The heats of adsorption of hydrogen on Ni‐SiO<sub>2</sub> catalysts

Under the conditions of decomposition of formic acid on a nickel‐silica catalyst a substantial amount of formate is present on the metal surface. This was demonstrated by infrared spectroscopy and can also be derived from calorimetrically determined heats of adsorption. Mass‐spectrometric analysis of the gas remaining after adsorption of formic acid vapour at room temperature showed that a monolayer of the approximate composition Ni(OOCH)2 is formed. The role of the formate as an intermediate in the catalytic reaction was definitely confirmed by comparing infrared spectrokinetic data on the separate adsorption and desorption reactions with kinetic data on the overall catalytic reaction. Rate and activation energy of the desorptionreaction from a fully covered surface are found to be equal to those of the catalytic reaction. The. formation of intermediate formate implies removal of hydrogen from the HCOOH molecule in two separate steps. This is in agreement with the complete absence of CH4 from the primary products of thecatalytic decomposition of acetic acid and methyl formate. The reaction products formed in the decomposition of HCOOH via the formate ion present a strong analogy with those of the pyrolysis of bulk nickel formate. Mass‐spectrometric analysis showed that both reactions yield H2, CO2, H2O and CO as primary products. In both cases the CO/CO2 ratio is substantially higher than corresponds to the water gas equilibrium.As HCOOH decomposition on other metal catalysts is also very likely to be dominated by the properties of their formates, the respective catalytic activities were related to the stability of these formates. Experimental data confirm the expectation that the highest catalytic activities are shown by metals with formats in a relatively narrow range of stabilities.

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