Mechanism Studies On Methanol Steam Reforming Reaction On Different Types Of Pd-based Related Catalysts

Due to the over-exploitation of traditional non-renewable resources,the energy crisis and environmental pollution become more and more serious.It is urgent to find new energy alternatives.As a kind of green and clean secondary energy,hydrogen energy is the inevitable result of energy development and one of the options of carbon peak and carbon neutralization.It has been widely used in industrial fields,including synthetic ammonia,water production,metal smelting and oil refining.However,the storage and transportation problems limit its wide usage.Methanol steam reforming（MSR）reaction is one of the most mature and widely used hydrogen production methods with simple devices,high hydrogen purity and few by-products.Compared with the reforming technology of other hydrocarbon compounds,methanol has the highest hydrogen/carbon ratio,and can be reformed at low temperature due to the lack of strong C-C bond.MSR reaction is an endothermic reaction with a reaction enthalpy of 49.7 k J/mol,and the required barrier is high,it is essential to find an effective catalyst to improve the reaction performance.Common catalysts include Cu-based and VIII transition metal catalysts.Although Cu-based catalysts exhibit high catalytic activity and CO₂selectivity,they are prone to be sintered and deactivation at higher temperature.The stability of Group VIII metal catalysts is better than that of Cu-based catalysts,but their activity is lower,which is easy to produce CO and poison the catalyst.Because alloy catalysts such as Pd Zn alloy and Ni Cu alloy have better activity,selectivity and thermal stability than single metal catalysts,they can be used as a substitute for pure metal catalysts.In addition,supported metal catalysts can improve the metal dispersion and specific surface area,provide more active sites,and reduce the possibility of sintering of active components.In this paper,the mechanisms of MSR reaction of three types of catalysts（including alloy catalysts,single-atom catalysts and dual-atom catalysts）have been systematically studied by using density functional theory,and their activity and selectivity have been compared.Our works provide some theoretical support for the diversified development of catalysts.The main research contents and results are as follows:（1）The mechanisms of MSR reaction on Pd Cu（111）and Pt Cu（111）alloy surfaces were calculated.Firstly,the structures of all intermediate species involved in the MSR reaction were optimized and their most stable configurations were obtained.Then two common reaction paths are analyzed including directly generating CO₂through formate path（CH₃OH firstly decomposed into CH₂O,and CH₂O combines with the hydroxyl decomposed by H₂O to generate CH₂OOH,and then gradually dehydrogenate to generate CHOOH,and continue dehydrogenation to generate CO₂）and the first decomposition of methanol and then undergoing water-gas shift reaction path（CH₃OH is continuously decomposed to generate CO,then CO and the hydroxyl group decomposed by H₂O is converted into CO₂and H₂）.The reaction energies and activation energies of all elementary reactions in these two paths were calculated,and the reactivity and selectivity of the two catalysts were compared.The results show that the paths of MSR reaction on these two catalysts are the same,which is that methanol is decomposed into CO first,and then converted into CO₂.The decisive step is the combination of CO and OH,and the required activation energies are 1.116 e V and 1.022 e V,respectively.It can be seen that Pt Cu alloy is a catalyst with higher activity for MSR reaction.（2）The mechanisms of MSR reaction on Ni₁/Zn O（100）and Pd₁/Zn O（100）single-atom catalysts were calculated.Firstly,the structures of all intermediate species involved in the MSR reaction were optimized and their most stable configurations were obtained.Then the reaction energy and activation energy of all the elementary reactions in the two common paths were analyzed,and the reaction activity and selectivity of the two catalysts were compared.The optimization results show that the adsorption energies of all species are more stable on Pd₁/Zn O than on Ni₁/Zn O,except that the adsorption energies of the two initial reactants H₂O and CH₃OH are almost same.This may because the d band center of Pd is closer to the Fermi level than that of Ni,the adsorption intensity is higher.The mechanism results show that the best reaction path on Ni₁/Zn O is relatively clear,which is mainly to directly generate CO₂without passing through the intermediate species of CO.The decisive step is the combination of CH₂O and OH to generate CH₂OOH,and the required activation energy is 0.844 e V.On Pd₁/Zn O,the activation energies required in the path of direct CO₂generation and first decomposition then WGS reaction are 1.190 e V and1.152 e V,respectively.So both of these two paths are possible,and the latter is slightly advantageous by comparison.To sum up,we believe that Ni₁/Zn O is a very promising MSR catalyst with higher activity and CO₂selectivity.（3）The mechanisms of MSR reaction on Pt Zn/Ti O₂（110）and Pd Zn/Ti O₂（110）heteronuclear diatomic catalysts were calculated.Firstly,the structures of all intermediate species involved in the MSR reaction were optimized and their most stable configurations were obtained.Then the reaction energy and activation energy of all elementary reactions in the two paths were analyzed,and the reaction activity and selectivity of the two catalysts were compared.Finally,the catalytic effect of these two catalysts was compared with that of the homo-nuclear diatomic catalyst Pt₂/Ti O₂.The results show that the optimum reaction paths of MSR reaction on these two heteronuclear diatomic catalysts are the same,which is CH₃OH→CH₃O→CH₂O→CH₂OOH→CHOOH→CHOO→CO₂.The rate-determining steps are CH₂O+OH→CH₂OOH,and their activation energies are 1.175 e V（Pt Zn/Ti O₂）and 1.294 e V（Pd Zn/Ti O₂）,respectively.As compared to traditional alloy catalysts,our catalysts not only improve the reaction activity,but also greatly reduce the use of precious metals and the production cost,by reason of the synergistic effect and interfacial interaction between metal diatoms and the surface of Ti O₂surface.On the other hand,compared with the homo-nuclear diatomic catalyst Pt₂/Ti O₂,the introduction of Zn atom in Pt Zn/Ti O₂leads to a change in the direction of electron transfer,reducing the adsorption intensity of intermediate species,and further avoiding the risk of CO poisoned the catalyst.Our work provides a theoretical guidance for MSR reaction with hetero-nuclear diatomic catalysts.