selective partial hydrogenation 
 of multi - unsaturated hydrocarbons , 
 particularly ,-unsaturated ketones and aldehydes , is 
 of a pivotal importance for numerous applications of heterogeneous 
 catalysis related to fine chemical and pharmaceutical industries . 
 among the most useful target products in transformations of multi - unsaturated 
 oxygenates
are unsaturated alcohols produced by heterogeneous chemoselective 
 hydrogenation of the c = o bond in unsaturated ketones and aldehydes .
generally , thermodynamics favors hydrogenation of the c = c 
 bond in these compounds to form the unwanted product , i.e. , saturated 
 aldehydes or ketones .
therefore , chemoselective hydrogenation of the 
 c = o bond requires manipulation of kinetic effects by means 
 of a suitable catalyst .
a variety of powdered supported 
 metal catalysts have been already 
 investigated to understand the activity and selectivity of this reaction . as key structural parameters controlling the chemoselectivity , a 
 series of ideas conceiving the amount of steric hindrance to adsorption 
 via the c = c bond , presence of surface 
 modifiers , or alloying with other metals have been put forward .
complementarily , some 
 theoretical work and model studies have been presented in order to 
 understand the chemoselectivity of the underlying elementary processes . despite these efforts , a deep fundamental understanding of this 
 reaction and the parameters governing its activity and selectivity 
 is still missing .
particularly , it remains unclear how the c = o 
 bond is activated on a transition metal surface and what are the structures 
 of the surface intermediates formed under the reaction conditions . 
 to extract this information on the reaction intermediates
would be 
 particularly important for approaching a rational design of new catalytic 
 materials for this class of reactions . in this article , we present 
 a mechanistic study on selective hydrogenation 
 of the smallest ,-unsaturated aldehyde acrolein over 
 model pd surfaces  both single crystal pd(111 ) and pd nanoparticles 
 supported on an planar oxide support  under well - defined ultrahigh - vacuum 
 ( uhv ) conditions .
previously , acrolein was reported to be hydrogenated 
 almost exclusively on the c = c bond over powdered pd catalysts 
 forming propanal . in this article , we 
 show for the first time that selective hydrogenation of the c = o 
 bond in acrolein with nearly 100% selectivity is possible over pd(111 ) . 

however , this process requires a very distinct modification of the 
 surface with a spectator species that turns the surface selective 
 toward unsaturated alcohol formation .
this densely packed overlayer 
 of spectator species is formed from acrolein during the initial stages 
 of surface reaction . by applying a combination of multimolecular beam 
 techniques and in situ infrared reflection 
absorption 
 spectroscopy ( iras ) , we were able to identify the chemical nature 
 of the spectator and the reactive surface intermediate and for the 
 first time experimentally follow the simultaneous evolution of the 
 reactive intermediate on the surface and formation of the product 
 in the gas phase . with this study
it has been possible to directly 
 identify the reaction intermediate that leads to the formation of 
 the unsaturated alcohol as a final product and obtain atomistic - level 
 insights into the chemoselective hydrogenation chemistry of acrolein . 

spectator species were found to play an important role in governing 
 chemoselectivity  the observation that might be of great interest 
 for development of new chemo- and enantioselective powdered catalysts 
 such as e.g. ligand - modified nanoparticles .
all experiments 
 were performed at the fritz - haber - institut , berlin , 
 in a uhv apparatus that has been described in detail previously .
two effusive doubly differentially pumped multichannel 
 array source operated at room temperature were used to supply acrolein 
 and h2 .
beam intensities of 4.8  10 molecules 
 cm s for h2 and 
 0.6  10 , 1.5  10 , or 4.5  
 10 molecules cms for acrolein ( fluka , 98% purity ) were used in these experiments .
the pd(111 ) single crystal was cleaned prior to use by repeated 
 cycles of ar ion bombardment at room temperature , annealing 
 at 1000 k and oxidation in 1  10 mbar of 
 o2 at 750 k to remove residual carbon .
the supported 
 pd / fe3o4 model catalyst was 
 prepared as follows : the thin ( 100  ) fe3o4 film was grown on a pt(111 ) single crystal surface by repeated 
 cycles of fe ( > 99.99% , goodfellow ) physical vapor deposition and 
 subsequent 
 oxidation .
pd particles ( > 99.9% , goodfellow ) 
 were grown by physical vapor deposition using a commercial evaporator 
 ( focus , efm3 , flux calibrated by a quartz microbalance ) while keeping 
 the sample temperature fixed at 115 k. during pd evaporation the sample 
 was biased to 850 v in order to avoid the creation of defects by metal 
 ions .
the resulting surfaces were then annealed to 600 k and stabilized 
 via cycles of oxygen ( 8  10 mbar for 1000 
 s ) and co ( 8  10 mbar for 3000 s ) exposures 
 at 500 k until the pd particles reached a stable geometry with an 
 average size of 12 nm .
residual oxygen was removed by 
 heating in co ( 1  10 mbar for 300 s ) followed 
 by flash - annealing in uhv to 485 k. the stm image of the resulting 
 pd / fe3o4/pt(111 ) model catalyst is shown in 
 figure si-1 of the supporting information .
that surface displays pd particles with an average diameter of 
 12 nm containing approximately 8100 atoms each and covering the support 
 uniformly with an island density of about 6  10 islands cm .
the majority of the particles are 
 well - shaped crystallites grown in the ( 111 ) orientation and are predominantly 
 terminated by ( 111 ) facets ( 80% ) , but a small fraction of 
 ( 100 ) facets ( 20% ) are also exposed .
iras data were 
 acquired using a vacuum fourier - transform infrared 
 ( ft - ir ) spectrometer ( bruker ifs 66v / s ) with a spectral resolution 
 of 2 cm and using a mid - infrared ( mir ) polarizer 
 and p - polarized ir light .
an automated quadrupole mass spectrometer 
 ( qms ) system ( abb extrel ) was employed for the continuous monitoring 
 of the partial pressures of the reactants ( acrolein : parent ion at 
 56 amu ; h2 : at 2 amu ) and products ( propanal : parent ion 
 and main fragment at 58 amu ; propenol : parent ion at 58 amu , main 
 fragment at 57 amu , and further prominent fragment at 31 amu ; propanol : 
 parent ion at 60 amu and main fragment at 31 amu ) .
selective partial hydrogenation of acrolein was investigated on 
 two well - defined surfaces prepared in situ under 
 uhv conditions : ( i ) a single crystal pd(111 ) and ( ii ) 12 nm sized 
 pd nanoparticles supported on a planar model fe3o4/pt(111 ) oxide support .
the catalytic 
 activity of these surfaces was probed under isothermal conditions 
 by using molecular beams with the simultaneous 
 monitoring of the species evolving on the catalytic surface turning 
 over by iras .
the two investigated catalyst systems were found 
 to show very different 
 selectivity in partial acrolein hydrogenation .
figure 1 shows the formation rates of competing reaction 
 pathways resulting in selective hydrogenation of either the c = c bond to form propanal ( figure 1a , b ) or the c = o 
 bond 
 to form the unsaturated alcohol ( figure 1c , d ) on both model pd nanoparticles ( left 
 side ) and pd(111 ) ( right side ) at different temperatures . for each 

reported curve , the surface was pre - exposed to a high flux continuous 
 h2 beam . at time
zero the second molecular beam was opened 
 to give a series of acrolein pulses , and the formation rates of reaction 
 products were recorded in the gas phase by quadrupole mass spectrometry . 
 for all investigated catalysts , a short induction period preceded 
 the onset of product formation . on the pd nanoparticles we observed 
 the only product 
propanal  in the first few pulses , after 
 which the reaction rate dropped to zero ; no hydrogenation of c = o 
 bond was detected .
this behavior is in a good agreement with the results 
 of the earlier studies on powdered pd catalysts under ambient conditions , 
 showing that essentially only the c = c bond can be hydrogenated . in separate experiments carried out on the pristine fe3o4 support
very surprisingly , pd(111 ) 
 showed very high catalytic activity toward the desired reaction product 
the propenol 
 formation rate exhibits clear temperature dependence with a maximum 
 of conversion at 270 k. at this temperature , about 40% of incoming 
 acrolein molecules were converted to propenol .
note that the most 
 likely product of c = o bond hydrogenation is allyl alcohol ( h2c = ch  ch2oh ) and not methyl 
 vinyl alcohol ( ch3ch = ch  oh ) as the 
 latter one could be transformed to propanal in the gas phase via keto 
since we do not observe any propanal evolution pattern 
 coinciding with unsaturated alcohol formation , the production of less 
 stable methyl vinyl alcohol is rather unlikely . to obtain further 
 atomistic - level insight into this catalytic process
, 
 we carried out this experiment on the pd(111 ) surface using a continuous 
 exposure of both h2 and acrolein via molecular beams and 
 simultaneously recorded the evolution of the species on the surface 
 turning over by iras .
propanal 
 ( upper 
 row ) and propenol ( lower row)on 12 nm sized supported pd nanoparticle 
 ( a , c ) and pd(111 ) ( b , d ) during continuous dosing of h2 and pulsed dosing of acrolein at different temperatures .
figure 2a shows 
 the evolution of the gas phase products  propanal and propenol  over 
 pd(111 ) with a continuous reactants exposure at 270 k. specifically , 
 the surface was pre - exposed with h2 , and then the acrolein 
 beam was switched on at time indicated as zero while the h2 beam was kept continuously running .
simultaneously , the acquisition 
 of ir spectra started with the time resolution of one spectrum per 
 45 s. selected ir spectra are shown in figure 2b ( labeled 1 to 4 ) , which correspond to the 
 time regions 14 indicated in figure 2a . in accordance with the reactivity behavior 
 in a pulsed experiment ( figure 1d ) , an onset of the propenol formation rate is observed after 
 an induction period of 24 s , in which about one acrolein molecule 
 per four pd atoms on average is irreversibly adsorbed onto the surface . 
 at the end of the induction period the propenol formation rate
quickly 
 rises followed by a reactivity decrease after 95 s. the first 
 three ir spectra ( 13 ) shown in figure 2b are obtained during the initial induction 
 period and the period of the maximal activity , while spectrum 4 was 
 collected during the period of decreasing reactivity .
the lowest ir 
 spectrum in figure 2b shows the composition of the surface after its complete deactivation . 
 as a reference for an intact molecule , the ir spectrum of acrolein 
 adsorbed on pd(111 ) at 100 k
analysis of the ir spectra obtained on the pd(111 ) surface under 
 reaction conditions allows us to determine the composition of the 
 active surface turning over and the nature of the reactive intermediate , 
 resulting in formation of propenol .
first , the bands at 
 1660 and 1755 cm correspond to the stretching 
 vibration of the c = o bond .
the vibrational band of c = c bond in acrolein 
 ( 1625 cm ) lies very close to the c = o vibration 
 and appears just as a weak shoulder .
while 
 the band at 1660 cm originates from the adsorbed 
 intact acrolein molecule , in which the c = o bond is still conjugated 
 to the c = c double bond ( also see comparison with the uppermost spectrum 
 of intact acrolein ) , the higher frequency band at 1755 cm is indicative of the surface species containing the c = o bond 
 not conjugated to the c = c bond anymore .
the appearance of this vibration under reaction conditions points 
 to the formation of the oxopropyl surface species , resulting from 
 the partial hydrogenation of acrolein molecule with only one h atom 
 attached to the c = c bond .
one of the possible structures of 
 this species is shown in figure 2b next to the vibrational band at 1755 cm .
the present data do not allow us to make a more precise conclusion 
 on whether the original acrolein molecule was partly hydrogenated 
 on the second or third carbon atom to form this species .
remarkably , 
 this band already appears at very early stages of the reaction , grows 
 in intensity , and remains intense even after the reaction rate recorded 
 in the gas phase vanishes .
this observation strongly suggests that 
 this species is not the reaction intermediate leading to the final 
 gas phase product propenol but is merely a spectator ( referred in 
 the following as spectator i ) .
( a ) formation rate of propenol ( blue line ) 
 and propanal ( black 
 line ) on pd(111 ) at 270 k under continuous exposure of h2 and acrolein .
( b ) ir spectra obtained for a monolayer of molecularly 
 adsorbed acrolein at 100 k on pristine pd(111 ) ( green line ) and on 
 pd surface turning over at 270 k during a continuous exposure to acrolein 
 and h2 .
spectra 14 correspond to the regions 14 
 indicated in ( a ) . the lowest spectrum is obtained after 450 s of acrolein 
 exposure after the propenol formation rate has dropped to zero .
( c ) 
 ir spectra obtained on pd(111 ) surface turning over with higher time 
 resolution during the induction period and period of growing reactivity .
note that this frequency is present neither 
 in adsorbed intact acrolein on pd ( figure 2b ) nor in acrolein ice and therefore can not be related to any prominent vibration 
 of the molecularly adsorbed acrolein .
further , this band appears only 
 under the reaction conditions : in the presence of h2 in 
 the temperature range 220290 k. the most striking observation 
 of this study is that the evolution of this vibrational band shows 
 strong correlation with the evolution of propenol in the gas phase . 

indeed , this band starts to appear in the region 1 , which comprises 
 the induction period and the region of growing reaction rate ( figure 2a ) , and then grows 
 in intensity in regions of the highest reactivity 2 and 3 .
consecutively , 
 the intensity of this band strongly decreases in region 4 accompanied 
 by the decrease of the propenol formation rate in the gas phase and 
 completely disappears in the lowest spectrum of figure 2b showing the region of zero reactivity . 
 a few other ir bands in the region of chx stretching and bending vibrations can also be correlated to the 
 gas phase formation rate of propenol .
the observed strong correlation 
 between the gas phase formation 
 rate of propenol and the evolution of the vibrational band at 1120 
 cm unambiguously shows that the corresponding 
 surface species is the surface intermediate that is directly involved 
 in the selective hydrogenation of the c = o bond . to the best 
 of our knowledge
, this is the first direct experimental observation 
 of such a direct correlation between the evolution of the product 
 in the gas phase and the formation of the corresponding reaction intermediate 
 on the surface obtained under the well - defined and isothermal conditions .
the vibrational range 10501200 cm is 
 typical for the stretching vibration of a single c 
previously , a large class of oxygen coordinated surface 
 adsorbates has been observed in this vibrational region . in our case ,
the most likely species related to the band at 1120 
 cm is a propenoxy group ch2=ch  ch 
opd , 
 in which the c  o entity is attached to pd through the o atom 
 to form a single c 
the high intensity 
 of this band , exceeding even the most intense c = o vibration 
 in acrolein ( 1660 cm ) and oxopropyl species ( 1755 
 cm ; regions 2 and 3 , figure 2b ) , additionally supports formation of a 
 single c 
o bond , which has a large dipole moment that can explain 
 very high ir intensity .
the vibrational band at 2990 cm , which also can be directly correlated to the evolution of propenol 
 in the gas phase , might be indicative of the stretching vibration 
 of the c  h entity , in which c is involved in the c = c 
 double bond , suggesting that the reaction 
 intermediate contains a c = c bond . 

opd bond 
 indicates that c  o entity is not lying flat on the surface 
 since otherwise this vibration would be not seeing due to the metal 
 surface selection rule .
this consideration 
 also implies that the c = c bond can not be in the very close 
 proximity to pd and is most likely is not directly involved into the 
 interaction with the surface .
( a ) correlation between the formation rate of 
 propenol in the gas 
 phase ( blue line ) and the evolution of the integral intensity of the 
 band at 1120 cm related to the surface reaction 
 imtermediate ( black symbols ) measured over pd(111 ) at 270 k. ( b ) the 
 corresponding ir spectra obtained on the pd(111 ) surface turning over .
the most likely reaction intermediate 
 consistent with all observed 
 vibrational signatures is shown in figure 2b next to the band at 1120 cm .
it contains the c = c bond and is attached to pd via o forming 
 the c 
this intermediate can be 
 formed though adsorption of acrolein via the c = o bond and the 
 addition of one h atom at the c next to o. only one additional step 
the 
 insertion of the second h atom into the pdo bond  is 
 required to form propenol .
the third prominent band appears 
 at 1330 cm during the period of highest reactivity 
 and steadily grows in intensity , 
 remaining intense even after the complete stop of the reaction .
this 
 band was previously related to formation of ethylidyne and ethylidyne - like 
 species .
this species can be considered 
 only as the second type of spectator ( spectator ii ) or a surface poison .
ir spectra obtained with 6 times higher time resolution 
 clearly show that the spectator i is formed on the surface prior the 
 onset of formation of the reaction intermediate ( figure 2c ) .
the first two spectra , 
 corresponding to the induction period ( 016 s ) , directly show 
 that first spectator i ( band at 17551790 cm ) is formed followed by formation of the propenoxy reaction intermediate 
 ( 1120 cm ) close to the onset of propenol evolution 
 in the gas phase .
figure 3a shows the gas phase formation rate of propenol ( blue line ) 
 together with the integral intensity of the vibration band 1120 cm ( black squares ) of all ir spectra obtained with a 
 high time resolution .
as clearly seen , 
 the integral intensity of the vibrational band 1120 cm , related to the proposed reaction intermediate , directly follows 
 the evolution of the product in the gas phase .
this strong correlation 
 between the evolution of the reaction rate in the gas phase and the 
 band at 1120 cm is a clear indication for the 
 interrelation of the gas phase product and the proposed reaction intermediate .
( a ) formation 
 rate of propenol ( blue line ) and propanal ( black 
 line ) on 12 nm sized supported pd nanoparticles at 270 k under continuous 
 exposure of h2 and acrolein .
it is important to underline that the surface reaction intermediate 
 is formed not on the clean pd(111 ) surface but on the surface strongly 
 modified with spectator i ( oxopropyl species ) . indeed , about one acrolein 
 molecule per four surface pd atoms was accumulated on the pd(111 ) 
 surface to form a dense overlayer of spectator i species prior to 
 the onset of propenol formation .
most likely , such strong geometrical 
 confinement of an adsorption site for acrolein on the spectator i - covered 
 surface prevents the competing hydrogenation of the c = c bond 
 and allows acrolein to adsorb only via o and to activate the c = o 
 bond .
obviously , the clean pd surface is not capable of activating 
 the c = o bond toward selective hydrogenation , and the strong 
 modification of the surface by spectator i is required to trigger 
 the desired selective chemistry .
recently , modification of pd(111 ) 
 with self - assembled hydrocarbon - based monolayers was reported to result 
 in similar effects leading to strong enhancement of chemoselectivity 
 of epoxybutene partial hydrogenation .
potentially , two effects might account for deactivation of 
 the 
 surface after the period of high reactivity .
we found that formation 
 of the spectator ii  strongly bound ethylidine - like species 
 ( band at 1330 cm)is correlated with deactivation 
 of the catalytic surface .
it might be speculated that it blocks the 
 surface sites that are relevant for the formation of oxopropyl reaction 
 intermediate .
second , dissociative hydrogen adsorption can potentially 
 be inhibited on the surfaces strongly covered with various hydrocarbon 
 species , which might result in decreasing 
 overall hydrogenation activity .
however , the latter effect should 
 play rather minor role under the ambient pressure conditions .
it is important to emphasize that a clear - cut conclusion on whether 
 a surface species is a reaction intermediate or not is possible only 
 in case when a direct correlation between the product formation 
 rate in the gas phase and the evolution of this surface 
 species followed by an appropriate surface sensitive technique 
 is observed .
it should be also pointed out that spectroscopic differentiation 
 between the reaction intermediate and the spectator species is possible 
 only if ( i ) the surface process is carried out in such a way that 
 the reaction intermediate follows a different time dependence than 
 the spectator and ( ii ) a direct correlation between the product formation 
 rate in the gas phase and the evolution of this surface species can 
 be established .
such situation can be achieved e.g. in the transient 
 regime applied in this study , in which the spectators were accumulated 
 on the surface , while the concentration of the reaction intermediate 
 was increasing and then decreasing .
this fact allowed us to establish 
 the direct correlation between the time evolution of the gas phase 
 product propenol and the surface reaction intermediate propenoxy group 
 ( figure 3 ) .
the more 
 traditional way of carrying out the reaction under the steady state 
 conditions would result in constant concentrations of all surface 
 species and with this prevent their clear assignment to either spectators 
 or a reaction intermediate . to the best of our knowledge ,
this is 
 the first example in model catalytic studies when such direct correlation 
 between the product formation rate in the gas phase and the evolution 
 of this surface species was observed and with this both requirements 
 discussed above were met . to understand the absence of selective 
 acrolein hydrogenation to 
 unsaturated alcohol over pd nanoparticles
, we carried out an identical 
 spectroscopic investigation on the evolution of surface species during 
 the reaction over oxide supported pd model catalyst with the simultaneous 
 monitoring the gas phase products .
figure 4a shows the results of these experiments ; 
 for a detailed description we refer the reader to the supporting information .
the ir spectra obtained 
 on the pd nanoparticles turning over are dominated by the features 
 in the range 18001960 cm , which can be 
 clearly related to an accumulation of co molecules on the surface . 

note that co can not desorb from pd nanoparticles at our low reaction 
 temperatures .
this observation suggests 
 that acrolein undergoes decarbonylation on pd nanoparticles , which 
 was also observed previously on powdered catalysts .
thus , the observed behavior strongly suggests that acrolein decomposes 
 on pd nanoparticles under the reaction conditions forming co molecules 
 that block the surface and prevent formation of well - ordered spectator 
 i overlayers required for selective acrolein hydrogenation to propenol . 
 since the pd clusters are mostly terminated by ( 111 ) facets that are not active in acrolein decomposition 
 as observed for pd(111 ) , most likely edges , corners , ( 100 ) facets , 
 and the other low - coordinated surface sites of pd nanoparticles are 
 responsible for acrolein decarbonylation .
note that these low coordinated 
 surface sites represent only 20% of the surface sites , while the rest 
 of the surface is ( 111 ) facets .
since the entire particle becomes 
 covered by co in the course of the reaction , strong coupling between 
 the ( 111 ) facets and the low coordinated sites by co surface diffusion 
 must be inferred to explain the observed co accumulation .
interestingly , 
 not only alcohol formation but also hydrogenation of the c = c 
 bond , which is generally discussed to be easy even over carbon - containing 
 surfaces , is prevented on the surface covered by co. model of the chemical 
 composition of the surfaces turning over 
 ( a ) pd(111 ) and ( b ) pd nanoparticles . on pd(111 ) , a dense overlayer 
 of spectator ( oxopropyl ) species is formed at the initial stages of 
 reaction , which allows activation of c = o double bond and renders 
 the surface chemoselective toward unsaturated alcohol formation . in 
 contrast , on pd nanoparticles acrolein readily decarbonylates forming 
 co molecules , which prevent hydrogenation of c = o bond and result 
 in formation of the undesired product propanal .
summarizing , the mechanisms of selective partial 
 hydrogenation 
 of acrolein over two model surfaces 
pd(111 ) and pd nanoparticles 
 supported on fe3o4/pt(111 ) film  were 
 investigated using a combination of molecular beam techniques with in situ iras under well - defined uhv conditions .
the desired 
 reaction pathway  selective hydrogenation of the c = o bond in acrolein to form 
 propenol  was 
 observed over pd(111 ) surface with nearly 100% selectivity , while 
 only c = c bond hydrogenation occurred over oxide supported pd 
 nanoparticles .
the selectivity in hydrogenation of the c = o 
 bond was found to critically depend on the presence of an overlayer 
 of spectator species formed at the initial stages of the reaction 
 ( figure 5a ) .
most likely , 
 the spectator , rendering the surface chemoselective , results from 
 the addition of one h atom to the c = c bond of acrolein to form 
 oxopropyl species . after a dense overlayer of the spectator
is formed , 
 acrolein adsorbs on this modified surface via the c = o bond 
 and can be selectively hydrogenated to an unsaturated alcohol propenol . 

the nature of the corresponding surface reaction intermediate was 
 established spectroscopically . by monitoring the surface species
during 
 the course of the reaction via iras , we were able for the first time 
 to experimentally follow the simultaneous evolution of the reactive 
 intermediate on the surface and formation of the product in the gas 
 phase . with this a direct assignment of one of the surface species 
 to a reaction intermediate
was achieved , while the other surface species 
 were identified as spectators . on supported pd nanoparticles , formation 
 of a spectator overlayer
was found to be prevented by strong acrolein 
 decarbonylation , and the surface was observed to be active only for 
 hydrogenation of the c = c bond ( figure 5b ) . obtained atomistic - level insights 
 into chemoselective hydrogenation 
 chemistry of acrolein highlight the exceptional importance of spectator 
 species which are usually formed on the catalytically active surface 
 under reaction conditions .
related effects are expected to play a 
 key role in controlling chemoselectivity in hydrogenation of all types 
 of ,-unsaturated aldehydes and ketones and hold a great 
 potential for further development of new chemo- and enantioselective 
 powdered catalysts such as e.g. ligand - modified nanoparticles .
 we present a mechanistic study on 
 selective hydrogenation of acrolein 
 over model pd surfaces  both single crystal pd(111 ) and pd nanoparticles 
 supported on a model oxide support . 
 we show for the first time that 
 selective hydrogenation of the c = o bond in acrolein to form 
 an unsaturated alcohol is possible over pd(111 ) with nearly 100% selectivity . 
 
 however , this process requires a very distinct modification of the 
 pd(111 ) surface with an overlayer of oxopropyl spectator species that 
 are formed from acrolein during the initial stages of reaction and 
 turn the metal surface selective toward propenol formation . by applying 
 pulsed multimolecular beam experiments and in situ infrared reflection  
 absorption spectroscopy , we identified 
 the chemical nature of the spectator and the reactive surface intermediate 
 ( propenoxy species ) and experimentally followed the simultaneous evolution 
 of the reactive intermediate on the surface and formation of the product 
 in the gas phase . 
Introduction
Experimental Section
Results and Discussion
Conclusions