Low-temperature partial oxidation of methane over Pd–Ni bimetallic catalysts supported on CeO2

Low-temperature partial oxidation of methane over PdeNi bimetallic catalysts supported on CeO 2 Shiva Fazlikeshteli, Xavier Vendrell ** , Jordi Llorca * Institute of Energy Technologies, Department of Chemical Engineering and Barcelona Research Center in Multiscale Science and Engineering, Universitat Polit ecnica de Catalunya, EEBE, Eduard Maristany 16, 08019, Barcelona, Spain highlights graphical abstract Bimetallic PdeNi/CeO 2 prepared by ball milling show excellent performance for POM. Maximum methane conversion and syngas with 0.12 wt% Pd and 1.38 wt% Ni. Superior stability for PdeNi/CeO 2 ball milled at 50 Hz for 20 min. Ball milling yields an impressive dispersion of metal species on CeO 2 surface. article info Article history: Received 1 February 2022 Received in revised form 26 June 2022 Accepted 3 July 2022 Available online 13 August 2022 Keywords: Methane partial oxidation Bimetallic catalysts Ceria catalysts Palladium Nickel Mechanochemistry abstract Monometallic Pd and Ni and bimetallic PdeNi catalysts supported on CeO 2 are prepared via mechanochemical and conventional incipient wetness impregnation methods and tested for the production of syngas by the partial oxidation of methane. Compared with monometallic Ni/CeO 2 and Pd/CeO 2 , bimetallic PdeNi/CeO 2 catalysts show considerable higher methane conversion and syngas yield. Additionally, the bimetallic catalysts prepared by ball milling produce syngas at lower temperature. Different preparation parameters, such as metal loading, Pd/Ni ratio, milling energy, milling time and order of incorporation of the metals are examined. The best performance is obtained with a bimetallic catalyst prepared at 50 Hz for 20 min with only 0.12 wt% Pd and 1.38 wt% Ni. Stability tests demonstrate superior stability for bimetallic PdeNi/CeO 2 catalysts prepared by a mechanochemical approach. From the characterization results, this is explained in terms of an impressive dispersion of metal species with a strong interaction with the surface of CeO 2 . ©2022 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). *Corresponding author. ** Corresponding author. E-mail addresses: xavier.vendrell.villafr[email protected] (X. Vendrell), [email protected] (J. Llorca). Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy 48 (2023) 12024e12035 https://doi.org/10.1016/j.ijhydene.2022.07.020 0360-3199/©2022 The Author(s). Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. This is an open access article under the CC BY-NCND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Introduction Methane is a major component of natural gas (>90%) and its conversion into syngas (a valuable feedstock composed by a mixture of gases dominated by the presence of carbon monoxide CO and hydrogen) has attracted increasing attention in the last few decades for the production of numerous chemical products and fuels [1e3]. Depending on the application, the catalytic syngas production from methane can be realized through three general routes: (1) steam reforming of methane (SRM) (Eq. (1)), (2) dry reforming of methane (DRM) (Eq. (2)), and (3) partial oxidation of methane (POM) (Eq. (3))[4e6]. CH 4 þH 2 O⟶CO þ3H 2 DH 298K ¼206 kJ mol 1 (Eq. 1) CH 4 þCO 2 ⟶2H 2 þ2CO DH 298K ¼247 kJ mol 1 (Eq. 2) CH 4 þ1/2O 2 /CO þ2H 2 DH 298K ¼36 kJ mol 1 (Eq. 3) In contrast to SRM and DRM methods, POM is a mildly exothermic reaction. For this reason, POM is seen as an energy-saving process, and is considered the most economic technology for syngas production [7,8]. Moreover, the molar ratio of H 2 /CO in the POM reaction is close to 2, which is suitable for the production of methanol and higher hydrocarbons through the Fischer-Tropsch process [9,10]. Besides, POM can handle a large volume of the feed gas with only a small amount of catalyst [11,12]. Usually, the POM process into syngas occurs in two steps [4,13]. In the first step, CH 4 is combusted by O 2 to produce CO 2 and H 2 O (Eq. (4)), and then the remaining unreacted CH 4 is reformed with water (Eq. (5)) and CO 2 (Eq. (6)) to produce CO and H 2 [13,14]. These reactions are always accompanied by the water gas-shift equilibrium (Eq. (7))[13]. CH 4 þ2O 2 %CO 2 þ2H 2 ODH 298 ¼803 kJ mol 1 (Eq. 4) CH 4 þH 2 O%CO þ3H 2 DH 298 ¼þ206 kJ mol 1 (Eq. 5) CH 4 þCO 2 %2CO þ2H 2 DH 298 ¼þ247 kJ mol 1 (Eq. 6) CO 2 þH 2 %CO þH 2 ODH 298 ¼41 kJ mol 1 (Eq. 7) After nearly 100 years of investigation on the catalytic partial oxidation of methane into syngas, numerous catalyst formulations have been tested. They can be divided basically into three main types: (1) nickel, cobalt, or iron catalysts, (2) transition metal carbide catalysts, and (3) noble metal catalysts [15,16]. Currently, the industrial catalysts used for POM are primarily nickel and noble metal supported catalysts [1]. Nickel-based catalysts have been widely investigated due to their low cost; however, they are easily deactivated owing to carbon deposition and metal loss at temperatures higher than 700 C[4,5,17]. To improve thermal stability and activity and decrease carbon deposition, other components like noble metals (Pd, Pt, Rh, Ru, etc.) are added to Ni catalysts. These noble metals are more active, less sensitive to coke formation and have higher capacity to oxidize hydrocarbons, but are expensive. Therefore, bimetallic catalysts with Ni and noble metals have been proposed to promote methane conversion [15,17]. Palladium is one of the most effective noble metals used for methane conversion processes [18e20]. The support plays also an important role on the POM process [21,22]. Most catalysts show high activity when metals are dispersed on a reducible oxide support [4]. Cerium oxide (CeO 2 ) is one of the most significant reducible oxide supports used in industrial catalysis as it reduces carbon deposition and has a remarkable oxygen storage/exchange capacity due to facile oxygen vacancy formation and change of oxidation states between Ce 3þ and Ce 4þ [19,23]. The main objective of the present investigation is to develop an effective bimetallic NiePd/CeO 2 catalytic system for POM to produce syngas at low temperature. To that end, a series of monometallic Ni, Pd, and PdeNi bimetallic catalysts supported on CeO 2 were prepared, characterized and tested. Two different methods were used to prepare the catalysts, dry ball milling (BM) and conventional incipient wetness impregnation (IWI). The BM technique is a green technology with enormous potential for the preparation of catalysts; it has an easy operation and it is fast, cost-effective and environmentally friendly [24e26]. Materials and methods Preparation of CeO 2 Cerium nitrate hexahydrate (Ce(NO 3 ) 3 $6H 2 O, 99.5%) and palladium (II) nitrate (Pd(NO 3 ) 2 , 93%) were purchased from Alfa Aesar. Nickel (II) nitrate hexahydrate (Ni(NO 3 ) 2 $6H 2 O, 98%) was obtained from Fisher chemical. Ammonia solution (NH 3 , 28%) was obtained from Scharlab. All reagents were used without further purification. Cerium dioxide (CeO 2 ) was obtained by adding dropwise NH 3 to an aqueous solution of Ce(NO 3 ) 3 $6H 2 O until the pH reached a value between 9 and 10 and a yellowish precipitate was obtained. After that, the precipitated was filtered and washed thoroughly with deionized water. The precipitate was dried overnight at 90 C and calcined at 650 C for 4 h (5 C min 1 ). Preparation of catalysts by mechanochemical method Monometallic Pd(x)/CeO 2 /BM (x ¼0.5, and 1 wt%) and Ni(y)/ CeO 2 /BM (y ¼0.5 wt%) catalysts were prepared via ball mill (labeled as BM). Ni(NO 3 ) 2 $6H 2 O and Pd(NO 3 ) 2 were used as Ni and Pd precursors, respectively. The desired amounts of metal nitrates were mixed directly with CeO 2 in a zirconium oxide vessel using a Fritsch Pulverisette 23 mini-mill apparatus and one zirconium oxide ball of 15 mm diameter (ball to powder ratio, BPR ¼10.2). Two different routes were used to prepare the bimetallic Pd(x)-Ni(y)/CeO 2 /BM catalysts (x ¼0.06e1 wt%, y¼0.5 to 1.44 wt%; x þy¼1.5) by the BM method: (1) co-BM and (2) sequential-BM. In the co-BM method, the bimetallic PdeNi/CeO 2 /BM catalysts were prepared in one-step by milling together CeO 2 and the metal precursors. In the sequentialBM method, firstly, one of the metal precursors was mixed with cerium oxide by BM, and in the next step the other precursor was added and subjected again to BM. These samples were labeled as PdeCeO 2 /BM/Ni/BM or NieCeO 2 /BM/Pd/BM international journal of hydrogen energy 48 (2023) 12024e12035 12025 depending if ceria was first milled with the Pd precursor or with the Ni precursor, respectively. The effect of ball mill frequency (15, 30 and 50 Hz) and milling time (5e40 min) was also investigated. All the fresh catalysts were used without any further treatment. Preparation of catalysts by incipient wetness impregnation Monometallic Pd, Ni, and bimetallic PdeNi catalysts were also prepared by conventional incipient wetness impregnation for comparison (labeled as IWI). For the preparation of monometallic Ni(y)/CeO 2 /IWI (y ¼0.5 wt%) and Pd(x)/CeO 2 /IWI (x ¼0.5, and 1 wt%), Ni(NO 3 ) 2 $6H 2 O and Pd(NO 3 ) 2 aqueous solutions were used as precursors. Samples were dried at 90 C and calcined at 650 C for 4 h (5 Cmin 1 ). Two different routes were used to prepare the bimetallic Pd(x)-Ni(y)/CeO 2 /IWI catalysts (x ¼0.06e1 wt%, y ¼0.5 to 1.44 wt%; x þy¼1.5) by the IWI method: (1) co-IWI and (2) sequential-IWI. In the co-IWI method, the PdeNi/CeO 2 /IWI catalysts were prepared in one-step, while in sequential-IWI two subsequent impregnations were carried out, with a calcination step at 650 C for 2 h between each impregnation. These samples were labeled as NieCeO 2 /IWI/Pd/IWI or PdeCeO 2 /IWI/Ni/IWI depending if the first impregnation was carried out with the Ni or the Pd precursor, respectively. No further treatments were performed on the calcined samples before the catalytic test. Catalytic tests The catalytic performance of the catalysts for the POM process was evaluated in a continuous-flow fixed-bed quartz tubular reactor. Reactions were carried out at atmospheric pressure between 300 and 550 C using a mixture of CH 4 :air:N 2 ¼4:11:85 (molar, CH 4 /O 2 ¼1.73) and F/W ¼60 L h 1 g 1 (gas hourly space velocity GHSV ¼12 10 3 h 1 ). Typically, 0.1 g of catalyst was mixed with silicon carbide to obtain a fixed bed volume of 0.5 cm 3 . The reactor was heated in an electrical furnace (Carbolite CTF) and the temperature in the center of the catalyst bed was measured with a type K thermocouple; no axial thermal gradients were measured. CH 4 conversion ðXCH4Þand syngas selectivities ðSH2;SCOÞwere evaluated from 300 C to 550 C in steps of 50 C. At each temperature, a dwell time of 45 min was applied, thus ensuring steady-state conditions. The reaction products were analyzed online every 4 min with a gas chromatograph (Varian CP-4900) equipped with Molecular Sieve of 5  A, Plot U and Stabilwax columns for a complete analysis of products. The methane conversion ðXCH4Þ(Eq. (8)), selectivity of hydrogen ðSH2Þ(Eq. (9)), selectivity of carbon monoxide (SCO) (Eq. (10)), and yield of syngas ðYsgÞ (Eq. (11)) were calculated according to the following equations: xCH4ð%Þ¼FinCH4FoutCH4 FinCH4 100 (Eq. 8) where FinCH4and FoutCH4are the inlet and outlet molar flow of methane, respectively SH2ð%Þ¼ moles of H2produced moles of ðH2þCO þCO2Þproduced 100 (Eq. 9) SCOð%Þ¼ moles of CO produced moles of ðH2þCO þCO2Þproduced 100 (Eq. 10) Ysgð%Þ¼xCH4ðSH2þSCOÞ 100 (Eq. 11) Catalyst characterization Characterization of the surface was accomplished by X-ray photoelectron spectroscopy (XPS) with a SPECS system equipped with a XR50 source operating at 250 W and a Phoibos 150 MCD-9 detector. The pass energy of the high-resolution spectra was set at 0.1 eV. Binding energy (BE) values were referred to the Ce 4þ 3d 5/2 peak at 916.9 eV. The microstructure of the catalysts was investigated with high resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray analysis (EDX) using a FEI TECNAI F20 instrument operated at 200 kV. Raman spectroscopy was performed using a Renishaw inViaQontor confocal Raman microscope equipped with a 532.1 ±0.3 nm laser with a nominal 100 mW output power directed through a specially adapted Leica DM2700 M microscope (x50 magnification). Spectra were acquired in two ranges, 50-800 cm 1 and 1000-2000 cm 1 , with an exposure time of 0.5 s, 1% of maximum laser power and 18 repetitions. Temperature programmed reduction (TPR-H 2 ) was performed with a Chemstar apparatus with TCD detector. Samples were first treated at 450 C in Ar (50 mL min 1 ,10C min 1 ), and then TPR was carried out from 30 to 850 C (10 C min 1 ) under 10% H 2 and kept at this temperature for 30 min. Results and discussion Characterization of fresh catalysts Raman spectroscopy The Raman spectra recorded for the bimetallic PdeNi/CeO 2 catalysts prepared by BM and IWI methods before the catalytic tests are shown in Fig. 1. In all cases, the Raman were dominated by the symmetrical stretching mode of the CeO 2 lattice structure (F 2g mode) at ~464 cm 1 [27]. Additionally, for the bimetallic PdeNi/CeO 2 catalyst prepared by BM, Raman spectra indicated a weak band at ~1050 cm 1 , which corresponds to the symmetric n 1 stretching mode of the nitrate anion [28,29]. The NO 3band was absent in the bimetallic catalysts prepared by IWI because nitrate residues disappeared following the calcination treatment performed at 650 C. Also, for the bimetallic PdeNi/CeO 2 catalyst prepared by IWI, a broad band from ~530 to 620 cm 1 is observed, which can be ascribed to NiO species [29], although a defect-induced vibrational mode of ceria lattice defects in the ceria structure, such as oxygen vacancies, cannot be discarded (~595 cm 1 ,D band) [30]. Transmission electron microscopy (HRTEM) With the aim of understanding the microstructure of the catalysts, the PdeNi dispersion and the possible interactions between metal and support, high resolution transmission international journal of hydrogen energy 48 (2023) 12024e1203512026 electron microscopy (HRTEM) and energy-dispersive X-ray analysis (EDX) were used (Fig. 2). A representative image of the monometallic Pd/CeO 2 catalyst prepared by BM is shown in Fig. 2a. The sample is very homogeneous and contains subnanometric Pd entities (some of them indicated with arrows in the image), which are well dispersed over the CeO 2 support. A larger Pd particle is highlighted with a dashed square. The Fourier Transform (FT) image of this area is shown in the inset and shows spots at 1.9  A from CeO 2 (220) planes and at 2.2  A from Pd (111) planes. The EDX spectrum recorded in the same area confirms the nature of the particle and shows the simultaneous occurrence of Ce, Pd and O (the Cu signal originates from the TEM grid). Fig. 2b corresponds to the bimetallic PdeNi/CeO 2 catalysts prepared by IWI method. It is difficult to distinguish any Pd, Ni or PdeNi particle in TEM analysis, which indicates an excellent dispersion of Ni and Pd on the CeO 2 support. The EDX spectrum recorded from the area enclosed by the square shows only weak Pd signals; no signals of Ni are observed, probably due to the low loading of Ni (0.5 wt %). Fig. 2c shows the microstructure of the bimetallic PdeNi/ CeO 2 catalysts prepared by the BM method. The mechanochemical preparation of PdeNi/CeO 2 creates an amorphous layer around the ceria crystallites (marked between arrows). This shell exhibits an average thickness of about 2e4 nm. The EDX spectrum recorded from the area enclosed by the square conclusively identified both Pd and Ni in this amorphous shell. The amorphous shell is similar to that recently reported in Pd/ CeO 2 catalysts prepared by BM [31] and corresponds to an unprecedented architecture. X-ray photoelectron spectroscopy (XPS) The surface composition of the bimetallic PdeNi/CeO 2 catalysts prepared by ball milling was analyzed by X-ray Fig. 1 eRaman spectra of bimetallic catalysts with 1 wt% Pd and 0.5 wt% Ni supported on CeO 2 prepared by BM at 50 Hz for 20 min (black), and IWI (red). (For interpretation of the references to color/colour in this figure legend, the reader is referred to the Web version of this article.) Fig. 2 eHRTEM images of (a) monometallic Pd (1)/CeO 2 catalyst prepared by BM at 50 Hz for 20 min; (b) bimetallic Pd (1)-Ni(0.5)/CeO 2 catalyst prepared by IWI; (c) bimetallic Pd (1)-Ni(0.5)/CeO 2 catalyst prepared by BM at 50 Hz for 20 min. international journal of hydrogen energy 48 (2023) 12024e12035 12027 photoelectron spectroscopy (XPS). Fig. 3a shows the Ce 3d spectrum. The peaks marked as v (882.7 eV), v 2 (889 eV), v 3 (899.8 eV), u (901.1 eV), u 2 (907.1eV) and u 3 (916.9 eV) correspond to the presence of Ce 4þ species, and those labeled as v 0 (881.6 eV), v 1 (885 eV), u 0 (898.4 eV), and u 1 (903 eV) correspond to Ce 3þ species [32,33]. About 40% of cerium at the surface appears as Ce 3þ species. Fig. 3b shows the Pd 3d spectrum. A single doublet corresponding to the 3d 5/2 and 3d 3/2 splitting is present, with a binding energy for 3d 5/2 at 335.3 eV, which corresponds well to reduced Pd. The surface Pd/Ce and Ni/Ce atomic ratios are 0.09 and 0.04, respectively, which points to an excellent dispersion of the two metals over the ceria support. Temperature programmed reduction (H 2 -TPR) The H 2 -TPR profiles recorded for the investigated samples are shown in Fig. 4. All the samples show a high-temperature reduction peak located above 700 C, which corresponds to the bulk reduction of CeO 2 , as reported elsewhere [34]. However, it is worth mentioning that the high-temperature peak is shifted to lower temperatures when samples are prepared by the BM method with respect to analogous samples prepared by IWI. The Ni/CeO 2 /IWI catalyst shows a broad H 2 consumption at ~450 C, which corresponds to the reduction of NiO strongly interacting with the CeO 2 support [35,36], according to an enhanced metal-support interaction following calcination [37,38]. In contrast, the H 2 consumption on the Ni/ CeO 2 catalyst prepared by BM appears at much lower temperature (~185 C) and with higher intensity, which can be assigned to the reduction of well-dispersed NiO species [39]. It is interesting to note that the ball mill method notably improves the metal reducibility at lower temperature. The monometallic catalyst Pd/CeO 2 prepared by BM method a broad reduction peak centered at ~285 C, which can be assigned to the reduction of PdO species anchored on CeO 2 [40,41]. The bimetallic PdeNi/CeO 2 catalysts prepared by both IWI and BM methods show similar H 2 -TPR profiles with a broad H 2 consumption at about ~250 C, due to PdOeNiO reduction. Compared to Pd/CeO 2 monometallic catalysts, in the bimetallic PdeNi/CeO 2 catalysts the reduction appears at a lower temperature, indicating that the addition of Ni facilitates the reduction of PdO. In other words, there is a strong synergy between both metals, which is in accordance to the HRTEM results (Fig. 2c). Fig. 3 eCe 3d (a) and Pd 3d (b) X-ray photoelectron spectra of the bimetallic Pd (1)-Ni(0.5)/CeO 2 /BM catalyst (50 Hz for 20 min). Fig. 4 eH 2 -TPR profiles corresponding to Pd (1)/CeO 2 /BM (black), Ni(0.5)/CeO 2 /BM (green), Ni(0.5)/CeO 2 /IWI (orange), Pd (1)-Ni(0.5)/CeO 2 /IWI (blue) and Pd (1)-Ni(0.5)/CeO 2 /BM (red). Ball mill conditions were 50 Hz for 20 min. (For interpretation of the references to color/colour in this figure legend, the reader is referred to the Web version of this article.) international journal of hydrogen energy 48 (2023) 12024e1203512028 Catalytic tests The temperature-dependent catalytic performance of the different catalysts prepared followed similar patterns. As a representative example, Fig. 5 shows the molar flow rates of reactants (methane and oxygen) and products (syngas, water and carbon dioxide) from 300 to 550 C of bimetallic PdeNi/CeO 2 catalysts prepared by BM (Fig. 5a) and IWI (Fig. 5b) methods. According to thermodynamics, the conversion of methane increases progressively with temperature. However, significant differences are observed between BM and IWI catalysts. For the bimetallic catalyst prepared by BM, all the oxygen is consumed at 450 C and from this temperature the consumption of methane runs parallel to the consumption of water and the production of syngas is observed. Therefore, this performance below 450 C obeys to the equation: CH 4 þ2O 2 /CO 2 þ2H 2 O. At higher temperatures, when all the oxygen is consumed, the reaction that takes place is the methane steam reforming reaction: CH 4 þH 2 O%CO þ3H 2 , where the conversion of methane is accompanied by the consumption of H 2 Oandthe production of syngas increases drastically. Simultaneously, the dry reforming of methane likely occurs: CH 4 þCO 2 %2CO þ 2H 2 . This mechanisms corresponds to the well-known “combustion and reforming reaction (CRR)”mechanism outlined in the introduction section [16,42], although a detailed kinetic analysis out of thermodynamic control would be necessary to discuss the reaction mechanism in more detail [43,44]. Similar trend distribution and appearance of products are obtained when the bimetallic catalyst was prepared by the conventional IWI method, confirming that a similar reaction mechanism takes place independently of the preparation method. However, all the reactions occur at higher temperatures, indicating lower catalytic activity. Considering the two consecutive steps involved in the POM reaction and given that below 450 C the production of syngas is not significant, only the performance of the catalysts at temperatures above 450 C will be considered further. Table 1 compiles the catalytic results of all prepared samples in terms of methane conversion ðxCH4Þ,H 2 selectivity ðSH2Þ, CO selectivity ðSCOÞand yield of syngas ðYsgÞ. Bare CeO 2 results are also included as a blank test, as well as chemical equilibrium values. The CeO 2 support in the absence of Pd and Ni is totally inactive for POM under the reaction conditions tested. Moreover, the catalytic activity results for the catalysts with 0.5 wt % Ni supported on CeO 2 prepared by either IWI or BM methods showed low methane conversion and the formation of syngas was not observed at all. On the other hand, it is clear that the catalytic activity of Pd/CeO 2 is much greater than Ni/CeO 2 ,as the methane conversion values for Pd/CeO 2 catalysts at 550 C doubled those obtained over Ni/CeO 2 catalysts and the syngas yield was 42e48%. The milling parameters had negligible effect on the catalytic performance of the monometallic catalysts, as methane conversion at 550 C oscillated between 51 and 55% and syngas yield from 42 to 47%. From Table 1, it can be clearly seen that the cooperative effect between Pd and Ni in the bimetallic catalysts is remarkable, with a dramatic increase in both the methane conversion and the syngas yield compared to monometallic catalysts. At 550 C, bimetallic PdeNi/CeO 2 catalysts show methane conversion levels from 62 to 75%, selectivity towards H 2 of 64e66%, selectivity towards CO of 19e24%, and syngas yield between 52 and 67%. These results demonstrate that the synergy between Pd and Ni supported on CeO 2 yields catalysts with improved activity and selectivity in the POM with respect to monometallic Ni/CeO 2 and Pd/CeO 2 . The preparation method (BM vs. IWI), the order of incorporation of the metals (co-BM/co-impregnation vs. sequential BM/IWI) play an important role on the catalytic performance of the bimetallic PdeNi/CeO 2 catalysts. Fig. 6a shows the methane conversion at different temperatures for the bimetallic Pd (1)-Ni(0.5)/CeO 2 catalysts prepared by coimpregnation and sequential-impregnation. Interestingly, at low temperature (450 C) the methane conversion of the catalysts prepared by sequential impregnation is much higher than that of the catalyst prepared by co-impregnation, regardless of the order of addition used for Pd and Ni. In contrast, at high temperatures (500 and 550 C), the methane conversion attained by the catalyst prepared by coimpregnation is higher than those of the catalysts prepared Fig. 5 eMolar flowrates recorded for bimetallic catalysts Pd (1)-Ni(0.5)/CeO 2 prepared by BM at 50 Hz for 20 min (a) and IWI (b) methods. GHSV ¼12 £10 3 h ¡1 , F/W ¼60 L h ¡1 g ¡1 . international journal of hydrogen energy 48 (2023) 12024e12035 12029 by sequential impregnation. No remarkable differences were observed between bimetallic catalysts prepared by co-BM or sequential-BM (Fig. 6b). In all cases, the methane conversion was similar at each temperature. In any case, the addition of Pd first gives better catalytic results than when Ni is first incorporated, which points to a better catalytic performance when Pd interacts strongly with CeO 2 , as opposed to Ni. On the other hand, the effect of milling conditions (frequency and time) on the catalytic activity of bimetallic PdeNi/ CeO 2 /BM catalyst at different temperatures is shown in Fig. 6c and d. The energy applied during the ball milling in terms of frequency of vibration has a strong effect on catalytic activity; the higher the energy the better the catalytic activity of the catalyst (Fig. 6c). Therefore, it is clear that high-energy milling has a beneficial influence on the number and quality of the active sites [45,46]. This work adds an additional evidence that ball milling is able to create unprecedented architectures that can exhibit outstanding catalytic activity. The ball milling time is another important factor that can affect catalytic performance. Fig. 6d shows the effect of the milling time (from 5 to 40 min) on the methane conversion values. A good compromise between methane conversion and milling time is found for the catalyst prepared using 20 min of milling time. Accordingly, a ball milling frequency of 50 Hz for 20 min appears as the optimal synthesis conditions. An important aspect regarding bimetallic catalysts is the relative amount of the two metals involved. In our case, it should be also considered that the cost of Pd is much higher than that of Ni, which obviously has a direct impact on the final cost of the catalyst. Fig. 7 shows the methane conversion and syngas yield at 450 C and 550 C exhibited by PdeNi/ CeO 2 /BM and PdeNi/CeO 2 /IWI catalysts containing different amounts of Pd and Ni but keeping the total metal loading at 1.5 wt% (Pd wt.% from 0.06 to 1% and Ni wt.% from 0.5 to 1.44%). Two aspects merit particular attention. First, it is very interesting to observe that at 450 C both the methane conversion and syngas yield are much higher for all the catalysts prepared by BM with respect to their respective counterparts prepared by the IWI method. This clearly demonstrates that very reactive and specific active sites are created by the BM method, which are highly active for the POM process at low temperature. At 550 C, the catalytic performance of PdeNi/ CeO 2 /BM and PdeNi/CeO 2 /IWI catalysts are similar. Second, the effect of the Pd amount on the catalytic performance is lower than expected and depends also on the preparation method. For the PdeNi/CeO 2 /IWI catalysts, a trend is observed Table 1 eMethane conversion, hydrogen and carbon monoxide selectivity values and syngas yields obtained over bare CeO 2 , Ni/CeO 2 , Pd/CeO 2 and PdeNi/CeO 2 bimetallic catalysts prepared by incipient wetness impregnation (IWI) and dry ball milling (BM) using different synthesis parameters and metal loading values. Reaction conditions: CH 4 :air:N 2 ¼4:11:85, F/ W¼60 L h ¡1 g ¡1 , GHSV ¼12 £10 3 h ¡1 . Sample Hz min wt.%Pd wt.%Ni x450C CH4x550C CH4S550C H2S550C CO Y550C syngas Equilibrium - - - - 51.3 77.9 67.2 26.3 70.9 CeO 2 /BM 50 10 0 0 0 0 0 0 0 Ni/CeO 2 /BM 50 10 0 0.5 3.2 22 0 0 0 Ni/CeO 2 /IWI - - 0 0.5 3 18.6 0 0 0 Pd/CeO 2 /BM 50 10 0.5 0 32.6 52.7 63.3 17.7 42.7 Pd/CeO 2 /IWI - - 0.5 0 39.7 51 63.8 23 44.5 Pd/CeO 2 /BM 50 10 1 0 30.5 53.2 64.8 19.3 44.8 Pd/CeO 2 /IWI - - 1 0 34.5 57.4 64.5 19.4 48.2 Pd/CeO 2 /BM 50 20 1 0 32.7 55.6 65.2 20.2 47.6 Pd/CeO 2 /BM 15 10 1 0 35.2 54.9 63.5 18 44.7 Pd/CeO 2 /BM 30 10 1 0 31.4 51 64 18.2 41.9 PdeNi/CeO 2 /BM 15 10 1 0.5 36.4 62.4 64.2 19.2 52.1 PdeNi/CeO 2 /BM 30 10 1 0.5 39.9 67.1 64.9 20.5 57.4 PdeNi/CeO 2 /BM 50 10 1 0.5 44 71.8 65.5 21.7 62.7 PdeCeO 2 /BM/Ni/BM 50 10 1 0.5 42.5 71.6 65.5 22 62.6 NieCeO 2 /BM/Pd/BM 50 10 1 0.5 41.6 68 64.9 20.5 58.1 PdeNi/CeO 2 /BM 50 5 1 0.5 37.5 67.2 65.5 20.7 57.9 PdeNi/CeO 2 /BM 50 15 1 0.5 38 66.3 65.5 21.2 57.5 PdeNi/CeO 2 /BM 50 20 1 0.5 42.1 72.4 66 22 63.7 PdeNi/CeO 2 /BM 50 40 1 0.5 41.4 71.5 65.8 21.7 62.6 PdeNi/CeO 2 /BM 50 20 0.75 0.75 37.9 67.4 65.4 21.7 58.7 PdeNi/CeO 2 /BM 50 20 0.5 1 35.5 68.2 65.3 21.7 59.4 PdeNi/CeO 2 /BM 50 20 0.25 1.25 37.7 70.6 65.9 23.8 63.4 PdeNi/CeO 2 /BM 50 20 0.12 1.38 40.8 69.5 65.9 23.6 62.2 PdeNi/CeO 2 /BM 50 20 0.06 1.44 36.6 69.5 66 23.1 62 NieCeO 2 /IWI/Pd/IWI - - 1 0.5 38.5 51.5 64.6 19.5 54 PdeCeO 2 /IWI/Ni/IWI - - 1 0.5 42.2 71.7 66 21.8 63 PdeNi/CeO 2 /IWI - - 1 0.5 12.6 75.8 66.1 22.8 67.4 PdeNi/CeO 2 /IWI - - 0.75 0.75 11.6 74.8 66.2 22.5 66.3 PdeNi/CeO 2 /IWI - - 0.5 1 7.7 74.4 66.3 22.8 66.3 PdeNi/CeO 2 /IWI - - 0.25 1.25 6.9 70 66 21.8 63 PdeNi/CeO 2 /IWI - - 0.12 1.38 2.8 67.7 65.8 23 60.2 PdeNi/CeO 2 /IWI - - 0.06 1.44 3.3 65.4 66 22.9 62.6 international journal of hydrogen energy 48 (2023) 12024e1203512030 between methane conversion and Pd content: the higher the Pd amount the higher the conversion of methane. This is not the case of the PdeNi/CeO 2 /BM catalysts, where both the methane conversion and syngas yield are rather unsensitive of the relative amounts of Pd and Ni in the range studied. It should be highlighted that the Ni/CeO 2 /BM sample showed Fig. 6 eConversion of methane at 450, 500 and 550 C over PdeNi/CeO 2 catalysts prepared by IWI (a), PdeNi/CeO 2 catalysts prepared by BM at 50 Hz for 10 min (b), PdeNi/CeO 2 /BM prepared at different milling frequency for 10 min (c), and PdeNi/ CeO 2 /BM prepared at 50 Hz for different milling time (d). GHSV ¼12 £10 3 h ¡1 , F/W ¼60 L h ¡1 g ¡1 . Fig. 7 eConversion of methane (a) and yield of syngas (b) for the bimetallic Pd(x)-Ni(y)/CeO 2 catalysts containing different amounts of Pd and Ni prepared by BM (black) and IWI (red) methods at 450 C and 550 C. All catalysts contain a total metal loading of 1.5 wt%. GHSV ¼12 £10 3 h ¡1 , F/W ¼60 L h ¡1 g ¡1 . (For interpretation of the references to color/colour in this figure legend, the reader is referred to the Web version of this article.) international journal of hydrogen energy 48 (2023) 12024e12035 12031 low methane conversion (3.2% at 450 C and 22% at 550 C, Table 1) and no syngas production, whereas with only 0.06 wt % Pd the methane conversion increased up to 36.6 and 69.5% at 450 and 550 C, respectively, and syngas yield was 62%. Certainly, the bimetallic PdeNi system appears as a particularly appealing metal combination for POM. It has recently been shown by using DFT calculations that the substitution of Ce by Pd ions in the surface of the catalysts originates highly active and stable species based on Pd x Ce 1-x O d which are highly effective in the rapid CeH bond activation [47,48]. From the HRTEM images, we cannot conclude if Pd substitutes Ce in the ceria structure but, in any case, we have evidence that both Pd and Ni are highly dispersed at the subnanometric level. At low temperature (450 C), where the bimetallic PdeNi catalysts prepared by BM exhibit a particular high activity for POM as outlined above, the best performance (both methane conversion and syngas yield) was observed for PdeNi/CeO 2 / BM with 0.12 wt% Pd and 1.38 wt% Ni. Characterization of the catalysts after reaction The Raman spectra recorded on the samples PdeNi/CeO 2 /BM, and PdeNi/CeO 2 /IWI after the catalytic test discussed above are shown in Fig. 8. In all cases, the position of the F 2g band of CeO 2 did not shift after the catalytic test with respect to the values recorded before reaction (Fig. 1), and the residual nitrate signals in the sample PdeNi/CeO 2 /BM disappeared after reaction due to decomposition. In addition, the two characteristic graphite D and G bands at about ~1345 and ~1595 cm 1 [49,50] were observed in the Raman spectra of the catalysts after reaction, being their relative contribution in the spectra considerably more intense in the case of the bimetallic sample prepared by impregnation. Therefore, less coke deposition occurred on the bimetallic sample prepared by ball milling. Catalyst PdeNi/CeO 2 /BM was studied by HRTEM and EDX (Fig. 9). Interestingly, a large number of voids are identified in the ceria crystallites, which were not visible in the sample before reaction (Fig. 2c). These are tentatively ascribed to clusters of oxygen vacancies, probably in contact with the metals, created under the reducing environment created by syngas during POM. On the other hand, even if the amorphous shell is no longer observed after the catalytic test, Pd and or Ni particles escape detection in HRTEM but are present in EDX analyses, indicating that the high dispersion of the metals is maintained during the catalytic reaction. Also, and in accordance with the Raman results, coke is identified by its characteristic spacing at 3.8  A, which corresponds to the (0001) interplanar spacing of poorly ordered graphite. However, the amount of deposited coke is rather low. Stability tests In order to prove the long-term performance of the catalysts, stability tests for more than 100 h at 550 C were carried out. Since bimetallic catalysts showed good methane conversion and selectivity to syngas (Table 1), PdeNi/CeO 2 catalysts with 1 wt% Pd and 0.5 Ni loading prepared by both IWI and BM Fig. 8 eRaman spectra of bimetallic Pd (1)-Ni(0.5)/CeO 2 catalysts prepared by BM at 50 Hz for 20 min (black), and IWI (red) after reaction (CH 4 :air:N 2 ¼4:11:85, F/ W¼60 L h ¡1 g ¡1 , GHSV ¼12 £10 3 h ¡1 ). (For interpretation of the references to color/colour in this figure legend, the reader is referred to the Web version of this article.) Fig. 9 eHRTEM images of the bimetallic Pd (1)-Ni(0.5)/CeO 2 /BM catalyst (50 Hz for 20 min) after the catalytic test. international journal of hydrogen energy 48 (2023) 12024e1203512032