Reactivity of Carbon Species Formed on Pt/ZrO2 Catalyst in Methane
Conversion
Ruth L. Martins1, Mariana M.V.M. Souza1, Maria Auxiliadora Baldanza1, Martin
Schmal1,2
1
Nucat / COPPE / UFRJ, Rio de Janeiro, Brazil. 2EQ / UFRJ, Rio de Janeiro, Brazil.
1
e-mail address: [email protected]
Introduction
In the last years the direct conversion of methane, the main component of natural gas,
into more valuable chemicals, aroused strong interest in catalysis research. By far, the
most promising route has been oxidative coupling, first in two steps [1], and after, in
only one [2], although the parallel formation of carbon oxides results into poor
selectivity to C2 products. Another approach of direct methane conversion is the twostep reaction sequences, involving transition metal catalyst and hydrogen [3-6].
Methane is believed to be adsorbed and dissociated on transition metal surfaces
forming CHx adsorbed species and H2. This CHx species can be polymerized and in a
second step, in the presence of H2, be hydrogenated into higher hydrocarbons.
Moderate temperatures favor the formation of CHx adspecies, instead of carbon
deposits, and the efficient H2 removal governs the H- deficiency of the CHx species
and the C-C bonding formation, which are precursors of the higher hydrocarbons.
Methane high flow rates [6] and membrane-assisted process [7] are alternatives used
experimentally to ensure the metal surfaces free from H adspecies, both resulting in a
very low conversion. Paréja et al. [8] made use of H acceptor in order to increase
yields. In this paper it was reported a comparative study about the interaction of CH4
with 5% Pt/ZrO2 catalyst by using different techniques. In an attempt to go more
deeply into the mechanism of the methane interaction with the surface catalyst, two
types of experiments were performed: “in situ” Infrared measurements of methane
adsorption and temperature programmed surface reaction of hydrogen (TPSRH) of
chemisorbed methane pulses. Complementary characterization of catalysts by
pyridine and piperidine chemisorptions were also carried out.
Results and Discussion
Figure 1 represents pyridine adsorbed irreversibly at 423K on the surface of 5%
Pt/ZrO2 reduced previously at 773K. Bands at 1443 (vs), 1480 (w) and 1607 cm-1
appeared and was assigned to pyridine coordinated to Lewis acid sites on ZrO2
surface. By heating at 573K the wafer in a closed system, a new band at 1541 cm-1
was created, besides the increase in intensity of the 1480 cm-1 band, and was
attributed to pyridine protonated by Brönsted acid sites. This new band was stable up
to 673K in vacuum of 10-5 Torr. Figure 2 represents piperidine irreversible adsorbed
at 423K on the same catalyst. After heating the wafer at 573K, in a closed system,
piperidine was dehydrogenated to pyridine, which remained at surface attached to
Lewis and the new Brönsted sites just created. Both bands were stable up to 673 in
vacuum of 10-5 Torr.
0,5
5% Pt / ZrO2 reduced at 773K 1443
1,0
0,8
Absorbance, u.a.
Absorbance u.a.
0,4
423K, vacuum
0,3
0,2
1541
623K, close system
0,0
1800
0,4
673K, vacuum
0,0
4000
1700
1600
1500
1400
1300
-1
373K, vacuum
0,6
0,2
0,1
5% Pt / ZrO2 reduced at 773K
623K, closed system
673K, vacuum
3500
3000
2500
2000
1500
1000
-1
cm
cm
Figure 1- Pyridine on 5% Pt/ZrO2
Figure 2- Piperidine on 5% Pt/ZrO2
In figure 3 the Infrared spectra of CH4 as well as CO2 chemisorbed on 5% Pt/ZrO2
pre-reduced at 773K are presented. In the CH4 adsorption, as the wafer was heated in
a closed system, bands at 1613, 1564, 1441 and 1331 cm-1 appeared and were
attributed to bicarbonates and carbonates species formed by decomposition of
methane. The same bands were observed after exposed the wafer just reduced to CO2.
Figure 4 represents the TPSRH experiment of CH4 followed by H2 pulses on 5% Pt/
ZrO2 reduced at 773K. These results were discussed with the Infrared experiments.
0
4000
3000
2000
Figure 3- CH4 and CO2 on 5% Pt/ZrO2
1000
Figure 4- TPSRH of CH4 on 5% Pt/ZrO2
References
1. G.E. Keller, J. Catal., 73 (1982) 9.
2. T.Ito, J.H. Lunsford, Nature, 314 (1985) 721.
3. M. Belgued, P. Paréja. A. Amariglio, H. Amariglio, Nature, 352 (1991) 789.
4. T. Koerts, R.A. van Santen , J. Chem. Soc. Chem. Commun., (1991) 1281.
5. T. Koerts, M.J.A.G. Deelen, R.A. van Santen, J. Catal., 138 91992) 101.
6. M. Belgued, H. Amariglio, P. Paréja, A. Amariglio, J. Saint-Just, Catal. Today, 13 (1992) 437.
7. M. Belgued, A. Amariglio, P. Paréja, H. Amariglio, J. Catal, 159 (1996) 441.
8. O. garnier, J. Shu, B.P.A. Grandjean, Ind. Eng. Chem. Res., 36 (1997) 553.
9. P. Paréja, M. Mercy, J.C. Gachon, A. Amariglio, H. Amariglio, Ind. Eng. Chem Res., 38 (1999)
1163.