Transition-metal oxides methanation

In 1990, Schroder and Schwarz reported that gas-phase FeO" " directly converts methane to methanol under thermal conditions [21]. The reaction is efficient, occuring at 20% of the collision rate, and is quite selective, producing methanol 40% of the time (FeOH+ + CH3 is the other major product). More recent experiments have shown that NiO and PtO also convert methane to methanol with good efficiency and selectivity [134]. Reactions of gas-phase transition metal oxides with methane thus provide a simple model system for the direct conversion of methane to methanol. These systems capture the essential chemistry, but do not have complicating contributions from solvent molecules, ligands, or multiple metal sites that are present in condensed-phase systems. 

Methane-to-methanol conversion by gas-phase transition metal oxide cations has been extensively studied by experiment and theory see reviews by Schroder, Schwarz, and co-workers [18, 23, 134, 135] and by Metz [25, 136]. We have used photofragment spectroscopy to study the electronic spectroscopy of FeO" " [47, 137], NiO [25], and PtO [68], as well as the electronic and vibrational spectroscopy of intermediates of the FeO - - CH4 reaction. [45, 136] We have also used photoionization of FeO to characterize low lying, low spin electronic states of FeO [39]. Our results on the iron-containing molecules are presented in this section. 

The large amounts of natural gas (mainly methane) found worldwide have led to extentive research programs in the area of the direct conversion of methane [1-3]. Ihe oxidative transformation of methane (OTM) is an important route for the effective utilization of the abundant natural gas resources. How to increase catalyst activity is a common problem on the activation of methane. The oxidation of methane over transition m al oxides is always high active, but its main product is CO2, namely the product of deep oxidation. It is because transition metal oxides have high oxidative activity. So, they were usually used as the main corrqtonent of catalysts for the conqilete oxidation of alkane[4]. The strong oxidative activity of CH4 over tran on metal oxides such as NiO indicates that the activation of C-H bond over transition metal oxides is much easier than that over alkaline earth metal oxides and rare earth metal oxides. Furthermore, the activation of C-H bond is the key step of OTM reaction. It is the reason that we use transition metal oxides as the mam conq>onent of the OTM catalysts. However, we have to reahze that the selectivity of OTM over transition metal oxides is poor. 

Metal oxide-mediated oxidation of methane using air as a primary oxidant is an alternative way to produce N2-free syngas. The concept is based on the oxidation of methane by transition metal oxides in high-oxidation state yielding syngas and corresponding metal oxide in a low-oxidation state ... 

Early in the nineties Ruiz et al. reported enhanced catalyst activities and increased selectivities to alkenes and higher hydrocarbons upon addition of V, Mg, and Ce oxides to Co-based F-T catalysts.These variations were attributed to electronic effects induced by the transition metal oxide. Similar results were obtained by Bessel et al. using a Cr promoter in Co/ZSM-5 catalysts.This group observed that the addition of Cr improved the catalyst activity, and shifted the selectivity from methane to higher, generally more olefinic, hydrocarbons. Based on H2 and CO chemisorption, as well as TPR and TPD results, they suggested that the promotion was caused by an interaction between the transition metal oxide and the cobalt oxide, which inhibits... 

Rhodium is a unique metal since it can catalyze several transformations.222,223 It is an active methanation catalyst and yields saturated hydrocarbons on an inert support. Methanol is the main product in the presence of rhodium on Mg(OH)2. Transition-metal oxides as supports or promoters shift the selectivity toward the formation of C2 and higher oxygenates. 

As for the complete oxidation of propene, propane and methane, Nieuwenhuys and coworkers studied the influence of metal oxides additives on the catalytic activity of Au/Al203 [109-115], The addition of 3d transition metal oxides (MnOx, CoOx or FeOx), which were active by themselves, or ceria that was poorly active by itself promoted the catalytic activity of Au/Al203 in the total oxidation of propene [112]. The most active catalyst was Au/Ce0x/Al203, with a T95 at 497 K and with a high stability. In these cases, ceria and the transition metal oxides may act as co-catalysts and the role is twofold it stabilizes the Au NPs against sintering (ceria)... 

The stability of the adsorption of a hydrocarbon on the active site prior to C-H bond dissociation apparently strongly depends on the nature of the site and has not yet been unequivocally established. None of the studies on oxidative coupling of methane on non-transition metal oxides reported a stable methane preadsorption.34 Transition metal oxides, however, may reveal different behaviour and there exists already theoretical evidence for stable methane adsorption on transition metal atoms and complexes.35 Also the stability of the so-called encounter complexes has been qualitatively predicted to increase the reactivity of transition metal MO species... 

Lanthanides as modifiers to other oxides in aluminas In zirconias In iron oxide Lanthanide oxides in mixed oxides With aluminas With iron oxides With other transition metal oxides To maintain surface area To increase oxidation rates To increase methanation rates For conduction in electrocatalysis For ammonia synthesis promotion To provide sulfur oxides (SO.,) control For dehydrogenation in carbon monoxide reactions For oxidation... 

One example is a catalyst consisting of gold on cobalt oxide particles supported on a mechanical mixture of zirconia-stabilised ceria, zirconia and titania, that survived 773 K for 157 h, with some deactivation [145]. Grisel and Nieuwenhuys [38,127] have shown that the addition of transition metal oxides to form Au/MO j/ADOs catalysts, massively suppresses Au particle sintering in methane oxidation tests up to 973 K. Also, Seker and Gulari [30] found that Au/ADOs catalysts survive rigorous pre-treatments of 873 K in air for 24 h, followed by several cycles of 423-773 K and they were then kept... 

This work is devoted to the synthesis of Zr02 by various methods, the synthesis of zirconium-containing pentasils and Zr02 - -zeolite based binary carriers. These materials were used as carriers of transition metal oxides (chromium, cobalt) and their catalytic properties were characterized in the selective reduction of NO by methane and propane-butane mixture, the acidic properties of the samples were investigated by thermoprogrammed desorption and IR-spectroscopy methods. 

It has been established from these studies that the different catalytic properties of transition metal oxides (chromium, cobalt) on zirconium dioxide are attributed to their different acidic properties determined by TPDA and IR-spectroscopy. The most active catalyst is characterized by strong acidic Bronsted centers. The cobalt oxide deposited by precipitation on the zirconium-containing pentasils has a considerable oxidative activity in the reaction N0+02 N02, and for SCR-activity the definite surface acidity is necessary for methane activation. Among the binary systems, 10% CoO/(65% H-Zeolite - 35% Z1O2)... 

Catalytic total oxidation of volatile organic compounds (VOC) is widely used to reduce emissions of air pollutants. Besides supported noble metals supported transition metal oxides (V, W, Cr, Mn, Cu, Fe) and oxidic compounds (perovskites) have been reported as suitable catalysts [1,2]. However, chlorinated hydrocarbons (CHC) in industrial exhaust gases lead to poisoning and deactivation of the catalysts [3]. Otherwise, catalysts for the catalytic combustion of VOCs and methane in natural gas burning turbines to avoid NO emissions should be stable at higher reaction temperatures and resists to thermal shocks [3]. Therefore, the development of chemically and thermally stable, low cost materials is of potential interest for the application as total oxidation catalysts. 

The decomposition of methane on a catalyst (with rare earth or transition metal oxides as suitable components) at 1100 C provides spherical carbon structures with a rather uniform diameter of ca. 210nm. These are, however, not classical carbon onions as the individual shells are not closed. They are rather objects made from graphitic units stacked up one over another that, to some extent, resemble classical soot particles. 

An Overview. During the past decade, much work has been reported ii.volving attempts to activate methane and convert it to ethylene and other higher hydrocarbons. Two main routes have been proposed. The first involves passing methane at high temperatures (800 c+) over transition metal oxides at variable oxidation states. The oxides are reduced to lower valence oxides, and the methane is oxidatively dehydrogenated and coupled to form ethane and ethylene as well as water and carbon dioxide. 

Fig. 1. Activity patterns of first row transition metal oxides at 300° C for (1) homo-molecular exchange of oxygen, (2) oxidation of hydrogen, (3) oxidation of methane and (4) nitrogen oxide decomposition. Adapted from reference [10] with permission.

Whereas the Mobil process starts with syn gas based methyl alcohol, Olah s studies were an extension of the previously discussed electrophilic functionalization of methane and does not involve any zeolite-type catalysts. It was found that bifunctional acidic-basic catalysts such as tungsten oxide on alumina or related supported transition metal oxides or oxyfluorides such as alumina or related supported transition metal oxides or oxyfluorides such as tantalum or zirconium oxyfluoride are capable of condensing methyl chloride, methyl alcohol (dimethyl ether), methyl mercaptan (dimethyl sulfide), primarily to ethylene (and propylene) (equation 65) . 

Transition metal. Transition metal oxides with high melting points have certain influence on methanation reaction (Table 6.58). From Table 6.58, Fe, Co, Ni, Mn, Zr, Mo, W and A1 have the inhibition role on methanation except Ni,... 

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