A method of calculating adsorption enthalpy distribution using ammonia temperature-programmed desorption spectrum under adsorption equilibrium conditions
A method of calculating adsorption enthalpy distribution using ammonia temperature-programmed desorption spectrum under adsorption equilibrium conditions
复制标题
吸附平衡条件下氨程序升温脱附谱计算吸附焓分布的方法
DOI：
10.1016/s0926-860x(97)00190-7
复制
发表时间：
1997
影响因子：
5.5
通讯作者：
K. Hashimoto
中科院分区：
文献类型：
--
作者：
T. Masuda;Yoshihiro Fujikata;S. Mukai;K. Hashimoto
利用完全吸附平衡条件下测定的固体催化剂上氨的程序升温解吸（TPD）谱，提出了一种计算氨吸附焓密度分布函数的方法ac-TPD法。TPD光谱是用含有氨的气流测量的，与从催化剂上解吸的氨相比，氨的浓度相对较大。在这些条件下，在实验过程中，催化剂颗粒内部和气相中的氨浓度几乎保持不变。这种情况意味着留在催化剂上的氨量处于平衡状态，并且取决于氨的温度和浓度。因此，可以假设氨在酸位上的吸附平衡可以用Langmuir-Hinshelwood型方程表示，该方程具有吸附焓和与酸位强度相对应的指前吸附平衡常数。总的TPD谱由不同酸强度的酸位谱之和表示。在此假设下，脱附温度与氨的吸附焓有关，并推导出TPD总谱与吸附焓密度分布函数之间的关系。该方法得到的分布函数可用于评价固体催化剂的酸性。利用所得分布计算了TPD实验中氨解吸所需的热流密度。结果表明，该数值与差示扫描量热仪直接测得的值吻合较好。用dc-TPD方法比较了吸附焓(Q)的密度分布与氨解吸活化能(E)的分布。通过比较得到了Q和E的关系，发现Q值比E值小约15 kJ mol−1。
A method, called the ac-TPD method, was presented for calculating the density distribution function of the adsorption enthalpy of ammonia by utilizing the temperature-programmed desorption (TPD) spectrum of ammonia from solid catalysts measured under complete adsorption equilibrium conditions. TPD spectra were measured using a gas stream containing ammonia, at a relatively large concentration compared to the amount of ammonia desorbed from the catalyst. Under these conditions, the concentrations of ammonia both inside the catalyst particles and in the gas phase are kept almost constant during the experiment. This situation meant that the amount of ammonia remaining on the catalyst was at an equilibrium state and dependent on the temperature and concentration of ammonia. Therefore, it could be assumed that the adsorption equilibrium of ammonia on acid sites can be expressed by a Langmuir-Hinshelwood type equation, which has an adsorption enthalpy and a pre-exponential adsorption equilibrium constant corresponding to the strength of an acid site. The overall TPD spectrum is represented by the sum of the spectra from acid sites with different acid strengths. Under these assumptions, the desorption temperature was related to the adsorption enthalpy of ammonia, and a relationship was derived between the overall TPD spectrum and the density distribution function of the adsorption enthalpy. The distribution function obtained by this method can be used for evaluating acidic properties of solid catalysts. The heat flux required for the desorption of ammonia from acid sites during the TPD experiment was calculated using the obtained distribution. The value was found to agree well with that directly measured by a differential scanning calorimetric apparatus. The density distribution of the adsorption enthalpy (Q) was compared to the distribution of activation energy for desorption of ammonia (E) by a method, called dc-TPD, which was reported previously by us. This comparison gave the relationship between Q and E, and it was found that the Q value was smaller than the E value by about 15 kJ mol−1.