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Volcano curves for homologous series reactions: Oxidation of small alkanes
Applied Catalysis A, General 587 (2019) 117255
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Volcano curves for homologous series reactions: Oxidation of small alkanes a
a
b
Garam Lee , Weiqing Zheng , Ivan C. Lee , Dionisios G. Vlachos a b
a,⁎
T
Department of Chemical and Biomolecular Engineering, Catalysis Center for Energy Innovation, University of Delaware, Newark, DE, 19716-3110, USA U.S.Army Research Laboratory, Sensors and Electron Devices Directorate, 2800 Powder Mill Road, Adelphi, MD, 20783, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Alkanes Homologous series Catalytic combustion Platinum Palladium Ag-Pd Bimetallics
Volcano curves are typically developed and used for predicting new catalysts for a single reaction and one substrate. The concept of using the volcano curve to predict catalysts for an entire homologous series of reactions has been unexplored. Herein the catalytic activity of seven monometallic catalysts (Pt, Pd, Rh, Ag, Ni, Cu, and Co/Al2O3) and three Ag-Pd/Al2O3 bimetallic catalysts is evaluated in the total oxidation of small alkanes (methane, ethane, propane, and isobutane) in the 280–400 °C temperature range under fuel lean and rich conditions. We show that hysteresis in activity, with three distinct kinetic regimes, is a common phenomenon of alkane oxidation over all catalysts studied when varying the oxygen concentration, and the size of the hysteresis loop depends on the oxophilicity of the catalyst and the reducing ability of the hydrocarbon. Expectedly, the concept of the universality of the volcano curve for a homologous series is valid but only when a suitable surrogate substrate is chosen. Hydrocarbons with two or more carbon atoms can serve as surrogates of the alkane homologous series, whereas methane is not. Interestingly and consistent with the hysteresis, the feed composition controls the catalyst oxidation state and potentially impacts the optimal catalyst descriptors used to determine new catalysts. The predicted 1:3 Ag-Pd catalyst is indeed superior to single metals for the homologous series under fuel lean conditions for ethane and larger alkanes. It is inferior to Pt under fuel rich conditions and better than Pd and Pt for methane rich conditions. A method for qualitative inference of the catalyst structure, based on the volcano curve and the oxidation state of the catalyst, is proposed.
1. Introduction
for this idea. For example, complete oxidation of hydrocarbons with 6–20 carbon atoms over 1% Pt/Al2O3 show that there is a correlation between light-off temperature and the number of carbons: heavy alkanes are oxidized easier than light alkanes since the CeH bond activation becomes more facile with increasing carbon number based on the CeH bond dissociation energy [9]. To the best of our knowledge, studies on universality of volcano curves across substrates are lacking. Our goal in this paper is to explore this concept of universality of volcano curves. The specific family of reactions studied herein is oxidation of hydrocarbons because (1) it is important in pollution abatement [10–13], portable power generation [14], and partial oxidation (where complete oxidation is unwanted) [15,16], and (2) the oxidation state of the catalyst may change with fuel and/or feed composition making the problem more interesting. Specifically, we study the oxidation of small hydrocarbons (methane, ethane, propane, and isobutane) over seven single-metal catalysts (Ag, Co, Cu, Ni, Pd, Pt, Rh) and three Ag-Pd bimetallics [8] under varying oxygen mole fraction. The kinetics of methane and propane oxidation over Pt, Pd, and Ag-Pd as a function of oxygen concentration is also investigated to relate the oxidation state of the catalyst to the volcano curves.
The volcano curve, based on Sabatier’s principle [1], relates the activity of a catalyst with a single descriptor, typically the binding energy of a key species. The optimal descriptor value (near the maximum of the volcano curve) is often matched to that on bimetallics in order to develop new materials, such as core-shell structures. Volcano relationships have been observed for several reactions, such as ammonia decomposition [2–4], isopropanol dehydrogenation [5], and desulfurization [6,7]. In our recent work, we found that the rate of propane total oxidation over monometallic catalysts is correlated with two, instead of one, descriptors, namely the oxygen and carbon binding energies [8]. A core-shell Ag-Pd bimetallic catalyst was computationally predicted and experimentally confirmed to be superior to Pt. An open question in discovering new materials is whether the concept of homologous series holds for volcano curves, i.e., whether a volcano developed for a single model compound and the identified optimal catalyst is optimal for an entire homologous series of compounds. The fact that molecules in a homologous series may have the same rate-determining step and a similar mechanism provides support
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Corresponding author. E-mail address: [email protected] (D.G. Vlachos).
https://doi.org/10.1016/j.apcata.2019.117255 Received 28 June 2019; Received in revised form 26 August 2019; Accepted 12 September 2019 Available online 14 September 2019 0926-860X/ © 2019 Elsevier B.V. All rights reserved.
Applied Catalysis A, General 587 (2019) 117255
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2. Experimental section
2.3. Kinetic experiments
2.1. Catalyst preparation and characterization
All kinetic experiments were performed at ambient pressure. Before each test, the catalyst was reduced in situ, under 10% H2/He flow (total flow rate of 300 cm3 min−1) at 500 °C for 1 h. Then, the reactor was cooled down to the reaction temperature under He, and then the hydrocarbon and oxygen were introduced to the reaction chamber. Homologous series experiments were performed at an O2/alkane equivalence ratio of 0.5 (fuel lean, oxygen excess) and 2 (fuel rich, oxygen deficient) in the 280–400 °C temperature range (Table S4). The equivalence ratio, defined as the molar ratio of fuel to oxygen over that at stoichiometric (see supporting information for the relevant equation), is a standard way of defining the feed composition and indicates clearly the deviation of a feed composition from the stoichiometric point. The hydrocarbon flow rate and the total flow rate were held constant during all oxidation experiments at 1.5 cm3 min−1 and 300 cm3 min−1, respectively, and the oxygen flow rate was varied (balanced by inert) for each hydrocarbon to maintain a fixed equivalence ratio. Hysteresis experiments were conducted at 3 cm3 min−1 of methane or propane and a constant weight hourly space velocity (WHSV) of 45000 mL g−1·hr−1 by varying the flow rate of O2 (Table S5). Pt, Pd, and 1:3 Ag-Pd were loaded in six channels of the HTE reactor (two channels for each catalyst for reproducibility of data). The remaining three channels were loaded with the same weight of SiC (blank measurements). After pretreatment, methane and oxygen were introduced to the reactor at an O2/C3H8 ratio of 2. Once steady-state was reached, the ratio was increased stepwise, allowing 30 min for steady state at each composition (time dependent experiments indicated that this time is sufficient to reach steady state). Subsequently, the O2/C3H8 ratio was decreased stepwise in a similar fashion to probe hysteresis. In this study, the TOF is the average from two channels. Like our previous study on propane oxidation [17], the conversion in channels containing the same catalyst shows very good reproducibility.
The single-metal catalysts (Ag, Co, Cu, Ni, Pd, Pt, Rh) were prepared by incipient wetness impregnation of commercial γ-Al2O3 (3 μm, Alfa Aesar) powder with an aqueous precursor solution of the suitable salt (Table S1). Ag-Pd bimetallic catalysts with three different molar ratios (1:10, 1:3, and 3:1) were prepared using incipient wetness co-impregnation. All catalysts were dried at 100 °C for 10 h and then calcined at 290 °C for 2 h with a ramp rate of 5 °C/min in ambient air. In order to avoid mass and heat transfer effects due to the high reaction rate and exothermicity of alkane oxidation reaction, the synthesized catalysts were diluted 20-fold with SiC (2 μm, Alfa Aesar). The diluted catalysts were then pressed into pellets, crushed, and sieved to a final size of 105–150 μm. It was shown in our earlier work that this procedure, along with feed dilution and a microreactor with large thermal mass (see below), eliminate transport effects and ensure isothermal conditions [8]. The turnover frequency (TOF) is reported in units of molecules of alkane reacted per catalyst site per second and was calculated based on the alkane differential conversion (conversion less than 20% and often much lower). The number of active sites was obtained through CO chemisorption (Table S2) using AMI-200ip (Altamira Instruments, USA). 0.5 g of each catalyst was loaded into a quartz reactor, reduced under 10% H2/He at 500 °C for 1 h, and purged in He for 1 h. Then, the catalyst was cooled down in He flow to 30 °C for CO chemisorption. The mean particle size of catalysts was also determined via extensive transmission electron microscope (TEM) and found to be comparable to that of chemisorption (Fig. S1–S8 and Table S3). Given the accuracy of chemisorption experiments and the uncertainty in quantification of surface area from the assumed ratio of the probe molecule (CO) to the exposed surface site, TEM is a good tool to interrogate the chemisorption findings. For example, the particle size estimated using CO chemisorption and TEM over Ag and Cu based catalysts is consistent (Table S3 in the SI). X-ray photoelectron spectroscopy (XPS) analysis was carried out on a monochromatic aluminum K-α X-ray photoelectron spectrometer (ThermoFisher K-alpha+, US). Samples for XPS measurements were prepared in a glove-box and transferred through a vacuum transfer module (ThermoFisher, US) without exposing them to air. Two representative conditions at 340 °C were chosen to probe the surface oxidation states of two catalysts, namely of Ni/Al2O3 and 1:3 Ag-Pd/Al2O3 catalyst: (1) after running the chemistry at O2/CH4 = 4 (fuel-lean conditions) for 1 h and (2) after conditions 1 followed by flashing the catalyst with inert gas and changing the composition to O2/ CH4 = 2 (the stoichiometric point) for 30 min. The Pt 4f peaks overlap with those of the Al (support) and cannot be separated due to the low loading (1%) of Pt; thus no XPS data is reported for Pt.
3. Results and discussion 3.1. Homologous series experiments The catalytic activity of monometallic catalysts and three nominal compositions of Ag-Pd bimetallic catalysts was investigated in the 280–400 °C temperature range at an equivalence ratio of 0.5 (O2/methane ratio 4, O2/ethane ratio 7, O2/propane ratio 10, O2/isobutane ratio 13). The experimental TOF of small hydrocarbons at 340 °C as a function of the binding energy of carbon and oxygen, obtained from DFT calculations conducted in [8], is plotted in Figs. 1 and 2. Pt/Al2O3 shows the highest activity while Co/Al2O3 shows the lowest activity. Consistent with our prior work [8], a volcano-type relation is observed between the TOF and the carbon and oxygen binding energies for all hydrocarbons. The necessity of two descriptors is illustrated by differences in the activity of Cu and Pt. Specifically, these catalysts have similar O binding energy but dissimilar C binding energy. The TOF increases with increasing carbon number, consistent with the increased reactivity of larger hydrocarbons, correlated with the CeH bond dissociation energy. Fig. S9 shows the TOF for the oxidation of all hydrocarbons over single metal catalysts as a function of temperature. Fig. S11 shows the same volcano-type relation at different temperatures, indicating that temperature has no apparent effect on the catalyst-activity ranking over the studied temperature range. Under fuel lean conditions (excess O2), Pt is superior, and Pd is the second-best catalyst for all alkanes. The optimal properties (O and C binding energies) are the same for all alkanes providing support to the homologous series concept. Fig. S12 shows the corresponding contour plots of catalyst activity vs. the carbon and oxygen binding energies. The TOF under fuel rich conditions, at an equivalence ratio of 2 (O2/
2.2. Instrumentation Details of the high-throughput experiment (HTE) system are given in Peela et al. [17]. The reactant (alkane and oxygen) and inert (helium) gas flow were controlled using RS-485 mass flow controllers (MKS Instrument, USA). The exit temperatures of all nine channels were measured with 0.16 cm K-type thermocouple. One of the nine reactor effluents was selected using a 10-port selector (VICI Instruments, USA), and the eight remaining effluents were vented. The tubes after the selector valve were heated at 160 °C to avoid condensation of water formed during reaction, which was then condensed using an ice trap prior to the gas chromatograph (GC). The product effluent was analyzed with a four-channel micro-GC (G3581, Agilent Technologies, USA) with three different capillary columns: molecular sieve 5A, porapak-U, and AlOx (Agilent Technologies, USA).
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Fig. 1. Experimental methane (Δ), ethane (•), propane (□), and isobutane (♦) TOF vs. DFT-calculated binding energy of carbon at 340 °C. Equivalence ratio is held constant at 0.5. Lines connect the points. The DFT values were taken from [8]. The inset is a zoom around the peak TOF.
Fig. 4. Experimental methane (Δ), ethane (•), propane (□), and isobutene (♦) TOF vs. DFT-calculated binding energies of oxygen at 340 °C. Equivalence ratio is held constant at 2. Lines connect the points. The DFT values were taken from [8]. The inset is a zoom around the peak TOF.
methane ratio 1, O2/ethane ratio 1.75, O2/propane ratio 2.5, and O2/ isobutane ratio 3.25), is plotted as a function of carbon and oxygen binding energies in Figs. 3 and 4. Volcanos are again observed. Pt is the most active catalyst for all hydrocarbons but methane, for which, Pd is the most active catalyst. Given the results for fuel lean and rich conditions, our results indicate that feed composition may change the optimal catalyst properties and unexpectedly, methane is not a surrogate of the alkane homologous series. We return to this point below. In our previous work [8], a library of 120 shell-core bimetallic catalysts was generated using 5 host metals as a core and 12 guest metals as a monolayer (ML) in both surface (top) and subsurface (second layer below the surface) configurations. The binding energy of carbon and oxygen on these 120 ML bimetallic catalysts was calculated using DFT calculations. The binding energies of the bimetallic catalysts were compared to those of the peak carbon and oxygen binding energies determined for propane total oxidation. Then, the stability of the promising bimetallic catalysts at high oxygen coverage was assessed. Active and stable catalysts predicted computationally were tested experimentally. The Ag-Pd bimetallic, with carbon and oxygen binding energies (−6.8 and −4.4 eV, respectively) close to the optimal values (−7.2 and −4.4 eV, respectively), showed superior catalytic experimental activity than the most active Pt catalyst under fuel lean conditions [8], consistent with model predictions. The Ag-Pd bimetallic was not tested for other hydrocarbons and other feed compositions. In order to test the universality of the Ag-Pd prediction for the homologous series, the rate of the total oxidation of additional small alkanes was measured over Pt/Al2O3, Pd/Al2O3, Ag/Al2O3, and Ag-Pd/ Al2O3. Fig. 5 shows the TOF at 340 °C (for other temperatures, see Fig. S13) under fuel lean conditions common to catalytic combustion and abetment. The Ag-Pd bimetallic catalyst (1:3 and 3:1 nominal composition) shows superior activity to Pt for the oxidation of ethane, propane, and isobutane but not for methane for which, the rates on bimetallics are slightly lower but comparable to Pt and Pd and superior to Ag. Consistent with the reactivity trends, Table 1 also shows that the activation energy of the total oxidation of ethane, propane, and isobutane over the 1:3 Ag-Pd bimetallic is slightly lower than the activation energy over Pt. Our kinetics results in a forthcoming publication are consistent with the literature. They indicate that a better catalyst discovered for a model compound using the volcano curve should generally be better for the entire homologous series, presumably due to the CeH bond activation being rate-determining in the total oxidation of all alkanes [9]. However, our data suggests that exceptions may exist for some members of the homologous series (in our case methane). We rationalize the reason for the methane anomaly below and propose that
Fig. 2. Experimental methane (Δ), ethane (•), propane (□), and isobutane (♦) TOF vs. DFT-calculated binding energy of oxygen at 340 °C. Equivalence ratio is held constant at 0.5. Lines connect the points. The DFT values were taken from [8]. The inset is a zoom around the peak TOF.
Fig. 3. Experimental methane (Δ), ethane (•), propane (□), and isobutane (♦) TOF vs. DFT-calculated binding energies of carbon at 340 °C. Equivalence ratio is held constant at 2. Lines connect the points. The DFT values were taken from [8]. The inset is a zoom around the peak TOF.
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Fig. 5. Methane, ethane, propane, and isobutane TOF over Pt/Al2O3, Pd/Al2O3, Ag/Al2O3, and three nominal compositions (1:3, 1:10, and 3:1) of Ag-Pd/Al2O3 at 340 °C. Feed composition: 1.5 cm3 min−1 hydrocarbon; O2 concentration varies for each hydrocarbon to maintain an equivalence ratio of 0.5.
Fig. 7. Steady-state propane TOF as a function of equivalence ratio on (•) Pt/ Al2O3, (◼) Pd/Al2O3, and (▲) 1:3 Ag-Pd/Al2O3 at 340 °C. Propane flow rate and WHSV of 45000 mL·g−1·hr−1 hr−1 are held constant. Arrows indicate the direction of oxygen partial pressure change (stoichiometric point at equivalence ratio = 1). Fuel-lean region is represented by the shaded area. Closed symbols for decreasing equivalence ratio from high values and open symbols for increasing equivalence ratio from low values.
Table 1 Apparent activation energies of methane, ethane, propane, and isobutane over Pt, Pd, and 1:3 Ag-Pd/Al2O3 for an equivalence ratio of 0.5. Arrhenius graphs in Fig. S10. Catalyst
1% Pt/Al2O3 1% Pd/Al2O3 1:3 Ag-Pd/Al2O3
(Figs. 6 and 7). All catalysts exhibit three distinct kinetic regimes for both alkanes. The reaction rate increases with decreasing equivalence ratio starting from high equivalence ratio before it reaches a maximum. This maximum occurs at an equivalence ratio of 0.8 for methane and 1.19 (Pt), 0.83 (Pd), and 0.77 (1:3 Ag-Pd) for propane. The reaction rate is positive-order in oxygen concentration. The fuel TOF decreases with further decreasing the equivalence ratio below this maximum. At sufficiently low equivalence ratios, the reaction rate is zero-order kinetics in O2. Upon increasing the equivalence ratio from low values, the zeroorder kinetics persist and hysteresis is found up to a maximum equivalence ratio. Our results are consistent with previously reported distinct kinetic regimes observed in methane oxidation over Pd [18] and Pt [19] that are demarcated by the oxygen concentration. Under fuel rich conditions, it has been suggested that metallic Pd is dominant and is a more effective than PdO for the dissociative adsorption of methane [20]. Our data is consistent with the recent observation by O’Brien et al. of the existence of hysteresis in propane oxidation over Pt [21], who attributed the hysteresis and different activity regimes to different PtO species [21]. In our study, Pd and Ag-Pd also exhibit distinct kinetic regimes and hysteresis with varying oxygen concentration. These results suggest that propane oxidation has a similar reaction pathway as methane [19] and ethane [22] oxidation. Combined with recent studies, we propose that hysteresis is a general phenomenon of alkane oxidation over many catalysts and is associated with a change in the catalyst from being oxidized (sufficiently fuel lean conditions) to being reduced (sufficiently fuel rich conditions). XPS spectra after reaction (air free) to probe the oxidation state of the Pd surface of the Ag-Pd catalyst at two feed compositions are shown in Fig. 8. The Pd 3d XPS core-level spectra at 340 °C and excess O2 show that the Pd surface is dominated by PdO. A lower O2 partial pressure (O2/CH4 = 2; stoichiometric point) results in a shoulder peak at around 335.7 eV characteristic of the formation of metallic Pd on the surface [23]. In contrast to the Ag-Pd catalyst, Fig. 8a shows that the surface of Ni (a more oxophilic metal) remains fully oxidized (NiO) when swapping the composition from fuel lean to stoichiometric. Obviously, the oxidation state of a catalyst depends on the feed composition and the catalyst oxophilicity. This finding has important ramifications for understanding reactivity and the associated volcano curves. The hysteresis size is a function of both the oxophilicity of the catalyst and the hydrocarbon fuel (Figs. 6 and 7). The larger the fuel, the higher its reactivity, consistent with the CeH bond dissociation
Apparent Activation Energy (kJ/mol) Methane
Ethane
Propane
Isobutane
43.4 65.7 49.5
41.3 64.1 40.6
40.8 63.0 39.9
39.1 60.7 37.6
propane, or any another alkane larger than methane, is a suitable surrogate of hydrocarbon oxidation. 3.2. Hysteresis in methane and propane oxidation over Pt/Al2O3, Pd/ Al2O3, and Ag-Pd/Al2O3 catalysts In order to rationalize the difference between methane and other alkanes, the kinetics of propane and methane oxidation over a select number of catalysts (Pt/Al2O3, Pd/Al2O3, and 1:3 Ag-Pd/Al2O3 bimetallics) was investigated at 340 °C for different equivalence ratios
Fig. 6. Steady-state methane TOF as a function of equivalence ratio on (•) Pt/ Al2O3, (◼) Pd/Al2O3, and (▲) 1:3 Ag-Pd/Al2O3 at 340 °C. Methane flow rate and WHSV of 45000 mL·g−1·hr−1 hr−1 are held constant. Arrows indicate the direction of oxygen partial pressure change (stoichiometric point at equivalence ratio = 1). Fuel-lean region is represented by the shaded area. Closed symbols for decreasing equivalence ratio from high values and open symbols for increasing equivalence ratio from low values. 4
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Fig. 8. XPS results of Ni/Al2O3 (a) and 1:3 Ag-Pd/Al2O3 (b) collected after reaction at different O2/CH4 ratios at 340 °C without air exposure. Data for calcined and reduced catalysts are also shown for reference.
Under fuel lean conditions, the catalysts are oxides. Thus, the reactivity data should correlate well with the formation energy of the bulk oxide (see indeed Fig. S14). Given that both Ag2O and PdO are on the left of Pt on the volcano curve, a mixed oxide could not explain the observed activity. Rather, the superior performance of the Ag-Pd catalyst indicates some core-shell oxide structure. The XPS data (Fig. 8b) supports a PdO rich surface. Obviously better control of the synthesis to ensure uniform catalyst composition in all nanoparticles and operando characterization of the structure and oxidation state of these catalysts are essential to improve our understanding and to better assess our ability to predict catalysts under oxidation conditions.
argument, and the easier is to reduce the metal oxide. The more oxophilic the catalyst is, the harder is for the hydrocarbon to reduce the catalyst. Interesting crossings in catalyst activity are observed with varying feed composition, indicating that composition, unlike temperature, the feed composition is a much more influential variable for predicting optimal catalyst when the oxidation state of the catalyst can vary, as can happen for oxidation reactions. We expect similar behavior when the catalyst can form nitrides, carbides, etc. when relevant reagents are used. Our results are consistent with literature reports. For example, Pt was reported as the most active catalyst for total oxidation of alkanes with a carbon number greater than 1 under fuel lean conditions, and Pd as the most active catalyst for total oxidation of methane under fuel rich conditions [24–30]. In contrast, Pt is more active at intermediate to high oxygen concentrations at 550 °C [31]. We postulate that difference in catalyst ranking for methane oxidation maybe due to different catalyst oxidation state, which should depend on feed composition, operating temperature, and possibly the history of the catalyst. Given that the stoichiometric ratio varies with changing hydrocarbon, the partial pressure of O2 at the same equivalence ratio is not the same. For example, for an equivalence ratio of 2 (fuel rich), the ratio of O2 to fuel partial pressures increases with the hydrocarbon size as follows: O2/methane ratio 1, O2/ethane ratio 1.75, O2/propane ratio 2.5, and O2/isobutane ratio 3.25, as stated above. In the case of methane, the oxygen partial pressure is even less than that of the larger hydrocarbons, and yet the catalyst reduction is harder, i.e., this indicates that the partial pressure of O2 is the not the key driver for the observed difference; rather, it is the reactivity of the alkane and its ability to reduce the oxide that drive this behavior. Finally, while Ag-Pd is the best catalyst for propane oxidation under slightly fuel rich conditions, it is better than Pd but inferior to Pt under most fuel rich conditions (Fig. 6). We propose that this reactivity data along with information on the oxidation state can be used to infer an approximate structure of the catalyst. Based on the interpolation principle with respect to the C binding energy (Fig. 3), one would not expect a mixed alloy of Ag-Pd to be better than Pd given that both Pd and Ag are to the right of Pt on the volcano curve. In contrast, core-shell Ag-Pd structures can possess both the C and O binding energies and could be comparable or exceed the Pt-activity [8]. Thus, our data under fuel rich conditions support indirectly (based on observed activities, DFT binding energies, and the metallic state of the catalyst) a core-shell bimetallic structure coexisting with a fraction of single metal nanoparticles and/or non ideal core-shell structures, rationalizing the inferior performance of Ag-Pd compared to Pt. Presumably, the performance should be even better for ideal core-shell bimetallic structures.
4. Conclusions In this work, the concept of universality of volcano curves for a homologous series was exploited. Specifically, small alkane oxidation was studied over seven monometallic and three Ag-Pd bimetallic catalysts in the 280–400 °C temperature range under both fuel lean and rich conditions. Hysteresis appears to be a common phenomenon of the entire alkane series over all catalysts studied. More oxophilic catalysts exhibit a larger hysteresis, and larger hydrocarbons, which are more reactive and thus better reducing agents, show smaller hysteresis. More quantitative correlation of the hysteresis characteristics with catalyst traits and feed molecular fingerprints will require much more data than those obtained here; such collection of data will be amenable to machine learning methods to reveal correlations and causalities. Taken together with prior EXAFS and XPS data, hysteresis is attributed to the varying oxidation state of catalysts with varying feed composition and potentially the ability of the hydrocarbon to reduce the catalyst. This leads to a complex dependence of the relative activity of catalysts on fuel/oxygen ratio and hydrocarbon fuel. As a result, interesting crossovers in relative catalyst activity are observed. For example, for propane, under fuel lean (oxygen excess) conditions, the activity ranks AgPd > Pt > Pd, whereas under slightly fuel rich (oxygen deficient) conditions, Ag-Pd > Pt∼Pd and under richer conditions Pt > AgPd > Pd. For methane, under fuel lean conditions the ranking is Pt > Pd∼Ag-Pd and under fuel rich conditions Ag-Pd > Pd > Pt. With a proper surrogate, the universality of the homologous series holds indeed valid as expected, i.e., a superior catalyst predicted for a model compound works for the entire homologous series. We propose that propane studied more extensively here, beyond methane, is a surrogate of the reactivity of the alkane homologous series but other alkanes with 2 or more carbons should suffice too. Unexpectedly, methane is not a representative compound of the homologous series due to its lower reactivity and inability to reduce the catalyst at lower 5
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References
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Schmidt, Production of syngas by direct catalytic oxidation of methane, Science 259 (1993) 343–346. [16] J.T. Jankowiak, M.A. Barteau, Ethylene epoxidation over silver and copper–silver bimetallic catalysts: I. Kinetics and selectivity, J. Catal. 236 (2005) 366–378. [17] N.R. Peela, I.C. Lee, D.G. Vlachos, Design and fabrication of a high-throughput microreactor and its evaluation for highly exothermic reactions, Ind. Eng. Chem. Res. 51 (2012) 16270–16277. [18] Y.-H. Chin, E. Iglesia, Elementary Steps, the role of chemisorbed oxygen, and the effects of cluster size in catalytic CH4–O2 reactions on palladium, J. Phys. Chem. C 115 (2011) 17845–17855. [19] Y.-H. Chin, C. Buda, M. Neurock, E. Iglesia, Reactivity of chemisorbed oxygen atoms and their catalytic consequences during CH4–O2 catalysis on supported Pt clusters, J. Am. Chem. Soc. 133 (2011) 15958–15978. [20] J.N. Carstens, S.C. Su, A.T. Bell, Factors affecting the catalytic activity of Pd/ ZrO2for the combustion of methane, J. 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Acknowledgements G.L., I.C.L., and D.G.V. were sponsored by the Army Research Laboratory for their catalyst synthesis and characterization, kinetic experiments, and DFT under the Cooperative Agreement Number W911NF-14-2-0041. The views and conclusions contained herein are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. government. The U.S. government is authorized to reproduce and distribute reprints for government purposes notwithstanding any copyright notation hereon. The microscopy and XPS studies were conducted by W.Z.; the latter was supported by ARL and the former by the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award no. DESC0001004. The authors acknowledge useful discussions with Alexander Mironenko. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2019.117255.
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Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
Volcano curves for homologous series reactions: Oxidation of small alkanes a
a
b
Garam Lee , Weiqing Zheng , Ivan C. Lee , Dionisios G. Vlachos a b
a,⁎
T
Department of Chemical and Biomolecular Engineering, Catalysis Center for Energy Innovation, University of Delaware, Newark, DE, 19716-3110, USA U.S.Army Research Laboratory, Sensors and Electron Devices Directorate, 2800 Powder Mill Road, Adelphi, MD, 20783, USA
A R T I C LE I N FO
A B S T R A C T
Keywords: Alkanes Homologous series Catalytic combustion Platinum Palladium Ag-Pd Bimetallics
Volcano curves are typically developed and used for predicting new catalysts for a single reaction and one substrate. The concept of using the volcano curve to predict catalysts for an entire homologous series of reactions has been unexplored. Herein the catalytic activity of seven monometallic catalysts (Pt, Pd, Rh, Ag, Ni, Cu, and Co/Al2O3) and three Ag-Pd/Al2O3 bimetallic catalysts is evaluated in the total oxidation of small alkanes (methane, ethane, propane, and isobutane) in the 280–400 °C temperature range under fuel lean and rich conditions. We show that hysteresis in activity, with three distinct kinetic regimes, is a common phenomenon of alkane oxidation over all catalysts studied when varying the oxygen concentration, and the size of the hysteresis loop depends on the oxophilicity of the catalyst and the reducing ability of the hydrocarbon. Expectedly, the concept of the universality of the volcano curve for a homologous series is valid but only when a suitable surrogate substrate is chosen. Hydrocarbons with two or more carbon atoms can serve as surrogates of the alkane homologous series, whereas methane is not. Interestingly and consistent with the hysteresis, the feed composition controls the catalyst oxidation state and potentially impacts the optimal catalyst descriptors used to determine new catalysts. The predicted 1:3 Ag-Pd catalyst is indeed superior to single metals for the homologous series under fuel lean conditions for ethane and larger alkanes. It is inferior to Pt under fuel rich conditions and better than Pd and Pt for methane rich conditions. A method for qualitative inference of the catalyst structure, based on the volcano curve and the oxidation state of the catalyst, is proposed.
1. Introduction
for this idea. For example, complete oxidation of hydrocarbons with 6–20 carbon atoms over 1% Pt/Al2O3 show that there is a correlation between light-off temperature and the number of carbons: heavy alkanes are oxidized easier than light alkanes since the CeH bond activation becomes more facile with increasing carbon number based on the CeH bond dissociation energy [9]. To the best of our knowledge, studies on universality of volcano curves across substrates are lacking. Our goal in this paper is to explore this concept of universality of volcano curves. The specific family of reactions studied herein is oxidation of hydrocarbons because (1) it is important in pollution abatement [10–13], portable power generation [14], and partial oxidation (where complete oxidation is unwanted) [15,16], and (2) the oxidation state of the catalyst may change with fuel and/or feed composition making the problem more interesting. Specifically, we study the oxidation of small hydrocarbons (methane, ethane, propane, and isobutane) over seven single-metal catalysts (Ag, Co, Cu, Ni, Pd, Pt, Rh) and three Ag-Pd bimetallics [8] under varying oxygen mole fraction. The kinetics of methane and propane oxidation over Pt, Pd, and Ag-Pd as a function of oxygen concentration is also investigated to relate the oxidation state of the catalyst to the volcano curves.
The volcano curve, based on Sabatier’s principle [1], relates the activity of a catalyst with a single descriptor, typically the binding energy of a key species. The optimal descriptor value (near the maximum of the volcano curve) is often matched to that on bimetallics in order to develop new materials, such as core-shell structures. Volcano relationships have been observed for several reactions, such as ammonia decomposition [2–4], isopropanol dehydrogenation [5], and desulfurization [6,7]. In our recent work, we found that the rate of propane total oxidation over monometallic catalysts is correlated with two, instead of one, descriptors, namely the oxygen and carbon binding energies [8]. A core-shell Ag-Pd bimetallic catalyst was computationally predicted and experimentally confirmed to be superior to Pt. An open question in discovering new materials is whether the concept of homologous series holds for volcano curves, i.e., whether a volcano developed for a single model compound and the identified optimal catalyst is optimal for an entire homologous series of compounds. The fact that molecules in a homologous series may have the same rate-determining step and a similar mechanism provides support
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Corresponding author. E-mail address: [email protected] (D.G. Vlachos).
https://doi.org/10.1016/j.apcata.2019.117255 Received 28 June 2019; Received in revised form 26 August 2019; Accepted 12 September 2019 Available online 14 September 2019 0926-860X/ © 2019 Elsevier B.V. All rights reserved.
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2. Experimental section
2.3. Kinetic experiments
2.1. Catalyst preparation and characterization
All kinetic experiments were performed at ambient pressure. Before each test, the catalyst was reduced in situ, under 10% H2/He flow (total flow rate of 300 cm3 min−1) at 500 °C for 1 h. Then, the reactor was cooled down to the reaction temperature under He, and then the hydrocarbon and oxygen were introduced to the reaction chamber. Homologous series experiments were performed at an O2/alkane equivalence ratio of 0.5 (fuel lean, oxygen excess) and 2 (fuel rich, oxygen deficient) in the 280–400 °C temperature range (Table S4). The equivalence ratio, defined as the molar ratio of fuel to oxygen over that at stoichiometric (see supporting information for the relevant equation), is a standard way of defining the feed composition and indicates clearly the deviation of a feed composition from the stoichiometric point. The hydrocarbon flow rate and the total flow rate were held constant during all oxidation experiments at 1.5 cm3 min−1 and 300 cm3 min−1, respectively, and the oxygen flow rate was varied (balanced by inert) for each hydrocarbon to maintain a fixed equivalence ratio. Hysteresis experiments were conducted at 3 cm3 min−1 of methane or propane and a constant weight hourly space velocity (WHSV) of 45000 mL g−1·hr−1 by varying the flow rate of O2 (Table S5). Pt, Pd, and 1:3 Ag-Pd were loaded in six channels of the HTE reactor (two channels for each catalyst for reproducibility of data). The remaining three channels were loaded with the same weight of SiC (blank measurements). After pretreatment, methane and oxygen were introduced to the reactor at an O2/C3H8 ratio of 2. Once steady-state was reached, the ratio was increased stepwise, allowing 30 min for steady state at each composition (time dependent experiments indicated that this time is sufficient to reach steady state). Subsequently, the O2/C3H8 ratio was decreased stepwise in a similar fashion to probe hysteresis. In this study, the TOF is the average from two channels. Like our previous study on propane oxidation [17], the conversion in channels containing the same catalyst shows very good reproducibility.
The single-metal catalysts (Ag, Co, Cu, Ni, Pd, Pt, Rh) were prepared by incipient wetness impregnation of commercial γ-Al2O3 (3 μm, Alfa Aesar) powder with an aqueous precursor solution of the suitable salt (Table S1). Ag-Pd bimetallic catalysts with three different molar ratios (1:10, 1:3, and 3:1) were prepared using incipient wetness co-impregnation. All catalysts were dried at 100 °C for 10 h and then calcined at 290 °C for 2 h with a ramp rate of 5 °C/min in ambient air. In order to avoid mass and heat transfer effects due to the high reaction rate and exothermicity of alkane oxidation reaction, the synthesized catalysts were diluted 20-fold with SiC (2 μm, Alfa Aesar). The diluted catalysts were then pressed into pellets, crushed, and sieved to a final size of 105–150 μm. It was shown in our earlier work that this procedure, along with feed dilution and a microreactor with large thermal mass (see below), eliminate transport effects and ensure isothermal conditions [8]. The turnover frequency (TOF) is reported in units of molecules of alkane reacted per catalyst site per second and was calculated based on the alkane differential conversion (conversion less than 20% and often much lower). The number of active sites was obtained through CO chemisorption (Table S2) using AMI-200ip (Altamira Instruments, USA). 0.5 g of each catalyst was loaded into a quartz reactor, reduced under 10% H2/He at 500 °C for 1 h, and purged in He for 1 h. Then, the catalyst was cooled down in He flow to 30 °C for CO chemisorption. The mean particle size of catalysts was also determined via extensive transmission electron microscope (TEM) and found to be comparable to that of chemisorption (Fig. S1–S8 and Table S3). Given the accuracy of chemisorption experiments and the uncertainty in quantification of surface area from the assumed ratio of the probe molecule (CO) to the exposed surface site, TEM is a good tool to interrogate the chemisorption findings. For example, the particle size estimated using CO chemisorption and TEM over Ag and Cu based catalysts is consistent (Table S3 in the SI). X-ray photoelectron spectroscopy (XPS) analysis was carried out on a monochromatic aluminum K-α X-ray photoelectron spectrometer (ThermoFisher K-alpha+, US). Samples for XPS measurements were prepared in a glove-box and transferred through a vacuum transfer module (ThermoFisher, US) without exposing them to air. Two representative conditions at 340 °C were chosen to probe the surface oxidation states of two catalysts, namely of Ni/Al2O3 and 1:3 Ag-Pd/Al2O3 catalyst: (1) after running the chemistry at O2/CH4 = 4 (fuel-lean conditions) for 1 h and (2) after conditions 1 followed by flashing the catalyst with inert gas and changing the composition to O2/ CH4 = 2 (the stoichiometric point) for 30 min. The Pt 4f peaks overlap with those of the Al (support) and cannot be separated due to the low loading (1%) of Pt; thus no XPS data is reported for Pt.
3. Results and discussion 3.1. Homologous series experiments The catalytic activity of monometallic catalysts and three nominal compositions of Ag-Pd bimetallic catalysts was investigated in the 280–400 °C temperature range at an equivalence ratio of 0.5 (O2/methane ratio 4, O2/ethane ratio 7, O2/propane ratio 10, O2/isobutane ratio 13). The experimental TOF of small hydrocarbons at 340 °C as a function of the binding energy of carbon and oxygen, obtained from DFT calculations conducted in [8], is plotted in Figs. 1 and 2. Pt/Al2O3 shows the highest activity while Co/Al2O3 shows the lowest activity. Consistent with our prior work [8], a volcano-type relation is observed between the TOF and the carbon and oxygen binding energies for all hydrocarbons. The necessity of two descriptors is illustrated by differences in the activity of Cu and Pt. Specifically, these catalysts have similar O binding energy but dissimilar C binding energy. The TOF increases with increasing carbon number, consistent with the increased reactivity of larger hydrocarbons, correlated with the CeH bond dissociation energy. Fig. S9 shows the TOF for the oxidation of all hydrocarbons over single metal catalysts as a function of temperature. Fig. S11 shows the same volcano-type relation at different temperatures, indicating that temperature has no apparent effect on the catalyst-activity ranking over the studied temperature range. Under fuel lean conditions (excess O2), Pt is superior, and Pd is the second-best catalyst for all alkanes. The optimal properties (O and C binding energies) are the same for all alkanes providing support to the homologous series concept. Fig. S12 shows the corresponding contour plots of catalyst activity vs. the carbon and oxygen binding energies. The TOF under fuel rich conditions, at an equivalence ratio of 2 (O2/
2.2. Instrumentation Details of the high-throughput experiment (HTE) system are given in Peela et al. [17]. The reactant (alkane and oxygen) and inert (helium) gas flow were controlled using RS-485 mass flow controllers (MKS Instrument, USA). The exit temperatures of all nine channels were measured with 0.16 cm K-type thermocouple. One of the nine reactor effluents was selected using a 10-port selector (VICI Instruments, USA), and the eight remaining effluents were vented. The tubes after the selector valve were heated at 160 °C to avoid condensation of water formed during reaction, which was then condensed using an ice trap prior to the gas chromatograph (GC). The product effluent was analyzed with a four-channel micro-GC (G3581, Agilent Technologies, USA) with three different capillary columns: molecular sieve 5A, porapak-U, and AlOx (Agilent Technologies, USA).
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Fig. 1. Experimental methane (Δ), ethane (•), propane (□), and isobutane (♦) TOF vs. DFT-calculated binding energy of carbon at 340 °C. Equivalence ratio is held constant at 0.5. Lines connect the points. The DFT values were taken from [8]. The inset is a zoom around the peak TOF.
Fig. 4. Experimental methane (Δ), ethane (•), propane (□), and isobutene (♦) TOF vs. DFT-calculated binding energies of oxygen at 340 °C. Equivalence ratio is held constant at 2. Lines connect the points. The DFT values were taken from [8]. The inset is a zoom around the peak TOF.
methane ratio 1, O2/ethane ratio 1.75, O2/propane ratio 2.5, and O2/ isobutane ratio 3.25), is plotted as a function of carbon and oxygen binding energies in Figs. 3 and 4. Volcanos are again observed. Pt is the most active catalyst for all hydrocarbons but methane, for which, Pd is the most active catalyst. Given the results for fuel lean and rich conditions, our results indicate that feed composition may change the optimal catalyst properties and unexpectedly, methane is not a surrogate of the alkane homologous series. We return to this point below. In our previous work [8], a library of 120 shell-core bimetallic catalysts was generated using 5 host metals as a core and 12 guest metals as a monolayer (ML) in both surface (top) and subsurface (second layer below the surface) configurations. The binding energy of carbon and oxygen on these 120 ML bimetallic catalysts was calculated using DFT calculations. The binding energies of the bimetallic catalysts were compared to those of the peak carbon and oxygen binding energies determined for propane total oxidation. Then, the stability of the promising bimetallic catalysts at high oxygen coverage was assessed. Active and stable catalysts predicted computationally were tested experimentally. The Ag-Pd bimetallic, with carbon and oxygen binding energies (−6.8 and −4.4 eV, respectively) close to the optimal values (−7.2 and −4.4 eV, respectively), showed superior catalytic experimental activity than the most active Pt catalyst under fuel lean conditions [8], consistent with model predictions. The Ag-Pd bimetallic was not tested for other hydrocarbons and other feed compositions. In order to test the universality of the Ag-Pd prediction for the homologous series, the rate of the total oxidation of additional small alkanes was measured over Pt/Al2O3, Pd/Al2O3, Ag/Al2O3, and Ag-Pd/ Al2O3. Fig. 5 shows the TOF at 340 °C (for other temperatures, see Fig. S13) under fuel lean conditions common to catalytic combustion and abetment. The Ag-Pd bimetallic catalyst (1:3 and 3:1 nominal composition) shows superior activity to Pt for the oxidation of ethane, propane, and isobutane but not for methane for which, the rates on bimetallics are slightly lower but comparable to Pt and Pd and superior to Ag. Consistent with the reactivity trends, Table 1 also shows that the activation energy of the total oxidation of ethane, propane, and isobutane over the 1:3 Ag-Pd bimetallic is slightly lower than the activation energy over Pt. Our kinetics results in a forthcoming publication are consistent with the literature. They indicate that a better catalyst discovered for a model compound using the volcano curve should generally be better for the entire homologous series, presumably due to the CeH bond activation being rate-determining in the total oxidation of all alkanes [9]. However, our data suggests that exceptions may exist for some members of the homologous series (in our case methane). We rationalize the reason for the methane anomaly below and propose that
Fig. 2. Experimental methane (Δ), ethane (•), propane (□), and isobutane (♦) TOF vs. DFT-calculated binding energy of oxygen at 340 °C. Equivalence ratio is held constant at 0.5. Lines connect the points. The DFT values were taken from [8]. The inset is a zoom around the peak TOF.
Fig. 3. Experimental methane (Δ), ethane (•), propane (□), and isobutane (♦) TOF vs. DFT-calculated binding energies of carbon at 340 °C. Equivalence ratio is held constant at 2. Lines connect the points. The DFT values were taken from [8]. The inset is a zoom around the peak TOF.
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Fig. 5. Methane, ethane, propane, and isobutane TOF over Pt/Al2O3, Pd/Al2O3, Ag/Al2O3, and three nominal compositions (1:3, 1:10, and 3:1) of Ag-Pd/Al2O3 at 340 °C. Feed composition: 1.5 cm3 min−1 hydrocarbon; O2 concentration varies for each hydrocarbon to maintain an equivalence ratio of 0.5.
Fig. 7. Steady-state propane TOF as a function of equivalence ratio on (•) Pt/ Al2O3, (◼) Pd/Al2O3, and (▲) 1:3 Ag-Pd/Al2O3 at 340 °C. Propane flow rate and WHSV of 45000 mL·g−1·hr−1 hr−1 are held constant. Arrows indicate the direction of oxygen partial pressure change (stoichiometric point at equivalence ratio = 1). Fuel-lean region is represented by the shaded area. Closed symbols for decreasing equivalence ratio from high values and open symbols for increasing equivalence ratio from low values.
Table 1 Apparent activation energies of methane, ethane, propane, and isobutane over Pt, Pd, and 1:3 Ag-Pd/Al2O3 for an equivalence ratio of 0.5. Arrhenius graphs in Fig. S10. Catalyst
1% Pt/Al2O3 1% Pd/Al2O3 1:3 Ag-Pd/Al2O3
(Figs. 6 and 7). All catalysts exhibit three distinct kinetic regimes for both alkanes. The reaction rate increases with decreasing equivalence ratio starting from high equivalence ratio before it reaches a maximum. This maximum occurs at an equivalence ratio of 0.8 for methane and 1.19 (Pt), 0.83 (Pd), and 0.77 (1:3 Ag-Pd) for propane. The reaction rate is positive-order in oxygen concentration. The fuel TOF decreases with further decreasing the equivalence ratio below this maximum. At sufficiently low equivalence ratios, the reaction rate is zero-order kinetics in O2. Upon increasing the equivalence ratio from low values, the zeroorder kinetics persist and hysteresis is found up to a maximum equivalence ratio. Our results are consistent with previously reported distinct kinetic regimes observed in methane oxidation over Pd [18] and Pt [19] that are demarcated by the oxygen concentration. Under fuel rich conditions, it has been suggested that metallic Pd is dominant and is a more effective than PdO for the dissociative adsorption of methane [20]. Our data is consistent with the recent observation by O’Brien et al. of the existence of hysteresis in propane oxidation over Pt [21], who attributed the hysteresis and different activity regimes to different PtO species [21]. In our study, Pd and Ag-Pd also exhibit distinct kinetic regimes and hysteresis with varying oxygen concentration. These results suggest that propane oxidation has a similar reaction pathway as methane [19] and ethane [22] oxidation. Combined with recent studies, we propose that hysteresis is a general phenomenon of alkane oxidation over many catalysts and is associated with a change in the catalyst from being oxidized (sufficiently fuel lean conditions) to being reduced (sufficiently fuel rich conditions). XPS spectra after reaction (air free) to probe the oxidation state of the Pd surface of the Ag-Pd catalyst at two feed compositions are shown in Fig. 8. The Pd 3d XPS core-level spectra at 340 °C and excess O2 show that the Pd surface is dominated by PdO. A lower O2 partial pressure (O2/CH4 = 2; stoichiometric point) results in a shoulder peak at around 335.7 eV characteristic of the formation of metallic Pd on the surface [23]. In contrast to the Ag-Pd catalyst, Fig. 8a shows that the surface of Ni (a more oxophilic metal) remains fully oxidized (NiO) when swapping the composition from fuel lean to stoichiometric. Obviously, the oxidation state of a catalyst depends on the feed composition and the catalyst oxophilicity. This finding has important ramifications for understanding reactivity and the associated volcano curves. The hysteresis size is a function of both the oxophilicity of the catalyst and the hydrocarbon fuel (Figs. 6 and 7). The larger the fuel, the higher its reactivity, consistent with the CeH bond dissociation
Apparent Activation Energy (kJ/mol) Methane
Ethane
Propane
Isobutane
43.4 65.7 49.5
41.3 64.1 40.6
40.8 63.0 39.9
39.1 60.7 37.6
propane, or any another alkane larger than methane, is a suitable surrogate of hydrocarbon oxidation. 3.2. Hysteresis in methane and propane oxidation over Pt/Al2O3, Pd/ Al2O3, and Ag-Pd/Al2O3 catalysts In order to rationalize the difference between methane and other alkanes, the kinetics of propane and methane oxidation over a select number of catalysts (Pt/Al2O3, Pd/Al2O3, and 1:3 Ag-Pd/Al2O3 bimetallics) was investigated at 340 °C for different equivalence ratios
Fig. 6. Steady-state methane TOF as a function of equivalence ratio on (•) Pt/ Al2O3, (◼) Pd/Al2O3, and (▲) 1:3 Ag-Pd/Al2O3 at 340 °C. Methane flow rate and WHSV of 45000 mL·g−1·hr−1 hr−1 are held constant. Arrows indicate the direction of oxygen partial pressure change (stoichiometric point at equivalence ratio = 1). Fuel-lean region is represented by the shaded area. Closed symbols for decreasing equivalence ratio from high values and open symbols for increasing equivalence ratio from low values. 4
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Fig. 8. XPS results of Ni/Al2O3 (a) and 1:3 Ag-Pd/Al2O3 (b) collected after reaction at different O2/CH4 ratios at 340 °C without air exposure. Data for calcined and reduced catalysts are also shown for reference.
Under fuel lean conditions, the catalysts are oxides. Thus, the reactivity data should correlate well with the formation energy of the bulk oxide (see indeed Fig. S14). Given that both Ag2O and PdO are on the left of Pt on the volcano curve, a mixed oxide could not explain the observed activity. Rather, the superior performance of the Ag-Pd catalyst indicates some core-shell oxide structure. The XPS data (Fig. 8b) supports a PdO rich surface. Obviously better control of the synthesis to ensure uniform catalyst composition in all nanoparticles and operando characterization of the structure and oxidation state of these catalysts are essential to improve our understanding and to better assess our ability to predict catalysts under oxidation conditions.
argument, and the easier is to reduce the metal oxide. The more oxophilic the catalyst is, the harder is for the hydrocarbon to reduce the catalyst. Interesting crossings in catalyst activity are observed with varying feed composition, indicating that composition, unlike temperature, the feed composition is a much more influential variable for predicting optimal catalyst when the oxidation state of the catalyst can vary, as can happen for oxidation reactions. We expect similar behavior when the catalyst can form nitrides, carbides, etc. when relevant reagents are used. Our results are consistent with literature reports. For example, Pt was reported as the most active catalyst for total oxidation of alkanes with a carbon number greater than 1 under fuel lean conditions, and Pd as the most active catalyst for total oxidation of methane under fuel rich conditions [24–30]. In contrast, Pt is more active at intermediate to high oxygen concentrations at 550 °C [31]. We postulate that difference in catalyst ranking for methane oxidation maybe due to different catalyst oxidation state, which should depend on feed composition, operating temperature, and possibly the history of the catalyst. Given that the stoichiometric ratio varies with changing hydrocarbon, the partial pressure of O2 at the same equivalence ratio is not the same. For example, for an equivalence ratio of 2 (fuel rich), the ratio of O2 to fuel partial pressures increases with the hydrocarbon size as follows: O2/methane ratio 1, O2/ethane ratio 1.75, O2/propane ratio 2.5, and O2/isobutane ratio 3.25, as stated above. In the case of methane, the oxygen partial pressure is even less than that of the larger hydrocarbons, and yet the catalyst reduction is harder, i.e., this indicates that the partial pressure of O2 is the not the key driver for the observed difference; rather, it is the reactivity of the alkane and its ability to reduce the oxide that drive this behavior. Finally, while Ag-Pd is the best catalyst for propane oxidation under slightly fuel rich conditions, it is better than Pd but inferior to Pt under most fuel rich conditions (Fig. 6). We propose that this reactivity data along with information on the oxidation state can be used to infer an approximate structure of the catalyst. Based on the interpolation principle with respect to the C binding energy (Fig. 3), one would not expect a mixed alloy of Ag-Pd to be better than Pd given that both Pd and Ag are to the right of Pt on the volcano curve. In contrast, core-shell Ag-Pd structures can possess both the C and O binding energies and could be comparable or exceed the Pt-activity [8]. Thus, our data under fuel rich conditions support indirectly (based on observed activities, DFT binding energies, and the metallic state of the catalyst) a core-shell bimetallic structure coexisting with a fraction of single metal nanoparticles and/or non ideal core-shell structures, rationalizing the inferior performance of Ag-Pd compared to Pt. Presumably, the performance should be even better for ideal core-shell bimetallic structures.
4. Conclusions In this work, the concept of universality of volcano curves for a homologous series was exploited. Specifically, small alkane oxidation was studied over seven monometallic and three Ag-Pd bimetallic catalysts in the 280–400 °C temperature range under both fuel lean and rich conditions. Hysteresis appears to be a common phenomenon of the entire alkane series over all catalysts studied. More oxophilic catalysts exhibit a larger hysteresis, and larger hydrocarbons, which are more reactive and thus better reducing agents, show smaller hysteresis. More quantitative correlation of the hysteresis characteristics with catalyst traits and feed molecular fingerprints will require much more data than those obtained here; such collection of data will be amenable to machine learning methods to reveal correlations and causalities. Taken together with prior EXAFS and XPS data, hysteresis is attributed to the varying oxidation state of catalysts with varying feed composition and potentially the ability of the hydrocarbon to reduce the catalyst. This leads to a complex dependence of the relative activity of catalysts on fuel/oxygen ratio and hydrocarbon fuel. As a result, interesting crossovers in relative catalyst activity are observed. For example, for propane, under fuel lean (oxygen excess) conditions, the activity ranks AgPd > Pt > Pd, whereas under slightly fuel rich (oxygen deficient) conditions, Ag-Pd > Pt∼Pd and under richer conditions Pt > AgPd > Pd. For methane, under fuel lean conditions the ranking is Pt > Pd∼Ag-Pd and under fuel rich conditions Ag-Pd > Pd > Pt. With a proper surrogate, the universality of the homologous series holds indeed valid as expected, i.e., a superior catalyst predicted for a model compound works for the entire homologous series. We propose that propane studied more extensively here, beyond methane, is a surrogate of the reactivity of the alkane homologous series but other alkanes with 2 or more carbons should suffice too. Unexpectedly, methane is not a representative compound of the homologous series due to its lower reactivity and inability to reduce the catalyst at lower 5
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References
temperatures. Interestingly, the optimal catalyst may change with feed composition (under reducing and oxidizing conditions) due to dependence of the oxidation state of a catalyst on the feed composition. This hypothesis was confirmed by control air-free XPS experiments on Ni and Ag-Pd catalysts. Our results further indicate, consistent with recent studies, that the oxidation state of the catalyst and chemisorbed oxygen play a key role in small alkane oxidation. Oxidation and hydrodeoxygenation for converting biomass are obvious families of reactions where optimal catalyst properties should be sensitive to feed composition and potentially to temperature. The volcano curves combined with information about the catalyst oxidation state provide indirect inference of the structure of two component catalysts under working conditions. Our work opens up interesting questions for future work. For example, whether the concept of homologous series extends to other reactions and classes of catalysts and different supports. We hypothesize that it generally does for each family of reactions (homologous series) over the same class of catalysts, e.g., metals on carbon, metals on an irreducible support, Bronsted or Lewis acids, etc. This hypothesis is based on the rationale that the mechanism and rate-determining step remain the same for a homologous series on the same class of catalysts. However, exceptions may be expected, as accumulating knowledge indicates that the rate-determining step is not always the same across the same class of catalysts. Furthermore, change of catalyst support, environment, or confinement may enable new elementary steps or active sites across catalysts; under such circumstances, a single volcano curve may not hold for an entire homologous series. Yet, catalyst screening of a few substrates of a homologous series, as done here, should provide a clear signature of mechanistic and/or catalyst and active site change across the homologous series. Our work underscores that descriptors beyond the catalyst may be needed to describe complex systems. We propose that the CeH bond dissociation energy or in general other molecular chemistry-dependent descriptors, revealed for example from microkinetic analysis, should augment catalyst descriptors considered so far in constructing volcano curves. Given the multidimensional nature of descriptors, simple machine learning methods to simultaneously exploit catalyst and feed correlations are encouraged; to the best of our knowledge, such combined substrate and catalyst descriptors do not currently exist but our studies support their existence. This will require generation of more ‘data’ than done in this work.
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Acknowledgements G.L., I.C.L., and D.G.V. were sponsored by the Army Research Laboratory for their catalyst synthesis and characterization, kinetic experiments, and DFT under the Cooperative Agreement Number W911NF-14-2-0041. The views and conclusions contained herein are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. government. The U.S. government is authorized to reproduce and distribute reprints for government purposes notwithstanding any copyright notation hereon. The microscopy and XPS studies were conducted by W.Z.; the latter was supported by ARL and the former by the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award no. DESC0001004. The authors acknowledge useful discussions with Alexander Mironenko. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2019.117255.
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