Oxidative Addition of Water to the Ru^II Catalyst K[Ru^II(Hedta)(CO)]: Homogeneous Catalysis of the Water‐Gas Shift Reaction under Ambient Conditions

Abstract

The reaction of CO with H 2 0 to form CO, and H, [Eq. (a)] is a very important reaction"] and is a principal method for obtaining H2 for the Haber-Bosch synthesis of NH3. Reaction (a), commonly referred to as the water-gas shift (WGS) reaction, is virtually the reductive decomposition of water by CO.CO + H 2 0 e C 0 2 + Hz + 42 kJ mol-' (a)The conventional heterogeneous catalysts for the watergas shift consist of chromium-activated iron oxide ( Fe30,/Cr,03) or cobalt-molybdenum oxide catalyst (CoO/MoO,); the reaction is carried out at 400-460°C and 200 atm of CO or CO + H,.Though the WGS reaction has been carried out under homogeneous c a t a l y~i s ,~~-'~~ only a few attempts have been made to perform the reaction under milder conditions (180°C and 35 atm of CO) using metal carbonyls of the type M(C0)6 (M = Fe, Cr, Mo, W, Rh, etc.) in alkaline or acidic medium. The turnover number reported"] is 83.3 moles of C02/H2 per mole of catalyst per hour.We report here the catalysis of reaction (a) by I(IRu"(Hedta)(CO)] at 20-80°C and 1-35 atm of CO. The effect of CO pressure (expressed as concentration of dissolved CO) on the rate of CO conversion into COz and HZ is shown in Figure 1. The rate u depends linearly on CO c -Fig. 1. Effect of concentration c M] of dissolved CO on the rate u [lo-' M min-'1 of the water-gas shift reaction (1 mmol catalyst, 30°C. 100 mL H20).pressure; moreover, the rate observed at 1 atm of CO pressure (a0.00012 M concentration of dissolved CO) indicates the feasibility of the reaction even under ambient conditions. The effect of temperature on the rate of CO conversion was studied between 20 and 80°C, as shown in the plot of -Inv versus 1/T in Figure 2; an activation energy E, of 6.15 kJ mol-' was found. Since the reaction is exothermic, it is not favored by higher temperatures. In fact, we observed formaldehyde formation at higher temperature (see Scheme 1).for the water-gas shift reaction are 50°C and 15 atm of CO, for which the turnover number is 350 moles of C02/H2 per mole of catalyst per hour. 5.801 \ 5. 781 5.76 i ;-.. : : : I , , , \ 5 6 6 3.0 3-1 3-2 3-3 3.L 3-5 2 [ 1 0-3K-'] T Fig. 2. Effect of temperature on the rate u of the water-gas shift reaction (1 mmol catalyst, 15 atm CO, 100 mL H20).The water-gas shift reaction was also conducted at 1 atm of CO and 20°C. A 75% conversion of CO into CO, and H, was determined after a contact time of 4 h with 3 x lop3 M catalyst concentration. Figure 3 shows that the rate of CO conversion increases with increasing catalyst concentration for the WGS reaction conducted at 1 atm of 0 0.5 1.0 1-5 2.0 2.5 3.0 ~'[rnrnoll -Fig. 3. Effect of catalyst concentration c'on the rate u [lo-" M min-'1 of the water-gas shift reaction at 20°C and 1 atm of CO (100 mL H20).CO and 20°C with a contact time of 4 h. For 0.25-1.0 mmol catalyst, the reaction showed a first-order dependence on catalyst concentration. Beyond a catalyst concentration of 1.0 mmol, saturation kinetics with respect to catalyst concentration was observed. The first-order rate constant calcul...

A mechanistic model for the water‐gas shift reaction over noble metal substituted ceria

“…Noble metals like Pt, Au, Pd, etc. have been reported to catalyze WGS reaction at low temperatures 6, 8, 9, 11–20. Apart from this, bimetal substituted compounds have also been tested 14, 18, 21.…”