Surface ionization of CsCl molecules in the learning process

. The paper presents the results of studying the catalytic properties of "metal-carbon" systems using the method of surface ionization of CsCl molecules. Using the Ir-C system as an example, it was shown that the poisoning effect of the same poison (carbon) strongly depends on the structure of the layer formed on the catalyst. In metals that do not form bulk carbides, the carbon monolayer completely poisons the catalytic properties of these metals. It is shown that individual atoms and clusters of samarium deposited on iridium coated with a monolayer of graphite lead to efficient catalytic dissociation of CsCl molecules. The mechanism of formation of a carbon film on the surface of a metal obtained by diffusion from the volume, which differs from the mechanism of formation of a carbon film obtained on the surface of a metal by adsorption, has been elucidated.


Introduction
The release of carbon on the surface of metals accompanies many of the most important processes of heterogeneous catalysis, physical electronics, metal physics, vacuum and electrovacuum technology. With the release of carbon on the surface of metals placed in a vacuum, one often encounters during the operation of many devices of technical electronics. The ways in which carbon enters the surface can be different: segregation of impurity dissolved carbon, catalytic decomposition of hydrocarbon molecules that are part of the residual gas of devices or let into devices (for example, in elemental and chemical analysis of substances using mass spectrometers). Carbon is released on the surface of metal catalysts in industrial heterogeneous catalysis during the processing of carbon-containing products. The release of carbon can lead to a sharp change in the adsorption, emission and catalytic properties of metals. It is obvious that knowledge of the patterns of growth of carbon films on metals and their influence on the complex of properties of these metals will help the correct use of these metals when they are used in contact with carbon-containing media.
Film systems consisting of atoms, metal clusters on graphite substrates are effective catalysts. Such supported metal catalysts are widely used in the practice of heterogeneous catalysis. With their help, it is possible to increase the resistance of metal particles to sintering, maintain the availability of the catalyst for reactants, and achieve a larger specific surface area. It is of great scientific and practical interest to relate the activity of catalysts to the degree of dispersion of the metal in the film. Such data are important for understanding the nature of the active centers of metal catalysts.

Methods
To study the properties of film systems, a considerable number of experimental methods are used, in particular: the method of thermal desorption spectroscopy, the method of modulation of the molecular flow, the method of electronic Auger spectroscopy, the method of slow electron diffraction, the TEPI method (a combination of thermionic emission and surface ionization of atoms), the electronic effect Schottky, the method of ion delay curves, the method of surface ionization (SI) of CsCl molecules.
In the educational process in physical universities, the necessary information about these methods is given, except for the method of PI of CsCl molecules, about which detailed information is given in [1]. In this work, methods for the surface ionization of atoms and molecules of alkali metal salts are developed, with the help of which a complex of processes on the surface and in the volume of metal-carbon film systems, which play a significant role in the use of these systems in emission electronics, physical chemistry, electrochemistry, and heterogeneous catalysis, is studied.
This research method was further developed in [2] to study the properties of a carbon film. It was based on the fact that molecules do not dissociate on the saturated valence surface of the graphite layer, but dissociate at individual atoms containing free valences (defects in the graphite layer, edges of graphite islands, and carbon atoms). The use of CsCl as model molecules makes it possible to obtain a high efficiency of ion formation and provides a high sensitivity of the method. The CsCl molecule is the simplest diatomic molecule with only one decay channel. The use of CsCl molecules makes it possible to apply an original sensitive method for studying the activity of catalysts for dissociation reactions, which is based on the detection of the ionic component during desorption from the surface of dissociation products. Under the conditions of these experiments, each Cs atom was desorbed in the form of Cs+ ions, which greatly simplified the detection of dissociation products.
When a flow with a density ν of cesium-chlorine molecules is directed to the surface, two successive processes take place on it: 1) catalytic dissociation CsCl→Cs+Cl; 2) surface ionization of the cesium atom Cs→ Cs+ +e With full ionization, the current density of cesium ions desorbing from the surface is j=eν, where = (ν -νСeCl)/ν is the degree of dissociation of molecules on the surface, (νСeCl is the flux density of desorbing CsCl molecules). Experiments have shown that for CsCl molecules on many metals (W, Mo, Re, Ir, Pt, Rh) =1. Finding  for the surface under study was reduced to measuring two cesium ion currents: I1 at PI of CsCl molecules on a clean iridium surface ( =1) and I2 at PI of the same flow of molecules on the surface under study, and  = I2/I1 (=β -surface ionization coefficient).
Using the proposed method, the catalytic activity in the reactions of dissociation of film systems Ir-C, Pt-C, Rh-C, Pd-C, Re-C was studied at different temperatures and degrees of surface coverage with carbon. These metals are widely used in industrial catalysis, and carbon is the most common and difficult to remove impurity to them. In addition, rhenium and iridium are heat-resistant and stable thermal emitters that are used as thermal emitters in the electronics industry, while palladium is used as a filter in the production of pure hydrogen.
Below we consider some of the results obtained by this method by combining this method with others in the study of the properties of metal-carbon systems.
Previously [1], it was believed that the adsorption properties of Pt with respect to the molecules of alkali haloionic salts differ significantly from the properties of refractory metals W, Re, Mo, and Ta. In [1], it was suggested that these properties of platinum are caused by surface contamination. Therefore, measures were taken to obtain emitters with a clean surface. In addition to platinum, iridium, a metal of the platinum group, was also used in experiments on the PI of Cs and CsCl atoms. The choice of iridium as the emitter was due to two reasons. First, it was important to find out whether anomalies would be observed in the PI of molecules on other than platinum platinum group metals. Secondly, iridium has a much higher melting point than platinum, and if the anomalies in the behavior of molecules are caused by surface contamination, then iridium is much easier to clean from them by hightemperature heat treatment in vacuum. Ease of surface cleaning from contamination, high stability of thermionic properties, good mechanical properties of iridium emitters give them great advantages over platinum emitters. The experiments were carried out at various pressures of the residual gas in the device from 5 10-6 Torr to 10-8 Torr. As a result of the analysis of the obtained results, it was shown that: 1) regularities of PI of salt molecules on platinum group metals with a clean surface practically do not differ from similar regularities of ionization on other refractory metals (W, Mo, Ta, Re); 2) the observed practical absence of ionization of CsCl molecules on platinum metals is characteristic only of these metals with a contaminated surface; 3) since the PI of molecules on platinum and iridium emitters with both clean and contaminated surfaces proceeds in a similar way, it is preferable to use iridium emitters in further experiments, which are easily cleaned of impurities and exhibit stable thermionic properties.
4) the absence of ionization of molecules of alkali-haloionic salts on platinum metals is caused by the presence of carbon atoms on the surface.
In [1], for the first time, adsorption and the initial stages of carbon condensation on a metal surface were studied using the molecular beam technique. In this work, a textured iridium ribbon was used as a substrate with an exit to the surface of the (III) face. Iridium as a substrate is convenient because: it does not form bulk carbides; there is no diffusion of carbon into the bulk of the metal. Mass spectrometric technique was used in the work, and the method of studying the film system was the thermionic method, which includes thermionic emission and surface ionization of atoms (SEPI). The TEPI method made it possible to judge the contrast of the Ir (III) -C film system from the work function and made it possible to reveal phase transitions of the first order and exfoliation in carbon.
Carbon emitted from pyrographite tapes was sprayed in portions onto the surface of iridium, the temperature of which was maintained in the range of 1600-1800 K. After spraying the next portion of carbon, ion and electron currents from the surface were measured (to analyze the surface, a stream of indium atoms was directed to it).
Thus, the so-called current isotherms were obtained. Based on these results, plots of the work function (electronic (φe) and ionic (φi)) versus deposition time t were plotted (Fig. 1). As a result, it was shown that when carbon atoms are deposited on the surface of iridium to a certain critical degree of coverage θc, the film system is uniform in work function (region I in Fig. 1). In the region θк˂θ˂1, the Ir-C film system becomes inhomogeneous. With this in mind, the following conclusion was made.
In the range of coatings θ˂θc, the film consists of a two-dimensional adsorbed carbon gas phase. In the region θc˂θ˂1, the carbon film consists of two phases: from a two-dimensional gas phase with a coating θc and from a two-dimensional condensed phase with a coating θ=1. The formation of two-dimensional islands after reaching θ=θc is attributed by the authors to a first-order phase transition of the condensation type. It is stated that two-dimensional islands and a carbon monolayer on the surface of face (III) have the structure of the graphite basal plane. Indeed, this is confirmed by OES experiments, which show that the carbon film on the surface (III) of iridium, as well as on the surface of other low-index faces of platinum metals, is in the form of graphite [3]. In the theoretical part of the kinetics of the formation of a carbon film, it is assumed that all carbon atoms entering the metal surface, free from graphite islands, stick to it [1]. In contrast to metal, the graphite film is valence-saturated and the bond with carbon atoms is weakened. There are two possible sinks for carbon atoms falling into the second layer onto graphite islands: rolling off the layer and attaching to the edge of the island; migration over the island surface followed by desorption from it. The migration time depends on the temperature, and therefore, as the temperature decreases, the formation of a multilayer carbon film can be observed. In the study of the Ir (III)-C system [1] by the method of PI of CsCl molecules, it was found that there is a latent period in the action of the carbon poison, during which carbon accumulates on the surface, but the catalyst is not poisoned. At this time, carbon is on the surface in the form of a two-dimensional gas. Catalyst poisoning begins only when twodimensional carbon islands with a graphite structure are formed in a two-dimensional gas. If the degree of dissociation of CsCl molecules on metal catalysts is of the order of unity, then on carbon islands it decreases by a factor of 103-105. Thus, using the Ir-C system as an example, it was shown that the poisoning effect of the same poison strongly depends on the structure of the layer formed on the catalyst.
In [1,2,4], the regularities of dissolution and segregation of carbon in rhenium, rhodium, and palladium were studied using the CsCl PI method. Directly heated palladium ribbons (40x0.5x0.1 mm3), rhodium wires (70 mm long and ∅ 100 µm), and rhenium wires (70 mm long and 150 µm ∅) served as emitters and adsorbents. The carbon coating was created by cracking benzene vapor on the surface of the samples or by absorption of carbon atoms emitted by heated pyrographite strips.
Fluxes of Cs atoms and CsCl molecules arrived at the central part of the emitter from Knudsen cells. The thermionic current from this section of the emitter was measured in a flat collector circuit equipped with an antidynatron grid. The ions formed at the emitter by PI could be registered by the same collector, or they entered the input of the mass analyzer and were registered in its collector circuit containing an ion-electron multiplier. The flow of neutral particles desorbed from the surface of the emitter could be ionized by electrons; the resulting ions were analyzed by a mass spectrometer.
The emitter temperatures were determined with an optical micropyrometer, and in the non-pyrometric region, they were estimated using extrapolation from the filament current. In the case of palladium, the temperature of the ribbon was determined using a platinumplatinum-rhodium thermocouple. It is shown that the features in the course of these processes in Rh, Re, and Pd coincide in general terms and, apparently, are common for metals and do not form bulk carbides. The processes occurring during the carburization of Rh, Re and Pd kept in a heated state (rhodium at a carburization temperature of ∼1500 K, rhenium at ∼1800 K, and palladium at ∼1300 K) in benzene vapors (at РС6Н6∼(1∻5) 10-5 torus) can be represented as follows. Benzene molecules are adsorbed on the metal surface with high sticking coefficients. Probably, hydrogen atoms are first split off from the adsorbed benzene molecule, and then the more strongly bound carbon skeleton is destroyed. The released carbon atoms can simultaneously participate in two processes: dissolution and construction of a carbon film in the adlayer. Experience shows that as long as n2˂n2m (n2 is the concentration of carbon in the interstitial plane closest to the surface, n2m is the same at the limiting solubility), only one sink works -dissolution, and the metal surface, even with a significant benzene flux density, remains practically free from carbon. This indicates that carbon at these carburizing temperatures diffuses rather quickly into the volume of the metal. When n2 is compared with n2m, dissolution ends and carbon accumulation begins on the surface. The maximum thickness of carbon having a graphite structure thus obtained on the metal surface is one monolayer. This is explained by the fact that benzene molecules do not dissociate on the valence-saturated surface of the graphite layer and quickly desorb from it.
In metal samples containing a constant amount of carbon, with a change in temperature, carbon is redistributed between the surface and the bulk of the metal (Fig. 2). At T˃Tk, when n2˂n2m (where n2m is the limiting solubility of carbon at Tk), almost all carbon is in the dissolved state (=1). At T˂Tk, when n2˃n2m, excess carbon from the bulk of the metal moves to the surface, to grain boundaries and lattice defects (=1). It is shown that a monolayer of graphite on the surface of these metals leads to severe poisoning of the metal catalyst in the dissociation reaction.
An analysis of the results (Fig. 2) shows that the amount of carbon in the volume of rhenium is sufficient with a decrease in temperature (from 1800 K to 900 K) to fill its surface not only with one monolayer, but with several layers of a carbon film. If further growth of the carbon film is formed on top of the first layer, then this could be detected by the PI of CsCl molecules, but this method denies this variant of the growth of the carbon film [2]. Excluding this option, we put forward the following assumption that the second layer of the film begins to form from below the first layer. To do this, the first layer of graphite must be raised from the surface of the sample to a considerable distance (∼4 (A) ), and there is no electron exchange between them, but only van der Waals interaction.
An analysis of the experimental results [2] obtained using the TDS, TE, and PI methods of CsCl molecules showed that the graphite film is elevated above the iridium surface, and the valence-unsaturated edges of the graphite islands are closed by metal atoms. This suggests that graphite films can be bent, which is confirmed by the example of fullerenes having a spherical shape [5][6][7].
In the part of work [2] devoted to the study of the catalytic dissociation of CsCl molecules from individual atoms and clusters of samarium deposited on a passive support (iridium coated with a monolayer of graphite), it was shown that individual atoms and clusters of samarium deposited on iridium coated with a monolayer of graphite leads to very strong provoking effect. So, for example, almost complete dissociation of CsCl molecules can be obtained with such small coatings of the Ir-C surface with samarium as Q˂10-2. Two types of experiments were carried out in this work. We studied the change in β over time after deposition of a certain dose of samarium and with a continuous supply of samarium to the surface. It was found that even small doses of samarium (1011 cm-2) deposition on Ir-C at T≈8500 K leads to a sharp increase, by five orders of magnitude. The main part of this formation is probably due to the two-dimensional samarium gas, which is indicated by the closeness of the fast decay time in the dependences β(t) after the flow is blocked by the lifetime of samarium atoms Ir-C. The slow part of the decrease in the dependence β(t) is observed at large t, which can be explained by assuming that some of the samarium atoms aggregate into clusters that slowly dissolve. The dissociation of molecules occurs, probably, both at the edges of the clusters and on the surface, but due to the low work function of samarium (2.7 eV), Cs+ ions are not desorbed from the surface of the clusters. The assumption about the formation of clusters is supported by the fact that complete cleaning of the surface from samarium required heating the film to 1300 K.

Conclusion
It is reasonable to assume that such small doses of samarium, which do not change the electrical properties of the substrate, do not cause a long-range change in its catalytic properties. The increase in β should then be associated with two reasons: with the effective contact catalytic dissociation of molecules and active centers and with the effective migration of molecules to these centers. The analysis showed that in the surface activated CsClSm complex, the dissociation activation energy is less than 2 eV, while for a free CsCl molecule it is 4.56 eV. The use of iridium coated with a monolayer of graphite and deposited with active centers as a model catalyst opens up the possibility of studying the nature of elementary catalytic acts. The advantages of a valence-saturated Ir-C substrate include a simple, well-defined surface topography, a low concentration of defects in the surface layer, and the absence of dissolved carbon in iridium. Active sites can be dosed onto such a substrate, their concentration, chemical composition, and structure can be varied over a wide range, and these properties can be compared with the activity of the catalyst.
Using the high sensitivity of the dissociation of CsCl molecules to atoms and clusters located on the graphite layer, the initial stages of the formation of a carbon film on iridium were studied in [2] using the catalytic dissociation of CsCl molecules. It is shown that at a given temperature Т1 (more precisely, a narrow temperature range of ∼ 50 degrees in the region Т1) above which a graphite film of only monoatomic thickness grows on the surface of iridium. At T˂T1, a multilayer carbon coating grows on the iridium surface. It is shown that the multilayer carbon film does not grow by the layer-by-layer growth mechanism. The question of the topography of a carbon film obtained by vacuum condensation on iridium at T˂T1 is considered. A multilayer carbon film consists of a bottom solid part (a number of two-dimensional layers of graphite) and an upper non-continuous part several atomic layers thick, containing many graphite "mountains".