ARTICLE pubs.acs.org/Langmuir
Effect of Reduction Treatment on Copper Modified Activated Carbons on NOx Adsorption at Room Temperature Benoit Levasseur,† Eugene Gonzalez-Lopez,‡ Joseph A. Rossin,§ and Teresa J. Bandosz*,† †
The City College of New York and The Graduate School of CUNY, 160 Convent Avenue, New York, New York 10031, United States John Jay College of Criminal Justice, 899 10th Avenue, New York, New York 10019, United States § Guild Associates, Inc., 5750 Shier-Rings Road, Dublin, Ohio 43016, United States ‡
ABSTRACT: Activated carbon was impregnated with copper salt and then exposed to reductive environment using hydrazine hydrate or heat treatment under nitrogen at 925 °C. On the obtained samples, adsorption of NO2 was carried out at dynamic conditions at ambient temperature. The adsorbents before and after exposure to nitrogen dioxide were characterized by X-ray diffraction (XRD), thermal analysis, scanning electron microcopy/energy dispersive X-ray spectroscopy (SEM-EDX), X-ray photoelectron spectroscopy (XPS), N2sorption at 196 °C, and potentiometric titration. Copper loading improved the adsorption capacity of NO2 as well as the retention of NO formed in the process of NO2 reduction on the carbon surface. That improvement is linked to the presence of copper metal and its high dispersion on the surface. Even though both reduction methods lead to the reduction of copper, different reactions with the carbon surface take place. Heat treatment results in a significant percentage of metallic copper and a reduction of oxygen functional groups of the carbon matrix, whereas hydrazine, besides reduction of copper, leads to an incorporation of nitrogen. The results suggest that NO2 mainly is converted to copper nitrates although the possibility to its reduction to N2 is not ruled out. A high capacity on hydrazine treated samples is linked to the high dispersion of metallic copper on the surface of this carbon.
’ INTRODUCTION Nowadays, an increasing number of diesel engines working at high air to fuel ratio in order to reduce CO2 emissions makes NOx release to the atmosphere an important environmental concern. Nitrogen dioxide contributes to the formation of smog and acid rain, and its toxicity may cause respiratory problems. Besides environmental repercussions, nitrogen oxides are also considered as toxic industrial compounds (TICs), and therefore, special precautions have to be taken in order to separate them from air at ambient conditions to prevent the damage caused by accidental/purposeful emission to the atmosphere or confined spaces. Removal of nitrogen oxides has been extensively studied over the past decades. The most recent studies focused on several depollution processes such as selective catalytic reduction (SCR)13 or NOx storage reduction (NSR).4,5 In spite of the promising results in NOx reduction, these processes suffer from high temperatures required for the process and the storage, transportation, and use of ammonia as a reducing agent. As an alternative of SCR and NSR processes, adsorption in a fixed bed seems to be a promising option, provided the adsorbent can be prepared in the form of a monolith so as not to impose an excessive pressure drop on the process. Among adsorbents tested, carbonaceous materials including activated carbons (ACs),68 activated carbon fibers (ACFs),9 or soot10,11 were found as promising filtration media. The studies suggest that surface chemistry is an important parameter to achieve high NOx r 2011 American Chemical Society
adsorption. Another aspect to consider is the hard to avoid reduction of NO2 at low temperatures on the surface of carbonaceous materials, which leads to the formation of NO as well as to oxidation of the adsorbent surface.9,12 As a general mechanism of adsorption, formation of both oxygen and nitrogen surface complexes such as C(ONO) and C(ONO2) during the NO2 adsorption process has been clearly indicated by Jeguirim and co-workers.12 They suggested that NO2 is mainly adsorbed on preexisting oxygen complexes. Even though the virgin carbons can exhibit some capacity for NOx removal, modifications of their surface with compounds changing their acid/base character13,14 or treatments leading to an incorporation of heteroatoms to the carbon matrix15,16 have been investigated and enhancements in the adsorption capacity have been reported. Taking into account the oxidation of the adsorbent surface induced by NO2, an incorporation of the metallic particles can be considered as another way to alter/enhance surface interactions with preserving the properties of the carbon matrix. Several studies address the impregnation of carbons with nitrate or acetate precursors,1722 but only few of them mention reduced metals (Pt, Ag).19,23 Among transition metals, copper appears to be a promising candidate for NOx adsorption. It was found that Received: December 13, 2010 Revised: March 23, 2011 Published: April 06, 2011 5354
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Langmuir copper has a high ability to convert NO into N2 at moderate temperature in the presence or absence of O2.17 Tighe and coworkers suggested that doping of activated charcoals with copper significantly increases the rate at which NO is reduced to N2 above 550 °C.19 Regarding NO adsorption, Adelman and coworkers indicated that preadsorption of NO2 provides new sites for adsorption of NO on Cu-ZSM5.24 They also found that copper nitrates are the major products formed during NO2 adsorption process on these materials at room temperature.24 Such species were also found on Cu/TiO225 and Cu-zeolites.26,27 Considering the above, the objective of this study is an evaluation of the adsorptive performance of activated carbon modified with copper in the process of NOx removal at ambient conditions. The carbons are heated at high temperature in an inert atmosphere or exposed to hydrazine hydrate, since we target the reduction of both copper species and certain functional groups. The results are discussed in terms of active species chemistry, their distribution on the surface, and the mechanism leading to their deposition/specific chemistry as well as in terms of their effects on the NOx retention. Even though many papers have discussed NOx adsorption on carbons, they mainly address the adsorption at elevated temperatures. Therefore, the effective removal of these species at ambient conditions still needs substantial optimization.
’ EXPERIMENTAL SECTION Materials. Nine different samples were prepared from wood-based activated carbon BAX-1500 manufactured by Mead Westvaco. The initial carbon is referred to as B. The raw material was impregnated with copper nitrate (Cu(NO3)2 3 5H2O) using an incipient wetness impregnation method. Predetermined amounts of copper nitrates were dissolved in the amount of water equal to 110% of the pore BAX-1500 pore volume. The solutions were then dropwise added to the activated carbons, and the samples were dried at ambient temperature overnight. As a next step, the samples were heated up to 250 °C in N2 for 2 h and denoted as 5Cu/B and 15Cu/B, where the numbers reflect the amount of copper in wt %. The subsamples were heated at 925 °C in N2 for 2 h. They are referred to as 5Cu/BT and 15Cu/BT. An amount of 5 g of either initial or copper modified carbons was mixed with 10 mL of a 40% solution of hydrazine hydrate at pH 10 (set by addition of NaOH). The suspension was stirred for 4 h at room temperature, then washed 5 times with 250 mL of distilled water to remove an excess of hydrazine, and finally dried overnight at 150 °C. Such samples are referred to as 5Cu/ BH and 15Cu/BH. With the exception of 15Cu/BH, all samples were in the form of 23 mm granules. In the case of 15Cu/BH, a powder was obtained after the reduction reaction. Methods. NO2 Breakthrough Capacity. The NO2 sorption capacities were measured in a laboratory-scale, fixed-bed reactor system, at room temperature and in dynamic conditions. In a typical test, 1000 ppm NO2 in nitrogen went through a fixed bed of adsorbent with a total inlet flow rate of 450 mL/min. The adsorbent bed was packed into a glass column (length 370 mm, internal diameter 9 mm, 23 mm carbon particles) and consisted of about 2 cm3. In the case of 15Cu/BH, to build the adsorbent bed with powdered solid, glass beads were used in order to prevent any pressure drop. The concentrations of NO2 and NO in the outlet gas were measured using an electrochemical sensor (RAE Systems, MultiRAE Plus PGM-50/5P). The adsorption capacity of each adsorbent was calculated in milligrams per gram of adsorbent by integration of the area above the breakthrough curve. The tests were conducted until the concentrations of NO2 and NO reached the electrochemical sensors’ upper limit values of 20 and 200 ppm, respectively. After
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Table 1. NO2 Adsorption Capacities, NO Retention, and pH Values (Initial and Exhausted) for the Carbons Studied sample
pH initial NO2 ads (mgNO2/gmat) NO released (%) pH final
B
5.7
35
17
2.1
5Cu/B 15Cu/B
6.4 6.3
40 57
14 12
2.5 2.5
BT
8.6
50
10
2.2
5Cu/BT
7.3
59
10
2.3
15Cu/BT
7
121
8
2.1
BH
5.9
29
18
2.3
5Cu/BH
6.8
51
8
2.2
15Cu/BH
7
206
5
2.6
the breakthrough tests, all samples were exposed to a flow of carrier air only (180 mL/min) to evaluate the strength of NO2 retention. The suffix -ED is added to the name of the samples after exposure to NO2. Surface pH. The samples were first dried and ground to obtain a powder, and then 0.4 g was added to 20 mL of distilled water and strirred overnight. The pH of the suspension was then measured. Thermal Analysis. Thermogravimetric (TG) curves and their derivatives (DTG) were obtained using a TA Instruments thermal analyzer. The samples (initial and exhausted) were previously dried in oven at 100 °C to remove moisture and then heated up to 1000 °C, with a heating rate of 10 °C/min under a nitrogen flow of 100 mL/min. The ash content was determined as a residue after burning the samples at 800 °C in air. Adsorption of Nitrogen. Nitrogen isotherms were measured at 196 °C using an ASAP 2010 instrument (Micromeritics). Prior to each measurement, initial and exhausted samples were outgassed at 120 °C. The surface area, SBET, the total pore volume, Vt, the micropore volume, Vmic (DubininRadushkevich method28), and the mesopore volume, Vmes, were obtained from the isotherms. Pore size distributions were calculated using density functional theory.29 XRD. X-ray diffraction (XRD) measurements were conducted using standard powder diffraction procedures. The adsorbents (initial and exhausted) were ground with methanol in a small agate mortar. The mixture was smear-mounted onto a glass slide and then analyzed using Cu KR radiation generated in a Philips X’Pert X-ray diffractometer. A diffraction experiment was run on a standard glass slide for the background correction. SEM-EDX. Scanning electron microscopy (SEM) and electron dispersive X-ray spectroscopy (EDX) were performed on a Zeiss Supra 55 instrument with a resolution of of 5 nm at 30 kV. Analyses were performed on a sample powder previously dried. For EDX, the analysis was done at magnification 5K and the content of elements on the surface was calculated. XPS Analysis. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Perkin-Elmer Phi 570 ESCA/SAM instrument employing Mg KR X-rays. All binding energies are referenced to the C 1s photoelectron peak at 284.6 eV. Activated carbon particles were crushed into a fine powder for analysis. In this manner, the analysis is considered as more representative of the bulk sample than an analysis of the granules’ external surface. Potentiometric Titration. Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator (Metrohm). The instrument was set at the mode when the equilibrium pH was collected. Subsamples of the materials studied of about 0.100 g in 50 mL of 0.01 M NaNO3 were placed in a container thermostatted at 298 K and equilibrated overnight with the electrolyte solution. To eliminate the influence of atmospheric CO2, the suspension was 5355
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Figure 1. NO2 Breakthrough curves for the samples studied. continuously saturated with N2. The carbon suspension was stirred throughout the measurements. Volumetric standard NaOH (0.1 M) was used as the titrant. The experiments were done in the pH range of 310. Each sample was titrated with base after acidifying the sample suspension. The experimental data was transformed into a proton binding isotherm, Q, representing the total amount of protonated sites.30
’ RESULTS AND DISCUSSION Measured NOx breakthrough curves and the calculated capacities are shown in Figure 1 and Table 1, respectively. Regardless of the reduction treatment applied, the addition of copper leads to an enhancement in the NO2 adsorption capacity. For both reduction methods, the NO2 capacities increase with the increase 5356
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Table 2. Parameters of the Porous Structure for the Samples Studied SBET (m2/g)
Vmic (cm3/g)
Vt (cm3/g)
Vmic/Vt (%)
B
2011
0.728
1.515
48
B ED BT
1675 1590
0.602 0.627
1.204 1.055
50 59
BT ED
1234
0.487
0.840
58
BH
2024
0.739
1.52
49
BH ED
1782
0.693
1.40
50
5Cu/B
1753
0.644
1.29
50
5Cu/B ED
1680
0.608
1.254
49
5Cu/BT
1339
0.517
0.927
56
5Cu/BT ED 5Cu/BH
1217 1930
0.392 0.707
0.709 1.422
56 50
5Cu/BH ED
1822
0.662
1.35
49
15Cu/B
1163
0.458
0.782
59
15Cu/B ED
1028
0.401
0.681
59
925
0.376
0.607
62
sample
15Cu/BT 15Cu/BT ED
892
0.360
0.605
60
15Cu/BH
1756
0.647
1.28
50
15Cu/BH ED
1179
0.443
0.853
52
in the weight percentage of copper added to the B carbon. During the adsorption of NO2, NO has been detected in the gas outlet in the case of all materials, which is related to the reactive nature of NO2 adsorption.18 The amounts of NO released in the outlet gas during the NO2 breakthrough experiments show a decreasing trend with an increase in the amount of copper deposited on the surface (Table 1). The reduction by heat treatment increases the NO2 adsorption capacities from about 30% for the BT carbon to over 200% for 15Cu/BT compared to the untreated samples. Regarding the percentage of NO released during the NO2 adsorption, a decrease of 4% is found for each carbon sample modified with copper. On the other hand, a decrease of 7% is noticed for the initial carbon. It is interesting that the treatment with hydrazine decreases the NO2 capacity of the BH carbon of about 17% compared to the B sample and has no visible effect on the 5Cu/BH material. On the other hand, for 15Cu/BH, a strong enhancement in the capacity of about 400% is found compared to 15 Cu/B. To determine in detail how NOx reacts with the surface and to understand the contribution of both copper and carbon, the porosity and surface chemistry of the initial and exhausted samples have to be analyzed. Information about the porosity of our materials is derived form the analysis of nitrogen adsorption isotherms at 196 °C. Parameters of the porous structure calculated from these isotherms are collected in Table 2. The BAX-1500 carbon has a very high surface area, and pore volume and can be considered as a micro/mesoporosus adsorbent. Introduction of copper decreases the parameters of the porous structure of about 25% to 45% depending on the copper loading. The loss of specific surface area and volume of pores is more visible for 15Cu/B than for 5Cu/B. The deposited copper species likely block some pores since the decreases are much greater than the percent of copper addition. A decrease in the porosity of the heat-treated samples is caused by shrinking of the carbon matrix owing to the high temperature treatment16 and/or changes in the chemistry and distribution of copper species. Nevertheless, the decrease in the porosity is more pronounced
Figure 2. Pore size distributions for the selected samples.
Table 3. Ash Content and Surface Composition Determined by EDX and XPS Analyses on the Cu/B Samples before and after Adsorption atom % from EDX atom % from XPS sample B
ash content (wt %) C
O
Cu
0.7
82
18
0
5Cu/B
6.2
90
8
1.1
5Cu/BT 5Cu/BH
6.3 5.5
91 86
15Cu/B
17.1
84
BH
15Cu/BH ED a
O Cu N 6.7a 10
7.5 13
0.9 0.8
11
4.2 84
14
1.7 0.3
82.3 15.2 1.5 1 17.4
89
5.7
4.9 91.7 6.5 1.3 0.45
14.2
81
14
4.6 86.4 11.1 1.1 1.4
15Cu/BT ED 15Cu/BH
91
a
90
15Cu/B ED 15Cu/BT
C
85 88
13
2
0
10.8 0.2 1
From ref 35.
for the samples containing copper, especially for Cu/BT than for the carbon without metal. This can be linked to changes in the agglomeration/physical location of copper when reduced and melted at 700 °C.31 On the contrary, treatment with hydrazine hydrate had a beneficial effect on the samples' porosity, regardless the content of copper. As shown in 5357
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Langmuir Figure 2, a significant increase in the volume of pores is noticed. Such a beneficial effect of hydrazine on the porosity of activated carbon has already been observed on carbon
Figure 3. XRD analysis of the carbons modified by copper.
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impregnated with nickel.32 This can be explained either by removing of some copper during washing or by formation of additional pores during reduction. The latter scenario seems to be more plausible. The exposure to NO2 generally decreases the porosity, which is the result of the reactive nature of NO2 adsorption.18 The loss of the specific surface area and volume of pores is much more pronounced for B, BT, and BH than for the copper containing carbon materials with the exception of 15Cu/BH. This suggests that the impregnation with copper limits the reaction between the carbon matrix and NO2. Comparison of the pore size distributions (for clarity reasons, in Figure 2, only the distributions for 5Cu/BH and 15Cu/BH are presented) before and after exposure to NO2 indicates that the whole range of pores are affected by NO2 . For 15Cu/ BH, exceptionally large loss in the porosity (around 30%) was found. Similar changes after the exposure to NO2 are usually linked to the extensive oxidation of the carbon surface18 or deposition of a significant quantity of surface reaction products. The latter reason can be predominant, since twice more NO2 was retained on the surface of this carbon than on its heat treated counterpart. Another reason can be in extensive reduction of the surface with hydrazine, which created new sites for surface oxidation by NO2. That oxidation might lead to the destruction of the pore walls. Since the porosity cannot be directly linked the adsorptive behavior of our carbons and in fact such a relationship is not expected in
Figure 4. SEM images of B (A), 5Cu/B (B), 5Cu/BT (C), 5Cu/BH (D), 15Cu/B (E), 15Cu/BT (F), and 15Cu/BH (G). 5358
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Figure 5. EDX maps of activated carbon modified with copper before exposure to NO2.
the case of reactive adsorption, other factors such a chemistry or dispersion of active phase have to be analyzed in detail. As seen from Table 3, where the ash content is reported, the samples after the treatment with hydrazine have about 25% less inorganic matter than those after the heat treatment. This supports the hypothesis about the removal of only a very small quantity of soluble copper species, if any, during the sample preparation. Although some discrepancies are noticed owing to the limited sensitivity of the gravimetric methods, the results confirm the intended copper loading and indicate that the majority of copper was reduced to metal when samples were burned in air in the presence of carbon. The XRD diffraction patterns of unexposed carbons are shown in Figure 3. Since the patterns for the samples with 5% copper are similar to that for the B carbon owing to the high dispersion of copper species and/or lack of the crystallinity, only the B sample and the 15Cu/B series are shown. Both B
and 15Cu/B are found to be amorphous. However, new signals ascribed to metallic copper appear on the diffraction patterns for the reduced samples. Calculation of the particle size from the Scherrer equation33 provides the average copper particles size to be 500 and 300 nm on 15Cu/BT and 15/ CuBH, respectively. Owing to a favorable copper reduction potential (þ 0.337 V for Cu þ2 and 0.521 for Cu1þ), the heat treatment at 925 °C under N2 resulted in a reduction/partial reduction of copper oxides and likely formation of carbonates in their reactions with released CO2 . On the other hand, during the treatment with hydrazine hydrate at alkaline environment, the following reaction on copper species was expected (eq 1): 2Cu2þ þ N2 H4 þ 4OH f 2Cuð0Þ þ N2 þ 4H2 O
ð1Þ
When hydrazine is used, nitrogen can be incorporated in the 5359
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Figure 6. Cu 2p spectra of 15Cu/B, 15Cu/BT, and 15Cu/BH before and after exposure to NO2.
reaction of the epoxide (ethers) with hydrazine leading to the formation of an aminoaziridine moiety:34
An increase in the size of the copper particles, between 300 nm for 15Cu/BH and 500 nm for 15Cu/BT, is the result of a sintering process which occurs during the heat treatment of the latter sample. The visibly different slope of the breakthrough curve for 15Cu/BT (Figure 1) suggests a slower mass transfer rate due to the potential blocking of the pores by large copper particles. Therefore, small copper particles would be very beneficial for our applications. They would provide better contact with the adsorbate and do not cause the drastic blocking of the pore space. SEM micrographs and EDX analysis presented in Figure 4 and Table 3 support the X-ray diffraction data. For comparison, also the surface of BAX-1500 carbon is presented (Figure 4A). Deposition of 15% copper is seen as irregular particles with sizes between 50 and 500 nm (Figure 4E). In the case of 5% cooper, the particles are much more homogeneous with sizes of about 25 nm (Figure 4B). Apparently, the heat treatment changes the chemistry of copper species, and, owing to the reduction process, the “balls” of quite uniform sizes of about 300 nm are seen on the surface of 15 Cu/BT (Figure 4F). The sizes in the case of 5Cu/ BT are about 70 nm (Figure 4C). The larger particle sizes in the case of the reduced samples with 5% copper than those for the unreduced samples can be explained by metal melting and its aggregation/reactions on the surface. A completely different surface topography is revealed in the case of samples after the
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hydrazine treatment (Figure 4D and G). Not only the “balls” cannot be seen but the surface looks much “cleaner”. The only plausible explanation is either the removal of some copper during the washing after the hydrazine treatment or the migration of copper to the small pores (not visible with the magnification used) along with the hydrazine solution where the reduction took place. Based on the analysis of the ash content, the former scenario is rather likely to take place. The particles seen on the surface are likely those unreduced copper oxides. The results discussed above suggest that the increase in the NO2 adsorption, observed on our samples, might be attributed to the presence of metallic copper dispersed on the surface of materials. Its content, distribution, and dispersion should also play a role, since higher NO2 adsorption capacity was observed on 15Cu/BH than on 15Cu/BT. The distribution of copper and oxygen is seen on the elements maps presented in Figure 5. For purposes of discussion, distribution refers to the physical location of the elements across the surface exposed to the X-ray source. Table 3 lists the atomic percent of these elements on a particular fragment of the scanned surface. A very high ratio of copper to oxygen on the surface of 15 Cu/BT (∼0.85) supports the reduction of copper oxide by the carbon surface and/or reduction of carbon oxygen groups. For the samples after the hydrazine treatment, these ratios are about 0.06 for 5 Cu/BH and 0.25 for 15 Cu/BH. Much lower ratio for the samples with a low copper content can be linked to the contribution of the oxygen from surface groups to the total oxygen content. As clearly seen from the analysis of the element maps (Figure 5), the copper particles are distributed homogeneously on the surface of all materials. Apparently, heat treatment reduces copper to Cu(0), and the relatively uniform “balls” seen on the surfaces of the heat-treated carbon materials represent these species. Interestingly, the results suggest that some cooper is also in the reduced form on the carbon impregnated with copper 5Cu/B and not exposed to any reduction procedure. The surface composition given by XPS indicates that the amounts of carbon, oxygen, and copper on 15Cu/B and 15Cu/ BH are equal, which supports the results discussed above that washing samples after hydrazine treatment did not result in a significant leaching of copper. The discrepancies between EDX and XPS analyses are related to the limitations in surface sensitivity of both methods and their ability to detect elements from a particular fragment of the surface, which have a rather heterogeneous nature in the case of activated carbons. Surface composition in atomic percent derived from the XPS analysis is presented in Table 3. The discrepancy with EDX results can be only related to the specificity of these surface analysis methods. The reason for so much copper detected in the latter experiments can be due to our focusing on the aggregates of an inorganic phase visible in the surface (Figure 4). On both 15Cu/BT and 15Cu/BH, a decrease in the amount of oxygen is found in comparison with 15Cu/B, which indicates a reduction of the metal and/or functional groups of BAX. This effect is especially visible for 15Cu/BT, suggesting the strong effect of the reduction treatment on surface chemistry. After NO2 adsorption, an increase of oxygen on the surface is found which confirms the oxidative nature of NO2 adsorption. On the surface of 15Cu/B, a residual nitrogen likely coming from the nitrate precursor is detected. After hydrazine treatment, the amount of nitrogen increased to 1.4%, supporting the incorporation of nitrogen via reactions of oxygen groups with hydrazine hydrate.34,36 5360
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Figure 7. Deconvoluted Cu 2p3/2 XPS spectra of 15Cu/B, 15Cu/BT, and 15Cu/BH before and after exposure to NO2.
Table 4. Relative Amounts of Oxygen and Copper Species (Based on Deconvolution Curves; in Percent of Total Oxygen or Copper) for the Selected Samples before and after Adsorption sample
a
CdO
COH
COC
Cu(0)
Cu(II)
100
Ba
65
35
—
BH 15Cu/B
30 37
59 52
11 11
0
15Cu/B ED
49
40
11
0
100
15Cu/BT
40
39
21
79
21
15Cu/BT ED
36
34
30
33
57
15Cu/BH
43
43
14
60
40
15Cu/BH ED
41
45
14
31
59
From ref 35.
The deconvoluted spectra for Cu2p for the initial samples and those after reduction are presented in Figures 6 and 7. The
relative amounts of each species obtained from deconvolution are summarized in Table 4. The Cu 2p spectrum of 15Cu/B presents a single spin orbital pair with Cu 2p3/2 and Cu 2p1/2 at 934.5 and 953.8 eV, suggesting that copper is present as Cu(II).37,38 That Cu(II) is confirmed by the appearance of a shakeup satellite at the binding energy higher of 10 eV.37 After the heat treatment, the Cu 2p spectrum changed and the single spin orbital pairs Cu 2p3/2 and Cu 2p1/2 are shifted to lower binding energy at 932.7 and 952.4 eV, respectively. The deconvolution of the main Cu 2p signal at 932.7 eV indicates that 79% of the copper is metallic. Residual Cu(II) is also seen as a shakeup signal at 944.7 eV.37 The Cu 2p spectrum of Cu/BH is very similar to that for Cu/BT with copper present as Cu(0) and Cu(II). This is in agreement with EDX analysis and confirms that the particles observed on SEM pictures of Cu/BH materials (Figure 4D and G) are indeed unreduced copper. Deconvolution of this spectrum indicates that 60% of the copper is in a metallic form. Exposure of 15Cu/B to NO2 does not change the chemistry 5361
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Figure 8. Deconvoluted O 1s XPS spectra of 15Cu/B, 15Cu/BT, and 15Cu/BH before and after exposure to NO2.
of copper. On the other hand, in the case of the reduced carbons, the oxidation of the copper particles is clearly seen. In the case of 15Cu/BT ED and 15Cu/BH ED, only 33% and 31% copper is still metallic which proves the involvement of the copper particles in reactions with NO2. Deconvolutions of O 1s spectra for 15Cu/B, 15Cu/BT, and 15Cu/BH, in Figure 8, show CdO groups (531.2 eV), CO or COC (533.3 eV) and adsorbed water molecules (535.7536.5 eV).39,40 After heat treatment, a strong decrease in the amount of CO and/or COC groups from 52% to 39% is observed. Interestingly, no changes are observed for CdO groups, which suggests their regeneration upon exposure to the atmospheric oxygen. After hydrazine treatment, the relative amounts of CO decreased. The distribution of oxygen in other groups did not change significantly. The N 1s spectrum of 15Cu/BH revealed a weak signal at the characteristic binding energy of the CN bond41 which confirms
the reaction with ethers (eq 2). The presence of nitrates is seen for 15 Cu/B ED at 406.2 eV.42 Interestingly, there is no clear signal from these species on the spectra of 15Cu/BT ED and 15Cu/BH ED, which might be related to either their low nitrogen content or location of surface reaction products in the small pores. Lack of atomic nitrogen may also indicate that NOx was reduced to N2. Even though NO was formed and released to the atmosphere as a result of NO2 conversion, that amount was less than that of NO2 delivered to the surface during the breakthrough experiments Proton binding curves for B, BT, and BH are shown in Figure 9. The analysis cannot be carried out on other samples owing the possible reaction of copper species with acid. The positive Q represents a proton uptake (basic surface) and negative proton release (acidic surface). BT appears as the least acidic carbon followed by BH and B. The results suggest that the 5362
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Figure 9. Proton binding curves for the B series before and after exposure to NO2.
surface oxidation is the main mechanism of a relatively high adsorption on BT since new adsorption centers arisen as a result of the reduction/partial reduction of the original oxygen containing groups. Treatment with hydrazine reduced some groups of the B sample, but only in the range of carbonyls and phenols. This agrees with the expected lack of hydrazine reactivity with carboxylic groups.34 One has to remember that in our experimental window we can detect only acids with pKa between 3 and 10 and such species as ethers cannot be accounted for. After the exposure to NO2, the surface of all carbons studied by potentiometric titration becomes much more acidic as a result of surface oxidation. The difference between the proton binding curves before and after adsorption on B and BT is more significant than that on BH, which is linked to smaller adsorption on the BH sample. The changes in surface chemistry caused by the exposure to NO2 were also monitored using thermal analysis. DTG curves in nitrogen for the samples after the NO2 exposure are presented in Figure 10, where peaks represent a weight loss. It is important to keep in mind that the initial BAX-1500 carbon was manufactured at 600 °C so any interpretation above this temperature is not meaningful. The first peak seen on all curves below 100 °C represents desorption of water. Even though the DTG curves are very complex, some new features can be clearly distinguished. For the B carbon after thermal reduction (Figure 9), the amount of acidic groups significantly decreases.10 Even after the hydrazine treatment, the groups decompose which might be the result of the destruction of nitrogen containing groups and those oxygen containing which did not react with hydrazine (carboxylic). After the exposure to NO2, the surface gets reoxidzied to a great extent, especially for the B/T on which a significant amount of acidic groups in the broad range of thermal stability is formed.10,12 Interestingly, the hydrazine treated B after NO2 adsorption sample shows changes only in the temperature range less than 500 °C. This indicates the differences in the mechanisms of surface reactions between the samples related to the specificity of surface chemical groups.
Figure 10. DTG curves in nitrogen for the studied samples before and after exposure to NO2.
In the case of 5Cu/B and 15Cu/B DTG curves, two peaks slightly after 200 °C and at 400 °C are visible. They can be linked the reduction of Cu(II) to Cu(I).43 This is confirmed by the fact that these signals are not revealed after heating or significantly decreased after the hydrazine treatment. In the case of 5Cu/BT, the results indicate that copper carbonate CuCO3 is also formed since a peak corresponding to the decomposition of this species is 5363
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visible on the DTG curve at 500 °C.44 This is owing to the release of significant quantities if CO2 during the heat treatment from the reduced carboxylic groups. After adsorption of NO2, a major peak is revealed slightly after 200 °C on the 5Cu/B carbons and is assigned to the decomposition of copper nitrate.43,45 On the 15Cu/B materials, decomposition of surface reaction products is shifted to lower temperature and two peaks are distinguished at 170 and 220 °C. This may be due to the presence of copper nitrate and nitrite in pores of various sizes. The decomposition temperatures of Cu(NO3)2 and Cu(NO2)2 are 256 and 120 °C,46 respectively. Analyzing the performance of the materials studied and taking into account their surface features, a mechanism of reactive adsorption is proposed. In the following equations, the asterisk (*) stands for the active reduced sites and (O) for the active oxidized sites. On the reduced carbon matrix, the following processes take place:
Reaction of NO2 leads first to an oxidation of the carbon active sites and formation of carboxylic acids and/or lactones (as found on DTG curves). Finally, the complete oxidation of the surface results in the release of NO, NO2, as well as CO and CO2, resulting in a decrease in porosity (Table 2). When B carbon is used without any post-treatment, many active sites are occupied by oxygen. Consequently, the complete oxidation of the surface occurs faster which results in the smaller adsorption than that of BT. Moreover, the reaction between hydrazine and epoxide groups may leads to the formation an aminoaziridine moiety (eq 2). To our knowledge, no study in the literature indicates a possible reaction between NO2 and hydrazone groups or aminiaziridine moiety function. However, the strong interactions between amino groups and NOx is very well-known;47 therefore, it is plausible to assume that those groups may react with NO2 and NO during adsorption. When copper is introduced to the surface, the mechanism of NO2 adsorption is different since the adsorption of NO2 on oxidized copper leads to the formation of stable copper nitrates (or nitrites) (eq 4). NO2
NO2
C Cu sf C CuðOÞ sf C CuðNO3 Þ
ð4Þ
Therefore, the decrease in the porosity is less visible in this case than on the B carbons,and the retention of NO2 increased with the loading of copper. When the surface is reduced after the heat treatment, the high adsorption capacity is measured since the copper needs to be first oxidized by NO2 to allow the formation of copper salts. Nevertheless, owing to the presence of the large agglomerates of copper (Figures 4 and 5), its potential is not fully used. In the case of activated carbon modified with copper and post-treated with hydrazine, the enhancement of their adsorption capacities is found owing to the increase in the porosity after the treatment. That increase in porosity has to result in an increase in the copper, which was seen on the EDX maps. On these carbons, there are no big agglomerates of copper as in the case of 15 Cu/BT.
’ CONCLUSIONS The results presented in this paper showed that the enhanced NO2 adsorption can be achieved at room temperature on activated carbon impregnated with copper. Two different reduction treatments of the carbon surface result in differences in the surface chemistry. The heat treatment causes a complete reduction of oxygen functional groups but also leads to a decrease in porosity. By contrast, the hydrazine treatment not only preserves the porous structure, but in the case of Cu/BH materials the porosity increases with an increase in the copper content. Moreover, a reaction between some oxygen containing groups of the carbon surface and hydrazine leads to the incorporation of hydrazone and aminoaziridine functional groups. The reactivity of these groups with NOx is however not clear and requires further investigation. The addition of copper improves the adsorption capacity for NO2. It also increases the retention of NO formed during the NO2 adsorption process. Adsorption capacities increase after reductive post-treatments. This is due to the reduction of copper and formation of new active sites. Their presence causes the carbon matrix to be more resistant to oxidation by NO2, and copper nitrate/nitrite are the main products of surface reactions. Reduction of the surface leads to well dispersed copper metals which can be oxidized by NO2 and bind significant quantities of nitrogen oxide during adsorption at ambient conditions. It is also possible that some nitrogen oxide is reduced to N2 on our materials. Although there is a possibility of regeneration of our materials by reductive treatment, one may expect a decrease in the capacity owing to the decrease in the porosity as a result of the carbon surface oxidation during the NO2 reactive adsorption. ’ AUTHOR INFORMATION Corresponding Author
*Telephone: 212-650-6017. Fax: 212-650-6107. E-mail: tbandosz@ ccny.cuny.edu.
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