Coating Platinum Nanoparticles with Methyl Radicals: Effects on Properties and Catalytic Implications

Abstract

It was recently reported that the reaction of methyl radicals with Pt(0) nanoparticles (NPs), prepared by the reduction of Pt(SO4)2 with NaBH4, is fast and yields as the major product stable (Pt(0)-NPs)-(CH3)n and as side products, in low yields, C2H6, C2H4, and some oligomers. We decided to study the effect of this coating on the properties of the Pt(0)-NPs. The results show that the coating can cover up to 75% of the surface Pt(0) atoms. The rate constant of the reaction, k((.)CH3+Pt(0)-NPs), decreases with the increase in the surface coverage, leading to competing reaction paths in the solution, which gradually become dominant, affecting the composition of the products. The methyl coating also affects the zeta potential, the UV spectra, and the electrocatalytic reduction of water in the presence of the NPs. Thus, the results suggest that binding alkyl radicals to Pt(0) surfaces might poison the NPs catalytic activity. When the Pt(0)-NPs are prepared by the reduction of a different precursor salt, PtCl6(2-), nearly no C2 H4 and oligomers are formed and the methyl coating covers a larger percentage of the surface Pt(0) atoms. The difference is attributed to the morphology of the Pt(0)-NPs: those prepared from Pt(SO4)2 are twinned nanocrystals, whereas those prepared from PtCl6(2-) consist mostly of single crystals. Thus, the results indicate that the side products, or most of them at least, are formed on the twinned Pt(0) nanocrystal edges created between (111) facets. In addition, the results show that Pt(0)-NPs react very differently compared with other noble metals, for example, Au(0) and Ag(0); this difference is attributed in part to the difference in the bond strength, (M(0)-NP)-CH3, and should be considered in heterogeneous catalytic processes involving alkyl radicals as intermediates.

Pd0‐ and Au0‐Nanoparticles Catalyze the Reduction of Perchlorate by ·C(CH3)2OH Radicals

“…The pH values of these suspensions were 8.9 ± 0.3 and 6.9 ± 0.3 for the Pd 0 ‐NPs and the Au 0 ‐NPs, respectively. Their final particle concentrations, calculated from these measurements, in a procedure identical to that described previously, were 1.25 × 10 –7 m for the Pd 0 ‐NPs and 1.73 × 10 –7 m for the Au 0 ‐NPs , …”

Effect of Hydrogen Pretreatment of Platinum Nanoparticles on their Catalytic Properties: Reactions with Alkyl Radicals – A Mechanistic Study

“…[15,16] Methyl radicals are proposed to be the surface reaction intermediates in the partial oxidation of methane to methanol and formaldehyde. [17] Recently, [18,19] it was shown that platinum nanoparticles (Pt 0 -NPs) suspended in aqueous solutions react with methyl radicals differently than with other noblemetal NPs such as gold and silver, [20,21] and the major reaction with Pt 0 -NPs is [Eq. (1)]:…”

“…Thes urface layer of methyl groups can be released as methane through reaction of (Pt 0 -NPs)-(CH 3 ) n with H 2 . [19] Among the various chemical properties of Pt and Pd nanostructures, their interaction with Hi se specially important, because Pd is regarded as am aterial for hydrogen storage [22] and because Pt and Pd NPs are widely used to catalyzer eactions involving elementary steps of hydrogenationand dehydrogenation. [23,24] Hydrogen adsorption on Pt 0 is known to occur at different sites [25][26][27] on the surface of the bulk metal and Pt 0 -NPs to be in equilibrium in water with H 3 O + , [28] to cause changes in the surface structures, [29,30] andt oa ffect the catalytic properties of the platinum catalysts [31] and the electrocatalysts.…”

“…This yield is actually observed in the blank samples that do not contain as cavenger (in this currents tudy,a queous solutionsi nthe absence of Pt 0 -NPs, see captionofF igure 1), as reported previously. [19,20] Pt 0 -NPs suspensions prepared by reduction of Pt(SO 4 ) 2 with NaBH 4 containing 0.010 m DMSO were g irradiated (at ad ose of 330 Gy) after severalt reatment procedures, samples A-D (see the Experimental Section).…”

“…Of special interesta re the results obtained for sample B. Short-term treatmentw ith H 2 betweeni rradiations is known to releases ome of the methyl groups bound to the surface through Equation (10): [19] Scheme1.Reduction of oxidized specieso nt he Pt-NP surface after dihydrogen treatment (note that H 3 O + that mightb ef ormed in these reactions does not affect significantly the pH owingtot he borate bufferformed duringt he synthesiso fthe NPs). If n > lÀm,t his reaction releases occupied sites for Reaction (1), and therefore, one expects G t (B) < G t (D) and G(C 2 H 6 ) B < G(C 2 H 6 ) D ,a so bserved.…”