Trimetalic heterogeneous catalyst for dehydrogenation of formic acid with enhanced CO tolerance

Abstract

Hydrogen energy is considered to be a promising alternative for the sustainable and environmentally friendly solution of the global energy problem. One of the major obstacles of hydrogen energy applications is to maintain safe and efficient storage of hydrogen which can also be achieved chemically using suitable carrier materials. Formic acid (HCOOH, FA) can be utilized as a hydrogen carrier due to its low molecular weight (46 g/mol) and high hydrogen density (%4.4 weight). FA is a stable, non-flammable, and non-toxic biomass side-product rendering it a perfect candidate for an alternative hydrogen vector. Design of novel heterogeneous catalysts which can substitute the existing homogeneous catalytic systems may allow overcoming catalyst isolation and recovery costs and associated logistical problems hindering their applications in on-board operations. FA can be catalytically decomposed via dehydrogenation and dehydration reactions. Selective dehydrogenation of FA is crucial because, the production of CO from dehydration mechanism can suppress the activity of the catalyst by blocking/poisoning the precious metal sites. Consequently, development of CO-resistant, selective, catalytically active, and reusable heterogeneous catalysts has a great significance. In the current work, a new material that can produce H2(g) from FA under ambient conditions in the absence of additives with high CO-poisoning tolerance will be introduced, which is comprised of Pd-based trimetallic active centers functionalized with Ag and Cr in addition to amine-functionalized MnOx promoters dispersed on a SiO2 support surface. A novel trimetallic FA dehydrogenation catalyst was prepared and studied using analytical, ex-situ and in-situ spectroscopic techniques and compared to the results obtained for monometallic, bimetallic and active site-free counterparts. Trimetallic catalysts were found to reveal superior catalytic activity and stability compared to all of the currently investigated catalysts. Structural and catalytic properties of the trimetallic catalysts were investigated as a function of metal loadings. Structural characterization of the synthesized materials was carried out by Raman spectroscopy, Inductively-Coupled Plasma Optical Emission Spectroscopy (ICP-OES), X-ray Diffraction (XRD), Brunauer, Emmett and Teller (BET) Specific Surface Area Analysis, Transmission Electron Microscopy (TEM), High Resolution TEM (HRTEM), Scanning Transmission Electron Microscopy (STEM), and STEM/Energy Dispersive X-Ray (EDX), High-Angle Annular Dark Field (HAADF)/STEM. In addition, interaction of the catalyst surfaces with reactants and products were also monitored via in-situ FTIR spectroscopy for functional characterization. Detailed in-situ FTIR spectroscopic experiments were also performed using HCOOD, DCOOH and DCOOD in order to understand the nature of the adsorbed species, products and catalytic inhibitors.