Precise Observation of Growth of Surface Oxide Layer for Pd and Cu Nanoparticles During Oxidative/Reductive Gases Cyclic Flow Studied by Real-Time-Resolved XAFS Spectroscopy∗

We have demonstrated in situ and real-time-resolved X-ray absorption fine structure (XAFS) observation for oscillatory creation and removal reactions of surface oxide layer of Pd and Cu metal nanoparticles during cyclic flow of oxidative and reductive gases every 10 s. Pd and Cu K-edges XAFS spectra were collected at about 1-2 Hz. Precise observation of edge position was utilized for monitoring surface oxidation and reduction reactions. Reaction rate of surface oxide layer creation by NO for Pd metal nanoparticle is faster in the case of H2/NO cyclic flow than in CO/NO. Fast and two-step surface oxidation and reduction reactions were revealed in Cu metal nanoparticles. [DOI: 10.1380/ejssnt.2016.48]


I. INTRODUCTION
Metal nanoparticles have been widely used as heterogeneous catalysts for surface chemical reaction because of their large specific surface area.However, metal nanoparticles are expected to not only provide highencounter fields but show peculiar performance originated in nanometer scale.Many studies about metal nanoparticles have revealed that various and unique structural changes are induced in metal nanoparticles themselves by changing temperature and/or atmosphere [1][2][3][4][5][6][7][8].These results indicate that observation of metal nanoparticles under only normal atmospheric condition may cause limited understandings of metal nanoparticles especially about the mechanism of surface catalytic reaction.Besides, irreversible structural change of metal nanoparticle are usually brought about by surrounding gas changes.This shows us the importance of real-time-resolved observation.Therefore, in situ and real-time-resolved observation of metal nanoparticles during atmospheric change is needed to understand the property of metal nanoparticles and precise mechanism of surface catalytic chemical reaction.
CO/NO catalytic reaction on the surface of metal nanoparticles is one of the most widely studied systems in the field of heterogeneous catalysts for gas-solid phases [9][10][11][12][13].Both of CO and NO are harmful gases emerged from the exhaust of automobiles and are converted to the nonharmful CO 2 and N 2 gases via the automotive catalysts.Pd nanoparticles which are used for this catalytic reaction have been revealed to show the various surface structural changes during reaction, such as surface oxidation, lattice constant expansion and dispersion of nanoparticles [7,8].In order to study the origin of structural transformations of Pd metal nanoparticles, we selected in situ and realtime-resolved approach.
We have developed the real-time-resolved X-ray absorption fine structure (XAFS) spectroscopy observation system and applied this technique for the reaction of nanoparticles in order to study atomic and electronic structure changes by catalytic reaction and fluctuation of surrounding gas [14][15][16][17][18]. Continuous XAFS spectra can be collected by using dispersive XAFS optics [19,20].The feature that there are no moving components during taking spectra enables us to deal with high-precision measurement as well as high-rate measurement.We have utilized the advantage of dispersive XAFS technique for the chemical reaction of metal nanoparticles.Continuous observation with high relative precision and high frame rate provides many absorption spectra having a slight shift each other.Surface structural change is insignificant as compared with the whole of metal nanoparticle even that the diameter of nanoparticle is not so long.We can get the information of slight shift of surface structure by only high-precision measurement of XAFS spectra.Consistent and fast analysis of XAFS spectra is important to obtain full information from many data sets.
In this study, we employed the in situ and real-timeresolved observation of oxidation and reduction reaction of Pd nanoparticles on alumina support by alternately changing oxidative and reductive gases every 10 seconds.Surface structural change of Pd metal nanoparticles were observed by XAFS spectra at a rate of about 1 Hz.Realtime-resolved XAFS observation by dispersive optics were performed for estimation of surface reduction and oxidation reactions.We have succeeded to get high-precision information and revealed the reaction rate of the surface oxidative and reductive reaction on metal nanoparticles.

II. EXPERIMENTS
Pd K-edge XAFS spectra were measured by the dispersive mode of Laue configuration at the bending magnet beamline BL14B1 and BL28B2 of SPring-8.The optics of both beamlines are similar.Cu K-edge XAFS spectra were measured by the dispersive mode of Bragg configuration at the bending magnet beamline BL14B1.Rh-coated mirrors were set to 5 mrad for the Bragg configuration, while mirrors were not used for Laue configuration.A curved silicon crystal called as a polychromator was exposed to white X rays to emerge energy-dispersed X rays.Dispersed X rays were obtained by Si(422) and Si(111) reflection plane in the case of Laue and Bragg configuration, respectively.The polychromator was bent horizontally by setting it in a curved copper block cooled by water in the case of Laue configuration.For Bragg configuration, both sides of Si wafer were independently grabbed and bent to make an asymmetry curvature and bottom of the wafer was dipped in an In-Ga cooled pool.From the curvature with a radius of 2000 mm, X rays with an energy range over 1000 eV were generated.Samples were set to the focal point of X rays.Gd 2 O 2 S(Tb) was exposed to re-dispersed X rays from the sample position and emitted lights were collected using a charge coupled device (CCD) camera (640 × 480 channels, 12 bits).The intensities in the vertical direction (∼200 channels) were summed up to produce a one-dimensional spectroscopy.The horizontal focus size of X ray was measured to be 0.05 mm in full width at half maximum (FWHM) and the vertical size is widened to about 3 mm height for accumulating the intensity of transmitted X rays.
Powdered γ-Al 2 O 3 was used for preparation of metal nanoparticles by the impregnation method with dilute aqueous palladium nitric acid, Pd(NO 3 ) 2 .Following drying and calcination at 500 • C, Pd(4 wt%)/Al 2 O 3 sample was obtained.Dilute aqueous copper nitric acid, Cu(NO 3 ) 2 , was used for preparing Cu case.Following drying and calcination at 650 • C, Cu(3 wt%)/Al 2 O 3 sample was obtained.90 and 30 mg of powdered Pd and Cu samples were set into a cylindrical sample holder (ϕ10 × ϕ7 × 10 mm 3 ) and pressed by hand in order to make a disk pellet (ϕ = 7 mm), respectively.The sample holder with pellet was placed in a in situ flow type XAFS cell [2].Gas flow and catalytic reaction were checked by a quadrupole mass spectrometer (OmniStar GSD, Pfeiffer Vacuum GmbH) whose capillary was directly attached to the in situ XAFS cell.
All XAFS spectra were collected under in situ and realtime-resolved mode.Observation temperature was set to 400 • C. Hydrogen and carbon monoxide were used for reductive gases and oxygen and nitrogen monoxide were used for oxidative gases.All gas concentration were 5% with He balance.Total flow rate was set to 50 cc/min.Oxidative gas and reductive gas were flowed alternately by 10 s.

III. RESULTS AND DISCUSSION
Dispersive optics are expected to bring about highprecision data set of real-time-resolved XAFS spectra because there are no mechanical motion components.We will show the typical result of real-time-resolved XAFS spectra at Pd K-edge by dispersive optics as Fig. 1.Oxygen and hydrogen gases were alternately dosed for Pd(4 wt%) metal nanoparticles on aluminum oxide every 10 s at 400 • C. We have collected XAFS spectra every 0.9 s.We can see the oscillatory oxidation and reduction reactions of Pd nanoparticle by the change of spectra induced by the cyclic flow of oxygen and hydrogen.All spectra were observed under high precision with clear fine structures.In this study, we focused on the change of spectra around the edge jump which is sensitive for the oxidation and reduction reaction of surface of Pd metal nanoparticles.The edge position was evaluated by fitting spectra with a simple step function.
Figure 2 shows the variations of the edge position dur- Oscillation of the edge position in Pd K-edge XAFS spectra during cyclic flow of hydrogen and oxygen gases is displayed in Fig. 3(a).The positive shift of the edge position indicates that the surface oxide layer grows by oxygen gas and the negative shift of the edge position indicates that the surface oxide layer is removed by dosing of hydrogen gas.In this condition, H 2 (5%) is enough for recovering the complete Pd metal state in 10 s.Oscillatory change of the edge position is represented with a linear and similar slopes both oxidative and reductive reactions.This means that the reaction rates of the creation of surface oxide layer by oxygen gas dosing and the removal of the surface oxide layer by hydrogen gas dosing are almost similar.The change of the edge position during oscillation is about 1.5 eV which is corresponds to the amount of the surface oxide layer created in 10 s.Mean diameter of Pd nanoparticles was estimated to about 5 nm by CO pulse adsorption technique.Difference of the edge position between Pd metal and PdO is about 4 eV (Fig. 2), which indicates that the thickness of the surface oxide layer by 10 s oxygen flow is about 0.4 nm.
Different situation is observed by cyclic flow of hydrogen and nitrogen monoxide, which is seen in Fig. 3(b).Reaction rate of the creation of surface oxide layer differs from that of the removal of surface oxide layer in the case of H 2 /NO cycling, which makes contrast with the case of H 2 /O 2 cycling (Fig. 3(a)).The edge position shows about 0.3 eV positive shift due to the creation of surface oxide layer induced by 10 seconds NO flow, while this oxide layer was rapidly removed by hydrogen dosing.Complicated change of the edge position was found just after the removal of the surface oxide layer, though detail structure model is unknown at this stage.
Change of the edge position by CO/NO cyclic flow (Fig. 3(c)) is compared with the H 2 /NO cyclic flow shown in Fig. 3(b).Oxidative gas is nitrogen monoxide in both cases, but reaction rates of creation of surface oxide layer by NO dosing in the two cases are largely different.The edge position shifts about 0.3 eV in the case of hydrogen gas as reductive one, while about 0.15 eV in the case of carbon monoxide, which means that reaction of creation of surface oxide layer by NO dosing proceeds faster in the case of hydrogen as a reductive gas than in the case of carbon monoxide.It is necessary to consider the surface adsorption state and catalytic reaction by changing surrounding gases as well as the surface oxidation and reduction reactions of metal nanoparticles in order to understand the difference in the reaction rate for the growth of surface oxide layer.Pd nanoparticles usually absorb hydrogen atoms into the interstitial sites when nanoparticles were exposed by hydrogen gases [21].However, although such hydrogenation reaction occur at room temperature, hydrogen atoms are desorbed from Pd metal nanoparticles over 100 • C and do not reside at both inside and the surface of metal nanoparticles any longer.At a temperature of 400 • C in this experiment, clean surface of Pd metal nanoparticles is thought to be kept even under hydrogen atmosphere.On the other hand, in the case of CO dosing, surface of Pd metal nanoparticles is covered by CO molecules even at 400 • C [14].The difference in reaction rates for the growth of the surface oxide layer of Pd metal nanoparticles should originate from the difference in surface adsorption manner of hydrogen and carbon monoxide.CO surface adsorption also brings about surface catalytic reaction of CO and NO and consumption of dosed NO gas.CO/NO cyclic flow shows slower creation of surface oxide layer of Pd nanoparticles than H 2 /NO cyclic flow.
Cu metal nanoparticle is one of the candidates for substitution of the precious metal nanoparticle which is used as an automotive catalyst [22].We studied the surface oxidation and reduction reaction of Cu metal nanoparticles for comparison with the case of Pd metal nanoparticles.The oscillatory change of the edge position of Cu K-edge XAFS spectra for Cu metal nanoparticles during cyclic CO/NO flow is summarized in Fig. 4. XAFS spectra were collected every 0. lar with the case of Pd metal nanoparticles.Comparing with the results of Pd metal nanoparticles displayed in Fig. 3(c), we can realize that both of surface oxidation reaction and reduction reaction are faster in Cu nanoparticles than in Pd nanoparticles.Besides, two types of reaction are observed in the case of Cu metal nanoparticles.Just after the change of surrounding gas from the reductive gas to the oxidative gas, abrupt change occurs in about two seconds.After that, slower change is presented.The faster change in the first step is assigned to the oxidation of surface layer, and the slower change in the second step is assigned to the oxidation of the inner layer of the metal nanoparticles.Similar two-step change is also recognized in the reduction step.Surface layer of Cu nanoparticles has revealed to have different oxidation property with the inner side of the particle.Surface oxide layer in Cu nanoparticles is easily created and easily re-moved by CO/NO successive flow.Fast creation of oxide layer on the metal nanoparticles may cause the degradation of catalysis of nanoparticles, however, fast removal of the surface oxide layer is connected to the revival of catalysis.The results about Cu metal nanoparticles indicate that Cu nanoparticles have a potential of CO/NO catalytic reaction.Further experiments including different gas composition and species are needed to understand the property of Cu metal nanoparticles as CO/NO catalyst especially in long-time use where agglomeration of metal nanoparticles are main issue of deterioration of catalysis.

IV. CONCLUSIONS
We have demonstrated in situ and real-time-resolved observation of structure change of Pd and Cu metal nanoparticles during alternative flow of oxidative and reductive gases.Edge position of XAFS spectra is useful for the evaluation of surface oxidation and reduction reactions.CO/NO cyclic flow shows slower surface oxide layer creation than H 2 /NO cyclic flow for Pd metal nanoparticles.This difference was discussed with the mechanism of surface adsorption reaction.Cu metal nanoparticles show faster creation and removal reactions of surface oxide layer than Pd metal nanoparticles.

FIG. 2 .FIG. 3 .
FIG.2.Change of the edge position in Pd K-edge XAFS spectra of Pd(4 wt%)/Al2O3 by 10 s cyclic flow of CO(5%)/O2(5%) at 400 • C. First flow of reductive CO gas was started at 10 s and first flow of oxidative O2 was started at 40 s.After that, alternately dosing of CO/O2 every 10 s was performed.Sample was set to completely metal state before the first gas dosing.Most of Pd atoms were oxidized at 200 s after of the start.
4 s for Cu case.Mean diameter of Cu metal nanoparticles was estimated to about 5 nm by N 2 O pulse adsorption technique, which is simihttp://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology FIG. 4. Oscillatory change of the edge position of Cu Kedge XAFS spectra of Cu(3 wt%)/Al2O3 by 10 s cyclic flow of CO(5%)/NO(5%) at 400 • C. Measurement time of one XAFS spectroscopy is 0.4 s.Two-step reaction is observed both in the reductive and oxidative processes.