A Thermodynamic Investigation of Ni on Thin-Film Titanates (ATiO3)

Abstract

Thin, ~1-nm films of CaTiO3, SrTiO3, and BaTiO3 were deposited onto MgAl2O4 by Atomic Layer Deposition (ALD) and then studied as catalyst supports for ~5 wt % of Ni that was added to the perovskite thin films by Atomic Layer Deposition. Scanning Transmission Electron Microscopy demonstrated that both the Ni and the perovskites uniformly covered the surface of the support following oxidation at 1073 K, even after redox cycling, but large Ni particles formed following a reduction at 1073 K. When compared to Ni/MgAl2O4, the perovskite-containing catalysts required significantly higher temperatures for Ni reduction. Equilibrium constants for Ni oxidation, as determined from Coulometric Titration, indicated that the oxidation of Ni shifted to lower PO2 on the perovskite-containing materials. Based on Ni equilibrium constants, Ni interactions are strongest with CaTiO3, followed by SrTiO3 and BaTiO3. The shift in the equilibrium constant was shown to cause reversible deactivation of the Ni/CaTiO3/MgAl2O4 catalyst for CO2 reforming of CH4 at high CO2 pressures, due to the oxidation of the Ni.

A Study of How LaFeO3 and CaTiO3 Supports Affect the Oxidation, Hydrogenation, and Methane Steam Reforming Activity of Pt and Ni Catalysts

“…In addition to the features associated with MgAl 2 O 4 and CaTiO 3 , the reduced sample exhibits a peak for metallic Ni while the oxidized sample does not. STEM−EDS on identical samples were reported previously, 22 and unlike the case for Pt/CaTiO 3 /MgAl 2 O 4 , there was no evidence of movement of the Ti toward the reduced Ni. This is further evidence that the apparent decomposition of the perovskite in Pt/CaTiO 3 /MgAl 2 O 4 is driven by interactions with the Pt.…”

“…This conclusion is supported by the aforementioned observation that metal–perovskite interactions are metal specific (e.g., CaTiO 3 influences Pd and Pt differently). Finally, thermodynamic measurements of Ni–NiO equilibria for Ni catalysts supported on LaFeO 3 and ATiO 3 (A = Ca, Sr, and Ba) showed changes consistent with the perovskites affecting the chemical state of the Ni.…”

Ni Ingress and Egress in SrTiO3 Single Crystals of Different Facets

“…Adsorbed Ni particles/clusters formed by the egress Ni atoms from a perovskite lattice such as CaTiO 3 , , LaFeO 3 , and LaMnO 3 ,, show intriguing catalytic properties. However, the mechanism of interaction between the Ni and its hosting perovskite, in general, remains unclear.…”

“…Perovskite-type oxides (ABO 3 ) are widely used as active components in electronic and optical devices, sensors, and heterogeneous catalysts. − One particularly well-studied perovskite is SrTiO 3 (STO), due to its dielectric properties, high-temperature stability, and tunability of properties by doping. , In its native form, pure STO is a semiconductor, , which can be modified by substitutionally doping the lattice with transition metal ions, such as Ni, resulting in a decrease in the bandgap . Addition of transition metal atoms in the form of adatoms and clusters on the surface of STO have also been shown to result in systems exhibiting interesting catalytic properties. − However, while most studies consider bulk properties of Ni–STO, as well as other doped STO systems, to explain its material properties, a clear understanding of the chemistry and catalysis requires a knowledge of different STO facet effects, which is still nebulous. Therefore, depending on the design interests, whether that be Ni stabilization on the surface of STO or facilitation or suppression of Ni ingress (ions move into the material from an adsorbed state on the surface into the lattice surface, subsurface, or bulk) or Ni egress (ions move out of the material from the bulk, subsurface, or the surface out into an adsorbed state onto of the surface), understanding the nuances involved in these processes across different facets is vital for synthesizing the desired material for a given application.…”

Two-Dimensional Perovskite Crystals Formed by Atomic Layer Deposition of CaTiO3 on γ-Al2O3

“…It is interesting to ask why the results on SiO 2 are so different from those on Al 2 O 3 and MgAl 2 O 4 [ 21 , 25 ]. Clearly, the perovskite films are not epitaxially matched to the crystal structures of Al 2 O 3 (cubic, a = 0.7912 nm) or MgAl 2 O 4 (cubic, a = 0.8081 nm) [ 12 ].…”