Engineering topics

Gonçalves, Renato V.

Publications and source records attributed to Gonçalves, Renato V..

3 records

DOE OSTI2021

Zeolitic-Imidazolate Framework Derived Intermetallic Nickel Zinc Carbide Material as a Selective Catalyst for CO 2 to CO Reduction at High Pressure

The conversion of CO 2 into CO is an important step in CO 2 utilization to achieve clean fuels and value-added chemicals. Herein, we explored the pyrolysis of zeolitic imidazolate framework-8 (ZIF-8) loaded with different amounts of Ni 2+ to obtain Ni-Zn carbide (Ni 3 ZnC) embedded in N-doped carbon. Ni is present in the intermetallic compound, while Zn excess remains on the N-doped carbon. The Ni 3 ZnC phase catalyzes the selective hydrogenation of CO 2 into CO via the reverse water gas shift reaction, reaching 100 % CO selectivity at ~30 % CO 2 conversion at 450 °C and atmosphere pressure (CO 2 :H 2 =1:4, GHSV=30000 mL g cat -1 h -1 ). The methanation reaction of CO 2 /CO, which is usually favored over Ni catalysts, is suppressed. The selectivity to CO at the expense of CH 4 is related to the stability of chemisorbed CO in the Ni 3 ZnC surface, which is lower compared to Ni surfaces. The Ni 3 ZnC@NC catalyst is selective towards CO over a wide range of conditions, including high pressure, that is usually required for the conversion of CO to hydrocarbons and alcohols via the Fisher-Tropsch synthesis (FTS) process. Contrarily, a classical Ni/SiO 2 catalyst prepared by impregnation produces CH 4 under high pressure.

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DOE OSTI2021

Optimizing Active Sites for High CO Selectivity during CO 2 Hydrogenation over Supported Nickel Catalysts

The undesired formation of CH 4 during the hydrogenation of CO 2 remains a great challenge with direct impact on selectivity towards CO or CH 3 OH. In this study, the selectivity of a supported Ni catalyst prepared by traditional impregnation method was found to change after a first CO 2 hydrogenation reaction cycle from 100 to 800 °C. The usually high CH 4 formation was suppressed leading to full selectivity towards CO. This behavior was also observed after the catalyst was treated under methane or propane atmospheres at elevated temperatures. In-situ spectroscopic studies revealed that the accumulation of carbon species on the catalyst surface at high temperature leads to a nickel carbide-like phase. The catalyst regains its high selectivity to CH 4 production after carbon depletion from the surface of the Ni particles by oxidation. However, the selectivity readily shifts back towards CO formation after a new temperature programmed CO 2 hydrogenation cycle. The fraction of weakly adsorbed CO species increases on the carbide-like surface when compared to a clean nickel surface, explaining the higher selectivity towards CO formation. This easy protocol of changing the surface of a common Ni catalyst to gain selectivity represents an important step for the commercial use of CO 2 hydrogenation to CO process together with Fischer-Tropsch applications.

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DOE OSTI2020

Structure and activity of supported bimetallic NiPd nanoparticles: influence of preparation method on CO 2 reduction

Here, bimetallic Ni-Pd and monometallic reference catalysts were prepared by decomposing organometallic precursors, Ni(cod) 2 and Pd 2 (dba) 3 , leading to nanoparticles with sizes ranging from 3 to 6 nm. Two different synthesis procedures were followed: i) solution synthesis using capping ligand (hexadecylamine) followed by impregnation of pre-formed nanoparticles on SiO 2 , called Sol-immobilization (SI); and 2) direct precursor decomposition onto SiO 2 , without stabilizer, called Direct Decomposition (DD). Samples prepared by SI procedure are alloyed bimetallic nanoparticles, whereas samples obtained by DD one show phase segregation. Interestingly, DD samples show better activity for CO 2 hydrogenation into CO (reverse water-gas shift reaction - RWGS) than SI ones. The best compromise between activity for CO 2 activation (at lower temperature) and CO selectivity was achieved with Ni DD and NiPd DD catalysts. Moreover, the addition of palladium increased the concentration of surface undercoordinated sites, which chemisorb CO weakly, thus improving activity and selectivity, in opposition to other samples that chemisorb CO strongly, in multiband configuration. In the presence of Pd, different decomposition rates drive the formation of smaller and more active Ni clusters. The knowledge acquired here on the influence of synthesis conditions on the catalytic properties of Ni-Pd catalysts should guide us to better catalysts for CO 2 transformations into valuable products.

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