Reactivity of Surface Carbon on Nickel Catalysts - American Chemical

13

Downloaded by PENNSYLVANIA STATE UNIV on August 6, 2013 | http://pubs.acs.org Publication Date: October 25, 1983 | doi: 10.1021/bk-1983-0202.ch013

Reactivity of Surface Carbon on Nickel Catalysts: Temperature-Programmed Surface Reaction with Hydrogen and Water J. G. McCARTY, P. Y. HOU, D. SHERIDAN, and H . WISE SRI International, Menlo Park, C A 94025

The nature of the carbon deposits formed on an alumina-supported nickel catalyst have been characterized by their reactivity with H and H O during temperature-programmed surface reaction (TPSR). Carbon deposits formed by exposure to hydrocarbons and carbon monoxide exhibit, depending primarily on the temperature during deposition, seven reactive carbon states during TPSR with 1-atm H , including two very reactive states of chemisorbed carbon, a carbon film, nickel carbide, and two types of filamentary carbon. Filamentary carbon was identified by transmission and scanning electron microscopy, and Ni C was identified from x-ray d i f f r a c t i o n measurements. These results suggest that the deactivation of nickel catalysts i s due to the accumulation of a carbonaceous film at low temperature, the rapid formation of filamentary carbon at moderate temperatures, and the formation of encapsulating carbon layers at very high temperature. 2

2

2

3

Carbon can e x i s t on the metal surfaces o f n i c k e l c a t a l y s t s i n a v a r i e t y of forms. Hydrocarbon exposure to n i c k e l c r y s t a l l i t e s at elevated temperature (> 700 K) can r a p i d l y produce a mass o f long-growing carbon filaments (ly 2) as i d e n t i f i e d i n numerous experiments analyzed by t r a n s m i s s i o n e l e c t r o n microscopy (TEM). Yet very r e a c t i v e forms o f s u r f a c e carbon can e x i s t , s i n c e carbon atoms chemisorbed on n i c k e l surfaces apparently p l a y a c e n t r a l r o l e i n the mechanism o f s e v e r a l n i c k e l - c a t a l y z e d r e a c t i o n s , such as hydrocarbon s y n t h e s i s , (3 ,4, 5) hydrocarbon steam reforming, (6, 7) and hydrogenolysis ( 8 ) . In a previous study (9) we used temperature-programmed surface r e a c t i o n (TPSR) with 1 atm hydrogen to determine the

0097-6156/82/0202-0253$08.50/0 © 1982 American Chemical Society

In Coke Formation on Metal Surfaces; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

COKE FORMATION

254

r e a c t i v i t y o f carbon deposited on alumina-supported n i c k e l . In that study s e v e r a l forms o f v e r y r e a c t i v e carbon were d i s t i n guished by t h e i r r e l a t i v e r e a c t i v i t y as determined by the tempera ture a t maximum g a s i f i c a t i o n r a t e with hydrogen. The two most r e a c t i v e forms o f s u r f a c e carbon (α' , α) were a t t r i b u t e d to chemisorbed carbon atoms followed in order o f r e a c t i v i t y by the i n i t i a l l a y e r s o f n i c k e l c a r b i d e (γ) and f i n a l l y a l a y e r o f amorphous carbon ( β ) . These carbon forms were deposited by exposure to both CO and 0 Η ^ over a temperature range from 473 Κ t o 750 K. In recent work, supported by the F o s s i l Fuels D i v i s i o n o f the U.S. Department o f Energy, we have extended the range o f exposure c o n d i t i o n s f o r carbon d e p o s i t i o n by CO and 0^1^ decomposition on N i / A ^ O ^ s u r f a c e s and more f u l l y explored the nature o f such deposits. The TPSR s t u d i e s with 1 atm H as r e a c t a n t were a l s o extended by s t u d i e s with 0.03 atm H 0 in He d i l u e n t as the reac tant gas. The b u l k carbon d e p o s i t s produced by CO o r C H, expo sure were a l s o examined with x-ray d i f f r a c t i o n and t r a n s m i s s i o n and scanning e l e c t r o n microscopy so that the r e a c t i v i t y o f each form o f carbon was i d e n t i f i e d with i t s morphology.


2

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2

2

Experimental D e t a i l s M i c r o r e a c t o r System f o r TPSR. The r e a c t i v i t y o f carbon d e p o s i t s on reforming c a t a l y s t s has been s t u d i e d w i t h a quartz m i c r o r e a c t o r system using the TPSR technique ( F i g u r e 1). A s m a l l bed (10 t o 40 mg) o f a powdered c a t a l y s t i s placed on the porous f r i t t e d d i s k o f a 1-cm q u a r t z m i c r o r e a c t o r . The c a t a l y s t bed can be heated up t o 1375 Κ by r a d i a t i o n and c o n v e c t i o n from a 12-mm by 25-mm c o i l of 0.81-mm-diameter Nichrome wire, which i s suspended around the m i c r o r e a c t o r . The wire i s e l e c t r i c a l l y heated with a programmable DC power supply t h a t i s c o n t r o l l e d by a custom-desig ned temperature programmer. A steam o f dry N purges a i r from c o n t a c t with the heater wire to extend i t s l i f e t i m e and to c o o l the r e a c t o r . The c a t a l y s t bed can be cooled from 1375 Κ t o below 500 Κ in l e s s than one minute, and when a steady stream of n i t r o gen i s passed through a Dewar f l a s k o f l i q u i d n i t r o g e n , the bed can be cooled to below 180 K. The programmer can heat the r e a c t o r at l i n e a r from 0.1 t o 4 K s"" . The c a t a l y s t bed temperature i s measured with an unshielded 0.075-mm Chromel-Alumel o r 0.25-mm P t 10% Rh/Pt thermocouple that penetrates the bed. The thermocouple output i s compensated f o r ambient temperature with an Omega MCJ reference junction. The thermal response time o f the c a t a l y s t bed thermocouple system i s about one second. 2

1

The TPSR experiments were conducted a t one atmosphere p r e s sure with a stream o f helium o r r e a c t i v e gas mixtures flowing through the c a t a l y s t bed. High p u r i t y r e a c t a n t gases, i n c l u d i n g hydrogen, carbon monoxide and l i g h t hydrocarbons, could be blended with a helium c a r r i e r before i n t r o d u c t i o n i n t o the r e a c t o r . We p u r i f i e d the helium c a r r i e r by passing i t through a bed o f copper turnings a t 573 Κ and then through a l i q u i d N~ c o o l e d molecular


3.

MCCARTY ET A L .

Surface

Carbon

on Nickel

Catalysts


Vacuum

Cooling N

2

M

X

Reactor

Variable

Assembly

Leak T o Quadrupole

Valve

Mass S p e c t r o m e t e r

Figure 1.

TPSR and pulsedflowsystem.


256

COKE FORMATION


s i e v e t r a p . Hydrogen, i n i t i a l l y 99.99% p u r i t y , was f u r t h e r p u r i f i e d by d i f f u s i o n through a Ag/Pd a l l o y tube. The other r e a c t a n t gas mixtures, 3.0-vol% CO in helium, 1.0-vol% C 2 H 2 in helium were used as r e c e i v e d from the s u p p l i e r . Steam was generated at the d e s i r e d rate by a s p e c i a l v a p o r i z e r fed by a s y r i n g e pump. Water was d e l i v e r e d through a s y r i n g e to a l a y e r o f g l a s s thread wrapped around a g l a s s c y l i n d e r l o c a t e d i n s i d e a l a r g e r c o n c e n t r i c g l a s s vessel. The feed r a t e of water was adjusted to a t t a i n the d e s i r e d vaporizaton rate. A c a r t r i d g e heater i n s i d e the c y l i n d e r provided the heat necessary f o r steady steam e v a p o r a t i o n . We c o n t i n u o u s l y monitored the composition o f the m i c r o r e a c t o r e f f l u e n t gas with a quadrupole mass spectrometer (EAI Quad 300C) by sampling a small p o r t i o n o f the e f f l u e n t gas stream with a var iable leak valve (Granville P h i l l i p s ) . The pressure, i n d i c a t e d by a nude i o n i z a t i o n gauge in the mass spectrometer chamber, was con t r o l l e d a t 9 χ 10 t o r r with an automatic s e r v o - c o n t r o l l e d pres sure c o n t r o l l e r ( G r a n v i l l e P h i l l i p s ) . The mass spectrometer sys tem was c a l i b r a t e d by i n j e c t i n g known amounts of pure gases i n t o a He c a r r i e r stream. A c o n v e r s i o n f a c t o r was obtained f o r each gas, g i v i n g q u a n t i t a t i v e e x p r e s s i o n of the TPSR r a t e data in terms of e i t h e r c o n c e n t r a t i o n in the e f f l u e n t gas o r net r e a c t i o n r a t e from the product o f e f f l u e n t c o n c e n t r a t i o n and c a r r i e r flow r a t e . In l a t e r experiments, 0.1-vol% argon was added (premixed) to the helium c a r r i e r as an i n t e r n a l standard f o r c a l i b r a t i n g the mass spectrometer. The mass spectrometer could monitor up to eight i n d i v i d u a l i o n mass peaks with an automatic mass peak s e l e c t o r (Vacuumetries) and transmit the s i g n a l from selectedmass fragments to r e c o r d e r s and an e l e c t r o n i c i n t e g r a t o r ( S p e c t r a p h y s i c s ) o r , more r e c e n t l y , to a l a b o r a t o r y microcomputer (Dual Systems). The mass spectrometer s e n s i t i v i t y during TPSR was about 30 ppm f o r H 2 and CO and 10 ppm f o r other gases. By c a r e f u l l y measuring the mass 32 peak, we kept the l e v e l of oxygen i m p u r i t y in the pure H 2 and He c a r r i e r streams reaching the r e a c t o r always below 1 ppm and t y p i c a l l y at 0.1 ppm. TPSR C h a r a c t e r i z a t i o n of Deposited Carbon. In an experiment to analyze the TPSR c h a r a c t e r i s t i c s of a carbon d e p o s i t , 15 to 35 mg of powdered (170/250 T y l e r mesh) c a t a l y s t was placed on the porous d i s k o f a quartz m i c r o r e a c t o r . T y p i c a l l y , a f r e s h sample of c a t a l y s t was reduced f o r 15 hr at 773 Κ in 1 atm flowing H2 and a p r e v i o u s l y reduced c a t a l y s t was heated to 773 Κ f o r a t l e a s t 1 hr before a TPSR experiment. Following r e d u c t i o n , the c a t a l y s t was flushed with pure He and cooled or heated to a predetermined carbon d e p o s i t i o n temperature. Carbon was deposited by exposure to a stream o f d i l u t e hydrocarbon ( t y p i c a l l y 0.14-vol% C0H4) in helium using a s p e c i a l 10-port switching v a l v e ( F i g u r e 1 ) . Following carbon d e p o s i t i o n , the c a t a l y s t was cooled to room temperature while the bed was flushed with pure He, and the He c a r r i e r (now in r e a c t o r by-pass mode) was r e p l a c e d by the TPSR reactant gas stream (1-atm H2 or 0.03-atm H2O in He). A f t e r the


13.

MCCARTY ET A L .

Surface

Carbon

on Nickel

Catalysis

257

c a t a l y s t bed cooled to ambient temperature, the r e a c t i v e gas was switched through the c a t a l y s t bed and temperature programming began. During TPSR, the r a t e o f g a s i f i c a t i o n o f the carbon deposit (as determined by continuous measurement o f the e f f l u e n t gas composition) was recorded as a f u n c t i o n i f time. The data are represented by the c a l c u l a t e d gas production r a t e versus bed temperature as in c o n v e n t i o n a l thermal a n a l y s i s . TPSR a n a l y s i s was performed f o r v a r i o u s c o n d i t i o n s o f exposure temperature, ex posure d u r a t i o n , and exposure gas (CO, 0 Η ^ or C 2 H 2 ) f o r both 1atm H and 0.03-atm H 0 in He as the g a s i f y i n g r e a c t a n t .


2

2

2

E l e c t r o n Microscopy. Some of the carbon produced on G-56H by C H^ exposure has been a s s o c i a t e d with f i l a m e n t a r y carbon growing from n i c k e l c r y s t a l l i t e s through observations o f deposited carbon using TEM and SEM. For TEM, samples were sandwiched between Formvar-coated 200-mesh n i c k e l g r i d s . Kodak e l e c t r o n image p l a t e s were exposed f o r 2 s a t an a c c e l e r a t i n g v o l t a g e o f 80,000 eV on a P h i l i p s 200 e l e c t r o n microscope. A m a g n i f i c a t i o n range o f 3,000200,000 diameters was used. For SEM, the samples were mounted in c o l l o i d a l s i l v e r on microscope chucks and observed in the Cambridge Mark I l a SEM instrument. The m a g n i f i c a t i o n range was 100 to 50,000 diameters. 2

C a t a l y s t s . The n i c k e l c a t a l y s t s used in the TPSR s t u d i e s G56H and G-65, both were commercial (united C a t a l y s t s Incorporated) h i g h metal-weight-loading c a t a l y s t s with alumina supports. The 17-wt% N i / A l 0 q c a t a l y s t (G-56H) had a r e l a t i v e l y low metal sur face area (4 n r / g Ni) but r e s i s t e d s i n t e r i n g since i t o n l y l o s t 20% o f i t s surface area a f t e r heating to 1273 Κ in 1 atm Η · The 25-wt% N i / A l 0 (G-65) had a modest surface area (29 m /g Ni) and contained some CaO (see Table I f o r a summary o f the c a t a l y s t properties). 2

2

2

2

3

Table I PROPERTIES OF NICKEL CATALYSTS

Metal Loading

Catalyst

Support

G-56H/UCI G-65/UCI

2°3 A1 0 + CaAl 0

A 1

2

3

2

4

17 25

Total Surface Area (m /g c a t )

Specific Metal Area (m /g Ni)

-

4 28

2

2

56

The metal surface area was determined by a d s o r p t i o n of CO a t 300 Κ presuming 1.1 χ 1 0 molecules CO/cm . 1 5


COKE FORMATION

258


Results Carbon Deposited on N i c k e l v i a CgH^ Exposure; TPSR w i t h 1atm Carbon deposited on alumina-supported n i c k e l (17 wt%) f o l l o w i n g ethylene exposure a t v a r i o u s temperatures has a wide range of a c t i v i t y f o r r e a c t i o n with H2 ( F i g u r e 2 ) . D i f f e r e n t s t a t e s of carbon are i d e n t i f i e d by maxima in the r a t e of CH^ pro d u c t i o n . The temperature o f the rate maximum (T ) f o r a p a r t i c u l a r carbon s t a t e was g e n e r a l l y found to be independent of both the amount of carbon in that s t a t e and the temperature of d e p o s i t i o n . Thus Τ w i l l be taken as c h a r a c t e r i s t i c o f the r e a c t i v e s t a t e of carbon in a d e p o s i t . I n c r e a s i n g the temperature of carbon deposi t i o n s v i a CoH/ produced carbon s t a t e s with higher Τ and hence lower r e a c t i v i t y with Η2· The TPSR (H ) r e s u l t s d i d not d i f f e r s i g n i f i c n a t l y f o r e i t h e r of the alumina-supported n i c k e l c a t a l y s t s as seen by Τ f o r the v a r i o u s carbon s t a t e s (Table I I ) . T h i s was true f o r both TPSR (H ) with 1-atm H and TPSR (H 0) with 0.03-atm H 0 in He. Exposure o f G-65 to a l i q u o t s o f C H^ in He at low temperature (573 K) produced s e v e r a l v e r y r e a c t i v e s t a t e s o f carbon. A small amount of the carbon d e p o s i t , i . e . the α' carbon s t a t e (9) with l e s s than one monolayer coverage which i s taken to be equal to the CO uptake at 300 K, had a peak temperature at 400 Κ and even had an observable CH^ p r o d u c t i o n r a t e at room temperature (300 K ) . The ct TPSR peak with H2 was o f t e n obscured by a l a r g e r peak a t 495 i 20 Κ r e p r e s e n t i n g the α carbon s t a t e . Below 600 Κ the α s t a t e was populated up to about one monolayer before g i v i n g r i s e to another s t a t e (the γ carbon s t a t e ) produced at very h i g h exposure (Figure 2 ) . Because the γ carbon s t a t e could be populated to an extent e q u i v a l e n t to 10 monolayers, i t seems obvious t h a t t h i s s t a t e was a bulk s t a t e and was e i t h e r bulk n i c k e l c a r b i d e , N13C, or some form of f r e e carbon. Based on x-ray d i f f r a c t i o n a n a l y s i s , and the f a c t that t h i s s t a t e was never populated a t 773 Κ (N13C i s known to decompose above 600 Κ (10)), the γ carbon s t a t e was most l i k e l y Ni^C. X-ray d i f f r a c t i o n measurements of reduced 17-wt% N i / A ^ O ^ always show m e t a l l i c n i c k e l l i n e s even a f t e r d e p o s i t i o n of l a r g e amounts o f carbon a t 773 Κ and higher temperatures. However, f o l lowing long exposure to C2H4 a t 573 K, the n i c k e l x-ray d i f f r a c t i o n l i n e s showed a pronounced decrease in i n t e n s i t y . D i f f r a c t i o n l i n e s f o r Ni^C were not observed probably because most of the n i c k e l c r y s t a l l i t e s were transformed i n t o amorphous N13C o r c r y s t a l l i t e s of N i C w i t h small domain s i z e (10). Another form o f carbon (the β carbon s t a t e ) was a l s o observed f o l l o w i n g the C0H4 exposure at 573 Κ to both the 25-wt% Ν ΐ / Α ^ Ο β and 17-wt% Ni/A±203 c a t a l y s t s . The β carbon s t a t e had a peak tem perature that increased from 660 Κ at low coverage to 740 Κ a t h i g h coverage. The amount of carbon deposited i n t o the β carbon state was observed not to exceed 4 monolayers. T h i s s t a t e has not been p o s i t i v e l y i d e n t i f i e d , but i t s coverage i s l i m i t e d to a few 2

2

2

2

2

2

T

3



13.

MCCARTY ET A L .

300

400

500

Surface

600

Carbon

700

on Nickel

800

900

Catalysts

1000

259

1100

1200

T E M P E R A T U R E (K) Figure 2. TPSR with hydrogen of carbon deposited on G-56H by exposure to ethylene. Key to carbon deposition temperature: — , 573 K; , 773 K; , 1073 K; and , 1273 K.


1300

In Coke Formation on Metal Surfaces; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983. 2

2

2

2

2

4

4

CH /H 4

CH /H CH /H 4

CH /H

773

873

973

1073

1273

-

C0 /H 0 C0 /H 0 C0 /H 0 C0 /H 0 C0/H 0 CO/H 0

973

1073

1273

1073

1273

2

2

2

2

2

2

2

2

2

2

-

-

-

-

860

-

815

850

-

830

-

860

855

-

-

873

2

855

525

-

-

515

400

925

950

935

945

-

985

1005

965

-

-

385

-

955

850

510

-

950 835

-

510

-

C0 /H 0 2

-

815

895

-

505

-

655

-

-

830

1095

1070

1115

-

1150

1130

1160

1090

-

-

-

-

-

865

1105 1115

-

850

1265

-

1280

-

1280

1255

1220

-

-

-

-

1215

1295

1210

-

(K) a t Maximum R a t e ε Carbon δ' δ

395

-

773

4

4

2

CH /H CH /H

573

2

505

2

co /o

773

-

575

-

630

-

2

2

2

C0 /H 0

773

2

4

2

625

C0 /H 0

685

505

573

395

2

4

CH /H

480

Temperature α β

CH /H

400

α'

773

Product/ R e a c t a n t Gas

573

Deposition T e m p e r a t u r e (K)

δ' and δ s t a t e s n o t r e s o l v e d .

G-56H

G-65

Catalyst

TPSR CHARACTERIZATION OF CARBON DEPOSITED ON NICKEL CATALYSTS FOLLOWING ETHYLENE EXPOSURE

Table I I


2

Os Ο

13.

MCCARTY ET AL.

Surface

Carbon

on

Nickel

Catalysts

261

monolayers, and t h i s makes i t l i k e l y to be an amorphous carbon f i l m ( p o s s i b l y with some hydrogen present (11). The β carbon state i s not populated a t 873 Κ and higher temperature (see Table I I I , f o r 17-wt% N i / A l 0 ) . A number of l e s s r e a c t i v e forms of carbon are produced f o l lowing C H^ exposure to the Ni/A^CU c a t a l y s t s a t 773 Κ and higher temperature. At 773 K, ( F i g u r e 2; exposure produces a b u l k form of carbon (the δ carbon s t a t e ) with Τ a t 875 ± 20 K. At 873 Κ two forms of carbon are produced ( F i g u r e s 3 and 4) on the 17-wt% Ni c a t a l y s t f o l l o w i n g steady exposure to a stream of 1.0v o l % C 2 H 4 in helium. One form i s the δ carbon s t a t e ; the other form with Τ a t 960 ± 15 Κ ( a l s o a bulk form) i s the δ carbon s t a t e . Following C H^ exposure a t 973 Κ the δ carbon s t a t e appears as a shoulder on the much l a r g e r peak r e p r e s e n t i n g the g a s i f i c a t i o n of the δ carbon state ( F i g u r e 4) and at 1073 Κ t h i s s t a t e i s no longer present ( F i g u r e s 4 and 5 ) . Both the δ' and δ s t a t e s are i d e n t i f i e d with filamentous carbon by e l e c t r o n microscopy. Transmission e l e c t r o n micrographs of G-56H, a f t e r a r e l a t i v e l y long exposure (10 min) to 0.1-atm C9H4 a t 875 K, show the e x t e n s i v e formation of filamentous carbon (Figures 6a through 6e). The micrographs show a wide d i s t r i b u t i o n of filament diameters from 70 Â to 1000 Â, with approximately bimodal d i s t r i b u t i o n centered at 100 Â and 800 Â. The TEM micrographs are very s i m i l a r to the c l a s s i c carbon filament p i c tures published by Baker and H a r r i s (1) and others ( 2 ) , (12-19). The f i l a m e n t s appear to have narrow hollow i n t e r i o r s with low d e n s i t y (more transparent) cores and denser ( l e s s transparent) outer s h e l l s , and o f t e n have a v e r y dense p a r t i c l e (presumably a n i c k e l c r y s t a l l i t e ) at t h e i r t i p s ( F i g u r e 6b). The SEM r e s u l t s showed the e n t i r e c a t a l y s t p a r t i c l e s are completely coated w i t h a t h i c k mat of carbon f i l a m e n t s ( F i g u r e s 6c through 6e). 2

3

2

1


1

1

2

The e l e c t r o n micrographs of the 17-wt% Ni c a t a l y s t (G-56H) a f t e r 100-s exposure to 0.004-atm C H^ a t 1073 Κ a r e s i m i l a r to the r e s u l t s obtained a t 875 K, except that the extent o f carbon formation at 1073 Κ was s u b s t a n t i a l l y l e s s : ~ 3% weight gain versus > 50% weight g a i n a t 875 K. The r a t e o f f i l a m e n t growth was f a s t e r at 873 Κ than at 1073 K, and the average filament length was much longer a t the lower temperature. Again, l a r g e (800 Â) and small (100 Â) carbon f i l a m e n t s were produced; however, many metal p a r t i c l e s appeared to be completely encapsulated by carbon f i l m s . At 1273 K, 1073 K, and 873 Κ (with > 100-s exposure) C H exposure produces yet another form of carbon (ε carbon s t a t e ) with very low c h a r a c t e r i s t i c r e a c t i v i t y with hydrogen ( F i g u r e 2 ) . The peak temperature of the ε carbon s t a t e was about 1120 K, although the TPSR (H ) peak produced by t h i s s t a t e v a r i e d some f 40 Κ and was o f t e n d i f f i c u l t to r e s o l v e from the peak produced by the δ carbon s t a t e . A unique f e a t u r e of the CH^ p r o d u c t i o n during TPSR w i t h 1-atm H 2 at temperatures above 1100 Κ in g e n e r a l , and f o r the peak 2

2

2


4


Beta

3

carbon

carbon

temperature

p

(T )

for

Equilibrium

limited for 9

TPSR w i t h H .

+

1120

15 20^^

X

X

X

X

573

limit.

2

is

X

X

X

X

X

X

X

X

X

1073

i s H,

X

X

1273

T e m p e r a t u r e (K)

reactant

873

CH^, the

X

X

X

X

773

C H ^ Exposure

The TPSR p r o d u c t

+

960

K/s.

1

+ 30 ,

+

8 7 5 + 20

660

550

+

25

(K)

,

TPSR

480

ρ

0

15

τ

2

410 +

4

CH,/H

low coverage

=0.9

T^ f o r

rate

coverage;

heating

carbon

(TEM)

( T E M , SEM)

carbon

Increases with increasing

Peak

Platelet

ε

(?)

Encapsulating

carbon

film

δ

Filamentous

carbon

carbide

Nickel

Ύ

Chemisorbed

Chemisorbed

f

Identification

a

a

State

III

P O P U L A T I O N OF T P S R STATES FOR CARBON D E P O S I T E D ON AN ALUMINA-SUPPORTED N I C K E L CATALYST ( G - 5 6 H ) FOLLOWING E T H Y L E N E EXPOSURE

Table


McCARTY ET A L .


ι

300

400

Surface

ι

ι

500

600

Carbon

on Nickel

Catalysts

Γ

700

800

900

1000

1100

1200 1

T E M P E R A T U R E (K) Figure 3.

TPSR with hydrogen of carbon deposited on G-56H at 873 Κ increasing exposure to ethylene.


COKE FORMATION


264

300

400

500

600

700

800

900

1000

1100

1200

1300

T E M P E R A T U R E (K) Figure 4. TPSR with hydrogen of the δ' carbon state deposited on G-56H by exposure to ethylene. Key to carbon deposition temperature: , 773 K; —, 873 K; , 973 K; and - -, 1073 K.



13.

McCARTY ET A L .

300

400

500

Surface

600

Carbon

700

on Nickel

800

900

Catalysts

1000

1100

265

1200


TPSR with hydrogen of carbon deposited on G-56H at 1073 Κ by increasing exposure to ethylene.


1300

COKE FORMATION


266

Figure 6.

Electron micrographs of carbon deposited on G-56H by decomposition of ethylene at 875 Κ and 1073 K.

Key: a, low-magnification (2340X) transmission electron micrograph (TEM) of carbon deposited at 875 K; b, high-magnification (21J75X) TEM of the same sample as in a; c, low magnification (65 X) scanning electron micrograph (SEM) of a G-56H catalyst particle exposed to ethylene at 875 K; d, medium-magnification (3,250X) SEM of the protrusion arrowed in c; and e, SEM micrograph of the same sample at near the limit of resolution (32,500x).


13.

Surface

McCARTY ET A L .

Carbon

on Nickel

Catalysts

267

r e p r e s e n t i n g the ε carbon s t a t e in p a r t i c u l a r , i s that the r a t e not o n l y decreases with i n c r e a s i n g temperature as expected but in creases again upon c o o l i n g . Apparently the c o n c e n t r a t i o n of CH^ in the e f f l u e n t gas i s l i m i t e d by the opposing r e a c t i o n s o f carbon g a s i f i c a t i o n with H and methane decomposition, i . e . , the methanecarbon e q u i l i b r i u m . It i s a l s o apparent that a t 1 K s heating r a t e s and 1 cm s " gas flow r a t e s , the TPSR (H ) experiments can not completely g a s i f y the ε carbon state except when very l i t t l e i s present (< 5 μπιοί) . G a s i f i c a t i o n with steam does not approach the e q u i l i b r i u m c o n c e n t r a t i o n s under TPSR c o n d i t i o n s and allows a more complete examination o f thee carbon s t a t e . Carbon Deposited on N i c k e l C a t a l y s t s by Exposure t o Carbon Monoxide. In a previous TPSR study (9) carbon was d e p o s i ted on a t 25-wt% N i / A l 0 c a t a l y s t (G-65) by exposure to CO a t temperatures between 550 and 610 K. TPSR (H ) o f carbon produced by the d i s s o c i a t i o n ^ i s p r o p o r t i o n a t i o n of chemisorbed CO showed the presence o f l a r g e α and β s t a t e s ( F i g u r e 7 ) . Although some chemi sorbed CO may have c o n t r i b u t e d to the α s t a t e s , i t was, c o n c l u s i v e l y shown that most o f the α carbon state was more r e a c t i v e than a monolayer of chemisorbed CO. In t h i s study, we extended the depo s i t i o n o f carbon by CO exposure to higher temperatures to d e t e r mine i f the δ and δ filament carbon s t a t e s in a d d i t i o n to the α and β s t a t e s can be populated by a hydrogen-free source. The TPSR ( H ) r e s u l t s ( F i g u r e 8) c l e a r l y show the presence o f the f i l ament carbon ( δ carbon) s t a t e a t 773 K. Comparison o f the peak temperatures f o r carbon deposited by C H^ and CO exposure (Table IV) shows l i t t l e d i f f e r e n c e in the r e a c t i v i t y o f the carbon states. 2

3

1


2

2

3

2

1

2

1

2

Table IV TPSR, CARBON STATES ON G-56H AFTER EXPOSURE TO C H,, AND CO 0

Carbon State α' a β Y δ' δ ε

Identification R e a c t i v e carbon Chemisorbed carbon Carbon f i l m Ni C Filament carbon Encapsulating carbon P l a t e l e t carbon 3

Temperature (K a t Maximum Rate [T ] ) CO Exposure CH Exposure 2

/f

410 ± 15 480 ± 25 660 ± 30 -555 875 ± 20 960 ± 15 1120 ± 20

380 ± 15 460 ± 20 660 ± 20 -480 850 db 40

Methane production from TPSR with 1-atm hydrogen.


—

COKE FORMATION

268

°· 1 Downloaded by PENNSYLVANIA STATE UNIV on August 6, 2013 | http://pubs.acs.org Publication Date: October 25, 1983 | doi: 10.1021/bk-1983-0202.ch013

0 5

I I I I I I I I

300 400 500

600

I

I

I

700

TEMPERATURE Figure 7. TPSR with hydrogen of carbon deposited on G-65 by CO exposure at 550 K. Key to relative carbon deposit: a, 0.48; b, 1.19; and c, 3.14 normalized to CO adsorption at 300 K.



MCCARTY ET A L .

Surface

Carbon

on Nickel

Catalysis

1 300

500

700

900

1100


TPSR with hydrogen of carbon deposited on Ni/Al O CO. Key: , 573 K; and , 773 K. 2

s

by exposure


COKE FORMATION

270

TPSR w i t h 0.03-atm HÔ o f Carbon Deposited on N i c k e l v i a C H Exposure. The g a s i f i c a t i o n of carbon d e p o s i t s with 0.03-atm steam ( F i g u r e 9 and Table V) shows that the δ s t a t e carbon r e a c t s more r a p i d l y with 0.03-atm H 0 than with 1-atm H , as i n d i c a t e d by the lower temperature (some 50 to 120 K) a t which the g a s i f i c a t i o n r a t e reaches a maximum. The y i e l d o f carbon deposited above 873 Κ during exposure to 0.01-atm C H^ decreases with i n c r e a s i n g c a t a l y s t temperatures seen by the TPSR (H 0) r e s u l t s ( F i g u r e 9 ) , which i s in agreement with the TPSR ( H ) r e s u l t s . However, TPSR ( H 0 ) , u n l i k e TPSR ( H ) , does not r e s o l v e the low temperature s t a t e s ( F i g u r e 9) and l i t t l e o r no g a s i f i c a t i o n i s seen below 475 K. 2

2

7|

2

2

2

2

2


2

Table V COMPARISON OF H 0 AND H TPSR RESULTS AFTER EXPOSURE OF G-56H TO ETHANE 2

TPSR States δ' δ ε

2

Temperature (K) at Maximum Rate (T ) 0.03-atm H 0 1-atm H 2

825 870 1000

Carbon Formation on N i c k e l

875 960 1120

2

T e n t a t i v e I d e n t i f i c a t on of Carbon Species Filament carbon E n c a p s u l a t i n g carbon P y r o l y t i c carbon

Catalysts

Morphology of Carbon Deposits on N i c k e l . The morphology o f carbon deposited on n i c k e l by exposure to hydrocarbons and CO v a r i e s d r a m a t i c a l l y with d e p o s i t i o n c o n d i t i o n s , as shown p r e v i o u s l y by numerous i n v e s t i g a t i o n s with e l e c t r o n microscopy ( 1_). The most dramatic f i n d i n g of these s t u d i e s i s the growing carbon f i l a m e n t s with n i c k e l c r y s t a l l i t e s attached a t t h e i r t i p s ( 1 ) , (2^,) ( 6 ) , (12-20). The carbon f i l a m e n t s ( o f t e n hollow) can grow r a p i d l y , p u l v e r i z i n g c a t a l y s t p e l l e t s ( 2 1 ) and forming a t h i c k , v i s c o u s mat of carbon. The mechanism of formation o f filamentous carbon i s b e l i e v e d to be the c o n c e n t r a t i o n - d r i v e n (20) o r temperature-driven d i f f u s i o n o f carbon through e n t r a i n e d , small (~ 10 nm) nickel c r y s t a l l i t e s . Because carbon i s c o n t i n u o u s l y removed by d i f f u s i o n from the surface r e g i o n s where i t f i r s t d e p o s i t s and i s t r a n s f e r r e d to the growing f i l a m e n t , the s u r f a c e where carbon i s f i r s t deposited need not d e a c t i v a t e . Thus coking proceeds r a p i d l y and c o n t i n u o u s l y , as long as carbon continues to move from the ex posed metal surface i n t o the growing f i l a m e n t . The TEM data, e s p e c i a l l y a t higher temperature, a l s o show that the f i l a m e n t s may stop growing and that a carbon s h e l l may encapsulate the metal crystallites. Such a p a s s i v a t i n g s h e l l stops f u r t h e r f i l a m e n t


MCCARTY ET AL.

13.

Surface

Carbon

on Nickel

Catalysis

271


0.30

0.25

0.20

h

< oc 0.15 Ο Ι Ο Ζ) α ο oc

ο.ιο

μ

ο ο

0.05

500

600

700

800

900

1000

1100

1200

1300

T E M P E R A T U R E (Κ) Figure 9. TPSR with 0.03 atm H 0 in He of carbon deposited on G-56H by exposure to ethylene. Key to carbon deposition temperature: , 1273 K; , 1073 K; - -, 973 K; , 873 K; and - · ·, 773 K. 2


COKE FORMATION


272

growth and presumably slows any c a t a l y z e d r e a c t i o n s such as hydrocarbon steam r e f o r m i n g . Other types o f carbonaceous d e p o s i t s found on n i c k e l surfaces f o l l o w i n g exposure to hydrocarbons include e n c a p s u l a t i n g polymer f i l m s p o s s i b l y c o n t a i n i n g hydrogen) formed a t low temperature, (20) c r y s t a l l i n e f l a k e o r p l a t e l e t carbon formed on f l a t metal surfaces a t h i g h temperatures (1_), (20), (22), and amorphous car bon f i l m s a l s o formed a t h i g h temperature (1_). In a d d i t i o n , pyrol y t i c carbon ( e s s e n t i a l l y formed in the gas phase) can c o l l e c t on the e x t e r i o r s u r f a c e s o f c a t a l y s t p a r t i c l e s and may e v e n t u a l l y coat the c a t a l y s t p a r t i c l e s w i t h a l a y e r o f carbon (20) and block the pores that lead to t h e i r i n t e r i o r s (23), (24). A d e s c r i p t i o n of the v a r i o u s forms and p r o p e r t i e s o f carbonaceous d e p o s i t s a r e summarized in Table VI. Given the wide range in morphology, we expected, as observed in our TPSR s t u d i e s , that these v a r i o u s car bon forms would have q u i t e d i f f e r e n t r e a c t i v i t i e s with H and H 0. 2

2

We have been a b l e to show that p a r t i c u l a r forms o f carbon have c h a r a c t e r i s t i c TPSR s p e c t r a in flowing H and H 0/He mix tures. Such carbon s t a t e s can be q u a n t i t a t i v e l y i d e n t i f i e d by t h e i r c h a r a c t e r i s t i c TPSR s p e c t r a . In p a r t i c u l a r , the a and α s t a t e s have been i d e n t i f i e d as i n d i v i d u a l carbon atoms, probably chemisorbed a t two d i f f e r e n t s i t e s (perhaps t e r r a c e and ledge or step s i t e s (25). We made t h i s i d e n t i f i c a t i o n because (9) (1) these s t a t e s a r e v e r y r e a c t i v e with hydrogen to produce methane, as we expect f o r C atoms; (2) they are populated only up to about monolayer q u a n t i t i e s in terms o f CO a d s o r p t i o n capacity; (3) they are not hydrocarbon fragments be cause they are populated by CO d i s s o c i a t i o n as w e l l as hydrocarbon decomposition; and (4) t h e i r r e a c t i v i t y i s o n l y a l i t t l e g r e a t e r than the γ carbon s t a t e , which we b e l i e v e i s bulk n i c k e l carbide because there are no n i c k e l l i n e s in the x-ray d i f f r a c t i o n o f G56H c a t a l y s t with a carbon d e p o s i t composed o f the γ carbon state. Thus the α and γ carbon s t a t e s both have metal-carbon bonding r a t h e r than carbon-carbon bonding. 2

2

?

I d e n t i f y i n g the β carbon s t a t e s as the polymeric "beta" carbon f i l m mentioned in the l i t e r a t u r e (6) i s l e s s c e r t a i n than assignments f o r α , a, and γ carbon TPSR s t a t e s . Rostrup-Nielsen (6), (11) and others (25) have r e f e r r e d to t h i s s t a t e on n i c k e l c a t a l y s t s as a polymeric s u r f a c e carbonaceous d e p o s i t , which in cludes w i t h i n i t a c o n s i d e r a b l e amount o f hydrogen. The formation of the β carbon s t a t e by CO exposure suggests that t h i s f i l m may not c o n t a i n hydrogen; however, t h e r e are s e v e r a l reasons to b e l i e v e that our β carbon s t a t e i s a surface hydrocarbon polymeric f i l m : (1) the maximum amount o f the β carbon s t a t e was produced by C H^ decomposition a t moderate temperature (573 K); (2) the β c a r bon state was o n l y observed a f t e r TPSR in H o f the α ( o r γ) c a r bon state and in the case of CO exposure may be formed by t r a n s formation o f the α o r γ carbon s t a t e s during TPSR; (3) f i n a l l y , the maximum amount o f the β carbon s t a t e was observed was 3 to 4 times the amount o f CO adsorbed on f r e s h l y reduced 25 wt% N i / A ^ O ^ 1

2

2


13.

MCCARTY ET AL.

Surface

Carbon

on

273

Catalysts

( G - 6 5 ) , o r about 4 . 5 χ Ι Ο carbon atoms per cn^ N i s u r f a c e area (assuming 1 . 1 χ 1 0 molecules adsorbed CO/cm Ni a t 2 9 8 K)· Such a mass o f carbon i s l e s s than expected f o r a bulk phase but more than expected f o r a s u r f a c e l a y e r , andtherefore we c o n s i d e r i t a polymeric hydrocarbon s u r f a c e f i l m . The TPSR r e s u l t s s t r o n g l y suggest that the δ' carbon s t a t e i s a s s o c i a t e d with the l o n g , r a p i d l y growing carbon f i l a m e n t s obser ved by TEM. Filamentous carbon was observed in t r a n s m i s s i o n e l e c t r o n micrographs of c a t a l y s t s exposed to CoH/ a t 8 7 3 Κ and 1 0 7 3 K, but not f o r those exposed a t 5 7 3 Κ and 1 2 7 3 Κ ( F i g u r e 2 ) . X-ray d i f f r a c t i o n s t u d i e s o f the same samples showed no l i n e s a t t r i b u t a ble to g r a p h i t i z e d carbon. The d e t a i l e d TPSR ( H ) s t u d i e s ( F i g u r e 4 ) c l e a r l y showed that the p o p u l a t i o n of the lower δ s t a t e ap pears only with the higher temperature δ s t a t e , but at 1 0 7 3 and 1 2 7 3 Κ the δ s t a t e appears alone. T h i s o b s e r v a t i o n l e a d s to our s p e c u l a t i o n that theô' carbon s t a t e i s the s o f t , more r e a c t i v e carbon observed (I) occupying the core of f i l a m e n t s , whereas δ carbon i s the hard outer s h e l l of f i l a m e n t s and perhaps i s a l s o the e n c a p s u l a t i n g carbon o f t e n observed ( 1 9 ) surrounding n i c k e l p a r t i c l e s d u r i n g high temperature exposure to hydrocarbons. TPSR and g r a v i m e t r i c observations o f the r a p i d decrease in the rate of δ carbon s t a t e formation with i n c r e a s i n g time exposure at 1 0 7 3 Κ a l s o support the view t h a t the δ carbon s t a t e could i n c l u d e the encapsulating form o f carbon. 1

5

2

1 5


Nickel

2

?

Whether the δ' or δ carbon s t a t e s are formed depends e n t i r e l y on the temperature during carbon d e p o s i t i o n . There i s a gradual t r a n s i t i o n between d e p o s i t i o n i n t o δ carbon and d e p o s i t i o n i n t o δ carbon f o r C H a expousre to N i / A l 0 o with temperature between 7 7 3 Κ and 1 0 7 3 Κ ( F i g u r e 2 ) . The nature of the ε carbon s t a t e seen d u r i n g TPSR w i t h H 0 i s u n c e r t a i n . TPSR w i t h H could not be used to r e s o l v e the ε carbon s t a t e from f r e e g r a p h i t i c carbon because at 1 1 0 0 Κ and higher temperatures, CH^ decomposition becomes favored over carbon g a s i f i c a t i o n by H?. The ε s t a t e seen d u r i n g TPSR with 0 . 0 3 - a t m H~0 was o f t e n small amounting to s e v e r a l monolayers carbon, and d i f f i c u l t to r e p r o d u c i b l y populate. For t h i s reason, we s p e c u l a t e t h a t the ε s t a t e may r e p r e s e n t a f i l m of p l a t e l e t carbon ( 2 2 ) , a h i g h l y g r a p h i t i z e d monolayer o f carbon o f t e n observed f o l l o w i n g p r e c i p i t a t i o n from carbon d i s s o l v e d in l a r g e specimens o f m e t a l l i c nickel ( 2 6 ) . However, the ε s t a t e could a l s o represent H 0 g a s i f i c a t i o n of carbon c a t a l y z e d by the support m a t e r i a l s , such as 2°3 e s p e c i a l l y CaO, a known c a t a l y s t f o r H 0 g a s i f i c a t i o n of carbon (2_7, 2 8 ) . The formation o f c a t a l y s t carbon as carbon f i l a m e n t s or en c a p s u l a t i n g carbon could profoundly i n f l u e n c e carbon f o u l i n g dur ing reforming. Carbon f i l a m e n t s can grow v e r y r a p i d l y , b l o c k the c a t a l y s t pores, and u l t i m a t e l y lead to r e s t r i c t e d flow and a l a r g e pressure drop through the r e a c t o r (Table V I ) . The r e s t r i c t e d flow could lead to increased p y r o l y s i s of unsaturated hydrocarbons and f u r t h e r carbon f o u l i n g . E n c a p s u l a t i n g carbon would lead to a much 1

2

2

2

2

2

A 1

o

r

2


COKE FORMATION

274

Table


Carbon TPSR S t a t e

VI. CHARACTERISTICS

Carbon Morphology Carbon

δ'

OF DEACTIVATING

CARBON DEPOSITS

Mechanism o f F o r m a t i o n

film

Slow p o l y m e r i z a t i o n o f C H r a d i c a l s on N i s u r f a c e s i n t o an amorphous f i l m (containing some H) X

Filaments, whisker-like growth, h e l i c a l (δ ) growth

D i f f u s i o n of carbon through N i c r y s t a l l i t e and p r e c i p i t a t i o n at favored s i t e s creating long (often hollow) f i l a m e n t s c a r r y i n g the c r y s tallite

Encapsulating carbon s h e l l

Formation s i m i l a r to f i l a m e n t carbon except carbon l a y e r s p r e c i p i t a t e and grow on a l l c r y s t a l planes

Platelet carbon

P r e c i p i t a t i o n of ( c r y s t a l l i n e ) g r a p h i t i c carbon from dissolved carbon

Pyrolytic carbon

Thermal c r a c k i n g of hydro c a r b o n and d e p o s i t i o n o f c a r b o n p r e c u r s o r s on e x t e r i o r of c a t a l y s t

Soot

Homogeneous n u c l e a t i o n and growth of c a r b o n p a r t i c l e s

1


ON

13.

McCARTY ET A L .

NICKEL HYDROCARBON

Effect Catalyst


sible

Carbon

on

rever

deactivation carbon

Low

temperature

Low H 0 / C

(< 625 K)

High

• C a t a l y s t breakdown

Low H 2 O / C

• Rapid

A r o m a t i c s and o l e f i n s feed

increase

in

•

deposit

Deactivation

• Slow

Low

temperature

2, 6, 20

in

(> 625 K)

1, 2, 6 19,

20, 21

ratio in

activity

High

carbon

References

ratio

• No d e a c t i v a t i o n

r e a c t o r ΔΡ

275

CATALYSTS

A r o m a t i c s and o l e f i n s feed

buildup

• Massive

Catalysts

Parameters Favoring Deposition

2

• Little

on Nickel

REFORMING AND SYNTHESIS

and R e a c t o r

• Progressive,

Surface

temperature

Low H 0 / C 2

(> 800 K)

2, 17, 18, 19 22

ratio

buildup Rapid

rate

of carbon

deposition Deactivation

High

Small

Low

carbon

temperature

(> 900 K)

1, 25

(> 900 K)

1, 20

pressure

buildup Aging Difficult remove

deposit

Encapsulation catalyst

of

High

temperature

particles High

Deactivation pore

deposit

to

blocking

due t o Low

void

High

I n c r e a s e ΔΡ

pressure

Low s p a c e Acidic Increasing Large

ΔΡ

carbon

deposit

fraction

H2O/C

velocity

catalyst

Low

reforming

Low

space

support

activity

20

velocity

High

temperature

High

pressure


COKE FORMATION

276

reduced l e v e l of c a t a l y s t i c a c t i v i t y f o r H 0 reforming with a cor responding increase in the r e s i d e n c e time f o r r e a c t i v e hydrocar bons, again l e a d i n g to p y r o l y s i s . 2

The Reaction Network. The chemisorbed carbon s t a t e s are l i k e l y reforming i n t e r m e d i a t e s . Chemisorbed carbon i s formed by the a b s o r p t i o n and r a p i d d i s s o c i a t i o n of C H^ and i s removed as CO by r e a c t i o n with s u r f a c e oxygen, which i s produced in t u r n by d i s s o c i a t i v e a d s o r p t i o n of H 0 or C 0 ( F i g u r e 10). Given the h i g h r e a c t i v i t y of chemisorbed carbon, the α ( i n c l u d i n g a,) s t a t e i s probably an intermediate in the production o f other forms o f c a t a l y s t carbon, δ and ε, and at lower temperature δ , β, and γ. A s i m i l a r set of equations would apply to other r e a c t i v e hydrocar bons, other hydrocarbon products, and CH^. 2


2

2

1

The s e l e c t i v i t y between reforming and carbon growth i s d e t e r mined in t h i s mechanistic scheme by the thermochemical p o t e n t i a l of chemisorbed carbon, C ( a ) . I f there i s a high c o n c e n t r a t i o n o f surface oxygen from the d i s s o c i a t i v e a d s o r p t i o n of H 0 and r e l a t i v e l y low c o n c e n t r a t i o n of C ( a ) , then we expect o n l y reforming products and no bulk carbon formation. However, i f the C (a) f o r mation r a t e i s v e r y h i g h and the 0(a) c o n c e n t r a t i o n low, ?hen the thermochemical p o t e n t i a l of C (a) c o u l d be high enough to produce f i l a m e n t carbon C^(a). I f fiïament carbon i s produced, and subsequently the c o n c e n t r a t i o n of C (a) a g a i n f a l l s due e i t h e r to lower p a r t i a l pressure of C H^(g) or a g r e a t e r c o n c e n t r a t i o n of 0(a) v i a increased p a r t i a l pressure of H 0 ( g ) , then C^(a) c o u l d be consumed by the r e v e r s a l o f the α too t r a n s f o r m a t i o n . Other r e a c t i o n s important to reforming are a l s o considered in the r e a c t i o n network in Figure 10, i n c l u d e the w a t e r - g a s - s h i f t r e a c t i o n and i t s r e v e r s e , the r e v e r s i b l e a d s o r p t i o n and decompo s i t i o n of water, the d e s o r p t i o n and a d s o r p t i o n o f reforming pro ducts l i k e CO, C0 , and H , and the formation of hydrocarbons l i k e CH^. The formation of d i s s o l v e d carbon, oxygen, and hydrogen in bulk n i c k e l i s a l s o considered. D i s s o l v e d C, 0, o r Η may be important in the t r a n s p o r t of those elements to or from i n t e r f a c e s with other s o l i d phase (carbon, c a r b i d e s , oxides, s u p p o r t ) . The p o s s i b l e formation of NiO from H 0 i s a l s o shown. F i n a l l y , an important r e a c t i o n to c o n s i d e r i s the formation of a d e a c t i v a t i n g l a y e r o f carbons (δ o r ε carbon s t a t e s ) . 2

2

2

2

2

2

Carbon G a s i f i c a t i o n Rates. Because the reforming r a t e s we observed during t h i s work were o f t e n c o n t r o l l e d by d i f f u s i o n , i t was not p o s s i b l e to determine i n d i v i d u a l r e a c t i o n r a t e s and rate constants. However, from the TPSR measurements we were able to estimate r a t e constants f o r the g a s i f i c a t i o n of c a t a l y s t and nonc a t a l y s t carbons. These r a t e s are l i s t e d in Table V I I along with s e l e c t e d r e s u l t s taken from the l i t e r a t u r e (29, 30, 31). We found that the c a t a l y s t carbon g a s i f i c a t i o n r a t e s were f i r s t order in carbon amounts up to e q u i v a l e n t (CO adsorption) monolayer


13.

MCCARTY ET A L .


GAS PHASE

Surface

Carbon

on Nickel

Catalysts

SURFACE STATES

277

BULK STATES

H(a) CH(g)

CH(a)

4

3

H(g) 9

H(a)

Alkanes (g)

CH (a) 2

0(a)

J) H(a)

Aromatics Alkenes (g) Alkynes

CH(a) H(a)

δ'(5)

C(d) C(a) CO(g)

δ (s) NiC(s)

0(a)

3

e(s)

CO(a) C0 (g)

C(a)

2

o(a)

: NiO(s)

H(a) H0(g)

OH(a)

2

H(a) H(g) 9

0(d)

0(a) H(a)

H(d)

Figure 10. Hydrocarbon synthesis and reforming reaction network on nickel surfaces. Key: a, adsorbed phase; g, gas phase; s, solid phase; and d, bulk solution.


COKE FORMATION

278


Table V I I .

Carbon State α

X

lCT^-atm

Y

1-atm H

2

β

1-atm H

2

1-atm H

2

δ

?

δ'

δ\δ

δ ,δ 1

Product Gas

Reactant Gas 1

3

X

2

10" -atm

Carbon G a s i f i c a t i o n Rate

H

CH. 4

2

CH. 4 CH. 4 CH. 4 co

H 0 2

2

1-atm Ho 1.5x1(^+9 ^ - a t m H 0 2

Activation Energy (kJ-mol- ) 1

CO

Pre-Exponential Factor (S-l-ymolC "™) 1

3

X

ΙΟ,,

3

X

io

130 + 40

2

X

io ±

108 + 20

7

X

ιο^ ·

210 + 40

2

X

1 0

3

X

io

4

7

71 + 10 84

134

CH

Λ

Parameters

6

1

+

9

3

1

5

10 1.9 ±

185

8

X

10

182 + 20

1

X

1 0

2

X

10

X

δ

1-atm H

CH. 4 CH. 4

2

δ(?)

1

X

10-3-atm

H

δ

3

X

10-2-atm

H 0

G(soot)

3

X

10" -atm

G(char) G(char)

5

X

10-3±l-atm

G(char)

1-atm H

2

C 0

2

2

1-atm

2

2

2

H 0 2

H 0 2

220 + 40 212 + 45

11±2.6

172

2

X

102

4 CO

238 + 30

3

X

107+1.6

CO

146

8

X

ιοί

227

2

X

109

C H

H 0

6±l-2

χ

CO χ

The u n c e r t a i n t y in p r e - e x p o n e n t i a l -actor d i r e c t l y f o l l o w s the u n c e r t a i n t y in a c t i v a t i o n e n e r g y w i t h i n t h e s p e c i f i c t e m p e r a t u r e range. ^ A s s i g n e d same a s α s t a t e .


13.

MCCARTY ET AL.

Surface

Carbon

on Nickel


f r o m Temperature-Programmed S u r f a c e

Experimental Methodt

Temperature Range ( K )

Catalysts

279

Reactions

Reaction Order Carbon

Reaction O r d e r Gas

Literature Reference

TPSR/HRV

450·-520

1

TPSR

530·-550

< 1

-

9

TPSR/HRV

680- -720

< 1

-

9

TPSR/IR

630- -760

§

TPSR/IR

700- -750

§

§

-

GA

773- -927

1

2

29

GA

821- -895

1

0

29

TPSR/IR

810- -820

§

-

TEM

975- -1240

-

-

TPSR/IR

710- -810

§

•

-

900- -1400

§

-

31

920- -1240

§

•

-

GA

820- -1400

§

< 1

31

GA

1003·-1123

1

0-1

38

GA TPSR/IR

± 1

~0.2

9

-

22

TPSR/HRV - v a r i a t i o n o f h e a t i n g r a t e w i t h Methods i n c l u d e : TPSRj TPSR- IR - i n i t i a l r a t e s w i t h TPSR; GA - m i c r o b a l a n c e s t u d i e s ; TEM - movement o f N i p a r t i c l e s o n a c a r b o n s u b s t r a t e d u r i n g TEM s t u d i e s . I n i t i a l rates only.

Âssumed f i r s t

order.


COKE

280

FORMATION

q u a n t i t i e s , and n e a r l y f i r s t order (n = 0.5 to 1.0) in carbon up to 30 monolayers f o r filament carbon. We d i d not attempt to determine r e c t i o n orders f o r g a s i f i c a t i o n with HÔ and H9; however, we d i d note that the h y d r o g a s i f i c a t i o n of the α carbon state was n e a r l y zero order with respect to H2 p a r t i a l pressure. At higher temperature (~ 800 K) we expect the r e a c t i o n order f o r hydrogen to i n c r e a s e due to the d e s o r p t i o n of hydrogen, and indeed F i g u r e i r e d o (20) reports second-order re a c t i o n in H 2 f o r h y d r o g a s i f i c a t i o n of carbon deposited on n i c k e l f o i l by decomposition of propylene at 720 ± 30 K. Under F i g u r e i r d o ' s c o n d i t i o n s , we would expect the δ carbon s t a t e to comprise most of the d e p o s i t . The r e a c t i o n order f o r steam g a s i f i c a t i o n o f such carbon f i l a m e n t d e p o s i t s v a r i e s between 0 and 1 depending on temperature (29, 30). Apparent a c t i v a t i o n energies f o r g a s i f i c a t i o n of the f i l a m e n t carbon s t a t e s were a l s o determined from i n i t i a l r a t e s o f the TPSR r e s u l t s (Table V I I ) . Because the r a t e s were f i r s t order in our s t u d i e s ( i . e . , d i r e c t l y p r o p o r t i o n a l to the mass o f deposited carbon), p l o t s of the logarithm of normalized r a t e s ( r a t e d i v i d e d by remaining carbon s t a t e mass) v s . r e c i p r o c a l temperature f o r the l e a d i n g edge of the TPSR curves have slopes p r o p o r t i o n a l to the apparent a c t i v a t i o n energy ( E ) . Our values of E are g e n e r a l l y in good agreement with the l i t e r a t u r e values given the experimen t a l u n c e r t a i n t i e s in t h i s type o f data. Rates e x t r a p o l a t e d to 1273 Κ are very l a r g e f o r the δ' carbon s t a t e ; thus under t y p i c a l high temperature a d i a b a t i c steam reforming c o n d i t i o n s we do not expect t h i s form of carbon to be present. However, the δ carbon state i s not so r e a d i l y g a s i f i e d and could be s t a b l e and accumu l a t e under ATR c o n d i t i o n s .


1

a

a

It i s l i k e l y that the δ carbon TPSR on Ν Ι / Α ^ Ο β represents g a s i f i c a t i o n of r e l a t i v e l y dense carbon in contact with n i c k e l surfaces. In f a c t , the t r a n s i t i o n from low d e n s i t y f i l a m e n t (δ) to encapsulating s h e l l c a t a l y s t carbon i s gradual (see F i g u r e 3 ) . At 1073 Κ and higher temperatures the range of carbon forma t i o n decreased r a p i d l y (< 50 s) probably because of the formation of a carbon s h e l l around the n i c k e l c r y s t a l l i t e s . G a s i f i c a t i o n o f dense c a t a l y s t carbon s h e l l s or tubes may be s i m i l a r to c a t a l y t i c g a s i f i c a t i o n of a graphite substrate by small nickel particles. D i r e c t c o n t r o l l e d atmosphere e l e c t r o n micros copy (CAEM) of n i c k e l - c a t a l y z e d h y d r o g a s i f i c a t i o n of c r y s t a l l i n e g r a p h i t e (22, 23) has provided many i n t e r e s t i n g observations about the nature of the g a s i f i c a t i o n process; one i s that the n i c k e l c r y s t a l l i n e s s o f t e n and change shape as they s t a r t to g a s i f y g r a p h i t e at temperatures w e l l below the melting point of n i c k e l (~ 100 K) (32). The i n t e r a c t i o n of g r a p h i t e lowers the s u r f a c e energy of the n i c k e l c r y s t a l l i t e s with diameters o f ~ 100 nm, and at 1075 Κ the n i c k e l d i s s o l v e s i n t o the graphite s u b s t r a t e . This may e x p l a i n why long f i l a m e n t s can be g a s i f i e d by metal c r y s t a l l i t e s ; the "wetting" a c t i o n o f the carbon keeps the metal in contact with the filament even as g a s i f i c a t i o n proceeds. With


13.

McCARTY E T A L .

281

Surface Carbon on Nickel Catalysts

metal carbon c o n t a c t assured, g a s i f i c a t i o n proceeds as long as the reactant gas has d i f f u s i o n a l access to the c r y s t a l l i t e . Another important CAEM o b s e r v a t i o n i s that some n i c k e l c r y s t a l s d e a c t i v a t e a t elevated temperature 1175 Κ and become immobile (22, 32). This phenomena was a t t r i b u t e d to d i s s o l u t i o n o f carbon i n t o the n i c k e l c r y s t a l l i t e s and i t s eventual p r e c i p i t a t i o n as g r a p h i t e p l a t e l e t s , s i m i l a r to o b s e r v a t i o n s on w e l l - d e f i n e d n i c k e l (33), platinum, (34) and i r o n (35, 36) s u r f a c e s . Under CAEM pres sure (< 1 - t o r r H ) p l a t e l e t carbon i s not f u l l y g a s i f i e d and r e mains a hollow s h e l l as the n i c k e l c r y s t a l d i s s o l v e s away a t 1225 K. S i m i l a r phenomena may g i v e r i s e to the ε TPSR carbon s t a t e , which a t 1-atm H2 o r 2 0 - t o r r l O i s g a s i f i e d a t higher tempera t u r e . In H 0, n i c k e l c r y s t a l l i t e s observed i n CAEM a r e s t a b l e and do not d i s s o l v e i n t o the bulk a t elevated temperature. However, even i n the presence o f 1 0 , the i n i t i a l step i n c r a c k i n g open a l a y e r of p l a t e l e t carbon o r the carbon s h e l l surrounding an immo b i l i z e d Ni c r y s t a l l i t e may be p e n e t r a t i o n by f l u i d metal. The r a t e s o f n i c k e l - c a t a l y z e d g a s i f i c a t i o n o f g r a p h i t e are i n reasonable accord with our TPSR r e s u l t s f o r the δ carbon s t a t e . For the δ carbon s t a t e , our E was 182 ± 2 0 kJ m o l " i n good agree ment with the r e s u l t s o f Keep e t a l . (22) 220 i 40 k J mol"" . Because o f the d i f f e r e n c e s i n H2 pressure and technique, i t i s d i f f i c u l t to compare a c t u a l r a t e s . Tomita e t a l . (37) observed s u b s t a n t i a l l y lower apparent a c t i v a t i o n energies 130 db 12 kJ mol" f o r nickel-impregnated carbons, but those r a t e s may have been l i m i t e d by d i f f u s i o n (22).


2

2

1

a

1

Literature Cited 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

Baker, R. T. K.; Harris, P. S., The Formation of Filamentous Carbon, i n "Chemistry and Physics of Carbon; "Walker, P., Jr.; Thrower, P. Α., Eds.; Marcel Dekker: New York, 1978, Vol. 14. Trimm, D. L. Catal. Rev. 1977, 16, 155. Ekerdt, J. G.; Bell, A. T. J. Catal. 1977, 48, 155. Ponec, V. Catal. Rev. S c i . Eng. 1978 18, 151. Niemantsverdriet, J. W.; van der Kraan, A. M. J. Catal. 1981, 72, 385. Rostrup-Nielsen, J. R.; Trimm, D. L. J. Catal. 1977, 48, 155. Munster, P.; Grabke, H. J. J. Catal. 1981, 72, 279. Martin, G. A. J. Catal. 1979, 60, 345. McCarty, J. G.; Wise, H. J. Catal. 1979,57,406. Nagakura, S. J. Phys. Soc. Japan 1957, 12, 482; 1958, 13, 1005. Rostrup-Nielson, private communication. Baker, R. T. K. Catal. Rev. 1979,19,161. Hofer, L. J. E.; Sterling, E.; McCartney, J. T. J. Phys. Chem. 1955, 59, 1153. Benshaw, G. D.; Roscoe, C.; Walker P. L. J. Catal., 22, 394.


282

COKE


15.

FORMATION

Baker, R. T. K.; Barber, Μ. Α.; Harris P. S.; Feates, F. S.; Waite, R. J. J. Catal. 1977, 26, 51. 16. Bohm, H. P. Carbon 1973, 11 583. 17. Baird, T.; Fryter, J. B.; Grant, B. Carbon 1974, 12, 591. 18. Audier, M.; Coulon, M.; Oberlin, A. Carbon, 1980,18,73. 19. Audier, M.; Oberlin, Α.; Oberlin, M; Coulon, M.; Bonnetain, L. Carbon 1981, 19, 217. 20. Rostrup-Nielsen, J. R.; Trimm, D. L. J. Catal. 1977,48,155. 21. Rostrup-Nielsen, J. R.; Tottrup, P. B., Steam Reforming of Heavy Feedstocks i n "Symposium on Science of Catalysis and Its Application i n Industry;" FPDIL Sindri, India, 22-24 February 1979, paper 39 p. 397. 22. Keep, C. W.; Terry, S.; Wells, M. J. Catal. 1980, 66, 451. 23. Froment, G. F. Proc. Int. Congr. Catal., 6th, The Chemical Society: Letchworth, England, p. 976. 24. Beeckman, J. W.; Froment, G. F. Chem. Eng. S c i . 1980, 35, 805. 25. Jackson, S. D.; Thomson, S. J. Webb, G. J. Catal. 1981, 70, 249. 26. Isett, L. E.; Blakley, J. M. Surf. S c i . 1976, 58, 397. 27. McKee, D. W. Fuel, 1980, 59, 308. 28. Otto, K.; Bartosiewicz, L.; Shelef, M. Carbon 1979, 17, 351. 29. Figueiredo, J. L. Carbon 1981, 19, 146. 30. Bernardo, C. Α.; Trimm, D. L. Carbon 1979, 17, 115. 31. Wen, C. Y. Dutta, S. i n "Coal Conversion Technology;" Wen, C. Y.; Lee, E. S., Eds.; Addison Wesley, 1979. 32. Baker, R. T. K.; Sherwood, R. D. J. Catal. 1981, 70, 198. 33. Derbyshire, F. J.; Presland, Α.; Trimm, D. L. Carbon 1978, 13, 111. 34. Baker, R. T. K.; Sherwood, R. D., Dumesic, J. A. J. Catal. 1980, 66, 56. 35. Baker, R. T. K.; Feats, F. S.; Harris, P. S. Carbon 1972, 10, 93. 36. Brown, Μ. Α.; Hill, M. P. Carbon 1981, 19, 51. 37. Tomita, Α.; Sato, N.; Tamari, Y. Carbon 1974, 12, 143 38. Chihara, K.; Matsui, I.; Smith, J. M. AIChE Journal 1981, 220. R E C E I V E D August 15,

1982.


Reactivity of Surface Carbon on Nickel Catalysts - American Chemical

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