Evidence of Subsurface Carbon and Enhanced Ce Activation on a Palladium Supported Catalyst for Methane Dry Reforming

MIGUEL D. SÁNCHEZ; IGNACIO O. COSTILLA; CARLOS E. GIGOLA; BASTIAN MEI; MARTIN MUHLER

Munich

Congreso; 15th Internacional Congress on Catalysis; 2012

Society for Chemical Engineering and Biotechnology

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. References [1] J.H. Bitter, K. Seshan, J.A. Lercher, J. Catal. 171 (1997) 279. [2] J. Erdöhelyi, F. Cserényi, Solymosi, J. Catal. 141 (1993) 287. [3] H.Y. Wang, E. Ruckenstein, Appl. Catal. A 204 (2000) 143. [4] A. Yamaguchi, E. Iglesia, J. Catal. 274, 1 (2010) 52. [5] P.G. Schulz, M.G. Gonzalez, C.E. Quincoces, C.E. Gigola. Ind. Eng. Chem. Res. 44 (2005) 9020. [6] C.E. Gigola, M.S. Moreno, I. Costilla, M.D. Sánchez, Appl. Surf. Sci. 254 (2007) 325. [7] M.S. Moreno, F. Wang, M. Malac, T. Kasama, C.E. Gigola, I. Costilla and M.D. Sánchez. J. Appl. Phy. 105 (2009) 083531. [8] M.S. Ahn, S.G. Jeon, H. Lee, K.Y. Park and Y.G. Shul. Appl. Cat. A: Gen. 193 (2000) 87.