Heats of Adsorption on Supported Pd - ACS Publications - American

8 CO, O , and H2 Heats of Adsorption on Supported Pd 2

Downloaded by NANYANG TECHNOLOGICAL UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: February 10, 1986 | doi: 10.1021/bk-1986-0298.ch008

M. Albert Vannice and Pen Chou Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802

Palladium dispersed on SiO , η-Al2Ο3, SiO -Al2O3, and TiO , was characterized by chemisorption measurements to determine H2, O2, and CO uptakes. Integral, isothermal heats of adsorption were then obtained using a modified differential scanning calorimeter. Values of ∆H(ad) were typically near 55 ± 10 kcal/ mole O2 and 25 ± 5 kcal/mole CO. Values for H2 appeared to be around 20 kcal/mole H after a correction was made for a baseline perturbation caused by the difference in thermal conductivities between H2 and Ar. The∆H(ad)values on Pd/TiO2 reduced at 773K were not markedly different from those on the "typical" Pd catalysts, and no significant support effect on heats of adsorption was found for Pd. For all three gases, heats of adsorption increased as Pd crystallite size decreased from 6 nm to below 2 nm. Pd hydride formation was routinely determined by chemisorption and calorimetric measurements and heats of formation of the β-phase hydride were consistently near 10 kcal/mole H2 absorbed, independent of support and crystallite size, which is in good agreement with literature values for bulk palladium. 2

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Since the r e p o r t by Tauster et a l . that high temperature reduction (HTR) near 773K markedly decreased the H and CO c h e m i s o r p t i o n c a p a c i t y of Group VIII metals dispersed on T i 0 ( 1 ) , many studies have been devoted to this phenomenon. One popular e x p l a n a t i o n i s t h a t bond s t r e n g t h s between t h e adsorbate and the metal are decreased as a consequence of an e l e c t r o n i c e f f e c t caused by some form of e l e c t r o n transfer between the metal and the support (2-4). Another, which does not require a decrease i n heats of a d s o r p t i o n , i s that chemisorption i s decreased because of physical blockage of metal surface s i t e s created by the migration of a T i 0 species onto the metal surface (5^7). In t h i s study we wanted to conduct direct calorimetric measurements of CO and H heats of a d s o r p t i o n on Pd d i s p e r s e d on T i 0 along with those obtained for " t y p i c a l " Pd cata2

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0097-6156/ 86/0298-0076$06.00/0 © 1986 American Chemical Society

In Strong Metal-Support Interactions; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

8.

VANNICE AND CHOU

77

Heats of Adsorption on Supported Pd

l y s t s u t i l i z i n g S i 0 , AI2O3 and S i 0 - A l 0 3 as supports. A compari son of these results should help determine whether a decreased heat of adsorption of CO and H i s the primary cause for lower chemisorp tion uptakes. It was also of interest to measure these i s o t h e r m a l , i n t e g r a l heats of adsorption because these values describe surfaces which are equilibrated at high gas pressures and, therefore, r e l a t e to s u r f a c e s which can e x i s t under r e a c t i o n c o n d i t i o n s . Another reason for t h i s investigation was the l a r g e v a r i a t i o n i n methana t i o n turnover frequency which has been found for t h i s family of Pd catalysts (8), and the correlation which has been observed between heats of adsorption and a c t i v i t y ( 3 ) . It was therefore of interest to determine whether any r e l a t i o n s h i p e x i s t e d between these two parameters for this family of Pd c a t a l y s t s . 2

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Experimental C a t a l y s t Preparation. A number of the catalysts used i n t h i s study were prepared from P d C l ( V e n t r o n C o r p . ) and η - Α 1 0 3 (Exxon Research & E n g i n e e r i n g C o . ) , S i 0 - A l 0 3 (Davison Grade 979, 13% alumina), T i 0 (P-25 from Degussa C o . , 80J anatase and 20? r u t i l e ) , and S i 0 (Davidson Grade 57) using an impregnation technique (8). The reported surface areas f o r each support are 245, 400, 50 and 220 m^ g~ , r e s p e c t i v e l y . To achieve maximum dispersion, an ion exchange technique with Pd(NH3)jj(N03) 2 H2O was a l s o used f o r s e v e r a l samples ( H ) ) . The f i n a l Pd weight loadings of the cata l y s t s were determined by both neutron a c t i v a t i o n a n a l y s i s and atomic emission spectroscopy. 2

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Chemisorption Measurements Apparatus. The uptake measurements were made i n a h i g h vacuum system consisting of a glass manifold connected to an Edwards Model E02 o i l - d i f f u s i o n pump backed by a mechanical pump with l i q u i d n i t r o g e n t r a p s at the i n l e t of each pump. An u l t i m a t e dynamic vacuum near 4 χ 10~? t o r r (1 t o r r = 133 Pa) was o b t a i n a b l e , as measured by a G r a n v i l l e P h i l l i p s Model 260-002 i o n i z a t i o n gauge. Isotherm pressures and temperatures were measured by a Texas Instru ments Model 145 Precision Gauge and a D o r i c d i g i t a l t r e n d i c a t o r , respectively. A more d e t a i l e d d e s c r i p t i o n of the gases, t h e i r p u r i f i c a t i o n , and the adsorption system i s given elsewhere (11). P r o c e d u r e s and P r e t r e a t m e n t s . The " t y p i c a l " Pd c a t a l y s t s ( P d / n - A l 0 3 , P d / S i 0 and P d / S i 0 - A l 0 3 ) were given a pretreatment consisting of a 1 h reduction i n flowing H or a mixture of 20% H + 80* He at 448K, 573K or 673K. Previous work had indicated that 448K was s u f f i c i e n t f o r r e d u c t i o n O ) , but we found that b e t t e r r e s u l t s were sometimes obtained at the higher temperatures (12). The P d / T i 0 catalysts were given either a low temperature r e d u c t i o n (LTR) at 448K or a h i g h temperature reduction (HTR) at 773K i n a flowing gas mixture of 2 0 Ï H and 8 0 Ï He following the procedure of Tauster et a l . {Y). The P d / T i 0 (LTR) sample was treated i n 20% 0 at 573K for 1 h after each CO heat of adsorption measurement, p r i o r to another (LTR) s t e p , to s t a b i l i z e the sample and to f a c i l i t a t e complete removal of the CO. After these pretreatments, a d s o r p t i o n was r a p i d as expected for nonactivated adsorption, and the pressure 2

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STRONG METAL-SUPPORT INTERACTIONS

t y p i c a l l y s t a b i l i z e d within 15 min for the i n i t i a l point and within 2-3 min for the succeeding uptakes at higher pressure.


Calorimetric Measurements Isothermal energy changes were measured using a modified PerkinElmer DSC-2C d i f f e r e n t i a l scanning calorimeter with an I n t r a c o o l e r II (Model 319-0207) which allows subambient runs down to 200K. The gas handling system preceding the calorimeter c o n t r o l l e d the flows of argon, helium, hydrogen and carbon monoxide and provided switching c a p a b i l i t y between gas streams, as shown i n F i g u r e 1. Ultra high p u r i t y argon (99.999% from MG S c i e n t i f i c Gases) was further p u r i f i e d by passing i t f i r s t through a d r y i n g tube c o n t a i n i n g 5A molecule sieve (Supelco I n c . ) , and then through an Oxytrap (Alltech A s s o c i a t e s ) before use as a purge gas i n the c a l o r i m e t e r . The helium ( 9 9 . 9 9 9 9 Î from MG S c i e n t i f i c ) was p u r i f i e d i n a s i m i l a r fashion. An Elhygen Mark V Hydrogen Generator produced u l t r a - p u r e hydrogen ( l e s s than 10 ppb t o t a l i m p u r i t i e s ) by e l e c t r o l y t i c a l l y d i s s o c i a t i n g d e i o n i z e d water and then d i f f u s i n g t h e h y d r o g e n through a t h i n p a l l a d i u m membrane. The carbon monoxide (99.99* from Matheson) was passed through a molecular s i e v e t r a p h e l d at 383K to remove any metal c a r b o n y l s . The gases were regulated by Tylan mass flow c o n t r o l l e r s (Model FC260), and a d i g i t a l T y l a n R020A readout box provided control and monitoring for the four mass flow c o n t r o l l e r s . Two of these controllers were capable of measuri n g between 0 and 50 cm min"" argon f l o w , the t h i r d one was designed f o r e i t h e r hydrogen or carbon monoxide f l o w c o n t r o l between 0 and 10 cm3 min"* , depending upon which valve was opened, and the fourth regulated He flows up to 10 cm3 m i n " . To enhance s e n s i t i v i t y and accuracy by m i n i m i z i n g b a s e l i n e p e r t u r b a t i o n s a f t e r switching from the purge gas to the gas stream containing the adsorbate, various e v o l u t i o n a r y m o d i f i c a t i o n s were made to the gas handling system and the calorimeter i t s e l f , and the f i n a l flow design i s shown i n F i g u r e 1. The changes to the DSC included: 1) p e r f o r a t i o n of the platinum sample holder covers to enhance gas mixing; 2) removal of the flow s p l i t t e r i n s i d e the DSC and i n s e r t i o n of a needle v a l v e i n each l i n e (to the sample side and to the r e f e r e n c e s i d e ) ; 3) r e g u l a t i o n of flows by mass flow c o n t r o l l e r s ; 4) u t i l i z a t i o n of adjustable He/Ar r a t i o s i n the purge stream; and 5) enclosure of the e n t i r e aluminum b l o c k , the c o v e r , and the d r a f t s h i e l d under a blanket of flowing N (99% from Linde) to eliminate the p o s s i b i l i t y of oxygen ( a i r ) d i f f u s i n g through any tiny leaks to the sample. To provide optimum performance, the f o l l o w i n g procedure was used. Needle valve 5 c o n t r o l l i n g gas flow to the atmosphere was adjusted so that equal pressure drops were attained through both l o o p s of the s w i t c h i n g v a l v e . This eliminated a p e r t u r b a t i o n d u r i n g switching due to a pressure d i f f e r e n t i a l . Adjustment of the two needle valves i n l i n e s 9 and 10 balanced the flows through the sample and reference c a v i t i e s at a constant o v e r a l l flow r a t e . The purge gas to the DSC was comprised of a constant Ar flow of - 36 cm3/min which bypassed the switching valve plus an additional component which passed through the switching valve. For the experiments with H , s w i t c h i n g occurred from 8 cm3 min"" He to 4 cnw min"* H 3

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VANNICE AND CHOU

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back t o t h e He. These He flow r a t e s (STP) minimized b a s e l i n e p e r t u r b a t i o n and o f f s e t due to d i f f e r e n c e s between the thermal c o n d u c t i v i t y of the mixture and pure A r , a problem which was more severe with the H m i x t u r e s . A f t e r these m o d i f i c a t i o n s , v e r y r e p r o d u c i b l e r e s u l t s w i t h m i n i m a l b a s e l i n e c o r r e c t i o n were obtained, as i n d i c a t e d i n F i g u r e 2 by the b a s e l i n e t r a c e s a f t e r purging both the sample and the reference sides and readsorbing the gas. The s i g n a l output from the DSC and i t s time i n t e g r a l were monitored on a 2-pen integrating recorder (Linear Instruments Model 252A). The energy c a l i b r a t i o n was conducted using various weights of indium and d i f f e r e n t ordinate (mcal/s) ranges, chart speeds, and h e a t i n g r a t e s to determine a c a l i b r a t i o n constant of Κ • 0.0442 ± .0002 s/count f o r the recorder i n t e g r a t o r . As a check, another Κ value was determined based on the weight of the chart paper under the curve. Both provided excellent c o r r e l a t i o n s , so the former was used i n most c a s e s . E i g h t ordinate s e n s i t i v i t i e s between 0.1 and 20 meals"" could be used, but with the sample sizes used here (0.03 - 0.15 g) and the t y p i c a l gas uptakes that o c c u r r e d , most runs could be e a s i l y recorded on the 5 or 10 meal s"" f u l l - s c a l e range. A f t e r admittance of H , 0 or CO, the b a s e l i n e was corrected by subtracting by switching back to the purge gas f o r 1 h , r e i n t r o d u c i n g the adsorbate m i x t u r e , and obtaining the baseline trace after weakly absorbed gas had been removed. For the runs with hydrogen, t h i s i n v o l v e d the determination of the energy associated with β-Pd hydride formation, as shown i n Figure 2(a). The samples placed i n the DSC were taken from a l a r g e r sample of that p a r t i c u l a r c a t a l y s t , which had been previously pretreated and c h a r a c t e r i z e d i n the c h e m i s o r p t i o n system then s t o r e d i n a desiccator. Each sample was then given another pretreatment i n the DSC i n pure H or a 2 0 Î H /80> Ar mixture, flowing at 40 cm3 m i n " and was heated at 40K min"" to the desired temperature. Because of the g r e a t e r thermal c o n d u c t i v i t y of H , s i g n i f i c a n t d e v i a t i o n s occurred between the actual cavity temperature and the temperature indicated on the DSC when pure H was u s e d . For example, with a flow of pure H , a maximum temperature of 713K was achieved rather than the indicated 773K. A c a l i b r a t i o n between ΔΤ and the H mole f r a c t i o n allowed the actual temperature to be obtained. F i n a l l y , to check f o r p o s s i b l e contamination e f f e c t s , t h e purge time at high temperature i n the DSC system was varied from 20 min to 660 min as a N blanket was kept around the c a v i t y . Small apparent i n c r e a s e s i n AH( j) f o r H were observed as purge time increased up to 60 m i n , which were a t t r i b u t e d to an i n c r e a s e i n d e s o r p t i o n from the s u r f a c e , but a d d i t i o n a l purging to 180 min produced no change, within experimental error, and a 660 min purge r e s u l t e d i n a 5% i n c r e a s e i n the energy change. Consequently, 60 min was chosen as an optimum time f o r complete removal of hydrogen from the surface after reduction.


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Results A t y p i c a l set of DSC traces i s i l l u s t r a t e d i n Figure 2. The d i f f e r ence i n areas under the t r a c e obtained d u r i n g a d s o r p t i o n and the b a s e l i n e t r a c e r e p r e s e n t s the energy change associated with the


1

In Strong Metal-Support Interactions; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986. 2

Figure 1. D i f f e r e n t i a l Scanning Calorimetry System: 1—Hoke valves, 2—Mass flow c o n t r o l l e r s , 3~Pressure gauge, 4—Four-way switching valve, 5—Needle valves, 6—Supelco Drying tubes, 7—Oxytraps, 8—Molecular sieve trap, 9—Line to sample cavity, 10—Line to reference c a v i t y , 11—N purge streams through p l a s t i c shrouds, 12—Draft s h i e l d , 13—Aluminum block.


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T3 *T3

C/3 C

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70 Ο Ζ Ο

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VANNICE AND CHOU



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6 TIME

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Figure 2. Energy Change During Adsorption at 300K on 1.95% P d / S i 0 - A l 0 3 (T = 673K) 2

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chemisorption process as the clean Pd surface was equilibrated with the adsorbate at 75 torr (0.1 atm). In a l l cases the a d s o r p t i o n process was e s s e n t i a l l y complete i n less than 2 minutes, i n agree ment with our chemisorption experiments which showed that chemisorp tion on the Pd surface was very r a p i d . A s e t of a d s o r p t i o n isotherms i s shown i n F i g u r e 3 . The d i f f e r e n c e i n uptakes at 75 t o r r was used to determine the amount of gas i r r e v e r s i b l y adsorbed on the Pd s u r f a c e ; however, b o t h i s o t h e r m s are q u i t e p a r a l l e l over a wide pressure range. The isotherm i n Figure 3a c l e a r l y shows the onset of fl-phase Pd h y d r i d e f o r m a t i o n at pressures above 10 torr but the amount of chemisorbed hydrogen on either the β-phase or α-phase hydride remains c o n s t a n t . The uptake from the lower isotherm, corrected for physical adsorp tion on the support, represents the extent of bulk hydride forma tion. A set of i s o t h e r m a l , i n t e g r a l AH( j) values i n kcal/mole i s l i s t e d i n Table I f o r a series of Pd c a t a l y s t s with an average Pd c r y s t a l l i t e s i z e near 3 ± 1 nm, as measured from H chemisorption. More complete representations of measured AH( j) values f o r CO, 0 , and H are shown i n Figures 4-6, r e s p e c t i v e l y . ac

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Table I .

Integral Heats of Adsorption on Supported Palladium (300K) DiaTRed meter (°K) (nm)

Catalyst 2.1? Pd/Si0 0.48? Pd/Si0 2

2

1.95? P d / S i 0 - A l 0 1.95? P d / S i 0 - A l 0 2

2

3

2

2

3

1.80? P d / n - A l 0 3 2

2.03? Pd/Ti0 2.03? Pd/Ti0

2

(0

2

2.03? Pd/Ti0

2

2

0573K)

H

(ad)

(a) H,

o

2

CO

573 573

1.7 2.4

32 35

70 60

29 30

448 673

3.2 4.0

32 33

55 50

22 22

673

3.4

31

61

20

448 448

3.7 3.6

33 32

71 62

24

773

(3.7)

26

—

17

(a) Apparent values prior to any correction

F i n a l l y , a series of values for the apparent enthalpy of forma t i o n of β-phase Pd hydride, along with the bulk Pd h y d r i d e r a t i o s , i s l i s t e d i n Table I I . The bulk ratios were obtained by dividing the hydrogen uptake at 75 t o r r from the second isotherm by the number of bulk Pd atoms, i . e . , the t o t a l number of Pd atoms minus the surface Pd atoms, as determined by chemisorbed hydrogen.



VANNICE AND CHOU


0

_L

50

_L

100

150

200

250

PRESSURE (Torr)

Figure 3. Gas Uptake (umole/g c a t . ) at 300K on 1.95% P d / S i 0 - A l 0 (T = 673K). 2

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I

I

I

I

SUPPORT 40

Si0 S1O2-AI2O3 2

—

a-Ai o Ti0

30

20

I0

•

••

• Τ

•

•

I

2

•-

• • I

4 Pd

Figure 4. (300K).

3

•

—

0

2

2

I

TR ( ° K ) 448 573 or 673 V • Δ Ο

I

6

I 8

I

ΙΟ

12

PARTICLE SIZE (nm)

Heat o f a d s o r p t i o n o f CO on s u p p o r t e d

palladium



1— SUPPORT

Tr

Si0 V Si0 -Al 0 Δ α-Αΐ ο Ο Ti0 • 2

2

2

2

( ° K )

573 or 673 Τ A

4 4 8

3

3


2

•

_L 4

Pd

F i g u r e 5. palladium

_L 6 PARTICLE

12

SIZE (nm)

Tr

Si0 V SiO^AfeOs Δ a-Ai o Ο Ti0 • 2

2

( ° K )

573 or 673 • •

4 4 8

·

3

2

35 ^

A

Heat o f a d s o r p t i o n o f oxygen on s u p p o r t e d (300K).

SUPPORT

Ô Ε

10

8

•

• β

4

*Ί 30|β χ"

Heats of Adsorption on Supported Pd - ACS Publications - American

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