Bismuth molybdates surface

Much work has been invested to reveal the mechanism by which propylene is catalytically oxidized to acrolein over the heterogeneous catalyst surface. Isotope labeling experiments by Sachtler and DeBoer revealed the presence of an allylic intermediate in the oxidation of propylene to acrolein over bismuth molybdate. In these experiments, propylene was tagged once at Ci, another time at C2 and the third time at C3. 

In the case of selective oxidation catalysis, the use of spectroscopy has provided critical Information about surface and solid state mechanisms. As Is well known( ), some of the most effective catalysts for selective oxidation of olefins are those based on bismuth molybdates. The Industrial significance of these catalysts stems from their unique ability to oxidize propylene and ammonia to acrylonitrile at high selectivity. Several key features of the surface mechanism of this catalytic process have recently been descrlbed(3-A). However, an understanding of the solid state transformations which occur on the catalyst surface or within the catalyst bulk under reaction conditions can only be deduced Indirectly by traditional probe molecule approaches. Direct Insights Into catalyst dynamics require the use of techniques which can probe the solid directly, preferably under reaction conditions. We have, therefore, examined several catalytlcally Important surface and solid state processes of bismuth molybdate based catalysts using multiple spectroscopic techniques Including Raman and Infrared spectroscopies, x-ray and neutron diffraction, and photoelectron spectroscopy. 

Selective oxidation and ammoxldatlon of propylene over bismuth molybdate catalysts occur by a redox mechanism whereby lattice oxygen (or Isoelectronlc NH) Is Inserted Into an allyllc Intermediate, formed via or-H abstraction from the olefin. The resulting anion vacancies are eventually filled by lattice oxygen which originates from gaseous oxygen dlssoclatlvely chemisorbed at surface sites which are spatially and structurally distinct from the sites of olefin oxidation. Mechanistic details about the... 

Crystal structures of the bismuth molybdate and of the mixed iron and cobalt solid solution molybdate samples were controlled by X-ray diffraction (10). The chemical compositions of the samples were determined by atomic absorption and their surface areas measured by nitrogen adsorption using the BET method. 

XPS has been used to characterize the three mixtures containing respectively 7,25, and 50 weight % of Bi2Mo30i2 (Table II samples J,K and L). These samples have been characterized before and after catalytic reaction (table III). Bi, Mo, Fe, Co and O have been analyzed. The Mo/0 ratio remains equal to 0.25 for all the samples, before and after catalysis which confirms that no new phase was formed since the molybdates suspected to have formed, have a much lower Mo/0 ratio (0.17 for Bi2Mo06 and Bi3FeMo20i2). Concerning the Bi/(Fe+Co) ratio, it can first be observed that before catalysis this ratio was always lower than that calculated from chemical analysis. This can be explained by the difference between the particles size of the bismuth molybdate and the iron and cobalt molybdates which is in a ratio of more than 30 as calculated from differences in surface area values, 0.3 and 9 to 22 m. g Secondly the Bi/(Fe+Co) ratio increased systematically after catalysis which could be explained by the decrease in size of the bismuth molybdate particles or by the covering of the iron and cobalt molybdate particles by the bismuth molybdate or by both effects. 

These results suggest that the (101) superstructure observed on the (001) -phase at the catalyst s operating temperature is closely related to Bi2M02O9. A quantification of the microanalysis of the jS-preparation shows a Bi-deficiency. Similar results are observed in the reaction of the a-phase in propylene. In a C3 H6-O2 mixture under working conditions both phases show the presence of this superstructure similar to the jS-structure. The ETEM results are consistent with XPS and Raman data which show that the surface structure of the active bismuth molybdate is close to the jS-phase and that the jS-phase is more active (Matsurra et al 1980, Burrington et al 1983). In these studies dramatic increases in the activity... 

Investigations into the scheelite-type catalyst gave much valuable information on the reaction mechanisms of the allylic oxidations of olefin and catalyst design. However, in spite of their high specific activity and selectivity, catalyst systems with scheelite structure have disappeared from the commercial plants for the oxidation and ammoxidation of propylene. This may be attributable to their moderate catalytic activity owing to lower specific surface area compared to the multicomponent bismuth molybdate catalyst having multiphase structure. 

The specific activity of pure bismuth molybdate, a-phase Bi2(Mo04)3 or y-phase Bi2Mo06, is fairly high. However, owing to its low surface area, the activity per unit weight of the catalyst is not so prominent. 

The addition of divalent metal cation, M(II) with ionic radius smaller than 0.8 A (Ni2+, Co2+, Fe2+, Mg2+, Mn2+, etc.), to the pure bismuth molybdate increases the specific surface area of the catalyst system, but the specific activity of the tricomponent system, Mo-Bi-M(II)-0 never exceeds that of pure bismuth molybdate. 

In conclusion, to make an excellent catalyst system, it is important to activate bismuth molybdate by both the divalent and trivalent metal cations with ionic radii smaller than 0.8 A at the same time. A part of the increasing activity of the Mo-Bi-M(II)-M(III)-0 system compared to the pure bismuth molybdate comes from the increase in surface area and the remains arise from the increase in specific activity. Semiquantitative evaluations of tri- and tetracomponent bismuth molybdates are listed in Table V in comparison with simple bismuth molybdate catalyst. 

Surface analyses were investigated mainly by using XPS (Fig. 7). It was clearly indicated that many composite oxides found by XRD are located un-homogeneously in the catalyst particle. Molybdenum and bismuth are undoubtedly concentrated in the surface layer of the catalyst particle and divalent and trivalent metal cations are found in the bulk of the catalyst. As a result, it is clear that bismuth molybdates, especially its a-phase, is located on the surface of each particle, and metal molybdates of divalent and trivalent cations are situated in the bulk of the catalyst. 

Strictly speaking, it is difficult to conclude which model is most reasonable. However, summing up the results obtained by the surface analyses, it is sure at least that bismuth molybdates are concentrated on the surface of the catalyst particle. Our investigations for Mo-Bi-Co2+-Fe3+-0 also support the conclusion mentioned above, and the core-shell structure proposed by Wolfs et al. may be essentially reasonable. However, since small amounts of divalent and trivalent metal cations are observed in the surface layers, the shell structure may be incompletely constructed. The epitaxial effect has been assumed on the condensation of bismuth molybdates on the divalent and trivalent metal molybdates on the basis of the fact that the y-phase of bismuth molybdate is mainly formed on NiMo04 but the a-phase is predominant on other divalent and trivalent metal molybdates (46). The... 

Fig. 13. The catalytic activity forming acrolein per unit surface area of the supported bismuth molybdate catalysts (52). (Q) BbMojCWCoMoO, ( ) Bi2Mo3On/Co]i izFei.ijMoO -...

The same synergy effect between bismuth molybdates and mixed iron and cobalt molybdates on the mechanical mixture of both particles was reported by Millet et al. (98). However, it was also found that the surface of mixed iron and cobalt molybdate particle was changed during catalysis and a thin layer of bismuth molybdate was formed on the surface of mixed iron and cobalt molybdates after the reaction. It is doubtful that pure mechanical mixture shows the synergy effect for propylene oxidation, and it seems likely that propylene was mainly oxidized on the thin layer of bismuth molybdates formed on the mixed iron and cobalt molybdate in the experiment reported by Millet et al. (98). 

Molybdenum and bismuth are indispensable elements, forming the a-phase of bismuth molybdate, which is located mainly on the surface of the catalyst particle and constitutes the reaction site of propylene. 

The lower activation energy of the multicomponent bismuth molybdate catalyst suggests that some structural or electronic modification of the active component, Bi2(Mo04)3, is given by M(II) and M(III) molybdates, which also contribute to the enlargement of the surface area of bismuth molybdate. 

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