Nanosized Catalytic Particles for the Decomposition of Green Propellants as Substitute for Hydrazine: Application of Catalysis and Nanocatalysis in Green Propulsion

1. Introduction

Nanoparticles, particularly gold nanoparticles, have a rich historical use dating back millennia in China and Egypt. Women employed them for both medicinal and aesthetic purposes, while artists utilized these particles in the creation and embellishment of various items such as the renowned Lycurgus cup from the fifth century AD. Colloidal nanoparticles gained attention in treatises from the 17th century onwards (Antonii, 1916). The first scientific exploration of light’s effect emerged in the mid-19th century through Faraday’s influential publication (Faraday, 1857), later complemented by Mie’s fundamental explanation of the plasmon about half a century later (Mie, 1908; Daniel & Astruc, 2004). In the realm of catalysis, the inception of research dates back to 1941 when papers were published on poly(vinyl) alcohol-protected palladium and platinum nanoparticle hydrogenation catalysts. These catalysts were prepared through the reduction of metal salts by H₂ (Rampino & Nord, 1941). This early catalytic research reflects the pioneering work of Paul Sabatier, who, as a Chemistry Nobel Prize laureate in 1912, discovered catalyzed hydrogenation using finely divided nickel particles. These nickel particles were prepared by the reduction of nickel oxide or hydroxide with H₂ (Sabatier, 1913). A significant breakthrough in the 20th century occurred in 1987 when Haruta discovered a remarkable enhancement in the catalytic activity of gold nanoparticles for the oxidation of CO to CO₂ by O₂. This improvement was particularly pronounced when the gold nanoparticles were smaller than 5 nm (Haruta et al., 1987). While the nanoworld encompasses dimensions ranging from 1 to 100 nm and beyond in optics (plasmon), nanomedicine, nanotechnology, and nanoscience as a whole (Astruc, 2004), Haruta’s groundbreaking finding redirected nanocatalysis research towards the smallest nanoparticles. Nevertheless, there are notable exceptions. For example, when large gold nanoparticles are exposed to visible light irradiation, plasmonic excitation occurs, generating hot electrons. These hot electrons, in turn, activate semiconductors like TiO₂ for substrate transformation. Presently, it is understood that even subnanoparticles composed of late transition metals exhibit significant activity. However, the peak of catalytic activity is likely achieved for particles containing between 12 and 20 metal atoms, with sizes approaching 1 nm or slightly below (Imaoka et al., 2015). Advancements in characterization techniques, such as aberration-corrected electron microscopy and X-ray absorption spectroscopy, have facilitated these insights. These modern methods enable the observation not only of individual metal atoms but also of subnanoclusters, providing detailed insights into their immediate surroundings. Small nanoparticles are commonly referred to as nanoclusters, and for those even smaller than one nanometer, they are termed subnanoclusters. However, there exists a continuum of situations between molecules and solid states, spanning from small clusters defined by molecular orbitals to larger nanoparticles defined by energy band structures. The distinctions among these various types arise from factors such as the number of metal atoms, the nature of the ligands, and the degree of dispersity. Strictly speaking, the term “cluster” or “nanocluster” is reserved for molecularly precise polymetallic entities with known X-ray crystal structures for their ligands. On the other hand, the term “nanoparticles” is applied to mixtures of more or less polydisperse large nanoclusters, as determined by the histogram obtained from transmission electron microscopy measurements. Clusters with metal-metal bonds typically consist of a few metal atoms, although there are instances of larger clusters. The X-ray crystal structures of metal clusters, particularly those crucial for theoretical investigations, began to emerge in the late 1970s. Subsequent catalytic studies revealed the lack of structural integrity in the cluster framework during catalysis (Chini, 1980). Over the past decade, this subfield has experienced a renaissance, particularly following the discovery of nanoclusters with structurally perfect definitions. These nanoclusters feature thiolate ligands identical to those found in the most well-known gold nanoparticles (Astruc, 2004; Zhu et al., 2008). Recent catalytic studies of these well-defined nanoclusters have been particularly prolific and noteworthy, given that the cluster framework is precisely characterized for theoretical calculations, contributing valuable insights to structure-reactivity relationships. However, a significant focus in catalytic research involves small metal nanoparticles situated at the interface of homogeneous and heterogeneous catalysis. These nanoparticles are only loosely stabilized by weak ligands such as polymers, dendrimers, ionic liquids, inorganic molecules, solids, and solvents. They are frequently chosen for catalytic investigations due to the simplicity of their preparation and their impressive catalytic performances, facilitated by the easy access of substrates to the nanoparticle surfaces through weak ligand displacement [11]. This trend has led to a merging of the former communities of heterogeneous catalysis into the nanoparticle community, despite the existence of numerous efficient high-temperature processes developed throughout the 20th century [12]. A unifying concept across both heterogeneous and homogeneous catalysis communities is the crucial role played by catalytic nanoparticle supports (Astruc et al., 2005). In the realm of heterogeneous catalysis, templates such as zeolites have been extended to microstructured silica, alumina, and other oxides, allowing the stabilization of subnanoclusters down to the nanometer scale (Polshettiwar et al., 2010).