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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"), nic el remains the favored catalyst
for the reaction by virtue of its life, high activity, selectivity to!ards methane
formation in preference to other hydrocarbons, and it s comparatively lo! cost#
$upported nic el catalysts are preferred on account of their thermal and mechanical
stability# In this paper, %i&Al' catalyst is discussed at length and subse*uently its
effect on methanation reaction along !ith the mechanism is also explained# + - of
%i&alumina catalysts gives evidence that they have ' sites for reaction, surface %i
and a form of %iAl ' . #/ethanation of 0 proceeds via dissociation on nic el catalyst
and the surface carbon species thus formed are hydrogenated to methane# 0 '
methanation proceeds via conversion of 0 ' to 0 via the reverse !ater gas shift
reaction follo!ed by 0 methanation#
Introduction:
-eaction bet!een hydrogen and carbon monoxide can lead to a variety of useful
products depending on reaction conditions, ratio of the gases in the feed and the type
of catalyst# Among the reactions of greatest interest are those producing methane,
paraffins, alcohols, especially methanol, and olefins# / ethane from hydrogen and
carbon monoxide ("methanation") is receiving particular attention because it is an
essential step in one process for the manufacture of substitute natural gas ($%G) gas
from coal# /ethanation 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 lo! level to prevent catalyst
poisoning# +o effect this, after conversion of carbon monoxide to carbon dioxide by
!ater gas shift and absorption of the latter, residual oxides of carbon are removed by
methanation, a simple and relatively inexpensive process# +he methane produced is
inert to ammonia synthesis catalysts#
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+he reaction bet!een carbon monoxide and hydrogen over a nic el catalyst to
produce methane !as first reported by $abatier and $enderens (123', 1234) in the
early part of this century, and since then, despite much research, nic el had continued
as the ma5or catalyst for the reaction because of its high activity, selectivity for
methane formation and lo! cost#
Catalyst Preparation
$upported nic el catalysts are !idely used in a number of industrial processes such as
hydrogenation, methanation# +he activity of the supported catalysts is strongly
dependent on the preparation method used and on the choice of reagents and support#
+here are many methods !hich can be used to prepare the catalyst depending upon
the re*uirements# +here are various methods of preparation of %i&Al ' catalyst# +!omost important methods !ill be discussed in brief6
Wet Impregnation method 6 +he retreated 78alumina is added to the solution of
%i(% )' #9: ' in deminerali;ed !ater and the mixture is dried in desiccator for 9
hours# +hen through a programmed increase in temperature from 113 o0 to 433 o0 ,
the mixture is calcined at 433 o0 for 9 hours# Instead of using %ic el nitrate, %ic el
acetate can also be used#
Coprecipitation method: +he coprecipitated %i&A1' catalysts !ere prepared by
co8precipitation of %i (% )' #9: ' and Al (% ) in an a*ueous solution !ith %a :
at ambient temperature# +he precipitates, matured for '. hrs, !ere vacuum filtered
and subse*uently !ashed until free of nitrate# +he air dried samples (134<0, .= h)
!ere ground in an agate mortar and the po!ders obtained !ere air dried again (134<0,
9 h) >1?#
Co-crystallization Method: %ic el (II) nitrate hexahydrate in de8ioni;ed !ater !as
added, !ith stirring, to a gelatinous solution of alumina in nitric acid previously
prepared by the slo! dissolution of alumina in concentrated nitric acid# +he resultant
solution !as neutrali;ed (@inal p: ), !ith continuous stirring, by the careful drop8
!ise addition of ammonia# After standing overnight !ater !as slo!ly removed (.= h)
on a rotary evaporator and the resulting solid dried in a vacuum oven >'?#
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Atomic Layer pita!y Method: Vapori;ed %ic el acetylacetonate !as chemisorbed
on a porous alumina support, and the produced surface complex !as then air treated
to remove the ligand residues > ?#
%i8alumina catalysts are also synthesi;ed by one-step sol-gel method using micelle
complex comprising lauric acid and nic el ion as a template !ith metal source andusing aluminum sec8butoxide as an aluminum source >.?#
Mathematical Modeling o" Impregnation Process #$%:
-ecent studies have tried to *uantify the impregnation method for preparation of
%i&Al' # +he hypotheses adopted to solve the mathematical model !ere based on the
one8dimensional single8pore model !ith cylindrical geometry, concentration gradientonly in the axial direction of the pore (i#e# radial gradients inside the pellet) and time8
dependent plug8flo! velocity of the penetrating li*uid#
+he partial differential e*uation corresponding to mass balance is6
'
'
z
c D
z c
vt c
p ∂∂=∂
∂+∂∂ BBBBBBBBBBBBBB# (1)
+he effect of the concentration gradient close to the pore !all is explained by the
mechanisms of impregnant removal of the solution# +hus, a term describing the
impregnating removal rate of system V (c, θ ) is added to C*# (1)6
),('
'
θ cV z
c D
z c
vt c
p +∂∂=∂
∂+∂∂ BBBBBB## (')
+o non dimensionali;e C*# ('), the follo!ing definitions are used6
, L
z =Γ
occ=ψ ,
Lt t =τ ,
' L
Dt =α ,
L
t vu
p=
),( θ ψ ψ τ ψ
oO
L cV ct u =
Γ ∂∂+
∂∂ (@irst term of -:$ is neglected since D EE1)
Fith6
(3, H) 1 and (3,3) 1
After some algebraic steps, !e arrive at follo!ing t!o e*uations6
−−−=Γ ∂
∂+∂∂
)1('
1'&1 θ
θ ψ
ψ τ τ
ψ
L K
K
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−−=∂∂
)1( θ θ
ψ τ τ
θ
L K K
!here ' mtK&r, L 'cs&rMco, K K co
+hese three variables describe the impregnation process is observed6 K , reduced mass
transfer coefficientNη, relative capacity of adsorption of the pore !all and K L,adsorption e*uilibrium constant#
+able 1 sho!s the values of these constants obtained experimentally#
Catalyst Characterization:
Wet chemical analysis: +he amount of nic el in the oxide form is determined by
acidic extraction of nic el from the sample, follo!ed by dimethylglyoxime
precipitation >9?#
Total surface area: +he surface area is determined by dynamic lo! temperature
nitrogen adsorption in helium as a carrier# +he data !ere interpreted using the OC+
e*uation and an effective % ' cross8sectional area of 19#' nm ' >9?#
Porosity: +he total porosity (P) !as calculated from apparent (a) and true (d)
densities, P (Q) &' -a(d) 133, determined pycnometrically using mercury and
ben;ene vapors as !or ing fluids >9?#
Pore size distribution: ore si;e distribution is determined by mercury porosimetry,
pressure range up to 1433 bars >9?#
X-ray diffraction: Identification of crystalline compounds and determination of
average crystallite si;e are carried out by R8ray diffraction (R-S) using a G#C#
diffractometer !ith %i filter and 0u D radiation# +he mean crystallite si;e (S) is
related to the pure R8ray broadening (T) by the $cherrer formula
S U&T cos
+he si;e (S) is defined as (volume) 1& leading to a value 3#24 !hen T is defined as
the half8maximum line!idth# +he half8maximum line!idth from %i (''3) reflecting
plane is employed for the alumina8supported #+he instrumental line broadening is
determined from the half8maximum line!idth of a single silicon crystal >9?#
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In Fig. 1 R-S diffractograms of catalysts and y8A1'3 are sho!n# +he
diffractogram of the 14Q catalyst sho!s strong %i lines at ' values of # ,
. # and 9'#2W) confirming a presence of XfreeW nic el oxide# @or both catalysts no
distinct nic el aluminate lines are observed# n the diffractogram of the reduced 4Q
catalyst one can observe lines characteristic for nic el crystallites at '= ..#4 and41#=W > ?#
SE studies: +he catalysts are examined in a scanning electron microscope !ith
resolution of about 4338 1333 A# +he samples are coated !ith gold to about 1338'33
A thic ness prior to observation(@ig ', ,.,4 ) >9? #
*emperature Programmed +eduction &*P+): + - determines the number of
reducible species present in the catalyst and reveals the temperature at !hich the
reduction occurs# + - analysis begins by flo!ing analysis gas (typically hydrogen in
an inert carrier gas such as nitrogen or argon#) over the sample, usually at ambient
temperature# Fhile the gas is flo!ingN the temperature of the sample is increased
linearly !ith time >=?# (T min81)
+he -ate of reaction is monitored6
1# Oy measuring concentration or pressure changes in the gas phase (reactants or
products)#
'# Oy observing !eight changes of the solid through a micrometer microscope#
+ - had advantages over other characteri;ation techni*ues, such as6
It is highly sensitive and does not depend on any specific property of the solid
other than its reducibility#
%on destructive, as solid does not have to be dissolved#
Cven the condition that the solid must be reducible is not mandatory# (better
than spectroscopic and R8ray techni*ues)#
+he apparatus is inexpensive !hen compared to R ray and spectroscopic
techni*ues#
-obust and minimal maintenance re*uired#
@ig 9 , sho!s + - of %i&Al ' made from three different precursors %ic el nitrate,
%ic el 0hloride and acetone solution of %ic el8acetylacetonate (%iAA)# %i (% )'
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catalyst is easily reduced and sho!s the first pea in the '43Y 43 o0 range, !hereas
for %i0l and %iAA catalysts the first pea occurs at ca# .330#
+he three catalysts also demonstrate t!o other pea s of : ' upta e at ca# 9330 and
430# +he first + - pea corresponds to the reduction of %i # +he t!o latter pea s
are due to the reduction of nic el aluminate# $o it is confirmed from the + - profilesthat %i&alumina catalysts have ' sites for reaction, surface %i and a form of
%iAl' . # n reduction of surface %i !ill form nic el crystallites, but reduction of
%i'Z ions in octahedral sites in the alumina lattice !ill form %i atoms surrounded by
oxygens of he alumina lattice# +hus there appear t!o forms of reduced nic el on
%i&Al' catalysts >2?# (Fig 7)
,ispersion o" Ni on .-Al / 0:
Sispersion of %i & 78Al' system has been extensively investigated and the state of
the dispersed nic el oxide has been proposed# %i 'Z ions preferentially incorporated
into the tetrahedral surface vacancies at lo! %i loadings, and both the tetrahedral
and octahedral nic el oxide species !ere present at the high %i loadings# $o, t!o
inds of surface vacant sites e#g# octahedral and tetrahedral sites exist on the exposed
plane of 78Al' >13?# (@ig = )
Methanation
:ydrogenation of 0 and 0 ' to methane on nic el catalysts are important reaction
occurring in purification of ammonia feeds and methanation of coal derived gas# Ooth
reactions are also of interest in the production of process heat or po!er from
reclaimable !aste streams containing dilute carbon oxides or from nuclear reactor
steam reformed 0 8: ' streams as part of a so called Xheat pipelinesW#
+he reactions involved are 0 Z : ' 0: . Z : ' [: 8.2 cal&mol
0 ' Z .: ' 0: . Z : ' [: 8 2 cal&mol
/ethanation of 0 and 0 ' occurs effectively on various transition metal catalysts# It
!as observed that 0 is dissociated on nic el catalyst in methanation of 0 and the
surface carbon species thus formed are hydrogenated to methane# 0 ' methanation
mechanism could be categori;ed into t!o (i) conversion of 0 ' to 0 via the reverse!ater gas shift reaction follo!ed by 0 methanation and (ii) direct hydrogenation of
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0 ' to methane by a mechanism distinct from 0 methanation# Out various studies
sho!ed that the first mechanism may be correct and that 0 ' methanation may
proceed via 0o' dissociation to 0 and atomic oxygen follo!ed by further
dissociation to 0 to carbon intermediate !hich is hydrogenated to methane >11?#
C Methanation Mechanism C / Methanation Mechanism: ' (g) Z' $ ⇔ ' :8$ (1)
0 (g) Z $ ⇔ 0 8$
(')
0 8$ Z$ ⇔ 08$ Z 8$ ( )
08$ Z :8$ ⇔ 0:8$ Z $ (.)
0:8$ Z :8$ ⇔ 0: ' 8$ Z $ (4)
0: ' 8$ Z:8$ ⇔ 0: 8$ Z $ (9)0: 8$ Z:8$ ⇔ 0: . 8$ Z $ ( )
0: . 8$ ⇔ 0: . (g) Z$ (=)
8$ Z:8$ ⇔ :8$ Z$ (2)
:8$ Z :8$ ⇔ : ' 8$ Z$ (13)
: ' 8$ ⇔ : ' (g) Z$ (11)
: ' (g) Z' $ ⇔ ' :8$ (1)
0 ' Z ' $ ⇔ 0 8$ Z 8$ (')
0 8$ ⇔ 0 (g) Z $ ( )
0 8$ Z$ ⇔ 08$ Z 8$ (.)
08$ Z :8$ ⇔ 0:8$ Z $ (4)
0:8$ Z :8$ ⇔ 0: ' 8$ Z $ (9)
0: ' 8$ Z:8$ ⇔ 0: 8$ Z $ ( )0: 8$ Z:8$ ⇔ 0: . 8$ Z $ (=)
0: . 8$ ⇔ 0: . (g) Z$ (2)
8$ Z:8$ ⇔ :8$ Z$ (13)
:8$ Z :8$ ⇔ : ' 8$ Z$ (11)
: ' 8$ ⇔ : ' (g) Z$ (1')
%o! based on this mechanism given above !e can derive many different langmuir8:insel!!ood expressions# @or example consider mechanism of 0 ' methanation
1) Assuming 0 ' adsorption to be the rate determining step !e get 6
''&1'
'&11
''
'
)1( H K
P Lk r CO
+=
') Assuming : ' adsorption to be rate determining step !e get6
''&1'
'&11
''
1
)1( CO
H
P K
P Lk
r +
=
:ere,
r rate
K total concentration of surface sites#
:' partial pressure of hydrogen
0 ' partial pressure of 0arbon di8oxide
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$imilarly one can get lots of rate expressions considering other steps as rate
determining steps#
-ecent studies sho! that there exist mechanisms in !hich both the sites present on
%i&Al' play different role# Ket this t!o sites be site 1 (M) and site ' (\)#
(i) 0 /ethanation60 Z M ⇔ 0 M (1)
3#4 : ' Z M ⇔ :M (')
0 M Z \ M Z 0\ ( ) (rate determining step)
0\ Z ' : ' 0: . Z \ (.) (fast)
M Z : ' : ' Z M (4) (fast)
In this mechanism 0 molecules compete !ith hydrogen atoms for type 1 sites (M)at the nic el surface and 0 dissociates to a type ' site (\), !hich are al!ays free at
the conditions used during the study# -eaction is the rate determining step,
reactions 1 and ' are so fast that 0 M and :M are in e*uilibrium !ith the gas8phase
species 0 and : ' , and reactions . and 4 are so fast that the coverages of 0\ and
M are negligible >1'?#
(ii) 0 ' methanation 6
: ' Z 'M ':M (1)
0 ' Z '\ 0\ Z \ (')
':M Z 0\ : ' Z 'A Z 0A ( )
':M Z \ : ' Z'M Z \ (.)
: ' Z 0\ : ' 0\ (4)
: ' Z : ' 0\ 0: . Z\ (9)
+he rate expression is derived using some assumption li e steps(1) and (') are
much slo!er than steps( )8(9) #+hus !e can say that the reaction is on18half order
in both reactants, provided N(i)the coverage is relatively lo! and (ii) slo!
dissociation follo!s rapid adsorption#
""ect o" addition o" promoters on Ni( Al / 0 "or methanation reaction #'0%
-ecent studies have characteri;ed the effect of cerium, lanthanum and ;irconium onnic el&alumina catalysts !ith respect to 0 and 0 ' hydrogenation# +he
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3: Ni-Ca aluminate on 8-A' / 90 hydrate: A blend of po!dered nic el oxide, calcium
aluminate and alumina hydrate !as !ater8sprayed and 0atalyst and rolled to form
pellets, !hich !ere calcined to a highly active catalyst#
+here exist a !ide range of catalysts for methanation# /any transition metals li e %ic el, Iron, and 0obalt are also active catalysts but %i catalysts are more selective
and efficient >14?#
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+e"erences:
1# :aber^ J#, Oloc ,J#:#, Selmon, O#, ure _ Appl# 0hem#, Vol# 9 , %os =&2, 1224
1'4 8 1 39
'# Savid Jac son , $#, Fillis Janice, elly Gordon J#, /cKellan Gavin S#, Febb
Geo#, /ather $ue, /oyes, -# O, $ydney $impson, Fells, # O#, Fhyman-obin#, hys# 0hem# 0hem# hys#, 1222, 1, '4 8'4=3
# Kindblad /arina, eter Kindfors Kars, $untola +uomo, 0atalysis Ketters '
(122.) ' 8 9
.# rompiec, $#, /ro!iec8Oia`o, J#, $ util , #, Su o!ic; , A#, a5a; , K#,
Jar;e;bs i , A#O#, Journal of %on80rystalline $olids 14 ,('33 ) '2 3
5. Assaf,C#/#, Jesus K#0# Assaf,J#/ . 0hemical Cngineering Journal 2., ('33 )
2 2=
9# Gavalas, G# -#, hichit ul, 0#, Voec s, G# C#, Journal of 0atalysis ==, (12=.),
4.89.
# -yn o!s i J#/, ary5c;a , +#, Keni /#, Applied 0atalysis A6 General, 139
(122 ) 8='
=# Jones, A#, /c%icol, O# S#, +emperature rogrammed reduction for solid materials
0haracteri;ation,Se er,12=9
2# Oecerra, A# /#, 0astro8Kuna, A# C#, J# 0hil# 0hem# $oc#, 43, % ' ('334),
.948.92
13# Fang Jun, Song Kin, uhai :u, heng Guishan, heng :u, i 0hen
Journal of $olid $tate 0hemistry 14 , ('331) ' .8'='
11# Featherbee,G#S# ,Oartholome!,0#:#,J# 0atal# , (12='), .938. '
1'# $ehested, Jens, Sahl $ ren, Jacobsen Joachim, -ostrup8%ielsen Jens -#,
J# hys# 0hem# O ('334), 132, '. '8'. =
1 # na ,K#, $to`ec i, #, ielin s i, J# , 0atalysis +oday 131, ('334) 94 1
1. # Go r e , #, feifer, #, $chubert, #, 0atalysis +oday 113 ,('334) 1 ' 1 2
14# :andboo of methanation catalyst, G# :# Fatson, @ebruary 12=3, lCA 0oal
-esearch, Kondon
19# +su5i, /#, odama, + #, oshida, +#, itayama, #, +amaura, #, Journal of
0atalysis 19., (1229)# 14 '1
1 # 0hang,@#F#, uo,/#$#, +say,/#+#, :sieh,/#0#, Applied 0atalysis A6 General
'. ('33 ) 32 '3
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APP N,I
*able ':
7ig4 ': +, patterns "or &a) Ni ; &b) '$< Ni ( .-Al / 0 &c) $< Ni ( .-Al / 0; &d)$< Ni (-Al / 0 catalyst reduced in the *P+ process &b=; c=; d=: subtracted +,patterns)> &?) .-Al / 0
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7ig4 34 5canning electron micrograph o" Ni (@A'/90-?$9 at !$9994
7ig $4 5canning electron micrograph o" Ni (a-A'/90-'9$9 at /9;9994
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7ig D: *P+ pro"iles4 a) NiAl/ 3; b) Ni ; c) Ni (Al/ 0 catalyst precursor&'940<
Ni)
7ig B: *P+ pro"iles o" Ni catalyst "rom di""erent precursors4&Ni B4B<)
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7ig ?: *entati2e model o" the sur"ace-dispersed nickel o!ide species "ormed onthe &''9) plane o" .-Al / 0 support @ith a Ni /E ion incorporating in a tetrahedral
2acant site4
7ig4 F4 Acti2ity o" un-promoted and Ce-; La- and Gr-promoted catalysts:&A) C hydrogenation and &H) C / hydrogenation4