MgO Catalysts

C. Morterra, A. Zecchina and G. Costa (Editors), Structure and Reactiuity of Surfaces 01989 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlanh

739

ACTIVITY AND CHARACTERIZATION OF ALKALI DOPED Ni/MgO CATALYSTS

A. PARMALIANA~, F. FRUSTERI

1

, F.

ARENA',

I s t i t u t o d i Chimica I n d u s t r i a l e

N. MONDELLO' and

F;.

GIORDANO'

U n i v e r s i t a d i Messina, 98100 Messina

* I s t i t u t o CNR-TAE, S a l i t a S . ~ u ac 39, 98126 S. Lucia, Messina

ABSTRACT Ni/Mg0 c a t a l y s t s have been p r e p a r e d and t h e i r r e a c t i v i t y i n methane steam r e f o r m i n g r e a c t i o n has been e v a l u a t e d as a f u n c t i o n o f p r e t r e a t m e n t and r e a c t i o n c o n d i t i o n s . The e f f e c t s o f a l k a l i d o p i n g ( L i and K) on t h e c a t a l y t i c f u n c t i o n a l i t y and s u r f a c e p r o p e r t i e s have been i n v e s t i g a t e d . The c a t a l y s t s were c h a r a c t e r i z e d by d i f f e r e n t t e c h n i q u e s ( H c h e m i s o r p t i o n , TPR, TEM and 2 SEM w i t h EDAX). A l k a l i s t r o n g l y a f f e c t c a t a l y t i c a c t i v i t y w i t h t h e L i b e i n g more t o x i c t h a n K. A good c o r r e l a t i o n was found between metal s u r f a c e a r e a and c a t a l y t i c a c t i v i t y o f doped c a t a l y s t s . An i n t e r p r e t a t i o n o f t h e b e h a v i o u r o f a l k a l i doped c a t a l y s t s , based on t h e g e o m e t r i c e f f e c t o f a l k a l i , i s proposed.

INTRODUCTION The Ni/MgO system i s

currently

receiving

considerable

attention

mostly

w i t h r e s p e c t t o t h e f o r m a t i o n o f NiO-MgO s o l i d s o l u t i o n s and t o t h e i r p e c u l i a r physico-chemical

and c a t a l y t i c p r o p e r t i e s

(refs.

1-41.

The p r o m o t i n g r o l e

o f MgO on t y p i c a l supported c a t a l y s t s f o r t h e steam r e f o r m i n g o f hydrocarbons has been a l s o i n v e s t i g a t e d ( r e f s . oxidation,

3,5).

As d e a c t i v a t i o n due t o s i n t e r i n g ,

p o i s o n i n g and c o k i n g l i m i t s t h e l i f e t i m e of

Ni-based

catalysts,

emphasis has been p u t on t h e e f f e c t s e x e r t e d by MgO i n such processes ( r e f s .

4-5).

A c c o r d i n g t o R o s t r u p - N i e l s e n MgO as a b a s i c promoter o f N i c a t a l y s t

s t r o n g l y reduces t h e c o k i n g r a t e ( r e f .

5).

The i n f l u e n c e o f MgO on t h e N i O

r e d u c i b i l i t y , mean s i z e o f t h e N i c r y s t a l l i t e s , c a t a l y t i c a c t i v i t y and mechanic a l s t r e n g t h had been i n v e s t i g a t e d by B o r o w i e c k i ( r e f . 6 ) . I t has been c l a i m e d t h a t a l k a l i added t o t h e c a t a l y s t cause a decrease o f t h e s p e c i f i c a c t i v i t y by more t h a n one o r d e r of

magnitude w h i l s t e x e r t i n g a promoter e f f e c t i n

t h e g a s i f i c a t i o n o f t h e coke p r e c u r s o r ( r e f . 5 ) . Recent advances i n t h e m o l t e n c a r b o n a t e f u e l c e l l s (MCFC) have i n d i c a t e d t h e o p p o r t u n i t y o f i m p r o v i n g t h e e f f i c i e n c y o f t h e system b y p r o d u c i n g t h e hydrogen f u e l

d i r e c t l y "on t h e c e l l " ,

by i n t e r n a l methane steam r e f o r m i n g

740

(ref.

7). As a r e s u l t o f many years o f continous research we have proposed

and t e s t e d a Ni/MgO c a t a l y s t which takes advantage o f t h e unique p r o p e r t i e s o f t h e support ( a w e l l c r y s t a l l i z e d MgO "smoke") and o f an our own preparation procedure.

As the working c a t a l y s t must be r e s i s t a n t t o t h e a l k a l i coming

e u t e c t i c mixture) t h e r e f o r e one aim from MCFC e l e c t r o l y t e ( L i CO -K CO 2 3 2 3 ' o f our studies was t o i n v e s t i g a t e t h e surface properties and c a t a l y t i c features o f pure and a l k a l i doped (K,

L i ) Ni/MgO i n t h e methane steam reforming.

I n t h e present work we r e p o r t some p r e l i m i n a r y r e s u l t s obtained from physicalchemical analysis (BET, prepared"

TEM,

catalysts i n t h e i r

SEM,

TPR,

chemisorption etc.) o f t h e "as2 r e l a t i o n s h i p s w i t h c a l c i n a t i o n and reduction H

c o n d i t i o n as w e l l as w i t h c a t a l y t i c properties. EXPERIMENTAL Catalyst preparation Supported

n i c k e l c a t a l y s t s were prepared by impregnation t o

incipient

wetness under continuos s t i r r i n g o f MgO "smoke" powder (UBE Ind. Ltd, Japan) according t o a procedure described i n d e t a i l s elsewhere ( r e f . 8 ) . The c a t a l y s t s were d r i e d overnight a t 393 K,

then calcined under an a i r stream f o r 16

h a t 673 K (standard c o n d i t i o n s ) . Aliquots o f t h e above c a t a l y s t s were f u r t h e r c a l c i n e d f o r 6 h a t 873, 1073 and 1273 K. The a l k a l i doped c a t a l y s t s were prepared by wet impregnation o f t h e f i n i t e o r uncalcined c a t a l y s t s w i t h isopropanol solutions o f t h e K o r L i acetate. A f t e r d r y i n g f o r 10 h a t 353 K t h e samples were calcined under an a i r stream f o r 16 h a t 673 K. The N i , K and L i contents were determined by atomic absorption. Catalysts c h a r a c t e r i z a t i o n ( i 1 Hydrogen chemisorption and t o t a l surface area measurements. The hydrogen chemisorption was determined i n a conventional gas volumetric apparatus equipped w i t h a turbomolecular vacuum pump. A f t e r reduction, i n a H

2

stream

f o r 2 h a t 673 K and f u r t h e r f o r 1 h a t 998 K, the sample (1-2 g) was outgassed a t 998 K down t o a pressure o f

(13.3

mPa) and t h e r e a f t e r

adsorption isotherms were determined 2 a t 298 K i n t h e range o f 0-6 kPA. The monolayer coverage was estimated by

cooled t o

298 K i n f l o w i n g He.

torr

H

back e x t r a p o l a t i o n t o zero pressure o f t h e s t r a i g h t

linear portion o f the

741 isotherm.

The metal surface area (MSA),

t h e metal dispersion ( D )

and t h e

average c r y s t a l l i t e s i z e were determined according t o Mustard and Bartholomew ( r e f . 9). The t o t a l surface area o f t h e c a t a l y s t s was measured by BET method using a Sorptomatic 1800 Carlo Erba N2 adsorption apparatus. (ii)

Temperature programmed r e d u c t i o n (TPR).

performed i n a conventional

The TPR measurements were

quartz

U-tube r e a c t o r using a p u r i f i e d H -N 2 2 m i x t u r e (6.15% H2) f l o w i n g a t 54 Ncm3min-' over a 200 mg sample d r i e d i n

N2 a t 673 K f o r 2 h and then cooled t o r.t. Measurements were made from r.t. t o 1223 K and a t a heating r a t e o f 10 K min-l.

From t h e change i n H 2 concentration, .detected by a thermal c o n d u c t i v i t y detector (TCO), t h e consumpt i o n o f H was determined w i t h high r e l i a b i l i t y from t h e i n t e g r a t e d peak 2 areas. The experimental operating variables used i n TPR were chosen according t o t h e c r i t e r i a suggested by Monti and Baiker ( r e f . 10). ( i i i 1 Electron microscopy. Transmission e l e c t r o n microscopy (TEM) analysis was performed by a P h i l i p s CM 12/STEM e l e c t r o n microscope operating a t 120 KV ( r e s o l u t i o n b e t t e r than 1 nm). The samples were u l t r a s o n i c a l l y dispersed

i n ethanol and deposited on a t h i n holey-carbon coated copper g r i d . Scanning e l e c t r o n microscopy (SEM) micrographs were made by a P h i l i p s 525 e l e c t r o n microscope equipped w i t h a EOAX PV 9900 energy d i s p e r s i v e X-ray spectrometer f o r microanalysis. Apparatus and Procedure The methane steam reforming r e a c t i o n was evaluated a t atmospheric pressure i n a continous "down-flow"

fixed-bed quartz microreactor (1= 200 mm;

4 mm). The d i l u e n t n i t r o g e n (PPL grade,

SIO product), was c a r e f u l l y purged

by passing i t a t r.t. through an O2 t r a p ( A l l t e c h product), sieve

bed and f i n a l l y over

methane (2.5

a 4A molecular

a Gas P u r i f i e r System (Supelco product). The

Messer Grieshein product,

further purification.

i.d.=

purity>99,9

Val%) was used without

The n i t r o g e n and t h e methane were bubbled together

i n t o a t h e r m o s t a t i c a l l y c o n t r o l l e d ( T = 2 0.01

K ) H20 s a t u r a t o r a t 357.6 K

t o g i v e a f i x e d volume r a t i o s : R= H20/CH4 o f 2.54 and R'= N /CH o f 1. The 2 4 r e a c t i o n mixtures was f e d i n t o t h e r e a c t o r containing t h e c a t a l y s t (0.02 g, 40-70 mesh) d i l u t e d w i t h same-sized carborundum ( l / l O , v o l / v o l ) t o ensure quasi-ideal

conditions f o r

mass and heat t r a n s f e r .

Analysis was performed

742

b y an " o n - l i n e "

gas-chromatograph

(ATC/f s e r i e s 410 C a r l o Erba, TCD d e t e c t o r )

equipped w i t h two s t a i n l e s s s t e e l columns ( 1 . r e s p e c t i v e l y molecular He as

2.5 mm;

4mm) c o n t a i n i n g

s i e v e 5A and Poropak Q and o p e r a t e d a t 347 K w i t h

t h e c a r r i e r gas ( 6 0 Ncrn3min-').

Before

the reaction,

was reduced i n a hydrogen f l o w o f ca, 50 Ncm3 min-' subsequently

i.d.=

the catalyst

f o r 2 h a t 673 K and

f o r 1 h a t 998 K w i t h t h e same hydrogen f l o w ,

then cooled

t o r e a c t i o n T i n a H atmosphere. The steam r e f o r m i n g o f methane was s t u d i e d 2 in t h e t e m p e r a t u r e range 873-923 K and space v e l o c i t y expressed as GHSV -1 -1 -1 (NlCH4h Nlcat= h ) ) r a n g i n g f r o m 37,500 to150,OOO h The c o n v e r s i o n of rne-

.

t h a n e (mol%) was t a k e n as a measure o f t h e c a t a l y t i c a c t i v i t y . f o r m a t i o n was observed.

No carbon

I n a s e r i e s o f experiments t h e methane c o n v e r s i o n

was k e p t below 10% t o o b t a i n r e l i a b l e d i f f e r e n t i a l k i n e t i c data.

RESULTS AND D I S C U S S I O N Cat a1y s t s c h a r a c t e r i z a t ion Hydrogen c h e m i s o r p t i o n and BET d a t a f o r t h e pure and f o r t h e a l k a l i - d o p e d Ni/MgO c a t a l y s t s a r e l i s t e d i n Table 1. The low values o f t h e metal d i s p e r s i o n (ca. 5%) f o r t h e MPF7 and MPF12 c a t a l y s t s w e l l agree w i t h t h o s e r e p o r t e d ( r e f s . 9,111

f o r the air-calcined e t c . 1. This 2 ( > l o wt%), t h e N i

h i g h l y N i - l o a d e d c a t a l y s t s on d i f f e r e n t c a r r i e r s (Si02, A1203, T i 0 o b s e r v a t i o n seems t o i n d i c a t e t h a t a t h i g h Ni d e p o s i t e d on s u p p o r t

builds

up as

content

a m u l t i l a y e r which i s i n s e n s i t i v e t o

t h e n a t u r e o f t h e support.

TABLE 1 Hydrogen c h e m i s o r p t i o n and BET d a t a f o r " p u r e " and a l k a l i doped N i w catalys%sa. __._

Catalyst MPF7 MPF12 MPF12-A MPF12-B

Ni (WtX)

16.5 17.9 17.9 17.9

A l k a l i metal (WtX)

--1 (Li) 1 (K)

H uptake 2 (pmol g - l cat __ 56.2 77.1 6.5 43.5

(%I

M S A (mzN/giNi

3.8 5.1 0.4 2.8

25.0 34.3 2.9 19.3

D

BETsurface area (m2gcet) 29.2 28.2 28.9 28.7

a A l l samples were c a l c i n e d i n an a i r s t r e a m a c c o r d i n g t o t h e s t a n d a r d c o n d i t i o n s and t h e r e a f t e r reduced,as

indicated i n the experimental section,in

f a r 1 h a t 998 K.

flowing H

2

f a r 2 h a t 6 7 3 K and s u b s e q u e n t l y

743

The same l o a d i n g o f d i f f e r e n t a l k a l i e s ( 1 w t % ) added t o t h e c a t a l y s t i m p a i r s t h e c h e m i s o r p t i o n c a p a c i t y towards t h e H2 a l t h o u g h t o a d i f f e r e n t e x t e n t : t h e L i appears t o be t h e most t o x i c as MSA i s c u t by as much as 92% w i t h respect

t o original

c a t a l y s t a g a i n s t a 45% c u t f o r t h e K-doped c a t a l y s t .

A p l a u s i b i l e reason o f t h i s r e s u l t c o u l d be f o u n d i n t h e d i f f e r e n t m o l e c u l a r

radius

of

the

poisoning

species

and t h e r e f o r e a geometric f a c t o r

could

be i n v o k e d t o e x p l a i n t h e observed losses i n the z t i v e metal area. On the c o n t r a r y t h e BET s u r f a c e a r e a o f t h e c a t a l y s t s i s u n a f f e c t e d f r o m t h e presence o f a l k a l i . Moreover no changes i n metal d i s p e r s i o n and BET s u r f a c e area were observed on aged c a t a l y s t s (200 h t i m e on stream a t 898 K ) . The e f f e c t s o f t h e Tc and a l k a l i d o p i n g on c a t a l y s t s r e d u c i b i l i t y were assessed

through

TPR

experiments.

Results

a r e summarized i n Table 2 i n

terms o f t h e temperature o f t h e maximum peaks (T,) d e r i v e d f r o m t h e H2 consumption

and t h e N i O r e d u c i b i l i t y

( a t T o f r e d u c t i o n up t o 1223 K ) b o t h on

p u r e and doped c a t a l y s t s . TABLE 2 Temperature programmed r e d u c t i o n o f Ni/MgO c a t a l y s t s . E f f e c t o f a l k a l i doping and c a l c i n a t i o n temperature.

Catalyst

Composition

Calcination temperature (K) and time ( h ) 17.9%Ni/MgO 673 (16) * 17.9XN i/MgO S.C.+ 873 (6) 17.9%Ni/MgO S.C.+1073 (6) 17.9%Ni/HgO S.C.+1273 (6) 17.9%Ni/l%Li/MgO S.C.+ 673 (6) 17.9%Ni/l%K/MgO S.C.+ 673 (6)

MPF12 MPFlZ-1 MPF 12-2 MPF12-3 MPF12-A MPF12-8

T,1 (K) 543 543 nil nil 549 548

T,2

T,3

(K) 643 708 nil nil 670 670

(K)

(K)

(%)

903 933 938 nil 908 888

1047 1101 1223

77.4 67.5 22.0 5.0 100.0 77.0

Tm4 NiO Reduction

nil 1123 948

*standard conditions (s.c.)

From t h e above i s e v i d e n t

that:

i)

t h e r e d u c t i o n i s c h a r a c t e r i z e d by

onset

o f f o u r d i f f e r e n t peaks which have been a t t r i b u t e d t o d i f f e r e n t s p e c i e s

(ref.

4);

ii) upon i n c r e a s e o f t h e c a l c i n a t i o n t e m p e r a t u r e ( T c ) , f r o m 673

t o 1273 K, t h e Tm o f a l l t h e s e peaks a r e s h i f t e d t o h i g h e r values; i i i ) t h e Tm, peak, the N i

ascribed t o Ni3+

(ref.

r e d u c i b i l i t y decreases

t o 5% f o r a Tc o f 1273 K.

41, d i s a p p e a r s a t Tc h i g h e r t h a n 873 K; i v ) m o n o t o n i c a l l y f r o m 77% f o r

a Tc o f 673 K

744

The TPR p r o f i l e s o f t h e samples a i r c a l c i n e d a t d i f f e r e n t Tc are shown i n Fig.

1.

TPR r e s u l t s f o r t h e pure c a t a l y s t s are i n t e r p r e t e d i n terms

o f a progressive d i f f u s i o n o f Ni2'

i o n s i n t o the MgO s t r u c t u r e ( w i t h inherent

formation o f s o l i d s o l u t i o n s ) increasing f o r an increase o f Tc,at o f t h e " f r e e " N i O (T

of

i3 and Tm

(ref.

mz

t h e expense

1. The progressive s h i f t towards higher temperatures

w i t h increase o f Tc denotes, i n accordance w i t h t h e l i t e r a t u r e ,

41, a more abundant presence o f N i

2+

ions i n thesub7sWaelayer and

w i t h i n t h e MgO l a t t i c e .

b

0

u

N

I

C

/ 573

n3

973

1173

T( K ) F i g . 1. TPR p r o f i l e s f o r c a t a l y s t MPF12 a i r - c a l c i n e d a t d i f f e r e n t temperature. a) MPF12 (673 K); b ) MPF12-1 (873 K); C ) MPF12-2 (1073 K);d)MPF12-3(127X). Fi g . 2 . TPR p r o f i l e s f o r Ni/MgO c a t a l y s t s . E f f e c t o f a l k a l i doping. a) MPF12-A (17.9%Ni/l%Li/MgO); b ) MPFlZ-B (17.9%Ni/lZK I M g O ) ; c ) MPF12 (17.%Ni/MgO).

The TPR p r o f i l e s (Fig. 2) o f L i and K doped N i c a t a l y s t s are vely

n o t d i f f e r e n t from those o f t h e undoped sample,although

qualitati-

t h e i r reducibi-

K modified (MPF12-B) shows t h e same

l i t y i s quantitatively

affected: t h e

degree o f r e d u c i b i l i t y

(77%) as t h e "pure"

catalyst, opposite t o t h e L i

doping which enhances, t i l l f u l l reduction, t h e N i r e d u c i b i l i t y . This behaviour looks s i m i l a r t o t h a t observed on t h e N i / A l 2 O 3

system

by Narayanan and Uma ( r e f . 12) who have a t t r i b u t e d i t t o a decreased a c i d i t y of L i modified system t h a t helps N i 2 + reduction. I n our case, as t h e a c i d i t y of

t h e c a t a l y t i c system Ni/MgO i s very low, any eventual chemical e f f e c t

o f a l k a l i should has been more marked i n case o f t h e K doped c a t a l y s t thus denying any chemical e f f e c t . Moreover t h e formation

of

a "true"

compound and/or

a s o l i d solution

i n c o r p o r a t i n g L i 0 i n the MgO l a t t i c e looks very u n l i k e l y , on account o f 2 the moderate Tc (673 K ) , as also proven by t h e f a c t t h a t t h e reduction's p a t t e r n o f t h e Li-doped c a t a l y s t i s n o t modified. Most l i k e l y t h e L i promotes t h e r e d u c t i o n o f N i 2 + i o n s located on t h e surface allowing a r a p i d formation o f metal n u c l e i ( N i l which,

i n turn,

enhance r e d u c i b i l i t y v i a a nucleation

mechanism as suggested f o r metal oxide reduction ( r e f . 13). Therefore t h e TPR r e s u l t s could be i n t e r p r e t e d t a k i n g i n t o account t h e prominent surface e f f e c t o f a l k a l i doping i n accordance w i t h H2 chemisorption r e s u l t s . TEM micrographs

o f the

support show a quasi-perfect

cubic

structure

o f t h e c o n s t i t u e n t p a r t i c l e s most l i k e l y due t o t h e p a r t i c u l a r preparation method.

Because o f very f i n e

dimensions,

i t s exposed geometric

area i s

equal t o t h a t determined from BET and thus i t can be c l a s s i f i e d as having an "open"

structure.

This i s confirmed by t h e very uniform d i s t r i b u t i o n

o f N i O evidenced by maping t h e sample w i t h EDX analysis. A l k a l i doping(sanp l e MPF12-B) a f f e c t s c a t a l y s t surface as shown i n Fig. 3.

Fig. 3. SEM micrographs o f MPF12-B c a t a l y s t

"as prepared"; ( a ) x 40, (b) x 325.

746

The surface appears covered by dark i s l a n d s which, f r o m EOX microanalysis, have been i d e n t i f i e d as due t o K p a r t i c l e s evenly d i s t r i b u t e d a l l over t h e surface, (Fig.

Furthermore,

examining a r e g i o n o f MPF12-B a t h i g h m a g n i f i c a t i o n

3 ( b ) ) i t i s evident t h a t even i f l o a d i n g o f t h e K i s low ( 1 w t % )

i t masks ca. h a l f o f t h e exposed surface. This observation i s w e l l i n agree-

ment w i t h chemisorption data which f o r t h e K-doped c a t a l y s t

MPF12-B i s

about t h e h a l f o f t h a t o f t h e pure N i c a t a l y s t (MPF12). I n order t o b e t t e r d e f i n e t h e n a t u r e o f surface e f f e c t e x e r t e d by t h e a l k a l i doping f u r t h e r work must be done.

I n p a r t i c u l a r i t seems necessary

t o evaluate whether a l k a l i covers p h y s i c a l l y t h e N i a c t i v e surface making i t inaccessible f o r H

chemisorption ("geometric e f f e c t " ) o r i f they modify 2 t h e N i a c t i v e surface by an "ensemble e f f e c t " , a f f e c t i n g H2 chemisorption and r e d u c i b i 1it y o f N i /MgO system. C a t a l y t i c a c t i v i t y data ( i )Pure c a t a l y s t s .

E q u i l i b r i u m conversion on pure c a t a l y s t s

a t 898K

i s f u l l y a t t a i n e d already a t GHSV o f 37,500 h - l (Table 3 ) . TABLE 3 E f f e c t s o f c a l c i n a t i o n temperature and GHSV on a c t i v i t y o f Ni/MgO c a t a l y s t s f o r t h e steam r e f o r m i n g o f methane a t TR= 873-923K; R= H O/CH =2.54and R0=N2/CHc1. 2 4 Catalyst

Tc

MPF7 MPF7 MPF7-1 MPF7-2 MPF7-3 MPF7 MPFl2 MPF12 MPF12 MPFlZ

(K) 673 673 873 1073 1273 673 673 673 673 673

*n.a.

GHSV

TR

CH4 conv.

(h-') 150,000 150,000 150,000 150,000 150,000 150,000 37,500 75,000 150,000 150,000

(K) 873 898 898 898 898 923 898 898 898 923

% 43.0 46.2 31 .O 8.0 n.a.* 56.4 67.8 52.5 46.1 54.5

Equilibrium CH4 conv. % 66 68 68 68 68 70 68 68 68 70

= non a c t i v e

The i n f l u e n c e o f t h e Tc and t h e o t h e r experimental v a r i a b l e s was i n v e s t i g a t e d a t h i g h e r GHSV (Table 3 ) .

Although,

as expected,

t h e conversion decreased

747

w i t h i n c r e a s i n g GHSV, h i g h c a t a l y t i c a c t i v i t i e s were observed

s t i l l at

GHSV

as h i g h as 150,000 h - l i n t h e whole t e m p e r a t u r e range i n v e s t i g a t e d (873-923K). Air-calcination

at

Tc

f r o m 673 t o

1273 K caused a p r o g r e s s i v e decrease

o f t h e c o n v e r s i o n f r o m t h e 46.2% f o r Tc o f 673 K t o z e r o f o r Tc o f 1273K. T h i s a l l o w s t o assess a s t r a i g h t c o r r e l a t i o n between a c t i v i t y and N i r e d u c i b i l i t y insofar,

as s t a t e d above,

h i g h e r Tc promote m i g r a t i o n o f N i O i n s i d e

t h e MgO

s t r u c t u r e t h u s l o w e r i n g t h e f r a c t i o n of

exposed.

As F i g .

4 shows,

r e d u c i b l e and a c t i v e N i

the catalyst exhibits high s t a b i l i t y indicating

absence o f c o k i n g and s i n t e r i n g . ( i i ) Doped c a t a l y s t s . As t o t h e e f f e c t s o f a l k a l i ( T a b l e 41, i t i s e v i d e n t t h a t L i depresses t h e c a t a l y t i c a c t i v i t y more s t r o n g l y t h a n K. T h i s observat i o n ‘ i s c o n s i s t e n t w i t h p r e v i o u s H c h e m i s o r p t i o n d a t a d e n o t i n g a geometric 2 e f f e c t o f t h e a l k a l i doping. Moreover i n t e r p r e t a t i o n w e l l complies w i t h r e s u l t s o f a s e r i e s o f experiments performed under d i f f e r e n t i a l conditions (GHSV~150,OOO h - ’ ) t h r o u g h which r e l i a b l e values o f t h e t u r n o v e r number, TON (molec M-’.S-’), CH4 s have been d e r i v e d . As F i g . 5 shows, TON i s independent f r o m t h e MSA f o r b o t h t h e doped and t h e undoped c a t a l y s t s d e n o t i n g t h a t a l k a l i render inaccessible t o

TABLE 4 C a t a l y t i c a c t i v i t y o f a l k a l i doped Ni/MgO c a t a l y s t s (Tc=673K) on nethane steam r e f o r m i n g a t T = 898 K. R

Catalyst MPFl2 MPF12-A MPFl2-A MPFl2-6 MPFl2-B MPF12-6 a hPF 12-61 a MPF12-61

A l k a l i content (wt%)

--

1(Li) 1( L i 1 1(K) 1(K) 1(K) 1 (K) 1 (K)

GHSV (h-’)

CH4 Conv.

150,000

46.1 26.8 2.7 52.3 45.1 28.2 28.0 5.0

75,000 150,000 37,500 75,000 150,000 75,000 150,000

(%I

a T h i s sample was prepared by i m p r e g n a t i o n o f t h e u n c a l c i n e d MPF12 c a t . w i t h i s o p r o p a n o l s o l u t i o n o f K a c e t a t e .

748

7 -

'u aa

3-

cn '3

= 2..

A-

Y

*-f

'0

c c

L: 0

0

'MSA Cm2fi.g"l(il

Reaction time Chi

F i g . 4. Methane conversion versus r e a c t i o n time a t T= 898 K a t d i f f e r e n t GHSV f o r MPF12 c a t a l y s t . GHSV: (0) 75,000 h - l , ( A ) 150,000 h-1. Fig. 5 . TON versus metal surface area. TR= 898 K . ( ( 0 ) MPF12.

1 MPF12-A; ( A ) MPF12-B;

H

and CH some f r a c t i o n of t h e a c t i v e s i t e s without hindering t h e r e a c t i v i 2 4 t y o f t h e adjacent sites.As t o t h e lower a c t i v i t y o f t h e MPF12-B1 sample i t r e f l e c t s a d i f f e r e n t poisoning e f f e c t depending upon t h e previous h i s t o r y

of

t h e sample.

An explanation could be found on t h e d i f f e r e n c e s i n t h e

i n t e r a c t i o n between K and N i species i n t h e uncalcined and c a l c i n e d sample. F u r t h e r studies, now in progress,

have been purported t o shed more l i g h t

on t h i s subject. REFERENCES H i g h f i e l d , A. B a s s i and F.S.

J.G.

Stone, i n G. P o n c e l e t , P. Grange and P.A.

Jacobs (Eds), Prepa-

r a t i o n o f C a t a l y s t s 111, E l s e v i e r , Amsterdam, 1983, p. 181.

A.

Zecchina, G. Spoto, S. C o l u c c i a and E. G u g l i e l m i n o t t i , J. Chem. SOC. Faraday Trans I. 80 (1984)

1891-1901. 1. B o r o u i e c k i , Appl. Catal.,

31 (1987) 207-220.

G.C.

Bond and S.P.

J.R.

R o s t r u p - N i e l s e n , Steam R e f o r m i n g C a t a l y s t s , D a n i s h T e c h n i c a l Press, Copenaghen, 1975, p.81.

Sarsan, Appl. C a t a l . 38 (1988) 365-377.

1. B o r o u i e c k i , Appl. C a t a l . 10 (1984) 273-289.

A.J.

Appleby, "Advanced F u e l C e l l Technology" EPRI J.,

A. Parmaliana,

F. F r u s t e r i .

P o l o A l t o CA-USA,

rence, Wien J u l y 20-24 1986, Perganon Press, d x f o r d , 1986, Vo1.3, D.G.

10 11 12 13

D.R.M. Y.J.

M u s t a r d and C.H.

Oecember 1984, p. 63.

P. T s i a k a r a s and N. Giordano, Proc. 6 t h World Hydrogen Energy Confe-

p. 1252.

Bartholomeu, J. C a t a l . 67 (1981) 186-206.

M o n t i and A. B a i k e r , J. C a t a l . 83 (1983) 323-335. Huang and J.A.

Schuarz, Appl. C a t a l . 30 (1987) 239-253.

S. Narayanan and K. Uma, J. Chem, Soc. Faraday Trans I, 81 (1985) 2733-2744. N.W.

H u r s t , S.J.

Gentry and C.Jones,

C a t a l . Rev. Sci. Eng. 24 (2)(1982) 233-309.