Metal Oxide-Based Catalysts for the Autothermal Reforming of

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Metal oxide based catalysts for the autothermal reforming of glycerol Faezeh Sabri, Raphael O. Idem, and Hussameldin Ibrahim Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04582 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 28, 2018

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Industrial & Engineering Chemistry Research

Metal oxide based catalysts for the autothermal reforming of glycerol Faezeh Sabri, Raphael Idem, Hussameldin Ibrahim* Clean Energy Technologies Research Institute, Process Systems Engineering, Faculty of Engineering and Applied Science, University of Regina, 3737 Wascana Parkway, Regina, SK, S4S 0A2, Canada. *Corresponding Author. Tel.: 1.306.337.3347; fax: 1.306.585.4855. E-mail address: [email protected]

Abstract In this study hydrogen production from the auto-thermal reforming (ATR) of glycerol was investigated in a packed bed tubular reactor (PBTR) using nickel-based catalysts with theoretical composition of 5% Ni/Ce0.5Zr0.33M0.16O2-δ, where M is the promoter element selected from Mg, Ca, Y, La, or Gd. The structural, textural, and physicochemical characteristics of the catalysts were investigated using various characterization techniques. The catalytic activity was evaluated in a temperature range from 450°C to 700°C, steam-to-glycerol (S/G) ratio of 6, 9, and 12, and oxygen-to-glycerol (O/G) ratio of 0.2, 0.5, and 0.8 at atmospheric pressure. Among all the catalyst formulations prepared in the current study, the 5Ni/CeZrGd exhibited the best catalytic performance and stability compared to the other promoter elements. For 5Ni/CeZrGd catalysts, it was found that, until 600 °C, conversion increased rapidly with the increase in temperature to reach 82 mol.% glycerol conversion and more than 70 mol.% H2 selectivity, which can be optimal operation conditions for industrial applications. Furthermore, the increase of O/G ratio showed similar trends. Although adding more S/G in the feed did not show any noticeable increase in glycerol conversion, it resulted in lower hydrogen concentration in the reformate product due to the dilution effect of steam.

Keywords Glycerol, Autothermal reforming, Nickel-based catalysts, Hydrogen production

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1. Introduction The continuously increasing energy consumption, has led to an enormous demand for fossil fuel worldwide as the main energy source. However, flue gases of most fossil fuels cause pollution directly affecting not only human health but also the environment and leading to many severe effects such as global warming. Owing to these serious disadvantages, it is necessary to discover other alternative energy resources that are abundant and environmentally friendly. Hydrogen is considered as a promising candidate to replace the presently used energy resources.1 It is one of the promising alternatives for energy production due to its ability to dramatically decrease oil consumption, greenhouse gas (GHG) emissions, and tail pipe pollution. Therefore, hydrogen as a promising clean energy resource is emerging for widespread future use. Hydrogen can be produced from non-renewable fossil sources such as natural gas, naphtha, and coal, as well as renewable sources such as biomass, bio-diesel, bio-oil, and biogas.2 Recent years have witnessed a fast growth in biodiesel production as a sustainable alternative to fossil fuel based energy sources, which is projected to make up for 20% of future energy.3 Crude glycerol is generated as a byproduct in biodiesel production plants; therefore the deployment of glycerol to produce valuable products can significantly enhance the economic feasibility of the biodiesel industry. Moreover, the CO2 discharged in utilizing glycerol is absorbed in the growth of the biomasses, and there is no net CO2 release into the atmosphere. In this context, glycerol can be effectively applied to produce hydrogen, which holds a series of advantages as a fuel. Glycerol can be converted into hydrogen through different processes such as auto-thermal reforming (ATR), partial oxidation, dry reforming, gasification, aqueous-phase reforming, and supercritical water reforming.2,4 Most studies have focused on the steam reforming process for H2 production by employing a variety of noble-based metal catalysts.5 Due to the exothermic nature of the oxidation reactions, partial oxidation was suggested in order to overcome the high-energy intensity of the steam reforming process, hence leading to more compact reformers. The autothermal processes (ATR) combines the advantages of steam reforming and partial oxidation processes by feeding fuel, oxygen, and steam together into the reactor in the presence of a catalyst. Glycerol can be reformed to hydrogen under ATR conditions to achieve high H2 selectivity by adjusting the feed/air and feed/steam ratios, as well as the catalyst used.6 The comparison between conventional steam reforming and ATR of glycerol process over ɣ-Al2O3 supported Ni/Pd/Cu/K catalysts was performed by Swami and coworkers.7 Their study indicated


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that the ATR process can produce higher amounts of H2 under the same conditions compared to steam reforming routes. However, higher temperatures favor H2 production in both the ATR and steam reforming processes. In summary, the study demonstrated that reforming of glycerol by the ATR process produced high selectivity toward H2 while all minor products were less than 2% under optimal conditions. Hydrogen generation through the ATR of glycerol was also investigated by Dauenhauer et al. over Rh-CeO2/Al2O3 prepared using the incipient wetness technique.8 The Rh-CeO2/Al2O3 catalysts exhibited higher H2 selectivity and it was concluded that the addition of steam suppressed CO formation. Therefore, the role of the catalyst development and the optimization of operating conditions in the ATR of glycerol can significantly influence H2 selectivity, glycerol conversion, and coke formation.6 Recently, the CexZr1-xO2 mixture has been considered as a good support composition for Ni-based catalysts for reforming processes.1 The use of ZrO2 and CeO2 supports with a transition metal catalyst can considerably improve the oxygen storage capacity, redox properties, thermal stability, and catalytic activity in reforming processes.1 CexZr1-xO2 can be prepared by different techniques such as urea hydrolysis,9 co-precipitation,10 sol–gel techniques,11 and surfactant-assisted methods.12 Among the above noted synthesis methods, the surfactant-assisted approach is considered the most attractive technique because of the soft template effect, catalyst reproducibility, and the ability to fine-tune the structure.13 It was concluded that the surfactant assisted route technique can be applied to prepare solid solutions with high specific surface area and thermal stability, which makes the solid solutions, good candidates to use at high temperatures.12,14 However, the presence of steam in the reaction media can deactivate the CexZr1−xO2 system due to the inherent hydrophilic nature of these supports.10,11,15-21 This issue can be overcome by the incorporation of a promoter element “M” into the Ce−Zr lattice to adjust the hydrophilicity and improve the support hydrothermal stability. In the current work, Ni-based catalysts supported on Ce0.5Zr0.33M0.17O2‑δ (where, M = Mg, Ca, Y, La, and Gd) prepared using the surfactant-assisted method were investigated for hydrogen production using glycerol ATR process. The role of the metal promoters, on the physicochemical properties of the catalyst and the catalytic activity was investigated. Moreover, the reaction temperature, steam/glycerol (S/G) ratio and oxygen /glycerol (O/G) ratio were varied in order to optimize the operating conditions and achieve the highest conversion and H2 selectivity.



2. Materials and methods 2.1. Support preparation Cetyl trimethyl ammonium bromide (CTAB) surfactant was employed in current work to prepare a variety of ceria-based ternary oxide supports using surfactant-assisted route under basic conditions as described elsewhere.22 The nominal compositions of each support were Ce0.5Zr0.33M0.17O2-δ (CZM), where M is Ca, Gd, La, Mg, or Y. Nitrate precursors of the different metal ions (Ce, Zr, and M) were dissolved separately in deionized (DI) water and then were mixed together. Another solution of a predetermined amount of surfactant (CTAB) in DI water was prepared by heating at 60 °C while stirring. Then these two solutions were mixed together to obtain a resultant mixture with S/M ratio of 0.5. Aqueous ammonia (25 vol. %) was slowly added to the aforementioned mixture while stirring at 60 °C until to maintain a basic condition during the precipitation process (pH of about 12). A yellow−brown colloidal slurry was formed which was then heated using an air-circulated oven at 90 °C for 5 days. The resultant precipitate was cooled, washed repeatedly with warm DI water, and then dried at 120°C for 12 hours. Finally, the obtained CZM supports were calcined at 650°C for 3 hours in an air environment.

2.2. Ni impregnation into the support A standard wet impregnation technique was used to load 5 wt.% Ni into the prepared CZM supports. Typically for Ni impregnation, a calculated amount of the support was immersed in a predetermined volume of 0.1 M Ni(NO3)2 solution in a round bottom flask. Then, the mixture was heated in a silicon oil bath at 80°C under constant stirring, in order to remove the excess water. The dried powders were calcined at 650°C in an air environment for 3 hours. Next the powders were ground to a fine grain size and pellets were made using a hydraulic press to obtain 0.80 mm particle size catalysts.

2.3. Catalyst characterizations The pore size distribution and the specific BET surface area were measured by N2 physisorption experiment at 77 K using Micromeritics ASAP 2010 apparatus. Before starting the analysis, all samples were degassed for 4 hours at 180°C under vacuum. Pore size distribution and average pore volume were measured using the desorption branch of the N2-isotherm.


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The metal dispersion and metallic surface were measured through hydrogen chemisorption experiment at 35°C using Micromeritics ASAP 2010. The catalyst samples were dried at 120°C prior to analysis and then reduced in situ in flowing H2 gas at 700oC for 2 hours, followed by evacuation at 700°C for 1 hour before cooling down to 35°C. During the chemisorption experiment, initially, the sample is dried followed by reduction with hydrogen, evacuated, then cooled to the analysis temperature (35°C), and finally evacuated before performing actual measurements. The metallic surface area (SNi) was estimated based on the number of hydrogen molecules adsorbed in the monolayer per gram of catalyst. The Ni dispersion (D %) was then estimated as the percentage of one Ni atom with respect to the total Ni atoms in the catalysts.13,23 A coupled plasma-mass spectroscopy (ICP-MS) technique was used to measure the total loaded Ni over each prepared support. The temperature programmed desorption (H2-TPR) measurements were performed using Quantachrome ChemBET 3000 equipped with a thermal conductivity detector (TCD). For each measurement, prior to analysis, 50 mg sample was degassed at 180°C in an inert atmosphere for 2 hours. The reducibility of the catalysts prepared were evaluated in a temperature range from ambient to 1050°C at a heating rate of 15°C/min using 5%H2/balance N2 as the reactive gas (flow rate = 45 mL/min). Powder X-ray diffraction (XRD) patterns of both the supports and catalysts were recorded on a Bruker Discover diffractometer using Ni-filtered Cu Kα (0.154056 nm) as the radiation source. The intensity data were collected over a 2θ range of 10 to 90° with a step size of 0.02° using a counting time of 1 s per point. UV-Vis Diffuse reflectance analysis were conducted using Shimadzu UV-probe (UV2600/2700) instrument to obtain information about the electronic structure of the support and catalyst, which was done based on the reflection of light in the ultraviolet and visible regions (200-800 nm). The amount of carbon deposited on the catalyst surface during reforming processes as a function of temperature was measured by thermogravimetric analysis (TGA). All TGA profiles presented in this study were obtained on a Shimadzu TGA-50 equipped with FC-60A flowmeter with a



precision of 0.1 mg. The samples were heated from ambient temperature to 1050°C under 4.87%O2/bal. N2 at a flow rate of 20 mL/min and a heating rate of 15°C/min.

2.4. Catalytic Activity Measurement Catalytic testing was carried out in a packed bed tubular reactor (PBTR) (0.5 in. inner diameter) made of Inconel 625.22 The reactor was placed vertically inside a programmable tubular furnace that was heated electrically and a sliding thermocouple (type K with accuracy of 0.1 oC) was used to measure the catalyst bed temperature. In each run, 250 mg of the catalyst particles were mixed with 7.6 grams of 0.78 mm sized α-Alumina beads to have a catalyst bed height of 4.5 cm. Prior to each experiment, the catalyst was reduced at 700°C for 2 hours in a reducing gas mixture of 5%H2 balance N2. The catalyst pre-treatment involved the partial reduction of nickel oxide (NiO) to metallic nickel species (Ni). The feed was composed of glycerol and water to maintain S/G molar ratio of 6, 9, and 12, while air supply was adjusted to provide O/G molar ratios of 0.2, 0.5, and 0.8. The activity evaluation tests were performed at five different temperatures, namely 700°C, 600°C, 550°C, 500°C, and 450°C. In order to approach plug flow conditions and minimize back mixing and channeling, the ratio of catalyst bed length to catalyst particle size (L/Dp) was kept at 56, and the ratio of the reactor inner diameter to particle size (D/Dp) was kept at 16.24 The product stream from the reactor was cooled down and the gas/liquid products were separated in knockout trap. The gaseous products coming from the knockout trap were thus analyzed using an online gas chromatograph (GC) (Agilent 6890 N) equipped with TCD. In order to identify the role of the catalyst in the ATR process, non-catalytic (thermal) experiments were carried out using the same reactor setup without the use of the catalyst. In order to maintain the same residence time for the thermal reactions as that of the catalytic reactions, a packed bed height of 4.5 cm was maintained using 0.78 mm sized α-Alumina as inert materials.

2.5. Catalytic activity criteria The stoichiometric balance reactions used in this work for the ATR of glycerol are shown in the following equations22:


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C3H8O3 + 6H2O+ 0.5 O2 ↔ 2CO2+ CO+ 5H2+ 5H2O

(1)

∆Hr (25oC) = -77.8KJ/mol C3H8O3 + 6H2O+ 0.8 O2 ↔ 2CO2+ CO+ 4.4H2+5.6H2O

(2)

∆Hr (25°C) = -222.8KJ/mol C3H8O3 + 6H2O+ 0.2 O2↔2CO2+ CO+ 4.6H2+ 5.4H2O

(3)

∆Hr (25°C) = -174.52KJ/mol C3H8O3 + 9H2O+ 0.2 O2 ↔2CO2+ CO+ 5.6H2+7.4H2O

(4)

∆Hr (25°C) = +67.28.8KJ/mol C3H8O3 + 9H2O+ 0.5 O2 ↔ 2CO2+ CO+ 5H2+ 8H2O

(5)

∆Hr (25°C) = -77.8KJ/mol C3H8O3 + 9H2O+ 0.8 O2 ↔ 2CO2+ CO+ 4.4H2+ 8.6 H2O

(6)

∆Hr (25°C) = -222.8KJ/mol C3H8O3 +12H2O+ 0.2 O2 ↔ 2CO2+ CO+ 5.6H2+10.4H2O

(7)

∆Hr (25°C) = +67.28KJ/mol C3H8O3 +12H2O+ 0.5 O2↔2CO2+ CO+ 5H2+ 11H2O

(8)

∆Hr (25°C) = -77.8KJ/mol C3H8O3 +12H2O+ 0.8 O2 ↔ 2CO2+ CO+ 4.4H2+11.6H2O

(9)

∆Hr (25°C) = -228.8KJ/mol The hydrogen yield is defined as follows: Hydrogen yield =

( ) ( )

× 100 [%mole]

(10)

Where the theoretical molar flow rate of hydrogen represents the moles of hydrogen expected from the feed at absolute glycerol conversion (100%) with no side reaction(s). Moreover the conversion is defined as follows: Conversion =

( ! ") ( " )#

× 100 [%mole]

(11)

Here, the Cout components found in the gas products are CO2, CO, and C2H6. However, the hydrogen selectivity is defined as follows:



( )

Hydrogen selectivity = $" % "! $ )

×100 [%mole]

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(12)

3. Results and Discussion 3.1. Catalyst Characteristics The specific surface area, specific pore volume, and average pore diameters of all the catalysts and supports employed in the present study are listed in Table 1. All support samples have exhibited reasonably high specific surface areas (> 116 m2/g). The average pore diameter was in the range of 60-95 Å, and the total pore volume was in the range from 0.23 to 0.53 cm3/g. The results clearly indicate that the use of surfactants for catalyst preparation significantly improved the support porosity in agreement with previous findings.21 The incorporation of the cetyltrimethylammonium-cation [(C16H33)N+(CH3)3] in the basic synthesis media (pH > 11) has reduced the surface tension of water molecules that are trapped in the hydrous support pores and essentially reduced the shrinkage and pore collapse and stabilized the framework, thereby imparting a high specific surface area.13,25 The overall porosity of the support, has considerably decreased after the impregnation of 5 wt. % Ni into the ternary oxide support, which is a common phenomenon observed in the case of supported metal oxide catalysts. This observed decrease is generally because of the penetration of the dispersed nickel oxide into the pores of the support. It could be observed that, all the ternary oxide catalysts prepared exhibited larger pore volumes and average pore sizes compared to the binary oxide catalyst (5Ni/CeZr). The presence of wider pore sizes in the catalyst provide better access of the reactants to the active sites within the catalyst surface and reduces the mass transfer limitations.26

Table 1

The results of metallic surface area and metal dispersion of the active nickel component in the final prepared monometallic catalysts measured by H2 chemisorption are summarized in Table 2. The Ni surface area of the catalyst 5Ni/CeZrCa recorded the highest value while 5Ni/CeZrGd


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shown a value of about 24 m2/g Ni despite the fact that the surface area of the Gd-based catalyst is higher than the Ca-based catalyst. It could be concluded that apart from the support total surface area, the nature of the metal promoter of the support has a great impact on the resulting dispersion percentage and the metallic Ni surface area. This could be explained by the fact that the Ni particles have formed larger agglomerates on the Gd-based support as compared to the Ca-based support. It’s worthy to mention that the catalytic activity of the Ni-supported catalysts is not only affected by the Ni surface area but also impacted by the oxygen storage capacity (OSC) of the support as well as the catalyst reducibility. The ICP-MS analyses results are also given in Table 2. In a volumetric H2 chemisorption measurement, known amounts of hydrogen are dosed and then adsorbed at different partial pressures, generating a chemisorption isotherm. In order to remove weakly adsorbed species, the isothermal measurement is repeated after applying an evacuation step at the analysis temperature. The chemically bonded reactive gas is represented by the difference between the two isotherms and is employed to calculate the active metal surface area. This information can be combined with information on metal loading to estimate the metal dispersion. The relative measurement of chemically bound hydrogen was used to distinguish the catalyst tested in this study. Ni dispersion and Ni surface area are strong functions of the catalyst formulation. The descending order of Ni dispersion was found to be 5Ni/CeZrCa > 5Ni/CeZrY> 5Ni/CeZrLa > 5Ni/CeZrGd > 5Ni/CeZrMg.

Table 2

Figure 1 displays the H2-TPR results of the CeZrGd support as well as the 5Ni/CeZrGd catalyst. As observed in Figure 1, the TPR profile of the pure support (CeZrGd) exhibited broad H2 consumption peak in the temperature range of 600-700 °C and two other peaks in the temperature range of 800-920 °C. These peaks can be referred to the reduction of both surface and bulk oxygen anions 26. On the other hand, the corresponding Ni impregnated (5Ni/CeZrGd) catalyst showed an additional sharp peak in the temperature range 400-500 °C, which could be attributed to the reduction of the active metal NiO to Ni. Previous studies has shown that pristine NiO samples has a sharp reduction peak at about 455 °C, ascribed to the transformation of Ni2+ to Ni0 species.27 During the reduction process, the removal of lattice oxygen can occur with the



help of higher mobility of the surface oxygen ions. Generally, in the low temperature region, the coordinately unsaturated surface capping oxygen ions can be easily removed. On the other hand, bulk oxygen needs to be transported to the surface prior to its reduction. Therefore, the bulk reduction occurs at a higher temperature compared to the surface reduction. After the complete reduction of the surface sites, the bulk reduction process starts to happen and pristine ZrO2 does not reveal any sign of reduction below 1000 °C because of its refractory nature.28

Figure 1

UV-Vis Diffuse reflectance technique was employed to study the structural features, oxygen exchange properties, and redox behavior of CeZrGd and 5Ni/CeZrGd as shown in Figures 2a and Figure 2b respectively. For the case of the support (Figure 2a), strong interactions were observed between the Zr4+ and Ce4+/Ce+3 ion pairs, which increases the efficiency of the redox processes at lower temperatures. ZrO2 addition to the ceria support improves the thermal stability and modifies the surface acid-base properties of exposed Ce4+ and Zr4+. The diffuse reflectance spectroscopy (DRS) spectra of CeZrGd sample shows two absorption peaks at 275 and 229 nm attributed to the ZrO2 in the support. Another peak was seen at 679 nm that is attributed to the low coordination of surface charge transition between Ce3+ to Ce4+ 27. Furthermore, as shown in Figure 2b for 5Ni/CeZrGd catalyst, it was found that the observed asymmetric UV band at 240 nm and a shoulder around 310 nm indicating its origin from O2- to Ni2+ charge transitions. The shoulder is due to O2- to Ce4+/3+/Zr4+ transitions. The weak bands of the octahedrally coordinated Ni2+ were also observed at 725, and 653 nm.27,28 All these findings imply that Ni2+ is well dispersed over the surface of the CeZrGd support in the final catalyst.

Figure 2

The XRD patterns of all the supports and catalysts studied in this work are shown in Figures 3. Both the supports and catalysts exhibited diffraction patterns identical to the pristine ceria;


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therefore, confirming the existence of a single-phase cubic fluorite-type structure. The XRD profile of the pristine NiO is also shown in order to identify the existence or absence of crystalline NiO in the nickel-impregnated catalyst samples (5Ni/CeZrM). Major peaks at 2Ɵ = 37o and 44o corresponding to the crystalline Ni phase were observed on all the catalysts prepared indicating the successful impregnation process and confirming the presence of Ni in the lattice structure of the CeZrM supports. Figure 3

3.2. Catalytic Activity Evaluation 3.2.1. Effect of promoter element on the catalytic activity The catalytic activities of the 5Ni/CeZrM (M = Ca, Gd, La, Mg, Y) catalysts were investigated and compared under identical operating conditions in a PBTR. Initially, the activity was evaluated at 500 oC, and the corresponding results are presented in Figures 4 and 5. As noted from the results, all the catalysts exhibited almost steady activity in terms of glycerol conversion and H2 selectivity. Among the five catalysts tested, 5Ni/CeZrGd showed the highest value of 48% and 58% for glycerol conversion and H2 selectivity respectively. The catalyst with the lowest catalytic activity was 5Ni/CeZrMg with glycerol conversion of 32% and H2 selectivity of 33%. Figure 4 Figure 5

The impact of promoter elements (Ca, Gd, La, Mg, and Y) on the gas product distribution for the ATR of glycerol was also studied at 500°C as shown in Figures 6. It is clear that there were noticeable differences in the reformate gas compositions for different promoter elements. Furthermore, 5Ni/CeZrGd catalyst exhibited the highest H2 yield and the lowest CO concentration, while on the other hand; 5Ni/CeZrMg catalyst has shown the highest CO concentration.



Figure 6 The term Carbon Propensity Factor (CPF) is used here to measure the rate of coke formation on all given catalyst formulations with different promoter elements at 500 °C reaction temperature. The values are presented in Table 3. According to the results, it is noted that carbon propensity factor is related to the type of promoter element employed. The descending order of CPF as a function of the promoter element M is as follows: La>Y>Mg>Gd>Ca. From the results, it can be concluded that Ca and Gd are the most coke tolerant promoter among the other promoter elements employed in the current study. The observed catalytic improvements due to the use of metal promoters could be ascribed to the fact that the phase separation of the Ce1-xZrxO2 lattice especially at high reaction temperatures is reduced due to the presence of the metal promoters in the framework and hence adding a stabilizing effect to the catalyst.28 The superior performance of 5Ni/CeZrGd could be explained by the catalyst reducibility as could be deduced from the H2TPR profiles displayed in Figure 1.

Table 3

3.2.2. Identification of Thermal and Catalytic Effects Figure 7 represents the catalytic performance obtained for the ATR of glycerol feed for the case of non-catalytic thermal operation compared to catalytic ATR using the catalyst 5Ni/CeZrGd. According to Figure 7, it is clearly observed that the use of the catalyst has a significant impact on the glycerol conversion especially at 600 °C where the magnitude of the catalytic effect is at maximum compared to the other temperatures. An average glycerol conversion of 82% at 600 °C was recorded when employing 5Ni/CeZrGd catalyst as compared to 30% glycerol conversion. The improvements in glycerol conversion due to the use of the catalyst were 1.6, 2.8, 2.2, and 1.8 folds at 700 °C, 600 °C, 500 °C, and 450 °C, respectively.

Figure 7


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In order to further demonstrate the significant role played by the catalysts the ATR of glycerol was conducted at different temperatures (450 oC to 700 oC) in the absence of the catalyst. The results obtained are given in Figures 8 and 9. The glycerol conversion increased with increasing temperature due to the endothermic nature of the steam reforming component of the ATR process. However, the H2 selectivity demonstrated close values at different temperatures. It is important to mention, here, that, no pressure build up was noted while a catalyst was employed and the H2 selectivity increased with temperature.

Figure 8 Figure 9

3.2.3. Effect of Reaction Temperature The effect of reaction temperature on the product gas composition for the case of non-catalytic ATR as well as the catalytic ATR over 5Ni/CeZrGd catalyst is presented in Figure 10. As observed from Figure 10 (a), the H2 and CO concentration of the reformate gas increased with an increase in reaction temperature from 450 °C to 700 °C. On the other hand, the concentration of CO2 in the reformate gas has gradually decreased with an increase in the reaction temperature to reach its minimum value at 700°C. Moreover, the concentration of CO in the reformate gas was higher than H2 throughout the temperature range studied. The above variation in the relative composition of CO, CO2, and H2 can be explained by the occurrence of water gas shift (WGS) and reverse water gas shift (RWGS) reactions at equilibria.29 At lower reaction temperatures, CO is converted to CO2 by a WGS reaction (Eq. 13) and hence additional H2 is produced. However, the WGS reaction is exothermic in nature, and with the increase of the reaction temperature, the RWGS reaction occurs, resulting in a gradual increase in CO and a gradual decrease in CO2 concentrations. CO + H2O → CO2 + H2

(13)

The effect of reaction temperature on the reformate gas composition for the catalytic ATR of glycerol over 5Ni/CeZrGd catalyst is shown in Figure 10 (b). As can be observed, the H2



concentration has considerably increased with an increase in reaction temperature, while CO2 concentration has significantly decreased with no noticeable change on the CO concentration. In the ATR reaction, at lower temperatures, CO is converted to CO2 by a water gas shift (WGS) reaction (CO + H2O → CO2 + H2) and additional H2 is produced. However, as WGS is an exothermic reaction, with the increase of reaction temperature, the reverse water gas shift reaction (RWGS) happens, causing a gradual increase in CO and a gradual decrease in CO2. However, the increase in H2 concentration due to the increase in reaction rate with the temperature increase and the decrease in H2 concentration due to the RWGS reaction with the increase in reaction temperature counter-balance each other to keep the H2 concentration almost constant from 550 °C to 700 °C. It is to be noted that in the case of catalytic reaction, the percentage of CO produced at higher temperature is comparatively less varied than that of the corresponding non-catalytic reaction, which implies that the 5Ni/CeZrGd catalyst influences the water gas shift (WGS) reaction kinetics and hence the conversion and product distribution.

Figure 10

The influence of operating temperature on the catalytic performance over catalyst 5Ni/CeZrGd for the ATR of glycerol was investigated in the temperature range from 450 °C to 700 °C, and the corresponding results are presented in Figures 11. The results revealed the significant role of reaction temperature on glycerol conversion mainly due to the endothermic nature of the ATR process and to the fact that glycerol reforming is more favorable at higher reaction temperatures. The performance of the catalyst 5Ni/CeZrGd was found to be stable at all different temperatures applied in the current work, which is corroborated by previous findings.30

Figure 11


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3.2.4. Effect of Calcinations Temperature on Catalytic Activity To investigate the effects of the catalyst calcination temperature for 5Ni/CeZrGd catalyst, three different calcinations temperature namely, 550 oC, 600 oC and 650 oC were employed in the current study as shown in Figure 12. It can be noted that increasing the catalyst calcination temperature from 550 oC to 650 oC only slightly increased the glycerol conversion with little impact on the H2 selectivity. The results thus indicate that calcinations can be performed at 550 o

C instead of higher temperatures of 650 oC in order to reduce the catalyst production costs.

Similar studies has shown a significant increase in glycerol conversion and H2 selectivity upon increasing the calcination temperature as was reported for the case of Ni/Al2O3 catalysts.31 This negligible effect of the calcination temperature could be due to the small range of calcination temperature values studied in this work as compared to the study by Dieuzeide and coworkers.31

Figure 12

3.2.5. A Comparison between the Steam Reforming and ATR of Glycerol For comparison between conventional steam reforming and the ATR of glycerol, the 5Ni/CeZrGd catalyst was tested at 500 °C for 6 hours in the presence and absence of oxygen in the main feed stream to the reactor. As can be seen from Figure 13, that the ATR process exhibited higher glycerol conversion and H2 selectivity as compared to the steam reforming route. Similar findings were reported by Swami and Abraham for the effect of oxygen content on the reforming of glycerol over ɣ-Al2O3 supported Ni/Pd/Cu/K catalyst.7 Based on their study, the amount of H2 generated from the ATR process would be less based on thermodynamics. But, the observed results here demonstrate that reforming of glycerol by the auto-thermal process can produce high selectivity to synthesis of H2 in comparison to the steam reforming process. Although the mole fraction of hydrogen was almost identical for the steam reforming and ATR processes, the hydrogen yield in the case of ATR is still higher as shown in Figure 13. This could be explained by the fact that the addition of oxygen has increased the glycerol conversion and hence the conversion of glycerol to more gaseous products and the breakdown of C1-C4 hydrocarbons.7 However our findings were in contradiction to Rioche and coworkers32 who



observed a reduction in hydrogen yield as a result of the addition of little oxygen to the feed stream to the reactor. This could be attributed to the high oxygen content in the bio-oil feedstock used in their study. Therefore, a careful optimization of the role of oxygen content on the catalytic performance is required as will be discussed in the next section.

Figure 13

3.2.6. Effect of Oxygen-to-glycerol ratio (O/G) The effect of oxygen-to-glycerol ratio (O/G) on the ATR of glycerol over 5Ni/CeZrGd catalyst at different steam-to-glycerol (S/G) ratios (6, 9, and 12) was examined and the corresponding results are given in Figures 14, 15, and 16. It was noted that the conversion of glycerol considerably increased with an increase in O/G ratio, at all the S/G ratios investigated with an exception at a temperature of 600 oC where no noticeable trend was observed. This could be ascribed to the fact that increasing the amount of oxygen favors the partial oxidation reaction at the expense of the steam reforming reaction and hence generates higher energies. It could be postulated that initially, all the available oxygen has reacted with glycerol via the partial oxidation reaction followed by the reaction of steam with the remaining glycerol via steam reforming reaction; as a result, the conversion is increased. The above variation in the conversion can be explained by the occurrence of water−gas shift (WGS) and reverse water−gas shift (RWGS) reaction equilibria. Generally, the addition of oxygen to the reformer is a significant factor, which has a direct impact on the heat requirement to sustain the ATR process. In ATR reactions, the system can be operated without requiring external heat input by adjusting the O/G ratio in the feed stream. According to the results, H2 selectivity is not affected significantly at S/G=9 (Figure 15) with an increase in O/G ratio, since the increased oxidation makes more unreacted feed steam, and, hence, the water gas shift reaction was more pronounced. Figure 14 Figure 15


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Figure 16

3.2.7. Effect of Steam-to-glycerol ratio (S/G) The S/G ratio is a critical parameter that can affect the conversion and H2 selectivity for the ATR process.33 Typically, excess steam can be applied to overcome the equilibrium limitations of the steam reforming reaction, enhancing the extent of hydrogen production.34 In the current study the S/G ratio was varied between 6, 9, and 12 at different O/G ratios and the glycerol conversion and H2 selectivity are presented in Figure 17, 18, and 19. The glycerol conversion remained almost unchanged except for S/G-=12 at three ratios of O/G=0.2, 0.5 and 0.8 at corresponding temperatures of 550 ˚C and 600 ˚C. It is difficult to define any trend for these ratios. Hydrogen selectivity has slightly decreased by increasing S/G ratio in the range studied. The reduction in hydrogen selectivity could be attributed to the dilution effect of the unreacted steam.

Figure 17 Figure 18 Figure 19

4. Conclusions The synthesis, characterizations and catalytic testing of a portfolio of ceria-based ternary oxides 5Ni/CeZrM (M = Ca, Mg, Gd, La, Y) catalysts were studied for the ATR of glycerol at different operating conditions. The catalysts were prepared using surfactant-assisted method with a S/M molar ratio of 0.5. Among all the tested catalysts, 5Ni/CeZrGd presents a low CPF, highest surface area, pore/volume with the best performance at all the tested reaction temperatures. Therefore, parametric analysis studies were performed on this catalyst to investigate the effects of S/G ratio and O/G ratios on glycerol conversion and H2 selectivity. The glycerol conversion has proportionally increased with increasing the reaction temperature; hence, the S/G and O/G



molar ratios at different operation temperature were optimized to achieve the maximum thermodynamically allowed glycerol conversion and hydrogen selectivity over the 5Ni/CeZrGd catalyst. In comparison with non-catalytic reactions, 5Ni/CeZrGd catalyzed ATR process has shown higher activity up to almost three times compared to the non-catalytic experiments at the same conditions. Applying 5Ni/CeZrGd catalyst for the ATR of glycerol resulted in conversion and selectivity of 82% and >70%, at 600 °C respectively. In comparison to steam reforming process ATR of glycerol exhibited superior catalytic performance. The results also indicated that CO formation, which can cause a poisoning problem in low-temperature fuel cells, has increased with increasing the reformer temperature and was suppressed by increasing the S/G ratio.

Acknowledgements The authors gratefully acknowledge the financial support from the Faculty of Graduate Studies and Research (University of Regina), the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), and the Clean Energy Technologies Research Institute (CETRi).

References (1) Mohamedali, M.; Henni, A.; Ibrahim, H. Hydrogen Production from Oxygenated Hydrocarbons: Review of Catalyst Development, Reaction Mechanism and Reactor Modeling. In Hydrogen Production Technologies, Mehmet Sankir, N. D. S., Ed. John Wiley & Sons, Inc.: NJ, USA, 2017; pp 1-76. (2) Wang, W. Thermodynamic Analysis of Glycerol Partial Oxidation for Hydrogen Production. Fuel Process. Technol. 2010, 91, 1401-1408. (3) Liu, Y.; Farrauto, R.; Lawal, A. Autothermal Reforming of Glycerol in a Dual Layer Monolith Catalyst. Chem. Eng. Sci. 2013, 89, 31-39. (4) Vaidya, P. D.; Rodrigues, A. E. Glycerol Reforming for Hydrogen Production: A Review. Chem. Eng. Technol. 2009, 32, 1463-1469. (5) Vaidya, P. D.; Rodrigues, A. E. Insight into Steam Reforming of Ethanol to Produce Hydrogen for Fuel Cells. Chem. Eng. J. 2006, 117, 39-49. (6) Jimmy, U.; Mohamedali, M.; Ibrahim, H. Thermodynamic Analysis of Autothermal Reforming of Synthetic Crude Glycerol (SCG) for Hydrogen Production. ChemEngineering 2017, 1, 4. (7) Swami, S. M.; Abraham, M. A. Integrated Catalytic Process for Conversion of Biomass to Hydrogen. Energy Fuels 2006, 20, 2616-2622.


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(25) Khan, M. F. A.; Khan, A.; Ibrahim, H.; Idem, R. Kinetic Study of the Catalytic Partial Oxidation of Synthetic Diesel over 5 wt % Ni/Ce0.5Zr0.33Ca0.085Y0.085O2-δ Catalyst for Hydrogen Production. Energy Fuels 2012, 26, 5421-5429. (26) Kumar, P.; Sun, Y.; Idem, R. Comparative Study of Ni-based Mixed Oxide Catalyst for Carbon Dioxide Reforming of Methane. Energy Fuels 2008, 22, 3575-3582. (27) Khan, A.; Smirniotis, P. G. Relationship Between Temperature-Programmed Reduction Profile and Activity of Modified Ferrite-Based Catalysts for WGS Reaction. J. Mol. Catal. A: Chem. 2008, 280, 43-51. (28) Montoya, J. A.; Romero-Pascual, E.; Gimon, C.; Del Angel, P.; Monzon, A. Methane Reforming with CO2 Over Ni/ZrO2-CeO2 Catalysts Prepared by Sol-Gel. Catal. Today 2000, 63, 71-85. (29) Sengupta, P.; Khan, A.; Zahid, M. A.; Ibrahim, H.; Idem, R. Evaluation of the Catalytic Activity of Various 5Ni/Ce0.5Zr0.33M0.17O2-δ Catalysts for Hydrogen Production by the Steam Reforming of a Mixture of Oxygenated Hydrocarbons. Energy Fuels 2012, 26, 816-828. (30) Kumar, P.; Idem, R. A Comparative Study of Copper-Promoted Water-Gas-Shift (WGS) Catalysts. Energy Fuels 2007, 21, 522-529. (31) Dieuzeide, M. L.; Iannibelli, V.; Jobbagy, M.; Amadeo, N. Steam Reforming of Glycerol Over Ni/Mg/γ-Al2O3 Catalysts. Effect of Calcination Temperatures. Int. J. Hydrogen Energy 2012, 37, 14926-14930. (32) Rioche, C.; Kulkarni, S.; Meunier, F. C.; Breen, J. P.; Burch, R. Steam Reforming of Model Compounds and Fast Pyrolysis Bio-oil on Supported Noble Metal Catalysts. Appl. Catal., B 2005, 61, 130-139. (33) Authayanun, S.; Arpornwichanop, A.; Paengjuntuek, W.; Assabumrungrat, S. Thermodynamic Study of Hydrogen Production from Crude Glycerol Autothermal Reforming for Fuel Cell Applications. Int. J. Hydrogen Energy 2010, 35, 6617-6623. (34) Ashrafi, M.; Proll, T.; Pfeifer, C.; Hofbauer, H. Experimental Study of Model Biogas Catalytic Steam Reforming: 1. Thermodynamic Optimization. Energy Fuels 2008, 22, 41824189.

Figure Legends Figure 1. H2-TPR profiles for CeZrGd support and 5Ni/CeZrGd catalyst Figure 2. UV-diffuse reflectance spectra of (a) CeZrGd and (b) 5Ni/CeZrGd Figure 3. XRD pattern of CeZrM supports and 5Ni/CeZrM catalysts Figure 4. Catalytic activity of glycerol ATR over 5Ni/CeZrM (M = Ca, Mg, La, Gd, Y) catalysts at 500°C, O/G = 0.5, and S/G = 6 Figure 5. Hydrogen selectivity over 5Ni/CeZrM (M = Ca, Mg, La, Gd, Y) catalysts at 500°C, O/G = 0.5, and S/G = 6 Figure 6. Effect of promoter element on product distribution for the ATR of glycerol at 500 °C


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Figure 7. Identification of catalytic and thermal effect for the catalyst 5Ni/CeZrGd at (a) 450 °C, (b) 500 °C, (c) 600 °C, and (d) 700 °C Figure 8. Glycerol conversion of glycerol ATR without catalyst at different reaction temperature Figure 9. H2 Selectivity of glycerol ATR without catalyst at different reaction temperature Figure 10. Influence of reaction temperature on the reformate gas composition (H2, CO2, and CO) for (a) non-catalytic reaction and (b) catalytic reaction with 5Ni/CeZrGd Figure 11. Effect of reaction temperature on glycerol conversion for the catalyst 5Ni/CeZrGd Figure 12. The effect of the catalyst calcination temperature on the catalytic performance at reaction temperature of 600 °C Figure 13. Comparison between the steam reforming and ATR of glycerol over 5Ni/CeZrGd catalyst at a reaction temperature of 500 oC Figure 14. The effect of Oxygen-to-glycerol (O/G) ratio on (a) Glycerol Conversion and (b) H2 selectivity at S/G=6 Figure 15. The effect of Oxygen-to-glycerol (O/G) ratio on (a) Glycerol Conversion and (b) H2 selectivity at S/G=9 Figure 16. The effect of Oxygen-to-glycerol (O/G) ratio on (a) Glycerol Conversion and (b) H2 selectivity at S/G=12 Figure 17. The effect of Steam-to-glycerol (O/G) ratio on (a) Glycerol Conversion and (b) H2 selectivity at O/G=0.2 Figure 18. The effect of Steam-to-glycerol (O/G) ratio on (a) Glycerol Conversion and (b) H2 selectivity at O/G=0.5 Figure 19. The effect of Steam-to-glycerol (O/G) ratio on (a) Glycerol Conversion and (b) H2 selectivity at O/G=0.8



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Tables with Headings Table 1. Porosity characterization results of the catalysts and supports obtained from N2Physisorption. Catalyst Specific BET Surface Area Specific Pore V. / S. Area Pore Size (m2/g)

Pore Volume

(A0)

(A0)

(cm3/g) 5Ni/CeZrCa

116.4

0.23

60.00

19.8

5Ni/CeZrGd

197.4

0.53

94.71

26.8

5Ni/CeZrMg

154.65

0.45

73.52

29.1

5Ni/CeZrY

193.34

0.48

91.97

24.8

5Ni/CeZrLa

162.48

0.50

105.6

30.77

5Ni/CeZr

215.2

0.3

51

13.9

Table 2. Ni dispersion, Ni surface area, Ni surface area, and Ni mass fraction (ICP-MS) of the catalysts obtained by H2-Chemisorption. Catalyst Ni Surface Ni Surface Ni Ni Mass Fraction (ICP-MS Area

Area

Dispersion

Result)

(m2/g)

(m2/g Ni)

(%)

(%)

5Ni/CeZrCa

3.18

76.28

11.46

5.5

5Ni/CeZrGd

1.18

23.56

3.54

7.8

5Ni/CeZrLa

3.19

75.86

9.57

7.3

5Ni/CeZrMg

1.32

8.76

0.44

7.7

5Ni/CeZrY

3.19

63.73

9.57

5.0

Table 3. The CPFs of 5Ni/CeZrM with different promoter elements at 500°C Catalyst 5N/CeZrM CPF from(TGA)[mg of carbon(g of catalyst -1) h-1] 5Ni/CeZrCa

44.48

5Ni/CeZrGd

47.25

5Ni/CeZrMg

52.06

5Ni/CeZrY

57.60


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5Ni/CeZrLa

64.61

Figure 1

Figure 2



Figure 3

Figure 4


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Figure 5

Figure 6



Figure 7


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Figure 8

Figure 9



Figure 10

Figure 11


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Figure 12

Figure 13



Figure 14

Figure 15


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Figure 16

Figure 17



Figure 18

Figure 19


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Graphical Abstract 338x190mm (96 x 96 DPI)


Metal Oxide-Based Catalysts for the Autothermal Reforming of

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