Georgian Scientists/ . 4 N 4, 2022 248 Georgian Scientists Vol. 4 Issue 4, 2022 https://doi.org/10.52340/gs.2022.04.04.27 Physical Chemistry Partipication of Oxygen Hydroxyl Groups, Covered the Aluminum Oxide Surface in the Reaction of Catalytic Oxidation of Carbon Monoxide on Pd/Al2O3 System Catalysts Giorgi Kakhniashvili, Revaz Kokhreidze, Irakli Nakhutsrishvili V.Chavchanidze Institute of Cybernetics of Georgian Technical University, 5 Z.Anjapharidze, 0186, Tbilisi, Georgia ABSTRACT Using oxygen enriched with the isotope , the contribution of various forms of catalyst oxygen to the oxidation process was studied. It turned out that, contrary to expectations, the content of the isotopic label in the formed is significantly lower than expected and at the initial stage of the reaction is close to the natural content of . Taking into account that the pretreatment of the catalyst excluded the existence of any noticeable amount of adsorbed oxygen on the surface of both palladium and the support, it was concluded that is oxidized due to oxygen , and the study of the SIMS spectra showed that oxygen is involved in the process of oxidation hydroxyl cover . Key Words: Palladium, carbon monoxide, oxidation, catalysis 1. INTRODUCTION The most effective way to clean car exhaust gases from carbon monoxide is its post-oxidation to . The highest catalytic activity in this reaction is exhibited by the supported platinum group metals. The study of the mechanism of this reaction and the nature of the interaction of the metal with the support will make it possible to select the most efficient catalyst. In this work, was studied the mechanism of the reaction of low-temperature oxidation of on catalysts, which are palladium deposited in an amount of mass 0.003 - 0.1% on the surface of . Georgian Scientists/ . 4 N 4, 2022 249 The peculiarity of the catalytic properties of metals deposited on an oxide substrate, as compared with bulk metals, are usually explained by a change in the state as a result of their interaction with the support and by their high degree of dispersity [1]. 2. MATERIALS AND METHODS Supported palladium catalysts were prepared by impregnating the – support with a solution, subsequent drying and reduction of to the metallic state with sodium formate. The reaction was carried out in a circulating-static setup in an excess of oxygen and an initial concentration of ~12 % and a pressure of = 0,4 in the temperature range of 360–435 K. The analysis of the reaction mixture and isotopic composition was carried out during the reaction by mass spectrometry. The standard treatment of the samples consisted in their evacuation (P 0.1 Pa) at a temperature of 673 K. The oxygen enriched in the isotope (~80 .%) used in the study as a reagent was obtained by electrolysis of water, containing . Secondary ion mass spectroscopy (SIMS) was used to study the nature of the reactive ( ) recorded for = 17, 19, 43, 45, 70, 72, which correspond to fragments +, +, , , , . At T 373K, values of catalytic activity close to stationary are established within 15–30 s. Their activity, calculated per unit mass of palladium, than that of Pd black studied under comparable conditions. In order to study the nature of reactive oxygen species in the CO oxidation reaction on the above catalysts, oxygen enriched in the isotope (~80 .%) was used as a reagent [2], also was using the method of secondary ion mass spectroscopy (SIMS). At 373 , values of catalytic activity close to stationary are established within 15–30 sec. Their activity, calculated per unit mass of palladium than, that of black studied under comparable conditions. 2. RESULT AND DISCUSSIONS Considering the extremely low activity of , compared to copper ( ) and iron ( ) oxides in the reaction of oxygen isotope exchange [3, 4] indicates a low reactivity of aluminum oxide oxygen, the use of oxygen enriched in the isotope in the oxidation reaction led to an unexpected result. Assuming, according to [5], that interacts with oxygen adsorbed on the palladium surface, it was natural to expect the appearance of with a high content of the isotope . Nonetheless, concentration of the isotopic label ( ) remains low even at the relative high consumption of oxygen during the reaction (Fig. 1). To test the possible effect of oxygen isotopic exchange on the content of the isotopic label in the formed , oxidation was carried out by freezing during the reaction. In this case, there is no significant change in the value of , which allows us to assume that there is no noticeable transfer of the isotope label between and the catalyst under the reaction conditions. This assumption is also Georgian Scientists/ . 4 N 4, 2022 250 confirmed by the results of experiments in which the change in the value of in , containing the isotopic label ( ), was determined in one pass through the catalyst layer, which was carried out by refreezing from a trap located in front of the catalyst into a trap behind the reactor. Fig.1 Change in the content of the isotope in carbon dioxide during oxidation with labeled oxygen: 0.1% (upper curve) and 0.04% (lower curve) In those cases when such refreezing was carried out in a flow of a mixture of + 2, the change in was relatively small (Tab.1). When determining the final value of , we took into account changes in due to formed in the reaction + 2. This change was determined in the course of so-called blank experiments by mixing the formed with the ), preliminarily frozen in a trap, the amount of which corresponded to standard experimental conditions. Table 1 Sample , , .% 378 - - 0,07 42,5 22,0 388 0,4 2,66 0,16 42,5 40,88 Thus, under the conditions of the oxidation reaction, it is possible to exclude a significant effect of oxygen isotopic exchange on the content of in formed in the reaction. Then, for the system, the low value of observed in the reaction can be explained only by the participation in the formation of of oxygen atoms of the catalyst, which had a natural isotopic composition before the start of the reaction. The participation oxygen of studied catalysts in the oxidation reaction suggests the possibility of carrying out this reaction in the mode of catalyst reduction with carbon monoxide. It turned out that the oxidation reaction actually proceeds on these catalysts in the absence of oxygen in the gas phase, but its rate becomes noticeable only at 623–673 K. In this case, the formation of is accompanied by hydrogen evolution. This fact leads to the natural conclusion that the oxygen- containing form interacting with is the hydroxyl groups of the support. It is natural to assume that the oxidation of in the catalytic mode can be carried out with the participation of the OH groups of the support. This assumption is confirmed by the analysis of the SIMS 0 2 4 6 8 10 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 , ,t% , min, Georgian Scientists/ . 4 N 4, 2022 251 spectra of samples preliminarily enriched with an isotopic label by repeated experiments on the oxidation of with labeled oxygen (423 K) under freezing conditions. Enrichment of samples with an isotopic label under these conditions occurs as a result of the reoxidation of oxygen-containing centers with labeled oxygen ( ) reacting with . From the SIMS spectra, the relative content of the isotope in various oxygen-containing fragments formed during the bombardment of both initial samples (tab. 2 a) and samples enriched with an isotope label (Tab. 2 b) with argon ions was estimated. An increased content of the isotope label was found only in OH fragments, which indicates the occurrence of the oxidation reaction on catalysts under the studied conditions through the interaction of molecules with oxygen atoms of the hydroxyl cover . Table 2 Sample + + + a) 0,58 0,21 0,014 b) 0,74 0,18 0,012 Thus, the oxidation of on supported palladium both in the reduction and catalysis regimes can proceed through the interaction of with oxygen atoms of OH groups, however, the temperature ranges for the implementation of these regimes differ by more than 250°C. We see the reason for this difference in the fact that in the catalysis regime there is no breaking of bonds in OH groups, and the formation of and the regeneration of OH groups are carried out by redistribution of bonds in the intermediate complex with the participation of adsorbed oxygen. Since, in the case under consideration, interacts directly with OH groups and not with oxygen under the reaction conditions, adsorption is accompanied by the formation of carboxyl-type structures [4, 5]. The formation of such structures is apparently the stage that determines the reaction rate. In this case, the role of adsorbed oxygen is reduced to the reoxidation of palladium atoms reduced in previous events and the initiation of the decomposition of carboxyl structures with the formation of and the regeneration of OH groups. The possibility of such a mechanism is consistent with a similar phenomenon of initiation of the decomposition of surface structures of the carbonate-carboxylate type by molecular oxygen [6, 7]. A similar mechanism has been proposed in the photocatalytic oxidation of CO using a supported platinum catalyst [8]. REFERENCES 1. H. Li, M. Shen, J. Wang, H.Wang, J. Wang (2020), Ind. Eng. Chem. Res., 59, 1477. 2. Xiaoying Ye, Brian Luke, Thorkell Andresson and Josip Blonder: 18O Stable Isotope Labeling in MS-based Proteomics. Briefings in functional genomics and proteomics. 2009, vol 8. no 2. 136-144; Georgian Scientists/ . 4 N 4, 2022 252 3. A.V. Bagazeeva, V. B. Vykhodetsb, E. V. Vykhodetsc, T. E. Kurennykhb, A. Ya. Fishmand T. M. Deminaa, A. I. Medvedeva, and A. M. Murzakaeva: Oxygen Isotope Exchange in Nanosized Powders of the Al2O3 Oxide. THE PHYSICS OF METALS AND METALLOGRAPHY Vol. 113 No. 5 (2012), p 513-519; 4. CRAYTON J. YAPP: Oxygen isotope effects associated with the solid-state cu-FeOOH to ar-Fez03 phase transformation. (1990) Geochimica et Cosmochimico Acla Vol. 54, pp. 229-236; 5. V.A. Shwets, V.E. Shubin, V.B. Kazansky (1982), Proc. Mater. III All-Union Conf. Mechanism of Catalytic Reactions. Novosibirsk, 122. 6. P. R. Chen, M.S.Hoang, K.Y.Lai, H.S.Chen (2022), Nanomaterials, 12, 573. 7. . , (1974), , 214, 1361, 216, 599. 8. K. Czupryn, I. Kocemba, J. Rynkowski; Photocatalytic CO oxidation with water over Pt/TiO2 catalysts (2018), React. Kinet. Mech. Cat., 124, 187-201. (II) , , , 0186 , . 5 . , , . , , , (SIMS) , .