周期性调制分子束具有时间分辨的特点,可用来分辨表面反应的不同机制。前人曾用单分子束研究过CO在Pt(111),Pd(111)与Pt(多晶)上氧化问题,本文运用混合调制与非调制(直流)分子束研究CO在Pd(多晶)上氧化的动力学过程。实验所用仪器为分子束—表面散射装置(图1),它由分子束源1,束调制室2及散射超高真空室3等组成。到达样品表面的分子束最大强度为10`(-4)帕,超高室背景极限压强为10~(-8)帕,活性样品为多晶Pd箔(8mm×8mm×0.2mm),其纯度为99.99％,散射束用四极质谱计、微电流放大器及锁相放大器来同时检测讯号幅度与相位滞后。对调制分子束,由LH与ER两种不同机制的表面反应速率方程求得的产物相位滞后φ与表面温度T的关系是不同的。在低温(T<550K)下,φ_(LH)~1随温度增加而增加,而φ_(ER)~1随温度增加而减少;在高温(T>600K)下,φ_(LH)~h随温度增加而减少,而φ_(ER)~h则与温度无关。根据CO与O_2混合调制分子束的实验结果,产物CO_2的相位滞后φ与温度T的关系在低温区与高温区都是与LH机制相符合的(图2)。运用图2曲线1′的数据求得lnk与1/T的关系(图3)。可分别求得该表面反应过程的活化能E_(LM)=21.1kcal/mole(低温区)与E_(LH)=25.0kcal/mole(高温区)。将本文所用混合调制分子束(图2曲线1,1′)与前人所用单调制分子束的实验结果比较后发现:混合束的特性曲线(J～T,φ～T)的低温部分与调制氧束的特性曲线相当,而其高温部分与调制CO束的特性曲线相当,而中温部分则是混合束所特有的,它相当于覆盖度的过渡区,即CO_2的产生率由低温区的θ_(co)为速率限制因素到高温区的θ_O为速率限制因素的过渡区。实验测定了混合比P_(O_2)/P_(CO)的变化对特性曲线的影响。当P_(O_2)/P_(CO)比值增加(如图2由曲线1,1′→曲线2,2′),则CO_2产生率J的极大值移向低温区。当P_(O_2)/P_(CO)比值进一步增加至大于1(如图2曲线3.3′),则J极大值进一步移向低温区,使J～T关系仅有一个极大值,φ-T关系没有极值出现,即混合束的低温特性消失。在LH机制的基础上,对直流混合束的速率方程用与调制混合束在低温与高温区同样的条件可求出CO_2产生率R与表面温度T的关系,用计算机对直流分子束的实验数据进行拟合(图4),可得该表面反应过程的活化能E_(LH)=25.0kcal/mole。将调制与直流分子束数据相结合,该表面代应的活化能可表为E_(LH)=23.0±2.0kcal/mole。 Periodically modulated molecular beams are time-resolved and can be used to discern different mechanisms of surface reactions. Previous studies on the oxidation of CO on Pt (111), Pd (111) and Pt (polycrystalline) have been carried out using single-molecule molecular beam. In this paper, Oxidation kinetics. The instrument used in the experiment is the molecular beam-surface scattering device (Fig. 1), which consists of the molecular beam source 1, the beam modulation chamber 2 and the scattering ultra-high vacuum chamber 3. The maximum intensity of the molecular beam reaching the sample surface is 10 ((-4) Pa, the ultimate pressure in the ultrahigh chamber is 10 -8 Pa, and the active sample is a polycrystalline Pd foil (8 mm × 8 mm × 0.2 mm) For 99.99%, scattered beam with quadrupole mass spectrometer, micro-current amplifier and lock-in amplifier to simultaneously detect signal amplitude and phase lag. The relationship between the phase lag φ and the surface temperature T obtained by the surface reaction rate equation of the modulated molecular beam and the two different mechanisms of LH and ER is different. At low temperature (T <550K), φ LH increased with the increase of temperature, while φ ER decreased with the increase of temperature. With the increase of temperature (T> 600K) Temperature increases and decreases, and φ_ (ER) ~ h is temperature independent. According to the experimental results of CO and O 2 mixed molecular beam modulation, the relationship between the phase lag φ of CO 2 and the temperature T is consistent with the LH mechanism in both low and high temperature regions (Figure 2). The relationship between lnk and 1 / T is obtained using the data of curve 1 ’in FIG. 2 (FIG. 3). The activation energy of the surface reaction can be obtained separately E LM LM = 21.1 kcal / mole and E LH = 25.0 kcal / mole. Comparing the experiment results of the mixed modulated molecular beam (curve 1, 1 ’) used in this paper with the monomolecular molecular beam used in the previous study, we found that the low temperature part of the characteristic curve (J ~ T, φ ~ T) The characteristic curve of oxygen beam is equivalent, while the high temperature part is equivalent to the characteristic curve of modulated CO beam, while the middle temperature part is unique to mixed beam, which is equivalent to the transitional region of coverage, that is, the production rate of CO 2 changes from θ_ (co) is the rate limiting factor to the high temperature region θ_O as a rate-limiting transition zone. The influence of the mixture ratio of P_ (O_2) / P_ (CO) on the characteristic curve was experimentally determined. When the ratio of P_ (O_2) / P_ (CO) increases (as shown in Fig. 2 from curve 1, 1 ’to curve 2, 2’), the maximum value of CO 2 production rate J moves to the low temperature region. When the ratio of P_ (O_2) / P_ (CO) further increases to more than 1 (as shown in Fig. 2, curve 3.3 ’), the J maximum moves further to the low temperature region so that there is only one maximum between J and T, T relationship does not appear extreme, that is, the low temperature characteristics of the mixed beam disappears. On the basis of the LH mechanism, the relationship between the CO 2 generation rate R and the surface temperature T can be obtained for the DC hybrid beam rate equation using the same conditions as the modulated mixed beam at low temperature and high temperature range. The experimental data of the DC molecular beam (Figure 4), the activation energy of the surface reaction process can be obtained E_ (LH) = 25.0 kcal / mole. Combining the modulation with the DC molecular beam data, the activation energy corresponding to this surface can be expressed as E LH = 23.0 ± 2.0 kcal / mole.