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Carbon Nanotube-Supported Metal Catalysts for NOx Reduction Using Hydrocarbon Reductants: Gas Switching and Adsorption Studies Eduardo Santillan-Jimenez,† Mark Crocker,*,† Agustín Bueno-Lopez,‡ and Concepcion Salinas-Martínez de Lecea‡ † ‡
Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40511, United States Department of Inorganic Chemistry, University of Alicante, Alicante E-03080, Spain
bS Supporting Information ABSTRACT: The selective catalytic reduction of NOx with hydrocarbons (HC-SCR) on functionalized multiwalled carbon nanotube (fMWCNT)-supported metal catalysts was investigated using a transient technique, together with kinetic and adsorption measurements. Results from the transient studies provide an explanation for the characteristic volcano shape of the NOx conversion curves: below Tmax, the temperature of maximum NOx conversion, the catalyst surface is covered by hydrocarbonaceous species, which results in the suppression of NOx reduction activity. Above Tmax, O2 adsorption becomes prevalent, favoring oxidation of both NO and the hydrocarbon. In an effort to understand the origin of the superior NOx reduction activity shown by 3:1 PtRh/ fMWCNTs as compared to Pt/fMWCNTs, Temperature Programmed Desorption (TPD) measurements were undertaken. Results indicate that hydrocarbon and/or hydrocarbon-derived species are more strongly adsorbed on the alloy than on Pt alone, while NO adsorption is weaker on the alloy than on Pt. This is suggested to give rise to a higher concentration of partially oxidized hydrocarbon intermediates on the surface of the PtRh catalyst at the temperature of maximum deNOx activity, leading to higher NOx reduction activity.
1. INTRODUCTION Supported platinum group metals (PGMs) are among the best catalysts found to date for the low temperature selective catalytic reduction of NOx with hydrocarbons (HC-SCR) under lean (oxygen-rich) conditions, based on the high NOx conversions levels they afford, their hydrothermal stability, and their resistance to poisons. However, catalysts of this type also show two important drawbacks, namely, a narrow temperature window of operation and an objectionably high selectivity toward N2O. In an attempt to gain fundamental insights that might help to address these limitations, the HC-SCR reaction mechanism on PGMcatalysts has been extensively investigated. Indeed, a survey of the scientific literature pertaining to this topic can prove confusing, given the great variation within the experimental parameters used in the various studies, the results obtained, and the conclusions drawn. Nevertheless, it is possible to find recurrent motifs that allow the proposed reaction mechanisms to be classified into two distinct categories: those involving NO decomposition and subsequent oxygen removal by the hydrocarbon reductant and those involving the formation of CN bonds. NO decomposition, followed by HC-assisted surface reduction, is undoubtedly the most straightforward mechanism proposed for the HC-SCR reaction. It was first proposed by Burch et al. in 1994,1 who suggested that NO dissociatively adsorbs on reduced Pt sites forming nitrogen and oxygen adatoms. Nads species can then combine and produce N2 or react with weakly adsorbed NO and yield N2O. The hydrocarbon subsequently reacts with Oads (clean off step) to afford combustion products along with reduced Pt sites, thus closing the catalytic cycle. Evidence supporting this mechanism comes from different kinds r 2011 American Chemical Society
of experiments, some of the most compelling data having been obtained through transient Temporal Analysis of Products (TAP) and Steady-State Isotopic Transient Kinetic Analysis (SSITKA).14 Other kinetic data, such as the power law order of reaction with respect to oxygen and the hydrocarbon are also consistent with a mechanism in which Pt is in a reduced state.57 Indeed, C3H6 adsorption occurs even under a large excess of oxygen, thereby keeping Pt completely reduced. The latter has been confirmed by experiments in which CO2 evolution is observed subsequent to the elimination of the hydrocarbon from the system.2 Most authors opposing the NO decomposition mechanism are proponents of mechanisms in which the hydrocarbon reductant reacts with oxygen adatoms (produced via the dissociative adsorption of O2) to yield oxygenated surface intermediates, which can then react selectively with adsorbed or gas phase NOx to produce the HC-SCR reaction products.8 The main evidence for these mechanisms comes from the detection of hydrocarbonderived surface intermediates via infrared spectroscopy, although the reactions of model organic compounds on HC-SCR catalysts have also been invoked. Mechanisms involving the formation of CN bonds can be further classified depending on the nature of the intermediate deemed crucial for the occurrence of the net HC-SCR reaction. Most mechanisms fall into one of two main categories: those involving cyanide (CN) or isocyanate Received: January 10, 2011 Accepted: May 6, 2011 Revised: April 20, 2011 Published: May 06, 2011 7191
dx.doi.org/10.1021/ie200054u | Ind. Eng. Chem. Res. 2011, 50, 7191–7200
Industrial & Engineering Chemistry Research (NCO) surface species as an intermediate914 and those involving organo-nitro and related adspecies.1517 Although the HC-SCR mechanism on PGM-containing catalysts is an actively investigated and highly debated topic, it is recognized by most authors that the experimental conditions as well as the nature and the state of the catalyst may have an important role in determining the type of mechanism that is followed. Moreover, almost all workers agree on the fact that it is possible for several mechanisms to operate simultaneously. Indeed, Nikolopoulos et al. found kinetic evidence which led them to conclude that a NO decomposition mechanism and a mechanism involving a CN-type intermediate may operate in parallel.18 In recent years, a new mechanistic model that may help to resolve this lingering controversy has started to emerge. In effect, this approach reconciles the mechanism involving NO decomposition followed by HC-assisted surface reduction with the mechanisms involving carbon and nitrogen-containing intermediates by claiming that the clean off step can be effectively accomplished by CxHyOzNw species.19 Further, these new types of mechanism have been proposed based on experimental evidence obtained through techniques alternative to infrared spectroscopy, addressing one of the main objections presented against schemes involving nitrogen-containing organic intermediates (Burch has claimed that the very detection of these species via IR spectroscopy proves that they are not real intermediates, since a true intermediate would react milliseconds after its formation2). Most notably, García-Cortes et al. have recently reported a mechanism involving partially oxidized intermediates (whose combustion by gaseous O2 constitutes the clean off step) based on the study of reaction transients.20 A transient is created by switching one gas out of a reactant stream and replacing it with an equal volume and flow of an inert gas, while keeping both temperature and pressure constant. By tracking the concentration of the reactants and products before, during and after the formation of the transient, insight on the nature, concentration, and reactivity of surface species can be gained. Relevant information can also be gathered from the reverse switch, i.e., substituting a reagent for an inert gas. Indeed, the state of the catalyst surface can be probed in considerable detail by submitting the various HC-SCR reactants to this methodology and performing the experiments at temperatures slightly below and above that of maximum NOx conversion (Tmax). However, this strategy has an important limitation, namely, the inability to distinguish between reactive intermediates and unreactive spectator species. Because of the latter, this technique does not directly evince the mechanism of the HCSCR reaction. However, it does provide valuable information regarding the species present on the surface of a working catalyst,3 which in turn can prove useful in the development of a reaction scheme. The preparation, characterization, and evaluation of multiwalled carbon nanotube (MWCNT)-based HC-SCR catalysts have recently been reported.21 These formulations show higher activity in NOx reduction with propene than Pt/Al2O3, while also showing comparable activity and superior resistance to support oxidation than Pt/activated carbon. Further, it has been observed that acid treating the nanotube support and using a 3:1 PtRh alloy instead of Pt as the catalytically active phase results in further improvements in deNOx performance. The objective of the present study was to develop an understanding of the HC-SCR reaction on MWCNT-supported PGMs catalysts consistent with
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these observations. Given that the high absorbance of MWCNTs in the infrared region makes the study of MWCNTs-supported catalysts via IR spectroscopy difficult, the state of the surface of a working Pt/fMWCNT catalyst has been investigated using a transient technique. Further, transient studies were complemented with experiments performed in order to determine relevant kinetic parameters for the HC-SCR reaction on the most promising MWCNT-based catalysts as well as by submitting the latter to C3H6 and NO temperature programmed desorption measurements.
2. EXPERIMENTAL SECTION 2.1. Kinetic Studies. Relevant kinetic parameters were determined using an experimental setup employed to perform catalyst evaluation studies in previous work.21 In order to operate the reactor in a differential manner, NO and propene conversions were restricted to no more than 20%. At the end of each kinetic run, the conditions of the first data point were duplicated to check for possible catalyst deactivation. For the determination of apparent activation energies, the temperature was varied in a nonsequential manner in order to avoid systematic errors. Similarly, when testing the effect of gas composition on the rate, the concentration of the species of interest was varied nonsequentially, while the concentrations of the other species were kept constant. Apparent activation energies were calculated from a series of twelve Arrhenius plots obtained by permutation of the following reactant concentrations: [NO] = 250, 500, 800, and 1200 ppm; [C3H6] = 500, 1000, and 1500 ppm. Reaction orders in NO were calculated from the slope of loglog plots of NO reduction rates vs initial NO concentrations, using the following reactant concentrations: [NO] = 100, 250, 500, 800, and 1200 ppm; [C3H6] = 1000 ppm. Reaction orders in propene were calculated from the slope of loglog plots of NO reduction rates vs initial C3H6 concentrations, using the following reactant concentrations: [C3H6] = 250, 500, 1000, 1500, and 2000 ppm; [NO] = 500 ppm. In these studies, 1 g of the catalyst being tested was packed into a ∼2.5 cm bed between plugs of glass wool. The gas mixture was flowed downward through the catalyst bed and consisted of NO and propene (in the concentrations stated above), 10% O2 and 10% H2O balanced with N2. The total flow rate was kept at 1667 cm3 3 min1 (STP), which yielded a W/F value of 0.01 g 3 h 3 dm3 (where W is the weight of the catalyst and F is the total flow rate), corresponding to a space velocity of ca. 50,000 h1. 2.2. Transient Experiments. A 2 wt % Pt/fMWCNT catalyst was selected to study the surface state of a working HC-SCR catalyst by means of a transient technique. Details of the preparation, characterization, and evaluation of this catalyst21 as well as of the gas switching apparatus used in this study20 can be found elsewhere. Given that the feasibility of using fMWCNTsupported PGMs as HC-SCR catalysts is fully contingent on the ability of the carrier to withstand exposure to oxidizing conditions during operation, experiments were undertaken to validate the use of fMWCNTs as a support for HC-SCR catalysts. The results of these experiments, which have been reported earlier,21 indicate that fMWCNTs offer satisfactory resistance to oxidation up to a temperature of at least 300 °C. Experiments were performed in a horizontal tubular quartz reactor (5 mm internal diameter). The catalyst (34.5 mg) was packed into a ∼1 cm bed between plugs of quartz wool. The gas mixture flowing through the catalyst bed consisted of 2350 ppm NO, 2350 ppm C3H6, and 7192
dx.doi.org/10.1021/ie200054u |Ind. Eng. Chem. Res. 2011, 50, 7191–7200
Industrial & Engineering Chemistry Research
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Table 1. Kinetic Data for 2 wt % Pt/fMWCNTs and 1.5 wt % Pt 0.5 wt % Rh/fMWCNTs reaction order in NO catalyst
a
190 °C
apparent activation energy (kJ/mol)
200 °C
reaction order in C3H6 190 °C
200 °C
2 wt %Pt/fMWCNTs
57 ( 4
0.21
0.21
0.31
0.64
1.5 wt % - 0.5 wt % Rh/fMWCNTs
58 ( 8
0.13
0.28
0.32
0.46a
0.39 at 205 °C.
10% O2 balanced with He. The gas flows were regulated with mass flow controllers (0.1 mL/min sensitivity), and the total flow rate was kept at 300 cm3 3 min1 (STP), corresponding to a W/F value of 0.002 g 3 h 3 dm3. A four-way electro valve allowed for the switching of a reactant gas with an equal flow of an inert gas (1% Ar in He) and vice versa, while a high precision regulation valve and two high sensitivity pressure transducers allowed for the switches to be performed without a concomitant pressure variation in the system. Temperature was kept constant using a controller with a sensitivity of 0.1 °C/min and two thermocouples (one of which was embedded in the upstream quartz wool plug, while the other was kept between the reactor and the furnace). The outlet gases were continuously analyzed using a Pfeiffer OmniStar mass spectrometer with a residence time of 10 ms. In a typical experiment, a gas mixture containing the reactant gases was flowed through the catalyst bed, and the temperature was slowly increased until both the NO and C3H6 conversion levels were deemed satisfactory. To ensure the reactor operated in a differential manner (below Tmax, the temperature of maximum NOx conversion), the maximum NOx conversion level was kept at 15%, the corresponding propene conversion being no more than 25%. For experiments performed above Tmax, the NOx and propene conversions were respectively
