Monitoring the influence of steam on highly-active rhodium catalyst during the combined reforming of biogas by transient and steady-state operando spectroscopic studies

Ca alysis Science & Technology PAPER Ci e his: Ca al. Sci. Technol.,2024, 14,3514 Recei ed 20 h Feb ua y 2024, Accep ed 6 h May 2024 DOI: 10.1039/d4cy00236a sc.li/ca alysis Moni o ing he in luence o s eam on highly- ac i e hodium ca alys du ing he combined e o ming o biogas by ansien and s eady-s a e ope ando spec oscopic s udies Vic o ia Ga cilaso, Rubén Blay-Roge , Mi iam González-Cas año, Luis F. Bobadilla, Miguel A. Cen eno and José A. Od iozola * The impac de i ed om inco po a ing wa e in o CH 4 /CO 2 biogas s eam o he gene a ion o syngas was in es iga ed o e he Rh/MgAl 2 O 4 ca alys using ope ando s eady-s a e and ansien DRIFT spec oscopy coupled wi h MS. The inco po a ion o s eam esul ed in imp o ed CH 4 con e sion a es and a ained syngas s eams wi h highe H 2 /CO a ios. I was demons a ed ha in he p esence o s eam, he gene a ion o CH x O species h ough he eac ion o CO*wi h ac i e *OH species is a o ed a he me al suppo su ace. Besides, he enhanced esis ance deli e ed by wa e molecules owa ds deac i a ing he coking phenomena was associa ed wi h easie ca bonaceous decomposi ion and he exposi ion o he e y ac i e Rh (100) su aces o me hane decomposi ion. The Rh/MgAl 2 O 4 ca alys was demons a ed o be an e ec i e ca alys o he p oduc ion o H 2 - ich syngas s eams. Mo e impo an ly, he insigh s epo ed he ein p o ide new e idences ega ding he impac o s eam on biogas e o ming eac ions. 1. In oduc ion O e he las se e al yea s, he p oduc ion o biogas om he anae obic diges ion o biomass was es has a ac ed conside able in e es as a enewable ca bon sou ce. 1,2 As biogas is composed mainly o CH 4 and CO 2 , he p oduc ion o syngas by he e o ming o biogas and he subsequen ans o ma ion in o liquid uels ia Fische –T opsch syn hesis (FTS) is he mos sui able p ocess and he mos cos -e ec i e om he indus ial poin o iew. 3,4 Howe e , he chemical composi ion o aw biogas depends on he ype o eeds ock, design o he diges e - e men e and ope a ing condi ions. 5–7 Typically, biogas con ains 60–75 ol% o me hane and 19–33 ol% o CO 2 along wi h mois u e besides o he con aminan s such as sulphu compounds, halogena ed compounds, ammonia, o ganic silicon compounds (siloxanes), a s, and pa icula e ma e . 8–10 The mos usual echnologies applied o emo e con aminan s om aw biogas du ing he upg ada ion p ocess include memb ane echnology, wa e sc ubbing, p essu e swing adso p ion (PSA), c yogenic sepa a ion and biological echniques. 11 Appa en ly, he p esence o s eam in biogas is no a d awback bu a majo ad an age o e o ming p ocesses. 12 Al hough he o ma ion o ca bon is he modynamically limi ed a ypical e o ming empe a u es, he deposi ion o coke on he ca alys su ace is una oidable in he d y e o ming eac ion o me hane. 13 The o ma ion o ca bon can be minimized due o he p esence o s eam, which p o ides a highe oxidan le el h oughou he ca bon gasi ica ion eac ion. Unlike he d y e o ming o model biogas, in which he syngas ob ained is always p esen a a H 2 /CO a io lowe han one, he p esence o wa e p oduces a syngas mo e lexible by adjus ing he CO 2 /H 2 O a io o ma ch he desi ed alue close o 2 equi ed o he downs eam p oduc ion o he chemical alue-added ia Fische –T opsch syn hesis. 14 In his scena io, he combina ion o d y e o ming o me hane (DRM) and s eam e o ming o me hane (SRM) has been explo ed ex ensi ely and i is known as he bi- e o ming o me hane 15 o also he combined e o ming o me hane. 16 This app oach has a ac ed g ea in e es om he indus ial poin o iew and p o ides an oppo uni y o ad anced esea ch owa d he comme cializa ion o his echnology. 17,18 Ca alys s used o combined e o ming p ocesses a e usually based on nickel, bu hei s abili y is limi ed by he me al sin e ing and/o coke deposi ion p ocesses. 19,20 S oud e al. 21 p oposed an Sn–Ni/Ce–Al ca alys able o enhance bo h me hane con e sion and ca aly ic s abili y in he p esence o s eam in he bi- e o ming o me hane compa ed o he same ca alys es ed unde d y- e o ming condi ions. On he o he hand, MgO and Al 2 O 3 ha e been p obed as he mos p omising 3514 |Ca al. Sci. Technol., 2024, 14,3514–3523 This jou nal is © The Royal Socie y o Chemis y 2024 Depa amen o de Química Ino gánica and Ins i u o de Ciencia de Ma e iales de Se illa, Cen o Mix o Uni e sidad de Se illa –CSIC, A . Amé ico Vespucio 49, 41092 Se illa, Spain. E-mail: [email p o ec ed] Open Access A icle. Published on 21 May 2024. Downloaded on 11/25/2024 3:33:14 PM. This a icle is licensed unde a C ea i e Commons A ibu ion 3.0 Unpo ed Licence. View A icle Online View Jou nal | View Issue Ca al. Sci. Technol., 2024, 14,3514–3523 | 3515This jou nal is © The Royal Socie y o Chemis y 2024 suppo s o he d y e o ming o me hane. I is well known ha he inc eased basici y o he suppo a ou s he adso p ion o CO 2 , acili a ing he gasi ica ion o ca bon o o m CO, as Zhao e al. 22 epo ed in a ecen s udy. He ein, a bime allic Ni–Co/MgAl 2 O 4 ca alys showed a high s abili y o e mo e han 30 h unde e o ming a mosphe es wi hou e idence o ca bon deposi s in he p esence o H 2 OandCO 2 .Ina mic okine ic s udy, Geng e al. 23 a gued ha oxygen a oms om s eam p omo ed me hane con e sion by swi ching he eac ion pa h om CH* 3+*→CH* 2+H* o CH* 3+O*→CH* 2+ OH*whils es ic ing he gene a ion o C*species, which cons ic ed he gene a ion o ca bon deposi s. Wi h mos o he s udies ocusing on Ni ca alys s, he inco po a ion o adequa e s eam concen a ions could inc ease he H 2 :CO a ios and imp o e he ca alys s abili y by a ou ing he gasi ica ion o ca bon deposi s. 24 In his sense, he la ge OH concen a ions o e Ni ca alys s gene a ed in he p esence o s eam we e p oposed o enhance he gasi ica ion o ca bon deposi s gene a ing CHO*species. 25 S ill, deac i a ion issues ela ed o me al sin e ing and he pa ial oxida ion o he ac i e si es ha e also been epo ed. 26,27 Al hough hei limi ed a ailabili y and high cos es ain hei use o la ge-scale applica ions, noble me als like P , Ru, Pd and Rh a ain highe con e sions and majo coke esis ance han ansi ion me als. 13 Among hem, Rh was demons a ed o be he mos ac i e owa ds he con e sion o me hane when suppo ed on CeO 2 /Al 2 O 3 . 13 O e di e en suppo s and eac ion condi ions, Rh me al also exhibi ed he highes u no e equency (TOF) compa ed o hei homologues based on Ru and P . 28 Maes i e al. 29 p oposed a consis en mic okine ic model on Rh ca alys ha poin ed ou he mechanis ic analogies be ween s eam and d y e o ming. They p oposed ha me hane consump ion p oceeds ia decomposi ion and subsequen ca bon oxida ion by OH* ollowing a sequen ial ou e (CH 4 →C*→CO*), and he ole o ei he CO 2 o H 2 O is o p o ide he main oxidize (OH*). Ros up-Nielsen and Bak Hansen 30 also obse ed ha eplacing s eam by ca bon dioxide ha dly modi ied he e o ming mechanism. Mo eo e , hey demons a ed ha Ru and Rh ca alys s a e highly s able o ca bon- ee ope a ion, p esen ing high e o ming a es combined wi h low coke o ma ion a es. Ope ando di use e lec ance in a ed Fou ie ans o m spec oscopy (DRIFTS) has been widely employed as a powe ul ool o elucida e he eac ion mechanism and gain insigh s in o he na u e o su ace in e media es. In a p e ious wo k, 31 we s udied he ac i i y o he Rh/MgAl 2 O 4 ca alys owa d he biogas e o ming eac ion using ansien and s eady-s a e DRIFTS expe imen s, p oposing a eac ion mechanism in which he dissocia i e adso p ion o CO 2 akes place ini ially and he ac i e oxygen species o med on he me allic si es acili a es C–Hbond ac i a ion. Al hough nume ous a icles ha e been published on he d y e o ming o me hane eac ion on di e en suppo ed Rh ca alys s, he e is no a clea and unambiguous explana ion o he eal e ec o s eam on he na u e o he eac ion. The sca ce s udies ocusing on he ole o s eam and OH su ace g oups on he eac ion mechanism and cons i u ed in e media es migh be associa ed wi h he low concen a ion es ablished unde DRM a mosphe es. 32 Conside ing he impac o la ge OH co e ages gene a ed in he p esence o s eam and he s uc u e sensi i i y cha ac e o e o ming p ocesses, 33,34 es ablishing he ole o s eam o ca aly ic sys ems aside o Ni me al is ex emely con enien . The p esen s udy con ibu es a new pe spec i e o unde s and he e ec o s eam in he ac i a ion p ocess o CO 2 and CH 4 du ing he e o ming o biogas by means o ansien expe imen s and DRIFTS coupled MS s udies o e he Rh/MgAl 2 O 4 ca alys . The in eg a ion o bo h ansien and s eady-s a e ope ando expe imen s p o ides a comp ehensi e ision o he ca alys su ace p ope ies, and hei co ela ion wi h ca aly ic pe o mance is e y use ul o achie ing he a ional design o ca alys s. 2. Expe imen al de ails A ca alys based on 1 w % o Rh dispe sed on MgAl 2 O 4 suppo was used in his s udy. The p epa a ion me hod and cha ac e iza ion de ails o his ca alys ha e been desc ibed in a p e ious wo k. 31 This ca alys was labelled as Rh/MgAl 2 O 4 . Thebiogas e o ming eac ionwasca iedou ina comme cial compu e ized mic oac i i y e e ence ca aly ic eac o om PID Eng & Tech employing a s ainless-s eel eac o wi h 9 mm in e nal diame e . A ixed-bed composed by 50 mg o ca alys dilu ed in SiC (bo h sie ed in 100–200 μm) o ensu e a good hea and mass ans e was placed a he cen e o he eac o in di ec con ac wi h a he mocouple. Be o e ca alys es ing, he sample was in si u educed in a low o 50% H 2 /N 2 a 750 °C o 3 h (100 mL min −1 ). Then, 50 mL min −1 o ni ogen was used o pu ge he eac o sys em o 30 min be o e eac ion expe imen s a 750 °C. The eac ion was pe o med a 750 °C and 1 ba , main aining a o al low a e o 100 mL min −1 wi h a weigh hou ly space eloci y (WHSV) equal o 200 L h −1 g −1 and a ying amoun s o s eam. Reac an gas mix u es consis ing o CH 4 ,CO 2 , and N 2 wi h a CH 4 /CO 2 mola a io equal o 1.5 we e in oduced using mass- low con olle s om B onkho s . Va iable amoun s o H 2 O we e in oduced using a HPLC pump (Gilbson Inc se ies) and e apo a ed o each concen a ions o 8, 12 and 17 ol% o s eam. In o de o main ain he space eloci y and he pa ial p essu es o CO 2 and CH 4 , he low o ni ogen was adjus ed as a unc ion o he s eam inco po a ed. The e o ming es s we e pe o med unde iso he mal condi ions, and e e y concen a ion o s eam was main ained o 2 h o gua an ee s eady-s a e condi ions. The e luen gases we e analysed on-line using a mic o-gas ch oma og aph (Va ian 4900 model) equipped wi h wo columns: Po aplo Q and molecula sie e 5A. The con e sions o me hane and CO 2 we e calcula ed on he basis o he di e ence be ween he inpu and ou pu o me hane and CO 2 , espec i ely. Ope ando DRIFTS-MS expe imen s we e pe o med in a high empe a u e eac ion chambe (P aying Man is om Ha ick) i ed wi h ZnSe windows and connec ed o a The mo Nicole Nexus FTIR spec ome e equipped wi h a liquid N 2 -cooled MCT de ec o . Spec a we e ob ained wi h a spec al esolu ion o 4 cm −1 and an a e age o 128 scans. The whole op ical pa h Ca alysis Science & Technology Pape Open Access A icle. Published on 21 May 2024. Downloaded on 11/25/2024 3:33:14 PM. This a icle is licensed unde a C ea i e Commons A ibu ion 3.0 Unpo ed Licence. View A icle Online 3516 |Ca al. Sci. Technol.,2024,14,3514–3523 This jou nal is © The Royal Socie y o Chemis y 2024 was con inuously pu ged wi h a low o wa e apou - and CO 2 - ee ni ogen, and he backg ound spec um was collec ed using an aluminium e lec i e mi o . Typically, abou 80 mg o sample was pu in he DRIFTS cell o ensu e ha he gases pass h ough he ca alys packed-bed mimicking he plug- low condi ion o he eac ion. P io o he measu emen s, he sample was ac i a ed in si u a 750 °C o 1hin50%H 2 /A a a o al low a e o 50 mL min −1 and hencooleddown o he eac ion empe a u e by pu ging wi h A . The spec um o he ac i a ed ca alys was used as he e e ence. The d y biogas e o ming eac ion was pe o med by eeding in o he cell a gas mix u e o 10 ol% CO 2 and 15 ol% CH 4 balanced in a gon wi h a o al low a e o 50 mL min −1 . Analogously, he bi- e o ming eac ion o biogas was ca ied ou using a mix u e o 10 ol% o CO, 15 ol% CH 4 and 10 ol% o H 2 O in a gon wi h a o al low a e o 50 mL min −1 . Fo i , an adequa e amoun o liquid wa e was ed con inuously using a HPLC pump and apo ising he liquid in a homemade e apo a o . In bo h he eac ions, he empe a u e was inc eased om 550 °C o750°C wi h in e als o 50 °C, and each empe a u e was main ained o 30 min o achie e s eady-s a e condi ions. Fo he ansien expe imen s, he eed composi ion was empo a ily a ied ollowing wo di e en sequences: 1) 10%CO 2 /A →A → 15%CH 4 /A , and 2) 10%CO 2 /10%H 2 O/A →10%H 2 O/A → 15%CH 4 /10%H 2 O/A . The di e en mix u es we e in oduced unde iso he mal condi ions a 550 °Ce e y20min.A ou -way al e was ins alled o swi ch he gas mix u es. The gas composi ion exi ing he DRIFTS cell we e sen o a mass spec ome e (PFEIFFER MS Vacuum P isma Plus) using a 1/ 16-inch s ainless s eel capilla y ubing hea ed a 150 °C o analysis o a oid condensa ion. The con inuous da a acquisi ion o MS was eco ded using he Quade a so wa e, ollowing mainly he signals m/z= 2, 15, 18, 28 and 44. 3. Resul s and discussion 3.1. E ec o s eam in he biogas e o ming eac ion The e ec o di e en s eam concen a ions in he biogas e o ming eac ion o e Rh/MgAl 2 O 4 ca alys was pe o med unde iso he mal condi ions a 750 °C, eeding a CH 4 /CO 2 mola a io o 1.5. Fig. 1a shows ha he p esence o s eam p omo es he me hane con e sion bu has a con a y e ec on CO 2 con e sion. The gene al eac ion o bi- e o ming o me hane can be ep esen ed by a combina ion o he s eam e o ming and d y e o ming eac ions, as shown in he ollowing equa ions. D y e o ming o me hane: CH 4 +CO 2 →2CO + 2H 2 (1) S eam e o ming o me hane: CH 4 +2H 2 O→CO + 3H 2 (2) Bi- e o ming o me hane: 3CH 4 +CO 2 +2H 2 O→4CO + 8H 2 (3) Addi ionally, a a ie y o di e en pa allel eac ions also can occu , including he ollowing. 35 Wa e gas shi eac ion (WGS): CO + H 2 O↔CO 2 +H 2 (4) Me hane decomposi ion: CH 4 ↔C+2H 2 (5) Boudoua d eac ion: CO 2 +C↔2CO (6) Beggs eac ion: C + H 2 O↔H 2 + CO (7) CO 2 me hana ion: CO 2 +4H 2 →CH 4 +2H 2 O (8) CO me hana ion: CO + 3H 2 →CH 4 +H 2 O (9) The enhancemen o he CH 4 con e sion wi h he amoun o wa e in he eed is mainly due o he pa icipa ion o he s eam e o ming eac ion (eqn (2)), whe eas he lowe CO 2 con e sion is ela ed o he CO 2 p oduced by he WGS eac ion (eqn (4)). An inc ease in he concen a ion o wa e a ou s he WGS eac ion agains he d y e o ming o me hane, esul ing in a nega i e CO 2 balance, as p edic ed he by he modynamics. 15,21 Fu he mo e, Fig. 1b shows ha he H 2 /CO mola a io inc eases om 1.1 o 1.7 by inc easing he ol% o wa e , which is s ic ly ela ed o he p oduc ion o hyd ogen and he consump ion o CO by he WGS eac ion and he gasi ica ion o ca bonaceous deposi s (eqn (7)). In a collabo a ion wo k o ou g oup, S oud e al. 21 epo ed a simila H 2 /CO a io o 1.6 o a eed o CH 4 /CO 2 /H 2 O = 1/1/1 (33 ol% o H 2 O) a 700 °C on a bime allic Ni–Sn suppo ed ca alys . Mo eo e , i was demons a ed ha he in oduc ion o wa e in he bi- e o ming eac ion a ou s he gasi ica ion o ca bonaceous species deposi ed on he ca alys su ace. Roh e al. 36 ound an op imal CO 2 :H 2 O:CH 4 eed a io o ob ain he syngas wi h a H 2 /CO mola a io o 2 sui able o Fische –T opsch syn hesis. The op imal concen a ion o wa e epo ed by hese au ho s was 25 ol% o s eam o a CH 4 /CO 2 a io o 2.5. In ano he p e ious wo k, we ound Fig. 1 E ec o he s eam concen a ion on he pe o mance o Rh/ MgAl 2 O 4 in e ms o a) CO 2 and CH 4 con e sion, and b) H 2 /CO mola a io du ing he biogas e o ming eac ion es ed unde WHSV = 200 L h −1 g −1 ,T= 750 °C and CH 4 /CO 2 mola a io = 1.5. Ca alysis Science & TechnologyPape Open Access A icle. Published on 21 May 2024. Downloaded on 11/25/2024 3:33:14 PM. This a icle is licensed unde a C ea i e Commons A ibu ion 3.0 Unpo ed Licence. View A icle Online Ca al. Sci. Technol., 2024, 14,3514–3523 | 3517This jou nal is © The Royal Socie y o Chemis y 2024 ha an H 2 /CO a io close o 2 can be achie ed using a eed o 28 ol% o wa e and a CH 4 /CO 2 a io o 2.5 o e a Ni–Ru- based ca alys . In he cu en wo k, he eac ion was pe o med using a ypical biogas composi ion wi h a CH 4 / CO 2 a io o 1.5 and concen a ions o wa e below 20 ol%. Ne e heless, he endency obse ed o he H 2 /CO a io wi h he wa e concen a ion (Fig. 1b) sugges s ha wa e concen a ion o abou 8 o 18 ol% should esul in indus ially ele an H 2 /CO mola a ios. 3.2. Ope ando DRIFTS-MS s udies Ope ando DRIFTS-MS s udies we e pe o med o be e unde s and he in luence o s eam and he in e media e species o med du ing he d y e o ming and bi- e o ming o biogas eac ion o e Rh/MgAl 2 O 4 ca alys . A comp ehensi e ision o he eac ion mechanism p o ides a use ul guidance o design an op imal ca alys in e ms o he ac i i y, selec i i y, and s abili y. 3.2.1. In si u educ ion p e ea men . The sample was educed in 50%H 2 /A a 750 °C o 1 h o clean he ca alys su ace, ensu ing he comple e educ ion o Rh pa icles. Fig. 2 gi es an accoun o he e olu ion o he DRIFT spec a du ing he ac i a ion ea men as a unc ion o he empe a u e. In he ini ial s age a RT, he esh sample p esen s a cha ac e is ic band a 1647 cm −1 assigned o he de o ma ion mode o adso bed wa e on he su ace 37 and se e al bands in he 1600–1300 cm −1 spec al egion ha can be a ibu ed o esidual ca bona e-like species o med by he in e ac ion wi h he a mosphe ic CO 2 o he hyd oxyla ed MgAl 2 O 4 su ace. 38 Hea ing up he sample in he educ ion low, he physiso bed wa e disappea ed comple ely a empe a u es nea 140 °C and on inc easing he empe a u e abo e 450 °C, one band a 3720 cm −1 appea ed, which is asc ibable o “ ee”hyd oxyls, disclosing he comple e dehyd a ion o he su ace by he elimina ion o he b oad band cen ed a ca. 3500 cm −1 associa ed wi h he H-bonded hyd oxyls o weakly adso bed wa e . 39 The e olu ion o he bands assigned o ca bona e species du ing he p e ea men sugges s he di e en he mal s abili y and eac i i y owa ds hyd ogen. I is no ewo hy ha as he empe a u e inc eases abo e 200 °C, an al e a ion o he ca bona e ea u es is obse ed, and new bands appea ed a 2010 and 1915 cm −1 , which can be assigned o Rh-ca bonyl species wi h di e en coo dina ion geome ies, espec i ely. 40 I is well known ha he he mal decomposi ion and/o educ ion o ca bona e species akes place in he p esence o hyd ogen o e noble me als, leading o he o ma ion o Rh-ca bonyl in e media e species and CO gas a empe a u es o 200–300 °C. 41 Because Rh is an op imal ac i e me al o he me hana ion eac ion, he dissocia ion o CO on Rh pa icles o o m “C”species and hei subsequen hyd ogena ion o o m me hane gas should no be uled ou . 42 In his sense, he disappea ance o Rh- ca bonyl species as he empe a u e is inc eased abo e 300 °C sugges s he possible me hana ion o adso bed CO. Ne e heless, he low concen a ion o me hane o med is oo low o be de ec ed using an IR spec ome e . When he empe a u e eached 750 °C, all ca bona e species we e emo ed, indica ing ha he p e ea men is e ec i e o ob ain a clean su ace. We can conclude ha appa en ly he ca alys su ace only con ains “ ee”hyd oxyls and Rh me allic pa icles highly dispe sed a e educ ion p e ea men o 1 h a 750 °C. 3.2.2. S eady-s a e d y and bi- e o ming o biogas. The in luence o s eam was in es iga ed by ope ando DRIFTS-MS on he Rh-based ca alys unde eac ion condi ions. Fo his pu pose, 10 ol% s eam was selec ed and compa ed wi h he Fig. 2 E olu ion o he DRIFT spec a wi h he empe a u e in he 4000–3100 and 2100–1300 cm −1 egions du ing he educ ion p e ea men a 750 °C o 1 h in 50%H 2 /A o e he Rh ca alys . Ca alysis Science & Technology Pape Open Access A icle. Published on 21 May 2024. Downloaded on 11/25/2024 3:33:14 PM. This a icle is licensed unde a C ea i e Commons A ibu ion 3.0 Unpo ed Licence. View A icle Online 3518 |Ca al. Sci. Technol.,2024,14,3514–3523 This jou nal is © The Royal Socie y o Chemis y 2024 d y condi ions. Fig. 3 shows he e olu ion o he DRIFT spec a wi h empe a u e in he absence (Fig. 3a) and p esence (Fig. 3b) o wa e in he eac ion mix u e. As shown Fig. 3a, a e in oducing he eac ing gas mix u e o CO 2 and CH 4 ee o wa e a 550 °C, a p ominen peak a 2360 cm −1 associa ed wi h he asymme ic s e ching (ν as )o CO 2 gas and wo bands a 3016 and 1303 cm −1 associa ed wi h he o a ional ine s uc u e o me hane gas appea . 43 The bands appea ing a 2030–2008 cm −1 and 1920– 1860 cm −1 indica e he o ma ion o linea ly adso bed CO and b idged CO species on Rh si es, sugges ing ha CO 2 is dissocia ed in o CO and “O”species. 40 Wi h inc easing empe a u e, he appea ance o he cha ac e is ic Pb anch be ween 2140 and 2200 cm −1 o he gaseous CO ib a ional- o a ional spec um is mo e app eciable, which indica es he elease o CO gas. 43 Addi ionally, wo complex pai o bands cen ed a 1595 and 1360 cm −1 we e iden i ied a lowe empe a u es. As he empe a u e inc eases, hese wo bands e ol e oge he , gi ing a se o bands a 1630, 1535, 1377 and 1330 cm −1 a 750 °C. These la e bands a e associa ed wi h he p esence o esidual ca bonaceous species adso bed on he suppo su ace. 44,45 Rega ding he assignmen o he bands a 1595 and 1360 cm −1 , some au ho s ha e assigned he band a 1595 cm −1 o he ν as (OCO) ib a ional mode o he o ma e species. Ne e heless, he wo cha ac e is ic bands o he o ma e species a 1395 and 1375 cm −1 asc ibed o CH de o ma ion and ν s (OCO), espec i ely, we e absen in ou expe imen al da a (Fig. 3). Addi ionally, a band a 2900– 2800 cm −1 asc ibed o he ν(C–H) s e ching o o ma e should appea in he spec a. Al hough his egion is o e lapped wi h he o a ional ine s uc u e o gaseous me hane, he band ela ed o he C–H s e ching o o ma e is ha dly obse able. The e o e, we canno a ibu e hese bands o o ma e species. I is mo e easonable o assume ha he bands a 1595–1360 cm −1 a e a ibu able o monoden a e ca bona es wi h Δν 3 o 100–200 cm −1 o med on he suppo 's basic si es. 46,47 These species a e he mally labile ca bona es, which a e decomposed o educed as he empe a u e inc eases. In he p esence o wa e (Fig. 3b), he bands associa ed wi h gaseous CO 2 and CH 4 we e also obse ed, al hough impo an di e ences we e no iced in he adso bed species. In he 1700–1200 cm −1 spec al egion, i is wo h no icing ha he bands a 1595 and 1360 cm −1 anished apidly wi h he empe a u e, whe eas he bands a 1630, 1535, 1377 and 1330 cm −1 we e ha dly obse ed. This clea ly e eals ha s eam a ou s he gasi ica ion/decomposi ion o ca bonaceous species. On he o he hand, he p esence o CO ca bonyl adso bed species o e Rh 0 si es cha ac e ized by he bands a 2040–2028 cm −1 and 1920–1860 cm −1 , assigned o CO adso bed linea ly and mul i-bonded ca bonyl species, espec i ely, was also obse ed. 40,48 Thus, CO 2 dissocia ion also akes place unde bi- e o ming condi ions. Ne e heless, he in a ed spec a associa ed wi h he ca bonyl species we e clea ly a ec ed by he eac ion a mosphe e. The di e ences in he adso bed CO species can p o ide in o ma ion abou he species and si es esponsible o he ac i a ion o CO 2 and CH 4 in he p esence and absence o s eam. In o de o assess he in luence o he s eam on he CO adso bed species on Rh in de ail, Fig. 4 ocuses on he di e en e olu ions no iced on inc easing he eac ion empe a u e o he 2100–1800 cm −1 egion. I can be no iced ha he inc ease in he empe a u e om 450 °C o 750 °C caused a sligh dec ease in he in ensi y o he CO adso p ion peaks associa ed wi h linea ly adso bed CO wi h a displacemen o lowe wa enumbe s om 2030 o 2008 cm −1 in he absence o wa e (Fig. 4a), while in p esence o s eam, he equi alen bands we e p og essi ely emo ed wi h he empe a u e and displaced om 2048 o 2028 cm −1 (Fig. 4b). The ib a ional equency depends on he CO co e age, and he highe equency obse ed a high CO co e ages is due o he dipole–dipole coupling be ween he CO adso bed molecules on neighbou ing si es. In he p esence o s eam, dipole–dipole coupling accoun s o he edshi obse ed o he ca bonyl bands due o he lowe CO co e age on Rh pa icles when he empe a u e is inc eased. Ne e heless, Fig. 3 E olu ion o DRIFT spec a unde he eac ion condi ions as a unc ion o empe a u e on he educed sample. Spec a we e eco ded in he eac ion mix u e wi h CH 4 /CO 2 = 1.5 in he absence o wa e (a) and he p esence o 10 ol% o wa e (b). Ca alysis Science & TechnologyPape Open Access A icle. Published on 21 May 2024. Downloaded on 11/25/2024 3:33:14 PM. This a icle is licensed unde a C ea i e Commons A ibu ion 3.0 Unpo ed Licence. View A icle Online Ca al. Sci. Technol., 2024, 14,3514–3523 | 3519This jou nal is © The Royal Socie y o Chemis y 2024 he displacemen o ca bonyl equencies obse ed unde d y condi ions (Fig. 4a) canno be ela ed o dipole–dipole coupling and CO co e age phenomena. Simul aneously, he band a 2030 cm −1 shi s o 2008 cm −1 and p ese es i s in ensi y, meaning ha he elec onic densi y on he hodium si es inc eases unde hese condi ions. Acco dingly, he o ma ion o ca bon species on he su ace mus be expec ed, sugges ing ha CO adso bs on Rh si es close o ca bonaceous deposi s ha inc ease he back-dona ion on he me al su ace. 38,49–51 This obse a ion is cohe en conside ing ha he absence o wa e led o he co e ing o Rh pa icles by ca bonaceous deposi s, whe eas he wa e pa icipa es in he ca bon gasi ica ion, cleaning he Rh su ace si es. On he o he side, i is also no iced om Fig. 5a and b ha no shi o he bands a ibu ed o b idge-bonded CO on Rh (1860 and 1920 cm −1 ) was obse ed wi h he inc ease in he eac ion empe a u e. The band a 1860 cm −1 has been asc ibed o 1:2 CO–Rh species while he peak a 1920 cm −1 is ela ed o a 3:2 s oichiome y. 52 These s oichiome ies depend on he ype o he hodium en i onmen and clea ly e eal ha he p esence o s eam is a ec ing he coo dina ion o Rh si es. Acco ding o La alley e al., 53 hese wo bands a e ce ainly ela ed o CO adso p ion on wo hodium aces such as Rh (100) and Rh (111). The mino p esence o he band a 1920 cm −1 unde d y condi ions may be en a i ely explained by he blocking o b idged adso p ion si es on he co esponding hodium ace. Wang e al. 54 s udied CH 4 dehyd ogena ion on Rh (111), Rh (110) and Rh (100) su aces using densi y unc ional heo y (DFT) calcula ions. They ound ha CH 4 dehyd ogena ion on he Rh (100) su ace is mo e a ou able in compa ison wi h ha on Rh (111) and Rh (110) su aces. On his basis, ca bonaceous deposi s o med in he absence o s eam could p e e en ially supp ess CO-b idged adso p ion on Rh (100) su ace si es. Rega ding he e olu ion o he gas-phase analysed on-line by MS in he absence and p esence o s eam (Fig. 5a and b), subs an ial o ma ion o H 2 and CO was obse ed a empe a u es abo e 450 °C du ing he d y e o ming and he bi- e o ming o biogas eac ion on he Rh ca alys . As he empe a u e inc eased, he con e sion o me hane is highe and he signal o H 2 (m/z= 2) is inc eased. In he absence o Fig. 4 E olu ion o he DRIFT spec a in he 2100–1800 cm −1 egion wi h he empe a u e o he educed sample unde he eac ion condi ions in he absence o wa e (a) and p esence o wa e (b). Fig. 5 E olu ion o he gas-phase analysed on-line by MS as a unc ion o ime-on-s eam and empe a u e in he absence (a) and p esence (b) o s eam: m/z=2(H 2 ), m/z= 15 (CH 4 ), m/z=18(H 2 O), m/z= 28 (CO) and m/z= 44 (CO 2 ). Ca alysis Science & Technology Pape Open Access A icle. Published on 21 May 2024. Downloaded on 11/25/2024 3:33:14 PM. This a icle is licensed unde a C ea i e Commons A ibu ion 3.0 Unpo ed Licence. View A icle Online 3520 |Ca al. Sci. Technol., 2024, 14,3514–3523 This jou nal is © The Royal Socie y o Chemis y 2024 wa e a 750 °C, he signi ican dec ease o me hane con e sion and H 2 concen a ion was associa ed wi h ca alys deac i a ion due o he o ma ion o ca bon deposi s o CO species s ongly bonded o Rh pa icles. In ag eemen wi h he ca aly ic ac i i y esul s, ope a ing in he p esence o wa e esul ed in highe H 2 concen a ion and H 2 /CO a ios close o 2. Rema kably, no signs o ca alys deac i a ion we e no iced in he p esence o wa e , and ai ly s able hyd ogen p oduc ion o he en i e empe a u e window was ob ained. 3.2.3. DRIFTS-MS ansien s udies. The kine ic esponses o he su ace species analysed unde ansien condi ions a e di e en om hose measu ed om s eady-s a e expe imen s and p o ide di ec in o ma ion abou he na u e and co e age o su ace in e media es. The ansien expe imen s we e ca ied ou using di e en combina ions o eac an s o analyse he e ec o s eam. Fi s ly, we examined he CO 2 ac i a ion om gas s eams con aining 10%CO 2 /A , and hen we moni o ed he e olu ion o he in e media es o med on pu ging wi h A and subsequen ly by swi ching a gas s eam con aining 15%CH 4 /A . In o de o s udy he impac o s eam, we pe o med an analogous expe imen in which he eed composi ion was empo a ily a ied ollowing he sequence 1) 10%CO 2 /10%H 2 O/A →2) 10%H 2 O/A →3) 15%CH 4 /10%H 2 O/ A . In bo h s udies, each s eam mix u e was main ained o 20 min unde iso he mal condi ions a 550 °C. Fig. 6a and b display he e olu ion o he IR spec a sequen ially eco ded by swi ching he h ee s eam e luen s e e y 20 min in d y and we condi ions, espec i ely. As can be no iced, unde CO 2 s eam, simila ea u es ela ed o ca bonyl adso bed species and ca bona es we e obse ed independen ly o he p esence o wa e . The p esence o bo h linea (2020–2000 cm −1 ) and b idged (1920–1860 cm −1 ) Fig. 6 E olu ion o he ansien DRIFT spec a obse ed a 550 °C unde he sequen ial swi ching o he eed gas in he absence and p esence o s eam: a) 10%CO 2 /A →A →15%CH 4 /A , and b) 10%CO 2 /10%H 2 O/A →10%H 2 O/A →15%CH 4 /10%H 2 O/A . Ca alysis Science & TechnologyPape Open Access A icle. Published on 21 May 2024. Downloaded on 11/25/2024 3:33:14 PM. This a icle is licensed unde a C ea i e Commons A ibu ion 3.0 Unpo ed Licence. View A icle Online Ca al. Sci. Technol., 2024, 14,3514–3523 | 3521This jou nal is © The Royal Socie y o Chemis y 2024 ca bonyls unde bo h s eam condi ions indica es ha CO 2 is dissocia ed on he me allic si es and ha wa e plays no a signi ican ole in he di ec dissocia ion o CO 2 . Upon swi ching he CO 2 s eam o a pu ge s eam, signi ican di e ences we e obse ed in he absence and p esence o wa e . As shown in Fig. 6a, he IR bands a ibu ed o he ca bona e species disappea ed almos comple ely, whe eas he in ensi ies o he bands a ibu ed o he adso bed ca bonyl species dec eased only pa ially du ing he A pu ging s ep. Meanwhile, Fig. 6b e eals ha in he p esence o s eam, he me allic ca bonyls and ca bona es disappea ed e y as . Mo eo e , i is ema kable o no ice he nega i e e olu ion o ca bonyls and he simul aneous eme gence o wo bands a 1690 and 1420 cm −1 . These wo bands a e assigned o CO s e ching and CH bending in he CH x O species. 55–57 These indings clea ly e eal ha CH x O species a e eac ion in e media es a o ed in he p esence o s eam. We can assume ha he gene a ion o CH x O species occu s h ough he eac ion o CO*wi h he ac i e *OH species o med likely a he me al suppo in e ace. Subsequen ly, by swi ching he gas eed om pu ge o CH 4 s eam, he impac o s eam is e en mo e e iden . F om Fig. 6a, i is e iden ha he bands ela ed o ca bonyls and ca bona es comple ely disappea whe eas esidual bands a 1580 and 1345 cm −1 a e de eloped. These ea u es a e asc ibed o ca bonaceous species. 58,59 The obse ed nega i e band o ca bonyl could be ela ed o he ans o ma ion o linea ly adso bed CO species in o –COH adso bed species. The e o e, hese obse a ions clea ly e eal ha he ca alys 's su ace is a co e ing o accumula ed ca bon o med by me hane dissocia ion in he absence o s eam. In con as , one can see in Fig. 6b ha wi h he inco po a ion o CH 4 /H 2 O/A s eam, CO 2 and CO gases we e eleased. Fu he mo e, bands asc ibed o ca bonyl species adso bed o e Rh me al si es and bands a 1620, 1595, 1360 and 1330 cm −1 asc ibed o ca bona es wi h di e en coo dina ion geome y we e also de ec ed. The o ma ion o CO and CO 2 indica e ha gasi ica ion and WGS eac ions a e happening on he ca alys su ace. Finally, Fig. 7a and b display he e olu ion o gaseous p oduc s moni o ed by on-line mass spec ome y (MS). Ion cu en s we e measu ed o he m/z alues o he H 2 O, CO 2 CH 4 ,H 2 and CO and H 2 O. Fi s ly, i mus be no iced ha a la ge amoun and s able p oduc ion o hyd ogen is achie ed in he CH 4 /H 2 O/A s eam, whe eas in he absence o s eam, he p oduc ion o hyd ogen decays apidly due o he accumula ion o ca bonaceous deposi s, in line wi h he abo e discussion. 3.3. Mechanis ic insigh s The obse ed indings enable a ionalizing he impac o s eam on he eac ion pa hway p oposed o he Rh/MgAl 2 O 4 ca alys in he absence and p esence o wa e (Fig. 8a and b). Unde bo h eac ion condi ions, CO 2 and CH 4 we e dissocia i ely adso bed o e he me al su ace. The esul ing CH x species eac , ei he o e he me al su ace o a he me al suppo in e ace, wi h close ac i e O*a oms, p oducing CH x O species, which decompose o CO and H 2 . S ill, ema kable di e ences conce ning he ype o su ace species de eloped on he su ace and Rh–CO in e ac ions we e obse ed. In ou p e ious wo k, we ound ha he dissocia i e adso p ion o CO 2 o e Rh me al su aces was associa ed wi h he pa ial oxida ion and e-dispe sion o Rh a oms. 31 The lowe -coo dina ed Rh clus e we e p oposed o p omo e he gene a ion o s able b idged ca bonyls h ough he decomposi ion o uns able geminal ca bonyls. In ac , i is gene ally accep ed ha he na u e and geome y o he ac i e me al de e mines he dissocia ion o CH 4 and he o e all ca alys pe o mance. He ein, we obse ed ha b idged ca bonyl bands we e de eloped o e wo di e en Rh Fig. 7 E olu ion o he gas-phase analysed on-line by MS as a unc ion o ime-on-s eam unde he sequen ial swi ching o he eed gas in he absence a) and p esence o s eam b). Ca alysis Science & Technology Pape Open Access A icle. Published on 21 May 2024. Downloaded on 11/25/2024 3:33:14 PM. This a icle is licensed unde a C ea i e Commons A ibu ion 3.0 Unpo ed Licence. View A icle Online 3522 |Ca al. Sci. Technol., 2024, 14,3514–3523 This jou nal is © The Royal Socie y o Chemis y 2024 su ace adso p ion si es. In his sense, he disappea ance o s able b idged ca bonyl migh unde mine he ca alys pe o mance by blocking he Rh ac i e si es h ough he gene a ion o ca bon species ia Boudoua d's and me hane c acking eac ions. The ca alys s' deac i a ion de i ed om he coke deposi s gene a ed o e he Rh su ace in he absence o wa e ag ees wi h he hyd ogen d op obse ed in he MS analysis a highe empe a u es (Fig. 5a). In he p esence o s eam, he absence o ca bonaceous deposi s obse ed on he ca alys s su ace and he g ea e CH 4 con e sion a es sugges ha wa e molecules acili a e he decomposi ion o eac ion in e media es in o H 2 and CO. F om he ope ando DRIFT-MS esul s, he hyd oxyla ed su ace could imp o e he eac ion o he CO*species wi h he nea by OH species, gene a ing CH x O species ha subsequen ly decomposes in o he eac ion p oduc s. Besides, he p esence o wa e should also p omo e he o wa d WGS eac ion, emo ing CO species om he me al su ace, he eby hinde ing coke gene a ion ia he Boudoua d eac ion. Rema kably, he inco po a ion o wa e molecules also cons ic ed he blocking o Rh ac i e si es. In his sense, s eam could p omo e he exposi ion o Rh su ace ac i e o me hane decomposi ion o some ex en and inhibi he accumula ion o s able ca bonaceous species o e highly dispe sed Rh si es. In all, he inco po a ion o wa e molecules enhances he pe o mance o he Rh/MgAl 2 O 4 ca alys by p omo ing he gene a ion o CH x O species, enhancing he decomposi ion o ca bonaceous species and inhibi ing he accumula ion o ca bon deposi s o e he ac i e Rh me al su aces. 4. Concluding ema ks This wo k analyses he bene i s a ained by inco po a ing wa e in o DRM s eam o e he Rh/MgAl 2 O 4 ca alys by combining s eady-s a e and ansien ope ando DRIFT coupled wi h MS spec ome y. The ca aly ic ac i i y esul s e idenced he gene a ion o syngas wi h highe H 2 /CO a ios because o he p omo ion o CH 4 con e sion and lowe CO 2 con e sions due o he impac o he o wa d WGS eac ion. The inco po a ion o wa e a ec ed he cons i u ed Rh nanopa icles and eac ion in e media es bo h in na u e and concen a ion. Hence, he hyd oxyla ed su ace a ained in he p esence o wa e enhanced he eac i i y o CH x species wi h he ac i e O*species, he eby enhancing he gene a ion o CH x O species, he p oposed majo eac ion in e media e owa ds H 2 p oduc ion. Mo eo e , he p esence o s eam p omo ed he decomposi ion o ca bonaceous species and cons ic ed he coking phenomena o e he me al su ace, hus enhancing he ca alys li e ime. Con lic s o in e es The e a e no con lic s o decla e. Acknowledgemen s D a. Mi iam González acknowledges o he EMERGIA p og am o he Jun a de Andalucía by he EMC21_00052 con ac . The au ho s acknowledge inancial suppo om Spanish Minis y o Science h ough he p ojec s NICER- BIOFUELS (PLEC2021-008086) inanced by Nex Gene a ion Eu opean Funds and SMART-FTS (PID2021-126876OB-I00). Re e ences 1P.Ve maandS.K.Saman a,inP oceedings o he Fi s In e na ional Con e ence on Recen Ad ances in Bioene gy Resea ch, 2016, pp. 227–243. 2 M. González-Cas año, M. H. Kou , J. González-A ias, F. M. Baena-Mo eno and H. A ellano-Ga cia, J. En i on. Manage., 2021, 300,1–8. 3 L. Yang, X. Ge, C. Wan, F. Yu and Y. Li, Renewable Sus ainable Ene gy Re ., 2014, 40, 1133–1152. 4 M. Taba abaei, M. Aghbashlo, E. Valijanian, H. Kazemi Sha ia Panahi, A.-S. Nizami, H. Ghana a i, A. Sulaiman, S. Mi mohamadsadeghi and K. Ka imi, Renewable Ene gy, 2020, 146, 1392–1407. 5 P. Fe ei a-Apa icio, M. J. Beni o and J. L. Sanz, Ca al. Re .: Sci. Eng., 2005, 47, 491–588. 6 S. Rasi, J. Leh inen and J. Rin ala, Renewable Ene gy, 2010, 35, 2666–2673. 7 M. Gus a sson and N. S ensson, J. Cleane P od., 2021, 278, 123535. 8 P. S. Roy, J. Song, K. Kim, C. S. Pa k and A. S. K. Raju, J. CO2 U il., 2018, 25, 275–282. 9 I. Ullah Khan, M. Ha iz Dza an O hman, H. Hashim, T. Ma suu a, A. F. Ismail, M. Rezaei-Dash A zhandi and I. Wan Azelee, Ene gy Con e s. Manage., 2017, 150, 277–294. 10 P. Ta i a, T. R. Reina, M. González-Cas año and H. A ellano- Ga cía, Ene gy Fuels, 2022, 36(15), 8267–8273. 11 S. Sa ke , J. J. Lamb, D. R. Hjelme and K. M. Lien, Fuel, 2018, 226, 686–697. 12 J. He, N. Han, M. Xia, T. Sun and H. Ghaebi, In . J. Hyd ogen Ene gy, 2023, 48, 21161–21175. 13 H. Zhang, Z. Sun and Y. H. Hu, Renewable Sus ainable Ene gy Re ., 2021, 149, 111330. 14 A. M. Ál a ez, M. Á. Cen eno and J. A. Od iozola, Top. Ca al., 2016, 59, 303–313. Fig. 8 P oposed eac ion pa hways in he absence a) and p esence o s eam b). Ca alysis Science & TechnologyPape Open Access A icle. Published on 21 May 2024. Downloaded on 11/25/2024 3:33:14 PM. This a icle is licensed unde a C ea i e Commons A ibu ion 3.0 Unpo ed Licence. View A icle Online