Synthesis of heterogeneous catalysts withwell shaped platinum particles to controlreaction selectivityIlkeun Lee, Ricardo Morales, Manuel A. Albiter, and Francisco Zaera*
Department of Chemistry, University of California, Riverside, CA 92521
Edited by Gabor A. Somorjai, University of California, Berkeley, CA, and approved August 18, 2008 (received for review June 12, 2008)
Colloidal and sol-gel procedures have been used to prepare het-erogeneous catalysts consisting of platinum metal particles withnarrow size distributions and well defined shapes dispersed onhigh-surface-area silica supports. The overall procedure was de-veloped in three stages. First, tetrahedral and cubic colloidal metalparticles were prepared in solution by using a procedure derivedfrom that reported by El-Sayed and coworkers [Ahmadi TS, WangZL, Green TC, Henglein A, El-Sayed MA (1996) Science 272:1924–1926]. This method allowed size and shape to be controlledindependently. Next, the colloidal particles were dispersed ontohigh-surface-area solids. Three approaches were attempted: (i) insitu reduction of the colloidal mixture in the presence of thesupport, (ii) in situ sol-gel synthesis of the support in the presenceof the colloidal particles, and (iii) direct impregnation of theparticles onto the support. Finally, the resulting catalysts wereactivated and tested for the promotion of carbon–carbon double-bond cis-trans isomerization reactions in olefins. Our results indi-cate that the selectivity of the reaction may be controlled by usingsupported catalysts with appropriate metal particle shapes.
On the basis of their kinetic behavior, catalytic reactions areoften classified as either mild or demanding (1–3). Demand-
ing reactions—such as the oxidation of CO, NO, or hydrocarbons;the synthesis of ammonia; and most oil processing conversions—usually require high temperatures and pressures, and involve smallconcentrations of intermediates similar to those identified undervacuum. The performance of these reactions often dependsstrongly on the structure of the catalyst used (4, 5). In contrast, mildreactions—in particular, hydrogenations and isomerizations of un-saturated hydrocarbons—take place under less-demanding temper-ature and pressure conditions. Mild reactions have historically beenconsidered structure-insensitive (6–8), but that conclusion has beendrawn from studies on reactivity vs. metal dispersion that usedill-defined supported catalysts (9, 10) and has been questioned bymore recent studies using better catalytic models (11). For instance,both experimental (12–14) and theoretical (15) studies on theselective catalytic hydrogenation of CAO bonds in unsaturatedaldehydes have suggested that such reactions may be promoted byclose-packed (111) surfaces. In another example, the dehydroge-nation of cyclohexene was found to be faster on Pt (111) than onPt (100) single-crystal surfaces (16). Our recent surface-scienceinvestigations on the isomerization of unsaturated olefins (17–19)strongly suggest that selectivity toward the formation of the cisisomer may be favored by Pt (111) facets. Additional surface-science reports on the conversion of alkyl and alkene adsorbatesunder vacuum conditions (20–25), as well as studies with morerealistic model systems (26, 27), point to a potential structuresensitivity in the conversion of other olefins and unsaturatedhydrocarbons.
These results not only suggest that mild reactions are structure-sensitive, but also highlight the separate roles that particle size andparticle shape may play in defining activity and selectivity in thosecatalytic processes. Accordingly, the independent control of particle
size and shape during catalyst manufacture may offer avenues forthe design of highly selective catalysts. The self-assembly and nanotechnologies developed during the last few years may now placesuch catalysts within reach. Indeed, several colloidal (28–30) anddendrimer (31) methodologies are already available for makingmetal particles with well defined sizes in solution. The control ofparticle shape is more challenging but has also been achievedrecently, largely by controlling the concentration ratio of thecapping polymer material to the metal cation used during synthesis(32–35). Tetrahedral, cubic, irregular-prismatic, icosahedral, andcubo-octahedral particles have been produced in this way. Alter-native routes have also been proposed for controlling both size andshape in metal colloidal particles (36–38).
The next challenge is to extend the liquid-phase chemistrymentioned above to the preparation of well defined supportedcatalysts. This requires the resolution of a number of issues,including the proper dispersion of the colloidal particles throughoutthe pores of suitable high-surface-area supports and the removal ofthe organic material used to stabilize the particles in solution toexpose the clean metal surface. A particularly important issue inconnection with the latter step is the need to avoid the loss of thewell defined size and shape of the particles or the occurrence ofparticle agglomeration (sintering). Once proper particle dispersionand activation are accomplished, the performance of the resultingcatalysts needs to be tested to identify clear correlations betweenactivity and/or selectivity and the size and/or shape of the particles.We in our laboratory have been working to resolve these issues andhere report on key results indicating the feasibility of this approach.Specifically, the selective promotion of trans-to-cis carbon–carbondouble-bond isomerizations in olefins was achieved by using cata-lysts with tetrahedral Pt particles exposing high fractions of (111)facets. The loss of those facets upon high-temperature treatmentleads to a reversal to the thermodynamically expected preferencefor cis-to-trans conversion.
ResultsSynthesis of Colloidal Platinum Particles. Supported platinum cata-lysts with well defined shapes were prepared and tested by using thestrategy shown in Scheme 1. The initial step focused on the synthesisof colloidal platinum particles with different, well defined sizes andshapes. The starting point for this synthetic effort was the protocoldeveloped by El-Sayed and coworkers (32, 39, 40), who identifieda shape-controlled growth mechanism based on differences be-tween the rate of the catalytic reduction process of Pt2� on (111)vs. (100) facets, a competition between the Pt2� reduction and the
Author contributions: F.Z. designed research; I.L., R.M., and M.A.A. performed research; I.L.and F.Z. analyzed data; and F.Z. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
*To whom correspondence should be addressed. E-mail: [email protected].
This article contains supporting information online at www.pnas.org/cgi/content/full/0805691105/DCSupplemental.
capping process on the different nanoparticle surfaces, and aconcentration-dependent buffer action of the polymer itself. Theseauthors have reported the selective production of cubic, tetrahedral,and truncated octahedral platinum nanoparticles by changing theratio of the concentration of the capping polymer to that of theplatinum cation in the solution used for the reductive synthesis ofthe colloidal nanoparticles.
We conducted a series of experiments to determine the optimumparameters to be used in preparing our tetrahedral and cubic Ptparticles. The best tetrahedral Pt particles were obtained by usingH2PtCl6 and poly(N-vinyl-2-pyrrolidone) (PVP, average Mw �360,000), and the best cubic particles employed K2PtCl4 and sodiumpolyacrylate (SPA, average Mw � 2,100). The specifics of thissynthetic approach are provided in the supporting information (SI)Materials and Methods. Our own experience in these studies indi-cated that, indeed, the most critical factor in controlling bothparticle shape and size is the concentration ratio between thetransition metal precursor and the capping agent. Use of lowconcentrations of the polymer led to narrower size distributions,within 10% of the mean, but not necessarily to well defined particleshapes. The concentration ratio is more critical with H2PtCl6, butsmaller amounts of the polymer are required for the preparation oftetrahedral Pt nanoparticles. Also, in the case of the cubic particles,using an excess of the polymer leads instead to the growth ofoctahedral particles. The lighter the capping polymer, the higherthe concentration needed: 10 times higher concentration ratios ofPVP of average Mw � 40,000 than of PVP of average Mw � 360,000
are needed to prepare similar tetrahedral Pt nanoparticles. In ourexperiments, the optimum ratios for making tetrahedral and cubicparticles were determined to be [H2PtCl6]:[PVP, average Mw �360,000] � 1:0.0005 and [K2PtCl4]:[SPA, average Mw � 2,100] �1:0.75, respectively. These ratios work in only a limited range ofprecursor concentrations—somewhere between 8 � 10�5 and 4 �10�4 M—but within that range the concentration can be used tocontrol the size of the particles (between �3 and 10 nm). Otherfactors, such as pH and reducing time, also must be carefullycontrolled to obtain high-quality shapes and highly monodispersednanoparticles. Slower reduction rates appear to lead to narrowersize distributions.
Fig. 1 shows representative transmission electron microscopy(TEM) images of Pt colloidal particles with cubic, tetrahedral, andstar-like (tetrapod) shapes obtained by this colloidal syntheticprocedure. The average sizes of tetrahedral and cubic Pt nanopar-ticles from TEM images were estimated at 4.8 � 0.3 and 5.0 � 0.3nm, respectively, and the purity of the shapes (fraction of tetrahe-dral or cubic particles over total) at 83% and 87%, respectively. Amore detailed analysis is provided in Fig. S2. The nature of theexposed surfaces—(111) and (100) facets for the tetrahedral andcubic particles, respectively—was confirmed by electron diffraction(see Fig. S3).
We placed particular emphasis on lowering the concentration ofthe capping polymers as much as possible, to minimize the problemsassociated with their removal after dispersion of the particles on thehigh-surface-area solids (see below). The final ratios of polymer to
Scheme 1. Methodology used for the preparation of supported platinum catalysts with well defined particle shapes and sizes.
Fig. 1. TEM images of different platinum colloidal particles obtained by tuning the conditions used for their assembly. Both particle size and shape can becontrolled in this way. Note the high-quality cubic (A), tetrahedral (B), and star-like (tetrapod) (C) shapes obtained in the three cases reported here [withpreferential exposed (100) and (111) facets in the first two cases].
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metal precursor used here are significantly lower than those re-ported in the literature [i.e., 1:0.0005 vs. 1:0.035 for the tetrahedralparticles (41)]. In further addressing this issue, it will be importantto test other capping polymers.
Particle Dispersion on High-Surface-Area Supports. The proceduresdeveloped for the deposition and activation of the platinum nano-particles onto high-surface-area solid supports are perhaps the mostcritical aspect of the preparation of our catalysts because thestabilizing capping polymers must be removed with no significantloss of particle size or shape. Three basic approaches were explored,as indicated in Scheme 1. The first was to grow the Pt particles insitu in the presence of a porous xerogel by carrying the reductionstep after the addition of the high-surface-area support. Unfortu-nately, all our attempts to prepare catalytic samples by followingthis procedure failed because significant metal particle aggregationwas observed and because particle penetration inside the porousstructure of the support was quite limited (Fig. 2A).
Our second approach was to synthesize the solid support in situin the presence of the colloidal particles, to encapsulate the latterwithin the pores of the former during the growth process (42). Theresults obtained from preliminary experiments using this method-ology are encouraging. In one test, ammonia and ammoniumfluoride were first mixed with a colloidal solution obtained byleaving the metal precursor plus capping polymer mixture in thedark for several hours and then adding an appropriate amount oftetraethyl orthosilicate (TEOS) to initiate the gelling of a silica solid(43, 44). The resulting xerogels were dried under vacuum withoutaging until gray powders were obtained. A TEM image of theresulting catalyst, with its tetrahedral particles embedded in the insitu-grown sol-gel silica, is shown in Fig. 2B. Sol-gel chemistry istypically carried out in ethanol; however, because ethanol interfereswith the stability of the colloidal particles, water was chosen hereas the solvent. We also learned that careful control of the concen-trations and total amounts of all of the chemicals is necessary tomatch the colloidal and gelling chemistries. More studies areneeded in this area. A similar approach has been used successfullyby Somorjai and coworkers (37, 45) to incorporate platinumnanoparticles synthesized by alcohol reduction into mesoporousSBA-15 silica.
The main shortcoming of the second procedure is that theresulting xerogels typically display inferior pore size and shapedistributions. We estimate that in our synthesis �50% of the porestructure shrunk during the drying process. Consequently, weexplored a third synthetic route in which the Pt particles weredispersed within the porous material by simple impregnation,followed by either evaporation or filtration (46, 47). The resultingdry catalysts displayed a gray color, indicating metal deposition, andshowed good dispersion in the TEM images (Fig. 2C). This ap-proach was also briefly tested for the dispersion of Pt particles ona homemade SBA-15 mesoporous sample. That solid is crystalline,with one-dimensional pores of �5–7 nm in diameter (as indicated
by TEM), and only allows the incorporation of nanoparticles withsizes of up to �5 nm; the larger particles were deposited only on theoutside surfaces (see Fig. S4).
Catalyst Activation. After preparation, the catalysts were washedwith either water or alcohol, dried, and annealed and/or calcined inair for 1 h to produce the desired dispersed metal catalysts. Asmentioned before, gentle calcination treatments are needed tominimize the potential changes in size or shape of the Pt particles.Interestingly, past TEM experiments with unsupported platinumcolloidal particles have indicated that although heating to 550 K issufficient to remove the capping polymers, the metal particlesretain their shape even after annealing at 625 K (48). In the presentstudy, the thermal chemistry of Pt colloidal particles deposited onxerogel supports was explored by using transmission IR absorptionspectroscopy, X-ray photoelectron spectroscopy (XPS), and TEM.Representative IR spectra are shown in Fig. 3. Those data, whichcorrespond to cubic Pt particles dispersed on silica gel, indicate thatmost (�95%) of the SPA capping polymer can be removed byheating to �625 K, even after deposition of the colloidal particleson the solid; the key vibrational peak for the asymmetricOACOOO stretch, �a(OACOOO), of SPA at 1,558 cm�1 dis-appears completely by 675 K. However, some polymer decompo-sition does occur, as manifested by the persistence of a few of theCOH stretching bands in the IR traces (data not shown), andsubsequent oxidation–reduction (OR) cycles are needed to remove
Fig. 3. Transmission IR spectra for 0.5 wt % cubic colloidal Pt particles dispersedonaxerogel silica solidatdifferent stagesofdepositionandactivation.Referencespectra for the SPA capping polymer and the silica support are provided to aid inthe peak assignment. (Left) Evidence for the deposition of the colloidal particleson the silica; (Right) Evidence for the successful removal of all hydrocarbonresidues after calcination and reduction at 625 K.
Fig. 2. TEM of 0.5 wt % tetrahedral colloidal Pt particles dispersed on porous silica supports, prepared by three different procedures. (A) Precipitation in situ byreduction in the presence of a high-surface-area silica xerogel powder. (B) Growing of the silica xerogel in situ using a sol-gel method. (C) Impregnation of a poroussilica xerogel powder with the Pt colloidal particles, followed by filtration. Of the three methods, C appears to be the best for obtaining well dispersed catalysts.
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the residual carbonaceous deposits (1 h for each half cycle under 1ml/s flow of either O2 or H2 at the same temperature of calcination).This chemistry is confirmed by XPS, which also indicates the extentto which the hydrogen treatment is able to reduce the Pt particles(Fig. 4): the Pt 4f7/2 peak at 75.1-eV binding energy due to theoxidized platinum seen in the fresh sample is significantly dimin-ished (from 30% to 10% of the total signal) after the reductiontreatment.
The evolution of the shape of the supported particles as afunction of the severity of the cleaning procedure was also tested byTEM. Fig. 5 shows a typical sequence of TEM images for 1.0 wt %tetrahedral Pt nanoparticles deposited by impregnation on a silicaxerogel, for the fresh sample and after calcination to varioustemperatures between 375 and 575 K. Minor shape changes alreadytake place after calcination at 375 K, but not until 575 K do theparticles lose their original shape and become rounded; thus, the
calcination temperature must be kept below 575 K to preserve thedesired particle shape. Lowering the concentration of the polymerused during the colloidal preparation step, as mentioned in theprevious section, could be tested as a way to minimize the severityof the conditions needed to clean the supported particles. Displace-ment of the original capping polymers by other easier-to-removeligands once the particles have been grown may also help minimizethis contamination problem.
Catalytic Performance. The catalytic performance of the materialsmade by using the procedures described above was evaluated.Specifically, the selectivity for the isomerization of olefins wasprobed by measuring the kinetics of isomerization of both cis- andtrans-2-butene in the presence of a small amount of hydrogen. Theability to control selectivity in these reactions may have significantpractical implications because, for instance, the trans fatty acids thatare produced during the partial hydrogenation of natural oils toedible fats are believed to have serious negative health effects (49,50). Although the conversion of alkenes by transition metals is oneof the oldest and most studied systems in catalysis (51–53), keyissues such as the dependence of activity on structure remainunresolved (54). As mentioned in our Introduction, results fromrecent surface-science work in our laboratory have indicated that,at least on Pt (111) single-crystal surfaces, the isomerization oftrans-2-butene to its cis isomer is easier than the opposite cis-to-trans conversion (17–19). A kinetic study on the catalytic conversionof butenes on single-crystal surfaces (55) has also suggested differ-ent selectivities on Pt (111) vs. Pt (100) surfaces.
Fig. 6 displays the ratios of the rates for trans-to-cis vs. cis-to-transconversions obtained with both tetrahedral and cubic supportedparticles as a function of the temperature used for calcinationduring the catalyst pretreatment. For the cubic particles, compa-rable rates are seen for both isomerizations—the conversion ofcis-2-butene to its trans isomer and the reverse trans-to-cis trans-formation—with all catalysts, regardless of the calcination temper-ature used. With the tetrahedral particles, on the other hand, notonly are the rates of reaction quite different, but their relative valueschange dramatically as the calcination temperature is increased. Onthe catalysts treated at low temperatures (�500 K), the trans-to-cisconversion occurs approximately twice as fast as the oppositecis-to-trans conversion. In contrast, the trans-to-cis reaction domi-
Fig. 4. Pt 4f XPS data for samples prepared by deposition of 0.5 wt % tetra-hedral colloidal Pt particles on a silica xerogel. Some Pt signal is lost uponcalcination (middle trace), but regained with almost complete reduction to themetal state after reduction at 625 K.
A B C
FED
Fig. 5. TEM of 1.0 wt % tetrahedral colloidal Pt particles dispersed on a porous silica xerogel, freshly prepared (A) and after calcination to 375 K (B), 425 K (C), 475K (D), 525 K (E), and 575 K (F). The platinum particles retain their initial tetrahedral shape after the low-temperature treatments but are clearly transformed into largerand rounder shapes upon calcination at 575 K. These high temperatures are, therefore, to be avoided in the catalyst activation stage.
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nates after catalyst calcination at temperatures �500 K, being asmuch as three to four times faster than the cis-to-trans conversion.These changes correlate well with the changes seen in the TEMimages in Fig. 5 and indicate that the preference for trans-to-cisconversion is lost once the particles are calcined to temperatureshigh enough to affect their shape and reduce the fraction of (111)facets exposed. The inference is that it is the (111) facets of theplatinum tetrahedral particles that promote the nonthermodynamicisomerization of trans olefins to their cis counterparts, as previouslyhypothesized.
Finally, the integrity of the catalysts after their exposure tothe catalytic conditions was checked with TEM. It is wellknown that exposure of supported metal particles to reactiveenvironments can induce surface reconstruction (56) andcould in this case reshape the original particles, reduce the highfraction of (111) facets exposed, and remove their selectivity.However, this does not seem to be the case here. The similarityof the TEM images obtained before and after the catalyticreaction indicates minimal changes in the shape of the particles(Fig. 7). It appears that, in this case, the catalytic conditionsare sufficiently mild to avoid major surface reconstruction onthe platinum-supported particles.
DiscussionIn this study, we have shown that colloidal chemistry can beused to prepare catalysts consisting of dispersed metal parti-cles with well defined sizes and shapes. Some operationaloptimization in terms of catalyst deposition and pretreatmentmay still be needed, but the results summarized above clearlyprove the viability of this approach. Perhaps more interest-ingly, it was also found that particles with different shapes maypromote different reactions selectively. The example providedhere refers to double-bond isomerizations, but similar behav-ior is conceivable with other important catalytic reactions. Theone caveat is that, because of the limited stability of thedeposited particles, only mild reactions can be expected tobenefit from the use of these shape-controlled catalysts.Nevertheless, this includes a large number of processes ofpotential interest to industry, including the hydrogenation andisomerization of many unsaturated hydrocarbons (olefins,aldehydes, ketones, organic acids, imines, etc.), as well aselectrocatalysis such as that involved in fuel cells. It is alsoworth mentioning that these colloidal synthetic methods, inconjunction with other self-assembly procedures, offer anarray of options for the preparation of specific catalysts. Forexample, the use of crystalline mesoporous materials such asSBA-15 or MCM-21 could add shape selectivity, and bimetalliccolloidal particles could be used to control the nature of theactive sites in these catalysts.
The concept of using colloidal methods to control size and shapein supported catalysts has been explored by others (57). Theusefulness of the resulting catalysts has also been tested, mostly forelectrocatalysis (35, 58–60), but in a few instances also for gas-phasecatalytic conversions (61–64). In fact, recent designs of shape-selective catalysts have been implemented industrially for reform-ing and hydrogen peroxide synthesis (65). However, to the best ofour knowledge, ours is the first systematic and comprehensive studyof the evolution of the shape of the particles throughout the variouspreparation and activation steps needed to make and use thesesupported heterogeneous catalysts. It is also, we believe, the firsttime that particle shape has been shown to control selectivity in acatalytic process.
Materials and MethodsThe following chemicals were used in the synthesis of the nanoparticles:dihydrogen hexachloroplatinate hexahydrate (H2PtCl6�6H2O, 99.9% purity;Alfa Aesar); potassium tetrachloroplatinate (K2PtCl4, 99.99% purity), poly(N-vinyl-2-pyrrolidone) (PVP, [CH2CH(NC3H6CO)]n, average Mw � 40,000 and360,000), sodium polyacrylate (SPA, [CH2CH(CO2Na)]n, average Mw � 2,100),2-methyl-2-propanethiol [(CH3)3CSH, 99% purity], toluene (C6H5CH3, 99.5%purity), 1,3,5-trimethylbenzene [(C6H3(CH3)3, 99.2% purity], all from Aldrich;Pluronic P123 triblock copolymer (BASF); and ammonium hydroxide (30%ammonia), ammonium fluoride (NH4F, 98% purity), and tetraethyl orthosili-cate (TEOS, 98% purity), all from Acros. In addition, the following gases wereused: argon (ultrahigh purity), hydrogen (ultrahigh purity), oxygen (ultrahighpurity), air (high purity), and nitrogen (high purity), all from Airgas; andcis-2-butene (�95% purity) and trans-2-butene (�95% purity), from MathsonTri-Gas. All compounds were used as received.
UV-visible (UV-vis) absorption spectra between 190 and 1,000 nm were re-corded by using a Varian Cary 50 UV-vis spectrometer. Transmission IR absorptionspectra of the catalysts (pressed into pellets) were acquired with a Bruker Tensor27 and a deuterated triglycine sulfate (DTGS) detector. For the colloidal samples,a commercial ZnSe attenuated total reflection (ATR) accessory (PIKE Technolo-gies) was used to minimize the contribution from background water. A spectralresolution of 4 cm�1 was used in all cases.
The TEM experiments were performed on a Philips TECNAI 12 TEM (20–120 kVacceleratingvoltage).Thecolloidal samplesweredepositeddirectlyontoCugridscovered with a holey carbon support film (Ted Pella), and the catalysts weredispersed in Milli-Q water by ultrasonication and deposited from the resultingsuspension onto the Cu grids.
The XPS data were collected in an ultrahigh vacuum chamber equippedwith an Al K� (h� � 1,486.6 eV) x-ray excitation source and a Leybold EA11
Fig. 6. Kinetic data for the conversion of cis- and trans-2-butenes withhydrogen on catalysts prepared by impregnation of 1.0 wt % Pt nanoparticleson silica xerogel as a function of calcination temperature. Shown are the ratiosof the initial rates for the conversion of the trans to the cis isomer vs. those ofthe cis-to-trans isomerization for catalysts made with tetrahedral and withcubic particles. The rates of both cis-to-trans and trans-to-cis conversions arecomparable for the cubic particles over the calcination temperature rangestudied here, but with the tetrahedral particles, a dramatic switch in selectivityis seen at �500 K.
Fig. 7. TEM images from a catalyst prepared by using tetrahedral colloidalparticles, before and after being used for the catalytic conversion of 2-butenes inexperiments similar to those reported in Fig. 6. The initial catalyst corresponds toa 1.0 wt % load of Pt nanoparticles on a silica xerogel after calcination to 475 K.The similarity of the two images suggests minimal deterioration of the shape ofthe supported Pt particles upon their exposure to the catalytic conditions.
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electron energy analyzer with multichannel detection (66). A constantbandpass energy of 100.8 eV was used, corresponding to a spectral reso-lution of �2.0 eV.
The catalytic activity of the Pt-supported catalysts was investigated by using a150-ml stainless-loop batch reactor (67, 68). The composition of the gas mixturewas determined periodically by gas chromatography, using a 23% SP-1700 on
80/100 Chromosorb PAW column and a flame ionization detector. Additionaldetails on the operation of the batch reactor are provided in SI Materials andMethods.
ACKNOWLEDGMENTS. Funding for this work was provided by the NationalScience Foundation.
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