CONCENTRATION POLARIZTION TO MEASURE NANO-PORE
ACCESSIBILITY
Miguel Solsona
1*, Anne-Eva Nieuwelink
2*, Vasileios A. Papadimitriou
1, Jan C.T. Eijkel
1,
Florian Meirer
2, Leon Abelmann
1,3, Wouter Olthuis
1, Bert M. Weckhuysen
2and
Albert van den Berg
11
MESA+ Institute for Nanotechnology, MIRA Institute for Biomedical Technology and Technical
medicine, University of Twente, Drienerlolaan 5, 7522 NB Enschede, The Netherlands
2
Inorganic Chemistry and Catalysis Group, Debye Institute for NanoMaterials Science, Faculty of
Science, Utrecht University, 3584 CG Utrecht, The Netherlands
3
KIST Europe, Campus E7, Saarbrücken, Germany
ABSTRACT
Understanding the deactivation process in porous catalysts is of tremendous economic significance. We demonstrate the feasibility of using the concentration polarization method to obtain the pore accessibility distribution at single particle level for the first time. By using this technique we could measure the pore blockage caused by Fe in fluid catalytic cracking particles as evidenced by correlating it to the activity. We expect this new single particle technique to increase our understanding of the deactivation processes occurring in porous catalysts.
KEYWORDS: Concentration polarization, heterogeneous porosity, catalysts INTRODUCTION
Micro-nanoporous materials are used in many applications, such as energy conversion and storage, desalination and catalysis. Catalysts need their high porosity to increase the available surface area with active sites. Fluid cata-lytic cracking (FCC) particles have a size between 50 – 150 µm and possess a heterogeneous pore network. They continuously cut long chain hydrocarbons into smaller molecules to produce gasoline and chemicals. FCC particles become deactivated by Fe, Ni and V. While Ni and V decrease the product yield by catalyzing side reactions, Fe is known to deactivate by blocking the pores at the first 2 micrometers from the surface. Several single FCC particle spectroscopic studies have been focused on measuring the catalytic activity [1] or the distribution of Fe, Ni and V [2], however, there is not yet an experimental method that physically demonstrates their pore blockage.
There are different techniques used to measure nanoporosity, however pore accessibility is difficult to measure since it could be very heterogeneous within the particles. Concentration polarization occurs when current is passed through an ion-permselective matrix, and is used for desalination or to concentrate ionic solutions. Due to the sur-face charge of the particles an electrical double layer (EDL) is formed and when the pores have a similar dimension as the EDL the transport through the pore becomes counter-ion selective. When the pore becomes completely blocked, the transport through becomes impossible. Many studies have focused on homogeneous porosity [3], while heterogeneous porosity is still under research. We propose to use the concentration polarization to probe pore block-age at single particle level and verify it by comparing it to the catalytic activity.
EXPERIMENTAL
FCC particles were sorted into 5 different Fe content levels with a homemade magnetophoretic chip, increas-ing from outlet 1 to outlet 5. Stainincreas-ing method: The catalytic activity of the sorted FCC particles was analyzed with furfuryl alcohol staining. The sorted FCC particles were dried for 5h at 600 C° (ramp of 5C°/min) and mixed with a 10 vol% furfuryl alcohol solution. A Nikon A1 confocal fluorescence microscope with a 561 nm laser was used to measure their fluorescence. Per outlet, the average fluorescence intensity of all particles was calculated.
Concentration polarization method:A solution with 10 mM Bodipy and 10mM PBS was used and a voltage difference of 80 V was applied from one inlet to the counter-outlet. A fluorescence filter and a Grasshopper3 GS3-U3-23S6M high speed camera were used to record each experiment for 30 s. Sorted FCC particles were introduced in a PDMS chip consisting of 4 inlet/outlets pairs with channels 40 µm wide and 100 µm high, Figure 1a,b. In order to have a complete overview of the porosity of the particle, 2 particles per outlet were introduced in 2 different chips, and 4 measurements per particle in different directions were made. 25 s after the start of each experiment the
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fluorescence intensity along the particle and in the electric field direction was analyzed and the highest intensity point was taken and averaged with all the experiments at each outlet, Figure 1c,d.
Figure 1: (a) (b) PDMS chip were the particles were introduced, (c) example of a conc. pol. experimental result and (d) data got from the (c).
RESULTS AND DISCUSSION
Figure 2 shows the experimental results of both methods for low (1) to high (5) Fe content level, indicating a general good agreement. The non-linear relation between both methods and the deactivation due to pore blocking is induced by the Fe distribution. Outlets 3, 4 and 5 have more Fe but localized in small spots leaving the rest of the particle accesible for furfuryl alcohol. Small differences between both methods encountered in outlet 3 could be related to the interparticle heterogeneity found in the staining results in the mentioned outlet. Besides, the small difference in outlet 5 could be caused by the intraparticle heterogeneity due to big Fe spots located in these particles.
Figure 2: (a) Average fluorescence intensity of furfuryl alcohol per outlet by measuring respectively 93,80,40,70,32 particles and (b) concentration polarization of sorted FCC particles.
CONCLUSION
We have demonstrated that concentration polarization results are correlated to furfuryl alcohol staining results. For the first time we have thus physically proven that catalyst deterioration is related to physical blocking of pores by accumulated Fe.
ACKNOWLEDGEMENTS
This work was supported by the Netherlands Center for Multiscale Catalytic Energy Conversion (MCEC), an NWO Gravitation program funded by the Ministry of Education, Culture and Science of the government of the Netherlands.
REFERENCES
[1] I.L.C. Buurmans, J.R. Martínez, W.V. Knowles, D.v.d. Beek, J.A. Bergwerff, E.T.C. Vogt and B. M. Weck-huysen, Nat. Chem. 3, 2011.
[2] F. Meirer, S. Kalirai, D. Morris, S. Soparawalla, Y. Liu, G. Mesu, J. Andrews, and B. M. Weckhuysen, Sci. Adv., 1, e1400199, 2015.
[3] A. Mani, T. A. Zangle, J.G. Santiago. Langmuir. 25, p. 3898-3908, 2009.
CONTACT
* Equal contributing and sharing first authorship
* M.S: m.solsona@utwente.com * A-E.N: A.Nieuwelink@uu.com
1 2 3 4 5 0 1x102 2x102 3x102 4x102 5x102 6x102 7x102 a.u. Outlets
Furfuryl alcohol staining
1 2 3 4 5 6,0x104 7,0x104 8,0x104 9,0x104 1,0x105 1,1x105 1,2x105 1,3x105 1,4x105 1,5x105 Concentration polarization a.u. Outlets FCC Particle 0 50 100 150 200 250 300 0 2E4 4E4 6E4 8E4 1E5 1E5 1E5 2E5 a.u. Distance d Highest peak a b c d FCC Particle 1498
