Two-Channel Decomposition of Methanol on Pt Nanoclusters Supported on a Thin Film of Al₂O₃/NiAl(100)

Abstract

The decomposition of methanol on Pt nanoclusters grown from vapor deposition onto an ordered Al 2 O 3 / NiAl(100) thin film was investigated under ultrahigh vacuum conditions with various surface probe techniques. The Pt clusters had mean diameter near 2.3 nm and height 0.4 nm before their coalescence; consisting of phase fcc, the clusters grew with their facets either ( 111) or ( 001) parallel to the θ-Al 2 O 3 (100) surface, depending on the temperature of growth. More than half the adsorbed monolayer of methanol on the Pt clusters decomposed via two channels: dehydrogenation to CO and C−O bond scission. The dehydrogenation was dominant and induced first at low-coordinated Pt sites, at 150 K on Pt(001) clusters and 200 K on Pt(111) clusters, whereas both lowcoordinated and some terrace Pt sites exhibited reactivity, despite the cluster size. On average, one CO was produced per surface Pt site, for a monolayer of methanol on either Pt(111) or Pt(001) clusters. In the other reaction, scission of the C−O bond occurred primarily in methanol itself and began about 250 K; the intermediate methyl preferentially formed methane on combining with atomic H from dehydrogenated methanol. No preferential reaction site for the C−O bond scission is indicated, but this process showed a remarkable dependence on the size and lattice parameter of the clusters: the probability of C−O bond scission decreased when the size increased and the lattice parameter decreased.

The decomposition of methanol on Au–Pt bimetallic clusters supported by a thin film of Al2O3/NiAl(100)

“…The results show that the alloying of Pt with Au altered little the dehydrogenation of methanol on Pt. The dehydrogenation occurred primarily on Pt sites of the bimetallic clusters; as on Pt clusters, 17 the process was initiated about 150 K and completed about 350 K. The average quantities of CO and hydrogen produced per surface Pt on the bimetallic clusters were similar to those on Pt clusters. 17 These properties were retained for varied surface composition and structures of the bimetallic clusters.…”

“…The dehydrogenation occurred primarily on Pt sites of the bimetallic clusters; as on Pt clusters, 17 the process was initiated about 150 K and completed about 350 K. The average quantities of CO and hydrogen produced per surface Pt on the bimetallic clusters were similar to those on Pt clusters. 17 These properties were retained for varied surface composition and structures of the bimetallic clusters. The electronic structures of Pt modied on alloying and the Au-Pt intermixed surface sites affected little the dehydrogenation on Pt.…”

“…We rst compared the TPD spectra of CD 3 OD, CD 2 O, CO, D 2 and CD 4 from methanol-d 4 (CD 3 OD) on 1.0 ML Pt clusters with those from Au-Pt bimetallic clusters formed by deposition of 1.0 ML Au onto 1.0 ML Pt clusters. The desorption of CD 3 OD (top of Fig 1a and b) shows two distinct maxima, centered about 130 and 200 K. As the former feature never saturated and was observed on varied surfaces, 17,[30][31][32][33] it is assigned primarily to desorption of multilayer methanol-d 4 ; the latter feature is ascribed to desorption of monolayer methanol-d 4 from Pt sites and the alumina lm, as methanol on both Pt and alumina surfaces desorbs about 200 K. 17,33 The feature, centered about 130 K, of the bimetallic clusters (Fig. 1b) involved desorption of methanol-d 4 also from Au sites, as methanol-d 4 on Au sites desorbed near 150 K. 33 The cracking pattern of the desorbing methanol-d 4 dominated the CD 2 O, CO and D 2 spectra in the same temperature regime (the second to fourth curves of Fig.…”

Investigation of Thermal Stability and Reactivity of Rh Nanoclusters on an Ultrathin Alumina Film

“…Figure 7a exemplifies the C 1s spectra with those from 5.0-L methanol adsorbed on annealed (450 K) 1.0-MLe Rh clusters (mean diameter about 1.8 nm) on the alumina at 125 K and annealed to selected temperatures. As most multilayer methanol already desorbed at 125 K [31,55,56], monolayer methanol became predominant on the surface. The line initially about binding energy (BE) 287.4 eV is assigned to C 1s of methanol adsorbed on both the Rh clusters and alumina film; the feature about 285.0 eV is contributed by the C 1s signals of contaminative elemental carbon, according to earlier measurements for CO dissociation.…”

Adsorption of an Au atom and dimer on a thin θ-Al2O3/NiAl(100) film: dependence on the thickness of the θ-Al2O3 film

“…Al 2 O 3 is a popular support for catalysts; a thin Al 2 O 3 /NiAl(100) lm has been studied and used as a model system. 15,[32][33][34][35][36][37][38][39][40][41] We have performed simulations with alumina slabs of 1-5 atomic layers atop a NiAl(100) slab of 11 or 12 atomic layers. The alumina slabs have the structure of q-Al 2 O 3 and a surface termination of the (001) facet to match previous experimental results.…”