Microreactors technology for hydrogen purification: Effect of the catalytic layer thickness on CuOx/CeO2-coated microchannel reactors for the PROX reaction

1 Microreactors technology for hydrogen purification: Effect of the catalytic layer thickness on CuOx/CeO2-coated microchannel reactors for the PROX reaction O.H. Laguna*, M. González Castaño, M.A. Centeno, J.A. Odriozola Instituto de Ciencia de Materiales de Sevilla, Centro Mixto CSIC – Universidad de Sevilla, Avenida Américo Vespucio 49, 41092 Seville, Spain *Corresponding author e-mail: [email protected] Keywords Microreactor; Process intensification; CO-PROX; Catalytic layer thickness Abstract Two blocks of microreactors composed by 100 microchannels and coated, respectively, with 150 and 300 mg of a CuOx/CeO2 catalyst, were prepared and tested in the preferential oxidation of CO in presence of H2 (PROX). The deposition of different amount of catalyst resulted in different catalytic layer thicknesses thus modifying the catalytic performances of the microreactor. The evaluation of the main reaction variables (the space velocity, the O2-to-CO ratio and the presence of H2O and/or CO2 in the stream) was performed over both microreactors and compared to that of the parent powder catalyst. The least loaded microreactor, with a coating thickness around 10 µm, presented the highest CO conversion and selectivity levels at temperatures below 160 ºC. This result evidences i) the improvement of the catalytic performances got by the structuration of the powder catalyst and ii) the importance of the selection of the adequate thickness of the catalytic layer on the microreactor, which have not to exceed and optimal value. An adequate coating thickness allows minimizing the mass and heat transport limitations, thus resulting in the enhancement of the catalytic performance during the PROX reaction. 2 1. Introduction Currently, the need for alternative energy sources motives the development of new technologies based on fuels different to those based in petroleum. In this sense, many efforts have been focused in the use of H2 for transport and portable applications [1, 2]. However the production of H2 is a multi-stage process including reforming of hydrogen-containing fuels, such as hydrocarbons and alcohols, followed by successive cleaning up steps for up-taking the CO, such us the high and low temperature water-gas-shift (HT and LT-WGS) and the preferential oxidation of CO (PROX). These processes should reduce the CO levels around 1% in the case of the WGS, and then below 50 ppm with the PROX, in order to ensure a longer lifetime of the electrochemical devices where the H2 fuelled is converted in electric power. In this sense, highly active catalysts in the CO abatement reactions are required. Actually many systems with different supports and active phases have been tested in CO oxidations reactions [2-7]. The real application of the H2 technology in automotive and portable devices strongly depends on the incorporation of the different reactions involved into the engine of these systems. Besides this, the size of the compact fuel processors where the different reactions occur must be adequate for the efficiency during the transport [1]. In this regard, the microreactors are an interesting option if their diverse advantages are considered [8, 9]. The microreactors can be considered as small scale chemical reactors that achieve high conversion rates within a reduced volume [10, 11]. These devices allow the process intensification because they are able to enhance the heating exchange, being especially attractive for exothermic reactions where a strict temperature control has to be achieved. Moreover, the total flow of the feeding and the pressure in the inlet can be considerably increased, compared with those of the common packed-bed reactors. In addition, a more direct transition from the laboratory to the industrial production scale is easily achievable for microreactors [8, 9]. Several works have presented interesting progresses on the application of microreactors in different reactions [12, 13]. For example, Jang et al. [14] 3 studied a micro-methanol steam reformer and established that the geometry of the manifold and the channel size, among other variables, had a crucial influence on the efficiency and consequently must be optimized for every microreactor. This agrees with the results presented by Mathieu-Potvin et al. [10] that analysed, by a numerical simulation study, the optimal geometry of catalytic microreactors. They conclude that the competition between diffusion, advection and kinetic phenomena dictate the design of the microreactors. In this sense, some adimensional numbers must be considered, such us the Schmidt and the Bejan number that are related to the behaviour of the fluid into the channels. Additionally, it was pointed out that the simulation of the microreactor behaviour requires the optimization of the mass of catalyst introduced into the microchannels and of the total flow used, in order to achieve the adequate aspect ratio that improves the maximum conversion. Among the different variables considered during the designing and manufacturing of microreactors, the optimization of the amount of loaded catalyst is a capital parameter for obtaining the maximum catalytic performance and consequently, the control of the catalytic layer thickness results determinant. This variable may strongly influence the heat and mass transport phenomena on the catalytic behaviour of the microreactor. In this sense, the modification of the products distribution by the variation of the selectivity and the decreasing of the heat transport could be noticeable if the catalytic layer thickness is outside the optimal value. The efficient releasing of heat during the reaction is crucial for exothermic processes such as PROX. Microreactors may improve the releasing of the heat produced in such exothermal reactions avoiding the formation of hot spots and allowing a strict control of the reaction temperature. For instance, recently Llorca et al. [15] have presented the study of a silicon micromonolith containing ca. 40000 regular channels of 3.3 µm in diameter per square millimeter containing Au/TiO2 as the catalysts coated in the walls of the channels, for the PROX reaction. The authors demonstrated that although their silicon microreactor did not achieve high CO conversions levels at low temperatures (150 – 240 ºC), this system was considerably more efficient than a conventional 4 400 cpsi (cells per square inch) cordierite monoliths, which presented a poor heat transfer during the process. Ouyang et al. [16] have showed the lower radial and axial temperature gradients in microreactors compared to those of packed bed reactors, which minimize the extend of the reverse WGS reaction, and favour higher CO conversions. However, if the amount of catalyst coated in the walls of the microchannels is very high, this better heat releasing rate may be less obvious, and the occurrence of some mass transport limitations inside the catalytic layer cannot be fully discarded. This problem was studied by Potemkin et al. [17] over a microreactor loaded with a 5 wt.% Cu/CeO2-X catalyst. The authors proposed the calculation of an internal effectiveness factor (ηCO), determined as the ratio of the reaction rate calculated assuming the existence of mass transfer limitations to that calculated in the assumption of zero mass transfer limitations. The analysis of the ηCO factor for different thickness of catalytic layers suggested that the lowest contribution of mass transport limitations in microreactors is produced when the thickness does not exceed ~20 µm. Obviously, we must take into account that this value is approximate and only valid for the specific catalyst used in that work, since the textural properties of the catalyst (pore size distribution, etc.) may control the kind and rate of diffusion of the reagents and products along the channels. From all above, it is clear that the development of microreactors still requires additional studies where many variables have to be optimized in order to generate a complete knowledge about the designing, manufacturing and applying of microreactors as catalytic systems. The present work pretends to analyse different catalytic experimental results obtained with two blocks of microreactors, composed by 100 microchannels, coated with different total amounts of a CuOx/CeO2 catalyst (150 and 300 mg) for the PROX reaction. 2. Materials and methods 2.1. Synthesis of the CuOx/CeO2 catalyst and manufacturing of the microreactors 5 Recently we have published the experimental procedures for the synthesis and the characterization by means of N2 adsorption-desorption, XRD, Raman spectroscopy and H2-TPR studies of the CuOx/CeO2 powder catalyst [18, 19]. The manufacturing process of our microreactor prototype coated with 300 mg of catalyst [4, 18-20] is summarized in Figure 1. In the present work, this microreactor is named as MR300 and will be compared with a new block coated with 150 mg of catalyst, denoted as MR150. 2.2. Catalytic activity measurements Prior to every catalytic test the catalysts were treated under 30 mL/min total flow of 21% O2 in N2 at 300 ºC for 2 h. For the powder catalyst, the PROX reaction was carried out in a fixed-bed cylindrical stainless steel reactor with internal diameter of 9 mm under the experimental conditions presented in [21]. For these experiments, 100 mg of catalyst (particle size= 100 < ϕ < 200 μm) were employed and diluted with crushed glass (in the same particle size) forming a bed of about 50 mm in length. The CO conversion and the selectivity to CO conversion were calculated according to Eq. 1 and Eq. 2, respectively. Fin and Fout refer to molar flow rates at the reactor inlet and outlet, respectively [4, 20]. CO conversion (%) in CO out CO in CO F FF , ,, 100)(    (Eq.1) Selectivity to CO conversion (%) )(2 100)( ,, ,, 22 outOinO out CO in CO FF FF     (Eq.2) In the case of microreactors, the same reaction set-up was used; however the cylindrical reactor was replaced by two stainless steel cases connecting the inlet and outlet positions of the microchannels, and allowing the contact between the coated walls of the channels and the feed-stream according to that described in [4, 20]. The temperature was continuously monitored by 4 K-type thermocouples. Two of them were placed in contact with the metallic block at 6 the inlet and outlet positions of some central microchannels. The other two sensors recorded the temperature at lateral positions in the walls of the microreactor [4]. The feed-stream compositions and the λ values used in the catalytic tests are presented in Table 1. The λ value was calculated for every mixture of reaction according to Eq. 3 [22].   .% .%2 2 COvol volO    (Eq. 3) A first comparison of the catalytic performances of the three catalysts studied, the powder and the MR150 and MR300 microreactors, was carried out employing the feed-stream A and maintaining the flow-to-catalyst weight ratio in 60 L·h-1·g-1. Additionally, the influence of the space velocity (30, 60 and 120 L·h- 1·g-1) was analyzed for both microreactors applying the feed-stream B. The influence of the λ value was also evaluated by using the feed-streams B, C and D at a constant space velocity of 60 L·h-1·g-1. Finally, the effect of the presence of CO2 and/or H2O was studied with the feed-stream compositions A, E, F, and G at a constant space velocity of 60 L·h-1·g-1. 3. Results and discussion 3.1. Comparison of the CuOx/CeO2 powder catalyst and the microreactors MR150 and MR300 for the PROX reaction The catalytic results obtained for the CuOx/CeO2 powder catalyst and for the two microreactors under a space velocity of 60 L·h-1·g-1 are presented in Figure 2. In all three catalysts, the CO conversion increases with temperature near to full conversion at 160ºC and above (Figure 2A). However at temperatures below 160 ºC, different behaviours are appreciable. MR150 presents the highest 7 catalytic activity while the MR300 exhibits the lowest CO conversion levels. The powder catalyst presents an intermediate behaviour. The selectivity to CO conversion (Figure 2B) decreases with the temperature in the three cases. Both microreactors present similar selectivities each other but higher than that of the powder catalyst, which suggests that the H2 consumption reactions (H2 oxidation and/or R-WGS) would be inhibited in them. From here, it is clear that the microreactor loaded with the lowest amount of catalyst (MR150) presents better catalytic performances than those of the powder catalyst. Besides this, although both microreactors has a similar selectivity to CO2, the highest CO conversion levels showed by the lowest loaded microreactor (MR150), makes it the system with the best performance. The observed differences in CO conversion and selectivity among the three evaluated systems may be attributed to the existence and different extension of mass and heat transport phenomena. The highest catalytic activity of the MR150 microreactor, must be related with the best efficiency in the release of the heat produced by the exothermic reactions implied (CO and H2 oxidations and WGS). This allows a better thermal control into the microchannels and ensures the isothermal conditions at every evaluated temperature, avoiding the hot spots formation. Consequently, although the difference between the apparent activation energies for the CO and H2 oxidation reactions is not so big (36.9 and 110 kJ/mol respectively) [21], the achieved strict control of the temperature in the MR150 delays the appearance of the H2 oxidation reaction, especially at lower temperatures. Our results are in good agreement with the highly efficient heat transfer that has been widely described for other microreactors employed in the PROX reaction, some of them coated with CuOx/CeO2 catalysts [16, 23, 24]. For instance Snytnikov et al. [25] have studied a copper-cerium oxide catalyst for the PROX reaction as a powder catalyst in a fixed-bed reactor and coated in microreactors. They have proposed that in the case of microreactors, the releasing of the heat produced during the oxidation reaction is very fast because of the direct contact between the catalytic layer and the metal substrate. However, in the case of the fixed-bed 8 reactor, the presence of quartz, which is used as diluent, may present heat transport limitations due to its rather low thermal conductivity. A similar effect could occur in our case for the powder catalysts, since it was tested in a fixedbed reactor and diluted with ground glass. Besides this, the superior catalytic behaviour of the MR150 microreactor respect to that of the powder catalyst may be also associated to the diminution of the mass transfer limitations in the structured systems. Probably, the contact between the catalyst and the reaction mixture is different in both situations and, additionally, the presence of ground glass with the powder catalyst in the fixedbed reactor also represents an additional barrier for the adequate mass transport. Despite the advantage of the MR150 structured system, the lower catalytic performance at temperatures below 160ºC of the MR300, even inferior to that of the powder catalyst highlights the need to optimize the amount of coated catalyst in the microreactors, that consequently means the need to optimize the thickness of the catalytic layer. As said in the introduction section, the relevance of optimizing of the catalytic layer thickness was emphasized by Potemkin et al. [17] who demonstrated that that optimum values of the internal effectiveness factor for the CO oxidation, ηCO, which assures no mass limitations, for the PROX reaction over a microreactor coated with a Cu/CeO2-x catalyst in the 170–230ºC temperature range, are achieved when the washcoated thickness did not exceed 20 µm. 3.2. Effect of the space velocity over the microreactors In order to evaluate the influence of the space velocity on the catalytic performances of the structured systems, three total flow to weight of catalyst ratios were studied (30, 60 and 120 L·h-1·g-1), keeping the same composition of the reactive feed and with a λ = 1 value. The results of CO conversion and selectivity to CO oxidation are presented in Figure 3. 9 The MR150 microreactor presents a maximum in the CO conversion at around 140 ºC whatever the space velocity used, although the value of such maximum conversion decreased when increasing the space velocity. Moreover, higher the space velocity of the experiment, lower the catalytic activity of the system at temperatures below 140 ºC. On the other hand, the MR300 microreactor shows a remarkable increasing of the CO conversion with the temperature, no matter the space velocity, up to 160 ºC. At higher temperatures, the catalytic activity tends to stabilize. In every case, the conversion curves shift to higher temperatures when increasing the space velocity. The block coated with 300 mg of catalyst exhibits higher conversions than those of the MR150 microreactor at high temperatures. However, in this temperature range, the MR150 preserves a superior selectivity to CO oxidation. For both microreactors, the increment of the space velocity results in the decrease of the catalytic activity, especially at temperatures below 160 ºC. Considering that maldistributions of feed-stream into the channels of the studied microchannels blocks may be discarded according to previous CFD studies carried out for designing the microreactors and the cases for inserting them in the reaction system [26], the low CO conversion shown by both blocks, especially at low temperatures occurs because the active sites are working under demanding conditions (a high amount of CO molecules to be converted per unit of time and an insufficient thermal activation), showing their full potential. Snytnikov et al. [23] reported similar observations studying microreactors coated with Cu/CeO2-x for the PROX reaction. They concluded that, when the flow rate is increased, the minimum outlet CO concentration is increased and the optimum reactor working temperature, corresponding to the minimum CO concentration emission, was shifted to higher values. 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Manufacturing of the microreactor prototype: (a) Micromilled plates; (b) Joining of the plates; c) Final microchannels block Figure 2. Catalytic activity of the three evaluated systems (CuOx/CeO2 powder, MR150 and MR300) in the PROX reaction: (A) CO conversion; B) Selectivity to CO oxidation Figure 3. Catalytic activity of the microreactors (MR150 and MR300) modifying the space velocity: (A) CO conversion; (B) Selectivity to CO oxidation Figure 4. Catalytic activity during the PROX reaction of MR150 and MR300 modifying the λ value in the feed-stream: (A) CO oxidation; (B) Selectivity to CO oxidation Figure 5. Catalytic activity of the MR150 and MR300 including CO2 and/or H2O in the feed-stream: (A) CO oxidation; (B) Selectivity to CO oxidation Figure 6. Effectiveness factor as a function of the temperature during the PROX reaction for the CuOx/CeO2 powder catalysts, MR150 and MR300 20 Figure 1 21 Figure 2  120 140 160 180 200 0 10 20 30 40 50 60 70 80 90 100 CuOx/CeO2 (powder) MR150 MR300 Conversion of CO (%) Temperature (ºC) (B) (A) Space velocity = 60 L·h-1·g-1 Feed-stream A (Table 1)  = 2 120 140 160 180 200 0 10 20 30 40 50 60 70 80 90 100 Space velocity = 60 L·h-1·g-1 Feed-stream A (Table 1)  = 2 CuOx/CeO2 (powder) MR150 MR300 Selectivity to CO oxidation (%) Temperature (ºC) 22 Figure 3  120 140 160 180 200 0 10 20 30 40 50 60 70 80 90 100 30L·h-1·g-1 60L·h-1·g-1 120 L·h-1·g-1 CO conversion (%) Temperature (ºC) = 1 Feed-stream B (Table 1) MR150 MR300 120 140 160 180 200 0 10 20 30 40 50 60 70 80 90 100 = 1 Feed-stream B (Table 1) 30L·h-1·g-1 60L·h-1·g-1 120 L·h-1·g-1 Selectivity to CO oxidation Temperature (ºC) (A) (B) MR150 MR300 23 Figure 4  120 140 160 180 200 0 10 20 30 40 50 60 70 80 90 100 CO conversion (%) Temperature ºC  = 1.5  = 2.0  = 3.0 MR150 MR300 60 L· h -1 · g -1 120 140 160 180 200 0 10 20 30 40 50 60 70 80 90 100 60 L· h -1 · g -1 Selectivity to CO oxidation (%) Temperature ºC  = 1.5  = 2.0  = 3.0 MR150 MR300 (A) (B) 24 Figure 5  125 150 175 200 225 250 0 10 20 30 40 50 60 70 80 90 100 H2O CO2MR300 CO conversion (%) Temperature (ºC) MR150 0% 0% 2% 0% 0% 10% 2% 10% 60 L·h-1·g-1  = 2 125 150 175 200 225 250 0 10 20 30 40 50 60 70 80 90 100 60 L·h-1·g-1  = 2 Selectivity to CO oxidation (%) Temperature (ºC) CO2H2OMR150 MR300 0% 0% 2% 0% 0% 10% 2% 10% (A) (B) 25 Figure 6  120 140 160 180 200 0 1 CO Temperature (ºC) CuOx/CeO2 (powder) MR150 MR300