(M=Fe, Co, Ni) Nanowires for Efficient Catalytic Hydrogen Generation

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Energy, Environmental, and Catalysis Applications

A Universal Strategy for Ultrathin Pt-M (M=Fe, Co, Ni) Nanowires for Efficient Catalytic Hydrogen Generation Shuxing Bai, Bolong Huang, Qi Shao, and Xiaoqing Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05873 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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ACS Applied Materials & Interfaces

A Universal Strategy for Ultrathin Pt-M (M=Fe, Co, Ni) Nanowires for Efficient Catalytic Hydrogen Generation Shuxing Bai,1 Bolong Huang,2 Qi Shao,1 and Xiaoqing Huang1* 1

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu, 215123, China.

2

Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China. *

To whom correspondence should be addressed. E-mail: [email protected]

KEYWORDS: methanol reformation, hydrogen, platinum, iron, ultrafine nanowires

ABSTRACT: Methanol (CH3OH) reformation with water (H2O) to in-situ release hydrogen (H2) is regarded as a hopeful H2 production approach for polymer electrolyte membrane fuel cells (PEMFCs), while developing highly efficient CH3OH reformation catalysts still remains a great challenge. Herein, a series of Pt-based ultrafine nanowires (UNWs) with high surface atoms ratio are used as highly active and stable catalysts for CH3OH reformation to H2. By tuning Pt3M (M = Fe, Co, Ni), support and the composition of the PtxFe UNWs, the optimized Pt4Fe UNWs/Al2O3 exhibits excellent catalytic behaviors with the high H2 turnover frequency (TOF) reaching to 2035.8 h-1, more than 4 times higher than that of Pt UNWs/Al2O3. The reaction mechanism investigated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) turns out that the production of H2 undergoes the CH3OH decomposition to *CO and gas-shift reaction of *CO with H2O. Combing with the XPS result and the

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density functional theory (DFT) calculations, the high CH3OH reformation activity of Pt4Fe UNWs/Al2O3 is attributable to synergism between Pt and Fe, which facilitates H2 desorption and intermediate HCOO* and *COO formations via the reaction between *CO and OH-.

INTRODUCTION The increasing consumption of fossil fuels contributes serious environmental issues,1,2 due to excessive emissions of NOx, COx, and hydrocarbons. Hydrogen (H2), as a renewable clean energy resource, is considered as one of the most potential alternative energy carriers and served as the anode fuel in polymer electrolyte membrane fuel cells (PEMFCs) to produce electricity, but it is difficult to transport and handle caused by its inherent properties, which largely limits its practical applications.3,4 Methanol (CH3OH) is expected to be one of the most promising H2 storage materials for PEMFCs, considering that it has high energy density as a liquid and can in-situ produce H2 by catalytic reforming with water (H2O).5,6 However, the conventional Cu-based catalysts for CH3OH reformation conduct under high temperatures (210-330 °C) and high pressures (2.5-5.0 MPa), which are unfit as in-situ H2 generation catalysts to supply H2 for PEMFCs.7 In fact, high H2 production can be obtained by the reaction of strong alkali (KOH) and neat CH3OH at low temperature,8 but treatment of byproduct (K2CO3) and stoichiometric alkali consumption result in more complex reaction system and operation cost. Therefore, there is an urgent demand yet significant challenge to exploit efficient catalysts for CH3OH reformation to generate H2 with low CO production (< 10 ppm) under neutral condition.9 Many researchers so far have focused on platinum (Pt)-based catalysts,10,11 due to their excellent ability for CH3OH reformation, while its high price and scarce reserves heavily restrict its practical applications.12 To improve intrinsic activity and atomic utilization of Pt, many researchers concentrate on controlling the structures of Pt-based catalysts, given that their catalytic properties are always


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determined by their structures.13,14 Unfortunately, since plenty of Pt atoms in the bulk phase could not participate in the catalytic reaction, low atomic utilization of various Pt-based nanostructures limit catalytic activity enhancements.15,16 Decreasing the diameter of nanowires (NWs) to several atomic layers (sub-2 nm) can offer an opportunity for exposing Pt atoms to a great extent.17,18 Meanwhile, due to high surface atoms ratio, unsaturated surface coordination and active surface, ultrafine NWs (UNWs) are expected to exhibit enhanced performance.19,20 Herein, Pt-based UNWs, synthesized by a facile method, are served as highly active and stable CH3OH reformation catalysts to produce H2. After series of catalysis optimization, the 1 wt% Pt4Fe UNWs/Al2O3 represents high H2 turnover frequency (TOF) of 2035.8 h-1 and excellent long-term stability. Combined the result of the surface electronic properties of different Pt-based UNWs/Al2O3, the higher H2 production of Pt4Fe UNWs/Al2O3 is due to more electron density of surface Pt atoms. Mechanism study by Fourier transform infrared spectroscopy (FT-IR) identifies that the reaction pathway includes CH3OH adsorption and activation, CH3OH decomposition to H2 and *CO, the reaction of *CO species with*OH by H2O dissociation to form the HCOO* and *COO, finally to produce H2 and CO2. Considering that the strong adsorption of *CO leads to the low H2 production in the neat CH3OH decomposition without H2O, the presence of H2O facilitates CH3OH decomposition as well as gas-shift reaction. The density functional theory (DFT) calculations indicate that the enhanced CH3OH reformation activity on Pt4Fe UNWs/Al2O3 stems from favorable HCOO* formation by desirable adsorption-switching at the Fe sites and an easy desorption of H2 due to electron transfer between Pt and Fe to weaken the Pt-H bond.

EXPERIMENTAL SECTION


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Synthesis of PtxFe Ultrafine Nanowires (UNWs): In a typical preparation of Pt3Fe UNWs, platinum (II) acetylacetonate (Pt(acac)2, 10.0 mg), Iron (II) acetylacetonate (Fe(acac)2, 4.2 mg), molybdenum hexacarbonyl (Mo(CO)6, 9 mg), cetyl trimethyl ammonium bromide (CTAB, 109.5 mg), 3 mL oleylamine (OAm) and 2 mL diphenyl ether (DPE) were added into a vial (volume: 35 mL). After the vial had been capped, the mixture was ultrasonicated for 120 min. The resulting homogeneous mixture was heated from room temperature to 180 ºC in 30 min and maintained at 180 ºC for 5 h in an oil bath, before it cooled to room temperature. The resulting colloidal products were collected by centrifugation and washed three times with a cyclohexane/ethanol mixture. The synthetic conditions for Pt3Co UNWs and Pt3Ni UNWs were similar to those of Pt3Fe UNWs except changing Fe(acac)2 to Cobalt(II) carbonate hydroxide (2CoCO3·3Co(OH)2, 0.9 mg) and nickel (II) acetylacetonate (Ni(acac)2, 2.1 mg), respectively. The synthetic conditions for Pt UNWs, Pt6Fe UNWs, Pt4Fe UNWs, and Pt2Fe UNWs UNWs were similar to those of Pt3Fe UNWs except changing the amounts of Fe(acac)2 to 0, 2.1, 3.2, and 5.3 mg, respectively. Characterizations. The morphologies of the UNWs were determined by transmission electron microscope (Hitachi, HT7700) at 120 kV. High resolution transmission electron microscopy (HRTEM) and high angle annular dark field scanning TEM (HAADF-STEM) were conducted on an FEI Tecnai F20 transmission electron microscope at an acceleration voltage of 200 kV. X-ray diffraction (XRD) patterns were collected using an X’Pert-Pro X-ray powder diffractometer equipped with a Cu radiation source (λ = 0.15406 nm). Scanning electron microscopy energy dispersive X-ray (SEM-EDX) spectroscopy was performed on a scanning electron microscope (Hitachi, S-4700). All the X-ray photoelectron spectroscopy (XPS) spectra of the Pt-based UNWs were collected by XPS (Thermo Scientific, ESCALAB 250 XI). The concentrations of all the catalysts were determined by the inductively coupled plasma atomic emission spectroscopy (ICP-AES) (710-ES, Varian).


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Catalytic tests. The methanol (CH3OH) reformation was performed in a 60 mL stainless-steel autoclave. After the additions of 10 mL H2O, 10 mL CH3OH and 5 mg catalyst into a Teflon inlet, the autoclave was pressurized with N2 (2 MPa). The reaction was performed at 200 ºC with stirring at 800 rpm for 1 h. After the completion of the reaction, the gaseous mixture was analyzed using a gas chromatograph (Agilent GC-7890B) equipped with a hayesep Q column connected to a thermal conductivity detector (TCD) and a 5A molsieve column connected to a methane reformer with nickel catalyst and a flame ionization detector (FID). The tests were repeated three times for each catalyst. The yields of H2 and the H2 turnover frequency based on the Pt (TOFPt) are calculated using Equation (1) and (2). Yieds(µmol g-1 s-1) = nH2(µmol)/(mcat(g)×t(s))

(1)

TOFPt(h-1) = nH2(mol)/(nPt(mol)×t(h)) = nH2(mol)×MPt(g mol-1)/( mcat(g)×Pt wt%×t(h)) (2) Owing to high solubility of CO2 in the liquid phase at high pressures, the concentration of CO2 could not be accurately detected. Therefore, the CO2 concentrations (CCO2, ppm) are calculated using Equation (3) based on the following chemical equations. CH3OH + H2O → CO2 + 3H2 CH3OH → CO + 2H2

(a)

(b)

CO2 + 4H2 → CH4 + 2H2O

(c)

CCO2(ppm) = (CH2(ppm)+4×CCH4(ppm)-2×CCO(ppm))/3-CCH4(ppm))

(3)

The selectivity of CO are calculated using Equation (4) SCO(%) = CCO(ppm)/(CCO(ppm)+CCO2(ppm)+CCH4(ppm))

(4)

RESULTS AND DISCUSSION


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Figure 1. (a) HAADF-STEM image, (b) TEM image, (c) HRTEM image, (d) XRD pattern, and (e) SEM-EDX spectrum of Pt4Fe UNWs. (f) TEM image of Pt4Fe UNWs/Al2O3.

The PtxFe UNWs (x = 2, 3, 4, 6) were prepared by using a facile wet-chemical method. Typically, platinum (II) acetylacetonate (Pt(acac)2), iron (II) acetylacetonate (Fe(acac)2), molybdenum hexacarbonyl (Mo(CO)6) and cetyltrimethyl ammonium bromide (CTAB) were dissolved in a mixed solvent of oleylamine and diphenyl ether (DPE) by ultrasonication. The solution was gradually heated to 180 ºC and maintained at 180 ºC for 5 h in an oil-bath. The Pt/Fe molar ratios in PtxFe UNWs were handily tuned by altering the amount of Fe(acac)2 supplied. The synthesis of Pt3Co UNWs and Pt3Ni UNWs was


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similar to that of PtxFe NWs except for substituting Fe(acac)2 with Co and Ni precursors, respectively. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images of Pt4Fe UNWs (Figure 1a&Figure S1) show that the product takes the form of homogeneous UNWs with an average diameter of ~ 1.7 nm and the aspect ratio of ~ 13 (inset of Figure 1a). As showed in Figure 1b&Figure S2-S3, the TEM images of different PtxFe UNWs and Pt3M (M = Fe, Co, Ni) UNWs clearly demonstrate the high yield and purity of these UNWs. As can be seen from the high-resolution TEM (HRTEM) image of Pt4Fe UNWs in Figure 1c, the 0.224 nm spacing is corresponding to the (111) facet of Pt4Fe UNWs, which is between that of Pt (0.226 nm) and Pt3Fe (0.222 nm).21 The X-ray diffraction (XRD) patterns of these PtxM UNWs (Figure 1d&Figure S4) show broad diffraction peaks at near 40°, due to the small diameter of these UNWs.22 The overall Pt to M atomic ratios in PtxFe UNWs and Pt3M UNWs were measured by scanning electron microscopy energy-dispersive X-ray spectroscopy (SEMEDX, Figure 1e&Figure S5-S6) and X-ray photoelectron spectroscopy (XPS, Figure S1c-S1d). The Ptbased UNWs were then loaded on different supports (Al2O3, TiO2, CeO2, C, and SiO2) for catalytic evaluations (Figure 1f&Figures S7). Generally, given full play of Pt atoms as active site, the main factors influencing catalytic behaviors contain alloying, support, catalyst composition, as well as mass loading.22-27 The alloy effect is firstly investigated and Al2O3 is selected as the support for the CH3OH reformation. After completion of the reaction, the gaseous mixture was analyzed by gas chromatograph (Figure S8-S9). It was informed that trace amounts of CO and methane, and abundant of H2 and CO2 as gas-phase products were detected. As shown in Figure 2a, the ranking for H2 yields of CH3OH reformation of Pt UNWs/Al2O3 and Pt3M UNWs/Al2O3 is as following: Pt3Fe > Pt3Co > Pt3Ni > Pt (Figure 2a&Table S1), indicating that alloying hastens the CH3OH reformation to H2. To precisely compare the intrinsic activity, Pt-based TOFs (TOFsPt) were assessed, supposing Pt atoms as the active sites for the CH3OH reformation.10 The


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overall activities of all the Pt3M UNWs/Al2O3 exhibited activity improvement by a factor of 1.4-4.1 over Pt UNWs/Al2O3. Considering that supports are also critical for performance improvement by supplying extra active sites and changing the electrical properties,25,26 the catalytic behaviors for CH3OH reformation of Pt3Fe UNWs over different supports were also measured (Figure 2b&Figure S10b). H2 yields for Pt3Fe UNWs/CeO2, Pt3Fe UNWs/SiO2, Pt3Fe UNWs/C, Pt3Fe UNWs/TiO2, and Pt3Fe UNWs/Al2O3 are 7.3, 10.8, 17.2, 19.4, and 20.0 µmol g-1 s-1, respectively. Among these supports, Al2O3 and TiO2 exhibit the greatest improvement for CH3OH reformation of Pt3Fe UNWs, in which abundant Lewis acids on the surface of Al2O3 and TiO2 likely provide active sites for H2O dissociation.28 We also examine the effect of composition of PtxFe alloy UNWs on CH3OH reformation activity. As shown in Figure 2c, H2 yields exhibit a typical volcano-shaped curve with the ratio of Pt to Fe, as 20.8, 22.6, 20.0, and 15.4 µmol g-1 s-1 are observed for the Pt6Fe UNWs/Al2O3, Pt4Fe UNWs/Al2O3, Pt3Fe UNWs/Al2O3, and Pt2Fe UNWs/Al2O3, respectively. It is evident that the Pt4Fe UNWs/Al2O3 exhibits the best CH3OH reformation activity to H2, with the TOFPt of 2035.8 h-1 being 1.23, 1.15, and 1.28 times higher than that of Pt6Fe UNWs/Al2O3, Pt3Fe UNWs/Al2O3, and Pt2Fe UNWs/Al2O3, respectively (Table S1&Figure S10c). The mass loading is usually regarded as an important factor for noble metal atomic utilization and subsequently cost.29 As the Pt mass loading increasing from 0.2 wt% to 1 wt%, the yields of H2 increase rapidly and then the yields of H2 increase slightly with further increasing to 3 wt% (Figure 2d). To balance high H2 production and high Pt atomic utilization, 1 wt% Pt loading is appropriate for CH3OH reformation.


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Figure 2. The achieved H2 yields and selectivity of CO of (a) Pt UNWs/Al2O3 and Pt3M UNWs/Al2O3, (b) Pt3Fe UNWs over different supports, (c) PtxFe UNWs/Al2O3 with different Pt/Fe ratios, and (d) Pt4Fe UNWs/Al2O3 with different Pt loadings in CH3OH reformation at 200 ºC for 1 h. Error bars correspond to the deviations from three independent experiments.

The influence of temperature on catalytic activity was also investigated (Figure 3a&Figure S11). As the temperature increase, H2 yield improved straightly and the CO selectivity at 140 ºC ~ 200 ºC was all below 0.3 %, suggesting that high reaction temperature is favorable to CH3OH reformation to H2. As can be noted from the Arrhenius plot on Pt4Fe UNWs/Al2O3 (Figure 3b), the apparent activation energy (Ea) value is ~ 78.2 kJ mol-1. The low Ea deduces low reaction energy barrier and high reactivity for CH3OH reformation, which is beneficial to the H2 generation.10 Accordingly, catalytic behaviors of the


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Pt4Fe UNWs/Al2O3 were further investigated by exploring H2 yield with the reaction time (Figure 3c), where H2 yield increases linearly to 0.41 mmol in initial 1 h, and then increases tardiness to 0.52 mmol in the following 4h, indicating 1 h is the optimum reaction time for Pt4Fe UNWs/Al2O3. Finally, the stability of Pt4Fe UNWs/Al2O3 was explored. In ten reaction rounds, the Pt4Fe UNWs/Al2O3 reaches a total turnover number (TToN) of surpassing 14808 for each Pt atom, producing ~ 0.59 molH2 gcat-1 (Figure 3d). After ten rounds of successive reactions, the UNWs structures of Pt4Fe UNWs/Al2O3 integrally retained (Figure S12). On contrast, only TToN of 1049 for each Pt atom was made for the commercial Pt/C (20 wt%) and severe aggregation was observed after ten rounds (Figure S13-S14). Therefore, excellent catalytic activity and long-term stability of CH3OH reformation are achieved on Pt4Fe UNWs/Al2O3, which are of crucial importance for practical applications.1 As the surface state of the catalyst is of great important for the catalytic behavior, the surface electronic properties and Pt valence state of different Pt-based UNWs/Al2O3 were characterized by XPS (Figure 4). Pt 4d spectrum of Pt UNWs/Al2O3 clearly demonstrate the presence of two chemical environments for Pt atoms with Pt 4d5/2 at 313.9 and 317.0 eV attributed to Pt0 and Pt2+, respectively.30 Compared with the binding energy (BE) of Pt0 in Pt UNWs/Al2O3, the decrease of BEs in Pt3M UNWs/Al2O3 (313.7-313.8 eV) (Figure 4a&Table S2) caused by the charge transfer from M to Pt, which induces the higher electron density of Pt atoms.21,31 The ranking for the BEs of Pt0 4d5/2 in Pt UNWs/Al2O3 and Pt3M UNWs/Al2O3 is listed as follows: Pt3Fe (313.7 eV) = Pt3Co (313.7 eV) < Pt3Ni (313.8 eV) < Pt (313.9 eV), which is in relation to electronegativity of Pt (2.28), Fe (1.83), Co (1.88), Ni (1.91). Compared this ranking with the TOFPt (Figure S10a), it is clearly shown that the lower BEs of Pt0 4d5/2 result in the better catalytic activities for CH3OH reformation. In other words, Pt atoms with the more electron density are favorable for CH3OH decomposition.32 Additionally, according to Pt 4d spectra of PtxFe UNWs/Al2O3 with different Pt/Fe ratios (Figure 4b), BEs of Pt0 4d5/2 in PtxFe


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UNWs/Al2O3 decrease with the increase of Fe content, which further confirmed the charge transfer from Fe to Pt.

Figure 3. (a) The achieved H2 yields and selectivity of CO and (b) the Arrhenius plot for the CH3OH reformation of Pt4Fe UNWs/Al2O3 at different temperatures for 1h. (c) Time course of CH3OH reformation catalysed by Pt4Fe UNWs/Al2O3 at 200 ºC. (d) TToN achieved by Pt4Fe UNWs/Al2O3 over ten rounds of successive reactions. Error bars correspond to the deviations from three independent experiments.

On account of the complicacy of CH3OH reformation to H2, the reaction mechanism investigated at the atomic level is still a grand challenge.5 To this end, we studied the conversion of reaction intermediate species by DRIFTS. Infrared (IR) spectra after exposure Pt4Fe UNWs/Al2O3 to pure CH3OH or reaction atmosphere of CH3OH + H2O are displayed in Figure 5. After exposing Pt4Fe


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UNWs/Al2O3 to CH3OH (Figure 5a&Figure S15a), a complex series of adsorption species were observed. The peaks appear at 3706, 2983, 1058 cm-1 and 3664, 2973, 1033 cm-1, which assigning to two kinds of chemisorption of CH3OH.33 The peaks assigned to *CH2OH (3679, 1008 cm-1), *OCH3 (2950, 2922, 2825 cm-1), *HCHO (2966, 1457 cm-1), *HCOH (1344 cm-1), and *CO (2052, 1802 cm-1) were observed, respectively.33 After removal of CH3OH gases, as time prolonging, the peaks of adsorption CH3OH, *CH2OH, *OCH3, and *HCHO species decline, while those of *CO boost. Combined with IR spectrum CO on Pt4Fe UNWs/Al2O3 (Figure S16), the peaks of gas CO at 2172 and 2117 cm-1 are not detected during CH3OH decomposition on Pt4Fe UNWs/Al2O3, indicating that the desorption of *

CO to form gas CO needs higher temperature.34 In other words, in the absence of H2O, the strong

adsorption of *CO leads the low H2 production (6.5µmol g-1 s-1) by neat CH3OH decomposition to H2 and CO on Pt4Fe UNWs/Al2O3. In principle, the introduction of H2O in the CH3OH reformation is to diminish the amount of CO and avoid formation of coke.35,36 To gain more insight, under reaction condition (CH3OH + H2O, 200 ºC) the peaks of adsorption CH3OH were weakened substantially and the peaks of intermediate species enhanced. Meanwhile, new IR peaks at 2372, 2335, 1652, 1397, 924, and 916 cm-1 appeared (Figure 5b&Figure S15b). The peak of formate (HCOO*) species appeared at 1397 cm-1.37 Peaks at 2372, 2335 cm-1 and 1652, 924, 916 cm-1 are assigned to CO2 and bidentate carbonate (*COO) species, respectively.37 On the basis of these results, the reaction pathway can be summarized as follows: *CO species react with*OH by H2O dissociation to form the HCOO* and *COO, consequent to produce CO2, during gas-shift reaction. Overall, the presence of H2O facilitates CH3OH decomposition as well as diminishes the amount of CO by gas-shift reaction.


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Figure 4. Pt 4d XPS curves of (a) Pt UNWs/Al2O3 and Pt3M UNWs/Al2O3, and (b) PtxFe UNWs/Al2O3 with different Pt/Fe ratios.

Due to the tremendous influence of alloying and Pt/Fe ratio on CH3OH reformation (Figure 2a&Figure 2c), the species on various catalysts are investigated under the reaction condition (Figure S17). For all catalysts, types of these species are analogous, indicating the similar reaction pathway and the same active sites for CH3OH reformation. On Pt UNWs/Al2O3 and Pt3M UNWs/Al2O3 (Figure S17a), the intensities of two kinds of chemisorption CH3OH have basically been the same, while ranking for the intensity of these intermediate species (*CH2OH, *OCH3, *HCHO, *HCOH, and *CO) are as follows: Pt3Fe > Pt3Ni > Pt3Co > Pt, which is consistent with ranking of the activity of CH3OH reformation. Thus alloying enhances CH3OH reformation by upgrading the capacity of Pt atoms for CH3OH decomposition. In order to deeply study the association of reaction intermediate species and catalytic behaviors, signal intensity of *CO species (2052 cm-1) and the H2 TOFPt values of Pt3M UNWs/Al2O3 and PtxFe UNWs/Al2O3 are summarized in Figure S18, where similar variation trends


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were observed, suggesting that the surface properties determine the catalysis by changing the strength of intermediate species.

Figure 5. IR spectra of (a) CH3OH decomposition and (b) CH3OH reformation after Pt4Fe UNWs/Al2O3 exposed to CH3OH and CH3OH + H2O at 200 ºC, respectively.

To study how the interactions between Pt atoms and Fe atoms in Pt4Fe UNWs and H2O facilitate CH3OH reformation reaction, projected density of states (PDOS) and the crucial reaction steps in CH3OH reformation were performed using DFT calculations (DFT models and calculations setup were


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described in supporting information). The electronic properties of the catalyst Pt4Fe have been illustrated in terms of the PDOS on the d-orbitals (Figure 6a). The d-bands of Pt-sites in the bulk are almost evenly distributed from -6 eV to 0 eV relative to the Fermi level (EF). However, the Pt sites on the surface show obviously different behavior. The surface Pt-5d level from Pt4Fe has been evidently modified by the incorporation of the Fe. The dominant peak of the Pt on the surface without neighboring to the Fe sites is locating at about 1.2 eV below the EF. The d-orbital levels of Pt sites on the surface next to the Fe sites have been suppressed by the Fe-4d6 electrons and peak of Pt-5d shows a downshift of 2.78 eV. The Fe sites show nearly the same d-orbital levels wherever it stays within the bulk or on the surface region, since the level variation has only 0.3~0.5 eV in difference. We further compared the electronic properties between Pt4Fe and PtFe (Figure 6b). We find that the Fe-3d orbital levels of bonding and anti-bonding states are almost unchanged, with only 0.3 ~ 0.5 eV shift shown by the peak of the antibonding states. The Pt-5d level on the surface with Fe nearest neighbored behaves different trend. The uniform Pt-bonding orbital (PtFe) will be split into two bands (Pt4Fe) with energy difference of 2.8 eV measured from peak to peak. The dominant Pt bonding orbital level within Pt4Fe has 1.7 eV lower than the one in PtFe. The pathway for energetic evolutions has been studied in terms of free energy change of chemisorption (∆G) (Figure 6c). The initial reactants are given as [CH3OH + H2O] considered from the experiments. The collective adsorption of CH3OH and H2O shows chemisorption of -0.21 eV. We find the decomposition of CH3OH into CO + 2H2 are rather energetic favorable with -3.58 eV gain in energy. The H2O* decomposition further gains -0.60 eV in energy and leads the system to the lowest energy level. We find that the potential determining step (PDS) occurs at the transformation into the [HCOO* + 2H2 + H]. This step shows the barrier of 3.29 eV. Following step shows a change in adsorption point from HCOO* to *COO with barrier of 2.93 eV. We find that the change of H + H into H2 does not involve


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evident energy change compared with [HCOO* + 2H2 + H] to [*COO + 3H2]. The final step CO2 + 3 H2 has an energy of -1.11 eV, indicating an overall spontaneous process and energetically favorable for such proposed route. We notice that the Fe site favors bonding with long-pair p-π electrons from OH- and CO*, thus the CO* actively bonding with Fe sites on the Pt4Fe surface, as well as the OH-. The down shifting of the Pt-5d orbital level where overlaps with Fe-3d bonding orbital level will further transfer the electrons from Fe-3d to Pt-5d bonding level to form electron-rich center (Pt0) on the surface Pt-site. This downshift will also facilitate the weakening of the binding energy between Pt-H. The proton (H+) can thus have fast charge exchange and H + H → H2 with an easy desorption. The schematic mechanism diagram has been summarized in Figure 6d. Therefore, the Pt4Fe is a unique catalyst to transform the CH3OH into CO2 and H2 with H2O assistance.

Figure 6. (a) PDOS of different Pt and Fe sites in the Pt4Fe system. (b) PDOS comparison on different surface Pt and Fe sites between Pt4Fe and PtFe. (c) Free energy pathway is shown for the transition from CH3OH + H2O to CO2 + 3H2 on the Pt4Fe surface. (d) Schematic mechanism is shown to highlight the significance of the downshifting of Pt-5d orbital level by existence of Fe within Pt4Fe system. Pt-Surface N. Fe is the Pt on the surface next to the Fe.


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CONCLUSIONS To summarize, Pt-based UNWs were used as highly efficient CH3OH reformation catalysts for H2 generation. By streamlining the alloying element, composition and supports, Pt4Fe UNWs/Al2O3 displayed the highest H2 yield and excellent durability. XPS results established that the lower BE of surface Pt atoms with more electron density is in favors of CH3OH reformation to H2. Depending on the results of DRIFTS, the pathway of CH3OH reformation includes the CH3OH decomposition and gasshift reaction. Furthermore, H2O not only facilitates CH3OH decomposition but also diminishes the amount of CO by gas-shift reaction. Comparing the variation of BEs of Pt0 4d5/2, *CO signal intensity, and the H2 TOFPt values, it is confirmed that controlling surface electronic properties boosts the catalytic performance via enhancing the adsorption of crucial intermediate species. The DFT results reveal that Fe-3d site in Pt4Fe UNWs favors bonding with lone-pair (O-2p-π) electrons from OH- and CO* to form HCOO*, and effective adsorption exchange from HCOO* to *COO. The downshifting of Pt-5d orbital level due to the charge transfer from Fe to Pt induces the weakening of the binding energy of Pt-H to facilitate the H2 desorption. The present work presents a promising strategy for controlling surface electronic properties and tuning adsorption of crucial intermediate species to boost the CH3OH reformation to H2. ASSOCIATED CONTENT Supporting Information. Figure S1-18 and Table S1-2. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author [email protected]


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Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the Ministry of Science and Technology (2016YFA0204100, 2017YFA0208200), the National Natural Science Foundation of China (21571135), Young Thousand Talented Program, the start-up supports from Soochow University, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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TOC


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(M=Fe, Co, Ni) Nanowires for Efficient Catalytic Hydrogen Generation

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