Science

Direct propane dehydrogenation (PDH) to propylene is a desirable commercial reaction but is highly endothermic and severely limited by thermodynamic equilibrium. Oxidative routes that oxidatively remove hydrogen as water have safety and cost challenges. ...

Gas-phase autoxidation—regenerative peroxy radical formation following intramolecular hydrogen shifts—is known to be important in the combustion of organic materials. The relevance of this chemistry in the oxidation of organics in the atmosphere has received less attention due, in part, to the lack of kinetic data at relevant temperatures. Here, we...

Nature advance online publication 22 March 2017. doi:10.1038/nature21672

Authors: Lili Lin, Wu Zhou, Rui Gao, Siyu Yao, Xiao Zhang, Wenqian Xu, Shijian Zheng, Zheng Jiang, Qiaolin Yu, Yong-Wang Li, Chuan Shi, Xiao-Dong Wen & Ding Ma

Polymer electrolyte membrane fuel cells (PEMFCs) running on hydrogen are attractive alternative power supplies for a range of applications, with in situ release of the required hydrogen from a stable liquid offering one way of ensuring its safe storage and transportation before use. The use of methanol is particularly interesting in this regard, because it is inexpensive and can reform itself with water to release hydrogen with a high gravimetric density of 18.8 per cent by weight. But traditional reforming of methanol steam operates at relatively high temperatures (200–350 degrees Celsius), so the focus for vehicle and portable PEMFC applications has been on aqueous-phase reforming of methanol (APRM). This method requires less energy, and the simpler and more compact device design allows direct integration into PEMFC stacks. There remains, however, the need for an efficient APRM catalyst. Here we report that platinum (Pt) atomically dispersed on α-molybdenum carbide (α-MoC) enables low-temperature (150–190 degrees Celsius), base-free hydrogen production through APRM, with an average turnover frequency reaching 18,046 moles of hydrogen per mole of platinum per hour. We attribute this exceptional hydrogen production—which far exceeds that of previously reported low-temperature APRM catalysts—to the outstanding ability of α-MoC to induce water dissociation, and to the fact that platinum and α-MoC act in synergy to activate methanol and then to reform it.

Publication date: March 2017
Source:Journal of Catalysis, Volume 347
Author(s): W. Damion Williams, Jeffrey P. Greeley, W. Nicholas Delgass, Fabio H. Ribeiro
Linear scaling relations and Brønsted-Evans-Polanyi (BEP) relations help to elucidate trends in activation energies and adsorption energies on different metal surfaces. In this paper, Density Functional Theory (DFT) calculations available in the literature are utilized to analyze these trends and their effect on the reactivity of transition metals for the low temperature water-gas shift reaction (CO+H2O↔CO2 +H2). The importance of OCO bond formation in water-gas shift is shown for metals not limited by water dissociation. In addition, the CO binding energy is shown to be an important parameter, as CO can crowd out the free sites which participate in adsorption steps, water dissociation, and carboxyl decomposition. From these results, we propose a catalyst design strategy to combine metals that adsorb O weakly, such as Au clusters or Pt nanoparticles, with supports that exhibit strong enough interactions with oxygen to be capable of easily dissociating water.

Graphical abstract

Abstract

Low temperature water–gas shift (WGS) catalysts, Pd nanoparticles supported on α-MnO₂ nanorods termed Pd/α-MnO₂ and Pt nanoparticles supported on α-MnO₂ nanorods termed Pt/α-MnO₂ were synthesized by introducing Pd or Pt precursor to well-prepared α-MnO₂ nanorods through precipitation deposition with a following annealing at 300 °C. They are quite active for WGS in the temperature range of 140–350 °C. Activation energies for WGS on Pd/α-MnO₂ and Pt/α-MnO₂ are 45.3 and 56.4 kJ/mol respectively, comparable to precious metal supported on CeO₂ and TiO₂ for WGS. Surface chemistries of the two catalysts during WGS were tracked with ambient pressure X-ray photoelectron spectroscopy. Different from the preservation of the surface and bulk phase of other oxide support such as CeO₂, TiO₂ in CeO₂- or TiO₂-based WGS catalysts, both surface and bulk of α-MnO₂ nanorods of Pd/α-MnO₂ and Pt/α-MnO₂ are transited to MnO during WGS. In-situ studies identified oxygen vacancies of the formed MnO support during WGS and the metallic state of Pd and Pt nanoparticles supported on the nonstoichiometric MnO.

Graphical Abstract

Publication date: December 2014
Source:Journal of Catalysis, Volume 320
Author(s): Mira Skaf , Samer Aouad , Sara Hany , Renaud Cousin , Edmond Abi-Aad , Antoine Aboukaïs
Two silver–cerium oxide samples (Ag 10wt.%) were prepared by two different methods: impregnation and deposition–precipitation. The XRD, EPR, XPS, and TPR techniques were used for physicochemical characterization. Catalysts were tested in C3H6, CO, and carbon black oxidation reactions. The impregnated catalyst showed better performance compared to the one prepared by deposition–precipitation in the different reactions. The EPR technique allowed the identification of three different Ag²⁺ sites in the impregnated solid along with the distinction of the Ag²⁺ isotopes (¹⁰⁷Ag²⁺ and ¹⁰⁹Ag²⁺) ions. The impregnated catalyst reduced at lower temperatures compared to the one prepared by deposition–precipitation. The catalytic activity of the impregnated solid was attributed to the presence of Ag²⁺ species that enhance the redox potential of the solid by creating three different redox couples: Ag²⁺/Ag⁺, Ag²⁺/Ag⁰, and Ag⁺/Ag⁰.

Graphical abstract