Copper-Based Water Gas Shift Catalysts for Hydrogen Rich Syngas

Article Cite This: Energy Fuels XXXX, XXX, XXX-XXX

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Copper-Based Water Gas Shift Catalysts for Hydrogen Rich Syngas Production from Biomass Steam Gasification Charlotte Lang, Xavier Sécordel, and Claire Courson* ICPEES, Équipe “Énergie et Carburants pour un Environnement Durable” UMR CNRS 7515, ECPM Université de Strasbourg, 25 rue Becquerel, 67087 Strasbourg Cedex 2, France S Supporting Information *

ABSTRACT: Copper-based catalysts deposited on foams were developed to perform the water gas shift reaction at low temperature (150−300 °C) at the outlet of a biomass gasifier with limited pressure drop. Different synthesis methods were used such as wet impregnation and urea-nitrates combustion methods for the preparation of copper/ceria/foam catalysts containing 4.5−7.0 wt % of copper as the active phase and 7.0−9.0 wt % of ceria as the wash coat. The latter were characterized in order to evaluate present phases, their crystallite size, and ceria lattice parameter by X-ray diffraction (XRD) analysis, specific surface area by N2 adsorption technique (BET method), and reducibility by temperature programed reduction (TPR). The activity of the catalysts was studied in the water gas shift reaction at low temperature (150−300 °C) and at a high gas hourly space velocity (between 3600 and 9500 h−1) as a function of preparation parameters (synthesis method and catalyst composition) and operating parameters (temperature and prereduction step). It appeared that the wet impregnation method is the optimized studied method because it led to similar activity to those obtained with a more complex synthesis method (urea-nitrate combustion). The optimized copper and ceria contents are 5.5 and 9.0 wt %, respectively, 30 ppi foam. The study of the temperature effect highlighted the optimum at 300 °C without the sintering of metallic copper nor ceria crystallites. Catalytic tests showed in situ activation of the catalyst at 300 °C. So, the prereduction step was unnecessary at this temperature. Finally, the more promising catalyst (5.5 wt % of Cu and 9.0 wt % of CeO2) was tested at the outlet of a high temperature water gas shift reactor, and the conditions of the two coupled reactors (high and low reaction temperatures and steam to carbon (S/C) molar ratio) were optimized permitting the reaching of thermodynamic values for methane conversion with 500 °C in the high temperature reactor and 350 °C and a S/C of 3 in the low temperature reactor. makes for also very interesting catalysts11−14 for WGS reaction, especially the copper−ceria catalysts due to its high stability15−18 and because ceria is able to dissociate readily H2O when it is partially reduced.19 Both metallic Cu and oxygen vacancies in CeO2 are needed for the formation of catalytically active sites for the WGS reaction.20 Moreover, ceria particle size variations can also have an important influence on the WGS reaction activity because of its effect on the amount of oxygen storage available for the reaction. Thus, it is fundamental to study both particle sizes of metal and ceria support.21 The use of ceria as a wash coat should permit the increasing of the very low specific surface area of the alumina ceramic foams then improving the dispersion of copper oxide particles and its anchorage.9 Ceria’s presence can also play an important role in the activation of the adsorbed species during the WGS reaction22 and limit carbon formation during catalytic tests because of the high oxygen mobility of its structure.23−27 Various methods of CuO−CeO2 catalyst synthesis were studied28,29 as sol−gel peroxide method, citric acid assisted synthesis, and coprecipitation, the latter producing the best WGS reaction catalyst under specific testing conditions. However, to synthesize WGS catalysts supported on ceramic

1. INTRODUCTION Hydrogen is widely considered as the clean fuel of the future, because of its high mass based energy density. Its production goes through reforming and/or gasification processes leading to CO as a coproduct. The water gas shift (WGS) reaction converts it while producing additional hydrogen, making WGS a very important process. WGS is a mildly exothermic and reversible reaction, and thermal equilibrium is reached faster at higher temperatures. Then, the reaction being equilibrium limited at high temperature and kinetically limited at low temperature, industrial WGS processes usually consist of two catalytic reaction steps:1−3 a high temperature step (HT, 300− 500 °C) working with an iron-based catalyst3−6 and a low temperature step (LT, 200−300 °C) accelerated by a copperbased catalyst.3,5 This configuration was also validated by computer simulation as the most appropriate one in a wide temperature range.7 Some characteristics as available oxygen vacancies, activity in water dissociation, and low CO adsorption strength8 are needed for the development of these catalysts. However, WGS catalysts also met another difficulty linked to their shaping: the pressure drop induced by their use in a fixed bed reactor. This parameter can be limited by the use of WGS catalyst supported on ceramic foam.9,10 Currently, Cu-ZnO-Al2O3 based catalysts, stabilizing the copper surface area are almost exclusively used in industrial LTS operations.5 However, ceria doped with transition metals © XXXX American Chemical Society

Received: June 21, 2017 Revised: September 13, 2017 Published: October 10, 2017 A

DOI: 10.1021/acs.energyfuels.7b01765 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Catalysts Nomenclature catalyst name

porosity (ppi)

ceria content (wt %) ± 0.1 wt %

Cu nitrate concentration (mol L‑1)

Cu content (wt %) ± 0.1 wt %

Cu/CeO2 (%)

4.5Cu/7CeO2 4.5Cu/7CeO2UNC 4.5Cu/9CeO2 5.5Cu/9CeO2 7.0Cu/9CeO2

30 30 45 45 45

7.0 7.0 9.0 9.0 9.0

1.65 1.21 1.35 1.65 2.83

4.5 4.5 4.5 5.5 7.0

65 65 50 60 80

2.1.3. Urea-Nitrate Combustion Method (UNC). The UNC method31,32 has been developed as a simple and fast route for the synthesis of ultrafine, nanocrystalline CuO−CeO2 catalysts in a single step. However, this method which is not yet developed for the supported form did not result in a satisfactory deposition on our ceramic foams. It was decided to use it for the deposition on copper after ceria deposition by wet impregnation (Figure 1).

foams and then to transpose the synthesis method on a large scale, the easier method to implement while maintaining appropriate dispersion and anchorage of the deposits is the wet impregnation method. The ceria wash coat and the addition of copper can be easily performed by wet impregnation as previously validated.9,30 Another method adapted to large scale synthesis is the urea-nitrate combustion method31,32 that permits the deposition of cerium and copper in a single step. These synthesis methods are compared in term of present phases, their crystallite size, and ceria lattice parameter by X-ray diffraction (XRD) analysis, specific surface area by N 2 adsorption technique (BET method), reducibility by temperature programed reduction (TPR), and efficiency (CO conversion and H2 gain) in WGS reaction under low temperature (between 150 and 300 °C) and at a high gas hourly space velocity (between 3600 and 9500 h−1). In the context of the synthesis gas purification, the LT-WGS reaction step follows a HT-WGS reactor. This configuration further improves CO conversion and increases the H2 gain in the gas mixture by combining the advantages linked to the use of a HT catalyst (less sensitive to pollutants) and a LT catalyst (efficient in kinetic improvement).3,5,7 This combination is studied with optimized HT catalyst10 and LT catalysts (this work) in order to adjust the operating parameters such as the reaction temperatures and the steam to carbon (S/C) ratio. This work highlights the interest of copper/ceria/foam catalysts developed for the first time for the LT WGS reaction. It studies the influence of various preparation parameters (synthesis method, copper content, and foam porosity) and operating parameters (reaction temperature, prereduction) for this reaction and in the coupling of HT and LT reactors on ceria particles size in the samples characterized after testing.

Figure 1. Urea-nitrate combustion method. This method was studied on a 30 ppi foam on which 7.0 wt % of CeO2 was deposited in order to compare it with the wet impregnation synthesis (Table 1). An aqueous solution of copper nitrate (C = 1.21 mol L−1) was mixed with urea (urea/nitrate weight ratio of 4.17). The wash coated foam was plunged (15 s) in this solution then dried (140 °C for 8 h) to remove water and allow gel formation in the pores (Figure 1), after which, the foam was placed in a preheated oven (140 °C) where it was calcined (3 °C min−1, 500 °C for 1 h). Calcination caused the gel to burn with high gas emission and led to the formation of copper oxide on the wash coated foam and to the 4.5CuUNC/7CeO2 catalyst (Table 1). Thus, these catalyst series permit the studying of the effects of synthesis method, of copper content, and of foam porosity. 2.2. Characterization Techniques. 2.2.1. X-ray Diffraction (XRD). X-ray diffraction (XRD) patterns were obtained thanks to a Bruker AXS-D8 Advanced using Cu K radiation (λ = 1.5406 Å) for the crystalline phases identification (step = 0.06°, time per step = 2 s) in a 2θ range of 20−70°. The diffraction spectra have been indexed by comparison with the JCPDS files (Joint Committee on Powder Diffraction Standards). The wash coat and catalyst crystallite sizes were determined by Debye−Scherrer equation from the width at halfheight (FWMH) of the more intense and better deconvoluted ray of each. The error made on the value of the crystallite size is lower than 1 nm. The lattice parameter (a) characteristic of the face centered cubic structure of ceria was calculated as follows [eq 1]:

2. EXPERIMENTAL SECTION The alumina ceramic cylinder foams (20 mm outer diameter, 20 mm length), with two different porosities (45 and 30 pores per inch (ppi)) provided by Pall Filtersystems GmbH, were used as support for the copper phase in WGS catalysts. The ceria and copper contents were evaluated by weight difference measured during each deposition step (after calcination) on the alumina ceramic cylinder foams, and the uncertainties were evaluated as being lower than 0.1 wt %. 2.1. Synthesis Methods. First, the foam was wash coated by ceria, and then, the copper phase was added by wet impregnation (WI) or by the urea-nitrates combustion method (UNC). 2.1.1. Wash Coating. The wash coating step consists of a wet impregnation of a cerium nitrate aqueous solution (0.87 mol L−1), a drying (100 °C for 52 h), and a calcination (3 °C min−1, 400 °C for 4 h). These conditions were previously optimized9 to obtain catalysts containing 9.0 (named 9CeO2) and 7.0 wt % (named 7CeO2) of ceria on 45 and 30 ppi foam porosity, respectively. 2.1.2. Wet Impregnation (WI). The wash coated foam was impregnated by a copper nitrate aqueous solution with various concentrations (from 1.35 to 2.83 mol L−1) then dried (100 °C for 52 h) and calcined (3 °C min−1, 400 °C for 4 h). This usual method permitted the obtaining of catalysts (Table 1) containing copper in the range from 4.5 to 7.0 wt % depending on the foam porosity.

a = d h2 + k 2 + l 2

(1)

with d being the inter-reticular distance and h, k, and l being the Miller indexes. The error made on the value of the ceria lattice parameter is lower than 0.005 Å. 2.2.2. Specific Surface Area (BET). The specific surface area of wash coated and catalytic foams was obtained by BET method on nitrogen adsorption measurements at 77 K using a Micromeritics ASAP 2420 instrument. Before the taking of each measurement, the samples were degassed overnight under vacuum, at 250 °C. 2.2.3. Temperature-Programmed Reduction (TPR). The temperature-programmed reduction (TPR) was performed with a Micromeritics Autochem II Chemisorption Analyzer on 150 to 500 mg of B


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Energy & Fuels sample under 10%H2/Ar with a total gas flow of 3 L h−1. The temperature was raised from room temperature to 900 °C at a rate of 15 °C min−1, and a thermal conductivity detector (TCD) continuously measured the hydrogen consumption of the samples until returning to the baseline to determine the reduction temperature range and the reducibility (reduction percentage) of the catalysts. The reducibility was calculated as the ratio of the hydrogen consumed by the reduction of the sample to the hydrogen needed for its total reduction (theoretical). 2.3. Reactivity Tests. 2.3.1. Experimental Device and Operating Conditions. The experimental device used for the reactivity tests with LT WGS catalysts was previously described.9 Reactivity tests were performed with a foam catalyst (sample mass between 5.63 and 5.89 g corresponding to between 0.26 to 0.40 g of copper oxide), under a gas mixture similar to that obtained at the biomass gasifier outlet (dry composition: 47 vol % H2, 27 vol % CO, 19 vol % CO2, 2 vol % CH4, and 5 vol % N2) with an addition of water (using an HPLC pump) up to a S/C ratio of 2.00 as recommended in the literature.15,33,34 The total flow rate including water varies from 11.7 to 30.9 NL h−1 to adjust the gas hourly space velocity (GHSV) in the range between 3600 and 9500 h−1 imposed by the reactor size on the pilot scale. The influence of preparation parameters such as synthesis method (WI or UNC) and catalysts composition (amounts of CuO) were studied. The operating parameters that come into play in this study are the reaction temperature which was studied between 150 and 300 °C and the prereduction. The influence of the prereduction studied whether a prereduction was necessary for the activation of our catalysts or if these were completely reduced in situ during the reaction. The optimized copper-based catalyst was then used in the presence of a reaction mixture corresponding to that obtained from the HT reactor containing the optimized iron-based catalyst (5.8 wt %Fe/5.0 wt %CeO2 on a 45 ppi foam).9 The gas mixture at the HT reactor inlet was the one described above for the LT catalyst study. The GHSV was equal to 3600 h−1. Various operating parameters were studied, such as the HT reactor temperature, LT reactor temperature, and S/C ratio. The reactivity results in WGS reaction are reported in terms of CO conversion, calculated as the fraction of CO consumed to CO upstream of the reactor [eq 2]

COconv =

COin − COout × 100 COin

increase in temperature with an S/C ratio of 2.00 leads to a decrease in CO conversion and H2 gain.

Table 2. CO Conversion and H2 Gain Calculated at the Thermodynamic Equilibrium (eq.) for Various Temperatures at an S/C = 2.00

H 2out − H 2in × 100 H 2in

CH4out + CO2out + COout × 100 CH4in + CO2in + COin

H2 gain eq. (%)

99.4 98.1 95.3 90.5

57.2 56.4 54.8 52.1

Table 3. CO Conversion and H2 Gain Calculated at the Thermodynamic Equilibrium (eq.) for Different Temperatures of the HT and LT Reactors and for Different S/C Ratios in the LT Reactor reactor temperature (°C)

(2)

HT

LT

S/C in LT reactor

CO conversion (eq.) (%)

H2 gain (eq.) (%)

450 500 550 500 500 500

300 300 300 250 350 350

2.00 2.00 2.00 2.00 2.00 3.00

81.5 83.3 84.8 90.6 74.6 81.8

46.8 47.9 48.8 52.1 42.9 47.1

It appears that an increase in the HT reactor temperature (from 450 to 550 °C) leads to an enhancement of the thermodynamic CO conversion (from 81.5 to 84.8%) and H2 gain (from 46.8 to 48.8%). An increase in the LT reactor temperature (from 250 to 350 °C) causes a reduction in the thermodynamic CO conversion (from 90.6 to 74.6%) and H2 gain (from 52.1 to 42.9%). Finally, a rise of the S/C ratio in the LT reactor (from 2.00 to 3.00) provokes an increase in the CO conversion (from 74.6 to 81.8%) and H2 gain (from 42.9 to 47.1%).

(3)

3. RESULTS 3.1. Characterization Results. 3.1.1. X-ray Diffraction (XRD). The diffractograms of the 4.5Cu/7CeO 2 and 4.5CuUNC/7CeO2 catalysts are compared (Figure 2) to the diffractogram of the corresponding wash coated foam (7CeO2) to study the influence of the synthesis method. They show the presence of alumina, ceria, and cupric oxide. Copper is only observed in the CuO phase (89-5895 JCPDS file). The CuO crystallite size could not be calculated because of the overlap of its diffraction lines with those of ceria or alumina (Figure 2), but the broad peaks indicate small crystallites. The lattice parameter and crystallite size of ceria of these catalysts can be evaluated and then compared to the corresponding wash coated foam (Table 4) according to the

where the H2in and H2out are the H2 moles upstream and downstream of the reactor, respectively. The values of CO conversion and H2 content were always compared to the thermodynamic values calculated under the same conditions (Prosim Plus software). Carbon balance (CB) is determined by the following equation [eq 4]: CB =

CO conversion eq. (%)

150 200 250 300

The CO conversions and H2 gains at the thermodynamic equilibrium are much greater than in the case of the high temperature (HT) WGS reactor.9,10 This shows the importance of using the LT WGS reactor after a HT WGS reactor for further optimizing CO conversion and hydrogen production.3,35 CO conversion and H2 gain at the thermodynamic equilibrium were calculated by combining two balanced reactors (Table 3) for the coupling of HT and LT reactors. The temperature of the reactors and the composition of the reaction mixture were imposed.

where COin and COout are the CO moles upstream and downstream of the reactor, respectively. The comparison of H2 content downstream of the reactor (values reported in the Supporting Information) and upstream of the reactor (values reported in the Supporting Information) also permitted the evaluation of the catalytic reactivity in WGS reaction by the H2 gain as the fraction of H2 produced to H2 in the inlet feed [eq 3]

G H2 =

temperature (°C)

(4)

where the CH4out and CO2out are the CH4 and CO2 moles downstream of the reactor, respectively, and the CH4in and CO2in are the CH4 and CO2 moles upstream of the reactor, respectively. 2.3.2. Thermodynamic Values. The thermodynamic limitations for the different conditions were calculated on the Prosim Plus simulation software. For low temperature (LT) WGS reaction (Table 2), an C


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that of the corresponding wash coated foam (9CeO2), indicating a possible integration of copper in the ceria lattice that could be confirmed by TPR analysis. After foam wash coating (9CeO2), nanocrystalline ceria was observed. The copper addition slightly reduces the ceria crystallite size (from 11 to 9−10 nm) regardless of the copper content added by wet impregnation on the 45 ppi foams (9CeO2, 4.5Cu/9CeO2, and 7.0Cu/9CeO2) as previously observed with the 30 ppi foams (4.5Cu/7CeO2). 3.1.2. Specific Surface Measurement by Brunauer− Emmett−Teller Method (BET). The specific surface analyses were carried out on the wash coated foams and on the copper catalysts to observe their evolution after copper addition (Table 5). Figure 2. Diffractograms of catalysts (a) 4.5Cu/7CeO2 and (b) 4.5CuUNC/7CeO2, and (c) corresponding wash coated foam (7CeO2). References: ◆, cupric oxide 77-0199, ▲, ceria 65-923, ●, alumina 10-0173.

Table 5. Specific Surface Area (BET Method) on Nude Foam, Wash Coated Foams, and Catalysts

used synthesis method (WI or UNC). After foam wash coating (7CeO2), nanocrystalline ceria was observed. Table 4. Crystallite Size and Lattice Parameter after Copper Addition Compared to Those of the Corresponding Wash Coated Foam wash coated foam or catalyst

ceria lattice parameter (Å) ± 0.005 Å

ceria crystallite size (nm) ± 1 nm

7CeO2 4.5Cu/7CeO2 4.5CuUNC/7CeO2 9CeO2 4.5Cu/9CeO2 7.0Cu/9CeO2

5.41 5.42 5.40 5.41 5.40 5.40

11 10 8 11 9 10

samples

specific surface area (m2 g‑1) ± 1 m2 g‑1

nude foam 7CeO2 4.5Cu/7CeO2 4.5CuUNC/7CeO2 9CeO2 4.5Cu/9CeO2 7.0Cu/9CeO2

350 °C) associated with the reduction of the surface oxygen.20 For the copper catalysts, the presence of ceria allows the reduction of CuO to Cu0 at a lower temperature (150−350 °C) than in the case of bulk copper oxide (150−450 °C).22,36 The presence of oxygen deficiencies causes a decrease in the reduction temperature of cupric oxide to metallic copper. A reduction temperature below 300 °C would be very advantageous in order to be able to use the catalyst without a prereduction step (in situ activation). The copper oxide addition by the UNC method (4.5CuUNC/7CeO2) makes it possible to achieve lower reduction temperatures (150−270 °C) than impregnation (4.5Cu/7CeO2) with the use of a 30 ppi foam (210−350 °C). The reduction profile consists of three main zones: the first zone at very low temperatures (145−180 °C) indicates the presence of well dispersed CuO particles as well as the reduction of the surface CeO2.16,22 The second zone associated with the reduction of Cu2+ to Cu+ (centered on 220−225 °C) represents the major part of hydrogen consumption. The third zone associated with the reduction of Cu+ to Cu0 (centered around 255 °C) occurs at low temperature compared to bulk copper oxide, indicating a greater participation of the oxygen mobility of the ceria. With the 4.5Cu/7CeO2 catalyst, the maximum reduction occurs at a higher temperature (310 °C), and it is more difficult to differentiate the copper reduction steps between 210 and 280 °C (shoulder peak). The copper oxide present in the 4.5Cu/7CeO2 catalyst prepared by wet impregnation is totally reduced at 350 °C (Table 6). The copper addition by UNC allows a lower reducibility (86%) for the 4.5CuUNC/7CeO2 catalyst that can be linked to the possible integration of copper in the ceria lattice observed by XRD. The reduction profiles of the 4.5Cu/9CeO2 and 7.0Cu/ 9CeO2 catalysts (Figure 3) are not similar which indicates different interactions between copper and ceria. The least loaded copper catalyst (4.5Cu/9CeO2) presents similar profile than 4.5CuUNC/7CeO2 with the same three main reduction zones associated with well dispersed CuO particles reduction, reduction of bigger CuO particles into Cu+,

Figure 4. Influence of synthesis method (a) WI (4.5Cu/7CeO2) and (b) UNC (4.5CuUNC/7CeO2) on CO conversion compared to value at the thermodynamic equilibrium.

of the test, but the catalyst deactivates progressively to reach 33.5% of CO conversion after 8 h. The WI method permits the obtaining of a catalyst which is activated during the first hours of testing. Initially, CO conversion is low (11.5%) but it increases rapidly and begins to level off around 35.0% after 3 h of test. E


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the reduction of copper oxide into the active phase as seen by TPR and which would allow the activation of the adsorbed species during the reaction.22 In Cu−CeO2 systems, the existence of a synergistic Cu−Ovacancy in ceria interaction has been proved. This interaction improves the chemical activity of Cu, and the presence of Cu facilitates the formation of oxygen vacancies in ceria under reaction conditions.20 The lowest copper content does not allow optimal CO conversion and H2 gain because the contact time between gas and catalyst is not sufficient. Finally, the optimum copper content is 5.5 wt % which allows a CO conversion of 42.9% and an H2 gain of 17.6%. These values remains far from the thermodynamic ones due to high GHSV (9500 h−1). 3.2.3. Influence of the Reaction Temperature. This test series was carried out at various temperatures between 150 and 300 °C, with a GHSV of 9500 h−1 and a water content adjusted to have an S/C molar ratio of 2.00. The 4.5Cu/9CeO2 catalyst was prereduced before the reactivity step to be sure that the active phase (Cu0) was formed despite the low reaction temperature. In fact, from the TPR results, CuO was reduced in Cu0 under reductive atmosphere between 150 and 300 °C (see TPR Figure 3). The increase in temperature causes both an increase in CO conversion and an H 2 gain remaining far from the thermodynamic values (Figure 6). Despite the presence of

The same tendency is observed for H2 gain (Figure 5). A H2 gain of 17.6% is obtained after stabilization for the wet

Figure 5. Influence of synthesis method (a) WI (4.5Cu/7CeO2) and (b) UNC (4.5CuUNC/7CeO2) on H2 gain compared to value at the thermodynamic equilibrium.

impregnated catalyst (4.5Cu/7CeO2). The H2 gain obtained with the catalyst prepared by UNC (4.5CuUNC/7CeO2) is only slightly higher (average of 8 h at 20.3%). Finally, the 4.5CuUNC/7CeO2 catalyst presents similar CO conversion and H2 gain than 4.5Cu/7CeO2 catalyst but does not need an activation period or a prereduction to reach these values. In fact, the UNC synthesis method leads to a lower reduction temperature (see TPR section 3.1.3 and Figure 3) and, then, to an earlier activated catalyst because it is easily reduced. However, this low reduction temperature is also associated with high sintering and coking risks which could explain the slow deactivation of this catalyst (CO conversion from 44% to 32%). Even if the 4.5CuUNC/7CeO2 catalyst presents a higher initial CO conversion than the 4.5Cu/7CeO2 catalyst, its deactivation with time of stream compared to the very stable behavior of 4.5Cu/7CeO2 could lead it to a lower level, and then the UNC method does not offer a tangible advantage. Moreover, the WI method is much easier to implement and would be more easily transposable on a larger scale than the UNC method. 3.2.2. Influence of Copper Content. The tests were carried out at 300 °C, with prereduction, the GHSV is 9500 h−1, and the amount of water is adjusted to have an S/C molar ratio of 2.00. The influence of the copper content is studied (Table 7) in the presence of catalysts containing the same ceria content and various copper contents (4.5Cu/9CeO2, 5.5Cu/9CeO2, and 7.0Cu/9CeO2 catalysts). An increase in copper content does not imply an increase in CO conversion or H2 gain as expected. In fact, the largest amount of copper (7.0 wt %) leads to the lowest CO conversion and H2 gain. Excessive copper content can mean a thicker copper layer and less intimate contact between the ceria and copper. Ceria plays the role of oxygen storage which helps

Figure 6. Influence of reaction temperature on CO conversion and H2 gain and their respective values at the thermodynamic equilibrium (eq.).

the Cu0 active phase, low temperatures (150, 200, and 250 °C) do not permit large CO conversions and H2 gains to be obtained. The kinetics of the WGS reaction at these temperatures is too slow and limiting. These results are in accordance with the other works.37 The kinetics of the reaction increases with temperature and allows higher CO conversions and H2 gains when reaching 300 °C. This shows the importance of using the low temperature reactor as a second step which further optimizes CO conversion and hydrogen production as previously demonstrated.3 3.2.4. Influence of Prereduction. The reactions were carried out at a temperature of 300 °C, a GHSV of 9500 h−1, and a water content adjusted to have an S/C molar ratio of 2.00. CO conversion and H2 gain are compared in the presence of the 7Cu/9CeO2 catalyst with or without prereduction (Figure 7). Prereduction allows a higher CO conversion at the beginning of the test (CCO = 41.2%) than for the test without prereduction (CCO = 28.3%). During the test without prereduction, an activation zone appears for which the conversion of CO increases and is probably associated with

Table 7. Influence of Copper Content on CO Conversion and H2 Gain Compared to Values at the Thermodynamic Equilibrium catalysts

CO conversion (%)

H2 gain (%)

4.5Cu/9CeO2 5.5Cu/9CeO2 7.0Cu/9CeO2 thermodynamic equilibrium

35.7 42.9 25.7 90.5

15.4 17.6 15.0 52.1 F


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All of the cupric oxide is reduced in metallic copper during the reactivity test at a temperature of 300 °C in the presence of an S/C ratio of 2.00 leading to the activation of the catalyst even without prereduction. This behavior permits the avoiding of a reduction step on an industrial scale. Table 8 presents the lattice parameter and crystallite size of ceria and the crystallite size of metallic copper after catalytic test. The lattice parameter of ceria generally increases during reactivity test indicating a copper integration in this structure favored by the reaction temperature and the reducing atmosphere. The synthesis method has a weak influence on the ceria crystallite size (Table 8) which slightly increases after reactivity. Finally, the catalyst prepared by UNC reduces at a lower temperature than the catalyst prepared by WI, which explains why it activates faster in situ. However, their catalytic activities are similar, which is consistent with the small difference observed between the copper crystallite sizes after a stream time of 8 h. The copper content has a higher influence on the crystallite size of metallic copper which is maintained between 31 and 35 nm for copper content between 4.5 and 5.5 wt % and increases to 42 nm for the most copper loaded catalyst (7.0Cu/9CeO2). For this catalyst, the higher copper content was not associated with a larger ceria lattice parameter. Thus, the copper excess which has not integrate the ceria structure leads to larger metallic copper particles. It was previously observed that 5.5Cu/9CeO2 led to the best CO conversion (Table 7). The catalyst containing less copper (4.5Cu/9CeO2) does not have sufficient activity. A higher copper content leads to the formation of larger crystallites and therefore to a lesser activity. An increase in the reactivity temperature (Table 8) does not lead to an increase in the ceria lattice parameter nor in the crystallite size, but their values are higher than before reactivity test (5.40 Å and 9 nm, respectively). The lowest copper crystallite size is observed at 300 °C which is also the temperature leading to the best results (Figure 6). Metallic copper is the only phase observed after testing with and without prereduction. The reactivity test after a prereduction step does not cause any significant increase in ceria crystallite size (9 nm; Table 8). Larger ceria and metallic copper crystallite sizes are observed in the absence of the prereduction step. The prereduction step is therefore not necessary because the catalyst is reduced in situ in the presence of the reducing reaction mixture and a suitable temperature (300 °C), but it permits the stabilizing of crystallite sizes during the reactivity test. 3.4. Coupling of WGS Reactions at High and Low Temperatures. The WGS reaction is generally carried out by coupling a high temperature reactor with a low temperature reactor. The HT10 and LT catalysts (this work) were tested, independently, for optimization. The coupling tests, associating a HT reactor followed by a LT reactor, were carried out in the presence of the optimized catalysts (5.8 wt %Fe/5.0 wt %CeO2 on a 45ppi foam and 5.5 wt %Cu/9.0 wt %CeO2 on a 45 ppi foam, respectively) and started from the operating conditions determined in the previous study.10 The operating parameters were adjusted again to obtain the most advantageous CO conversion and H2 gain. 3.4.1. Optimization of the HT Operating Conditions. The influence of the temperature of the HT reactor was studied in

Figure 7. Influence of prereduction (prered) step on CO conversion and H2 gain.

the reduction of CuO to Cu0 under reaction flow. CO conversions with and without prereduction become similar after this activation zone. The H2 gain is only slightly greater in the presence of prereduction (an average of 17.6% compared to an average of 15.4% without prereduction). The catalyst activation by a prereduction step is more difficult to achieve on industrial scale because it requires a controlled reducing atmosphere and time during which the purification unit is not functional. In our case, the reaction mixture is sufficiently reducing to cause, at 300 °C, an in situ catalyst reduction at the beginning of the test and to stabilize the CO conversion at a similar value than that observed after prereduction. Moreover, the H2 gain differs little (2.2%) between the two test conditions. 3.3. After Test Characterization by XRD. The diffractograms of the catalysts after test (Figure 8b−e) regardless of the

Figure 8. Diffractograms of (a) fresh catalyst and after test catalysts (b) 4.5Cu/7CeO2, (c) 4.5CuUNC/7CeO2, (d) 4.5Cu/9CeO2, and (e) 7.0Cu/9CeO2. References: check mark, metallic copper 70-3039; ×, cuprous oxide 89-5895; ◆, cupric oxide 77-0199; ▲, ceria 65-5923; and ●, alumina 10-0173.

synthesis method show, as unique copper phase, the Cu0 one (2θ = 50.3°), which is the active phase in the WGS reaction. The disappearance of the CuO phase is observable (2θ = 35.5° and 38.7°) compared to the fresh catalyst (Figure 8a). The XRD does not highlight any difference between the 4.5CuUNC/7CeO2 catalyst and the 4.5Cu/7CeO2 catalyst prepared by WI. The 7.0Cu/9CeO2 catalyst is the only one to discretely present the Cu2O main rays (2θ = 36.4° and 42.3°) depending on the copper content. This behavior can be correlated with the higher reduction temperature of this catalyst (see Figure 3) and to its lower activity (Table 7). G


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Table 8. Lattice Parameter of Ceria and Crystallite Size of Ceria and Metallic Copper in Various Catalysts after Catalytic Test (and before Test between Brackets), in 4.5Cu/9CeO2 Catalyst as a Function of Reaction Temperature, and in 7.0Cu/9CeO2 Catalyst as a Function of Prereduction Step catalyst

conditions: temperature (°C) prereduction

4.5Cu/7CeO2 4.5CuUNC/7CeO2 4.5Cu/9CeO2

300 300 150 200 250 300 300 300 yes

5.5Cu/9CeO2 7.0Cu/9CeO2

ceria lattice parameter (Å) 5.42 5.41 5.43 5.42 5.42 5.41 5.43 5.42 5.41

(5.42) (5.40) (5.40) (5.40) (5.40) (5.40) (5.40) (5.40) (5.40)

crystallite size of CeO2 (nm)

crystallite size of Cu0 (nm)

9 (10) 9 (8) 11 (9) 11 (9) 11 (9) 10 (9) 11 (9) 11 (10) 10 (10)

32 31 36 31 34 35 31 42 39

the presence of a LT reactor at 300 °C and an S/C ratio of 2.00 in each of the reactors (Figure 9).

Figure 10. Influence of the temperature of the LT reactor on the CO conversion and H2 gain and their respective values at the thermodynamic equilibrium (eq.).

Figure 9. Influence of the temperature of the HT reactor on the CO conversion and H 2 gain and their respective values at the thermodynamic equilibrium (eq.).

The influence of the S/C ratio of the LT reactor was studied in the presence of a HT reactor at 500 °C and a LT reactor at 350 °C (Table 9).

The increase in the temperature of the HT reactor leads to an enhancement in the CO conversion between 450 and 500 °C and then to a very slight increase between 500 and 550 °C. The difference between experimental and thermodynamic CO conversions is lower for the higher temperatures (500 and 550 °C) than for 450 °C. The H2 gain also increases with the temperature of the HT reactor and gradually approaches the thermodynamic values (Figure 9). With the CO conversion difference between 500 and 550 °C being low (59.3 and 61.3%, respectively), it was decided to perform the coupling optimizations with a HT temperature reactor of 500 °C. This temperature is a good compromise between an interesting CO conversion, a low energy consumption, and the stability of the catalytic activity compared to a higher temperature. 3.4.2. Optimization of the LT Operating Conditions. The influence of the temperature of the LT reactor is studied in the presence of the HT reactor at 500 °C and an S/C ratio of 2.00 (Figure 10). An increase in the temperature of the LT reactor (between 250 and 350 °C) leads to a sharp improvement of the CO conversion and H2 gain which then approximate thermodynamic values. The CO conversion and H2 gain obtained at 350 °C are the closest to the thermodynamic values. Therefore, this temperature is selected for the study of the S/C ratio in the LT reactor.

Table 9. Influence of S/C Ratio of the LT Reactor on the CO Conversion and H2 Gain, Compared to the Values at the Thermodynamic Equilibrium (eq.) S/C ratio in LT reactor

CO conversion (%)

CO conversion eq. (%)

H2 gain (%)

H2 gain eq. (%)

2.00 3.00

70.7 81.4

74.6 81.8

25.1 30.3

42.9 47.1

An increase in the S/C ratio from 2.00 to 3.00 leads to an enhancement of CO conversion and H2 gain. For S/C = 3.00, the experimental CO conversion reaches a thermodynamic value, and the H2 gain tends to level out. This behavioral difference can be explained by the presence of a secondary reaction which consumes CO without producing H2. This reaction of CO dismutation (2 CO ⇆ C + CO2) that implies the formation of carbon during the reactivity test could explain the CB of 97% observed under these conditions.

4. CONCLUSION The comparison of the synthesis method (WI and UNC) for the addition of copper on the wash coated foams by XRD permitted the identifying of the same phases present in the catalysts 4.5Cu/7CeO2 and 4.5CuUNC/7CeO2, with higher ceria crystallite size and lattice parameter (calculated from XRD) for the WI method. As expected, ceria wash coat led to H


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would like to thank the European Commission (299732) for its financial support. The authors are very grateful to Daniel Schawartz, a native Englishman, who took the time to reread this paper.

an increase in specific surface area. However, the addition of copper with both methods slightly reduces this property. TPR highlighted differences between the two synthesis methods: the 4.5Cu/7CeO2 catalyst permitted higher reducibility but at higher temperature than 4.5CuUNC/7CeO2 catalyst. That confirms stronger copper-ceria interactions in the 4.5Cu/7CeO2 catalyst assumed by the integration of a part of copper in the ceria lattice. The efficiency (CO conversion and H2 gain) of these catalysts was similar after 2 h on stream but with different behaviors before. The 4.5Cu/7CeO2 catalyst needed time to be totally activated under the reductive reactional atmosphere while the 4.5CuUNC/7CeO2 catalyst deactivated during this period probably due to its lower reduction temperature and to a slight loss of copper by integration in the ceria lattice. The UNC synthesis method does not present an advantage compared to the WI method. The study of the copper content effect led to the identifying of ideal copper and ceria contents (5.5 wt % of Cu and 9.0 wt % of CeO2 on 45 ppi foam). For the catalyst containing a higher copper amount, the sintering of metallic copper crystallites induced weaker activity. The study of the temperature effect in a LT reactor highlighted an increase in the CO conversion and the H2 gain with the temperature rise with the optimum at 300 °C without the sintering of metallic copper nor ceria crystallites. Catalytic tests showed in situ activation of the catalyst at 300 °C. So, the prereduction step was unnecessary at this temperature. The HT and LT coupling reactivity tests made it possible to determine the ideal operating conditions for the use of the optimized catalysts. It is possible under certain conditions to achieve thermodynamics: the optimum conditions are a temperature of 500 °C and an S/C ratio of 2.00 for the HT reactor and a temperature of 350 °C and an S/C ratio of 3.00 for the LT, which allows a CO conversion of 81.4% (thermodynamic equilibrium) and a H2 gain of 30.3%. Therefore, this work highlights the efficiency of copper/ceria/ foam catalysts developed for the first time for the LT WGS reaction and the associated conditions permitting stable activity without ceria particles size increasing.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b01765. Tables listing partial pressures of dry inlet and outlet gases used for results presented Tables 7 (Table A) and 9 (Table B) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +33 368852770. ORCID

Claire Courson: 0000-0001-6811-8484 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out under an EC Project (Contract UNIfHY ID 299732 − Program PF7-2012/2016). The authors I


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J


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