Substitution of W⁵⁺in monophosphate tungsten bronzes by combinationsMⁿ⁺/W⁶⁺

Abstract

A series of hitherto unknown mixed-metal phosphates of the monophosphate tungsten bronze structure family [MPTB; (WO3)2m(PO2)4] have been obtained by solution combustion synthesis followed by annealing (ϑ = 850°C) at appropriate oxygen pressures. These new phosphates show substitution of W5+ by either M3+1/3W6+2/3 (M: V, Cr, Fe, Mo) or Ti4+1/2W6+1/2. Members of the MPTB structural series with m = 2 [e.g. CrIII4/3WVI8/3O12(PO2)4; TiIV6/3WVI6/3O12(PO2)4] and m = 4 [e.g. Cr4/3W20/3O24(PO2)4] have been obtained. In the course of our investigation the crystal structure of WOPO4 (MPTB with m = 2: W4O12(PO2)4) has been re-determined from X-ray single-crystal data, showing monoclinic instead of the orthorhombic symmetry reported in literature (P21/m, Z = 1, 80 parameters, 1832 independent reflections R1 = 0.027, wR2 = 0.063). The crystal structures of MoIII4/3WVI8/3O12(PO2)4 and CrIII4/3WVI8/3O12(PO2)4 (MPTBs with m = 2) were also refined from single-crystal data {(Mo/W (Cr/W): P21/m, Z = 1, 80(86) parameters, 1782(1769) independent reflections, R1 = 0.035(0.059), wR2 = 0.081(0.146)}. These refinements indicate statistical distribution of MIII and WVI over the metal sites. By selected area electron diffraction the unit cell dimensions of CrIII4/3WVI8/3O12(PO2)4 and CrIII4/3WVI20/3O24(PO2)4 derived from XRPD and SXRD are confirmed. HRTEM images of Cr4/3W20/3O24(PO2)4 are in agreement with its assumed close structural relation to W8O24(PO2)4 and show an highly ordered atomic arrangement.

Highly robust, novel aluminum counter cation-based monophosphate tungsten bronze electro-catalysts for oxygen evolution in acidic solution

“…The band gaps of the Al-based MPTB are indirect in nature and the values of E g decrease by changing the calcination atmosphere from N 2 to O 2 due to the formation of the 2D-layered structure of the orthorhombic phase of AlO7. The catalytic performances of all MPTBs towards electrochemical OER varied widely as established by comparing η onset , Tafel slope and TOF and were revealed to be associated with the ratios of the W 5+ : W 6+ states in the MPTBs, which entirely depend on the counter-cations 48 and calcination 49 conditions. The robustness of AlO7 in catalysis is rationally achieved due to the unusual stability of W 5+ in MPTB created by the incorporation of Al 3+ , as explained earlier.…”

Mixed‐Metal Monophosphate Tungsten Bronzes Containing Rhodium and Iridium

“…For the MPTB (WO 3 ) 4 (PO 2 ) 4 (m = 2) mixed-metal substitution variants with M: V III, IV , Cr III , Fe III , Mo III, IV and Ti IV have been reported. [10] In addition to the described MPTBs at m = 2, for M: Sc, V, Cr, Fe, Mo, Ru, Rh, In and Ir the ortho-pyrophosphates M III-(WO 2 ) 2 (P 2 O 7 )(PO 4 ) of identical sum formula (with respect to the MPTBs) but different structure were obtained. [10] In this paper we report on synthesis, thermal behavior and characterization of several mixed-metal (Rh,W)-and (Ir,W)-MPTBs.…”

“…[10] In addition to the described MPTBs at m = 2, for M: Sc, V, Cr, Fe, Mo, Ru, Rh, In and Ir the ortho-pyrophosphates M III-(WO 2 ) 2 (P 2 O 7 )(PO 4 ) of identical sum formula (with respect to the MPTBs) but different structure were obtained. [10] In this paper we report on synthesis, thermal behavior and characterization of several mixed-metal (Rh,W)-and (Ir,W)-MPTBs. For characterization results from XRPD, SXRD, electron microscopy, UV/vis-spectroscopy, and magnetic measurements are presented.…”

Synthesis and Characterization of Niobium and Tungsten Substituted VOPO4

“…On the other hand, substitution of "Nb" by "W" in α-NbOPO4, S.G. P4/n (Longo et al 1996) lead to (Nb V 0.787W V 0.213)OPO4, S.G. P4/nmm (Leclaire et al 1997) which is very much related to the host structure. It is worth to be mentioned that the high temperature polymorph β-NbOPO4 (Leclaire et al 1986) is isotypic to WOPO4 (Wang et al 1989, andRoy et al 2016). However, information on solid solution formation among VOPO4 NbOPO4 and WOPO4 were lacking in literature.…”