The hydrogenation of alkenes by heterogeneous
catalysts has been
studied for 80 years. The foundational mechanism was proposed by Horiuti
and Polanyi in 1934 and consists of three steps: (i) alkene adsorption
on the surface of the hydrogenated metal catalyst, (ii) hydrogen migration
to the β-carbon of the alkene with formation of a σ-bond
between the metal and α-C, and finally (iii) reductive elimination
of the free alkane. Hundreds of papers have appeared on the topic,
along with a number of variations on the Horiuti–Polanyi mechanism.
The second step is highly reversible, leading to extensive deuterium–hydrogen
exchange when D2(g) is used. This paper describes the investigation
of gas-phase reactions between deuterium and 1-butene using a supported
palladium catalyst under ambient laboratory conditions and how the
results are consistent with the Horiuti–Polanyi mechanism.
An Excel spreadsheet for modeling the extent and distribution of deuteration
within butane-d
x
is described.
Interested readers could develop a laboratory or research experience
based on results presented here. The results are also suitable for
inclusion in an upper-division chemistry course in which organometallic
chemistry or reaction mechanisms involving heterogeneous catalysts
are discussed. The catalyst tubes are inexpensive and easy to construct.
Analysis of the butane produced by 1H NMR and GC–MS
leads to numerous conclusions in support of the Horiuti–Polanyi
mechanism.
Crystalline manganese
oxides have attracted the most attention
in aqueous zinc-ion batteries due to their diverse nanostructures
and low cost. However, extensive studies on amorphous manganese oxides
are lacking. Herein, we report a mesoporous amorphous manganese oxide
(UCT-1-250) as a cathode material with high capacity (222 mAh g–1), good cyclability (57% capacity retention after
200 cycles), and an acceptable discharge plateau (between 1.2 and
1.4 V). An approach to mechanistic studies was performed by comparison
of UCT-1-250 and other crystalline manganese oxides through electrochemical,
elemental, and structural analyses. An in situ conversion to ZnMn2O4 spinel phase after initial cycling contributes
to the high performance. The irreversible capacity fading is due to
the formation of the woodruffite phase.
Propane and propene oxidations on M1 phase MoVTeNb mixed oxide catalysts exhibit relatively high selectivity to acrolein and acrylic acid. We probe the ability of the reactant molecules to access the catalytic sites inside the heptagonal pores of these oxides and analyze elementary steps that limit selectivity. Measured propane/cyclohexane activation rate ratios on MoVTeNbO are nearly an order of magnitude higher than non‐microporous VOx/SiO2, which suggests significant contribution of M1 phase pores to propane activation because both molecules react via homologous rate‐limiting C−H activation. Density functional theory suggests that desired C3H8 dehydrogenation and C3H6 allylic oxidation to acrolein and acrylic acid are limited by C−H activation steps, while less valuable oxygenates form via steps limited by C−O bond formation. Calculated activation barriers for C−O formation are invariably higher than C−H activation when these activations occur inside the pores, suggesting that reactions restricted within the pores are highly selective to desired products. These results demonstrate the role of pore confinement and a framework to assess selectivity limitation in hydrocarbon oxidations involving a complex network of sequential and parallel steps.
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