Chemistry, spectroscopy and the role of supported vanadium oxides in heterogeneous catalysis

TLDR: In this paper, an overview of the current knowledge available about vanadium oxide-based catalysts is given, including the molecular structure of supported vanadium oxides under hydrated, dehydrated and reduced conditions, including parameters which influence the molecular structures formed at the surface of the support oxide.

About: This article is published in Catalysis Today.The article was published on 2003-02-28 and is currently open access. It has received 821 citations till now. The article focuses on the topics: Vanadium oxide & Vanadium.

Figures

Table 3 Spectroscopic techniques for studying supported vanadium oxides and the obtained information on coordination states, and dispersion[61–65]

Table 5 Surface vanadium oxide species, as detected by Raman spectroscopy, on different hydrated amorphous inorganic oxides[183]

Fig. 11. Three possible V–O bonds involved in the selective oxidation of methanol to formaldehyde: (a) monomeric VO4 species and (b) polymeric VO4 species characterized by V=O, V–O–V and V–O–support bonds.

Fig. 10. Reaction mechanism for the selective oxidation of methanol to formaldehyde over supported vanadium oxide catalysts.

Table 1 Industrial catalytic processes using vanadium oxides[6–13]

Fig. 1. Overview of the importance of vanadium in supported metal oxide catalysis. The numbers are based on an extensive open literature search in the period 1967–2000.

Frequently asked questions

Q1. What are the contributions mentioned in the paper "Chemistry, spectroscopy and the role of supported vanadium oxides in heterogeneous catalysis" ?

In this review, an overview is given of the current knowledge available about vanadium oxide-based catalysts. The review starts with the importance of vanadium in heterogeneous catalysis, a discussion of the molecular structure of vanadium in water and in the solid state and an overview of the spectroscopic techniques enabling to study the chemistry of supported vanadium oxides. 

Q2. What are the future works in "Chemistry, spectroscopy and the role of supported vanadium oxides in heterogeneous catalysis" ?

It is hoped that this goal can be achieved in the future by applying in situ spectroscopic techniques and appropriate spectroscopic reaction cells. Therefore, further research has to be directed towards the use of an intelligent combination of preferably in situ spectroscopic techniques delivering both molecular and electronic information about the supported vanadium oxides. Theoretical calculations of relevant cluster models of supported vanadium oxide catalysts can be in this respect very helpful. 

Q3. What is the IEP of the supported vanadium oxides?

Upon heating in air, the water molecules adsorbed on the support and around the supported vanadiumoxides are removed and the supported vanadium oxides are oxidized mainly to the +5 oxidation state. 

Q4. What is the way to determine the surface acidity of a catalyst?

The adsorption of basic molecules, such as ammonia and pyridine, is a useful method to determine the surface acidity of the catalyst as it enables to distinguish between Brönsted and Lewis acid sites. 

Q5. What is the role of vanadium oxides in catalysis?

Their synthesis and molecular design require a profound knowledge of solution chemistry, solid-state chemistry and inorganic chemistry, and their application as catalysts results from the specific interaction between the support oxide and the vanadium oxide. 

Q6. How can adsorption of NO be determined?

Oxidation states of supported vanadium oxides, namely V3+ and V4+, can be determined by adsorption of NO at low temperature (e.g. −180 ◦C). 

Q7. What is the role of vanadium in aerospace?

Vanadium is also an essential element in aerospace industry, as it guarantees low density, high strength and resistance to high operating temperatures and stress. 

Q8. What is the effect of the presence of additives on the chemistry of vanadium oxide?

The higher the H+ concentration near the surface, the more the equilibrium reactions in aqueous solutions are driven towards the formation of more polymerized vanadium oxide species. 

Q9. What is the importance of a spectroscopic tool in a heterogeneous?

Fundamental knowledge about the chemistry of supported vanadium oxides is of key importance in heterogeneous catalysis and spectroscopic tools are necessary to built up this knowledge. 

Q10. What is the role of vanadium in the destruction of zeolite Y?

It is clear that metavanadic acid can be considered as a catalyst that facilitates the removal of sodium ions from zeolite Y and this results in the formation of NaOH, which is quite potent for zeolite Y destruction. 

Q11. What is the role of supported vanadium oxides in catalysis?

Supported vanadium oxide catalysts are very complex inorganic materials that play an important role in heterogeneous catalysis in both the gas and the liquid phase. 

Q12. What is the main reason for the formation of a monolayer of vanadium oxides?

Heating of, e.g. a physical mixture of V2O5 and Al2O3, will lead to the spreading of vanadium oxides over the alumina support and the formationof different surface vanadium oxide configurations. 

Q13. How many species of vanadium oxides are known to exist in aqueous solutions?

V5+ hydrolyses in a very complex way and 12 different species—monomeric as well as polymeric species—are known to exist in aqueous solutions. 

Q14. What is the effect of additives on the IEP of the supports?

The effect of the presence of additives, such as P2O5 and K2O, can alter the IEP of the supports or form vanadium phosphate or potassium vanadate compounds, and consequently, change the vanadium oxide speciation [185]. 

Q15. What is the effect of the support oxide on the mono-oxo V=O bond?

The support oxide has some effect on the mono-oxo V=O bond length, which varies in the range 1.60–1.79 Å depending on the support oxide material. 

Q16. What is the effect of changing the specific support oxide composition on the TOF of the corresponding?

Since changing the specific support oxide composition dramatically influences the TOF of the corresponding supported vanadium oxide catalysts for the selective oxidation of methanol to formaldehyde (Table 7), it strongly suggests that the oxygen in the V–O–support bond is critical for this catalytic oxidation reaction. 

Q17. How can the authors deduce the V=O bond distance from the Raman spectra?

The V=O bond distance can also be deduced from the corresponding Raman stretching frequencies by applying empirical correlations based on the diatomic approximations as developed by Hardcastle and Wachs [86]. 

Q18. How does the Raman band of the vanadium oxide species change with increasing vanadium?

It can be seen that the Raman band at 930 cm−1, due to the presence of polymeric vanadium oxides, gradually increases with increasing vanadium oxide loading.