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Continuous Synthesis of Cu/ZnO/Al 2 O 3 Nanoparticles in a Co-precipitation Reaction Using a Silicon Based Microfluidic Reactor Microfluidic synthesis of

: CuO/ZnO/Al 2 O 3 catalysts were continuously synthesized in a microfluidic reactor, analyzed by X-ray diffraction (XRD), physisorption (BET), chemisorption, electron microscopy and X-ray absorption spectroscopy (XAS), and tested for methanol synthesis from CO-rich synthesis gas. The results were compared to those obtained from CuO/ZnO and CuO/ZnO/Al 2 O 3 produced by conventional co-precipitation in a batch reactor. The predominant phase of the aged precursor from microfluidic co-precipitation was identified as zincian malachite. After calcination the microfluidically synthesized catalyst exhibited smaller CuO crystallites, larger BET surface area, a rather uniform morphology and a homogeneous distribution of Cu and Zn compared to catalysts prepared by batch co-precipitation. H 2 -temperature programmed reduction (TPR) showed that Cu species in CuO/ZnO/Al 2 O 3 from microfluidic co-precipitation were easier reducible. In situ Cu and Zn K-edge XAS during the TPR indicated reduction of Cu 2+ to Cu 0 between 150 °C and 240 °C, without detectable reduction of Zn. N 2 O pulse chemisorption evidenced enlarged Cu surface area of the nanoparticles from the microfluidic synthesis. Based on activity tests in methanol synthesis, at 250 °C the microfluidically synthesized Cu/ZnO/Al 2 O 3 catalysts showed better performance than the catalyst from batch preparation when 1 mol% CO 2 was present in the synthesis gas. Dimethyl ether formed as a side product. As the microreactor is specially designed for high X-ray transmission with a thin Si/glass observation window, this study opens interesting perspectives for investigating the formation of catalyst precursors at the early stage of precipitation in future.


Introduction
Methanol (MeOH) is not only an essential industrial bulk chemical but can also be used as fuel additive or precursor for clean fuels. 1-4 Due to its high energy density methanol plays an important role in chemical energy and hydrogen storage including fuel-cell applications. [5][6][7][8] The most prominent catalytic system for industrial methanol synthesis since decades is based on Cu/ZnO/(Al 2 O 3 ) and enormous efforts have been dedicated to the enhancement of its catalytic activity based on rational design via different preparation methods. 3,5,[9][10][11][12][13][14][15][16][17][18][19] One widely applied method for the synthesis of Cu/ZnO/(Al 2 O 3 ) is co-precipitation of soluble copper, zinc and aluminum precursors using e.g. sodium carbonate. 9,[20][21][22] Specific synthesis parameters such as pH, temperature, ageing time and mixing conditions play key roles in the resulting catalytic performance. 5,9,21,23 Fundamental studies in this field e.g. by Behrens et al. 20 show that co-precipitation temperature between 60 °C and 70 °C and pH values between 6-7 as well as the ageing time and temperature are crucial to achieve optimum material properties. Sufficient ageing time is required for meso-structuring of the precipitate. 9,24 These parameters directly influence the formation of the -in this context important -zincian malachite (Cu x Zn y ) 2 (OH) 2 CO 3 phase. Calcination, typically in the range of 320 °C to 430 °C, promotes further nano-structuring during the phase transition to the mixed metal oxide. 20,25 Most preparation methods focus on batch procedures. However, for larger quantities and industrial applications a semi-batch or continuous process would be advantageous. [26][27][28][29] Catalytic performance of Cu/ZnO/(Al 2 O 3 ) is particularly influenced by the surface area of copper (SA Cu ) and the area of reactive interface with ZnO in a nanoparticulate and porous structure. 30 Metallic Cu clusters/particles are considered to be the most active sites for methanol synthesis, strongly improved by ZnO, 2,25 and Al 2 O 3 as a structural promoter 25,29 reducing NP sintering. Fast and homogeneous mixing of the soluble metal precursors and the precipitating agent are essential for the most uniform precipitate. 27 This is difficult to obtain in conventional stirred batch reactors where initial precipitation may already start before the reactants have been efficiently mixed, resulting in spatial and temporal material gradients. Every single added drop of reactants induces concentration changes during the entire process time. Redissolution, reprecipitation and ion exchange during ageing, washing and drying of the catalyst precursor may additionally influence the catalytic properties. 20,25,29 In contrast, during co-precipitation in a continuous microflow 29 the precipitates are formed in a small volume. 27,31 This results in defined and stable preparation conditions, once they are established. In the last decade, micromixing techniques have been increasingly applied for precipitation reactions. 26,27,29,32 Advanced micromixing devices such as microfluidic reactors, T-mixers and confined impinging jet reactors are designed for fast and homogeneous mixing based on high mass transfer and short residence time. [27][28][29] In earlier studies, we used a novel microfluidic reactor especially designed for X-ray spectroscopic in situ studies to investigate colloidal noble metal NP formation during fast reduction reactions, specifically focusing on the critical early stages of the process. [33][34][35]36 The experiments described here aimed at extending the application range of this type of microfluidic device towards coprecipitation reactions, focusing on Cu/ZnO/Al 2 O 3 catalysts for methanol synthesis as a case study.
For this purpose, two streams of the reactants were injected separately into the micromixers integrated in the microfluidic chip. For comparison, Cu/ZnO and Cu/ZnO/Al 2 O 3 catalysts from conventional batch synthesis under conditions adopted from the literature were also characterized and tested in order to illustrate potential advantages of microfluidic mixing during co-precipitation.  3 were dissolved in 38 mL distilled water with 2.5 mL of concentrated nitric acid. 23 mL of 1.6 M aqueous solution of Na 2 CO 3 was used as a precipitating agent. The two solutions were injected dropwise into the batch reactor using individual syringe pumps (neMESYS, Cetoni GmbH, with 50 mL glass syringes).

Materials
The flow rates for injecting metal precursor and precipitating agent solutions were 1 and 0.56 mL min -1 , respectively. The batch reactor used was a round-bottom flask containing 100 mL distilled water stirred at ~ 65 °C. The pH of the precipitate was kept constant at 6-7. Subsequently, the precipitate was aged for 3 h, filtered and washed several times. This process was followed by drying overnight at 70 °C and calcination at 330 °C for 3 h. This procedure resulted in 2 g of catalyst. For synthesizing Cu/ZnO the same procedure was applied, cf. Table 1.

Microfluidic synthesis of Cu/ZnO/Al 2 O 3
A nitrate-derived ternary Cu/ZnO/Al 2 O 3 catalyst was prepared by co-precipitation of aqueous metal nitrate solution with Na 2 CO 3 in a microfluidic chip (Fig. 1

) fabricated in collaboration with
GeSiM GmbH and the Institute of Semiconductors and Microsystems at Technische Universität Dresden (IHM TUD). It is made of Si-bonded glass and was specifically designed for in situ characterization using X-ray spectroscopic and scattering techniques. 36 A metal nitrate solution with a molar Cu:Zn:Al ratio of 60:30:10 was prepared by dissolving the precursors in 40 ml distilled water (total metal ion concentration 0.3 M). 1 ml concentrated nitric acid was added to the solution resulting in pH 1. The Na 2 CO 3 concentration was 0.36 M. To avoid heavy deposition of precipitates in the channel and clogging, the precursors were diluted. However, the metal ratio was kept at the same level as for the batch synthesis. Each of the two reactant solutions (40 mL) was loaded in a 50 mL glass syringe and injected into the microfluidic chip at a flow rate of 22 mL min -1 (neMESYS syringe pump, Cetoni GmbH), and the precipitate was transferred into a flask containing 100 mL distilled water under stirring at 65 °C. After ~15 min a color change from blue to turquoise was observed. The pH of the precipitate was constant at 6-7, and the same ageing, washing, drying and calcination conditions as in the previous section were applied. This procedure provided 0.8 g catalyst.

Catalytic testing: methanol synthesis
MeOH synthesis was performed using the CuO/ZnO and CuO/ZnO/Al 2 O 3 pre-catalysts. The precatalysts were pressed and sieved into fractions of 250-500 µm particle size before use. To minimize hot spot formation and potential sintering, the pre-catalyst was mixed with SiC (Mineraliengrosshandel Hausen GmbH, Telfs, Austria) in a mass ratio of 1:5. The reaction was performed in a continuous-flow, stainless-steel fixed bed reactor (460 mm length, 12 mm inner diameter) which was filled with 5 g of the pre-catalyst/SiC mixture. The temperature was measured at the outside of the reactor wall, details about the equipment are reported elsewhere. 37 Prior to the experiments, pre-catalysts were heated to 100 °C in Ar for 1 h. Catalyst reduction was performed by heating to 200 °C in 5% H 2 in Ar with a heating rate of 0.33 °C min −1 , followed first by increasing the H 2 content to 50% and then the temperature to 240 °C with a heating rate of 0.  Table S1. Reaction products were analyzed using gas chromatography on an Agilent G1530A equipped with RT®-Msieve 5A and RT®-U-BOND columns, a thermal conductivity detector (TCD) and a flame ionization detector (FID). The performance indicators productivity and selectivity were calculated using the equations provided in the supporting information (SI).

Inductively coupled plasma-optical emission spectroscopy (ICP-OES)
The weight loadings of Al, Cu and Zn were determined by ICP-OES spectroscopy (Optima 4300 DV, PerkinElmer). 10 mg of the sample (accuracy ± 0.01 mg) was dissolved in 4 ml hydrochloric acid and 4 ml sulfuric acid at 240 °C for 6 h in the pressure digestion vessel DAB-2 (Berghof).
Elemental analysis was performed with four different calibration solutions and an internal standard (Sc) using three wavelengths of each chemical element for calculation.

Physisorption (BET surface area)
The specific surface area of the catalysts was determined by nitrogen physisorption at -196 °C on a Belsorp mini II apparatus using the Brunauer-Emmett-Teller (BET) method. Prior to the measurements the samples were degassed at 300 °C in vacuum.

Temperature programmed reduction in H 2 (H 2 -TPR) and Chemisorption
For chemisorption analysis, a Micromeritics AutoChem 2950 HP equipped with a thermal conductivity detector (TCD) and an MKS Cirrus 2 mass spectrometer (MS) was used. 160 mg of a sample were placed in a U-shaped quartz glass reactor between two quartz wool plugs. The sample was dried under Ar flow of 30 mL min -1 in situ by heating it from room temperature to 250 °C at 5 °C min -1 and keeping the temperature constant at 250 °C for 2 h.
Afterwards, the sample was cooled down to 50 °C and temperature-programmed reduction (TPR) at ambient pressure in a flow of 5% H 2 in Ar (30 mL min -1 ) through the sample was started. The temperature was increased at a rate of 1 °C min -1 up to 250 °C and kept at 250 °C for 30 min. The effluent gas passed through a cold trap at 0 °C to condense any water formed. The dry gas was then analyzed by the TCD and the MS in series. The H 2 signal of the MS (m/z = 2) was calibrated with 5% H 2 in Ar before and after each measurement and used for quantification of the overall H 2 consumption during TPR.
Subsequently, N 2 O pulse chemisorption was performed. The sample was cooled to room temperature and flushed with 30 mL min -1 He for 30 min. Afterwards, 40 pulses with 0.32 mL of 30% N 2 O in He were dosed onto the sample. The effluent gas from the sample passed through a cooling trap at -196 °C to separate any N 2 O not converted to N 2 . N 2 formed by partial Cu oxidation was then analyzed with the TCD and the MS in series. The TCD data were used to determine the metallic Cu surface area and Cu dispersion (Cu atomic area 0.068 nm², stoichiometry factor 2.0).

X-ray absorption spectroscopy (XAS)
XAS measurements were performed at the CAT-ACT beamline at the KIT synchrotron light source. 38 CAT-ACT´s superconducting multipole wiggler and a double crystal Si (111) monochromator provide high intensity (flux ~ 5x10 10 photons/s/100 mA) at the Cu K-edge energy.
Each catalyst was diluted with Al 2 O 3 in order to optimize the absorption step. Finely pressed and sieved (100-200 μm) catalysts were loaded in 1.5 mm quartz capillaries (20 μm wall thickness, catalyst bed length 6 mm) which serve as in situ reactor cells. The capillaries were carefully leaktested by flowing He gas. TPR was performed in a temperature range between 20 °C and 260 °C (heating rate 1 °C min -1 ) in a 5% H 2 in He flow (50 mL min -1 ). X-ray absorption near edge structure (XANES) scans at the Cu and Zn K-edges were recorded continuously during the mentioned TPR conditions and cooling down. Extended X-ray absorption fine structure (EXAFS) scans were recorded at 20 °C and 260 °C. For energy calibration at the respective edges, spectra of Cu metal foil and ZnO were recorded simultaneously with the samples. EXAFS data analysis was performed using the software packages Athena and Artemis. 39 Athena was first used to process the raw data (absorption coefficient (E) vs. E) by background subtraction, normalization, converting (E) data to (k) data and Fourier transforming the resulting spectrum from k-space to R-space. For the EXAFS background extraction, the input parameter R bkg , was set to 1.0 Å. EXAFS fitting was performed using Artemis by generating theoretical models from available crystallographic data of reference compounds, i.e. CuO and Cu. The models were fitted to the experimental data in k-space and R-space to determine energy shifts (ΔE 0 ) and structural parameters, including changes in the scattering path length (ΔR), passive electron reduction factor (S 0 2 ), coordination number (N) and relative mean-square displacement of the atoms (Debye-Waller factor, σ 2 ).
In situ XANES spectra were analyzed by linear combination fitting (LCF) using the spectra measured at 20 °C and 260 °C as internal references.

Transmission electron microscopy (TEM)
The catalyst powder samples were directly dispersed on Cu grids covered with holey carbon film.
Morphology and microstructure of the catalysts were characterized by high angle annular darkfield (HAADF) scanning transmission electron microscopy (STEM) and high resolution transmission electron microscopy (HRTEM), and their composition was analyzed by energy dispersive X-ray analysis (EDX) using an EDAX S-UTW EDX detector in a FEI Titan 80-300 microscope operating at 300 kV at the Karlsruhe Nano Micro Facility (KNMF). Analysis of STEM-EDX maps was carried out by using the TEM Image & Analysis (TIA 4.7 SP3) software.

Comparison of Cu/ZnO/(Al 2 O 3 ) produced in the microfluidic and batch reactor
The ternary Cu,Zn,(Al) hydroxycarbonate precursor materials were co-precipitated at constant pH in the magnetically stirred-batch reactor and the microfluidic reactor and then, after ageing, washing and drying, analyzed by XRD. In both cases the acquired XRD patterns clearly show reflections of zincian malachite [(Cu 1-x Zn x ) 2 (OH) 2 CO 3 ] with x < 0.3 (Fig. 2a). The XRD data of the precipitates from the batch reactor show additional reflections attributed to gerhardtite [Cu 2 (OH) 3 NO 3 ], 20 whereas the diffraction patterns of the precipitates from the microfluidic reactor indicate presence of rosasite phase with poor crystallinity. 29 The formation of different phases is attributed to the efficient micromixing effect (homogeneous mixing in short time) and the spatially controlled nucleation of the primary precipitates in small volumes flowing in the microchannel.
The pre-catalysts, including the Cu/ZnO-BR, calcined at 330 °C were also investigated by XRD.
Reflections of Al 2 O 3 were not observed for the Al containing materials due to its amorphous phase structure. 24,42 The results in Fig. 2b  Thus, better catalytic performance is expected from Cu/ZnO/Al 2 O 3 -MF. Finally, the samples were studied by transmission electron microscopy combined with EDX mapping. Fig. 4 shows TEM images of calcined Cu/ZnO produced in the batch reactor. STEM measurements combined with EDX mapping (Fig. 5a, S1, S4a and Table S2)  nanoparticles produced in the batch reactor are shown in Fig. 5b, S2, S4b and Table S3, which showed uniform distributions of O, Al, Cu and Zn as well as relatively uniform Cu:Zn ratio in the nanoparticles of this catalyst. This more uniform distribution compared to the Cu/ZnO-BR sample is attributed to the presence of alumina which prevents random sintering of nanoparticles. 25 Interestingly, metal distribution in the calcined Cu/ZnO/Al 2 O 3 -MF is somewhat more uniform (lower variance in Cu:Zn ratios at different positions) and the morphology looks more finely structured compared to nanoparticles produced in the batch reactor (Fig. 5c, S3, S4c and Table   S4). indicating Cu in +2 oxidation state. Another feature is the shoulder S attributed to square planar geometry 43 that has been found to be weaker in the spectrum of the microfluidically synthesized sample (MF) indicating higher geometrical distortion. The EXAFS analysis (   (Table 4). In this case, no contribution from Cu(I) was observed during the TPR which is similar to the results recently reported by Frei et al., 45 however there are examples in literature where relatively low (<10%) contribution from Cu(I) were also reported. 23 Fig. 7c shows the derivative XANES spectra at Zn K-edge during the same TPR to 260 C. The changes observed in the Zn K-edge in Fig. 7b are weak, and to highlight these the spectra are shown as derivative. The reduction of the copper oxide phase that occurs between 150 and 240 °C does not strongly affect the Zn K-edge XANES profile, showing that the segregation and reduction of the Cu 2+ to Cu 0 metal particles is an independent phenomenon. The decrease in the white line intensity accompanied by a shift of the edge to lower energies have been interpreted as indicators of the transformation of Zn(II) from octahedral to tetrahedral coordination. 46,47 These effects are more evident in the corresponding first derivatives that display a regular shift in energy and decrease in the intensity of the Zn 2+ 1s→4p peak from ∼9662.5 eV to ∼9662.0 eV. 48 As the reduction temperature increases, the Zn containing phase is undergoing a regular transformation from the mixed aluminate phase in which Zn possesses an octahedral geometry to a highly dispersed and amorphous ZnO phase with Zn in tetrahedral coordination. Also, the appearance of the ∼9658 eV feature, typical of metallic Zn, 45 is a clear indication that Zn(0) formation is just initiated at the end of the TRP experiment. By comparing the results of LCF of Cu K XANES spectra of TPR obtained for the two catalysts (Fig. 7d) it can be observed that the reduction of Cu species from the batch reactor starts at lower temperature than of those from the microreactor.

Methanol synthesis
After activation, catalysts prepared by the batch and continuous microfluidic precipitation were compared in order to assess their catalytic performance during methanol synthesis from CO-rich syngas. Reaction conditions are summarized in By adding a small amount of CO 2 to the feed (CO/CO 2 = 14:1) CO 2 formation via WGSR can be suppressed in the investigated operation range, leading to nearly 100 % MeOH selectivity (see Table 5), also the water produced through reaction 8 inhibits the formation of DME. At 230 °C and in presence of 1 mol% CO 2 , methanol productivity over the Cu/ZnO/Al 2 O 3 -BR is by a factor of 1.5 higher than over the Cu/ZnO/Al 2 O 3 -MF, whereas at 250 °C in the same gas mixture the catalyst from the microfluidic reactor produces >10% more methanol compared to its counterpart from the batch reactor.
Overall, the Cu/ZnO/Al 2 O 3 -MF catalyst also shows the highest observed methanol productivity (21.9 mmol h -1 g -1 cat at 250 °C, CO/CO 2 = 14) in all experiments, suggesting that the microfluidic co-precipitation method has a promoting effect on catalytic activity under these conditions, while further optimization of the MF synthesis is needed to control the DME selectivity.

Conclusion
A continuously operated microfluidic reactor was used to synthesize a nitrate-derived The catalyst synthesized in the batch reactor demonstrates higher MeOH productivity at 230 °C, whereas at 250 °C the microfluidically synthesized catalyst provides higher MeOH and DME yields. Hence, the study shows that a continuous synthesis using a micromixer is an attractive process for preparation of highly active methanol synthesis catalyst, as it is well-controllable and scalable. The used microfluidic reactor with optically and X-ray transmissible windows, furthermore, offers a possibility to investigate the early stages of methanol catalyst formation during co-precipitation with a multitude of X-ray spectroscopic and scattering techniques.

Conflicts of interest
There are no conflicts to declare.