Pd Supported on Carbon Nitride Boosts the Direct Hydrogen Peroxide Synthesis

Abstract

Herein, the development of an improved Pd on carbon nitride catalyst for direct H2O2 synthesis from the elements is reported. Microcalorimetric CO chemisorption is used to characterize the chemical speciation of the Pd-selective and -unselective sites. Selectivity trends among the samples suggest that a bare metal surface with a differential heat of CO chemisorption ranging between 140 and 120 kJ mol–1 is responsible for the total O2 hydrogenation, while a maximum threshold value of differential heat of CO chemisorption of approximately 70 kJ mol–1 is necessary for the partial hydrogenation of O2 to H2O2. Such a low differential heat of CO chemisorption indicates a low exposed metallic Pd surface subjected to electron withdrawal from the surrounding ligands: i.e., the N functional groups on the carbon support. With respect to N-containing carbon nanotubes, carbon nitrides provide the following: a higher concentration of N sites, a flexible network of π-conjugated polymeric subunits with sp3 linking subunits, and a flakelike morphology with high exposure of reactive C edge terminations. This results in a more effective kinetic stabilization of the electronically modified Pd species.

Direct Synthesis of H2O2 on Pd Based Catalysts: Modelling the Particle Size Effects and the Promoting Role of Polyvinyl Alcohol

“…Analogous observations were reported for Pd catalysts supported on carbon nanotubes prepared by a similar SI procedure and tested for the direct synthesis of H 2 O 2 in harsher conditions than those used here,,,, leading to a faster rate of Pd sintering.…”

“…The indication, however, is that possibly terrace sites are involved in this hydrogenolysis reaction. PVA, in this case, could lower this reaction by limiting the adsorption of H 2 on the surface …”

“…To improve the safety of the process while operating in the flammability range, microreactors,, and membrane reactors,, allowing to operate in an intrinsically safe way, have been proposed. For improving the selectivity of the process, the use high H 2 /O 2 ratios, bimetallic catalysts, promoters, surface modifiers as polyvinyl alcohol (PVA) and oxidative pretreatments have been proposed.…”

“…The procedure is easy to implement and good dispersions of Pd are possible. However, when PVA is removed by in‐situ oxidation, there is a rapid deactivation of the catalysts and sintering of the Pd particles ,,…”

Effect of Heat Treatment on the Nitrogen Content and Its Role on the Carbon Dioxide Adsorption Capacity of Highly Ordered Mesoporous Carbon Nitride

“…All XANES spectra were normalized at the maximum intensity for comparison purposes. [33,34] The intensity ratio of p*/s* peaks at the carbon and nitrogen Ke dge as af unction of carbonization temperature is shown in Figure 8C andD .Atahigh carbonization temperature of 1000 8C, the significant decrease in the p*/s*r atio in both carbon and nitrogen Ke dges is the sign of deformationo ft he sp 2 -graphite-like MCN structure. In the nitrogen K-edge XANES spectra ( Figure 8B), three typical p*r esonances are observeda t3 99.3,4 00.5, and 402.3 eV;t hese correspond to nitrogen atoms with two carbon neighbors in ap yridine-like configuration (p* C-N-C (N1)), sp 2 /sp 3 -CNH bonding configuration (p* C-N-H (N2)), and sp 3 -NÀ3C bridging among three structuralu nits (p* N-3C (N3)).…”