Remote Fluorination by Spillover of Atomic Fluorine - The Journal of

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Remote Fluorination by Spillover of Atomic Fluorine Norbert S. Chilingarov, Andrey Yakovlevich Borschevsky, Boris Vasil'evich Romanovsky, and Lev Nikolaevich Sidorov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05464 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on November 10, 2018

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The Journal of Physical Chemistry

Remote Fluorination by Spillover of Atomic Fluorine Norbert S. Chilingarova, Andrey Ya. Borschevskya*, Boris V. Romanovskya, Lev N. Sidorova a

Department of Chemistry, M. V. Lomonosov Moscow State University, Leninskiye Gory 1/3,

Moscow 119991, Russian Federation Corresponding author: tel: +7 4959395396, fax: +7 4959391240, e-mail: [email protected] Abstract The phenomenon of atomic fluorine spillover was studied by comparing fluorination of an isolated platinum plate and of one welded to a nickel plate. Fluorination through the gas phase was performed by atomic fluorine generated by thermal decomposition of spatially separated TbF4. The reactions were carried out in a Knudsen effusion cell and monitored mass spectrometrically with subsequent microscopic investigation of the fluorinated Pt and Ni surfaces. We demonstrate that the contact with Ni strongly enhances the rate of Pt fluorination due to additional influx of adsorbed fluorine atoms across the nonstoichiometric NiF2x layer that forms on the Ni surface. Related earlier studies with iron and cerium tri- and tetrafluorides suggest that diffusion of atomic fluorine to the zone of its final consumption (e. g. Pt) across vacancy-rich intermediary surface layers of transition metal fluorides is a common phenomenon. Furthermore, such layers appear to suppress atomic fluorine recombination. We compare our present findings with the literature data on the spillover of hydrogen and oxygen. Introduction The diffusive transfer of active species, such as H, O, N atoms or NCO radicals, from one phase, where they form, to another phase, with which they react chemically, was experimentally discovered more than 50 years ago1 and later named “spillover phenomenon”2. Over the past decades, the spillover effect has been studied in more detail, both experimentally and theoretically. 1 ACS Paragon Plus Environment

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The investigations have been aimed mainly to shed light on atomic hydrogen spillover since it plays a key role in many heterogeneous catalytic processes. In particular, some supported metals (Pt, Pd, Ni) capable of dissociative adsorption of molecular hydrogen, being mixed with transition metal oxides, such as MoO3, V2O5, Fe2O3, Co3O4, were shown to reduce them by hydrogen at the temperatures much lower than occurs usually35. The results of these experimental observations were commonly considered as direct evidence of the spillover effect, i. e. the transfer of hydrogen atoms from their place of generation to the reducible oxide along the support. A number of publications have appeared recently, in which the role of the support in the delivery of the active species to the reaction zone has been analyzed68. In particular, the authors of work, published recently8, have described the results of their experiment in which they arranged the nanoparticles of metallic platinum and iron oxide, separated from each other by 045 nm. Two different oxide supports were used, namely, reducible TiO2 and nonreducible Al2O3. The reduction of iron oxide by hydrogen was observed only when using titanium oxide as a support. In such a way, the spillover effect or, in other words, the flow of atomic hydrogen from the generation zone along the direction of the reduction zone was shown clearly to depend in a crucial way on the reducibility or non-reducibility of the support. The present research is focused on the process of remote fluorination which takes place when the zones of atomic fluorine generation and its intake are spatially separated from each other, so that the delivery of active F atoms to the point of their irreversible consumption can be realized by several means. In the first case, the diffusive transfer in the homogeneous gas phase was realized for a platinum hexafluoride and krypton difluoride preparation9,10. Atomic fluorine was obtained by the thermal dissociation of F2 molecules in the hot zone and delivered to the cold zone by gas phase diffusion. Another method of obtaining atomic fluorine is thermolysis of transition metals higher 2 ACS Paragon Plus Environment

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fluorides. It was shown that during thermal decomposition of TbF4, atomic fluorine is released, and the appearance of molecular fluorine is the result of its recombination11. The peculiarity of this source of atomic fluorine is that there are two ways of moving atoms into the reaction zone. Surface diffusion is added to transfer through the gas phase, atomic fluorine can move along the surface of the carrier towards the object of fluorination (heterogeneous remote fluorination), and in some cases the diffusion path proves to be more effective than the gas phase path12,13. With remote fluorination, the distance to which atomic fluorine moves along the surface of the carrier reaches several millimeters, which makes it easy to understand the role of the chemical reaction of active particles with the surface of the carrier, followed by the formation on the carrier of a nonstoichiometric compound forming the vacancy conducting layer. We believe that in catalysis a similar mechanism of moving active particles along the carrier surface is realized. Understanding this mechanism will undoubtedly be useful for the application of any kind of spillover. In the present work, heterogeneous remote fluorination is considered. Experimental Experimental studies of the interaction of atomic fluorine with metallic platinum surfaces were carried out using the Knudsen effusion method with mass spectral registration of the evaporative products. A detailed description is given in12. A nickel Knudsen effusion cell 12 mm in height, inner diameter of 8 mm, and an effusion orifice of 9.6102 mm2 in area was used as a reactor. The cell was preliminarily passivated by molecular fluorine at 740 K and p(F2) ≈ 5 bar for ~10 hrs, so that a NiF2 protective layer formed on the inner surface of the cell. As a source of atomic fluorine, terbium tetrafluoride TbF4 was used, the thermal decomposition of which proceeds with the atomic fluorine liberation. At 630 K atomic fluorine dominates in the vapor phase of TbF4 and there is no equilibrium between F and F2 in Knudsen chamber11. 3 ACS Paragon Plus Environment


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Pt

Ni

Pt Ni

a

b

Fig. 1a. Nickel substrate with welded Pt plate.

Fig. 1b. Combined Pt/Ni plate used in exp. 3.

The surface structure of the Pt and Ni plates were analyzed by the SEM-EDX technique using SUPRA 50 VP (LEO, FRG) apparatus equipped with a secondary electron detector. The elemental composition of the surface layers was determined using the energy-dispersive X-ray detector INCA X-MAX (Oxford Instruments, UK) installed on a microscope. Particular attention was paid to the analysis of the layers formed during fluorination on the surface of platinum and nickel14. According to the data of this work Pt layer is a solid solution of Pt2F6PtF4. In the first experiment, a nickel plate with a size of 720.2 mm3, to which a platinum plate with a size of 320.1 mm3 was fixed with spot welding, as shown in Fig. 1a, was placed in the reactor. The width of the strip, free from contact with the nickel surface, was 1 mm, so that the areas accessible for gaseous components were different for two sides of the welded Pt plate. The combined Pt/Ni plate was mounted on a platform made of nickel foil (see Fig. 3). A platform with a Pt/Ni plate as well as TbF4 was loaded into the cell in an atmosphere of dry argon. The contact of the plate with the TbF4 phase was eliminated. The temperature of the cell was in the range 598– 648 K. Table 1. Partial pressures (atm) of the observed gas phase species at 623 K. Sample

p(F)

p(F)/p(F2)

p(F2)

exp.

calc.

p(PtF4)

p(PtF6)

Pt/Ni (exp. 1)

1.2×106

9.6×107

1.2

0.2

11×107

5.2×107

Pt (exp. 2)

1.5×105

5.0×106

3.0

0.06

9.0×106

3.2×106

4 ACS Paragon Plus Environment

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Pt/Ni (exp. 3)

6.6×106

9.0×106

0.7

0.07

13×106

4.4×106

The partial pressures of molecular and atomic fluorine as well as the pressures of platinum tetra- and hexafluorides are given in Table 1 (exp. 1). A fundamentally important result of the experiment is the fact that the thickness of the layer on the smaller side of the platinum plate (Fig. 2a) turned out to be twice as large as on the side with the larger area (Fig. 2b). The thicknesses in both figures are indicated by the line segments.

a



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b Fig. 2. SEM image of the layer formed on the smaller (a) and larger (b) side of the platinum plate (exp. 1); d(NiF2) = 1.0m; d(PtF4Pt2F6) = 8m (smaller area); d(PtF4Pt2F6) = 4m (larger area). For more detailed information on the role of nickel, fluorination of pure platinum, as well as platinum in the combined Pt/Ni plate, were investigated. The performance of the experiments was similar to that described above. Fluorination was carried out in the same effusion cell using TbF4 as a thermal source of atomic fluorine; the temperature (623 K) and the duration of fluorination (11 hrs) were the same. A platinum plate (exp. 2) was placed in the previously used nickel platform with minimal contact with passivated nickel (Fig. 3).


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Slots

Ni holder

Pt Ni chamber

Fig. 3. Effusion cell with installed nickel holder. The design of the combined Pt/Ni plate in exp. 3 (Fig. 1b) was different from that used in the first experiment: the platinum plate (3.54.00.1 mm3) was welded to the center of the nickel plate (foil, d = 0.2 mm). The Pt/Ni plate was placed directly into the reactor, and its contact with the inner surface of the reactor was minimized. Just as in exp. 1, the contact of the plates with the TbF4 phase was eliminated. In all three experiments both atomic and molecular fluorine, as well as volatile platinum tetraand hexafluorides, were detected in the effusion beam. The partial pressures of these components at 623 K, derived from the appropriate ion currents11, are listed in Table 1; the ratios of F/F2 experimental partial pressures in comparison with thermodynamically calculated ones are also given.

Results and Discussion The main and unexpected result of this work is the presence of a diffusion flux of atomic fluorine over the surface of weakly fluorinated nickel into the reaction zone with platinum, instead of the expected further fluorination of Ni and the formation of a passivating NiF2 layer, as would be 7 ACS Paragon Plus Environment


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necessary from thermodynamic considerations. The formation of a thick layer of fluorinated platinum is illustrated in Fig. 4. G a s Thin NiF2-x layer

P h a s e F(g)

F(g)

F(ads)

F(g)

Platinum fluoride Atomic Fluorine Flux Pt plate

Ni plate

Fig. 4. Formation of a layer of fluorinated Pt by the migration of atomic fluorine on the Ni surface. A. Surface layer of nickel difluoride The common feature of experiments 1 and 3 is the formation of a NiF2 layer on the nickel surface. However the morphology of these layers differs significantly from passivated Ni surface where the phase of difluoride completely covers the surface of the metal (see Fig. 5a). In our case after 11 hrs of interaction of atomic fluorine with Pt/Ni plate at T = 623 K a thin layer of nickel difluoride is formed (see Fig. 5b).


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a

b Fig. 5. (a) SEM image of the phase of nickel difluoride formed after passivation of the nickel 9 ACS Paragon Plus Environment


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surface; (b) SEM image of a thin layer of nickel difluoride formed on the Ni part of combined Pt/Ni plate (exp. 3). The characteristic texture of the metal clearly manifests itself through a thin layer of NiF2. In addition, according to the energy-dispersive X-ray detector INCA X-MAX the F/Ni atomic ratio is 0.9 (for comparison: for the layer of passivated Ni surface this ratio is 3.0), which show the thickness of the NiF2 layer in the combined Pt/Ni plate is miserable and the formation of a deficient NiF2x vacancy layer is realized. It is worth noticing that the indicated above experimental F/Ni molar ratios for NiF2 (3 and 0.9) refer to the thick and the thin layers, respectively. The theoretical value is, of course, 2. Nevertheless, the EDX method gives 3 because of discrimination for light elements (common feature of the method). When a thin layer is being analyzed (the thickness is less than the depth of penetration of irradiation into the sample) the fraction of Ni increases and the ratio decreases. The NiF2x vacancy layer seems to be a very efficient transport medium for adsorbed atomic fluorine migration resulted in the influx of fluorine to form a Pt2F6PtF4 layer on the platinum surface. Respectively the flow of fluorine from the NiF2x surface layer to the reaction zone (Pt) decreases or even stops further fluorination (passivation) of the nickel substrate. B. Surface layer of Pt2F6PtF4 As can be seen from Table 1, in all three experiments at 623 K after 23 hrs the same components were present in the gas phase and the partial pressures p(F), p(F2), p(PtF4) and p (PtF6) as well as the ratios p(F)/p(F2) have close values. Only a weak decrease in the ratio p(F)/p(F2) with time was noticed. However, the SEM/EDX analysis of the platinum surface showed significant differences for the pure Pt plate (exp. 2) and the combined Pt/Ni plates (exp. 1 and 3). In exp. 2 at 623 K after 11 hrs contact of the pure Pt plate with the gas phase, a thin transparent layer of Pt2F6PtF4 formed on its surface (70 nm). This layer is visible on the SEM images of the surface at a relatively large magnification (Fig. 6a). The platinum plate retained a metallic luster after 10 ACS Paragon Plus Environment

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fluorination. In addition, because of the small thickness of the layer, the characteristic surface texture of the metal is clearly seen. This thin transparent layer may be considered as an intermediate product of the reaction of atomic fluorine with Pt accompanied by the formation of PtF6(g). During the time of fluorination a stationary state of the reaction system is achieved when the flux of F atoms from the gas phase to the Pt plate equals the reverse flux from the plate in the form of tetrafluoride and hexafluoride.

a



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b Fig. 6. (a) SEM image of a thin Pt2F6PtF4 layer formed on the platinum plate (exp. 2); (b) SEM image of a thick Pt2F6PtF4 layer formed on the Pt part of combined Pt/Ni plate (exp. 3). In exp. 3, when a platinum plate is reinforced on a nickel plate as a substrate (see Fig. 3) under the same fluorination conditions (temperature, time, partial pressures of F, F2, PtF4, PtF6) a noticeable thickening (from 70 nm to 3 μm, i.e., an increase of 40 times roundly) of the Pt2F6PtF4 layer on the Pt plate surface is observed (Fig. 6b), while the process itself has not undergone any noticeable changes (see Table. 1). Such a thickening could occur only due to the surface flow of F atoms from the nickel substrate to the Pt plate as the flow of gas phase F atoms does not change. The growth of the layer proceeds uniformly over its entire area which indicates free movement of fluorine atoms on the surface layer. The diffusive transfer of fluorine atoms between two phases is described in15. A continuous flow of gaseous atomic fluorine was directed to a mixture of crystals of cobalt difluoride and pieces of platinum foil. In the initial period of fluorination at 570720 K only one volatile product PtF6 was 12 ACS Paragon Plus Environment

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formed, whereas the formation of CoF4, the second volatile product, occurred much later and was accompanied by a significant decrease in the partial pressure of PtF6. The authors relate this result to the change in the diffusion flux of atomic fluorine between these phases. C. Remote fluorination In experiments with iron trifluoride it was found that at 9801100 K FeF3 evaporates without decomposition from the Knudsen platinum cell13. However, if FeF3 is not placed directly in the Pt cell, but in the platinum liner which is closely adherent to the inner wall of the nickel cell, which was not previously passivated, its thermolysis at 1050 K is accompanied not only by the formation of the FeF2 phase inside the cell, but also by the formation of a NiF2 layer on the wall of the cell between the outer surface of the platinum liner and the inner surface of the nickel cell. Recently, an unusual behavior of cerium tetrafluoride during its sublimation from the Knudsen platinum cell was discovered12. The effusion beam, leaving the chamber, consisted of CeF4 molecules only, and there were no other fluorides as well as atomic and molecular fluorine. After complete evaporation of cerium tetrafluoride, registered mass spectrometrically, there still remained at least 20% of the initial cerium in the form of the CeF3 phase, which is much less volatile than CeF4. The entire inner surface of the platinum cell was covered with a layer of trifluoride, the thickness of which increased in the direction of the effusion hole. The last observation indicated that atomic fluorine, formed as a result of the thermolysis of CeF4, left the chamber, moving along the surface of the edges of the effusion hole and subsequently the flow of atoms F spread over the outer surface of the Pt cell, fluorinating not only platinum itself, but also the metallic container. The formation of CeF3 as a result of the thermal decomposition of CeF4 and the transfer of atomic fluorine to a potential object of fluorination, i. e. the outer surface of the platinum cell and the holder, provides a very clear picture of the migration path of F atoms along the inner surface of the effusion chamber. The thickness of the formed CeF3 layer gives a qualitative characteristic of the 13 ACS Paragon Plus Environment


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flux density of atomic fluorine and indicates its increase when moving towards the object of fluorination. In other words, collection of fluorine atoms takes place from the entire inner surface of the platinum cell, coated with CeF4, and the diffusion flux of fluorine atoms to the reaction zone is marked by CeF3. D. Spillover phenomena in remote fluorination, oxidation and reduction The interaction of atomic fluorine with metallic nickel results in the formation of a deficient NiF2x vacancy layer. This layer seems to be not only a very efficient transport medium for atomic fluorine but also it decreases or even stops further fluorination (passivation) of the nickel substrate due to drain of F atoms from the NiF2x surface layer to the reaction zone (Pt). In the case of atomic oxygen, a mechanical contact of a platinum tablet 40 nm thick with a carbon sheet of 100 nm resulted in the burn out of the carbon under the Pt piece at 277C in pure O27. However, there is no carbon oxidation if a tablet of non-reducible Al2O3 was inserted between the Pt and the carbon. Quite the contrary, if the tablet of СеО2x was placed there, the carbon burn out occurred not only under the Pt tablet, but also around the cerium oxide layer. Thus the nature of the surface that is capable or unable to transfer oxygen atoms plays the crucial role in this case. A similar situation takes place in the case of heterogeneous remote fluorination where support has to be able to transfer fluorine atoms. In both cases the active species delivering to a point of consumption were provided by non-stoichiometric layers having fluorine or oxygen deficit. Another case is observed, when the sublimation of CeF412 or FeF313 takes place from a platinum effusion cell that is tightly inserted into a steel or nickel holder. In both cases there is a negligible amount of atomic or molecular fluorine in the gas phase. The inner surface of the cell is covered with a layer of higher fluorides, and near the effusion orifice fluorides contact an external reducing agent  the steel or metallic nickel surface. This is a region of atomic fluorine consumption or by the terminology of the authors12, a point of fluorine drain. At any case the density of fluorine 14 ACS Paragon Plus Environment

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atoms on a surface (Fs) goes to zero in this region and as a result there is a gradient of the Fs concentration in the effusion cell and surface diffusion of atoms to the point of fluorine drain is on. In case of CeF4 sublimation12 the layer, formed on the inner surface of a platinum cell is the adsorption layer of CeF4 but after the loss of fluorine atoms it is fluorine deficient layer CeF4x and after full evaporation of CeF4 the inner surface of the platinum cell covered by hard volatile CeF3. The density distribution of this layer gives the paths of atomic fluorine diffusion. It would be of interest to compare the spillover of atomic hydrogen above the reducible titanium oxide as a substrate, with that of atomic fluorine within the F deficient layer of higher cerium and iron fluorides. Both processes seem to be very similar, at least from a pure phenomenological point of view. In both cases a diffusion transfer ensures that the active species is delivered from its place of origin to the point of loss. Likewise, the diffusion is efficacious only if it occurs using some vacancy layer as a proper way for the active species to go from the generation phase to the consumption one without significant recombination. This is the case in both H and F spillover. Nevertheless, there is a difference in the formation of such vacancy layers. Hydrogen spillover takes place over a surface layer with vacancies, so to say, "inborn" or resulted from partial surface reduction of a reducible oxide like TiO2. In contrast, the vacancy layer transporting atomic fluorine is created from Ce and Fe fluorides as the volatile compounds are deposited on the support (on the inner side of the cell) from the gaseous phase. Conclusion Summarizing the above discussed cases of remote solid phase fluorination (Fs), one can conclude that the fluorination is facilitated by vacancy-rich layers of non-stoichiometric compounds. Those layers form on the surface of the intermediary substrate that enables efficient delivery of active fluorine atoms (or other active species, such as atomic hydrogen in heterogeneous catalysis) 15 ACS Paragon Plus Environment


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from their source to the spatially separated zone of their consumption whilst also inhibiting their recombination into far less reactive molecules. To form a non-stoichiometric layer, the active species must react either with the surface of the intermediary substrate itself or with some auxiliary adsorbed phase. The flow of atomic fluorine toward the consumption zone is governed by the gradients of the fluorine chemical potential between the source compound (e. g. FeF3, CeF4, TbF4), the intermediary layer, and the target fluorinated phase (Pt, Fe, Ni), as well as by the integral fluorine conductivity in the intermediary layer and through its interface with the target phase. Importantly, the fluorine chemical potential and diffusion coefficient in the intermediary layer are uniquely related to its stoichiometry. Therefore, the stoichiometry of the surface layer usually varies in the initial period of the process and then reaches a stationary state governed by a detailed balance of the three fluxes of fluorine – the adsorption flux from the source compound, the stoichiometrydependent diffusion flux through the intermediary layer, and the consumption flux to the target fluorinated phase.

Acknowledgments This work was supported by the Russian Foundation for Basic Research (grant No. 16-0300678). The authors acknowledge partial support from M. V. Lomonosov Moscow State University Program of Development. We are also grateful to Zoran Mazej and Igor Shlyapnikov for providing the sample of TbF4, Ilya Ioffe, Michael Temerin and Natalia Lukonina for helpful discussion. References 1.

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13. Chilingarov, N. S.; Skokan, E. V.; Rau, J. V.; Sidorov, L. N. A Mass-Spectrometric Study of Iron Trifluoride Decomposition. Russ. J. Phys. Chem. 1994, 68, 11831189. 14. Tressaud, A.; Pintchovski, F.; Lozano, L.; Wold, A.; Hagenmuller, P. Un Nouveau Fluorure de Platine: PtIIPtIVF6. Mat. Res. Bull. 1976, 11, 689694. 15. Chilingarov, N. S.; Rau, J. V.; Nikitin, A. V.; Sidorov, L. N. A Mass-Spectrometric Study of Fluorination in the CoF2(s)F and CoF2(s)Pt(met)F Systems. Russ. J. Phys. Chem. 1997, 71, 14551459. TOC Graphic


Remote Fluorination by Spillover of Atomic Fluorine - The Journal of

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