CNT Catalysts

Article pubs.acs.org/EF

Electrochemical CO2 Reduction to Fuels Using Pt/CNT Catalysts Synthesized in Supercritical Medium Carlos Jiménez, Jesús García, Rafael Camarillo, Fabiola Martínez,* and Jesusa Rincón Universidad de Castilla-La Mancha, Facultad de Ciencias Ambientales y Bioquímica, Departamento de Ingeniería Química, Avenida Carlos III, s/n, 45071 Toledo, Spain ABSTRACT: The electrochemical reduction of CO2 in the gas phase has been carried out in a solid polymer electrolyte type cell (25 cm2 geometric area) in continuous operation mode using carbon nanotube-supported platinum catalysts (Pt/CNT). The main novelty of this work relies on the use of supercritical media (supercritical CO2) for Pt deposition on CNT. Supercritical synthesis has allowed obtaining small Pt nanoparticles divided into two modal distributions (for 3−4 nm and 8−9 nm, respectively) with a high deposition efficiency (about 80%). The main reaction products of the electrocatalytic conversion of CO2 have been formic acid (59−89%), methane (2−33%), CO (3−11%), methanol (0−1.9%), and small amounts of acetone, isopropanol, and methyl acetate. The CO2 conversion rate multiplies almost by four when increasing current density, although selectivity barely changes. Lower temperature promotes further reduction of CO2 to methane (33% of selectivity) to the detriment of formic acid and CO. However, increases of temperature favor mainly formic acid production (up to 89%) as well as methanol formation (1.9%) at the expense of methane. In addition, low CO2 flow rate favors production of methane and methanol (in a lesser extent) to the detriment of CO and formic acid. The maximum CO2 conversion rate attained has been 2.8 × 10−2 mmol h−1 for the highest current density studied. Attending to the selectivity, a low CO2 flow rate favored the production of fuels such as methane (5.2 × 10−3 mmol h−1) and methanol (3.6 × 10−4 mmol h−1). These results indicate that the supercritical synthesis of the Pt/CNT electrocatalyst improves the results reported in the literature up to now.

1. INTRODUCTION The last Intergovernmental Panel on Climate Change (IPCC 2014) issued an alert about the continuous increase of greenhouse gases in the earth’s atmosphere and recommended taking immediate and efficient actions to reduce CO2 emissions in order to keep our climate safe. Among these actions, two are nowadays the most interesting from a technical point of view: substitution of fossil fuels by alternative and cleaner technologies (avoiding CO2 production) or trying to reduce the amount of CO2 that reaches the atmosphere after combustion has occurred. In the second case, storage and recycling are being developed in parallel nowadays. In both cases, CO2 has to be captured, but in the first case, it is stored in a stable zone in the land1−3 while, in the second one, it is converted into valuable products. From the recycling point of view, gas conversion into liquid fuels (easy to store and transport) is beginning to attract followers in the last few years4−10 because it is the option with larger potential to reduce anthropogenic CO2 emissions. In this context, electrocatalytic technologies have been demonstrated to be a very promising alternative, since CO2 has been converted into different fuel organic compounds (formic acid, methanol, etc.) with high current efficiency.11−16 The electrochemical process is usually done in the liquid phase, normally water,17−19 as the conductivity is an important parameter when using electrochemistry and, in an aqueous solution, the amount of electrolyte can be controlled to reduce voltage and, then, energy consumption. However, the very low solubility of CO2 in water, around 30 mM at 1 atm and ambient temperature,20 discourages the use of this process in the aqueous phase. In addition, after the CO2 reduction, a highly © 2017 American Chemical Society

diluted water solution containing the liquid fuels is formed, so the recovery of these chemicals is expensive.7 To overcome these drawbacks, CO2 has been introduced directly in the gas phase into the cell21−26 using gas diffusion electrodes (GDE) with ion exchange membranes, which reduce mass transfer limitations and increase process efficiency.27,28 On the other hand, it should be noted that the group of Centi29 has compared the efficiency of the electroreduction of CO2 in liquid (hydrogenocarbonate aqueous solution) or gas phase, using the same electrochemical cell. They have found that the electroreduction in aqueous medium leads to a different product distribution (formic acid and acetic acid, versus methanol, acetaldehyde, ethanol, acetone, and isopropanol when the reduction is carried out in gas phase). In addition, they have observed that in the gas phase the conversion rate of CO2 to valuable compounds is an order of magnitude higher than in the aqueous phase. Although these results are encouraging, there are not currently many works studying the gas phase approach, and if they do, it is a very small scale. Thus, most works use electrodes surfaces of only 1 cm2.10,18,22,30 In addition, the conversion process is usually made in batch mode,18,31 despite the fact that industrial applications require continuous operation. On the other hand, one of the most important challenges to effectively accomplish the electrocatalytic reduction of CO2 is to develop new catalysts endowed with high capacity in activating the CO2 molecule, high selectivity toward a Received: November 14, 2016 Revised: January 25, 2017 Published: January 31, 2017 3038

DOI: 10.1021/acs.energyfuels.6b03017 Energy Fuels 2017, 31, 3038−3046

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Figure 1. Experimental setup for deposition of platinum nanoparticles onto carbon nanotubes (Pt/CNT) using supercritical CO2 and H2 as solvent and reducing agent, respectively.

liquids.47,48 Therefore, since its density approaches that of liquids, it can be a good solvent for organometallic compounds (metal precursors) and their organic decomposition products. Consequently, precursor transport occurs in solution and reduction occurs at the solution/solid interface at lower temperatures and higher reagent concentrations than those of vapor-phase techniques such as CVD. Further, the use of scCO2 as solvent facilitates desorption of ligand decomposition products. On the other hand, the low viscosity and high diffusivity of scCO2 (relative to liquids), its zero surface tension, and its complete miscibility with gaseous reducing agents, such as, for example, H2, avoid problems associated with liquid phase reductions (poor mass transfer and deposition rates). Another interesting quality is the fact that their properties can be adjusted with simple changes in temperature and/or pressure.49−51 To sum up, the process combines the advantages of CVD and liquid-phase epitaxy while minimizing the drawbacks of each.52 According to this, it may be said that metal deposition efficiency may be promoted by appropriate adjustment of pressure, temperature, and cosolvent (type and concentration) in both impregnation (precursor dissolution followed by its adsorption to substrate) and reduction steps.53,54 In this way, the desired density at which the impregnation should be performed is reached (i.e., the density of the supercritical solution leading to both its rapid penetration into the pores and the wanted partitioning of precursor between supercritical solvent and substrate). Moreover, metal particle size and uniform metal deposition throughout the pores are further governed by independent control of the transport (via solution) and reduction of metal precursor adsorbed on substrate (via chemical reducing agent). Taking all this into account, the main objective of this work is to study the electrochemical reduction of carbon dioxide in the gas phase using CNT supported platinum catalysts synthesized in supercritical media. To our knowledge, there are no works in the literature concerning the use of this type of electrocatalysts

compound of interest, and adequate durability. In this sense, an interesting review regarding the electrochemistry of CO2 on carbon electrodes32 has pointed out that nanostructured carbon based electrodes (such as CNT among others) are more favorable, since their interfaces promote electron/ion transport and inhibit active catalysts from mechanical and chemical degradation. It should be noted that the use of electrocatalysts based on carbon nanotubes doped with metals (e.g., Fe or Pt) has led to long-chain hydrocarbons (C9 to C10).7,22,33 This result has been attributed to the effect of nanoconfinement and the local increase of CO2 and intermediates concentration promoted by the use of nanostructures.22,34,35 Regarding metal deposition, it has been carried out successfully using different techniques, such as chemical vapor deposition (CVD),36 impregnation,37 deposition−precipitation,38 or a sonochemical approach,39 among others. Generally, conventional methods of synthesis of these catalysts involve the use of large amounts of organic solvents, which is a major drawback from an environmental point of view, due to the large amount of wastes generated. An alternative with great potential consists of performing the metal deposition in supercritical media, which can be carried out using a much lower amount of organic solvents, which is very interesting from an environmental point of view.40 Further, its application does not entail any health risk and ensures a superior product quality.41,42 Thus, various groups43−46 have synthesized metal catalysts supported on carbon nanotubes using this technique. Basically, the supercritical fluid deposition of metals involves the chemical reduction of organometallic compounds (metal precursors) soluble in supercritical carbon dioxide (scCO2) at low temperature (about 100−200 °C). The keys to this technique are the physicochemical properties of the solvent. A supercritical fluid (SCF) presents solvent properties similar to liquids but also exhibits transport properties similar to gases, as they are much less compressible and have better transport properties (viscosity, diffusivity, thermal conductivity, ...) than 3039


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Figure 2. Experimental setup for electrocatalytic reduction of CO2. The cell is equipped with a temperature controller, and the electrical current is applied using a potentiostat-galvanostat (PGSTAT302N, Metrohm AUTOLAB), that is also used to measure and register voltage values. 2.2. Analytical System. 2.2.1. Characterization of Pt/CNT catalyst. The catalyst morphology and size distribution of Pt nanoparticles deposited on the CNT has been determined by TEM (transmission electronic microscopy) in a Jeol 2100 TEM microscope operating at 200 kV equipped with a side entry double-tilt (±25°) sample holder and EDS detector (Oxford Link) in an external lab ́ Renovables, Universidad de Castilla-La (Instituto de Energias Mancha). The amount of platinum deposited on the CNT during the synthesis has been measured through ICP-AES (inductively coupled plasma, atomic emission spectrometry) using an ICP-AES Liberty Sequential (VARIAN). The catalyst was also characterized by XRD using an X-ray diffractometer (Philips, X́ Pert MPD) in an external lab (IRICA, Universidad de Castilla-La Mancha). 2.2.2. CO2 electroreduction. The analytical method used to identify and quantify the CO2 reduction products was developed in a previous work of our group.55 Briefly, it is described below. A gas chromatograph (GC 7890A supplied by Agilent) with a system to concentrate the samples (GC Sampler 80) equipped with FID and TCD detectors has been used for the quantification of reaction products during the electroreduction of CO 2. The configuration of the GC implies five columns, four packed sieves (3Ft 1/8 2 mm HayeSep Q 80/100 Ni; HP-MOLESIEVE; 6Ft 1/8 2 mm HayeSep Q 80/100 Ni; 6Ft 1/8 2 mm MolSieve 5A 60/80 Ni) together with a column HP-PLOT/Q. The analysis of the liquid reaction products involves the use of the GC Sampler 80 to concentrate products by adsorbing them on a fiber (SPME (Solid Phase Microextraction) Fiber Carboxen (SUPELCO)). Compounds adsorbed on the fiber are desorbed inside the injection port of the GC, swept through the column HP-PLOT/Q, and analyzed by FID. Previous calibration with liquid standards (formic acid, methanol, ethanol, acetone, methyl acetate, and isopropanol) in different concentration ranges allows identification and quantification of liquid reaction products according to their retention times and FID signal. To quantify products contained in the gas sample stream released from the cold trap, GC is configured with two gas sampling valves (Pneumatic Control Modules) connected to five columns (previously mentioned), two detectors (FID and TCD), and a methanizer. The calibration is carried out using an ad hoc gas standard supplied by Air Liquide, which contains carbon dioxide, carbon monoxide, hydrogen, methane, ethane, propane, isobutene, isopentane, ethylene,

for the electrochemical conversion of CO2. On the other hand, very few works have addressed the study of electrocatalytic reduction of CO2 in the gas phase. In this work, we have used a continuous electrochemical cell, which includes a solid polymer electrolyte (Nafion membrane) assembled to gas diffusion electrodes (GDE), to reduce CO2 in the gas phase. The main reaction products have been identified and quantified. Further, the effect of current density, temperature, and CO2 flow rate on reaction parameters (CO2 conversion rate and product distribution) has been established.

2. EXPERIMENTAL SECTION 2.1. Experimental Setup. 2.1.1. Synthesis of Pt/CNT catalyst using supercritical CO2. The experimental setup used to accomplish the deposition of Pt nanoparticles onto carbon nanotubes (Pt/CNT), which have been used as electrocatalyst for the reduction process of CO2, is shown in Figure 1. This setup includes an ad hoc 100 mL stainless steel reactor (DEMEDE Engineering and Research, Spain) able to work at 250 °C and 300 bar. It is equipped with a temperature controller (TOHO, TTM-204). A pressure pump (P-50, Thar SFC, supplied by Productos de Instrumentación, S.A., Spain) allows increasing the pressure in the reactor, which is measured and regulated by several manometers and valves. A thermostatic bath (JP-Selecta, FRIGITERM) chills CO2 before its pumping. A secondary circuit is used to add H2 to the reactor during the synthesis experiment. The experimental setup also has a 1 μm stainless steel filter installed in the depressurization line to avoid particle entrainment. 2.1.2. CO2 electroreduction. The experimental setup to carry out the electrocatalytic reduction of CO2 (Figure 2) consists of an electrochemical cell (single cell PaxiTech supplied by MICROBEAM, electrode geometric area: 25 cm2), which is analogous to a PEM (proton exchange membrane) fuel cell. Pt/CNT electrocatalyst is coated on carbon cloth (used as Gas Diffusion Layer) and assembled (by hot pressing) to both sides of a Nafion proton exchange membrane. The electrochemical flow cell is divided into anodic and cathodic compartments, which are comprised of one graphite monopolar plate each with a single channel (1 mm × 1 mm) serpentine flow field. A peristaltic pump (DINKO-D21-V) circulates a KHCO3 aqueous solution through the anodic zone. A mass flow controller (BROOKS, SLA5850) allows fixing the CO2 gas flow rate flowing through the cathodic zone of the cell. 3040


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Energy & Fuels propylene, and nitrogen. Hydrocarbons are analyzed by FID, whereas CO is conducted through the methanizer/FID, and H2 is detected by TCD. The make-up gas is nitrogen, and the time of the chromatographic method is 45 min. 2.3. Experimental Procedure. 2.3.1. Synthesis of Pt/CNT catalyst using supercritical CO2. Commercial multiwalled carbon nanotubes were used for this research (supplied by SIGMA-Aldrich; ≥98% carbon basis, O.D. × I.D. × L: 10 nm ±1 nm × 4.5 nm ± 0.5 nm × 3 ∼ 6 μm, TEM; surface area 280−350 m2/g (BET)). Platinum acetylacetonate (98%, ACROS ORGANICS) has been used as the metal precursor of Pt nanoparticles. These reagents were used as received. The synthesis of Pt deposited onto CNT catalyst is carried out as follows:56 First, the metal precursor (0.25 g), carbon nanotubes (0.25 g), and 5 mL of methanol are introduced inside the reactor. The metal precursor is then dissolved in supercritical CO2 and adsorbed on CNTs at 200 °C and 100 bar during 1 h. Next, hydrogen (5% H2 in CO2) is introduced inside the reactor and pressure is increased up to 240 bar by pumping CO2. These conditions are held for 30 min to reduce the metal precursor to its metallic form onto the CNT. Once the reduction time has been completed, the system has to be very slowly depressurized to avoid the entrainment of the catalyst. Finally, the reactor and the filter are washed with acetone to collect the catalyst. The solution obtained is then filtered by vacuum using a nylon filter (0.45 μm, Merck Millipore). Next, the filter with the particles is dried in an oven at 40 °C during 24 h to remove traces of acetone. The reproducibility of this supercritical synthesis has been tested through duplicate synthesis experiments and subsequent characterization of the material obtained. The experimental conditions used in this work for support impregnation (precursor dissolution in scCO2 followed by its adsorption on CNTs) and subsequent metal deposition via reduction of the organometallic precursor (100 bar, 200 °C, and 60 min for impregnation and 240 bar, 200 °C, and 30 min for reduction) were selected on the basis of reports by Ye et al.56−59 Other conditions are found in the literature for these steps.52,60 Specifically, in the reviews by Zhang and Erkey (2006)53 and Erkey (2009),54 pressures in the range 150−300 bar (mode 280 bar) and temperatures of 80−100 °C (mode 80 °C) have been reported for the impregnation stage. Instead, for the reduction of the metal precursor adsorbed on a substrate, pressures in the range 1−300 bar (mode 280 bar) and temperatures of 40−1000 °C (mode 300 °C) have been used. Further, reduction times reported are also quite different (2−24 h; mode 6 h). Regarding metal loads and particle sizes published in these reviews, typical values are in the range 10−30 wt % of Pt and below 5 nm, respectively. When comparing conditions in the literature to those of this work, one would expect our method to yield, in a shorter period of time, metal loads and metal particle sizes of the same magnitude order. Effectively, since in our method a cosolvent (methanol) is added to CO2, the precursor solubility in the mixture of CO2 and methanol can approach that in CO2 at typical conditions of the impregnation step (higher pressure and lower temperature than those used in this work). As a sequel, the metal loads on CNTs will be more or less similar, since the adsorption of a precursor on a substrate depends on its concentration in the fluid phase. Regarding particle size, analogous values are also expected, mainly because it is thought that the high surface of the CNTs will cause the particles to disperse and the short reaction time will limit particle growth. 2.3.2. CO2 electroreduction. The manufacturing of the membraneelectrodes assembly (MEA) includes the preparation of the electrodes (gas diffusion electrodes), the assembly of the electrodes to both sides of the protonic exchange membrane, and its setup in the electrochemical reactor. For the preparation of the electrodes, a catalytic ink is deposited on carbon cloth (Toray TM Carbon Paper 0.28 mm thick, TeflonTM treated (20 (±5) wt %) supplied by Quintech) using an airbrush. The catalytic inks are prepared by dispersing the catalyst (Pt/CNT synthesized using the supercritical method explained in section 2.2.1)

into methanol with a 5% Nafion solution using ultrasound. The platinum load is 0.4 mg Pt cm−2 in each electrode (anode and cathode). A Nafion membrane 117 (supplied by Sigma-Aldrich, 180 μm thickness) is used for the membrane-electrodes assembly. A hydraulic press (Specac, UK) is employed to make the assembly (5.1 tons and 130 °C for 10 min). Before each electroreduction experiment, GC is calibrated for gaseous product analysis by injecting the gas standard. During the electrochemical assays, temperature is fixed using a temperature controller. Then, a carbonate solution (0.1 M KHCO3, 99.5% supplied by SIGMA-Aldrich) is pumped into the anodic compartment (5 mL min−1). In the cathodic compartment, the carbon dioxide (99.998%, supplied by CONTSE, Spain) flow rate is fixed by the mass flow controller and fed into the cell previously humidified and preheated. Then, current intensity is selected using the potentiostat, and voltage values are measured and registered over 120 min of experiment. The stream leaving the cathodic zone is circulated through a cold trap (5 °C) to collect all liquid products formed along the experiment (120 min to ensure that the steady state is achieved and that enough amount of liquid products is formed). The gas phase continuously flowing from the cold trap is driven through a pipe with a heating cord and a particle filter to the gas sampling valves of the GC-FID-TCD, where it is injected at 45 and 120 min of experiment. After 120 min of experiment, the volume of sample collected in the cold trap is divided into two aliquots. An aliquot of 1 mL is directly preconcentrated using an automatic system (GC Sampler 80, Agilent) that allows concentrating the volatile products and adsorbing them on a SPME fiber. Then, the fiber content is desorbed and injected in the GC, so the products adsorbed on it are released and analyzed with the GCFID. In this chromatographic analysis, carbonic acid (from CO2 absorption) in the sample masks formic acid, so it cannot be quantified. For this reason, a second aliquot of sample is subjected to 24 h of magnetic stirring followed by 2 h of N2 sparge, to ensure desorption of CO 2 from the liquid. Then, this aliquot is preconcentered and analyzed by GC-FID using the same chromatographic method. Consequently, formic acid standards are subjected to the same procedure (24 h stirring and 2h of N2 sparging) before calibration. After each experiment the electrochemical cell is cleaned by pumping osmotized water through anodic and cathodic compartments. Then a flow of N2 is circulated through the cathodic compartment to remove the rest of the water from the channels. Several blank experiments were carried out to check that the products of the electrocatalytic reduction of CO2 were not detected in two cases: (1) in the absence of CO2 flow in the cathode (with N2 flow and cell voltage applied) or (2) in the presence of CO2 flowing through the cathode with no voltage applied across the cell. A negative response was obtained in both cases.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Pt/CNT Catalysts Using Supercritical CO2. The Pt/CNT catalysts synthesized using the supercritical method described in section 2.3.1 have been characterized in order to know its composition and morphology. The XRD diffractogram for the synthesized catalyst is shown in Figure 3. The diffraction lines at about 2θ = 39.62, 46.02, 67.34, 81.34, and 85.78 can be attributed to the characteristic (111), (200), (220), (311), and (222) planes of the facecentered cubic (fcc) crystal lattice of platinum,61,62 confirming that the synthesis method achieves the deposition of Pt metallic species onto the carbon support. Figure 4 shows the TEM images for the platinum nanoparticles deposited onto the carbon nanotubes. It can be seen that the Pt particles (black zones in the micrographs) are correctly deposited on the carbon nanotubes (gray fibers shown in the images). Further, it can also be observed that they are 3041


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Figure 3. X-ray diffractogram of the Pt/CNT catalyst synthesized using supercritical CO2.

Figure 5. Histogram of Pt particles size distribution on CNT (deposited in supercritical CO2) obtained from TEM images.

even formed inside the nanotubes. Regarding their size, they are very small, and their particle size distribution is quite homogeneous. The Pt particles size distribution has also been analyzed from these TEM images. In Figure 5 can be seen the histogram with the size distribution of platinum particles deposited onto the carbon nanotubes. 150 particles in 10 different regions have been analyzed to measure the particle size distribution. Moreover, individual particles have been analyzed, not considering the particles coalescing, which can distort the results. The results obtained with this quantitative analysis are consistent with the results previously observed in the images, with a narrow size distribution of the Pt nanoparticles deposited on the carbon nanotubes divided into two modal distributions (for 3−4 nm and 8−9 nm, respectively). These two modal distributions observed in the histogram can be attributed to Pt nanoparticles deposited inside the CNT (3−4 nm) whose inner diameter is 4.5 nm ±0.5 nm, and particles deposited on the CNT with near normal distribution around 8−9 nm. The average diameter of the particles has also been calculated with a result of 6.9 nm and a standard deviation of 2.6.

In addition, inductively coupled plasma has been used to measure the amount of metal deposited on the carbon nanotubes. The result shows that the platinum concentration in the catalyst is 26 wt %. Taking into account the Pt precursor:CNT ratio used, the efficiency of the deposition of Pt is 78.5%. Duplicate synthesis experiments have allowed calculation of the standard deviation of the Pt load in the synthesized material to be 2.1%. Definitely, these results confirm that the synthesis method using supercritical CO2 can be successfully used to deposit Pt nanoparticles onto carbon nanotubes, with a high efficiency, a narrow particle size distribution, and a very small particle diameter. In comparison to classical catalysts synthesis methods, the great advantage of using supercritical fluids as reaction media is that the amount of organic solvent needed is much lower. Further, as CO2 is used at the supercritical state, it can be easily separated from the synthesized catalysts, and subsequently recovered, so no emissions are produced. 3.2. CO2 Electroreduction. This section is devoted to the analysis of experimental results obtained in the CO2 electroreduction using the Pt/CNT catalysts characterized above. In

Figure 4. TEM images of the Pt/CNT catalyst synthesized using supercritical CO2. Scale: (a) 0.2 μm, (b) 50 nm, (c) 20 nm, (d) 10 nm. 3042


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Energy & Fuels particular, the effect of current density, temperature, and CO2 f low rate on the characteristic parameters of the catalytic reaction (CO2 reduction rate and products distribution) are investigated. Results obtained in experiments performed are shown in Figure 6. Formic acid, methane, carbon monoxide, methanol, acetone, isopropanol, methyl acetate, and hydrogen have been obtained as reduction products. Hydrogen has not been included in the discussion, as it is not produced from the reduction of carbon

dioxide but from water reduction through a competitive reaction. Some other compounds that are usually obtained by other authors11,17,22 have also been taken into account for this study (ethane, ethanol, or ethylene, as described in the Analytical system section) but they have not been detected at the experimental conditions studied. Formic acid is the compound mostly formed (between 60 and 90% of carbon dioxide converted), followed by methane (2−36%), CO (3−11%), and methanol (0−1.9%). The selectivity toward the compounds produced in minor proportion (including acetone, isopropanol, and methyl acetate) is between 0 and 1%. The production rate of methyl acetate is so small that it is not detected in some experiments. Consistent with the reaction mechanisms described by eqs 1 and 2,63 formic acid is usually one of the first reaction products formed in CO2 reduction due to its low overpotential. Pt

CO2 + e− → CO2* −

(1)

CO2* − + e− + H + → HCOO−

(2)

Nevertheless, it should be noted that, according to the literature,20 the formation mechanism of the first reduction product (formic acid or CO) is promoted or not depending on both the metal used as catalyst and the adsorption stability of radical CO2*−ads (first reaction intermediate) on that metal (Figure 7). Once these first CO2*−ads radicals are formed, they can remain in stable forms (CO2Hads* or COads*) and be desorbed as reaction products (formic acid (HCOO−aq) or COg) or be further reduced to other products, such as methanol, methane, or other species according to the reaction scheme shown in Figure 7 through different mechanisms suggested in the literature.64−66 Therefore, the higher formic acid production rate (pathway A in the figure), as compared to other CO2 reduction products, is not surprising, since radical CO2*−ads is not stable on the platinum surface and tends to quickly desorb to form mainly formic acid.20,67 Thus, behind the formation rate of formic acid is that of methane and carbon monoxide, as part of CO2*− radicals formed are not desorbed and then they can be further reduced on the metal surface (pathway B in the figure). Then, methanol is mainly formed between the minority reduction products. Finally, from the reaction products distribution observed, it can be deduced that, in general, Pt promotes reduction of CO to methane against CO desorption under most of the experimental conditions studied. These observations are in accordance with the results reported by Kuhl et al.,64 who related the CO binding energy of several metals with their behavior in the electrochemical reduction of CO2 to methane/ methanol. It was shown that Pt is within the group of metals which exhibit tight CO binding energy, what is traduced in an effective CO2 activation while slow CO desorption. Thus, the high CO coverage on this metal during CO2 reduction implies that the reduction of CO adsorbed is the rate-determining step in the production of reduced compounds (such as methane/ methanol).64 As can be seen in Figure 6a, selectivity barely changes with current density, but CO2 conversion rate increases almost four times from 7.5 × 10−3 to 2.8 × 10−2 mmol h−1. This higher conversion rate of CO2 leads to an increase mainly in formic acid, methane, CO, and methanol (specially from 8 to 16 mAcm−2) production, that keeps the selectivity almost constant at 16 and 24 mA cm−2. For the rest of the products, a clear

Figure 6. Distribution of products and CO2 reduction rate during the electroreduction of CO2. Experiments carried out in a PEM type cell with 25 cm2 surface electrode geometric area, working in gas phase and in continuous mode ([KHCO3]anode: 0.1 M). Influence of (a) Current density (CO2 flow rate: 0.05 dm3 min−1; temperature: 60 °C); (b) Temperature (current density: 16 mA cm−2; CO2 flow rate: 0.05 dm3 min−1); and (c) CO2 flow rate (current density: 16 mA cm−2; temperature: 60 °C). 3043


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Figure 7. Simplified scheme of formation of products from carbon dioxide electroreduction (Adapted from Kuhl et al, 2014;60 Hatsukade et al, 2014;61 and Viswanathan, 201362).

88%) and CO (3 to 8%) increases with CO2 flow rate. In addition, methane and methanol production decrease with carbon dioxide flow rate (methane decreases an order of magnitude from 5.2 × 10−3 to 4.5 × 10−4 mmol h−1, whereas methanol is even not produced at 0.08 S dm3 min−1). It seems that high CO2 flow rate favors the quick desorption of intermediates formed during the reduction process (CO2Hads* by pathway A and COads* by pathway B), and their diffusion out of the electrocatalyst (through the gas diffusion layer), so mainly the first reaction products are formed (formic acid and CO). On the other way, at low CO2 flow rate (0.02 S dm3 min−1) pathway B is promoted with further reduction of CO to methane and methanol, whose selectivity values are 28% and 2%, respectively. These results suggest that the less turbulence near the gas diffusion electrodes favors CO2 conversion to more reduced products (against CO2Hads* and COads* desorption and their diffusion out of the catalytic layer). At intermediate CO2 flow rate (0.05 S dm3 min−1), none of the mechanisms seem to be particularly promoted, leading to a minimum of global conversion rate of CO2. According to the results reported above, it seems that the use of supercritical fluids for the deposition of platinum onto CNTs allows obtaining significant improvement in CO2 conversion rates (in terms of mmol h−1 mg Pt−1 of products formed) compared to conventional synthesis methods. Thus, Centi et al. (2007)22 used Pt/C commercial catalysts and a similar cell configuration (semihalf continuous cell) with an electrode geometric area of 1 cm2 and a Pt loading of 0.5 mg cm−2, obtaining lower formation rates of reduction products. It has to be remarked that they obtained a completely different products distribution, including methane, ethane, ethylene, isopropanol, as well as the formation of long-chain hydrocarbons up to C8− 9, depending on the operation conditions. Nevertheless, the production rates reported for these compounds were below 1 × 10−5 mmol h−1 mg Pt−1 whereas in this work the production rate is about 28.5 × 10−5 mmol h−1 mg Pt−1, which is almost 30 times higher than those obtained with the commercial Pt/C catalyst. In subsequent works, professor Centi’s group have increased the efficiency of the process by using CNT as support material, with production rates of around 7 mmol h−1 mg Pt−130 and 13 mmol h−1 mg Pt−169 (these values have been estimated from the corresponding literature). However, these values are still smaller than those obtained in our system, using the supercritical synthesis method of Pt/CNT catalyst. Finally, according to the results presented above, it may be stated that, within the experimental conditions tested in this

tendency cannot be observed, although they seem to decrease or keep constant with increasing current density. Methyl acetate has only been detected at 16 mA cm−2 and in a very small amount (6.5 × 10−8 mmol h−1). On the other hand, the influence of the temperature inside the electrochemical cell is shown in Figure 6b. In this case, an increase can be observed in the percentage of formic acid and methanol formed (in terms of selectivity) when increasing temperature. In addition, the selectivity toward methane decreases while it barely changes for CO. Methyl acetate production is as low that it is only detected at 60 and 80 °C, with production rates around 1 × 10−8 mmol h−1. In the case of isopropanol, very low production rates are obtained (between 5 × 10−7 and 2 × 10−4 mmol h−1). Therefore, when increasing temperature from 40 to 80 °C, the formation of CO2Hads* (against COads*) and the subsequent desorption of formic acid (CO2 reduction pathway A in Figure 7) are mainly promoted, growing up from 59 to 89% the selectivity of CO2 converted to formic acid (from 8.8 × 10−3 to 1.2 × 10−2 mmol h−1). On the contrary, pathway B (through COads*) globally diminishes with increasing temperature (from 41 to 11% of total CO2 conversion), although within this pathway B it is remarkable the abrupt decrease of methane production from 33 to 2% of selectivity (what is equivalent to 4.8 × 10−3 and 2.1 × 10−4 mmol h−1, less than half at 80 °C) and the significant increase of methanol production from 0.3 to 1.9% of CO2 converted (4.0 × 10−5 to 2.6 × 10−4 mmol h−1) multiplying by 6 its production rate. CO remains barely constant (∼7%) with temperature. To sum up, higher temperature promotes mainly the formation of formic acid (by pathway A), and methanol production against methane (by pathway B). In general, it could be said that higher temperature values favor the reaction pathways implying lower number of protons (with the exception of CO desorption, which is not affected by temperature68). This could be explained by the worsening of transport of protons through the proton exchange membrane with increasing temperature, whose behavior becomes deficient at 80 °C. In addition, it may also be said that higher temperatures increase the diffusivity of liquid products (formic acid and methanol) out of the catalyst layer through the gas diffusion layer. Moreover, although selectivity seems to be very influenced by temperature, as has been discussed, the CO2 conversion rate barely changes when increasing temperature. Finally, the influence of carbon dioxide flow rate is shown in Figure 6c. As can be seen, selectivity toward formic acid (66 to 3044


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Energy & Fuels work, the optimal conditions for CO2 valorization in the gas phase, mainly into formic acid (2.2 × 10−2 mmol h−1), using Pt/CNT catalyst in gas diffusion electrodes, seem to be 24 mA cm−2, 0.05 S dm3 CO2 min−1, and 60 °C. At these conditions a maximum CO2 conversion rate of 2.8 × 10−2 mmol CO2 h−1 is attained. Attending to the selectivity of the CO2 conversion to fuels such as methane and methanol (with lower oxidation state), favorable operating conditions for electroreduction of CO2 are 16 mA cm−2, 0.02 S dm3 CO2 min−1, and 60 °C, resulting in 5.2 × 10−3 mmol methane h−1 and 3.6 × 10−4 mmol methanol h−1, with a global conversion rate of 1.9 × 10−2 mmol CO2 h−1. These are interesting results, improving CO2 conversion rates obtained by other authors, which is indicative of the advantage of using supercritical media in the synthesis of electrocatalysts for CO2 conversion. Nevertheless, a larger effort has to be done to create new catalysts able to improve the selectivity and global conversion rate of CO2. The use of more efficient metals (Pb, Cu, ...) and new carbon supports (such as graphene) should allow increasing both parameters, so future works will focus on those two aspects.

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values mainly promote the formation of formic acid, with low selectivity values for methane, and very low ones for methanol. However, low CO2 flow rate favors the selectivity toward production of methane and methanol (in a lesser extent) to the detriment of CO and formic acid. • Pt/CNT catalysts synthesized in supercritical media promote the electrocatalytic conversion of CO2 at a significant rate (near 0.03 mmol h−1). Further, finetuning of the experimental conditions allows modifying the selectivity to valuable compounds such as formic acid, methane, or methanol.

AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Fax: +34 902204130. Phone: +34 902204100, extension: 5446. ORCID

Fabiola Martínez: 0000-0001-7780-0805 Notes

The authors declare no competing financial interest.

4. CONCLUSIONS From the results showed in this work, the following conclusions can be obtained: • Deposition of Pt nanoparticles on carbon nanotubes may be accomplished in supercritical CO2, with metal deposition yield above 78%. This technique leads to a homogeneous distribution of Pt nanoparticles on CNT and deposition of some nanoparticles inside of them. The histogram of particle size frequency exhibits a two modal like distribution, with the highest frequency of 8− 9 nm (20% of particles), and 15% of particles with 3−4 nm that can be attributed to particles deposited inside the CNT. This material (Pt/CNT) can be successfully used as electrocatalyst in the electrochemical reduction of CO2. • The electrocatalytic reduction of CO2 may be carried out in the gas phase in an electrochemical cell with solid polymer electrolyte (Nafion membrane) and gas diffusion electrodes (25 cm2 geometric area), using Pt/ CNT as electrocatalyst. The main reduction products from CO2 conversion are formic acid (59−89% of selectivity), methane (2−33%), CO (3−11%), methanol (0−1.9%), and small amounts of acetone, isopropanol, and methyl acetate. • Current density has been varied between 8 and 24 mA cm−2, leading to an increase of CO2 conversion rate almost four times higher (from 7.5 × 10−3 to 2.8 × 10−2 mmol h−1), although selectivity barely changed. • The influence of temperature has been studied between 40 and 80 °C, and CO2 conversion rate hardly has changed within this interval. However, selectivity has been significantly affected: at lower temperature, the selectivity was promoted toward methane (33% selectivity). Higher temperature (80 °C) lead to predominant formic acid production (above 89% selectivity), and selectivity toward methanol production has been favored (1.9% of selectivity) against methane (whose selectivity decreased to 2%). • The CO2 flow rate has been varied between 0.02 and 0.08 S dm3 min−1, and it has been found that higher

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ACKNOWLEDGMENTS The authors gratefully acknowledge Ministerio de Ciencia e Innovación of Spain, and Junta de Comunidades de Castilla-La Mancha for the financial support of this work through projects CTM2011-26564, PAI08-0195-3614, and PEII10-0310-5840.

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ABBREVIATIONS CNT = carbon nanotubes CVD = chemical vapor deposition FID = flame ionization detector GC = gas chromatograph GDE = gas diffusion electrodes ICP-AES = inductively coupled plasma atomic emission spectrometry MEA = membrane electrode assembly PEM = polymer electrolyte membrane SPME = solid phase microextraction TCD = thermal conductivity detector TEM = transmission electronic microscopy REFERENCES

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