Catalysts Amorphous Surface

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

Vanadium phosphates have been established as selective hydrocarbon oxidation catalysts for more than 40 years. Their primary use commercially has been in the production of maleic anhydride (MA) from n-butane. During this period, improvements in the yield of MA have been sought. Strategies to achieve these improvements have included the addition of secondary metal ions to the catalyst, optimization of the catalyst precursor formation, and intensification of the selective oxidation process through improved reactor technology. The mechanism of the reaction continues to be an active subject of research, and the role of the bulk catalyst structure and an amorphous surface layer are considered here with respect to the various V-P-O phases present. The active site of the catalyst is considered to consist of V and V couples, and their respective incidence and roles are examined in detail here. The complex and extensive nature of the oxidation, which for butane oxidation to MA is a 14-electron transfer process, is of broad importance, particularly in view of the applications of vanadium phosphate catalysts to other processes. A perspective on the future use of vanadium phosphate catalysts is included in this review. 

Furthermore, these catalysts were found to be active from the start of operation, whereas catalysts derived from V0HP04 7iH20 take several hours to achieve full activity. These observations were considered to support the proposal that the active catalyst comprises an amorphous surface layer—and the crystalline vanadyl pyrophosphate that has been so well studied may be nothing more than an elaborate support (Figure 28). 

Hutchings and coworkers [72-74] have prepared vanadium phosphate catalysts using supercritical precipitation methods. These materials were found to be amorphous by XRD and electron diffraction, but showed activity comparable to standard vanadium phosphate catalysts. This demonstrates that an amorphous surface layer can be the active phase in these catalysts and that the crystalline VPP that has been so well studied may be nothing more than an elaborate support. 

Nano-grained Ni/ZrOj and Ni/ZrOj-Sm Oj catalysts were prepared from amorphous Ni-Zr and Ni-Zr-Sm alloys by oxidation-reduction treatment. Their catalytic activity for methanation of carbon dioxide was examined as a function of precursor alloy composition and temperature. The addition of samarium is effective in enhancing the activity of the nickel-rich catalysts, but not effective for the zirconium-rich catalysts. The surface area and hydrogen uptake of the nickel-rich catalysts are increased by the samarium addition. In addition, tetragonal zirconia, the formation of which is beneficial to the catalytic activity, is stabilized and formed predominantly by the addition of samarium to the nickel-rich catalysts, although monoclinic zirconia is also formed in the zirconium-rich catalysts. As a consequence, the higher conversion of carbon dioxide is obtained on the Ni-Zr-Sm catalysts with relatively high nickel contents. 

The purpose of heat treating a solid precursor is to remove volatiles (typically water) and to convert the solid to a desirable amorphous or crystallite phase. It is during heat treatment that the precursor converts to a physically robust and chemically active catalyst. It affects such properties of the catalyst as surface acidity, number of active sites, surface area, pore structure, and crush strength. Several operations that involve heat treatment include drying, calcination, reduction, and stabilization and they are frequently employed in multiple steps before and after forming catalyst pellets. 

BET results in figure 1 show that sol-gel technique yields catalysts with surface areas that are approximately twice of coprecipitated ones. This may be due to formation of a second amorphous phase of M0O3 in sol-gel catalysts [6] instead occupation of lattice interstices, by Mo excess, which occurs in coprecipitated catalysts [16]. Industrial catalysts present a lower surface area than coprecipitated catalyst with the same atomic ratio Mo/Fe (=3) which can be attributed to the severe calcination step. 

Transmission electron microscopy of the Sn-Cr-0 catalyst corroborates these observations. For example, the TEM micrograph of the catalyst with a Sn Cr ratio of 0.015 at 600° shows only the presence of small crystallites of tin(IV) oxide and no cluomium-containing phase although cliromium is detected by EDXa analysis. We deduce therefore that the cliromium is present in an amorphous surface layer of an as yet unknown composition on the crystallites of the tin(IV) oxide. 

Catalyst Development. Traditional slurry polypropylene homopolymer processes suffered from formation of excessive amounts of low grade amorphous polymer and catalyst residues. Introduction of catalysts with up to 30-fold higher activity together with better temperature control have almost eliminated these problems (7). Although low reactor volume and available heat-transfer surfaces ultimately limit further productivity increases, these limitations are less restrictive with the introduction of more finely suspended metallocene catalysts and the emergence of industrial gas-phase fluid-bed polymerization processes. 

Fig. 11. The loss of carbon rapidly increases with the increase of temperature. Heating of the catalysts in open air for 30 minutes at 973 K leads to the total elimination of carbon from the surface. The gasification of amorphous carbon proceeds more rapidly than that of filaments. The tubules obtained after oxidation of carbon-deposited catalysts during 30 minutes at 873 K are almost free from amorphous carbon. The process of gasification of nanotubules on the surface of the catalyst is easier in comparison with the oxidation of nanotubes containing soot obtained by the arc-discharge method[28, 29]. This can be easily explained, in agreement with Ref [30], by the surface activation of oxygen of the gaseous phase on Co-Si02 catalyst.

Zeolites as cracking catalysts are characterized hy higher activity and better selectivity toward middle distillates than amorphous silica-alumina catalysts. This is attrihuted to a greater acid sites density and a higher adsorption power for the reactants on the catalyst surface. 

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