Oxidation catalysts active sites

Banares, M.A., Martfnez-Huerta, M.V., Gao, X., Fierro, J.L.G., and Wachs, I.E., in "Metal oxide catalysts active sites, intermediates and reaction mechanisms", Symposium, 220th ACS National Meeting, Washington, USA (2000d). 

Catalysts include oxides, mixed oxides (perovskites) and zeolites [3]. The latter, transition metal ion-exchanged systems, have been shown to exhibit high activities for the decomposition reaction [4-9]. Most studies deal with Fe-zeolites [5-8,10,11], but also Co- and Cu-systems exhibit high activities [4,5]. Especially ZSM-5 catalysts are quite active [3]. Detailed kinetic studies, and those accounting for the influence of other components that may be present, like O2, H2O, NO and SO2, have hardly been reported. For Fe-zeolites mainly a first order in N2O and a zero order in O2 is reported [7,8], although also a positive influence of O2 has been found [11]. Mechanistic studies mainly concern Fe-systems, too [5,7,8,10]. Generally, the reaction can be described by an oxidation of active sites, followed by a removal of the deposited oxygen, either by N2O itself or by recombination, eqs. (2)-(4). 

The photocatalytic oxidation of alcohols constitutes a novel approach for the synthesis of aldehydes and acid from alcohols. Modification of Ti02 catalyst with Pt and Nafion could block the catalyst active sites for the oxidation of ethanol to CO2. Incorporation of Pt resulted in enhanced selectivity towards formate (HCOO ad)-Blocking of active sites by Nafion resulted in formation of significantly smaller amounts of intermediate species, CO2 and H2O, and accumulation of photogenerated electrons. The IR experimental teclmique has been extended to Attenuated Total Reflectance (ATR), enabling the study of liquid phase photocatalytic systems. 

Based on the experimental data and some speculations on detailed elementary steps taking place over the catalyst, one can propose the dynamic model. The model discriminates between adsorption of carbon monoxide on catalyst inert sites as well as on oxidized and reduced catalyst active sites. Apart from that, the diffusion of the subsurface species in the catalyst and the reoxidation of reduced catalyst sites by subsurface lattice oxygen species is considered in the model. The model allows us to calculate activation energies of all elementary steps considered, as well as the bulk... 

Figure 12.20 A designer C H oxidation catalyst the positions the reactive CH-bond over the catalyst active site using molecular recognition. The ibuprofen substrate is oxidised to the 2-(4-Isobutyryl-phenyl) -propionic acid product in > 98 % selectivity (reproduced by permission of The Royal Society of Chemistry).

P. E. Sinclair, C. R. A. Catlow, Quantum chemical study of the mechanism of partial oxidation reactivity on titanosilicate catalysts Active site formation, oxygen transfer, and catalyst deactivation, J. Phys. Chem. B 103 (1999) 1084. 

The scientific community interest has been focused in recent years especially on H2 catalytic production by partial oxidation of methane, due to the large diffusion of natural gas as primary feedstock. Coke formation and its deposition on catalyst active sites represent, as well as for SR process, the main barrier to be abated for a practical utilization of CPO in hydrogen production plants. 

The phosphorus deactivation curve is typical type C, and, according to the Wheeler model, this is associated with selective poisoning of pore mouths. Phosphorus distribution on the poisoned catalyst is near the gas-solid interface, i.e. at pore mouths, which confirms the Wheeler model of pore mouth poisoning for type C deactivation curves. Thus we may propose that in the fast oxidative reactions with which we are dealing, transport processes within pores will control the effectiveness of the catalyst. Active sites at the gas-solid interface will be controlled by relatively fast bulk diffusional processes, whereas active sites within pores of 20-100 A present in the washcoat aluminas on which the platinum is deposited will be controlled by the slower Knudsen diffusion process. Thus phosphorus poisoning of active sites at pore mouths will result in a serious loss in catalyst activity since reactant molecules must diffuse deeper into the pore structure by the slower Knudsen mass transport process to find progressively fewer active sites. 

A serious drawback of the process is the catalyst deactivation, which results in low reaction rates and the necessity of a relatively high catalyst/substrate ratio. There are numerous reports in the literature on catalyst deactivation, attributed variously to overoxidation of the catalyst [3-5], irreversible adsorption of by-products [6-8] or dissolution and re-deposition of Pt [5]. It has been suggested that the over-oxidation of active sites can be avoided by working at low and constant level of dissolved oxygen [9]. 

Nevertheless, the light-off temperatines determined on laboratory tests are inferior by about 100°C to those detennined on engine bench tests or on vehicle under similar conditions [6]. This difference could only be the consequence of major phenomena that have not yet been identified. It was thought tliat, under real conditions, there could be mixture effects due to interactions between the catalyst active sites and hydrocarbons belonging to different families, in the presence of CO, NO, O2, CO2, H2O and SO2. Thus, a study was undertaken to determine the influence of the nature of various HC on the oxidation reactions of CO and HC by O2 and NO. Special emphasis was directed toward HC able to strongly coordinate to tlie catalyst siufaces. 

Many problems may occur when natural organic matters are present in the water, since they can occupy the catalyst active sites causing much lower decomposition efficiency. A combination of adsorption and oxidative destruction techniques may become a useful method to overcome the above problem. Ilisz et al. [367] used a combination of r/02-based photocatalysis and adsorption processes to test the decomposition of 2-chlorophenol (2-CP). The group created three systems which are presented below ... 

Figure 11.8 Mechanism of redox reaction catalyzed by NAD dependent lactate dehydrogenase Lactate dehydrogenase (EC 1.1.1.27) is a tetrameric enzyme which catalyzes the reversible redox reaction between L-lactate and pyruvate via ordered kinetic sequence. The hydride ion is transferred to the proR side of the 4 position of NAD. His 195 acts as an acid-base catalyst removing the proton from lactate during oxidation. The active site loop (residues 98-110) carries Argl09 which helps stabilize the transition state during hydride transfer and contacts required for the substrate specificity.

The simplest mechanism for interpreting critical phenomena in heterogeneous catalysis is the Langmuir adsorption mechanism, also referred to as the Langmuir-Hinshelwood mechanism. This mechanism includes three elementary steps (1) adsorption of one type of gas molecule on a catalyst active site (2) adsorption of a different type of gas molecule on another active site (3) reaction between these two adsorbed species. For the oxidation of carbon monoxide on platinum, this mechanism can be written as follows ... 

Schwidder M, Heikens S, De Toni A, Geisler S, Bemdt M, Briickner A, Griinert W (2008) The role of NO2 in the Selective Catalytic Reduction of Nitrogen Oxides over Fe-ZSM-5 Catalysts - Active Sites ftn the Conversion of NO and of NO/NO2 mixtures. J Catal 259... 

There are several advantages for the use of S-ZrOj as a catalyst support in PEMFC applications. Because of its hydrophilicity, it has been suggested that this type of fuel cell catalyst would be well suited for low-relative humidity conditions and possibly simplify fuel cell components to operate without the use of a humidifier. Due to the proton conductivity across the surface of the material, less Nafion iono-mer needs to be cast to form the TPBs. Platinum utilization increases as the S-ZrOj support acts as both the platinum and proton conductor and better gas diffusion to the catalyst site results from the decreased blockage of Nafion ionomer (Liu et al., 2006a,b). It is beheved that within porous carbon catalyst supports, platinum deposited within the pores may not have proton conductivity due to the perfluorosul-fonated ionomer unahle to penetrate into the pores. Thus, a TPB which is necessary for a catalyst active site will not be formed. Therefore, the S-ZrOj support has an additional benefit over porous carbon material supports in that by using the S-ZrOj as a support for platinum catalysts, the surface of the support can act as a proton conductor and platinum deposited anywhere on the surface of the support will provide immediate access to the electron and proton pathways thereby requiring less Nafion. Thus the use of S-ZrOj in fuel cell MEA components may potentially lower the cost of materials substantially, as the catalytic metals and membrane materials are among the most costly in a PEMFC. However, like most metallic oxides, the downside of their use stems from their relatively low electron conductivity and low surface areas that results in poor platinum dispersion. 

Only the surface layers of the catalyst soHd ate generaHy thought to participate in the reaction (125,133). This implies that while the bulk of the catalyst may have an oxidation state of 4+ under reactor conditions, the oxidation state of the surface vanadium may be very different. It has been postulated that both V" " and V " oxidation states exist on the surface of the catalyst, the latter arising from oxygen chemisorption (133). Phosphoms enrichment is also observed at the surface of the catalyst (125,126). The exact role of this excess surface phosphoms is not weH understood, but it may play a role in active site isolation and consequently, the oxidation state of the surface vanadium. 

Catalytic Oxidation. Catalytic oxidation is used only for gaseous streams because combustion reactions take place on the surface of the catalyst which otherwise would be covered by soHd material. Common catalysts are palladium [7440-05-3] and platinum [7440-06-4]. Because of the catalytic boost, operating temperatures and residence times are much lower which reduce operating costs. Catalysts in any treatment system are susceptible to poisoning (masking of or interference with the active sites). Catalysts can be poisoned or deactivated by sulfur, bismuth [7440-69-9] phosphoms [7723-14-0] arsenic, antimony, mercury, lead, zinc, tin [7440-31-5] or halogens (notably chlorine) platinum catalysts can tolerate sulfur compounds, but can be poisoned by chlorine. 

ButylatedPhenols and Cresols. Butylated phenols and cresols, used primarily as oxidation inhibitors and chain terrninators, are manufactured by direct alkylation of the phenol using a wide variety of conditions and acid catalysts, including sulfuric acid, -toluenesulfonic acid, and sulfonic acid ion-exchange resins (110,111). By use of a small amount of catalyst and short residence times, the first-formed, ortho-alkylated products can be made to predominate. Eor the preparation of the 2,6-substituted products, aluminum phenoxides generated in situ from the phenol being alkylated are used as catalyst. Reaction conditions are controlled to minimise formation of the thermodynamically favored 4-substituted products (see Alkylphenols). The most commonly used is -/ fZ-butylphenol [98-54-4] for manufacture of phenoHc resins. The tert-huty group leaves only two rather than three active sites for condensation with formaldehyde and thus modifies the characteristics of the resin. 

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