Adsorbed States of Hydrogen on Platinum: A New Perspective

Abstract The interaction of hydrogen with platinum is enormously important in many areas of catalysis. The most significant of these are in polymer electrolyte membrane fuel cells (PEMFC), in which carbon‐supported platinum is used to dissociate hydrogen gas at the anode. The nature of adsorbed hydrogen on platinum has been studied for many years on single‐crystal surfaces, on high‐surface area‐platinum metal (Raney platinum and platinum black), and on supported catalysts. Many forms of vibrational spectroscopy have played a key role in these studies, however, there is still no clear consensus as to the assignment of the spectra. In this work, ab initio molecular dynamics (AIMD) and lattice dynamics were used to study a 1.1 nm nanoparticle, Pt44H80. The results were compared to new inelastic neutron scattering spectra of hydrogen on platinum black and of a carbon‐supported platinum fuel cell catalyst and an assignment scheme that rationalises all previous data is proposed.

Abstract: The interaction of hydrogen with platinum is enormously importanti nm any areas of catalysis. The most significant of these are in polymer electrolyte membrane fuel cells (PEMFC), in which carbon-supported platinum is used to dissociate hydrogen gas at the anode.T he nature of adsorbed hydrogen on platinum has been studied for many years on single-crystal surfaces, on high-surface area-platinum metal (Raney platinum and platinum black), and on supported catalysts. Many forms of vibrational spectroscopy have played ak ey role in these studies, however,t here is still no clear consensus as to the assignment of the spectra.I nt his work,a bi nitio molecular dynamics (AIMD) and lattice dynamics were used to study a1 .1 nm nanoparticle, Pt 44 H 80 .T he results were compared to new inelasticn eutron scattering spectra of hydrogen on platinum black and of ac arbon-supported platinum fuel cell catalyst and an assignments cheme that rationalises all previousd ata is proposed.
Platinum-based catalysts are widely used throughout industry. [1] Major applicationsi nclude:t he reduction of nitroarenes to aromatic aminoarenesf or use in polyurethane manufacture, [2] as ac omponent in the three-way automotive catalyst, [3] as the anode in polymer electrolyte membrane fuel cells (PEMFC) [4] ,a nd in chemotherapy. [5] Many of the uses of platinum arise from the facile [6] dissociation of dihydrogen at the catalysts urface. Thus, it is not surprising that the nature of adsorbed hydrogen on platinum surfaces has been studied for decades. [7] Vibrational spectroscopy has played ak ey role in these studies and many forms of vibrational spectroscopy have been used to investigate adsorbed hydrogeno ns ingle-crystal surfaces[ in ultra-high vacuum( UHV) [8] and on electrodes [9] ], on high-surface-area platinum metal [10] and on supported catalysts. [11] However,t here is still no clear consensus as to the assignment of the spectra.
Ar ecent computational study has shown [12] that an initially ideal Pt 44 octahedron,w ith only {111}f acets, undergoes considerable reconstruction as hydrogen is added to produce a Pt 44 H 80 C 2h tetradecahedron of fcc packing, with 8{111}f acets, 6{100} facets and 18 apex Pt atoms (see Figure 1). This structure has 18 on-top,4 4twofold, 18 threefold, 0f ourfold coordinatedh ydrogen and no subsurface hydrogen. The last observationi sc onsistent with the extremely small solubility of hydrogen in platinum, [13] in marked in contrast to the high hydrogen storage capability of palladium, [14] irrespectiveo fw hether it is am etal black or supported nanoparticles.
In the present work, we have calculated the vibrationald ensity of states (VDOS) of hydrogen on this reconstructed1 .1 nm platinum nanoparticle by both lattice dynamics (which uses the harmonic approximation) and by ab initio molecular dynamics (AIMD,w hich includesa nharmonicity).   [12] (Pt = dark blue, on-toph ydrogen = white,t wofold hydrogen = red, threefold hydrogen = yellow).
[a] Dr.S.F . Parker no selection rules in INS spectroscopy;however,t here is ab ias, such that modes that involve displacement of 1 Ha re those observed. [15] (This is explained in more detail in the Supporting Information). INS spectroscopy is findingi ncreasing use in studies of catalysts. [16] It is apparent that the lattice dynamics and AIMD calculations produce similarr esults. (The difference in the mode intensities between the lattice dynamics and the AIMD calculation is explained in the Supporting Information). Decomposing the spectra into the separate contributions from on-top,t wofold and threefold sites, Figure 3, shows why this is the case: both methods predict very similar transition energies. It has been suggested [8e,f] that hydrogen in the threefold site experiences as ignificantly anharmonic potential, the similarity of the lattice dynamics (a harmonic calculation) and the AIMD (which includes anharmonicity)s uggests this is not the case.
We note that the calculation was for a1 .1 nm particle, whereas af uel cell catalyst will have platinum particles in the of approximately 3 AE 1nm, because it has been shown [17] that this is the optimum size for both the hydrogen oxidation reaction and the oxygen reduction reaction. The platinum black sample shown in Figure 2c onsists of primary crystallites of varying size in the 3t o1 0nmr ange, which are grown together to form strongly boundl arge polydisperse aggregatesa nd loosely co-adherent agglomerates. [18] However,t he excellent agreement between the model system and the experimental data demonstrates that in both cases the spectra are dominated by the twofold bridge sites. Previous work [11e] has shown that the main peak at approximately 500 cm À1 narrows as the average particle size increases from 3t o5nm, but the overall profilei sr etained.
The relative contributiono ft he on-top hydrogen will vary depending on the conditions, in particulari ti so nly present with an overpressureo fh ydrogen. The PtÀHs tretchm ode has been observed by infrared [11a,b] and INS spectroscopies [6] on supported metal catalysts. Figure4 shows infrared spectra of the on-top PtÀHs pecies on av ariety of supports:aPt (1 %)/ Al 2 O 3 hydrogenation catalyst, aP t(10 %)/C fuel cell catalyst and the standard catalystE uroPt1 [Pt (6 %)/SiO 2 ]. [19] The associated bending mode has not been detected previously,b ecause it is forbidden in the infrared spectrum by the metal surface selection rule. This is irrelevant for INS spectroscopy and Figure 5 shows the INS spectrum at the temperature of 15 K( at which    [18] The latter,(c), is ad ifference spectrum generated by:{catalyst+ +H 2 }À{catalyst+ +D 2 }. H 2 is asolid) of aPt(58 %)/C catalyst with H 2 present, Figure 5a, and the same sample after briefly pumping at 77 K, which removes the H 2 (and hence the on-top hydrogen) but leaves the high coordinations ites untouched, Figure 5b.T he features associated with solid H 2 disappear and am ode at 480 cm À1 is attenuated, which is assigned to the PtÀHb ending mode.
We note that the absence of anyf ourfold coordination of hydrogen is consistent with single-crystal studies of Pt(100) [8g] accordingt ow hich on the unreconstructed surface only twofold bridging sites are proposed.T his probablyb ecause the PtÀPt diagonal distance in the fourfold site is 3.924 ,w hich is too long for the hydrogen to span. For the edge site the PtÀPt distance is only 2.775 ,s ot he hydrogen bridgest he edges of the fourfolds ite. The twofold site has three modes: an out-ofplane bend, in-plane asymmetric, and symmetric Pt-H stretch. Inspection of the mode visualisations show that these occur in the ranges:5 50-650, 900-1120, 1385-1585 cm À1 ,a lthough there is extensive mixingw ith the on-top hydrogen bending mode and the asymmetrics tretch of the threefolds ite. (This is readily seen in Figure 3, in the 400-800 cm À1 region of which the same feature occurs in two or all three of the partial spectra, for example, modes at 430 and 550 cm À1 .) The only reported values for the modes of twofold coordinated hydrogen on platinum are at 1190-1230 cm À1[8g] and9 50 cm À1[11c] in reasonable agreement with our results.
Our result, that the INS spectrum can be assigned to am ixture of on-top, twofold and threefold sites, accountsf or all of the literature on the spectroscopy of adsorbed hydrogeno n nanoparticulate platinum. Figure S1 (Supporting Information) shows ac ompilation from the literaturea nd the spectra show ar emarkable degree of similarity,i rrespective of whether the platinum is present as high surface area metal [10a-c] or as as upported catalyst. [6, 10d, 11d,e] This strongly suggestst hat, in general, for adsorbed hydrogen on platinum nanoparticles, most of the hydrogen is in twofold sites. This hasn ot been recognised previously,t he spectra are generally assigned to mostly threefold hydrogen. The importance of the twofold sites is that these are proposed [20] to be the most active sites for the hydrogen oxidation reaction.

Experimental Section
Commercial high-purity platinum black (98.44 %; CAS No. 7440-06-4) was purchased from Umicore Precious Metals Chemistry.T he product specification is based on gravimetric analysis and inductively coupled plasma spectroscopy/optical emission spectral analysis (ICP-OES). The Brunauer-Emmett-Teller (BET) surface area in is ca. 25 m 2 g À1 .T he presence of traces of alkaline elements (Na, K) is noted in the specification. The Pt (58 %)/C catalyst was prepared by wet impregnation. High purity carbon black with an itrogen surface area of ca. 60 m 2 g À1 was used as the support. The preparation procedures for the INS measurements have been described previously. [6] Pt (1 %)/Al 2 O 3 and Pt (10 %)/C were purchased from Alfa Aesar.I NS spectra were recorded with the TOSCA spectrometer. [21] (ISIS, Chilton, UK) and IN1-Lagrange (ILL, Grenoble, France). [22] The IN1-Lagrange data is available at:h ttps://doi.ill.fr/10.5291/ILL-DATA.7-05-441. Diffuse reflectance infrared spectra were recorded by using aS pectra-Tech Collector with an environmental chamber fitted with KBr windows and either aD igilab FTS-60A or aB ruker Vertex70 FTIR spectrometer.T he computational studies are described in the Supporting Information.