Spectroscopy of Hydrothermal Reactions 17. Kinetics of the Surface-Catalyzed Water−Gas Shift Reaction with Inadvertent Formation of Ni(CO)4

The rate constants and Arrhenius parameters for the reaction of CO in H 2 O were determined at 230−270 °C and 27.4 MPa by the use of a titanium flow reactor with real-time detection by infrared spectroscopy through sapphire windows. These rate measurements appear to be the first below the critical temperature of water. The zeroth-order kinetics model produced an Arrhenius activation energy of 32 ± 3 kcal/mol, which is in the range of previously reported values at higher temperatures, but the preexponential factor [ln( A, mol kg -1 s -1 )] of 20.5 is much larger. The higher overall reaction rate is consistent with heterogeneous catalysis by the reactor surfaces considering (1) the zeroth-order kinetics, (2) the high A factor, (3) the activation energy in the range for the water-catalyzed reactions, and (4) the previously determined dependence of the decomposition rate of the putative formic acid intermediate on the metal used to construct the cell. Extremely toxic Ni(CO) 4 was observed to form as a result of extraction of Ni from slightly corroded 316 stainless steel tubes that connected the cell/reactor to the flow control system. Ni(CO) 4 formed under somewhat limited conditions, but its occurrence forewarns of the potential hazard of hydrothermal processing when a high CO concentration might be present in a nickel-containing reaction vessel.