Combining Electronic and Geometric Effects of ZnO-Promoted Pt

Apr 26, 2016 - (b) Dai , Y. Q.; Lim , B.; Yang , Y.; Cobley , C. M.; Li , W. Y.; Cho , E. C.; Grayson , B.; Fanson , P. T.; Campbell , C. T.; Sun , Y...
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Letter pubs.acs.org/acscatalysis

Combining Electronic and Geometric Effects of ZnO-Promoted Pt Nanocatalysts for Aqueous Phase Reforming of 1‑Propanol Yu Lei,*,†,‡ Sungsik Lee,§ Ke-Bin Low,¶ Christopher L. Marshall,⊥ and Jeffrey W. Elam*,† †

Energy Systems Division, §X-ray Science Division, and ⊥Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States ‡ Department of Chemical and Materials Engineering, University of Alabama in Huntsville, Huntsville, Alabama 35899, United States ¶ Research Resources Center, University of Illinois at Chicago, Chicago, Illinois 60607, United States S Supporting Information *

ABSTRACT: Compared with Pt/Al2O3, sintering-resistant Pt nanoparticle catalysts promoted by ZnO significantly improved the reactivity and selectivity toward hydrogen formation in the aqueous phase reforming (APR) of 1-propanol. The improved performance was found to benefit from both the electronic and geometric effects of ZnO thin films. In situ small-angle X-ray scattering and scanning transmission electron microscopy showed that ZnO-promoted Pt possessed promising thermal stability under APR reaction conditions. In situ X-ray absorption spectroscopy showed clear charge transfer between ZnO and Pt nanoparticles. The improved reactivity and selectivity seemed to benefit from having both Pt-ZnO and Pt-Al2O3 interfaces. KEYWORDS: aqueous phase reforming, biomass, atomic layer deposition, ALD overcoat, platinum catalysts

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exhibited promising reactivity in APR of 1-propanol for hydrogen production. We used two strategies to synthesize ZnO-promoted Pt catalysts (Figure 1) to assess both the electronic and geometric

eforming of biomass-derived feedstock, including monooxygenates, polyoxygenates, and carboxylic acids, can be developed into a useful source for obtaining H2.1 As an extension of the well-established steam reforming of fossil fuels, steam reforming of biomass-derived feedstock is operated at high temperature (873−1073 K),2 while aqueous phase reforming (APR) is carried out in the liquid phase at 473− 574 K under high pressure (25−64 bar).3 Supported Pt-based nanocatalysts are active catalysts in these reactions. Despite past studies, a general challenge for utilizing Pt nanocatalysts is their low stability under harsh reaction conditions. The stability of the nanosized catalysts can be enhanced by adapting the catalyst support materials with matched lattice parameters, proper support-metal interaction, and nanostructured support spatial confinement.4 Another strategy developed recently is to coat the nanoparticles with an overlayer to prevent catalysts from sintering. Colloidal methods5 and atomic layer deposition (ALD)6 have been used to prepare such protecting layers. Because of its self-limiting surface reaction nature, the ALD oxide thin film of proper choice can selectively passivate the defect sites on the catalysts, while leaving favorable sites exposed for enhanced selectivity and reactivity.7 In addition, ALD yields highly dispersed supported preciousmetal nanoparticles on various supports.8 Consequently, ALD is an ideal tool for one-batch preparation of the catalysts support, overcoating oxides, and small precious-metal nanoparticles from nanometer to subnanometer length scales. In this work, we take advantages of ALD to manipulate the electronic and geometric effects of ZnO-promoted Pt catalysts that © XXXX American Chemical Society

Figure 1. Preparation of the ZnO-promoted Pt catalysts with inverse spatial arrangement.

effects of ZnO. The ZnO-promoted Pt catalysts were prepared by performing six ALD ZnO cycles9 on a spherical Al2O3 (NanoDur Al2O3, Alfa-Aesar) substrate, followed by one Pt ALD cycle10 (Pt/ZnO/Al2O3) and vice versa (ZnO/Pt/Al2O3). Unlike the spatial arrangement in the first strategy, where Pt nanoparticles were only in contact with the thin ZnO film, Pt nanoparticles were in contact with both ZnO and Al2O3 in the ZnO/Pt/Al2O3 architecture. Moreover, in the ZnO/Pt/Al2O3 arrangement, the ZnO overcoat may serve as a protecting layer to prevent Pt sintering. Using this approach, we produced PtReceived: April 3, 2016

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DOI: 10.1021/acscatal.6b00963 ACS Catal. 2016, 6, 3457−3460

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anomalous small-angle X-ray scattering (ASAXS) measurements (Table S1). The spatial arrangement of the catalyst designed in Figure 1 is illustrated in the STEM images. For Pt/ZnO/Al2O3 (Figures 2a and 2b), the Pt nanoparticles are supported on ZnO and ZnO covers the Al2O3 surface. The lattice fringes show an interplanar spacing of 0.26 nm, which corresponds to the ZnO (002) lattice planes, and the Pt nanoparticles show a distinct Pt/vacuum interface. In contrast, STEM images of ZnO/Pt/ Al2O3 (Figures 2c−e) show clearly that the Pt nanoparticles are encapsulated by a ZnO thin film (1−2 nm), which is consistent with the XRF results. Figures 2e and 2f show the ZnO thin film of wurtzite structure with a hexagonal unit cell viewed from the [0001] direction. The encapsulated Pt nanoparticles appear as regions of enhanced brightness (white circle), because of the high Z number of Pt. We assessed the overall accessibility of the platinum surface on the fresh catalyst samples using in situ CO adsorption XANES spectra at the Pt L3 edge (see Figure S1 in the Supporting Information). For the ZnO/Pt/Al2O3, most of the Pt surface was covered by ZnO as the Pt L3 XANES spectrum only slightly broadened after CO adsorption. The strong interaction between Pt and ZnO was observed in the ALD catalyst by synchrotron X-ray-based differential pair distribution function (d-PDF, Figure S2 in the Supporting Information). The Pt/Al2O3 has an average Pt−Pt bond distance of 2.72 Å, contracting slightly from the normal bond distance of 2.75 Å observed in bulk Pt. The Pt−Pt bond distance of Pt/ZnO/Al2O3 catalysts relaxed back to 2.75 Å, which was a clear indication of strong Pt−ZnO interaction. This strong Pt and ZnO interaction is also observed by in situ XANES spectra measured in helium (Figure S3 in the Supporting Information). The white line reflecting the 1s → 5p transition shifted to higher photon energy by 2.5 and 2.2 eV for Pt/ZnO/Al2O3 and ZnO/Pt/Al2O3, respectively, compared to the XANES for Pt/Al2O3. This shift indicates a significant Zn → Pt charge transfer. The catalytic properties of the ALD catalysts were evaluated for the APR of 1-propanol to produce H2 as a model reaction (see Figure 3, as well as Table S2 in the Supporting Information). These studies were carried out at 523 K at a system pressure of 64 bar in a trickle-bed reactor. A 5 wt % 1propanol/water solution was used as the feed. The gas-phase effluent was composed of propanal, propane, ethane, and CO2, while only propanal and unreacted 1-propanol were detected in the liquid-phase effluent. No CO was detected in the gas phase, suggesting that any CO product was rapidly converted to CO2 via the Pt-catalyzed water−gas-shift (WGS) reaction.3b Therefore, the C2H6:CO2 molar ratio is close to 1 in the product. The highest activity was achieved from the ZnO/Pt/Al2O3 catalyst, with a rate of 0.44 mol of H2 formed per mole of Pt per minute. This catalyst also exhibited stable catalytic performance (see Figure 3b). Pt/Al2O3 and Pt/ZnO/Al2O3 were less active, with reaction rates of 0.36 and 0.25 mol of H2 formed per mole of Pt per minute, respectively. The H2 formation rate from Pt/ Al2O3 is ca. 1.4 times larger than that from Pt/ZnO/Al2O3, suggesting that Pt on Al2O3 is more active than Pt on ZnO for the APR of 1-propanol. Similarly, the hydrogen turnover frequency from Pt/Al2O3 was reported to be ca. 3 times greater than that of Pt/ZnO for APR of ethylene glycol at 498 K.13 The H2 selectivity decreased in the order

based catalysts that resist sintering in the APR of 1-propanol at 523 K under a system pressure of 64 bar, while enhancing the catalytic activity of the Pt nanoparticles with the proper spatial arrangement of catalyst and its promoter. X-ray fluorescence spectroscopy (XRF) was used to determine the weight loading of Pt and ZnO on the catalysts (see Table S1 in the Supporting Information). After one Pt ALD cycle, the Pt loading on spherical Al2O3 was 1.6 wt %, similar to our previous study.11 Pt ALD can exhibit nonlinear growth during the initial ALD cycles, and this is strongly dependent on the starting surface.8b,12 Indeed, the Pt loading for Pt/ZnO/Al2O3 was 2.0 wt %, higher on the bare Al2O3 substrate. The ZnO loading was 23.8 and 24.5 wt % for Pt/ ZnO/Al2O3 and ZnO/Pt/Al2O3, respectively. Assuming a ZnO density of 5.61 g/cm3 and a Al2O3 surface area of 45 m2/g, the ZnO film thickness following six ZnO ALD cycles was 1.3 nm, indicating an average ZnO ALD growth rate of ca. 2.2 Å/cycle, in agreement with previous studies.9 Figure 2 displays aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images for the Pt ALD nanocatalyst samples. The Pt nanoparticles are uniformly dispersed on the spherical Al2O3 with an average diameter of 0.9 ± 0.1 and 0.9 ± 0.2 for Pt/ZnO/Al2O3 and ZnO/Pt/Al2O3, respectively. The diameter of the Pt nanoparticle was verified using synchrotron-based

Figure 2. Aberration-corrected HAADF-STEM images of (a, b) Pt/ ZnO/Al2O3 and (c, d, e) ZnO/Pt/Al2O3; (f) atomic structure of wurtzite ZnO thin film viewed from the [0001].

ZnO/Pt/Al 2O3 > Pt/ZnO/Al 2O3 > Pt/Al 2O3 3458

DOI: 10.1021/acscatal.6b00963 ACS Catal. 2016, 6, 3457−3460

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suppresses the C−O bond cleavage and hydrogen consumption to form propane. These reductions in propane formation with the addition of ZnO might be caused by the Zn → Pt charge transfer. The presence of PtZn alloys has been correlated with the reactivity of Pt/ZnO catalysts in catalytic conversion of methanol such as steam reforming and selective methanol dehydrogenation to methyl formate.14 ZnO/Pt/Al2O3 has the highest selectivity to propanal (68%) and low selectivity to C2H6 (20.7%) and CO2 (10.7%). Without ZnO, Pt/Al2O3 produced the largest percentage of C2H6 (48.3%) and CO2 (23.1%). Propanal was produced through the second reaction pathway: dehydrogenation of 1-propanol. The C−C activation and cleavage subsequently occurred on the catalyst surface, forming ethane and CO. The C−C bond is less activated on the ZnO/Pt/Al2O3 surface. We ascribe this to a possible steric hindrance on the surface for C2H5−CHO activation caused by the overcoating ZnO layer. The enhanced activity and H2 selectivity from ZnO/Pt/ Al2O3 derived from both the modified electronic and morphological structures. Figure 4 shows the aberration-

Figure 4. Aberration-corrected HAADF-STEM images of the ALD ZnO/Pt/Al2O3 catalysts after APR of 1-proponal carried out at 523 K and under a system pressure of 64 bar. Focusing was adjusted to allow (a) Pt and (b) ZnO to be in-focus, respectively.

corrected HAADF-STEM images of ZnO/Pt/Al2O3 catalysts after the APR of 1-proponal. In contrast to the as-deposited structure (Figure 2), the Pt nanoparticles are no longer completely encapsulated, but still possess both Pt-ZnO and Pt− Al2O3 interfaces. The partially overcoated ZnO suppressed propane formation, which consumed H 2. The Pt-Al2 O3 combination, on the other hand, enhanced the activity of the catalyst for dehydrogenation of 1-propanol for highly selective H2 production with the least amount of CO2 formation. The thermal stability of Pt catalyst samples was analyzed using STEM. Histograms of the size distributions of Pt nanoparticles based on STEM images taken before and after testing and from multiple locations are shown in Figure S5 in the Supporting Information. The average diameter of the Pt/ Al2O3 catalyst increased from 1.0 ± 0.3 nm to 2.4 ± 0.7 nm following the APR testing with largest particles up to 8 nm in diameter. The ripening mechanisms for the unpromoted Pt/ Al2O3 catalyst included Ostwald ripening (Figure S6 in the Supporting Information) and coalescence (Figure S7 in the Supporting Information) of Pt nanoparticles under the APR conditions. The Pt/ZnO/Al2O3 and ZnO/Pt/Al2O3 catalysts showed greater stability under APR conditions, with average diameters of only 1.4 ± 0.4 and 1.2 ± 0.3 nm, respectively, for the spent catalysts.

Figure 3. Reactivity and selectivity of APR of 1-propanol. (a) Alkane selectivity (black), H2 selectivity (red) in gas-phase effluent, and H2 formation rate (blue bar). (b) Performance of ZnO/Pt/Al2O3 with time-on-stream in the APR of 1-propanol at 473 K under 64 bar system pressure. Carbon selectivity in gas phase effluent for (c) Pt/ Al2O3, (d) Pt/ZnO/Al2O3, and (e) ZnO/Pt/Al2O3.

while the alkane selectivity showed the opposite trend (Figure 3a). The ZnO/Pt/Al2O3 is most selective to H2 formation (50.5%). Without the ZnO promoter, Pt/Al2O3 has the highest alkane selectivity of 56.2%. Figures 3c−e shows the carbon selectivity obtained from the three catalysts in the APR of 1-propanol. According to the reaction mechanism (see Figure S4 in the Supporting Information), the direct deoxygenation of 1-propanol will consume 1 hydrogen molecule and produce 1 propane molecule. The propane selectivity from the Pt/Al2O3 catalyst is 7.9%. With the ZnO promoter, the propane selectivity decreased to 0.9% and 0.6% from Pt/ZnO/Al2O3 and ZnO/Pt/ Al2 O 3 catalysts, respectively. This indicates that ZnO 3459

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The enhanced thermal stability imparted by the ALD ZnO was confirmed using ASAXS measurements (Figure S8 in the Supporting Information). The average diameter of the Pt nanoparticles in the Pt/Al2O3 catalyst increased from 1.5 nm to 1.6 nm at 473 K and 2 nm at 573 K after 30 min annealing in 1propanol vapor. For Pt/ZnO/Al2O3 and ZnO/Pt/Al2O3, there was no obvious size change of Pt nanoparticles between 473 K and 573 K. This enhanced thermal stability may result from favorable epitaxy between the Pt and the ZnO. Despite the different crystal structure of ZnO (P63mc, a = b = 3.25 Å, c = 5.20 Å) and Pt (Fm3m, a = 3.92 Å), a combined low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) study showed that six unit cells of Pt(111) surface only mismatch with five ZnO(0001) cells by 3%.15 In conclusion, the introduction of ZnO to the Pt/Al2O3 catalysts enhanced the thermal stability of Pt nanoparticles of ca. 0.9 nm in diameter. In addition, ZnO tuned the H2 selectivity in the APR of 1-propanol via Zn → Pt charge transfer. In comparison to the Pt/ZnO/Al2O3 catalyst, ZnO/ Pt/Al2O3 showed the higher reactivity and H2 selectivity, as it benefited from having both Pt-ZnO and Pt−Al2O3 interfaces under the reaction conditions. These results demonstrate the possibility to tune the activity and selectivity, as well as the thermal stability, of Pt nanocatalysts by varying the spatial arrangement of the promoter and the nanoparticles.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b00963. Additional details on materials and experiments, XANES spectra, STEM images, ASAXS, and catalytic performance (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: 1-256-824-6527. E-mail: [email protected] (Y. Lei). *Tel.: 1-630-252-3520. E-mail: [email protected] (J. W. Elam). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported as part of the Institute for Atom-Efficient Chemical Transformations (IACT), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH11357. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. We thank Dr. Haiyan Zhao for helping with PDF experiments.

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REFERENCES

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DOI: 10.1021/acscatal.6b00963 ACS Catal. 2016, 6, 3457−3460