Molybdena reduced

Effect of Catalyst The catalysts used in hydrotreating are molybdena on alumina, cobalt molybdate on alumina, nickel molybdate on alumina or nickel tungstate. Which catalyst is used depends on the particular application. Cobalt molybdate catalyst is generally used when sulfur removal is the primary interest. The nickel catalysts find application in the treating of cracked stocks for olefin or aromatic saturation. One preferred application for molybdena catalyst is sweetening, (removal of mercaptans). The molybdena on alumina catalyst is also preferred for reducing the carbon residue of heating oils. 

Again, points on the curve were the measured acrolein production rates, and the line is the predicted production rate based on the current and the stoichiometry according to eq 9. At higher conversions, we observed significant amounts of CO2 and water, sufficient to explain the difference between the acrolein production and the current. It should be noted that others have also observed the electrochemical production of acrolein in a membrane reactor with molybdena in the anode. The selective oxidation of propylene to acrolein with the Cu—molybdena— YSZ anode can only be explained if molybdena is undergoing a redox reaction, presumably being oxidized by the electrolyte and reduced by the fuel. By inference, ceria is also likely acting as a catalyst, but for total oxidation. 

The simplest interpretation of these results is that ceria and molybdena act as catalysts in the TPB region, as shown in Figure 17. Either molybdena or ceria are oxidized by 0 coming through the electrolyte and then subsequently reduced by the fuel. According to this picture, reaction at the TPB is a simple redox process with a nonconventional oxygen source. Because molybdena is selective for the oxidation of propylene to acrolein, while ceria is nonselective, the products formed in cells with these two catalysts are different. 

Fra. 29. Spectra of cobalt-molybdena-alumina catalyst and related compositions, C oMo04, and a coprecipitated cobalt alumina catalyst (all three samples were calcined in air) also spectrum of cobalt-alumina catalyst reduced in hydrogen. 

Infrared spectra of chemisorbed NO. Chemisorption of NO on partially reduced molybdena based catalysts has been shown to be a usefull technique in evaluating M0O3 exposures on such a catalyst surface (see, e. g. 16)). However, the application of this technique to a series of catalysts which differ in M0O3 loading, or even in preparation method, requires a carefull control of the degree of reduction. As NO appears to be chemisorbed mainly at Mo sites, the catalysts must be quantitatively reduced according to the reaction ... 

The earliest NMR studies of oxide surfaces (362-364) involved wide-line proton NMR of adsorbed organic species. For example, Petrakis and Kiviat (363), who studied the adsorption of pyridine and thiophene on molybdena-modified alumina, found that chemisorbed and physisorbed species can be readily distinguished. When physically adsorbed, both compounds exhibited liquid-like NMR behavior with high molecular mobility even at low temperatures. Chemisorbed pyridine was much more rigidly held with essentially only a rotation about the C2 molecular axis persisting to - 130°C. Pyridine was sorbed both physically and chemically, and pretreatment of the surface was not particularly significant in this respect. By contrast, thiophene was physisorbed only on surfaces previously reduced with hydrogen, and underwent a reaction on calcined but unreduced surfaces. 

The individual techniques used to characterize molybdena catalysts are now considered. Table II presents a listing of articles concerning the characterization of molybdena catalysts. Unless otherwise specified, we implicitly refer to Mo and/or Co supported on an activated alumina, commonly y-AlaOs. Most work has been done on the calcined (oxidized) state of the catalyst because of ease of sample handling. Reduced and sulfided catalysts are more difficult to work with since for meaningful results, exposure of these samples to air or moisture should be rigorously avoided. Therefore, sample transfer or special in situ treatment facilities must be provided. 

In summary, the presence of a relatively strong Mo5+ signal found on A1203 catalysts but not on Si02 catalysts can be considered as additional evidence for an interaction between the molybdena and the alumina, which permits stabilization of the Mo5+ state in the reduced and sulfided catalyst. An H-containing specie seems a reasonable hypothesis for this state. The Co may be in tetrahedral or octahedral environment or both. 

Molybdena catalysts have been used for a large number of catalytic reactions—the literature is extensive in its use. We will limit our discussion to only the most common reactions occurring over reduced or sulfided catalyst. Furthermore, only those studies which attempt to relate catalyst activity to catalyst properties will be covered here. 

Increases Mo reduction. The molybdena catalyst reduces more in the presence of Co and activity depends on extent of reduction. As discussed earlier, there is considerable disagreement on the former point and the latter is unproved, especially for sulfided catalysts. 

No one particular site or specie has been identified as the active site for the various reactions which occur on reduced molybdena catalysts. It may be that different sites are used for different types of reactions, e.g., hydrogenation and isomerization. Phase studies and measurement of Mo... 

Payen et al. (1986) investigated the reduction of alumina-supported molybdena and ascribed a Raman band at 760 cm-1 to reduced supported molybdenum oxide. The transformations could be reversed by reoxidization (Payen et al., 1986). Mestl and Srinivasan (1998) described some reduced phases formed from bulk molybdena, whereas reduced dispersed vanadia and chromia catalysts do not show Raman bands (Airaksinen et al., 2005 Banares et al., 2000a Gasior et al., 1988 Weckhuysen and Wachs, 1996). 

Supported dispersed metal oxides can also be reduced by alcohols. Hu and Wachs (1995) reported Raman bands of surface-reduced molybdena that was generated through contact with methanol. Zhao and Wachs (2006) recently investigated V205/Nb205 catalysts during propene oxidation to acrolein and detected a previously unknown Raman band at 978 cm-1, which was tentatively assigned to a surface V4+ species. 

Indeed, Lund and Dumesic (5-8) reported that the water-gas shift activity of an iron oxide catalyst is reduced by several orders of magnitude when supported on silica. Strong interactions between molybdena and alumina have been documented for the calcined states of hydrotreating catalysts (e.g., 9-11). Also, interaction is manifested in many mixed oxides by enhanced acidity, compared to the acidities of the pure component oxides (12-14). 

Another degree of modification of the catalysts can be achieved by introduction of components which on one hand affect the dispersion of the noble metal similarly to the ceria discussed earlier, but also possess catalytic activities of their own. One example of such an additive explored in depth at Ford Research is molybdenum oxide. Molybdena, similar to ceria, forms a two-dimensional phase on 7-AI2O3 and thereby also affects the Pt dispersion and its catalytic properties. Platinum, in turn, affects strongly the reducibility of molybdena, as shown in Fig. 4, using ESCA to characterize the oxidation state after reduction in the absence and presence of Pt [7]. 

A direct confirmation of this behavior is obtained by TPR [8], One can deduce that in the presence of Pt the average oxidation state of surface molybdenum ions will be lower in an operating catalyst. It is possible to postulate the existence of surface complexes of the type PtMoOx, where below 600°C x may range from 0 to 2 depending on the reaction temperature. Recent preliminary EXAFS results seems to corroborate such a picture [9J. Conversely, one can consider the surface Pt in such complexes as being more oxidized (electron deficient) than when dispersed in the absence of modifiers such as molybdena or ceria. This is found to affect the catalytic properties of Pt. A similar behavior prevails in other systems as well. For instance, it was recently reported that addition of ceria to a Pd/AlgC catalyst results in a Pd surface state which is more difficult to reduce [10]. 

Problem 9.11 Metal-oxide catalyzed polymerization of ethylene was carried out in benzene solution in a stirred autoclave with a suspension of hydrogen-reduced molybdena-alumina catalyst (Friedlander and Oita, 1957). The pressure was maintained nearly constant by repressuring the autoclave with ethylene as it was consumed in the polymerization process. Temperatures of 200-275°C were studied. The ethylene concentration in solution was controlled by adjusting the pressure (in the range 625 to 1000 psi) at any particular temperature. The ethylene uptake rate (rate of pressure drop, dPIdt) was mea-... 

Rather than survey all of the possible modifications that can be made to an alumina surface, we will focus on a subset involved in two different types of surface-catalyzed chemical reactions, namely, the partial oxidation of ethylene to ethylene oxide (EO) and hydrodesulfurization (HDS) processes. Both of these catalytic systems have functional points in common, in that alumina serves as a support (a-alumina for the EO process and 7-alumina for the HDS process) and alkali-metal salts serve as promoters for both reactions. To illustrate this commonality, this section will be divided into three parts (1) the adsorption of alkali-metal salts to 7-alumina, as reflected in the Rb and Cs solid-state NMR spectroscopy of these systems (2) the absorption of ethylene to silver supported on aluminas in the presence and absence of cesium salts, as followed by C NMR spectroscopy, and (3) the solid-state Mo NMR of fresh and reduced/ sulfided molybdena-alumina catalysts. 

In re-forming, molybdena on alumina is alternately subjected to oxidizing and reducing atmospheres which may contain sulfur compounds. To gain more basic information about the interactions of the catalyst with hydrogen, water vapor, hydrogen sulfide, sulfur dioxide, and sulfur trioxide, a series of adsorption studies were carried out. Various equilibrium conditions were calculated from thermodyuamic data (9) to interpret further the complex chemistry evidenced by these physicochemical studies. 

The losses of weight have been associated with valence changes of the molybdenum with time as shown in Fig. 4. As can be seen, it would take extremely long times to reduce the molybdena to M0O2. 

The latter observation was reaffirmed by determining the average valence of the molybdena in a catalyst (8.7 % M0O3) that had been reduced in a flow of hydrogen for 60 hrs. at 480° and 1 atm. A wet-chemical method of analysis was used in which the reduced molybdenum was oxidized by ceric sulfate, excess ceric ion being back titrated with ferrous sulfate. It was found that the average valence of the molybdenum corresponded to M0O2.36 -... 

The values of ka, ka, Wa, and Wa and the molybdena contents for the various catalysts are listed in Table IV. The initial rate of adsorption (kaWa) was always greater than the initial rate of desorption (kaWa). In addition, more water could be adsorbed than desorbed. The last column indicates the fraction of water that was retained and could not be desorbed. Although their rate constants were smaller, the reduced catalysts adsorbed and retained more water than the oxidized forms. 

When first reduced in H2 at 482° and then contacted with SO2, the sulfur contents of the catalysts were proportional to the surface area and independent of molybdena content. 

Pretreatment involving the partial reduction of the oxide with hydrogen can similarly produce significant effects that vary with the metal oxide used. The effect that prereduction has on supported chromia and molybdenum oxides is widely different. On a chromia catalyst, the reduction step leads to the formation of Br0nsted sites, which then catalyze the isomerization reaction via a cationic intermediate. On a reduced molybdena catalyst, metathesis-type mechanisms dominate, with the cationic mechanism proceeding only on a fully oxidized molybdena surface. 

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