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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#



+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 >'?#



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



−−=∂∂

)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?#



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 (% )'



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



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



$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



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?#



+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),

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1'# $ehested, Jens, Sahl $ ren, Jacobsen Joachim, -ostrup8%ielsen Jens -#,

J# hys# 0hem# O ('334), 132, '. '8'. =

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1. # Go r e , #, feifer, #, $chubert, #, 0atalysis +oday 113 ,('334) 1 ' 1 2

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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



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



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<)



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

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