Interaction of gas phase atomic hydrogen with Pt(111): Direct evidence for the formation of bulk hyd Interaction of gas phase atomic hydrogen with Pt(111): Direct evidence for the formation of bulk hyd

Interaction of gas phase atomic hydrogen with Pt(111): Direct evidence for the formation of bulk hyd

  • 期刊名字:中国科学B辑(英文版)
  • 文件大小:156kb
  • 论文作者:JIANG ZhiQuan,HUANG WeiXin,BAO
  • 作者单位:Department of Chemical Physics,State Key Laboratory of Catalysis
  • 更新时间:2020-09-15
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论文简介

Science in China series b: Chem2007in China presser verlaInteraction of gas phase atomic hydrogen with Pt(111):Direct evidence for the formation of bulk hydrogenspeclesJIANG ZhiQuan. HUANG WeiXin't bAo XinHeDepartment of Chemical Physics, University of Science and Technology of China, Hefei 230026, China2 State Key laboratory of Catalysis Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, ChinaEmploying hot tungsten filament to thermal dissociate molecular hydrogen, we generated gas phaseatomic hydrogen under ultra-high vacuum(UHV)conditions and investigated its interaction with Pt(111)surface. Thermal desorption spectroscopy (TDs)results demonstrate that adsorption of molecularhydrogen on Pt(111)forms surface Had species whereas adsorption of atomic hydrogen forms not onlysurface Had species but also bulk Had species. Bulk Had species is more thermal-unstable than surfaceHad species on Pt(111), suggesting that bulk Had species is more energetic. This kind of weakly-adsorbed bulk Had species might be the active hydrogen species in the Pt-catalyzed hydrogenationreactionsPt(111) surface, bulk H species, thermal desorption spectroscopyHeterogeneous catalysis plays important roles in chemi- cause the concentration of the formed activecal industry and environmental protection, however, too low to be detected under UhV conditions. Thereforbecause of its complexity, most fundamental under- the key to overcome the "pressure gap"is to form destanding of heterogeneous catalysis is achieved by in- tectable active species in the practical catalytic reactiovestigating catalytic reactions over model catalysts(sur- on the surface of model catalysts under UHV conditionsface science studies) under ultra-high vacuum(UHVPt, Pd, and Ni catalysts demonstrate excellent cata-conditions. Although surface science studies can give lytic activity for the hydrogenation of unsaturated hyunambiguous information on the investigated system drocarbons. It has been reported that the raney ni cata-nism and reaction kinetivarious gaps exist between the conditions employed in ethylene and that its activity is directly related with theface science studies and those in practical heteroge- Had content within the catalyst, 7. But the surface hadneous catalytic reaction, one of which is the great dif- species formed via the adsorption of molecular hydrogenference in the employed reaction pressures, namely, the on Ni(lll) cannot react with co-adsorbed ethylene to"pressure gap"existing between surface science studies produce ethane under UHV conditions. Ceyer et al. 8-121of model catalysts and practical heterogeneous catalytic employed highly-energetic gas phase atomic hydrogenreactions-3. Many heterogeneous catalytic reactions to interact with Ni(111) under UHV conditions, whichcan facilely occur under certain reaction pressure, butcannot under UHV conditions 4. 5. The"pressure gap'doi:10.1007/s11426-007-2029-xarises mainly because the reactants cannot form theCorrespondingauthor(email:huangwx(@ustc.edu.cn)tive species in the practical catalytic reaction on the sur-ported by the National Natural Science Foundation of China (Grant Noface of model catalysts under UHV conditions, or be-0503027), Talent Program of Chinese Academy of Sciences, and China Postdotoral Science Foundation( Grant No. 2005038479)www.scichina.comwww.springerlink.comSci China SelYH中国煤化工CNMHG5OInO. 1191-96bulk); comparing surface Had species, bulk Had species heating rate of 5 K/s was employedinteracts with Ni(lll) more weakly; but the reactionHighly-pure gases(H2 and O2) were admitted in theresults proved that bulk Had species in Ni(111) shows UHV chamber via the controllable leak valve. One leakhigh catalytic activity for hydrogenation and is thevalve for H2 dosing ended with a stainless steel tubetive hydrogen species for Ni-catalyzed hydrogenation with an outer diameter of 3 mm to direct the gas, in frontreaction. These results for the first time demonstrate that of which a home-made tungsten coil(tungsten filamentthe active species in practical catalytic reactions can be diameter: 0.2 mm; coil diameter: 4 mm; coil length: 3lyprepared under UHV conditions by emcm)was positioned. The tungsten coil could be controlploying highly-nergetic reactants to interact with model lably heated by a power supply with the constant-currentcatalysts. And in such a way it is likely to overcome the mode. The Pt(111) surface was positioned about 5 cm inpressure gap "so that the molecular-level understanding front of the tungsten coil when exposed to H2. Duringof heterogeneous catalysis can be achieved. Experimen- the exposure of gas phase atomic hydrogen, H2 was ad-tal results of catalytic hydrogenations over Pt and pd mitted into the UHV chamber to a desirable pressure andmodel catalysts also suggest that weakly-adsorbed Had then the tungsten coil was heated so that H2 would bespecies, instead of strongly-adsorbed surface Had Species, cracked into atomic hydrogen when passing through theis the active species). Under UHV conditions, un- hot tungsten coil. We could not measure the temperaturesaturated hydrocarbons do not hydrogenate with surface of the tungsten coil due to the lack of thermocouple at-Had species on Pd(111)and Pd(100)7 8, but the same tached to the tungsten coil. Therefore, the tungsten coilhydrogenation reaction can proceed on Pd nanoparticles, temperature was expressed in a relative value by theon which weakly-adsorbed subsurface Had species, applied voltage and current of the power supply. It isplaying a key role in the catalytic hydrogenation reac- noteworthy that the sample temperature might increasetion, can form via molecular hydrogen adsorption 5,6. by 10-15 K because of the irradiation of the hot tungSo far there is no report studying the weakly-adsorbed sten coil during the exposure of gas phase atomic hy-Had species on Pt surfaces. In this work, we employed drogen. The exposure herein was reported in Langmuirthermal desorption spectroscopy(TDs) to study the in-(L)(1 L=1.33x10 Pas)teraction of highly-energetic gas phase atomic hydrogenwith Pt(111)surface and reported for the first time the 2 Results and discussiondirect evidence of the formation of weakly-adsorbedbulk Had species on Pt(ll1)The concentration variation of H2 in gas phase wasmonitored by QMs during the exposure of gas phase1 Experimentalatomic hydrogen(Figure 1). After the admission andstabilization of H2 in the UHV chamber with a pressureExperiments were performed in an ELS-22 electron en- of 80x10 Pa, quickly heating the tungsten coil(heat-ergy loss spectrometer(Leybold-Heraeus)equipped with ing voltage: 20.7 V, heating current: 6.0 A)quickly de-an Auger electron spectroscope(AES), quadrupole mass creased the concentration of gas phase H2, which soonspectrometer(QMS). The base pressure of the UHV reached equilibrium again. This demonstrates that achamber was -5x10 Pa, which could be quickly im- fraction of H, undergoes thermal-dissociation and formsroved by a Ti sublimation pump a Pt(lll) sample was gas phase atomic hydrogegen when passing through themounted on the sample holder by two 0.5 mm-thick Ta hot tungsten coil. Ceasing the heating immediately ter-filaments. The sample temperature, measured by the minates the thermal-cracking reaction of H2 and theNiCr-NiAl thermocouple welded on the backside of the concentration of gas phase H2 restores to the originalsample, could be controlled between 100 and 1200 K via value(the H2 concentration changes slightly before andresistively heating and liquid N2 cooling. Prior to ex- after heating the tungsten coil because of the drift of theperiments, the Pt(lll) was cleaned by repeated cycles of leak valve). As shown in Figure 1, the hot tungsten coiloxidation, ion sputtering and annealing at 1200 K until (heating voltage: 20.7 V, heating current: 6.0 A)is notAES could not detect any contaminants. During TDs very efficient to thermal crack H2 to produce gas phaseJIANG ZhiQuan et al. Sci China Ser B-Chem February 2007中国煤化工CNMHGatomic hydrogen(another likely reason accounting forhydrogen recombines to form H2 during its athe low efficiency is that the formed gas phase atomH/Pt(lll)Therefore when the tungsten coil was heated, the dosedgas was actually the mixture of gas phase atomic hydrogen and molecular hydrogen. The curve shown inFigure 1 could be reproduced very well, thus the experimental condition shown in Figure I was adopted forle exposure of gas phase atomic hydrogen throughoutour experiments. The exposure of gas phase atomic hydrogen was calculated by the H2 pressure timing thelasting time of the hot tungsten coil. aes did not detect200300400500600the tungsten on Pt(lll) even after long time exposure ofT(K)gas phase atomic hydrogen, suggesting that no tungstenFigure 2 H2-TDS spectra following various exposures of H on Pt(lll)contaminant occurred under our experimental condi- at 140 K Exposure of H2: a, O L; b, 0.3 L;c,1.0 L;d, 3.0 L; e,10 L; f,50L,g,100Lh,300Ltionsdesorption peak at various exposuresxposure of molecular hydrogen. Fithe H2-TDS spectra following various exposures of gasphase atomic hydrogen on Pt(lll) at 120 K. ThebH2-TDS spectrum following small exposure of gas phaseatomic hydrogen (3.0 L) is similar to those followingexposure of molecular H,, suggesting the formation ofsurface Had species. Increasing the exposure of gasphase atomic hydrogen to 30 L results in the saturationof surface Had species on Pt(111); meanwhile, a newdesorption feature appears at 176 K. This new desorpTition feature is assigned to the recombination of a newHad species denoted as weakly-adsorbed Had species be-Figure 1 The concentration variation of molecular hydrogen in the UHVhamber during the exposure of gas phase atomic hydrogen. Heatingcause it is less thermal-stable than surface Had speciesoUtage of the tungsten coil: 20.7 V, heatingof the tungsten coilThe weakly-adsorbed Had species is not observed when6.0A.molecular hydrogen adsorbs on Pt(111), indicating thatthe interaction of gas phase atomic hydrogen with Pt(111)Figure 2 presents the H2-TDS spectra after various leads to the formation of the weakly-adsorbed Had speexposures of H2 on Pt(lll)at 140 K. After a dose of 0.3 cies. The desorption peak corresponding to theL H2, the H2-TDS spectrum gives a single symmetric weakly-adsorbed Had species grows continuously withdesorption peak at 430 K; with the increasing H2 expo- the increasing exposure of gas phase atomic hydrogensure, the desorption peak grows and the desorption tem- and does not saturate even after an exposure of 300 Lperature shifts to lower temperature. These observations gas phase atomic hydrogen. This observation stronglydemonstrate a typical two-order desorption kinetics, suggests that the weakly-adsorbed Had species is locatedproving that H2 dissociatively chemisorbs on Pt(lll) to on the subsurface or bulk of Pt(111). As shown in Figureform surface Had species that, upon heating, recombines 3(a), both the initial desorption temperature and the peakto form H2 desorbing from the surface). The surfaceThe surface temperature of the H2-TDS spectra slightly shift toHad species tends to saturate after a dose of 600 L H2, higher temperature with the increasing exposure of gasand the corresponding TDS spectrum gives a single de- phase atomic hydrogen. This is because the samplesorption peak at 334 K.temperature increases slightly due to the irradiation ofThe H2-TDS spectra following exposure of gas phase the hot tungsten coil during the continuous TDs ex-atomic hydrogen to Pt(lll)differ from those following periments. And the longer the dosing time, the higherJIANG ZhiQuan et al. Sci China Ser B-Chem February 200V凵中国煤化工CNMHGH/Pt(lll!Pt()18 amu2 amuH+, exposure300L4006008001000|20020040060(7(K)T《K)Figure 3 TDS spectra following various exposures of (H+H2)on Pt(111)at 120 K. Heating voltage of the tungsten: 20.7 V, heating current of the tunsten coil: 6.0Asample temperatureassign the weakly-adsorbed species observed in Tds toThe desorption peak of water from the surface was bulk Had species. Bulk Had species recombinatively de-also detected at a dose of gas phase atomic hydrogen sorbs from Pt(1l1) at a temperature at least 120 K lowerlarger than 30 L, located at 298 K(a dose of 100 L) and than surface Had species, demonstrating that bulk Hadat 216 K(a dose of 300 L), as shown in Figure 3(b). species has a smaller desorption activation energy thanThese two desorption peaks correspond to the desorption surface Had species, namely, bulk Had speecies is moreof dissociatively-adsorbed water and associatively- energetic than surface Had species. Thus it is anticipatedadsorbed water from Pt(111), respectively/20 We pro- that bulk Had species behaves a highepose that the water desorption peak arises from adsorp ity than surface Had species. Bulk Had species is 63tion of water on Pt(1ll) coming from the chamber wall kJ/mol more energetic than surface Had species onwarmed by the heat transfer from the hot tungsten coil Ni(lll), and the reaction results confirm that bulk hadduring the long-time exposure of gas phase atomic hy- species is the true active species for the catalytic hydro-drogen. Comparison between Figure 3(a)and(b)clearly genation reaction[ 2. Indirect evidence also suggestedshows that, following a dose of 100 L gas phase atomic that weakly-adsorbed Had species, instead ofhydrogen, the H2 desorption peak corresponding to the strongly-adsorbed surface Had species, be active for theeakly-adsorbed Hhydrogenation reactions catalyzed by Pt catalysts 31whereas only a weak water desorption peak appears at Our results for the first time evidence the formation of298 K. This proves that the observed H2 desorption peak weakly-adsorbed bulk Had species on Pt(111)of weakly-adsorbed Had species does not result from theLegare also calculated the phase diagram of Pt-Hfragmentation of the desorbed water.at various H2 pressure and concluded that subsurThe TDs results show that, under UHV conditions, face/bulk Had species could form on Pt(lll) under pracexposing Pt(lll) to gas phase atomic hydrogen forms tical hydrogenation reaction conditions. However, ex-not only strongly-adsorbed surface Had species but also perimental results clearly show that H2 adsorption onweakly-adsorbed bulk Had species, whereas exposing Pt(lll)only forms surface Had species under UHV conPt(lll) to molecular hydrogen only results in the forma- ditions. Actually this reflects the existence of"pressuretion of strongly-adsorbed surface Had species. Legare!) gap"between studies under UHV conditions and thoseemployed density functional theory to calculate the under practical reaction conditions. Upon H2 adsorptionHad/Pt(111)system, and the results suggest that Had the activation barrier for the formation of bulk Had spepreferentially occupies the most stable surface sites; cies is higher than that of surface Had species 2.Underhowever, beyond 1 ML(monolayer) Had, Had will dis- UHV conditions, the energy of H2 can only afford thesolve into the subsurface/bulk of Pt(lll). Therefore, we formation of surface Had species and is not enough forJIANG ZhiQuan et al. Sci China Ser B-Chem February 2007 voltHe中国煤化工CNMHGthe formation of Had species; or even though bulk Had into gas phase atomic hydrogen can be improved by inspecies can be formed, its concentration is too low to be creasing the heating power applied to the tungsten coildetected. However, even though the ratio of H2 mole- so as to prepare enough bulk Had species at low H2 ecules being energetic enough to form bulk Had species in posures and eliminate the water desorption signalsthe total H2 molecules under practical reaction condi- shown in Figure 3(b). Meanwhile, because the sampletions is the same as that under UHV conditions, enough temperature was not affected much by irradiation of thebulk Had species can be formed to participate the hydro- hot tungsten coil, the desorption temperature of bulk Hadgenation reaction because of the huge amounts of the species downshifts to lower temperature with the intotal H2 molecules under practical reaction conditions. creasing desorption amounts, which demonstrates theThe case is different when gas phase atomic hydrogen, second-order kinetics of the recombinative desorption ofinstead of molecular hydrogen, is employed as reactants. bulk Had speciesBecause gas phase atomic hydrogen is 218.4 kJ/molmore energetic than molecular hydrogen(1/2 dissocia100L.P(I)tion energy of H2), when interacting with Pt(111),gasphase atomic hydrogen is energetic enough to overcomeany activation energy to form all likely adsorbed Hadspecies (surface Had species and bulk Had species)Tungsten filamentsurely including the active species for practical catalyticheating powerreactions. Gland et al. 21-25 found that gas phase atomic20.7V,6.0Ahydrogen can undergo ring-opening reaction directly179V5.5Awith cycloalkanes adsorbed on metal single crystal surfaces and that the ring-opening reaction followsEley-Rideal mechanism. Therefore, even under UHV200300405006007008009001001001200conditions, interaction of gas phase atomic hy7(k)with Pt(lll)can lead to the formation of detectable bulkFigure 4 TDS spectra following a dose of 100 L H2 passing through theHhad species. The fact that employing gas phase atomic tungsten coil heated with different powershydrogen can prepareclecatalytic hydrogenation reactions on the model catalysurface under UHV conditions suggests that highl3 Conclusionsenergetic reactants (gas phase atomic hydrogen, gasphase atomic oxygen, radicals, etc. )can be employed toGas phase atomic hydrogen was successfully generatedinteract with model catalysts to prepare actual active via the thermal cracking of molecular hyoogen underspecies for realistic catalytic reactions under UHV con- UHV conditions. Interaction of gas phase atomic hyditions so that the""pressure gap"might be overcomedrogen with Pt(111) forms not only surface Had speciesand molecular-level understanding of heterogeneousbut also bulk Had species whereas molecular hydrogencatalysis can be achievedadsorption only forms surface Had species on Pt(lll)Finally we compared the H2-TDS spectra following a The desorption activation barrier of bulk Had Species isdose of 100 L H2 passing through the tungsten coilsmaller than that of surface Had species, demonstratingheated by various heating powers(Figure 4 ) a dose of that bulk Had species is more energetic and chemically100 L H2 only forms surface Had species on Pt(111). reactive than surface Had species In our future work, weHeating the tungsten coil(heating voltage: 17.9 V; heat- will investigate the hydrogenation activity of surface hading current: 5.5 A)gives rise to an obvious desorption species and bulk Had species on Pt(111)and identify thepeak originating from bulk Had species in the TDS spec- actual active hydrogen species so as to understand thetrum; further increasing the heating power (heating reaction mechanism in the catalytic hydrogenation reac-voltage: 20.7 V; heating current: 6.0 A)intensifies the tions at the molecular leveldesorption peak originating from bulk Had species. 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