IFISUR   23398
INSTITUTO DE FISICA DEL SUR
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Evidence of Subsurface Carbon and Enhanced Ce Activation on a Palladium Supported Catalyst for Methane Dry Reforming
Autor/es:
MIGUEL D. SÁNCHEZ; IGNACIO O. COSTILLA; CARLOS E. GIGOLA; BASTIAN MEI; MARTIN MUHLER
Lugar:
Munich
Reunión:
Congreso; 15th Internacional Congress on Catalysis; 2012
Institución organizadora:
Society for Chemical Engineering and Biotechnology
Resumen:
In the last two
decades, the reforming reaction of CH4 with CO2 has
motivated numerous studies in search of active and selective catalysts. The
increase in H2 demand, the existence of natural gas deposits with
high CO2 content and the availability of landfill gas have recently
renewed the interest on methane reforming catalysts. Since the dry reforming is
a highly endothermic reaction, it has to be performed at high temperature to
achieve a good level of conversion. Under this operating condition, rapid
deactivation is a serious problem due to metal sintering and the formation and
accumulation of carbonaceous deposits. It was demonstrated that metal supported
catalysts like Pt/ZrO2, Rh/Al2O3 and Rh/MgO
[1?3] are active and selective for a dry reforming reaction, but the high cost
of these metals limit their potential application. Less expensive metals like
Ni and Pd, can be used for dry reforming with the addition of promoters such as
LaO3, CaO, and CeO2, which prevent or reduce carbon
formation. The use of Pd as an active metal is particularly attractive due to
its remarkable activity for the decomposition of CH4 at high
temperature [4]. We have shown [5] that the addition of 2.5 wt% of Ce to a Pd(1
wt%)/α-Al2O3 low dispersion catalyst practically
eliminates the carbon deposition, with a slight increase in the CO/H2
ratio. Recently, we investigated in detail the Pd-Ce interaction by HRTEM, XPS
and EELS, verifiying that the CeOx forms small crystallites around
the large Pd particles [6, 7].
In the present
work we have carried out the preparation, characterization and testing of a low
loaded (0.11 wt%) highly dispersed Pd catalyst (Pd011) supported on α-Al2O3
that was modified by the addition of 0.6 wt% Ce (Ce06Pd011). Pd and Ce were
deposited by impregnation with Pd(C45H7O2)2
and Ce(NH4)2(NO3)6 solutions
followed by calcination in air and H2 reduction The samples were
characterized by H2 chemisorption, FTIR of adsorbed CO, TEM and
X-ray Photoelectron Spectroscopy (XPS).
Both, FTIR and
chemisorption results indicated a high Pd dispersion (>65%) for Pd011. The
presence of a Ce-Pd interaction upon Ce addition and reduction was evidenced by
a shift of the linear and multiple coordinated IR bands to higher wavenumbers
despite a marked decrease in intensity. Figure 1 shows the comparison of CH4
conversions for the Pd011 and Ce06Pd011 catalysts at 650oC. A
noticeable activation process on both catalysts and a fivefold increase in the
final catalytic activity for the promoted catalyst were observed. The steady
state of CH4 and CO2 conversions were close to
equilibrium. The CO2 conversion was larger due to the presence of
the RWGS reaction and the CO/H2 ratio was 1.3. A sample of the Pd011
catalyst after 7 hours on the stream was analysed by means of XPS (Figure 2).
The result was consistent with the formation of a Pd-carbide phase [8] as it is
evidenced by the components at 283.9 eV in the C 1s region and at 337.4 eV
found in the Pd 3d spectrum. According to the TEM results, a similar increase
in particle size due to thermal sintering was observed on Pd011 and Ce06Pd011.
Figure 1. CH4
conversion as a function of time for Pd011 and Ce06Pd011 catalysts. T = 650oC
; SV = 5.800 h-1 ; CH4/CO2/Ar = 25/25/50
Figure 2. XPS spectra for the catalyst Pd011 after 7 hours on stream.
In summary, we
successfully prepared highly dispersed Pd supported catalysts for CH4
reforming with CO2 which exhibited an activation process that could
be related to the formation of a Pd-carbide phase as shown by the XPS results.
The Ce modified catalyst showed a similar activation process. However, the
Ce-Pd interaction observed on the fresh catalyst seems to play a role in the
reaction mechanism leading to an enhanced catalytic activity.
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