Catalytic autoignition of higher alkane partial oxidation ... · PDF fileCatalytic autoignition of higher alkane partial oxidation on Rh ... Received 23 May 2005; ... the ignition

Catalytic autoignition of higher alkane partial oxidation on Rh-coated foams

Kenneth A. Williams, Lanny D. Schmidt *

Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455-0132, USA

Received 23 May 2005; received in revised form 11 September 2005; accepted 29 September 2005

Available online 17 November 2005

Abstract

Through online mass spectrometry it is demonstrated that steady-state production of syngas (CO and H2) can be attained within 5 s after

admitting large alkanes (i-octane, n-octane, n-decane, or n-hexadecane) and air into a short-contact-time reactor by using an automotive fuel

injector and initially preheating the Rh-coated catalyst above the respective catalytic autoignition temperature for each fuel. Minimum catalytic

autoignition temperatures on Rh were �260 8C for n-octane and 240 8C for i-octane and n-decane. In contrast, catalytic autoignition of n-

hexadecane indirectly occurred at temperatures (>220 8C) lower than those of the other fuels investigated because of exothermic homogeneous

chemistry that preheated the catalyst (30–60 8C) to a temperature (�280 8C) sufficient for surface lightoff.

Additionally, the ignition kinetics for the large alkanes were determined and compared with those of methane. The step controlling surface

ignition possessed an apparent activation energy of �78 kJ/mol that was not significantly different between fuels ( p > 0.05). However, a

significant difference was found between the ignition preexponential for methane, O(104 s�1), and the other large alkanes, O(106 �1). The

dominant energetic step for large alkane surface ignition is hypothesized to be oxygen desorption at saturation coverage as has been suggested for

methane.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Reaction kinetics; Transient experiments; Partial oxidation; Rhodium catalyst; Catalysis; Liquid fuels; Alkanes; Surface ignition; Start-up; Lightoff

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Applied Catalysis A: General 299 (2006) 30–45

1. Introduction

Catalytic reforming of heavy hydrocarbon fuels (e.g.,

gasoline, diesel, or jet fuel) to produce a hydrogen-rich

reformate has generated great interest for NOx abatement in

diesel engines and electricity production in fuel cells.

Transportation fuels are attractive because of their high energy

density and widespread distribution infrastructure. While

traditional steam reforming (SR) may be suitable for fixed

power applications, its slow start-up and endothermic operation

are not attractive for efficient mobile applications. Catalytic

partial oxidation (CPO) at short contact times, another route to

syngas production, can be carried out at millisecond contact

times on Rh-based catalysts with greater than 90% fuel

conversion and over 80% hydrogen selectivity for large alkanes

(n-octane, i-octane, n-decane, and n-hexadecane) and diesel

fuel [1,2]. Autothermal reforming (ATR), which combines CPO

and SR, has been studied for many of the aliphatic (i-octane, n-

decane, n-dodecane, and n-hexadecane) and aromatic (hexene

* Corresponding author. Tel.: +1 612 625 9391; fax: +1 612 626 7246.

E-mail address: [email protected] (L.D. Schmidt).

0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2005.09.039

and toluene) components of transportation fuels as well as

combinations thereof [3–6]. Besides improved start-up and

transient performance, thermodynamic analyses have shown

CPO and ATR can be more efficient than SR alone for

reforming applications [7], leading most mobile application

developers to concentrate on ATR.

Multiple criteria have been deemed crucial for the efficient

start-up of mobile fuel processors including start-up time as

well as energy usage and carbon formation during start-up [8].

These issues are related to the lightoff temperature of the

catalyst and the fuel used. Multiple strategies have been devised

to reduce start-up time including heating the catalyst

electrically [9] and using homogeneous combustion to heat

the catalyst from room temperature to lightoff in seconds,

which has been demonstrated for alkanes from methane to i-

octane [10–12]. The latter technique has been shown to effect

steady-state production of syngas within 10 s from start-up

[13]. While this technique produces a rich hydrogen stream that

is suitable for a solid oxide fuel cell, the reformate’s high CO

content is not suitable for a polymer electrode membrane

(PEM) fuel cell. Research is ongoing to produce a PEM fuel-

cell grade hydrogen stream in a mobile reformer within 30 s

from start-up [14,15]. Fuel effects on start-up energy and

K.A. Williams, L.D. Schmidt / Applied Catalysis A: General 299 (2006) 30–45 31

Nomenclature

Ai,D preexponential factor for desorption of species i

(molecules/site/s)

E activation energy (J/mol)

Eign activation energy for ignition (J/mol)

k apparent first-order rate constant (s�1)

kapp,o apparent preexponential (s�1)

ki,A rate of adsorption of species i at zero coverage

(molecules/m2/s)

ki,D rate coefficient for desorption of species i (mole-

cules/m2/s)

ko preexponential (s�1)

ni,A order of adsorption for species i

ni,D order of desorption for species i

NAv Avogadro constant (6.02214 � 1023 molecules/

mol)

p statistical probability used in significance testing

P pressure (Pa)

rapp apparent rate (mol/m3/s)

ri,A rate of adsorption for species i (molecules/m2/s)

ri,D rate of desorption for species i (molecules/m2/s)

R ideal gas constant (8.314151 J/mol/K)

Si,o zero coverage sticking coefficient for species i

Sv specific surface area (m2/m3)

t time (s)

tidt ignition delay time (s)

tidt,10% ignition delay time based on 10% O2 conversion

(s)

tidt,25% ignition delay time based on 25% O2 conversion

(s)

T temperature (K)

Tai,min minimum gas-phase autoignition temperature

(K)

Tb normal boiling point (K)

Tbf back face catalyst temperature (K)

Tcai,min minimum catalytic autoignition temperature (K)

Tff front face catalyst temperature (K)

Ti initial preheat temperature (K)

Tign,min minimum ignition temperature (K)

TL/O lightoff temperature (K)

Tsat saturation temperature (K)

Wi molar mass of species i (kg/mol)

Greek letters

a statistical significance level

g ratio of oxygen to fuel in gas-phase

G active site density of Rh catalyst

(1.64 � 1019 sites/m2)

ui site coverage of species i

s stoichiometric ratio of oxygen to fuel for surface

reaction during ignition

tgas characteristic time for gas-phase ignition (s)

tsurf characteristic time for surface ignition (s)

Subscripts

F fuel

O oxygen

surf surface

V vacant

reactor volume have been investigated for a number of liquid

fuels and methane [16]. Use of an upstream cool-flame mode

to vaporize and mix liquid fuels with air and water to lower

external preheat requirements has also been proposed [17–

19]. Micro-scale reactor experiments with an ATR feedstock

(fuel/air/water) and a Pt/Rh catalyst indicate that aliphatic

hydrocarbons possess lower lightoff temperatures than

aromatic hydrocarbons, and short-chain aliphatics have

lower lightoff temperatures than longer chain aliphatics

[8]. Lightoff tests for both alkane and aromatic fuels with

and without water co-feed reveal the production of some

surface carbon (0.5–3% of carbon fed in the first 30 s of start-

up) [8].

To reduce start-up time, it may be favorable to start the fuel

processor in CPO mode (exothermic) and then add in water to

transition to ATR mode (thermally neutral) [15]. However, in

order to partially oxidize transportation fuels (which contain a

large percentage of higher alkanes), the fuels must be

vaporized and mixed with air before the catalyst while

avoiding homogeneous autoignition. For normal alkanes, the

minimum homogeneous autoignition temperature (Tai,min)

decreases asymptotically from �600 8C for methane to

�200 8C for alkanes larger than undecane (C11H24), which

is lower than the normal boiling point (Tb) for fuels larger than

undecane (Fig. 1). Saturation temperatures for combustion (C/

O = variable) and fuel reforming stoichiometries (C/O = 1 and

2) indicate that, given enough time (equilibrium time), fuel can

be vaporized and mixed with air belowTai,min. However, during

start-up the mixing time is constrained to be extremely short. In

the absence of exothermic homogeneous chemistry, subse-

quent surface chemistry requires the catalyst to be initially

heated near the heterogeneous lightoff temperature (TL/O,surf).

On Rh, avoiding homogeneous chemistry with Rh presents a

challenge for alkanes larger than hexane since their TL/O,surf are

higher than Tai,min; this phenomenon implies that the ignition

bifurcation diagrams for higher alkane gas-phase and surface

chemistry overlap, a situation not found for lower alkanes such

as methane and ethane [20–22]. To avoid homogeneous

ignition before the catalyst, the fuel must be vaporized and

mixed with air on a time scale shorter than the gas-phase

ignition delay time.

To better understand the surface processes at work during

higher alkane CPO, transient studies of catalyst performance

(such as during lightoff) must complement steady-state

characterization. Although global mechanisms have been

proposed for the steady-state high temperature surface kinetics

of higher alkanes and aromatics [3,4,23,24], a significant

knowledge gap exists between the surface mechanism of higher

alkane versus small alkane CPO on noble metals, especially

during ignition. For example, while Pt and Rh mean-field

surface chemistry and the accompanying interactions between

K.A. Williams, L.D. Schmidt / Applied Catalysis A: General 299 (2006) 30–4532

Fig. 1. Temperatures pertinent to successful vaporization and mixing of C1–C16

alkanes with air and alkane CPO ignition. Normal boiling points [53] for

alkanes monotonically rise from �162 8C (methane) to 281 8C (hexadecane).

However, for alkanes that are liquids at standard conditions the saturation

temperatures (Tsat) needed to maintain C/O ratios between combustion and C/

O = 2.0 in air are much lower than the corresponding boiling points. Minimum

autoignition temperatures for gas-phase chemistry asymptotically decrease

from near 600 8C (methane) to �200 8C (hexadecane) [54,55]. Surface ignition

temperatures for alkanes on Pt (C/O = 1.0) range from �430 8C (methane) to

175 8C (butane) [20,22]; on Rh these temperatures asymptotically decrease

from 430 8C (methane) to 250 8C (hexane) [31,22,1,11].

Fig. 2. Schematic of fuel-injected short-contact-time reactor. Permanent gases

and liquid fuels were delivered to the top of the reactor and entered the catalyst,

while products left the catalyst and entered a back heat shield at 21.5 cm.

Effluent gases flowed out of the reactor into heated stainless steel tubing for

either analysis with the mass spectrometer system or incineration. Temperature

was measured with thermocouples at the catalyst front (Tff) and back face (Tbf),

and the external side of the reactor was wrapped with ceramic blanket insulation

to attenuate radial heat losses.

surface and gas-phase chemistry are known relatively well for

methane [25–31] and ethane [32–34], no analogous treatment

has been proposed for higher alkanes.

In this work, the start-up and ignition behaviors of higher

alkane fuels (2,2,4-trimethyl-pentane also known as i-octane,

n-octane, n-decane, and n-hexadecane) on Rh-coated foams

were investigated under CPO conditions using online mass

spectrometry to better understand the chemistry governing

lightoff. In particular, the minimum surface autoignition

temperature as well as the ignition delay time as a function

of initial surface preheat temperature were determined for each

fuel in a near adiabatic reactor. From this analysis, the kinetic

parameters governing lightoff for each liquid fuel were

determined for the first time to the best of our knowledge

and compared with those of methane. Additionally, the effect of

carbon surface coverage on lightoff and the roles of gas-phase

and surface chemistry during start-up were studied.

2. Materials and methods

2.1. Experimental apparatus

High-purity reactant gases (O2 and Ar) were fed through

calibrated mass flow controllers, premixed through 0.64 cm

inside diameter (i.d.) stainless steel tubing, and delivered to the

top of the reactor through a quartz endcap with a 0.64 cm i.d.

side port at �108 kPa. The reactor consisted of a quartz tube

approximately 19 mm i.d. � 21 mm outside diameter

(o.d.) � 40 cm long (Fig. 2). The first 20 cm of the reactor

outer wall were wrapped with resistive heating tape (1.27 cm

� 122 cm) supplying 210 W at 120 V and 1.75 A. Liquid fuels

(>99% pure) were admitted at the top of the reactor using a

calibrated automotive fuel injector that sprayed fuel in a hollow

conical pattern on the reactor inner wall, which vaporized on

the heated inner wall and mixed partially with air in the first

18 cm of the reactor. Computational fluid dynamics simulations

show that temperature and concentration gradients develop in

the upstream section of the reactor that allow fuel vaporization

and partial mixing with air but avoid fuel autoignition (results

to be published). Vaporized fuel and air were mixed by flowing

through a blank ceramic monolith (static mixer) placed

approximately 18–19 cm downstream. The static mixer also

largely equilibrates the radial temperature gradient in the gas-

phase at 19 cm (results to be published).

Reactants entered the front heat shield at 20 cm downstream

and then the catalyst at 21 cm. Products left the catalyst and

entered a back heat shield at 21.5 cm. The catalyst, mixer, and

heat shields were 80 pores per linear inch (ppi) a-alumina foam

monoliths. The catalytic monolith was 5 mm long � 18 mm

diameter, whereas the mixer and heat shields were 10 mm

long � 18 mm diameter. All four monoliths were wrapped in

alumino-silicate paper to make a tight seal with the reactor wall.

Catalysts were prepared by coating foams with g-alumina wash


coat (3–5 wt.%) to increase effective surface area and then by

repeated coating with Rh(NO3)3 solution to achieve final Rh

loadings of about 5 wt.%. A detailed account of catalyst

preparation has been previously reported [35].

Reactor temperature was measured at the catalyst front (Tff)

and back face (Tbf) with a 0.25 mm diameter type K

thermocouple (<0.3 s response time constant). Thermocou-

ples were also placed between the heating tape and quartz tube

approximately 10 cm downstream to monitor the reactor wall

temperature and between the static mixer and front heat shield

to measure upstream preheat temperature. The entire external

side of the reactor was wrapped with ceramic blanket

insulation to attenuate radial heat losses. Effluent gases flowed

out of the reactor into heated stainless steel tubing. The outlet

flow was split after the bottom reactor endcap so that the

majority of the flow was directed to an incinerator, whereas the

remainder traveled to a differentially pumped quadrupole mass

spectrometer (QMS) system for rapid analysis (�0.5 s

response time constant). A detailed description of the QMS

system and its calibration as well as diagnostic analysis of the

measurement lag and response times can be found in a previous

work [13].

Experimental control and data acquisition were computer

automated using LabVIEW1 that allowed quick changes in

individual flow rates and QMS scan rate as well as all

experimental data to be electronically recorded in a data file

for analysis. Simulated air flow rates (79% Ar/21% O2, or

hereafter ‘‘air ’’) were 5–10 standard liters per minute (slpm), and

fuel flow rates were adjusted accordingly to create C/O ratios

between 0.6 and 1.4. Assuming a fully developed velocity profile

in the reactor tube, delay time before injected fuel reached the

catalyst was estimated as 0.1–0.3 s for air flow rates between 5

and 10 slpm. Given the tortuous 80 ppi catalyst geometry with

�83% porosity [36], steady-state gas hourly space velocities

(GHSV) ranged from 2.8 � 105 to 8.1 � 105 h�1.

2.2. Experimental procedure

Before the start of ignition experiments, air was preheated

resistively by the reactor wall upstream of the catalyst, which

raised the catalyst temperature to 200–400 8C. After preheating

to the desired temperature, masses 1–50 were scanned at 5 Hz

for approximately 10 s to establish baseline signals for O2

(mass 32) and Ar (mass 40). After 10 s, fuel was injected

(t = 0 s), and the start-up behavior of the catalyst was recorded

for approximately 60 s by which time chemical and thermal

steady states were attained. Other masses recorded included H2

(mass 2), CH4 (15), C2H4 (26), CO (28), and CO2 (44). Steady-

state product compositions were also measured by gas

chromatography [2].

In all experiments, catalyst temperatures greater than

1200 8C were avoided to minimize metal sintering and

vaporization of Rh metal oxide. Typically, steady-state back

face temperatures between 800 and 1100 8C were attained, and

a measurable amount of carbon was formed on the catalyst

(<1–5% of catalyst weight depending on alkane size) based on

carbon burnoff measurements with the QMS. To shut down the

reactor, O2 flow rate was first cut in half to lower the back face

temperature to 800 8C, and then fuel flow was stopped causing a

100–200 8C exotherm over the 10–20 s carbon burnoff. Oxygen

flow rate was then reset back to its original value, the catalyst

was cooled in air to the desired preheat temperature, and the

next run was performed. Although stopping the fuel flow in the

presence of oxygen may render the reactant mixture explosive,

no flames or explosions were observed since the time fuel and

air spend completely mixed upstream of the catalyst is shorter

than the ignition delay time as briefly mentioned in Section 2.1

(results to be published).

The effect of surface carbon on catalyst start-up perfor-

mance was also studied by preserving catalyst carbon coverage

after a 60 s run. After 60 s of runtime, O2 and fuel input were

stopped, and the catalyst was allowed to cool in Ar only, thus

avoiding the carbon burnoff exotherm. Once temperature was

reduced to the desired preheat value, O2 flow was re-initiated,

and the next run was performed. For each fuel and flow rate

studied, 2–3 experiments were run where each consisted of 10–

20 start-ups with 1 catalyst over a range of preheat temperatures

(typically between 200 and 400 8C).

2.3. Analysis of ignition parameters

Three measured parameters were used to characterize the

two-stage surface ignition of the various fuels on Rh: (1)

minimum catalytic autoignition temperature (Tcai,min), (2)

lightoff temperature (TL/O), and (3) ignition delay time (tidt).

Determination of these parameters is demonstrated for

methane (Fig. 3); methane was fed to the reactor with a

fast-response mass flow controller, and preheat was provided

by a controllable tube furnace. Tcai,min was defined as the

lowest catalyst preheat temperature that supported exothermic

surface chemistry leading to lightoff within 60 s (Fig. 3A). For

an initial 305 8C preheat temperature, exothermic ignition

chemistry occurred at the back face once methane flow was

activated (t = 0 s in Fig. 3A); an induction period was observed

where back face temperature continued to rise slowly with time

until TL/O (�350 8C) was reached at �14 s. The front face

temperature decreased during the induction period indicating

ignition occurred at the back of the catalyst. After lightoff, the

back face temperature rose rapidly towards steady-state

marking the transition from a kinetically limited to mass-

transfer or flux limited regime. In contrast, preheating to

295 8C did not support ignition: once methane flow was

activated (t = 0 s) both the front and back face continuously

cooled over the next 20 s (Fig. 3A, inset). Determination of

Tcai,min is strongly dependent on the experimental set-up and

heuristic definition of the allowable transition time. In these

experiments, the maximum time allowed for the system to

ignite was 60 s; if lightoff was not observed within 60 s, the run

was stopped, and a higher temperature was selected for the next

run.

tidt was defined as the time required for 10% oxygen

conversion. To measure tidt for a given preheat temperature,

QMS oxygen signal was transformed into oxygen conversion

and plotted versus time (Fig. 3B). For each experiment, the near


Fig. 3. Demonstration of measured ignition parameters with methane (C/

O = 1.0, 310 8C preheat, and 5 slpm air flow rate). (A) Before methane addition

(t < 0 s), the back face temperature (Tbf) is �308 8C and the front face

temperature (Tff) is 300 8C. Upon introducing methane (t > 0 s), Tbf rises from

310 to 350 8C during the first 14 s, whereas Tff decreases from 300 to 285 8C in

the same time frame. Upon reaching TL/O (�350 8C), Tbf rapidly rises to steady

state and makes the transition from kinetically limited to mass-transfer limited

as noted by (dT/dt)bf (the derivative of Tbf with respect to time calculated using a

central difference approximation). Approximately 3 s after back face lightoff,

the front face responds. By varying the initial preheat temperature Tcai,min was

found to be �300 8C for methane. (A, inset) No ignition occurs below 300 8C.

(B) O2 conversion reaches 10% conversion 13.7 s (tidt) after CH4 is introduced

to the catalyst in good agreement with the time for lightoff (�14 s) determined

from the temperature data in panel (A).

linear portion of the oxygen conversion data during lightoff was

identified, regression was performed, and tidt was calculated.

Signal noise (standard deviation in O2 conversion from �5 to

0 s) for oxygen conversion was typically 4–6%; uncertainty

(error bar) in the tidt calculation was estimated as the signal

noise divided by the slope of the least-squares fit line. As an

example, for 305 8C initial preheat methane tidt was

�13.7 � 0.6 s (Fig. 3B) in good agreement with the lightoff

induction period determined from temperature data (�14 s

from Fig. 3A).

2.4. Determination of ignition kinetics

Assuming plug flow, first-order reaction with respect to

oxygen, quasi-steady state in surface coverages before lightoff,

decoupling of flow and temperature from steady-state chemistry

[31], and constant surface temperature, the rate constant

governing lightoff can be approximated as

k ¼ �lnð1 � XO2Þ

tidt

¼ �lnð1 � 0:1Þtidt

� 0:105

tidt

(1)

where tidt represents the residence time needed for 10% oxygen

conversion. The kinetic parameters governing ignition were

calculated from an Arrhenius plot of tidt versus an exponential

function of the catalyst preheat temperature using non-linear

regression:

tidt �0:105

k¼ 0:105

ko

exp

�E

RTi

�(2)

Since tidt was based on the 10% oxygen conversion criterion,

accuracy error of the constant surface temperature assumption

was up to �8% for a given run depending on the proximity of Ti

to TL/O (e.g., Fig. 3).

2.5. Statistical analysis

Confidence intervals (a = 0.05) for E and ko were

determined using non-linear regression. Systematic deviations

between the curve fit and data were tested by residual analysis

through the use of a runs test ( p < 0.05) [37]. Analysis of

covariance [38] was used to test if the E and ko determined from

regression were significantly different ( p < 0.05) between

fuels and flow rates used. At Ti greater than 400 8C for the

higher alkanes and 525 8C for methane, tidt values system-

atically deviated from the linear trend for the lower temperature

data in a given run indicating that the measured ignition

response time may have been significantly limited by the

analytical response time of the QMS. As a result, these data

were not used in determining kinetic parameters.

3. Results

3.1. Effect of preheat temperature on start-up

3.1.1. i-Octane and n-octane

Fig. 4 displays the reactor start-up behavior for an initial

300 8C preheat and 10 slpm air flow rate with i-octane as fuel.

Only air is fed to the reactor until t = 0 s, when the fuel injector

is actuated, and fuel and air are fed to the reactor. From 0 to 3 s,

back face temperature rises from 300 to 310 8C and then rises

rapidly from 3 to 20 s reaching �1080 8C at steady state

(Fig. 4A). Once fuel is admitted, O2 conversion reaches

completion within 5 s and is accompanied by CO2, CO, and H2

production (Fig. 4B). CO2 production reaches a maximum at

2.5–3 s then decreases slightly towards its steady-state value

(Fig. 4C). H2 and CO reach steady state at �8 s (Fig. 4B). i-

Octane conversion is near 100% at steady state (data not

shown).

Initial preheat temperature was also varied to determine

minimum temperature needed for surface ignition (Fig. 5). For


Fig. 4. Effect of 300 8C preheat on catalyst temperature and effluent species

profiles during start-up for i-octane/air feed (C/O = 1) at 10 slpm air flow rate:

(A) Tbf; (B) O2, CO2, CO, and H2 molar flow rates; (C) magnified view of panel

(B) from 0 to 5 s.

Fig. 5. Effect of varying preheat temperature on start-up time for i-octane fuel

(same feed stoichiometry and flow rate as Fig. 4). (A) Preheating the catalyst to

230 8C is not sufficient to produce ignition chemistry within 20 s after fuel is fed

to the reactor, and Tbf decreases slightly in the same time span. Above 240 8Cthe catalyst lights off on a time scale exponentially dependent on preheat

temperature. (B and C) H2 and CO flow rates track Tbf and reach steady state

within 10 s for preheat temperatures above 260 8C.


Table 1

Minimum catalytic autoignition and lightoff temperatures for C1–C10 on Rh

Fuel Tcai,min (8C)a,b TL/O (8C)a,b

Methane 305 � 8 360 � 9

i-Octane 242 � 5 250 � 5

n-Octane 258 � 5 265 � 5

n-Decane 241 � 2 252 � 10

Data reported are for a 5 wt.% Rh-coated foam with �3–5% wash coat and C/

O = 1.0.a Values are mean � standard deviation.b No significant differences between air flow rates of 5–10 slpm ( p > 0.05).


profiles during start-up for n-decane/air feed (C/O = 1) at 10 slpm air flow rate:

(A) Tbf; (B) O2, C2H4, CO2, CO, and H2 molar flow rates; (C) magnified view of

panel (B) from 0 to 5 s.

230 8C preheat, no catalyst reactivity is observed, and Tbf drops

slightly after 20 s (Fig. 5A). For 240 8C preheat, the back face

lights off after approximately 6 s, and as preheat is further

increased, lightoff occurs after 1–2 s. Tff lagged Tbf during

ignition for all preheat temperatures capable of lightoff

indicating ignition started at the catalyst back face; however,

for preheat above 300 8C this difference was negligible (data

not shown). H2 and CO production tracked Tbf quite well and

reached steady state within 15 s for even the lowest preheat

temperature capable of ignition (240 8C). For higher preheat,

steady-state syngas production could be attained in as little as

6 s after the fuel was turned on (Fig. 5B and C). For all preheat

temperatures studied, the maximum in CO2 production always

occurred before syngas reached steady state and then CO2

decayed to steady state. From the i-octane preheat experiments,

Tcai,min was found to be �242 8C, and TL/O was 250 8C(Table 1).

Experiments with n-octane showed similar behavior to i-

octane as preheat temperature was varied (data not shown).

Oxygen consumption after ignition was followed by a peak and

then decay in CO2 production followed by CO and H2 reaching

steady state in less than 10 s. However, differences in the

minimum ignition temperatures between i-octane and n-octane

were observed (Table 1). Tcai,min and TL/O for n-octane were

�258 and 265 8C, respectively.

3.1.2. n-Decane


295 8C preheat and 10 slpm air flow rate with n-decane. From 0

to 2 s, Tbf rises from 295 to 315 8C and then rises rapidly from 2

to 10 s reaching �970 8C at steady state (Fig. 6A). Species

profiles during this time span are similar to the results shown for

i-octane; however, ethylene is also a major product of decane at

C/O = 1.0. Once the fuel injector is actuated, O2 flow

immediately starts to decrease and is accompanied by

production of CO2, H2O, CO, H2, and C2H4 (Fig. 6B). CO2

production peaks at �2.5 s, then decreases to steady state

(Fig. 6C). Syngas production reaches steady state within 10 s.

C2H4 response is significantly slower than H2 and CO and

levels off after 20 s. n-Decane conversion is approximately 85–

90% at steady state.

Varying initial preheat temperature gave results qualitatively

similar to i-octane (Fig. 7). For 230 8C preheat, no catalyst

reactivity is observed, and Tbf drops slightly after 20 s

(Fig. 7A). For 250 8C preheat, the back face lights off after


Fig. 7. Effect of varying preheat temperature on start-up time for n-decane fuel (same feed stoichiometry and flow rate as Fig. 6). (A) Preheating the catalyst to 230 8C is

not sufficient to produce ignition chemistry within 20 s after fuel is fed to the reactor, and Tbf decreases slightly in the same time span. Above 240 8C the catalyst lights off

on a time scale exponentially dependent on preheat temperature. (B and C) H2 and CO flow rates trackTbf and reach steady state within 10 s for preheat temperatures above

260 8C. (D) C2H4 flow rate typically lags syngas production by 2–5 s and reaches steady state no quicker than 20 s for preheat between 250 and 320 8C.

approximately 5 s, and as preheat is further increased, lightoff

occurs after 1–2 s. For preheat above 250 8C, H2 and CO

production reaches steady state within 10 s (Fig. 7B and C).

C2H4 flow rate typically lags syngas production by 2–5 s and

reaches steady state no quicker than 20 s for preheat between

250 and 320 8C (Fig. 7D). Ignition behavior for n-decane was

very similar to n-octane and i-octane for all preheat

temperatures studied as the maximum in CO2 production

occurred first followed by syngas during lightoff. Furthermore,

Tff lagged Tbf during lightoff indicating ignition started at the

catalyst back face; however, for preheat above 300 8C the

difference was negligible (data not shown). From these preheat

experiments, Tcai,min was 241 8C, and TL/O was 252 8C(Table 1).

3.1.3. n-Hexadecane


315 8C preheat and 5 slpm air flow rate with n-hexadecane as

fuel. From 0 to 2 s, Tbf rises from 315 to 340 8C and then rises

rapidly from 2 to 10 s reaching over 1000 8C at steady state

(Fig. 8A). Species profiles develop similarly to the other

alkanes with major products being CO2, H2, and CO at steady

state. Once the fuel injector is actuated, O2 flow immediately

starts to decrease and is accompanied by production of CO2,

H2O, CO, and H2 (Fig. 8B). CO2 production peaks at �3.5 s

then decreases to steady state (Fig. 8C). Syngas production

reaches steady state within 10 s. n-Hexadecane conversion is

�90% at steady state.

In contrast to the other alkanes investigated, n-hexadecane

was reactive at temperatures below 240 8C preheat for this

experimental setup. Below 240 8C all other alkanes showed a

drop in Tbf after 20 s from fuel admission, and no lightoff

occurred within 60 s; n-hexadecane showed an increase in Tbf

on the route to lightoff for preheat temperatures as low as

220 8C (Fig. 9A). For a 227 8C preheat, after fuel admission Tbf

continually rose during the first 30 s from 227 to 245 8C(Fig. 9A). O2 conversion rose from 0 to 5–10% during this same

period, and some CO and CO2 production was observed (data

not shown). An energy balance confirmed that 5–10% O2

conversion to CO and CO2 agreed well with the observed

temperature rise. After 38 s, lightoff occurred (not shown). For

235 8C preheat, Tbf rose from 235 to 255 8C from 0 to 20 s, and

an accompanying 10% O2 conversion was attained after 5 s,

which stayed constant until lightoff at 22 s (Fig. 9A). H2 and

CO production tracked Tbf quite well and reached steady-state

values within 15 s for preheat temperatures above 280 8C(Fig. 9B and C).

Control experiments substituting a blank monolith for the

Rh-coated monolith reproduced the same temperature and

oxygen conversion behavior over a large range of preheat

temperatures (220–360 8C): typically Tbf rose 30–100 8C over

the 60 s after fuel admission, and an accompanying 10% O2



profiles during start-up for n-hexadecane/air feed (C/O = 1) at 5 slpm air flow

rate: (A) Tbf; (B) O2, CO2, CO, and H2 molar flow rates; (C) magnified view of

panel (B) from 0 to 6 s.

Fig. 9. Effect of varying preheat temperature on start-up time for n-hexadecane

fuel (same feed stoichiometry and flow rate as Fig. 8). (A) Preheating the

catalyst to 227 8C is sufficient to produce ignition chemistry within 20 s after

fuel is fed to the reactor, and Tbf increases in the same time span by 20 8C.

Lightoff eventually occurs at �38 s (not shown). For 235 8C preheat, Tbf rises

from 235 to 255 8C from 0 to 20 s, and an accompanying 10% O2 conversion is

attained after 5 s (not shown), which remains constant until lightoff at 22 s. (B

and C) H2 and CO flow rates track Tbf and reach steady state within 15 s for

preheat temperatures above 280 8C.


Table 2

Minimum ignition and lightoff temperatures for C16H34

C/O Surface Tign,min (8C)a,b,c TL/O (8C)a,b

0.63 Rh 223 � 3 290 � 5

1 Rh 225 � 5 285 � 5

0.63 Blank (control) 219 � 4 N/A

1 Blank (control) 222 � 5 N/A

Data reported are for a 5 wt.% Rh-coated foam with �3–5% wash coat and C/

O = 1.0. N/A: not applicable.a Values are mean � standard deviation.b No significant differences between air flow rates of 5–10 slpm ( p > 0.05).c Tign,min is used for n-hexadecane instead of Tcai,min since a blank foam gave

the same minimum ignition temperatures (for 10% O2 conversion) as a Rh-

coated foam.

Fig. 10. Surface ignition kinetic parameters for C1–C10 alkanes from Arrhehius

plots. Feed stoichiometry was C/O = 1.0, and ignition criterion was 10% O2

conversion. Ignition delay times are plotted as an exponential function of 1/T

with high confidence using non-linear regression; curve deviations from the data

are insignificant ( p > 0.05). See Table 3 for results of statistical analysis. (A)

Least-squares analysis for 5 slpm air flow rate on CH4 (E = 74 kJ/mol,

ko = 3.7 � 104 s�1), n-C8H18 (E = 85 kJ/mol, ko = 2.8 � 106 s�1), and n-

C10H22 (E = 80 kJ/mol, ko = 2.3 � 106 s�1). (B) Least-squares analysis for

10 slpm air flow rate on n-C8H18 (E = 83 kJ/mol, ko = 4.6x106 s�1), i-C8H18

(E = 75 kJ/mol, ko = 8.9 � 105 s�1), and n-C10H22 (E = 75 kJ/mol,

ko = 1.0 � 106 s�1). Kinetic parameters were statistically invariant ( p > 0.05)

between fuels at 10 slpm air flow rate so tidt values for i-octane and n-decane (and

best-fit regression line) are scaled by a constant for ease of viewing.

conversion was observed after 3–6 s, which remained

constant over the next 60 s. Therefore, the observed exotherm

was kinetically controlled and did not make the transition to a

mass-transfer limited condition (indicative of a cool-flame

type regime). Control experiments indicated that the initial

10% O2 conversion observed was not due to Rh surface

chemistry but gas-phase chemistry (slow oxidation) assisted

either directly (surface-assisted initiation) or indirectly

(enhanced mixing of fuel and air) by the tortuous blank

alumina foam. From these preheat experiments, Tign,min was

found to be �220 8C regardless of whether the catalyst was

coated with Rh; TL/O was �280–290 8C for the Rh-coated

foam (Table 2).

3.1.4. Effect of temperature versus fuel

Overall, there is little difference in the range of temperatures

presented in Figs. 4–9 (<5%). The only appreciable difference

is the air flow rate (5 slpm) for hexadecane experiments

compared to the other alkane experiments (10 slpm). This flow

rate modification was made because the steady-state tempera-

ture for hexadecane with 10 slpm air was above 1200 8C. If a

quantitative comparison of surface ignition behavior is desired

between fuels, Figs. 10 and 11 highlight the differences

between fuels and flow rates used (see Section 3.2). In contrast,

Figs. 4–9 show a quantitative comparison between temporal

species and temperature profiles as initial preheat temperature

is varied for a given fuel.

3.2. Ignition kinetics

3.2.1. Methane, n-octane, i-octane, and n-decane on Rh

From the variable preheat experiments, the kinetic para-

meters governing ignition were determined by regression

(Fig. 10). Results from this analysis are shown in Table 3. For a

5 slpm air flow rate (Fig. 10A), no significant difference was

found for the apparent activation energy (slope) between

methane, n-octane, and n-decane; however, the preexponential

factors (ordinate intercept) were significantly different. For a

10 slpm air flow rate (Fig. 10B), no significant differences were

found for either kinetic parameter between n-octane, i-octane,

and n-decane. Since activation energy was statistically invariant

to the fuel or flow rate studied, the pooled value was 78 kJ/mol.

3.2.2. n-Hexadecane: gas-phase versus surface ignition

Initial gas-phase chemistry, which consumed approximately

10% O2 with either a Rh or blank foam over a large range of

preheat temperatures (Section 3.1.3), had a significant effect on

the analysis of the ignition kinetics for n-hexadecane. Further

analysis with a 25% O2 conversion criterion was performed to

decouple the possible gas-phase chemistry from the kinetics of


Fig. 11. Gas-phase vs. surface chemistry during ignition for n-hexadecane. C/

O = 0.63, and ignition criterion was 10 or 25% O2 conversion. See Table 4 for

results of statistical analysis. (A) tidt values for 10% conversion with a blank

foam are denoted by white squares. For a 5 slpm air flow rate and 10% O2

conversion, there is no significant difference in the activation energy between a

Rh foam and blank foam indicating initial O2 consumption is due to exothermic

gas-phase chemistry. When using the 25% O2 conversion criterion, two linear

regions (high T and low T) are found with significantly different activation

energies ( p < 0.05). E for the high T region is statistically similar to the

previous 10% O2 conversion curves. However, in the low T region the activation

energy is identical (77 kJ/mol, p < 0.05) to E calculated for C1–C10 alkanes

indicating surface chemistry. Subtracting tidt,10% from tidt,25% decouples the

initial gas-phase chemistry from surface chemistry and gives E and ko values

(85 kJ/mol and 3.9 � 106 s�1, respectively) similar to those reported in Table 3

for C1–C10. (B) tidt values for 10% conversion with a blank foam are denoted by

white circles. The same phenomenon is observed at 10 slpm air flow. The

intersection of the two linear regimes for 25% O2 conversion occurs at the

approximate TL/O for n-hexadecane; above TL/O the characteristic time for

surface ignition (tsurf) is much less than the time for 10% O2 consumption in the

gas-phase. Below TL/O the opposite situation is observed.

Table 3

Kinetic parameters for surface ignition of C1–C10 alkanes

Fuel Air flow

(slpm)

ko,mean

(s�1)

ko, 95%

CIa

Emean (kJ/mol)

� 95% CIa

Methane 5 3.7 � 104** 1.3 � 104,

1.0 � 105

74 � 6.0

n-Octane 5 2.8 � 106** 9.2 � 104,

8.6 � 107

85 � 16

n-Octane 10 4.6 � 106 3.8 � 105,

5.7 � 107

83 � 12

i-Octane 10 8.9 � 105 2.5 � 104,

3.2 � 107

75 � 17

n-Decane 5 2.3 � 106** 1.1 � 105,

4.9 � 107

80 � 14

n-Decane 10 1.0 � 106 2.5 � 104,

4.2 � 107

75 � 17

Pooled value N/A 78

Feed stoichiometry was C/O = 1.0, and ignition criterion was 10% oxygen

conversion.a 95% CI (confidence interval).

** Value is significantly different from other 5 slpm air flow values in column

( p < 0.05).

surface ignition. For C/O = 1.0, no difference in the ignition

kinetics was measured with either the 10 or 25% O2 conversion

criteria. To further suppress gas-phase chemistry, a C/O ratio of

�0.6 was used; typically gas-phase ignition delay times

increase as C/O ratios are decreased because of the feed dilution

effect.

For a 5 slpm air flow rate, 10% O2 conversion, and C/

O = 0.6, there was no significant difference in the activation

energy between a Rh foam and blank foam confirming that

initial O2 consumption was not due to Rh surface chemistry

(Fig. 11A and Table 4). When using the 25% O2 conversion

criterion, two linear regions (high T and low T) were found

with significantly different activation energies and preexpo-

nentials. Activation energy for the high temperature region

was statistically similar to the previous 10% O2 conversion

curves. However, in the low temperature region the activation

energy (Table 4) was identical ( p > 0.05) to E calculated for

C1–C10 alkanes (Table 3) indicative of Rh chemistry.

Subtracting tidt,10% from tidt,25% decoupled the initial gas-

phase chemistry from surface chemistry and gave E and ko

values (Table 4, data set #7) similar to those reported in Table 3

for the other alkanes.

Similar behavior was observed for 10 slpm air flow (Fig. 11B

and Table 4). The intersection of the two linear regimes for 25%

O2 conversion occurs at the approximate TL/O for n-hexadecane;

above TL/O the characteristic time for surface ignition (tsurf) is

much less than the time for 10% O2 consumption in the gas-phase

(tgas). Below TL/O, the magnitudes of tsurf and tgas become

similar to causing the two distinct linear regimes.

3.2.3. Effect of previous burnoff on subsequent start-up

with n-decane

For all alkanes investigated the activation energy for surface

lightoff on a Rh catalyst that had undergone carbon burnoff


Table 4

Kinetic parameters for ignition of C16H34

Data set no. C/O Air flow

(slpm)

XO2criterion/

comment

ko,mean

(s�1)

ko, 95%

CIa

Emean (kJ/mol)

� 95% CIa

1 1 5 10% 4.4 � 100 1.4 � 100, 1.4 � 101 21 � 5.4

2 1 5 25% 3.9 � 100 1.6 � 100, 9.3 � 100 22 � 4.1

3 1 10 10% 1.8 � 100 2.5 � 10�1, 1.3 � 101 16 � 9.1

4 0.63 5 10% 1.1 � 101 3.4 � 100, 3.7 � 101 26 � 5.4

5 0.63 5 25%/high Tb 4.6 � 101 7.9 � 100, 2.7 � 102 35 � 8.8

6 0.63 5 25%/low Tc 4.1 � 105 3.7 � 104, 4.6 � 106 77 � 11**

7 0.63 5 25–10%/all T 3.9 � 106 4.7 � 105, 3.2 � 107 85 � 9.6**

8 0.63 10 25%/high Tb 2.6 � 100 5.2 � 10�1, 1.3 � 101 20 � 7.9

9 0.63 10 25%/low Tc 2.3 � 106 1.3 � 105, 4.1 � 107 83 � 13**

10 0.63 5 10%/blankd 2.5 � 100 6.6 � 10�2, 9.2 � 101 20 � 17

11 0.63 10 10%/blankd 3.4 � 100 4.9 � 10�1, 2.4 � 101 21 � 8.8

a 95% CI (confidence interval).b High T indicates Ti > 290 8C tidt values were used in the regression analysis.c Low T indicates Ti < 290 8C tidt values were used in the regression analysis.d Blank indicates the data correspond to a blank foam instead of a Rh-coated foam.

** E for data sets #6, 7 and 9 are significantly different ( p < 0.05) from all other experiments (1–5, 8, 10, and 11) but are not significantly different from each other.

Fig. 12. Effect of previous surface burnoff on subsequent ignition kinetics with

n-decane. C/O = 1.0, air flow rate was 5 slpm, and ignition criterion was 10% O2

conversion. Least-squares analysis on previously burned off surface gives

E = 84 kJ/mol and ko = 3.6 � 106 s�1. Least-squares analysis for no burnoff

condition gives E = 42 kJ/mol and ko = 4.2 � 102 s�1. These results indicate

that carbon burnoff has a significant effect ( p < 0.05) on the kinetic parameters

for n-decane. See Table 5 for results of statistical analysis.

Table 5

Effect of surface burnoff on kinetic parameters for n-decane ignition

C/O With

burnoff?

ko,mean

(s�1)a

ko,

95% CIa

Emean (kJ/mol)

� 95% CIa

1 Yes 3.6 � 106 1.3 � 105,

9.9 � 107

84 � 16

1.4 Yes 1.8 � 106 1.3 � 105,

2.5 � 107

81 � 12

Pooled value 2.4 � 106 82

1 No 4.2 � 102 1.5 � 101,

1.2 � 104

42 � 16

1.4 No 2.0 � 102 2.9 � 101,

1.4 � 103

39 � 9.0

Pooled value 2.7 � 102 40

Feed flow rate was 5 slpm, and ignition criterion was 10% oxygen conversion.

Kinetic ignition parameters for burned off surfaces were significantly different

( p < 0.05) than those for surfaces that did not receive burnoff. No significant

difference was found between C/O = 1.0 and 1.4 for same surface condition

prior to start-up.a 95% CI (confidence interval).

subsequent to ignition has been shown statistically invariant.

Experiments were also performed to see if carbon coverage

had a significant effect on ignition kinetics for C/O = 1.0 and

1.4. Not burning off carbon accumulated on the catalyst

surface after a 60 s run had a significant effect on the lightoff

kinetics (Fig. 12 and Table 5). However, kinetic parameters

were not statistically different between C/O feed ratios of 1.0

and 1.4 for ignition experiments performed on the same type

of surface.

4. Discussion

The present study has shown there is little difference in

the qualitative lightoff behavior of short-chain (methane) and

long-chain alkanes (n-octane or i-octane, n-decane, and

n-hexadecane) on Rh-coated monoliths. During the induction

period CO2 and H2O (H2O not shown) production occurs,

which is more apparent at very low preheat temperatures near

Tcai,min. After CO2 reaches a maximum, H2 and CO rapidly rise

towards steady state while CO2 decreases to steady state.

Ignition starts at the back face of the catalyst for all fuels

investigated in agreement with the numerical study of methane

lightoff using multi-step surface chemistry [31]. Initially

heating the catalyst 10–20 8C above Tcai,min allows steady-state


syngas production within 5 s after fuel admission. While these

results were obtained on ceramic foam supports, use of a

metallic support that possesses a faster thermal time constant

may further decrease start-up time.

Previous lightoff/ignition experiments [20,31,39,8] and

associated numerical models [20,40,41] do not address the

induction period observed during autothermal ignition studies.

These studies give indirect estimates of the kinetics governing

surface chemistry during ignition since the data sets contain no

transient information on the induction time leading to lightoff

as a function of initial preheat temperature. No quantitative

analysis of the ignition kinetics for higher alkanes is available

in the literature, nor has an experimental transient assessment of

the ignition kinetics been performed directly with small alkanes

to the best of our knowledge. In this work, the ignition delay

time, which is the time span between when fuel and oxygen

surface competition begins and lightoff (defined here as 10%

oxygen conversion), has been measured as a function of initial

surface temperature.

The fit of Eq. (2) to lightoff data possessed no systematic

deviation from the data ( p > 0.05) suggesting that surface

coverages do not change appreciably over the ignition delay

time (Figs. 10–12). This also implies that the ignition delay

times were accurately measured by the QMS over a large range

of initial surface temperatures. Furthermore, kinetic parameters

were extracted from the data with high confidence. From the

regression analysis, the apparent Eign on Rh was 77 � 15 kJ/

mol, which was statistically invariant between fuels (methane,

n-octane and i-octane, n-decane, and n-hexadecane) and

flow rates. A significant difference was found between the

ignition ko for methane, O(104 s�1), and the other large alkanes,

O(106 s�1).

4.1. Controlling step for surface lightoff

A mathematical model [41] has been previously proposed to

fit lightoff temperature versus feed stoichiometry data from

stagnation flow or catalytic wire experiments, and it is helpful

to examine the present results in light of this surface reaction

model. Assuming the site coverages are in quasi-steady state,

the apparent reaction rate prior to lightoff can be expressed as

(see [41] for derivation of Eqs. (3)–(7))

rsurf ¼ rF;A � rF;D ¼ 1

sðrO;A � rO;DÞ (3)

The rates of adsorption (without activation) and desorption

(with activation) for species i can be defined, respectively,

as

ri;A ¼ ki;Auni;AV ¼ Si;o pXiNAvffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

2pWiRTp u

ni;AV (4)

ri;D ¼ ki;Duni;Di ¼ GAi;D exp

��Ei;D

RT

�uni;Di (5)

For an initially oxygen covered surface (uO � 1, uF � 1,

uV � 0), the reaction rate for O2 consumption prior to lightoff

is

rsurf ¼ 2skF;AunF;A

V (6)

If nO,A and nF,A are equal, and nO,A is 2, then the apparent

reaction rate is

rsurf ¼ 2sGAO;DkF;A

kO;A � skF;Aexp

��EO;D

RT

�(7)

Note the units of the above rate are molecules/m2/s. To convert

this rate to an apparent homogeneous rate (mol/m3/s) corre-

sponding to the measured rate constant (s�1) in Eq. (1) the

following conversion is made

rapp ¼ Sv

NAv

rsurf

¼ 2sSvGAO;DRT

NAv

SF;o

gffiffiffiffiffiWF

p

SO;offiffiffiffiffiffiffiWO2

p � sg

SF;offiffiffiffiffiWF

pexp

��EO;D

RT

�CO2

¼ kapp;o exp

��EO;D

RT

�CO2

(8)

Comparing Eq. (1) with Eq. (8) leads to the analogy that ko and

E in Eq. (2) are kapp,o and EO,D in Eq. (8).

In order for lightoff to occur, a sufficient number of empty

sites must be available [42]. With methane, it has been proposed

that before ignition O2 readsorption is fast (no significant

activation barrier for adsorption), and methane adsorption is the

controlling ignition step [39]. While methane adsorption may

be slightly activated (up to 22 kJ/mol [30]), typically O2 and

CH4 adsorption is treated as non-activated [41]. This would

indicate that the controlling activation energy for methane

ignition is the desorption of O2 to free surface sites. Since E was

invariant between fuels in this study, O2 desorption appears to

be the controlling energetic event for alkane lightoff.

Previously, the apparent kinetics for O2 desorption have

been measured for a Pt/Rh catalyst over a range of surface

coverages with dramatic results [43]. Decreasing the oxygen

surface coverage from 1.0 to 0.95 caused the apparent

activation energy and preexponential for desorption to increase

from 69 to 219 kJ/mol and from 107 to 1013 s�1, respectively

[43]. The rapid change in surface bonding energy between

adsorbed oxygen and metal surface with changing surface

coverage has been confirmed by density functional theory

calculations for Rh(1 1 1) [44]. E measured in this work (77 kJ/

mol) is in good agreement with the apparent O2 desorption

energy measured when the Pt/Rh surface is saturated with O2

(69 kJ/mol from [43]) indicating that the surface in the present

work may be close to O2 saturation prior to ignition. In addition,

E in this work matches extremely well with the value of

activation energy for O2 desorption at saturation coverage

(75.2 kJ/mol) as part of multi-step methane surface chemistry


on Rh [31]; this value was used to accurately simulate methane

TL/O experimental data. A similar value of E (79 kJ/mol) for O2

desorption at saturation coverage was used in multi-step H2 and

CH4 surface chemistry on Pt [45,30], based on the net repulsive

interaction (134 kJ/mol) found between adsorbed oxygen on Pt

at high uO [46].

The 2 orders of magnitude difference between ko for

methane and the large alkanes may be explained by differences

between the sticking probabilities and/or activation energies in

the fuel adsorption steps. TL/O is 360 8C for methane and

�260 8C for all other alkanes investigated. In order to give the

same rate constant for the higher alkanes as with methane at a

100 8C lower temperature for the same feed stoichiometry (C/

O = 1.0, s = 2, and g = 0.5–8 depending on fuel) with Eq. (8),

SF,o for the higher alkanes must be 20–40 times higher than

SCH4;o assuming fuel adsorption is not activated. If fuel

adsorption is assumed unequally activated between methane

and the higher alkanes (methane activation energy higher),

energetic compensation occurs and the calculated difference

between the sticking coefficients decreases. However, since no

difference was detected in the apparent ignition activation

energy between fuels, large differences in the sticking

coefficients seem to better explain the differences between

ko for methane and the larger alkanes.

4.2. Comparison with previous experimental ignition

studies

The terms ignition and lightoff are often used interchange-

ably in the literature. In this work, a surface temperature

capable of catalytic autoignition possesses enough energy to

either directly lightoff or indirectly produce a sufficient amount

of exothermic chemistry to further raise the surface temperature

high enough to undergo lightoff. Starting the ignition

experiment at a catalyst temperature below Tcai,min results in

catalyst cooling because of the increased heat load from the

added fuel, and no reaction occurs. TL/O is defined as the

temperature where a stepwise transition occurs from kinetically

limited to mass-transfer limited operation [47,39]. Previous

studies have measured TL/O with C1–C4 alkanes on various

noble metals (most commonly with Pt, Rh, or a mixture of

both). Measurements with a Pt foil (in stagnation flow

geometry) [22] and with a Rh-coated honeycomb monolith

[31] found TL/O was 430 8C for methane at C/O � 1.0; similar

measurements in a micro-scale reactor with Rh gave 480 8C,

while in a near-adiabatic reactor with a Rh-coated metallic

monolith TL/O was 360–380 8C [39]. Although some mention is

made about the limited exothermic chemistry occurring before

lightoff [39,31], no estimates for tidt were made in any of these

studies.

Lightoff temperature measurements were also conducted

with various large aliphatics and aromatics with water co-feed

in an isothermal micro-scale reformer (with heating ramp, not

autothermal) and adiabatic (autothermal) reactor with Pt/Rh

catalyst on a yttria stabilized zirconia foam [8]. However, it is

unclear what the quantitative criterion was for lightoff in this

study. Experiments with the micro-scale reformer showed that

lightoff temperature varied from 155 to 180 8C for straight

chain n-alkanes from C6 to C10. i-Octane possessed a higher

lightoff temperature (210 8C) than any of the n-alkanes.

Differences between the apparatus/procedure in some of the

above experiments and the set-up in the present work may

explain the discrepancies between the respective results. In this

work, the temperatures of the incoming gaseous feed and

catalyst surface are in equilibrium before fuel is added similar

to the adiabatic reactor in [39]; for methane, excellent

agreement is observed for methane TL/O (360–380 8C) between

[39] and the present study. Other experimental geometries

[22,31] incorporated a cold (25 8C) feed impinging on a

resistively heated surface and report higher temperatures for

methane TL/O on Rh (430 8C) possibly because of the thermally

non-equilibrated condition. Not surprisingly, whether the

catalyst is heated in a premixed fuel/air feed [39] or only an

air feed until just a few seconds before the ignition experiment

(this work) makes no difference in TL/O for a rich methane/air

feed. This observation further suggests that the surface is

covered with O2 prior to ignition for small alkane fuels as has

been previously discussed [20,48,22,49,31,39]. Little quanti-

tative comparison can be made between the higher alkane

lightoff results in the present work and [8] because of

differences in feed composition and reactor configuration.

While both studies agree that higher alkane/air mixtures will

catalytically ignite for an initial preheat temperature of 325 8C,

the minimum autoignition temperatures on the Pt/Rh catalyst

were not characterized in [8].

4.3. Comparison with previous numerical ignition studies

Numerical analyses have been used to extract ignition

kinetics indirectly from the TL/O versus feed stoichiometry

data in [20]. Using a Langmuir–Hinshelwood mechanism

coupled with energy balance and mathematical description of

ignition (turning point), a simplified model was used to

deduce the apparent preexponential (ko) and activation

energy (E) of ignition for C1–C4 alkanes on Pt [20]. Apparent

activation energy (apparent E for the overall reaction plus

hydrocarbon heat of adsorption minus oxygen heat of

adsorption) varied from 110 kJ/mol (methane) to 80 kJ/mol

(n-propane and i-butane). Inclusion of E for the overall

surface reaction in Eign contradicts other numerical studies

that show the lightoff temperature to be independent of

surface reaction rate parameters (directly shown in [41] and

indirectly mentioned in [42,48]). For rich methane ignition

on Pt, studies indicate that the important kinetic

parameters governing ignition temperature are those for

adsorption of methane and adsorption/desorption of O2

[48,40,41].

A more sophisticated analysis [41] of the data set for

methane ignition on Pt [20] used non-activated second-order

adsorption for fuel and oxygen and second-order desorption of

oxygen; model results indicated that E and ko for O2 desorption

are �190 kJ/mol and 7.5 � 1013 s�1, and the ratio of the zero

coverage sticking coefficients for O2/CH4 is approximately 5.9.

Although the model fits the data well, use of lightoff


temperature data (steady state) as a function of stoichiometry to

determine ignition kinetics still presents a challenge in

determining the initial kinetics governing lightoff since

typically O2 desorption is a strong function of surface

coverage [46], and coverages are unknown experimentally

at the ignition temperature. The difference between the

ignition activation energy found in the present work (77 kJ/

mol) and in [41] (190 kJ/mol) in light of the results of [43]

suggests there may be up to a 5% decrease in O2 coverage

between ignition and lightoff (i.e., between the non-reactive O2

covered surface and the surface at the time when the turning

point criterion occurs).

4.4. Effect of C/O ratio and carbon burnoff with n-decane

For an initially oxygen covered surface prior to lightoff, it is

expected that increasing the fuel to oxygen ratio in the feed

should generally decrease TL/O up to a certain limit, as shown

for alkanes from C1 to C4 on Pt [20]. However, experiments

with decane showed that changing the C/O ratio from 1 to 1.4

produced no difference in Tcai,min, TL/O, or the ignition kinetics

in this study (Table 5). Since an injector was used to deliver

higher alkane fuels, the fuel was vaporized, mixed with air, and

then contacted the catalyst. During the initial seconds of the

experiment, the C/O ratio the catalyst ‘‘experienced’’ was a

broad range; the catalyst saw ratios from 0 up to the steady-state

value. Based on this assessment, the C/O ratio was not a

constant quantity during reactor start-up, and therefore similar

ignition kinetics were obtained from different steady-state C/O

ratios (Table 5).

Experiments testing the effect of carbon burnoff on the

subsequent lightoff run were significant (Fig. 12 and

Table 5). A partially carbon covered catalyst lights off

faster at lower preheat temperatures and slower at higher

preheat temperatures than a burned-off catalyst. This

phenomenon may appear at first to be the homogeneous

oxidation of the carbon before surface lightoff occurs. Once

the carbon is burned off, free sites may become available for

O2 adsorption and then surface ignition can occur. However,

the measured activation energy (�35–40 kJ/mol) is much

lower than those measured for the homogeneous ignition of

coke compounds and decoking of reformers (130–170 kJ/

mol) [50–52]. A more plausible explanation is that by not

performing burnoff, the next ignition run is forced to be

covered by fuel species instead of the typical oxygen

coverage found with alkanes. Oxygen is then forced to

compete for sites with the adsorbed unsaturated (possible

polyolefin) carbon deposits (the opposite of the situation

discussed in Section 4.1). For example, the activation

energies in Table 5 for ignition experiments without previous

carbon burnoff are in good agreement with those measured

for ethylene and propylene on Pt (35–50 kJ/mol) [20]; in

addition, the ignition preexponential without burnoff drops

�4 orders of magnitude as compared to the preexponential

for a burned off surface (Table 5). This difference is similar

to the measured differences in the ignition preexponential

between using alkanes (initially oxygen covered surface)

and olefins (initially fuel covered surface) as fuel on

Pt [20].

5. Conclusions

This work has demonstrated that steady-state production of

syngas (CO and H2) can be attained within 5 s after admitting

large alkanes (i-octane, n-octane, n-decane, or n-hexadecane)

and air to a short-contact-time reactor by using an automotive

fuel injector and initially preheating the Rh-coated catalyst

above each fuel’s respective catalytic autoignition temperature.

Tcai,min with Rh was 240–250 8C for n-octane, i-octane, and n-

decane, and �300 8C for methane. In contrast, ignition of n-

hexadecane occurred at lower temperatures (220 8C and

greater) because of an indirect two-stage process where

exothermic homogeneous reactions preheated the catalyst by

30–60 8C to temperatures (�280 8C) sufficient for surface

lightoff.

The catalytic autoignition kinetics for large alkanes were

determined experimentally and compared with those of

methane using rapid-response QMS. The controlling step for

surface ignition possessed an apparent activation energy of

�77 kJ/mol, which was not significantly different between

fuels ( p > 0.05), and a preexponential on the order of 106 s�1

for higher alkanes and 104 s�1 for methane. Catalytic

autoignition from Tcai,min is a visible two-stage process:

starting the reactor between Tcai,min and TL/O effects sufficient

exothermic surface chemistry to raise the catalyst temperature

high enough to undergo lightoff (transition from kinetically

limited to mass-transfer limited operation). The differences

between Tcai,min and TL/O varied from �60 8C for methane to

10 8C for higher alkanes with the ceramic foam monoliths used

in this study.

Through this work, some of the similarities and differences

in ignition behavior between alkanes of various size have been

demonstrated. Transient characterization has helped in under-

standing the surface processes at work during the ignition of

higher alkane CPO. However, transportation fuels are not single

components but rather a diverse mixture of aliphatics and

aromatics. Further transient experiments are needed to better

understand the differences in the lightoff kinetics between

aliphatics and aromatics as well as aromatic–aliphatic mixtures.

Future work will employ temperature programmed oxidation

and reduction experiments coupled with transient character-

ization to identify the reactivity of the catalyst carbon coverage

versus temperature and its composition (C or C and H) as a

function of start-up time. This information is crucial towards

building mean-field descriptions of the surface chemistry

governing these compounds that can be used in predictive

models of reactor performance.

Acknowledgments

Funding was provided in part by the National Science

Foundation under grant CTS-0211890. K.A.W. gratefully

acknowledges funding through a National Science Foundation

Graduate Research Fellowship (2001–2004).


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