Enhanced Stability of CaO and/or La2O3 Promoted Pd/Al2O3 Egg-Shell Catalysts in Partial Oxidation of Methane to Syngas

An egg-shell Pd/Al2O3 catalyst showed higher activity than a regular Pd/Al2O3 catalyst in the partial oxidation of methane to syngas, but a common problem of this reaction is the catalyst deactivation on stream. We attempted to modify the egg-shell catalyst via impregnation with some metal oxide additives. Although the addition of MgO did not show any beneficial effect, the addition of CaO and/or La2O3 significantly improved the stability due to the suppression of carbon deposition and phase transformation of the Al2O3 support. The catalysts were characterized by X-ray diffraction (XRD), N2 adsorption-desorption, and thermogravimetric analysis (TGA).


Introduction
Synthesis gas, a valuable feedstock for the synthesis of ammonia, methanol, dimethyl ether, acetic acid, etc., is mainly produced by steam reforming of methane [1][2][3]. In recent years, increasing attention has been paid to the catalytic partial oxidation of methane (POM) for syngas production because of less energy consumption, lower investment, and more desirable H 2 /CO ratio for some downstream processes [4,5].

OPEN ACCESS
The catalysts investigated in the POM processes are mainly supported Ni catalysts and noble metal (e.g., Rh, Pt, Ru, Pd, or Ir) catalysts [6,7]. Ni-based catalysts are cheap, but they deactivate easily due to carbon deposition and sintering above 700 °C [8]. Noble metal-based catalysts have much higher activity, and are less susceptible to catalyst deactivation [9,10].
Among noble metal catalysts, Pd catalysts supported on Al 2 O 3 , SiO 2 , MgO, CaO, and rare earth metal oxides have been used in POM [11,12]. The catalytic activity and stability can be improved by adding some promoters. For instance, Ryu et al. investigated the addition of CeO 2 , BaO, and SrO on the catalytic performance of Pd/Al 2 O 3 for POM, and found that the optimum catalyst was a triply promoted Pd catalyst, namely Pd(2)/CeO 2 (23)/BaO(11)/SrO(0.8)/Al 2 O 3 [13].
Because POM is performed under very high space velocity, the inner diffusion and reaction inside the pores of the catalysts are limited [14]. Therefore, it is desirable to disperse the active species on the outer surface of the catalyst as far as possible. Egg-shell catalysts can satisfy such a need. Besides, the cost of catalysts can be significantly lowered when adopting the egg shell design. In recent years, Rh [15,16] and Ni [14,17,18] egg-shell catalysts have been investigated in POM. For instance, Chen and co-workers compared the performance of egg-shell and uniform Ni/Al 2 O 3 catalysts in POM, and found that egg-shell Ni/Al 2 O 3 was more active due to the distribution of nickel component in the outer region of particles [17].
Herein, a Pd/Al 2 O 3 egg-shell catalyst and its modified versions were tested in POM. The Pd/Al 2 O 3 egg-shell catalyst showed higher activity than a conventional Pd/Al 2 O 3 , and the addition of CaO and/or La 2 O 3 additives can significantly improve the stability on stream due to suppressed carbon deposition and phase transformation of the Al 2 O 3 support. Figure 1 shows the photos of the prepared egg-shell catalysts before reduction, highlighting the cross section. The outer surface was dark brown and the inner part of the particles was white, suggesting the accumulation of Pd species on the catalyst surface. The average thickness of the Pd-rich eggshell was approximately 0.2 mm. The catalysts after reduction also showed the egg-shell structure, as seen from the grey-pale black color of the shell while white color at the center. Table 1 summarizes the specific surface areas of support (calcined at 1,000 °C) and egg-shell catalyst samples reduced at 750 °C. The surface area of Pd/Al 2 O 3 egg-shell catalyst was lower than its corresponding Al 2 O 3 support, probably due to the blockage of the pores by loading Pd as well as some degradation of the support as a result of the catalyst preparation procedure [19]. The addition of MgO, CaO, La 2 O 3 , or CaO and La 2 O 3 into Pd/Al 2 O 3 could retain a higher surface area.  Figure 2 shows the performance of Pd/Al 2 O 3 egg-shell catalyst in POM reaction as a function of reaction temperature. The CH 4 conversion increased with the reaction temperature and reached 72% at 750 °C. For comparison, the CH 4 conversion on a regular Pd/Al 2 O 3 catalyst with the same Pd loading (0.2%) was 49% at 750 °C, demonstrating the advantage of the egg shell design. The CH 4 conversion on Pd/Al 2 O 3 egg-shell catalyst decreased when the reaction temperature was above 750 °C, due to the accelerated carbon deposition at high temperatures. Figure 3 shows the CH 4 conversion on various egg-shell catalysts at 750 °C as a function of time on stream. The initial CH 4 conversions on these catalysts were comparable, ranging from 67% to 73%. However, the deactivation behaviors were quite different. Pd/Al 2 O 3 egg-shell catalyst deactivated quickly on stream, as seen by the dropping of CH 4 conversion from 72% to 40% after operation for 10 h. The addition of MgO barely changed the quick deactivation, whereas the addition of CaO and/or La 2 O 3 significantly suppressed the deactivation. In particular, Pd/La 2 O 3 /CaO/Al 2 O 3 egg-shell catalyst showed the best stability: the CH 4 conversion changed from 70% to 62% after operation for 10 h.   Figure 4 shows the H 2 selectivity and CO selectivity on Pd/La 2 O 3 /CaO/Al 2 O 3 egg-shell catalyst as a function of time on stream. The H 2 selectivity and CO selectivity were about 85% and 91%, respectively.

Results and Discussion
One reason for catalyst deactivation in the POM process is the deposition of carbon [20]. Indeed, the outer surface of the spent catalysts turned into black, indicating the deposition of carbon during the course of reaction. Therefore, TGA was employed to characterize the spent catalysts. As shown in Figure 5, all the samples showed an obvious weight loss from ca. 600 °C to 750 °C, attributing to the oxidation of deposited carbon, most likely graphitic carbon [21]. Interestingly, the amounts of deposited coke were different with different spent egg-shell catalysts: those on Pd/CaO/Al 2   Another reason for catalyst deactivation in POM process is the phase transformation of Al 2 O 3 [22]. Figure 6 shows the XRD patterns of the support (calcined at 1,000 °C) and egg-shell catalysts reduced at 750 °C. The characteristic diffraction peaks of Al 2 O 3 support calcined at 1,000 °C ( Figure 6a

Catalyst Preparation
Preparation of PdO/Al 2 O 3 , PdO/MgO/Al 2 O 3 , and PdO/CaO/Al 2 O 3 egg-shell samples: γ-Al 2 O 3 Raschig rings purchased from Yixing YiPu Catalyst Company (Yixing, China) were pre-calcined at 1,000 °C for 2 h and then crushed to 1.0-1.3 mm particles. Al 2 O 3 particles (5 g) were impregnated with an aqueous solution of Pd precursor by the previously described preparation method [19]. The obtained materials were calcined at 500 °C for 2 h to obtain PdO/Al 2

Catalyst Characterization
The powder X-ray diffraction (XRD) patterns were recorded on a Bruker Advance diffractometer (Bruker, Karlsruhe, Germany) with a Cu Kα monochromatic radiation operated at 40 kV and 40 mA, using a scanning range 2θ between 10° and 90°.
The BET surface areas of the support materials and catalysts were measured by N 2 adsorption at −196 °C using a Micromeritics ASAP 2020 instrument (Micromeritics, Norcross, GA, USA). The samples were outgassed under N 2 flow at 300 °C for 2 h before the measurements.
The amount of carbon deposited on the catalysts after reaction was determined on a Pyris Diamond TG instrument (PerkinElmer, Waltham, MA, USA) by heating the samples from room temperature to 1,000 °C (heating rate 10 °C/min) in a flowing air (25 mL/min).

Catalytic Test
The POM reaction was carried out in a continuous flow reactor apparatus equipped with a quartz tube reactor operated at atmospheric pressure. The flow rate of gases was controlled by mass flow controllers. The catalysts (0.5 g) were firstly reduced in a 60 mL/min H 2 flow at 750 °C for 30 min, then a 60 mL/min Ar flow was introduced to flush the system. Afterwards, a mixture of CH 4 (99.9%, 277.8 mL/min) and O 2 (99.999%, 138.9 mL/min) with a molar ratio of 2:1 at weight hour space velocity (WHSV) of 5 × 10 4 mL·g −1 ·h −1 was introduced. The product gases were analyzed online by a gas chromatograph equipped with a TCD detector and TDX-01 packed column. Water was trapped before gases entered the gas chromatograph.

Conclusions
A series of Pd/Al 2 O 3 -based egg-shell catalysts were tested in the partial oxidation of methane. The addition of MgO, CaO, or La 2 O 3 additives had no dramatic effect on the initial CH 4 conversion. The addition of MgO additive had no effect on the catalyst stability, whereas the addition of CaO and/or La 2 O 3 significantly improved the catalyst stability because they suppressed the formation of carbon deposit and phase transformation of the Al 2 O 3 support. More experiments should be done in the future to understand why the addition of certain additives could suppress the deposition of carbon.