Stability of a NiAl2O4 Derived Catalyst in the Ethanol Steam Reforming in Reaction-Regeneration Cycles: Effect of Reduction Temperature
Abstract
This research was funded by the Ministry of Science and Innovation of the Spanish Government (grant RTI2018-100771-B-I00 and PhD grant BES-2019-090943 for S.I.-V. funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe”); the European Commission (HORIZON H2020-MSCA RISE 2018. Contract No. 823745); the Department of Education, Universities and Investigation of the Basque Government, grant number IT1645-22.
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Citation: Iglesias-Vázquez, S.; Valecillos, J.; Remiro, A.; Bilbao, J.; Gayubo, A.G. Stability of a NiAl2O4 Derived Catalyst in the Ethanol Steam Reforming in Reaction-Regeneration Cycles: Effect of Reduction Temperature. Catalysts 2022,12, 550. https://doi.org/ 10.3390/catal12050550 Academic Editor: Binlin Dou Received: 26 April 2022 Accepted: 16 May 2022 Published: 17 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). catalysts Article Stability of a NiAl2O4Derived Catalyst in the Ethanol Steam Reforming in Reaction-Regeneration Cycles: Effect of Reduction Temperature Sergio Iglesias-Vázquez , JoséValecillos * , Aingeru Remiro , Javier Bilbao and Ana Guadalupe Gayubo Department of Chemical Engineering, University of the Basque Country (UPV/EHU), P.O. Box 644, 48080 Bilbao, Spain ; [email protected] (S.I.-V.); aingeru.r[email protected] (A.R.); javier[email protected] (J.B.); [email protected] (A.G.G.) *Correspondence: [email protected]; Tel.: +34-946-01-53-41 Abstract: The catalyst regeneration is still a challenge to make the ethanol steam reforming (ESR) process feasible for sustainable H 2 production. NiAl 2 O 4 spinel derived catalysts are highly active and selective for ESR, but they require avoiding irreversible deactivation to ensure their regeneration. Their stability depends on the catalyst structure, and herein we report different Ni/Al 2 O 3 -NiAl 2 O 4 catalysts obtained upon reduction of a NiAl 2 O 4 spinel at 700, 750, or 850 ◦ C. The catalysts were tested in ESR reaction-regeneration cycles, with reaction at 600 ◦ C and regeneration by coke combustion at 850 ◦ C followed by reduction at the corresponding temperature. The fresh, spent, and regenerated catalysts were characterized using X-ray diffraction, N 2 physisorption, temperature programmed reduction and oxidation, and scanning electron microscopy. The irreversible deactivation is due to Ni volatilization and catalyst particle fragmentation. These phenomena are prompted by a high filamentous carbon deposition favored by the Al 2 O 3 content in the catalyst. The reduction in the 700–750 ◦ C range is optimum for controlling the Al 2 O 3 content, increasing the NiAl 2 O 4 /Al 2 O 3 ratio in the resulting catalyst. These catalysts show a period of partial reversible deactivation by coke with a change in the H 2 formation mechanism reaching a pseudo-stable state with a H 2 yield of 40% and a reproducible performance in successive reaction-regeneration cycles. Keywords: hydrogen; ethanol steam reforming (ESR); Ni catalyst; NiAl 2 O 4 spinel; catalyst deactivation; coke; catalyst regeneration; reduction temperature 1. Introduction Hydrogen is a promising energy vector [ 1 ] and raw material for the synthesis of commodities [ 2 ], complying with sustainability precepts when it is obtained from renewable raw materials. H 2 production from ethanol via steam reforming (ESR) is a suitable alternative since ethanol can be obtained sustainably from biomass (bio-ethanol) by fermentation/hydrolysis with no need to separate the water and is easy to handle with relative low risks in comparison with other feedstock for the H 2 production [ 3 , 4 ]. Moreover, the ESR fundamentals are similar to those of CH 4 steam reforming (MSR) which is the basis of the largest technology for H 2 production from natural gas extended worldwide [ 5 ], representing an advantage for making an easier adaptation and transition to sustainable H2production from ethanol. The ESR process involves several reactions in a complex series-parallel scheme, in which ethanol reforming (Equation (1)) and water gas shift (Equation (2)) are the main reactions giving the global steam reforming reaction represented by Equation (3) [ 3 , 6 – 8 ]. However, ethanol may undergo dehydrogenation (Equation (4)), dehydration (Equation (5)), and decomposition (Equation (6)) yielding up more H 2 or intermediates (ethylene and acetaldehyde). Acetaldehyde and ethylene may undergo steam reforming ( Equations (7) and (8) , respectively) and decomposition (Equations (9) and (10), respectively). CH 4 may be formed Catalysts 2022,12, 550. https://doi.org/10.3390/catal12050550 https://www.mdpi.com/journal/catalysts
Catalysts 2022,12, 550 2 of 16 from some decomposition reactions (Equations (6) and (9)) or methanation (Equation (11)), and simultaneously may undergo steam reforming (reverse of Equation (11)) and decomposition (Equation (12)). CO may undergo disproportionation (Boudouard reaction, Equation (13)). The carbon formed from decomposition (Equations (10) and (12)) or CO disproportionation may undergo gasification (Equation (14)). C2H6O+H2O→2CO + 4H2(1) CO + H2OCO2+ H2(2) C2H6O + 3H2O→2CO2+ 6H2(3) C2H6O→C2H4O+H2(4) C2H6OC2H4+ H2O (5) C2H6O→CH4+CO+H2(6) C2H4O+H2O→2CO + 3H2(7) C2H4+ 2H2O→2CO + 3H2(8) C2H4O→CH4+ CO (9) C2H4→2C + H2(10) CO + 3H2CH4+ H2O (11) CH4→C + 2H2(12) 2CO C + CO2(13) C+H2O→CO + H2(14) The most suitable catalysts in terms of activity and costs for the ESR process are based on Co or Ni supported on resilient oxides, being Ni/Al 2 O 3 catalysts the widest studied [ 7 ]. Ni is highly active for breaking C-C bonds and absorbing and dissociating water [ 9 , 10 ]. On the other hand, Al 2 O 3 , often combined with La 2 O 3 or CeO 2 , is an appropriate support for a high Ni dispersion, which improves the catalytic performance and provides good mechanical strength to the catalyst particles for operation in most types of reactors. However, Ni catalysts are prone to suffer deactivation due mainly to carbon (coke) formation and deposition that blocks the access to active sites, and sintering of active sites that decreases the surface area or amount of active sites [ 11 , 12 ]. The deactivation by carbon deposition might be reversible since carbon deposits are eliminated by combustion and the catalyst might recover its activity. This catalyst regeneration capacity is a key factor to make the ESR processes feasible for large-scale operations. The carbon elimination by combustion is a critical step for a successful catalyst regeneration since the uncontrolled combustion may sinter Ni sites due to the large amount of heat released [ 13 ], causing the irreversible deactivation of the catalyst. Thus, many catalyst formulations have been proposed to the end of decreasing the carbon formation, increasing the catalyst lifetime and making possible its regeneration. Accordingly, Montero et al. [ 14 ] demonstrated the reproducible performance of a Ni/ α Al 2 O 3 -La 2 O 3 catalyst in ESR reaction-regeneration cycles (reaction at 500 and 700 ◦ C) upon an equilibration treatment consisting of a reaction-regeneration cycle with a reaction temperature of 700 ◦ C. Campos et al. [ 15 ] showed that a 1.0%Rh10%Ni/15%La 2 O 3 10%CeO 2 alumina supported catalyst can increase the H 2 selectivity with a stable operation and regeneration capacity for two ESR reaction-regeneration cycles at 500 ◦ C. Contreras et al. [ 16 ] demonstrated that various Ni, Co and Ni-Co catalysts supported on hydrotalcite (Mg and Al oxides) stabilized with W oxides have a good regeneration capacity in various ESR reaction-regeneration cycles at 600 ◦ C. Boudadi et al. [ 17 ] found a good activity recovery of various Ni catalysts with modified supports, including Ni/La-Al 2 O 3 , Ni/La/TiO 2 -Al 2 O 3 , and Ni/La-clay, in ESR
Catalysts 2022,12, 550 3 of 16 reaction-regeneration cycles at 500 ◦ C. Di Michele et al. [ 18 ] proved Ni/MgAl 2 O 4 is a stable catalyst for the ESR reaction with low carbon formation leading to an easy regeneration. A remarkable and reproducible Ni catalyst easily prepared from the reduction of NiAl 2 O 4 spinel is quite active for the steam reforming of bio-oil [ 19 , 20 ] and ethanol [ 21 ]. Accordingly, the NiAl 2 O 4 spinel precursor is obtained by co-precipitation of Ni and Al sources followed by calcination, and then the synthesized spinel is completely reduced in H 2 to obtain a Ni/Al 2 O 3 catalyst with high Ni dispersion. This catalyst has been proven to have a moderate carbon deposition in the bio-oil steam reforming (BSR) and recovers its activity when it is used in reaction-regeneration cycles [ 22 ]. The regeneration of this catalyst consists of two steps: (i) coke elimination by combustion at 850 ◦ C in air, which also reconstructs the NiAl 2 O 4 spinel structure; and (ii) reduction of the spinel at 850 ◦ C in H 2 resulting in the highly dispersed Ni/Al 2 O 3 catalyst. This performance in the stability and regeneration capacity makes it a promising alternative over others for the steam reforming of oxygenates, since the formulation is simple and reproducible. However, the use of this catalyst in the ESR leads to a high carbon formation due to the presence of acidic Al 2 O 3 that favors the ethanol dehydration reaction yielding ethylene (Equation (5)), and favoring the subsequent reactions of oligomerization, aromatization and condensation into carbon structures (coke) (Equation (10)) [ 21 ]. This high carbon formation and nature may affect the regeneration capacity of this catalyst due to the possible uncontrollable combustion and other issues that cause an irreversible deactivation. In this work, we propose to attenuate the deactivation and achieve the regeneration of the catalyst derived from NiAl 2 O 4 spinel, by decreasing the Al 2 O 3 presence to control the carbon formation. The simple and reproducible strategy to this end is decreasing the reduction temperature of the spinel, which leads to an incomplete reduction of the Ni species resulting in a Ni/Al 2 O 3 -NiAl 2 O 3 catalyst with lower Al 2 O 3 content than in the Ni/Al 2 O 3 catalyst prepared from the spinel reduction at 850 ◦ C. This strategy has been effective for the catalyst equilibration in the glycerol aqueous-phase reforming [ 23 ], and it is studied in this work for the ESR in reaction-regeneration cycles. 2. Results 2.1. Fresh Catalyst Properties Figure 1a shows the temperature programmed reduction (TPR) profile of the NiAl 2 O 4 synthetized. The TPR profile evidences that the Ni reduction takes place between 500 and 900 ◦ C with a maximum at 800 ◦ C, characteristic of the reduction of Ni species in the spinel structure. Likewise, the absence of reduction peaks at lower temperatures indicates that no free nickel oxides are significantly present. This is verified by the XRD pattern (plotted in Figure 1b), evidencing the absence of NiO crystalline phases and the only presence of NiAl 2 O 4 spinel. Thus, the reduction of all Ni species results in Ni crystals supported on Al 2 O 3 , which is possible at reduction temperatures above 850 ◦ Cfor4h[ 19 ]. Likewise, the Ni content calculated from the TPR profile is 34.7 wt% in NiAl 2 O 4 or 38.7 wt% in Ni/Al 2 O 3 , being very close to the stoichiometric values of 33.2 and 36.5 wt%, respectively. The catalysts have been prepared by reducing the NiAl 2 O 4 spinel precursor at 700, 750 and 850 ◦ C, and they are named according to their reduction temperature, as R-700, R-750 and R-850, respectively. These catalysts have been characterized by several techniques to determine their more relevant properties. Figure 1b shows the X-ray diffraction (XRD) patterns of the catalysts and the NiAl 2 O 4 precursor to determine the crystalline phases present in each material. The NiAl 2 O 4 shows diffraction peaks at 2 θ = 37.2, 45.3, 59.9 and 65.7 ◦ corresponding to a typical spinel cubic structure (PDF 01-071-0965) [ 21 , 23 ]. Upon reduction at 850 ◦ C, the NiAl 2 O 4 is almost completely converted into reduced Ni crystals (2 θ = 44.6, 51.9 and 76.5 ◦ ) (PDF 04-010-6148) and Al 2 O 3 (2 θ = 37.5, 45.9 and 66.9 ◦ ) (PDF 04-005-4662), being a Ni/Al 2 O 3 catalyst (R-850). In contrast, the catalysts obtained upon reduction at 700 ◦ C (R-700) or 750 ◦ C (R-750) show peaks of the NiAl 2 O 4 , reduced Ni crystals and Al 2 O 3 , indicating that a partial Ni reduction takes place since the reduction temperature is insufficient for a complete reduction according to the TPR data (Figure 1a).
Catalysts 2022,12, 550 4 of 16 Thus, these catalysts are composed of reduced Ni crystals supported on NiAl 2 O 4 and Al 2 O 3 (Ni/Al 2 O 3 -NiAl 2 O 4 catalysts). The average Ni crystal size was determined from the XRD data using the Scherrer equation (with diffraction peak at 51.9 ◦ ), and the results (listed in Table 1) show that the Ni crystal size increases with increasing reduction temperatures. Likewise, the content of reduced Ni crystals was estimated from the TPR data according to the reduction temperature employed, and the results (listed in Table 1) confirm that this content increases with increasing reduction temperatures, as expected. Therefore, the decrease in the reduction temperature leads to a lower content of reduced Ni crystals but it also shortens the extent of Ni sintering, which results in lower average Ni crystal sizes. Catalysts2022,12,5504of16 (XRD)patternsofthecatalystsandtheNiAl2O4precursortodeterminethecrystalline phasespresentineachmaterial.TheNiAl2O4showsdiffractionpeaksat2θ=37.2,45.3, 59.9and65.7°correspondingtoatypicalspinelcubicstructure(PDF01‐071‐0965)[21,23]. Uponreductionat850°C,theNiAl2O4isalmostcompletelyconvertedintoreducedNi crystals(2θ=44.6,51.9and76.5°)(PDF04‐010‐6148)andAl2O3(2θ=37.5,45.9and66.9°) (PDF04‐005‐4662),beingaNi/Al2O3catalyst(R‐850).Incontrast,thecatalystsobtained uponreductionat700°C(R‐700)or750°C(R‐750)showpeaksoftheNiAl2O4,reducedNi crystalsandAl2O3,indicatingthatapartialNireductiontakesplacesincethereduction temperatureisinsufficientforacompletereductionaccordingtotheTPRdata(Figure1a). Thus,thesecatalystsarecomposedofreducedNicrystalssupportedonNiAl2O4and Al2O3(Ni/Al2O3‐NiAl2O4catalysts).TheaverageNicrystalsizewasdeterminedfromthe XRDdatausingtheScherrerequation(withdiffractionpeakat51.9°),andtheresults (listedinTable1)showthattheNicrystalsizeincreaseswithincreasingreductiontem‐ peratures.Likewise,thecontentofreducedNicrystalswasestimatedfromtheTPRdata accordingtothereductiontemperatureemployed,andtheresults(listedinTable1)con‐ firmthatthiscontentincreaseswithincreasingreductiontemperatures,asexpected. Therefore,thedecreaseinthereductiontemperatureleadstoalowercontentofreduced NicrystalsbutitalsoshortenstheextentofNisintering,whichresultsinloweraverage Nicrystalsizes. 300 400 500 600 700 800 900 1000 0 2 4 6 8 10 (a) Ni reduced (mg/(g min)) Temperature (ºC) 30 40 50 60 70 80 NiAl 2 O 4 R-700 R-750 R-850 21 13 3 3 2 Intensity (a.u.) Diffraction angle (º) 12 1 1. NiAl 2 O 4 2. Al 2 O 3 3. Ni (b) Figure1.CharacterizationoftheNiAl2O4precursorandcatalystsobtainedatdifferentreduction temperatures.(a)TPRprofileoftheNiAl2O4;(b)XRDpatternsoftheNiAl2O4andcatalysts. Table1.MainpropertiesoftheNiAl2O4spinelandderivedcatalystsatdifferentreductiontemper‐ atures. SampleNiContent (wt%) ReducedNi1 (wt%) SBET (m2g−1) Vpore (cm3g−1) Dpore (nm) NiCrystal Size (nm) Acidity (mmolg−1) NiAl2O4spinel34.7**‐ 78.30.1778.50‐ ‐ R‐700catalyst38.7*11.876.70.19710.59.100.026 R‐750catalyst38.7*17.774.00.20510.912.00.029 R‐850catalyst38.7*38.770.30.21212.617.00.044 SBET,specificsurfaceareacalculatedusingtheBrunauer‐Emmett‐Teller(BET)theory;Vpore,totalvol‐ umeofpores;Dpore,averageporediameter.*AmountofreducedNiintheNi/Al2O3orNi/Al2O3‐ NiAl2O4catalyst.**AmountofNiintheNiAl2O4spinel.1EstimatedfromtheTPRdata. ThetexturalpropertieslistedinTable1alsoevidencetheeffectofthetransitionof theNiAl2O4spineltoNi/Al2O3‐NiAl2O4catalysts.Accordingly,thespecificsurfacearea (SBET)decreaseswithincreasingreductiontemperatures,whereasthetotalporevolume (Vpore)andaverageporediameter(Dpore)increase.Thecatalystaciditywasdeterminedby Figure 1. Characterization of the NiAl 2 O 4 precursor and catalysts obtained at different reduction temperatures. (a) TPR profile of the NiAl2O4; (b) XRD patterns of the NiAl2O4and catalysts. Table 1. Main properties of the NiAl 2 O 4 spinel and derived catalysts at different reduction temperatures. Sample Ni Content (wt%) Reduced Ni 1 (wt%) SBET (m2g−1) Vpore (cm3g−1) Dpore (nm) Ni Crystal Size (nm) Acidity (mmol g−1) NiAl2O4spinel 34.7 ** - 78.3 0.177 8.50 - - R-700 catalyst 38.7 * 11.8 76.7 0.197 10.5 9.10 0.026 R-750 catalyst 38.7 * 17.7 74.0 0.205 10.9 12.0 0.029 R-850 catalyst 38.7 * 38.7 70.3 0.212 12.6 17.0 0.044 S BET , specific surface area calculated using the Brunauer-Emmett-Teller (BET) theory; V pore , total volume of pores; D pore , average pore diameter. * Amount of reduced Ni in the Ni/Al 2 O 3 or Ni/Al 2 O 3 -NiAl 2 O 4 catalyst. ** Amount of Ni in the NiAl2O4spinel. 1Estimated from the TPR data. The textural properties listed in Table 1also evidence the effect of the transition of the NiAl 2 O 4 spinel to Ni/Al 2 O 3 -NiAl 2 O 4 catalysts. Accordingly, the specific surface area (S BET ) decreases with increasing reduction temperatures, whereas the total pore volume (V pore ) and average pore diameter (D pore ) increase. The catalyst acidity was determined by means of NH 3 adsorption, and the results are listed in Table 1. As seen, the acidity increases with increasing reduction temperatures from 0.026 mmol g −1 at 700 ◦ C to 0.044 mmol g −1 at 850 ◦ C, which should be related to the increasing Al 2 O 3 content, with the presence of acidic γ-Al2O3phases [21]. In summary, the reduction at 850 ◦ C is sufficient to reduce all of the Ni species in the NiAl 2 O 4 spinel and obtain a Ni/Al 2 O 3 catalyst (R-850 catalyst), whereas the reduction at 700 and 750 ◦ C partially reduces the Ni species in the NiAl 2 O 4 spinel leading to obtain Ni/Al 2 O 3 -NiAl 2 O 4 catalysts (R-700 and R-750 catalysts). Therefore, the contents of reduced Ni species (active for the ESR reaction) and acidic Al 2 O 3 decrease as the reduction temperature decreases. In consequence, for the ESR reaction-regeneration cyclic tests, the space time is adjusted by using higher catalyst amounts when the reduction temperature
Catalysts 2022,12, 550 5 of 16 is lower, for providing similar amount of reduced Ni species in the catalytic bed with the purpose of obtaining comparable values of ethanol conversion and H2yield. 2.2. Catalyst Performance in ESR Reaction-Regeneration Cycles The performance of the catalysts in reaction-regeneration cycles was studied quantifying the time on stream (TOS) evolution of the ethanol conversion and product yields for the ESR using the R-850 (Figure 2), R-750 (Figure 3) and R-700 (Figure 4) catalysts. The main products observed are H 2 , CO, CO 2 , CH 4 , and C 2 H 4 and carbon deposited on the catalyst. The carbon yield was estimated from C balance taking into account the carbonaceous components in the feed and effluent streams. Catalysts2022,12,5506of16 Incontrast,fortheR‐700andR‐750catalysts,theinitialvaluesofH2,CO,CO2,and CH4yieldsandtheirTOSevolutionsinthesecondandthirdreactionsarequitecompara‐ blewiththoseofthefirstreactionforeachcatalyst.However,theTOSevolutionofthe C2H4yieldnotablydiffers,withanincreasingtrendinthevaluesandapparentprolonged periodofthemaximumvalueinthesuccessivecycles,whichmakestheethanolconver‐ sionalsoincreasecomparedwiththefirstreactionforeachcatalyst. ItshouldbenotedthattheH2yieldisalmostconstantupon4honstream,whereas theyieldsofotherproductschangeasaconsequenceofapartialcatalystdeactivationfor theextentofvariousreactions(Equations(1)–(14)).Thisobservationisconsistentwiththe H2formationmechanismoncatalystsderivedfromaNiAl2O4spinelproposedinaprevi‐ ouswork[21].Accordingly,thepartialcatalystdeactivationsequentiallyaffectstheH2 andcarbonformationfromethylene(Equation(10))thatexplainstheincreaseintheC2H4 yieldoverTOS,andtheethanoldehydration(Equation(5))thatexplainsthemaximum observedintheC2H4yieldintheR‐700andR‐750catalysts.Thereformingreactionscata‐ lyzedbyNisiteskeepconstanttheH2yield,andthesesiteskeeptheiraccessibilityfor thesereactionssincetheyalsocatalyzethecarbongasification[24]. 0123456 0 0.2 0.4 0.6 0.8 1.0 Conversion or yield Time on stream (h) Conversion H 2 CO 2 CO 2 CH 4 C 2 H 4 C 2 H 4 O C first reaction regeneration regeneration 01234 second reaction Time on stream (h) 01234 third reaction Time on stream (h) Figure2.TOSevolutionoftheethanolconversionandproductsyieldwithR‐850catalystinESR reaction‐regenerationcycles. 0123456 0 0.2 0.4 0.6 0.8 1.0 Conversion H2 CO2 CO CH4 C2H4 C2H4O C Conversion or yield Time on stream (h) first reaction regeneration second reaction regeneration third reaction 01234 Time on stream (h) 01234 Time on stream (h) Figure3.TOSevolutionoftheethanolconversionandproductsyieldwithR‐750catalystinESR reaction‐regenerationcycles. Figure 2. TOS evolution of the ethanol conversion and products yield with R-850 catalyst in ESR reaction-regeneration cycles. Catalysts2022,12,5506of16 Incontrast,fortheR‐700andR‐750catalysts,theinitialvaluesofH2,CO,CO2,and CH4yieldsandtheirTOSevolutionsinthesecondandthirdreactionsarequitecompara‐ blewiththoseofthefirstreactionforeachcatalyst.However,theTOSevolutionofthe C2H4yieldnotablydiffers,withanincreasingtrendinthevaluesandapparentprolonged periodofthemaximumvalueinthesuccessivecycles,whichmakestheethanolconver‐ sionalsoincreasecomparedwiththefirstreactionforeachcatalyst. ItshouldbenotedthattheH2yieldisalmostconstantupon4honstream,whereas theyieldsofotherproductschangeasaconsequenceofapartialcatalystdeactivationfor theextentofvariousreactions(Equations(1)–(14)).Thisobservationisconsistentwiththe H2formationmechanismoncatalystsderivedfromaNiAl2O4spinelproposedinaprevi‐ ouswork[21].Accordingly,thepartialcatalystdeactivationsequentiallyaffectstheH2 andcarbonformationfromethylene(Equation(10))thatexplainstheincreaseintheC2H4 yieldoverTOS,andtheethanoldehydration(Equation(5))thatexplainsthemaximum observedintheC2H4yieldintheR‐700andR‐750catalysts.Thereformingreactionscata‐ lyzedbyNisiteskeepconstanttheH2yield,andthesesiteskeeptheiraccessibilityfor thesereactionssincetheyalsocatalyzethecarbongasification[24]. 0123456 0 0.2 0.4 0.6 0.8 1.0 Conversion or yield Time on stream (h) Conversion H 2 CO 2 CO 2 CH 4 C 2 H 4 C 2 H 4 O C first reaction regeneration regeneration 01234 second reaction Time on stream (h) 01234 third reaction Time on stream (h) Figure2.TOSevolutionoftheethanolconversionandproductsyieldwithR‐850catalystinESR reaction‐regenerationcycles. 0123456 0 0.2 0.4 0.6 0.8 1.0 Conversion H2 CO2 CO CH4 C2H4 C2H4O C Conversion or yield Time on stream (h) first reaction regeneration second reaction regeneration third reaction 01234 Time on stream (h) 01234 Time on stream (h) Figure3.TOSevolutionoftheethanolconversionandproductsyieldwithR‐750catalystinESR reaction‐regenerationcycles. Figure 3. TOS evolution of the ethanol conversion and products yield with R-750 catalyst in ESR reaction-regeneration cycles.
Catalysts 2022,12, 550 6 of 16 Catalysts2022,12,5507of16 0123456 0 0.2 0.4 0.6 0.8 1.0 Conversion H2 CO2 CO Conversion or yield Time on stream (h) CH4 C2H4 C2H4O C first reaction regeneration second reaction regeneration third reaction 01234 Time on stream (h) 01234 Time on stream (h) Figure4.TOSevolutionoftheethanolconversionandproductsyieldwithR‐700catalystinESR reaction‐regenerationcycles. 2.3.CatalystCharacterizationafterReaction‐RegenerationCycles Thecatalystswerecharacterizedafterthethirdreactionusingseveraltechniques withthepurposeofevaluatingthecarbondepositionandtherecoveryofthecatalyst properties. 2.3.1.CarbonDeposition Thecontentandcombustioncharacteristicsofdepositedcarbonweredeterminedby subjectingsamplesofthespentcatalyststotemperatureprogrammedoxidation(TPO). Figure5showstheTPOprofilesofthethreespentcatalystsafterthethirdreaction,in whichauniquecombustionpeakisobservedforeachcatalyst.Thedepositedcarbonstarts toburnabove400°Candmostofthecarbonspeciesburnat525°CfortheR‐850catalyst andat545°CfortheR‐700andR‐750catalysts.Thus,thecarboncontent(estimatedfrom theareaofeachTPOprofile)isnotoriouslyhigherfortheR‐850catalyst(2.37g(gcata‐ lyst)−1)thanfortheR‐700(0.12g(gcatalyst)−1)orR‐750(0.15g(gcatalyst)−1)catalysts.The valuesoftheaveragecarbonformationrate(rC)[25,26],(alsoindicatedinFigure5)are significantlylowerforR‐700andR‐750catalyststhanforR‐850catalyst.Likewise,thehigh combustiontemperatureatthemaximumcombustionrate(maximumpeakpositionin theTPOprofile)indicatesthatthecarbondepositedonthethreecatalystsishighlystruc‐ turedandcondensedwithalowH/Cratio,expectedlycomposedofcarbonfilaments [21,27].TheshifttohighercombustiontemperaturesforthecarbonformedontheR‐700 andR‐750catalystsindicatesthatthecarbonstructuresaremorerefractorythanthose formedontheR‐850catalyst[28]. 300 400 500 600 700 0 0.02 0.04 0.06 0.08 C C = 0.12 g/g r C = 0.0032 g/(g g h) C C = 0.15 g/g r C = 0.0040 g/(g g h) R-700 R-750 R-850 dTG (g/(min g)) Temperature (ºC) C C = 2.37 g/g r C = 0.063 g/(g g h) Figure5.TPOprofileofthethreespentcatalystsafterthethirdESRreaction.CCisthecarboncontent referredtothecatalystmassandrCisthecarbonformationratereferredtothecatalystmass,Cmass fed,andtimeonstream. Figure 4. TOS evolution of the ethanol conversion and products yield with R-700 catalyst in ESR reaction-regeneration cycles. Comparing the performance of the catalysts in the first reaction, the initial ethanol conversion and H 2 , CO, CO 2 , CH 4 and C 2 H 4 yields are similar. The TOS evolution of the H 2 , CO, CO 2 and CH 4 yields is also quite similar for the three catalysts, showing a significant decrease in the first 2 h on stream and more stable behavior afterwards, which indicates a partial catalyst deactivation for the reactions forming these products. The ethanol conversion decreases faster and reaches lower values in the pseudo-stationary state as the reduction temperature decreases (in this catalyst order: R700 > R750 > R850). Above 4 h on stream, the three catalysts reach a pseudo-stable state with constant product yields, in particular that of H 2 . The data evidence that the deactivation rate for the H 2 formation reactions is similar for the three catalysts. However, the TOS evolution of the C 2 H 4 and carbon yields is different among the three catalysts. For the R-850 catalyst, the C 2 H 4 and carbon yields increase with TOS and keep a stable behavior up to 4 h on stream and afterwards the C 2 H 4 yield slightly increases coinciding with the decrease in the carbon yield. In contrast, for the R-700 and R-750 catalysts, the C 2 H 4 yield increases, reaches a maximum value (much lower than the values observed for the R-850 catalyst) and decreases afterwards, whereas the carbon yield is negligible. The slower decrease and higher values in the ethanol conversion with increasing reduction temperatures is due to the higher presence of acidic Al 2 O 3 (Table 1) that catalyzes the ethanol dehydration yielding more C2H4[21]. Upon the first reaction, the corresponding spent catalyst was subjected to the regeneration procedure described in Section 4(combustion with air at 850 ◦ C for coke removal and reconstruction of the spinel, followed by spinel reduction with H 2 -N 2 stream at different temperatures of 700, 750 or 850 ◦ C for the R-700, R-750 and R-850 catalysts, respectively). The whole procedure is repeated for a third reaction and the results are also shown in Figures 2–4. For the R-850 catalyst, the TOS evolution of the product yields completely changes from the first to the second and third reactions, with the product distribution of the second and third reactions being almost identical (reproducible behavior upon the regenerations). Accordingly, the initial H 2 , CO, CO 2 and CH 4 yields are almost comparable with those of the fresh catalyst, but they rapidly decrease with TOS reaching much lower values in the second and third reactions in comparison with the first reaction. Likewise, the C 2 H 4 and carbon yields notably increase in the second and third reactions, making the ethanol conversion also increase in comparison with the first use of the catalyst. In contrast, for the R-700 and R-750 catalysts, the initial values of H 2 , CO, CO 2 , and CH 4 yields and their TOS evolutions in the second and third reactions are quite comparable with those of the first reaction for each catalyst. However, the TOS evolution of the C 2 H 4
Catalysts 2022,12, 550 7 of 16 yield notably differs, with an increasing trend in the values and apparent prolonged period of the maximum value in the successive cycles, which makes the ethanol conversion also increase compared with the first reaction for each catalyst. It should be noted that the H 2 yield is almost constant upon 4 h on stream, whereas the yields of other products change as a consequence of a partial catalyst deactivation for the extent of various reactions (Equations (1)–(14)). This observation is consistent with the H 2 formation mechanism on catalysts derived from a NiAl 2 O 4 spinel proposed in a previous work [ 21 ]. Accordingly, the partial catalyst deactivation sequentially affects the H 2 and carbon formation from ethylene (Equation (10)) that explains the increase in the C 2 H 4 yield over TOS, and the ethanol dehydration (Equation (5)) that explains the maximum observed in the C 2 H 4 yield in the R-700 and R-750 catalysts. The reforming reactions catalyzed by Ni sites keep constant the H 2 yield, and these sites keep their accessibility for these reactions since they also catalyze the carbon gasification [24]. 2.3. Catalyst Characterization after Reaction-Regeneration Cycles The catalysts were characterized after the third reaction using several techniques with the purpose of evaluating the carbon deposition and the recovery of the catalyst properties. 2.3.1. Carbon Deposition The content and combustion characteristics of deposited carbon were determined by subjecting samples of the spent catalysts to temperature programmed oxidation (TPO). Figure 5shows the TPO profiles of the three spent catalysts after the third reaction, in which a unique combustion peak is observed for each catalyst. The deposited carbon starts to burn above 400 ◦ C and most of the carbon species burn at 525 ◦ C for the R-850 catalyst and at 545 ◦ C for the R-700 and R-750 catalysts. Thus, the carbon content (estimated from the area of each TPO profile) is notoriously higher for the R-850 catalyst (2.37 g (g catalyst) −1 ) than for the R-700 (0.12 g (g catalyst) −1 ) or R-750 (0.15 g (g catalyst) −1 ) catalysts. The values of the average carbon formation rate (r C ) [ 25 , 26 ], (also indicated in Figure 5) are significantly lower for R-700 and R-750 catalysts than for R-850 catalyst. Likewise, the high combustion temperature at the maximum combustion rate (maximum peak position in the TPO profile) indicates that the carbon deposited on the three catalysts is highly structured and condensed with a low H/C ratio, expectedly composed of carbon filaments [ 21 , 27 ]. The shift to higher combustion temperatures for the carbon formed on the R-700 and R-750 catalysts indicates that the carbon structures are more refractory than those formed on the R-850 catalyst [28]. Catalysts2022,12,5507of16 0123456 0 0.2 0.4 0.6 0.8 1.0 Conversion H2 CO2 CO Conversion or yield Time on stream (h) CH4 C2H4 C2H4O C first reaction regeneration second reaction regeneration third reaction 01234 Time on stream (h) 01234 Time on stream (h) Figure4.TOSevolutionoftheethanolconversionandproductsyieldwithR‐700catalystinESR reaction‐regenerationcycles. 2.3.CatalystCharacterizationafterReaction‐RegenerationCycles Thecatalystswerecharacterizedafterthethirdreactionusingseveraltechniques withthepurposeofevaluatingthecarbondepositionandtherecoveryofthecatalyst properties. 2.3.1.CarbonDeposition Thecontentandcombustioncharacteristicsofdepositedcarbonweredeterminedby subjectingsamplesofthespentcatalyststotemperatureprogrammedoxidation(TPO). Figure5showstheTPOprofilesofthethreespentcatalystsafterthethirdreaction,in whichauniquecombustionpeakisobservedforeachcatalyst.Thedepositedcarbonstarts toburnabove400°Candmostofthecarbonspeciesburnat525°CfortheR‐850catalyst andat545°CfortheR‐700andR‐750catalysts.Thus,thecarboncontent(estimatedfrom theareaofeachTPOprofile)isnotoriouslyhigherfortheR‐850catalyst(2.37g(gcata‐ lyst)−1)thanfortheR‐700(0.12g(gcatalyst)−1)orR‐750(0.15g(gcatalyst)−1)catalysts.The valuesoftheaveragecarbonformationrate(rC)[25,26],(alsoindicatedinFigure5)are significantlylowerforR‐700andR‐750catalyststhanforR‐850catalyst.Likewise,thehigh combustiontemperatureatthemaximumcombustionrate(maximumpeakpositionin theTPOprofile)indicatesthatthecarbondepositedonthethreecatalystsishighlystruc‐ turedandcondensedwithalowH/Cratio,expectedlycomposedofcarbonfilaments [21,27].TheshifttohighercombustiontemperaturesforthecarbonformedontheR‐700 andR‐750catalystsindicatesthatthecarbonstructuresaremorerefractorythanthose formedontheR‐850catalyst[28]. 300 400 500 600 700 0 0.02 0.04 0.06 0.08 C C = 0.12 g/g r C = 0.0032 g/(g g h) C C = 0.15 g/g r C = 0.0040 g/(g g h) R-700 R-750 R-850 dTG (g/(min g)) Temperature (ºC) C C = 2.37 g/g r C = 0.063 g/(g g h) Figure5.TPOprofileofthethreespentcatalystsafterthethirdESRreaction.CCisthecarboncontent referredtothecatalystmassandrCisthecarbonformationratereferredtothecatalystmass,Cmass fed,andtimeonstream. Figure 5. TPO profile of the three spent catalysts after the third ESR reaction. C C is the carbon content referred to the catalyst mass and r C is the carbon formation rate referred to the catalyst mass, C mass fed, and time on stream.
Catalysts 2022,12, 550 8 of 16 To investigate the extent of carbon deposition and the carbon morphology, samples of the spent catalyst were analyzed by means of scanning electron microscopy (SEM) using a backscattered electron (BSE) or secondary electron (SE) detectors, with the equipment described in Section 4. Figure 6shows the BSE-SEM images of the spent catalysts after the third reaction, which provide insights into the extent of carbon deposition on the catalyst particles according to the brightness intensity levels. All of the spent catalyst particles show a strong carbon deposition on the external surface based on the almost homogeneous dark color of all samples. Remarkably, the size and textural appearance of some R-850 catalyst particles (Figure 6a) notably changed in comparison with those of the R-700 and R-750 catalysts (Figure 6c,d). Some R-850 catalyst particles are larger than the fresh catalyst (0.15–0.25 mm) and with a highly rough and mesoand macro-porous texture characteristic of carbon structures. Likewise, some bright small fragments can be seen on the external surface (magnified in Figure 6b), which is indicative of the presence of catalyst fragments (Ni and Al would give this brightness intensity) over the carbon deposited. The particle sizes of the spent R-750 (Figure 6c) and R-700 (Figure 6d) catalysts did not significantly change by the carbon deposition. Catalysts2022,12,5508of16 Toinvestigatetheextentofcarbondepositionandthecarbonmorphology,samples ofthespentcatalystwereanalyzedbymeansofscanningelectronmicroscopy(SEM)us‐ ingabackscatteredelectron(BSE)orsecondaryelectron(SE)detectors,withtheequip‐ mentdescribedinSection4.Figure6showstheBSE‐SEMimagesofthespentcatalysts afterthethirdreaction,whichprovideinsightsintotheextentofcarbondepositiononthe catalystparticlesaccordingtothebrightnessintensitylevels.Allofthespentcatalystpar‐ ticlesshowastrongcarbondepositionontheexternalsurfacebasedonthealmosthomo‐ geneousdarkcolorofallsamples.Remarkably,thesizeandtexturalappearanceofsome R‐850catalystparticles(Figure6a)notablychangedincomparisonwiththoseoftheR‐700 andR‐750catalysts(Figure6c,d).SomeR‐850catalystparticlesarelargerthanthefresh catalyst(0.15–0.25mm)andwithahighlyroughandmeso‐andmacro‐poroustexture characteristicofcarbonstructures.Likewise,somebrightsmallfragmentscanbeseenon theexternalsurface(magnifiedinFigure6b),whichisindicativeofthepresenceofcatalyst fragments(NiandAlwouldgivethisbrightnessintensity)overthecarbondeposited.The particlesizesofthespentR‐750(Figure6c)andR‐700(Figure6d)catalystsdidnotsignif‐ icantlychangebythecarbondeposition. Figure6.BSE‐SEMimagesofthecatalystparticlesafterthethirdESRreaction:(a)R‐850catalyst (×50);(b)R‐850catalyst(×100);(c)R‐750catalyst(×50);(d)R‐700catalyst(×50). Figure7showstheSE‐SEMimagesprovidingmoredetailsofthecarbonmorphology bymagnifyingsomeareasofrepresentativeparticlesofeachsample.Thephotosevidence thattheformedanddepositedcarbonismostlycomposedofcarbonfilamentsastypically expectedfortheESR[21,27–29],beingparticularlyabundantinallofthespentR‐850cat‐ alystparticles(Figure7a,b).AnapparentcatalystfragmentiscapturedinFigure7b,sug‐ gestingthesefragmentsareonthetipofcarbonfilaments.Ontheotherhand,inthepar‐ ticlesofthespentR‐700andR‐750catalysts,twodifferentsurfacemorphologiescanbe distinguished,onesimilartothatoftheR‐850catalystwithabundantcarbonfilaments (Figure7c,e)andanotherwithanincipientpresenceofcarbonfilamentsandapparent amorphousmassofcarbon(Figure7d,f). Figure 6. BSE-SEM images of the catalyst particles after the third ESR reaction: ( a ) R-850 catalyst (×50); (b) R-850 catalyst (×100); (c) R-750 catalyst (×50); (d) R-700 catalyst (×50). Figure 7shows the SE-SEM images providing more details of the carbon morphology by magnifying some areas of representative particles of each sample. The photos evidence that the formed and deposited carbon is mostly composed of carbon filaments as typically expected for the ESR [ 21 , 27 – 29 ], being particularly abundant in all of the spent R-850 catalyst particles (Figure 7a,b). An apparent catalyst fragment is captured in Figure 7b, suggesting these fragments are on the tip of carbon filaments. On the other hand, in the particles of the spent R-700 and R-750 catalysts, two different surface morphologies can be distinguished, one similar to that of the R-850 catalyst with abundant carbon filaments (Figure 7c,e) and another with an incipient presence of carbon filaments and apparent amorphous mass of carbon (Figure 7d,f).
Catalysts 2022,12, 550 9 of 16 Catalysts2022,12,5509of16 Figure7.SE‐SEMimagesofthecatalystparticlesafterthethirdESRreaction:(a,b)R‐850catalyst; (c,d)R‐750catalyst;(e,f)R‐700catalyst. 2.3.2.RegeneratedCatalystProperties Toverifythespinelreconstructionabilityfromthespentcatalystsbycombustion/cal‐ cinationat850°C,samplesofthereconstructedNiAl 2 O 4 spinelfromtheusedR‐700,R‐ 750andR‐850catalystswerecharacterizedbyusingTPRandXRD(Figure8).TheTPR profiles(Figure8a)arecomparedwiththatoftheNiAl 2 O 4 precursor.Inthereconstructed NiAl 2 O 4 spinels,particularlyfromtheusedR‐850catalyst,theNireductiontakesplacein twotemperatureranges,400–600and600–950°C,indicatingthepresenceoftwoNispe‐ cies.TheXRDpatterns(Figure8b)verifythepresenceoftwocrystallinephases,NiAl 2 O 4 spinel(PDF01‐071‐0965)andNiO(PDF01‐080‐5508),beingthespinelstructurepredom‐ inantinallofthesamplesandthepresenceofNiOmorenotoriousinthespinelrecon‐ structedfromthespentR‐850catalyst.Thus,theNireductionat400–600°Cmaybeasso‐ ciatedtothereductionofNiO,whereastheNireductionintheNiAl 2 O 4 spineltakesplace at600–950°Cwithmaximaataround810–820°C.Interestingly,thereductionpeakand XRDpeaksassociatedtotheNiAl 2 O 4 spineltendtobenarrowerforthealloftherecon‐ structedNiAl 2 O 4 spinelsthanforthespinelprecursor,whichisindicativeofmorehomo‐ geneousNiAl 2 O 4 structures.Likewise,theTPRprofilesevidencesthatthetotalamountof reducedNi(listedinTable2)islowerforthereconstructedNiAl 2 O 4 spinelsthanforthe spinelprecursor.Therefore,thedecreaseintheNicontentwithrespecttothefreshcatalyst indicatesapartialNilossaftertheESRreactioncycles.Theselossesare3.90,9.50,and 14.5%fortheR‐700,R‐750,andR‐850catalysts,respectively.Presumably,theNilossis Figure 7. SE-SEM images of the catalyst particles after the third ESR reaction: ( a , b ) R-850 catalyst; (c,d) R-750 catalyst; (e,f) R-700 catalyst. 2.3.2. Regenerated Catalyst Properties To verify the spinel reconstruction ability from the spent catalysts by combustion/calcination at 850 ◦ C, samples of the reconstructed NiAl 2 O 4 spinel from the used R-700, R-750 and R-850 catalysts were characterized by using TPR and XRD (Figure 8). The TPR profiles (Figure 8a) are compared with that of the NiAl 2 O 4 precursor. In the reconstructed NiAl 2 O 4 spinels, particularly from the used R-850 catalyst, the Ni reduction takes place in two temperature ranges, 400–600 and 600–950 ◦ C, indicating the presence of two Ni species. The XRD patterns (Figure 8b) verify the presence of two crystalline phases, NiAl 2 O 4 spinel (PDF 01-071-0965) and NiO (PDF 01-080-5508), being the spinel structure predominant in all of the samples and the presence of NiO more notorious in the spinel reconstructed from the spent R-850 catalyst. Thus, the Ni reduction at 400–600 ◦ C may be associated to the reduction of NiO, whereas the Ni reduction in the NiAl 2 O 4 spinel takes place at 600–950 ◦ C with maxima at around 810–820 ◦ C. Interestingly, the reduction peak and XRD peaks associated to the NiAl 2 O 4 spinel tend to be narrower for the all of the reconstructed NiAl 2 O 4 spinels than for the spinel precursor, which is indicative of more homogeneous NiAl 2 O 4 structures. Likewise, the TPR profiles evidences that the total amount of reduced Ni (listed in Table 2) is lower for the reconstructed NiAl 2 O 4 spinels than for the spinel precursor. Therefore, the decrease in the Ni content with respect to the fresh catalyst indicates a partial Ni loss after the ESR reaction cycles. These losses are 3.90, 9.50, and 14.5% for the R-700, R-750, and R-850 catalysts, respectively. Presumably, the Ni loss is due to the
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