Preparation Of Ni/CeO₂ And Ni/N-CeO₂ And Photothermal Catalytic CO₂ Reduction

Carbon dioxide（CO₂）reduction into other value-added chemical materials,such as carbon monoxide（CO）and methane（CH₄）etc.can effectively solve the energy and environmental issues caused by CO₂ emissions.However,in the current CO₂ reduction research,the CO₂ conversion rate and the products selectivity are usually low.Therefore,searching for CO₂ conversion catalysts with high activity and high selectivity is one of the hot research topics.In this work,two photothermal catalysts（Ni/Ce O₂ and Ni/N-Ce O₂）were synthesized,and their CO₂ reduction performances and related mechanisms were comparatively investigated.The main research results are as follows.Firstly,a series of Ni_x/Ce O₂（x=1,5,10,15,20 wt%）catalysts with different Ni loading amounts of nickel nanoparticles were synthesized through a simple two-step method,and their photothermal CO₂ reduction performances were evaluated.All Ni_x/Ce O₂ catalysts showed about 30%CO selectivity and 70%CH₄ selectivity,and the optimal Ni loading amount was 10 wt%.Upon a 300 W Xenon lamp irradiation,the Ni₁₀/Ce O₂ obtained a photothermal temperature of 340℃,realizing the maximum yields of CO(8.1 mmol·g_cat^-1·h^-1)and CH₄(20.5 mmol·g_cat^-1·h^-1).Secondly,Ni₁₀/N_y-Ce O₂（y=N/Ce（mol）=0.25,1.0,3.0,5.0,6.0,7.0;y is the molar ratio of the amount of N and Ce precursor added）catalysts were synthesized via a three-step method.Evaluation of the photothermal CO₂ reduction performances found that all the Ni₁₀/N_y-Ce O₂catalysts showed almost 100%of CO selectivity and the best performance was obtained over Ni₁₀/N_5.0-Ce O₂.With a photothermal temperature of 350℃ upon a 300W Xenon lamp irradiation,the Ni₁₀/N_5.0-Ce O₂ realized the maximum CO yield of 20.9mmol·g_cat^-1·h^-1.Based on the above results,CO₂ temperature program desorption（CO₂-TPD）,in situ Fourier transform infrared（FT-IR）spectroscopy,and density functional theory（DFT）were employed to comparatively clarify the reaction mechanisms of photothermal CO₂ reduction over Ni/Ce O₂ and Ni/N-Ce O₂.The results showed that Ni/Ce O₂ preferred form CH₄ through CHO^*and CH₃O^*intermediates,while Ni/N-Ce O₂ preferred to form CO through HCOO^*intermediates.On the one hand,N-doping enhanced the CO₂ adsorption and weakened the hydrogenolysis capacity.On the other hand,the formation of N-H bonds due to N-doping decreased hydrogenation capacity by weakening the domination of activated hydrogen on the surface of Ni nanoparticles,thereby inhibited the methanation reaction.This study demonstrates that elemental doping could be a feasible method to effectively modulate the product selectivity of photthermal CO₂ reduction.