Reduction of Cu-ZnO water-gas shift catalysts in presence of water
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Cu/ZnO/Al2O3 catalysts are used to catalyze the low temperature water-gas shift (WGS) step in the ammonia synthesis process. Deactivation of the catalyst due to low mechanical strength has been experienced by Yara, which most likely is due to grain growth without sintering. This can take place through formation of less stable phases during hydrothermal conditions and capillary condensation. The commercial Cu/ZnO/Al2O3 catalyst is derived from various hydroxycarbonate precursors which typically include malachite, zincian malachite, rosasite, aurichalcite and hydrotalcite. It is suspected that the hydrotalcite and zincian malachite phases reconstruct at high partial pressures of water as a consequence of oxidation and the memory effect of layered double hydroxides. X-ray diffraction (XRD) can be used to detect formation of these phases. However, detection of these phases from XRD of the commercial catalyst is difficult because this catalyst is dominated by metallic copper and CuO. This makes it difficult to recognize the diffraction patterns of hydrotalcite and zincian malachite. Therefore, phase pure samples of hydrotalcite and zincian malachite should be used as references, as detection is easier in phase pure samples. Temperature programmed reduction (TPR) of a CuO sample was performed to study the reduction behavior of pure CuO. Hydrotalcite precursors of Cu, Zn and Al were prepared by the constant pH co-precipitation method. A Cu:Zn:Al ratio of 70:30:60 was used. The precursor samples were calcined to form mixed oxide catalysts. XRD was used to characterize the synthesized samples and one sample of zincian malachite prepared by Yara. Crystallite sizes were estimated using Scherrer's equation. CuO/ZnO/Al2O3 samples were reduced at 20 barg with water present and characterized by XRD. The results were compared to the diffraction patterns of the phase pure hydrotalcite and zincian malachite. Additional samples were reduced at various conditions to study the effect of pressure and water on the results. The TPR result showed that phase pure CuO most likely is directly reduced to metallic copper in one step around 200-260 °C. XRD confirmed that a phase pure hydrotalcite with the formula Cu3Zn3Al2(OH)16CO3*4H2O was synthesized with the co-precipitation method when a pH of 8 was used during precipitation and aging. Diffraction patterns of hydrotalcite samples calcined at 330 °C and 500 °C revealed a decomposed structure and a starting formation of CuO. The structure crystallized after calcination at 700 °C and formed a mixed oxide catalyst consisting of CuO and ZnAl2O4. Reconstructed hydrotalcite was detected in two samples reduced at 20 barg with water present. One sample showed reflections of the (003), (006) and (015) planes of hydrotalcite and the other sample had diffraction peaks which corresponded to the (015) and (110) planes. A sample reduced at ambient pressure with water present showed reflections of the (003), (006) and (012) planes of the hydrotalcite structure. These results confirm that hydrotalcite can reconstruct in presence of water due to the memory effect. The pressure does not seem to have any impact on the reconstruction of hydrotalcite. Reconstructed zincian malachite phases were not detected in any of the samples. This indicate that hydrotalcites are easier reconstructed than zincian malachite, due to the structural memory of layered double hydroxides. The crystallite sizes of copper oxides obtained from the phase pure hydrotalcite sample were estimated to be 10 nm. This confirms that decomposition of hydrotalcite precursors results in small crystallites which give catalysts with highly dispersed active surface atoms.