Hydrodesulfurization over model sulfide cluster-derived catalysts

There is growing interest in the use of organometallic clusters as precursors to model heterogeneous catalysts. This interest is partially derived from the expectation that "solid-state metal complex catalysts" will be uniform in composition and structure thus more amenable to characterization than catalysts prepared using conventional techniques. This research used a series of sulfide clusters as precursors to model heterogeneous catalysts in an effort to enhance our understanding of the active sites in hydro-desulfurization catalysts. Alumina supported catalysts derived from the sulfide clusters Cp2Mo2([mu]-SH)2([mu]-S)2, Cp2Mo2Co2([mu]3-S)2([mu]4-S)(CO)4, and Cp2Mo2Fe2([mu]3-S)2(CO)8 (Cp=cyclopentadienyl) shared important catalytic and spectroscopic features with a commercial catalyst and unsupported MoS2. These strong similarities implied that the active sites in the sulfide cluster-derived catalysts could be used as models of the sites in conventionally prepared hydrodesulfurization catalysts. The active sites in the bimetallic sulfide cluster-derived catalysts appeared to be highly dispersed ensembles containing Mo[delta]+ ([delta]2 and C3 hydrocarbons while the commercial catalyst produced mostly C4 hydrocarbons. These results suggested that C-C bond hydrogenolysis preceded C-S bond cleavage over the sulfide cluster-derived catalysts. More subtle differences between the product distributions of the cluster-derived catalysts have been attributed to differences in the nature of sulfide ligands in their precursors.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29586/1/0000675.pd

Characterization of HDS/HDN active sites in cluster-derived and conventionally-prepared sulfide catalysts

The catalytic and sorptive properties of a series of sulfide cluster-derived catalysts were compared to those of an analogous series of conventionally-prepared sulfides. The cluster-derived and conventionally-prepared catalysts had similar hydrotreatment activities and chemisorbed similar amounts of O2 and NO. From infrared spectroscopy, it was observed that the clusters retained much of their character upon adsorption, and that the CO ligands were lost upon heating up to 350 K. Nitric oxide chemisorbed onto the bimetallic cluster-derived catalysts was initially associated with the Co sites then shifted to the Mo sites upon heating, indicating that the promoter and Mo were in close proximity. For the conventionally-prepared materials, NO adsorption suggested that the promoted and unpromoted sites behaved independently. Nitric oxide was more strongly held to the unpromoted sites in both the cluster-derived and conventionally-prepared materials. Thiophene adsorbed onto the cluster-derived and conventionally-prepared catalysts gave rise to nearly identical infrared spectra. Upon heating, the thiophene peaks disappeared and were replaced by peaks that we have attributed to a linear olefin, which is noteworthy since 1-butene was the primary product from thiophene HDS. Nitric oxide easily displaced adsorbed thiophene from the cluster-derived materials, indicating that NO and thiophene adsorbed onto the same sites.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31379/1/0000292.pd

Characterization of Structural and Electronic Transitions During Reduction and Oxidation of Ru(acac)3 Flow Battery Electrolytes by using X‐ray Absorption Spectroscopy

Metal acetylacetonates possess several very attractive electrochemical properties; however, their cyclabilities fall short of targets for use in nonaqueous redox flow batteries. This paper describes structural and compositional changes during the reduction and oxidation of ruthenium(III) acetylacetonate [Ru(acac)3], a representative acetylacetonate. Voltammetry, bulk electrolysis, and in situ X‐ray absorption spectroscopy (XAS) results are complemented by those from density functional theory (DFT) calculations. The reduction of Ru(acac)3 in acetonitrile is highly reversible, producing a couple at −1.1 V versus Ag/Ag+. In situ XAS and DFT indicate the formation of [Ru(acac)3]− with Ru−O bonds lengthened relative to Ru(acac)3, nearly all of the charge localized on Ru, and no ligand shedding. The oxidation of Ru(acac)3 is quasireversible, with a couple at 0.7 V. The initial product is likely [Ru(acac)3]+; however, this species is short‐lived, converting to a product with a couple at −0.2 V, a structure that is nearly identical to that of Ru(acac)3 within 3 Å of Ru, and approximately 70 % of the charge extracted from Ru (balance from acetylacetone). This non‐innocence likely contributes to the instability of [Ru(acac)3]+. Taken together, the results suggest that the stabilities and cyclabilities of acetylacetonates are determined by the degree of charge transfer to/from the metal.Track changes: The structural and electronic changes of Ru(acac)3 during oxidation and reduction are characterized using bulk electrolysis and in situ X‐ray absorption spectroscopy. Reduction is found to be reversible with minimal structural changes, and the electrons being stored entirely on the ruthenium. Oxidation results in a rapid side reaction as a result of electrons extracted from the ligand.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134821/1/celc201600360-sup-0001-misc_information.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134821/2/celc201600360_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134821/3/celc201600360.pd

The structure and function of supported molybdenum nitride hydrodenitrogenation catalysts

A series of [gamma]-Al2O3 supported Mo nitrides were prepared by the temperature programmed reaction of supported molybdates with NH3. Several loadings, heating rates, and space velocities were employed in an attempt to vary the properties of the nitride. The structure and composition of the supported nitrides depended on the structure of the precursor oxide and the conditions employed in nitriding the oxide. The ease with which the oxide was nitrided improved with increasing Mo loading. Nitride domains in the low-loaded materials (4 and 8 wt% Mo) were highly dispersed and X-ray amorphous while the higher-loaded catalysts (16 wt% Mo) consisted of [gamma]-Mo2N crystallites. The response of the high-loaded materials to the various nitriding conditions was similar to that observed for unsupported [gamma]-Mo2N. The supported Mo nitrides were active for the hydrodenitrogenation of pyridine. In fact, their activities were superior to that of a commercial sulfided Ni---Mo hydrotreating catalyst and comparable to those of the unsupported Mo nitrides. The catalytic properties of the supported Mo nitrides depended on the size and composition of the Mo nitride domains. We believe that the most active sites were located at the perimeters of two-dimensional, raft-like domains. Regions near the perimeter also appeared to be nitrogen deficient. Lower activity sites were associated with the [gamma]-Mo2N crystallite surfaces.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31902/1/0000855.pd

Selective Hydrogenation of Furfural in a Proton Exchange Membrane Reactor Using Hybrid Pd/Pd Black on Alumina

Invited for this month’s cover picture are the groups of Dr. Peter Pintauro (Vanderbilt University, Tennessee, USA), Dr. Levi Thompson (University of Delaware, Delaware, USA), and Dr. William Tarpeh (Stanford University, California, USA). The cover picture shows the controlled variation of furfural hydrogenation product speciation based on varying cathode formulations of hybrid Pd black and Pd on alumina support. Read the full text of the article at 10.1002/celc.201901314.“The performance of different cathode compositions is evaluated at different current densities (which varies with hydrogen production) in terms of production rate, faradaic efficiency, and selectivity. To isolate the influences of the electrocatalyst in the hybrid catalyst, the performance of electrocatalyst Pd black is evaluated separately. These four variations of the hybrid cathode are investigated to test the hypothesis that the addition of the metal loaded on metal oxide to the electrocatalyst enhances the production rate for hydrogenated products compared to electrodes with only an electrocatalyst…“ Learn more about the story behind the research featured on the front cover in this issue’s Cover Profile. Read the corresponding Article at 10.1002/celc.201901314.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/152714/1/celc201901737.pd

Carbon-hydrogen bond activation over tungsten carbide catalysts

Tungsten carbides with surface areas ranging from 4 to 39 m2/g were prepared by the temperature-programmed carburization of tungsten oxide and nitride powders with pure CH4 or a 48.9% CH4 in H2 mixture. Oxygen uptakes on the carbides were low most likely because of the presence of graphitic carbon at the surface. Nevertheless, the carbides were active and selective for the dehydrogenation of butane at temperatures between 623 and 723 K and atmospheric pressure with and without H2 in the reactant feed. With H2 in the feed, the higher surface area carbides ([ges] 36 m2/g) were as active as a Pt-Sn/[gamma]-Al2O3 catalyst, but their selectivities were different. Without H2 in the feed, the selectivities of the carbides were similar to those of the Pt-Sn/[gamma]-Al2O3 catalyst, but their reaction rates were 1-3 orders of magnitude lower. The high surface area materials were also active for the hydrogenolysis of butane. Because the catalytic properties of the carbides varied with the average particle size, we concluded that butane dehydrogenation was structure-sensitive over these materials, and suspected that this behavior was due to variations in the surface stoichiometry as well as the particle faceting.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31381/1/0000294.pd

The use of sulfide cluster-derived catalysts to understand the promotional effect in hydrotreatment catalysis

Sulfide cluster-derived ensembles are promising models of the active sites in commercial hydrotreatment catalysts. A series of sulfide clusters were adsorbed intact onto high-surface-area Γ-alumina, magnesium oxide and activated carbon supports, then pretreated to produce highly dispersed catalytic ensembles with sizes similar to those of their precursor clusters. The activities of the bimetallic cluster-derived catalysts were significantly higher than those of the monometallic catalysts. We took this as evidence that direct interactions between molybdenum and the promoter element cause the promotional effect observed in commercial hydrotreatment catalysts. The hydrodesulfurization and hydrodenitrogenation activities correlated with the extent of molybdenum reduction. Our results suggested that the active sites in promoted hydrotreatment catalysts are centered on molecular-scale ensembles containing molybdenum, sulfur and the promoter element.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/38298/1/590060507_ftp.pd

Feasibility of a Supporting‐Salt‐Free Nonaqueous Redox Flow Battery Utilizing Ionic Active Materials

Nonaqueous redox flow batteries (NAqRFBs) are promising devices for grid‐scale energy storage, but high projected prices could limit commercial prospects. One route to reduced prices is to minimize or eliminate the expensive supporting salts typically employed in NAqRFBs. Herein, the feasibility of a flow cell operating in the absence of supporting salt by utilizing ionic active species is demonstrated. These ionic species have high conductivities in acetonitrile (12–19 mS cm−1) and cycle at 20 mA cm−2 with energy efficiencies (>75 %) comparable to those of state‐of‐the‐art NAqRFBs employing high concentrations of supporting salt. A chemistry‐agnostic techno‐economic analysis highlights the possible cost savings of minimizing salt content in a NAqRFB. This work offers the first demonstration of a NAqRFB operating without supporting salt. The associated design principles can guide the development of future active species and could make NAqRFBs competitive with their aqueous counterparts.Salt‐free cell: Decreasing the contribution of salt costs to the total electrolyte cost for nonaqueous redox flow batteries is essential for economic viability. A nonaqueous flow battery utilizing ionic active materials completely removes the need for a supporting salt. The cell cycling performance and area‐specific specific resistance are comparable to those of state‐of‐the‐art nonaqueous flow cells with high salt concentrations.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137469/1/cssc201700028-sup-0001-misc_information.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137469/2/cssc201700028.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137469/3/cssc201700028_am.pd

Temperature-programmed desorption of H2 from molybdenum nitride thin films

The thermal desorption of hydrogen from [beta]-Mo16N7, [gamma]-Mo2N and [delta]-MoN thin films was investigated. Hydrogen adsorption at [approximate]270 K produced two hydrogen desorption peaks, one at [approximate]370 K (designated as the [beta]1 site) and the other at [approximate]800 K (designated as the [beta]2 site). We concluded that the [beta]1 peak was due to a low-energy surface site and the [beta]2 peak to a subsurface site. Desorption from the [beta]1 state obeyed first-order kinetics. The hydrogen saturation capacity increased in the following order: [beta]-Mo16N72N. This variation in saturation coverage tracked with the nominal molybdenum atom surface density. Nitrogen also desorbed from the Mo nitride surfaces during the H2 temperature-programmed desorption experiments producing a low-temperature peak at [approximate]370 K and several high-temperature peaks in the range 500-900 K. The amount of nitrogen that desorbed increased with increasing H2 dose, suggesting a hydrogen-induced nitrogen desorption process. Hydrogen may have weakened the Mo-N bond thereby facilitating the desorption of nitrogen. We believe that the low-temperature peak was due to nitrogen bound to Mo atoms at the surface. The high-temperature peaks were probably a consequence of hydrogen that diffused into the surface altering the Mo-N bonding in the subsurface region. Thermal desorption characteristics of the Mo nitride films were similar to those of a series of bulk [gamma]-Mo2N powders that we previously investigated. Apparent energies for hydrogen desorption from the low-temperature sites were near 25 kcal/mol, and both the thin films and powders possessed high-temperature subsurface binding sites. The most striking observation was the similarity between the characters of the [delta]-MoN film and the Mo nitride powder with the highest hydrodenitrogenation activity. This similarity suggested that the high activity was due to the presence of [delta]-MoN-like structures at or near the bulk powder surface.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31475/1/0000397.pd

Molybdenum nitride catalysts : I. Influence of the synthesis factors on structural properties

Effects of the synthesis parameters on the structural properties of molybdenum nitride catalysts, prepared by the temperature-programmed reaction of MoO3 with NH3, have been examined. Molybdenum trioxide was heated in flowing NH3 through two linear heating segments (623 to 723 K then 723 to 973 K) with different space velocities in a 23 factorial design. The temperature limits for these heating segments were defined based on the results of in situ X-ray diffraction analysis of the gas-solid reaction. The resulting catalysts were characterized using BET surface area analysis, environmental scanning electron microscopy, ex situ X-ray diffraction, and oxygen chemisorption. The primary bulk phase present was [gamma]-Mo2N. Some of the lower surface area catalysts also contained MoO2 and Mo, but there was no evidence of nitrides other than [gamma]-Mo2N. The catalysts consisted of micrometersized, plate-like aggregates of nanometer-sized crystallites, and possessed surface areas ranging up to [approximate]140 m2/g depending on the synthesis and reduction conditions employed. Statistical analysis of the results revealed that the space velocity individually and the heating rates combined had the most significant effects on the structural properties. The production of catalysts with surface areas in excess of 50 m2/g required the use of slow heating rates during the first segment and high space velocities. We concluded that the key to producing the highest surface area Mo nitrides was channeling the reaction through HxMoO3 (x 2OyN1-y intermediates. Passivation of the materials immediately following synthesis appeared to produce an oxynitride at the surface. Reduction of the passivated materials in H2 at temperatures up to 673 K caused a significant increase in the surface area and O2 uptake. The O2 uptake for the low and medium surface area catalysts varied linearly with the BET surface area and corresponded to an O:Mo stoichiometry of approximately 1:5. The oxygen site density for the highest surface area nitride was lower than those for the lower surface area catalysts, presumably due to differing surface structures.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31907/1/0000860.pd