Syngas Production from Carbon Dioxide Reforming of Ethanol over Ir

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Syngas production from carbon dioxide reforming of ethanol over Ir/Ce0.75Zr0.25O2 catalyst: Effect of calcination temperatures Fengzuo Qu, Yichen Wei, Weijie Cai, Hao Yu, Yi Li, Shaoyin Zhang, and Congming Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03945 • Publication Date (Web): 05 Jan 2018 Downloaded from http://pubs.acs.org on January 7, 2018

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Syngas production from carbon dioxide reforming of ethanol over Ir/Ce0.75Zr0.25O2 catalyst: Effect of calcination temperatures Fengzuo Qu1, Yichen Wei1, Weijie Cai*1,3, Hao Yu4, Yi Li2, Shaoyin Zhang1, Congming Li3* 1

Faculty of Light Industry and Chemical Engineering, Dalian Polytechnic University,

116023 Dalian, China 2

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of

Chemistry, Jilin University, 130012 Changchun, China 3

State Key Laboratory Breeding Base of Coal Science and Technology Co-founded by

Shanxi Province and the Ministry of Science and Technology, Taiyuan University of Technology, 030024 Taiyuan, China 4

College of Chemical and Environmental Engineering, Shandong University of

Science and Technology, 266590 Qingdao, China.

*Corresponding author: [email protected]; [email protected] Fax: +86-41186322228 Phone: +86-41186324482

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Abstract Ir/Ce0.75Zr0.25O2 catalysts synthesized by facile co-precipitation method were calcined at various temperatures. The influence of calcination temperature on their physicochemical features and catalytic behavior for carbon dioxide reforming of ethanol was investigated. Several key factors such as Ir dispersion, reducibility, the oxygen vacancies as well as Ir-support interaction declined with increasing the calcination temperature, which result into the decrease of ethanol conversion and less intrinsic turnover frequency (TOF). Among the catalysts tested, IrCeZr550 sample exhibited satisfactory activity and maintained stable performance as long as 90h time-on-stream. In contrast, full ethanol conversion was only achieved at 750oC for the more sintered IrCeZr850 catalyst, thereby higher than that of the others. Moreover, stability test of IrCeZr850 elucidated that ethanol conversion continuously decreased from 87% to 62% and the molar ratio of H2/CO increased from 1.16 to 1.32 due to the inhibition of reverse water gas shift reaction. For the deactivated IrCeZr850 catalyst, characterization results including HRTEM, XRD, Raman and TPO revealed that the observed deactivation might be caused by the combination of the encapsulated carbon formation because of less oxygen defects and the remarkable sintering of active Ir species due to weaker Ir-support interaction. This established structure-activity relationship might provide insight on the development of suitable catalysts for syngas production from ethanol dry reforming.

Keywords: Calcination temperature; Ir/Ce0.75Zr0.25O2; Ethanol; Dry reforming.

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1. Introduction Nowadays, syngas production (a mixture of H2 and CO) from carbon dioxide reforming of methane has been extensively investigated considering this process converted the undesirable CO2 (greenhouse gas) into value-added chemicals [1,2]. The downstream syngas is regarded as suitable feedstocks in Fischer-Tropsch process to synthesize hydrocarbons, carbonate, ester and/or liquid fuels etc as pointed out in literature [3,4]. However, this process were unsustainable due to the employment of natural gas. With this concern, great interest has been arisen to explore the alternative routes for syngas production. Among the various renewable feedstocks, ethanol possessed several merits including the relatively high hydrogen content, availability, non-toxicity, easy storage and safety etc. Moreover, the development of the second generation bio-ethanol production from non edible lignocellulosic biomass made it more economic and competitive [5,6]. Therefore, ethanol dry reforming process (EDR) might be a green and environmental-friendly technique to produce syngas. Recently, EDR process has been only scarcely investigated and the achievement is limited [7-9]. Herein, thermodynamic analysis has been conducted by several groups suggesting the feasibility of this green process [10-12]. So far, the design of an efficient catalytic system for EDR is still basically a trial-and-error approach [13-17]. The main challenge is the requirement of the high reaction temperature and the quick deactivation caused by active metal sintering and/or coke accumulation. Stable, active and selective catalysts are desirable for its practical industrial applications. Noble metal catalysts are well known regarding their highly catalytic behavior in any type of reactions, especially when coke formation has to be avoided [18,19]. For ethanol conversion, a series of noble metals including Rh, Ru, Pd, Ir, and Pt have been extensively studied. It was normally supported on the redox materials like CeO2-based systems, which were able to store/release oxygen [20,21]. Herein, Ir supported catalysts (Ir/CeO2, Ir/Ce0.75Zr0.25O2 etc) exhibited the promising activity towards hydrogen production from ethanol and/or propane as described in our previous literature [22-24]. The outstanding catalytic performance was probably attributed to its strong Ir-support interaction and sufficient interfacial areas which could efficiently 3


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promote ethanol activation and inhibit the sintering of Ir species. Furthermore, a high oxygen storage capacity (OSC) of support might facilitate CO2 adsorption and its subsequent dissociation into active O atoms. This could dynamically remove the formed coke on the catalyst surface Apart from the nature of active metal and support [25], other factors such as the synthesis conditions and calcination temperature are also considered as important parameters determining catalytic behavior. Valle et al. [26] pointed out that the calcination/reduction conditions played an important role on the activity and stability of the Ni/La2O3-Al2O3 catalyst for steam reforming of raw bio-oil/ethanol. Garcia-Vargas et al. [27] also reported that the various calcination atmosphere corresponded to the remarkable difference in methane dry reforming process. Similarly, calcination temperature greatly affected the textural features of Ni/CeO2-ZrO2 catalyst as reported by Arslan et al. [28]. Given the above background together with our previous study about Ir-based catalysts, the aim of this work was to explore the influence of calcination temperature of Ir/Ce0.75Zr0.25O2 catalyst on its catalytic performance during ethanol dry reforming. The change of physicochemical properties was comprehensively evaluated by various characterization technique such as XRD, HRTEM, SEM, H2-TPR, CO2-TPD, Raman etc. Catalyst activity and stability under EDR conditions was also tested in order to establish a correlation between calcination temperature and catalytic behavior. 2. Experimental 2.1 Preparation of the Ir/Ce0.75Zr0.25O2 catalysts Ir/Ce0.75Zr0.25O2 catalysts calcined at different temperatures were synthesized by urea assisted co-precipitation method selecting H2IrCl6, (NH4)2Ce(NO3)6 and ZrOCl2 as precursors [22]. The obtained catalysts were labeled as IrCeZr550, IrCeZr700 and IrCeZr850, respectively, based on the calcination temperature. 2.2 Catalyst characterization The actual elemental compositions of the catalysts were measured by an ICP-OES apparatus (Perkin Elmer, Optima 8000). Prior to analysis, the samples were dissolved by HF acid. 4


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The specific surface areas were determined by N2 adsorption-desorption isotherms using a Micromeritics ASAP 2010 apparatus. X-ray diffraction patterns (XRD) were recorded on a Shimadzu diffractometer (D/max3B) to estimate the crystal structure of the synthesized catalysts and calculate the mean crystallite sizes according to Scherrer equation. Hydrogen temperature programmed reduction (H2-TPR) was conducted using a Micromeritics AutoChem II 2920. Generally, the samples were pretreated at 300oC for 1h under a N2 flow. After cooling down to room temperature and introducing the reduction agent (5vol%H2/N2), temperature was then heated to 800°C at a rate of 10oC/min. The dispersion of active Ir species was evaluated by H2 chemisorption method [25]. The tests were conducted using a static chromatographic apparatus at -85oC in order to avoid the influence of H2 spillover [29,30]. The catalysts were pre-reduced at 500oC for 1h using a 5vol% H2/N2 flow and then cooled down to room temperature under nitrogen. After placing the reactor into an acetone-dry ice mixture (-85oC), H2 pulses were consecutively injected until adsorption saturation, which determined the total H2 chemisorption (HCT). The second pulse procedure was carried out after flushing the sample for 15min under N2 flow to remove the physically adsorbed H2. The consumed H2 was denoted as HCP. Finally, the chemisorbed H2 (HCC) was quantified as following (Eq1): HCC = HCT - HCP

Eq 1

The dispersion of metallic Ir was further calculated assuming that the adsorption stoichiometry was one H atom for one surface Ir atom. The morphology structure of the catalysts were determined using SEM on a JSM-7800F and elemental mappings were obtained with a silicon drift detector (50mm2, X-Max50 Oxford Instruments). Transmission electron microscopy (TEM) was conducted on a JEM-2100 operating at 200kV to analysis morphology features. Prior to analysis, catalysts were ultrasonically dispersed in ethanol solution and then deposited onto a mesh grid with carbon film. 5


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Raman spectra were collected on a invia Raman microscope (Renishaw) equipped with an Ar Laser at 532nm and 10-40s exposure time. The intensity and position was achieved by Lorentzian fitting model. The region of the recorded spectra was from 100 to 4000cm-1. CO2-TPD tests were conducted to estimate the basic sites on the investigated catalyst surface. Prior to analysis, samples were firstly insitu reduced at 500oC for 1h with 5vol% H2/N2 flow and then cooled down to room temperature under N2. A 10vol% CO2/N2 flow was injected for 30min and then purged with N2 for 20min in order to remove the physically adsorbed CO2. The temperature was progressively risen up to 700°C at 10oC/min. The desorbed CO2 signal was on-line monitored by a mass spectrometry (Inficon quadrupole). The formation of the carbonaceous deposits during stability tests was quantified by temperature programmed oxidation (TPO) of the used catalyst. The used catalyst were loaded in a microreactor and the system was heated from 25 to 800oC at a rate of 10oC /min with a flow of 5.0% O2/N2 mixture. The effluents were analyzed by the on-line mass spectrometry (Inficon quadrupole). 2.3 Catalytic activity tests CO2 reforming of ethanol was carried out in a conventional fixed-bed steel reactor (8mm of internal diameter). Typically, 200mg of catalyst (40-60mesh) were mounted in the reactor and sandwiched by quartz wool. Prior to reaction, sample was in-situ reduced with a 5vol% H2/N2 flow (50mL/min) at 500oC for 1h. The reactions were conducted with stoichiometric feed composition of ethanol/CO2 (1/1, molar ratio). Absolute ethanol (﹥99.5%) was injected into a vaporizer by a micro-pump (Series III type) and the vapors were blended with a mixture flow of CO2 and N2 precisely controlled by mass-flow controllers (ethanol/CO2/N2 = 30/30/40, molar ratio). The space velocity of the reaction was set at 10000mL gcat-1h-1. N2 stream was introduced as both diluted gas and internal standard gas in order to quantify ethanol conversion and product yields. The effluent gas was analyzed on-line by gas chromatograph. H2, CO and CO2 were separated by a packed column (HayeSep D) and analyzed by a thermal conductivity detector (TCD) using He as carrier gas. Hydrocarbons and 6


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oxygenates were separated with a capillary column (INNOWAX) and analyzed with a flame ionization detector (FID). Ethanol conversion was calculated according to: XEtOH = (MEtOH)inlet-(MEtOH)outlet/ (MEtOH) inlet

Eq 2

where (MEtOH)inlet referred to the moles of injected ethanol (MEtOH)outlet referred to the moles of unreacted ethanol in the effluent gas Molar concentrations of products in the outlet gas were calculated based on: C*x = (Mx)outlet /∑(Mi)outlet

Eq 3

where (Mi)outlet was the moles of products (labeled as i) in the outlet product gas including H2, CO, CH4, acetaldehyde, acetone (except CO2). To compare the intrinsic rates of the as-prepared catalysts, ethanol turn-over frequency (TOF) were determined according to: TOF= (rEtOH × MIr)/(XIr × DIr)

Eq 4

where rEtOH was ethanol conversion rate (mol gcat-1 s-1) MIr was the relative atomic mass of iridium (192g/mol) XIr was the actual Ir loading analyzed by ICP analysis DIr was the iridium dispersion calculated by H2-chemisorption 3. Results and discussion 3.1 Characterization of the fresh catalysts 3.1.1 Physical properties (BET, ICP) Typical N2 adsorption-desorption isotherms of the as-prepared catalysts, which are calcined at various temperatures, are presented in Fig 1. The profiles of IrCeZr550 and IrCeZr700 catalysts presented type IV isotherms indicating the formation of mesoporous solid. Hysteresis loops could be classified as type H2 loops. On the contrary, mesoporous structure was remarkable distorted with increasing calcination temperature to 850oC. Similar results were proposed in the literature for the CeO2-ZrO2 composite oxide [28]. Additionally, as shown in Table 1, a significant decline of specific surface area (from 130 to 35m2/g) was observed when the calcination temperature increased from 550 to 850oC. Consequently, the pore volume decreased from 0.134 up to 0.102cm3/g. Additionally, the average pore diameters 7


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deriving from the BJH curves reversibly increased from 3.66 to 5.56nm. Similarly, the surface area loss of CeO2-based materials with increasing calcination temperature was also pointed out because of the growth of meal oxide sizes [31,32]. It should be noted that all the catalysts possessed higher surface area than that of Ir/CeO2 samples (from 123 to 18m2/g) as described in our previous results [33]. This suggested that the incorporation of Zr greatly enhanced the thermal stability of composite oxides due to a change in crystal structure [34]. Specially, the doped Zr4+ might act as "islands" between the Ce4+ species and thereby prevented its aggregation during the calcination treatment. ICP-AES analysis elucidated that the actual loading of Ir species was ca.2wt% in close to the targeted values and the molar ratio of Ce/Zr was around 3 in all cases. Additionally, the dispersion of metallic Ir (DIr) determined from H2 pulse chemisorption drastically declined from 58% to 17% with increasing calcination temperature. The particle sizes of Ir species were semi-quantified from the Ir dispersions as described in [29]. dIr = 91/DIr

Eq 5

As shown in Table 1, IrCeZr550 catalyst presented the highest Ir dispersion and the smallest Ir particle size. Hence, the higher surface area of CeZr support inhibited the aggregation of Ir species and resulted in the higher dispersion. In addition, it was recognized that CeO2-based supports possessed O defects and thereby the tighter bonding of active metal nanoparticles with such O vacancies could remarkably inhibit its migration and aggregation during the calcination process, which gave rise to the higher dispersion. Hence, IrCeZr550 sample with higher Ir dispersion revealed better Ir-support interaction. Sanchez et al. [35] also investigated the interaction of the O defects in ceria-based materials with noble metals including Ir, Pd, Rh etc and corresponded to a strong metal-support interaction model, which was termed as metal nesting, via the filling of the O vacancies with the metallic atoms. A similarly anchoring mechanism for Au/CeO2 system was reported by Shen et al. [36]. In summary, catalyst calcination temperature possessed a great effect on the textural properties of the as-prepared catalysts. 8


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3.1.2 XRD profiles Fig 2a shows XRD patterns of the catalysts calcined at various temperatures ranging from 550 to 850oC. Notably, in all cases, only several reflections centered at 28.7, 33.3, 47.9 and 56.8o respectively were observed. They were generally assigned to (111), (200), (220) and (311) crystal planes corresponding to the c-CeO2 fluorite structure (JCPDS 34394) [37,38]. On the other hand, no obvious diffraction peaks attributed to ZrO2 species were detected implying the incorporation of Zr species into the CeO2 lattice matrix to form a CeZr solid solution. It was evidenced by a positive shift of the peaks in comparison with pure CeO2 (JCPDS 34394), which was caused by the lattice shrinking because of the replacement of Ce4+ (0.097nm) by smaller Zr4+ ions (0.083nm). Similar results on CeO2-ZrO2 composite oxides were also reported by other groups [39,40]. The absence of characteristic ZrO2 bands was also not rule out the reason of small concentration of Zr species. With increasing calcincation temperature, a remarkable increase in the intensity of the diffraction peaks was noted as a consequence of crystallite growth and the decreased concentration of lattice defects upon heating [41]. The mean crystallite sizes were calculated using the ceria (111) crystal plane according to Scherrer equation and the results were also presented in Table 1. While the crystal size of CeZr support was ca.5.1nm, it remarkably increased to 12nm at 850oC. Furthermore, the growth of support sizes resulted in loss of surface area, which was in line with the BET analysis. Generally, a close relationship between crystal size and surface area was established in which lower the particle size, larger the surface area. In terms of active metal Ir, no detection of characteristic reflexes for Ir species were observed because of its low content, or, more probably, to its high dispersion as early reported by our group [22,24]. The external surface area (Sexternal) was calculated using the following equation, assuming the particles to be spherical: Sexternal = 6000/(ρCeZr×dCeZr)

Eq 6

Where ρCeZr corresponded to the density of the composite oxides dCeZr was the average crystallite sizes calculated from XRD profiles 9


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(Debye-Scherrer equation). Interestingly, the obtained values of Sexternal were larger than those of BET surface areas, whatever the calcination temperature. Hence, it could be deduced that the mixed oxide grains were typically polycrystalline and partial grain boundaries might be not accessible for the adsorbed N2 gas [29]. 3.1.3 Raman spectra The as-prepared IrCeZr catalysts were further characterized by Raman spectroscopy in order to provide some insight into the crystal structure. As shown in Fig 3a, the samples treated at various thermal conditions displayed similar profiles. Particularly, a strong band at ca.469cm-1 was attributed to the F2g vibration mode of the face-centered cubic CeO2 structure (Ce-O-Ce streching). It was caused because of the symmetric breathing mode of the O atoms around Ce4+ ions [42,43]. The weak peak near 641cm-1 was assigned to the presence of some vacancies linked to O defects (Frenkel-type) in the CeO2 framework. Laguna et al. [44] reported that the incorporation of Zr into CeO2 lattice enhanced the formation of O vacancies. It was believed that an oxygen ion was displaced from its lattice position to an interstitial position, hereby resulting in a defect at its initial position [45]. It was apparent that the intensity of this band remarkably declined with increasing calcination temperature, which indicated the decrease of O vacancies concentration. Mamontov et al. [46] also pointed out that the higher calcination temperature inhibited the formation of O vacancies in CeO2 materials because of the recombination of the interstitial ions with the defects. The area ratio of bands at 469, 641cm-1 (labeled as A641/A469) could be selected to quantitatively evaluate the O defects: the higher the area ratio, the higher the concentration of O vacancies [47,48]. As presented in Fig 3b, the value of A641/A469 decreased in the following sequence: IrCeZr550 (1.53)﹥IrCeZr700 (0.67) ﹥IrCeZr850 (0.27), revealing the larger amount of O defects in IrCeZr550 catalysts. Generally, O vacancies played an important role to greatly improve the catalyst activity and stability in various reactions including ethanol and/or CO2 chemical conversion [49]. It was assumed that the formed vacancies might facilitate the adsorption/dissociation of CO2 to supply active O species for the dynamic removal of 10


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carbon. In addition, the presence of O defects could efficiently improve the dispersion of noble metals as evidenced by several groups [35,50]. The tighter bonding of Ir nanoparticles with such O vacancies could remarkably inhibit its migration and aggregation during the calcination process, which gave rise to the higher dispersion. The interaction of the O defects in ceria-based materials with other noble metals including Pd, Rh etc has been proposed and related to a strong metal-support interaction, via the filling of the O vacancies with the metallic atoms [44]. This result was in consistent with the H2-chemisorption tests, which indicated that IrCeZr550 sample possessed the highest dispersion of Ir species. It’s interesting to note that the relative intensity of the band greatly relied on the calcination temperature. Upon heating from 550 to 850oC, the peak at 469cm-1 became more sharp and symmetrical, which was associated to the higher crystallinity of CeZr composite oxides caused by the sintering of the catalyst. The most intense and narrow band was achieved with IrCeZr850 catalyst, which possessed the most ordered structure due to the rearrangement of atoms. Graham et al. [51] proposed an inverse trend between the particle size of CeO2 and the width of the F2g peak. They pointed out that the F2g peak became broader with the decrease of CeO2 crystallite sizes. Our XRD results was in consistent with this observation regarding the highest crystallite size of IrCeZr850 sample as presented in Table1. This also partly explained the smaller A641/A469 value for IrCeZr850 catalyst implying its less O defects. As reported by Kosacki et al. [52], it was apparent that the smaller crystallite sizes was related with the higher concentration of O vacancies in the catalyst. They considered that the smaller particles might reduce the formation enthalpy of the O defects promoting the presence of more Ce3+ species. Another interesting feature was that no characteristic bands assigned to ZrO2 phase and/or Ir species could be observed in all cases. 3.1.4 Catalyst reducibility (H2-TPR) The reducibility of the as-prepared catalysts were evaluated by H2-TPR tests and the reduction profiles are shown in Fig 4. In terms of IrCeZr550 catalyst, two main peaks at low and high temperature were observed. The low temperature one at 186oC 11


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was ascribed to the combination reduction of IrO2 species and the partial surface CeO2 which located near by the Ir species. It was accepted that after dissociation on metallic species, active H atoms could spill over to support surface and reduce it at relatively lower temperatures, which was termed as H2 spillover phenomenon [43]. The high temperature peak at ca.369oC might be assigned to the reduction of surface CeO2 away from the Ir species. It should be noted that increasing catalyst calcination temperature resulted in a positive shift to higher reduction temperature and a decline of the intensity. This implied that the severe calcination conditions could prevent H2 spillover process to reduce support, probably because of the less Ir-support interaction as evidenced and discussed in the Raman results. Moreover, IrCeZr850 catalyst possessed the higher ordered crystalline structure with less oxygen mobility, which inhibited its reduction property. It was assumed that the consumed hydrogen was mainly attributed to the Ir and/or CeO2 reduction because it was difficult for Zr4+ ions to be reduced in the H2-TPR tests at this temperature region [53]. Nevertheless, the total H2 consumption for each sample was quantified to evaluate its reducibility percentage according to the method described in [44]. This factor was defined as the ratio between the measured H2 consumption and the theoretical value to reduce all of the Ce4+ ions. It was regarded as an additional criterion to compare the metal-support interaction of the investigated catalysts. As shown in Fig 4 (insert), the reduction degree declined with increasing calcination temperature. This was in good agreement with the fact that the higher calcination temperature diminished the concentration of the easily reducible CeO2 species located where the interaction between active Ir species and support occurred [44]. Hence, the larger reduction percentage was attributed to the stronger metal-support interaction. Precisely, the total hydrogen consumption of the Ir-based catalysts decreased in the sequence: IrCeZr550 (1.21mmol/g) ﹥ IrCeZr700 (1.05mmol/g) ﹥ IrCeZr850 (0. 71mmol/g). Interestingly, H2 consumption of the first peak at low temperature for IrCeZr850 sample (0.083mmol/g) was much less than the theoretical value for the reduction of IrO2 (0.198mmol/g). This implied the presence of bulk Ir species which weakly interacted with the CeZr mixed oxide and were reduced at higher temperature. 12


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Based on the results of XRD, Raman and TPR, it might be deduced that increasing calcination temperature could decrease the amount of O vacancies and the metal-support interaction, which were key factors for catalyst behavior. 3.1.5 Basic sites (CO2-TPD) CO2-TPD tests were conducted to evaluate the basicity of the catalysts according to the desorbed temperature of CO2. As shown in Fig 5, three CO2 desorption bands observed at 100-200oC, 200-400oC and 550-700oC (labeled as LT, MT and HT peaks respectively), could be observed. Generally, they were assigned to weak, moderate and strong basic sites [25,54,55]. Herein, the first desorption peak corresponded to the interaction of CO2 with weakly basic hydroxyl groups on CeZr mixed oxides. On the other hand, the MT band was assigned to the bidentate carbonate species, which formed on the moderate basic sites. Another HT peak was attributed to the adsorption of CO2 on the strong basic sites (low-coordination oxygen anions) [56]. Following the above discussion, it could be proposed that IrCeZr550 catalyst exhibited the various types of basic sites, especially possessing more moderate and strong ones. In sharp contrast, almost no surface or bulk basicity were identified for the highly sintered IrCeZr850 sample. More precisely, the total amount of basic sites was quantified (Fig 6) and declined in the following rank: IrCeZr550﹥IrCeZr700﹥IrCeZr850. It revealed that the concentration of basic sites on IrCeZr550 catalyst was almost four times higher than that on IrCeZr850 sample. It could be deduced that increasing calcination temperature significantly eliminated catalyst basicity probably due to the decrease of Ir-support interaction and the diminishment of O defects [55]. Noticeably, catalyst basicity was critical for the chemical capture and dissociation of CO2 during ethanol dry reforming, which resulted in the promising performance. 3.1.6 Morphological analysis (HRTEM, SEM) HRTEM measurements were conducted to evaluate the morphology and structure. As presented in Fig 7, it was apparent that increasing calcination temperature led to an obvious increase of catalyst particle sizes, which was in accordance with XRD analysis. Furthermore, extended IrO2 particles were not observed on IrCeZr550 and 13


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IrCeZr700 samples, which was probably attributed to its high dispersion. On the contrary, in case of IrCeZr850 sample, the isolated cluster of Ir species (3-5nm) were clearly observed at the edge of sintered support. It was similar with ones that calculated from Ir dispersion as shown in Table 1. It was considered that active metals with smaller particle sizes could improve the activity of ethanol reforming [57]. SEM images (Fig 8) revealed an array of micro and nanoparticles in all catalysts. In addition, elemental mapping analysis presented the relatively uniform distribution of active Ir species. However, increasing calcination temperature led to the significant aggregation of particles, which was in good agreement with the XRD results. 3.2 Catalytic activity The effect of calcination temperature on catalyst activity for ethanol dry reforming was determined and the results are summarized in Fig 9. Obviously, rising calcination temperature led to the decline of ethanol conversion. For IrCeZr850 sample, only ca.87% ethanol conversion was achieved even at 700oC and complete ethanol conversion was obtained as high as 750oC. As previously discussed in the section of fresh catalyst characterization, great changes occurred in the catalyst structures with increasing calcination temperature. The serious sintering of active metal Ir and support for IrCeZr850 resulted in less Ir-support interaction, which probably hindered the dissociation capacity of C-C bond in ethanol as discussed in previous literature [24]. Kugai et al. [58] also reported that the electronic transfer (metal-support interaction) from the reduced support to the active metallic sites might control the crucial steps of C-C breaking at the Rh-CeO2 interface for ethanol reforming process. Moreover, less amount of oxygen vacancies in IrCeZr850 catalyst also prevented the surface migration of the intermediate ad-species along the support surface to the active Ir species and their sequential cleavage at Ir-support interface [24]. In terms of IrCeZr850 sample, the smaller Ir dispersion determined by H2-chemisroption method supplied insufficient active metallic sites for ethanol conversion. Towards to the product distribution, H2 concentration progressively decreased with reaction temperature and the molar concentration of CO in the outlet gas followed the reverse trend. This might be caused by the occurrence of reverse water gas shift 14


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reaction (RWGS), which was thermodynamically favorable at high temperature. CO2 + H2 → CO + H2O

Eq 7

However, IrCeZr850 sample presented less CO concentration at above 700oC probably suggesting the inhibition of RWGS process. Analogous tests for the RWGS reaction over Ir-based catalysts have also been carried out at 700oC. The experimental results revealed that CO2 conversion decreased in the following trend: IrCeZr550 (54%)﹥IrCeZr700 (48%) ﹥IrCeZr850 (37%). This might be caused by the decline of O defects with increasing calcination temperature. Liu et al. [59] considered the dissociation of CO2 by O vacancies in ceria based materials. Noticeably, more undesirable byproducts such as acetone was observed on IrCeZr850 catalyst. It was formed via aldolisation processes (Eq 8) and only disappeared at as high as 750oC. 2C2H6O → C3H6O + CO + 3H2

Eq 8

More importantly, there was still much methane formation (18.5mol%) for IrCeZr850 sample at 700oC, which reduced syngas yield. This implied its relatively less capacity for methane dry reforming (Eq 9). CH4 + CO2 → 2CO + 2H2

Eq 9

It was assumed that both the smaller amount of O defects and less Ir-support interaction after the severe calcination conditions hindered the adsorption and/or activation of methane and CO2. The above results unambiguously pointed out the important role of calcination temperature for catalyst behavior. Moreover, a non-catalytic gas-phase CO2 reforming of ethanol was also studied at 600oC in absence of catalyst and selecting quartz as inserted solid. As expected, ethanol conversion was only reached as low as 36%. On the contrary, the formation of much acetone (26mol%) and acetaldehyde (13mol%) revealed the difficulty of C-C bond dissociation for ethanol molecule under gas-phase reaction. Fatsikostas et al. [60] also proposed that ethanol conversion only started from 600oC and ethanol dehydrogenation to acetaldehyde (Eq 10) was the prevailing route. C2H6O → C2H4O + H2

Eq 10

Hence, the aforementioned results clearly elucidated that the surface reactions 15


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which occurred on catalyst were remarkably faster and more selective than those of the gas phase. Drif et al. [8] reported that ethanol dry reforming could not occur at 800oC without catalyst. The apparent turnover frequency (TOF) was estimated in order to compare intrinsic activity of Ir species under various calcination temperatures. It was calculated from the initial ethanol conversion and the concentration of available surface Ir atoms measured from the H2-chemisorption. Typically, the reactions were conducted at lower temperature, and higher GHSV to keep the reactions under the real kinetic conditions in order to eliminate the effect of internal and external diffusion and/or the thermodynamic influence. It was evident that TOF values of Ir based catalysts (Table 2) decreased in the order: IrCeZr550 > IrCeZr700 > IrCeZr850. As pointed out in our previous literature [24], ethanol conversion mainly occurred at the Ir-support interface where 1) Ir species corresponded to the dissociation of the C-C bond in ethanol and 2) the O vacancies in CeO2-ZrO2 support could efficiently capture CO2 and form active O species to oxidize carbonaceous intermediate fragments in order to fulfill the dynamic removal of coke. Therefore, the strong Ir-support interaction and the adequate interfacial sites between active metal and support as well as the larger amount of O defects in IrCeZr550 sample could accelerate these cooperative reaction steps and improve ethanol conversion. 3.3 Stability test The development of stable catalysts is regarded as one of the most important issues in syngas production from ethanol dry reforming, particularly for stoichiometric feed compositions where no excess CO2 was accessible to remove coke deposits that were detrimental to catalyst stability. Herein, the effect of time on stream on catalytic behavior was investigated over a period of 90h. As illustrated in Fig 10, catalyst stability seemed to be a function of calcination temperature. For IrCeZr550 catalyst, complete ethanol conversion was observed over the whole testing period, which was probably because of the stronger Ir-support interaction and/or more amounts of O defects as revealed by TPR and Raman analysis. On the contrary, a significant deactivation over IrCeZr850 catalyst occurred during the stability test. Ethanol 16


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conversion finally dropped to ca.73% and 62%, respectively. In the meantime, a remarkable increase of methane concentration over IrCeZr850 sample implied that methane dry reforming reaction became unfavorable. Predictably, the drastic sintering of Ir and supports greatly declined the concentration of interface Ir/support sites, which prevented the C-C dissociation. Another important property was the formation of ethylene on IrCeZr850 catalyst (not shown). It was a typical product of ethanol dehydration (Eq 11), that was favorably formed on acid sites. C2H6O → C2H4 + H2O

Eq 11

Hence, this indicated the less basicity of the sintered IrCeZr850 as revealed by CO2-TPD measurement. With respect to the syngas composition, no obvious change for the molar ratio of H2/CO (still kept at ca.1.0) was noticed for IrCeZr500 catalyst. However, H2/CO ratio increased from 1.16 to 1.32 for IrCeZr850 sample implying the unfavorable occurrence of RWGS reaction because the deactivation of catalyst prevented the adsorption/dissociation of CO2. The stability of the promising IrCeZr550 system was also tested under conditions where ethanol conversion was only partial. At 650 °C (Fig11), ethanol conversion was kept constant along the 90h on stream. Meanwhile, the outlet gas compositions remained very stable. The molar concentrations of H2, CO, CH4 and C3H6O in the outlet product gas were ca.42%, 37%, 18% and 3% respectively, without any acetaldehyde and ethylene formation. Therefore it was quite likely that the IrCeZr550 catalyst displayed an excellent stability for ethanol dry reforming, even at partial ethanol conversion and with stoichiometric feed compositions. 3.4 Characterization of used catalysts Catalyst deactivation during ethanol reforming process has been normally assigned to coke encapsulation as well as active metal sintering [61]. Hence, several characterization tests such as XRD, HRTEM and Raman were carried out to determine the structure change and/or carbonaceous deposits of the used catalysts. XRD: XRD patterns of the aged IrCeZr catalysts are presented in Fig 2b. In the case of IrCeZr550 and IrCeZr700 samples, no diffraction reflexes corresponding to Ir species were observed as before testing. This provided evidence that no serious 17


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aggregation of Ir particles occurred during the aging tests. On the contrary, the crystallite size of support increased to ca.8.3 and 13.2nm, respectively, relying on the catalyst calcination temperature. However, the observed band at ca.40.6o for IrCeZr850 sample was assigned to IrO2 species (JCPDS 15-0870) implying the sintering of active metal Ir [62]. Moreover, the crystallite size of support drastically increased to 19.8nm. The slight agglomeration behavior for IrCeZr550 sample might be attributed to its higher Ir-support interaction, which prevented the sintering of catalyst particles. For IrCeZr850 catalyst, the weaker Ir-support interaction after severe calcination treatment probably corresponded to the remarkable active metal sintering. This result evidenced that the calcination temperature played an important role in catalyst thermal stability and metal-support interaction and affected catalyst performance. HRTEM: The morphology change of the used catalysts was also evaluated by HRTEM characterization and the results were shown in Fig 12. The shape of catalyst particles shifted from near spherical particles in the fresh sample to polygonal crystallites after the stability tests. In all cases, the increase of particle sizes occurred. Interestingly, the aged IrCeZr550 catalyst appeared to be less agglomerated than others, in line with the above XRD analysis. As for Ir species, they could be observed on the aged samples because the support particles were sintered enough to distinguish Ir phase from support. The observed Ir particles in IrCeZr550 sample showed a mean size in the ca.2nm range, exactly close to that for the fresh catalyst. This implied that the existence of a strong Ir-support interaction inhibited the sintering of Ir particles during the reaction. It could therefore elucidated that the active Ir species still maintained a high dispersion on the catalyst surface, which kept its stable performance. In addition, no evidence of obvious coke formation was achieved because its adequate O vacancies might supply active O species by CO2 dissociation to dynamically remove carbon deposits. In contrast, for the used IrCeZr700 and IrCeZr850 catalysts, the appearance of Ir phase within sizes of 5-10nm indicated the serious sintering of active Ir species, which might partly correspond to the noted deactivation during the stability tests. On 18


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the other hand, a thin layer of encapsulating coke was formed around the surface of the aged IrCeZr850 sample, which might cover the active sites and lead to quick catalyst deactivation. Predictably, the less density of O defects for IrCeZr850 catalyst inhibited the dissociation of CO2 and the removal of coke. It was generally recognized that oxygen migration was accelerated by the presence of microdomains in the CeZr composite oxide because the defects (Frenkel types) could form preferential routes for O2- migration [45]. On the contrary, these microdomains were no more interconnected if catalyst was calcined at higher calcination temperature. This prevented the O mobility and led to coke accumulation on the catalyst surface. Raman: The types of coke were further estimated by Raman characterization considering that it was a powerful technique for determining the structure of carbonaceous materials. As presented in Fig 13, two obvious peaks centered at ca.1340 (D band) and 1582cm-1 (G band) were observed in all cases, which corresponded to the vibrations of carbon atoms in the amorphous coke and the in-plane carbon–carbon stretching vibrations (E2g) in the ordered graphite carbon, respectively [63-65]. Additionally, the intensity of the IrCeZr850 catalyst was the highest indicating the largest coke formation. Generally, the degree of graphitization of the carbon deposits might be evaluated by the area ratio (IG/ID); a higher degree of graphitization was associated with a higher IG/ID ratio. The ID/IG intensity ratio calculated by the curve fitting of Raman spectra declined in the following order: IrCeZr550 (2.16)﹥IrCeZr700 (1.67)﹥IrCeZr850 (1.52). Moreover, the total amounts of coke were quantified by TPO experiments for each aged catalyst and resulted in the sequence: IrCeZr550 (0.13mg C gcat − 1h − 1) < IrCeZr700 (0.21mg C gcat − 1h − 1) < IrCeZr850 (0.34mg C gcat 1h 1). Hence, it could be deduced from the aforementioned −

−

results that IrCeZr sample calcined at less temperature might prevent the coke formation and decrease its graphitization degree. During the 90h on-stream aging tests, only IrCeZr550 catalyst presented satisfactory stability under stoichiometric feed composition. Both the efficient removal of carbonaceous deposits from Ir phase because of an optimized Ir-support interface and the resistant sintering of Ir species due to a strong Ir-support interaction 19


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might explain its stable behavior. For IrCeZr850 sample, a weaker Ir-support interaction would diminish the efficiency of coke remove, which led to deactivated carbon accumulation as evidenced and discussed by HRTEM and Raman. This result was also related to less concentration of O vacancies after deep sintering, which hindered the adsorption/dissociation of CO2 to supply active O atoms. Moreover, the sintering of Ir species reduced the amount of accessible active sites. As deduced from the above discussion, it was concluded that calcination temperature was a key point for catalyst performance, controlling critical features including active metal dispersion, metal-support interaction and reducibility of the active species. 4. Conclusions In this work, the effect of calcination temperature on CO2 reforming of ethanol was investigated for IrCeZr catalysts. The results revealed that catalyst activity, stability as well as ethanol TOF relied to a great extent on calcination temperature. Typically, IrCeZr550 catalyst exhibited a promising performance with high activity and stability. Increasing calcination temperature to 850oC caused a remarkable decline of Ir-support interaction, Ir dispersion, catalyst reducibility and the dissociation capacity of C-C bond, which resulted in less ethanol conversion and more byproduct formation such as methane and acetone. In addition, a continuous deactivation was noted all along the testing period for IrCeZr850 catalyst. The characterization results of the used catalysts elucidated that the quick deactivation for IrCeZr850 sample was attributed to the partial blockage of active sites by coke, thereby hindering the adsorption of ethanol and/or CO2 reactants. The serious sintering of active Ir species during the reaction also corresponded to the observed deactivation. To sum up, a clear structure-activity relationship was established for the reference catalysts. Acknowledgements The authors sincerely acknowledge the financial support from Natural Science Foundation of Liaoning Province (Grant number 20170540073), Open project from State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University (Grant number 2017-17), General project from Department of education of 20


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Liaoning Province (2017J026) and Open project from State Key Laboratory Breeding Base of Coal Science and Technology Co-founded by Shanxi Province and the Ministry of Science and Technology, Taiyuan University of Technology (Grant number mkx201704).

21


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[52] Kosacki, I.; Suzuki, T.; Anderson, H. U.; Colomban, P. Raman scattering and lattice defects in nanocrystalline CeO2 thin films. Solid. State. Ion. 2002, 149, 99-105. [53] Dong, X. F.; Zou, H. B.; Lin, W. M. Effect of preparation conditions of CuO-CeO2-ZrO2 catalyst on CO removal from hydrogen-rich gas. Int. J. Hydrogen Energy. 2006, 31, 2337-2344. [54] Wurzler, G. T.; Rabelo-Neto, R. C.; Mattos, L. V.; Fraga, M.; Noronha, F. B. Steam reforming of ethanol for hydrogen production over MgO-supported Ni-based catalysts. Appl. Catal. A 2016, 518, 115-128. [55] Podila, S.; Driss, H.; Zaman, S. F.; Alhamed, Y. A.; Alzahrani, A. A.; Daous, M. A.; Petrov, L. A. Hydrogen generation by ammonia decomposition using Co/MgO-La2O3 catalyst: Influence of support calcination atmosphere. J. Mol. Catal. A 2016, 414, 130-139. [56] Sato, S.; Takahashi, R.; Kobune, M.; Gotoh, H. Basic properties of rare earth oxides. Appl. Catal. A. 2009, 356, 57-63. [57] Zhao, X. X.; Lu, G. X. Modulating and controlling active species dispersion over Ni-Co bimetallic catalysts for enhancement of hydrogen production of ethanol steam reforming. Int. J. Hydrogen Energy. 2016, 41, 3349-3362. [58] Kugai, J.; Subramani, V.; Song, C.; Engelhard, M. H.; Chin, Y. H. Effects of nanocrystalline CeO2 supports on the properties and performance of Ni-Rh bimetallic catalyst for oxidative steam reforming of ethanol. J. Catal. 2006, 238, 430-440. [59] Liu, Y. J.; Li, Z. F.; Xu, H. B.; Han, Y. Y. Reverse water-gas shift reaction over ceria nanocube synthesized by hydrothermal method. Catal. Commun. 2016, 76, 1-6. [60] Fatsikostas, A. N.; Verykios, X. E. Reaction network of steam reforming of ethanol over Ni-based catalysts. J. Catal. 2004, 225, 439-452. [61] Mattos, L. V.; Jacobs, G.; Davis, B. H.; Noronha, F. B. Production of hydrogen from ethanol: Review of reaction mechanism and catalyst deactivation. Chem. Rev. 2012, 112, 4094-4123. [62] Kim, H. W.; Shim, S. H.; Myung, J. H.; Lee, C. Annealing effects on the structural properties of IrO2 thin films. Vacuum. 2008, 82, 1400-1403. [63] Cai W. J.; Homs, N.; Ramirez de la Piscina, P. Efficient hydrogen production 27


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from bio-butanol oxidative steam reforming over bimetallic Co-Ir/ZnO catalysts. Green. Chem. 2012, 14, 1035-1043. [64] Galetti, A. E.; Gomez, M. F.; Arrua, L. A.; Abello, M. C. Ethanol steam reforming over Ni/ZnAl2O4-CeO2. Influence of calcination atmosphere and nature of catalytic precursor. Appl. Catal. A. 2011, 408, 78-86. [65] Mondal, T.; Pant, K. K.; Dalai, A. K. Oxidative and non-oxidative steam reforming of crude bio-ethanol for hydrogen production over Rh promoted Ni/CeO2-ZrO2 catalyst. Appl. Catal. A. 2015, 499, 19-31. 

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Figure captions Fig 1 N2 adsorption-desorption isotherms of the catalysts. Fig 2 XRD patterns of the IrCeZr catalysts (a) fresh; (b) used. Fig 3 Raman spectra of the IrCeZr catalysts. Fig 4 H2-TPR profiles of the catalysts. Fig 5 Comparison of CO2-TPD profiles over the investigated catalysts. Fig 6 Quantified basic sites of the IrCeZr catalysts. Fig 7 HRTEM images of the catalysts: (A and B) IrCeZr550; (C and D) IrCeZr700; (E and F) IrCeZr850. Fig 8 SEM and elemental mapping images of the catalysts: (A and B) IrCeZr550; (C and D) IrCeZr700; (E and F) IrCeZr850. Fig 9 Effect of reaction temperature on ethanol conversion and product distribution for ethanol dry reforming over IrCeZr catalysts. Reaction conditions: Ethanol/CO2 =1/1 (molar ratio), GHSV=10000mL/gcat·h. Figure 10 Changes in outlet gas composition for ethanol dry reforming over IrCeZr catalysts. Reaction conditions:700oC, Ethanol/CO2=1/1 (molar ratio), GHSV=10000 mL/gcat·h. Figure 11 Changes in outlet gas composition for ethanol dry reforming over IrCeZr550 catalyst. Reaction conditions: 650oC, Ethanol/CO2=1/1 (molar ratio), GHSV=10000 mL/gcat·h. Fig 12 HRTEM images of the used catalysts: (A and B) IrCeZr550; (C and D) IrCeZr700; (E and F) IrCeZr850. Fig 13 Raman spectra of the used IrCeZr catalysts.

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Fig 1 N2 adsorption-desorption isotherms of the catalysts.

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Fig 2 XRD patterns of the IrCeZr catalysts (a) fresh; (b) used.

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Fig 3 Raman spectra of the IrCeZr catalysts.

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Fig 4 H2-TPR profiles of the catalysts.

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Fig 5 Comparison of CO2-TPD profiles over the investigated catalysts.

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Fig 6 Quantified basic sites of the IrCeZr catalysts.

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Fig 7 HRTEM images of the catalysts: (A and B) IrCeZr550; (C and D) IrCeZr700; (E and F) IrCeZr850.

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Fig 8 SEM and elemental mapping images of the catalysts: (A and B) IrCeZr550; (C and D) IrCeZr700; (E and F) IrCeZr850.

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Fig 9 Effect of reaction temperature on ethanol conversion and product distribution for ethanol dry reforming over IrCeZr catalysts. Reaction conditions: Ethanol/CO2 =1/1 (molar ratio), GHSV=10000mL/gcat·h. 38


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Figure 10 Changes in outlet gas composition for ethanol dry reforming

over

IrCeZr catalysts. Reaction conditions:700oC, Ethanol/CO2=1/1 (molar ratio), GHSV=10000 mL/gcat·h.

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Figure 11 Changes in outlet gas composition for ethanol dry reforming over IrCeZr550 catalyst. Reaction conditions: 650oC, Ethanol/CO2=1/1 (molar ratio), GHSV=10000 mL/gcat·h.

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Fig 12 HRTEM images of the used catalysts: (A and B) IrCeZr550; (C and D) IrCeZr700; (E and F) IrCeZr850.

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Fig 13 Raman spectra of the used IrCeZr catalysts.

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Table 1 Physicochemical properties of the IrCeZr catalysts. Catalyst

IrCeZr550 IrCeZr700 IrCeZr850

XIra (wt%) 1.91 1.84 1.87

DIrb

dIrc

(%)

58 39 17

(nm) 1.6 2.3 5.3

SBETd (m2/g)

Sexternale Vpored Dpored dCeO2f (m2/g) (cm3/g) (nm) fresh (nm) 172 0.134 4.14 5.1 135 0.123 6.28 6.5 73 0.102 11.6 12

130 78 35

a

Ir content from ICP analysis. Ir dispersion obtained from H2 chemisorption c Calculated from DIr d Measured by N2 adsorption-desorption. e Calculated from CeO2 particle size f Calculated from the XRD patterns (Debye-Scherrer equation). b

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dCeO2f used

(nm) 8.3 13.2 19.8

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Table 2 Intrinsic TOF values of the IrCeZr catalysts Catalyst Ethanol conversion (%) TOF (s-1) IrCeZr550 19.5 0.378 IrCeZr700 7.1 0.212 IrCeZr850 2.3 0.146 T: 400oC; GHSV=300000mLgcat-1h-1

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Graphic Abstract

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Highlights ► Ethanol dry reforming on IrCeZr catalyst calcined at various temperature was studied. ►Catalyst calcination temperature greatly affected the physicochemical properties. ► IrCeZr550 presented the best catalytic activity and stability.

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Syngas Production from Carbon Dioxide Reforming of Ethanol over Ir

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