Carbon Dioxide Hydrogenation by Means of Plasmonic Resonance Activation in Silica Aerogel Media

ma e ials A icle Ca bon Dioxide Hyd ogena ion by Means o Plasmonic Resonance Ac i a ion in Silica Ae ogel Media Se gio Muñoz 1, Alexande Na a e e 2,*, Ángel Ma ín1, Roland Di meye 2 and M. JoséCoce o 1 1BioEcoUVa, Bioeconomy Resea ch Ins i u e, High P essu e P ocesses G oup, Depa men o Chemical Enginee ing and En i onmen al Technology, Uni e si y o Valladolid, C/P ado de la Magdalena s/n, 47011 Valladolid, Spain; [email p o ec ed] (S.M.); [email p o ec ed] (Á.M.); mjcoce [email p o ec ed] (M.J.C.) 2Ins i u e o Mic o P ocess Enginee ing, Ka ls uhe Ins i u e o Technology (KIT), He mann- on-Helmhol z-Pla z 1, 76344 Eggens ein-Leopoldsha en, Ge many; [email p o ec ed] *Co espondence: alexande [email p o ec ed]; Tel.: +49-721-608-26653 Recei ed: 29 Sep embe 2018; Accep ed: 25 Oc obe 2018; Published: 30 Oc obe 2018   Abs ac : Su ace Plasmon Resonance can be used o ac i a e zinc oxide/coppe ca alys s in o de o pe o m he ca bon dioxide hyd ogena ion eac ion by means o ligh ene gy, a oiding high- empe a u e p ocesses. The syn hesis and imp egna ion me hods ha e been designed o ill glass mic o eac o s wi h ZnO/Cu nanopa icles suppo ed on anspa en silica ae ogels o maximize he ligh abso bed by he ca alys . A LED de ice su ounding he glass mic o eac o s p o ided whi e ligh o ac i a e he ca alys homogeneously h oughou he eac o . Tempe a u e, p essu e, amoun o ca alys and gases low we e s udied as possible a iables o enhance he p ocess ying o maximize CO 2 con e sion a es, achie ing he bes esul s wo king a high p essu es. The use o anspa en SiO 2 Ae ogels as suppo s o pho oca aly ic gas phase eac ions e en unde high-p essu e condi ions is demons a ed. Keywo ds: CO2hyd ogena ion; pho oca alysis; su ace plasmon esonance; sola uels 1. In oduc ion Ca bon dioxide con e sion in o use ul p oduc s is a ac ing much a en ion in he las yea s as a way o educe he g eenhouse e ec and i s d ama ic consequences o he plane [ 1 , 2 ]. Among he op ions o pe o m his CO 2 con e sion, pho oca aly ic p ocesses using sola ene gy ep esen an in e es ing way o ans o m CO2due o he use o a enewable ene gy. CO2can be ans o med in o me hanol, which is a chemical commodi y and can s o e he ene gy om he sun [ 3 , 4 ]. T ans o ming CO 2 in o me hanol using sola ene gy gene a es, he e o e, wo bene i s a he same ime: he educ ion o CO2emissions and he s o age o ene gy om enewable sou ces. Pho oca aly ic CO 2 ans o ma ion usually shows low con e sions because he e iciency o he p ocess is limi ed [ 5 ]. One o he easons is ela ed o he low amoun o ligh ene gy ha mos o he ca alys s abso b in he isible o ul a iole band. Su ace Plasmon Resonance (SPR) is a phenomenon exhibi ed by some me allic nanos uc u es, which esul s in a highe ligh ene gy abso bance ha can be con olled wi h he shape and size o he nanopa icles. One a ea whe e he SPR e ec has ound applica ion is in biosensing [ 6 , 7 ]. Howe e , in he las ew yea s, i s applica ion has ex ended o pho oca alysis [ 8 ], which is he main opic o his pape . Commonly, gold and sil e nanopa icles ha e been s udied o enhance SPR e ec [ 9 , 10 ], al hough o he me als such as coppe also p oduce his e ec [ 11 ]. Coppe has been widely used o ca bon dioxide hyd ogena ion in indus y combined wi h he use o zinc oxide in o de o p oduce me hanol wi h high selec i i y [ 12 ]. Fo his eason, Ma e ials 2018,11, 2134; doi:10.3390/ma11112134 www.mdpi.com/jou nal/ma e ials Ma e ials 2018,11, 2134 2 o 13 a combina ion o bo h ma e ials Cu/ZnO could be also used o pe o m ca bon dioxide hyd ogena ion by means o sola ene gy using SPR as a way o enhance he e iciency o he p ocess and o maximize he CO2 ans o ma ion. SPR e ec depends on coppe nanopa icles size, whose a ia ion can modi y he abso p ion wa eleng h o isual ligh [ 13 ]. Ca bon dioxide hyd ogena ion is pe o med in he con ac a ea be ween zinc oxide and coppe nanopa icles [ 14 ], gi ing a c ucial impo ance o maximize his con ac a ea wi h he syn hesis me hod chosen. Fo his eason, he syn hesis me hod mus p oduce disc e e coppe nanopa icles whose inal size can be con olled, and hen hese coppe nanopa icles mus be deposi ed on o zinc oxide nano ods a oiding agglome a ion. Ca bon dioxide and hyd ogen can p oduce me hanol di ec ly in a one-s ep eac ion. Howe e , he con e sion is inc eased when he ans o ma ion is ca ied ou in wo s eps [ 15 ]. Fi s , ca bon dioxide is ans o med in ca bon monoxide (endo he mic), and a e his CO again wi h hyd ogen p oduce me hanol (exo he mic). In his wo k, glass mic ochannels we e selec ed o pe o m he ca bon dioxide hyd ogena ion because hey p o ided g ea homogenei y in he ligh dis ibu ion h ough he eac o [ 16 ]. Glass mic o eac o s a oided ligh abso p ion in e e ences, allowing o s udy he in luence o he ma e ial p ope ies and eac ion condi ions on CO2con e sion. In o de o ix he ca alys inside he mic ochannels, a suppo was equi ed. This suppo mus p o ide high su ace a eas in o de o load he bime allic ca alys and, a he same ime, allow ha enough ligh is ansmi ed o he ca alys o ac i a e i . Silica ae ogels a e mic o-mesopo ous ma e ials wi h high su ace a eas (400–1500 m 2 /g) and high isible anspa ency [ 17 ], p ope ies ha placed hem as he bes op ion o be used as a suppo in his wo k. The e o e, a bime allic ca alys ZnO/Cu suppo ed in silica ae ogels can be ac i a ed wi h isual ligh and be used o pe o m ca bon dioxide hyd ogena ion in glass mic ochannels. In a p e ious wo k, he iabili y o he idea was es ed wi h good esul s in a se up desc ibed in [ 18 ], opening he possibili y o imp o e he p ocess op imizing he eac ion pa ame e s. Wi h he pu pose o inc easing CO 2 con e sion a e, a comple e s udy o he in luence o some eac ion a iables ( empe a u e, p essu e, ca alys amoun and eac an s p opo ion) has been pe o med in his wo k. Wi h his wo k, we aim o open new oppo uni ies o he in eg a ion o plasmonic pho oca alys s in u u e scalable eac ion sys ems o p oduce use ul compounds as such me hanol using sola ene gy. 2. Ma e ials and Me hods 2.1. Reagen s The chemicals used du ing his s age we e: Zinc ace a e dihyd a e (>98%), oleylamine (70%), e ame hyl o hosilica e (98%), ammonia (30%), ie hylene glycol (99%) we e pu chased om Sigma-Ald ich (S . Louis, MO, USA). E hylene glycol (99.5%) (Me ck, Kenilwo h, NJ, USA). Coppe ace a e monohyd a e (99.9%) was pu chased om Al a Aesa (Ha e hill, MA, USA). Me hanol (99.8%) was pu chased om Pan eac (Ba celona, Spain). 2.2. Syn hesis o Plasmonic Ca alys Following he me hod p oposed by Tan e al. (2013) [ 19 ], zinc oxide nano ods we e de eloped i s ly, and hen coppe nanopa icles we e deposi ed on o in a second s ep, con olling he inal size o he nanopa icles and he wa eleng h whe e isual ligh was abso bed. Oleylamine/zinc ace a e a io was he mos impo an pa ame e ha de ined he inal shape o zinc oxide nanopa icles. In o de o p oduce nano ods, 3 mmol o zinc ace a e we e mixed wi h 1.3 mL o oleylamine in a h ee-necked lask. A e degassing he mix u e a 80 ◦ C o 45 min in a acuum a mosphe e, a ni ogen s eam was in oduced in he sys em and he empe a u e was inc eased o 220 ◦ C o 15 min. A e his, he inal p oduc ob ained was a whi e pas e s uck o he bo om o he lask. 10 mL o e hanol we e used wi h s i ing o emo e he solid p oduc , and hen he mix u e Ma e ials 2018,11, 2134 3 o 13 was cen i uged (cen i uge Kubo a 5100, Tokyo, Japan) a 5000 pm in o de o isola e he p ecipi a e. The solid p oduc was washed again wice wi h 5 mL o e hanol o ensu e a pe ec emo al o he eac an s. ZnO nano ods p epa ed we e edispe sed in 20 mL o ie hylene glycol o wo hou s by sonica ion, and hen s i ed unde oom condi ions o e nigh . A e ha , a dual glycol sys em was p epa ed o deposi coppe oxide nanopa icles on o he ZnO nano ods. Coppe ace a e (1.5 mmol) was added in a second po o 4 mL o e hylene glycol, using sonica ion o 1h o c ea e a homogeneous solu ion. E hylene glycol (2 mL) was also added o he zinc oxide essel, and he solu ion was ans e ed o a h ee-necked lask again o be degassed a ambien empe a u e o 10 min. Then, he coppe ace a e solu ion was placed in an addi ion unnel connec ed o he h ee-necked lask wi h he coppe oxide, which was hea ed a 190 ◦ C unde ni ogen pu ging. Wi h he aim o a oiding agglome a ion, coppe ace a e solu ion was added slowly d op by d op du ing 10 min. A e his, he solu ion was cooled, washed and cen i uged h ee imes wi h isop opanol a 5000 pm. The mass o he solid ob ained was hen egis e ed. 2.3. Syn hesis and Imp egna ion o Silica Ae ogels The silica gels we e syn hesized ollowing he me hod used by Sanz-Mo al e al. (2014) [ 20 ]. The p ecu so s used we e e ame hyl o hosilica e (TMOS), me hanol, wa e and ammonia in a 1:2.3:3.84:0.012 mola a io. The amoun o ammonia was he con olling pa ame e o he gela ion ime. Depending on he ca alys load, which also a ec ed he gela ion ime, mola a io TMOS: ammonia was a ied om 1:0.012 o 1:0.12 in o de o achie e he igh gela ion ime. We adjus ed he amoun o silica p ecu so added in acco dance o he equi ed ca aly ic load (7.0%, 10.0%, 13.0%) based on he mass o Cu/ZnO ca alys ob ained in he p e ious s ep. Silica ae ogels also ep esen ed a bene i o subsequen imp egna ion in he glass mic ochannels. Ae ogels we e p oduced h ough he sol-gel ou e, mixing all he p ecu so s in a liquid s a e unde s i ing o gene a e he hyd ogel. These p ecu so s included me hanol, which was used o suspend he ca alys nanopa icles by sonica ion. A e ha , me hanol was mixed wi h he o he p ecu so s gene a ing he silica ne , while e aining he Cu/ZnO ca alys inside. This silica s uc u e was in a liquid o m o a ew seconds, which allowed he imp egna ion in glass mic o eac o s by using jus a sy inge. Then, he gela ion p ocess ook place di ec ly inside he mic o eac o s ob aining a hyd ogel well a ached o he walls o he mic ochannels and a oiding p oblems o in oducing a solid ca alys in a na ow mic ochannel. In his way, gela ion ime became he key ac o o he ca alys imp egna ion inside he mic o eac o s. The injec ion wi h he sy inge in a liquid s a e equi ed gela ion imes no oo sho . Howe e , i he gela ion p ocess we e oo slow, he ca alys would p ecipi a e and he inal s uc u e would no be homogeneous. Fi s , me hanol wi h he ca alys nanopa icles was mixed wi h TMOS in a small glass and s i ed o a ew minu es. A he same ime, a second glass beake wi h wa e and ammonia was also s i ed and co e ed o a oid ammonia e apo a ion. Then, bo h solu ions we e mixed and he sy inge was used o ill he mic o eac o . A e a ew seconds, he liquid u ned in o he hyd ogel wi h he ca alys inside showing good adhesion and homogenei y along he en i e leng h o he glass mic o eac o (15 cm). 2.4. Ae ogels Supe c i ical D ying Be o e ca ying ou he d ying wi h supe c i ical CO 2 , an in e media e s ep was equi ed o aging he hyd ogels [21]. In his s age, he mic o eac o s we e placed in a essel illed wi h me hanol and closed o one week. The aim was o emo e he wa e om he s uc u e p oducing alcogels, which we e d ied la e wi h supe c i ical CO 2 . In o de o ensu e no wa e emained in he s uc u e, me hanol was enewed wice du ing he week. A e ha , he alcogels we e eady o be d ied wi h supe c i ical CO 2 . The expe imen al plan o ae ogels d ying (Figu e 1) had an o en o hea he sys em a 45 ◦ C, and CO 2 was pumped a 110 ba Ma e ials 2018,11, 2134 4 o 13 abo e he supe c i ical poin [ 20 ]. Inside he o en, he e was a high-p essu e essel ha was illed wi h me hanol be o e mic ochannels we e placed inside. Ma e ials 2018, 11, x FOR PEER REVIEW 4 o 13 comple e sys em o 45 min. As me hanol has no a high solubili y in supe c i ical CO2, ou d ying cycles we e pe o med, enewing CO2 be ween each cycle o ensu e a comple e d ying o me hanol. The inal p oduc s we e he ae ogels co ec ly o med and wi h g ea adhe ence o he mic ochannel walls. Cooling O en High P essu e Vessel Ven CO 2 Pump PI PI TI Figu e 1. Silica ae ogels d ying plan . 2.5. Expe imen al Reac ion Plan A e mic o eac o s we e p epa ed o pe o m he eac ion, hey we e ca ied o he expe imen al se up whe e ca bon dioxide hyd ogena ion was pe o med. This plan (Figu e 2) was used in p e ious wo k o es he eac o concep wi h good esul s [18]. H2 CO2 Mic oGC FIC FIC O en LED Ven PIC TIC Figu e 2. Ca bon dioxide hyd ogena ion expe imen al plan . The expe imen al se up was designed o be able o con ol he eac ion pa ame e s ha can ha e an in luence on he ca bon dioxide ans o ma ion, such as p essu e, empe a u e and gas inle lows. The plan had wo low mass me e /con olle s (EL-Flow F-200, B onkho s , The Ne he lands) o H2 and CO2 wi h anges up o 1 mL/min. The empe a u e was con olled by placing he mic o eac o s inside a gas ch oma og aphy o en (Agilen 7890, San a Cla a, CA, USA) and he p essu e was con olled by a p essu e me e /con olle (EL-P ess se ies, B onkho s , The Ne he lands) loca ed a he exi o he o en. The mic o eac o consis ed o a 0.5 mm ID glass capilla y wi h an ex e nal diame e o 5 mm (Scho Du an, Mainz, Ge many). The mic o eac o was su ounded by a se o LEDs (Supe b igh , inspi ed LED), which p o ided a nominal powe o 9780 W/m2 o whi e ligh in he mos homogeneous way o illumina e co ec ly he en i e eac o [18]. The gas ou le s eam was measu ed wi h a Mic o Gas Ch oma og aph (CP-4900, Va ian, Palo Al o, CA, USA) equipped wi h wo columns: a po aplo 10 m and a 5A molsie e. Figu e 1. Silica ae ogels d ying plan . When he high-p essu e essel was closed, he uppe al e was sligh ly opened o in oduce he supe c i ical ca bon dioxide and o inc ease slowly he p essu e (i he e we e a sudden change o p essu e inside he essel, he ae ogels would be b oken). A e he p essu e in he essel eached 110 ba , he lowe al e was also opened in o de ha ca bon dioxide s a ed o ci cula e h ough he comple e sys em o 45 min. As me hanol has no a high solubili y in supe c i ical CO 2 , ou d ying cycles we e pe o med, enewing CO 2 be ween each cycle o ensu e a comple e d ying o me hanol. The inal p oduc s we e he ae ogels co ec ly o med and wi h g ea adhe ence o he mic ochannel walls. 2.5. Expe imen al Reac ion Plan A e mic o eac o s we e p epa ed o pe o m he eac ion, hey we e ca ied o he expe imen al se up whe e ca bon dioxide hyd ogena ion was pe o med. This plan (Figu e 2) was used in p e ious wo k o es he eac o concep wi h good esul s [18]. Ma e ials 2018, 11, x FOR PEER REVIEW 4 o 13 comple e sys em o 45 min. As me hanol has no a high solubili y in supe c i ical CO2, ou d ying cycles we e pe o med, enewing CO2 be ween each cycle o ensu e a comple e d ying o me hanol. The inal p oduc s we e he ae ogels co ec ly o med and wi h g ea adhe ence o he mic ochannel walls. Cooling O en High P essu e Vessel Ven CO 2 Pump PI PI TI Figu e 1. Silica ae ogels d ying plan . 2.5. Expe imen al Reac ion Plan A e mic o eac o s we e p epa ed o pe o m he eac ion, hey we e ca ied o he expe imen al se up whe e ca bon dioxide hyd ogena ion was pe o med. This plan (Figu e 2) was used in p e ious wo k o es he eac o concep wi h good esul s [18]. H2 CO2 Mic oGC FIC FIC O en LED Ven PIC TIC Figu e 2. Ca bon dioxide hyd ogena ion expe imen al plan . The expe imen al se up was designed o be able o con ol he eac ion pa ame e s ha can ha e an in luence on he ca bon dioxide ans o ma ion, such as p essu e, empe a u e and gas inle lows. The plan had wo low mass me e /con olle s (EL-Flow F-200, B onkho s , The Ne he lands) o H2 and CO2 wi h anges up o 1 mL/min. The empe a u e was con olled by placing he mic o eac o s inside a gas ch oma og aphy o en (Agilen 7890, San a Cla a, CA, USA) and he p essu e was con olled by a p essu e me e /con olle (EL-P ess se ies, B onkho s , The Ne he lands) loca ed a he exi o he o en. The mic o eac o consis ed o a 0.5 mm ID glass capilla y wi h an ex e nal diame e o 5 mm (Scho Du an, Mainz, Ge many). The mic o eac o was su ounded by a se o LEDs (Supe b igh , inspi ed LED), which p o ided a nominal powe o 9780 W/m2 o whi e ligh in he mos homogeneous way o illumina e co ec ly he en i e eac o [18]. The gas ou le s eam was measu ed wi h a Mic o Gas Ch oma og aph (CP-4900, Va ian, Palo Al o, CA, USA) equipped wi h wo columns: a po aplo 10 m and a 5A molsie e. Figu e 2. Ca bon dioxide hyd ogena ion expe imen al plan . The expe imen al se up was designed o be able o con ol he eac ion pa ame e s ha can ha e an in luence on he ca bon dioxide ans o ma ion, such as p essu e, empe a u e and gas inle lows. The plan had wo low mass me e /con olle s (EL-Flow F-200, B onkho s , The Ne he lands) o H 2 and CO 2 wi h anges up o 1 mL/min. The empe a u e was con olled by placing he mic o eac o s inside a gas ch oma og aphy o en (Agilen 7890, San a Cla a, CA, USA) and he Ma e ials 2018,11, 2134 5 o 13 p essu e was con olled by a p essu e me e /con olle (EL-P ess se ies, B onkho s , The Ne he lands) loca ed a he exi o he o en. The mic o eac o consis ed o a 0.5 mm ID glass capilla y wi h an ex e nal diame e o 5 mm (Scho Du an, Mainz, Ge many). The mic o eac o was su ounded by a se o LEDs (Supe b igh , inspi ed LED), which p o ided a nominal powe o 9780 W/m 2 o whi e ligh in he mos homogeneous way o illumina e co ec ly he en i e eac o [ 18 ]. The gas ou le s eam was measu ed wi h a Mic o Gas Ch oma og aph (CP-4900, Va ian, Palo Al o, CA, USA) equipped wi h wo columns: a po aplo 10 m and a 5A molsie e. 2.6. Ca alys Cha ac e iza ion Ligh abso bance es s o he ZnO/Cu ca alys we e pe o med using a UV-Vis Spec opho ome e (Shimadzu UV 2550, Shimadzu, Kyo o, Japan). T ansmission Elec on Mic oscopy was ca ied ou wi h a JEOL JEM-2100F UHR (JEOL, Tokyo, Japan) o ca alys nanopa icles, and wi h a JEOL JEM-FS2200 HRP (JEOL, Tokyo, Japan) o silica nanocomposi es. The chemical s uc u e o he ae ogel was s udied by Fou ie T ans o m In a ed Spec oscopy (FTIR model TENSOR om B uke , Bille ica, MA, USA). Accele a ed Su ace A ea and Po osime y Sys ems (ASAP 2020 and 2420 om Mic ome i ics, A lan a, GA, USA) we e used o calcula e he speci ic su ace a ea and o de e mine he ni ogen iso he mal adso p ion—deso p ion cu e. 3. Resul s and Discussion 3.1. Ca alys Cha ac e iza ion T ansmission Elec on Mic oscopy was used o check he co ec o ma ion o ZnO nano ods, and o p o e ha coppe nanopa icles we e deposi ed on o nano ods su ace (Figu e 3) be o e loading he ca alys in he ae ogel. Ma e ials 2018, 11, x FOR PEER REVIEW 5 o 13 2.6. Ca alys Cha ac e iza ion Ligh abso bance es s o he ZnO/Cu ca alys we e pe o med using a UV-Vis Spec opho ome e (Shimadzu UV 2550, Shimadzu, Kyo o, Japan). T ansmission Elec on Mic oscopy was ca ied ou wi h a JEOL JEM-2100F UHR (JEOL, Tokyo, Japan) o ca alys nanopa icles, and wi h a JEOL JEM-FS2200 HRP (JEOL, Tokyo, Japan) o silica nanocomposi es. The chemical s uc u e o he ae ogel was s udied by Fou ie T ans o m In a ed Spec oscopy (FTIR model TENSOR om B uke , Bille ica, MA, USA). Accele a ed Su ace A ea and Po osime y Sys ems (ASAP 2020 and 2420 om Mic ome i ics, A lan a, GA, USA) we e used o calcula e he speci ic su ace a ea and o de e mine he ni ogen iso he mal adso p ion—deso p ion cu e. 3. Resul s and Discussion 3.1. Ca alys Cha ac e iza ion T ansmission Elec on Mic oscopy was used o check he co ec o ma ion o ZnO nano ods, and o p o e ha coppe nanopa icles we e deposi ed on o nano ods su ace (Figu e 3) be o e loading he ca alys in he ae ogel. Figu e 3. TEM images o ZnO nano ods (le ) and coppe nanopa icles deposi ed on o ZnO nano ods ( igh ). These TEM images con i med ha ZnO nano ods we e co ec ly c ea ed and disc e e coppe nanopa icles we e o med on he ZnO su ace wi hou agglome a ion as i was expec ed. Mo eo e , he imp egna ion o he ca alys inside he silica ne was also s udied in o de o ensu e ca alys was p ope ly loaded (Figu e 4). Figu e 3. TEM images o ZnO nano ods (le ) and coppe nanopa icles deposi ed on o ZnO nano ods ( igh ). These TEM images con i med ha ZnO nano ods we e co ec ly c ea ed and disc e e coppe nanopa icles we e o med on he ZnO su ace wi hou agglome a ion as i was expec ed. Mo eo e , he imp egna ion o he ca alys inside he silica ne was also s udied in o de o ensu e ca alys was p ope ly loaded (Figu e 4). The dis ibu ion o silica, zinc and coppe in he s uc u e was ob ained by using da k ield echnique selec ing he e lec ions o each elemen . Silica was he main componen and appea ed h oughou he s uc u e, while zinc oxide nanopa icles we e ound all oge he inside he silica s uc u e. Mos o he coppe nanopa icles we e loca ed in he zinc oxide a ea, indica ing ha hey we e co ec ly deposi ed on o ZnO nano ods. Howe e , an impo an amoun o coppe nanopa icles we e ound away om zinc oxide nano ods gi en ha hey we e no co ec ly deposi ed, a ec ing he Ma e ials 2018,11, 2134 6 o 13 e iciency o he p ocess. This means ha he Cu/ZnO ca alys nanos uc u e was a ec ed du ing i s in oduc ion in o he ae ogel ne . This also sugges s he possibili y o maldis ibu ion o he ca alys inside he ae ogel. Ma e ials 2018, 11, x FOR PEER REVIEW 6 o 13 Figu e 4. TEM image o a silica ae ogel wi h ca alys and da k ield o iden i y silica (Si), zinc (Zn) and coppe (Cu) dis ibu ion. The dis ibu ion o silica, zinc and coppe in he s uc u e was ob ained by using da k ield echnique selec ing he e lec ions o each elemen . Silica was he main componen and appea ed h oughou he s uc u e, while zinc oxide nanopa icles we e ound all oge he inside he silica s uc u e. Mos o he coppe nanopa icles we e loca ed in he zinc oxide a ea, indica ing ha hey we e co ec ly deposi ed on o ZnO nano ods. Howe e , an impo an amoun o coppe nanopa icles we e ound away om zinc oxide nano ods gi en ha hey we e no co ec ly deposi ed, a ec ing he e iciency o he p ocess. This means ha he Cu/ZnO ca alys nanos uc u e was a ec ed du ing i s in oduc ion in o he ae ogel ne . This also sugges s he possibili y o maldis ibu ion o he ca alys inside he ae ogel. Ligh abso p ion o his bime allic ca alys was es ed by UV-Vis Spec opho ome y (Figu e 5) in o de o p o e bo h ma e ials we e abso bing ligh in he isible band (coppe ) and ul a iole band (zinc oxide). ZnO nano ods exhibi ed a peak a 370 nm, and Cu nanopa icles induced a peak a ound 600 nm, which co esponded o he abso p ion o he ange be ween o ange and ed colou [22]. The b oad de ini ion o a peak was due o he a iable size o nanopa icles, mo ing in a na ow ange o sizes bu no enough o de ine a clea peak. Si Cu Z n Figu e 4. TEM image o a silica ae ogel wi h ca alys and da k ield o iden i y silica (Si), zinc (Zn) and coppe (Cu) dis ibu ion. Ligh abso p ion o his bime allic ca alys was es ed by UV-Vis Spec opho ome y (Figu e 5) in o de o p o e bo h ma e ials we e abso bing ligh in he isible band (coppe ) and ul a iole band (zinc oxide). ZnO nano ods exhibi ed a peak a 370 nm, and Cu nanopa icles induced a peak a ound 600 nm, which co esponded o he abso p ion o he ange be ween o ange and ed colou [ 22 ]. The b oad de ini ion o a peak was due o he a iable size o nanopa icles, mo ing in a na ow ange o sizes bu no enough o de ine a clea peak. FTIR analysis was used o s udy he in luence o ca alys loading on he s uc u e o he ae ogel. Wi h his pu pose, silica ae ogel wi hou ca alys was i s analysed, in o de o ob ain he IR spec a o he silica ma e ial and iden i y all i s chemical bonds. A e ha , wo di e en amoun s o ca alys load (7.0 and 10.0% in weigh ) we e analysed o s udy i hei addi ion a ec ed he silica s uc u e o he nanopa icles we e loca ed inside he po ous s uc u e wi hou c ea ing new bonds (Figu e 6). The main peaks o he silica s uc u e we e iden i ied in he silica ae ogel spec a. The mos in ense and b oad peak was ound a 1050 cm −1 , wi h a shoulde a a ound 1200 cm −1 due o Si-O-Si asymme ic s e ching ib a ions [ 23 ]. The symme ic s e ching ib a ions o Si-O-Si we e obse ed a 800 cm −1 . When he ca alys was imp egna ed in he ae ogel, no signi ican peaks appea ed on he spec a, and he silica s uc u e peaks we e s ill co ec ly o med, concluding ha ca alys loads did no a ec he chemical s uc u e o he ae ogel. Ma e ials 2018,11, 2134 7 o 13 Ma e ials 2018, 11, x FOR PEER REVIEW 7 o 13 Figu e 5. UV-Vis abso p ion spec a o ZnO/Cu ca alys . FTIR analysis was used o s udy he in luence o ca alys loading on he s uc u e o he ae ogel. Wi h his pu pose, silica ae ogel wi hou ca alys was i s analysed, in o de o ob ain he IR spec a o he silica ma e ial and iden i y all i s chemical bonds. A e ha , wo di e en amoun s o ca alys load (7.0 and 10.0% in weigh ) we e analysed o s udy i hei addi ion a ec ed he silica s uc u e o he nanopa icles we e loca ed inside he po ous s uc u e wi hou c ea ing new bonds (Figu e 6). Figu e 6. FTIR spec a o silica ae ogel wi hou and wi h ca alys load. The main peaks o he silica s uc u e we e iden i ied in he silica ae ogel spec a. The mos in ense and b oad peak was ound a 1050 cm−1, wi h a shoulde a a ound 1200 cm−1 due o Si-O-Si asymme ic s e ching ib a ions [23]. The symme ic s e ching ib a ions o Si-O-Si we e obse ed a 800 cm−1. When he ca alys was imp egna ed in he ae ogel, no signi ican peaks appea ed on he spec a, and he silica s uc u e peaks we e s ill co ec ly o med, concluding ha ca alys loads did no a ec he chemical s uc u e o he ae ogel. Same samples we e analysed o de e mine he su ace a ea and po e olume depending on ca alys load. Bo h p ope ies we e calcula ed om N2 iso he m adso p ion—deso p ion cu e, ha showed o all he samples a ype IV iso he m cu e ha is usual o mesopo ous silica ae ogels [24] (Figu e 7). Almos no di e ences we e ound when 7.0% and 10.0% ca alys we e loaded in he ae ogel. 10.0% Ca alys 7.0% Ca alys Silica ae ogel Figu e 5. UV-Vis abso p ion spec a o ZnO/Cu ca alys . Ma e ials 2018, 11, x FOR PEER REVIEW 7 o 13 Figu e 5. UV-Vis abso p ion spec a o ZnO/Cu ca alys . FTIR analysis was used o s udy he in luence o ca alys loading on he s uc u e o he ae ogel. Wi h his pu pose, silica ae ogel wi hou ca alys was i s analysed, in o de o ob ain he IR spec a o he silica ma e ial and iden i y all i s chemical bonds. A e ha , wo di e en amoun s o ca alys load (7.0 and 10.0% in weigh ) we e analysed o s udy i hei addi ion a ec ed he silica s uc u e o he nanopa icles we e loca ed inside he po ous s uc u e wi hou c ea ing new bonds (Figu e 6). Figu e 6. FTIR spec a o silica ae ogel wi hou and wi h ca alys load. The main peaks o he silica s uc u e we e iden i ied in he silica ae ogel spec a. The mos in ense and b oad peak was ound a 1050 cm−1, wi h a shoulde a a ound 1200 cm−1 due o Si-O-Si asymme ic s e ching ib a ions [23]. The symme ic s e ching ib a ions o Si-O-Si we e obse ed a 800 cm−1. When he ca alys was imp egna ed in he ae ogel, no signi ican peaks appea ed on he spec a, and he silica s uc u e peaks we e s ill co ec ly o med, concluding ha ca alys loads did no a ec he chemical s uc u e o he ae ogel. Same samples we e analysed o de e mine he su ace a ea and po e olume depending on ca alys load. Bo h p ope ies we e calcula ed om N2 iso he m adso p ion—deso p ion cu e, ha showed o all he samples a ype IV iso he m cu e ha is usual o mesopo ous silica ae ogels [24] (Figu e 7). Almos no di e ences we e ound when 7.0% and 10.0% ca alys we e loaded in he ae ogel. 10.0% Ca alys 7.0% Ca alys Silica ae ogel Figu e 6. FTIR spec a o silica ae ogel wi hou and wi h ca alys load. Same samples we e analysed o de e mine he su ace a ea and po e olume depending on ca alys load. Bo h p ope ies we e calcula ed om N 2 iso he m adso p ion—deso p ion cu e, ha showed o all he samples a ype IV iso he m cu e ha is usual o mesopo ous silica ae ogels [ 24 ] (Figu e 7). Almos no di e ences we e ound when 7.0% and 10.0% ca alys we e loaded in he ae ogel. The BET su ace a ea calcula ed was 845.3 m 2 /g o silica ae ogels, and i was sligh ly educed o 843.4 and 842.7 m 2 /g o 7.0 and 10.0% ca alys load, espec i ely. This indica ed ha he p esence o ca alys did no ha e a big in luence on he ex u al p ope ies o he silica ae ogel. Howe e , he inclusion o he ca alys in he po ous s uc u e had an in luence on he po e olume. Fo silica ae ogels he BJH po e olume calcula ed was 2.91 cm 3 /g, being educed o 2.46 and 2.29 cm 3 /g o 7 and 10% ca alys load, espec i ely. Loading mo e ca alys led o lowe po e olume. I is in e es ing ha al hough he po e olume is educed he su ace a ea emains cons an , in spi e o he inc ease o ca aly ic ma e ial loaded. This e ec could be owed o he al eady obse ed maldis ibu ion (e.g., agglome a ion) o he ca alys inside he ae ogel s uc u e which inc eases a highe loads. This could, in e ec , lea e mos o he su ace a ea ee. This sugges s ha mo e s udies a e necessa y o he imp o emen o he dispe sion o he ma e ial and i s cha ac e iza ion. Ma e ials 2018,11, 2134 8 o 13 Ma e ials 2018, 11, x FOR PEER REVIEW 8 o 13 Figu e 7. Iso he m adso p ion—deso p ion cu e o silica ae ogels. The BET su ace a ea calcula ed was 845.3 m2/g o silica ae ogels, and i was sligh ly educed o 843.4 and 842.7 m2/g o 7.0 and 10.0% ca alys load, espec i ely. This indica ed ha he p esence o ca alys did no ha e a big in luence on he ex u al p ope ies o he silica ae ogel. Howe e , he inclusion o he ca alys in he po ous s uc u e had an in luence on he po e olume. Fo silica ae ogels he BJH po e olume calcula ed was 2.91 cm3/g, being educed o 2.46 and 2.29 cm3/g o 7 and 10% ca alys load, espec i ely. Loading mo e ca alys led o lowe po e olume. I is in e es ing ha al hough he po e olume is educed he su ace a ea emains cons an , in spi e o he inc ease o ca aly ic ma e ial loaded. This e ec could be owed o he al eady obse ed maldis ibu ion (e.g., agglome a ion) o he ca alys inside he ae ogel s uc u e which inc eases a highe loads. This could, in e ec , lea e mos o he su ace a ea ee. This sugges s ha mo e s udies a e necessa y o he imp o emen o he dispe sion o he ma e ial and i s cha ac e iza ion. 3.2. Ca bon Dioxide Hyd ogena ion A e he p oo o concep p e iously done wo king a 20 ba and 50 and 70 °C [18], a comple e s udy o he in luence o di e en pa ame e s has been done. The a iables whose in luence has been s udied we e he p ehea ing empe a u e o gases, he p essu e, he gas inle mass lows and he amoun o ca alys . The esul s a e p esen ed as a unc ion o he ime elapsed since he low (and illumina ion) was s a ed. 3.2.1. In luence o Tempe a u e The i s pa ame e s udied was he empe a u e o gases p ehea ing. Visual ligh was he main sou ce o ene gy o he ca bon dioxide hyd ogena ion, al hough gases empe a u e could also ha e an in luence on he p ocess. In o de o analyze his a iable, se e al expe imen s we e pe o med. Fi s o all, an ini ial expe imen wo king wi hou connec ing he LEDs was ca ied ou in o de o analyze i he eac ion could be pe o med only wi h empe a u e. Wo king always a 20 ba , 10.0% ca alys loaded and lows o 1 mL/min o hyd ogen and 0.33 mL/min o ca bon dioxide, expe imen s a 50, 100 and 150 °C we e done and he conclusion was he same in all he cases: no con e sion o ca bon dioxide was p oduced. A e his, he same expe imen s we e pe o med u ning on he ligh s. In his case, ca bon dioxide s a ed o be ans o med (Figu e 8). Figu e 8 showed ha p ehea ing o gases had almos no in luence on he CO2 con e sion a e and he esul s we e almos cons an . This caused p oximi y be ween he da a poin s, and hus e o ba s we e excluded. Howe e , he gases low con ol sys em did no allow o a oid p ehea ing. Flow 0 500 1000 1500 2000 0 0.2 0.4 0.6 0.8 1 Quan i y Adso bed (cm³/g STP) Rela i e P essu e (P/Po) Adso p ion Deso p ion Figu e 7. Iso he m adso p ion—deso p ion cu e o silica ae ogels. 3.2. Ca bon Dioxide Hyd ogena ion A e he p oo o concep p e iously done wo king a 20 ba and 50 and 70 ◦ C [ 18 ], a comple e s udy o he in luence o di e en pa ame e s has been done. The a iables whose in luence has been s udied we e he p ehea ing empe a u e o gases, he p essu e, he gas inle mass lows and he amoun o ca alys . The esul s a e p esen ed as a unc ion o he ime elapsed since he low (and illumina ion) was s a ed. 3.2.1. In luence o Tempe a u e The i s pa ame e s udied was he empe a u e o gases p ehea ing. Visual ligh was he main sou ce o ene gy o he ca bon dioxide hyd ogena ion, al hough gases empe a u e could also ha e an in luence on he p ocess. In o de o analyze his a iable, se e al expe imen s we e pe o med. Fi s o all, an ini ial expe imen wo king wi hou connec ing he LEDs was ca ied ou in o de o analyze i he eac ion could be pe o med only wi h empe a u e. Wo king always a 20 ba , 10.0% ca alys loaded and lows o 1 mL/min o hyd ogen and 0.33 mL/min o ca bon dioxide, expe imen s a 50, 100 and 150 ◦ C we e done and he conclusion was he same in all he cases: no con e sion o ca bon dioxide was p oduced. A e his, he same expe imen s we e pe o med u ning on he ligh s. In his case, ca bon dioxide s a ed o be ans o med (Figu e 8). Figu e 8showed ha p ehea ing o gases had almos no in luence on he CO 2 con e sion a e and he esul s we e almos cons an . This caused p oximi y be ween he da a poin s, and hus e o ba s we e excluded. Howe e , he gases low con ol sys em did no allow o a oid p ehea ing. Flow con olle s could no s abilize pe ec ly lows when he gases we e cold, and hey wo ked much be e wi h a small hea ing. Fo his eason, i was decided o wo k always a 50 ◦ C o he nex expe imen s. Ma e ials 2018,11, 2134 9 o 13 Ma e ials 2018, 11, x FOR PEER REVIEW 9 o 13 con olle s could no s abilize pe ec ly lows when he gases we e cold, and hey wo ked much be e wi h a small hea ing. Fo his eason, i was decided o wo k always a 50 °C o he nex expe imen s. Figu e 8. In luence o p ehea ing empe a u e on he eac ion. 3.2.2. In luence o Ca alys Amoun The nex a iable s udied was he amoun o ca alys . Va ia ion o he amoun inside he ae ogel was eally easy, because he ca alys was di ec ly dilu ed in he me hanol needed as a p ecu so o he silica ae ogel. Howe e , 13.0% ca alys loaded in he ae ogel was he maximum amoun loaded, due o gela ion p oblems o highe loads. These highe loads equi ed longe gela ion imes, which p omo ed he p ecipi a ion o he ca alys , and as a esul , he ca alys was no well dis ibu ed h oughou he silica ne . Because o his, expe imen s wi h 7.0, 10.0 and 13.0% ca alys deposi ed we e pe o med, wo king always a 20 ba , 50 °C and lows o 1ml/min o hyd ogen and 0.33 mL/min o ca bon dioxide (Figu e 9). Figu e 9. In luence o ca alys load on he eac ion a 20 ba (7.0% ■, 10.0% •, 13.0% ▲). Da a poin s ep esen he a e age o 3 expe imen s, wi h e o ba s co esponding o a 95% con idence in e al. Figu e 9 showed ha CO2 con e sion a es we e highe when he amoun o ca alys dec eased. Using mo e ca alys did no help o imp o e he p ocess, a aining he opposi e e ec . Al hough me allic nanopa icles did no p ecipi a e, hey could be agglome a ed inside he silica ne . Agglome a ion p ocess en ailed a educ ion in he a ea used o ca bon dioxide hyd ogena ion, and CO2 Con e sion a e (mmol/gca min) CO2 Con e sion a e (mmol/gca min) Figu e 8. In luence o p ehea ing empe a u e on he eac ion. 3.2.2. In luence o Ca alys Amoun The nex a iable s udied was he amoun o ca alys . Va ia ion o he amoun inside he ae ogel was eally easy, because he ca alys was di ec ly dilu ed in he me hanol needed as a p ecu so o he silica ae ogel. Howe e , 13.0% ca alys loaded in he ae ogel was he maximum amoun loaded, due o gela ion p oblems o highe loads. These highe loads equi ed longe gela ion imes, which p omo ed he p ecipi a ion o he ca alys , and as a esul , he ca alys was no well dis ibu ed h oughou he silica ne . Because o his, expe imen s wi h 7.0, 10.0 and 13.0% ca alys deposi ed we e pe o med, wo king always a 20 ba , 50 ◦ C and lows o 1ml/min o hyd ogen and 0.33 mL/min o ca bon dioxide (Figu e 9). Ma e ials 2018, 11, x FOR PEER REVIEW 9 o 13 con olle s could no s abilize pe ec ly lows when he gases we e cold, and hey wo ked much be e wi h a small hea ing. Fo his eason, i was decided o wo k always a 50 °C o he nex expe imen s. Figu e 8. In luence o p ehea ing empe a u e on he eac ion. 3.2.2. In luence o Ca alys Amoun The nex a iable s udied was he amoun o ca alys . Va ia ion o he amoun inside he ae ogel was eally easy, because he ca alys was di ec ly dilu ed in he me hanol needed as a p ecu so o he silica ae ogel. Howe e , 13.0% ca alys loaded in he ae ogel was he maximum amoun loaded, due o gela ion p oblems o highe loads. These highe loads equi ed longe gela ion imes, which p omo ed he p ecipi a ion o he ca alys , and as a esul , he ca alys was no well dis ibu ed h oughou he silica ne . Because o his, expe imen s wi h 7.0, 10.0 and 13.0% ca alys deposi ed we e pe o med, wo king always a 20 ba , 50 °C and lows o 1ml/min o hyd ogen and 0.33 mL/min o ca bon dioxide (Figu e 9). Figu e 9. In luence o ca alys load on he eac ion a 20 ba (7.0% ■, 10.0% •, 13.0% ▲). Da a poin s ep esen he a e age o 3 expe imen s, wi h e o ba s co esponding o a 95% con idence in e al. Figu e 9 showed ha CO2 con e sion a es we e highe when he amoun o ca alys dec eased. Using mo e ca alys did no help o imp o e he p ocess, a aining he opposi e e ec . Al hough me allic nanopa icles did no p ecipi a e, hey could be agglome a ed inside he silica ne . Agglome a ion p ocess en ailed a educ ion in he a ea used o ca bon dioxide hyd ogena ion, and CO2 Con e sion a e (mmol/gca min) CO2 Con e sion a e (mmol/gca min) Figu e 9. In luence o ca alys load on he eac ion a 20 ba (7.0%  , 10.0% • , 13.0% N ). Da a poin s ep esen he a e age o 3 expe imen s, wi h e o ba s co esponding o a 95% con idence in e al. Figu e 9showed ha CO 2 con e sion a es we e highe when he amoun o ca alys dec eased. Using mo e ca alys did no help o imp o e he p ocess, a aining he opposi e e ec . Al hough me allic nanopa icles did no p ecipi a e, hey could be agglome a ed inside he silica ne . Agglome a ion p ocess en ailed a educ ion in he a ea used o ca bon dioxide hyd ogena ion, and his could lead o a dec ease in he CO 2 con e sion a e. In he same way, loading mo e ca alys