Atomic layer deposition - Sequential self-limiting surface reactions for advanced catalyst "bottom-up" synthesis

Catalyst synthesis with precise control over the structure of catalytic active sites at the atomic level is of essential importance for the scientific understanding of reaction mechanisms and for rational design of advanced catalysts with high performance. Such precise control is achievable using atomic layer deposition (ALD). ALD is similar to chemical vapor deposition (CVD), except that the deposition is split into a sequence of two self-limiting surface reactions between gaseous precursor molecules and a substrate. The unique self-limiting feature of ALD allows conformal deposition of catalytic materials on a high surface area catalyst support at the atomic level. The deposited catalytic materials can be precisely constructed on the support by varying the number and type of ALD cycles. As an alternative to the wet-chemistry based conventional methods, ALD provides a cycle-by-cycle "bottom-up" approach for nanostructuring supported catalysts with near atomic precision. In this review, we summarize recent attempts to synthesize supported catalysts with ALD. Nucleation and growth of metals by ALD on oxides and carbon materials for precise synthesis of supported monometallic catalyst are reviewed. The capability of achieving precise control over the particle size of monometallic nanoparticles by ALD is emphasized. The resulting metal catalysts with high dispersions and uniformity often show comparable or remarkably higher activity than those prepared by conventional methods. For supported bimetallic catalyst synthesis, we summarize the strategies for controlling the deposition of the secondary metal selectively on the primary metal nanoparticle but not on the support to exclude monometallic formation. As a review of the surface chemistry and growth behavior of metal ALD on metal surfaces, we demonstrate the ways to precisely tune size, composition and structure of bimetallic metal nanoparticles. The cycle-by-cycle "bottom up" construction of bimetallic (or multiple components) nanoparticles with near atomic precision on supports by ALD is illustrated. Applying metal oxide ALD over metal nanoparticles can be used to precisely synthesize nanostructured metal catalysts. In this part, the surface chemistry of Al₂O₃ ALD on metals is specifically reviewed. Next, we discuss the methods of tailoring the catalytic performance of metal catalysts including activity, selectivity and stability, through selective blocking of the low-coordination sites of metal nanoparticles, the confinement effect, and the formation of new metal-oxide interfaces. Synthesis of supported metal oxide catalysts with high dispersions and "bottom up" nanostructured photocatalytic architectures are also included. Therein, the surface chemistry and morphology of oxide ALD on oxides and carbon materials as well as their catalytic performance are summarized.