SYNCHRONOUS ELECTROSYNTHESIS OF HYDROGEN PEROXIDE AND OZONE 1 . Topological model

The regularities of the influence of the modification of the surface layer of tissue carbon and graphite fibrous materia on the composition, the amount of superficial functional oxygen-containing groups by different methods have been studied. The relationship between the electrocatalytic and corrosion characteristics of cathodes from tissue carbon and graphite fibrous materia has been established during electrosynthesis of H2O2. They were tested during electrosynthesis of H2O2 in a diaphragm cell with a current load of up to 100 A. A topological model of synchronous electrosynthesis hydrogen peroxide and ozone was developed.


Introduction
Hydrogen peroxide (H 2 O 2 ) today is one of the most perspective products among other chemicals.Interest to it is due to successful combination of physical and chemical properties and high ecological safety of decomposition productswater and oxygen.
In the world market of industrial H 2 O 2 production three main technologies prevail [1]: by anthraquinone, by isopropyl and electrochemical one.In Russia 94% of the market of production of H 2 O 2 is carried out by an isopropyl method (KHIMPROM) and only 3% of the marketby electrochemical method.On this technology H 2 O 2 is produced through oxidation on Pt anodes solutions of sulfuric acid or its salts to persulphates which then hydrolyzed and distilled with sharp steam, receiving H 2 O 2 .
Power and operational expenses at the same time are high.By data [1] they make up to 40% of prime cost of H 2 O 2 .Besides, the traditional electrochemical technology ecologically is also dangerous because of formation of considerable volumes of molecular hydrogen and application of Pb as the cathodic material.However the electrochemical technology allows to receive the cleanest H 2 O 2 , with a wide range of concentration from 30% to 98%.Sales market of the electrochemical H 2 O 2pharmaceutics, medicine, electronics and radio engineering, the space equipment etc.
In this regard the problem of improvement of electrochemical production of H 2 O 2 is very relevant.

Experiment part
Experiments were carried out in the laboratory electrochemical cell with cathode made from the carbongraphite fibrous material of tissue structure reinforced in the Ni grid (UGVM t -Ni) (Figure 1).In the experiments processes of modifying surface of UGVM t and also electrosynthesis of H 2 O 2 were investigated.The cathodic and anodic spaces are divided by a cation-exchange membrane MF-SK.
Composition of the surface layer of UGVM t was investigated by method of the reverse titration by the bases of NaHCO 3 , Na 2 CO 3 , NaOH [2].Several functional groups with various acidity are distinguished as results of sodium-hydrogen exchange.
In Figure 2 the moduleelectrolyzer for current loading up to 100 A is shown in which we carried out synchronous receiving O 3 and H 2 O 2 .
Catalytic covering of the anode was Pt, and the cathode material -UGVM t -Ni.The anode and cathodic spaces are divided by cationexchange membrane.As anolyte 5 m NaClO 4 and as catholyte -1% NaOH solutions were used.All applied solutions were prepared from reactants of the brand "chemically pure".

Results and discussion
Earlier in [3,4] we considered the possibility of intensification of technology of electrochemical production of H 2 O 2 .Main attention was concentrated on creation of highly effective, cheap electrode materials.The following industrially released carbon materials were investigated: for anodeglassy carbon, and for cathodecarbon-graphite fibrous material of fabric structure.The surface of the specified electrode materials was exposed to chemical and electrochemical modifying that allows to change purposefully composition of the surface layer, its electrocatalytic and corrosion characteristics during the process of electrosynthesis of H 2 O 2 .
For example, in electrosynthesis of H 2 O 2 on UGVM t -Ni cathode in solutions: 1% of NaOH + 0.1 g/l of MgSO 4 + 10 -3 M C 6 H 4 (OH) 2 under the impact of generated on anode ozoneoxygen mix abnormally high current efficiency was reached for H 2 O 2 (696.9-911%).It was suggested that the mixed way of electrosynthe-sis of H 2 O 2 was realized, including the direct electro-reduction of oxygen-containing gas catalyzed by redox processes in superficial oxygen-containing groups of UGVM t , glassy carbon and chemical processes of oxidation.
In the paper [5] without hydroquinone C 6 H 4 (OH) 2 the synchronous electrosynthesis of O 3 and H 2 O 2 on glassy carbon (SU-20) anodes and cathodes from UGVM t -Ni with total current efficiency of oxidizers production more than 130% was established.
The influence of content and feed rate of oxigen-containing mixture on electrosynthesis of hydrogen peroxide was studied.It is established that injecting ozoneoxygen mix instead of pure oxygen or air accelerates electrosynthesis of hydrogen peroxide and creates almost non-corroded cathode.Such technical solution provides increase in content of oxygen in a catholyte without use of additional pressure and equipment.
In development of our researches [3-5] there was investigated the influence of conditions of modification of UGVM t surface on its chemistry, governing electrocatalytic (current efficiency, %) and corrosion (g/Ah) characteristics during electrosynthesis of H 2 O 2 in standard conditions (when injecting pure air in catholyte).Content of the formed surface functional oxygen-containing groups (SFOG) is presented in the Table 1.It is necessary to pay attention to the experimental fact that the rate of formation of H 2 O 2 in the presence of additive 0.001 m C 6 H 4 (OH) 2 becomes seven times higher, that confirms suggestion about complex mechanism of hydrogen peroxide electro-synthesis by combined electrochemical and anthraquinone ways.
Functioning of electrochemical productions is impossible without their optimization, creation of mathematical model of the electrolyzer, separate technological processes of electrolysis [6].High-quality and optimum control of technology requires use of the mathematical model of process of electrolysis which is adequately describing warm and a mass exchange in the electrolyzer, changes of material balance, electrobalance, to the hydrodynamics engineer of solutions.Creation of mathematical model of electrolysisa way of modification of the existing production of hydrogen peroxide, introduction new technical the solution of process, improvement of quality of management of technology.
Work [7] in which aluminum electrolysis process is investigated is known, key parameters of process are determined, their interrelations are revealed, the most essential communications are chosen.The topological model of process of electrolysis of aluminum is developed.On the basis of the developed topological model the method of the structural and parametrical analysis of systems of automatic control of production is considered.In other work [8] by means of a topological method on the basis of the structural count the mathematical model of process of anode effect in aluminum production is created.
In work [8] the topological model of process of electrosynthesis of nadserny acid is created.Cathodic process is in which release of molecular hydrogen.
The growing need of the market for peroxide of hydrogen demands an intensification, modernization of productions.The technology of obtaining hydrogen peroxide combining electrochemical and anthraquinone methods is relevant Therefore a perspective priority is development of topological model of process of synchronous electrosynthesis of hydrogen peroxide and ozone that will allow to create mathematical model.On the basis of the received topological model the main quantitative characteristics of synchronous electrosynthesis of peroxide of hydrogen and ozone will be calculated.
In Figure 3 the simplified topological scheme of synchronous electrochemical production of O 3 and H 2 O 2 is submitted.The topological model of system shown in Figure 3 contains information on structure of physical streams.
For drawing up topological model of technological process of simultaneous generation several inorganic oxiding components the structure of streams 1-5 (which can be received by analysis of local processes in nodes) is required.
In anode nodeanode reactions: For example, ozone-oxygen mix is formed on anode in node (+) and will be present only in stream 2. Hydrogen peroxide is formed in node (-), i.e. in cathodic space, and will be present only in stream 4. Ions of H + are formed in anode space and will be present in stream 5.The combination of block diagrams of streams of substances shown in Fig. 4 gives the general block diagram provided on Figure 3.
From the analysis of the processes proceeding in nodes (diffusion, filtration and evaporations of streams in the first approximation can be neglected) it is possible to construct topological schemes for separate components (Figure 4).
The general scheme already contains information on what substances are contained in group streams 1-5.For example, the stream 4 contains substances: H 2 O, 2 HO  OH -.The structure of streams of substances in reactions in each node and system in general can be represented graphically.Such problem is solved on the basis of the data of stoichiometric equations of the reactions proceeding in separate nodes.The equations specify the directions of streams in education reactions (products of reactions) or consumption (initial substances).The example of the scheme of transformation of substances is shown in Figure 5.
At combination of block diagrams of processes of transfer and transformation we receive the full technological scheme (Figure 6).This scheme unambiguously and fully reflects the nature of material processes in system: appearance and disappearances of substances in separate nodes as a result of processes of different physical nature.Fig. 6.Full technological scheme of process of electrosynthesis of O 3 and H 2 O 2 .P a , P kcorrespond to anode and cathodic reactions.Numbersto transfer streams according to Figures 3,4; continuousin solution; dottedin the gas phase.
These elements characterizing dynamics of material processes in technological system are the basis for drawing up mathematical model of the technology (which describes material transformations) having the form of the equations of balance of substances.
By drawing up the balance equations the use of block diagrams considerably facilitates work and guarantees an exception of mistakes.
Equation of balance O 3 + O 2 in node (+) according to graph of Figure 6 is: where g pA and g 2 mass streams of O 3 and O 2 , kg/s, i.e. production rate in anode reaction of generation of ozoneoxygen mix and removal from the node in stream 2.
The considered block diagrams reflect technological topology of concrete processes and they are called process graphs.Graphs contain the same information, as the balance equations in general view, so actually they are a graphic form of algebraic equations of balance of substances: Except material, in the technological scheme power processes proceed.As the partial problem of the analysis of power balance of the electrochemical device usually the balance of thermal streams is considered which determine solution of temperature in working area.Structure of thermal streams of anode and cathodic reactions (q), radiation and convection (m) are represented graphically in Figure 7.

Conclusions
1.The regularities of the effect of modification of the surface layer of UGVM t on the amount of surface functional oxygen-containing groups (SFOG) are studied.
2. The interrelation between electrocatalytic, corrosion characteristics of fabric UGVM t -Ni in the course of electrosynthesis of H 2 O 2 and quantity of SFOG is established.
3. Tests of cathodes made from the fabric UGVM t -Ni reinforced by Ni in the course of electrosynthesis of H 2 O 2 in the diaphragm electrolyzer for current loading up to 100 A are carried out.
4. The topological structural model of synchronous electrochemical synthesis of O 3 and H 2 O 2 is created.Fig. 7. Structure of thermal (q A , m Aanode and q K , m Kcathodic) technological system of producing O 3 and H 2 O 2 ; numbersas in Figure 3.

Fig. 2 .
Fig. 2. The moduleelectrolyzer for current loading up to 100 A.

Table 1 .
Content of the formed surface functional oxygen-containing groups (SFOG).Electrocatalytic and corrosion characteristics of UGVM t -Ni in direct electrosynthesis of H 2 O 2 in 10% NaOH, the speed of injecting air 120-160 ml/h,

Table 2 .
Production of hydrogen peroxide.The stream of O 3 -O 2 mix generated on anode is injected in cathodic space with a rate 4 l/h.Concentration of O 3 is 5-6 vol.%.The cathode material is UGVM t -Ni As we see, the correlation between current efficiency (% H 2 O 2 ) and the number of phenolic and hydroxyl groups on the surface of UGVM t -Ni is observed.Electrocatalytic properties of UGVM t -Ni are highest after modifying by anode oxidation, and corrosionthe worst, than at electrochemical fluoration.It is possible to assume that complex modification by anode oxidation and fluoration the surface of UGVM t creates hydrophobic and hydrophilic conditions under which the speed of diffusion of H 2 O 2 into main solution volume is higher, than in fiber pores with reduction H 2 O 2 into H 2 O.We also performed tests of the diaphragm electrolyzer for current loading up to 100 A in the course of synchronous synthesis of O 3 and H 2 O 2 .Results of tests are presented in the

Table 2 .
From the data of Table2it follows that fabric UGVM t -Ni cathodes are perspective for producing diluted alkaline H 2 O 2 solutions.We will note that replacement of usual Pb cathodes by the combined cathodes from UGVM t -Ni allows us to receive instead of H 2 alkaline H 2 O 2 solutions as a commodity product and to avoid Pb pollution of environment.