SCR

Two-stage catalyst system for selective catalytic reduction of NO x byNH 3at low temperaturesMin Kang a ,Dae Jung Kim b ,*,Eun Duck Park c ,Ji Man Kim a ,Jae Eui Yie d ,Seoung Hyun Kim e ,Louisa Hope-Weeks b ,Edward M.Eyring faDepartment of Chemistry and Sungkyunkwan Advanced Institute of Nano Technology,Sungkyunkwan University,Suwon 440-746,South KoreabDepartment of Chemistry and Biochemistry,Texas Tech University,Lubbock,TX 769409,USAcDepartment of Chemical Engineering and Division of Energy Systems Research,Ajou University,Suwon 443-749,South KoreadDepartment of Applied Chemistry,Ajou University,Suwon 443-749,South KoreaeDepartment of Environmental Engineering and Biotechnology,Myongji University,Yongin 449-728,South KoreafDepartment of Chemistry,University of Utah,Salt Lake City,UT 84112,USAReceived 27October 2005;received in revised form 30June 2006;accepted 19July 2006Available online 6September 2006AbstractThree catalysts,1wt.%Pt/Al 2O 3,20wt.%Cu/Al 2O 3,and 1wt.%Pt–20wt.%Cu/Al 2O 3were evaluated for the oxidation of NO by O 2and the selective catalytic reduction (SCR)of NO x by NH 3in the absence and presence of H 2O.The Pt/Al 2O 3catalyst exhibited the largest values in NO oxidation efficiency and NO x removal efficiency among the three catalysts.The presence of H 2O caused a drop in the NO oxidation and the SCR activities.The SCR performance was significantly improved when Pt/Al 2O 3and Cu/Al 2O 3catalysts were employed sequentially in a reactor.The highest SCR performance was achieved using two separate stages:oxidation of NO by O 2over the Pt/Al 2O 3catalyst followed by reduction of NO 2by NH 3over the Cu/Al 2O 3catalyst.Under the condition with H 2O,the two-stage catalyst system showed NO x removal efficiency over 80%at low temperatures below 2008C,which was the highest value among the catalyst systems used in this study.The physical and chemical properties of the catalysts were determined using nitrogen adsorption,XRD,TPR,and pulse CO chemisorption experiments.#2006Elsevier B.V .All rights reserved.Keywords:NH 3SCR;NO oxidation;Cu catalyst;Pt catalyst;Two-stage catalyst system1.IntroductionNO x including NO and NO 2impacts on the formation of tropospheric ozone,photochemical smog and acid rain,and thus NO x removal from combustion sources has been a major environmental concern for protecting air quality.A large number of research studies addressing the conversion of NO x into N 2and H 2O have focused on developing catalyst systems.Highly effective NO x removal can be achieved by noble metal based three-way catalysts,a technique now widely applied for the purification of automobile emissions.The three-way catalyst exhibits high NO x removal efficiency when a stoichiometric air-to-fuel ratio prevails while it is much lesseffective for controlling NO x emissions from combustion processes,when a large amount of oxygen is present [1,2].One of the most promising techniques for removing NO x in the presence of a great excess of oxygen is selective catalytic reduction (SCR),in which catalysts are generally employed with reducing agents such as hydrocarbons (HCs)or ammonia (NH 3)[3–6].The SCR performance over a catalyst can be improved when both NO and NO 2are present rather than when only NO is present in emissions.Furthermore,the NO x removal efficiency can be enhanced through the combination of an oxidation catalyst of NO to NO 2by O 2and a reduction catalyst of NO x (NO,NO 2)to N 2and H 2O by a reducing agent.Iwamoto et al.[7]reported that combined catalysts based on metal ion-exchanged zeolites were employed with C 2H 4as the reductant in an HC-SCR reaction,but the NO x removal efficiency was slightly improved by maintaining the same operating temperatures of the oxidation and reduction catalysts./locate/apcatbApplied Catalysis B:Environmental 68(2006)21–27*Corresponding author.Tel.:+18067425073x272;fax:+18067421289.E-mail address:kim8682@ (D.J.Kim).0926-3373/$–see front matter #2006Elsevier B.V .All rights reserved.doi:10.1016/j.apcatb.2006.07.013To enhance the SCR performance of the combined catalyst system,the oxidation and reduction catalysts should be operated at different temperatures to show their maximum activity because of the differences in the catalytic character-istics of the two catalysts.Recently,Madia et al.[8]applied this idea to an NH3-SCR process.They found that NO x conversion was increased when Pt/cordierite was used as an oxidation catalyst and V2O5–WO3/TiO2was employed as a reduction catalyst.Epling et al.[9]showed that the efficiency of NO x conversion was increased by using NO x storage/reduction (NSR)catalyst in which NO x storage materials such as Ba were added.Kim et al.[10]reported that NO x removal efficiency was increased using a combined system consisting of a pulse corona discharge and a Co-ZSM-5catalyst.They found that NO2 formed by the pulse corona discharge improved the NO x removal over the catalyst.Gieshoff et al.[11]claimed that the addition of the Pt-based oxidation catalyst to the V2O5based SCR catalyst increased the NO x removal efficiency.The present work reports a combined catalyst system having a high SCR performance at low temperatures through the combinations of the three catalysts,Pt/Al2O3,Cu/Al2O3,and Pt–Cu/Al2O3.Characteristics required for oxidation of NO by O2and conversion of NO x by NH3over the catalysts are studied. The impact of water vapor on the NO x oxidation and SCR performances of the catalyst system is investigated.The physical and chemical properties of the catalysts were evaluated using nitrogen adsorption,X-ray diffraction (XRD),temperature-programmed reduction(TPR)and pulse CO chemisorption experiments.2.Experimental2.1.Catalyst preparationAbout1wt.%Pt/Al2O3was a commercial catalyst (Aldrich),and it was used in this study after calcination in air at4508C for2h.About20wt.%Cu/Al2O3was prepared by an incipient wetness impregnation method.An ethanol solution containing copper nitrate hydrate(Aldrich,Cu(NO3)2Áx H2O) was mixed with g-alumina(BET surface area=201.9m2/g), and the mixture was evacuated in a rotary evaporator at608C for2h.The slurry obtained was dried at1008C for24h,and calcined in air at5008C for4h.The alumina supported Pt–Cu catalyst was prepared with the co-impregnation of1wt.%Pt and20wt.%Cu on the g-alumina.Precursors of Pt and Cu were copper nitrate hydrate and tetraammineplatinum nitrate (Aldrich,Pt(NH3)4(NO3)2).The drying and calcination conditions for the slurry obtained from this co-impregnation were the same as for the incipient wetness impregnation method.2.2.Characterization of catalystsNitrogen adsorption experiments were carried out using an Autosorb-1instrument(Quantachrom)at liquid nitrogen temperature.The catalyst samples were preliminarily degassed at1008C overnight.BET surface areas of the catalysts were calculated from the nitrogen adsorption isotherms obtained.X-ray diffraction(XRD)tests were performed on a Rigaku D/MAC-III instrument using Cu K a radiation.The samples were scanned in the2u range of10–808.The XRD patterns of the samples were analyzed by a reference library search.Temperature-programmed reduction(TPR)spectra were recorded with a Micromeritics ASAP2010C TPR instrument equipped with a TCD detector.The TPR experiments were carried out in the range of30–9008C at a temperature ramp of 58C/min in a30ml/minflow of10%H2in Ar.Pulse CO chemisorption tests were performed in the same instrument as used in the TPR tests.The catalyst samples were pre-reduced in a30ml/minflow of H2at1008C for1h.The samples were cooled down to308C,and the0.1cm3pulses of a mixture of5%CO in N2were supplied to the samples until saturation of CO was reached.The amounts of CO chemisorbed were calculated from the data obtained from the pulse CO chemisorption tests.2.3.Activity testsActivity tests for the catalysts were carried out in a tubular quartz reactor(8mm i.d.).Before activity tests,a catalyst sample was treated with a mixture of5%O2in N2(30ml/min) at4008C for2h,and purged with N2at the same temperature for1h.After that,the sample was set to a predetermined temperature for the activity test.The inlet gases supplied to the reactor containing the sample were500ppm NO,500ppm NH3,5%O2,and N2.Water vapor(3.5%H2O)was generated by passing N2through the heated saturator containing de-ionized water,and loaded in the gas stream.Theflow rates of the inlet gases were controlled by massflow controllers(MKS). The gas hourly space velocity(GHSV)for all catalysts and combined catalyst systems was constant set at25,000hÀ1.The concentrations of NO and NO2in the inlet and outlet of the reactor were analyzed using a NO/NO2combustion gas analyzer(Euroton).3.Results and discussion3.1.CharacterizationTable1reports BET surface areas of the three catalysts: 1wt.%Pt/Al2O3,20wt.%Cu/Al2O3and1wt.%Pt–20wt.% Cu/Al2O3.The Pt/Al2O3catalyst had a larger BET surface area than the other catalysts.The BET surface area of the Pt–Cu/ Al2O3catalyst was slightly higher than that of the Cu/Al2O3 catalyst.Fig.1shows XRD patterns for the three catalysts. XRD patterns of Pt species on the Pt/Al2O3catalyst were not detected,indicating that Pt species were well dispersed with small particle sizes.The XRD results of two Cu-based catalysts revealed that CuO crystallites were formed on the catalysts. The average diameters of CuO crystallites were calculated using the widths at half-height of the peaks at2u=35.488in the XRD patterns and the Scherrer equation.As presented in Table1,the average diameter of CuO crystallites formed on theM.Kang et al./Applied Catalysis B:Environmental68(2006)21–27 22Cu/Al 2O 3catalyst is obviously smaller than the Pt–Cu/Al 2O 3catalyst.Fig.2shows TPR profiles of the three catalysts.The Pt/Al 2O 3catalyst exhibited two reduction peaks in the TPR curves.The first peak having a maximum intensity at 1978C is probably due to the reduction of Pt oxides strongly interacting with the support [12,13].The second peak at about 3808C may be attributed to a strong H 2chemisorption on metallic Pt [12].The Cu/Al 2O 3catalyst showed a main reduction peak at 2138C with a shoulder peak at about 1138C.The shoulder peak may be the reduction of well dispersed copper oxides on alumina surface [14,15].The main peak can be attributed to the reduction of bulk CuO particles [15].Interestingly,the Pt–Cu/Al 2O 3catalyst shows the same TPR pattern as the Cu/Al 2O 3catalyst.This result suggests that the TPR profile for the Pt–Cu/Al 2O 3catalyst is dominated by the reduction of Cu oxides.The amount of CO chemisorbed on the Pt/Al 2O 3catalyst,as shown in Table 1,is higher than on the Pt–Cu/Al 2O 3catalyst.Rioux and Vannice [16]reported that the amount of CO chemisorbed was decreased with increasing doping of Cu to a Pt-based catalyst.They insisted that copper species covered the Pt surface and decreased the number of available surface Pt atoms.The results obtained from XRD,TPR,and CO chemisorption tests suggest that Pt species on the Pt–Cu/Al 2O 3catalyst may be covered by copper oxide particles.3.2.Catalytic activityThe oxidation tests of NO to NO 2over the three catalysts:1wt.%Pt/Al 2O 3,20wt.%Cu/Al 2O 3and 1wt.%Pt–20wt.%Cu/Al 2O 3,were conducted in a gas mixture of 500ppm NO,5%O 2,and N 2.Fig.3shows the conversion of NO to NO 2over the three catalysts in the absence and presence of H 2O and the ratio of NO 2/NO x obtained at the thermodynamic equilibrium of the following reaction [8]:2NO þO 2¼2NO 2(1)The molar ratio of NO 2/NO x is largely decreased at tempera-tures over 2008C with decreasing NO 2concentration.Under the feed gas without H 2O,the conversion of NO to NO 2over the Pt/Al 2O 3catalyst is increased with rising temperature in a range of 100–2508C,and it shows the highest value of 70%at 2508C.After that,the conversion is significantly decreased because of the restriction of the thermodynamic equilibrium (Fig.3a).The conversion of NO to NO 2over the Cu/Al 2O 3catalyst is slowly increased with increasing temperature,and the maximum conversion is 4%at 4008C (Fig.3b).The Pt–Cu/Al 2O 3catalyst behaves according to the same pattern as the Cu/Al 2O 3catalyst in the NO oxidation (Fig.3c),indicating that the addition of Pt to the Cu/Al 2O 3catalyst does not make any contribution to enhancing the NO oxidation.This result confirms that most Pt particles may not be exposed at the surface of the Pt–Cu/Al 2O 3M.Kang et al./Applied Catalysis B:Environmental 68(2006)21–2723Fig.1.XRD patterns of the three catalysts:(a)1wt.%Pt/Al 2O 3;(b)20wt.%Cu/Al 2O 3;(c)1wt.%Pt–20wt.%Cu/Al 2O 3.Fig.2.Temperature-programmed reduction of the three catalysts:(a)1wt.%Pt/Al 2O 3;(b)20wt.%Cu/Al 2O 3;(c)1wt.%Pt–20wt.%Cu/Al 2O 3.Table 1BET surface areas,average crystalline diameters of Pt and Cu compounds,and the amounts of CO chemisorbed on the three catalysts:(a)1wt.%Pt/Al 2O 3;(b)20wt.%Cu/Al 2O 3;(c)1wt.%Pt–20wt.%Cu/Al 2O 3CatalystS a (m 2/g)CuO crystallite diameter b (nm)Co chemisorbed c (m mol/g cat.)1wt.%Pt/Al 2O 3148–3820wt.%Cu/Al 2O 39045–1wt.%Pt–20wt.%Cu/Al 2O 3108542a BET surface areas calculated from nitrogen isotherms.b CuO crystallite diameters calculated using the widths at half-height of the peaks at 2u =35.488in the XRD patterns and the Scherrer equation.cThe amounts of CO chemisorbed obtained from the pulse CO chemisorption tests.catalyst.This is consistent with the results of the XRD,TPR,and pulse CO chemisorption experiments described earlier.To elucidate the effect of H 2O on the conversion of NO to NO 2,3.5%water vapor was added to the gas stream.As shown in Fig.3,the introduction of H 2O causes the maximum NOconversion to decrease on a catalyst.The NO conversion for Pt/Al 2O 3is greatly decreased with addition of H 2O at the temperature range of 150–2508C (Fig.3a).However,for two Cu-based catalysts,the NO conversion is almost unchanged in the presence of H 2O at the same temperature range (Fig.3b and c).This result indicates that the Pt catalyst is more affected by H 2O than the Cu catalyst.Crocoll and Kureti [17]have reported that the decrease in the NO conversion on the Pt/Al 2O 3catalyst in the presence of H 2O is attributed to the irreversible H 2O adsorbed on the Pt surface.Mulla et al.[18]have suggested that for the Pt/Al 2O 3catalyst,H 2O may make the migration of impurities from the support to block the Pt sites,and consequently reduce the NO conversion.The conversions of NO and NO x to N 2and H 2O by 500ppm NH 3over the three catalysts in the absence and presence of H 2O are shown in Fig.4.In this study,NO and NO x conversions on a catalyst are defined asNO conversion ð%Þ¼ 1ÀNO outNO in Â100(2)NO x conversion ð%Þ¼ 1ÀðNO out þNO 2out ÞNO in Â100(3)Here,NO in is the concentration of NO at the inlet of the reactor,and NO out and NO 2out are the concentrations of NO and NO 2formed by the oxidation of NO at the outlet of the reactor,respectively.As shown in Fig.4,all catalysts show a similar pattern in NO x conversion in the absence of H 2O.The NO x conversion is increased with temperature,and reaches the largest value at 2008C.The NO x conversion is largely decreased with increasing temperature at temperatures over 2008C.It can be seen in Fig.4that for all catalysts,the NO x conversion is smaller than the NO conversion at temperatures over 2508C.Slavinskaya et al.[19]have reported that ammonia over a catalyst is preferentially reacted with oxygen at high temperatures and converted to NO x such as NO and N 2O.This reaction has been reported to occur more easily over metals such as Pt,Pd,Ni,Fe and Cu [19,20].Therefore,the significant decrease in the NO x conversion at high temperatures can be attributed to the depletion of ammonia and the increase of NO x concentration by the oxidation of ammonia to NO x .Comparing Fig.4a with Fig.4b,the maximum value of the NO x conversion over the Pt/Al 2O 3catalyst in the absence of H 2O is larger than that over the Cu/Al 2O 3catalyst.However,the NO x conversions over the Pt/Al 2O 3catalyst at temperatures below 1508C or over 3508C are lower than over the Cu/Al 2O 3catalyst.The results indicate that the NO x removal window of the Pt/Al 2O 3catalyst is narrower than that of the Cu/Al 2O 3catalyst.In the standard SCR reaction,NO is easily converted to N 2and H 2O by NH 3:4NO þ4NH 3þO 2!4N 2þ6H 2O(4)When both NO and NO 2are present,the following reaction proceeds much more rapidly than the above reaction (4):4NH 3þ2NO þ2NO 2!4N 2þ6H 2O(5)M.Kang et al./Applied Catalysis B:Environmental 68(2006)21–2724Fig.3.NO oxidation over the three catalysts in the absence and presence of H 2O:(a)1wt.%Pt/Al 2O 3;(b)20wt.%Cu/Al 2O 3;(c)1wt.%Pt–20wt.%Cu/Al 2O 3.Wallin et al.[21]have reported that the presence of NO2in the gas mixture enhanced the SCR performance of a catalyst.Sun et al.[22]have also proposed a reaction mechanism for the reduction of a mixture of NO and NO2by NH3.They have investigated that the reaction proceeds swiftly through the formation of ammonium nitrite,NH4NO2,which easily decom-poses to N2and H2O near1008C.Blanco et al.[23]have reported that the SCR reaction is faster and the SCR perfor-mance is higher in a mixture of NO and NO2than in pure NO.Therefore,it can be inferred that the difference in the maximum NO x conversion between the two catalysts is probably due to the difference in the amount of NO2produced from the oxidation of NO by O2.The Pt–Cu/Al2O3catalyst shows lower NO x removal efficiency than the Cu/Al2O3 catalyst.It is probable that the decrease in the dispersion of CuO with the addition of Pt causes the decrease in the SCR performance of the Pt–Cu/Al2O3catalyst.As shown in Fig.4,for the three catalysts the maximum NO x conversions at2008C are slightly decreased in the presence of H2O,but the NO x conversions at temperatures below or above 2008C are significantly decreased with the addition of H2O. Ramis et al.[24]have proposed that ammonia is adsorbed and activated on Lewis acid sites of a catalyst in the standard SCR reaction.Many researchers[24–26]have found that the presence of H2O converts some of the Lewis acid sites of the catalyst to Brønsted acid sites.The loss of the Lewis acid sites by the addition of H2O may cause the decrease in the SCR performance at temperatures below2008C.Moreover,as shown in Fig.3,the presence of H2O inhibits the conversion of NO to NO2at the low temperatures.The side SCR reaction(5) in the presence of H2O is less effective than in the absence of H2O.This may contribute to the decrease in the SCR activity at the low temperatures.Madia et al.[26]has reported that H2O at high temperatures gives rise to the surface concentration of the Lewis acid sites. They have insisted that ammonia is selectively oxidized on the Lewis acid sites,and the SCR selectivity is decreased with the increase of the Lewis acid sites by the presence of H2O. Therefore,the partial loss of SCR activity at temperatures over 2008C could be due to the increase of selective catalytic oxidation(SCO)of ammonia.bined catalyst systemsTo enhance the SCR performance,various combined catalyst systems using two catalysts:1wt.%Pt/g-Al2O3and 20wt.%Cu/Al2O3,were tested.Fig.5shows the SCR performances of two-bed catalyst systems in the absence of H2O.For the catalyst system of Fig.5a(system A),thefirst bed and second bed consisted of the Cu/Al2O3catalyst and the Pt/ Al2O3catalyst,respectively.The catalyst system of Fig.5b (system B)was in the reverse order of Fig.5a.The two catalysts had the same loading in the reactor.Both catalyst systems showed a wider NO x removal window than the single catalyst system having only the Pt catalyst.It is likely that the Cu/Al2O3 catalyst played a key role in the improvement of the NO removal window.The system B,as shown in Fig.5,has a larger value in the maximum NO x conversion than the system A has.This result implies that NO2formed on the Pt/Al2O3catalyst promotes the NO x removal efficiency over the Cu/Al2O3catalyst.To substantiate this speculation,a two-stage catalyst system using the two catalysts was developed as shown in Fig.6.M.Kang et al./Applied Catalysis B:Environmental68(2006)21–2725 Fig.4.NO and NO x conversions over the three catalysts in the absence andpresence of H2O:(a)1wt.%Pt/Al2O3;(b)20wt.%Cu/Al2O3;(c)1wt.%Pt–20wt.%Cu/Al2O3.The Pt/Al 2O 3catalyst was employed as the oxidation catalyst for the oxidation of NO by O 2,and the Cu/Al 2O 3catalyst was used as the reduction catalyst of NO x by NH 3,which was injected after the Pt/Al 2O 3catalyst.The NO and NO x removal efficiencies in the two-stage catalyst system in the absence and presence of H 2O are shown in Fig.7.The reaction temperature in the oxidation catalyst (1wt.%Pt/Al 2O 3)was set at a predetermined temperature between 100and 2508C,and the reaction temperature in theSCR catalyst (20wt.%Cu/Al 2O 3)was subsequently varied in a temperature range of 100–4008C.The NO and NO x removal efficiencies over the Cu/Al 2O 3catalyst in the absence of H 2O were increased with increasing bed temperature of the Pt/Al 2O 3catalyst.According to the result of Fig.3,the oxidation efficiency of NO over the Pt/Al 2O 3catalyst in the absence of H 2O was increased with increasing temperature in a temperature range of 100–2508C.The results,as shown in Fig.7,clearly indicate that the SCR performance over the Cu/Al 2O 3catalyst is significantly influenced by the concentration of NO 2formed over the Pt/Al 2O 3catalyst.When the bed temperature of the oxidation catalyst was maintained at 2508C,the SCR catalyst showed the highest NO and NO x conversions at low temperatures below 2008C.The NO and NO x removal efficiencies of the two-stage catalyst system at low tempera-tures below 2008C was much higher than those of the two-bed catalyst systems shown in Fig.5.To investigate the effect of H 2O on the SCR performance of the two-stage catalyst system,water vapor was supplied to theM.Kang et al./Applied Catalysis B:Environmental 68(2006)21–2726Fig.5.NO and NO x conversions in the two-bed catalyst systems in the absence of H 2O:(a)20wt.%Cu/Al 2O 3in the first bed and 1wt.%Pt/Al 2O 3in the second bed;(b)1wt.%Pt/Al 2O 3in the first bed and 1wt.%Pt/Al 2O 3in the secondbed.Fig.6.The schematic diagram for a two-stage catalyst system:1wt.%Pt/Al 2O 3as an oxidation catalyst of NO by O 2in the first stage and 20wt.%Cu/Al 2O 3as a reduction catalyst of NO x by NH 3in the secondstage.Fig.7.NO and NO x conversions for a two-stage catalyst system with respect to the bed temperature of the oxidation catalyst in the absence and presence of H 2O:(a)1008C without H 2O;(b)2008C without H 2O;(c)2508C without H 2O;(d)2508C with H 2O.gas stream before the oxidation catalyst.The bed temperature of the SCR catalyst was maintained at a determined temperature in the range of100–4008C,while the bed temperature of the oxidation catalyst was set to2508C.As shown in Fig.5,for the two-stage catalyst system the maximum NO x conversion at2008C was unchanged in the presence of H2O,and the NO x conversions at temperatures below and over 2008C were slightly decreased.In particular,in the presence of H2O,the two-stage catalyst system showed NO x removal efficiency over80%at low temperatures below2008C,which was the highest value among the catalyst systems used in this study.4.ConclusionsIn this study,the three catalysts:1wt.%Pt/Al2O3,20wt.% Cu/Al2O3,and1wt.%Pt–20wt.%Cu/Al2O3were prepared to investigate the oxidation of NO and the selective catalytic reduction(SCR)of NO x by NH3in the absence and presence of H2O.The addition of Pt to the Cu/Al2O3catalyst increased the average crystallite size of CuO,and consequently decreased the SCR performance of the Cu/Al2O3catalyst.The Pt/Al2O3 catalyst showed the highest NO oxidation and NO x removal efficiencies among the three catalysts.The addition of H2O inhibited both the NO oxidation and the SCR reaction,and consequently led to a decrease in the SCR performance of the catalyst.The two-stage catalyst system,in which Pt/Al2O3and Cu/ Al2O3catalysts were employed as the oxidation catalyst of NO and the reduction catalyst of NO x by NH3,showed the highest SCR performance at low temperatures below2008C in the absence and presence of H2O among the catalyst systems used in this present work.The SCR performance of the two-stage catalyst system was strongly influenced by the concentration of NO2formed over the oxidation catalyst.AcknowledgementsDr.Dae Jung Kim and Prof.Edward M.Eyring are grateful for partialfinancial support by the U.S.Department of Energy through the Consortium for Fossil Fuel Science(Contract 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