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

Abstract

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 nickelcontaining reaction vessel.

Uncatalyzed and wall‐catalyzed forward water–gas shift reaction kinetics

“…Furthermore, no metal carbonyls were detected immediately downstream of the trap, and analysis of loosely bound carbon deposits upstream of the reaction zone (that is, perhaps carried away by the flow) (Figure 10) did not show the presence of nickel or iron. Thus the most likely source of the nickel (and iron) deposited in the preheating section is the formation of nickel (and iron) carbonyl in the presence of the stainless steel tubing immediately preceding the preheating section under the CO‐rich conditions as reported by Miksa and Brill 25. These carbonyls would then readily decompose in the hotter quartz tubing.…”

A reaction cell with sample laser heating forin situsoft X-ray absorption spectroscopy studies under environmental conditions

“…The reactor cell and gas supply system have to be made from materials that do not interact with the reactant gases, which could lead to the formation of compounds that can contaminate the surfaces, e.g. Fe and Ni carbonyls generated from stainless steel (Sykes et al, 1993;Miksa & Brill, 2001). In our design these problems are avoided by coating all the materials in contact with the reactant gases with gold, and by using copper and brass for the gas lines and connections.…”

The Role of the Reactor Wall in Hydrothermal Biomass Conversions

“…By examining the water–gas shift reaction under hydrothermal conditions, Brill and Miksa35 have shown that the extremely toxic Ni(CO) 4 was formed by extraction of Ni from a slightly corroded 316 stainless steel (SS316) tube. These results are consistent with metals leaching from the reactor.…”