Pretreatment of Carbon Supports

Without pretreatment, the surfaces of carbon supports are nearly neutral. The surface structure of carbon supports can be altered by various oxidation treatments. Such treatments lead to the formation of the O-containing groups. The summary of methods used for oxidative treatments of carbons was given 

Carbons and Carbon-Supported Catalysts in Hydroprocessing By Edward Furimsky Edward Furimsky, 2008 

Support Surf, area, m lg pH Neutralization of acidibase surface sites (meqvjg)  

Martin-Gullon et al subjected AC supports to various treatments with the aim to increase porosity and surface area, to introduce O-containing surface groups and to decrease ash content. The increase in surface area and porosity 

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Catalyst performance may be influenced by oxidative pretreatment of carbon supports. The AC samples shown in Table 12 were used for the preparation of the Mo/AC and NiMo/AC catalysts. The activity of the catalysts was tested in the flow reactor at 3 MPa and 623 K using the 7% solution of pyridine in cyclohexane. Figure 39 shows that for the catalyst consisting of the pretreated supports, the overall conversion (to C5 hydrocarbons and piperidine) decreased, indicating an enhanced deactivation of catalyst. However, in the presence of H2S, the activity difference was much less evident. For pretreated... 

The feasibility of carbon-supported nickel-based catalysts as the alternative to the platinum catalyst is studied in this chapter. Carbon-supported nickel (Ni/C, 10 wt-metal% [12]), ruthenium (Ru/C, 10 wt-metal% [12]), and nickel-ruthenium composite (Ni-Ru/C, 10 wt-metal%, mixed molar ratio of Ni/Ru 0.25,1,4, 8, and 16 [12]) catalysts were prepared similarly by the impregnation method. Granular powders of the activated carbon without the base pretreatment were stirred with the NiCl2, RuC13, and NiCl2-RuCl3 aqueous solutions at room temperature for 24 h, respectively. Reduction and washing were carried out in the same way as done for the Pt/C catalyst. Finally, these nickel-based catalysts were evacuated at 70°C for 10 h. 

For farther improvement of hydrogen enzyme electrode the commercial carbon filament materials were used as an electrode matrix. Such type of materials are accessible and well characterized, that provides the reproducibility of the results. A procedure for hydrogen enzyme electrode preparation included the pretreatment of electrode support with sulfuric acid followed by enzyme immobilization. This procedure is a critical step, since initially carbon filament material is completely hydrophobic [9]. 

In recent years a simplifying attempt to overcome this complexity was to analyze carbon by TPD and to integrate the total CO and CO2 emission and to correlate the results with sample pretreatment and chemical reactivity [33]. The limited validity of such an approach is apparent. As is illustrated below, the chemically complex surfaces which are not described by such crude correlations are those with the highest catalytic activity. In applications of carbons as catalyst support it is immediately apparent that the details of the car-bon-to-metal interaction depend crucially on such details of surface chemistry. This explains the enormous number of carbon supports commercially used (several thousands). A systematic effort to understand these relationships on the basis of modern analytical capabilities is still missing. 

Depending on the material of the microreactor, different methods of catalyst deposition are feasible. Most of the mentioned techniques are possible for metallic microreactors especially sol-gel synthesis is very commonly used as a coating method. By the use of ceramic microreactors, several pretreatments such as anodic oxidation cannot be carried out in turn the deposition of carbon supports has been... 

Little is known about the stability of the acidic sites under hydroprocessing conditions. Apparently, these sites comprise C-O bonds. It is unlikely that such sites may survive a prolonged exposure to high temperatures and H2 pressures. In this regard, no information could be found in the scientific literature. Thus, the long-term performance of carbon-supported catalysts deserves additional attention. It is, however, believed that suitable pretreatments may facilitate the adsorption of active metals during impregnation. 

Fuertes and Menendez (2002) and Centeno and Fuertes (1999, 2001) have published a series of studies using this precursor. Centeno and Fuertes (1999) have spin coated a small amount of a novolak-type phenolic resin on the surface of carbon supports. The membranes were then carbonized in a tubular furnace from 500 to 1000°C in vacuum. The resulting membranes had O2/N2 selectivity of around 10 and CO2/CH4 selectivity of 160. This work was later extended and in that case (Centeno and Fuertes, 2001 Fuertes and Menendez, 2002) a novalak-type phenolic resin was deposited on the inner face of a ceramic tubular membrane used for ultrafiltration. The membrane was subsequently pyro-lyzed to 700°C. In some cases an oxidative pretreatment was used before pyrolysis or an oxidative posttreatment after pyrolysis. The resulting membranes had O2 permeabilities around 100 Barrers and O2/N2 selectivities around 12 at 25°C. Films dip coated with resin three times had lower permeability and only slightly higher selectivities than those dipped only once. For hydrocarbon mixtures, the separation performance was increased by several treatments air oxidation of the resin, air oxidation of the carbon, or chemical vapor deposition (CVD) posttreatment of the carbon. 

Fe/Ir catalysts In situ Fe and Ir Mossbauer spectroscopy of silica-supported Fe/Ir catalysts with different iron to iridium ratios following pretreatment in hydrogen show that the reduction of the Fe component is enhanced by the presence of Ir metal. The presence of Ir was found to increase the catalytic activity in hydrogenation of carbon monoxide and also to influence selectivity... 

Before preparing these carbon-supported Pt-based catalysts, a support pretreatment toward granular activated carbon with an aqueous solution of NaOH (pH 14) was carried out by immersing for 24 h to promote the anion exchange between the ligand chloride of impregnated metal precursers (K2PtCl4) and the aqueous hydroxide ion (OH ) inside the micropores of the activated carbon [33]. 

The volume of carbon, sand, ion exchange resin, or other media contained in a pressure tank and used as pretreatment process for water. Requirements for bed depth, expansion, and support, etc, are governed by design criteria for each process. 

There are many considerations that must be taken into account when choosing a particular carbon, or carbon structure, as an electrocatalyst support. In hot phosphoric acid at cathodic potentials, the carbon surface is capable of being oxidized to carbon dioxide. The degree of oxidation will depend on the pretreatment of the carbon (for instance, the degree of graphitization), on the carbon precursor, and the provenance. There are two important parameters that will govern the primary oxidation rate for any given carbon material in an electrochemical environment. These are electrode potential (the carbon corrosion is an electrochemical process and therefore will increase rapidly as the electrode potential is raised) and temperature. 

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