No abstract

Thermoelectrics interconvert heat to electricity and are of great interest in waste heat recovery, solid-state cooling and so on. Here we assessed the potential of SnS2 and SnSe2 as thermoelectric materials at the temperature gradient from 300 to 800 K. Reflecting the crystal structure, the transport coefficients are highly anisotropic between a and c directions, in particular for the electrical conductivity. The preferred direction for both materials is the a direction in TE application. Most strikingly, when 800 K is reached, SnS2 can show a peak power factor (PF) of 15.50 μW cm(-1) K(-2) along the a direction, while a relatively low value (11.72 μW cm(-1) K(-2)) is obtained in the same direction of SnSe2. These values are comparable to those observed in thermoelectrics such as SnSe and SnS. At 300 K, the minimum lattice thermal conductivity (κmin) along the a direction is estimated to be about 0.67 and 0.55 W m(-1) K(-1) for SnS2 and SnSe2, respectively, even lower than the measured lattice thermal conductivity of Bi2Te3 (1.28 W m(-1) K(-1) at 300 K). The reasonable PF and κmin suggest that both SnS2 and SnSe2 are potential thermoelectric materials. Indeed, the estimated peak ZT can approach 0.88 for SnSe2 and a higher value of 0.96 for SnS2 along the a direction at a carrier concentration of 1.94 × 10(19) (SnSe2) vs. 2.87 × 10(19) cm(-3) (SnS2). The best ZT values in SnX2 (X = S, Se) are comparable to that in Bi2Te3 (0.8), a typical thermoelectric material. We hope that this theoretical investigation will provide useful information for further experimental and theoretical studies on optimizing the thermoelectric properties of SnX2 materials.

Oxidative coupling of toxic pollutant CO to form the platform raw chemical material dimethyl oxalate (DMO) has been industrialized however the catalytic mechanism has been unknown so far. The reaction mechanism of CO oxidative coupling to form DMO on a Pd(111) surface has been investigated using density functional theory (DFT) and in situ diffuse reflectance infrared (DRIR) spectroscopy. DFT calculations and in situ DRIRS measurements indicate that two co-adsorbed intermediates COOMe and OCCO, initiate the reaction. C-C coupling occurs earlier due to a low coupling barrier and small steric hindrance. The results also suggest that Pd(111) is selective towards DMO over DMC, and that CO pre-adsorption and CO in excess effectively enhance the yield of DMO. The microscopic elucidation of this important reaction suggests improvements in coal-to-EG (CTEG) production which can be applied in practice to effectively enhance the yield and reduce the cost. The results may help with further fine-tuning and designing of high-efficient noble metal catalysts.