Understanding the Reactions of CO2, NO, and N2O with Activated

Review pubs.acs.org/EF

Understanding the Reactions of CO2, NO, and N2O with Activated Carbon Catalyzed by Binary Mixtures Sónia A. C. Carabineiro*,† and L. Sousa Lobo*,‡ †

Laboratory of Catalysis and Materials (LCM), Associate Laboratory LSRE/LCM, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal ‡ Requimte Research Center, Chemistry Department, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal ABSTRACT: The carbon bulk diffusion mechanism and the catalytic carbon “nanoworm” model are used to understand the role of the phases prevailing in the catalyst particles and the causes of the synergistic effects observed in gasification rates of activated carbon by CO2, NO, and N2O. The kinetics were isothermally studied with the help of a TGA apparatus. Single and binary catalysts were used. The metals were deposited on carbon by incipient wetness. Synergistic effects were observed with some binary systems. Carbon gasification by CO2 and NO were particularly promoted by the addition of Ba combined with V, and gasification by N2O was promoted by Mn combined with Ba. The changes of catalyst phases with temperature were observed by in situ X-ray diffraction. Temperature-programmed experiments using NO showed N2, N2O, CO2, and CO as reaction products, while experiments using N2O showed only CO2 and N2. With CO2, only CO formation was detected.

■

(iii) for temperatures higher than 500 °C, the amount of N2 was constant, while CO was the main product. Compared to the amount of research carried out on NO, N2O has been practically disregarded.1n,t,4 In fact, the role of N2O was not even considered until recently. However, it is originated by the same reactions that produce NO, such as combustion of fossil fuels (like automotive processes) and production of adipic and nitric acids along with conversion of biomass.1n,t,5 The impact of N2O is important due to its very large duration in the atmosphere (150 years), therefore it has an important contribution to ozone layer reduction.1c,n,t,4−6 Several studies show that the N2O abatement rate is larger than that of NO on activated carbon with no catalyst.1c,7 RodriguezMirasol et al.1c,4 showed that N2 and CO2 were obtained in a 2:1 stoichiometry as follows:

INTRODUCTION Environmental problems are a worldwide concern, and control of CO2 is among the highest priorities. Elimination of nitric oxide (NO) from gas streams of several industrial foundations has also become progressively more significant recently.1 The carbon particles in diesel car exhaust are an important problem in many cities in Europe. The catalyzed reduction of nitric oxide using carbon materials as reductants has been largely studied.1a,c−q,s It is known that the process is dependent on the catalyst composition and the carbon material.1j McKee used thermogravimetric analysis (TGA) and microscopy to study the gasification of carbon with O2, CO2, H2O vapor, and H2 using different metals (alkaline, alkaline earth, and transition) as catalysts. He proposed that the reaction includes a cycle of oxidation and reduction and concluded that the metal’s oxidation state rules its behavior and the capacity of carbon to reduce the metal precursor.2 It is reported that the capacity of the catalyst to form a liquid film over the surface of carbon promotes the contact between catalyst and carbon, facilitating reactivity.1b,n,o,q−t,2,3 Suuberg et al. stated that, for lower temperatures, complexes of carbon and oxygen are formed through the dissociative chemisorption of NO and that their decomposition at elevated temperatures can lead to the formation of CO2 and CO.1m Illán-Gomez et al. claimed that catalysts consisting of alkaline, alkaline earth, and transition metals can improve the chemisorption of NO and play a significant part in an oxidation− reduction cycle, transferring the oxygen from the surface of the catalyst to the surface of carbon and yielding CO2 and CO.1e−j This process is similar to what was proposed for different reactions of gasification of carbon.2 These authors observed three stages of behavior according to the temperature range:1e−j (i) only N2 and/or N2O were found at lower temperatures, and oxygen was kept on the surface of the carbon or the catalyst; (ii) for temperatures higher than 300 °C, N2 was still present, CO2 started to appear, and NO reduction rate enlarged; and © XXXX American Chemical Society

N2O(g) + C(s) → N2(g) + C(O) N2O(g) + C(O) → N2(g) + CO2(g)

These complexes decompose to liberate CO2 and produce vacant sites.1c,4,6c,8 The role of the catalysts is to enlarge the reaction rate and diminish the temperature of decomposition and chemisorb N2O dissociatively.1c,4,6c,8 Carbon dioxide is mainly responsible for global heating, causing climate alternations. CO2 can be used in carbon gasification producing CO and chemical feed stocks, avoiding its release to the atmosphere.1r,9 When CO2 reacts with carbon, carbon monoxide is produced, according to the so-called reverse Boudouard reaction (RB): CO 2(g) + C(s) → 2CO(g) Received: May 2, 2016 Revised: July 25, 2016

A

DOI: 10.1021/acs.energyfuels.6b01051 Energy Fuels XXXX, XXX, XXX−XXX

Review

Energy & Fuels Several metals (like those of group VIII, alkali, and alkaline earth) can be efficient catalysts in reducing the reaction temperature of the RB reaction.9a,c−e,g Several mechanisms have been proposed to explain the catalytic activity of carbonates of alkaline-earth metals in the RB reaction, including cycles of carbonate and oxide where the carbonate is reacted with carbon and originates CO and a metal oxide, the latter reacting with CO2 to restore the carbonate.2b,9a,f,10 However, the 12 mechanisms proposed in the past to explain catalytic carbon gasification10 fail to explain several observed kinetic features: (a) In isothermal carbon gasification studies, linearity is usually observed in the conversion/time register (Figure 1).

Figure 2. Catalyst nanoparticles. (A) Porous carbon cut showing catalyst nanoparticles (moving under reaction conditions). (B) Analogy with a woodworm and detail of a single carbon-worm particle. It shows the diffusion of carbon atoms through the particle in one direction and the particle itself moving in the opposite direction, keeping tight contact with carbon (into a step, corresponding to a crystallographic direction). Adapted from ref 11.

(the carbon bulk diffusion proceeds more rapidly as there is a shorter distance to travel). When surface reaction is slower and rate controlling, the larger particles move faster (there is a larger area for the gas/catalytic surface reaction).11a,b (c) When the temperature is raised, the catalytic effect frequently begins at the Tamman temperature. This is consistent with the need to have a larger active catalyst/ carbon contact by shape adjustment, similar to a sintering process, to allow easy carbon dissolution. All of these features are explained by the worm mechanism but not by the other 12 mechanisms previously proposed,10 as listed on Table 1. This will be commented on below.

Figure 1. (a) Linearity up to 95% in carbon gasification by CO2 catalyzed by several metals and alloys at 600 °C. A CASA front is assumed. Important synergistic effects can be observed, joining V with Fe, Mg, or Cu (adapted from ref 1n). (b) Linearity up to 90% in catalytic carbon gasification by NO at 800 °C using various metals and alloys, showing the important synergistic effects observed using Cu−V and Fe−V combinations (adapted from ref 1n). W0 is the initial carbon weight and W is the current carbon weight at reaction time t.

Table 1. Success of the Various Mechanisms in Explaining the Behavior of Observed Kinetics and Geometry Effects

The “worm mechanism” explains linearity: a large amount of carbon-worms operate with a continuous advance (the overall “contact active surface area (CASA)”) (Figure 2).11a,b (b) The particles of the catalyst have movements that depend on their size, as seen under transmission electron microscopy (TEM) and scanning electron microscopy (SEM) in situ studies. However, in some systems small particles move faster, and in other systems larger particles move faster. This has a consistent explanation: when the step of carbon bulk diffusion becomes slower and rate determining, the smaller particles move faster

observed behavior

C bulk diffusion

12 other mechanisms

moving catalyst particles rates of movement vs size starting temperature vs Tamman temperature catalyst contact: zigzag vs armchair front kinetic linearity (channeling) order of reaction vs rate-determining step

yes yes yes yes yes yes

no no no yes no no

This paper presents a review of the work on the catalytic carbon gasification by NO, N2O, and CO2 using binary catalysts on commercial activated carbon and discussed assuming the carbon bulk diffusion/worm mechanism.11b B


Review

Energy & Fuels

■

EXPERIMENTAL SECTION

Samples of charcoal activated GR MERCK were loaded with Ba, Co, Cu, Fe, Mg, Mn, Ni, Pb, and V precursor (acetate) solutions using the incipient wetness method (4 wt %). Binary mixtures (4 + 4%) of all the metal pair combinations were prepared. Kinetic experiments were done isothermally on a CI Electronics MK II thermobalance system that allowed the continuous recording of weight changes. Conversion of the gases was carried out in a fixed bed reactor connected to a GC−MS (Fisons MD800) apparatus to analyze the outlet gases. The X-ray diffraction (XRD) experiments were done in situ using a Rigaku D/max III C diffractometer equipped with a high-temperature special chamber. The reactions conditions used for XRD were the same as those of the thermogravimetric studies. More details on the experimental procedures can be found in previous publications.1n−t The crystal phases found after a pretreatment carried out in inert conditions at 500 °C were the following: BaCO3/BaO/BaO2/Ba, Co2O4/CoO/Co, CuO/Cu, Fe2O3/Fe3O4, MgO/MgO2, Mn3O4/ MnO2, Pb3O4/PbO, NiO/Ni, and V2O5/V6O13 for samples loaded with Ba, Co, Cu, Fe, Mg, Mn, Pb, Ni, and V, respectively.1n

■

RESULTS AND DISCUSSION Kinetic experiments were done isothermally to obtain the initial reaction rates for NO, N2O, and CO2 carbon gasification using binary catalyst mixtures impregnated by incipient wetness. All mixtures were shown to have at least an additive effect. Synergistic effects were observed with some binary catalyst systems. Synergistic effects in NO conversion were found to be promoted by the addition of Ba1o and V1s to various transition metals, as shown in Table 2. Ba + Pb and V + Fe mixtures exhibit the highest effects, and rates as a function of temperature are shown in the Arrhenius plots (Figures 3a and b). The values found for the apparent activation energy are in the

Figure 3. Arrhenius plots for carbon gasification in NO for (a) the carbon parent sample and samples doped with Ba, Pb, and Ba + Pb mixtures (adapted from ref 1o) and for (b) the carbon parent sample and samples doped with V, Fe, and V + Fe mixtures (adapted from ref 1s) with their respective activation energy (Ea) values.

Table 2. Selected Initial Rates of Gasification in NO of the Parent Carbon Sample and Materials Impregnated with Individual Catalysts and Selected Ba and V Mixtures at 450 and 800 °C and Synergistic Effectsa 450 °C catalytic system

initial rate

uncatalyzed Ba Co Cu Fe Mg Mn Ni Pb V V + Co V + Cu V + Fe V + Ba Ba + Mn Ba + Cu Ba + Pb Ba + Fe

0.0 0.1 1.5 1.4 0.1 0.9 0.7 0.2 0.1 0.1 1.5 5.0 3.0 0.2 2.4 3.5 2.0 1.0

Table 3. Selected Initial Rates of Gasification in N2O of the Parent Carbon Sample and Materials Impregnated with Individual Catalysts and Selected Ba and Mn Mixtures at 400 and 600 °C and Synergistic Effectsa

800 °C

synergistic effect

low 3.4 20.2 3.0 2.4 12.6 7.8

initial rate 5.2 8.1 33.3 13.4 15.8 4.2 8.9 53.4 2.7 5.2 55.0 52.0 83.6 72.1 52.2 51.1 158.0 137.2

400 °C

synergistic effect

1.9 2.8 4.0 5.4 3.0 2.4 14.6 5.7

Gasification: × 105 s−1, 0.5% in Ar. Values at 450 °C taken from ref 1p and values at 800 °C taken from refs 1n and 1p. Synergistic effects: alloy rate divided by the expected rate (addition of the two individual rates). a

catalytic system

initial rate

uncatalyzed Ba Co Cu Fe Mg Mn Ni Pb V Ba + Cu Ba + Fe Ba + Pb Ba + Mn Mn + Cu Mn + Ni Mn + Co Mn + Pb Mn + Fe

0.0 1.0 2.7 1.4 0.2 0.2 1.5 1.2 0.2 0.2 3.1 7.3 7.3 17.4 5.3 4.0 4.3 1.7 1.6

600 °C

synergistic effect

initial rate

synergistic effect

1.3 6.2 6.4 7.0 1.8 1.5 low low low

1.4 43.9 34.1 17.6 6.2 9.0 11.9 20.4 11.4 7.6 124.1 149.1 149.1 233.9 115.7 77.9 141.4 126.1 117.4

1.9 2.4 2.4 3.7 4.0 2.1 2.4 5.6 5.4

Gasification: × 105 s−1, 0.5% in Ar. Synergistic effects: alloy rate divided by the expected rate (addition of the two individual rates). a

C


Review

Energy & Fuels

Figure 4. Arrhenius plots for carbon gasification in N2O for (a) the carbon parent sample and samples doped with Ba, Fe, and Ba + Fe mixtures (adapted from ref 1o) and for (b) the carbon parent sample and samples doped with Mn, Ba, and Mn + Ba mixtures (adapted from ref 1t) with their respective activation energy (Ea) values.

Table 4. Selected Initial Rates of Gasification in CO2 of the Parent Carbon Sample and Materials Impregnated with Individual Catalysts and Selected Ba and V Mixtures at 600 and 900 °C and Synergistic Effectsa 600 °C catalytic system

initial rate

uncatalyzed Ba Co Cu Fe Mg Mn Ni Pb V V + Cu V + Fe V + Mg V + Pb V + Ba Ba + Cu Ba + Fe

0.0 1.6 2.7 0.1 1.1 1.9 1.9 2.9 1.5 0.1 2.1 7.5 10.8 4.1 6.1 4.8 11.5

900 °C

synergistic effect

initial rate

13.2 6.4 5.4 2.6 3.5 2.9 4.3

38.0 156.3 207.2 38.7 83.3 140.8 109.2 208.0 57.5 117.1 299.8 296.7 513.5 543.6 306.85 212.84 385.80

synergistic effect

Figure 5. Arrhenius plots for carbon gasification in CO2 for (a) the carbon parent sample and samples doped with V, Cu, and Cu + V mixtures (adapted from ref 1r), (b) the carbon parent sample and samples doped with V, Fe, and Fe + V mixtures (adapted from ref 1r), and (c) the carbon parent sample and samples doped with Ba, Fe, and Fe + Ba mixtures with their respective activation energy (Ea) values.

reaction (139 kJ mol−1) shows a significant reduction when Ba (74 kJ mol−1) and V (43 kJ mol−1) are used as catalysts, as shown in Figures 3a and b.1m,s When Fe is used (alone), the apparent activation energy is not much different from that of the process in the absence of catalyst (131 kJ mol−1), as it can be seen in Figure 3b. Above 700 °C, the activation energy is lower, indicating a change in the rate-determining step (55 kJ mol−1). However, when V is added to Fe, a significant decrease is observed (61 kJ mol−1).1s The conversion of N2O was promoted by Ba1o and Mn1t mixtures as shown in Table 3. The apparent activation energy values found are between 55 and 127 kJ mol−1, as shown in the Arrhenius plots in Figures 4a and b. Also, a large reduction in the activation energy was seen when a catalyst was present, similar to that in the case of the NO reaction. External limitations

1.9 1.5 2.0 3.1 low low 1.6

Gasification: × 105 s−1. Synergistic effect: alloy rate divided by the expected rate (addition of the two individual rates). a

interval of 43−139 kJ mol−1 and lie within the range reported in the literature.1c,m The activation energy of the noncatalyzed D


Review

Energy & Fuels

124 kJ mol−1, respectively). Similar results were found for the mixtures, and 101 kJ mol−1 was the smallest value obtained for the mixtures Fe + V1r and Ba + Fe. Temperature-programmed experiments showed that the products detected with NO were N2, N2O, CO2, and CO. Using N2O, the gas products were CO2 and N2, and using CO2, only CO was detected. Figure 6 shows the reaction products of NO with uncatalyzed carbon (a) as well as with materials loaded with V (b), Fe (c), Ba (d), and mixtures of Ba + Fe (e) and V + Fe (f). Above 300 °C, CO2 was detected. When a catalyst was present (Figures 6b−d), CO2 started to be detected below 300 °C, compared with the uncatalyzed reaction (Figure 6a). Figures 6e and f show that, for the binary catalysts, the amount of CO2 was larger above 300 °C, while N2 was found at lower temperatures as oxygen coming from the dissociative chemisorption of NO is held by the catalyst. A larger rate of CO2 evolution is related to a greater NO conversion. It was found that for N2O conversion, the sole products detected were N2 and CO2 in all studied cases, in agreement with the results found in the literature.1c,n,o,q,t,4,6c CO2

to mass transfer were found above 600 °C, as also shown in the Arrhenius plots: the activation energy was nearly null. When a catalyst was present, limitations were found for larger rates in comparison with the reaction with no catalyst. CO2 conversion increased in the presence of V1r and Ba, as shown in Table 4. Figures 5a−c also show, again, an important diminution of the activation energy for catalytic reactions (101−200 kJ mol−1) compared to the that for the uncatalyzed case (290 kJ mol−1). Similar results were obtained with all other systems. This differs from some studies reported in the literature, which state that reactions with and without catalyst showed comparable activation energies.9e,12 The result found for the reaction with no catalyst (290 kJ mol−1) is similar to what was expected from the literature.9e,12,13 Silva and Lobo remarked in 1986 that 130 kJ mol−1was most likely the lowest value of activation energy ever published for CO2-activated carbon reaction catalyzed by Mo, and the activation energy of the uncatalyzed process was reported to be 272 kJ mol−1 up to 850 °C.13a The results found for Ba and Fe catalysts were comparable to those of the reaction catalyzed by Mo (117 and

Figure 6. Reaction products in temperature programmed reaction in a NO atmosphere for the carbon parent sample (a) and samples doped with V (b), Fe (c), Ba (d), Ba + Fe (e), and V + Fe (f) mixtures (profiles a−c and f were adapted from ref 1s and profiles d and e were adapted from ref 1o). E


Review

Energy & Fuels

Figure 7. Reaction products in temperature programmed reaction in a N2O atmosphere for the carbon parent sample (a) and samples doped with Ba (b), Mn (c), Fe (d), Mn + Ba (e), and Ba + Fe mixtures (profiles a−c and e werew adapted from ref 1t).

formation occurs only above 200 °C, as shown in Figure 7, while N2 is present over all of the temperature ranges. Production of CO2 and N2 become steady at ∼380 °C. For higher temperatures, the product ratio is nearly 2:1 in accordance with the stoichiometry:1c,6c

CO was the sole gasification product detected along with some CO2 that did not react (Boudouard reaction) in all cases.1r Active solid phases must play a key part in the reaction of NO, N2O, and CO2 with carbon.1e−j,n,o,q−t Figures 8−11 show examples of XRD patterns obtained at various temperatures using several metals and their binary mixtures as catalysts. The peak shifts with increasing temperature must reflect an expansion of the crystal lattices possibly due to increased dissolution of H, C, O, and N. The solubility of C, O, and N in metals has been known since the 1920s. The solubility of H in metals has been known since 1864. Interstitial solubility of those atoms is facilitated due to the ratio of the covalent radius solute/solvent being lower than 0.60.15 Some peaks of platinum are observed in the XRD spectra, which come from the sample holder that is exposed to the X-ray beam as the carbon is consumed during the experiment or if a piece of the catalyst drops down when the sample is prepared or the pretreatment is carried out. This is possible because the sample holder is placed in the vertical position once inside the special chamber for the high-temperature experiments.

2N2O(g) + C = 2N2(g) + CO2(g)

For temperatures higher than 750 °C, a certain amount of CO is detected, as stated by Rodriguez-Mirasol and co-workers.1c Carbon monoxide is produced from the reaction of carbon dioxide with activated carbon with no catalyst. Higher temperatures are favorable for the BR reaction.1n,2b The distribution of products is not much affected when the catalyst is changed. The temperatures for N2O conversion are inferior to those of the single catalysts (Figures 7b−d) and even lower temperatures were found for the binary mixtures (Figures 7e and f), in comparison with those for the reaction without a catalyst (Figure 7a). The obtained data show that the catalytic route leads to the same distribution of the gaseous products. Different authors reported comparable results using other carbon supported catalysts.1c,4,8,14 Using CO2, F


Review

Energy & Fuels

to the sum of the rates obtained with each individual catalyst. The synergistic effect is thus quantified by dividing the rate of the combined (alloy) effect by the expected rate (corresponding to the addition of the two separate rates). In the present paper, we observed two particular metals exhibiting important synergistic effects: barium and vanadium. There was no evidence by in situ XRD of new phases being formed by mixing metals. The same active phases were observed using a metal alone or combined. However, the addition of another metal frequently increases the rate. Could this be due to a metal interaction during the preparation phase by incipient wetness, leading to smaller catalyst particles? Or is there a change of a solid-state phase at one of the sides of the nanoparticles? Is there a liquid film catalyst in some cases? This will be discussed below. Let us consider vanadium. The melting points and Tamman temperatures (TTa) of the metal and its main oxides are shown in Table 5. The TTa of V2O5 is very low: 208 °C. V2O5 is reported to be a good carbon gasification catalyst with a low Tamman temperature that facilitates the catalyst-carbon contact. Several researchers suggested that V2O5 is able to form a liquid film on carbon surfaces,2,3b,16 but this only occurs above the melting point of this oxide (690 °C). Sintering-like contact should not be taken as a liquid phase behavior. V2O5 being reduced to V6O13 causes a reduction of the catalyst’s melting point that might facilitate the contact even more. However, the active contact between catalyst particles and carbon is just one side of the problem. The other side is the surface catalysis activity. There is no structural gas−liquid surface catalysis. Effective gas−solid catalysis is required. An external Fe2O3 solid film may explain the very high synergistic effect with the mixture V−Fe. XRD experiments obtained in N2, NO, N2O, and CO2 (Figures 8a−d, respectively) demonstrated the presence of V2O5 and V6O13 in reaction conditions. As temperature increases, the XRD peaks start to be less strong for those vanadium oxides. Phase changes of catalysts are also observed in Figure 9b for the V + Fe mixture. It was suggested that the particles of the oxides have a fluid-like performance.16b Some authors stated that the reaction between V2O5 and carbon, which caused the reduction of the former to V6O11, originated processes of “pitting and channeling” on graphite. However, this must be seen as an initial change sustained during the steady-state reaction. So the CASA front or worm mechanism is a more consistent explanation. The V−Fe alloy shows formation of a σ-phase structure at 50% composition between 400 and 1200 °C.17 It could be proposed that the occurrence of a σ-phase (a phase without definite stoichiometric composition) could explain the synergistic effect. However, the orders of reaction were found to be in the range of 0.6−0.9, which excludes a bulk diffusion control regime. Interplay of separate V and Fe nanoparticles could be advanced. This possibility will be explored in future research work. The more plausible way to explain the synergistic effect consists in the acceleration of the external surface reaction of NO or CO2 with surface emerging carbon atoms dissolved in the catalyst. A flux of carbon atoms must be constantly diffusing due to the chemical activity gradient operating, following Fick’s law. The catalytic effect observed for the NO conversion for both V and Ba mixtures has been previously explained1o,s by the existence of oxidation−reduction reactions that caused reduction of the oxides by contact with carbon in some points, originating oxides of lower oxidation state according to the following

Figure 8. In situ XRD data obtained in N2 (a), NO (b), N2O (c), and CO2 (d) upon heating samples doped with vanadium at several temperatures (adapted from ref 1q).

Synergistic Effects. A synergistic effect is present when the reaction rate obtained using a mixture of two metals is superior G


Review

Energy & Fuels

Table 5. Melting Points and Tamman Temperatures of K, Ca, and Eight Transition Metals in Addition to a Selection of Oxides/ Carbonates metal K

a

mp

TTa

64

oxide, low T

mp

TTa

K2O K2CO3 CaCO3 V2O5 CrO CrO2 MnO2 BaO

740 891 1339 690 300 375 535 1923

233 309 533 208

Ca V Cr

839 1890 1857

280 808 792

Mn Ba Fe

1245 727 1538

486 227 632

Co

1495

611

Co3O4

895

311

Ni Cu

1455 1083

591 405

Ni2O3 Cu2O3

600 (400)*a

163

Zn

420

73

131 825

oxide, high T

mp

TTa

CaO V2O3 Cr2O3

2613 1940 2435

1170 833 1081

Mn3O4 BaCO3 FeO Fe2O3 CoO Co2O3 NiO Cu2O CuO ZnO

1567 811 1377 1540 1933 1900 1955 1232 1326 1975

647 269 552 633 830 813 841 479 526 851

* - Unstable, decomposes.

Figure 9. In situ XRD data obtained in NO upon heating samples doped with Ba + Fe (a) and V + Fe (b) mixtures at several temperatures (profile a was adapted from ref 1o, and profile b was adapted from ref 1s).

Figure 10. In situ XRD data obtained in N2O upon heating samples doped with Ba + Fe (a) and Mn + Ba(b) mixtures at several temperatures (profile a was adapted from ref 1o, and profile b was adapted from ref 1t).

oxidation−reduction mechanism: geometry in the performance of the particles of the catalyst during the reaction, as seen under in situ electron microscopy. Let us now consider the effect of Ba. In the case of Ba mixtures, BaO can react with CO2 to form a carbonate.1n,p,9a Figure 9a shows that the formation of carbonate does occur. This will be discussed further ahead. In Table 5, a list of melting points and Tamman temperatures of several metals, active as catalysts in gasification, can be found. The metal oxide reduction can facilitate the metal catalytic activity in several systems when the active contact catalyst carbon or the external surface activity

Mx + y NO → MxOy + y/2N2 MxOy + y/2C → Mx + y/2CO2

This represents a cycle. Most of the mechanisms proposed in the past for carbon gasification include such cycles.2b,10 However, we recognize that cycles in a steady-state process under stable operational conditions contradict thermodynamics and solid-state molecules do not exist, just atoms and ions do. In addition, those mechanisms do not explain the role of H


Review

Energy & Fuels

With catalyst nanoparticles, in most stages, the solid-state initial transformations should be fast, and a steady-state carbon gasification rate is quickly established, as shown in Figure 1 (kinetic linearity). Baker et al. published observations of graphite gasification reactions using in situ TEM and SEM and also combining metals: Pt−Rh, Pt−Ir (using O2), and Pt−Ag (using H2).18 They discussed the occurrence of increased activity by a change in the way particles sinter above the Tamman temperature: by the occurrence of changes of shape of the particles, by a change from nonwetting to wetting conditions, or by change of the nature of the particles surface (preferential migration of one of the metals).19 We are dealing with solid-state chemistry at the nano level. The operation of a catalytic effect with two different phases at the two “sides” of the nanoparticle has been discussed in catalyzed gasification and formation of carbon, assuming a steady-state kinetic and thermodynamic equilibrium adjusting the thickness each phase, as shown in Figure 12 and discussed below.1p,11b

Figure 11. In situ XRD data obtained in CO2 upon heating samples doped with Ba (a) and the Ba + Fe (b) mixture at several temperatures (profile a adapted from ref 1r).

is much better. Illán-Gomez and co-workers stated that the easiness of the redox reaction (leading to lower or metallic oxidation states) is related with either the oxide lattice energy or the oxide free energy of formation.1i In that report, the catalyst inactivity was attributed to its oxidation. Insignificant gas reduction took place.1i,j Our paper showed that treatment at 500 °C carried out in N2 allowed the reduction of the catalysts by carbon. Figures 10a and b depict the reduction of oxides by carbon in N2O (BaCO3/BaO/BaO2/Ba, Fe2O3/Fe3O4, and Mn3O4/ MnO2). Zhu et al. also claimed that a catalyst redox cycle could explain the catalyzed N2O conversion reaction.6c However, those cycles are not possible in a continuous stable solid-state reaction, as mentioned above and further discussed below. It was shown earlier that, for the Cu + Mn mixture (not shown), the increase in temperature does not make the CuO peaks smaller (for the XRD patterns of the single catalyst), which is different from what was found for the Mn + Cu mixture.1t As for the results obtained with CO2 (Figure 11), because lower oxidation oxides were also detected (BaCO3/BaO/BaO2/Ba and Fe2O3/Fe3O4, for example), it is plausible that the oxides catalyze the C−CO2 bond not by oxidation/reduction cycles but by adjusting the solid phases prevailing at each side of the catalyst particle. To understand the process, we must identify the phases stable at each reaction stage: (a) at reaction temperature, but before gas admission, (b) under steady-state reaction, and (c) prevailing after full conversion of carbon. This can be established by on site and in operando XRD. There may be problems when the first stage reactions are slow and interact for a long time with the second stage. This occurs more likely with Fe. The temperature time transformation (TTT) diagram of Fe shows a fast transformation temperature at 525 °C, but a very sluggish transformation at higher and lower temperatures.

Figure 12. (A) Catalyst nanoparticles with two different solid phases prevailing at each side during reaction with H2O or CO2: hydroxide or oxide (gas side) and carbonate or metal (carbon side), depending on experimental conditions (gas composition, pressure, and temperature). For example, BaCO3/BaO in CO2 gasification. (B) In some cases, just a surface layer may be formed to help carbon solubilization (internal layer) or be the gasification reaction catalyst (external layer).

Bulk Diffusion/Worm Mechanism and the Operative Solid-State Phases. There is much kinetic evidence that the carbon bulk diffusion mechanism (carbon nanoworms) takes place, as referred to above and discussed in some detail elsewhere.11 Figures 12 and 13 summarize the geometry and kinetics. The phases in equilibrium with solid carbon and the gas phase may be different. Confirmation of this fact can be found in recent XRD data of carbon gasification by CO2.20 Figure 12 shows two alternatives for a two-phase/two side active catalyst. The kinetic model based on the contact active surface area (CASA) front was previously discussed in detail.11 The carbon activity differences (Δa1 and Δa2) and equilibrium thickness (l1 and l2) of the two phases will adjust according to the respective carbon diffusivities (D1 and D2) so that an equal carbon flux is D Δa / l maintained:11 D1 = Δa 1 / l1 . The scale of the initial solid-state 2 2 2 reactions and sustained carbon bulk diffusion is very small. Use of the Scherrer formula for in situ XRD spectra indicated average sizes in the range 7.5−23 nm.1n The kinetics can better be understood drawing an Arrhenius plot as schematized in Figure 13. The bending between the straight-line portions of the plot signifies either a modification in the rate-determining step (upward pointing “knee”) or else a change in the faster parallel route (downward pointing “knee”). I


Review

Energy & Fuels

easier C bulk diffusion, or to a reduction in the size of the particles. Further study on the solid-state metal interaction behavior will be an important tool to increase the gasification efficiency. A comprehensive kinetic study is an essential basis to understand the mechanism of chemical reactions. In reactions involving solid phases, kinetics must be interpreted by taking geometry into account. With catalyst nanoparticles involved and acting with a dual function, the role of geometry is essential.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS SACC acknowledges Fundaçaõ para a Ciência e a Tecnologia (FCT) for Investigador FCT program (Project IF/01381/ 2013/CP1160/CT0007) with financing from the European Social Fund and the Human Potential Operational Program. This work was cofinanced by project POCI-01-0145-FEDER006984 from the Associate Laboratory LSRE-LCM funded by FEDER funds through COMPETE2020 (Programa Operacional Competitividade e Internacionalizaçaõ (POCI)) and by national funds through FCT. The assistance of Vitor Teodoro for some illustrations is acknowledged.

Figure 13. Arrhenius plot of the catalytic gasification rate showing the kinetic features of the alternative rate-determining steps: (I) carbon bulk diffusion, (II) surface reaction, and (III) external gas film diffusion. A simplified view of a carbon-worm catalyst particle shows the three main steps (single solid phase particle). The dashed line represents the rate with a higher pressure (cf. reaction order above).

■

REFERENCES

(1) (a) Okuhara, T.; Tanaka, K. Adsorption and Reduction of Nitrogen Monoxide by Potassium-doped Carbon. J. Chem. Soc., Faraday Trans. 1 1986, 82, 3657−3666. (b) Baker, R. T. K.; Lund, C. R. F.; Chludzinski, J. J. Catalytic gasification of graphite by barium in steam, carbon dioxide, oxygen, and hydrogen. J. Catal. 1984, 87 (1), 255−264. (c) Rodriguez-Mirasol, J.; Ooms, A. C.; Pels, J. R.; Kapteijn, F.; Moulijn, J. A. NO and N2O decomposition over coal char at fluidized-bed combustion conditions. Combust. Flame 1994, 99 (3−4), 499−507. (d) Illan-Gomez, M. J.; Linares-Solano, A.; Salinas-Martinez de Lecea, C.; Calo, J. M. Nitrogen oxide (NO) reduction by activated carbons. 1. The role of carbon porosity and surface area. Energy Fuels 1993, 7 (1), 146−154. (e) Illan-Gomez, M. J.; Linares-Solano, A.; Radovic, L. R.; Salinas-Martinez de Lecea, C. NO reduction by activated carbons 2. Catalytic effect of potassium. Energy Fuels 1995, 9 (1), 97−103. (f) Illan-Gomez, M. J.; Linares-Solano, A.; Radovic, L. R.; Salinas-Martinez de Lecea, C. NO reduction by activated carbons 0.3. Influence of catalyst loading on the catalytic effect of potassium. Energy Fuels 1995, 9 (1), 104−111. (g) Illan-Gomez, M. J.; Linares-Solano, A.; Radovic, L. R.; Salinas-Martinez de Lecea, C. NO reduction by activated carbons. 4. Catalysis by calcium. Energy Fuels 1995, 9 (1), 112−118. (h) Illan-Gomez, M. J.; Linares-Solano, A.; Radovic, L. R.; Salinas-Martinez de Lecea, C. NO reduction by activated carbons. 5. Catalytic effect of iron. Energy Fuels 1995, 9 (3), 540−548. (i) IllanGomez, M. J.; Linares-Solano, A.; Salinas-Martinez de Lecea, C. NO Reduction by Activated Carbon. 6. Catalysis by Transition Metals. Energy Fuels 1995, 9 (6), 976−983. (j) Illán-Gómez, M. J.; LinaresSolano, A.; Radovic, L. R.; Salinas-Martínez de Lecea, C. NO Reduction by Activated Carbons. 7. Some Mechanistic Aspects of Uncatalyzed and Catalyzed Reaction. Energy Fuels 1996, 10 (1), 158− 168. (k) Marquez-Alvarez, C.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A. Removal of no over carbon-supported copper catalysts. I. Reactivity of no with graphite and activated carbon. Carbon 1996, 34 (3), 339−346. (l) Mehandjiev, D.; Khristova, M.; Bekyarova, E. Conversion of no on co-impregnated active carbon. Carbon 1996, 34 (6), 757−762. (m) Aarna, I.; Suuberg, E. M. A review of the kinetics of the nitric oxide carbon reaction. Fuel 1997, 76 (6), 475−491. (n) Carabineiro, S.

An external film diffusion control corresponds to an almost flat profile, as seen in Figure 13 (N2O gasification above 600 °C). The expected orders of reaction are also shown in Figure 13. The observed orders of reaction in this study were 1 for CO2 gasification and between 0 and 1 for the two nitrogen oxides. The use of molecules to explain solid-state reactions is not correct. There are no solid-state molecules, just atoms or ions. In solid-state reactions, chemical equations can be used just to represent the stoichiometry of a complete solid transformation reaction. Solid-state reaction mechanisms are based in Fick’s second law.15,21 Growing Importance of This Area. Fischer−Tropsch synthesis, methanation, and blast furnace use are expanding. The biorefinery concept is contributing to that expansion. Diesel soot combustion is still a problem. Coke-based fuel cells19,22 are a very promising process to recycle CO2, producing energy with high performance (>60%). Graphite etching is a new and interesting use of carbon gasification.19,22d As a consequence, catalytic carbon gasification is an increasingly important process. Remarks on Behavior Observed and Improvement. CO2, NO, and N2O conversions were studied using binary catalyst mixtures supported on activated carbon. Kinetic linearity was generally observed from the very beginning, which indicates that the changes in the initial solid-state were quick, so that the first law of Fick with a constant geometry was operating a few minutes from the beginning. NO and CO2 conversion were found to be better promoted by the addition of Ba and V to other metals, while with N2O, the best catalytic effect is obtained with Mn and Ba mixtures with other metals. The synergistic effects observed for the binary mixtures may be due to a faster surface reaction, an J


Review

Energy & Fuels A. Studies of Conversion of NO, N2O, CO2 and Adsorption/ Desorption of SO2 Using Binary Catalysts Supported on Activated Carbon. Ph.D. Thesis, New University of Lisbon, Lisbon, Portugal, 2000. (o) Carabineiro, S. A.; Fernandes, F. B.; Vital, J. S.; Ramos, A. M.; Silva, I. F. Uncatalyzed and catalyzed NO and N2O reaction using various catalysts and binary barium mixtures supported on activated carbon. Catal. Today 1999, 54 (4), 559−567. (p) Carabineiro, S. A.; Fernandes, F. B.; Vital, J. S.; Ramos, A. M.; Silva, I. F. Modelling of uncatalyzed and barium catalyzed NO reduction by activated carbon. In Stud. Surf. Sci. Catal.; Avelino Corma, F. V. M. S. M., José Luis, G. F., Eds.; Elsevier: 2000; Vol. 130, pp 1421−1426. (q) Carabineiro, S. A.; Brás Fernandes, F.; Ramos, A. M.; Vital, J.; Silva, I. F. Vanadium as a catalyst for NO, N2O and CO2 reaction with activated carbon. Catal. Today 2000, 57 (3−4), 305−312. (r) Carabineiro, S. A.; McKee, D. W.; Silva, I. F. Uncatalyzed and catalyzed CO2 reaction using metal catalysts and binary vanadium mixtures supported on activated carbon. Carbon 2001, 39 (3), 451−463. (s) Carabineiro, S. A.; Fernandes, F. B.; Vital, J. S.; Ramos, A. M.; Fonseca, I. M. NO conversion using binary vanadium mixtures supported on activated carbon. Appl. Catal., B 2003, 44 (3), 227−235. (t) Carabineiro, S. A.; Fernandes, F. B.; Vital, J. S.; Ramos, A. M.; Fonseca, I. M. N2O conversion using manganese binary mixtures supported on activated carbon. Appl. Catal., B 2005, 59 (3−4), 181−186. (2) (a) McKee, D. W. Metal oxides as catalysts for the oxidation of graphite. Carbon 1970, 8 (5), 623−635. (b) McKee, D. W. Chemistry and Physics of Carbon; Marcel Dekker: New York, 1981; Vol. 16. (3) (a) Silva, I. F.; Lobo, L. S. Uncatalyzed and MoO3-catalyzed carbon-oxygen reaction: A kinetic study. J. Catal. 1990, 126 (2), 489− 495. (b) Silva, I. F.; McKee, D. W.; Lobo, L. S. A Kinetic andin SituXRD Study of Carbon Reactions Catalyzed by Nickel, Cobalt, Molybdenum, and Their Mixtures. J. Catal. 1997, 170 (1), 54−61. (c) Silva, I. F.; Palma, C.; Klimkiewicz, M.; Eser, S. Kinetics, in situ Xray diffraction and environmental scanning electron microscopy of activated charcoal gasification catalyzed by vanadium oxide, molybdenum oxide and their eutectic alloy. Carbon 1998, 36 (7−8), 861−868. (4) Kapteijn, F.; Rodriguez-Mirasol, J.; Moulijn, J. A. Heterogeneous catalytic decomposition of nitrous oxide. Appl. Catal., B 1996, 9 (1− 4), 25−64. (5) (a) Li, Y.; Armor, J. N. Simultaneous, catalytic removal of nitric oxide and nitrous oxide. Appl. Catal., B 1993, 3 (1), 55−60. (b) Gonçalves, F.; Figueiredo, J. L. Development of carbon supported metal catalysts for the simultaneous reduction of NO and N2O. Appl. Catal., B 2004, 50 (4), 271−278. (6) (a) Armor, J. N. Environmental catalysis. Appl. Catal., B 1992, 1 (4), 221−256. (b) Dandekar, A.; Vannice, M. A. Decomposition and reduction of N2O over copper catalysts. Appl. Catal., B 1999, 22 (3), 179−200. (c) Zhu, Z. H.; Lu, G. Q. M.; Zhuang, Y. H.; Shen, D. X. A comparative study of N2O conversion to N-2 over Co/AC and Cu/ AC catalysts. Energy Fuels 1999, 13 (4), 763−772. (7) Zhu, Z. H.; Radovic, L. R.; Lu, G. Q. Effects of acid treatments of carbon on N2O and NO reduction by carbon-supported copper catalysts. Carbon 2000, 38 (3), 451−464. (8) Zhu, Z. H.; Lu, G. Q. Catalytic Conversion of N2O to N2 over Potassium Catalyst Supported on Activated Carbon. J. Catal. 1999, 187 (2), 262−274. (9) (a) McKee, D. W. Catalytic effects of alkaline earth carbonates in the carbon-carbon dioxide reaction. Fuel 1980, 59 (5), 308−314. (b) Wood, B. J.; Sancier, K. M. The Mechanism of the Catalytic Gasification of Coal Char: A Critical Review. Catal. Rev.: Sci. Eng. 1984, 26 (2), 233−279. (c) Cazorla-Amorós, D.; Linares-Solano, A.; de Lecea, C. S.-M.; Joly, J. P. Calcium-carbon interaction study: Its importance in the carbon-gas reactions. Carbon 1991, 29 (3), 361− 369. (d) Meijer, R.; Weeda, M.; Kapteijn, F.; Moulijn, J. A. Catalyst loss and retention during alkali-catalyzed carbon gasification in CO2. Carbon 1991, 29 (7), 929−941. (e) Pérez-Florindo, A.; CazorlaAmorós, D.; Linares-Solano, A. CO2-Carbon gasification catalyzed by alkaline-earths: Comparative study of the metal-carbon interaction and of the specific activity. Carbon 1993, 31 (3), 493−500. (f) Ohme, H.;

Suzuki, T. Mechanisms of CO2 Gasification of Carbon Catalyzed with Group VIII Metals. 1. Iron-Catalyzed CO2 Gasification. Energy Fuels 1996, 10 (4), 980−987. (g) Xiaoding, X.; Moulijn, J. A. Mitigation of CO2 by Chemical Conversion: Plausible Chemical Reactions and Promising Products. Energy Fuels 1996, 10 (2), 305−325. (10) Li, K.; Khanna, R.; Zhang, J.; Liu, Z.; Sahajwalla, V.; Yang, T.; Kong, D. The evolution of structural order, microstructure and mineral matter of metallurgical coke in a blast furnace: A review. Fuel 2014, 133, 194−215. (11) (a) Lobo, L. Catalytic Carbon Gasification: Review of Observed Kinetics and Proposed Mechanisms or Models - Highlighting Carbon Bulk Diffusion. Catal. Rev.: Sci. Eng. 2013, 55, 210−54. (b) Lobo, L. S. Intrinsic kinetics in carbon gasification: Understanding linearity, “nanoworms” and alloy catalysts. Appl. Catal., B 2014, 148−149, 136−143. (12) Kapteijn, F.; Porre, H.; Moulijn, J. A. CO2 gasification of activated carbon catalyzed by earth alkaline elements. AIChE J. 1986, 32 (4), 691−695. (13) (a) Silva, I. F.; Lobo, L. S. Study of CO2 gasification of activated carbon catalyzed by molybdenum oxide and potassium carbonate. Fuel 1986, 65 (10), 1400−1403. (b) DeGroot, W. F.; Richards, G. N. Relative rates of carbon gasification in oxygen, steam and carbon dioxide. Carbon 1989, 27 (2), 247−252. (14) Zhu, Z. H.; Lu, G. Q.; Yang, R. T. New Insights into AlkaliCatalyzed Gasification Reactions of Carbon: Comparison of N2O Reduction with Carbon over Na and K Catalysts. J. Catal. 2000, 192 (1), 77−87. (15) Barrer, R. Diffusion in and through Solids; Cambridge University Press: Cambridge, 1941. (16) (a) Chu, X.; Schmidt, L. D.; Chen, S. G.; Yang, R. T. Catalyzed Carbon Gasification Studied by Scanning Tunneling Microscopy and Atomic Force Microscopy. J. Catal. 1993, 140 (2), 543−556. (b) Pan, Z. J.; Yang, R. T. Catalytic behavior of transition metal oxide in graphite gasification by oxygen, water, and carbon dioxide. J. Catal. 1991, 130 (1), 161−172. (17) (a) Smith, J. F. The Fe−V (Iron-Vanadium) system. Bull. Alloy Phase Diagrams 1984, 5 (2), 184−194. (b) Dubiel, S. M.; Cieslak, J. Sigma-phase in Fe-Cr and Fe-V alloy systems and its physical properties. Crit. Rev. Solid State Mater. Sci. 2011, 36 (4), 191−208. (18) (a) Baker, R. T. K.; Sherwood, R. D. Catalytic oxidation of graphite by iridium and rhodium. J. Catal. 1980, 61 (2), 378−389. (b) Baker, R. T. K.; Sherwood, R. D.; Dumesic, J. A. Catalytic hydrogenation of graphite by platinum, iridium, and platinum-iridium. J. Catal. 1980, 66 (1), 56−64. (19) Ci, L.; Xu, Z.; Wang, L.; Gao, W.; Ding, F.; Kelly, K.; Yakobson, B.; Ajayan, P. Controlled nanocutting of graphene. Nano Res. 2008, 1 (2), 116−122. (20) Shen, Y.; Ding, M.; Ge, X.; Chen, M. Catalytic CO2 Gasification of Rice Husk Char for Syngas and Silica-Based Nickel Nanoparticles Production. Ind. Eng. Chem. Res. 2015, 54 (36), 8919−8928. (21) Budnikov, P.; Ginstling, A. Principles of Solid State Chemistry, Reactions in Solids; Gordon and Breach Sc. Publishers: New York, 1968. (22) (a) Tang, Y.; Liu, J. Effect of anode and Boudouard reaction catalysts on the performance of direct carbon solid oxide fuel cells. Int. J. Hydrogen Energy 2010, 35 (20), 11188−11193. (b) Bai, Y.; Liu, Y.; Tang, Y.; Xie, Y.; Liu, J. Direct carbon solid oxide Fuel Cella potential high performance battery. Int. J. Hydrogen Energy 2011, 36 (15), 9189−9194. (c) Gür, T. M. Critical Review of Carbon Conversion in “Carbon Fuel Cells. Chem. Rev. 2013, 113 (8), 6179−6206. (d) Cheng, G.; Calizo, I.; Hight Walker, A. R. Metalcatalyzed etching of graphene governed by metal−carbon interactions: A comparison of Fe and Cu. Carbon 2015, 81, 678−687.

K


Understanding the Reactions of CO2, NO, and N2O with Activated

Recommend Documents