Copper-based Catalyst for Water Gas Shift Reaction: Influence of Surface Area on Rates of Reaction
Divya Nair, Noor Dayana Abd Halim, Nurbiha A. Shukor, Nurul Aini Md Desa,
Yau Seet Ting
Abstract
Copper-based catalyst is used in water gas shift reaction to increase carbonyl, CO conversion. CO conversion produces hydrogen for industrial purposes. During the CO conversion, the copper-based catalyst deactivated due to the formation of copper crystallite. The surface area of copper-based catalyst is related to the rate of reaction of water gas shift reaction. This paper will report on the influence of surface area on the rate of reaction. It will also discuss the effect of promoter on the catalytic activity. The rate of reaction is measured in terms of rate per area of copper and rate per mass of catalyst.
1.0 Introduction
The water gas shift (WGS) reaction (CO + H2O CO2 + H2) plays an important role in carbonyl conversion and manufacturing hydrogen. Hydrogen is largely used in the production of hydrogen fuel cells. As stated by Pradhan et al [3], the success of hydrogen economy lies on the production of polymer electrolyte membrane (PEM) fuel cells where hydrogen is used primarily.
Water gas shift reaction is where carbon monoxide, CO is reacted with water to produce carbon dioxide and hydrogen gas. The significance of this reaction is to reduce the composition of CO in air by converting it to CO2. At the same time it produces hydrogen gas in a large scale for industrial purposes. This reaction which is highly exothermic, is a part of steam reforming of hydrocarbons. The reformated gas that comes out of a steam or autothermal reformer mostly contains a very high content of carbon monoxide [3].
Copper-based catalyst has been used in commercial WGS reaction to increase the conversion rate of CO to CO2 and H2. However, Mellor et al [2] mentioned that doubts arise in aspects of the nature of the active sites and also the role of additional components such as zinc oxide, alumina, and/ or ceria in WGS reaction. Cai et al [5] also discussed about the controversy of the oxidation state of copper, the role of zinc oxide, the active site for the reaction and the role of support, alumina.
Therefore, our current investigation is to develop an understanding of the copper based catalyst carried out under low temperature shift, LTS with the influence of the surface area on the reaction rate.
2.0 Experimental Procedure
2.1 Catalyst preparation
2.1.1 Alloy Preparation
According to Mellor et al [1], the Cu-Al and Cu-Ce binary alloys were prepared by firstly melting the required amount of copper and then adding required amount of aluminium. The alloy melt was stirred thoroughly and rapidly quench by pouring into agitated cold water. In some cases, a similar procedure was adopted to prepare the Cu-Zn-Al and Cu-Ce-Al ternary alloys. Both Cu-Al and Cu-Ce melt were made as described above, and cooled to below 420°C (melting point of Zn) and below -795°C (melting point of Ce). This procedure helps to avoid rapid vaporization of the lower melting Zn and Ce metal. Rapid stirring and cold water quenching of the melt completed the process.
2.1.2 Caustic leaching
Raney copper catalysts were prepared from alloys by a procedure similar to that adopted by Friedrich et al in Mellor et al [1]. During these experiments (preparation of CuO-Al2O3, CuO-CeO-Al2O3, CuO-CeO2 and Cuo-ZnO-Al2O3 catalyst), 20 g of alloy particles 0.50-1.180 mm in diameter were placed in 111 g of de-ionized water at 50
3.5°C. A solution containing 111 mL of 14.1M aqueous sodium hydroxide solution was added dropwise over 1 h to achieve a final leach concentration of 7.06 M. The extraction times used in the preparation of catalyst from ternary alloys of Cu-Zn-Al and Cu-Ce-Al were 1.0; 1.5; 2.0; 3.0 and 19.5 h. Binary alloy of Cu-Al and Cu-Ce were extracted for 1.5 h. The catalyst particles were then thoroughly washed with de-ionized water or distilled water until the pH of the wash water was 7. Then the copper-based catalysts were dried in air at 120°C for 12 h, crushed and calcined in air in a muffle furnace at 500°C for 5 h. Prior to testing, copper-based catalysts prepared were re-sieved in order to remove small copper fines generated during the leach reaction.
3.0 Results & Discussion
Table 1: Summary for the kinetics for water-gas shift reaction on Copper catalyst
| Catalyst | Rate per area of Cu* / 10-6 mol m-2 s-1 | Rate per mass of catalyst* / 10-6 mol g-1 s-1 | Reaction Order | |||
| COa | H2Ob | CO2c | H2d | |||
| 8% CuO-Al2O3 | 0.80 | 2.4 | 0.9 | 0.8 | -0.7 | -0.8 |
| 8% CuO-15%CeO2-Al2O3 | 0.83 | 0.75 | 0.7 | 0.6 | -0.6 | -0.6 |
| 8% CuO-CeO2 | - | - 0.11 | 0.9 | 0.4 | -0.6 | -0.6 |
| 40% CuO-ZnO-Al2O3 | 0.79 | 7.6 | 0.8 | 0.8 | -0.9 | -0.9 |
*Rates of reaction for the WGS reaction on Cu based catalysts at 200ยบC, 1 atm total pressure,
7% CO, 8.5% CO2, 22 % H2O, 37 % H2, and 25% Ar
aConcentration range: 5 to 25% CO and balance Ar to 33%; 8.5% CO2, 22 % H2O, 37 % H2.
bConcentration range: 10 to 46% H2O and balance Ar to 47.5%; 7% CO, 8.5% CO2, 37 % H2.
cConcentration range: 5 to 30% CO2 and balance Ar to 34%; 7% CO, 22 % H2O, 37 % H2.
dConcentration range: 25 to 60% H2 and balance Ar 62.5%; 7% CO, 8.5% CO2, 22 % H2O.
(Taken from Kinetic of Water Gas Shift Reaction on Copper Catalyst: Application to Fuel Cell Reformers [6]).
According to Pradhan et al [3], LTS reaction will result in higher CO conversion. This is due to the fact that WGS is an exothermic reaction. High temperature reactor converts bulk carbon monoxide from 2-3 vol % whereas low temperature unit further reduces CO level to less than 0.3 vol %.
High conversion rate of CO requires high surface area of copper-based catalyst. However, copper-based catalyst on its own will deactivate due to copper crystallite sintering. Therefore, there is a need to prepare Cu-based catalysts supported on various metal based oxides to resist copper crystallization. The crystallized copper is related to the surface area of the copper-based catalyst. When the size of the copper crystallite decreases, the surface area increases, resulting in higher rate of CO conversion.
A research conducted by Mellor et al [1] discovered that an increase in ZnO loading improved the contact between the copper surface and ZnO crystallites, thus, significantly improving the stability of copper-based catalyst. It was reported that ZnO played a role in assisting the copper crystals stabilizing process by acting as a suitable spacer material.
The role of aluminium is to produce high surface area of copper-based catalyst. However, according to Mellor [1] residual alumina in the active catalyst showed no beneficial effects. On the other hand, when copper is supported on CeO2,
This experiment was carried out using copper-based catalyst promoted by different types of metal oxide to generate new active sites. According to an experiment carried out by Cai et al [5], it has been proven that the maximum use of active sites exhibit higher activity rate.
Based on Table 1, Koryabkina et al [6] explained that the power rate law expression serves as an indication for the common reaction mechanism whereby the temperature and concentration at the same conditions could be obtained for different samples.
Based on Table 1, the reaction rates per surface area of Cu are the same on all four samples. Looking at the rate of mass of catalyst, CuO-ZnO-Al2O3 showed the highest activity rate. Catalysts supported by Ce showed lower rates per unit of mass and lower copper surface area. However, Ce did not show any promotional effect in WGS reaction at the conditions tested, contrary to the effects shown on Pt or Pd. The constancy of rate per unit of copper surface area in table 1 indicates that the reaction occurs on copper only. Ce and ZnO do not affect the rates of reaction.
4.0 Conclusion
Based on the experimental data, it can be concluded that high surface area of copper-based catalyst will result in a higher rate of reaction. The surface area of copper is influenced by the type of metal oxide it is promoted with. According to the data used in this report, the copper-based catalyst promoted by CuO-ZnO-Al2O3 gave the highest reaction rate.
