[Show abstract][Hide abstract] ABSTRACT: The kinetics of catalytic oxidation of methane (1–3% in air) over a palladium oxide (PdO) surface was investigated by wire microcalorimetry at atmospheric pressure and over the temperature range from 560 to 800 K. Wire surface structures and compositions were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, and atom force microscopy. It was found that a porous PdO layer with a constant thickness of 1–2 μm was formed on the Pd wire after it was heat treated in nitrogen followed by air at elevated temperatures. Under the condition of the experiment, the reaction was found to be in the pseudo-first-order regime with respect to the methane concentration. The apparent rate constant of methane oxidation on PdO was determined to be kapp(cm/s) = (3.2 ± 0.8) × 104e–(62.8±1.6)(kJ/mol)/RT for 600 < T < 740 K. Experimental data were analyzed using a gas–surface reaction model proposed previously. Analysis shows that the overall catalytic oxidation rate is governed by equilibrium adsorption/desorption of molecular oxygen, which determines the density of surface palladium sites and dissociative adsorption of methane on these sites. The equilibrium constant of O2 adsorption and desorption was estimated from literature values of desorption energy and molecular parameters of adsorbed oxygen atoms. The rate coefficient of methane dissociative adsorption was estimated to be k16(cm/s) = (7.7 ± 1.6) × 104e–(59.9±1.2)(kJ/mol)/RT, derived from the equilibrium constant of oxygen adsorption over the same temperature range.
The Journal of Physical Chemistry C 09/2013; 117(38):19499-19507. DOI:10.1021/jp4058302 · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: For the first time, it was found that dry-grinding the mixture containing technical grade PdO and multi-walled carbon nanotubes (MWCNTs) can generate a catalyst with ultrahigh electrocatalytic activity for ethanol oxidation reaction (EOR). The as-prepared catalysts were denoted as PdO/MWCNTs. For a comparison, the graphene and graphite supported PdO samples were also prepared using the same process, leading to the formation of PdO/graphene and PdO/graphite catalysts respectively. The structural details, the morphologies as well as the particle sizes of the prepared catalysts are mainly characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Although no novel diffraction peaks were observed in the XRD patterns of the resulting samples, the morphologies of the samples after the dry-grinding process have changed greatly as compared to the starting materials of PdO. The results also indicate that the PdO/MWCNTs catalysts show the smallest particle sizes among all the prepared catalysts. The catalytic activities of the prepared PdO/MWCNTs catalysts towards EOR are examined by electrochemical measurements, and the results obtained from cyclic voltammery (CV) test demonstrated that the PdO/MWCNTs catalyst delivers a forward peak current density for EOR of 5029 mA mg−1 at a scan rate of 50 mV s−1, which is about 2.1 times higher than the reported value (2361 mA mg−1) obtained on the (Pd/C) catalyst. After detailed analysis, it is thought that the easier hydrogen evolution process on the PdO/MWCNTs catalyst is regarded as the main reason for its excellent electrochemical performance as compared to other catalysts, i.e., PdO/graphene and PdO/graphite. Most interestingly, the as-prepared catalyst has electrocatalytic activity for both methanol oxidation reaction and formic acid oxidation, which were also explored approximately in this preliminary work.
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