Surface Segregation in Iron Molybdate Catalysts

House, Matthew Peter; Shannon, M. D.; Bowker, Michael

doi:10.1007/s10562-008-9467-8

Cited by 33 publications

(41 citation statements)

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“…At very high conversion and temperature CO 2 is seen, and can become dominant, but is likely to be because of secondary CO oxidation. This behaviour is similar to what is observed for 1.5: 1 Mo:Fe catalysts, and for MoO 3 itself, which are highly selective to formaldehyde over a wide temperature range, 4,8,13 though the conversion for MoO 3 itself is much lower than for iron molybdate and only reaches 100% at much higher temperatures. 4,13 Data for iron oxide are in contrast to those in Fig.…”

Section: Site Distribution a Specific Examplesupporting

confidence: 84%

“…7 is a function of bulk loading and we do not at this point know if the latter is the same as the surface concentration of Mo. Indeed, we 4,8,10 and others 5 -7,11,12 have shown that there is a strong tendency of Mo to segregate to the surface of these materials, that is, the surface concentration is higher than the bulk concentration. This may especially be important for the very low mole fraction of Mo in Fig.…”

Section: Site Distribution a Specific Examplementioning

confidence: 84%

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Selectivity determinants for dual function catalysts: applied to methanol selective oxidation on iron molybdate

Bowker

House

Alshehri

et al. 2015

Catalysis, Structure & Reactivity

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Evolution of the IRAS spectrum with temperature after adsorbing methanol at room temperature. The bands at 2930 and 2820 cm 21 are due to the methoxy species C -H stretches, while that at 2870 is due to the formate.Here, we report a simple, quantitative model to describe the behaviour of bi-cationic oxide catalysts, in terms of selectivity variation as a function of increased loading of one cation into a sample of the other. We consider its application to a particular catalytic system, namely the selective oxidation of methanol, which proceeds with three main C1 products, namely CO 2 , CO, and H 2 CO. The product selectivity varies in this order as Mo is added in increasing amounts to an iron oxide catalyst, and the product selectivity is determined by the distribution of dual sites and single sites of each species.

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Section: Site Distribution a Specific Examplesupporting

confidence: 84%

Section: Site Distribution a Specific Examplementioning

confidence: 84%

Selectivity determinants for dual function catalysts: applied to methanol selective oxidation on iron molybdate

Bowker

House

Alshehri

et al. 2015

Catalysis, Structure & Reactivity

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“…[14] More [21] According to our results, this zone supports an outermost layer containing almost exclusively Mo oxide species. Conventional XPS data (Table 1) are in disagreement with these findings for NiMoO 4 and AlVO 4 while a Mo surface enrichment found for Fe 2 (MoO 4 ) 3 seems to track the thicker Mo enrichment zone found by House et al [21] Some examples that prevent the tempting generalization that the surfaces of all stoichiometric bulk molybdates or vanadates are more or less completely covered by a Mo or V oxide overlayer are given in Figure 4 and Figure S4 in the Supporting Information. Neither with CoMoO 4 nor with MnMoO 4 is there any significant change of the signal shapes …”

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confidence: 70%

Anomalous Surface Compositions of Stoichiometric Mixed Oxide Compounds

et al. 2010

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Surface-oxide films are present in many types of oxidecontaining materials, such as grain boundaries in ceramics, [1] interfaces in ceramic-ceramic [2] and metal-oxide systems, [3] and affect their materials and transport properties. In heterogeneous catalysis, the properties of the outermost surface layer are of prime importance because they control the catalytic performance. Although bulk mixed-metal oxide catalysts are widely used in industrial selective oxidation processes, [4,5] not much is known about their outermost surface composition. Models based on surfaces derived from a truncation of the bulk structure have dominated discussion on catalytic reaction mechanisms and active sites (reviewed, for example, in Ref. [6]). This view has been questioned by several recent studies reporting the surface enrichment and depletion phenomena in solid-oxide solutions (e.g., Co x Ni 1Àx O [7] ), the identification of TiO 2 -rich overlayers on reconstructed SrTiO 3 (001) model surfaces, [8] and evidence for the formation of amorphous oxide overlayers in which there is surface enrichment of one of the components under selective oxidation reaction conditions. [9,10] However, the development of realistic concepts on reactant activation, surface reaction mechanisms, and the design of advanced catalytic materials are still hampered by the lack of detailed knowledge of the surface composition and structure of bulk mixed-metal oxides.For such studies, X-ray photoelectron spectroscopy (XPS) with laboratory sources is of limited value because its average sampling depth of 1-3 nm results in a signal where the outermost surface layer only contributes on the order of 30 %. Synchrotron radiation allows for increasing the surface sensitivity of XPS by decreasing excitation and, hence, photoelectron kinetic energies. Exclusive information on the outermost surface layer, however, is only given by low-energy ion scattering (LEIS) because ions penetrating below the surface become largely neutralized.[11]The surfaces of stoichiometric bulk mixed-metal molybdates and vanadates have also been characterized through their interactions with probe molecules, for example, CH 3 OH, [12][13][14][15] which allows CH 3 O* and intact CH 3 OH* intermediates on different surface cations to be discriminated by IR spectroscopy. For such materials, combined methanol chemisorption and oxidation kinetic studies suggested a strong surface enrichment of MoO x or VO x . [12,14,15] In methanol oxidation studies, similar catalytic turnover frequencies were found over bulk mixed-metal oxides and related supported metal oxides (e.g., Fe 2 (MoO 4 ) 3 and MoO 3 /Fe 2 O 3 ), which supports the idea of surface MoO x enrichment of the bulk phases. [16][17][18][19] These observations, however, are qualitative as exposed metal oxide ions of low catalytic activity would not be detected by the test reaction. Thus, we have undertaken a study of the outermost surface compositions of such compounds by LEIS and excitation-energy resolved XPS (ERXPS). The LEIS was applied in sp...

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“…Here it can be seen that MoO 3 itself behaves in a very similar manner to the catalyst, whereas haematite is very different-it is a combustor of the adsorbed methanol yielding CO 2 and H 2 into the gas phase. More definitively acSTEM (aberration-corrected scanning transmission electron microscopy) can be used with a very small electron probe of *1 nm in the EELS mode to determine the composition of nanoparticles of the iron molybdate catalyst [17]. The results are shown in Fig.…”

Section: Rule 3 Preferential Cation Segregation Occurs and Is Crucialmentioning

confidence: 99%

“…Over the last few years we have reported on the nature of the kinetics of this reaction, including addressing the active site and the relationship between structure and reactivity [12][13][14][15][16][17][18][19][20]. In what follows we bring this work together in the form of some 'rules' for the selective of oxidation of methanol on iron molybdate, but it is likely that at least some of the rules apply to other catalytic materials and selective oxidation processes.…”

Section: Introductionmentioning

confidence: 99%

Rules for Selective Oxidation Exemplified by Methanol Selective Oxidation on Iron Molybdate Catalysts

Bowker

2015

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A number of simple rules are proposed which dictate for high selectivity to formaldehyde during methanol oxidation on iron molybdate catalysts. The reaction is of the Mars-van Krevelen type, that is, it is surface lattice oxygen that is the active species. However, we also show that the material is quite a flexible one, in that the bulk oxygen anions become mobile enough at moderate temperatures to exchange rapidly with the surface vacancies that are created during the reaction. Further, and essential to high selectivity, is the fact that Mo is preferentially segregated to the surface layer of the catalyst, with no Fe present there. Finally, it can also be shown that the important oxidation state for the cations at the surface is the highest one (6 ? for Mo), with lower oxidation states being detrimental to selectivity. It is likely that these basic rules are also important for a range of other catalytic processes/catalysts.

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Surface Segregation in Iron Molybdate Catalysts

Cited by 33 publications

References 17 publications

Selectivity determinants for dual function catalysts: applied to methanol selective oxidation on iron molybdate

Selectivity determinants for dual function catalysts: applied to methanol selective oxidation on iron molybdate

Anomalous Surface Compositions of Stoichiometric Mixed Oxide Compounds

Rules for Selective Oxidation Exemplified by Methanol Selective Oxidation on Iron Molybdate Catalysts

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