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

Citation: Valecillos, J.; Landa, L.; Elordi, G.; Remiro, A.; Bilbao, J.; Gayubo, A.G. Are Rh Catalysts a Suitable Choice for Bio-Oil Reforming? The Case of a Commercial Rh Catalyst in the Combined H2O and CO2Reforming of Bio-Oil. Catalysts 2024,14, 571. https://doi.org/10.3390/ catal14090571 Academic Editors: Georgios Bampos, Paraskevi Panagiotopoulou and Eleni A. Kyriakidou Received: 30 July 2024 Revised: 22 August 2024 Accepted: 26 August 2024 Published: 29 August 2024 Copyright: © 2024 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 Are Rh Catalysts a Suitable Choice for Bio-Oil Reforming? The Case of a Commercial Rh Catalyst in the Combined H2O and CO2Reforming of Bio-Oil JoséValecillos * , Leire Landa, Gorka Elordi , 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] (L.L.); gorka.elor[email protected] (G.E.); aingeru.r[email protected] (A.R.); javier[email protected] (J.B.) *Correspondence: [email protected] (J.V.); [email protected] (A.G.G.) Abstract: Bio-oil combined steam/dry reforming (CSDR) with H 2 O and CO 2 as reactants is an attractive route for the joint valorization of CO 2 and biomass towards the sustainable production of syngas (H 2 + CO). The technological development of the process requires the use of an active and stable catalyst, but also special attention should be paid to its regeneration capacity due to the unavoidable and quite rapid catalyst deactivation in the reforming of bio-oil. In this work, a commercial Rh/ZDC (zirconium-doped ceria) catalyst was tested for reaction–regeneration cycles in the bio-oil CSDR in a fluidized bed reactor, which is beneficial for attaining an isothermal operation and, moreover, minimizes catalyst deactivation by coke deposition compared to a fixed-bed reactor. The fresh, spent, and regenerated catalysts were characterized using either N 2 physisorption, H 2 -TPR, TPO, SEM, TEM, or XRD. The Rh/ZDC catalyst is initially highly active for the syngas production (yield of 77% and H 2 /CO ratio of 1.2) and for valorizing CO 2 (conversion of 22%) at 700 ◦ C, with space time of 0.125 g catalyst h (g oxygenates ) −1 and CO 2 /H 2 O/C ratio of 0.6/0.5/1. The catalyst activity evolves in different periods that evidence a selective deactivation of the catalyst for the reforming reactions of the different compounds, with the CH 4 reforming reactions (with both steam and CO 2 ) being more rapidly affected by catalyst deactivation than the reforming of hydrocarbons or oxygenates. After regeneration, the catalyst’s textural properties are not completely restored and there is a change in the Rh–support interaction that irreversibly deactivates the catalyst for the CH 4 reforming reactions (both SR and DR). As a result, the coke formed over the regenerated catalyst is different from that over the fresh catalyst, being an amorphous mass (of probably turbostractic nature) that encapsulates the catalyst and causes rapid deactivation. Keywords: bio-oil; steam reforming; dry reforming; syngas; coke deactivation; regeneration; irreversible deactivation 1. Introduction Syngas, a blend of H 2 and CO, is a basic (petro)chemical platform for the syntheses of alcohols (mainly methanol), ethers, carboxylic acids, other various carbonyl compounds, synthetic fuels, ammonia, and urea, and is also a fuel employed in gas engines for energy generation. Its production is still highly dependent on fossil resources, mostly by reforming of natural gas and petroleum derivatives, with a significant contribution to the global CO 2 emissions. Hence, sustainable production options are urged for the transition towards chemical and energy industries that are completely clean and renewable. Among many alternatives, the reforming of biomass and its derivatives replacing the traditional fossilbased feedstock is an attractive option as it meets the goal of net zero CO 2 emissions [ 1 ]. Its feasibility for a prompt implementation takes advantage of starting from an existing reforming technology with the challenge of making improvements to minimize costs and environmental impacts. Catalysts 2024,14, 571. https://doi.org/10.3390/catal14090571 https://www.mdpi.com/journal/catalysts Catalysts 2024,14, 571 2 of 18 One promising route is the use of lignocellulose biomass wastes, which does not interfere with food chains and can be processed by fast pyrolysis in simple and decentralized facilities yielding large quantities of bio-oil (a complex mixture of oxygenates comprising carboxylic acids, aldehydes, alcohols, ketones, esters, furfurals, phenols, and saccharides [ 2 , 3 ]). The subsequent bio-oil reforming may be targeted at the production of syngas with suitable H 2 /CO ratios for the syntheses of fuels or chemicals, depending on the reforming strategy [ 4 , 5 ]. The CO 2 reforming, commonly known as dry reforming (DR), has been proposed as an attractive alternative reforming strategy that allows the production of useful syngas from bio-oil oxygenates (C n H m O k ) while valorizing CO 2 simultaneously (Equation (1)), avoiding the excessive side CO 2 production when using the steam reforming (SR) strategy [ 4 ]. The reverse water gas shift (r-WGS) reaction also contributes to the CO 2 conversion at high temperatures (reverse Equation (2)). Nevertheless, the inherent H 2 O content in the bio-oil, which varies depending on its origin, also promotes the SR of oxygenates (Equations (3) and (4)), and thus a combined steam/dry reforming (CSDR) takes place. Moreover, in the conversion of bio-oil, the decomposition/cracking of oxygenates into H 2 , CO, CO 2 , CH 4 , hydrocarbons (C a H b ), other oxygenates (C x H y O z ), and carbon (coke) should be considered, as represented by Equation (5). Therefore, the conversion of CH 4 and hydrocarbons by DR (Equations (6) and (7), respectively) and SR ( Equations (8) and (9) , respectively) also contributes to the overall kinetic scheme. Likewise, coke formation may be favored by CH 4 decomposition (Equation (10)), hydrocarbon decomposition ( Equation (11) ), and the CO disproportionation (Boudouard) reaction ( Equation (12) ), whereas its gasification may occur with steam (Equation (13)) or CO 2 (reverse Equation (12)). Consequently, the co-feeding of CO 2 is expected to favor coke removal. 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 catalysts for bio-oil reforming are commonly based on non-noble or noble metals, or bimetallic compositions [ 6 – 11 ]. Nevertheless, the CSDR of real bio-oil has been scarcely studied experimentally on Ni catalysts (non-noble metal catalysts) [ 12 – 14 ], showing promising results, and no studies with a noble metal catalyst have been reported so far. However, it could be hypothesized that the use of noble metal catalysts, such as Rh catalysts, may improve the performance of Ni catalysts grounded in previous studies reporting a remarkable activity for the H 2 /syngas production from real bio-oil by conventional SR [ 15 , 16 ], oxidative SR (OSR) [ 17 – 19 ], and sequential cracking [ 20 , 21 ]. Likewise, Rh catalysts have been successfully used in the DR of CH 4 [ 22 – 24 ] and ethanol [ 25 ], whose results may be extrapolated to the case of bio-oil, in particular due to the role of CH 4 as a reaction inter- Catalysts 2024,14, 571 3 of 18 mediate in bio-oil reforming. The advantage of noble metal catalysts relies on their high dehydrogenation and oxidation capacities, which are translated into high yields of H 2 and CO/CO 2 without CH 4 production (CH 4 is converted) and low coke formation, preventing catalyst deactivation. Additionally, Rh sites may also adsorb and dissociate CO 2 , which is a paramount step in DR reactions [ 26 ]. Likewise, catalyst support may play a role in the catalysis by providing acidic or basic sites, hydrophilicity, or oxygen mobility [ 22 ]. For example, the use of CeO 2 , TiO 2 , and ZrO 2 provides the latter features enhancing H 2 O or CO 2 adsorption and dissociation generating OH or CO species facilitating their conversions, which would globally enhance the performance of the bio-oil CSDR. Kartavova et al. [ 27 ] highlight the importance of the support on the surface distribution and crystal size of Rh in the catalysts used in the cyclohexane ring opening reaction. CeO 2 is an interesting support because of its oxygen storage and mobility capacity due to the fast Ce 4+ /Ce 3+ redox cycling. Its limited thermal stability is improved by the formation of the Ce x Zr 1−x O 2 solid solution, with excellent performance as a support for chromium oxide in the dehydrogenation of propane with CO2[28]. In spite of the high activity of Rh catalysts, reversible and irreversible deactivation has been observed in the OSR and SR of bio-oil [ 19 , 29 , 30 ]. The reversible deactivation is associated with the formation and deposition of coke encapsulating the active sites, whereas the irreversible deactivation has been related to irreversible structural changes in the catalyst due to the harsh reaction conditions (high temperature and presence of steam). These changes mainly involve the aging of the support partially occluding Rh sites and possible changes in the Rh structure. Likewise, Rh sintering is a cause of deactivation at 750 ◦ C in the OSR bio-oil [ 19 ]. In other catalytic systems for different applications, it has been found that Rh catalysts are irreversibly deactivated by the partial encapsulation of Rh nanoparticles by CeO 2 [ 31 , 32 ], which has also been reported for various noble metals [ 33 ]. In the case of reforming strategies, it is believed that high steam concentrations may favor this phenomenon of irreversible deactivation of Rh catalysts, which represents a drawback of using these catalysts. In this work, we have studied the performance of a commercial catalyst made of Rh supported on zirconium-doped ceria (Rh/ZDC) for the CSDR of a real bio-oil targeted at syngas production, with focus on its activity, deactivation, and regeneration capacity. It is expected that the low steam concentration in the CSDR strategy lessens the phenomenon of irreversible deactivation. As previously mentioned, the novelty of this work is stressed by the use of a noble metal catalyst for this process with a feed of real bio-oil, with a formulation that is expected to enhance the CSDR performance. The bio-oil CSDR tests were carried out in a fluidized bed reactor at 700 ◦ C, comprising a reaction/regeneration cycle. To investigate the causes of deactivation, the fresh, deactivated, and regenerated catalyst samples have been analyzed by means of temperature-programmed reduction (H 2 -TPR), N 2 physisorption, temperature-programmed oxidation (TPO), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The discussion of the results is aimed at understanding the reversibility and irreversibility of catalyst deactivation. 2. Results The results are divided in three main subsections. The first one summarizes the findings for the performance of the Rh/ZDC catalyst in the CSDR of bio-oil at 700 ◦ C considering the operation in reaction–regeneration cycles and a comparison with the conventional SR of bio-oil at the same conditions. The second one collects the data obtained from the characterization of the fresh and spent catalysts by using different techniques in order to shed light on the carbon formation that causes a reversible catalyst deactivation. The last subsection explores the reasons for the irreversible catalyst deactivation observed for the regenerated catalyst, based on the analysis of the Rh sites by H 2 -TPR measurements. Catalysts 2024,14, 571 4 of 18 2.1. Performance of the Rh Catalyst in Bio-Oil Reforming Figure 1shows the performance of the Rh/ZDC catalyst in the CSDR of bio-oil at 700 ◦C in two successive reactions with a catalyst regeneration in between. The performance is analyzed in terms of the evolution over time on stream of the product yields (Figure 1a for the reaction with fresh catalyst (first reaction) and Figure 1c for the reaction with the regenerated catalyst (second reaction)) and syngas yield or H 2 /CO ratio (Figure 1b and Figure 1d for the first and second reactions, respectively). In these data, the oxygenate conversion is estimated as the total yield of the carbon gaseous products (sum of all carbonaceous products after subtracting the molar flow rate of CO 2 in the feed), since the estimated coke content and yield are very low according to the TPO analysis shown in the next subsection. Catalysts 2024, 14, x FOR PEER REVIEW 4 of 18 the characterization of the fresh and spent catalysts by using different techniques in order to shed light on the carbon formation that causes a reversible catalyst deactivation. The last subsection explores the reasons for the irreversible catalyst deactivation observed for the regenerated catalyst, based on the analysis of the Rh sites by H2-TPR measurements. 2.1. Performance of the Rh Catalyst in Bio-Oil Reforming Figure 1 shows the performance of the Rh/ZDC catalyst in the CSDR of bio-oil at 700 °C in two successive reactions with a catalyst regeneration in between. The performance is analyzed in terms of the evolution over time on stream of the product yields (Figure 1a for the reaction with fresh catalyst (first reaction) and Figure 1c for the reaction with the regenerated catalyst (second reaction)) and syngas yield or H2/CO ratio (Figure 1b and 1d for the first and second reactions, respectively). In these data, the oxygenate conversion is estimated as the total yield of the carbon gaseous products (sum of all carbonaceous products after subtracting the molar flow rate of CO2 in the feed), since the estimated coke content and yield are very low according to the TPO analysis shown in the next subsection. 050 100 150 200 250 300 350 -0.2 0 0.2 0.4 0.6 0.8 1.0 Conversion or Yield Time on stream (min) (a) 050 100 150 200 250 300 350 -0.2 0 0.2 0.4 0.6 0.8 1.0 (c) Conversion or Yield Time on stream (min) Conversion Oxygenates 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 stream (min) 0 0.5 1.0 1.5 2.0 2.5 H2/CO ratio 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 stream (min) 0 0.5 1.0 1.5 2.0 2.5 H2/CO ratio H2/CO ratio direction to read data Figure 1. Performance of the Rh/ZDC catalyst in the CSDR of bio-oil at 700 °C, CO2/H2O/C ratio of 0.6/0.5/1, space time (referred to the mass flow of oxygenates in the bio-oil), and 0.125 gcatalyst h (goxygenates)−1: Evolution of the conversion and product yields over time on stream for (a) the fresh catalyst and (c) the regenerated catalyst; evolution of the syngas yield and H2/CO ratio over time on stream for (b) the fresh catalyst and (d) the regenerated catalyst. The initial oxygenate conversion is around 0.77 with the fresh catalyst (Figure 1a). This low value evidences that the space time used is low to reach an equilibrium state at these reaction conditions (particularly because of the low S/C ratio), and therefore the maximum product yields may not have been achieved, although this situation is adequate to study the catalyst deactivation. In spite of this, the initial yield values of H2 and CO Figure 1. Performance of the Rh/ZDC catalyst in the CSDR of bio-oil at 700 ◦ C, CO 2 /H 2 O/C ratio of 0.6/0.5/1, space time (referred to the mass flow of oxygenates in the bio-oil), and 0.125 g catalyst h (g oxygenates ) −1 : Evolution of the conversion and product yields over time on stream for (a) the fresh catalyst and (c) the regenerated catalyst; evolution of the syngas yield and H 2 /CO ratio over time on stream for (b) the fresh catalyst and (d) the regenerated catalyst. The initial oxygenate conversion is around 0.77 with the fresh catalyst (Figure 1a). This low value evidences that the space time used is low to reach an equilibrium state at these reaction conditions (particularly because of the low S/C ratio), and therefore the maximum product yields may not have been achieved, although this situation is adequate to study the catalyst deactivation. In spite of this, the initial yield values of H 2 and CO provide evidence for the high activity of Rh for the bio-oil CSDR, which has also been demonstrated for the OSR of bio-oil [ 17 – 19 ]. The comparison of the results in Figure 1a,b Catalysts 2024,14, 571 5 of 18 with those obtained on a Ni catalyst (Ni/Al 2 O 3 catalyst derived from a NiAl 2 O 4 spinel) under the same conditions [ 12 ] reveals that Rh exhibits notably high activity considering that less active sites are actually present in the Rh catalyst (2 wt% Rh vs. 36 wt% Ni). Thus, the initial yield and H 2 /CO ratio of the syngas are almost 90% and 1.2, respectively, with this Rh catalyst compared to 92% and 0.9 with the Ni catalyst, and the CO 2 conversion is 22% against 28%. As a comparison, Figure S1 in the Supplementary Materials shows the results obtained with the Rh/ZDC catalyst at the same conditions as in Figure 1but without CO 2 co-feeding (that is, under SR conditions with a low S/C ratio of 0.5). When comparing the SR and CSDR strategies, a slightly higher oxygenate conversion is observed in the SR reaction (around 80%, Figure S1a) than in the CSDR reaction (Figure 1a). The main difference between both reforming strategies is the increase in the CO yield in the CSDR reaction (from 62% in Figure S1a to 85% in Figure 1a), hence yielding slightly more syngas (near 90% in CSDR reaction, compared to 80% in SR reaction) with a H 2 /CO ratio of about 1.2 (Figure 1b) in comparison to a H 2 /CO ratio of 2 for the SR strategy (Figure S1b). The product distribution (H 2 , CO, CO 2 , CH 4 , and hydrocarbons) observed may be explained considering the DR (Equation (1)) and SR (Equations (3) and (4)) of oxygenates, as well as the catalytic cracking of oxygenates (Equation (5)), as the dominant routes at this high temperature, generating H 2 and various gaseous intermediates (CO, CO 2 , CH 4 and hydrocarbons). Then, the SR or DR of CH 4 and hydrocarbons (Equations (6)–(9)) generates CO and H 2 , and the CO 2 may be transformed into CO and H 2 O by the r-WGS reaction (reverse Equation (2)), which is thermodynamically favored at this high temperature and low S/C ratio [ 4 , 5 ]. In spite of its high catalytic activity, the Rh catalyst undergoes deactivation during both the CSDR and SR reactions, which is clearly evidenced by the drop of the H 2 and CO yields over time on stream (Figure 1a,b). The activity decay must be mainly attributed to coke formation, which may proceed through the decomposition of oxygenates (Equation (5)), CH 4 (Equation (10)), and hydrocarbons (Equation (11)), considering the high temperature (700 ◦ C) and low S/C ratio employed [ 2 ]. Simultaneously, coke gasification may take place with CO 2 (reverse Equation (12)) or steam (Equation (13)). Curiously, the evolution of the product yields draws five well-distinguished periods, as has also been observed in the OSR and SR of bio-oil with the same catalyst [ 19 , 29 ] and in Figure S1. Thus, for the CSDR strategy, the first period (around 125 min) shows a quite stable behavior of the catalyst, with high oxygenate conversion and complete conversion of intermediates, CH 4 , and hydrocarbons, by SR or DR reactions, generating the highest H 2 and CO yields with the maximum CO 2 conversion boosted by the r-WGS reaction. The second period (~125–175 min) shows a decrease in the H 2 and CO yields and in the CO 2 conversion and an increase in the CH 4 yield, indicating the selective deactivation of the catalyst for the SR or DR of CH 4 and the r-WGS reaction. The third period ( ~175–225 min ) describes a pseudo steady state where the conversion of hydrocarbons by SR and DR reactions remains complete. Subsequently, in the fourth period (~225–275 min), the complete catalyst deactivation for the SR and DR reactions of hydrocarbons and oxygenates occurs, evidenced by the sharp decrease in the oxygenate conversion and the H 2 and CO yields and increase in the hydrocarbon yield. In this period, the CO 2 conversion falls to negative values, reaching a minimum and then increasing up to zero, which is translated into a maximum for the CO 2 yield, which suggests a selective deactivation of the DR of hydrocarbons over their SR reaction. The fifth period (above 275 min) presumably corresponds to the yields obtained in the thermal reaction of the bio-oil oxygenates in the presence of both steam and CO 2 . Consequently, the syngas yield decreases over time following the same trend of these five periods (Figure 1b), whereas the H 2 /CO ratio is almost constant during the first two periods, slightly increases in the third period, and decays noticeably in the fourth period when the catalyst undergoes complete deactivation. These periods are also observed in the SR of bio-oil (Figure S1), but in this case CO 2 is only a product. Particularly, the CO 2 yield increases during the second period (partial deactivation for CH 4 conversion) and keeps a higher stable value in the third period in comparison with the first period. This result suggests that the r-WGS reaction (which is Catalysts 2024,14, 571 6 of 18 thermodynamically favored in these reaction conditions) is also affected by deactivation in the second period. This behavior can also be observed in the CSDR reaction (Figure 1a), though the CO 2 yield values are negative. Strikingly, the H 2 /CO ratio evolution follows a trend similar to that of the CO 2 yield, which confirms the partial catalyst deactivation for the r-WGS reaction in the second period. After the CSDR reaction with the fresh catalyst (Figure 1a), the spent catalyst was regenerated in situ (in the reactor) by coke combustion with air at 600 ◦ C and subsequent reduction in a H 2 /N 2 flow at 700 ◦ C. Upon regeneration (Figure 1c), the catalyst does not fully recover its initial activity, which is particularly evidenced by the absence of two different deactivation periods described above. In turn, the reaction with the regenerated catalyst initiates at a stable period in which hydrocarbons are fully converted by SR or DR reactions, whereas CH 4 is not converted (third period in Figure 1a), followed by the complete catalyst deactivation (fourth period in Figure 1a). The syngas yield (Figure 1d) is initially stable and then decreases over time on stream as described by the third and fourth periods in Figure 1b, and the H 2 /CO ratio also slightly increases and then decreases following the trend described by the CO 2 yield. This evidences that the Rh/ZDC catalyst undergoes both reversible and irreversible deactivation. The former is due to the deposition of coke so that the catalyst partially recovers its reforming activity after regeneration. The latter is presumably associated with irreversible changes in the catalyst structure or metal sites. In the following sections, we further investigate the causes of both deactivation phenomena by analyzing the spent catalyst from each reaction test. 2.2. Chacaractarization of Deactivated Catalyst Samples The coke formed and deposited on the catalyst during each reaction was analyzed by TPO, SEM, and TEM in order to determine its amount and/or nature. Additionally, N 2 physisorption was also employed to analyze the effect of coke deposition on the catalyst textural properties. The possible changes in the crystalline structure of the deactivated catalyst were analyzed by XRD analysis. Finally, the H 2 -TPR profiles of the fresh and regenerated catalyst were compared to analyze the Rh sites. In general, the spent catalyst samples are identified with reference to their reaction test: CSDR-1 for the first CSDR reaction (with the fresh catalyst) and CSDR-2 for the second CSDR reaction (with the regenerated catalyst). 2.2.1. Coke Formation Figure 2shows the TPO profiles for the spent catalyst samples, in which the peaks are associated with the combustion of coke. The corresponding coke content (determined by integration of each curve) is indicated next to each profile. The TPO profiles are noticeably different for the two samples, showing combustion peaks centered at different temperatures. The CSDR-1 sample shows two separate peaks centered at 300 and 400 ◦ C, and the CSDR-2 sample shows two overlapping peaks that are approximately centered at 415 and 430 ◦ C. Likewise, the coke content is significantly lower in the CSDR-1 sample (93.6 mg g −1 ) than in the CSDR-2 sample (302 mg g −1 ). The temperature position of the combustion peaks may be related to the nature of carbon and to the possible catalytic effect of the catalyst components on the carbon combustion. Thus, the peaks at lower temperatures can be associated with the combustion of poorly developed coke (amorphous carbon, most probably encapsulating Rh sites) and the peaks at higher temperatures are associated with structured coke (carbon nanostructures or turbostratic/graphite carbon) [ 34 , 35 ]. Additionally, both Ce (in the ZDC support) and Rh species can catalyze the carbon combustion [ 36 ], which results in combustion peaks at lower temperature than those obtained for Ni catalysts in different bio-oil reforming strategies [12,17,37]. Catalysts 2024,14, 571 7 of 18 Catalysts 2024, 14, x FOR PEER REVIEW 7 of 18 Additionally, both Ce (in the ZDC support) and Rh species can catalyze the carbon combustion [36], which results in combustion peaks at lower temperature than those obtained for Ni catalysts in different bio-oil reforming strategies [12,17,37]. 200 250 300 350 400 450 500 0 10 20 30 40 CSDR-1 CSDR-2 DTG (mg (g min)-1) Temperature (ºC) 93.6 coke content (mg/g) = 302 Figure 2. TPO profiles of the spent catalyst samples. Coke content is indicated close to each profile. To discern the nature of the coke, the two spent catalyst samples have been analyzed by SEM (with a backscattered electron (BSE) or secondary electron (SE) detector) and TEM. Figure 3 shows the BSE-SEM images for the fresh and spent catalyst samples, which provide qualitative information on the degree of coke deposition on the external surface. Accordingly, the brightness intensity is indicative of the presence of heavy (bright) or light (dark) elements [2,12,35], and for this case, a high or dense presence of coke on the catalyst surface is indicated by darker particles since coke contains the lightest element (C) in these samples in comparison with the catalyst components (Rh, Ce, and Zr). In general, all the particles exhibit relatively bright intensities similar to those of the fresh catalyst (Figure 3a), indicating a poor coke deposition on the catalyst surface or the deposition of carbon with low density, although the CSDR-1 sample also exhibits some dark particles. Figure 3. BSE-SEM images of the (a) fresh catalyst and spent catalyst samples from the (b) 1st CSDR reaction (with fresh catalyst) and (c) 2nd CSDR reaction (with regenerated catalyst). Figure 2. TPO profiles of the spent catalyst samples. Coke content is indicated close to each profile. To discern the nature of the coke, the two spent catalyst samples have been analyzed by SEM (with a backscattered electron (BSE) or secondary electron (SE) detector) and TEM. Figure 3shows the BSE-SEM images for the fresh and spent catalyst samples, which provide qualitative information on the degree of coke deposition on the external surface. Accordingly, the brightness intensity is indicative of the presence of heavy (bright) or light (dark) elements [ 2 , 12 , 35 ], and for this case, a high or dense presence of coke on the catalyst surface is indicated by darker particles since coke contains the lightest element (C) in these samples in comparison with the catalyst components (Rh, Ce, and Zr). In general, all the particles exhibit relatively bright intensities similar to those of the fresh catalyst (Figure 3a), indicating a poor coke deposition on the catalyst surface or the deposition of carbon with low density, although the CSDR-1 sample also exhibits some dark particles. Catalysts 2024, 14, x FOR PEER REVIEW 7 of 18 Additionally, both Ce (in the ZDC support) and Rh species can catalyze the carbon combustion [36], which results in combustion peaks at lower temperature than those obtained for Ni catalysts in different bio-oil reforming strategies [12,17,37]. 200 250 300 350 400 450 500 0 10 20 30 40 CSDR-1 CSDR-2 DTG (mg (g min)-1) Temperature (ºC) 93.6 coke content (mg/g) = 302 Figure 2. TPO profiles of the spent catalyst samples. Coke content is indicated close to each profile. To discern the nature of the coke, the two spent catalyst samples have been analyzed by SEM (with a backscattered electron (BSE) or secondary electron (SE) detector) and TEM. Figure 3 shows the BSE-SEM images for the fresh and spent catalyst samples, which provide qualitative information on the degree of coke deposition on the external surface. Accordingly, the brightness intensity is indicative of the presence of heavy (bright) or light (dark) elements [2,12,35], and for this case, a high or dense presence of coke on the catalyst surface is indicated by darker particles since coke contains the lightest element (C) in these samples in comparison with the catalyst components (Rh, Ce, and Zr). In general, all the particles exhibit relatively bright intensities similar to those of the fresh catalyst (Figure 3a), indicating a poor coke deposition on the catalyst surface or the deposition of carbon with low density, although the CSDR-1 sample also exhibits some dark particles. Figure 3. BSE-SEM images of the (a) fresh catalyst and spent catalyst samples from the (b) 1st CSDR reaction (with fresh catalyst) and (c) 2nd CSDR reaction (with regenerated catalyst). Figure 3. BSE-SEM images of the (a) fresh catalyst and spent catalyst samples from the (b) 1st CSDR reaction (with fresh catalyst) and (c) 2nd CSDR reaction (with regenerated catalyst). Further, to explore the coke deposition, Figure 4shows the SE-SEM images at higher zooms for all the samples. The Rh/ZDC catalyst shows a granular surface typical of porous materials (in this case the ZDC support), showing some compacted areas. The majority of the particles in the CSDR-1 sample are bright, even though there are some carbon filaments Catalysts 2024,14, 571 8 of 18 on the surface and some granular and compacted areas that may be the surface of the ZDC support (Figure 4b). The dark particles in the CSDR-1 sample show an abundant presence of carbon filaments (Figure 4c). On the other hand, the CSDR-2 sample shows a smooth granular-like surface (apparently different from that of the catalyst support) with some carbon filaments (Figure 4d), and when this surface is zoomed in upon (Figure 4e), it seems to be a blend of an amorphous mass of carbon with some nanotubes trapped in it. Catalysts 2024, 14, x FOR PEER REVIEW 8 of 18 Further, to explore the coke deposition, Figure 4 shows the SE-SEM images at higher zooms for all the samples. The Rh/ZDC catalyst shows a granular surface typical of porous materials (in this case the ZDC support), showing some compacted areas. The majority of the particles in the CSDR-1 sample are bright, even though there are some carbon filaments on the surface and some granular and compacted areas that may be the surface of the ZDC support (Figure 4b). The dark particles in the CSDR-1 sample show an abundant presence of carbon filaments (Figure 4c). On the other hand, the CSDR-2 sample shows a smooth granular-like surface (apparently different from that of the catalyst support) with some carbon filaments (Figure 4d), and when this surface is zoomed in upon (Figure 4e), it seems to be a blend of an amorphous mass of carbon with some nanotubes trapped in it. Figure 4. SE-SEM images of the (a) fresh catalyst and spent catalyst samples from the (b,c) 1st CSDR reaction (with fresh catalyst) and (d,e) 2nd CSDR reaction (with regenerated catalyst). TEM has been useful to confirm the presence of these carbon types as shown in Figure 5, from which it should be considered that only tiny fragments of the samples can be analyzed without distinguishing between the external/internal surface of the particle. The Rh/ZDC catalyst shows grains of different grey tones, with the darker ones presumably being Rh crystals as this is the densest catalyst component. The CSDR-1 sample shows the presence of both carbon filaments (Figure 5b) and amorphous/turbostratic carbon encapsulating the catalyst components (Figure 5c). On the other hand, the CSDR-2 sample barely shows a mass of carbon covering the catalyst components in spite of having the highest coke content. The difficulty to observe the carbon on this latter sample is perhaps Figure 4. SE-SEM images of the (a) fresh catalyst and spent catalyst samples from the (b,c) 1st CSDR reaction (with fresh catalyst) and (d,e) 2nd CSDR reaction (with regenerated catalyst). TEM has been useful to confirm the presence of these carbon types as shown in Figure 5, from which it should be considered that only tiny fragments of the samples can be analyzed without distinguishing between the external/internal surface of the particle. The Rh/ZDC catalyst shows grains of different grey tones, with the darker ones presumably being Rh crystals as this is the densest catalyst component. The CSDR-1 sample shows the presence of both carbon filaments (Figure 5b) and amorphous/turbostratic carbon encapsulating the catalyst components (Figure 5c). On the other hand, the CSDR-2 sample barely shows a mass of carbon covering the catalyst components in spite of having the highest coke content. The difficulty to observe the carbon on this latter sample is perhaps due to its nature, apparently a thin layer of carbon mass covering the entire surface evenly, as well as the limitation of the technique to analyze this type of solid. Catalysts 2024,14, 571 9 of 18 Catalysts 2024, 14, x FOR PEER REVIEW 9 of 18 due to its nature, apparently a thin layer of carbon mass covering the entire surface evenly, as well as the limitation of the technique to analyze this type of solid. Figure 5. TEM images of the (a) fresh catalyst and spent catalyst samples from the (b,c) 1st CSDR reaction (with fresh catalyst) and (d) 2nd CSDR reaction (with regenerated catalyst). The textural properties of the fresh and spent catalyst samples (Table 1) evidence the impact of coke deposition. The formation of both carbon filaments and amorphous carbon in the CSDR reaction with the fresh catalyst (CSDR-1 sample) causes the BET surface area to decrease by 33%, with a strong impact on the microporous surface area and on the volume, which both decrease by about 62%. On the other hand, the formation of a blend of amorphous/turbostratic carbon and small carbon filaments in the CSDR reaction with the regenerated catalyst (CSDR-2 sample) causes a strong decrease in the BET surface area (about 74%) and in the total volume of pores (by 83%) without affecting the microporous surface area. This suggests that in the reaction with the fresh catalyst, the amorphous carbon may be deposited on the microporous surface, whereas all the carbon is deposited on the external surface of the catalyst particles in the reaction with the regenerated catalyst. It should be noted that the reduced specific surface is a notable limitation of the CeO2 support, as pointed out in the literature [28], and it is more pronounced in this reaction due to coke deposition. Table 1. Textural properties of fresh, spent, and regenerated Rh/ZDC catalyst samples. Sample SBET (m2 g−1) Smicro (m2 g−1) Vpore (cm3 g−1) Vmicro (cm3 g−1) dpore (nm) Fresh 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 1st regeneration (CSDR-1) 39.6 2.16 0.20 1.00 × 10−3 20.2 Figure 5. TEM images of the (a) fresh catalyst and spent catalyst samples from the (b,c) 1st CSDR reaction (with fresh catalyst) and (d) 2nd CSDR reaction (with regenerated catalyst). The textural properties of the fresh and spent catalyst samples (Table 1) evidence the impact of coke deposition. The formation of both carbon filaments and amorphous carbon in the CSDR reaction with the fresh catalyst (CSDR-1 sample) causes the BET surface area to decrease by 33%, with a strong impact on the microporous surface area and on the volume, which both decrease by about 62%. On the other hand, the formation of a blend of amorphous/turbostratic carbon and small carbon filaments in the CSDR reaction with the regenerated catalyst (CSDR-2 sample) causes a strong decrease in the BET surface area (about 74%) and in the total volume of pores (by 83%) without affecting the microporous surface area. This suggests that in the reaction with the fresh catalyst, the amorphous carbon may be deposited on the microporous surface, whereas all the carbon is deposited on the external surface of the catalyst particles in the reaction with the regenerated catalyst. It should be noted that the reduced specific surface is a notable limitation of the CeO 2 support, as pointed out in the literature [ 28 ], and it is more pronounced in this reaction due to coke deposition. Table 1. Textural properties of fresh, spent, and regenerated Rh/ZDC catalyst samples. Sample SBET (m2g−1) Smicro (m2g−1) Vpore (cm3g−1) Vmicro (cm3g−1) dpore (nm) Fresh 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 1st regeneration (CSDR-1) 39.6 2.16 0.20 1.00 × 10 −320.2 S BET = BET specific surface area; S micro = microporous specific surface area; V pore = specific volume of pores; Vmicro = specific volume of micropores; dpore = average pore diameter. Catalysts 2024,14, 571 16 of 18 Acknowledgments: The authors are thankful for the technical and human support provided by SGIker (UPV/EHU/ERDF, EU). Conflicts of Interest: The authors declare no conflicts of interest. References 1. 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