The effect of modifying Pd/Al2O3 catalysts by atomic layer deposition of 1 nm ZrO2 films was studied. For deposition on oxidized, PdO/Al2O3 catalysts, TEM imaging, EDS mapping, and metal-dispersion measurements confirmed the presence of the thin ZrO2 over both the Al2O3 support and the metal particles. The ZrO2 films were surprisingly stable, forming a well-crystallized phase only above 1173 K. The ZrO2 coating over the PdO particles created a semicore–shell-like structure that stabilized the metal against sintering in air at 1073 K. Steady-state, methane oxidation rates on unmodified PdO/Al2O3 decreased with increasing catalyst calcination temperature, but rates on the ZrO2-covered surfaces increased with increasing calcination temperature.

The concept of self-regenerating or "smart" catalysts, developed to mitigate the problem of supported metal particle coarsening in high-temperature applications, involves redispersing large metal particles by incorporating them into a perovskite-structured support under oxidizing conditions and then exsolving them as small metal particles under reducing conditions. Unfortunately, the redispersion process does not appear to work in practice because the surface areas of the perovskite supports are too low and the diffusion lengths for the metal ions within the bulk perovskite too short. Here, we demonstrate reversible activation upon redox cycling for CH oxidation and CO oxidation on Pd supported on high-surface-area LaFeO, prepared as a thin conformal coating on a porous MgAlO support using atomic layer deposition. The LaFeO film, less than 1.5 nm thick, was shown to be initially stable to at least 900 °C. The activated catalysts exhibit stable catalytic performance for methane oxidation after high-temperature treatment.

Atomic layer deposition (ALD) offers exciting possibilities for controlling the structure and composition of surfaces on the atomic scale in heterogeneous catalysts and solid oxide fuel cell (SOFC) electrodes. However, while ALD procedures and equipment are well developed for applications involving flat surfaces, the conditions required for ALD in porous materials with a large surface area need to be very different. The materials (e.g., rare earths and other functional oxides) that are of interest for catalytic applications will also be different. For flat surfaces, rapid cycling, enabled by high carrier-gas flow rates, is necessary in order to rapidly grow thicker films. By contrast, ALD films in porous materials rarely need to be more than 1 nm thick. The elimination of diffusion gradients, efficient use of precursors, and ligand removal with less reactive precursors are the major factors that need to be controlled. In this review, criteria will be outlined for the successful use of ALD in porous materials. Examples of opportunities for using ALD to modify heterogeneous catalysts and SOFC electrodes will be given.

Precise control of electron density at catalyst active sites enables regulation of surface chemistry for the 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/HfO 2 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. Temperatureprogrammed surface reaction of thermocatalytic isopropanol (IPA) dehydration to propene on the charged alumina surface revealed a shift in the propene formation peak temperature of up to ΔT peak ∼50 °C relative to the uncharged film, consistent with a 16 kJ mol −1 (0.17 eV) reduction in the apparent activation energy. Electrical characterization of the thin amorphous alumina film by ultraviolet photoelectron spectroscopy and scanning tunneling microscopy indicates that the film is a defective semiconductor with an appreciable density of in-gap electronic states. Density functional theory calculations of IPA binding on the pentacoordinate aluminum active sites indicate significant binding energy changes (ΔBE) up to 60 kJ mol −1 (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.

Al 2 O 3 powders were modified by Atomic Layer Deposition (ALD) of CeO 2 to produce composite catalyst supports for Pd. The weight of the support was found to increase linearly with the number of ALD cycles. This, together with TEM images, indicated that the CeO 2 grows as a dense, conformal film, with a growth rate of 0.02 nm per cycle. The films showed good thermal stability under oxidizing conditions. XRD measurements on a sample with 0.28 g CeO 2 /g Al 2 O 3 showed no evidence for crystalline CeO 2 until calcination above 1073 K. Water-gas-shift rates on 1-wt% Pd catalysts supported on the CeO 2 ALD-modified Al 2 O 3 were essentially identical to rates on conventional Pd-CeO 2 catalysts and much higher than rates on a catalyst in which Pd was supported on Al 2 O 3 with CeO 2 added by infiltration. The WGS rates, together with results from FTIR and COO 2 pulse studies, suggest that all of the Pd is in contact with CeO 2 on the ALD-prepared supports and that it should be possible to prepare high-surface-area, functional supports using ALD.