Fe-Cr catalyst

Andreev and coworkers—promoting effect of Cu addition on Fe-Cr shift catalysts, and Raney Cu/Zn catalysis described by an associative mechanism. Andreev and coworkers240 added promoters to traditional high temperature shift Fe-Cr catalyst, and found that Cu and Co moved the maximum in the CO conversion curve to lower temperatures. No explanation was provided for the effect. 

In 1991 a 15000t/a ammonia plant in Hubei Province, China came on stream. The hydrocarbon feed was converted mainly to H2 and CO under a pressure of about 0.8 MPa. The activity of a conventional Fe-Cr catalyst was not good enough when the inlet temperature of reaction bed was about 200°C... 

The iron-chrome catalyst was improved by addition of Cu to increase activity and selectivity (by suppressing CH4 generation). The possible elimination of Cr from Fe-based WGS catalysts has also been addressed.However, Fe-Cr catalysts have been used for a long time and a change of catalyst by industry will inevitably be slow. 

The mechanism and kinetics of the WGS reaction over Fe-Cr catalysts have been the subject of numerous publications. Despite intense investigations, still there is no full agreement as to the reaction mechanism. The two competing approaches are a redox (regenerative) mechanism first proposed by Kulkova and Temkin as early as 1949 which presumes reduction of an oxide center (O) by a CO molecule yielding CO2 and a vacant surface center ( ), followed by reoxidation of the vacant center by water that produces hydrogen and regenerates the oxide center for the catalytic cycle. 

To investigate the effect of H S (Bohlboro and Jorgensen, 1970 Bohlboro, 1969), the same type of commercial Fe-Cr catalyst was used as in previous experiments. Two H2S levels, based on wet feed, were used ... 

Chinchen et al. investigated [3] the stability of the Fe-Cr catalyst in both the reactors. There are two stages of deactivation. First it was observed in the initial stage which is a fast decay while the other one is a slow decay which occurs in the later stages of the reaction. The Arrhenius plots show that the activation energy for the water-gas shift reaction had not changed significantly between the new catalyst and a catalyst which had been operated for 9 months at about 470 °C. However, it is clear that there is a 30% decrease in the activity. The main reason for the deactivation is the loss of surface area. 

Andreev et al. [12] synthesized first row transition metal doped Fe-Cr catalysts. They synthesized 5% Zn, Mn, Cu and Co Fe-Cr catalysts by co-precipitation method. Cu- and Co-oxides proved to be catalytic promoters of interesting relevance. In the late 1980s Sud-Chemie developed Fe-Cr-Cu catalysts for the low steam to CO ratio applications [11]. Operating iron-chromia catalysts at lower steam to CO ratios leads to the formation of iron carbides and iron carbides are very active catalysts for Fisher-Tropsch reactions. Figure 2.1 compares the methane formation across the cOTiventional Fe-Cr catalysts and Cu-promoted Fe-Cr catalysts. It is seen that Cu promotion suppresses the methane formatiOTi. They proposed that Cu suppresses the C-O cleavage and prevents the formation of iron carbides. 

Idakiev et al. [15] also explained the promotional effect of copper. Initially, they investigated the effect of temperature on the activity of 5% Cu and 15% Cu-doped Fe-Cr catalysts. The CO cmiversion increases with increasing Cu loading. Also, the degree of conversimi of carbon monoxide on the unpromoted iron-chromia catalyst sharply decreases with the increase of the space velocity. On the copper oxide promoted samples, it remains almost unchanged. However,... 

Li et al. [34] synthesized Fe-Al-Ce catalysts and compared with Fe-Cr catalysts and evaluated for high-temperature WGS reaction under simulated coal derived syngas at a steam to CO ratio of 1. The Fe-Al-Ce catalyst exhibits better activity compared to Fe-Cr catalyst. 

Kuijpers et al. [5] synthesized Cu/Si02 catalysts using deposition-precipitation method. They proposed that the reaction is structure sensitive and follows reduction-oxidation mechanism. Xue et al. [6] compared Cu-Zn catalysts with Pt/Zr02 and Fe-Cr catalysts. The catalytic activity results of those catalysts at various temperatures are presented in Figure 3.1. They carried out the reaction... 

Hossain et al. [23] reported Cu-Fe-Cr and Cu-Fe-Mn catalysts and they compared with commercial catalysts. Cu-Fe-Mn catalyst showed significantly higher CO conversion than the commercial Cu-Zn0/Al203 catalyst, while the activity of Cu-Fe-Cr catalyst is lower than that of the commercial catalyst. Thouchprasitchai et al. [24] reported Cu-Fe, Cu-Zn and Cu-Fe-Zn catalysts. The ternary catalyst shows better activity compared to binary catalysts. Takehira et al. [25] investigated alkali metal-doped Cu/Zn catalysts for low-temperature WGSR. The highest activity is obtained for Cu/MgO/ZnO catalysts. The Cu/MgO ZnO catalyst... 

Then we incorporated Cu into the Fe/M catalysts (M = Cr, Mn, Co, Ni, Cu, Zn and Ce) since Cu is a promoter for the Fe/Cr catalysts for the high-temperature WGS reaction in the industries [3-5]. Interestingly, Cu acts as a promoter for all the modified ferrite catalysts except Fe/Ce catalyst. It acts as an inhibitor for the Fe/Ce catalyst. These results show that all of the copper co-doped ferrites behave like Fe/Cr/Cu except Fe/Ce/Cu, which behaves differently for high-temperature WGS reaction. We explained this interesting behaviour of copper... 

Brunetti et al. also foimd that with Fe-Cr catalysts and at temperatures more than 400 °C and feed pressures 15 bar the membrane reactor only require 9% of the catalyst volume of the traditional reactor [11b]. 

Pinaccia et al. [25] investigated the tubular Pd membrane reactor at higher temperatures, i.e., above 400 °C and higher pressures (100-800 kPa). They did permeation tests for 1200 h at 400 °C and the membrane exhibits excellent stability. After permeation they evaluated membrane reactor for WGS reaction using commercial Fe-Cr catalyst in the syngas mixture. They are able to achieve 85% CO conversion and 82% H2 recovery with 97% purity. 

Bohlbro [45] investigated the kinetics of commercial Fe-Cr catalyst by varying the concentration of one component and keeping the other component s concentration constant. They used the power law expression. Bohlbro found that the power law expression provided fairly good accuracy for the shift reaction on a Fe-Cr catalyst in the temperature region 330-500 °C and the rate equation is... 

Bohlbro et al. [48] also investigated the kinetics of commercial Fe-Cr catalyst in the presence of H2S. They investigated two H2S levels, namely 50-100 and 2000 ppm. They found the catalytic activity is considerably low by the addition of 2000 ppm. The reaction orders are shown in Table 9.4. 

TABLE 9.4 Reaction Orders of Fe-Cr Catalysts in the Presence of H2S H2S (ppm) CO order H2O order CO2 order H2 order... 

The HTS usually is an iron oxide - chromium oxide based catalyst. Also reaction promoters such as Cu may be present in catalyst composition. Operational temperatures vary from 31CFC to 450°C. Inlet temperatures are usually kept at 350°C to prevent the catalyst bed temperature from damage. Exit CO concentrations are in the order of 2% to 4%. Industrial reactors can operate from atmospheric pressure to 8375 kPa. Sulfur is a poison for Fe-Cr catalysts. 

The activity of the Fe/Cr catalyst can be improved by adding Cu as a promoter, able to decrease the activation energy (Ea) as well as the process selectivity toward methane (Lee et al., 2013). 

The preparative chemistry of the Cu-ZnO catalyst, with or without AI2O3 or Cr203, has been smdied extensively in literature, and it is still a subject of interest because of the namre of the precursor mixture (Uchida, Isogai, Oba, Hasegawa, 1967). Similar to Fe-Cr catalysts for the HTS process, Cu-based ones also require an activation procedure before the reaction by thermal treatments with temperatures in the range 453-533 K. 

Other promoters such as Ag, B, Ba, Ce, Co, Cu, Hg, Pb, Rh, and Zn (243-246) are reported to improve the performance of the Fe/Cr catalyst used in the WGS reaction. However, one of the disadvantages, during preparation and disposal of the Fe/Cr-based catalysts, is the content of Cr of about 2 wt% (245), which is highly toxic and hazardous to health. Rangel and co-workers (247,248) performed various studies on Cr-free/Fe-based catalysts and reported that Fe-Th-Cu catalyst is promising for industrial applications (249). Other catalytic systems, such as Fe-Al-Cu, are also reported to show attractive properties (250). 

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