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

Low- empe a u e pa ial oxida ion o me hane o e PdeNi bime allic ca alys s suppo ed on CeO 2 Shi a Fazlikesh eli, Xa ie Vend ell ** , Jo di Llo ca * Ins i u e o Ene gy Technologies, Depa men o Chemical Enginee ing and Ba celona Resea ch Cen e in Mul iscale Science and Enginee ing, Uni e si a Poli  ecnica de Ca alunya, EEBE, Edua d Ma is any 16, 08019, Ba celona, Spain highligh s g aphical abs ac Bime allic PdeNi/CeO 2 p epa ed by ball milling show excellen pe - o mance o POM. Maximum me hane con e sion and syngas wi h 0.12 w % Pd and 1.38 w % Ni. Supe io s abili y o PdeNi/CeO 2 ball milled a 50 Hz o 20 min. Ball milling yields an imp essi e dispe sion o me al species on CeO 2 su ace. a icle in o A icle his o y: Recei ed 1 Feb ua y 2022 Recei ed in e ised o m 26 June 2022 Accep ed 3 July 2022 A ailable online 13 Augus 2022 Keywo ds: Me hane pa ial oxida ion Bime allic ca alys s Ce ia ca alys s Palladium Nickel Mechanochemis y abs ac Monome allic Pd and Ni and bime allic PdeNi ca alys s suppo ed on CeO 2 a e p epa ed ia mechanochemical and con en ional incipien we ness imp egna ion me hods and es ed o he p oduc ion o syngas by he pa ial oxida ion o me hane. Compa ed wi h mono- me allic Ni/CeO 2 and Pd/CeO 2 , bime allic PdeNi/CeO 2 ca alys s show conside able highe me hane con e sion and syngas yield. Addi ionally, he bime allic ca alys s p epa ed by ball milling p oduce syngas a lowe empe a u e. Di e en p epa a ion pa ame e s, such as me al loading, Pd/Ni a io, milling ene gy, milling ime and o de o inco po a ion o he me als a e examined. The bes pe o mance is ob ained wi h a bime allic ca alys p epa ed a 50 Hz o 20 min wi h only 0.12 w % Pd and 1.38 w % Ni. S abili y es s demons a e supe io s abili y o bime allic PdeNi/CeO 2 ca alys s p epa ed by a mechanochemical app oach. F om he cha ac e iza ion esul s, his is explained in e ms o an imp essi e dispe sion o me al species wi h a s ong in e ac ion wi h he su ace o CeO 2 . ©2022 The Au ho (s). Published by Else ie L d on behal o Hyd ogen Ene gy Publica ions LLC. This is an open access a icle unde he CC BY-NC-ND license (h p:// c ea i ecommons.o g/licenses/by-nc-nd/4.0/). *Co esponding au ho . ** Co esponding au ho . E-mail add esses: xa ie . end ell. illa [email p o ec ed] (X. Vend ell), [email p o ec ed] (J. Llo ca). A ailable online a www.sciencedi ec .com ScienceDi ec jou nal homepage: www.else ie .com/loca e/he in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e12035 h ps://doi.o g/10.1016/j.ijhydene.2022.07.020 0360-3199/©2022 The Au ho (s). Published by Else ie L d on behal o Hyd ogen Ene gy Publica ions LLC. This is an open access a icle unde he CC BY-NC- ND license (h p://c ea i ecommons.o g/licenses/by-nc-nd/4.0/). In oduc ion Me hane is a majo componen o na u al gas (>90%) and i s con e sion in o syngas (a aluable eeds ock composed by a mix u e o gases domina ed by he p esence o ca bon mon- oxide CO and hyd ogen) has a ac ed inc easing a en ion in he las ew decades o he p oduc ion o nume ous chemical p oduc s and uels [1e3]. Depending on he applica ion, he ca aly ic syngas p oduc ion om me hane can be ealized h ough h ee gene al ou es: (1) s eam e o ming o me hane (SRM) (Eq. (1)), (2) d y e o ming o me hane (DRM) (Eq. (2)), and (3) pa ial oxida ion o me hane (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 con as o SRM and DRM me hods, POM is a mildly exo he mic eac ion. Fo his eason, POM is seen as an ene gy-sa ing p ocess, and is conside ed he mos economic echnology o syngas p oduc ion [7,8]. Mo eo e , he mola a io o H 2 /CO in he POM eac ion is close o 2, which is sui able o he p oduc ion o me hanol and highe hyd o- ca bons h ough he Fische -T opsch p ocess [9,10]. Besides, POM can handle a la ge olume o he eed gas wi h only a small amoun o ca alys [11,12]. Usually, he POM p ocess in o syngas occu s in wo s eps [4,13]. In he i s s ep, CH 4 is combus ed by O 2 o p oduce CO 2 and H 2 O (Eq. (4)), and hen he emaining un eac ed CH 4 is e o med wi h wa e (Eq. (5)) and CO 2 (Eq. (6)) o p oduce CO and H 2 [13,14]. These eac ions a e always accompanied by he wa e gas-shi equilib ium (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) A e nea ly 100 yea s o in es iga ion on he ca aly ic pa ial oxida ion o me hane in o syngas, nume ous ca alys o mula ions ha e been es ed. They can be di ided basically in o h ee main ypes: (1) nickel, cobal , o i on ca alys s, (2) ansi ion me al ca bide ca alys s, and (3) noble me al ca a- lys s [15,16]. Cu en ly, he indus ial ca alys s used o POM a e p ima ily nickel and noble me al suppo ed ca alys s [1]. Nickel-based ca alys s ha e been widely in es iga ed due o hei low cos ; howe e , hey a e easily deac i a ed owing o ca bon deposi ion and me al loss a empe a u es highe han 700 C[4,5,17]. To imp o e he mal s abili y and ac i i y and dec ease ca bon deposi ion, o he componen s like noble me als (Pd, P , Rh, Ru, e c.) a e added o Ni ca alys s. These noble me als a e mo e ac i e, less sensi i e o coke o ma ion and ha e highe capaci y o oxidize hyd oca bons, bu a e expensi e. The e o e, bime allic ca alys s wi h Ni and noble me als ha e been p oposed o p omo e me hane con e sion [15,17]. Palladium is one o he mos e ec i e noble me als used o me hane con e sion p ocesses [18e20]. The suppo plays also an impo an ole on he POM p ocess [21,22]. Mos ca alys s show high ac i i y when me als a e dispe sed on a educible oxide suppo [4]. Ce ium oxide (CeO 2 ) is one o he mos signi ican educible oxide suppo s used in indus ial ca alysis as i educes ca bon deposi ion and has a ema kable oxygen s o age/exchange capaci y due o acile oxygen a- cancy o ma ion and change o oxida ion s a es be ween Ce 3þ and Ce 4þ [19,23]. The main objec i e o he p esen in es iga ion is o de elop an e ec i e bime allic NiePd/CeO 2 ca aly ic sys em o POM o p oduce syngas a low empe a u e. To ha end, a se ies o monome allic Ni, Pd, and PdeNi bime allic ca alys s suppo ed on CeO 2 we e p epa ed, cha ac e ized and es ed. Two di e en me hods we e used o p epa e he ca alys s, d y ball milling (BM) and con en ional incipien we ness imp eg- na ion (IWI). The BM echnique is a g een echnology wi h eno mous po en ial o he p epa a ion o ca alys s; i has an easy ope a ion and i is as , cos -e ec i e and en i onmen- ally iendly [24e26]. Ma e ials and me hods P epa a ion o CeO 2 Ce ium ni a e hexahyd a e (Ce(NO 3 ) 3 $6H 2 O, 99.5%) and palladium (II) ni a e (Pd(NO 3 ) 2 , 93%) we e pu chased om Al a Aesa . Nickel (II) ni a e hexahyd a e (Ni(NO 3 ) 2 $6H 2 O, 98%) was ob ained om Fishe chemical. Ammonia solu ion (NH 3 , 28%) was ob ained om Scha lab. All eagen s we e used wi hou u he pu i ica ion. Ce ium dioxide (CeO 2 ) was ob ained by adding d opwise NH 3 o an aqueous solu ion o Ce(NO 3 ) 3 $6H 2 O un il he pH eached a alue be ween 9 and 10 and a yellowish p ecipi a e was ob ained. A e ha , he p ecipi a ed was il e ed and washed ho oughly wi h deion- ized wa e . The p ecipi a e was d ied o e nigh a 90 C and calcined a 650 C o 4 h (5 C min 1 ). P epa a ion o ca alys s by mechanochemical me hod Monome allic Pd(x)/CeO 2 /BM (x ¼0.5, and 1 w %) and Ni(y)/ CeO 2 /BM (y ¼0.5 w %) ca alys s we e p epa ed ia ball mill (labeled as BM). Ni(NO 3 ) 2 $6H 2 O and Pd(NO 3 ) 2 we e used as Ni and Pd p ecu so s, espec i ely. The desi ed amoun s o me al ni a es we e mixed di ec ly wi h CeO 2 in a zi conium oxide essel using a F i sch Pul e ise e 23 mini-mill appa a us and one zi conium oxide ball o 15 mm diame e (ball o powde a io, BPR ¼10.2). Two di e en ou es we e used o p epa e he bime allic Pd(x)-Ni(y)/CeO 2 /BM ca alys s (x ¼0.06e1 w %, y¼0.5 o 1.44 w %; x þy¼1.5) by he BM me hod: (1) co-BM and (2) sequen ial-BM. In he co-BM me hod, he bime allic PdeNi/CeO 2 /BM ca alys s we e p epa ed in one-s ep by mill- ing oge he CeO 2 and he me al p ecu so s. In he sequen ial- BM me hod, i s ly, one o he me al p ecu so s was mixed wi h ce ium oxide by BM, and in he nex s ep he o he p e- cu so was added and subjec ed again o BM. These samples we e labeled as PdeCeO 2 /BM/Ni/BM o NieCeO 2 /BM/Pd/BM in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e12035 12025 depending i ce ia was i s milled wi h he Pd p ecu so o wi h he Ni p ecu so , espec i ely. The e ec o ball mill equency (15, 30 and 50 Hz) and milling ime (5e40 min) was also in es iga ed. All he esh ca alys s we e used wi hou any u he ea men . P epa a ion o ca alys s by incipien we ness imp egna ion Monome allic Pd, Ni, and bime allic PdeNi ca alys s we e also p epa ed by con en ional incipien we ness imp egna ion o compa ison (labeled as IWI). Fo he p epa a ion o mono- me allic Ni(y)/CeO 2 /IWI (y ¼0.5 w %) and Pd(x)/CeO 2 /IWI (x ¼0.5, and 1 w %), Ni(NO 3 ) 2 $6H 2 O and Pd(NO 3 ) 2 aqueous solu ions we e used as p ecu so s. Samples we e d ied a 90 C and calcined a 650 C o 4 h (5 Cmin 1 ). Two di e en ou es we e used o p epa e he bime allic Pd(x)-Ni(y)/CeO 2 /IWI ca - alys s (x ¼0.06e1 w %, y ¼0.5 o 1.44 w %; x þy¼1.5) by he IWI me hod: (1) co-IWI and (2) sequen ial-IWI. In he co-IWI me hod, he PdeNi/CeO 2 /IWI ca alys s we e p epa ed in one-s ep, while in sequen ial-IWI wo subsequen imp egna- ions we e ca ied ou , wi h a calcina ion s ep a 650 C o 2 h be ween each imp egna ion. These samples we e labeled as NieCeO 2 /IWI/Pd/IWI o PdeCeO 2 /IWI/Ni/IWI depending i he i s imp egna ion was ca ied ou wi h he Ni o he Pd p e- cu so , espec i ely. No u he ea men s we e pe o med on he calcined samples be o e he ca aly ic es . Ca aly ic es s The ca aly ic pe o mance o he ca alys s o he POM p ocess was e alua ed in a con inuous- low ixed-bed qua z ubula eac o . Reac ions we e ca ied ou a a mosphe ic p essu e be ween 300 and 550 C using a mix u e o CH 4 :ai :N 2 ¼4:11:85 (mola , CH 4 /O 2 ¼1.73) and F/W ¼60 L h 1 g 1 (gas hou ly space eloci y GHSV ¼12 10 3 h 1 ). Typically, 0.1 g o ca alys was mixed wi h silicon ca bide o ob ain a ixed bed olume o 0.5 cm 3 . The eac o was hea ed in an elec ical u nace (Ca boli e CTF) and he empe a u e in he cen e o he ca alys bed was measu ed wi h a ype K he mocouple; no axial he mal g adien s we e measu ed. CH 4 con e sion ðXCH4Þand syngas selec i i ies ðSH2;SCOÞwe e e alua ed om 300 C o 550 C in s eps o 50 C. A each empe a u e, a dwell ime o 45 min was applied, hus ensu ing s eady-s a e con- di ions. The eac ion p oduc s we e analyzed online e e y 4 min wi h a gas ch oma og aph (Va ian CP-4900) equipped wi h Molecula Sie e o 5  A, Plo U and S abilwax columns o a comple e analysis o p oduc s. The me hane con e sion ðXCH4Þ(Eq. (8)), selec i i y o hyd ogen ðSH2Þ(Eq. (9)), selec i i y o ca bon monoxide (SCO) (Eq. (10)), and yield o syngas ðYsgÞ (Eq. (11)) we e calcula ed acco ding o he ollowing equa ions: xCH4ð%Þ¼FinCH4Fou CH4 FinCH4 100 (Eq. 8) whe e FinCH4and Fou CH4a e he inle and ou le mola low o me hane, espec i ely SH2ð%Þ¼ moles o H2p oduced moles o ðH2þCO þCO2Þp oduced 100 (Eq. 9) SCOð%Þ¼ moles o CO p oduced moles o ðH2þCO þCO2Þp oduced 100 (Eq. 10) Ysgð%Þ¼xCH4ðSH2þSCOÞ 100 (Eq. 11) Ca alys cha ac e iza ion Cha ac e iza ion o he su ace was accomplished by X- ay pho oelec on spec oscopy (XPS) wi h a SPECS sys em equipped wi h a XR50 sou ce ope a ing a 250 W and a Phoibos 150 MCD-9 de ec o . The pass ene gy o he high- esolu ion spec a was se a 0.1 eV. Binding ene gy (BE) alues we e e e ed o he Ce 4þ 3d 5/2 peak a 916.9 eV. The mic os uc u e o he ca alys s was in es iga ed wi h high esolu ion ans- mission elec on mic oscopy (HRTEM) and ene gy-dispe si e X- ay analysis (EDX) using a FEI TECNAI F20 ins umen ope a ed a 200 kV. Raman spec oscopy was pe o med using a Renishaw inViaQon o con ocal Raman mic oscope equip- ped wi h a 532.1 ±0.3 nm lase wi h a nominal 100 mW ou pu powe di ec ed h ough a specially adap ed Leica DM2700 M mic oscope (x50 magni ica ion). Spec a we e acqui ed in wo anges, 50-800 cm 1 and 1000-2000 cm 1 , wi h an exposu e ime o 0.5 s, 1% o maximum lase powe and 18 epe i ions. Tempe a u e p og ammed educ ion (TPR-H 2 ) was pe o med wi h a Chems a appa a us wi h TCD de ec o . Samples we e i s ea ed a 450 C in A (50 mL min 1 ,10C min 1 ), and hen TPR was ca ied ou om 30 o 850 C (10 C min 1 ) unde 10% H 2 and kep a his empe a u e o 30 min. Resul s and discussion Cha ac e iza ion o esh ca alys s Raman spec oscopy The Raman spec a eco ded o he bime allic PdeNi/CeO 2 ca alys s p epa ed by BM and IWI me hods be o e he ca aly ic es s a e shown in Fig. 1. In all cases, he Raman we e domi- na ed by he symme ical s e ching mode o he CeO 2 la ice s uc u e (F 2g mode) a ~464 cm 1 [27]. Addi ionally, o he bime allic PdeNi/CeO 2 ca alys p epa ed by BM, Raman spec a indica ed a weak band a ~1050 cm 1 , which co e- sponds o he symme ic n 1 s e ching mode o he ni a e anion [28,29]. The NO 3band was absen in he bime allic ca alys s p epa ed by IWI because ni a e esidues dis- appea ed ollowing he calcina ion ea men pe o med a 650 C. Also, o he bime allic PdeNi/CeO 2 ca alys p epa ed by IWI, a b oad band om ~530 o 620 cm 1 is obse ed, which can be asc ibed o NiO species [29], al hough a de ec -induced ib a ional mode o ce ia la ice de ec s in he ce ia s uc u e, such as oxygen acancies, canno be disca ded (~595 cm 1 ,D band) [30]. T ansmission elec on mic oscopy (HRTEM) Wi h he aim o unde s anding he mic os uc u e o he ca alys s, he PdeNi dispe sion and he possible in e ac ions be ween me al and suppo , high esolu ion ansmission in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e1203512026 elec on mic oscopy (HRTEM) and ene gy-dispe si e X- ay analysis (EDX) we e used (Fig. 2). A ep esen a i e image o he monome allic Pd/CeO 2 ca alys p epa ed by BM is shown in Fig. 2a. The sample is e y homogeneous and con ains sub- nanome ic Pd en i ies (some o hem indica ed wi h a ows in he image), which a e well dispe sed o e he CeO 2 suppo . A la ge Pd pa icle is highligh ed wi h a dashed squa e. The Fou ie T ans o m (FT) image o his a ea is shown in he inse and shows spo s a 1.9  A om CeO 2 (220) planes and a 2.2  A om Pd (111) planes. The EDX spec um eco ded in he same a ea con i ms he na u e o he pa icle and shows he simul aneous occu ence o Ce, Pd and O ( he Cu signal o ig- ina es om he TEM g id). Fig. 2b co esponds o he bime allic PdeNi/CeO 2 ca alys s p epa ed by IWI me hod. I is di icul o dis inguish any Pd, Ni o PdeNi pa icle in TEM analysis, which indica es an excellen dispe sion o Ni and Pd on he CeO 2 suppo . The EDX spec um eco ded om he a ea enclosed by he squa e shows only weak Pd signals; no signals o Ni a e obse ed, p obably due o he low loading o Ni (0.5 w %). Fig. 2c shows he mic os uc u e o he bime allic PdeNi/ CeO 2 ca alys s p epa ed by he BM me hod. The mechano- chemical p epa a ion o PdeNi/CeO 2 c ea es an amo phous laye a ound he ce ia c ys alli es (ma ked be ween a ows). This shell exhibi s an a e age hickness o abou 2e4 nm. The EDX spec um eco ded om he a ea enclosed by he squa e conclusi ely iden i ied bo h Pd and Ni in his amo phous shell. The amo phous shell is simila o ha ecen ly epo ed in Pd/ CeO 2 ca alys s p epa ed by BM [31] and co esponds o an unp eceden ed a chi ec u e. X- ay pho oelec on spec oscopy (XPS) The su ace composi ion o he bime allic PdeNi/CeO 2 ca a- lys s p epa ed by ball milling was analyzed by X- ay Fig. 1 eRaman spec a o bime allic ca alys s wi h 1 w % Pd and 0.5 w % Ni suppo ed on CeO 2 p epa ed by BM a 50 Hz o 20 min (black), and IWI ( ed). (Fo in e p e a ion o he e e ences o colo /colou in his igu e legend, he eade is e e ed o he Web e sion o his a icle.) Fig. 2 eHRTEM images o (a) monome allic Pd (1)/CeO 2 ca alys p epa ed by BM a 50 Hz o 20 min; (b) bime allic Pd (1)-Ni(0.5)/CeO 2 ca alys p epa ed by IWI; (c) bime allic Pd (1)-Ni(0.5)/CeO 2 ca alys p epa ed by BM a 50 Hz o 20 min. in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e12035 12027 pho oelec on spec oscopy (XPS). Fig. 3a shows he Ce 3d spec um. The peaks ma ked as (882.7 eV), 2 (889 eV), 3 (899.8 eV), u (901.1 eV), u 2 (907.1eV) and u 3 (916.9 eV) co e- spond o he p esence o Ce 4þ species, and hose labeled as 0 (881.6 eV), 1 (885 eV), u 0 (898.4 eV), and u 1 (903 eV) co espond o Ce 3þ species [32,33]. Abou 40% o ce ium a he su ace appea s as Ce 3þ species. Fig. 3b shows he Pd 3d spec um. A single double co esponding o he 3d 5/2 and 3d 3/2 spli ing is p esen , wi h a binding ene gy o 3d 5/2 a 335.3 eV, which co esponds well o educed Pd. The su ace Pd/Ce and Ni/Ce a omic a ios a e 0.09 and 0.04, espec i ely, which poin s o an excellen dispe sion o he wo me als o e he ce ia suppo . Tempe a u e p og ammed educ ion (H 2 -TPR) The H 2 -TPR p o iles eco ded o he in es iga ed samples a e shown in Fig. 4. All he samples show a high- empe a u e educ ion peak loca ed abo e 700 C, which co esponds o he bulk educ ion o CeO 2 , as epo ed elsewhe e [34]. How- e e , i is wo h men ioning ha he high- empe a u e peak is shi ed o lowe empe a u es when samples a e p epa ed by he BM me hod wi h espec o analogous samples p epa ed by IWI. The Ni/CeO 2 /IWI ca alys shows a b oad H 2 con- sump ion a ~450 C, which co esponds o he educ ion o NiO s ongly in e ac ing wi h he CeO 2 suppo [35,36], ac- co ding o an enhanced me al-suppo in e ac ion ollowing calcina ion [37,38]. In con as , he H 2 consump ion on he Ni/ CeO 2 ca alys p epa ed by BM appea s a much lowe em- pe a u e (~185 C) and wi h highe in ensi y, which can be assigned o he educ ion o well-dispe sed NiO species [39]. I is in e es ing o no e ha he ball mill me hod no ably im- p o es he me al educibili y a lowe empe a u e. The monome allic ca alys Pd/CeO 2 p epa ed by BM me hod a b oad educ ion peak cen e ed a ~285 C, which can be assigned o he educ ion o PdO species ancho ed on CeO 2 [40,41]. The bime allic PdeNi/CeO 2 ca alys s p epa ed by bo h IWI and BM me hods show simila H 2 -TPR p o iles wi h a b oad H 2 consump ion a abou ~250 C, due o PdOeNiO educ ion. Compa ed o Pd/CeO 2 monome allic ca alys s, in he bime- allic PdeNi/CeO 2 ca alys s he educ ion appea s a a lowe empe a u e, indica ing ha he addi ion o Ni acili a es he educ ion o PdO. In o he wo ds, he e is a s ong syne gy be ween bo h me als, which is in acco dance o he HRTEM esul s (Fig. 2c). Fig. 3 eCe 3d (a) and Pd 3d (b) X- ay pho oelec on spec a o he bime allic Pd (1)-Ni(0.5)/CeO 2 /BM ca alys (50 Hz o 20 min). Fig. 4 eH 2 -TPR p o iles co esponding o Pd (1)/CeO 2 /BM (black), Ni(0.5)/CeO 2 /BM (g een), Ni(0.5)/CeO 2 /IWI (o ange), Pd (1)-Ni(0.5)/CeO 2 /IWI (blue) and Pd (1)-Ni(0.5)/CeO 2 /BM ( ed). Ball mill condi ions we e 50 Hz o 20 min. (Fo in e p e a ion o he e e ences o colo /colou in his igu e legend, he eade is e e ed o he Web e sion o his a icle.) in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e1203512028 Ca aly ic es s The empe a u e-dependen ca aly ic pe o mance o he di e en ca alys s p epa ed ollowed simila pa e ns. As a ep esen a i e example, Fig. 5 shows he mola low a es o eac an s (me hane and oxygen) and p oduc s (syngas, wa e and ca bon dioxide) om 300 o 550 C o bime allic PdeNi/CeO 2 ca alys s p epa ed by BM (Fig. 5a) and IWI (Fig. 5b) me hods. Acco ding o he modynamics, he con e sion o me hane in- c eases p og essi ely wi h empe a u e. Howe e , signi ican di e ences a e obse ed be ween BM and IWI ca alys s. Fo he bime allic ca alys p epa ed by BM, all he oxygen is consumed a 450 C and om his empe a u e he consump ion o me hane uns pa allel o he consump ion o wa e and he p oduc ion o syngas is obse ed. The e o e, his pe o mance below 450 C obeys o he equa ion: CH 4 þ2O 2 /CO 2 þ2H 2 O. A highe empe a u es, when all he oxygen is consumed, he eac ion ha akes place is he me hane s eam e o ming e- ac ion: CH 4 þH 2 O%CO þ3H 2 , whe e he con e sion o me hane is accompanied by he consump ion o H 2 Oand he p oduc ion o syngas inc eases d as ically. Simul aneously, he d y e o ming o me hane likely occu s: CH 4 þCO 2 %2CO þ 2H 2 . This mechanisms co esponds o he well-known “com- bus ion and e o ming eac ion (CRR)”mechanism ou lined in he in oduc ion sec ion [16,42], al hough a de ailed kine ic analysis ou o he modynamic con ol would be necessa y o discuss he eac ion mechanism in mo e de ail [43,44]. Simila end dis ibu ion and appea ance o p oduc s a e ob ained when he bime allic ca alys was p epa ed by he con en ional IWI me hod, con i ming ha a simila eac ion mechanism akes place independen ly o he p epa a ion me hod. Howe - e , all he eac ions occu a highe empe a u es, indica ing lowe ca aly ic ac i i y. Conside ing he wo consecu i e s eps in ol ed in he POM eac ion and gi en ha below 450 C he p oduc ion o syngas is no signi ican , only he pe o mance o he ca alys s a empe a u es abo e 450 C will be conside ed u he . Table 1 compiles he ca aly ic esul s o all p epa ed samples in e ms o me hane con e sion ðxCH4Þ,H 2 selec i i y ðSH2Þ, CO selec- i i y ðSCOÞand yield o syngas ðYsgÞ. Ba e CeO 2 esul s a e also included as a blank es , as well as chemical equilib ium alues. The CeO 2 suppo in he absence o Pd and Ni is o ally inac i e o POM unde he eac ion condi ions es ed. Mo e- o e , he ca aly ic ac i i y esul s o he ca alys s wi h 0.5 w % Ni suppo ed on CeO 2 p epa ed by ei he IWI o BM me hods showed low me hane con e sion and he o ma ion o syngas was no obse ed a all. On he o he hand, i is clea ha he ca aly ic ac i i y o Pd/CeO 2 is much g ea e han Ni/CeO 2 ,as he me hane con e sion alues o Pd/CeO 2 ca alys s a 550 C doubled hose ob ained o e Ni/CeO 2 ca alys s and he syngas yield was 42e48%. The milling pa ame e s had negligible e - ec on he ca aly ic pe o mance o he monome allic ca a- lys s, as me hane con e sion a 550 C oscilla ed be ween 51 and 55% and syngas yield om 42 o 47%. F om Table 1, i can be clea ly seen ha he coope a i e e ec be ween Pd and Ni in he bime allic ca alys s is ema kable, wi h a d ama ic inc ease in bo h he me hane con e sion and he syngas yield compa ed o monome allic ca alys s. A 550 C, bime allic PdeNi/CeO 2 ca alys s show me hane con e sion le els om 62 o 75%, selec i i y owa ds H 2 o 64e66%, selec i i y owa ds CO o 19e24%, and syngas yield be ween 52 and 67%. These esul s demons a e ha he syne gy be ween Pd and Ni suppo ed on CeO 2 yields ca alys s wi h imp o ed ac i i y and selec i i y in he POM wi h espec o monome allic Ni/CeO 2 and Pd/CeO 2 . The p epa a ion me hod (BM s. IWI), he o de o inco - po a ion o he me als (co-BM/co-imp egna ion s. sequen ial BM/IWI) play an impo an ole on he ca aly ic pe o mance o he bime allic PdeNi/CeO 2 ca alys s. Fig. 6a shows he me hane con e sion a di e en empe a u es o he bime- allic Pd (1)-Ni(0.5)/CeO 2 ca alys s p epa ed by co- imp egna ion and sequen ial-imp egna ion. In e es ingly, a low empe a u e (450 C) he me hane con e sion o he ca - alys s p epa ed by sequen ial imp egna ion is much highe han ha o he ca alys p epa ed by co-imp egna ion, ega dless o he o de o addi ion used o Pd and Ni. In con as , a high empe a u es (500 and 550 C), he me hane con e sion a ained by he ca alys p epa ed by co- imp egna ion is highe han hose o he ca alys s p epa ed Fig. 5 eMola low a es eco ded o bime allic ca alys s Pd (1)-Ni(0.5)/CeO 2 p epa ed by BM a 50 Hz o 20 min (a) and IWI (b) me hods. GHSV ¼12 £10 3 h ¡1 , F/W ¼60 L h ¡1 g ¡1 . in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e12035 12029 by sequen ial imp egna ion. No ema kable di e ences we e obse ed be ween bime allic ca alys s p epa ed by co-BM o sequen ial-BM (Fig. 6b). In all cases, he me hane con e sion was simila a each empe a u e. In any case, he addi ion o Pd i s gi es be e ca aly ic esul s han when Ni is i s inco po a ed, which poin s o a be e ca aly ic pe o mance when Pd in e ac s s ongly wi h CeO 2 , as opposed o Ni. On he o he hand, he e ec o milling condi ions ( e- quency and ime) on he ca aly ic ac i i y o bime allic PdeNi/ CeO 2 /BM ca alys a di e en empe a u es is shown in Fig. 6c and d. The ene gy applied du ing he ball milling in e ms o equency o ib a ion has a s ong e ec on ca aly ic ac i i y; he highe he ene gy he be e he ca aly ic ac i i y o he ca alys (Fig. 6c). The e o e, i is clea ha high-ene gy milling has a bene icial in luence on he numbe and quali y o he ac i e si es [45,46]. This wo k adds an addi ional e idence ha ball milling is able o c ea e unp eceden ed a chi ec u es ha can exhibi ou s anding ca aly ic ac i i y. The ball milling ime is ano he impo an ac o ha can a ec ca aly ic pe o mance. Fig. 6d shows he e ec o he milling ime ( om 5 o 40 min) on he me hane con e sion alues. A good comp omise be ween me hane con e sion and milling ime is ound o he ca alys p epa ed using 20 min o milling ime. Acco dingly, a ball milling equency o 50 Hz o 20 min ap- pea s as he op imal syn hesis condi ions. An impo an aspec ega ding bime allic ca alys s is he ela i e amoun o he wo me als in ol ed. In ou case, i should be also conside ed ha he cos o Pd is much highe han ha o Ni, which ob iously has a di ec impac on he inal cos o he ca alys . Fig. 7 shows he me hane con e sion and syngas yield a 450 C and 550 C exhibi ed by PdeNi/ CeO 2 /BM and PdeNi/CeO 2 /IWI ca alys s con aining di e en amoun s o Pd and Ni bu keeping he o al me al loading a 1.5 w % (Pd w .% om 0.06 o 1% and Ni w .% om 0.5 o 1.44%). Two aspec s me i pa icula a en ion. Fi s , i is e y in e es ing o obse e ha a 450 C bo h he me hane con- e sion and syngas yield a e much highe o all he ca alys s p epa ed by BM wi h espec o hei espec i e coun e pa s p epa ed by he IWI me hod. This clea ly demons a es ha e y eac i e and speci ic ac i e si es a e c ea ed by he BM me hod, which a e highly ac i e o he POM p ocess a low empe a u e. A 550 C, he ca aly ic pe o mance o PdeNi/ CeO 2 /BM and PdeNi/CeO 2 /IWI ca alys s a e simila . Second, he e ec o he Pd amoun on he ca aly ic pe o mance is lowe han expec ed and depends also on he p epa a ion me hod. Fo he PdeNi/CeO 2 /IWI ca alys s, a end is obse ed Table 1 eMe hane con e sion, hyd ogen and ca bon monoxide selec i i y alues and syngas yields ob ained o e ba e CeO 2 , Ni/CeO 2 , Pd/CeO 2 and PdeNi/CeO 2 bime allic ca alys s p epa ed by incipien we ness imp egna ion (IWI) and d y ball milling (BM) using di e en syn hesis pa ame e s and me al loading alues. Reac ion condi ions: CH 4 :ai :N 2 ¼4:11:85, F/ W¼60 L h ¡1 g ¡1 , GHSV ¼12 £10 3 h ¡1 . Sample Hz min w .%Pd w .%Ni x450C CH4x550C CH4S550C H2S550C CO Y550C syngas Equilib ium - - - - 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 in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e1203512030 be ween me hane con e sion and Pd con en : he highe he Pd amoun he highe he con e sion o me hane. This is no he case o he PdeNi/CeO 2 /BM ca alys s, whe e bo h he me hane con e sion and syngas yield a e a he unsensi i e o he ela i e amoun s o Pd and Ni in he ange s udied. I should be highligh ed ha he Ni/CeO 2 /BM sample showed Fig. 6 eCon e sion o me hane a 450, 500 and 550 C o e PdeNi/CeO 2 ca alys s p epa ed by IWI (a), PdeNi/CeO 2 ca alys s p epa ed by BM a 50 Hz o 10 min (b), PdeNi/CeO 2 /BM p epa ed a di e en milling equency o 10 min (c), and PdeNi/ CeO 2 /BM p epa ed a 50 Hz o di e en milling ime (d). GHSV ¼12 £10 3 h ¡1 , F/W ¼60 L h ¡1 g ¡1 . Fig. 7 eCon e sion o me hane (a) and yield o syngas (b) o he bime allic Pd(x)-Ni(y)/CeO 2 ca alys s con aining di e en amoun s o Pd and Ni p epa ed by BM (black) and IWI ( ed) me hods a 450 C and 550 C. All ca alys s con ain a o al me al loading o 1.5 w %. GHSV ¼12 £10 3 h ¡1 , F/W ¼60 L h ¡1 g ¡1 . (Fo in e p e a ion o he e e ences o colo /colou in his igu e legend, he eade is e e ed o he Web e sion o his a icle.) in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e12035 12031 low me hane con e sion (3.2% a 450 C and 22% a 550 C, Table 1) and no syngas p oduc ion, whe eas wi h only 0.06 w % Pd he me hane con e sion inc eased up o 36.6 and 69.5% a 450 and 550 C, espec i ely, and syngas yield was 62%. Ce ainly, he bime allic PdeNi sys em appea s as a pa icu- la ly appealing me al combina ion o POM. I has ecen ly been shown by using DFT calcula ions ha he subs i u ion o Ce by Pd ions in he su ace o he ca alys s o igina es highly ac i e and s able species based on Pd x Ce 1-x O d which a e highly e ec i e in he apid CeH bond ac i a ion [47,48]. F om he HRTEM images, we canno conclude i Pd subs i u es Ce in he ce ia s uc u e bu , in any case, we ha e e idence ha bo h Pd and Ni a e highly dispe sed a he subnanome ic le el. A low empe a u e (450 C), whe e he bime allic PdeNi ca alys s p epa ed by BM exhibi a pa icula high ac i i y o POM as ou lined abo e, he bes pe o mance (bo h me hane con e sion and syngas yield) was obse ed o PdeNi/CeO 2 / BM wi h 0.12 w % Pd and 1.38 w % Ni. Cha ac e iza ion o he ca alys s a e eac ion The Raman spec a eco ded on he samples PdeNi/CeO 2 /BM, and PdeNi/CeO 2 /IWI a e he ca aly ic es discussed abo e a e shown in Fig. 8. In all cases, he posi ion o he F 2g band o CeO 2 did no shi a e he ca aly ic es wi h espec o he alues eco ded be o e eac ion (Fig. 1), and he esidual ni- a e signals in he sample PdeNi/CeO 2 /BM disappea ed a e eac ion due o decomposi ion. In addi ion, he wo cha ac- e is ic g aphi e D and G bands a abou ~1345 and ~1595 cm 1 [49,50] we e obse ed in he Raman spec a o he ca alys s a e eac ion, being hei ela i e con ibu ion in he spec a conside ably mo e in ense in he case o he bime allic sample p epa ed by imp egna ion. The e o e, less coke deposi ion occu ed on he bime allic sample p epa ed by ball milling. Ca alys PdeNi/CeO 2 /BM was s udied by HRTEM and EDX (Fig. 9). In e es ingly, a la ge numbe o oids a e iden i ied in he ce ia c ys alli es, which we e no isible in he sample be o e eac ion (Fig. 2c). These a e en a i ely asc ibed o clus e s o oxygen acancies, p obably in con ac wi h he me als, c ea ed unde he educing en i onmen c ea ed by syngas du ing POM. On he o he hand, e en i he amo phous shell is no longe obse ed a e he ca aly ic es , Pd and o Ni pa icles escape de ec ion in HRTEM bu a e p esen in EDX analyses, indica ing ha he high dispe sion o he me als is main ained du ing he ca aly ic eac ion. Also, and in acco - dance wi h he Raman esul s, coke is iden i ied by i s cha - ac e is ic spacing a 3.8  A, which co esponds o he (0001) in e plana spacing o poo ly o de ed g aphi e. Howe e , he amoun o deposi ed coke is a he low. S abili y es s In o de o p o e he long- e m pe o mance o he ca alys s, s abili y es s o mo e han 100 h a 550 C we e ca ied ou . Since bime allic ca alys s showed good me hane con e sion and selec i i y o syngas (Table 1), PdeNi/CeO 2 ca alys s wi h 1 w % Pd and 0.5 Ni loading p epa ed by bo h IWI and BM Fig. 8 eRaman spec a o bime allic Pd (1)-Ni(0.5)/CeO 2 ca alys s p epa ed by BM a 50 Hz o 20 min (black), and IWI ( ed) a e eac ion (CH 4 :ai :N 2 ¼4:11:85, F/ W¼60 L h ¡1 g ¡1 , GHSV ¼12 £10 3 h ¡1 ). (Fo in e p e a ion o he e e ences o colo /colou in his igu e legend, he eade is e e ed o he Web e sion o his a icle.) Fig. 9 eHRTEM images o he bime allic Pd (1)-Ni(0.5)/CeO 2 /BM ca alys (50 Hz o 20 min) a e he ca aly ic es . in e na ional jou nal o hyd ogen ene gy 48 (2023) 12024e1203512032