Supported metal oxides molecular structures

Vibrational spectroscopic studies of heterogeneously catalyzed reactions refer to experiments with low area metals in ultra high vacuum (UHV) as well as experiments with high area, supported metal oxides over wide ranges of pressure, temperature and composition [1]. There is clearly a need for this experimental diversity. UHV studies lead to a better understanding of the fundamental structure and chemistry of the surface-adsorbate system. Supported metals and metal oxides are utilized in a variety of reactions. Their study leads to a better understanding of the chemistry, kinetics and mechanisms in the reaction. Unfortunately, the most widely used technique for determining adsorbate molecular structure in UHV,... 

The present chapter will primarily focus on oxidation reactions over supported vanadia catalysts because of the widespread applications of these interesting catalytic materials.5 6,22 24 Although this article is limited to well-defined supported vanadia catalysts, the supported vanadia catalysts are model catalyst systems that are also representative of other supported metal oxide catalysts employed in oxidation reactions (e.g., Mo, Cr, Re, etc.).25 26 The key chemical probe reaction to be employed in this chapter will be methanol oxidation to formaldehyde, but other oxidation reactions will also be discussed (methane oxidation to formaldehyde, propane oxidation to propylene, butane oxidation to maleic anhydride, CO oxidation to C02, S02 oxidation to S03 and the selective catalytic reduction of NOx with NH3 to N2 and H20). This chapter will combine the molecular structural and reactivity information of well-defined supported vanadia catalysts in order to develop the molecular structure-reactivity relationships for these oxidation catalysts. The molecular structure-reactivity relationships represent the molecular ingredients required for the molecular engineering of supported metal oxide catalysts. 

Banares, M.A. and Wachs, I.E. (2002) Molecular structures of supported metal oxide catalysts under different environments. /. Raman Spectrosc., 33, 359. 

As more Raman spectra of supported metal oxide catalysts appeared in the literature, many contradictory models for the dispersed metal oxide structure were proposed. It was observed in 1983-1984 by Wang and Hall (1983), Chan et al. (1984), and Stencel et al. (1984) that supported Re207, M0O3, and WO3-V2O5 were in hydrated states during ambient Raman measurements. However, the molecular structures of the various hydrated dispersed metal oxide species on oxide supports were not fully understood at that time. 

The Raman investigation of niobium species in aqueous solutions of niobium oxalate (Jehng and Wachs, 1991) nicely showed the dependence of their constitution on pH and concentration. The PZC theory was successfully applied to predict the hydrated, molecular structures of multicomponent supported metal oxide species, such as iron-molybdenum, iron-vanadium, molybdenum-vanadium, tungsten-vanadium, and sodium-vanadium oxide species (Vuurman et al., 1991 Wachs et al., 1993). 

It is emphasized that the PZC theory for the prediction of the molecular structure of hydrated polyoxo anions holds true only under ambient conditions when the oxide surfaces are extensively hydrated. This condition is not satisfied when the supported metal oxide catalysts are heated... 

The molecular structure of supported metal oxides under selective catalytic reduction (SCR) conditions was reported to be the same as that under conditions leading to catalyst dehydration (Wachs et al., 1996). Raman... 

VO-CH3) and 665 cm"1 (V-O-CH3 vibrations). (B) The intensity of the Raman bands assigned to V-OCH3 methyl vibrations at 2930 and 2830 cm"1 increase with respect to those of the Si-OCH3 vibrations at 2960 and 2860 cm"1 with surface vanadium coverage. (Adapted from M.A. Banares, I.E. WachsJ. Raman Spectrosc. 33, 359 (2002) Molecular Structures of Supported Metal Oxide Catalysts Under Different Environments ). 

Oxidation Reactions over Supported Metal Oxide Catalysts Molecular/Electronic Structure-Activity/Selectivity Relationships... 

Supported Nb205 [54], Ta20s [55] and WO3 [26] catalysts typically possess almost no redox potential and exclusively behave as surface acid sites. Other than their acidic properties, these supported metal oxides possess similar molecular and electronic structural characteristics as the redox surface sites discussed above. 

Supported metal oxide catalysts are a new class of catalytic materials that are excellent oxidation catalysts when redox surface sites are present. They are ideal catalysts for investigating catalytic molecular/electronic structure-activity selectivity relationships for oxidation reactions because (i) the number of catalytic active sites can be systematically controlled, which allows the determination of the number of participating catalytic active sites in the reaction, (ii) the TOP values for oxidation studies can be quantitatively determined since the number of exposed catalytic active sites can be easily determined, (iii) the oxide support can be varied to examine the effect of different types of ligand on the reaction kinetics, (iii) the molecular and electronic structures of the surface MOj, species can be spectroscopically determined under all environmental conditions for structure-activity determination and (iv) the redox surface sites can be combined with surface acid sites to examine the effect of surface Bronsted or Lewis acid sites. Such fundamental structure-activity information can provide insights and also guide the molecular engineering of advanced hydrocarbon oxidation metal oxide catalysts such as supported metal oxides, polyoxo metallates, metal oxide supported zeolites and molecular sieves, bulk mixed metal oxides and metal oxide supported clays. 

Strength (FLS) empirical approach are discussed in Section 3 as methods for determining the molecular structures of metal-oxide species from their Raman spectra. The state-of-the-art in Raman instrumentation as well as new instrumental developments are discussed in Section 4. Sampling techniques typically employed in Raman spectroscopy experiments, ambient as well as in situ, are reviewed in Section S. The application of Raman spectroscopy to problems in heterogeneous catalysis (bulk mixed-oxide catalysts, supported metal-oxide catalysts, zeolites, and chemisorption studies) is discussed in depth in Section 6 by selecting a few recent examples from the literature. The future potential of Raman spectroscopy in heterogeneous catalysis is discussed in the fmal section. 

Under dehydrated conditions, the adsorbed moisture is removed and the in situ Raman spectra of the surface metal oxides differ markedly showing that the structures of the dehydrated species are very different from those of their hydrated counterparts (see references above in Section 6.2.2). These changes are not surprising since the influence of the net zero surface charge of the oxide support can only be exerted in an aqueous medium. For the dehydrated surface metal oxides, however, essentially the same molecular structures are seen on all the oxide supports for each supported metal oxide. ... 

Titanium oxide monolayer on y-AljOj is a potential support for noble metals [1-4]. Many studies have shown that two-dimensional transition metal oxide overlayers are formed when one metal oxide (Vj05, Nb205, MoOj, etc.) is deposited on an oxide support (AljOj, TiO, etc.) [5-7]. The influence of the molecular structures of surface metal oxide species on the catalytic properties of supported metal oxide catalyst has been examined [8-9]. It has been demonstrated that the formation and location of the surface metal oxide species are controlled by the surface hydroxyl chemistry. Moreover, thin-layer oxide catalysts have been synthesized on alumina by impregnation technique with alkoxide precursor [10]. It has been found for titanium oxide, by using Raman spectroscopy, that a monolayer structure is formed for titanium contents below 17% and that polymeric titanium oxide surface species only posses Ti-O-Ti bonds and not Ti=0 bonds. Titanium is typically ionic in its oxy-compounds, and while it can exist in lower oxidation states, the ionic form TF is generally observed in octahedral coordination [11-12]. However, there is no information available about the Ti coordination and structure of this oxide in a supported monolayer. In this work we have studied the structural evolution of the titanium oxy-hydroxide overlayer obtained from alkoxide precursor, during calcination. 

In summary, the Raman studies have provided a deeper understanding of the molecular structure and reactivity properties of bulk metal oxide catalysts during selective oxidation reactions. However, the fundamental insights have primarily been limited to the bulk properties of the bulk metal oxide catalysts. In order to obtain surface information about metal oxide catalysts with Raman spectroscopy (essentially a bulk characterization technique), it is necessary to look at chemisorbed species on the surface of bulk metal oxides (see Sec. VIII) or highly dispersed metal oxide systems such as supported metal oxide catalysts. 

In summary, in a very short period of time, Raman spectroscopy has significantly advanced the catalysis science of supported metal oxide catalysts, well-defined model systems, as well as industrial catalysts and allowed for the establishment of molecular structure-reactivity/selectivity relationships. This fundamental information allows for the molecular engineering of supported metal oxide catalysts from fundamental principles [132]. 

Metal oxide catalytic materials currently find wide application in the petroleum, chemical, and environmental industries, and their uses have significantly expanded since the mid-20th century (especially in environmental applications) [1,2], Bulk mixed metal oxides are extensively employed by the chemical industries as selective oxidation catalysts in the synthesis of chemical intermediates. Supported metal oxides are also used as selective oxidation catalysts by the chemical industry, as environmental catalysts, to selectively transform undesirable pollutants to nonnox-ious forms, and as components of catalysts employed by the petroleum industry. Zeolite and molecular sieve catalytic materials are employed as solid acid catalysts in the petroleum industry and as aqueous selective oxidation catalysts in the chemical industry, respectively. Zeolites and molecular sieves are also employed as sorbents for separation of gases and to trap toxic impurities that may be present in water supplies. Significant molecular spectroscopic advances in recent years have finally allowed the nature of the active surface sites present in these different metal oxide catalytic materials to be determined in different environments. This chapter examines our current state of knowledge of the molecular structures of the active surface metal oxide species present in metal oxide catalysts and the influence of different environments upon the structures of these catalytic active sites. 

It is important to know the molecular structures of the active sites present in supported metal oxide catalysts in order to fully understand their fundamental characteristics. Supported metal oxide catalysts consist of an active metal oxide... 

The molecular structures of the hydrated surface metal oxides on oxide supports have been determined in recent years with various spectroscopic characterization methods (Raman [34,37,40 3], IR [43], UV-Vis [44,45], solid stateNMR [32,33], and EXAFS/XANES [46-51]). These studies found that the surface metal oxide species possess the same molecular strucmres that are present in aqueous solution at the same net pH values. The effects of vanadia surface coverage and the different oxide supports on the hydrated surface vanadia molecular structures are shown in Table 1.2. As the value of the pH at F ZC of the oxide support decreases, the hydrated surface vanadia species become more polymerized and clustered. Similarly, as the surface vanadia coverage increases, which decreases the net pH at PZC, the hydrated surface vanadia species also become more polymerized and clustered. Consequently, only the value of the net pH at PZC of a given hydrated supported metal oxide system is needed to predict the hydrated molecular structure(s) of the surface metal oxide species. 

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