釩鈦觸媒孔隙擴散限制及表面酸性之研究

Abstract

本研究分別對SCR除硝技術常用之釩鈦觸媒做一物理化學特性分析。利用效率因子法求得真實化學活性、有效反應常數及反應特徵時間等參數以分析觸媒在SCR反應時所受到的孔隙擴散限制。研究結果顯示在一般SCR操作溫度時，效率因子僅約百分之幾而已，反應受到孔隙內擴散限制的影響非常大。在相同的比表面積下，可藉由增加孔徑這個物理結構改善方法使有效擴散係數提高，進而使去除效率提昇。 化學部分的研究在於探討觸媒表面受到操作參數如水氣、V2O5含量及溫度的影響。研究以in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)來了解上述參數對觸媒表面的Bronsted及Lewis酸性基的影響。研究結果顯示，水氣會與NH3競爭吸附於只含Lewis酸性基的TiO2上，但是競爭吸附並不發生於釩鈦觸媒上，主要是水氣吸附於釩鈦觸媒上的Lewis酸性基後，轉化為Bronsted酸性基。但此一現象造成反應所需的活性基減少而使去除效率降低。因此，釩鈦觸媒上的酸性基會隨操作調件不同而改變。一般而言，Lewis酸性基主要存在於乾燥以及低V2O5含量的釩鈦觸媒上，且其受溫度的影響較小。相反地，Bronsted酸性基則是主要存在於含有水氣及高V2O5含量的釩鈦觸媒上，但是極易受到溫度升高而消失。

An evaluation of physicochemical characteristic of vanadia-titania catalyst for NO removal by selective catalytic reduction (SCR) process was conducted in this study. The intrinsic rate constant, effective rate constant and characteristic time were obtained using a mathematical model employing the effective factor method to describe physical and chemical characteristic of SCR reaction. The result showed that the physical enhancement was obvious by increasing pore radius that resulted in a higher effective diffusivity. One the contrast, decreasing pore radius that resulted in lower effective diffusivity and reduced the NO removal efficiency although it has higher surface area. The effects of operating parameters (moisture, vanadia content, working temperature) on the roles of Bronsted and Lewis acids acting in the SCR reaction were specified by the observed NO removals and the spectra of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The results indicated that H2O competed with NH3 molecules to be adsorbed on the pure titania catalyst that contained only Lewis acid sites. But competition did not occur over vanadia-based catalyst. Both Bronsted and Lewis acid sites appeared on vanadia-based catalyst in the presence of H2O. But a lower catalytic activity was observed and this may be due to less reaction sites are available as compared to that under dry conditions where only Lewis acid sites were found. It is concluded that surface acid sites are varied according to different operation conditions. Lewis acidity was dominant under the condition of low vanadia content and in the absence of H2O molecules. It is also less irrelevant to temperature. In contrast, Bronsted acidity was dominant under the condition of high vanadia content and with the presence of H2O molecules. It is also highly affected by temperature.