Activity of Carbon-Supported Catalysts

The direct determination of catalyst activity is desirable, although results of other tests may be used as guidance for selecting the testing conditions. In order 

The wealth of information on the activity of carbon-supported catalysts obtained at a near atmospheric pressure of H2 has been noted. It may appear that the value of such results is somehow limited. However, in most cases, the activity was determined in parallel with characterization of active phase by spectroscopic techniques and other methods. The low H2 pressure ensured the state of active phase and reaction network that could not be identified under high H2 pressure conditions. Thus, the primary stages of hydroprocessing reactions could be better described under low H2 pressure. 

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It is demonstrated above that the presence of oxygen surface groups is beneficial in many aspects of the preparation, dispersion and activity of carbon-supported catalysts. However, if one considers that one of the main advantages of carbon supports with respect to conventional oxidic supports, like alumina and silica, is the ease of reducibility of the metal on the support, the presence of oxygen groups could be detrimental. However, carbon is an ideal support for this type of study because, in contrast to alumina and silica, the carbon-active phase interaction is weak and the behavior of the catalyst is basically governed by the chemical nature of the active phase. Thus, the effect of adding other metals or promoters may be studied without the problem of undesirable effects with the support. 

Derbyshire F, de Beer VHJ, Abotsi GMK, Scaroni AW, Solar JM, Skrovanek DJ. The influence of surface functionality on the activity of carbon-supported catalysts. Appl Catal I986 27(l) l 17-131. 

Rapoport s findings have been confirmed in the authors laboratory where the actions of carbon-supported catalysts (5% metal) derived from ruthenium, rhodium, palladium, osmium, iridium, and platinum, on pyridine, have been examined. At atmospheric pressure, at the boiling point of pyridine, and at a pyridine-to-catalyst ratio of 8 1, only palladium was active in bringing about the formation of 2,2 -bipyridine. It w as also found that different preparations of palladium-on-carbon varied widely in efficiency (yield 0.05-0.39 gm of 2,2 -bipyridine per gram of catalyst), but the factors responsible for this variation are not knowm. Palladium-on-alumina was found to be inferior to the carbon-supported preparations and gave only traces of bipyridine,... 

Finally, a simple method for a rapid evaluation of the activity of high surface area electrocatalysts is to observe the electrocatalytic response of a dispersion of carbon-supported catalyst in a thin layer of a recast proton exchange membrane.This type of electrode can be easily obtained from a solution of Nafion. As an example. Fig. 11 gives the comparative... 

There is increased interest in the use of Ru-based systems as catalysts for oxygen reduction in acidic media, because these systems have potential applications in practicable direct methanol fuel cell systems. The thermolysis of Ru3(CO)i2 has been studied to tailor the preparation of such materials [123-125]. The decarbon-ylation of carbon-supported catalysts prepared from Ru3(CO)i2 and W(CO)6, Mo(CO)is or Rh(CO)is in the presence of selenium has allowed the preparation of catalysts with enhanced activity towards oxygen reduction, when compared with the monometallic ruthenium-based catalyst [126],... 

Activity of carbon-supported cobalt-molybdenum catalysts. 

In the case of methanol oxidation, it has been reported that the catalytic activity of carbon-supported Pt catalysts with similar particle size shows a maximum at about pHzPC = 6. This behavior has been interpreted by considering that, besides enhancing dispersion, the metal-support interaction reduces the number of active sites for methanol oxidation. In fact, as the pHzpc deviates from neutrality, the number of acid or basic functional groups on the catalyst progressively increases. This was confirmed by the shift of the Pt-oxide reduction peak potential in the catalysts with high metal-support interaction, as determined by pHzpc measurements. 

The metal catalysts are oxidized during storage in air and have to be reduced (activated) with hydrogen before use. Activation of carbon-supported Pt metals occurs smoothly at ambient temperature but reduction of Cu, Ni, Co, or Fe oxides requires at least 150-200 °C. Major roles of the support (usually AI2O3, SiO, or carbon) and additives are to stabilize the high surface area of the metal and possibly contribute to acid or base-catalyzed reactions. The amount of catalyst in a batch reactor is usually between 5 and 30 % (jntm), relative to the alcohol. 

The possibility of having a catalyst for diatomic oxygen (O2) reduction with a monolayer of Pt on Ru that approaches the activity of carbon-supported Pt has been demonstrated. 

We have found that cathodes with significantly reduced Pt loading seem to benefit from the use of carbon-supported catalysts. A comparison of the activity of unsupported Pt cathode catalyst with the activity of carbon-supported Pt catalyst (40% Pt by weight) indicates that carbon-supported catalysts outperform unsupported catalysts as long as Pt loading remains below ca. 1 mg cm . Also, carbon-supported Pt and Pt-X cathodes both require careful optimization of the ionomer content in the catalyst layer, with the best results obtained at a weight fraction of recast Nafion between 30 and 40%. 

It seems clear that a large surface area formed by accessible pores is important for obtaining highly dispersed and active catalysts. However, there are other carbon characteristics that have to be taken into account to explain the catalytic behavior of carbon-supported catalysts. One of the most important is the chemical composition of its surface. 

Electrostatic interactions between the carbon surface and the active-phase precursors have also to be taken into account in the preparation of carbon-supported catalysts. The presence of oxygen functionalities on the carbon surface, which can be produced upon the activation process (for activated carbons) and/or by subsequent oxidation treatments, renders it amphoteric. This implies that it can be more or less charged, positively or negatively, depending on the pH of the surrounding solution. Preparation variables such as the polarity of the solvent, the pH of the solution, the anionic or cationic nature of the metal precursor, and the isoelectric point (lEP) of the carbon support determine the extent of precursor-support interaction and, in this way, the total uptake and dispersion of the active phase in the final catalyst [17,20,37]. Thus, for carbons containing acidic surface groups and, as a consequence, a low isoelectric point, best results in the preparation of supported catalysts are achieved when a cationic precursor is used in basic media. Under these conditions, the acidic complexes (-COOH, -OH) are deprotonated (-COO , -0 ) in such a way that... 

Several other methods have been employed for the preparation of carbon-supported catalysts, although to a lesser extent that impregnation methods. Nakamura et al. [38] prepared molybdenum catalysts for ethene homologation by physical deposition of gaseous [Mo(CO)6]. Their supports were commercial activated carbons that were subjected to different treatments to modify then-surface. The authors compared these supports with oxidic supports and concluded that the interaction between the metal carbonyl and the carbon supports were weaker. Furthermore, they observed that oxidation of the carbon surface was effective in enhancing the catalytic activity of Mo/C, and they ascribed this effect to the contribution of the surface oxygen groups to the partial oxidation of decomposed [Mo(CO)6]. 

Work by Prins, de Beer, and co-workers represents a great contribution to our knowledge of the structure of carbon-supported hydrotreating catalysts as well as of the kinetics and mechanisms of these reactions. These authors first reported that the activity of carbon-supported molybdenum and tungsten catalysts was higher than that of their counterparts supported on alumina or silica [80,81]. They... 

The relative inertness of the carbon surface is of paramount importance when carbon materials are going to be used as supports for hydrogenation catalysts. These systems usually consist of more than one metallic phase (bimetallic systems) and even by metals promoted by metal oxides. The carbon inertness facilitates interaction between the metals and/or between the metals and the promoters, yielding more active and selective catalysts than those supported on other common supports. These aspects will be illustrated by examples of the application of carbon-supported catalysts to the hydrogenation of carbon oxides. 

After reviewing the literature we conclude that in all three methods—wet impregnation, incipient-wetness impregnation, and ion adsorption—two major factors govern the final dispersion of the active phase precursor-support interaction and pore structure. These two issues are discussed in the next two subsections. Note that we discuss primarily that literature which adds fundamental insights in the preparation of carbon-supported catalysts. 

The results published in the literature demonstrate the effect of nitrogen incorporation as a key parameter for the adsorptive properties of carbon materials [73-78] and for the catalytic activity and dispersion of carbon-supported catalysts [79], It has been reported that the presence of nitrogen functionalities within the carbon matrix enhances the processes of hydrogen sulfide and SO2 removal by activated carbons [74, 75, 78]. 

The results obtained with bulk alloys can be also translated to more practical carbon supported PtM catalysts. In general, a certain increase in the mass activity (A/gpt) for the ORR reaction was found in PtM (M = Cr, V, Mn, Ni, Co) as compared to Pt/C. Again, it is difficult to extract accurate conclusions to explain the role of the foreign metal on the activity of carbon supported bimetallic (or polymetallic) electrocatalysts for the ORR since features such as particle size and size distribution, or metal loading cannot always be eliminated by normalization procedures. Paulus et studied... 

In another work, Shukla and co-authors [27] studied the electrocatalytic activity of carbon-supported Pt-Au alloy catalysts, with different atomic ratios, to improve the oxygen reduction reaction (ORR) kinetics and methanol tolerance, in a direct methanol fuel ceU. The electrocatalysts were prepared by codeposition of Pt and Au nanoparticles onto a carbon support. 

Pt particles supported on high surface-area carbon substrates to form supported catalyst (abbreviated as Pt/C) is the most widely used ORR catalyst at the current state of technology. The surface characteristics of Pt particles supported on carbon are similar to that of Pt polycrystalline electrode surface, which can be observed by their surface CVs. However, the ORR activity of Pt/C is different from what can be observed for polycrystalline Pt surface. This difference is possibly due to some differences in the superficial structure, or a too strong adsorption of oxygenated species on very small particles. In addition, the ORR activity of carbon-supported Pt particles might also be affected by the electronic properties of Pt atoms from carbon. 

Fundamental anode catalyst research is imperative for improved direct formic acid fuel cell (DFAFC) performance and stability such that an intimate understanding of the interplay between structural, morphological, and physicochemical properties is needed. The primary base catalysts found to be active for formic acid electrooxidation are either platinum (Pt) or palladium (Pd). The cyclic voltammograms in Fig. 4.1 compare the activity of carbon-supported Pt to Pd towards formic acid electrooxidation. The anodic (forward) scan, relevant to DFAFC performance, is relatively inactive on Pt/C until the applied potential... 

Although the primary focus of this review was carbon and carbon-supported catalysts, attempts have been made to identify the difference in the effect of carbon supports compared with the oxidic supports, particularly that of y-Al203. It has been noted that many studies had the same objective. For this purpose, the difference in catalyst activity and stability was estimated using both model compounds and real feeds under variable conditions. The conditions applied during the preparation of carbon-supported catalysts have... 

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. 

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