Abstract
Multimetallic materials formed by surface or near surface alloying of elements often exhibit functional properties compared to their parent metal alone. They have shown improved catalytic activity, stability, and selectivity in wide range of applications such as catalysis, electrocatalysis, hydrogen storage etc. Further, the topology (e.g., particle size, morphology, porosity) exhibited by nanoparticles and nanoporous metals play an important role is determining the functional properties. With different shapes, different facets are exposed with distinct electronic and geometric structures, leading to variation in adsorption energy of reactants and intermediates, offering versatility of a multimetallic nanocatalyst. Despite much recent effort to develop approaches to synthesize multimetallic materials, simultaneous control of both surface morphology and composition remains a challenge, particularly at the large scale required for industrial applications, and with high surface area substrate. Atomic Layer Electroless Deposition (ALED) is a scalable approach to surface-modified metal powders in which elements more noble than the surface hydrides of the substrate metal are deposited layer-by-layer in a surface-limited fashion. In previous studies, conformal deposition of Pt and Rh as adlayers have been grown on Pd powders and Pd/C. For better understanding of this technique, we herein expand the scope of ALED, demonstrating that high surface area Pt powder is a viable substrate for controlled deposition of Pd adlayers. Two different growth mechanisms have been proposed based on bulk and surface Pd atomic fractions obtained from atomic absorption spectroscopy and X-ray photoelectron spectroscopy, respectively. Further, spectral simulations were performed to strengthen the proposed growth mechanisms, favoring conformal growth in initial deposition followed by island formation in subsequent cycles. Observation of multiple pathways suggest a means of controlling adlayer surface morphology of ALED materials, in which an initial cycle of deposition sets the fractional coverage and subsequent cycles tune adlayer thickness. Also, nanoporous Pt powders prepared by chemical reductions of metal salts in soft templates in the presence of a nonionic surfactant, are explored as an ALED substrate. Finally, the potential feature of ALED to tune the surface morphology of a deposited adlayer metal is studied, controlling nucleation versus growth by changing the rate of addition of the Pd adlayer precursor solution to prepared Pt-H substrate using a syringe pump. Spectroscopy provided promising preliminary indications that the growth mechanism is altered under varied conditions. The change in surface morphology is further probed by examining the product distribution of a well-known, surface-sensitive reaction, using catalysts prepared under these different conditions. Conformal vs. island growth of Pd on Pt substrate demonstrated differences in single and double hydrogenation of phenylacetylene to styrene and ethylbenzene, respectively, providing evidence for variations in surface morphology.