Catalytic performance of highly dispersed Ni/TiO2 for dry and steam reforming of methane

Article abstract of DOI:10.1039/c3ra45961f

The present study reports a sonochemical-assisted synthesis of a highly active and coke resistant Ni/TiO₂ catalyst for dry and steam reforming of methane. The catalyst was characterized using XRD, TEM, XPS, BET analyzer and TGA/DTA techniques. The TEM analysis showed that Ni nanoparticles were uniformly dispersed on TiO₂ surface with a narrow size distribution. The catalyst prepared via this approach exhibited excellent activity and stability for both the reactions compared to the reference catalyst prepared from the conventional wet impregnation method. For dry reforming, 86% CH₄ conversion and 84% CO₂ conversion was obtained at 700 °C. Nearly 92% CH₄ conversion and 77% CO selectivity was observed under a H₂O/CH₄ ratio of 1.2 at 700 °C for the steam reforming reaction. In particular, the present catalyst is extremely active and resistant to coke formation for steam reforming at low steam/carbon ratios. There is no significant modification of Ni particles size and no coke deposition, even after a long term reaction, demonstrating its potential applicability as an industrial reformate for hydrogen production. The detailed kinetic studies have been presented for steam reforming and the mechanism involving Langmuir-Hinshelwood kinetics with adsorptive dissociation of CH ₄ as a rate determining step has been used to correlate the experimental data.

Full text of DOI:10.1039/c3ra45961f

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Page 1 of 37
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DOI: 10.1039/C3RA451961F
Catalytic performance of highly dispersed Ni/TiO2
for dry and steam reforming of methane
Vijay M. Shinde, Giridhar Madras*
Department of Chemical Engineering,
Indian Institute of Science, Bangalore-560 012. INDIA
*Corresponding author Tel.: +91-80-22932321; Fax: +91-80-23601310.
E-mail: giridhar@chemeng.iisc.ernet.in

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Abstract
The present study reports a sonochemical assisted synthesis of highly active and coke
resistant Ni/TiO2 catalyst for dry and steam reforming of methane. The catalyst was
characterized using XRD, TEM, XPS, BET analyzer and TGA/DTA techniques. TEM analysis
showed that Ni nanoparticles were uniformly dispersed on TiO2 surface with a narrow size
distribution. The catalyst prepared via this approach exhibited excellent activity and stability for
both the reactions than the reference catalyst prepared from the conventional wet impregnation
method. For dry reforming, 86% CH4 conversion and 84% CO2 conversion was obtained at
700⁰C. Nearly 92% CH4 conversion and 77% CO selectivity was observed under H2O/CH4 ratio
of 1.2 at 700⁰C for the steam reforming reaction. In particular, the present catalyst is extremely
active and resistant to coke formation for steam reforming at low steam/carbon ratio. There is no
significant modification of Ni particles size and no coke deposition, even after long term
reaction, demonstrating its potential applicability in industrial reformate for hydrogen
production. The detailed kinetic studies have been presented for steam reforming and the
mechanism involving Langmuir-Hinshelwood kinetics with adsorptive dissociation of CH4 as a
rate determining step has been used to correlate the experimental data.
Keywords: dry reforming; steam reforming; hydrogen production; coke deposition; nickel
catalyst.

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1. Introduction
The steam reforming of methane is a well established industrial process for the
1 2
,
production of hydrogen and synthetic gas . Ni based catalysts are commonly used in industrial
reforming reaction due to their low cost and reasonably good activity. However, these catalysts
deactivate due to sintering and coke deposition.3 Therefore, excess steam (H2O/CH4>3) is
introduced in the feedstock to prevent coke deposition. Though excess steam favors higher
conversion, unnecessary generation of steam hampers the process economy and increases the
H2/CO ratio. This make the stream unsuitable for downstream processes such as Fischer-Tropsch
synthesis.4 Further, it increases the size of reactor and the required amount of catalyst. It also
leads to deactivation of the catalyst due to oxidation of Ni particle.5 Therefore, the inhibition of
coke deposition under low H2O/CH4 ratio on the Ni-based catalysts is one of the biggest
challenges in the steam reforming process.
Several studies discuss enhancing the stability of Ni based catalysts for the reforming
reaction. Promoters such as alkaline earth oxide (MgO or CaO) are often used to lower the
coking propensity and provide higher stability against sintering.6, 7 However, the addition of
these promoters impedes the reduction of NiO leading to the decrease in the activity. It has been
observed that the promotion with K or Ca increase the formation of NiAl2O4 phase, which is
reducible above 700⁰C.8 Horiuchi et al. reported that the addition of alkaline metal suppressed
the reforming activity of Ni with the markedly suppression of coke deposition for dry reforming
reaction.7 Therefore, it is better to modify Ni-based catalysts without compromising their
activity.
It is evident that the reducible supports (CeO2, TiO2) provide better stability and coke
resistance in comparison with their non-reducible supports counterparts (Al2O3, SiO2).9-11 There

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is a direct correlation between oxygen storage capacity (OSC) and coke deposition propensity:
higher the OSC, lower is the coke deposition on the catalyst.12 Ceria and modified ceria
compounds are well known for their reversible exchange of lattice oxygen during the reaction.13-
15
However, CeO2 support is vulnerable to sintering and loses its OSC at high temperature.16
TiO2 exhibits lower OSC compared to CeO2 but is stable at high temperature. The dry reforming
was studied over Ni supported on various supports and specific activities followed the order:
Ni/TiO2 > Ni/C > Ni/SiO2 > Ni/MgO.17 Therefore, TiO2 seems to be good alternative support for
CeO2 and Al2O3.
The activity of the catalyst often depends on the size and extent of metal dispersion.18, 19
Small particles increases metal dispersion and also provides more steps/kinks on the surface.20, 21
The energy barrier for methane dissociation, a rate determining step, over step sites is much
lower than stair sites. Therefore, the rate of reaction increases with the extent of dispersion of
active phase.22, 23 Further, small particles below a critical size have also been reported to be more
resistant to coke formation and the spillover of steam on the support is a key parameter between
particle size and rate of coke deposition.20, 24 The highly dispersed metal particles also tend to
minimize surface energy by increasing interaction with the support and hence minimizing
sintering.22 Therefore, the synthesis of homogeneous and highly dispersed Ni nanosized particles
with a significant metal support interaction is essential for stable performance.
The preparation method often influences structure and morphology of the catalyst.
Recently, it was shown that a sonochemical assisted method produces uniformly dispersed
nanoparticles with a narrow size distribution in range of 8-9 nm.25, 26 Here, we report a
sonochemical assisted synthesis of highly active and coke resistant Ni/TiO2 catalyst for dry and
steam reforming of methane. In contrast to the conventional wet impregnation method, the metal

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precursor of active phase and support are added together to an aqueous solution and irradiated
using high intensity ultrasonic horn resulting in a very fine dispersion of active phase. The
catalytic performance of the material was investigated by performing dry and steam reforming
reactions. High activity and stability was manifested for both the reactions. The intrinsic kinetics
over a wide range of temperature was studied for the steam reforming reaction and the effect of
inlet concentration of reactants and products on rate of reaction was investigated. A Langmuir-
Hinshelwood mechanism was used to correlate the experimental data.
2. Experimental
2.1Synthesis and characterization
Titanyl nitrate (TiO(NO3)2) solution, and nickel nitrate (S.D Fine, India) were used as
starting materials for the synthesis of 15 % Ni/TiO2. A white colored TiO(OH)2 was precipitated
by the controlled hydrolysis of 5 ml of titanium isopropoxide (Alfa Aesar, India) under ice cold
(4°C). 10 ml HNO3 was added to obtain a clear solution of TiO(NO3)2. In another precursor
solution, 0.99 g of nickel nitrate was dissolved in 20 ml of distilled water and both the solutions
were mixed together. The resulting solution was sonicated for 3 h using a high intensity Ti-horn
probe of 25 mm diameter (50 KHz, and 125 W/cm2 at 60% efficiency). The powder was
separated, washed with water-ethanol mixture, and dried in hot air oven at 120⁰C. The catalytic
activity of 15% Ni/TiO2 catalyst, synthesized by conventional wet impregnation method was
synthesized and compared with the catalyst synthesized via sonication method. For this purpose,
pure known weight of TiO2 (prepared by sonication method) was dispersed in water and nickel
nitrate equivalent to 15 wt % was added. Ni2+ ions were reduced to Ni metals by hydrazine
hydrate (S.D Fine, India) solution at room temperature. The solid was separated and dried at
120° C. Both the powders were further calcined at 700⁰ C for 1 h. The catalyst synthesized via

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sonochemical method is designated as 15% Ni/TiO2 (sonic) while the catalyst synthesized via
impregnation method is designated as 15% Ni/TiO2 (imp).
X-ray diffraction (XRD) patterns were recorded on a Philips X’Pert diffractometer with
Cu-Kα radiation (λ=1.54178 Å) in the 2θ range of 20-80°. Transmission electron microscopy
(TEM; FEI Technai 20) was used to study morphology and microstructures of the catalyst. The
TEM specimen was prepared by dropping a trace amount of sample dispersed in ethanol on a
carbon coated grid (300 mesh). XPS spectra were recorded on a Thermo Scientific Multilab 2000
instrument with monochromatized Al-Kα X-rays (1486.6 eV). The binding energies were charge
corrected using the C 1s peak observed at 285 eV. The BET area measurement was carried out
with a Quantachrome NOVA 1000 gas adsorption analyzer. Prior to the measurement, the
sample was degassed at 150⁰C for 4 h under vacuum. The amount of carbon deposited during
long term stability test was determined using thermo-gravimetric analysis and differential
thermal analysis (TGA/DTA). The experiment was performed on a Mettler Toledo thermal
analyzer under O2 flow of 30 ml/min with a heating rate of 10⁰ C/min.
2.2 Catalytic studies
Dry reforming of CH4 was studied in a fixed bed reactor under atmospheric pressure. The
quartz reactor (4 mm ID and length of 30 cm) was heated in an electric furnace and the
temperature of bed was controlled by a K-type thermocouple positioned in the center of the
catalyst bed. 75 mg of catalyst was packed between two glass wool plugs in the center of reactor.
The feed mixture consisting of 2 % CH4, 2 % CO2 and balance of N2 keeping the total flow rate
at 100 ml/min (GHSV of 95500 h-1 is based on the catalyst bed volume of 0.0628 cm3). The
oxidized catalyst in air showed the formation of inactive NiTiO3 phase and no activity for both
the reactions. Therefore, the catalyst was reduced at 650⁰C for 2 h with pure H2 at a flow of 20

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ml/min before the reaction. The product was analyzed using an on-line gas chromatograph
(Mayura Analytical Bangalore, India) equipped with a TCD and FID (incorporating a
methanator). The conversions (X) and the H2/CO ratio were calculated as follows
CH
 CH

4 in


4 out
(1)
XCH % 
100
100
 
4
CH
4 in
CO  CO

2 in


2 out
(2)
(3)
XCO % 
 
2
CO
2 in
H2
mol of H2 produced
ratio 
CO
mol of CO produced
The bracketed quantity represents the concentration of the component in the product
stream. The activity of the catalyst was measured under steady state at various temperatures. In
order to ensure steady state, the temperature of the reactor was set at desired value and the gases
were allowed to flow over the catalyst continuously. After 15 min, four readings at the same
temperature were averaged. The temperature of the reactor was then set to the next high
temperature and the same procedure was repeated. The average of the four readings was taken
for the calculation and standard deviation of reported conversion was less than 3%. The
experimental data was collected under the absence of any external and internal diffusion
limitation.
The steam reforming reaction was carried over 150 mg of catalyst diluted by required
amount of glass beads. The feed mixture consisting of 3 vol % of CH4 and balance of N2 was
passed at rate of 100 ml/min. This corresponds to the gas hourly space velocity of 48000 h-1
(based on the catalyst bed volume of 0.125 cm3). Water was fed to the steam generator using a
HPLC pump (Waters 515) at flow rate of 0.1 ml/min and the generated vapor (3.6 ml/min) was
mixed with reaction mixture before entering the reactor. A moisture trap was kept at the outlet of

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the reactor to condense any water from the product gas stream. Prior to reaction, the catalyst was
reduced in pure H2 with a flow rate of 20 ml/min for 2 h at 650⁰C. The CH4 conversion (X) and
CO selectivity were calculated as follows
CO  CO

  2 
XCH % 
100
(4)
(5)
 
4
CO  CO  CH

  2   4 
CO


CO selectivity (%) =
100
CO  CO
  2 

The rate of formation of (CO + CO2) was nearly same to the rate of disappearance of CH4 which
indicates that the rate of carbon formation was negligible over the catalyst.
3. Results and discussion
3.1. Structural studies
XRD patterns of both the catalysts before and after the steam reforming reaction are
shown in Fig.1. 15% Ni/TiO2 (sonic) catalyst shows well resolved peaks characteristic of a rutile
structure of TiO2. The peaks observed at 2θ = 76.4, 51.8 and 44.5° can be assigned to the (220),
(200), (111) planes of Ni metal, respectively. However, 15% Ni/TiO2 (imp) catalyst shows peaks
due to metallic Ni (111) along with anatase phases of TiO2. The small peak at 27.4⁰ indicates
that the small amount of rutile phase is also present in 15% Ni/TiO2 (imp) catalyst. The average
crystallite size of Ni was determined by the peak broadening of the (111) reflection in the XRD
patterns, using the Scherrer formula and was found to be 13 nm and 19 nm respectively, for 15%
Ni/TiO2 (sonic) and 15% Ni/TiO2 (imp) catalyst. Further, the close observation of XRD patterns
of 15% Ni/TiO2 (sonic) catalyst shows that the peak of rutile TiO2 phase observed at 27.6⁰ is
shifted to higher values by 0.4⁰ due to decrease in d-spacing. This shows partial substitution of
Ni into TiO2 lattice during synthesis. In contrast, no shift in XRD pattern of 15% Ni/TiO2 (imp)

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catalyst was observed. Therefore, partial incorporation of Ni into TiO2 lattice is possible during
synthesis.
It is well known that Ni/TiO2 catalyst deactivates due to the formation of NiTiO3 phase
and oxidation of Ni species, which make the catalyst more difficult to reduce during the reaction.
Therefore, the XRD patterns were also recorded after the reaction to observe the changes in the
catalyst structure. The XRD patterns of 15% Ni/TiO2 (sonic) shows that the rutile structure of
TiO2 was retained after the reaction and reflections either due to NiO or NiTiO3 were not
observed indicating catalyst is stable and not oxidized during the reaction. For Ni/TiO2 (imp)
catalyst, peaks either due to NiO or NiTiO3 is also absent. However, the formation of the rutile
phase was found to be high after the reaction.
Bright field images of 15% Ni/TiO2 (sonic) and 15% Ni/TiO2 (imp) catalyst are shown in
Fig. 2. In the as synthesized 15% Ni/TiO2 (sonic) catalyst (Fig 2a), Ni particles are spherical and
uniformly distributed over TiO2 support and no aggregation of particles was observed. The
average particle size of Ni species was 8-10 nm. After calcination at 700⁰C, the size of Ni
nanoparticles slightly increased. The average particle size of Ni is about 14-16 nm, which is
similar to the size calculated from the broadening of XRD diffraction patterns. After 16 h of
steady state test reaction, the presence of appreciable carbon was not observed (Fig. 2c), which is
an agreement with TGA/DTA (~0.6 wt %) results. The TEM image shows that Ni particles
retained physical contact with TiO2 support and no considerable agglomeration of Ni species was
observed. However, the average particle size Ni metal is between 16-18 nm. This shows that
there is no appreciable increase in the size of Ni particles (compared to calcinated catalyst)
during the reaction. In contrast, 15% Ni/TiO2 (imp) catalyst form large nanoparticles compared
to 15% Ni/TiO2 (sonic) catalyst (Fig. 2d). The BET surface area for the catalyst before and after

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the steam reforming reaction was found to be 62 and 49 m2/ g, respectively. This reduction in
surface areas may be due to an increase in particles size.
Fig.3 shows XPS of both the catalysts. The NiO species is characterized by the peak at binding
energy of 854.6 eV along with the broad satellite at around 860 eV.27 A peak at binding energy
of 852.6 eV corresponds to Ni0 metal.28 The main Ni (2p) peak was deconvoluted corresponding
to the Ni2+ and Ni0 states. Ti (2p3/2) binding energies are observed at ~ 458.9 eV in both the
catalysts is correspond to a Ti ion in the +4 state.29 The spectra of the spent catalyst were very
broad, indicating the partial reduction of Ti4+ ions to the Ti3+ state. The Ti3+ ion in Ti2O3 is
observed at 458.2 eV.29 It was found that the binding energy of Ni (2p) and Ti (2p) peaks in 15%
Ni/TiO2 (sonic) catalyst were shifted slightly to higher value compared to 15% Ni/TiO2 (imp)
catalyst. The shift in binding energy of Ni (2p) in 15% Ni/TiO2 (sonic) catalyst is due to the
different chemical environment of the substituted Ni ions in the catalyst compared to that of the
pure Ni ions in NiO. The metal-support interaction has previously been observed after steam
treatment of Ni/Al2O3 catalyst and a shift in binding energy of Ni 2p and Al 2p peaks towards
higher value was observed after steam pretreatment.30 The surface concentration of Ni in both
the catalysts was estimated from the intensities of Ni (2p) and Ti (2p) peak. The relative surface
concentration was calculated from
IM   D

M 
M
M
Concentration, CM 
(6)
I
  D





M
M
M
M
IM, λM, σM and DM is the integral intensity of Ni (2p) and Ti (2p) peak, mean escape depths of the
respective photoelectrons, photoionization cross section and geometric factor, respectively. The
photoionization cross-section values and mean escape depths were taken from the literature.31, 32
The geometric factor was taken as 1, since the maximum intensity in this spectrometer is
obtained at 90o. The relative surface concentration of Ni species obtained for 15% Ni/TiO2

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(sonic) and 15% Ni/TiO2 (ionic) was 26 % and 19% at, respectively, which is much higher than
15 wt % (15 wt % corresponds to 17% at) taken in the preparation. After the reaction, the relative
surface concentration of Ni species for 15% Ni/TiO2 (sonic) catalyst was found to be 18%.
3.2 Catalytic performance for dry reforming
The activity of the catalyst for the dry reforming was expressed in terms of conversion of
reactants. The variation of CH4 and CO2 conversions over 15% Ni/TiO2 (sonic) catalyst with
temperature is shown in Fig. 4(a). Both the conversions increased with increasing temperature. In
addition, CO2 conversion was similar to CH4 conversion, which indicates that the contribution of
reverse water gas shift reaction (CO2 + H2 ↔ CO + H2O) is negligible. Nearly 86% CH4
conversion and 84% CO2 conversion was observed at 700°C. The catalysts with the low Ni
loading namely, 5% and 10% over TiO2 were also synthesized and tested for dry reforming
reaction. Both CH4 and CO2 conversions were found to be lower than 15% Ni/TiO2 catalyst. At
700⁰C, 31% CH4 and 34% CO2 conversion was obtained over 5% Ni/TiO2 catalyst while only
59% CH4 and 56% CO2 conversion was obtained over 10% Ni/TiO2 catalyst.
The catalyst performance of 15% Ni/TiO2 (imp) was also compared against the catalyst
synthesized via the conventional wet impregnation method. Ni loading and amount of catalyst in
both cases were kept constant. CH4 and CO2 conversions as a function of temperature over 15%
Ni/TiO2 (imp) are depicted in Fig. 4(c). Nearly, 64% CH4 conversion and 72% CO2 conversion
was obtained at 700°C in the presence of the impregnated catalyst. At high temperature, the rate
of reaction is controlled by diffusion of reactant. Therefore, the rate of reaction was expressed in
mol g-1s-1 and the performance of both the catalysts was compared at low temperature. It was
found that the reaction rates were higher for 15% Ni/TiO2 (sonic) catalyst. Therefore, the catalyst
synthesized by sonication method exhibits higher activity than the catalyst synthesized via

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conventional wet impregnation. The enhancement in reforming activities of 15% Ni/TiO2 (sonic)
catalyst is due to intimate contact between Ni and TiO2 support, as evidenced from XPS studies.
It must be noted that XRD and TEM studies showed that Ni in the catalyst synthesized by
sonication method had smaller crystallites size and high metal dispersion. Therefore, the
enhancement in the activity of the catalyst is related to the intimate contact of Ni and TiO2
support and fine dispersion of the active species. Further, despite an equimolar amount of CH4
and CO2 in the feed, CO2 conversion was higher compared to conversion of CH4 for the
temperature range over 15% Ni/TiO2(imp) catalyst. This indicates that the extent of occurrence
of the reverse water gas shift reaction is higher over the catalyst.
The H2/CO ratio is an important for downstream processes. Fig. 4 (b) and Fig. 4 (d) show
the H2/CO molar ratio, respectively, for 15% Ni/TiO2 (sonic) and 15% Ni/TiO2 (imp) catalyst at
various reaction temperatures. The H2/CO ratio increased with an increase in temperature for
both the catalysts. In particular, 15% Ni/TiO2 (sonic) catalyst reached H2/CO ratio close to 1
above 650°C whereas 15% Ni/TiO2 (imp) catalyst showed H2/CO ratio lower than 1. The low
value of H2/CO ratio (<1) is due to the strong contribution of reverse water gas shift. The
simultaneous occurrence of reverse water gas shift reaction consumes additional H2 and produces
extra CO, which lowers the H2/CO ratio.
3.3 Kinetic study of steam reforming
Fig. 5(a) shows the variation of CH4 conversion and CO selectivity with temperature over
15% Ni/TiO2 (sonic) catalyst. Both CH4 conversion and CO selectivity increased with increase in
temperature. Nearly 92% CH4 conversion with 77% CO selectivity was observed at 700°C. The
CH4 conversion below 400°C was small and a sharp rise in the CH4 conversion was observed
above 400°C indicating high energy barrier for this reaction. Further, at low temperature, CO

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selectivity is low, which implies a large contribution from WGS reaction. The effect of steam
concentration on the CO selectivity is also depicted in Fig. 5(b). The CO selectivity decreased
with an increase in the inlet steam concentration due to promotion of the WGS activity.
The rate of reaction and activation energy for reforming over 15% Ni/TiO2 (sonic)
catalyst was measured by performing experiments with different amounts of catalyst loading
keeping the total flow rate constant at 100 ml/min. The mixture consisting of 3% of CH4, 3% of
H2O and balance of N2 with the total flow rate of 100 ml/min was used. All experiments were
performed under isothermal and differential conditions at atmospheric pressure over a
temperature range of 450-550⁰C and the rate of reaction at various temperatures was calculated
using following equation.
F  x
W
x
rate(r) 

(7)
W / F
F, W and x is flow of the gas in mol/s, weight of the catalyst in g and fractional CH4 conversion,
respectively. Fig. 6(a) shows the variation of W/FCO with the fractional conversion of CH4 at
various temperatures. The plot of fractional conversion (x) with W/FCO is linear up to 40%
conversion and non linearity at higher CH4 conversion indicates that the differential reactor
approach is not valid at high temperature. Therefore, the rates of reaction were calculated from
the slope of linear portion. The variation of rate of reaction with temperature is shown in Fig.
6(b). The apparent activation energy was calculated from the Arrhenius plot (see inset of Fig. 6
(b)) and found to be 105 kJ/mol.
The effect of concentration of CH4, steam and CO on the rate of reaction was also
investigated to understand the reaction kinetics. The rate of reaction was measured in a
differential reactor in the absence of any transport artifacts under atmospheric pressure. All of
the experiments were carried over 50 mg of catalyst with a total flow of 100 ml/min and reactor

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temperature was varied such that the differential reactor approach (CH4 conversion ~25%) was
always maintained. The concentration of CH4 was varied between 1 to 4 % keeping the steam
concentration constant at 3.6 %. For another set of experiments, the concentration of steam was
varied between 1 and 8% keeping the methane concentration constant at 3%. The effect of CO
concentration on the rate of the reaction was also independently examined. The concentration of
CO was varied in the range of 0.25 to 0.75%, while the inlet concentrations of CH4 (3%) and
steam (3.6%) were kept constant.
Fig. 7(a) shows the variation of rate of reaction with concentration of CH4 at various
reaction temperatures. The rate of reaction increases with the concentration of CH4 at all
temperatures. The order of reaction with respect to concentration of CH4 was determined by
plotting the variation of rate of reaction with concentration on a log-log scale. In general, the
reaction is first order with CH4 concentration for all temperatures and it is consistent with rate
determining step, namely CH4 chemisorption. Fig. 7(b) shows the variation of rate of reaction
with steam concentration. A strong negative effect of steam concentration on rate of reaction was
observed. This effect is attributed to an optimum concentration of steam and CH4 coverage on
the catalyst surface. The negative order of reaction with steam concentration indicates that CH4
and steam undergo competitive adsorption on the catalyst for the same active sites. However, no
maximum in the rate of reaction was observed when CH4 concentration was varied. This
observation is in line with a reaction scheme in which the rate limiting step is the activation of
CH4 molecules. The effect of inlet concentration of CO on the steam reforming is also depicted
in Fig. 7(c). The rate of reaction decreases with increasing inlet concentration of CO. This effect
is due to the chemisorption of CO interfering with CH4 chemisorption on the same active surface
sites. The qualitative trends presented above elucidate major mechanistic aspects of the steam

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reforming reaction. Further, the measured rates reflect intrinsic kinetics without any mass
transfer effects. Therefore, these observations were used to propose the reaction mechanism for
steam reforming reaction.
3.4 Kinetic model
The kinetics of steam reforming has been studied extensively over noble metals (Ru, Rh,
Pt) and nickel based catalysts. However, there is no agreement in the reaction mechanism and the
corresponding rate expression. Several expressions including Langmuir-Hinshelwood, power
laws, and expressions based on micro kinetic analysis have been proposed to describe the kinetic
of reforming reaction.6, 33 This is likely due to the different nature of the active species, identity
of support, catalyst morphology (shape and size), different experimental conditions (synthesis
method and testing condition etc.) and metal loading.20 In most of the studies, the dissociative
methane adsorption reaction is assumed to be a rate determining step.8, 32 Steam and dry
reforming over Ru catalyst supported on various supports have been studied and a first order
kinetic equation on CH4 concentration was proposed. No dependence on the nature of support,
metal loading and concentration of H2O or CO2 was observed.19, 34 The effect of various forms of
support on the activity of the reaction has also been studied. Berman et al. have reported a
negative reaction order in steam concentration, and speculated that the adsorption of steam
occurs on Al2O3/MnOx support.35 The kinetic model involving CO2 interaction with the support
was proposed for dry reforming of CH4 over Ru/La2O3 catalyst.36 On the other hand, no
influence of the support on the reaction rate has also been reported.22, 37
The reforming reaction was studied over the nickel foil at 900°C at atmospheric pressure
and the rate of reaction is satisfactory described by a simple first order equation ( rate  kP ),
CH4
where
P
is the partial pressure of the CH4 and k is the rate constant.38 However, a simple first
CH4

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order equation cannot be used to describe the kinetics of this reaction because the inlet partial
pressure of H2O has a significant influence on the rate of reaction. The high CO coverage was
observed over the catalyst during the reaction.5 The present study also showed that the negative
effect of concentration of H2O and CO on the reaction. Therefore, the development of an
improved model with the effect of concentration of steam and products on the overall rate of
reaction is necessary. It should be noted that the kinetic models for methane reforming are quite
different from each other and depend on the experimental conditions.20
Based on our observations and available literature, we have proposed a set of elementary
steps and the kinetic expression was derived using a Langmuir-Hinshelwood kinetic by
considering the CH4 dissociative adsorption as the rate limiting step.39
CH4 *K1 C*2H2
(8)
K2
H2O 2*OH *H *
(9)
k3
OH **O*H *
(10)
(11)
(12)
(13)
k4
C*O*CO*
k5
CO*CO*
k6
H *H *H2  2*
Equation (8) represents the decomposition of CH4 into the adsorbed surface containing
species (CH3*, CH2*, CH* and H*). The rate of hydrogen removal in subsequent steps is much
higher than that of the initial C-H bond rupture. Therefore, subsequent H-abstraction from CH4
molecules is represented by a single equation. The negative order with respect to H2O shows that
there is competitive adsorption between CH4 and steam for the same active sites. Therefore,
equation (8) and (9) assume that the activation of CH4 and steam compete for same active sites.

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The rapid steam adsorption gives the surface oxygen species (equation (10)) and the carbon
species formed on the surface. This reacts rapidly with surface oxygen species resulting in
product, CO (equation (11)).40 Further, high WGS indicate that CO adsorption over the catalyst
surface is considerable. The rate of reaction decreases with increasing inlet concentration of CO
due to interference of CO with CH4 chemisorption for the same surface sites (equation (12)).
The rate of reaction is first order with respect to CH4 concentration and this implies that
the dissociative adsorption of CH4 is a rate determining step. Therefore, while deriving kinetic
expression, it was explicitly assumed that the dissociative adsorption of CH4 (equation 8) is a
rate limiting step. Solving the above set of elementary steps, the following rate expression is
obtained.
K1C
CH4
rate 
(14)
2
K C  K2C  K5CCO


1
CH4
H2O
The above kinetic expression is consistent with the observations that the dissociative CH4
adsorption step, a rate determining step and activation of H2O molecules takes place over a
single metal surface atom without involvement of the support. The values of K1, K2 and K5 were
obtained simultaneously using non-linear regression. Fig. 8 shows the predicted rate of reaction
corresponding to the experimental observed reaction rate. The model without inhibition term due
to CO adsorption was also tried to fit the data but this resulted in poor fitting. Therefore, the
adsorption of CO over the catalyst is substantial and cannot be ignored. The high activity of the
catalyst for WGS reaction is also manifested this fact. The optimized values for K1, K2 and K5
are 68 exp(-4400/T), 5400 exp(-4900/RT) and 1200 exp(-1600/RT), respectively.

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Stability and deactivation studies
The dependence of catalytic activity on time on stream over 15% Ni/TiO2 (sonic) catalyst
was studied at 650⁰ C for 16 h for both dry and steam reforming reactions. Fig. 9 (a) shows the
variation of CH4, CO2 conversions and H2/CO ratio over 16 h for dry reforming reaction. The
stable conversion value for CH4 and CO2 were 65% and 67%, respectively. A small drop (about
2-3% ) in both the conversions was observed for the initial few hours, which might be due to
some modification of the catalyst surface causing instability in the carbon deposition.41 However,
the H2/CO ratio (~0.96) was nearly stable over 16 h reaction period. The stability of 15%
Ni/TiO2 (imp) catalyst was also studied at 650⁰ C for 16 h for dry reforming reaction. Fig. 9 (b)
shows the variation of CH4, CO2 conversions and H2/CO ratio over 16 h. The time on stream test
shows that 15% Ni/TiO2 (imp) catalyst is less stable and more than 30% loss of initial activity
was observed during first 4 h of reaction. Fig. 9(c) shows the stability for steam reforming
reaction. There was a slight decrease in CH4 conversion and CO selectivity for the initial period.
However, a stable CH4 conversion (~75) and CO selectivity (~62) was observed after 4 h.
Therefore, the catalyst has high activity and excellent stability for both the reactions.
TGA analysis was used to estimate the deposited coke over the catalyst. Fig. 10 shows
the TGA/DTA analysis performed in the presence of pure O2 of the spent catalyst on stream. The
thermogram was divided into three different temperatures region. The first region below 260⁰C
showed a minimal weight loss and this can be ascribed to the loss of absorbed moisture and
volatile species, such as reactants and products.42 The second region between 300⁰C and 600⁰C
showed an increase in weight of the catalyst due to oxidation of the Ni particles. Finally, the
third region above 600⁰C showed decrease in the weight of the catalyst due to oxidation of
deposited coke with different degree of graphitization. The amount of coke deposited on the

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spent catalyst was found to be around 0.6 wt%. The catalyst synthesized by sonochemical
method showed less coke deposition due to stronger metal support interactions, contributing to
good reaction stability. Meanwhile, it has been reported that relatively large Ni particles are
vulnerable to coke formation.43 Therefore, nanosized Ni particles and strong metal support
interactions facilitate carbon gasification via H2O dissociation and oxygen spillover to Ni
particles.
5. Conclusions
The highly dispersed supported Ni nanoparticles over TiO2 were successfully prepared
via sonochemical assisted method and the performance of the material was investigated for dry
and steam reforming of methane. The catalyst synthesized by the present approach exhibited
excellent catalytic activity and stability for both the reactions than the catalyst prepared by
incipient wetness impregnation method. For dry reforming, 86% CH4 and 84% CO2 conversion
was obtained at 700⁰C. Nearly 92% CH4 with 77% CO selectivity was observed at 700⁰C for
steam reforming reaction. The high activity and stability of the catalyst is attributed to the fine
dispersion (due to small metallic Ni clusters) of Ni species. The present catalyst is remarkably
active and stable even after long period and no appreciable coke deposition was observed.
Therefore, the present approach is a simple and feasible for the synthesis of supported metal
catalyst, which can be used to enhancing the catalytic performance.
Acknowledgments
Authors gratefully acknowledge the financial support from Gas authority of India Limited.

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Figure captions
Fig. 1 XRD patterns of (a) 15% Ni/TiO2 (imp) before reaction (b) 15% Ni/TiO2 (imp) after
reaction, (c) 15% Ni/TiO2 (sonic) before reaction and (d) 15% Ni/TiO2 (sonic) after reaction
Fig. 2 Bright field images of 15% Ni/TiO2 (sonic) catalyst (a) as-synthesized (b) calcinated at
700⁰ C for 1 h (c) after reaction and (d) 15% Ni/TiO2 (imp) catalyst
Fig. 3 Core level XPS of (a) Ni 2p and (b) Ti 2p.The numbers (1), (2) and (3) represent spectra
for 15% Ni/TiO2 (imp) catalyst, 15% Ni/TiO2 (sonic) catalyst before and after the reaction,
respectively
Fig. 4 Catalytic activity for the dry reforming (a) CH4 conversion and CO2 conversion (b) H2/CO
ratio as a function of temperature for 15% Ni/TiO2 (sonic) catalyst and (c) CH4 conversion and
CO2 conversion (d) H2/CO ratio as a function of temperature for 15% Ni/TiO2 (imp) catalyst
Fig. 5 Variation of (a) CH4 conversion and (b) CO selectivity with H2O/CH4 ratio at various
temperature over 15% Ni/TiO2 (sonic) catalyst for steam reforming reaction
Fig. 6 (a) Variation of fractional conversion of CH4 with W/FCH4 and (b) rate of reaction as
function of temperature for steam reforming reaction over 15% Ni/TiO2 (sonic) catalyst
Fig. 7 (a) The effect of concentration of CH4 on the rate of methane conversion at a constant
steam concentration of 2.7% (b) effect of H2O concentration on the rate of methane conversion

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at a constant methane concentration of 3% (c) effect of CO concentration on the rate of methane
conversion at constant methane (3%) and steam concentration (2.7%), respectively
Fig. 8 Comparison between experimentally measured rate and calculated rate from the model for
steam reforming reaction
Fig. 9 Time on steam (a) CH4, CO2 conversion and H2/CO ratio for dry reforming of methane
over 15 % Ni/TiO2 (sonic) catalyst, (b) CH4, CO2 conversion and H2/CO ratio for dry reforming
of methane over 15 % Ni/TiO2 (imp) catalyst and (c) CH4 conversion and CO selectivity for
steam reforming reaction over 15 % Ni/TiO2 (sonic) catalyst
Fig. 10 TGA-DTA plot for the catalyst under pure oxygen after on stream in steam reforming
reaction over 15 % Ni/TiO2 (sonic) catalyst

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*
*
*
(d)
*

*

*
*
*

*
*
*
(c)

*
*


*
*


*
*
*
*
o
o
*
(b)
o
o
o
o
o
o
o

o
*
(a)

o
*

o

*
*
*
20
30
40
50
60
70
80
2(deg.)
Fig. 1 XRD patterns of (a) 15% Ni/TiO2 (imp) before reaction (b) 15% Ni/TiO2 (imp) after
reaction, (c) 15% Ni/TiO2 (sonic) before reaction and (d) 15% Ni/TiO2 (sonic) after reaction (o-
anatase, *-rutile and #- Ni metal)

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Fig. 2 Bright field images of 15% Ni/TiO2 (sonic) catalyst (a) as-synthesized (b) calcinated at
700⁰ C for 1 h (c) after reaction and (d) 15% Ni/TiO2 (imp) catalyst

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0
2+
Ni (2p)
Ni (2p)
(a)
*
0
2+
Ni (2p)
satellite
Ni (2p)
*
*
0
0
Ni (2p)
Ni (2p)
*
852
855
858
861
864
867
Binding Energy (eV)
4+
Ti (2p)
(b)
3+
Ti (2p)
(3)
4+
Ti (2p)
(2)
4+
Ti (2p)
(1)
456
458
460
462
464
466
Binding Energy (eV)
Fig. 3 Core level XPS of (a) Ni 2p and (b) Ti 2p.The numbers (1), (2) and (3) represent spectra
for 15% Ni/TiO2 (imp) catalyst, 15% Ni/TiO2 (sonic) catalyst before and after the reaction,
respectively

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100
80
60
40
20
0
CH
4
(a)
CO
2
400
450
500
550
600
650
700
1.0
(b)
0.8
0.6
0.4
0.2
0.0
400
450
500
550
600
650
700
Temperature ( C)
100
80
60
40
20
0
CH
4
(c)
CO
2
400
450
500
550
600
650
700
1.0
0.8
0.6
0.4
0.2
0.0
(d)
400
450
500
550
600
650
700
Temperature (°C)
Fig. 4 Catalytic activity for the dry reforming (a) CH4 conversion and CO2 conversion (b) H2/CO
ratio as a function of temperature for 15% Ni/TiO2 (sonic) catalyst and (c) CH4 conversion and
CO2 conversion (d) H2/CO ratio as a function of temperature for 15% Ni/TiO2 (imp) catalyst

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(Gas composition: 2% CH4, 2% CO2 and balance N2 with total gas flow 100 ml/min, over 75 mg
of catalyst. Dotted line represents the equilibrium conversion)