The adsorption and catalyticoxidation of CO on Pt are among the most frequently examined surfaceprocesses due to their environmental and industrial relevance. Ptexhibits superior catalytic properties for various applications, suchas (preferential) CO oxidation for emission control or cleaning ofhydrogen streams for fuel cells.¹⁻⁵ Despite efforts to replace expensive Pt by cheaper materials, itsactivity can typically not be matched. Thus, the focus is rather onreducing the Pt amount, e.g., by using Pt atoms, clusters, and smallnanoparticles⁶⁻¹³ (or alloys and core–shell structures^14,15) on suitable support materials. It is, however, still challengingto obtain detailed knowledge about increasingly smaller nanoparticles,especially about their inherent activity and metal/support interaction.¹⁶⁻²¹

In recent years, significant advances have been made in modelcatalysis,enabling surface characterization at (near) atmospheric pressure,overcoming the “pressure gap”,²²⁻³¹ but bridging the “materials gap” is evenly important.Previous single-crystal studies have provided fundamental insight,but they cannot fully mimic nanoparticles^25,32,33 (with support effects being apparently inaccessible),which is why more realistic model systems are required, such as oxide-supportednanoparticles/islands^11,16,25,34 or inverse systems.³⁵⁻³⁷

In thiscontribution, we present Pt/ZrO₂ model catalystsprepared by atomic layer deposition (ALD) that were examined at mbarpressures byoperando sum frequency generation (SFG)spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy(NAP-XPS), with simultaneous mass spectrometry (MS) product analysis,and complemented by density functional theory (DFT) calculations.

ALD has been widely used in industrial manufacturing,^38,39 especially for dielectrics and microelectronics, and is receivingincreasing attention for (upscalable) catalyst preparation.⁴⁰ The current model catalysts consist of a zirconiafilm, ALD-grown (400 cycles) on a Si (100) wafer, and Pt depositsprepared by different numbers of ALD cycles (10–250; see transmissionelectron microscopy (TEM) images inFigure1a andFigure S3). Whereas the zirconia ALD generated a uniform 42 nm-thick oxidesupport, using a few (10, 50) Pt cycles produced small Pt particlesup to 8 nm in size (Figure1a, right panel). Upon applying 125 or 250 deposition cycles,the Pt particles coalesced into islands, finally forming a homogeneousPt film of uniform ∼10 nm thickness (Figure1a, left panel). These nucleation and growthprocesses⁴¹ allow for the preparation ofdifferent well-defined Pt/ZrO₂ model catalysts rangingfrom isolated Pt nanoparticles to bulk-like thin films. Herein, the50 and 250 cycle samples are the most informative ones (other samplesare described in theSupporting Information). The ALD approach to catalyst synthesis is not new,^42,43 but the combination withoperando surface spectroscopy(SFG and NAP-XPS carried out in high-pressure cells with simultaneousMS gas-phase analysis) and DFT calculations provides a complementarypicture.

The standard cleaning used for single crystals in ultrahigh vacuum(UHV), i.e., sputtering/annealing, could not be applied as it wouldhave destroyed the ALD samples. Inspired by the (re-)activation oftechnological catalysts, all samples were thus cleaned from carbonaceousresidues by heating in 10 mbar O₂ to 400 °C and in20 mbar CO/O₂ (1:1) to 300 °C.

The Pt morphologywas then addressed by polarization-dependentSFG of CO adsorption (10 mbar CO at 150 °C;Figure1b; see theSupporting Information for SFG theory and fit values). Thespectra show the on-top CO resonance region, as no other binding geometrieswere observed. Two polarization combinations were employed: ppp (hasits maximum intensity for C=O bonds parallel to the macroscopicsurface normal; black inFigure1b) and ssp (has its maximum intensity for C=Obonds inclined with respect to the macroscopic surface normal, i.e.,around 30–40° depending on molecular polarizability; redinFigure1b).^44,45 Due to the angular dependence, the resulting intensity ratioI_ppp/I_ssp for COis expected to decrease with increasing bond inclination of the molecules.^46,47 Because the Pt film of the 250 cycle sample consisted of planarislands with a uniform height of about 10 nm, adsorbed CO was mostlyperpendicular to the ZrO₂/Si(100) surface so that the COpeak intensity was high in ppp and very low in ssp (ratio of 17.4).In light of our previous study of CO/Pt(111),⁴⁷ assuming an identical optical interface model, this would correspondto an average CO tilt angle of ∼5° (relative to the macroscopicsurface normal), although this value is just meant to show a trend.In contrast, the 50 ALD cycle Pt film consisted of small particles(about 8 nm) with multiple facets, many of which are no longer parallelto the substrate. On these inclined facets, CO still adsorbs perpendicularly,but the CO bonds are inclined with respect to the macroscopic surfacenormal. Accordingly, the SFG intensity is lower in ppp, resultingin a much lowerI_ppp/I_ssp ratio of 2.3. Following the sameassumptions as above, the average CO tilt angle would be ∼40°(note that this again is just to show the trend). This rough estimateagrees with the facet inclination and ratio from TEM images.

Apart from the intensity, the peak position and peak symmetry/asymmetryare noteworthy. The peak position depends on the coordination of thePt adsorption site and the CO surface coverage (inducing chemicaland dipole–dipole interactions).^27,48,49 As the coverage should be (nearly) the same underidentical pressure and temperature conditions, the 3 cm^–1 difference points to slightly rougher surfaces for the nanoparticlesample. The (a)symmetry of an SFG signal depends on the amplitudesA_r orA_nr and phasedifference ϕ between resonant (adsorbed CO) and nonresonantsignal contributions⁵⁰⁻⁵² (see theSupporting Information). The nonresonant backgroundA_nr mayoriginate from Pt surface defects (changing the electron localizationat the surface) and electronic contributions of the ZrO₂/Si(100) substrate. Comparing the spectra of the 250 and 50 cyclesamples, the intensities of the resonant and nonresonant contributionsare much more similar in the case of the particulate film (smallerparticle size, more defects, and more metal/oxide interface), whilethe resonance phase relative to the nonresonant background, as obtainedby means of the quantitative deconvolution of the data, is found tobe similar for both CO-Pt systems. This leads to a more asymmetricline shape for the 50 cycle Pt, directly reflecting its surface morphology.

In order to further characterize the ALD-prepared model catalysts,they were compared to Pt single crystals at different temperatures.Figure2 shows the SFG pppspectra of smooth (UHV annealed to 800 °C) Pt(111), 250 and 50ALD cycle Pt/ZrO₂, and sputtered Pt(111) in 10 mbar CO.Respective fitting values are given in theSupporting Information (our system has a spectral accuracy of 2 cm^–1). At 175 °C, the characteristic on-top CO onPt(111) was observed at 2092 cm^–1, matching thesaturation coverage.^49,53 The 250 cycle sample exhibiteda continuous Pt surface (Figure1 andFigures S2 and S3),but was still rougher than the annealed Pt(111), as indicated by theredshifted wavenumber (2089 cm^–1) and increasedpeak asymmetry. Adsorbed CO on the 50 cycle Pt/ZrO₂ sample,consisting of 8 nm (connected) particles, exhibited a similar wavenumber,indicating identical coordination, but lower intensity and higherasymmetry due to the inclined facets. The higher number of low-coordinated(step/kink) sites^54,55 on the ALD samples was confirmedby comparison with the CO spectra of sputtered Pt(111) (2081 cm^–1) and Pt(110) (2075 cm^–1) (Figure S9), showing even lower wavenumbers.^53,56 Apparently, the ALD Pt catalysts exhibit roughness intermediatebetween the annealed and sputtered Pt(111).

Now, turning to the CO adsorption at higher temperatures (225/275°C), the decreased CO coverage induced a redshift.^24,48,49,53 For Pt(111), the on-top CO signal redshifted, but the intensitywas similar to that at 175 °C. Analogously, the CO signals ofthe ALD samples and sputtered Pt(111) exhibited small redshifts at225 °C (and a minor intensity loss). However, at 275 °C,the rougher surfaces showed a pronounced intensity loss, peak shift,and phase alteration. Previously, a similar observation on polycrystallinePt foil⁵⁷ was explained by CO desorption,but CO is more strongly bonded to steps/defects than terraces.^58,59 Furthermore, the spectral changes were irreversible upon cooldownin CO (seeFigure S10) and ϕ changedsignificantly, ruling out simple adsorption/desorption and rathersuggesting a permanent modification/blocking of the adsorption sites.It has been reported that stepped Pt surfaces or small Pt particles/clustersmay cause CO dissociation, forming a carbon overlayer,⁶⁰ in line with the current observation. Indeed,the CO spectrum of the 250 cycle Pt at 10 mbar/225 °C agreesquite well with the reported values of Pt(557) at 30 mbar/250 °C,which showed CO dissociation at 275 °C. The dissociation hypothesisis supported by the fact that adding O₂ to CO and heatingreversed the spectral change by reoxidizing carbon to CO₂ (Figure S11). Nevertheless, CO dissociationon Pt has been controversially discussed for a long time.

Thismotivated the DFT calculations of CO dissociation on smoothand rough Pt surfaces (Figure3). CO dissociation on Pt(111) and Pt(211) is strongly endothermicand barriers >3 eV have been reported in the low coverage limit,⁶¹ making this process improbable. For a more faciledissociation, the adsorbed state of CO should be destabilized, whereasthe final state needs to be stabilized. A destabilization of the adsorbedstate is achieved by increasing the coverage (gas pressure), whereasthe final state is stabilized by CO₂ formation. In particular,direct CO₂ formation according to the Boudouard reaction(CO* + CO* → CO₂ + C*) hinders the backward C–Oassociation reaction. Similar contributions (local coverage and finalstate) to favoring the Boudouard reaction have been reported for PtSn⁶¹ and Cu.^62,63 For efficient CO–COcoupling and reaction, the C weight of the 2π* orbital on onereacting molecule should overlap with the O weight on the other molecule.This may be accomplished on stepped and kinked surfaces, so we consideredtwo model structures: a Pt adatom coordinated with two CO moleculeson Pt(100) and Pt(410). The barriers were evaluated at coverages obtainedfrom a thermodynamic analysis (Figures S14 and S15). The barriers for the reactions are 1.8 and 2.1 eV fordissociation at the adatom and Pt(410), respectively. For the reactionat the adatom (Figure3b–d), one of the CO molecules on the Pt adatom is reactingwith a CO on the (100) facet. The transition state is a bent O–C–Ostructure (Figure3c), whereas the final state (Figure3d) is gas-phase CO₂ and the remaining carbonatom is in a highly coordinated position. A fourfold coordinated Con Pt(100) is (per carbon atom) as stable as graphite, thus stabilizingthe final state.⁶⁴ The reaction path onPt(410) is different as both reacting CO molecules are below the step(Figure3e–g).The 2π*–2π* match is, in this case, enabled byan initial bending of the CO close to the step, and the final stateagain has a fourfold coordinated C atom. Accordingly, on the rougherPt surfaces, CO dissociation is facilitated by a barrier that is lowerthan 1 eV as compared to the smooth surfaces.

Turning to CO oxidationon the Pt/ZrO₂ model catalysts,Figure4a,b shows the SFGppp spectra acquired in a reaction mixture of 10 mbar CO and 20 mbarO₂ from 150 to 500 °C. At 150 °C, the Pt particleswere on-top CO-covered (poisoned) and thus inactive, with 2091 cm^–1 indicating high-coordination sites for the 250 cyclesample. The 50 cycle sample showed a 10 cm^–1 redshift,indicating lower coordination/higher roughness, with lower intensityand more asymmetry, due to the nanoparticle morphology. Upon temperatureincrease, the peak of adsorbed CO decreased and redshifted (due todecreasing CO coverage) and finally disappeared when the Pt surfaceswere fully oxygen-covered and thus active: at 400 °C for thePt thin film, but at 450 °C for the Pt nanoparticles. The temperature-dependentshifts were 2091–2080 cm^–1 for the smootherPt film and 2081–2074 cm^–1 for the rougherPt nanoparticles, the latter wavenumbers indicating stronger bondingon the rougher surfaces. The ignition temperatures were corroboratedby the simultaneously acquired CO₂-MS traces inFigure4c,d and are in accordancewith values reported in the literature^31,65 (note thatMS measurements without simultaneous SFG, “laser-off”,ruled out any laser-induced effects;Figure S16). As long as adsorbed CO is present, the oxidation reaction is largelyinhibited, as the Pt surface can only be in a single stable state,either CO-poisoned (inactive) or O-covered (active).⁶⁶ Accordingly, both SFG and MS indicated that about 50 °Chigher temperature was required for ignition on the rougher (50 cycle)Pt nanoparticles. This is unexpected, because rougher Pt surfacesare generally considered to be more active in CO oxidation,^9,66 as their low-coordinated sites (steps, kinks, and edges) bind bothoxygen and CO stronger than terraces. The O-covered active state canbe more easily established on rougher surfaces (indicated by the lowerignition temperature and higher CO tolerance) despite the higher COoxidation barriers⁶⁷ (according to theBrønsted–Evans–Polanyi relation⁶⁸). However, in the current case, the inherent activity isnot the only important factor, as shown in the following.

To further examine the reactiononset, NAP-XPS was applied as thesecondoperando technique (which again rules outlaser-induced effects). Due to technical limitations, the pressurewas limited to 1 mbar CO and 2 mbar O₂. The C 1s spectraacquired during CO oxidation (Figure S17) detected graphitic carbon (284.7 eV),⁶⁹ adsorbed CO (286.2 eV),^59,70 a weak shoulder (around288 eV; likely carbonate on zirconia), and gas-phase CO (∼291eV).Figure4e,f displaysthe fitted peak areas vs (increasing) temperature for adsorbed COand carbon. In analogy to SFG, for smoother (250 cycles) Pt films,CO fully disappears at a temperature 50 °C lower than for rougher(50 cycles) Pt nanoparticles (the absolute temperatures are lowerdue to the 10-fold lower pressure).

However, NAP-XPS showedthat much more carbon was present duringthe reaction on the 50 cycle (rough) Pt nanoparticles, which evenincreased during the first two temperature steps, clearly indicatingCO disproportionation (Figure4f). Atomic carbon apparently poisons the (low-coordinated)active sites for oxygen activation until it is removed by oxygen athigher temperatures. This effect explains the higher reaction onsettemperature of the Pt nanoparticles despite their presumably moreactive surface. In contrast, the smoother 250 cycle Pt film was muchless affected by C poisoning, yielding a lower reaction onset temperature.After ignition, both Pt surfaces were O-covered and showed the expectedhysteresis upon lowering the temperature (Figure S19). CO readsorbed at a comparably lower temperature paralleledby less coking (due to cooling in an oxygen-containing atmosphere).Throughout all experiments, theoperando Pt 4f spectrarevealed that Pt remained metallic (Figure S17).

In summary, combining the ALD model catalyst preparation,operando SFG/NAP-XPS/MS spectroscopy, and DFT calculationsenabled us to build another bridge across the “materials andpressure gap”. Few ALD Pt deposition cycles produced Pt nanoparticleswith multiple inclined facets, whereas more deposition cycles (≥125)led to more uniform Pt films. The polarization-dependent SFG revealedthe molecular orientation of CO (relative to the macroscopic surfacenormal) and thus both the morphology and roughness of different ALD-grownPt model catalysts. Upon CO adsorption at mbar pressure around 275°C, Pt(111) did not show CO disproportionation, whereas rougherPt particles/films and sputtered Pt(111) did. According to the DFTcalculations, direct CO dissociation is unfeasible even at steppedPt surfaces. Dissociation instead occurs at high coverages via a disproportionationreaction at low-coordinated sites that structurally promote CO–COcoupling and stabilize the remaining C atom. The effect of surfaceroughness on the CO oxidation was monitored at mbar pressure and elevatedtemperature by correlating theoperando SFG and NAP-XPSspectra with the MS reactivity data. Different from the general expectation,the reaction onset temperature was higher for the smaller/rougherPt nanoparticles than for the smooth Pt surfaces. The rougher surfaceswere poisoned by carbon coking, detected by NAP-XPS, and explainedby DFT calculations via CO disproportionation on favorable sites.Only after the removal of the carbon deposits did the rough Pt surfacesbecome active, but at 50 °C higher temperature than for smoothPt. Upon cooldown, the smooth Pt films exhibited a wider hysteresiswindow and were hardly affected by CO disproportionation. Future studiesof ALD Pt particles and films on different support materials shouldreveal whether reducible supports facilitate activation at lower temperatureand reduce/suppress the initial carbon poisoning.