Noble metal ions in CeO₂and TiO₂: synthesis, structure and catalytic properties

Abstract

In past four decades, CeO 2 has been recognized as an attractive material in the area of auto exhaust catalysis because of its unique redox properties. In presence of CeO 2 , catalytic activity of noble metals supported on Al 2 O 3 gets enhanced due to higher dispersion of noble metals in their ionic form. In last several years, we have been exploring an entirely new approach of dispersing noble metal ions on CeO 2 and TiO 2 matrices for redox catalysis. In this article, dispersing of noble metal ions by solution combustion as well as other methods over CeO 2 and TiO 2 resulting mainly in −δ (M = Pd, Pt, Rh and Ru) catalysts, structure of these materials, their catalytic properties toward different types of catalysis, structure−property relation and mechanism of catalytic reactions are reviewed. In these catalysts, noble metal ions are incorporated into substrate matrix to a certain limit in a solid solution form. Lower valent noble metal ion substitution in CeO 2 and TiO 2 creates noble metal ionic sites and oxide ion vacancies that act as adsorption sites for redox catalysis. It has been demonstrated that these new generation noble metal ionic catalysts (NMIC) have been found to be catalytically more active than conventional respective nano crystalline noble metal catalysts dispersed on oxide supports. 5 at.% and hence, substitution of only 1−2 at.% noble metal ion in CeO 2 is sufficient to develop a catalyst. The underlying principle of doping aliovalent metal ions into CeO 2 or TiO 2 lattice is to retain their parent structures. In this sense, new-age advanced catalysts such as Ce 1−x M x O 2−δ (M = Pd, Pt, Rh, Cu, Ag and Au), Ce 1−x−y A x M y O 2−δ (A = Ti, Zr, Sn and Fe; M = Pt and Pd) and Ti 1−x M x O 2−δ (M = Pd, Pt, Rh and Ru) retaining their parent fluorite and anatase structures have been prepared in our laboratory employing solution combustion method. 20−23 Thus, synthesis of these single phase oxides is a new conceptwhich is the basis of metal ion catalysts. In these catalysts, noble metal ions and corresponding oxide ion vacancies are supposed to be the active sites. Since, the adsorption sites in these catalysts are noble metal ions such as Pd 2+ , Pt 2+ , Rh 3+ and Ru 4+ , we have named these materials as noble metal ionic catalysts (NMIC). In last several years, we have been studying several exhaust catalytic reactions such as NO reduction, CO and hydrocarbon oxidation, selective catalytic reduction of NO, three-way catalytic reaction, water gas shift (WGS) reaction, preferential oxidation of CO (CO−PROX), H 2 + O 2 recombination reaction, hydrogenation and Heck reaction over these noble metal ionic catalysts and it has been demonstrated that these catalysts are much more active for several catalytic reactions compared to other conventional supported metal catalysts. In this review, at first, we mainly discuss about background, synthesis and characterization of these noble metal ionic catalysts (NMIC). Then we emphasize on our detailed work related to exhaust catalysis comparing activities with t...

Recent Progress in Well‐Controlled Synthesis of Ceria‐Based Nanocatalysts towards Enhanced Catalytic Performance

“…[ 123 ] At a structural level, there should be changes in the atomic environment of the parent CeO 2 structure after substitution of metal ions. The substitution of noble metal ions into CeO 2 lattice further augments these properties.…”

Review of Recent Studies on Solution Combustion Synthesis of Nanostructured Catalysts

“…It is suggested that noble metal ionic sites and oxide ion vacancies are developed from noble metal‐ion substitution in ceria and titania and behave as adsorption sites for redox catalysis. It was shown that the as‐synthesized noble metal ionic catalysts evaluated to be more active than traditional noble metal nanocatalysts dispersed on oxide supports …”

Multifunctional properties of ceria nanocubes synthesized by a hydrothermal method

“…Cerium oxide-based materials have been extensively investigated and widely used in various applications, such as oxygen storage, catalysts, gas sensors, UV blockers, electronics and corrosion resistance, and the functional layer is made up of solid oxide fuel cells [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Cerium oxides have been one of the most widely used heterogeneous catalysts for several technologically important reactions [2][3][4][18][19][20]. Their catalytic traits can be attributed to two of their main properties, namely oxygen storage capacity of ceria due to the propensity of Ce to change between Ce 3+ and Ce 4+ and high mobility [21].…”

“…It is a very simple and ideal reaction in heterogeneous catalysis and has been used as a model reaction for probing the oxidation activity of several catalysts. CO oxidation has been carried out over different types of CeO 2 -based oxide catalysts [3,4].…”