Design, Synthesis, and Properties of Chemical Vapor Deposited Diamond on Inorganic Oxide Catalysts: Experimentation and Simulation

The effects of a porous diamond coating on the physical and chemical properties of catalytic inorganic oxides have been studied with experimentation and simulation. A layer of diamond 20–50 µm thick has been grown on pellets of silica alumina, using voltage-biased, hot-filament chemical vapor deposition. The morphology and thickness of the coating is examined by scanning electron microscopy. Nitrogen and argon porosimetry indicate that the surface area, pore volume, and pore-size distribution of the diamond-coated silica alumina are virtually identical to those of the native oxide (surface area = 73 m²/g, pore volume = 0.38 cm³/g r̄ = 34 Å). Dehydration of methanol over the diamond-coated and uncoated silica alumina pellets leads to experimentally equivalent levels of conversion over the full range of temperatures tested (∼20% at 175 °C to ∼80% at 475 °C). Despite these physical similarities at the pellet level, thermal conductivities were measurably enhanced by the diamond coating. This effect is clear from the ratio of the thermal conductivities of the coated and uncoated pellets (50-µm film λ_ctdλ_uc = 2.4, 20-µm film λ_ctd/λ_uc = 1.6). Simulation was used to consider the extent of the effect that a porous diamond coating could have on heat transport through a packed bed. Oxidation of o-xylene was simulated, for diamond-coated and uncoated vanadium pentoxide catalysts, using the two-dimensional, heterogeneous model of DeWasch and Froment and with thermal conductivities based on our own experimental results with diamond on silica alumina. The results of the simulation indicate that, for a fixed bed of diamond-coated particles, temperature gradients could be reduced and selectivity increased versus those obtained with uncoated particles.