Methane Reforming with Carbon Dioxide. The Behavior of Pd/α-Al₂O₃ and Pd−CeO_x/α-Al₂O₃ Catalysts

Abstract

The catalytic reforming of methane with carbon dioxide on Pd (∼1%)/R-Al 2 O 3 and Pd (∼1%)-CeO x /R-Al 2 O 3 catalysts was studied at 627 °C using a stoichiometric feed mixture. Characterization of fresh and used samples was carried out by TEM and TG analyses. Preliminary experiments in the 600-730 °C temperature range demonstrated that, on Pd/R-Al 2 O 3 , the conversion of CO 2 was larger than that of CH 4 because of the reverse water-gas shift (RWGS) reaction, although the CO/H 2 ratio was close to 1. This behavior is attributed to the predominant CH 4 decomposition reaction that leads to carbon deposition and catalyst deactivation. The decline in activity was also due to palladium sintering, a process favored by the reducing atmosphere generated by the presence of CO and H 2 . Kinetic information obtained in experiments carried out under differential conditions was used to evaluate the importance of mass-and heat-transport limitations. It is shown that intraparticle mass transfer and interparticle heat resistances can be controlled by using small catalyst particles and diluting the catalyst bed with an inert solid, respectively. On the other hand, gas-solid temperature gradients are expected if the reforming reaction is carried out at >600 °C at low flow rates. The catalytic activity, selectivity, and stability of Pd/R-Al 2 O 3 and Pd-CeO x /R-Al 2 O 3 were tested in 24-h runs. It was found that a Pd/R-Al 2 O 3 sample with an average particle size of 5 nm exhibits an initial activity comparable to that of Pt/ZrO 2 catalysts and a high and constant selectivity to H 2 (CO/H 2 ≈ 1). However, the catalytic activity decreased by 50% after a reaction time of 24 h due, in part, to metal sintering but mainly to carbon formation. A presintered Pd/R-Al 2 O 3 sample with an average particle size of 19 nm deactivates more rapidly because of a rapid accumulation of carbon deposits. The process of carbon deposition can be suppressed to a very large extent by Ce addition, although the CO/H 2 ratio becomes larger (CO/H 2 ≈ 1.3). In addition, the presence of Ce reduces the sintering process.

Role of MgO over γ-Al2O3-supported Pd catalysts for carbon dioxide reforming of methane

“…Addition of MgO notably influences surface properties of the catalyst, including thermal stability of Pd oxides, metallic surface dispersion and particle size, metal sintering and carbon resistances. These surface properties could essentially account for the catalytic performance of the catalysts [8,[14][15][16][17][18][19][20][21][22]62,63]. The XRD results identified that MgO mainly exist as amorphous type on catalysts with Mg loading no more than 7 wt%.…”

“…One of the effective ways is to introduce a second metal to the catalysts. Addition of Ce to Pd/␣Al 2 O 3 distinctly increased activity and stability of catalyst at 627 • C due to suppressing carbon deposition and decreasing Pd sintering [18]. Alkaline earth and rare earth metals (Ba and La) were also found to improve the catalytic stability of Pd/Al 2 O 3 in carbon dioxide reforming of methane at 750 • C [21], which was ascribed to the decrease of the deposited coke amount.…”

“…Among Group VIII metals, Pd shows the strongest ability to activate the C H bond of CH 4 [14], and the C H bond activation is suggested as the elemental reaction step in the reforming process [3]. Supported Pd catalysts, such as Pd/SiO 2 , Pd/Al 2 O 3 and Pd/TiO 2 , have shown high conversion activities of CH 4 and CO 2 at relatively low temperatures (250-700 • C) [15][16][17][18]. On the other hand, deactivation of Pd catalysts is still a problem, caused by coking deposition on their active metal surfaces and by structural transformations, sintering and recombination of the active components under reaction conditions.…”

“…Carbon deposits usually develop on the surfaces of metal particles through two reactions: (i) methane decomposition to carbon and hydrogen (CH 4 → C + 2H 2 ), and (ii) disproportionation of carbon monoxide to carbon and carbon dioxide (2CO → C + CO 2 ) [3,4]. Many publications have reported that non-promoted Pd catalysts suffer significant deactivation due to metal sintering and severe carbon deposition [18][19][20]. Considerable efforts are dedicated to overcoming these drawbacks.…”

Role of CeO2 in Rh/α-Al2O3 Catalysts for CO2 Reforming of Methane

“…Supported noble metals have promising high catalytic performances and low sensitivities to carbon deposition during the reforming process. In the literature it is found that the supported Pt [3][4][5][6], Rh [7,8] and Pd [9] catalysts are active and stable for this reaction. However, it is important, by the low availability and high cost, to develop active catalysts with very low noble metal content.…”