Alumina Graphene Catalytic Condenser for Programmable Solid Acids

Abstract

Precise control of electron density at catalyst active sites enables regulation of surface chemistry for optimal rate and selectivity to products. Here, an ultrathin catalytic film of amorphous alumina (4 nm) was integrated into a catalytic condenser device that enabled tunable electron depletion from the alumina active layer and correspondingly stronger Lewis acidity. The catalytic condenser had the following structure: amorphous alumina/graphene/HfO2 dielectric (70 nm)/p-type Si. Application of positive voltages up to +3 V between graphene and the p-type Si resulted in electrons flowing out of the alumina; positive charge accumulated in the catalyst. Temperature programmed surface reaction of thermocatalytic isopropanol dehydration to propene on the charged alumina surface revealed a shift in the propene formation peak temperature of up to ΔT(peak)~50 ⁰C relative to the uncharged film, consistent with a 16 kJ/mol (0.17 eV) reduction in the apparent activation energy. Electrical characterization of the thin amorphous alumina film by ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling microscopy (STM) indicates the film is a defective semiconductor with an appreciable density of in-gap electronic states. Density functional theory calculations of isopropanol binding on the pentacoordinate aluminum active sites indicate significant binding energy changes (ΔBE) up to 60 kJ/mol (0.62 eV) for 0.125 e- depletion per active site, supporting the experimental findings. Overall, the results indicate that continuous and fast electronic control of thermocatalysis can be achieved with the catalytic condenser device.

Platinum Graphene Catalytic Condenser for Millisecond Programmable Metal Surfaces

“…From the plan view obtained by scanning electron microscopy (SEM) as shown in Figures 1d and S5, the surface of the device comprised regions of single (light gray) and multilayer (dark gray) graphene with several hexagonal-shaped domains visible and consistent with previous results. 21 The presence of these layers of graphene was confirmed by Raman spectra collected over 20 spot sites in Figures 1f and S6. More importantly, SEM-EDX verified that both carbon (C) and Pt were evenly distributed across the device surface (Figure 1d).…”

“…We have previously characterized ALD-grown 70 nm HfO 2 films and determined that they remain amorphous at or below 350 °C; XRD peaks associated with crystalline HfO 2 only appear when heated to 400 °C or above. 21 The crystalline phase of HfO 2 exhibits a lower dielectric constant (∼17 vs 26 for amorphous HfO 2 ), and the crystallized HfO 2 film is more likely to exhibit pinholes that conduct current between the active catalyst layer and the underlying electrode. 44−48 Operating the catalytic condenser at temperatures below 400 °C allows for broad application of Pt active sites to many catalytic reactions while also achieving the highest amount of charge condensation at the active site.…”

“…As a platform technology, the catalytic condenser is a general approach for active site design that can tune the electronic state of both metals and metal oxides. 21 Instead of controlling the energies of surface species and transition states through active site design alone, a continuum of surface electronic states are accessible on a catalytic condenser via applied voltage, V CAT , which were previously not available in the discrete atomic structure of surfaces. Catalysts can be optimized by selection of a specific applied potential that optimizes rate, selectivity, or active site stability for applications that use Pt catalysts including emissions control, hydrogenation, and catalytic reforming.…”

“…1 cm 2 graphene sheet was transferred using an established polymethyl methacrylate (PMMA)-based method to provide a conductive sheet across the insulating layer. 40,41 While a gold contact was added to earlier catalytic condenser devices, 21 this was not included for the current device with the Pt/graphene active layer to avoid its participation in carbon monoxide adsorption. Finally, Pt was deposited on graphene by electron beam evaporation of a pure Pt source and reduced in flowing H 2 at 150 °C in a tube furnace for an hour.…”

“…19,20 We have recently described the method of a 'catalytic condenser' that uses a high-k dielectric HfO 2 layer to separate a conductive silicon electrode from a conductive active layer of ultrathin amorphous alumina on graphene. 21 In this example, the amorphous alumina layer is a solid Lewis acid catalyst, while the supporting graphene rapidly distributes charge across the device surface. When a positive voltage bias is applied to the active alumina/graphene layer relative to the silicon electrode, the alumina catalyst active sites are depleted of electrons, making them more acidic and faster at dehydrating alcohols such as isopropyl alcohol.…”