CuZn-Based Catalyst

As noted above, experimental evidence is consistent with both redox-type and formate-intermediate mechanisms for the CuZn-catalyzed LTS reaction. It is also noted that slight differences in experimental conditions from one study to another could be the basis of important differences in catalyst surface configuration, leading to different elementary reactions observed.9 

Kinetic parameters were measured by Koryabkina et al.16 on a Cu-based LTS catalyst under the conditions potentially relevant for fuel cell applications (220 °C, latm, and a feed comprising 7% CO, 8.5% C02, 22% H20, 37% H2, and 25% Ar by volume). The data suggested a redox-type mechanism shown in Equations 6.22-6.29, with the rate-determining step involving the formation of adsorbed C02 as shown in step VII  

The kinetic expression as shown in Equations 6.19 and 6.20 was found applicable, with partial reaction orders l = 0.8, m = 0.8, n = -0.7, and q = -0.8 and an activation energy of 77.7kJ/mol. The values for the exponents n and q suggest a strong inhibition effect form the reaction products H2 and C02.16 

---

In situ spectroscopic studies have identified a variety of species, such as formate, dioxymethylene, carbonate, and methoxide, to coexist under methanol synthesis conditions on Cu/ZnO-based catalysts [22, 23], Fourier transform infrared spectroscopy studies of CuZn-based catalysts under H2/C02 identified the presence of formate bound to both Cu and ZnO, whereas methoxide was found on ZnO only. Carbonates were found to form via C02 adsorption on ZnO [24] and partially oxidized Cu [23], and were quickly converted into formate via Cu-activated hydrogen. Upon exposure to CO mixtures, only zinc-bound formate was observed [22], The hydrogenation of these formates to methoxide is thought to be rate determining in methanol synthesis. 

Figure 5.3.7 The variation of CO and methanol production rates with space velocity (A) and methanol synthesis rates on promoted CuZn-based catalysts derived from hydrotalcite and Malachite precursors (B). Reaction conditions 200 mg catalyst, 30 bar, 230°C, 3 1 H2 C02.

CuZn-based catalysts can be used, and different operating methods are possible. The most common reformer reactions can be seen in (21.1H21.4). 

Commercially, methanol is produced from the hydrogenation of CO (syngas) over heterogeneous CuZn oxide based catalysts using fixed bed reactors (Equation 11). 

Many catalytic formulation are proposed for the hydroconversion of CO2, most of them are based on promoted copper-zinc oxides given by the long industrial experience on methanol synthesis from syngas (CO+CO2+H2) [3-6]. Specific methanol catalysts working for CO2 are proposed including promoted Cu-Zn catalysts [3,6], zirconia supported systems [7] as well as copper associated with stabilized rare earth oxides [8,9]. In the last case Cu-LaZr and CuZn-Lcfer catalysts were proposed and showed interesting catalytic properties in the methanol formation. 

For example, a nonpyrophoric and proprietary catalyst has been developed by Engelhard as a replacement for the CuZn-based LTS catalyst. This catalyst also possesses features such as direct activation under process gas conditions, similar activity as the traditional catalyst, no safety and stability concerns upon air exposure, and good stability against temperature cycling and aging.39... 

Different types of catalysts exhibit shift activity. Commercially available high temperature (HT) FeCr and low temperature (LT) CuZn are generally employed. Among some interesting new developments is the Co/Mo based catalyst. This type is completely insensitive to sulphur and certain formulations are claimed to possess good activity at both high and low temperatures. The... 

There is also a need to develop better water gas shift catalysts (7, 33, 34), especially catalysts that operate at temperatures ranging from 200 to 300 °C. Commercial shift catalysts based on FeCr and CuZn oxides are available, but are not designed for the rapid startups and frequent exposure to oxidizing conditions that will be experienced during normal operation of fuel processors developed for transportation applications. These commercial catalysts have fixed size, high density, and are susceptible to contaminant poisoning by ingredients found in... 

---