Atmospheric-Pressure Atomic Layer Deposition Of Platinum Nanoclusters On Titania Nanoparticles

Pt-based catalysts are widely used, for example in petrochemical processes, fuel cells and emission control in cars. For optimizing the catalyst performance – its activity, selectivity, and stability – while using a minimum amount of scarce and expensive Pt, it is crucial to maintain excellent control over the morphology and distribution of the Pt clusters during their synthesis process. In the past years, atomic layer deposition (ALD) has appeared as an appealing nanofabrication technique for the preparation of precisely tailored heterogeneous catalysts, see e.g. [1].

We will present a novel approach to deposit Pt on nanoparticles in a fluidized-bed ALD reactor at atmospheric pressure. So far, ALD deposition of Pt has only been shown at vacuum. We will demonstrate that the precision characteristics of ALD can be maintained at atmospheric pressure, effectively proving the potential of the process for catalyst fabrication on an industrially relevant scale.

Utilizing (trimethyl)methylcyclopentadienyl platinum (IV) and ozone as precursors we were able to grow Pt islands on TiO₂ (Aeroxide P-25) nanoparticles at 250^oC under atmospheric pressure conditions. The nanoparticles were fluidized with nitrogen at a superficial gas velocity of 4.2 cm/s. By applying sub-saturating pulses we achieved ~95% precursor utilization, while the use of a drastic oxidizing agent like ozone ensured that the aimed metal loading could be reached in only a few reaction cycles. Increasing the number of performed cycles resulted in a monotonic increase in the metal loading (Fig. 1), while the obtained particles were highly dispersed (average size of only 1.5nm) and exhibited a narrow particle size distribution (Fig. 2).

The use of relatively mild temperature conditions, the very low impurities inclusion in the materials and the absence of solvent use during the fabrication procedure are rendering ALD an attractive technique for the fabrication and tailoring of materials. We will demonstrate that there is even more space for improvement in the process scale-up by showing coating results obtained in a continuous pneumatic ALD reactor that also operates in atmospheric pressure. Proving that the results obtained in the batch process can be reproduced in the continuous reactor for processing powder is an important development in ALD technology.

[1] King, D.M., Liang, X., Weimer, A.W., Powder Technol. 221 (2012) 13

[2] Goulas, A., van Ommen, J.R., J. Mater. Chem. A 1 (2013) 4647

Fig. 1 Pt loading measured (ICP-OES) (o) and predicted for ideal ALD (X), and specific surface area (u) as a function of ALD cycles. [2]

Fig. 2 Pt particle size distributions for nanoclusters deposited with 1 and 5 ALD cycles (inset: TEM images of Pt nanoclusters on the P-25 support nanoparticles). [2]