Hydrogen Production via the Catalytic Partial Oxidation of Ethanol on

Jun 14, 2019 - To produce hydrogen for automotive exhaust gas aftertreatment systems, the catalytic partial oxidation of ethanol over a platinum–rho...
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Hydrogen Production via the Catalytic Partial Oxidation of Ethanol on a Platinum−Rhodium Catalyst: Effect of the Oxygen-to-Ethanol Molar Ratio and the Addition of Steam B. Sawatmongkhon,*,†,‡ K. Theinnoi,†,‡ T. Wongchang,‡,§ C. Haoharn,†,‡ C. Wongkhorsub,†,‡ and A. Tsolakis∥

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†

College of Industrial Technology, King Mongkut’s University of Technology North Bangkok, 1518 Pracharat 1 Road, Wongsawang, Bangsue, Bangkok 10800, Thailand ‡ Research Centre for Combustion Technology and Alternative Energy (CTAE), Science and Technology Research Institute, King Mongkut’s University of Technology North Bangkok, Bangkok 10800, Thailand § Department of Mechanical and Automotive Engineering Technology (MAet), Faculty of Engineering and Technology, King Mongkut’s University of Technology North Bangkok (Rayong Campus), Nonglalok, Bankhai, Rayong 21120, Thailand ∥ School of Engineering, Mechanical and Manufacturing Engineering, University of Birmingham, Birmingham B15 2TT, U.K. S Supporting Information *

ABSTRACT: To produce hydrogen for automotive exhaust gas aftertreatment systems, the catalytic partial oxidation of ethanol over a platinum−rhodium catalyst supported on alumina is examined via experimental studies as well as thermodynamic analysis. The research focuses on the effects of the ethanol concentration, oxygen-to-ethanol molar ratio, and water content of ethanol on the ethanol conversion and product yield (e.g., H2, CO, CO2, and CH4). The hot spot temperature and position and the temperature profile along the monolithic catalyst are also analyzed as a function of the inlet gas composition. Different surface chemical reactions (e.g., partial oxidation and steam reforming of ethanol, water−gas shift, and hydrocarbon cracking) are employed to explain the phenomena that take place during ethanol reforming. The process follows the indirect reforming pathway, which involves the exothermic oxidation of ethanol to produce H2O, CO2, and heat, followed by endothermic steam reforming to generate CO and H2. The temperature profile inside the catalyst depends critically on the amount of ethanol supplied and the oxygen-to-ethanol molar ratio. The ethanol conversion, hydrogen production, and selectivity toward hydrogen and methane depend strongly on the operating conditions. The addition of steam has a slightly positive effect on the hydrogen formation and temperature profile. step,6,7 and (7) the prevention of active site blocking by strongly adsorbed species (e.g., nitrates) at low temperatures.8−10 In systems designed to eliminate the particulate matter (PM), a small amount of hydrogen (e.g., 500−2500 ppm) can improve the production of NO2 by the diesel oxidation catalyst;11−13 the formed NO2, which is more active than oxygen, then oxidizes the PM to regenerate the diesel particulate filter.14,15 Furthermore, the utilization of hydrogen in combustion engines also enhances the autoignition of some alternative hydrocarbon fuels16 and avoids the PM−NOx trade-off.17−20 Hydrogen has also been beneficial in gasoline engines. Tsolakis and co-workers studied the potential of hydrogen-rich reformate exhaust gas recirculation (REGR) to improve the performance of gasoline direct injection engines. The authors found that the use of REGR (1) improved the indicated engine efficiency, (2) extended the dilution limit, (3) reduced NOx emissions, (4) reduced knock, (5) decreased both the mass and number of PM emissions, (6) increased combustion stability inside the cylinder due to the high

1. INTRODUCTION Hydrogen (H2) is considered a future energy vector for transportation. Unlike traditional fossil fuels, hydrogen is a zero-emission energy carrier when it is utilized in conjunction with oxygen in fuel cells to power electric vehicles. Furthermore, hydrogen is a highly important feed stock for the production of methanol, fine chemicals, and liquid hydrocarbons via the Fischer−Tropsch process. Moreover, it is used both to enhance the combustion stability and to reduce the nitrogen oxide (NOx) emissions of stationary gas turbines using a lean premixed combustion technology.1 Indeed, the hydrogen substitution to methane (the main constituent of natural gas) is beneficial in terms of increase in both laminar burning velocity2 and flame resistance to strain-induced extinction.3,4 In addition, hydrogen has been used in aftertreatment systems for both diesel and gasoline engines. In diesel engines, NOx reduction systems such as hydrocarbonselective catalytic reduction, hydrogen promotes several processes: (1) the oxidation of NO to nitrate;5−7 (2) the oxidation of NO to NO2;5−8 (3) the partial oxidation of hydrocarbons to acetate;5−7 (4) the oxidation of hydrocarbons to COx;9 (5) the oxidation of acetate with the NO + O2 mixture;5,6 (6) the enhancement of the rate-determining © 2019 American Chemical Society

Received: May 6, 2019 Revised: June 11, 2019 Published: June 14, 2019 6742

DOI: 10.1021/acs.energyfuels.9b01398 Energy Fuels 2019, 33, 6742−6753

Article

Energy & Fuels Table 1. Possible Reactions Taking Place during the Partial Oxidation of Ethanol R 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

C2H5OH + 1/2O2 → 2CO + 3H2 C2H5OH + 3/2O2 → 2CO2 + 3H2 C2H5OH + 3O2 → 2CO2 + 3H2O C2H5OH + H2O → 2CO + 4H2 C2H5OH + 3H2O → 2CO2 + 6H2 C2H5OH → CH3CHO + H2 C2H5OH + 1/2O2 → CH3CHO + H2O CH3CHO → CH4 + CO CO + 3H2 → CH4 + H2O CO2 + 4H2 → CH4 + 2H2O C2H5OH → C2H4 + H2O C2H4 → 2C + 2H2 CH4 → C + 2H2 2CO → C + CO2 CH4 + 1/2O2 → CO + 2H2 CH4 + O2 → CO2 + 2H2 CH4 + 2O2 → CO2 + 2H2O CO + 1/2O2 → CO2 CO + H2O → CO2 + H2 CO2 + H2 → CO + H2O C2H5OH → CO + CH4 + H2

diffusivity and flame speed of hydrogen, and (7) allowed spark timing to be retarded, which increased the time available for fuel−air mixing.21−23 Ethanol is an alternative and renewable energy carrier that can be produced by the fermentation of plant materials. It has low toxicity, low risk of explosion,24 and high energy density and is carbon neutral and easy to store and handle. In automotive applications, the direct utilization of hydrogen is inappropriate because of its low volumetric energy density. As a result, on-board hydrogen production from ethanol via reforming processes is more suitable. The production of hydrogen via fuel reforming is considered to have higher energy efficiency than many hydrogen production processes.25 Endothermic steam reforming provides the maximum theoretical yield of hydrogen;26 therefore, it is commercially employed to generate hydrogen from hydrocarbons. However, endothermic steam reforming requires complex systems and a supply of heat from an external source; additionally, the catalyst deactivation problem hinders on-board hydrogen production. The partial oxidation of ethanol is an attractive alternative because it does not require an external heat source or heat exchanger. This process can be continuing spontaneously in the short-contact-time mode.27−30 Furthermore, it utilizes a simple design, is compact, and allows fast start-up and quick response to a transient load, which are necessary for automotive applications. The reaction pathways of the partial oxidation of methane on a rhodium-based catalyst were studied systematically by Horn et al.28,31 By using the capillary technique, the spatial concentration of the products and the temperature profile were measured along 10 mm of the catalyst length. The authors found that all the O2 was completely consumed within 2 mm of the entrance. The reforming of CO2 was not observed during the study. An exo−endothermic reaction mechanism, which is also called the indirect pathway, consisting of two sequential reaction zones, the oxidation zone and the reforming zone, was proposed. In the oxidation zone, O2 and CH4 were consumed and H2O was generated, and H2 and CO

ΔhR (kJ/mol)

reaction

13.8 −552.2 −1277.6 255.6 173.2 68.6 −173.2 −18.9 −205.9 −164.7 45.4 −52.4 74.6 −172.5 −35.9 −318.9 −802.5 −283.0 −41.2 41.2 49.7

ethanol partial oxidation ethanol partial oxidation ethanol complete oxidation ethanol steam reforming ethanol steam reforming ethanol dehydrogenation selective oxidation of ethanol acetaldehyde decomposition methanation methanation ethanol dehydration carbon formation methane decomposition Boudouard reaction methane partial oxidation methane partial oxidation methane complete oxidation carbon monoxide oxidation water−gas shift reverse water−gas shift ethanol decomposition

were partly formed. The reaction temperature increased rapidly during the highly exothermic oxidation of CH4. A small amount of CO2 was produced only when O2 was present in the reactant (the CO2 concentration was constant after O2 had been completely converted). In the endothermic reforming zone, O2 was absent; H2 and CO were mainly produced via the steam reforming reaction, and the operating temperature gradually decreased. In a recent work, a study of the sequence of the elementary steps for methane partial oxidation confirmed the indirect pathway.32,33 Acetaldehyde, an important intermediate during ethanol reforming, is generated from either the dehydrogenation34−37 or the selective oxidation of ethanol,38,39 according to R6 and R7 given in Table 1, respectively, depending on the operating temperature and the metal catalyst or support material used. In the oxidative steam reforming of ethanol over rhodium supported on a CeO2 and Al2O3 catalyst, acetaldehyde was mainly formed via the selective oxidation of ethanol at low temperatures.39 Moreover, the oxygen storage/release capacity of CeO2 promoted the transformation of acetaldehyde into the final products at high temperatures. Methane is an unwanted byproduct whose formation reduces the opportunity for ethanol to be converted to hydrogen. It is formed through three main pathways: decomposition of acetaldehyde,37,40−42 methanation,36 and decomposition of ethanol,41,43 according to R8, R10, and R21 given in Table 1, respectively. Simson et al. reported that the formation of methane decreased when the space velocity was increased from 50 000 to 100 000 h−1. Moreover, the amount of methane measured was far from the calculated equilibrium value.36 Several works in the literature have dealt with the steam reforming of ethanol in order to produce a high concentration of hydrogen. In contrast, the study of partial oxidation of ethanol is limited because of the requirement for pure oxygen. Unlike fuel cell applications, in which a large amount of hydrogen is necessary,44,45 in automotive systems, only a small amount of hydrogen is needed. Consequently, air can be used 6743

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respectively. The calculation constraints were constant pressure and enthalpy. With constant enthalpy, the equilibrium temperature was estimated based on the assumption that the generated heat was not lost through the monolith wall. Therefore, heat can be accumulated inside the catalyst channel and the temperature can be correspondingly increased. The seven possible products were hydrogen, carbon monoxide, carbon dioxide,30 methane (CH4),49 acetaldehyde (CH3CHO),49 ethylene (C2H4),50 and carbon (C). Table 1 presents potential reaction pathways for the species formed during the partial oxidation of ethanol. The corresponding change in Gibbs free energy, ΔGR, as a function of temperature was evaluated using the van’t Hoff equation.51 2.3. Experimental Study. The reforming was carried out in a stainless steel laboratory-scale reactor as schematically depicted in Figure S1. The prepared monolithic catalyst was placed vertically inside the reactor without reduction by H2. Two uncoated monoliths of 10 mm in length were placed in front of and behind the active catalyst to serve as heat shields to reduce radiative heat loss. The gap between the active monolith and the reactor was sealed using glass wool. A stainless steel tube of 2 mm diameter was placed inside the catalyst and then a movable k-type thermocouple of 1 mm diameter was placed inside the tube to monitor the temperature profile along the catalyst with a spatial resolution of 2 mm. The reactor was placed inside an electric furnace to provide the required inlet temperature. The temperature was set at a constant value of 300 °C to represent the exhaust temperature of a passenger vehicle. O2 (99.9% purity) and N2 (the balance gas, 99.9% purity) were independently supplied from gas bottles. The inlet composition was controlled by flow meters. The volume flow rate of nitrogen was kept constant at 5 L/min. Therefore, the gas hourly space velocity (GHSV) had a high value (∼70 000 h−1) to eliminate the mass-transfer limitation. Ethanol (99.9% purity) was used as the hydrogen source in the partial oxidation study. In the studies of the effect of water addition, ethanol was blended with the designated amount of distilled water to obtain water concentrations of 2, 4, 8, and 10 vol %. The liquid reactant was fed into an evaporator whose temperature was maintained at 250 °C. The injection rate of the liquid was in the range 44−137 cm3/h. A stainless steel tube was used to link the evaporator and reactor; the tube was wrapped with a heat tape and its temperature was maintained at 125 °C to prevent condensation. The produced gases were passed through a water trap. After the temperature at the reference point (10 mm from the inlet of the active catalyst) was stabilized, the dry gas was collected in a gas sampling bag. The temperature profile was obtained by averaging three measurements of the temperature inside the catalyst. The product gas was analyzed offline using a Shimadzu GC-2014 gas chromatograph equipped with a flame ionization detector and a thermal conductivity detector to determine the gas composition. The repeatability was verified by performing an experiment with one operating condition and then the same experimental condition was carried out 1 day later. The results were repeatable.

as the oxidizer rather than pure oxygen in the partial oxidation process. Moreover, the presence of carbon monoxide, which is harmful to fuel cells, does not negatively affect the automotive systems. In on-board hydrogen generation systems constructed for use in automotive applications, the externally supplied heat needed to initiate the reforming process is provided by the exhaust gas, which has a temperature of 300−400 °C. The catalytic partial oxidation of medium-grade ethanol is expected to occur spontaneously when the oxygen-to-ethanol molar ratio at the reforming catalyst is controlled. In a gasoline engine, the exhaust gas contains little oxygen; thus, external air is mixed with the exhaust gas to achieve the desired oxygen composition. In a diesel engine, in which the exhaust gas contains a large amount of oxygen, ethanol is added to achieve the required oxygen-to-ethanol molar ratio at the catalyst. Using these strategies, compact on-board hydrogen production systems can be designed. This work is focused on the production of hydrogen via the catalytic partial oxidation of ethanol. The aim of the study is to obtain a high hydrogen yield and avoid overly high catalyst temperatures. Therefore, the hot spot temperature and the temperature profile inside the catalyst are studied as a function of the inlet composition (ethanol concentration, molar ratio of oxygen to ethanol, and water content of ethanol). Furthermore, the effect of the operating conditions on the ethanol reforming activities (ethanol conversion, hydrogen production, methane formation, and chemical reactions taking place during the reforming) and product selectivity are investigated. Thermodynamic analysis and experimental studies are carried out to explain the reforming phenomena.

2. METHODS 2.1. Catalyst Preparation. The catalyst used in this work was prepared via incipient wetness impregnation. The support was formulated by dissolving a calculated amount of cerium(III) nitrate hexahydrate (Sigma-Aldrich, 99% trace metals basis) to obtain a ceria loading of 20% (by mass). The precursor solution was then added dropwise to powdered alumina (Sigma-Aldrich) with a Brunauer− Emmett−Teller surface area of 155 m2/g. The solvent was removed by drying at 110 °C for 8 h in an oven. Then, the dry support was calcined in air at 600 °C at a heating rate of 10 °C/min for 2 h. A dispersion of 2% platinum and 1% rhodium (by mass) was prepared using the same support preparation. A suitable quantity of chloroplatinic acid solution (Sigma-Aldrich) was dropped on the prepared support; drying and calcination were then conducted in the same manner. Finally, the appropriate amount of a rhodium(III) nitrate solution (Sigma-Aldrich) was added to the prepared catalyst and the catalyst was dried and calcined. To form a monolithic catalyst bed, a substrate with a diameter and length of 13 and 30 mm, respectively, was cut from a standard cordierite honeycomb (2MgO· 2Al2O3·5SiO2). The cells of the honeycomb had a square shape with a high density of 900 cpsi and a wall thickness of 0.15 mm. An aqueous suspension of the prepared powdered catalyst was uniformly coated onto the substrate with a washcoat thickness of approximately 0.03 mm. The corresponding geometric surface area of the catalyst was 3541 m2/m3. The monolithic catalyst was dried in an oven at 110 °C for 8 h. 2.2. Thermodynamic Analysis. Thermodynamic analysis was conducted using the total Gibbs free-energy minimization method.46,47 Equilibrium calculations were performed using the free software GASEQ.48 The starting compounds were ethanol, oxygen, steam, and nitrogen (balance gas). The analysis was carried out at different ethanol concentrations (4−12 vol %), oxygen-to-ethanol molar ratios (0.5−3.5), and steam contents (0−10 vol % in ethanol). The initial temperature and pressure were 300 °C and 1 bar,

3. RESULTS AND DISCUSSION The effect of the ethanol and oxygen concentrations on the catalytic partial oxidation activity was investigated by adding different ethanol compositions and oxygen-to-ethanol molar ratios. The ethanol concentrations ranged from 4 to 10% (mol %) and the oxygen-to-ethanol molar ratios ranged from 0.7 to 2. Moreover, the impact of the addition of water to ethanol (2−10% water by volume) on the reforming activity was also examined. The ethanol conversion and product selectivity are defined as XC2H5OH =

SH 2 = 6744

nCin2H5OH − nCout 2 H5OH nCin2H5OH

n H2 /3 nCin2H5OH DOI: 10.1021/acs.energyfuels.9b01398 Energy Fuels 2019, 33, 6742−6753

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of hydrogen reaches the maximum value when the oxygen-toethanol molar ratio is between 0.7 and 1. This is an interesting point. As shown in Table 1, hydrogen can be produced from ethanol and oxygen via R1 and R2. At oxygen-to-ethanol molar ratios higher than 0.5 (the stoichiometric ratio for R1), the surplus oxygen is exothermally consumed by ethanol to produce additional hydrogen (R2), extra heat (R2 and R3), and water (R3). The generated heat and water promote the endothermic steam reforming reactions (R4 and R5) to produce additional hydrogen. However, the hydrogen yield is dramatically reduced at high oxygen-to-ethanol molar ratios because ethanol is competitively consumed by oxygen through complete combustion with a high reaction rate (R3); additionally, the produced hydrogen is depleted by reaction with other substances such as CO2 via the reverse water−gas shift process (R20). A considerable amount of carbon monoxide is generated in the oxygen-to-ethanol molar ratio range of 0.5 and 1, which coincides with the formation of hydrogen. This implies that in this range, hydrogen and carbon monoxide are formed together by the same chemical reactions. The content of carbon monoxide gradually declines at oxygento-ethanol molar ratios between 1 and 2 while the hydrogen concentration is substantially reduced. This suggests that at least one process consumes hydrogen and produces carbon monoxide (e.g., the reverse water−gas shift). Interestingly, the steady drop of carbon monoxide and obvious increase of carbon dioxide at oxygen-to-ethanol molar ratios of 1−2 indicate that the major path of carbon dioxide creation does not utilize carbon monoxide as the starting compound; therefore, the main route of carbon dioxide formation should be complete oxidation. A large amount of water is formed when the oxygen-to-ethanol molar ratio is between 1 and 2. This confirms that the key process in the oxygen-to-ethanol molar ratio range of 1−2 is complete oxidation. The spontaneity of selected chemical reactions for ethanol reforming provided in Table 1 is shown in Figure 3 as a function of the operating temperature. All reactions that are not shown in the figure are spontaneous from 100 to 1000 °C. Fundamentally, a chemical reaction proceeds spontaneously when the change of Gibbs free energy is negative. The figure shows that methane cannot be formed through the methanation processes (R9 and R10) at temperatures higher than 700 °C. The steam reforming reactions (R4 and R5) and the ethanol dehydrogenation to form acetaldehyde (R6) can take place at temperatures above 300 °C. At temperatures higher than 700 °C, the water−gas shift (R19) cannot proceed, but the reverse water−gas shift (R20) can occur. 3.2. Experimental Studies. 3.2.1. Partial Oxidation of Pure Ethanol. As illustrated in Figure 4, the conversion of ethanol increased with the oxygen-to-ethanol molar ratio. Although there is evidence that byproducts (mainly acetaldehyde and ethylene,50,52−54 including diethyl ether29) are formed during the partial oxidation of ethanol, in this work, the products were assumed to be only H2, CO, CO2, CH4, and H2O (not measured). Although the equilibrium calculation predicted that the conversion of ethanol would be almost complete, the experiments demonstrated that the transformation of ethanol into the product gases was limited (less than 80%). The rest was possibly in the form of unreacted ethanol, acetaldehyde, or ethylene. The results revealed that at a short contact time (GHSV ≈ 70 000 h−1), the ethanol partial oxidation over the catalyst took place far from the equilibrium

Figure 1. Equilibrium temperature for the reforming of ethanol at different ethanol concentrations and oxygen-to-ethanol molar ratios.

SCO =

SCO2 =

SCH4 =

nCO/2 nCin2H5OH nCO2 /2 nCin2H5OH nCH4 /2 nCin2H5OH

The number of moles of ethanol at the outlet, nCout , was 2 H5OH calculated by balancing the number of carbon atoms in the product gas. The hydrogen selectivity was 100% when one molecule of ethanol (containing six atoms of hydrogen) was converted into three molecules of hydrogen. In the same manner, the selectivity of the C-containing molecules (e.g., CO, CO2, and CH4) was 100% if two C-containing molecules were generated from one molecule of ethanol. 3.1. Thermodynamic Analysis. The temperatures at the equilibrium state of ethanol transformation at different ethanol fractions and oxygen-to-ethanol molar ratios are presented in Figure 1. Ratios of 0.5 and 3 are stoichiometric for the partial oxidation and the complete combustion of ethanol, according to R1 and R3, respectively. The operating temperature depended on both the ethanol and oxygen compositions. The calculations indicated that complete oxidation is dominant at oxygen-to-ethanol molar ratios larger than 0.8. Ethanol is probably consumed by excess oxygen via complete combustion, which is extremely exothermic, resulting in a substantial increase in the reaction temperature. The temperatures predicted from the thermodynamic analysis were used as a guide to design the experimental conditions to avoid excessively high operating temperatures. The main products (e.g., H2, CO, CO2, and H2O) are illustrated as a function of the ethanol concentration and oxygen-to-ethanol molar ratio provided in Figure 2. The conversion of ethanol is almost complete, and small amounts of methane, ethylene, and atomic carbon are found at equilibrium for all conditions (results not shown). Depending on the quantity of ethanol, the formation 6745

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Figure 2. Equilibrium product distributions for the reforming of ethanol at different ethanol concentrations and oxygen-to-ethanol molar ratios: (a) H2; (b) CO; (c) CO2; and (d) H2O. Calculation constraints: constant pressure and enthalpy.

Figure 4. Ethanol conversion as a function of the ethanol content and oxygen-to-ethanol molar ratios for ethanol contents of 10% (◯), 8% (△), 6% (□), and 4% (◇).

Figure 3. Change in the Gibbs free energy of selected reactions as a function of operating temperature. 6746

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The maximum temperatures (hot spot) inside the catalyst during ethanol reforming are shown in Figure 5. The temperature increased substantially with the ethanol fraction and oxygen-to-ethanol molar ratio. The experimentally measured temperatures were considerably lower than those calculated in the thermodynamic analysis, indicating that the actual process had not yet reached equilibrium or that the produced heat was lost to the surroundings. Theoretically, the partial oxidation of ethanol (R1) is stoichiometric at an oxygen-to-ethanol molar ratio of 0.5 and slightly endothermic. However, to provide the required heat to drive the spontaneous reforming, a superstoichiometric oxygen-toethanol molar ratio is necessary. The extra oxygen reacts exothermically with ethanol via the complete oxidation reaction to produce a large amount of heat. The strong dependence of the operating temperature on the oxygen-toethanol molar ratio indicated that ethanol and oxygen are adsorbed effectively on the catalyst sites and react rapidly and exothermically through the complete oxidation reaction.28,30,31 Figures 4 and 5 suggest a direct relationship between the ethanol conversion and the operating temperature. With higher oxygen-to-ethanol molar ratios, the reaction temperature increases notably, resulting in higher ethanol conversion according to the Arrhenius law. The ethanol conversion is greater than 50% when the hot spot temperature is greater

Figure 5. Maximum temperature (hot spot) inside the catalyst as a function of the ethanol content and oxygen-to-ethanol molar ratio for ethanol contents of 10% (◯), 8% (△), 6% (□), and 4% (◇).

state. Greater ethanol conversion could be obtained if the contact time was increased by reducing the GHSV.55

Figure 6. Product gas concentrations as a function of the ethanol content and oxygen-to-ethanol molar ratio: (a) H2; (b) CO; (c) CO2; and (d) CH4 for ethanol contents of 10% (◯), 8% (△), 6% (□), and 4% (◇). 6747

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Figure 7. Product selectivity as a function of the ethanol content and oxygen-to-ethanol molar ratio: (a) H2; (b) CO; (c) CO2; and (d) CH4 for ethanol contents of 10% (◯), 8% (△), 6% (□), and 4% (◇).

than 650 °C. Higher ethanol conversion is expected when the operating temperature of the process is increased by adding more oxygen. However, an overly high operating temperature can cause damage to the catalyst. The effect of the oxygen-to-ethanol molar ratio and amount of ethanol supplied on the contents of the gas products is presented in Figure 6. Increased ethanol concentration and oxygen-to-ethanol molar ratio at the inlet enhanced the production of hydrogen at the outlet.28,56 For an inlet ethanol concentration of 10%, higher hydrogen production can be confidently expected at oxygen-to-ethanol molar ratios higher than 1.4. However, when the ratio is greater than 1.4, the hot spot temperature can easily reach 1000 °C. To prevent damage to the catalyst, excessive temperatures should be avoided. For a given ethanol concentration, the equilibrium (maximum) hydrogen yield was obtained at an oxygen-to-ethanol molar ratio higher than that predicted by the equilibrium calculation. For example, at 4% ethanol, a maximum hydrogen production of 8.82 vol % was estimated to occur at an oxygen-to-ethanol molar ratio of 1.0 in the equilibrium calculations (Figure 2a), but as shown in Figure 6a, only 0.2 vol % hydrogen was obtained at an oxygen-to-ethanol molar ratio of 1.0 while 7.07 vol % hydrogen was obtained at a ratio of 2.0. This suggests that the hydrogen production may reach its equilibrium value at higher operating temperatures (high oxygen-to-ethanol molar ratios).

The production of carbon monoxide as a function of the inlet ethanol content and oxygen-to-ethanol molar ratio is presented in Figure 6b. According to the partial oxidation reaction (R1), 1 mol of ethanol is transformed into 2 mol of carbon monoxide and 3 mol of hydrogen. This means that the concentrations of these products should be similar. In a study of partial oxidation on an inert porous medium without catalyst materials, similar contents of carbon monoxide and hydrogen were found in the product gas.57 However, in this work, the quantity of carbon monoxide was much lower than that of hydrogen. This indicates that at least one chemical reaction that consumed the generated carbon monoxide occurred. Interestingly, at oxygen-to-ethanol molar ratios that resulted in hot spot temperatures higher than 700 °C, the concentration of carbon monoxide was sharply reduced. According to Figure 3, the water−gas shift reaction (R19) is not spontaneous at temperatures higher than 700 °C. This indicates that the water−gas shift reaction is not the major reaction consuming the carbon monoxide.32 If this were the key carbon monoxide-consuming reaction, the carbon monoxide concentration would increase at temperatures higher than 700 °C; instead, the concentration of carbon monoxide declined sharply at temperatures beyond 700 °C. Thus, the reaction responsible for the carbon monoxide consumption should be spontaneous and highly active at temperatures higher than 700 °C. According to Table 1, the oxidation of carbon monoxide (R18) is the only candidate. This reaction is 6748

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Figure 8. Temperature profile inside the catalyst as a function of the oxygen-to-ethanol molar ratio for ethanol concentrations of (a) 4; (b) 6; (c) 8; and (d) 10%.

the catalyst temperature (oxygen-to-ethanol molar ratio). Acetaldehyde, which is derived from ethanol, can decompose on rhodium sites to produce methane, according to R8 given in Table 1.49 The selectivity of the conversion of ethanol to H2, CO, CO2, and CH4 is given in Figure 7. As shown in Figure 7a, the selectivity for H2 depended directly on the amount of ethanol supplied and the operating temperature. Fascinatingly, the H2 selectivity approached the same values at the same oxygen-toethanol molar ratios when the hot spot temperature was higher than 700 °C. This indicates that the water−gas shift played a vital role in the formation of hydrogen at temperatures lower than 700 °C. However, at temperatures over 700 °C, the selectivity of hydrogen was influenced only by the concentration of oxygen, regardless of the ethanol content. The selectivity for CO and CO2 are displayed in Figure 7b,c, respectively. Interestingly, at high operating temperatures, the selectivity for CO2 was higher than that for CO, indicating that the complete oxidation of ethanol (to produce CO2) was preferable to partial oxidation (to form CO) at high temperatures or, alternatively, that a chemical reaction that transforms CO into CO2 occurred at high temperatures. As previously discussed, the production of CO2 via the oxidation of CO was dominant at temperatures over 700 °C. The selectivity for methane as a function of the oxygen-toethanol molar ratio and the amount of ethanol supplied is

exothermic and spontaneous throughout the temperature range 100−1000 °C. As illustrated in Figure 6c, the carbon dioxide content increased with increasing ethanol and oxygen input and with the operating temperature. Thus, carbon dioxide was formed via exothermic reactions in which oxygen is a coreactant. Horn et al.31 found that carbon dioxide was produced via complete oxidation at the entrance of the catalyst (the oxidation zone), after which its quantity did not change along the catalyst length. The authors concluded that neither dry reforming nor the water−gas shift (which causes the generation of carbon dioxide) occurred during the catalytic partial oxidation of methane on the rhodium catalyst. The absence of dry reforming during partial oxidation has been suggested by several researchers.28,30,31 However, carbon dioxide can participate in the partial oxidation of methane via the reverse water−gas shift.30 As described in the preceding paragraph, the oxidation of carbon monoxide led to an increase in operating temperature and, therefore, the formation of carbon dioxide, especially at temperatures higher than 700 °C. The creation of the unwanted product methane is shown in Figure 6d. The formation of methane reduces the opportunity for ethanol to be converted to hydrogen. The formation of methane followed the same trend as the generation of hydrogen, that is, the amount of methane formed increases directly with an increase in the hydrogen source (ethanol) and 6749

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Figure 10. Effect of the water content on the hydrogen and methane selectivity at different ethanol concentrations and oxygen-to-ethanol molar ratios: (a) H2; (b) CH4. Ethanol content of 8% and oxygen-toethanol molar ratio of 1.8 (△); ethanol content of 8% and oxygen-toethanol molar ratio of 1.6 (□); ethanol content of 10% and oxygento-ethanol molar ratio of 1.4 (◯).

Figure 9. Effect of the water content on the amount of hydrogen and methane produced at different ethanol concentrations and oxygen-toethanol molar ratios: (a) H2; (b) CH4 at an ethanol content of 8% and oxygen-to-ethanol molar ratio of 1.8 (△), an ethanol content of 8% and oxygen-to-ethanol molar ratio of 1.6 (□), and an ethanol content of 10% and oxygen-to-ethanol molar ratio of 1.4 (◯).

limitation.28 Below the hot spot, the temperature dropped gradually until the end of the catalyst, especially at high ethanol concentrations and oxygen-to-ethanol molar ratios. According to the exo−endothermic model, endothermic steam reforming occurs below the oxidation zone. The majority of the produced hydrogen is generated in this region.30 The production of hydrogen is high when the temperature in the oxidation zone is high because steam reforming is thermodynamically favored. The temperature at the entrance deviated from the assigned inlet temperature of 300 °C because of backward heat transfer. 3.2.2. Effect of the Addition of Water. A small amount of water is always found in the medium-grade bioethanol. Purifying the ethanol involves additional cost. Therefore, with the intention of using medium-grade ethanol as the hydrogen source, the effect of water on the partial oxidation of ethanol is examined in this section. The influence of the water content of ethanol on the formation of hydrogen and methane is illustrated in Figure 9. A small fraction of water (2−4 vol %) enhanced the production of both hydrogen and methane. Different theories regarding the effect of steam have been proposed in the literature. In the presence of ceria on noble metal catalysts, steam promoted the water−gas shift activity, which resulted in an increase in the reaction temperature and hydrogen production via the catalytic

given in Figure 7d. The selectivity was strongly correlated with the operating temperature. Houtman and Barteau49 found that the selectivity for methane was dependent on the initial coverage of the catalyst with acetaldehyde and that methane was generated with a selectivity of 50% when the first layer of the catalyst was saturated with acetaldehyde. The temperature profiles inside the catalyst for different ethanol contents and oxygen-to-ethanol molar ratios are illustrated in Figure 8. Hot spots, which are the result of the highly exothermic complete oxidation reaction, can be clearly observed at high ethanol concentrations and oxygen-to-ethanol molar ratios. The hot spot position tended to move toward the entrance of the catalyst as the amounts of ethanol and oxygen were increased. First, a high rate of adsorption occurs because of the high concentration of the reactants. Subsequently, complete oxidation predominates until the limited quantity of oxygen is entirely consumed. Normally, a catalytic reaction cycle consists of three consecutive steps: adsorption of the reactants, surface reactions, and desorption of the products. Therefore, the downward shift of the hot spot toward the exit of the catalyst at low reactant concentrations suggests that the adsorption of oxygen or ethanol was the rate-controlling step of the complete oxidation process. The hot spot was located behind the oxidation zone because of the heat transport 6750

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ethanol conversion and hydrogen selectivity but slightly reduced methane selectivity.55 The effect of the addition of a small amount of water to the ethanol on the selectivity of hydrogen and methane in this work is presented in Figure 10. The water clearly had a promotion effect on the hydrogen selectivity, especially at an ethanol content of 8%. However, at an ethanol concentration of 10%, the hydrogen selectivity was slightly reduced by the addition of water. As shown in Figure 11, the maximum temperature at 10% ethanol content was lower than 850 °C, whereas it was higher than 850 °C for 8% ethanol (because of the higher oxygen-to-ethanol molar ratio). This indicated that the addition of water promoted the steam reforming of ethanol at temperatures higher than 850 °C. Fascinatingly, the addition of a small amount of water affected the selectivity toward methane slightly. The impact of water content on the catalytic ethanol reforming is illustrated via the temperature profiles along the inside of the catalyst, which are shown in Figure 11. The position of the hot spot tended to shift downward as the amount of water was increased. With the addition of water, some of the active sites were competitively occupied by the added steam, leading to a lower availability of active sites for the adsorption of other reactants (e.g., ethanol and oxygen); therefore, a longer length of the catalyst was required to compensate for the reforming process. The temperature after the hot spot dropped sharply at an oxygen-to-ethanol molar ratio of 1.6 (Figure 11a) compared to that at a ratio of 1.8 (Figure 11b). Because of the lower availability of the oxidizer (oxygen) at the lower oxygen-to-ethanol molar ratio of 1.6, more ethanol remained for endothermic steam reforming in the consecutive reforming zone; therefore, a greater decline in the temperature was observed. However, as was previously discussed, at higher oxygen-to-ethanol molar ratios, higher operating temperatures occurred, resulting in greater hydrogen production (Figure 9) and selectivity (Figure 10). At a relatively high ethanol concentration of 10% (Figure 11c), the addition of water had a small effect on the temperature profile and the product distribution (Figure 9) and selectivity (Figure 10). Interestingly, comparing the results for 8% ethanol and an oxygen-to-ethanol molar ratio of 1.6 (Figure 11a) with those for 10% ethanol and an oxygen-to-ethanol molar ratio of 1.4 (Figure 11c), these two samples exhibited a similar maximum temperature (Figure 5) and hydrogen production (Figure 6a) in the absence of water. However, the higher ethanol fraction was less sensitive to the addition of water. As less oxygen was present in this system, a larger quantity of ethanol remained to be transformed into hydrogen via the slow steam reforming reaction. A greater amount of hydrogen production could possibly be obtained if the process was carried out in a longer catalyst.

Figure 11. Effect of the water content on the temperature profile inside the catalyst at different ethanol concentrations and oxygen-toethanol molar ratios: (a) ethanol content of 8% and oxygen-toethanol molar ratio of 1.6; (b) ethanol content of 8% and oxygen-toethanol molar ratio of 1.8; and (c) ethanol content of 10% and oxygen-to-ethanol molar ratio of 1.4 for water contents of 10% (−), 8% (◇), 4% (△), 2% (+), and 0% (◯).

4. CONCLUSIONS The catalytic partial oxidation of ethanol on a monolithic platinum−rhodium catalyst with an alumina support was investigated at different ethanol contents and oxygen-toethanol molar ratios. An understanding of the chemical processes occurring during ethanol reforming was obtained via thermodynamic analysis and measurements of the gases produced and the temperature profiles inside the catalyst. The oxygen-to-ethanol molar ratio plays a crucial role in determining the reaction temperature and radically affects the conversion of ethanol, formation of hydrogen, and selectivity toward hydrogen. For a given ethanol concentration,

partial oxidation of ethanol.58 In the catalytic partial oxidation of methane on a rhodium catalyst, the addition of water affected the product distribution through the water−gas shift, and its effect on the steam reforming was negligible; the rate of the steam reforming reaction was zero order with respect to the amount of water added.30 In the catalytic oxidative steam reforming of bioethanol, increased water content was reported to result in improved 6751

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Toroidal Vortex Structures. Int. J. Hydrogen Energy 2012, 37, 16201−16213. (4) Di Sarli, V.; Di Benedetto, A. Effects of Non-Equidiffusion on Unsteady Propagation of Hydrogen-Enriched Methane/air Premixed Flames. Int. J. Hydrogen Energy 2013, 38, 7510−7518. (5) Satokawa, S.; Shibata, J.; Shimizu, K.-i.; Satsuma, A.; Hattori, T. Promotion Effect of H2 on the Low Temperature Activity of the Selective Reduction of NO by Light Hydrocarbons over Ag/Al2O3. Appl. Catal., B 2003, 42, 179−186. (6) Shibata, J.; Shimizu, K.-i.; Satokawa, S.; Satsuma, A.; Hattori, T. Promotion Effect of Hydrogen on Surface Steps in SCR of NO by Propane over Alumina-Based Silver Catalyst as Examined by Transient FT-IR. Phys. Chem. Chem. Phys. 2003, 5, 2154−2160. (7) Sazama, P.; Capek, L.; Drobna, H.; Sobalik, Z.; Dedecek, J.; Arve, K.; Wichterlova, B. Enhancement of Decane-SCR-NOx over Ag/alumina by Hydrogen. Reaction Kinetics and in Situ FTIR and UV−vis Study. J. Catal. 2005, 232, 302−317. (8) Houel, V.; Millington, P.; Rajaram, R.; Tsolakis, A. Promoting Functions of H2 in Diesel-SCR over Silver Catalysts. Appl. Catal., B 2007, 77, 29−34. (9) Shimizu, K.-i.; Satsuma, A.; Hattori, T. Catalytic Performance of Ag-Al2O3 Catalyst for the Selective Catalytic Reduction of NO by Higher Hydrocarbons. Appl. Catal., B 2000, 25, 239−247. (10) Creaser, D.; Kannisto, H.; Sjöblom, J.; Ingelsten, H. H. Kinetic Modeling of Selective Catalytic Reduction of NOx with Octane over Ag-Al2O3. Appl. Catal., B 2009, 90, 18−28. (11) Herreros, J. M.; Gill, S. S.; Lefort, I.; Tsolakis, A.; Millington, P.; Moss, E. Enhancing the Low Temperature Oxidation Performance over a Pt and a Pt−Pd Diesel Oxidation Catalyst. Appl. Catal., B 2014, 147, 835−841. (12) Azis, M. M.; Auvray, X.; Olsson, L.; Creaser, D. Evaluation of H2 Effect on NO Oxidation over a Diesel Oxidation Catalyst. Appl. Catal., B 2015, 179, 542−550. (13) Wang, W.; Herreros, J. M.; Tsolakis, A.; York, A. P. E. Increased NO2 Concentration in the Diesel Engine Exhaust for Improved Ag/ Al2O3 Catalyst NH3-SCR Activity. Chem. Eng. J. 2015, 270, 582−589. (14) Theinnoi, K.; Gill, S. S.; Tsolakis, A.; York, A. P. E.; Megaritis, A.; Harrison, R. M. Diesel Particulate Filter Regeneration Strategies: Study of Hydrogen Addition on Biodiesel Fuelled Engines. Energy Fuels 2012, 26, 1192−1201. (15) Di Sarli, V.; Landi, G.; Lisi, L.; Saliva, A.; Di Benedetto, A. Catalytic Diesel Particulate Filters with Highly Dispersed Ceria: Effect of the Soot-Catalyst Contact on the Regeneration Performance. Appl. Catal., B 2016, 197, 116−124. (16) Tsolakis, A.; Megaritis, A.; Wyszynski, M. L. Application of Exhaust Gas Fuel Reforming in Compression Ignition Engines Fueled by Diesel and Biodiesel Fuel Mixtures. Energy Fuels 2003, 17, 1464− 1473. (17) Tsolakis, A.; Megaritis, A.; Wyszynski, M. L. Low Temperature Exhaust Gas Fuel Reforming of Diesel Fuel. Fuel 2004, 83, 1837− 1845. (18) Abu-Jrai, A.; Rodríguez-Fernández, J.; Tsolakis, A.; Megaritis, A.; Theinnoi, K.; Cracknell, R. F.; Clark, R. H. Performance, Combustion and Emissions of a Diesel Engine Operated with Reformed EGR. Comparison of Diesel and GTL Fuelling. Fuel 2009, 88, 1031−1041. (19) Ishida, M.; Yamamoto, S.; Ueki, H.; Sakaguchi, D. Remarkable improvement of NOx−PM trade-off in a diesel engine by means of bioethanol and EGR. Energy 2010, 35, 4572−4581. (20) Banerjee, R.; Roy, S.; Bose, P. K. Hydrogen-EGR Synergy as a Promising Pathway to Meet the PM−NOx−BSFC Trade-off Contingencies of the Diesel Engine: A Comprehensive Review. Int. J. Hydrogen Energy 2015, 40, 12824−12847. (21) Fennell, D.; Herreros, J.; Tsolakis, A. Improving Gasoline Direct Injection (GDI) Engine Efficiency and Emissions with Hydrogen from Exhaust Gas Fuel Reforming. Int. J. Hydrogen Energy 2014, 39, 5153−5162. (22) Bogarra, M.; Herreros, J. M.; Tsolakis, A.; York, A. P. E.; Millington, P. J. Study of Particulate Matter and Gaseous Emissions in

the equilibrium (maximum) hydrogen production was observed experimentally at an oxygen-to-ethanol molar ratio higher than that estimated using the thermodynamic calculations. At operating temperatures above 700 °C, the water−gas shift reaction is not spontaneous; thus, the hydrogen selectivity depends only on the oxygen-to-ethanol molar ratio and converges to a similar value regardless of the ethanol concentration. Compared to hydrogen, the amount of carbon monoxide produced is relatively low because it is consumed via the water−gas shift reaction at temperatures lower than 700 °C and is oxidized by oxygen to form carbon dioxide at temperatures higher than 700 °C. The selectivity toward methane depends directly on the operating temperature. The hot spot position is controlled by the adsorption rate of ethanol and oxygen. The hot spot moves backward toward the catalyst entrance when the ethanol and oxygen contents are increased. The temperature profiles along the catalyst confirm the existence of an exothermic oxidation zone followed by an endothermic steam reforming region. The addition of water to the ethanol (2−10 vol %) promotes hydrogen formation and selectivity and has a small effect on methane formation and selectivity. On the one hand, oxygen plays an important role as the heat and water provider. On the other hand, it consumes ethanol and creates side products other than the desired product, hydrogen. Therefore, optimization of the amount of oxygen provided is necessary in future works in order to optimize hydrogen production and avoid overly high hot spot temperatures. The results obtained in this work highlight the opportunity to design a compact onboard hydrogen production system to produce hydrogen from the medium-grade ethanol.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.9b01398. Catalytic partial oxidation of ethanol (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

B. Sawatmongkhon: 0000-0001-8972-4387 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the College of Industrial Technology, King Mongkut’s University of Technology North Bangkok (grant no. Res-CIT0219/2018).

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REFERENCES

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