Hydrogenation of carbonaceous adsorbed species on a zirconia aero-gel catalyst in presence of platinum

Spillover and Migration of Surface Species on Catalysts Can Li and Qin Xin, editors 9 1997 Elsevier Science B.V. All rights reserved.

81

H y d r o g e n a t i o n o f c a r b o n a c e o u s a d s o r b e d species on a z i r c o n i a aero-gel catalyst in p r e s e n c e o f p l a t i n u m H. Kalies a, D. Bianchi b and G. M. Pajonk b aon leave from the University of Leipzig, Germany bLACE UMR 5634 CNRS UCB-Lyon 1, 43 Bid du 11 Novembre 1918-69622 VILLEURBANNE, France

The adsorption of hydrogen and carbon dioxide on pure Z r O 2 aerogel and on 0.5 wt % Pt/ZrO2 aerogel are studied by various transient experiments (TPD, TPR, Isothermal hydrogenation of adsorbed species) using either a mass spectrometer or a FTIR spectrometer as detector. No hydrogen chemisorption is observed on ZrO 2 while 0.5 ~mol Hz/g of catalyst are adsorbed on Pt/ZrO2. When CO2 is adsorbed at room temperature on the zirconia aerogel, five carbonate species are detected: bridged carbonate, hydroxyl carbonate, bidentate carbonate, ionic carbonate and carboxylate. The same species, formed on the ZrO2 support, are observed on Pt/ZrO 2 after adsorption of CO2. On pure ZrO 2 the Temperature Programmed Hydrogenation (TPH) of the carbonate species leads neither to hydrocarbon compounds (ex: CH4) nor to new adsorbed species. In isothermal condition, at 473 K, the same rate of disappearance of the carbonate species are observed in H2 and in He. During the TPH of the carbonate species formed on Pt/ZrO2, the formation of C H 4 is detected and new adsorbed species are observed (methoxy group and CO adsorbed on Pt particles). At 473 K the rate of disappearance of the carbonate species in H 2 is higher than the rate of desorption in helium. This shows that the presence of the Pt particles leads to the hydrogenation of the adsorbed carbonate species formed on the support (H 2 spillover).

1. I N T R O D U C T I O N Among new interesting materials for catalytic reactions, zirconia has clearly demonstrated that even used alone (or as a support) it develops a rich and complex surface chemistry(I,2,3,4). When zirconia is sulfated and contains some precious metals it constitutes a very promising superacid catalyst, which exhibits the application of the concepts of H 2 spillover as recently described by Shishido and Hattori (5,6) and Tanaka et al(7). Pt/ZrO2 has been shown to spill over hydrogen at relatively high temperature (8,9), and moreover in this laboratory, one of us activated pure ZrO2 aerogle by H2 spillover in order to hydrogenate ethylene and cyclohexene in particular (10). This paper is intended to give insight in the mechanism of the C O 2 / H 2 conversion on catalysts based upon zirconia (11 and references therein).

82 Considerations on the many possible mechanisms of H 2 spillover can be found in ref. 12 and 13 respectively.

2. E X P E R I M E N T A L

2.1. Catalyst preparation and physical properties Zirconia aerogel was prepared according to the autoclave method as described in ref. 14 starting with Zr(IV) isopropylate. A Pt-zirconia aerogel sample was obtained by impregnating a Za32 aerogel by an aqueous solution of chloroplatinic acid. All samples were calcined at 400~ in O2 to remove all remaining adsorbed species due to their preparations. XRD patterns (Siemens D 500) show that when prepared, ZIO 2 and Pt-ZrO 2 samples were amorphous, while once calcined they only exhibit traces of baddeleyite, a result similar to that registered by Baiker and Gasser for the preparation of Pd supported catalysts from amorphous PdZr alloys (I 5). The Pt content was measured by AES (ARL 3520 ICP), it was of 0.5 % Pt (in weight). The Pt dispersion determined by H2-TPD was found to be 4 % (see below) in agreement with the value quoted by Akubuiro et al. (16) for the same type of Pt-ZrO 2 catalysts preparations. The BET aeras of calcined pure zirconia and Pt-zirconia samples were 105 -and-113 m2/g respectively.

2.2. Transient experiments Temperature programmed techniques such as TPD and TPR were performed in a quartz micro reactor of 1 cm 3 of total volume. The outlet gases were analysed by a mass spectrometer equipped with a capillary inlet device and detection was performed either with a quadrupole or electromagnetic mass spectrometer. A second analysis system consisted in an IR cell, made in stainless steel of 1 cm3 volume and working between 293-900K. FTIR spectra were obtained with a Nicolet 5DXC spectrometer under dynamic conditions (100-2000 ml/mn). Before each experiment all the samples were treated in He at 450~ and cooled down to the reaction temperature in He.

then in 02 for 5 mn

83

3. R E S U L T S

3.1. H 2 adsorption

The samples were introduced in the quartz micro reactor and reduced for lh at 613K by H 2, a purge with He for 10 mn followed, and then the temperature was decreased to 298K and H 2 was adsorbed (1 atm for 10 mn, 90 ml/mn) and the temperature was raised after a switch to He (60 ml/mn), at a rate of 50K/mn. ZaO 2 did not adsorb H 2 at all while on Pt-ZrO 2 two desorption peaks were recorded at 373 and 873K respectively (figure 1 curve a). It was shown that after the initial treatment, if the Pt-ZrO 2 sample is first heated at 873K in He, then cooled down to 613K and contacted by H 2 for lh, again heated at 873K in He and finally cooled down to 298K before being submitted to H 2 adsorption, only one peak was recorded (maximum at a temperature = 373K, figure 1 curve

b). 0.6.E E. 0 . 4 9

-

a

_,a..

9- 0 . 2

~

-

b

,

0 ,

100

200 300 400 Temperature

500 (~

600

Figure 1 : H2-TPD of Pt-ZrO 2 (a) chemisorption at 298K (b) chemisorption at 298K after treatment at 873K The first peak at 373K is attributed to H 2 chemisorbed on the Pt part of the catalyst, hence the above mentioned Pt dispersion of 4 %, while the second one at 873K involved the OH groups of the support (3).

84 The FTIR runs showed that on both types of samples, only two bands remained at 3679 and 3770 cm ~ after the H 2 adsorption experiments. They are known to be belong to terminal and bridged OH groups on zirconia (17) (see below).

3.2.

CO 2

adsorption

After the pretreatment, CO 2 is adsorbed (from a 5 % CO2/He mixture) at 298K. The samples are purged with He and the TPD spectra are recorded (rate range 300K-400K/mn) using the quadrupole mass spectrometer. It was shown that both TPD spectra (ZJO 2 in fig. 2 and Pt-ZrO2 not shown) are identical beetwen 323 and 623K. No CO or other species are recorded (fig.2). The total amount of at least five CO2 desorbed species is 167 ~tmol/g. It seems clear that Pt does not contribute to the formation of any other adsorbed species than these registered on pure zirconia.

400

210~

~

E 300 e5 o

E

200 100

l

I

100 200

i

i

i

300 400 500 Temperature (~

2

600

Figure 2 9 CO2-TPD of CO 2 adsorbed on ZrO 2 at 298K The FTIR spectra recorded after CO 2 adsorption at 298K on both solids were the same, in agreement with the TPD results which showed that Pt does not introduce variations in the adsorption of CO 2 on ZK) 2 (fig. 3). The adsorption of CO 2 leads to the appearance of IR bands at 1225, 1325, 1425, 1445, 1575, 1620 and 1723 cm ~. The intensity of the hydroxyl group on ZrO 2 at 3770 cm" decreases and simultaneously a new IR band appears at 3615 cm -~, (fig. 4) The intensity of the second hydroxyl group at 3677 cm ~ is not affected by CO 2 adsorption,

85

t~

0

0,25 ~1

tt')

(a)

l

,._q 0 ,...Q

(f)

<

Figure 3 : CO2-TPD-FTIR spectra on ZrO 2 (a) TPD at 298K (b) . 338K (C) 393K (d) . 453K (e) . 498K (f) . 538K

I

1900

!

1700

!

1500

I

1300

!

1100

Wavenumber (cm- 1)

(fig. 4). A TPD in helium is then realised at a rate of 15K/mn between 298 and 538K, the IR bands at 1225, 1325, 1620, 1723 et 3615 cm-' decrease in intensity and lead to a more simple spectrum with IR bands at 1556, 1445, 1425 and 1325 cm -t. The assignement of the various bands to adsorbed species has been given in a previous paper (11). The IR bands at 3615, 1620 and 1225 which disappear during the TPD at 498K come from bidentate bicarbonate species. The IR band at 1723 cm -l, recorded at 298K, is attributed to bridged carbonate species. The IR bands remaining during the TPD at temperatures higher than 498K, are assigned to three species : the band at 1445 corresponds to the ionic carbonate, the bands at 1556 and 1325 cm ~ corresponds to the bidentate surface carbonate while the band at 1425 cm -t may be ascribed either to a surface carboxylate species or to a polydentate carbonate.

86

(b)-. :-(a) o ~

E

r.~

(c)-.-

C~ t,-,

(a)->

2%

0

(b)

t~

t~ I

4000

tO t~ I

!

I

3600 3200 Wavenumber (cm- 1)

Figure 4 9FTIR spectra of adsorption and desorption of CO 2 on ZrO 2 (a) blank Co) CO 2 adsorbed at 298K (c) TPD at 453K

3.3. Hydrogenation of adsorbed CO 2 s p e c i e s TPR and FTIR techniques were applied to study the interaction between the adsorbed CO 2 species and hydrogen. On the pure zirconia only CO 2 was recorded during the TPR (300K/mn) while on Pt-ZrO 2 (figure 5) CO 2 (124 ~tmol/g cat), CO (7 lxmol/g cat at 553K) and CH4 (45 ptmol/g cat at 610K) were found as reaction products. The total amount of C1 detected (CO 2, CO and CH,) was very close to that of CO 2 adsorbed (167 versus 176 gmol/g) on the same solid.

87

400 I I I I

E 300 60 i

r

O ..a_

\

~

200

,,

; ]

It <-CH4(,e)

\, ,, h c,o(-)',

r

100

/ ) 2'

ff_.._._..r162-!

I\k\

~

!

!

|

i

|

100 200 300 400 500 600 Temperature (~ Figure 5 9H2-TPR spectra of CO 2 adsorbed on Pt-ZrO 2 The study of the hydrogenation of the adsorbed species by FTIR on the two solids gives more details on the reaction mechanism. As the hydrogenation is only detected at T>473K on the PtJZ10 2 catalyst, the following procedure is used for the FTIR experiment : after the initial treatment, Pt/ZaO 2 is cooled to 473K where the COffHe mixture is introduced, after a purge with He (spectrume fig. 3), hydrogen is then introduced in the IR cell. During the first minute in He the fast disappearance of the IR bands of the bidentate bicarbonate is registered. The intensities of the remaing IR bands decrease slowly in He. Figure 6a gives the decrease of the ratio A/Ao of the intensity of the IR bands in the range (1850-1200 cm~) during the helium contact and after the hydrogen introduction. It can be noted that the rate of disappearance of the carbonate species increases under hydrogen. This means that on Pt/ZrO2, the rate of hydrogenation of the caa'bonate species adsorbed on ZrO 2, is higher than the rate of desorption. Figure 6b gives the results obtained on ZrO~ according to the same previous experimental procedure. During the desorption step in He, the rate of disappearance of the carbonate is identical to that recorded with the Pt/ZrO2 solid. The subsequent introduction of H 2 does not change the rate of disappearance of the carbonate species.

88

_~ He'>H2 o

<-,<-< 0.6

~ ~

-

;< k 'i

0 -.~

-

rr

0.2

\

-

X

"X-.

-me,.. - .,~ . . . .

I

0

1000

~...

......... /

I

I"K

2000 Time" (s)

3000

Figure 6a 9 Interaction of H 2 with carbonate species on Pt-ZrO 2 --

I

I

I

I

I

i_ b He_>H2 , 'V

%

0

<

0.6

_

~

-

%

0 t~

_

e.~

_

rr "o

G. G

0.2

_

_ "~'"~"

U

!

0

I

1000

I

"~"

"" 'o..~.

t

2000 Time" (s)

....

~-~... ~2

I

9

3000

Figure 6b 9 Interaction of H 2 with carbonate species adsorbed on ZrO 2 However another difference between the two solids is the formation of new IR bands on R/ZrO 2 solid during the hydrogenation process. An IR band at 2049 and a shoulder at 1990 cm' are formed immediately after the H 2 introduction (figure 7a). These bands are due to the adsorption of CO species on the Pt particles of the catalyst (linear and bridged species). Indeed two similar bands were found by Schild et al. during CO 2 hydrogenation over Ni/zirconia catalysts (18). Three other weak bands are detected at 2934 cm' and 2831 cm' (figure 7b) and 1144 cm' (not shown) during the hydrogenation. Those three IR bands are attributed to methoxy groups formed on the ZaO 2 support (19). The change of the adsorbance of those IR bands with time on stream was recorded and showed that the evolution of those species follows the same prof'fle with a maximum with time on stream (fig. 8).

89

0.012

0.012

,~

(e) tD

(D

G

O

r,r

d2~

<

<

I

2100

I

!

2000 1900 1800 W a v e n u m b e r ( c m - 1)

3000

Figure 7a : FFIR spectra of CO adsorbed on Pt- ZrO2 (a) CO2/He treatment at 473K (b) He treatment (40s) at 473K (c) H2 at 473 (40s) (d) He at 473K (180s) (e) H2 at 473K (300s)

2900 2800 2700 W a v e n u m b e r ( c m - 1)

Figure 7b " FTIR spectra of the methoxy group (continued)

8

~ 6

.

~4

"<-a

- - 2 2 ~"~~ \ *.

0 2000

4000 Time-(s)

6000

Figure 8 9 IR band intensity of the carbonaceous adsorbed species with time on stream, under H 2, on Pt-ZrO 2 at 473K (a) carbonates ( 1800-1200 cm ~) (b) CO (2100-1900 cm ~ x 25) (c) CH 30 (3000 - 2800 cm -~ x 25)

90

4. D I S C U S S I O N Six main observations can be made from this investigation. a) Pt is very poorly dispersed at the surface of zirconia, it contributes to less than 0 . 1 % of the total area, b) the rate of carbonates disappearence in H2, is faster with Pt-ZrO 2 than with ZrO2, c) the methoxy group can be considered as the last adsorbed species in the process going from the carbonate adsorbed species to methane (TPR results), d) other intermediates such as formate groups are not detected which means that the rate of formation of the methoxy group is fast compared to that of its hydrogenation into CH4, e) the methoxy group is an intermediate compound formed during hydrogenation of CO 2 species which transforms into CH4 at comparable rates, f) the coincidence of the maximum coverages of CO (on Pt) and CH30 indicate that both species are formed by parallel reactions. To explain the above results it is proposed that H 2 is chemisorbed on Pt particles in a dissociative way, monohydrogen species then spill over to Z.a32 and interact with carbonate species. The nature of the spilling species (electrically charged or not) is yet not identified (20). Since CO is produced, the site transfer process involving the migration of CH~O species from Pt to the support cannot be ruled out (21, 12, 13) without further investigation. It also cannot be ruled out that H 2 spillover may displace CO 2 from ZrO z to Pt (on PtZaO2) where a reverse water gas shift reaction may develop the formation of CO and its further methanation.

5. C O N C L U S I O N On Pt-ZrO 2, CO 2 is chemisorbed on the support while H z is dissociatively adsorbed on Pt only and spills over to ZtO 2 where the H 2 spiltover species react with adsorbed carbonate species, finally releasing CH 4+CO as reaction products. The process can be summarized by the following scheme: (1) R + A/B (2) R' + A/B

:.

RB+A ;- R'A + R'B

hydrogen spillover

91

(3) RB + R'B WhereR'CO,,

,~ CH4,CO + B B-ZaO 2

A-Pt

R"H 2

REFERENCES

.

2. .

.

5. 6. 7. .

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