4 research outputs found
The Influence of Functionals on Density Functional Theory Calculations of the Properties of Reducible Transition Metal Oxide Catalysts
Reducible
transition metal oxides (RTMOs) comprise an important class of catalytic
materials that are used for the selective oxidation and electro- and
photochemical splitting of water, and as supports for metal nanoparticles.
It is, therefore, highly desirable to model the properties of these
materials accurately using density functional theory (DFT) in order
to understand how oxide structure and performance are related and
to guide the search for materials exhibiting superior performance.
Unfortunately, accurate description of the structural and electronic
properties of RTMOs using DFT has proven particularly challenging.
The M06-L density functional, which has been shown to be broadly accurate
for calculations of gas phase clusters, has recently become available
to researchers carrying out calculations in the solid state, but its
performance in determining the properties RTMOs has been little investigated.
The aim of this work was to assess the performance of the M06-L functional
for describing the structural and electronic properties of a family
of RTMOs: MoO<sub>2</sub>, MoO<sub>3</sub>, and Bi<sub>2</sub>Mo<sub>3</sub>O<sub>12</sub>. Lattice constants, band gaps, and densities
of states calculated using the M06-L functional are compared to those
obtained from DFT+U. We have also used the M06-L functional to determine
the reaction barrier for propene activation over Bi<sub>2</sub>Mo<sub>3</sub>O<sub>12</sub>, the rate-limiting step in the oxidation of
propene to acrolein. We find that while DFT calculations carried out
with the M06-L functional are roughly five times more expensive computationally
than those performed with DFT+U, the results obtained using the M06-L
functional provide sensible results for all properties investigated,
while avoiding the necessary trade-off between accurate electronic
structure and accurate thermochemistry that occurs in DFT+U
Kinetic Relevance of Surface Reactions and Lattice Diffusion in the Dynamics of CeâZr Oxides ReductionâOxidation Cycles
Reductionâoxidation cycles in oxides are ubiquitous
in oxygen
storage and transport, chemical looping processes, and fuel cells.
O-atom addition and removal are mediated by coupling reactions of
oxidants and reductants at surfaces with diffusion of O-atoms within
oxide crystals, with either or both processes as limiting steps. CeO2âZrO2 solid solutions (CZO) are ubiquitous
in practice. They are used here to illustrate general experimental
strategies and reactionâdiffusion formalisms for nonideal systems
that enable assessments of the kinetic relevance of the steps that
mediate O-atom addition and removal in these materials; these experiments
are described within the context of models that describe the driving
forces for reaction and diffusion rigorously in terms of oxygen chemical
potentials (ÎŒO). These strategies assess the rate
consequences of varying the fluid phase redox potential, through changes
in the identity and pressures of the reactants and products used in
redox cycles (O2; CO/CO2; H2/H2O; N2O/N2), of introducing dispersed
metal nanoparticles that capture and react lattice O-atoms in CZO
using CO or H2, and of imposing intervening dwells without
reaction within redox cycles. O-removal rates depend on reductant
pressures, even when CO/CO2 and H2/H2O ratios are chosen to maintain the same surface ÎŒO if surface reactions were quasi-equilibrated. These data, taken
together with significant rate enhancements in O-removal when Pt nanoparticles
are present at CZO crystal surfaces and with similar rates before
and after inert dwells, demonstrate that reduction rates by both CO
and H2 are limited by surface reactions without the presence
of consequential spatial gradients in ÎŒO within CZO
crystals. In contrast, O-addition rates to partially reduced CZO crystals
are similar for N2O and O2 reactants and are
not affected by the presence of Pt nanoparticles; O-addition rates
are significantly higher after intervening inert dwells during CZO
oxidation, indicative of spatial gradients in ÎŒO,
which relax during nonreactive periods. These methods and models,
illustrated here for CZO redox cycles at conditions relevant to oxygen
storage practice, allow systematic assessments of the kinetic relevance
of lattice diffusion and surface reactions for systems that use solids
for the reversible storage and release of atoms, irrespective of the
identity of the solids or the atoms (e.g., O, H, N, and S)
Identifying the Unique Properties of 뱉Bi<sub>2</sub>Mo<sub>3</sub>O<sub>12</sub> for the Activation of Propene
In
order to understand the remarkable activity of α-Bi<sub>2</sub>Mo<sub>3</sub>O<sub>12</sub> for selective oxidation and ammoxidation
of propene, the propene activation ability of four molybdenum-based
mixed metal oxidesîžBi<sub>2</sub>Mo<sub>3</sub>O<sub>12</sub>, PbMoO<sub>4</sub>, Bi<sub>2</sub>Pb<sub>5</sub>Mo<sub>8</sub>O<sub>32</sub>, and MoO<sub>3</sub>îžwas investigated using density
functional theory. Propene activation is considered to occur via abstraction
of a hydrogen atom from the methyl group of physisorbed propene by
lattice oxygen. For each material, the apparent activation energy
was estimated by summing the heat of adsorption of propene, the CâH
bond dissociation energy, and the hydrogen attachment energy (HAE)
for hydrogen addition to lattice oxygen; this sum provides a lower
bound for the apparent activation energy. It was found that two structural
features of oxide surfaces are essential to achieve low activation
barriers: under-coordinated surface cation sites enable strong propene
adsorption, and suitable 5- or 6-coordinate geometries at molybdenum
result in favorable HAEs. The impact of molybdenum coordination on
HAE was elucidated by carrying out a molecular orbital analysis using
a cluster model of the molybdate unit. This effort revealed that,
in 5- and 6-coordinate molybdates, oxygen donor atoms <i>trans</i> to molybdenyl oxo atoms destabilize the molybdate prior to H addition
but stabilize the molybdate after H addition, thereby providing an
HAE âŒ15 kcal/mol more favorable than that on 4-coordinate molybdate
oxo atoms. Bi<sup>3+</sup> cations in Bi<sub>2</sub>Mo<sub>3</sub>O<sub>12</sub> thus promote catalytic activity by providing both
strong adsorption sites for propene and forcing molybdate into 5-coordinate
geometries that lead to particularly favorable values of the HAE
Sintering-Resistant Single-Site Nickel Catalyst Supported by MetalâOrganic Framework
Developing supported single-site
catalysts is an important goal
in heterogeneous catalysis since the well-defined active sites afford
opportunities for detailed mechanistic studies, thereby facilitating
the design of improved catalysts. We present herein a method for installing
Ni ions uniformly and precisely on the node of a Zr-based metalâorganic
framework (MOF), NU-1000, in high density and large quantity (denoted
as Ni-AIM) using atomic layer deposition (ALD) in a MOF (AIM). Ni-AIM
is demonstrated to be an efficient gas-phase hydrogenation catalyst
upon activation. The structure of the active sites in Ni-AIM is proposed,
revealing its single-site nature. More importantly, due to the organic
linker used to construct the MOF support, the Ni ions stay isolated
throughout the hydrogenation catalysis, in accord with its long-term
stability. A quantum chemical characterization of the catalyst and
the catalytic process complements the experimental results. With validation
of computational modeling protocols, we further targeted ethylene
oligomerization catalysis by Ni-AIM guided by theoretical prediction.
Given the generality of the AIM methodology, this emerging class of
materials should prove ripe for the discovery of new catalysts for
the transformation of volatile substrates