

<Dp>1 = 2.3 nm

Aggregates

N3 = 4.2x106 (cm-3)

σ3 = 1.36

<Dp>3 = 98 nm

dN

/dlo

gD

p (#

/cm

3 )

Particle diameter, Dp (nm)

Tf-1 = 911 K

Fig. 8. Representative particle size distributions (symbols) measured during Run 3.Lines are fitted into a tri-log-normal function whereas thinner lines indicate thethree separate particle size modes.

0

5

10

15

20

25

30

35

40

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

700 800 900 1000 1100

Med

ian

dia

met

er, <

Dp>

(n

m)

Are

a-to

-vo

lum

e ra

tio

, ζ(c

m-1

)

Furnace temperature, Tf-1 (K)

Ignition

Run 3

Run 4

Run 3Run 4Ignition

Fig. 9. Particle median diameter hDpi and area-to-volume ratio f (STP) measured atthe exit of the flow reactor during Run 3 and Run 4, as a function of the furnacetemperature of Zone I, Tf-1. Squares and circles represent Run 3 and Run 4,respectively.

428 T. Shimizu et al. / Combustion and Flame 157 (2010) 421–435

of 10 g/cm3. This gives a mean diameter of 20 nm, which is close tothe experimental observation (�25 nm). Of course, the initial non-uniform distribution of the aerosol could cause the local Pd(v) con-centration to exceed the initial concentration of 2 � 1014 cm�3.This would result in an effectively shortened coagulation time forparticle production.

The measured PSDFs can be fitted into a tri-log-normaldistribution,

dNd log Dp

¼X3

k¼1

Nkffiffiffiffiffiffiffi2pp

logrk

exp � ½logðDp=hDpikÞ�2

2ðlogrkÞ2

( )ð6Þ

where Dp is the particle diameter, Nk, hDpik and rk are the numberdensity, the median diameter, and the geometric standard deviationof the kth size mode, respectively. Fig. 8 presents a sample of the fitto data taken from Run 3. Aside from the fact that the PSDF param-eters are roughly invariant with respect to the furnace temperature(especially the dominant, second mode), the most important featurehere is that r2 = 1.53, which is slightly larger but close to that of aself-preserved distribution function of 1.46 [49]. Hence, the parti-cles measured are conclusively those from nucleation and growthout of the gas-phase precursors.

Fig. 10. TEM images of nanoparticles sampled in Run 8 (left) and Run 9 (right). The furnatwo runs look alike with particles of nearly identical size and dispersion.

As we discussed earlier, the PSDFs are not sensitive to the tem-perature for Tf > 800 K. Below that temperature, however, this isnot the case. Fig. 9 shows that below 800 K the median particlediameter hDpi rises to 40 nm in some cases, whereas the surfacearea-to-volume ratio f (integrated from the distribution function)remains relatively constant. The rise in the median diameter atlow temperatures was probably caused by coke formation or car-bon deposition, since the aerosol was injected into the reactor ina nitrogen stream and locally the precursor flow was extremelyfuel rich for a period of time. Meanwhile, toluene undergoes pyro-lysis either homogeneously or catalyzed by dissociation of thePd(THD)2 and Pd/PdO clusters formed thereafter. Regardless, theobserved variations in the median size below 800 K do not affectour interpretation of the experiment, since ignition was observedabove that temperature.

4.4. Particle composition

To confirm that the particles observed are palladium and toexamine the fine structure of these particles, we carried out TEManalysis on particles collected in Runs 8 and 9, in which the furnacetemperature was held constant and equal to 773 and 973 K,respectively. Fig. 10 shows the TEM images from the two runs.

ce temperature Tf was kept at Tf = 773 K and 973 K, respectively. The images for the

Fig. 11. HRTEM images of nanoparticles sampled in Run 8 (left) and Run 9 (right). The furnace temperature Tf was kept at Tf = 773 K and 973 K, respectively. Most particleswere found to be rich in Pd metal.

0.04

0.06

0.08

0.10

Mo

le f

ract

ion

CH4

T. Shimizu et al. / Combustion and Flame 157 (2010) 421–435 429

Both TEM grids were exposed to the exhaust flow for 6 min. Thetwo images were almost identical; an observation consistent withthe mobility size measurement. Most of the particles are smallerthan the median diameter hDpi observed by SMPS. This could bedue to the fact that the current particle collection mechanism relieson the particle diffusion towards the grid. The collection efficiencywould be, therefore, biased against larger particles. More detailedparticle structure and composition were probed by high resolutionTEM and XPS, as described in a separate paper [18]. Only key re-sults are summarized here. Fig. 11 shows HRTEM images collectedfor particles representative of the samples. The fact that the imagesshow lattice spacings that clearly correspond to the Pd(1 1 1) inter-plane spacing indicates that bulk of the particles is crystalline,metallic Pd. XPS analyses, however, show that an oxide coatingseveral atomic layers thick is present on the surface of the particlescollected in the 973 K experiment. In the 773 K experiment, theparticles have nearly identical bulk crystalline Pd structure, but al-most no surface oxide [18]. This observation suggests that oxidegrowth is kinetically controlled, and that formation of the oxidemay be a critical step in ignition, which occurs at 973 K, but not773 K. This observation is consistent with the literature on oxideformation for bulk Pd surfaces over much longer residence time[8,9], and also with the observation that PdO is more catalyticallyactive than Pd [2]. An appreciable amount of SiO2 or partially oxi-dized silicon was also detected in the particle sample [18]. Controlexperiments show that the source of the silicon contamination wasthe toluene solvent used to dissolve the Pd(THD)2 precursor. Be-cause silica is a stable, chemically inert oxide, its participation di-rectly in the catalysis process is quite unlikely. This is also becausethe Pd particles are relatively large. The presence of this silica con-tamination should not have much effect on the catalytic chemistryof the Pd particles. Indeed, noncatalytic runs with the toluene sol-vent only show the ignition temperature to be nearly identical tothat without toluene injection.

0.00

0.02

700 800 900 1000 1100 1200

Furnace temperature, Tf-1 (K)

CO2

Fig. 12. CH4 and CO2 mole fractions measured during Run 5 as a function of thefurnace temperature of Zone I, Tf-1. The wall of the reactor tube used for Run 5 wascontaminated by wall coating of condensed-phase material from Pd(THD)2

decomposition.

4.5. Possible wall effect

SMPS, TEM and XPS evidence all points to a mechanism of igni-tion induced by the catalytic activity of the Pd/PdO nanoparticlesproduced in situ from the decomposition of the Pd(THD)2/tolueneaerosol. An issue that remains to be addressed is the role of con-densed matter deposition on the wall and the subsequent wallcatalysis. As we discussed earlier, it is nearly impossible to elimi-nate wall catalysis completely due to the finite surface-to-volume

ratio of the reactor. Hence, our effort was directed at minimizingthe effect of wall catalysis.

The precursor atomizer was designed specifically for that pur-pose. Because of the use of multiple impactors and hence the rela-tively small Stokes’ number, the aerosol follows the flow closely;and the droplets have little possibility to impinge onto the reactorwall before they disintegrate. In addition, the unburned reactantgas (CH4/O2/N2) acts as a sheath surrounding the aerosol injectiontube at the inlet of the reactor. This design further prevents theaerosol from reaching the reactor wall, at least before coming incontact with the unburned gas. Finally, as nanoparticles begin toform in the gas flow, the fact that the wall temperatures are higherthan the centerline temperature means that thermophoretic ef-fects tend to drive particles away from the walls. Under ignitionconditions, heat release in the gas-phase may reverse this tendencyto some extent, but because the experiments always were scannedfrom non-reactive conditions toward ignition, thermophoresisshould help prevent catalyst deposition up to the ignition point,and thus the ignition temperatures or concentrations should notbe strongly affected by wall catalysis. Nonetheless, there still re-mains a minor possibility that the actual ignition is induced by cat-alytic reaction on the reactor wall.

430 T. Shimizu et al. / Combustion and Flame 157 (2010) 421–435

To examine the influence of catalysis on the reactor wall inde-pendently, an experiment (Run 5) was carried out without inject-ing the Pd(THD)2 precursor but using a quartz tube with a visiblecoating of catalyst from a prior experiment. Fig. 12 shows theCO2 and CH4 profiles as a function of the furnace temperature. Itis seen that the ‘‘contaminated” tube caused the methane molefraction to drop, starting at around Tf-1 = 730 K. It reached a plateauat around Tf-1 = 1000 K. Interestingly, the ignition occurs atTf-1 � 1180 K (Table 2), about 30 K higher than experiments con-ducted in clean, unused tubes (Runs 1 and 2).

We interpret the above observation as follows. In Run 5, wallcatalysis indeed occurs, leading to the formation of the plateausin the concentration profiles. Ignition at a furnace temperature�1180 K is caused by the homogeneous process. A higher ignitiontemperature is caused by a reduced methane concentration due tocatalytic conversion on the wall. Hence, it is clear that in this casewall catalysis actually retarded the ignition process. More impor-tantly, the catalytic ignition studies in Runs 3 and 4 (Fig. 6) showconstant CH4 concentration over the entire temperature range upto the point of ignition. The absence of any significant drop inCH4 concentration prior to ignition indicates that wall catalysiswas indeed suppressed.

4.6. Mechanism of particle formation and catalytic activity

Summarizing the above results, we may now advance a phe-nomenological description of the particle formation mechanismand its catalytic activities. The kinetic processes of catalyst particleformation may be described by the following processes:

1. 1–10 lm-diameter toluene droplets containing Pd(THD)2

vaporize, and the Pd(THD)2 decomposes, generating Pd vaporor Pdi clusters,

2. Pd(v) + Pd(v) ? Pd2,3. Pd(v) + Pd2 ? Pd3,4. Pd2 + Pd2 ? Pd4,

0

1x109

2x109

100

5

10

15

20

dN

/dlo

gD

p (

#/cm

3 )

Parti

Time

(ms)

0

1x109

2x109

100

5

10

15

20

dN

/dlo

gD

p (

#/cm

3 )

Parti

Time

(ms)

Fig. 13. Selected particle size distribution functions at various testing times measured atwhile the catalyst concentration was gradually increased, leading to an increase in the

5. . . .. . .,6. Pdi + Pdj ? Pdi+j.

This coalescence process leads to the formation of Pd nanopar-ticles, as observed in Figs. 7–11. The surface of the Pd particles be-comes oxidized, especially for temperatures above �973 K, leadingto enhanced catalytic activity, which ignites the gas mixture at re-duced temperature.

What remains unclear is why the oxide does not form at anearly stage of cluster and particle formation. The reason may bethermodynamic and/or kinetic in nature. It is possible that forsmall particles the formation of palladium oxides is not favorablethermodynamically. A more plausible cause for the lack of bulkPdO formation is the finite time required to mix the reactantsand diffuse oxygen to the surface. As the aerosol is injected intothe reactor and undergoes evaporation and decomposition, thisflow stream is not yet mixed locally with oxygen in the sheath.Hence the initial particle nucleation process takes place in spatialregions depleted of oxygen. If oxygen is present locally, it wouldhave been depleted by toluene decomposition and oxidation, asevidenced by the early rise of CO2 in Fig. 6. The burst of nucleationfollowed by coagulation causes the Pd(v) to condense onto existingparticle surface rapidly, before the particles have an opportunity tomix by diffusion with the outer sheath of oxygen. The result is aparticle with a Pd core that oxidizes slowly at low temperatures,and forms a real oxide layer only near or above �973 K, as con-firmed by TEM and XPS results. Although additional work at theelementary level is needed to confirm the above analysis, we arenow in a position to advance a chemical model to explain the igni-tion phenomenon observed.

4.7. Detailed kinetic modeling

To facilitate model comparison, Runs 6 and 7 were made, inwhich the furnace temperature was held constant. These runs werenecessary because we were able to measure the temperature pro-

100

cle diameter, <Dp> (nm)

100

cle diameter, <Dp> (nm)

Ignition

the exit of the flow reactor during Run 7. Furnace temperature Tf was kept at 973 Ksurface-to-volume ratio f. The mixture ignited 20 min after the start of the run.

10

15

20

0.00

0.01

0.02

0 5 10 15 20 25

Tf = 973 K

Med

ian

dia

met

er, <

Dp>

(n

m)

Are

a-to

-vo

lum

e ra

tio

, ζ(c

m-1

)

Test time (min)

Ignition

Fig. 14. Particle median diameter hDpi and area-to-volume ratio f measured at theexit of the flow reactor during Run 7. Round and rectangular symbols represent hDpiand f (STP), respectively.

T. Shimizu et al. / Combustion and Flame 157 (2010) 421–435 431

files along the reactor only under the condition of steady reactoroperation. Under the condition of transient furnace temperature,the rise in the gas temperature lags further behind as comparedto the temperature profiles obtained under steady operating condi-tion and shown in Figs. 3 and 4. For this reason, while the results ofRuns 3 and 4 are useful for examining the response of methaneignition to furnace temperature, they cannot be modeled withcertainty.

For background, noncatalytic ignition of methane (Run 6), weincrease the furnace temperature in 5 K steps. The reactor was al-lowed to equilibrate for 30 min at each furnace temperature, until

0

50

100

150

0 40 80 120 160 200 240 280 320

Time (ms)

H2

CH3OH

Mo

le F

ract

ion

(P

PM

)

0

5000

10000

15000

20000

CH4

CO

CO2

Mo

le F

ract

ion

(P

PM

)

0

200

400

600

800

1000

CH2O

C2H6

C2H4

Mo

le F

ract

ion

(P

PM

)

0 50

Tim

Fig. 15. Experimental (symbols [50]) and computed species profiles during homogene0.02CH4–0.205O2–0.775N2 at 6 atm and �953 K; middle panels: 0.02CH4–0.205O2–0.77�974 K.

ignition is observed. For the mixture composition given as F-1 inTable 1, the ignition was observed at Tf = 1113 K with an uncer-tainty of ±5 K. This temperature is about 35 K lower than that ofRuns 1 and 2, which is reasonable considering that the furnacetemperature is transient in those runs. In run 7, the furnace tem-perature was held fixed at 973 K. The Pd(THD)2 loading was in-creased gradually while the Pd particle size distribution wasmeasured continuously. Fig. 13 shows selected PSDFs data at vari-ous times over the course of the run. The median diameter ob-served continued to increase, as seen in Fig. 14, while the totalnumber density remained nearly the same over the 20-min exper-iment period. When the catalytic loading reached 1.3 � 10�2 cm�1,i.e., the highest loading shown in Fig. 13, ignition occurred. At thatpoint, the atomic Pd-to-CH4 ratio was 2 � 10�4 and the surfacearea-to-gas volume ratio f was 0.013 cm�1, as shown in Table 2.As before, the PSDFs are trimodal and the main mode is log-nor-mal. This set of the experiments was reproducible, and was usedas the basis of modeling studies in what follows.

To verify the accuracy of USC Mech II for gas-phase methaneoxidation under conditions comparable to the current flow reactorexperiments, we first report the simulation results and comparethem to the turbulent flow reactor data of Hunter et al. [50]. Inthose experiments, methane was highly diluted and oxidized inan air-like mixture at pressures of 6 and 10 atm. The temperaturealong the reactor axis was nearly constant and ranged from 950 to975 K. As shown in Fig. 15, major and minor species concentrationswere predicted by USC Mech II accurately.

Additional computational cases are summarized in Table 4.Simu-1 and -2 correspond to Run 6 and Simu-3–6 represent Run7. In these simulations, the measured background temperatures(Figs. 1 and 2) were used as a model input, i.e., the first term onthe right-hand side of Eq (3). Heat release is accounted for in thesecond term of that equation. Of course, this description is not

100 150 200

H2

CH3OH

e (ms)

CH4

CO

CO2

CH2O

C2H6

C2H4

0 50 100 150 200 250

H2

CH3OH

Time (ms)

CH4

CO

CO2

CH2O

C2H6 C2H4

ous, non-catalytic oxidation of methane in a turbulent flow reactor. Left panels:5N2 at 10 atm and �953 K; right panels: 0.021CH4–0.205O2–0.774N2 at 6 atm and

Table 4Summary of numerical simulations.

No. Ignition type BackgroundTemperature input

Flow conditionin Table 1

Computed furnacetemperature at ignition Tf-ig (K)

Difference frommeasureda (K)

Simu-1 Non-Catalytic Tb,center F-1 1127 +14Simu-2 Non-Catalytic Tb,wall F-1 1039 �74Simu-3 Catalyticd Tb,center F-2b 956 �17Simu-4 Catalyticd Tb,wall F-2b 880 �93Simu-5 Catalyticd Tb,center F-2c 960 �13Simu-6 Catalyticd Tb,wall F-2c 885 �88

a Run 6 and Run 7 in Table 2 are used for comparisons with non-catalytic and catalytic cases, respectively.b Toluene was converted to CO2 and H2O, i.e. cold gas velocity = 52.8 cm/s, mole fractions (CH4/O2/CO2/H2O/N2) = 0.083/0.356/0.047/0.027/0.486, and / = 0.47.c Toluene was included as a reactant.d hDpi = 19 nm and f = 0.013 cm�1 (STP), the ignition conditions in Run 7 of Table 2.

432 T. Shimizu et al. / Combustion and Flame 157 (2010) 421–435

mathematically rigorous since local heat release would affect thefirst term because of sensitivity of heat conduction and convectiontowards local temperature. This uncertainty, however, is expectedto be smaller than the non-uniform temperature in the reactor. Inthe simulations, we used both the measured centerline and near-wall temperatures as model input and treated them as the two lim-iting cases.

Table 4 shows that the predicted furnace temperature at whichthe ignition occurs is only 14 K higher than the experimental valuewhen the centerline temperature was used as a model input (also,see Fig. 16). Considering all of the experimental and model uncer-tainties, the close agreement is perhaps fortuitous, but encouragingnonetheless. If the near-wall temperature was used in the simula-tion, the furnace temperature of ignition was predicted to be 74 Klower than the observation. The results computed with the twolimiting temperatures bracket the experimental value, as expected.Considering that the wall quenching of the free radicals was notconsidered in either computational case, the current results areconsidered as more than acceptable. In what follows, only compu-tations made with the centerline temperature will be discussed,though computational results using the near-wall temperaturesare reported in Table 4 for all cases.

Fig. 16 shows the CH4 and CO2 mole fraction profiles computedas a function of the furnace temperature, comparing the non-cata-

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

800 900 1000 1100 1200

CH4

CO2

Mo

le f

ract

ion

Furnace temperature, Tf (K)

Non-CatalyticSimu-1

CatalyticExp. Run 7

Non-CatalyticExp. Run 6

CatalyticSimu-3

CatalyticSimu-5

Fig. 16. Computational results of CH4 and CO2 mole fractions at the reactor exit as afunction of furnace temperature Tf for both non-catalytic (Simu-1) and catalytic(Simu-3 and -5) cases. The model used the centerline temperature Tb,center as theinput. For Simu-3, toluene was assumed to be completely oxidized to CO2 and H2Obefore ignition, whereas for Simu-5 toluene was included as a reactant. For bothcatalytic cases, Pd particles with hDpi = 19 nm and f = 0.013 cm�1 (STP) wereassumed.

lytic case (Simu-1) with the catalytic cases (Simu-3 and -5). Alsoshown in the figure are the corresponding experimental values.As seen, the computed temperatures are within 20 K of the exper-imental values. Under comparable conditions, the model predicts areduction of 171 K in the furnace temperature at a catalyst loadingof 2 � 10�4 (atomic Pd-to-CH4 ratio) and a corresponding surfacearea-to-gas volume ratio of 0.013 cm�1, whereas the experimentalvalues give 140 K. The simulation results are expected to be sensi-tive to the local temperature inside the flow reactor, but since thistemperature is not a constant, it was not easy to quantify thissensitivity.

Because toluene is used as the solvent for Pd(THD)2, its impacton the gas-phase reaction kinetics is considered in the simulation.USC Mech II predicts the laminar flame speed and ignition delay oftoluene-oxidizer mixtures reasonably well [51]. However, themodel includes a limited number of reactions for toluene oxidationunder the current experimental conditions. Hence, the simulationwas conducted by assuming all of the carbon in toluene was con-verted to CO2 (Simu-3) and by assuming all of the toluene remainsas the reactant at the onset of the reaction (Simu-5). Unlike theexperimental observation, Simu-5 did not yield noticeable tolueneconsumption prior to ignition, as seen in Fig. 16. The reason, ofcourse, is gas-phase reactions and/or catalytic processes not ac-counted for by the model. On the other hand, the computed igni-tion temperature is not at all sensitive to the toluene chemistry,because Simu-3 and 5 yielded basically the same results. Overall,the combined gas-phase and gas-surface model predicts the exper-imental results well.

4.8. Mechanism of methane ignition catalyzed by Pd particles

Analysis of the computational results indicates that the ignitionprocess involves both thermal and radical runaway. Fig. 17 pre-sents the evolution of gas-phase species concentration and catalystsurface coverage just before ignition (Simu-3) against the axial po-sition of the flow reactor and the reaction time. The model predictsthe surface to be nearly saturated by O(S), in agreement with theXPS results at 973 K. For the reactor conditions used in Simu-3,the concentration of O(S) was found to be independent of the ini-tial surface composition. Simulations by assuming the initial Pdsurface being fully Pd(S) or fully saturated by O(S) yielded virtuallyidentical ignition temperatures. Additional tests were made byintroducing the Pd particles at different reaction times. The resultsshow that as long as the particles were ‘‘injected” computationallyin the first half of the reactor, the computed ignition temperatureremains the same.

As seen in Fig. 17, the initial rise in temperature is caused byheating of the gas by the furnace until about 50 cm from the inlet.The subsequent rise in temperature is largely the result of heat re-lease due to catalytic reactions. This may be seen in the top panel

0.0

0.1

0.2

0.3

0.4

200

400

600

800

1000

1200

Residence time, t (sec)

Mo

le f

ract

ion

Tcenter

Tb,center

O2

CH4

CO2H2O

Gas

tem

per

atu

re,

T (

K)

0.00 0.15 0.27 0.36 0.44 0.51 0.58 0.64

10-12

10-10

10-8

10-6

10-4

HO

OH

H2

HO2

CO

CH2O

Mo

le f

ract

ion

10-9

10-7

10-5

10-3

10-1

0.96

0.97

0.98

0.99

1.00

0 10 20 30 40 50 60 70Distance from reactor inlet, x (cm)

Su

rfac

e si

te f

ract

ion

, θθ

O(S

) si

te f

ract

ion

, θ ΟΟ

O(S)

OH(S)

CO(S)

H2O(S)

C(S)

Pd(S)

CH3(S)

H(S)

Fig. 17. Gas-phase species mole fraction, surface site fraction h, reacting centerline temperature Tcenter and non-reacting, background centerline Tb,center as a function of thedistance from reactor inlet and residence time, computed for the furnace temperature Tf = 954.7 K (Simu-3: hDpi = 19 nm and f = 0.013 cm�1 (STP)).

T. Shimizu et al. / Combustion and Flame 157 (2010) 421–435 433

of Fig. 17 by comparing the centerline temperature profiles be-tween the background and catalytic oxidation. From 50 to 60 cm,almost all CH4 consumption and CO2 production is the result ofcatalysis. At the same time, the reaction becomes autocatalytic,in that the temperature rise causes an increase in the catalyticreaction rate. At about 70 cm from the inlet, the temperature isbrought to �1000 K, at which point the increased desorption ofoxygen leads to a rapid rise in other surface sites and the overallreaction rate. The temperature now increases rapidly. Above1000 K, gas-phase reaction starts to play a role, as evidenced bythe rapid rise in the gas-phase H, O, OH, and HO2 concentrations.

To understand the impact of surface reactions on ignition, wecarried out sensitivity analysis for the simulation discussed above.The sensitivity coefficient defined as �DTf-ig/hTf-igi, where DTf-ig isthe change in the furnace temperature of ignition resulting fromperturbing the pre-factors by a factor of 2, and hTf-igi is an averagebefore and after perturbation. The results are shown in Fig. 18. It is

seen that the key steps are adsorption and desorption of O2 (R1)and the oxidative dissociation of CH4 on a vacant site (R11). Theseresults are in agreement with what Fujimoto et al. [13] reported forCH4 oxidation from 500 to 800 K over PdO surfaces. The overall cat-alytic reaction rate appears to be limited by this reversible adsorp-tion and desorption process, which controls the density of vacantPd sites needed for CH4 oxidative dissociation on catalyst surfaces.Water or OH poisoning is predicted to play a minor role under theconditions studied. Around 900 K, the major pathway for waterformation is the surface metathesis reaction between two surfaceOH sites.

The numerical results reported in Fig. 17 are complicated by thevariable centerline temperature. To better understand the ignitionmechanism, we carried out calculations for stoichiometric meth-ane oxidation in air under the adiabatic and isobaric conditionsat an initial temperature of 900 K and 1 atm pressure. The com-puted temperature–time histories are shown in Fig. 19. The calcu-

-0.06 -0.04 -0.02 0.00 0.02 0.04 0.06

R1f

R1b

R2f

R2b

R5f

R5b

R11

Sensitivity coefficient, -ΔTf-ig/<Tf-ig>

O2+2Pd(S) 2O(S)

O2+2Pd(S)2O(S)

H2O+Pd(S) H2O(S)

H2O(S) H2O+Pd(S)

2OH(S) H2O(S)+O(S)

H2O(S)+O(S) 2OH(S)

CH4+Pd(S)+O(S)

CH3(S)+OH(S)

Fig. 18. Sensitivity coefficients, �DTf-ig/hTf-igi, computed for Simu-3. A positivesensitivity coefficient indicates lowered ignition temperature. Reactions with noise-level sensitivities are omitted.

10-3

10-2

10-1

100

101

102

103

600 800 1000 1200 1400 1600

Igni

tion

dela

y tim

e, τ

(sec

)

Initial temperature, T0 (K)

Catalytic

Non-Catalytic

p = 1 atm

Fig. 20. Ignition delay time computed as a function of initial temperature of astoichiometric methane/air mixture under adiabatic and isobaric conditions. Thecatalytic case used hDpi = 19 nm and f = 0.013 cm�1 (STP).

434 T. Shimizu et al. / Combustion and Flame 157 (2010) 421–435

lation for catalytic oxidation used typical Pd loading and size, i.e.,hDpi = 19 nm and the surface-to-volume ratio equal to 0.013 cm�1

(STP). Under this Pd loading, the ignition delay time is shortenedby about two orders of magnitude compared to homogeneous,non-catalytic methane oxidation. For the catalytic case, analysisshows that the initial temperature rise at around 10�2 s is entirelycaused by heat release from catalytic reactions. Interestingly thetemperature levels off at around 1400 K just before it takes offagain some 10�2 s later. This behavior is explored in detail, asshown in the inset of Fig. 19. Without considering gas-phase reac-tions, the predicted temperature plateaus at around 1400 K, far be-low the adiabatic flame temperature. In other words, the catalyticreaction shuts itself off at that temperature because surfacedesorption of O2 becomes so rapid that the catalytic activity is lost.Yet because the catalytic reactions had brought the gas tempera-ture to �1400 K, close to the cross-over temperature of H + O2

800

1000

1200

1400

1600

1800

2000

2200

10-4 10-3 10-2 10-1 100

Tem

per

atu

re, T

(K

)

Time, t (sec)

Catalytic

Non-Catalytic

Heat release due to catalytic oxidation

Ignitiondue togas-phaseoxidation1000

1200

1400

1600

1800

0.02 0.03

T (K

)

t

Fig. 19. Temperature–time histories computed for a stoichiometric methane/airmixture under adiabatic and isobaric conditions, showing the difference in ignitiononset between catalytic and noncatalytic ignition of the mixture with initialtemperature T0 = 900 K and pressure p = 1 atm. The catalytic case used a medianparticle diameter hDpi = 19 nm and the surface-to-volume ratio f = 0.013 cm�1

(STP). The inset also shows the temperature history computed without consideringgas-phase reactions.

chain branching and termination under that pressure, homoge-neous gas-phase reactions become active. Eventually ignition oc-curs because of both gas-phase thermal and radical runaways.Hence, the present analysis suggests a two-step process to ignition.That is, catalytic reactions result in initial heat release and raise thegas temperature. Although the temperature rise would shut off thecatalytic activity at some point, gas-phase radical chain branchingtakes over eventually and completes the ignition process.

To assess the influence of initial temperature on the ignition de-lay time, we extend the above calculation to a range of 700–1500 K. Fig. 20 shows that under the level of Pd loading discussedearlier, the ignition delay time is shortened by two orders of mag-nitude from 700 to about 1200 K. When the initial temperature isat least 1400 K, the catalytic and noncatalytic ignition delay isidentical because the catalyst activity is suppressed for reasons al-ready discussed and also because gas-phase reaction kinetics tendto dominate at high temperatures. Over the range of temperatureshown in Fig. 20 and up to 40 atm pressure (results not shownhere), the ignition mechanism remains the same. That is, we ob-serve a sequential, two-step process in which catalytic reactiondominates initially, but gas-phase reactions take over after cata-lytic processes are deactivated by the rising temperature.

5. Conclusions

Freely suspended palladium nanoparticles were generatedin situ in a laminar flow reactor from an aerosol composed ofPd(THD)2 diluted in toluene. Mixing the aerosol with a stream ofmethane, oxygen, and nitrogen (equivalence ratio / � 0.5) undera heated condition can lead to a notable reduction in the ignitiontemperature of the unburned mixture, compared to homogeneousmethane oxidation without palladium particles. Measurement ofthe particle size distribution shows that the median particle diam-eter is around 20 nm, and the surface-to-gas volume fraction is�0.02 cm�1 (STP). The observed size distribution is consistent witha particle formation mechanism dominated by homogeneousnucleation and particle size growth by coagulation. TEM/XPS anal-ysis shows that the particles have a crystalline palladium core anda few atomic layers of PdO on the particle surface for particles ob-served near or above 973 K furnace temperature, but very littleoxide was formed at 773 K. Coupled with the observation that cat-

T. Shimizu et al. / Combustion and Flame 157 (2010) 421–435 435

alytic ignition occurred near 973 K, Pd oxidation kinetics must playa critical role in the catalytic process.

Numerical simulation using a detailed, combined gas-phase andgas-surface reaction model shows that the experimental results arewell captured by the model. Simulation results suggested that thesurfaces of Pd nanoparticles give rise to catalytic reactions at atemperature which would otherwise not lead to substantial gas-phase reactions. The heat release resulting from catalysis eventu-ally brings the gas to a temperature where the catalytic reactionis overtaken by the gas-phase reactions, which eventually leadsto thermal and radical runaway and fuel ignition.

The key surface reaction steps were found to be the reversibleoxygen adsorption/desorption, which governed the vacant Pd sitedensity and the overall rate of the catalytic reaction. Methane oxi-dative dissociation on a vacant Pd site is the rate-limiting step forcatalytic conversion of methane to CO and hence the heat release.

Acknowledgements

The Utah and USC groups gratefully acknowledge support forthis work from the Air Force Office of Scientific Research throughthe MURI program (FA9550-08-1-0400). The Utah, USC, TDA, andReaction Systems efforts were also supported by AFOSR STTR con-tracts (FA8650-06-C-2673 and FA9550-07-0106) to TDA, with sub-contracts to Utah and USC. In addition, the authors thank Mr. BrianWindecker of TDA Research for design and assembly of the catalysttest apparatus.

References

[1] M.J. Lewis, J. Propul. Power 17 (2001) 1214–1221.[2] D. Ciuparu, M.R. Lyubovsky, E. Altman, L.D. Pfefferle, A. Datye, Catal. Rev. 44

(2002) 593–649.[3] D. Ciuparu, L. Pfefferle, Appl. Catal. A 209 (2001) 415–428.[4] K. Persson, L.D. Pfefferle, W. Schwartz, A. Ersson, S.G. Jaras, Appl. Catal. B 74

(2007) 242–250.[5] Y.K. Rao, Stoichiometry and Thermodynamics of Metallurgical Processes,

Cambridge Press, Cambridge, 1985.[6] G. Zheng, E.I. Altman, Surf. Sci. 462 (2000) 151–168.[7] T. Schalow, B. Brandt, M. Laurin, S. Schauermann, S. Guimond, H. Kuhlenbeck, J.

Libuda, H.-J. Freund, Surf. Sci. 600 (2006) 2528–2542.[8] A.K. Datye, J. Bravo, T.R. Nelson, P. Atanasova, M. Lyubovsky, L. Pfefferle, Appl.

Catal. A 198 (2000) 179–196.[9] M.M. Wolf, H.Y. Zhu, W.H. Green, G.S. Jackson, Appl. Catal. A: Gen. 244 (2003)

323–340.[10] Å. Johansson, M. Försth, A. Rosén, Surf. Sci. 529 (2003) 247–266.[11] J.C.G. Andrae, A. Johansson, P. Bjornbom, A. Rosen, Surf. Sci. 563 (2004) 145–

158.[12] S.L. Tait, Z. Dohnalek, C.T. Campbell, B.D. Kay, Surf. Sci. 591 (2005) 90–107.[13] K. Fujimoto, F.H. Ribeiro, M. Avalos-Borja, E. Iglesia, J. Catal. 179 (1998) 431–

442.[14] K. Sekizawa, M. Machida, K. Eguchi, H. Arai, J. Catal. 142 (1993) 655–663.[15] R.W. Sidwell, H.Y. Zhu, R.J. Kee, D.T. Wickham, C. Schell, G.S. Jackson, Proc.

Combust. Inst. 29 (2002) 1013–1020.

[16] H. Wang, X. You, A.V. Joshi, S.G. Davis, A. Laskin, F. Egolfopoulos, C.K. Law,High-Temperature Combustion Reaction Model of H2/CO/C1–C4 Compounds,2007, <http://ignis.usc.edu/USC_Mech_II.htm>.

[17] M. Lashdaf, H. Hatanpaa, M. Tiitta, J. Therm. Anal. Calorim. 64 (2001) 1171–1182.

[18] B. Van Devener, S.L. Anderson, T. Shimizu, H. Wang, J. Nabity, J. Engel, J. Yu, D.Wickham, S. Williams, J. Phys. Chem. A, submitted for publication.

[19] M.E. Coltrin, R.J. Kee, F.M. Rupley, Int. J. Chem. Kinet. 23 (2004) 1111–1128.[20] S.G. Davis, C.K. Law, H. Wang, Combust. Flame 119 (1999) 375–399.[21] A. Laskin, H. Wang, Chem. Phys. Lett. 303 (1999) 43–49.[22] A. Laskin, H. Wang, C.K. Law, Int. J. Chem. Kinet. 32 (2000) 589–614.[23] Z. Qin, V. Lissianski, H. Yang, W.C. Gardiner Jr., S.G. Davis, H. Wang, Proc.

Combust. Inst. 28 (2000) 1663–1669.[24] H. Wang, Int. J. Chem. Kinet. 33 (2001) 698–706.[25] C.K. Law, C.J. Sung, H. Wang, T.F. Lu, AIAA J. 41 (2003) 1629–1646.[26] S.G. Davis, A.V. Joshi, H. Wang, F. Egolfopoulos, Proc. Combust. Inst. 30 (2005)

1283–1292.[27] A.V. Joshi, H. Wang, Int. J. Chem. Kinet. 38 (2006) 57–73.[28] X.Q. You, H. Wang, E. Goos, C.J. Sung, S.J. Klippenstein, J. Phys. Chem. A 111

(2007) 4031–4042.[29] S.G. Davis, C.K. Law, H. Wang, J. Phys. Chem. A 103 (1999) 5889–5899.[30] S.-A. Seyed-Reihani, G.S. Jackson, Chem. Eng. Sci. 59 (2004) 5937–5948.[31] J.F. Kramer, S.-A.S. Reihani, G.S. Jackson, Proc. Combust. Inst. 29 (2002) 989–

996.[32] O. Deutschmann, R. Schmidt, F. Behrendt, J. Warnatz, Proc. Combust. Inst. 26

(1996) 1747–1754.[33] K. Yagi, D. Sekiba, H. Fukutani, Surf. Sci. 442 (1999) 307–317.[34] Y. Cao, Z.-X. Chen, Surf. Sci. 600 (2006) 4572–4583.[35] E. Broclawik, J. Haber, A. Endou, A. Stirling, R. Yamauchi, M. Kubo, A. Miyamoto,

J. Mol. Catal. A 119 (1997) 35–44.[36] E. Broclawik, R. Yamauchi, A. Endou, M. Kubo, A. Miyamoto, J. Chem. Phys. 104

(1996) 4098–4104.[37] A.B. Mhadeshwar, H. Wang, D.G. Vlachos, J. Phys. Chem. B 107 (2003) 12721–

12733.[38] Z.G. Li, H. Wang, Phys. Rev. E 68 (2003) 061206.[39] Z.G. Li, H. Wang, Phys. Rev. E 68 (2003) 061207.[40] H. Wang, Transport Properties of Small Spherical Particles, in: S. S. Sadhal,

(Ed.), Interdisciplinary Transport Phenomena V: Fluid, Thermal, Biological,Materials & Space Sciences, 2008.

[41] B. Zhao, Z.W. Yang, Z.G. Li, M.V. Johnston, H. Wang, Proc. Combust. Inst. 30(2005) 1441–1448.

[42] B. Zhao, Z. Yang, M.V. Johnston, H. Wang, A.S. Wexler, M. Balthasar, M. Kraft,Combust. Flame 133 (2003) 173–188.

[43] B. Zhao, Z.W. Yang, J.J. Wang, M.V. Johnston, H. Wang, Aerosol. Sci. Technol. 37(2003) 611–620.

[44] J. Singh, R.I.A. Patterson, M. Kraft, H. Wang, Combust. Flame 145 (2006) 117–127.

[45] B. Zhao, K. Uchikawa, H. Wang, Proc. Combust. Inst. 31 (2007) 851–860.[46] A.D. Abid, N. Heinz, E. Tolmachoff, D. Phares, C. Campbell, H. Wang, Combust.

Flame 154 (2008) 775–788.[47] A.D. Abid, E.D. Tolmachoff, D.J. Phares, H. Wang, Y. Liu, A. Laskin, Proc.

Combust. Inst. 32 (2009) 681–688.[48] H. Wang, A.D. Abid, Size distribution and chemical composition measurements

of nascent soot formed in premixed ethylene flames, in: H. Bockhorn, A.D’Anna, A.F. Sarofim, H. Wang (Eds.), Combustion Generated FineCarbonaceous Particles – Proceedings of an International Workshop held inVilla Orlandi, Anacapri (May 13–16, 2007), Karlsruhe University Press,Karlsruhe, 2009, pp. 367–384.

[49] J.D. Landgrebe, S.E. Pratsinis, Ind. Eng. Chem. Res. 28 (1989) 1474–1481.[50] T.B. Hunter, H. Wang, T.A. Litzinger, M. Frenklach, Combust. Flame 97 (1994)

201–224.[51] Z.M. Djurisic, A.V. Joshi, H. Wang, Detailed kinetic modeling of benzene and

toluene combustion, in: The Second Joint Meeting of the US Sections of theCombustion Institute, Berkeley, CA, March 2001 (paper 238).