tube Catalysts

Article pubs.acs.org/JPCC

Novel Pulse Electrodeposited Co−Cu−ZnO Nanowire/tube Catalysts for C1−C4 Alcohols and C2−C6 (Except C5) Hydrocarbons from CO and H2 Mayank Gupta,† Viviane Schwartz,‡ Steven H. Overbury,‡ Karren More,‡ Harry M. Meyer, III,‡ and James J. Spivey*,† †

Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States

‡

ABSTRACT: Co−Cu−ZnO nanowire/tube catalysts were synthesized using pulse electrodeposition technique from a single aqueous electrolyte solution using a template synthesis technique. They were then tested as catalysts for the hydrogenation of CO to alcohols and higher hydrocarbons. Nanowires/ tubes were grown inside the pores of membranes using a three-step sequential deposition process. First, a low current of −6.9 mA/cm2 was applied for 300 ms for Cu deposition, then a high current density of −11.5 mA/cm2 for t ms (t = 500, 600, 750 ms) was applied for Co deposition, and finally no current was applied for 1200 ms so that the ions near the cathode replenish. The surface had a significantly different composition than the bulk. On the surface, there was more Co, less Cu, and more Zn. The catalyst showed the alcohol (C1−C4) selectivity of 20.9 %C at H2/CO = 3/1, GHSV = 16 000 scc/h gcat, temperature = 270 °C, pressure = 15 bar, and time-onstream = 65 h. materials.14−16 The two main reasons for these distinct properties17,18 are as follows: (1) an increased surface to volume ratio which results in corresponding increase in their activity as catalysts and (2) the dominance of quantum effects which significantly changes a material’s optical, magnetic, or electrical properties.19−21 Diversified synthesis techniques including lithography,17 molecular beam epitaxy (MBE),22 thermal evaporation,23 ion beam sputtering,24 chemical vapor deposition,25−27 pulse laser deposition,28 vapor−liquid−solid (VLS) mechanism,29 sol−gel method,30 pulse electrodeposition of nanowires/tubes,13,31 surfactant-assisted growth,32 and hydrothermal approach,33,34 have been employed to synthesize these nanostructures. Electrodeposition is one of the promising technologies because of its versatility and suitability for deposition in trenches of small dimensions. Electrodeposition can be used to control the aspect ratio (length-to-diameter ratio) of the metal nanowires by monitoring the total amount of passed charge, which is not possible with other methods.35,36 Nanowires with multiple segments of different metal/alloys in a controlled sequence can be fabricated by applying suitable current/potential (pulse electrodeposition) in an electrolyte solution containing different metal ions. However, careful selection of electrolytes, additives, and

1. INTRODUCTION There is a need for alternative fuels because of limited oil supply,1 increased oil demand,1 and persistent increase in oil prices.2 One alternative fuel/additive is a mixture of methanol and higher alcohols. Another important application of this mixture is that it can be used for transporting hydrogen. These alcohols can then be steam reformed or partially oxidized to produce a hydrogen-rich reformate.3−5 One of the ways to produce these alcohols is via catalytic conversion of CO and H2. To date, rhodium-based catalysts have been the most promising but their prohibitive cost and limited supply hinder their ability to be used as industrial catalysts.6 Thus, much less expensive Cu-based catalysts7,8 are an attractive option. Generally, heterogeneous catalysts are prepared by conventional methods such as coprecipitation and impregnation. To further increase the selectivity of these catalysts toward desired products, novel methods that can engineer the surface and molecular environment of the catalysts, are needed. Novel catalyst preparation methods have been found to be promising.9−11 One such method is pulse electrodeposition of nanowires/ tubes.12,13 This method consists of a number of independent synthesis variables (electrolyte composition, temperature, pulse schemes, etc.) that allows better control over surface properties of multimetallic catalysts than conventional methods. Fabrication of one-dimensional (1D) metals and metal oxide nanostructures are the subject of increasing attention due to their distinct properties compared with the corresponding bulk © 2012 American Chemical Society

Received: December 12, 2011 Published: April 18, 2012 10924

dx.doi.org/10.1021/jp301965s | J. Phys. Chem. C 2012, 116, 10924−10933

The Journal of Physical Chemistry C

Article

pulse schemes is necessary when elements have very large difference in their reduction potentials such as Cu and Co. The template-based approach has been extensively investigated to fabricate nanowires/tubes. Possin reported the electrodeposition of Sn, In, and Zn nanowires using radiation track etched mica membranes for the first time.37 A comprehensive overview of the template based preparation of wide variety of nanowires including metals, polymers, semiconductors, and carbon with detailed chemistry had been explored by various researchers.38−40 Individual metal/oxide nanowires of Co, Cu, and Zn had been fabricated by several research groups using variety of templates due to its promising applications in piezoelectric devices, sensors, and solar cells.41−48 Combination of such metal/oxide nanowires might yield novel materials having superior applications. For example, Oh et al.49 electrochemically deposited n-p and/or p-n nanocolumnar junction structures of Cu2O, ZnO, on Ni nanowires using a commercially available alumina template. In one of our previous works, nanowires of Cu-ZnO and Mn−Cu−ZnO50 were synthesized by pulse and direct electrodeposition. These nanowires were active catalysts for the synthesis of alcohols from CO hydrogenation. Mn−Cu−ZnO nanowire catalysts seem to be promising because of high selectivity (15.7%) toward higher alcohols at low reaction pressure (10 bar). Addition of manganese to Cu−ZnO nanowires increased the selectivity toward C2−C4 alcohols from 5.4% to 15.7%, while suppressing the formation of methane. Here, we chose Co−Cu−ZnO because these types of catalysts have shown to be promising for mixed alcohols. However, there is no clear consensus on how Co and Cu interact. Pure Cu catalysts adsorb CO associatively and are well-known for synthesis of methanol;51 whereas pure Co catalysts adsorb CO dissociatively and are active for the synthesis of hydrocarbons.52 Because both types of CO adsorption are needed to form higher alcohols, intimate mixing of Co and Cu atoms holds promise for the formation of higher alcohols,53 possibly by interaction between Co and Cu to form a CoCu surface alloy.54 Several researchers have found Co−Cu−ZnO catalysts to be promising for ethanol and higher alcohols.53,55,56 In the present study, we synthesized, characterized, and tested (as catalysts) Co−Cu−ZnO nanowires/tubes using electrodeposition into nanopores of polycarbonate track etch (PCTE) membranes.57,58

The composition and pH of the electrolyte solution used in this work are shown in Table 1. The electrolyte was magnetically Table 1. Composition of the Electrolyte electrolyte

initial pH

Cu(NO3)2 (M)

Zn(NO3)2 (M)

Co(NO3)2(M)

CoCuZn

4.2

0.002

0.025

0.01

stirred at 320 rpm. The cell was kept inside a water bath to maintain the temperature of the electrolyte solution at 60 ± 2 °C. All of the experiments were carried out with a VersaSTAT3 potentiostat/galvanostat (AMETEK Princeton Applied Research, TN). Three types of current pulse schemes were applied in order to obtain different metal/oxide compositions and atomic environments. A low current to deposit Cu, followed by a high current to deposit Co and then an off-time was given to allow the ions to be replenished near the cathode. Figure 1 is an example of one of the pulse schemes used and Table 2 gives all of the details of the pulses applied.

Figure 1. Pulse scheme.

Table 2. Detailed Pulse Schemes catalyst

low current density (mA/cm2) /time(ms)

high current density (mA/cm2) /time(ms)

off-time (ms)

A B C

−6.9/300 −6.9/300 −6.9/300

−11.5/500 −11.5/600 −11.5/750

1200 1200 1200

After the electrodeposition experiment, membranes were dissolved in methylene chloride (CH2Cl2) and then sonicated to separate the entangled nanowires/tubes. Nanowires were separated from CH2Cl2 by centrifugation and then using acetone they were transferred to aluminum dishes. The nanowires were then dried in the oven at 110 °C for 12 h. Imaging and compositional analyses were performed using a Hitachi HF3300 high-resolution transmission-scanning transmission electron microscope (TEM/STEM), which was operated at 300 kV. This instrument is also equipped with a secondary electron detector (SE) and a Bruker X-Flash silicon drift detector (SDD) for energy dispersive spectrometry (EDS). Samples for TEM were prepared by embedding the nanowires in epoxy (Gatan G-1) and placing between two silicon wafers. Cross sections of this “sandwich” structure were prepared by mechanically grinding, dimpling, and ion milling to perforation such that several thin, individual nanowires could be viewed longitudinally and in cross-section. Bulk elemental compositions were determined using a Perkin-Elmer 2000 DV ICPOES. X-ray diffraction (XRD) patterns were obtained with an

2. EXPERIMENTAL SECTION Polycarbonate track etch (PCTE) membranes (Sterlitech Corporation, WA), having an average pore diameter of 400 nm and thickness of ca. 25 μm, were used as templates for the synthesis of Co−Cu−ZnO nanowires/tubes via pulse electrodeposition. Experiments were carried out in a typical three-electrode cell.50 A thin gold film was sputter coated on one side of the membrane which later served as the working electrode for the pulse electrodeposition process. The coated membrane with open pores facing upward was placed on a Cu plate held inside a polyetheretherketone (PEEK) holder. The deposition of nanowires takes place inside these pores. The counter electrode and the reference electrode were a platinized Ti-mesh and a saturated calomel electrode (SCE), respectively. The three electrode holder was immersed in a 5 L container, the whole assembly served as the electrochemical cell. It was filled with freshly prepared electrolyte containing nitrates of Co, Cu, and Zn dissolved in deionized water at room temperature. More details about the experimental setup and method can be found in this work.50 10925



Article

controlled regime. All the potentials reported in the present study are the reversible electrode potential (E) vs SCE, calculated using the Nernst equation. The deposition of Cu takes place through the following reaction:

automated X-ray powder diffractometer (Bruker/Siemens D5000, Cu Kα radiation). XPS analyses were done using a ThermoScientific model K-α X-ray Photoelectron Spectrometer (XPS) system. Samples were mounted onto nonconducting double-sided tape which were fixed to glass slides. Mounted samples were introduced into the XPS analysis chamber (base pressure 6 × 10−10 mbar) though a turbo-pumped load-lock. Experiments were conducted using monochromatic Al-Kα X-rays and detecting photoemitted electrons using a hemispherical electron energy analyzer and multichannel plate detector. Survey spectra were collected at a pass energy of 200 eV over the range 0−1300 eV binding energy, while core level spectra were collected at 50 eV pass energy. Sample charging induced by the photoemission process was alleviated using a charge compensation system consisting of both low energy Ar-ion and low energy electrons. CO hydrogenation studies were performed in a tubular fixed bed reactor (AMI-200R-HP) manufactured by Altamira Instruments, Inc. Catalysts were placed inside a glass lined reactor tube (0.25″ OD, 0.15″ ID, 12″ length) using quartz wool. First, 10% O2 in He was passed through the catalyst for 2 h at 400 °C to oxidize any carbon left after dissolution of the polycarbonate membrane. Then, the catalyst (physically mixed with γ-Al2O3 supplied by Alfa Aesar) was reduced using 100% H2 at 320 °C for 2 h. The reaction was performed at 15 bar, H2/CO = 3/1, GHSV = 16 000 scc/h gcat, and 270 °C. The product stream was analyzed by a GC-FID (GC-2014) manufactured and supplied by Shimadzu Scientific Instruments, Inc.

Cu 2 + + 2e → Cu 0

(E = + 0.01 V)

(1)

where E is the reversible reduction potential. Region B has a plateau that is indicative of the mass transport controlled deposition of Cu. The following side reaction could also occur in this region: 2H+ + 2e → H 2

(E = −0.37 V)

(2)

Region C represents the sudden increase in current after −0.54 V, which could be due to following reactions: Cu(OH)2 + 2e → Cu 0 + 2OH− Co2 ++ 2e → Co0

(E = − 0.55 V)

(E = − 0.59 V)

Co(OH)2 + 2e → Co0 + 2OH−

(3) (4)

(E = − 1.03 V)

(5)

The following undesirable reaction can also take place in this region. Mainly, this is the reduction of water to form hydrogen. 2H 2O + 2e → H 2 + 2OH−

(E = −1.01 V)

(6)

At the electrode surface, the pH of the solution changes from acidic to alkaline due to the reduction of nitrate ions.59,61,62 In fact, Zhang et al.61 performed the in situ experiments using a microsensor and found that the local pH at the electrode increased from 4.5 to 8.3 during the electrodeposition of mixed Er-ZnO/(OH)2 using a nitrate electrolyte possibly due to the following reaction:

3. RESULTS AND DISCUSSION The electrodeposition of different elements (such as Cu and Co) with desired composition is difficult because they have disparate reduction potentials. In addition to these two elements, we have deposition of ZnO which makes it even more complex because ZnO deposits chemically rather electrochemically.59,60 To achieve desired composition of the nanowires/tubes, it is necessary to know the different reactions occurring at the cathode in the potential range studied. Figure 2 shows the polarization curve for the electrolyte used to electrodeposit nanowires/tubes using a gold sputtered mem-

NO−3 + H 2O + 2e− → NO−2 + 2OH−

(E = − 0.26 V) (7)

Deposition of ZnO is not electrochemical rather chemical. Therefore, it cannot be represented by any of the regions of the polarization curve. ZnO is deposited in the nanowires/tubes via the following reaction sequence:59,60 Zn 2 + + 2OH− → Zn(OH)2

(8)

Zn(OH)2 → ZnO + H 2O

(9)

The above reactions and their corresponding E values give an idea to construct pulse schemes (Figure 1, Table 2) to achieve desired morphology and concentrations. First, a low current was applied to deposit Cu (major) and Co (minor) and then a high current density was applied to deposit Co. ZnO will be deposited both times because of the pH change.13 An off-time was given to allow ions to replenish on the cathode. 3.1. Image Analysis. 3.1.1. SEM. Figure 3 shows nanowires/tubes produced using different current schemes. The nanowires are 425 ± 10 nm thick and 0.5−8 μm long. The wide range of length is due to the sonication process during the dissolution of membranes in CH2Cl2. Although it is an essential step during sample preparation, the process causes the nanowires/tubes to disperse. This separation is useful for increasing the exposed surface area. More surface area increases the specific catalytic activity. In general, most of the nanostructures for 500 and 600 ms are nanowires as evident from SEM micrographs. Nevertheless, at 750 ms most of nanostructures are nanotubes, which are formed when there is excessive hydrogen generation due to reactions 2 and 6 at

Figure 2. Polarization for the CoCuZn electrolyte using a gold sputter coated membrane.

brane as a cathode. The potential scan rate was 10 mV/s. Region A corresponds to the electrodeposition of Cu in kinetically 10926



Article

Figure 3. SEM micrographs of nanostructures at on-time (a) 500, (b) 600, and (c) 750 ms.

Figure 4. Cross-sectional view of nanowires at on-time (a) 500, (b) 600, and (c) 750 ms.

higher potentials.50,63 Higher current is required for longer time to include more Co in the nanowire/tube. TEM-STEM cross-sectional images for nanowires synthesized for 500, 600, and 750 ms are shown in Figure 4a−c, respectively. Nanowires are porous due to hydrogen generation during electrodeposition. Porosity is helpful in increasing the surface of area of nanowire/tube catalysts. 3.2. Cross-Sectional Composition Maps of Nanowires/ Tubes. Most of the reported literature on nanowires do not discuss the distribution of its elements.13,16,64 Properties of nanomaterials can change significantly depending upon the distribution of their constituent elements. Figure 5 shows a series of elemental maps acquired for crosssectioned Cu−Co−Zn nanowires synthesized for 500, 600, and 750 ms. At any given condition, many of the nanowires exhibited a core−shell type structure, where Cu was found predominantly in the core and Zn formed the shell (see

Figure 5b, 600 ms). However, some nanowires were composed only of Zn (see Figure 5a, 500 ms) and others exhibited a more random elemental distribution (see Figure 5c, 750 ms). In all nanowires produced, the Cu and Zn were highly phaseseparated. It should be noted that oxygen was also associated with Zn, indicating the formation of Zn-oxide phase (s). Although there was a relatively inhomogeneous distribution of the Cu and Zn in some nanowires, their bulk and surface compositions were reproducible. Figure 5(a),(b) has well-defined cross sections as expected for nanowires. However, Figure 5(c) does not have a welldefined cross-section but a hollow center as expected for a nanotube. The diameter is much smaller than 400 nm and there is no material in some parts. As mentioned earlier, this sample also had some nanowires. It was not possible to get a crosssection of nanotubes after trying several times. We are not aware of any studies where cross-section of a nanowire and/or 10927



Article

longitudinal sections of nanowires for 500, 600, and 750 ms. It is evident that the composition is nonuniform for deposition time of 500 and 750 ms. Deposition time of 600 ms produces more uniform nanowires. It is clear that some parts of nanowires contained more Cu and some more Zn which could be due to nonuniform transportation of ions into the pores of the membrane. Another reason could be side reactions (reactions 2 and 6) that can significantly affect the distribution of ions in the solution. Side reactions cannot be avoided because reduction potential of Co is more positive than water reduction. There would be more side reactions when on-time is 750 ms; therefore making final morphology mostly that of a nanotube instead of a nanowire. 3.4. Bulk Composition (ICP). Figure 7 shows the variation in wt% Cu, Co, and Zn for the “bulk” batches of nanowires

Figure 7. Bulk composition (ICP) of nanowires/tubes.

measured by ICP as a function of increasing the on-times for high current density. Co content increased with on-time from 0.52 to 0.82% when the on-time was increased from 500 to 600 ms. Interestingly, there is a further 3-fold increase when the on-time increased from 600 to 750 ms. This is expected because deposition of Co is kinetically controlled. However, the Cu content decreases and then increases with the increase in on-time. This decrease is expected with an increase in on-time because of enhanced deposition of Co. But the reason for its increase when the on-time was increased from 600 to 750 ms is not clear. Cu content decreased from 47.1% to 33.1% when the on-time increased from 500 to 600 ms; whereas it increased from 33.1% to 44.1% when the on-time increased from 600 to

Figure 5. Longitudinal compositional map of nanowires/tubes at ontime (a) 500, (b) 600, and (c) 750 ms.

tube (by electrodeposition) is reported. Even though the crosssectional image does not clearly indicate that the morphology is nanotube, the SEM images do. Therefore, we believe that the nanostructures are nanotubes. 3.3. Longitudinal Composition Maps of Nanowires. Figure 6 shows EDS elemental maps collected from

Figure 6. Cross-sectional compositional map of nanowires/tubes at on-time (a) 500, (b) 600, and (c) 750 ms. 10928



Article

750 ms. Zn content increased from 52.3% to 66.1% and then decreased from 66.1% to 56.2% with an increase in on-time because of Cu deposition. 3.5. Surface Composition (XPS). 3.5.1. Survey Scan. Figure 8 shows the surface composition of the nanowires as

Figure 9. Different environments near and away from the pore wall.

Figure 8. Surface composition (XPS) of nanowires/tubes.

measured by XPS; the surface composition parallels the bulk ICP measurements for each time, except that the Co and Zn show much higher surface concentrations and Cu shows lower surface preference. Co content increased with on-time. The increase in Co surface concentration was more than 4-fold when the on-time increased from 500 to 750 ms. Greater Co surface concentration is necessary for greater selectivity toward higher alcohols and higher hydrocarbons because it catalyzes chain growth.65 However, a balance between Co and Cu concentration is required to enhance the higher alcohol selectivity because Co promotes the formation of both higher hydrocarbons and higher alcohols.66 Cu surface concentration decreases and then increases with an increase in on-time, similar to the bulk composition. The Cu surface concentration decreased from 16.6% to 12.9% when the on-time increased from 500 to 600 ms whereas it increased from 12.9% to 24.6% when the on-time increased from 600 to 750 ms. The reason for this trend is not clear. There is no significant change in Zn when on-time was increased from 500 to 600 ms; however, it decreased from 78.9% to 53.4% when the on-time was increased from 600 to 750 ms. Direct comparison between bulk and surface concentrations shows that there is more Co on the surface, less Cu on the surface, and more Zn on the surface at any given on-time. This variation in the compositions is attributed to the different atomic environment for the bulk and the surface species during the time of deposition. The surface of the nanowires was formed due to the deposition of ions near the pore wall but the bulk material of the nanowires was formed due to the deposition within the pores, as shown schematically in Figure 9. Pore walls are coated with polyvinylpyrrolidone (PVP) to make them hydrophilic. PVP has different affinity toward different ions.67,68 The order is Co2+ > Zn2+ > Cu2+, explaining why the surface has more Co and Zn than the bulk. 3.5.2. High Resolution Scan. To determine the oxidation states and bonding of the species present on the surface, high resolution scans of XPS were performed. In general, the

Figure 10. High resolution spectra of (a) carbon and (b) oxygen.

calculated compositions from the survey and core level data for the as-received samples are in good agreement. Note that in Figures 10 and 11, spectra have been normalized to constant intensity to emphasize line shape and binding energy positions. The C 1s spectra [Figure 10 (a)] of all the samples look similar and consist of a large peak at ∼284.5 eV attributed to C−C and C−H bonds, and three smaller features at ∼286, ∼288, and ∼290 eV. These smaller peaks are all oxidized forms of carbon and are generally assigned to C−O, O−C−O, 10929



Article

Figure 11. High resolution spectra of (a) copper, (b) zinc, and (c) cobalt.

and −CO3 (carbonate) species respectively. All are typical forms of carbon observed on metallic and oxide surfaces. There are corresponding features in the O 1s [Figure 10 (b)] spectra to support this. The O 1s spectra show a main feature at ∼530 eV corresponding to metal oxide bonding (Zn−O predominantly). The O 1s spectra are composed of a complex mix of (at least) 5 different species. The apparent shift in the peak maximum

is simply a shifting of the maximum of the entire O 1s “envelope” due to an increase in the amount of the four higher BE components as compared to the lowest BE component, i.e., the red component which is related to Zn−O bonding. The green is attributed to Cu−O bonding, while the three yellow peaks are attributed to the various forms of oxidized C. The ratio of the green-to-red varies similar to the Cu/Zn ratio. 10930



Article

were composed of a well crystalline material containing ZnO and metallic Cu.Cu is in metallic form as expected from its electrochemistry (reaction 1). However, XPS shows that Cu is in oxidized form, which means that the surface of nanowires/ tubes was oxidized during sample storage and transfer. Zn is present as ZnO, consistent with theory and XPS. No Co is observed so it is either amorphous or doped into the ZnO lattice.69 The gold peak arises from the precursor membrane that was gold coated before the electrodeposition experiments. There is no significant difference between the nanowires/ tubes having different on-times. This is in agreement with XPS results which state that the different on-times do not have any effect on phases of the nanowires/tubes. 3.7. CO Hydrogenation Studies. Here, we present preliminary reaction results using one set of Co−Cu−ZnO nanowires/tubes (on-time 600 ms) to convert CO and H2 to mixed alcohols and C2−C6 hydrocarbons. The main undesired products were methane and CO2. The selectivity to C2−C4 alcohol synthesis is favored thermodynamically as pressure increases.70 The use of pressures as low as 10 bar50,71 and as high as 400 bar72 have been reported in the literature.73 Here, we investigate the selectivities of various products at H2/CO = 3/1, 15 bar, GHSV = 16 000 scc/h gcat, and temperature = 270 °C. The CO hydrogenation reactions were carried out at differential conditions to avoid the condensation of products in the reactor line, and therefore the CO conversion was very low (Table 3). At 15 bar, the C1−C4 alcohol selectivity was 20.92% while selectivity for C2−C6 (except C5) was 31.4%. Selectivities for CO2 and methanol were 12.9 and 13.1%, respectively.

Figure 11 shows the spectra for Cu, Co, and Zn. The Cu spectra [Figure 11 (a)] show a major peak at ∼932 eV corresponding to Cu1+ for all the samples. Electrochemical deposition of oxides of Cu is not possible at the conditions applied (see reactions 1,3,4, and 5) so that the surface of the nanowire must have oxidized during exposure to room conditions. The same observation was made for Mn−Cu− ZnO nanowires in our previous work.13 The Cu 2p spectra have two features. The first one is a narrow peak at ∼932 eV and is attributed to either Cu 1+ or Cu0. The second feature is made up of three components: a main peak at ∼934 eV and two satellite peaks at higher BE. These three components are characteristic of Cu2+ (CuO). The ratio of Cu1+ (or Cu0) to Cu2+ for 500 and 600 ms is significantly different but it is much less for 750 ms and is in agreement with the larger Cu−O peak in the O 1s spectrum. This is due to higher oxidation of sample with 750 ms as they were mostly nanotubes. The Zn spectra [Figure 11 (b)] look similar to each other and indicate that Zn is in an oxidized form (this was confirmed by also collecting the Zn Auger feature at ∼500 eV and comparing the separation of the Zn Auger from the Zn 2p 3/2, the socalled Auger parameter.) The inclusion of Zn in the nanowires/ tubes as ZnO is expected, as discussed earlier. The position of the main Co feature [Figure 11 (c)] at ∼781 eV and a satellite at ∼5.5 eV higher BE both indicate Co in a 2+ valence state. The formation of Co2+ cannot be due to electrodeposition. It is either chemically deposited or oxidized during sample preparation, transportation, and storage. It can be inferred from the above findings that pulse schemes (changes in on-time during higher current density) did not affect the phases of material electrodeposited unlike its composition (both bulk and surface). Phases can change if the applied current density is different and/or off-time is different. 3.6. XRD. Figure 12 shows the XRD patterns for the nanowire/tubes at varying on-times. All of the nanowires/tubes

4. CONCLUSIONS Co−Cu−ZnO nanowires/tubes having different composition and morphology have been prepared by pulse electrodeposition. Different on-times resulted in different bulk and surface composition. More compositional uniformity was obtained when the on-time for higher current density was 60 ms, which resulted in mostly a core−shell type of structure. Compositional maps revealed that nanowires/tubes were heterogeneous in nature. Co content (both bulk as well as surface) increased significantly when the on-time was increased. Bulk and surface concentration of Cu decreased and then increased with the increase in on-time but it was less at the surface than the bulk. Zn bulk and surface concentration showed no trend with on-time. High resolution XPS showed that Co, Cu, and Zn are present as Cu2+, Co2+, Zn2+, respectively. We believe that the presence of ZnO is due to synthesis process but the presence of CuO and CoO could be due to the oxidation of Cu and Co during transfer and storage of the samples. XRD showed that nanowire/tube catalysts were composed of a well crystalline material containing ZnO and metallic Cu. However, Co was not observed.

Figure 12. XRD patterns of nanowires/tubes at different on-times.

Table 3. Selectivities of Various Products over Nanowire/Tube Catalysts with 600 ms On-Timea selectivity (%C) pressure (bar)

methanol

C2−C4 alcohols

methane

C2−C6 hydrocarbonsb

CO2

15 (after 65 h)

13.1

7.82

34.8

31.4

12.9

CO conversion 0.65

Reaction conditions: H2/CO = 3/1, GHSV = 16 000 scc/h gcat, temperature = 270 °C. Error: ± 7% bWithout C5. Selectivity (%C) = (NiCi × 100)/(∑ NiCi), where Ni is the number of carbon atoms in product and Ci is concentration (mol %). a

10931



Article

(15) Ohgai, T.; Mizumoto, M.; Nomura, S.; Kagawa, A. Mater. Manuf. Proc. 2007, 22, 440−443. (16) Piraux, L.; Encinas, A.; Vila, L.; Matefi-Tempfli, S.; MatefiTempfli, M.; Darques, M.; Elhoussine, F.; Michotte, S. J. Nanosci. Nanotechnol. 2005, 5, 372−389. (17) Chang, T. H. P.; Kern, D. P.; Kratschmer, E.; Lee, K. Y.; Luhn, H. E.; McCord, M. A.; Rishton, S. A.; Vladimirsky, Y. IBM J. Res. Dev. 1988, 32, 462−493. (18) Khomutov, G. B. Adv. Colloid Interface Sci. 2004, 111, 79−116. (19) Kaur, R.; Verma, N. K.; Chakarvarti, S. K. J. Mater. Sci. 2007, 42, 3588−3591. (20) Sulitanu, N. D. NATO Sci. Ser., II: Math., Phys. Chem. 2004, 169, 363−376. (21) Fert, A.; Piraux, L. J. Magn. Magn. Mater. 1999, 200, 338−358. (22) Debnath, R. K.; Meijers, R.; Richter, T.; Stoica, T.; Calarco, R.; Luth, H. App. Phys. Lett. 2007, 90, 123117. (23) Suleiman, K. M.; Huang, X.; Liu, J.; Tang, M. Smart Mater. Struct. 2007, 16, 89−92. (24) Oates, T. W. H.; Keller, A.; Noda, S.; Facsko, S. App. Phys. Lett. 2008, 93, 063106. (25) Malandrino, G.; Finocchiaro, S. T.; Nigro, R. L.; Bongiorno, C.; Spinella, C.; Fragala, I. L. Chem. Mater. 2004, 16, 5559−5561. (26) Kim, H. W.; Kim, N. H. App. Phys. A: Mater. Sci. Process. 2005, 81, 763−765. (27) Park, W. I.; Kim, D. H.; Jung, S. W.; Yi, G. C. App. Phys. Lett. 2002, 80, 4232−4234. (28) Zhang, T.; Shen, Y.; Hu, W.; Sun, J.; Wu, J.; Ying, Z.; Xu, N. J. Vac. Sci. Technol. B 2007, 25, 1823−1826. (29) Bae, S. Y.; Seo, H. W.; Park, J. H. J. Phys. Chem. B 2004, 108, 5206−5210. (30) Chen, Y. W.; Liu, Y. C.; Lu, S. X.; Xu, C. S.; Shao, C. L.; Wang, C.; Zhang, J. Y.; Lu, Y. M.; Shen, D. Z.; Fan, X. W. J. Chem. Phys. 2005, 123, 134701. (31) Pullini, D.; Busquets, D.; Ruotolo, A.; Innocenti, G.; Amigo, V. J. Magn. Magn. Mater. 2007, 316, e242−e245. (32) Xu, C.; Xu, G.; Liu, Y.; Wang, G. Solid State Commun. 2002, 122, 175−179. (33) Cao, M.; Wang, Y.; Guo, C.; Qi, Y.; Hu, C.; Wang, E. J. Nanosci. Nanotechnol. 2004, 4, 824−828. (34) Zheng, D.; Sun, S.; Fan, W.; Yu, M.; Fan, C.; Cao, G.; Yin, Z.; Song, X. J. Phys. Chem. B 2005, 109, 16439−16443. (35) Baba, N.; Masuda, H. 1993, 5, 61−66. (36) Kaur, R.; Verma, N. K.; Chakarvarti, S. K. J. Mater. Sci. 2007, 42, 8083−8087. (37) Possin, G. E. Rev. Sci. Instrum. 1970, 41, 772−774. (38) Hulteen, J. C.; Martin, C. R. J. Mater. Chem. 1997, 7, 1075− 1087. (39) Martin, C. R. Science (Washington, D. C.) 1994, 266, 1961− 1966. (40) Al-Salman, R.; Mallet, J.; Molinari, M.; Fricoteaux, P.; Martineau, F.; Troyon, M.; El Abedin, S. Z.; Endres, F. Phys. Chem. Chem. Phys. 2008, 10, 6233−6237. (41) Cheng, C. L.; Lin, J. S.; Chen, Y. F. Mater. Lett. 2008, 62, 1666− 1669. (42) Fang, D.; Huang, K.; Liu, S.; Qin, D. Electrochem. Commun. 2009, 11, 901−904. (43) Wang, J. G.; Tian, M. L.; Kumar, N.; Mallouk, T. E. Nano Lett. 2005, 5, 1247−1253. (44) Wu, M. S. App. Phys. Lett. 2005, 87, 153102. (45) de Jongh, P. E.; Vanmaekelbergh, D.; Kelly, J. J. Chem. Mater. 1999, 11, 3512−3517. (46) Liu, X.; Zhou, Y. J. Mater. Res. 2005, 20, 2371−2378. (47) Fan, Z.; Lu, J. G. J. Nanosci. Nanotechnol. 2005, 5, 1561−1573. (48) Tian, Z. R.; Voigt, J. A.; Liu, J.; McKenzie, B.; Mcdermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. Nat. Mater. 2003, 2, 821−826. (49) Oh, J.; Tak, Y.; Lee, J. Electrochem. Solid-State Lett. 2005, 8, C81−C84. (50) Gupta, M.; Spivey, J. J. Catal. Today 2009, 147, 126−132. (51) Klier, K. Adv. Catal. 1982, 31, 243−313.

Nanowires/tubes were then used as catalysts for CO hydrogenation reaction. The present work demonstrates that Co−Cu−ZnO nanowires/tubes are promising catalysts. The catalysts show moderate selectivity toward C1−C4 alcohols and C1−C6 (except C5) hydrocarbons at low reaction pressure (15 bar). Methane remains the main undesired product at the reaction conditions studied. Nevertheless, this study was not aimed toward optimizing the process variables during CO hydrogenation and therefore only preliminary results are presented here. To understand how these catalysts are different from conventional catalysts, future work would include the effect of different parameters during CO hydrogenation coupled with experimental studies such as in situ FTIR and in situ XRD to help gain insight about surface species and their molecular environment.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This material is based upon work supported as part of the Center for Atomic Level Catalyst Design, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0001058. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy. Research supported in part by ORNL’s Shared Research Equipment (ShaRE) User Facility, which is sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy. The authors are also thankful to Kim Hutchings (NCSU) for ICP, and Wanda Leblanc (LSU) for XRD.

■

REFERENCES

(1) U.S. Department of Energy. Strategic Significance of America’s Oil Shale Resource; US Government Printing Office: Washington, DC, 2004. (2) U.S. Department of Energy. U.S. Energy Information Administration: Washington, DC, 2009. (3) de la Piscina, P. R.; Homs, N. Chem. Soc. Rev. 2008, 37, 2459− 2467. (4) Spivey, J. J.; Egbebi, A. Chem. Soc. Rev. 2007, 36, 1514−1528. (5) Subramani, V.; Song, C. Catalysis 2007, 20, 65−106. (6) Subramani, V.; Gangwal, S. K. Energy Fuels 2008, 22, 814−839. (7) Slaa, J. C.; Van Ommen, J. G.; Ross, J. R. H. Catal. Today 1992, 15, 129−148. (8) Elliott, D. J. J. Catal. 1988, 111, 445−449. (9) Pan, X.; Fan, Z.; Chen, W.; Ding, Y.; Luo, H.; Bao, X. Nat. Mater. 2007, 6, 507−511. (10) Zhang, H.-B.; Dong, X.; Lin, G.-D.; Liang, X.-L.; Li, H.-Y. Chem. Commun. (Cambridge, United Kingdom) 2005, 5094−5096. (11) Subramanian, N. D.; Balaji, G.; Kumar, C. S. S. R.; Spivey, J. J. Catal. Today 2009, 147, 100−106. (12) Gupta, M.; Podlaha, E. J. J. Appl. Electrochem. 2010, 40, 1429− 1439. (13) Gupta, M.; Pinisetty, D.; Flake, J. C.; Spivey, J. J. J. Electrochem. Soc. 2010, 157, D473−D478. (14) Tourillon, G.; Pontonnier, L.; Levy, J. P.; Langlais, V. Electrochem. Solid-State Lett. 2000, 3, 20−23. 10932



Article

(52) Dry, M. E. J. Chem. Technol. Biotechnol. 2001, 77, 43−50. (53) Baker, J. E.; Burch, R.; Golunski, S. E. Appl. Catal. 1989, 53, 279−297. (54) Bailliard-Letournel, R. M.; Gomez Cobo, A. J.; Mirodatos, C.; Primet, M.; Dalmon, J. A. Catal. Lett. 1989, 2, 149−156. (55) Boz, I. Catal. Lett. 2003, 87, 187−194. (56) Cao, R.; Pan, W. X.; Griffin, G. L. Langmuir 1988, 4, 1108− 1112. (57) Apel, P. Radiat. Meas. 2001, 34, 559−566. (58) Kaur, R.; Verma, N. K.; Kumar, S.; Chakarvarti, S. K. J. Mater. Sci. 2006, 41, 3723−3728. (59) Switzer, J. A. Am. Ceram. Soc. Bull. 1987, 66, 1521−1524. (60) Izaki, M. J. Electrochem. Soc. 1999, 146, 4517−4521. (61) Zhang, J.-M.; Feng, X.-M. Mater. Lett. 2008, 62, 3224−3227. (62) Puippe, J. C.; Ibi, N. J. Appl. Electrochem. 1980, 10, 775−784. (63) Chakarvarti, S. K.; Vetter, J. J. Micromech. Microeng. 1993, 3, 57−59. (64) Gu, J.-j.; Liu, L.-h.; Li, H.-t.; Xu, Q.; Sun, H.-y. J. Alloys Compd. 2010, 508, 516−519. (65) Xu, R.; Yang, C.; Wei, W.; Li, W.-h.; Sun, Y.-h.; Hu, T.-d. J. Mol. Catal. A Chem. 2004, 221, 51−58. (66) Chu, W.; Kieffer, R.; Kiennemann, A.; Hindermann, J. P. Appl. Catal. A: Gen. 1995, 121, 95−111. (67) Guner, A.; Sevil, A. U.; Guven, O. J. Appl. Poym. Sci. 1998, 68, 891−895. (68) Caykara, T.; Inam, R. J. Appl. Polym. Sci. 2003, 89, 2013−2018. (69) Vincent, R.; Cherns, D.; Nguyen, X. N.; Ursaki, V. V. Nanotechnology 2008, 19, 475702/475701−475702/475710. (70) Xu, X.; Doesburg, E. B. M.; Scholten, J. J. F. Catal. Today 1987, 2, 125−170. (71) Mouaddib, N.; Perrichon, V.; Martin, G. A. Appl. Catal. A: Gen. 1994, 118, 63−72. (72) Boz, I.; Sahibzada, M.; Metcalfe, I. S. Ind. Eng. Chem. Res. 1994, 33, 2021−2028. (73) Gupta, M.; Smith, M. L.; Spivey, J. J. ACS Catal. 2011, 1, 641− 656.

10933


tube Catalysts

Recommend Documents