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Mechanistic studies of the homogeneous catalysis of the water gas shift reaction by rhodium carbonyl iodide

Journal Article · · J. Am. Chem. Soc.; (United States)
DOI:https://doi.org/10.1021/ja00523a019· OSTI ID:5400829
; ;
The water gas shift reaction, H/sub 2/O + CO = H/sub 2/ + CO/sub 2/, is effectively catalyzed at low temperatures and pressures by a catalyst system based on rhodium carbonyl iodide. The results of semiquantitative kinetic studies and of quenching experiments are reported. In the temperature range 80 to 100/sup 0/C the catalytic reaction shows first-order dependence on the partial pressure of CO and inverse dependence on acid and iodide concentrations. The major species in solution are rhodium (III) carbonyls. However, below 65/sup 0/C the reaction is independent of CO pressure, and exhibits second-order dependence on iodide concentration and positive dependence on acid concentration. Rhodium (I) species now become predominant in solution. The temperature dependence of the rate shows corresponding behavior. Above 80/sup 0/C the Arrhenius plot is linear with a slope which yields an apparent activation energy of 9.3 kcal mol/sup -1/. However, below 70/sup 0/C it is concave downwards with a slope at 55/sup 0/C corresponding to 25.8 kcal mol/sup -1/. Limiting behavior is not reached on the low-temperature side. These results are explained in terms of a change in the rate-limiting step of the catalysis. At high temperature CO/sub 2/ production from reduction of Rh(III) species by CO is viewed as rate determining, while at lower temperature hydrogen production from oxidation of Rh(I) species by protons is thought to be limiting.Possible intermediates are discussed and a mechanistic scheme is proposed. A steady-state kinetic analysis and assumptions about rate-determining steps show the mechanism to be consistent with the observed behavior of the system. 46 references, 3 figures, 3 tables.
Research Organization:
Univ. of Rochester, NY
OSTI ID:
5400829
Journal Information:
J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 102:3; ISSN JACSA
Country of Publication:
United States
Language:
English

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Related Subjects

08 HYDROGEN
080104* -- Hydrogen-- Production-- Water Gas Processes
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201 -- Chemical & Physicochemical Properties
400301 -- Organic Chemistry-- Chemical & Physicochemical Properties-- (-1987)
ACTIVATION ENERGY
CARBON COMPOUNDS
CARBON DIOXIDE
CARBON MONOXIDE
CARBON OXIDES
CARBONYLS
CATALYSTS
CATALYTIC EFFECTS
CHALCOGENIDES
CHEMICAL REACTION KINETICS
CHEMICAL REACTION YIELD
CHEMICAL REACTIONS
DATA
DATA FORMS
ENERGY
EXPERIMENTAL DATA
HALIDES
HALOGEN COMPOUNDS
HYDROGEN COMPOUNDS
HYDROGEN PRODUCTION
INFORMATION
IODIDES
IODINE COMPOUNDS
ISOLATED VALUES
KINETICS
LOW PRESSURE
MEDIUM TEMPERATURE
NUMERICAL DATA
OXIDES
OXYGEN COMPOUNDS
PRESSURE DEPENDENCE
QUANTITY RATIO
REACTION KINETICS
RHODIUM COMPOUNDS
SHIFT PROCESSES
TEMPERATURE DEPENDENCE
TIME DEPENDENCE
TRANSITION ELEMENT COMPOUNDS
WATER
WATER GAS PROCESSES
YIELDS