Can microwave assisted in situ reduction of supported Pt nanoparticles challenge the chemical method in controlling the dispersion profile-catalytic performance relationship?

Abstract

In this study, Pt (of 0.3, 0.6 and 0.9 wt % loadings) was supported on mesoporous silica surface via microwave-assisted solution (MAS) method or rotary chemical evaporation (RCE) method in the in situ reduction step. The as-synthesized Pt nanocatalysts were characterized through XRD, XRF, TGA/ DSC, TEM, N 2 -adsorption-desorption, H 2 -pulse titration and electrical conductivity techniques. The samples prepared by MAS method exhibited higher surface area and a better dispersion profile of Pt NPs, of average sizes not exceeding 10 nm with increasing the Pt loading. In contrast, although RCE method showed higher efficacy in decomposing the used precursor, uneven distribution of larger Pt nanoparticles (≥ 33 nm) was displayed. Electrical properties in terms of AC conductivity and dielectric constant confirmed the enhancement of even distribution of smaller Pt NPs with higher concentration of grain boundaries affected by microwave electromagnetic radiations. Highly mobile electrons and lattice vibrations (phonons) were favored, as compared with aggregated NPs produced during RCE method. The TOF values calculated for reactions to selectively produce ethylene (from ethanol) or benzene (from cyclohexane) decreased with Pt loadings on catalyst samples synthesized by MAS method. The highly dispersed NPs (of 3-7 nm) seemed to be responsible of the activity in both reactions, tending probably to be structure insensitive. However, samples reduced by RCE method, with enlarged average sizes of surface Pt NPs (approaching 15.5 nm), exhibited increasing TOF values with Pt loading, i.e., turning the reactions most probably to be structure sensitive.

Supported nanocatalysts: recent developments in microwave synthesis for application in heterogeneous catalysis

“…209,210 The conversion was by the dispersion behavior of PtNPs. 121 The selectivity toward ethylene (dehydration) at 450 °C for 0.6 and 0.9 wt% Pt loaded catalysts prepared by MW method was higher (50-53 %) compared to those (47 %) for samples prepared by RCE method.…”

“…The above sections revealed that a variety of supported nanocatalysts has been prepared by MW assisted methods. The supported nano catlaysts included simple metal catalysts such as Cu, [81][82][83]105,132 Ni, 105,118,136 Co, 105,119,120 Fe, 84 Ru, 85,86,[95][96][97][98][99]141 Ag, 112,116 Au, 112,129,131,142 Pt, [87][88][89][90][91][92]106,111,121,142,145,146 Ti, 122 Zr, 123 Pd, 93,[106][107][108]127,128,130,133,134,…”

“…In this regard, anchoring or mobilization of nanocatalyst to inert or catalytically active support could help to improve the catalyst life time through inhibiting leaching, enhancing stability, less poisoning and maintaining the nano-size distribution. 81,87,88,[90][91][92]95,99,111,112,132,141,142,145,146,158,162,166,[171][172][173][175][176][177][178][179][180]182,183,186,190,198,199 MW assisted reduction in presence of reducing agents or under H 2 pressure, 98,[106][107][108][109]116,121,[127][128][129][130]133,134,161,…”

“…113,114 Alternatively, mechanochemical ball milling (BM) technique was used to prepare BM-2%Ag@AlSBA-15 and BM-2%Au@AlSBA-15 nanocatalysts developed by others. 115 The detailed characterization in 121 The increase in Pt loading to 0.9 wt% slightly increased the particle size to <10 nm. Comparatively, rotary chemical evaporation (RCE) method produced larger (≥ 20 nm)…”

The Catalytic Performance of Fe2O3-CeO2 Nanocomposite in Ethanol Conversion

“…The catalytic activity experiments for ethanol conversion were expressed in the terms of yield, total conversion and % selectivity calculated as follows (34) : Ethanol conversion (wt%) = Σ yield% of each component in the products Component yield (wt%) = quantity of the component/total quantity of the produced components. Fractional selectivity of each component = yield (wt%)/ total conversion x 100 Figure 1 illustrates the XRD patterns of the as prepared (35) .…”