**2.2 Cr+6 oxides**

Bulk chromates hold CrO4 coordination in isolated mono-chromate (CrO4), dichromate (Cr2O7) that termed as dimer, tri-chromate (Cr3O10) which is designated as trimer, and tetr-achromate (Cr4O13) which is named as tetramer with infinite chain CrO3 (polychromate or metachromate) structures [22]. In contrast to the respective bulk vanadates, bulk non-CrO4 comprising structures are unidentified as an example CrO5 and CrO6 (see **Figure 2**). The crystalline structure of CrO3 is assembled up of countless chains via connecting CrO4 entities comprised of two short bonds (0.160 nm) and two extended bonds (0.175 nm). These entities are lonely apprehended with each other via van der Waal interactions. Infrequent short MP of CrO3 is 197 °C reveals weak van der Waal forces between poly-chromate chains. Bulk CrO3 attaining faint thermal stability is also reflected in its superficial lessening and the decomposition to respective bulk Cr2O3, which contains only Cr with +3 oxidation state as cations. The Cr with an oxidation state of +6 is generally unchanging through the existence of non-reducible cations that include As, K, P, Rb, and Na. Chromium oxy-halides that correspond to gas-phase are also recognized and vibration of mono-oxo F4Cr = O are detected around 1028 cm−1, while the vibrations associated with di-oxo F2Cr(=O)2 are identified around 1006 cm−1 for *v*s as well as 1016 cm−1 for *v*as. Additionally, vibrations of di-oxo Cl2Cr(=O)2 are noticed around 984 cm−1 for *v*s as well as 994 cm−1 for *v*as. Lastly, vibrations of tri-oxo CsBrCr(=O)3 around 908 for *v*s, 933, 947, and 955 cm−1 for *v*as [23]. These vibrational frequency swings as a function of the M = O bonds are pointedly away from the expected value that was imagined for dissimilar halide ligands as the gas-phase vanadyl oxy-halide complexes swing downward to 23 cm−1 by reflecting the shift from F-Cl ligands and downward to 10 cm−1 by considering the shift from Cl-Br ligands. Thus, aggregation of the amount of chromyl bonds swings leads to the corresponding vibrations to inferior wavenumbers and gradually upturns the

**7**

**2.3 Re+7 oxides**

**Figure 2.**

**2.4 Mo+6 oxides**

*Rational Design and Advance Applications of Transition Metal Oxides*

of CrO4 units with different extents of polymerization [24, 25].

*Structures of (a) dehydrated isolated and (b) polymeric surface monoxo CrO4 species [15].*

sum of vibrational bands. In summary, inorganic chemistry of Cr with respect to structural analysis that owns an oxidation state of +6 chromates essentially consists

Spectroscopic measurements of the dehydrated supported chromates with EXAFS or XANES, UV–vis, and chemiluminescence, exposed that dehydrated surface chromates hold CrO4 coordination and are stabilized as Cr(+6) at prominent temperatures through oxide supports under monolayer surface exposure. Above the monolayer surface coverage, the excess chromium oxide that resides on the surface chromium monolayer becomes reduced at elevated temperatures in the oxidizing environments and forms Cr(+3) Cr2O3 crystallites. Thus, the surface species of Cr with oxidation of +6 are lonely steady around elevated temperatures by coordination to the oxide substrates. For non-SiO2 supports, the Raman measurements and the IR fingerprints reveal two resilient bands around 1005–1010 cm−1, as well as 1020–1030 cm − 1 and the corresponding overtone, ranges for these two bands in vibrational regions of 1986–1995 plus 2010–2015 cm−1. The vibrational alteration is reliable with di-oxo functionality however; it lies faintly on the higher side [26, 27].

The bulk rhenium regarding its inorganic chemistry that possesses +7 oxidation states is slightly sparse. Numerous ortho-rhenate compounds covering isolated units of ReO4 which are somewhat common: KReO4, NaReO4, and NH4ReO4. Bulk Re2O7 holds a layered structure comprising of interchanging groups of ReO4 and ReO6, along with subunits of rings that are constituted two groups of both ReO4 and ReO6. The weak bonding among rhenium oxide groups in the layered structure of Re2O7 consequences in the effective vaporization of Re2O7 dimers that hold two groups of ReO4 bridged through one O atom for example gaseous O3Re–O–ReO3. The supreme possible surface ReOx attention on oxide supports is permanently reduced than monolayer attention due to the surface ReOx species association to produce volatile dimers (Re2O7) at extreme surface coverage. Moreover, crystalline Re2O7 is not ever perceived as this MO is not stable to higher calcination values along with the introduction to ambient moisture. Therefore, mono-layer ReOx with its surface coverage is not ever gotten because crystalline Re2O7 and volatilization does certainly not exist. Hence, supported ReOx catalysts are exceptional between the sustained MO catalysts. In this materialization, only surface ReOx attention below single-layer can

be accomplished deprived of the occurrence of crystallites [15, 28, 29].

Bulk polymolybdate chains typically comprise MoO6 coordinated units that are different from the chains of polyvanadate as well as polychromate and these chains are respectively possessed with VO4 and CrO4 groups. This reveals the liking of molybdates for greater coordination groups in comparison with vanadates and chromates units in the respective polymeric structures. Yet, certain exemptions occur to this tendency in the structural chemistry of bulk molybdate. Short coordinated molybdates exist in the dimer of MoO4 which is MgMo2O7 and in the chain of

*DOI: http://dx.doi.org/10.5772/intechopen.96568*

**Figure 1.** *Structures of (a) dehydrated isolated and (b) polymeric surface monoxo VO4 species [15].* *Rational Design and Advance Applications of Transition Metal Oxides DOI: http://dx.doi.org/10.5772/intechopen.96568*

#### **Figure 2.**

*Transition Metal Compounds - Synthesis, Properties, and Application*

The oxyhalide vibrations of F2V O2

(see **Figure 1**).

**2.2 Cr+6 oxides**

cations (e.g., Na3VO4, and Na4V2O7,). Bulk vanadate's (VO6) exhibits quite collective structures, which are normally, bring into extended structures of vanadia. As an example, decavandate cluster present in Na6V10O28 contains five discrete distorted sites of VO6 [20]. The extremely distorted VO6 structures typically retain one V=O (terminal bond) of the type mono-oxo) having bond lengths ranges from 0.158 to 0.162 nm. In certain greatly distorted oxides of VO6, the sixth oxygen is positioned far away from vanadium atom in such a way that these compounds are efficiently reflected to hold VO5 coordination. Numerous gas-phase X3V=O mono-oxo halide classes are also recognized and vanadyl vibrations ranges in the order of 1025–1058 cm−1 that owns growing electronegativity of respective halides species which follows the sequence Br *<* Cl *<* F [21].

<sup>−</sup> and Cl2V O2

Bulk chromates hold CrO4 coordination in isolated mono-chromate (CrO4), dichromate (Cr2O7) that termed as dimer, tri-chromate (Cr3O10) which is designated as trimer, and tetr-achromate (Cr4O13) which is named as tetramer with infinite chain CrO3 (polychromate or metachromate) structures [22]. In contrast to the respective bulk vanadates, bulk non-CrO4 comprising structures are unidentified as an example CrO5 and CrO6 (see **Figure 2**). The crystalline structure of CrO3 is assembled up of countless chains via connecting CrO4 entities comprised of two short bonds (0.160 nm) and two extended bonds (0.175 nm). These entities are lonely apprehended with each other via van der Waal interactions. Infrequent short MP of CrO3 is 197 °C reveals weak van der Waal forces between poly-chromate chains. Bulk CrO3 attaining faint thermal stability is also reflected in its superficial lessening and the decomposition to respective bulk Cr2O3, which contains only Cr with +3 oxidation state as cations. The Cr with an oxidation state of +6 is generally unchanging through the existence of non-reducible cations that include As, K, P, Rb, and Na. Chromium oxy-halides that correspond to gas-phase are also recognized and vibration of mono-oxo F4Cr = O are detected around 1028 cm−1, while the vibrations associated with di-oxo F2Cr(=O)2 are identified around 1006 cm−1 for *v*s as well as 1016 cm−1 for *v*as. Additionally, vibrations of di-oxo Cl2Cr(=O)2 are noticed around 984 cm−1 for *v*s as well as 994 cm−1 for *v*as. Lastly, vibrations of tri-oxo CsBrCr(=O)3 around 908 for *v*s, 933, 947, and 955 cm−1 for *v*as [23]. These vibrational frequency swings as a function of the M = O bonds are pointedly away from the expected value that was imagined for dissimilar halide ligands as the gas-phase vanadyl oxy-halide complexes swing downward to 23 cm−1 by reflecting the shift from F-Cl ligands and downward to 10 cm−1 by considering the shift from Cl-Br ligands. Thus, aggregation of the amount of chromyl bonds swings leads to the corresponding vibrations to inferior wavenumbers and gradually upturns the

*Structures of (a) dehydrated isolated and (b) polymeric surface monoxo VO4 species [15].*

detected at two reading firstly, at 970/962 and secondly at 970/959 cm−1. As a conclusion, bulk vanadium that owns +5 oxidation state and holds rich inorganic chemistry is assembled up from the coordinated structures of VO4, VO5, and VO6

<sup>−</sup> that belongs to di-oxo are

**6**

**Figure 1.**

*Structures of (a) dehydrated isolated and (b) polymeric surface monoxo CrO4 species [15].*

sum of vibrational bands. In summary, inorganic chemistry of Cr with respect to structural analysis that owns an oxidation state of +6 chromates essentially consists of CrO4 units with different extents of polymerization [24, 25].

Spectroscopic measurements of the dehydrated supported chromates with EXAFS or XANES, UV–vis, and chemiluminescence, exposed that dehydrated surface chromates hold CrO4 coordination and are stabilized as Cr(+6) at prominent temperatures through oxide supports under monolayer surface exposure. Above the monolayer surface coverage, the excess chromium oxide that resides on the surface chromium monolayer becomes reduced at elevated temperatures in the oxidizing environments and forms Cr(+3) Cr2O3 crystallites. Thus, the surface species of Cr with oxidation of +6 are lonely steady around elevated temperatures by coordination to the oxide substrates. For non-SiO2 supports, the Raman measurements and the IR fingerprints reveal two resilient bands around 1005–1010 cm−1, as well as 1020–1030 cm − 1 and the corresponding overtone, ranges for these two bands in vibrational regions of 1986–1995 plus 2010–2015 cm−1. The vibrational alteration is reliable with di-oxo functionality however; it lies faintly on the higher side [26, 27].
