11 research outputs found
Pathways for H<sub>2</sub> Activation on (Ni)-MoS<sub>2</sub> Catalysts
The activation of H<sub>2</sub> and
H<sub>2</sub>S on (Ni)ĀMoS<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> leads
to the formation of SH
groups with acid character able to protonate 2,6-dimethylpyridine.
The variation in concentrations of SH groups induced by H<sub>2</sub> and H<sub>2</sub>S adsorption shows that both molecules dissociate
on coordinatively unsaturated cations and neighboring S<sup>2ā</sup>. In the studied materials, one sulfur vacancy and four SH groups
per 10 metal atoms exist at the active edges of MoS<sub>2</sub> under
the conditions studied. H<sub>2</sub>āD<sub>2</sub> exchange
studies show that Ni increases the concentration of active surface
hydrogen by up to 30% at the optimum Ni loading, by increasing the
concentration of H<sub>2</sub> and H<sub>2</sub>S chemisorption sites
CarbonāCarbon Bond Scission Pathways in the Deoxygenation of Fatty Acids on Transition-Metal Sulfides
The mechanism of
the deoxygenation of fatty acids on transition-metal
sulfides was determined on the basis of kinetic data obtained with
fatty acids, their reaction intermediates (aldehyde and alcohol),
and reactants of restricted reactivity (adamantanyl-substituted carboxylic
acids). Deoxygenation on MoS<sub>2</sub> proceeds exclusively via
hydrogenolysis to aldehyde, followed by hydrogenation to the corresponding
alcohol, consecutive dehydration to the olefin, and hydrogenation
to the alkane. In contrast, the selectivity on Ni-MoS<sub>2</sub> and
on Ni<sub>3</sub>S<sub>2</sub> substantially shifts toward carbon
oxide elimination routes: i.e., direct production of C<sub><i>n</i>ā1</sub> olefins and alkanes. The carbon losses
occur by decarbonylation of a ketene intermediate, which forms only
on sites associated with Ni. The rate determining steps are the cleavage
of the CāC bond and the removal of oxygen from the surface
below and above, respectively, 2.5 MPa of H<sub>2</sub>. The different
reaction pathways catalyzed by MoS<sub>2</sub> and Ni-MoS<sub>2</sub> are attributed to a preferred deprotonation of a surface acyl intermediate
formed upon the adsorption of the fatty acid on Ni-MoS<sub>2</sub>. The shift in mechanism is concluded to originate from the higher
basicity of sulfur induced by nickel
Mechanistic Pathways for Methylcyclohexane Hydrogenolysis over Supported Ir Catalysts
H/D
isotope effects on methylcyclohexane hydrogenolysis over Ir/Al<sub>2</sub>O<sub>3</sub> catalysts were examined by combining measured
rates with theoretical estimates provided by partition function based
analyses. Normal H/D isotope effects (<i>r</i><sub>H</sub>/<i>r</i><sub>D</sub> > 1) were observed for endocyclic
and exocyclic CāC bond hydrogenolysis. Hydrogenolysis is concluded
to occur via stepwise dehydrogenation followed by cleavage of the
CāC bond and subsequent hydrogenation of the cleaved entities.
The so-called āmultipletā mechanism (i.e., the CāC
bond of a flat-lying physisorbed cyclic molecule is cleaved upon the
attack of a coadsorbed H atom) is unequivocally excluded. For ring-opening,
either CāC bond cleavage or CāHĀ(D) bond reformation
may be rate-determining, due to their indistinguishable isotope effects
under the studied conditions. CāHĀ(D) bond dissociation does
not control the rate of CāC bond hydrogenolysis. For the exocyclic
cleavage of the methyl group, a higher degree of unsaturation of the
surface intermediate and the potential impact of mobile H atoms on
large Ir particles are noted
Deoxygenation of Palmitic Acid on Unsupported Transition-Metal Phosphides
Highly active bulk
transition-metal phosphides (WP, MoP, and Ni<sub>2</sub>P) were synthesized
for the catalytic hydrodeoxygenation of
palmitic acid, hexadecanol, hexadecanal, and microalgae oil. The specific
activities positively correlated with the concentration of exposed
metal sites, although the relative rates changed with temperature
due to activation energies varying from 57 kJ mol<sup>ā1</sup> for MoP to 142 kJ mol<sup>ā1</sup> for WP. The reduction
of the fatty acid to the aldehyde occurs through a LangmuirāHinshelwood
mechanism, where the rate-determining step is the addition of the
second H to the hydrocarbon. On WP, the conversion of palmitic acid
proceeds via R-CH<sub>2</sub>COOH ā R-CH<sub>2</sub>CHO ā
R-CH<sub>2</sub>CH<sub>2</sub>OH ā R-CHCH<sub>2</sub> ā
R-CH<sub>2</sub>CH<sub>3</sub> (hydrodeoxygenation). Decarbonylation
of the intermediate aldehyde (R-CH<sub>2</sub>COOH ā R-CH<sub>2</sub>CHO ā R-CH<sub>3</sub>) was an important pathway on
MoP and Ni<sub>2</sub>P. Conversion via dehydration to a ketene, followed
by its decarbonylation, occurred only on Ni<sub>2</sub>P. The rates
of alcohol dehydration (R-CH<sub>2</sub>CH<sub>2</sub>OH ā
R-CHCH<sub>2</sub>) correlate with the concentrations of Lewis acid
sites of the phosphides
Overcoming the Rate-Limiting Reaction during Photoreforming of Sugar Aldoses for H<sub>2</sub>āGeneration
Photoreforming of
sugars on metalloaded semiconductors is an attractive
process for H<sub>2</sub>-generation. However, the reaction proceeds
typically with rapidly decreasing rates. We identified that this decrease
is due to kinetic constraints rather than to catalyst deactivation.
Thus, the nature of the rate-limiting reaction was elucidated by investigation
of the reaction pathways and oxidation mechanisms during photoreforming
of sugar aldoses on TiO<sub>2</sub> decorated with Rh, Pd, or Pt.
Using selective isotope labeling it is shown that ring opening of
aldoses via direct hole transfer to the chemisorbed oxygenates yields
primary formate esters. Under pH-neutral and acidic conditions, formates
convert to the consecutive aldose intermediate through light-driven,
redox-neutral hydrolysis. The slow kinetics of this step, which requires
interaction with negative and positive photogenerated charges, leads
to blocking of active sites at the photoanode and enhanced electronāhole
recombination. Stable H<sub>2</sub>-evolution and sugar conversion
over time is achieved through alkalinization of the aqueous-phase
due to fast OH<sup>ā</sup>-induced hydrolytic cleavage of formate
intermediates
Effects of the Support on the Performance and Promotion of (Ni)MoS<sub>2</sub> Catalysts for Simultaneous Hydrodenitrogenation and Hydrodesulfurization
MoS<sub>2</sub> and Ni-promoted MoS<sub>2</sub> catalysts supported
on Ī³-Al<sub>2</sub>O<sub>3</sub>, siliceous SBA-15, and Zr-
and Ti-modified SBA-15 were explored for the simultaneous hydrodesulfurization
(HDS) of dibenzothiophene (DBT) and hydrodenitrogenation (HDN) of <i>o</i>-propylaniline (OPA). In all cases, OPA reacted preferentially
via initial hydrogenation, and DBT was converted through direct sulfur
removal. HDN and HDS activities of MoS<sub>2</sub> catalysts are determined
by the dispersion of the sulfide phase. Ni promotion increased its
dispersion and activity for DBT HDS and also increased the rate of
HDN via enhancing the rate of hydrogenation. On nonpromoted MoS<sub>2</sub> catalysts, HDS was strongly inhibited by NH<sub>3</sub>,
and the addition of Ni dramatically reduced this inhibiting effect.
The conclusion is that HDS is proportional to the concentration of
Mo and Ni on the edges of sulfide particles. In contrast, the direct
hydrodenitrogenation of OPA occurs only on accessible Mo cations and,
hence, decreases with increasing Ni substitution. The nature of the
support influences the dispersion of the nonpromoted catalysts as
well as the decoration degree of Ni on the edges of the NiāMoāS
phase. Furthermore, the acidity of the support influences the acidity
of the supported sulfide phase, which may play an important role in
HDN
Kinetic Coupling of Water Splitting and Photoreforming on SrTiO<sub>3</sub>āBased Photocatalysts
Coupling
the proton reduction of overall water splitting with oxidation
of oxygenated hydrocarbons (photoreforming) on Al-doped SrTiO<sub>3</sub> decorated with cocatalysts enables efficient photocatalytic
H<sub>2</sub> generation along with oxygenate conversion, while decreasing
the accumulation of harmful byproducts such as formaldehyde. Net H<sub>2</sub> evolution rates result from the contributions of the individual
rates of water oxidation, oxygenate oxidation, and the back-reaction
of H<sub>2</sub> and O<sub>2</sub> to water. The latter reaction is
suppressed by a RhCrO<sub><i>x</i></sub> cocatalyst or by
high concentrations of oxygenates in the case of Rh cocatalyst, whereas
the rates of organic oxidation depend on their molecular structure.
In the absence of the back-reaction to water, the H<sub>2</sub> evolution
rates are independent of the oxygenate type and concentration because
the rates of water splitting compensate the variations in the rates
of oxygenate conversion. Under such conditions of suppressed back-reaction,
the selectivities to water and oxygenate oxidation, both occurring
with the same quantum efficiencies, depend on the oxygenate type and
concentration. The dominant pathways for organic transformations are
ascribed to the action of intermediates generated at the semiconductor
during water oxidation and O<sub>2</sub> evolution. On a semiconductor
without cocatalyst, the O<sub>2</sub> produced during overall water
splitting is reductively activated to participate in oxidation of
organics without consuming evolved H<sub>2</sub>
Enabling Overall Water Splitting on Photocatalysts by CO-Covered Noble Metal Co-catalysts
Photocatalytic
overall water splitting requires co-catalysts that
efficiently promote the generation of H<sub>2</sub> but do not catalyze
its reverse oxidation. We demonstrate that CO chemisorbed on metal
co-catalysts (Rh, Pt, Pd) suppresses the back reaction while maintaining
the rate of H<sub>2</sub> evolution. On Rh/GaN:ZnO, the highest H<sub>2</sub> production rates were obtained with 4ā40 mbar of CO,
the back reaction remaining suppressed below 7 mbar of O<sub>2</sub>. The O<sub>2</sub> and H<sub>2</sub> evolution rates compete with
CO oxidation and the back reaction. The rates of all reactions increased
with increasing photon absorption. However, due to different dependencies
on the rate of charge carrier generation, the selectivities for O<sub>2</sub> and H<sub>2</sub> formation increased in comparison to CO
oxidation and the back reaction with increasing photon flux and/or
quantum efficiency. Under optimum conditions, the impact of CO to
prevent the back reaction is identical to that of a Cr<sub>2</sub>O<sub>3</sub> layer covering the active metal particle
Influence of the Molecular Structure on the Electrocatalytic Hydrogenation of Carbonyl Groups and H<sub>2</sub> Evolution on Pd
We investigated the electrocatalytic hydrogenation (ECH)
of model
aldehydes and ketones over carbon-supported Pd in the aqueous phase.
We propose reaction mechanisms based on kinetic measurements and on
spectroscopic and electrochemical characterization of the working
catalyst. The reaction rates of ECH and of the H2 evolution
reaction (HER) vary with the applied electric potential following
trends that strongly depend on the organic substrate. The intrinsic
rates of hydrogenation and H2 evolution are influenced,
in opposing ways, by the sorption of the reacting organic substrate.
Strong interactions, that is, higher standard free energies of adsorption
of the organic compound, induce high hydrogenation rates. The fast
hydrogenation kinetics produces a hydrogen-depleted environment that
kinetically hinders the HER and the bulk phase transition of Pd to
a H-rich bulk Pd hydride, which is triggered by the applied potential
in the absence of reacting organic compounds. As a consequence of
strong organicāmetal interactions, hydrogenation dominates
at low overpotential. However, the coverages of organic substrates
on the metal surface decrease, and the rates of H2 evolution
surpass those of hydrogenation with increasingly negative electric
potential. We determined the range of electric potential favoring
hydrogenation on Pd and quantitatively deconvoluted the effects of
the sorption of the organic compound, and of the rates of proton-coupled
electron transfers, on the kinetics of both ECH and HER. The results
indicate that electrocatalysis offers hydrogenation pathways for polar
molecules which are different and, in some cases, faster than those
dominating in the absence of an external electric potential
Impact of the Environment of BEA-Type Zeolites for Sorption of Water and Cyclohexanol
The (mutual) interactions of water and cyclohexanol with
the pore
walls and functional groups of BrĆønsted acidic zeolites of the
BEA type (H-BEA) have been investigated. Upon reaction with BrĆønsted
acid sites, water forms hydrated hydroxonium ions limited in size
by the sorption free energy, creating in this way domains occupied
by water. Organic molecules, such as cyclohexanol, occupy the remaining
unoccupied volume. The pore size of the zeolite H-BEA stabilizes hydrated
hydroxonium ions (H+(H2O)10) that
are two H2O molecules larger than those formed in the smaller
pore zeolite H-ZSM-5. Increasing the density of hydroxonium ions by
increasing the concentration of aluminum in the zeolite gradually
leads to less negative standard free energy of adsorbed cyclohexanol.
The increasing proximity of positive charges of the hydroxonium ions
induces a higher excess chemical potential of the sorbed molecule,
which is manifested in a weakened interaction strength with the zeolite
pores