On the Reactivity of Carbon Supported Pd Nanoparticles during NO

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On the Reactivity of Carbon Supported Pd Nanoparticles during NO Reduction: Unraveling a Metal-Support Redox Interaction Marcus Vinicius Castegnaro, Alex Sandre Kilian, Ione Baibich, Maria do Carmo Martins Alves, and Jonder Morais Langmuir, Just Accepted Manuscript • DOI: 10.1021/la401460c • Publication Date (Web): 18 May 2013 Downloaded from http://pubs.acs.org on May 20, 2013

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On the Reactivity of Carbon Supported Pd Nanoparticles during NO Reduction: Unraveling a Metal-Support Redox Interaction Marcus V. Castegnaro†, Alex S. Kilian†, Ione M. Baibich§, Maria C. M. Alves§ and Jonder Morais†* †

Instituto de Física, Universidade Federal do Rio Grande do Sul (UFRGS), Avenida Bento

Gonçalves, 9500, Bairro Agronomia, CP 15051, CEP 91501-970, Porto Alegre, RS, Brazil, §

Instituto de Química, Universidade Federal do Rio Grande do Sul (UFRGS), Avenida Bento

Gonçalves, 9500, Bairro Agronomia, CP 15003, CEP 91501-970, Porto Alegre, RS, Brazil.

ABSTRACT

Pd nanoparticles (NPs) were successfully obtained by the reduction of PdCl2 with L-ascorbic acid, whose morphology was revealed by HRTEM to be a worm-like system, formed by linked crystallite clusters with an average short-axis diameter of 5.42 nm. In-situ UV-visible absorption measurements were used to monitor their formation, while XPS and XRD characterization confirmed the NPs’ metallic state. A straightforward way to support the obtained Pd NPs on activated carbon (AC) was used to prepare a catalyst for NO decomposition reaction. The Pd/AC catalysts proved to be highly active in the temperature range of 323 K to 673 K, and a redox

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mechanism is proposed, where the catalyst´s active sites are oxidized by NO and reduced by carbon, emitting CO2 and enhancing their capacity to absorb and dissociate NO.

KEYWORDS: Pd nanoparticles, green synthesis, activated carbon, NO decomposition, NOx, XPS, HRTEM. 1. INTRODUCTION Synthesis and characterization of metal nanoparticles (NPs) have gained ground in materials research, especially due to their unique properties, which differ widely from those presented by corresponding bulk materials 1. Since the main properties of metal NPs are basically determined by their size, shape, composition and crystal and electronic structure, many syntheses have been developed, whose resulting NPs present electrical, magnetic, optical or catalytic characteristics suitable for specific applications in several fields of science and technology 1-4. As in other fields of science and technology, nanoscience is beginning to give attention to the development of greener and more sustainable methods to synthesize their materials. These efforts are part of a movement known as Green Chemistry, which has gained strength in both academic and industrial research since the 90s. Green chemistry is defined as the design of chemical products and processes aiming to reduce or eliminate the use and generation of hazardous substances 5. The principles of Green Chemistry have already been applied to the design of a wide range of products and industrial processes

5,6

in order to minimize the impacts

on health and the environment, reducing losses and costs and preventing pollution. In turn, Green nanoscience/nanotechnology involves the application of Green Chemistry principles to the project of nanoscale products, the development of methods for nanomaterials production and to the application of resulting nanomaterials 7,8.


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Concerning the synthesis of NPs, the use of nontoxic chemicals, environmentally benign solvents, and renewable materials are some of the key issues in a green synthetic strategy 9. Furthermore, a green synthesis should consume the least amount of solvent and reagents, with high efficiency and its design must prevent the production of harmful waste or byproducts while consuming as little energy as possible. Among the different methods developed to synthesize metal NPs

1-4

, the wet chemical

reduction is one of the most used, since it is a very controlled process, whose parameters can be easily assorted. This method is mainly achieved by reduction of a metal ion salt solution. Some synthetic methods of wet chemical reduction, compatible with green chemistry principles, have been reported, especially for noble metal NPs. Raveendran et al.

9,10

prepared

Au, Ag, and Au–Ag NPs in water, using glucose as the reducing agent and starch as the protecting agent. Liu and coworkers

11

synthesized Au nanocrystals in water, using glucose as

both the reducing and stabilizing agent. Similarly, Xiong and coworkers 12 employed L-ascorbic acid to obtain Cu NPs in water. Nadagouda et al. 13 used coffee and tea extract to obtain Ag and Pd NPs in an aqueous solution. Transition metal NPs have been studied more due to their numerous applications in industry, medicine and pharmacy

1-4

. Among the transition metals, palladium plays a major role in many

industrial applications due to its catalytic properties, particularly in fine chemistry. In organic synthesis, for example, Pd is used for catalyzing carbon-carbon bond forming 14. Therefore, Pd is widely employed in Heck, Suzuki and Sonogashira reactions

14-17

, and it also serves as the

primary catalyst for low temperature control of exhaust gases from vehicles 18-23. Essentially, all of these catalysts require the use of metallic Pd in a finely divided state and are supported on materials such as silica, alumina, titania, carbon or zeolites.


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In the present work, a simple, environmentally friendly and cost effective method for obtaining Pd NPs is proposed. Deionized water is utilized as a solvent for the reduction of PdCl2 at room temperature using L-ascorbic acid (a renewable, inexpensive, nontoxic and eco-friendly compound) as the reducing agent and trisodium citrate (a biocompatible compound employed as an emulsifier and preservative of several food products) as the stabilizing agent. The Pd NPs were characterized by in-situ UV-visible spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Moreover, we applied the Pd NPs as catalyst for NO decomposition, a reaction that has a crucial role in environmental catalysis 24,25, since NO accounts for more than 95% of the nitrogen oxides (NOx) emissions from automobiles and power plants

26-28

. The main motivations for

research in NOx abatement are their noxious effects on human health and on the environment. It is well know that NOx act on tropospheric ozone photolysis and on photo-oxidation pollution, especially on “smog” and ground-level ozone formation, by interacting with volatile organic compounds

26-28

. In addition, they play a leading role in acid rain formation, which affects both

terrestrial and aquatic life. Due to these hazards related to the presence of NOx in the environment, international regulations have been proposed to control the emissions of these compounds and research on NO abatement has become crucial. Although NO is thermodynamically unstable at room temperature (2NO N2 + O2, ∆Gf0 = - 86.6 kJmol-1), it does not decompose due to its high activation energy reaction (364 kJmol-1)

25

. Therefore, it is

necessary to use catalysts to promote the decomposition of NO into harmless compounds, namely N2 and O2. Among the main catalytic systems studied for this purpose, the direct decomposition of NO over supported transition metal catalysts is one of the most attractive 25,27.


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Numerous studies have been done regarding this process, resulting in the development of a wide range of catalysts, most of them obtained by impregnation of a large surface area of support, such as Al2O3

21,22

, SiO2

22

, zeolite

23

, TiO2 29, and activated carbon (AC) 21,30 with a transition

metal precursor solution followed by a reduction process, which generally involves the use of high temperatures and reducing atmospheres. Lastly, we proposed a synthesis of a catalyst based on the adsorption of the Pd NPs on AC. The AC is an inexpensive material that can be easily obtained from carbon-containing materials, such as wood, sawdust, fruit stones, nutshells, waste from petrochemical industries, etc. The use of AC as a support for catalysts used in NO decomposition reaction has been already reported 21,30-32

. Nevertheless, the route presented in our work is simpler, more cost effective and

environmentally friendly, since it does not require any heat treatment in inert or reducing atmospheres as the usual methods employed to obtain heterogeneous catalysts

14-27

. Moreover,

our proposed catalyst has an additional advantage that the metallic particles, on which surface the decomposition of NO will take place, were well characterized prior to and after the interaction with the supporting material.

2. EXPERIMENTAL METHODS 2.1. Synthesis of Pd NPs In a typical synthesis, a 0.06 mmol solution of PdCl2 (Vetec), a 0.6 mmol solution of citric acid (trisodium citrate, C6H5O7Na3.2H2O, Sigma-Aldrich), and a 10 mmol solution of L-ascorbic acid (C6H8O6, Vetec) were mixed under constant stirring at room temperature. Deionized water was used as a solvent to prepare the solutions. During the first 5 minutes, the initial light orange color of the precursor solution mixture turned black, indicating the formation of Pd NPs. After 10


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minutes, no more color changes were observed. Then the solution was left to rest at room temperature, without any inert gas protection for 80 more minutes. The colloidal solution obtained by the method presented here was stable for more than 6 months, and no precipitation was observed during this time.

2.2. Characterization of Pd NPs In-situ UV-visible spectra were acquired in a Shimadzu UV-1601PC spectrophotometer. For that an aliquot of 3.5 mL was taken from the final solution and placed in a quartz cell with a path length of 1 cm. One spectrum was collected every 1 minute for 120 minutes over the wavelength range of 200-1000 nm, using a 0.5 nm step size. An identical cell filled with deionized water was used to subtract the baseline. The sample for TEM and HRTEM analysis was prepared by placing a drop of the colloidal solution on a carbon-coated copper grid (300 mesh, SPI Supplies) and left to dry at room temperature in a desiccator. TEM images were obtained (JEOL model JEM-1200 EX II) at an accelerating voltage of 100 kV. About 750 NPs from different parts of the image were taken into account (ImageJ 1.45) in order to estimate the size distribution of the NPs, which were fitted (OriginPro 8) considering a Gaussian distribution. HRTEM analyses were made (JEOL JEM2010) with an accelerating voltage of 200kV. The resulting HRTEM images were analyzed (ImageJ 1.45) for lattice spacing measurement. The XPS measurements were performed at the LNLS (Brazilian Synchrotron Light Laboratory) at the SXS beamline

33,34

. The sample was obtained by direct centrifugation of the

colloid (4000 rpm for 30 minutes) followed by washing with deionized water (twice) and isopropyl alcohol (three times). The resulting powder was placed on carbon tape and the


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measurements were made at room temperature using a 45° takeoff angle. The spectra were excited with 1840 eV photons and collected using a hemispherical electron analyzer (Physical Electronics model 10-360), adjusted at a pass energy of 23.5 eV and an energy step size of 0.1 eV. The acquisition time was 500 ms per point. The analyzer’s energy calibration was made using the Ag 3d5/2 peak at 368.3 eV 35 ,measured from a standard Ag foil. To check and correct charging effects, the C 1s peak at 284.5 eV

35

was used as a reference. All peaks were adjusted

with an asymmetric Gaussian-Lorentzian sum function (75% Gaussian and 25% Lorentzian) and a Shirley background 36 using XPSPeak 4.1.

2.3. Preparation of Pd/AC catalysts The as-prepared colloid was adsorbed in activated charcoal (Sigma-Aldrich, untreated powder, particle size: 100 - 400 mesh). For this purpose, AC was added to the colloid and stirred for 15 minutes. Then, the solution was centrifuged (4000 rpm) and washed with deionized water (twice) and isopropyl alcohol (three times). The resultant powder was dried at room temperature for 24h in a desiccator. Two palladium loadings (1.5 and 3 wt %) were used in this work.

2.4. Characterization of Pd/AC catalysts XRD patterns of the 3 wt % Pd/AC catalyst were collected (Diffraktometer D500 Siemens) using Cu K α radiation (λ = 1.5418 Å) working at 40 kV and 17.5 mA. The step size was 0.05° and the acquisition time was 1 s per point, ranging from 20º to 90º. The reference standards were obtained with PCPDFWIN, version 2.1, using the JCPDS-ICDD database. Peak positions and FWHM were determined by fitting (OriginPro 8) the peaks with a Lorentzian function.


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Scanning electron micrographs (obtained with the signal of backscattered electrons) and analysis of the elemental composition by EDXS of the catalysts were performed (JEOL model JIB-4500) using an accelerating voltage of 15 kV. The as-prepared catalysts were dispersed on a carbon tape before measurements. The specific surface area of the support and the catalysts, previously degassed at 473 K during 12h under vacuum, was determined (Gemini Micromeritics) by the BET multipoint technique and the porous volume and size were obtained using the BJH method. Catalytic tests during the NO decomposition reaction were carried out in a fixed-bed quartz reactor. Prior to reaction, the catalysts were reduced with H2 (28 mL min-1) at the reaction temperature for 30 min. The flow rate of NO (500 ppm in He) was adjusted to 120 mL min-1, giving a space velocity of 15 000 h-1. The effluent gases were analyzed by in-situ IR absorption in an FTIR spectrometer (MB100-BOMEM), equipped with a multiple-reflection gas cell (7.0 m path length and 2.1 L volume). The stretching bands of NO (1955-1790 cm-1), NO2 (1658-1565 cm-1), N2O (2266-2159 cm-1) and CO2 (2397-2283 cm-1) were monitored every 5 minutes. In order to calculate the NO conversions from the IR data, a method of treating the measured absorbance values was used to determine the NO concentration

37

. The tests occurred in two

steps: (i) reactions were performed with the 3 wt % Pd/AC catalyst at temperatures of 323-673 K, in order to study how the activity of the catalyst depends on the temperature and set the temperatures at which the catalyst has the most interesting behavior; and (ii) in order to compare the two metal loadings, tests were performed with a 1.5 wt % Pd/AC catalyst at the temperatures found in step (i). All NO decomposition experiments were done in duplicate and the reproducibility of results was verified.


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3. RESULTS AND DISCUSSION 3.1. In-situ UV-visible measurements The UV-visible spectra of reference solutions were acquired (Figure 1(a)) in order to verify the contributions of the employed reagents on UV-visible absorption of the resultant colloidal solution. The spectrum of the PdCl2 solution shows a peak near 400 nm related to Pd2+ ions´ absorption

38-40

. This absorption also appears after the addition of the citric acid solution,

indicating the presence of a Pd2+ complex. The spectrum of the L-ascorbic acid solution presents a strong absorbance at wavelengths smaller than 300 nm, which is present in all spectra obtained after addition of the reducing agent, as shown in Figure 1(b). The first spectrum obtained after the addition of the L-ascorbic acid solution shows the strong absorption characteristic of Lascorbic acid and the beginning of the disappearance of the peak near 400 nm, indicating the reduction of Pd2+ ions to Pd0 38-41. This initial reduction can also be seen in the first and second spectra in Figure 1(b). The third spectrum in the same figure was acquired 5 minutes after the start of the reaction and does not show the absorption peak of Pd2+ ions. This fast conversion agrees with the reaction´s behavior, as described in the synthesis section. As the reaction evolves, the solution turns darker and the absorption from the UV to visible region increases and no additional peaks, such as those from surface plasmon resonance, are observed. It is well known that metal NPs absorb photons in the UV-visible region due to a characteristic coherent oscillation of the conduction band electrons, induced by interaction with incident light. However, some transition metals, such as those from the palladium group, do not show surface plasmon peaks due to d–d interband transitions 38,40,41. Figure 1(b) shows that there are no significant changes in the UV-visible spectra after 60 minutes of reaction, indicating the complete reduction to metallic Pd.


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Figure 1. UV-visible measurements: (a) for the reference solutions and for the reaction, 1 minute after the addition of the reducing agent; (b) for the selected spectra collected during Pd NPs formation and also for the PdCl2 and citric acid mixture, immediately prior to the addition of the reducing agent.

3.2. TEM and HRTEM analysis Figure 2 shows a representative TEM image of as-prepared Pd NPs. The image exhibits a high concentration of particles in a wire bundle arrangement typical for Pd NPs


42-46

. The histogram

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(Figure 2), made with 750 counts, reveals an average short-axis diameter distribution of 5.42 nm, with a standard deviation of 1.30 nm and relative standard deviation of 0.24.

Figure 2. Selected TEM micrograph and corresponding short-axis diameter distribution of the Pd NPs HRTEM analysis (Figure 3) shows that the particles are formed by small aggregated domains, whose high crystallinity can be inferred from the atomic planes observed. Figure 3 (a) shows a selected image of the sample, where crystallites are linearly aggregated, forming worm-like nanostructures 43,44. This could be observed in several parts of the TEM grid (Figure 2). Figure 3 (b) shows a more detailed section of a worm-like assembly. A higher magnification image (Figure 3(c)) clearly displays the different crystal plane orientations of the aggregated domains in a single worm-like particle. The distances between adjacent fringes in three crystallites agree with the Pd fcc interplanar spacing, despite of a 5 % lattice expansion. The main advantage of obtaining the worm-like structure is the availability of numerous exposed metallic sites, since it presents a variety of Pd surfaces with different crystallographic


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orientations. Therefore, the number of possible active sites would be much higher if compared to simpler structures.

(a)

(b)

Figure 3. (a) A closer look at the wire bundle arrangement. (b) HRTEM image of a region where the linear aggregation of small crystallites with different crystal plane orientations to form the worm-like NPs can be seen.

3.3. XPS measurements Figure 4 displays the Pd 3d region of the XPS spectrum, presenting two asymmetrical peaks, which corresponds to the Pd 3d5/2 (335 eV) and Pd 3d3/2 (340.3 eV). The binding energy values and the spin-orbit splitting (5.3 eV) are in agreement with the literature for Pd-Pd bonds on Pd 35,45-51

. As reported

46-49

, the Pd component has a 3d-electron shake-up satellite at 342.2 eV that

was taken into account for a better fit of the experimental result. Another peak was observed at 336.4 eV, which corresponds to Pd-O bonds 46,50,51. The ratio between the Pd-Pd and Pd-O peak areas in the spectrum is about 6:1. This ratio implies that the Pd NPs are present in its metallic


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chemical state and that the PdO component is caused by surface oxidation, probably due to exposure of the sample to air. In addition, we collected XPS spectra after the dispersion of the NPs on the AC (Figure S1 of the supporting information). It confirmed that the chemical states of the Pd atoms on the catalyst surface was not modified due to the process of catalyst preparation.

Figure 4. XPS spectra of the Pd 3d region. The open circles indicate the experimental points and the overlying continuous black line represents the sum of the three components used in the fitting. The continuous gray line represents the Shirley background. The vertical line indicates the binding energies of the observed chemical components.

3.4. XRD measurements The crystal structures of Pd NPs supported on activated carbon were investigated by XRD. The full-scale pattern obtained is shown in Figure 5, where one can see the five characteristic diffraction peaks of metallic palladium in the fcc phase. There is also a broad peak around 24º, which is typical of amorphous solids, a contribution of the AC used as support for the NPs.


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The results of data fitting are given in Table 1, as well as the average size of crystallites, which was estimated by applying Scherrer's equation (supposing spherical crystallites) for the diffraction peaks related to the (111), (200), (220), (311) and (222) crystal planes of fcc Pd. By averaging the values of D for each plane, an average size of 8.02 nm for Pd crystallites was obtained. As might be expected, this value is larger than the mean short-axis diameter obtained by TEM analysis, insofar as Pd crystallites in worm-like particles are elongated and not spherical, as observed in the HRTEM image (Figure 3(c)).

Figure 5. XRD pattern of 3 wt % Pd/AC catalyst. As shown in Table 1, the positions observed for fcc Pd peaks are dislocated to smaller angles when compared to the values found in the database, assigned for bulk Pd. This result indicates that Pd interatomic distances are expanded on Pd NPs (by comparing the dhkl values found for experimental data and for bulk Pd from the database), i.e., this means that the Pd lattice parameter observed in that sample is larger than that of bulk Pd. The reduction of the lattice parameter with the decrease of the particle size of metallic NPs has been reported in many experimental and theoretical works opposite behavior

39,52,54,55

52,53

. However, in some cases, Pd clusters showed the

. By calculating the Pd lattice parameter, (a) using the interplanar


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distances of each crystal plane, both for experimental data (dhklc) as well as for the database (dhkld), and averaging the obtained values, it can be found that (a) assumes mean values of 3.8938 Å and 3.8887 Å for the calculation based on experimental data and on the database, respectively. Thereby, the relative expansion of (a) observed in our work (0.13%) is not conclusive, since it could be due to experimental errors and approximations in the calculation, although it is corroborated by the HRTEM results (Figure 3(c)). The interplanar distances obtained by HRTEM (Figure 3(c)) and XRD (Table 1) analysis display a small difference as result of the distinct resolution of these techniques and approximations assumed in the Scherrer formula. Based on the experimental data, we can ascertain that the catalyst consists of a metallic phase (fcc Pd0 with small crystallites) supported on AC. Table 1. Results of XRD analysis. hkl

2θa(º)

2θb(º)

dhklc (Å)

dhkld (Å)

FWHMe(º) Df (nm)

111

40.063

40.118

2.248

2.245

0.843

9.91

200

46.524

46.658

1.949

1.944

1.100

7.77

220

67.987

68.119

1.375

1.375

1.208

7.84

311

81.964

82.098

1.174

1.173

1.428

7.29

222

86.523

86.617

1.124

1.123

1.478

7.30

a Peak positions obtained by fitting XRD data. b Peak positions from JCPDS 46-1043. c Interplanar distances calculated using the peak position from fitted data. d Interplanar distances calculated using the.peak position from JCPDS 46-1043. e FWHM obtained by fitting XRD data. f Average size of Pd crystallites calculated via Scherrer's equation using the FWHM and peak position obtained from data fitting.


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3.5. SEM-EDXS analysis and textural characterization Table 2 shows the elemental composition of the two metal loading catalysts obtained by EDXS analysis, the specific surface area obtained by the BET method, and the porous volume and size obtained by the BJH method. Table 2. Pd loadings and textural characteristics of the activated carbon and Pd/AC catalysts.

Sample

Pd (wt%)

Specific surface area (m2g-1)

Pore volume (cm3g-1)

Pore size (Å)

1.5 wt.% Pd/AC

1.4 ± 0.6

779

0.20

18

3 wt.% Pd/AC

3.4 ± 0.9

756

0.19

17

AC

-

762

0.20

17

It can be observed in Table 2 that the Pd loadings obtained by EDXS are close to those added to the support. The relative difference between the observed Pd loading and the amounts of Pd added to support are about – 5.3 % and + 13% for the 1.5 wt % Pd/AC and 3 wt % for the Pd/AC catalyst, respectively. These differences are consistent with EDXS limitations for low elementary concentrations. By comparing the three textural properties, one notices that both catalysts presented analogous characteristics. Moreover, the comparison between the properties of the catalysts and the AC indicates that the textural characteristics of the support were not modified after the impregnation with the colloidal solution of Pd NPs. It also can be inferred that the NPs are hosted on the external surface of CA, since no changes were observed in pore volume and size.


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Additionally, we have acquired SEM images (Figure S2 in the supporting information), which shows that the NPs are dispersed on the support’s surface as agglomerates of wires with sizes in the range between few nanometers and about 10 micrometers. Thus, we verified that the aggregation of the NPs (observed by TEM in Figure 2) remains after been supported on the AC, which assures that all their active sites will be always exposed to the reactive gases.

3.6. Catalytic tests The influence of the reaction temperature on NO conversion using 3 wt % Pd/AC as the catalyst can be seen in Figures 6 (a) and 6 (b). A noteworthy result is that the catalyst maintained the total conversion (100%) for more than 30 minutes at all tested temperatures, even at 323 K, which is slightly higher than room temperature. Most of the published results on the direct catalytic NO decomposition presented a total conversion in tests carried out only at high temperatures

21-27,56

. One may observe in Figure 6 (a) that the longest time of total conversion

was obtained for the catalytic test performed at 373 K. This experimentally verified behavior was attributed to the competition between two thermally activated processes

56

: (i) NO decomposition and (ii) desorption of non-dissociated NO plus

recombination of nitrogen and oxygen atoms adsorbed on the catalyst. Such processes are more efficient at higher temperatures, and (ii) would contribute to increase the measured NO concentration (i.e., they decrease the NO conversion). As result, we have observed that the time of total conversion was longer at 373 K than at lower or higher temperatures. Moreover, the deactivation rate was slower at 373 K than at the other temperatures presented in Figure 6 (a).


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Figure 6. NO conversion versus time of reaction in the temperature ranges of: (a) 323 – 473 K and (b) 523 – 673 K. In order to compare the two loadings (1.5 and 3 wt % Pd/AC), both catalysts were used in reactions at 373 and 573 K (Figure 7). It can be seen that the 1.5 wt % Pd/AC presented better activity toward NO decomposition at 373 K and worked at maximum conversion for about 150 min, which is a better result when compared to reported similar tests performed at higher temperatures

22,23

. The fact that the catalysts with lower Pd loading presented a longer time of

total conversion at 373 K was probably due to the inefficiency of the reduction process at such low temperature. Due to that inefficient reduction step, as greater is the Pd amount present on the catalyst, greater is the number of oxidized sites. Thereby, in the beginning of the reaction, the catalyst with more Pd (3 wt %) had more oxidized clusters exposed to NO than the lower one


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(1.5 wt %) and so, it was less able to adsorb and dissociate NO molecules. As a consequence, the 1.5 wt % Pd/CA catalyst was able to maintain the total NO conversion for a longer time than the 3 wt % Pd/CA. In the test performed at 573 K, the 3 wt % Pd/AC catalyst maintained high activity for a longer time than the 1.5 wt % Pd/AC. At this temperature, the reduction step is more efficient, so both catalysts started the reaction in the metallic reduced state. As a consequence, the catalyst with more Pd presented the best results, since it has more metallic active sites. Thus, the catalyst with lower Pd loading is more active for NO decomposition at 373 K, whereas 3 wt % Pd/AC is more active at 573 K. In addition, both catalysts presented total conversion for longer times at 373 K than at 573 K, despite the fact that slower deactivation rates were observed at 573 K.”

Figure 7. Influence of Pd loading on the activity of Pd/AC catalysts in NO decomposition reactions carried out at 373 and 573 K.

During NO decomposition, concomitantly to formation of N2 and O2, N2O and NO2 formation might occur by recombination of N and O adsorbed on the catalyst´s surface as shown in the following reactions 28,56:


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NOads + Nads N2O

(1)

NOads + Oads NO2

(2)

In monometallic catalysts, oxygen from dissociated NO stays bonded to the surface of the catalysts, poisoning the active sites and further inhibiting NO dissociation and, if there is no promoter to reduce active metallic sites, the catalyst deactivates 21,56, promoting N2O production and inhibiting N2 and O2 formation, as well as NO2 formation. In this study, only NO2 traces were found, in agreement with the behavior expected for a monometallic catalyst. The N2O production was used as an indirect measure of selectivity because the higher the N2O concentration observed, the lower the reaction selectivity will be to N2 and O2 formation. Figure 8 shows the concentrations of N2O observed during the experiments carried out with 1.5 and 3 wt % Pd/AC at 373 and 573 K.

Figure 8. Influence of Pd loading and temperature on selectivity of Pd/AC catalysts expressed as N2O formation

At 573 K, the amounts of N2O formed during reactions with both catalysts were lower than those formed at 373 K. By comparing the curves corresponding to reactions carried out at 573 K in Figures 7 and 8, one can see that the more active catalyst formed higher amounts of N2O. At


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373 K, both catalysts formed similar maximum concentrations of N2O. Whereas, 1.5 wt % Pd/AC worked at a total conversion rate for a longer time, the reaction using it as a catalyst formed a total amount of N2O higher than that formed during the reaction with 3 wt % Pd/AC. Therefore, it can be said of Pd/AC catalysts that the higher their activity for NO decomposition, the higher the amounts of N2O formed. The comparison between the curves obtained for NO conversion and for N2O formation during tests with 1.5 wt % Pd/AC at 373K (Figure 9(a)) confirms that the oxidation of the catalyst causes its deactivation. While NO conversion is total (i.e., while the concentration of NO measured at the exhaust of the reactor was zero), a progressive increase on N2O formation is observed, which reflects in a progressive oxidation of the catalyst, since oxygen retention promotes the reaction (1) 56. At approximately 150 minutes of reaction, the oxygen poisoning of the catalyst starts to inhibit NO adsorption and dissociation as a result of catalyst deactivation. The deactivation process can be clearly observed in both NO conversion and N2O formation curves in Figure 9(a). An equivalent analysis can be made for the 3 wt % Pd/AC behavior at 373 K, by comparing the black curves shown in Figures 7 and 8.


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Figure 9. The catalytic behavior of 1.5 wt % Pd/AC: (a) at 373 K expressed via the relation between NO conversion and N2O formation; (b) at 573 K expressed via the relation between NO conversion and the formation of CO2 and N2O. At higher reaction temperatures, Pd/AC catalysts take part in a redox mechanism, where the catalyst´s active sites are oxidized by NO and reduced by carbon, emitting oxygen-containing products (CO and CO2) 21,31,32. In fact, the formation of significant amounts of CO2 was observed in this study. For both catalysts, the CO2 formation was observed in reactions carried out at 573 K while no CO2 was detected in the effluent gas from reactions at 373 K. This temperaturedependent behavior agrees with the NO conversion curves: when CO2 formation is observed, the catalysts are involved in a regeneration process, resulting in a lower deactivation rate, whereas, at lower temperatures, the deactivation is not prevented via reduction by carbon. The relationship between CO2 formation and the regeneration of a catalyst can be clearly seen by observing the curves relative to 1.5 wt% Pd/AC in the reaction carried out at 573 K (Figures 9b). One observes that the onset of CO2 formation (at just after 150 minutes of reaction) coincides with an increase in NO conversion and N2O formation. In the reaction carried out at 573 K with 3 wt % Pd/AC (result presented as supporting information), the redox mechanism has maintained the NO


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conversion rate greater than 50 % for more than 225 minutes, which is a promising result for industrial applications.

4. CONCLUSIONS An effortless green route was employed to obtain Pd NPs by reduction of PdCl2 with Lascorbic acid. In-situ UV-visible absorption measurements confirmed the quick reduction of Pd2+ ions to metallic Pd during formation of NPs. By TEM and HRTEM analyses, it was concluded that the sample consists of worm-like nanoparticles with an average short-axis diameter distribution of 5.42 nm, formed by small crystallite clusters. Surface analysis (by XPS) and longrange order characterization (by XRD) confirmed the formation of Pd0. Also by XRD data, it was verified that Pd is present in its fcc phase and has an average crystallite size of 8.02 nm. A very straightforward and efficient way to support the obtained Pd NPs on AC was used to prepare a catalyst for use in NO decomposition reactions. Two metal loadings (1.5 and 3 wt % Pd/AC) were tested in a temperature range from 323 K to 673 K. Both Pd/AC catalysts proved to be active in the full temperature range tested, and as efficient as similar reported systems with the great advantage of working at lower temperatures, and therefore more cost effective. By comparing the behavior of the two variations of a Pd/AC catalyst in reactions carried out at 373 and 573 K, it was observed that 1.5 wt % Pd/AC is more active for NO decomposition at 373 K, whereas 3 wt % Pd/AC is more active at 573 K. Furthermore, both catalysts presented total conversion for a longer time at 373 K than at 573 K, and the catalysts´ deactivation rates presented at 573 K were slower. In fact, at 573 K, Pd/AC catalysts participate in a redox mechanism, where the catalyst´s active sites are oxidized by NO and reduced by carbon, emitting CO2 and enhancing their capacity to adsorb and dissociate NO. This process is able to prevent the deactivation of Pd/AC catalysts, improving their activity for NO decomposition and, thus,


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making them an interesting system for theoretical and practical applications. In our future investigations, NPs of other noble metals will be supported on AC and tested as catalysts for NO decomposition reaction. Therefore, we plan to find the conditions for optimizing the activity of these eco-friendly catalysts and to better understand the redox mechanism observed in the present work.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. Phone number: +55 51 3308-6525. Fax number: +55 51 3308-6510. E-mail: [email protected].

ACKNOWLEDGMENTS This work was funded by CNPq, CNANO-UFRGS and LNLS (SXS-7648 and SXS-9965 proposals). The authors would like to thank the LNLS and CME-UFRGS staffs for the support given, Dr. J. H. Z. dos Santos for textural analysis, J. Alexandre, D. C. A. Ribeiro and F. Bernardi for important contributions in sample preparation and characterization, and G. Machado for fruitful discussions on XRD analysis. M. V. Castegnaro thanks CAPES for his MA fellowship.

Supporting Information: XPS spectra of Pd 3d region of the 3 wt % Pd/CA; SEM image of the 3 wt % Pd/CA; The catalytic behavior of 3 wt % Pd/AC at 573 K expressed via the relation between NO conversion and the formation of CO2 and N2O; and the original IR spectra collected at 5 and 250 minutes of


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the reaction with the 3 wt % Pd/CA catalyst at 373 K. This material is available free of charge via the Internet at http://pubs.acs.org

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On the Reactivity of Carbon Supported Pd Nanoparticles during NO

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