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The Quest to Reliable Structure-Property Relationships in Methanol Steam Reforming

Subject Area Solid State and Surface Chemistry, Material Synthesis
Term from 2017 to 2021
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 313854401
 
Final Report Year 2021

Final Report Abstract

Within the project, single-phase (Cu 9 Zr 2 , Cu 51 Zr 14 , Cu 8 Zr 3 , Cu 10 Zr 7 and CuZr 2 ) and two-phase (with 1:1 phase ratio) bulk intermetallic Cu-Zr compounds were synthesised. Arc melting and subsequent annealing in Mo-crucibles in evacuated quartz glass ampoules proved the best synthesis route. The high oxophilicity of zirconium posed a challenge to the synthesis of nearly oxygen-free samples, which could be achieved by using zirconium in the form of bars obtained by the Van-Arkel-de-Boer process and removing the Zr-layer affected by oxygen diffusion. All samples were characterised by XRD, metallographically polished and investigated by optical and electron microscopy (including EDXS), concerning their phase and elemental composition. Subsequently, the materials were used as precursors for Cu/ZrO 2 materials to investigate the influence of the precursor on the Cu-ZrO 2 interface and, thus, on the catalytic properties. Decomposition of the single- and two-phase precursors under methanol steam reforming conditions in a water/methanol mixture (1:1 molar ratio) was monitored by DTA/TG/MS. In all cases the decomposition products are identified as Cu, m-ZrO 2 and traces of t-ZrO 2 by XRD. Cu-rich samples decompose initially at a lower rate than zirconium-rich samples. During the decomposition, the rate of the latter decreases strongly due to formation of a ZrO 2 -layer on the surface, which hinders diffusion of the gas phase and thus limits the oxidation rate. Interestingly, the presence of a second phase in the sample decreases the onset temperature for the decomposition and increases the rate. This is attributed to the phase boundaries between the two phases, which seem to activate the decomposition. For the catalytic tests, intermetallic samples were loaded into the reactor and decomposed in situ. During the decomposition under MSR conditions, the evolution of the Cu-ZrO 2 interface leads to strong activation and increase in CO 2 -selectivity. Running the catalysis above 300 °C leads to rather fast deactivation of the samples, which is accompanied by a loss of CO 2 -selectivity – both indications for sintering of the copper particles leading to a loss of Cu-ZrO 2 interface concentration. A very different picture is obtained at 300 °C where the materials show higher stability, which goes hand-in-hand with an excellent CO 2 -selectivity of up to 99.4%. To identify the species present in the near-surface region under reaction conditions, nearambient pressure XPS under operando conditions were conducted on the samples. The samples were characterised in the intermetallic state under UHV conditions as reference point and then heated in water/methanol atmosphere. With ongoing decomposition, the formation of Cu and ZrO 2 was identified in accordance with the XRD results. In addition, an oxidised Cu-species (Cu ox ) was detected, which is present in too small amounts to be detected by XRD. Analysis of the catalytic data during the XPS measurements revealed the beneficial effect of twophase samples concerning their catalytic activity and selectivity, which was in all cases higher than the according materials derived from single-phase precursors. The catalytic properties do not correlate with the Cu-, ZrO 2 - or Cu ox -concentration detected in the most surface-sensitive measurements, which is in accordance with the Cu-ZrO 2 interface as active region for methanol steam reforming. Thus, the decomposition of two-phase intermetallic precursors leads to materials with superior catalytic properties in the methanol steam reforming. Highest activity and selectivity is observed for Cu-rich samples, while Zr-rich samples possess higher stability against sintering likely caused by epitaxial effects. Further research will be devoted to the optimisation of the decomposition conditions to combine the excellent catalytic properties with higher stability. The overall results within the project could only be obtained because of the exemplary cooperation and interaction with PD Simon Penner at the University of Innsbruck, which has been very successful and productive.

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