Ce0. 5Zr0. 33M0

Article pubs.acs.org/EF

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 Protyai Sengupta, Ataullah Khan, Md. Abu Zahid, Hussameldin Ibrahim, and Raphael Idem* Process Systems Engineering, Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan S4S 0A2, Canada ABSTRACT: A portfolio of nickel-based catalysts with nominal composition 5Ni/Ce0.5Zr0.33M0.17O2‑δ [where M is the promoter element(s) selected from Mg, Ca, Y, La, CaMg, or Gd] was prepared and examined for their catalyst activity for the steam reforming of an equimolar liquid mixture of six oxygenated hydrocarbons (ethanol, 1-propanol, 1-butanol, lactic acid, ethylene glycol, and glycerol) at different temperatures in the range of 500−700 °C at atmospheric pressure in a packed-bed tubular reactor (PBTR). The Ce0.5Zr0.33M0.17O2‑δ supports were prepared by a surfactant-assisted route. Nickel was impregnated over the supports by the wet-impregnation method. The physicochemical and textural characteristics of the catalysts were evaluated by means of various characterization techniques. Among the portfolio of catalysts evaluated, the ones containing Ca, CaMg, Mg, or Gd promoter exhibited steady activity at all of the temperatures evaluated. To deconvolute the thermal effect from the catalytic effect, a number of thermal noncatalytic experiments were conducted in the absence of any catalyst at different temperatures (450−700 °C). Correlations have also been established between the catalytic performance and catalyst characteristics to identify the parameters that influence the catalytic behavior, which could aid in catalyst improvement. It was found that catalytic activity increased with an increase in the active metal reducibility, ratio of pore volume/surface area (PV/SA), and active metal dispersion but decreased with an increase in the carbon propensity factor (CPF).

1. INTRODUCTION Hydrogen chemically combines with virtually all elements. This renders hydrogen as one of the most used chemicals in a wide range of industrial applications.1 Hydrogen can be produced from nonrenewable fossil sources, such as natural gas, naphtha, coal, etc., and renewable sources, such as biomass, bio-oil, biogas, etc.2 However, if a global cycle of clean and sustainable production of energy is envisaged, a novel eco-friendly reservoir of hydrogen is needed. In this context, oxygenated hydrocarbons (biomass derived) satisfy most of these requirements because these are easy to produce, safe to handle, transport, and store, and commonly available as industrial byproducts,3,4 such as bioethanol,5 glycerol,5 water-soluble fraction of bio-oil,6 etc. Among the various reformation technologies, steam reforming is the most viable choice for hydrogen production from oxygenated hydrocarbons, because oxygenated hydrocarbonbased feedstocks inherently contain large quantities of water.7,8 Steam reforming of oxygenated hydrocarbons has been widely investigated on supported monometallic Pd,9 Rh,9−12 Ru,9,13−16 Ir,5 Pt,4,9,11,17 and Ni4,7,18−31 catalytic systems. According to the literature, noble metal catalysts, such as Pt, Pd, Ru, and Rh, are active and stable for steam reforming, but cost is a major drawback for their use in industrial applications. For this reason, Ni-based catalysts are becoming the most lucrative catalysts for steam reforming at industrial scale.32 Nickel has been deposited on various materials, such as MgO,28 MgO− CeO2,29 Al2O3,19,23 Mg-, Zr-, Ce-, and La-modified Al2O3,30 Laand Ce-modified Al2O3,31 La2O3,19 Y2O3,19 etc. A number of catalyst preparation methods, such as co-precipitation, precipitation, impregnation, or sol−gel methods, are conventionally used for catalyst preparation, and some of these methods yield © 2012 American Chemical Society

catalysts, which perform reasonably well for the steam reforming of ethanol and glycerol. However, according to a recent study by Palmeri et al.,33 different oxygenated hydrocarbons with different functional groups behave differently in a steam-reforming process over a commercial Ni/Al2O3 catalyst. On the basis of their study, it was found that coke formation is more pronounced for compounds with hydroxyl groups than carbonyl compounds. Furthermore, the branched hydrocarbons were found to rapidly deactivate the catalyst.33 Finally, it was also concluded that obtaining a working catalyst for the steam reforming of multiple oxygenated hydrocarbon components was difficult.33 There are limited studies found in the literature on catalytic steam reforming of oxygenated hydrocarbon mixtures. This work aims to develop a working catalyst for the steam reforming of multiple oxygenated hydrocarbon components. Currently, the CexZr1−xO2 system has been considered as a favorable support material for the Ni-based catalyst.34−43 The addition of ZrO2 to CeO2 has been found to improve the oxygen storage capacity, redox property, thermal stability, and catalytic activity.34−37 According to Diagne et al.,34 the Zr content positively affects the structure and redox properties of the ceria−zirconia mixed oxides. CexZr1−xO2 can be prepared by various methods, including urea hydrolysis,38 co-precipitation,39 incipient wetness impregnation,39 sol−gel techniques,40 and the surfactant-assisted method.41 Among these methods, the surfactant-assisted method has attracted considReceived: November 24, 2011 Revised: January 6, 2012 Published: January 11, 2012 816

dx.doi.org/10.1021/ef201854t | Energy Fuels 2012, 26, 816−828

Energy & Fuels

Article

(NO3)2·6H2O salts were dissolved in DI water and hydrolyzed with aqueous ammonia until precipitation was complete. The resulting precipitate was filtered, oven-dried at 120 °C overnight, and calcined at 650 °C for 3 h in an air environment. A nominal 5 wt % Ni was loaded over the prepared CZM and CZCaMg supports using a standard wet impregnation method to obtain the final 5N/CZM and 5N/CZCaMg catalysts. In a typical impregnation, a calculated amount of catalyst support was immersed in a predetermined volume of 0.1 M Ni(NO3)2 solution in a roundbottomed flask. The mixture was subjected to slow heating in a silicon oil bath at 80 °C under continuous stirring to remove excess water. The dried powders thus obtained were calcined at 650 °C in an air environment for 3 h. For another parametric study, 10 wt % Ni was loaded over the CZMg support by following the above detailed procedure to obtain the 10N/CZMg catalyst. All of the catalysts reported in this study were prepared by analogous procedures to allow for a direct comparison of their catalytic properties. 2.2.1. Catalyst Characterization. Surface Area and Pore Size Analysis. The Brunauer−Emmett−Teller (BET) specific surface area, pore volume, and average pore size analyses for all catalysts were obtained by N2 physisorption at a liquid N2 temperature using a Micromeritics ASAP 2010 apparatus. Prior to analysis, all of the samples were degassed for 4 h at 180 °C under ultrahigh vacuum. Some of the samples were analyzed at least 3 times to establish repeatability. The observed percent deviation in these measurements was ≤ ±5%. 2.2.2. Metallic Surface Area and Metal Dispersion Measurements. The metallic surface area and metal dispersion in the catalyst samples were estimated by hydrogen chemisorption at 35 °C using a Micromeritics ASAP 2010 instrument.43−45 Prior to analysis, the catalyst samples were dried at 120 °C and then reduced in situ in flowing H2 gas [ultrahigh power (UHP) grade] at 700 °C for 2 h (to obtain the reduced state formed during the course of a typical catalytic run), followed by evacuation at 700 °C for 1 h before cooling to 35 °C (analysis temperature). A vast majority of researchers working on the development of Ni-based commercial catalysts have employed 25−35 °C (room temperature) for H2 chemisorption experiments. The reasons for employing ambient temperature (35 °C) in H 2 chemisorption analysis are that (1) there is usually little or no physically adsorbed gas remaining and little spill over, (2) between 273 and 323 K, the isobar is comparatively flat, so that close control of the temperature is unnecessary, and (3) the common assumption that H/ Nis is about unity under these conditions is quite well-justified.45 The nickel surface area (SNi) and dispersion (DNi) were calculated as per the procedure published elsewhere.43−45 The total nickel amount in the catalyst was measured by inductively coupled plasma−mass spectrometry (ICP−MS) analysis used in calculating DNi. The H2 chemisorption analysis was repeated for a few of the samples to check reproducibility. The percent deviation in these measurements was < ±2%. 2.2.3. Temperature-Programmed Reduction (TPR) Measurements. The H2−TPR measurements of various support and catalyst samples were performed on a Quantachrome ChemBET 3000 unit equipped with a thermal conductivity detector (TCD). Prior to TPR measurements, 50 mg of samples was degassed at 180 °C in an inert atmosphere (N2 UHP grade) for 2 h. The reducibility of the supports and that of the catalysts prepared in the current study were studied using the TPR technique 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). The total reactive gas consumed during TPR analysis was measured. The H2 uptake as a function of TCD response versus temperature was used to plot the TPR profile. For reference purposes, the TPR profiles of pristine NiO and CeO2 were also studied. A few samples were analyzed using TPR at least twice to establish reproducibility. The deviation in the Tmax values was found to be less than ±5 °C. 2.2.4. Temperature-Programmed Oxidiation (TPO) Measurements. The O2−TPO analyses of all of the used catalyst samples were performed on a Quantachrome ChemBET 3000 unit equipped with a TCD. In a typical TPO experiment, 150 mg of used catalyst

erable interest because of the effective soft template effect, reproducibility, and simple maneuverability.41 As indicated by Idem et al.,42 this method can be used to prepare solid solutions with high specific surface area and thermal stability, which favors the application of the solid solutions at high temperatures. However, because of the inherent hydrophilic nature, the CexZr1−xO2 system is prone to deactivation in the presence of steam.43 This problem can be overcome by the incorporation of a promoter element “M” into the Ce−Zr lattice system. In the present study, the Ni-based catalysts supported on Ce0.5Zr0.33M0.17O2‑δ (where, M = Mg, Ca, Y, La, Gd, and CaMg) supports prepared using the surfactant-assisted method were screened for steam reforming of the oxygenated hydrocarbon mixture. To optimize surfactant use during the course of support preparation and to reduce the generation of chemical wastes, a separate study was undertaken, where the Ce0.5Zr0.33Mg0.17O2‑δ support was prepared by varying the surfactant/metal (S/M) molar ratios from 0 to 1.25. Finally, the effect of active metal loading on the resultant catalytic activity was evaluated by preparing two formulations of Ce0.5Zr0.33Mg0.17O2‑δ with 5 and 10% Ni loading, respectively. The results of the above-described investigations are presented and discussed in this paper.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Idem et al. reported the use of different kinds of surfactants, such as cationic, anionic, amphoteric, and non-ionic (oligomeric) surfactants, for the preparation of ceria-based mixed oxides.42 From the above study, it was established that cationic surfactants, particularly, cetyl trimethyl ammonium bromide (CTAB), yields better ceria-based mixed oxides of high surface area and high thermal stability.42 Therefore, the CTAB surfactant was used all through subsequent work to prepare the various ceria-based ternary and quaternary mixed oxide supports by the surfactant-assisted route. Ternary oxide supports were prepared using a surfactant-assisted method under basic conditions. The nominal compositions of each support were Ce0.5Zr0.33M0.17O2‑δ (CZM), where M is a transition, non-transition, or inner-transition metal ion, such as Ca, Gd, La, Mg, or Y. Nitrate precursors were employed to prepare all of the above catalysts. In a typical batch preparation, calculated amounts of nitrate precursors of various metal ions were dissolved separately in 500 mL of deionized (DI) water under rigorous stirring and were subsequently mixed together. In a separate beaker, a predetermined amount of surfactant (CTAB) was dissolved in 1000 mL of DI water at 60 °C under rigorous stirring. The above two solutions were mixed together to obtain a resultant mixture solution. The molar ratio of S/M (Ce + Zr + M) was kept constant at 1.25. Aqueous ammonia (25 vol %) was gradually added to the aforementioned mixture solutions under vigorous stirring at 60 °C until precipitation was complete (pH 11.8). The addition of ammonia induced the precipitation of gelatinous yellow−brown colloidal slurry. The slurry was then transferred into Pyrex glass bottles, sealed, and aged hydrothermally in an air-circulated oven for 5 days at 90 °C. The mixture was then cooled, and the resulting precipitate was filtered and washed repeatedly with warm DI water. The resulting cakes were oven-dried at 120 °C for 12 h and finally calcined at 650 °C for 3 h in an air environment. The quaternary mixed oxide support Ce0.5Zr0.33Ca0.08Mg0.08O2‑δ(CZCaMg) was also prepared using the same procedure as described above, keeping the S/M (Ce + Zr + Ca + Mg) molar ratio constant at 1.25. To optimize the S/M molar ratio, a parametric study was undertaken, wherein three other Ce0.5Zr0.33Mg0.17O2‑δ supports were prepared using the same method described above but using S/M (Ce + Zr + Mg) molar ratios of 0.3, 0.5, and, 0.8. The Ce0.5Zr0.33Mg0.17O2‑δ mixed oxide support was also prepared by the conventional co-precipitation method, where the requisite quantities of Ce(NO 3 ) 3 ·6H 2 O, ZrO(NO 3 ) 2 ·H 2 O, and Mg817


Energy & Fuels

Article

The oxygenated hydrocarbon mixture conversion and H2 selectivity are defined as follows, with all of the above calculated in units of mol %:

samples was recovered and analyzed. The carbon propensity factors (CPFs, amount of carbon accumulated in milligram per gram of catalyst per hour) were studied using the TPO technique in a temperature range from ambient to 1050 °C at a heating rate of 15 °C/min using 5% O2/balance He as the reactive gas (flow rate = 30 mL/min). The gas production as a function of TCD response versus temperature was used to plot the TPO profile. To determine the carbon propensity, the areas under the TPO profiles was integrated. A few samples were analyzed using TPO at least twice to establish reproducibility. The experimental error in these measurements was ≤ ±2%. 2.3. Activity Evaluation. Activity evaluation studies were carried out in a packed-bed tubular reactor (PBTR) (1/2 in. inner diameter) made of Inconel 625. The reactor was placed vertically inside a programmable tubular furnace, which was heated electrically. The catalyst bed temperature was measured by means of a sliding thermocouple dipped inside the catalyst bed, the tip of which was placed in the center of the bed. In a typical reaction, 250 mg of the catalyst particles (0.78 mm) was mixed with 7.6 g of α-alumina beads of the same size to make a catalyst bed height of 4.5 cm. Prior to each run, the catalyst was reduced at 700 °C for 2 h using a gas mixture of 5% H2/balance N2 (100 mL/min). All of the gases were regulated through precalibrated mass flow controllers with a digital readout unit. The feed was composed of an equimolar mixture of ethanol, 1propanol, 1-butanol, lactic acid, ethylene glycol, and glycerol. Two times the stoichiometric amount of water (as per eq. 1)was used in the current work to prevent undesired catalyst coking. The steam/carbon ratio was 2.8. The feed flow rate was regulated at 100 μL/min through a motorized syringe pump to maintain the steam/carbon ratio of 2.8. The activity evaluation tests were performed at four different temperatures, namely, 500, 550, 600, and 700 °C. To approach plug-flow conditions and minimize back mixing and channeling, certain operating criteria, namely, ratio of the catalyst bed height/ catalyst particle size (L/Dp) = 56 and ratio of the inside diameter of the reactor to particle size (D/Dp) = 16, were used. The product reformate stream coming from the reactor was passed through a double-pipe heat exchanger, where the product mixture was passed inside the inner tube, while water, at 16 °C, was passed through the outer tube. An ice-cooled glass separator was used to condense the liquid product. The product gases were analyzed with an online gas chromatograph (Agilent 6890 N, Canada) equipped with a TCD and Hayesep Q and Molecular Sieve A columns. To deduce the actual catalytic effect (exclusive of the thermal effect) of any given catalyst, properly planned thermal (catalyst-free) runs were performed at different temperatures (450, 500, 550, 600, and 700 °C) on the same experimental setup under identical operating conditions (as detailed above), including residence time, alumina bed height, feed flow rate, and gas hourly space velocity. 2.4. Activity Evaluation Criteria. The steam-reforming process of an equimolar mixture of the six oxygenated hydrocarbon components used as feed in the current work (Table 1) can be represented by the following overall equation:

conversion =

H2 selectivity =

vol %

mol %

4.542 5.820 7.115 5.842 4.335 5.734 66.611

1.87 1.87 1.87 1.87 1.87 1.87 88.78

(2)

F(H2 actual) × 100 conversion × F(H2 theoretical)

(mol %)

(3) −1

Turnover frequencies with regard to H2 produced (TOFH2) (s ) are defined as the ratio of the number of H2 molecules produced per second to the total number of catalytically active Ni atoms present in the reactor. Herein, it is assumed that all surface nickel atoms accessible by the H2 chemisorption technique are catalytically active.

TOFH2 = (number of molecules of H 2 produced per second) /(number of surface Ni atoms present in the reactor (PBTR)) (s−1)

(4)

3. RESULTS AND DISCUSSION 3.1. Catalyst Characteristics. The textural characteristics of the supports as well as that of the catalysts prepared in this study are summarized in Table 2. The specific surface area, specific pore volume, and average pore size of the various CZM supports and 5N/CZM catalysts (where M = Ca, CaMg, Gd, La, Mg, and Y) were estimated using the N2 physisorption technique. From Table 2, it is found that the CZMg support prepared by the co-precipitation method (S/M = 0) shows a specific surface area of 90.5 m2/g. Interestingly, all other support samples prepared by the surfactant-assisted method with various S/M ratios exhibit reasonably high specific surface areas (>127 m2/g). From the results, it is clear that the use of the surfactant during the course of the catalyst preparation leads to significant improvement in the specific surface area. The average pore diameter varied in the range of 43−84 Å (mesopore sized), and the cumulative specific pore volume varied from 0.15 to 0.5 cm3/g of catalyst. The development of higher specific surface areas can be attributed to the method of preparation adapted in the current work, as detailed in the earlier publications.41−43 In a typical surface-assisted preparation under highly basic medium, usually at pH > 11.0, the surface hydroxyl protons [CeZrM(O−−H+)4] are exchanged with the cetyltrimethylammonium cation [(C16H33)N+(CH3)3], resulting in the incorporation of the surfactant cations into a hydrous ternary mixed hydroxide gel. This incorporation decreases the interfacial energy and eventually decreases the surface tension of water that exists in the hydrous support pores. As a result, the degree of shrinkage and pore collapse that would occur in the hydrous support during drying and calcination is reduced, thereby imparting a high specific surface area to the sample.43 The same argument is equally applicable to the quaternary mixed oxide preparation. Upon impregnation of a nominal 5−10 wt % Ni over the surface of supports, the specific surface areas and cumulative specific pore volume decreased. This is a general phenomenon observed in the case of supported catalysts when an active component is

Table 1. Oxygenated Hydrocarbon (Oxy-HC) Mixture Feed Composition ethanol 1-propanol 1-butanol lactic acid ethylene glycol glycerol water

× 100

Here, the C-1 components that are found in the reformate gaseous effluent are CO2 and CO, where F is the molar flow (mol/time). The hydrogen selectivity is defined as follows:

(1)

feed components

F(C atoms in feed in)

(mol %)

C2.8H7.3O1.8 + 3.8H2O = 2.8CO2 + 7.5H2 ΔH298 = 180.1 kJ/mol

F(C atoms in C‐1 componentsout)

818


Energy & Fuels

Article

Table 2. Textural Characterization catalyst (xN/ CZM)

BET surface area (m2/g)

pore volume (cm3/g)

average pore size (Å)

5N/CZCa 5N/CZCaMg 5N/CZGd 5N/CZLa 5N/CZMg 5N/CZY 10N/CZMg

108.8 133.0 124.5 141.0 124.7 173.6 109.0

0.22 0.26 0.2 0.27 0.29 0.25 0.25

61.3 56.4 49.3 61.1 71.2 43.5 71.3

CZCa CZCaMg CZGd CZLa CZMg CZY

127.5 150.7 146.8 156.0 146.4 188.2

0.28 0.31 0.24 0.35 0.37 0.5

60.3 58.8 48.7 65.9 78.0 83.1

S/M S/M S/M S/M S/M

= = = = =

0 0.3 0.5 0.8 1.25

67.4 116.3 119.6 123.2 124.7

0.16 0.16 0.22 0.2 0.29

69.1 43.0 55.9 48.1 71.2

S/M S/M S/M S/M S/M

= = = = =

0 0.3 0.5 0.8 1.25

90.5 134.2 139.7 146.4 146.4

0.18 0.22 0.27 0.26 0.37

66.8 46.8 56.7 50.8 78.0

pore volume/ surface area (Å)

Ni surface area (m2/g)

Ni dispersion (%)

Catalyst: xN/CZM, S/M = 1.25 20.1 2.1 8.5 19.6 1.6 5.5 16.1 1.7 6.6 19.0 1.5 5.7 23.0 2.9 12.5 14.4 1.9 9.3 22.7 3.3 4.9 Support: CZM, S/M = 1.25 21.7 20.5 16.3 22.3 25.1 26.6 Catalyst: 5N/CZMg, Different S/M Ratios 23.6 1.0 3.6 14.1 3.0 11.2 18.7 2.8 10.5 16.4 3.5 12.2 23.0 2.9 12.5 Support: CZMg, Different S/M Ratios 19.5 16.1 19.9 17.6 25.1

impregnated on its surface. The observed decrease is mainly due to penetration of the dispersed nickel oxide into the pores of the support. The measurements of pore volume per unit surface area can also be found in Table 2. Interestingly, the supports and corresponding catalysts possessing higher pore volume per surface area (>14.1 × 10−10 m) exhibit exceptionally good performance, as detailed in later sections. From Table 2, it is observed that the catalyst 5N/CZY has the highest specific surface area. The catalyst 5N/CZCa shows the lowest value for specific surface area. Results presented in Table 2 also show that, with the addition of the surfactant, the textural characteristics of the resulting supports and corresponding catalysts improve significantly, thus establishing the fact that the use of the surfactant (CTAB) in the preparation stage imparts excellent textural characteristics to the resultant support. Interestingly, it is noted from Table 2 that beyond a certain S/M ratio, 0.5 in the current case, the textural and physicochemical characteristics of the resulting supports and corresponding catalysts remain almost identical. From the above observation, it can be inferred that it is unnecessary to increase the S/M molar ratio beyond 0.5. Restricting the S/M ratio to 0.5 will result in reduction in the costs of catalyst manufacturing and waste disposal. The H2 chemisorption technique was employed to estimate the nickel surface area and nickel dispersion in the various catalysts; the observed findings are given in Table 2. It is important to emphasize that all of the catalyst formulations were prepared using a standard wet impregnation method. The actual mass fraction of Ni present in the catalysts was determined by the ICP−MS technique. The corresponding results are also given in Table 2. Chemisorption analysis results

Ni mass fraction (ICP−MS) (wt %)

nickel reducibility 1/Tmax (from TPR) (10−3 °C−1)

3.6 4.4 3.9 3.8 3.4 3.1

2.30 2.28 2.09 2.08 2.39 2.19 2.33

4.2 4.0 4.1 4.4 3.4

2.35 2.36 2.37 2.38 2.39

were combined with information obtained on metal loading from the ICP−MS results to calculate the metal dispersion. The relative measurement of chemically bound hydrogen was used to distinguish the various catalyst formulations investigated in the current study (Table 2). The nickel surface area and nickel dispersion in the case of 5N/CZMg prepared with a S/M ratio of 0 was found to be the lowest. Ni dispersion improves significantly with the addition of the surfactant. Moreover, it was observed that Ni dispersion and Ni surface area are a strong function of the catalyst formulation (i.e., promoter “M” dependent). The descending order of Ni dispersion was found to be 5N/CZMg > 5N/CZY > 5N/CZCa > 5N/CZGd > 5N/ CZLa > 5N/CZCaMg prepared with a S/M ratio of 1.25. On the other hand, an increase in Ni loading from 5 to 10 wt % on the CZMg support decreases the Ni dispersion significantly, as noted from Table 2. A generic profiling of the TPR peaks was performed using a representative support, “CZCaMg”, and catalyst, “5N/ CZCaMg”. These were indexed using pristine CeO2 and NiO samples, as shown in Figure 1A. Pristine NiO shows a sharp reduction peak at about 455 °C, which can be attributed to the transformation of Ni2+ to Ni0 species. In the case of pristine CeO2, the peak at the lower temperature (Tmax = 577 °C) was ascribed to the reduction of the surface oxygen species, and the other broad peak at the higher temperature (Tmax = 754 °C) was due to the reduction of bulk oxygen species.46 The higher mobility of the surface oxygen ions helps in the removal of lattice oxygen during the reduction process. The coordinately unsaturated surface capping oxygen ions can be easily removed in the low-temperature region. However, bulk oxygen needs to be transported to the surface before its reduction. Con819


Energy & Fuels

Article

Figure 1. Identification of TPR peaks and TPR pattern of titled CZMg supports and 5N/CZMg catalysts with different S/M ratios.

Figure 2. TPR patterns of titled CZM supports and 5N/CZM catalysts.

820


Energy & Fuels

Article

Figure 3. TPO patterns of titled used 5N/CZM catalysts after reaction at 500, 600, and 700 °C reaction temperature. TPO patterns of various used 5N/CZMg catalysts prepared with different S/M ratios and employed at 500 °C.

is observed that the Tmax values for the reduction of NiO to Ni decreases with the increase in the S/M ratio. A detailed analysis of the obtained results is shown in Table 2, from which it is inferred that, with an increasing S/M ratio, the reducibility of the resultant catalyst formulation improves, because reducibility is inversely related to the Tmax value. However, the increase is not significant in value in comparison to the change in reducibility because of different promoter elements. For instance, an increase of the S/M ratio from 0 to 1.25 in the case of 5N/CZMg promotes reducibility from 2.35 × 10−3 to 2.39 × 10−3 °C−1, while the change of the promoter element from La to Mg improves the reducibility of 5N/CZM (S/M = 1.25) (where M = Ca, Gd, La, Mg, and Y) from 2.08 × 10−3 to 2.39 × 10−3 °C −1. From the above results, it can be inferred that Ni reducibility changes drastically on account of a change in the promoter element, as compared to the change in the S/ M ratio. The CPF is defined as the amount of carbon formed on a unit amount of given catalyst surface per unit time under certain reaction conditions. The CPF depends upon many factors, such as the feed composition, operating variables, such as the temperature, pressure, space velocity, duration of reactor operation, and, most importantly, the catalyst formulation. Some catalysts have an inherent ability to generate carbon on their surface, while others have the ability to rapidly gasify surface carbon. Accordingly, the CPFs pertaining to various catalyst formulations, each employed at different operating conditions, were evaluated in the current work by TPO analyses in the ambient to 900 °C temperature range, using 5% O2/balance He as the reactive gas mixture. From Figure 3, it is

sequently, the bulk reduction takes place at a higher temperature compared to the surface reduction. The bulk reduction begins only after the complete reduction of the surface sites.47 According to the literature, pristine ZrO2 does not show any sign of reduction below 1000 °C, because of its refractory nature. As observed in Figure 2, the TPR profiles of the pure supports “CZM” (where M = Ca, Gd, La, Mg, and Y) exhibit broad H2 consumption peaks in the temperature range of 600−700 °C and two other peaks in the temperature range of 800−920 °C (1050 °C for La). These peaks can be attributed to the reduction of surface and bulk oxygen anions, respectively. However, all of the corresponding Ni-impregnated “5N/CZM” (where M = Ca, Gd, La, Mg, and Y) catalysts show an additional sharp peak in the temperature range of 400−500 °C because of the reduction of active metal NiO to Ni. The Tmax values observed for the reduction of NiO to Ni in the various catalyst formulations can be found in Figure 2 and Table 2. The Ni reducibility is calculated using eq 5, wherein the reducibility (1/Tmax) is found to be inversely proportional to the Tmax value obtained from the TPR experiment. Ni reducibility=1/Tmax Ni peak(TPR)

(°C−1)

(5)

Figure 2 shows a comparative analysis of the TPR profiles of all of the “5N/CZM” catalysts tested in the current study. The results reveal that the catalyst 5N/CZMg is easily reducible, while the catalyst 5N/CZLa is difficult to reduce compared to other formulations tested in the current work. The TPR profiles of various CZMg and 5N/CZMg formulations, which were prepared by varying the S/M ratio from 0 to 1.25 are compiled in panels B and C of Figure 1, respectively. From Figure 1C, it 821


Energy & Fuels

Article

clear that, at a certain operating temperature (i.e., 500, 600, and 700 °C), the TPO peaks are identical for all of the catalyst formulations evaluated, which implies that the same type of carbon species are formed on the catalyst surface. However, this occurs to varying extents. Integration of the peak areas of the TPO profiles can be used as a direct measure of coking or carbon propensity (carbon affinity). In general, there are two types of carbons formed on the catalyst surface during the course of a typical steam-reforming reaction: low-temperature amorphous filamental (whisker) carbon and high-temperature crystalline graphitic carbon. However, the same catalyst formulation evaluated at different operating temperatures exhibits different types of TPO profiles, which point to the formation of different types of carbon species. Generally, the TPO profiles of catalysts tested at 500 °C exhibit three TPO peaks. The first, sharp peak is between 425 and 550 °C; a second, broad peak occurs between 600 and 650 °C; and the third, sharp peak is between 850 and 900 °C, as shown in Figure 3A. According to Bartholomew,48 the first lowtemperature peak is due to Ni carbide formation by the amorphous carbon. The second peak, in the moderatetemperature range, is due to the presence of less reactive coke on support sites, and the high-temperature peak is due to graphite-like carbon formation.48,49 The most common carbongenerating side reactions are listed below. ΔH973 = −171 kJ/mol

2CO = CO2 + C(s) CO + H2 = H2O + C(s)

Table 3. CPFs of 5N/CZM Catalysts at Different Reaction Temperatures catalyst

500 °C

5N/CZCa 5N/CZCaMg 5N/CZGd 5N/CZLa 5N/CZMg 10N/CZMg 5N/CZY

37.5 40.2 44.8

550 °C

600 °C

700 °C

24.6

7.4

26.2 35.8 18.0

7.0 13.0 4.0 9.0 4.0

in the S/M ratio beyond 0.5 does not lead to any significant improvement in the decoking ability of the 5N/CZMg catalyst. From Table 3, it can also be noted that CPFs are strongly related and dependent upon the operating variables, followed by the type of promoter element “M” employed. The descending order of CPF as a function of the promoter element “M” is as follows: La > Y > Gd > Ca > Mg. At low temperatures, the CPF is high. The CPF drastically decreases with an increase in the temperature. For example, for 5N/ CZMg, when the reaction temperature is changed from 500 to 700 °C, the CPF reduces from 30.7 to 4.0 mg of C (g of catalyst)−1 h−1. From the above observations, it can be concluded that Mg is the most coke-tolerant promoter among the other promoter elements tested in the current study. 3.2.1. Catalytic Activity Evaluation. Effect of the Promoter Element on the Catalytic Activity. The performance of the 5N/CZM (M = Ca, Gd, La, Mg, and Y) catalysts developed in the current study was evaluated by comparing their catalytic activities under identical operating conditions in a PBTR. Initially, the activity was evaluated at 700 °C, and the corresponding results are presented in panels A, D, and G of Figure 4. As noted from the above panels, all of the catalysts exhibit steady activity with >92 mol % oxy-HC conversion, >82 mol % H2 selectivity, and >2.26 s−1 TOF. Among the five catalysts tested, 5N/CZMg shows the highest value for oxygenated hydrocarbon conversion (98 mol %), while 5N/ CZLa shows the lowest conversion (92 mol %). On the other hand, 5N/CZLa shows the highest TOF value (4.16 s−1), while 5N/CZMg shows the lowest TOF (2.26 s−1). As indicated earlier, the degree of Ni dispersion is promoter-dependent. A lower Ni dispersion (5.7%) in the 5N/CZLa catalyst results in lowering the population of surface Ni sites available for reaction, which in turn results in an increase in the TOF value. A converse argument would apply in the case of the 5N/CZMg catalyst with the highest Ni dispersion (12.5%) and exhibiting the lowest TOF value. Because all of the catalysts evaluated at 700 °C showed steady and identical activities, these were further evaluated at 600 °C and the corresponding results are shown in panels B, E, and H of Figure 4. Among the five catalysts screened, four catalysts, namely, 5N/CZM (M = Ca, Gd, Mg, and Y) exhibit stable performance with >85 mol % oxy-HC conversion, >80 mol % H2 selectivity, and >2.18 s−1 TOF. Among the four

(6)

ΔH973 = −136 kJ/mol

ΔH973 = 89 kJ/mol

5NCZM, S/M = 1.25

30.7 25.8 49.9 47.7 27.8 5N/CZMg with Different S/M Ratios S/M = 1.25 30.7 25.8 18.0 S/M = 0.8 31.3 S/M = 0.5 31.5 S/M = 0.3 40.2 S/M = 0 42.6

(7)

CH 4 = 2H2 + C(s)

CPF (from TPO) [mg of carbon (g of catalyst)−1 h−1]

(8)

Because the first two reactions are exothermic in nature, they are more favored at lower temperatures. For instance, reaction 6 is not favored above 701 °C, and reaction 7 is not favored above 673 °C.50 At 500 °C, these reactions are more likely to occur, because significant amounts of carbon were found after the reaction. Moreover, there is potential for the formation of whisker-like carbon above 450 °C, which lowers the activity of the catalyst.48 The ceria−zirconia-based binary oxides are known to reduce coke formation.37 From Figure 3B, it is clear that the TPO profiles after the reaction at 600 °C demonstrate a sharp peak between 600 and 700 °C and a few other less prominent peaks at higher temperatures. Similarly, the catalysts, after 6.5 h of reaction at 700 °C, exhibited a sharp TPO peak and a second, broad peak between 700 and 750 °C, which are visible in Figure 3C. These peaks belong to less reactive graphite-like carbon that converts from the amorphous phase to graphite-like phase at higher temperatures. These observations indicate that different kinds of carbon are formed at different temperatures, and the TPO profiles differ slightly for different catalysts employed at the same operating temperature. In the case of 5N/CZMg catalysts prepared with different S/M ratios but employed at 500 °C operating temperature, similar TPO profiles were noted, with the only exception being S/M = 1.25, which shows the first sharp peak at a relatively low temperature, as shown in Figure 3D; this could be attributed to the occurrence of a large average pore size (71.2 Å), which allows the feed molecules to come in contact with high amounts of surface Ni species, and thereby promoting Ni carbide formation.48 The CPF values obtained for the various catalysts employed at different operating temperatures are presented in Table 3. From Table 3, it is possible to generalize that a change 822


Energy & Fuels

Article

Figure 4. Performance evaluation of titled 5N/CZM catalysts for the steam-reforming reaction of mixtures of oxygenated hydrocarbons.

Figure 5. Long-term TOS stability studies on the titled 5N/CZMg catalyst at 500 °C reaction temperature.

stable catalysts, 5N/CZMg was the most active, with 94 mol %

the least active. Vice versa, 5N/CZGd showed the highest TOF

conversion and 80 mol % H2 selectivity, while 5N/CZGd was

(3.33 s−1), and 5N/CZMg showed the lowest TOF (2.18 s−1). 823


Energy & Fuels

Article

Figure 6. Identification of thermal and catalytic effects for typical steam reforming of mixtures of oxygenated hydrocarbons over the 5N/CZMg (S/ M = 1.25) catalyst at various reaction temperatures.

Figure 7. Influence of the temperature on the noncatalytic and catalytic product distribution for a typical steam reforming of mixtures of oxygenated hydrocarbons.

nature of the steam-reforming process. Figure 6 depicts the thermal and catalytic effects separately, using the results obtained for steam reforming of oxygenated hydrocarbon feed without and with catalyst 5N/CZMg (S/M = 1.25), respectively. From Figure 6, it is observed that, at 600 °C, the magnitude of the catalytic effect is at a maximum, as compared to the other temperatures (i.e., 500, 550, and 700 °C). At 600 °C, the overall conversion with catalyst 5N/CZMg was 92%, and without catalyst, it was 26%, which means that the catalyst improved the conversion 3.5-fold. At 700, 550, and 500 °C, the catalyst improved the conversions by 2.5-, 3.2-, and 2.8-fold, respectively. On the basis of the above observations, it can be concluded that 600 °C operating temperature is optimal for obtaining high feed conversions and also making maximal use of the catalytic effect. 3.2.3. Effect of the Reaction Temperature on the Product Gas Distribution. The influence of the reaction temperature on the reformate gas composition for both noncatalytic and catalytic reactions are presented in panels A and B of Figure 7, respectively. As observed from Figure 7A, the H2 concentration of the reformate gas initially increased with an increase in the reaction temperature from 450 to 550 °C and then significantly decreased with a further increase in the reaction temperature up to 700 °C. From the CO2 concentration profiles at various reaction temperatures, it can be noted that the concentration of CO2 in the reformate gas gradually increased with an increase in the reaction temperature to reach its maximum value at 550 °C, after which it dropped drastically with a further increase in the reaction temperature. On the other hand, increasing the

To test the feasibility of employing the currently developed catalyst for membrane reactor applications, four catalysts that performed well at 600 °C along with an additional 5N/ CZCaMg catalyst were further tested at 500 °C operating temperature, and the results obtained are shown in panels C, F, and I of Figure 4. At this temperature, four catalysts showed stability with more than 26 mol % oxy-HC conversion, >60 mol % H2 selectivity, and >0.66 s−1 TOF. These are 5N/CZM (M = Ca, CaMg, Gd, and Mg). The 5N/CZY catalyst experienced a steady decay with time on stream (TOS) operation. Among the four stable catalysts, 5N/CZMg showed the highest conversion (33 mol %), with 65 mol % H2 selectivity, while 5N/CZGd showed the lowest conversion (26 mol %), with 60 mol % H2 selectivity. Among the stable catalysts, 5N/CZCaMg showed the highest TOF value (0.96 s−1), while 5N/CZMg showed the lowest TOF value (0.66 s−1). An extended TOS stability study was performed on the 5N/CZMg catalyst for ∼11 h TOS at 500 °C. The corresponding results are presented in Figure 5. From the figure, it is clear that the catalyst showed steady performance in terms of oxy-HC conversion, H2 selectivity, and TOF parameters. 3.2.2. Identification of Thermal and Catalytic Effects. To ascertain the true impact of the catalyst on the reforming reaction and to distinguish the thermal effect from the catalytic effect, a systematic study was undertaken, wherein the steam reforming of the oxygenated hydrocarbon mixture was conducted at different temperatures in the absence of a catalyst. The oxygenated hydrocarbon conversion increased with an increasing temperature, because of the endothermic 824


Energy & Fuels

Article

Figure 8. Influence of Ni loading on the catalytic activity for a typical steam reforming of mixtures of oxygenated hydrocarbons.

Figure 9. Influence of the S/M ratio on the catalytic activity for a typical steam reforming of mixtures of oxygenated hydrocarbons.

reaction temperature from 450 to 550 °C resulted in a decrease in the CO concentration, as shown in Figure 7A. Surprisingly, a further increase in the reaction temperature from 550 to 700 °C resulted in a sharp increase in the CO concentration. 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) reaction equilibria. As observed from Figure 7B, the H2 concentration of the reformate gas increases with an increase in the reaction temperature. Most importantly, increasing the reaction temperature from 500 to 550 °C results in a 4 mol % increase in the H2 concentration in the reformate effluent gas; however, a further increase in the operating temperature from 550 to 700 °C does not enhance the hydrogen concentration. From the CO2 concentration profiles at various reaction temperatures, it can be noted that the concentration of CO2 in the reformate gas gradually decreased with an increase of the reaction

temperature. Increasing the reaction temperature from 500 to 700 °C results in a 9 mol % decrease in the CO2 concentration. However, the CO concentrations gradually increased with the reaction temperature. At lower temperatures, CO is converted to CO2 by a WGS reaction (CO + H2O → CO2 + H2) and additional H2 is produced. However, because the WGS reaction is exothermic in nature, 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. The results thus indicate that the 5N/CZM catalyst influences the WGS reaction equilibria. 3.2.4. Effect of Ni Loading on the Catalytic Activity. The influence of Ni loading on the steam reforming of the oxygenated hydrocarbon mixture was tested by increasing the Ni loading from 5 to 10% over the CZMg support. The reactions were carried out at 500 °C. Surprisingly, only a meager 3% increase in oxy-HC conversion was noted, while the 825


Energy & Fuels

Article

H2 selectivity and TOF remained the same for both formulations, as shown in panels A, B, and C of Figure 8, respectively. A relatively low metal surface area limits the catalytic activity at higher Ni loadings, as per the characterization results. 3.2.5. Effect of the S/M Ratio on the Catalytic Activity. A major goal of the current work was to optimize the S/M ratio, to minimize the catalyst production and catalyst waste disposal costs without having to compromise the catalytic performance. The 5N/CZMg catalyst was prepared using five different S/M ratios, namely, 0, 0.3, 0.5, 0.8, and 1.25 (Table 2), which were subsequently evaluated at 500 °C reaction temperature. All of the catalysts showed steady performance with steady oxy-HC conversion, as noted in Figure 9A, steady H2 selectivity, as presented in Figure 9B, and steady TOF, as presented in Figure 9C. From Figure 9D, it is clear that, for S/M = 0, the catalyst shows 23.5% oxy-HC conversion, which improves with an increase in the S/M molar ratio. It attains maxima (35 mol %) with S/M = 0.5. After this, a further increase in the S/M molar ratio does not lead to any improvement in the catalytic activity. However, the H2 selectivity value did not significantly change with the change in the S/M molar ratio, as represented in Figure 9E. From Figure 9F, it is observed that, upon the addition of the surfactant, the TOF value did not significantly change with the change in the S/M molar ratio. From these panels, it can be easily concluded that, for this specific application, the S/M molar ratio = 0.5 is optimal for preparing the catalyst support “CZM”. These findings lower the catalyst production and waste disposal costs significantly. 3.3. Structure−Activity Relationships (SARs). Knowledge of any possible relationships between the inherent characteristics and the resultant activity is essential to tailor design the catalyst toward the reaction of interest, in this case, steam reforming of a liquid mixture of oxygenated hydrocarbons. In this study, this was performed using various characterization techniques, such as N2 physisorption, TPR, TPO, and H2 chemisorption, to determine various catalyst characteristics, such as BET surface area, pore volume, average pore size, reducibility, CPF, and nickel dispersion. The obtained characteristics of the catalysts were correlated with the observed catalytic activity evaluated under analogous operating conditions. The resulting relationships, referred to as SARs, are required to provide a better understanding of the catalytic, thermodynamic, and mass-transfer phenomena involved in the reforming process. In the current study, the activity of a catalyst is defined as the oxy-HC conversion observed over a certain catalyst after 5.5 h TOS operation under a certain set of operating conditions. The activity of all of the catalysts employed in this work was determined at all of the operating temperatures investigated and plotted against a certain inherent characteristic, such as Ni reducibility, pore volume/surface area (PV/SA), and Ni dispersion, to formulate the SARs. From Figure 10A, it is clear that the activity of the catalyst increases with the increase in the PV/SA value for all of the reaction temperatures studied. Figure 10B represents the relationship between Ni dispersion and catalytic activity at different temperatures. For all of the temperatures investigated, the catalytic activity increases with the increase in Ni dispersion. Figure 10C represents the relationship between the catalytic activity and the Ni reducibility at different reaction temperatures. From Figure 10C, it is evident that the activity of the catalyst increased with the increase in Ni reducibility for all of

Figure 10. Structure−activity correlation plots for a typical steam reforming of mixtures of oxygenated hydrocarbons: (A) activity versus PV/SA, (B) activity versus Ni dispersion, and (C) activity versus reducibility.

the reaction temperatures evaluated. From panels A, B, and C of Figure 10, it can be concluded that high PV/SA, high Ni dispersion, and high Ni reducibility are the most important catalyst traits that are essential for the progress of the oxy-HC steam-reforming reaction. Appropriate empirical correlations have been derived for each reaction temperature using the data of the SAR plots (panels A−C of Figure 10) by assuming linearity (y = mx + c) and thus using the linear regression analysis technique. The model parameters, slope (m) and intercept (c), are shown in 826


Energy & Fuels

Article

Table 4. As noted from Table 4, the values of m generally decreased with an increase in the reaction temperature for all of

shown from thermal (noncatalytic) reactions, that 5N/CZMg has the ability to increase the oxy-HC conversion by 3.5-fold at 600 °C operating temperature. Also, an increase in Ni loading from 5 to 10% did not contribute to improvement in the catalytic activity. A S/M molar ratio of 0.5 was found to be optimal for the current application. Any further increase in the S/M molar ratio does not contribute to improvement in the catalytic activity. The surfactant-assisted route yields better catalysts when compared to the conventional co-precipitation route. It was also shown that the addition of the surfactant up to a S/M molar ratio of 0.5 enhanced the catalyst specific surface area, specific pore volume, and Ni dispersion significantly. Furthermore, TPO analysis showed that the rate of coking (CPF) decreases with an increasing reaction temperature and the type of promoter element “M” employed. High reducibility, high PV/SA, high Ni surface area, low CPF, and high Ni dispersion are critical characteristics that are necessary for the progress of the steam reforming of oxygenated hydrocarbon mixtures.

Table 4. Values of the Parameters (m and c) of the Empirical Correlations Derived Using the SAR Plots (Figure 10) as a Function of the Reaction Temperature model parameters SAR plot catalytic activity versus PV/SA catalytic activity versus Ni reducibility catalytic activity versus Ni dispersion

temperature (°C) 500 600 700 500 600 700 500 600 700

m 1.1 0.6 0.34 25612.2 23063.5 14305.4 0.78 1.1 0.71

± ± ± ± ± ± ± ± ±

c 0.1 0.4 0.32 5024.7 3002.8 4986.8 0.5 0.25 0.25

7.2 78.0 89.5 −28.2 38.1 64.2 23.3 79.8 89.8

± ± ± ± ± ± ± ± ±

2.6 7.6 6.0 11.4 6.6 11.0 4.4 2.2 2.2

■ ■

the characteristics evaluated, except with the anomaly in the activity versus dispersion series. The observed anomaly can be attributed to intrinsic experimental error(s) involved possibly with the ICP−MS and H2 chemisorption analyses. Conversely, Table 4 shows a definite increase of c with temperature. The above models can be employed to differentiate the various ternary (quaternary) mixed oxide formulations devised in the current work, in terms of the differentiating ingredient (dopant) “M”, and can also be used by other researchers for predictive and comparative purposes. There are similarities in the surface areas of the developed 5Ni/Ce0.5Zr0.33M0.17O2‑δ (M = Mg, Ca, La, Y, Gd, and CaMg) catalysts and Ni/Al2O3 commercial catalyst.51 This establishes the fact that surface area alone is not the only criterion (characteristic) responsible for the resultant catalytic activity. In fact, it would be best to consider that the right combination of all relevant characteristics is responsible for the resultant overall activity and that the percentage contribution from each characteristic to the overall activity is not necessarily the same. On the basis of our studies, it is noted that the absence of any desired characteristic in a given catalyst formulation leads to a poor performance or even deactivation.43 From the current investigation, it is quite apparent that reasonably high surface area, high reducibility, and high nickel dispersion are the most prominent characteristics that lead to excellent catalytic activity in the oxygenated hydrocarbon(s) steam-reforming process. The catalysts devised in the current work exhibit high activity as well as improved tolerance to coking based on a combination of catalyst characteristics, such as high reducibility, high metal dispersion, high OSC, reasonably high surface areas, and excellent steam tolerance.43 All of the above-mentioned characteristics, which are essential for the progress of the steam-reforming reaction at temperatures as low as 500 °C and with feed as complex as employed in the current work, can be attributed to the components and process of making the supports.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ACKNOWLEDGMENTS The financial support provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) Strategic Projects is gratefully acknowledged.

■

REFERENCES

(1) Ramachandran, R.; Menon, R. K. Int. J. Hydrogen Energy 1998, 23, 593−598. (2) Holladay, J. D.; Hu, J.; King, D. L.; Wang, Y. Catal. Today 2009, 139, 244−260. (3) Cavallaro, S.; Freni, S. Int. J. Hydrogen Energy 1996, 21, 465−469. (4) Iriondo, A.; Barrio, V. L.; Cambra, J. F.; Arias, P. L.; Güemez, M. B.; Navarro, R. M.; Sanchez-Sanchez, M. C.; Fierro, J. L. G. Catal. Commun. 2009, 10, 1275−1278. (5) Zhang, B.; Tang, X.; Li, Y.; Xu, Y.; Shen, W. Int. J. Hydrogen Energy 2007, 32, 2367−2373. (6) Oasmaa, A.; Meier, D. J. Anal. Appl. Pyrolysis 2005, 73, 323−334. (7) Navarro, R. M.; Pena, M. A.; Fierro, J. L. G. Chem. Rev. 2007, 107, 3952−3991. (8) Pompeo, F.; Santori, G.; Nichio, N. N. Int. J. Hydrogen Energy 2010, 35, 8912−8920. (9) Fierro, V.; Akdim, O.; Mirodatos, C. Green Chem. 2003, 5, 20− 24. (10) Le Valant, A.; Garron, A.; Bion, N.; Duprez, D.; Epron, F. Int. J. Hydrogen Energy 2011, 36, 311−318. (11) Rioche, C.; Kulkarni, S.; Meunier, F. C.; Breen, J. P.; Burch, R. Appl. Catal., B 2005, 61, 130−139. (12) Mizuno, T.; Matsumura, Y.; Nakajima, T.; Mishima, S. Int. J. Hydrogen Energy 2003, 28, 1393−1399. (13) Liguras, D. K.; Kondarides, D. I.; Verykios, X. E. Appl. Catal., B 2003, 43, 345−354. (14) Hirai, T.; Ikenaga, N.; Miyake, T.; Suzuki, T. Energy Fuels 2005, 19, 1761−1762. (15) Vaidya, P. D.; Rodrigues, A. E. Chem. Eng. J. 2006, 117, 39−49. (16) Basagiannis, A. C.; Verykios, X. E. Catal. Today 2007, 127, 256− 264. (17) Yamazaki, T.; Kikuchi, N.; Katoh, M.; Hirose, T.; Salto, H.; Yoshlkawa, T.; Wada, M. Appl. Catal., B 2010, 99, 81−88. (18) Comas, J.; Marino, F.; Laborde, M.; Amadeo, N. Chem. Eng. J. 2004, 98, 61−68. (19) Sun, J.; Qiu, X. P.; Wu, F.; Zhu, W. T. Int. J. Hydrogen Energy 2005, 30, 437−445.

4. CONCLUSION Among the portfolio of 5N/CZM catalysts developed in the current work, catalysts where M = Ca, CaMg, Mg, and Gd perform well at all of the reaction temperatures tested. Among all of the tested catalysts, 5N/CZMg showed the highest oxyHC conversion values at all of the tested temperatures. It was 827


Energy & Fuels

Article

(51) Mäki-Arvela, P.; Tiainen, L.-P.; Lindblad, M.; Demirkan, K.; Kumar, N.; Sjöholm, R.; Ollonqvist, T.; Väyrynen, J.; Salmi, T.; Murzin , D.Yu. Appl. Catal. A: Gen. 2003, 241, 271−288.

(20) Liberator, J. W. C.; Ribeiro, R. U.; Zanchet, D.; Noronha, F. B.; Bueno, J. M. C. Appl. Catal., A 2007, 327, 197−204. (21) Zhang, L.; Li, W.; Liu, J.; Guo, C.; Wang, Y.; Zhang, J. Fuel 2009, 88, 511−518. (22) Adhikari, S.; Fernando, S.; Haryanto, A. Energy Fuels 2007, 21, 2306−2310. (23) Akande, A. J.; Idem, R. O.; Dalai, A. K. Appl. Catal., A 2005, 287, 159−175. (24) Buffoni, I. N.; Pompeo, F.; Santori, G. F.; Nichio, N. N. Catal. Commun. 2009, 10, 1656−1660. (25) Bimbela, F.; Oliva, M.; Ruiz, J; Garcıa, L.; Arauzo, J. J. Anal. Appl. Pyrolysis 2009, 85, 204−213. (26) Frusteri, F.; Freni, S.; Chiodo., V.; Donato, S.; Bonura, S.; Cavallaro, S. Int. J. Hydrogen Energy 2006, 31, 2193−2199. (27) Hu, X.; Lu, G. Appl. Catal., B 2009, 88, 376−385. (28) Freni, S.; Cavallaro, S.; Mondello, N.; Spadaro, L.; Frusteri, F. J. Power Sources 2002, 108, 53−57. (29) Shi, Q.; Liu, C; Chen, W. J. Rare Earths 2009, 27, 948−954. (30) Iriondo, A.; Barrio, V. L.; Cambra, J. F.; Arias, P. L.; Guemez, M. B.; Navarro, R. M.; Sanchez, M. C.; Fierro, L. G. Chem. Mater. Sci. 2008, 49, 46−58. (31) Montini, T.; Singh, R.; Das, P.; Lorenzut, B.; Bertero, N.; Riello, P.; Benedetti, A.; Giambastiani, G.; Bianchini, C.; Zinoviev, S.; Miertus, S.; Fornasiero, P. ChemSusChem 2010, 3, 619−628. (32) Ni, M.; Leung, D. Y. C.; Leung, M. K. H. Int. J. Hydrogen Energy 2007, 32, 3238−3247. (33) Palmeri, N.; Chiodo, V.; Freni, S.; Frusteri, F.; Bart, J. C. J.; Cavallaro, S. Int. J. Hydrogen Energy 2008, 33, 6627−6634. (34) Diagne, C.; Idriss, H.; Kiennemann, A. Catal. Commun. 2002, 3, 565−571. (35) Roh, H. S.; Potdar, H. S.; Jun, K. W.; Kim, J. W.; Oh, Y. S. Appl. Catal., A 2004, 276, 231239. (36) Erdohelyi, A.; Raskó, J.; Kecskés, T.; Tóth, M.; Domok, M.; Baán, K. Catal. Today 2006, 116, 367−376. (37) de Lima, S. M.; Silva, A. M.; da Cruz, I. O.; Jacobs, G.; Davis, B. H.; Mattos, L. V.; Noronha, F. B. Catal. Today 2008, 138, 162−168. (38) Pengpanich, S.; Meeyoo, V.; Rirksomboon, T.; Bunyakiat, K. Appl. Catal., A 2002, 234, 221−233. (39) Biswas, P.; Kunzru, D. Chem. Eng. J. 2008, 136, 41−49. (40) Youn, M. H.; Seo, J. G.; Cho, K. M.; Park, S.; Park, D. R.; Jung, J. C.; Song, I. K. Int. J. Hydrogen Energy 2008, 33, 5052−5059. (41) Terribile, D.; Trovarelli, A.; Llorca, J.; Leitenbrug, C.; Dolcetti, G. Catal. Today 1998, 43, 79−88. (42) (a) Idem, R. O.; Kumar, P.; Sun, Y. Catalysts for hydrogen production. U.S. Patent 7,824,656 B2, Nov 2, 2010. (b) Kumar, P.; Sun, Y.; Idem, R. O. Energy Fuels 2008, 22, 3575−31582. (43) (a) Sukonket, T.; Khan, A.; Saha, B.; Ibrahim, H.; Tantayanon, S.; Kumar, P.; Idem, R. Energy Fuels 2011, 25, 864−877. (b) Khan, A.; Sukonket, T.; Saha, B.; Idem, R. Energy Fuels 2012, 26, 365−379. (c) Idem, R. O.; Khan, A.; Ibrahim, H.; Tontiwachwuthikul, P.; Sukonket, T.; Khan, F.; Sengupta, P.; Zahid, A.; Saha, B. Catalyst for feedstock and process flexible hydrogen production. WO Patent 2011/ 106867 A1, 2011. (44) (a) Iglesia, E.; Boudart, M. J. Catal. 1983, 81, 204−213. (b) Tsay, M. T.; Chang, F. W. Appl. Catal., A 2000, 203, 15−22. (45) (a) Ding, R.-G.; Yan, Z.-F. Prepr. Pap.Am. Chem. Soc., Div. Fuel Chem. 2002, 47, 103. (b) Hoang-Van, C.; Kachaya, Y.; Teichner, S. J.; Arnaud, Y.; Dalmon, J. A. Appl. Catal. 1989, 46, 281. (c) Richardson, J. T.; Cale, T. S. J. Catal. 1986, 102, 419. (46) Khan, A.; Smirniotis, P. G. J. Mol. Catal. A: Chem. 2008, 280, 43−51. (47) Montoya, J. A.; Romero-Pascual, E.; Gimon, C.; Del Angel, P.; Monzon, A. Catal. Today 2000, 63, 71−85. (48) Bartholomew, C. H. Appl. Catal., A 2001, 212, 17−60. (49) Penkova, A.; Bobadilla, L.; Ivanova, S.; Dominguez, M. I.; Romero-Sarria, F.; Roger, A. C.; Centeno, M. A.; Odriozola, J. A. Appl. Catal., A 2011, 392, 184−191. (50) Kauppi, E. I.; Kaila, R. K.; Linnekoski, J. A.; Krause, A. O. I.; Niemelä, M. K. V. Int. J. Hydrogen Energy 2010, 35, 7759−7767. 828


Ce0. 5Zr0. 33M0

Recommend Documents