Relations between Synthesis and Microstructural Properties of Copper/Zinc Hydroxycarbonates

Abstract

Cu/Zn Hydroxycarbonates obtained by co‐precipitation of Cu2+ and Zn2+ with Na2CO3 have been investigated regarding phase formation and thermal decomposition in two series with varying Cu/Zn ratios prepared according to the decreasing pH and constant pH method. Hydrozincite, aurichalcite and (zincian)‐malachite were found to form at differing Cu/Zn ratios for both series. For the constant pH preparation the Cu/Zn ratio in zincian‐malachite was close to the nominal values whereas excess values were found for the decreasing pH samples. The degree of crystallinity as well as the thermal decomposition temperatures were lower for the constant pH series. All samples containing aurichalcite revealed an unexpected decomposition step at high temperatures evolving exclusively CO2. The differences in composition and microstucture were traced back to the different pathways of solid formation for the two preparation methods. Substantial changes were observed during the post‐precipitation processes of ageing and washing. The effects were studied in detail on samples with a cation ratio of Cu/Zn 70:30 mol %. Ageing of the precipitates in their own solutions is accompanied by a spontaneous crystallization of the initially amorphous solids. The decreasing pH sample develops from a hydroxy‐rich material comprising basic copper nitrate (gerhardtite) as an intermediate. Only small changes in the chemistry of the samples were detected for the constant pH precipitation. The findings are summarised into a scheme of solid formation processes that explains the phenomenon of a “chemical memory” of the precipitates when they are converted into Cu/ZnO model catalysts.

Microstructural characterization of Cu/ZnO/Al2O3 catalysts for methanol steam reforming—A comparative study

“…The short-range order structure (<2 Å) of this resulting ZnO phase mainly corresponds to disordered ZnO. Hydroxide and carbonate species may persist after mild thermal treatment (calcination, reduction) in the catalytically active catalyst [25]. Thus, the diminished amplitude of the 1st Zn-Zn shell obtained after reduction of sample CZA-1 may be caused by a superimposition of the spectra of different Zn species in addition to a highly disordered ZnO phase (Fig.…”

“…Such a modified zinc oxide serves as a growth inhibitor of copper particles [25], [51] and [52]. Hence, mild reduction in hydrogen results in copper dissolved in ZnO and copper metal particles bonded epitaxially to the surface of the ZnO matrix [50].…”

“…Compared to the role of ZnO as a structural promoter in Cu/ZnO/Al 2 O 3 catalysts [12], [13], [14], [15], [16], [17], [18], [19], [20] and [21], alumina is known to inhibit thermal sintering of metal particles and, therefore, imparts chemical and thermal stability required for an industrially used catalyst [22], [23] and [24]. The precursor preparation is governing the electronic and structural properties of the final copper catalyst (chemical memory effect [25]). Here we present microstructural and catalytic characteristics of various laboratory prepared ternary Cu/ZnO/Al 2 O 3 catalysts under MSR conditions.…”

Role of Lattice Strain and Defects in Copper Particles on the Activity of Cu/ZnO/Al2O3 Catalysts for Methanol Synthesis

“…We speculate at this point that the relevant nonequilibrium structure of active Cu is generated during synthesis explaining the phenomenon of "chemical memory". [27] The kinetic stabilization of metastable structures during long-term operation th er sition (Figure 2), Cu partic ces (Figure 1). In parallel, the geometric and the surface electronic structure of highly active systems will be analyzed with synchtrotron-based techniques to further refine the structural properties of the active Cu metal phase under reaction conditions.…”

The Microstructure of Copper Zinc Oxide Catalysts: Bridging the Materials Gap

“…Precipitate ageing, for instance, strongly influences the activity of the resulting Cu/ZnO catalysts. 14,15,16,17 The superior activity of Cu/ZnO catalysts obtained from precipitates aged for more than 30 min was shown to correlate with the increasing microstrain in the copper nanoparticles. 18 Apparently, extrapolating structure-activity relationships revealed for idealized copper model systems to the microstructure and catalytic properties of industrial catalysts needs to be conducted with care.…”