In situ DRIFT studies on methane transformation in absence of oxygen

It is generally accepted that methane is mainly activated on metallic surfaces. The electron donation from the HOMO of CH4 to the lowest unfilled molecular orbitals of metal surface should dominate dissociative CH4 adsorption. However,the fact that the carburization of Mo species by CH4 is thermodynamically possible, therefore CH4 can interact with Mo vacancies according to:

Mo[ ]Mo + CH4 ↔ Mo[C]Mo + 2H2 (Eq-38)

In this state, carbon can be extracted from the site, producing CO and regenerating the vacancy, as shown:

Where (♦) and (■) represent adsorption sites on Mo oxide. Thus, the formation of CO from CH4 would not be a simple dual site reaction involving adsorbed carbon and lattice oxygen, but involves a solid-state reaction. This mechanism would be consistent with the high activation energy of CH4 conversion on the supported catalyst.

Accordingly, the reduction of Mo by H2 at high temperature providesreactive H atoms able to form OH groups and oxygen vacancies on the surface, particularly those present in the bulk, are the driving force for CH4 activation. Furthermore, hydroxyls are also formed during the activation step due to the reaction of the lattice oxygen with hydrogen as dihydrogen arising from methane dehydrogenation or as water arising from the initial decomposition of methane.

It comes therefore that these rather basic OH groups are required for formate formation supporting the following reaction steps:

H2(g) + 2Mo → 2Mo-H (Eq-43)

Mo-H + O^2– → Mo + OH^– + e^– (Eq-44)

CO + OH^– ↔ HCOO ^– (Eq-45)

Based on these results, it can be inferred that the loss of OH groups during CH4 conversion does not necessarily implicate these species in the reaction to produce CO as the generation of formate species from CO would also account for the consumption of hydroxyls (Fig. 34).

Lin et al. carried out a comparative FTIR study on the interaction of CH4 with silica, alumina, and HZSM-5. The results demonstrated that OH groups played a very important role in CH4

adsorption. When an interaction between the OH groups and CH4 took place, the band shift of the OH groups varied and the strength of the interaction decreased in accordance with the order of their acidities (Si–OH–Al > Al–OH > Si-OH). The authors considered the possibility that CH4 is activated by interacting with a proton leading to a heterolytic cleavage of a C–H bond of CH4 [110].

Recent studies using supported and unsupported Mo compounds indicate that interactions with methane at temperature around 700°C lead to the formation of Mo2C, which is considered as the active site for the formation of CH2 and CH3 fragments. These carbides may be destroyed by reaction with air or CO2 [111-113].

However, it is well-known that alumina support has the lowest oxygen mobility and non-reducible alumina support is unable to store carbonaceous adspecies, the present results show that molybdenum oxide can activate methane and oxidize it into surface formates and carbon

monoxide but methane conversion is low. Consequently, this CH4 partialoxidation involves the interaction of methane with lattice oxygen anions and surface OH groups created by the previous reduction step and/or upon the initial decomposition of CH4.

Iglesia and co-workers [114] claimed that selective silanation of external acid sites on HZSM-5 by using large organosilane molecules could decrease the content of acid sites as well as the number of MoOx species retained on the external surface, which were regarded as key factors for coke formation during MDA. On samples prepared using silica-modified HZSM-5, acid sites, MoOx precursors, and active MoCx species formed during the CH4

reaction at 677°C were found to predominately reside within the zeolite channels, where spatial constraints could inhibit the bimolecular chain-growth pathways. Consequently, the selectivity of hydrocarbons on a 4% Mo/silica-modified HZSM-5 increased by about 30% in comparison with that on a 4% Mo/HZSM-5.

On ceria containing catalysts, to explain the fractional presence of CO2 and CO a mechanism can be proposed (Figs. 35 and 36), as well as taking into account the carbon exchange between CH4 and the surface. Surface reactions describe the proposed mechanism:

(Eq-46)

Meanwhile, the CO2 could adsorb on the carbon filled-site from the reaction above, extract the carbon to produce CO, then the carbon is removed from the active surface replenishing a vacancy according to the reverse Boudouard reaction:

(Eq-47) Moreover, ceria could simultaneously undergo a redox reaction in the presence of CO2:

CO

Where (■) represents adsorption sites on ceria, on the other hand, the CO could also reduce the ceria by reversing reaction, and further undergo CO disproportionation as the consequence of the occurrence of the Boudouard equilibrium:

(Eq-50)

However, such higher methane conversions are not necessarily related to the larger amounts of coke and faster deactivation of the catalyst, because deactivation also critically depends on the rate with which the coke can be removed by CO2 under reaction conditions when CO2 can react with the CHx species as well as with CH4:

CO2 + CHx → 2CO + (x/2) H2 (Eq-51)

Therefore, in comparison with the CH4 reaction on Mo/Al2O3 the presence of CeO2

contributes to the rapid activation of CH4, thereby accelerating the carbon gasification reaction to produce CO2. Thus, this supports the conclusion that CO2 is readily dissociated to CO and adsorbed oxygen over reduced CeO2 by filling up the oxygen vacancies in Ce³⁺

species. The occurrence of Ce⁴⁺/Ce³⁺ redox couple generates oxygen vacancies and releases free electrons. Free electrons transfer readily from Ce³⁺ to π* orbital of CO2 to activate CO2. The increased CO2 then decomposes to CO and active surface oxygen, reacts with the CHx

species and enhances the catalytic activity of CH4 decomposition since with the aid of Ce⁴⁺/Ce³⁺ redox couple, CO2 is more readily activated to release more surface oxygen, and the rate of carbon elimination has been accelerated.

Analogous observations were made by Darujati et al. who found that Ce promotion dramatically improves the stability of the Mo2C/γ-Al2O3 catalysts. They claimed that Ce acted to increase the oxidation resistance of Mo2C and avoid coking by CO2 activation via the redox reaction (Eq-48), thereby helping to prevent oxidation of Mo2C by CO2. From their study the addition of ceria promoter to Mo2C/γ-Al2O3 catalyst appears to alter the dry methane reforming (DMR) mechanism proposed earlier for bulk Mo2C catalysts by enhancing relatively strong CO2 adsorption and the role of ceria was found to influence the redox reactions on the surface as well as the activity and stability of the catalyst [115, 116].

On the other hand, although the presence of CO and CHX as well as carbonates over Rh/Al2O3

and Rh/Mo/Al2O3 catalysts has been observed by Anderson et al. during IR study on CH4

decomposition at 400°C, which also facilitated carbide formation [117].

R. Wang et al. have reported that an interaction between Rh and CeO2 was induced by high temperature reduction, which resulted in the creation of oxygen vacancies in ceria. They concluded that the CO2 activation in CH4/CO2 reforming should be mainly favoured by availability of Ce⁴⁺/Ce³⁺ redox couple in Rh–CeO2/Al2O3 catalyst, which was rather slow process on Rh/Al2O3 catalyst. The Ce³⁺ species readily promoted CO2 dissociation into CO and surface oxygen. The higher catalytic activity and coke resistance of Rh–CeO2/Al2O3 were

associated with the presence of the two-redox couples favoring the activation of both CH4 and CO2. Their catalytic test results showed that the promotional effect of CeO2 on CO2

conversion was much higher than that on CH4 conversion [118].

Given that oxygen mobility is high in ceria, allowing substantial reduction of the bulk, and given that CO2 gas is present under reaction conditions, one need not invoke surface mobility of adsorbed CO or CO2 in order to explain the apparently high coverage of carbonate.

Formation of CO2 takes place in two distinct moments: a fraction of the CO is rapidly oxidized to CO2 and the CO2 desorbs readily from the catalysts, while the remaining fraction of CO/CO2 is slightly adsorbed and accumulated on the catalyst forming various carbonate species (Eqs. 47-51).

When ceria and molybdena have been reduced at high temperature (>700°C), oxygen vacancies, particularly those present in the bulk, seem to be the driving force for CH4

activation. Therefore, when CeO2 is exposed to H2 or CO, oxygen vacancies VO2- with two electrons trapped have been created. Such a vacancy is a neutral entity with respect to the surface lattice of ceria. It can easily lose an electron by spontaneous ionization becoming singly positively charged with respect to the solid as follows [118-121]:

O

^{2(lattice− )+H2 →H2O+}

V

^{O2− (Eq-52)}

Reduction of CeO2 with hydrogen is generally thought to occur via a stepwise mechanism, first reduction of the outer most layers of Ce⁴⁺ (surface reduction), then reduction of the inner Ce⁴⁺ layers (bulk reduction) at higher temperatures. A few mechanisms have been put forward to account for this behaviour that comprises sequential steps of: (i) dissociation of chemisorbed hydrogen with formation of OH groups, (ii) formation of anionic vacancies with desorption of water by recombination of H and OH (with concomitant reduction of Ce⁴⁺ to Ce³⁺) and (iii) diffusion of surface anionic vacancies into the bulk. This picture is consistent with results obtained by Trovarelli and others upon the temperature programmed reduction with hydrogen of high surface area CeO2 when the TPR profile showed two well-defined peaks centred at approximately 600 and 800°C [119, 120]. Hence, the ability of ceria to be

easily reduced to nonstoichiometric oxides is related to the properties of fluorite structured-mixed valence oxides to deviate from stoichiometry (Fig. 12).

When considering the hydrogen conversion, the dissociation is faster and occurs at much lower temperature (200°C) than for the alkanes (> 600°C). Hence, the cleavage of the H-H bond is likely fast and is probably not a rate-limiting step such as the cleavage of C-H bond of the alkanes. It may also appear on different active sites. Therefore, the conversion of hydrogen is able to maintain its level with time, despite the production of water.

As it was pointed out previously, CH4 transformation over Mo/CeO2 can be explained by invoking a redox mechanism with a simple redox route for CH4 oxidation, which utilizes oxygen activated from the support in a typical reduction/oxidation mechanism (Mars Van Krevelen type) in which the catalyst undergoes a partial reduction by methane [124-127].

Oxygen storage is therefore important because it provides an alternative route for the oxidation of CH4. An alternative redox route involves oxygen from the support, which reacts with methane to form adsorbed CO2 in the form of carbonates. Decomposition of carbonates is then stimulated which provides also reoxidation of the support:

2

However, Ce⁴⁺/Ce³⁺ redox couple facilitates the elimination of CHx species by partial oxidation, resulting in higher methane conversion and lower amounts of coke. This partial oxidation of CHx species over Mo/CeO2 will continuously result in the creation of oxygen vacancies and Ce⁴⁺/Ce³⁺ redox couple. Thus, CH4 decomposition acts as the supplier of a hydrogen pool, while Ce⁴⁺/Ce³⁺ redox couple promotes CO2 activation by accepting electrons and replenishing the oxygen vacancies. This demonstrates that the reoxidized ceria can be reduced again by methane, regenerating the oxygen vacancies and releasing free electrons, so the creation of oxygen vacancies of ceria is a reversible process in the reaction atmosphere.

Accordingly, the presence of ceria in the catalysts as either promoter or support leads to significantly higher methane conversion especially on Mo/CeO2 by decreasing in C storage capacity and therefore an increase in the CO2 release upon the decomposition step. This effect is likely to favour carbon trapping via carbonates as shown previously by DRIFT spectroscopy. However, the mean CO concentration was not affected by the ceria or tin since it is essentially controlled by the enthalpy of desorption from the metal phase [117].

Regarding the intimate atomic mechanism involved in oxidation of carbon, several authors pointed out the importance of redox properties of the catalyst. That is, the effectiveness of the catalyst can be related to its ability to deliver oxygen from the lattice to carbon reactant in a wide temperature range. Recently, it has been reported that the use of supports based on CeO2

confers interesting properties to CH4 decomposition catalysts due to high availability of surface oxygen and high surface reducibility. Nevertheless, analysis discrepancies in the outcome of the results from different laboratories derive from synthesis and treatment procedures [116-121, 145, 146].

For instance, Craciun et al. studied the CH4 + H2O reaction over Rh, Pt and Pd supported on ceria catalysts [145]. In their study, they proposed a mechanism, which involved a surface reaction of the adsorbed oxygen on the ceria with the dissociated methane on the surface.

Their study led to the conclusion that oxygen transfer from the ceria to the noble metals was the rate determining step, where the participation of lattice oxygen and catalyst reducibility have shown to improve overall performances.

Similar results over Pt/CeO2 were reported by Otsuka et al. who found that the oxidation of CH4 by CeO2 was thermodynamically available at above 600°C. The reduction degree of CeO2 was significantly improved from 3.5% to 17.1% in the presence of Pt after the reaction with CH4 [146].

Concerning the CH4 dissociation on Mo/SnO2, the generation of formaldehyde intermediate upon CH4 reaction may indicate that the Mo/SnO2 has a high concentration of Lewis acid sites (Fig. 38). However, since the free CH2O was not detected in the gas phase that undoubtedly appears at around 1730 cm^-1 [175-179], the absence of formaldehyde may be due to its low concentration and/or due to surface reactions by extracting lattice oxygen to form additionally CO and CO2. These steps are represented by the following reaction steps:

CH4 + 2MO → CH2O + H2O + 2M (Eq-57)

CH2O + MO → CO + H2O + M (Eq-58)

CH2O + 2MO → CO2 + H2O + 2M (Eq-59) Where MO represents metal oxide surface site. Moreover, CO and H2 can also be produced via the pyrolytic decomposition of CH2O:

CH2O → CO + H2 (Eq-60)

Meanwhile, exclusion of a direct pathway from CH4 to CH2O allows CO and CO2 to be treated as a single product (CO)x in the analysis of the kinetics of CH2O formation and consumption. Since the interconversion of CO and CO2 was not investigated, no attempt was made to include the effects of this process in the modelling of CH2O formation and consumption on Mo/SnO2. The following mechanistic reaction pathway can be proposed:

, 2

Accordingly, it is argued that step 1, hydrogen abstraction, is stated to be favoured by lattice oxygen possessing more negative charge. The general trends observed with formaldehyde selectivity were explained on the basis of electrophilicity of adsorbed oxygen enhancing the rate of step 2 with respect to step 3. Consequently, it can be stated that acid-base bifunctional catalysts would be more effective for formaldehyde production [100-106].

Niwa and Igarashi correlated the acidity and reducibility to the catalytic behaviour of Mo/SnO2 system in the oxidative dehydrogenation of methanol converted into formaldehyde selectively. They found that the generation of acid sites is also strongly affected by the calcination temperature of tin oxide that affects by its turn the formation of acid sites on the loaded molybdenum oxide [100].

Smith and Ozkan [190, 191] studied the partial oxidation of methane to formaldehyde over MoO3 samples exposing different relative amounts of (010) and (100) plane areas. Their experimental characterization studies suggest that the Mo=O sites residing preferentially on the side planes could be promoting to the formation of formaldehyde, while the bridging sites Mo–O–Mo mainly on the basal plane were more likely to lead to complete oxidation of CH4. S. Chempath and A. T. Bell studied the partial oxidation of CH4 at 700ºC on Mo/SiO2

catalyst. They found that CH2O is the only initial product. As the CH4 conversion increased, the CH2O selectivity decreased and the selectivity for CO and CO2 increased [192].

Finally, the present results permit to infer that Mo/SnO2 showed the highest CH4 conversion among the catalysts studied leading almost to complete CH4 oxidation (Fig. 38). On the other hand, the results also demonstrate the activity of the molybdena and tin lattice oxygen and its participation in the reaction under study conditions.

One may suggest that the high catalytic activity of the Mo/SnO2 system towards CH4 might be associated with the dissolution of Mo ions in the SnO2 crystals, since this dissolution was observed at the end of the experiment and H2SO4 was used to clean the sample cup due to the adhesive form of the sample.

The activation of methane is believed to be the key step in the conversion of methane.

According to Lunsford, methane activation occurs homolytically via the abstraction of a hydrogen atom by oxygen anions present on oxide surface [153-155]:

[ ]− + → − + ^⋅

− O CH Cat OH CH₃

Cat ₄ (Eq-61)

This first hypothesis has been substantiated by the presence of methyl radicals on the surface.

The methyl radicals so formed on the surface could react with the catalyst and produce methoxide ions, which on a subsequent reaction with water yield methanol. Further oxidation of methanol or dehydrogenation of methoxide ion leads to the formation of formaldehyde.

However, the second hypothesis has been proposed by Sokolovskii and co-workers [193].

They suggested that methane activation proceeds heterolytically via the participation of acid-base centers giving place to a proton detachment and the formation of a metal–methyl compound, where the methyl results in being negatively charged:

−

The coordinately unsaturated metal and ion paired with a strong nucleophile (O^2-) ion may act as an active center. Further oxidation of these surface methyl anions would lead to methyl radicals, which then dimerize:

At the same time, the authors mentioned the possibility that some methyl radicals could escape to the basal plane and dimerize subsequently in ethane or, in presence of additional electron–hole pair excitations, to produce formaldehyde. In addition, the barrier to methyl radical diffusion over O^2- sites is approximately the same as the desorption energy (0.4 eV).

Unless CH2O is desorbed, additional electron–hole pair excitations should lead to further dehydrogenation and surface reduction with the formation of CO and CO2.

Moreover, the results of B. Irigoyen et al. [194] obtained from their theoretical and computational study of the CH4–MoO3 chemical interaction suggest that while the hydrogen abstraction requires less energy for each consecutive step the overall process remains endothermic. The different sequences and sites for hydrogen abstraction from methane, analyzed over different layers exposing molybdenum and oxygen atoms, allowed them to conclude that the heterolytic H-abstraction is an energetically more favorable process in comparison to the homolytic one. Their results indicate that despite an important energy barrier being necessary for the first C–H bond activation, the overall oxidation process is kinetically more favoured in the heterolytic mechanism.

Other studies suggest that the activation of methane occurs on super acid catalysts as well as on organometallic complexes at low temperatures via heterolytic cleavage of the C-H bond of CH4 [195-197]. In addition, the controlled activation of the C–H bond of methane and the formation of the C–C bond have been extremely important and common topics in transition

Other studies suggest that the activation of methane occurs on super acid catalysts as well as on organometallic complexes at low temperatures via heterolytic cleavage of the C-H bond of CH4 [195-197]. In addition, the controlled activation of the C–H bond of methane and the formation of the C–C bond have been extremely important and common topics in transition

In document Molibdén tartalmú katalizátorok felületkémiai tulajdonságainak vizsgálata  (Pldal 93-104)