Performance Improvement of Nano-Catalysts by Promoter-Induced Defects in the Support Material: Methanol Synthesis over Cu/ZnO:Al

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1 Supporting information for Performance Improvement of Nano-Catalysts by Promoter-Induced Defects in the Support Material: Methanol Synthesis over Cu/ZnO:Al By Malte Behrens, Stefan Zander, Patrick Kurr, Nikolas Jacobsen, Jürgen Senker, Gregor Koch, Thorsten Ressler, Richard W. Fischer, and Robert Schlögl Experimental details: Preparation of reference compounds: Binary Cu,Al and (Zn),Ga reference samples have been prepared by coprecipitation from nitrate solutions. The Cu,Al samples have been onbainted by decreasing ph co-precipitation and aged in the mother liquor until the color of the precipitate changed from blue to bluish green. XRD patterns were recorded and the position of the 20-1 peak of zincian malachite was evaluated by single peak fitting. The ZnO:Ga reference sample for XANES measurements was prepared analogously to the catalyst samples by constant ph coprecipitation and calcined at 330 C in air. The zincite crystal structure was confirmed by XRD and the incorporation of Ga 3+ into the zincite lattice was indicated by a yellow color of the sample. The ZnGa 2O 4 sample was prepared accordingly, but with a different Zn:Ga ratio and by calcination at 800 C. The pure Gallia references were prepared according as described in [L. Li, W. Wei, M. Behrens, Solid Sate Sci. 14, 971 (2012)]. Thermogravimetric analysis: TGA-MS was performed with a NETZSCH STA449 thermobalance under a controlled flow of 20% O2 in Ar with approximately 20 mg of sample at a heating rate of 2 C/min. The gases evolved were monitored with a quadrupole mass spectrometer (QMS200 Omnistar, Balzers) coupled to the thermal balance via a quartz capillary. Note on the crystallinity of the precursor materials: The amorphous fractions have been analyzed by the Rietveld spiking method using a known amount (ca. 50 wt%) of an internal standard (NIST-certified ZnO). These measurements have been done for the samples containing 2.5% and 4% Al resulting in formally 16 and 21 wt.% of amorphous material in the precursors. The relative trend is in accordance with the proposed model of Al incorporation that suggests presence of Al(OH) 3 in case of the sample containing 4% Al, but the absolute values seem to be overestimated. Considering the long ageing time in the reactor, an almost complete crystallization was expected. We note that the error of this method is rather large, in particular if the dramatic difference in scattered intensity between the samples the standard is considered: If mixed with the strongly scattering ZnO standard, the signature of the catalyst precursor tends to vanish in the background. Even for the standard ZnO, the NIST-certified crystallinity determined by diffraction was only 95%. The other 5% are not necessarily foreign phases, but domains of ZnO that scatter only weakly and might be associated with surfaces and pores. We thus conclude that with this method 100% crystallinity cannot be expected for a nano-structured and S1

2 porous material like the catalyst precursor. In the light of these considerations, ca. 80% crystallinity seems reasonable compared with 95% of the standard. The spiking experiment has excluded the presence of major amounts of an amorphous phase. The effect of the Al promoter on the crystal structure of the zm precursor (Fig. 2) is a strong indication that it is not present as an amorphous by-phase for low Al concentrations. The completely safe exclusion of any amorphous fraction (a little bit of finely distributed aluminium hydroxide) however remains difficult. We can however conclude that if such material exists, its amount must be small and - more important - it does not interfere with the correlations shown in Figure 2 that clearly show that the changes in the crystalline part of the precursor are linked to the catalytic properties. Note on Ga K-edge XAFS analysis: No attempt was made to exploit the structural information of the EXAFS region for two reasons:1. The amount of Ga is low and the weak features of the Ga-K EXAFS are superimposed to those of the neighboring Zn K-EXAFS, which due to the higher amount of Zn can lead to significant oscillations of the absorption coefficient even several kev beyond the Zn K-edge in region of the Ga EXAFS. 2. The experimental Ga K- is characterized by a single shell of neighbors around the Ga absorber, which is not unusual for highly disordered or nano-sized materials. From the semi-quantitative XANES analysis, it is known that for a refinement at least three different Ga phases have to be taken into account. All these phases exhibit Ga-O shells in the same range of distances. It is thus concluded that for a fit of the EXAFS region to extract structural parameters, the number of parameters to be refined would be too high and too correlated compared to the limited experimental observation to give a reliable diagnostic insight. S2

3 7.5 ph Turbidity / a.u Ageing time / min Figure S1: Evolution of ph and turbidity during Cu,Zn,Al precursor ageing for samples with different Al content. The constant ph co-precipitation was finished at t = 0 min and ph was allowed to vary freely. The ph minima and turbidity maxima are associated with the crystallization of the zm precursor phase. Ageing was stopped approximately 30 min after the crystallization (13 % Al: black, 6.5 % Al: red, 4.0 % Al: green, 3.3 % Al: blue, and 2.5 % Al: orange) Intensity / a.u θ Figure S2: Powder XRD patterns of the catalyst precursor materials showing the presence of zm (grey bar graph) as the major phase and hydrotalcite (red bar graph) for higher Al contents. Color code as in Fig. S1. S3

4 100 mass loss / % m/e=18 m/e=44 norm. ion current (m/e=18, m/e=44) Temperature / C Figure S3: TGA-MS data in synthetic air showing similar thermal decomposition behaviour of all samples except for the Al-richest, which contains a larger fraction of the hydrotalcite phase. The sharp water signal near 100 C is due to the emission of inter-layer water molecules of this phase. All samples contain similar fraction of so-called hightemperature carbonate, which remains in the CuO/ZnO/Al 2O 3 product upon calcination at 330 C. S4

5 Intensity [a.u.] θ [ ] Figure S4: Representative Rietveld fit of the crystalline CuZnAl precursor containing 3.3% Al (Data points: experimental data; red line: calculated pattern; green tick marks: position of Bragg reflections; grey line: difference curve). Malachite was used as structural model [F. Zigan, W. Joswig, H.D. Schuster, S.A. Mason, Z. Krist. 145, 412 (1977)]. The quality of this fit is representative for refinement of other samples, in which additionally hydrotalcite (Alrich samples) and aurichalcite (Al-free sample) were found as structure by-phases. The R wp-values of the Rietveld refinement ranged from 6.95 to 8.18%. It is noted that the accurate determination of the exact weight fractions in the phase mixture is difficult due to the low amounts of some components, the generally low crystallinity and the high noise of the XRD patterns. However, the general trends within the series of samples as shown in Fig. 2 are regarded as reliable, while the absolute values of individual samples depend on the used fitting constraints and have to be compared with care. S5

6 d (20-1) / Å Al content / % Figure S5: Plot of the d-spacing of the (20-1) planes as a function of nominal Al-content (metal base) for Zn-free malachite precursors. The decrease in d-spacing confirms the incorporation of Al 3+ on Cu 2+ sites. Due to the charge mismatch, the incorporation of Al is only possible until a certain limit between 2 and 3%. After exceeding this limit, CuAl-hydrotalcite crystallizes as a by-phase and acts as a sink for Al causing again an increase of d(20-1) of the Aldepleted malachite phase. S6

7 Table S1*: Results of linear combination fit of the Ga-K-edge XANES of CuO/ZnO/3%Ga 2O 3 and the experimental Ga-oxide-reference spectra. The references were α-ga 2O 3, β-ga 2O 3, γ-ga 2O 3, ZnGa 2O 4-spinel and a ZnO/3%Ga 2O 3 sample. γ-ga 2O 3 seemed excluded after several runs as it did not seem to be present in significant amounts. The fitting procedure was applied in the fitting region of -20 to 150 ev (related to the Ga-K-edge) for all possible combinations of the four references. The E 0-shifts were smaller than 0.3 ev in all cases. Comparing the R-values, it was obvious that no satisfying fits were possible without using the Ga-doped ZnO reference. Especially simulating the regions around and ev needed the contribution of this reference (see also Fig. S7). As a conclusion, Ga 3+ should also be present in the ZnO lattice of the ternary Cu/ZnO/Ga 2O 3 catalyst. Fit-Nr. R-value (*10-3 ) α-ga 2O 3 β-ga 2O 3 ZnGa 2O 4 ZnO/3%Ga % 0% 45% 36% % - 45% 36% % 59% 29% % 36% % % % 0% - 48% % 65% % 35% 65% % - 86% % - 53% % 49% - - * It is noted that the exact quantification of the phase composition should be treated with care because of the differences in crystallinity and particle size of the used references and the promoter species in the catalyst. The qualitative information on the presence of absence of major components is regarded as reliable. S7

8 ZnO/3%Ga 2 O 3 µ(e) Simulated XANES CuO/ZnO/3%Ga 2 O % ZnGa 2 O % β Ga 2 O 3 0 % α-ga 2 O 3 Difference Energy [ev] Figure S6: Ga-K-edge XANES of CuO/ZnO/3%Ga 2O 3 and the best linear combination fit without using the ZnO:Ga reference sample. Strong deviations between experimental and simulated curve can be seen (encircled areas in the regions around and ev) implying that some of Ga 3+ is present in the ZnO lattice of the ternary Cu/ZnO/Ga 2O 3 catalyst. S8