Abstract
Bimetallic Pd-Au catalysts were prepared on the porous nanocrystalline silicon (PSi) for the first time. The catalysts were tested in the reaction of direct hydrogen peroxide synthesis and characterized by standard structural and chemical techniques. It was shown that the Pd-Au/PSi catalyst prepared from conventional H2[PdCl4] and H[AuCl4] precursors contains monometallic Pd and a range of different Pd-Au alloy nanoparticles over the oxidized PSi surface. The PdAu2/PSi catalyst prepared from the [Pd(NH3)4][AuCl4]2 double complex salt (DCS) single-source precursor predominantly contains bimetallic Pd-Au alloy nanoparticles. For both catalysts the surface of bimetallic nanoparticles is Pd-enriched and contains palladium in Pd0 and Pd2+ states. Among the catalysts studied, the PdAu2/PSi catalyst was the most active and selective in the direct H2O2 synthesis with H2O2 productivity of 0.5 at selectivity of 50% and H2O2 concentration of 0.023 M in 0.03 M H2SO4-methanol solution after 5 h on stream at −10°C and atmospheric pressure. This performance is due to high activity in the H2O2 synthesis reaction and low activities in the undesirable H2O2 decomposition and hydrogenation reactions. Good performance of the PdAu2/PSi catalyst was associated with the major part of Pd in the catalyst being in the form of the bimetallic Pd-Au nanoparticles. Porous silicon was concluded to be a promising catalytic support for direct hydrogen peroxide synthesis due to its inertness with respect to undesirable side reactions, high thermal stability, and conductivity, possibility of safe operation at high temperatures and pressures and a well-established manufacturing process.
Highlights
In the general field of industrial chemistry the role of heterogeneous catalysis is difficult to overestimate: the advances in large-scale catalytic processes, such as ammonia synthesis, sulphuric acid and nitric acid processes, ethylene and propylene production, acetic acid, and methanol syntheses and several others were responsible for the rapid development of human civilization in the twentieth-century
The search for alternative supports is still actual, especially in the scope of further reaction translation from lab-scale batch reactors to more practical and scalable continuous flow multiphase reactors, in which the application of conventional catalysts powders is limited by the need of its fixing and mass-/heat-transfer problem. In this regard we investigated the properties of porous nanocrystalline silicon powder (PSi) as a support for Pd-Au direct H2O2 synthesis catalysts
We would like to point out key features of double complex salt (DCS) application as a precursor and porous silicon as a support for direct H2O2 synthesis catalysts:
Summary
In the general field of industrial chemistry the role of heterogeneous catalysis is difficult to overestimate: the advances in large-scale catalytic processes, such as ammonia synthesis, sulphuric acid and nitric acid processes, ethylene and propylene production, acetic acid, and methanol syntheses and several others were responsible for the rapid development of human civilization in the twentieth-century. In the twenty-first century the challenges for catalysis are different but, arguably, even harder than those that were solved till : selective activation of C-H bonds in saturated alkanes, efficient and selective activation of CO2, efficient splitting of water, highly selective functionalization of complex molecules for pharmaceutical and speciality chemicals, etc. These challenges have one common aspect—the requirement for much better control of reaction selectivity in the traditionally “difficult” reactions, such as sp C-H activation, as an example. Advanced synthesis techniques are under development: from modified impregnation and adsorption (Munnik et al, 2015) to a novel approach of nanoparticles biosynthesis which presents an alternative eco-friendly and potentially precise way for synthesis of heterogeneous catalysts (Hulkoti and Taranath, 2014)
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