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The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e9
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The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process V. Palma, D. Pisano, M. Martino* University of Salerno, Department of Industrial Engineering, Via Giovanni Paolo II 132, 84084 Fisciano, SA, Italy
article info
abstract
Article history:
Aluminum foam based structured catalysts were prepared and tested for the CO water gas
Received 7 November 2016
shift reaction, and the influence of the textural properties of the carriers, over the per-
Received in revised form
formances, were evaluated. An estimation of the kinetic parameters, by applying the
31 March 2017
LangmuireHinshelwood model, was also provided. Three aluminum foams with different
Accepted 2 April 2017
porosity were used in the preparation of the catalysts; the carriers were previously
Available online xxx
washcoated with an alumina-based slurry, subsequently, the support and the active component, were loaded by impregnation with an aqueous solution of the precursor salts.
Keywords:
The good stability of the structured catalysts to the mechanical stress, was demonstrated
Water gas shift
with the results of the ultrasonic adhesion test. The results of the activity tests showed no
Process intensification
direct correlation between the porosity of the carriers and the performance of the catalysts,
Aluminum foam
suggesting the occurrence of other phenomena. The estimation of the effective thermal
Structured catalyst
conductivity of the three aluminum foams indicated a clear correlation between CO con-
Thermal conductivity
version and thermal conductivity of the carriers, highlighting the effect of the redistribution of the generated heat of reaction along the catalytic bed, on the performance of the catalyst. However, the direct dependence of the thermal conductivity of the foams from the thickness of the intersection between the fibers, was recently demonstrated, confirming the dependence of the performance of these catalysts from the textural properties of the carriers. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In exothermic reactions under adiabatic reactions, the equilibrium conversion decreases with the increasing of the temperature, at the same time the rate of reaction increase with the temperature so, for a single-stage process, for each degree of conversion exists a temperature, below the equilibrium, to which correspond the maximum reaction rate [1]. For this
kind of reaction, the temperature (T) increases linearly with the conversion (X) so, the adiabatic reaction pathway results in a straight line with DT/DX as gradient. A widely used strategy to achieve the maximum productivity, for these kind of reactions, is a multi-stage process, taking advantage of fast kinetics at high temperature and high conversion at low temperature, resulting in a reaction pathway made by straight lines of an adiabatic bed followed by the vertical lines of intermediate cooling. The best example
* Corresponding author. Fax: þ39 089 964057. E-mail addresses: [email protected] (V. Palma), [email protected] (M. Martino). http://dx.doi.org/10.1016/j.ijhydene.2017.04.003 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Palma V, et al., The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.003
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of multi-stage process is the water gas shift reaction (WGS). Currently performed into a two-stage, the WGS process consists of a high temperature step (HTS), carried on Fe/Cr-based catalysts [2] in the temperature range 623e873 K [3], and a low temperature step (LTS) carried on Cu/Zn-based catalysts [4] in the temperature range 453e653 K [5]. This kind of configuration is extremely profitable, allowing to reduce the carbon monoxide (CO) percentage to less than 0.5% to the output of the LTS stage, however, the growing demand of hydrogen for fuel cell application, make it necessary much higher purity (CO <100 ppm), attainable exclusively by a third stage of methanation or preferential oxidation or alternatively by a hydrogen perm-selective membrane reactor [6]. Despite the three-stage process is efficient in providing the desired purity, at the same time it is extremely expensive; for this reason the CO-WGS is the most studied reaction in the field of the hydrocarbon chemistry, in particular the attention of the researcher focused on the preparation of more and more efficient catalysts, possibly active in a wide range of temperature, with the intent of realize a single stage process. Unfortunately, at the moment we are far from achieving this result. The more efficient catalysts used are essentially those proposed in 1914 by Bosch and Wild [2] for the HTS and in 1967 by Dienes [4] for LTS; it is therefore evident that the WGS process is one of the best candidate to the process intensification [7]. The philosophy underpinning the process intensification, is the optimization of fixed and operative costs in industrial plants, for example by substituting inefficient apparatus, reducing the plant sizes, optimizing the catalytic systems, combining more reaction steps in a reduced number of plants, improve the energy process management [8]. A crucial aspect in the design, as well as in the process intensification of an industrial plant, is the optimization of the transport phenomena, in particular for what concern the mass and heat transfer limitations [9]. It was demonstrated that the use of heat-conducting composite plates as catalysts, can maximize the CO conversion in tubular reactors for WGS process [10]. Recently van Dij K et al. [11] proposed the idea of the “isothermal adiabatic reactor”, obtained by dissipating the reaction heat throughout the catalytic bed by thermal conduction. The use of structured carriers with high thermal conductivity, in the catalysts for WGS, can flatten the thermal profile, by retro-diffusion of the heat from the outlet to the inlet of the catalytic bed, with an increase at the inlet temperature and a decrease at the outlet temperature, with a net increase of the conversion [12]; moreover, an improved heat transfer allows a safer control of catalytic reactors, preventing undesired phenomena, such as thermal hotspots and runaway reactions [13,14]. In a comparative study, on the heat transfer on different types of open cell foams [15] (alumina, aluminum, ferritic and FeCrAlloy foams), we showed that the aluminum foams are the most promising carriers for a possible use in “isothermal adiabatic reactors”, showing excellent heat exchange and low pressure drops [16]. The structured catalysts are usually prepared by deposition of the active phase on the carrier, however the physiochemical characteristics of the foams make it necessary the use of a washcoat to enhance the surface area and the adhesion of the active part; the best adhesions are normally obtained with alumina-based washcoat [17]. For what concern the catalytic
formulation, the conventionally used catalysts, are poorly compatible with fuel processor systems [18], so the most promising catalysts for WGS applications, seem to be the noble metals-based, such as Rhodium [19], Ruthenium [20], Gold [21], Platinum [22] and Palladium [23]. Gold catalysts are the most active at low temperature for LTS, however deactivate rapidly for sintering phenomena so, an anchoring to the support seems to be mandatory [24]. Pt-based catalysts seem to highly active and stable at the same time, resulting the best choice for the preparation of a high performance catalyst [25,26]. In this paper we report a study on the effect of the structural characteristics of aluminum foams carriers, on the WGS activity of structured catalysts obtained by deposition on different aluminum foams, of an alumina washcoat coated with a platinum/ceria formulation.
Materials and methods The structured catalysts were prepared by deposition of an alumina washcoat, obtained by dispersing of ɣ-alumina powder in a colloidal solution of pseudoboehmite. The washcoat coating has the double task to increase the specific surface area of the support and to act as binder between the aluminum carrier and the ceria support [17]. The washcoated aluminum foam was impregnated with ceria nitrate precursor in order to completely plate the alumina with a ceria support; finally, the active component was charged by wet impregnation of the structured precursor with tetramine platinum nitrate. The structured catalysts thus prepared were tested in a stainless steel tubular reactor, for the WGS reaction in “quasi adiabatic” conditions.
Carriers preparation The effect of the structural characteristic of the carrier was studied by comparing the performance of the corresponding structured catalysts obtained with aluminum foams (provided by ERG Materials and Aerospace) with different porosity (10, 20 and 40 pores per inch, PPI) and same relative density (10e12%). The alumina foam carriers were obtained by cutting and shaping the corresponding commercial foam, in order to obtain a cylinder of 17 mm of diameter and 102 mm of length. The desired carriers were previously treated with a 0,1 N HCl solution for 20 min in order to remove the passivated layer and enhance the rugosity of the aluminum surface, then washed with deionized water, dried in an oven at 120 C for 2 h and calcined at 500 C for 3 h.
Washcoat preparation The ɣ-alumina (PURALOX SCCa 150/200, provided by SASOL) used in the preparation of the washcoat slurry was pulverized by grinding with a Retsch RM100 mill, until reaching a particle size distribution of less than 10 mm [27]. The colloidal solution was obtained by mixing a solution of methylcellulose (viscosity 4,000 cP, provided by SigmaeAldrich) and a solution of a pseudoboehmite (Pural SB, provided by SASOL), acidified at pH ¼ 4 with nitric acid (65%, provided by Carlo Erba Reagenti). The milled ɣ-alumina was suspended in the colloidal solution
Please cite this article in press as: Palma V, et al., The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.003
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and mixed vigorously under mechanical stirring, obtaining the desired slurry. The composition of the slurry was: 1 wt% of methylcellulose, 4.6 wt% of pseudoboehmite and 15.4 wt% of ɣ-alumina.
Catalysts preparation The structured catalysts were prepared in three steps, a dipcoating in order to load the alumina washcoat and two subsequent impregnations with ceria support before, then the active component. The suitable shaped and treated aluminum foam was immersed into the washcoat suspension for 20 min, then the excess of washcoat was removed by centrifuge at 3000 rpm with an CWS 4236 centrifuge [28]. The resulting foam was dried in an oven at 120 C for 2 h and calcined at 450 C for 3 h. This procedure was repeated further until the weight of the washcoat reached the desired value (a loading of about 40 wt% with respect to the total weight of the washcoated foam). The ceria support was loaded by impregnation of the washcoated foam with an appropriate solution of cerium(III) nitrate (Ce(NO3)3*6H2O, 99.9%-Ce, provided by Strem Chemicals), for 20 min, then dried at 120 C for 2 h and calcined at 450 C for 3 h. The title of the cerium(III) nitrate solution was previously calculated, on the basis of a black test, in order to obtain the 20 wt% of ceria loading with respect to the amount of the washcoat. The same procedure was followed for the loading of platinum; the sample was impregnated with the appropriate solution of tetrammineplatinum(II) nitrate (Pt(NH3)4(NO3)2, 99%, provided by Strem Chemicals) for 20 min, dried at 120 C for 2 h and calcined at 450 C for 3 h, obtaining a Pt loading of 1 wt% with respect the amount of washcoat and ceria. The theoretical final formula of the structured catalysts was: 1%Pt/ 20%CeO2/g-Al2O3/Al-foam. As reference, an aliquot of the washcoat slurry was dried and calcined at 450 C for 3 h, impregnated with a cerium(III) nitrate solution and a tetrammineplatinum(II) nitrate solution, dried at 120 C and calcined at 450 C for 3 h at each step.
Catalysts characterization The prepared catalysts were characterized by a series of physical-chemical analytical techniques. The ɣ-alumina particles size distribution was checked by optical microscope analysis; a sample of about 20 mg was dispersed in 20 ml of ethylene glycol, the mixture was sonicated for about 5 min and then evaluated by optical microscopy (Philips PCVC750K Camera Video). The chemical composition was evaluated by means of ARL QUANT'X ED-XRF spectrometer (Thermo Scientific). An aliquot of the sample was grinded, the resulting powders were compressed to form a tablet that was analyzed by the Fundamental parameters method for evaluating the elemental composition. The stability of the washcoated foams to mechanical stress was evaluated by ultrasound adherence test [29]; the samples were dipped in a beaker containing 200 mL of petroleum ether (provided by Carlo Erba reagenti) and the beaker placed in a ultrasonic bath CP104 (EIA S.p.A.) filled with distilled water. The tests were performed at 25 C by applying the 60% of rated power for six cycles of 5 min. The specific surface area (B.E.T.) measurements were evaluated
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with a Costech Sorptometer 1040 (Costech International), by dynamic N2 adsorption measurement at 196 C, the samples were previously treated at 150 C for 30 min in a flow of helium. The crystal phases and the crystallite dimension were evaluated by X-ray Powder Diffraction (XRD) with a D8 Advance (Brucker); the samples were finely ground and sieved, then analyzed with a Cu Ka radiation source (35 kV; 40 mA) in the 2q range 20e80 , (Stp ¼ 737; Stp size ¼ 0,0814; t/Stp 0,5 s). The structural changes, attributed to the impregnation steps, were evaluated by Raman spectroscopy by an inVia Raman Microscope (Renishaw), equipped with a 514 nm Ar ion laser operating at 25 mW, in the 200e2000 cm1 range. Scanning electron microscopy (SEM) (Assing.mod. LEO 420) at an accelerating voltage of 20 kV, was used to estimate the strut diameters and pore size of the foams to evaluate the washcoat layer. The H2-TPR experiments were carried out in the reactor described below, also used for the catalytic tests, in the temperature range of 20e450 C, with a heating rate of 10 C/min and a reducing stream of 1000 Ncc/min composed by 5 vol% of H2 in N2. The washcoat density was measured by pycnometer. The simplified Montebelli method was applied to estimate the thickness of the washcoat layer [30]. The catalyst kinetic parameters were estimated by applying the LangmuireHinshelwood kinetic model, as reported in the literature for this type of reaction [40]. The parameters were obtained by using the Euler method. The estimated parameters were: - the kinetic constant: k1 (mol/gcat/min) - the adsorption constants: KCO (), KH2O (), KCO2 ().
Catalytic activity tests The catalytic activity tests were carried out with the intent of comparing the catalytic activity of the catalyzed foams, previously reduced in the H2-TPR experiments. The tests were performed at atmospheric pressure in the temperature range 200e400 C, by feeding a reactive mixture composed by 8 vol% of CO, 30 vol% of H2O and N2 balance, in a stainless steel tubular reactor with an internal diameter of 22 mm and a length of 40 cm (all the gases were provided by SOL S.p.A.). The catalyzed foams were surrounded by a thermo expanding pad with a thickness of 2.5 mm, while the Weight Hourly Space Velocity (WHSV) was set to 1.2 h1, with respect to the CO mass flow rate.
Catalytic tests in “quasi adiabatic” conditions The catalytic tests in “quasi-adiabatic” conditions were performed on the pre-reduced catalysts, at the same conditions of pressure, with the same reactive mixture and in the same reactor indicated in Section “Catalytic activity tests”, while the inlet temperature was fixed at 260 C, the WHSV at 2.4 1 gCO*g1 and the linear flow velocity equal to 18.2 cm*s1. cat*h
Experimental apparatus The experimental apparatus was conceived with the intent to approach as much as possible an adiabatic system, avoiding
Please cite this article in press as: Palma V, et al., The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.003
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both radial and axial thermal dispersions. The thermal dispersions were minimized with the use of an appropriate heating system; this system provided three main heating zones: the reagents input area, the furnace for the reactor and the reaction products output area. The reagent input area was developed by heating a stainless steel tube (70 cm of length and internal diameter equal to 6.35 mm), connected to the reactor through a connecting fitting; the temperature of the reagent mixture supplied was monitored and adjusted by a thermocouple connected to a PID controller, in order to preheat the reactive mixture at the same temperature of the inlet section of the catalyst and to guarantee a perfect vaporization of water as well as mixing of the reaction mixture. The furnace was provided with three independent thermally controlled zones, in order to realize a large isothermal zone in the center of the reactor. The reaction products output area was developed by heating a stainless steel tube (70 cm of length and internal diameter equal to 6.35 mm), connected to the reactor through a connecting fitting; the temperature of the reaction products was monitored and adjusted by a thermocouple connected to a PID controller, in order to heat the mixture at the same temperature of the outlet section on the catalyst. With this set-up of the system, we attempted to minimize the driving force, or the temperature difference between the reactive mixture and the surface of the reactor. In preliminary experiments we monitored the thermal profile of the reactor in function of the thermic parameters applied, by filling the reactor with quartz powder and by sliding a thermocouple along the axial direction at the center of the reactor. On the basis of the registered thermal profile we chose the region of the reactor in which to insert the catalyst. The composition of the mixture of the reaction products was continuously monitored, on dry basis, by passing the mixture through a refrigerator Julabo F12, by an ABB system (infrared detector “Uras 14” for CO, CO2 and CH4 and thermal conductivity detector “Caldos 17” for H2).
Characterization results The particle size distribution of the ɣ-alumina is a crucial aspect used in the preparation of a good washcoat [31]; the technical data sheet of the commercial Puralox SCCa150/200 present in our laboratory certify a particle size distribution
extremely inappropriate for our purpose (d50 ¼ 60e150 mm), so a pulverizing step was realized as told in Section “Washcoat preparation”. The powder thus obtained was then checked by optical microscopy following the protocol described in Section “Catalysts characterization”; in Fig. 1 a comparison between the imagine before and after the milling, at the optical microscopy, of the ɣ-alumina was reported, the maximum size found in the analyzed sample was much lower than 10 mm. The ED-XRF results showed a good agreement with the theoretical ceria and platinum loading, confirming the reproducibility of the coating techniques (Table 1). The ultrasound adherence texts [32] were performed on the three washcoated foams, designed as WF40 (washcoated 40 PPI foam), WF20 (washcoated 20 PPI foam) and WF10 (washcoated 10 PPI foam). The results showed a good resistance to the mechanical stress of the washcoat layers. The weight losses (Fig. 2) have been calculated in function of the amount of the washcoat layer only; the losses occurred in the first 15 min, while there appeared to be a correlation between the weight loss and the porosity of the foam, however, the overall weight loss were, in all cases, in the order of 5e8%, in agreement with similar formulations reported in the literature. The SSAB.E.T. obtained for the structured catalysts confirmed the large increase of the specific surface area of the foams (Table 2), assessable in two order of magnitude; if compared with those obtained for ɣ-alumina, the SSAB.E.T. of the catalysts, indicated a low effect of the repeated cycles of calcination on the reduction of the surface area of the washcoat. The XRD patterns of the catalytic washcoat showed the presence of ɣ-alumina and the cubic ceria fluorite type and, vice versa, no evidence of peaks ascribable to the platinum, probably due to the low loading. The crystallite sizes were calculated by Scherrer equation (k ¼ 0.9); the overlapping of the peaks of alumina and ceria, in the final catalysts, made difficult a calculation on the former, so for ɣ-alumina the calculation was made on the washcoat powder, anyway for all the samples the crystallite size were in the order of few nanometers both for alumina and ceria, on average 6 nm for the former and 7 nm for the latter. The presence of platinum was confirmed also from the comparison of the Raman spectra before and after the platinum loading over the ceriacoated derivatives. All the ceria-coated derivatives showed
Fig. 1 e Imaging from optical microscopy of ɣ-alumina before (a) and after milling (b). Please cite this article in press as: Palma V, et al., The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.003
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Table 1 e Composition of the structured catalysts. Sample
PtCeWF40 PtCeWF20 PtCeWF10
Foam
Catalytic washcoat composition (%)
PPI
Density (%)
Al2O3
CeO2
Pt
40 20 10
10e12 10e12 10e12
78.99 79.07 79.56
20.02 19.95 19.45
0.99 0.98 0.99
Table 2 e H2 uptake in TPR experiments and specific surface area. Sample
TPR Experimental H2 Theoretical H2 uptake (mmol/g) uptake (mmol/g)
PtCeWF40 PtCeWF20 PtCeWF10 Pural SB Puralox SCCa150/200
0.25 0.16 0.12 e e
0.05 0.05 0.05 e e
SSAB.E.T. (m2/g)
61 57 55 211 132
Fig. 2 e Washcoat weight loss vs. time during the ultrasonic treatment of the washcoated foams. Fig. 3 e H2-TPR profiles of structured catalysts. the characteristic peak centered at 464 cm1 of the triply degenerate F2g mode of the fluorite type cubic ceria, due to the symmetric breathing vibration of oxygen around Ce4þ ions in the octahedral sites (CeO8) [33] and the almost imperceptible bands centered at 262 cm1 (second-order transverse acoustic mode 2TA) and 596 cm1 (defect induced mode D), attributed to the oxygen vacancies [34]. The platinum loading induced the appearance of a broad band 675 cm1, attributed to the formation of a metal oxide phase strongly interacting with the ceria surface [35], or alternatively to a distortion of the ceria structure caused by the insertion of platinum ions in the ceria lattice [36]. The H2-TPR experiments showed for all the structured catalysts two main peak regions, the first one, in the 130e150 C region, was mainly attributed to the PtOx species reduction, the second one in the 300e360 C region attrib-
aluminum foams. The results of these calculations are shown in Table 3, in which there are foam density (by picnometer measures), strut diameters (by SEM images) and washcoat layers (by Montebelli method).
Kinetic evaluation The kinetic experiments were made at three different temperatures: 220 C, 230 C and 240 C, by varying the reactants flow rate (800 Ncc/min, 600 Ncc/min and 400 Ncc/min). The feeding mixture was composed by 8 vol% of CO, 30 vol% of H2O and N2 balance. Four grams of catalytic powder were used for these experiments. The equation describing the rate of reaction was derived from the LangmuireHinshelwood model (1).
2 k1 KCO KH2O yCO yH2O yH2 yCO2 Keq 1 þ KCO yCO þ KH2O yH2O þ KCO2 yCO2
uted to the surface reduction of ceria (Fig. 3) [37]. The extremely low temperature reduction of the first peak was attributed to the ceria/alumina system [38], while the different reduction temperatures were attributed to the dispersion degree of platinum. The hydrogen uptake (Table 2) was much higher than the theoretical in all cases, also by considering the lonely first peak, suggesting the occurrence of the spillover effect [39]. Through the Montebelli method it was possible to calculate the layer of the washcoat deposited on the different
(1)
where the yi are the molar fractions of the components of the reaction. The Euler method was used in order to discretize the differential equation of the mass balance; then, through the least square method, applied between the model and experimental values, it was possible to estimate the kinetic parameters at the different conditions used for these experiments. The calculated parameters were similar to other ones already reported in the literature [40]. The results of the kinetic
Please cite this article in press as: Palma V, et al., The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.003
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Table 3 e Foams densities, strut mean diameters and washcoat layers. Sample
Foam density
Strut mean diameter
Washcoat layer
PtCeWF40 PtCeWF20 PtCeWF10
0.83 g/cc 0.85 g/cc 0.70 g/cc
300 mm 350 mm 500 mm
120 mm 290 mm 400 mm
calculations are shown in Table 4, including the parameters of the Arrhenius equation. Fig. 4 e Temperature dependence of the CO conversion of the structured catalysts.
Experimental results In this section the results of the activity tests in non-adiabatic and quasi-adiabatic conditions were reported.
Activity tests As previously mentioned, the activity tests were performed at a Weight Hourly Space Velocity (WHSV) was set to 1.2 h1, with respect to the CO mass flow rate, calculated with respect to the lonely catalytic washcoat, in the condition described in Section “Catalytic activity tests”, on the pre-reduced catalysts. The catalytic performances were estimated in terms of CO conversion (XCO) (2) and selectivity to H2 (YH2) (3), in order to evaluate the water gas shift and methanation contribution. . out Fin XCO ¼ Fin CO FCO CO YH2 ¼ Fout H2
(2)
. out Fin CO FCO
(3)
Where Fin and Fout are respectively the molar rate supplied to the reaction system and the molar rate at the outlet, of the species. The CO conversion profile (Fig. 4) was very similar for the three structured catalysts, in the above-described conditions, at high temperature. All the structured catalysts approached to the equilibrium from 255 to 260 C above; below 250 C the conversion decreased rapidly, however remaining near 40% at 220 C for the PtCeWF10 and PtCeWF40 catalysts, while the PtCeWF20 catalyst showed slightly worse performance at low temperatures. The selectivity to hydrogen and carbon dioxide, close to the equilibrium below 350 C, evidenced the occurrence of the methanation reaction only above this temperature (Fig. 5), however, since the amount of methane produced was in the order of few ppm, for all the studied catalysts, this side reaction resulted negligible, confirming published reports on similar catalytic systems [40].
The high conversion and selectivity obtained at low temperature make this catalyst a serious candidate in view of design of a catalytic system for a single stage process [41].
Evaluation of the activity tests in “quasi-adiabatic” conditions To guarantee the “quasi-adiabatic” conditions, as we previously explained, we realized a multiple heating system in order to minimize both the axial and the radial thermal dispersion, moreover the catalysts were surrounded with a thermo expanding pad avoiding the direct contact with the internal wall of the reactor, as well as avoiding the gas stream by-pass of the catalyst. The internal thermal profile of the reactor was monitored by replacing the catalyst with glass quartz, by sliding a thermocouple in the center of the reactor and recording the temperature change every centimeter, applying the same condition of reaction in terms of water and total volumetric flow rate used in the activity tests. The as obtained profile showed an almost flatter temperature from the middle to the end, while the expansion effect of the reacting mixture at the entrance of the reactor generated a gradient of 20 C. On the basis of these results we decided to conduct the tests in the flat temperature profile zone of the reactor.
Table 4 e Results of the kinetic modeling. Parameter k1 KCO KH2O KCO2
Pre-exponential factor
Activation energy
2.72 Eþ09 2.78 Eþ07 4.92 Eþ08 1.46 Eþ00
68.6 kJ/mol 74.0 kJ/mol 35.8 kJ/mol 5.8 kJ/mol
Fig. 5 e Temperature dependence of the H2 selectivity of the structured catalysts.
Please cite this article in press as: Palma V, et al., The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.003
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The tests in “quasi-adiabatic” condition were performed at a double space velocity with respect the catalytic tests reported in the previous section, realizing a total gas flow rate greater than 2600 Ncc/min (see Section “Catalytic tests in “quasi adiabatic” conditions”). The procedure used provided a preliminary phase in which the system was brought in a thermal equilibrium condition; a mixture of water and nitrogen, at the desired total gas flow rate, was vaporized and superheated at the temperature of 260 C and sent to the preheated catalyst. The so set system was considered at the thermal equilibrium when the temperature at the inlet and at the outlet of the catalyst was stable at 260 C (±0.2 C) for 30 min. At the thermal equilibrium the nitrogen was replaced, by a valves system, with a mixture of carbon monoxide and nitrogen, with the desired volumetric ratio and with the same total gas flow rate, inducing the water gas shift reaction. The resulting reaction mixture and the temperature profile at the inlet and at the outlet of the catalyst was monitored until the system reached a new thermal equilibrium, and maintained it for at least 30 min. This procedure was applied to the three structured catalysts, obtaining a characteristic difference of temperature (DT) between the inlet and the outlet of the catalytic bed for each catalyst, that was correlated to the CO conversion. The activity tests showed that at a lower temperature difference DT correspond a higher CO conversion, therefore the trend in the conversion was PtCeWF10 > PtCeWF40 > PtCeWF20, corresponding to an opposite trend for the DT (Fig. 6). The as obtained results were explained in terms of different thermal conductivity of the three foam carriers, however from this trend was not possible to find a direct correlation between the porosity of the carrier and the thermal conductivity. Indeed, the textural properties can be very effective in generating the foam thermal conductivity value. In particular, when aluminum foams are manufactured, a foaming agent is inserted inside the fused aluminum, and then it is vaporized, generating the porous structure. This structure is composed
Fig. 6 e Correlation CO conversion vs. temperature difference DT between the inlet (lower temperature) and the outlet (higher temperature) of the catalytic bed for the three structured catalysts.
Table 5 e Thermal conductivities of the tested foams. Sample PtCeWF40 PtCeWF20 PtCeWF10
Thermal conductivity 30,7 W/m/K 8,3 W/m/K 31,5 W/m/K
by a series of solid ligaments with a lump of metal at the intersection between the fibers. Bhattacharya et al. [42] demonstrated that the thermal conductivity of aluminum open cell foams depends strongly on the lump of metal between the fibers, as well as the fibers geometrical dimensions themselves. By applying the Bhattacharya method, it was possible to estimate the effective thermal conductivity of the three aluminum foams samples. The estimated values are reported in Table 5: As it can bee seen in Table 5, the more conductive foam was the 10 PPI sample, followed by the 40 PPI and the 20 PPI. The results obtained by this calculation reflect strongly the performance obtained in “quasi-adiabatic” conditions, revealing that the textural properties of the aluminum foams determine the effective thermal conductivity, and consequently the tendency to make the redistribution of the WGS heat of reaction throughout the catalytic bed.
Conclusion In this paper we presented the results of a study on the influence of the textural properties, of structured carriers, on the performances of aluminum foam based catalysts, in the CO water gas shift reaction. A comparison on the activity of structured catalysts, with the same catalytic formulation, but different porosity of the carrier was showed. In the preparation of the catalysts, the aluminum foams were firstly coated with a primer of alumina with high surface area, subsequently catalyzed with a Pt/CeO2 formulation. The ultrasound adherence tests showed a good resistance of the alumina layer to the mechanical stress, while the screening on the activity of the catalysts showed similar dependence of the CO conversion versus the temperature. By applying a much higher space velocity, a quasi isothermal adiabatic reactor was realized, and the results of these tests showed that to a reduced difference of temperature on the catalytic bed corresponds a higher CO conversion. These results were explained as a direct consequence of the thermal conductivity of the foams carriers; the back diffusion of the heat of the reaction throughout the carrier, flattens the thermal profile over the catalytic bed, increasing the temperature at the inlet and decreasing the temperature at the outlet. The redistribution of the heat over the bed allows to increase the CO conversion and the reaction rate. A correlation between the textural characteristics of the carriers and the catalytic performance was also showed. The estimation of the effective thermal conductivity of the carriers, confirmed the direct correlation with the CO conversion and, at the same time, the dependence of the thermal conductivity of the carriers from the dimensions of the lump of metal fibers in the foams with different porosity, suggested
Please cite this article in press as: Palma V, et al., The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.003
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in previous study by Bhattacharya et al., makes the thermal conductivity itself, a link between the performance of the catalysts and the textural properties. Moreover, the catalyst kinetic parameters were estimated by applying the LangmuireHinshelwood kinetic model, by using the Euler method. These tests were made at three different flowing conditions, by varying also the inlet temperature of the reactants, from 220 C to 240 C. By applying the Arrhenius equation, it was possible to estimate the kinetic parameters, whose values appeared to be in strong agreement with other results already reported in the literature.
Acknowledgments The authors acknowledge funding by the Italian Ministry of Education, University and Research, Rome (MIUR, Progetti di Ricerca Scientifica di Rilevante Interesse Nazionale, prot. num.: 2010XFT2BB) within the project IFOAMS (“Intensification of Catalytic Processes for Clean Energy, Low-Emission Transport and Sustainable Chemistry using Open-Cell Foams as Novel Advanced Structured Materials”).
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The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process V. Palma, D. Pisano, M. Martino* University of Salerno, Department of Industrial Engineering, Via Giovanni Paolo II 132, 84084 Fisciano, SA, Italy
article info
abstract
Article history:
Aluminum foam based structured catalysts were prepared and tested for the CO water gas
Received 7 November 2016
shift reaction, and the influence of the textural properties of the carriers, over the per-
Received in revised form
formances, were evaluated. An estimation of the kinetic parameters, by applying the
31 March 2017
LangmuireHinshelwood model, was also provided. Three aluminum foams with different
Accepted 2 April 2017
porosity were used in the preparation of the catalysts; the carriers were previously
Available online xxx
washcoated with an alumina-based slurry, subsequently, the support and the active component, were loaded by impregnation with an aqueous solution of the precursor salts.
Keywords:
The good stability of the structured catalysts to the mechanical stress, was demonstrated
Water gas shift
with the results of the ultrasonic adhesion test. The results of the activity tests showed no
Process intensification
direct correlation between the porosity of the carriers and the performance of the catalysts,
Aluminum foam
suggesting the occurrence of other phenomena. The estimation of the effective thermal
Structured catalyst
conductivity of the three aluminum foams indicated a clear correlation between CO con-
Thermal conductivity
version and thermal conductivity of the carriers, highlighting the effect of the redistribution of the generated heat of reaction along the catalytic bed, on the performance of the catalyst. However, the direct dependence of the thermal conductivity of the foams from the thickness of the intersection between the fibers, was recently demonstrated, confirming the dependence of the performance of these catalysts from the textural properties of the carriers. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction In exothermic reactions under adiabatic reactions, the equilibrium conversion decreases with the increasing of the temperature, at the same time the rate of reaction increase with the temperature so, for a single-stage process, for each degree of conversion exists a temperature, below the equilibrium, to which correspond the maximum reaction rate [1]. For this
kind of reaction, the temperature (T) increases linearly with the conversion (X) so, the adiabatic reaction pathway results in a straight line with DT/DX as gradient. A widely used strategy to achieve the maximum productivity, for these kind of reactions, is a multi-stage process, taking advantage of fast kinetics at high temperature and high conversion at low temperature, resulting in a reaction pathway made by straight lines of an adiabatic bed followed by the vertical lines of intermediate cooling. The best example
* Corresponding author. Fax: þ39 089 964057. E-mail addresses: [email protected] (V. Palma), [email protected] (M. Martino). http://dx.doi.org/10.1016/j.ijhydene.2017.04.003 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Palma V, et al., The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.003
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of multi-stage process is the water gas shift reaction (WGS). Currently performed into a two-stage, the WGS process consists of a high temperature step (HTS), carried on Fe/Cr-based catalysts [2] in the temperature range 623e873 K [3], and a low temperature step (LTS) carried on Cu/Zn-based catalysts [4] in the temperature range 453e653 K [5]. This kind of configuration is extremely profitable, allowing to reduce the carbon monoxide (CO) percentage to less than 0.5% to the output of the LTS stage, however, the growing demand of hydrogen for fuel cell application, make it necessary much higher purity (CO <100 ppm), attainable exclusively by a third stage of methanation or preferential oxidation or alternatively by a hydrogen perm-selective membrane reactor [6]. Despite the three-stage process is efficient in providing the desired purity, at the same time it is extremely expensive; for this reason the CO-WGS is the most studied reaction in the field of the hydrocarbon chemistry, in particular the attention of the researcher focused on the preparation of more and more efficient catalysts, possibly active in a wide range of temperature, with the intent of realize a single stage process. Unfortunately, at the moment we are far from achieving this result. The more efficient catalysts used are essentially those proposed in 1914 by Bosch and Wild [2] for the HTS and in 1967 by Dienes [4] for LTS; it is therefore evident that the WGS process is one of the best candidate to the process intensification [7]. The philosophy underpinning the process intensification, is the optimization of fixed and operative costs in industrial plants, for example by substituting inefficient apparatus, reducing the plant sizes, optimizing the catalytic systems, combining more reaction steps in a reduced number of plants, improve the energy process management [8]. A crucial aspect in the design, as well as in the process intensification of an industrial plant, is the optimization of the transport phenomena, in particular for what concern the mass and heat transfer limitations [9]. It was demonstrated that the use of heat-conducting composite plates as catalysts, can maximize the CO conversion in tubular reactors for WGS process [10]. Recently van Dij K et al. [11] proposed the idea of the “isothermal adiabatic reactor”, obtained by dissipating the reaction heat throughout the catalytic bed by thermal conduction. The use of structured carriers with high thermal conductivity, in the catalysts for WGS, can flatten the thermal profile, by retro-diffusion of the heat from the outlet to the inlet of the catalytic bed, with an increase at the inlet temperature and a decrease at the outlet temperature, with a net increase of the conversion [12]; moreover, an improved heat transfer allows a safer control of catalytic reactors, preventing undesired phenomena, such as thermal hotspots and runaway reactions [13,14]. In a comparative study, on the heat transfer on different types of open cell foams [15] (alumina, aluminum, ferritic and FeCrAlloy foams), we showed that the aluminum foams are the most promising carriers for a possible use in “isothermal adiabatic reactors”, showing excellent heat exchange and low pressure drops [16]. The structured catalysts are usually prepared by deposition of the active phase on the carrier, however the physiochemical characteristics of the foams make it necessary the use of a washcoat to enhance the surface area and the adhesion of the active part; the best adhesions are normally obtained with alumina-based washcoat [17]. For what concern the catalytic
formulation, the conventionally used catalysts, are poorly compatible with fuel processor systems [18], so the most promising catalysts for WGS applications, seem to be the noble metals-based, such as Rhodium [19], Ruthenium [20], Gold [21], Platinum [22] and Palladium [23]. Gold catalysts are the most active at low temperature for LTS, however deactivate rapidly for sintering phenomena so, an anchoring to the support seems to be mandatory [24]. Pt-based catalysts seem to highly active and stable at the same time, resulting the best choice for the preparation of a high performance catalyst [25,26]. In this paper we report a study on the effect of the structural characteristics of aluminum foams carriers, on the WGS activity of structured catalysts obtained by deposition on different aluminum foams, of an alumina washcoat coated with a platinum/ceria formulation.
Materials and methods The structured catalysts were prepared by deposition of an alumina washcoat, obtained by dispersing of ɣ-alumina powder in a colloidal solution of pseudoboehmite. The washcoat coating has the double task to increase the specific surface area of the support and to act as binder between the aluminum carrier and the ceria support [17]. The washcoated aluminum foam was impregnated with ceria nitrate precursor in order to completely plate the alumina with a ceria support; finally, the active component was charged by wet impregnation of the structured precursor with tetramine platinum nitrate. The structured catalysts thus prepared were tested in a stainless steel tubular reactor, for the WGS reaction in “quasi adiabatic” conditions.
Carriers preparation The effect of the structural characteristic of the carrier was studied by comparing the performance of the corresponding structured catalysts obtained with aluminum foams (provided by ERG Materials and Aerospace) with different porosity (10, 20 and 40 pores per inch, PPI) and same relative density (10e12%). The alumina foam carriers were obtained by cutting and shaping the corresponding commercial foam, in order to obtain a cylinder of 17 mm of diameter and 102 mm of length. The desired carriers were previously treated with a 0,1 N HCl solution for 20 min in order to remove the passivated layer and enhance the rugosity of the aluminum surface, then washed with deionized water, dried in an oven at 120 C for 2 h and calcined at 500 C for 3 h.
Washcoat preparation The ɣ-alumina (PURALOX SCCa 150/200, provided by SASOL) used in the preparation of the washcoat slurry was pulverized by grinding with a Retsch RM100 mill, until reaching a particle size distribution of less than 10 mm [27]. The colloidal solution was obtained by mixing a solution of methylcellulose (viscosity 4,000 cP, provided by SigmaeAldrich) and a solution of a pseudoboehmite (Pural SB, provided by SASOL), acidified at pH ¼ 4 with nitric acid (65%, provided by Carlo Erba Reagenti). The milled ɣ-alumina was suspended in the colloidal solution
Please cite this article in press as: Palma V, et al., The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.003
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and mixed vigorously under mechanical stirring, obtaining the desired slurry. The composition of the slurry was: 1 wt% of methylcellulose, 4.6 wt% of pseudoboehmite and 15.4 wt% of ɣ-alumina.
Catalysts preparation The structured catalysts were prepared in three steps, a dipcoating in order to load the alumina washcoat and two subsequent impregnations with ceria support before, then the active component. The suitable shaped and treated aluminum foam was immersed into the washcoat suspension for 20 min, then the excess of washcoat was removed by centrifuge at 3000 rpm with an CWS 4236 centrifuge [28]. The resulting foam was dried in an oven at 120 C for 2 h and calcined at 450 C for 3 h. This procedure was repeated further until the weight of the washcoat reached the desired value (a loading of about 40 wt% with respect to the total weight of the washcoated foam). The ceria support was loaded by impregnation of the washcoated foam with an appropriate solution of cerium(III) nitrate (Ce(NO3)3*6H2O, 99.9%-Ce, provided by Strem Chemicals), for 20 min, then dried at 120 C for 2 h and calcined at 450 C for 3 h. The title of the cerium(III) nitrate solution was previously calculated, on the basis of a black test, in order to obtain the 20 wt% of ceria loading with respect to the amount of the washcoat. The same procedure was followed for the loading of platinum; the sample was impregnated with the appropriate solution of tetrammineplatinum(II) nitrate (Pt(NH3)4(NO3)2, 99%, provided by Strem Chemicals) for 20 min, dried at 120 C for 2 h and calcined at 450 C for 3 h, obtaining a Pt loading of 1 wt% with respect the amount of washcoat and ceria. The theoretical final formula of the structured catalysts was: 1%Pt/ 20%CeO2/g-Al2O3/Al-foam. As reference, an aliquot of the washcoat slurry was dried and calcined at 450 C for 3 h, impregnated with a cerium(III) nitrate solution and a tetrammineplatinum(II) nitrate solution, dried at 120 C and calcined at 450 C for 3 h at each step.
Catalysts characterization The prepared catalysts were characterized by a series of physical-chemical analytical techniques. The ɣ-alumina particles size distribution was checked by optical microscope analysis; a sample of about 20 mg was dispersed in 20 ml of ethylene glycol, the mixture was sonicated for about 5 min and then evaluated by optical microscopy (Philips PCVC750K Camera Video). The chemical composition was evaluated by means of ARL QUANT'X ED-XRF spectrometer (Thermo Scientific). An aliquot of the sample was grinded, the resulting powders were compressed to form a tablet that was analyzed by the Fundamental parameters method for evaluating the elemental composition. The stability of the washcoated foams to mechanical stress was evaluated by ultrasound adherence test [29]; the samples were dipped in a beaker containing 200 mL of petroleum ether (provided by Carlo Erba reagenti) and the beaker placed in a ultrasonic bath CP104 (EIA S.p.A.) filled with distilled water. The tests were performed at 25 C by applying the 60% of rated power for six cycles of 5 min. The specific surface area (B.E.T.) measurements were evaluated
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with a Costech Sorptometer 1040 (Costech International), by dynamic N2 adsorption measurement at 196 C, the samples were previously treated at 150 C for 30 min in a flow of helium. The crystal phases and the crystallite dimension were evaluated by X-ray Powder Diffraction (XRD) with a D8 Advance (Brucker); the samples were finely ground and sieved, then analyzed with a Cu Ka radiation source (35 kV; 40 mA) in the 2q range 20e80 , (Stp ¼ 737; Stp size ¼ 0,0814; t/Stp 0,5 s). The structural changes, attributed to the impregnation steps, were evaluated by Raman spectroscopy by an inVia Raman Microscope (Renishaw), equipped with a 514 nm Ar ion laser operating at 25 mW, in the 200e2000 cm1 range. Scanning electron microscopy (SEM) (Assing.mod. LEO 420) at an accelerating voltage of 20 kV, was used to estimate the strut diameters and pore size of the foams to evaluate the washcoat layer. The H2-TPR experiments were carried out in the reactor described below, also used for the catalytic tests, in the temperature range of 20e450 C, with a heating rate of 10 C/min and a reducing stream of 1000 Ncc/min composed by 5 vol% of H2 in N2. The washcoat density was measured by pycnometer. The simplified Montebelli method was applied to estimate the thickness of the washcoat layer [30]. The catalyst kinetic parameters were estimated by applying the LangmuireHinshelwood kinetic model, as reported in the literature for this type of reaction [40]. The parameters were obtained by using the Euler method. The estimated parameters were: - the kinetic constant: k1 (mol/gcat/min) - the adsorption constants: KCO (), KH2O (), KCO2 ().
Catalytic activity tests The catalytic activity tests were carried out with the intent of comparing the catalytic activity of the catalyzed foams, previously reduced in the H2-TPR experiments. The tests were performed at atmospheric pressure in the temperature range 200e400 C, by feeding a reactive mixture composed by 8 vol% of CO, 30 vol% of H2O and N2 balance, in a stainless steel tubular reactor with an internal diameter of 22 mm and a length of 40 cm (all the gases were provided by SOL S.p.A.). The catalyzed foams were surrounded by a thermo expanding pad with a thickness of 2.5 mm, while the Weight Hourly Space Velocity (WHSV) was set to 1.2 h1, with respect to the CO mass flow rate.
Catalytic tests in “quasi adiabatic” conditions The catalytic tests in “quasi-adiabatic” conditions were performed on the pre-reduced catalysts, at the same conditions of pressure, with the same reactive mixture and in the same reactor indicated in Section “Catalytic activity tests”, while the inlet temperature was fixed at 260 C, the WHSV at 2.4 1 gCO*g1 and the linear flow velocity equal to 18.2 cm*s1. cat*h
Experimental apparatus The experimental apparatus was conceived with the intent to approach as much as possible an adiabatic system, avoiding
Please cite this article in press as: Palma V, et al., The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.003
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both radial and axial thermal dispersions. The thermal dispersions were minimized with the use of an appropriate heating system; this system provided three main heating zones: the reagents input area, the furnace for the reactor and the reaction products output area. The reagent input area was developed by heating a stainless steel tube (70 cm of length and internal diameter equal to 6.35 mm), connected to the reactor through a connecting fitting; the temperature of the reagent mixture supplied was monitored and adjusted by a thermocouple connected to a PID controller, in order to preheat the reactive mixture at the same temperature of the inlet section of the catalyst and to guarantee a perfect vaporization of water as well as mixing of the reaction mixture. The furnace was provided with three independent thermally controlled zones, in order to realize a large isothermal zone in the center of the reactor. The reaction products output area was developed by heating a stainless steel tube (70 cm of length and internal diameter equal to 6.35 mm), connected to the reactor through a connecting fitting; the temperature of the reaction products was monitored and adjusted by a thermocouple connected to a PID controller, in order to heat the mixture at the same temperature of the outlet section on the catalyst. With this set-up of the system, we attempted to minimize the driving force, or the temperature difference between the reactive mixture and the surface of the reactor. In preliminary experiments we monitored the thermal profile of the reactor in function of the thermic parameters applied, by filling the reactor with quartz powder and by sliding a thermocouple along the axial direction at the center of the reactor. On the basis of the registered thermal profile we chose the region of the reactor in which to insert the catalyst. The composition of the mixture of the reaction products was continuously monitored, on dry basis, by passing the mixture through a refrigerator Julabo F12, by an ABB system (infrared detector “Uras 14” for CO, CO2 and CH4 and thermal conductivity detector “Caldos 17” for H2).
Characterization results The particle size distribution of the ɣ-alumina is a crucial aspect used in the preparation of a good washcoat [31]; the technical data sheet of the commercial Puralox SCCa150/200 present in our laboratory certify a particle size distribution
extremely inappropriate for our purpose (d50 ¼ 60e150 mm), so a pulverizing step was realized as told in Section “Washcoat preparation”. The powder thus obtained was then checked by optical microscopy following the protocol described in Section “Catalysts characterization”; in Fig. 1 a comparison between the imagine before and after the milling, at the optical microscopy, of the ɣ-alumina was reported, the maximum size found in the analyzed sample was much lower than 10 mm. The ED-XRF results showed a good agreement with the theoretical ceria and platinum loading, confirming the reproducibility of the coating techniques (Table 1). The ultrasound adherence texts [32] were performed on the three washcoated foams, designed as WF40 (washcoated 40 PPI foam), WF20 (washcoated 20 PPI foam) and WF10 (washcoated 10 PPI foam). The results showed a good resistance to the mechanical stress of the washcoat layers. The weight losses (Fig. 2) have been calculated in function of the amount of the washcoat layer only; the losses occurred in the first 15 min, while there appeared to be a correlation between the weight loss and the porosity of the foam, however, the overall weight loss were, in all cases, in the order of 5e8%, in agreement with similar formulations reported in the literature. The SSAB.E.T. obtained for the structured catalysts confirmed the large increase of the specific surface area of the foams (Table 2), assessable in two order of magnitude; if compared with those obtained for ɣ-alumina, the SSAB.E.T. of the catalysts, indicated a low effect of the repeated cycles of calcination on the reduction of the surface area of the washcoat. The XRD patterns of the catalytic washcoat showed the presence of ɣ-alumina and the cubic ceria fluorite type and, vice versa, no evidence of peaks ascribable to the platinum, probably due to the low loading. The crystallite sizes were calculated by Scherrer equation (k ¼ 0.9); the overlapping of the peaks of alumina and ceria, in the final catalysts, made difficult a calculation on the former, so for ɣ-alumina the calculation was made on the washcoat powder, anyway for all the samples the crystallite size were in the order of few nanometers both for alumina and ceria, on average 6 nm for the former and 7 nm for the latter. The presence of platinum was confirmed also from the comparison of the Raman spectra before and after the platinum loading over the ceriacoated derivatives. All the ceria-coated derivatives showed
Fig. 1 e Imaging from optical microscopy of ɣ-alumina before (a) and after milling (b). Please cite this article in press as: Palma V, et al., The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.003
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Table 1 e Composition of the structured catalysts. Sample
PtCeWF40 PtCeWF20 PtCeWF10
Foam
Catalytic washcoat composition (%)
PPI
Density (%)
Al2O3
CeO2
Pt
40 20 10
10e12 10e12 10e12
78.99 79.07 79.56
20.02 19.95 19.45
0.99 0.98 0.99
Table 2 e H2 uptake in TPR experiments and specific surface area. Sample
TPR Experimental H2 Theoretical H2 uptake (mmol/g) uptake (mmol/g)
PtCeWF40 PtCeWF20 PtCeWF10 Pural SB Puralox SCCa150/200
0.25 0.16 0.12 e e
0.05 0.05 0.05 e e
SSAB.E.T. (m2/g)
61 57 55 211 132
Fig. 2 e Washcoat weight loss vs. time during the ultrasonic treatment of the washcoated foams. Fig. 3 e H2-TPR profiles of structured catalysts. the characteristic peak centered at 464 cm1 of the triply degenerate F2g mode of the fluorite type cubic ceria, due to the symmetric breathing vibration of oxygen around Ce4þ ions in the octahedral sites (CeO8) [33] and the almost imperceptible bands centered at 262 cm1 (second-order transverse acoustic mode 2TA) and 596 cm1 (defect induced mode D), attributed to the oxygen vacancies [34]. The platinum loading induced the appearance of a broad band 675 cm1, attributed to the formation of a metal oxide phase strongly interacting with the ceria surface [35], or alternatively to a distortion of the ceria structure caused by the insertion of platinum ions in the ceria lattice [36]. The H2-TPR experiments showed for all the structured catalysts two main peak regions, the first one, in the 130e150 C region, was mainly attributed to the PtOx species reduction, the second one in the 300e360 C region attrib-
aluminum foams. The results of these calculations are shown in Table 3, in which there are foam density (by picnometer measures), strut diameters (by SEM images) and washcoat layers (by Montebelli method).
Kinetic evaluation The kinetic experiments were made at three different temperatures: 220 C, 230 C and 240 C, by varying the reactants flow rate (800 Ncc/min, 600 Ncc/min and 400 Ncc/min). The feeding mixture was composed by 8 vol% of CO, 30 vol% of H2O and N2 balance. Four grams of catalytic powder were used for these experiments. The equation describing the rate of reaction was derived from the LangmuireHinshelwood model (1).
2 k1 KCO KH2O yCO yH2O yH2 yCO2 Keq 1 þ KCO yCO þ KH2O yH2O þ KCO2 yCO2
uted to the surface reduction of ceria (Fig. 3) [37]. The extremely low temperature reduction of the first peak was attributed to the ceria/alumina system [38], while the different reduction temperatures were attributed to the dispersion degree of platinum. The hydrogen uptake (Table 2) was much higher than the theoretical in all cases, also by considering the lonely first peak, suggesting the occurrence of the spillover effect [39]. Through the Montebelli method it was possible to calculate the layer of the washcoat deposited on the different
(1)
where the yi are the molar fractions of the components of the reaction. The Euler method was used in order to discretize the differential equation of the mass balance; then, through the least square method, applied between the model and experimental values, it was possible to estimate the kinetic parameters at the different conditions used for these experiments. The calculated parameters were similar to other ones already reported in the literature [40]. The results of the kinetic
Please cite this article in press as: Palma V, et al., The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.003
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Table 3 e Foams densities, strut mean diameters and washcoat layers. Sample
Foam density
Strut mean diameter
Washcoat layer
PtCeWF40 PtCeWF20 PtCeWF10
0.83 g/cc 0.85 g/cc 0.70 g/cc
300 mm 350 mm 500 mm
120 mm 290 mm 400 mm
calculations are shown in Table 4, including the parameters of the Arrhenius equation. Fig. 4 e Temperature dependence of the CO conversion of the structured catalysts.
Experimental results In this section the results of the activity tests in non-adiabatic and quasi-adiabatic conditions were reported.
Activity tests As previously mentioned, the activity tests were performed at a Weight Hourly Space Velocity (WHSV) was set to 1.2 h1, with respect to the CO mass flow rate, calculated with respect to the lonely catalytic washcoat, in the condition described in Section “Catalytic activity tests”, on the pre-reduced catalysts. The catalytic performances were estimated in terms of CO conversion (XCO) (2) and selectivity to H2 (YH2) (3), in order to evaluate the water gas shift and methanation contribution. . out Fin XCO ¼ Fin CO FCO CO YH2 ¼ Fout H2
(2)
. out Fin CO FCO
(3)
Where Fin and Fout are respectively the molar rate supplied to the reaction system and the molar rate at the outlet, of the species. The CO conversion profile (Fig. 4) was very similar for the three structured catalysts, in the above-described conditions, at high temperature. All the structured catalysts approached to the equilibrium from 255 to 260 C above; below 250 C the conversion decreased rapidly, however remaining near 40% at 220 C for the PtCeWF10 and PtCeWF40 catalysts, while the PtCeWF20 catalyst showed slightly worse performance at low temperatures. The selectivity to hydrogen and carbon dioxide, close to the equilibrium below 350 C, evidenced the occurrence of the methanation reaction only above this temperature (Fig. 5), however, since the amount of methane produced was in the order of few ppm, for all the studied catalysts, this side reaction resulted negligible, confirming published reports on similar catalytic systems [40].
The high conversion and selectivity obtained at low temperature make this catalyst a serious candidate in view of design of a catalytic system for a single stage process [41].
Evaluation of the activity tests in “quasi-adiabatic” conditions To guarantee the “quasi-adiabatic” conditions, as we previously explained, we realized a multiple heating system in order to minimize both the axial and the radial thermal dispersion, moreover the catalysts were surrounded with a thermo expanding pad avoiding the direct contact with the internal wall of the reactor, as well as avoiding the gas stream by-pass of the catalyst. The internal thermal profile of the reactor was monitored by replacing the catalyst with glass quartz, by sliding a thermocouple in the center of the reactor and recording the temperature change every centimeter, applying the same condition of reaction in terms of water and total volumetric flow rate used in the activity tests. The as obtained profile showed an almost flatter temperature from the middle to the end, while the expansion effect of the reacting mixture at the entrance of the reactor generated a gradient of 20 C. On the basis of these results we decided to conduct the tests in the flat temperature profile zone of the reactor.
Table 4 e Results of the kinetic modeling. Parameter k1 KCO KH2O KCO2
Pre-exponential factor
Activation energy
2.72 Eþ09 2.78 Eþ07 4.92 Eþ08 1.46 Eþ00
68.6 kJ/mol 74.0 kJ/mol 35.8 kJ/mol 5.8 kJ/mol
Fig. 5 e Temperature dependence of the H2 selectivity of the structured catalysts.
Please cite this article in press as: Palma V, et al., The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.003
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The tests in “quasi-adiabatic” condition were performed at a double space velocity with respect the catalytic tests reported in the previous section, realizing a total gas flow rate greater than 2600 Ncc/min (see Section “Catalytic tests in “quasi adiabatic” conditions”). The procedure used provided a preliminary phase in which the system was brought in a thermal equilibrium condition; a mixture of water and nitrogen, at the desired total gas flow rate, was vaporized and superheated at the temperature of 260 C and sent to the preheated catalyst. The so set system was considered at the thermal equilibrium when the temperature at the inlet and at the outlet of the catalyst was stable at 260 C (±0.2 C) for 30 min. At the thermal equilibrium the nitrogen was replaced, by a valves system, with a mixture of carbon monoxide and nitrogen, with the desired volumetric ratio and with the same total gas flow rate, inducing the water gas shift reaction. The resulting reaction mixture and the temperature profile at the inlet and at the outlet of the catalyst was monitored until the system reached a new thermal equilibrium, and maintained it for at least 30 min. This procedure was applied to the three structured catalysts, obtaining a characteristic difference of temperature (DT) between the inlet and the outlet of the catalytic bed for each catalyst, that was correlated to the CO conversion. The activity tests showed that at a lower temperature difference DT correspond a higher CO conversion, therefore the trend in the conversion was PtCeWF10 > PtCeWF40 > PtCeWF20, corresponding to an opposite trend for the DT (Fig. 6). The as obtained results were explained in terms of different thermal conductivity of the three foam carriers, however from this trend was not possible to find a direct correlation between the porosity of the carrier and the thermal conductivity. Indeed, the textural properties can be very effective in generating the foam thermal conductivity value. In particular, when aluminum foams are manufactured, a foaming agent is inserted inside the fused aluminum, and then it is vaporized, generating the porous structure. This structure is composed
Fig. 6 e Correlation CO conversion vs. temperature difference DT between the inlet (lower temperature) and the outlet (higher temperature) of the catalytic bed for the three structured catalysts.
Table 5 e Thermal conductivities of the tested foams. Sample PtCeWF40 PtCeWF20 PtCeWF10
Thermal conductivity 30,7 W/m/K 8,3 W/m/K 31,5 W/m/K
by a series of solid ligaments with a lump of metal at the intersection between the fibers. Bhattacharya et al. [42] demonstrated that the thermal conductivity of aluminum open cell foams depends strongly on the lump of metal between the fibers, as well as the fibers geometrical dimensions themselves. By applying the Bhattacharya method, it was possible to estimate the effective thermal conductivity of the three aluminum foams samples. The estimated values are reported in Table 5: As it can bee seen in Table 5, the more conductive foam was the 10 PPI sample, followed by the 40 PPI and the 20 PPI. The results obtained by this calculation reflect strongly the performance obtained in “quasi-adiabatic” conditions, revealing that the textural properties of the aluminum foams determine the effective thermal conductivity, and consequently the tendency to make the redistribution of the WGS heat of reaction throughout the catalytic bed.
Conclusion In this paper we presented the results of a study on the influence of the textural properties, of structured carriers, on the performances of aluminum foam based catalysts, in the CO water gas shift reaction. A comparison on the activity of structured catalysts, with the same catalytic formulation, but different porosity of the carrier was showed. In the preparation of the catalysts, the aluminum foams were firstly coated with a primer of alumina with high surface area, subsequently catalyzed with a Pt/CeO2 formulation. The ultrasound adherence tests showed a good resistance of the alumina layer to the mechanical stress, while the screening on the activity of the catalysts showed similar dependence of the CO conversion versus the temperature. By applying a much higher space velocity, a quasi isothermal adiabatic reactor was realized, and the results of these tests showed that to a reduced difference of temperature on the catalytic bed corresponds a higher CO conversion. These results were explained as a direct consequence of the thermal conductivity of the foams carriers; the back diffusion of the heat of the reaction throughout the carrier, flattens the thermal profile over the catalytic bed, increasing the temperature at the inlet and decreasing the temperature at the outlet. The redistribution of the heat over the bed allows to increase the CO conversion and the reaction rate. A correlation between the textural characteristics of the carriers and the catalytic performance was also showed. The estimation of the effective thermal conductivity of the carriers, confirmed the direct correlation with the CO conversion and, at the same time, the dependence of the thermal conductivity of the carriers from the dimensions of the lump of metal fibers in the foams with different porosity, suggested
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in previous study by Bhattacharya et al., makes the thermal conductivity itself, a link between the performance of the catalysts and the textural properties. Moreover, the catalyst kinetic parameters were estimated by applying the LangmuireHinshelwood kinetic model, by using the Euler method. These tests were made at three different flowing conditions, by varying also the inlet temperature of the reactants, from 220 C to 240 C. By applying the Arrhenius equation, it was possible to estimate the kinetic parameters, whose values appeared to be in strong agreement with other results already reported in the literature.
Acknowledgments The authors acknowledge funding by the Italian Ministry of Education, University and Research, Rome (MIUR, Progetti di Ricerca Scientifica di Rilevante Interesse Nazionale, prot. num.: 2010XFT2BB) within the project IFOAMS (“Intensification of Catalytic Processes for Clean Energy, Low-Emission Transport and Sustainable Chemistry using Open-Cell Foams as Novel Advanced Structured Materials”).
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Please cite this article in press as: Palma V, et al., The influence of the textural properties of aluminum foams as catalyst carriers for water gas shift process, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.003