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Effect of oxygen addition, reaction temperature and thermal treatments on syngas production from biogas combined reforming using Rh/alumina catalysts Andrea Navarro-Puyueloa , Inés Reyeroa , Ainara Morala , Fernando Bimbelaa,* , Miguel A. Bañaresb , Luis M. Gandíaa a Grupo de Reactores Químicos y Procesos para la Valorización de Recursos Renovables, Institute for Advanced Materials (InaMat), Departamento de Ciencias, Edificio de los Acebos, Universidad Pública de Navarra, Campus de Arrosadía, E-31006 Pamplona, Spain b Catalytic Spectroscopy Laboratory, Instituto de Catálisis y Petroleoquímica, CSIC, Marie Curie 2, E-28049 Madrid, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 May 2019 Received in revised form 4 July 2019 Accepted 30 July 2019 Available online xxx
Dry reforming and partial oxidation of biogas were studied using 0.5 wt.% Rh/Al2O3 catalysts, both inhouse prepared and commercial. The effects of O2 addition on syngas yield and biogas conversion were studied at 700 C using different O2/CH4 ratios in the gas feeding stream: 0 (dry reforming), 0.12, 0.25, 0.45 and 0.50. The highest CH4 conversion, H2 yield and H2/CO molar ratio were obtained with an O2/CH4 ratio of 0.45, even though simultaneous valorization of both CH4 and CO2 could be best attained when the O2/CH4 ratio was 0.12. Increased biogas conversions and syngas yields were obtained by increasing reaction temperatures between 650 and 750 C. A detrimental influence on catalytic activity could be observed when the catalyst was subjected to calcination. Increasing the hold time of the thermal conditioning of the catalyst under inert flow altered Rh dispersion, though had no significant impact on catalyst performance in the dry reforming of methane at 700 C and 150 N L CH4/(gcat h). Characterization of spent samples after reaction by Raman spectroscopy revealed the presence of carbonaceous deposits of different nature, especially on the commercial (named as Rh com) and calcined (Rh calc) catalysts, though oxygen addition in the biogas feed significantly reduced the amount of these deposits. The Rh catalysts that had not been calcined after impregnation (Rh prep) did not present any noticeable characteristic peaks in the G and D bands. In particular, scanning transmission electron microscopy (STEM) images of the spent Rh prep sample revealed the presence of very highly dispersed Rh nanoparticles after reaction, of particle sizes of about 1 nm, and no noticeable C deposits. Combined oxy-CO2 reforming of biogas using highly dispersed and low metal-loading Rh/Al2O3 catalysts with low O2 dosage in the reactor feed can be used to effectively transform biogas into syngas. © 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Biogas reforming Dry reforming Partial oxidation Rh catalyst Syngas
Introduction Biogas production has experienced a significant boost in the last decades as a consequence of the implementation of more efficient and environmentally friendly technologies for waste management. According to a recent report by the World Bioenergy Association based on data from the International Energy Agency (IEA) [1], world biogas production reached 58.7 billion Nm3 in 2014, with an average annual growth production rate of 11.2%.
* Corresponding author. E-mail address:
[email protected] (F. Bimbela).
Biogas is mainly composed of CH4 and CO2, together with some minor compounds. Its exact composition may differ between production plants and depends on several factors, including the organic substrate and the production technology, among others [2,3]. In the case of Spain and other countries from the European Union, the main source for biogas production is the anaerobic digestion of organic residues in landfills [4]. For this reason, landfill biogas valorization is relentlessly gaining interest for the production of energy and value-added compounds. There are different possibilities for biogas exploitation. As concerns energy applications, heat and power production by cogeneration is one of the most usual [5]. Another alternative implies an enrichment of the methane content by means of different conditioning and purification processes, thus obtaining a
https://doi.org/10.1016/j.jiec.2019.07.051 1226-086X/© 2019 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Navarro-Puyuelo, et al., Effect of oxygen addition, reaction temperature and thermal treatments on syngas production from biogas combined reforming using Rh/alumina catalysts, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.07.051
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gas stream denominated biomethane [5,6]. However, there are other interesting alternatives, such as its transformation into syngas by means of reforming reactions. Syngas or synthesis gas, a mixture of H2 and CO, is a key intermediate in the production of biofuels and valuable chemicals of industrial relevance, though its production is one of the most expensive steps of its processing such as in the case of the Fischer-Tropsch synthesis [5,7,8]. Catalytic dry reforming of methane has been widely studied and several interesting reviews are available in the literature [9–14]. This reaction allows the simultaneous utilization of the two main components of biogas (CH4 and CO2), though this route presents two major disadvantages: strong endothermicity and catalysts’ deactivation by coke deposition [15,16]. In order to overcome these inconveniences, some strategies have been proposed, such as the “combined reforming” processes. They consist in coupling dry reforming with one or more reactions, as steam reforming by adding H2O, called “bi-reforming” [17]; partial oxidation by adding O2, usually known as “oxy-CO2 reforming” [18]; or even by adding both simultaneously, a process denominated “tri-reforming” [19]. These alternatives present several advantages compared to dry reforming, such as alleviating coke deposition, higher flexibility on tuning the H2/CO molar ratio obtained, higher methane conversion as well as high selectivity to H2 and CO. Besides, oxy-CO2 reforming allows reducing the energy requirements, operating in a safer manner than the partial oxidation of methane and with higher energy efficiency if compared to the dry reforming reaction [20,21]. Oxy-CO2 reforming could be advantageous for the valorization of biogas streams that already contain small amounts of oxygen, for instance, in the case of landfill biogas, which is usually extracted and collected from landfills by vacuum pumping. Ni-based catalysts are typically used in dry reforming, autothermal reforming and partial oxidation of methane reactions due to their high catalytic activity and relative low cost [15,22–26]. However, these catalysts present a rapid deactivation by coke formation and sintering of the active phase that aggravates deactivation by coke [16,27]. In addition they can also suffer an easy re-oxidation of the active phase in the presence of oxidizing species, which can be an important drawback for the combined reforming with oxygen. An alternative to Ni catalysts is the use of noble metal catalysts, such as Rh, which allows to obtain superior performances with higher resistance to coke deposition on dry reforming [10,11,28–31], combined oxy-steam reforming [32,33] and partial oxidation of methane [34–36]. Bimetallic RhNi systems have also shown promising performances both in the dry reforming of methane and ethanol [37,38]. Steam reforming of biogas using metal-foam-coated Rh-Pd catalysts has also been explored [39,40]. H2-rich syngas from steam reforming of methane and heavier hydrocarbons can also been obtained using highly active and stable Rh catalysts [41,42]. Noble metals have the disadvantage of their high cost and limited availability, but an adequate design of the catalyst, particularly using a low active phase load, and a rational process development could overcome these obstacles and result in technically and economically viable technologies at industrially relevant scales. Regarding the use of Rh catalysts for oxy-CO2 reforming of methane, there are not many studies to date. The works by Cimino et al. [43] and Tsyganok et al. [44] were already discussed in our previous work [3]. In recent years, renewed interest in this topic has been observed. Lau et al. have studied the effect of different experimental conditions, such as reaction temperature, Gas Hourly Space Velocity (GHSV) and O2/CH4 molar ratio on the performance of 2% Pt–1% Rh/Al2O3 on cordierite monoliths modified with CeO2ZrO2 [21]. Rh has also been used as promoter of Ni–Co catalysts in order to reduce coke deposition [45]. Finally, Chen et al. performed a numerical investigation on combined reforming with oxygen on biogas from different origins, as landfill, sewage and farm biogas
streams, with a 5 wt.% Rh/Al2O3 catalyst on a spiral Swiss-roll reactor [46]. In our previous work, a preliminary screening of Rh catalysts for biogas valorization into syngas through combined reforming with oxygen was developed using a series of 0.5 wt.% Rh catalysts prepared on Al2O3, SiO2 and CeO2. It was found that Rh/ Al2O3 outperformed the rest of the catalysts tested, including a 0.5 wt.% Pt/Al2O3 one [3]. The present work aims to delve more deeply into biogas valorization through combined oxy-CO2 reforming of biogas using Rh/Al2O3 catalysts. In particular, it has been studied the effect of thermal preconditioning on the performance of a 0.5 wt.% Rh/Al2O3 catalyst prepared in the laboratory, and compared to the performance of a commercial catalyst used as reference. Furthermore, different O2/CH4 molar ratios and reaction temperatures have been tested aiming to investigate their influence on CH4 and CO2 conversions, as well as on the yield to syngas while simultaneously seeking to reduce carbon deposition on the catalysts surface. Special focus has been put on the stability of the catalysts and on the nature of the carbonaceous deposits formed. Ultimately, the general objective of this research line is to develop suitable rhodium catalysts with low metal content for syngas production via combined dry reforming of biogas with oxygen. Materials and methods Catalysts preparation and characterization 0.5 wt.% Rh/Al2O3 catalysts were prepared by the incipient wetness impregnation technique. The g-Al2O3 support (Spheralite 505, Procatalyse) was grinded and sieved to obtain a particle size distribution between 100 and 200 mm, and subsequently calcined at 750 C for 6 h under air before impregnation with Rh nitrate solution (Rh(NO3)3 (III), 10% (w/v) Rh in 20–25 wt.% HNO3, Acros Organics). To prepare a typical batch of 5 g of 0.5 wt.% Rh catalyst, 4.975 g of support were used in the impregnation. In order to prepare the Rh solution, 2.5 mL of the Rh precursor was diluted in deionized water up to a final volume of 25 mL, and the total volume of this solution used in the catalyst preparation was determined by taking into account the pore volume of the support. The solid obtained after impregnation was dried overnight at 105 C and named Rh prep. Part of this catalyst was further calcined at 750 C for 6 h under an air stream and labelled as Rh calc. A 0.5 wt.% Rh/ Al2O3 commercial catalyst (Johnson Matthey) was also tested and used as reference (Rh com). Fresh catalyst samples were characterized by means of different techniques, including N2 adsorption-desorption, temperatureprogrammed reduction (TPR), CO pulse chemisorption, X-ray diffraction (XRD) and transmission electron microscopy (TEM) or scanning transmission electron microscopy (STEM) combined with energy-dispersive X-ray spectroscopy (EDS). In addition, some spent catalyst samples were characterized by Raman spectroscopy. All high purity gases and gas mixtures used in the characterization were supplied by Nippon Gases Spain. N2 adsorption-desorption isotherms were determined in a Micromeritics Gemini V 2380 static volumetric analyser at 77 K. The samples were previously degassed at 200 C for 2 h under a N2 gas flow. The specific surface area was determined by the Brunauer-Emmett-Teller (BET) method, whereas the specific pore volume and the average pore size were calculated by the Barrett– Joyner–Halenda (BJH) method. TPR and CO pulse chemisorption analyses were conducted in a Micromeritics AUTOCHEM II 2920 apparatus equipped with a thermal conductivity detector (TCD). In a typical TPR run, about 100 mg of catalyst was reduced using 25 N mL/min of H2 diluted in Ar (5 vol.% H2) from room temperature through 930 C under a 5 C/
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min temperature ramp. As for the CO chemisorption, the samples were firstly subjected to heating under a 50 N mL/min He flow from room temperature up to 700 C under a 10 C/min temperature ramp to reproduce the steps followed prior to performing the catalytic tests. Different dwell times at the final temperature of 700 C have been tested: 0, 30 and 60 min. Next, the samples were cooled down while maintaining the He flow until the chemisorption temperature (30 C) was reached. Dynamic pulse chemisorption was readily carried out using 10 vol.% CO in He. A stoichiometry factor of 1:1 was assumed for CO chemisorption on Rh sites. XRD analyses were carried out at the Servicio de Apoyo a la Investigación of the Universidad de Zaragoza (Zaragoza, Spain) using a D-Max 2500 Rigaku diffractometer with CuKα radiation at 40 kV and 80 mA and scanning 2u from 5 to 95 . Raman spectra of the used catalysts samples were collected using a Renishaw System 1000 equipped with an Ar laser (514 nm), a cooled Charge Coupled Device (CCD) detector (73 C) and an Edge type filter. The spectral resolution was 3 cm1, and the spectra acquisition consisted of four accumulations of 30 s. TEM/STEM-EDS analyses were carried out using a Tecnai F30 (FEI company) microscope in the Laboratorio de Microscopías Avanzadas (LMA) at the Instituto de Nanociencia de Aragón (INA) of the Universidad de Zaragoza. It is a 300 kV Field Emission Gun (FEG) microscope equipped with a SuperTwin1 lens and a 2000 2000 Ultrascan CCD camera (Gatan) that allows a point resolution of 1.9 Å. For Z-contrast imaging in scanning transmission electron microscopy (STEM) mode, the microscope has a HighAngle Annular Dark Field (HAADF) detector. Catalytic tests All the catalysts were tested in a fixed-bed quartz tubular reactor (8 mm of internal diameter) at atmospheric pressure, coupled to an on-line gas analysis system, which consisted of a gas chromatograph (GC) (Agilent 6890 N) equipped with two chromatographic columns connected in series, an HPINNOWAX and a MolSieve 5A, and a TCD detector. The online analysis were carried out at 85 C, every 5 min, and using N2 as internal standard. The catalytic bed typically consisted in a mixture of 20 mg of 0.5 wt.% Rh catalysts in powder form (100–200 mm particle size fraction) and 1 g of high-purity α-Al2O3 (99.5%, Strem Chemicals), used as inert filler. The catalytic tests were carried out between 650 and 750 C with various O2/CH4 molar ratios in the gas feed, ranging between 0 and 0.5. The gas hourly space velocity (GHSV) was fixed at 150 N L CH4/(gcat h) in all runs. The reaction temperature was reached after a 10 C/min temperature ramp under He flow. Once the reaction temperature was reached, a dwell time of 30 min was held before entering the feed stream into the reactor, except in those additional runs that were conducted to study the effect of the dwell time on the catalytic performance. In the latter case, dwell times of 0 and 60 min were also set. A synthetic biogas mixture composed of 54 mol% of CH4, 40 mol% of CO2 and 6 mol% of N2 was directly used as feed in the dry reforming tests. In addition, high purity synthetic air was co-fed in the oxy-CO2 runs and He (99.999%) was used as carrier in the GC. To verify the repeatability of experiments
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conducted during the catalytic testing, five replicates of two of the experimental conditions were done using the commercial Rh catalyst at 700 C. The O2/CH4 ratios selected to analyse the repeatability were 0 (dry reforming) and 0.45 (oxy-CO2 reforming). Confidence level was set at 95% (α = 0.05) and Student’s t distribution (n ¼ 5 : tn1; a =2 ¼2:776 ) was assumed for the error in the determination of the confidence intervals for CH4 and CO2 conversion values using data from the GC analyses. It was found that the analytical uncertainty in our data was equal to or lower than 5% in all cases. Results and discussion Catalysts characterization The results obtained from the N2 adsorption/desorption analyses of the three catalysts tested and the calcined Al2O3 support are shown in Table 1. The calcined Al2O3 support and the Rh prep catalyst presented very similar specific surface areas (160 m2/g), pore volumes (0.44 cm3/g) and average pore sizes (7.3 nm). Conversely, the catalyst subjected to calcination, Rh calc, showed a decrease in the specific surface area and a slight increase in the average pore size. As for the commercial catalyst, Rh com, the sample had lower values of the specific surface area (99 m2/g) and pore volume (0.25 cm3/g) than the other samples, although having a very similar pore size to that of the Rh prep catalysts. In the XRD analyses (see Supplemental information Fig. S1), all three catalysts only showed the characteristic diffraction peaks related to the g-Al2O3 crystalline phase. The absence of Rh species diffraction peaks could be attributed most probably to the low metal loading on the catalysts [47]. All these results are in agreement with others previously reported by our group [3]. Table 2 presents the results from the CO chemisorption analyses, including some data already published in our previous work for the sake of comparison [3]. In this study, the commercial catalyst presented the greatest metal dispersion, having a very high value of around 87% in agreement with the evidences found by transmission electron microscopy (TEM) [3]. Regarding the catalysts prepared in the laboratory, the Rh calc sample had a lower dispersion and metallic surface area and larger particle diameters than the Rh prep one. The TPR profiles (not depicted) did not show any characteristic H2 consumption peak that could be attributed to the reduction of Rh species. However, the significant increase in metal dispersion and metallic surface area observed when the dwell time under He flow was raised from 0 to 30 min for all Rh catalysts can be explained by the existence of oxidized Rh surface species in the original samples [48]. The low Rh loading on the samples, the small amount of sample that must be used and the limitations of the instrument could lead to a very small H2 consumption over time, which could be on the verge of the detection limits for the TCD detector in the TPR analyses. As will be further discussed below, it can be concluded that modifying the thermal treatment carried out prior to conducting the CO chemisorption analyses may alter the state of the surface Rh atoms producing changes in metal dispersion, metallic surface area and active particle diameter. It can be proposed that a metastable situation occurs when the Rh catalysts are subjected to high
Table 1 Results from the N2 adsorption/desorption measurements. Sample
BET surface area (m2/g)
Pore volume (cm3/g)
Average pore size (nm)
Calcined Al2O3 support Rh prep Rh calc Rh com
160 167 145 99
0.44 0.45 0.48 0.25
7.3 7.5 8.5 6.9
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Table 2 CO chemisorption results. Sample Rh Rh Rh Rh Rh Rh Rh Rh Rh Rh a
prep prep prep prep calca calc calc calc coma com
Dwell time at 700 C under He flow (min)
Metal dispersion (%)
Metallic surface area (m2/g sample)
Metallic surface area (m2/g metal)
Active particle diameter (nm)
0 30 60 120 0 30 60 120 0 30
32.7 63.6 47.7 29.1 28.0 52.9 38.7 27.3 36.2 86.8
0.72 1.40 1.1 0.64 0.62 1.16 0.85 0.60 0.80 1.91
144.0 280.0 210.1 128.2 123.6 232.7 170.4 120.1 159.2 382.0
3.4 1.7 2.3 3.8 3.9 2.1 2.8 4.0 3.0 1.3
Data already published in Moral et al. [3].
temperature values as those typically used in the reforming of biogas. On the one hand, the presence of oxidized Rh surface species results in lower metallic surface areas when no dwell time is held at 700 C. Therefore, selecting relatively short dwell times at the reaction temperature is advantageous for significantly increasing the number of Rh metallic sites available on the catalyst surface, since that allows for reducing the oxidized Rh surface species thus increasing the metallic surface area. However, subjecting the catalysts to more intense or longer thermal treatments can be detrimental on the metallic surface area available, since solid diffusion phenomena may be initiated, which could eventually lead to the migration of Rh clusters from the surface to the lattice of alumina support, thus markedly affecting metal dispersion by diminishing the number of metallic Rh sites available on the catalyst surface. This could explain the lower metal dispersion and metallic surface areas of the Rh calc catalyst compared to those of the Rh prep, and subsequently this could be linked to a poorer catalytic performance both in terms of activity and stability, in agreement with the results from the catalytic tests that will be discussed later. Fig. 1 shows a representative TEM image of the fresh Rh prep sample. The lighter areas from the images taken in STEM mode could be attributed to Rh species as confirmed by the energydispersive X-ray spectroscopy (EDS) analyses made in various points of the sample. Very homogeneously and highly dispersed Rh
species could be seen in all the images, with particle sizes typically ranging around 4 nm, thus showing a similar size Rh particles distribution to that observed in the TEM images of the Rh calc sample [3]. It is interesting that despite not having evidences of sintering and having similar Rh particle sizes and the same Rh loading in both Rh prep and Rh calc catalysts, the CO chemisorption revealed a higher metal dispersion and metallic surface area for the Rh prep. This could be explained by the different interaction between CO molecules and surface Rh atoms in the catalysts, which could also be a consequence of a stronger interaction of the Rh clusters with the alumina support when the calcination is carried out. Previous studies found that depending on the type of surface Rh species, the CO molecules can be adsorbed very differently even at the same metal loading and resulting in variations in the stoichiometry of CO chemisorbed on Rh sites. Yates et al. [49,50] concluded that up to three different kinds of CO adsorbed species on surface Rh sites can be found in Rh/Al2O3 systems. Single atom sites would be responsible for the chemisorption of CO molecules in a CO:Rh stoichiometry of 2:1, while Rh atoms within a Rh cluster could result in chemisorbed CO molecules that could range from a CO:Rh stoichiometry of 1:1 to values of 1:2, the latter corresponding to a bridge configuration of CO chemisorbed species on Rhx sites where Rh atoms are coordinated to other Rh atoms in the form of clusters. Therefore, this change in the stoichiometry depending on the coordination of Rh atoms in the clusters could explain the results obtained in the CO chemisorption experiments (Table 2). Consequently, the stoichiometry for CO chemisorption on Rh is probably an intermediate value between the CO:Rh values of 1:1 and 1:2. The coexistence of both linear and bridged CO species adsorbed on reduced 2% Rh/ Al2O3 powder catalysts were identified by Cao et al. using infrared diffuse reflectance (DRIFTS) spectroscopy [51]. The authors found that exposure of the catalyst to a reducing atmosphere at increasing temperatures resulted in an augmented proportion of bridged CO species adsorbed on Rh metallic sites. Effect of catalyst calcination on the catalytic performance. Biogas dry reforming
Fig. 1. TEM image of the Rh prep fresh sample.
The effect of calcination on the reforming activity of Rhimpregnated catalysts was first studied in biogas dry reforming (which corresponds to an O2/CH4 ratio in the biogas feed equal to 0). Biogas dry reforming was conducted at 700 C and 150 N L CH4/ (gcat h). The evolution of methane (Fig. 2a) and carbon dioxide (Fig. 2b) conversions over reaction time is depicted for the three catalysts tested: Rh com, Rh prep and Rh calc. Control runs using bare alumina support particles under the same experimental conditions did not reveal any catalytic activity of the alumina standalone.
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Fig. 2. Evolution of (a) CH4 and (b) CO2 conversion over reaction time under dry reforming of synthetic biogas at 700 C and 150 N L CH4/(gcat h).
The commercial Rh catalyst, Rh com, and the catalyst prepared in the laboratory without the calcination step, Rh prep, presented very similar performances, with a slight decay in activity from initial CH4 and CO2 conversion levels down to values of CH4 and CO2 conversions around 45% and 67%, respectively, after 120 min of reaction. Similar tendencies were found for H2 (44%) and CO (51%) yields. In comparison, the calcined catalyst, Rh calc, showed a slightly lower catalytic activity and stability, with a decay on CH4 and CO2 conversions from 50 to 34% and from 71 to 54%, respectively. However, in all the cases very stable values were obtained for H2 and CO selectivity (around 1) and H2/CO molar ratios (0.95). The differences found between the Rh prep and the Rh calc catalysts evidence a detrimental effect of calcination on the catalyst performance, which could be explained by alterations on the porous structure and by a partial sintering of the Rh crystallites during the treatment at high temperatures. These results are in agreement with previous studies in the literature that reported a decrease on the catalytic performance of the partial oxidation of methane caused by the calcination of Rh/Al2O3 catalysts at high temperatures [52]. Finally, all the catalysts suffered a slight decay in their performance over time, probably due to deactivation by carbon deposition on the catalysts surface, as it could be observed in the characterization of the spent samples, which will be discussed later on. Interestingly, the dry reforming tests carried out with the Rh prep using different dwell times before reaction (0, 30 and 60 min, see Supplemental information Fig. S2) did not reveal any impact on the performance of this catalyst in spite of the notable changes in metal dispersion and metallic surface area that were found in the characterization by means of CO chemisorption when the dwell time was varied. This may be attributed to the high reforming activity displayed by Rh even at loadings as low as 0.5 wt.% and to the moderate structure sensitivity of the dry reforming reaction on the Rh/Al2O3 system, which could result in having no apparent changes in the catalyst performance regardless of the metal dispersion or metal particle size [53,54]. In fact, when the Rh crystallites are small (below 4 nm) and well dispersed, they can be more prone to deactivation by carbon deposition and to redispersion and eventually sintering [54]. As will be discussed later in the characterization of spent samples by TEM and Raman spectroscopy, the Rh prep catalyst did not show sintering after being used in the dry reforming tests carried out at 700 C, though deactivation by carbon deposits could be found. Influence of the O2/CH4 molar ratio The influence of feeding oxygen with different O2/CH4 molar ratios was studied at 700 C and 150 N L CH4/(gcat h). Figs. 3 and 4 show the results obtained for the evolution of the CH4 and CO2 conversions over time with the different mixtures fed into the reactor: (a) O2/CH4 = 0.12; (b) O2/CH4 = 0.25; (c) O2/CH4 = 0.45; and
(d) O2/CH4 = 0.5. A ratio of 0.5 corresponds to the stoichiometric amount of O2 needed for the partial oxidation of all the methane fed in, while a ratio of 0.12 corresponds to the stoichiometric amount needed for the partial oxidation of the methane in excess with respect to the stoichiometric amount that would completely react with CO2 in the dry reforming of the feed. Generally speaking, increasing the O2/CH4 molar ratio resulted in an increase in the CH4 conversion, as well as in augmented H2 yields (see Supplemental information Fig. S3) and H2/CO ratios, at the expense of a diminution in the CO2 conversion, the CO yield (see Supplemental information Fig. S4) and the selectivity to H2 and CO, which can be attributed to the partial and complete oxidation of methane reactions. A ratio of 0.45 yielded the highest CH4 conversions and H2 yields, even though a 0.12 ratio was selected as the best feeding conditions in order to have notable CH4 conversions while maintaining high CO2 conversion levels and a stable H2/CO ratio of 1.15. The selection of an O2/CH4 of 0.12 is in agreement with a thermodynamic simulation study reported in the literature, that established the optimum O2/CH4 ratio between 0.1 and 0.2 [20]. Regarding the results obtained with the different catalysts, Rh com showed the best catalytic activity and stability, followed by the catalysts prepared in the laboratory without calcination, Rh prep, which presented a slight decay on their activity over reaction time due to carbon deposition. Under combined reforming conditions, it can be observed again the detrimental effect of calcination on the catalyst performance, obtaining the lowest conversions and yields. It is also interesting the evolution of the conversions obtained with the Rh calc catalyst at O2/CH4 ratio of 0.5. The rapid decay of the conversions and yields at the first minutes, together with an increment on the CO2/CO ratio until values around 2, could be caused by the prevalence of the complete oxidation of methane over the partial oxidation reaction. With lower O2/CH4 ratios, the role exerted by the dry reforming and partial oxidation of methane reactions is notably incremented, which allows for a significant rise in CO2 conversion, as shown in Fig. 4d, together with a decrease in the CO2/CO ratio down to 1.45 and an increase in the values of syngas yield. Effect of the reaction temperature The catalyst prepared without the calcination step, Rh prep, was tested under dry (O2/CH4 = 0) and combined reforming (O2/CH4 = 0.12) conditions at reaction temperatures in the range between 650 and 750 C and 150 N L CH4/(gcat h). Figs. 5 and 6 present the evolution of the methane and carbon dioxide conversions over reaction time obtained in these tests. Under dry reforming conditions, increasing the reaction temperature resulted in an enhancement of CH4 conversion up to values around 53% after 2 h at 750 C, as well as for CO2 conversions and H2 and CO yields (See Supp. Info. Fig S3). At all three studied reaction temperatures, the H2/CO ratios obtained
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Fig. 3. Influence of the O2/CH4 molar ratio on the evolution of methane conversion over time.
Fig. 4. Influence of the O2/CH4 molar ratio on the evolution of carbon dioxide conversion over time.
were very similar and stable over time (around 0.95), even though at 750 C a ratio of 1 was reached. This temperature effect was also observed under combined reforming conditions, obtaining a CH4 conversion of 70% at 750 C. In addition, adding O2 at 650 C produced a significant decrease of CO2 conversion and H2 and CO yields, around 15%, which is comparatively larger than the observed decrease in CH4 conversion that together with a lower selectivity to syngas and higher CO2/CO ratios can be attributed to an increased selectivity toward the complete oxidation of methane at relatively low reaction temperatures. This would be compatible with a kinetic effect associated to a lower activation energy of methane combustion over Rh compared with CH4 partial oxidation
and CO2 reforming. In this regard, the combination of an enhanced oxygen mobility on the catalyst support at temperatures around 650 C and the presence of active surface O species adsorbed on the metal particle have been proposed in reaction mechanisms reported previously [55,56]. However, the H2/CO ratio obtained under combined reforming conditions was maintained stable in values around 1.15 in all the range of temperatures studied. The effect of the reaction temperature had been previously reported on the dry reforming of methane with noble [57] and transition metal catalysts [58]. Besides, in other studies on the combined reforming of methane, it was observed an increment in the CH4 and CO2 conversion with increasing reaction temperature,
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conversion by CO2 dissociation but also favouring the promotion of the reverse water–gas shift reaction [59], that could result in the consumption of H2 and in an increased CO production that would lower the H2/CO ratio. In consequence, the choice of higher reaction temperatures for favouring the simultaneous conversion of CH4 and CO2 must be balanced with a careful analysis of the optimal conditions for increasing the selectivity and hence the yield to syngas, as well as the H2/CO ratio of the product gas. Catalysts stability
Fig. 5. Influence of the reaction temperature on the evolution of methane conversion on Rh prep over time in dry (a) and combined (b) reforming biogas with Rh prep catalyst.
Catalysts stability tests were carried out under dry reforming (O2/CH4 = 0) and combined reforming (O2/CH4 = 0.12) conditions at 700 C and 150 N L CH4/(gcat h) during 8 h. The evolutions of the CH4 and CO2 conversions are depicted in Figs. 7 and 8 for the three catalysts studied. Regarding the dry reforming tests (Fig. 7a), the Rh prep catalyst presented the best performance showing the highest CH4 conversion, around 41% after 8 h, whilst the Rh calc yielded lower conversions, with a final value of 33%. Both catalysts presented a similar deactivation trend over time, with a decay in CH4 conversion of around 15%, while this deactivation was more accentuated in the case of the Rh com (25%). The commercial catalyst showed similar performance to that the Rh prep sample in the first 80 min of reaction and then fell down to values that were closer to those obtained with the Rh calc catalyst. The decay in activity observed on the three catalysts could be attributed to carbon deposition on the catalysts surface, as will be discussed later. However, under combined reforming conditions, the Rh com catalyst yielded higher and more stable CH4 conversion values throughout, with values around 63% during 8 h of reaction, followed by the Rh prep and the Rh calc catalysts, with final CH4 conversions around 51 and 43%, respectively. It can be observed that feeding an O2/CH4 ratio of 0.12, improved the stability of the catalysts, with a smaller loss on CH4 conversion over time, below 8% after 8 h of reaction. These CH4 conversion trends described under dry and combined reforming conditions were also observed for CO2 conversion and H2 and CO yields, even though H2/ CO ratio in all cases were very stable over time, with values around 0.95 in dry and 1.15 in combined reforming conditions for the three catalysts tested. Catalysts characterization after reaction
Fig. 6. Influence of the reaction temperature on the evolution of carbon dioxide conversion on Rh prep over time in dry (a) and combined (b) reforming biogas with Rh prep catalyst.
though the H2/CO ratio decreased at temperatures above 800 C [18]. Such results could be a consequence of the activation of CO2 on the catalyst surface, which could result in enhanced CO2
In order to gain further insight on the catalysts’ deactivation, some spent samples of catalysts were characterized after reaction by Raman spectroscopy and by STEM-EDS. Fig. 9 shows the Raman spectra obtained for spent catalyst samples after 2 h of reaction under dry (O2/CH4 = 0) and combined reforming (O2/CH4 = 0.12) conditions at 700 C and 150 N L CH4/(gcat h), respectively. Regarding the dry reforming samples (Fig. 9a), both the characteristic bands corresponding to carbon deposits of more graphitic nature (G) at 1595 cm1 and to more amorphous carbon (D) at 1333 cm1, respectively, can be seen even though the proportion of these bands was slightly different for each catalyst. The ratio between the intensities of D and G peaks (ID/IG) provides information about the size of carbon domains [60] and the disorder degree of the carbon [61], obtaining similar values for Rh com and Rh calc, 0.8 and 1.0, respectively. This is in agreement with our previous findings, which revealed the coexistence of carbon deposits with different disorder degree in both catalysts [3]. However, in the case of the Rh prep, the Raman spectra only showed a weak band, with significantly lower intensity in comparison to the other two catalysts. The band is at~1590 cm1, which is associated with incipient carbon deposits of more amorphous nature. This is in agreement with the superior performance observed (Figs. 7 and 8). These results can explain
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Fig. 7. Role of O2/CH4 ratio on the evolution of methane conversion over reaction time in dry (a) and combined (b) reforming of biogas at 700 C and 150 N L CH4/ (gcat h).
Fig. 8. Role of O2/CH4 ratio on the evolution of carbon dioxide conversion over reaction time in dry (a) and combined (b) reforming of biogas at 700 C and 150 N L CH4/(gcat h).
the different loss of activity observed in all cases, which could be attributed to deactivation by coke formation. Rh catalysts supported on Al2O3 can stabilize reactive Cβ carbon species of
amorphous nature [62], which upon ageing may evolve into more graphitized and less reactive C species. Interestingly, under combined reforming conditions (Fig. 9b), a remarkable decrease in the characteristic signals of carbon is apparent, especially on the Rh com and Rh prep catalysts, which hardly presented G and D bands. The Rh calc catalyst presented peaks with an ID/IG ratio similar to that found in the dry reforming spent samples, though with lower overall intensity. These results show the beneficial effect of adding O2 to reduce the carbon deposition in the biogas reforming to syngas. The formation of ethylene (C2H4(g)) is favoured over the oxidation of methane to COX species when controlled dosing of O2 is conducted [63], which could explain the results found. According to this, the amorphous carbon species formed on the surface could react and be desorbed, to be subsequently adsorbed on the catalyst particles downstream and yield more adsorbed C deposits that would evolve into graphitized forms. This would explain the gradual but less steep decay in activity observed in the combined reforming tests and the qualitatively lower intensity of the signal in the Raman spectra of the catalysts used. For the spent catalysts under dry and combined reforming (O2/ CH4 = 0.12) at 700 C after 2 and 8 h, thermogravimetric analyses (TGA) were carried out in order to quantify the total amount of carbon deposits. However, the quantification was not possible because the weight loss was extremely low, as a consequence of the characteristic low carbon deposition on Rh catalysts that typically impede to quantify the carbon deposited by TGA analysis, neither at dry reforming conditions, which are more prone to carbon formation [64,65]. Fig. 10 shows STEM images of the Rh prep spent samples after reaction. All three spent samples presented a similar morphology after reaction regardless of the O2/CH4 ratio used. The STEM images revealed the presence of very highly dispersed Rh nanoparticles, having typical particle sizes of 1 nm or less, which correspond to the brighter spots seen in the images. The samples did not show noticeable C deposits, which is in agreement with the relatively low amounts of C found in the Raman spectroscopy and in the TGA analyses. Rh prep catalyst stands out in the present study by a wellbalanced performance in terms of activity, selectivity and stability. Its performance is similar and in some aspects even slightly superior to the catalyst taken as reference. The resistance of Rh prep toward carbon deposits formation is remarkable. The lack of relevant information (e.g. preparation method, whether it was calcined or not, etc.) regarding the commercial catalyst complicates making more in-depth comparisons between these solids. In both cases Rh presents high dispersions with particle sizes within the 2–4 nm range. On the other hand, Rh prep has a specific surface area (167 m2/g) that almost doubles that of Rh com (99 m2/g). Perhaps this could be related with a higher concentration of active sites for CO2 activation on the support in the case of Rh prep. Specific characterization through operando Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and kinetic studies are being planned in order to gain insight in the catalytic properties of this interesting catalyst. Furthermore, if the results presented in this study are compared to previous works on biogas dry reforming under the same reaction conditions (700 C) and using different Ni and Rh catalysts, it can be concluded that the performance of the Rh catalysts in this study displays similar levels of methane conversion of those already published in the literature, but at a much higher spatial velocity, which is a notable outcome of this study. Fig. 11 shows the comparison made with results from previous studies in the literature. It can be noted that the spatial velocity used in this study is the highest among those that can be directly compared. The remarkable performance of the Rh catalysts prepared in this study
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Fig. 9. Raman spectra of the spent catalysts after 2 h of reaction at 700 C under (a) dry reforming (O2/CH4 = 0) and (b) combined reforming (O2/CH4 = 0.12) conditions.
Fig. 10. STEM images of the Rh prep catalyst after reaction using different O2/CH4 ratios (a) O2/CH4 = 0 (dry reforming), (b) O2/CH4 = 0.12 and c) O2/CH4 = 0.45.
Fig. 11. Literature comparison of biogas dry reforming at 700 C.
is reflected on the capacity for attaining similar methane conversion levels at much higher GHSV values. The performance of the Rh prep catalyst is somewhat lower than that obtained by De Caprariis et al. [29] (45% of overall methane conversion, present study, versus 56%, De Caprariis et al.), but the GHSV in the present study is 1.7 times higher, which clearly shows that the performance of the Rh catalysts in this study ranks among the best results in the available literature. Conclusions 0.5 wt.% Rh/Al2O3 catalysts have been studied in the dry and combined reforming of biogas. The home-made catalyst presented a good performance, and gave results that were close to those achieved with a commercial catalyst taken as reference, even reaching higher conversions and yields in dry reforming stability tests after 8 h of reaction. The thermal pretreatment of the catalysts before reaction was found to have an influence on metal dispersion of the Rh/Al2O3 catalysts. A careful choice of conditions must be done for effectively activating the catalysts. Subjecting the catalysts to moderate dwell times to an inert flow at the temperatures usually selected for biogas
reforming resulted in increased metal dispersions and higher available metallic surface areas, as a consequence of effectively reducing oxidized Rh surface species. However, the catalyst calcination at 750 C for 6 h prior to their use in reaction had a detrimental effect on the catalyst activity under dry and combined reforming conditions. Another factor that had an important effect on the reforming activity was the reaction temperature, resulting in enhanced conversions and yields with increasing temperatures in the range studied, between 650–750 C. Adding O2 to the reaction feed increases CH4 conversion, H2 yield and H2/CO molar ratio, reaching a maximum at an O2/CH4 molar ratio of 0.45, though at such conditions total combustion of methane seemingly prevailed over partial oxidation at relatively low reaction temperatures, and CO2 conversion was significantly lowered. Consequently, a 0.12 O2/CH4 ratio can be considered more adequate because notable CH4 conversions were obtained while high CO2 conversions were maintained. Besides, combined reforming of biogas increased catalysts’ stability and reduced carbon deposition, due to the partial elimination of carbon deposits promoted by the oxidizing conditions in the combined reforming reaction. Overall, it can be concluded that processes based on combined reforming strategies using Rh/Al2O3 catalysts with low Rh content could constitute a promising alternative for biogas valorization into syngas. Acknowledgements The authors thank the Spanish Ministerio de Economía, Industria y Competitividad (MINECO) (ENE2015-66975-C3), Spanish Ministerio de Ciencia, Innovación y Universidades (RTI2018096294-B-C31), and the European Regional Development Fund (ERDF/FEDER) for the financial support. MINECO and ERDF/FEDER are also thanked for the pre-doctoral aid (BES-2016-077866) awarded to Andrea Navarro and for funding the contract of Dr. Ainara Moral. The Universidad Pública de Navarra (UPNA) is also acknowledged for the post-doctoral aid awarded to Inés Reyero.
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Please cite this article in press as: A. Navarro-Puyuelo, et al., Effect of oxygen addition, reaction temperature and thermal treatments on syngas production from biogas combined reforming using Rh/alumina catalysts, J. Ind. Eng. Chem. (2019), https://doi.org/10.1016/j.jiec.2019.07.051