In this perspective, we argue that transition metal carbides such as molybdenum and tungsten hold great potential for the catalytic conversions of future feedstocks due to their ability to remain active in the presence of impurities in the feedstock.
The influence of the support‐oxygen groups and Pt particle size on the catalytic performance of Pt/AC for the aerobic oxidation of α‐D‐glucose to gluconic acid (glycolate) was studied. Surface‐oxygen groups were introduced by treating the activated carbon support with diluted HNO3 without significantly affecting the support porosity. The platinum particle size could be decreased on both the treated and untreated support by adding an additional calcination step to the synthesis. The presence of oxygen‐containing groups is shown to be highly beneficial (∼4 fold increase in the turnover frequency) only for the smallest Pt particle size (1.8–2.5 nm, determined by TEM). For the catalyst with the larger Pt size (3.4–3.6 nm), the presence of additional oxygen‐contacting groups does not significantly enhance the activity. Since the size of the smaller Pt particles is close to the product/substrate molecular diameter (glucose/gluconic acid, ∼0.9 nm) the observed effect can be attributed to the effective repulsion by the negatively charged oxygen groups in close proximity to the glycolate reaction product. The increase in activity originates from the resulting enhanced desorption of glycolate by alleviating the product inhibition presence due to the strong interaction of glycolate with Pt.
Solid sorbents are essential for developing technologies
that directly
capture CO2 from air. In solid sorbents, metal oxides and/or
alkali metal carbonates such as potassium carbonate (K2CO3) are promising active components owing to their high
thermal stability, low cost, and ability to chemisorb the CO2 present at low concentrations in air. However, this chemisorption
process is likely limited by internal diffusion of CO2 into
the bulk of K2CO3. Therefore, the size of the
K2CO3 particles is expected to be an important
factor in determining the kinetics of the sorption process during
CO2 capture. To date, the effects of particle size on supported
K2CO3 sorbents are unknown mainly because particle
sizes cannot be unambiguously determined. Here, we show that by using
a series of techniques, the size of supported K2CO3 particles can be established. We prepared size-tuned carbon-supported
K2CO3 particles by tuning the K2CO3 loading. We further used melting point depression of K2CO3 particles to collectively estimate the average
K2CO3 particle sizes. Using these obtained average
particle sizes, we show that the particle size critically affects
the efficiency of the sorbent in CO2 capture from air and
directly affects the kinetics of CO2 sorption as well as
the energy input needed for the desorption step. By evaluating the
mechanisms involved in the diffusion of CO2 and H2O into K2CO3 particles, we relate the microscopic
characteristics of sorbents to their macroscopic performance, which
is of interest for industrial-scale CO2 capture from air.
The potential of carbon supported Mo and W carbides to replace Pt is shown for the hydrogenation of cinnamaldehyde. Although the carbide catalysts are 4-6 times less active, both the...
For mixed MoW carbide catalysts, the relationship between synthesis conditions, evolution of (mixed) phases, extent of mixing, and catalytic performance of supported Mo/W carbides remains unclear. In this study, we prepared a series of carbon nanofibersupported mixed Mo/W-carbide catalysts with varying Mo and W compositions using either temperature-programmed reduction (TPR) or carbothermal reduction (CR). Regardless of the synthesis method, all bimetallic catalysts (Mo:W bulk ratios of 1:3, 1:1, and 3:1) were mixed at the nanoscale, although the Mo/W ratio in individual nanoparticles varied from the expected bulk values. Moreover, the crystal structures of the produced phases and nanoparticle sizes differed depending on the synthesis method. When using the TPR method, a cubic carbide (MeC 1−x ) phase with 3−4 nm nanoparticles was obtained, while a hexagonal phase (Me 2 C) with 4−5 nm nanoparticles was found when using the CR method. The TPR-synthesized carbides exhibited higher activity for the hydrodeoxygenation of fatty acids, tentatively attributed to a combination of crystal structure and particle size.
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