108756-21-4Relevant articles and documents
Mechanistic studies of the catalytic reduction of nitric oxide by carbon monoxide in the presence of platinum(II), copper(I), and copper(II)
Sun,Kong,Cheng
, p. 1998 - 2004 (2008/10/08)
A rapid reduction of NO by CO to give N2O and CO2 is catalyzed at low temperatures and pressures by an aqueous system comprised of K2PtCl4, CuCl2, CuCl, and HCl. Two platinum species, PtCl3(CO) and PtCl42-, were observed and characterized spectroscopically. The former may also be isolated as (AsPh4)[PtCl3(CO)] by quenching the reacting catalyst solution with AsPh4+Cl-. During the course of catalysis, CuCl absorbs gaseous CO to yield CuCl(CO), which then reacts with PtCl42- to give PtCl3(CO)-. No carbonylation of PtCl42- and hence no catalysis occur in the absence of CuCl. Isotopic labeling studies by using 18O-enriched water as the solvent for the catalysis show that the CO2 evolved contains nearly the same 18O abundance as in the solvent. In contrast, no 18O from water is incorporated inio the product N2O. These observations clearly establish that both oxygens in CO2 are from water and exclude a direct oxygen transfer from NO to CO to yield CO2. Moreover, the high 18O content in CO2 strongly implies the presence of a rapid process of oxygen scrambling between coordinated CO and water. Indeed, PtCl3(CO) undergoes oxygen exchange with 18O-enriched water extremely rapidly without significant decomposition to give CO2 in the catalyst solution. The reaction of PtCl3(CO)- with CO to give CO2 is also too slow to account for the CO2 production. However, by reaction with NO, PtCl3(CO)- in the CuCl-CuCl2-HCl system evolves CO2 and N2O rapidly. The presence of CuCl2 or CuCl in the solution is required for the rapid gas production. This reaction is thought to be the process for N2O and CO2 production in the catalysis. Consistent with the observed behavior of the system, the results of kinetic studies may be explained in terms of two kinetically distinguishable processes. One is the carbonylation of PtCl42- of which the rate shows first-order dependence on each of [PtCl42-], PCO, and [CuCl]. The other process is the reaction of PtCl3(CO) with NO and water. Its rate is first order on [PtCl3(CO)-], PNO, and [CuCl2]. For the reaction of PtCl42- with CO, the enthalpy of activation is 9.1 ± 0.3 kcal mol-1 and the entropy of activation is -.30.2 ± 0.1 cal K-1 mol-1 The corresponding values for the reaction of PtCl3(CO) with NO and H2O are -1.5 ± 0.3 kcal mol-1 and -65.9 ± 0.3 cal K-1 mol-1. On the basis of these two processes, a mechanism for the observed catalysis is proposed.
