Are Rh Catalysts a Suitable Choice for Bio-Oil Reforming? The Case of a Commercial Rh Catalyst in the Combined H2O and CO2 Reforming of Bio-Oil

Ci a ion: Valecillos, J.; Landa, L.; Elo di, G.; Remi o, A.; Bilbao, J.; Gayubo, A.G. A e Rh Ca alys s a Sui able Choice o Bio-Oil Re o ming? The Case o a Comme cial Rh Ca alys in he Combined H2O and CO2Re o ming o Bio-Oil. Ca alys s 2024,14, 571. h ps://doi.o g/10.3390/ ca al14090571 Academic Edi o s: Geo gios Bampos, Pa aske i Panagio opoulou and Eleni A. Ky iakidou Recei ed: 30 July 2024 Re ised: 22 Augus 2024 Accep ed: 26 Augus 2024 Published: 29 Augus 2024 Copy igh : © 2024 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). ca alys s A icle A e Rh Ca alys s a Sui able Choice o Bio-Oil Re o ming? The Case o a Comme cial Rh Ca alys in he Combined H2O and CO2Re o ming o Bio-Oil JoséValecillos * , Lei e Landa, Go ka Elo di , Ainge u Remi o , Ja ie Bilbao and Ana Guadalupe Gayubo * Depa men o Chemical Enginee ing, Uni e si y o he Basque Coun y (UPV/EHU), P.O. Box 644, 48080 Bilbao, Spain; [email p o ec ed] (L.L.); go ka.elo [email p o ec ed] (G.E.); ainge u. [email p o ec ed] (A.R.); ja ie [email p o ec ed] (J.B.) *Co espondence: [email p o ec ed] (J.V.); [email p o ec ed] (A.G.G.) Abs ac : Bio-oil combined s eam/d y e o ming (CSDR) wi h H 2 O and CO 2 as eac an s is an a ac i e ou e o he join alo iza ion o CO 2 and biomass owa ds he sus ainable p oduc ion o syngas (H 2 + CO). The echnological de elopmen o he p ocess equi es he use o an ac i e and s able ca alys , bu also special a en ion should be paid o i s egene a ion capaci y due o he una oidable and qui e apid ca alys deac i a ion in he e o ming o bio-oil. In his wo k, a comme cial Rh/ZDC (zi conium-doped ce ia) ca alys was es ed o eac ion– egene a ion cycles in he bio-oil CSDR in a luidized bed eac o , which is bene icial o a aining an iso he mal ope a ion and, mo eo e , minimizes ca alys deac i a ion by coke deposi ion compa ed o a ixed-bed eac o . The esh, spen , and egene a ed ca alys s we e cha ac e ized using ei he N 2 physiso p ion, H 2 -TPR, TPO, SEM, TEM, o XRD. The Rh/ZDC ca alys is ini ially highly ac i e o he syngas p oduc ion (yield o 77% and H 2 /CO a io o 1.2) and o alo izing CO 2 (con e sion o 22%) a 700 ◦ C, wi h space ime o 0.125 g ca alys h (g oxygena es ) −1 and CO 2 /H 2 O/C a io o 0.6/0.5/1. The ca alys ac i i y e ol es in di e en pe iods ha e idence a selec i e deac i a ion o he ca alys o he e o ming eac ions o he di e en compounds, wi h he CH 4 e o ming eac ions (wi h bo h s eam and CO 2 ) being mo e apidly a ec ed by ca alys deac i a ion han he e o ming o hyd oca bons o oxygena es. A e egene a ion, he ca alys ’s ex u al p ope ies a e no comple ely es o ed and he e is a change in he Rh–suppo in e ac ion ha i e e sibly deac i a es he ca alys o he CH 4 e o ming eac ions (bo h SR and DR). As a esul , he coke o med o e he egene a ed ca alys is di e en om ha o e he esh ca alys , being an amo phous mass (o p obably u bos ac ic na u e) ha encapsula es he ca alys and causes apid deac i a ion. Keywo ds: bio-oil; s eam e o ming; d y e o ming; syngas; coke deac i a ion; egene a ion; i e- e sible deac i a ion 1. In oduc ion Syngas, a blend o H 2 and CO, is a basic (pe o)chemical pla o m o he syn heses o alcohols (mainly me hanol), e he s, ca boxylic acids, o he a ious ca bonyl compounds, syn he ic uels, ammonia, and u ea, and is also a uel employed in gas engines o ene gy gene a ion. I s p oduc ion is s ill highly dependen on ossil esou ces, mos ly by e o ming o na u al gas and pe oleum de i a i es, wi h a signi ican con ibu ion o he global CO 2 emissions. Hence, sus ainable p oduc ion op ions a e u ged o he ansi ion owa ds chemical and ene gy indus ies ha a e comple ely clean and enewable. Among many al e na i es, he e o ming o biomass and i s de i a i es eplacing he adi ional ossil- based eeds ock is an a ac i e op ion as i mee s he goal o ne ze o CO 2 emissions [ 1 ]. I s easibili y o a p omp implemen a ion akes ad an age o s a ing om an exis ing e o ming echnology wi h he challenge o making imp o emen s o minimize cos s and en i onmen al impac s. Ca alys s 2024,14, 571. h ps://doi.o g/10.3390/ca al14090571 h ps://www.mdpi.com/jou nal/ca alys s Ca alys s 2024,14, 571 2 o 18 One p omising ou e is he use o lignocellulose biomass was es, which does no in e e e wi h ood chains and can be p ocessed by as py olysis in simple and decen- alized acili ies yielding la ge quan i ies o bio-oil (a complex mix u e o oxygena es comp ising ca boxylic acids, aldehydes, alcohols, ke ones, es e s, u u als, phenols, and saccha ides [ 2 , 3 ]). The subsequen bio-oil e o ming may be a ge ed a he p oduc ion o syngas wi h sui able H 2 /CO a ios o he syn heses o uels o chemicals, depending on he e o ming s a egy [ 4 , 5 ]. The CO 2 e o ming, commonly known as d y e o m- ing (DR), has been p oposed as an a ac i e al e na i e e o ming s a egy ha allows he p oduc ion o use ul syngas om bio-oil oxygena es (C n H m O k ) while alo izing CO 2 simul aneously (Equa ion (1)), a oiding he excessi e side CO 2 p oduc ion when using he s eam e o ming (SR) s a egy [ 4 ]. The e e se wa e gas shi ( -WGS) eac ion also con ibu es o he CO 2 con e sion a high empe a u es ( e e se Equa ion (2)). Ne e he- less, he inhe en H 2 O con en in he bio-oil, which a ies depending on i s o igin, also p omo es he SR o oxygena es (Equa ions (3) and (4)), and hus a combined s eam/d y e o ming (CSDR) akes place. Mo eo e , in he con e sion o bio-oil, he decomposi- ion/c acking o oxygena es in o H 2 , CO, CO 2 , CH 4 , hyd oca bons (C a H b ), o he oxy- gena es (C x H y O z ), and ca bon (coke) should be conside ed, as ep esen ed by Equa ion (5). The e o e, he con e sion o CH 4 and hyd oca bons by DR (Equa ions (6) and (7), espec- i ely) and SR ( Equa ions (8) and (9) , espec i ely) also con ibu es o he o e all kine ic scheme. Likewise, coke o ma ion may be a o ed by CH 4 decomposi ion (Equa ion (10)), hyd oca bon decomposi ion ( Equa ion (11) ), and he CO disp opo iona ion (Boudoua d) eac ion ( Equa ion (12) ), whe eas i s gasi ica ion may occu wi h s eam (Equa ion (13)) o CO 2 ( e e se Equa ion (12)). Consequen ly, he co- eeding o CO 2 is expec ed o a o coke emo al. CnHmOk+ xCO2→(n + x)CO + (m/2 −(x + k −n))H2+(x+k−n)H2O (1) CO + H2O↔CO2+ H2(2) CnHmOk+ (n −k)H2O→nCO + (n + m/2 −k)H2(3) CnHmOk+ (2n −k)H2O→nCO2+ (2n + m/2 −k)H2(4) CnHmOk→CxHyOz+ (CO, CO2, CH4, CaHb, H2) + C(coke) (5) CH4+ CO2↔2CO + 2H2(6) CaHb+ aCO2↔(2a)CO + (b/2)H2(7) CH4+ H2O↔CO + H2(8) CaHb+ aH2O↔aCO + (a + b/2)H2(9) CH4→2H2+ C (10) CaHb→(b/2)H2+ aC (11) 2CO ↔C + CO2(12) C+H2O→CO + H2(13) The ca alys s o bio-oil e o ming a e commonly based on non-noble o noble me als, o bime allic composi ions [ 6 – 11 ]. Ne e heless, he CSDR o eal bio-oil has been sca cely s udied expe imen ally on Ni ca alys s (non-noble me al ca alys s) [ 12 – 14 ], showing p omis- ing esul s, and no s udies wi h a noble me al ca alys ha e been epo ed so a . Howe e , i could be hypo hesized ha he use o noble me al ca alys s, such as Rh ca alys s, may imp o e he pe o mance o Ni ca alys s g ounded in p e ious s udies epo ing a ema k- able ac i i y o he H 2 /syngas p oduc ion om eal bio-oil by con en ional SR [ 15 , 16 ], oxida i e SR (OSR) [ 17 – 19 ], and sequen ial c acking [ 20 , 21 ]. Likewise, Rh ca alys s ha e been success ully used in he DR o CH 4 [ 22 – 24 ] and e hanol [ 25 ], whose esul s may be ex apola ed o he case o bio-oil, in pa icula due o he ole o CH 4 as a eac ion in e - Ca alys s 2024,14, 571 3 o 18 media e in bio-oil e o ming. The ad an age o noble me al ca alys s elies on hei high dehyd ogena ion and oxida ion capaci ies, which a e ansla ed in o high yields o H 2 and CO/CO 2 wi hou CH 4 p oduc ion (CH 4 is con e ed) and low coke o ma ion, p e en ing ca alys deac i a ion. Addi ionally, Rh si es may also adso b and dissocia e CO 2 , which is a pa amoun s ep in DR eac ions [ 26 ]. Likewise, ca alys suppo may play a ole in he ca alysis by p o iding acidic o basic si es, hyd ophilici y, o oxygen mobili y [ 22 ]. Fo example, he use o CeO 2 , TiO 2 , and Z O 2 p o ides he la e ea u es enhancing H 2 O o CO 2 adso p ion and dissocia ion gene a ing OH o CO species acili a ing hei con e sions, which would globally enhance he pe o mance o he bio-oil CSDR. Ka a o a e al. [ 27 ] highligh he impo ance o he suppo on he su ace dis ibu ion and c ys al size o Rh in he ca alys s used in he cyclohexane ing opening eac ion. CeO 2 is an in e es ing suppo because o i s oxygen s o age and mobili y capaci y due o he as Ce 4+ /Ce 3+ edox cycling. I s limi ed he mal s abili y is imp o ed by he o ma ion o he Ce x Z 1−x O 2 solid solu ion, wi h excellen pe o mance as a suppo o ch omium oxide in he dehyd ogena ion o p opane wi h CO2[28]. In spi e o he high ac i i y o Rh ca alys s, e e sible and i e e sible deac i a ion has been obse ed in he OSR and SR o bio-oil [ 19 , 29 , 30 ]. The e e sible deac i a ion is associa ed wi h he o ma ion and deposi ion o coke encapsula ing he ac i e si es, whe eas he i e e sible deac i a ion has been ela ed o i e e sible s uc u al changes in he ca alys due o he ha sh eac ion condi ions (high empe a u e and p esence o s eam). These changes mainly in ol e he aging o he suppo pa ially occluding Rh si es and possible changes in he Rh s uc u e. Likewise, Rh sin e ing is a cause o deac i a ion a 750 ◦ C in he OSR bio-oil [ 19 ]. In o he ca aly ic sys ems o di e en applica ions, i has been ound ha Rh ca alys s a e i e e sibly deac i a ed by he pa ial encapsula ion o Rh nanopa icles by CeO 2 [ 31 , 32 ], which has also been epo ed o a ious noble me als [ 33 ]. In he case o e o ming s a egies, i is belie ed ha high s eam concen a ions may a o his phenomenon o i e e sible deac i a ion o Rh ca alys s, which ep esen s a d awback o using hese ca alys s. In his wo k, we ha e s udied he pe o mance o a comme cial ca alys made o Rh suppo ed on zi conium-doped ce ia (Rh/ZDC) o he CSDR o a eal bio-oil a ge ed a syngas p oduc ion, wi h ocus on i s ac i i y, deac i a ion, and egene a ion capaci y. I is expec ed ha he low s eam concen a ion in he CSDR s a egy lessens he phenomenon o i e e sible deac i a ion. As p e iously men ioned, he no el y o his wo k is s essed by he use o a noble me al ca alys o his p ocess wi h a eed o eal bio-oil, wi h a o mula ion ha is expec ed o enhance he CSDR pe o mance. The bio-oil CSDR es s we e ca ied ou in a luidized bed eac o a 700 ◦ C, comp ising a eac ion/ egene a ion cycle. To in es iga e he causes o deac i a ion, he esh, deac i a ed, and egene a ed ca alys samples ha e been analyzed by means o empe a u e-p og ammed educ ion (H 2 -TPR), N 2 physiso p ion, empe a u e-p og ammed oxida ion (TPO), X- ay di ac ion (XRD), scanning elec on mic oscopy (SEM), and ansmission elec on mic oscopy (TEM). The discussion o he esul s is aimed a unde s anding he e e sibili y and i e e sibili y o ca alys deac i a ion. 2. Resul s The esul s a e di ided in h ee main subsec ions. The i s one summa izes he indings o he pe o mance o he Rh/ZDC ca alys in he CSDR o bio-oil a 700 ◦ C conside ing he ope a ion in eac ion– egene a ion cycles and a compa ison wi h he con en ional SR o bio-oil a he same condi ions. The second one collec s he da a ob ained om he cha ac e iza ion o he esh and spen ca alys s by using di e en echniques in o de o shed ligh on he ca bon o ma ion ha causes a e e sible ca alys deac i a ion. The las subsec ion explo es he easons o he i e e sible ca alys deac i a ion obse ed o he egene a ed ca alys , based on he analysis o he Rh si es by H 2 -TPR measu emen s. Ca alys s 2024,14, 571 4 o 18 2.1. Pe o mance o he Rh Ca alys in Bio-Oil Re o ming Figu e 1shows he pe o mance o he Rh/ZDC ca alys in he CSDR o bio-oil a 700 ◦C in wo successi e eac ions wi h a ca alys egene a ion in be ween. The pe o mance is analyzed in e ms o he e olu ion o e ime on s eam o he p oduc yields (Figu e 1a o he eac ion wi h esh ca alys ( i s eac ion) and Figu e 1c o he eac ion wi h he egene a ed ca alys (second eac ion)) and syngas yield o H 2 /CO a io (Figu e 1b and Figu e 1d o he i s and second eac ions, espec i ely). In hese da a, he oxygena e con e sion is es ima ed as he o al yield o he ca bon gaseous p oduc s (sum o all ca bonaceous p oduc s a e sub ac ing he mola low a e o CO 2 in he eed), since he es ima ed coke con en and yield a e e y low acco ding o he TPO analysis shown in he nex subsec ion. Ca alys s 2024, 14, x FOR PEER REVIEW 4 o 18 he cha ac e iza ion o he esh and spen ca alys s by using di e en echniques in o de o shed ligh on he ca bon o ma ion ha causes a e e sible ca alys deac i a ion. The las subsec ion explo es he easons o he i e e sible ca alys deac i a ion obse ed o he egene a ed ca alys , based on he analysis o he Rh si es by H2-TPR measu emen s. 2.1. Pe o mance o he Rh Ca alys in Bio-Oil Re o ming Figu e 1 shows he pe o mance o he Rh/ZDC ca alys in he CSDR o bio-oil a 700 °C in wo successi e eac ions wi h a ca alys egene a ion in be ween. The pe o mance is analyzed in e ms o he e olu ion o e ime on s eam o he p oduc yields (Figu e 1a o he eac ion wi h esh ca alys ( i s eac ion) and Figu e 1c o he eac ion wi h he egene a ed ca alys (second eac ion)) and syngas yield o H2/CO a io (Figu e 1b and 1d o he i s and second eac ions, espec i ely). In hese da a, he oxygena e con e sion is es ima ed as he o al yield o he ca bon gaseous p oduc s (sum o all ca bonaceous p od- uc s a e sub ac ing he mola low a e o CO2 in he eed), since he es ima ed coke con en and yield a e e y low acco ding o he TPO analysis shown in he nex subsec- ion. 050 100 150 200 250 300 350 -0.2 0 0.2 0.4 0.6 0.8 1.0 Con e sion o Yield Time on s eam (min) (a) 050 100 150 200 250 300 350 -0.2 0 0.2 0.4 0.6 0.8 1.0 (c) Con e sion o Yield Time on s eam (min) Con e sion Oxygena es CO2 YieldH2 CO CO2 CH4 HC 050 100 150 200 250 300 350 0 0.2 0.4 0.6 0.8 1.0 (b) Syngas yield Time on s eam (min) 0 0.5 1.0 1.5 2.0 2.5 H2/CO a io 050 100 150 200 250 300 350 0 0.2 0.4 0.6 0.8 1.0 Syngas (H2+CO) (d) Syngas yield Time on s eam (min) 0 0.5 1.0 1.5 2.0 2.5 H2/CO a io H2/CO a io di ec ion o ead da a Figu e 1. Pe o mance o he Rh/ZDC ca alys in he CSDR o bio-oil a 700 °C, CO2/H2O/C a io o 0.6/0.5/1, space ime ( e e ed o he mass low o oxygena es in he bio-oil), and 0.125 gca alys h (goxy- gena es)−1: E olu ion o he con e sion and p oduc yields o e ime on s eam o (a) he esh ca alys and (c) he egene a ed ca alys ; e olu ion o he syngas yield and H2/CO a io o e ime on s eam o (b) he esh ca alys and (d) he egene a ed ca alys . The ini ial oxygena e con e sion is a ound 0.77 wi h he esh ca alys (Figu e 1a). This low alue e idences ha he space ime used is low o each an equilib ium s a e a hese eac ion condi ions (pa icula ly because o he low S/C a io), and he e o e he maximum p oduc yields may no ha e been achie ed, al hough his si ua ion is adequa e o s udy he ca alys deac i a ion. In spi e o his, he ini ial yield alues o H2 and CO Figu e 1. Pe o mance o he Rh/ZDC ca alys in he CSDR o bio-oil a 700 ◦ C, CO 2 /H 2 O/C a io o 0.6/0.5/1, space ime ( e e ed o he mass low o oxygena es in he bio-oil), and 0.125 g ca alys h (g oxygena es ) −1 : E olu ion o he con e sion and p oduc yields o e ime on s eam o (a) he esh ca alys and (c) he egene a ed ca alys ; e olu ion o he syngas yield and H 2 /CO a io o e ime on s eam o (b) he esh ca alys and (d) he egene a ed ca alys . The ini ial oxygena e con e sion is a ound 0.77 wi h he esh ca alys (Figu e 1a). This low alue e idences ha he space ime used is low o each an equilib ium s a e a hese eac ion condi ions (pa icula ly because o he low S/C a io), and he e o e he maximum p oduc yields may no ha e been achie ed, al hough his si ua ion is adequa e o s udy he ca alys deac i a ion. In spi e o his, he ini ial yield alues o H 2 and CO p o ide e idence o he high ac i i y o Rh o he bio-oil CSDR, which has also been demons a ed o he OSR o bio-oil [ 17 – 19 ]. The compa ison o he esul s in Figu e 1a,b Ca alys s 2024,14, 571 5 o 18 wi h hose ob ained on a Ni ca alys (Ni/Al 2 O 3 ca alys de i ed om a NiAl 2 O 4 spinel) unde he same condi ions [ 12 ] e eals ha Rh exhibi s no ably high ac i i y conside ing ha less ac i e si es a e ac ually p esen in he Rh ca alys (2 w % Rh s. 36 w % Ni). Thus, he ini ial yield and H 2 /CO a io o he syngas a e almos 90% and 1.2, espec i ely, wi h his Rh ca alys compa ed o 92% and 0.9 wi h he Ni ca alys , and he CO 2 con e sion is 22% agains 28%. As a compa ison, Figu e S1 in he Supplemen a y Ma e ials shows he esul s ob ained wi h he Rh/ZDC ca alys a he same condi ions as in Figu e 1bu wi hou CO 2 co- eeding ( ha is, unde SR condi ions wi h a low S/C a io o 0.5). When compa ing he SR and CSDR s a egies, a sligh ly highe oxygena e con e sion is obse ed in he SR eac ion (a ound 80%, Figu e S1a) han in he CSDR eac ion (Figu e 1a). The main di e ence be ween bo h e o ming s a egies is he inc ease in he CO yield in he CSDR eac ion ( om 62% in Figu e S1a o 85% in Figu e 1a), hence yielding sligh ly mo e syngas (nea 90% in CSDR eac ion, compa ed o 80% in SR eac ion) wi h a H 2 /CO a io o abou 1.2 (Figu e 1b) in compa ison o a H 2 /CO a io o 2 o he SR s a egy (Figu e S1b). The p oduc dis ibu ion (H 2 , CO, CO 2 , CH 4 , and hyd oca bons) obse ed may be explained conside ing he DR (Equa ion (1)) and SR (Equa ions (3) and (4)) o oxygena es, as well as he ca aly ic c acking o oxygena es (Equa ion (5)), as he dominan ou es a his high empe a u e, gene a ing H 2 and a ious gaseous in e media es (CO, CO 2 , CH 4 and hyd oca bons). Then, he SR o DR o CH 4 and hyd oca bons (Equa ions (6)–(9)) gene a es CO and H 2 , and he CO 2 may be ans o med in o CO and H 2 O by he -WGS eac ion ( e e se Equa ion (2)), which is he modynamically a o ed a his high empe a u e and low S/C a io [ 4 , 5 ]. In spi e o i s high ca aly ic ac i i y, he Rh ca alys unde goes deac i a ion du ing bo h he CSDR and SR eac ions, which is clea ly e idenced by he d op o he H 2 and CO yields o e ime on s eam (Figu e 1a,b). The ac i i y decay mus be mainly a ibu ed o coke o ma ion, which may p oceed h ough he decomposi ion o oxygena es (Equa ion (5)), CH 4 (Equa ion (10)), and hyd oca bons (Equa ion (11)), conside ing he high empe a u e (700 ◦ C) and low S/C a io employed [ 2 ]. Simul aneously, coke gasi ica ion may ake place wi h CO 2 ( e e se Equa ion (12)) o s eam (Equa ion (13)). Cu iously, he e olu ion o he p oduc yields d aws i e well-dis inguished pe iods, as has also been obse ed in he OSR and SR o bio-oil wi h he same ca alys [ 19 , 29 ] and in Figu e S1. Thus, o he CSDR s a egy, he i s pe iod (a ound 125 min) shows a qui e s able beha io o he ca alys , wi h high oxygena e con e sion and comple e con e sion o in e media es, CH 4 , and hyd oca bons, by SR o DR eac ions, gene a ing he highes H 2 and CO yields wi h he maximum CO 2 con e sion boos ed by he -WGS eac ion. The second pe iod (~125–175 min) shows a dec ease in he H 2 and CO yields and in he CO 2 con e sion and an inc ease in he CH 4 yield, indica ing he selec i e deac i a ion o he ca alys o he SR o DR o CH 4 and he -WGS eac ion. The hi d pe iod ( ~175–225 min ) desc ibes a pseudo s eady s a e whe e he con e sion o hyd oca bons by SR and DR eac- ions emains comple e. Subsequen ly, in he ou h pe iod (~225–275 min), he comple e ca alys deac i a ion o he SR and DR eac ions o hyd oca bons and oxygena es occu s, e idenced by he sha p dec ease in he oxygena e con e sion and he H 2 and CO yields and inc ease in he hyd oca bon yield. In his pe iod, he CO 2 con e sion alls o nega i e alues, eaching a minimum and hen inc easing up o ze o, which is ansla ed in o a max- imum o he CO 2 yield, which sugges s a selec i e deac i a ion o he DR o hyd oca bons o e hei SR eac ion. The i h pe iod (abo e 275 min) p esumably co esponds o he yields ob ained in he he mal eac ion o he bio-oil oxygena es in he p esence o bo h s eam and CO 2 . Consequen ly, he syngas yield dec eases o e ime ollowing he same end o hese i e pe iods (Figu e 1b), whe eas he H 2 /CO a io is almos cons an du ing he i s wo pe iods, sligh ly inc eases in he hi d pe iod, and decays no iceably in he ou h pe iod when he ca alys unde goes comple e deac i a ion. These pe iods a e also obse ed in he SR o bio-oil (Figu e S1), bu in his case CO 2 is only a p oduc . Pa icula ly, he CO 2 yield inc eases du ing he second pe iod (pa ial deac i a ion o CH 4 con e sion) and keeps a highe s able alue in he hi d pe iod in compa ison wi h he i s pe iod. This esul sugges s ha he -WGS eac ion (which is Ca alys s 2024,14, 571 6 o 18 he modynamically a o ed in hese eac ion condi ions) is also a ec ed by deac i a ion in he second pe iod. This beha io can also be obse ed in he CSDR eac ion (Figu e 1a), hough he CO 2 yield alues a e nega i e. S ikingly, he H 2 /CO a io e olu ion ollows a end simila o ha o he CO 2 yield, which con i ms he pa ial ca alys deac i a ion o he -WGS eac ion in he second pe iod. A e he CSDR eac ion wi h he esh ca alys (Figu e 1a), he spen ca alys was egene a ed in si u (in he eac o ) by coke combus ion wi h ai a 600 ◦ C and subsequen educ ion in a H 2 /N 2 low a 700 ◦ C. Upon egene a ion (Figu e 1c), he ca alys does no ully eco e i s ini ial ac i i y, which is pa icula ly e idenced by he absence o wo di e en deac i a ion pe iods desc ibed abo e. In u n, he eac ion wi h he egene a ed ca alys ini ia es a a s able pe iod in which hyd oca bons a e ully con e ed by SR o DR eac ions, whe eas CH 4 is no con e ed ( hi d pe iod in Figu e 1a), ollowed by he comple e ca alys deac i a ion ( ou h pe iod in Figu e 1a). The syngas yield (Figu e 1d) is ini ially s able and hen dec eases o e ime on s eam as desc ibed by he hi d and ou h pe iods in Figu e 1b, and he H 2 /CO a io also sligh ly inc eases and hen dec eases ollowing he end desc ibed by he CO 2 yield. This e idences ha he Rh/ZDC ca alys unde goes bo h e e sible and i e e sible deac i a ion. The o me is due o he deposi ion o coke so ha he ca alys pa ially eco e s i s e o ming ac i i y a e egene a ion. The la e is p esumably associa ed wi h i e e sible changes in he ca alys s uc u e o me al si es. In he ollowing sec ions, we u he in es iga e he causes o bo h deac i a ion phenomena by analyzing he spen ca alys om each eac ion es . 2.2. Chaca ac a iza ion o Deac i a ed Ca alys Samples The coke o med and deposi ed on he ca alys du ing each eac ion was analyzed by TPO, SEM, and TEM in o de o de e mine i s amoun and/o na u e. Addi ionally, N 2 physiso p ion was also employed o analyze he e ec o coke deposi ion on he ca alys ex u al p ope ies. The possible changes in he c ys alline s uc u e o he deac i a ed ca alys we e analyzed by XRD analysis. Finally, he H 2 -TPR p o iles o he esh and egene a ed ca alys we e compa ed o analyze he Rh si es. In gene al, he spen ca alys samples a e iden i ied wi h e e ence o hei eac ion es : CSDR-1 o he i s CSDR eac ion (wi h he esh ca alys ) and CSDR-2 o he second CSDR eac ion (wi h he egene a ed ca alys ). 2.2.1. Coke Fo ma ion Figu e 2shows he TPO p o iles o he spen ca alys samples, in which he peaks a e associa ed wi h he combus ion o coke. The co esponding coke con en (de e mined by in eg a ion o each cu e) is indica ed nex o each p o ile. The TPO p o iles a e no iceably di e en o he wo samples, showing combus ion peaks cen e ed a di e en empe a u es. The CSDR-1 sample shows wo sepa a e peaks cen e ed a 300 and 400 ◦ C, and he CSDR-2 sample shows wo o e lapping peaks ha a e app oxima ely cen e ed a 415 and 430 ◦ C. Likewise, he coke con en is signi ican ly lowe in he CSDR-1 sample (93.6 mg g −1 ) han in he CSDR-2 sample (302 mg g −1 ). The empe a u e posi ion o he combus ion peaks may be ela ed o he na u e o ca bon and o he possible ca aly ic e ec o he ca alys componen s on he ca bon combus ion. Thus, he peaks a lowe empe a u es can be associa ed wi h he combus ion o poo ly de eloped coke (amo phous ca bon, mos p obably encapsula ing Rh si es) and he peaks a highe empe a u es a e associa ed wi h s uc u ed coke (ca bon nanos uc u es o u bos a ic/g aphi e ca bon) [ 34 , 35 ]. Addi ionally, bo h Ce (in he ZDC suppo ) and Rh species can ca alyze he ca bon combus ion [ 36 ], which esul s in combus ion peaks a lowe empe a u e han hose ob ained o Ni ca alys s in di e en bio-oil e o ming s a egies [12,17,37]. Ca alys s 2024,14, 571 7 o 18 Ca alys s 2024, 14, x FOR PEER REVIEW 7 o 18 Addi ionally, bo h Ce (in he ZDC suppo ) and Rh species can ca alyze he ca bon com- bus ion [36], which esul s in combus ion peaks a lowe empe a u e han hose ob ained o Ni ca alys s in di e en bio-oil e o ming s a egies [12,17,37]. 200 250 300 350 400 450 500 0 10 20 30 40 CSDR-1 CSDR-2 DTG (mg (g min)-1) Tempe a u e (ºC) 93.6 coke con en (mg/g) = 302 Figu e 2. TPO p o iles o he spen ca alys samples. Coke con en is indica ed close o each p o ile. To disce n he na u e o he coke, he wo spen ca alys samples ha e been analyzed by SEM (wi h a backsca e ed elec on (BSE) o seconda y elec on (SE) de ec o ) and TEM. Figu e 3 shows he BSE-SEM images o he esh and spen ca alys samples, which p o ide quali a i e in o ma ion on he deg ee o coke deposi ion on he ex e nal su ace. Acco dingly, he b igh ness in ensi y is indica i e o he p esence o hea y (b igh ) o ligh (da k) elemen s [2,12,35], and o his case, a high o dense p esence o coke on he ca alys su ace is indica ed by da ke pa icles since coke con ains he ligh es elemen (C) in hese samples in compa ison wi h he ca alys componen s (Rh, Ce, and Z ). In gene al, all he pa icles exhibi ela i ely b igh in ensi ies simila o hose o he esh ca alys (Figu e 3a), indica ing a poo coke deposi ion on he ca alys su ace o he deposi ion o ca bon wi h low densi y, al hough he CSDR-1 sample also exhibi s some da k pa icles. Figu e 3. BSE-SEM images o he (a) esh ca alys and spen ca alys samples om he (b) 1s CSDR eac ion (wi h esh ca alys ) and (c) 2nd CSDR eac ion (wi h egene a ed ca alys ). Figu e 2. TPO p o iles o he spen ca alys samples. Coke con en is indica ed close o each p o ile. To disce n he na u e o he coke, he wo spen ca alys samples ha e been analyzed by SEM (wi h a backsca e ed elec on (BSE) o seconda y elec on (SE) de ec o ) and TEM. Figu e 3shows he BSE-SEM images o he esh and spen ca alys samples, which p o ide quali a i e in o ma ion on he deg ee o coke deposi ion on he ex e nal su ace. Acco dingly, he b igh ness in ensi y is indica i e o he p esence o hea y (b igh ) o ligh (da k) elemen s [ 2 , 12 , 35 ], and o his case, a high o dense p esence o coke on he ca alys su ace is indica ed by da ke pa icles since coke con ains he ligh es elemen (C) in hese samples in compa ison wi h he ca alys componen s (Rh, Ce, and Z ). In gene al, all he pa icles exhibi ela i ely b igh in ensi ies simila o hose o he esh ca alys (Figu e 3a), indica ing a poo coke deposi ion on he ca alys su ace o he deposi ion o ca bon wi h low densi y, al hough he CSDR-1 sample also exhibi s some da k pa icles. Ca alys s 2024, 14, x FOR PEER REVIEW 7 o 18 Addi ionally, bo h Ce (in he ZDC suppo ) and Rh species can ca alyze he ca bon com- bus ion [36], which esul s in combus ion peaks a lowe empe a u e han hose ob ained o Ni ca alys s in di e en bio-oil e o ming s a egies [12,17,37]. 200 250 300 350 400 450 500 0 10 20 30 40 CSDR-1 CSDR-2 DTG (mg (g min)-1) Tempe a u e (ºC) 93.6 coke con en (mg/g) = 302 Figu e 2. TPO p o iles o he spen ca alys samples. Coke con en is indica ed close o each p o ile. To disce n he na u e o he coke, he wo spen ca alys samples ha e been analyzed by SEM (wi h a backsca e ed elec on (BSE) o seconda y elec on (SE) de ec o ) and TEM. Figu e 3 shows he BSE-SEM images o he esh and spen ca alys samples, which p o ide quali a i e in o ma ion on he deg ee o coke deposi ion on he ex e nal su ace. Acco dingly, he b igh ness in ensi y is indica i e o he p esence o hea y (b igh ) o ligh (da k) elemen s [2,12,35], and o his case, a high o dense p esence o coke on he ca alys su ace is indica ed by da ke pa icles since coke con ains he ligh es elemen (C) in hese samples in compa ison wi h he ca alys componen s (Rh, Ce, and Z ). In gene al, all he pa icles exhibi ela i ely b igh in ensi ies simila o hose o he esh ca alys (Figu e 3a), indica ing a poo coke deposi ion on he ca alys su ace o he deposi ion o ca bon wi h low densi y, al hough he CSDR-1 sample also exhibi s some da k pa icles. Figu e 3. BSE-SEM images o he (a) esh ca alys and spen ca alys samples om he (b) 1s CSDR eac ion (wi h esh ca alys ) and (c) 2nd CSDR eac ion (wi h egene a ed ca alys ). Figu e 3. BSE-SEM images o he (a) esh ca alys and spen ca alys samples om he (b) 1s CSDR eac ion (wi h esh ca alys ) and (c) 2nd CSDR eac ion (wi h egene a ed ca alys ). Fu he , o explo e he coke deposi ion, Figu e 4shows he SE-SEM images a highe zooms o all he samples. The Rh/ZDC ca alys shows a g anula su ace ypical o po ous ma e ials (in his case he ZDC suppo ), showing some compac ed a eas. The majo i y o he pa icles in he CSDR-1 sample a e b igh , e en hough he e a e some ca bon ilamen s Ca alys s 2024,14, 571 8 o 18 on he su ace and some g anula and compac ed a eas ha may be he su ace o he ZDC suppo (Figu e 4b). The da k pa icles in he CSDR-1 sample show an abundan p esence o ca bon ilamen s (Figu e 4c). On he o he hand, he CSDR-2 sample shows a smoo h g anula -like su ace (appa en ly di e en om ha o he ca alys suppo ) wi h some ca bon ilamen s (Figu e 4d), and when his su ace is zoomed in upon (Figu e 4e), i seems o be a blend o an amo phous mass o ca bon wi h some nano ubes apped in i . Ca alys s 2024, 14, x FOR PEER REVIEW 8 o 18 Fu he , o explo e he coke deposi ion, Figu e 4 shows he SE-SEM images a highe zooms o all he samples. The Rh/ZDC ca alys shows a g anula su ace ypical o po ous ma e ials (in his case he ZDC suppo ), showing some compac ed a eas. The majo i y o he pa icles in he CSDR-1 sample a e b igh , e en hough he e a e some ca bon ila- men s on he su ace and some g anula and compac ed a eas ha may be he su ace o he ZDC suppo (Figu e 4b). The da k pa icles in he CSDR-1 sample show an abundan p esence o ca bon ilamen s (Figu e 4c). On he o he hand, he CSDR-2 sample shows a smoo h g anula -like su ace (appa en ly di e en om ha o he ca alys suppo ) wi h some ca bon ilamen s (Figu e 4d), and when his su ace is zoomed in upon (Figu e 4e), i seems o be a blend o an amo phous mass o ca bon wi h some nano ubes apped in i . Figu e 4. SE-SEM images o he (a) esh ca alys and spen ca alys samples om he (b,c) 1s CSDR eac ion (wi h esh ca alys ) and (d,e) 2nd CSDR eac ion (wi h egene a ed ca alys ). TEM has been use ul o con i m he p esence o hese ca bon ypes as shown in Figu e 5, om which i should be conside ed ha only iny agmen s o he samples can be an- alyzed wi hou dis inguishing be ween he ex e nal/in e nal su ace o he pa icle. The Rh/ZDC ca alys shows g ains o di e en g ey ones, wi h he da ke ones p esumably being Rh c ys als as his is he denses ca alys componen . The CSDR-1 sample shows he p esence o bo h ca bon ilamen s (Figu e 5b) and amo phous/ u bos a ic ca bon encap- sula ing he ca alys componen s (Figu e 5c). On he o he hand, he CSDR-2 sample ba ely shows a mass o ca bon co e ing he ca alys componen s in spi e o ha ing he highes coke con en . The di icul y o obse e he ca bon on his la e sample is pe haps Figu e 4. SE-SEM images o he (a) esh ca alys and spen ca alys samples om he (b,c) 1s CSDR eac ion (wi h esh ca alys ) and (d,e) 2nd CSDR eac ion (wi h egene a ed ca alys ). TEM has been use ul o con i m he p esence o hese ca bon ypes as shown in Figu e 5, om which i should be conside ed ha only iny agmen s o he samples can be analyzed wi hou dis inguishing be ween he ex e nal/in e nal su ace o he pa icle. The Rh/ZDC ca alys shows g ains o di e en g ey ones, wi h he da ke ones p esumably being Rh c ys als as his is he denses ca alys componen . The CSDR-1 sample shows he p esence o bo h ca bon ilamen s (Figu e 5b) and amo phous/ u bos a ic ca bon encapsula ing he ca alys componen s (Figu e 5c). On he o he hand, he CSDR-2 sample ba ely shows a mass o ca bon co e ing he ca alys componen s in spi e o ha ing he highes coke con en . The di icul y o obse e he ca bon on his la e sample is pe haps due o i s na u e, appa en ly a hin laye o ca bon mass co e ing he en i e su ace e enly, as well as he limi a ion o he echnique o analyze his ype o solid. Ca alys s 2024,14, 571 9 o 18 Ca alys s 2024, 14, x FOR PEER REVIEW 9 o 18 due o i s na u e, appa en ly a hin laye o ca bon mass co e ing he en i e su ace e enly, as well as he limi a ion o he echnique o analyze his ype o solid. Figu e 5. TEM images o he (a) esh ca alys and spen ca alys samples om he (b,c) 1s CSDR eac ion (wi h esh ca alys ) and (d) 2nd CSDR eac ion (wi h egene a ed ca alys ). The ex u al p ope ies o he esh and spen ca alys samples (Table 1) e idence he impac o coke deposi ion. The o ma ion o bo h ca bon ilamen s and amo phous ca bon in he CSDR eac ion wi h he esh ca alys (CSDR-1 sample) causes he BET su ace a ea o dec ease by 33%, wi h a s ong impac on he mic opo ous su ace a ea and on he olume, which bo h dec ease by abou 62%. On he o he hand, he o ma ion o a blend o amo phous/ u bos a ic ca bon and small ca bon ilamen s in he CSDR eac ion wi h he egene a ed ca alys (CSDR-2 sample) causes a s ong dec ease in he BET su ace a ea (abou 74%) and in he o al olume o po es (by 83%) wi hou a ec ing he mic opo ous su ace a ea. This sugges s ha in he eac ion wi h he esh ca alys , he amo phous ca - bon may be deposi ed on he mic opo ous su ace, whe eas all he ca bon is deposi ed on he ex e nal su ace o he ca alys pa icles in he eac ion wi h he egene a ed ca alys . I should be no ed ha he educed speci ic su ace is a no able limi a ion o he CeO2 suppo , as poin ed ou in he li e a u e [28], and i is mo e p onounced in his eac ion due o coke deposi ion. Table 1. Tex u al p ope ies o esh, spen , and egene a ed Rh/ZDC ca alys samples. Sample SBET (m2 g−1) Smic o (m2 g−1) Vpo e (cm3 g−1) Vmic o (cm3 g−1) dpo e (nm) F esh 57.5 2.17 0.24 1.31 × 10−3 17.0 CSDR-1 38.4 0.82 0.15 0.49 × 10−3 15.9 CSDR-2 14.8 2.16 0.04 0.85 × 10−3 10.9 1s egene a ion (CSDR-1) 39.6 2.16 0.20 1.00 × 10−3 20.2 Figu e 5. TEM images o he (a) esh ca alys and spen ca alys samples om he (b,c) 1s CSDR eac ion (wi h esh ca alys ) and (d) 2nd CSDR eac ion (wi h egene a ed ca alys ). The ex u al p ope ies o he esh and spen ca alys samples (Table 1) e idence he impac o coke deposi ion. The o ma ion o bo h ca bon ilamen s and amo phous ca bon in he CSDR eac ion wi h he esh ca alys (CSDR-1 sample) causes he BET su ace a ea o dec ease by 33%, wi h a s ong impac on he mic opo ous su ace a ea and on he olume, which bo h dec ease by abou 62%. On he o he hand, he o ma ion o a blend o amo phous/ u bos a ic ca bon and small ca bon ilamen s in he CSDR eac ion wi h he egene a ed ca alys (CSDR-2 sample) causes a s ong dec ease in he BET su ace a ea (abou 74%) and in he o al olume o po es (by 83%) wi hou a ec ing he mic opo ous su ace a ea. This sugges s ha in he eac ion wi h he esh ca alys , he amo phous ca bon may be deposi ed on he mic opo ous su ace, whe eas all he ca bon is deposi ed on he ex e nal su ace o he ca alys pa icles in he eac ion wi h he egene a ed ca alys . I should be no ed ha he educed speci ic su ace is a no able limi a ion o he CeO 2 suppo , as poin ed ou in he li e a u e [ 28 ], and i is mo e p onounced in his eac ion due o coke deposi ion. Table 1. Tex u al p ope ies o esh, spen , and egene a ed Rh/ZDC ca alys samples. Sample SBET (m2g−1) Smic o (m2g−1) Vpo e (cm3g−1) Vmic o (cm3g−1) dpo e (nm) F esh 57.5 2.17 0.24 1.31 × 10 −317.0 CSDR-1 38.4 0.82 0.15 0.49 × 10 −315.9 CSDR-2 14.8 2.16 0.04 0.85 × 10 −310.9 1s egene a ion (CSDR-1) 39.6 2.16 0.20 1.00 × 10 −320.2 S BET = BET speci ic su ace a ea; S mic o = mic opo ous speci ic su ace a ea; V po e = speci ic olume o po es; Vmic o = speci ic olume o mic opo es; dpo e = a e age po e diame e . Ca alys s 2024,14, 571 16 o 18 Acknowledgmen s: The au ho s a e hank ul o he echnical and human suppo p o ided by SGIke (UPV/EHU/ERDF, EU). Con lic s o In e es : The au ho s decla e no con lic s o in e es . Re e ences 1. Bachmann, M.; Völke , S.; Kleineko e, J.; Ba dow, A. Syngas om Wha ? Compa a i e Li e-Cycle Assessmen o Syngas P oduc ion om Biomass, CO2, and S eel Mill O -Gases. ACS Sus ain. Chem. Eng. 2023,11, 5356–5366. [C ossRe ] 2. Ga cía-Gómez, N.; Valecillos, J.; Valle, B.; Remi o, A.; Bilbao, J.; Gayubo, A.G. 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