Methanation on Nickel-Alumina Catalyst
Anchal Jatale
Abstract
Although all the Group VIII metals are catalytically active in the hydrogenation of
carbon oxides to form methane ("methanation"), nickel remains the favored catalyst
for the reaction by virtue of its life, high activity, selectivity towards methane
formation in preference to other hydrocarbons, and it’s comparatively low cost.
Supported nickel catalysts are preferred on account of their thermal and mechanical
stability. In this paper, Ni/Al2O3 catalyst is discussed at length and subsequently its
effect on methanation reaction along with the mechanism is also explained. TPR of
Ni/alumina catalysts gives evidence that they have 2 sites for reaction, surface NiO
and a form of NiAl2O4.Methanation of CO proceeds via dissociation on nickel
catalyst and the surface carbon species thus formed are hydrogenated to methane. CO2
methanation proceeds via conversion of CO2 to CO via the reverse water gas shift
reaction followed by CO methanation.
Introduction:
Reaction between hydrogen and carbon monoxide can lead to a variety of useful
products depending on reaction conditions, ratio of the gases in the feed and the type
of catalyst. Among the reactions of greatest interest are those producing methane,
paraffins, alcohols, especially methanol, and olefins. Methane from hydrogen and
carbon monoxide ("methanation") is receiving particular attention because it is an
essential step in one process for the manufacture of substitute natural gas (SNG) gas
from coal. Methanation is practiced also for another purpose. In the manufacture of
ammonia by the catalytic reaction of hydrogen and nitrogen the content of oxides of
carbon in the synthesis gas must be reduced to a very low level to prevent catalyst
poisoning. To effect this, after conversion of carbon monoxide to carbon dioxide by
water gas shift and absorption of the latter, residual oxides of carbon are removed by
methanation, a simple and relatively inexpensive process. The methane produced is
inert to ammonia synthesis catalysts.
The reaction between carbon monoxide and hydrogen over a nickel catalyst to
produce methane was first reported by Sabatier and Senderens (1902, 1905) in the
early part of this century, and since then, despite much research, nickel had continued
as the major catalyst for the reaction because of its high activity, selectivity for
methane formation and low cost.
Catalyst Preparation
Supported nickel catalysts are widely used in a number of industrial processes such as
hydrogenation, methanation. The activity of the supported catalysts is strongly
dependent on the preparation method used and on the choice of reagents and support.
There are many methods which can be used to prepare the catalyst depending upon
the requirements. There are various methods of preparation of Ni/Al2O3 catalyst. Two
most important methods will be discussed in brief:
Wet Impregnation method: The Pretreated γ-alumina is added to the solution of
Ni(NO3)2 .6H2O in demineralized water and the mixture is dried in desiccator for 6
hours. Then through a programmed increase in temperature from 110oC to 500 oC ,
the mixture is calcined at 500oC for 6 hours. Instead of using Nickel nitrate, Nickel
acetate can also be used.
Coprecipitation method: The coprecipitated Ni/A12O3 catalysts were prepared by
co-precipitation of Ni (NO3)2 .6H2O and Al (NO3)3 in an aqueous solution with NaOH
at ambient temperature. The precipitates, matured for 24 hrs, were vacuum filtered
and subsequently washed until free of nitrate. The air dried samples (105°C, 48 h)
were ground in an agate mortar and the powders obtained were air dried again (105°C,
6 h) [1].
Co-crystallization Method: Nickel (II) nitrate hexahydrate in de-ionized water was
added, with stirring, to a gelatinous solution of alumina in nitric acid previously
prepared by the slow dissolution of alumina in concentrated nitric acid. The resultant
solution was neutralized (Final pH 7), with continuous stirring, by the careful drop-
wise addition of ammonia. After standing overnight water was slowly removed (48 h)
on a rotary evaporator and the resulting solid dried in a vacuum oven [2].
Atomic Layer Epitaxy Method: Vaporized Nickel acetylacetonate was chemisorbed
on a porous alumina support, and the produced surface complex was then air treated
to remove the ligand residues [3].
Ni-alumina catalysts are also synthesized by one-step sol-gel method using micelle
complex comprising lauric acid and nickel ion as a template with metal source and
using aluminum sec-butoxide as an aluminum source [4].
Mathematical Modeling of Impregnation Process [5]:
Recent studies have tried to quantify the impregnation method for preparation of
Ni/Al2O3. The hypotheses adopted to solve the mathematical model were based on the
one-dimensional single-pore model with cylindrical geometry, concentration gradient
only in the axial direction of the pore (i.e. radial gradients inside the pellet) and time-
dependent plug-flow velocity of the penetrating liquid.
The partial differential equation corresponding to mass balance is:
c c 2c
vp D 2 ……………………………………. (1)
t z z
The effect of the concentration gradient close to the pore wall is explained by the
mechanisms of impregnant removal of the solution. Thus, a term describing the
impregnating removal rate of system V(c, θ) is added to Eq. (1):
c c 2c
vp D 2 V (c, ) ……………….. (2)
t z z
To non dimensionalize Eq. (2), the following definitions are used:
z c t Dt v pt
, , , 2 , u
L co tL L L
t
u L V (co , ) (First term of RHS is neglected since α <<1)
cO
With:
ψ (0, τ) = 1 and ψ(0,0) = 1
After some algebraic steps, we arrive at following two equations:
1
1/ 2 K
2 K L (1 )
K
K L (1 )
where K = 2kmtL/r, η = 2cs/r*co, KL = KL’ co
These three variables describe the impregnation process is observed: K, reduced mass
transfer coefficient; η, relative capacity of adsorption of the pore wall and KL,
adsorption equilibrium constant.
Table 1 shows the values of these constants obtained experimentally.
Catalyst Characterization:
Wet chemical analysis: The amount of nickel in the oxide form is determined by
acidic extraction of nickel from the sample, followed by dimethylglyoxime
precipitation [6].
Total surface area: The surface area is determined by dynamic low temperature
nitrogen adsorption in helium as a carrier. The data were interpreted using the BET
equation and an effective N2 cross-sectional area of 16.2 nm2 [6].
Porosity: The total porosity (Q) was calculated from apparent (a) and true (d)
densities, Q (%) = (1 -a/d) 100, determined pycnometrically using mercury and
benzene vapors as working fluids [6].
Pore size distribution: Pore size distribution is determined by mercury porosimetry,
pressure range up to 1500 bars [6].
X-ray diffraction: Identification of crystalline compounds and determination of
average crystallite size are carried out by X-ray diffraction (XRD) using a G.E.
diffractometer with Ni filter and CuKα radiation. The mean crystallite size (D) is
related to the pure X-ray broadening (β) by the Scherrer formula
D = Kλ/β cosθ
The size (D) is defined as (volume) 1/3 leading to a value K = 0.95 when β is defined
as the half-maximum linewidth. The half-maximum linewidth from NiO (220)
reflecting plane is employed for the alumina-supported .The instrumental line
broadening is determined from the half-maximum linewidth of a single silicon crystal
[6].
In Fig. 1 XRD diffractograms of catalysts and y-A1203 are shown. The
diffractogram of the 15% catalyst shows strong NiO lines at 2θ values of 37.3,
43.3 and 62.9”) confirming a presence of “free” nickel oxide. For both catalysts no
distinct nickel aluminate lines are observed. On the diffractogram of the reduced 5%
catalyst one can observe lines characteristic for nickel crystallites at 28= 44.5 and
51.8” [7].
SEM studies: The catalysts are examined in a scanning electron microscope with
resolution of about 500- 1000 A. The samples are coated with gold to about 100-200
A thickness prior to observation(Fig 2,3,4,5 ) [6] .
Temperature Programmed Reduction (TPR): TPR determines the number of
reducible species present in the catalyst and reveals the temperature at which the
reduction occurs. TPR analysis begins by flowing analysis gas (typically hydrogen in
an inert carrier gas such as nitrogen or argon.) over the sample, usually at ambient
temperature. While the gas is flowing; the temperature of the sample is increased
linearly with time [8]. (β K min-1)
The Rate of reaction is monitored:
1. By measuring concentration or pressure changes in the gas phase (reactants or
products).
2. By observing weight changes of the solid through a micrometer microscope.
TPR had advantages over other characterization techniques, such as:
It is highly sensitive and does not depend on any specific property of the solid
other than its reducibility.
Non destructive, as solid does not have to be dissolved.
Even the condition that the solid must be reducible is not mandatory. (better
than spectroscopic and X-ray techniques).
The apparatus is inexpensive when compared to X ray and spectroscopic
techniques.
Robust and minimal maintenance required.
Fig 6 , shows TPR of Ni/Al2O3 made from three different precursors Nickel nitrate,
Nickel Chloride and acetone solution of Nickel-acetylacetonate (NiAA). Ni (NO3)2
catalyst is easily reduced and shows the first peak in the 250±350oC range, whereas
for NiCl and NiAA catalysts the first peak occurs at ca. 400C.
The three catalysts also demonstrate two other peaks of H2 uptake at ca. 600C and
750C. The first TPR peak corresponds to the reduction of NiO. The two latter peaks
are due to the reduction of nickel aluminate. So it is confirmed from the TPR profiles
that Ni/alumina catalysts have 2 sites for reaction, surface NiO and a form of
NiAl2O4.On reduction of surface NiO will form nickel crystallites, but reduction of
Ni2+ ions in octahedral sites in the alumina lattice will form Ni atoms surrounded by
oxygens of he alumina lattice. Thus there appear two forms of reduced nickel on
Ni/Al2O3 catalysts [9]. (Fig 7)
Dispersion of NiO on γ-Al2O3:
Dispersion of NiO/ γ-Al2O3 system has been extensively investigated and the state of
the dispersed nickel oxide has been proposed. Ni2+ ions preferentially incorporated
into the tetrahedral surface vacancies at low NiO loadings, and both the tetrahedral
and octahedral nickel oxide species were present at the high NiO loadings. So, two
kinds of surface vacant sites e.g. octahedral and tetrahedral sites exist on the exposed
plane of γ-Al2O3 [10]. (Fig 8 )
Methanation
Hydrogenation of CO and CO2 to methane on nickel catalysts are important reaction
occurring in purification of ammonia feeds and methanation of coal derived gas. Both
reactions are also of interest in the production of process heat or power from
reclaimable waste streams containing dilute carbon oxides or from nuclear reactor
steam reformed CO-H2 streams as part of a so called “heat pipelines”.
The reactions involved are
CO + 3H2 CH4 + H2O ΔH= -49 kcal/mol
CO2 + 4H2 CH4 + H2O ΔH= -39 kcal/mol
Methanation of CO and CO2 occurs effectively on various transition metal catalysts. It
was observed that CO is dissociated on nickel catalyst in methanation of CO and the
surface carbon species thus formed are hydrogenated to methane. CO2 methanation
mechanism could be categorized into two (i) conversion of CO2 to CO via the reverse
water gas shift reaction followed by CO methanation and (ii) direct hydrogenation of
CO2 to methane by a mechanism distinct from CO methanation. But various studies
showed that the first mechanism may be correct and that CO2 methanation may
proceed via Co2 dissociation to CO and atomic oxygen followed by further
dissociation to CO to carbon intermediate which is hydrogenated to methane [11].
CO Methanation Mechanism CO2 Methanation Mechanism
H2(g) +2 S 2 H-S (1) H2(g) +2 S 2 H-S (1)
CO(g) + S CO-S (2) CO2 + 2 S CO-S +O-S (2)
CO-S +S C-S + O-S (3) CO-S CO(g) + S (3)
C-S + H-S CH-S + S (4) CO-S +S C-S + O-S (4)
CH-S + H-S CH2-S + S (5) C-S + H-S CH-S + S (5)
CH2-S +H-S CH3-S + S (6) CH-S + H-S CH2-S + S (6)
CH3-S +H-S CH4-S + S (7) CH2-S +H-S CH3-S + S (7)
CH4-S CH4(g) +S (8) CH3-S +H-S CH4-S + S (8)
O-S +H-S OH-S +S (9) CH4-S CH4(g) +S (9)
OH-S + H-S H2O-S +S (10) O-S +H-S OH-S +S (10)
H2O-S H2O(g) +S (11) OH-S + H-S H2O-S +S (11)
H2O-S H2O(g) +S (12)
Now based on this mechanism given above we can derive many different langmuir-
Hinselwwood expressions. For example consider mechanism of CO2 methanation
1) Assuming CO2 adsorption to be the rate determining step we get :
k 2 L2 PCO 2
r
(1 K1 H 2 ) 2
1/ 2 1/ 2
2) Assuming H2 adsorption to be rate determining step we get:
k1 L2 PH 2
r
(1 K1 PCO 2 ) 2
1/ 2 1/ 2
Here,
r=rate
L=total concentration of surface sites.
PH2=partial pressure of hydrogen
PCO2=partial pressure of Carbon di-oxide
Similarly one can get lots of rate expressions considering other steps as rate
determining steps.
Recent studies show that there exist mechanisms in which both the sites present on
Ni/Al2O3 play different role. Let this two sites be site 1 (*) and site 2 (#).
(i) CO Methanation:
CO + * CO* (1)
0.5 H2 + * H* (2)
CO* + # O* + C# (3) (rate determining step)
C# + 2 H2 CH4 + # (4) (fast)
O* + H2 H2O + * (5) (fast)
In this mechanism CO molecules compete with hydrogen atoms for type 1 sites (*)
at the nickel surface and CO dissociates to a type 2 site (#), which are always free at
the conditions used during the study. Reaction 3 is the rate determining step,
reactions 1 and 2 are so fast that CO* and H* are in equilibrium with the gas-phase
species CO and H2, and reactions 4 and 5 are so fast that the coverages of C# and
O* are negligible [12].
(ii) CO2 methanation :
H2 + 2* 2H* (1)
CO2 + 2# OC# + O# (2)
2H* + OC# H2O + 2A’ + CA (3)
2H* + O# H2O +2* + # (4)
H2 + C# H2C# (5)
H2 + H2C# CH4 +# (6)
The rate expression is derived using some assumption like steps(1) and (2) are
much slower than steps(3)-(6) .Thus we can say that the reaction is on1-half order
in both reactants, provided ;(i)the coverage is relatively low and (ii) slow
dissociation follows rapid adsorption.
Effect of addition of promoters on Ni/ Al2O3 for methanation reaction [13]
Recent studies have characterized the effect of cerium, lanthanum and zirconium on
nickel/alumina catalysts with respect to CO and CO2 hydrogenation. The
examinations of O2 and CO adsorption demonstrate that the promoters decrease rate
of adsorption but do not affect quantity of the adsorption. On the other side, the
promoters do not affect TP desorption of hydrogen and TP hydrogenation of pre-
adsorbed oxygen which indicates that the promoters do not modify the state of
hydrogen and oxygen adsorbed on nickel.
Fig 9, shows that in contrary to zirconium, cerium and lanthanum considerably
increase conversion of CO and CO2 to methane
Highly selective methanation [14]:
In the present, selective methanation became attractive in order to reduce the CO
content to less than 100 ppm in hydrogen-rich reforming gases for fuel cell
applications. One way to do highly selective methanation is to use micro channel
reactors. Studies showed that a Ru/Al2O3 and Ru/SiO2 catalyst were successfully
applied to micro reactors to reduce the CO content by methanation in a model gas
mixture that contains the major product components of a natural gas reformer or of a
partial oxidizer used for hydrogen production.
Other catalysts for methanation:
As methanation is one of the very important reactions there are various other catalysts
which can be used for hydrogenation of Co and CO2 in effective manner. Few of
them are:
1: Ni/SiO2 : The catalyst was obtained by contacting silica with a solution of nickel
nitrate hexamine. The solid was dried, then crushed to powder, and reduced at 650
ْC for 15 h at 2ْC/min heating rate in flowing hydrogen (5 liters /h) [15].
2: Ultrafine Ni (II) Ferrite Catalysts: prepared by Ni2+ substitution for Fe2+ in
magnetite. Ni (II) ferrites were prepared by two different methods [16]:
(i) Coprecipitation of Ni2, Fe2, and Fe3+ at 60ْC followed by heating to
300ْC
(ii) Oxidation of aqueous suspension of Fe2+ and Ni2+ hydroxides at 65ْC
3: Ni/ RHA-Al2O3: Rice husk ash and aluminum nitrate were used in preparation of
rice husk ash-alumina support. Then Nickel nitrate is used and the catalyst with
desired loading is prepared using wet impregnation [17].
4: Ni-Ca aluminate on β-A1203 hydrate: A blend of powdered nickel oxide, calcium
aluminate and alumina hydrate was water-sprayed and Catalyst and rolled to form
pellets, which were calcined to a highly active catalyst.
There exist a wide range of catalysts for methanation. Many transition metals like
Nickel, Iron, and Cobalt are also active catalysts but Ni catalysts are more selective
and efficient [15].
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APPENDIX
Table 1:
Fig. 1: XRD patterns for (a) NiO, (b) 15% NiO/ γ-Al2O3 (c) 5% NiO/ γ-Al2O3, (d)
5% NiO/-Al2O3 catalyst reduced in the TPR process (b’, c’, d’: subtracted XRD
patterns); (8) γ-Al2O3
Fig. 2. Scanning electron micrograph of wA1203 at x 20,000.
Fig. 3. Scanning electron micrograph of NiO/α(-A1203-750 at x5000.
Fig. 4. Scanning electron micrograph of NiO/wA1203-850 at x5000.
Fig 5. Scanning electron micrograph of NiO/a-A1203-1050 at ~20,000.
Fig 6: TPR profiles. a) NiAl2O4, b) NiO, c) NiO/Al2O3 catalyst precursor(10.3%
Ni)
Fig 7: TPR profiles of Ni catalyst from different precursors.(Ni 7.7%)
Fig 8: Tentative model of the surface-dispersed nickel oxide species formed on
the (110) plane of γ-Al2O3 support with a Ni2+ ion incorporating in a tetrahedral
vacant site.
Fig. 9. Activity of un-promoted and Ce-, La- and Zr-promoted catalysts:
(A) CO hydrogenation and (B) CO2 hydrogenation.