(398bh) Tailoring Pore Topology to Polymorphism By Engineering Metal Oxide Interfaces during Templating of Nanostructure Materials

The rational engineering of porous metal oxide catalysts requires identification of versatile strategies for simultaneously tailoring properties ranging from porosity to polymorphism across a diverse materials palette. The low surface energies characteristic of most oxides enable increased atomic surface mobility that makes oxides susceptible to low-temperature structural (chemical, topological), and, thereby, functional reconfiguration. In this talk, we will introduce a phenomenon [1] whereby the interface between metal oxides can be exploited as a means for preserving both the structure and the traditionally metastable metal oxide polymorphs at temperatures well in excess of those where complete transformation to the coarsened, thermodynamically stable polymorph occurs in bulk powders. Specifically, we will employ titania as a surrogate metal oxide and will demonstrate the preservation of the traditionally metastable anatase crystal structure along a two-dimensional oxide interface at temperatures well in excess of those that normally trigger a full polymorphic transformation to rutile in higher dimensionality crystalline powders.

We will then extend this polymorphic stabilization phenomenon to cases of three-dimensionally distributed interfaces. Specifically, we will show how convectively assembled colloidal crystal templates, comprised of size-tunable (ca. 15-50 nm) silica nanoparticles, enable versatile sacrificial templating of three-dimensionally ordered mesoporous (3DOm) metal oxides (MO_x) at both mesoscopic and microscopic size-scales [2]. Beyond scaffolding porous materials with high surface areas and tunable pore sizes, we will identify a synergistic, template-mediated interfacial and volumetric mechanism for stabilizing metastable 3DOm metal oxide polymorphs upon high-temperature processing and for tuning fractional polymorphism [3]. Mechanistic investigations suggest that this polymorph stabilization is derived from the combined effects of the template-replica (MO_x/SiO₂) interface and simultaneous interstitial confinement that limits the degree of coarsening during high temperature calcination of the template-replica composite. We will employ ZrO₂ (i.e. tetragonal ZrO₂) as a surrogate metal oxide for establishing the generalizability of 3DOm templating for tailoring both the mesostructure (i.e. pore size and surface area) and the crystallography (i.e. polymorphism) of porous oxides. This work ultimately identifies a facile yet versatile templating strategy for realizing nanostructured oxide catalysts and supports with (i) surface areas that are more than an order of magnitude larger than untemplated control samples, (ii) pore diameters and volumes that can be tuned across a continuum of size-scales, and (iii) selectable polymorphism.

References

[1] D. G. Gregory, L. Lu, C. J. Kiely, M. A. Snyder, J. Phys. Chem. C, 121(8) (2017) 4434-4442

[2] M. A. Snyder, MRS Bulletin, 41(9) (2016) 683-688.

[3] D. G. Gregory, Q. Guo, L. Lu, C.J. Kiely, M. A. Snyder, submitted (2017).